"But with regard
to the material world, we can at least go so far as this-we can perceive that
events are brought about not by insulated interpositions of Divine power,
exerted in each particular case, but by the establishment of general
laws."
W. Whewell: Bridgewater Treatise.
"To conclude,
therefore, let noe man out of a weak conceit of sobriety, or an ill-applied
moderation, think or maintain, that a man can search too far or be too well
studied in the book of God's word, or in the book of God's works; divinity or
philosophy; but rather let men endeavour an endless progress or proficience in
both."
Bacon: Advancement of Learning
Down, Bromley, Kent,
October 1st, 1859.
Introduction
Chapter 1: VARIATION
UNDER DOMESTICATION
Causes of Variability -- Effects of Habit -- Correlation of Growth --
Inheritance -- Character of Domestic Varieties -- Difficulty of distinguishing
between Varieties and Species -- Origin of Domestic Varieties from one or more
Species -- Domestic pigeons, their Differences and Origin -- principle of
Selection anciently followed, its Effects -- Methodical and Unconscious
Selection -- Unknown Origin of our Domestic Productions -- Circumstances
favourable to Man's power of Selection
Chapter 2: VARIATION
UNDER NATURE
Variability -- Individual differences -- Doubtful species -- Wide ranging, much
diffused, and common species vary most -- Species of the larger genera in any
country vary more than the species of the smaller genera -- Many of the species
of the larger genera resemble varieties in being very closely, but unequally,
related to each other, and in having restricted ranges
Chapter 3: STRUGGLE FOR
EXISTENCE
Bears on natural selection -- The term used in a wide sense -- Geometrical powers
of increase -- Rapid increase of naturalised animals and plants -- Nature of
the checks to increase -- Competition universal -- Effects of climate --
Protection from the number of individuals -- Complex relations of all animals
and plants throughout nature -- Struggle for life most severe between
individuals and varieties of the same species; often severe between species of
the same genus -- The relation of organism to organism the most important of
all relations
Chapter 4: NATURAL
SELECTION
Natural Selection -- its power compared with man's selection -- its power on
characters of trifling importance -- its Power at all ages and on both sexes --
Sexual Selection -- On the generality of intercrosses between individuals of
the same species -- Circumstances favourable and unfavourable to Natural
Selection, namely, intercrossing, isolation, number of individuals -- Slow
action -- Extinction caused by Natural Selection -- Divergence of Character,
related to the diversity of inhabitants of any small area, and to
naturalisation -- Action of Natural Selection, through Divergence of Character
and Extinction, on the descendants from a common parent -- Explains the
Grouping of all organic beings
Chapter 5: LAWS OF
VARIATION
Effects of external conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlation of
growth -- Compensation and economy of growth -- False correlations -- Multiple,
rudimentary, and lowly organised structures variable -- Parts developed in an
unusual manner are highly variable: specific character more variable than
generic: secondary sexual characters variable -- Species of the same genus vary
in an analogous manner -- Reversions to long -- lost characters -- Summary
Chapter 6: DIFFICULTIES
ON THEORY
Difficulties on the theory of descent with modification -- Transitions --
Absence or rarity of transitional varieties -- Transitions in habits of life --
Diversified habits in the same species -- Species with habits widely different
from those of their allies -- Organs of extreme perfection -- Means of
transition -- Cases of difficulty -- Natura non facit saltum -- Organs of small
importance -- Organs not in all cases absolutely perfect -- The law of Unity of
Type and of the Conditions of Existence embraced by the theory of Natural
Selection
Chapter 7: INSTINCT
Instincts comparable with habits, but different in their origin -- Instincts
graduated -- Aphides and ants -- Instincts variable -- Domestic instincts,
their origin -- Natural instincts of the cuckoo, ostrich, and parasitic bees --
Slave -- making ants -- Hive -- bee, its cell -- making instinct --
Difficulties on the theory of the Natural Selection of instincts -- Neuter or
sterile insects -- Summary
Chapter 8: HYBRIDISM
Distinction between the sterility of first crosses and of hybrids -- Sterility
various in degree, not universal, affected by close interbreeding, removed by
domestication -- Laws governing the sterility of hybrids -- Sterility not a
special endowment, but incidental on other differences -- Causes of the
sterility of first crosses and of hybrids -- Parallelism between the effects of
changed conditions of life and crossing -- Fertility of varieties when crossed
and of their mongrel offspring not universal -- Hybrids and mongrels compared
independently of their fertility -- Summary
Chapter 9: ON THE
IMPERFECTION OF THE GEOLOGICAL RECORD
On the absence of intermediate varieties at the present day -- On the nature of
extinct intermediate varieties; on their number -- On the vast lapse of time,
as inferred from the rate of deposition and of denudation -- On the poorness of
our palaeontological collections -- On the intermittence of geological
formations -- On the absence of intermediate varieties in any one formation --
On their sudden appearance in the lowest known fossiliferous strata
Chapter 10: ON THE
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
On the slow and successive appearance of new species -- On their different
rates of change -- Species once lost do not reappear -- Groups of species
follow the same general rules in their appearance and disappearance as do
single species -- On Extinction -- On simultaneous changes in the forms of life
throughout the world -- On the affinities of extinct species to each other and
to living species -- On the state of development of ancient forms -- On the
succession of the same types within the same areas -- Summary of preceding and
present chapters
Chapter 11:
GEOGRAPHICAL DISTRIBUTION
Present distribution cannot be accounted for by differences in physical
conditions -- Importance of barriers -- Affinity of the productions of the same
continent -- Centres of creation -- Means of dispersal, by changes of climate
and of the level of the land, and by occasional means -- Dispersal during the
Glacial period co -- extensive with the world
Chapter 12:
GEOGRAPHICAL DISTRIBUTION cont'd
Distribution of fresh -- water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relations of the inhabitants of islands to those of the nearest mainland -- On
colonisation from the nearest source with subsequent modification -- Summary of
the last and present chapters
Chapter 13: MUTUAL
AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS
CLASSIFICATION, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups -- MORPHOLOGY,
between members of the same class, between parts of the same individual --
EMBRYOLOGY, laws of, explained by variations not supervening at an early age,
and being inherited at a corresponding age -- RUDIMENTARY ORGANS; their origin
explained -- Summary
Chapter 14:
RECAPITULATION AND CONCLUSION
Recapitulation of the difficulties on the theory of Natural Selection --
Recapitulation of the general and special circumstances in its favour -- Causes
of the general belief in the immutability of species -- How far the theory of
natural selection may be extended -- Effects of its adoption on the study of
Natural history -- Concluding remarks
I WILL here give a
brief sketch of the progress of opinion on the Origin of Species. Until
recently the great majority of naturalists believed that species were immutable
productions, and had been separately created. This view has been ably
maintained by many authors. Some few naturalists, on the other hand, have
believed that species undergo modification, and that the existing forms of life
are the descendants by true generation of pre-existing forms. passing over
allusions to the subject in the classical writers,Aristotle, in his 'Physicae
Auscultationes' (lib. 2, cap. 8, s. 2), after remarking that rain does not fall
in order to make the corn grow, any more than it falls to spoil the farmer's
corn when threshed out of doors, applies the same argument to organization: and
adds (as translated by Mr Clair Grece, who first pointed out the passage to
me), 'So what hinders the different parts [of the body] from having this merely
accidental relation in nature? as the teeth, for example, grow by necessity,
the front ones sharp, adapted for dividing, and the grinders flat, and
serviceable for masticating the food; since they were not made for the sake of
this, but it was the result of accident. And in like manner as to the other
parts in which there appears to exist an adaptation to an end. Wheresoever,
therefore, all things together(that is all the parts of one whole) happened
like as if they were made for the sake of something, these were preserved,
having been appropriately constituted by an internal spontaneity, and
whatsoever things were not thus constituted, perished, and still perish. We
here see the principle of natural selection shadowed forth, but how little
Aristotle fully comprehended the principle, is shown by his remarks on the
formation of the teeth. the first author who in modern times has treated it in
a scientific spirit was Buffon. But as his opinions fluctuated greatly at
different periods, and as he does not enter on the causes or means of the
transformation of species, I need not here enter on details.
Lamarck was the first
man whose conclusions on the subject excited much attention. This
justly-celebrated naturalist first published his views in 1801; he much
enlarged them in 1809 in his "philosophie Zoologique," and
subsequently, in 1815, in the Introduction to his "Hist. Nat. des Animaux
sans Vertébres.' In these works he upholds the doctrine that species, including
man, are descended from other species. He first did the eminent service of
arousing attention to the probability of all change in the organic, as well as
in the inorganic world, being the result of law, and not of miraculous
interposition. Lamarck seems to have been chiefly led to his conclusion on the
gradual change of species, by the difficulty of distinguishing species and varieties,
by the almost perfect gradation of forms in certain groups, and by the analogy
of domestic productions. With respect to the means of modification, he
attributed something to the direct action of the physical conditions of life,
something to the crossing of already existing forms, and much to use and
disuse, that is, to the effects of habit. To this latter agency he seemed to
attribute all the beautiful adaptations in nature; - such as the long neck of
the giraffe for browsing on the branches of trees. But he likewise believed in
a law of progressive development; and as all the forms of life thus tend to
progress, in order to account for the existence at the present day of simple
productions, he maintains that such forms are now spontaneously generated. I
have taken the date of the first publication of Lamarck from Isid. Geoffroy
Saint-Hilaire's ('Hist. Nat. Générale,' tom. ii. p. 405, 1859) excellent
history of opinion on this subject. In this work a full account is given of
Buffon's conclusions on the same subject. It is curious how largely my
grandfather, Dr Erasmus Darwin, anticipated the views and erroneous grounds of
opinion of Lamarck in his Zoonomia, (vol. i. pp. 500-510), published in 1794.
According to Isid. Geoffroy there is no doubt that Goethe was an extreme
partisan of similar views, as shown in the Introduction to a work written in
1794 and 1795, but not published till long afterwards: he has pointedly
remarked ('Goethe als Naturforscher,' von Dr Karl Medinge s. 34) that the
future question for naturalists will be how, for instance, cattle got their
horns, and not for what they are used. It is rather a singular instance of the
manner in which similar views arise at about the same time, that Goethe in
Germany, Dr Darwin in England, and Geoffroy Saint-Hilaire (as we shall
immediately see) in France; came to the same conclusion on the origin of
species, in the years 1794-5.
Geoffroy Saint-Hilaire,
as is stated in his 'Life,' written by his son, suspected, as early as 1795,
that what we call species are various degenerations of the same type. It was
not until 1828 that he published his conviction that the same forms have not
been perpetuated since the origin of all things. Geoffroy seems to have relied
chiefly on the conditions of life, or the 'monde ambiant' as the cause of
change. He was cautious in drawing conclusions, and did not believe that
existing species are now undergoing modification; and, as his son adds,
"C'est donc un problème à réserver entièrement à l'avenir, supposé meme
que l'avenir doive avoir prise sur lui.'
In 1813, Dr W. C. Wells
read before the Royal Society 'An Account of a White female, part of whose skin
resembled that of a Negro'; but his paper was not published until his famous
'Two Essays upon Dew and Single Vision' appeared in 1818. In this paper he
distinctly recognises the principle of natural selection, and this is the first
recognition which has been indicated; but he applies it only to the races of
man, and to certain characters alone. After remarking that negroes and
mulattoes enjoy an immunity from certain tropical diseases, he observes,
firstly, that all animals tend to vary in some degree, and, secondly, that
agriculturists improve their domesticated animals by selection; and then, he
adds, but what is done in this latter case ' by art, seems to be done with
equal efficacy, though more slowly, by nature, in the formation of varieties of
mankind, fitted for the country which they inhabit. Of the accidental varieties
of man, which would occur among the first few and scattered inhabitants of the
middle regions of Africa, some one would be better fitted than the others to
bear the diseases of the country. This race would consequently multiply, while
the others would decrease; not only from their inability to sustain the attacks
of disease, but from their incapacity of contending with their more vigorous
neighbours. The colour of this vigorous race I take for granted, from what has
been already said, would be dark. But the same disposition to form varieties
still existing, a darker and a darker race would in the course of time occur:
and as the darkest would be the best fitted for the climate, this would at
length become the most prevalent; if not the only race, in the particular
country in which it had originated.' He then extends these same views to the
white inhabitants of colder climates. I am indebted to Mr Rowley, of the United
States, for having called my attention, through Mr Brace, to the above passage
in Dr Wells' work.
The Hon. and Rev. W.
Herbert, afterwards Dean of Manchester, in the fourth volume of the
'Horticultural Transactions,, 1822, and in his work on the 'Amaryllidaceae'
(1837, pp. 19, 339), declares that "horticultural experiments have
established, beyond the possibility of refutation, that botanical species are
only a higher and more permanent class of varieties.' He extends the same view
to animals. The Dean believes that single species of each genus were created in
an originally highly plastic condition, and that these have produced, chiefly
by intercrossing, but likewise by variation, all our existing species.
In 1826 professor
Grant, in the concluding paragraph in his well-known paper ('Edinburgh
philosophical journal,, vol. xiv. p. 283) on the Spongilla, clearly declares
his belief that species are descended from other species, and that they become
improved in the course of modification. This same view was given in his 55th
Lecture, published in the 'Lancet' in 1834.
In 1831 Mr patrick
Matthew published his work on 'Naval Timber and Arboriculture,' in which he
gives precisely the same view on the origin of species as that (presently to be
alluded to) propounded by Mr Wallace and myself in the "Linnean journal,'
and as that enlarged in the present volume. Unfortunately the view was given by
Mr Matthew very briefly in scattered passages in an Appendix to a work on a
different subject, so that it remained unnoticed until Mr Matthew himself drew
attention to it in the 'Gardener's Chronicle,' on April 7th, 1860. The
differences of Mr Matthew's view from mine are not of much importance; he seems
to consider that the world was nearly depopulated at successive periods, and
then re-stocked; and he gives as an alternative, that new forms may be
generated ' without the presence of any mould or germ of former aggregates.' I
am not sure that I understand some passages; but it seems that he attributes
much influence to the direct action of the conditions of life. He clearly saw,
however, the full force of the principle of natural selection.
The celebrated geologist
and naturalist, Von Buch, in his excellent 'Description physique des Isles
Canaries' (1836, p. 147), clearly expresses his belief that varieties slowly
become changed into permanent species, which are no longer capable of
intercrossing.
Rafinesque, in his 'New
Flora of North America,' published in 1836, wrote (p. 6) as follows: -'All
species might have been varieties once, and many varieties are gradually
becoming species by assuming constant and peculiar characters'; but farther on
(p. 18) he adds, 'except the original types or ancestors of the genus.
In 1843-44 professor
Haldeman ('Boston journal of Nat. Hist. U. States, vol. iv. p. 468) has ably
given the arguments for and against the hypothesis of the development and
modification of species: he seems to lean towards the side of change.
The "Vestiges of
Creation' appeared in 1844. In the tenth and much improved edition (1853) the
anonymous author says (p. 155): -'The proposition determined on after much
consideration is, that the several series of animated beings, from the simplest
and oldest up to the highest and most recent, are, under the providence of God,
the results, first, of an impulse which has been imparted to the forms of life,
advancing them, in definite times, by generation, through grades of
organisation terminating in the highest dicotyledons- and vertebrata, these
grades being few in number, and generally marked by intervals of organic
character, which we find to be a practical difficulty in ascertaining
affinities; second, of another impulse connected with the vital forces,
tending, in the course of generations, to modify organic structures in
accordance with external circumstances, as food, the nature of the habitat, and
the meteoric agencies, these being the ''adaptations'' of the natural
theologian.' The author apparently believes that organisation progresses by
sudden leaps, but that the effects produced by the conditions of life are
gradual. He argues with much force on general grounds that species are not
immutable productions. But I cannot see how the two supposed "impulses'
account in a scientific sense for the numerous and beautiful co- adaptations
which we see throughout nature; I cannot see that we thus gain any insight how,
for instance, a woodpecker has become adapted to its peculiar habits of Life.
The work, from its powerful and brilliant style, though displaying in the
earlier editions little accurate knowledge and a great want of scientific
caution, immediately had a very wide circulation. In my opinion it has done
excellent service in this country in calling attention to the subject, in
removing prejudice, and in thus preparing the ground for the reception of
analogous views.
In 1846 the veteran
geologist N. J. d'Omalius d'Halloy published in an excellent though short paper
("Bulletins de l'Acad. Roy Bruxelles,' tom. xiii. p. 581) his opinion that
it is more probable that new species have been produced by descent with
modification than that they have been separately created: the author first
promulgated this opinion in 1831.
professor Owen, in 1849
('Nature of Limbs,' p. 86), wrote as follows:- "The archetypal idea was
manifested in the flesh under diverse such modifications, upon this planet,
long prior to the existence of those animal species that actually exemplify it.
To what natural laws or secondary causes the orderly succession and progression
of such organic phenomena may have been committed, we, as yet, are ignorant.'
In his Address to the British Association, in 1858, he speaks (p. li.) of
"the axiom of the continuous operation of creative power, or of the
ordained becoming of living things.' Farther on (p. xc.), after referring to
geographical distribution, he adds, 'These phenomena shake our confidence in
the conclusion that the Apteryx of New Zealand and the Red Grouse of England
were distinct creations in and for those islands respectively. Always, also, it
may be well to bear in mind that by the word '' creation'' the zoologist means
'" a process he knows not what.'' He amplifies this idea by adding that
when such cases as that of the Red Grouse are enumerated by the zoologists as
evidence of distinct creation of the bird in and for such islands, he chiefly
expresses that he knows not how the Red Grouse came to be there, and there
exclusively; signifying also, by this mode of expressing such ignorance, his
belief that both the bird and the islands owed their origin to a great first
Creative Cause.' If we interpret these sentences given in the same Address, one
by the other, it appears that this eminent philosopher felt in 1858 his
confidence shaken that the Apteryx and the Red Grouse first appeared in their
respective homes, 'he knew not how,' or by some process 'he knew not what.'
This Address was
delivered after the papers by Mr Wallace and myself on the Origin of Species,
presently to & referred to, had been read before the Linnean Society. When
the first edition of this work was published, I was so completely deceived, as
were many others, by such expressions as 'the continuous operation of creative
power,' that I included professor Owen with other palaeontologists as being
firmly convinced of the immutability of species; but it appears ('Anat. of
Vertebrates,' vol. iii. p. 796) that this was on my part a preposterous error.
In the last edition of this work I inferred, and the inference still seems to
me perfectly just, from a passage beginning with the words 'no doubt the
type-form,' &c. (Ibid. vol. i. p. xxxv.), that professor Owen admitted that
natural selection may have done something in the formation of a new species;
but this it appears (Ibid. vol. nl. p. 798) is inaccurate and without evidence.
I also gave some extracts from a correspondence between professor Owen and the
Editor of the 'London Review,' from which it appeared manifest to the Editor as
well as to myself, that professor Owen Claimed to have promulgated the theory
of natural selection before I had done so; and I expressed my surprise and
satisfaction at this announcement; but as far as it is possible to understand
certain recently published passages Ibid. vol. iii. p. 798) I have either
partially or wholly again fallen into error. It is consolatory to me that
others find professor Owen's controversial writings as difficult to understand
and to reconcile with each other, as I do. As far as the mere enunciation of
the principle of natural selection is concerned, it is quite immaterial whether
or not professor Owen preceded me, for both of us, as shown in this historical
sketch, were long ago preceded by Dr Wells and Mr Matthews.
N. Isidore Geoffroy
Saint-Hilaire, in his lectures delivered in 1850 (of which a Résumé appeared in
the 'Revue et Nag. de Zoolog.,' jan. 1851), briefly gives his reason for
believing that specific characters "sont fixés, pour chaque espèce, tant
qu'elle se perpétue au milieu des mèmes circonstances: ils se modifient, si les
circonstances ambiantes viennent à changer.' 'En résumé, l'observation des
animaux sauvages démontre déjà la variabilité limité des espèces. Les expériences
sur les animaux sauvages devenus domestiques, et sur les animaux domestiques
redevenus s auvages, la démontrent plus clairement encore. Ces memes ex' - ces
prouvent, de plus, que les différences produites peuvent peuvent etre de valenr
générique.' In his 'Hist. Nat. Générale' (tom. ii.p. 430, 1859) he amplifies
analogous conclusions.
From a circular lately
issued it appears that Dr Ereke, in 1851 ("Dublin Medical press,' p. 322),
propounded the doctrine that all organic beings have descended from one
primordial form. His grounds of belief and treatment of the subject are wholly
different from mine; but as Dr Freke has now (1861) published his Essay on the
'Origin of Species by means of Organic Affinity,' the difficult attempt to give
any idea of his views would be superfluous on my part.
Mr Herbert Spencer, in
an Essay (originally published in the ' Leader,' March, 1852, and republished
in his 'Essays,' in 1858), has contrasted the theories of the Creation and the
Development of organic beings with remarkable skill and force. He argues from
the analogy of domestic productions, from the changes which the embryos of many
species undergo, from the difficulty of distinguishing species and varieties,
and from the principle of general gradation, that species have been modified;
and he attributes the modification to the change of circumstances. The author
(1855) has also treated psychology on the principle of the necessary
acquirement of each mental power and capacity by gradation.
ln 1852 N. Naudin, a
distinguished botanist, expressly stated, in an admirable paper on the Origin
of Species ('Revue Horticole, p. 102; since partly republished in the
'Nouvelles Archives du Muséum,' tom. i. p. 171), his belief that species are
formed in an analogous manner as varieties are under cultivation; and the latter
process he attributes to man's power of selection. But he does not show how
selection acts under nature. He believes, like Dean Herbert, that species, when
nascent, were more plastic than at present. He lays weight on what he calls the
principle of firiality, 'puissance mystérieuse, indéterminée; fatalité pour les
uns; pour les autrese volonté providentielle, dont l'action incessante sur les ètres
vivants détermine, à toutes les époques de l'existence du monde, la forme, le
volume, et la durée de chacun d'eux, en raison de sa destinée dans l'ordre de
choses dont il fait partie. C'est cette puissance qui harmonise chaque membre à
l'ensemble, en l'appropriant à la fonction qu'il doit remplir dans l'organisme
général de la nature, fonction qui est pour lui sa raison d'ètre.' From
references in Bronn's 'Untersuchungen über die Entwickenlungs-Gesetze,' it
appears that the celebrated botanist and palaeontologist Unger published, in
1852, his belief that species undergo development and modification. Dalton, likewise,
in pander and Dalcon's work on Fossil Sloths, expressed, in 1821 a similar
belief. Similar views have, as is well known, been maintained by Oken in his
mystical 'Natur-philosophie.' From other references in Godron's work 'Sur l'Espéce,'
it seems that Bory St Vincent, Burdach, Poiret, and Fries, have all admitted
that new species are continually being produced. I may add, that of the
thirty-four authors named in this Historical Sketch, who believe in the
modification of species, or at least disbelieve in separate acts of creation,
twenty-seven have written on special branches of natural history or geology.
In 1853 a celebrated
geologist, Count Keyserling ('Bulletin de la Soc. Gêolog.,' 2nd Ser., tom. x.
p. 357), suggested that as new diseases, supposed to have been caused by some
miasma, have arisen and spread over the world, so at certain periods the germs
of existing species may have been chemically affected by circumambient
molecules of a particular nature, and thus have given rise to new forms.
In this same year,
1853, Dr Schaaffhausen published an excellent pamphlet ('Verhand. des
Naturhist. Vereins der preuss. Rheinlands,' &c.), in which he maintains the
development of organic forms on the earth. He infers that many species have
kept true for long periods, whereas a few have become modified. The distinction
of species he explains by the destruction of intermediate graduated forms.
'Thus living plants and animals are not separated from the extinct by new
creations, but are to be regarded as their descendants through continued
reproduction.'
A well-known French
botanist, N. Lecoq, writes in 1854 ('Etudes sur Géograph. Bot.,' tom. i. p.
250), 'On voit que nos recherches sur la fixité ou la variation de l'espèce,
nous conduisent directement aux idées émises, par deux hommes justement célèbres,
Geoffroy Saint-Hilaire et Goethe.' Some other passages scattered through N.
Lecoq's large work, make it a little doubtful how far he extends his views on
the modification of species.
The 'Philosophy of
Creation' has been treated in a masterly manner by the Rev. Baden Powell, in
his "Essays on the Unity of Worlds,' 1855. Nothing can be more striking
than the manner in which he shows that the introduction of new species is 'a
regular, not a casual phenomenon,' or, as Sir John Herschel expresses it, 'a
natural in contradistinction to a miraculous' process.
The third volume of the
'Journal of the Linnean Society' contains papers, read July 1st, 1858, by Mr
Wallace and myself, in which, as stated in the introductory remarks to this
volume, the theory of Natural Selection is promulgated by Mr Wallace with
admirable force and clearness.
Von Baer, towards whom
all zoologists feel so profound a respect, expressed about the year 1859 (see
prof. Rudolph Wagner, a "Zoologisch-Anthropologische Untersuchugen,' 1861,
s. 51) his conviction, chiefly grounded on the laws of geographical
distribution, that forms now perfectly distinct have descended from a single
parent-form.
In June, 1859,
professor Huxley gave a lecture before the Royal Institution on the 'persistent
Types of Animal Life.' Referring to such cases, he remarks, "It is
difficult to comprehend the meaning of such facts as these, if we suppose that
each species of animal and plant, or each great type of organisation, was
formed and placed upon the surface of the globe at long intervals by a distinct
act of creative power; and it is well to recollect that such an assumption is
as unsupported by tradition or revelation as it is opposed to the general
analogy of nature. If, on the other hand, we view 'Persistent Types' in
relation to that hypothesis which supposes the species living at any time to be
the result of the gradual modification of pre-existing species a hypothesis
which, though unproven, and sadly damaged by some of its supporters, is yet the
only one to which physiology lends any countenance; their existence would seem
to show that the amount of modification which living beings have undergone
during geological time is but very small in relation to the whole series of
changes which they have suffered.'
In December, 1859, Dr
Hooker published his 'Introduction to the Australian Flora.' In the first part
of this great work he admits the truth of the descent and modification of
species, and supports this doctrine by many original observations.
The first edition of
this work was published on November 24th, 1859, and the second edition on
January 7th, 1860.
WHEN on board H.M.S.
Beagle, as naturalist, I was much struck with certain facts in the distribution
of the inhabitants of South America, and in the geological relations of the
present to the past inhabitants of that continent. These facts seemed to me to
throw some light on the origin of species -- that mystery of mysteries, as it
has been called by one of our greatest philosophers. On my return home, it
occurred to me, in 1837, that something might perhaps be made out on this
question by patiently accumulating and reflecting on all sorts of facts which
could possibly have any bearing on it. After five years' work I allowed myself
to speculate on the subject, and drew up some short notes; these I enlarged in
1844 into a sketch of the conclusions, which then seemed to me probable: from
that period to the present day I have steadily pursued the same object. I hope
that I may be excused for entering on these personal details, as I give them to
show that I have not been hasty in coming to a decision.
My work is now nearly
finished; but as it will take me two or three more years to complete it, and as
my health is far from strong, I have been urged to publish this Abstract. I
have more especially been induced to do this, as Mr Wallace, who is now
studying the natural history of the Malay archipelago, has arrived at almost
exactly the same general conclusions that I have on the origin of species. Last
year he sent to me a memoir on this subject, with a request that I would
forward it to Sir Charles Lyell, who sent it to the Linnean Society, and it is
published in the third volume of the journal of that Society. Sir C. Lyell and
Dr Hooker, who both knew of my work - the latter having read my sketch of 1844
- honoured me by thinking it advisable to publish, with Mr Wallace's excellent
memoir, some brief extracts from my manuscripts.
This Abstract, which I
now publish, must necessarily be imperfect. I cannot here give references and
authorities for my several statements; and I must trust to the reader reposing
some confidence in my accuracy. No doubt errors will have crept in, though I
hope I have always been cautious in trusting to good authorities alone. I can
here give only the general conclusions at which I have arrived, with a few
facts in illustration, but which, I hope, in most cases will suffice. No one
can feel more sensible than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my conclusions have been
grounded; and I hope in a future work to do this. For I am well aware that
scarcely a single point is discussed in this volume on which facts cannot be
adduced, often apparently leading to conclusions directly opposite -- to those
at which I have arrived. A fair result can be obtained only by fully stating
and balancing the facts and arguments on both sides of each question and this
cannot possibly be here done.
I much regret that want
of space prevents my having the satisfaction of acknowledging the generous
assistance which I have received from very many naturalists, some of them
personally unknown to me. I cannot, however, let this opportunity pass without
expressing my deep obligations to Dr Hooker, who for the last fifteen years has
aided me in every possible way by his large stores of knowledge and his
excellent judgement.
In considering the Origin
of Species, it is quite conceivable that a naturalist, reflecting on the mutual
affinities of organic beings, on their embryological relations, their
geographical distribution, geological succession, and other such facts, might
come to the conclusion that each species had not been independently created,
but had descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it could be
shown how the innumerable species inhabiting this world have been modified so
as to acquire that perfection of structure and coadaptation which most justly
excites our admiration Naturalists continually refer to external conditions,
such as climate, food, &c., as the only possible cause of variation. In one
very limited sense, as we shall hereafter see, this may be true; but it is
preposterous to attribute to mere external conditions, the structure, for
instance, of the woodpecker, with its feet, tail, beak, and tongue, so
admirably adapted to catch insects under the bark of trees. In the case of the
misseltoe, which draws its nourishment from certain trees, which has seeds that
must be transported by certain birds, and which has flowers with separate sexes
absolutely requiring the agency of certain insects to bring pollen from one
flower to the other, it is equally preposterous to account for the structure of
this parasite, with its relations to several distinct organic beings, by the
effects of external conditions, or of habit, or of the volition of the plant
itself.
The author of the
'Vestiges of Creation' would, I presume, say that, after a certain unknown
number of generations, some bird had given birth to a woodpecker, and some
plant to the misseltoe, and that these had been produced perfect as we now see
them; but this assumption seems to me to be no explanation, for it leaves the
case of the coadaptations of organic beings to each other and to their physical
conditions of life, untouched and unexplained.
It is, therefore, of
the highest importance to gain a clear insight into the means of modification
and coadaptation. -At the commencement of my observations it seemed to me
probable that a careful study of domesticated animals and of cultivated plants
would offer the best chance of making out this obscure problem. or have I been
disappointed; in this and in all other perplexing cases I have invariably found
that our knowledge, imperfect though it be, of variation under domestication,
afforded the best and safest clue. I may venture to express my conviction of
the high value of such studies, although they have been very commonly neglected
by naturalists.
From these
considerations, I shall devote the first chapter of his Abstract to Variation
under Domestication. We shall thus see that a large amount of hereditary
modification is at least possible, and, what is equally or more important, we
shall see how great is the power of man in accumulating by his Selection
successive slight variations, I will then pass on to the variability of species
in a state of nature; but I shall, unfortunately, be compelled to treat this
subject far too briefly, as it can be treated properly only by giving long
catalogues of facts. We shall, however, be enabled to discuss what
circumstances are most favourable to variation. In the next chapter the
Struggle for Existence amongst all organic beings throughout the world, which
inevitably follows from their high geometrical powers of increase, will be
treated of. This is the doctrine of Malthus, applied to the whole animal and vegetable
kingdoms. As many more individuals of each species are born than can possibly
survive; and as, consequently, there is a frequently recurring struggle for
existence, it follows that any being, if it vary however slightly in any manner
profitable to itself, under the complex and sometimes varying conditions of
life, will have a better chance of surviving, and thus be naturally selected.
From the strong principle of inheritance, any selected variety will tend to
propagate its new and modified form.
This fundamental
subject of Natural Selection will be treated at some length in the fourth
chapter; and we shall then see how Natural Selection almost inevitably causes
much Extinction of the less improved forms of life and induces what I have
called Divergence of Character. In the next chapter I shall discuss the complex
and little Known laws of variation and of correlation of growth. In the four
succeeding chapters, the most apparent and gravest difficulties on the theory
will be given: namely, first, the difficulties of transitions, or understanding
how a simple being or a simple organ can be changed and perfected into a highly
developed being or elaborately constructed organ; secondly the subject of
Instinct, or the mental powers of animals, thirdly, Hybridism, or the
infertility of species and the fertility of varieties when intercrossed; and
fourthly, the imperfection of the Geological Record. In the next chapter I
shall consider the geological succession of organic beings throughout time; in
the eleventh and twelfth, their geographical distribution throughout space; in
the thirteenth, Their classification or mutual affinities, both when mature and
in an embryonic condition In the last chapter I shall give a brief
recapitulation of the whole work, and a few concluding remarks.)
No one ought to feel
surprise at much remaining as yet unexplained in regard to the origin of
species and varieties, if he makes due allowance for our profound ignorance in
regard to the mutual relations of all the beings which live around us. Who can
explain why one species ranges widely and is very numerous, and why another
allied species has a narrow range and is rare? Yet these relations are of the
highest importance, for they determine the present welfare, and, as I believe, the
future success and modification of every inhabitant of this world. Still less
do we know of the mutual relations of the innumerable inhabitants of the world
during the many past geological epochs in its history) Although much remains
obscure, and will long remain obscure, I can entertain no doubt, after the most
deliberate study and dispassionate judgement of which I am capable, that the
view which most naturalists entertain, and which I formerly entertained --
namely, that each species has been independently created -- is erroneous. I am
fully convinced that species are not immutable; but that those belonging to
what are called the same genera are lineal descendants of some other and
generally extinct species, in the same manner as the acknowledged varieties of
any one species are the descendants of that species. Furthermore, I am
convinced that Natural Selection has been the main but not exclusive means of
modification.
Causes of Variability- Effects of Habit - Correlation of Growth -
Inheritance - Character of Domestic Varieties - Difficulty of distinguishing
between Varieties and Species - Origin of Domestic Varieties from one or more
Species - Domestic pigeons, their Differences and Origin - principle of
Selection anciently followed, its Effects - Methodical and Unconscious
Selection - Unknown Origin of our Domestic Productions - Circumstances
favourable to Man's power of Selection WHEN
we look to the individuals of the same variety or sub-variety of our older
cultivated plants and animals, one of the first points which strikes us, is,
that they generally differ much more from each other, than do the individuals
of any one species or variety in a state of nature. When we reflect on the vast
diversity of the plants and animals which have been cultivated, and which have
varied during all ages under the most different climates and treatment, I think
we are driven to conclude that this greater variability is simply due to our
domestic productions having been raised under conditions of life not so uniform
as, and somewhat different from, those to which the parent-species have been
exposed under nature. There is, also, I think, some probability in the view
propounded by Andrew Knight, that this variability may be partly connected with
excess of food. It seems pretty clear that organic beings must be exposed
during several generations to the new conditions of life to cause any
appreciable amount of variation; and that when the organisation has once begun
to vary, it generally continues to vary for many generations. No case is on
record of a variable being ceasing to be variable under cultivation. Our oldest
cultivated plants, such as wheat, still often yield new varieties: our oldest
domesticated animals are still capable of rapid improvement or modification.
It has been disputed at
what period of time the causes of variability, whatever they may be, generally
act; whether during the early or late period of development of the embryo, or
at the instant of conception. Geoffroy St Hilaire's experiments show that
unnatural treatment of the embryo causes monstrosities; and monstrosities
cannot be separated by any clear line of distinction from mere variations. But
I am strongly inclined to suspect that the most frequent cause of variability
may be attributed to the male and female reproductive elements having been
affected prior to the act of conception. Several reasons make me believe in
this; but the chief one is the remarkable effect which confinement or
cultivation has on the functions of the reproductive system; this system
appearing to be far more susceptible than any other part of the organization,
to the action of any change in the conditions of life. Nothing is more easy
than to tame an animal, and few things more difficult than to get it to breed
freely under confinement, even in the many cases when the male and female
unite. How many animals there are which will not breed, though living long
under not very close confinement in their native country! This is generally
attributed to vitiated instincts; but how many cultivated plants display the
utmost vigour, and yet rarely or never seed! In some few such cases it has been
found out that very trifling changes, such as a little more or less water at
some particular period of growth, will determine whether or not the plant sets
a seed. I cannot here enter on the copious details which I have collected on
this curious subject; but to show how singular the laws are which determine the
reproduction of animals under confinement, I may just mention that carnivorous
animals, even from the tropics, breed in this country pretty freely under
confinement, with the exception of the plantigrades or bear family; whereas,
carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. Many
exotic plants have pollen utterly worthless, in the same exact condition as in
the most sterile hybrids. When, on the one hand, we see domesticated animals
and plants, though often weak and sickly, yet breeding quite freely under
confinement; and when, on the other hand, we see individuals, though- taken
young from a state of nature, perfectly tamed, long-lived, and healthy (of
which I could give numerous instances), yet having their reproductive system so
seriously affected by unperceived causes as to fail in acting, we need not be
surprised at this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.
Sterility has been said
to be the bane of horticulture; but on this view we owe variability to the same
cause which produces sterility; and variability is the source of all the
choicest productions of the garden. I may add, that as some organisms will
breed most freely under the most unnatural conditions (for instance, the rabbit
and ferret kept in hutches), showing that their reproductive system has not
been thus affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly -- perhaps hardly more than in a state of
nature.
A long list could
easily be given of 'sporting plants;' by this term gardeners mean a single bud
or offset, which suddenly assumes a new and sometimes very different character
from that of the rest of the plant. Such buds can be propagated by grafting, &c.,
and sometimes by seed. These 'sports' are extremely rare under nature, but far
from rare under cultivation; and in this case we see that the treatment of the
parent has affected a bud or offset, and not the ovules or pollen. But it is
the opinion of most physiologists that there is no essential difference between
a bud and an ovule in their earliest stages of formation; so that, in fact,
'sports' support my view, that variability may be largely attributed to the
ovules or pollen, or to both, having been affected by the treatment of the
parent prior to the act of conception. These cases anyhow show that variation
is not necessarily connected, as some authors have supposed, with the act of
generation.
Seedlings from the same
fruit, and the young of the same litter, sometimes differ considerably from
each other, though both the young and the parents, as Muller has remarked, have
apparently been exposed to exactly the same conditions of life; and this shows
how unimportant the direct effects of the conditions of life are in comparison
with the laws of reproduction, and of growth, and of inheritance; for had the
action of the conditions been direct, if any of the young had varied, all would
probably have varied in the same manner. To judge how much, in the case of any
variation, we should attribute to the direct action of heat, moisture, light,
food, &c., is most difficult: my impression is, that with animals such
agencies have produced very little direct effect, though apparently more in the
case of plants. Under this point of view, Mr Buckman's recent experiments on
plants seem extremely valuable. When all or nearly all the individuals exposed
to certain conditions are affected in the same way, the change at first appears
to be directly due to such conditions; but in some cases it can be shown that
quite opposite conditions produce similar changes of structure. Nevertheless
some slight amount of change may, I think, be attributed to the direct action
of the conditions of life - as, in some cases, increased size from amount of
food, colour from particular kinds of food and from light, and perhaps the
thickness of fur from climate.
Habit also has a
deciding influence, as in the period of flowering with plants when transported
from one climate to another. In animals it has a more marked effect; for
instance, I find in the domestic duck that the bones of the wing weigh less and
the bones of the leg more, in proportion to the whole skeleton, than do the
same bones in the wild- duck; and I presume that this change may be safely
attributed to the domestic duck flying much less, and walking more, than its
wild parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison with the
state of these organs in other countries, is another instance of the effect of
use. Not a single domestic animal can be named which has not in some country
drooping ears; and the view suggested by some authors, that the drooping is due
to the disuse of the muscles of the ear, from the animals not being much
alarmed by danger, seems probable.
There are many laws
regulating variation, some few of which can be dimly seen, and will be
hereafter briefly mentioned. I will here only allude to what may be called
correlation of growth. Any change in the embryo or larva will almost certainly
entail changes in the mature animal. In monstrosities, the correlations between
quite distinct parts are very curious; and many instances are given in Isidore
Geoffroy St Hilaire's great work on this subject. Breeders believe that long
limbs are almost always accompanied by an elongated head. Some instances of
correlation are quite whimsical; thus cats with blue eyes are invariably deaf;
colour and constitutional peculiarities go together, of which many remarkable
cases could be given amongst animals and plants. From the facts collected by
Heusinger, it appears that white sheep and pigs are differently affected from
coloured individuals by certain vegetable poisons. Hairless dogs have imperfect
teeth; long-haired and coarse-haired animals are apt to have, as is asserted,
long or many horns; pigeons with feathered feet have skin between their outer
toes; pigeons with short beaks have small feet, and those with long beaks large
feet. Hence, if man goes on selecting, and thus augmenting, any peculiarity, he
will almost certainly unconsciously modify other parts of the structure, owing
to the mysterious laws of the correlation of growth.
The result of the
various, quite unknown, or dimly seen laws of variation is infinitely complex
and diversified. It is well worth while carefully to study the several
treatises published on some of our old cultivated plants, as on the hyacinth,
potato, even the dahlia, &c.; and it is really surprising to note the endless
points in structure and constitution in which the varieties and sub varieties
differ slightly from each other. The whole organization seems to have become
plastic, and tends to depart in some small degree from that of the parental
type.
Any variation which is
not inherited is unimportant for us. But the number and diversity of
inheritable deviations of structure, both those of slight and those of
considerable physiological importance, is endless. Dr Prosper Lucas's treatise,
in two large volumes, is the fullest and the best on this subject. No breeder
doubts how strong is the tendency to inheritance: like produces like is his
fundamental belief: doubts have been thrown on this principle by theoretical
writers alone. When a deviation appears not unfrequently, and we see it in the
father and child, we cannot tell whether it may not be due to the same original
cause acting on both; but when amongst individuals, apparently exposed to the
same conditions, any very rare deviation, due to some extraordinary combination
of circumstances, appears in the parent -- say, once amongst several million
individuals -- and it reappears in the child, the mere doctrine of chances
almost compels us to attribute its reappearance to inheritance. Every one must
have heard of cases of albinism, prickly skin, hairy bodies, &c. appearing
in several members of the same family. If strange and rare deviations of
structure are truly inherited, less strange and commoner deviations may be
freely admitted to be inheritable. perhaps the correct way of viewing the whole
subject, would be, to look at the inheritance of every character whatever as
the rule, and non-inheritance as the anomaly.
The laws governing
inheritance are quite unknown; no one can say why the same peculiarity in
different individuals of the same species, and in individuals of different
species, is sometimes inherited and sometimes not so; why the child often
reverts in certain characters to its grandfather or grandmother or other much
more remote ancestor; why a peculiarity is often transmitted from one sex to
both sexes or to one sex alone, more commonly but not exclusively to the like
sex. It is a fact of some little importance to use that peculiarities appearing
in the males of our domestic breeds are often transmitted either exclusively,
or in a much greater degree, to males alone. A much more important rule, which
I think may be trusted, is that, at whatever period of life a peculiarity first
appears, it tends to appear in the offspring at a corresponding age, though sometimes
earlier. In many cases this could not be otherwise: thus the inherited
peculiarities in the horns of cattle could appear only in the offspring when
nearly mature; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and some
other facts make me believe that the rule has a wider extension, and that when
there is no apparent reason why a peculiarity should appear at any particular
age, yet that it does tend to appear in the offspring at the same period at
which it first appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of course
confined to the first appearance of the peculiarity, and not to its primary
cause, which may have acted on the ovules or male element; in nearly the same
manner as in the crossed offspring from a short-horned cow by a long-horned
bull, the greater length of horn, though appearing late in life, is clearly due
to the male element.
Having alluded to the
subject of reversion, I may here refer to a statement often made by naturalists
- namely, that our domestic varieties, when run wild, gradually but certainly
revert in character to their aboriginal stocks. Hence it has been argued that
no deductions can be drawn from domestic races to species in a state of nature.
I have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be great difficulty
in proving its truth: we may safely conclude that very many of the most
strongly-marked domestic varieties could not possibly live in a wild state. In
many cases we do not know what the aboriginal stock was, and so could not tell
whether or not nearly perfect reversion had ensued. It would be quite
necessary, in order to prevent the effects of intercrossing, that only a single
variety should be turned loose in its new home. Nevertheless, as our varieties
certainly do occasionally revert in some of their characters to ancestral forms,
it seems to me not improbable, that if we could succeed in naturalising, or
were to cultivated during many generations, the several races, for instance, of
the cabbage, in very poor soil (in which case, however, some effect would have
to be attributed to the direct action of the poor soil), that they would to a
large extent, or even wholly, revert to the wild aboriginal stock. Whether or
not the experiment would succeed, is not of great importance for our line of
argument; for by the experiment itself the conditions of life are changed. If
it could be shown that our domestic varieties manifested a strong tendency to
reversion, -- that is, to lose their acquired characters, whilst kept under
unchanged conditions, and whilst kept in a considerable body, so that free
intercrossing might check, by blending together, any slight deviations of
structure, in such case, I grant that we could deduce nothing from domestic
varieties in regard to species. But there is not a shadow of evidence in favour
of this view: to assert that we could not breed our cart and race-horses, long
and short-horned cattle and poultry of various breeds, and esculent vegetables,
for an almost infinite number of generations, would be opposed to all
experience. I may add, that when under nature the conditions of life do change,
variations and reversions of character probably do occur; but natural
selection, as will hereafter be explained, will determine how far the new
characters thus arising shall be preserved.
When we look to the
hereditary varieties or races of our domestic animals and plants, and compare
them with species closely allied together, we generally perceive in each
domestic race, as already remarked, less uniformity of character than in true
species. Domestic races of the same species, also, often have a somewhat
monstrous character; by which I mean, that, although differing from each other,
and from the other species of the same genus, in several trifling respects,
they often differ in an extreme degree in some one part, both when compared one
with another, and more especially when compared with all the species in nature
to which they are nearest allied. With these exceptions (and with that of the
perfect fertility of varieties when crossed, - a subject hereafter to be discussed),
domestic races of the same species differ from each other in the same manner
as, only in most cases in a lesser degree than, do closely-allied species of
the same genus in a state of nature. I think this must be admitted, when we
find that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by some competent judges as mere varieties,
and by other competent judges as the descendants of aboriginally distinct
species. If any marked distinction existed between domestic races and species,
this source of doubt could not so perpetually recur. It has often been stated
that domestic races do not differ from each other in characters of generic
value. I think it could be shown that this statement is hardly correct; but
naturalists differ most widely in determining what characters are of generic
value; all such valuations being at present empirical. Moreover, on the view of
the origin of genera which I shall presently give, we have no right to expect
often to meet with generic differences in our domesticated productions.
When we attempt to
estimate the amount of structural difference between the domestic races of the
same species, we are soon involved in doubt, from not knowing whether they have
descended from one or several parent-species. This point, if could be cleared
up, would be interesting; if, for instance, could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their
kind so truly, were the offspring of any single species, then such facts would
have great weight in making us doubt about the immutability of the many very
closely allied and natural species - for instance, of the many foxes -
inhabiting different quarters of the world. I do not believe, as we shall
presently see, that all our dogs have descended from any one wild species; but,
in the case of some other domestic races, there is presumptive, or even strong,
evidence in favour of this view.
It has often been
assumed that man has chosen for domestication animals and plants having an
extraordinary inherent tendency to vary, and likewise to withstand diverse
climates. I do not dispute that these capacities have added largely to the
value of most of our domesticated productions; but how could a savage possibly
know, when he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the little
variability of the ass or guinea-fowl, or the small power of endurance of
warmth by the reindeer, or of cold by the common camel, prevented their
domestication? I cannot doubt that if other animals and plants, equal in number
to our domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to breed for an
equal number of generations under domestication, they would vary on an average
as largely as the parent species of our existing domesticated productions have
varied.
In the case of most of
our anciently domesticated animals and plants, I do not think it is possible to
come to any definite conclusion, whether they have descended from one or
several species. The argument mainly relied on by those who believe in the
multiple origin of our domestic animals is, that we find in the most ancient
records, more especially on the monuments of Egypt, much diversity in the
breeds; and that some of the breeds closely resemble, perhaps are identical
with, those still existing. Even if this matter fact were found more strictly
and generally true than seems to me to be the case, what does it show, but that
some of our breeds originated there, four or five thousand ?!?years ago?!?? But
Mr Horner's researches have rendered it in some degree probable that man
sufficiently civilized to have manufactured pottery existed in the valley of
the Nile thirteen or fourteen thousand years ago; and who will pretend to say
how long before these ancient periods, savages, like those of Tierra del Fuego
or Australia, who possess a semi-domestic dog, may not have existed in Egypt?
The whole subject must,
I think, remain vague; nevertheless, I may, without here entering on any
details, state that, from geographical and other considerations, I think it
highly probable that our domestic dogs have descended from several wild
species. In regard to sheep and goats I can form no opinion. I should think,
from facts communicated to me by Mr Blyth, on the habits, voice, and
constitution, &c., of the humped Indian cattle, that these had descended
from a different aboriginal stock from our European cattle; and several
competent judges believe that these latter have had more than one wild parent.
With respect to horses, from reasons which I cannot give here, I am doubtfully
inclined to believe, in opposition to several authors, that all the races have
descended from one wild stock. Mr Blyth, whose opinion, from his large and
varied stores of knowledge, I should value more than that of almost any one,
thinks that all the breeds of poultry have proceeded from the common wild
Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds of
which differ considerably from each other in structure, I do not doubt that
they all have descended from the common wild duck and rabbit.
The doctrine of the
origin of our several domestic races from several aboriginal stocks, has been
carried to an absurd extreme by some authors. They believe that every race
which breeds true, let the distinctive characters be ever so slight, has had
its wild prototype. At this rate there must have existed at least a score of
species of wild cattle, as many sheep, and several goats in Europe alone, and
several even within Great Britain. One author believes that there formerly
existed in Great Britain eleven wild species of sheep peculiar to it ! When we
bear in mind that Britain has now hardly one peculiar mammal, and France but
few distinct from those of Germany and conversely, and so with Hungary, Spain,
&c., but that each of these kingdoms possesses several peculiar breeds of
cattle, sheep, &c., we must admit that many domestic breeds have originated
in Europe; for whence could they have been derived, as these several countries
do not possess a number of peculiar species as distinct parent-stocks? So it is
in India. Even in the case of the domestic dogs of the whole world, which I
fully admit have probably descended from several wild species, I cannot doubt
that there has been an immense amount of inherited variation. Who can believe
that animals closely resembling the Italian greyhound, the bloodhound, the bull-dog,
or Blenheim spaniel, &c. -- so unlike all wild Canidae -- ever existed
freely in a state of nature? It has often been loosely said that all our races
of dogs have been produced by the crossing of a few aboriginal species; but by
crossing we can get only forms in some degree intermediate between their
parents; and if we account for our several domestic races by this process, we
must admit the former existence of the most extreme forms, as the Italian
greyhound, bloodhound, bull-dog, &c., in the wild state. Moreover, the
possibility of making distinct races by crossing has been greatly exaggerated.
There can be no doubt that a race may be modified by occasional crosses, if
aided by the careful selection of those individual mongrels, which present any
desired character; but that a race could be obtained nearly intermediate
between two extremely different races or species, I can hardly believe. Sir J.
Sebright expressly experimentised for this object, and failed. The offspring
from the first cross between two pure breeds is tolerably and sometimes (as I
have found with pigeons) extremely uniform, and everything seems simple enough;
but when these mongrels are crossed one with another for several generations,
hardly two of them will be alike, and then the extreme difficulty, or rather
utter hopelessness, of the task becomes apparent. Certainly, a breed
intermediate between two verydistinct breeds could not be got without extreme
care and long-continued selection; nor can I find a single case on record of a
permanent race having been thus formed.
On the Breeds of the
Domestic pigeon. Believing that it is always best to study some special group,
I have, after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured with skins
from several quarters of the world, more especially by the Hon. W. Elliot from
India, and by the Hon. C. Murray from Persia. Many treatises in different
languages have been published on pigeons, and some of them are very important,
as being of considerably antiquity. I have associated with several eminent
fanciers, and have been permitted to join two of the London pigeon Clubs. The
diversity of the breeds is something astonishing. Compare the English carrier and
the short-faced tumbler, and see the wonderful difference in their beaks,
entailing corresponding differences in their skulls. The carrier, more
especially the male bird, is also remarkable from the wonderful development of
the carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a wide
gape of mouth. The short-faced tumbler has a beak in outline almost like that
of a finch; and the common tumbler has the singular and strictly inherited
habit of flying at a great height in a compact flock, and tumbling in the air
head over heels. The runt is a bird of great size, with long, massive beak and
large feet; some of the sub-breeds of runts have very long necks, others very
long wings and tails, others singularly short tails. The barb is allied to the
carrier, but, instead of a very long beak, has a very short and very broad one.
The pouter has a much elongated body, wings, and legs; and its enormously
developed crop, which it glories in inflating, may well excite astonishment and
even laughter. The turbit has a very short and conical beak, with a line of
reversed feathers down the breast; and it has the habit of continually
expanding slightly the upper part of the oesophagus. The Jacobin has the
feathers so much reversed along the back of the neck that they form a hood, and
it has, proportionally to its size, much elongated wing and tail feathers. The
trumpeter and laugher, as their names express, utter a very different coo from
the other breeds. The fantail has thirty or even forty tail-feathers, instead
of twelve or fourteen, the normal number in all members of the great pigeon
family; and these feathers are kept expanded, and are carried so erect that in
good birds the head and tail touch; the oil-gland is quite aborted. Several
other less distinct breeds might have been specified.
In the skeletons of the
several breeds, the development of the bones of the face in length and breadth
and curvature differs enormously. The shape, as well as the breadth and length
of the ramus of the lower jaw, varies in a highly remarkable manner. The number
of the caudal and sacral vertebrae vary; as does the number of the ribs,
together with their relative breadth and the presence of processes. The size
and shape of the apertures in the sternum are highly variable; so is the degree
of divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in strict
correlation with the length of beak), the size of the crop and of the upper
part of the oesophagus; the development and abortion of the oil-gland; the
number of the primary wing and caudal feathers; the relative length of wing and
tail to each other and to the body; the relative length of leg and of the feet;
the number of scutellae on the toes, the development of skin between the toes,
are all points of structure which are variable. The period at which the perfect
plumage is acquired varies, as does the state of the down with which the
nestling birds are clothed when hatched. The shape and size of the eggs vary.
The manner of flight differs remarkably; as does in some breeds the voice and
disposition. Lastly, in certain breeds, the males and females have come to
differ to a slight degree from each other.
Altogether at least a
score of pigeons might be chosen, which if shown to an ornithologist, and he
were told that they were wild birds, would certainly, I think, be ranked by him
as well-defined species. Moreover, I do not believe that any ornithologist
would place the English carrier, the short-faced tumbler, the runt, the barb,
pouter, and fantail in the same genus; more especially as in each of these
breeds several truly-inherited sub-breeds, or species as he might have called
them, could be shown him.
Great as the
differences are between the breeds of pigeons, I am fully convinced that the
common opinion of naturalists is correct, namely, that all have descended from
the rock-pigeon (Columba livia), including under this term several geographical
races or sub-species, which differ from each other in the most trifling
respects. As several of the reasons which have led me to this belief are in
some degree applicable in other cases, I will here briefly give them. If the
several breeds are not varieties, and have not proceeded from the rock- pigeon,
they must have descended from at least seven or eight aboriginal stocks; for it
is impossible to make the present domestic breeds by the crossing of any lesser
number: how, for instance, could a pouter be produced by crossing two breeds
unless one of the parent-stocks possessed the characteristic enormous crop? The
supposed aboriginal stocks must all have been rock-pigeons, that is, not
breeding or willingly perching on trees. But besides C. livia, with its
geographical sub-species, only two or three other species of rock-pigeons are
known; and these have not any of the characters of the domestic breeds. Hence
the supposed aboriginal stocks must either still exist in the countries where
they were originally domesticated, and yet be unknown to ornithologists; and
this, considering their size, habits, and remarkable characters, seems very
improbable; or they must have become extinct in the wild state. But birds
breeding on precipices, and good fliers, are unlikely to be exterminated; and
the common rock-pigeon, which has the same habits with the domestic breeds, has
not been exterminated even on several of the smaller British islets, or on the
shores of the Mediterranean. Hence the supposed extermination of so many
species hawing similar habits with the rock-pigeon seems to me a very rash
assumption. Moreover, the several above-named domesticated breeds have been
transported to all parts of the world, and, therefore, some of them must have
been carried back again into their native country; but not one has ever become
wild or feral, though the dovecot-pigeon, which is the rock- pigeon in a very
slightly altered state, has become feral in several places. Again, all recent
experience shows that it is most difficult to get any wild animal to breed
freely under domestication; yet on the hypothesis of the multiple origin of our
pigeons, it must be assumed that at least seven or eight species were so
thoroughly domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.
An argument, as it
seems to me, of great weight, and applicable in several other cases, is, that
the above- specified breeds, though agreeing generally in constitution, habits,
voice, colouring, and in most parts of their structure, with the wild
rock-pigeon, yet are certainly highly abnormal in other parts of their
structure: we may look in vain throughout the whole great family of Columbidae
for a beak like that of the English carrier, or that of the short-faced
tumbler, or barb; for reversed feathers like those of the jacobin; for a crop
like that of the pouter; for tail-feathers like those of the fantail. Hence it
must be assumed not only that half-civilized man succeeded in thoroughly
domesticating several species, but that he intentionally or by chance picked
out extraordinarily abnormal species; and further, that these very species have
since all become extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree.
Some facts in regard to
the colouring of pigeons well deserve consideration. The rock-pigeon is of a
slaty-blue, and has a white rump (the Indian sub-species, C. intermedia of
Strickland, having it bluish); the tail has a terminal dark bar, with the bases
of the outer feathers externally edged with white; the wings have two black
bars: some semi- domestic breeds and some apparently truly wild breeds have,
besides the two black bars, the wings chequered with black. These several marks
do not occur together in any other species of the whole family. Now, in every
one of the domestic breeds, taking thoroughly well-bred birds, all the above
marks, even to the white edging of the outer tail- feathers, some times concur
perfectly developed. Moreover, when two birds belonging to two distinct breeds
are crossed, neither of which is blue or has any of the above-specified marks,
the mongrel offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly black
barbs, and they produced mottled brown and black birds; these I again crossed
together, and one grandchild of the pure white fantail and pure black barb was
of as beautiful a blue colour, with the white rump, double black wing-bar, and
barred and white-edged tail-feathers, as any wild rock- pigeon I We can
understand these facts, on the well-known principle of reversion to ancestral
characters, if all the domestic breeds have descended from the rock-pigeon. But
if we deny this, we must make one of the two following highly improbable
suppositions. Either, firstly, that all the several imagined aboriginal stocks
were coloured and marked like the rock- pigeon, although no other existing
species is thus coloured and marked, so that in each separate breed there might
be a tendency to revert to the very same colours and markings. Or, secondly,
that each breed, even the purest, has within a dozen or, at most, within a
score of generations, been crossed by the rock-pigeon: I say within a dozen or
twenty generations, for we know of no fact countenancing the belief that the
child ever reverts to some one ancestor, removed by a greater number of
generations. In a breed which has been crossed only once with some distinct
breed, the tendency to reversion to any character derived from such cross will
naturally become less and less, as in each succeeding generation there will be
less of the foreign blood; but when there has been no cross with a distinct
breed, and there is a tendency in both parents to revert to a character, which
has been lost during some former generation, this tendency, for all that we can
see to the contrary, may be transmitted undiminished for an indefinite number
of generations. These two distinct cases are often confounded in treatises on
inheritance.
Lastly, the hybrids or
mongrels from between all the domestic breeds of pigeons are perfectly fertile.
I can state this from my own observations, purposely made on the most distinct
breeds. Now, it is difficult, perhaps impossible, to bring forward one case of
the hybrid offspring of two animals clearly distinct being themselves perfectly
fertile. Some authors believe that long-continued domestication eliminates this
strong tendency to sterility: from the history of the dog I think there is some
probability in this hypothesis, if applied to species closely related together,
though it is unsupported by a single experiment. But to extend the hypothesis
so far as to suppose that species, aboriginally as distinct as carriers,
tumblers, pouters, and fantails now are, should yield offspring perfectly
fertile, inter se, seems to me rash in the extreme.
From these several
reasons, namely, the improbability of man having formerly got seven or eight
supposed species of pigeons to breed freely under domestication; these supposed
species being quite unknown in a wild state, and their becoming nowhere feral;
these species having very abnormal characters in certain respects, as compared with
all other Columbidae, though so like in most other respects to the rock-pigeon;
the blue colour and various marks occasionally appearing in all the breeds,
both when kept pure and when crossed; the mongrel offspring being perfectly
fertile; -- from these several reasons, taken together, I can feel no doubt
that all our domestic breeds have descended from the Columba livia with its
geographical sub-species.
In favour of this view,
I may add, firstly, that C. livia, or the rock-pigeon, has been found capable
of domestication in Europe and in India; and that it agrees in habits and in a
great number of points of structure with all the domestic breeds. Secondly,
although an English carrier or short-faced tumbler differs immensely in certain
characters from the rock-pigeon, yet by comparing the several sub-breeds of
these breeds, more especially those brought from distant countries, we can make
an almost perfect series between the extremes of structure. Thirdly, those
characters which are mainly distinctive of each breed, for instance the wattle
and length of beak of the carrier, the shortness of that of the tumbler, and
the number of tail-feathers in the fantail, are in each breed eminently
variable; and the explanation of this fact will be obvious when we come to
treat of selection. Fourthly, pigeons have been watched, and tended with the
utmost care, and loved by many people. They have been domesticated for
thousands of years in several quarters of the world; the earliest known record
of pigeons is in the fifth Egyptian dynasty, about 3000 B.C., as was pointed
out to me by professor Lepsius; but Mr Birch informs me that pigeons are given
in a bill of fare in the previous dynasty. In the time of the Romans, as we
hear from Pliny, immense prices were given for pigeons; 'nay, they are come to
this pass, that they can reckon up their pedigree and race.' pigeons were much
valued by Akber Khan in India, about the year l600; never less than 20,000
pigeons were taken with the court. 'The monarchs of Iran and Turan sent him
some very rare birds;' and, continues the courtly historian, 'His Majesty by
crossing the breeds, which method was never practised before, has improved them
astonishingly.' About this same period the Dutch were as eager about pigeons as
were the old Romans. The paramount importance of these considerations in
explaining the immense amount of variation which pigeons have undergone, will
be obvious when we treat of Selection. We shall then, also, see how it is that
the breeds so often have a somewhat monstrous character. It is also a most
favourable circumstance for the production of distinct breeds, that male and
female pigeons can be easily mated for life; and thus different breeds can be
kept together in the same aviary.
I have discussed the
probable origin of domestic pigeons at some, yet quite insufficient, length;
because when I first kept pigeons and watched the several kinds, knowing well
how true they bred, I felt fully as much difficulty in believing that they
could ever have descended from a common parent, as any naturalist could in
coming to a similar conclusion in regard to the many species of finches, or
other large groups of birds, in nature. One circumstance has struck me much;
namely, that all the breeders of the various domestic animals and the
cultivators of plants, with whom I have ever conversed, or whose treatises I
have read, are firmly convinced that the several breeds to which each has
attended, are descended from so many aboriginally distinct species. Ask, as I
have asked, a celebrated raiser of Hereford cattle, whether his cattle might
not have descended from long horns, and he will laugh you to scorn. I have
never met a pigeon, or poultry, or duck, or rabbit fancier, who was not fully
convinced that each main breed was descended from a distinct species. Van Moms,
in his treatise on pears and apples, shows how utterly he disbelieves that the
several sorts, for instance a Ribston-pippin or Codlin- apple, could ever have
proceeded from the seeds of the same tree. Innumerable other examples could be
given. The explanation, I think, is simple: from long-continued study they are
strongly impressed with the differences between the several races; and though
they well know that each race varies slightly, for they win their prizes by
selecting such slight differences, yet they ignore all general arguments, and
refuse to sum up in their minds slight differences accumulated during many
successive generations. May not those naturalists who, knowing far less of the
laws of inheritance than does the breeder, and knowing no more than he does of
the intermediate links in the long lines of descent, yet admit that many of our
domestic races have descended from the same parents -- may they not learn a
lesson of caution, when they deride the idea of species in a state of nature
being lineal descendants of other species?
Selection. Let us now
briefly consider the steps by which domestic races have been produced, either
from one or from several allied species. Some little effect may, perhaps, be
attributed to the direct action of the external conditions of life, and some
little to habit; but he would be a bold man who would account by such agencies
for the differences of a dray and race horse, a greyhound and bloodhound, a
carrier and tumbler pigeon. One of the most remarkable features in our
domesticated races is that we see in them adaptation, not indeed to the
animal's or plant's own good, but to man's use or fancy. Some variations useful
to him have probably arisen suddenly, or by one step; many botanists, for
instance, believe that the fuller's teazle, with its hooks, which cannot be
rivalled by any mechanical contrivance, is only a variety of the wild Dipsacus;
and this amount of change may have suddenly arisen in a seedling. So it has
probably been with the turnspit dog; and this is known to have been the case
with the ancon sheep. But when we compare the dray-horse and race-horse, the
dromedary and camel, the various breeds of sheep fitted either for cultivated
land or mountain pasture, with the wool of one breed good for one purpose, and
that of another breed for another purpose; when we compare the many breeds of
dogs, each good for man in very different ways; when we compare the gamecock,
so pertinacious in battle, with other breeds so little quarrelsome, with
'everlasting layers' which never desire to sit, and with the bantam so small
and elegant; when we compare the host of agricultural, culinary, orchard, and
flower-garden races of plants, most useful to man at different seasons and for
different purposes, or so beautiful in his eyes, we must, I think, look further
than to mere variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed, in several cases,
we know that this has not been their history. The key is man's power of
accumulative selection: nature gives successive variations; man adds them up in
certain directions useful to him. In this sense he may be said to make for
himself useful breeds.
The great power of this
principle of selection is not hypothetical. It is certain that several of our
eminent breeders have, even within a single lifetime, modified to a large
extent some breeds of cattle and sheep. In order fully to realize what they
have done, it is almost necessary to read several of the many treatises devoted
to this subject, and to inspect the animals. Breeders habitually speak of an
animal's organisation as something quite plastic, which they can model almost
as they please. If I had space I could quote numerous passages to this effect
from highly competent authorities. Youatt, who was probably better acquainted
with the works of agriculturalists than almost any other individual, and who
was himself a very good judge of an animal, speaks of the principle of selection
as 'that which enables the agriculturist, not only to modify the character of
his flock, but to change it altogether. It is the magician's wand, by means of
which he may summon into life whatever form and mould he pleases.' Lord
Somerville, speaking of what breeders have done for sheep, says: - 'It would
seem as if they had chalked out upon a wall a form perfect in itself, and then
had given it existence.' That most skilful breeder, Sir john Sebright, used to
say, with respect to pigeons, that 'he would produce any given feather in three
years, but it would take him six years to obtain head and beak.' In Saxony the
importance of the principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade: the sheep are placed on a table and
are studied, like a picture by a connoisseur; this is done three times at
intervals of months, and the sheep are each time marked and classed, so that
the very best may ultimately be selected for breeding.
What English breeders
have actually effected is proved by the enormous prices given for animals with
a good pedigree; and these have now been exported to almost every quarter of
the world. The improvement is by no means generally due to crossing different
breeds; all the best breeders are strongly opposed to this practice, except
sometimes amongst closely allied sub-breeds. And when a cross has been made,
the closest selection is far more indispensable even than in ordinary cases. If
selection consisted merely in separating some very distinct variety, and
breeding from it, the principle would be so obvious as hardly to be worth
notice; but its importance consists in the great effect produced by the
accumulation in one direction, during successive generations, of differences
absolutely inappreciable by an uneducated eye -- differences which I for one
have vainly attempted to appreciate. Not one man in a thousand has accuracy of
eye and judgement sufficient to become an eminent breeder. If gifted with these
qualities, and he studies his subject for years, and devotes his lifetime to it
with indomitable perseverance, he will succeed, and may make great
improvements; if he wants any of these qualities, he will assuredly fail. Few
would readily believe in the natural capacity and years of practice requisite
to become even a skilful pigeon-fancier.
The same principles are
followed by horticulturists; but the variations are here often more abrupt. No
one supposes that our choicest productions have been produced by a single
variation from the aboriginal stock. We have proofs that this is not so in some
cases, in which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be quoted.
We see an astonishing improvement in many florists' flowers, when the flowers
of the present day are compared with drawings made only twenty or thirty years
ago. When a race of plants is once pretty well established, the seed- raisers
do not pick out the best plants, but merely go over their seed-beds, and pull
up the 'rogues,' as they call the plants that deviate from the proper standard.
With animals this kind of selection is, in fact, also followed; for hardly any
one is so careless as to allow his worst animals to breed.
In regard to plants,
there is another means of observing the accumulated effects of selection --
namely, by comparing the diversity of flowers in the different varieties of the
same species in the flower-garden; the diversity of leaves, pods, or tubers, or
whatever part is valued, in the kitchen-garden, in comparison with the flowers
of the same varieties; and the diversity of fruit of the same species in the
orchard, in comparison with the leaves and flowers of the same set of
varieties. See how different the leaves of the cabbage are, and how extremely
alike the flowers; how unlike the flowers of the heartsease are, and how alike
the leaves; how much the fruit of the different kinds of gooseberries differ in
size, colour, shape, and hairiness, and yet the flowers present very slight
differences. It is not that the varieties which differ largely in some one
point do not differ at all in other points; this is hardly ever, perhaps never,
the case. The laws of correlation of growth, the importance of which should
never be overlooked, will ensure some differences; but, as a general rule, I
cannot doubt that the continued selection of slight variations, either in the
leaves, the flowers, or the fruit, will produce races differing from each other
chiefly in these characters.
It may be objected that
the principle of selection has been reduced to methodical practice for scarcely
more than three-quarters of a century; it has certainly been more attended to
of late years, and many treatises have been published on the subject; and the
result, I may add, has been, in a corresponding degree, rapid and important.
But it is very far from true that the principle is a modern discovery. I could
give several references to the full acknowledgement of the importance of the
principle in works of high antiquity. In rude and barbarous periods of English
history choice animals were often imported, and laws were passed to prevent
their exportation: the destruction of horses under a certain size was ordered,
and this may be compared to the 'roguing' of plants by nurserymen. The
principle of selection I find distinctly given in an ancient Chinese
encyclopaedia. Explicit rules are laid down by some of the Roman classical
writers. From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes cross their
dogs with wild canine animals, to improve the breed, and they formerly did so,
as is attested by passages in pliny. The savages in South Africa match their
draught cattle by colour, as do some of the Esquimaux their teams of dogs.
Livingstone shows how much good domestic breeds are valued by the negroes of
the interior of Africa who have not associated with Europeans. Some of these
facts do not show actual selection, but they show that the breeding of domestic
animals was carefully attended to in ancient times, and is now attended to by
the lowest savages. It would, indeed, have been a strange fact, had attention
not been paid to breeding, for the inheritance of good and bad qualities is so
obvious.
At the present time,
eminent breeders try by methodical selection, with a distinct object in view,
to make a new strain or sub-breed, superior to anything existing in the
country. But, for our purpose, a kind of Selection, which may be called
Unconscious, and which results from every one trying to possess and breed from
the best individual animals, is more important. Thus, a man who intends keeping
pointers naturally tries to get as good dogs as he can, and afterwards breeds
from his own best dogs, but he has no wish or expectation of permanently
altering the breed. Nevertheless I cannot doubt that this process, continued
during centuries, would improve and modify any breed, in the same way as
Bakewell, Collins, &c., by this very same process, only carried on more
methodically, did greatly modify, even during their own lifetimes, the forms
and qualities of their cattle. Slow and insensible changes of this Kind could
never be recognised unless actual measurements or careful drawings of the
breeds in question had been made long ago, which might serve for comparison. In
some cases, however, unchanged or but little changed individuals of the same
breed may be found in less civilised districts, where the breed has been less
improved. There is reason to believe that King Charles's spaniel has been
unconsciously modified to a large extent since the time of that monarch. Some
highly competent authorities are convinced that the setter is directly derived
from the spaniel, and has probably been slowly altered from it. It is known
that the English pointer has been greatly changed within the last century, and
in this case the change has, it is believed, been chiefly effected by crosses
with the fox- hound; but what concerns us is, that the change has been effected
unconsciously and gradually, and yet so effectually, that, though the old
Spanish pointer certainly came from Spain, Mr Barrow has not seen, as I am
informed by him. any native dog in Spain like our pointer.
By a similar process of
selection, and by careful training, the whole. body of English racehorses have
come to surpass in fleetness and size the parent Arab stock, so that the
latter, by the regulations for the Goodwood Races, are favoured in the weights
they carry. Lord Spencer and others have shown how the cattle of England have
increased in weight and in early maturity, compared with the stock formerly
kept in this country. By comparing the accounts given in old pigeon treatises
of carriers and tumblers with these breeds as now existing in Britain, India,
and persia, we can, I think, clearly trace the stages through which they have
insensibly passed, and come to differ so greatly from the rock-pigeon.
Youatt gives an
excellent illustration of the effects of a course of selection, which may be
considered as unconsciously followed, in so far that the breeders could never
have expected or even have wished to have produced the result which ensued --
namely, the production of two distinct strains. The two flocks of Leicester
sheep kept by Mr Buckley and Mr Burgess, as Mr Youatt remarks, 'have been
purely bred from the original stock of Mr Bakewell for upwards of fifty years.
There is not a suspicion existing in the mind of any one at all acquainted with
the subject that the owner of either of them has deviated in any one instance
from the pure blood of Mr Bakewell's flock, and yet the difference between the
sheep possessed by these two gentlemen is so great that they have the
appearance of being quite different varieties.
If there exist savages
so barbarous as never to think of the inherited character of the offspring of
their domestic animals, yet any one animal particularly useful to them, for any
special purpose, would be carefully preserved during famines and other
accidents, to which savages are so liable, and such choice animals would thus
generally leave more offspring than the inferior ones; so that in this case
there would be a kind of unconscious selection going on. We see the value set
on animals even by the barbarians of Tierra del Fuego, by their killing and
devouring their old women, in times of dearth, as of less value than their
dogs.
In plants the same
gradual process of improvement, through the occasional preservation of the best
individuals, whether or not sufficiently distinct to be ranked at their first
appearance as distinct varieties, and whether or not two or more species or
races have become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of the
heartsease, rose, pelargonium, dahlia, and other plants, when compared with the
older varieties or with their parent-stocks. No one would ever expect to get a
first-rate heartsease or dahlia from the seed of a wild plant. No one would
expect to raise a first- rate melting pear from the seed of a wild pear, though
he might succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears, from
pliny's description, to have been a fruit of very inferior quality. I have seen
great surprise expressed in horticultural works at the wonderful skill of
gardeners, in having produced such splendid results from such poor materials;
but the art, I cannot doubt, has been simple, and, as far as the final result
is concerned, has been followed almost unconsciously. It has consisted in
always cultivating the best known variety, sowing its seeds, and, when a
slightly better variety has chanced to appear, selecting it, and so onwards.
But the gardeners of the classical period, who cultivated the best pear they
could procure, never thought what splendid fruit we should eat; though we owe
our excellent fruit, in some small degree, to their having naturally chosen and
preserved the best varieties they could anywhere find.
A large amount of
change in our cultivated plants, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a vast number of cases we
cannot recognise, and therefore do not know, the wild parent-stocks of the
plants which have been longest cultivated in our flower and kitchen gardens. If
it has taken centuries or thousands of years to improve or modify most of our
plants up to their present standard of usefulness to man, we can understand how
it is that neither Australia, the Cape of Good Hope, nor any other region
inhabited by quite uncivilised man, has afforded us a single plant worth
culture. It is not that these countries, so rich in species, do not by a
strange chance possess the aboriginal stocks of any useful plants, but that the
native plants have not been improved by continued selection up to a standard of
perfection comparable with that given to the plants in countries anciently
civilised.
In regard to the
domestic animals kept by uncivilised man, it should not be overlooked that they
almost always have to struggle for their own food, at least during certain
seasons. And in two countries very differently circumstanced, individuals of
the same species, having slightly different constitutions or structure, would
often succeed better in the one country than in the other, and thus by a
process of 'natural selection,' as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains what has been
remarked by some authors, namely, that the varieties kept by savages have more
of the character of species than the varieties kept in civilised countries.
On the view here given
of the all-important part which selection by man has played, it becomes at once
obvious, how it is that our domestic races show adaptation in their structure
or in their habits to man's wants or fancies. We can, I think, further
understand the frequently abnormal character of our domestic races, and
likewise their differences being so great in external characters and relatively
so slight in internal parts or organs. Man can hardly select, or only with much
difficulty, any deviation of structure excepting such as is externally visible;
and indeed he rarely cares for what is internal. He can never act by selection,
excepting on variations which are first given to him in some slight degree by
nature. No man would ever try to make a fantail, till he saw a pigeon with a
tail developed in some slight degree in an unusual manner, or a pouter till he
saw a pigeon with a crop of somewhat unusual size; and the more abnormal or
unusual any character was when it first appeared, the more likely it would be
to catch his attention. But to use such an expression as trying to make a
fantail, is, I have no doubt, in most cases, utterly incorrect. The man who
first selected a - with a slightly larger tail, never dreamed what the
descendants of that pigeon would become through long-continued, partly
unconscious and partly methodical s-election. Perhaps the parent bird of all
fantails had only fourteen tail-feathers somewhat expanded, like the present
java fantail, or like individuals of other and distinct breeds, in which as
many as seventeen tail- feathers have been counted. perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now does the
upper part of its oesophagus, -- a habit which is disregarded by all fanciers,
as it is not one of the points of the breed.
Nor let it be thought that
some great deviation of structure would be necessary to catch the fancier's
eye: he perceives extremely small differences, and it is in human nature to
value any novelty, however slight, in one's own possession. Nor must the value
which would formerly be set on any slight differences in the individuals of the
same species, be judged of by the value which would now be set on them, after
several breeds have once fairly been established. Many slight differences
might, and indeed do now, arise amongst pigeons, which are rejected as faults
or deviations from the standard of perfection of each breed. The common goose
has not given rise to any marked varieties; hence the Thoulouse and the common
breed, which differ only in colour, that most fleeting of characters, have
lately been exhibited as distinct at our poultry-shows.
I think these views
further explain what has sometimes been noticed -- namely that we know nothing
about the origin or history of any of our domestic breeds. But, in fact, a
breed, like a dialect of a language, can hardly be said to have had a definite
origin. A man preserves and breeds from an individual with some slight
deviation of structure, or takes more care than usual in matching his best
animals and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a distinct name,
and from being only slightly valued, their history will be disregarded. When
further improved by the same slow and gradual process, they will spread more
widely, and will get recognised as something distinct and valuable, and will
then probably first receive a provincial name. In semi-civilised countries,
with little free communication, the spreading and knowledge of any new
sub-breed will be a slow process. As soon as the points of value of the new
sub-breed are once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend, -- perhaps more at one period than at
another, as the breed rises or falls in fashion, -- perhaps more in one
district than in another, according to the state of civilisation of the
inhabitants -- slowly to add to the characteristic features of the breed,
whatever they may be. But the chance will be infinitely small of any record
having been preserved of such slow, varying, and insensible changes.
I must now say a few
words on the circumstances, favourable, or the reverse, to man's power of
selection. A high degree of variability is obviously favourable, as freely
giving the materials for selection to work on; not that mere individual
differences are not amply sufficient, with extreme care, to allow of the
accumulation of a large amount of modification in almost any desired direction.
But as variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by a large
number of individuals being kept; and hence this comes to be of the highest
importance to success. On this principle Marshall has remarked, with respect to
the sheep of parts of Yorkshire, that 'as they generally belong to poor people,
and are mostly in small lots, they never can be improved.' On the other hand,
nurserymen, from raising large stocks of the same plants, are generally far
more successful than amateurs in getting new and valuable varieties. The
keeping of a large number of individuals of a species in any country requires
that the species should be placed under favourable conditions of life, so as to
breed freely in that country. When the individuals of any species are scanty,
all the individuals, whatever their quality may be, will generally be allowed
to breed, and this will effectually prevent selection. But probably the most
important point of all, is, that the animal or plant should be so highly useful
to man, or so much valued by him, that the closest attention should be paid to
even the slightest deviation in the qualities or structure of each individual.
Unless such attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to vary just
when gardeners began to attend closely to this plant. No doubt the strawberry
had always varied since it was cultivated, but the slight varieties had been
neglected. As soon, however, as gardeners picked out individual plants with
slightly larger, earlier, or better fruit, and raised seedlings from them, and
again picked out the best seedlings and bred from them, then, there appeared
(aided by some crossing with distinct species) those many admirable varieties
of the strawberry which have been raised during the last thirty or forty years.
In the case of animals
with separate sexes, facility in preventing crosses is an important element of
success in the formation of new races, -- at least, in a country which is
already stocked with other races. In this respect enclosure of the land plays a
part. Wandering savages or the inhabitants of open plains rarely possess more
than one breed of the same species. pigeons can be mated for life, and this is
a great convenience to the fancier, for thus many races may be kept true,
though mingled in the same aviary; and this circumstance must have largely
favoured the improvement and formation of new breeds. pigeons, I may add, can
be propagated in great numbers and at a very quick rate, and inferior birds may
be freely rejected, as when killed they serve for food. On the other hand,
cats, from their nocturnal rambling habits, cannot be matched, and, although so
much valued by women and children, we hardly ever see a distinct breed kept up;
such breeds as we do sometimes see are almost always imported from some other
country, often from islands. Although I do not doubt that some domestic animals
vary less than others, yet the rarity or absence of distinct breeds of the cat,
the donkey, peacock, goose, &c., may be attributed in main part to
selection not having been brought into play: in cats, from the difficulty in
pairing them; in donkeys, from only a few being kept by poor people, and little
attention paid to their breeding; in peacocks, from not being very easily
reared and a large stock not kept; in geese, from being valuable only for two
purposes, food and feathers, and more especially from no pleasure having been
felt in the display of distinct breeds.
To sum up on the origin
of our Domestic Races of animals and plants. I believe that the conditions of
life, from their action on the reproductive system, are so far of the highest
importance as causing variability. I do not believe that variability is an
inherent and necessary contingency, under all circumstances, with all organic
beings, as some authors have thought. The effects of variability are modified
by various degrees of inheritance and of reversion. Variability is governed by
many unknown laws, more especially by that of correlation of growth. Something
may be attributed to the direct action of the conditions of life. Something
must be attributed to use and disuse. The final result is thus rendered
infinitely complex. In some cases, I do not doubt that the intercrossing of
species, aboriginally distinct, has played an important part in the origin of
our domestic productions. when in any country several domestic breeds have once
been established, their occasional intercrossing, with the aid of selection,
has, no doubt, largely aided in the formation of new sub-breeds; but the
importance of the crossing of varieties has, I believe, been greatly
exaggerated, both in regard to animals and to those plants which are propagated
by seed. In plants which are temporarily propagated by cuttings, buds, &c.,
the importance of the crossing both of distinct species and of varieties is
immense; for the cultivator here quite disregards the extreme variability both
of hybrids and mongrels, and the frequent sterility of hybrids; but the cases
of plants not propagated by seed are of little importance to us, for their
endurance is only temporary. Over all these causes of Change I am convinced
that the accumulative action of Selection, whether applied methodically and
more quickly, or unconsciously and more slowly, but more efficiently, is by far
the predominant power.
Variability - Individual differences - Doubtful species - Wide ranging,
much diffused, and common species vary most - Species of the larger genera in
any country vary more than the species of the smaller genera - Many of the
species of the larger genera resemble varieties in being very closely, but
unequally, related to each other, and in having restricted ranges BEFORE applying the principles arrived at in
the last chapter to organic beings in a state of nature, we must briefly
discuss whether these latter are subject to any variation. To treat this
subject at all properly, a long catalogue of dry facts should be given; but
these I shall reserve for my future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one definition has as
yet satisfied all naturalists; yet every naturalist knows vaguely what he means
when he speaks of a species. Generally the term includes the unknown element of
a distinct act of creation. The term 'variety' is almost equally difficult to
define; but here community of descent is almost universally implied, though it
can rarely be proved. We have also what are called monstrosities; but they
graduate into varieties. By a monstrosity I presume is meant some considerable
deviation of structure in one part, either injurious to or not useful to the
species, and not generally propagated. Some authors use the term 'variation' in
a technical sense, as imploring a modification directly due to the physical
conditions of life; and 'variations' in this sense are supposed not to be
inherited: but who can say that the dwarfed condition of shells in the brackish
waters of the Baltic, or dwarfed plants on Alpine summits, or the thicker fur
of an animal from far northwards, would not in some cases be inherited for at
least some few generations? and in this case I presume that the form would be
called a variety.
Again, we have many
slight differences which may be called individual differences, such as are
known frequently to appear in the offspring from the same parents, or which may
be presumed to have thus arisen, from being frequently observed in the
individuals of the same species inhabiting the same confined locality. No one
supposes that all the individuals of the same species are cast in the very same
mould. These individual differences are highly important for us, as they afford
materials for natural selection to accumulate, in the same manner as man can
accumulate in any given direction individual differences in his domesticated
productions, These individual differences generally affect what naturalists
consider unimportant parts; but I could show by a long catalogue of facts, that
parts which must be called important, whether viewed under a physiological or
classificatory point of view, sometimes vary in the individuals of the same
species. I am convinced that the most experienced naturalist would be surprised
at the number of the cases of variability, even in important parts of
structure, which he could collect on good authority, as I have collected,
during a course of years. It should be remembered that systematists are far
from pleased at finding variability in important characters, and that there are
not many men who will laboriously examine internal and important organs, and
compare them in many specimens of the same species. I should never have
expected that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species; I should
have expected that changes of this nature could have been effected only by slow
degrees: yet quite recently Mr Lubbock has shown a degree of variability in
these main nerves in Coccus, which may almost be compared to the irregular
branching of the stem of a tree. This philosophical naturalist, I may add, has
also quite recently shown that the muscles in the larvae of certain insects are
very far from uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank that
character as important (as some few naturalists have honestly confessed) which
does not vary; and, under this point of view, no instance of any important part
varying will ever be found: but under any other point of view many instances
assuredly can be given.
There is one point
connected with individual differences, which seems to me extremely perplexing:
I refer to those genera which have sometimes been called 'protean' or
'polymorphic,' in which the species present an inordinate amount of variation;
and hardly two naturalists can agree which forms to rank as species and which
as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants,
several genera of insects, and several genera of Brachiopod shells. In most
polymorphic genera some of the species have fixed and definite characters.
Genera which are polymorphic in one country seem to be, with some few
exceptions, polymorphic in other countries, and likewise, judging from
Brachiopod shells, at former periods of time. These facts seem to be very
perplexing, for they seem to show that this kind of variability is independent
of the conditions of life. I am inclined to suspect that we see in these polymorphic
genera variations in points of structure which are of no service or disservice
to the species, and which consequently have not been seized on and rendered
definite by natural selection, as hereafter will be explained.
Those forms which
possess in some considerable degree the character of species, but which are so
closely similar to some other forms, or are so closely linked to them by
intermediate gradations, that naturalists do not like to rank them as distinct
species, are in several respects the most important for us. We have every
reason to believe that many of these doubtful and closely-allied forms have
permanently retained their characters in their own country for a long time; for
as long, as far as we know, as have good and true species. practically, when a
naturalist can unite two forms together by others having intermediate
characters, he treats the one as a variety of the other, ranking the most
common, but sometimes the one first described, as the species, and the other as
the variety. But cases of great difficulty, which I will not here enumerate,
sometimes occur in deciding whether or not to rank one form as a variety of
another, even when they are closely connected by intermediate links; nor will
the commonly- assumed hybrid nature of the intermediate links always remove the
difficulty. In very many cases, however, one form is ranked as a variety of
another, not because the intermediate links have actually been found, but
because analogy leads the observer to suppose either that they do now somewhere
exist, or may formerly have existed; and here a wide door for the entry of
doubt and conjecture is opened.
Hence, in determining
whether a form should be ranked as a species or a variety, the opinion of
naturalists having sound judgement and wide experience seems the only guide to
follow. We must, however, in many cases, decide by a majority of naturalists,
for few well-marked and well-known varieties can be named which have not been
ranked as species by at least some competent judges.
That varieties of this
doubtful nature are far from uncommon cannot be disputed. Compare the several
floras of Great Britain, of France or of the United States, drawn up by
different botanists, and see what a surprising number of forms have been ranked
by one botanist as good species, and by another as mere varieties. Mr H. C.
Watson, to whom I lie under deep obligation for assistance of all kinds, has
marked for me 182 British plants, which are generally considered as varieties,
but which have all been ranked by botanists as species; and in making this list
he has omitted many trifling varieties, but which nevertheless have been ranked
by some botanists as species, and he has entirely omitted several highly
polymorphic genera. Under genera, including the most polymorphic forms, Mr
Babington gives 251 species, whereas Mr Bentham gives only 112, -- a difference
of 139 doubtful forms! Amongst animals which unite for each birth, and which
are highly locomotive, doubtful forms, ranked by one zoologist as a species and
by another as a variety, can rarely be found within the same country, but are
common in separated areas. How many of those birds and insects in North America
and Europe, which differ very slightly from each other, have been ranked by one
eminent naturalist as undoubted species, and by another as varieties, or, as
they are often called, as geographical races! Many years ago, when comparing,
and seeing others compare, the birds from the separate islands of the Galapagos
Archipelago, both one with another, and with those from the American mainland,
I was much struck how entirely vague and arbitrary is the distinction between
species and varieties. On the islets of the little Madeira group there are many
insects which are characterized as varieties in Mr Wollaston's admirable work,
but which it cannot be doubted would be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded as
varieties, but which have been ranked as species by some zoologists. Several
most experienced ornithologists consider our British red grouse as only a
strongly-marked race of a Norwegian species, whereas the greater number rank it
as an undoubted species peculiar to Great Britain. A wide distance between the
homes of two doubtful forms leads many naturalists to rank both as distinct
species; but what distance, it has been well asked, will suffice? if that
between America and Europe is ample, will that between the Continent and the
Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must be
admitted that many forms, considered by highly-competent judges as varieties,
have so perfectly the character of species that they are ranked by other
highly-competent judges as good and true species. But to discuss whether they
are rightly called species or varieties, before any definition of these terms
has been generally accepted, is vainly to beat the air.
Many of the cases of
strongly-marked varieties or doubtful species well deserve consideration; for
several interesting lines of argument, from geographical distribution,
analogical variation, hybridism, &c., have been brought to bear on the
attempt to determine their rank. I will here give only a single instance, --
the well-known one of the primrose and cowslip, or primula veris and elatior.
These plants differ considerably in appearance; they have a different flavour
and emit a different odour; they flower at slightly different periods; they
grow in somewhat different stations; they ascend mountains to different
heights; they have different geographical ranges; and lastly, according to very
numerous experiments made during several years by that most careful observer Gärtner,
they can be crossed only with much difficulty. We could hardly wish for better
evidence of the two forms being specifically distinct. On the other hand, they
are united by many intermediate links, and it is very doubtful whether these
links are hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents, and
consequently must be ranked as varieties.
Close investigation, in
most cases, will bring naturalists to an agreement how to rank doubtful forms.
Yet it must be confessed, that it is in the best-known countries that we find
the greatest number of forms of doubtful value. I have been struck with the
fact, that of any animal or plant in a state of nature be highly useful to man,
or from any cause closely attract his attention, varieties of it will almost
universally be found recorded. These varieties, moreover, will be often ranked
by some authors as species. Look at the common oak, how closely it has been
studied; yet a German author makes more than a dozen species out of forms,
which are very generally considered as varieties; and in this country the
highest botanical authorities and practical men can be quoted to show that the
sessile and pedunculated oaks are either good and distinct species or mere
varieties.
When a young naturalist
commences the study of a group of organisms quite unknown to him, he is at
first much perplexed to determine what differences to consider as specific, and
what as varieties; for he knows nothing of the amount and kind of variation to
which the group is subject; and this shows, at least, how very generally there
is some variation. But if he confine his attention to one class within one
country, he will soon make up his mind how to rank most of the doubtful forms.
His general tendency will be to make many species, for he will become
impressed, just like the pigeon or poultry-fancier before alluded to, with the
amount of difference in the forms which he is continually studying; and he has
little general knowledge of analogical variation in other groups and in other
countries, by which to correct his first impressions. As he extends the range
of his observations, he will meet with more cases of difficulty; for he will
encounter a greater number of closely-allied forms. But ff his observations be
widely extended, he will in the end generally be enabled to make up his own
mind which to call varieties and which species; but he will succeed in this at
the expense of admitting much variation, -- and the truth of this admission
will often be dispute by other naturalists. When, moreover, he comes to study
allied forms brought from countries not now continuous, in which case he can
hardly hope to find the intermediate links between his doubtful forms, he will
have to trust almost entirely to analogy, and his difficulties will rise to a
climax.
Certainly no clear line
of demarcation has as yet been drawn between species and sub-species -- that
is, the forms which in the opinion of some naturalists come very near to, but
do not quite arrive at the rank of species; or, again, between sub-species and
well-marked varieties, or between lesser varieties and individual differences.
These differences blend into each other in an insensible series; and a series
impresses the mind with the idea of an actual passage.
Hence I look at
individual differences, though of small interest to the systematist, as of high
importance for us, as being the first step towards such slight varieties as are
barely thought worth recording in works on natural history. And I look at
varieties which are in any degree more distinct and permanent, as steps leading
to more strongly marked and more permanent varieties; and at these latter, as
leading to sub-species, and to species. The passage from one stage of
difference to another and higher stage may be, in some cases, due merely to the
long-continued action of different physical conditions in two different
regions; but I have not much faith in this view; and I attribute the passage of
a variety, from a state in which it differs very slightly from its parent to
one in which it differs more, to the action of natural selection in
accumulating (as will hereafter be more fully explained) differences of
structure in certain definite directions. Hence I believe a well- marked
variety may be justly called an incipient species; but whether this belief be
justifiable must be judged of by the general weight of the several facts and
views given throughout this work.
It need not be supposed
that all varieties or incipient species necessarily attain the rank of species.
They may whilst in this incipient state become extinct, or they may endure as
varieties for very long periods, as has been shown to be the case by Mr
Wollaston with the varieties of certain fossil land-shells in Madeira. If a
variety were to flourish so as to exceed in numbers the parent species, it
would then rank as the species, and the species as the variety; or it might
come to supplant and exterminate the parent species; or both might co-exist,
and both rank as independent species. But we shall hereafter have to return to
this subject.
From these remarks it
will be seen that I look at the term species, as one arbitrarily given for the
sake of convenience to a set of individuals closely resembling each other, and
that it does not essentially differ from the term variety, which is given to
less distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied arbitrarily, and
for mere convenience sake.
Guided by theoretical
considerations, I thought that some interesting results might be obtained in
regard to the nature and relations of the species which vary most, by
tabulating all the varieties in several well-worked floras. At first this
seemed a simple task; but Mr H. C. Watson, to whom I am much indebted for
valuable advice and assistance on this subject, soon convinced me that there
were many difficulties, as did subsequently Dr Hooker, even in stronger terms.
I shall reserve for my future work the discussion of these difficulties, and
the tables themselves of the proportional numbers of the varying species. Dr
Hooker permits me to add, that after having carefully read my manuscript, and
examined the tables, he thinks that the following statements are fairly well
established. The whole subject, however, treated as it necessarily here is with
much brevity, is rather perplexing, and allusions cannot be avoided to the
'struggle for existence,' 'divergence of character,' and other questions,
hereafter to be discussed.
Alph. De Candolle and
others have shown that plants which have very wide ranges generally present
varieties; and this might have been expected, as they become exposed to diverse
physical conditions, and as they come into competition (which, as we shall
hereafter see, is a far more important circumstance) with different sets of
organic beings. But my tables further show that, in any limited country, the
species which are most common, that is abound most in individuals, and the
species which are most widely diffused within their own country (and this is a
different consideration from wide range, and to a certain extent from
commonness), often give rise to varieties sufficiently well-marked to have been
recorded in botanical works. Hence it is the most flourishing, or, as they may
be called, the dominant species, -- those which range widely over the world,
are the most diffused in their own country, and are the most numerous in
individuals, -- which oftenest produce well-marked varieties, or, as I consider
them, incipient species. And this, perhaps, might have been anticipated; for,
as varieties, in order to become in any degree permanent, necessarily have to
struggle with the other inhabitants of the country, the species which are
already dominant will be the most likely to yield offspring which, though in
some slight degree modified, will still inherit those advantages that enabled
their parents to become dominant over their compatriots.
If the plants
inhabiting a country and described in any Flora be divided into two equal
masses, all those in the larger genera being placed on one side, and all those
in the smaller genera on the other side, a somewhat larger number of the very
common and much diffused or dominant species will be found on the side of the
larger genera. This, again, might have been anticipated; for the mere fact of
many species of the same genus inhabiting any country, shows that there is
something in the organic or inorganic conditions of that country favourable to
the genus; and, consequently, we might have expected to have found in the
larger genera, or those including many species, a large proportional number of
dominant species. But so many causes tend to obscure this result, that I am
surprised that my tables show even a small majority on the side of the larger
genera. I will here allude to only two causes of obscurity. Fresh-water and
salt-loving plants have generally very wide ranges and are much diffused, but
this seems to be connected with the nature of the stations inhabited by them,
and has little or no relation to the size of the genera to which the species
belong. Again, plants low in the scale of organisation are generally much more
widely diffused than plants higher in the scale; and here again there is no
close relation to the size of the genera. The cause of lowly-organised plants
ranging widely will be discussed in our chapter on geographical distribution.
From looking at species
as only strongly-marked and well-defined varieties, I was led to anticipate
that the species of the larger genera in each country would oftener present
varieties, than the species of the smaller genera; for wherever many closely
related species (i.e. species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now forming.
where many large trees grow, we expect to find saplings. Where many species of
a genus have been formed through variation, circumstances have been favourable
for variation; and hence we might expect that the circumstances would generally
be still favourable to variation. On the other hand, if we look at each species
as a special act of creation, there is no apparent reason why more varieties
should occur in a group having many species, than in one having few.
To test the truth of
this anticipation I have arranged the plants of twelve countries, and the
coleopterous insects of two districts, into two nearly equal masses, the
species of the larger genera on one side, and those of the smaller genera on
the other side, and it has invariably proved to be the case that a larger
proportion of the species on the side of the larger genera present varieties,
than on the side of the smaller genera. Moreover, the species of the large
genera which present any varieties, invariably present a larger average number
of varieties than do the species of the small genera. Both these results follow
when another division is made, and when all the smallest genera, with from only
one to four species, are absolutely excluded from the tables. These facts are
of plain signification on the view that species are only strongly marked and
permanent varieties; for whenever many species of the same genus have been
formed, or where, if we may use the expression, the manufactory of species has
been active, we ought generally to find the manufactory still in action, more
especially as we have every reason to believe the process of manufacturing new
species to be a slow one. And this certainly is the case, if varieties be
looked at as incipient species; for my tables clearly show as a general rule
that, wherever many species of a genus have been formed, the species of that
genus present a number of varieties, that is of incipient species, beyond the
average. It is not that all large genera are now varying much, and are thus
increasing in the number of their species, or that no small genera are now
varying and increasing; for if this had been so, it would have been fatal to my
theory; inasmuch as geology plainly tells us that small genera have in the
lapse of time often increased greatly in size; and that large genera have often
come to their maxima, declined, and disappeared. All that we want to show is,
that where many species of a genus have been formed, on an average many are
still forming; and this holds good.
There are other
relations between the species of large genera and their recorded varieties
which deserve notice. We have seen that there is no infallible criterion by
which to distinguish species and well-marked varieties; and in those cases in
which intermediate links have not been found between doubtful forms,
naturalists are compelled to come to a determination by the amount of
difference between them, judging by analogy whether or not the amount suffices
to raise one or both to the rank of species. Hence the amount of difference is
one very important criterion in settling whether two forms should be ranked as
species or varieties. Now Fries has remarked in regard to plants, and Westwood
in regard to insects, that in large genera the amount of difference between the
species is often exceedingly small. I have endeavoured to test this numerically
by averages, and, as far as my imperfect results go, they always confirm the
view. I have also consulted some sagacious and most experienced observers, and,
after deliberation, they concur in this view. In this respect, therefore, the
species of the larger genera resemble varieties, more than do the species of
the smaller genera. Or the case may be put in another way, and it may be said,
that in the larger genera, in which a number of varieties or incipient species
greater than the average are now manufacturing, many of the species already
manufactured still to a certain extent resemble varieties, for they differ from
each other by a less than usual amount of difference.
Moreover, the species
of the large genera are related to each other, in the same manner as the
varieties of any one species are related to each other. No naturalist pretends
that all the species of a genus are equally distinct from each other; they may
generally be divided into sub-genera, or sections, or lesser groups. As Fries
has well remarked, little groups of species are generally clustered like
satellites around certain other species. And what are varieties but groups of
forms, unequally related to each other, and clustered round certain forms --
that is, round their parent-species? Undoubtedly there is one most important
point of difference between varieties and species; namely, that the amount of
difference between varieties, when compared with each other or with their
parent-species, is much less than that between the species of the same genus.
But when we come to discuss the principle, as I call it, of Divergence of
Character, we shall see how this may be explained, and how the lesser differences
between varieties will tend to increase into the greater differences between
species.
There is one other
point which seems to me worth notice. Varieties generally have much restricted
ranges: this statement is indeed scarcely more than a truism, for if a variety
were found to have a wider range than that of its supposed parent-species,
their denominations ought to be reversed. But there is also reason to believe,
that those species which are very closely allied to other species, and in so
far resemble varieties, often have much restricted ranges. For instance, Mr H.
C. Watson has marked for me in the well-sifted London Catalogue of plants (4th
edition) 63 plants which are therein ranked as species, but which he considers
as so closely allied to other species as to be of doubtful value: these 63
reputed species range on an average over 6.9 of the provinces into which Mr
Watson has divided Great Britain. Now, in this same catalogue, 53 acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas, the
species to which these varieties belong range over 14.3 provinces. So that the
acknowledged varieties have very nearly the same restricted average range, as
have those very closely allied forms, marked for me by Mr Watson as doubtful
species, but which are almost universally ranked by British botanists as good
and true species.
Finally, then,
varieties have the same general characters as species, for they cannot be
distinguished from species, -- except, firstly, by the discovery of intermediate
linking forms, and the occurrence of such links cannot affect the actual
characters of the forms which they connect; and except, secondly, by a certain
amount of difference, for two forms, if differing very little, are generally
ranked as varieties, notwithstanding that intermediate linking forms have not
been discovered; but the amount of difference considered necessary to give to
two forms the rank of species is quite indefinite. In genera having more than
the average number of species in any country, the species of these genera have
more than the average number of varieties. In large genera the species are apt
to be closely, but unequally, allied together, forming little clusters round
certain species. Species very closely allied to other species apparently have
restricted ranges. In all these several respects the species of large genera
present a strong analogy with varieties. And we can clearly understand these
analogies, if species have once existed as varieties, and have thus originated:
whereas, these analogies are utterly inexplicable if each species has been
independently created.
We have, also, seen
that it is the most flourishing and dominant species of the larger genera which
on an average vary most; and varieties, as we shall hereafter see, tend to
become converted into new and distinct species. The larger genera thus tend to
become larger; and throughout nature the forms of life which are now dominant
tend to become still more dominant by leaving many modified and dominant
descendants. But by steps hereafter to be explained, the larger genera also
tend to break up into smaller genera. And thus, the forms of life throughout
the universe become divided into groups subordinate to groups.
Bears on natural selection - The term used in a wide sense - Geometrical
powers of increase - Rapid increase of naturalised animals and plants - Nature
of the checks to increase - Competition universal - Effects of climate -
Protection from the number of individuals - Complex relations of all animals
and plants throughout nature - Struggle for life most severe between
individuals and varieties of the same species; often severe between species of
the same genus - The relation of organism to organism the most important of all
relations BEF0RE entering on
the subject of this chapter, I must make a few preliminary remarks, to show how
the struggle for existence bears on Natural Selection. It has been seen in the
last chapter that amongst organic beings in a state of nature there is some individual
variability; indeed I am not aware that this has ever been disputed. It is
immaterial for us whether a multitude of doubtful forms be called species or
sub-species or varieties; what rank, for instance, the two or three hundred
doubtful forms of British plants are entitled to hold, if the existence of any
well-marked varieties be admitted. But the mere existence of individual
variability and of some few well-marked varieties, though necessary as the
foundation for the work, helps us but little in understanding how species arise
in nature. How have all those exquisite adaptations of one part of the
organisation to another part, and to the conditions of life, and of one
distinct organic being to another being, been perfected? We see these beautiful
co-adaptations most plainly in the woodpecker and misseltoe; and only a little
less plainly in the humblest parasite which clings to the hairs of a quadruped
or feathers of a bird; in the structure of the beetle which dives through the
water; in the plumed seed which is wafted by the gentlest breeze; in short, we
see beautiful adaptations everywhere and in every part of the organic world.
Again, it may be asked,
how is it that varieties, which I have called incipient species, become
ultimately converted into good and distinct species, which in most cases
obviously differ from each other far more than do the varieties of the same
species? How do those groups of species, which constitute what are called
distinct genera, and which differ from each other more than do the species of
the same genus, arise? All these results, as we shall more fully see in the
next chapter, follow inevitably from the struggle for life. Owing to this
struggle for life, any variation, however slight and from whatever cause
proceeding, if it be in any degree profitable to an individual of any species,
in its infinitely complex relations to other organic beings and to external
nature, will tend to the preservation of that individual, and will generally be
inherited by its offspring. The offspring, also, will thus have a better chance
of surviving, for, of the many individuals of any species which are
periodically born, but a small number can survive. I have called this
principle, by which each slight variation, if useful, is preserved, by the term
of Natural Selection, in order to mark its relation to man's power of
selection. We have seen that man by selection can certainly produce great
results, and can adapt organic beings to his own uses, through the accumulation
of slight but useful variations, given to him by the hand of Nature. But
Natural Selection, as we shall hereafter see, is a power incessantly ready for
action, and is as immeasurably superior to man's feeble efforts, as the works
of Nature are to those of Art.
We will now discuss in
a little more detail the struggle for existence. In my future work this subject
shall be treated, as it well deserves, at much greater length. The elder De
Candolle and Lyell have largely and philosophically shown that all organic
beings are exposed to severe competition. In regard to plants, no one has
treated this subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge. Nothing
is easier than to admit in words the truth of the universal struggle for life,
or more difficult -- at least I have found it so -- than constantly to bear
this conclusion in mind. Yet unless it be thoroughly engrained in the mind, I
am convinced that the whole economy of nature, with every fact on distribution,
rarity, abundance, extinction, and variation, will be dimly seen or quite
misunderstood. We behold the face of nature bright with gladness, we often see
superabundance of food; we do not see, or we forget, that the birds which are
idly singing round us mostly live on insects or seeds, and are thus constantly
destroying life; or we forget how largely these songsters, or their eggs, or
their nestlings are destroyed by birds and beasts of prey; we do not always
bear in mind, that though food may be now superabundant, it is not so at all
seasons of each recurring year.
I should premise that I
use the term Struggle for Existence in a large and metaphorical sense,
including dependence of one being on another, and including (which is more
important) not only the life of the individual, but success in leaving progeny.
Two canine animals in a time of dearth, may be truly said to struggle with each
other which shall get food and live. But a plant on the edge of a desert is
said to struggle for life against the drought, though more properly it should
be said to be dependent on the moisture. A plant which annually produces a
thousand seeds, of which on an average only one comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds which
already clothe the ground. The missletoe is dependent on the apple and a few
other trees, but can only in a far-fetched sense be said to struggle with these
trees, for if too many of these parasites grow on the same tree, it will languish
and die. But several seedling missletoes, growing close together on the same
branch, may more truly be said to struggle with each other. As the missletoe is
disseminated by birds, its existence depends on birds; and it may
metaphorically be said to struggle with other fruit-bearing plants, in order to
tempt birds to devour and thus disseminate its seeds rather than those of other
plants. In these several senses, which pass into each other, I use for
convenience sake the general term of struggle for existence.
A struggle for
existence inevitably follows from the high rate at which all organic beings
tend to increase. Every being, which during its natural lifetime produces
several eggs or seeds, must suffer destruction during some period of its life,
and during some season or occasional year, otherwise, on the principle of
geometrical increase, its numbers would quickly become so inordinately great
that no country could support the product. Hence, as more individuals are
produced than can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or with the
individuals of distinct species, or with the physical conditions of life. It is
the doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase of
food, and no prudential restraint from marriage. Although some species may be
now increasing, more or less rapidly, in numbers, all cannot do so, for the world
would not hold them.
There is no exception
to the rule that every organic being naturally increases at so high a rate,
that if not destroyed, the earth would soon be covered by the progeny of a
single pair. Even slow-breeding man has doubled in twenty-five years, and at
this rate, in a few thousand years, there would literally not be standing room
for his progeny. Linnaeus has calculated that if an annual plant produced only
two seeds - and there is no plant so unproductive as this -- and their seedlings
next year produced two, and so on, then in twenty years there would be a
million plants. The elephant is reckoned to be the slowest breeder of all known
animals, and I have taken some pains to estimate its probable minimum rate of
natural increase: it will be under the mark to assume that it breeds when
thirty years old, and goes on breeding till ninety years old, bringing forth
three pairs of young in this interval; if this be so, at the end of the fifth
century there would be alive fifteen million elephants, descended from the
first pair.
But we have better
evidence on this subject than mere theoretical calculations, namely, the
numerous recorded cases of the astonishingly rapid increase of various animals
in a state of nature, when circumstances have been favourable to them during
two or three following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of the
world: if the statements of the rate of increase of slow-breeding cattle and
horses in South America, and latterly in Australia, had not been well
authenticated, they would have been quite incredible. So it is with plants:
cases could be given of introduced plants which have become common throughout
whole islands in a period of less than ten years, Several of the plants now
most numerous over the wide plains of La plata, clothing square leagues of
surface almost to the exclusion of all other plants, have been introduced from
Europe; and there are plants which now range in India, as I hear from Dr
Falconer, from Cape Comorin to the Himalaya, which have been imported from
America since its discovery. In such cases, and endless instances could be
given, no one supposes that the fertility of these animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very favourable, and that
there has consequently been less destruction of the old and young, and that
nearly all the young have been enabled to breed. In such cases the geometrical
ratio of increase, the result of which never fails to be surprising, simply
explains the extraordinarily rapid increase and wide diffusion of naturalised
productions in their new homes.
In a state of nature
almost every plant produces seed, and amongst animals there are very few which
do not annually pair. Hence we may confidently assert, that all plants and
animals are tending to increase at a geometrical ratio, that all would most
rapidly stock every station in which they could any how exist, and that the
geometrical tendency to increase must be checked by destruction at some period
of life. Our familiarity with the larger domestic animals tends, I think, to
mislead us: we see no great destruction falling on them, and we forget that
thousands are annually slaughtered for food, and that in a state of nature an
equal number would have somehow to be disposed of.
The only difference
between organisms which annually produce eggs or seeds by the thousand, and
those which produce extremely few, is, that the slow-breeders would require a
few more years to people, under favourable conditions, a whole district, let it
be ever so large. The condor lays a couple of eggs and the ostrich a score, and
yet in the same country the condor may be the more numerous of the two: the
Fulmar petrel lays but one egg, yet it is believed to be the most numerous bird
in the world, One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one; but this difference does not determine how many
individuals of the two species can be supported in a district. A large number
of eggs is of some importance to those species, which depend on a rapidly
fluctuating amount of food, for it allows them rapidly to increase in number.
But the real importance of a large number of eggs or seeds is to make up for
much destruction at some period of life; and this period in the great majority
of cases is an early one. If an animal can in any way protect its own eggs or
young, a small number may be produced, and yet the average stock be fully kept
up; but if many eggs or young are destroyed, many must be produced, or the
species will become extinct. It would suffice to keep up the lull number of a
tree, which lived on an average for a thousand years, of a single seed were
produced once in a thousand years, supposing that this seed were never
destroyed, and could be ensured to germinate in a fitting place. So that in all
cases, the average number of any animal or plant depends only indirectly on the
number of its eggs or seeds.
In looking at Nature,
it is most necessary to keep the foregoing considerations always in mind --
never to forget that every single organic being around us may be said to be
striving to the utmost to increase in numbers; that each lives by a struggle at
some period of its life; that heavy destruction inevitably falls either on the
young or old, during each generation or at recurrent intervals. Lighten any
check, mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount. The face of Nature may be
compared to a yielding surface, with ten thousand sharp wedges packed close
together and driven inwards by incessant blows, sometimes one wedge being
struck, and then another with greater force.
What checks the natural
tendency of each species to increase in number is most obscure. Look at the
most vigorous species; by as much as it swarms in numbers, by so much will its
tendency to increase be still further increased. We know not exactly what the
checks are in even one single instance. Nor will this surprise any one who
reflects how ignorant we are on this head, even in regard to mankind, so
incomparably better known than any other animal. This subject has been ably
treated by several authors, and I shall, in my future work, discuss some of the
checks at considerable length, more especially in regard to the feral animals
of South America. Here I will make only a few remarks, just to recall to the
reader's mind some of the chief points. Eggs or very young animals seem
generally to suffer most, but this is not invariably the case. With plants
there is a vast destruction of seeds, but, from some observations which I have
made, I believe that it is the seedlings which suffer most from germinating in
ground already thickly stocked with other plants. Seedlings, also, are
destroyed in vast numbers by various enemies; for instance, on a piece of
ground three feet long and two wide, dug and cleared, and where there could be
no choking from other plants, I marked all the seedlings of our native weeds as
they came up, and out of the 357 no less than 295 were destroyed, chiefly by
slugs and insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more vigorous
plants gradually kill the less vigorous, though fully grown, plants: thus out
of twenty species growing on a little plot of turf (three feet by four) nine
species perished from the other species being allowed to grow up freely.
The amount of food for
each species of course gives the extreme limit to which each can increase; but
very frequently it is not the obtaining food, but the serving as prey to other
animals, which determines the average numbers of a species. Thus, there seems
to be little doubt that the stock of partridges, grouse, and hares on any large
estate depends chiefly on the destruction of vermin. If not one head of game
were shot during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game than at
present, although hundreds of thousands of game animals are now annually
killed. On the other hand, in some cases, as with the elephant and rhinoceros,
none are destroyed by beasts of prey: even the tiger in India most rarely dares
to attack a young elephant protected by its dam.
Climate plays an
important part in determining the average numbers of a species, and periodical
seasons of extreme cold or drought, I believe to be the most effective of all
checks. I estimated that the winter of 1854-55 destroyed four-fifths of the
birds in my own grounds; and this is a tremendous destruction, when we remember
that ten per cent. is an extraordinarily severe mortality from epidemics with
man. The action of climate seems at first sight to be quite independent of the
struggle for existence; but in so far as climate chiefly acts in reducing food,
it brings on the most severe struggle between the individuals, whether of the
same or of distinct species, which subsist on the same kind of food. Even when
climate, for instance extreme cold, acts directly, it will be the least
vigorous, or those which have got least food through the advancing winter,
which will suffer most. When we travel from south to north, or from a damp
region to a dry, we invariably see some species gradually getting rarer and
rarer, and finally disappearing; and the change of climate being conspicuous,
we are tempted to attribute the whole effect to its direct action. But this is
a very false view: we forget that each species, even where it most abounds, is
constantly suffering enormous destruction at some period of its life, from
enemies or from competitors for the same place and food; and if these enemies
or competitors be in the least degree favoured by any slight change of climate,
they will increase in numbers, and, as each area is already fully stocked with
inhabitants, the other species will decrease. When we travel southward and see
a species decreasing in numbers, we may feel sure that the cause lies quite as
much in other species being favoured, as in this one being hurt. So it is when
we travel northward, but in a somewhat lesser degree, for the number of species
of all kinds, and therefore of competitors, decreases northwards; hence in
going northward, or in ascending a mountain, we far oftener met with stunted
forms, due to the directly injurious action of climate, than we do in
proceeding southwards or in descending a mountain. When we reach the Arctic
regions, or snow-capped summits, or absolute deserts, the struggle for life is
almost exclusively with the elements.
That climate acts in
main part indirectly by favouring other species, we may clearly see in the
prodigious number of plants in our gardens which can perfectly well endure our
climate, but which never become naturalised, for they cannot compete with our
native plants, nor resist destruction by our native animals.
When a species, owing
to highly favourable circumstances, increases inordinately in numbers in a
small tract, epidemics -- at least, this seems generally to occur with our game
animals -- often ensue: and here we have a limiting check independent of the
struggle for life. But even some of these so-called epidemics appear to be due
to parasitic worms, which have from some cause, possibly in part through
facility of diffusion amongst the crowded animals, been disproportionably
favoured: and here comes in a sort of struggle between the parasite and its
prey.
On the other hand, in
many cases, a large stock of individuals of the same species, relatively to the
numbers of its enemies, is absolutely necessary for its preservation. Thus we
can easily raise plenty of corn and rape-seed, &c., in our fields, because
the seeds are in great excess compared with the number of birds which feed on
them; nor can the birds, though having a superabundance of food at this one
season, increase in number proportionally to the supply of seed, as their
numbers are checked during winter: but any one who has tried, knows how
troublesome it is to get seed from a few wheat or other such plants in a
garden; I have in this case lost every single seed. This view of the necessity
of a large stock of the same species for its preservation, explains, I believe,
some singular facts in nature, such as that of very rare plants being sometimes
extremely abundant in the few spots where they do occur; and that of some
social plants being social, that is, abounding in individuals, even on the
extreme confines of their range. For in such cases, we may believe, that a
plant could exist only where the conditions of its life were so favourable that
many could exist together, and thus save each other from utter destruction. I
should add that the good effects of frequent intercrossing, and the ill effects
of close interbreeding, probably came into play in some of these cases; but on
this intricate subject I will not here enlarge.
Many cases are on
record showing how complex and unexpected are the checks and relations between
organic beings, which have to struggle together in the same country. I will
give only a single instance, which, though a simple one, has interested me. In
Staffordshire, on the estate of a relation where I had ample means of
investigation, there was a large and extremely barren heath, which had never
been touched by the hand of man; but several hundred acres of exactly the same
nature had been enclosed twenty-five years previously and planted with Scotch
fir. The change in the native vegetation of the planted part of the heath was
most remarkable, more than is generally seen in passing from one quite
different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not counting
grasses and carices) flourished in the plantations, which could not be found on
the heath. The effect on the insects must have been still greater, for six
insectivorous birds were very common in the plantations, which were not to be
seen on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done, with the
exception that the land had been enclosed, so that cattle could not enter. But
how important an element enclosure is, I plainly saw near Farnham, in Surrey.
Here there are extensive heaths, with a few clumps of old Scotch firs on the
distant hill-tops: within the last ten years large spaces have been enclosed,
and self-sown firs are now springing up in multitudes, so close together that
all cannot live. When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to several points
of view, whence I could examine hundreds of acres of the unenclosed heath, and
literally I could not see a single Scotch fir, except the old planted clumps.
But on looking closely between the stems of the heath, I found a multitude of
seedlings and little trees, which had been perpetually browsed down by the
cattle. In one square yard, at a point some hundreds yards distant from one of
the old clumps, I counted thirty-two little trees; and one of them, judging
from the rings of growth, had during twenty-six years tried to raise its head
above the stems of the heath, and had failed. No wonder that, as soon as the
land was enclosed, it became thickly clothed with vigorously growing young
firs. Yet the heath was so extremely barren and so extensive that no one would
ever have imagined that cattle would have so closely and effectually searched
it for food.
Here we see that cattle
absolutely determine the existence of the Scotch fir; but in several parts of
the world insects determine the existence of cattle. Perhaps Paraguay offers
the most curious instance of this; for here neither cattle nor horses nor dogs
have ever run wild, though they swarm southward and northward in a feral state;
and Azara and Rengger have shown that this is caused by the greater number in
Paraguay of a certain fly, which lays its eggs in the navels of these animals
when first born. The increase of these flies, numerous as they are, must be
habitually checked by some means, probably by birds. Hence, if certain
insectivorous birds (whose numbers are probably regulated by hawks or beasts of
prey) were to increase in Paraguay, the flies would decrease - then cattle and
horses would become feral, and this would certainly greatly alter (as indeed I
have observed in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we just have seen in Staffordshire,
the insectivorous birds, and so onwards in ever-increasing circles of
complexity. We began this series by insectivorous birds, and we have ended with
them. Not that in nature the relations can ever be as simple as this. Battle
within battle must ever be recurring with varying success; and yet in the
long-run the forces are so nicely balanced, that the face of nature remains
uniform for long periods of time, though assuredly the merest trifle would often
give the victory to one organic being over another. Nevertheless so profound is
our ignorance, and so high our presumption, that we marvel when we hear of the
extinction of an organic being; and as we do not see the cause, we invoke
cataclysms to desolate the world, or invent laws on the duration of the forms
of life I
I am tempted to give
one more instance showing how plants and animals, most remote in the scale of
nature, are bound together by a web of complex relations. I shall hereafter
have occasion to show that the exotic Lobelia fulgens, in this part of England,
is never visited by insects, and consequently, from its peculiar structure,
never can set a seed. Many of our orchidaceous plants absolutely require the
visits of moths to remove their pollen-masses and thus to fertilise them. I
have, also, reason to believe that humble-bees are indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not visit
this flower. From experiments which I have tried, I have found that the visits
of bees, if not indispensable, are at least highly beneficial to the
fertilisation of our clovers; but humble-bees alone visit the common red clover
(Trifolium pratense), as other bees cannot reach the nectar. Hence I have very
little doubt, that if the whole genus of humble- bees became extinct or very
rare in England, the heartsease and red clover would become very rare, or
wholly disappear. The number of humble-bees in any district depends in a great
degree on the number of field-mice, which destroy their combs and nests; and Mr
H. Newman, who has long attended to the habits of humble-bees, believes that
'more than two thirds of them are thus destroyed all over England.' Now the
number of mice is largely dependent, as every one knows, on the number of cats;
and Mr Newman says, 'Near villages and small towns I have found the nests of
humble-bees more numerous than elsewhere, which I attribute to the number of
cats that destroy the mice.' Hence it is quite credible that the presence of a
feline animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain flowers
in that district!
In the case of every
species, many different checks, acting at different periods of life, and during
different seasons or years, probably come into play; some one check or some few
being generally the most potent, but all concurring in determining the average
number or even the existence of the species. In some cases it can be shown that
widely- different checks act on the same species in different districts. When
we look at the plants and bushes clothing an entangled bank, we are tempted to
attribute their proportional numbers and kinds to what we call chance. But how
false a view is this! Every one has heard that when an American forest is cut
down, a very different vegetation springs up; but it has been observed that the
trees now growing on the ancient Indian mounds, in the Southern United States,
display the same beautiful diversity and proportion of kinds as in the
surrounding virgin forests. What a struggle between the several kinds of trees
must here have gone on during long centuries, each annually scattering its
seeds by the thousand; what war between insect and insect - between insects, snails,
and other animals with birds and beasts of prey - all striving to increase, and
all feeding on each other or on the trees or their seeds and seedlings, or on
the other plants which first clothed the ground and thus checked the growth of
the trees! Throw up a handful of feathers, and all must fall to the ground
according to definite laws; but how simple is this problem compared to the
action and reaction of the innumerable plants and animals which have
determined, in the course of centuries, the proportional numbers and kinds of
trees now growing on the old Indian ruins!
The dependency of one
organic being on another, as of a parasite on its prey, lies generally between
beings remote in the scale of nature. This is often the case with those which
may strictly be said to struggle with each other for existence, as in the case
of locusts and grass-feeding quadrupeds. But the struggle almost invariably
will be most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to the same
dangers. In the case of varieties of the same species, the struggle will
generally be almost equally severe, and we sometimes see the contest soon
decided: for instance, if several varieties of wheat be sown together, and the
mixed seed be resown, some of the varieties which best suit the soil or
climate, or are naturally the most fertile, will beat the others and so yield
more seed, and will consequently in a few years quite supplant the other
varieties. To keep up a mixed stock of even such extremely close varieties as
the variously coloured sweet-peas, they must be each year harvested separately,
and the seed then mixed in due proportion, otherwise the weaker kinds will
steadily decrease in numbers and disappear. So again with the varieties of
sheep: it has been asserted that certain mountain-varieties will starve out
other mountain-varieties, so that they cannot be kept together. The same result
has followed from keeping together different varieties of the medicinal leech.
It may even be doubted whether the varieties of any one of our domestic plants
or animals have so exactly the same strength, habits, and constitution, that
the original proportions of a mixed stock could be kept up for half a dozen
generations, if they were allowed to struggle together, like beings in a state
of nature, and if the seed or young were not annually sorted.
As species of the same
genus have usually, though by no means invariably, some similarity in habits
and constitution, and always in structure, the struggle will generally be more
severe between species of the same genus, when they come into competition with
each other, than between species of distinct genera. We see this in the recent
extension over parts of the United States of one species of swallow having
caused the decrease of another species. The recent increase of the
missel-thrush in parts of Scotland has caused the decrease of the song-thrush.
How frequently we hear of one species of rat taking the place of another
species under the most different climates! In Russia the small Asiatic cockroach
has everywhere driven before it its great congener. One species of charlock
will supplant another, and so in other cases. We can dimly see why the
competition should be most severe between allied forms, which fill nearly the
same place in the economy of nature; but probably in no one case could we
precisely say why one species has been victorious over another in the great
battle of life.
A corollary of the
highest importance may be deduced from the foregoing remarks, namely, that the
structure of every organic being is related, in the most essential yet often
hidden manner, to that of all other organic beings, with which it comes into
competition for food or residence, or from which it has to escape, or on which
it preys. This is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite which clings to the
hair on the tiger's body. But in the beautifully plumed seed of the dandelion,
and in the flattened and fringed legs of the water-beetle, the relation seems
at first confined to the elements of air and water. Yet the advantage of plumed
seeds no doubt stands in the closest relation to the land being already thickly
clothed by other plants; so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of its legs, so well
adapted for diving, allows it to compete with other aquatic insects, to hunt
for its own prey, and to escape serving as prey to other animals.
The store of nutriment
laid up within the seeds of many plants seems at first sight to have no sort of
relation to other plants. But from the strong growth of young plants produced
from such seeds (as peas and beans), when sown in the midst of long grass, I
suspect that the chief use of the nutriment in the seed is to favour the growth
of the young seedling, whilst struggling with other plants growing vigorously
all around.
Look at a plant in the
midst of its range, why does it not double or quadruple its numbers? We know
that it can perfectly well withstand a little more heat or cold, dampness or
dryness, for elsewhere it ranges into slightly hotter or colder, damper or
drier districts. In this case we can clearly see that if we wished in
imagination to give the plant the power of increasing in number, we should have
to give it some advantage over its competitors, or over the animals which
preyed on it. On the confines of its geographical range, a change of
constitution with respect to climate would clearly be an advantage to our
plant; but we have reason to believe that only a few plants or animals range so
far, that they are destroyed by the rigour of the climate alone. Not until we
reach the extreme confines of life, in the arctic regions or on the borders of
an utter desert, will competition cease. The land may be extremely cold or dry,
yet there will be competition between some few species, or between the
individuals of the same species, for the warmest or dampest spots.
Hence, also, we can see
that when a plant or animal is placed in a new country amongst new competitors,
though the climate may be exactly the same as in its former home, yet the
conditions of its life will generally be changed in an essential manner. If we
wished to increase its average numbers in its new home, we should have to
modify it in a different way to what we should have done in its native country;
for we should have to give it some advantage over a different set of
competitors or enemies.
It is good thus to try
in our imagination to give any form some advantage over another. probably in no
single instance should we know what to do, so as to succeed. It will convince
us of our ignorance on the mutual relations of all organic beings; a conviction
as necessary, as it seems to be difficult to acquire. All that we can do, is to
keep steadily in mind that each organic being is striving to increase at a
geometrical ratio; that each at some period of its life, during some season of
the year, during each generation or at intervals, has to struggle for life, and
to suffer great destruction. When we reflect on this struggle, we may console
ourselves with the full belief, that the war of nature is not incessant, that
no fear is felt, that death is generally prompt, and that the vigorous, the
healthy, and the happy survive and multiply.
Natural Selection - its power compared with man's selection - its power
on characters of trifling importance - its Power at all ages and on both sexes
- Sexual Selection - On the generality of intercrosses between individuals of
the same species - Circumstances favourable and unfavourable to Natural
Selection, namely, intercrossing, isolation, number of individuals - Slow
action - Extinction caused by Natural Selection - Divergence of Character,
related to the diversity of inhabitants of any small area, and to
naturalisation - Action of Natural Selection, through Divergence of Character
and Extinction, on the descendants from a common parent - Explains the Grouping
of all organic beings HOW will
the struggle for existence, discussed too briefly in the last chapter, act in
regard to variation? Can the principle of selection, which we have seen is so
potent in the hands of man, apply in nature? I think we shall see that it can
act most effectually. Let it be borne in mind in what an endless number of
strange peculiarities our domestic productions, and, in a lesser degree, those
under nature, vary; and how strong the hereditary tendency is. Under
domestication, it may be truly said that the, whole organisation becomes in
some degree plastic. Let it be borne in mind how infinitely complex and
close-fitting are the mutual relations of all organic beings to each other and
to their physical conditions of life. Can it, then, be thought improbable,
seeing that variations useful to man have undoubtedly occurred, that other
variations useful in some way to each being in the great and complex battle of
life, should sometimes occur in the course of thousands of generations? If such
do occur, can we doubt (remembering that many more individuals are born than
can possibly survive) that individuals having any advantage, however slight,
over others, would have the best chance of surviving and of Procreating their
hind? On the other hand, we may feel sure that any variation in the least
degree injurious would be rigidly destroyed. This preservation of favourable
variations and the rejection of injurious variations, I call Natural Selection.
Variations neither useful nor injurious would not be affected by natural
selection, and would be left a fluctuating element, as perhaps we see in the
species called polymorphic.
We shall best
understand the Probable course of natural selection by taking the case of a
country undergoing some Physical change, for instance, of climate. The
proportional numbers of its inhabitants would almost immediately undergo a
change, and some species might become extinct. We may conclude, from what we
have seen of the intimate and complex manner in which the inhabitants of each
country are bound together, that any change in the numerical proportions of
some of the inhabitants, independently of the change of climate itself, would
most seriously affect many of the others. If the country were open on its
borders, new forms would certainly immigrate, and this also would seriously disturb
the relations of some of the former inhabitants. Let it be remembered how
powerful the influence of a single introduced tree or mammal has been shown to
be. But in the case of an island, or of a country partly surrounded by
barriers, into which new and better adapted forms could not freely enter, we
should then have Places in the economy of nature which would assuredly be
better filled up, if some of the original inhabitants were in some manner
modified; for, had the area been open to immigration, these same places would
have been seized on by intruders. In such case, every slight modification,
which in the course of ages chanced to arise, and which in any way favoured the
individuals of any of the species, by better adapting them to their altered conditions,
would tend to be preserved; and natural selection would thus have free scope
for the work of improvement.
We have reason to
believe, as stated in the first chapter, that a change in the conditions of
life, by specially acting on the reproductive system, causes or increases
variability; and in the foregoing case the conditions of life are supposed to
have undergone a change, and this would manifestly be favourable to natural
selection, by giving a better chance of Profitable variations occurring; and
unless profitable variations do occur, natural selection can do nothing. Not
that, as I believe, any extreme amount of variability is necessary; as man can
certainly Produce great results by adding up in any given direction mere
individual differences, so could Nature, but far more easily, from having
incomparably longer time at her disposal. Nor do I believe that any great
physical change, as of climate, or any unusual degree of isolation to check
immigration, is actually necessary to produce new and unoccupied places for
natural selection to fill up by modifying and improving some of the varying
inhabitants. For as all the inhabitants of each country are struggling together
with nicely balanced forces, extremely slight modifications in the structure or
habits of one inhabitant would often give it an advantage over others; and
still further modifications of the same kind would often still further increase
the advantage. No country can be named in which all the native inhabitants are
now so perfectly adapted to each other and to the physical conditions under
which they live, that none of them could anyhow be improved; for in all
countries, the natives have been so far conquered by naturalised productions,
that they have allowed foreigners to take firm possession of the land. And as
foreigners have thus everywhere beaten some of the natives, we may safely
conclude that the natives might have been modified with advantage, so as to
have better resisted such intruders.
As man can produce and
certainly has produced a great result by his methodical and unconscious means
of selection, what may not nature effect? Man can act only on external and
visible characters: nature cares nothing for appearances, except in so far as
they may be useful to any being. She can act on every internal organ, on every
shade of constitutional difference, on the whole machinery of life. Man selects
only for his own good;, Nature only for that of the being which she tends.
Every selected character is fully exercised by her; and the being is placed
under well-suited conditions of life. Man keeps the natives of many climates in
the same country; he seldom exercises each selected character in some peculiar
and fitting manner; he feeds a long and a short beaked pigeon on the same food;
he does not exercise a long- backed or long-legged quadruped in any peculiar
manner; he exposes sheep with long and short wool to the same climate. He does
not allow the most vigorous males to struggle for the females. He does not
rigidly destroy all inferior animals, but protects during each varying season,
as far as lies in his power, all his productions. He often begins his selection
by some half-monstrous form; or at least by some modification prominent enough
to catch his eye, or to be plainly useful to him. Under nature, the slightest
difference of structure or constitution may well turn the nicely-balanced scale
in the struggle for life, and so be preserved. How fleeting are the wishes and
efforts of man! how short his time! and consequently how poor will his Products
be, compared with those accumulated by nature during whole geological periods.
Can we wonder, then, that nature's productions should be far 'truer' in
character than man's productions; that they should be infinitely better adapted
to the most complex conditions of life, and should Plainly bear the stamp of
far higher workmanship?
It may be said that
natural selection is daily and hourly scrutinising, throughout the world, every
variation, even the slightest; rejecting that which is bad, preserving and
adding up all that is good; silently and insensibly working, whenever and
wherever opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see nothing of
these slow changes in progress, until the hand of time has marked the long
lapses of ages, and then so imperfect is our view into long past geological
ages, that we only see that the forms of life are now different from what they
formerly were.
Although natural
selection can act only through and for the good of each being, yet characters
and structures, which we are apt to consider as of very trifling importance,
may thus be acted on. When we see leaf-eating insects green, and bark-feeders
mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour
of heather, and the black-grouse that of Peaty earth, we must believe that
these tints are of service to these birds and insects in preserving them from
danger. Grouse, if not destroyed at some period of their lives, would increase
in countless numbers; they are known to suffer largely from birds of prey; and
hawks are guided by eyesight to their prey, -- so much so, that on parts of the
Continent persons are warned not to keep white pigeons, as being the most liable
to destruction. Hence I can see no reason to doubt that natural selection might
be most effective in giving the proper colour to each kind of grouse, and in
keeping that colour, when once acquired, true and constant. Nor ought we to
think that the occasional destruction of an animal of any particular colour
would produce little effect: we should remember how essential it is in a flock
of white sheep to destroy every lamb with the faintest trace of black. In
plants the down on the fruit and the colour of the flesh are considered by
botanists as characters of the most trifling importance: yet we hear from an
excellent horticulturist, Downing, that in the United States smooth-skinned
fruits suffer far more from a beetle, a curculio, than those with down; that purple
plums suffer far more from a certain disease than yellow plums; whereas another
disease attacks yellow-fleshed peaches far more than those with other coloured
flesh. If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state of
nature, where the trees would have to struggle with other trees and with a host
of enemies, such differences would effectually settle which variety, whether a
smooth or downy, a yellow or purple fleshed fruit, should succeed.
In looking at many
small points of difference between species, which, as far as our ignorance
permits us to judge, seem to be quite unimportant, we must not forget that
climate, food, &c., probably produce some slight and direct effect. It is,
however, far more necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is modified
through variation, and the modifications are accumulated by natural selection for
the good of the being, will cause other modifications, often of the most
unexpected nature.
As we see that those
variations which under domestication appear at any particular period of life,
tend to reappear in the offspring at the same period; -- for instance, in the
seeds of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of
poultry, and in the colour of the down of their chickens; in the horns of our
sheep and cattle when nearly adult; -- so in a state of nature, natural
selection will be enabled to act on and modify organic beings at any age, by
the accumulation of profitable variations at that age, and by their inheritance
at a corresponding age. If it profit a plant to have its seeds more and more
widely disseminated by the wind, I can see no greater difficulty in this being
effected through natural selection, than in the cotton-planter increasing and
improving by selection the down in the pods on his cotton- trees. Natural
selection may modify and adapt the larva of an insect to a score of
contingencies, wholly different from those which concern the mature insect.
These modifications will no doubt affect, through the laws of correlation, the
structure of the adult; and probably in the case of those insects which live
only for a few hours, and which never feed, a large part of their structure is
merely the correlated result of successive changes in the structure of their
larvae. So, conversely, modifications in the adult will probably often affect
the structure of the larva; but in all cases natural selection will ensure that
modifications consequent on other modifications at a different period of life,
shall not be in the least degree injurious: for if they became so, they would
cause the extinction of the species.
Natural selection will
modify the structure of the young in relation to the parent, and of the parent
in relation to the young. In social animals it will adapt the structure of each
individual for the benefit of the community; if each in consequence profits by
the selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the good of
another species; and though statements to this effect may be found in works of
natural history, I cannot find one case which will bear investigation. A
structure used only once in an animal's whole life, if of high importance to
it, might be modified to any extent by natural selection; for instance, the
great jaws possessed by certain insects, and used exclusively for opening the
cocoon -- or the hard tip to the beak of nestling birds, used for breaking the
egg. It has been asserted, that of the best short-beaked tumbler-pigeons more
perish in the egg than are able to get out of it; so that fanciers assist in
the act of hatching. Now, if nature had to make the beak of a full-grown pigeon
very short for the bird's own advantage, the process of modification would be
very slow, and there would be simultaneously the most rigorous selection of the
young birds within the egg, which had the most powerful and hardest beaks, for
all with weak beaks would inevitably perish: or, more delicate and more easily
broken shells might be selected, the thickness of the shell being known to vary
like every other structure.
Sexual Selection.
Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, the same fact probably occurs under
nature, and if so, natural selection will be able to modify one sex in its
functional relations to the other sex, or in relation to wholly different
habits of life in the two sexes, as is sometimes the case with insects. And
this leads me to say a few words on what I call Sexual Selection. This depends,
not on a struggle for existence, but on a struggle between the males for
possession of the females; the result is not death to the unsuccessful
competitor, but few or no offspring. Sexual selection is, therefore, less
rigorous than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most progeny. But
in many cases, victory will depend not on general vigour, but on having special
weapons, confined to the male sex. A hornless stag or spurless cock would have
a poor chance of leaving offspring. Sexual selection by always allowing the
victor to breed might surely give indomitable courage, length to the spur, and
strength to the wing to strike in the spurred leg, as well as the brutal
cock-fighter, who knows well that he can improve his breed by careful selection
of the best cocks. How low in the scale of nature this law of battle descends,
I know not; male alligators have been described as fighting. bellowing, and
whirling round, like Indians in a war-dance, for the possession of the females;
male salmons have been seen fighting all day long; male stag-beetles often bear
wounds from the huge mandibles of other males. The war is, perhaps, severest
between the males of polygamous animals, and these seem oftenest provided with
special weapons. The males of carnivorous animals are already well armed;
though to them and to others, special means of defence may be given through
means of sexual selection, as the mane to the lion, the shoulder-pad to the
boar, and the hooked jaw to the male salmon; for the shield may be as important
for victory, as the sword or spear.
Amongst birds, the
contest is often of a more peaceful character. All those who have attended to
the subject, believe that there is the severest rivalry between the males of
many species to attract by singing the females. The rock-thrush of Guiana,
birds of paradise, and some others, congregate; and successive males display
their gorgeous plumage and perform strange antics before the females, which
standing by as spectators, at last choose the most attractive partner. Those
who have closely attended to birds in confinement well know that they often
take individual preferences and dislikes: thus Sir R. Heron has described how
one pied peacock was eminently attractive to all his hen birds. It may appear
childish to attribute any effect to such apparently weak means: I cannot here
enter on the details necessary to support this view; but if man can in a short
time give elegant carriage and beauty to his bantams, according to his standard
of beauty, I can see no good reason to doubt that female birds, by selecting,
during thousands of generations, the most melodious or beautiful males,
according to their standard of beauty, might produce a marked effect. I
strongly suspect that some well-known laws with respect to the plumage of male
and female birds, in comparison with the plumage of the young, can be explained
on the view of plumage having been chiefly modified by sexual selection, acting
when the birds have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or seasons,
either by the males alone, or by the males and females; but I have not space
here to enter on this subject.
Thus it is, as I
believe, that when the males and females of any animal have the same general
habits of life, but differ in structure, colour, or ornament, such differences
have been mainly Caused by sexual selection; that is, individual males have
had, in successive generations, some slight advantage over other males, in
their weapons, means of defence, or charms; and have transmitted these
advantages to their male offspring. Yet, I would not wish to attribute all such
sexual differences to this agency: for we see peculiarities arising and
becoming attached to the male sex in our domestic animals (as the wattle in
male carriers, horn-like protuberances in the cocks of certain fowls, &c.),
which we cannot believe to be either useful to the males in battle, or
attractive to the females. We see analogous cases under nature, for instance,
the tuft of hair on the breast of the turkey-cock, which can hardly be either
useful or ornamental to this bird; -- indeed, had the tuft appeared under domestication,
it would have been called a monstrosity.
Illustrations of the
action of Natural Selection. In order to make it clear how, as I believe,
natural selection acts, I must beg permission to give one or two imaginary
illustrations. Let us take the case of a wolf, which preys on various animals,
securing some by craft, some by strength, and some by fleetness; and let us
suppose that the fleetest prey, a deer for instance, had from any change in the
country increased in numbers, or that other prey had decreased in numbers,
during that season of the year when the wolf is hardest pressed for food. I can
under such circumstances see no reason to doubt that the swiftest and slimmest
wolves would have the best chance of surviving, and so be preserved or selected,
-- provided always that they retained strength to master their prey at this or
at some other period of the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that man can improve the
fleetness of his greyhounds by careful and methodical selection, or by that
unconscious selection which results from each man trying to keep the best dogs
without any thought of modifying the breed.
Even without any change
in the proportional numbers of the animals on which our wolf preyed, a cub
might be born with an innate tendency to pursue certain kinds of prey. Nor can
this be thought very improbable; for we often observe great differences in the
natural tendencies of our domestic animals; one cat, for Instance, taking to
catch rats, another mice; one cat, according to Mr St john, bringing home
winged game, another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats rather
than mice is known to be inherited. Now, if any slight innate change of habit
or of structure benefited an individual wolf, it would have the best chance of
surviving and of leaving offspring. Some of its young would probably inherit
the same habits or structure, and by the repetition of this process, a new
variety might be formed which would either supplant or coexist with the
parent-form of wolf. Or, again, the wolves inhabiting a mountainous district,
and those frequenting the lowlands, would naturally be forced to hunt different
prey,; and from the continued preservation of the individuals best fitted for
the two sites, two varieties might slowly be formed. These varieties would
cross and blend where they met; but to this subject of intercrossing we shall
soon have to return. I may add, that, according to Mr pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains in the United States,
one with a light greyhound-like form, which pursues deer, and the other more
bulky, with shorter legs, which more frequently attacks the shepherd's flocks.
Let us now take a more
complex case. Certain plants excrete a sweet juice, apparently for the sake of
eliminating something injurious from their sap: this is effected by glands at
the base of the stipules in some Leguminosae, and at the back of the leaf of
the common laurel. This juice, though small in quantity, is greedily sought by
insects. Let us now suppose a little sweet juice or nectar to be excreted by
the inner bases of the petals of a flower. In this case insects in seeking the
nectar would get dusted with pollen, and would certainly often transport the
pollen from one flower to the stigma of another flower. The flowers of two
distinct individuals of the same species would thus get crossed; and the act of
crossing, we have good reason to believe (as will hereafter be more fully
alluded to), would produce very vigorous seedlings, which consequently would
have the best chance of flourishing and surviving. Some of these seedlings
would probably inherit the nectar-excreting power. Those in individual flowers
which had the largest glands or nectaries, and which excreted most nectar,
would be oftenest visited by insects, and would be oftenest crossed; and so in
the long-run would gain the upper hand. Those flowers, also, which had their
stamens and pistils placed, in relation to the size and habits of the
particular insects which visited them, so as to favour in any degree the
transportal of their pollen from flower to flower, would likewise be favoured
or selected. We might have taken the case of insects visiting flowers for the
sake of collecting pollen instead of nectar; and as pollen is formed for the
sole object of fertilisation, its destruction appears a simple loss to the
plant; yet if a little pollen were carried, at first occasionally and then
habitually, by the pollen-devouring insects from flower to flower, and a cross
thus effected, although nine-tenths of the pollen were destroyed, it might
still be a great gain to the plant; and those individuals which produced more
and more pollen, and had larger and larger anthers, would be selected.
When our plant, by this
process of the continued preservation or natural selection of more and more
attractive flowers, had been rendered highly attractive to insects, they would,
unintentionally on their part, regularly carry pollen from flower to flower;
and that they can most effectually do this, I could easily show by many
striking instances. I will give only one -- not as a very striking case, but as
likewise illustrating one step in the separation of the sexes of plants,
presently to be alluded to. Some holly-trees bear only male flowers, which have
four stamens producing rather a small quantity of pollen, and a rudimentary
pistil; other holly-trees bear only female flowers; these have a full-sized
pistil, and four stamens with shrivelled anthers, in which not a grain of
pollen can be detected. Having found a female tree exactly sixty yards from a
male tree, I put the stigmas of twenty flowers, taken from different branches,
under the microscope, and on all, without exception, there were pollen-grains,
and on some a profusion of pollen. As the wind had set for several days from
the female to the male tree, the pollen could not thus have been carried. The
weather had been cold and boisterous, and therefore not favourable to bees,
nevertheless every female flower which I examined had been effectually
fertilised by the bees, accidentally dusted with pollen, having flown from tree
to tree in search of nectar. But to return to our imaginary case: as soon as
the plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence. No
naturalist doubts the advantage of what has been called the 'physiological
division of labour;' hence we may believe that it would be advantageous to a
plant to produce stamens alone in one flower or on one whole plant, and pistils
alone in another flower or on another plant. In plants under culture and placed
under new conditions of life, sometimes the male organs and sometimes the
female organs become more or less impotent; now if we suppose this to occur in
ever so slight a degree under nature, then as pollen is already carried
regularly from flower to flower, and as a more complete separation of the sexes
of our plant would be advantageous on the principle of the division of labour,
individuals with this tendency more and more increased, would be continually
favoured or selected, until at last a complete separation of the sexes would be
effected.
Let us now turn to the
nectar-feeding insects in our imaginary case: we may suppose the plant of which
we have been slowly increasing the nectar by continued selection, to be a
common plant; and that certain insects depended in main part on its nectar for
food. I could give many facts, showing how anxious bees are to save time; for
instance, their habit of cutting holes and sucking the nectar at the bases of
certain flowers, which they can, with a very little more trouble, enter by the
mouth. Bearing such facts in mind, I can see no reason to doubt that an
accidental deviation in the size and form of the body, or in the curvature and
length of the proboscis, &c., far too slight to be appreciated by us, might
profit a bee or other insect, so that an individual so characterised would be
able to obtain its food more quickly, and so have a better chance of living and
leaving descendants. Its descendants would probably inherit a tendency to a
similar slight deviation of structure. The tubes of the corollas of the common
red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty
glance appear to differ in length; yet the hive- bee can easily suck the nectar
out of the incarnate clover, but not out of the common red clover, which is
visited by humble-bees alone; so that whole fields of the red clover offer in
vain an abundant supply of precious nectar to the hive-bee. Thus it might be a
great advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, I have found by experiment that the
fertility of clover greatly depends on bees visiting and moving parts of the
corolla, so as to push the pollen on to the stigmatic surface. Hence, again, if
humble-bees were to become rare in any country, it might be a great advantage
to the red clover to have a shorter or more deeply divided tube to its corolla,
so that the hive- bee could visit its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one after the
other, modified and adapted in the most perfect manner to each other, by the
continued preservation of individuals presenting mutual and slightly favourable
deviations of structure.
I am well aware that
this doctrine of natural selection, exemplified in the above imaginary
instances, is open to the same objections which were at first urged against Sir
Charles Lyell's noble views on 'the modern changes of the earth, as
illustrative of geology;' but we now very seldom hear the action, for instance,
of the coast-waves, called a trifling and insignificant cause, when applied to
the excavation of gigantic valleys or to the formation of the longest lines of
inland cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each profitable
to the preserved being; and as modern geology has almost banished such views as
the excavation of a great valley by a single diluvial wave, so will natural
selection, if it be a true principle, banish the belief of the continued
creation of new organic beings, or of any great and sudden modification in
their structure.
On the Intercrossing of
Individuals. I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two individuals
must always unite for each birth; but in the case of hermaphrodites this is far
from obvious. Nevertheless I am strongly inclined to believe that with all
hermaphrodites two individuals, either occasionally or habitually, concur for
the reproduction of their kind. This view, I may add, was first suggested by
Andrew Knight. We shall presently see its importance; but I must here treat the
subject with extreme brevity, though I have the materials prepared for an ample
discussion. All vertebrate animals, all insects, and some other large groups of
animals, pair for each birth. Modern research has much diminished the number of
supposed hermaphrodites, and of real hermaphrodites a large number pair; that
is, two individuals regularly unite for reproduction, which is afl that
concerns us. But still there are many hermaphrodite animals which certainly do
not habitually pair, and a vast majority of plants are hermaphrodites. What
reason, it may be asked, is there for supposing in these cases that two
individuals ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.
In the first place, I
have collected so large a body of facts, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of another
strain, gives vigour and fertility to the offspring; and on the other hand,
that close interbreeding diminishes vigour and fertility; that these facts
alone incline me to believe that it is a general law of nature (utterly
ignorant though we be of the meaning of the law) that no organic being
self-fertilises itself for an eternity of generations; but that a cross with
another individual is occasionally -- perhaps at very long intervals --
indispensable.
On the belief that this
is a law of nature, we can, I think, understand several large classes of facts,
such as the following, which on any other view are inexplicable. Every
hybridizer knows how unfavourable exposure to wet is to the fertilisation of a
flower, yet what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable, the
fullest freedom for the entrance of pollen from another individual will explain
this state of exposure, more especially as the plant's own anthers and pistil
generally stand so close together that self-fertilisation seems almost
inevitable. Many flowers, on the other hand, have their organs of
fructification closely enclosed, as in the great papilionaceous or pea-family;
but in several, perhaps in all, such flowers, there is a very curious
adaptation between the structure of the flower and the manner in which bees
suck the nectar; for, in doing this, they either push the flower's own pollen
on the stigma, or bring pollen from another flower. So necessary are the visits
of bees to papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits be
prevented. Now, it is scarcely possible that bees should fly from flower to
flower, and not carry pollen from one to the other, to the great good, as I
believe, of the plant. Bees will act like a camel-hair pencil, and it is quite
sufficient just to touch the anthers of one flower and then the stigma of another
with the same brush to ensure fertilisation; but it must not be supposed that
bees would thus produce a multitude of hybrids between distinct species; for if
you bring on the same brush a plant's own pollen and pollen from another
species, the former will have such a prepotent effect, that it will invariably
and completely destroy, as has been shown by Gärtner, any influence from the
foreign pollen.
When the stamens of a
flower suddenly spring towards the pistil, or slowly move one after the other towards
it, the contrivance seems adapted solely to ensure self- fertilisation; and no
doubt it is useful for this end: but, the agency of insects is often required
to cause the stamens to spring forward, as Kölreuter has shown to be the case
with the barberry; and curiously in this very genus, which seems to have a
special contrivance for self-fertilisation, it is well known that if very
closely-allied forms or varieties are planted near each other, it is hardly
possible to raise pure seedlings, so largely do they naturally cross. In many
other cases, far from there being any aids for self-fertilisation, there are
special contrivances, as I could show from the writings of C. C. Sprengel and
from my own observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a really
beautiful and elaborate contrivance by which every one of the infinitely
numerous pollen-granules are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready to receive them; and as
this flower is never visited, at least in my garden, by insects, it never sets
a seed, though by placing pollen from one flower on the stigma of another, I
raised plenty of seedlings; and whilst another species of Lobelia growing close
by, which is visited by bees, seeds freely. In very many other cases, though
there be no special mechanical contrivance to prevent the stigma of a flower
receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can
confirm, either the anthers burst before the stigma is ready for fertilisation,
or the stigma is ready before the pollen of that flower is ready, so that these
plants have in fact separated sexes, and must habitually be crossed. How
strange are these facts! How strange that the pollen and stigmatic surface of
the same flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to each other!
How simply are these facts explained on the view of an occasional cross with a
distinct individual being advantageous or indispensable!
If several varieties of
the cabbage, radish, onion, and of some other plants, be allowed to seed near
each other, a large majority, as I have found, of the seedlings thus raised
will turn out mongrels: for instance, I raised 233 seedling cabbages from some
plants of different varieties growing near each other, and of these only 78
were true to their kind, and some even of these were not perfectly true. Yet
the pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant. How, then,
comes it that such a vast number of the seedlings are mongrelised? I suspect
that it must arise from the pollen of a distinct variety having a prepotent
effect over a flower's own pollen; and that this is part of the general law of
good being derived from the intercrossing of distinct individuals of the same
species. When distinct species are crossed the case is directly the reverse,
for a plant's own pollen is always prepotent over foreign pollen; but to this
subject we shall return in a future chapter.
In the case of a
gigantic tree covered with innumerable flowers, it may be objected that pollen
could seldom be carried from tree to tree, and at most only from flower to
flower on the same tree, and that flowers on the same tree can be considered as
distinct individuals only in a limited sense. I believe this objection to be
valid, but that nature has largely provided against it by giving to trees a
strong tendency to bear flowers with separated sexes. When the sexes are
separated, although the male and female flowers may be produced on the same
tree, we can see that pollen must be regularly carried from flower to flower;
and this will give a better chance of pollen being occasionally carried from
tree to tree. That trees belonging to all Orders have their sexes more often
separated than other plants, find to be the case in this country; and at my
request Dr Hooker tabulated the trees of New Zealand, and Dr Asa Gray those of
the United States, and the result was as I anticipated. On the other hand, Dr
Hooker has recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees simply to
call attention to the subject.
Turning for a very
brief space to animals: on the land there are some hermaphrodites, as
land-mollusca and earth- worms; but these all pair. As yet I have not found a
single case of a terrestrial animal which fertilises itself. We can understand
this remarkable fact, which offers so strong a contrast with terrestrial
plants, on the view of an occasional cross being indispensable, by considering
the medium in which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects and of the
wind in the case of plants, by which an occasional cross could be effected with
terrestrial animals without the concurrence of two individuals. Of aquatic
animals, there are many self- fertilising hermaphrodites; but here currents in
the water offer an obvious means for an occasional cross. And, as in the case
of flowers, have as yet failed, after consultation with one of the highest authorities,
namely, professor Huxley, to discover a single case of an hermaphrodite animal
with the organs of reproduction so perfectly enclosed within the body, that
access from without and the occasional influence of a distinct individual can
be shown to be physically impossible. Cirripedes long appeared to me to present
a case of very great difficulty under this point of view; but I have been
enabled, by a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck
most naturalists as a strange anomaly that, in the case of both animals and
plants, species of the same family and even of the same genus, though agreeing
closely with each other in almost their whole organisation, yet are not rarely,
some of them hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as function is
concerned, becomes very small.
From these several
considerations and from the many special facts which I have collected, but
which am not here able to give, I am strongly inclined to suspect that, both in
the vegetable and animal kingdoms, an occasional intercross with a distinct
individual is a law of nature. am well aware that there are, on this view, many
cases of difficulty, some of which am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two individuals is an
obvious necessity for each birth; in many others it occurs perhaps only at long
intervals; but in none, as I suspect, can self-fertilisation go on for
perpetuity.
Circumstances
favourable to Natural Selection. This is an extremely intricate subject. A
large amount of inheritable and diversified variability is favourable, but
believe mere individual differences suffice for the work. A large number of
individuals, by giving a better chance for the appearance within any given
period of profitable variations, will compensate for a lesser amount of
variability in each individual, and is, believe, an extremely important element
of success. Though nature grants vast periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic beings
are striving, it may be said, to seize on each place in the economy of nature,
if any one species does not become modified and improved in a corresponding
degree with its competitors, it will soon be exterminated.
In man's methodical
selection, a breeder selects for some definite object, and free intercrossing
will wholly stop his work. But when many men, without intending to alter the
breed, have a nearly common standard of perfection, and all try to get and
breed from the best animals, much improvement and modification surely but
slowly follow from this unconscious process of selection, notwithstanding a
large amount of crossing with inferior animals. Thus it will be in nature; for
within a confined area, with some place in its polity not so perfectly occupied
as might be, natural selection will always tend to preserve all the individuals
varying in the right direction, though in different degrees, so as better to
fill up the unoccupied place. But if the area be large, its several districts
will almost certainly present different conditions of life; and then if natural
selection be modifying and improving a species in the several districts, there
will be intercrossing with the other individuals of the same species on the confines
of each. And in this case the effects of intercrossing can hardly be
counterbalanced by natural selection always tending to modify all the
individuals in each district in exactly the same manner to the conditions of
each; for in a continuous area, the conditions will generally graduate away
insensibly from one district to another. The intercrossing will most affect
those animals which unite for each birth, which wander much, and which do not
breed at a very quick rate. Hence in animals of this nature, for instance in
birds, varieties will generally be confined to separated countries; and this I
believe to be the case. In hermaphrodite organisms which cross only
occasionally, and likewise in animals which unite for each birth, but which
wander little and which can increase at a very rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there maintain
itself in a body, so that whatever intercrossing took place would be chiefly
between the individuals of the same new variety. A local variety when once thus
formed might subsequently slowly spread to other districts. On the above
principle, nurserymen always prefer getting seed from a large body of plants of
the same variety, as the chance of intercrossing with other varieties is thus
lessened.
Even in the case of
slow-breeding animals, which unite for each birth, we must not overrate the
effects of intercrosses in retarding natural selection; for I can bring a
considerable catalogue of facts, showing that within the same area, varieties
of the same animal can long remain distinct, from haunting different stations,
from breeding at slightly different seasons, or from varieties of the same kind
preferring to pair together.
Intercrossing plays a
very important part in nature in keeping the individuals of the same species,
or of the same variety, true and uniform in character. It will obviously thus
act far more efficiently with those animals which unite for each birth; but I
have already attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if these
take place only at long intervals, I am convinced that the young thus produced
will gain so much in vigour and fertility over the offspring from long-continued
self- fertilisation, that they will have a better chance of surviving and
propagating their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If there exist organic
beings which never intercross, uniformity of character can be retained amongst
them, as long as their conditions of life remain the same, only through the
principle of inheritance, and through natural selection destroying any which
depart from the proper type; but if their conditions of life change and they
undergo modification, uniformity of character can be given to their modified
offspring, solely by natural selection preserving the same favourable
variations.
Isolation, also, is an
important element in the process of natural selection. In a confined or
isolated area, if not very large, the organic and inorganic conditions of life
will generally be in a great degree uniform; so that natural selection will
tend to modify all the individuals of a varying species throughout the area in the
same manner in relation to the same conditions. Intercrosses, also, with the
individuals of the same species, which otherwise would have inhabited the
surrounding and differently circumstanced districts, will be prevented. But
isolation probably acts more efficiently in checking the immigration of better
adapted organisms, after any physical change, such as of climate or elevation
of the land, &c.; and thus new places in the natural economy of the country
are left open for the old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution. Lastly, isolation,
by checking immigration and consequently competition, will give time for any
new variety to be slowly improved; and this may sometimes be of importance in
the production of new species. If, however, an isolated area be very small,
either from being surrounded by barriers, or from having very peculiar physical
conditions, the total number of the individuals supported on it will
necessarily be very small; and fewness of individuals will greatly retard the
production of new species through natural selection, by decreasing the chance
of the appearance of favourable variations.
If we turn to nature to
test the truth of these remarks, and look at any small isolated area, such as
an oceanic island, although the total number of the species inhabiting it, will
be found to be small, as we shall see in our chapter on geographical
distribution; yet of these species a very large proportion are endemic, -- that
is, have been produced there, and nowhere else. Hence an oceanic island at
first sight seems to have been highly favourable for the production of new
species. But we may thus greatly deceive ourselves, for to ascertain whether a
small isolated area, or a large open area like a continent, has been most
favourable for the production of new organic forms, we ought to make the
comparison within equal times; and this we are incapable of doing.
Although do not doubt
that isolation is of considerable importance in the production of new species,
on the whole I am inclined to believe that largeness of area is of more
importance, more especially in the production of species, which will prove
capable of enduring for a long period, and of spreading widely Throughout a great
and open area, not only will there be a better chance of favourable variations
arising from the large number of individuals of the same species there
supported, but the conditions of life are infinitely complex from the large
number of already existing species; and if some of these many species become
modified and improved, others will have to be improved in a corresponding
degree or they will be exterminated. Each new form, also, as soon as it has
been much improved, will be able to spread over the open and continuous area,
and will thus come into competition with many others. Hence more new places
will be formed, and the competition to fill them will be more severe, on a
large than on a small and isolated area. Moreover, great areas, though now
continuous, owing to oscillations of level, will often have recently existed in
a broken condition, so that the good effects of isolation will generally, to a
certain extent, have concurred. Finally, I conclude that, although small
isolated areas probably have been in some respects highly favourable for the
production of new species, yet that the course of modification will generally
have been more rapid on large areas; and what is more important, that the new
forms produced on large areas, which already have been victorious over many
competitors, will be those that will spread most widely, will give rise to most
new varieties and species, and will thus play an important part in the changing
history of the organic world.
We can, perhaps, on
these views, understand some facts which will be again alluded to in our
chapter on geographical distribution; for instance, that the productions of the
smaller continent of Australia have formerly yielded, and apparently are now
yielding, before those of the larger. Europaeo-Asiatic area. Thus, also, it is
that continental productions have everywhere become so largely naturalised on
islands. On a small island, the race for life will have been less severe, and
there will have been less modification and less extermination. Hence, perhaps,
it comes that the flora of Madeira, according to Oswald Heer, resembles the
extinct tertiary flora of Europe. All fresh-water basins, taken together, make
a small area compared with that of the sea or of the land; and, consequently,
the competition between fresh-water productions will have been less severe than
elsewhere; new forms will have been more slowly formed, and old forms more
slowly exterminated. And it is in fresh water that we find seven genera of
Ganoid fishes. remnants of a once preponderant order: and in fresh water we
find some of the most anomalous forms now known in the world, as the
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain
extent orders now widely separated in the natural scale. These anomalous forms
may almost be called living fossils; they have endured to the present day, from
having inhabited a confined area, and from having thus been exposed to less
severe competition.
To sum up the
circumstances favourable and unfavourable to natural selection, as far as the
extreme intricacy of the subject permits. I conclude, looking to the future,
that for terrestrial productions a large continental area, which will probably
undergo many oscillations of level, and which consequently will exist for long
periods in a broken condition, will be the most favourable for the production
of many new forms of life, likely to endure long and to spread widely. For the
area will first have existed as a continent, and the inhabitants, at this
period numerous in individuals and kinds, will have been subjected to very
severe competition. when converted by subsidence into large separate islands,
there will still exist many individuals of the same species on each island:
intercrossing on the confines of the range of each species will thus be
checked: after physical changes of any kind, immigration will be prevented, so
that new places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. when, by renewed
elevation, the islands shall be re-converted into a continental area, there
will again be severe competition: the most favoured or improved varieties will
be enabled to spread: there will be much extinction of the less improved forms,
and the relative proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be a fair field for
natural selection to improve still further the inhabitants, and thus produce
new species.
That natural selection
will always act with extreme slowness, I fully admit. Its action depends on
there being places in the polity of nature, which can be better occupied by
some of the inhabitants of the country undergoing modification of some kind.
The existence of such places will often depend on physical changes, which are
generally very slow, and on the immigration of better adapted forms having been
checked. But the action of natural selection will probably still oftener depend
on some of the inhabitants becoming slowly modified; the mutual relations of
many of the other inhabitants being thus disturbed. Nothing can be effected,
unless favourable variations occur, and variation itself is apparently always a
very slow process. The process will often be greatly retarded by free
intercrossing. Many will exclaim that these several causes are amply sufficient
wholly to stop the action of natural selection. do not believe so. On the other
hand, I do believe that natural selection will always act very slowly, often
only at long intervals of time, and generally on only a very few of the
inhabitants of the same region at the same time. I further believe, that this
very slow, intermittent action of natural selection accords perfectly well with
what geology tells us of the rate and manner at which the inhabitants of this
world have changed.
Slow though the process
of selection may be, if feeble man can do much by his powers of artificial
selection, I can see no limit to the amount of change, to the beauty and
infinite complexity of the coadaptations between all organic beings, one with
another and with their physical conditions of life, which may be effected in
the long course of time by nature's power of selection.
Extinction. This subject
will be more fully discussed in our chapter on Geology; but it must be here
alluded to from being intimately connected with natural selection. Natural
selection acts solely through the preservation of variations in some way
advantageous, which consequently endure. But as from the high geometrical
powers of increase of all organic beings, each area is already fully stocked
with inhabitants, it follows that as each selected and favoured form increases
in number, so will the less favoured forms decrease and become rare. Rarity, as
geology tells us, is the precursor to extinction. We can, also, see that any
form represented by few individuals will, during fluctuations in the seasons or
in the number of its enemies, run a good chance of utter extinction. But we may
go further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must become
extinct. That the number of specific forms has not indefinitely increased,
geology shows us plainly; and indeed we can see reason why they should not have
thus increased, for the number of places in the polity of nature is not
indefinitely great, -- not that we have any means of knowing that any one
region has as yet got its maximum of species. probably no region is as yet
fully stocked, for at the Cape of Good Hope, where more species of plants are
crowded together than in any other quarter of the world, some foreign plants
have become naturalised, without causing, as far as we know, the extinction of
any natives.
Furthermore, the
species which are most numerous in individuals will have the best chance of
producing within any given period favourable variations. We have evidence of
this, in the facts given in the second chapter, showing that it is the common
species which afford the greatest number of recorded varieties, or incipient
species. Hence, rare species will be less quickly modified or improved within
any given period, and they will consequently be beaten in the race for life by
the modified descendants of the commoner species.
From these several
considerations I think it inevitably follows, that as new species in the course
of time are formed through natural selection, others will become rarer and
rarer, and finally extinct. The forms which stand in closest competition with
those undergoing modification and improvement, will naturally suffer most. And
we have seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms, -- varieties of the same species, and species of the same
genus or of related genera, -- which, from having nearly the same structure,
constitution, and habits, generally come into the severest competition with
each other. Consequently, each new variety or species, during the progress of
its formation, will generally press hardest on its nearest kindred, and tend to
exterminate them. We see the same process of extermination amongst our
domesticated productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds of cattle,
sheep, and other animals, and varieties of flowers, take the place of older and
inferior kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these 'were swept away by the
short-horns' (I quote the words of an agricultural writer) 'as if by some
murderous pestilence.'
Divergence of
Character. The principle, which I have designated by this term, is of high
importance on my theory, and explains, as I believe, several important facts.
In the first place, varieties, even strongly-marked ones, though having
somewhat of the character of species -- as is shown by the hopeless doubts in many
cases how to rank them -- yet certainly differ from each other far less than do
good and distinct species. Nevertheless, according to my view, varieties are
species in the process of formation, or are, as have called them, incipient
species. How, then, does the lesser difference between varieties become
augmented into the greater difference between species? That this does
habitually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas varieties, the
supposed prototypes and parents of future well-marked species, present slight
and ill-defined differences. Mere chance, as we may call it, might cause one
variety to differ in some character from its parents, and the offspring of this
variety again to differ from its parent in the very same character and in a
greater degree; but this alone would never account for so habitual and large an
amount of difference as that between varieties of the same species and species
of the same genus.
As has always been my
practice, let us seek light on this head from our domestic productions. We
shall here find something analogous. A fancier is struck by a pigeon having a
slightly shorter beak; another fancier is struck by a pigeon having a rather
longer beak; and on the acknowledged principle that 'fanciers do not and will
not admire a medium standard, but like extremes,' they both go on (as has
actually occurred with tumbler-pigeons) choosing and breeding from birds with
longer and longer beaks, or with shorter and shorter beaks. Again, we may
suppose that at an early period one man preferred swifter horses; another
stronger and more bulky horses. The early differences would be very slight; in
the course of time, from the continued selection of swifter horses by some
breeders, and of variety and then several mixed varieties of wheat have been
sown on equal spaces of ground. Hence, if any one species of grass were to go
on varying, and those varieties were continually selected which differed from
each other in at all the same manner as distinct species and genera of grasses
differ from each other, a greater number of individual plants of this species
of grass, including its modified descendants, would succeed in living on the
same piece of ground. And we well know that each species and each variety of
grass is annually sowing almost countless seeds; and thus, as it may be said,
is striving its utmost to increase its numbers. Consequently, I cannot doubt
that in the course of many thousands of generations, the most distinct
varieties of any one species of grass would always have the best chance of
succeeding and of increasing in numbers, and thus of supplanting the less
distinct varieties; and varieties, when rendered very distinct from each other,
take the rank of species.
The truth of the
principle, that the greatest amount of life can be supported by great
diversification of structure, is seen under many natural circumstances. In an
extremely small area, especially if freely open to immigration, and where the
contest between individual and individual must be severe, we always find great
diversity in its inhabitants. For instance, found that a piece of turf, three
feet by four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to eighteen
genera and to eight orders, which shows how much these plants differed from
each other. So it is with the plants and insects on small and uniform islets;
and so in small ponds of fresh water. Farmers find that they can raise most
food by a rotation of plants belonging to the most different orders: nature
follows what may be called a simultaneous rotation. Most of the animals and
plants which live close round any small piece of ground, could live on it
(supposing it not to be in any way peculiar in its nature), and may be said to
be striving to the utmost to live there; but, it is seen, that where they come
into the closest competition with each other, the advantages of diversification
of structure, with the accompanying differences of habit and constitution,
determine that the inhabitants, which thus jostle each other most closely,
shall, as a general rule, belong to what we call different genera and orders.
The same principle is
seen in the naturalisation of plants through man's agency in foreign lands. It
might have been expected that the plants which have succeeded in becoming
naturalised in any land would generally have been closely allied to the
indigenes; for these are commonly looked at as specially created and adapted
for their own country. It night, also, perhaps have been expected that
naturalised plants would have belonged to a few groups more especially adapted
to certain stations in their new homes. But the case is very different; and Alph.
De Candolle has wall remarked in his great and admirable work, that floras gain
by naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single instance:
in the last edition of Dr Asa Gray's 'Manual of the Flora of the Northern
United States,' 260 naturalised plants are enumerated, and these belong to 162
genera. We thus see that these naturalised plants are of a highly diversified
nature. They differ, moreover, to a large extent from the indigenes, for out of
the 162 genera, no less than l00 genera are not there indigenous, and thus a
large proportional addition is made to the genera of these States.
By considering the
nature of the plants or animals which have struggled successfully with the
indigenes of any country, and have there become naturalised, we can gain some
crude idea in what manner some of the natives would have had to be modified, in
order to have gained an advantage over the other natives; and we may, I think, at
least safely infer that diversification of structure, amounting to new generic
differences, would have been profitable to them.
The advantage of
diversification in the inhabitants of the same region is, in fact, the same as
that of the physiological division of labour in the organs of the same
individual body -- a subject so well elucidated by Milne Edwards. No
physiologist doubts that a stomach by being adapted to digest vegetable matter
alone, or flesh alone, draws most nutriment from these substances. So in the
general economy of any land, the more widely and perfectly the animals and
plants are diversified for different habits of life, so will a greater number
of individuals be capable of there supporting themselves. A set of animals,
with their organisation but little diversified, could hardly compete with a set
more perfectly diversified in structure. It may be doubted, for instance,
whether the Australian marsupials, which are divided into groups differing but
little from each other, and feebly representing, as Mr Waterhouse and others
have remarked, our carnivorous, ruminant, and rodent mammals, could
successfully compete with these well-pronounced orders. In the Australian
mammals, we see the process of diversification in an early and incomplete stage
of development.
After the foregoing
discussion, which ought to have been much amplified, we may, I think, assume
that the modified descendants of any one species will succeed by so much the better
as they become more diversified in structure, and are thus enabled to encroach
on places occupied by other beings. Now let us see how this principle of great
benefit being derived from divergence of character, combined with the
principles of natural selection and of extinction, will tend to act.
The accompanying
diagram will aid us in understanding this rather perplexing subject. Let A to L
represent the species of a genus large in its own country; these species are
supposed to resemble each other in unequal degrees, as is so generally the case
in nature, and as is represented in the diagram by the letters standing at
unequal distances. have said a large genus, because we have seen in the second
chapter, that on an average more of the species of large genera vary than of
small genera; and the varying species of the large genera present a greater
number of varieties. We have, also, seen that the species, which are the
commonest and the most widely-diffused, vary more than rare species with
restricted ranges. Let (A) be a common, widely-diffused, and varying species,
belonging to a genus large in its own country. The little fan of diverging
dotted lines of unequal lengths proceeding from (A), may represent its varying
off-spring. The variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear simultaneously, but
often after long intervals of time; nor are they all supposed to endure for
equal periods. Only those variations which are in some way profitable will be
preserved or naturally selected. And here the importance of the principle of
benefit being derived from divergence of character comes in; for this will
generally lead to the most different or divergent variations (represented by
the outer dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there marked by
a small numbered letter, a sufficient amount of variation is supposed to have
been accumulated to have formed a fairly well-marked variety, such as would be
thought worthy of record in a systematic work.
The intervals between
the horizontal lines in the diagram, may represent each a thousand generations;
but it would have been better if each had represented ten thousand generations.
After a thousand generations, species (A) is supposed to have produced two
fairly well-marked varieties, namely a/1 and m/1. These two varieties will
generally continue to be exposed to the same conditions which made their
parents variable, and the tendency to variability is in itself hereditary,
consequently they will tend to vary, and generally to vary in nearly the same
manner as their parents varied. Moreover, these two varieties, being only
slightly modified forms, will tend to inherit those advantages which made their
common parent (A) more numerous than most of the other inhabitants of the same
country; they will likewise partake of those more general advantages which made
the genus to which the parent-species belonged, a large genus in its own
country. And these circumstances we know to be favourable to the production of
new varieties.
If, then, these two
varieties be variable, the most divergent of their variations will generally be
preserved during the next thousand generations. And after this interval,
variety a[s2]s is supposed in the diagram to have produced variety a 2, which
will, owing to the principle of divergence, differ more from (A) than did
variety a1. Variety m1 is supposed to have produced two varieties, namely m 2
and s 2, differing from each other, and more considerably from their common
parent (A). We may continue the process by similar steps for any length of
time; some of the varieties, after each thousand generations, producing only a
single variety, but in a more and more modified condition, some producing two
or three varieties, and some failing to produce any. Thus the varieties or
modified descendants, proceeding from the common parent (A), will generally go
on increasing in number and diverging in character. In the diagram the process
is represented up to the ten-thousandth generation, and under a condensed and
simplified form up to the fourteen-thousandth generation.
But must here remark
that do not suppose that the process ever goes on so regularly as is
represented in the diagram, though in itself made somewhat irregular. I am far
from thinking that the most divergent varieties will invariably prevail and
multiply:. a medium form may often long endure, and may or may not produce more
than one modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not perfectly
occupied by other beings; and this will depend on infinitely complex relations.
But as a general rule, the more diversified in structure the descendants from
any one species can be rendered, the more places they will be enabled to seize
on, and the more their modified progeny will be increased. In our diagram the
line of succession is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently distinct to be
recorded as varieties. But these breaks are imaginary, and might have been
inserted anywhere, after intervals long enough to have allowed the accumulation
of a considerable amount of divergent variation.
As all the modified
descendants from a common and widely-diffused species, belonging to a large
genus, will tend to partake of the same advantages which made their parent
successful in life, they will generally go on multiplying In number as well as
diverging in character: this is represented in the diagram by the several
divergent branches proceeding from (A). The modified offspring from the later
and more highly improved branches in the lines of descent, will, it is probable,
often take the place of, and so destroy, the earlier and less improved
branches: this is represented in the diagram by some of the lower branches not
reaching to the upper horizontal lines. In some cases I do not doubt that the
process of modification will be confined to a single line of descent, and the
number of the descendants will not be increased; although the amount of
divergent modification may have been increased in the successive generations.
This case would be represented in the diagram, if all the lines proceeding from
(A) were removed, excepting that from a1 to a[s10]s In the same way, for
instance, the English race-horse and English pointer have apparently both gone
on slowly diverging in character from their original stocks, without either
having given off any fresh branches or races.
After ten thousand
generations, species (A) is supposed to have produced three forms, a[s10]s
f[s10]s, and m[s10]s, which, from having diverged in character during the
successive generations, will have come to differ largely, but perhaps
unequally, from each other and from their common parent. if we suppose the
amount of change between each horizontal line in our diagram to be excessively
small, these three forms may still be only well-marked varieties; or they may
have arrived at the doubtful category of sub- species; but we have only to
suppose the steps in the process of modification to be more numerous or greater
in amount, to convert these three forms into well-defined species: thus the
diagram illustrates the steps by which the small differences distinguishing
varieties are increased into the larger differences distinguishing species. By
continuing the same process for a greater number of generations (as shown in
the diagram in a condensed and simplified manner), we get eight species, marked
by the letters between a[s14]s and m[s14]s, all descended from (A). Thus, as I
believe, species are multiplied and genera are formed.
In a large genus it is
probable that more than one species would vary. In the diagram I have assumed
that a second species (I) has produced, by analogous steps, after ten thousand
generations, either two well-marked varieties (w[s10]s and z[s10]s) or two
species, according to the amount of change supposed to be represented between the
horizontal lines. After fourteen thousand generations, six new species, marked
by the letters n[s14]s to z[s14]s, are supposed to have been produced. In each
genus, the species, which are already extremely different in character, will
generally tend to produce the greatest number of modified descendants; for
these will have the best chance of filling new and widely different places in
the polity of nature: hence in the diagram I have chosen the extreme species
(A), and the nearly extreme species (I), as those which have largely varied,
and have given rise to new varieties and species. The other nine species
(marked by capital letters) of our original genus, may for a long period
continue transmitting unaltered descendants; and this is shown in the diagram
by the dotted lines not prolonged far upwards from want of space.
But during the process
of modification, represented in the diagram, another of our principles, namely
that of extinction, will have played an important part. As in each fully
stocked country natural selection necessarily acts by the selected form having
some advantage in the struggle for life over other forms, there will be a
constant tendency in the improved descendants of any one species to supplant
and exterminate in each stage of descent their predecessors and their original
parent. For it should be remembered that the competition will generally be most
severe between those forms which are most nearly related to each other in
habits, constitution, and structure. Hence all the intermediate forms between
the earlier and later states, that is between the less and more improved state
of a species, as well as the original parent-species itself, will generally
tend to become extinct. So it probably will be with many whole collateral lines
of descent, which will be conquered by later and improved lines of descent. If,
however, the modified offspring of a species get into some distinct country, or
become quickly adapted to some quite new station, in which child and parent do
not come into competition, both may continue to exist.
If then our diagram be
assumed to represent a considerable amount of modification, species (A) and all
the earlier varieties will have become extinct, having been replaced by eight
new species (a[s14]s to m[s14]s); and (I) will have been replaced by six
(n[s14]s to z[s14]s) new species.
But we may go further
than this. The original species of our genus were supposed to resemble each
other in unequal degrees, as is so generally the case in nature; species (A)
being more nearly related to B, C, and D, than to the other species; and
species (I) more to G, H, K, L, than to the others. These two species (A) and
(I), were also supposed to be very common and widely diffused species, so that
they must originally have had some advantage over most of the other species of
the genus. Their modified descendants, fourteen in number at the
fourteen-thousandth generation, will probably have inherited some of the same
advantages: they have also been modified and improved in a diversified manner
at each stage of descent, so as to have become adapted to many related places
in the natural economy of their country. It seems, therefore, to me extremely
probable that they will have taken the places of, and thus exterminated, not
only their parents (A) and (I), but likewise some of the original species which
were most nearly related to their parents. Hence very few of the original
species will have transmitted offspring to the fourteen-thousandth generation.
We may suppose that only one (F), of the two species which were least closely
related to the other nine original species, has transmitted descendants to this
late stage of descent.
The new species in our
diagram descended from the original eleven species, will now be fifteen in
number. Owing to the divergent tendency of natural selection, the extreme
amount of difference in character between species a[s14]s and z[s14]s will be
much greater than that between the most different of the original eleven
species. The new species, moreover, will be allied to each other in a widely
different manner. Of the eight descendants from(A) the three marked a[s14]s,
q[s14]s, p[s14]s, will be nearly related from having recently branched off from
a[s14]s; b[s14]s and f[s14]s, from having diverged at an earlier period from
a[s5]s, will be in some degree distinct from the three first-named species; and
lastly, o[s14]s, e[s14]s, and m[s14]s, will be nearly related one to the other,
but from having diverged at the first commencement of the process of
modification, will be widely different from the other five species, and may
constitute a sub-genus or even a distinct genus. The six descendants from (I)
will form two sub-genera or even genera. But as the original species (I)
differed largely from (A),standing nearly at the extreme points of the original
genus, the six descendants from (d will, owing to inheritance, differ
considerably from the eight descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The intermediate
species, also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct, and have
left no descendants. Hence the six new species descended from d), and the eight
descended from (A), will have to be ranked as very distinct genera, or even as
distinct sub-families.
Thus it is, as I
believe, that two or more genera are produced by descent, with modification,
from two or more species of the same genus. And the two or more parent- species
are supposed to have descended from some one species of an earlier genus. In
our diagram, this is indicated by the broken lines, beneath the capital
letters, converging in sub-branches downwards towards a single point; this
point representing a single species, the supposed single parent of our several
new sub-genera and genera.
It is worth while to
reflect for a moment on the character of the new species F[s14]s, which is
supposed not to have diverged much in character, but to have retained the form
of (F), either unaltered or altered only in a slight degree. In this case, its
affinities to the other fourteen new species will be of a curious and
circuitous nature. Having descended from a form which stood between the two
parent-species (A) and (q, now supposed to be extinct and unknown, it will be
in some degree intermediate in character between the two groups descended from
these species. But as these two groups have gone on diverging in character from
the type of their parents, the new species (F[s14]s) will not be directly
intermediate between them, but rather between types of the two groups; and
every naturalist will be able to bring some such case before his mind.
In the diagram, each
horizontal line has hitherto been supposed to represent a thousand generations,
but each may represent a million or hundred million generations, and likewise a
section of the successive strata of the earth's crust including extinct
remains. We shall, when we come to our chapter on Geology, have to refer again
to this subject, and think we shall then see that the diagram throws light on
the affinities of extinct beings, which, though generally belonging to the same
orders, or families, or genera, with those now living, yet are often, in some
degree, intermediate in character between existing groups; and we can
understand this fact, for the extinct species lived at very ancient epochs when
the branching lines of descent had diverged less.
I see no reason to
limit the process of modification, as now explained, to the formation of genera
alone. If, in our diagram, we suppose the amount of change represented by each
successive group of diverging dotted lines to be very great, the forms marked
a2[s14]s to p[s14]s, those marked b[s14]s and f[s14]s, and those marked o[s14]s
to m[s14]s, will form three very distinct genera. We shall also have two very
distinct genera descended from (I) and as these latter two genera, both from
continued divergence of character and from inheritance from a different parent,
will differ widely from the three genera descended from (A), the two little
groups of genera will form two distinct families, or even orders, according to
the amount of divergent modification supposed to be represented in the diagram.
And the two new families, or orders, will have descended from two species of
the original genus; and these two species are supposed to have descended from
one species of a still more ancient and unknown genus.
We have seen that in
each country it is the species of the larger genera which oftenest present
varieties or incipient species. This, indeed, might have been expected; for as
natural selection acts through one form having some advantage over other forms
in the struggle for existence, it will chiefly act on those which already have
some advantage; and the largeness of any group shows that its species have
inherited from a common ancestor some advantage in common. Hence, the struggle
for the production of new and modified descendants, will mainly lie between the
larger groups, which are all trying to increase in number. One large group will
slowly conquer another large group, reduce its numbers, and thus lessen its
chance of further variation and improvement. Within the same large group, the
later and more highly perfected sub-groups, from branching out and seizing on
many new places in the polity of Nature, will constantly tend to supplant and
destroy the earlier and less improved sub-groups. Small and broken groups and
sub-groups will finally tend to disappear. Looking to the future, we can
predict that the groups of organic beings which are now large and triumphant,
and which are least broken up, that is, which as yet have suffered least
extinction, will for a long period continue to increase. But which groups will
ultimately prevail, no man can predict; for we well know that many groups,
formerly most extensively developed, have now become extinct. Looking still
more remotely to the future, we may predict that, owing to the continued and
steady increase of the larger groups, a multitude of smaller groups will become
utterly extinct, and leave no modified descendants; and consequently that of
the species living at any one period, extremely few will transmit descendants
to a remote futurity. I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of the more
ancient species having transmitted descendants, and on the view of all the
descendants of the same species making a class, we can understand how it is
that there exist but very few classes in each main division of the animal and
vegetable kingdoms. Although extremely few of the most ancient species may now
have living and modified descendants, yet at the most remote geological period,
the earth may have been as well peopled with many species of many genera,
families, orders, and classes, as at the present day.
Summary of Chapter. If
during the long course of ages and under varying conditions of life, organic
beings vary at all in the several parts of their organisation, and think this
cannot be disputed; if there be, owing to the high geometrical powers of
increase of each species, at some age, season, or year, a severe struggle for
life, and this certainly cannot be disputed; then, considering the infinite
complexity of the relations of all organic beings to each other and to their
conditions of existence, causing an infinite diversity in structure,
constitution, and habits, to be advantageous to them, think it would be a most
extraordinary fact if no variation ever had occurred useful to each being's own
welfare, in the same way as so many variations have occurred useful to man. But
if variations useful to any organic being do occur, assuredly individuals thus
characterised will have the best chance of being preserved in the struggle for
life; and from the strong principle of inheritance they will tend to produce
offspring similarly characterised. This principle of preservation, have called,
for the sake of brevity, Natural Selection. Natural selection, on the principle
of qualities being inherited at corresponding ages, can modify the egg, seed,
or young, as easily as the adult. Amongst many animals, sexual selection will
give its aid to ordinary selection, by assuring to the most vigorous and best
adapted males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles with other males.
Whether natural
selection has really thus acted in nature, in modifying and adapting the
various forms of life to their several conditions and stations, must be judged
of by the general tenour and balance of evidence given in the following
chapters. But we already see how it entails extinction; and how largely
extinction has acted in the world's history, geology plainly declares. Natural
selection, also, leads to divergence of character; for more living beings can
be supported on the same area the more they diverge in structure, habits, and
constitution, of which we see proof by looking at the inhabitants of any small
spot or at naturalised productions. Therefore during the modification of the
descendants of any one species, and during the incessant struggle of all
species to increase in numbers, the more diversified these descendants become,
the better will be their chance of succeeding in the battle of life. Thus the
small differences distinguishing varieties of the same species, will steadily
tend to increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is
the common, the widely-diffused, and widely-ranging species, belonging to the
larger genera, which vary most; and these will tend to transmit to their
modified off-spring that superiority which now makes them dominant in their own
countries. Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate forms of
life. On these principles, I believe, the nature of the affinities of all
organic beings may be explained. It is a truly wonderful fact -- the wonder of
which we are apt to overlook from familiarity -- that a;; animals and all
plants throughout all time and space should be related to each other in group
subordinate to group, in the manner which we everywhere behold -- namely,
varieties of the same species most closely related together, species of the
same genus less closely and unequally related together, forming sections and
sub-genera, species of distinct genera much less closely related, and genera
related in different degrees, forming sub-families, families, orders, sub-
classes, and classes. The several subordinate groups in any class cannot be
ranked in a single file, but seem rather to be clustered round points, and
these round other points, and so on in almost endless cycles. On the view that
each species has been independently created, I can see no explanation of this
great fact in the classification of all organic beings; but, to the best of my
judgment, it is explained through inheritance and the complex action of natural
selection, entailing extinction and divergence of character, as we have seen
illustrated in the diagram.
The affinities of all
the beings of the same class have sometimes been represented by a great tree. I
believe this simile largely speaks the truth. The green and budding twigs may
represent existing species; and those produced during each former year may
represent the long succession of extinct species. At each period of growth all
the growing twigs have tried to branch out on all sides, and to overtop and
kill the surrounding twigs and branches, in the same manner as species and
groups of species have tried to overmaster other species in the great battle
for life. The limbs divided into great branches, and these into lesser and lesser
branches, were themselves once, when the tree was small, budding twigs; and
this connexion of the former and present buds by ramifying branches may well
represent the classification of all extinct and living species in groups
subordinate to groups. Of the many twigs which flourished when the tree was a
mere bush, only two or three, now grown into great branches, yet survive and
bear all the other branches; so with the species which lived during long-past
geological periods, very few now have living and modified descendants. From the
fist growth of the tree, many a limb and branch has decayed and dropped off;,
and these lost branches of various sizes may represent those whole orders,
families, and genera which have now no living representatives, and which are
known to us only from having been found in a fossil state. As we here and there
see a thin straggling branch springing from a fork low down in a tree, and
which by some chance has been favoured and is still alive on its summit, so we
occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in
some small degree connects by its affinities two large branches of life, and
which has apparently been saved from fatal competition by having inhabited a
protected station. As buds give rise by growth to fresh buds, and these, if
vigorous, branch out and overtop on all sides many a feebler branch, so by
generation I believe it has been with the great Tree of Life, which fills with
its dead and broken branches the crust of the earth, and covers the surface
with its ever branching and beautiful ramifications.
Effects of external conditions - Use and disuse, combined with natural
selection; organs of flight and of vision - Acclimatisation - Correlation of
growth - Compensation and economy of growth - False correlations - Multiple,
rudimentary, and lowly organised structures variable - Parts developed in an
unusual manner are highly variable: specific character more variable than
generic: secondary sexual characters variable - Species of the same genus vary
in an analogous manner - Reversions to long-lost characters - Summary I HAVE hitherto sometimes spoken as if the
variations -- so common and multiform in organic beings under domestication,
and in a lesser degree in those in a state of nature -- had been due to chance.
This, of course, is a wholly incorrect expression, but it serves to acknowledge
plainly our ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to produce
individual differences, or very slight deviations of structure, as to make the
child like its parents. But the much greater variability, as well as the
greater frequency of monstrosities, under domestication or cultivation, than
under nature, leads me to believe that deviations of structure are in some way
due to the nature of the conditions of life, to which the parents and their
more remote ancestors have been exposed during several generations. I have
remarked in the first chapter -- but a long catalogue of facts which cannot be
here given would be necessary to show the truth of the remark -- that the
reproductive system is eminently susceptible to changes in the conditions of
life; and to this system being functionally disturbed in the parents, I chiefly
attribute the varying or plastic condition of the offspring. The male and
female sexual elements seem to be affected before that union takes place which
is to form a new being. In the case of 'sporting, plants, the bud, which in its
earliest condition does not apparently differ essentially from an ovule, is
alone affected. But why, because the reproductive system is disturbed, this or
that part should vary more or less, we are profoundly ignorant. Nevertheless,
we can here and there dimly catch a faint ray of light, and we may feel sure
that there must be some cause for each deviation of structure, however slight.
How much direct effect
difference of climate, food, &c., produces on any being is extremely
doubtful. My impression is, that the effect is extremely small in the case of
animals, but perhaps rather more in that of plants. We may, at least, safely
conclude that such influences cannot have produced the many striking and
complex co- adaptations of structure between one organic being and another,
which we see everywhere throughout nature. Some little influence may be
attributed to climate, food, &c.: thus, E. Forbes speaks confidently that
shells at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from greater
depths. Gould believes that birds of the same species are more brightly
coloured under a clear atmosphere, than when living on islands or near the
coast. So with insects, Wollaston is convinced that residence near the sea
affects their colours. Moquin-Tandon gives a list of plants which when growing
near the sea-shore have their leaves in some degree fleshy, though not
elsewhere fleshy. Several other such cases could be given.
The fact of varieties
of one species, when they range into the zone of habitation of other species,
often acquiring in a very slight degree some of the characters of such species,
accords with our view that species of all kinds are only well-marked and
permanent varieties. Thus the species of shells which are confined to tropical
and shallow seas are generally brighter-coloured than those confined to cold
and deeper seas. The birds which are confined to continents are, according to
Mr Gould, brighter-coloured than those of islands. The insect-species confined
to sea-coasts, as every collector knows, are often brassy or lurid. plants
which live exclusively on the sea-side are very apt to have fleshy leaves. He
who believes in the creation of each species, will have to say that this shell,
for instance, was created with bright colours for a warm sea; but that this
other shell became bright-coloured by variation when it ranged into warmer or
shallower waters.
When a variation is of
the slightest use to a being, we cannot tell how much of it to attribute to the
accumulative action of natural selection, and how much to the conditions of
life. Thus, it is well known to furriers that animals of the same species have
thicker and better fur the more severe the climate is under which they have
lived; but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during many
generations, and how much to the direct action of the severe climate? for it
would appear that climate has some direct action on the hair of our domestic
quadrupeds.
Instances could be
given of the same variety being produced under conditions of life as different
as can well be conceived; and, on the other hand, of different varieties being
produced from the same species under the same conditions. Such facts show how indirectly
the conditions of life must act. Again, innumerable instances are known to
every naturalist of species keeping true, or not varying at all, although
living under the most opposite climates. Such considerations as these incline
me to lay very little weight on the direct action of the conditions of life.
Indirectly, as already remarked, they seem to play an important part in
affecting the reproductive system, and in thus inducing variability; and
natural selection will then accumulate all profitable variations, however
slight, until they become plainly developed and appreciable by us.
Effects of Use and
Disuse. From the facts alluded to in the first chapter, I think there can be
little doubt that use in our domestic animals strengthens and enlarges certain
parts, and disuse diminishes them; and that such modifications are inherited.
Under free nature, we can have no standard of comparison, by which to judge of
the effects of long-continued use or disuse, for we know not the parent-forms;
but many animals have structures which can be explained by the effects of
disuse. As professor Owen has remarked, there is no greater anomaly In nature
than a bird that cannot fly; yet there are several in this state. The
logger-headed duck of South America can only flap along the surface of the
water, and has its wings in nearly the same condition as the domestic Aylesbury
duck. As the larger ground-feeding birds seldom take flight except to escape
danger, I believe that the nearly wingless condition of several birds, which
now inhabit or have lately inhabited several oceanic islands, tenanted by no
beast of prey, has been caused by disuse. The ostrich indeed inhabits
continents and is exposed to danger from which it cannot escape by flight, but
by kicking it can defend itself from enemies, as well as any of the smaller
quadrupeds. We may imagine that the early progenitor of the ostrich had habits
like those of a bustard, and that as natural selection increased in successive
generations the size and weight of its body, its legs were used more, and its
wings less, until they became incapable of flight.
Kirby has remarked (and
I have observed the same fact) that the anterior tarsi, or feet, of many male
dung-feeding beetles are very often broken off; he examined seventeen specimens
in his own collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described as not
having them. In some other genera they are present, but in a rudimentary
condition. In the Ateuchus or sacred beetle of the Egyptians, they are totally
deficient. There is not sufficient evidence to induce us to believe that
mutilations are ever inherited; and I should prefer explaining the entire
absence of the anterior tarsi in Ateuchus, and their rudimentary condition in
some other genera, by the long- continued effects of disuse in their
progenitors; for as the tarsi are almost always lost in many dung-feeding
beetles, they must be lost early in life, and therefore cannot be much used by these
insects.
In some cases we might
easily put down to disuse modifications of structure which are wholly, or
mainly, due to natural selection. Mr Wollaston has discovered the remarkable
fact that 200 beetles, out of the 550 species inhabiting Madeira, are so far
deficient in wings that they cannot fly; and that of the twenty-nine endemic
genera, no less than twenty-three genera have all their species in this
condition! Several facts, namely, that beetles In many parts of the world are
very frequently blown to sea and perish; that the beetles in Madeira, as
observed by Mr Wollaston, lie much concealed, until the wind lulls and the sun
shines; that the proportion of wingless beetles is larger on the exposed
Dezertas than in Madeira itself; and especially the extraordinary fact, so
strongly insisted on by Mr Wollaston, of the almost entire absence of certain
large groups of beetles, elsewhere excessively numerous, and which groups have
habits of life almost necessitating frequent flight; -- these several considerations
have made me believe that the wingless condition of so many Madeira beetles is
mainly due to the action of natural selection, but combined probably with
disuse. For during thousands of successive generations each individual beetle
which flew least, either from its wings having been ever so little less
perfectly developed or from indolent habit, will have had the best chance of
surviving from not being blown out to sea; and, on the other hand, those
beetles which most readily took to flight will oftenest have been blown to sea
and thus have been destroyed.
The insects in Madeira
which are not ground-feeders, and which, as the flower-feeding coleoptera and
lepidoptera, must habitually use their wings to gain their subsistence, have,
as Mr Wollaston suspects, their wings not at all reduced, but even enlarged.
This is quite compatible with the action of natural selection. For when a new
insect first arrived on the island, the tendency of natural selection to
enlarge or to reduce the wings, would depend on whether a greater number of
individuals were saved by successfully battling with the winds, or by giving up
the attempt and rarely or never flying. As with mariners ship-wrecked near a
coast, it would have been better for the good swimmers if they had been able to
swim still further, whereas it would have been better for the bad swimmers if
they had not been able to swim at all and had stuck to the wreck.
The eyes of moles and
of some burrowing rodents are rudimentary in size, and in some cases are quite
covered up by skin and fur. This state of the eyes is probably due to gradual
reduction from disuse, but aided perhaps by natural selection. In South
America, a burrowing rodent, the tuco- tuco, or Ctenomys, Is even more
subterranean in its habits than the mole; and I was assured by a Spaniard, who
had often caught them, that they were frequently blind; one which I kept alive
was certainly in this condition, the cause, as appeared on dissection, having
been inflammation of the nictitating membrane. As frequent inflammation of the
eyes must be injurious to any animal, and as eyes are certainly not
indispensable to animals with subterranean habits, a reduction in their size
with the adhesion of the eyelids and growth of fur over them, might in such case
be an advantage; and if so, natural selection would constantly aid the effects
of disuse.
It is well known that
several animals, belonging to the most different classes, which inhabit the
caves of Styria and of Kentucky, are blind. In some of the crabs the foot-
stalk for the eye remains, though the eye is gone; the stand for the telescope
is there, though the telescope with its glasses has been lost. As it is
difficult to imagine that eyes, though useless, could be in any way injurious
to animals living in darkness, I attribute their loss wholly to disuse. In one
of the blind animals, namely, the cave-rat, the eyes are of immense size; and
professor Silliman thought that it regained, after living some days in the
light, some slight power of vision. In the same manner as in Madeira the wings
of some of the insects have been enlarged, and the wings of others have been
reduced by natural selection aided by use and disuse, so in the case of the
cave-rat natural selection seems to have struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other inhabitants
of the caves, disuse by itself seems to have done its work.
It is difficult to
imagine conditions of life more similar than deep limestone caverns under a
nearly similar climate; so that on the common view of the blind animals having
been separately created for the American and European caverns, close similarity
in their organisation and affinities might have been expected; but, as Schiodte
and others have remarked, this is not the case, and the cave- insects of the
two continents are riot more closely allied than might have been anticipated
from the general resemblance of the other inhabitants of North America and
Europe. On my view we must suppose that American animals, having ordinary
powers of vision, slowly migrated by successive generations from the outer
world into the deeper and deeper recesses of the Kentucky caves, as did
European animals into the caves of Europe. We have some evidence of this
gradation of habit; for, as Schiodte remarks, 'animals not far remote from
ordinary forms, prepare the transition from light to darkness. Next follow
those that are constructed for twilight; and, last of all, those destined for
total darkness.' By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have more or less
perfectly obliterated its eyes, and natural selection will often have effected
other changes, such as an increase in the length of the antennae or palpi, as a
compensation for blindness. Notwithstanding such modifications, we might expect
still to see in the cave-animals of America, affinities to the other
inhabitants of that continent, and in those of Europe, to the inhabitants of
the European continent. And this is the case with some of the American
cave-animals, as I hear from professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding country. It
would be most difficult to give any rational explanation of the affinities of
the blind cave-animals to the other inhabitants of the two continents on the
ordinary view of their independent creation. That several of the inhabitants of
the caves of the Old and New Worlds should be closely related, we might expect
from the well-known relationship of most of their other productions. Far from
feeling any surprise that some of the cave-animals should be very anomalous, as
Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is the
case with the blind proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved, owing to
the less severe competition to which the inhabitants of these dark abodes will
probably have been exposed.
Acclimatisation. Habit
is hereditary with plants, as in the period of flowering, in the amount of rain
requisite for seeds to germinate, in the time of sleep, &c., and this leads
me to say a few words on acclimatisation. As it is extremely common for species
of the same genus to inhabit very hot and very cold countries, and as I believe
that all the species of the same genus have descended from a single parent, if
this view be correct, acclimatisation must be readily effected during
long-continued descent. It is notorious that each species is adapted to the
climate of its own home: species from an arctic or even from a temperate region
cannot endure a tropical climate, or conversely. So again, many succulent
plants cannot endure a damp climate. But the degree of adaptation of species to
the climates under which they live is often overrated. We may infer this from
our frequent inability to predict whether or not an imported plant will endure
our climate, and from the number of plants and animals brought from warmer
countries which here enjoy good health. We have reason to believe that species
in a state of nature are limited in their ranges by the competition of other
organic beings quite as much as, or more than, by adaptation to particular
climates. But whether or not the adaptation be generally very close, we have
evidence, in the case of some few plants, of their becoming, to a certain
extent, naturally habituated to different temperatures, or becoming
acclimatised: thus the pines and rhododendrons, raised from seed collected by
Dr Hooker from trees growing at different heights on the Himalaya were found in
this country to possess different constitutional powers of resisting cold. Mr
Thwaites informs me that he has observed similar facts in Ceylon, and analogous
observations have been made by Mr H. C. Watson on European species of plants
brought from the Azores to England. In regard to animals, several authentic
cases could be given of species within historical times having largely extended
their range from warmer to cooler latitudes, and conversely; but we do not
positively know that these animals were strictly adapted to their native
climate, but in all ordinary cases we assume such to be the case; nor do we
know that they have subsequently become acclimatised to their new homes.
As I believe that our
domestic animals were originally chosen by uncivilised man because they were
useful and bred readily under confinement, and not because they were
subsequently found capable of far-extended transportation, I think the common
and extraordinary capacity in our domestic animals of not only withstanding the
most different climates but of being perfectly fertile (a far severer test)
under them, may be used as an argument that a large proportion of other
animals, now in a state of nature, could easily be brought to bear widely
different climates. We must not, however, push the foregoing argument too far,
on account of the probable origin of some of our domestic animals from several
wild stocks: the blood, for instance, of a tropical and arctic wolf or wild dog
may perhaps be mingled in our domestic breeds. The rat and mouse cannot be
considered as domestic animals, but they have been transported by man to many
parts of the world, and now have a far wider range than any other rodent,
living free under the cold climate of Faroe in the north and of the Falklands
in the south, and on many islands in the torrid zones. Hence I am inclined to
look at adaptation to any special climate as a quality readily grafted on an
innate wide flexibility of constitution, which is common to most animals. On
this view, the capacity of enduring the most different climates by man himself
and by his domestic animals, and such facts as that former species of the
elephant and rhinoceros were capable of enduring a glacial climate, whereas the
living species are now all tropical or sub-tropical in their habits, ought not
to be looked at as anomalies, but merely as examples of a very common
flexibility of constitution, brought, under peculiar circumstances, into play.
How much of the
acclimatisation of species to any peculiar climate is due to mere habit, and
how much to the natural selection of varieties having different innate
constitutions, and how much to both means combined, is a very obscure question.
That habit or custom has some influence I must believe, both from analogy, and
from the incessant advice given in agricultural works, even in the ancient
Encyclopaedias of China, to be very cautious in transposing animals from one
district to another; for it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially fitted for
their own districts: the result must, I think, be due to habit. On the other
hand, I can see no reason to doubt that natural selection will continually tend
to preserve those individuals which are born with constitutions best adapted to
their native countries. In treatises on many hinds of cultivated plants,
certain varieties are said to withstand certain climates better than others:
this is very strikingly shown in works on fruit trees published in the United
States, in which certain varieties are habitually recommended for the northern,
and others for the southern States; and as most of these varieties are of
recent origin, they cannot owe their constitutional differences to habit. The
case of the jerusalem artichoke, which is never propagated by seed, and of
which consequently new varieties have not been produced, has even been advanced
-- for it is now as tender as ever it was -- as proving that acclimatisation
cannot be effected! The case, also, of the kidney-bean has been often cited for
a similar purpose, and with much greater weight; but until some one will sow,
during a score of generations, his kidney-beans so early that a very large
proportion are destroyed by frost, and then collect seed from the few
survivors, with care to prevent accidental crosses, and then again get seed
from these seedlings, with the same precautions, the experiment cannot be said
to have been even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has been
published how much more hardy some seedlings appeared to be than others.
On the whole, I think
we may conclude that habit, use, and disuse, have, in some cases, played a
considerable part in the modification of the constitution, and of the structure
of various organs; but that the effects of use and disuse have often been
largely combined with, and sometimes overmastered by, the natural selection of
innate differences.
Correlation of Growth.
I mean by this expression that the whole organisation is so tied together
during its growth and development, that when slight variations in any one part
occur, and are accumulated through natural selection, other parts become
modified. This is a very important subject, most imperfectly understood. The
most obvious case is, that modifications accumulated solely for the good of the
young or larva, will, it may safely be concluded, affect the structure of the
adult; in the same manner as any malconformation affecting the early embryo,
seriously affects the whole organisation of the adult. The several parts of the
body which are homologous, and which, at an early embryonic period, are alike,
seem liable to vary in an allied manner: we see this in the right and left
sides of the body varying in the same manner; in the front and hind legs, and
even in the jaws and limbs, varying together, for the lower jaw is believed to
be homologous with the limbs. These tendencies, I do not doubt, may be mastered
more or less completely by natural selection: thus a family of stags once
existed with an antler only on one side; and if this had been of any great use
to the breed it might probably have been rendered permanent by natural
selection.
Homologous parts, as
has been remarked by some authors, tend to cohere; this is often seen in
monstrous plants; and nothing is more common than the union of homologous parts
in normal structures, as the union of the petals of the corolla into a tube.
Hard parts seem to affect the form of adjoining soft parts; it is believed by
some authors that the diversity in the shape of the pelvis in birds causes the
remarkable diversity in the shape of their kidneys. Others believe that the
shape of the pelvis in the human mother influences by pressure the shape of the
head of the child. In snakes, according to Schlegel, the shape of the body and
the manner of swallowing determine the position of several of the most
important viscera.
The nature of the bond
of correlation is very frequently quite obscure. M. Is. Geoffroy St Hilaire has
forcibly remarked, that certain malconformations very frequently, and that
others rarely coexist, without our being able to assign any reason. what can be
more singular than the relation between blue eyes and deafness in cats, and the
tortoise- shell colour with the female sex; the feathered feet and skin between
the outer toes in pigeons, and the presence of more or less down on the young
birds when first hatched, with the future colour of their plumage; or, again,
the relation between the hair and teeth in the naked Turkish dog, though here
probably homology comes into play? With respect to this latter case of
correlation, I think it can hardly be accidental, that if we pick out the two
orders of mammalia which are most abnormal in their dermal coverings, viz.
Cetacea (whales) and Edentata (armadifloes, scaly ant- eaters, &c.), that
these are likewise the most abnormal in their teeth.
I know of no case
better adapted to show the importance of the laws of correlation in modifying
important structures, independently of utility and, therefore, of natural
selection, than that of the difference between the outer and inner flowers in
some Compositous and Umbelliferous plants. Every one knows the difference in
the ray and central florets of, for instance, the daisy, and this difference is
often accompanied with the abortion of parts of the flower. But, in some
Compositous plants, the seeds also differ in shape and sculpture; and even the
ovary itself, with its accessory parts, differs, as has been described by
Cassini. These differences have been attributed by some authors to pressure,
and the shape of the seeds in the ray-florets in some Compositae countenances
this idea; but, in the case of the corolla of the UinbelliferÆ, it is by no
means, as Dr Hooker informs me, in species with the densest heads that the
inner and outer flowers most frequently differ. It might have been thought that
the development of the ray-petals by drawing nourishment from certain other
parts of the flower had caused their abortion; but in some Compositae there is
a difference in the seeds of the outer and inner florets without any difference
in the corolla. possibly, these several differences may be connected with some
difference in the flow of nutriment towards the central and external flowers:
we know, at least, that in irregular flowers, those nearest to the axis are
oftenest subject to peloria, and become regular. I may add, as an instance of
this, and of a striking case of correlation, that I have recently observed in
some garden pelargiums, that the central flower of the truss often loses the
patches of darker colour in the two upper petals; and that when this occurs,
the adherent nectary is quite aborted; when the colour is absent from only one
of the two upper petals, the nectary is only much shortened.
With respect to the
difference in the corolla of the central and exterior flowers of a head or
umbel, I do not feel at all sure that C. C. Sprengel's idea that the ray-
florets serve to attract insects, whose agency is highly advantageous in the
fertilisation of plants of these two orders, is so far-fetched, as it may at
first appear: and if it be advantageous, natural selection may have come into
play. But in regard to the differences both in the internal and external
structure of the seeds, which are not always correlated with any differences in
the flowers, it seems impossible that they can be in any way advantageous to
the plant: yet in the Umbelliferae these differences are of such apparent
importance -- the seeds being in some cases, according to Tausch, orthospermous
in the exterior flowers and coelospermous in the central flowers, -- that the
elder De Candolle founded his main divisions of the order on analogous
differences. Hence we see that modifications of structure, viewed by
systematists as of high value, may be wholly due to unknown laws of correlated
growth, and without being, as far as we can see, of the slightest service to
the species.
We may often falsely
attribute to correlation of growth, structures which are common to whole groups
of species, and which in truth are simply due to inheritance; for an ancient
progenitor may have acquired through natural selection some one modification in
structure, and, after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted to a whole
group of descendants with diverse habits, would naturally be thought to be correlated
in some necessary manner. So, again, I do not doubt that some apparent
correlations, occurring throughout whole orders, are entirely due to the manner
alone in which natural selection can act. For instance, Alph. De Candolle has
remarked that winged seeds are never found in fruits which do not open: I
should explain the rule by the fact that seeds could not gradually become
winged through natural selection, except in fruits which opened; so that the
individual plants producing seeds which were a little better fitted to be
wafted further, might get an advantage over those producing seed less fitted
for dispersal; and this process could not possibly go on in fruit which did not
open.
The elder Geoffroy and
Goethe propounded, at about the same pen-od, their law of compensation or
balancement of growth; or, as Goethe expressed it, 'in order to spend on one
side, nature is forced to economise on the other side.' I think this holds true
to a certain extent with our domestic productions: if nourishment flows to one
part or organ in excess, it rarely flows, at least in excess, to another part;
thus it is difficult to get a cow to give much milk and to fatten readily. The
same varieties of the cabbage do not yield abundant and nutritious foliage and
a copious supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our poultry,
a large tuft of feathers on the head is generally accompanied by a diminished
comb, and a large beard by diminished wattles. With species in a state of
nature it can hardly be maintained that the law is or universal application;
but many good observers, more especially botanists, believe in its truth. I
will not, however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being largely
developed through natural selection and another and adjoining part being
reduced by this same process or by disuse, and, on the other hand, the actual
withdrawal of nutriment from one part owing to the excess of growth in another
and adjoining part.
I suspect, also, that
some of the cases of compensation which have been advanced, and likewise some
other facts, may be merged under a more general principle, namely, that natural
selection is continually trying to economise in every part of the organisation.
If under changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be seized on
by natural selection, for it will profit the individual not to have its
nutriment wasted in building up an useless structure. I can thus only
understand a fact with which I was much struck when examining cirripedes, and
of which many other instances could be given: namely, that when a cirripede is
parasitic within another and is thus protected, it loses more or less
completely its own shell or carapace. This is the case with the male Ibla, and
in a truly extraordinary manner with the proteolepas: for the carapace in all
other eirripedes consists of the three highly-important anterior segments of
the head enormously developed, and furnished with great nerves and muscles; but
in the parasitic and protected proteolepas, the whole anterior part of the head
is reduced to the merest rudiment attached to the bases of the prehensile
antennae. Now the saving of a large and complex structure, when rendered
superfluous by the parasitic habits of the proteolepas, though effected by slow
steps, would be a decided advantage to each successive individual of the
species; for in the struggle for life to which every animal is exposed, each
individual proteolepas would have a better chance of supporting itself, by less
nutriment being wasted in developing a structure now become useless.
Thus, as I believe,
natural selection will always succeed in the long run in reducing and saving
every part of the organisation, as soon as it is rendered superfluous, without
by any means causing some other part to be largely developed in a corresponding
degree. And, conversely, that natural selection may perfectly well succeed in
largely developing any organ, without requiring as a necessary compensation the
reduction of some adjoining part.
It seems to be a rule,
as remarked by Is. Geoffroy St Hilaire, both in varieties and in species, that
when any part or organ is repeated many times in the structure of the same
individual (as the vertebrae in snakes, and the stamens in polyandrous flowers)
the number is variable; whereas the number of the same part or organ, when it
occurs in lesser numbers, is constant. The same author and some botanists have
further remarked that multiple parts are also very liable to variation in
structure. Inasmuch as this 'vegetative repetition,' to use prof. Owen's
expression, seems to be a sign of low organisation; the foregoing remark seems
connected with the very general opinion of naturalists, that beings low in the
scale of nature are more variable than those which are higher. I presume that
lowness in this case means that the several parts of the organisation have been
but little specialised for particular functions; and as long as the same part
has to perform diversified work, we can perhaps see why it should remain
variable, that is, why natural selection should have preserved or rejected each
little deviation of form less carefully than when the part has to serve for one
special purpose alone. In the same way that a knife which has to cut all sorts
of things may be of almost any shape; whilst a tool for some particular object
had better be of some particular shape. Natural selection, it should never be
forgotten, can act on each part of each being, solely through and for its
advantage.
Rudimentary parts, it
has been stated by some authors, and I believe with truth, are apt to be highly
variable. We shall have to recur to the general subject of rudimentary and
aborted organs; and I will here only add that their variability seems to be
owing to their uselessness, and therefore to natural selection having no power
to check deviations in their structure. Thus rudimentary parts are left to the
free play of the various laws of growth, to the effects of long-continued
disuse, and to the tendency to reversion.
A part developed in any
species in an extraordinary degree or manner, in comparison with the same part
in allied species, tends to be highly variable. Several years ago I was much
struck with a remark, nearly to the above effect, published by Mr Waterhouse. I
infer also from an observation made by professor Owen, with respect to the
length of the arms of the ourang-outang, that he has come to a nearly similar
conclusion. It is hopeless to attempt to convince any one of the truth of this
proposition without giving the long array of facts which I have collected, and
which cannot possibly be here introduced. I can only state my conviction that
it is a rule of high generality. I am aware of several causes of error, but I
hope that I have made due allowance for them. It should be understood that the
rule by no means applies to any part, however unusually developed, unless it be
unusually developed in comparison with the same part in closely allied species.
Thus, the bat's wing is a most abnormal structure in the class mammalia; but
the rule would not here apply, because there is a whole group of bats having
wings; it would apply only if some one species of bat had its wings developed
in some remarkable manner in comparison with the other species of the same
genus. The rule applies very strongly In the case of secondary sexual characters,
when displayed in any unusual manner. The term, secondary sexual characters,
used by Hunter, applies to characters which are attached to one sex, but are
not directly connected with the act of reproduction. The rule applies to males
and females; but as females more rarely offer remarkable secondary sexual
characters, it applies more rarely to them. The rule being so plainly
applicable in the case of secondary sexual characters, may be due to the great
variability of these characters, whether or not displayed in any unusual manner
-- of which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; and I may here add, that I particularly attended to Mr
Waterhouse's remark, whilst investigating this Order, and I am fully convinced
that the rule almost invariably holds good with cirripedes. I shall, in my
future work, give a list of the more remarkable cases; I will here only briefly
give one, as it illustrates the rule in its largest application. The opercular
valves of sessile cirripedes (rock barnacles) are, in every sense of the word,
very important structures, and they differ extremely little even in different
genera; but in the several species of one genus, pyrgoma, these valves present
a marvellous amount of diversification: the homologous valves in the different
species being sometimes wholly unlike in shape; and the amount of variation in
the individuals of several of the species is so great, that it is no
exaggeration to state that the varieties differ more from each other in the
characters of these important valves than do other species of distinct genera.
As birds within the
same country vary in a remarkably small degree, I have particularly attended to
them, and the rule seems to me certainly to hold good in this class. I cannot
make out that it applies to plants, and this would seriously have shaken my
belief in its truth, had not the great variability in plants made it
particularly difficult to compare their relative degrees of variability.
When we see any part or
organ developed in a remarkable degree or manner in any species, the fair
presumption is that it is of high importance to that species; nevertheless the
part in this case is eminently liable to variation. why should this be so? On
the view that each species has been independently created, with all its parts
as we now see them, I can see no explanation. But on the view that groups of
species have descended from other species, and have been modified through
natural selection, I think we can obtain some light. In our domestic animals,
if any part, or the whole animal, be neglected and no selection be applied,
that part (for instance, the comb in the Dorking fowl) or the whole breed will
cease to have a nearly uniform character. The breed will then be said to have
degenerated. In rudimentary organs, and in those which have been but little
specialized for any particular purpose, and perhaps in polymorphic groups, we
see a nearly parallel natural case; for in such cases natural selection either
has not or cannot come into full play, and thus the organisation is left in a
fluctuating condition. But what here more especially concerns us is, that in
our domestic animals those points, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to variation.
Look at the breeds of the pigeon; see what a prodigious amount of difference
there is in the beak of the different tumblers, in the beak and wattle of the
different carriers, in the carriage and tail of our fantails, &c., these
being the points now mainly attended to by English fanciers. Even in the
sub-breeds, as in the short-faced tumbler, it is notoriously difficult to breed
them nearly to perfection, and frequently individuals are born which depart
widely from the standard. There may be truly said to be a constant struggle
going on between, on the one hand, the tendency to reversion to a less modified
state, as well as an innate tendency to further variability of all kinds, and,
on the other hand, the power of steady selection to keep the breed true. In the
long run selection gains the day, and we do not expect to fail so far as to
breed a bird as coarse as a common tumbler from a good short-faced strain. But
as long as selection is rapidly going on, there may always be expected to be
much variability in the structure undergoing modification. It further deserves
notice that these variable characters, produced by man's selection, sometimes
become attached, from causes quite unknown to us, more to one sex than to the
other, generally to the male sex, as with the wattle of carriers and the
enlarged crop of pouters.
Now let us turn to
nature. When a part has been developed in an extraordinary manner in any one
species, compared with the other species of the same genus, we may conclude
that this part has undergone an extraordinary amount of modification, since the
period when the species branched off from the common progenitor of the genus.
This period will seldom be remote in any extreme degree, as species very rarely
endure for more than one geological period. An extraordinary amount of
modification implies an unusually large and long-continued amount of
variability, which has continually been accumulated by natural selection for
the benefit of the species. But as the variability of the extraordinarily-
developed part or organ has been so great and long-continued within a period
not excessively remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation, which have
remained for a much longer period nearly constant. And this, I am convinced, is
the case. That the struggle between natural selection on the one hand, and the
tendency to reversion and variability on the other hand, will in the course of
time cease; and that the most abnormally developed organs may be made constant,
I can see no reason to doubt. Hence when an organ, however abnormal it may be,
has been transmitted in approximately the same condition to many modified
descendants, as in the case of the wing of the bat, it must have existed,
according to my theory, for an immense period in nearly the same state; and
thus it comes to be no more variable than any other structure. It is only in
those cases in which the modification has been comparatively recent and
extraordinarily great that we ought to find the generative variability, as it
may be called, still present in a high degree. For in this case the variability
will seldom as yet have been fixed by the continued selection of the
individuals varying in the required manner and degree, and by the continued
rejection of those tending to revert to a former and less modified condition.
The principle included
in these remarks may be extended. It is notorious that specific characters are
more variable than generic. To explain by a simple example what is meant. If
some species in a large genus of plants had blue flowers and some had red, the
colour would be only a specific character, and no one would be surprised at one
of the blue species varying into red, or conversely; but if all the species had
blue flowers, the colour would become a generic character, and its variation
would be a more unusual circumstance. I have chosen this example because an
explanation is not in this case applicable, which most naturalists would
advance, namely, that specific characters are more variable than generic,
because they are taken from parts of less physiological importance than those
commonly used for classing genera. I believe this explanation is partly, yet
only indirectly, true; I shall, however, have to return to this subject in our
chapter on Classification. It would be almost superfluous to adduce evidence in
support of the above statement, that specific characters are more variable than
generic; but I have repeatedly noticed in works on natural history, that when
an author has remarked with surprise that some important organ or part, which
is generally very constant throughout large groups of species, has differed
considerably in closely-allied species, that it has, also, been variable in the
individuals of some of the species. And this fact shows that a character, which
is generally of generic value, when it sinks in value and becomes only of specific
value, often becomes variable, though its physiological importance may remain
the same. Something of the same kind applies to monstrosities: at least Is.
Geoffroy St Hilaire seems to entertain no doubt, that the more an organ
normally differs in the different species of the same group, the more subject
it is to individual anomalies.
On the ordinary view of
each species having been independently created, why should that part of the
structure, which differs from the same part in other independently-created
species of the same genus, be more variable than those parts which are closely
alike in the several species? I do not see that any explanation can be given.
But on the view of species being only strongly marked and fixed varieties, we
might surely expect to find them still often continuing to vary in those parts
of their structure which have varied within a moderately recent period, and
which have thus come to differ. Or to state the case in another manner: -- the
points in which all the species of a genus resemble each other, and in which
they differ from the species of some other genus, are called generic
characters; and these characters in common I attribute to inheritance from a
common progenitor, for it can rarely have happened that natural selection will
have modified several species, fitted to more or less widely- different habits,
in exactly the same manner: and as these so-called generic characters have been
inherited from a remote period, since that period when the species first
branched off from their common progenitor, and subsequently have not varied or
come to differ in any degree, or only in a slight degree, it is not probable
that they should-vary at the present day. On the other hand, the points in
which species differ from other species of the same genus, are called specific
characters; and as these specific characters have varied and come to differ
within the period of the branching off of the species from a common progenitor,
it is probable that they should still often be in some degree variable, -- at
least more variable than those parts of the organisation which have for a very
hong period remained constant.
In connexion with the
present subject, I will make only two other remarks. I think it will be
admitted, without my entering on details, that secondary sexual characters are
very variable; I think it also will be admitted that species of the same group
differ from each other more widely in their secondary sexual characters, than
in other parts of their organisation; compare, for instance, the amount of
difference between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference between their
females; and the truth of this proposition will be granted. The cause of the
original variability of secondary sexual characters is not manifest; but we can
see why these characters should not have been rendered as constant and uniform
as other parts of the organisation; for secondary sexual characters have been
accumulated by sexual selection, which is less rigid in its action than
ordinary selection, as it does not entail death, but only gives fewer offspring
to the less favoured males. Whatever the cause may be of the variability of
secondary sexual characters, as they are highly variable, sexual selection will
have had a wide scope for action, and may thus readily have succeeded in giving
to the species of the same group a greater amount of difference in their sexual
characters, than in other parts of their structure.
It is a remarkable
fact, that the secondary sexual differences between the two sexes of the same
species are generally displayed in the very same parts of the organisation in
which the different species of the same genus differ from each other. Of this
fact I will give in illustration two instances, the first which happen to stand
on my list; and as the differences in these cases are of a very unusual nature,
the relation can hardly be accidental. The same number of joints in the tarsi
is a character generally common to very large groups of beetles, but in the
Engidae, as Westwood has remarked, the number varies greatly; and the number
likewise differs in the two sexes of the same species: again in fossorial
hymenoptera, the manner of neuration of the wings is a character of the highest
importance, because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the same
species. This relation has a clear meaning on my view of the subject: I look at
all the species of the same genus as having as certainly descended from the
same progenitor, as have the two sexes of any one of the species. Consequently,
whatever part of the structure of the common progenitor, or of its early
descendants, became variable; variations of this part would it is highly
probable, be taken advantage of by natural and sexual selection, in order to
fit the several species to their several places in the economy of nature, and
likewise to fit the two sexes of the same species to each other, or to fit the
males and females to different habits of life, or the males to struggle with
other males for the possession of the females.
Finally, then, I
conclude that the greater variability of specific characters, or those which
distinguish species from species, than of generic characters, or those which
the species possess in common; -- that the frequent extreme variability of any
part which is developed in a species in an extraordinary manner in comparison
with the same part in its congeners; and the not great degree of variability in
a part, however extraordinarily it may be developed, if it be common to a whole
group of species; -- that the great variability of secondary sexual characters,
and the great amount of difference in these same characters between closely
allied species; -- that secondary sexual and ordinary specific differences are
generally displayed in the same parts of the organisation, -- are all
principles closely connected together. All being mainly due to the species of
the same group having descended from a common progenitor, from whom they have
inherited much in common, -- to parts which have recently and largely varied
being more likely still to go on varying than parts which have long been
inherited and have not varied, -- to natural selection having more or less
completely, according to the lapse of time, overmastered the tendency to
reversion and to further variability, -- to sexual selection being less rigid
than ordinary selection, -- and to variations in the same parts having been
accumulated by natural and sexual selection, and thus adapted for secondary
sexual, and for ordinary specific purposes.
Distinct species
present analogous variations; and a variety ofone species often assumes some of
the characters of an alliedspecies, or reverts to some of the characters of an
early progenitor. These propositions will be most readily understood by looking
to our domestic races. The most distinct breeds of pigeons, in countries most
widely apart, present sub-varieties with reversed feathers on the head and
feathers on the feet, -- characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct races.
The frequent presence of fourteen or even sixteen tail-feathers in the pouter,
may be considered as a variation representing the normal structure of another
race, the fantail. I presume that no one will doubt that all such analogous
variations are due to the several races of the pigeon having inherited from a
common parent the same constitution and tendency to variation, when acted on by
similar unknown influences. In the vegetable kingdom we have a case of
analogous variation, in the enlarged stems, or roots as commonly called, of the
Swedish turnip and Ruta baga, plants which several botanists rank as varieties
produced by cultivation from a common parent: if this be not so, the case will
then be one of analogous variation in two so-called distinct species; and to
these a third may be added, namely, the common turnip. According to the
ordinary view of each species having been independently created, we should have
to attribute this similarity in the enlarged stems of these three plants, not
to the vera causa of community of descent, and a consequent tendency to vary in
a like manner, but to three separate yet closely related acts of creation.
With pigeons, however,
we have another case, namely, the occasional appearance in all the breeds, of
slaty-blue birds with two black bars on the wings, a white rump, a bar at the
end of the tail, with the outer feathers externally edged near their bases with
white. As all these marks are characteristic of the parent rock-pigeon, I
presume that no one will doubt that this is a case of reversion, and not of a
new yet analogous variation appearing in the several breeds. We may I think
confidently come to this conclusion, because, as we have seen, these coloured
marks are eminently liable to appear in the crossed offspring of two distinct
and differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue, with
the several marks, beyond the influence of the mere act of crossing on the laws
of inheritance.
No doubt it is a very
surprising fact that characters should reappear after having been lost for
many, perhaps for hundreds of generations. But when a breed has been crossed
only once by some other breed, the offspring occasionally show a tendency to
revert in character to the foreign breed for many generations -- some say, for
a dozen or even a score of generations. After twelve generations, the
proportion of blood, to use a common expression, of any one ancestor, is only 1
in 2048; and yet, as we see, it is generally believed that a tendency to
reversion is retained by this very small proportion of foreign blood. In a
breed which has not been crossed, but in which both parents have lost some character
which their progenitor possessed, the tendency, whether strong or weak, to
reproduce the lost character might be, as was formerly remarked, for afl that
we can see to the contrary, transmitted for almost any number of generations.
When a character which has been lost in a breed, reappears after a great number
of generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations distant, but that in
each successive generation there has been a tendency to reproduce the character
in question, which at last, under unknown favourable conditions, gains an
ascendancy. For instance, it is probable that in each generation of the
barb-pigeon, which produces most rarely a blue and black-barred bird, there has
been a tendency in each generation in the plumage to assume this colour. This
view is hypothetical, but could be supported by some facts; and I can see no
more abstract improbability in a tendency to produce any character being
inherited for an endless number of generations, than in quite useless or
rudimentary organs being, as we all know them to be, thus inherited. Indeed, we
may sometimes observe a mere tendency to produce a rudiment inherited: for
instance, in the common snapdragon (Antirrhinum) a rudiment of a fifth stamen
so often appears, that this plant must have an inherited tendency to produce
it.
As all the species of
the same genus are supposed, on my theory, to have descended from a common
parent, it might be expected that they would occasionally vary in an analogous
manner; so that a variety of one species would resemble in some of its
characters another species; this other species being on my view only a
well-marked and permanent variety. But characters thus gained would probably be
of an unimportant nature, for the presence of all important characters will be
governed by natural selection, in accordance with the diverse habits of the
species, and will not be left to the mutual action of the conditions of life
and of a similar inherited constitution. It might further be expected that the
species of the same genus would occasionally exhibit reversions to lost
ancestral characters. As, however, we never know the exact character of the
common ancestor of a group, we could not distinguish these two cases: if, for
instance, we did not know that the rock-pigeon was not feather-footed or
turn-crowned, we could not have told, whether these characters in our domestic
breeds were reversions or only analogous variations; but we might have inferred
that the blueness was a case of reversion, from the number of the markings,
which are correlated with the blue tint, and which it does not appear probable
would all appear together from simple variation. More especially we might have
inferred this, from the blue colour and marks so often appearing when distinct
breeds of diverse colours are crossed. Hence, though under nature it must
generally be left doubtful, what cases are reversions to an anciently existing
character, and what are new but analogous variations, yet we ought, on my
theory, sometimes to find the varying offspring of a species assuming
characters (either from reversion or from analogous variation) which already
occur in some members of the same group. And this undoubtedly is the case in nature.
A considerable part of
the difficulty in recognising a variable species in our systematic works, is
due to its varieties mocking, as it were, come of the other species of the same
genus. A considerable catalogue, also, could be given of forms intermediate
between two other forms, which themselves must be doubtfully ranked as either
varieties or species, that the one in varying has assumed some of the
characters of the other, so as to produce the intermediate form. But the best
evidence is afforded by parts or organs of an important and uniform nature
occasionally varying so as to acquire, in some degree, the character of the
same pard or organ in an allied species. I have collected a long list of such
cases; but here, as before, I lie under a great disadvantage in not being able
to give them. I can only repeat that such cases certainly do occur, and seem to
me very remarkable.
I will, however, give
one curious and complex case, not indeed as affecting any important character,
but from occurring in several species of the same genus, partly under
domestication and partly under nature. It is a case apparently of reversion.
The ass not rarely has very distinct transverse bars on its legs, like those of
a zebra: it has been asserted that these are plainest in the foal. and from
inquiries which I have made, I believe this to be true. It has also been
asserted that the stripe on each shoulder is sometimes double. The
shoulder-stripe is certainly very variable in length and outline. A white ass,
but not and albino, has been described without either spinal or
shoulder-stripe; and these stripes are sometimes very obscure, or actually
quite lost, in dark-coloured asses. The koulan of Pallas is said to have been
seen with a double shoulder-stripe; but traces of it, as stated by Mr Blyth and
others, occasionally appear: and I have been informed by Colonel Poole that
foals of this species are generally striped on the legs, and faintly on the
shoulder. The quagga, though so plainly barred like a zebra over the body, is without
bars on the legs; but Dr Gray has figured one specimen with very distinct
zebra-like bars on the hocks.
With respect to the
horse, I have collected cases in England of the spinal stripe in horses of the
most distinct breeds, and of all colours; transverse bars on the legs are not
rare in duns, mouse-duns, and in one instance in a chestnut: a faint
shoulder-stripe may sometimes be seen in duns, and I have seen a trace in a bay
horse. My son made a careful examination and sketch for me of a dun Belgian
cart-horse with a double stripe on each shoulder and with leg-stripes; and a
man, whom I can implicitly trust, has examined for me a small dun Welch pony
with three short parallel stripes on each shoulder.
In the north-west part
of India the Kattywar breed of horses is so generally striped, that, as I hear
from Colonel Poole, who examined the breed for the Indian Government, a horse
without stripes is not considered as purely-bred. The spine is always striped;
the legs are generally barred; and the shoulder-stripe, which is sometimes
double and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes quite
disappear in old horses. Colonel poole has seen both gray and bay Kattywar
horses striped when first foaled. I have, also, reason to suspect, from
information given me by Mr W. W. Edwards, that with the English race- horse the
spinal stripe is much commoner in the foal than in the full-grown animal.
Without here entering on further details, I may state that I have collected
cases of leg and shoulder stripes in horses of very different breeds, in
various countries from Britain to Eastern China; and from Norway in the north
to the Malay Archipelago in the south. In all parts of the world these stripes
occur far oftenest in duns and mouse-duns; by the term dun a large range of
colour is included, from one between brown and black to a close approach to
cream-colour.
I am aware that Colonel
Hamilton Smith, who has written on this subject, believes that the several
breeds of the horse have descended from several aboriginal species -- one of
which, the dun, was striped; and that the above-described appearances are all
due to ancient crosses with the dun stock. But I am not at all satisfied with
this theory, and should be loth to apply it to breeds so distinct as the heavy
Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race, &c.,
inhabiting the most distant parts of the world.
Now let us turn to the
effects of crossing the several species of the horse-genus. Rollin asserts,
that the common mule from the ass and horse is particularly apt to have bars on
its legs. I once saw a mule with its legs so much striped that any one at first
would have thought that it must have been the product of a zebra; and Mr W. C.
Martin, in his excellent treatise on the horse, has given a figure of a similar
mule. In four coloured drawings, which I have seen, of hybrids between the ass
and zebra, the legs were much more plainly barred than the rest of the body;
and in one of them there was a double shoulder-stripe. In Lord Moreton's famous
hybrid from a chestnut mare and male quagga, the hybrid, and even the pure
offspring subsequently produced from the mare by a black Arabian sire, were
much more plainly barred across the legs than is even the pure quagga. Lastly,
and this is another most remarkable case, a hybrid has been figured by Dr Gray
(and he informs me that he knows of a second case) from the ass and the
hemionus; and this hybrid, though the ass seldom has stripes on its legs and
the hemionus has none and has not even a shoulder-stripe, nevertheless had all
four legs barred, and had three short shoulder-stripes, like those on the dun
Welch pony, and even had some zebra-like stripes on the sides of its face. With
respect to this last fact, I was so convinced that not even a stripe of colour
appears from what would commonly be called an accident, that I was led solely
from the occurrence of the face-stripes on this hybrid from the ass and hemionus,
to ask Colonel poole whether such face-stripes ever occur in the eininently
striped Kattywar breed of horses, and was, as we have seen, answered in the
affirmative.
What now are we to say
to these several facts? We see several very distinct species of the horse-genus
becoming, by simple variation, striped on the legs like a zebra, or striped on
the shoulders like an ass. In the horse we see this tendency strong whenever a
dun tint appears -- a tint which approaches to that of the general colouring of
the other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We see this
tendency to become striped most strongly displayed in hybrids from between
several of the most distinct species. Now observe the case of the several
breeds of pigeons: they are descended from a pigeon (including two or three
sub-species or geographical races) of a bluish colour, with certain bars and
other marks; and when any breed assumes by simple variation a bluish tint,
these bars and other . marks invariably reappear; but without any other change
of form or character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks to
reappear in the mongrels. I have stated that the most probable hypothesis to
account for the reappearance of very ancient characters, is -- that there is a
tendency in the young of each successive generation to produce the long-lost
character, and that this tendency, from unknown causes, sometimes prevails. And
we have just seen that in several species of the horse-genus the stripes are
either plainer or appear more commonly in the young than in the old. Call the
breeds of pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the horse-genus! For
myself, I venture confidently to look back thousands on thousands of
generations, and I see an animal striped like a zebra, but perhaps otherwise
very differently constructed, the common parent of our domestic horse, whether
or not it be descended from one or more wild stocks, of the ass, the hemionus,
quagga, and zebra.
He who believes that
each equine species was independently created, will, I presume, assert that
each species has been created with a tendency to vary, both under nature and
under domestication, in this particular manner, so as often to become striped
like other species of the genus; and that each has been created with a strong
tendency, when crossed with species inhabiting distant quarters of the world,
to produce hybrids resembling in their stripes, not their own parents, but
other species of the genus. To admit this view is, as it seems to me, to reject
a real for an unreal, or at least for an unknown, cause. It makes the works of
God a mere mockery and deception; I would almost as soon believe with the old
and ignorant cosmogonists, that fossil shells had never lived, but had been
created in stone so as to mock the shells now living on the sea-shore.
Summary. Our ignorance
of the laws of variation is profound. Not in one case out of a hundred can we
pretend to assign any reason why this or that part differs, more or less, from
the same part in the parents. But whenever we have the means of instituting a
comparison, the same laws appear to have acted in producing the lesser
differences between varieties of the same species. and the greater differences
between species of the same genus. The external conditions of life, as climate
and food, &c., seem to have induced some slight modifications. Habit in
producing constitutional differences, and use in strengthening, and disuse in
weakening and diminishing organs, seem to have been more potent in their
effects. Homologous parts tend to vary in the same way, and homologous parts
tend to cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed, perhaps
it tends to draw nourishment from the adjoining parts; and every part of the
structure which can be saved without detriment to the individual, will be
saved. Changes of structure at an early age will generally affect parts
subsequently developed; and there are very many other correlations of growth,
the nature of which we are utterly unable to understand. Multiple parts are
variable in number and in structure, perhaps arising from such parts not having
been closely specialized to any particular function, so that their
modifications have not been closely checked by natural selection. It is
probably from this same cause that organic beings low in the scale of nature
are more variable than those which have their whole organisation more
specialized, and are higher in the scale. Rudimentary organs, from being
useless, will be disregarded by natural selection, and hence probably are
variable. Specific characters -- that is, the characters which have come to
differ since the several species of the same genus branched off from a common
parent -- are more variable than generic characters, or those which have long
been inherited, and have not differed within this same period. In these remarks
we have referred to special parts or organs being still variable, because they
have recently varied and thus come to differ; but we have also seen in the
second Chapter that the same principle applies to the whole individual; for in
a district where many species of any genus are found -- that is, where there
has been much former variation and differentiation, or where the manufactory of
new specific forms has been actively at work -- there, on an average, we now
find most varieties or incipient species. Secondary sexual characters are
highly variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally been
taken advantage of in giving secondary sexual differences to the sexes of the
same species, and specific differences to the several species of the same
genus. Any part or organ developed to an extraordinary size or in an
extraordinary manner, in comparison with the same part or organ in the allied
species, must have gone through an extraordinary amount of modification since
the genus arose; and thus we can understand why it should often still be
variable in a much higher degree than other parts; for variation is a long-
continued and slow process, and natural selection will in such cases not as yet
have had time to overcome the tendency to further variability and to reversion
to a less modified state. But when a species with any extraordinarily-developed
organ has become the parent of many modified descendants -- which on my view
must be a very slow process, requiring a long lapse of time -- in this case,
natural selection may readily have succeeded in giving a fixed character to the
organ, in however extraordinary a manner it may be developed. Species
inheriting nearly the same constitution from a common parent and exposed to
similar influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters of their
ancient progenitors. Although new and important modifications may not arise
from reversion and analogous variation, such modifications will add to the
beautiful and harmonious diversity of nature.
whatever the cause may
be of each slight difference in the offspring from their parents -- and a cause
for each must exist -- it is the steady accumulation, through natural
selection, of such differences, when beneficial to the individual, that gives
rise to all the more important modifications of structure, by which the
innumerable beings on the face of this earth are enabled to struggle with each
other, and the best adapted to survive.
Difficulties on the theory of descent with modification - Transitions -
Absence or rarity of transitional varieties - Transitions in habits of life -
Diversified habits in the same species - Species with habits widely different
from those of their allies - Organs of extreme perfection - Means of transition
- Cases of difficulty - Natura non facit saltum - Organs of small importance -
Organs not in all cases absolutely perfect - The law of Unity of Type and of
the Conditions of Existence embraced by the theory of Natural Selection LONG before having arrived at this part
of my work, a crowd of difficulties will have occurred to the reader. Some of
them are so grave that to this day I can never reflect on them without being
staggered; but, to the best of my judgment, the greater number are only apparent,
and those that are real are not, I think, fatal to my theory.
These difficulties and
objections may be classed under the following heads: -Firstly, why, if species
have descended from other species by insensibly fine gradations, do we not
everywhere see innumerable transitional forms? Why is not all nature in
confusion instead of the species being, as we see them, well defined?
Secondly, is it
possible that an animal having, for instance, the structure and habits of a
bat, could have been formed by the modification of some animal with wholly
different habits? Can we believe that natural selection could produce, on the
one hand, organs of trifling importance, such as the tail of a giraffe, which
serves as a fly-flapper, and, on the other hand, organs of such wonderful
structure, as the eye, of which we hardly as yet fully understand the
inimitable perfection?
Thirdly, can instincts
be acquired and modified through natural selection? What shall we say to so
marvellous an instinct as that which leads the bee to make cells, which have
practically anticipated the discoveries of profound mathematicians?
Fourthly, how can we
account for species, when crossed, being sterile and producing sterile
offspring, whereas, when varieties are crossed, their fertility is unimpaired?
The two first heads
shall be here discussed -- Instinct and Hybridism in separate chapters.
On the absence or
rarity of transitional varieties. As natural selection acts solely by the
preservation of profitable modifications, each new form will tend in a
fully-stocked country to take the place of, and finally to exterminate, its own
less improved parent or other less-favoured forms with which it comes into
competition. Thus extinction and natural selection will, as we have seen, go hand
in hand. Hence, if we look at each species as descended from some other unknown
form, both the parent and all the transitional varieties will generally have
been exterminated by the very process of formation and perfection of the new
form.
But, as by this theory
innumerable transitional forms must have existed, why do we not find them
embedded in countless numbers in the crust of the earth? It will be much more
convenient to discuss this question in the chapter on the Imperfection of the
geological record; and I will here only state that I believe the answer mainly
lies in the record being incomparably less perfect than is generally supposed;
the imperfection of the record being chiefly due to organic beings not
inhabiting profound depths of the sea, and to their remains being embedded and
preserved to a future age only in masses of sediment sufficiently thick and
extensive to withstand an enormous amount of future degradation; and such
fossiliferous masses can be accumulated only where much sediment is deposited
on the shallow bed of the sea, whilst it slowly subsides. These contingencies
will concur only rarely, and after enormously long intervals. Whilst the bed of
the sea is stationary or is rising, or when very little sediment is being
deposited, there will be blanks in our geological history. The crust of the
earth is a vast museum; but the natural collections have been made only at
intervals of time immensely remote.
But it may be urged
that when several closely-allied species inhabit the same territory we surely
ought to find at the present time many transitional forms. Let us take a simple
case: in travelling from north to south over a continent, we generally meet at
successive intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land. These
representative species often meet and interlock; and as the one becomes rarer
and rarer, the other becomes more and more frequent, till the one replaces the
other. But if we compare these species where they intermingle, they are
generally as absolutely distinct from each other in every detail of structure
as are specimens taken from the metropolis inhabited by each. By my theory
these allied species have descended from a common parent; and during the
process of modification, each has become adapted to the conditions of life of
its own region, and has supplanted and exterminated its original parent and all
the transitional varieties between its past and present states. Hence we ought
not to expect at the present time to meet with numerous transitional varieties
in each region, though they must have existed there, and may be embedded there
in a fossil condition. But in the intermediate region, having intermediate
conditions of life, why do we not now find closely-linking intermediate
varieties? This difficulty for a long time quite confounded me. But I think it
can be in large part explained.
In the first place we
should be extremely cautious in inferring, because an area is now continuous,
that it has been continuous during a long period. Geology would lead us to
believe that almost every continent has been broken up into islands even during
the later tertiary periods; and in such islands distinct species might have
been separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land and of
climate, marine areas now continuous must often have existed within recent
times in a far less continuous and uniform condition than at present. But I
will pass over this way of escaping from the difficulty; for I believe that
many perfectly defined species have been formed on strictly continuous areas; though
I do not doubt that the formerly broken condition of areas now continuous has
played an important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species
as they are now distributed over a wide area, we generally find them tolerably
numerous over a large territory, then becoming somewhat abruptly rarer and
rarer on the confines, and finally disappearing. Hence the neutral territory
between two representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending mountains, and
sometimes it is quite remarkable how abruptly, as Alph. De Candolle has
observed, a common alpine species disappears. The same fact has been noticed by
Forbes in sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all- important elements of
distribution, these facts ought to cause surprise, as climate and height or
depth graduate away insensibly. But when we bear in mind that almost every
species, even in its metropolis, would increase immensely in numbers, were it
not for other competing species; that nearly all either prey on or serve as
prey for others; in short, that each organic being is either directly or
indirectly related in the most important manner to other organic beings, we
must see that the range of the inhabitants of any country by no means
exclusively depends on insensibly changing physical conditions, but in large
part on the presence of other species, on which it depends, or by which it is
destroyed, or with which it comes into competition; and as these species are
already defined objects (however they may have become so), not blending one
into another by insensible gradations, the range of any one species, depending
as it does on the range of others, will tend to be sharply defined. Moreover,
each species on the confines of its range, where it exists in lessened numbers,
will, during fluctuations in the number of its enemies or of its prey, or in
the seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.
If I am right in
believing that allied or representative species, when inhabiting a continuous
area, are generally so distributed that each has a wide range, with a
comparatively narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not essentially differ
from species, the same rule will probably apply to both; and if we in
imagination adapt a varying species to a very large area, we shall have to
adapt two varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist in lesser
numbers from inhabiting a narrow and lesser area; and practically, as far as I
can make out, this rule holds good with varieties in a state of nature. I have
met with striking instances of the rule in the case of varieties intermediate
between well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr Watson, Dr Asa Gray, and Mr Wollaston, that
generally when varieties intermediate between two other forms occur, they are
much rarer numerically than the forms which they connect. Now, if we may trust
these facts and inferences, and therefore conclude that varieties linking two
other varieties together have generally existed in lesser numbers than the
forms which they connect, then, I think, we can understand why intermediate
varieties should not endure for very long periods; - why as a general rule they
should be exterminated and disappear, sooner than the forms which they
originally linked together.
For any form existing
in lesser numbers would, as already remarked, run a greater chance of being
exterminated than one existing in large numbers; and in this particular case
the intermediate form would be eminently liable to the inroads of closely
allied forms existing on both sides of it. But a far more important consideration,
as I believe, is that, during the process of further modification, by which two
varieties are supposed on my theory to be converted and perfected into two
distinct species, the two which exist in larger numbers from inhabiting larger
areas, will have a great advantage over the intermediate variety, which exists
in smaller numbers in a narrow and intermediate zone. For forms existing in
larger numbers will always have a better chance, within any given period, of
presenting further favourable variations for natural selection to seize on,
than will the rarer forms which exist in lesser numbers. Hence, the more common
forms, in the race for life, will tend to beat and supplant the less common
forms, for these will be more slowly modified and improved. It is the same
principle which, as I believe, accounts for the common species in each country,
as shown in the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what I mean
by supposing three varieties of sheep to be kept, one adapted to an extensive
mountainous region; a second to a comparatively narrow, hilly tract; and a
third to wide plains at the base; and that the inhabitants are all trying with
equal steadiness and skill to improve their stocks by selection; the chances in
this case will be strongly in favour of the great holders on the mountains or
on the plains improving their breeds more quickly than the small holders on the
intermediate narrow, hilly tract; and consequently the improved mountain or
plain breed will soon take the place of the less improved hill breed; and thus
the two breeds, which originally existed in greater numbers, will come into
close contact with each other, without the interposition of the supplanted, intermediate
hill-variety.
To sum up, I believe
that species come to be tolerably well-defined objects, and do not at any one
period present an in extricable chaos of varying and intermediate links:
firstly, because new varieties are very slowly formed, for variation is a very
slow process, and natural selection can do nothing until favourable variations
chance to occur, and until a place in the natural polity of the country can be
better fled by some modification of some one or more of its inhabitants. And such
new places will depend on slow changes of climate, or on the occasional
immigration of new inhabitants, and, probably, in a still more important
degree, on some of the old inhabitants becoming slowly modified, with the new
forms thus produced and the old ones acting and reacting on each other. So
that, in any one region and at any one time, we ought only to see a few species
presenting slight modifications of structure in some degree permanent; and this
assuredly we do see.
Secondly, areas now
continuous must often have existed within the recent period in isolated
portions, in which many forms, more especially amongst the classes which unite
for each birth and wander much, may have separately been rendered sufficiently
distinct to rank as representative species. In this case, intermediate
varieties between the several representative species and their common parent,
must formerly have existed in each broken portion of the land, but these links
will have been supplanted and exterminated during the process of natural
selection, so that they will no longer exist in a living state.
Thirdly, when two or
more varieties have been formed in different portions of a strictly continuous
area, intermediate varieties will, it is probable, at first have been formed in
the intermediate zones, but they will generally have had a short duration. For
these intermediate varieties will, from reasons already assigned (namely from
what we know of the actual distribution of closely allied or representative
species, and likewise of acknowledged varieties), exist in the intermediate
zones in lesser numbers than the varieties which they tend to connect. From
this cause alone the intermediate varieties will be liable to accidental
extermination; and during the process of further modification through natural
selection, they will almost certainly be beaten and supplanted by the forms
which they connect; for these from existing in greater numbers will, in the
aggregate, present more variation, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to
any one time, but to all time, if my theory be true, numberless intermediate
varieties, linking most closely all the species of the same group together,
must assuredly have existed; but the very process of natural selection
constantly tends, as has been so often remarked, to exterminate the parent
forms and the intermediate links Consequently evidence of their former
existence could be found only amongst fossil remains, which are preserved, as
we shall in a future chapter attempt to show, in an extremely imperfect and
intermittent record.
On the origin and
transitions of organic beings with peculiarhabits and structure. It has been
asked by the opponents of such views as I hold, how, for instance, a land
carnivorous animal could have been converted into one with aquatic habits; for
how could the animal in its transitional state have subsisted? It would be easy
to show that within the same group carnivorous animals exist having every
intermediate grade between truly aquatic and strictly terrestrial habits; and
as each exists by a struggle for life, it is clear that each is well adapted in
its habits to its place in nature. Look at the Mustela vison of North America,
which has webbed feet and which resembles an otter in its fur, short legs, and
form of tail; during summer this animal dives for and preys on fish, but during
the long winter it leaves the frozen waters, and preys like other polecats on
mice and land animals If a different case had been taken, and it had been asked
how an insectivorous quadruped could possibly have been converted into a flying
bat, the question would have been far more difficult, and I could have given no
answer. Yet I think such difficulties have very little weight.
Here, as on other
occasions, I lie under a heavy disadvantage, for out of the many striking cases
which I have collected, I can give only one or two instances of transitional
habits and structures in closely allied species of the same genus; and of diversified
habits, either constant or occasional, in the same species. And it seems to me
that nothing less than a long list of such cases is sufficient to lessen the
difficulty in any particular case like that of the bat.
Look at the family of
squirrels; here we have the finest gradation from animals with their tails only
slightly flattened, and from others, as Sir J. Richardson has remarked, with
the posterior part of their bodies rather wide and with the skin on their
flanks rather full, to the so-called flying squirrels; and flying squirrels
have their limbs and even the base of the tail united by a broad expanse of
skin, which serves as a parachute and allows them to glide through the air to
an astonishing distance from tree to tree. We cannot doubt that each structure
is of use to each kind of squirrel in its own country, by enabling it to escape
birds or beasts of prey, or to collect food more quickly, or, as there is
reason to believe, by lessening the danger from occasional falls. But it does
not follow from this fact that the structure of each squirrel is the best that
it is possible to conceive under all natural conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey immigrate,
or old ones become modified, and all analogy would lead us to believe that some
at least of the squirrels would decrease in numbers or become exterminated,
unless they also became modified and improved in structure in a corresponding
manner. Therefore, I can see no difficulty, more especially under changing
conditions of life, in the continued preservation of individuals with fuller
and fuller flank-membranes, each modification being useful, each being
propagated, until by the accumulated effects of this process of natural
selection, a perfect so-called flying squirrel was produced.
Now look at the
Galeopithecus or flying lemur, which formerly was falsely ranked amongst bats.
It has an extremely wide flank-membrane, stretching from the corners of the jaw
to the tail, and including the limbs and the elongated fingers: the flank
membrane is, also, furnished with an extensor muscle. Although no graduated
links of structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Lemuridae, yet I can see no difficulty in
supposing that such links formerly existed, and that each had been formed by
the same steps as in the case of the less perfectly gliding squirrels; and that
each grade of structure had been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might be greatly
lengthened by natural selection; and this, as far as the organs of flight are
concerned, would convert it into a bat. In bats which have the wing-membrane
extended from the top of the shoulder to the tail, including the hind-legs, we
perhaps see traces of an apparatus originally constructed for gliding through
the air rather than for flight.
If about a dozen genera
of birds had become extinct or were unknown, who would have ventured to have
surmised that birds might have existed which used their wings solely as
flappers, like the logger-headed duck (Micropterus of Eyton); as fins in the
water and front legs on the land, like the penguin; as sails, like the ostrich;
and functionally for no purpose, like the Apteryxi Yet the structure of each of
these birds is good for it, under the conditions of life to which it is
exposed, for each has to live by a struggle; but it is not necessarily the best
possible under all possible conditions. It must not be inferred from these
remarks that any of the grades of wing-structure here alluded to, which perhaps
may all have resulted from disuse, indicate the natural steps by which birds
have acquired their perfect power of flight; but they serve, at least, to show
what diversified means of transition are possible.
Seeing that a few
members of such water-breathing classes as the Crustacea and Mollusca are
adapted to live on the land, and seeing that we have flying birds and mammals,
flying insects of the most diversified types, and formerly had flying reptiles,
it is conceivable that flying-fish, which now glide far through the air,
slightly rising and turning by the aid of their fluttering fins, might have
been modified into perfectly winged animals. If early transitional state they
had been inhabitants of the open ocean, and had used their incipient organs of
flight exclusively, as far as we know, to escape being devoured by other fish?
When we see any
structure highly perfected for any particular habit, as the wings of a bird for
flight, we should bear in mind that animals displaying early transitional
grades of the structure will seldom continue to exist to the present day, for
they will have been supplanted by the very process of perfection through
natural selection. Furthermore, we may conclude that transitional grades
between structures fitted for very different habits of life will rarely have
been developed at an early period in great numbers and under many subordinate
forms. Thus, to return to our imaginary illustration of the flying-fish, it
does not seem probable that fishes capable of true flight would have been
developed under many subordinate forms, for taking prey of many kinds in many ways,
on the land and in the water, until their organs of flight had come to a high
stage of perfection, so as to have given them a decided advantage over other
animals in the battle for life. Hence the chance of discovering species with
transitional grades of structure in a fossil condition will always be less,
from their having existed in lesser numbers, than in the case of species with
fully developed structures.
I will now give two or
three instances of diversified and of changed habits in the individuals of the
same species. When either case occurs, it would be easy for natural selection
to fit the animal, by some modification of its structure, for its changed
habits, or exclusively for one of its several different habits. But it is
difficult to tell, and immaterial for us, whether habits generally change first
and structure afterwards; or whether slight modifications of structure lead to
changed habits; both probably often change almost simultaneously. Of cases of
changed habits it will suffice merely to allude to that of the many British
insects which now feed on exotic plants, or exclusively on artificial
substances. Of diversified habits innumerable instances could be given: I have
often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,
hovering over one spot and then proceeding to another, like a kestrel, and at
other times standing stationary on the margin of water, and then dashing like a
kingfisher at a fish. In our own country the larger titmouse (parus major) may
be seen climbing branches, almost like a creeper; it often, like a shrike,
kills small birds by blows on the head; and I have many times seen and heard it
hammering the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne swimming for hours
with widely open mouth, thus catching, like a whale, insects in the water Even
in so extreme a case as this, if the supply of insects were constant, and if
better adapted competitors did not already exist in the country, I can see no
difficulty in a race of bears being rendered, by natural selection, more and
more aquatic in their structure and habits, with larger and larger mouths, till
a creature was produced as monstrous as a whale.
As we sometimes see
individuals of a species following habits widely different from those both of
their own species and of the other species of the same genus, we might expect,
on my theory, that such individuals would occasionally have given rise to new
species, having anomalous habits, and with their structure either slightly or
considerably modified from that of their proper type. And such instances do
occur in nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and for seizing insects in the chinks of the
bark? Yet in North America there are woodpeckers which feed largely on fruit,
and others with elongated wings which chase insects on the wing; and on the
plains of La plata, where not a tree grows, there is a woodpecker, which in
every essential part of its organisation, even in its colouring, in the harsh
tone of its voice, and undulatory flight, told me plainly of its close
blood-relationship to our common species; yet it is a woodpecker which never
climbs a tree!
Petrels are the most
aerial and oceanic of birds, yet in the quiet Sounds of Tierra del Fuego, the
puffinuria berardi, in its general habits, in its astonishing power of diving,
its manner of swimming, and of flying when unwillingly it takes flight, would
be mistaken by any one for an auk or grebe; nevertheless, it is essentially a
petrel, but with many parts of its organisation profoundly modified. On the
other hand, the acutest observer by examining the dead body of the water-ouzel
would never have suspected its sub-aquatic habits; yet this anomalous member of
the strictly terrestrial thrush family wholly subsists by diving, -- grasping
the stones with its feet and using its wings under water.
He who believes that
each being has been created as we now see it, must occasionally have felt
surprise when he has met with an animal having habits and structure not at all
in agreement. What can be plainer than that the webbed feet of ducks and geese
are formed for swimming; yet there are upland geese with webbed feet which rarely
or never go near the water; and no one except Audubon has seen the frigate-
bird, which has all its four toes webbed, alight on the surface of the sea. On
the other hand, grebes and coots are eminently aquatic, although their toes are
only bordered by membrane. What seems plainer than that the long toes of
grallatores are formed for walking over swamps and floating plants, yet the
water-hen is nearly as aquatic as the coot; and the landrail nearly as
terrestrial as the quail or partridge. In such cases, and many others could be
given, habits have changed without a corresponding change of structure. The
webbed feet of the upland goose may be said to have become rudimentary in
function, though not be structure. In the frigate-bird, the deeply-scooped membrane
between the toes shows that structure has begun to change.
He who believes in
separate and innumerable acts of creation will say, that in these cases it has
pleased the Creator to cause a being of one type to take the place of one of
another type; but this seems to me only restating the fact in dignified
language. He who believes in the struggle for existence and in the principle of
natural selection, will acknowledge that every organic being is constantly
endeavouring to increase in numbers; and that if any one being vary ever so
little, either in habits or structure, and thus gain an advantage over some
other inhabitant of the country, it will seize on the place of that inhabitant,
however different it may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with webbed feet, either
living on the dry land or most rarely alighting on the water; that there should
be long-toed corncrakes living in meadows instead of in swamps; that there
should be woodpeckers where not a tree grows; that there should be diving
thrushes, and petrels with the habits of auks.
Organs of extreme
perfection and complication. To suppose that the eye, with all its inimitable
contrivances for adjusting the focus to different distances, for admitting
different amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I freely
confess, absurd in the highest possible degree. Yet reason tells me, that if numerous
gradations from a perfect and complex eye to one very imperfect and simple,
each grade being useful to its possessor, can be shown to exist; if further,
the eye does vary ever so slightly, and the variations be inherited, which is
certainly the case; and if any variation or modification in the organ be ever
useful to an animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural selection,
though insuperable by our imagination, can hardly be considered real. How a
nerve comes to be sensitive to light, hardly concerns us more than how life
itself first originated; but I may remark that several facts make me suspect
that any sensitive nerve may be rendered sensitive to light, and likewise to
those coarser vibrations of the air which produce sound.
In looking for the
gradations by which an organ in any species has been perfected, we ought to
look exclusively to its lineal ancestors; but this is scarcely ever possible,
and we are forced in each case to look to species of the same group, that is to
the collateral descendants from the same original parent-form, in order to see
what gradations are possible, and for the chance of some gradations having been
transmitted from the earlier stages of descent, in an unaltered or little
altered condition. Amongst existing Vertebrata, we find but a small amount of
gradation in the structure of the eye, and from fossil species we can learn
nothing on this head In this great class we should probably have to descend far
beneath the lowest known fossiliferous stratum to discover the earlier stages,
by which the eye has been perfected.
In the Articulata we
can commence a series with an optic nerve merely coated with pigment, and
without any other mechanism; and from this low stage, numerous gradations of
structure, branching off in two fundamentally different lines, can be shown to
exist, until we reach a moderately high stage of perfection. In certain
crustaceans, for instance, there is a double cornea, the inner one divided into
facets, within each of which there is a lens shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and which
properly act only by excluding lateral pencils of light, are convex at their
upper ends and must act by convergence; and at their lower ends there seems to
be an imperfect vitreous substance. With these facts, here far too briefly and
imperfectly given, which show that there is much graduated diversity in the
eyes of living crustaceans, and bearing in mind how small the number of living
animals is in proportion to those which have become extinct, I can see no very
great difficulty (not more than in the case of many other structures) in
believing that natural selection has converted the simple apparatus of an optic
nerve merely coated with pigment and invested by transparent membrane, into an
optical instrument as perfect as is possessed by any member of the great
Articulate class.
He who will go thus
far, if he find on finishing this treatise that large bodies of facts,
otherwise inexplicable, can be explained by the theory of descent, ought not to
hesitate to go further, and to admit that a structure even as perfect as the
eye of an eagle might be formed by natural selection, although in this case he
does not know any of the transitional grades. His reason ought to conquer his
imagination; though I have felt the difficulty far too keenly to be surprised
at any degree of hesitation in extending the principle of natural selection to
such startling lengths.
It is scarcely possible
to avoid comparing the eye to a telescope. We know that this instrument has
been perfected by the long-continued efforts of the highest human intellects;
and we naturally infer that the eye has been formed by a somewhat analogous
process. But may not this inference be presumptuous? Have we any right to
assume that the Creator works by intellectual powers like those of man? If we
must compare the eye to an optical instrument, we ought in imagination to take
a thick layer of transparent tissue, with a nerve sensitive to light beneath,
and then suppose every part of this layer to be continually changing slowly in
density, so as to separate into layers of different densities and thicknesses,
placed at different distances from each other, and with the surfaces of each
layer slowly changing in form. Further we must suppose that there is a power
always intently watching each slight accidental alteration in the transparent
layers; and carefully selecting each alteration which, under varied
circumstances, may in any way, or in any degree, tend to produce a distincter
image. We must suppose each new state of the instrument to be multiplied by the
million;, and each to be preserved till a better be produced, and then the old
ones to be destroyed. In living bodies, variation will cause the slight
alterations, generation will multiply them almost infinitely, and natural
selection will pick out with unerring skill each improvement. Let this process
go on for millions on millions of years; and during each year on millions of
individuals of many kinds; and may we not believe that a living optical
instrument might thus be formed as superior to one of glass, as the works of
the Creator are to those of man?
If it could be
demonstrated that any complex organ existed, which could not possibly have been
formed by numerous, successive, slight modifications, my theory would
absolutely break down. But I can find out no such case. No doubt many organs
exist of which we do not know the transitional grades, more especially if we
look to much-isolated species, round which, according to my theory, there has
been much extinction. Or again, if we look to an organ common to all the
members of a large class, for in this latter case the organ must have been
first formed at an extremely remote period, since which all the many members of
the class have been developed; and in order to discover the early transitional
grades through which the organ has passed, we should have to look to very
ancient ancestral forms, long since become extinct.
We should be extremely
cautious in concluding that an organ could not have been formed by transitional
gradations of some kind. Numerous cases could be given amongst the lower
animals of the same organ performing at the same time wholly distinct
functions; thus the alimentary canal respires, digests, and excretes in the
larva of the dragon- fly and in the fish Cobites. In the Hydra, the animal may
be turned inside out, and the exterior surface will then digest and the stomach
respire. In such cases natural selection might easily specialise, if any
advantage were thus gained, a part or organ, which had performed two functions,
for one function alone, and thus wholly change its nature by insensible steps.
Two distinct organs sometimes perform simultaneously the same function in the
same individual; to give one instance, there are fish with gills or branchiae
that breathe the air dissolved in the water, at the same time that they breathe
free air in their swimbladders, this latter organ having a ductus pneumaticus
for its supply, and being divided by highly vascular partitions. In these
cases, one of the two organs might with ease be modified and perfected so as to
perform all the work by itself, being aided during the process of modification
by the other organ; and then this other organ might be modified for some other
and quite distinct purpose, or be quite obliterated.
The illustration of the
swimbladder in fishes is a good one, because it shows us clearly the highly
important fact that an organ originally constructed for one purpose, namely
flotation, may be converted into one for a wholly different purpose, namely
respiration. The swimbladder has, also, been worked in as an accessory to the
auditory organs of certain fish, or, for I do not know which view is now
generally held, a part of the auditory apparatus has been worked in as a
complement to the swimbladder. All physiologists admit that the swimbladder is
homologous, or 'ideally similar,' in position and structure with the lungs of
the higher vertebrate animals: hence there seems to me to be no great
difficulty in believing that natural selection has actually converted a
swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly
doubt that all vertebrate animals having true lungs have descended by ordinary
generation from an ancient prototype, of which we know nothing, furnished with
a floating apparatus or swimbladder. We can thus, as I infer from professor Owen's
interesting description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the orifice of the
trachea, with some risk of falling into the lungs, notwithstanding the
beautiful contrivance by which the glottis is closed. In the higher Vertebrata
the branchiae have wholly disappeared -- the slits on the sides of the neck and
the loop-like course of the arteries still marking in the embryo their former
position. But it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some quite distinct purpose:
in the same manner as, on the view entertained by some naturalists that the
branchiae and dorsal scales of Annelids are homologous with the wings and
wing-covers of insects, it is probable that organs which at a very ancient
period served for respiration have been actually converted into organs of
flight.
In considering
transitions of organs, it is so important to bear in mind the probability of
conversion from one function to another, that I will give one more instance.
pedunculated cirripedes have two minute folds of skin, called by me the
ovigerous frena, which serve, through the means of a sticky secretion, to
retain the eggs until they are hatched within the sack. These cirripedes have
no branchiae, the whole surface of the body and sack, including the small
frena, serving for respiration. The Balanidae or sessile cirripedes, on the
other hand, have no ovigerous frena, the eggs lying loose at the bottom of the
sack, in the well-enclosed shell; but they have large folded branchiae. Now I
think no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchiae of the other family; indeed, they graduate
into each other. Therefore I do not doubt that little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly aided
the act of respiration, have been gradually converted by natural selection into
branchiae, simply through an increase in their size and the obliteration of
their adhesive glands. If all pedunculated cirripedes had become extinct, and
they have already suffered far more extinction than have sessile cirripedes,
who would ever have imagined that the branchiae in this latter family had
originally existed as organs for preventing the ova from being washed out of
the sack?
Although we must be
extremely cautious in concluding that any organ could not possibly have been
produced by successive transitional gradations, yet, undoubtedly, grave cases
of difficulty occur, some of which will be discussed in my future work.
One of the gravest is
that of neuter insects, which are often very differently constructed from
either the males or fertile females; but this case will be treated of in the
next chapter. The electric organs of fishes offer another case of special
difficulty; it is impossible to conceive by what steps these wondrous organs
have been produced; but, as Owen and others have remarked, their intimate structure
closely resembles that of common muscle; and as it has lately been shown that
Rays have an organ closely analogous to the electric apparatus, and yet do not,
as Matteuchi asserts, discharge any electricity, we must own that we are far
too ignorant to argue that no transition of any kind is possible.
The electric organs
offer another and even more serious difficulty; for they occur in only about a
dozen fishes, of which several are widely remote in their affinities. Generally
when the same organ appears in several members of the same class, especially if
in members having very different habits of life, we may attribute its presence
to inheritance from a common ancestor; and its absence in some of the members
to its loss through disuse or natural selection. But if the electric organs had
been inherited from one ancient progenitor thus provided, we might have
expected that all electric fishes would have been specially related to each
other. Nor does geology at all lead to the belief that formerly most fishes had
electric organs, which most of their modified descendants have lost. The
presence of luminous organs in a few insects, belonging to different families
and orders, offers a parallel case of difficulty. Other cases could be given;
for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with a sticky gland at the end, is the
same in Orchis and Asclepias, -- genera almost as remote as possible amongst
flowering plants. In all these cases of two very distinct species furnished
with apparently the same anomalous organ, it should be observed that, although
the general appearance and function of the organ may be the same, yet some
fundamental difference can generally be detected. I am inclined to believe that
in nearly the same way as two men have sometimes independently hit on the very
same invention, so natural selection, working for the good of each being and
taking advantage of analogous variations, has sometimes modified in very nearly
the same manner two parts in two organic beings, which owe but little of their
structure in common to inheritance from the same ancestor.
Although in many cases
it is most difficult to conjecture by what transitions an organ could have
arrived at its present state; yet, considering that the proportion of living
and known forms to the extinct and unknown is very small, I have been
astonished how rarely an organ can be named, towards which no transitional
grade is known to lead. The truth of this remark is indeed shown by that old
canon in natural history of 'Natura non facit saltum.' We meet with this
admission in the writings of almost every experienced naturalist; or, as Milne
Edwards has well expressed it, nature is prodigal in variety, but niggard in
innovation. Why, on the theory of Creation, should this be so? Why should all
the parts and organs of many independent beings, each supposed to have been
separately created for its proper place in nature, be so invariably linked
together by graduated steps? Why should not Nature have taken a leap from
structure to structure? On the theory of natural selection, we can clearly
understand why she should not; for natural selection can act only by taking
advantage of slight successive variations; she can never take a leap, but must
advance by the shortest and slowest steps.
Organs of little
apparent importance. As natural selection acts by life and death, -- by the
preservation of individuals with any favourable variation, and by the
destruction of those with any unfavourable deviation of structure, -- I have
sometimes felt much difficulty in understanding the origin of simple parts, of
which the importance does not seem sufficient to cause the preservation of
successively varying individuals. I have sometimes felt as much difficulty,
though of a very different kind, on this head, as in the case of an organ as
perfect and complex as the eye.
In the first place, we
are much too ignorant in regard to the whole economy of any one organic being,
to say what slight modifications would be of importance or not. In a former
chapter I have given instances of most trifling characters, such as the down on
fruit and the colour of the flesh, which, from determining the attacks of
insects or from being correlated with constitutional differences, might
assuredly be acted on by natural selection. The tail of the giraffe looks like
an artificially constructed fly-flapper; and it seems at first incredible that
this could have been adapted for its present purpose by successive slight
modifications, each better and better, for so trifling an object as driving
away flies; yet we should pause before being too positive even in this case,
for we know that the distribution and existence of cattle and other animals in
South America absolutely depends on their power of resisting the attacks of
insects: so that individuals which could by any means defend themselves from
these small enemies, would be able to range into new pastures and thus gain a
great advantage. It is not that the larger quadrupeds are actually destroyed
(except in some rare cases) by the flies, but they are incessantly harassed and
their strength reduced, so that they are more subject to disease, or not so
well enabled in a coming dearth to search for food, or to escape from beasts of
prey.
Organs now of trifling
importance have probably in some cases been of high importance to an early
progenitor, and, after having been slowly perfected at a former period, have
been transmitted in nearly the same state, although now become of very slight
use; and any actually injurious deviations in their structure will always have
been checked by natural selection. Seeing how important an organ of locomotion
the tail is in most aquatic animals, its general presence and use for many
purposes in so many land animals, which in their lungs or modified
swim-bladders betray their aquatic origin, may perhaps be thus accounted for. A
well-developed tail having been formed in an aquatic animal, it might
subsequently come to be worked in for all sorts of purposes, as a fly-flapper,
an organ of prehension, or as an aid in turning, as with the dog, though the
aid must be slight, for the hare, with hardly any tail, can double quickly
enough.
In the second place, we
may sometimes attribute importance to characters which are really of very
little importance, and which have originated from quite secondary causes,
independently of natural selection. We should remember that climate, food,
&c., probably have some little direct influence on the organisation; that
characters reappear from the law of reversion;, that correlation of growth will
have had a most important influence in modifying various structures; and
finally, that sexual selection will often have largely modified the external
characters of animals having a will, to give one male an advantage in fighting
with another or in charming the females. Moreover when a modification of
structure has primarily arisen from the above or other unknown causes, it may
at first have been of no advantage to the species, but may subsequently have
been taken advantage of by the descendants of the species under new conditions
of life and with newly acquired habits.
To give a few instances
to illustrate these latter remarks. If green woodpeckers alone had existed, and
we did not know that there were many black and pied kinds, I dare say that we
should have thought that the green colour was a beautiful adaptation to hide
this tree-frequenting bird from its enemies; and consequently that it was a
character of importance and might have been acquired through natural selection;
as it is, I have no doubt that the colour is due to some quite distinct cause,
probably to sexual selection. A trailing bamboo in the Malay Archipelago climbs
the loftiest trees by the aid of exquisitely constructed hooks clustered around
the ends of the branches, and this contrivance, no doubt, is of the highest
service to the plant; but as we see nearly similar hooks on many trees which
are not climbers the hooks on the bamboo may have arisen from unknown laws of
growth, and have been subsequently taken advantage of by the plant undergoing
further modification and becoming a climber. The naked skin on the head of a
vulture is generally looked at as a direct adaptation for wallowing in
putridity; and so it may be, or it may possibly be due to the direct action of
putrid matter; but we should be very cautious in drawing any such inference,
when we see that the skin on the head of the clean-feeding male turkey is
likewise naked. The sutures in the skulls of young mammals have been advanced
as a beautiful adaptation for aiding parturition, and no doubt they facilitate,
or may be indispensable for this act; but as sutures occur in the skulls of
young birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been
taken advantage of in the parturition of the higher animals.
We are profoundly
ignorant of the causes producing slight and unimportant variations; and we are
immediately made conscious of this by reflecting on the differences in the
breeds of our domesticated animals in different countries, -- more especially
in the less civilized countries where there has been but little artificial
selection. Careful observers are convinced that a damp climate affects the
growth of the hair, and that with the hair the horns are correlated. Mountain
breeds always differ from lowland breeds; and a mountainous country would
probably affect the hind limbs from exercising them more, and possibly even the
form of the pelvis; and then by the law of homologous variation, the front
limbs and even the head would probably be affected. The shape, also, of the
pelvis might affect by pressure the shape of the head of the young in the womb.
The laborious breathing necessary in high regions would, we have some reason to
believe, increase the size of the chest; and again correlation would come into
play. Animals kept by savages in different countries often have to struggle for
their own subsistence, and would be exposed to a certain extent to natural
selection, and individuals with slightly different constitutions would succeed
best under different climates; and there is reason to believe that constitution
and colour are correlated. A good observer, also, states that in cattle
susceptibility to the attacks of flies is correlated with colour, as is the
liability to be poisoned by certain plants; so that colour would be thus
subjected to the action of natural selection. But we are far too ignorant to
speculate on the relative importance of the several known and unknown laws of
variation; and I have here alluded to them only to show that, ff we are unable
to account for the characteristic differences of our domestic breeds, which
nevertheless we generally admit to have arisen through ordinary generation, we
ought not to lay too much stress on our ignorance of the precise cause of the
slight analogous differences between species. I might have adduced for this
same purpose the differences between the races of man, which are so strongly
marked; I may add that some little light can apparently be thrown on the origin
of these differences, chiefly through sexual selection of a particular kind,
but without here entering on copious details my reasoning would appear
frivolous.
The foregoing remarks
lead me to say a few words on the protest lately made by some naturalists,
against the utilitarian doctrine that every detail of structure has been
produced for the good of its possessor. They believe that very many structures
have been created for beauty in the eyes of man, or for mere variety. This
doctrine, if true, would be absolutely fatal to my theory. Yet I fully admit
that many structures are of no direct use to their possessors. physical
conditions probably have had some little effect on structure, quite
independently of any good thus gained. Correlation of growth has no doubt
played a most important part, and a useful modification of one part will often
have entailed on other parts diversified changes of no direct use. So again
characters which formerly were useful, or which formerly had arisen from
correlation of growth, or from other unknown cause, may reappear from the law
of reversion, though now of no direct use. The effects of sexual selection,
when displayed in beauty to charm the females, can be called useful only in
rather a forced sense. But by far the most important consideration is that the
chief part of the organisation of every being is simply due to inheritance; and
consequently, though each being assuredly is well fitted for its place in
nature, many structures now have no direct relation to the habits of life of
each species. Thus, we can hardly believe that the webbed feet of the upland
goose or of the frigate- bird are of special use to these birds; we cannot
believe that the same bones in the arm of the monkey, in the fore leg of the
horse, in the wing of the bat, and in the flipper of the seal, are of special
use to these animals. We may safely attribute these structures to inheritance.
But to the progenitor of the upland goose and of the frigate-bird, webbed feet
no doubt were as useful as they now are to the most aquatic of existing birds.
So we may believe that the progenitor of the seal had not a flipper, but a foot
with five toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse, and bat,
which have been inherited from a common progenitor, were formerly of more
special use to that progenitor, or its progenitors, than they now are to these
animals having such widely diversified habits. Therefore we may infer that
these several bones might have been acquired through natural selection,
subjected formerly, as now, to the several laws of inheritance, reversion,
correlation of growth, &c. Hence every detail of structure in every living
creature (making some little allowance for the direct action of physical
conditions) may be viewed, either as having been of special use to some
ancestral form, or as being now of special use to the descendants of this form
- either directly, or indirectly through the complex laws of growth.
Natural selection
cannot possibly produce any modification in any one species exclusively for the
good of another species; though throughout nature one species incessantly takes
advantage of, and profits by, the structure of another. But natural selection
can and does often produce structures for the direct injury of other species,
as we see in the fang of the adder, and in the ovipositor of the ichneumon, by
which its eggs are deposited in the living bodies of other insects. If it could
be proved that any part of the structure of any one species had been formed for
the exclusive good of another species, it would annihilate my theory, for such
could not have been produced through natural selection. Although many
statements may be found in works on natural history to this effect, I cannot
find even one which seems to me of any weight. It is admitted that the
rattlesnake has a poison-fang for its own defence and for the destruction of its
prey; but some authors suppose that at the same time this snake is furnished
with a rattle for its own injury, namely, to warn its prey to escape. I would
almost as soon believe that the cat curls the end of its tail when preparing to
spring, in order to warn the doomed mouse. But I have not space here to enter
on this and other such cases.
Natural selection will
never produce in a being anything injurious to itself, for natural selection
acts solely by and for the good of each. No organ will be formed, as Paley has
remarked, for the purpose of causing pain or for doing an injury to its
possessor. If a fair balance be struck between the good and evil caused by each
part, each will be found on the whole advantageous. After the lapse of time,
under changing conditions of life, if any part comes to be injurious, it will
be modified; or if it be not so, the being will become extinct, as myriads have
become extinct.
Natural selection tends
only to make each organic being as perfect as, or slightly more perfect than,
the other inhabitants of the same country with which it has to struggle for
existence. And we see that this is the degree of perfection attained under
nature. The endemic productions of New Zealand, for instance, are perfect one
compared with another; but they are now rapidly yielding before the advancing
legions of plants and animals introduced from Europe. Natural selection will
not produce absolute perfection, nor do we always meet, as far as we can judge,
with this high standard under nature. The correction for the aberration of
light is said, on high authority, not to be perfect even in that most perfect
organ, the eye. If our reason leads us to admire with enthusiasm a multitude of
inimitable contrivances in nature, this same reason tells us, though we may
easily err on both sides, that some other contrivances are less perfect. Can we
consider the sting of the wasp or of the bee as perfect, which, when used
against many attacking animals, cannot be withdrawn, owing to the backward
serratures, and so inevitably causes the death of the insect by tearing out its
viscera?
If we look at the sting
of the bee, as having originally existed in a remote progenitor as a boring and
serrated instrument, like that in so many members of the same great order, and
which has been modified but not perfected for its present purpose, with the
poison originally adapted to cause galls subsequently intensified, we can
perhaps understand how it is that the use of the sting should so often cause
the insect's own death: for if on the whole the power of stinging be useful to
the community, it will fulfil all the requirements of natural selection, though
it may cause the death of some few members. If we admire the truly wonderful
power of scent by which the males of many insects find their females, can we
admire the production for this single purpose of thousands of drones, which are
utterly useless to the community for any other end, and which are ultimately
slaughtered by their industrious and sterile sisters? It may be difficult, but
we ought to admire the savage instinctive hatred of the queen-bee, which urges
her instantly to destroy the young queens her daughters as soon as born, or to
perish herself in the combat; for undoubtedly this is for the good of the
community;, and maternal love or maternal hatred, though the latter fortunately
is most rare, is all the same to the inexorable principle of natural selection.
If we admire the several ingenious contrivances, by which the flowers of the
orchis and of many other plants are fertilised through insect agency, can we
consider as equally perfect the elaboration by our fir-trees of dense clouds of
pollen, in order that a few granules may be wafted by a chance breeze on to the
ovules?
Summary of Chapter. We
have in this chapter discussed some of the difficulties and objections which
may be urged against my theory. Many of them are very grave; but I think that
in the discussion light has been thrown on several facts, which on the theory
of independent acts of creation are utterly obscure. We have seen that species
at any one period are not indefinitely variable, and are not finked together by
a multitude of intermediate gradations, partly because the process of natural
selection will always be very slow, and will act, at any one time, only on a
very few forms; and partly because the very process of natural selection almost
implies the continual supplanting and extinction of preceding and intermediate
gradations. Closely allied species, now living on a continuous area, must often
have been formed when the area was not continuous, and when the conditions of
life did not insensibly graduate away from one part to another. When two
varieties are formed in two districts of a continuous area, an intermediate
variety will often be formed, fitted for an intermediate zone; but from reasons
assigned, the intermediate variety will usually exist in lesser numbers than
the two forms which it connects; consequently the two latter, during the course
of further modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus generally
succeed in supplanting and exterminating it.
We have seen in this
chapter how cautious we should be in concluding that the most different habits
of life could not graduate into each other; that a bat, for instance, could not
have been formed by natural selection from an animal which at first could only
glide through the air.
We have seen that a
species may under new conditions of life change its habits, or have diversified
habits, with some habits very unlike those of its nearest congeners. Hence we
can understand bearing in mind that each organic being is trying to live
wherever it can live, how it has arisen that there are upland geese with webbed
feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief
that an organ so perfect as the eye could have been formed by natural
selection, is more than enough to stagger any one; yet in the case of any
organ, if we know of a long series of gradations in complexity, each good for
its possessor, then, under changing conditions of life, there is no logical
impossibility in the acquirement of any conceivable degree of perfection
through natural selection. In the cases in which we know of no intermediate or
transitional states, we should be very cautious in concluding that none could
have existed, for the homologies of many organs and their intermediate states
show that wonderful metamorphoses in function are at least possible. For
instance, a swim-bladder has apparently been converted into an air-breathing
lung. The same organ having performed simultaneously very different functions,
and then having been specialised for one function; and two very distinct organs
having performed at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely facilitated
transitions.
We are far too
ignorant, in almost every case, to be enabled to assert that any part or organ
is so unimportant for the welfare of a species, that modifications in its
structure could not have been slowly accumulated by means of natural selection.
But we may confidently believe that many modifications, wholly due to the laws
of growth, and at first in no way advantageous to a species, have been
subsequently taken advantage of by the still further modified descendants of
this species. We may, also, believe that a part formerly of high importance has
often been retained (as the tail of an aquatic animal by its terrestrial
descendants), though it has become of such small importance that it could not,
in its present state, have been acquired by natural selection, -- a power which
acts solely by the preservation of profitable variations in the struggle for life.
Natural selection will
produce nothing in one species for the exclusive good or injury of another;
though it may well produce parts, organs, and excretions highly useful or even
indispensable, or highly injurious to another species, but in all cases at the
same time useful to the owner. Natural selection in each well-stocked country,
must act chiefly through the competition of the inhabitants one with another,
and consequently will produce perfection, or strength in the battle for life,
only according to the standard of that country. Hence the inhabitants of one
country, generally the smaller one, will often yield, as we see they do yield,
to the inhabitants of another and generally larger country. For in the larger
country there will have existed more individuals, and more diversified forms,
and the competition will have been severer, and thus the standard of perfection
will have been rendered higher. Natural selection will not necessarily produce
absolute perfection; nor, as far as we can judge by our limited faculties, can
absolute perfection be everywhere found.
On the theory of
natural selection we can clearly understand the full meaning of that old canon
in natural history, 'Natura non facit saltum.' This canon, if we look only to
the present inhabitants of the world, is not strictly correct, but if we
include all those of past times, it must by my theory be strictly true.
It is generally
acknowledged that all organic beings have been formed on two great laws - Unity
of Type, and the Conditions of Existence. By unity of type is meant that
fundamental agreement in structure, which we see in organic beings of the same
class, and which is quite independent of their habits of life. On my theory,
unity of type is explained by unity of descent. The expression of conditions of
existence, so often insisted on by the illustrious Cuvier, is fully embraced by
the principle of natural selection. For natural selection acts by either now
adapting the varying parts of each being to its organic and inorganic conditions
of life; or by having adapted them during long-past periods of time: the
adaptations being aided in some cases by use and disuse, being slightly
affected by the direct action of the external conditions of life, and being in
all cases subjected to the several laws of growth. Hence, in fact, the law of
the Conditions of Existence is the higher law; as it includes, through the
inheritance of former adaptations, that of Unity of Type.
Instincts comparable with habits, but different in their origin -
Instincts graduated - Aphides and ants - Instincts variable - Domestic
instincts, their origin - Natural instincts of the cuckoo, ostrich, and
parasitic bees - Slave-making ants - Hive-bee, its cell-making instinct -
Difficulties on the theory of the Natural Selection of instincts - Neuter or
sterile insects - Summary THE
subject of instinct might have been worked into the previous chapters; but I
have thought that it would be more convenient to treat the subject separately,
especially as so wonderful an instinct as that of the hive-bee making its cells
will probably have occurred to many readers, as a difficulty sufficient to
overthrow my whole theory. I must premise, that I have nothing to do with the
origin of the primary mental powers, any more than I have with that of life
itself. We are concerned only with the diversities of instinct and of the other
mental qualities of animals within the same class.
I will not attempt any
definition of instinct. It would be easy to show that several distinct mental
actions are commonly embraced by this term; but every one understands what is
meant, when it is said that instinct impels the cuckoo to migrate and to lay
her eggs in other birds' nests. Am action, which we ourselves should require
experience to enable us to perform, when performed by an animal, more
especially by a very young one, without any experience, and when performed by
many individuals in the same way, without their knowing for what purpose it is
performed, is usually said to be instinctive. But I could show that none of
these characters of instinct are universal. A little dose, as Pierre Huber
expresses it, of judgment or reason, often comes into play, even in animals
very low in the scale of nature.
Frederick Cuvier and
several of the older metaphysicians have compared instinct with habit. This
comparison gives, I think, a remarkably accurate notion of the frame of mind
under which an instinctive action is performed, but not of its origin. How
unconsciously many habitual actions are performed, indeed not rarely in direct
opposition to our conscious will! yet they may be modified by the will or
reason. Habits easily become associated with other habits, and with certain
periods of time and states of the body. When once acquired, they often remain
constant throughout life. Several other points of resemblance between instincts
and habits could be pointed out. As in repeating a well- known song, so in
instincts, one action follows another by a sort of rhythm; if a person be
interrupted in a song, or in repeating anything by rote, he is generally forced
to go back to recover the habitual train of thought: so p. Huber found it was
with a caterpillar, which makes a very complicated hammock; for if he took a
caterpillar which had completed its hammock up to, say, the sixth stage of
construction, and put it into a hammock completed up only to the third stage,
the caterpillar simply re-performed the fourth, fifth, and sixth stages of
construction. If, however, a caterpillar were taken out of a hammock made up,
for instance, to the third stage, and were put into one finished up to the
sixth stage, so that much of its work was already done for it, far from feeling
the benefit of this, it was much embarrassed, and, in order to complete its
hammock, seemed forced to start from the third stage, where it had left off,
and thus tried to complete the already finished work.
If we suppose any
habitual action to become inherited -- and I think it can be shown that this
does sometimes happen - then the resemblance between what originally was a
habit and an instinct becomes so close as not to be distinguished. If Mozart,
instead of playing the pianoforte at three years old with wonderfully little
practice, had played a tune with no practice at all, be might truly be said to
have done so instinctively. But it would be the most serious error to suppose
that the greater number of instincts have been acquired by habit in one
generation, and then transmitted by inheritance to succeeding generations. It
can be clearly shown that the most wonderful instincts with which we are
acquainted, namely, those of the hive-bee and of many ants, could not possibly
have been thus acquired.
It will be universally
admitted that instincts are as important as corporeal structure for the welfare
of each species, under its present conditions of life. Under changed conditions
of life, it is at least possible that slight modifications of instinct might be
profitable to a species; and if it can be shown that instincts do vary ever so
little, then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that may be
profitable. It is thus, as I believe, that all the most complex and wonderful
instincts have originated. As modifications of corporeal structure arise from,
and are increased by, use or habit, and are diminished or lost by disuse, so I
do not doubt it has been with instincts. But I believe that the effects of
habit are of quite subordinate importance to the effects of the natural
selection of what may be called accidental variations of instincts; - that is
of variations produced by the same unknown causes which produce slight
deviations of bodily structure.
No complex instinct can
possibly be produced through natural selection, except by the slow and gradual
accumulation of numerous, slight, yet profitable, variations. Hence, as in the
case of corporeal structures, we ought to find in nature, not the actual
transitional gradations by which each complex instinct has been acquired - for
these could be found only in the lineal ancestors of each species -- but we
ought to find in the collateral lines of descent some evidence of such
gradations; or we ought at least to be able to show that gradations of some
kind are possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little observed
except in Europe and North America, and for no instinct being known amongst
extinct species, how very generally gradations, leading to the most complex
instincts, can be discovered. The canon of 'Natura non facit saltum' applies
with almost equal force to instincts as to bodily organs. Changes of instinct
may sometimes be facilitated by the same species having different instincts at
different periods of life, or at different seasons of the year, or when placed
under different circumstances, &c.; in which case either one or the other
instinct might be preserved by natural selection. And such instances of
diversity of instinct in the same species can be shown to occur in nature.
Again as in the case of
corporeal structure, and conformably with my theory, the instinct of each
species is good for itself, but has never, as far as we can judge, been
produced for the exclusive good of others. One of the strongest instances of an
animal apparently performing an action for the sole good of another, with which
I am acquainted, is that of aphides voluntarily yielding their sweet excretion
to ants: that they do so voluntarily, the following facts show. I removed all
the ants from a group of about a dozen aphides on a dock-plant, and prevented
their attendance during several hours. After this interval, I felt sure that
the aphides would want to excrete. I watched them for some time through a lens,
but not one excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennae; but not one
excreted. Afterwards I allowed an ant to visit them, and it immediately seemed,
by its eager way of running about, to be well aware what a rich flock it had
discovered; it then began to play with its antennae on the abdomen first of one
aphis and then of another; and each aphis, as soon as it felt the antennae,
immediately lifted up its abdomen and excreted a limpid drop of sweet juice,
which was eagerly devoured by the ant. Even the quite young aphides behaved in
this manner, showing that the action was instinctive, and not the result of experience.
But as the execretion is extremely viscid, it is probably a convenience to the
aphides to have it removed; and therefore probably the aphides do not
instinctively excrete for the sole good of the ants. Although I do not believe
that any animal in the world performs an action for the exclusive good of
another of a distinct species, yet each species tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily structure of
others. So again, in some few cases, certain instincts cannot be considered as
absolutely perfect; but as details on this and other such points are not
indispensable, they may be here passed over.
As some degree of
variation in instincts under a state of nature, and the inheritance of such
variations, are indispensable for the action of natural selection, as many
instances as possible ought to have been here given; but want of space prevents
me. I can only assert, that instincts certainly do vary -- for instance, the
migratory instinct, both in extent and direction, and in its total loss. So it
is with the nests of birds, which vary partly in dependence on the situations
chosen, and on the nature and temperature of the country inhabited, but often
from causes wholly unknown to us: Audubon has given several remarkable cases of
differences in nests of the same species in the northern and southern United
States. Fear of any particular enemy is certainly an instinctive quality, as
may be seen in nestling birds, though it is strengthened by experience, and by
the sight of fear of the same enemy in other animals. But fear of man is slowly
acquired, as I have elsewhere shown, by various animals inhabiting desert
islands; and we may see an instance of this, even in England, in the greater
wildness of all our large birds than of our small birds; for the large birds
have been most persecuted by man. We may safely attribute the greater wildness
of our large birds to this cause; for in uninhabited islands large birds are
not more fearful than small; and the magpie, so wary in England, is tame in
Norway, as is the hooded crow in Egypt.
That the general
disposition of individuals of the same species, born in a state of nature, is
extremely diversified, can be shown by a multitude of facts. Several cases
also, could be given, of occasional and strange habits in certain species,
which might, if advantageous to the species, give rise, through natural
selection, to quite new instincts. But I am well aware that these general
statements, without facts given in detail, can produce but a feeble effect on
the reader's mind. I can only repeat my assurance, that I do not speak without
good evidence.
The possibility, or
even probability, of inherited variations of instinct in a state of nature will
be strengthened by briefly considering a few cases under domestication. We
shall thus also be enabled to see the respective parts which habit and the
selection of so-called accidental variations have played in modifying the
mental qualities of our domestic animals. A number of curious and authentic
instances could be given of the inheritance of all shades of disposition and
tastes, and likewise of the oddest tricks, associated with certain frames of
mind or periods of time. But let us look to the familiar case of the several
breeds of dogs: it cannot be doubted that young pointers (I have myself seen a
striking instance) will sometimes point and even back other dogs the very first
time that they are taken out; retrieving is certainly in some degree inherited
by retrievers; and a tendency to run round, instead of at, a flock of sheep, by
shepherd-dogs. I cannot see that these actions, performed without experience by
the young, and in nearly the same manner by each individual, performed with
eager delight by each breed, and without the end being known, - for the young
pointer can no more know that he points to aid his master, than the white
butterfly knows why she lays her eggs on the leaf of the cabbage, - I cannot
see that these actions differ essentially from true instincts. If we were to
see one kind of wolf, when young and without any training, as soon as it
scented its prey, stand motionless like a statue, and then slowly crawl forward
with a peculiar gait; and another kind of wolf rushing round, instead of at, a
herd of deer, and driving them to a distant point, we should assuredly call
these actions instinctive. Domestic instincts, as they may be called, are
certify far less fixed or invariable than natural instincts; but they have been
acted on by far less rigorous selection, and have been transmitted for an
incomparably shorter period, under less fixed conditions of life.
How strongly these
domestic instincts, habits, and dispositions are inherited, and how curiously
they become mingled, is well shown when different breeds of dogs are crossed.
Thus it is known that a cross with a bull-dog has affected for many generations
the courage and obstinacy of greyhounds; and a cross with a greyhound has given
to a whole family of shepherd-dogs a tendency to hunt hares. These domestic instincts,
when thus tested by crossing, resemble natural instincts, which in a like
manner become curiously blended together, and for a long period exhibit traces
of the instincts of either parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a trace of its wild parentage
only in one way, by not coming in a straight line to his master when called.
Domestic instincts are
sometimes spoken of as actions which have become inherited solely from
long-continued and compulsory habit, but this, I think, is not true. No one
would ever have thought of teaching, or probably could have taught, the
tumbler-pigeon to tumble, -- an action which, as I have witnessed, is performed
by young birds, that have never seen a pigeon tumble. We may believe that some
one pigeon showed a slight tendency to this strange habit, and that the
long-continued selection of the best individuals in successive generations made
tumblers what they now are; and near Glasgow there are housetumblers, as I hear
from Mr Brent, which cannot fly eighteen inches high without going head over
heels. It may be doubted whether any one would have thought of training a dog
to point, bad not some one dog naturally shown a tendency in this line; and
this is known occasionally to happen, as I once saw in a pure terrier. When the
first tendency was once displayed, methodical selection and the inherited
effects of compulsory training in each successive generation would soon
complete the work; and unconscious selection is still at work, as each man
tries to procure, without intending to improve the breed, dogs which will stand
and hunt best. On the other hand, habit alone in some cases has sufficed; no
animal is more difficult to tame than the young of the wild rabbit; scarcely
any animal is tamer than the young of the tame rabbit; but I do not suppose
that domestic rabbits have ever been selected for tameness; and I presume that
we must attribute the whole of the inherited change from extreme wildness to
extreme tameness, simply to habit and long- continued close confinement.
Natural instincts are
lost under domestication: a remarkable instance of this is seen in those breeds
of fowls which very rarely or never become 'broody,' that is, never wish to sit
on their eggs. Familiarity alone prevents our seeing how universally and
largely the minds of our domestic animals have been modified by domestication.
It is scarcely possible to doubt that the love of man has become instinctive in
the dog. All wolves, foxes, jackals, and species of the cat genus, when kept
tame, are most eager to attack poultry, sheep, and pigs; and this tendency has
been found Incurable in dogs which have been brought home as puppies from
countries, such as Tierra del Fuego and Australia, where the savages do not
keep these domestic animals. How rarely, on the other hand, do our civilised
dogs, even when quite young, require to be taught not to attack poultry, sheep,
and pigs! No doubt they occasionally do make an attack, and are then beaten;
and if not cured, they are destroyed; so that habit, with some degree of
selection, has probably concurred in civilising by inheritance our dogs. On the
other hand, young chickens have lost, wholly by habit, that fear of the dog and
cat which no doubt was originally instinctive in them, in the same way as it is
so plainly instinctive in young pheasants, though reared under a hen. It is not
that chickens have lost all fear, but fear only of dogs and cats, for if the
hen gives the danger-chuckle, they will run (more especially young turkeys)
from under her, and conceal themselves in the surrounding grass or thickets;
and this is evidently done for the instinctive purpose of allowing, as we see
in wild ground-birds, their mother to fly away. But this instinct retained by our
chickens has become useless under domestication, for the mother-hen has almost
lost by disuse the power of flight.
Hence, we may conclude,
that domestic instincts have been acquired and natural instincts have been lost
partly by habit, and partly by man selecting and accumulating during successive
generations, peculiar mental habits and actions, which at first appeared from
what we must in our ignorance call an accident. In some cases compulsory habit
alone has sufficed to produce such inherited mental changes; in other cases
compulsory habit has done nothing, and all has been the result of selection,
pursued both methodically and unconsciously; but in most cases, probably, habit
and selection have acted together.
We shall, perhaps, best
understand how instincts in a state of nature have become modified by
selection, by considering a few cases. I will select only three, out of the
several which I shall have to discuss in my future work, -- namely, the
instinct which leads the cuckoo to lay her eggs in other birds' nests; the
slave-making instinct of certain ants; and the comb-making power of the
hive-bee: these two latter instincts have generally, and most justly, been
ranked by naturalists as the most wonderful of all known instincts.
It is now commonly
admitted that the more immediate and final cause of the cuckoo's instinct is,
that she lays her eggs, not daily, but at intervals of two or three days; so
that, if she were to make her own nest and sit on her own eggs, those first
laid would have to be left for some time unincubated, or there would be eggs
and young birds of different ages in the same nest. If this were the case, the
process of laying and hatching might be inconveniently long, more especially as
she has to migrate at a very early period; and the first hatched young would
probably have to be fed by the male alone. But the American cuckoo is in this
predicament; for she makes her own nest and has eggs and young successively
hatched, all at the same time. It has been asserted that the American cuckoo
occasionally lays her eggs in other birds' nests; but I hear on the high
authority of Dr Brewer, that this is a mistake. Nevertheless, I could give
several instances of various birds which have been known occasionally to lay
their eggs in other birds' nests. Now let us suppose that the ancient
progenitor of our European cuckoo had the habits of the American cuckoo; but
that occasionally she laid an egg in another bird's nest. If the old bird
profited by this occasional habit, or if the young were made more vigorous by
advantage having been taken of the mistaken maternal instinct of another bird,
than by their own mother's care, encumbered as she can hardly fail to be by
having eggs and young of different ages at the same time; then the old birds or
the fostered young would gain an advantage. And analogy would lead me to
believe, that the young thus reared would be apt to follow by inheritance the
occasional and aberrant habit of their mother, and in their turn would be apt
to lay their eggs in other birds' nests, and thus be successful in rearing
their young. By a continued process of this nature, I believe that the strange
instinct of our cuckoo could be, and has been. generated. I may add that,
according to Dr Gray and to some other observers, the European cuckoo has not
utterly lost all maternal love and care for her own offspring.
The occasional habit of
birds laying their eggs in other birds' nests, either of the same or of a
distinct species, is not very uncommon with the Gallinaceae; and this perhaps
explains the origin of a singular instinct in the allied group of ostriches.
For several hen ostriches, at least in the case of the American species, unite
and lay first a few eggs in one nest and then in another; and these are hatched
by the males. This instinct may probably be accounted for by the fact of the
hens laying a large number of eggs; but, as in the case of the cuckoo, at
intervals of two or three days. This instinct, however, of the American ostrich
has not as yet been perfected; for a surprising number of eggs lie strewed over
the plains, so that in one day's hunting I picked up no less than twenty lost
and wasted eggs.
Many bees are
parasitic, and always lay their eggs in the nests of bees of other kinds. This
case is more remarkable than that of the cuckoo; for these bees have not only
their instincts but their structure modified in accordance with their parasitic
habits; for they do not possess the pollen- collecting apparatus which would be
necessary if they had to store food for their own young. Some species,
likewise, of Sphegidae (wasp-like insects) are parasitic on other species; and
M. Fabre has lately shown good reason for believing that although the Tachytes
nigra generally makes its own burrow and stores it with paralysed prey for its
own larvae to feed on, yet that when this insect finds a burrow already made
and stored by another sphex, it takes advantage of the prize, and becomes for
the occasion parasitic. In this case, as with the supposed case of the cuckoo,
I can see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose nest and
stored food are thus feloniously appropriated, be not thus exterminated.
Slave-making instinct.
This remarkable instinct was first discovered in the Formica (polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated father.
This ant is absolutely dependent on its slaves; without their aid, the species
would certainly become extinct in a single year. The males and fertile females
do no work. The workers or sterile females, though most energetic and
courageous in capturing slaves, do no other work. They are incapable of making
their own nests, or of perhaps fancied that, after all, they had been victorious
in their late combat.
At the same time I laid
on the same place a small parcel of the pupae of another species, F. flava,
with a few of these little yellow ants still clinging to the fragments of the
nest. This species is sometimes, though rarely, made into slaves, as has been
described by Mr Smith. Although so small a species, it is very courageous, and
I have seen it ferociously attack other ants. In one instance I found to my
surprise an independent community of F. flava under a stone beneath a nest of
the slave-making F. sanguinea; and when I had accidentally disturbed both
nests, the little ants attacked their big neighbours with surprising courage.
Now I was curious to ascertain whether F. sanguinea could distinguish the pupae
of F. fusca, which they habitually make into slaves, from those of the little
and furious F. flava, which they rarely capture, and it was evident that they
did at once distinguish them: for we have seen that they eagerly and instantly
seized the pupae of F. fusca, whereas they were much terrified when they came
across the pupae, or even the earth from the nest of F. flava, and quickly ran
away; but in about a quarter of an hour, shortly after all the little yellow
ants had crawled away, they took heart and carried off the pupae.
One evening I visited
another community of F. sanguinea, and found a number of these ants entering
their nest, carrying the dead bodies of F. fusca (showing that it was not a
migration) and numerous pupae. I traced the returning file burthened with
booty, for about forty yards, to a very thick clump of heath. whence I saw the
last individual of F. sanguinea emerge, carrying a pupa; but I was not able to
find the desolated nest in the thick heath. The nest, however, must have been
close at hand, for two or three individuals of F. fusca were rushing about in
the greatest agitation, and one was perched motionless with its own pupa in its
mouth on the top of a spray of heath over its ravaged home.
Such are the facts,
though they did not need confirmation by me, in regard to the wonderful
instinct of making slaves. Let it be observed what a contrast the instinctive
habits of F. sanguinea present with those of the F. rufescens. The latter does
not build its own nest, does not determine its own migrations, does not collect
food for itself or its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on the other hand,
possesses much fewer slaves, and in the early part of the summer extremely few.
The masters determine when and where a new nest shall be formed, and when they
migrate, the masters carry the slaves. Both in Switzerland and England the
slaves seem to have the exclusive care of the larvae, and the masters alone go
on slave-making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly the
slaves, tend, and milk as it may be called, their aphides; and thus both
collect food for the community. In England the masters alone usually leave the
nest to collect building materials and food for themselves, their slaves and
larvae. So that the masters in this country receive much less service from
their slaves than they do in Switzerland.
By what steps the
instinct of F. sanguinea originated I will not pretend to conjecture. But as
ants, which are not slave-makers, will, as I have seen, carry off pupae of
other species, if scattered near their nests, it is possible that pupae
originally stored as food might become developed; and the ants thus
unintentionally reared would then follow their proper instincts, and do what
work they could. If their presence proved useful to the species which had
seized them -- if it were more advantageous to this species to capture workers
than to procreate them -- the habit of collecting pupae originally for food
might by natural selection be strengthened and rendered permanent for the very
different purpose of raising slaves. When the instinct was once acquired, if
carried out to a much less extent even than in our British F. sanguinea, which,
as we have seen, is less aided by its slaves than the same species in
Switzerland, I can see no difficulty in natural selection increasing and
modifying the instinct -- always supposing each modification to be of use to the
species -- until an ant was formed as abjectly dependent on its slaves as is
the Formica rufescens.
Cell-making instinct of
the Hive-Bee. I will not here enter on nearly related to the latter: it forms a
nearly regular waxen comb of cylindrical cells, in which the young are hatched,
and, in addition, some large cells of wax for holding honey. These latter cells
are nearly spherical and of nearly equal sizes, and are aggregated into an
irregular mass. But the important point to notice, is that these cells are
always made at that degree of nearness to each other, that they would have
intersected or broken into each other, if the spheres had been completed; but
this is never permitted, the bees building perfectly flat walls of wax between
the spheres which thus tend to intersect. Hence each cell consists of an outer
spherical portion and of two, three, or more perfectly flat surfaces, according
as the cell adjoins two, three or more other cells. When one cell comes into
contact with three other cells, which, from the spheres being nearly of the
same size, is very frequently and necessarily the case, the three flat surfaces
are united into a pyramid; and this pyramid, as Huber has remarked, is
manifestly a gross imitation of the three-sided pyramidal basis of the cell of
the hive-bee. As in the cells of the hive-bee, so here, the three plane
surfaces in any one cell necessarily enter into the construction of three
adjoining cells. It is obvious that the Melipona saves wax by this manner of
building; for the flat walls between the adjoining cells are not double, but
are of the same thickness as the outer spherical portions, and yet each flat
portion forms a part of two cells.
Reflecting on this
case, it occurred to me that if the Melipona had made its spheres at some given
distance from each other, and had made them of equal sizes and had arranged
them symmetrically in a double layer, the resulting structure would probably
have been as perfect as the comb of the hive-bee. Accordingly I wrote to
professor Miller, of Cambridge, and this geometer has kindly read over the
following statement, drawn up from his information, and tells me that it is
strictly correct:.
If a number of equal
spheres be described with their centres placed in two parallel layers; with the
centre of each sphere at the distance of radius X /sqrt[2] or radius X 1.41421
(or at some lesser distance), from the centres of the six surrounding spheres
in the same layer; and at the same distance from the centres of the adjoining
spheres in the other and parallel layer; then, if planes of intersection
between the several spheres in both layers be formed, there will result a
double layer of hexagonal prisms united together by pyramidal bases formed of
three rhombs; and the rhombs and the sides of the hexagonal prisms will have
every angle identically the same with the best measurements which have been
made of the cells of the hive- bee.
Hence we may safely
conclude that if we could slightly modify the instincts already possessed by
the Melipona, and in themselves not very wonderful, this bee would make a
structure as wonderfully perfect as that of the hive-bee. We must suppose the
Melipona to make her cells truly spherical, and of equal sizes; and this would
not be very surprising, seeing that she already does so to a certain extent,
and seeing what perfectly cylindrical burrows in wood many insects can make,
apparently by turning round on a fixed point. We must suppose the Melipona to
arrange her cells in level layers, as she already does her cylindrical cells;
and we must further suppose, and this is the greatest difficulty, that she can
somehow judge accurately at what distance to stand from her fellow-labourers
when several are making their spheres; but she is already so far enabled to
judge of distance, that she always describes her spheres so as to intersect
largely; and then she unites the points of intersection by perfectly flat
surfaces. We have further to suppose, but this is no difficulty, that after
hexagonal prisms have been formed by the intersection of adjoining spheres in
the same layer, she can prolong the hexagon to any length requisite to hold the
stock of honey; in the same way as the rude humble-bee adds cylinders of wax to
the circular mouths of her old cocoons. By such modifications of instincts in
themselves not very wonderful, -- hardly more wonderful than those which guide
a bird to make its nest, -- I believe that the hive-bee has acquired, through
natural selection, her inimitable architectural powers.
But this theory can be
tested by experiment. Following the example of Mr Tegetmeier, I separated two
combs, and put between them a long, thick, square strip of wax: the bees
instantly began to excavate minute circular pits in it; and as they deepened
these little pits, they made them wider and wider until they were converted
into shallow basins, appearing to the eye perfectly true or parts of a sphere,
and of about the diameter of a cell. It was most interesting to me to observe
that wherever several bees had begun to excavate these basins near together,
they had begun their work at such a distance from each other, that by the time
the basins had acquired the above stated width (i.e. about the width of an
ordinary cell), and were in depth about one sixth of the diameter of the sphere
of which they formed a part, the rims of the basins intersected or broke into
each other. As soon as this occurred, the bees ceased to excavate, and began to
build up flat walls of wax on the lines of intersection between the basins, so
that each hexagonal prism was built upon the festooned edge of a smooth basin,
instead of on the straight edges of a three-sided pyramid as in the case of
ordinary cells.
I then put into the
hive, instead of a thick, square piece of wax, a thin and narrow, knife-edged
ridge, coloured with vermilion. The bees instantly began on both sides to
excavate little basins near to each other, in the same way as before; but the
ridge of wax was so thin, that the bottoms of the basins, if they had been
excavated to the same depth as in the former experiment, would have broken into
each other from the opposite sides. The bees, however, did not suffer this to
happen, and they stopped their excavations in due time; so that the basins, as
soon as they had been a little deepened, came to have flat bottoms; and these
flat bottoms, formed by thin little plates of the vermilion wax having been
left ungnawed, were situated, as far as the eye could judge, exactly along the
planes of imaginary intersection between the basins on the opposite sides of
the ridge of wax. In parts, only little bits, in other parts, large portions of
a rhombic plate had been left between the opposed basins, but the work, from
the unnatural state of things, had not been neatly performed. The bees must
have worked at very nearly the same rate on the opposite side of the ridge of
vermilion wax, as they circularly gnawed away and deepened the basins on both
sides, in order to have succeeded in thus leaving flat plates between the
basins, by stopping work along the intermediate planes or planes of
intersection.
Considering how
flexible thin wax is, I do not see that there Is any difficulty in the bees,
whilst at work on the two sides of a strip of wax, perceiving when they have
gnawed the wax away to the proper thinness, and then stopping their work. In
ordinary combs it has appeared to me that the bees do not always succeed in
working at exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just-commenced cell, which were slightly
concave on one side, where I suppose that the bees had excavated too quickly,
and convex on the opposed side, where the bees had worked less quickly. In one
well- marked instance, I put the comb back into the hive and allowed the bees
to go on working for a short time and again examined the cell, and I found that
the rhombic plate had been completed, and had become perfectly flat: it was
absolutely impossible, from the extreme thinness of the little rhombic plate,
that they could have affected this by gnawing away the convex side; and I suspect
that the bees in such cases stand in the opposed cells and push and bend the
ductile and warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.
From the experiment of
the ridge of vermilion wax, we can clearly see that if the bees were to build
for themselves a thin wall of wax, they could make their cells of the proper
shape, by standing at the proper distance from each other, by excavating at the
same rate, and by endeavouring to make equal spherical hollows, but never
allowing the spheres to break into each other. Now bees, as may be clearly seen
by examining the edge of a growing comb, do make a rough, circumferential wall
or rim all round the comb; and they gnaw into this from the opposite sides,
always working circularly as they deepen each cell. They do not make the whole
three-sided pyramidal base of any one cell at the same time, but only the one
rhombic plate which stands on the extreme growing margin, or the two plates, as
the case may be; and they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these statements differ from
those made by the justly celebrated elder Huber, but I am convinced of their
accuracy; and if I had space, I could show that they are conformable with my
theory.
Huber's statement that
the very first cell is excavated out of a little parallel-sided wall of wax, is
not, as far as I have seen, strictly correct; the first commencement having
always been a little hood of wax; but I will not here enter on these details.
We see how important a part excavation plays in the construction of the cells;
but it would be a great error to suppose that the bees cannot build up a rough
wall of wax in the proper position -- that is, along the plane of intersection
between two adjoining spheres. I have several specimens showing clearly that
they can do this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in position to
the planes of the rhombic basal plates of future cells. But the rough wall of
wax has in every case to be finished off, by being largely gnawed away on both
sides. The manner in which the bees build is curious; they always make the
first rough wall from ten to twenty times thicker than the excessively thin
finished wall of the cell, which will ultimately be left. We shall understand
how they work, by supposing masons first to pile up a broad ridge of cement,
and then to begin cutting it away equally on both sides near the ground, till a
smooth, very thin wall is left in the middle; the masons always piling up the
cut-away cement, and adding fresh cement, on the summit of the ridge. We shall
thus have a thin wall steadily growing upward; but always crowned by a gigantic
coping. From all the cells, both those just commenced and those completed,
being thus crowned by a strong coping of wax, the bees can cluster and crawl
over the comb without injuring the delicate hexagonal walls, which are only
about one four-hundredth of an inch in thickness; the plates of the pyramidal
basis being about twice as thick. By this singular manner of building, strength
is continually given to the comb, with the utmost ultimate economy of wax.
It seems at first to
add to the difficulty of understanding how the cells are made, that a multitude
of bees all work together; one bee after working a short time at one cell going
to another, so that, as Huber has stated, a score of individuals work even at
the commencement of the first cell. I was able practically to show this fact,
by covering the edges of the hexagonal walls of a single cell, or the extreme
margin of the circumferential rim of a growing comb, with an extremely thin
layer of melted vermilion wax; and I invariably found that the colour was most
delicately diffused by the bees - as delicately as a painter could have done
with his brush -- by atoms of the coloured wax having been taken from the spot
on which it had been placed, and worked into the growing edges of the cells all
round. The work of construction seems to be a sort of balance struck between
many bees, all instinctively standing at the same relative distance from each
other, all trying to sweep equal spheres, and then building up, or leaving
ungnawed, the planes of intersection between these spheres. It was really
curious to note in cases of difficulty, as when two pieces of comb met at an
angle, how often the bees would entirely pull down and rebuild in different
ways the same cell, sometimes recurring to a shape which they had at first
rejected.
When bees have a place
on which they can stand in their proper positions for working, - for instance,
on a slip of wood, placed directly under the middle of a comb growing downwards
so that the comb has to be built over one face of the slip -- in this case the
bees can lay the foundations of one wall of a new hexagon, in its strictly
proper place, projecting beyond the other completed cells. It suffices that the
bees should be enabled to stand at their proper relative distances from each
other and from the walls of the last completed cells, and then, by striking
imaginary spheres, they can build up a wall intermediate between two adjoining
spheres; but, as far as I have seen, they never gnaw away and finish off the angles
of a cell till a large part both of that cell and of the adjoining cells has
been built. This capacity in bees of laying dowm under certain circumstances a
rough wall in its proper place between two just-commenced cells, is important,
as it bears on a fact, which seems at first quite subversive of the foregoing
theory; namely, that the cells on the extreme margin of wasp-combs are
sometimes strictly hexagonal; but I have not space here to enter on this
subject. Nor does there seem to me any great difficulty in a single insect (as
in the case of a queen-wasp) making hexagonal cells, if she work alternately on
the inside and outside of two or three cells commenced at the same time, always
standing at the proper relative distance from the parts of the cells just
begun, sweeping spheres or cylinders, and building up intermediate planes. It
is even conceivable that an insect might, by fixing on a point at which to
commence a cell, and then moving out- side, first to one point, and then to
five other points, at the proper relative distances from the central point and
from each other, strike the planes of intersection, and so make an isolated
hexagon: but I am not aware that any such case has been observed; nor would any
good be derived from a single hexagon being built, as in its construction more
materials would be required than for a cylinder.
As natural selection
acts only by the accumulation of slight modifications of structure or instinct,
each profitable to the individual under its conditions of life, it may
reasonably be asked, how a long and graduated succession of modified
architectural instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I think the
answer is not difficult: it is known that bees are often hard pressed to get
sufficient nectar; and I am informed by Mr Tegetmeier that it has been
experimentally found that no less than from twelve to fifteen pounds of dry
sugar are consumed by a hive of bees for the secretion of each pound of wax; so
that a prodigious quantity of fluid nectar must be collected and consumed by
the bees in a hive for the secretion of the wax necessary for the construction
of their combs. Moreover, many bees have to remain idle for many days during the
process of secretion. A large store of honey is indispensable to support a
large stock of bees during the winter; and the security of the hive is known
mainly to depend on a large number of bees being supported. Hence the saving of
wax by largely saving honey must be a most important element of success in any
family of bees. Of course the success of any species of bee may be dependent on
the number of its parasites or other enemies, or on quite distinct causes, and
so be altogether independent of the quantity of honey which the bees could
collect. But let us suppose that this latter circumstance determined, as it
probably often does determine, the numbers of a humble-bee which could exist in
a country; and let us further suppose that the community lived throughout the
winter, and consequently required a store of honey: there can in this case be
no doubt that it would be an advantage to our humble-bee, if a slight
modification of her instinct led her to make her waxen cells near together, so
as to intersect a little; for a wall in common even to two adjoining cells,
would save some little wax. Hence it would continually be more and more
advantageous to our humble-bee, if she were to make her cells more and more
regular, nearer together, and aggregated into a mass, like the cells of the
Melipona; for in this case a large part of the bounding surface of each cell
would serve to bound other cells, and much wax would be saved. Again, from the
same cause, it would be advantageous to the Melipona, if she were to make her
cells closer together, and more regular in every way than at present; for then,
as we have seen, the spherical surfaces would wholly disappear, and would all
be replaced by plane surfaces; and the Melipona would make a comb as perfect as
that of the hive-bee. Beyond this stage of perfection in architecture, natural
selection could not lead; for the comb of the hive-bee, as far as we can see,
is absolutely perfect in economising wax.
Thus, as I believe, the
most wonderful of all known instincts, that of the hive-bee, can be explained
by natural selection having taken advantage of numerous, successive, slight
modifications of simpler instincts; natural selection having by slow degrees,
more and more perfectly, led the bees to sweep equal spheres at a given
distance from each other in a double layer, and to build up and excavate the
wax along the planes of intersection. The bees, of course, no more knowing that
they swept their spheres at one particular distance from each other, than they
know what are the several angles of the hexagonal prisms and of the basal
rhombic plates. The motive power of the process of natural selection having
been economy of wax; that individual swarm which wasted least honey in the
secretion of wax, having succeeded best, and having transmitted by inheritance
its newly acquired economical instinct to new swarms, which in their turn will
have had the best chance of succeeding in the struggle for existence.
No doubt many instincts
of very difficult explanation could be opposed to the theory of natural
selection, -- cases, in which we cannot see how an instinct could possibly have
originated; cases, in which no intermediate gradations are known to exist;
cases of instinct of apparently such trifling importance, that they could hardly
have been acted on by natural selection; cases of instincts almost identically
the same in animals so remote in the scale of nature, that we cannot account
for their similarity by inheritance from a common parent, and must therefore
believe that they have been acquired by independent acts of natural selection.
I will not here enter on these several cases, but will confine myself to one
special difficulty, which at first appeared to me insuperable, and actually
fatal to my whole theory. I allude to the neuters or sterile females in
insect-communities:. for these neuters often differ widely in instinct and in
structure from both the males and fertile females, and yet, from being sterile,
they cannot propagate their kind.
The subject well
deserves to be discussed at great length, but I will here take only a single
case, that of working or sterile ants. How the workers have been rendered
sterile is a difficulty; but not much greater than that of any other striking
modification of structure; for it can be shown that some insects and other
articulate animals in a state of nature occasionally become sterile; and if
such insects had been social, and it had been profitable to the community that
a number should have been annually born capable of work, but incapable of
procreation, I can see no very great difficulty in this being effected by
natural selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males and
the fertile females in structure, as in the shape of the thorax and in being
destitute of wings and sometimes of eyes, and in instinct. As far as instinct
alone is concerned, the prodigious difference in this respect between the
workers and the perfect females, would have been far better exemplified by the
hive-bee. If a working ant or other neuter insect had been an animal in the
ordinary state, I should have unhesitatingly assumed that all its characters
had been slowly acquired through natural selection; namely, by an individual
having been born with some slight profitable modification of structure, this
being inherited by its offspring, which again varied and were again selected,
and so onwards. But with the working ant we have an insect differing greatly
from its parents, yet absolutely sterile; so that it could never have
transmitted successively acquired modifications of structure or instinct to its
progeny. It may well be asked how is it possible to reconcile this case with
the theory of natural selection?
First, let it be
remembered that we have innumerable instances, both in our domestic productions
and in those in a state of nature, of all sorts of differences of structure
which have become correlated to certain ages, and to either sex. We have
differences correlated not only to one sex, but to that short period alone when
the reproductive system is active, as in the nuptial plumage of many birds, and
in the hooked jaws of the male salmon. We have even slight differences in the
horns of different breeds of cattle in relation to an artificially imperfect
state of the male sex; for oxen of certain breeds have longer horns than in
other breeds, in comparison with the horns of the bulls or cows of these same
breeds. Hence I can see no real difficulty in any character having become
correlated with the sterile condition of certain members of ;nsect-communities:
the difficulty lies in understanding how such correlated modifications of
structure could have been slowly accumulated by natural selection.
This difficulty, though
appearing insuperable, is lessened, or, as I believe, disappears, when it is
remembered that selection may be applied to the family, as well as to the
individual, and may thus gain the desired end. Thus, a well-flavoured vegetable
is cooked, and the individual is destroyed; but the horticulturist sows seeds
of the same stock, and confidently expects to get nearly the same variety;
breeders of cattle wish the flesh and fat to be well marbled together; the
animal has been slaughtered, but the breeder goes with confidence to the same
family. I have such faith in the powers of selection, that I do not doubt that
a breed of cattle, always yielding oxen with extraordinarily long horns, could
be slowly formed by carefully watching which individual bulls and cows, when
matched, produced oxen with the longest horns; and yet no one ox could ever
have propagated its kind. Thus I believe it has been with social insects: a
slight modification of structure, or instinct, correlated with the sterile
condition of certain members of the community, has been advantageous to the
community: consequently the fertile males and females of the same community
flourished, and transmitted to their fertile offspring a tendency to produce
sterile members having the same modification. And I believe that this process
has been repeated, until that prodigious amount of difference between the
fertile and sterile females of the same species has been produced, which we see
in many social insects.
But we have not as yet
touched on the climax of the difficulty;, namely, the fact that the neuters of
several ants differ, not only from the fertile females and males, but from each
other, sometimes to an almost incredible degree, and are thus divided into two
or even three castes. The castes, moreover, do not generally graduate into each
other, but are perfectly well defined; being as distinct from each other, as
are any two species of the same genus, or rather as any two genera of the same
family. Thus in Eciton, there are working and soldier neuters, with jaws and
instincts extraordinarily different: in Cryptocerus, the workers of one caste
alone carry a wonderful sort of shield on their heads, the use of which is
quite unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they have an
enormously developed abdomen which secretes a sort of honey, supplying the
place of that excreted by the aphides, or the domestic cattle as they may be
called, which our European ants guard or imprison.
It will indeed be
thought that I have an overweening confidence in the principle of natural
selection, when I do not admit that such wonderful and well-established facts
at once annihilate my theory. In the simpler case of neuter insects all of one
caste or of the same kind, which have been rendered by natural selection, as I
believe to be quite possible, different from the fertile males and females, --
in this case, we may safely conclude from the analogy of ordinary variations,
that each successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few alone; and
that by the long-continued selection of the fertile parents which produced most
neuters with the profitable modification, all the neuters ultimately came to
have the desired character. On this view we ought occasionally to find
neuter-insects of the same species, in the same nest, presenting gradations of
structure; and this we do find, even often, considering how few neuter-insects
out of Europe have been carefully examined. Mr F. Smith has shown how
surprisingly the neuters of several British ants differ from each other in size
and sometimes in colour; and that the extreme forms can sometimes be perfectly
linked together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often happens that the larger or
the smaller sized workers are the most numerous; or that both large and small
are numerous, with those of an intermediate size scanty in numbers. Formica
flava has larger and smaller workers, with some of intermediate size; and, in
this species, as Mr F. Smith has observed, the larger workers have simple eyes
(ocelli), which though small can be plainly distinguished, whereas the smaller
workers have their ocelli rudimentary. Having carefully dissected several
specimens of these workers, I can affirm that the eyes are far more rudimentary
in the smaller workers than can be accounted for merely by their proportionally
lesser size; and I fully believe, though I dare not assert so positively, that
the workers of intermediate size have their ocelli in an exactly intermediate
condition. So that we here have two bodies of sterile workers in the same nest,
differing not only in size, but in their organs of vision, yet connected by
some few members in an intermediate condition. I may digress by adding, that if
the smaller workers had been the most useful to the community, and those males
and females had been continually selected, which produced more and more of the
smaller workers, until all the workers had come to be in this condition; we
should then have had a species of ant with neuters very nearly in the same
condition with those of Myrmica. For the workers of Myrruica have not even
rudiments of ocelli, though the male and female ants of this genus have
well-developed ocelli.
I may give one other
case: so confidently did I expect to find gradations in important points of
structure between the different castes of neuters in the same species, that I
gladly availed myself of Mr F. Smith's offer of numerous specimens from the
same nest of the driver ant (Anomma) of West Africa. The reader will perhaps
best appreciate the amount of difference in these workers, by my giving not the
actual measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house of whom many
were five feet four inches high, and many sixteen feet high; but we must
suppose that the larger workmen had heads four instead of three times as big as
those of the smaller men, and jaws nearly five times as big. The jaws,
moreover, of the working ants of the several sizes differed wonderfully in
shape, and in the form and number of the teeth. But the important fact for us
is, that though the workers can be grouped into castes of different sizes, yet
they graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as Mr
Lubbock made drawings for me with the camera lucida of the jaws which I had
dissected from the workers of the several sizes.
With these facts before
me, I believe that natural selection, by acting on the fertile parents, could
form a species which should regularly produce neuters, either all of large size
with one form of jaw, or all of small size with jaws having a widely different
structure; or lastly, and this is our climax of difficulty, one set of workers
of one size and structure, and simultaneously another set of workers of a
different size and structure; -- a graduated series having been first formed,
as in the case of the driver ant, and then the extreme forms, from being the
most useful to the community, having been produced in greater and greater
numbers through the natural selection of the parents which generated them;
until none with an intermediate structure were produced.
Thus, as I believe, the
wonderful fact of two distinctly defined castes of sterile workers existing in
the same nest, both widely different from each other and from their parents,
has originated. We can see how useful their production may have been to a
social community of insects, on the same principle that the division of labour
is useful to civilised man. As ants work by inherited instincts and by
inherited tools or weapons, and not by acquired knowledge and manufactured
instruments, a perfect division of labour could be effected with them only by
the workers being sterile; for had they been fertile, they would have
intercrossed, and their instincts and structure would have become blended. And
nature has, as I believe, effected this admirable division of labour in the
communities of ants, by the means of natural selection. But I am bound to
confess, that, with all my faith in this principle, I should never have
anticipated that natural selection could have been efficient in so high a
degree, had not the case of these neuter insects convinced me of the fact. I
have, therefore, discussed this case, at some little but wholly insufficient length,
in order to show the power of natural selection, and likewise because this is
by far the most serious special difficulty, which my theory has encountered.
The case, also, is very interesting, as it proves that with animals, as with
plants, any amount of modification in structure can be effected by the
accumulation of numerous, slight, and as we must call them accidental,
variations, which are in any manner profitable, without exercise or habit
having come into play. For no amount of exercise, or habit, or volition, in the
utterly sterile members of a community could possibly have affected the
structure or instincts of the fertile members, which alone leave descendants. I
am surprised that no one has advanced this demonstrative case of neuter
insects, against the well-known doctrine of Lamarck.
Summary. I have
endeavoured briefly in this chapter to show that the mental qualities of our
domestic animals vary, and that the variations are inherited. Still more
briefly I have attempted to show that instincts vary slightly in a state of
nature. No one will dispute that instincts are of the highest importance to
each animal. Therefore I can see no difficulty, under changing conditions of
life, in natural selection accumulating slight modifications of instinct to any
extent, in any useful direction. In some cases habit or use and disuse have
probably come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of difficulty,
to the best of my judgment, annihilate it. On the other hand, the fact that
instincts are not always absolutely perfect and are liable to mistakes; - that
no instinct has been produced for the exclusive good of other animals, but that
each animal takes advantage of the instincts of others; -- that the canon in
natural history, of 'natura non facit saltum' is applicable to instincts as
well as to corporeal structure, and is plainly explicable on the foregoing
views, but is otherwise inexplicable, -- all tend to corroborate the theory of
natural selection.
This theory is, also,
strengthened by some few other facts in regard to instincts; as by that common
case of closely allied, but certainly distinct, species, when inhabiting
distant parts of the world and living under considerably different conditions
of life, yet often retaining nearly the same instincts. For instance, we can
understand on the principle of inheritance, how it is that the thrush of South
America lines its nest with mud, in the same peculiar manner as does our
British thrush: how it is that the male wrens (Troglodytes) of North America,
build 'cocknests,' to roost in, like the males of our distinct Kitty-wrens, --
a habit wholly unlike that of any other known bird. Finally, it may not be a
logical deduction, but to my imagination it is far more satisfactory to look at
such instincts as the young cuckoo ejecting its foster- brothers, - ants making
slaves, -- the larvae of ichneumonidae feeding within the live bodies of
caterpillars, -- not as specially endowed or created instincts, but as small
consequences of one general law, leading to the advancement of all organic
beings, namely, multiply, vary, let the strongest live and the weakest die.
Distinction between the sterility of first crosses and of hybrids -
Sterility various in degree, not universal, affected by close interbreeding,
removed by domestication - Laws governing the sterility of hybrids - Sterility
not a special endowment, but incidental on other differences - Causes of the
sterility of first crosses and of hybrids - Parallelism between the effects of
changed conditions of life and crossing - Fertility of varieties when crossed
and of their mongrel offspring not universal - Hybrids and mongrels compared
independently of their fertility - Summary THE
view generally entertained by naturalists is that species, when intercrossed,
have been specially endowed with the quality of sterility, in order to prevent
the confusion of all organic forms. This view certainly seems at first
probable, for species within the same country could hardly have kept distinct
had they been capable of crossing freely. The importance of the fact that
hybrids are very generally sterile, has, I think, been much underrated by some
late writers. On the theory of natural selection the case is especially
important, inasmuch as the sterility of hybrids could not possibly be of any
advantage to them, and therefore could not have been acquired by the continued
preservation of successive profitable degrees of sterility. I hope, however, to
be able to show that sterility is not a specially acquired or endowed quality,
but is incidental on other acquired differences.
ln treating this
subject, two classes of facts, to a large extent fundamentally different, have
generally been confounded together; namely, the sterility of two species when
first crossed, and the sterility of the hybrids produced from them.
Pure species have of
course their organs of reproduction in a perfect condition, yet when
intercrossed they produce either few or no offspring. Hybrids, on the other
hand, have their reproductive organs functionally impotent, as may be clearly
seen in the state of the male element in both plants and animals; though the
organs themselves are perfect in structure, as far as the microscope reveals.
In the first case the two sexual elements which go to form the embryo are
perfect; in the second case they are either not at all developed, or are
imperfectly developed. This distinction is important, when the cause of the
sterility, which is common to the two cases, has to be considered. The
distinction has probably been slurred over, owing to the sterility in both
cases being looked on as a special endowment, beyond the province of our
reasoning powers.
The fertility of
varieties, that is of the forms known or believed to have descended from common
parents, when intercrossed, and like wise the fertility of their mongrel
offspring, is, on my theory, of equal importance with the sterility of species;
for it seems to make a broad and clear distinction between varieties and
species.
First, for the
sterility of species when crossed and of their hybrid offspring. It is
impossible to study the several memoirs and works of those two conscientious
and admirable observers, Kölreuter and Gärtner, who almost devoted their lives
to this subject, without being deeply impressed with the high generality of
some degree of sterility. Kölreuter makes the rule universal; but then he cuts
the knot, for in ten cases in which he found two forms, considered by most
authors as distinct species, quite fertile together, he unhesitatingly ranks
them as varieties. Gärtner, also, makes the rule equally universal; and he
disputes the entire fertility of Kölreuter's ten cases. But in these and in
many other cases, Gärtner is obliged carefully to count the seeds, in order to
show that there is any degree of sterility. He always compares the maximum
number of seeds produced by two species when crossed and by their hybrid
offspring, with the average number produced by both pure parent-species in a
state of nature. But a serious cause of error seems to me to be here
introduced: a plant to be hybridised must be castrated, and, what is often more
important, must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimentised on by Gärtner
were potted, and apparently were kept in a chamber in his house, That these
processes are often injurious to the fertility of a plant cannot be doubted;
for Gärtner gives in his table about a score of cases of plants which he
castrated, and artificially fertilised with their own pollen, and (excluding
all cases such as the Leguminosae, in which there is an acknowledged difficulty
in the manipulation) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gärtner during several years repeatedly crossed
the primrose and cowslip, which we have such good reason to believe to be
varieties, and only once or twice succeeded in getting fertile seed; as he found
the common red and blue pimpernels (Anagallis arvensis and coerulea), which the
best botanists rank as varieties, absolutely sterile together; and as he came
to the same conclusion in several other analogous cases; it seems to me that we
may well be permitted to doubt whether many other species are really so
sterile, when intercrossed, as Gärtner believes.
It is certain, on the
one hand, that the sterility of various species when crossed is so different in
degree and graduates away so insensibly, and, on the other hand, that the
fertility of pure species is so easily affected by various circumstances, that
for all practical purposes it is most difficult to say where perfect fertility
ends and sterility begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever lived, namely, Kölreuter
and Gärtner, should have arrived at diametrically opposite conclusions in
regard to the very same species. It is also most instructive to compare -- but
I have not space here to enter on details -- the evidence advanced by our best
botanists on the question whether certain doubtful forms should be ranked as
species or varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during different
years. It can thus be shown that neither sterility nor fertility affords any
clear distinction between species and varieties; but that the evidence from
this source graduates away, and is doubtful in the same degree as is the
evidence derived from other constitutional and structural differences.
In regard to the
sterility of hybrids in successive generations; though Gärtner was enabled to
rear some hybrids, carefully guarding them from a cross with either pure parent,
for six or seven, and in one case for ten generations, yet he asserts
positively that their fertility never increased, but generally greatly
decreased. I do not doubt that this is usually the case, and that the fertility
often suddenly decreases in the first few generations. Nevertheless I believe
that in all these experiments the fertility has been diminished by an
independent cause, namely, from close interbreeding. I have collected so large
a body of facts, showing that close interbreeding lessens fertility, and, on
the other hand, that an occasional cross with a distinct individual or variety
increases fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent- species, or other allied
hybrids, generally grow in the same garden, the visits of insects must be
carefully prevented during the flowering season: hence hybrids will generally
be fertilised during each generation by their own individual pollen; and I am
convinced that this would be injurious to their fertility, already lessened by
their hybrid origin. I am strengthened in this conviction by a remarkable
statement repeatedly made by Gärtner, namely, that if even the less fertile
hybrids be artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation, sometimes
decidedly increases, and goes on increasing. Now, in artificial fertilisation
pollen is as often taken by chance (as I know from my own experience) from the
anthers of another flower, as from the anthers of the flower itself which is to
be fertilised; so that a cross between two flowers, though probably on the same
plant, would be thus effected. Moreover, whenever complicated experiments are
in progress, so careful an observer as Gärtner would have castrated his
hybrids, and this would have insured in each generation a cross with the pollen
from a distinct Bower, either from the same plant or from another plant of the
same hybrid nature. And thus, the strange fact of the increase of fertility in
the successive generations of artificially fertilised hybrids may, I believe,
be accounted for by close interbreeding having been avoided.
Now let us turn to the
results arrived at by the third most experienced hybridiser, namely, the Hon.
and Rev. W. Herbert. He is as emphatic in his conclusion that some hybrids are
perfectly fertile -- as fertile as the pure parent-species -- as are Kölreuter
and Gärtner that some degree of sterility between distinct species is a
universal law of nature. He experimentised on some of the very same species as
did Gärtner. The difference in their results may, I think, be in part accounted
for by Herbert's great horticultural skill, and by his having hothouses at his
command. Of his many important statements I will here give only a single one as
an example, namely, that 'every ovule in a pod of Crinum capense fertilised by
C. revolutum produced a plant, which (he says) I never saw to occur in a case
of its natural fecundation.' So that we here have perfect, or even more than
commonly perfect, fertility in a first cross between two distinct species.
This case of the Crinum
leads me to refer to a most singular fact, namely, that there are individual
plants, as with certain species of Lobelia, and with all the species of the
genus Hippeastrum, which can be far more easily fertilised by the pollen of
another and distinct species, than by their own pollen. For these plants have
been found to yield seed to the pollen of a distinct species, though quite
sterile with their own pollen, notwithstanding that their own pollen was found
to be perfectly good, for it fertilised distinct species. So that certain
individual plants and all the individuals of certain species can actually be
hybridised much more readily than they can be self-fertilised! For instance, a
bulb of Hippeastrum aulicum produced four flowers; three were fertilised by
Herbert with their own pollen, and the fourth was subsequently fertilised by
the pollen of a compound hybrid descended from three other and distinct
species: the result was that 'the ovaries of the three first flowers soon
ceased to grow, and after a few days perished entirely, whereas the pod impregnated
by the pollen of the hybrid made vigorous growth and rapid progress to
maturity, and bore good seed, which vegetated freely.' ln a letter to me, in
1839, Mr Herbert told me that he had then tried the experiment during five
years, and he continued to try it during several subsequent years, and always
with the same result. This result has, also, been confirmed by other observers
in the case of Hippeastrum with its sub-genera, and in the case of some other
genera, as Lobelia, Passiflora and Verbascum. Although the plants in these
experiments appeared perfectly healthy, and although both the ovules and pollen
of the same flower were perfectly good with respect to other species, yet as
they were functionally imperfection their mutual self-action, we must infer
that the plants were in an unnatural state. Nevertheless these facts show on
what slight and mysterious causes the lesser or greater fertility of species
when crossed, in comparison with the same species when self- fertilised,
sometimes depends.
The practical
experiments of horticulturists, though not made with scientific precision,
deserve some notice. It is notorious in how complicated a manner the species of
Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, &c., have been
crossed, yet many of these hybrids seed freely. For instance, Herbert asserts
that a hybrid from Calceolaria integrifolia and plantaginea, species most
widely dissimilar in general habit,' reproduced itself as perfectly as if it
had been a natural species from the mountains of Chile.' I have taken some
pains to ascertain the degree of fertility of some of the complex crosses of
Rhododendrons, and I am assured that many of them are perfectly fertile. Mr C.
Noble, for instance, informs me that he raises stocks for grafting from a
hybrid between Rhod. Ponticum and Catawbiense, and that this hybrid 'seeds as
freely as it is possible to imagine.' Had hybrids, when fairly treated, gone on
decreasing in fertility in each successive generation, as Gärtner believes to
be the case, the fact would have been notorious to nurserymen. Horticulturists
raise large beds of the same hybrids, and such alone are fairly treated, for by
insect agency the several individuals of the same hybrid variety are allowed to
freely cross with each other, and the injurious influence of close
interbreeding is thus prevented. Any one may readily convince himself of the
efficiency of insect-agency by examining the flowers of the more sterile kinds
of hybrid rhododendrons, which produce no pollen, for he will find on their
stigmas plenty of pollen brought from other flowers.
In regard to animals,
much fewer experiments have been carefully tried than with plants. If our
systematic arrangements can be trusted, that is if the genera of animals are as
distinct from each other, as are the genera of plants, then we may infer that
animals more widely separated in the scale of nature can be more easily crossed
than in the case of plants; but the hybrids themselves are, I think, more
sterile. I doubt whether any case of a perfectly fertile hybrid animal can be
considered as thoroughly well authenticated. It should, however, be borne in
mind that, owing to few animals breeding freely under confinement, few
experiments have been fairly tried: for instance, the canary-bird has been
crossed with nine other finches, but as not one of these nine species breeds
freely in confinement, we have no right to expect that the first crosses
between them and the canary, or that their hybrids, should be perfectly
fertile. Again, with respect to the fertility in successive generations of the
more fertile hybrid animals, I hardly know of an instance in which two families
of the same hybrid have been raised at the same time from different parents, so
as to avoid the ill effects of close interbreeding. On the contrary, brothers
and sisters have usually been crossed in each successive generation, in
opposition to the constantly repeated admonition of every breeder. And in this
case, it is not at all surprising that the inherent sterility in the hybrids
should have gone on increasing. If we were to act thus, and pair brothers and
sisters in the case of any pure animal, which from any cause had the least
tendency to sterility, the breed would assuredly be lost in a very few
generations.
Although I do not know
of any thoroughly well- authenticated cases of perfectly fertile hybrid
animals, I have some reason to believe that the hybrids from Cervulus vaginalis
and Reevesii, and from phasianus colchicus with p. torquatus and with p.
versicolor are perfectly fertile. The hybrids from the common and Chinese geese
(A. cygnoides), species which are so different that they are generally ranked
in distinct genera, have often bred in this country with either pure parent,
and in one single instance they have bred inter se. This was effected by Mr
Eyton, who raised two hybrids from the same parents but from different hatches;
and from these two birds he raised no less than eight hybrids (grandchildren of
the pure geese) from one nest. In India, however, these cross-bred geese must
be far more fertile; for I am assured by two eminently capable judges, namely
Mr Blyth and Capt. Hutton, that whole flocks of these crossed geese are kept in
various parts of the country; and as they are kept for profit, where neither pure
parent- species exists, they must certainly be highly fertile.
A doctrine which
originated with Pallas, has been largely accepted by modern naturalists;
namely, that most of our domestic animals have descended from two or more
aboriginal species, since commingled by intercrossing. On this view, the
aboriginal species must either at first have produced quite fertile hybrids, or
the hybrids must have become in subsequent generations quite fertile under
domestication. This latter alternative seems to me the most probable, and I am
inclined to believe in its truth, although its rests on no direct evidence. I
believe, for instance, that our dogs have descended from several wild stocks;
yet, with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; and analogy makes me greatly doubt,
whether the several aboriginal species would at first have freely bred together
and have produced quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite fertile together; but
from facts communicated to me by Mr Blyth, I think they must be considered as
distinct species. On this view of the origin of many of our domestic animals,
we must either give up the belief of the almost universal sterility of distinct
species of animals when crossed; or we must look at sterility, not as an
indelible characteristic, but as one capable of being removed by domestication.
Finally, looking to all
the ascertained facts on the intercrossing of plants and animals, it may be
concluded that some degree of sterility, both in first crosses and in hybrids,
is an extremely general result; but that it cannot, under our present state of
knowledge, be considered as absolutely universal.
Laws governing the
Sterility of first Crosses and of Hybrids. We will now consider a little more
in detail the circumstances and rules governing the sterility of first crosses
and of hybrids. Our chief object will be to see whether or not the rules
indicate that species have specially been endowed with this quality, in order
to prevent their crossing and blending together in utter confusion. The
following rules and conclusions are chiefly drawn up from Gärtner's admirable
work on the hybridisation of plants. I have taken much pains to ascertain how
far the rules apply to animals, and considering how scanty our knowledge is in
regard to hybrid animals, I have been surprised to find how generally the same
rules apply to both kingdoms.
It has been already
remarked, that the degree of fertility, both of first crosses and of hybrids,
graduates from zero to perfect fertility. It is surprising in how many curious
ways this gradation can be shown to exist; but only the barest outline of the
facts can here be given. When pollen from a plant of one family is placed on
the stigma of a plant of a distinct family, it exerts no more influence than so
much inorganic dust. From this absolute zero of fertility, the pollen of
different species of the same genus applied to the stigma of some one species,
yields a perfect gradation in the number of seeds produced, up to nearly
complete or even quite complete fertility; and, as we have seen, in certain
abnormal cases, even to an excess of fertility, beyond that which the plant's
own pollen will produce. So in hybrids themselves, there are some which never
have produced, and probably never would produce, even with the pollen of either
pure parent, a single fertile seed: but in some of these cases a first trace of
fertility may be detected, by the pollen of one of the pure parent-species
causing the flower of the hybrid to wither earlier than it otherwise would have
done; and the early withering of the flower is well known to be a sign of
incipient fertilisation. From this extreme degree of sterility we have self-
fertilised hybrids producing a greater and greater number of seeds up to
perfect fertility.
Hybrids from two
species which are very difficult to cross, and which rarely produce any
offspring, are generally very sterile; but the parallelism between the
difficulty of making a first cross, and the sterility of the hybrids thus
produced -- two classes off acts which are generally confounded together -- is
by no means strict. There are many cases, in which two pure species can be united
with unusual facility, and produce numerous hybrid- offspring, yet these
hybrids are remarkably sterile. On the other hand, there are species which can
be crossed very rarely, or with extreme difficulty, but the hybrids, when at
last produced, are very fertile. Even within the limits of the same genus, for
instance in Dianthus, these two opposite cases occur.
The fertility, both of
first crosses and of hybrids, is more easily affected by unfavourable
conditions, than is the fertility of pure species. But the degree of fertility
is likewise innately variable; for it is not always the same when the same two
species are crossed under the same circumstances, but depends in part upon the
constitution of the individuals which happen to have been chosen for the
experiment. So it is with hybrids, for their degree of fertility is often found
to differ greatly in the several individuals raised from seed out of the same
capsule and exposed to exactly the same conditions.
By the term systematic
affinity is meant, the resemblance between species in structure and in
constitution, more especially in the structure of parts which are of high
physiological importance and which differ little in the allied species. Now the
fertility of first crosses between species, and of the hybrids produced from
them, is largely governed by their systematic affinity. This is clearly shown
by hybrids never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied species
generally uniting with facility. But the correspondence between systematic
affinity and the facility of crossing is by no means strict. A multitude of
cases could be given of very closely allied species which will not unite, or
only with extreme difficulty; and on the other hand of very distinct species
which unite with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering effort shave failed to
produce between extremely close species a single hybrid. Even within the limits
of the same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of almost
any other genus; but Gärtner found that N. acuminata, which is not a
particularly distinct species, obstinately failed to fertilise, or to be
fertilised by, no less than eight other species of Nicotiana. Very many
analogous facts could be given.
No one has been able to
point out what kind, or what amount, of difference in any recognisable
character is sufficient to prevent two species crossing. It can be shown that
plants most widely different in habit and general appearance, and having
strongly marked differences in every part of the flower, even in the pollen, in
the fruit, and in the cotyledons, can be crossed. Annual and perennial plants,
deciduous and evergreen trees, plants inhabiting different stations and fitted
for extremely different climates, can often be crossed with ease.
By a reciprocal cross
between two species, I mean the case, for instance, of a stallion-horse being
first crossed with a female-ass, and then a male-ass with a mare: these two
species may then be said to have been reciprocally crossed. There is often the
widest possible difference in the facility of making reciprocal crosses. Such
cases are highly important, for they prove that the capacity in any two species
to cross is often completely independent of their systematic affinity, or of
any recognisable difference in their whole organisation. On the other hand,
these cases clearly show that the capacity for crossing is connected with
constitutional differences imperceptible by us, and confined to the
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Kölreuter. To give an
instance: Mirabilis jalappa can easily be fertilised by the pollen of M.
longiflora, and the hybrids thus produced are sufficiently fertile; but Kölreuter
tried more than two hundred times, during eight following years, to fertilise
reciprocally M. longiflora with the pollen of M. jalappa, and utterly failed.
Several other equally striking cases could be given. Thuret has observed the
same fact with certain sea-weeds or Fuci. Gärtner, moreover, found that this
difference of facility in making reciprocal crosses is extremely common in a
lesser degree. He has observed it even between forms so closely related (as
Matthiola annua and glabra) that many botanists rank them only as varieties. It
is also a remarkable fact, that hybrids raised from reciprocal crosses, though
of course compounded of the very same two species, the one species having first
been used as the father and then as the mother, generally differ infertility in
a small, and occasionally in a high degree.
Several other singular
rules could be given from Gärtner: for instance, some species have a remarkable
power of crossing with other species; other species of the same genus have a
remarkable power of impressing their likeness on their hybrid offspring; but
these two powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate character between
their two parents, always closely resemble one of them; and such hybrids,
though externally so like one of their pure parent-species, are with rare
exceptions extremely sterile. So again amongst hybrids which are usually
intermediate in structure between their parents, exceptional and abnormal
individuals sometimes are born, which closely resemble one of their pure
parents; and these hybrids are almost always utterly sterile, even when the
other hybrids raised from seed from the same capsule have a considerable degree
of fertility. These facts show how completely fertility in the hybrid is
independent of its external resemblance to either pure parent.
Considering the several
rules now given, which govern the fertility of first crosses and of hybrids, we
see that when forms, which must be considered as good and distinct species, are
united, their fertility graduates from zero to perfect fertility, or even to
fertility under certain conditions in excess. That their fertility, besides
being eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in the
first cross and in the hybrids produced from this cross. That the fertility of
hybrids is not related to the degree in which they resemble in external
appearance either parent. And lastly, that the facility of making a first cross
between any two species is not always governed by their systematic affinity or
degree of resemblance to each other. This latter statement is clearly proved by
reciprocal crosses between the same two species, for according as the one
species or the other is used as the father or the mother, there is generally
some difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from reciprocal
crosses often differ in fertility.
Now do these complex
and singular rules indicate that species have been endowed with sterility
simply to prevent their becoming confounded in nature? I think not. For why
should the sterility be so extremely different in degree, when various species
are crossed, all of which we must suppose it would be equally important to keep
from blending together? Why should the degree of sterility be innately variable
in the individuals of the same species? Why should some species cross with
facility, and yet produce very sterile hybrids; and other species cross with
extreme difficulty, and yet produce fairly fertile hybrids? Why should there
often be so great a difference in the result of a reciprocal cross between the
same two species? Why, it may even be asked, has the production of hybrids been
permitted? To grant to species the special power of producing hybrids, and then
to stop their further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems to be a
strange arrangement.
The foregoing rules and
facts, on the other hand, appear to me clearly to indicate that the sterility
both of first crosses and of hybrids is simply incidental or dependent on
unknown differences, chiefly in the reproductive systems, of the species which
are crossed. The differences being of so peculiar and limited a nature, that,
in reciprocal crosses between two species the male sexual element of the one
will often freely act on the female sexual element of the other, but not in a
reversed direction. It will be advisable to explain a little more fully by an
example what I mean by sterility being incidental on other differences, and not
a specially endowed quality. As the capacity of one plant to be grafted or
budded on another is so entirely unimportant for its welfare in a state of
nature, I presume that no one will suppose that this capacity is a specially
endowed quality, but will admit that it is incidental on differences in the
laws of growth of the two plants. We can sometimes seethe reason why one tree
will not take on another, from differences in their rate of growth, in the
hardness of their wood, in the period of the flow or nature of their sap,
&c.; but in a multitude of cases we can assign no reason whatever. Great
diversity in the size of two plants, one being woody and the other herbaceous,
one being evergreen and the other deciduous, and adaptation to widely different
climates, does not always prevent the two grafting together. As in
hybridisation, so with grafting, the capacity is limited by systematic
affinity, for no one has been able to graft trees together belonging to quite
distinct families; and, on the other hand, closely allied species, and
varieties of the same species, can usually, but not invariably, be grafted with
ease. But this capacity, as in hybridisation, is by no means absolutely
governed by systematic affinity. Although many distinct genera within the same
family have been grafted together, in other cases species of the same genus
will not take on each other. The pear can be grafted far more readily on the
quince, which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of the
apricot and peach on certain varieties of the plum.
As Gärtner found that
there was sometimes an innate difference in different individuals of the same
two species incrossing; so Sagaret believes this to be the case with different
individuals of the same two species in being grafted together. As in reciprocal
crosses, the facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common goose-berry, for instance, cannot be
grafted on the currant, whereas the currant will take, though with difficulty,
on the gooseberry.
We have seen that the
sterility of hybrids, which have their reproductive organs in an imperfect
condition, is a very different case from the difficulty of uniting two pure
species, which have their reproductive organs perfect; yet these two distinct
cases run to a certain extent parallel. Something analogous occurs in grafting;
for Thouin found that three species of Robinia, which seeded freely on their
own roots, and which could be grafted with no great difficulty on another
species, when thus grafted were rendered barren. On the other hand, certain
species of Sorbus, when grafted on other species, yielded twice as much fruit
as when on their own roots. We are reminded by this latter fact of the
extraordinary case of Hippeastrum, Lobelia, &c., which seeded much more
freely when fertilised with the pollen of distinct species, than when self-fertilised
with their own pollen.
We thus see, that
although there is a clear and fundamental difference between the mere adhesion
of grafted stocks, and the union of the male and female elements in the act of
reproduction, yet that there is a rude degree of parallelism in the results of
grafting and of crossing distinct species. And as we must look at the curious
and complex laws governing the facility with which trees can be grafted on each
other as incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of first
crosses, are incidental on unknown differences, chiefly in their reproductive
systems. These differences, in both cases, follow to a certain extent, as might
have been expected, systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The facts by
no means seem to me to indicate that the greater or lesser difficulty of either
grafting or crossing together various species has been a special endowment;
although in the case of crossing, the difficulty is as important for the
endurance and stability of specific forms, as in the case of grafting it is
unimportant for their welfare.
Causes of the Sterility
of first Crosses and of Hybrids. We may now look a little closer at the
probable causes of the sterility of first crosses and of hybrids. These two
cases are fundamentally different, for, as just remarked, in the union of two
pure species the male and female sexual elements are perfect, whereas in
hybrids they are imperfect. Even in first crosses, the greater or lesser
difficulty in effecting a union apparently depends on several distinct causes.
There must sometimes be a physical impossibility in the male element reaching
the ovule, as would be the case with a plant having a pistil too long for the
pollen-tubes to reach the ovarium. It has also been observed that when pollen
of one species is placed on the stigma of a distantly allied species, though
the pollen- tubes protrude, they do not penetrate the stigmatic surface. Again,
the male element may reach the female element, but be incapable of causing an
embryo to be developed, as seems to have been the case with some of Thuret's
experiments on Fuci. No explanation can be given of these facts, any more than
why certain trees cannot be grafted on others. Lastly, an embryo may be
developed, and then perish at an early period. This latter alternative has not
been sufficiently attended to; but I believe, from observations communicated to
me by Mr Hewitt, who has had great experience in hybridising gallinaceous
birds, that the early death of the embryo is a very frequent cause of sterility
in first crosses. I was at first very unwilling to believe in this view; as hybrids,
when once born, are generally healthy and long-lived, as we see in the case of
the common mule. Hybrids, however, are differently circumstanced before and
afterbirth: when born and living in a country where their two parents can live,
they are generally placed under suitable conditions of life. But a hybrid
partakes of only half of the nature and constitution of its mother, and
therefore before birth, as long as it is nourished within its mother's womb or
within the egg or seed produced by the mother, it may be exposed to conditions
in some degree unsuitable, and consequently be liable to perish at an early
period; more especially as all very young beings seem eminently sensitive to
injurious or unnatural conditions of life.
In regard to the
sterility of hybrids, in which the sexual elements are imperfectly developed,
the case is very different. I have more than once alluded to a large body of
facts, which I have collected, showing that when animals and plants are removed
from their natural conditions, they are extremely liable to have their
reproductive systems seriously affected. This, in fact, is the great bar to the
domestication of animals. Between the sterility thus superinduced and that of
hybrids, there are many points of similarity. In both cases the sterility is
independent of general health, and is often accompanied by excess of size or
great luxuriance. In both cases, the sterility occurs in various degrees; in
both, the male element is the most liable to be affected; but sometimes the female
more than the male. In both, the tendency goes to a certain extent with
systematic affinity, or whole groups of animals and plants are rendered
impotent by the same unnatural conditions; and whole groups of species tend to
produce sterile hybrids. On the other hand, one species in a group will
sometimes resist great changes of conditions with unimpaired fertility; and
certain species in a group will produce unusually fertile hybrids. No one can
tell, till he tries, whether any particular animal will breed under confinement
or any plant seed freely under culture; nor can he tell,till he tries, whether
any two species of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations under conditions not
natural to them, they are extremely liable to vary, which is due, as I believe,
to their reproductive systems having been specially affected, though in a
lesser degree than when sterility ensues. So it is with hybrids, for hybrids in
successive generations are eminently liable to vary, as every experimentalist
has observed.
Thus we see that when
organic beings are placed under new and unnatural conditions, and when hybrids
are produced by the unnatural crossing of two species, the reproductive system,
independently of the general state of health, is affected by sterility in a
very similar manner. In the one case, the conditions of life have been
disturbed, though often in so slight a degree as to be inappreciable by us; in
the other case, or that of hybrids,the external conditions have remained the
same, but the organisation has been disturbed by two different structures and
constitutions having been blended into one. for it is scarcely possible that
two organisations should be compounded into one, without some disturbance
occurring in the development, or periodical action, or mutual relation of the
different parts and organs one to another, or to the conditions of life. When
hybrids are able to breed inter se, they transmit to their offspring from
generation to generation the same compounded organisation, and hence we need
not be surprised that their sterility, though in some degree variable, rarely
diminishes.
It must, however, be
confessed that we cannot understand, excepting on vague hypotheses, several facts
with respect to the sterility of hybrids; for instance, the unequal fertility
of hybrids produced from reciprocal crosses; or the increased sterility in
those hybrids which occasionally and exceptionally resemble closely either pure
parent. Nor do I pretend that the foregoing remarks go to the root of the
matter: no explanation is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted to show, is that in
two cases, in some respects allied, sterility is the common result, -- in the
one case from the conditions of life having been disturbed, in the other case
from the organisation having been disturbed by two organisations having been
compounded into one.
It may seem fanciful,
but I suspect that a similar parallelism extends to an allied yet very
different class of facts. It is an old and almost universal belief, founded, I
think, on a considerable body of evidence, that slight changes in the
conditions of life are beneficial to all living things. We see this acted on by
farmers and gardeners in their frequent exchanges of seed, tubers, &c.,
from one soil or climate to another, and back again. During the convalescence
of animals, we plainly see that great benefit is derived from almost any change
in the habits of life. Again, both with plants and animals, there is abundant
evidence, that across between very distinct individuals of the same species,
that is between members of different strains or sub-breeds, give vigour and
fertility to the offspring. I believe, indeed, from the facts alluded to in our
fourth chapter, that a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept under
the same conditions of life, always induces weakness and sterility in the
progeny.
Hence it seems that, on
the one hand, slight changes in the conditions of life benefit all organic
beings, and on the other hand, that slight crosses, that is crosses between the
males and females of the same species which have varied and become slightly
different, give vigour and fertility to the offspring. But we have seen that
greater changes, or changes of a particular nature, often render organic beings
in some degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce hybrids
which are generally sterile in some degree. I cannot persuade myself that this
parallelism is an accident or an illusion. Both series of facts seem to be
connected together by some common but unknown bond, which is essentially
related to the principle of life.
Fertility of Varieties
when crossed, and of their Mongrel off-spring. It may be urged, as a most
forcible argument, that there must be some essential distinction between
species and varieties, and that there must be some error in all the foregoing
remarks, inasmuch as varieties, however much they may differ from each other in
external appearance, cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably the case. But if we
look to varieties produced under nature, we are immediately involved in
hopeless difficulties; for if two hitherto reputed varieties be found in any
degree sterile together, they are at once ranked by most naturalists as
species. For instance, the blue and red pimpernel, the primrose and cowslip,
which are considered by many of our best botanists as varieties, are said by Gärtner
not to be quite fertile when crossed, and lie consequently ranks them as
undoubted species. If we thus argue in a circle, the fertility of all varieties
produced under nature will assuredly have to be granted.
If we turn to
varieties, produced, or supposed to have been produced, under domestication, we
are still involved in doubt. For when it is stated, for instance, that the
German Spas dog unites more easily than other dogs with foxes, or that certain
South American indigenous domestic dogs do not readily cross with European
dogs, the explanation which will occur to everyone, and probably the true one,
is that these dogs have descended from several aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic varieties, differing
widely from each other in appearance, for instance of the pigeon or of the
cabbage, is a remarkable fact; more especially when we reflect how many species
there are, which, though resembling each other most closely, are utterly
sterile when intercrossed. Several considerations, however, render the
fertility of domestic varieties less remarkable than at first appears. It can,
in the first place, be clearly shown that mere external dissimilarity between
two species does not determine their greater or lesser degree of sterility when
crossed; and we may apply the same rule to domestic varieties. In the second
place, some eminent naturalists believe that a long course of domestication
tends to eliminate sterility in the successive generations of hybrids, which
were at first only slightly sterile; and if this be so, we surely ought not to
expect to find sterility both appearing and disappearing under nearly the same
conditions of life. Lastly, and this seems to me by far the most important
consideration, new races of animals and plants are produced under domestication
by man's methodical and unconscious power of selection, for his own use and
pleasure: he neither wishes to select, nor could select, slight differences in
the reproductive system, or other constitutional difference correlated with the
reproductive system. He supplies his several varieties with the same food;
treats them in nearly the same manner, and does not wish to alter their general
habits of life. Nature acts uniformly and slowly during vast periods of time on
the whole organization, in any way which may be for each creature's own good;
and thus she may, either directly, or more probably indirectly, through
correlation, modify the reproductive system in the several descendants from any
one species. Seeing this difference in the process of selection, as carried on
by man and nature, we need not be surprised at some difference in the result.
I have as yet spoken as
if the varieties of the same species were invariably fertile when intercrossed.
But it seems to me impossible to resist the evidence of the existence of a
certain amount of sterility in the few following cases, which I will briefly abstract.
The evidence is at least as good as that from which we believe in the sterility
of a multitude of species. The evidence is, also, derived from hostile
witnesses, who in all other cases consider fertility and sterility as safe
criterions of specific distinction. Gärtner kept during several years a dwarf
kind of maize with yellow seeds, and a tall variety with red seeds, growing
near each other in his garden; and although these plants have separated sexes,
they never naturally crossed. He then fertilized thirteen flowers of the one
with the pollen of the other; but only a single head produced any seed, and
this one head produced only five grains. Manipulation in this case could not
have been injurious, as the plants have separated sexes. No one, I believe, has
suspected that these varieties of maize are distinct species; and it is
important to notice that the hybrid plants thus raised were themselves
perfectly fertile; so that even Gärtner did not venture to consider the two
varieties as specifically distinct.
Girou de Buzareingues
crossed three varieties of gourd, which like the maize has separated sexes, and
he asserts that their mutual fertilization is by so much the less easy as their
differences are greater. How far these experiments may be trusted, I know not;
but the forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.
The following case is
far more remarkable, and seems at first quite incredible; but it is the result
of an astonishing number of experiments made during many years on nine species
of Verbascum, by so good an observer and so hostile a witness, as Gärtner:
namely, that yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilized with pollen from their own coloured flowers. Moreover, he asserts
that when yellow and white varieties of one species are crossed with yellow and
white varieties of a distinct species, more seed is produced by the crosses
between the same coloured flowers, than between those which are differently
coloured. Yet these varieties of Verbascum present no other difference besides
the mere colour of the flower; and one variety can sometimes be raised from the
seed of the other.
From observations which
I have made on certain varieties of hollyhock, I am inclined to suspect that
they present analogous facts.
Kölreuter, whose
accuracy has been confirmed by every subsequent observer, has proved the
remarkable fact, that one variety of the common tobacco is more fertile, when
crossed with a widely distinct species, than are the other varieties. He
experimentized on five forms, which are commonly reputed to be varieties, and
which he tested by the severest trial, namely, by reciprocal crosses, and he
found their mongrel off spring perfect. fertile. But one of these five
varieties, when used either as father or mother, and crossed with the Nicotiana
glutinosa, always yielded hybrids not so sterile as those which were produced
from the four other varieties when crossed with N. glutinosa. Hence the
reproductive system of this one variety must have been in some manner and in
some degree modified.
From these facts; from
the great difficulty of ascertaining the infertility of varieties in a state of
nature, for a supposed variety if infertile in any degree would generally be
ranked as species; from man selecting only external characters in the
production of the most distinct domestic varieties, and from not wishing or being
able to produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think that the
very general fertility of varieties can be proved to be of universal
occurrence, or to form a fundamental distinction between varieties and species.
The general fertility of varieties does not seem to me sufficient to over throw
the view which I have taken with respect to the very general, but not
invariable, sterility of first crosses and of hybrids, namely, that it is not a
special endowment, but is incidental on slowly acquired modifications, more
especially in the reproductive systems of the forms which are crossed.
Hybrids and Mongrels
compared, independently of their fertility. Independently of the question of
fertility, the off spring of species when crossed and of varieties when crossed
may be compared in several other respects. Gärtner, whose strong wish was to
draw a marked line of distinction between species and varieties, could find
very few and, as it seems to me, quite unimportant differences between the
so-called hybrid offspring of species, and the so-called mongrel offspring of
varieties. And, on the other hand, they agree most closely in very many
important respects.
I shall here discuss
this subject with extreme brevity. The most important distinction is, that in
the first generation mongrels are more variable than hybrids; but Gärtner
admits that hybrids from species which have long been cultivated are often
variable in the first generation; and I have myself seen striking instances of
this fact. Gärtner further admits that hybrids between very closely allied
species are more variable than those from very distinct species; and this shows
that the difference in the degree of variability graduates away. When mongrels
and the more fertile hybrids are propagated for several generations an extreme
amount of variability in their offspring is notorious; but some few cases both
of hybrids and mongrels long retaining uniformity of character could be given.
The variability, however, in the successive generations of mongrels is,
perhaps, greater than in hybrids.
This greater
variability of mongrels than of hybrids does not seem to me at all surprising.
For the parents of mongrels are varieties, and mostly domestic varieties (very
few experiment shaving been tried on natural varieties), and this implies in
most cases that there has been recent variability; and therefore we might
expect that such variability would often continue and be super-added to that
arising from the mere act of crossing. These light degree of variability in
hybrids from the first cross or in the first generation, in contrast with their
extreme variability in the succeeding generations, is a curious fact and
deserves attention. For it bears on and corroborates the view which I have
taken on the cause of ordinary variability; namely, that it is due to the
reproductive system being eminently sensitive to any change in the conditions
of life, being thus often rendered either impotent or at least incapable of its
proper function of producing offspring identical with the parent-form. Now
hybrids in the first generation are descended from species (excluding those
long cultivated) which have not had their reproductive systems in any way affected,
and they are not variable; but hybrids themselves have their reproductive
systems seriously affected, and their descendants are highly variable.
But to return to our
comparison of mongrels and hybrids: Gärtner further insists that when any two species,
although most closely allied to each other, are crossed with a third species,
the hybrids are widely different from each other; whereas if two very distinct
varieties of one species are crossed with another species, the hybrids do not
differ much. But this conclusion, as far as I can make out, is founded on a
single experiment; and seems directly opposed to the results of several
experiments made by Kölreuter.
These alone are the
unimportant differences, which Gärtner is able to point out, between hybrid and
mongrel plants. On the other hand, the resemblance in mongrels and in hybrids
to their respective parents, more especially in hybrids produced from nearly
related species, follows according to Gärtner the same laws. When two species
are crossed, one has sometimes a prepotent power of impressing its likeness on
the hybrid; and so I believe it to be with varieties of plants. With animals
one variety certainly often has this prepotent power over another variety.
Hybrid plants produced from a reciprocal cross, generally resemble each other
closely; and so it is with mongrels from a reciprocal cross. Both hybrids and
mongrels can be reduced to either pure parent-form, by repeated crosses in
successive generations with either parent.
These several remarks
are apparently applicable to animals; but the subject is here excessively
complicated, partly owing to the existence of secondary sexual characters; but
more especially owing to prepotency in transmitting likeness running more
strongly in one sex than in the other, both when one species is crossed with
another, and when one variety is crossed with another variety. For instance, I
think these authors are right, who maintain that the ass has a prepotent power
over the horse, so that both the mule and the hinny more resemble the ass than
the horse; but that the prepotency runs more strongly in the male-ass than in
the female, so that the mule, which is the offspring of the male-ass and mare,
is more like an ass, than is the hinny, which is the offspring of the
female-ass and stallion.
Much stress has been
laid by some authors on the supposed fact, that mongrel animals alone are born
closely like one of their parents; but it can be shown that this does sometimes
occur with hybrids; yet I grant much less frequently with hybrids than with
mongrels. Looking to the cases which I have collected of cross-bred animals
closely resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly appeared
-- such as albinism, melanism, deficiency of tail or horns, or additional
fingers and toes; and do not relate to characters which have been slowly
acquired by selection. Consequently, sudden reversions to the perfect character
of either parent would be more likely to occur with mongrels, which are
descended from varieties often suddenly produced and semi-monstrous in
character, than with hybrids, which are descended from species slowly and
naturally produced. On the whole I entirely agree with Dr Prosper Lucas, who,
after arranging an enormous body of facts with respect to animals, comes to the
conclusion, that the laws of resemblance of the child to its parents are the
same, whether the two parents differ much or little from each other, namely in
the union of individuals of the same variety, or of different varieties, or of
distinct species.
Laying aside the
question of fertility and sterility, in all other respects there seems to be a
general and close similarity in the offspring of crossed species, and of
crossed varieties. If we look at species as having been specially created, and
at varieties as having been produced by secondary laws, this similarity would
be an astonishing fact. But it harmonizes perfectly with the view that there is
no essential distinction between species and varieties.
Summary of Chapter.
First crosses between forms sufficiently distinct to be ranked as species, and
their hybrids, are very generally, but not universally, sterile. The sterility
is of all degrees, and is often so slight that the two most careful
experimentalists who have ever lived, have come to diametrically opposite
conclusions in ranking forms by this test. The sterility is innately variable
in individuals of the same species, and is eminently susceptible of favourable
and unfavourable conditions. The degree of sterility does not strictly follow
systematic affinity, but is governed by several curious and complex laws. It is
generally different, and sometimes widely different, in reciprocal crosses
between the same two species. It is not always equal in degree in a first cross
and in the hybrid produced from this cross.
In the same manner as
in grafting trees, the capacity of one species or variety to take on another,
is incidental on generally unknown differences in their vegetative systems, so
in crossing, the greater or less facility of one species to unite with another,
is incidental on unknown differences in their reproductive systems. There is no
more reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature, than to
think that trees have been specially endowed with various and somewhat
analogous degrees of difficulty in being grafted together in order to prevent
them becoming inarched in our forests.
The sterility of first
crosses between pure species, which have their reproductive systems perfect,
seems to depend on several circumstances; in some cases largely on the early
death of the embryo. The sterility of hybrids, which have their reproductive
systems imperfect, and which have had this system and their whole organisation
disturbed by being compounded of two distinct species, seems closely allied to
that sterility which so frequently affects pure species, when their natural
conditions of life have been disturbed. This view is supported by a parallelism
of another kind; -- namely, that the crossing of forms only slightly different
is favourable to the vigour and fertility of their offspring; and that slight
changes in the conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree of
difficulty in uniting two species, and the degree of sterility of their
hybrid-offspring should generally correspond, though due to distinct causes;
for both depend on the amount of difference of some kind between the species
which are crossed. Nor is it surprising that the facility of effecting a first
cross, the fertility of the hybrids produced, and the capacity of being grafted
together -- though this latter capacity evidently depends on widely different
circumstances -- should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment; for
systematic affinity attempts to express all kinds of resemblance between all
species.
First crosses between
forms known to be varieties, or sufficiently alike to be considered as
varieties, and their mongrel offspring, are very generally, but not quite
universally, fertile. Nor is this nearly general and perfect fertility
surprising, when we remember how liable we are to argue in a circle with
respect to varieties in a state of nature; and when we remember that the
greater number of varieties have been produced under domestication by the
selection of mere external differences, and not of differences in the
reproductive system. In all other respects, excluding fertility, there is a
close general resemblance between hybrids and mongrels. Finally, then, the
facts briefly given in this chapter do not seem to me opposed to, but even
rather to support the view, that there is no fundamental distinction between
species and varieties.
On the absence of intermediate varieties at the present day - On the
nature of extinct intermediate varieties; on their number - On the vast lapse
of time, as inferred from the rate of deposition and of denudation - On the
poorness of our palaeontological collections - On the intermittence of
geological formations - On the absence of intermediate varieties in any one
formation - On their sudden appearance in the lowest known fossiliferous strata
IN the sixth chapter I
enumerated the chief objections which might be justly urged against the views
maintained in this volume. Most of them have now been discussed. One, namely
the distinctness of specific forms, and their not being blended together by
innumerable transitional links, is a very obvious difficulty. I assigned
reasons why such links do not commonly occur at the present day, under the
circumstances apparently most favourable for their presence, namely on an
extensive and continuous area with graduated physical conditions. I endeavoured
to show, that the life of each species depends in a more important manner on
the presence of other already defined organic forms, than on climate; and,
therefore. that the really governing conditions of life do not graduate away
quite insensibly like heat or moisture. I endeavoured, also, to show that
intermediate varieties, from existing in lesser numbers than the forms which
they connect, will generally be beaten out and exterminated during the course
of further modification and improvement. The main cause, however, of innumerable
intermediate links not now occurring everywhere throughout nature depends on
the very process of natural selection, through which new varieties continually
take the places of and exterminate their parent-forms. But just in proportion
as this process of extermination has acted on an enormous scale, so must the
number of intermediate varieties, which have formerly existed on the earth, be
truly enormous. Why then is not every geological formation and every stratum
full of such intermediate links? Geology assuredly does not reveal any such
finely graduated organic chain; and this, perhaps, is the most obvious and
gravest objection which can be urged against my theory. The explanation lies,
as I believe, in the extreme imperfection of the geological record.
In the first place it
should always be borne in mind what sort of intermediate forms must, on my
theory, have formerly existed. I have found it difficult, when looking at any
two species, to avoid picturing to myself, forms directly intermediate between
them. But this is a wholly false view; we should always look for forms
intermediate between each species and a common but unknown progenitor; and the
progenitor will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter pigeons have
both descended from the rock-pigeon; if we possessed all the intermediate
varieties which have ever existed, we should have an extremely close series
between both and the rock- pigeon; but we Should have no varieties directly
intermediate between the fantail and pouter; none, for instance, combining a
tail somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have become so much
modified, that if we had no historical or indirect evidence regarding their
origin, it would not have been possible to have determined from a mere
comparison of their structure with that of the rock-pigeon, whether they had
descended from this species or from some other allied species, such as C.
oenas.
So with natural
species, if we look to forms very distinct, for instance to the horse and
tapir, we have no reason to suppose that links ever existed directly
intermediate between them, but between each and an unknown common parent. The
common parent will have had in its whole organisation much general resemblance
to the tapir and to the horse; but in some points of structure may have
differed considerably from both, even perhaps more than they differ from each
other. Hence in all such cases, we should be unable to recognise the
parent-form of any two or more species, even if we closely compared the
structure of the Parent with that of its modified descendants, unless at the
same time we had a nearly perfect chain of the intermediate links.
It is just Possible by
my theory, that one of two living forms might have descended from the other;
for instance, a horse from a tapir; and in this case direct intermediate links
will have existed between them. But such a case would imply that one form had
remained for a very long Period unaltered, whilst its descendants had undergone
a vast amount of change; and the Principle of competition between organism and
organism, between child and Parent, will render this a very rare event; for in
all cases the new and improved forms of life will tend to supplant the old and
unimproved.
By the theory of
natural selection all living species have been connected with the
Parent-species of each genus, by differences not greater than we see between
the varieties of the same species at the present day; and these parent-
species, now generally extinct, have in their turn been similarly connected
with more ancient species; and so on backwards, always converging to the common
ancestor of each great class. So that the number of intermediate and
transitional links, between all living and extinct species, must have been
inconceivably great. But assuredly, if this theory be true, such have lived
upon this earth.
On the lapse of Time.
Independently of our not finding fossil remains of such infinitely numerous
connecting links, it may be objected, that time will not have sufficed for so
great an amount of organic change, all changes having been effected very slowly
through natural selection. It is hardly possible for me even to recall to the
reader, who may not be a practical geologist, the facts leading the mind feebly
to comprehend the lapse of time. He who can read Sir Charles Lyell's grand work
on the Principles of Geology, which the future historian will recognise as
having produced a revolution in natural science, yet does not admit how
incomprehensibly vast have been the past periods of time, may at once close
this volume. Not that it suffices to study the Principles of Geology, or to read
special treatises by different observers on separate formations, and to mark
how each author attempts to give an inadequate idea of the duration of each
formation or even each stratum. A man must for years examine for himself great
piles of superimposed strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend anything of the
lapse of time, the monuments of which we see around us.
It is good to wander
along lines of sea-coast, when formed of moderately hard rocks, and mark the
process of degradation. The tides in most cases reach the cliffs only for a
short time twice a day, and the waves eat into them only when they are charged
with sand or pebbles; for there is reason to believe that pure water can effect
little or nothing in wearing away rock. At last the base of the cliff is
undermined, huge fragments fall down, and these remaining fixed, have to be
worn away, atom by atom, until reduced in size they can be rolled about by the
waves, and then are more quickly ground into pebbles, sand, or mud. But how
often do we see along the bases of retreating cliffs rounded boulders, all
thickly clothed by marine productions, showing how little they are abraded and
how seldom they are rolled about! Moreover, if we follow for a few miles any
line of rocky cliff, which is undergoing degradation, we find that it is only
here and there, along a short length or round a promontory, that the cliffs are
at the present time suffering. The appearance of the surface and the vegetation
show that elsewhere years have elapsed since the waters washed their base.
He who most closely
studies the action of the sea on our shores, will, I believe, be most deeply
impressed with the slowness with which rocky coasts are worn away. The
observations on this head by Hugh Miller, and by that excellent observer Mr
Smith of Jordan Hill, are most impressive. With the mind thus impressed, let
any one examine beds of conglomerate many thousand feet in thickness, which,
though probably formed at a quicker rate than many other deposits, yet, from
being formed of worn and rounded pebbles, each of which bears the stamp of
time, are good to show how slowly the mass has been accumulated. Let him
remember Lyell's profound remark, that the thickness and extent of sedimentary
formations are the result and measure of the degradation which the earth's
crust has elsewhere suffered. And what an amount of degradation is implied by
the sedimentary deposits of many countries ! Professor Ramsay has given me the
maximum thickness, in most cases from actual measurement, in a few cases from
estimate, of each formation in different parts of Great Britain; and this is
the result:
Feet
Palaeozoic strata (not including igneous beds) 57,154
Secondary strata 13,190
Tertiary strata 2,240 -- making altogether 72,584 feet; that is, very nearly
thirteen and three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness on the
Continent. Moreover, between each successive formation, we have, in the opinion
of most geologists, enormously long blank periods. So that the lofty Pile of
sedimentary rocks in Britain, gives but an inadequate idea of the time which
has elapsed during their accumulation; yet what time this must have consumed!
Good observers have estimated that sediment is deposited by the great
Mississippi river at the rate of only 600 feet in a hundred thousand years.
This estimate may be quite erroneous; yet, considering over what wide spaces
very fine sediment is transported by the currents of the sea, the process of
accumulation in any one area must be extremely slow.
But the amount of
denudation which the strata have in many places suffered, independently of the
rate of accumulation of the degraded matter, probably offers the best evidence
of the lapse of time. I remember having been much struck with the evidence of
denudation, when viewing volcanic islands, which have been worn by the waves
and pared all round into Perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their formerly liquid
state, showed at a glance how far the hard, rocky beds had once extended into
the open ocean. The same story is still more plainly told by faults, -- those
great cracks along which the strata have been upheaved on one side, or thrown
down on the other, to the height or depth of thousands of feet; for since the
crust cracked, the surface of the land has been so completely planed down by
the action of the sea, that no trace of these vast dislocations is externally
visible.
The Craven fault, for
instance, extends for upwards of 30 miles, and along this line the vertical
displacement of the strata has varied from 600 to 3000 feet. Prof. Ramsay has
published an account of a downthrow in Anglesea of 2300 feet; and he informs me
that he fully believes there is one in Merionethshire of 12,000 feet; yet in
these cases there is nothing on the surface to show such prodigious movements;
the pile of rocks on the one or other side having been smoothly swept away. The
consideration of these facts impresses my mind almost in the same manner as
does the vain endeavour to grapple with the idea of eternity.
I am tempted to give
one other case, the well-known one of the denudation of the Weald. Though it
must be admitted that the denudation of the Weald has been a mere trifle, in
comparison with that which has removed masses of our Palaeozoic strata, in
Parts ten thousand feet in thickness, as shown in Prof. Ramsay's masterly
memoir on this subject. Yet it is an admirable lesson to stand on the North
Downs and to look at the distant South Downs; for, remembering that at no great
distance to the west the northern and southern escarpments meet and close, one
can safely picture to oneself the great dome of rocks which must have covered
up the Weald within so limited a period as since the latter part of the Chalk
formation. The distance from the northern to the southern Downs is about 22
miles, and the thickness of the several formations is on an average about 1100
feet, as I am informed by Prof. Ramsay. But If, as some geologists suppose, a
range of older rocks underlies the Weald, on the flanks of which the overlying
sedimentary deposits might have accumulated in thinner masses than elsewhere,
the above estimate would be erroneous; but this source of doubt probably would
not greatly affect the estimate as applied to the western extremity of the
district. If, then, we knew the rate at which the sea commonly wears away a
line of cliff of any given height, we could measure the time requisite to have
denuded the Weald. This, of course, cannot be done; but we may, in order to
form some crude notion on the subject, assume that the sea would eat into
cliffs 500 feet in height at the rate of one inch in a century. This will at
first appear much too small an allowance; but it is the same as if we were to
assume a cliff one yard in height to be eaten back along a whole fine of coast
at the rate of one yard in nearly every twenty-two years. I doubt whether any
rock, even as soft as chalk, would yield at this rate excepting on the most
exposed coasts; though no doubt the degradation of a lofty cliff would be more
rapid from the breakage of the fallen fragments. On the other hand, I do not
believe that any line of coast, ten or twenty miles in length, ever suffers
degradation at the same time along its whole indented length; and we must
remember that almost all strata contain harder layers or nodules, which from
long resisting attrition form a breakwater at the base. Hence, under ordinary
circumstances, I conclude that for a cliff 500 feet in height, a denudation of
one inch per century for the whole length would be an ample allowance. At this
rate, on the above data, the denudation of the Weald must have required
306,662,400 years; or say three hundred million years.
The action of fresh
water on the gently inclined Wealden district, when upraised, could hardly have
been great, but it would somewhat reduce the above estimate. On the other hand,
during oscillations of level, which we know this area has undergone, the
surface may have existed for millions of years as land, and thus have escaped
the action of the sea: when deeply submerged for Perhaps equally long Periods,
it would, likewise, have escaped the action of the coast-waves. So that in all
Probability a far longer Period than 300 million years has elapsed since the
latter part of the Secondary period.
I have made these few
remarks because it is highly important for us to gain some notion, however
imperfect, of the lapse of years. During each of these years, over the whole world,
the land and the water has been peopled by hosts of living forms. What an
infinite number of generations, which the mind cannot grasp, must have
succeeded each other in the long roll of years! Now turn to our richest
geological museums, and what a Paltry display we behold!
On the Poorness of our
Palaeontological collections. That our Palaeontological collections are very
imperfect, is admitted by every one. The remark of that admirable
Palaeontologist, the late Edward Forbes, should not be forgotten, namely, that
numbers of our fossil species are known and named from single and often broken
specimens, or from a few specimens collected on some one spot. Only a small
portion of the surface of the earth has been geologically explored, and no part
with sufficient care, as the important discoveries made every year in Europe
prove. No organism wholly soft can be preserved. Shells and bones will decay
and disappear when left on the bottom of the sea, where sediment is not
accumulating. I believe we are continually taking a most erroneous view, when
we tacitly admit to ourselves that sediment is being deposited over nearly the
whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil
remains. Throughout an enormously large proportion of the ocean, the bright
blue tint of the water bespeaks its purity. The many cases on record of a
formation conformably covered, after an enormous interval of time, by another
and later formation, without the underlying bed having suffered in the interval
any wear and tear, seem explicable only on the view of the bottom of the sea
not rarely lying for ages in an unaltered condition. The remains which do
become embedded, if in sand or gravel, will when the beds are upraised
generally be dissolved by the percolation of rain- water. I suspect that but
few of the very many animals which live on the beach between high and low
watermark are preserved. For instance, the several species of the ChthamalinÆ
(a sub-family of sessile cirripedes) coat the rocks all over the world in
infinite numbers: they are all strictly littoral, with the exception of a
single Mediterranean species, which inhabits deep water and has been found
fossil in Sicily, whereas not one other species has hitherto been found in any
tertiary formation: yet it is now known that the genus Chthamalus existed
during the chalk period. The molluscan genus Chiton offers a partially
analogous case.
With respect to the
terrestrial productions which lived during the Secondary and Palaeozoic
periods, it is superfluous to state that our evidence from fossil remains is
fragmentary in an extreme degree. For instance, not a land shell is known
belonging to either of these vast periods, with one exception discovered by Sir
C. Lyell in the carboniferous strata of North America. in regard to mammiferous
remains, a single glance at the historical table published in the Supplement to
Lyell's Manual, will bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is their rarity surprising,
when we remember how large a proportion of the bones of tertiary mammals have
been discovered either in caves or in lacustrine deposits; and that not a cave
or true lacustrine bed is known belonging to the age of our secondary or
palaeozoic formations.
But the imperfection in
the geological record mainly results from another and more important cause than
any of the foregoing; namely, from the several formations being separated from
each other by wide intervals of time. When we see the formations tabulated in
written works, or when we follow them in nature, it is difficult to avoid
believing that they are closely consecutive. But we know, for instance, from
Sir R. Murchison's great work on Russia, what wide gaps there are in that
country between the superimposed formations; so it is in North America, and in
many other parts of the world. The most skilful geologist, if his attention had
been exclusively confined to these large territories, would never have
suspected that during the periods which were blank and barren in his own
country, great piles of sediment, charged with new and peculiar forms of life,
had elsewhere been accumulated. And if in each separate territory, hardly any
idea can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be ascertained.
The frequent and great changes in the mineralogical composition of consecutive
formations, generally implying great changes in the geography of the
surrounding lands, whence the sediment has been derived, accords with the
belief of vast intervals of time having elapsed between each formation.
But we can, I think,
see why the geological formations of each region are almost invariably
intermittent; that is, have not followed each other in close sequence. Scarcely
any fact struck me more when examining many hundred miles of the South American
coasts, which have been upraised several hundred feet sufficiently extensive to
last for even a short geological period. Along the whole west coast, which is
inhabited by a peculiar marine fauna, tertiary beds are so scantily developed,
that no record of several successive and peculiar marine faunas will probably
be preserved to a distant age. A little reflection will explain why along the rising
coast of the western side of South America, no extensive formations with recent
or tertiary remains can anywhere be found, though the supply of sediment must
for ages have been great, from the enormous degradation of the coast-rocks and
from muddy streams entering the sea. The explanation, no doubt, is, that the
littoral and sub-littoral deposits are continually worn away, as soon as they
are brought up by the slow and gradual rising of the land within the grinding
action of the coast-waves.
We may, I think, safely
conclude that sediment must be accumulated in extremely thick, solid, or
extensive masses, in order to withstand the incessant action of the waves, when
first upraised and during subsequent oscillations of level. Such thick and
extensive accumulations of sediment may be formed in two ways; either, in
profound depths of the sea, in which case, judging from the researches of E.
Forbes, we may conclude that the bottom will be inhabited by extremely few
animals, and the mass when upraised will give a most imperfect record of the
forms of life which then existed; or, sediment may be accumulated to any
thickness and extent over a shallow bottom, if it continue slowly to subside.
In this latter case, as long as the rate of subsidence and supply of sediment
nearly balance each other, the sea will remain shallow and favourable for life,
and thus a fossiliferous formation thick enough, when upraised, to resist any
amount of degradation, may be formed.
I am convinced that all
our ancient formations, which are rich in fossils, have thus been formed during
subsidence. Since publishing my views on this subject in 1845, I have watched
the progress of Geology, and have been surprised to note how author after
author, in treating of this or that great formation, has come to the conclusion
that it was accumulated during subsidence. I may add, that the only ancient
tertiary formation on the west coast of South America, which has been bulky
enough to resist such degradation as it has as yet suffered, but which will
hardly last to a distant geological age, was certainly deposited during a
downward oscillation of level, and thus gained considerable thickness.
All geological facts
tell us plainly that each area has undergone numerous slow oscillations of
level, and apparently these oscillations have affected wide spaces.
Consequently formations rich in fossils and sufficiently thick and extensive to
resist subsequent degradation, may have been formed over wide spaces during
periods of subsidence, but only where the supply of sediment was sufficient to
keep the sea shallow and to embed and preserve the remains before they had time
to decay. On the other hand, as long as the bed of the sea remained stationary,
thick deposits could not have been accumulated in the shallow parts, which are
the most favourable to life. Still less could this have happened during the
alternate periods of elevation; or, to speak more accurately, the beds which
were then accumulated will have been destroyed by being upraised and brought
within the limits of the coast-action.
Thus the geological
record will almost necessarily be rendered intermittent. I feel much confidence
in the truth of these views, for they are in strict accordance with the general
principles inculcated by Sir C. Lyell; and E. Forbes independently arrived at a
similar conclusion.
One remark is here
worth a passing notice. During periods of elevation the area of the land and of
the adjoining shoal parts of the sea will be increased, and new stations will
often be formed; -- all circumstances most favourable, as previously explained,
for the formation of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other hand, during
subsidence, the inhabited area and number of inhabitants will decrease
(excepting the productions on the shores of a continent when first broken up
into an archipelago), and consequently during subsidence, though there will be
much extinction, fewer new varieties or species will be formed; and it is
during these very periods of subsidence, that our great deposits rich in
fossils have been accumulated. Nature may almost be said to have guarded
against the frequent discovery of her transitional or linking forms.
From the foregoing
considerations it cannot be doubted that the geological record, viewed as a
whole, is extremely imperfect; but if we confine our attention to any one
formation, it becomes more difficult to understand, why we do not therein find
closely graduated varieties between the allied species which lived at its
commencement and at its close. Some cases are on record of the same species
presenting distinct varieties in the upper and lower parts of the same
formation, but, as they are rare, they may be here passed over. Although each
formation has indisputably required a vast number of years for its deposition,
I can see several reasons why each should not include a graduated series of
links between the species which then lived; but I can by no means pretend to
assign due proportional weight to the following considerations.
Although each formation
may mark a very long lapse of years, each perhaps is short compared with the
period requisite to change one species into another. I am aware that two
palaeontologists, whose opinions are worthy of much deference, namely Bronn and
Woodward, have concluded that the average duration of each formation is twice
or thrice as long as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us coming to any just conclusion on
this head. When we see a species first appearing in the middle of any
formation, it would be rash in the extreme to infer that it had not elsewhere
previously existed. So again when we find a species disappearing before the
uppermost layers have been deposited, it would be equally rash to suppose that
it then became wholly extinct. We forget how small the area of Europe is
compared with the rest of the world; nor have the several stages of the same
formation throughout Europe been correlated with perfect accuracy.
With marine animals of
all kinds, we may safely infer a large amount of migration during climatal and
other changes; and when we see a species first appearing in any formation, the
probability is that it only then first immigrated into that area. It is well
known, for instance, that several species appeared somewhat earlier in the
palaeozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the European seas.
In examining the latest deposits of various quarters of the world, it has
everywhere been noted, that some few still existing species are common in the
deposit, but have become extinct in the immediately surrounding sea; or,
conversely, that some are now abundant in the neighbouring sea, but are rare or
absent in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the Glacial
period, which forms only a part of one whole geological period; and likewise to
reflect on the great changes of level, on the inordinately great change of
climate, on the prodigious lapse of time, all included within this same glacial
period. Yet it may be doubted whether in any quarter of the world, sedimentary
deposits, including fossil remains, have gone on accumulating within the same
area during the whole of this period. It is not, for instance, probable that
sediment was deposited during the whole of the glacial period near the mouth of
the Mississippi, within that limit of depth at which marine animals can
flourish; for we know what vast geographical changes occurred in other parts of
America during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial period
shall have been upraised, organic remains will probably first appear and
disappear at different levels, owing to the migration of species and to
geographical changes. And in the distant future, a geologist examining these
beds, might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is extending from before the glacial epoch
to the present day.
In order to get a
perfect gradation between two forms in the upper and lower parts of the same
formation, the deposit must have gone on accumulating for a very long period,
in order to have given sufficient time for the slow process of variation; hence
the deposit will generally have to be a very thick one; and the species
undergoing modification will have had to live on the same area throughout this
whole time. But we have seen that a thick fossiliferous formation can only be
accumulated during a period of subsidence; and to keep the depth approximately
the same, which is necessary in order to enable the same species to live on the
same space, the supply of sediment must nearly have counterbalanced the amount
of subsidence. But this same movement of subsidence will often tend to sink the
area whence the sediment is derived, and thus diminish the supply whilst the
downward movement continues. In fact, this nearly exact balancing between the
supply of sediment and the amount of subsidence is probably a rare contingency;
for it has been observed by more than one palaeontologist, that very thick
deposits are usually barren of organic remains, except near their upper or
lower limits.
It would seem that each
separate formation, like the whole pile of formations in any country, has
generally been intermittent in its accumulation. When we see, as is so often
the case, a formation composed of beds of different mineralogical composition,
we may reasonably suspect that the process of deposition has been much
interrupted, as a change in the currents of the sea and a supply of sediment of
a different nature will generally have been due to geographical changes
requiring much time. Nor will the closest inspection of a formation give any
idea of the time which its deposition has consumed. Many instances could be
given of beds only a few feet in thickness, representing formations, elsewhere
thousands of feet in thickness, and which must have required an enormous period
for their accumulation; yet no one ignorant of this fact would have suspected
the vast lapse of time represented by the thinner formation. Many cases could
be given of the lower beds of a formation having been upraised, denuded,
submerged, and then re-covered by the upper beds of the same formation, --
facts, showing what wide, yet easily overlooked, intervals have occurred in its
accumulation. In other cases we have the plainest evidence in great fossilised
trees, still standing upright as they grew, of many long intervals of time and
changes of level during the process of deposition, which would never even have
been suspected, had not the trees chanced to have been preserved: thus, Messrs
Lyell and Dawson found carboniferous beds 1400 feet thick in Nova Scotia, with
ancient root-bearing strata, one above the other, at no less than sixty-eight
different levels. Hence, when the same species occur at the bottom, middle, and
top of a formation, the probability is that they have not lived on the same
spot during the whole period of deposition, but have disappeared and
reappeared, perhaps many times, during the same geological period. So that if
such species were to undergo a considerable amount of modification during any
one geological period, a section would not probably include all the fine
intermediate gradations which must on my theory have existed between them, but
abrupt, though perhaps very slight, changes of form.
It is all-important to
remember that naturalists have no golden rule by which to distinguish species
and varieties; they grant some little variability to each species, but when
they meet with a somewhat greater amount of difference between any two forms,
they rank both as species, unless they are enabled to connect them together by
close intermediate gradations. And this from the reasons just assigned we can
seldom hope to effect in any one geological section. Supposing B and C to be
two species, and a third, A, to be found in an underlying bed; even if A were
strictly intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be most closely connected
with either one or both forms by intermediate varieties. Nor should it be
forgotten, as before explained, that A might be the actual progenitor of B and
C, and yet might not at all necessarily be strictly intermediate between them
in all points of structure. So that we might obtain the parent-species and its
several modified descendants from the lower and upper beds of a formation, and
unless we obtained numerous transitional gradations, we should not recognise
their relationship, and should consequently be compelled to rank them all as
distinct species.
It is notorious on what
excessively slight differences many palaeontologists have founded their
species; and they do this the more readily if the specimens come from different
sub-stages of the same formation. Some experienced conchologists are now
sinking many of the very fine species of D'Orbigny and others into the rank of
varieties; and on this view we do find the kind of evidence of change which on
my theory we ought to find. Moreover, if we look to rather wider intervals,
namely, to distinct but consecutive stages of the same great formation, we find
that the embedded fossils, though almost universally ranked as specifically different,
yet are far more closely allied to each other than are the species found in
more widely separated formations; but to this subject I shall have to return in
the following chapter.
One other consideration
is worth notice: with animals and plants that can propagate rapidly and are not
highly locomotive, there is reason to suspect, as we have formerly seen, that
their varieties are generally at first local; and that such local varieties do
not spread widely and supplant their parent-forms until they have been modified
and perfected in some considerable degree. According to this view, the chance
of discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes are
supposed to have been local or confined to some one spot. Most marine animals
have a wide range; and we have seen that with plants it is those which have the
widest range, that oftenest present varieties; so that with shells and other
marine animals, it is probably those which have had the widest range, far
exceeding the limits of the known geological formations of Europe, which have
oftenest given rise, first to local varieties and ultimately to new species;
and this again would greatly lessen the chance of our being able to trace the
stages of transition in any one geological formation.
It should not be
forgotten, that at the present day, with perfect specimens for examination, two
forms can seldom be connected by intermediate varieties and thus proved to be
the same species, until many specimens have been collected from many places;
and in the case of fossil species this could rarely be effected by
palaeontologists. We shall, perhaps, best perceive the improbability of our
being enabled to connect species by numerous, fine, intermediate, fossil links,
by asking ourselves whether, for instance, geologists at some future period
will be able to prove, that our different breeds of cattle, sheep, horses, and
dogs have descended from a single stock or from several aboriginal stocks; or,
again, whether certain sea-shells inhabiting the shores of North America, which
are ranked by some conchologists as distinct species from their European
representatives, and by other conchologists as only varieties, are really
varieties or are, as it is called, specifically distinct. This could be
effected only by the future geologist discovering in a fossil state numerous
intermediate gradations; and such success seems to me improbable in the highest
degree.
Geological research,
though it has added numerous species to existing and extinct genera, and has
made the intervals between some few groups less wide than they otherwise would
have been, yet has done scarcely anything in breaking down the distinction
between species, by connecting them together by numerous, fine, intermediate
varieties; and this not having been effected, is probably the gravest and most
obvious of all the many objections which may be urged against my views. Hence
it will be worth while to sum up the foregoing remarks, under an imaginary
illustration. The Malay Archipelago is of about the size of Europe from the
North Cape to the Mediterranean, and from Britain to Russia; and therefore
equals all the geological formations which have been examined with any
accuracy, excepting those of the United States of America. I fully agree with
Mr Godwin-Austen, that the present condition of the Malay Archipelago, with its
numerous large islands separated by wide and shallow seas, probably represents
the former state of Europe, when most of our formations were accumulating. The
Malay Archipelago is one of the richest regions of the whole world in organic
beings; yet if all the species were to be collected which have ever lived
there, how imperfectly would they represent the natural history of the world!
But we have every
reason to believe that the terrestrial productions of the archipelago would be
preserved in an excessively imperfect manner in the formations which we suppose
to be there accumulating. I suspect that not many of the strictly littoral
animals, or of those which lived on naked submarine rocks, would be embedded;
and those embedded in gravel or sand, would not endure to a distant epoch.
Wherever sediment did not accumulate on the bed of the sea, or where it did not
accumulate at a sufficient rate to protect organic bodies from decay, no
remains could be preserved.
in our archipelago, I
believe that fossiliferous formations could be formed of sufficient thickness
to last to an age, as distant in futurity as the secondary formations lie in
the past, only during periods of subsidence. These periods of subsidence would
be separated from each other by enormous intervals, during which the area would
be either stationary or rising; whilst rising, each fossilferous formation would
be destroyed, almost as soon as accumulated, by the incessant coast-action, as
we now see on the shores of South America. During the periods of subsidence
there would probably be much extinction of life; during the periods of
elevation, there would be much variation, but the geological record would then
be least perfect.
It may be doubted
whether the duration of any one great period of subsidence over the whole or
part of the archipelago, together with a contemporaneous accumulation of
sediment, would exceed the average duration of the same specific forms; and
these contingencies are indispensable for the preservation of all the
transitional gradations between any two or more species. If such gradations
were not fully preserved, transitional varieties would merely appear as so many
distinct species. It is, also, probable that each great period of subsidence
would be interrupted by oscillations of level, and that slight climatal changes
would intervene during such lengthy periods; and in these cases the inhabitants
of the archipelago would have to migrate, and no closely consecutive record of
their modifications could be preserved in any one formation.
Very many of the marine
inhabitants of the archipelago now range thousands of miles beyond its
confines; and analogy leads me to believe that it would be chiefly these
far-ranging species which would oftenest produce new varieties; and the
varieties would at first generally be local or confined to one place, but if
possessed of any decided advantage, or when further modified and improved, they
would slowly spread and supplant their parent-forms. When such varieties
returned to their ancient homes, as they would differ from their former state,
in a nearly uniform, though perhaps extremely slight degree, they would,
according to the principles followed by many palaeontologists, be ranked as new
and distinct species.
If then, there be some
degree of truth in these remarks, we have no right to expect to find in our
geological formations, an infinite number of those fine transitional forms,
which on my theory assuredly have connected all the past and present species of
the same group into one long and branching chain of life. We ought only to look
for a few links, some more closely, some more distantly related to each other;
and these links, let them be ever so close, if found in different stages of the
same formation, would, by most palaeontologists, be ranked as distinct species.
But I do not pretend that I should ever have suspected how poor a record of the
mutations of life, the best preserved geological section presented, had not the
difficulty of our not discovering innumerable transitional links between the
species which appeared at the commencement and close of each formation, pressed
so hardly on my theory.
On the sudden
appearance of whole groups of Allied Species. The abrupt manner in which whole
groups of species suddenly appear in certain formations, has been urged by
several palaeontologists, for instance, by Agassiz, Pictet, and by none more
forcibly than by professor Sedgwick, as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging to the same genera or
families, have really started into life all at once, the fact would be fatal to
the theory of descent with slow modification through natural selection. For the
development of a group of forms, all of which have descended from some one
progenitor, must have been an extremely slow process; and the progenitors must
have lived long ages before their modified descendants. But we continually
over-rate the perfection of the geological record, and falsely infer, because
certain genera or families have not been found beneath a certain stage, that
they did not exist before that stage. We continually forget how large the world
is, compared with the area over which our geological formations have been
carefully examined; we forget that groups of species may elsewhere have long
existed and have slowly multiplied before they invaded the ancient
archipelagoes of Europe and of the United States. We do not make due allowance
for the enormous intervals of time, which have probably elapsed between our
consecutive formations, -- longer perhaps in some cases than the time required
for the accumulation of each formation. These intervals will have given time
for the multiplication of species from some one or some few parent-forms; and
in the succeeding formation such species will appear as If suddenly created.
I may here recall a
remark formerly made, namely that it might require a long succession of ages to
adapt an organism to some new and peculiar line of life, for instance to fly
through the air; but that when this had been effected, and a few species had
thus acquired a great advantage over other organisms, a comparatively short
time would be necessary to produce many divergent forms, which would be able to
spread rapidly and widely throughout the world.
I will now give a few
examples to illustrate these remarks; and to show how liable we are to error in
supposing that whole groups of species have suddenly been produced. I may
recall the well-known fact that in geological treatises, published not many
years ago, the great class of mammals was always spoken of as having abruptly
come in at the commencement of the tertiary series. And now one of the richest
known accumulations of fossil mammals belongs to the middle of the secondary
series; and one true mammal has been discovered in the new red sandstone at
nearly the commencement of this great series. Cuvier used to urge that no
monkey occurred in any tertiary stratum; but now extinct species have been
discovered in India, South America, and in Europe even as far back as the
eocene stage. The most striking case, however, is that of the Whale family; as
these animals have huge bones, are marine, and range over the world, the fact
of not a single bone of a whale having been discovered in any secondary
formation, seemed fully to justify the belief that this great and distinct
order had been suddenly produced in the interval between the latest secondary
and earliest tertiary formation. But now we may read in the Supplement to
Lyell's ' Manual,' published in 1858, clear evidence of the existence of whales
in the upper greensand, some time before the close of the secondary period.
I may give another
instance, which from having passed under my own eyes has much struck me. In a
memoir on Fossil Sessile Cirripedes, I have stated that, from the number of
existing and extinct tertiary species; from the extraordinary abundance of the
individuals of many species all over the world, from the Arctic regions to the
equator, inhabiting various zones of depths from the upper tidal limits to 50
fathoms; from the perfect manner in which specimens are preserved in the oldest
tertiary beds; from the ease with which even a fragment of a valve can be
recognised; from all these circumstances, I inferred that had sessile
cirripedes existed during the secondary periods, they would certainly have been
preserved and discovered; and as not one species had been discovered in beds of
this age, I concluded that this great group had been suddenly developed at the
commencement of the tertiary series. This was a sore trouble to me, adding as I
thought one more instance of the abrupt appearance of a great group of species.
But my work had hardly been published, when a skilful palaeontologist, M.
Bosquet, sent me a drawing of a perfect specimen of an unmistakeable sessile
cirripede, which he had himself extracted from the chalk of Belgium. And, as if
to make the case as striking as possible, this sessile cirripede was a
Chthamalus, a very common, large, and ubiquitous genus, of which not one
specimen has as yet been found even in any tertiary stratum. Hence we now
positively know that sessile cirripedes existed during the secondary period;
and these cirripedes might have been the progenitors of our many tertiary and
existing species.
The case most
frequently insisted on by palaeontologists of the apparently sudden appearance
of a whole group of species, is that of the teleostean fishes, low down in the
Chalk period. This group includes the large majority of existing species.
Lately, Professor Pictet has carried their existence one sub-stage further
back; and some palaeontologists believe that certain much older fishes, of
which the affinities are as yet imperfectly known, are really teleostean.
Assuming, however, that the whole of them did appear, as Agassiz believes, at
the commencement of the chalk formation, the fact would certainly be highly
remarkable; but I cannot see that it would be an insuperable difficulty on my
theory, unless it could likewise be shown that the species of this group
appeared suddenly and simultaneously throughout the world at this same period.
It is almost superfluous to remark that hardly any fossil-fish are known from
south of the equator; and by running through Pictet's palaeontology it will be
seen that very few species are known from several formations in Europe. Some
few families of fish now have a confined range; the teleostean fish might
formerly have had a similarly confined range, and after having been largely
developed in some one sea, might have spread widely. Nor have we any right to
suppose that the seas of the world have always been so freely open from south
to north as they are at present. Even at this day, if the Malay Archipelago
were converted into land, the tropical parts of the Indian Ocean would form a
large and perfectly enclosed basin, in which any great group of marine animals
might be multiplied; and here they would remain confined, until some of the
species became adapted to a cooler climate, and were enabled to double the
southern capes of Africa or Australia, and thus reach other and distant seas.
From these and similar
considerations, but chiefly from our ignorance of the geology of other
countries beyond the confines of Europe and the United States; and from the
revolution in our palaeontological ideas on many points, which the discoveries
of even the last dozen years have effected, it seems to me to be about as rash
in us to dogmatize on the succession of organic beings throughout the world, as
it would be for a naturalist to land for five minutes on some one barren point
in Australia, and then to discuss the number and range of its productions.
On the sudden
appearance of groups of Allied Species in thelowest known fossiliferous strata.
There is another and allied difficulty, which is much graver. I allude to the
manner in which numbers of species of the same group, suddenly appear in the
lowest known fossiliferous rocks. Most of the arguments which have convinced me
that all the existing species of the same group have descended from one
progenitor, apply with nearly equal force to the earliest known species. For
instance, I cannot doubt that all the Silurian trilobites have descended from
some one crustacean, which must have lived long before the Silurian age, and
which probably differed greatly from any known animal. Some of the most ancient
Silurian animals, as the Nautilus, Lingula, &c., do not differ much from
living species; and it cannot on my theory be supposed, that these old species
were the progenitors of all the species of the orders to which they belong, for
they do not present characters in any degree intermediate between them. If,
moreover, they had been the progenitors or these orders, they would almost
certainly have been long ago supplanted and exterminated by their numerous and
improved descendants.
Consequently, if my
theory be true, it is indisputable that before the lowest Silurian stratum was
deposited, long periods elapsed, as long as, or probably far longer than, the
whole interval from the Silurian age to the present day; and that during these
vast, yet quite unknown, periods of time, the world swarmed with living
creatures.
To the question why we
do not find records of these vast primordial periods, I can give no
satisfactory answer. Several of the most eminent geologists, with Sir R.
Murchison at their head, are convinced that we see in the organic remains of
the lowest Silurian stratum the dawn of life on this planet. Other highly
competent judges, as Lyell and the late E. Forbes, dispute this conclusion. We
should not forget that only a small portion of the world is known with
accuracy. M. Barrande has lately added another and lower stage to the Silurian
system, abounding with new and peculiar species. Traces of life have been
detected in the Longmynd beds beneath Barrande's so-called primordial zone. The
presence of phosphatic nodules and bituminous matter in some of the lowest
azoic rocks, probably indicates the former existence of life at these periods.
But the difficulty of understanding the absence of vast piles of fossiliferous
strata, which on my theory no doubt were somewhere accumulated before the
Silurian epoch, is very great. If these most ancient beds had been wholly worn
away by denudation, or obliterated by metamorphic action, we ought to find only
small remnants of the formations next succeeding them in age, and these ought
to be very generally in a metamorphosed condition. But the descriptions which
we now possess of the Silurian deposits over immense territories in Russia and
in North America, do not support the view, that the older a formation is, the
more it has suffered the extremity of denudation and metamorphism.
The case at present
must remain inexplicable; and may be truly urged as a valid argument against
the views here entertained. To show that it may hereafter receive some
explanation, I will give the following hypothesis. From the nature of the
organic remains, which do not appear to have inhabited profound depths, in the
several formations of Europe and of the United States; and from the amount of
sediment, miles in thickness, of which the formations are composed, we may
infer that from first to last large islands or tracts of land, whence the
sediment was derived, occurred in the neighbourhood of the existing continents
of Europe and North America. But we do not know what was the state of things in
the intervals between the successive formations; whether Europe and the United
States during these intervals existed as dry land, or as a submarine surface
near land, on which sediment was not deposited, or again as the bed of an open
and unfathomable sea.
Looking to the existing
oceans, which are thrice as extensive as the land, we see them studded with
many islands; but not one oceanic island is as yet known to afford even a
remnant of any palaeozoic or secondary formation. Hence we may perhaps infer,
that during the palaeozoic and secondary periods, neither continents nor
continental islands existed where our oceans now extend; for had they existed
there, palaeozoic and secondary formations would in all probability have been
accumulated from sediment derived from their wear and tear; and would have been
at least partially upheaved by the oscillations of level, which we may fairly
conclude must have intervened during these enormously long periods. If then we
may infer anything from these facts, we may infer that where our oceans now
extend, oceans have extended from the remotest period of which we have any
record; and on the other hand, that where continents now exist, large tracts of
land have existed, subjected no doubt to great oscillations of level, since the
earliest silurian period. The coloured map appended to my volume on Coral
Reefs, led me to conclude that the great oceans are still mainly areas of
subsidence, the great archipelagoes still areas of oscillations of level, and
the continents areas of elevation. But have we any right to assume that things
have thus remained from eternity? Our continents seem to have been formed by a
preponderance, during many oscillations of level, of the force of elevation;
but may not the areas of preponderant movement have changed in the lapse of
ages? At a period immeasurably antecedent to the silurian epoch, continents may
have existed where oceans are now spread out; and clear and open oceans may
have existed where our continents now stand. Nor should we be justified in
assuming that If, for instance, the bed of the pacific Ocean were now converted
into a continent, we should there find formations older than the silurian
strata, supposing such to have been formerly deposited;, for it might well
happen that strata which had subsided some miles nearer to the centre of the
earth, and which had been pressed on by an enormous weight of superincumbent
water, might have undergone far more metamorphic action than strata which have
always remained nearer to the surface. The immense areas in some parts of the
world, for instance in South America, of bare metamorphic rocks, which must
have been heated under great pressure, have always seemed to me to require some
special explanation; and we may perhaps believe that we see in these large
areas, the many formations long anterior to the silurian epoch in a completely
metamorphosed condition.
The several
difficulties here discussed, namely our not finding in the successive
formations infinitely numerous transitional links between the many species
which now exist or have existed; the sudden manner in which whole groups of
species appear in our European formations; the almost entire absence, as at
present known, of fossiliferous formations beneath the Silurian strata, are all
undoubtedly of the gravest nature. We see this in the plainest manner by the
fact that all the most eminent palaeontologists, namely Cuvier, Owen, Agassiz,
Barrande, Falconer, E. Forbes, &c., and all our greatest geologists, as
Lyell, Murchison, Sedgwick, &c., have unanimously, often vehemently,
maintained the immutability of species. But I have reason to believe that one
great authority, Sir Charles Lyell, from further reflexion entertains grave
doubts on this subject. I feel how rash it is to differ from these great
authorities, to whom, with others, we owe all our knowledge. Those who think
the natural geological record in any degree perfect, and who do not attach much
weight to the facts and arguments of other kinds even in this volume, will
undoubtedly at once reject my theory. For my part, following out Lyell's
metaphor, I look at the natural geological record, as a history of the world
imperfectly kept, and written in a changing dialect; of this history we possess
the last volume alone, relating only to two or three countries. Of this volume,
only here and there a short chapter has been preserved; and of each page, only
here and there a few lines. Each word of the slowly-changing language, in which
the history is supposed to be written, being more or less different in the
interrupted succession of chapters, may represent the apparently abruptly
changed forms of life, entombed in our consecutive, but widely separated
formations. On this view, the difficulties above discussed are greatly
diminished, or even disappear.
On the slow and successive appearance of new species - On their
different rates of change - Species once lost do not reappear - Groups of
species follow the same general rules in their appearance and disappearance as
do single species - On Extinction - On simultaneous changes in the forms of
life throughout the world - On the affinities of extinct species to each other
and to living species - On the state of development of ancient forms - On the
succession of the same types within the same areas -- Summary of preceding and
present chapters LET us now see whether
the several facts and rules relating to the geological succession of organic
beings, better accord with the common view of the immutability of species, or
with that of their slow and gradual modification, through descent and natural
selection.
New species have
appeared very slowly, one after another, both on the land and in the waters.
Lyell has shown that it is hardly possible to resist the evidence on this head
in the case of the several tertiary stages; and every year tends to fill up the
blanks between them, and to make the percentage system of lost and new forms more
gradual. In some of the most recent beds, though undoubtedly of high antiquity
if measured by years, only one or two species are lost forms, and only one or
two are new forms, having here appeared for the first time, either locally, or,
as far as we know, on the face of the earth. If we may trust the observations
of Philippi in Sicily, the successive changes in the marine inhabitants of that
island have been many and most gradual. The secondary formations are more
broken; but, as Bronn has remarked, neither the appearance nor disappearance of
their many now extinct species has been simultaneous in each separate
formation.
Species of different
genera and classes have not changed at the same rate, or in the same degree. In
the oldest tertiary beds a few living shells may still be found in the midst of
a multitude of extinct forms. Falconer has given a striking instance of a
similar fact, in an existing crocodile associated with many strange and lost
mammals and reptiles in the sub-Himalayan deposits. The Silurian Lingula
differs but little from the living species of this genus; whereas most of the
other Silurian Molluscs and all the Crustaceans have changed greatly. The
productions of the land seem to change at a quicker rate than those of the sea,
of which a striking instance has lately been observed in Switzerland. There is
some reason to believe that organisms, considered high in the scale of nature,
change more quickly than those that are low: though there are exceptions to
this rule. The amount of organic change, as Pictet has remarked, does not
strictly correspond with the succession of our geological formations; so that
between each two consecutive formations, the forms of life have seldom changed
in exactly the same degree. Yet if we compare any but the most closely related
formations, all the species will be found to have undergone some change. When a
species has once disappeared from the face of the earth, we have reason to
believe that the same identical form never reappears. The strongest apparent exception
to this latter rule, is that of the so- called `colonies' of M. Barrande, which
intrude for a period in the midst of an older formation, and then allow the
pre- existing fauna to reappear; but Lyell's explanation, namely, that it is a
case of temporary migration from a distinct geographical province, seems to me
satisfactory.
These several facts
accord well with my theory. I believe in no fixed law of development, causing
all the inhabitants of a country to change abruptly, or simultaneously, or to
an equal degree. The process of modification must be extremely slow. The
variability of each species is quite independent of that of all others. Whether
such variability be taken advantage of by natural selection, and whether the
variations be accumulated to a greater or lesser amount, thus causing a greater
or lesser amount of modification in the varying species, depends on many
complex contingencies, -- on the variability being of a beneficial nature, on
the power of intercrossing, on the rate of breeding, on the slowly changing
physical conditions of the country, and more especially on the nature of the
other inhabitants with which the varying species comes into competition. Hence
it is by no means surprising that one species should retain the same identical
form much longer than others; or, if changing, that it should change less. We
see the same fact in geographical distribution; for instance, in the
land-shells and coleopterous insects of Madeira having come to differ
considerably from their nearest allies on the continent of Europe, whereas the
marine shells and birds have remained unaltered. We can perhaps understand the
apparently quicker rate of change in terrestrial and in more highly organised
productions compared with marine and lower productions, by the more complex
relations of the higher beings to their organic and inorganic conditions of
life, as explained in a former chapter. When many of the inhabitants of a
country have become modified and improved, we can understand, on the principle
of competition, and on that of the many all- important relations of organism to
organism, that any form which does not become in some degree modified and
improved, will be liable to be exterminated. Hence we can see why all the
species in the same region do at last, if we look to wide enough intervals of
time, become modified; for those which do not change will become extinct.
In members of the same
class the average amount of change, during long and equal periods of time, may,
perhaps, be nearly the same; but as the accumulation of long-enduring
fossiliferous formations depends on great masses of sediment having been
deposited on areas whilst subsiding, our formations have been almost
necessarily accumulated at wide and irregularly intermittent intervals; consequently
the amount of organic change exhibited by the fossils embedded in consecutive
formations is not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at hazard,
in a slowly changing drama.
We can clearly
understand why a species when once lost should never reappear, even if the very
same conditions of life, organic and inorganic, should recur. For though the
offspring of one species might be adapted (and no doubt this has occurred in
innumerable instances) to fill the exact place of another species in the
economy of nature, and thus supplant it; yet the two forms -- the old and the
new -- would not be identically the same; for both would almost certainly
inherit different characters from their distinct progenitors. For instance, it
is just possible, if our fantail-pigeons were all destroyed, that fanciers, by
striving during long ages for the same object, might make a new breed hardly
distinguishable from our present fantail; but if the parent rock-pigeon were
also destroyed, and in nature we have every reason to believe that the
parent-form will generally be supplanted and exterminated by its improved
offspring, it is quite incredible that a fantail, identical with the existing
breed, could be raised from any other species of pigeon, or even from the other
well- established races of the domestic pigeon, for the newly- formed fantail
would be almost sure to inherit from its new progenitor some slight
characteristic differences.
Groups of species, that
is, genera and families, follow the same general rules in their appearance and
disappearance as do single species, changing more or less quickly, and in a
greater or lesser degree. A group does not reappear after it has once disappeared;
or its existence, as long as it lasts, is continuous. I am aware that there are
some apparent exceptions to this rule, but the exceptions are surprisingly few,
so few, that E. Forbes, Pictet, and Woodward (though all strongly opposed to
such views as I maintain) admit its truth; and the rule strictly accords with
my theory. For as all the species of the same group have descended from some
one species, it is clear that as long as any species of the group have appeared
in the long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the same old and
unmodified forms. Species of the genus Lingula, for instance, must have
continuously existed by an unbroken succession of generations, from the lowest
Silurian stratum to the present day.
We have seen in the
last chapter that the species of a group sometimes falsely appear to have come
in abruptly; and I have attempted to give an explanation of this fact, which if
true would have been fatal to my views. But such cases are certainly
exceptional; the general rule being a gradual increase in number, till the
group reaches its maximum, and then, sooner or later, it gradually decreases.
If the number of the species of a genus, or the number of the genera of a
family, be represented by a vertical line of varying thickness, crossing the
successive geological formations in which the species are found, the line will
sometimes falsely appear to begin at its lower end, not in a sharp point, but
abruptly; it then gradually thickens upwards, sometimes keeping for a space or
equal thickness, and ultimately thins out in the upper beds, marking the
decrease and final extinction of the species. This gradual increase in number
of the species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can increase only
slowly and progressively; for the process of modification and the production of
a number of allied forms must be slow and gradual, -- one species giving rise
first to two or three varieties, these being slowly converted into species,
which in their turn produce by equally slow steps other species, and so on,
like the branching of a great tree from a single stem, till the group becomes
large.
On Extinction . We have
as yet spoken only incidentally of the disappearance of species and of groups
of species. On the theory of natural selection the extinction of old forms and
the production of new and improved forms are intimately connected together. The
old notion of all the inhabitants of the earth having been swept away at
successive periods by catastrophes, is very generally given up, even by those
geologists, as Elie de Beaumont, Murchison, Barrande, &c., whose general
views would naturally lead them to this conclusion. On the contrary, we have
every reason to believe, from the study of the tertiary formations, that
species and groups of species gradually disappear, one after another, first
from one spot, then from another, and finally from the world. Both single
species and whole groups of species last for very unequal periods; some groups,
as we have seen, having endured from the earliest known dawn of life to the
present day; some having disappeared before the close of the palaeozoic period.
No fixed law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that the
complete extinction of the species of a group is generally a slower process
than their production: if the appearance and disappearance of a group of
species be represented, as before, by a vertical line of varying thickness, the
line is found to taper more gradually at its upper end, which marks the
progress of extermination, than at its lower end, which marks the first
appearance and increase in numbers of the species. In some cases, however, the
extermination of whole groups of beings, as of ammonites towards the close of
the secondary period, has been wonderfully sudden.
The whole subject of
the extinction of species has been involved in the most gratuitous mystery.
Some authors have even supposed that as the individual has a definite length of
life, so have species a definite duration. No one I think can have marvelled
more at the extinction of species, than I have done. When I found in La Plata
the tooth of a horse embedded with the remains of Mastodon, Megatherium,
Toxodon, and other extinct monsters, which all co-existed with still living
shells at a very late geological period, I was filled with astonishment; for
seeing that the horse, since its introduction by the Spaniards into South
America, has run wild over the whole country and has increased in numbers at an
unparalleled rate, I asked myself what could so recently have exterminated the former
horse under conditions of life apparently so favourable. But how utterly
groundless was my astonishment! Professor Owen soon perceived that the tooth,
though so like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist would have
felt the least surprise at its rarity; for rarity is the attribute of a vast
number of species of all classes, in all countries. If we ask ourselves why
this or that species is rare, we answer that something is unfavourable in its
conditions of life; but what that something is, we can hardly ever tell. On the
supposition of the fossil horse still existing as a rare species, we might have
felt certain from the analogy of all other mammals, even of the slow-breeding
elephant, and from the history of the naturalisation of the domestic horse in
South America, that under more favourable conditions it would in a very few
years have stocked the whole continent. But we could not have told what the
unfavourable conditions were which checked its increase, whether some one or
several contingencies, and at what period of the horse's life, and in what
degree, they severally acted. If the conditions had gone on, however slowly,
becoming less and less favourable, we assuredly should not have perceived the
fact, yet the fossil horse would certainly have become rarer and rarer, and
finally extinct; -- its place being seized on by some more successful
competitor.
It is most difficult
always to remember that the increase of every living being is constantly being
checked by unperceived injurious agencies; and that these same unperceived
agencies are amply sufficient to cause rarity, and finally extinction. We see
in many cases in the more recent tertiary formations, that rarity precedes
extinction; and we know that this has been the progress of events with those
animals which have been exterminated, either locally or wholly, through man's
agency. I may repeat what I published in 1845, namely, that to admit that
species generally become rare before they become extinct -- to feel no surprise
at the rarity of a species, and yet to marvel greatly when it ceases to exist,
is much the same as to admit that sickness in the individual is the forerunner
of death -- to feel no surprise at sickness, but when the sick man dies, to
wonder and to suspect that he died by some unknown deed of violence.
The theory of natural
selection is grounded on the belief that each new variety, and ultimately each
new species, is produced and maintained by having some advantage over those
with which it comes into competition; and the consequent extinction of
less-favoured forms almost inevitably follows. It is the same with our domestic
productions: when a new and slightly improved variety has been raised, it at
first supplants the less improved varieties in the same neighbourhood; when
much improved it is transported far and near, like our short-horn cattle, and
takes the place of other breeds in other countries. Thus the appearance of new
forms and the disappearance of old forms, both natural and artificial, are
bound together. In certain flourishing groups, the number of new specific forms
which have been produced within a given time is probably greater than that of
the old forms which have been exterminated; but we know that the number of
species has not gone on indefinitely increasing, at least during the later
geological periods, so that looking to later times we may believe that the
production of new forms has caused the extinction of about the same number of
old forms.
The competition will
generally be most severe, as formerly explained and illustrated by examples,
between the forms which are most like each other in all respects. Hence the
improved and modified descendants of a species will generally cause the
extermination of the parent-species; and if many new forms have been developed
from any one species, the nearest allies of that species, i.e. the species of
the same genus, will be the most liable to extermination. Thus, as I believe, a
number of new species descended from one species, that is a new genus, comes to
supplant an old genus, belonging to the same family. But it must often have
happened that a new species belonging to some one group will have seized on the
place occupied by a species belonging to a distinct group, and thus caused its
extermination; and if many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be allied
forms, which will suffer from some inherited inferiority in common. But whether
it be species belonging to the same or to a distinct class, which yield their
places to other species which have been modified and improved, a few of the
sufferers may often long be preserved, from being fitted to some peculiar line
of life, or from inhabiting some distant and isolated station, where they have
escaped severe competition. For instance, a single species of Trigonia, a great
genus of shells in the secondary formations, survives in the Australian seas;
and a few members of the great and almost extinct group of Ganoid fishes still
inhabit our fresh waters. Therefore the utter extinction of a group is
generally, as we have seen, a slower process than its production.
With respect to the
apparently sudden extermination of whole families or orders, as of Trilobites
at the close of the palaeozoic period and of Ammonites at the close of the
secondary period, we must remember what has been already said on the probable
wide intervals of time between our consecutive formations; and in these
intervals there may have been much slow extermination. Moreover, when by sudden
immigration or by unusually rapid development, many species of a new group have
taken possession of a new area, they will have exterminated in a
correspondingly rapid manner many of the old inhabitants; and the forms which
thus yield their places will commonly be allied, for they will partake of some
inferiority in common.
Thus, as it seems to
me, the manner in which single species and whole groups of species become
extinct, accords well with the theory of natural selection. We need not marvel
at extinction; if we must marvel, let it be at our presumption in imagining for
a moment that we understand the many complex contingencies, on which the
existence of each species depends. If we forget for an instant, that each
species tends to increase inordinately, and that some check is always in
action, yet seldom perceived by us, the whole economy of nature will be utterly
obscured. Whenever we can precisely say why this species is more abundant in
individuals than that; why this species and not another can be naturalised in a
given country; then, and not till then, we may justly feel surprise why we
cannot account for the extinction of this particular species or group of
species.
On the Forms of Life
changing almost simultaneously throughout the World . Scarcely any
palaeontological discovery is more striking than the fact, that the forms of
life change almost simultaneously throughout the world. Thus our European Chalk
formation can be recognised in many distant parts of the world, under the most
different climates, where not a fragment of the mineral chalk itself can be
found; namely, in North America, in equatorial South America, in Tierra del
Fuego, at the Cape of Good Hope, and in the peninsula of India. For at these
distant points, the organic remains in certain beds present an unmistakeable
degree of resemblance to those of the Chalk. It is not that the same species
are met with; for in some cases not one species is identically the same, but
they belong to the same families, genera, and sections of genera, and sometimes
are similarly characterised in such trifling points as mere superficial
sculpture. Moreover other forms, which are not found in the Chalk of Europe,
but which occur in the formations either above or below, are similarly absent
at these distant points of the world. In the several successive palaeozoic
formations of Russia, Western Europe and North America, a similar parallelism
in the forms of life has been observed by several authors: so it is, according
to Lyell, with the several European and North American tertiary deposits. Even
if the few fossil species which are common to the Old and New Worlds be kept wholly
out of view, the general parallelism in the successive forms of life, in the
stages of the widely separated palaeozoic and tertiary periods, would still be
manifest. and the several formations could be easily correlated.
These observations,
however, relate to the marine inhabitants of distant parts of the world: we
have not sufficient data to judge whether the productions of the land and of
fresh water change at distant points in the same parallel manner. We may doubt
whether they have thus changed: if the Megatherium, Mylodon, Macrauclienia, and
Toxodon had been brought to Europe from La Plata, without any information in
regard to their geological position, no one would have suspected that they had
coexisted with still living sea-shells; but as these anomalous monsters
coexisted with the Mastodon and Horse, it might at least have been inferred
that they had lived during one of the latter tertiary stages.
When the marine forms
of life are spoken of as having changed simultaneously throughout the world, it
must not be supposed that this expression relates to the same thousandth or
hundred-thousandth year, or even that it has a very strict geological sense;
for if all the marine animals which live at the present day in Europe, and all
those that lived in Europe during the pleistocene period (an enormously remote
period as measured by years, including the whole glacial epoch), were to be
compared with those now living in South America or in Australia, the most
skilful naturalist would hardly be able to say whether the existing or the
pleistocene inhabitants of Europe resembled most closely those of the southern
hemisphere. So, again, several highly competent observers believe that the
existing productions of the United States are more closely related to those
which lived in Europe during certain later tertiary stages, than to those which
now live here; and if this be so, it is evident that fossiliferous beds
deposited at the present day on the shores of North America would hereafter be
liable to be classed with somewhat older European beds. Nevertheless, looking
to a remotely future epoch, there can, I think, be little doubt that all the
more modern marine formations, namely, the upper pliocene, the pleistocene and
strictly modern beds, of Europe, North and South America, and Australia, from
containing fossil remains in some degree allied, and from not including those
forms which are only found in the older underlying deposits, would be correctly
ranked as simultaneous in a geological sense.
The fact of the forms
of life changing simultaneously, in the above large sense, at distant parts of
the world, has greatly struck those admirable observers, MM. de Verneuil and
d'Archiac. After referring to the parallelism of the palaeozoic forms of life
in various parts of Europe, they add, `If struck by this strange sequence, we
turn our attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of species,
their extinction, and the introduction of new ones, cannot be owing to mere
changes in marine currents or other causes more or less local and temporary,
but depend on general laws which govern the whole animal kingdom.' M. Barrande
has made forcible remarks to precisely the same effect. It is, indeed, quite
futile to look to changes of currents, climate, or other physical conditions,
as the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has remarked,
look to some special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the relation
between the physical conditions of various countries, and the nature of their
inhabitants.
This great fact of the
parallel succession of the forms of life throughout the world, is explicable on
the theory of natural selection. New species are formed by new varieties
arising, which have some advantage over older forms; and those forms, which are
already dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or incipient
species; for these latter must be victorious in a still higher degree in order
to be preserved and to survive. We have distinct evidence on this head, in the
plants which are dominant, that is, which are commonest in their own homes, and
are most widely diffused, having produced the greatest number of new varieties.
It is also natural that the dominant, varying, and far-spreading species, which
already have invaded to a certain extent the territories of other species,
should be those which would have the best chance of spreading still further,
and of giving rise in new countries to new varieties and species. The process
of diffusion may often be very slow, being dependent on climatal and
geographical changes, or on strange accidents, but in the long run the dominant
forms will generally succeed in spreading. The diffusion would, it is probable,
be slower with the terrestrial inhabitants of distinct continents than with the
marine inhabitants of the continuous sea. We might therefore expect to find, as
we apparently do find, a less strict degree of parallel succession in the
productions of the land than of the sea.
Dominant species spreading
from any region might encounter still more dominant species, and then their
triumphant course, or even their existence, would cease. We know not at all
precisely what are all the conditions most favourable for the multiplication of
new and dominant species; but we can, I think, clearly see that a number of
individuals, from giving a better chance of the appearance of favourable
variations, and that severe competition with many already existing forms, would
be highly favourable, as would be the power of spreading into new territories.
A certain amount of isolation, recurring at long intervals of time, would
probably be also favourable, as before explained. One quarter of the world may
have been most favourable for the production of new and dominant species on the
land, and another for those in the waters of the sea. If two great regions had
been for a long period favourably circumstanced in an equal degree, whenever
their inhabitants met, the battle would be prolonged and severe; and some from
one birthplace and some from the other might be victorious. But in the course
of time, the forms dominant in the highest degree, wherever produced, would
tend everywhere to prevail. As they prevailed, they would cause the extinction
of other and inferior forms; and as these inferior forms would be allied in
groups by inheritance, whole groups would tend slowly to disappear; though here
and there a single member might long be enabled to survive.
Thus, as it seems to
me, the parallel, and, taken in a large sense, simultaneous, succession of the
same forms of life throughout the world, accords well with the principle of new
species having been formed by dominant species spreading widely and varying;
the new species thus produced being themselves dominant owing to inheritance,
and to having already had some advantage over their parents or over other
species; these again spreading, varying, and producing new species. The forms
which are beaten and which yield their places to the new and victorious forms,
will generally be allied in groups, from inheriting some inferiority in common;
and therefore as new and improved groups spread throughout the world, old
groups will disappear from the world; and the succession of forms in both ways
will everywhere tend to correspond.
There is one other
remark connected with this subject worth making. I have given my reasons for
believing that all our greater fossiliferous formations were deposited during
periods of subsidence; and that blank intervals of vast duration occurred
during the periods when the bed of the sea was either stationary or rising, and
likewise when sediment was not thrown down quickly enough to embed and preserve
organic remains. During these long and blank intervals ( suppose that the
inhabitants of each region underwent a considerable amount of modification and
extinction, and that there was much migration from other parts of the world. As
we have reason to believe that large areas are affected by the same movement,
it is probable that strictly contemporaneous formations have often been
accumulated over very wide spaces in the same quarter of the world; but we are
far from having any right to conclude that this has invariably been the case,
and that large areas have invariably been affected by the same movements. When
two formations have been deposited in two regions during nearly, but not
exactly the same period, we should find in both, from the causes explained in
the foregoing paragraphs, the same general succession in the forms of life; but
the species would not exactly correspond; for there will have been a little
more time in the one region than in the other for modification, extinction, and
immigration.
I suspect that cases of
this nature have occurred in Europe. Mr Prestwich, in his admirable Memoirs on
the eocene deposits of England and France, is able to draw a close general
parallelism between the successive stages in the two countries; but when lie
compares certain stages in England with those in France, although lie finds in
both a curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult to account
for, considering the proximity of the two areas, -- unless, indeed, it be
assumed that an isthmus separated two seas inhabited by distinct, but
contemporaneous, faunas. Lyell has made similar observations on some of the
later tertiary formations. Barrande, also, shows that there is a striking
general parallelism in the successive Silurian deposits of Bohemia and
Scandinavia; nevertheless he finds a surprising amount of difference in the
species. If the several formations in these regions have not been deposited
during the same exact periods, -- a formation in one region often corresponding
with a blank interval in the other, -- and if in both regions the species have
gone on slowly changing during the accumulation of the several formations and
during the long intervals of time between them; in this case, the several
formations in the two regions could be arranged in the same order, in
accordance with the general succession of the form of life, and the order would
falsely appear to be strictly parallel; nevertheless the species would not all
be the same in the apparently corresponding stages in the two regions.
On the Affinities of extinct
Species to each other, and to living forms . Let us now look to the mutual
affinities of extinct and living species. They all fall into one grand natural
system; and this fact is at once explained on the principle of descent. The
more ancient any form is, the more, as a general rule, it differs from living
forms. But, as Buckland long ago remarked, all fossils can be classed either in
still existing groups, or between them. That the extinct forms of life help to
fill up the wide intervals between existing genera, families, and orders,
cannot be disputed. For if we confine our attention either to the living or to
the extinct alone, the series is far less perfect than if we combine both into
one general system. With respect to the Vertebrata, whole pages could be filled
with striking illustrations from our great palaeontologist, Owen, showing how
extinct animals fall in between existing groups. Cuvier ranked the Ruminants
and Pachyderms, as the two most distinct orders of mammals; but Owen has
discovered so many fossil links, that he has had to alter the whole
classification of these two orders; and has placed certain pachyderms in the
same sub-order with ruminants: for example, he dissolves by fine gradations the
apparently wide difference between the pig and the camel. In regard to the
Invertebrata, Barrande, and a higher authority could not be named, asserts that
he is every day taught that palaeozoic animals, though belonging to the same
orders, families, or genera with those living at the present day, were not at
this early epoch limited in such distinct groups as they now are.
Some writers have
objected to any extinct species or group of species being considered as
intermediate between living species or groups. If by this term it is meant that
an extinct form is directly intermediate in all its characters between two
living forms, the objection is probably valid. But I apprehend that in a
perfectly natural classification many fossil species would have to stand
between living species, and some extinct genera between living genera, even
between genera belonging to distinct families. The most common case, especially
with respect to very distinct groups, such as fish and reptiles, seems to be,
that supposing them to be distinguished at the present day from each other by a
dozen characters, the ancient members of the same two groups would be
distinguished by a somewhat lesser number of characters, so that the two
groups, though formerly quite distinct, at that period made some small approach
to each other.
It is a common belief
that the more ancient a form is, by so much the more it tends to connect by
some of its characters groups now widely separated from each other. This remark
no doubt must be restricted to those groups which have undergone much change in
the course of geological ages; and it would be difficult to prove the truth of
the proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very distinct
groups. Yet if we compare the older Reptiles and Batrachians, the older Fish,
the older Cephalopods, and the eocene Mammals, with the more recent members of
the same classes, we must admit that there is some truth in the remark.
Let us see how far
these several facts and inferences accord with the theory of descent with
modification. As the subject is somewhat complex, I must request the reader to
turn to the diagram in the fourth chapter. We may suppose that the numbered
letters represent genera, and the dotted lines diverging from them the species
in each genus. The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal lines may
represent successive geological formations, and all the forms beneath the
uppermost line may be considered as extinct. The three existing genera,
a[s14]s, q[s14]s, p[s14]s, will form a small family; b[s14]s and f[s14]s a
closely allied family or sub-family; and o[s14]s, e[s14]s, m[s14]s, a third
family. These three families, together with the many extinct genera on the
several lines of descent diverging from the parent-form A, will form an order;
for all will have inherited something in common from their ancient and common
progenitor. On the principle of the continued tendency to divergence of
character, which was formerly illustrated by this diagram, the more recent any
form is, the more it will generally differ from its ancient progenitor. Hence
we can understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of character is a
necessary contingency; it depends solely on the descendants from a species
being thus enabled to seize on many and different places in the economy of
nature. Therefore it is quite possible, as we have seen in the case of some
Silurian forms, that a species might go on being slightly modified in relation
to its slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the diagram by
the letter F[s14]s.
All the many forms,
extinct and recent, descended from A, make, as before remarked, one order; and
this order, from the continued effects of extinction and divergence of
character, has become divided into several sub-families and families, some of
which are supposed to have perished at different periods, and some to have
endured to the present day.
By looking at the
diagram we can see that if many of the extinct forms, supposed to be embedded
in the successive formations, were discovered at several points low down in the
series, the three existing families on the uppermost line would be rendered
less distinct from each other. If, for instance, the genera a1, a[s5]s,
a[s10]s, m[s3]s, m[s6]s, m[s9]s were disinterred, these three families would be
so closely linked together that they probably would have to be united into one
great family, in nearly the same manner as has occurred with ruminants and
pachyderms. Yet he who objected to call the extinct genera, which thus linked
the living genera of three families together, intermediate in character, would
be justified, as they are intermediate, not directly, but only by a long and
circuitous course through many widely different forms. If many extinct forms
were to be discovered above one of the middle horizontal lines or geological
formations -- for instance, above No. VI. -- but none from beneath this line,
then only the two families on the left hand (namely, a[s14]s, &c., and
b[s14]s, gc.) would have to be united into one family; and the two other
families (namely, a[s14]s to f[s14]s now including five genera, and o[s14]s to
m[s14]s) would yet remain distinct. These two families, however, would be less
distinct from each other than they were before the discovery of the fossils.
If, for instance, we suppose the existing genera of the two families to differ
from each other by a dozen characters, in this case the genera, at the early
period marked VI., would differ by a lesser number of characters; for at this
early stage of descent they have not diverged in character from the common
progenitor of the order, nearly so much as they subsequently diverged. Thus it
comes that ancient and extinct genera are often in some slight degree
intermediate in character between their modified descendants, or between their
collateral relations.
In nature the case will
be far more complicated than is represented in the diagram; for the groups will
have been more numerous, they will have endured for extremely unequal lengths
of time, and will have been modified in various degrees. As we possess only the
last volume of the geological record, and that in a very broken condition, we
have no right to expect, except in very rare cases, to fill up wide intervals
in the natural system, and thus unite distinct families or orders. All that we
have a right to expect, is that those groups, which have within known
geological periods undergone much modification, should in the older formations
make some slight approach to each other; so that the older members should
differ less from each other in some of their characters than do the existing
members of the same groups; and this by the concurrent evidence of our best
palaeontologists seems frequently to be the case.
Thus, on the theory of
descent with modification, the main facts with respect to the mutual affinities
of the extinct forms of life to each other and to living forms, seem to me
explained in a satisfactory manner. And they are wholly inexplicable on any
other view.
On this same theory, it
is evident that the fauna of any great period in the earth's history will be
intermediate in general character between that which preceded and that which
succeeded it. Thus, the species which lived at the sixth great stage of descent
in the diagram are the modified offspring of those which lived at the fifth
stage, and are the parents of those which became still more modified at the
seventh stage; hence they could hardly fail to be nearly intermediate in
character between the forms of life above and below. We must, however, allow
for the entire extinction of some preceding forms, and for the coming in of
quite new forms by immigration, and for a large amount of modification, during
the long and blank intervals between the successive formations. Subject to
these allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas. I need
give only one instance, namely, the manner in which the fossils of the Devonian
system, when this system was first discovered, were at once recognised by
palaeontologists as intermediate in character between those of the overlying
carboniferous, and underlying Silurian system. But each fauna is not
necessarily exactly intermediate, as unequal intervals of time have elapsed
between consecutive formations.
It is no real objection
to the truth of the statement, that the fauna of each period as a whole is
nearly intermediate in character between the preceding and succeeding faunas,
that certain genera offer exceptions to the rule. For instance, mastodons and
elephants, when arranged by Dr Falconer in two series, first according to their
mutual affinities and then according to their periods of existence, do not
accord in arrangement. The species extreme in character are not the oldest, or the
most recent; nor are those which are intermediate in character, intermediate in
age. But supposing for an instant, in this and other such cases, that the
record of the first appearance and disappearance of the species was perfect, we
have no reason to believe that forms successively produced necessarily endure
for corresponding lengths of time: a very ancient form might occasionally last
much longer than a form elsewhere subsequently produced, especially in the case
of terrestrial productions inhabiting separated districts. To compare small
things with great: if the principal living and extinct races of the domestic
pigeon were arranged as well as they could be in serial affinity, this
arrangement would not closely accord with the order in time of their production,
and still less with the order of their disappearance; for the parent
rock-pigeon now lives; and many varieties between the rock-pigeon and the
carrier have become extinct; and carriers which are extreme in the important
character of length of beak originated earlier than short-beaked tumblers,
which are at the opposite end of the series in this same respect.
Closely connected with
the statement, that the organic remains from an intermediate formation are in
some degree intermediate in character, is the fact, insisted on by all
palaeontologists, that fossils from two consecutive formations are far more
closely related to each other, than are the fossils from two remote formations.
Pictet gives as a well-known instance, the general resemblance of the organic
remains from the several stages of the chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to have
shaken Professor Pictet in his firm belief in the immutability of species. He
who is acquainted with the distribution of existing species over the globe,
will not attempt to account for the close resemblance of the distinct species
in closely consecutive formations, by the physical conditions of the ancient
areas having remained nearly the same. Let it be remembered that the forms of
life, at least those inhabiting the sea, have changed almost simultaneously
throughout the world, and therefore under the most different climates and
conditions. Consider the prodigious vicissitudes of climate during the
pleistocene period, which includes the whole glacial period, and note how
little the specific forms of the inhabitants of the sea have been affected.
On the theory of
descent, the full meaning of the fact of fossil remains from closely
consecutive formations, though ranked as distinct species, being closely
related, is obvious. As the accumulation of each formation has often been
interrupted, and as long blank intervals have intervened between successive
formations, we ought not to expect to find, as I attempted to show in the last
chapter, in any one or two formations all the intermediate varieties between
the species which appeared at the commencement and close of these periods; but
we ought to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as they
have been called by some authors, representative species; and these we
assuredly do find. We find, in short, such evidence of the slow and scarcely
sensible mutation of specific forms, as we have a just right to expect to find.
On the state of
Development of Ancient Forms . There has been much discussion whether recent
forms are more highly developed than ancient. I will not here enter on this
subject, for naturalists have not as yet defined to each other's satisfaction
what is meant by high and low forms. But in one particular sense the more
recent forms must, on my theory, be higher than the more ancient; for each new
species is formed by having had some advantage in the struggle for life over
other and preceding forms. If under a nearly similar climate, the eocene
inhabitants of one quarter of the world were put into competition with the
existing inhabitants of the same or some other quarter, the eocene fauna or
flora would certainly be beaten and exterminated; as would a secondary fauna by
an eocene, and a palaeozoic fauna by a secondary fauna. I do not doubt that
this process of improvement has affected in a marked and sensible manner the
organisation of the more recent and victorious forms of life, in comparison
with the ancient and beaten forms; but I can see no way of testing this sort of
progress. Crustaceans, for instance, not the highest in their own class, may
have beaten the highest molluscs. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have seized on
places which must have been previously occupied, we may believe, if all the
animals and plants of Great Britain were set free in New Zealand, that in the
course of time a multitude of British forms would become thoroughly naturalized
there, and would exterminate many of the natives. On the other hand, from what
we see now occurring in New Zealand, and from hardly a single inhabitant of the
southern hemisphere having become wild in any part of Europe, we may doubt, if
all the productions of New Zealand were set free in Great Britain, whether any
considerable number would be enabled to seize on places now occupied by our
native plants and animals. Under this point of view, the productions of Great
Britain, may be said to be higher than those of New Zealand. Yet the most
skilful naturalist from an examination of the species of the two countries
could not have foreseen this result.
Agassiz insists that
ancient animals resemble to a certain extent the embryos of recent animals of
the same classes; or that the geological succession of extinct forms is in some
degree parallel to the embryological development of recent forms. I must follow
Pictet and Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it here after confirmed, at least in regard
to subordinate groups, which have branched off from each other within
comparatively recent times. For this doctrine of Agassiz accords well with the
theory of natural selection. In a future chapter I shall attempt to show that
the adult differs from its embryo, owing to variations supervening at a not
early age, and being inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered. continually adds, in the course of
successive generations, more and more difference to the adult.
Thus the embryo comes
to be left as a sort of picture, preserved by nature, of the ancient and less
modified condition of each animal. This view may be true, and yet it may never
be capable of full proof. Seeing, for instance, that the oldest known mammals,
reptiles, and fish strictly belong to their own proper classes, though some of
these old forms are in a slight degree less distinct from each other than are
the typical members of the same groups at the present day, it would be vain to
look for animals having the common embryological character of the Vertebrata,
until beds far beneath the lowest Silurian strata are discovered -- a discovery
of which the chance is very small.
On the succession of
the same Types within the same areas, during the later tertiary periods . Mr
Clift many years ago showed that the fossil mammals from the Australian caves
were closely allied to the living marsupials of that continent. In South
America, a similar relationship is manifest, even to an uneducated eye, in the
gigantic pieces of armour like those of the armadillo, found in several parts
of La Plata; and Professor Owen has shown in the most striking manner that most
of the fossil mammals, buried there in such numbers, are related to South
American types. This relationship is even more clearly seen in the wonderful
collection of fossil bones made by MM. Lund and Clausen in the caves of Brazil.
I was so much impressed with these facts that I strongly insisted, in 1839 and
1845, on this `law of the succession of types,' -- on `this wonderful
relationship in the same continent between the dead and the living.' Professor
Owen has subsequently extended the same generalisation to the mammals of the
Old World. We see the same law in this author's restorations of the extinct and
gigantic birds of New Zealand. We see it also in the birds of the caves of
Brazil. Mr Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well displayed
by them. Other cases could be added, as the relation between the extinct and
living land-shells of Madeira; and between the extinct and living brackish-water
shells of the Aralo-Caspian Sea.
Now what does this
remarkable law of the succession of the same types within the same areas mean?
He would be a bold man, who after comparing the present climate of Australia
and of parts of South America under the same latitude, would attempt to
account, on the one hand, by dissimilar physical conditions for the
dissimilarity of the inhabitants of these two continents, and, on the other
hand, by similarity of conditions, for the uniformity of the same types in each
during the later tertiary periods. Nor can it be pretended that it is an
immutable law that marsupials should have been chiefly or solely produced in
Australia; or that Edentata and other American types should have been solely
produced in South America. For we know that Europe in ancient times was peopled
by numerous marsupials; and I have shown in the publications above alluded to,
that in America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly of the
present character of the southern half of the continent; and the southern half
was formerly more closely allied, than it is at present, to the northern half.
In a similar manner we know from Falconer and Cautley's discoveries, that
northern India was formerly more closely related in its mammals to Africa than
it is at the present time. Analogous facts could be given in relation to the
distribution of marine animals.
On the theory of
descent with modification, the great law of the long enduring, but not
immutable, succession of the same types within the same areas, is at once
explained; for the inhabitants of each quarter of the world will obviously tend
to leave in that quarter, during the next succeeding period of time, closely
allied though in some degree modified descendants. If the inhabitants of one
continent formerly differed greatly from those of another continent, so will
their modified descendants still differ in nearly the same manner and degree.
But after very long intervals of time and after great geographical changes,
permitting much inter-migration, the feebler will yield to the more dominant
forms, and there will be nothing immutable in the laws of past and present
distribution.
It may be asked in
ridicule, whether I suppose that the megatherium and other allied huge monsters
have left behind them ln South America the sloth, armadillo, and anteater, as
their degenerate descendants. This cannot for an instant be admitted. These
huge animals have become wholly extinct, and have left no progeny. But in the
caves of Brazil, there are many extinct species which are closely allied in
size and in other characters to the species still living in South America; and
some of these fossils may be the actual progenitors of living species. It must
not be forgotten that, on my theory, all the species of the same genus have
descended from some one species; so that if six genera, each having eight
species, be found in one geological formation, and in the next succeeding
formation there be six other allied or representative genera with the same
number of species, then we may conclude that only one species of each of the
six older genera has left modified descendants, constituting the six new
genera. The other seven species of the old genera have all died out and have
left no progeny. Or, which would probably be a far commoner case, two or three
species of two or three alone of the six older genera will have been the
parents of the six new genera; the other old species and the other whole genera
having become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of South
America, still fewer genera and species will have left modified
blood-descendants.
Summary of the
preceding and present Chapters . I have attempted to show that the geological
record is extremely imperfect; that only a small portion of the globe has been
geologically explored with care; that only certain classes of organic beings
have been largely preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums, is absolutely as nothing
compared with the incalculable number of generations which must have passed
away even during a single formation; that, owing to subsidence being necessary
for the accumulation of fossiliferous deposits thick enough to resist future
degradation, enormous intervals of time have elapsed between the successive
formations; that there has probably been more extinction during the periods of
subsidence, and more variation during the periods of elevation, and during the
latter the record will have been least perfectly kept; that each single
formation has not been continuously deposited; that the duration of each
formation is, perhaps, short compared with the average duration of specific
forms; that migration has played an important part in the first appearance of
new forms in any one area and formation; that widely ranging species are those
which have varied most, and have oftenest given rise to new species; and that
varieties have at first often been local. All these causes taken conjointly,
must have tended to make the geological record extremely imperfect, and will to
a large extent explain why we do not find interminable varieties, connecting
together all the extinct and existing forms of life by the finest graduated
steps.
He who rejects these
views on the nature of the geological record, will rightly reject my whole
theory. For he may ask in vain where are the numberless transitional links
which must formerly have connected the closely allied or representative
species, found in the several stages of the same great formation. He may
disbelieve in the enormous intervals of time which have elapsed between our
consecutive formations; he may overlook how important a part migration must
have played, when the formations of any one great region alone, as that of
Europe, are considered; he may urge the apparent, but often falsely apparent,
sudden coming in of whole groups of species. He may ask where are the remains
of those infinitely numerous organisms which must have existed long before the
first bed of the Silurian system was deposited: I can answer this latter
question only hypothetically, by saying that as far as we can see, where our
oceans now extend they have for an enormous period extended, and where our
oscillating continents now stand they have stood ever since the Silurian epoch;
but that long before that period, the world may have presented a wholly
different aspect; and that the older continents, formed of formations older
than any known to us, may now all be in a metamorphosed condition, or may lie
buried under the ocean.
Passing from these
difficulties, all the other great leading facts in palaeontology seem to me
simply to follow on the theory of descent with modification through natural
selection. We can thus understand how it is that new species come in slowly and
successively; how species of different classes do not necessarily change
together, or at the same rate, or in the same degree; yet in the long run that
all undergo modification to some extent. The extinction of old forms is the
almost inevitable consequence of the production of new forms. We can understand
why when a species has once disappeared it never reappears. Groups of species
increase in numbers slowly, and endure for unequal periods of time; for the
process of modification is necessarily slow, and depends on many complex
contingencies. The dominant species of the larger dominant groups tend to leave
many modified descendants, and thus new sub-groups and groups are formed. As
these are formed, the species of the less vigorous groups, from their
inferiority inherited from a common progenitor, tend to become extinct
together, and to leave no modified offspring on the face of the earth. But the
utter extinction of a whole group of species may often be a very slow process,
from the survival of a few descendants, lingering in protected and isolated
situations. When a group has once wholly disappeared, it does not reappear; for
the link of generation has been broken.
We can understand how
the spreading of the dominant forms of life, which are those that oftenest
vary, will in the long run tend to people the world with allied, but modified,
descendants; and these will generally succeed in taking the places of those
groups of species which are their inferiors in the struggle for existence.
Hence, after long intervals of time, the productions of the world will appear
to have changed simultaneously.
We can understand how
it is that ali the forms of life, ancient and recent, make together one grand
system; for all are connected by generation. We can understand, from the
continued tendency to divergence of character, why the more ancient a form is,
the more it generally differs from those now living. Why ancient and extinct
forms often tend to fill up gaps between existing forms, sometimes blending two
groups previously classed as distinct into one; but more commonly only bringing
them a little closer together. The more ancient a form is, the more often,
apparently, it displays characters in some degree intermediate between groups
now distinct; for the more ancient a form is, the more nearly it will be
related to, and consequently resemble, the common progenitor of groups, since
become widely divergent. Extinct forms are seldom directly intermediate between
existing forms; but are intermediate only by a long and circuitous course
through many extinct and very different forms. We can clearly see why the
organic remains of closely consecutive formations are more closely allied to
each other, than are those of remote formations; for the forms are more closely
linked together by generation: we can clearly see why the remains of an
intermediate formation are intermediate in character.
The inhabitants of each
successive period in the world's history have beaten their predecessors in the
race for life, and are, in so far, higher in the scale of nature; and this may
account for that vague yet ill-defined sentiment, felt by many
palaeontologists, that organisation on the whole has progressed. If it should
hereafter be proved that ancient animals resemble to a certain extent the
embryos of more recent animals of the same class, the fact will be
intelligible. The succession of the same types of structure within the same
areas during the later geological periods ceases to be mysterious, and is
simply explained by inheritance.
If then the geological
record be as imperfect as I believe it to be, and it may at least be asserted
that the record cannot be proved to be much more perfect, the main objections
to the theory of natural selection are greatly diminished or disappear. On the
other hand, all the chief laws of palaeontology plainly proclaim, as it seems
to me, that species have been produced by ordinary generation: old forms having
been supplanted by new and improved forms of life, produced by the laws of
variation still acting round us, and preserved by Natural Selection.
Present distribution cannot be accounted for by differences in physical
conditions - Importance of barriers - Affinity of the productions of the same
continent - Centres of creation - Means of dispersal, by changes of climate and
of the level of the land, and by occasional means - Dispersal during the
Glacial period co-extensive with the world In
considering the distribution of organic beings over the face of the globe, the
first great fact which strikes us is, that neither the similarity nor the
dissimilarity of the inhabitants of various regions can be accounted for by
their climatal and other physical conditions. Of late, almost every author who
has studied the subject has come to this conclusion. The case of America alone
would almost suffice to prove its truth: for if we exclude the northern parts
where the circumpolar land is almost continuous, all authors agree that one of
the most fundamental divisions in geographical distribution is that between the
New and Old Worlds; yet if we travel over the vast American continent, from the
central parts of the United States to its extreme southern point, we meet with
the most diversified conditions; the most humid districts, arid deserts, lofty
mountains, grassy plains, forests, marshes, lakes, and great rivers, under
almost every temperature. There is hardly a climate or condition in the Old
World which cannot be paralleled in the New -- at least as closely as the same
species generally require; for it is a most rare case to find ?!?a group of
organisms confined to any small spot, having conditions peculiar in only a
slight degree; for instance, small areas in the Old World could be pointed out
hotter than any in the New World, yet these are not inhabited by a peculiar
fauna or flora. Notwithstanding this parallelism in the conditions of the Old
and New Worlds, how widely different are their living productions!
In the southern
hemisphere, if we compare large tracts of land in Australia, South Africa, and
western South America, between latitudes 25 and 3?!?5, we shall find parts
extremely similar in all their conditions, yet it would not be possible to
point out three faunas and floras more utterly dissimilar. Or again we may
compare the productions of South America south of lat. 35 with those north of
25, which consequently inhabit a considerably different climate, and they will
be found incomparably more closely related to each other, than they are to the
productions of Australia or Africa under nearly the same climate. Analogous
facts could be given with respect to the inhabitants of the sea.
A second great fact
which strikes us in our general review is, that barriers of any kind, or
obstacles to free migration, are related in a close and important manner to the
differences between the productions of various regions. We see this in the
great difference of nearly all the terrestrial productions of the New and Old
Worlds, excepting in the northern parts, where the land almost joins, and
where, under a slightly different climate, there might have been free migration
for the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same latitude:
for these countries are almost as much isolated from each other as is possible.
On each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts, and sometimes even
of large rivers, we find different productions; though as mountain chains,
deserts, &c., are not as impassable, or likely to have endured so long as
the oceans separating continents, the differences are very inferior in degree
to those characteristic of distinct continents.
Turning to the sea, we
find the same law. No two marine faunas are more distinct, with hardly a fish,
shell, or crab in common, than those of the eastern and western shores of South
and Central America; yet these great faunas are separated only by the narrow,
but impassable, isthmus of panama. Westward of the shores of America, a wide
space of open ocean extends, with not an island as a halting-place for
emigrants; here we have a barrier of another kind, and as soon as this is
passed we meet in the eastern islands of the pacific, with another and totally
distinct fauna. So that here three marine faunas range far northward and
southward, in parallel lines not far from each other, under corresponding
climates; but from being separated from each other by impassable barriers,
either of land or open sea, they are wholly distinct. On the other hand,
proceeding still further westward from the eastern islands of the tropical
parts of the pacific, we encounter no impassable barriers, and we have
innumerable islands as halting-places, until after travelling over a hemisphere
we come to the shores of Africa; and over this vast space we meet with no
well-defined and distinct marine faunas. Although hardly one shell, crab or
fish is common to the above-named three approximate faunas of Eastern and
Western America and the eastern pacific islands, yet many fish range from the
Pacific into the Indian Ocean, and many shells are common to the eastern
islands of the pacific and the eastern shores of Africa, on almost exactly
opposite meridians of longitude.
A third great fact,
partly included in the foregoing statements, is the affinity of the productions
of the same continent or sea, though the species themselves are distinct at
different points and stations. It is a law of the widest generality, and every
continent offers innumerable instances. Nevertheless the naturalist in
travelling, for instance, from north to south never fails to be struck by the
manner in which successive groups of beings, specifically distinct, yet clearly
related, replace each other. He hears from closely allied, yet distinct kinds
of birds, notes nearly similar, and sees their nests similarly constructed, but
not quite alike, with eggs coloured in nearly the same manner. The plains near
the Straits of Magellan are inhabited by one species of Rhea (American
ostrich), and northward the plains of La Plata by another species of the same
genus; and not by a true ostrich or emeu, like those found in Africa and
Australia under the same latitude. On these same plains of La Plata, we see the
agouti and bizcacha, animals having nearly the same habits as our hares and
rabbits and belonging to the same order of Rodents, but they plainly display an
American type of structure. We ascend the lofty peaks of the Cordillera and we
find an alpine species of bizcacha; we look to the waters, and we do not find
the beaver or musk-rat, but the coypu and capybara, rodents of the American
type. Innumerable other instances could be given. If we look to the islands off
the American shore, however much they may differ in geological structure, the
inhabitants, though they may be all peculiar species, are essentially American.
We may look back to past ages, as shown in the last chapter, and we find
American types then prevalent on the American continent and in the American
seas. We see in these facts some deep organic bond, prevailing throughout space
and time, over the same areas of land and water, and independent of their
physical conditions. The naturalist must feel little curiosity, who is not led
to inquire what this bond is.
This bond, on my
theory, is simply inheritance, that cause which alone, as far as we positively
know, produces organisms quite like, or, as we see in the case of varieties
nearly like each other. The dissimilarity of the inhabitants of different
regions may be attributed to modification through natural selection, and in a
quite subordinate degree to the direct influence of different physical
conditions. The degree of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been effected with
more or less ease, at periods more or less remote; -- on the nature and number
of the former immigrants; -- and on their action and reaction, in their mutual
struggles for life; -- the relation of organism to organism being, as I have
already often remarked, the most important of all relations. Thus the high
importance of barriers comes into play by checking migration; as does time for
the slow process of modification through natural selection. Widely-ranging
species, abounding in individuals, which have already triumphed over many
competitors in their own widely-extended homes will have the best chance of
seizing on new places, when they spread into new countries. In their new homes
they will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this principle
of inheritance with modification, we can understand how it is that sections of
genera, whole genera, and even families are confined to the same areas, as is
so commonly and notoriously the case.
I believe, as was
remarked in the last chapter, in no law of necessary development. As the
variability of each species is an independent property, and will be taken
advantage of by natural selection, only so far as it profits the individual in
its complex struggle for life, so the degree of modification in different
species will be no uniform quantity. If, for instance, a number of species,
which stand in direct competition with each other, migrate in a body into a new
and afterwards isolated country, they will be little liable to modification;
for neither migration nor isolation in themselves can do anything. These
principles come into play only by bringing organisms into new relations with
each other, and in a lesser degree with the surrounding physical conditions. As
we have seen in the last chapter that some forms have retained nearly the same
character from an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly modified.
On these views, it is
obvious, that the several species of the same genus, though inhabiting the most
distant quarters of the world, must originally have proceeded from the same
source, as they have descended from the same progenitor. In the case of those
species, which have undergone during whole geological periods but little
modification, there is not much difficulty in believing that they may have
migrated from the same region; for during the vast geographical and climatal
changes which will have supervened since ancient times, almost any amount of
migration is possible. But in many other cases, in which we have reason to
believe that the species of a genus have been produced within comparatively
recent times, there is great difficulty on this head. It is also obvious that
the individuals of the same species, though now inhabiting distant and isolated
regions, must have proceeded from one spot, where their parents were first
produced: for, as explained in the last chapter, it is incredible that
individuals identically the same should ever have been produced through natural
selection from parents specifically distinct.
We are thus brought to
the question which has been largely discussed by naturalists, namely, whether
species have been created at one or more points of the earth's surface.
Undoubtedly there are very many cases of extreme difficulty, in understanding
how the same species could possibly have migrated from some one point to the
several distant and isolated points, where now found. Nevertheless the
simplicity of the view that each species was first produced within a single
region captivates the mind. He who rejects it, rejects the vera causa of
ordinary generation with subsequent migration, and calls in the agency of a
miracle. It is universally admitted, that in most cases the area inhabited by a
species is continuous; and when a plant or animal inhabits two points so
distant from each other, or with an interval of such a nature, that the space
could not be easily passed over by migration, the fact is given as something
remarkable and exceptional. The capacity of migrating across the sea is more distinctly
limited in terrestrial mammals, than perhaps in any other organic beings; and,
accordingly, we find no inexplicable cases of the same mammal inhabiting
distant points of the world. No geologist will feel any difficulty in such
cases as Great Britain having been formerly united to Europe, and consequently
possessing the same quadrupeds. But if the same species can be produced at two
separate points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the same, so that
a multitude of European animals and plants have become naturalised in America
and Australia; and some of the aboriginal plants are identically the same at
these distant points of the northern and southern hemispheres? The answer, as I
believe, is, that mammals have not been able to migrate, whereas some plants,
from their varied means of dispersal, have migrated across the vast and broken
interspace. The great and striking influence which barriers of every kind have
had on distribution, is intelligible only on the view that the great majority
of species have been produced on one side alone, and have not been able to
migrate to the other side. Some few families, many sub- families, very many
genera, and a still greater number of sections of genera are confined to a
single region; and it has been observed by several naturalists, that the most
natural genera, or those genera in which the species are most closely related
to each other, are generally local, or confined to one area. What a strange
anomaly it would be, if, when coming one step lower in the series, to the
individuals of the same species, a directly opposite rule prevailed; and
species were not local, but had been produced in two or more distinct areas!
Hence it seems to me,
as it has to many other naturalists, that the view of each species having been
produced in one area alone, and having subsequently migrated from that area as
far as its powers of migration and subsistence under past and present
conditions permitted, is the most probable. Undoubtedly many cases occur, in
which we cannot explain how the same species could have passed from one point
to the other. But the geographical and climatal changes, which have certainly
occurred within recent geological times, must have interrupted or rendered
discontinuous the formerly continuous range of many species. So that we are
reduced to consider whether the exceptions to continuity of range are so
numerous and of so grave a nature, that we ought to give up the belief,
rendered probable by general considerations, that each species has been
produced within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same species,
now living at distant and separated points; nor do I for a moment pretend that
any explanation could be offered of many such cases. But after some preliminary
remarks, I will discuss a few of the most striking classes of facts; namely,
the existence of the same species on the summits of distant mountain-ranges,
and at distant points in the arctic and antarctic regions; and secondly (in the
following chapter), the wide distribution of freshwater productions; and
thirdly, the occurrence of the same terrestrial species on islands and on the
mainland, though separated by hundreds of miles of open sea. If the existence
of the same species at distant and isolated points of the earth's surface, can
in many instances be explained on the view of each species having migrated from
a single birthplace; then, considering our ignorance with respect to former
climatal and geographical changes and various occasional means of transport,
the belief that this has been the universal law, seems to me incomparably the
safest.
In discussing this
subject, we shall be enabled at the same time to consider a point equally
important for us, namely, whether the several distinct species of a genus,
which on my theory have all descended from a common progenitor, can have
migrated (undergoing modification during some part of their migration) from the
area inhabited by their progenitor. If it can be shown to be almost invariably
the case, that a region, of which most of its inhabitants are closely related
to, or belong to the same genera with the species of a second region, has
probably received at some former period immigrants from this other region, my
theory will be strengthened; for we can clearly understand, on the principle of
modification, why the inhabitants of a region should be related to those of
another region, whence it has been stocked. A volcanic island, for instance,
upheaved and formed at the distance of a few hundreds of miles from a
continent, would probably receive from it in the course of time a few
colonists, and their descendants, though modified, would still be plainly
related by inheritance to the inhabitants of the continent. Cases of this
nature are common, and are, as we shall hereafter more fully see, inexplicable
on the theory of independent creation. This view of the relation of species in
one region to those in another, does not differ much (by substituting the word
variety for species) from that lately advanced in an ingenious paper by Mr
Wallace, in which he concludes, that `every species has come into existence
coincident both in space and time with a pre-existing closely allied species.'
And I now know from correspondence, that this coincidence he attributes to
generation with modification.
The previous remarks on
`single and multiple centres of creation' do not directly bear on another
allied question, -- namely whether all the individuals of the same species have
descended from a single pair, or single hermaphrodite, or whether, as some
authors suppose, from many individuals simultaneously created. With those
organic beings which never intercross (if such exist), the species, on my
theory, must have descended from a succession of improved varieties, which will
never have blended with other individuals or varieties, but will have
supplanted each other; so that, at each successive stage of modification and
improvement, all the individuals of each variety will have descended from a
single parent. But in the majority of cases, namely, with all organisms which
habitually unite for each birth, or which often intercross, I believe that during
the slow process of modification the individuals of the species will have been
kept nearly uniform by intercrossing; so that many individuals will have gone
on simultaneously changing, and the whole amount of modification will not have
been due, at each stage, to descent from a single parent. To illustrate what I
mean: our English racehorses differ slightly from the horses of every other
breed; but they do not owe their difference and superiority to descent from any
single pair, but to continued care in selecting and training many individuals
during many generations.
Before discussing the
three classes of facts, which I have selected as presenting the greatest amount
of difficulty on the theory of `single centres of creation,' I must say a few
words on the means of dispersal.
Means of Dispersal. Sir
C. Lyell and other authors have ably treated this subject. I can give here only
the briefest abstract of the more important facts. Change of climate must have
had a powerful influence on migration: a region when its climate was different
may have been a high road for migration, but now be impassable; I shall,
however, presently have to discuss this branch of the subject in some detail.
Changes of level in the land must also have been highly influential: a narrow
isthmus now separates two marine faunas; submerge it, or let it formerly have
been submerged, and the two faunas will now blend or may formerly have blended:
where the sea now extends, land may at a former period have connected islands
or possibly even continents together, and thus have allowed terrestrial
productions to pass from one to the other. No geologist will dispute that great
mutations of level have occurred within the period of existing organisms.
Edward Forbes insisted that all the islands in the Atlantic must recently have
been connected with Europe or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and have united
almost every island to some mainland. If indeed the arguments used by Forbes
are to be trusted, it must be admitted that scarcely a single island exists
which has not recently been united to some continent. This view cuts the
Gordian knot of the dispersal of the same species to the most distant points,
and removes many a difficulty: but to the best of any judgement we are not
authorised in admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of great
oscillations of level in our continents; but not of such vast changes in their
position and extension, as to have united them within the recent period to each
other and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have served as
halting places for plants and for many animals during their migration. In the
coral-producing oceans such sunken islands are now marked, as I believe, by
rings of coral or atolls standing over them. Whenever it is fully admitted, as
I believe it will some day be, that each species has proceeded from a single
birthplace, and when in the course of time we know something definite about the
means of distribution, we shall be enabled to speculate with security on the
former extension of the land. But I do not believe that it will ever be proved
that within the recent period continents which are now quite separate, have
been continuously, or almost continuously, united with each other, and with the
many existing oceanic islands. Several facts in distribution, -- such as the
great difference in the marine faunas on the opposite sides of almost every
continent, -- the close relation of the tertiary inhabitants of several lands
and even seas to their present inhabitants, -- a certain degree of relation (as
we shall hereafter see) between the distribution of mammals and the depth of
the sea, -- these and other such facts seem to me opposed to the admission of
such prodigious geographical revolutions within the recent period, as are
necessitated in the view advanced by Forbes and admitted by his many followers.
The nature and relative proportions of the in?!?habitants of oceanic islands
likewise seem to me opposed to the belief of their former continuity with
continents. Nor does their almost universally volcanic composition favour the
admission that they are the wrecks of sunken continents; -- if they had
originally existed as mountain-ranges on the land, some at least of the islands
would have been formed, like other mountain-summits, of granite, metamorphic
schists, old fossiliferous or other such rocks, instead of consisting of mere
piles of volcanic matter.
I must now say a few
words on what are called accidental means, but which more properly might be
called occasional means of distribution. I shall here confine myself to plants.
In botanical works, this or that plant is stated to be ill adapted for wide
dissemination; but for transport across the sea, the greater or less facilities
may be said to be almost wholly unknown. Until I tried, with Mr Berkeley's aid,
a few experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of 87 kinds, 64
germinated after an immersion of 28 days, and a few survived an immersion of
137 days. For convenience sake I chiefly tried small seeds, without the capsule
or fruit; and as all of these sank in a few days, they could not be floated
across wide spaces of the sea, whether or not they were injured by the
salt-water. Afterwards I tried some larger fruits, capsules, &c., and some
of these floated for a long time. It is well known what a difference there is
in the buoyancy of green and seasoned timber; and it occurred to me that floods
might wash down plants or branches, and that these might be dried on the banks,
and then by a fresh rise in the stream be washed into the sea. Hence I was led
to dry stems and branches of 94 plants with ripe fruit, and to place them on
sea water. The majority sank quickly, but some which whilst green floated for a
very short time, when dried floated much longer; for instance, ripe hazel-nuts
sank immediately, but when dried, they floated for 90 days and afterwards when
planted they germinated; an asparagus plant with ripe berries floated for 23
days, when dried it floated for 85 days, and the seeds afterwards germinated:
the ripe seeds of Helosciadium sank in two days, when dried they floated for
above 90 days, and afterwards germinated. Altogether out of the 94 dried
plants, 18 floated for above 28 days, and some of the 18 floated for a very
much longer period. So that as 64/87 seeds germinated after an immersion of 28
days; and as 18/94 plants with ripe fruit (but not all the same species as in
the foregoing experiment) floated, after being dried, for above 28 days, as far
as we may infer anything from these scanty facts, we may conclude that the
seeds of 14/100 plants of any country might be floated by sea-currents during
28 days, and would retain their power of germination. In Johnston's physical
Atlas, the average rate of the several Atlantic currents is 33 miles per diem
(some currents running at the rate of 60 miles per diem); on this average, the
seeds of 14/100 plants belonging to one country might be floated across 924
miles of sea to another country; and when stranded, if blown to a favourable
spot by an inland gale, they would germinate.
Subsequently to my
experiments, M. Martens tried similar ones, but in a much better manner, for he
placed the seeds in a box in the actual sea, so that they were alternately wet
and exposed to the air like really floating plants. He tried 98 seeds, mostly
different from mine; but he chose many large fruits and likewise seeds from
plants which live near the sea; and this would have favoured the average length
of their flotation and of their resistance to the injurious action of the
salt-water. On the other hand he did not previously dry the plants or branches
with the fruit; and this, as we have seen, would have caused some of them to
have floated much longer. The result was that 18/98 of his seeds floated for 42
days, and were then capable of germination. But I do not doubt that plants
exposed to the waves would float for a less time than those protected from
violent movement as in our experiments. Therefore it would perhaps be safer to assume
that the seeds of about 10/100 plants of a flora, after having been dried,
could be floated across a space of sea 900 miles in width, and would then
germinate. The fact of the larger fruits often floating longer than the small,
is interesting; as plants with large seeds or fruit could hardly be transported
by any other means; and Alph. de Candolle has shown that such plants generally
have restricted ranges.
But seeds may be
occasionally transported in another manner. Drift timber is thrown up on most
islands, even on those in the midst of the widest oceans; and the natives of
the coral-islands in the Pacific, procure stones for their tools, solely from
the roots of drifted trees, these stones being a valuable royal tax. I find on
examination, that when irregularly shaped stones are embedded in the roots of
trees, small parcel s of earth are very frequently enclosed in their
interstices and behind them, -- so perfectly that not a particle could be
washed away in the longest transport: out of one small portion of earth thus
completely enclosed by wood in an oak about 50 years old, three dicotyledonous
plants germinated: I am certain of the accuracy of this observation. Again, I
can show that the carcasses of birds, when floating on the sea, sometimes
escape being immediately devoured; and seeds of many kinds in the crops of
floating birds long retain their vitality: peas and vetches, for instance, are
killed by even a few days' immersion in sea- water; but some taken out of the
crop of a pigeon, which had floated on artificial salt-water for 30 days, to my
surprise nearly all germinated.
Living birds can hardly
fail to be highly effective agents in the transportation of seeds. I could give
many facts showing how frequently birds of many kinds are blown by gales to
vast distances across the ocean. We may I think safely assume that under such
circumstances their rate of flight would often be 35 miles an hour; and some
authors have given a far higher estimate. I have never seen an instance of
nutritious seeds passing through the intestines of a bird; but hard seeds of
fruit will pass uninjured through even the digestive organs of a turkey. In the
course of two months, I picked up in my garden 12 kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them, which I
tried, germinated. But the following fact is more important: the crops of birds
do not secrete gastric juice, and do not in the least injure, as I know by
trial, the germination of seeds; now after a bird has found and devoured a
large supply of food, it is positively asserted that all the grains do not pass
into the gizzard for 12 or even 18 hours. A bird in this interval might easily
be blown to the distance of 500 miles, and hawks are known to look out for
tired birds, and the contents of their torn crops might thus readily get
scattered. Mr Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English coast
destroyed so many on their arrival. Some hawks and owls bolt their prey whole,
and after an interval of from twelve to twenty hours, disgorge pellets, which,
as I know from experiments made in the Zoological Gardens, include seeds
capable of germination. Some seeds of the oat, wheat, millet, canary, hemp,
clover, and beet germinated after having been from twelve to twenty-one hours
in the stomachs of different birds of prey; and two seeds of beet grew after
having been thus retained for two days and fourteen hours. Freshwater fish, I
find, eat seeds of many land and water plants: fish are frequently devoured by
birds, and thus the seeds might be transported from place to place. I forced
many kinds of seeds into the stomachs of dead fish, and then gave their bodies
to fishing-eagles, storks, and pelicans; these birds after an interval of many
hours, either rejected the seeds in pellets or passed them in their excrement;
and several of these seeds retained their power of germination. Certain seeds,
however, were always killed by this process.
Although the beaks and
feet of birds are generally quite clean, I can show that earth sometimes
adheres to them: in one instance I removed twenty-two grains of dry argillaceous
earth from one foot of a partridge, and in this earth there was a pebble quite
as large as the seed of a vetch. Thus seeds might occasionally be transported
to great distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions of
quails which annually cross the Mediterranean; and can we doubt that the earth
adhering to their feet would sometimes include a few minute seeds? But I shall
presently have to recur to this subject.
As icebergs are known
to be sometimes loaded with earth and stones, and have even carried brushwood,
bones, and the nest of a land-bird, I can hardly doubt that they must
occasionally have transported seeds from one part to another of the arctic and
antarctic regions, as suggested by Lyell; and during the Glacial period from
one part of the now temperate regions to another. In the Azores, from the large
number of the species of plants common to Europe, in comparison with the plants
of other oceanic islands nearer to the mainland, and (as remarked by Mr H. C.
Watson) from the somewhat northern character of the flora in comparison with
the latitude, I suspected that these islands had been partly stocked by
ice-borne seeds, during the Glacial epoch. At my request Sir C. Lyell wrote to
M. Hartung to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments of granite and other
rocks, which do not occur in the archipelago. Hence we may safely infer that icebergs
formerly landed their rocky burthens on the shores of these mid-ocean islands,
and it is at least possible that they may have brought thither the seeds of
northern plants.
Considering that the
several above means of transport, and that several other means, which without
doubt remain to be discovered, have been in action year after year, for
centuries and tens of thousands of years, it would I think be a marvellous fact
if many plants had not thus become widely transported. These means of transport
are sometimes called accidental, but this is not strictly correct: the currents
of the sea are not accidental, nor is the direction of prevalent gales of wind.
It should be observed that scarcely any means of transport would carry seeds
for very great distances; for seeds do not retain their vitality when exposed
for a great length of time to the action of seawater; nor could they be long
carried in the crops or intestines of birds. These means, however, would
suffice for occasional transport across tracts of sea some hundred miles in
breadth, or from island to island, or from a continent to a neighbouring
island, but not from one distant continent to another. The floras of distant
continents would not by such means become mingled in any great degree; but would
remain as distinct as we now see them to be. The currents, from their course,
would never bring seeds from North America to Britain, though they might and do
bring seeds from the West Indies to our western shores, where, if not killed by
so long an immersion in salt-water, they could not endure our climate. Almost
every year, one or two land-birds are blown across the whole Atlantic Ocean,
from North America to the western shores of Ireland and England; but seeds
could be transported by these wanderers only by one means, namely, in dirt
sticking to their feet, which is in itself a rare accident. Even in this case,
how small would the chance he of a seed falling on favourable soil, and coming
to maturity ! But it would be a great error to argue that because a well-
stocked island, like Great Britain, has not, as far as is known (and it would
be very difficult to prove this), received within the last few centuries,
through occasional means of transport, immigrants from Europe or any other
continent, that a poorly-stocked island, though standing more remote from the
mainland, would not receive colonists by similar means. I do not doubt that out
of twenty seeds or animals transported to an island, even ff far less well-
stocked than Britain, scarcely more than one would be so well fitted to its new
home, as to become naturalised. But this, as it seems to me, is no valid
argument against what would be effected by occasional means of transport,
during the long lapse of geological time, whilst an island was being upheaved
and formed, and before it had become fully stocked with inhabitants. On almost
bare land, with few or no destructive insects or birds living there, nearly
every seed, which chanced to arrive, would be sure to germinate and survive.
Dispersal during the
Glacial period. The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where the Alpine
species could not possibly exist, is one of the most striking cases known of
the same species living at distant points, without the apparent possibility of
their having migrated from one to the other. It is indeed a remarkable fact to
see so many of the same plants living on the snowy regions of the Alps or
pyrenees, and in the extreme northern parts of Europe; but it is far more
remarkable, that the plants on the White Mountains, in the United States of
America, are all the same with those of Labrador, and nearly all the same, as
we hear from Asa Gray, with those on the loftiest mountains of Europe. Even as
long ago as 1747, such facts led Gmelin to conclude that the same species must
have been independently created at several distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid attention
to the Glacial period, which, as we shall immediately see, affords a simple
explanation of these facts. We have evidence of almost every conceivable kind,
organic and inorganic, that within a very recent geological period, central
Europe and North America suffered under an Arctic climate. The ruins of a house
burnt by fire do not tel l their tale more plainly, than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and perched
boulders, of the icy streams with which their valleys were lately filled. So
greatly has the climate of Europe changed, that in Northern Italy, gigantic
moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic boulders, and rocks
scored by drifted icebergs and coast-ice, plainly reveal a former cold period.
The former influence of
the glacial climate on the distribution of the inhabitants of Europe, as
explained with remarkable clearness by Edward Forbes, is substantially as
follows. But we shall follow the changes more readily, by supposing a new
glacial period to come slowly on, and then pass away, as formerly occurred. As
the cold came on, and as each more southern zone became fitted for arctic
beings and ill-fitted for their former more temperate inhabitants, the latter
would be supplanted and arctic productions would take their places. The
inhabitants of the more temperate regions would at the same time travel
southward, unless they were stopped by barriers, in which case they would perish.
The mountains would become covered with snow and ice, and their former Alpine
inhabitants would descend to the plains. By the time that the cold had reached
its maximum, we should have a uniform arctic fauna and flora, covering the
central parts of Europe, as far south as the Alps and pyrenees, and even
stretching into Spain. The now temperate regions of the United States would
likewise be covered by arctic plants and animals, and these would be nearly the
same with those of Europe; for the present circumpolar inhabitants, which we
suppose to have everywhere travelled southward, are remarkably uniform round
the world. We may suppose that the Glacial period came on a little earlier or
later in North America than in Europe, so will the southern migration there
have been a little earlier or later; but this will make no difference in the
final result.
As the warmth returned,
the arctic forms would retreat northward, closely followed up in their retreat
by the productions of the more temperate regions. And as the snow melted from
the bases of the mountains, the arctic forms would seize on the cleared and
thawed ground, always ascending higher and higher, as the warmth increased,
whilst their brethren were pursuing their northern journey. Hence, when the warmth
had fully returned, the same arctic species, which had lately lived in a body
together on the lowlands of the Old and New Worlds, would be left isolated on
distant mountain-summits (having been exterminated on all lesser heights) and
in the arctic regions of both hemispheres.
Thus we can understand
the identity of many plants at points so immensely remote as on the mountains
of the United States and of Europe. We can thus also understand the fact that
the Alpine plants of each mountain-range are more especially related to the
arctic forms living due north or nearly due north of them: for the migration as
the cold came on, and the re-migration on the returning warmth, will generally
have been due south and north. The Alpine plants, for example, of Scotland, as
remarked by Mr H. C. Watson, and those of the pyrenees, as remarked by Ramond,
are more especially allied to the plants of northern Scandinavia; those of the
United States to Labrador,; those of the mountains of Siberia to the arctic
regions of that country. These views, grounded as they are on the perfectly
well- ascertained occurrence of a former Glacial period, seem to me to explain
in so satisfactory a manner the present distribution of the Alpine and Arctic
productions of Europe and America, that when in other regions we find the same
species on distant mountain-summits, we may almost conclude without other
evidence, that a colder climate permitted their former migration across the low
intervening tracts, since become too warm for their existence.
If the climate, since
the Glacial period, has ever been in any degree warmer than at present (as some
geologists in the United States believe to have been the case, chiefly from the
distribution of the fossil Gnathodon), then the arctic and temperate productions
will at a very late period have marched a little further north, and
subsequently have retreated to their present homes; but I have met with no
satisfactory evidence with respect to this intercalated slightly warmer period,
since the Glacial period.
The arctic forms,
during their long southern migration and re-migration northward, will have been
exposed to nearly the same climate, and, as is especially to be noticed, they
will have kept in a body together; consequently their mutual relations will not
have been much disturbed, and, in accordance with the principles inculcated in
this volume, they will not have been liable to much modification. But with our
Alpine productions, left isolated from the moment of the returning warmth,
first at the bases and ultimately on the summits of the mountains, the case
will have been somewhat different; for it is not likely that all the same
arctic species will have been left on mountain ranges distant from each other,
and have survived there ever since; they will, also, in all probability have
become mingled with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which during its
coldest period will have been temporarily driven down to the plains; they will,
also, have been exposed to somewhat different climatal influences. Their mutual
relations will thus have been in some degree disturbed; consequently they will
have been liable to modification; and this we find has been the case; for if we
compare the present Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the same, some
present varieties, some are ranked as doubtful forms, and some few are distinct
yet closely allied or representative species.
In illustrating what,
as I believe, actually took place during the Glacial period, I assumed that at
its commencement the arctic productions were as uniform round the polar regions
as they are at the present day. But the foregoing remarks on distribution apply
not only to strictly arctic forms, but also to many subarctic and to some few
northern temperate forms, for some of these are the same on the lower mountains
and on the plains of North America and Europe; and it may be reasonably asked
how I account for the necessary degree of uniformity of the sub-arctic and
northern temperate forms round the world, at the commencement of the Glacial
period. At the present day, the sub-arctic and northern temperate productions
of the Old and New Worlds are separated from each other by the Atlantic Ocean
and by the extreme northern part of the Pacific. During the Glacial period,
when the inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider spaces of
ocean. I believe the above difficulty may be surmounted by looking to still
earlier changes of climate of an opposite nature. We have good reason to
believe that during the newer Pliocene period, before the Glacial epoch, and
whilst the majority of the inhabitants of the world were specifically the same
as now, the climate was warmer than at the present day. Hence we may suppose
that the organisms now living under the climate of latitude 60, during the
pliocene period lived further north under the polar Circle, in latitude 66-67;
and that the strictly arctic productions then lived on the broken land still
nearer to the pole. Now if we look at a globe, we shall see that under the
polar Circle there is almost continuous land from western Europe, through
Siberia, to eastern America. And to this continuity of the circumpolar land,
and to the consequent freedom for intermigration under a more favourable
climate, I attribute the necessary amount of uniformity in the sub-arctic and
northern temperate productions of the Old and New Worlds, at a period anterior
to the Glacial epoch.
Believing, from reasons
before alluded to, that our continents have long remained in nearly the same
relative position, though subjected to large, but partial oscillations of
level, I am strongly inclined to extend the above view, and to infer that
during some earlier and still warmer period, such as the older Pliocene period,
a large number of the same plants and animals inhabited the almost continuous circumpolar
land; and that these plants and animals, both in the Old and New Worlds, began
slowly to migrate southwards as the climate became less warm, long before the
commencement of the Glacial period. We now see, as I believe, their
descendants, mostly in a modified condition, in the central parts of Europe and
the United States. On this view we can understand the relationship, with very
little identity, between the productions of North America and Europe, -- a
relationship which is most remarkable, considering the distance of the two
areas, and their separation by the Atlantic Ocean. We can further understand
the singular fact remarked on by several observers, that the productions of
Europe and America during the later tertiary stages were more closely related
to each other than they are at the present time; for during these warmer
periods the northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered impassable by
cold, for the inter-migration of their inhabitants.
During the slowly
decreasing warmth of the pliocene period, as soon as the species in common,
which inhabited the New and Old Worlds, migrated south of the polar Circle,
they must have been completely cut off from each other. This separation, as far
as the more temperate productions are concerned, took place long ages ago. And
as the plants and animals migrated southward, they will have become mingled in
the one great region with the native American productions, and have had to
compete with them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification, -- for
far more modification than with the Alpine productions, left isolated, within a
much more recent period, on the several mountain-ranges and on the arctic lands
of the two Worlds. Hence it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find very
few identical species (though Asa Gray has lately shown that more plants are
identical than was formerly supposed), but we find in every great class many
forms, which some naturalists rank as geographical races, and others as
distinct species; and a host of closely allied or representative forms which
are ranked by all naturalists as specifically distinct.
As on the land, so in
the waters of the sea, a slow southern migration of a marine fauna, which
during the pliocene or even a somewhat earlier period, was nearly uniform along
the continuous shores of the Polar Circle, will account, on the theory of
modification, for many closely allied forms now living in areas completely
sundered. Thus, I think, we can understand the presence of many existing and
tertiary representative forms on the eastern and western shores of temperate
North America; and the still more striking case of many closely allied
crustaceans (as described in Dana's admirable work), of some fish and other
marine animals, in the Mediterranean and in the seas of Japan, -- areas now separated
by a continent and by nearly a hemisphere of equatorial ocean.
These cases of
relationship, without identity, of the inhabitants of seas now disjoined, and
likewise of the past and present inhabitants of the temperate lands of North
America and Europe, are inexplicable on the theory of creation. We cannot say
that they have been created alike, in correspondence with the nearly similar
physical conditions of the areas; for if we compare, for instance, certain
parts of South America with the southern continents of the Old World, we see
countries closely corresponding in all their physical conditions, but with
their inhabitants utterly dissimilar.
But we must return to
our more immediate subject, the Glacial period. I am convinced that Forbes's
view may be largely extended. In Europe we have the plainest evidence of the
cold period, from the western shores of Britain to the Oural range, and
southward to the Pyrenees. We may infer, from the frozen mammals and nature of
the mountain vegetation, that Siberia was similarly affected. Along the
Himalaya, at points 900 miles apart, glaciers have left the marks of their
former low descent; and in Sikkim, Dr Hooker saw maize growing on gigantic
ancient moraines. South of the equator, we have some direct evidence of former
glacial action in New Zealand; and the same plants, found on widely separated
mountains in this island, tell the same story. If one account which has been
published can be trusted, we have direct evidence of glacial action in the
southeastern corner of Australia.
Looking to America; in
the northern half, ice-borne fragments of rock have been observed on the
eastern side as far south as lat. 36-37, and on the shores of the pacific,
where the climate is now so different, as far south as lat. 46; erratic
boulders have, also, been noticed on the Rocky Mountains. In the Cordillera of
Equatorial South America, glaciers once extended far below their present level.
In central Chile I was astonished at the structure of a vast mound of detritus,
about 800 feet in height, crossing a valley of the Andes; and this I now feel
convinced was a gigantic moraine, left far below any existing glacier. Further
south on both sides of the continent, from lat. 41 to the southernmost
extremity, we have the clearest evidence of former glacial action, in huge
boulders transported far from their parent source.
We do not know that the
Glacial epoch was strictly simultaneous at these several far distant points on
opposite sides of the world. But we have good evidence in almost every case,
that the epoch was included within the latest geological period. We have, also,
excel lent evidence, that it endured for an enormous time, as measured by
years, at each point. The cold may have come on, or have ceased, earlier at one
point of the globe than at another, but seeing that it endured for long at
each, and that it was contemporaneous in a geological sense, it seems to me
probable that it was, during a part at least of the period, actually
simultaneous throughout the world. Without some distinct evidence to the
contrary, we may at least admit as probable that the glacial action was
simultaneous on the eastern and western sides of North America, in the
Cordillera under the equator and under the warmer temperate zones, and on both
sides of the southern extremity of the continent. If this be admitted, it is
difficult to avoid believing that the temperature of the whole world was at
this period simultaneously cooler. But it would suffice for my purpose, if the
temperature was at the same time lower along certain broad belts of longitude.
On this view of the
whole world, or at least of broad longitudinal belts, having been
simultaneously colder from pole to pole, much light can be thrown on the
present distribution of identical and allied species. in America, Dr Hooker has
shown that between forty and fifty of the flowering plants of Tierra del Fuego,
forming no inconsiderable part of its scanty flora, are common to Europe,
enormously remote as these two points are; and there are many closely allied
species. On the lofty mountains of equatorial America a host of peculiar
species belonging to European genera occur. On the highest mountains of Brazil,
some few European genera were found by Gardner, which do not exist in the wide
intervening hot countries. So on the Silla of Caraccas the illustrious Humboldt
long ago found species belonging to genera characteristic of the Cordillera. On
the mountains of Abyssinia, several European forms and some few representatives
of the peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope
a very few European species, believed not to have been introduced by man, and
on the mountains, some few representative European forms are found, which have
not been discovered in the intertropical parts of Africa. On the Himalaya, and
on the isolated mountain-ranges of the peninsula of India, on the heights of
Ceylon, and on the volcanic cones of Java, many plants occur, either
identically the same or representing each other, and at the same time representing
plants of Europe, not found in the intervening hot lowlands. A list of the
genera collected on the loftier peaks of Java raises a picture of a collection
made on a hill in Europe l Still more striking is the fact that southern
Australian forms are clearly represented by plants growing on the summits of
the mountains of Borneo. Some of these Australian forms, as I hear from Dr
Hooker, extend along the heights of the peninsula of Malacca, and are thinly
scattered, on the one hand over india and on the other as far as Japan.
On the southern
mountains of Australia, Dr F. Mller has discovered several European species;
other species, not introduced by man, occur on the lowlands; and a long list
can be given, as I am informed by Dr Hooker, of European genera, found in
Australia, but not in the intermediate torrid regions. In the admirable
`Introduction to the Flora of New Zealand,' by Dr Hooker, analogous and
striking facts are given in regard to the plants of that large island. Hence we
see that throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern hemispheres, are
sometimes identically the same; but they are much oftener specifically
distinct, though related to each other in a most remarkable manner.
This brief abstract
applies to plants alone: some strictly analogous facts could be given on the
distribution of terrestrial animals. In marine productions, similar cases
occur; as an example, I may quote a remark by the highest authority, Prof.
Dana, that `it is certainly a wonderful fact that New Zealand should have a
closer resemblance in its crustacea to Great Britain, its antipode, than to any
other part of the world.' Sir J. Richardson, also, speaks of the reappearance
on the shores of New Zealand, Tasmania, &c., of northern forms of fish. Dr
Hooker informs me that twenty-five species of Algae are common to New Zealand
and to Europe, but have not been found in the intermediate tropical seas.
It should be observed
that the northern species and forms found in the southern parts of the southern
hemisphere, and on the mountain-ranges of the intertropical regions, are not
arctic, but belong to the northern temperate zones. As Mr H. C. Watson has
recently remarked, `In receding from polar towards equatorial latitudes, the
Alpine or mountain floras really become less and less arctic.' Many of the
forms living on the mountains of the warmer regions of the earth and in the
southern hemisphere are of doubtful value, being ranked by some naturalists as
specifically distinct, by others as varieties; but some are certainly
identical, and many, though closely related to northern forms, must be ranked
as distinct species.
Now let us see what
light can be thrown on the foregoing facts, on the belief, supported as it is
by a large body of geological evidence, that the whole world, or a large part
of it, was during the Glacial period simultaneously much colder than at
present. The Glacial period, as measured by years, must have been very long;
and when we remember over what vast spaces some naturalised plants and animals
have spread within a few centuries, this period will have been ample for any
amount of migration. As the cold came slowly on, all the tropical plants and
other productions will have retreated from both sides towards the equator,
followed in the rear by the temperate productions, and these by the arctic; but
with the latter we are not now concerned. The tropical plants probably suffered
much extinction; how much no one can say; perhaps formerly the tropics
supported as many species as we see at the present day crowded together at the
Cape of Good Hope, and in parts of temperate Australia. As we know that many
tropical plants and animals can withstand a considerable amount of cold, many might
have escaped extermination during a moderate fall of temperature, more
especially by escaping into the warmest spots. But the great fact to bear in
mind is, that all tropical productions will have suffered to a certain extent.
On the other hand, the temperate productions, after migrating nearer to the
equator, though they will have been placed under somewhat new conditions, will
have suffered less. And it is certain that many temperate plants, if protected
from the inroads of competitors, can withstand a much warmer climate than their
own. Hence, it seems to me possible, bearing in mind that the tropical
productions were in a suffering state and could not have presented a firm front
against intruders, that a certain number of the more vigorous and dominant
temperate forms might have penetrated the native ranks and have reached or even
crossed the equator. The invasion would, of course, have been greatly favoured
by high land, and perhaps by a dry climate; for Dr Falconer informs me that it
is the damp with the heat of the tropics which is so destructive to perennial
plants from a temperate climate. On the other hand, the most humid and hottest
districts will have afforded an asylum to the tropical natives. The mountain-
ranges north-west of the Himalaya, and the long line of the Cordillera, seem to
have afforded two great lines of invasion: and it is a striking fact, lately
communicated to me by Dr Hooker, that all the flowering plants, about forty-six
in number, common to Tierra del Fuego and to Europe still exist in North
America, which must have lain on the line of march. But I do not doubt that
some temperate productions entered and crossed even the lowlands of the tropics
at the period when the cold was most intense, -- when arctic forms had migrated
some twenty-five degrees of latitude from their native country and covered the
land at the foot of the pyrenees. At this period of extreme cold, I believe
that the climate under the equator at the level of the sea was about the same
with that now felt there at the height of six or seven thousand feet. During
this the coldest period, I suppose that large spaces of the tropical lowlands
were clothed with a mingled tropical and temperate vegetation, like that now
growing with strange luxuriance at the base of the Himalaya, as graphically
described by Hooker.
Thus, as I believe, a
considerable number of plants, a few terrestrial animals, and some marine
productions, migrated during the Glacial period from the northern and southern
temperate zones into the intertropical regions, and some even crossed the
equator. As the warmth returned, these temperate forms would naturally ascend
the higher mountains, being exterminated on the lowlands; those which had not
reached the equator, would re-migrate northward or southward towards their
former homes; but the forms, chiefly northern, which had crossed the equator,
would travel still further from their homes into the more temperate latitudes
of the opposite hemisphere. Although we have reason to believe from geological evidence
that the whole body of arctic shells underwent scarcely any modification during
their long southern migration and re-migration northward, the case may have
been wholly different with those intruding forms which settled themselves on
the intertropical mountains, and in the southern hemisphere. These being
surrounded by strangers will have had to compete with many new forms of life;
and it is probable that selected modifications in their structure, habits, and
constitutions will have profited them. Thus many of these wanderers, though
still plainly related by inheritance to their brethren of the northern or
southern hemispheres, now exist in their new homes as well-marked varieties or
as distinct species.
It is a remarkable
fact, strongly insisted on by Hooker in regard to America, and by Alph. de
Candolle in regard to Australia, that many more identical plants and allied
forms have apparently migrated from the north to the south, than in a reversed
direction. We see, however, a few southern vegetable forms on the mountains of
Borneo and Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the northern
forms having existed in their own homes in greater numbers, and having consequently
been advanced through natural selection and competition to a higher stage of
perfection or dominating power, than the southern forms. And thus, when they
became commingled during the Glacial period, the northern forms were enabled to
beat the less powerful southern forms. Just in the same manner as we see at the
present day, that very many European productions cover the ground in La Plata,
and in a lesser degree in Australia, and have to a certain extent beaten the
natives; whereas extremely few southern forms have become naturalised in any
part of Europe, though hides, wool, and other objects likely to carry seeds
have been largely imported into Europe during the last two or three centuries
from La plata, and during the last thirty or forty years from Australia.
Something of the same kind must have occurred on the intertropical mountains:
no doubt before the Glacial period they were stocked with endemic Alpine forms;
but these have almost everywhere largely yielded to the more dominant forms,
generated in the larger areas and more efficient workshops of the north. ln
many islands the native productions are nearly equalled or even outnumbered by
the naturalised; and if the natives have not been actually exterminated, their
numbers have been greatly reduced, and this is the first stage towards
extinction. A mountain is an island on the land; and the intertropical
mountains before the Glacial period must have been completely isolated; and I
believe that the productions of these islands on the land yielded to those
produced within the larger areas of the north, just in the same way as the
productions of real islands have everywhere lately yielded to continental
forms, naturalised by man's agency.
I am far from supposing
that all difficulties are removed on the view here given in regard to the range
and affinities of the allied species which live in the northern and southern
temperate zones and on the mountains of the intertropical regions. Very many
difficulties remain to be solved. I do not pretend to indicate the exact lines
and means of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise to new
groups of forms, and others have remained unaltered. We cannot hope to explain
such facts, until we can say why one species and not another becomes
naturalised by man's agency in a foreign land; why one ranges twice or thrice
as far, and is twice or thrice as common, as another species within their own
homes.
I have said that many
difficulties remain to be solved: some of the most remarkable are stated with
admirable clearness by Dr Hooker in his botanical works on the antarctic
regions. These cannot be here discussed. I will only say that as far as regards
the occurrence of identical species at points so enormously remote as Kerguelen
Land, New Zealand, and Fuegia, I believe that towards the close of the Glacial
period, icebergs, as suggested by Lyell, have been largely concerned in their
dispersal. But the existence of several quite distinct species, belonging to
genera exclusively confined to the south, at these and other distant points of
the southern hemisphere, is, on my theory of descent with modification, a far
more remarkable case of difficulty. For some of these species are so distinct,
that we cannot suppose that there has been time since the commencement of the
Glacial period for their migration, and for their subsequent modification to
the necessary degree. The facts seem to me to indicate that peculiar and very
distinct species have migrated in radiating lines from some common centre; and
I am inclined to look in the southern, as in the northern hemisphere, to a
former and warmer period, before the commencement of the Glacial period, when
the antarctic lands, now covered with ice, supported a highly peculiar and
isolated flora. I suspect that before this flora was exterminated by the
Glacial epoch, a few forms were widely dispersed to various points of the
southern hemisphere by occasional means of transport, and by the aid, as
halting-places, of existing and now sunken islands, and perhaps at the
commencement of the Glacial period, by icebergs. By these means, as I believe,
the southern shores of America, Australia, New Zealand have become slightly
tinted by the same peculiar forms of vegetable life.
Sir C. Lyell in a
striking passage has speculated, in language almost identical with mine, on the
effects of great alterations of climate on geographical distribution. I believe
that the world has recently felt one of his great cycles of change; and that on
this view, combined with modification through natural selection, a multitude of
facts in the present distribution both of the same and of allied forms of life
can be explained. The living waters may be said to have flowed during one short
period from the north and from the south, and to have crossed at the equator;
but to have flowed with greater force from the north so as to have freely
inundated the south. As the tide leaves its drift in horizontal lines, though
rising higher on the shores where the tide rises highest, so have the living
waters left their living drift on our mountain-summits, in a line gently rising
from the arctic lowlands to a great height under the equator. The various
beings thus left stranded may be compared with savage races of man, driven up
and surviving in the mountain-fastnesses of almost every land, which serve as a
record, full of interest to us, of the former inhabitants of the surrounding
lowlands.
Distribution of fresh-water productions - On the inhabitants of oceanic
islands - Absence of Batrachians and of terrestrial Mammals - On the relations
of the inhabitants of islands to those of the nearest mainland - On
colonisation from the nearest source with subsequent modification - Summary of
the last and present chapters AS
lakes and river-systems are separated from each other by barriers of land, it
might have been thought that fresh-water productions would not have ranged
widely within the same country, and as the sea is apparently a still more
impassable barrier, that they never would have extended to distant countries.
But the case is exactly the reverse. Not only have many fresh-water species,
belonging to quite different classes, an enormous range, but allied species
prevail in a remarkable manner throughout the world. I well remember, when
first collecting in the fresh waters of Brazil, feeling much surprise at the
similarity of the fresh-water insects, shells, &c., and at the
dissimilarity of the surrounding terrestrial beings, compared with those of
Britain.
But this power in
fresh-water productions of ranging widely, though so unexpected, can, I think,
in most cases be explained by their having become fitted, in a manner highly
useful to them, for short and frequent migrations from pond to pond, or from
stream to stream; and liability to wide dispersal- would follow from this
capacity as an almost necessary consequence. We can here consider only a few
cases. In regard to fish, I believe that the same species never occur in the
fresh waters of distant continents. But on the same continent the species often
range widely and almost capriciously; for two river-systems will have some fish
in common and some different. A few facts seem to favour the possibility of
their occasional transport by accidental means; like that of the live fish not
rarely dropped by whirlwinds in India, and the vitality of their ova when
removed from the water. But I am inclined to attribute the dispersal of
fresh-water fish mainly to slight changes within the recent period in the level
of the land, having caused rivers to flow into each other. Instances, also,
could be given of this having occurred during floods, without any change of
level. We have evidence in the loess of the Rhine of considerable changes of
level in the land within a very recent geological period, and when the surface
was peopled by existing land and fresh-water shells. The wide difference of the
fish on opposite sides of continuous mountain-ranges, which from an early
period must have parted river-systems and completely prevented their
inosculation, seems to lead to this same conclusion. With respect to allied
fresh-water fish occurring at very distant points of the world, no doubt there
are many cases which cannot at present be explained: but some fresh-water fish
belong to very ancient forms, and in such cases there will have been ample time
for great geographical changes, and consequently time and means for much
migration. In the second place, salt-water fish can with care be slowly
accustomed to live in fresh water; and, according to Valenciennes, there is
hardly a single group of fishes confined exclusively to fresh water, so that we
may imagine that a marine member of a fresh-water group might travel far along
the shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.
Some species of
fresh-water shells have a very wide range, and allied species, which, on my
theory, are descended from a common parent and must have proceeded from a
single source, prevail throughout the world. Their distribution at first
perplexed me much, as their ova are not likely to be transported by birds, and
they are immediately killed by sea water, as are the adults. I could not even
understand how some naturalised species have rapidly spread throughout the same
country. But two facts, which I have observed -- and no doubt many others
remain to be observed -- throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these little
plants adhering to its back; and it has happened to me, in removing a little
duck-weed from one aquarium to another, that I have quite unintentionally
stocked the one with fresh-water shells from the other. But another agency is
perhaps more effectual: I suspended a duck's feet, which might represent those
of a bird sleeping in a natural pond, in an aquarium, where many ova of fresh-
water shells were hatching; and I found that numbers of the extremely minute
and just hatched shells crawled on the feet, and clung to them so firmly that
when taken out of the water they could not be jarred off, though at a somewhat
more advanced age they would voluntarily drop off. These just hatched molluscs,
though aquatic in their nature, survived on the duck's feet, in damp air, from
twelve to twenty hours; and in this length of time a duck or heron might fly at
least six or seven hundred miles, and would be sure to alight on a pool or
rivulet, if blown across sea to an oceanic island or to any other distant
point. Sir Charles Lyell also informs me that a Dyticus has been caught with an
Ancylus (a fresh-water shell like a limpet) firmly adhering to it; and a
water-beetle of the same family, a Colymbetes, once flew on board the Beagle.
when forty-five miles distant from the nearest land: how much farther it might
have flown with a favouring gale no one can tell.
With respect to plants,
it has long been known what enormous ranges many fresh-water and even
marsh-species have, both over continents and to the most remote oceanic
islands. This is strikingly shown, as remarked by Alph. de Candolle, in large
groups of terrestrial plants, which have only a very few aquatic members; for
these latter seem immediately to acquire, as if in consequence, a very wide
range. I think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some quantity to
the feet and beaks of birds. Wading birds, which frequent the muddy edges of
ponds, if suddenly flushed, would be the most likely to have muddy feet. Birds
of this order I can show are the greatest wanderers, and are occasionally found
on the most remote and barren islands in the open ocean; they would not be
likely to alight on the surface of the sea, so that the dirt would not be
washed off their feet; when making land, they would be sure to fly to their
natural fresh-water haunts. I do not believe that botanists are aware how
charged the mud of ponds is with seeds: I have tried several little
experiments, but will here give only the most striking case: I took in February
three table-spoonfuls of mud from three different points, beneath water, on the
edge of a little pond; this mud when dry weighed only 6 3/4 ounces; I kept it
covered up in my study for six months, pulling up and counting each plant as it
grew; the plants were of many kinds, and were altogether 537 in number; and yet
the viscid mud was all contained in a breakfast cup ! Considering these facts,
I think it would be an inexplicable circumstance if water-birds did not
transport the seeds of fresh-water plants to vast distances, and if
consequently the range of these plants was not very great. The same agency may
have come into play with the eggs of some of the smaller fresh-water animals.
Other and unknown
agencies probably have also played a part. I have stated that fresh-water fish
eat some kinds of seeds, though they reject many other kinds after having
swallowed them; even small fish swallow seeds of moderate size, as of the
yellow water-lily and Potamogeton. Herons and other birds, century after
century, have gone on daily devouring fish; they then take flight and go to
other waters, or are blown across the sea; and we have seen that seeds retain
their power of germination, when rejected in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks on this
plant, I thought that its distribution must remain quite inexplicable; but
Audubon states that he found the seeds of the great southern water-lily
(probably, according to Dr Hooker, the Nelumbium luteum) in a heron's stomach;
although I do not know the fact, yet analogy makes me believe that a heron
flying to another pond and getting a hearty meal of fish, would probably reject
from its stomach a pellet containing the seeds of the Nelumbium undigested; or
the seeds might be dropped by the bird whilst feeding its young, in the same
way as fish are known sometimes to be dropped.
In considering these
several means of distribution, it should be remembered that when a pond or
stream is first formed, for instance, on a rising islet, it will be unoccupied;
and a single seed or egg will have a good chance of succeeding. Although there
will always be a struggle for life between the individuals of the species,
however few, already occupying any pond, yet as the number of kinds is small,
compared with those on the land, the competition will probably be less severe
between aquatic than between terrestrial species; consequently an intruder from
the waters of a foreign country, would have a better chance of seizing on a
place, than in the case of terrestrial colonists. We should, also, remember
that some, perhaps many, fresh-water productions are low in the scale of
nature, and that we have reason to believe that such low beings change or
become modified less quickly than the high; and this will give longer time than
the average for the migration of the same aquatic species. We should not forget
the probability of many species having formerly ranged as continuously as
fresh-water productions ever can range, over immense areas, and having
subsequently become extinct in intermediate regions. But the wide distribution
of fresh- water plants and of the lower animals, whether retaining the same
identical form or in some degree modified, I believe mainly depends on the wide
dispersal of their seeds and eggs by animals, more especially by fresh-water
birds, which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful gardener, thus
takes her seeds from a bed of a particular nature, and drops them in another
equally well fitted for them.
On the Inhabitants of
Oceanic Islands. We now come to the last of the three classes of facts, which I
have selected as presenting the greatest amount of difficulty, on the view that
all the individuals both of the same and of allied species have descended from
a single parent; and therefore have all proceeded from a common birthplace,
notwithstanding that in the course of time they have come to inhabit distant
points of the globe. I have already stated that I cannot honestly admit
Forbes's view on continental extensions, which, if legitimately followed out,
would lead to the belief that within the recent period all existing islands
have been nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in regard to
insular productions. In the following remarks I shall not confine myself to the
mere question of dispersal; but shall consider some other facts, which bear on
the truth of the two theories of independent creation and of descent with
modification.
The species of all
kinds which inhabit oceanic islands are few in number compared with those on
equal continental areas: Alph. de Candolle admits this for plants, and
Wollaston for insects. If we look to the large size and varied stations of New
Zealand, extending over 780 miles of latitude, and compare its flowering
plants, only 750 in number, with those on an equal area at the Cape of Good
Hope or in Australia, we must, I think, admit that something quite
independently of any difference in physical conditions has caused so great a
difference in number. Even the uniform county of Cambridge has 847 plants, and
the little island of Anglesea 764, but a few ferns and a few introduced plants
are included in these numbers, and the comparison in some other respects is not
quite fair. We have evidence that the barren island of Ascension aboriginally
possessed under half-a-dozen flowering plants; yet many have become naturalised
on it, as they have on New Zealand and on every other oceanic island which can
be named. In St Helena there is reason to believe that the naturalised plants
and animals have nearly or quite exterminated many native productions. He who
admits the doctrine of the creation of each separate species, will have to
admit, that a sufficient number of the best adapted plants and animals have not
been created on oceanic islands; for man has unintentionally stocked them from
various sources far more fully and perfectly than has nature.
Although in oceanic
islands the number of kinds of inhabitants is scanty, the proportion of endemic
species ( i.e. those found nowhere else in the world) is often extremely large.
If we compare, for instance, the number of the endemic land-shells in Madeira,
or of the endemic birds in the Galapagos Archipelago, with the number found on
any continent, and then compare the area of the islands with that of the
continent, we shall see that this is true. This fact might have been expected
on my theory for, as already explained, species occasionally arriving after
long intervals in a new and isolated district, and having to compete with new
associates, will be eminently liable to modification, and will often produce
groups of modified descendants. But it by no means follows, that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified having
immigrated with facility and in a body, so that their mutual relations have not
been much disturbed. Thus in the Galapagos Islands nearly every land-bird, but
only two out of the eleven marine birds, are peculiar; and it is obvious that
marine birds could arrive at these islands more easily than landbirds. Bermuda,
on the other hand, which lies at about the same distance from North America as
the Galapagos Islands do from South America, and which has a very peculiar
soil, does not possess one endemic land bird; and we know from Mr J. M. Jones's
admirable account of Bermuda, that very many North American birds, during their
great annual migrations, visit either periodically or occasionally this island.
Madeira does not possess one peculiar bird, and many European and African birds
are almost every year blown there, as I am informed by Mr E. V. Harcourt. So
that these two islands of Bermuda and Madeira have been stocked by birds, which
for long ages have struggled together in their former homes, and have become
mutually adapted to each other; and when settled in their new homes, each kind
will have been kept by the others to their proper places and habits, and will
consequently have been little liable to modification. Madeira, again, is
inhabited by a wonderful number of peculiar land-shells, whereas not one
species of sea-shell is confined to its shores: now, though we do not know how
seashells are dispersed, yet we can see that their eggs or larvae, perhaps
attached to seaweed or floating timber, or to the feet of wading-birds, might
be transported far more easily than landshells, across three or four hundred
miles of open sea. The different orders of insects in Madeira apparently
present analogous facts.
Oceanic islands are
sometimes deficient in certain classes, and their places are apparently
occupied by the other inhabitants; in the Galapagos Islands reptiles, and in
New Zealand gigantic wingless birds, take the place of mammals. In the plants
of the Galapagos Islands, Dr Hooker has shown that the proportional numbers of
the different orders are very different from what they are elsewhere. Such
cases are generally accounted for by the physical conditions of the islands;
but this explanation seems to me not a little doubtful. Facility of
immigration, I believe, has been at least as important as the nature of the
conditions.
Many remarkable little
facts could be given with respect to the inhabitants of remote islands. For
instance, in certain islands not tenanted by mammals, some of the endemic
plants have beautifully hooked seeds; yet few relations are more striking than
the adaptation of hooked seeds for transportal by the wool and fur of
quadrupeds. This case presents no difficulty on my view, for a hooked seed
might be transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds, would form an
endemic species, having as useless an appendage as any rudimentary organ, --
for instance, as the shrivelled wings under the soldered elytra of many insular
beetles. Again, islands often possess trees or bushes belonging to orders which
elsewhere include only herbaceous species; now trees, as Alph. de Candolle has
shown, generally have, whatever the cause may be, confined ranges. Hence trees
would be little likely to reach distant oceanic islands; and an herbaceous
plant, though it would have no chance of successfully competing in stature with
a fully developed tree, when established on an island and having to compete
with herbaceous plants alone, might readily gain an advantage by growing taller
and taller and overtopping the other plants. If so, natural selection would
often tend to add to the stature of herbaceous plants when growing on an
island, to whatever order they belonged, and thus convert them first into
bushes and ultimately into trees.
With respect to the
absence of whole orders on oceanic islands, Bory St Vincent long ago remarked
that Batrachians (frogs, toads, newts) have never been found on any of the many
islands with which the great oceans are studded. I have taken pains to verify
this assertion, and I have found it strictly true. I have, however, been
assured that a frog exists on the mountains of the great island of New Zealand;
but I suspect that this exception (if the information be correct) may be
explained through glacial agency. This general absence of frogs, toads, and
newts on so many oceanic islands cannot be accounted for by their physical
conditions; indeed it seems that islands are peculiarly well fitted for these
animals; for frogs have been introduced into Madeira, the Azores, and Mauritius,
and have multiplied so as to become a nuisance. But as these animals and their
spawn are known to be immediately killed by sea-water, on my view we can see
that there would be great difficulty in their transportal across the sea, and
therefore why they do not exist on any oceanic island. But why, on the theory
of creation, they should not have been created there, it would be very
difficult to explain.
Mammals offer another
and similar case. I have carefully searched the oldest voyages, but have not
finished my search; as yet I have not found a single instance, free from doubt,
of a terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like fox,
come nearest to an exception; but this group cannot be considered as oceanic,
as it lies on a bank connected with the mainland; moreover, icebergs formerly
brought boulders to its western shores, and they may have formerly transported
foxes, as so frequently now happens in the arctic regions. Yet it cannot be
said that small islands will not support small mammals, for they occur in many
parts of the world on very small islands, if close to a continent; and hardly
an island can be named on which our smaller quadrupeds have not become
naturalised and greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals; many
volcanic islands are sufficiently ancient, as shown by the stupendous
degradation which they have suffered and by their tertiary strata: there has
also been time for the production of endemic species belonging to other
classes; and on continents it is thought that mammals appear and disappear at a
quicker rate than other and lower animals. Though terrestrial mammals do not
occur on oceanic islands, a rial mammals do occur on almost every island. New
Zealand possesses two bats found nowhere else in the world: Norfolk Island, the
Viti Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes,
and Mauritius, all possess their peculiar bats. Why, it may be asked, has the
supposed creative force produced bats and no other mammals on remote islands?
On my view this question can easily be answered; for no terrestrial mammal can
be transported across a wide space of sea, but bats can fly across. Bats have
been seen wandering by day far over the Atlantic Ocean; and two North American
species either regularly or occasionally visit Bermuda, at the distance of 600
miles from the mainland. I hear from Mr Tomes, who has specially studied this
family, that many of the same species have enormous ranges, and are found on
continents and on far distant islands. Hence we have only to suppose that such
wandering species have been modified through natural selection in their new
homes in relation to their new position, and we can understand the presence of
endemic bats on islands, with the absence of all terrestrial mammals.
Besides the absence of
terrestrial mammals in relation to the remoteness of islands from continents,
there is also a relation, to a certain extent independent of distance, between
the depth of the sea separating an island from the neighbouring mainland, and
the presence in both of the same mammiferous species or of allied species in a
more or less modified condition. Mr Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago, which is
traversed near Celebes by a space of deep ocean; and this space separates two
widely distinct mammalian faunas. On either side the islands are situated on
moderately deep submarine banks, and they are inhabited by closely allied or
identical quadrupeds. No doubt some few anomalies occur in this great
archipelago, and there is much difficulty in forming a judgment in some cases
owing to the probable naturalisation of certain mammals through man's agency;
but we shall soon have much light thrown on the natural history of this
archipelago by the admirable zeal and researches of Mr Wallace. I have not as
yet had time to follow up this subject in all other quarters of the world; but
as far as I have gone, the relation generally holds good. We see Britain
separated by a shallow channel from Europe, and the mammals are the same on
both sides; we meet with analogous facts on many islands separated by similar
channels from Australia. The West Indian Islands stand on a deeply submerged
bank, nearly 1000 fathoms in depth, and here we find American forms, but the
species and even the genera are distinct. As the amount of modification in all
cases depends to a certain degree on the lapse of time, and as during changes
of level it is obvious that islands separated by shallow channels are more
likely to have been continuously united within a recent period to the mainland
than islands separated by deeper channels, we can understand the frequent
relation between the depth of the sea and the degree of affinity of the
mammalian inhabitants of islands with those of a neighbouring continent, -- an
explicable relation on the view of independent acts of creation.
All the foregoing
remarks on the inhabitants of oceanic islands, -- namely, the scarcity of kinds
-- the richness in endemic forms in particular classes or sections of classes,
-- the absence of whole groups, as of batrachians, and of terrestrial mammals
notwithstanding the presence of a rial bats, -- the singular proportions of
certain orders of plants, -- herbaceous forms having been developed into trees,
&c., -- seem to me to accord better with the view of occasional means of
transport having been largely efficient in the long course of time, than with
the view of all our oceanic islands having been formerly connected by
continuous land with the nearest continent; for on this latter view the
migration would probably have been more complete; and if modification be
admitted, all the forms of life would have been more equally modified, in accordance
with the paramount importance of the relation of organism to organism.
I do not deny that
there are many and grave difficulties in understanding how several of the
inhabitants of the more remote islands, whether still retaining the same
specific form or modified since their arrival, could have reached their present
homes. But the probability of many islands having existed as halting-places, of
which not a wreck now remains, must not be overlooked. I will here give a
single instance of one of the cases of difficulty. Almost all oceanic islands,
even the most isolated and smallest, are inhabited by land-shells, generally by
endemic species, but sometimes by species found elsewhere. Dr Aug. A. Gould has
given several interesting cases in regard to the land-shells of the islands of
the pacific. Now it is notorious that land-shells are very easily killed by
salt; their eggs, at least such as I have tried, sink in sea-water and are
killed by it. Yet there must be, on my view, some unknown, but highly efficient
means for their transportal. Would the just-hatched young occasionally crawl on
and adhere to the feet of birds roosting on the ground, and thus get
transported? It occurred to me that land-shells, when hybernating and having a
membranous diaphragm over the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea. And I found that several
species did in this state withstand uninjured an immersion in sea-water during
seven days: one of these shells was the Helix pomatia, and after it had again
hybernated I put it in sea-water for twenty days, and it perfectly recovered.
As this species has a thick calcareous operculum, I removed it, and when it had
formed a new membranous one, I immersed it for fourteen days in sea- water, and
it recovered and crawled away: but more experiments are wanted on this head.
The most striking and
important fact for us in regard to the inhabitants of islands, is their
affinity to those of the nearest mainland, without being actually the same
species. Numerous instances could be given of this fact. I will give only one,
that of the Galapagos Archipelago, situated under the equator, between 500 and
600 miles from the shores of South America. Here almost every product of the
land and water bears the unmistakeable stamp of the American continent. There
are twenty-six land birds, and twenty-five of those are ranked by Mr Gould as
distinct species, supposed to have been created here; yet the close affinity of
most of these birds to American species in every character, in their habits,
gestures, and tones of voice, was manifest. So it is with the other animals,
and with nearly all the plants, as shown by Dr Hooker in his admirable memoir
on the Flora of this archipelago. The naturalist, looking at the inhabitants of
these volcanic islands in the pacific, distant several hundred miles from the
continent, yet feels that he is standing on American land. Why should this be
so? why should the species which are supposed to have been created in the
Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to
those created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South American coast: in fact there is
a considerable dissimilarity in all these respects. On the other hand, there is
a considerable degree of resemblance in the volcanic nature of the soil, in
climate, height, and size of the islands, between the Galapagos and Cape de
Verde Archipelagos: but what an entire and absolute difference in their
inhabitants l The inhabitants of the Cape de Verde Islands are related to those
of Africa, like those of the Galapagos to America. I believe this grand fact
can receive no sort of explanation on the ordinary view of independent
creation; whereas on the view here maintained, it is obvious that the Galapagos
Islands would be likely to receive colonists, whether by occasional means of
transport or by formerly continuous land, from America; and the Cape de Verde
Islands from Africa; and that such colonists would be liable to modifications;
-- the principle of inheritance still betraying their original birthplace.
Many analogous facts
could be given: indeed it is an almost universal rule that the endemic
productions of islands are related to those of the nearest continent, or of
other near islands. The exceptions are few, and most of them can be explained.
Thus the plants of Kerguelen Land, though standing nearer to Africa than to
America, are related, and that very closely, as we know from Dr Hooker's
account, to those of America: but on the view that this island has been mainly
stocked by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its endemic plants
is much more closely related to Australia, the nearest mainland, than to any
other region: and this is what might have been expected; but it is also plainly
related to South America, which, although the next nearest continent, is so
enormously remote, that the fact becomes an anomaly. But this difficulty almost
disappears on the view that both New Zealand, South America, and other southern
lands were long ago partially stocked from a nearly intermediate though distant
point, namely from the antarctic islands, when they were clothed with
vegetation, before the commencement of the Glacial period. The affinity, which,
though feeble, I am assured by Dr Hooker is real, between the flora of the
south-western corner of Australia and of the Cape of Good Hope, is a far more
remarkable case, and is at present inexplicable: but this affinity is confined
to the plants, and will, I do not doubt, be some day explained.
The law which causes
the inhabitants of an archipelago, though specifically distinct, to be closely
allied to those of the nearest continent, we sometimes see displayed on a small
scale, yet in a most interesting manner, within the limits of the same
archipelago. Thus the several islands of the Galapagos Archipelago are
tenanted, as I have elsewhere shown, in a quite marvellous manner, by very
closely related species; so that the inhabitants of each separate island, though
mostly distinct, are related in an incomparably closer degree to each other
than to the inhabitants of any other part of the world. And this is just what
might have been expected on my view, for the islands are situated so near each
other that they would almost certainly receive immigrants from the same
original source, or from each other. But this dissimilarity between the endemic
inhabitants of the islands may be used as an argument against my views; for it
may be asked, how has it happened in the several islands situated within sight
of each other, having the same geological nature, the same height, climate,
&c., that many of the immigrants should have been differently modified,
though only in a small degree. This long appeared to me a great difficulty: but
it arises in chief part from the deeply-seated error of considering the
physical conditions of a country as the most important for its inhabitants;
whereas it cannot, I think, be disputed that the nature of the other
inhabitants, with which each has to compete, is at least as important, and
generally a far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found in other parts of the
world (having on one side for the moment the endemic species, which cannot be
here fairly included, as we are considering how they have come to be modified
since their arrival), we find a considerable amount of difference in the
several islands. This difference might indeed have been expected on the view of
the islands having been stocked by occasional means of transport -- a seed, for
instance, of one plant having been brought to one island, and that of another
plant to another island. Hence when in former times an immigrant settled on any
one or more of the islands, or when it subsequently spread from one island to
another, it would undoubtedly be exposed to different conditions of life in the
different islands, for it would have to compete with different sets of
organisms: a plant, for instance, would find the best-fitted ground more
perfectly occupied by distinct plants in one island than in another, and it
would be exposed to the attacks of somewhat different enemies. If then it
varied, natural selection would probably favour different varieties in the
different islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some species'
spreading widely and remaining the same.
The really surprising
fact in this case of the Galapagos Archipelago, and in a lesser degree in some
analogous instances, is that the new species formed in the separate islands
have not quickly spread to the other islands. But the islands, though in sight
of each other, are separated by deep arms of the sea, in most cases wider than
the British Channel, and there is no reason to suppose that they have at any
former period been continuously united. The currents of the sea are rapid and
sweep across the archipelago, and gales of wind are extraordinarily rare; so
that the islands are far more effectually separated from each other than they
appear to be on a map. Nevertheless a good many species, both those found in
other parts of the world and those confined to the archipelago, are common to
the several islands, and we may infer from certain facts that these have
probably spread from some one island to the others. But we often take, I think,
an erroneous view of the probability of closely allied species invading each
other's territory, when put into free intercommunication. Undoubtedly if one
species has any advantage whatever over another, it will in a very brief time
wholly or in part supplant it; but if both are equally well fitted for their
own places in nature, both probably will hold their own places and keep
separate for almost any length of time. Being familiar with the fact that many
species, naturalised through man's agency, have spread with astonishing
rapidity over new countries, we are apt to infer that most species would thus
spread; but we should remember that the forms which become naturalised in new
countries are not generally closely allied to the aboriginal inhabitants, but
are very distinct species, belonging in a large proportion of cases, as shown
by Alph. de Candolle, to distinct genera. In the Galapagos Archipelago, many
even of the birds, though so well adapted for flying from island to island, are
distinct on each; thus there are three closely- allied species of
mocking-thrush, each confined to its own island. Now let us suppose the
mocking-thrush of Chatham Island to be blown to Charles Island, which has its
own mocking-thrush: why should it succeed in establishing itself there? We may
safely infer that Charles Island is well stocked with its own species, for
annually more eggs are laid there than can possibly be reared;, and we may
infer that the mocking-thrush peculiar to Charles Island is at least as well
fitted for its home as is the species peculiar to Chatham Island. Sir C. Lyell
and Mr Wollaston have communicated to me a remarkable fact bearing on this
subject; namely, that Madeira and the adjoining islet of Porto Santo possess
many distinct but representative land- shells, some of which live in crevices
of stone; and although large quantities of stone are annually transported from
porto Santo to Madeira, yet this latter island has not become colonised by the
Porto Santo species: nevertheless both islands have been colonised by some
European land- shells, which no doubt had some advantage over the indigenous
species. From these considerations I think we need not greatly marvel at the
endemic and representative species, which inhabit the several islands of the
Galapagos Archipelago, not having universally spread from island to island. In
many other instances, as in the several districts of the same continent, pre-
occupation has probably played an important part in checking the commingling of
species under the same conditions of life. Thus, the south-east and south-west
corners of Australia have nearly the same physical conditions, and are united
by continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants.
The principle which
determines the general character of the fauna and flora of oceanic islands,
namely, that the inhabitants, when not identically the same, yet are plainly
related to the inhabitants of that region whence colonists could most readily
have been derived, -- the colonists having been subsequently modified and
better fitted to their new homes, -- is of the widest application throughout
nature. We see this on every mountain, in every lake and marsh. For Alpine
species, excepting in so far as the same forms, chiefly of plants, have spread
widely throughout the world during the recent Glacial epoch, are related to
those of the surrounding lowlands; -- thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, &c., all of strictly American
forms, and it is obvious that a mountain, as it became slowly upheaved, would
naturally be colonised from the surrounding lowlands. So it is with the
inhabitants of lakes and marshes, excepting in so far as great facility of
transport has given the same general forms to the whole world. We see this same
principle in the blind animals inhabiting the caves of America and of Europe.
Other analogous facts could be given. And it will, I believe, be universally
found to be true, that wherever in two regions, let them be ever so distant,
many closely allied or representative species occur, there will likewise be
found some identical species, showing, in accordance with the foregoing view,
that at some former period there has been intercommunication or migration
between the two regions. And wherever many closely-allied species occur, there
will be found many forms which some naturalists rank as distinct species, and
some as varieties; these doubtful forms showing us the steps in
the process of
modification.
This relation between
the power and extent of migration of a species, either at the present time or
at some former period under different physical conditions, and the existence at
remote points of the world of other species allied to it, is shown in another
and more general way. Mr Gould remarked to me long ago, that in those genera of
birds which range over the world, many of the species have very wide ranges. I
can hardly doubt that this rule is generally true, though it would be difficult
to prove it. Amongst mammals, we see it strikingly displayed in Bats, and in a
lesser degree in the Felidae and Canidae. We see it, if we compare the
distribution of butterflies and beetles. So it is with most fresh-water
productions, in which so many genera range over the world, and many individual
species have enormous ranges. It is not meant that in world-ranging genera all
the species have a wide range, or even that they have on an average a wide
range; but only that some of the species range very widely; for the facility
with which widely-ranging species vary and give rise to new forms will largely
determine their average range. For instance, two varieties of the same species
inhabit America and Europe, and the species thus has an immense range; but, if
the variation had been a little greater, the two varieties would have been
ranked as distinct species, and the common range would have been greatly
reduced. Still less is it meant, that a species which apparently has the
capacity of crossing barriers and ranging widely, as in the case of certain
powerfully-winged birds, will necessarily range widely; for we should never
forget that to range widely implies not only the power of crossing barriers,
but the more important power of being victorious in distant lands in the
struggle for life with foreign associates. But on the view of all the species
of a genus having descended from a single parent, though now distributed to the
most remote points of the world, we ought to find, and I believe as a general
rule we do find, that some at least of the species range very widely; for it is
necessary that the unmodified parent should range widely, undergoing
modification during its diffusion, and should place itself under diverse
conditions favourable for the conversion of its offspring, firstly into new
varieties and ultimately into new species.
In considering the wide
distribution of certain genera, we should bear in mind that some are extremely
ancient, and must have branched off from a common parent at a remote epoch; so
that in such cases there will have been ample time for great climatal and
geographical changes and for accidents of transport; and consequently for the
migration of some of the species into all quarters of the world, where they may
have become slightly modified in relation to their new conditions. There is,
also, some reason to believe from geological evidence that organisms low in the
scale within each great class, generally change at a slower rate than the
higher forms; and consequently the lower forms will have had a better chance of
ranging widely and of still retaining the same specific character. This fact,
together with the seeds and eggs of many low forms being very minute and better
fitted for distant transportation, probably accounts for a law which has long
been observed, and which has lately been admirably discussed by Alph. de
Candolle in regard to plants, namely, that the lower any group of organisms is,
the more widely it is apt to range.
The relations just
discussed, -- namely, low and slowly- changing organisms ranging more widely
than the high, -- some of the species of widely-ranging genera themselves
ranging widely, - such facts, as alpine, lacustrine, and marsh productions
being related (with the exceptions before specified) to those on the
surrounding low lands and dry lands, though these stations are so different --
the very close relation of the distinct species which inhabit the islets of the
same archipelago, -- and especially the striking relation of the inhabitants of
each whole archipelago or island to those of the nearest mainland, -- are, I
think, utterly inexplicable on the ordinary view of the independent creation of
each species, but are explicable on the view of colonisation from the nearest
and readiest source, together with the subsequent modification and better
adaptation of the colonists to their new homes.
Summary of last and
present Chapters. In these chapters I have endeavoured to show, that if we make
due allowance for our ignorance of the full effects of all the changes of
climate and of the level of the land, which have certainly occurred within the
recent period, and of other similar changes which may have occurred within the
same period; if we remember how profoundly ignorant we are with respect to the
many and curious means of occasional transport, -- a subject which has hardly
ever been properly experimentised on; if we bear in mind how often a species
may have ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that all the
individuals of the same species, wherever located, have descended from the same
parents, are not insuperable. And we are led to this conclusion, which has been
arrived at by many naturalists under the designation of single centres of
creation, by some general considerations, more especially from the importance
of barriers and from the analogical distribution of sub-genera, genera, and
families.
With respect to the
distinct species of the same genus, which on my theory must have spread from
one parent-source; if we make the same allowances as before for our ignorance,
and remember that some forms of life change most slowly, enormous periods of
time being thus granted for their migration, I do not think that the
difficulties are insuperable; though they often are in this case, and in that
of the individuals of the same species, extremely grave.
As exemplifying the
effects of climatal changes on distribution, I have attempted to show how
important has been the influence of the modern Glacial period, which I am fully
convinced simultaneously affected the whole world, or at least great meridional
belts. As showing how diversified are the means of occasional transport, I have
discussed at some little length the means of dispersal of fresh-water
productions.
If the difficulties be
not insuperable in admitting that in the long course of time the individuals of
the same species, and likewise of allied species, have proceeded from some one
source; then I think all the grand leading facts of geographical distribution
are explicable on the theory of migration (generally of the more dominant forms
of life), together with subsequent modification and the multiplication of new
forms. We can thus understand the high importance of barriers, whether of land
or water, which separate our several zoological and botanical provinces. We can
thus understand the localisation of sub-genera, genera, and families; and how
it is that under different latitudes, for instance in South America, the
inhabitants of the plains and mountains, of the forests, marshes, and deserts,
are in so mysterious a manner linked together by affinity, and are likewise
linked to the extinct beings which formerly inhabited the same continent.
Bearing in mind that the mutual relations of organism to organism are of the
highest importance, we can see why two areas having nearly the same physical
conditions should often be inhabited by very different forms of life; for
according to the length of time which has elapsed since new inhabitants entered
one region; according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers; according
or not, as those which entered happened to come in more or less direct
competition with each other and with the aborigines; and according as the
immigrants were capable of varying more or less rapidly, there would ensue in
different regions, independently of their physical conditions, infinitely
diversified conditions of life, -- there would be an almost endless amount of
organic action and reaction, -- and we should find, as we do find, some groups
of beings greatly, and some only slightly modified, -- some developed in great
force, some existing in scanty numbers -- in the different great geographical
provinces of the world.
On these same
principles, we can understand, as I have endeavoured to show, why oceanic
islands should have few inhabitants, but of these a great number should be
endemic or peculiar; and why, in relation to the means of migration, one group
of beings, even within the same class, should have all its species endemic, and
another group should have all its species common to other quarters of the
world. We can see why whole groups of organisms, as batrachians and terrestrial
mammals, should be absent from oceanic islands, whilst the most isolated islands
possess their own peculiar species of a rial mammals or bats. We can see why
there should be some relation between the presence of mammals, in a more or
less modified condition, and the depth of the sea between an island and the
mainland. We can clearly see why all the inhabitants of an archipelago, though
specifically distinct on the several islets, should be closely related to each
other, and likewise be related, but less closely, to those of the nearest
continent or other source whence immigrants were probably derived. We can see
why in two areas, however distant from each other, there should be a
correlation, in the presence of identical species, of varieties, of doubtful
species, and of distinct but representative species.
As the late Edward Forbes
often insisted, there is a striking parallelism in the laws of life throughout
time and space: the laws governing the succession of forms in past times being
nearly the same with those governing at the present time the differences in
different areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule are so
few, that they may fairly be attributed to our not having as yet discovered in
an intermediate deposit the forms which are therein absent, but which occur
above and below: so in space, it certainly is the general rule that the area
inhabited by a single species, or by a group of species, is continuous; and the
exceptions, which are not rare, may, as I have attempted to show, be accounted
for by migration at some former period under different conditions or by
occasional means of transport, and by the species having become extinct in the
intermediate tracts. Both in time and space, species and groups of species have
their points of maximum development. Groups of species, belonging either to a
certain period of time, or to a certain area, are often characterised by
trifling characters in common, as of sculpture or colour. in looking to the
long succession of ages, as in now looking to distant provinces throughout the
world, we find that some organisms differ little, whilst others belonging to a
different class, or to a different order, or even only to a different family of
the same order, differ greatly. in both time and space the lower members of
each class generally change less than the higher; but there are in both cases
marked exceptions to the rule. On my theory these several relations throughout
time and space are intelligible; for whether we look to the forms of life which
have changed during successive ages within the same quarter of the world, or to
those which have changed after having migrated into distant quarters, in both
cases the forms within each class have been connected by the same bond of
ordinary generation; and the more nearly any two forms are related in blood,
the nearer they will generally stand to each other in time and space; in both
cases the laws of variation have been the same, and modifications have been
accumulated by the same power of natural selection.
CLASSIFICATION, groups subordinate to groups - Natural system - Rules
and difficulties in classification, explained on the theory of descent with
modification - Classification of varieties - Descent always used in
classification - Analogical or adaptive characters - Affinities, general,
complex and radiating - Extinction separates and defines groups - MORPHOLOGY,
between members of the same class, between parts of the same individual -
EMBRYOLOGY, laws of, explained by variations not supervening at an early age,
and being inherited at a corresponding age - RUDIMENTARY ORGANS; their origin
explained - Summary FROM the first
dawn of life, all organic beings are found to resemble each other in descending
degrees, so that they can be classed in groups under groups. This
classification is evidently not arbitrary like the grouping of the stars in
constellations. The existence of groups would have been of simple signification,
if one group had been exclusively fitted to inhabit the land, and another the
water; one to feed on flesh, another on vegetable matter, and so on; but the
case is widely different in nature; for it is notorious how commonly members of
even the same subgroup have different habits. In our second and fourth
chapters, on Variation and on Natural Selection, I have attempted to show that
it is the widely ranging, the much diffused and common, that is the dominant
species belonging to the larger genera, which vary most. The varieties, or
incipient species, thus produced ultimately become converted, as I believe,
into new and distinct species; and these, on the principle of inheritance, tend
to produce other new and dominant species. Consequently the groups which are
now large, and which generally include many dominant species, tend to go on
increasing indefinitely in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as different
places as possible in the economy of nature, there is a constant tendency in
their characters to diverge. This conclusion was supported by looking at the
great diversity of the forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in naturalisation.
I attempted also to
show that there is a constant tendency in the forms which are increasing in
number and diverging in character, to supplant and exterminate the less
divergent, the less improved, and preceding forms. I request the reader to turn
to the diagram illustrating the action, as formerly explained, of these several
principles; and he will see that the inevitable result is that the modified
descendants proceeding from one progenitor become broken up into groups subordinate
to groups. In the diagram each letter on the uppermost line may represent a
genus including several species; and all the genera on this line form together
one class, for all have descended from one ancient but unseen parent, and,
consequently, have inherited something in common. But the three genera on the
left hand have, on this same principle, much in common, and form a sub-family,
distinct from that including the next two genera on the right hand, which
diverged from a common parent at the fifth stage of descent. These five genera
have also much, though less, in common; and they form a family distinct from
that including the three genera still further to the right hand, which diverged
at a still earlier period. And all these genera, descended from (A), form an
order distinct from the genera descended from (I). So that we here have many
species descended from a single progenitor grouped into genera; and the genera
are included in, or subordinate to, sub-families, families, and orders, all
united into one class. Thus, the grand fact in natural history of the
subordination of group under group, which, from its familiarity, does not
always sufficiently strike us, is in my judgement fully explained.
Naturalists try to
arrange the species, genera, and families in each class, on what is called the
Natural System. But what is meant by this system? Some authors look at it
merely as a scheme for arranging together those living objects which are most
alike, and for separating those which are most unlike; or as an artificial
means for enunciating, as briefly as possible, general propositions, -- that
is, by one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those common to
the dog-genus, and then by adding a single sentence, a full description is
given of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant by the
Natural System; they believe that it reveals the plan of the Creator; but
unless it be specified whether order in time or space, or what else is meant by
the plan of the Creator, it seems to me that nothing is thus added to our
knowledge. Such expressions as that famous one of Linnaeus, and which we often
meet with in a more or less concealed form, that the characters do not make the
genus, but that the genus gives the characters, seem to imply that something
more is included in our classification, than mere resemblance. I believe that
something more is included; and that propinquity of descent, -- the only known
cause of the similarity of organic beings, -- is the bond, hidden as it is by
various degrees of modification, which is partially revealed to us by our
classifications.
Let us now consider the
rules followed in classification, and the difficulties which are encountered on
the view that classification either gives some unknown plan of creation, or is
simply a scheme for enunciating general propositions and of placing together
the forms most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined the habits of
life, and the general place of each being in the economy of nature, would be of
very high importance in classification. Nothing can be more false. No one
regards the external similarity of a mouse to a shrew, of a dugong to a whale,
of a whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
`adaptive or analogical characters;' but to the consideration of these
resemblances we shall have to recur. It may even be given as a general rule,
that the less any part of the organisation is concerned with special habits,
the more important it becomes for classification. As an instance: Owen, in
speaking of the dugong, says, `The generative organs being those which are most
remotely related to the habits and food of an animal, I have always regarded as
affording very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for an essential
character.' So with plants, how remarkable it is that the organs of vegetation,
on which their whole life depends, are of little signification, excepting in
the first main divisions; whereas the organs of reproduction, with their
product the seed, are of paramount importance !
We must not, therefore,
in classifying, trust to resemblances in parts of the organisation, however
important they may be for the welfare of the being in relation to the outer
world. Perhaps from this cause it has partly arisen, that almost all
naturalists lay the greatest stress on resemblances in organs of high vital or
physiological importance. No doubt this view of the classificatory importance
of organs which are important is generally, but by no means always, true. But
their importance for classification, I believe, depends on their greater
constancy throughout large groups of species; and this constancy depends on
such organs having generally been subjected to less change in the adaptation of
the species to their conditions of life. That the mere physiological importance
of an organ does not determine the classificatory value, is almost shown by the
one fact, that in allied groups, in which the same organ, as we have every
reason to suppose, has nearly the same physiological value, its classificatory
value is widely different. No naturalist can have worked at any group without
being struck with this fact; and it has been most fully acknowledged in the
writings of almost every author. It will suffice to quote the highest
authority, Robert Brown, who in speaking of certain organs in the Proteaceae,
says their generic importance, `like that of all their parts, not only in this
but, as I apprehend, in every natural family, is very unequal, and in some
cases seems to be entirely lost.' Again in another work he says, the genera of
the Connaraceae `differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular aestivation. Any one of these
characters singly is frequently of more than generic importance, though here
even when all taken together they appear insufficient to separate Cnestis from
Connarus.' To give an example amongst insects, in one great division of the
Hymenoptera, the antennae, as Westwood has remarked, are most constant in
structure; in another division they differ much, and the differences are of
quite subordinate value in classification; yet no one probably will say that
the antennae in these two divisions of the same order are of unequal
physiological importance. Any number of instances could be given of the varying
importance for classification of the same important organ within the same group
of beings.
Again, no one will say
that rudimentary or atrophied organs are of high physiological or vital
importance; yet, undoubtedly, organs in this condition are often of high value
in classification. No one will dispute that the rudimentary teeth in the upper
jaws of young ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and pachyderms.
Robert Brown has strongly insisted on the fact that the rudimentary florets are
of the highest importance in the classification of the Grasses.
Numerous instances
could be given of characters derived from parts which must be considered of
very trifling physiological importance, but which are universally admitted as
highly serviceable in the definition of whole groups. For instance, whether or
not there is an open passage from the nostrils to the mouth, the only
character, according to Owen, which absolutely distinguishes fishes and
reptiles -- the inflection of the angle of the jaws in Marsupials -- the manner
in which the wings of insects are folded -- mere colour in certain Algae --
mere pubescence on parts of the flower in grasses -- the nature of the dermal
covering, as hair or feathers, in the Vertebrata. If the Ornithorhynchus had
been covered with feathers instead of hair, this external and trifling
character would, I think, have been considered by naturalists as important an
aid in determining the degree of affinity of this strange creature to birds and
reptiles, as an approach in structure in any one internal and important organ.
The importance, for
classification, of trifling characters, mainly depends on their being
correlated with several other characters of more or less importance. The value
indeed of an aggregate of characters is very evident in natural history. Hence,
as has often been remarked, a species may depart from its allies in several
characters, both of high physiological importance and of almost universal
prevalence, and yet leave us in no doubt where it should be ranked. Hence,
also, it has been found, that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is universally constant. The importance of an aggregate of
characters, even when none are important, alone explains, I think, that saying
of Linnaeus, that the characters do not give the genus, but the genus gives the
characters; for this saying seems founded on an appreciation of many trifling
points of resemblance, too slight to be defined. Certain plants, belonging to
the Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, `the greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus laugh
at our classification.' But when Aspicarpa produced in France, during several
years, only degraded flowers, departing so wonderfully in a number of the most
important points of structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus should still be retained
amongst the Malpighiaceae. This case seems to me well to illustrate the spirit
with which our classifications are sometimes necessarily founded.
Practically when
naturalists are at work, they do not trouble themselves about the physiological
value of the characters which they use in defining a group, or in allocating
any particular species. If they find a character nearly uniform, and common to
a great number of forms, and not common to others, they use it as one of high
value; if common to some lesser number, they use it as of subordinate value.
This principle has been broadly confessed by some naturalists to be the true
one; and by none more clearly than by that excellent botanist, Aug. St Hilaire.
If certain characters are always found correlated with others, though no
apparent bond of connexion can be discovered between them, especial value is
set on them. As in most groups of animals, important organs, such as those for
propelling the blood, or for a rating it, or those for propagating the race,
are found nearly uniform, they are considered as highly serviceable in
classification; but in some groups of animals all these, the most important
vital organs, are found to offer characters of quite subordinate value.
We can see why
characters derived from the embryo should be of equal importance with those
derived from the adult, for our classifications of course include all ages of
each species. But it is by no means obvious, on the ordinary view, why the
structure of the embryo should be more important for this purpose than that of
the adult, which alone plays its full part in the economy Of nature. Yet it has
been strongly urged by those great naturalists, Milne Edwards and Agassiz, that
embryonic characters are the most important of any in the classification of
animals; and this doctrine has very generally been admitted as true. The same
fact holds good with flowering plants, of which the two main divisions have
been founded on characters derived from the embryo, -- on the number and
position of the embryonic leaves or cotyledons, and on the mode of development
of the plumule and radicle. In our discussion on embryology, we shall see why
such characters are so valuable, on the view of classification tacitly including
the idea of descent.
Our classifications are
often plainly influenced by chains of affinities. Nothing can be easier than to
define a number of characters common to all birds; but in the case of
crustaceans, such definition has hitherto been found impossible. There are
crustaceans at the opposite ends of the series, which have hardly a character
in common; yet the species at both ends, from being plainly allied to others,
and these to others, and so onwards, can be recognised as unequivocally belonging
to this, and to no other class of the Articulata.
Geographical
distribution has often been used, though perhaps not quite logically, in
classification, more especially in very large groups of closely allied forms.
Temminck insists on the utility or even necessity of this practice in certain
groups of birds; and it has been followed by several entomologists and
botanists.
Finally, with respect
to the comparative value of the various groups of species, such as orders,
sub-orders, families, subfamilies, and genera, they seem to be, at least at
present, almost arbitrary. Several of the best botanists, such as Mr Bentham
and others, have strongly insisted on their arbitrary value. Instances could be
given amongst plants and insects, of a group of forms, first ranked by
practised naturalists as only a genus, and then raised to the rank of a
sub-family or family; and this has been done, not because further research has
detected important structural differences, at first overlooked, but because
numerous allied species, with slightly different grades of difference, have
been subsequently discovered.
All the foregoing rules
and aids and difficulties in classification are explained, if I do not greatly
deceive myself, on the view that the natural system is founded on descent with
modification;, that the characters which naturalists consider as showing true
affinity between any two or more species, are those which have been inherited
from a common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike.
But I must explain my
meaning more fully. I believe that the arrangement of the groups within each
class, in due subordination and relation to the other groups, must be strictly
genealogical in order to be natural; but that the amount of difference in the several
branches or groups, though allied in the same degree in blood to their common
progenitor, may differ greatly, being due to the different degrees of
modification which they have undergone; and this is expressed by the forms
being ranked under different genera, families, sections, or orders. The reader
will best understand what is meant, if he will take the trouble to referring to
the diagram in the fourth chapter. We will suppose the letters A to L to
represent allied genera, which lived during the Silurian epoch, and these have
descended from a species which existed at an unknown anterior period. Species
of three of these genera (A, F, and I) have transmitted modified descendants to
the present day, represented by the fifteen genera (a[s14]s to z[s14]s) on the
uppermost horizontal line. Now all these modified descendants from a single
species, are represented as related in blood or descent to the same degree;
they may metaphorically be called cousins to the same millionth degree; yet
they differ widely and in different degrees from each other. The forms
descended from A, now broken up into two or three families, constitute a
distinct order from those descended from I, also broken up into two families.
Nor can the existing species, descended from A, be ranked in the same genus
with the parent A; or those from I, with the parent I. But the existing genus
F[s14]s may be supposed to have been but slightly modified; and it will then
rank with the parent-genus F; just as some few still living organic beings belong
to Silurian genera. So that the amount or value of the differences between
organic beings all related to each other in the same degree in blood, has come
to be widely different. Nevertheless their genealogical arrangement remains
strictly true, not only at the present time, but at each successive period of
descent. All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I; so will it
be with each subordinate branch of descendants, at each successive period. If,
however, we choose to suppose that any of the descendants of A or of I have
been so much modified as to have more or less completely lost traces of their
parentage, in this case, their places in a natural classification will have
been more or less completely lost, -- as sometimes seems to have occurred with
existing organisms. All the descendants of the genus F, along its whole line of
descent, are supposed to have been but little modified, and they yet form a
single genus. But this genus, though much isolated, will still occupy its
proper intermediate position; for F originally was intermediate in character
between A and I, and the several genera descended from these two genera will
have inherited to a certain extent their characters. This natural arrangement
is shown, as far as is possible on paper, in the diagram, but in much too
simple a manner. If a branching diagram had not been used, and only the names
of the groups had been written in a linear series, it would have been still
less possible to have given a natural arrangement; and it is notoriously not
possible to represent in a series, on a flat surface, the affinities which we
discover in nature amongst the beings of the same group. Thus, on the view
which I hold, the natural system is genealogical in its arrangement, like a
pedigree; but the degrees of modification which the different groups have
undergone, have to be expressed by ranking them under different so-called
genera, sub-families, families, sections, orders, and classes.
It may be worth while
to illustrate this view of classification, by taking the case of languages. If
we possessed a perfect pedigree of mankind, a genealogical arrangement of the
races of man would afford the best classification of the various languages now
spoken throughout the world; and if all extinct languages, and all intermediate
and slowly changing dialects, had to be included, such an arrangement would, I
think, be the only possible one. Yet it might be that some very ancient language
had altered little, and had given rise to few new languages, whilst others
(owing to the spreading and subsequent isolation and states of civilisation of
the several races, descended from a common race) had altered much, and had
given rise to many new languages and dialects. The various degrees of
difference in the languages from the same stock, would have to be expressed by
groups subordinate to groups; but the proper or even only possible arrangement
would still be genealogical; and this would be strictly natural, as it would
connect together all languages, extinct and modern, by the closest affinities,
and would give the filiation and origin of each tongue.
In confirmation of this
view, let us glance at the classification of varieties, which are believed or
known to have descended from one species. These are grouped under species, with
sub-varieties under varieties; and with our domestic productions, several other
grades of difference are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same with varieties as
with species, namely, closeness of descent with various degrees of
modification. Nearly the same rules are followed in classifying varieties, as
with species. Authors have insisted on the necessity of classing varieties on a
natural instead of an artificial system; we are cautioned, for instance, not to
class two varieties of the pine-apple together, merely because their fruit,
though the most important part, happens to be nearly identical; no one puts the
swedish and common turnips together, though the esculent and thickened stems
are so similar. Whatever part is found to be most constant, is used in classing
varieties: thus the great agriculturist Marshall says the horns are very useful
for this purpose with cattle, because they are less variable than the shape or
colour of the body, &c.; whereas with sheep the horns are much less
serviceable, because less constant. In classing varieties, I apprehend if we
had a real pedigree, a genealogical classification would be universally
preferred; and it has been attempted by some authors. For we might feel sure,
whether there had been more or less modification, the principle of inheritance
would keep the forms together which were allied in the greatest number of
points. In tumbler pigeons, though some sub-varieties differ from the others in
the important character of having a longer beak, yet all are kept together from
having the common habit of tumbling; but the short-faced breed has nearly or quite
lost this habit; nevertheless, without any reasoning or thinking on the
subject, these tumblers are kept in the same group, because allied in blood and
alike in some other respects. If it could be proved that the Hottentot had
descended from the Negro, I think he would be classed under the Negro group,
however much he might differ in colour and other important characters from
negroes.
With species in a state
of nature, every naturalist has in fact brought descent into his
classification; for he includes in his lowest grade, or that of a species, the
two sexes; and how enormously these sometimes differ in the most important
characters, is known to every naturalist: scarcely a single fact can be
predicated in common of the males and hermaphrodites of certain cirripedes,
when adult, and yet no one dreams of separating them. The naturalist includes
as one species the several larval stages of the same individual, however much
they may differ from each other and from the adult; as he likewise includes the
so-called alternate generations of Steenstrup, which can only in a technical
sense be considered as the same individual. He includes monsters; he includes
varieties, not solely because they closely resemble the parent-form, but
because they are descended from it. He who believes that the cowslip is
descended from the primrose, or conversely, ranks them together as a single
species, and gives a single definition. As soon as three Orchidean forms
(Monochanthus, Myanthus, and Catasetum), which had previously been ranked as
three distinct genera, were known to be sometimes produced on the same spike,
they were immediately included as a single species. But it may be asked, what
ought we to do, if it could be proved that one species of kangaroo had been
produced, by a long course of modification, from a bear? Ought we to rank this
one species with bears, and what should we do with the other species? The
supposition is of course preposterous; and I might answer by the argumentum ad
hominem, and ask what should be done if a perfect kangaroo were seen to come
out of the womb of a bear? According to all analogy, it would be ranked with
bears; but then assuredly all the other species of the kangaroo family would
have to be classed under the bear genus. The whole case is preposterous; for
where there has been close descent in common, there will certainly be close
resemblance or affinity.
As descent has
universally been used in classing together the individuals of the same species,
though the males and females and larvae are sometimes extremely different; and
as it has been used in classing varieties which have undergone a certain, and
sometimes a considerable amount of modification, may not this same element of
descent have been unconsciously used in grouping species under genera, and
genera under higher groups, though in these cases the modification has been
greater in degree, and has taken a longer time to complete? I believe it has
thus been unconsciously used; and only thus can I understand the several rules
and guides which have been followed by our best systematists. We have no
written pedigrees; we have to make out community of descent by resemblances of
any kind. Therefore we choose those characters which, as far as we can judge,
are the least likely to have been modified in relation to the conditions of
life to which each species has been recently exposed. Rudimentary structures on
this view are as good as, or even sometimes better than, other parts of the
organisation. We care not how trifling a character may be -- let it be the mere
inflection of the angle of the jaw, the manner in which an insect's wing is
folded, whether the skin be covered by hair or feathers -- if it prevail
throughout many and different species, especially those having very different
habits of life, it assumes high value; for we can account for its presence in
so many forms with such different habits, only by its inheritance from a common
parent. We may err in this respect in regard to single points of structure, but
when several characters, let them be ever so trifling, occur together
throughout a large group of beings having different habits, we may feel almost
sure, on the theory of descent, that these characters have been inherited from
a common ancestor. And we know that such correlated Or aggregated characters
have especial value in classification.
We can understand why a
species or a group of species may depart, in several of its most important
characteristics, from its allies, and yet be safely classed with them. This may
be safely done, and is often done, as long as a sufficient number of
characters, let them be ever so unimportant, betrays the hidden bond of
community of descent. Let two forms have not a single character in common, yet
if these extreme forms are connected together by a chain of intermediate
groups, we may at once infer their community of descent, and we put them all
into the same class. As we find organs of high physiological importance --
those which serve to preserve life under the most diverse conditions of
existence -- are generally the most constant, we attach especial value to them;
but if these same organs, in another group or section of a group, are found to
differ much, we at once value them less in our classification. We shall
hereafter, I think, clearly see why embryological characters are of such high
classificatory importance. Geographical distribution may sometimes be brought
usefully into play in classing large and widely-distributed genera, because all
the species of the same genus, inhabiting any distinct and isolated region,
have in all probability descended from the same parents.
We can understand, on
these views, the very important distinction between real affinities and
analogical or adaptive resemblances. Lamarck first called attention to this
distinction, and he has been ably followed by Macleay and others. The
resemblance, in the shape of the body and in the fin-like anterior limbs,
between the dugong, which is a pachydermatous animal, and the whale, and
between both these mammals and fishes, is analogical. Amongst insects there are
innumerable instances: thus Linnaeus, misled by external appearances, actually
classed an homopterous insect as a moth. We see something of the same kind even
in our domestic varieties, as in the thickened stems of the common and swedish
turnip. The resemblance of the greyhound and racehorse is hardly more fanciful
than the analogies which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why analogical
or adaptive character, although of the utmost importance to the welfare of the
being, are almost valueless to the systematist. For animals, belonging to two
most distinct lines of descent, may readily become adapted to similar
conditions, and thus assume a close external resemblance; but such resemblances
will not reveal -- will rather tend to conceal their blood-relationship to
their proper lines of descent. We can also understand the apparent paradox,
that the very same characters are analogical when one class or order is
compared with another, but give true affinities when the members of the same
class or order are compared one with another: thus the shape of the body and
fin-like limbs are only analogical when whales are compared with fishes, being
adaptations in both classes for swimming through the water; but the shape of
the body and fin-like limbs serve as characters exhibiting true affinity
between the several members of the whale family; for these cetaceans agree in
so many characters, great and small, that we cannot doubt that they have
inherited their general shape of body and structure of limbs from a common ancestor.
So it is with fishes.
As members of distinct
classes have often been adapted by successive slight modifications to live
under nearly similar circumstances, -- to inhabit for instance the three
elements of land, air, and water, -- we can perhaps understand how it is that a
numerical parallelism has sometimes been observed between the sub-groups in
distinct classes. A naturalist, struck by a parallelism of this nature in any
one class, by arbitrarily raising or sinking the value of the groups in other classes
(and all our experience shows that this valuation -has hitherto been
arbitrary), could easily extend the parallelism over a wide range; and thus the
septenary, quinary, quaternary, and ternary classifications have probably
arisen.
As the modified
descendants of dominant species, belonging to the larger genera, tend to
inherit the advantages, which made the groups to which they belong large and
their parents dominant, they are almost sure to spread widely, and to seize on
more and more places in the economy of nature. The larger and more dominant
groups thus tend to go on increasing in size; and they consequently supplant
many smaller and feebler groups. Thus we can account for the fact that all
organisms, recent and extinct, are included under a few great orders, under
still fewer classes, and all in one great natural system. As showing how few
the higher groups are in number, and how widely spread they are throughout the
world, the fact is striking, that the discovery of Australia has not added a single
insect belonging to a new order; and that in the vegetable kingdom, as I learn
from Dr Hooker, it has added only two or three orders of small size.
In the chapter on
geological succession I attempted to show, on the principle of each group
having generally diverged much in character during the long-continued process
of modification, how it is that the more ancient forms of life often present
characters in some slight degree intermediate between existing groups. A few
old and intermediate parent-forms having occasionally transmitted to the
present day descendants but little modified, will give to us our so-called
osculant or aberrant groups. The more aberrant any form is, the greater must be
the number of connecting forms which on my theory have been exterminated and
utterly lost. And we have some evidence of aberrant forms having suffered
severely from extinction, for they are generally represented by extremely few
species; and such species as do occur are generally very distinct from each
other, which again implies extinction. The genera Ornithorhynchus and
Lepidosiren, for example, would not have been less aberrant had each been
represented by a dozen species instead of by a single one; but such richness in
species, as I find after some investigation, does not commonly fall to the lot
of aberrant genera. We can, I think, account for this fact only by looking at
aberrant forms as failing groups conquered by more successful competitors, with
a few members preserved by some unusual coincidence of favourable
circumstances.
Mr Waterhouse has
remarked that, when a member belonging to one group of animals exhibits an
affinity to a quite distinct group, this affinity in most cases is general and
not special: thus, according to Mr Waterhouse, of all Rodents, the bizcacha is
most nearly related to Marsupials; but in the points in which it approaches
this order, its relations are general, and not to any one marsupial species
more than to another. As the points of affinity of the bizcacha to Marsupials
are believed to be real and not merely adaptive, they are due on my theory to
inheritance in common. Therefore we must suppose either that all Rodents,
including the bizcacha, branched off from some very ancient Marsupial, which
will have had a character in some degree intermediate with respect to all
existing Marsupials; or that both Rodents and Marsupials branched off from a
common progenitor, and that both groups have since undergone much modification
in divergent directions. On either view we may suppose that the bizcacha has
retained, by inheritance, more of the character of its ancient progenitor than
have other Rodents; and therefore it will not be specially related to any one
existing Marsupial, but indirectly to all or nearly all Marsupials, from having
partially retained the character of their common progenitor, or of an early
member of the group. On the other hand, of all Marsupials, as Mr Waterhouse has
remarked, the phascolomys resembles most nearly, not any one species, but the
general order of Rodents. In this case, however, it may be strongly suspected
that the resemblance is only analogical, owing to the phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made nearly
similar observations on the general nature of the affinities of distinct orders
of plants.
On the principle of the
multiplication and gradual divergence in character of the species descended
from a common parent, together with their retention by inheritance of some
characters in common, we can understand the excessively complex and radiating
affinities by which all the members of the same family or higher group are
connected together. For the common parent of a whole family of species, now
broken up by extinction into distinct groups and sub-groups, will have
transmitted some of its characters, modified in various ways and degrees, to
all; and the several species will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the diagram
so often referred to), mounting up through many predecessors. As it is
difficult to show the blood-relationship between the numerous kindred of any
ancient and noble family, even by the aid of a genealogical tree, and almost
impossible to do this without this aid, we can understand the extraordinary
difficulty which naturalists have experienced in describing, without the aid of
a diagram, the various affinities which they perceive between the many living
and extinct members of the same great natural class.
Extinction, as we have
seen in the fourth chapter, has played an important part in defining and
widening the intervals between the several groups in each class. We may thus
account even for the distinctness of whole classes from each other -- for
instance, of birds from all other vertebrate animals - by the belief that many
ancient forms of life have been utterly lost, through which the early
progenitors of birds were formerly connected with the early progenitors of the
other vertebrate classes. There has been less entire extinction of the forms of
life which once connected fishes with batrachians. There has been still less in
some other classes, as in that of the Crustacea, for here the most wonderfully
diverse forms are still tied together by a long, but broken, chain of
affinities. Extinction has only separated groups: it has by no means made them;
for if every form which has ever lived on this earth were suddenly to reappear,
though it would be quite impossible to give definitions by which each group
could be distinguished from other groups, as all would blend together by steps
as fine as those between the finest existing varieties, nevertheless a natural
classification, or at least a natural arrangement, would be possible. We shall
see this by turning to the diagram: the letters, A to L, may represent eleven
Silurian genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and their
primordial parent, and every intermediate link in each branch and sub-branch of
their descendants, may be supposed to be still alive; and the links to be as
fine as those between the finest varieties. In this case it would be quite
impossible to give any definition by which the several members of the several
groups could be distinguished from their more immediate parents; or these
parents from their ancient and unknown progenitor. Yet the natural arrangement
in the diagram would still hold good; and, on the principle of inheritance, all
the forms descended from A, or from I, would have something in common. In a
tree we can specify this or that branch, though at the actual fork the two
unite and blend together. We could not, as I have said, define the several
groups; but we could pick out types, or forms, representing most of the
characters of each group, whether large or small, and thus give a general idea
of the value of the differences between them. This is what we should be driven
to, if we were ever to succeed in collecting all the forms in any class which
have lived throughout all time and space. We shall certainly never succeed in
making so perfect a collection: nevertheless, in certain classes, we are
tending in this direction; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
Finally, we have seen
that natural selection, which results from the struggle for existence, and
which almost inevitably induces extinction and divergence of character in the
many descendants from one dominant parent-species, explains that great and
universal feature in the affinities of all organic beings, namely, their
subordination in group under group. We use the element of descent in classing
the individuals of both sexes and of all ages, although having few characters
in common, under one species; we use descent in classing acknowledged
varieties, however different they may be from their parent; and I believe this
element of descent is the hidden bond of connexion which naturalists have
sought under the term of the Natural System. On this idea of the natural system
being, in so far as it has been perfected, genealogical in its arrangement,
with the grades of difference between the descendants from a common parent,
expressed by the terms genera, families, orders, &c., we can understand the
rules which we are compelled to follow in our classification. We can understand
why we value certain resemblances far more than others; why we are permitted to
use rudimentary and useless organs, or others of trifling physiological
importance; why, in comparing one group with a distinct group, we summarily
reject analogical or adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see how it is that all
living and extinct forms can be grouped together in one great system; and how
the several members of each class are connected together by the most complex
and radiating lines of affinities. We shall never, probably, disentangle the inextricable
web of affinities between the members of any one class; but when we have a
distinct object in view, and do not look to some unknown plan of creation, we
may hope to make sure but slow progress.
Morphology . We have
seen that the members of the same class, independently of their habits of life,
resemble each other in the general plan of their organisation. This resemblance
is often expressed by the term `unity of type;' or by saying that the several
parts and organs in the different species of the class are homologous. The
whole subject is included under the general name of Morphology. This is the
most interesting department of natural history, and may be said to be its very
soul. What can be more curious than that the hand of a man, formed for
grasping, that of a mole for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be constructed on the same
pattern, and should include the same bones, in the same relative positions?
Geoffroy St Hilaire has insisted strongly on the high importance of relative
connexion in homologous organs: the parts may change to almost any extent in
form and size, and yet they always remain connected together in the same order.
We never find, for instance, the bones of the arm and fore, arm, or of the
thigh and leg, transposed. Hence the same names can be given to the homologous
bones in widely different animals. We see the same great law in the
construction of the mouths of insect: what can be more different than the
immensely long spiral proboscis of a sphinx-moth, the curious folded one of a
bee or bug, and the great jaws of a beetle? -- yet all these organs, serving
for such different purposes, are formed by infinitely numerous modifications of
an upper lip, mandibles, and two pairs of maxillae. Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the flowers
of plants.
Nothing can be more
hopeless than to attempt to explain this similarity of pattern in members of
the same class, by utility or by the doctrine of final causes. The hopelessness
of the attempt has been expressly admitted by Owen in his most interesting work
on the `Nature of Limbs.' On the ordinary view of the independent creation of
each being, we can only say that so it is; - that it has so pleased the Creator
to construct each animal and plant.
The explanation is
manifest on the theory of the natural selection of successive slight
modifications, -- each modification being profitable in some way to the
modified form, but often affecting by correlation of growth other parts of the
organisation. In changes of this nature, there will be little or no tendency to
modify the original pattern, or to transpose parts. The bones of a limb might
be shortened and widened to any extent, and become gradually enveloped in thick
membrane, so as to serve as a fin; or a webbed foot might have all its bones,
or certain bones, lengthened to any extent, and the membrane connecting them
increased to any extent, so as to serve as a wing: yet in all this great amount
of modification there will be no tendency to alter the framework of bones or
the relative connexion of the several parts. If we suppose that the ancient
progenitor, the archetype as it may be called, of all mammals, had its limbs constructed
on the existing general pattern, for whatever purpose they served, we can at
once perceive the plain signification of the homologous construction of the
limbs throughout the whole class. So with the mouths of insects, we have only
to suppose that their common progenitor had an upper lip, mandibles, and two
pair of maxillae, these parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in structure and
function of the mouths of insects. Nevertheless, it is conceivable that the
general pattern of an organ might become so much obscured as to be finally
lost, by the atrophy and ultimately by the complete abortion of certain parts,
by the soldering together of other parts, and by the doubling or multiplication
of others, -- variations which we know to be within the limits of possibility.
In the paddles of the extinct gigantic sea- lizards, and in the mouths of
certain suctorial crustaceans, the general pattern seems to have been thus to a
certain extent obscured.
There is another and
equally curious branch of the present subject; namely, the comparison not of
the same part in different members of a class, but of the different parts or
organs in the same individual. Most physiologists believe that the bones of the
skull are homologous with -- that is correspond in number and in relative
connexion with -- the elemental parts of a certain number of vertebrae. The
anterior and posterior limbs in each member of the vertebrate and articulate
classes are plainly homologous. We see the same law in comparing the
wonderfully complex jaws and legs in crustaceans. It is familiar to almost
every one, that in a flower the relative position of the sepals, petals,
stamens, and pistils, as well as their intimate structure, are intelligible in
the view that they consist of metamorphosed leaves, arranged in a spire. In
monstrous plants, we often get direct evidence of the possibility of one organ
being transformed into another; and we can actually see in embryonic crustaceans
and in many other animals, and in flowers, that organs which when mature become
extremely different, are at an early stage of growth exactly alike.
How inexplicable are
these facts on the ordinary view of creation ! Why should the brain be enclosed
in a box composed of such numerous and such extraordinarily shaped pieces of
bone? As Owen has remarked, the benefit derived from the yielding of the
separate pieces in the act of parturition of mammals, will by no means explain
the same construction in the skulls of birds. Why should similar bones have
been created in the formation of the wing and leg of a bat, used as they are
for such totally different purposes? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have fewer
legs; or conversely, those with many legs have simpler mouths? Why should the
sepals, petals, stamens, and pistils in any individual flower, though fitted
for such widely different purposes, be all constructed on the same pattern ?
On the theory of
natural selection, we can satisfactorily answer these questions. In the
vertebrata, we see a series of internal vertebrae bearing certain processes and
appendages; in the articulata, we see the body divided into a series of
segments, bearing external appendages; and in flowering plants, we see a series
of successive spiral whorls of leaves. An indefinite repetition of the same
part or organ is the common characteristic (as Owen has observed) of all low or
little-modified forms; therefore we may readily believe that the unknown
progenitor of the vertebrata possessed many vertebrae; the unknown progenitor
of the articulata, many segments; and the unknown progenitor of flowering
plants, many spiral whorls of leaves. We have formerly seen that parts many
times repeated are eminently liable to vary in number and structure;
consequently it is quite probable that natural selection, during a long-
continued course of modification, should have seized on a certain number of the
primordially similar elements, many times repeated, and have adapted them to
the most diverse purposes. And as the whole amount of modification will have
been effected by slight successive steps, we need not wonder at discovering in
such parts or organs, a certain degree of fundamental resemblance, retained by
the strong principle of inheritance.
In the great class of
molluscs, though we can homologise the parts of one species with those of
another and distinct species, we can indicate but few serial homologies; that
is, we are seldom enabled to say that one part or organ is homologous with
another in the same individual. And we can understand this fact; for in
molluscs, even in the lowest members of the class, we do not find nearly so
much indefinite repetition of any one part, as we find in the other great
classes of the animal and vegetable kingdoms.
Naturalists frequently
speak of the skull as formed of metamorphosed vertebrae: the jaws of crabs as
metamorphosed legs; the stamens and pistils of flowers as metamorphosed leaves;
but it would in these cases probably be more correct, as professor Huxley has
remarked, to speak of both skull and vertebrae, both jaws and legs, &c., --
as having been metamorphosed, not one from the other, but from some common
element. Naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind -- vertebrae in the one case and legs in the other -- have
actually been modified into skulls or jaws. Yet so strong is the appearance of
a modification of this nature having occurred, that naturalists can hardly
avoid employing language having this plain signification. On my view these
terms may be used literally; and the wonderful fact of the jaws, for instance,
of a crab retaining numerous characters, which they would probably have
retained through inheritance, if they had really been metamorphosed during a
long course of descent from true legs, or from some simple appendage, is
explained.
Embryology. It has
already been casually remarked that certain organs in the individual, which
when mature become widely different and serve for different purposes, are in
the embryo exactly alike. The embryos, also, of distinct animals within the
same class are often strikingly similar: a better proof of this cannot be
given, than a circumstance mentioned by Agassiz, namely, that having forgotten
to ticket the embryo of some vertebrate animal, he cannot now tell whether it
be that of a mammal, bird, or reptile. The vermiform larvae of moths, flies,
beetles, &c., resemble each other much more closely than do the mature
insects; but in the case of larvae, the embryos are active, and have been
adapted for special lines of life. A trace of the law of embryonic resemblance,
sometimes lasts till a rather late age: thus birds of the same genus, and of
closely allied genera, often resemble each other in their first and second
plumage; as we see in the spotted feathers in the thrush group. In the cat
tribe, most of the species are striped or spotted in lines; and stripes can be
plainly distinguished in the whelp of the lion. We occasionally though rarely
see something of this kind in plants: thus the embryonic leaves of the ulex or
furze, and the first leaves of the phyllodineous acaceas, are pinnate or
divided like the ordinary leaves of the leguminosae.
The points of
structure, in which the embryos of widely different animals of the same class
resemble each other, often have no direct relation to their conditions of
existence. We cannot, for instance, suppose that in the embryos of the
vertebrata the peculiar loop-like course of the arteries near the branchial
slits are related to similar conditions, -- in the young mammal which is
nourished in the womb of its mother, in the egg of the bird which is hatched in
a nest, and in the spawn of a frog under water. We have no more reason to
believe in such a relation, than we have to believe that the same bones in the
hand of a man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one will suppose that the stripes on the whelp of a
lion, or the spots on the young blackbird, are of any use to these animals, or
are related to the conditions to which they are exposed.
The case, however, is
different when an animal during any part of its embryonic career is active, and
has to provide for itself. The period of activity may come on earlier or later
in life; but whenever it comes on, the adaptation of the larva to its
conditions of life is just as perfect and as beautiful as in the adult animal.
from such special adaptations, the similarity of the larvae or active embryos
of allied animals is sometimes much obscured; and cases could be given of the
larvae of two species, or of two groups of species, differing quite as much, or
even more, from each other than do their adult parents. In most cases, however,
the larvae, though active, still obey more or less closely the law of common
embryonic resemblance. Cirripedes afford a good instance of this: even the
illustrious Cuvier did not perceive that a barnacle was, as it certainly is, a
crustacean; but a glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, which differ widely in external appearance, have
larvae in all their several stages barely distinguishable.
The embryo in the
course of development generally rises in organisation: I use this expression,
though I am aware that it is hardly possible to define clearly what is meant by
the organisation being higher or lower. But no one probably will dispute that
the butterfly is higher than the caterpillar. In some cases, however, the
mature animal is generally considered as lower in the scale than the larva, as
with certain parasitic crustaceans. To refer once again to cirripedes: the
larvae in the first stage have three pairs of legs, a very simple single eye,
and a probosciformed mouth, with which they feed largely, for they increase
much in size. In the second stage, answering to the chrysalis stage of
butterflies, they have six pairs of beautifully constructed natatory legs, a
pair of magnificent compound eyes, and extremely complex antennae; but they
have a closed and imperfect mouth, and cannot feed: their function at this
stage is, to search by their well-developed organs of sense, and to reach by
their active powers of swimming, a proper place on which to become attached and
to undergo their final metamorphosis. When this is completed they are fixed for
life: their legs are now converted into prehensile organs; they again obtain a
well- constructed mouth; but they have no antennae, and their two eyes are now
reconverted into a minute, single, and very simple eye-spot. In this last and
complete state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some genera the
larvae become developed either into hermaphrodites having the ordinary
structure, or into what I have called complemental males: and in the latter,
the development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time, and is destitute of mouth, stomach, or other
organ of importance, excepting for reproduction.
We are so much
accustomed to see differences in structure between the embryo and the adult,
and likewise a close similarity in the embryos of widely different animals
within the same class, that we might be led to look at these facts as
necessarily contingent in some manner on growth. But there is no obvious reason
why, for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all the parts in proper proportion, as soon as any
structure became visible in the embryo. And in some whole groups of animals and
in certain members of other groups, the embryo does not at any period differ
widely from the adult: thus Owen has remarked in regard to cuttle-fish, `there
is no metamorphosis; the cephalopodic character is manifested long before the
parts of the embryo are completed;' and again in spiders, `there is nothing
worthy to be called a metamorphosis.' The larvae of insects, whether adapted to
the most diverse and active habits, or quite inactive, being fed by their
parents or placed in the midst of proper nutriment, yet nearly all pass through
a similar wormlike stage of development; but in some few cases, as in that of
Aphis, if we look to the admirable drawings by professor Huxley of the
development of this insect, we see no trace of the vermiform stage.
How, then, can we
explain these several facts in embryology, -- namely the very general, but not
universal difference in structure between the embryo and the adult; -- of parts
in the same individual embryo, which ultimately become very unlike and serve
for diverse purposes, being at this early period of growth alike; -- of embryos
of different species within the same class, generally, but not universally,
resembling each other; -- of the structure of the embryo not being closely
related to its conditions of existence, except when the embryo becomes at any
period of life active and has to provide for itself; -- of the embryo
apparently having sometimes a higher organisation than the mature animal, into
which it is developed. I believe that all these facts can be explained, as
follows, on the view of descent with modification.
It is commonly assumed,
perhaps from monstrosities often affecting the embryo at a very early period,
that slight variations necessarily appear at an equally early period. But we
have little evidence on this head -- indeed the evidence rather points the
other way; for it is notorious that breeders of cattle, horses, and various
fancy animals, cannot positively tell, until some time after the animal has
been born, what its merits or form will ultimately turn out. We see this
plainly in our own children; we cannot always tell whether the child will be
tall or short, or what its precise features will be. The question is not, at
what period of life any variation has been caused, but at what period it is
fully displayed. The cause may have acted, and I believe generally has acted,
even before the embryo is formed; and the variation may be due to the male and
female sexual elements having been affected by the conditions to which either
parent, or their ancestors, have been exposed. Nevertheless an effect thus
caused at a very early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which appears in old age
alone, has been communicated to the offspring from the reproductive element of
one parent. Or again, as when the horns of cross-bred cattle have been affected
by the shape of the horns of either parent. For the welfare of a very young
animal, as long as it remains in its mother's womb, or in the egg, or as long
as it is nourished and protected by its parent, it must be quite unimportant
whether most of its characters are fully acquired a little earlier or later in
life. It would not signify, for instance, to a bird which obtained its food
best by having a long beak, whether or not it assumed a beak of this particular
length, as long as it was fed by its parents. Hence, I conclude, that it is
quite possible, that each of the many successive modifications, by which each
species has acquired its present structure, may have supervened at a not very
early period of life; and some direct evidence from our domestic animals
supports this view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared at an extremely
early period.
I have stated in the
first chapter, that there is some evidence to render it probable, that at
whatever age any variation first appears in the parent, it tends to reappear at
a corresponding age in the offspring. Certain variations can only appear at
corresponding ages, for instance, peculiarities in the caterpillar, cocoon, or
imago states of the silk-moth; or, again, in the horns of almost full-grown
cattle. But further than this, variations which, for all that we can see, might
have appeared earlier or later in life, tend to appear at a corresponding age
in the offspring and parent. I am far from meaning that this is invariably the
case; and I could give a good many cases of variations (taking the word in the
largest sense) which have supervened at an earlier age in the child than in the
parent.
These two principles,
if their truth be admitted, will, I believe, explain all the above specified
leading facts in embryology. But first let us look at a few analogous cases in
domestic varieties. Some authors who have written on Dogs, maintain that the
greyhound and bulldog, though appearing so different, are really varieties most
closely allied, and have probably descended from the same wild stock; hence I
was curious to see how far their puppies differed from each other: I was told
by breeders that they differed just as much as their parents, and this, judging
by the eye, seemed almost to be the case; but on actually measuring the old
dogs and their six-days old puppies, I found that the puppies had not nearly
acquired their full amount of proportional difference. So, again, I was told
that the foals of cart and race-horses differed as much as the full-grown
animals; and this surprised me greatly, as I think it probable that the
difference between these two breeds has been wholly caused by selection under
domestication; but having had careful measurements made of the dam and of a
three-days old colt of a race and heavy carthorse, I find that the colts have
by no means acquired their full amount of proportional difference.
As the evidence appears
to me conclusive, that the several domestic breeds of pigeon have descended
from one wild species, I compared young pigeons of various breeds, within
twelve hours after being hitched; I carefully measured the proportions (but
will not here give details) of the beak, width of mouth, length of nostril and
of eyelid, size of feet and length of leg, in the wild stock, in pouters,
fantails, runts, barbs, dragons, carriers, and tumblers. Now some of these birds,
when mature, differ so extraordinarily in length and form of beak, that they
would, I cannot doubt, be ranked in distinct genera, had they been natural
productions. But when the nestling birds of these several breeds were placed in
a row, though most of them could be distinguished from each other, yet their
proportional differences in the above specified several points were
incomparably less than in the full-grown birds. Some characteristic points of
difference -- for instance, that of the width of mouth -- could hardly be
detected in the young. But there was one remarkable exception to this rule, for
the young of the short-faced tumbler differed from the young of the wild
rock-pigeon and of the other breeds, in all its proportions, almost exactly as
much as in the adult state.
The two principles
above given seem to me to explain these facts in regard to the later embryonic
stages of our domestic varieties. Fanciers select their horses, dogs, and
pigeons, for breeding, when they are nearly grown up: they are indifferent
whether the desired qualities and structures have been acquired earlier or
later in life, if the full- grown animal possesses them. And the cases just
given, more especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been accumulated by
man's selection, have not generally first appeared at an early period of life,
and have been inherited by the offspring at a corresponding not early period.
But the case of the short-faced tumbler, which when twelve hours old had
acquired its proper proportions, proves that this is not the universal rule;
for here the characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been inherited, not
at the corresponding, but at an earlier age.
Now let us apply these
facts and the above two principles -- which latter, though not proved true, can
be shown to be in some degree probable -- to species in a state of nature. Let
us take a genus of birds, descended on my theory from some one parent-species,
and of which the several new species have become modified through natural
selection in accordance with their diverse habits. Then, from the many slight
successive steps of variation having supervened at a rather late age, and
having been inherited at a corresponding age, the young of the new species of
our supposed genus will manifestly tend to resemble each other much more
closely than do the adults, just as we have seen in the case of pigeons. We may
extend this view to whole families or even classes. The fore-limbs, for
instance, which served as legs in the parent-species, may become, by a long
course of modification, adapted in one descendant to act as hands, in another
as paddles, in another as wings; and on the above two principles -- namely of
each successive modification supervening at a rather late age, and being
inherited at a corresponding late age -- the fore-limbs in the embryos of the
several descendants of the parent-species will still resemble each other
closely, for they will not have been modified. But in each individual new
species, the embryonic fore-limbs will differ greatly from the fore-limbs in
the mature animal; the limbs in the latter having undergone much modification
at a rather late period of life, and having thus been converted into hands, or
paddles, or wings. Whatever influence long-continued exercise or use on the one
hand, and disuse on the other, may have in modifying an organ, such influence
will mainly affect the mature animal, which has come to its full powers of
activity and has to gain its own living; and the effects thus produced will be
inherited at a corresponding mature age. Whereas the young will remain
unmodified, or be modified in a lesser degree, by the effects of use and
disuse.
In certain cases the
successive steps of variation might supervene, from causes of which we are
wholly ignorant, at a very early period of life, or each step might be
inherited at an earlier period than that at which it first appeared. In either
case (as with the short-faced tumbler) the young or embryo would closely
resemble the mature parent-form. We have seen that this is the rule of
development in certain whole groups of animals, as with cuttle-fish and
spiders, and with a few members of the great class of insects, as with Aphis.
With respect to the final cause of the young in these cases not undergoing any
metamorphosis, or closely resembling their parents from their earliest age, we
can see that this would result from the two following contingencies; firstly,
from the young, during a course of modification carried on for many
generations, having to provide for their own wants at a very early stage of
development, and secondly, from their following exactly the same habits of life
with their parents; for in this case, it would be indispensable for the
existence of the species, that the child should be modified at a very early age
in the same manner with its parents, in accordance with their similar habits.
Some further explanation, however, of the embryo not undergoing any
metamorphosis is perhaps requisite. If, on the other hand, it profited the
young to follow habits of life in any degree different from those of their
parent, and consequently to be constructed in a slightly different manner,
then, on the principle of inheritance at corresponding ages, the active young
or larvae might easily be rendered by natural selection different to any
conceivable extent from their parents. Such differences might, also, become
correlated with successive stages of development; so that the larvae, in the
first stage, might differ greatly from the larvae in the second stage, as we
have seen to be the case with cirripedes. The adult might become fitted for
sites or habits, in which organs of locomotion or of the senses, &c., would
be useless; and in this case the final metamorphosis would be said to be
retrograde.
As all the organic
beings, extinct and recent, which have ever lived on this earth have to be
classed together, and as all have been connected by the finest gradations, the
best, or indeed, if our collections were nearly perfect, the only possible
arrangement, would be genealogical. Descent being on my view the hidden bond of
connexion which naturalists have been seeking under the term of the natural
system. On this view we can understand how it is that, in the eyes of most
naturalists, the structure of the embryo is even more important for
classification than that of the adult. For the embryo is the animal in its less
modified state; and in so far it reveals the structure of its progenitor. In
two groups of animal, however much they may at present differ from each other
in structure and habits, if they pass through the same or similar embryonic
stages, we may feel assured that they have both descended from the same or
nearly similar parents, and are therefore in that degree closely related. Thus,
community in embryonic structure reveals community of descent. It will reveal
this community of descent, however much the structure of the adult may have
been modified and obscured; we have seen, for instance, that cirripedes can at
once be recognised by their larvae as belonging to the great class of
crustaceans. As the embryonic state of each species and group of species
partially shows us the structure of their less modified ancient progenitors, we
can clearly see why ancient and extinct forms of life should resemble the
embryos of their descendants, -- our existing species. Agassiz believes this to
be a law of nature; but I am bound to confess that I only hope to see the law
hereafter proved true. It can be proved true in those cases alone in which the
ancient state, now supposed to be represented in many embryos, has not been
obliterated, either by the successive variations in a long course of
modification having supervened at a very early age, or by the variations having
been inherited at an earlier period than that at which they first appeared. It
should also be borne in mind, that the supposed law of resemblance of ancient
forms of life to the embryonic stages of recent forms, may be true, but yet,
owing to the geological record not extending far enough back in time, may
remain for a long period, or for ever, incapable of demonstration.
Thus, as it seems to
me, the leading facts in embryology, which are second in importance to none in
natural history, are explained on the principle of slight modifications not
appearing, in the many descendants from some one ancient progenitor, at a very
early period in the life of each, though perhaps caused at the earliest, and
being inherited at a corresponding not early period. Embryology rises greatly
in interest, when we thus look at the embryo as a picture, more or less
obscured, of the common parent-form of each great class of animals.
Rudimentary, atrophied,
or aborted organs. Organs or parts in this strange condition, bearing the stamp
of inutility, are extremely common throughout nature. For instance, rudimentary
mammae are very general in the males of mammals: I presume that the
`bastard-wing' in birds may be safely considered as a digit in a rudimentary
state: in very many snakes one lobe of the lungs is rudimentary; in other
snakes there are rudiments of the pelvis and hind limbs. Some of the cases of
rudimentary organs are extremely curious; for instance, the presence of teeth
in foetal whales, which when grown up have not a tooth in their heads; and the
presence of teeth, which never cut through the gums, in the upper jaws of our
unborn calves. It has even been stated on good authority that rudiments of
teeth can be detected in the beaks of certain embryonic birds. Nothing can be
plainer than that wings are formed for flight, yet in how many insects do we
see wings so reduced in size as to be utterly incapable of flight, and not
rarely lying under wing-cases, firmly soldered together !
The meaning of
rudimentary organs is often quite unmistakeable: for instance there are beetles
of the same genus (and even of the same species) resembling each other most
closely in all respects, one of which will have full- sized wings, and another
mere rudiments of membrane; and here it is impossible to doubt, that the
rudiments represent wings. Rudimentary organs sometimes retain their
potentiality, and are merely not developed: this seems to be the case with the
mammae of male mammals, for many instances are on record of these organs having
become well developed in full-grown males, and having secreted milk. So again
there are normally four developed and two rudimentary teats in the udders of
the genus Bos, but in our domestic cows the two sometimes become developed and
give milk. In individual plants of the same species the petals sometimes occur
as mere rudiments, and sometimes in a well-developed state. In plants with
separated sexes, the male flowers often have a rudiment of a pistil; and Kölreuter
found that by crossing such male plants with an hermaphrodite species, the
rudiment of the pistil in the hybrid offspring was much increased in size; and
this shows that the rudiment and the perfect pistil are essentially alike in
nature.
An organ serving for
two purposes, may become rudimentary or utterly aborted for one, even the more
important purpose;, and remain perfectly efficient for the other. Thus in
plants, the office of the pistil is to allow the pollen- tubes to reach the
ovules protected in the ovarium at its base. The pistil consists of a stigma
supported on the style; but in some Compositae, the male florets, which of
course cannot be fecundated, have a pistil, which is in a rudimentary state,
for it is not crowned with a stigma; but the style remains well developed, and
is clothed with hairs as in other compositae, for the purpose of brushing the
pollen out of the surrounding anthers. Again, an organ may become rudimentary
for its proper purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or lung.
Other similar instances could be given.
Rudimentary organs in
the individuals of the same species are very liable to vary in degree of
development and in other respects. Moreover, in closely allied species, the
degree to which the same organ has been rendered rudimentary occasionally
differs much. This latter fact is well exemplified in the state of the wings of
the female moths in certain groups. Rudimentary organs may be utterly aborted;
and this implies, that we find in an animal or plant no trace of an organ,
which analogy would lead us to expect to find, and which is occasionally found in
monstrous individuals of the species. Thus in the snapdragon (antirrhinum) we
generally do not find a rudiment of a fifth stamen; but this may sometimes be
seen. In tracing the homologies of the same part in different members of a
class, nothing is more common, or more necessary, than the use and discovery of
rudiments. This is well shown in the drawings given by Owen of the bones of the
leg of the horse, ox, and rhinoceros.
It is an important fact
that rudimentary organs, such as teeth in the upper jaws of whales and
ruminants, can often be detected in the embryo, but afterwards wholly
disappear. It is also, I believe, a universal rule, that a rudimentary part or
organ is of greater size relatively to the adjoining parts in the embryo, than
in the adult; so that the organ at this early age is less rudimentary, or even
cannot be said to be in any degree rudimentary. Hence, also, a rudimentary
organ in the adult, is often said to have retained its embryonic condition.
I have now given the
leading facts with respect to rudimentary organs. In reflecting on them, every
one must be struck with astonishment: for the same reasoning power which tells
us plainly that most parts and organs are exquisitely adapted for certain
purposes, tells us with equal plainness that these rudimentary or atrophied
organs, are imperfect and useless. In works on natural history rudimentary
organs are generally said to have been created `for the sake of symmetry,' or
in order `to complete the scheme of nature;' but this seems to me no
explanation, merely a restatement of the fact. Would it be thought sufficient
to say that because planets revolve in elliptic courses round the sun,
satellites follow the same course round the planets, for the sake of symmetry,
and to complete the scheme of nature? An eminent physiologist accounts for the
presence of rudimentary organs, by supposing that they serve to excrete matter
in excess, or injurious to the system; but can we suppose that the minute
papilla, which often represents the pistil in male flowers, and which is formed
merely of cellular tissue, can thus act ? Can we suppose that the formation of
rudimentary teeth which are subsequently absorbed, can be of any service to the
rapidly growing embryonic calf by the excretion of precious phosphate of lime?
When a man's fingers have been amputated, imperfect nails sometimes appear on
the stumps: I could as soon believe that these vestiges of nails have appeared,
not from unknown laws of growth, but in order to excrete horny matter, as that
the rudimentary nails on the fin of the manatee were formed for this purpose.
On my view of descent
with modification, the origin of rudimentary organs is simple. We have plenty
of cases of rudimentary organs in our domestic productions, -- as the stump of
a tail in tailless breeds, -- the vestige of an ear in earless breeds, -- the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals, -- and the state of the
whole flower in the cauliflower. We often see rudiments of various parts in
monsters. But I doubt whether any of these cases throw light on the origin of
rudimentary organs in a state of nature, further than by showing that rudiments
can be produced; for I doubt whether species under nature ever undergo abrupt
changes. I believe that disuse has been the main agency; that it has led in
successive generations to the gradual reduction of various organs, until they
have become rudimentary, -- as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which have
seldom been forced to take flight, and have ultimately lost the power of
flying. Again, an organ useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small and exposed islands;
and in this case natural selection would continue slowly to reduce the organ,
until it was rendered harmless and rudimentary.
Any change in function,
which can be effected by insensibly small steps, is within the power of natural
selection; so that an organ rendered, during changed habits of life, useless or
injurious for one purpose, might easily be modified and used for another
purpose. Or an organ might be retained for one alone of its former functions.
An organ, when rendered useless, may well be variable, for its variations
cannot be checked by natural selection. At whatever period of life disuse or
selection reduces an organ, and this will generally be when the being has come
to maturity and to its full powers of action, the principle of inheritance at
corresponding ages will reproduce the organ in its reduced state at the same
age, and consequently will seldom affect or reduce it in the embryo. Thus we
can understand the greater relative size of rudimentary organs in the embryo,
and their lesser relative size in the adult. But if each step of the process of
reduction were to be inherited, not at the corresponding age, but at an
extremely early period of life (as we have good reason to believe to be
possible) the rudimentary part would tend to be wholly lost, and we should have
a case of complete abortion. The principle, also, of economy, explained in a
former chapter, by which the materials forming any part or structure, if not
useful to the possessor, will be saved as far as is possible, will probably
often come into play; and this will tend to cause the entire obliteration of a
rudimentary organ.
As the presence of
rudimentary organs is thus due to the tendency in every part of the
organisation, which has long existed, to be inherited -- we can understand, on
the genealogical view of classification, how it is that systematists have found
rudimentary parts as useful as, or even sometimes more useful than, parts of
high physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless in the
pronunciation, but which serve as a clue in seeking for its derivation. On the
view of descent with modification, we may conclude that the existence of organs
in a rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the ordinary doctrine
of creation, might even have been anticipated, and can be accounted for by the
laws of inheritance.
Summary. In this
chapter I have attempted to show, that the subordination of group to group in
all organisms throughout all time; that the nature of the relationship, by
which all living and extinct beings are united by complex, radiating, and
circuitous lines of affinities into one grand system; the rules followed and
the difficulties encountered by naturalists in their classifications; the value
set upon characters, if constant and prevalent, whether of high vital
importance, or of the most trifling importance, or, as in rudimentary organs,
of no importance; the vide opposition in value between analogical or adaptive
characters, and characters of true affinity; and other such rules; -- all
naturally follow on the view of the common parentage of those forms which are
considered by naturalists as allied, together with their modification through
natural selection, with its contingencies of extinction and divergence of
character. In considering this view of classification, it should be borne in
mind that the element of descent has been universally used in ranking together
the sexes, ages, and acknowledged varieties of the same species, however
different they may be in structure. If we extend the use of this element of
descent, -- the only certainly known cause of similarity in organic beings, --
we shall understand what is meant by the natural system: it is genealogical in
its attempted arrangement, with the grades of acquired difference marked by the
terms varieties, species, genera, families, orders, and classes.
On this same view of
descent with modification, all the great facts in Morphology become
intelligible, -- whether we look to the same pattern displayed in the
homologous organs, to whatever purpose applied, of the different species of a
class; or to the homologous parts constructed on the same pattern in each
individual animal and plant.
On the principle of
successive slight variations, not necessarily or generally supervening at a
very early period of life, and being inherited at a corresponding period, we
can understand the great leading facts in Embryology; namely, the resemblance
in all individual embryo of the homologous parts, which when matured will
become widely different from each other in structure and function; and the
resemblance in different species of a class of the homologous parts or organs,
though fitted in the adult members for purposes as different as possible.
Larvae are active embryos, which have become specially modified in relation to
their habits of life, through the principle of modifications being inherited at
corresponding ages. On this same principle -- and bearing in mind, that when
organs are reduced in size, either from disuse or selection, it will generally
be at that period of life when the being has to provide for its own wants, and
bearing in mind how strong is the principle of inheritance -- the occurrence of
rudimentary organs and their final abortion, present to us no inexplicable
difficulties; on the contrary, their presence might have been even anticipated.
The importance of embryological characters and of rudimentary organs in
classification is intelligible, on the view that an arrangement is only so far
natural as it is genealogical.
Finally, the several
classes of facts which have been considered in this chapter, seem to me to
proclaim so plainly, that the innumerable species, genera, and families of
organic beings, with which this world is peopled, have all descended, each
within its own class or group, from common parents, and have all been modified
in the course of descent, that I should without hesitation adopt this view,
even if it were unsupported by other facts or arguments.
Recapitulation of the difficulties on the theory of Natural Selection - Recapitulation
of the general and special circumstances in its favour - Causes of the general
belief in the immutability of species - How far the theory of natural selection
may be extended - Effects of its adoption on the study of Natural history -
Concluding remarks AS this whole
volume is one long argument, it may be convenient to the reader to have the
leading facts and inferences briefly recapitulated.
That many and grave
objections may be advanced against the theory of descent with modification through
natural selection, I do not deny. I have endeavoured to give to them their full
force. Nothing at first can appear more difficult to believe than that the more
complex organs and instincts should have been perfected not by means superior
to, though analogous with, human reason, but by the accumulation of innumerable
slight variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot be
considered real if we admit the following propositions, namely, -- that
gradations in the perfection of any organ or instinct, which we may consider,
either do now exist or could have existed, each good of its kind, -- that all
organs and instincts are, in ever so slight a degree, variable, -- and, lastly,
that there is a struggle for existence leading to the preservation of each
profitable deviation of structure or instinct. The truth of these propositions
cannot, I think, be disputed.
It is, no doubt,
extremely difficult even to conjecture by what gradations many structures have
been perfected, more especially amongst broken and failing groups of organic
beings; but we see so many strange gradations in nature, as is proclaimed by
the canon, `Natura non facit saltum,' that we ought to be extremely cautious in
saying that any organ or instinct, or any whole being, could not have arrived
at its present state by many graduated steps. There are, it must be admitted,
cases of special difficulty on the theory of natural selection; and one of the
most curious of these is the existence of two or three defined castes of
workers or sterile females in the same community of ants but I have attempted
to show how this difficulty can be mastered. With respect to the almost
universal sterility of species when first crossed, which forms so remarkable a
contrast with the almost universal fertility of varieties when crossed, I must
refer the reader to the recapitulation of the facts given at the end of the
eighth chapter, which seem to me conclusively to show that this sterility is no
more a special endowment than is the incapacity of two trees to be grafted
together, but that it is incidental on constitutional differences in the
reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the result, when the same two species are
crossed reciprocally; that is, when one species is first used as the father and
then as the mother.
The fertility of
varieties when intercrossed and of their mongrel offspring cannot be considered
as universal; nor is their very general fertility surprising when we remember
that it is not likely that either their constitutions or their reproductive
systems should have been profoundly modified. Moreover, most of the varieties
which have been experimentised on have been produced under domestication; and
as domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.
The sterility of
hybrids is a very different case from that of first crosses, for their
reproductive organs are more or less functionally impotent; whereas in first
crosses the organs on both sides are in a perfect condition. As we continually
see that organisms of all kinds are rendered in some degree sterile from their
constitutions having been disturbed by slightly different and new conditions of
life, we need not feel surprise at hybrids being in some degree sterile, for
their constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is supported by
another parallel, but directly opposite, class of facts; namely, that the
vigour and fertility of all organic beings are increased by slight changes in
their conditions of life, and that the offspring of slightly modified forms or
varieties acquire from being crossed increased vigour and fertility. So that,
on the one hand, considerable changes in the conditions of life and crosses
between greatly modified forms, lessen fertility; and on the other hand, lesser
changes in the conditions of life and crosses between less modified forms,
increase fertility.
Turning to geographical
distribution, the difficulties encountered on the theory of descent with
modification are grave enough. All the individuals of the same species, and all
the species of the same genus, or even higher group, must have descended from
common parents; and therefore, in however distant and isolated parts of the
world they are now found, they must in the course of successive generations
have passed from some one part to the others. We are often wholly unable even
to conjecture how this could have been effected. Yet, as we have reason to
believe that some species have retained the same specific form for very long
periods, enormously long as measured by years too much stress ought not to be
laid on the occasional wide diffusion of the same species; for during very long
periods of time there will always be a good chance for wide migration by many
means. A broken or interrupted range may often be accounted for by the
extinction of the species in the intermediate regions. It cannot be denied that
we are as yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern periods; and
such changes will obviously have greatly facilitated migration. As an example,
I have attempted to show how potent has been the influence of the Glacial
period on the distribution both of the same and of representative species
throughout the world. We are as yet profoundly ignorant of the many occasional
means of transport. With respect to distinct species of the same genus
inhabiting very distant and isolated regions, as the process of modification
has necessarily been slow, all the means of migration will have been possible
during a very long period; and consequently the difficulty of the wide
diffusion of species of the same genus is in some degree lessened.
As on the theory of
natural selection an interminable number of intermediate forms must have
existed, linking together all the species in each group by gradations as fine
as our present varieties, it may be asked, Why do we not see these linking
forms all around us? Why are not all organic beings blended together in an
inextricable chaos? With respect to existing forms, we should remember that we
have no right to expect (excepting in rare cases) to discover directly
connecting links between them, but only between each and some extinct and
supplanted form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life change
insensibly in going from a district occupied by one species into another
district occupied by a closely allied species, we have no just right to expect
often to find intermediate varieties in the intermediate zone. For we have
reason to believe that only a few species are undergoing change at any one
period; and all changes are slowly effected. I have also shown that the
intermediate varieties which will at first probably exist in the intermediate
zones, will be liable to be supplanted by the allied forms on either hand; and
the latter, from existing in greater numbers, will generally be modified and
improved at a quicker rate than the intermediate varieties, which exist in
lesser numbers; so that the intermediate varieties will, in the long run, be
supplanted and exterminated.
On this doctrine of the
extermination of an infinitude of connecting links, between the living and
extinct inhabitants of the world, and at each successive period between the
extinct and still older species, why is not every geological formation charged
with such links? Why does not every collection of fossil remains afford plain
evidence of the gradation and mutation of the forms of life? We meet with no
such evidence, and this is the most obvious and forcible of the many objections
which may be urged against my theory. Why, again, do whole groups of allied
species appear, though certainly they often falsely appear, to have come in
suddenly on the several geological stages? Why do we not find great piles of
strata beneath the Silurian system, stored with the remains of the progenitors
of the Silurian groups of fossils? For certainly on my theory such strata must
somewhere have been deposited at these ancient and utterly unknown epochs in
the world's history.
I can answer these
questions and grave objections only on the supposition that the geological
record is far more imperfect than most geologists believe. It cannot be
objected that there has not been time sufficient for any amount of organic
change; for the lapse of time has been so great as to be utterly inappreciable
by the human intellect. The number of specimens in all our museums is
absolutely as nothing compared with the countless generations of countless
species which certainly have existed. We should not be able to recognise a
species as the parent of any one or more species if we were to examine them
ever so closely, unless we likewise possessed many of the intermediate links
between their past or parent and present states; and these many links we could
hardly ever expect to discover, owing to the imperfection of the geological
record. Numerous existing doubtful forms could be named which are probably
varieties; but who will pretend that in future ages so many fossil links will
be discovered, that naturalists will be able to decide, on the common view,
whether or not these doubtful forms are varieties ? As long as most of the
links between any two species are unknown, if any one link or intermediate variety
be discovered, it will simply be classed as another and distinct species. Only
a small portion of the world has been geologically explored. Only organic
beings of certain classes can be preserved in a fossil condition, at least in
any great number. Widely ranging species vary most, and varieties are often at
first local, -- both causes rendering the discovery of intermediate links less
likely. Local varieties will not spread into other and distant regions until
they are considerably modified and improved; and when they do spread, if
discovered in a geological formation, they will appear as if suddenly created
there, and will be simply classed as new species. Most formations have been
intermittent in their accumulation; and their duration, I am inclined to
believe, has been shorter than the average duration of specific forms.
Successive formations are separated from each other by enormous blank intervals
of time; for fossiliferous formations, thick enough to resist future
degradation, can be accumulated only where much sediment is deposited on the
subsiding bed of the sea. During the alternate periods of elevation and of
stationary level the record will be blank. During these latter periods there
will probably be more variability in the forms of life; during periods of
subsidence, more extinction.
With respect to the
absence of fossiliferous formations beneath the lowest Silurian strata, I can
only recur to the hypothesis given in the ninth chapter. That the geological
record is imperfect all will admit; but that it is imperfect to the degree which
I require, few will be inclined to admit. If we look to long enough intervals
of time, geology plainly declares that all species have changed; and they have
changed in the manner which my theory requires, for they have changed slowly
and in a graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to each
other, than are the fossils from formations distant from each other in time.
Such is the sum of the
several chief objections and difficulties which may justly be urged against my
theory; and I have now briefly recapitulated the answers and explanations which
can be given to them. I have felt these difficulties far too heavily during
many years to doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly ignorant;
nor do we know how ignorant we are. We do not know all the possible
transitional gradations between the simplest and the most perfect organs it cannot
be pretended that we know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect the Geological Record is. Grave
as these several difficulties are, in my judgement they do not overthrow the
theory of descent with modification.
Now let us turn to the
other side of the argument. Under domestication we see much variability. This
seems to be mainly due to the reproductive system being eminently susceptible
to changes in the conditions of life so that this system, when not rendered
impotent, fails to reproduce offspring exactly like the parent-form.
Variability is governed by many complex laws, -- by correlation of growth, by
use and disuse, and by the direct action of the physical conditions of life.
There is much difficulty in ascertaining how much modification our domestic
productions have undergone; but we may safely infer that the amount has been
large, and that modifications can be inherited for long periods. As long as the
conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations. On the
other hand we have evidence that variability, when it has once come into play,
does not wholly cease; for new varieties are still occasionally produced by our
most anciently domesticated productions.
Man does not actually
produce variability; he only unintentionally exposes organic beings to new
conditions of life, and then nature acts on the organisation, and causes
variability. But man can and does select the variations given to him by nature,
and thus accumulate them in any desired manner. He thus adapts animals and
plants for his own benefit or pleasure. He may do this methodically, or he may
do it unconsciously by preserving the individuals most useful to him at the
time, without any thought of altering the breed. It is certain that he can
largely influence the character of a breed by selecting, in each successive generation,
individual differences so slight as to be quite inappreciable by an uneducated
eye. This process of selection has been the great agency in the production of
the most distinct and useful domestic breeds. That many of the breeds produced
by man have to a large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or aboriginal
species.
There is no obvious
reason why the principles which have acted so efficiently under domestication
should not have acted under nature. In the preservation of favoured individuals
and races, during the constantly-recurrent Struggle for Existence, we see the
most powerful and ever- acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is common
to all organic beings. This high rate of increase is proved by calculation, by
the effects of a succession of peculiar seasons, and by the results of
naturalisation, as explained in the third chapter. More individuals are born
than can possibly survive. A grain in the balance will determine which
individual shall live and which shall die, -- which variety or species shall
increase in number, and which shall decrease, or finally become extinct. As the
individuals of the same species come in all respects into the closest
competition with each other, the struggle will generally be most severe between
them; it will be almost equally severe between the varieties of the same
species, and next in severity between the species of the same genus. But the
struggle will often be very severe between beings most remote in the scale of
nature: The slightest advantage in one being, at any age or during any season,
over those with which it comes into competition, or better adaptation in
however slight a degree to the surrounding physical conditions, will turn the
balance.
With animals having
separated sexes there will in most cases be a struggle between the males for
possession of the females. he most vigorous individuals, or those which have
most successfully struggled with their conditions of life, will generally leave
most progeny. But success will often depend on having special weapons or means
of defence, or on the charms of the males; and the slightest advantage will lead
to victory.
As geology plainly
proclaims that each land has undergone great physical changes, we might have
expected that organic beings would have varied under nature, in the same way as
they generally have varied under the changed conditions of domestication. And
if there be any variability under nature, it would be an unaccountable fact if
natural selection had not come into play. It has often been asserted, but the
assertion is quite incapable of proof, that the amount of variation under
nature is a strictly limited quantity. Man, though acting on external
characters alone and often capriciously, can produce within a short period a
great result by adding up mere individual differences in his domestic
productions; and every one admits that there are at least individual
differences in species under nature. But, besides such differences, all
naturalists have admitted the existence of varieties, which they think
sufficiently distinct to be worthy of record in systematic works. No one can
draw any clear distinction between individual differences and slight varieties;
or between more plainly marked varieties and subspecies, and species. Let it be
observed how naturalists differ in the rank which they assign to the many
representative forms in Europe and North America.
If then we have under
nature variability and a powerful agent always ready to act and select, why
should we doubt that variations in any way useful to beings, under their
excessively complex relations of life, would be preserved, accumulated, and
inherited ? Why, if man can by patience select variations most useful to
himself, should nature fail in selecting variations useful, under changing
conditions of life, to her living products ? What limit can be put to this
power, acting during long ages and rigidly scrutinising the whole constitution,
structure, and habits of each creature, -- favouring the good and rejecting the
bad? I can see no limit to this power, in slowly and beautifully adapting each
form to the most complex relations of life. The theory of natural selection,
even if we looked no further than this, seems to me to be in itself probable. I
have already recapitulated, as fairly as I could, the opposed difficulties and
objections: now let us turn to the special facts and arguments in favour of the
theory.
On the view that
species are only strongly marked and permanent varieties, and that each species
first existed as a variety, we can see why it is that no line of demarcation
can be drawn between species, commonly supposed to have been produced by
special acts of creation, and varieties which are acknowledged to have been
produced by secondary laws. On this same view we can understand how it is that
in each region where many species of a genus have been produced, and where they
now flourish, these same species should present many varieties; for where the
manufactory of species has been active, we might expect, as a general rule, to
find it still in action; and this is the case if varieties be incipient
species. Moreover, the species of the large genera, which afford the greater
number of varieties or incipient species, retain to a certain degree the
character of varieties; for they differ from each other by a less amount of
difference than do the species of smaller genera. The closely allied species
also of the larger genera apparently have restricted ranges, and they are
clustered in little groups round other species -- in which respects they
resemble varieties. These are strange relations on the view of each species
having been independently created, but are intelligible if all species first
existed as varieties.
As each species tends
by its geometrical ratio of reproduction to increase inordinately in number;
and as the modified descendants of each species will be enabled to increase by
so much the more as they become more diversified in habits and structure, so as
to be enabled to seize on many and widely different places in the economy of
nature, there will be a constant tendency in natural selection to preserve the
most divergent offspring of any one species. Hence during a long-continued
course of modification, the slight differences, characteristic of varieties of
the same species, tend to be augmented into the greater differences
characteristic of species of the same genus. New and improved varieties will
inevitably supplant and exterminate the older, less improved and intermediate
varieties; and thus species are rendered to a large extent defined and distinct
objects. Dominant species belonging to the larger groups tend to give birth to
new and dominant forms; so that each large group tends to become still larger,
and at the same time more divergent in character. But as all groups cannot thus
succeed in increasing in size, for the world would not hold them, the more
dominant groups beat the less dominant. This tendency in the large groups to go
on increasing in size and diverging in character, together with the almost
inevitable contingency of much extinction, explains the arrangement of all the
forms of life, in groups subordinate to groups, all within a few great classes,
which we now see everywhere around us, and which has prevailed throughout all
time. This grand fact of the grouping of all organic beings seems to me utterly
inexplicable on the theory of creation.
As natural selection
acts solely by accumulating slight, successive, favourable variations, it can
produce no great or sudden modification; it can act only by very short and slow
steps. Hence the canon of `Natura non facit saltum,' which every fresh addition
to our knowledge tends to make more strictly correct, is on this theory simply
intelligible. We can plainly see why nature is prodigal in variety, though
niggard in innovation. But why this should be a law of nature if each species
has been independently created, no man can explain.
Many other facts are,
as it seems to me, explicable on this theory. How strange it is that a bird,
under the form of woodpecker, should have been created to prey on insects on
the ground; that upland geese, which never or rarely swim, should have been
created with webbed feet; that a thrush should have been created to dive and
feed on sub- aquatic insects; and that a petrel should have been created with
habits and structure fitting it for the life of an auk or grebe! and so on in endless
other cases. But on the view of each species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill- occupied place in nature, these
facts cease to be strange, or perhaps might even have been anticipated.
As natural selection
acts by competition, it adapts the inhabitants of each country only in relation
to the degree of perfection of their associates; so that we need feel no
surprise at the inhabitants of any one country, although on the ordinary view
supposed to have been specially created and adapted for that country, being
beaten and supplanted by the naturalised productions from another land. Nor
ought we to marvel if all the contrivances in nature be not, as far as we can
judge, absolutely perfect; and if some of them be abhorrent to our ideas of
fitness. We need not marvel at the sting of the bee causing the bee's own
death; at drones being produced in such vast numbers for one single act, and
being then slaughtered by their sterile sisters; at the astonishing waste of
pollen by our fir-trees; at the instinctive hatred of the queen bee for her own
fertile daughters; at ichneumonidae feeding within the live bodies of
caterpillars; and at other such cases. The wonder indeed is, on the theory of
natural selection, that more cases of the want of absolute perfection have not
been observed.
The complex and little
known laws governing variation are the same, as far as we can see, with the
laws which have governed the production of so-called specific forms. In both
cases physical conditions seem to have produced but little direct effect; yet
when varieties enter any zone, they occasionally assume some of the characters
of the species proper to that zone. In both varieties and species, use and
disuse seem to have produced some effect; for it is difficult to resist this
conclusion when we look, for instance, at the logger- headed duck, which has
wings incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, which is occasionally blind,
and then at certain moles, which are habitually blind and have their eyes
covered with skin; or when we look at the blind animals inhabiting the dark
caves of America and Europe. In both varieties and species correction of growth
seems to have played a most important part, so that when one part has been
modified other parts are necessarily modified. In both varieties and species
reversions to long-lost characters occur. How inexplicable on the theory of
creation is the occasional appearance of stripes on the shoulder and legs of
the several species of the horse-genus and in their hybrids ! How simply is
this fact explained if we believe that these species have descended from a
striped progenitor, in the same manner as the several domestic breeds of pigeon
have descended from the blue and barred rock-pigeon !
On the ordinary view of
each species having been independently created, why should the specific
characters, or those by which the species of the same genus differ from each
other, be more variable than the generic characters in which they all agree?
Why, for instance, should the colour of a flower be more likely to vary in any
one species of a genus, if the other species, supposed to have been created
independently, have differently coloured flowers, than if all the species of
the genus have the same coloured flowers? If species are only well-marked
varieties, of which the characters have become in a high degree permanent, we
can understand this fact; for they have already varied since they branched off
from a common progenitor in certain characters, by which they have come to be
specifically distinct from each other; and therefore these same characters
would be more likely still to be variable than the generic characters which
have been inherited without change for an enormous period. It is inexplicable
on the theory of creation why a part developed in a very unusual manner in any
one species of a genus, and therefore, as we may naturally infer, of great
importance to the species, should be eminently liable to variation; but, on my
view, this part has undergone, since the several species branched off from a
common progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But a part
may be developed in the most unusual manner, like the wing of a bat, and yet
not be more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long period;
for in this case it will have been rendered constant by long-continued natural
selection.
Glancing at instincts,
marvellous as some are, they offer no greater difficulty than does corporeal
structure on the theory of the natural selection of successive, slight, but
profitable modifications. We can thus understand why nature moves by graduated
steps in endowing different animals of the same class with their several
instincts. I have attempted to show how much light the principle of gradation
throws on the admirable architectural powers of the hive- bee. Habit no doubt
sometimes comes into play in modifying instincts; but it certainly is not
indispensable, as we see, in the case of neuter insects, which leave no progeny
to inherit the effects of long-continued habit. On the view of all the species
of the same genus having descended from a common parent, and having inherited
much in common, we can understand how it is that allied species, when placed under
considerably different conditions of life, yet should follow nearly the same
instincts; why the thrush of South America, for instance, lines her nest with
mud like our British species. On the view of instincts having been slowly
acquired through natural selection we need not marvel at some instincts being
apparently not perfect and liable to mistakes, and at many instincts causing
other animals to suffer.
If species be only
well-marked and permanent varieties, we can at once see why their crossed offspring
should follow the same complex laws in their degrees and kinds of resemblance
to their parents, -- in being absorbed into each other by successive crosses,
and in other such points, -- as do the crossed offspring of acknowledged
varieties. On the other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary laws.
If we admit that the
geological record is imperfect in an extreme degree, then such facts as the
record gives, support the theory of descent with modification. New species have
come on the stage slowly and at successive intervals; and the amount of change,
after equal intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played so
conspicuous a part in the history of the organic world, almost inevitably
follows on the principle of natural selection; for old forms will be supplanted
by new and improved forms. Neither single species nor groups of species
reappear when the chain of ordinary generation has once been broken. The
gradual diffusion of dominant forms, with the slow modification of their
descendants, causes the forms of life, after long intervals of time, to appear
as if they had changed simultaneously throughout the world. The fact of the
fossil remains of each formation being in some degree intermediate in character
between the fossils in the formations above and below, is simply explained by
their intermediate position in the chain of descent. The grand fact that all
extinct organic beings belong to the same system with recent beings, falling
either into the same or into intermediate groups, follows from the living and
the extinct being the offspring of common parents. As the groups which have
descended from an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in character
in comparison with its later descendants; and thus we can see why the more
ancient a fossil is, the oftener it stands in some degree intermediate between
existing and allied groups. Recent forms are generally looked at as being, in
some vague sense, higher than ancient and extinct forms; and they are in so far
higher as the later and more improved forms have conquered the older and less
improved organic beings in the struggle for life. Lastly, the law of the long
endurance of allied forms on the same continent, -- of marsupials in Australia,
of edentata in America, and other such cases, -- is intelligible, for within a
confined country, the recent and the extinct will naturally be allied by
descent.
Looking to geographical
distribution, if we admit that there has been during the long course of ages
much migration from one part of the world to another, owing to former climatal
and geographical changes and to the many occasional and unknown means of
dispersal, then we can understand, on the theory of descent with modification,
most of the great leading facts in Distribution. We can see why there should be
so striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both cases
the beings have been connected by the bond of ordinary generation, and the
means of modification have been the same. We see the full meaning of the
wonderful fact, which must have struck every traveller, namely, that on the
same continent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on deserts and marshes, most of the inhabitants within
each great class are plainly related; for they will generally be descendants of
the same progenitors and early colonists. On this same principle of former
migration, combined in most cases with modification, we can understand, by the
aid of the Glacial period, the identity of some few plants, and the close
alliance of many others, on the most distant mountains, under the most
different climates; and likewise the close alliance of some of the inhabitants
of the sea in the northern and southern temperate zones, though separated by
the whole intertropical ocean. Although two areas may present the same physical
conditions of life, we need feel no surprise at their inhabitants being widely
different, if they have been for a long period completely separated from each
other; for as the relation of organism to organism is the most important of all
relations, and as the two areas will have received colonists from some third
source or from each other, at various periods and in different proportions, the
course of modification in the two areas will inevitably be different.
On this view of
migration, with subsequent modification, we can see why oceanic islands should
be inhabited by few species, but of these, that many should be peculiar. We can
clearly see why those animals which cannot cross wide spaces of ocean, as frogs
and terrestrial mammals, should not inhabit oceanic islands; and why, on the
other hand, new and peculiar species of bats, which can traverse the ocean,
should so often be found on islands far distant from any continent. Such facts
as the presence of peculiar species of bats, and the absence of all other
mammals, on oceanic islands, are utterly inexplicable on the theory of
independent acts of creation.
The existence of
closely allied or representative species in any two areas, implies, on the
theory of descent with modification, that the same parents formerly inhabited
both areas; and we almost invariably find that wherever many closely allied
species inhabit two areas, some identical species common to both still exist.
Wherever many closely allied yet distinct species occur, many doubtful forms
and varieties of the same species likewise occur. It is a rule of high
generality that the inhabitants of each area are related to the inhabitants of
the nearest source whence immigrants might have been derived. We see this in
nearly all the plants and animals of the Galapagos archipelago, of Juan
Fernandez, and of the other American islands being related in the most striking
manner to the plants and animals of the neighbouring American mainland; and
those of the Cape de Verde archipelago and other African islands to the African
mainland. It must be admitted that these facts receive no explanation on the
theory of creation.
The fact, as we have
seen, that all past and present organic beings constitute one grand natural
system, with group subordinate to group, and with extinct groups often falling
in between recent groups, is intelligible on the theory of natural selection
with its contingencies of extinction and divergence of character. On these same
principles we see how it is, that the mutual affinities of the species and
genera within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification; -- why
adaptive characters, though of paramount importance to the being, are of hardly
any importance in classification; why characters derived from rudimentary
parts, though of no service to the being, are often of high classificatory
value; and why embryological characters are the most valuable of all. The real
affinities of all organic beings are due to inheritance or community of
descent. The natural system is a genealogical arrangement, in which we have to
discover the lines of descent by the most permanent characters, however slight
their vital importance may be.
The framework of bones
being the same in the hand of a man, wing of a bat, fin of the porpoise, and
leg of the horse, -- the same number of vertebrae forming the neck of the
giraffe and of the elephant, -- and innumerable other such facts, at once
explain themselves on the theory of descent with slow and slight successive
modifications. The similarity of pattern in the wing and leg of a bat, though
used for such different purposes, -- in the jaws and legs of a crab, -- in the
petals, stamens, and pistils of a flower, is likewise intelligible on the view
of the gradual modification of parts or organs, which were alike in the early
progenitor of each class. On the principle of successive variations not always
supervening at an early age, and being inherited at a corresponding not early
period of life, we can clearly see why the embryos of mammals, birds, reptiles,
and fishes should be so closely alike, and should be so unlike the adult forms.
We may cease marvelling at the embryo of an air-breathing mammal or bird having
branchial slits and arteries running in loops, like those in a fish which has
to breathe the air dissolved in water, by the aid of well-developed branchiae.
Disuse, aided sometimes
by natural selection, will often tend to reduce an organ, when it has become
useless by changed habits or under changed conditions of life; and we can
clearly understand on this view the meaning of rudimentary organs. But disuse and
selection will generally act on each creature, when it has come to maturity and
has to play its full part in the struggle for existence, and will thus have
little power of acting on an organ during early life; hence the organ will not
be much reduced or rendered rudimentary at this early age. The calf, for
instance, has inherited teeth, which never cut through the gums of the upper
jaw, from an early progenitor having well-developed teeth; and we may believe,
that the teeth in the mature animal were reduced, during successive
generations, by disuse or by the tongue and palate having been fitted by
natural selection to browse without their aid; whereas in the calf, the teeth
have been left untouched by selection or disuse, and on the principle of
inheritance at corresponding ages have been inherited from a remote period to
the present day. On the view of each organic being and each separate organ
having been specially created, how utterly inexplicable it is that parts, like
the teeth in the embryonic calf or like the shrivelled wings under the soldered
wing-covers of some beetles, should thus so frequently bear the plain stamp of
inutility ! Nature may be said to have taken pains to reveal, by rudimentary
organs and by homologous structures, her scheme of modification, which it seems
that we wilfully will not understand.
I have now
recapitulated the chief facts and considerations which have thoroughly
convinced me that species have changed, and are still slowly changing by the
preservation and accumulation of successive slight favourable variations. Why,
it may be asked, have all the most eminent living naturalists and geologists
rejected this view of the mutability of species ? It cannot be asserted that
organic beings in a state of nature are subject to no variation; it cannot be
proved that the amount of variation in the course of long ages is a limited
quantity; no clear distinction has been, or can be, drawn between species and
well-marked varieties. It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or that sterility is a
special endowment and sign of creation. The belief that species were immutable
productions was almost unavoidable as long as the history of the world was
thought to be of short duration; and now that we have acquired some idea of the
lapse of time, we are too apt to assume, without proof, that the geological
record is so perfect that it would have afforded us plain evidence of the
mutation of species, if they had undergone mutation.
But the chief cause of
our natural unwillingness to admit that one species has given birth to other
and distinct species, is that we are always slow in admitting any great change
of which we do not see the intermediate steps. The difficulty is the same as
that felt by so many geologists, when Lyell first insisted that long lines of
inland cliffs had been formed, and great valleys excavated, by the slow action
of the coast-waves. The mind cannot possibly grasp the full meaning of the term
of a hundred million years; it cannot add up and perceive the full effects of
many slight variations, accumulated during an almost infinite number of
generations.
Although I am fully
convinced of the truth of the views given in this volume under the form of an
abstract, I by no means expect to convince experienced naturalists whose minds
are stocked with a multitude of facts all viewed, during a long course of
years, from a point of view directly opposite to mine. It is so easy to hide
our ignorance under such expressions as the `plan of creation,' `unity of
design,' &c., and to think that we give an explanation when we only restate
a fact. Any one whose disposition leads him to attach more weight to
unexplained difficulties than to the explanation of a certain number of facts
will certainly reject my theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt on the immutability of
species, may be influenced by this volume; but I look with confidence to the
future, to young and rising naturalists, who will be able to view both sides of
the question with impartiality. Whoever is led to believe that species are
mutable will do good service by conscientiously expressing his conviction; for
only thus can the load of prejudice by which this subject is overwhelmed be
removed.
Several eminent
naturalists have of late published their belief that a multitude of reputed
species in each genus are not real species; but that other species are real,
that is, have been independently created. This seems to me a strange conclusion
to arrive at. They admit that a multitude of forms, which till lately they
themselves thought were special creations, and which are still thus looked at
by the majority of naturalists, and which consequently have every external
characteristic feature of true species, -- they admit that these have been
produced by variation, but they refuse to extend the same view to other and
very slightly different forms. Nevertheless they do not pretend that they can
define, or even conjecture, which are the created forms of life, and which are
those produced by secondary laws. They admit variation as a vera causa in one
case, they arbitrarily reject it in another, without assigning any distinction
in the two cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors seem no
more startled at a miraculous act of creation than at an ordinary birth. But do
they really believe that at innumerable periods in the earth's history certain
elemental atoms have been commanded suddenly to flash into living tissues? Do
they believe that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants created as
eggs or seed, or as full grown? and in the case of mammals, were they created
bearing the false marks of nourishment from the mother's womb? Although
naturalists very properly demand a full explanation of every difficulty from
those who believe in the mutability of species, on their own side they ignore
the whole subject of the first appearance of species in what they consider
reverent silence.
It may be asked how far
I extend the doctrine of the modification of species. The question is difficult
to answer, because the more distinct the forms are which we may consider, by so
much the arguments fall away in force. But some arguments of the greatest
weight extend very far. All the members of whole classes can be connected
together by chains of affinities, and all can be classified on the same
principle, in groups subordinate to groups. Fossil remains sometimes tend to
fill up very wide intervals between existing orders. Organs in a rudimentary
condition plainly show that an early progenitor had the organ in a fully
developed state; and this in some instances necessarily implies an enormous
amount of modification in the descendants. Throughout whole classes various
structures are formed on the same pattern, and at an embryonic age the species
closely resemble each other. Therefore I cannot doubt that the theory of
descent with modification embraces all the members of the same class. I believe
that animals have descended from at most only four or five progenitors, and
plants from an equal or lesser number.
Analogy would lead me
one step further, namely, to the belief that all animals and plants have
descended from some one prototype. But analogy may be a deceitful guide.
Nevertheless all living things have much in common, in their chemical
composition, their germinal vesicles, their cellular structure, and their laws
of growth and reproduction. We see this even in so trifling a circumstance as
that the same poison often similarly affects plants and animals; or that the
poison secreted by the gallfly produces monstrous growths on the wild rose or
oak-tree. Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed.
When the views
entertained in this volume on the origin of species, or when analogous views
are generally admitted, we can dimly foresee that there will be a considerable
revolution in natural history. Systematists will be able to pursue their
labours as at present; but they will not be incessantly haunted by the shadowy
doubt whether this or that form be in essence a species. This I feel sure, and
I speak after experience, will be no slight relief. The endless disputes
whether or not some fifty species of British brambles are true species will
cease. Systematists will have only to decide (not that this will be easy)
whether any form be sufficiently constant and distinct from other forms, to be
capable of definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a far more
essential consideration than it is at present; for differences, however slight,
between any two forms, if not blended by intermediate gradations, are looked at
by most naturalists as sufficient to raise both forms to the rank of species.
Hereafter we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known, or
believed, to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without quite rejecting
the consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher the
actual amount of difference between them. It is quite possible that forms now
generally acknowledged to be merely varieties may hereafter be thought worthy
of specific names, as with the primrose and cowslip; and in this case
scientific and common language will come into accordance. In short, we shall
have to treat species in the same manner as those naturalists treat genera, who
admit that genera are merely artificial combinations made for convenience. This
may not be a cheering prospect; but we shall at least be freed from the vain
search for the undiscovered and undiscoverable essence of the term species.
The other and more
general departments of natural history will rise greatly in interest. The terms
used by naturalists of affinity, relationship, community of type, paternity,
morphology, adaptive characters, rudimentary and aborted organs, &c., will
cease to be metaphorical, and will have a plain signification. When we no
longer look at an organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production of nature as
one which has had a history; when we contemplate every complex structure and
instinct as the summing up of many contrivances, each useful to the possessor,
nearly in the same way as when we look at any great mechanical invention as the
summing up of the labour, the experience, the reason, and even the blunders of
numerous workmen; when we thus view each organic being, how far more
interesting, I speak from experience, will the study of natural history become
!
A grand and almost
untrodden field of inquiry will be opened, on the causes and laws of variation,
on correlation of growth, on the effects of use and disuse, on the direct
action of external conditions, and so forth. The study of domestic productions
will rise immensely in value. A new variety raised by man will be a far more
important and interesting subject for study than one more species added to the
infinitude of already recorded species. Our classifications will come to be, as
far as they can be so made, genealogies; and will then truly give what may be
called the plan of creation. The rules for classifying will no doubt become
simpler when we have a definite object in view. We possess no pedigrees or
armorial bearings; and we have to discover and trace the many diverging lines
of descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect to
the nature of long-lost structures. Species and groups of species, which are
called aberrant, and which may fancifully be called living fossils, will aid us
in forming a picture of the ancient forms of life. Embryology will reveal to us
the structure, in some degree obscured, of the prototypes of each great class.
When we can feel
assured that all the individuals of the same species, and all the closely
allied species of most genera, have within a not very remote period descended
from one parent, and have migrated from some one birthplace; and when we better
know the many means of migration, then, by the light which geology now throws,
and will continue to throw, on former changes of climate and of the level of
the land, we shall surely be enabled to trace in an admirable manner the former
migrations of the inhabitants of the whole world. Even at present, by comparing
the differences of the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants of that continent in
relation to their apparent means of immigration, some light can be thrown on
ancient geography.
The noble science of
Geology loses glory from the extreme imperfection of the record. The crust of
the earth with its embedded remains must not be looked at as a well-filled
museum, but as a poor collection made at hazard and at rare intervals. The
accumulation of each great fossiliferous formation will be recognised as having
depended on an unusual concurrence of circumstances, and the blank intervals
between the successive stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We must be cautious
in attempting to correlate as strictly contemporaneous two formations, which
include few identical species, by the general succession of their forms of
life. As species are produced and exterminated by slowly acting and still
existing causes, and not by miraculous acts of creation and by catastrophes;
and as the most important of all causes of organic change is one which is
almost independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism, -- the improvement of one
being entailing the improvement or the extermination of others; it follows,
that the amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual time. A number of
species, however, keeping in a body might remain for a long period unchanged,
whilst within this same period, several of these species, by migrating into new
countries and coming into competition with foreign associates, might become
modified; so that we Must not overrate the accuracy of organic change as a
measure of time. During early periods of the earth's history, when the forms of
life were probably fewer and simpler, the rate of change was probably slower;
and at the first dawn of life, when very few forms of the simplest structure
existed, the rate of change may have been slow in an extreme degree. The whole
history of the world, as at present known, although of a length quite
incomprehensible by us, will hereafter be recognised as a mere fragment of
time, compared with the ages which have elapsed since the first creature, the
progenitor of innumerable extinct and living descendants, was created.
In the distant future I
see open fields for far more important researches. psychology will be based on
a new foundation, that of the necessary acquirement of each mental power and
capacity by gradation. Light will be thrown on the origin of man and his
history.
Authors of the highest
eminence seem to be fully satisfied with the view that each species has been
independently created. To my mind it accords better with what we know of the
laws impressed on matter by the Creator, that the production and extinction of
the past and present inhabitants of the world should have been due to secondary
causes, like those determining the birth and death of the individual. When I
view all beings not as special creations, but as the lineal descendants of some
few beings which lived long before the first bed of the Silurian system was
deposited, they seem to me to become ennobled. judging from the past, we may
safely infer that not one living species will transmit its unaltered likeness
to a distant futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species of each
genus, and all the species of many genera, have left no descendants, but have
become utterly extinct. We can so far take a prophetic glance into futurity as
to fortell that it will be the common and widely-spread species, belonging to
the larger and dominant groups, which will ultimately prevail and procreate new
and dominant species. As all the living forms of life are the lineal
descendants of those which lived long before the Silurian epoch, we may feel
certain that the ordinary succession by generation has never once been broken,
and that no cataclysm has desolated the whole world. Hence we may look with
some confidence to a secure future of equally inappreciable length. And as
natural selection works solely by and for the good of each being, all corporeal
and mental endowments will tend to progress towards perfection.
It is interesting to
contemplate an entangled bank, clothed with many plants of many kinds, with
birds singing on the bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these elaborately
constructed forms, so different from each other, and dependent on each other in
so complex a manner, have all been produced by laws acting around us. These
laws, taken in the largest sense, being Growth with Reproduction; inheritance
which is almost implied by reproduction; Variability from the indirect and
direct action of the external conditions of life, and from use and disuse; a
Ratio of Increase so high as to lead to a Struggle for Life, and as a
consequence to Natural Selection, entailing Divergence of Character and the
Extinction of less-improved forms. Thus, from the war of nature, from famine
and death, the most exalted object which we are capable of conceiving, namely,
the production of the higher animals, directly follows. There is grandeur in
this view of life, with its several powers, having been originally breathed
into a few forms or into one; and that, whilst this planet has gone cycling on
according to the fixed law of gravity, from so simple a beginning endless forms
most beautiful and most wonderful have been, and are being, evolved.
I am indebted to the
kindness of Mr. W. S. Dallas for this Glossary, which has been given because
several readers have complained to me that some of the terms used were
unintelligible to them. Mr. Dallas has endeavoured to give the explanations of
the terms in as popular a form as possible.
ABERRANT. --- Forms or
groups of animals or plants which deviate in important characters from their
nearest allies, so as not to be easily included in the same group with them,
are said to be aberrant.
ABERRATION (in Optics).
--- In the refraction of light by a convex lens the rays passing through
different parts of the lens are brought to a focus at slightly different
distances, --- this is called sphericalaberration; at the same time the
coloured rays are separated by the prismatic action of the lens and likewise
brought to a focus at different distances, --- this is chromatic aberration.
ABNORMAL. --- Contrary
to the general rule.
ABORTED. --- An organ
is said to be aborted, when its development has been arrested at a very early
stage.
ALBINISM. -- Albinos
are animals in which the usual colouring matters characteristic of the species
have not been produced in the skin and its appendages. Albinism is the state of
being an albino.
ALGAE. --- A class of
plants including the ordinary sea-weeds and the filamentous fresh-water weeds.
ALTERNATION OF
GENERATIONS. --- This term is applied to a peculiar mode of reproduction which
prevails among many of the lower animals, in which the egg produces a living
form quite different from its parent, but from which the parent-form is
reproduced by a process of budding, or by the division of the substance of the
first product of the egg.
AMMONITES. --- A group
of fossil, spiral, chambered shells, allied to the existing pearly Nautilus,
but having the partitions between the chambers waved in complicated patterns at
their junction with the outer wall of the shell.
ANALOGY. --- That
resemblance of structures which depends upon similarity of function, as in the
wings of insects and birds. Such structures are said to be analogous, and to be
analogues of each other.
ANIMAALCULE. -- A
minute animal: generally applied to those visible only by the microscope.
ANNELIDS. --- A class
of worms in which the surface of the body exhibits a more or less distinct
division into rings or segments, generally provided with appendages for
locomotion and with gills. It includes the ordinary marine worms, the
earthworms, and the leeches.
ANTENNÆ. --- Jointed
organs appended to the head in Insects, Crustacea and Centipedes, and not
belonging to the mouth.
ANTHERS. --- The
summits of the stamens of flowers, in which the pollen or fertilising dust is
produced.
APLACENTALIA,
APLACENTATA or Aplacental Mammals. See Mammalia.
ARCHETYPAL. --- Of or
belonging to the Archetype, or ideal primitive form upon which all the beings
of a group seem to be organised.
ARTICULATA. --- A great
division of the Animal Kingdom characterised generally by having the surface of
the body divided into rings called segments, a greater or less number of which
are furnished with jointed legs (such as Insects, Crustaceans and Centipedes).
ASYMMETRICAL. ---
Having the two sides unlike.
ATROPHIED. --- Arrested
in development at a very early stage.
BALANUS. --- The genus
including the common Acorn-shells which live in abundance on the rocks of the
sea-coast.
BATRACHIANS. -- A class
of animals allied to the Reptiles, but undergoing a peculiar metamorphosis, in
which the young animal is generally aquatic and breathes by gills. (Examples,
Frogs, Toads, and Newts.)
BOULDERS. --- Large
transported blocks of stone generally imbedded in clays or gravels.
BRACHIOPODA. --- A
class of marine Mollusca, or soft-bodied animals, furnished with a bivalve
shell, attached to submarine objects by a stalk which passes through an
aperture in one of the valves, and furnished with fringed arms, by the action
of which food is carried to the mouth.
BRANCHIÆ. --- Gills or
organs for respiration in water.
BRANCHIAL. ---
Pertaining to gills or branchiæ.
CAMBRIAN SYSTEM. --- A
Series of very ancient Palæozoic rocks, between the Laurentian and the
Silurian. Until recently these were regarded as the oldest fossiliferous rocks.
CANIDÆ. --- The
Dog-family, including the Dog, Wolf, Fox, Jackal, &c.
CARAPACE. --- The shell
enveloping the anterior part of the body in Crustaceans generally; applied also
to the hard shelly pieces of the Cirripedes.
CARBONIFEROUS. --- This
term is applied to the great formation which includes, among other rocks, the
coal-measures. It belongs to the oldest, or Palæozoic, system of formations.
CAUDAL. --- Of or
belonging to the tail.
CEPHALOPODS. --- The
highest class of the Mollusca, or Soft- bodied animals, characterised by having
the mouth surrounded by a greater or less number of fleshy arms or tentacles,
which, in most living species, are furnished with sucking- cups. (Examples,
Cuttle-fish, Nautilus.)
CETACEA. --- An order
of Mammalia, including the Whales, Dolphins, &c., having the form of the
body fish-like, the skin naked, and only the fore-limbs developed.
CHELONIA. --- An order
of Reptiles including the Turtles, Tortoises, &c.
CIRRIPEDES. --- An
order of Crustaceans including the Barnacles and Acorn-shells. Their young
resemble those of many other Crustaceans in form; but when mature they are
always attached to other objects, either directly or by means of a Stalk, and
their bodies are enclosed by a calcareous shell composed of several pieces, two
of which can open to give issue to a bunch Of curled, jointed tentacles, which
represent the limbs.
COCCUS. --- The genus
of Insects including the Cochineal. In these the male is a minute, winged fly,
and the female generally a motionless, berry-like mass.
COCOON. --- A case usually
of Silky material, in which insects are frequently enveloped during the second
or resting-stage (pupa) of their existence. The term 'cocoon-stage' is here
used as equivalent to 'pupa-stage.'
CŒLOSPERMOUS. --- A
term applied to those fruits of the Umbelliferæ which have the seed hollowed on
the inner face.
COLEOPTERA. ---
Beetles, an order of Insects, having a biting mouth and the first pair of wings
more or less horny, forming Sheaths for the second pair, and usually meeting in
a straight line down the middle of the back.
COLUMN. --- A peculiar
organ in the flowers of Orchids, in which the stamens, style and stigma (or the
reproductive parts) are united.
COMPOSITÆ or
COMPOSITOUS PLANTS. --- Plants in which the inflorescence consists of numerous
small flowers (florets) brought together into a dense head, the base of which
is enclosed by a common envelope. (Examples, the Daisy, Dandelion, &c.)
CONFERVÆ. --- The
filamentous weeds of fresh water.
CONGLOMERATE. --- A
rock made up of fragments of rock or pebbles, cemented together by some other
material.
COROLLA. --- The second
envelope of a flower usually composed of coloured, leaf-like organs (petals),
which may be united by their edges either in the basal part or throughout.
CORRELATION. --- The
normal coincidence of one phenomenon, character, &c., with another.
CORYMB. --- A bunch of
flowers in which those springing from the lower part of the flower stalk are
supported on long stalks so as to be nearly on a level with the upper ones.
COTYLEDONS. --- The
first or seed-leaves of plants.
CRUSTACEANS. --- A
class of articulated animals, having the skin of the body generally more or
less hardened by the deposition of calcareous matter, breathing by means of
gills. (Examples, Crab, Lobster, Shrimp, &c.)
CURCULIO. --- The old
generic term for the Beetles known as Weevils, characterised by their
four-jointed feet, and by the head being produced into a sort of beak, upon the
sides of which the antennæ are inserted.
CUTANEOUS. --- Of or
belonging to the skin.
DEGRADATION. --- The
wearing down of land by the action of the sea or of meteoric agencies.
DENUDATION. --- The wearing
away of the surface of the land by water.
DEVONIAN SYSTEM or
formation. --- A series of Palæozoic rocks, including the Old Red Sandstone.
DICOTYLEDONS or
DICOTYLEDONOUS PLANTS. --- A class of plants characterised by having two
seed-leaves, by the formation of new wood between the bark and the old wood
(exogenous growth) and by the reticulation of the veins of the leaves. The
parts of the flowers are generally in multiples of five.
DIFFERENTIATION. ---
The separation or discrimination of parts or organs which in simpler forms of
life are more or less united.
DIMORPHIC. --- Having
two distinct forms. --- Dimorphism is the condition of the appearance of the
same species under two dissimilar forms.
DIŒCIOUS. --- Having
the organs of the sexes upon distinct individuals.
DIORITE. -- A peculiar
form of Greenstone.
DORSAL. --- Of or
belonging to the back.
EDENTATA. --- A
peculiar order of Quadrupeds, characterised by the absence of at least the
middle incisor (front) teeth in both jaws. (Examples, the Sloths and
Armadillos.)
ELYTRA. --- The
hardened fore-wings of Beetles, serving as sheaths for the membranous
hind-wings, which constitute the true organs of flight.
EMBRYO. --- The young
animal undergoing development within the egg or womb.
EMBRYOLOGY. --- The
study of the development of the embryo.
ENDEMIC. --- Peculiar
to a given locality.
ENTOMOSTRACA. --- A
division of the class Crustacea, having all the Segments of the body usually
distinct, gills attached to the feet or organs of the mouth, and the feet
fringed with fine hairs. They are generally of Small size.
EOCENE. --- The
earliest of the three divisions of the Tertiary epoch of geologists. Rocks of
this age contain a small proportion of shells identical with species now living.
EPHEMEROUS INSECTS. --
Insects allied to the May-fly.
FAUNA. --- The totality
of the animals naturally inhabiting a certain country or region, or which have
lived during a given geological period.
FELIDÆ. --- The
Cat-family.
FERAL. --- Having become
wild from a state of cultivation or domestication.
FLORA. --- The totality
of the plants growing naturally in a country, or during a given geological
period.
FLORETS. --- Flowers
imperfectly developed in some respects, and collected into a dense spike or
head, as in the Grasses, the Dandelion, &c.
FŒTAL. --- Of or
belonging to the fœtus, or embryo in course of development.
FORAMINIFERA. --- A
class of animals of very low organisation, and generally of small size, having
a jelly- like body, from the Surface of which delicate filaments can be given
off and retracted for the prehension of external objects, and having a
calcareous or sandy shell, usually divided into chambers, and perforated with
small apertures.
FOSSILIFEROUS. ---
Containing fossils.
FOSSORIAL. --- Having a
faculty of digging. The Fossorial Hymenoptera are a group of Wasp-like Insects,
which burrow in sandy soil to make nests for their young.
FRENUM (pl. FRENA). ---
A small band or fold of skin.
FUNGI (Sing. FUNGUS).
--- A class of cellular plants, of which Mushrooms, Toadstools, and Moulds, are
familiar examples.
FURCULA. --- The forked
bone formed by the union of the collarb ones in many birds, such as the common
Fowl.
GALLINACEOUS BIRDS. ---
An order of Birds of which the common Fowl, Turkey, and Pheasant, are
well-known examples.
GALLUS. --- The genus
of birds which includes the common Fowl.
GANGLION. --- A
swelling or knot from which nerves are given off as from a centre.
GANOID FISHES. ---
Fishes covered with peculiar enamelled bony scales. Most of them are extinct.
GERMINAL VESICLE. --- A
minute vesicle in the eggs of animals, from which development of the embryo
proceeds.
GLACIAL PERIOD. --- A
period of great cold and of enormous extension of ice upon the surface of the
earth. It is believed that glacial periods have occurred repeatedly during the
geological history of the earth, but the term is generally applied to the close
of the Tertiary epoch, when nearly the whole of Europe was subjected to an
arctic climate.
GLAND. -- An organ
which secretes or separates some peculiar product from the blood or sap of
animals or plants.
GLOTTIS. -- The opening
of the windpipe into the œsophagus or gullet.
GNEISS. --- A rock
approaching granite in composition, but more or less laminated, and really
produced by the alteration of a sedimentary deposit after its consolidation.
GRALLATORES. --- The
so-called Wading-birds (Storks, Cranes, Snipes, &c.), which are generally
furnished with long legs, bare of feathers above the heel, and have no
membranes between the toes.
GRANITE. --- A rock
consisting essentially of crystal of felspar and mica in a mass of quarts.
HABITAT. --- The
locality in which a plant or animal naturally lives.
HEMIPTERA. --- An order
or sub-order of Insects, characterised by the possession of a jointed beak or
rostrum, and by having the fore-wings horny in the basal portion and membranous
at the extremity, where they cross each other. This group includes the various
species of Bugs.
HERMAPHRODITE. ---
Possessing the organs of both sexes.
HOMOLOGY. --- That
relation between parts which results from their development from corresponding
embryonic parts, either in different animals, as in the case of the arm of man,
the foreleg of a quadruped, and the wing of a bird; or in the same individual,
as in the case of the fore and hind legs in quadrupeds, and the segments or
rings and their appendages of which the body of a worm, a centipede, &c.,
is composed. The latter is called serial homology. The parts which stand in
such a relation to each other are said to be homologous, and one such part or
organ is called the homologue of the other. In different plants the parts of
the flower are homologous, and in general these parts are regarded as
homologous with leaves.
HOMOPTERA. --- An order
or sub-order of Insects having (like the Hemiptera) a jointed beak, but in
which the fore-wings are either wholly membranous or wholly leathery. The Cicadœ,
Frog-hoppers, and Aphides, are well-known examples.
HYBRID. --- The
offspring of the union of two distinct species.
HYMENOPTERA. --- An
order of insects possessing biting jaws and usually four membranous wings in
which there are a few veins. Bees and Wasps are familiar examples of this
group.
HYPERTROPHIED. ---
Excessively developed.
ICHNEUMONIDÆ. --- A
family of Hymenopterous insects, the members of which lay their eggs in the
bodies or eggs of other insects.
IMAGO. --- The perfect
(generally winged) reproductive state of an insect.
INDIGENS. --- The
aboriginal animal or vegetable inhabitants of a country or region.
INFLORESCENCE. --- The
mode of arrangement of the flowers of plants.
INFUSORIA. --- A class
of microscopic Animalcules, so called from their having originally been
observed in infusions of vegetable matters. They consist of a gelatinous
material enclosed in a delicate membrane, the whole or part of which is
furnished with short vibrating hairs (called cilia), by means of which the
animalcules swim through the water or convey the minute particles of their food
to the orifice of the mouth.
INSECTIVOROUS. ---
Feeding on Insects.
INVERTEBRATA, or
INVERTEBRATE ANIMALS. --- Those animals which do not possess a backbone or
spinal column.
LACUNÆ. --- Spaces left
among the tissues in some of the lower animals, and serving in place of vessels
for the circulation of the fluids of the body.
LAMIELLATED. ---
Furnished with lamellæ or little plates.
LARVA (pl. LARVÆ). ---
The first condition of an insect at its issuing from the egg, when it is
usually in the form of a grub, caterpillar, or maggot.
LARYNX. --- The upper
part of the windpipe opening into the gullet.
LAURENTIAN. --- A group
of greatly altered and very ancient rocks, which is greatly developed along the
course of the St. Laurence, whence the name. It is in these that the earliest
known traces of organic bodies have been found.
LEGUMINOSÆ. --- An
order of plants represented by the common Peas and Beans, having an irregular
flower in which one petal stands up like a wing, and the stamens and pistil are
enclosed in a sheath formed by two Other petals. The fruit is a pod (or
legume).
LEMURIDÆ. --- A group
of four-handed animals, distinct from the Monkeys and approaching the
Insectivorous Quadrupeds in some of their characters and habits. Its members
have the nostrils curved or twisted, and a claw instead of a nail upon the
first finger of the hind hands.
LEPIDOPTERA. --- An
order of Insects, characterised by the possession of a spiral proboscis, and of
four large more or less scaly wings. It includes the well-known Butterflies and
Moths.
LITTORAL. ---
Inhabiting the seashore.
LOESS. --- A marly
deposit of recent (Post-Tertiary) date, which occupies a great part of the
valley of the Rhine.
MALACOSTRACA. --- The
higher division of the Crustacea, including the ordinary Crabs, Lobsters,
Shrimps, &c., together with the Woodlice and Sand-hoppers.
MAMMALIA. --- The
highest class of animals, including the ordinary hairy quadrupeds, the Whales,
and Man, and characterised by the production of living young which are
nourished after birth by milk from the teats (Mammœ, Mammary glands) of the
mother. A striking difference in embryonic development has led to the division
of this class into two great groups; in one Of these, when the embryo has
attained a certain stage, a vascular connection, called the placenta, is formed
between the embryo and the mother; in the other this is wanting, and the young
are produced in a very incomplete state. The former, including the greater part
of the class, are called Placentalmammals; the latter, or Aplacental mammals,
include the Marsupials and Monotremes (Ornithorhynchus).
MAMMIFEROUS. Having
mammæ; or teats (See MAMMALIA).
MANDIBLES, in Insects.
--- The first or uppermost pair of jaws, which are generally solid, horny,
biting organs. In Birds the term is applied to both jaws with their horny
coverings. In Quadrupeds the mandible is properly the lower jaw.
MARSUPIALS. --- An
order of Mammalia in which the young are born in a very incomplete state of
development, and carried by the mother, while sucking, in a ventral pouch
(marsupium), such as the Kangaroos, Opossums, &c. (see MAMMALIA).
MAXILLÆ, in Insects.
--- The second or lower pair of jaws, which are composed of several joints and
furnished with peculiar jointed appendages called palpi, or feelers.
MELANISM. --- The
opposite of albinism; an undue development of colouring material in the skin
and its appendages.
METAMORPHIC ROCKS. ---
Sedimentary rocks which have undergone alteration, generally by the action of
heat, subsequently to their deposition and consolidation.
MOLLUSCA. --- One of
the great divisions of the Animal Kingdom, including those animals which have a
soft body, usually furnished with a shell, and in which the nervous ganglia, or
centres, present no definite general arrangement. They are generally known
under the denomination of "" shell-fish;'' the cuttle-fish, and the
common snails, whelks, oysters, mussels, and cockles, may serve as examples of
them.
MONOCOTYLEDONS, Or
MONOCOTYLEDONOUS PLANTS. --- Plants in which the seed sends up only a single
seed-leaf (or cotyledon); characterised by the absence of consecutive layers of
wood in the stem (endogenous growth), by the veins of the leaves being
generally straight, and by the parts of the flowers being generally in
multiples of three. (Examples, Grasses, Lilies, Orchids, Palms, &c.)
MORAINES. --- The
accummulations of fragments of rock brought down by glaciers.
MORPHOLOGY. --- The law
of form or structure independent of function.
MYSIS-STAGE. --- A
stage in the development of certain Crustaceans (Prawns), in which they closely
resemble the adults of a genus (Mysis) belonging to a slightly lower group.
NASCENT. --- Commencing
development.
NATATORY. --- Adapted
for the purpose of swimming.
NAUPLIUS-FORM. --- The
earliest stage in the development of many Crustacea, especially belonging to
the lower groups. In this stage the animal has a short body, with indistinct
indications of a division into segments, and three pairs of fringed limbs. This
form of the common fresh-water Cyclops was described as a distinct genus under
the name Of Nauplius.
NEURATION. --- The
arrangement of the veins or nervures in the wings of Insects.
NEUTERS. ---
Imperfectly developed females of certain social insects (such as Ants and
Bees), which perform all the labours of the community. Hence they are also
called workers.
NICTITATING MEMBRANE.
--- A semi-transparent membrane, which can be drawn across the eye in Birds and
Reptiles, either to moderate the effects of a strong light or to sweep
particles of dust, &c., from the surface of the eye.
OCELLI --- The simple
eyes or stemmata of Insects, usually situated on the crown of the head between
the great compound eyes.
ŒSOPHAGUS. --- The
gullet.
OOLITIC. --- A great
series Of secondary rocks, so called from the texture of some of its members, which
appear to be made up Of a mass of small egg-like calcareous bodies.
OPERCULUM. --- A
calcareous plate employed by many Mollusca to close the aperture of their
shell. The opercular valves of Cirripedes are those which close the aperture of
the shell.
ORBIT. --- The bony
cavity for the reception of the eye.
ORGANISM. --- An
organised being, whether plant or animal.
ORTHOSPERMOUS. --- A
term applied to those fruits of the Umbel, liferæ which have the seed straight.
OSCULANT. --- Forms or
groups apparently intermediate between and connecting other groups are said to
be osculant.
OVA. --- Eggs.
OVARIUM or OVARY (in
plants). --- The lower part of the pistil or female organ of the flower,
containing the ovules or incipient seeds; by growth after the other organs of
the flower have fallen, it usually becomes converted into the fruit.
OVIRGEROUS. ---
Egg-bearing.
OVULES (of plants). ---
The seeds in the earliest condition.
PACHYDERMS. --- A group
of Mammalia, so called from their thick skins, and including the Elephant,
Rhinoceros, Hippopotamus, &c.
PALÆOZOIC. --- The
oldest system of fossiliferous rocks.
PALPI. --- Jointed
appendages to some of the organs of the mouth in Insects and Crustacea.
PAPILIONACEÆ. --- An
order of Plants (see LEGUMINOSÆ). --- The flowers of these plants are called
papilionaceous, or butterfly-like, from the fancied resemblance of the expanded
superior petals to the wings of a butterfly.
PARASITE. --- An animal
or plant living upon or in, and at the expense of, another organism.
PARTHENOGENESIS. ---
The production Of living Organisms from unimpregnated eggs or seeds.
PEDUNCAULTAED. ---
Supported upon a stem or stalk. The pedunculated oak has its acorns borne upon
a footstalk.
PELORIA or PELORISM.
--- The appearance of regularity of structure in the flowers of plants which
normally bear irregular flowers.
PELVIS. --- The bony
arch to which the hind limbs of Vertebrate animals are articulated.
PETALS. --- The leaves
of the corolla, or second circle of organs in a flower. They are usually of
delicate texture and brightly coloured.
PHYLLODINEOUS. ---
Having flattened, leaf-like twigs or leafstalks instead of true leaves.
PIGMENT. --- The
colouring material produced generally in the superficial parts of animals. The
cells secreting it are called pigment-cells.
PINNATE. --- Bearing
leaflets on each side of a Central stalk.
PISTILS. --- The female
organs of a flower, which occupy a position in the centre of the other floral
organs. The pistil is generally divisible into the ovary or germen, the style
and the stigma.
PLACENTALIA,
PLACENTATA, or Placental Mammals. --- See MAMALIA.
PLANTIGRADES. ---
Quadrupeds which walk upon the whole sole of the foot, like the Bears.
PLASTIC. --- Readily
capable of change.
PLEISTOCENE PERIOD. ---
The latest portion of the Tertiary epoch.
PLUMULE (in plants).
--- The minute bud between the seed-leaves of newly-germinated plants.
PLUTONIC ROCKS. ---
Rocks supposed to have been produced by igneous action in the depths of the
earth.
POLLEN. --- The male
element in flowering plants; usually a fine dust produced by the anthers,
which, by contact with the stigma effects the fecundation of the seeds. This
impregnation is brought about by means of tubes (pollen- tubes) which issue
from the pollen-grains adhering to the stigma, and penetrate through the
tissues until they reach the ovary.
POLYANDROUS (flowers).
--- Flowers having many stamens.
POLYGAMOUS PLANTS. ---
Plants in which some flowers are unisexual and others hermaphrodite. The
unisexual (male and female) flowers, may be on the same or on different plants.
POLYMORPHIC. ---
Presenting many forms.
POLYZOARY. --- The
common structure formed by the cells of the Polyzoa, such as the well-known
Sea-mats.
PREHENSILE. --- Capable
of grasping.
PREPOTENT. --- Having a
superiority of power.
PRIMARIES. --- The
feathers forming the tip of the wing of a bird, and inserted upon that part
which represents the hand of man.
PROCESSES. ---
Projecting portions of bones, usually for the attachment of muscles, ligaments,
&c.
PROPOLIS. --- A
resinous material collected by the Hive-Bees from the opening buds of various
trees.
PROTEAN. ---
Exceedingly variable.
PROTOZOA. --- The
lowest great division of the Animal Kingdom. These animals are composed of a
gelatinous material, and show scarcely any trace of distinct organs. The
Infusoria, Foraminifera, and Sponges, with some other forms, belong to this
division.
PUPA (pl. PUPÆ). ---
The second stage in the development of an Insect, from which it emerges in the
perfect (winged) reproductive form. In most insects the pupal stage is passed
in perfect repose. The chrysalis is the pupal state of butterflies.
RADICLE. --- The minute
root of an embryo plant.
RAMUS. --- One half of
the lower jaw in the Mammalia. The portion which rises to articulate with the
skull is called the ascendingramus.
RANGE. --- The extent
of country over which a plant or animal is naturally spread. Range in time
expresses the distribution of a species or group through the fossiliferous beds
of the earth's crust.
RETINA. --- The
delicate inner coat of the eye, formed by nervous filaments spreading from the
optic nerve, and serving for the perception of the impressions produced by
light.
RETROGRESSION. ---
Backward development. When an animal, as it approaches maturity, becomes less
perfectly organised than might be expected from its early stages and known
relationships, it is said to undergo a retrograde development or metamorphosis.
RHIZOPODS. --- A class
of lowly organised animals (protozoa), having a gelatinous body, the surface of
which can be protruded in the form of root-like processes or filaments, which serve
for locomotion and the prehension of food. The most important order is that of
the Foraminifera.
RODENTS. --- The
gnawing Mammalia, such as the Rats, Rabbits, and Squirrels. They are especially
characterised by the possession of a single pair of chisel- like cutting teeth
in each jaw, between which and the grinding teeth there is a great gap.
RUBUS. --- The Bramble
Genus.
RUDIMENTARY. --- Very
imperfectly developed.
RUMINANTS. --- The
group of Quadrupeds which ruminate or chew the cud, such as oxen, sheep, and
deer. They have divided hoofs, and are destitute of front teeth in the upper
jaw.
SACRAL. --- Belonging
to the sacrum, or the bone composed usually of two or more united vertebræ to
which the sides of the pelvis in Vertebrate animals are attached.
SARCODE. --- The
gelatinous material of which the bodies of the lowest animals (Protozoa) are
composed.
SCUTELLÆ. --- The horny
plates with which the feet of birds are generally more or less covered,
especially in front.
SEDIMENTARY FORMATIONS.
--- Rocks deposited as sediments from water.
SEGMENTS. --- The
transverse rings of which the body of an articulate animal or Annelid is
composed.
SEPALS. --- The leaves
or segments of the calyx, or outermost envelope of an ordinary flower. They are
usually green, but sometimes brightly coloured.
SERRATURES. --- Teeth
like those of a saw.
SESSILE. --- Not
supported on a stem or footstalk.
SILURIAN SYSTEM. --- A
Very ancient system of fossiliferous rocks belonging to the earlier part of the
Palæozoic series.
SPECIALISATION. --- The
setting apart of a particular organ for the performance of a particular
function.
SPINAL CHORD. --- The
central portion of the nervous system in the Vertebrata, which descends from
the brain through the arches of the vertebræ, and gives off nearly all the
nerves to the various organs of the body.
STAMENS. --- The male
organs of flowering plants, standing in a circle within the petals. They
usually consist of a filament and an anther, the anther being the essential
part in which the pollen, or fecundating dust, is formed.
STERNUM. --- The
breast-bone.
STIGMA. --- The apical
portion of the pistil in flowering plants.
STIPULES. --- Small
leafy organs placed at the base of the footstalks of the leaves in many plants.
STYLE. --- The middle
portion of the perfect pistil, which rises like a column from the ovary and
supports the stigma at its summit.
SUBCUTANEOUS. ---
Situated beneath the skin.
SUCTORIAL. --- Adapted
for sucking.
SUTURES (in the skull).
--- The lines of junction of the bones of which the skull is composed.
TARSUS (pl. TARSI). ---
The jointed feet of articulate animals, such as Insects.
TELEOSTEAN FISHES. ---
Fishes of the kind familiar to us in the present day, having the skeleton
usually completely ossified and the scales horny.
TENTACULA or TENTACLES.
--- Delicate fleshy organs of prehension or touch possessed by many of the
lower animals.
TERTIARY. --- The
latest geological epoch, immediately preceding the establishment of the present
order of things.
TRACHEA. --- The
windpipe or passage for the admission of air to the lungs.
TRIDACTYLE. ---
Three-fingered, or composed of three movable parts attached to a common base.
TRILOBITES. --- A
peculiar group of extinct Crustaceans, somewhat resembling the Woodlice in
external form, and, like some of them, capable of rolling themselves up into a
ball. Their remains are found only in the Palæozoic rocks, and most abundantly
in those of Silurian age.
TRIMORPHIC. --- Presenting
three distinct forms.
UMBELLIFERÆ. --- An
order of plants in which the flowers, which contain five stamens and a pistil
with two styles, are supported upon footstalks which spring from the top of the
flower stem and spread out like the wires of an umbrella, so as to bring all
the flowers in the same head (umbel) nearly to the same level. (Examples,
Parsley and Carrot).
UNGULATA. --- Hoofed
quadrupeds.
UNICELLULAR. ---
Consisting of a single cell.
VASCULAR. ---
Containing blood-vessels
VERMIFORM. --- Like a
worm.
VERTEBRATA: or
VERTEBRATE ANIMALS. --- The highest division of the animal kingdom, so called
from the presence in most cases of a backbone composed of numerous joints or
vertebrœ, which constitutes the centre of the skeleton and at the same time
supports and protects the central parts of the nervous system.
WHORLS. --- The circles
or spiral lines in which the parts of plants are arranged upon the axis of
growth.
WORKERS. --- See
neuters.
ZOEA-STAGE. --- The
earliest stage in the development of many of the higher Crustacea, so called
from the name of Zoea applied to these young animals when they were supposed to
constitute a peculiar genus.
ZOOIDs. --- In many of
the lower animals (such as the Corals, Medusæ, &c.) reproduction takes
place in two ways, namely, by means of eggs and by a process of budding with or
without separation from the parent of the product of the latter, which is often
Very different from that of the egg. The individuality of the species is
represented by the whole of the form produced between two sexual reproductions;
and these forms, which are apparently individual animals, have been called
zooids.