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Darwin's " Origin of Species " has now passed out of 

It should, however ; be clearly understood that the edition 
which thus loses its legal protection is the imperfect edition 
which the author subsequently revised, and which was 
accordingly superseded. The complete and authorized 
edition of the work wiU not lose copyright fir some years. 

The only complete editions authorized by Mr. Darwin 
and his representatives are those pubRshed by Mr. Murray. 





\ / 







; * 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." 

WHFWBLL : Bridgewater Treatise. 

" The only distinct meaning of the word ' natural ' is stated, 

fixed, or settled; since what is natural as much requires and 

> presupposes an intelligent agent to render it so, i.e., to effect it 

continually or at stated times, as what is supernatural or 

miraculous does to effect it for once." 

BCTLER : Analogy of Revealed Religion. 

" To conclude, therefore, let no 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, 

Dovn. Beckenham, Kent, 


HISTORICAL SKETCH . . . . . . . . . . . . xiii 

INTRODUCTION .. .. .. .. .. -. .. .. 1 



Causes of Variability Effects of Habit and the use or disuse of Parts 
Correlated Variation 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 Principles of Selection, anciently followed, their 
Effects Methodical and Unconscious Selection Unknown Origin of our 
Domestic Productions Circumstances favourable to Man's power of 
Selection 5 



Variability Individual differences Doubtful species Wide ranging, much 
diffused, and common species, vary most Species of the larger genera in 
each country vary more frequently 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 30 



Its bearing on natural selection The term used in a wide sense Geometrical 
ratio 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 Page 44 



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 the 
results of 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 Advance 
in organisation Low forms preserved Convergence of Character 
Indefinite multiplication of species Summary . . . . . . 57 



Effects of changed conditions Use and disuse, combined with natural selection ; 
organs of flight and of vision Acclimatisation Correlated variation 
Compensation and economy of growth False correlations Multiple, 
rudimentary, and lowly organised structures variable Parts developed in 
an unusual manner are highly variable ; specific characters more variable 
than generic : secondary sexual characters variable Species of the same 
genus van- in an analogous manner Reversions to long-lost characters 
Summary . . . . . . . . . . . , . . . . 98 



Difficulties of the theory of descent with modification 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 Modes of transition Cases of 


difficulty Natura non facit saltum Organs of email 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 

Page 124 



Longevity Modifications not necessarily simultaneous Modifications ap- 
parently of no direct service Progressive development Characters of 
small functional importance, the most constant Supposed incompetence 
of natural selection to account for the incipient stages of useful structures 
Causes which interfere with the acquisition through natural selection 
of useful structures Gradations of structure with changed functions 
Widely different organs in members of the same class, developed from 
one and the same source Reasons for disbelieving in great and abrupt 
modifications 166 



Instincts comparable with habits, but different in their origin Instincts 
graduated Aphides and ants Instincts variable Domestic instincts, 
their oriin Natural instincts of the cuckoo, molothrus, ostrich, and 
parasitic bees Slave-making ants Hive-bee, its cell-making instinct 
Changes of instinct and structure not necessarily simultaneous Difficulties 
of the theory of the Natural Selection of instincts Neuter or sterile 
insects Summary 191 



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, not accumulated 
by natural selection Causes of the sterility of first crosses and of hybrid.* 
Parallelism between the effects of changed conditions of life and of 
crossing Dimorphism and Trimorphism Fertility of varieties when 
crossed, and of their mongrel offspring not universal Hybrids and 
mongrels compared independently of their fertility Summary . . 218 




On the absence of intermediate varieties at the present day On the nature 
of extinct intermediate varieties ; on their number On the lapse of 
time, as inferred from the rate of denudation and of deposition On the 
lapse f time as estimated by years On the poorness of our palaeonto- 
logical collections On the intermittence of geological formations On 
the denudation of granitic areas On the absence of intermediate varieties 
in any one formation On the sudden appearance of groups of species 
On their sudden appearance in the lowest known fossiliferous strata 
Antiquity of the habitable earth . . . . . . . . Page 247 


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 chapter . . . . . . . . . . 272 



Present distribution cannot be accounted for by differences in physical con- 
ditions 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 Alternate Glacial periods in the north and south 




Distribution of fresh-water productions On the inhabitants of oceanic islands 
Absence of Batrachians and of terrestrial Mammals On the relation 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 chapter . . . . . . . . . . . . 322 




CLASSIFICATION, groups subordinate to groups Natural system Rules and 
difficulties in classification, explained on the theory of descent with modifi- 
cation Classification of varieties Descent always used in classification 
Analogical or adaptive characters Affinities, general, complex, and 
radiating Extinction separates and defines groups MORPHOLOGY, be- 
tween members of the same class, between parts of the same individual 
EMBRYOLOGY, laws of, explained by variations not supervening at any early 
age, and being inherited at a corresponding age RUDIMENTARY ORGANS ; 
their origin explained Summary . . . . . . . . Page 340 


Recapitulation of the objections to the theory of Natural Selection Recapitu- 
lation 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 . . . . . . 379 

GLOSSARY OF SCIENTIFIC TERMS . . . . . . . . . . 405 

INDEX , 417 


The Diagram to front page 84, and to face the latter part of the Volume. 



NUMEROUS small corrections have been made in the last and 
present editions on various subjects, according as the evidence 
has become somewhat stronger or weaker. The more important 
corrections and some additions in the present volume are tabu- 
lated below, for the convenience of those interested in the sub- 
ject, and who possess the fifth edition. The second edition was 
little more than a reprint of the first. The third edition was 
largely corrected and added to, and the fourth and fifth still more 
largely. As copies of the present work will be sent abroad, it 
may be of use if I specify the state of the foreign editions. The 
third French and second German editions were from the third 
English, with some few of the additions given in the fourth edition. 
A new fourth French edition has been translated by Colonel 
Moulinie ; of which the first naif is from the fifth English, and 
the latter half from the present edition. A third German edition, 
under the superintendence of Professor Victor Carus, was from 
the fourth English edition ; a fifth, by the same author, from the 
sixth edition. The second American edition was from the English 
second, with a few of the additions given in the third; and a 
third American edition has been printed from the fifth English 
edition. The Italian is from the third, the Dutch and three 
Russian editions from the second English edition, and the 
Swedish from the fifth English edition. 



Chief Additions and Corrections. 





Influence of fortuitous destniction on natural selection. 



On the convergence of specific forma. 



Account of the Ground-Woodpecker of La Plata modified. 
On the modification of the eye. 



Transitions through the acceleration or retardation of the 

period of reproduction. 



Fifth Sixth 
Fdition. Edition. 

Chief Additions and Corrections. 










The account of the electric organ of fishes added to. 

Analogical resemblance between the eyes of Cephalopods and 

Claparede on the analogical resemblance of the hair-claspers 
of the Acaridre. 

The probable use of the rattle to the Rattle -snake. 

Helmholtz on the imperfection of the human eye. 

The first part of this new chapter consists of portions, in a 
much modified state, taken from chap. iv. of the former 
editions. The latter and larger part is new, and relates 
chiefly to the supposed incompetency of natural selection 
to account for the incipient stages of useful structures. 
There is also a discussion on the causes which prevent in 
many cases the acquisition through natural selection of 
useful structures. Lastly, reasons are given for disbelieving 
in great and sudden modifications. Gradations of character, 
often accompanied by changes of function, are likewise 
here incidentally considered. 

The statement with respect to young cuckoos ejecting their 
foster-brothers confirmed. 

On the cuckoo-like habits of the Molothrus. 

On fertile hybrid moths. 

The discussion on the fertility of hybrids not having been 
acquired through natural selection condensed and modified. 

On the causes of sterility of hybrids, added to and corrected. 

Pyrgoma found in the chalk. 

Extinct forms serving to connect existing groups. 

On earth adhering to the feet of migratory birds. 

On the wide geographical range of a species of Galaxias, a 
fresh-water fish. 

Discussion on analogical resemblances, enlarged and modified. 

Homological structure of the feet of certain marsupial animals. 

On serial homologies, corrected. 

Mr. E. Ray Lankester on morphology. 

On the asexual reproduction of Chironomus. 

On the origin of rudimentary parts, corrected. 

Recapitulation on the sterility of hybrids, corrected. 

Recapitulation on the absence of fossils beneath the Cam- 
brian system, corrected. 

Natural selection not the exclusive agency in the modification 
of species, as always maintained in this work. 

The belief in the separate creation of species generally held 
by naturalists, until a recent period. 




I WILL here give a brief sketch of the progress of opinion on the 
Origin of Species. Until recently the great majority of natural- 
ists 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-exist- 
ing forms. Passing over allusions to the subject in the c'assical 
writers,* 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 

Lamarck was the first man whose conclusions on the subject 
excited much attention. This justly-celebrated naturalist first 

* Aristotle, in his 'Physicse 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 organisation ; 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 necassity, 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 sponta- 
neity ; and whatsoever things were not thus constituted, perished, and still 
perish." We here see the principle of natural selection shadowed forth, but 
now little Aristotle fully comprehended the principle, is shown by his remarks 
ou the formation of the teeth. 


published his views in 1801 ; he much enlarged them in 1809 in 
his 'Philosophic Zoologique,' and subsequently, in 1815, in the 
Introduction to his ' Hist. Nat. des Animaux sans Yertebres.' 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 diffi- 
culty 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 condi- 
tions 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 seems to attribute all the beautiful adapta- 
tions 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.* 

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 per- 
])tuated 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 modifica- 
tion; and, as his son adds, "C'est done un probleme a reserve* 

* I have taken the date of the first publication of Lamarck from Isid. 
Geoffroy Saint- Hilaire's ('Hist. Nat. Generale,' torn. 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 Danvin, anticipated the views and erroneous 
grounds of opinion of Lamarck in his ' Zoonomia ' (vol. i. pp. 500-510), pub- 
lished 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 MedLig, 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 Geoffrey Saint-Hilaire (as we shall 
immediately see) in France, came to the same conclusion on the origin of 
soecies, in the years 1 794-5. 


entierement k 1'avenir, suppose meme que 1'avenir doive avoir 
prise sur lui." 

In 1813, Dr. W. C. Wells read before the Royal Society ' Aa 
Account of a White female, part of whose skin resembles 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 incapa- 
city 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 pre- 
valent, 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 ' Amaryllidaceaa ' (1837, pp. 19, 339), declares 
that " horticultural experiments have established, beyond the pos- 
sibility 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 jriv. 
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 
Tiew 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 excel- 
lent '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 inter- 

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 

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 
ap 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 dicoty- 
ledons 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 organi- 
sation 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, imme- 
diately 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 M. J. d'Omalius d'Halloy pub- 
lished in an excellent though short paper (' Bulletins de 1'Acad. 
Roy Bruxelles,' torn. xiii. p. 581) his opinion that it is more prob- 
able 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 Associa- 
tion, 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 dis- 
tribution, 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 zoologist 
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 hi& 
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 be 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 per- 
fectly 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. iii. 
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. 

M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 
1850 (of which a Resume appeared in the 'Revue et Mag. de 
Zoolog.,' Jan. 1851), briefly gives his reason for believing that 
specific characters " sont fixes, pour chaque espece, tant qu'elle se 
perpetue au milieu des memes circonstances : ils se modifient, si 
les circonstances ambiantes viennent a changer." " En resume, 
I" observation des animaux sauvages de"montre deja la variabilite 
limitee des especes. Les experiences sur les animaux sauvages 
devenus domestiques, et sur les animaux domestiques redevenus 
sauvages, la deinontrent plus clairement encore. Ces memes ex- 
periences prouvent, de plus, que les differences produites peuvent 


fitre de valeur gen&rique." In his ' Hist. Nat. Generale ' (torn. ii. 
p. 430, 1859) he amplifies analogous conclusions. 

From a circular lately issued it appears that Dr. Freke, 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 diffi- 
cult 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 

In 1852 M. 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 
Museum,' torn. 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 
finality, " puissance mysterieuse, indeterminee ; fatalite pour les 
uns ; pour les autres, volonte providentielle, dont 1'action inces- 
sante sur les etres vivants determine, a toutes les epoques de 
1'existence du monde, la forme, le volume, et la duree de chacun 
d'eux, en raison de sa destinee dans 1'ordre de choses dont il fait 
partie. C'est cette puissance qui harmonise chaque membre a 
1'ensemble, en 1'appropriant a la fonction qu'il doit remplir dans 
1'organisme general de la nature, fonction qui est pour lui sa raison 

* From references in Bronn's ' Untersuchungen iiber die Entwickelungs- 
Gesetze,' it appears that the celebrated botanist and palaeontologist linger 
published, in 1852, his belief that species undergo development and modifica- 
tion. Dalton, likewise, in Pander and Dalton's work on Fossil Sloths, ex- 
pressed, in 1821, a similar belief. Similar views have, as is well known, 
oeen maintained by Oken in his mystical ' Natur-Philosophie.' From other 


In 1853 a celebrated geologist, Count Keyserling (' Bulletin dc 
la Soc. Geolog.,' 2nd Ser., torn. 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 circum- 
ambient molecules of a particular nature, and thus have given 
rise to new forms. 

In this same year, 1853, Dr. Schaaffhausen published an excel- 
lent 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 inter- 
mediate 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, M. Lecoq, writes in 1854 
(' Etudes sur Geograph. Bot.,' torn. i. p. 250), " On voit que nos 
recherches sur la fixite ou la variation de 1'espece, nous conduisent 
directement aux idees emises, par deux hommes justement celebres, 
Geoffroy Saint-Hilaire et Goethe." Some other passages scattered 
through M. 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 Bev. 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 

The third volume of the ' Journal of the Linnean Society ' con- 
tains 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. Budolph Wagner, 
' Zoologisch-Anthropologische Untersuchungen, 5 1861, s. 51) his 
conviction, chiefly grounded on the laws of geographical distribu- 

references in Godron's work ' Sur 1'Espece,' 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. 


tiera, 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 L : fe.' 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 sup- 
porters, is yet the only one to which physiology lends any counte- 
nance ; 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. v 




WHEN on board H.M.S. ' Beagle,' as naturalist, I was much struck 
with certain facts in the distribution of the organic beings in- 
habiting South America, and in the geological relations of the 
present to the past inhabitants of that continent. These facts, as 
will be seen in the latter chapters of this volume, seemed 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 bear- 
ing 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 s^w 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 (1859) nearly finished ; but as it will take me 
many 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. In 1858 he sent 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 worlc 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 

This Abstract, which I now publish, must necessarily be im- 
perfect. 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 argu- 
ments on both sides of each question ; and this is here impossible. 

I much regret that want of space prevents my having the satis- 
faction 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 judgment. 

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 con- 
clusion that 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 justly excites our admiration. Naturalists 
continually refer to external conditions, such as climate, food, etc., 
as the only possible cause of variation. In one 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 admir- 
ably adapted to catch insects under the bark of trees. In the case 
of the mistletoe, 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. 

It is, therefore, of the highest importance to gain a clear insight 


into the means of modification and coadaptation. At the com- 
mencement 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. 
Nor 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 
this Abstract to Variation under Domestication. We shall thus 
see that a large amount of hereditary modification is at least pos- 
sible ; and, what is equally or more important, we shall see how 
great is the power of man in accumulating by his Selection succes- 
sive slight variations. I will then pass on to the variability of 
species in a state of nature ; but I shall, unfortunately, be com- 
pelled to treat this subject far too briefly, as it can be treated 
properly only by giving long catalogues of facts. We shall, how- 
ever, 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 the high geometrical ratio of their increase, will be 
considered. 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, conse- 
quently, there is a frequently recurrent 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 con- 
ditions 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 leads to what I have called Diver- 
gence of Character. In the next chapter I shall discuss the complex 
and little known laws of variation. In the five succeeding chap- 
ters, the most apparent and gravest difficulties in accepting the 
theory will be given : namely, first, the difficulties of transitions, 
or how a simple being or a simple organ can be changed and 
perfected into a highly developed being or into an elaborately con- 
structed 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 shal] 


consider the geological succession of organic beings throughout 
time ; in the twelfth and thirteenth, their geographical distribution 
jhroughout space ; in the fourteenth, 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 
tvork, and a few concluding remarks. 

No one ought to feel surprise at much lemaining as yet unex 
plained in regard to the origin of species and varieties, if he make 
due allowance for our profound ignorance in regard to the mutual 
relations of the many 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 dis- 
passionate judgment of which I am capable, that the view which 
most naturalists until recently entertained, 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 most important 
but not the exclusive, means of modification. 



Causes of variability Effects of habit and the use or disuse of parts Corre- 
lated variation 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 Principles of election, anciently followed, their effects 
Methodical and unconscious selection Unknown origin of our domestic 
productions Circumstances favourable to man's power of selection. 

Causes of Variability. 

WHEN we compare 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 more from 
each other than do the individuals of any one species or variety in 
a state of nature. And if 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 treat- 
ment, we are driven to conclude that this great variability is 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 had been exposed under nature. There 
is, also, some probability in the view propounded by Andrew 
Knight, that this variability may be partly connected with excess 
of food. It seems clear that organic beings must be exposed 
during several generations to new conditions to cause any great 
amount of variation ; and that, when the organisation has once 
begun to vary, it generally continues varying for many generations. 
No case is on record of a variable organism ceasing to vary under 
cultivation. Our oldest cultivated plants, such as wheat, still 
yield new varieties : our oldest domesticated animals are still 
capable of rapid improvement or modification. 

As far as I am able to judge, after long attending to the subject, 
the conditions of life appear to act in two ways, directly on the 
whole organisation or on certain parts alone, and indirectly by 
affecting the reproductive system. With respect to the direct 
action, we must bear in mind that in every case, as Professor 
Weismann has lately insisted, and as I have incidentally shown 
in my work on 'Variation under Domestication,' there are two 


factors : namely, the nature of the organism, and the nature of the 
conditions. The former seems to be much the more important ; 
for nearly similar variations sometimes arise under, as far as we 
can judge, dissimilar conditions ; and, on the other hand, dissimilar 
variations arise under conditions whichappear to be n early uniform. 
The effects on the offspring are either definite or indefinite. They 
may be considered as definite when all or nearly all the offspring 
of individuals exposed to certain conditions during several genera- 
tions are modified in the same manner. It is extremely difficult to 
come to any conclusion in regard to the extent of the changes 
which have been thus definitely induced. There can, however, be 
little doubt about many slight changes, such as size from the 
amount of food, colour from the nature of the food, thickness of the 
skin and hair from climate, etc. Each of the endless variations 
which we see in the plumage of our fowls must have had some 
efficient cause ; and if the same cause were to act uniformly during 
a long series of generations on many individuals, all probably 
would be modified in the same manner. Such facts as the complex 
and extraordinary out-growths which variably follow from the 
insertion of a minute drop of poison by a gall-producing insect, 
show us what singular modifications might result in the case of 
plants from a chemical change in the nature of the sap. 

Indefinite variability is a much more common result of changed 
conditions than definite variability, and has probably played a 
more important part in the formation of our domestic races. We 
see indefinite variability in the endless slight peculiarities which 
distinguish the individuals of the same species, and which cannot 
be accounted for by inheritance from either parent or from some 
more remote ancestor. Even strongly-marked differences occa 
sionally appear in the young of the same litter, and in seedlings 
from the same seed-capsule. At long intervals of time, out of 
millions of individuals reared in the same country and fed on nearly 
the same food, deviations of structure so strongly pronounced as to 
deserve to be called monstrosities arise ; but monstrosities cannot 
be separated by any distinct line from slighter variations. Al] 
such changes of structure, whether extremely slight or strongly 
marked, which appear amongst many individuals living together, 
may be considered as the indefinite effects of the conditions of life 
on each individual organism, in nearly the same manner as the 
chill affects different men in an indefinite manner, according to 
their state of body or constitution, causing coughs or colds, rheu 
matism, or inflammation of various organs. 

With respect to what I have called the indirect action of changed 
conditions, namely, through the reproductive system of being 
affected, we may infer that variability is thus induced, partly from 
the fact of this system being extremely sensitive to any change in 


the conditions, and partly from the similarity, as Kolreuter and 
others have remarked, between the variability which follows from 
the crossing of distinct species, and that which may be observed 
with plants and animals when reared under new or unnatural con- 
ditions. Many facts clearly show how eminently susceptible the 
reproductive system is to very slight changes in the surrounding 
conditions. Nothing is more easy than to tame an animal, and few 
things more difficult than to get it to breed freely under confine- 
ment, even when the male and female unite. How many animals 
there are which will not breed, though kept in an almost free state in 
their native country ! This is generally, but erroneously, attributed 
to vitiated instincts. Many cultivated plants display the utmost 
vigour, and yet rarely or never seed ! In some few cases it has 
been discovered that a very trifling change, such as a little more 
or less water at some particular period of growth, will determine 
whether or not a plant will produce seeds. I cannot here give the 
details which I have collected and elsewhere published on this 
curious subject ; but to show how singular the laws are which 
determine the reproduction of animals under confinement, I may 
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, which seldom produce young ; 
whereas carnivorous birds, with the rarest exceptions, hardly ever 
lay fertile eggs. Many exotic plants have pollen utterly worthless, 
in the same condition as in the most sterile hybrids. When, on the 
one hand, we see domesticated animals and plants, though often 
weak and sickly, breeding freely under confinement ; and when, 
on the other hand, we see individuals, though taken young from . 
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 to act, 
we need not be surprised at this system, when it does act under 
confinement, acting irregularly, and producing offspring somewhat 
unlike their parents. I may add, that as some organisms breed 
freely under the most unnatural conditions (for instance, rabbits 
and ferrets kept in hutches), showing that their reproductive 
organs are not easily affected ; so will some animals and plants 
withstand domestication or cultivation, and vary very slightly 
perhaps hardly more than in a state of nature. 

Some naturalists have maintained that all variations are con- 
nected with the act of sexual reproduction ; but this is certainly an 
error ; for I have given in another work a long list of "sporting 
plants," as they are called by gardeners ; that is, of plants which 
have suddenly produced a single bud with a new and sometimes 
widely different character from that of the other buds on the same 
plant. These bud-variations, as they may be named, can be pro- 


pagated by grafts, offsets, etc., and sometimes by seed. They occur 
rarely under nature, but are far from rare under culture. As a 
single bud out of the many thousands, produced year after year on 
the same tree under uniform conditions, has been known suddenly 
to assume a new character ; and as buds on distinct trees, growing 
under different conditions, have sometimes yielded nearly the same 
variety for instance, buds on peach-trees producing nectarines, 
and buds on common roses producing moss-roses we clearly see 
that the nature of the condition is of subordinate importance in 
comparison with the nature of the organism in determining each 
particular form of variation ; perhaps of not more importance 
than the nature of the spark, by which a mass of combustible 
matter is ignited, has in determining the nature of the flames. 

Effects of Habit and of the Use or Disuse of Parts ; Correlated 
Variation ; Inheritance. 

Changed habits produce an inherited effect, as in the period of 
the flowering of plants when transported from one climate to 
another. With animals the increased use or disuse of parts has 
had a more marked influence ; thus 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 this change may be safely attributed to the 
domestic duck flying much less, and walking more, than its wild 
parents. The great and inherited development of the udders in 
cows and goats in countries where they are habitually milked, in 
comparison with these organs in other countries, is probably 
another instance of the effects of use. Not one of our domestic 
animals can be named which has not in some country drooping 
ears ; and the view which has been suggested that the drooping 
is due to disuse of the muscles of the ear, from the animals being 
seldom much alarmed, seems probable. 

Many laws regulate variation, some few of which can be dimly 
seen, and will hereafter be briefly discussed. I will here only allude 
to what may be called correlated variation. Important changes in 
the embryo or larva will probably 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 
which are entirely white and have blue eyes are generally deaf ; but 
it has been lately stated by Mr. Tait that this is confined to the 
males. Colour and constitutional peculiarities go together, of which 
many remarkable cases could be given amongst animals and plants. 


From facts collected by Heusinger, it appears that white sheep and 
pigs are injured by certain plants, whilst dark-coloured individuals 
escape : Professor Wyman has recently communicated to me a good 
illustration of this fact ; on asking some fanners in Virginia how it 
was that all their pigs were black, they informed him that the pigs 
ate the paint-root (Lachnanthes), which coloured their bones pink, 
and which caused the hoofs of all but the black varieties to drop 
off ; and one of the " crackers " (i.e. Virginia squatters) added, "we 
select the black members of a litter for raising, as they alone have 
a good chance of living." 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 select- 
ing, and thus augmenting, any peculiarity, he will almost certainly 
modify unintentionally other parts of the structure, owing to the 
mysterious laws of correlation. 

The results of the various, unknown, or but dimly understood 
laws of variation are infinitely complex and diversified. It is well 
worth while carefully to study the several treatises on some of our 
old cultivated plants, as on the hyacinth, potato, even the dahlia, 
etc. ; and it is really surprising to note the endless points of struc- 
ture and constitution in which the varieties and sub-varieties differ 
slightly from each other. The whole organisation seems to have 
become plastic, and departs in a slight 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 import- 
ance, are 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 ; that like produces like is his 
fundamental belief : doubts have been thrown on this principle only 
by theoretical writers. When any deviation of structure often ap- 
pears, and we see it in the father and child, we cannot tell whether 
it may not be due to the same cause having acted on both ; but 
when amongst individuals, apparently exposed to the same condi- 
tions, any very rare deviation, due to some extraordinary combina- 
tion 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, etc., appearing in several members of the 
same family. If strange and rare deviations of structure are really 
inherited, less strange and commoner deviations may be freely 
aclmi tted 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 for the most part unknown. 
No one can say why the same peculiarity in different individuals of 
the same species, or in different species, is sometimes inherited and 
sometimes not so ; why the child often reverts in certain characters 
to its grandfather or grandmother or more remote ancestor ; why : 
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 importance to us, that peculiarities appearing in the 
males of our domestic breeds are often transmitted, either exclu- 
sively or in a much greater degree, to the males alone. A much 
more important rule, which I think maybe trusted, is that, at what- 
ever period of life a peculiarity first appears, it tends to reappear in 
the offspring at a corresponding age, though sometimes earlier. In 
many cases this could not be otherwise ; thus the inherited pecu- 
liarities 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 the primary cause which may have acted on 
the ovules or on the male element ; in nearly the same manner as 
the increased length of the horns in the offspring from a short- 
horned cow by a long-horned bull, 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 invariably revert in cha- 
racter 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 
necessary, in order to prevent the effects of intercrossing, that only 
a single variety should Ks-ve been 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 im- 
probable that if we could succeed in naturalising, or were to culti- 
vate, during many generations, the several races, for instance, of 
the cabbage, in veiy poor soil (in which case, however, some effect 
would have to be attributed to the definite 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 the same conditions, and whilst kept in a considerable body, 
so that free intercrossing might check, by blending together, any 
slight deviations in their 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 unlimited number of generations, would be 
opposed to all experience. 

Character of Domestic Varieties; difficulty of distinguishing 
between Varieties and Species ; origin of Domestic Varieties 
from one or more Species. 

When we look to the hereditary varieties or races of our domestic 
animals and plants, and compare them with closely allied species, 
we generally perceive in each domestic race, as already remarked, 
less uniformity of character than in true species. Domestic races 
often have a somewhat monstrous character ; by which I mean, 
that, although differing from each other, and from 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 the species under 
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 cf the same 
species differ from each other in the same manner as do the closely- 
allied species of the same genus in a state of nature, but the differ- 
ences in most cases are less in degree. This must be admitted as 
true, for the domestic races of many animals and plants have been 
ranked by some competent judges as the descendants of aboriginally 
distinct species, and by other competent judges as mere varieties. 
If any well-marked distinction existed between a domestic race and 
a species, this source of doubt would not so perpetually recur. It 
has often been stated that domestic races do not differ from each 
other in characters of generic value. It can be shown that this 



statement is not correct ; but naturalists differ much in determining 
what characters are of generic value ; all such valuations being 
present empirical. When it is explained how genera originate unde 
nature, it will be seen that we have no right to expect often to fir 
a generic amount of difference in our domesticated races. 

In attempting to estimate the amount of structural differenc 
between allied domestic races, we are soon involved in doubt, froi 
not knowing whether they are descended from one or several parei 
species. This point, if it could be cleared up, would be interesting ; 
if, for instance, it could be shown that the greyhound, bloodhounc 
terrier, spaniel, and bull-dog, which we all know propagate theii 
kind truly, were the offspring of any single species, then such fa 
would have great weight in making us doubt about the immut 
bility of the many closely allied natural species for instance, 
the many foxes inhabiting different quarters of the world. I 
not believe, as we shall presently see, that the whole amount 
<Ufference between the several breeds of the dog has been produc 
under domestication ; I believe that a small part of the differenc 
is due to their being descended from distinct species. In the 
of strongly marked races of some other domesticated species, the 
is presumptive or even strong evidence, that all are descended froi 
a single wild stock. 

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 tho 
little variability of the ass and goose, or the small power of endur- 
ance 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 produc- 
tions, 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 cai 
an average vary 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, it is not possible to come to any definite conclusion, whether 
they are descended from one or several wild species. The argu- 
ment mainly relied on by those who believe in the multiple origin 
of our domestic animals is, that we find in the most ancient times, 
on the monuments of Egypt, and in the lake-habitations of Switzer- 
land, much diversity in the breeds ; and that some of- these ancient j 
breeds closely resemble, or are even identical with, those still exist- 


ing. But this only throws far backwards the history of civilisa- 
tion, and shows that animals were domesticated at a much earlier 
period than has hitherto been supposed. The lake-inhabitants of 
Switzerland cultivated several kinds of wheat and barley, the pea, 
the poppy for oil, and flax ; and they possessed several domesticated 
animals. They also carried on commerce with other nations. All 
this clearly shows, as Heer has remarked, that they had at this 
early age progressed considerably in civilisation ; and this again 
implies a long continued previous period of less advanced civilisa- 
tion, during which the domesticated animals, kept by different 
tribes in different districts, might have varied and given rise to 
distinct races. Since the discovery of flint tools in the superficial 
formations of many parts of the world, all geologists believe that 
barbarian man existed at an enormously remote period ; and we 
know that at the present day there is hardly a tribe so barbarous, 
as not to have domesticated at least the dog. 

The origin of most of our domestic animals will probably for ever 
remain vague. But I may here state, that, looking to the domestic 
dogs of the whole world, I have, after a laborious collection of all 
known facts, come to the conclusion that several wild species of 
Canidae have been tamed, and that their blood, in some cases 
mingled together, flows in the veins of our domestic breeds. In 
regard to sheep and goats I can form no decided opinion. From 
facts communicated to me by Mr. Blyth, on the habits, voice, con- 
stitution, and structure of the humped Indian cattle, it is almost 
certain that they are descended from a different aboriginal stock 
from our European cattle ; and some competent judges believe that 
these latter have had two or three wild progenitors, whether or 
not these deserve to be called species. This conclusion, as well as 
that of the specific distinction between the humped and common 
cattle, may, indeed, be looked upon as established by the admirable 
researches of Professor Rutimeyer. With respect to horses, from 
reasons which I cannot here give, I am doubtfully inclined to 
believe, in opposition to several authors, that all the races belong 
to the same species. Having kept nearly all the English breeds of 
the fowl alive, having bred and crossed them, and examined their 
skeletons, it appears to me almost certain that all are the descend- 
ants of the wild Indian fowl, Gallus bankiva ; and this is the con- 
clusion of Mr. Blyth, and of others who have studied this bird in 
India. In regard to ducks and rabbits, some breeds of which 
differ much from each other, the evidence is clear that they are all 
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 proto- 


type. 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 eleven wild species of sheep peculiar to 
Great Britain ! When we bear in mind that Britain has now not 
one peculiar mammal, and France but few distinct from those of 
Germany, and so with Hungary, Spain, etc., but that each of these 
kingdoms possesses several peculiar breeds of cattle, sheep, etc., we 
must admit that many domestic breeds must have originated in 
Europe ; from whence otherwise could they have been derived ? 
So it is in India. Even in the case of the breeds of the domestic 
dog throughout the world, which I admit are descended from 
several wild species, it cannot be doubted that there has been an 
immense amount of inherited variation ; for who will believe that 
animals closely resembling the Italian greyhound, the bloodhound, 
the bull-dog, pug-dog, or Blenheim spaniel, etc. so unlike all wild 
Canidae ever existed in a state of nature 1 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 only 
get 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, etc., in the wild state. 
Moreover, the possibility of making distinct races by crossing has 
been greatly exaggerated. Many cases are on record, showing that 
a race may be modified by occasional crosses, if aided by the care- 
ful selection of the individuals which present the desired character ; 
but to obtain a race intermediate between two quite distinct races, 
would be very difficult. Sir J. Sebright expressly experimented 
with this object and failed. The offspring from the first cross 
between two pure breeds is tolerably and sometimes (as I have 
found with pigeons) quite uniform in character, and everything 
seems simple enough ; but when these mongrels are crossed one 
with another for several generations, hardly two of them are alike, 
and then the difficulty of the task becomes manifest. 

Breeds of the Domestic Pigeon, their Differences and Origin. 

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 considerable 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 espe- 
cially the male bird, is also remarkable from the wonderful develop- 
ment 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 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 long beak, 
has a very short and 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 short and conical beak, with a line of 
reversed feathers down the breast ; and it has the habit of con- 
tinually expanding, slightly, the upper part of the oesophagus. 
The Jacobin has the feathers so much reserved along the back of 
the neck that they form a hood ; and it has, proportionally to its 
size, 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 the members of 
the great pigeon family : 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 
be 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 caudal and sacral vertebrae vary in number ; 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 the wing and tail to each other and to the 


body ; the relative length of the leg and foot ; 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, and 
in some breeds the voice and disposition, differs remarkably. 
Lastly, in certain breeds, the males and females have come to 
differ in a slight degree in 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 be ranked by him as well-defined species. 
Moreover, I do not believe that any ornithologist would in this 
case 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 would call them, could be shown him. 

Great as are the differences between the breeds of the pigeon, 
I am fully convinced that the common opinion of naturalists is 
correct, namely, that all are 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 charac- 
teristic enormous crop ? The supposed aboriginal stocks must all 
have been rock-pigeons, that is, they did not breed or willingly 
perch 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 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 having similar habits with the 
rock-pigeon seems a very rash assumption. Moreover, the several 


above-named domesticated breeds have been transported to aU 
parts of the world, and, therefore, some of them must have been 
carried back again into their native country ; but not one has 
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 difficult to 
^ret wild animals 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 -civilised man, as to be quite prolific under 

An argument of great weight, and applicable in several other 
cases, is, that the above-specified breeds, though agreeing generally 
with the wild rock-pigeon in constitution, habits, voice, colouring, 
and in most parts of their structure, yet are certainly highly abnor- 
mal in other parts ; we may look in vain through 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-civilised man succeeded in thoroughly domesti- 
cating 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 are 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, with white loins ; 
but the Indian sub-species, C. intermedia of Strickland, has this 
part bluish. The tail has a terminal dark bar, with the outer 
feathers xternally edged at the base with white. The wings have 
two black bars. Some semi-domestic breeds, and some 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, sometimes 
concur perfectly developed. Moreover, when birds belonging to 
two or more distinct breeds are crossed, none of which are blue or 
have any of the above-specified marks, the mongrel offspring are 
very apt suddenly to acquire these characters. To give one instance 
out of several which I have observed : I crossed some white 
fantails, which breed very true, with some black barbs and it so 
happens that blue varieties of barbs are so rare that I never heard 
of an instance in England ; and the mongrels were black, brown, 
and mottled. I also crossed a barb with a spot, which is a white 
bird with a red tail and red spot on the forehead, and which 


notoriously breeds very true ; the mongrels were dusky and mottled. 
I then crossed one of the mongrel barb-fantails with a mongrel 
barb-spot, and they produced a bird of as beautiful a blue colour, 
with the white loins, double black wing-bar, and barred and white- 
edged tail-feathers, as any wild rock -pigeon ! We can understand 
these facts, on the well-known principle of reversion to ancestral 
characters, if all the domestic breeds are descended from the rock- 
pigeon. But if we deny this, we must make one of the two following 
highly improbable suppositions. Either, first, 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 no instance 
is known of crossed descendants reverting to an ancestor of foreign 
blood, removed by a greater number of generations. In a breed 
which has been crossed only once, the tendency to revert to any 
character derived from such a 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, and there is a 
tendency in the breed to revert to a character which was 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 of 
reversion are often confounded together by those who have written 
on inheritance. 

Lastly, the hybrids or mongrels from between all the breeds of 
the pigeon are perfectly fertile, as I can state from my own observa- 
tions, purposely made, on the most distinct breeds. Now, hardly 
any cases have been ascertained with certainty of hybrids from two 
quite distinct species of animals being perfectly fertile. Some 
authors believe that long-continued domestication eliminates this 
strong tendency to sterility in species. From the history of the 
dog, and of some other domestic animals, this conclusion is probably 
quite correct, if applied to species closely related to each other. 
But to extend it 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, would be rash in the 

From these several reasons, namely, the improbability of man 
having formerly made seven or eight supposed species of pigeons 
to breed freely under domestication ; these supposed species being 
quite unknown in a wild state, and their not having become 
anywhere feral ; these species presenting certain very abnormal 


characters, as compared with all other Columbidie, though so like 
the rock-pigeon in most respects ; the occasional re-appearance of 
the blue colour and various black marks in all the breeds, both 
when kept pure and when crossed ; and lastly, the mongrel off- 
spring being perfectly fertile ; from these several reasons, taken 
together, we may safely conclude that all our domestic breeds 
are descended from the rock-pigeon or Columba livia with its 
geographical sub-species. 

In favour of this view, I may add, firstly, that the wild C. livia 
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, that, although 
an English carrier or a short-faced tumbler differs immensely in 
certain characters from the rock-pigeon, yet that, by comparing 
the several sub-breeds of these two races, more especially those 
brought from distant countries, we can make, between them and 
the rock-pigeon, an almost perfect series ; so we can in some other 
cases, but not with all the breeds. Thirdly, those characters 
which are mainly distinctive of each breed are in each eminently 
variable, 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 ; and the explanation of this fact will be obvious 
when we 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 1600 ; 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 astonish- 
ingly." 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 varia- 
tion which pigeons have undergone, will likewise be obvious when 
we treat of Selection. We shall then, also, see how it is that the 
several 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, well knowing how truly they breed, 
I felt fully as much difficulty in believing that since they had been 
domesticated they had all proceeded from a common parent, as 
any naturalist could in coming to a similar conclusion in regard 
to the many species of finches, or other groups of birds, in nature. 
One circumstance has struck me much ; namely, that nearly all 
the breeders of the various domestic animals and the cultivators 
of plants, with whom I have 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, or both from a common parent-stock, 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 Mons, in his treatise on 
pears and apples, shows how utterly he disbelieves that the several 
sorts, for instance a Bibston-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 inter- 
mediate links in the long lines of descent, yet admit that many of 
our domestic races are 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 1 

Principles of Selection anciently followed,, and their Effects. 

Let us now briefly consider the steps by which domestic races 
have been produced, either from one or from several allied species. 
Some effect may be attributed to the direct and definite action of 
the external conditions of life, and some to habit ; but he would be 
a bold man who would account by such agencies for the differences 
between 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 sud- 


denly, or by one step ; many botanists, for instance, believe that 
the fuller's teasel, 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 com- 
pare 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 different ways ; when we com- 
pare the game-cock, 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 many cases, we know that this has not been their 
history. The key is man's power of accumulative selection : nature 
gives succossive variations ; man adds them up in certain direc- 
tions useful to him. In this sense he may be said to have made 
for himself useful breeds. 

The great power of this principle of selection is not hypotheti- 
cal. It is certain that several of our eminent breeders have, even 
within a single lifetime, modified to a large extent their breeds of 
cattle and sheep. In order fully to realise 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 plastic, which they can 
model almost as they please. If I had space I could quote nume- 
rous passages to this effect from highly competent authorities. 
Youatt, who was probably better acquainted with the works of 
agriculturists than almost any other individual, and who was 
himself a very good judge of animals, 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, speak- 
ing of what breeders hare 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." In Saxony the importance of the 
principle of selection in regard to merino sheep is so fully recog- 
nised, that men follow it as a trade : the sheep are placed on a table 
lid 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 

What English breeders have actually effected is proved by the 
enormous prices given for animals with a good pedigree ; and these 
have been exported to almost every quarter of the world. The im- 
provement 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 sepa- 
rating some very distinct variety, and breeding from it, the prin- 
ciple would be so obvious as hardly to be worth notice ; but its 
importance consists in the great effect produced by the accumula- 
tion 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 judgment 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 has not been 
so in several 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 im- 
provement 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, likewise followed ; for hardly any one is so 
careless as to breed from his worst animals. 

In regard to plants, there is another means of observing the 
accumulated effects of selection namely, by comparing the di- 
versity 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, I speak after careful observation, perhaps 
never, the case. The law of correlated variation, the importance 
of which should never be overlooked, will ensure some differences ; 
but, as a general rule, it cannot be doubted 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 
iate years, and many treatises have been published on the subject ; 
and the result has been, in a corresponding degree, rapid and im- 
portant. But it is very far from true that the principle is a modem 
discovery. I could give several references to works of high anti- 
quity, in which the full importance of the principle is acknow- 
ledged. 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 domesticated 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 states that good 
domestic breeds are highly valued by the negroes in 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. 

Unconsciow Selection. 

At the present time, eminent breeders try by methodical selec- 
tion, with a distinct object in view, to make a new strain or sub- 


breed, superior to anything of the kind in the country. But, for 
our purpose, a form 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. Never- 
theless we may infer that this process, continued during centuries, 
would improve and modify any breed, in the same way as Bake well, 
Collins, etc., by this very same process, only carried on more 
methodically, did greatly modify, even during their lifetimes, the 
forms and qualities of their cattle. Slow and insensible changes of 
this kind can never be recognised unless actual measurements or 
careful drawings of the breeds in question have been made long 
ago, which may serve for comparison. In some cases, however, 
unchanged, or but little changed individuals of the same breed 
exist in less civilised districts, where the breed has been less im- 
proved. 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 pro- 
bably 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 foxhound : 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. Borrow has not seen, as I am informed by him, 
any native dog in Spain like our pointer. 

By a simple process of selection, and by careful training, English 
racehorses have come to surpass in fleetness and size the parent 
Arabs, so that the latter, by the regulations for the Goodwood 
Races, are favoured in the weights Avhich 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 various old treatises of the former and present state of 
carrier and tumbler pigeons in Britain, India, and Persia, we can 
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 unconscious, in so far that 
the breeders could never have expected, or even wished, to produce 
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 pur- 
pose, would be carefully preserved during famines and other acci- 
dents, 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 dis- 
tinct varieties, and whether or not two or more species or races 
have become blended together by crossing, may plainly be recog- 
nised 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 hearts- 
ease or dahlia from the seed of a wild plant. No one would expect 
to raise a first-rate melting pear from the seed of the "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 has been simple, and, as far as the final result is con- 
cerned, has been followed almost unconsciously. It has consisted 
in always cultivating the best-known variety, sowing its seeds, and, 
when a slightly better variety chanced to appear, selecting it, and 
so onwards. But the gardeners of the classical period, who culti- 
vated the best pears which 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, thus slowly and unconsciously ac- 
cumulated, explains, as I believe, the well-known fact, that in a 
number of cases we cannot recognise, and therefore do not know, 


the wild parent-stocks of the plants which have been longest culti- 
vated 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 acquired by the plants in countries anciently 

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 why the varieties kept by savages, as has been remarked 
by some authors, have more of the character of true species than 
the varieties kept in civilised countries. 

On the view here given of the 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 
'LQ man's wants or fancies. We can, I think, further understand 
the frequently abnormal character of our domestic races, and like- 
wise 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 fan- 
tail, is, I have no doubt, in most cases, utterly incorrect. The man 
who first selected a pigeon with a slightly larger tail, never dreamed 
what the descendants of that pigeon would become through long- 
continued, partly unconscious and partly methodical, selection. 
Perhaps the parent-bird of all fan tails had only fourteen tail-feathers 
some what expanded, like the present Java fan tail, 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 fancy any novelty, however 
slight, in one's own possession. Nor must the value which would 
formerly have been set on any slight differences in the individuals 
of the same species, be judged of by the value which is now set on 
them, after several breeds have fairly been established. It is known 
that with pigeons many slight variations now occasionally appear, 
but these are rejected as faults or deviations from the standard of 
perfection in each breed. The common goose has not given rise to 
any marked varieties ; hence the Toulouse and the common breed, 
which differ only in colour, that most fleeting of characters, have 
lately been exhibited as distinct at our poultry -shows. 

These views appear to explain what has sometimes been noticed 
namely, that we know hardly anything 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 a distinct 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 animals slowly 
spread in the immediate neighbourhood. But they will as yet hardly 
have a distinct name, and from being only slightly valued, their 
history will have been disregarded. When further improved by the 
same slow and gradual process, they will spread more widely, and 
will be recognized as something distinct and valuable, and will then 
probably first receive a provincial name. In semi-civilised countries, 
with little free communication, the spreading of a new sub-breed 
would be a slow process. As soon as the points of value are once 
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. 

Circumstances favourable to Man's Power of Selection. 

I will 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 occa- 
sionally, the chance of their appearance will be much increased by 
a large number of individuals being kept. Hence, number is of 
the highest importance for success. On this principle Marshall 
formerly remarked, with respect to the sheep of parts of Yorkshire, 
" as they generally belong to poor people, and are mostly in small 
lots, they never can be improved." On the other hand, nurserymen, 
from keeping large stocks of the same plant, are generally far more 
successful than amateurs in raising new and valuable varieties. A 
large number of individuals of an animal or plant can be reared 
only where the conditions for its propagation are favourable. When 
the individuals are scanty, all will be allowed to breed, whatever 
their quality may be, and this will effectually prevent selection. 
But probably the most important element is that the animal or 
plant should be so highly valued by man, that the closest attention 
is paid to even the slightest deviations in its qualities or structure. 
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 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 (with some aid by 
crossing distinct species) those many admirable varieties of the 
strawberry were raised which have appeared during the last half- 

With animals, facility in preventing crosses is an important 
element 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 improved and kept true, though 
mingled in the same aviary; and this circumstance must have 
largely favoured the 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 easily matched, and, although so much valued by women 
and children, we rarely see a distinct breed long kept up ; such breeds 
as we do sometimes see are almost always imported from some other 
country. 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 ; for 
recently in certain parts of Spain and of the United States this 
animal has been surprisingly modified and improved by careful 
selection : 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 ; but the goose, under 
the conditions to which it is exposed when domesticated, seems to 
have a singularly inflexible organisation, though it has varied to 
a slight extent, as I have elsewhere described. 

Some authors have maintained that the amount of variation in 
our domestic productions is soon reached, and can never afterwards 
be exceeded. It would be somewhat rash to assert that the limit 
has been attained in any one case ; for almost all our animals and 
plants have been greatly improved in many ways within a recent 
period ; and this implies variation. It would be equally rash to 
assert that characters now increased to their usual limit, could not, 
after remaining fixed for many centuries, again vary under new 
conditions of life. No doubt, as Mr. Wallace has remarked with 
much truth, a limit will be at last reached. For instance, there 
must be a limit to the fleetness of any terrestrial animal, as this 
will be determined by the friction to be overcome, the weight of 
body to be carried, and the power of contraction in the muscular 
fibres. But what concerns us is that the domestic varieties of the 
same species differ from each other in almost every character, which 
man has attended to and selected, more than do the distinct species 
of the same genera. Isidore Geoffroy St. Hilaire has proved this 
in regard to size, and so it is with colour and probably with the 
length of hair. With respect to fleetness, which depends on many 
bodily characters, Eclipse was far fleeter, and a dray-horse is in- 
comparably stronger than any two natural species belonging to 
the same genus. So with plants, the seeds of the different varie- 
ties of the bean or maize probably differ more in size, than do the 
seeds of the distinct species in any one genus in the same two 
families. The same remark holds good in regard to the fruit of 
the several varieties of the plum, and still more strongly with the 
melon, as well as in many other analogous cases. 

To sum up on the origin of our domestic races of animals and 
plants. Changed conditions of life are of the highest importance 
in causing variability, both by acting directly on the organisation, 
and indirectly by affecting the reproductive system. It is not 
probable that variability is an inherent and necessary contingent, 


under all circumstances. The greater or less force of inheritance 
and reversion determine whether variations shall endure. Varia- 
bility is governed by many unknown laws, of which correlated 
growth is probably the most important. Something, but how 
much we do not know, may be attributed to the definite action of 
the conditions of life. Some, perhaps a great, effect may be attri- 
buted to the increased use or disuse of parts. The final result is 
thus rendered infinitely complex. In some cases the intercrossing 
of aboriginally distinct species appears to have played an important 
part in the origin of our breeds. When several breeds have once 
been formed in any country, their occasional intercrossing, with the 
aid of selection, has, no doubt, largely aided in the formation of 
new sub-breeds ; but the importance of crossing has been much 
exaggerated, both in regard to animals and to those plants which 
are propagated by seed. With plants which are temporarily propa- 
gated by cuttings, buds, &c., the importance of crossing is im- 
mense ; for the cultivator may here disregard the extreme variability 
both of hybrids and of mongrels, and the sterility of hybrids ; but 
plants not propagated by seed are of little importance to us, for 
their endurance is only temporary. Over all these causes of 
Change, the accumulative action of Selection, whether applied 
methodically and quickly, or unconsciously and slowly but more 
efficiently, seems to have been the predominant Power. 



Variability Individual differences Doubtful species Wide ranging, much 
diffused, and common species, vary most Species of the larger genera in 
each country vary more frequently 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 

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 
properly, a long catalogue of dry facts ought to be given ; but these 
I shall reserve for a future work. Nor shall I here discuss the 
various definitions which have been given of the term species. No 
one definition has satisfied all naturalists ; yet every naturalist 
knows vaguely what he means when he speaks of a species. Gene- 
rally 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 im- 
plied, though it can rarely be proved. We have also what are 
called monstrosities ; but they graduate into varieties. By a mon- 
strosity I presume is meant some considerable deviation of struc- 
ture, generally injurious, or not useful to the species. Some authors 
use the term " variation " in a technical sense, as implying a modi- 
fication directly due to the physical conditions of life ; and " varia- 
tions " 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 in- 
herited for at least a few generations 1 and in this case I presume 
that the form would be called a variety. 

It may be doubted whether sudden and considerable deviations 
of structure such as we occasionally see in our domestic produc- 
tions, more especially with plants, are ever permanently propagated 
in a state of nature. Almost every part of every organic being is 
so beautifully related to its complex conditions of life that it seems 
as improbable that any part should have been suddenly produced 
perfect, as that a complex machine should have been invented by 
man in a perfect state. Under domestication monstrosities some- 
times occur which resemble normal structures in widely different 
animals. Thus pigs have occasionally been born with a sort of 
proboscis, and if any wild species of the same genus had naturally 
possessed a proboscis, it might have been argued that this had 
appeared as a monstrosity ; but I have as yet failed to find, after 
diligent search, cases of monstrosities resembling normal structures 
in nearly allied forms, and these alone bear on the question. If 
monstrous forms of this kind ever do appear in a state of nature 
and are capable of reproduction (which is not always the case), as 
they occur rarely and singly, their preservation would depend on 
unusually favourable circumstances. They would, also, during the 
first and succeeding generations cross with the ordinary form, and 
thus their abnormal character would almost inevitably be lost. 
But I shall have to return in a future chapter to the preservation 
and perpetuation of single or occasional variations. 

Individual Differences. 

The many slight differences which appear in the offspring from 
the same parents, or which it may be presumed have thus arisen, 
from being observed in the individuals of the same species inhabit- 
ing the same confined locality, may be called individual differences. 
No one supposes that all the individuals of the same species are 
cast in the same actual mould. These individual differences are of 
the highest importance for us, for they are often inherited, as must 
be familiar to every one ; and they thus afford materials for natural 


selection to act on and accumulate, in the same manner as man 
accumulates 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 sur- 
prised 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 being 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. It would never have 
been 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 ; it might have bc^n thought that changes of this 
nature could have been effected only by slow degrees ; yet Sir J. 
Lubbock has shown a degree of variability in these main nerves in 
Coccus, which may almost be compared to th^ irregular branching 
of the stem of a tree. This philosophical naturalist, I may add, 
has also shown that the muscles in the larvae oi certain insects are 
far from uniform. Authors sometimes argue in a circle when they 
state that important organs never vary ; for these same authors 
practically rank those parts as important (as some few naturalists 
have honestly confessed) which do not vary ; and, under this point 
of view, no instance will ever be found of an important part vary- 
ing ; but under any other point of view many instances assuredly 
can be given. 

There is one point connected with individual differences, which 
is extremely perplexing : I refer to those genera which have been 
called " protean " or " polymorphic," in which the species present 
an inordinate amount of variation. With respect to many of these 
forms, hardly two naturalists agree whether to rank them as species 
or as varieties. We may instance Rubus, Rosa, and Hieracium 
amongst plants, several genera of insects and 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 a few exceptions, polymorphic in other countries, 
and likewise, judging from Brachiopod shells, at former periods of 
time. These facts are 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, at least in some of these 
polymorphic genera, variations which are of no service or disservice 
to the species, and which consequently have not been seized cm 


and rendered definite by natural selection, as hereafter to be 

Individuals of the same species often present, as is known to 
every one, great differences of structure, independently of varia- 
tion, as in the two sexes of various animals, in the two or three 
castes of sterile female or workers amongst insects, and in the 
immature and larval states of many of the lower animals. There 
are, also, cases of dimorphism and trimorphism, both with animals 
and plants. Thus, Mr. Wallace, who has lately called attention 
to the subject, has shown that the females of certain species of 
butterflies, in the Malayan archipelago, regularly appear under 
two or even three conspicuously distinct forms, not connected by 
intermediate varieties. Fritz Miiller has described analogous but 
more extraordinary cases with the males of certain Brazilian 
Crustaceans : thus, the male of a Tanais regularly occurs under 
two distinct forms ; one of these has strong and differently shaped 
pincers, and the other has antennae much more abundantly 
furnished with smell ing-hairs. Although in most of these cases, 
the two or three forms, both with animals and plants, are not now 
connected by intermediate gradations, it is probable that they 
were once thus connected. Mr. Wallace, for instance, describes 
a certain butterfly which presents in the same island a great range 
of varieties connected by intermediate links, and the extreme 
links of the chain closely resemble the two forms of an allied 
dimorphic species inhabiting another part of the Malay archi- 
pelago. Thus also with ants, the several worker-castes are 
generally quite distinct ; but in some cases, as we shall hereafter 
see, the castes are connected together by finely graduated varieties. 
So it is, as I have myself observed, with some dimorphic plants. 
It certainly at first appears a highly remarkable fact that the same 
female butterfly should have the power of producing at the 
same time three distinct female forms and a male ; and that an 
hermaphrodite plant should produce from the same seed-capsule 
three distinct hermaphrodite forms, bearing three different kinds 
of females and three -or even six different kinds of males. Never- 
theless these cases are only exaggerations of the common fact that 
the female produces offspring of two sexes which sometimes differ 
from each other in a wonderful manner. 

Doubtful Species. 

The forms which possess in some considerable degree the 
character of species, but which are so closely similar to 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 for a long time ; 
for as long, as far as we know, as have good and true species. 
Practically, when a naturalist can unite by means of intermediate 
links any two forms, 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 arise 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 
forms always remove the difficulty. In very many cases, however, 
one form is ranked as a variety of another, not because the inter- 
mediate 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 judg- 
ment 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 

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 assist- 
ance 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 the 
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, geographical races ! Mr. Wallace, in 
several valuable papers on the various animals, especially on the 


Lepidoptera, inhabiting the islands of the great Malayan archi- 
pelago, shows that they may be classed under four heads, namely, 
as variable forms, as local forms, as geographical races or sub- 
species, and as true representative species. The first or variable 
forms vary much within the limits of the same island. The local 
forms are moderately constant and distinct in each separate 
island ; but when all from the several islands are compared 
together, the differences are seen to be so slight and graduated, 
that it is impossible to define or describe them, though at the same 
time the extreme forms are sufficiently distinct. The geographical 
races or sub-species are local forms completely fixed and isolated ; 
but as they do not differ from each other by strongly marked and 
important characters, "there is no possible test but individual 
opinion to determine which of them shall be considered as species 
and which as varieties." Lastly, representative species fill the 
same place in the natural economy of each island as do the local 
forms and sub-species ; but as they are distinguished from each 
other by a greater amount of difference than that between the 
local forms and sub-species, they are almost universally ranked 
by naturalists as true species. Nevertheless, no certain criterion 
can possibly be given by which variable forms, local forms, sub- 
species, and representative species can be recognised. 

Many years ago, when comparing, and seeing others compare, 
the birds from the closely neighbouring islands of the Galapagos 
archipelago, 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 cha- 
racterised as varieties in Mr. Wollaston's admirable work, but 
which would certainly 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 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 them 
as distinct species ; but what distance, it has been well asked, 
will suffice ; if that between America and Europe is ample, will 
that between Europe and the Azores, or Madeira, or the Canaries, 
or between the several islets of these small archipelagos, be 
sufficient ? 

Mr. B. D. Walsh, a distinguished entomologist of the United 
States, has described what he calls Phytophagic varieties and 
Phytophagic species. Most vegetable-feeding insects live on one 
kind of plant or on one group of plants ; some feed indiscriini- 


nately on many kinds, but do not in consequence vary. In several 
cases, however, insects found living on different plants, have been 
observed by Mr. Walsh to present in their larval or mature state, 
or in both states, slight, though constant differences in colour, 
size, or in the nature of their secretions. In some instances the 
males alone, in other instances both males and females, have been 
observed thus to differ in a slight degree. When the differences 
are rather more strongly marked, and when both sexes and all 
ages are affected, the forms are ranked by all entomologists as 
good species. But no observer can determine for another, even if 
he can do so for himself, which of these Phytophagic forms ought 
to be called species and which varieties. Mr. Walsh ranks the 
forms which it may be supposed would freely intercross, as 
varieties ; and those which appear to have lost this power, as 
species. As the differences depend on the insects having long fed 
on distinct plants, it cannot be expected that intermediate links 
connecting the several forms should now be found. The naturalist 
thus loses his best guide in determining whether to rank doubtful 
forms as varieties or species. This likewise necessarily occurs 
with closely allied organisms, which inhabit distinct continents 
or islands. When, on the other hand, an animal or plant ranges 
over the same continent, or inhabits many islands in the same 
archipelago, and presents different forms in the different areas, 
there is always a good chance that intermediate forms will be 
discovered which will link together the extreme states ; and these 
are then degraded to the rank of varieties. 

Some few naturalists maintain that animals never present 
varieties ; but then these same naturalists rank the slightest 
difference as CA specific value ; and when the same identical form 
is met with in two distant countries, or in two geological forma- 
tions, they believe that two distinct species are hidden under the 
same dress. The term species thus comes to be a mere useless 
abstraction, implying and assuming a separate act of creation. It 
is certain that many forms, considered by highly-competent judges 
to be varieties, resemble species so completely in character, that 
they have been thus ranked by other highly-competent judges. 
But to discuss whether they ought to be 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 in the attempt to 
determine their rank ; but space does not here permit me to 
discuss them. Close investigation, in many cases, will no doubt 
bring naturalists to agree 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 them. I have been struck with the 
fact, that if any animal or plant in a state of nature be highly 
useful to man, or from any cause closely attracts his attention, 
varieties of it will almost universally be found recorded. These 
varieties, moreover, will often be 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 almost universally considered by other 
botanists to be 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. 

I may here allude to a remarkable memoir lately published by 
A. de Candolle, on the oaks of the whole world. No one ever 
had more ample materials for the discrimination of the species, 
or could have worked on them with more zeal and sagacity. He 
first gives in detail all the many points of structure which vary in 
the several species, and estimates numerically the relative frequency 
of the variations. He specifies above a dozen characters which 
may be found varying even on the same branch, sometimes 
according to age or development, sometimes without any assign- 
able reason. Such characters are not of course of specific value, 
but they are, as Asa Gray has remarked in commenting on this 
memoir, such as generally enter into specific definitions. De 
Candolle then goes on to say that he gives the rank of species 
to the forms that differ by characters never varying on the same 
tree, and never found connected by intermediate states. After 
this discussion, the result of so much labour, he emphatically 
remarks : " They are mistaken, who repeat that the greater part 
of our species are clearly limited, and that the doubtful species 
are in a feeble minority. This seemed to be true, so long as a 
genus was imperfectly known, and its species were founded upon 
a few specimens, that is to say, were provisional. Just as we 
come to know them better, intermediate forms flow in, and doubts 
as to specific limits augment." He also adds that it is the best 
known species which present the greatest number of spontaneous 
varieties and sub-varieties. The Quercus robur has twenty-eight 
varieties, all of which, excepting six, are clustered round three 
sub-species, namely, Q. pedunculata, sessiliflora, and pubescens. 
The forms which connect these three sub-species are compara- 
tively rare ; and, as Asa Gray again remarks, if these connecting 
forms which are now rare, were to become wholly extinct, the 
three sub-species would hold exactly the same relation to each 
other, as do the four or five provisionally admitted species which 
closely surround the typical Quercus robur. Finally, De Candolle 


admits that out of the 300 species, which will be enumerated in 
his Prodromus as belonging to the oak family, at least two-thirds 
are provisional species, that is, are not known strictly to fulfil 
the definition above given of a true species. It should be added 
that De Candolle no longer believes that species are immutable 
creations, but concludes that the derivative theory is the most 
natural one, "and the most accordant with the known facts in 
palaeontology, geographical botany and zoology, of anatomical 
structure and classification." 

When a young naturalist commences the study of a group of 
organisms quite unknown to him, he is at first much perplexed in 
determining what differences to consider as specific, and what as 
varietal ; for he knows nothing of the amount and kind of varia- 
tion 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 im- 
pressed, 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 varia- 
tion 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 if his observations 
be widely extended, he will in the end generally be able to make 
up his own mind : but he will succeed in this at the expense of 
admitting much variation, and the truth of this admission will 
often be disputed by other naturalists. When he comes to study 
allied forms brought from countries not now continuous, in which 
case he cannot hope to find intermediate links, he will be com- 
pelled 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 by an insen- 
sible 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 the highest importance for us, as being 
the first steps 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 towards more strongly-marked and permanent varieties ; 
and at the latter, as leading to sub-species, and then to species. 
The passage from one stage of difference to another may, in many 
cases, be the simple result of the nature of the organism and of 
the different physical conditions to which it has long been ex* 
posed ; but with respect to the more important and adaptive 
characters, the passage from one stage of difference to another, 
may be safely attributed to the cumulative action of natural 
selection, hereafter to be explained, and to the effects of the 
increased use or disuse of parts. A well-marked variety may 
therefore be called an incipient species ; but whether this belief 
is justifiable must be judged by the weight of the various facts 
and considerations to be given throughout this work. 

It need not be supposed that all varieties or incipient species 
attain the rank of species. They may 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, and with plants by Gaston de Saporta. 
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 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 arbi- 
trarily, for convenience' sake. . 

Wide-ranging, much diffused, and common Species vary most. 

Guided by theoretical considerations, I thought that some inte- 
resting 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 a future work 
the discussion of these difficulties, and the tables of the propor- 
tional numbers of the varying species. Dr. Hooker permits me 
to add that after having carefully read my manuscript, and ex- 
amined 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 

Alphonse de Candolle and others have shown that plants which 
have very wide ranges generally present varieties ; and this might 
have been expected, as they are exposed to diverse physical 
conditions, and as they come into competition (which, as we shall 
hereafter see, is an equally or more important circumstance) with 
different sets of organic beings. But my tables further show that, ; 
in any limited country, the species which are the most common, 
that is abound most in individuals, and the species which are most I 
widely diffused within their own country (and this is a different ) 
consideration from wide range, and to a certain extent from j' 
commonness), oftenest 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, are the most diffused in their | 
own country, and are the most numerous in individuals, which I 
oftenest produce well-marked varieties, or, as I consider them, | 
incipient species. And this, perhaps, might have been anticipated ; a 
for, as varieties, in order to become in any degree permanent, I 
necessarily have to struggle with the other inhabitants of the I 
country, the species which are already dominant will be the most I 
likely to yield offspring, which, though in some slight degree I 
modified, still inherit those advantages that enabled their parents f' 
to become dominant over their compatriots. In these remarks 
on predominance, it should be understood that reference is made 
only to the forms which come into competition with each other, 
and more especially to the members of the same genus or class 
having nearly similar habits of life. With respect to the number 
of individuals or commonness of species, the comparison of course 
relates only to the members of the same group. One of the 
higher plants may be said to be dominant if it be more numerous 
in individuals and more widely diffused than the other plants of 
the same country, which lire under nearly the same conditions. 
A plant of this kind is not the less dominant because some 
conferva inhabiting the water or some parasitic fungus is infinitely 
more numerous in individuals, and more widely diffused. But if 
the conferva or parasitic fungus exceeds its allies in the above 
respects, it will then be dominant within its own class. 

Species of the Larger Genera in each Country vary move frequently 
than the Species of the Smaller Genera. 

If the plants inhabiting a country, as described in any Flora, 
be divided into two equal masses, all those in the larger genera 
(i.e., those including many species) being placed on one side, and 


all those in the smaller genera on the other side, the former will 
be found to include a somewhat larger number of the very 
common and much diffused or dominant species. This 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 larger 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 generally have 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 varia- 
tion ; 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 presented 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, anc' when all the least genera, with from only one to four 


species, are altogether excluded from the tables. These facts are 
of plain signification on the view that species are only strongly- 
marked and permanent varieties ; for wherever 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 holds true, 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, decline, 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 certainly holds good. 

Many of tJie Species included within t/te Larger Genera resemble 
Varieties in being very closely^ but unequally, related to each 
other, and in having restricted ranges. 

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 when 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 confirm 
the view. I have also consulted some sagacious and 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 less than the usual 
amount of difference. 

Moreover, the species of the larger 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 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 ? Un- 
doubtedly there is one most important point of difference between 
varieties and species ; namely, that the amount of difference be- 
tween 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 tend 
to increase into the greater differences between species. 

There is one other point which is worth notice. Varieties gene- 
rally 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 
would be reversed. But there is reason to believe that the 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 Cata- 
logue 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 acknow- 
ledged 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 nearly the same 
restricted average range, as have the 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, varieties cannot be distinguished from species, except, 
first, by the discovery of intermediate linking forms ; and, secondly, 
by a certain indefinite amount of difference between them ; for two 
forms, if differing very little, are generally ranked as varieties, 
notwithstanding that they cannot be closely connected ; but the 



amount of difference considered necessary to give to any two forms 
the rank of species cannot be defined. 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 other species. Species very 
closely allied to other species apparently have restricted ranges. 
In all these respects the species of large genera present a strong 
analogy with varieties. And we can clearly understand these 
analogies, if species once existed as varieties, and thus originated : 
whereas, these analogies are utterly inexplicable if species are 
independent creations. 

We have, also, seen that it is the most flourishing or dominant 
species of the larger genera within each class which on an average 
yield the greatest number of varieties ; and varieties, as we shall 
hereafter see, tend to become converted into new and distinct 
species. Thus the larger genera 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. 


Its bearing on natural selection The terra used in a wide sense Geometrical 
ratio 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. 

BEFORE 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 indir 
vidual 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 con- 
ditions of life, and of one organic being to another being, been 
perfected 1 We see these beautiful co-adaptations most plainly in 
the woodpecker and the mistletoe ; 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 1 All these results, as we shall 
more fully see in the next chapter, follow from the struggle for 
Jife. Owing to this struggle, variations, however slight and from 
whatever cause proceeding, if they be in any degree profitable to 
the individuals of a species, in their infinitely complex relations to 
other organic beings and to their physical conditions of life, will 
tend to the preservation of such individuals, and will generally be 
inherited by the 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 sur- 
vive. I have called this principle, by which each slight variation, 
if useful, is preserved, by the term Natural Selection, in order to 
mark its relation to man's power of selection. But the expression 
often used by Mr. Herbert Spencer of the Survival of the Fittest 
is more accurate, and is sometimes equally convenient. We have 
seen that man by selection can certainly produce great results, 
and can adapt organic beings to his own uses, through the accu- 
mulation 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 will be treated, as it 
well deserves, at 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, 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 glad- 
ness, 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. 

The Term, Struggle for Existence, used in a large sense. 

I should premise that I use this term in a large and meta- 
phorical 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 pro- 
perly it should be said to be dependent on the moisture. A plant 
which annually produces a thousand seeds, of which only one of 
an average 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 mistletoe 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 languishes and dies. But several seedling mistletoes, 
growing close together on the same branch, may more truly be 
said to struggle with each other. As the mistletoe is disseminated 
by birds, its existence depends on them , and it may metaphori- 
cally be said to struggle with other fruit-bearing plants, in tempt- 
ing the birds to devour and thus disseminate its seeds. In these 
several senses, which pass into each other, I use for convenience' 
sake the general term of Struggle for Existence. 

Geometrical Ratio of Increase. 

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 indi- 
viduals 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 less than a 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 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 safest to assume that 
it begins breeding when thirty years old, and goes on breeding 
till ninety years old, bringing forth six young in the interval, 
and surviving till one hundred years old ; if this be so, after a 
period of from 740 to 750 years there would be nearly nineteen 
million elephants alive, descended from the first pair. 

But we have better evidence on this subject than mere theore- 
tical 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 
incredible. So it is with plants ; cases could be given of intro- 
duced plants which have become common throughout whole islands 
in a period of less than ten years. Several of the plants, such as 
the cardoon and a tall thistle, which are now the commonest over 
the wide plains of La Plata, clothing square leagues of surface 
almost to the exclusion of every other plant, 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 others could be given, no one supposes, that 
the fertility of the animals or plants has- been suddenly and tem- 
porarily increased in any sensible degree. The obvious explanation 
is that the conditions of life have been highly 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. 
Their geometrical ratio of increase, the result of which never fails 
to be surprising, simply explains their extraordinarily rapid in- 
crease and wide diffusion in their new homes. 

In a state of nature almost every full-grown plant annually pro- 
duces 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 rapidly stock every station in which they could anyhow 
exist, and that this 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, but we do not keep in 
mind 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 
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 pro- 
duced, or the species will become extinct. It would suffice to keep 
up the full number of a tree, which lived on an average for a 
thousand years, if 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 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. 

Nature of the Checks to Increase. 

The causes which check the natural tendency of each species to 
increase are most obscure. Look at the most vigorous species ; 
by as much as it swarms in numbers, by so much will it tend to 
increase still further. We know not exactly what the checks are 
even in a single instance. Nor will this surprise any one who 
reflects how ignorant we are on this head, even in regard to man- 
kind, although so incomparably better known than any other 
animal. This subject of the checks to increase has been ably 
treated by several authors, and I hope in a future work to discuss 
it 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, it appears 
that the seedlings 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 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 mown turf (three feet by four) nine species perished, from the 
other species being allowed to grow uf 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 bb 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 
shot. On the other hand, in some cases, as with the elephant, 
none are destroyed by beasts of prey ; for 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 seem to be the most effective of all checks. I estimated 
jchiefly from the greatly reduced numbers of nests in the spring) 
that the winter of 1854-5 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 individuals, 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 false view ; we forget that each 
species, even where it most abounds, is constantly suffering enor- 
mous 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 must 
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 northwards, or in ascending 
a mountain, we far oftener meet 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 Jjy favouring other 
species, we clearly see in the prodigious number of plants which 


in our gardens 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, in- 
creases 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 disproportionally 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 exist ; and that of some social plants being social, 
that is abounding in individuals, even on the extreme verge 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 the species from utter 
destruction. I should add that the good effects of intercrossing, 
and the ill effects of close interbreeding, no doubt come into play 
in many of these cases ; but I will not here enlarge on this 

Complex Relations of all Animals and Plants to each other in the 
Struggle for Existence. 

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, interested me. In Stafford- 
shire, on the estate of a relation, where I had ample means of in- 
vestigation, there was a large and extremely barren heath, which 
had never been touched by the hand of man ; but several acres of 
exactly the same nature had been enclosed twenty-five years pre- 



viously 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 com- 
mon in the plantations, which were not to be seen on the heath ; 
and the heath was frequented by two or three distinct insecti- 
vorous 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 of the land having 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 hundred yards distant from one of 
the old clumps, I counted thirty-two little trees ; and one of 
them, with twenty-six rings of growth, had, during many 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 Kengger have shown that this is 
caused by the greater number in Paraguay of a certain fly, Avhich 
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 other parasitic insects. 
Hence, if certain insectivorous birds were to decrease in Paraguay, 


the parasitic insects would probably increase ; and this would 
lessen the number of the navel-frequenting flies then cattle and 
horses would become feral, and this would certainly greatly alter 
(as indeed I have observed in parts of South America) the vegeta- 
tion : this again would largely affect the insects ; and this, as we 
have just seen in Staffordshire, the insectivorous birds, and so 
onwards in ever-increasing circles of complexity. Not that under 
nature the relations will ever be as simple as this. Battle within 
battle must be continually recurring with varying success ; and yet 
in the long-run the forces are so nicely balanced, that the face of 
nature remains for long periods of time uniform, though assuredly 
the merest trifle would 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 am tempted to give one more instance showing how plants and 
animals, 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 is never visited in my garden by 
insects, and consequently, from its peculiar structure, never sets a 
seed. Nearly all our orchidaceous plants absolutely require the 
visits of insects to remove their pollen-masses and thus to fertilise 
them. I find from experiments that humble-bees are almost indis- 
pensable to the fertilisation of the heartsease (Viola tricolor), for 
other bees do not visit this flower. I have also found that the 
visits of bees are necessary for the fertilisation of some kinds of 
clover ; for instance, 20 heads of Dutch clover (Trifolium repens) 
yielded 2,290 seeds, but 20 other heads protected from bees pro- 
duced not one. Again, 100 heads of red clover (T. pratense) pro- 
duced 2,700 seeds, but the same number of protected heads produced 
not a single seed. Humble-bees alone visit red clover, as other 
bees cannot reach the nectar. It has been suggested that moths 
may fertilise the clovers ; but I doubt whether they could do so in 
the case of the red clover, from their weight not being sufficient to 
depress the wing petals. Hence we may infer as highly probable 
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 measure upon the number of field-mice, which 
destroy their combs and nests ; and Col. 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 Col. 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 will concur 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 ancient 
Indian, ruins in the Southern United States, which must formerly 
have been cleared of trees, now display the same beautiful 
diversity and proportion of kinds as in the surrounding virgin 
forest. What a struggle must have gone on during long centuries 
between the several kinds of trees, 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, all feeding on each other, or on 
the trees, 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 fall to the ground 
according to definite laws ; but how simple is the problem where 
each shall fall compared to that of 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 likewise sometimes the case with those 
which may be strictly said to struggle with each other for 
existence, as in the case of locusts and grass-feeding quadrupeds. 
But the struggle will almost invariably 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 tk 


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 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 number 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 of our domestic plants or animals have so exactly 
the same strength, habits, and constitution, that the original 
proportions of a mixed stock (crossing being prevented) could be 
kept up for half-a-dozen generations, if they were allowed to 
struggle together, in the same manner as beings in a state of 
nature, and if the seed or young were not annually preserved in 
due proportion. 

Struggle for Life most severe between Individuals and Varieties of 
the same Species. 

As the species of the same genus usually have, though by no 
means invariably, much similarity in habits and constitution, and 
always in structure, the struggle will generally be more severe 
between them, if they come into competition with each other, than 
between the 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 in- 
crease 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. In Australia the 
imported hive-bee is rapidly exterminating the small, stingless 
native bee. One species of charlock has been known to supplant 
another species ; 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, tc 
that of all the other organic beings, with which it comes into com- 
petition 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 with 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, it 
may be suspected that the chief use of the nutriment in the seed 
is to favour the growth of the seedlings, 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 1 We know that it can perfectly well 
withstand a little more heat or cold, dampness or dryness, for else- 
where it ranges into slightly hotter or colder, damper or drier 
districts. In this case we can clearly see that if we wish 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 prey on it. On the confines of its geo- 
graphical 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 exclusively by the rigour of the climate. 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 laud 
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 we can see that when a plant or animal is placed in a 
new country amongst new competitors, the conditions of its life 
will generally be changed in an essential manner, although the 
climate may be exactly the same as in its former home. If its 
average numbers are to increase in its new home, we should have 
to modify it in a different way to what we should have had to do 
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 imagination to give to any one 
species an advantage over another. Probably in no single instance 


should we know what to do. This ought to convince us of our 
ignorance on the mutual relations of all organic beings ; a con- 
viction as necessary, as it is difficult to acquire. All that we 
can do, is to keep steadily in mind that each organic being is 
striving to increase in a geometrical ratio ; that each at some 
period of its life, during some season of the year, during 
generation or at intervals, has to struggle for life and to suffer 
great destruction. When we reflect on this struggle, we may con 
sole ourselves witlr"the full belief, that the war of nature is not 
incessant, that no fear is felt, that death is generally _prompt, 
and thar the vigorous, the healthy, and the happy survive and 


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 indi- 
viduals of the same species Circumstances favourable and unfavourable 
to the results of 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 
Advance in organisation Low forms preserved Convergence of character 
Indefinite multiplication of species Summary. 

How will the struggle for existence, briefly discussed 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 under 
nature 1 ? I think we shall see that it can act most efficiently. 
Let the endless number of slight variations and individual 
differences occurring in our domestic productions, and, in a lesser 
degree, in those under nature, be borne in mind ; as well as the 
strength of the hereditary tendency. Under domestication, it 
may be truly said that the whole organisation becomes in some 
degree plastic. But the variability, which we almost universally 
meet with in our domestic productions, is not directly produced, 
as Hooker and Asa Gray have well remarked, by man ; he can 
neither originate varieties, nor prevent their occurrence ; he can 
only preserve and accumulate such as do occur. Unintentionally 
he exposes organic beings to new and changing conditions of life, 
and variability ensues ; but similar changes of conditions might 



and do occur under nature. Lei it algo be borne in mind ^ w 
infinitely complex and close-fitting are the mutua i relations of all 
organic beings to each other jfoft to ^ e ^ r physical conditions of 
life ; and consequently whaV^ifinitely varied diversities of structure 
might be ( o 3gffo each being under changing conditions of life. 
Car?* i^' '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 occur in the course of many successive generations 1 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 kind ? On 
the other hand, we may feel sure that any variation in the least 
degree injurious would be rigidly destroyed. This preservation 
of favourable individual differences and variations, and the 
destruction of those which are injurious, I have called Natural 
Selection, or the Survival of the Fittest. Variations neither useful 
nor injurious would not be affected by natural selection, and 
would be left either a fluctuating element, as perhaps we see in 
certain polymorphic species, or would ultimately become fixed, 
owing to the nature of the organism and the nature of the 

Several writers have misapprehended or objected to the term 
Natural Selection. Some have even imagined that natural 
selection induces variability, whereas it implies only the preserva- 
tion of such variations as arise and are beneficial to the being 
under its conditions of life. No one objects to agriculturists 
speaking of the potent effects of man's selection ; and in this case 
the individual differences given by nature, which man for some 
object selects, must of necessity first occur. Others have objected 
that the term selection implies conscious choice in the animals 
which become modified ; and it has even been urged that, as 
plants have no volition, natural selection is not applicable to 
them ! In the literal sense of the word, no doubt, natural selection 
is a false term ; but who ever objected to chemists speaking of 
the elective affinities of the various elements ? and yet an acid 
cannot strictly be said to elect the base with which it in preference 
combines. It has been said that I speak of natural selection as 

-p an active power or Deity ; but who objects to an author speaking- 
of the attraction of gravity as ruling the movements of the 
planets ? Every one knows what is meant and is implied by such 
metaphorical expressions ; and they are almost necessary for 
brevity. So again it is difficult to avoid personifying the word 
Nature ; but I mean by Nature, only the aggregate action and 

'' product of many natural laws, and by laws the sequence of events 


as ascertained by us. With a little familiarity such superficial 
objections will be forgotten. 

We shall best understand the probable course of natural 
selection by taking the case of a country undergoing some slight 
physical change, for instance, of climate. The proportional 
numbers of its inhabitants will almost immediately undergo a 
change, and some species will probably 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 the 
inhabitants, independently of the change of climate itself, would 
seriously affect the others. If the country were open on its 
borders, new forms would certainly immigrate, and this would 
likewise 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 cases, slight modifications, which in any 
way favoured the individuals of any species, by better adapting 
them to their altered conditions, would tend to be preserved ; and 
natural selection would have free scope for the work of im- 

We have good reason to believe, as shown in the first chapter, 
that changes in the conditions of life give a tendency to increased 
variability; and in the foregoing cases the conditions have 
changed, and this would manifestly be favourable to natural 
selection, by affording a better chance of the occurrence of profit- 
able variations. Unless such occur, natural selection can do 
nothing. Under the term of " variations," it must never be for- 
gotten that mere individual differences are included. As man 
can produce a great result with his domestic animals and plants 
by adding up in any given direction individual differences, so 
could natural selection, but far more easily from having incom- 
parably longer time for action. Nor do I believe that any great 
physical change, as of climate, or any unusual degree of isolation 
to check immigration, is necessary in order that new and un- 
occupied places should be left, for natural selection to fill up by 
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 species would often give it an advantage over 


others ; and still further modifications of the same kind would 
often still further increase the advantage, as long as the species 
continued under the same conditions of life and profited by similar 
means of subsistence and defence. 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 be still better adapted or improved ; for 
in all countries, the natives have been so far conquered by 
naturalised productions, that they have allowed some foreigners 
to take firm possession of the land. And as foreigners have thus 
in every country beaten some of the natives, we may safely 
conclude that the natives might have been modified with 
advantage, so as to have better resisted the intruders. 

As man can produce, and certainly has produced, a great result 
by his methodical and unconscious means of selection, what may 
not natural selection effect 1 Man can act only on external and 
visible characters : Nature, if I may be allowed to personify the 
natural preservation or survival of the fittest, cares nothing for 
appearances, except in so far as they are useful to any being. 
She can act on every internal organ, on every shade of constitu- 
tional 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, as 
is implied by the fact of their selection. 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 the eye or to be plainly useful to him. Under 
nature, the slightest differences 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 be his 
results, compared with those accumulated by Nature during whole 
geological periods ! Can we wonder, then, that Nature's produc- 
tions 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 1 

It may metaphorically be said that natural selection is daily and 


hourly scrutinising, throughout the world, the slightest variations ; 
rejecting those that are bad, preserving and adding up all that are 
good ; silently and insensibly working, wlienever and wlierever 
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 lapse of ages, and then so imperfect is our view 
into long-past geological ages, that we see only that the forms of 
life are now different from what they formerly were. 

In order that any great amount of modification should be effected 
in a species, a variety when once formed must again, perhaps after 
a long interval of time, vary or present individual differences of 
the same favourable nature as before ; and these must be again 
preserved, and so onwards step by step. Seeing that individual 
differences of the same kind perpetually recur, this can hardly be 
considered as an unwarrantable assumption. But whether it is 
true, we can judge only by seeing how far the hypothesis accords 
with and explains the general phenomena of nature. On the other 
hand, the ordinary belief that the amount of possible variation is 
a strictly limited quantity is likewise a simple assumption. 

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, 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 
natural selection might be 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 a lamb with the faintest trace of 
black. We have seen how the colour of the hogs, which feed on 
the " paint-root " in Virginia, determines whether they shall live 
or die. 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, Down- 
ing, that in the United States smooth-skinned fruits suffer far 
more from a beetle, a Curculio, than those with down ; that purplo 
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 quite 
unimportant, we must not forget that climate, food, <fcc., have no 
doubt produced some direct effect. It is also necessary to bear in 
mind that, owing to the law of correlation, when one part varies, 
and the variations are accumulated through natural selection, 
other modifications, often of the most unexpected nature, will 

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 shape, size, and 
flavour of 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 variations profitable 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 ; and these modifications may effect, through 
correlation, the structure of the adult. So, conversely, modifica- 
tions in the adult may affect the structure of the larva ; but in all 
cases natural selection will ensure that they shall not be injurious : 
for if they were so, the species would become extinct. 

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 whole community ; if the community 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 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, used exclusively for 
opening the cocoon or the hard tip to the beak of unhatched 
birds, used for breaking the egg. It has been asserted, that of 
the best short-beaked tumbler-pigeons a greater number 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 all 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. 

It may be well here to remark that with all beings there must 
be much fortuitous destruction, which can have little or no 
influence on the course of natural selection. For instance a vast 
number of eggs or seeds are annually devoured, and these could 
be modified through natural selection only if they varied in some 
manner which protected them from their enemies. Yet many of 
these eggs or seeds would perhaps, if not destroyed, have yielded 
individuals better adapted to their conditions of life than any of 
those which happened to survive. So again a vast number of 
mature animals and plants, whether or not they be the best 
adapted to their conditions, must be annually destroyed by 
accidental causes, which would not be in the least degree mitigated 
by certain changes of structure or constitution which would in 
other ways be beneficial to the species. But let the destruction 
of the adults be ever so heavy, if the number which can exist in 
any district be not wholly kept down by such causes, or again 
let the destruction of eggs or seeds be so great that only a 
hundredth or a thousandth part are developed, yet of those 
which do survive, the best adapted individuals, supposing that 
there is any variability in a favourable direction, will tend to 
propagate their kind in larger numbers than the less well adapted. 
If the numbers be wholly kept down by the causes just indicated, 
as will often have been the case, natural selection will be powerless 
in certain beneficial directions ; but this is no valid objection to 
its efficiency at other times and in other ways ; for we are far 
from having any reason to suppose that many species ever undergo 
modification and improvement at the same time in the same 


Sexual Selection. 

Inasmuch as peculiarities often appear under domestication in 
one sex and become hereditarily attached to that sex, so no doubt 
it will be under nature. Thus it is rendered possible for the two 
sexes to be modified through natural selection in relation to 
different habits of life, as is sometimes the case ; or for one sex 
to be modified in relation to the other sex, as commonly occurs. 
This leads me to say a few words on what I have called Sexual 
Selection. This form of selection depends, not on a struggle for 
existence in relation to other organic beings or to external con- 
ditions, but on a struggle between the individuals of one sex, 
generally the males, for the possession of the other sex. 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 depends not so much on general 
vigour, as on having special weapons, confined to the male sex. 
A hornless stag or spurless cock would have a poor chance of 
leaving numerous 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, 
in nearly the same manner as does the brutal cockfighter by the 
careful selection of his best cocks. How low in the scale of nature 
the 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 observed fighting all day long ; male stag-beetles some- 
times bear wounds from the huge mandibles of other males ; the 
males of certain hymenopterous insects have been frequently seen 
by that inimitable observer M. Fabre, fighting for a particular 
female who sits by, an apparently unconcerned beholder of the 
struggle, and then retires with the conqueror. 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 of the lion, 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 dis- 


play with the most elaborate care, and show off in the best 
manner, their gorgeous plumage ; they likewise 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 in- 
dividual preferences and dislikes : thus Sir R. Heron has described 
how a pied peacock was eminently attractive to all his hen birds. 
I cannot here enter on the necessary details ; but if man can in 
a short time give beauty and an elegant carriage 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 genera- 
tions, the most melodious or beautiful males, according to their 
standard of beauty, might produce a marked effect. Some well- 
known laws, with respect to the plumage of male and female 
birds, in comparison with the plumage of the young, can partly 
be explained through the action of sexual selection on variations 
occurring at different ages, and transmitted to the males alone or 
to both sexes at corresponding ages ; 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, by individual males having 
had, in successive generations, some slight advantage over other 
males, in their weapons, means of defence, or charms, which they 
have transmitted to their male offspring alone. Yet, I would not 
wish to attribute all sexual differences to this agency : for we see 
in our domestic animals peculiarities arising and becoming 
attached to the male sex, which apparently have not been aug- 
mented through selection by man. The tuft of hair on the breast 
of the wild turkey-cock cannot be of any use, and it is doubtful 
whether it can be ornamental in the eyes of the female bird ; 
indeed, had the tuft appeared under domestication, it would have 
been called a monstrosity. 

Illustrations of tJie Action of Natural Selection, or the Survival 
of the Fittest. 

In order to make it clear how, as I believe, natural selection 
acts, I must beg permission to give one or two imaginary illustra- 
tions. 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 num- 
bers, or that other prey had decreased in numbers, during that 
season of the year when the wolf was hardest pressed for food. 
Under such circumstances 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 some other period of the year, when they were com- 
pelled to prey on other animals. I can see no more reason to doubt 
that this would be the result, than that man should be able to 
improve the fleetness of his greyhounds by careful and methodical 
selection, or by that kind of unconscious selection which follows 
from each man trying to keep the best dogs without any thought 
of modifying the breed. 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. 

It should be observed that, in the above illustration, I speak of 
the slimmest individual wolves, and not of any single strongly- 
marked variation having been preserved. In former editions of 
this work I sometimes spoke as if this latter alternative had 
frequently occurred. I saw the great importance of individual 
differences, and this led me fully to discuss the results of uncon-j 
scious selection by man, which depends on the preservation of all 
the more or less valuable individuals, and on the destruction of 
the worst. I saw, also, that the preservation in a state of nature 
of any occasional deviation of structure, such as a monstrosity, 
would be a rare event ; and that, if at first preserved, it would 
generally be lost by subsequent intercrossing with ordinary indi- 
viduals. Nevertheless, until reading an able and valuable article 
in the ' North British Review ' (1867), I did not appreciate how 
rarely single variations, whether slight or strongly-marked, could 
be perpetuated. The author takes the case of a pair of animals, 
producing during their lifetime two hundred offspring, of which, 
from various causes of destruction, only two on an average survive 
to pro-create their kind. This is rather an extreme estimate for 
most of the higher animals, but by no means so for many of the 
lower organisms. He then shows that if a single individual were 
born, which varied in some manner, giving it twice as good a 
chance of life as that of the other individuals, yet the chances 
would be strongly against its survival. Supposing it to survive 
and to breed, and that half its young inherited the favourable 
variation ; still, as the Reviewer goes on to show, the young 
would have only a slightly better chance of surviving and breed- 
ing ; and this chance would go on decreasing in the succeeding 
generations. The justice of these remarks cannot, I think, be 
disputed. If, for instance, a bird of some kind could procure its 
food more easily by having its beak curved, and if one were born 
with its beak strongly curved, and which consequently flourished, 
nevertheless there would be a very poor chance of this one indi- 


vidual perpetuating its kind to the exclusion of the common form ; 
but there can hardly be a doubt, judging by what we see taking 
place under domestication, that this result would follow from the 
preservation during many generations of a large number of indi- 
viduals with more or less strongly curved beaks, and from the 
destruction of a still larger number with the straightest beaks. 

It should not, however, be overlooked, that certain rather 
strongly marked variations, which no one would rank as mere 
individual differences, frequently recur owing to a similar organisa- 
tion being similarly acted on of which fact numerous instances 
could be given with our domestic productions. In such cases, if 
the varying individual did not actually transmit to its offspring 
its newly-acquired character, it would undoubtedly transmit to 
them, as long as the existing conditions remained the same, a still 
stronger tendency to vary in the same manner. There can also 
be little doubt that the tendency to vary in the same manner has 
often been so strong that all the individuals of the same species 
have been similarly modified without the aid of any form of selec- 
tion. Or only a third, fifth, or tenth part of the individuals may 
have been thus affected, of which fact several instances could be 
given. Thus Graba estimates that about one-fifth of the guille- 
mots in the Faroe Islands consist of a variety so well marked, 
that it was formerly ranked as a distinct species under the name 
of Uria lacrymans. In cases of this kind, if the variation were 
of a beneficial nature, the original form would soon be supplanted 
by the modified form, through the survival of the fittest. 

To the effects of intercrossing in eliminating variations of all 
kinds, I shall have to recur ; but it may be here remarked that 
most animals and plants keep to their proper homes, and do not 
needlessly wander about ; we see this even with migratory birds, 
which almost always return to the same spot. Consequently each 
newly-formed variety would generally be at first local, as seems 
to be the common rule with varieties in a state of nature ; so that 
similarly modified individuals would soon exist in a small body 
together, and would often breed together. If the new varietj* 
were successful in its battle for life, it would slowly spread from 
a central district, competing with and conquering the unchanged 
individuals on the margins of an ever-increasing circle. 

It may be worth while to give another and more complex 
illustration of the action of natural selection. Certain plants 
excrete sweet juice, apparently for the sake of eliminating some- 
thing injurious from the sap : this is effected, for instance, by 
glands at the base of the stipules in some Leguminosae, and at the 
backs of the leaves of the common laurel. This juice, though 
small in quantity, is greedily sought by insects ; but their visits 
do not in any way benefit the plant. Now, let us suppose that 


the juice or nectar was excreted from the inside of the flowers of 
a certain number of plants of any species. Insects in seeking the 
nectar would get dusted with pollen, and would often transport 
it from one flower to another. The flowers of two distinct in- 
dividuals of the same species would thus get crossed ; and the act 
of crossing, as can be fully proved, gives rise to vigorous seedlings, 
which consequently would have the best chance of flourishing and 
surviving. The plants which produced flowers with the largest 
glands or nectaries, excreting most nectar, would oftenest be 
visited by insects, and would oftenest be crossed ; and so in the 
long-run would gain the upper hand and form a local variety. 
The flowers, also, which had their stamens and pistils placed, in 
relation to the size and habits of the particular insect which 
visited them, so as to favour in any degree the transportal of the 
pollen, would likewise be favoured. 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 purpose of 
fertilisation, its destruction appears to be 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 
to be thus robbed ; and the individuals which produced more and 
more pollen, and had larger anthers, would be selected. 

When our plant, by the above process long continued, had been 
rendered highly attractive to insects, they would, unintentionally 
on their part, regularly carry pollen from flower to flower ; and 
that they do this effectually, I could easily show by many striking 
facts. I will give only one, as likewise illustrating one step in the 
separation of the sexes of plants. Some holly-trees bear only 
male flowers, which have four stamens producing a rather 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 a few pollen-grains, and on some a pro- 
fusion. 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 favour- 
able to bees, nevertheless every female flower which I examined 
had been effectually fertilised by the bees, which had 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 might 
be effected. It would take up too much space to show the various 
steps, through dimorphism and other means, by which the separa- 
tion of the sexes in plants of various kinds is apparently now in 
progress ; but I may add that some of the species of holly in 
North America, are, according to Asa Gray, in an exactly in- 
termediate condition, or, as he expresses it, are more or less 
dioaciously polygamous. 

Let us now turn to the nectar-feeding insects ; 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 with a very little 
more trouble, they can enter by the mouth. Bearing such facts 
in mind, it may be believed that under certain circumstances 
individual differences in the curvature or length of the proboscis, 
tc., too slight to be appreciated by us, might profit a bee or other 
insect, so that certain individuals would be able to obtain their 
food more quickly than others ; and thus the communities to 
which they belonged would flourish and throw off many swarms 
inheriting the same peculiarities. The tubes of the corolla of the 
common red and incarnate clovers (Trifolium pratense and incar- 
natum) 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 ID 
vain an abundant supply of precious nectar to the hive-bee. That 
this nectar is much liked by the hive-bee is certain ; for I have 
repeatedly seen, but only in the autumn, many hive-bees sucking 
the flowers through holes bitten in the base of the tube by 
humble-bees. The difference in the length of the corolla in the 


two kinds of clover, which determines the visits of the hive-bee, 
must be very trifling; for I have been assured that when red 
clover has been mown, the flowers of the second crop are some- 
what smaller, and that these are visited by many hive-bees. I 
do not know whether this statement is accurate ; nor whether 
another published statement can be trusted, namely, that the 
Ligurian bee, which is generally considered a mere variety of the 
common hive-bee, and which freely crosses with it, is able to 
reach and suck the nectar of the red clover. Thus, in a country 
where this kind of clover abounded, it might be a great advantage 
to the hive-bee to have a slightly longer or differently constructed 
proboscis. On the other hand, as the fertility of this clover 
absolutely depends on bees visiting the flowers, if humble-bees 
were to become rare in any country, it might be a great advantage 
to the plant to have a shorter or more deeply divided corolla, so 
that the hive-bees should be enabled to suck 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 
to each other in the most perfect manner, by the continued pre- 
servation of all the individuals which presented slight deviations 
of structure mutually favourable to each other. 

I am well aware that this doctrine of natural selection, 
exemplied in the above imaginary instances, is open to the same 
objections which were first urged against Sir Charles Lyell's noble 
views on "the modern changes of the earth, as illustrative of 
geology ; " but we now seldom hear the agencies which we see still 
at work, spokf-n of as trifling or insignificant, when used in 
explaining the excavation of the deepest valleys or the formation 
of long lines of inland cliffs. Natural selection acts only by the 
preservation and accumulation of 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 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 (with the exception of the curious and 
not well understood cases of parthenogenesis) unite for each 
birth ; but in the case of hermaphrodites this is far from obvious. 
Nevertheless there is reason to believe that with all hermaphro- 
dites two individuals, either occasionally or habitually, concur for 
the reproduction of their kind. This view was long ago doubtfully 
suggested by Sprengel, Knight and Kolreuter. 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 dis- 
cussion. 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 all that concerns us. 
But still there are many hermaphrodite animals which certainly 
do not habitually pair, and a vast majority of plants are herma- 
phrodites. 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, and 
made so many experiments, 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 that no organic being 
fertilises itself for a perpetuity of generations ; but that a cross 
with another individual is occasionally perhaps at long intervals 
of time 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 ! If an occasional cross be 
indispensable, notwithstanding that the plant's own anthers and 
pistil stand so near each other as almost to insure self-fertilisation, 
the fullest freedom for the entrance of pollen from another 
individual will explain the above state of exposure of the organs. 
Many flowers, on the other hand, have their organs of fructification 
closely enclosed, as in the great papilionaceous or pea-family ; but 
these almost invariably present beautiful and curious adaptations 
in relation to the visits of insects. So necessary are the visits of 
bees to many papilionaceous flowers, that their fertility is greatly 
diminished if these visits be prevented. Now, it is scarcely possible 
for insects to fly from flower to flower, and not to carry pollen 
from one to the other, to the great good of the plant. Insects act 
like a camel-hair pencil, and it is sufficient, to ensure fertilisation, 
just to touch with the same brush the anthers of one flower and 
then the stigma of another ; but it must not be supposed that bees 
would thus produce a multitude of hybrids between distinct 


species ; for if a plant's own pollen and that from another species 
are placed on the same stigma, the former is so prepotent that it 
invariably and completely destroys, as has been shown by Gartner, 
the influence of the foreign pollen. 

When the stamens of a flower suddenly spring towards the 
pistil, or slowly move one after the other towards it, the con- 
trivance 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 Kolreuter has 
shown to be the case with the barberry ; and in this very genus, 
which seems to have a special contrivance for self-fertilisation, it 
is well known that, if 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 numerous other cases, far from 
self-fertilisation being favoured, there are special contrivances 
which effectually prevent the stigma receiving pollen from its own 
flower, as I could show from the works of Sprengel and others, as 
well as from my own observations : for instance, in Lobelia 
fulgens, there is a really beautiful and elaborate contrivance by 
which all 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 raise plenty of seedlings. Another species of Lobelia, 
which is visited by bees, seeds freely in my garden. In very many 
other cases, though there is no special mechanical contrivance to 
prevent the stigma receiving pollen from the same flower, yet, as 
Sprengel, and more recently Hildebrand, and others, have 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 so-named dichogamous plants 
have in fact separated sexes, and must habitually be crossed. So 
it is with the reciprocally dimorphic and trimorphic plants pre- 
viously alluded to. 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 be in so many cases 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 majori 
of the seedlings thus raised turn out, as I have found, mongre' 
for instance, I raised 233 seedling cabbages from some plants of 
different varieties growing near each other, and of these only 76 




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 ; and the pollen of each flower readily 
gets on its own stigma without insect agency ; for I have found 
that plants carefully protected from insects produce the full 
number of pods. How, then, comes it that such a vast number of 
the seedlings are mongrelized ? It must arise from the pollen 
of a distinct variety having a prepotent effect over the 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 reversed, 
for a plant's own pollen is almost always prepotent over foreign 
pollen ; but to this subject we shall return in a future chapter. 

In the case of a large 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 flowers on the same tree can be considered as distinct indi- 
viduals 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, 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, I 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 informs me that 
the rule does not hold good in Australia : but if most of the 
Australian trees are dichogamous, the same result would follow 
as if they bore flowers with separated sexes. I have made these 
few remarks on treos simply to call attention to the subject. 

Turning for a brief space to animals : various terrestrial species 
are hermaphrodites, such as the land-mollusca and earth-worms ; 
but these all pair. As yet I have not found a single terrestrial 
animal which can fertilise itself. This remarkable fact, which 
offers so strong a contrast with terrestrial plants, is intelligible 
on the view of an occasional cross being indispensable ; for owing 
to the nature of the fertilising element there are no means, 
analogous to the action of insects and of the wind with 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 
the currents of water offer an obvious means for an occasional 


cross. As in the case of flowers, I have as yet failed, after con- 
sultation with one of the highest authorities, namely, Professor 
Huxley, to discover a single hermaphrodite animal with the 
organs of reproduction so perfectly enclosed 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, under this point of view, a case of great 
difficulty; but I have been enabled, by a fortunate chance, to 
prove that two individuals, though both are self -fertilising herma- 
phrodites, do sometimes cross. 

It must have struck most naturalists as a strange anomaly that, 
both with animals and plants, some species of the same family 
and even of the same genus, though agreeing closely with each 
other in their whole organisation, are hermaphrodites, and some 
unisexual. But if, in fact, all hermaphrodites do occasionally 
intercross, the difference between them and unisexual species is, 
as far as function is concerned, very small. 

From these several considerations and from the many special 
facts which I have collected, but which I am unable here to give, 
it appears that with animals and plants an occasional intercross 
between distinct individuals is a very general, if not universal, 
law of nature. 

Circumstances favourable for the production of new forms throiiyh 
Natural Selection. 

This is an extremely intricate subject. A great amount of 
variability, under which term individual differences are always 
included, will evidently be favourable. A large number of 
individuals, by giving a better chance within any given period for 
the appearance of profitable variations, will compensate for a 
lesser amount of variability in each individual, and is, I believe, 
a highly important element of success. Though Nature grants 
long periods of time for the work of natural selection, she does 
not grant an indefinite period ; for as all organic beings are 
striving to seize on each place in the economy of nature, if any 
one species does not become modified and improved in a cor- 
responding degree with its competitors, it will be exterminated. 
Unless favourable variations be inherited by some at least of the 
offspring, nothing can be effected by natural selection. The 
tendency to reversion may often check or prevent the work ; but 
as this tendency has not prevented man from forming by selection 
numerous domestic races, why should it prevail against natural 
selection ? 

In the case of methodical selection, a breeder selects for some 
definite object, and if the individuals be allowed freely to inter- 
cross, his work will completely fail. But when many men, without 


intending to alter the breed, have a nearly common standard of 
perfection, and all try to procure and breed from the best animals, 
improvement surely but slowly follows from this unconscious 
process of selection, notwithstanding that there is no separation 
of selected individuals. Thus it will be under nat'\re ; for within 
a confined area, with some place in the natural polity not perfectly 
occupied, all the individuals varying in the right direction, though 
in different degrees, will tend to be preserved. But if the area be 
large, its several districts will almost certainly present different 
conditions of life ; and then, if the same species undergoes modifi- 
cation in different districts, the newly -formed varieties will inter- 
cross on the confines of each. But we shall see in the sixth 
chapter that intermediate varieties, inhabiting intermediate 
districts, will in the long run generally be supplanted by one of 
the adjoining varieties. Intercrossing will chiefly affect those 
animals which unite for each birth and wander much, and which 
do not breed at a very quick rate. Hence with animals of this 
nature, for instance, birds, varieties will generally be confined to 
separated countries ; and this I find to be the case. With herma- 
phrodite organisms which cross only occasionally, and likewise 
with animals which unite for each birth, but which wander little 
and can increase at a rapid rate, a new and improved variety 
might be quickly formed on any one spot, and might there main- 
tain itself in a body and afterwards spread, so that the individuals 
of the new variety would chiefly cross together. On this principle, 
nurserymen always prefer saving seed from a large body of plants, 
as the chance of intercrossing is thus lessened. 

Even with animals which unite for each birth, and which do 
not propagate rapidly, we must not assume that free intercrossing 
would always eliminate the effects of natural selection ; for I can 
bring forward a considerable body of facts showing that within the 
same area, two varieties of the same animal may long remain dis- 
tinct, from haunting different stations, from breeding at slightly 
different seasons, or from the individuals of each variety preferring 
to pair together. 

Intercrossing plays a very important part in nature by 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, as 
already stated, we have reason to believe that occasional inter- 
crosses take place with all animals and plants. Even if these take 
place only at long intervals of time, 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 crosses, even at rare intervals, will be great. With 



respect to organic beings extremely low in the scale, which do not 
propagate sexually, nor conjugate, and which cannot possibly 
intercross, uniformity of character can be retained by them under 
the same conditions of life, only through the principle of inherit- 
ance, and through natural selection which will destroy any 
individuals departing from the proper type. If the conditions of 
life change and the form undergoes modification, uniformity of 
character can be given to the modified offspring, solely by natural 
selection preserving similar favourable variations. 

Isolation, also, is an important element in the modification of 
species through natural selection. In a confined or isolated area, 
if not very large, the organic and inorganic conditions of life will 
generally be almost uniform ; so that natural selection will tend 
to modify all the varying individuals of the same species in the 
same manner. Intercrossing with the inhabitants of the surround- 
ing districts will, also, be thus prevented. Moritz Wagner has 
lately published an interesting essay on this subject, and has 
shown that the service rendered by isolation in preventing crosses 
between newly-formed varieties is probably greater even than I 
supposed. But from reasons already assigned I can by no means 
agree with this naturalist, that migration and isolation are neces- 
sary elements for the formation of nev/ species. The importance 
of isolation is likewise great in preventing, after any physical 
change in the conditions, such as of climate, elevation of the land, 
&c., the immigration of better adapted organisms ; and thus new 
places in the natural economy of the district will be left open to 
be filled up by the modification of the old inhabitants. Lastly, 
isolation will give time for a new variety to be improved at a slow 
rate ; and this may sometimes be of much importance. If, how- 
ever, an isolated area be very small, either from being surrounded 
by barriers, or from having very peculiar physical conditions, the 
total number of the inhabitants will be small ; and this will retard 
the production of new species through natural selection, by 
decreasing the chances of favourable variations arising. 

The mere lapse of time by itself does nothing, either for or 
against natural selection. I state this because it has been errone- 
ously asserted that the element of time has been assumed by me 
to play an all -important part in modifying species, as if all the 
forms of life were necessarily undergoing change through some 
innate law. Lapse of time is only so far important, and its 
importance in this respect is great, that it gives a better chance 
of beneficial variations arising and of their being selected, accumu- 
lated, and fixed. It likewise tends to increase the direct action 
of the physical conditions of life, in relation to the constitution of 
each organism. 

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 
number of species inhabiting it is 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 in the world. Hence an oceanic island at first 
sight seems to have been highly favourable for the production of 
new species. But we may thus 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 isolation is of great importance in the production of 
new species, on the whole I am inclined to believe that largeness 
of area is still more important, especially for the production of 
species which shall 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 much more 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 extermi- 
nated. 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 other forms. 
Moreover, great areas, though now continuous, will often, owing 
to former oscillations of level, have 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 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, and will give rise to the 
greatest number of new varieties and species. They will thus 
play a more important part in the changing history of the organic 

In accordance with this view, we can, perhaps, understand 
some facts which will be again alluded to in our chapter on 
Geographical Distribution ; for instance, the fact of the produc- 
tions of the smaller continent of Australia now yielding befora 
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, we can understand how it is that 
the flora of Madeira, according to Oswald Heer, resembles to a 
certain extent 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. Consequently, the competition 
between fresh-water productions will have been less severe than 
elsewhere ; new forms will have been then more slowly produced, 
and old forms more slowly exterminated. And it is in fresh- 
water basins 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 at present widely sundered in the natural 
scale. These anomalous forms may be called living fossils ; they 
have endured to the present day, from having inhabited a confined 
area, and from having been exposed to less varied, and therefore 
less severe, competition. 

To sum up, as far as the extreme intricacy of the subject 
permits, the circumstances favourable and unfavourable for the 
production of new species through natural selection. I conclude 
that for terrestrial productions a large continental area, which 
has undergone many oscillations of level, will have been the most 
favourable for the production of many new forms of life, fitted to 
endure for a long time and to spread widely. Whilst the area 
existed as a continent, the inhabitants will have been numerous 
in individuals and kinds, and will have been subjected to severe 
competition. When converted by subsidence into large separate 
islands, there will still have existed many individuals of the same 
species on each island : intercrossing on the confines of the range 
of each new species will have been checked: after physical 
changes of any kind, immigration will have been prevented, so 
that new places in the polity of each island will have had to be 
filled up by the modification of the old inhabitants ; and time will 
have been allowed for the varieties in each to become well 
modified and perfected. When, by renewed elevation, the islands 
were reconverted into a continental area, there will again have 
been very severe competition: the most favoured or improved 
varieties will have been enabled to spread : there will have been 
much extinction of the less improved forms, and the relative 
porportional numbers of the various inhabitants of the reunited 
continent will again have been changed ; and again there will have 
been a fair field for natural selection to improve still further the 
inhabitants, and thus to produce new species. 

That natural selection generally acts with extreme slowness 
I fully admit. It can act only when there are places in the 
natural polity of a district which can be better occupied by th? 


modification of some of its existing inhabitants. The occurrence 
of such places will often depend on physical changes, which 
generally take place very slowly, and on the immigration of better 
adapted forms being prevented. As some few of the old inhabi- 
tants become modified, the mutual relations of others will often 
be disturbed ; and this will create new places, ready to be filled 
up by better adapted forms; but all this will take place very 
slowly. Although all the individuals of the same species differ in 
some slight degree from each other, it would often be long before 
differences of the right nature in various parts of the organisation 
might occur. The result would often be greatly retarded by free 
intercrossing. Many will exclaim that these several causes are 
amply sufficient to neutralise the power of natural selection. I 
do not believe so. But I do believe that natural selection will 
generally act very slowly, only at long intervals of time, and only 
on a few of the inhabitants of the same region. I further believe 
that these slow, intermittent results accord well with what 
geology tells us of the rate and manner at which the inhabitants 
of the world have changed. 

Slow though the process of selection may be, if feeble man can 
do much by artificial selection, I can see no limit to the amount 
of change, to the beauty and complexity of the coadaptations 
between all organic beings, one with another and with their 
physical conditions of life, which may have been affected in the 
long course of time through nature's power of selection, that is by 
the survival of the fittest. 

Extinction caused by Natural Selection. 

This subject will be more fully discussed in our chapter on 
Geology; but it must here be 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. Owing to the high geometrical rate 
of increase of all organic beings, each area is already fully stocked 
with inhabitants ; and it follows from this, that as the favoured 
forms increase in number, so, generally, will the less favoured 
decrease and become rare. Barity, as geology tells us, is the 
precursor to extinction. We can see that any form which is 
represented by few individuals will run a good chance of utter 
extinction, during great fluctuations in the nature of the seasons, 
or from a temporary increase in the number of its enemies. But 
we may go further than this; for, as new forms are produced, 
unless we admit that specific forms can go on indefinitely increas- 
ing in number, many old forms must become extinct. That the 
number of specific forms has not indefinitely increased, geology 
plainly tells us ; and we shall uresently attemot to show why it 


is that the number of species throughout the world has not become 
immeasurably great. 

We have seen that the species which are most numerous in 
individuals have the best chance of producing favourable varia- 
tions within any given period. We have evidence of this, in the 
facts stated in the second chapter, showing that it is the common 
and diffused or dominant species which offer the greatest number 
of recorded varieties. Hence, rare species will be less quickly 
modified or improved within any given period ; they will conse- 
quently be beaten in the race for life by the modified and im- 
proved 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 pro- 
gress 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, 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 far less from each other than do good and distinct species. 
Nevertheless, according to my view, varieties are species in the 
process of formation, or are, as I have called them, incipient species. 
How, then, does the lesser difference between varieties become aug- 
mented 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 a degree of difference as that between the 
species of the same genus. 

As has always been my practice, I have sought light on this 
head from our domestic productions. We shall here find some- 
thing analogous. It will be admitted that the production of races 
so different as short-horn and Hereford cattle, race and cart 
horses, the several breeds of pigeons, &c., could never have been 
effected by the mere chance accumulation of similar variations 
during many successive generations. In practice, a fancier is, for 
instance, 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 the sub-breeds of the tumbler- 
pigeon) 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 of history, the men of one nation or dis- 
trict required swifter horses, whilst those of another required 
stronger and bulkier horses. The early differences would be very 
slight ; but, in the course of time, from the continued selection of 
swifter horses in the one case, and of stronger ones in the other, 
the differences would become greater, and would be noted as 
forming two sub-breeds. Ultimately, after the lapse of centuries, 
these sub-breeds would become converted into two well-established 
and distinct breeds. As the differences became greater, the in- 
ferior animals with intermediate characters, being neither very 
swift nor very strong, would not have been used for breeding, and 
will thus have tended to disappear. Here, then, we see in man's 
productions the action of what may be called the principle of 
divergence, causing differences, at first barely appreciable, steadily 
to increase, and the breeds to diverge in character, both from each 
other and from their common parent. 

But how, it may be asked, can any analogous principle apply in 
nature 1 I believe it can and does apply most efficiently (though 
it was a long time before I saw how), from the simple circumstance 
that the more diversified the descendants from any one species 
become in structure, constitution, and habits, by so much will 
they be better enabled to seize on many and widely diversified 
places in the polity of nature, and so be enabled to increase in 


We can clearly discern this in the case of animals with simple 
habits. Take the case of a carnivorous quadruped, of which the 
number that can be supported in any country has long ago 
arrived at its full average. If its natural power of increase be 
allowed to act, it can succeed in increasing (the country not under- 
going any change in conditions) only by its varying descendants 
seizing on places at present occupied by other animals : some of 
them, for instance, being enabled to feed on new kinds of prey, 
either dead or alive; some inhabiting new stations, climbing 
trees, frequenting water, and some perhaps becoming less carni- 
vorous. The more diversified in habits and structure the descend- 
ants of our carnivorous animals become, the more places they 
will be enabled to occupy. What applies to one animal will apply 
throughout all time to all animals that is, if they vary for 
otherwise natural selection can effect nothing. So it will be with 
plants. It has been experimentally proved, that if a plot of 
ground be sown with one species of grass, and a similar plot be 
sown with several distinct genera of grasses, a greater number of 
plants and a greater weight of dry herbage can be raised in the 
latter than in the former case. The same has been found to hold 
good when one variety and 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 the varieties were continually 
selected which differed from each other in the same manner, 
though in a very slight degree, as do the distinct species and 
genera of grasses, a greater number of individual plants of this 
species, including its modified descendants, would succeed in 
living on the same piece of ground. And we know that each 
species and each variety of grass is annually sowing almost count- 
less seeds ; and is thus striving, as it may be said, to the utmost 
to increase in number. Consequently, in the course of many 
thousand generations, the most distinct varieties of any one 
species of grass would 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, especi- 
ally if freely open to immigration, and where the contest between 
individual and individual must be very severe, we always find 
great diversity in its inhabitants. For instance, I 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 : also 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 its nature not to be in any way peculiar), 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, 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 would succeed 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 might 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 well 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 naturalised genera, no less 
than 100 genera are not there indigenous, and thus a large pro- 
portional addition is made to the genera now living in the United 

By considering the nature of the plants or animals which have 
in any country struggled successfully with the indigenes, and 
have there become naturalised, we may gain some crude idea in 
what manner some of the natives would have to be modified, in 
order to gain an advantage over their compatriots ; and we may 
at least infer that diversification of structure, amounting to new 
generic differences, would be profitable to them. 

The advantage of diversification of structure 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 adapted to digest vegetable matter alone, 


or flesh aitme, 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 per- 
fectly 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-developed orders. In the Australian mammals, we see 
the process of diversification in an early and incomplete stage of 

The Probable Effects of the Action of Natural Selection through 
Divergence of Character and Extinction, on the Descendants of 
a Common Ancestor. 

After the foregoing discussion, which has been much com- 
pressed, we may assume that the modified descendants of any 
one species will succeed 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 benefit being derived from divergence of character, 
combined with the principles of natural selection and of extinc- 
tion, tends to act. 

The accompanying diagr^* 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. I have said a large genus, because 
as we saw in the second chapter, on an average more species vary 
in large genera than in 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 do the rare and restricted species. 
Let (A) be a common, widely-diffused, and varying species, belong- 
ing to a genus large in its own country. The branching and 
diverging dotted lines of unequal lengths proceeding from (A) 
may represent its varying offspring. 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 

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importance of the principle of benefit 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 form it into 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 or more generations. After a thousand 
generations, species (A) is supposed to have produced two fairly 
well-marked varieties, namely a 1 and m l . These two varieties will 
generally still be exposed to the same conditions which made their 
parents variable, and the tendency to variability is in itself here- 
ditary ; consequently they will likewise tend to vary, and commonly 
in nearly the same manner as did their parents. Moreover, these 
two varieties, being only slightly modified forms, will tend to 
inherit those advantages which made their parent (A) more nume- 
rous than most of the other inhabitants of the same country ; they 
will also partake of those more general advantages which made 
the genus to which the parent-species belonged, a large genus in 
its own country. And all these circumstances are 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 thou- 
sand generations. And after this interval, variety a 1 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 
a 1 . Variety m l is supposed to have produced two varieties, namely 
?/i 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 of 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 I must here remark that I do not suppose that the process 
ever goes on so regularly as is represented in the diagram, though 
in itself made somewhat irregular, nor that it goes on continuously ; 
it is far more probable that each form remains for long periods 
unaltered, and then again undergoes modification. Nor do I 


suppose that the most divergent varieties are invariably preserved : 
a medium form may often long endure, and may or may not pro- 
duce 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 
10 seize on, and the more their modified progeny will increase. 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 any- 
where, after intervals long enough to allow 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 diverg- 
ing 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 90 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 no doubt the process of modifica- 
tion will be confined to a single line of descent, and the number of 
modified descendants will not be increased ; although the amount 
of divergent modification may have been augmented. This case 
would be represented in the diagram, if all the lines proceeding 
from (A) were removed, excepting that from a 1 to a 10 . In the 
same way the English race-horse and English pointer have appa- 
rently 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 10 ,/ 10 , and m 10 , 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 ; 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 doubtful or at least 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 sim- 
plified manner), we get eight species, marked by the letters between 
a 14 and i 14 , 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 10 and 2 10 ) or two species, 
according to the amount of change supposed to be represented 
between the horizontal lines. After fourteen thousand genera- 
tions, six new species, marked by the letters n w to 2 14 , are supposed 
to have been produced. In any genus, the species which are 
already very different in character from each other, will generally 
tend to produce the greatest number of modified descendants ; for 
these will have the best chance of seizing on new and widely dif- 
ferent 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 long but unequal periods 
continue to transmit unaltered descendants ; and this is shown in 
the diagram by the dotted lines unequally prolonged upwards. 

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 pre- 
decessors and their original progenitor. For it should be remem- 
bered 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 states of the same 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. If, however, the 
modified offspring of a species get into some distinct country, or 
become quickly adapted to some quite new station, in which off- 
spring and progenitor 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, being replaced by eight new species 
(a u to t") ; and species (I) will be replaced by six (n u to 2 14 ) new 

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, 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 (E and F) 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 14 and 2 14 will be much 
greater than that between the most distinct 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 14 , q u , /> 14 , will be nearly related from having 
recently branched off from a 10 ; b l \ and / u , from having diverged 
at an earlier period from a 5 , will be in some degree distinct from 
the three first-named species ; and lastly, o 14 , c 14 , and m l \ 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 a distinct genus. 

The six descendants from (I) will form two sub-genera or genera. 
But as the original species (I) differed largely from (A), standing 
nearly at the extreme end of the original genus, the six descendants 
from (I) will, owing to inheritance alone, 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 corisidefa- 
tion), which connected the original species (A) and (I), have 
all become, excepting (F), extinct, and have left no descendant.-;. 
Hence the six new species descended from (I), and the eight 
descendants 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 
be 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 represents a species, the supposed progenitor 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 14 , 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. Being descended from a form which stood between the 
parent-species (A) and (I), now supposed to be extinct and 
unknown, it will be in some degree intermediate in character 
between the two groups descended from these two species. But 
as these two groups have gone on diverging in character from the 
type of their parents, the new species (F U ) will not V>e directly 
intermediate between them, but rather between types of the two 
groups ; and every naturalist will be able to call such cases 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 more generations ; it may also represent 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 I think we shall then see that the 
diagram throws light on the affinities of extinct beings, which, 
though generally belonging to the same orders, 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 various remote 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 the diagram, 
we suppose the amount of change represented by each successive 
group of diverging dotted lines to be great, the forms marked a 14 
to /> u , those marked 6 14 and /", and those marked o 14 to m l \ will 
form three very distinct genera. We shall also have two very 


distinct genera descended from (I), differing widely from the 
descendants of (A). These two groups of genera will thus form 
two distinct families, or orders, according to the amount of 
divergent modification supposed to be represented in the diagram. 
And the two new families, or orders, are descended from two 
species of the original genus, and these are supposed to be 
descended from some still more ancient and unknown form. 

We have seen that in each country it is the species belonging to 
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 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 have as yet suffered 
least extinction, will, for a long period, continue to increase. But 
which groups will ultimately prevail, no man can predict ; for we 
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 mil 
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 as, according to this view, extremely few of the 
wore ancient species have transmitted descendants to the present 
day, and, as all the descendants of the same species form a class, 
we can understand how it is that there exists so few classes 
in each main division of the animal and vegetable kingdoms. 
Although few of the most ancient species have left modified 
descendants, yet, at remote geological periods, the earth may have 
been almost as well peopled with species of many genera, families^ 
orders, and classes, as at the present time. 


On the Degree to which Organisation tends to advance. 

Natural Selection acts exclusively by the preservation and 
accumulation of variations, which are beneficial under the organic 
and inorganic conditions to which each creature is exposed at all 
periods of life. The ultimate result is that each creature tends to 
become more and more improved in relation to its conditions. This 
improvement inevitably leads to the gradual advancement of the 
organisation of the greater number of living beings throughout the 
world. But here we enter on a very intricate subject, for natu- 
ralists have not defined to each other's satisfaction what is meant 
by an advance in organisation. Amongst the vertebrata the degree 
of intellect and an approach in structure to man clearly come into 
play. It might be thought that the amount of change which the 
various parts and organs pass through in their development from 
the embryo to maturity would suffice as a standard of comparison ; 
but there are cases, as with certain parasitic crustaceans, in which 
several parts of the structure become less perfect, so that the 
mature animal cannot be called higher than its larva. Von Baer's 
standard seems the most widely applicable and the best, namely, 
the amount of differentiation of the parts of the same organic 
being, in the adult state as I should be inclined to add, and their 
specialisation for different functions ; or, as Milne Edwards would 
express it, the completeness of the division of physiological labour. 
But we shall see how obscure this subject is if we look, for instance, 
to fishes, amongst which some naturalists rank those as highest 
which, like the sharks, approach nearest to amphibians; whilst 
other naturalists rank the common bony or teleostean fishes as the 
highest, inasmuch as they are most strictly fish-like, and differ 
most from the other vertebrate classes. We see still more plainly 
the obscurity of the subject by turning to plants, amongst which 
the standard of intellect is of course quite excluded; and here 
some botanists rank those plants as highest which have every 
organ, as sepals, petals, stamens, and pistils, fully developed in 
each flower ; whereas other botanists, probably with more truth, 
look at the plants which have their several organs much modified 
and reduced in number as the highest. 

If we take as the standard of high organisation, the amount of 
differentiation and specialisation of the several organs in each being 
when adult (and this will include the advancement of the brain 
for intellectual purposes), natural selection clearly leads towards 
this standard : for all physiologists admit that the specialisation of 
organs, inasmuch as in this state they perform their functions 
better, is an advantage to each being ; and hence the accumulation 
of variations tending towards specialisation is within the scope of 
natural selection. On the other hand, we can see, bearing in mind 


that all organic beings are striving to increase at a high ratio and 
to seize on every unoccupied or less well occupied place in the 
economy of nature, that it is quite possible for natural selection 
gradually to fit a being to a situation in which several organs 
would be superfluous or useless : in such cases there would be 
retrogression in the scale of organisation. Whether organisation 
on the whole has actually advanced from the remotest geological 
periods to the present day will be more conveniently discussed in 
our chapter on Geological Succession. 

But it may be objected that if all organic beings thus tend to 
rise in the scale, how is it that throughout the world a multitude 
of the lowest forms still exist ; and how is it that in each great 
class some forms are far more highly developed than others ? Why 
have not the more highly developed forms everywhere supplanted 
and exterminated the lower ? Lamarck, who believed in an innate 
and inevitable tendency towards perfection in all organic beings, 
seems to have felt this difficulty so strongly, that he was led to 
suppose that new and simple forms are continually being produced 
by spontaneous generation. Science has not as yet proved the 
truth of this belief, whatever the future may reveal. On our theory 
the continued existence of lowly organisms offers no difficulty; for 
natural selection, or the survival of the fittest, does not necessarily 
include progressive development it only takes advantage of such 
variations as arise and are beneficial to each creature under its 
complex relations of life. And it may be asked what advantage, 
as far as we can see, would it be to an infusorian animalcule to 
an intestinal worm or even to an earth-worm, to be highly or- 
ganised. If it were no advantage, these forms would be left, by 
natural selection, unimproved or but little improved, and might 
remain for indefinite ages in their present lowly condition. And 
geology tells us that some of the lowest forms, as the infusoria and 
rhizopods, have remained for an enormous period in nearly their 
present state. But to suppose that most of the many now existing 
low forms have not in the least advanced since the first dawn of 
life would be extremely rash; for every naturalist who has dis- 
sected some of the beings now ranked as very low in the scale, 
must have been struck with their really wondrous and beautiful 

Nearly the same remarks are applicable if we look to the different 
grades of organisation within the same great group ; for instance, 
in the vertebrata, to the co-existence of mammals and fish amongst 
mammalia, to the co-existence of man and the ornithorhynchus 
amongst fishes, to the co-existence of the shark and the lancelet 
(Amphioxus), which latter fish in the extreme simplicity of its struc- 
ture approaches the invertebrate classes. But mammals and fish 
hardly come into competition with each other ; the advancement of 


the whole class of mammals, or of certain members in this class, to 
the highest grade would not lead to their taking the place of fishes. 
Physiologists believe that the brain must be bathed by warm blood 
to be highly active, and this requires aerial respiration ; so that 
warm-blooded mammals when inhabiting the water lie under a 
disadvantage in having to come continually to the surface to 
breathe. With fishes, members of the shark family would not 
tend to supplant the lancelet; for the lancelet, as I hear from 
Fritz Miiller, has as sole companion and competitor on the barren 
sandy shore of South Brazil, an anomalous annelid. The three 
lowest orders of mammals, namely, marsupials, edentata, and 
rodents, co-exist in South America in the same region with nume- 
rous monkeys, and probably interfere little with each other. 
Although organisation, on the whole, may have advanced and be 
still advancing throughout the world, yet the scale will always 
present many degrees of perfection ; for the high advancement of 
certain whole classes, or of certain members of each class, does not 
at all necessarily lead to the extinction of those groups with which 
they do not enter into close competition. In some cases, as we 
shall hereafter see, lowly organised forms appear to have been 
preserved to the present day, from inhabiting confined or peculiar 
stations, where they have been subjected to less severe competi- 
tion, and where their scanty numbers have retarded the chance of 
favourable variations arising. 

Finally, I believe that many lowly organised forms now exist 
throughout the world, from various causes. In some cases 
variations or individual differences of a favourable nature may 
never have arisen for natural selection to act on and accumulate. 
In no case, probably, has time sufficed for the utmost possible 
amount of development. In some few cases there has been what 
we must call retrogression of organisation. But the main cause 
lies in the fact that under very simple conditions of life a high 
organisation would be of no service, possibly would be of actual 
disservice, as being of a more delicate nature, and more liable to 
be put out of order and injured. 

Looking to the first dawn of life, when all organic beings, as 
we may believe, presented the simplest structure, how, it has been 
asked, could the first steps in the advancement or differentiation 
of parts have arisen ? Mr. Herbert Spencer would probably 
answer that, as soon as simple unicellular organism came by 
growth or division to be compounded of several cells, or became 
attached to any supporting surface, his law " that homologous 
units of any order become differentiated in proportion as their 
relations to incident forces become different " would come into 
action. But as we have no facts to guide us, speculation on the 
subject is almost useless. It is, however, an error to suppose that 


there would be no struggle for existence, and, consequently, no 
natural selection, until many forms had been produced : variations 
in a single species inhabiting an isolated station might be 
beneficial, and thus the whole mass of individuals might be 
modified, or two distinct forms might arise. But, as I remarked 
towards the close of the Introduction, no one ought to feel 
surprise at much remaining as yet unexplained on the origin of 
species, if we make due allowance for our profound ignorance 
on the mutual relations of the inhabitants of the world at the 
present time, and still more so during past ages. 

Convergence of CJiaracter. 

Mr. H. C. Watson thinks that I have overrated the importance 
of divergence of character (in which, however, he apparently 
believes), and that convergence, as it may be called, has likewise 
played a part. If two species, belonging to two distinct though 
allied genera, had both produced a large number of new and 
divergent forms, it is conceivable that these might approach each 
other so closely that they would have all to be classed under the 
same genus; and thus the descendants of two distinct genera 
would converge into one. But it would in most cases be extremely 
rash to attribute to convergence a close and general similarity of 
structure in the modified descendants of widely distinct forms. 
The shape of a crystal is determined solely by the molecular 
forces, and it is not surprising that dissimilar substances should 
sometimes assume the same form; but with organic beings we 
should bear in mind that the form of each depends on an infinitude 
of complex relation;?, namely on the variations which have arisen, 
these being due to causes far too intricate to be followed out, 
on the nature of the variations which have been preserved or 
selected, and this depends on the surrounding physical conditions, 
and in a still higher degree on the surrounding organisms with 
which each being has come into competition, and lastly, on 
inheritance (in itself a fluctuating element) from innumerable 
progenitors, all of which have had their forms determined through 
equally complex relations. It is incredible that the descendants 
of two organisms, which had originally differed in a marked 
manner, should ever afterwards converge so closely as to lead to 
a near approach to identity throughout their whole organisation. 
If this had occurred, we should meet with the same form, inde- 
pendently of genetic connection, recurring in widely separated 
geological formations ; and the balance of evidence is opposed to 
any such an admission. 

Mr. Watson has also objected that the continued action of 
natural selection, together with divergence of character, would 
tend to make an indefinite number of specific forms. As far as 


mere inorganic conditions are concerned, it seems probable that 
a sufficient number of species would soon become adapted to all 
considerable diversities of heat, moisture, <fec. ; but I fully admit 
that the mutual relations of organic beings are more important ; 
and as the number of species in any country goes on increasing, 
the organic conditions of life must become more and more 
complex. Consequently there seems at first sight no limit to the 
amount of profitable diversification of structure, and therefore no 
limit to the number of species which might be produced. We do 
not know that even the most prolific area is fully stocked with 
specific forms : at the Cape of Good Hope and in Australia, which 
support such an astonishing number of species, many European 
plants have become naturalised. But geology shows us, that 
from an early part of the tertiary period the number of species of 
shells, and that from the middle part of this same period the 
number of mammals, has not greatly or at all increased. What 
then checks an indefinite increase in the number of species ? The 
amount of life (I do not mean the number of specific forms) 
supported on an area must have a limit, depending so largely as 
it does on physical conditions ; therefore, if an area be inhabited 
by very many species, each or nearly each species will be repre- 
sented by few individuals; and such species will be liable to 
extermination from accidental fluctuations in the nature of the 
seasons or in the number of their enemies. The process of 
extermination in such cases would be rapid, whereas the production 
of new species must always- be slow. Imagine the extreme case 
of as many species as individuals in England, and the first severe 
winter or very dry summer would exterminate thousands on 
thousands of species. Rare species, and each species will become 
rare if the number of species in any country becomes indefinitely 
increased, will, on the principle often explained, present within a 
given period few favourable variations ; consequently, the process 
ot giving birth to new specific forms would thus be retarded. 
When any species becomes very rare, close interbreeding will 
help to exterminate it ; authors have thought that this comes into 
play in accounting for the deterioration of the Aurochs in 
Lithuania, of Red Deer in Scotland, and of Bears in Norway, &c. 
Lastly, and this I am inclined to think is the most important 
element, a dominant species, which has already beaten many 
competitors in its own home, will tend to spread and supplant 
many others. Alph. de Candolle has shown that those species 
which spread widely, tend generally to spread very widely; 
consequently, they will tend to supplant and exterminate several 
species in several areas, and thus check the inordinate increase of 
specific forms throughout the world. Dr. Hooker has recently 
ehown that in the S.E. corner of Australia, where, apparently, 


there are many invaders from different quarters of the globe, the 
endemic Australian species have been greatly reduced in number. 
How much weight to attribute to these several considerations 
I will not pretend to say; but conjointly they must limit in each 
country the tendency to an indefinite augmentation of specific 

Summary of Chapter. 

If under changing conditions of life organic beings present indi 
vidual differences in almost every part of their structure, and this 
cannot be disputed ; if there be, owing to their geometrical rate 
of increase, a severe struggle for life at some age, season, or year, 
and this certainly cannot be disputed ; then, considering the in- 
finite complexity of the relations of all organic beings to each 
other and to their conditions of life, causing an infinite diversity 
in structure, constitution, and habits, to be advantageous to them, 
it would be a most extraordinary fact if no variations had ever 
occurred useful to each being's own welfare, in the same manner 
as so many variations have occurred useful to man. But if varia- 
tions useful to any organic being ever do occur, assuredly indivi- 
duals thus characterised will have the best chance of being 
preserved in the struggle for life ; and from the strong principle 
of inheritance, these will tend to produce offspring similarly 
characterised. This principle of preservation, or the survival of 
the fittest, I have called Natural Selection. It leads to the im- 
provement of each creature in relation to its organic and inorganic 
conditions of life ; and consequently, in most cases, to what must 
be regarded as an advance in organisation. Nevertheless, low and 
simple forms will long endure if well fitted for their simple con- 
ditions of life. 

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 
have given 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 or rivalry with other males ; and these 
characters will be transmitted to one sex or to both sexes, accord- 
ing to the form of inheritance which prevails. 

Whether natural selection has really thus acted in 'adapting the 
various forms of life to their several conditions and stations, must 
be judged by the general tenor and balance of evidence given in 
the following chapters. But we have already seen how it entails 
extinction; and how largely extinction has acted in the world's 
history, geology plainly declares. Natural selection, also, leada 
to divergence of character ; for the more organic beings diverge in 


structure, habits, and constitution, by so much the more can a 
large number be supported on the area, of which we see proof by 
looking to the inhabitants of any small spot, and to the produc- 
tions naturalised in foreign lands. Therefore, during the modifi- 
cation of the descendants of any one species, and during the 
incessant struggle of all species to increase in numbers, the more 
diversified the descendants become, the better will be their chance 
of success in the battle for life. Thus the small differences dis- 
tinguishing varieties of the same species, steadily tend to increase, 
till they 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 within each 
class, which vary most ; and these tend to transmit to their modi- 
fied offspring 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, the 
nature of the affinities, and the generally well-defined distinctions 
between the innumerable organic beings in each class throughout 
the world, may be explained. It is a truly wonderful fact the 
wonder of which we are apt to overlook from familiarity that all 
animals and all plants throughout all time and space should be 
related to each other in groups, subordinate to groups, in the 
manner which we everywhere behold namely, varieties of the 
same species most closely related, species of the same genus less 
closely and unequally related, 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 clustered round 
points, and these round other points, and so on in almost endless 
cycles. If species had been independently created, no explana- 
tion would have been possible of this kind of classification ; but 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 former years 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 at all 
times overmastered 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 young, budding 
twigs; and this connection 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 the other branches ; so with the species which lived during 
long-past geological periods, very few have left living and modi- 
iied descendants. From the first growth of the tree, many a limb 
and branch has decayed and dropped off; and these fallen 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 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 Ornitho- 
rhynchus 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 
branc ;, 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 changed conditions Use and disuse, combined with natural selection ; 
organs of flight and of vision Acclimatisation Correlated variation 
Compensation and economy of growth False correlations Multiple, 
rudimentary, and lowly organised structures variable Parts developed in 
an unusual manner are highly variable : specific characters more variable 
than generic : secondary sexual characters variable Species of the same 

fenus vary in an analogous manner Eeversions to long-lost characters 

I HAVE hitherto sometimes spoken as if the variations so common 
and multiform with organic beings under domestication, and in a 
lesser degree with those under nature were due to chance. This, 
of course, is a wholly incorrect expression, but it serves to acknow- 


ledge plainly our ignorance of the cause of each particular varia- 
tion. Some authors believe it to be as much the function of the 
reproductive system to produce individual differences, or slight 
deviations of structure, as to make the child like its parents. But 
the fact of variations and monstrosities occurring much more fre- 
quently under domestication than under nature, and the greater 
variability of species having wide ranges than of those with 
restricted ranges, lead to the conclusion that variability is generally 
related to the conditions of life to which each species has been 
exposed during several successive generations. In the first chapter ' 
I attempted to show that changed conditions act in two ways, 
directly on the whole organisation or on certain parts alone, and 
indirectly through the reproductive system. In all cases there are 
fATC factors, the nature of the organism, which is much the most 
important of the two, and the nature of the conditions. The direct 
action of changed conditions leads to definite or indefinite results. 
In the latter case the organisation seems to become plastic, and 
we have much fluctuating variability. In the former case the 
nature of the organism is such that it yields readily, when sub- 
jected to certain conditions, and all, or nearly all the individuals 
become modified in the same way. 

It is very difficult to decide how far changed conditions, such as 
f climate, food, &c., have acted in a definite manner. There is 
reason to believe that in the course of time the effects have been 
greater than can be proved by clear evidence. But we may safely 
conclude that the innumerable complex co-adaptations of structure, 
which we see throughout nature between various organic beings, 
cannot be attributed simply to such action. In the following 
cases the conditions seem to have produced some slight definite 
effect : E. Forbes asserts that shells at their southern limit, and 
when living in shallow water, are more brightly coloured than 
those of the same species from further north or from a greater 
depth ; but this certainly does not always hold good. Mr. Gould 
believes that birds of the same species are more brightly coloured 
under a clear atmosphere, than when living near the coast or on 
islands; and Wollaston is convinced that residence near the sea 
affects the colours of insects. 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. These slightly varying 
organisms are interesting in as far as they present characters analo- 
gous to those possessed by the species which are confined to similar 

"When a variation is of the slightest use to any being, we cannot \ 
tell how much to attribute to the accumulative action of natural 
selection, and how much to the definite action of the conditions of / 
life. Thus, it is well known to furriers that animals of the same 


species have thicker and better fur the f urther north they live ; 
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 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 similar varieties being produced 
from the same species under external conditions of life as different 
as can well be conceived ; and, on the other hand, of dissimilar 
varieties being produced under apparently the same external 
conditions. 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 nc 
these incline me to lay less weight on the direct action of the 
surrounding conditions, than on a tendency to vary, due to causes 
of which we are quite ignorant. 

In one sense the conditions of life may be said, not only to 
cause variability, either directly or indirectly, but likewise to 
include natural selection, for the conditions determine whether 
this or that variety shall survive. But when man is the selecting 
agent, we clearly see that the two elements of change are distinct ; 
variability is in some manner excited, but it is the will of man 
which accumulates the variations in certain directions ; and it is 
this latter agency which answers to the survival of the fittest 
under nature. 

Effects of the increased Use and Disu&e of Parts, as 
controlled by Natural Selection. 

i From the facts alluded to in the first chapter, I think there can 
I be no doubt that use in our domestic animals has strengthened 
' and enlarged certain parts, and disuse diminished them ; and that 
"such modifications are inherited. Under free nature, we 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 possess structures which can be best 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 : it is a remarkable fact that the young birds, according to 
Mr. Cunningham, can fly, while the adults have lost this power. 
As the larger ground-feeding birds seldom take flight except to 
escape danger, it is probable that the nearly wingless condition of 
several birds, now inhabiting or which lately inhabited several 
oceanic islands, tenanted by no beasts 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 it can defend 
itself by kicking its enemies, as efficiently as many quadrupeds. 
We may believe that the progenitor of the ostrich genus had habits 
like those of the bustard, and that, as the size and weight of its 
body were increased during successive generations, its legs were 
used more, and its wings less, until they became incapable of 

Kirby has remarked (and I have observed the same fact) that 
the anterior tarsi, or feet, of many male dung-feeding beetles are 
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. The evidence that acci- 
dental mutilations can be inherited is at present not decisive ; but 
the remarkable cases observed by Brown-Sequard in guinea-pigs, 
of the inherited effects of operations, should make us cautious in 
denying this tendency. Hence it will perhaps be safest to look at 
the entire absence of the anterior tarsi in Ateuchus, and their 
rudimentary condition in some other genera, not as cases of 
inherited mutilations, but as due to the effects of long-continued 
disuse ; for as many dung-feeding beetles are generally found with 
their tarsi lost, this must happen early in life ; therefore the tarsi 
cannot be of much importance or 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 (but more are now known) 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 have all 
their species in this condition ! Several facts, namely, that beetles 
in many parts of the world are 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 pro- 
portion of wingless beetles is larger on the exposed Desertas than 
in Madeira itself ; and especially the extraordinary fact, so strongly 
insisted on by Mr. Wollaston, that certain large groups of beetles, 
elsewhere excessively numerous, which absolutely require the use 
of their wings, are here almost entirely absent ; these several con- 
siderations make me believe that the wingless condition of so 
many Madeira beetles is mainly due to the action of natural selec- 
tion, combined probably with disuse. For during many successive 
generations each individual beetle which flew least, either from its 
wings having been ever so little less oerfectlv developed or front 


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 would oftenest have been blown 
to sea, and thus destroyed. 

The insects in Madeira which are not ground-feeders, and 
which, as certain 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 rudi- 
mentary in size, and in some cases are quite covered 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 in- 
flammation of the nictitating membrane. As frequent inflam- 
mation of the eyes must be injurious to any animal, and as eyes 
are certainly not necessary to animals having 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 aid the effects of disuse. 

It is well known that several animals, belonging to the most 
different classes, which inhabit the caves of Carniola 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, their loss may be 
attributed to disuse. In one of the blind animals, namely, the 
cave-rat (Neotoma), two of which were captured by Professor 
Silliman at above half a mile distance from the mouth of the 
cave, and therefore not in the profoundest depths, the eyes were 
lustrous and of large size ; and these animals, as I am informed 
by Professor Silliman, after having been exposed for about a 


month to a graduated light, acquired a dim perception of 

It is difficult to imagine conditions of life more similar than 
deep limestone caverns under a nearly similar climate ; so that, in 
accordance with the old view of the blind animals having been 
separately created for the American and European caverns, very 
close similarity in their organisation and affinities might have 
oeen expected. This is certainly not the case if we look at the 
two whole faunas ; and with respect to the insects alone, Schiodte 
has remarked, "We are accordingly prevented from considering 
the entire phenomenon in any other light than something purely 
local, and the similarity which is exhibited in a few forms between 
the Mammoth cave (in Kentucky) and the caves in Carniola, 
otherwise than as a very plain expression of that analogy which 
subsists generally between the fauna of Europe and of North 
America." On my view we must suppose that American animals, 
having in most cases 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, " We accordingly 
look upon the subterranean faunas as small ramifications which 
have penetrated into the earth from the geographically limited 
faunas of the adjacent tracts, and which, as they extended them- 
selves into darkness, have been accommodated to surrounding 
circumstances. 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, and whose formation is quite peculiar." These 
remarks of Schiodte's, it should be understood, apply not to the 
same, but to distinct species. 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 com- 
pensation 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 
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 relation- 
ship of most of their other productions. As a blind species of 
Bathyscia is found in abundance on shady rocks far from caves, 
the loss of vision in the cave-species of this one genus has probably 
had no relation to its dark habitation ; for it is natural that an 
insect already deprived of vision should readily become adapted 
to dark caverns. Another blind genus (Anophthalmus) offers this 
remarkable peculiarity, that the species, as Mr. Murray observes, 
have not as yet been found anywhere except in caves ; yet those 
which inhabit the several caves of Europe and America are 
distinct; but it is possible that the progenitors of these several 
species, whilst they were furnished with eyes, may formerly have 
ranged over both continents, and then have become extinct, 
excepting in their present secluded abodes. Far from feeling 
surprise that some of the cave-animals should be very anomalous, 
as Agassiz has remarked in regard to the blind fish, the Ambly- 
opsis, and as is the case with the blind Proteus with reference to 
the reptiles of Eui'ope, I am only surprised that more wrecks of 
ancient life have not been preserved, owing to the less severe 
competition to which the scanty inhabitants of these dark abodes 
will have been exposed. 


Habit is hereditary with plants, as in the period of flowering, 
in the time of sleep, in the amount of rain requisite for seeds to 
germinate, &c., and this leads me to say a few words on acclima- 
tisation. As it is extremely common for distinct species belonging 
to the same genus to inhabit hot and cold countries, if it be true 
that all the species of the same genus are descended from a single 
parent-form, acclimatisation must be readily effected during a 
long course of 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 inf ?r 
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 different countries which are 
here perfectly healthy. We have reason to believe that species 
in a state of nature are closely 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 n$? 
this adaptation is in most cases very close, we have evidence with 
some few plants, of their becoming, to a certain extent, natur- 
ally habituated to different temperatures; that is, they become 


acclimatised : thus the pines and rhododendrons, raised from seed 
collected by Dr. Hooker from the same species growing at different 
heights on the Himalaya, were found to possess in this country 
different constitutional powers of resisting cold. Mr. Thwaites 
informs me that he has observed similar facts in Ceylon ; analogous 
observations have been made by Mr. H. C. Watson on European 
species of plants brought from the Azores to England; and I 
could give other cases. In regard to animals, several authentic 
instances could be adduced of species having largely extended, 
within historical times, 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, though in all ordinary 
cases we assume such to be the case ; nor do we know that they 
have subsequently become specially acclimatised to their new 
homes, so as to be better fitted for them than they were at first. 

As we may infer that our domestic animals were originally 
chosen by uncivilised man because they were useful and because 
they bred readily under confinement, and not because they were 
subsequently found capable of far-extended transportation, 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 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; for 
they live under the cold climate of Faroe in the north and of the 
Falklands in the south, and on many an island in the torrid zones. 
Hence adaptation to any special climate may be looked at as a 
quality readily grafted on an innate wide flexibility of constitution, 
common to most animals. On this view, the capacity of enduring 
the most different climates by man himself and by his domestic 
animals, and the fact of the extinct elephant and rhinoceros having 
formerly endured a glacial climate, whereas the living species are 
now all tropical or sub-tropical in their habits, ought not to bo 
looked at as anomalies, but as examples of a very common flexi- 
bility of constitution, brought, under peculiar circumstances, into 

How much of the acclimatisation of species to any peculiar 
climate is due to mere habit, and how much to the natural selec- 
tion 01 varieties having different innate constitutions, and how 


much to both means combined, is an 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 ia 
the ancient Encyclopaedias of China, to be very cautious in trans- 
porting animals from one district to another. And as 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, natural selection Avould inevitably tend to preserve those 
individuals which were born with constitutions best adapted to 
any country which they inhabited. In treatises on many kinds of 
cultivated plants, certain varieties are said to withstand certain 
climates better than others ; this is strikingly shown in works on 
fruit-trees published in the United States, in which certain varie- 
ties 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 in 
England by seed, and of which consequently new varieties have 
not been produced, has even been advanced, as proving that 
acclimatisation cannot be effected, for it is now as tender as ever 
it was ! The case, also, of the kidney-bean has been often cited 
for a similar purpose, and with much greater weight ; but until 
someone 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 acci- 
dental crosses, and then again get seed from these seedlings, with 
the same precautions, the experiment cannot be said to have been 
tried. Nor let it be supposed that differences in the constitution 
of seedling kidney-beans never appear, for an account has been 
published how much more hardy some seedlings are than others ; 
and of this fact I have myself observed striking instances. 

On the whole, we may conclude that habit, or use and disuse, 
have, in some cases, played a considerable part in the modification 
of the constitution and structure ; but that the effects have often 
been largely combined with, and sometimes overmastered by, the 
natural selection of innate variations. 

Correlated Variation. 

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 im- 
portant subject, most imperfectly understood, and no doubt wholly 
different classes of facts may be here easily confounded together. 
Ws shall presently see that simple inheritance often gives the false 


appearance of correlation. One of the most obvious real cases is, 
that variations of structure arising in the young or Lrvae naturally 
tend to affect the structure of the mature animal. The several 
parts of the body which are homologous, and which, at an early 
embryonic period, are identical in structure, and which are neces- 
sarily exposed to similar conditions, seem eminently liable to vary 
in a like 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 by some anatomists 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 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 struc- 
tures, as in the union of the petals into a tube. Hard parts seem 
to affect the form of adjoining soft parts : it is believed by some 
authors that with birds the diversity in the shape of the pelvis 
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 form of the body and the 
manner of swallowing determine the position and form of several 
of the most important viscera. 

The nature of the bond is frequently quite obscure. M. Is. 
Geoffroy St. Hilaire has forcibly remarked, that certain malcon- 
formations frequently, and that others rarely, co-exist, without 
our being able to assign any reason. What can be more singular 
than the relation in cats between complete whiteness and blue 
eyes with deafness, or between the tortoise-shell colour and the 
female sex ; or in pigeons between their feathered feet and skin 
betwixt the outer toes, or between the presence of more or less 
down on the young pigeon when first hatched, with the future 
colour of its plumage ; or, again, the relation between the hair and 
teeth in the naked Turkish dog, though here no doubt homology 
comes into play? With respect to this latter case of correlation, 
I think it can hardly be accidental, that the two orders of 
mammals which are most abnormal in their dermal covering, viz., 
Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, 
<fec.), are likewise on the whole the most abnormal in their teeth ; 
but there are so many exceptions to this rule, as Mr. Mivart has 
remarked, that it has little value. 

I know of no case better adapted to show the importance of the 



laws of correlation and variation, independently of utility and 
therefore of natural selection, than that of the difference between 
the outer and inner flowers in some Compositous and Umbelli- 
ferous plants. Every one is familiar with the difference between 
the ray and central florets of, for instance, the daisy, and this 
difference is often accompanied with the partial or complete 
abortion of the reproductive organs. But in some of these plants, 
the seeds also differ in shape and sculpture. These differences 
have sometimes been attributed to the pressure of the involucra 
on the florets, or to their mutual pressure, and the shape of the 
seeds in the ray florets of some Compositse countenances this 
idea; but with the Umbellifene, it is by no means, as Dr. Hooker 
informs me, the species with the densest heads which most 
frequently differ in their inner and outer flowers. It might have 
been thought that the development of the ray-petals by drawing 
nourishment from the reproductive organs causes their abortion ; 
but this can hardly be the sole cause, for in some Compositse the 
seeds of the outer and inner florets differ, without any difference 
in the corolla. Possibly these several differences may be connected 
with the different flow of nutriment towards the central and 
external flowers: we know, at least, that with irregular flowers, 
those nearest to the axis are most subject to peloria, that is to 
become abnormally symmetrical. I may add, as an instance of 
this fact, and as a striking case of correlation, that in many 
pelargoniums, the two upper petals in the central flower of the 
truss often lose their patches of darker colour; and when this 
occurs, the adherent nectary is quite aborted ; the central flower 
thus becoming peloric or regular. When the colour is absent 
from only one of the two upper petals, the nectary is not quite 
aborted but is much shortened. 

With respect to the development of the corolla, Sprengel's idea 
that the ray-florets serve to attract insects, whose agency is highly 
advantageous or necessary for the fertilisation of these plants, is 
highly probable ; and if so, natural selection may have come into 
play. But with respect to the seeds, it seems impossible that 
their differences in shape, which are not always correlated with 
any difference in the corolla, can be in any way beneficial : yet in 
the Umbelliferse these differences are of such apparent impor- 
tance the seeds being sometimes orthospermous in the exterior 
flowers and coelospermous in the central flowers, that the elder 
De Candolle founded his main divisions in the order on such 
characters. Hence modifications of structure, viewed by systema- 
tists as of high value, may be wholly due to the laws of variation 
and correlation, without being, as far as we can judge, of the 
slightest service to the species. 

We may often falsely attribute to correlated variation 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 in some necessary 
manner correlated. Some other correlations are apparently due 
to the manner in which natural selection can alone act. For 
instance, Alph. de Candolle has remarked that winged seeds are 
never found in fruits which do not open ; I should explain this 
rule by the impossibility of seeds gradually becoming winged 
through natural selection, unless the capsules were open; for in 
this case alone could the seeds, which were a little better adapted 
to be wafted by the wind, gain an advantage over others less 
well fitted for wide dispersal. 

Compensation and Economy of Growth. 

The elder Geoffroy and Goethe propounded, at about the same 
time, 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 
atropliied, the fruit itself gains largely in size and quality. la 
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 of 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 develoj)ed through natural selection . 
and another and adjoining part being reduced by this same- 
process or by disuse, and, on the other hand, the actual with- 
drawal 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 
iiave been advanced, and likewise some other facts, may be 
merged under a more general principle, namely, that natural 
selection is continually trying to economise every part of the 
organisation. If under changed conditions of life a structure, . 


before useful, becomes less useful, its diminution will be favoured, 
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 1 was much struck when examining cirripedes, 
and of which many analogous instances could be given : namely, 
that when a cirripede is parasitic within another cirripede 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 cirripedes 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. 
Xow the saving of a large and complex structure, when rendered 
superfluous, 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 would have a better chance of 
supporting itself, by less nutriment being wasted. 

Thus, as I believe, natural selection will tend in the long run 
to reduce any part of the organisation, as soon as it becomes, 
through changed habits, 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 an organ without requiring as 
a necessary compensation the reduction of some adjoining part. 

Multiple, Rudimentary, and Lowly-organised Structures are 

It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, 
both with varieties and species, that when any part or organ is 
repeated many times in the same individual (as the vertebrae in 
snakes, and the stamens in polyandrous flowers) the number is 
variable ; whereas the same part or organ, when it occurs in lesser 
numbers, is constant. The same author as well as some botanists 
have further remarked that multiple parts are extremely liable to 
vary in structure. As "vegetative repetition," to use Pro: 
Owen's expression, is a sign of low organisation, the forego! 
statements accord with the common opinion of naturalists, t 
beings which stand low in the scale of nature are more variab 
than those which are higher. I presume that lowness here means 
that the several parts of the organisation have been but litt 
specialised for particular functions ; and as long as the same 
has to perform diversified work, we can perhaps see why it shou 
remain variable, that is, why natural selection should not ha 
preserved or rejected each little deviation of form so carefully 


when the part has to serve for some one special purpose. In the 
same >vay that a knife which has to cut all sorts of things may be 
of almost any shape; whilst a tool for some particular purpose 
must be of some particular shape. Natural selection, it should 
never be forgotten, can act solely through and for the advantage 
of each being. 

Rudimentary parts, as it is generally admitted, are apt to be 
highly variable. We shall have to recur to this subject; and I 
will here only add that their variability seems to result from their 
uselessness, and consequently from natural selection having had 
no power to check deviations in their structure. 

A Part developed in any Species in an extraordinary degree or 
manner, in comparison with t/te same Part in allied Species, 
tends to be highly variable. 

Several years ago I was much struck by a remark, to the above 
effect, made by Mr. Waterhouse. Professor Owen, also, seems to 
have come to a nearly similar conclusion. It is hopeless to attempt 
to convince any one of the truth of the above proposition without 
giving the long array of facts which I have collected, and which 
cannot possibly be here introduced. I can only state my convic- 
tion 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 one species or in a few species in comparison with 
the same part in many closely allied species. Thus, the wing of a 
bat is a most abnormal structure in the class of mammals, but the 
rule would not apply here, because the whole group of bats possesses 
wings ; it would apply only if some one species had wings developed 
in a 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, 
relates 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 more rarely to the females, as they 
seldom offer remarkable secondary sexual characters. 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 ; I particularly attended to Mr. Water- 
house's remark, whilst investigating this Order, and I am fully 
tonvinced that the rule almost always holds good. I shall, in a 


future work, give a list of all the more remarkable cases ; I will 
.here give only one, as it illustrates the rule in its largest applica- 
tion. The opercular valves of sessile cerripedes (rock barnacles) 
are, in every sense of the word, very important structures, and 
they differ extremely little even in distinct 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 the same species is 
so great, that it is no exaggeration to state that the varieties of 
the same species differ more from each other in the characters 
derived from these important organs, than do the species belong- 
ing to other distinct genera. 

As with birds the individuals of the same species, inhabiting 
the same country, vary extremely little, I have particularly 
attended to them ; and the rule certainly seems to hold good in 
this class. I cannot make out that it applies to plants, and this 
would have seriously shaken my belief in its truth, had not the 
great variability in plants made it particularly difficult to com- 
pare their relative degrees of variability. 

When we see any part or organ developed in a remarkable 
degree or manner in a species, the fair presumption is that it is of 
high importance to that species: nevertheless it is in this case 
eminently liable to variation. Why should this be so 1 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 are descended from some other 
species, and have been modified through natural selection, I think 
we can obtain some light. First let me make some preliminary 
remarks. If, in our domestic animals, 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 uniform character : and the breed may be said to 
be degenerating. In rudimentary organs, and in those which 
have been but little specialised for any particular purpose, and 
perhaps in polymorphic groups, we see a nearly parallel case ; for 
in such cases natural selection either has not or cannot have come 
into full play, and thus the organisation is left in a fluctuating 
condition. But what here more particularly concerns us is, that 
those points in our domestic animals, which at the present time 
are undergoing rapid change by continued selection, are also 
eminently liable to variation. Look at the individuals of the same 
breed of the pigeon, and see what a prodigious amount of differ- 
ence there is in the beaks of tumblers, in the beaks and wattle of 
carriers, in the carriage and tail of fantaiis, tc., these being the 
points now mainly attended to by English fanciers. Even in the 


same sub-breed, as in that of the short-faced tumbfer, it is notori- 
ously difficult to breed nearly perfect birds, many departing widely 
from the standard. There may truly be said to be a constant 
struggle going on between, on the one hand, the tendency to 
reversion to a less perfect state, as well as an innate tendency to 
new variations, and, on the other hand, the i>ower of steady selec- 
tion to keep the breed true. In the long run selection gains the day, 
and we do not expect to fail so completely as to breed a bird as 
coarse as a common tumbler pigeon from a good short-faced strain. 
But as long as selection is rapidly going on, much variability in 
the parts undergoing modification may always be expected. 

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 several species branched off from the common 
progenitor of the genus. This period will seldom be remote in 
any extreme degree, as species rarely endure for more than one 
geological period. An extraordinary amount of modification 
implies an unusually large and long-continued amount of varia- 
bility, which has continually been accumulated by natural selec- 
tion 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, still expect 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 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 our theory, for an immense 
period in nearly the same state ; and thus it has come not to be 
more variable than any other structure. It is only in those cases 
in which the modification has been comparatively recent and extra- 
ordinarily 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 con- 
tinued 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. 


Specific CJiaracters more Variable titan Generic Characters. 

The principle discussed under the last heading may be applied 
to our present subject. It is notorious that specific characters 
are more variable than generic. To explain by a simple example 
what is meant : if in a large genus of plants some species 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 the explanation which most natural- 
ists would advance is not here applicable, 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 
point in the chapter on Classification. It would be almost 
superfluous to adduce evidence in support of the statement, that 
ordinary specific characters are more variable than generic ; but 
\vith respect to important characters, I have repeatedly noticed 
in works on natural history, that when an author remarks with 
surprise that some important organ or part, which is generally 
very constant throughout a large group of species, differs con- 
siderably in closely-allied species, it is often variable in the 
individuals of the same 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. Geoffrey St. Hilaire apparently entertains no doubt, that the 
more an organ normally differs in the different species of the same 
group, the more subject it is to anomalies in the individuals. 

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 that species are only strongly marked 
and fixed varieties, we might expect often to find them still 
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 allied genera, are called generic 
characters ; and these characters may be attributed to inheritance 


from a common progenitor, for it can rarely have happened that 
natural selection will have modified several distinct 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 before the period when the several 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 
since the period when the species branched off 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 long period remained constant. 

Secondary Sexual Characters Variable. I think it will be 
admitted by naturalists, without my entering on details, that 
secondary sexual characters are highly variable. It will also 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 the females. The cause of the 
original variability of these characters is not manifest ; but we 
can see why they should not have been rendered as constant and 
uniform as others, for they are 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 have 
succeeded in giving to the species of the same group a greater 
amount of difference in these than in other respects. 

It is a remarkable fact, that the secondary differences between 
the two sexes of the same species are generally displayed in the 
very same parts of the organisation in which the species of the 
same genus differ from each other. Of this fact I will give in 
illustration the two first instances 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 common to very large groups 
of beetles, but in the Engidse, as Westwood has remarked, the 
number varies greatly ; and the number likewise differs in the two 
sexes of the same species. Again in the fossorial hymenoptera, 
the neuration of the wings is a character of the highest import- 



ance, 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. Sir J. Lubbock has recently remarked, 
that several minute crustaceans offer excellent illustrations of this 
law. " In Pontella, for instance, the sexual characters are afforded 
mainly by the anterior antennae and by the fifth pair of legs : the 
specific differences also are principally given by these organs." 
This relation has a clear meaning on my view : I look at all the 
species of the same genus as having as certainly descended from 
a common progenitor, as have the two sexes of any one 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 to struggle with other males for the possession of the 

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 are possessed by all the 
species ; 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 slight 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 their great differ- 
ence in closely allied species ; that secondary sexual and ordinary 
specific differences are generally displayed in the same parts ot 
the organisation, are all principles closely connected together. 
All being mainly due to the species of the same group being the 
descendants of 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 having been 
thus adapted for secondary sexual, and for ordinary purposes. 

Distinct SjKcies present analogous Variations, so that a Variety 
of one Species often assumes a Character proper to an allied 
Species, or reverts to some of the Characters of an early Pro-, 
genitor. These propositions will be most readily understood by 


looking to our domestic races. The most distinct breeds of the 
pigeon, in countries widely apart, present sub-varieties with 
reversed feathers on the head, and with 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 as com- 
monly called roots, 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. Many similar cases of analogous variation have been 
observed by Naudin in the great gourd-family, and by various 
authors in our cereals. Similar cases occurring with insects under 
natural conditions have lately been discussed with much ability 
by Mr. Walsh, who has grouped them under his law of Equable 

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, white loins, a bar at the end of the 
tail, with the outer feathers externally edged near their basis 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, probably for hundreds 
of generations. But when a breed has been crossed only once 


by some other breed, the offspring occasionally show for many 
generations a tendency to revert in character to the foreign breed 
some say, for a dozen or even a score of generations. After twelve 
generations, the proportion of blood, to use a common expression, 
from 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 remnant of foreign blood. In a breed which has not been 
crossed but in which lot h parents have lost some character which 
their progenitor possessed, the tendency, whether strong or weak, 
to reproduce the lost character might, as was formerly remarked, 
for all that we can see to the contrary, be 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 one individual suddenly 
takes after an ancestor removed by some hundred generations, 
but that in each successive generation the character in question 
has been lying latent, and at last, under unknown favourable 
conditions, is developed. With the barb-pigeon, for instance, 
which very rarely produces a blue bird, it is probable that there is 
a latent tendency in each generation to produce blue plumage. 
The abstract improbability of such a tendency being transmitted 
through a vast number of generations, is not greater than that of 
quite useless or rudimentary organs being similarly transmitted. 
A mere tendency to produce a rudiment is indeed sometimes 
thus inherited. 

As all the species of the same genus are supposed to be 
descended from a common progenitor, it might be expected that 
they would occasionally vary in an analogous manner ; so that 
the varieties of two or more species would resemble each other, 
or that a variety of one species would resemble in certain 
characters another and distinct species, this other species being, 
according to our view, only a well-marked and permanent variety. 
But characters exclusively due to analogous variation would 
probably be of an unimportant nature, for the preservation of 
all functionally important characters will have been determined 
through natural selection, in accordance with the different habits 
of the species. It might further be expected that the species of 
the same genus would occasionally exhibit reversions to long lost 
characters. As, however, we do not know the common ancestor 
of any natural group, we cannot distinguish between revisionary 
and analogous characters. If, for instance, we did not know that 
the parent rock-pigeon was not feather-footed or turn-crowned, 
we could not have told, whether such characters in our domestic 
breeds were reversions or only analogous variations; but we 
might have inferred that the blue colour was a case of reversion 
from the number of the markings, which are correlated with this 


tint, and which would not probably have all appeared together 
from simple variation. More especially we might have inferred 
this, from the blue colour and the several marks so often appear- 
ing when differently coloured breeds are crossed. Hence, although 
under nature it must generally be left doubtful, what cases are 
reversions to formerly existing characters, and what are new but 
analogous variations, yet we ought, on our theory, sometimes to 
find the varying offspring of a species assuming characters which 
are already present in other members of the same group. And 
this undoubtedly is the case. 

The difficulty in distinguishing variable species is largely due 
to the varieties mocking, as it were, other species of the same 
genus. A considerable catalogue, also, could be given of forms 
intermediate between two other forms, which themselves can only 
doubtfully be ranked as species ; and this shows, unless all these 
closely allied forms be considered as independently created species, 
that they have in varying assumed somfi of the characters of the 
others. But the best evidence of analogous variations is afforded 
by parts or organs which are generally constant in character, but 
which occasionally vary so as to resemble, in some degree, the 
same part or organ in an allied species. I have collected a long 
list of such cases ; but here, as before, I lie under the great dis- 
advantage of not being able to give them. I can only repeat that 
such cases certainly 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 almost certainly of reversion. 
The ass sometimes has very distinct transverse bars on its legs, 
like those on the legs of the 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. The stripe on the shoulder is 
sometimes double, and is very variable in length and outline. 
A white ass, but not an 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. 
Mr. Blyth has seen a specimen of the hemionus with a distinct 
shoulder-stripe, though it properly has none; and I have been 
informed by Colonel Poole that the 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 ; I have myself seen a dun Devon- 
shire pony, and a small dun Welsh pony has been carefully 
described to me, both with three, 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 this 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 stipes are 
often 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. I have myself recently bred 
a foal from a bay mare (offspring of a Turkoman horse and a 
Flemish mare) by a bay English race-horse ; this foal when a 
week old was marked on its hinder quarters and on its forehead 
with numerous, very narrow, dark, zebra-like bars, and its legs 
were feebly striped : all the stripes soon disappeared completely. 
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 
of tenest 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 are 
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 this view may be 
safely rejected ; for it is highly improbable that the heavy Belgian 
cart-horse, Welsh ponies, Norwegian cobs, the lanky Kattywar 
race, fec., inhabiting the most distant parts of the world, should 
all have been crossed with one supposed aboriginal stock. 

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 ; according 


to Mr. Gosse, in certain parts of the United States about nine out 
of ten mules have striped legs. I once saw a mule with its legs 
so much striped that any one might have thought that it was a 
hybrid-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 Morton's famous hybrid, from a chestnut mare and male 
quagga, the hybrid, and even the pure offspring subsequently pro- 
duced from the same 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 only occasionally has stripes on his 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 Devonshire and Welsh ponies, 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 is commonly called chance, 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 occurred in the eminently 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 1 We see several 
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 invari- 
ably 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 cha- 
racters, is that there is a tendency in the young of each succes- 


sive 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 con- 
fidently 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 domestica- 
tion, in this particular manner, so as often to become striped like 
the 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 living on the sea-shore. 

Summary. Our ignorance of the laws of variation is profound. 
Not in one case out of a hnndred can we pretend to assign any 
reason why this or that part has varied. 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. Changed conditions generally induce mere fluc- 
tuating variability, but sometimes they cause direct and definite 
effects ; and these may become strongly marked in the course of 
time, though we have not sufficient evidence on this head. Habit 
in producing constitutional peculiarities and use in strengthening; 
and disuse in weakening and diminishing organs, appear in many 
cases to have been potent in their effects. Homologous parts tend 
to vary in the same manner, 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 detri- 
ment will be saved. Changes of structure at an early age may 
affect parts subsequently developed ; and many cases of correlated 


variation, the nature of which we are unable to understand, un- 
doubtedly occur. Multiple parts are variable in number and in 
structure, perhaps arising from such parts not having been closely 
specialised for any particular function, so that their modifications 
have not been closely checked by natural selection. It follows 
probably from this same cause, that organic beings low in the 
scale are more variable than those standing higher in the scale, 
and which have their whole organisation more specialised. Rudi- 
mentary organs, from being useless, are not regulated by natural 
selection, and hence 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 vari- 
able, 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 a 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 in that district and 
amongst these species, we now find, on an average, most varieties. 
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 two 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 our view must be 
a very slow process, requiring a long lapse of time in this case, 
natural selection has succeeded in giving a fixed character to the 
organ, in however extraordinary a manner it may have been 
developed. Species inheriting nearly the same constitution from 
a common parent, and exposed to similar influences, naturally 
tend to present analogous variations, or these same species may 
occasionally revert to some of the characters of their ancient pro- 
genitors. Although new and importaat 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 between 
the offspring and their parents and a cause for each must exist 
we have reason to believe that it is the steady accumulation of 
beneficial differences which has given rise to all the more impor- 
tant modifications of structure in relation to the habits of each 


Difficulties of the theory of descent with modification 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 Modes of transition Cases of difficulty 
datura 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 the reader has arrived at this part of my work, a 
crowd of difficulties will have occurred to him. Some of them are 
so serious that to this day I can hardly reflect on them without 
being in some degree 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 the theory. 

These difficulties and objections may be classed under the 
following heads: First, why, if species have descended from 
other species by fine gradations, do we not everywhere see 
innumerable transitional forms ? Why is not all nature in con- 
fusion, 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 other animal with widely different habits 
and structure ? Can we believe that natural selection could pro- 
duce, on the one hand, an organ of trifling importance, such as 
the tail of a giraffe, which serves as a fly -flapper, and, on the other 
hand, an organ so wonderful as the eye 1 

Thirdly, can instincts be acquired and modified through natural 
selection ? What shall we say to the instinct which leads the bee 
to make cells, and which has practically anticipated the discoveries 
of profound mathematicians 1 

Fourthly, how can we account for species, when crossed, being 


sterile and producing sterile offspring, whereas, when varieties 
are crossed, their fertility is unimpaired 1 

The two first heads will here be discussed ; some miscellaneous 
objections in the following chapter; Instinct and Hybridism in 
the two succeeding 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- 
form and other less-favoured forms with which it comes into 
competition. Thus extinction and natural selection go hand in 
hand. Hence, if we look at each species as descended from some 
unknown form, both the parent and all the transitional varieties 
will generally have been exterminated by the very process of the 
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 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 crust of the earth is a vast museum; 
hut the natural collections have been imperfectly made, and only 
at long intervals of time. 

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 ars 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 are 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 sup- 
planted and exterminated its original parent-form and all the 
transitional varieties between its past and present states. Hence 
we ought not to expect at the present time to nie^t 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 1 


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 most 
continents have 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 

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 com- 
parison 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 E. 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 see that the range of the inhabitants of any country 
by no means exclusively depends on insensibly changing physical 
conditions, but in a large part on the presence of other species, on 
which it lives, or by which it is destroyed, or with which it comes 
into competition ; and as these species are already defined objects, 
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 nature of the seasons, be extremely liable to utter 
extermination ; and thus its geographical range will come to be 
still more sharply denned. 

As allied or representative species, when inhabiting a continuous 
area, are generally distributed in such a manner 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 take a varying 
species inhabiting a very large area, we shall have to adapt two 
varieties to two large areas, and a third variety to a narrow inter- 
mediate 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 conclude that varieties linking two other varieties 
together generally have existed in lesser numbers than the forms 
which they connect, then 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 inter- 
mediate form would be eminently liable to the inroads of closely- 
allied forms existing on both sides of it. But it is a far more 
important consideration, that during the process of further modi- 
fication, by which two varieties are supposed 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 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 that 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 the 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 t\to 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 inextric- 
able chaos of varying and intermediate links : first, because new 
varieties are very slowly formed, for variation is a slow process, 
and natural selection can do nothing until favourable individual 
differences or variations occur, and until a place in the natural 
pclity of the country can be better filled 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 immigra- 
tion 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 to see only 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 as 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, inter- 
mediate varieties between the several representative species and 
their common parent, must formerly have existed within each 
isolated portion of the land, but these links during the process of 
natural selection will have been supplanted and exterminated, so 
that they will no longer be found 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 distri- 


bution 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 varieties, 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 closely 
together all the species of the same group, 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 attempt to show in a future 
chapter, in an extremely imperfect and intermittent record. 

On the Origin and Transitions of Organic Beings with peculiar 
Habits and Structure. It has been asked by the opponents of 
such views as I hold, how, for instance, could a land carnivorous 
animal 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 there now exist carnivorous animals 
presenting close intermediate grades from strictly terrestrial to 
aquatic habits ; and as each exists by a struggle for life, it is clear 
that each must be well adapted 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 the summer this animal dives for and preys on fish, but 
during the long winter it leaves the frozen waters, and preys, like 
other pole-cats, 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 to answer. Yet I think such 
difficulties have 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 allied species ; 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 diffi- 
culty in any particular case like that of the bat. 

Look at the family of squirrels ; here we have the finest grada- 
tion 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, to collect food more quickly, or, as there is reason 
to believe, to lessen 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 possible 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 exter- 
minated, 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 so-called flying lemur, which 
formerly was ranked amongst bats, but is now believed to belong 
to the Insectivora, An extremely wide flank-membrane stretches 
from the corners of the jaw to the tail, and includes the limbs 
with the elongated fingers. This flank-membrane is furnished 
with an extensor muscle. Although no graduated links of 
structure, fitted for gliding through the air, now connect the 
Galeopithecus with the other Insectivora, yet there is no difficulty 
in supposing that such links formerly existed, and that each was 
developed in the same manner as with the less perfectly gliding 
squirrels ; each grade of structure having been useful to its 
possessor, Nor can I see any insuperable difficulty in further 
believing that the membrane connected fingers and fore-arm of 
the Galeopithecus might have been greatly lengthened by natural 
selection ; and this, as far as the organs of flight are concerned, 
would have converted the animal into a bat. In certain bats in 
which the wing-membrane extends from the top of the shoulder 
to the tail and includes the hind-legs, we perhaps see traces of an 
api 9,ratus originally fitted for gliding through the air rather than 
for night. 

If about a dozen genera of birds were to become extinct, who 
would have ventured to surmise 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 as front- 
legs on the land, like the penguin ; as sails, like the ostrich ; and 
functionally for no purpose, like the Apteryx ? 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 be the 
result of disuse, indicate the steps by which birds actually 
acquired their perfect power of flight ; but they serve to show 
what diversified means of transition are at least 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 this had 
been effected, who would have ever imagined that in an early 
transitional state they had been the inhabitants of the open ocean, 
and had used their incipient organs of flight exclusively, as faf 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 have survived to the present day, for they will have been 
supplanted by their successors, which were gradually rendered 
more perfect through natural selection. Furthermore, we may 
conclude that transitional states 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 both of diversified and of 
changed habits in the individuals of the same species. In either 
case it would be easy for natural selection to adapt the structure 
of the animal to its changed habits, or exclusively to one of its 
several habits. It is, however, difficult to decide, and immaterial 


for us, whether habits generally change first and structure after- 
wards ; or whether slight modifications of structure lead to changed 
habits ; both probably often occurring 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 innu- 
merable 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 into it 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 sometimes, 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, almost like a whale, insects in the water. 

As we sometimes see individuals following habits different from 
those proper to their species and to the other species of the same 
genus, we might expect that such individuals would occasionally 
give rise to new species, having anomalous habits, and with their 
structure either slightly or considerably modified from that of 
their type. And such instances occur in nature. Can a more 
striking instance of adaptation be given than that of a woodpecker 
for climbing trees and 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. On the plains of La Plata, where hardly a tree grows, 
there is a woodpecker (Colaptes campestris) which has tv,-o toes 
before and two behind, a Jong pointed tongue, pointed tail- 
feathers, sufficiently stiff to support the bird in a vertical position 
on a post, but not so stiff as in the typical woodpeckers, and a 
straight strong beak. The beak, however, is not so straight or so 
strong as in the typical woodpeckers, but it is strong enough to 
bore into wood. Hence this Colaptes in all the essential parts of 
its structure is a woodpecker. Even in such trifling characters as 
the colouring, the harsh tone of the voice, and undulatory flight, 
its close blood-relationship to our common woodpecker is plainly 
declared ; yet, as I can assert, not only from my own observations 
but from those of the accurate Azara, in certain large districts it 
does not climb trees, and it makes its nest in holes in banks ! In 
certain other districts, however, this same woodpecker, as Mr. 
Hudson states, frequents trees, and bores holes in the trunk for 
its nest. I may mention as another illustration of the varied 
habits of this genus, that a Mexican Colaptes has been described 


by De Saussure as boring holes into hard wood in order to lay up 
a store of acorna 

Petrels are the most aerial and oceanic of birds, but in the quiet 
sounds of Tierra del Fuego, the Putfinuria berardi, in its general 
habits, in its astonishing power cf diving, in its manner of swim- 
ming and of Hying when made to take Hight, would be mistaken 
by any one for an auk or a grebe ; nevertheless it is essentially a 
petrel, but with many parts of its organisation profoundly modified 
in relation to its new habits of life ; whereas the woodpecker of 
La Plata has had its structure only slightly modified. In the case 
of the water-ouzel, the acutest observer by examining its dead 
body would never have suspected its sub-aquatic habits ; yet this 
bird, which is allied to the thrush family, subsists by diving 
using its wings under water, and grasping stones with its feet. All 
the members of the great order of Hymenopterous insects are 
terrestrial, excepting the genus Proctotrupes, which Sir John 
Lubbock has discovered to be aquatic in its habits ; it often enters 
the water and dives about by the use not of its legs but of its 
wings, and remains as long as four hours beneath the surface ; yet 
it exhibits no modification in structure in accordance with its 
abnormal habits. 

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 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 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 ocean. 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, not 
furnished with membrane of the Grallatores are formed for walk- 
ing over swamps and floating plants ? the water-hen and landrail 
are members of this order, yet the first is nearly as aquatic as the 
coot, and the second nearly as terrestrial a.s 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 almost rudi- 
mentary in function, though not in structure. In the frigate-bird, 
the deeply scooped membrane between the toes shows that struc- 
ture has begun to change. 

He who believes in separate and innumerable acts of creation 
may say, that in these cases it has pleased the Creator to cause a 
being of one type to take the place of one belonging to another 
type ; but this seems to me only re-stating tke 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 varies ever so little, either in habits or 
r, structure, and thus gains an advantage over some other inhabitant 
of the same country, it will seize on the place of that inhabitant, 
however different that may be from its own place. Hence it will 
cause him no surprise that there should be geese and frigate-birds 
with webbed feet, living on the dry land and 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 hardly a tree grows ; that there should be diving thrushes 
and diving Hymenoptera, 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 degree. When it was first 
said that the sun stood still and the world turned round, the 
.common sense of mankind declared the doctrine false ; but the old 
saying of Vox populi, vox Dei, as every philosopher knows, can- 
not be trusted in science. Reason tells me, that if numerous 
gradations from a simple and imperfect eye to one complex and 
perfect can be shown to exist, each grade being useful to its 
possessor, as is certainly the case ; if further, the eye ever varies 
and the variations be inherited, as is likewise certainly the case ; 
\j and if such variations should be useful to any 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, should not be considered 
as subversive of the theory. How a nerve comes to be sensitive 
to light, hardly concerns us more than how life itself originated ; 
but I may remark that, as some of the lowest organisms, in which 
nerves cannot be detected, are capable of perceiving light, it does 
not seem impossible that certain sensitive elements in their 
sarcode should become aggregated and developed into nerves, 
endowed with this special sensibility. 

In searching for the gradations through which an organ in any 
species has been perfected, we ought to look exclusively to its 
lineal progenitors ; but this is scarcely ever possible, and we are 
forced to look to other species and genera of the same group, that 
is to the collateral descendants from the same parent-form, in 
order to see what gradations are possible, and for the chance of 
some gradations having been transmitted in an unaltered or little 
altered condition. But the state of the same organ in distinct 


classes may incidentally throw light on the steps by which it has 
been perfected. 

The simplest organ which can be called an eye consists of an 
optic nerve, surrounded by pigment-cells and covered by translu- 
cent skin, but without any lens or other refractive body. We may, 
however, according to M. Jourdain, descend even a step lower and 
find aggregates of pigment-cells, apparently serving as organs of 
vision, without any nerves, and resting merely on sarcodic tissue. 
Eyes of the above simple nature are not capable of distinct vision, 
and serve only to distinguish light from darkness. In certain star- 
fishes, small depressions in the layer of pigment which surrounds 
the nerve are filled, as described by the author just quoted, with 
transparent gelatinous matter, projecting with a convex surface, 
like the cornea in the higher animals. He suggests that this serves 
not to form an image, but only to cencentrate the luminous rays 
and render their perception more easy. In this concentration of 
the rays we gain the first and by far the most important step 
towards the formation of a true, picture-forming eye ; for we have 
only to place the naked extremity of the optic nerve, which in 
pome of the lower animals lies deeply buried in the body, and in 
some near the surface, at the right distance from the concentrating 
apparatus, and an image will be formed on it. 

In the great class of the Articulata, we may start from an optic 
nerve simply coated with pigment, the latter sometimes forming 
a sort of pupil, but destitute of a lens or other optical contrivance. 
With insects it is now known that the numerous facets on the 
cornea of their great compound eyes form true lenses, and that 
the cones include curiously modified nervous filaments. But these 
organs in the Articulata are so much diversified that Miiller 
formerly made three main classes with seven subdivisions, besides 
a fourth main class of aggregated simple eyes. 

When we reflect on these facts, here given much too briefly, 
with respect to the wide, diversified, and graduated range of 
structure in the eyes of the lower animals ; and when we bear in 
mind how small the number of all living forms must be in com- 
] larison with those which have become extinct, the difficulty ceases 
to be very great in believing that natural selection may have 
converted the simple apparatus of an optic nerve, coated with 
pigment and invested by transparent membrane, into an optical 
instrument as perfect as is possessed by any member of the 
Articulate Class. 

He who will go thus far, ought not to hesitate to go one step 
further, if he finds on finishing this volume that large bodies of 
facts, otherwise inexplicable, can be explained by the theory of 
modification through natural selection ; he ought to admit that a 
structure even as perfect as an eagle's eye might thus be formed. 


although in this case he does not know the transitional states. 
It has been objected that in order to modify the eye and still 
preserve it as a perfect instrument, many changes would have 
to be effected simultaneously, which, it is assumed, could not be 
done through natural selection ; but as I have attempted to show 
in my work on the variation of domestic animals, it is not 
necessary to suppose that the modifications were all simultaneous, 
if they were extremely slight, and gradual. Different kinds of 
modification would, also, serve for the same general purpose : as 
Mr. Wallace has remarked, "if a lens has too short or too long 
a focus, it may be amended either by an alteration of curvature, 
or an alteration of density; if the curvature be irregular, and 
the rays do not converge to a point, then any increased regularity 
of curvature Avill be an improvement. So the contraction of the 
iris and the muscular movements of the eye are neither of them 
essential to vision, but only improvements which might have been 
added and perfected at any stage of the construction of the 
instrument." Within the highest division of the animal kingdom, 
namely, the Vertebrata, we can start from an eye so simple, that 
it consists, as in the lancelet, of a little sack of transparent skin, 
furnished with a nerve and lined with pigment, but destitute of any 
other apparatus. In fishes and reptiles, as Owen has remarked, 
"the range of gradations of dioptric structures is very great." 
It is a significant fact that even in man, according to the high 
authority of Virchow, the beautiful crystalline lens is formed 
in the embryo by an accumulation of epidermic cells, lying in 
a sack-like fold of the skin ; and the vitreous body is formed from 
embryonic sub-cutaneous tissue. To arrive, however, at a just 
conclusion regarding the formation of the eye, with all its marvel- 
lous yet not absolutely perfect characters, it is indispensable that 
the reason should conquer the imagination ; but I have felt the 
difficulty far too keenly to be surprised at others hesitating to 
extend the principle of natural selection to so startling a length. 

It is scarcely possible to avoid comparing the eye with 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 some- 
what analogous process. But may not this inference be pre- 
sumptuous? 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 imagin- 
ation to take a thick layer of transparent tissue, with spaces filled 
with fluid, and with a nerve sensitive to light beneath, and 
then suppose every part of this layer to be continually chang- 
ing 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, repre- 
sented by natural selection or the survival of the fittest, always 
intently watching each slight alteration in the transparent layers ; 
and carefully preserving each which, under varied circumstances, 
in any way or in any degree, tends to produce a distincter image. 
We must suppose each new state of the instrument to be multi- 
plied by the million ; each to be preserved until a better one is 
produced, and then the old ones to be all 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 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 >C 
glass, as the works of the Creator are to those of man 'i 

Modes of Transition. 

If it could be demonstrated that any complex organ existed, 
which could not possibly have been formed by numerous, succes-(r^i 
sive, slight modifications, my theory would absolutely break down, v 
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 the 
theoiy, there has been much extinction. Or again, if we take an 
organ common to all the members of a class, for in this latter case 
the organ must have been originally formed at a remote period, 
since which all the many members of the class have been de- 
veloped ; 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 in the larva of the dragon-fly and in the fish Cobites the 
alimentary canal respires, digests, and excretes. 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 selec- 
tion might specialise, if any advantage were thus gained, the whole 
or part of an organ, which had previously performed two functions, 
for one function alone, and thus by insensible steps greatly change 
its nature. Many plants are known which regularly produce at 
the same time differently constructed flowers ; and ft such plants 
were to produce one kind alone, a great change would be effected 
with comparative suddenness in the character of the species, li 


is, however, probable that the two sorts of flowers borne by the 
same plant were originally differentiated by finely graduated steps, 
which may still be followed in some few cases. 

Again, two distinct organs, or the same organ under two very 
different forms, may simultaneously perform in the same individual 
the same function, and this is an extremely important means of 
transition : to give one instance, there are fish with gills or bran- 
chiae that breathe the air dissolved in the water, at the same time 
that they breathe free air in their swimbladders, this latter organ 
being divided by highly vascular partitions and having a ductus 
pneumaticus for the supply of air. To give another instance from 
the vegetable kingdom : plants climb by three distinct means, by 
spirally twining, by clasping a support with their sensitive tendrils, 
and by the emission of aerial rootlets ; these three means are 
usually found in distinct groups, but some few species exhibit two 
of the means, or even all three, combined in the same individual. 
In all such cases one of the two organs might readily be modified 
and perfected so as to perform all the work, being aided during 
the progress of modification by the other organ; and then this 
other organ might be modified for some other and quite distinct 
purpose, or be wholly 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 widely different purpose, namely, 
respiration. The swimbladder has, also, been worked in as an 
accessory to the auditory organs of certain fishes. 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 is no reason to doubt that the swimbladder 
has actually been converted into lungs, or an organ used exclu- 
sively for respiration. 

According to this view it may be inferred that all vertebrate 
animals with true lungs are descended by ordinary generation 
from an ancient and unknown prototype, which was furnished 
with a floating apparatus or swimbladder. We can thus, as I 
infer from Owen's interesting description of these parts, under- 
stand 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 con- 
trivance by which the glottis is closed. In the higher Vertebrata 
the branchias have wholly disappeared but in the embryo the 
slits on the sides of the neck and the loop-like course of the 
arteries still mark their former position. But it is conceivable that 
the now utterly lost branchiae might have been gradually worked 
in by natural selection for some distinct purpose: for instance. 


Landois has shown that the wings of insects are developed from 
the tracheae ; it is therefore highly probable that in this great class 
organs which once served for respiration have been actually 
converted into organs for 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 another 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 of the sack, together 
with the small frena, serving for respiration. The Balauidae or 
sessile cirripedes, on the other hand, have no ovigerous frena, the 
eggs lying loose at the bottom of the sack, within the well-enclosed 
shell ; but they have, in the same relative position with the frena, 
large, much-folded membranes, which freely communicate with 
the circulatory lacunae of the sack and body, and which have been 
considered by all naturalists to act as 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 it need not be doubted 
that the two little folds of skin, which originally served as 
ovigerous frena, but which, likewise, very slightly aided in 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 cirri- 
pedes had become extinct, and they have suffered far more extinc- 
tion 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 1 

There is another possible mode of transition, namely, through 
the acceleration or retardation of the period of reproduction. This 
has lately been insisted on by Prof. Cope and others in the United 
States. It is now known that some animals are capable of repro- 
duction at a very early age, before they have acquired their perfect 
characters ; and if this power became thoroughly well developed 
in a species, it seems probable that the adult stage of development 
would sooner or later be lost ; and in this case, especially if the 
larva differed much from the mature form, the character of the 
species would be greatly changed and degraded. Again, not a 
few animals, after arriving at maturity, go on changing in character 
during nearly their whole lives. With mammals, for instance, the 
form of the skull is often much altered with age, of which Dr. 
Murie has given some striking instances with seals ; every one 
knows how the horns of stags become more and more branched, 
and the plumes of some birds become more finely developed, as 



they grow older. Prof. Cope states that the teeth of certain lizards 
change much in shape with advancing years; with crustaceans 
not only many trivial, but some important parts assume a new 
character, as recorded by Fritz Miilier, after maturity. In all 
such cases, and many could be given, if the age for reproduction 
were retarded, the character of the species, at least in its adult 
state, would be modified ; nor is it improbable that the previous 
and earlier stages of development would in some cases be hurried 
through and finally lost. Whether species have often or ever been 
modified through this comparatively sudden mode of transition, I 
can form no opinion ; but if this has occurred, it is probable that 
the differences between the young and the mature, and between 
the mature and the old, were primordially acquired by graduated 

Special Difficulties of the Theory of Natural Selection. 

Although we must be extremely cautious in concluding that 
any organ could not have been produced by successive, small 
transitional gradations, yet undoubtedly serious cases of difficulty 

One of the most serious is that of neuter insects, which are 
often 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 diffi- 
culty; for it is impossible to conceive by what steps these 
wondrous organs have been produced. But this is not surprising, 
for we do not even know of what use they are. In the Gymnotus 
and Torpedo they no doubt serve as powerful means of defence, 
and perhaps for securing prey; yet in the Eay, as observed by 
Matteucci, an analogous organ in the tail manifests but little 
electricity, even when the animal is greatly irritated; so little, 
that it can hardly be of any use for the above purposes. More- 
over, in the Ray, besides the organ just referred to, there is, as Dr. 
R. M'Donnell has shown, another organ near the head, not known 
to be electrical, but which appears to be the real homologue of the 
electric battery in the Torpedo. It is generally admitted that 
there exists between these organs and ordinary muscle a close 
analogy, in intimate structure, in the distribution of the nerves, 
and in the manner in which they are acted on by various reagents. 
It should, also, be especially observed that muscular contraction 
is accompanied by an electrical discharge ; and, as Dr. Radcliffe 
insists, "in the electrical apparatus of the torpedo during rest, 
there would seem to be a charge in every respect like that which 
is met with in muscle and nerve during rest, and the discharge of 
the torpedo, instead of being peculiar, may be only another form 
of the discharge which attends upon the action of muscle and 


motor nerve." Beyond this we cannot at present go in the way of 
explanation; but as we know so little about the uses of these 
organs, and as we know nothing about the habits and structure of 
the progenitors of the existing electric fishes, it would be extremely 
bold to maintain that no serviceable transitions are possible by 
which these organs might have been gradually developed. 

These organs appear at first to offer another and far more 
serious difficulty ; for they occur in about a dozen kinds of fish, of 
which several are widely remote in their affinities. When the 
same organ is found in several members of the same class, especially 
if in members 'having very different habits of life, we may generally 
attribute its presence to inheritance from a common ancestor; 
and its absence in some of the members to loss through disuse or 
natural selection. So that, if the electric organs had been inherited 
from some one ancient progenitor, we might have expected that 
all electric fishes would have been specially related to each other ; 
but this is far from the case. Nor does geology at all lead to tho 
belief that most fishes formerly possessed electric organs, which 
their modified descendants have now lost. But when we look 
at the subject more closely, we find in the several fishes provided 
with electric organs, that these are situated in different parts ot 
the body, that they differ in construction, as in the arrangement A 
of the plates, and, according to Pacini, in the process or means by 
which the electricity is excited and lastly, in being supplied witlr 
nerves proceeding from different sources, and this is perhaps the 
most important of all the differences. Hence in the several fishes 
furnished with electric organs, these cannot be considered as 
homologous, but only as analogous in function. Consequently 
there is no reason to suppose that they have been inherited from 
a common progenitor ; for had this been the case they would have 
closely resembled each other in all respects. Thus the difficulty 
of an organ, apparently the same, arising in several remotely 
allied species, disappears, leaving only the lesser yet still great 
difficulty; namely, by what graduated steps these organs have 
been developed in each separate group of fishes. 

The luminous organs which occur in a few insects, belonging to xi, 
widely different families, and which are situated in different parts 
of the body, offer, under our present state of ignorance, a difficulty 
almost exactly parallel with that of the electric organs. Other 
similar cases could be given; for instance in plants, the very 
i curious contrivance of a mass of pollen-grains, borne on a foot- 
stalk with an adhesive gland, is apparently the same in Orchis 
and Asclepias, genera almost as remote as is possible amongst 
Howering plants; but here again the parts are not homologous. 
In all cases of beings, far removed from each other in the scale of 
organisation, which are furnished with similar and peculiar organs, 


it will be found that although the general appearance and function 
of the organs may be the same, yet fundamental differences between 
them can always be detected. For instance, the eyes of cephalo- 
pods or cuttle-fish and of vertebrate animals appear wonderfully 
alike; and in such widely sundered groups no part of this re- 
semblance can be due to inheritance from a common progenitor. 
Mr. Mivart has advanced this case as one of special difficulty, but 
I am unable to see the force of his argument. An organ for vision 
must be formed of transparent tissue, and must include some sort 
of lens for throwing an image at the back of a darkened chamber. 
Beyond this superficial resemblance, there is hardly any real 
similarity between the eyes of cuttle-fish and vertebrates, as may 
be seen by consulting Hensen ; s admirable memoir on these organs 
in the Cephalopoda. It is impossible for me here to enter on 
details, but I may specify a few of the points of difference. The 
crystalline lens in the higher cuttle-fish consists of two parts, 
placed one behind the other like two lenses, both having a very 
different structure and disposition to what occurs in the vertebrata. 
The retina is wholly different, with an actual inversion of the 
elemental parts, and with a large nervous ganglion included within 
the membranes of the eye. The relations of the muscles are as 
different as it is possible to conceive, and so in other points. 
Hence it is not a little difficult to decide how far even the same 
terms ought to be employed in describing the eyes of the Cepha- 
lopoda and Vertebrata. It is, of course, open to any one to deny 
that the eye in either case could have been developed through the 
natural selection of successive slight variations; but if this be 
admitted in the one case, it is clearly possible in the other ; and 
fundamental differences of structure in the visual organs of two 
groups might have been anticipated, in accordance with this view 
of their manner of formation. As two men have sometimes inde- 
pendently hit on the same invention, so in the several foregoing 
cases it appears that natural selection, working for the good of 
each being, and taking advantage of all favourable variations, has 
produced similar organs, as far as function is concerned, in distinct 
organic beings, which owe none of their structure in common to 
inheritance from a common progenitor. 

Fritz Miiller, in order to test the conclusions arrived at in this 
volume, has followed out with much care a nearly similar line of 
argument. Several families of crustaceans include a few species, 
possessing an air-breathing apparatus and fitted to live out of the 
water. In two of these families, which were more especially 
examined by Miiller, and which are nearly related to each other, 
the species agree most closely in all important characters ; namely 
in their sense organs, circulating system, in the position of the tufts 
of hair within their complex stomachs, and lastly in the whole 


structure of the water-breathing branchiae, even to the micro- 
scopical hooks by which they are cleansed. Hence it might have 
been expected that in the few species belonging to both families 
which live on the land, the equally-important air-breathing appa- 
ratus would have been the same ; for why should this one apparatus, 
given for the same purpose, have been made to differ, whilst all 
the other important organs were closely similar or rather identical. 

Fritz Miiller argues that this close similarity in so many points 
of structure must, in accordance with the views advanced by me, 
be accounted for by inheritance from a common progenitor. But 
as the vast majority of the species in the above two families, as 
well as most other crustaceans, are aquatic in their habits, it is 
improbable in the highest degree, that their common progenitor 
should have been adapted for breathing air Miiller was thus led 
carefully to examine the apparatus in the air-breathing species ; 
and he found it to differ in each in several important points, as in 
the position of the orifices, in the manner in which they are opened 
and closed, and in some accessory details. Now such differences 
are intelligible, and might even have been expected, on the sup- 
position that species belonging to distinct families had slowly 
become adapted to live more and more out of water, and to breathe 
the air. For these species, from belonging to distinct families, 
would have differed to a certain extent, and in accordance with 
the principle that the nature of each variation depends on two 
factors, viz., the nature of the organism and that of the sur- 
rounding conditions, their variability assuredly would not have 
been exactly the same. Consequently natural selection would 
have had different materials or variations to work on, in order to 
arrive at the same functional result; and the structures thus 
acquired would almost necessarily have differed. On the hypo- 
thesis of separate acts of creation the whole case remains unintel- 
ligible. This line of argument seems to have had great weight in 
leading Fritz Miiller to accept the views maintained by me in 
this volume. 

Another distinguished zoologist, the late Professor Claparede, 
has argued in the same manner, and has arrived at the same 
result. He shows that there are parasitic mites (Acaridae), 
belonging to distinct sub-families and families, which are furnished 
with hair-claspers. These organs must have been independently 
developed, as they could not have been inherited from a common 
progenitor ; and in the several groups they are formed by the 
modification of the fore-legs, of the hind-legs, of the maxillse 
or lips, and of appendages on the under side of the hind part of 
the body. 

In the foregoing cases, we see the same end gained and the 


same function performed, in beings not at all or only remotely 
allied, by organs in appearance, though not in development, 
closely similar. On the other hand, it is a common rule through- 
out nature that the same end should be gained, even sometimes in 
the case of closely-related beings, by the most diversified means. 
How differently constructed is the feathered wing of a bird and 
the membrane-covered wing of a bat ; and still more so the four 
wings of a butterfly, the two wings of a fly, and the two wings 
with the elytra of a beetle. Bivalve shells are made to open and 
shut, but on what a number of patterns is the hinge constructed, 
from the long row of neatly interlocking teeth in a Xucula to the 
simple ligament of a Mussel ! Seeds are disseminated by their 
minuteness, by their capsule being converted into a light balloon- 
like envelope, by being embedded in pulp or flesh, formed of the 
most diverse parts, and rendered nutritious, as well as conspicuously 
coloured, so as to attract and be devoured by birds, by having 
hooks and grapnels of many kinds and serrated awns, so as to 
adhere to the fur of quadrupeds, and by being furnished with 
wings and plumes, as different in shape as they are elegant in 
structure, so as to be wafted by every breeze. I will give one 
other instance ; for this subject of the same end being gained by 
the most diversified means well deserves attention. Some authors 
maintain that organic beings have been formed in many ways for 
the sake of mere variety, almost like toys in a shop, but such a 
view of nature is incredible. With plants having separated sexes, 
and with those in which, though hermaphrodites, the pollen does 
not spontaneously fall on the stigma, some aid is necessary for 
their fertilisation. With several kinds this is effected by the 
pollen-grains, which are light and incoherent, being blown by 
the wind through mere chance on to the stigma ; and this is the 
simplest plan which can well be conceived. An almost equally 
simple, though very different, plan occurs in many plants in which 
A symmetrical flower secretes a few drops of nectar, and is con- 
sequently visited by insects ; and these carry the pollen from the 
anthers to the stigma. 

From this simple stage we may pass through an inexhaustible 
number of contrivances, all for the same purpose and effected in 
essentially the same manner, but entailing changes in every part 
of the flower. The nectar, may be stored in variously shaped 
receptacles, with the stamens and pistils modified in many ways, 
sometimes forming trap-like contrivances, and sometimes capable 
of neatly adapted movements through irritability or elasticity. 
From such structures we may advance till we come to such a case 
of extraordinary adaptation as that lately described by Dr. Criiger 
in the Coryanthes. This orchid has part of its labellum or lower 
lip hollowed out into a great bucket, into which drops of almost 


pure water continually fall from two secreting horns which stand 
above it ; and when the bucket is half full, the water overflows by 
a spout on one side. The basal part of the labellum stands over 
the bucket, and is itself hollowed out into a sort of chamber with 
two lateral entrances; within this chamber there are curious 
fleshy ridges. The most ingenious man, if he had not witnessed 
what takes place, could never have imagined what purpose all 
these parts serve. But Dr. Criiger saw crowds of large humble- 
bees visiting the gigantic flowers of this orchid, not in order to 
suck nectar, but to gnaw off the ridges within the chamber above 
the bucket ; in doing this they frequently pushed each other into 
the bucket, and their wings being thus wetted they could not fly 
away, but were compelled to crawl out through the passage formed 
by the spout or overflow. Dr. Criiger saw a "continual procession" 
of bees thus crawling out of their involuntary bath. The passage is 
narrow, and is roofed over by the column, so that a bee, in forcing 
its way out, first rubs its back against the viscid stigma and then 
against the viscid glands of the pollen-masses. The pollen-masses 
are thus glued to the back of the bee which first happens to crawl 
out through the passage of a lately expanded flower, and are thus 
carried away. Dr. Criiger sent me a flower in spirits of wine, 
with a bee which he had killed before it had quite crawled out 
with a pollen-mass still fastened to its back. When the bee, thus 
provided, flies to another flower, or to the same flower a second 
time, and is pushed by its comrades into the bucket and then 
crawls out by the passage, the pollen-mass necessarily comes first 
into contact with the viscid stigma, and adheres to it, and the 
flower is fertilised. Now at last we see the full use of every part 
of the flower, of the water-secreting horns, of the bucket half full 
of water, which prevents the bees from flying away, and forces 
them to crawl out through the spout, and rub against the properly 
placed viscid pollen-masses and the viscid stigma. 

The construction of the flower in another closely allied orchid, 
namely the Catasetum, is widely different, though serving the 
same end ; and is equally curious. Bees visit these flowers, like 
those of the Coryanthes, in order to gnaw the labellum ; in doing 
this they inevitably touch a long, tapering, sensitive projection, 
or, as I have called it, the antenna. This antenna, when touched, 
transmits a sensation or vibration to a certain membrane which is 
instantly ruptured; this sets free a spring by which the pollen- 
mass is shot forth, like an arrow, in the right direction, and adheres 
by its viscid extremity to the back of the bee. The pollen-mass 
of the male plant (for the sexes are separate in this orchid) is thus 
carried to the flower of the female plant, where it is brought into 
contact with the stigma, which is viscid enough to break certain 
elastic threads, and retaining the pollen, fertilisation is effected. 


How, it may be asked, in the foregoing and in innumerable 
other instances, can we understand the graduated scale of com- 
plexity and the multifarious means for gaining the same end. 
The answer no doubt is, as already remarked, that when two 
forms vary, which already differ from each other in some slight 
degree, the variability will not be of the same exact nature, and 
consequently the results obtained through natural selection for the 
same general purpose will not be the same. We should also bear 
in mind that every highly developed organism has passed through 
many changes ; and that each modified structure tends to be 
inherited, so that each modification will not readily be quite lost, 
but may be again and again further altered. Hence the structure 
of each part of each species, for whatever purpose it may serve, is 
the sum of many inherited changes, through which the species has 
passed during its successive adaptations to changed habits and 
conditions of life. 

Finally then, although in many cases it is most difficult even to 
conjecture by what transitions organs have arrived at their present 
state ; yet, considering how small the proportion of living and 
known forms is to the extinct and unknown, I have been 
astonished how rarely an organ can be named, towards which no 
transitional grade is known to lead. It certainly is true, that new 
organs appearing as if created for some special purpose, rarely or 
never appear in any being ; as indeed is shown by that old, but 
^ somewhat exaggerated, 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 
p expressed it, Nature is prodigal in variety, but niggard in inno- 
vation. Why, on the theory of Creation, should there be so much 
variety and so little real novelty ? 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 commonly 
linked together by graduated steps ? Why should not Nature take 
a sudden leap from structure to structure ? On the theory of 
natural selection, we can clearly understand why she should not ; 
for natural selection acts only by taking advantage of slight suc- 
cessive variations ; she can never take a great and sudden leap, 
but must advance by short and sure, though slow steps. 

S Organs of little apparent Imjxn'tance, as affected by Natural 


As natural selection acts by life and death, by the survival of 
the fittest, and by the destruction of the less well-fitted in- 
dividuals, I have sometimes felt great difficulty in understanding 
. the origin or formation of parts of little importance ; almost as 


great, though of a very different kind, as in the case of the most 
perfect and complex organs. 

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 very trifling characters, such as the down 
on fruit and the colour of its flesh, the colour of the skin and hair 
of quadrupeds, which, from being correlated with constitutional 
differences or from determining the attacks of insects, 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 
fitted, for so trifling an object as to drive 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 depend 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 advan- 
tage. It is not that the larger quadrupeds are actually destroyed 
(except in some rare cases) by 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 to 
existing species in nearly the same date, although now of very 
slight use ; but any actually injurious deviations in their structure 
would of course 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 in the case of the dog, though the aid in this latter respect 
must be slight, for the hare, with hardly any tail, can double still 
more quickly. 

In the second place, we may easily err in attributing importance 
to characters, and in believing that they have been developed 
through natural selection. We must by no means overlook the 
effects of the definite action of changed conditions of life, of 
so-called spontaneous variations, which seem to depend in a quite 



subordinate degree on the nature of the conditions, of the 
tendency to reversion to long-lost characters, of the complex laws 
of growth, such as of correlation, compensation, of the pressure of 
one part on another, etc., and finally of sexual selection, by which 
characters of use to one sex are often gained and then transmitted 
more or less perfectly to the other sex, though of no use to this 
sex. But structures thus indirectly gained, although at first of no 
advantage to a species, may subsequently have been taken 
advantage of by its modified descendants, under new conditions of 
life and newly acquired habits. 

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 adapta- 
tion to conceal this tree-frequenting bird from its enemies ; and 
consequently that it was a character of importance, and had been 
acquired through natural selection ; as it is, the colour is probably 
in chief part due to sexual selection. A trailing palm in the Malay 
Archipelago climbs the loftiest trees by the aid of exquisitely con- 
structed 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, and which, as there is reason to believe from the dis- 
tribution of the thorn-bearing species in Africa and South 
America, serve as a defence against browsing quadrupeds, so the 
spikes on the palm may at first have been developed for this 
object, and subsequently have been improved and taken advan- 
tage of by the plant, as it underwent further modification and 
became a climber. The naked skin on the head of a vulture is 
generally considered 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 cause of each slight variation 
or individual difference ; and we are immediately made conscious 
cf this by reflecting on the differences between the breeds of our 
domesticated animals in different countries, more especially in 
the less civilised countries where there has been but little 
methodical selection. Animals kept by savages in different 


countries often have to struggle for their own subsistence, and are 
exposed to a certain extent to natural selection, and individuals 
with slightly different constitutions would succeed best under 
different climates. With cattle susceptibility to the attacks of 
flies is correlated with colour, as is the liability to be poisoned by 
certain plants; so that even colour would be thus subjected to the 
action of natural selection. Some 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 the head would probably be affected. The 
shape, also, of the pelvis might affect by pressure the shape of 
certain parts of the young in the womb. The laborious breathing 
necessary in high regions tends, as we have good reason to believe, 
to increase the size of the chest ; and again correlation would come 
into play. The effects of lessened exercise together with abundant 
food on the whole organisation is probably still more important ; 
and this, as H. von Nathusius has lately shown in his excellent 
Treatise, is apparently one chief cause of the great modification 
which the breeds of swine have undergone. But we are far too 
ignorant to speculate on the relative importance of the several 
known and unknown causes of variation ; and I have made these 
remarks only to show that, if we are unable to account for the 
characteristic differences of our several domestic breeds, which 
nevertheless are generally admitted to have arisen through 
ordinary generation from one or a few parent-stocks, we ought 
not to lay too much stress on our ignorance of the precise cause 
of the slight analogous differences between true species. 

Utilitarian Doctrine, how far true : Beauty, how acquired. 

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 many structures have been 
created for the sake of beauty, to delight man or the Creator (but 
this latter point is beyond the scope of scientific discussion), or for 
the sake of mere variety, a view already discussed. Such doctrines, 
if true, would be absolutely fatal to my theory. I fully admit 
that many structures are now of no direct use to their possessors, 
and may never have been of any use to their progenitors ; but this 
does not prove that they were formed solely for beauty or variety- 
No doubt the definite action of changed conditions, and the 
various causes of modifications, lately specified, have all produced 
ao effect, probably a great effect, independently of any advantage 


thus gained. But a still more important consideration is that the 
chief part of the organisation of every living creature is due to 
inheritance; and consequently, though each being assuredly is 
well fitted for its place in nature, many structures have now no 
very close and direct relation to present habits of life. 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 similar 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 webbed feet no doubt were 
as useful to the progenitor of the upland goose and of the frigate- 
bird, as they now are to the most aquatic of living birds. So we 
may believe that the progenitor of the seal did not possess 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, were originally developed, 
on the principle of utility, probably through the reduction of more 
numerous bones in the fin of some ancient fish-like progenitor of 
the whole class. It is scarcely possible to decide how much 
allowance ought to be made for such causes of change, as the 
definite action of external conditions, so-called spontaneous varia- 
tions, and the complex laws of growth ; but with these important 
exceptions, we may conclude that the structure of every living 
creature either now is, or was formerly, of some direct or indirect 
use to its possessor. 

With respect to the belief that organic beings have been created 
beautiful for the delight of man, a belief which it has been 
pronounced is subversive of my whole theory, I may first remark 
that the sense of beauty obviously depends on the nature of the 
mind, irrespective of any real Quality in the admired object ; and 
that the idea of what is beautiful, is not innate or unalterable- 
We see this, for instance, in the men of different races admiring 
an entirely different standard of beauty in their women. If 
beautiful objects had been created solely for man's gratification, 
it ought to be shown that before man appeared, there was less 
beauty on the face of the earth than since he came on the stage. 
Were the beautiful volute and cone shells of the Eocene epoch, 
and the gracefully sculptured ammonites of the Secondary period, 
created that man might ages afterwards admire them in his 
cabinet ? Few objects are more beautiful than the minute 
siliceous cases of the diatomaceae: were these created that they 
might be examined and admired under the higher powers of the 
microscope? The beauty in this latter case, and in many others, 
is apparently wholly due to symmetry of growth. Flowers rank 
amongst the most beautiful productions of nature ; but they hav<* 


been rendered conspicuous in contrast with the green leaves, and 
in consequence at the same time beautiful, so that they may be 
easily observed by insects. I have come to this conclusion from 
finding it an invariable rule that when a flower is fertilised by the 
wind it never has a gaily-coloured corolla. Several plants habitu- 
ally produce two kinds of flowers ; one kind open and coloured so 
as to attract insects ; the other closed, not coloured, destitute of 
nectar, and never visited by insects. Hence we may conclude 
that, if insects had not been developed on the face of the earth, 
our plants would not have been decked with beautiful flowers, 
but would have produced only such poor flowers as we see on our 
fir, oak, nut and ash trees, on grasses, spinach, docks, and nettles, 
which are all fertilised through the agency of the wind. A similar 
line of argument holds good with fruits ; that a ripe strawberry or 
cherry is as pleasing to the eye as to the palate, that the gaily- 
coloured fruit of the spindle-wood tree and the scarlet berries of 
the holly are beautiful objects, will be admitted by every one. 
But tlu's beauty serves merely as a guide to birds and beasts, 
in order that the fruit may be devoured and the manured seeds 
disseminated: I infer that this is the case from having as yet 
found no exception to the rule that seeds are always thu? 
disseminated when embedded within a fruit of any kind (that is 
within a fleshy or pulpy envelope), if it be coloured of any brilliant 
tint, or rendered conspicuous by being white or black. 

On the other hand, I willingly admit that a great number of 
male animals, as all our most gorgeous birds, some fishes, reptiles, 
and mammals, and a host of magnificently coloured butterflies, 
have been rendered beautiful for beauty's sake ; but this has been 
effected through sexual selection, that is, by the more beautiful 
males having been continually preferred by the females, and not 
for the delight of man. So it is with the music of birds. We 
may infer from all this that a nearly similar taste for beautiful 
colours and for musical sounds runs through a large part of the 
animal kingdom. When the female is as beautifully coloured as 
the male, which is not rarely the case with birds and butterflies, 
the cause apparently lies in the colours acquired through sexual 
selection having been transmitted to both sexes, instead of to the 
males alone. How the sense of beauty in its simplest form that 
is, the reception of a peculiar kind of pleasure from certain colours, 
forms, and sounds was first developed in the mind of man and 
of the lower animals, is a very obscure subject. The same sort of 
difficulty is presented, if we enquire how it is that certain flavours 
and odours give pleasure, and others displeasure. Habit in all 
these cases appears to have come to a certain extent into play; 
but there must be some fundamental cause in the constitution of 
tUe nervous system in each species. 


Natural selection cannot possibly produce any modification in 
a species exclusively for the good of another species; though 
throughout nature one species incessantly takes advantage of, and 
profits by, the structures of others. But natural selection can 
and does often produce structures for the direct injury of other 
animals, 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 seenu 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 it is furnished with a rattle 
for its own injury, namely, to warn its prey. 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. It is a much 
more probable view that the rattlesnake uses its rattle, the cobra 
expands its frill, and the puff-adder swells whilst hissing so loudly 
and harshly, in order to alarm the many birds and beasts which 
are known to attack even the most venomous species. Snakes 
act on the same principle which makes the hen ruffle her feathers 
and expand her wings when a dog approaches her chickens ; but 
I have not space here to enlarge on the many ways by which 
animals endeavour to frighten away their enemies. 

Natural selection will never produce in a being any structure 
more injurious than beneficial to that being, 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 comes into competition. And we 
eee that this is the standard 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 
t:om Europe. Natural selection Avill not produce absolute per- 
fection. <ior 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 by Miiller not to be perfect even in that most perfect 
organ, the human eye. Helmholtz, whose judgment no one will 
dispute, after describing in the strongest terms the wonderful 
power of the human eye, adds these remarkable words : " That 
which we have discovered in the way of inexactness and imper- 
fection in the optical machine and in the image on the retina, is as 
nothing in comparison with the incongruities which we have just 
come across in the domain of the sensations. One might say that 
nature has taken delight in accumulating contradictions in order 
to remove all foundation from the theory of a pre-existing 
harmony between the external and internal worlds.' 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 con- 
trivances are less perfect. Can we consider the sting of the bee as 
perfect, which, when used against many kinds of enemies, cannot 
be withdrawn, owing to the backward serratures, and thus inevit- 
ably causes the death of the insect by tearing out its viscera ? 

If we look at the sting of the bee, as having existed in a remote 
progenitor, as a boring and serrated instrument, like that in so 
many members of the same great order, and that it has since been 
modified, but not perfected for its present purpose, with the 
poison originally adapted for some other object, such as to produce 
galls, since 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 social 
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 purpose, 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 to destroy the young 
queens, her daughters, as soon as they are 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 orchids and many other plants are fertilised 
through insect agency, can we consider as equally perfect the 
elaboration of dense clouds of pollen by our fir-trees, so that a few 
granules may be wafted by chance on to the ovules t 


Summary : the Law of Unity of Type and of the Conditions of 
Existence embraced by the Theory of Natural Selection. 

We have in this chapter discussed some of the difficulties and 
objections which may be urged against the theory. Many of 
them are serious ; but I think that in the discussion light has been 
thrown on several facts, which on the belief of independent acts of 
creation are utterly obscure. We have seen that species at any 
one period are not indefinitely variable, and are not linked , 
together by a multitude of intermediate gradations, partly because 
the process of natural selection is always very slow, and at any one 
time acts only on a few forms ; and partly because the very 
process of natural ' selection 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 liow cautious we should be in con- 
cluding 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 only 
glided through the air. 

We have seen that a species under new conditions of life may 
change its habits ; or it may have diversified habits, with some 
very unlike those of its nearest congeners. Hence we can under- 
stand, 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 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 inter- 
mediate or transitional states, we should be extremely cautious in 


concluding that none can have existed, for the metamorphoses of 
many organs show what wonderful changes in function are at 
least possible. For instance, a swimbladder has apparently been 
converted into an air-breathing lung. The same organ having 
performed simultaneously very different functions, and then 
having been in part or in whole specialised for one function ; and 
two 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 have seen that in two beings widely remote from each other 
in the natural scale, organs serving for the same purpose and 
in external appearance closely similar may have been separately 
and independently formed; but when such organs are closely 
examined, essential differences in their structure can almost always 
be detected; and this naturally follows from the principle of 
natural selection. On the other hand, the common rule through- 
out nature is infinite diversity of structure for gaining the same 
end; and this again naturally follows from the same great 

In many cases we are far too ignorant to be enabled to assert 
that a 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. In many other cases, 
modifications are probably the direct result of the laws of variation 
or of growth, independently of any good having been thus gained. 
But even such structures have often, as we may feel assured, 
been subsequently taken advantage of, and still further modified, 
for the good of species under new conditions of life. We may, 
also, believe that a part formerly of high importance has frequently 
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 means of 
natural selection. 

Natural selection can 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 again highly injurious to another species, but in all cases at the 
same time useful to the possessor. In each well-stocked country 
natural selection acts through the competition of the inhabitants, 
and consequently leads to success in the battle for life, only in 
accordance with the standard of that particular country. Hence 
the inhabitants of one country, generally the smaller one, often 
yield to the inhabitants of another and generally the 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 lead to 
absolute perfection ; nor, as far as we can judge by our limited 
faculties, can absolute perfection be everywhere predicated. 

On the theory of natural selection we can clearly understand 
the full meaning of that old canon in natural history, " Xatura 
non facit saltum." This canon, if we look to the present inhabi- 
tants alone of the world, is not strictly correct ; but if we include 
all those of past times, whether known or unknown, it must on 
this 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 agree- 
ment 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 past periods of time : 
the adaptations being aided in many cases by the increased use or 
disuse of parts, being affected by the direct action of the external 
conditions of life, and subjected in all cases to the several laws of 
growth and variation. Hence, in fact, the law of the Conditions 
of Existence is the higher law ; as it includes, through the inheri- 
tance of former variations and adaptations, that of Unity of Type. 



Longevity Modifications not necessarily simultaneous Modifications appa- 
rently of no direct service Progressive development Characters of small 
functional importance, the most constant Supposed incompetence of 
natural selection to account for the incipient stages of useful structures 
Causes which interfere with the acquisition through natural selection of 
useful structures Gradations of structure with changed functions Widely 
different organs in members of the same class, developed from one and the 
same source Reasons for disbelieving in great and abrupt modifications. 

I WILL devote this chapter to the consideration of various miscel- 
laneous objections which have been advanced against my views, 
as some of the previous discussions may thus be made clearer ; but 
it would be useless to discuss all of them, as many have been 


made by writers who have not taken the trouble to understand 
the subject. Thus a distinguished German naturalist has asserted 
that the weakest part of my theory is, that I consider all organic 
beings as imperfect : what I have really said is, that all are not 
as perfect as they might have been in relation to their conditions ; 
and tlu's is shown to be the case by so many native forms in many 
quarters of the world having yielded their places to intruding 
foreigners. Nor can organic beings, even if they were at any 
one time perfectly adapted to their conditions of life, have re- 
mained so, when their conditions changed, unless they themselves 
likewise changed ; and no one will dispute that the physical con- 
ditions of each country, as well as the numbers and kinds of its 
inhabitants, have undergone many mutations. 

A critic has lately insisted, with some parade of mathematical 
accuracy, that longevity is a great advantage to all species, so that 
he who believes in natural selection " must arrange his genea- 
logical tree " in such a manner that all the descendants have longer 
lives than their progenitors! Cannot our critic conceive that a 
biennial plant or one of the lower animals might range into a cold 
climate and perish there every winter ; and yet, owing to advan- 
tages gained through natural selection, survive from year to year 
by means of its seeds, or ova? Mr. E. Ray Lankester has recently 
discussed this subject, and he concludes, as far as its extreme 
complexity allows him to form a judgment, that longevity is 
generally related to the standard of each species in the scale of 
organisation, as well as to the amount of expenditure in reproduc- 
tion and in general activity. And these conditions have, it is 
probable, been largely determined through natural selection. 

It has been argued that, as none of the animals and plants of 
Egypt, of which we know anything, have changed during the last 
three or four thousand years, so probably have none in any part 
of the world. But, as Mr. G. H. Lewes has remarked, this line 
of argument proves too much, for the ancient domestic races 
figured on the Egyptian monuments, or embalmed, are closely 
similar or even identical with those now living; yet all naturalists 
admit that such races have been produced through the modifica- 
tion of their original types. The many animals which have 
remained unchanged since the commencement of the glacial period, 
would have been an incomparably stronger case, for these have 
been exposed to great changes of climate and have migrated over 
great distances ; whereas, in Egypt, during the last several thousand 
years, the conditions of life, as far as we know, ..have remained 
absolutely uniform. The fact of little or no modification having 
been effected since the glacial period would have been of some 
avail against those who believe in an innate and necessary law of 
development, but is powerless against the doctrine of natural 


selection or the survival of the fittest, which implies that when 
variations or individual differences of a beneficial nature happen 
to arise, these will be preserved ; but this will be effected only 
under certain favourable circumstances. 

The celebrated palaeontologist, Bronn, at the close of his German 
translation of this work, asks, how, on the principle of natural 
selection, can a variety live side by side with the parent species ? 
If both have become fitted for slightly different habits of life or 
conditions, they might live together ; and if we lay on one side 
polymorphic species, in which the variability seems to be of a 
peculiar nature, and all mere temporary variations, such as size, 
albinism, &c., the more permanent varieties are generally found, 
as far as I can discover, inhabiting distinct stations, such as 
high land or low land, dry or moist districts. Moreover, in the 
case of animals which wander much about and cross freely, their 
varieties seem to be generally confined to distinct regions. 

Bronn also insists that distinct species never differ from each 
other in single characters, but in many parts ; and he asks, how it 
always comes that many parts of the organisation should have 
been modified at the same time througli variation and natural 
selection ? But there is no necessity for supposing that all the 
parts of any being have been simultaneously modified. The most 
striking modifications, excellently adapted for some purpose, might, 
as was formerly remarked, be acquired by successive variations, if 
slight, first in one part and then in another ; and as they would be 
transmitted all together, they would appear to us as *if they had 
been simultaneously developed. The best answer, however, to the 
above objection is afforded by those domestic races which have 
been modified, chiefly through man's power of selection, for some 
special purpose. Look at the race and dray horse, or at the grey- 
hound and mastiff. Their whole frames and even their mental 
characteristics have been modified; but if we could trace each 
step in the history of their transformation, and the latter steps 
can be traced, we should not see great and simultaneous changes, 
but first one part and then another slightly modified and improved. 
Even when selection has been applied by man to some one 
character alone, of which our cultivated plants offer the best 
instances, it will invariably be found that although this one part, 
whether it be the flower, fruit, or leaves, has been greatly changed, 
almost all the other parts have been slightly modified. This may 
be attributed partly to the principle of correlated growth, and 
partly to so-called spontaneous variation. 

A much more serious objection has been urged by Bronn, and 
recently by Broca, namely, that many characters appear to be of 
no service whatever to their possessors, and therefore cannot have 
been influenced through natural selection. Bronn adduces the 


length of the ears and tails in the different species of hares and 
mice, the complex folds of enamel in the teeth of many animals, 
and a multitude of analogous cases. With respect to plants, this 
subject has been discussed by Nageli in an admirable essay. He 
admits that natural selection has effected much, but he insists 
that the families of plants differ chiefly from each other in mor- 
phological characters, which appear to be quite unimportant for 
the welfare of the species. He consequently believes in an innate 
tendency towards progressive and more perfect development. He 
specifies the arrangement of the cells in the tissues, and of the 
leaves on the axis, as cases in which natural selection could not 
have acted. To these may be added the numerical divisions in 
the parts of the flower, the position of the ovules, the shape of the 
seed, when not of any use for dissemination, &c. 

There is much force in the above objection. Nevertheless, we 
ought, in the first place, to be extremely cautious in pretending 
to decide what structures now are, or have formerly been, of use 
to each species. In the second place, it should always be borne 
in mind that when one part is modified, so will be other parts, 
through certain dimly seen causes, such as an increased or dimin- 
ished flow of nutriment to a part, mutual pressure, an early deve- 
loped part affecting one subsequently developed, and so forth, as 
well as through other causes which lead to the many mysterious 
cases of correlation, which we do not in the least understand. 
These agencies may be all grouped together, for the sake of brevity, 
under the expression of the laws of growth. In the third place, 
we have to allow for the direct and definite action of changed con- 
ditions of life, and for so-called spontaneous variations, in which 
the nature of the conditions apparently plays a quite subordinate 
part. Bud-variations, such as the appearance of a moss-rose on a 
common rose, or of a nectarine on a peach-tree, offer good instances 
of spontaneous variations ; but even in these cases, if we bear in 
mind the power of a minute drop of poison in producing complex 
galls, we ought not to feel too sure that the above variations are 
not the effect of some local change in the nature of the sap, due 
to some change in the conditions. There must be some efficient 
cause for each slight individual difference, as well as for more 
strongly marked variations which occasionally arise; and if the 
unknown cause were to act persistently, it is almost certain that 
all the individuals of the species would be similarly modified. 

In the earlier editions of this work I under-rated, as it now 
seems probable, the frequency and importance of modifications 
due to spontaneous variability. But it is impossible to attribute 
to this cause the innumerable structures which are so well adapted 
to the habits of life of each species. I can no more believe in this, 
than that the well-adapted form of a race-horse or greyhound, 


which before the principle of selection by man was well under- 
stood, excited so much surprise in the minds of the older naturalists, 
can thus be explained. 

It may be worth while to illustrate some of the foregoing remarks. 
With respect to the assumed inutility of various parts and organs, 
it is hardly necessary to observe that even in the higher and best- 
known animals many structures exist, which are so highly deve- 
loped that no one doubts that they are of importance, yet their 
use has not been, or has only recently been, ascertained. As Bronn 
gives the length of the ears and tail in the several species of mice 
as instances, though trifling ones, of differences in structure which 
can be of no special use, I may mention that, according to Dr. 
Schiibl, the external ears of the common mouse are supplied in 
an extraordinary manner with nerves, so that they no doubt serve 
as tactile organs ; hence the length of the ears can hardly be quite 
unimportant. We shall, also, presently see that the tail is a 
highly useful prehensile organ to some of the species; and its 
use would be much influenced by its length. 

With respect to plants, to which on account of Xageli's essay I 
shall confine myself in the following remarks, it will be admitted 
thrt the flowers of orchids present a multitude of curious structures, 
which a few years ago would have been considered as mere morpho- 
logical differences without any special function ; but they are now 
known to be of the highest importance for the fertilisation of the 
species through the aid of insects, and have probably been gained 
through natural selection. No one until lately would have 
imagined that in dimorphic and trimorphic plants the different 
lengths of the stamens and pistils, and their arrangement, could 
have been of any service, but now we know this to be the case. 

In certain whole groups of plants the ovules stand erect, and in 
others they are suspended ; and within the same ovarium of some 
few plants, one ovule holds the former and a second ovule the 
latter position. These positions seem at first purely morphological, 
or of no physiological signification ; but Dr. Hooker informs me that 
within the same ovarium, the upper ovules alone in some cases, and 
in other cases the lower ones alone are fertilised ; and he suggests 
that this probably depends on the direction in which the pollen- 
tubes enter the ovarium. If so, the position of the ovules, even 
when one is erect and the other suspended within the same 
ovarium, would follow from the selection of any slight deviations 
in position which favoured their fertilisation, and the production 
of seed. 

Several plants belonging to distinct orders habitually produce 
flowers of two kinds, the one open of the ordinary structure, the 
other closed and imperfect. These two kinds of flowers sometimes 
differ wonderfully in structure, yet may be seen to graduate into 


each other on the same plant. The ordinary and open flowers can 
be intercrossed ; and the benefits which certainly are derived from 
this process are thus secured. The closed and imperfect flowers 
are, however, manifestly of high importance, as they yield with 
the utmost safety a large stock of seed, with the expenditure of 
wonderfully little pollen. The two kinds of flowers often differ 
much, as just stated, in structure. The petals in the imperfect 
flowers almost always consist of mere rudiments, and the pollen- 
grains are reduced in diameter. In Ononis columnar five of the 
alternate stamens are rudimentary; and in some species of Viola 
three stamens are in this state, two retaining their proper function, 
but being of very small size. In six out of thirty of the closed 
flowers in an Indian violet (name unknown, for the plants have 
never produced with me perfect flowers), the sepals are reduced 
from the normal number of five to three. In one section of the 
Malpighiaceae the closed flowers, according to A. de Jussieu, are 
still further modified, for the five stamens which stand opposite to 
the sepals are all aborted, a sixth stamen standing opposite to a petal 
being alone developed ; and this stamen is not present in the ordi- 
nary flowers of these species ; the style is aborted ; and the ovaria 
are reduced from three to two. Now although natural selection 
may well have had the power to prevent some of the flowers from 
expanding, and to reduce the amount of pollen, when rendered by 
the closure of the flowers superfluous, yet hardly any of the above 
special modifications can have been thus determined, but must 
have followed from the laws of growth, including the functional 
inactivity of parts, during the progress of the reduction of the 
pollen and the closure of the flowers. 

It is so necessary to appreciate the important effects of the laws 
of growth, that I will give some additional cases of another kind, 
namely of differences in the same part or organ, due to differences 
in relative position on the same plant. In the Spanish chestnut, 
and in certain fir-trees, the angles of divergence of the leaves differ, 
according to Schacht, in the nearly horizontal and in the upright 
branches. In the common rue and some other plants, one flower, 
usually the central or terminal one, opens first, and has five sepals 
and petals, and five divisions to the ovarium ; whilst all the other 
flowers on the plant are tetramerous. In the British Adoxa the 
uppermost flower generally has two calyx-lobes with the other 
organs tetramerous, whilst the surrounding flowers generally have 
three calyx-lobes with the other organs pentamerous. In many 
Compositae and Umbelliferse (and in some other plants) the cir- 
cumferential flowers have their corollas much more developed 
than those of the centre ; and this seems often connected with the 
abortion of the reproductive organs. It is a more curious fact, 
previously referred to, that the achenes or seeds of the circum- 


ference and centre sometimes differ greatly in form, colour, and 
other characters. In Carthamus and some other Composite the 
central achenes alone are furnished with a pappus ; and in Hyoseris 
the same head yields achenes of three different forms. In certain 
Umbelliferee the exterior seeds, according to Tausch, are ortho- 
spermous, and the central one ccelospermous, and this is a character 
which was considered by De Candolle to be in other species of the 
highest systematic importance. Prof. Braun mentions a Fumari- 
aceous genus, in which the flowers in the lower part of the spike 
bear oval, ribbed, one-seeded nutlets; and in the upper part of 
the spike, lanceolate, two-valved, and two-seeded siliques. In 
these several cases, with the exception of that of the well developed 
ray-florets, which are of service in making the flowers conspicuous 
to insects, natural selection cannot, as far as we can judge, have 
come into play, or only in a quite subordinate manner. All these 
modifications follow from the relative position and inter-action of 
the parts ; and it can hardly be doubted that if all the flowers and 
leaves on the same plant had been subjected to the same external 
and internal condition, as are the flowers and leaves in certain 
positions, all would have been modified in the same manner. 

In numerous other cases we find modifications of structure, 
which are considered by botanists to be generally of a highly 
important nature, affecting only some of the flowers on the same 
plant, or occurring on distinct plants, which grow close together 
under the same conditions. As these variations seem of no special 
use to the plants, they cannot have been influenced by natural 
selection. Of their cause we are quite ignorant ; we cannot even 
attribute them, as in the last class of cases, to any proximate 
agency, such as relative position. I will give only a few instances. 
It is so common to observe on the same plant, flowers indifferently 
tetramerous, pentamerous, &c., that I need not give examples; 
but as numerical variations are comparatively rare when the parts 
are few, I may mention that, according to De Candolle, the flowers 
of Papaver bracteatum offer either two sepals with four petals 
(which is the common type with poppies), or three sepals with six 
petals. The manner in which the petals are folded in the bud is 
in most groups a very constant morphological character; but 
Professor Asa Gray states that with some species of Mimulus, 
the aestivation is almost as frequently that of the Rhinanthidese 
as of the Antirrhinidese, to which latter tribe the genus belongs. 
Aug. St. Hilaire gives the following cases : the genus Zanthoxylon 
belongs to a division of the Piutaceae with a single ovary, but in 
some species flowers may be found on the same plant, and even 
in the same panicle, with either one or two ovaries. In Helian- 
themum the capsule has been described as unilocular or 3-locular ; 
and in H. mutabile, "Une lame, 2^ us ou moins large, s'etend 


entre le pericarpe et le placenta." In the flowers of Saponaria 
officinalis, Dr. Masters has observed instances of both marginal 
and free central placentation. Lastly, St. Hilaire found towards 
the southern extreme of the range of Gomphia oleseformis two 
forms which he did not at first doubt were distinct species, but h\ 
subsequently saw them growing on the same bush ; and he then 
adds, " Voilk done dans un meme individu des loges et un style 
qui se rattachent tantot a un axe verticals et tantot a un 

We thus see that with plants many morphological changes may 
be attributed to the laws of growth and the inter-action of parts, 
independently of natural selection. But with respect to Nageli's 
doctrine of an innate tendency towards perfection or progressive 
development, can it be said in the case of these strongly pro- 
nounced variations, that the plants have been caught in the act of 
progressing towards a higher state of development ? On the con- 
trary, I should infer from the mere fact of the parts in question 
differing or varying greatly on the same plant, that such modifica- 
tions were of extremely small importance to the plants themselves, 
of whatever importance they may generally be to us for our 
classifications. The acquisition of a useless part can hardly be 
said to raise an organism in the natural scale ; and in the case of 
the imperfect, closed flowers above described, if any new principle 
has to be invoked, it must be one of retrogression rather than of 
progression ; and so it must be with many parasitic and degraded 
animals. We are ignorant of the exciting cause of the above 
specified modifications ; but if the unknown cause were to act 
almost uniformly for a length of time, we may infer that the result 
would be almost uniform ; and in this case all the individuals of 
the species would be modified in the same manner. 

From the fact of the above characters being unimportant for 
the welfare of the species, any slight variations which occurred in 
them would not have been accumulated and augmented through 
natural selection. A structure which has been developed through 
long-continued selection, when it ceases to be of service to a 
species, generally becomes variable, as we see with rudimentary 
organs ; for it will no longer be regulated by this same power of 
selection. But when, from the nature of the organism and of the 
conditions, modifications have been induced which are unim- 
portant for the welfare of the species, they may be, and apparently 
often have been, transmitted in nearly the same state to numerous, 
otherwise modified, descendants. It cannot have been of much 
importance to the greater number of mammals, birds, or reptiles, 
whether they were clothed with hair, feathers, or scales ; yet hair 
has been transmitted to almost all mammals, feathers to all birds, 
and scales to all true reptiles. A structure, whatever it may be, 


which is common to many allied forms, is ranked by us as of high 
systematic importance, and consequently is often assumed to be 
of high vital importance to the species. Thus, as I am inclined to 
believe, morphological differences, which we consider as important 
such as the arrangement of the leaves, the divisions of the 
flower or of the ovarium, the position of the ovules, &c. first 
appeared in many cases as fluctuating variations, which sooner or 
later became constant through the nature of the organism and of 
the surrounding conditions, as well as through the intercrossing 
of distinct individuals, but not through natural selection ; for as 
these morphological diameters do not affect the welfare of the 
species, any slight deviations in them could not have been 
governed or accumulated through this latter agency. It is a 
strange result which we thus arrive at, namely that characters of 
slight vital importance to the species, are the most important to 
the systematist ; but, as we shall hereafter see when we treat of 
the genetic principle of classification, this is by no means so 
paradoxical as it may at first appear. 

Although we have no good evidence of the existence in organic 
beings of an innate tendency towards progressive development, 
yet this necessarily follows, as I have attempted to show in the 
fourth chapter, through the continued action of natural selection. 
For the best definition which has ever been given of a high 
standard of organisation, is the degree to which the parts have 
been specialised or differentiated ; and natural selection tends 
towards this end, inasmuch as the parts are thus enabled to per- 
form their functions more efficiently. 

A distinguished zoologist, Mr. St. George Mivart, has recently 
collected all the objections which have ever been advanced by 
myself and others against the theory of natural selection, as pro- 
pounded by Mr. Wallace and myself, and has illustrated them 
with admirable art and force. When thus marshalled, they make 
a formidable array; and as it forms no part of Mr. Mivart's plan 
to give the various facts and considerations opposed to his conclu- 
sions, no slight effort of reason and memory is left to the reader, 
who may wish to weigh the evidence on both sides. When dis- 
cussing special cases, Mr. Mivart passes over the effects of the 
increased use and disuse of parts, which I have always maintained 
to be highly important, and have treated in my ' Variation under 
Domestication ' at greater length than, as I believe, any other 
writer. He likewise often assumes that I attribute nothing to 
variation, independently of natural selection, whereas in the Avork 
just referred to I have collected a greater number of well- 
established cases than can be found in any other work known to 
me. My judgment may not be trustworthy, but after reading 


with care Mr. Mivart's book, and comparing each section with 
what I have said on the same head, I never before felt so strongly 
convinced of the general truth of the conclusions here arrived at, 
subject, of course, in so intricate a subject, to much partial error. 

All Mr. Mivart's objections will be, or have been, considered in 
the present volume. The one new point which appears to have 
struck many readers is, " that natural selection is incompetent to 
account for the incipient stages of useful structures." This subject 
is intimately connected with that of the gradation of characters, 
often accompanied by a change of function, for instance, the 
conversion of a swim-bladder into lungs, points which were 
discussed in the last chapter under two headings. Nevertheless, 
I will here consider in some detail several of the cases advanced 
by Mr. Mivart, selecting those which are the most illustrative, as 
want of space prevents me from considering all. 

The giraffe, by its lofty stature, much elongated neck, fore-legs, 
head and tongue, has its whole frame beautifully adapted for 
browsing on the higher branches of trees. It can thus obtain food 
beyond the reach of the other Ungulata or hoofed animals 
inhabiting the same country ; and this must be a great advantage 
to it during dearths. The Niata cattle in S. America show us how 
small a difference in structure may make, during such periods, a 
great difference in preserving an animal's life. These cattle can 
browse as well as others on grass, but from the projection of the 
lower jaw they cannot, during the often recurrent droughts, browse 
on the twigs of trees, reeds, tkc., to which food the common cattle 
and horses are then driven ; so that at these times the Niatas 
perish, if not fed by their owners. Before coming to Mr. Mivart's 
objections, it may be well to explain once again how natural 
selection will act in all ordinary cases. Man has modified some of 
his animals, without necessarily having attended to special points 
of structure, by simply preserving and breeding from the fleetest 
individuals, as with the race-horse and greyhound, or as with the 
game-cock, by breeding from the victorious birds. So under 
nature with the nascent giraffe, the individuals which were the 
highest browsers and were able during dearths to reach even an 
inch or two above the others, will often have been preserved ; for 
they will have roamed over the whole country in search of food. 
That the individuals of the same species often differ slightly in the 
relative lengths of all their parts may be seen in many works of 
natural history, in which careful measurements are given. These 
slight proportional differences, due to the laws of growth and 
variation, are not of the slightest use or importance to most 
species. But it will have been otherwise with the nascent giraffe, 
considering its probable habits of life ; for those individuals 
which had some one part or several wrts of their bodies rather 


more elongated than usual, would generally have survived. These 
will have intercrossed and left offspring, either inheriting the same 
bodily peculiarities, or with a tendency to vary again in the same 
manner; whilst the individuals, less favoured in the same respects, 
will have been the most liable to perish. 

We here see that there is no need to separate single pairs, as 
man does, when he methodically improves a breed : natural 
selection will preserve and thus separate all the superior indi- 
viduals, allowing them freely to intercross, and will destroy all 
the inferior individuals. By this process long-continued, which 
exactly corresponds with what I have called unconscious selection 
by man, combined no doubt in a most important manner with the 
inherited effects of the increased use of parts, it seems to me 
almost certain that an ordinary hoofed quadruped might be 
converted into a giraffe. 

To this conclusion Mr. Mivart brings forward two objections. 
One is that the increased size of the body would obviously require 
an increased supply of food, and he considers it as "very problem- 
atical whether the disadvantages thence arising would not, in 
times of scarcity, more than counterbalance the advantages." But 
as the giraffe does actually exist in large numbers in S. Africa, and 
as some of the largest antelopes in the world, taller than an ox, 
abound there, why should we doubt that, as far as size is con- 
cerned, intermediate gradations could formerly have existed there, 
subjected as now to severe dearths. Assuredly the being able to 
reach, at each stage of increased size, to a supply of food, left 
untouched by the other hoofed quadrupeds of the country, would 
have been of some advantage to the nascent giraffe. Nor must we 
overlook the fact, that increased bulk would act as a protection 
against almost all beasts of prey excepting the lion ; and against 
this animal, its tall neck, and the taller the better, would, as 
Mr. Chauncey Wright has remarked, serve as a watch-tower. It 
is from this cause, as Sir S. Baker remarks, that no animal is 7nore 
difficult to stalk than the giraffe. This animal also uses its long 
neck as a means of offence or defence, by violently swinging its 
head armed with stump-like horns. The preservation of each 
'species can rarely be determined by any one advantage, but by the 
union of all, great and small. 

Mr. Mivart then asks (and this is his second objection), if 
natural selection be so potent, and if high browsing be so great 
an advantage, why has not any other hoofed quadruped acquired 
a long neck and lofty stature, besides the giraffe, and, in a lesser 
degree, the camel, guanaco, and macrauchenia ? Or, again, why 
has not any member of the group acquired a long proboscis ? 
With respect to S. Africa, which was formerly inhabited by 
numerous herds of the giraffe, the answer is not difficult, and can 


best be given by an illustration. In every meadow in England in 
which trees grow, we see the lower branches trimmed or planed 
to an exact level by the browsing of the horses or cattle ; and what 
advantage would it be, for instance, to sheep, if kept there, to 
acquire slightly longer necks 1 In every district some one kind of 
animal will almost certainly be able to browse higher than the 
others ; and it is almost equally certain that this one kind alone 
could have its neck elongated for this purpose, through natural 
selection and the effects of increased use. In S. Africa the com- 
petition for browsing on the higher branches of the acacias and 
other trees must be between giraffe and giraffe, and not with the 
other ungulate animals. 

Why, in other quarters of the world, various animals belonging 
to this same order have not acquired either an elongated neck or 
a proboscis, cannot be distinctly answered ; but it is as unreason- 
able to expect a distinct answer to such a question, as why some 
event in the history of mankind did not occur in one country, 
whilst it did in another. We are ignorant with respect to the 
conditions which determine the numbers and range of each 
species ; and we cannot even conjecture what changes of structure 
would be favourable to its increase in some new country. We 
can, however, see in a general manner that various causes might 
have interfered with the development of a long neck or proboscis. 
To reach the foliage of a considerable height (without climbing, 
for which hoofed animals are singularly ill-constructed) implies 
greatly increased bulk of body ; and we know that some areas 
support singularly few large quadrupeds, for instance S. America, 
though it is so luxuriant ; whilst S. Africa abounds with them to 
an unparalleled degree. Why this should be so, we do not know ; 
nor why the later tertiary periods should have been much more 
favourable for their existence than the present time. Whatever 
the causes may have been, we can see that certain districts and 
times would have been much more favourable than others for the 
development of so large a quadruped as the giraffe. 

In order that an animal should acquire some structure specially 
and largely developed, it is almost indispensable that several other 
parts should be modified and co-adapted. Although every part of 
the body varies slightly, it does not follow that the necessary parts 
should always vary in the right direction and to the right degree. 
With the different species of our domesticated animals we know 
that the parts vary in a different manner and degree ; and that 
some species are much more variable than others. Even if the 
fitting variations did arise, it does not follow that natural selection 
would be able to act on them, and produce a structure which 
apparently would be beneficial to the species. For instance, if the 
number of individual? existing in a country is determined chiefly 


through destruction by beasts of prey, by external or internal 
parasites, etc., as seems often to be the case, then natural 
selection will be able to do little, or will be greatly retarded, in 
modifying any particular structure for obtaining food. Lastly, 
natural selection is a slow process, and the same favourable con- 
ditions must long endure in order that any marked effect should 
thus be produced. Except by assigning such general and vague 
reasons, we cannot explain why, in many quarters of the world, 
hoofed quadrupeds have not acquired much elongated necks or 
other means for browsing on the higher branches of trees. 

Objections of the same nature as the foregoing have been 
advanced by many writers. In each case various causes, besides 
the general ones just indicated, have probably interfered with the 
acquisition through natural selection of structures, which it is 
thought would be beneficial to certain species. One writer asks, 
why has not the ostrich acquired the power of flight 1 ? But a 
moment's reflection will show what an enormous supply of food 
would be necessary to give to this bird of the desert force to move 
its huge body through the air. Oceanic islands are inhabited by 
bats and seals, but by no terrestrial mammals; yet as some of 
these bats are peculiar species, they must have long inhabited 
their present homes. Therefore Sir C. Lyell asks, and assigns 
certain reasons in answer, why have not seals and bats given birth 
on such islands to forms fitted to live on the land? But seals 
would necessarily be first converted into terrestrial carnivorous 
animals of considerable size, and bats into terrestrial insectivorous 
animals; for the former there would be no prey; for the bats 
ground-insects would serve as food, but these would already be 
largely preyed on by the reptiles or birds, which first colonise and 
abound on most oceanic islands. Gradations of structure, with 
each stage beneficial to a changing species, will be favoured only 
under certain peculiar conditions. A strictly terrestrial animal, 
by occasionally hunting for food in shallow water, then in streams 
or lakes, might at last be converted into an animal so thoroughly 
aquatic as to brave the open ocean. But seals would not find 
on oceanic islands the conditions favourable to their gradual 
reconversion into a terrestrial form. Bats, as formerly shown, 
probably acquired their wings by at first gliding through the air 
from tree to tree, like the so-called flying squirrels, for the sake of 
escaping from their enemies, or for avoiding falls ; but when the 
power of true flight had once been acquired, it would never be 
reconverted back, at least for the above purposes, into the less 
efficient power of gliding through the air. Bats might, indeed, 
like many birds, have had their wings greatly reduced in size, or 
completely lost, through disuse; but in this case it would be 
necessary that they should first have acquired the power of 


running quickly on the ground, by the aid of their hind legs 
alone, so as to compete with birds or other ground animals; 
and for such a change a bat seems singularly ill-fitted. These 
conjectural remarks have been made merely to show that a 
transition of structure, with each step beneficial, is a highly 
complex affair ; and that there is nothing strange in a transition 
not having occurred in any particular case. 

Lastly, more than one writer has asked, why have some animals 
had their mental powers more highly developed than others, as 
such development would be advantageous to all ? Why have not 
apes acquired the intellectual powers of man? Various causes 
could be assigned ; but as they are conjectural, and their relative 
probability cannot be weighed, it would be useless to give them. 
A definite answer to the latter question ought not to be expected, 
seeing that no one can solve the simpler problem why, of two 
races of savages, one has risen higher in the scale of civilisation 
than the other ; and this apparently implies increased brain-power. 

We will return to Mr. Mivart's other objections. Insects often 
resemble for the sake of protection various objects, such as green 
or decayed leaves, dead twigs, bits of lichen, flowers, spines, 
excrement of birds, and living insects ; but to this latter point I 
shall hereafter recur. The resemblance is often wonderfully close, 
and is not confined to colour, but extends to form, and even to 
the manner in which the insects hold themselves. The caterpillars 
which project motionless like dead twigs from the bushes on 
which they feed, offer an excellent instance of a resemblance 
of this kind. The cases of the imitation of such objects as the 
excrement of birds, are rare and exceptional. On this head, Mr. 
Mivart remarks, " As, according to Mr. Darwin's theory, there is 
a constant tendency to indefinite variation, and as the minute 
incipient variations will be in all directions, they must tend 
to neutralize each other, and at first to form such unstable 
modifications that it is difficult, if not impossible, to see how such 
indefinite oscillations of infinitesimal beginnings can ever build 
up a sufficiently appreciable resemblance to a leaf, bamboo, or 
other object, for Natural Selection to seize upon and perpetuate." 

But in all the foregoing cases the insects in their original state 
no doubt presented some rude and accidental resemblance to an 
object commonly found in the stations frequented by them. Nor 
is this at all improbable, considering the almost infinite number 
of surrounding objects and the diversity in form and colour of 
the hosts of insects which exist. As some rude resemblance is 
necessary for the first start, we can understand how it is that the 
larger and higher animals do not (with the exception, as far as 
I know, of one fish) resemble for the sake of protection special 
objects, but only the surface which commonly surrounds them, 


and this chiefly in colour. Assuming that an insect originally 
happened to resemble in some degree a dead twig or a decayed 
leaf, and that it varied slightly in many ways, then all the 
variations which rendered the insect at all more like any such 
object, and thus favoured its escape, would be preserved, whilst 
other variations would be neglected and ultimately lost; or, if 
they rendered the insect at all less like the imitated object, 
they would be eliminated. There would indeed be force in Mr. 
Mivart's objection, if we were to attempt to account for the above 
resemblances, independently of natural selection, through mere 
fluctuating variability ; but as the case stands there is none. 

Xor can I see any force in Mr. Mivart's difficulty with respect 
to "the last touches of perfection in the mimicry;" as in the 
case given by Mr. Wallace, of a walking-stick insect (Ceroxylus 
laceratus), which resembles "a stick grown over by a creeping 
moss or jungermannia." So close was this resemblance, that a 
native Dyak maintained that the foliaceous excrescences were 
really moss. Insects are preyed on by birds and other enemies, 
whose sight is probably sharper than ours, and every grade in 
resemblance which aided an insect to escape notice or detection, 
would tend towards its preservation; and the more perfect the 
resemblance so much the better for the insect. Considering the 
nature of the differences between the species in the group which 
includes the above Ceroxylus, there is nothing improbable in this 
insect having varied in the irregularities on its surface, and in 
these having become more or less green-coloured ; for in every 
group the characters which differ in the several species are the 
most apt to vary, whilst the generic characters, or those common 
to all the species, are the most constant. 

The Greenland whale is one of the most wonderful animals in 
the world, and the baleen, or whale-bone, one of its greatest 
peculiarities. The baleen consists of a row, on each side, of the 
upper jaw, of about 300 plates or laminae, which stand close 
together transversely to the longer axis of the mouth. Within the 
main row there are some subsidiary rows. The extremities and 
inner margins of all the plates are frayed into stiff bristles, which 
clothe the whole gigantic palate, and serve to strain or sift the 
water, and thus to secure the minute prey on which these great 
animals subsist. The middle and longest lamina in the Greenland 
whale is ten, twelve, or even fifteen feet in length ; but in the 
different species of Cetaceans there are gradations in length ; the 
middle lamina being in one species, according to Scoresby, four 
feet, in another three, in another eighteen inches, and in the 
Balaenoptera rostrata only about nine inches in length. The 
quality of the whale-bone also differs in the different species. 


With respect to the baleen, Mr. Mivart remarks that if it " had 
once attained such a size and development as to be at all useful, 
then its preservation and augmentation within serviceable limits 
would be promoted by natural selection alone. But how to obtain 
the beginning of such useful development 1 " In answer, it may 
be asked, why should not the early progenitors of the whales with 
baleen have possessed a mouth constructed something like the 
lamellated beak of a duck ? Ducks, like whales, subsist by sifting 
the mud and water ; and the family has sometimes been called 
Criblatores, or sifters. I hope that I may not be misconstrued 
into saying that the progenitors of whales did actually possess 
mouths lamellated like the beak of a duck. I wish only to show 
that this is not incredible, and that the immense plates of baleen 
in the Greenland whale might have been developed from such 
lamellae by finely graduated steps, each of service to its possessor. 

The beak of a shoveller-duck (Spatula clypeata) is a more 
beautiful and complex structure than the mouth of a whale. The 
upper mandible is furnished on each side (in the specimen 
examined by me) with a row or comb formed of 188 thin, elastic 
lamellae, obliquely bevelled so as to be pointed, and placed trans- 
versely to the longer axis of the mouth. They arise from the 
palate, and are attached by flexible membrane to the sides of the 
mandible. Those standing towards the middle are the longest, 
being about one-third of an inch in length, and they project '14 
of an inch beneath the edge. At their bases there is a short 
subsidiary row of obliquely transverse lamellae. In these several 
respects they resemble the plates of baleen in the mouth of a 
whale. But towards the extremity of the beak they differ much, 
as they project inwards, instead of straight downwards. The 
entire head of the shoveller, though incomparably less bulky, is 
about one-eighteenth of the length of the head of a moderately 
large Balaenoptera rostrata, in which species the baleen is only 
nine inches long ; so that if we were to make the head of the 
shoveller as long as that of the Balaenoptera, the lamellae would 
be six inches in length, that is, two-thirds of the length of the 
baleen in this species of whale. The lower mandible of the 
shoveller-duck is furnished with lamellae of equal length with 
those above, but finer ; and in being thus furnished it differs 
conspicuously from the lower jaw of a whale, which is destitute 
of baleen. On the other hand, the extremities of these lower 
lamellae are frayed into fine bristly points, so that they thus 
curiously resemble the plates of baleen. In the genus Prion, a 
member of the distinct family of the Petrels, the upper mandible 
alone is furnished with lamellae, which are well developed and 
project beneath the margin ; so that the beak of this bird resem- 
bles in this respect the inouth of a whale. 


From the highly developed structure of the shoveller's beak we 
may proceed (as I have learnt from information and specimens 
sent to me by Mr. Salvin), without any great break, as far as 
fitness for sifting is concerned, through the beak of the Merganetta 
armata, and in some respects through that of the Aix sponsa, to 
the beak of the common duck. In this latter species, the lamellae 
are much coarser than in the shoveller, and are firmly attached to 
the sides of the mandible ; they are only about 50 in number on 
each side, and do not project at all beneath the margin. They are 
square-topped, and are edged with translucent hardish tissue, as if 
for crushing food. The edges of the lower mandible are crossed 
by numerous fine ridges, which project very little. Although the 
beak is thus very inferior as a sifter to that of the shoveller, yet 
this bird, as every one knows, constantly uses it for this purpose. 
There are other species, as I hear from Mr. Salvin, in which the 
lamellae are considerably less developed than in the common 
duck ; but I do not know whether they use their beaks for sifting 
the water. 

Turning to another group of the same family. In the Egyptian 
goose (Chenalopex) the beak closely resembles that of the common 
duck ; but the lamellae are not so numerous, nor so distinct from 
each other, nor do they project so much inwards ; yet this goose, 
as I am informed by Mr. E. Bartlett, "uses its bill like a duck by 
throwing the waters out at the corners." Its chief food, however, 
is grass, which it crops like the common goose. In this latter bird, 
the lamellae of the upper mandible are much coarser than in the 
common duck, almost confluent, about 27 in number on each side, 
and terminating upwards in teeth-like knobs. The palate is also 
covered with hard rounded knobs. The edges of the lower 
mandible are serrated with teeth much more prominent, coarser, 
and sharper than in the duck. The common goose does not sift 
the water, but uses its beak exclusively for tearing or cutting 
herbage, for which purpose it is so well fitted, that it can crop 
grass closer than almost any other animal. There are othe 
species of geese, as I hear from Mr. Bartlett, in which the lamella 
are less developed than in the common goose. 

We thus see that a member of the duck family, with a beat 
constructed like that of the common goose and adapted solely foi 
grazing, or even a member with a beak having less well-developec 
lamellae, might be converted by small changes into a species lik 
the Egyptian goose, this into one like the common duck, and 
lastly, into one like the shoveller, provided with a beak almos 
exclusively adapted for sifting the water ; for this bird couk 
hardly use any part of its beak, except the hooked tip, for seizing 
or tearing solid food. The beak of a goose, as I may add, mighi 
also be converted by small changes into one provided with 


prominent, recurved teeth, like those of the Merganser (a member 
of the same family), serving for the widely different purpose ot 
securing live fish. 

Returning to the whales. The Hyperoodon bidens is destitute 
of true teeth in an efficient condition, but its palate is roughened, 
according to Lacepede, with small, unequal, hard points of horn. 
There is, therefore, nothing improbable in supposing that some 
early Cetacean form was provided with similar points of horn on 
the palate, but rather more regularly placed, and which, like the 
knobs on the beak of the goose, aided it in seizing or tearing its 
food. If so, it will hardly be denied that the points might have 
been converted through variation and natural selection into lamellae 
as well-developed as those of the Egyptian goose, in which case 
they would have been used both for seizing objects and for sifting 
the water ; then into lamellae like those of the domestic duck ; and 
so onwards, until they became as well constructed as those of the 
shoveller, in which case they would have served exclusively as a 
sifting apparatus. From this stage, in which the lamellae would 
be two-thirds of the length of the plates of baleen in the Balae- 
noptera rostrata, gradations, which may be observed in still-existing 
Cetaceans, lead us onwards to the enormous plates of baleen in 
the Greenland whale. Nor is there the least reason to doubt that 
each step in this scale might have been as serviceable to certain 
ancient Cetaceans, with the functions of the parts slowly changing 
during the progress of development, as are the gradations in the 
beaks of the different existing members of the duck-family. We 
should bear in mind that each species of duck is subjected to a 
severe struggle for existence, and that the structure of every part 
of its frame must be well adapted to its conditions of life. 

The Pleuronectidas, or Flat-fish, are remarkable for their asym- 
metrical bodies. They rest on one side, in the greater number 
of species on the left, but in some on the right side; and occa- 
sionally reversed adult specimens occur. The lower, or resting- 
surface, resembles at first sight the ventral surface of an ordinary 
fish : it is of a white colour, less developed in many ways than the 
upper side, with the lateral fins often of smaller size. But the 
eyes offer the most remarkable peculiarity; for they are both 
placed on the upper side of the head. During early youth, how- 
ever, they stand opposite to each other, and the whole body is 
then symmetrical, with both sides equally coloured. Soon the eye 
proper to the lower side begins to glide slowly round the head to 
the upper side ; but does not pass right through the skull, as was 
formerly thought to be the case. It is obvious that unless the 
lower eye did thus travel round, it could not be used by the fish 
whilst lying in its habitual position on one side. The lower eye 
would, also, have been liable to be abraded by the sandy bottom. 


That the Pleuronectidoe are admirably adapted by their flattened 
and asymmetrical structure for their habits of life, is manifest 
from several species, such as soles, flounders, &c., being extremely 
common. The chief advantages thus gained seem to be protection 
from their enemies, and facility for feeding on the ground. The 
different members, however, of the family present, as Schiodte 
remarks, "a long series of forms exhibiting a gradual transition 
from Hippoglossus pinguis, which does not in any considerable 
degree alter the shape in which it leaves the ovum, to the soles, 
which are entirely thrown to one side." 

Mr. Mivart has taken up this case, and remarks that a sudden 
spontaneous transformation in the position of the eyes is hardly 
conceivable, in which I quite agree with him. He then adds : " if 
the transit was gradual, then how such transit of one eye a minute 
fraction of the journey towards the other side of the head could 
benefit the individual is, indeed, far from clear. It seems, even, 
that such an incipient transformation must rather have been 
injurious." But he might have found an answer to this objection 
in the excellent observations published in 1867 by Malm. The 
Pleuronectidse, whilst very young and still symmetrical, with their 
eyes standing on opposite sides of the head, cannot long retain a 
vertical position, owing to the excessive depth of their bodies, the 
small size of their lateral fins, and to their being destitute of a 
swimbladder. Hence soon growing tired, they fall to the bottom 
on one side. Whilst thus at rest they often twist, as Malm 
observed, the lower eye upwards, to see above them ; and they do 
this so vigorously that the eye is pressed hard against the upper 
part of the orbit. The forehead between the eyes consequently 
becomes, as could be plainly seen, temporarily contracted in 
breadth. On one occasion Malm saw a young fish raise and 
depress the lower eye through an angular distance of about seventy 

We should remember that the skull at this early age is cartila- 
ginous and flexible, so that it readily yields to muscular action. 
It is also known with the higher animals, even after early youth, 
that the skull yields and is altered in shape, if the skin or muscles 
be permanently contracted through disease or some accident. 
With long-eared rabbits, if one ear lops forwards and downwards, 
its weight drags forward all the bones of the skull on the same 
side, of which I have given a figure. Malm states that the newly- 
hatched young of perches, salmon, and several other symmetrical 
fishes, have the habit of occasionally resting on one side at the 
bottom; and he has observed that they often then strain their 
lower eyes so as to look upwards; and their skulls are thus 
rendered rather crooked. These fishes, however, are soon able to 
hold themselves in a vertical position, and no permanent eflect is 


thus produced. With the Pleuronectidae, on the other hand, the 
older they grow the more habitually they rest on one side, owing 
to the increasing flatness of their bodies, and a permanent effect is> 
thus produced on the form of the head, and on the position of the 
eyes. Judging from analogy, the tendency to distortion would no 
doubt be increased through the principle of inheritance. Schiodte 
believes, in opposition to some other naturalists, that the Pleuro- 
nectidae are not quite symmetrical even in the embryo ; and if this 
be so, we could understand how it is that certain species, whilst 
young, habitually fall over and rest on the left side, and other 
species on the right side. Malm adds, in confirmation of the above 
view, that the adult Trachypterus arcticus, which is not a member 
of the Pleuronectidse, rests on its left side at the bottom, and swims 
diagonally through the water ; and in this fish, the two sides of 
the head are said to be somewhat dissimilar. Our great authority 
on Fishes, Dr. Giinther, concludes his abstract of Malm's paper, by 
remarking that " the author gives a very simple explanation of the 
abnormal condition of the Pleuronectoids." 

We thus see that the first stages of the transit of the eye from 
one side of the head to the other, which Mr. Mivart considers 
would be injurious, may be attributed to the habit, no doubt 
beneficial to the individual and to the species, of endeavouring 
to look upwards with both eyes, whilst resting on one side at the 
bottom. We may also attribute to the inherited effects of use the 
fact of the mouth in several kinds of flat-fish being bent towards 
the lower surface, with the jaw bones stronger and more effective 
on this, the eyeless side of the head, than on the other, for the 
sake, as Dr. Traquair supposes, of feeding with ease on the ground. 
Disuse, on the other hand, will account for the less developed con- 
dition of the whole inferior half of the body, including the lateral 
fins ; though Yarrell thinks that the reduced size of these fins is 
advantageous to the fish, as " there is so much less room for their 
action, than with the larger fins above." Perhaps the lesser 
number of teeth in the proportion of four to seven in the upper 
halves of the two jaws of the plaice, to twenty-five to thirty in the 
lower halves, may likewise be accounted for by disuse. From the 
colourless state of the ventral surface of most fishes and of many 
other animals, we may reasonably suppose that the absence of 
colour in flat-fish on the side, whether it be the right or left, 
which is undermost, is due to the exclusion of light. But it can- 
not be supposed that the peculiar speckled appearance of the upper 
side of the sole, so like the sandy bed of the sea, or the power in 
some species, as recently shown by Pouchet, of changing their 
colour in accordance with the surrounding surface, or the presence 
of bony tubercles on the upper side of the turbot, are due to the 
action of the light. Here natural selection has probably come into 


play, as well as in adapting the general shape of the body of these 
fishes, and many other peculiarities, to their habits of life. We 
should keep in mind, as I have before insisted, that the inherited 
effects of the increased use of parts, and perhaps of their disuse, 
will be strengthened by natural selection. For all spontaneous 
variations in the right direction will thus be preserved ; as will 
those individuals which inherit in the highest degree the effects 
of the increased and beneficial use of any part. How much to 
attribute in each particular case to the effects of use, and how 
much to natural selection, it seems impossible to decide. 

I may give another instance of a structure which apparently 
owes its origin exclusively to use or habit. The extremity of the 
tail in some American monkeys has been converted into a wonder- 
fully perfect prehensile organ, and serves as a fifth hand. A 
reviewer who agrees with Mr. Mivart in every detail, remarks on 
this structure : " It is impossible to believe that in any number of 
ages the first slight incipient tendency to grasp could preserve the 
lives of the individuals possessing it, or favour their chance of 
having and of rearing offspring." But there is no necessity for 
any such belief. Habit, and this almost implies that some benefit 
great or small is thus derived, would in all probability suffice for 
the work. Brehm saw the young of an African monkey (Cercopi- 
thecus) clinging to the under surface of their mother by their 
hands, and at the same time they hooked their little tails round 
that of their mother. Professor Henslow kept in confinement 
some harvest mice (Mus messorius) which do not possess a struc- 
turally prehensile tail; but he frequently observed that they 
curled their tails round the branches of a bush placed in the cage, 
and thus aided themselves in climbing. I have received an 
analogous account from Dr. Giinther, who has seen a mouse thus 
suspend itself. If the harvest mouse had been more strictly 
arboreal, it would perhaps have had its tail rendered structurally 
prehensile, as is the case with some members of the same order. 
Why Cercopithecus, considering its habits whilst young, has not 
become thus provided, it would be difficult to say. It is, however, 
possible that the long tail of this monkey may be of more service 
to it as a balancing organ in making its prodigious leaps, than as 
a prehensile organ. 

The mammary glands are common to the whole class of mam- 
mals, and are indispensable for their existence ; they must, there- 
fore, have been developed at an extremely remote period, and we 
can know nothing positively about their manner of development. 
Mr. Mivart asks : " Is it conceivable that the young of any animal 
was ever saved from destruction by accidentally sucking a drop 
of scarcely nutritious fluid from an accidentally hypertrophied 


cutaneous gland of its mother? And even if one was so, what 
chance was there of the perpetuation of such a variation ? " But 
the case is not here put fairly. It is admitted by most evolu- 
tionists that mammals are descended from a marsupial form ; 
and if so, the mammary glands will have been at first developed 
within the marsupial sack. In the case of the fish (Hippocampus) 
the eggs are hatched, and the young are reared for a time, within 
a sack of this nature ; and an American naturalist, Mr. Lockwood, 
believes from what he has seen of the development of the young, 
that they are nourished by a secretion from the cutaneous glands 
of the sack. Now with the early progenitors of mammals, almost 
before they deserved to be thus designated, is it not at least 
possible that the young might have been similarly nourished? 
And in this case, the individuals which secreted a fluid, in some 
degree or manner the most nutritious, so as to partake of the 
nature of milk, would in the long run have reared a larger 
number of well-nourished offspring, than would the individuals 
which secreted a poorer fluid ; and thus the cutaneous glands, 
which are the homologues of the mammary glands, would have 
been improved or rendered more effective. It accords with the 
widely extended principle of specialisation, that the glands over 
a certain space of the sack should have become more highly 
developed than the remainder ; and they would then have formed 
a breast, but at first without a nipple, as we see in the Ornitho- 
rhyncus, at the base of the mammalian series. Through what 
agency the glands over a certain space became more highly 
specialised than the others, I will not pretend to decide, whether 
in part through compensation of growth, the effects of use, or of 
natural selection. 

The development of the mammary glands would have been of 
no service, and could not have been effected through natural 
selection, unless the young at the same time were able to partake 
of the secretion. There is no greater difficulty in understanding 
how young mammals have instinctively learnt to suck the breast, 
than in understanding how unhatched chickens have learnt to 
break the egg-shell by tapping against it with their specially 
adapted beaks ; or how a few hours after leaving the shell they 
have learnt to pick up grains of food. In such cases the most 
probable solution seems to be, that the habit was at first acquired 
by practice at a more advanced age, and afterwards transmitted 
to the offspring at an earlier age. But the young kangaroo is 
said not to suck, only to cling to the nipple of its mother, who 
has the power of injecting milk into the mouth of her helpless, 
half-formed offspring. On this head Mr. Mivart remarks : " Did 
no special provision exist, the young one must infallibly be choked 
by the intrusion of the milk into the windpipe. But there is a 


special provision. The larynx is so elongated that it rises up into 
the posterior end of the nasal passage, and is thus enabled to 
give free entrance to the air for the lungs, while the milk passes 
harmlessly on each side of this elongated larynx, and so safely 
attains the gullet behind it." Mr. Mivart then asks how did 
natural selection remove in the adult kangaroo (and in most 
other mammals, on the assumption that they are descended from 
a marsupial form), " this at least perfectly innocent and harmless 
structure ? " It may be suggested in answer that the voice, which 
is certainly of high importance to many animals, could hardly have 
been used with full force as long as the larynx entered the nasal 
passage ; and Professor Flower has suggested to me that this 
structure would have greatly interfered with an animal swallowing 
solid food. 

We will now turn for a short space to the lower divisions of 
the animal kingdom. The Echinodennata (star-fishes, sea-urchins, 
tfcc.) are furnished with remarkable organs, called pedicellariae, 
which consist, when well developed, of a tridactyle forceps that 
is, of one formed of three serrated arms, neatly fitting together 
and placed on the summit of a flexible stem, moved by muscles. 
These forceps can seize firmly hold of any object ; and Alexander 
Agassiz has seen an Echinus or sea-urchin rapidly passing par- 
ticles of excrement from forceps to forceps down certain lines of 
its body, in order that its shell should not be fouled. But there 
is no doubt that besides removing dirt of all kinds, they subserve 
other functions ; and one of these apparently is defence. 

With respect to these organs, Mr. Mivart, as on so many 
previous occasions, asks : " What would be the utility of the 
first rudimentary beginnings of such structures, and how could 
such incipient buddings have ever preserved the life of a single 
Echinus 1 " He adds, " not even the sudden development of the 
snapping action could have been beneficial without the freely 
moveable stalk, nor could the latter have been efficient without 
the snapping jaws, yet no minute merely indefinite variations 
could simultaneously evolve these complex co-ordinations of 
structure; to deny this seems to do no less than to affirm a 
startling paradox." Paradoxical as this may appear to Mr. 
Mivart, tridactyle forcepses, immovably fixed at the base, but 
capable of a snapping action, certainly exist on some star-fishes ; 
and this is intelligible if they serve, at least in part, as a means 
of defence. Mr. Agassiz, to whose great kindness I am indebted 
for much information on the subject, informs me that there are 
other star-fishes, in which one of the three arms of the forceps is 
reduced to a support for the other two ; and again, other genera, 
in which the third arm is completely lost. In Echinoneus, the 
shell is described by M. Perrier as bearing two kinds of pedi- 


cellari;ie, one resembling those of Echinus, and the other those of 
Spatangus ; and such cases are always interesting as affording 
the means of apparently sudden transitions, through the abortion 
of one of the two states of an organ. 

With respect to the steps by which these curious organs have 
been evolved, Mr. Agassiz infers from his own researches and 
those of Mtiller, that both in star-fishes and sea-urchins the 
pedicellariae must undoubtedly be looked at as modified spines. 
This may be inferred from their manner of development in the 
individual, as well as from a long and perfect series of gradations 
in different species and genera, from simple granules to ordinary 
spines, to perfect tridactyle pedicellari#\ The gradation extends 
even to the manner in which ordinary spines and the pedicellariae 
with their supporting calcareous rods are articulated to the shell. 
In certain genera of star-fishes, "the very combinations needed 
to show that the pedicellariae are only modified branching spines " 
may be found. Thus we have fixed spines, with three equi- 
distant, serrated, moveable branches, articulated to near their 
bases ; and higher up, on the same spine, three other moveable 
branches. Now when the latter arise from the summit of a spine 
they form in fact a rude tridactyle pedicellaria, and such may be 
seen on the same spine together with the three lower branches. 
In this case the identity in nature between the arms of the pedi- 
cellariaa and the moveable branches of a spine, is unmistakable. 
It is generally admitted that the ordinary spines serve as a 
protection ; and if so, there can be no reason to doubt that those 
furnished with serrated and moveable branches likewise serve 
for the same purpose ; and they would thus serve still more 
effectively as soon as by meeting together they acted as a pre- 
hensile or snapping apparatus. Thus every gradation, from an 
ordinary fixed spine to a fixed pedicellaria, would be of service. 

In certain genera of star-fishes these organs, instead of being 
fixed or borne on an immovable support, are placed on the 
summit of a flexible and muscular, though short, stem ; and in 
this case they probably subserve some additional function besides 
defence. In the sea-urchins the steps can be followed by which a 
fixed spine becomes articulated to the shell, and is thus rendered 
moveable. I wish I had space here to give a fuller abstract of Mr. 
Agassiz's interesting observations on the development of the pedi. 
cellariae. All possible gradations, as he adds, may likewise be 
found between the pedicellariae of the star-fishes and the hooks 
of the Ophiurians, another group of the Echinodermata ; and 
again between the pedicellariae of sea-urchins and the anchors of 
the Holothuriae, also belonging to the same great class. 

Certain compound animals, or zoophytes as they have been 



termed, namely the Polyzoa, are provided with curious organs 
called avicularia. These differ much in structure in the different 
species. In their most perfect condition, they curiously resemble 
the head and beak of a vulture in miniature, seated on a neck and 
capable of movement, as is likewise the lower jaw or mandible. 
In one species observed by me all the avicularia on the same 
branch often moved simultaneously backwards and forwards, 
with the lower jaw widely open, through an angle of about 90, 
in the course of five seconds ; and their movement caused the 
whole polyzoary to tremble. When the jaws are touched with a 
needle they seize it so firmly that the branch can thus be 

Mr. Mivart adduces this case, chiefly on account of the supposed 
difficulty of organs, namely the avicularia of the Polyzoa and the 
pedicellariae of the Echinodermata, which he considers as " essen- 
tially similar," having been developed through natural selection 
in widely distinct divisions of the animal kingdom. But, as far 
as structure is concerned, I can see no similarity between tridac- 
tyle pedicellariae and avicularia. The latter resemble somewhat 
more closely the chelae or pincers of Crustaceans ; and Mr. Mivart 
might have adduced with equal appropriateness this resemblance 
as a special difficulty ; or even their resemblance to the head and 
beak of a bird. The avicularia are believed by Mr. Busk, Dr. 
Smitt, and Dr. Nitsche naturalists who have carefully studied 
this group to be homologous with the zooids and their cells 
which compose the zoophyte ; the moveable lip or lid of the cell 
corresponding with the lower and moveable mandible of the 
avicularium. Mr. Busk, however, does not know of any gradations 
now existing between a zooid and an avicularium. It is therefore 
impossible to conjecture by what serviceable gradations the one 
could have been converted into the other : but it by no means 
follows from this that such gradations have not existed. 

As the chelae of Crustaceans resemble in some degree the 
avicularia of Polyzoa, both serving as pincers, it may be worth 
while to show that with the former a long series of serviceable 
gradations still exists. In the first and simplest stage, the 
terminal segment of a limb shuts down either on the square 
summit of the broad penultimate segment, or against one whole 
side ; and is thus enabled to catch hold of an object ; but the 
limb still serves as an organ of locomotion. We next find one 
corner of the broad penultimate segment slightly prominent, 
sometimes furnished with irregular teeth ; and against these the 
terminal segment shuts down. By an increase in the size of this 
projection, with its shape, as well as that of the terminal segment, 
slightly modified and improved, the pincers are rendered more 
and more perfect, until we have at last an instrument as efficient 


as the chelae of a lobster ; and all these gradations can be actually 

Besides the avicularia, the Polyzoa possess curious organs called 
vibracula. These generally consist of long bristles, capable of 
movement and easily excited. In one species examined by me the 
vibracula were slightly curved and serrated along the outer 
margin ; and all of them on the same polyzoary often moved 
simultaneously ; so that, acting like long oars, they swept a branch 
rapidly across the object-glass of my microscope. When a branch 
was placed on its face, the vibracula became entangled, and they 
made violent efforts to free themselves. They are supposed to 
serve as a defence, and may be seen, as Mr. Busk remarks, " to 
sweep slowly and carefully over the surface of the polyzoary, 
removing what might be noxious to the delicate inhabitants of 
the cells when their tentacula are protruded." The avicularia, 
like the vibracula, probably serve for defence, but they also catch 
and kill small living animals, which it is believed are afterwards 
swept by the currents within reach of the tentacula of the zooids. 
Some species are provided with avicularia and vibracula ; some 
with avicularia alone, and a few with vibracula alone. 

It is not easy to imagine two objects more widely different in 
appearance than a bristle or vibraculum, and an avicularium like 
the head of a bird ; yet they are almost certainly homologous and 
have been developed from the same common source, namely a 
zooid with its cell. Hence we can understand how it is that these 
organs graduate in some cases, as I am informed by Mr. Busk, 
into each other. Thus with the avicularia of several species of 
Lepralia, the moveable mandible is so much produced and is so 
like a bristle, that the presence of the upper or fixed beak alone 
serves to determine its avicularian nature. The vibracula may 
have been directly developed from the lips of the cells, without 
having passed through the avicularian stage ; but it seems more 
probable that they have passed through this stage, as during the 
early stages of the transformation, the other parts of the cell with 
the included zooid could hardly have disappeared at once. In 
many cases the vibracula have a grooved support at the base, 
which seems to represent the fixed beak ; though this support in 
some species is quite absent. This view of the development of the 
vibracula, if trustworthy, is interesting ; for supposing that all the 
species provided with avicularia had become extinct, no one with 
the most vivid imagination would ever have thought that the 
vibracula had originally existed as part of an organ, resembling a 
bird's head or an irregular box or hood. It is interesting to see 
two such widely different organs developed from a common origin ; 
and as the moveable lip of the cell serves as a protection to the 
zooid, there is no difficulty in believing that all the gradations, by 


which the lip became converted first into the lower mandible of an 
avicularium and then into an elongated bristle, likewise served as 
a protection in different ways and under different circumstances. 

In the vegetable kingdom Mr. Mivart only alludes to two cases, 
namely the structure of the flowers of orchids, and the movements 
of climbing plants. With respect to the former, he says, " the 
explanation of their origin is deemed throughly unsatisfactory 
utterly insufficient to explain the incipient, infinitesimal begin- 
nings of structures which are of utility only when they are 
considerably developed." As I have fully treated this subject in 
another work, I will here give only a few details on one alone of 
the most striking peculiarities of the flowers of orchids, namely 
their pollinia. A pollinium when highly developed consists of a 
mass of pollen-grains, affixed to an elastic foot-stalk or caudicle, 
and this to a little mass of extremely viscid matter. The pollinia 
are by this means transported by insects from one flower to the 
stigma of another. In some orchids there is no caudicle to the 
pollen-masses, and the grains are merely tied together by fine 
threads ; but as these are not confined to orchids, they need not 
here be considered ; yet I may mention that at the base of the 
orchidaceous series, in Cypripedium, we can see how the threads 
were probably first developed. In other orchids the threads 
cohere at one end of the pollen-masses ; and this forms the first 
or nascent trace of a caudicle. That this is the origin of the 
caudicle, even when of considerable length and highly developed, 
we have good evidence in the aborted pollen-grains which can some- 
times be detected embedded within the central and solid parts. 

With respect to the second chief peculiarity, namely the little 
mass of viscid matter attached to the end of the caudicle, a long 
series of gradations can be specified, each of plain service to the 
plant. In most flowers belonging to other orders the stigma 
secretes a little viscid matte/. Now in certain orchids similar 
viscid matter is secreted, but in much larger quantities by one 
alone of the three stigmas ; and this stigma, perhaps in conse- 
quence of the copious secretion, is rendered sterile. When an 
insect visits a flower of this kind, it rubs off some of the viscid 
matter and thus at the same time drags away some of the pollen- 
grains. From this simple condition, which differs but little from 
that of a multitude of common flowers, there are endless 
gradations, to species in which the pollen-mass terminates in 
a very short, free caudicle, to others in which the caudicle 
becomes firmly attached to the viscid matter, with the sterile 
stigma itself much modified. In this latter case we have a 
pollinium in its most highly developed and perfect condition. He 
"/ho will carefully examine the flowers of orchids for himself will 


not deny the existence of the above series of gradations from a 
mass of pollen-grains merely tied together by threads, with the 
stigma differing but little from that of an ordinary flower, to a 
highly complex pollinium, admirably adapted for transportal by 
insects ; nor will he deny that all the gradations in the several 
species are admirably adapted in relation to the general structure 
of each flower for its fertilisation by different insects. In this, and 
in almost every other case, the enquiry may be pushed further 
backwards ; and it may be asked how did the stigma of an 
ordinary flower become viscid, but as we do not know the full 
history of any one group of beings, it is as useless to ask, as it is 
hopeless to attempt answering, such questions. 

We will now turn to climbing plants. These can be arranged 
in a long series, from those whick simply twine round a support, 
to those which I have called leaf -climbers, and to those provided 
with tendrils. In these two latter classes the stems have generally, 
but not always, lost the power of twining, though they retain the 
power of revolving, which the tendrils likewise possess. The 
gradations from leaf-climbers to tendril-bearers are wonderfully 
close, and certain plants may be indifferently placed in either 
class. But in ascending the series from simple twiners to leaf- 
climbers, an important quality is added, namely sensitiveness to 
a touch, by which means the foot-stalks of the leaves or flowers, 
or these modified and converted into tendrils, are excited to bend 
round and clasp the touching object. He who will read my 
memoir on these plants will, I think, admit that all the many 
gradations in function and structure between simple twiners and 
tendril-bearers are in each case beneficial in a high degree to the 
species. For instance, it is clearly a great advantage to a twining 
plant to become a leaf -climber ; and it is probable that every 
twiner which possessed leaves with long foot-stalks would have 
been developed into a leaf-climber, if the foot-stalks had 
possessed in any slight degree the requisite sensitiveness to 
a touch. 

As twining is the simplest means of ascending a support, and 
forms the basis of our series, it may naturally be asked how did 
plants acquire this power in an incipient degree, afterwards to be 
improved and increased through natural selection. The power of 
twining depends, firstly, on the stems whilst young bdng ex- 
tremely flexible (but this is a character common to many plants 
which are not climbers) ; and, secondly, on their continually 
bending to all points of the compass, one after the other in 
succession, in the same order. By this movement the stems are 
inclined to all sides, and are made to move round and round. As 
soon as the lower part of a stem strikes against any object and is 
stopped, the upper part still goes on bending and revolving, and 


thus necessarily twines round and up the support. The revolving 
movement ceases after the early growth of each shoot. As in 
many widely separated families of plants, single species and 
single genera possess the power of revolving, and have thus 
become twiners, they must have independently acquired it, and 
cannot have inherited it from a common progenitor. Hence I was 
ied to predict that some slight tendency to a movement of this 
kind would be found to be far from uncommon with plants which 
did not climb ; and that this had afforded the basis for natural 
selection to work on and improve. When I made this prediction, 
I knew of only one imperfect case, namely of the young flower- 
peduncles of a Maurandia which revolved slightly and irregularly, 
like the stems of twining plants, but without making any use of 
this habit. Soon afterwards Fritz Miiller discovered that the 
young stems of an Alisma and of a Linum, plants which do not 
climb and are widely separated in the natural system, revolved 
plainly, though irregularly ; and he states that he has reason to 
suspect that this occurs with some other plants. These slight 
movements appear to be of no service to the plants in question ; 
anyhow, they are not of the least use in the way of climbing, 
which is the point that concerns us. Nevertheless we can see that 
if the stems of these plants had been flexible, and if under the con- 
ditions to which they are exposed it had profited them to ascend 
to a height, then the habit of slightly and irregularly revolving 
might have been increased and utilised through natural selection, 
until they had become converted into well-developed twiningspecies. 
With respect to the sensitiveness of the foot-stalks of the leaves 
and flowers, and of tendrils, nearly the same remarks are applicable 
as in the case of the revolving movements of twining plants. As 
a vast number of species, belonging to widely distinct groups, are 
endowed with this kind of sensitiveness, it ought to be found in 
a nascent condition in many plants which have not become 
climbers. This is the case: I observed that the young flower- 
peduncles of the above Maurandia curved themselves a little 
towards the side which was touched. Morren found in several 
species of Oxalis that the leaves and their foot-stalks moved, 
especially after exposure to a hot sun, when they were gently and 
repeatedly touched, or when the plant was shaken. I repeated 
these observations on some other species of Oxalis with the same 
result ; in some of them the movement was distinct, but was best 
seen in the young leaves ; in others it was extremely slight. It is 
a more important fact that according to the high authority of 
Hofmeister, the young shoots and leaves of all plants move after 
being shaken ; and with climbing plants it is, as we know, only 
during the early stages of growth that the foot-stalks and tendrils 
are sensitive. 


It is scarcely possible that the above slight movements, due to 
a touch or shake, in the young and growing organs of plants, can 
be of any functional importance to them. But plants possess, in 
obedience to various stimuli, powers of movement, which are of 
manifest importance to them ; for instance, towards and more 
rarely from the light. in opposition to, and more rarely in the 
direction of, the attraction of gravity. When the nerves and 
muscles of an animal are excited by galvanism or by the absorp- 
tion of strychnine, the consequent movements may be called an 
incidental result, for the nerves and muscles have not been 
rendered specially sensitive to these stimuli. So with plants it 
appears that, from having the power of movement in obedience to 
certain stimuli, they are excited in an incidental manner by a 
touch, or by being shaken. Hence there is no great difficulty in 
admitting that in the case of leaf-climbers and tendril-bearers, it 
is this tendency which has been taken advantage of and increased 
through natural selection. It is, however, probable, from reasons 
which I have assigned in my memoir, that this will have occurred 
only with plants which had already acquired the power of revolving, 
and had thus become twiners. 

I have already endeavoured to explain how plants became 
twiners, namely, by the increase of a tendency to slight and 
irregular revolving movements, which were at first of no use to 
them; this movement, as well as that due to a touch or shake, 
being the incidental result of the power of moving, gained for 
other and beneficial purposes. Whether, during the gradual 
development of climbing plants, natural selection has been aided 
by the inherited effects of use, I will not pretend to decide ; but 
we know that certain periodical movements, for instance the 
so-called sleep of plants, are governed by habit. 

I have now considered enough, perhaps more than enough, of 
the cases, selected with care by a skilful naturalist, to prove that 
natural selection is incompetent to account for the incipient 
stages of useful structures; and I have shown, as I hope, that 
there is no great difficulty on this head. A good opportunity has 
thus been afforded for enlarging a little on gradations of structure, 
often associated with changed functions, an important subject, 
which was not treated at sufficient length in the former editions 
of this work. I will now briefly recapitulate the foregoing cases. 

With the giraffe, the continued preservation of the individuals 
of some extinct high-reaching ruminant, which had the longest 
necks, legs, &c., and could browse a little above the average 
height, and the continued destruction of those which could not 
browse so high, would have sufficed for the production of this 
remarkable quadruped; but the prolonged use of all the parts 


together with inheritance will have aided in an important manner 
in their co-ordination. With the many insects which imitate 
various objects, there is no improbability in the belief that an 
accidental resemblance to some common object was in each case 
the foundation for the work of natural selection, since perfected 
through the occasional preservation of slight variations which 
made the resemblance at all closer; and this will have been 
carried on as long as the insect continued to vary, and as long as 
a more and more perfect resemblance led to its escape from sharp- 
sighted enemies. In certain species of whales there is a tendency 
to the formation of irregular little points of horn on the palate ; 
and it seems to be quite within the scope of natural selection to 
preserve all favourable variations, until the points were converted 
first into lamellated knobs or teeth, like those on the beak of a 
goose, then into short lamellae, like those of the domestic ducks, 
and then into lamellae, as perfect as those of the shoveller-duck, 
and finally into the gigantic plates of baleen, as in the mouth 
of the Greenland whale. In the family of the ducks, the lamellae 
are first used as teeth, then partly as teeth and partly as a sifting 
apparatus, and at last almost exclusively for this latter purpose. 

With such structures as the above lamellae of horn or whalebone, 
habit or use can have done little or nothing, as far as we can 
judge, towards their development. On the other hand, the trans- 
portal of the lower eye of a flat-fish to the upper side of the head, 
and the formation of a prehensile tail, may be attributed almost 
wholly to continued use, together with inheritance. With respect 
to the mammae of the higher animals, the most probable conjecture 
is that primordially the cutaneous glands over the whole surface 
of a marsupial sack secreted a nutritious fluid; and that these 
glands were improved in function through natural selection, and 
concentrated into a confined area, in which case they would have 
formed a mamma. There is no more difficulty in understanding 
how the branched spines of some ancient Echinoderm, which 
served as a defence, became developed through natural selection 
into tridactyle pedicellariae, than in understanding the develop- 
ment of the pincers of crustaceans, through slight, serviceable 
modifications in the ultimate and penultimate segments of a limb, 
which was at first used solely for locomotion. In the avicularia 
and vibracula of the Polyzoa we have organs widely different in 
appearance developed from the same source; and with the 
vibracula we can understand how the successive gradations might 
have been of service. With the pollinia of orchids, the threads 
which originally served to tie together the pollen-grains, can be 
traced cohering into caudicles; and the steps can likewise be 
followed by which viscid matter, such as that secreted by the 
stigmas of ordinary flowers, and still subserving nearly but not 


quite the same purpose, became attached to the free ends of the 
caudicles; all these gradations being of manifest benefit to the 
plants in question. With respect to climbing plants, I need not 
repeat what has been so lately said. 

It has often been asked, if natural selection be so potent, why 
has not this or that structure been gained by certain species, to 
which it would apparently have been advantageous? But it is 
unreasonable to expect a precise answer to such questions, con- 
sidering our ignorance of the past history of each species, and of 
the conditions which at the present day determine its numbers 
and range. In most cases only general reasons, but in some few 
cases special reasons, can be assigned. Thus to adapt a species 
to new habits of life, mary co-ordinated modifications are almost 
indispensable, and it may often have happened that the requisite 
parts did not vary in the right manner or to the right degree. 
Many species must have been prevented from increasing in 
numbers through destructive agencies, which stood in no relation 
to certain structures, which we imagine would have been gained 
through natural selection from appearing to us advantageous to 
the species. In this case, as the struggle for life did not depend 
on such structures, they could not have been acquired through 
natural selection. In many cases complex and long-enduring 
conditions, often of a peculiar nature, are necessary for the 
development of a structure; and the requisite conditions may 
seldom have concurred. The belief that any given structure, 
which we think, often erroneously, would have been beneficial to 
a species, would have been gained under all circumstances through 
natural selection, is opposed to what we can understand of its 
manner of action. Mr. Mivart does not deny that natural selec- 
tion has effected something ; but he considers it as " demonstrably 
insufficient " to account for the phenomena which I explain by its 
agency. His chief arguments have now been considered, and the 
others will hereafter be considered. They seem to me to partake 
little of the character of demonstration, and to have little weight 
in comparison with those in favour of the power of natural 
selection, aided by the other agencies often specified. I am 
bound to add, that some of the facts and arguments here used by 
me, have been advanced for the same purpose in an able article 
lately published in the ' Medico-Chirurgical Review.' 

At the present day almost all naturalists admit evolution under 
some torm. Mr. Mivart believes that species change through " an 
internal force or tendency," about which it is not pretended that 
anything is known. That species have a capacity for change will 
be admitted by all evolutionists; but there is no need, as it 
seems to me, to invoke any internal force beyond the tendency to 
ordinary variability, which through the aid of selection by man 


has given rise to many well-adapted domestic races, and which 
through the aid of natural selection would equally well give rise 
by graduated steps to natural races or species. The final result 
will generally have been, as already explained, an advance, but in 
some few cases a retrogression, in organisation. 

Mr. Mivart is further inclined to believe, and some naturalists 
agree with him, that new species manifest themselves "with 
suddenness and by modifications appearing at once." For instance, 
lie supposes that the differences between the extinct three-toed 
Hipparion and the horse arose suddenly. He thinks it difficult to 
believe that the wing of a bird " was developed in any other way 
than by a comparatively sudden modification of a marked and 
important kind ;" and apparently he would extend the same view 
to the wings of bats and pterodactyles. This conclusion, which 
implies great breaks or discontinuity in the series, appears to me 
improbable in the highest degree. 

Every one who believes in slow and gradual evolution, will of 
course admit that specific changes may have been as abrupt and 
as great as any single variation which we meet with under nature, 
or even under domestication. But as species are more variable 
when domesticated or cultivated than under their natural 
conditions, it is not probable that such great and abrupt variations 
have often occurred under nature, as are known occasionally to 
arise under domestication. Of these latter variations several 
may be attributed to reversion; and the characters which thus 
reappear were, it is probable, in many cases at first gained 
in a gradual manner. A still greater number must be called 
monstrosities, such as six-fingered men, porcupine men, Ancon 
sheep, Niata cattle, &c. ; and as they are widely different in 
character from natural species, they throw very little light on 
our subject. Excluding such cases of abrupt variations, the few 
which remain would at best constitute, if found in a state of 
nature, doubtful species, closely related to their parental types. 

My reasons for doubting whether natural species have changed 
as abruptly as have occasionally domestic races, and for entirely 
disbelieving that they have changed in the wonderful manner 
indicated by Mr. Mivart, are as follows. According to our 
experience, abrupt and strongly marked variations occur in our 
domesticated productions, singly and at rather long intervals of 
time. If such occurred under nature, they would be liable, as 
formerly explained, to be lost by accidental causes of destruction 
and by subsequent inter-crossing ; and so it is known to be under 
domestication, unless abrupt variations of this kind are specially 
preserved and separated by the care of man. Hence in order that 
a new species should suddenly appear in the manner supposed by 
Mr. Mivart, it is almost necessary to believe, in opposition to all 


analogy, that several wonderfully changed individuals appeared 
simultaneously within the same district. This difficulty, as in 
the case of unconscious selection by man, is avoided on the theory 
of gradual evolution, through the preservation of a large number 
of individuals, which varied more or less in any favourable 
direction, and of the destruction of a large number which varied 
in an opposite manner. 

That many species have been evolved in an extremely gradual 
manner, there can hardly be a doubt. The species and even the 
genera of many large natural families are so closely allied together, 
that it is difficult to distinguish not a few of them. On every 
continent in proceeding from north to south, from lowland to 
upland, &c., we meet with a host of closely related or representative 
species; as we likewise do on certain distinct continents, which 
we have reason to believe were formerly connected. But in 
making these and the following remarks, I am compelled to 
allude to subjects hereafter to be discussed. Look at the many 
outlying islands round a continent, and see how many of their 
inhabitants can be raised only to the rank of doubtful species. 
So it is if we look to past times, and compare the species which 
have just passed away with those still living within the same 
areas; or if we compare the fossil species embedded in the sub- 
stages of the same geological formation. It is indeed manifest 
that multitudes of species are related in the closest manner to 
other species that still exist, or have lately existed; and it will 
hardly be maintained that such species have been developed in an 
abrupt or sudden manner. Nor should it be forgotten, when we 
look to the special parts of allied species, instead of to distinct 
species, that numerous and wonderfully fine gradations can be 
traced, connecting together widely different structures. 

Many large groups of facts are intelligible only on the principle 
that species have been evolved by very small steps. For instance 
the fact that the species included in the larger genera are more 
closely related to each other, and present a greater number of 
varieties than do the species in the smaller genera. The former 
are also grouped in little clusters, like varieties round species; 
and they present other analogies with varieties, as was shown in 
our second chapter. On this same principle we can understand 
how it is that specific characters are more variable than generic 
characters ; and how the parts which are developed in an extra- 
ordinary degree or manner are more variable than other parts of 
the same species. Many analogous facts, all pointing in the same 
direction, could be added. 

Although very many species have almost certainly been 
produced by steps not greater than those separating fine varieties ; 
yet it may be maintained that some have been developed in a 


different and abrupt manner. Such an admission, however, ought 
not to be made without strong evidence being assigned. The 
vague and in some respects false analogies, as they have been 
shown to be by Mr. Chauncey Wright, which have been advanced 
in favour of this view, such as the sudden crystallisation of 
inorganic substances, or the falling of a facetted spheroid from 
one facet to another, hardly deserve consideration. One class of 
facts, however, namely, the sudden appearance of new and distinct 
forms of life in our geological formations supports at first sight 
the belief in abrupt development. But the value of this evidence 
depends entirely on the perfection of the geological record, in 
relation to periods remote in the history of the world. If the 
record is as fragmentary as many geologists strenuously assert, 
there is nothing strange in new forms appearing as if suddenly 

Unless we admit transformations as prodigious as those 
advocated by Mr. Mivart, such as the sudden development of the 
wings of birds or bats, or the sudden conversion of a Hipparion 
into a horse, hardly any light is thrown by the belief in abrupt 
modifications on the deficiency of connecting links in our geological 
formations. But against the belief in such abrupt changes, 
embryology enters a strong protest. It is notorious that the 
wings of birds and bats, and the legs of horses or other quadrupeds, 
are undistinguishable at an .early embryonic period, and that they 
become differentiated by insensibly fine steps. Embryological 
resemblances of all kinds can be accounted for, as we shall 
hereafter see, by the progenitors of our existing species having 
varied after early youth, and having transmitted their newly 
acquired characters to their offspring, at a corresponding age. The 
embryo is thus left almost unaffected, and serves as a record of 
the past condition of the species. Hence it is that existing species 
during the early stages of their development so often resemble 
ancient and extinct forms belonging to the same class. On this 
view of the meaning of embryological resemblances, and indeed 
on any view, it is incredible that an animal should have undergone 
such momentous and abrupt transformations, as those above 
indicated ; and yet should not bear even a trace in its embryonic 
condition of any sudden modification ; every detail in its structure 
being developed by insensibly fine steps. 

He who believes that some ancient form was transformed 
suddenly through an internal force or tendency into, for instance, 
one furnished with wings, will be almost compelled to assume, 
in opposition to all analogy, that many individuals varied 
simultaneously. It cannot be denied that such abrupt and great 
changes of structure are widely different from those which most 
species apparently have undergone. He will further be compelled 


to believe that many structures beautifully adapted to all the 
other parts of the same creature and to the surrounding conditions, 
have been suddenly produced ; and of such complex and wonderful 
co-adaptations, he will not be able to assign a shadow of an 
explanation. He will be forced to admit that these great and 
sudden transformations have left no trace of their action on the 
embryo. To admit all this is, as it seems to me, to enter into the 
realms of miracle, and to leave those of Science. 



Instincts comparable with habits, but different in their origin .fnstincts 
graduated Aphides and ants Instincts variable Domestic instincts, 
their origin Natural instincts of the cuckoo, raolothrus, ostrich, and 
parasitic bees Slave-making ants Hive-bee, its cell-making instinct 
Changes of instinct and structure not necessarily simultaneous Difficulties 
of the theory of the Natural Selection of instincts Neuter or sterile 
insects Summary. 

MANY instincts are so wonderful that their development will 
probably appear to the reader a difficulty sufficient to overthrow 
my whole theory. I may here premise, that I have nothing to do 
with the origin of the 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 faculties in animals of the same 

I will not attempt any definition of instinct. It would be easy 
to show that several distinct mental actions are commonly em- 
braced 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. An action, which we ourselves 
require experience to enable us to perform, when performed by an 
animal, more especially by a very young one, without 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 are universal. A little dose of judgment or reason, as 
Pierre Huber expresses it, often comes into play, even with 
animals low in the scale of nature. 

Frederick Cuvier and several of the older metaphysicians have 
compared instinct with habit. This comparison gives, I think, an 
accurate notion of the frame of mind under which an instinctive 


action is performed, but not necessarily 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, with certain periods of time, and states of the 
body. When once acquired, they often remain constant through- 
out 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 deriving any benefit from 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 it 
can be shown that this does sometimes happen then the resem- 
blance 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, he might truly 
be said to have done so instinctively. But it would be a 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 acquired by habit. 

It will be universally admitted that instincts are as important 
as corporeal structures 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 was 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 in many cases of subordinate importance to the effects of 
the natural selection of what may be called spontaneous 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 dis- 
covered. 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 the 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 to 
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, as 
was first observed by Huber, 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, 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. It is certain, from 
the observations of Huber, that the aphides show no dislike to 
the ants : if the latter be not present they are at last compelled to 
eject their excretion. But as the excretion is extremely viscid, it 
is no doubt a convenience to the aphides to have it removed ; 
therefore probably they do not excrete solely for the good of the 
ants. Although there is no evidence that any animal performs an 
action for the exclusive good of another species, yet each tries to 
take advantage of the instincts of others, as each takes advantage 
of the weaker bodily structure of other species. So again 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 
be 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 the 
nests of the same species in the northern and southern United 
States. Why, it has been asked, if instinct be variable, has it not 
granted to the bee " the ability to use some other material when 
wax was deficient " ? But what other natural material could bees 
use ? They will work, as I have seen, with wax hardened with 
vermilion or softened with lard. Andrew Knight observed that 
his bees, instead of laboriously collecting propolis, used a cement 
of wax and turpentine, with which he had covered decorticated trees. 
It has lately been shown that bees, instead of searching for pollen, 
will gladly use a very different substance, namely oatmeal. 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. The 
fear of man is slowly acquired, as I have elsewhere shown, by the 
various animals which inhabit desert islands ; and we see an 
instance of this even in England, in the greater wildness of all 
our large birds in comparison with 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 mental qualities of animals of the same kind, born in 
a state of nature, vary much, could be shown by many facts. 
Several cases could also be adduced of occasional and strange 
habits in wild animals, which, if advantageous to the species, 
might have given rise, through natural selection, to new instincts. 
But I am well aware that these general statements, without the 
facts in detail, will produce but a feeble effect on the reader's 
mind. I can only repeat my assurance, that I do not speak 
without good evidence. 

Inlierited Changes of Habit or Instinct in Domesticated Animals. 

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 be 
enabled to see the part which habit and the selection of so-called 
spontaneous variations have played in modifying the mental 
qualities of our domestic animals. It is notorious how much 
domestic animals vary in their mental qualities. With cats, for 
instance, one naturally takes to catching rats, and another mice, 
and these tendencies are known to be inherited. One cat, 
according to Mr. St. John, always brought home game-birds, 
another hares or rabbits, and another hunted on marshy ground 
and almost nightly caught woodcocks or snipes. A number of 
curious and authentic instances could be given of various shades 
of disposition and of taste, and likewise of the oddest tricks, 
associated with certain frames of mind or periods of time, being 
inherited. But let us look to the familiar case of the breeds of the 
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 behold one 
kind /of wolf, when young and without any training, as soon as it 
scented its prey, stand motionless like a statue, and than 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 certainly far less 
fixed than natural instincts ; but they have been acted on by far 
less rigorous selection, and have been transmitted for an incom- 
parably 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 genera- 
tions 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 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 house-tumblers, 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, had 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 : the act 
of pointing is probably, as many have thought, only the exag- 
gerated pause of an animal preparing to spring on its prey. When 
the first tendency to point 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 in progress, as each man tries to procure, 
without intending to improve the breed, dogs which stand and 
hunt best. On the other hand, habit alone in some cases has 
sufficed; hardly any 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 can hardly suppose that domestic 
rabbits have often been selected for tameness alone ; so that we 
must attribute at least the greater part of the inherited change 


from extreme wildness to extreme tameness, 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 largely and how per- 
manently the minds of our domestic animals have been modified. 
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 and some degree of selection have 
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 ; for I am informed by Captain Hutton that the young 
chickens of the parent-stock, the Gallus bankiva, when reared in 
India under a hen, are at first excessively wild. So it is with 
young pheasants reared in England 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 under domestication 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 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 
habit and selection have probably concurred. 

Special Instincts, 
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, namely, the instinct which leads 
the cuckoo to lay her eggs in other birds' nests ; the slave-making 
instinct of certain ants ; and the cell-making power of the hive- 
bee. These two latter instincts have generally and justly been 
ranked by naturalists as the most wonderful of all known 

Instincts of the Cuckoo. It is supposed by some naturalists that 
the more immediate cause of the instinct of the cuckoo 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 
migrates at^tl 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 both asserted and denied that the American 
cuckoo occasionally lays her eggs in other birds' nests ; but I have 
lately heard from Dr. Merrell, of Iowa, that he once found in 
Illinois a young cuckoo together with a young jay in the nest 
of a Blue jay (Garrulus cristatus) ; and as both were nearly full 
feathered, there could be no mistake in their identification. I 
could also 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, and that she occasionally 
laid an egg in another bird's nest. If the old bird profited by this 
occasional habit through being enabled to migrate earlier or 
through any other cause; or if the young were made more 
vigorous by advantage being taken of the mistaken instinct of 
another species than when reared by their own mother, encum- 
bered as she could 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 us 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 more successful in rearing their young. 
By a continued process of this nature, I believe that the strange 
instinct of our cuckoo has been generated. It has, also, recently 
been ascertained on sufficient evidence, by Adolf Miiller, that 
the cuckoo occasionally lays her eggs on the bare ground, sits on 
them, and feeds her young, This rare event is probably a 


case of reversion to the long-lost, aboriginal instinct of nidi- 

It has been objected that I have not noticed other related 
instincts and adaptations of structure in the cuckoo, which are 
spoken of as necessarily co-ordinated. But in all cases, speculation 
on an instinct known to us only in a single species, is useless, for 
we have hitherto had no facts to guide us. Until recently the 
instincts of the European and of the non-parasitic American 
cuckoo alone were known ; now, owing to Mr. Ramsay's observa^ 
tions, we have learnt something about three Australian species, 
which lay their eggs in other birds' nests. The chief points to be 
referred to are three: first, that the common cuckoo, \vith rare 
exceptions, lays only one egg in a nest, so that the large and vora- 
cious young bird receives ample food. Secondly, that the eggs 
are remarkably small, not exceeding those of the skylark, a bird 
about one-fourth as large as the cuckoo. That the small size of 
the egg is a real case of adaptation we may infer from the fact of 
the non-parasitic American cuckoo laying full-sized eggs. Thirdly, 
that the young cuckoo, soon after birth, has the instinct, the 
strength, and a properly shaped back for ejecting its foster- 
brothers, which then perish from cold and hunger. This has been 
boldly called a beneficent arrangement, in order that the young 
cuckoo may get sufficient food, and that its foster-brothers may 
perish before they had acquired much feeling ! 

Turning now to the Australian species; though these birds 
generally lay only one egg in a nest, it is not rare to find two and 
even three eggs in the same nest. In the Bronze cuckoo the eggs 
vary greatly in size, from eight to ten lines in length. Now if it 
had been of an advantage to this species to have laid eggs even 
smaller than those now laid, so as to have deceived certain foster- 
parents, or, as is more probable, to have been hatched within a 
shorter period (for it is asserted that there is a relation between 
the size of eggs and the period of their incubation), then there is 
no difficulty in believing that a race or species might have been 
formed which would have laid smaller and smaller eggs ; for these 
would have been more safely hatched and reared. Mr. Ramsay 
remarks that two of the Australian cuckoos, when they lay their 
eggs in an open nest, manifest a decided preference for nests 
containing eggs similar in colour to their own. The European 
species apparently manifests some tendency towards a similar 
instinct, but not rarely departs from it, as is shown by her laying 
her dull and pale-coloured eggs in the nest of the Hedge-warbler 
with bright greenish-blue eggs. Had our cuckoo invariably dis- 
played the above instinct, it would assuredly have been added to 
those which it is assumed must all have been acquired together. 
The eggs of the Australian Bronze cuckoo vary, according to Mr. 


Ramsay, to an extraordinary degree in colour; so that in this 
respect, as well as in size, natural selection might have secured 
and fixed any advantageous variation. 

In the case of the European cuckoo, the offspring of the foster- 
parents are commonly ejected from the nest within three days 
after the cuckoo is hatched ; and as the latter at this age is in 
a most helpless condition, Mr. Gould was formerly inclined to 
believe that the act of ejection was performed by the foster- 
parents themselves. But he has now received a trustworthy 
account of a young cuckoo which was actually seen, whilst still 
blind and not able even to hold up its own head, in the act of 
ejecting its foster-brothers. One of these was replaced in the 
nest by the observer, and was again thrown out. With respect to 
the means by which this strange and odious instinct was acquired, 
if it were of great importance for the young cuckoo, as is probably 
the case, to receive as much food as possible soon after birth, I 
can see no special difficulty in its having gradually acquired, 
during successive generations, the blind desire, the strength, and 
structure necessary for the work of ejection; for those young 
cuckoos which had such habits and structure best developed 
would be the most securely reared. The first step towards the 
acquisition of the proper instinct might have been mere uninten- 
tional restlessness on the part of the young bird, when somewhat 
advanced in age and strength ; the habit having been afterwards 
improved, and transmitted to an earlier age. I can see no more 
difficulty in this, than in the unhatched young of other birds 
acquiring the instinct to break through their own shells; or 
than in young snakes acquiring in their upper jaws, as Owen has 
remarked, a transitory sharp tooth for cutting through the tough 
egg-shell. For if each part is liable to individual variations at all 
ages, and the variations tend to be inherited at a corresponding or 
earlier age, propositions which cannot be disputed, then the 
instincts and structure of the young could be slowly modified as 
surely as those of the adult ; and both cases must stand or fall 
together with the whole theory of natural selection. 

Some species of Molothrus, a widely distinct genus of American 
birds, allied to our starlings, have parasitic habits like those of the 
cuckoo; and the species present an interesting gradation in the 
perfection of their instincts. The sexes of Molothrus badius are 
stated by an excellent observer, Mr. Hudson, sometimes to live 
promiscuously together in flocks, and sometimes to pair. They 
either build a nest of their own, or seize on one belonging to some 
other bird, occasionally throwing out the nestlings of the stranger. 
They either lay their eggs in the nest thus appropriated, or oddly 
enough build one for themselves on the top of it. They usually 
sit on their own eggs and rear their own young ; but Mr. Hudson 


says it is probable that they are occasionally parasitic, for he has 
seen the young of this species following old birds of a distinct 
kind and clamouring to be fed by them. The parasitic habits of 
another species of Molothrua., the M. bonariensis, are much more 
highly developed than those of the last, but are still far from 
perfect. This bird, as far as it is known, invariably lays its eggs 
in the nests of strangers; but it is remarkable that several 
together sometimes commence to build an irregular untidy nest of 
their own, placed in singularly ill-adapted situations, as on the 
leaves of a large thistle. They never, however, as far as Mr. 
Hudson has ascertained, complete a nest for themselves. They 
often lay so many eggs from fifteen to twenty in the same 
foster-nest, that few or none can possibly be hatched. They have, 
moreover, the extraordinary habit of pecking holes in the eggs, 
whether of their own species or of their foster-parents, which they 
find in the appropriated nests. They drop also many eggs on the 
bare ground, which are thus wasted. A third species, the M. 
pecoris of North America, has acquired instincts as perfect as 
those of the cuckoo, for it never lays more than one egg in a 
foster-nest, so that the young bird is securely reared. Mr. 
Hudson is a strong disbeliever in evolution, but he appears to 
have been so much struck by the imperfect instincts of the 
Molothrus bonariensis that he quotes my words, and asks, "Must 
we consider these kabits, not as especially endowed or created 
instincts, but as small consequences of one general law, namely, 
transition 1 " 

Various birds, as has already been remarked, occasionally lay 
their eggs in the nests of other birds. This habit is not very 
uncommon with the Gallinaceae, and throws some light on the 
singular instinct of the ostrich. In this family several hen-birds 
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 with the cuckoo, at intervals of two or three days. 
The instinct, however, of the American ostrich, as in the case of 
the Molothrus bonariensis, 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 regularly lay their eggs in the 
nests of other kinds of bees. This case is more remarkable than 
that of the cuckoo ; for these bees have not only had their instincts 
but their structure modified in accordance with their parasitic 
habits; for they do not possess the pollen-collecting apparatus 
which would have been indispensable if they had stored up food 
for their own young. Some species of Sphegidse (wasp-like 


insects) are likewise parasitic ; 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, 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 that of the Molothrus or 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 feloniously appropriated, be not thus exterminated. 

Slave-making instinct. This remarkable instinct was first dis- 
covered 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 of any kind, and 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 feeding their own larvae. When the old nest is found incon- 
venient, and they have to migrate, it is the slaves which determine 
the migration, and actually carry their masters in their jaws. So 
utterly helpless are the masters, that when Huber shut up thirty 
of them without a slave, but with plenty of the food which they like 
best, and with their own larvse and pupse to stimulate them to 
work, they did nothing ; they could not even feed themselves, and 
many perished of hunger. Huber then introduced a single slave 
(F. fusca), and she instantly set to work, fed and saved the 
survivors ; made some cells and tended the larvae, and put all to 
rights. What can be more extraordinary than these well-ascer- 
tained facts 1 If we had not known of any other slave-making ant, 
it would have been hopeless to speculate how so wonderful an 
instinct could have been perfected. 

Another species, Fonnica sanguinea, was likewise first discovered 
by P. Huber to be a slave-making ant. This species is found in 
the southern parts of England, and its habits have been attended 
to by Mr. F. Smith, of the British Museum^ to whom I am much 
indebted for information on this and other subjects. Although 
fully trusting to the statements of Huber and Mr. Smith, I tried 
to approach the subject in a sceptical frame of mind, as any one 
may well be excused for doubting the existence of so extraordinary 
an instinct as that of making slaves. Hence, I will give the obser- 
vations which I made in some little detail. I opened fourteen 
nests of F. sanguinea, and found a few slaves in all. Males and 
fertile females of the slave species (F. fusca) are found only in their 
own proper communities, and have never been observed in the 
nests of F. sanguinea. The slaves are black and not above half 


the size of their red masters, so that the contrast in their appear- 
ance is great. When the nest is slightly disturbed, the slaves 
occasionally come out, and like their masters are much agitated 
and defend the nest : when the nest is much disturbed, and the 
larvae and pupae are exposed, the slaves work energetically together 
with their masters in carrying them away to a place of safety. 
Hence, it is clear, that the slaves feel quite at home. During the 
months of June and July, on three successive years, I watched for 
many hours several nests in Surrey and Sussex, and never saw a 
slave either leave or enter a nest. As, during these months, the 
slaves are very few in number, I thought that they might behave 
differently when more mimerous ; but Mr. Smith informs me that 
he has watched the nests at various hours during May, June, and 
August, both in Surrey and Hampshire, and has never seen the 
slaves, though present in large numbers in August, either leave or 
enter the nest. Hence he considers them as strictly household 
slaves. The masters, on the other hand, may be constantly 
seen bringing in materials for the nest, and food of all kinds. 
During the year 1860, however, in the month of July, I came 
across a community with an unusually large stock of slaves, and I 
observed a few slaves mingled with their masters leaving the nest, 
and marching along the same road to a tall Scotch-fir-tree, twenty- 
five yards distant, which they ascended together, probably in search 
of aphides or cocci. According to Huber, who had ample oppor- 
tunities for observation, the slaves in Switzerland habitually work 
with their masters in making the nest, and they alone open and 
close the doors in the morning and evening ; and, as Huber 
expressly states, their principal office is to search for aphides. This 
difference in the usual habits of the masters and slaves in the two 
countries, probably depends merely on the slaves being captured 
in greater numbers in Switzerland than in England. 

One day I fortunately witnessed a migration of F. sanguinea 
from one nest to another, and it was a most interesting spectacle 
to behold the masters carefully carrying their slaves in their jaws 
instead of being carried by them, as in the case of F. rufescens. 
Another day my attention was struck by about a score of the slave- 
makers haunting the same spot, and evidently not in search of 
food ; they approached and were vigorously repulsed by an inde- 
pendent community of the slave-species (F. fusca) ; sometimes as 
many as three of these ants clinging to the legs of the slave-making 
F. sanguinea. The latter ruthlessly killed their small opponents, 
and carried their dead bodies as food to their nest, twenty-nine 
yards distant; but they were prevented from getting any pupae to 
rear as slaves. I then dug up a small parcel of the pupae of F. 
fusca from another nest, and put them down on a bare spot near 
the place of combat ; they were eagerly seized and carried off by 



the tyrants, who 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 their 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 pupee 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 returning home and entering their 
nests, carrying the dead bodies of F. fusca (showing that it was 
not a migration) and numerous pupae. I traced a long file of ants 
burthened with booty, for about forty yards back, 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, an 
image of despair 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 continental 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 san- 
guinea, on the other hand, possesses much fewer slaves, and in th& 
early part of the summer extremely few : the masters determine 
when and where a new nest shall be formed, and wnen 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 Switzer- 
land 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 larvaj. So that the masters in this country receive much 
less service from their slaves than they do in Switzerland. 

By Avhat 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 the pupae of other species, if scattered 
near their nests, it is possible that such pupse originally stored as 
food might become developed ; and the foreign ants thus uninten- 
tionally 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. san- 
guinea, which, as we have seen, is less aided by its slaves than the 
same species in Switzerland, natural selection might increase and 
modify 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 
minute details on this subject, but will merely give an outline of 
the conclusions at which I have arrived. He must be a dull man 
who can examine the exquisite structure of a comb, so beautifully 
adapted to its end, without enthusiastic admiration. We hear 
from mathematicians that bees have practically solved a recondite 
problem, and have made their cells of the proper shape to hold the 
greatest possible amount of honey, with the least possible con- 
sumption of precious wax in their construction. It has been 
remarked that a skilful workman with fitting tools and measures, 
would find it very difficult to make cells of wax of the true form, 
though this is effected by a crowd of bees working in a dark hive. 
Granting whatever instincts you please, it seems at first quite 
inconceivable how they can make all the necessary angles and 
planes, or even perceive when they are correctly made. But the 
difficulty is not nearly so great as it at first appears : all this 
beautiful work can be shown, I think, to follow from a few simple 

I was led to investigate this subject by Mr. Waterhouse, who 
has shown that the form of the cell stands in close relation to the 


presence of adjoining cells ; and the following view may, perhaps, 
be considered only as a modification of his theory. Let us look 
to the great principle of gradation, and see whether Nature does 
not reveal to us her method of work. At one end of a short 
series we have humble-bees, which use their old cocoons to hold 
honey, sometimes adding to them short tubes of wax, and likewise 
making separate and very irregular rounded cells of wax. At the 
other end of the series we have the cells of the hive-bee, placed in 
a double layer: each cell, as is well known, is an hexagonal 
prism, with the basal edges of its six sides bevelled so as to join 
an inverted pyramid, of three rhombs. These rhombs have certain 
angles, and the three which form the pyramidal base of a single 
cell on one side of the comb enter into the composition of the 
bases of three adjoining cells on the opposite side. In the series 
between the extreme perfection of the cells of the hive-bee and 
the simplicity of those of the humble-bee we have the cells of the 
Mexican Melipona domestica, carefully described and figured by 
Pierre Huber. The Melipona itself is intermediate in structure 
between the hive and humble bee, but more 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 flat surfaces, according 
as the cell adjoins two, three, or more other cells. When one cell 
rests on 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 base 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, and what is more 
important, labour, 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 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 V 2, or radius x I'4i421 (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. But I hear from 
Prof. Wyman, who has made numerous cireful measurements, 
that the accuracy of the workmanship of the bee has been greatly 
exaggerated ; so much so, that whatever the typical form of the 
cell may be, it is rarely, if ever, realised. 

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 have the power of forming 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 many insects make in wood, 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 
to a certain extent ; and then she unites the points of intersection 
by perfectly flat surfaces. By such modifications of instincts 
which in themselves are 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, rectangular strip of wax: the bees 
instantly began to excavate minute circular pits in it; and aa 


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 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 
scalloped 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, rectangular piece of 
wax, a thin and narrow, knife-edged ridge, colou^d 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 bases; and these flat 
bases, formed by thin little plates of the vermilion wax 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 some parts, only small 
portions, in other parts, large portions of a rhombic plate were 
thus 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 in circularly 
gnawing away and deepening the basins on both sides of the ridge 
of vermilion wax, in order to have thus succeeded in leaving flat 
plates between the basins, by stopping work at the planes or 

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 plate, that they 
could have effected this by gnawing away the convex side; and 
I suspect that the bees in such cases stand on opposite sides 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 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 this away 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 that 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 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 
giound, 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. These walls, as Professor Miller has kindly 
ascertained for me, vary greatly in thickness ; being, on an average 
of twelve measurements made near the border of the comb, 3 ^r of 
an inch in thickness; whereas the basal rhomboidal plates are 
thicker, nearly in the proportion of three to two, having a mean 
thickness, from twenty-one measurements, of -^1-^ of an inch. By 
the above 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 com- 
mencement 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 it 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 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 trom the walls of the last 
somyleted 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 adjoin- 
ing cells has been built. This capacity in bees of laying down 
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 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 were to 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 

As natural selection acts only by the accumulation of slight 
modifications of structure or instinct, each profitable to the indi- 
vidual 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 construc- 
tion, could have profited the progenitors of the hive-bee ? I think 
the answer is not difficult : cells constructed like those of the bee 
or the wasp gain in strength, and save much in labour and space, 
and in the materials of which they are constructed. With respect 
to the formation of wax, it is known that bees are often hard 
pressed to get sufficient nectar, and I am informed by Mr. Teget- 
meier that it has been experimentally proved that from twelve to 
fifteen pounds of dry sugar are consumed by a hive of bees for the 
secretion of a 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 clays 
during the process of secretion. A large store of honey is indis- 
pensable 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 and the time consumed in collecting the 
honey must be an important element of success to any family of 
bees. Of course the success of the species may be dependent on 
the number of its enemies, or parasites, or on quite distinct causes, 
and so be altogether independent of the quantity of honey which 
the bees can collect. But let us suppose that this latter circum- 
stance determined, as it probably often has determined, whether a 
bee allied to our humble-bees could exist in large numbers in any 
country; and let us further suppose that the community lived 



through 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 imaginary humble-bee, if a slight modification in her instincts 
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 labour and wax. Hence it would continually be more 
and more advantageous to our humble-bees, if they were to make 
their 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 the 
adjoining cells, and much labour and 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 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 labour and 

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 par- 
ticular 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 the construction of cells of due strength and of the proper 
size and shape for the larvie, this being effected with the greatest 
possible economy of labour and wax ; that individual swarm which 
thus made the best cells Avith least labour, and least waste of 
honey in the secretion of wax, having succeeded best, and having 
transmitted their newly acquired economical instincts to new 
swarms, which in their turn will have had the best chance of suc- 
ceeding in the struggle for existence. 

Objections to the Theory of Natural Selection as applied to 
Instincts: Neuter and Sterile Insects. 

It has been objected to the foregoing view of the origin of 
instincts that "the variations of structure and of instinct must 
have been simultaneous and accurately adjusted to each other as 
A modification in the one without an immediate corresponding 


change in the other would have been fatal." The force of this 
objection rests entirely on the assumption that the changes in the 
instincts and structure are abrupt. To take as an illustration the 
case of the larger titmouse (Parus major) alluded to in a previous 
chapter ; this bird often holds the seeds of the yew between its 
feet on a branch, and hammers with its beak till it gets at the 
kernel. Now what special difficulty would there be in natural 
selection preserving all the slight individual variations in the 
shape of the beak, which were better and better adapted to break 
open the seeds, until a beak was formed, as well constructed for 
this purpose as that of the nut-hatch, at the same time that habit, 
or compulsion, or spontaneous variations of taste, led the bird to 
become more and more of a seed-eater ? In this case the beak is 
supposed to be slowly modified by natural selection, subsequently 
to, but in accordance with, slowly changing habits or taste ; but 
let the feet of the titmouse vary and grow larger from correlation 
with the beak, or from any other unknown cause, and it is not 
improbable that such larger feet would lead the bird to climb 
more and more until it acquired the remarkable climbing instinct 
and power of the nuthatch. In this case a gradual change of 
structure is supposed to lead to changed instinctive habits. To 
take one more case : few instincts are more remarkable than that 
which leads the swift of the Eastern Islands to make its nest wholly 
of inspissated saliva. Some birds build their nests of mud, believed 
to be moistened with saliva ; and one of the swifts of North America 
makes its nest (as I have seen) of sticks agglutinated with saliva, 
and even with flakes of this substance. Is it then very improbable 
that the natural selection of individual swifts, which secreted more 
and more saliva, should at last produce a species with instincts 
leading it to neglect other materials, and to make its nest exclu- 
sively of inspissated saliva ? And so in other cases. It must, how- 
ever, be admitted that in many instances we cannot conjecture 
whether it was instinct or structure which first varied. 

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 have originated ; cases, in which 
no intermediate gradations are known to exist ; cases of instincts 
of 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 pro- 
genitor, and consequently must believe that they were indepen- 
dently acquired through 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 the 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 especial difficulty in this having been effected through 
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 
wonderful difference in this respect between the workers and the 
perfect females, would have been better exemplified by the hive- 
bee. If a working ant or other neuter insect had been an ordinary 
animal, I should have unhesitatingly assumed that all its 
characters had been slowly acquired through natural selection; 
namely, by individuals having been born with slight profitable 
modifications, which were inherited by the offspring; and that 
these again varied and again were 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 1 

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 inherited structure which are corre- 
lated with certain ages, and with either sex. We have differences 
correlated not only with one sex, but with that short period 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 the oxen of other 
breeds, relatively to the length of the horns in both the bulls and 
cows of these same breeds. Hence I can see no great difficulty in 
any character becoming correlated with the sterile condition of 
certain members of insect-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. Breeders of cattle wish the flesh and 
fat to be well marbled together : an animal thus characterised has 
been slaughtered, but the breeder has gone with confidence to the 
same stock and has succeeded. Such faith may be placed in the 
power of selection, that a breed of cattle, always yielding oxen 
with extraordinarily long horns, could, it is probable, be formed 
by carefully watching which individual bulls and cows, when 
matched, produced oxen with the longest horns ; and yet no one 
ox would ever have propagated its kind. Here is a better and 
real illustration: according to M. Verlot, some varieties of the 
double annual Stock from having been long and carefully selected 
to the right degree, always produce a large proportion of seedlings 
bearing double and quite sterile flowers ; but they likewise yield 
some single and fertile plants. These latter, by which alone the 
variety can be propagated, may be compared with the fertile male 
and female ants, and the double sterile plants with the neuters of 
the same community. As with the varieties of the stock, so with 
social insects, selection has been applied to the family, and not to 
the individual, for the sake of gaining a serviceable end. Hence 
we may conclude that slight modifications of structure or of 
instinct, correlated with the sterile condition of certain members 
of the community, have proved advantageous : consequently the 
fertile males and females have flourished, and transmitted to their 
fertile offspring a tendency to produce sterile members with the 
same modifications. This process must have been repeated many 
times, 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 acme 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 commonly 
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 and 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 the 
theory. In the simpler case of neuter insects all of one caste, 
which, as I believe, have been rendered different from the fertile 
males and females through natural selection, we may conclude 
from the analogy of ordinary variations, that the successive, 
slight, profitable modifications did not first arise in all the 
neuters in the same nest, but in some few alone; and that by 
the survival of the communities with females which produced 
most neuters having the advantageous modification, all the 
neuters ultimately came to be thus characterised. According to 
this view we ought occasionally to find in the same nest neuter 
insects, presenting gradations of structure; and this we do find, 
even not rarely, considering how few neuter insects out of Europe 
have been carefully examined. Mr. F. Smith has shown that the 
neuters of several British ants differ surprisingly from each other 
in size and sometimes in colour ; and that the extreme forms can 
be linked together by individuals taken out of the same nest : I 
have myself compared perfect gradations of this kind. It some- 
times happens that the larger or the smaller sized workers are the 
most numerous; or that both large and small are numerous, 
whilst those of an intermediate size are scanty in numbers. 
Formica flava has larger and smaller workers, with some few 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 dis- 
sected 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 inter- 
mediate condition. So that here we 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 were in this condition; we should then have had a 
species of ant with neuters in nearly the same condition as those 


of Myrmica. For the workers of Myrmica have not even rudi- 
ments 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 occasionally 
to find gradations of important structures 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 workersi 
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 in 
addition 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 Sir J. Lubbock made drawings for me, with the 
camera lucida, of the jaws which I dissected from the workers of 
the several sizes. Mr. Bates, in his interesting ' Naturalist on the 
Amazons,' has described analogous cases. 

With these facts before me, I believe that natural selection, by 
acting on the fertile ants or parents, could form a species which 
should regularly produce neuters, all of large size with one form 
of jaw, or all of small size with widely different jaws ; or lastly, 
and this is the greatest 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 first been 
formed, as in the case of the driver ant, and then the extreme 
forms having been produced in greater and greater numbers, 
through the survival of the parents which generated them, until 
none with an intermediate structure were produced. 

An analogous explanation has been given by Mr. Wallace, of 
the equally complex case, of certain Malayan Butterflies regularly 
appearing under two or even three distinct female forms ; and by 
Fritz Miiller, of certain Brazilian crustaceans likewise appearing 
under two widely distinct male forms. But this subject need not 
here be discussed. 

I have now explained how, 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 ants, on the same 
principle that the division of labour is useful to civilised man. 
Ants, however, work by inherited instincts and by inherited 
organs or tools, whilst man works by acquired knowledge and 
manufactured instruments. But I must confess, that, with all 
my faith in natural selection, I should never have anticipated 
that this principle could have been efficient in so high a degree, 
had not the case of these neuter insects led me to this conclusion. 
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 may be effected by the accumulation 
of numerous, slight, spontaneous variations, which are in any way 
profitable, without exercise or habit having been brought into 
play. For peculiar habits confined to the workers or sterile 
females, however long they might be followed, could not possibly 
affect the males and fertile females, which alone leave descendants. 
I am surprised that no one has hitherto advanced this demonstra- 
tive case of neuter insects, against the well-known doctrine of 
inherited habit, as advanced by Lamarck, 


I have endeavoured in this chapter briefly 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 there is no real difficulty, under changing 
conditions of life, in natural selection accumulating to any extent 
slight modifications of instinct which are in any way useful. In 
many 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 can be shown to have been produced 
for the good of other animals, though animals take 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 


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 under- 
stand, on the principle of inheritance, how it is that the thrush 
of tropical South America lines its nest with mud, in the same 
peculiar manner as does our British thrush; how it is that the 
Hornbills of Africa and India have the same extraordinary instinct 
of plastering up and imprisoning the females in a hole in a tree, 
with only a small hole left in the plaster through which the males 
feed them and their young when hatched ; how it is that the male 
wrens (Troglodytes) of North America build " cock -nests," to roost 
in, like the males of our 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 ichneumonidse 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, not accumulated 
by natural selection Causes of the sterility of first crosses and of hybrids 
Parallelism between the effects of changed conditions of life and of 
crossing Dimorphism and trimorphism Fertility of varieties when 
crossed and of their mongrel offspring not universal Hybrids and 
mongrels compared independently of their fertility Summary. 

THE view commonly entertained by naturalists is that species, 
when intercrossed, have been specially endowed with sterility, in 
order to prevent their confusion. This view certainly seems at 
first highly probable, for species living together could hardly have 
been kept distinct had they been capable of freely crossing. The 
subject is in many ways important for us, more especially as the 
sterility of species when first crossed, and that of their hybrid 
offspring, cannot have been acquired, as I shall show, by the 
preservation of successive profitable degrees of sterility. It is aa 


incidental result of differences in the reproductive systems of the 

In treating this subject, two classes of facts, to a large extent 
fundamentally different, have generally been confounded ; namely, 
the sterility of 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 repro- 
ductive organs functionally impotent, as may be clearly seen in 
the state of the male element in both plants and animals ; though 
the formative 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 probably has 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 be descended from common parents, when crossed, and likewise 
the fertility of their mongrel offspring, is, with reference to my 
theory, of equal importance with the sterility of species; for it 
seems to make a broad and clear distinction between varieties 
and species. 

Degrees of Sterility. 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, Kolreuter and Gartner, who almost devoted 
their lives to this subject, without being deeply impressed with 
the high generality of some degree of sterility. Kolreuter 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. Gartner, also, makes the rule equally universal; and 
he disputes the entire fertility of Kb'lreuter's ten cases. But in 
these and in many other cases, Gartner 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 first crossed, and the maximum 
produced by their hybrid offspring, with the average number pro- 
duced by both pure parent-species in a state of nature. But 
causes of serious error here intervene : 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 experimented on by 
Gartner were potted, and were kept in a chamber in his house. 
That these processes are often injurious to the fertility of a plant 
cannot be doubted ; for Gartner 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 Leguminosai, 
in which there is an acknowledged difficulty in the manipulation) 
half of these twenty plants had their fertility in some degree 
impaired. Moreover, as Gartner repeatedly crossed some forms, 
such as the common red and blue pimpernels (Anagallis arvensis 
and coerulea), which the best botanists rank as varieties, and 
found them absolutely sterile, we may doubt whether many 
species are really so sterile, when intercrossed, as he believed. 

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 Kolreuter and Gartner, arrived at 
diametrically opposite conclusions in regard to some of the very 
same forms. It is also most instructive to compare but I have 
not space here to enter into 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 observer 
from experiments made during different years. It can thus be 
shown that neither sterility nor fertility affords any certain dis- 
tinction between species and varieties. The evidence from this 
source graduates away, and is doubtful in the same degree as 
is the evidence derived from other constitutional and structural 

In regard to the sterility of hybrids in successive generations ; 
though Gartner 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 increases, but generally decreases greatly 
ind suddenly. With respect to this decrease, it may first be 
noticed that when any deviation in structure or constitution is 
common to both parents, this is often transmitted in an aug- 
mented degree to the offspring; and both sexual elements in 
hybrid plants are already affected in some degree. But I believe 
that their fertility has been diminished in nearly all these cases 
by an independent cause, namely, by too close interbreeding. I 
have made so many experiments and collected so many facts. 


showing on the one hand that an occasional cross with a distinct 
individual or variety increases the vigour and fertility of the 
offspring, and on the other hand that very close interbreeding 
lessens their vigour and fertility, that I cannot doubt the correct- 
ness of this conclusion. Hybrids are seldom raised by experi- 
mentalists 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, if left to themselves, will generally be fertilised 
during each generation by pollen from the same flower ; and this 
would probably be injurious to their fertility, already lessened by 
their hybrid origin, I am strengthened in this conviction by a 
remarkable statement repeatedly made by Gartner, 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 from manipulation, sometimes decidedly 
increases, and goes on increasing. Now, in the process of 
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 
often on the same plant, would be thus effected. Moreover, 
whenever complicated experiments are in progress, so careful an 
observer as Gartner would have castrated his hybrids, and this 
would have ensured in each generation a cross with pollen from 
a distinct flower, either from the same plant or from another plant 
of the same hybrid nature. And thus, the strange fact of an 
increase of fertility in the successive generations of artificially 
fertilised hybrids, in contrast with those spontaneously self-fer- 
tilised, may, as I believe, be accounted for by too close inter- 
breeding having been avoided. 

Now let us turn to the results arrived at by a third most 
experienced hybridiser, namely, the Hon. and Eev. W. Herbert. 
He is as emphatic in his conclusion that some hybrids are perfectly 
fertile as fertile as the pure parent-species as are Kol renter and 
Gartner that some degree of sterility between distinct species is a 
universal law of nature. He experimented on some of the very 
same species as did Gartner. The difference in their results may, 
I think, be in part accounted for by Herbert's great horticultural 
skill, and by his having hot-houses 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 Crinurn capense 
fertilised by C. revolutum produced a plant, which I never saw to 
occur in a case of its natural fecundation." So that here we have 
perfect or even more than commonly perfect fertility, in a first 
cross between two distinct species. 


This cas of the Crinum leads me to refer to a singular fact, 
namely, tat individual plants of certain species of Lobelia, 
Verbascur and Passiflora, can easily be fertilised by pollen from 
a distinct pecies, but not by pollen from the same plant, though 
this pollei can be proved to be perfectly sound by fertilising other 
plants or species. In the genus Hippeastrum, in Corydalis as 
shown by Professor Hildebrand, in various orchids as shown by 
Mr. Scott ind Fritz Miiller, all the individuals are in this peculiar 
condition. So that with some species, certain abnormal in- 
dividuals, ind in other species all the individuals, can actually 
be hybridsed much more readily than they can be fertilised by 
pollen froi the same individual plant ! To give one instance, a 
bulb of Hppeastrum aulicum produced four flowers ; three were 
fertilised >y Herbert with their own pollen, and the fourth was 
subsequenly fertilised by the pollen of a compound hybrid 
descended from three distinct species : the result was that 
" the ovarBS of the three first flowers soon ceased to grow, and 
after a fev days perished entirely, whereas the pod impreg- 
nated by he pollen of the hybrid made vigorous growth and 
rapid prog-ess to maturity, and bore good seed, which vegetated 
freely." JV^r. Herbert tried similar experiments during many years, 
and always with the same result. These cases serve to show on 
what slight and mysterious causes the lesser or greater fertility of 
a species 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 widel/ dissimilar in general habit, "reproduces itself as 
perfectly as if it had been a natural species from the mountains 
of Chili." 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 hybr .d between Rhod. ponticum and catawbiense, and that 
this hybrid 1" seeds as freely as it is possible to imagine." Had 
hybrids, when fairly treated, always gone on decreasing in fertility 
in each successive generation, as Gartner believed to be the case, 
the fact would have been notorious to nurserymen. Horticul- 
turists raise large beds of the same hybrid, and such alone are 
fairly treated, for by insect agency the several individuals are 
allowed to cross freely 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 hybri Rhodo- 
dendrons, 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 ben care- 
fully tried than with plants. If our systematic arrangements can 
be trusted, that is, if the genera of animals are as distact from 
each other as are the genera of plants, then we may ifer that 
animals more widely distinct in the scale of nature can b crossed 
more easily than in the case of plants ; but the hybrids tlemselves 
are, I think, more sterile. It should, however, be borne in mind 
that, owing to few animals breeding freely under corinement, 
few experiments have been fairly tried : for instance, tb canary- 
bird has been crossed with nine distinct species of finchs, but, as 
not one of these breeds freely in confinement, we have n> right to 
expect that the first crosses between them and the canay, or that 
then- hybrids, should be perfectly fertile. Again, with % espect to 
the fertility in successive generations of the more fertle hybrid 
animals, I hardly know of an instance in which two kmilies of 
the same hybrid have been raised at the same time fron 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 slould have 
gone on increasing. 

Although I know of hardly any thoroughly well-authenticated 
cases of perfectly fertile hybrid animals, I have reasor. to believe 
that the hybrids from Cervulus vaginalis and ReevesL, and from 
Phasianus colchicus with P. torquatus, are perfectly fertile. M. 
Quatrefages states that the hybrids from two moths (Bombyx 
cynthia and arrindia) were proved in Paris to be fertile inter se 
for eight generations. It has lately been asserted that two such 
distinct species as the hare and rabbit, when they can be got to 
breed together, produce offspring, which are highly fertile when 
crossed with one of the parent-species. The hybrids from the 
common and Chinese geese (A. cygnoides), species which are so 
different that they are generally ranked in distinct ienera, 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 or perfectly fertile. 

With our domesticated animals, the various races when crossed 
together are quite fertile ; yet in many cases they are descended 
from two or more wild species. From this fact we must conclude 
either that the aboriginal parent-species at first produced perfectly 
fertile hybrids, or that the hybrids subsequently reared under 
domestication became quite fertile. This latter alternative, 
which was first propounded by Pallas, seems by far the most 
probable, and can, indeed, hardly be doubted. It is, for instance, 
almost certain that our dogs are descended from several wild 
stocks; yet, with perhaps the exception of certain indigenous 
domestic dogs of South America, all are quite fertile together; 
but analogy makes me greatly doubt, whether the several abori- 
ginal species would at first have freely bred together and have 
produced quite fertile hybridu. So again I have lately acquired 
decisive evidence that the crossed offspring from the Indian 
humped and common cattle are inter se perfectly fertile; and 
from the observations by Riitimeyer on their important osteo- 
logical differences, as well as from those by Mr. Blyth on their 
differences in habits, voice, constitution, &c., these two forms 
must be regarded as good and distinct species. The same remarks 
may be extended to the two chief races of the pig. We must, 
therefore, either give up the belief of the universal sterility of 
species when crossed ; or we must look at this sterility in animals, 
not as an indelible characteristic, but as one capable of being 
removed by domestication. 

Finally, considering all the ascertained facts on the inter- 
crossing 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 laws governing 
the sterility of first crosses and of hybrids. Our chief object will 
be to see whether or not these laws indicate that species have 
been specially endowed with this quality, in order to prevent 
their crossing and blending together in utter confusion The 
following conclusions are drawn up chiefly from Gartner's ad- 
mirable work on the hybridisation of plants. I have taken much 
pains to ascertain how far they 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 

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 ; 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 applied to the 
stigma of some one species of the same genus, 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 produces. So in hybrids themselves, there 
are some which never have produced, and probably never would 
produce, even with the pollen of the pure parents, 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. 

The hybrids raised 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 
of facts which are generally confounded together is by no means 
strict. There are many cases, in which two pure species, as in the 
genus Verbascum, can be united with unusual facility, and 
produce numerous hybrid-offspring, yet these hybrids are re- 
markably feterile. 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 that of pure species. 
But the fertility of first crosses is likewise innately variable ; for 
it is not always the same in degree when the same two species are 
crossed under the same circumstances ; it 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 the same 

By the term systematic affinity is meant, the general resem 
bianco between species in structure and constitution. Now the 


fertility of first crosses, 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 efforts have 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 Gartner 
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. 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 female-ass being first crossed by a stallion, and then 
a mare by a male-ass ; 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, that is of any difference in their structure 
or constitution, excepting in their reproductive systems. The 
diversity of the result in reciprocal crosses between the same 
two species was long ago observed by Kolreuter. To give an 
instance : Mirabilis jalapa can easily be fertilised by the pollen 
of M. longiflora, and the hybrids thus produced are sufficiently 
fertile ; but Kolreuter tried more than two hundred times, during 
eight following years, to fertilise reciprocally M. longiflora witk 
the pollen of M. jalapa, and utterly failed. Several other equallj 


striking cases could be given. Thuret has observed the same fact 
with certain sea-weeds or Fuci. Gartner, moreover, found that 
this difference of facility in making reciprocal crosses is extremely 
common in a lesser degree. He has observed it even between 
closely related forms (as Matthiola annua and glabra) which 
many botanists rank 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, though they 
rarely differ in external characters, yet generally differ in fertility 
in a small, and occasionally in a high degree. 

Several other singular rules could be given from Gartner: 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 inter- 
mediate 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 the fertility of a hybrid 
may be independent of its external resemblance to either pure 

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 unfavour- 
able 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 the difference in the result of 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 
tome difference, and occasionally the widest possible difference, in 


the facility of effecting an union. The hybrids, moreover, pro- 
duced 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 produc- 
tion of hybrids been permitted 1 To grant to species the special 
power of producing hybrids, and then to stop their further pro- 
pagation by different degrees of sterility, not strictly related to 
the facility of the first union between their parents, seems 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 
ui their reproductive systems ; the differences being of so peculiar 
and limited a nature, that, in reciprocal crosses between the same 
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 differ- 
ences, and not a specially endowed quality. As the capacity of 
one plant to be grafted or budded on another is unimportant for 
their 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 see the 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, <fcc. ; 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, 
do not always prevent the two grafting together. As in hybridisa- 
tion, so with grafting, the capacity is limited by systematic affinity, 
for no one has been able to graft together trees 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 Gartner found that there was sometimes an innate difference 
in different individuals of the same two species in crossing ; so 
Sageret 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 gooseberry, 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 different case 
from the difficulty of uniting two pure species, which have their re- 
productive organs perfect ; yet these two distinct classes of cases run 
to a large 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 a fourth species, when thus grafted were rendered 
barren. On the other hand, certain species of Sorbus, when graf ted 
on other species yielded twice as much fruit as when on their own 
roots. We are reminded by this latter fact of the extraordinary 
cases of Hippeastrum, Passiflora, fec., which seed much more freely 
when fertilised with the pollen of a distinct species, than when 
fertilised with pollen from the same plant. 

We thus see, that, although there is a clear and great 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 vegeta- 
tive systems, so I believe that the still more complex laws govern- 
ing the facility of first crosses are incidental on unknown differences 
in their reproductive systems. These differences in both cases, 
follow to a certain extent, as might have been expected, systematic 
affinity, by which term every kind of resemblance and dissimilarity 
between organic beings is attempted to be expressed. The facts 
by no means seem to indicate that the greater or lesser difficulty 
of either grafting or crossing 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. 

Origin and Causes of the Sterility of first Crosses and 
of Hybrids. 

At one time it appeared to me probable, as it has to others, that 
the sterility of first crosses and of hybrids might have been slowly 
acquired through the natural selection of slightly lessened degrees 
of fertility, which, like any other variation, spontaneously appeared 
in certain individuals of one variety when crossed with those of 
another variety. For it would clearly be advantageous to two 
varieties or incipient species, if they could be kept from blending, 
on the same principle that, when man is selecting at the same time 
two varieties, it is necessary that he should keep them separate. In 
the first place, it may be remarked that species inhabiting distinct 
regions are often sterile when crossed ; now it could clearly have 
been of no advantage to such separated species to have been 
rendered mutually sterile, and consequently this could not have 
been effected through natural selection ; but it may perhaps be 
argued, that, if a species was rendered sterile with some one com- 
patriot, sterility with other species would follow as a necessary 
contingency. In the second place, it is almost as much opposed 
to the theory of natural selection as to that of special creation, 
that in reciprocal crosses the male element of one form should have 
been rendered utterly impotent on a second form, whilst at the 
same time the male element of this second form is enabled freely 
to fertilise the first form; for this peculiar state of the repro- 
ductive system could hardly have been advantageous to either 

In considering the probability of natural selection having come 
into action, in rendering species mutually sterile, the greatest 
difficulty will be found to lie in the existence of many graduated 
steps from slightly lessened fertility to absolute sterility. It may 
be admitted that it would profit an incipient species, if it were 
rendered in some slight degree sterile when crossed with its parent 
form or with some other variety ; for thus fewer bastardised and 
deteriorated offspring would be produced to commingle their blood 
with the new species in process of formation. But he who will 
take the trouble to reflect on the steps by which this first degree 
of sterility could be increased through natural selection to that 
high degree which is common with so many species, and which is 
universal with species which have been differentiated to a generic 
or family rank, will find the subject extraordinarily complex. 
After mature reflection it seems to me that this could not have 
been effected through natural selection. Take the case of any two 
species which, when crossed, produced few and sterile offspring; 


now, what is there which could favour the survival of those indi- 
viduals which happened to be endowed in a slightly higher degree 
with mutual infertility, and which thus approached by one small 
step towards absolute sterility ? Yet an advance of this kind, if 
the theory of natural selection be brought to bear, must have in- 
cessantly occurred with many species, for a multitude are mutually 
quite barren. With sterile neuter insects we have reason to believe 
that modifications in their structure and fertility have been slowly 
accumulated by natural selection, from an advantage having been 
thus indirectly given to the community to which they belonged 
over other communities of the same species; but an individual 
animal not belonging to a social community, if rendered slightly 
sterile when crossed with some other variety, would not thus itself 
gain any advantage or indirectly give any advantage to the other 
individuals of the same variety, thus leading to their preservation. 
But it would be superfluous to discuss this question in detail ; 
for with plants we have conclusive evidence that the sterility of 
crossed species must be due to some principle, quite independent 
of natural selection. Both Gartner and Kolreuter have proved 
that in genera including numerous species, a series can be formed 
from species which when crossed yield fewer and fewer seeds, to 
species which never produce a single seed, but yet are affected by 
the pollen o* certain other species, for the germen swells. It is 
here manifestly impossible to select the more sterile individuals, 
which have already ceased to yield seeds ; so that this acme of 
sterility, when the germen alone is affected, cannot have been 
gained through selection ; and from the laws governing the various 
grades of sterility being so uniform throughout the animal and 
vegetable kingdoms, we may infer that the cause, whatever it may 
be, is the same or nearly the same in all cases. 

We will now look a little closer at the probable nature of the 
differences between species which induce sterility in first crosses 
and in hybrids. In the case of first crosses, the greater or less 
difficulty in effecting an union and in obtaining offspring appar- 
ently 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 the pollen of one species is placed on the stigma of a dis- 
tantly 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 
pheasants and fowls, that the early death of the embryo is a very 
frequent cause of sterility in first crosses. Mr. Salter has recently 
given the results of an examination of about 500 eggs produced 
from various crosses between three species of Gallus and their 
hybrids; the majority of these eggs had been fertilised; and in 
the majority of the fertilised eggs, the embryos had either been 
partially developed and had then perished, or had become nearly 
mature, but the young chickens had been unable to break through 
the shell. Of the chickens which were born, more than four-fifths 
died within the first few days, or at latest weeks, " without any 
obvious cause, apparently from mere inability to live ; " so that 
from the 500 eggs only twelve chickens were reared. With plants, 
hybridised embryos probably often perish in a like manner; at 
least it is known that hybrids raised from very distinct species 
are sometimes weak and dwarfed, and perish at an early age ; of 
which fact Max Wichura has recently given some striking cases 
with hybrid willows. It may be here worth noticing that in some 
cases of parthenogenesis, the embryos within the eggs of silk 
moths which had not been fertilised, pass through their early 
stages of development and then perish like the embryos produced 
by a cross between distinct species. Until becoming acquainted 
with these facts, I was unwilling to believe in the frequent early 
death of hybrid embryos ; for hybrids, when once born, are gener- 
ally healthy and long-lived, as we see in the case of the common 
mule. Hybrids, however, are differently circumstanced before and 
after birth : when born and living in a country where their two 
parents live, they are generally placed under suitable conditions 
of life. But a hybrid partakes of only half of the nature and con- 
stitution of its mother ; it may therefore before birth, as long as it 
is nourished within its mother's womb, or within the egg or seed 
produced by the mother, 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 are eminently sensitive 
to injurious or unnatural conditions of life. But after all, the 
cause more probably lies fn some imperfection in the original act 
of impregnation, causing the embryo to be imperfectly developed, 
rather than in the conditions to which it is subsequently exposed. 
In regard to the sterility of hybrids, in which the sexual 
elements are imperfectly developed, the case is somewhat different. 
I have more than once alluded to a large body of facts showing 
that, when anbnals 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 superin- 
duced 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, for whole groups of animals 
and plants are rendered impotent by the same unnatural condi- 
tions ; 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 exotic 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 seems to 
be partly due to their reproductive systems having been specially 
affected, though in a lesser degree than when sterility ensues. So 
it is with hybrids, for their offspring 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 un- 
natural crossing of two species, the reproductive system, indepen- 
dently of the general state of health, is affected in a very similar 
manner. In the one case, the conditions of life have been dis- 
turbed, though often in so slight a degree as to be inappreciable 
by us ; in the other case, or that of hybrids, the external condi- 
tions have remained the same, but the organisation has been dis- 
turbed by two distinct structures and constitutions, including of 
course the reproductive systems, having been blended into one. 
For it is scarcely possible that two organisations should be com- 
pounded into one, without some disturbance occurring in the 
development, or periodical action, or mutual relations 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, does not diminish ; it is even apt 
to increase, this being generally the result, as before explained, of 
too close interbreeding. The above view of the sterility of hybrid* 
being caused by two constitutions being compounded into one has 
been strongly maintained by Max Wichura. 


It must, however, be owned that we cunnot understand, on the 
above or any other view, several facts with respect to the sterility 
of hybrids ; for instance, the unequal fertility of hybrids pro- 
duced 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 being compounded into one. 

A similar parallelism holds good with an allied yet very 
different class of facts. It is an old and almost universal belief 
founded on a considerable body of evidence, which I have else- 
where given, 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, <fcc., 
from one soil or climate to another, and back again. During the 
convalescence of animals, great benefit is derived from almost 
any change in their habits of life. Again, both with plants and 
animals, there is the clearest evidence that a cross between indi- 
viduals of the same species, which differ to a certain extent, gives 
vigour and fertility to the offspring ; and that close interbreeding 
continued during several generations between the nearest relations, 
if these be kept under the same conditions of life, almost always 
leads to decreased size, weakness, or sterility. 

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 been subjected to slightly 
different conditions, or which have slightly varied, give vigour 
and fertility to the offspring. But, as we have seen, organic 
beings long habituated to certain uniform conditions under a 
state of nature, when subjected, as under confinement, to a con- 
siderable change in their conditions, very frequently are rendered 
more or less sterile ; and we know that a cross between two forms, 
that have become widely or specifically different, produce hybrids 
which are almost always in some degree sterile. I am fully per- 
suaded that this double parallelism is by no means an accident or 
an illusion. He who is able to explain why the elephant and a 
multitude of other animals are incapable of breeding when kept 
under only partial confinement in their native country, will be 
able to explain the primary cause of hybrids being so generally 
sterile. He will at the same time be able to explain how it is 



that the races of some of our domesticated animals, which have 
often been subjected to new and not uniform conditions, are quite 
fertile together, although they are descended from distinct species, 
which would probably have been sterile if aboriginally crossed. 
The above two parallel series of facts seem to be connected to- 
gether by some common but unknown bond, which is essentially 
related to the principle of life ; this principle, according to Mr. 
Herbert Spencer, being that life depends on, or consists in, the 
incessant action and reaction of various forces, which, as through- 
out nature, are always tending towards an equilibrium ; and when 
this tendency is slightly disturbed by any change, the vital forces 
gain in power. 

Reciprocal Dimorphism and Trlmorpkism. 

This subject may be here briefly discussed, and will be found 
to throw some light on hybridism. Several plants belonging to 
distinct orders present two forms, which exist in about equal 
numbers and which differ in no respect except in their repro- 
ductive organs ; one form having a long pistil with short stamens, 
the other a short pistil with long stamens ; the two having 
differently sized pollen-grains. With trimorphic plants there 
are three forms likewise differing in the lengths of their pistils 
and stamens, in the size and colour of the pollen-grains, and in 
some other respects ; and as in each of the three forms there are 
two sets of stamens, the three forms possess altogether six sets of 
stamens and three kinds of pistils. These organs are so pro- 
portioned in length to each other, that half the stamens in two 
of the forms stand on a level with the stigma of the third form. 
Now I have shown, and the result has been confirmed by other 
observers, that, in order to obtain full fertility with these plants, 
it is necessary that the stigma of the one form should be fertilised 
by pollen taken from the stamens of corresponding height in 
another form. So that with dimorphic species two unions, which 
may be called legitimate, are fully fertile ; and two, which may 
be called illegitimate, are more or less infertile. With trimorphic 
species six unions are legitimate, or fully fertile, and twelve are 
illegitimate, or more or less infertile. 

The infertility which may be observed in various dimorphic 
and trimorphic plants, when they are illegitimately fertilised, that 
is by pollen taken from stamens not corresponding in height with 
the pistil, differs much in degree, up to absolute and utter 
sterility ; just in the same manner as occurs in crossing distinct 
species. As the degree of sterility in the latter case depends in 
an eminent degree on the conditions of life being more or less 
favourable, so I have found it with illegitimate unions. It is 
well known that if pollen of a distinct species be placed on th& 


stigma of a flower, and its own pollen be afterwards, even after 
a considerable interval of time, placed on the same stigma, its 
action is so strongly prepotent that it generally annihilates the 
effect of the foreign pollen ; so it is with the pollen of the several 
forms of the same species, for legitimate pollen is strongly pre- 
potent over illegitimate pollen, when both are placed on the same 
stigma. I ascertained this by fertilising several flowers, first 
illegitimately, and twenty-four hours afterwards legitimately, with 
pollen taken from a peculiarly coloured variety, and all the seed- 
lings were similarly coloured ; this shows that the legitimate 
pollen, though applied twenty-four hours subsequently, had wholly 
destroyed or prevented the action of the previously applied ille- 
gitimate pollen. Again, as in making reciprocal crosses between 
the same two species, there is occasionally a great difference in 
the result, so the same thing occurs with trimorphic plants ; for 
instance, the mid-styled form of Lythrum salicaria was illegiti- 
mately fertilised with the greatest ease by pollen from the longer 
stamens of the short-styled form, and yielded many seeds ; but 
the latter form did not yield a single seed when fertilised by the 
longer stamens of the mid -sty led form. 

In all these respects, and in others which might be added, the 
forms of the same undoubted species when illegitimately united 
behave in exactly the same manner as do two distinct specie* 
when crossed. This led me carefully to observe during four years 
many seedlings, raised from several illegitimate unions. The 
chief result is that these illegitimate plants, as they may be called, 
are not fully fertile. It is possible to raise from dimorphic- 
species, both long-styled and short-styled illegitimate plants, and 
from trimorphic plants all three illegitimate forms. These can 
then be properly united in a legitimate manner. When this is 
done, there is no apparent reason why they should not yield as. 
many seeds as did their parents when legitimately fertilised. But 
such is not the case. They are all infertile, in various degrees ; 
some being so utterly and incurably sterile that they did not 
yield during four seasons a single seed or even seed-capsule. The 
sterility of these illegitimate plants, when united with each other 
in a legitimate manner, may be strictly compared with that of 
hybrids when crossed inter se. If, on the other hand, a hybrid is 
crossed with either pure parent-species, the sterility is usually 
much lessened : and so it is when an illegitimate plant is fertilised 
by a legitimate plant. In the same manner as the sterility of 
hybrids does not always run parallel with the difficulty of making 
the first cross between the two parent-species, so the sterility of 
certain illegitimate plants was unusually great, whilst the sterility 
of the union from which they were derived was by no means great. 
With hybrids raised from the same seed-capsule the degree of 


sterility is innately variable, so it is in a marked manner with 
illegitimate plants. Lastly, many hybrids are profuse and per- 
sistent flowerers, whilst other and more sterile hybrids produce 
few flowers, and are weak, miserable dwarfs ; exactly similar 
cases occur with the illegitimate offspring of various dimorphic 
and trimorphic plants. 

Altogether there is the closest identity in character and 
behaviour between illegitimate plants and hybrids. It is hardly 
an exaggeration to maintain that illegitimate plants are hybrids, 
produced within the limits of the same species by the improper 
union of certain forms, whilst ordinary hybrids are produced from 
an improper union between so-called distinct species. We have 
also already seen that there is the closest similarity in all respects 
between first illegitimate unions and first crosses between distinct 
species. This will perhaps be made more fully apparent by an 
illustration ; we may suppose that a botanist found two well- 
marked varieties (and such occur) of the long-styled form of the 
trimorphic Lythrum salicaria, and that he determined to try by 
crossing whether they were specifically distinct. He would find 
that they yielded only about one-fifth of the proper number of 
seed, and that they behaved in all the other above specified 
respects as if they had been two distinct species. But to make 
the case sure, he would raise plants from his supposed hybridised 
seed, and he would find that the seedlings were miserably dwarfed 
and utterly sterile, and that they behaved in all other respects 
like ordinary hybrids. He might then maintain that he had 
actually proved, in accordance with the common view, that his 
two varieties were as good and as distinct species as any in the 
world ; but he would be completely mistaken. 

The facts now given on dimorphic and trimorphic plants are 
important, because they show us, first, that the physiological test 
of lessened fertility, both in first crosses and in hybrids, is no 
safe criterion of specific distinction ; secondly, because we may 
conclude that there is some unknown bond which connects the 
infertility of illegitimate unions with that of their illegitimate 
offspring, and we are led to extend the same view to first crosses 
and hybrids ; thirdly, because we find, and this seems to me of 
especial importance, that two or three forms of the same species 
may exist and may differ in no respect whatever, either in 
structure or in constitution, relatively to external conditions, and 
yet be sterile when united in certain ways. For we must re- 
member that it is the union of the sexual elements of individuals 
of the same form, for instance, of two long-styled forms, which 
results in sterility ; whilst it is the union of the sexual elements 
proper to two distinct forms which is fertile. Hence the case 
appears at first sight exactly the reverse of what occurs, in the 


ordinary unions of the individuals of the same species and with 
crosses between distinct species. It is, however, doubtful whether 
this is really so ; but I will not enlarge on this obscure subject. 

We may, however, infer as probable from the consideration of 
dimorphic and trimorphic plants, that the sterility of distinct 
species when crossed and of their hybrid progeny, depends ex- 
clusively on the nature of their sexual elements, and not on any 
difference in their structure or general constitution. We are also 
led to this same conclusion by considering reciprocal crosses, in 
which the male of one species cannot be united, or can be united 
with great difficulty, with the female of a second species, whilst 
the converse cross can be effected with perfect facility. That 
excellent observer, Gartner, likewise concluded that species when 
crossed are sterile owing to differences confined to their repro- 
ductive systems. 

Fertility of Varieties when Crossed, and of their Mongrel 
Offspring, not universal. 

It may be urged, as an overwhelming argument, that there 
must be some essential distinction between species and varieties, 
inasmuch as the latter, however much they may differ from each 
other in external appearance, cross with perfect facility, and yield 
perfectly fertile offspring. With some exceptions, presently to 
be given, I fully admit that this is the rule. But the subject is 
surrounded by difficulties, for, looking to varieties produced under 
nature, if two forms hitherto reputed to be 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, 
which are considered by most botanists as varieties, are said by 
Gartner to be quite sterile when crossed, and he 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 some 
doubt. For when it is stated, for instance, that certain South 
American indigenous domestic dogs do not readily unite with 
European dogs, the explanation which will occur to every one, 
and probably the true one, is that they are descended from 
aboriginally distinct species. Nevertheless the perfect fertility 
of so many domestic races, differing widely from each other in 
appearance, for instance those 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 considera* 
tions, however, render the fertility of domestic varieties lesa 


remarkable. In the first place, it may be observed that the 
amount of external difference between two species is no sure 
guide to their degree of mutual sterility, so that similar differ- 
ences in the case of varieties would be no sure guide. It is 
certain that with species the cause lies exclusively in differences 
in their sexual constitution. Now the varying conditions to 
which domesticated animals and cultivated plants have been 
subjected, have had so little tendency towards modifying the 
reproductive system in a manner leading to mutual sterility, that 
we have good grounds for admitting the directly opposite doctrine 
of Pallas, namely, that such conditions generally eliminate this 
tendency ; so that the domesticated descendants of species, which 
in their natural state probably would have been in some degree 
sterile when crossed, become perfectly fertile together. With 
plants, so far is cultivation from giving a tendency towards 
sterility between distinct species, that in several well-authenti- 
cated cases already alluded to, certain plants have been affected 
in an opposite manner, for they have become self -impotent whilst 
still retaining the capacity of fertilising, and being fertilised by, 
other species. If the Pallasian doctrine of the elimination of 
sterility through long-continued domestication be admitted, and 
it can hardly be rejected, it becomes in the highest degree im- 
probable that similar conditions long-continued should likewise 
induce this tendency ; though in certain cases, with species having 
a peculiar constitution, sterility might occasionally be thus caused. 
Thus, as I believe, we can understand why with domesticated 
animals varieties have not been produced which are mutually 
sterile ; and why with plants only a few such cases, immediately 
to be given, have been observed. 

The real difficulty in our present subject is not, as it appears 
to me, why domestic varieties have not become mutually infertile 
when crossed, but why this has so generally occurred with natural 
varieties, as soon as they have been permanently modified in a 
sufficient degree to take rank as species. We are far from pre- 
cisely knowing the cause ; nor is this surprising, seeing how pro- 
foundly ignorant we are in regard to the normal and abnormal 
action of the reproductive system. But we can see that species, 
owing to their struggle for existence with numerous competitors, 
will have been exposed during long periods of time to more uni- 
form conditions, than have domestic varieties ; and this may well 
make a wide difference in the result. For we know how commonly 
wild animals and plants, when taken from their natural condi- 
tions and subjected to captivity, are rendered sterile ; and the 
reproductive functions of organic beings which have always lived 
under natural conditions would probably in like manner be 
eminently sensitive to the influence of an unnatural cross. 


Domesticated productions, on the other hand, which, as shown 
by the mere fact of their domestication, were not originally highly 
sensitive to changes in their conditions of life, and which can now 
generally resist with undiminished fertility repeated changes of 
conditions, might be expected to produce varieties, which would 
be little liable to have their reproductive powers injuriously 
affected by the act of crossing with other varieties which had 
originated in a like manner. 

I have as yet spoken as if the varieties of the same species were 
invariably fertile when intercrossed. But it is 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. Gartner 
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 fertilised thirteen flowers of 
the one kind with 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 im- 
portant to notice that the hybrid plants thus raised were them- 
selves jwfectly fertile ; so that even Gartner 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 fertilisation 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 experimented on are ranked by Sageret, who 
mainly founds his classification by the test of infertility, as 
varieties, and Naudin has come to the same conclusion. 

The following case is far more remarkable, and seems at first 
incredible ; but it is the result of an astonishing number of 
experiments made during many years on nine species of Verbas- 
cum, by so good an observer and so hostile a witness as Gartner : 
namely that the yellow and white varieties when crossed produce 
less seed than the similarly coloured varieties of the same species. 
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 similarly 
coloured flowers, than between those which are differently coloured. 
Mr. Scott also has experimented on the species and varieties of 


Verbascum ; and although unable to confirm Gartner's results on 
the crossing of the distinct species, he finds that the dissimilarly 
coloured varieties of the same species yield fewer seeds, in the 
proportion of 86 to 100, than the similarly coloured varieties. 
Yet these varieties differ in no respect except in the colour of 
their flowers ; and one variety can sometimes be raised from the 
seed of another. 

Kolreuter, whose accuracy has been confirmed by every sub- 
sequent observer, has proved the remarkable fact, that one 
particular variety of the common tobacco was more fertile than 
the other varieties, when crossed with a widely distinct species. 
He experimented 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 offspring perfectly 
fertile. But one of these five varieties, when used either as the 
father or mother, and crossed with the Nicotiana glutinosa, 
always yielded hybrids not so sterile as those which were pro- 
duced 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 it can no longer be maintained that varieties 
when crossed are invariably quite fertile. From the great diffi- 
culty of ascertaining the infertility of varieties in a state of nature, 
for a supposed variety, if proved to be infertile in any degree, 
would almost universally be ranked as a species; from man. 
attending only to external characters in his domestic varieties, 
and from such varieties not having been exposed for very long 
periods to uniform conditions of life; from these several con- 
siderations we may conclude that fertility does not constitute a 
fundamental distinction between varieties and species when 
crossed. The general sterility of crossed species may safely be 
looked at, not as a special acquirement or endowment, but as 
incidental on changes of an unknown nature in their sexual 

Hybrids and Mongrels compared, independently of their fertility. 

Independently of the question of fertility, the offspring of species 
and of varieties when crossed may be compared in several other 
respects. Gartner, whose strong wish it was to draw a distinct 
line 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 
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 Gartner admits that hybrids 
from species Avhich have long been cultivated are often variable in 
the first generation ; and I have myself seen striking instances of 
this fact. Gartner further admits that hybrids between very 
closely allied species are more variable than those from very dis- 
tinct 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 the offspring in both cases is notorious ; but some 
few instances of both hybrids and mongrels long retaining a 
uniform character could be given. The variability, however, in 
the successive generations of mongrels is, perhaps, greater than in 

This greater variability in mongrels than in hybrids does not 
eeem at all surprising. For the parents of mongrels are varieties, 
and mostly domestic varieties (very few experiments having been 
tried on natural varieties), and this implies that there has been 
recent variability, which would often continue and would augment 
that arising from the act of crossing. The slight variability of 
hybrids in the first generation, in contrast with that in the 
succeeding generations, is a curious fact and deserves attention. 
For it bears on the view which I have taken of one of the causes 
of ordinary variability; namely, that the reproductive system 
from being eminently sensitive to changed conditions of life, fails 
under these circumstances to perform its proper function of pro- 
ducing offspring closely similar in all respects to the parent-form. 
Now hybrids in the first generation are descended from species 
(excluding those long-cultivated) which have not had their repro- 
ductive 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 cf mongrels and hybrids: 
Gartner states that mongrels are more liable than hybrids to 
revert to either parent-form ; but this, if it be true, is certainly 
only a difference in degree. Moreover, Gartner expressly states 
that hybrids from long cultivated plants are more subject to 
reversion than hybrids from species in their natural state; and 
this probably explains the singular difference in the results arrived 
at by different observers: thus Max Wichura doubts whether 
hybrids ever revert to their parent-forms, and he experimented on 
uncultivated species of willows; whilst Naudin, on the other 
hand, insists in the strongest terms on the almost universal 
tendency to reversion in hybrids, and he experimented chiefly on 
cultivated plants. Gartner further states 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 Kolreuter. 

Such alone are the unimportant differences which Gartner is 
able to point out between hybrid and mongrel plants. On the 
other hand, the degrees and kinds of resemblance in mongrels and 
in hybrids to their respective parents, more especially in hybrids 
produced from nearly related species, follow according to Gartner 
the same laws. When two species are crossed, one has sometimes 
a prepotent power of impressing its likeness on the hybrid. So I 
believe it to be with varieties of plants; and 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 mongrel plants from 
a reciprocal cross. Both hybrids and mongrels can be reduced to 
either pure parent-form, by repeated crosses in successive genera- 
tions with either parent. 

These several remarks are apparently applicable to animals; 
but the subject is here much 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 those authors are right 
who maintain that the ass has a prepotent power over the horse, 
so that both the mule and the hinny resemble more closely the 
ass than the horse ; but that the prepotency runs more strongly in 
the male than in the female ass, so that the mule, which is the 
off-spring 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 it is only with mongrels that the offspring are not inter- 
mediate in character, but closely resemble one of their parents ; 
but this does sometimes occur with hybrids, yet I grant much less 
frequently 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 through selection. A tendency 
to sudden reversions to the perfect character of either parent 
would, also, be much 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 con- 
clusion that the laws of resemblance of the child to its parents are 
the same, whether the two parents differ little or much from each 
other, namely, in the union of individuals of the same variety, or 
of different varieties, or of distinct species. 

Independently of 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 harmonises 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 most careful experimentalists have arrived at diametrically 
opposite conclusions in ranking forms by this test. The sterility 
is innately variable in individuals of the same species, and is 
eminently susceptible to the action of favourable and unfavourable 
conditions. The degree of sterility does not strictly follow 
systematic affinity, but is governed by several curious and com- 
plex 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 hybrids produced 
from this cross. 

In the same manner as in grafting trees, the capacity in one 
species or variety to take on another, is incidental on differences, 
generally of an unknown nature, 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 
their 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 
their inarching in our forests. 

The sterility of first crosses and of their hybrid progeny has not 
been acquired through natural selection. In the case of first 
crosses it seems to depend on several circumstances; in some 
instances in chief part on the early death of the embryo. In the 
case of hybrids, it apparently depends on their whole organisation 

246 SUMMARY. [CHAP. 12. 

having been disturbed by being compounded from two distinct 
forms ; the sterility being closely allied to that which so frequently 
affects pure species, when exposed to new and unnatural conditions 
of life. He who will explain these latter cases will be able to 
explain the sterility of hybrids. This view is strongly supported 
by a parallelism of another kind: namely, that, firstly, slight 
changes in the conditions of life add to the vigour and fertility of 
all organic beings ; and secondly, that the crossing of forms, which 
have been exposed to slightly different conditions of life or which 
have varied, favours the size, vigour, and fertility of their offspring. 
The facts given on the sterility of the illegitimate unions of 
dimorphic and trimorphic plants and of their illegitimate progeny, 
perhaps render it probable that some unknown bond in all cases 
connects the degree of fertility of first unions with that of their 
offspring. The consideration of these facts on dimorphism, as 
well as of the results of reciprocal crosses, clearly leads to the con- 
clusion that the primary cause of the sterility of crossed species is 
confined to differences in their sexual elements. But why, in the 
case of distinct species, the sexual elements should so generally 
have become more or less modified, leading to their mutual 
infertility, we do not know; but it seems to stand in some close 
relation to species having been exposed for long periods of time to 
nearly uniform conditions of life. 

It is not surprising that the difficulty in crossing any two 
species, and the sterility of their hybrid offspring, should in most 
cases correspond, even if due to distinct causes : for both depend 
on the amount of difference between the species which are crossed. 
Nor is it surprising that the facility of effecting a first cross, and 
the fertility of the hybrids thus 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 
subjected to experiment ; for systematic affinity includes re- 
semblances of all kinds. 

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, as is so often stated, invariably fertile. 
Nor is this almost universal and perfect fertility surprising, when 
it is remembered 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 domestica- 
tion by the selection of mere external differences, and that they 
have not been long exposed to uniform conditions of life. It 
should also be especially kept in mind, that long-continued 
f^mestication tends to eliminate sterility, and is therefore little 
likely to induce this same quality. Independently of the question 


of fertility, in all other respects there is the closest general 
resemblance between hybrids and mongrels, in their variability, 
in their power of absorbing each other by repeated crosses, and in 
their inheritance of characters from both parent-forms. Finally, 
then, although we are as ignorant of the precise cause of the 
sterility of first crosses and of hybrids as we are why animals and 
plants removed from their natural conditions become sterile, yet 
the facts given in this chapter do not seem to me opposed to the 
belief that species aboriginally existed as varieties. 


On the absence of intermediate varieties at the present day On the nature of 
extinct intermediate varieties ; on their number On the lapse of time, as 
inferred from the rate of denudation and of deposition On the lapse of 
time as estimated by years On the poorness of our palaoontological col- 
lections On the intermittence of geological formations On the denuda- 
tion of granitic areas On the absence of intermediate varieties in any one 
formation On the sudden appearance of groups of species On their 
sudden appearance in the lowest known fossiliferous strata Antiquity of 
the habitable earth. 

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 distinct- 
ness 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 oa 
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 exist- 
ing in lesser numbers than the forms which they connect, will 
generally be beaten out and exterminated during the course ot 
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 selec- 
tion, through which new varieties continually take the places of 
and supplant 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, 
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 serious objection which 
can be urged against the 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 the 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 un- 
known 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 are both 
descended from the rock-pigeon; if we possessed all the inter- 
mediate 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 ex- 
panded 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, C. livia, whether they had descended from this 
species or from some other allied form, 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 directly intermediate between them ever existed, 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 the 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 Jife tend to supplant the old and un- 
improved forms. 

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 natural and domestic 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 forms ; 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 as- 
suredly, if this theory be true, such have lived upon the earth. 

On, tlie Lapse of Time, as inferred from the rate of Deposition and 
extent of Denudation. 

Independently of our not finding fossil remains of such infinitely 
numerous connecting links, it may be objected that time cannot 
have sufficed for so great an amount of organic change, all changes 
having been effected slowly. It is hardly possible for me to recall 
to the reader who is not a practical geologist, the facts leading the 
mind feebly to comprehend the lapse of time. He who can read 
Sir Charles Ly ell's grand work on the Principles of Geology, which 
the future historian will recognise as having produced a revolu- 
tion in natural science, and yet does not admit how 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 of each stratum. We can best 
gain some idea of past time by knowing the agencies at work, and 
learning how deeply the surface of the land has been denuded, and 
how much sediment has been deposited. As Lyell has well re- 
marked, the extent and thickness of our sedimentary formations 
are the result and the measure of the denudation which the earth's 
crust has elsewhere undergone Therefore a man should examino 
for himself the great piles of superimposed strata, and watch the 
rivulets bringing down mud, and the waves wearing away the sea- 
cliffs, in order to comprehend something about the duration of 
past time, the monuments of which we see all around us. 

It is good to wander along the 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 ot 


pebbles ; for there is good evidence that pure water effects nothing 
in wearing away rock. At last the base of the cliff is undermined, 
huge fragments fall down, and these, remaining fixed, have to bo 
worn away atom by atom, until after being reduced in size they 
can be rolled about by the waves, and then they 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 else- 
where years have elapsed since the waters washed their base. 

We have, however, recently learnt from the observations of 
Ramsay, in the van of many excellent observers of Jukes, 
Geikie, Croll, and others, that subaerial degradation is a much 
more important agency that coast-action, or the power of the 
waves. The whole surface of the land is exposed to the chemical 
action of the air and of the rain-water with its dissolved carbonic 
acid, and in colder countries to frost ; the disintegrated matter is 
carried down even gentle slopes during heavy rain, and to a 
greater extent than might be supposed, especially in arid districts, 
by the wind; it is then transported by the streams and rivers, 
which when rapid deepen their channels, and triturate the 
fragments. On a rainy day, even in a gently undulating country, 
we see the effects of subaerial degradation in the muddy rills 
which flow down every slope. Messrs. Ramsay and Whitaker 
have shown, and the observation is a most striking one, that the 
great lines of escarpment in the Wealden district and those 
ranging across England, which formerly were looked at as ancient 
sea-coasts, cannot have been thus formed, for each line is com- 
posed of one and the same formation, whilst our sea-cliffs are 
everywhere formed by the intersection of various formations. 
This being the case, we are compelled to admit that the escarp- 
ments owe their origin in chief part to the rocks of which they 
are composed having resisted subaerial denudation better than 
the surrounding surface ; this surface consequently has been 
gradually lowered, with the lines of harder rock left projecting. 
Nothing impresses the mind with the vast duration of time, 
according to our ideas of time, more forcibly than the conviction 
thus gained that subaerial agencies which apparently have so 
little power, and which seem to work so slowly, have produced 
great results. 

When thus impressed with the slow rate at which the land is 
away through subaerial and littoral action, it is good, ia 


order to appreciate the past duration of time, to consider, on the 
one hand, the masses of rock which have been removed over many 
extensive areas, and on the other hand the thickness of our sedi- 
mentary formations. I remember having been much struck 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 told still more plainly 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, and it makes no great difference whether the 
upheaval was sudden, or, as most geologists now believe, was slow 
and effected by many starts, the surface of the land has been so 
completely planed down 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 varies from 600 to 3000 feet. Professor Ramsay has 
published an account of a downthrow in Anglesea of 2300 feet ; 
and he informs me that he fully believes that there is one in. 
Merionethshire of 12,000 feet ; yet in these cases there is nothing 
on the surface of the land to show such prodigious movements; 
the pile of rocks on either side of the crack having been smoothly 
swept away. 

On the other hand, in all parts of the world the piles of sedi- 
mentary strata are of wonderful thickness. In the Cordillera I 
estimated one mass of conglomerate at ten thousand feet; and 
although conglomerates have probably been accumulated at a 
quicker rate than finer sediments, yet from being formed of worn 
and rounded pebbles, each of which bears the stamp of time, they 
are good to show how slowly the mass must have been heaped 
together. Professor Ramsay has given me the maximum thick- 
ness, from actual measurement in most cases, of the successive 
formations in different parts of Great Britain ; and this is the 
result : 


Paheozoic 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 the 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, blank 
periods of enormous length. So that the lofty pile of sedimen- 


tary rocks in Britain gives but an inadequate idea of the time 
which has elapsed during their accumulation. The consideration 
of these various facts impresses the mind almost in the same 
manner as does the vain endeavour to grapple with the idea of 

Nevertheless this impression is partly false. Mr. Croll, in an 
interesting paper, remarks that we do not err " in forming too 
"great a conception of the length of geological periods," but in 
estimating them by years. When geologists look at large and 
complicated phenomena, and then at the figures representing 
several million years, the two produce a totally different effect on 
the mind, and the figures are at once pronounced too small. In 
regard to subaerial denudation, Mr. Croll shows, by calculating 
the known amount of sediment annually brought down by certain 
rivers, relatively to their areas of drainage, that 1000 feet of solid 
rock, as it became gradually disintegrated, would thus be removed 
from the mean level of the whole area in the course of six million 
years. This seems an astonishing result, and some considerations 
lead to the suspicion that it may be too large, but even if halved 
or quartered it is still very surprising. Few of us, however, know 
what a million really means : Mr. Croll gives the following 
illustration: take a narrow strip of paper, 83 feet 4 inches in 
length, and stretch it along the wall of a large hall ; then mark off 
at one end the tenth of an inch. This tenth of an inch will 
represent one hundred years, and the entire strip a million years. 
But let it be borne in mind, in relation to the subject of this 
work, what a hundred years implies, represented as it is by a 
measure utterly insignificant in a hall of the above dimensions. 
Several eminent breeders, during a single lifetime, have so largely 
modified some of the higher animals, which propagate their kind 
much more slowly than most of the lower animals, that they have 
formed what well deserves to be called a new sub-breed. Few 
men have attended with due care to any one strain for more than 
half a century, so that a hundred years represents the work of 
two breeders in succession. It is not to be supposed that species 
in a state of nature ever change so quickly as domestic animals 
under the guidance of methodical selection. The comparison 
would be in every way fairer with the effects which follow from 
unconscious selection, that is the preservation of the most useful 
or beautiful animals, with no intention of modifying the breed ; 
but by this process of unconscious selection, various breeds have 
been sensibly changed in the course of two or three centuries. 

Species, however, probably change much more slowly, and 
within the same country only a few change at the same time. 
This slowness follows from all the inhabitants of the same 
country being already so well adapted to each other, that new 


places in the polity of nature do not occur until after long 
intervals, due to the occurrence of physical changes of some kind, 
or through the immigration of new forms. Moreover variations or 
individual differences of the right nature, by which some of the 
inhabitants might be better fitted to their new places under the 
altered circumstances, would not always occur at once. Unfor- 
tunately we have no means f of determining, according to the 
standard of years, how long a period it takes to modify a species ; 
but to the subject of time we must return. 

On the Poorness of Palceont oloyical Collections. 

Now let us turn to our richest geological museums, and what a 
paltry display we behold ! That our collections are imperfect is 
admitted by every one. The remark of that admirable palaeonto- 
logist, Edward Forbes, should never be forgotten, namely, that very 
many 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 im- 
portant discoveries made every year in Europe prove. No 
organism wholly soft can be preserved. Shells and bones decay 
and disappear when left on the bottom of the sea, where sediment 
is not accumulating. We probably take a quite erroneous view, 
when we assume 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 pro- 
portion 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 immense 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 charged with carbonic acid. Some 
of the many kinds of animals which live on the beach between 
high and low water mark seem to be rarely preserved. For 
instance, the several species of the Chthamalinae (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 this 
has been found fossil in Sicily, whereas not one other species has 
hitherto been found in any tertiary formation: yet it is known 
that the genus Chthamalus existed during the Chalk period. 
Lastly, many great deposits requiring a vast length of time for 


their accumulation, are entirely destitute of organic remains, 
without our being able to assign any reason: one of the most 
striking instances is that of the Flysch formation, which consists 
of shale and sandstone, several thousand, occasionally even six 
thousand feet in thickness, and extending for at least 300 miles 
from Vienna to Switzerland ; and although this great mass has 
been most carefully searched, no fossils, except a few vegetable 
remains, have been found. 

With respect to the terrestrial productions which lived during 
the Secondary and Palaeozoic periods, it is superfluous to state 
that our evidence is fragmentary in an extreme degree. For 
instance, until recently not a land-shell was known belonging to 
either of these vast periods, with the exception of one species 
discovered by Sir C. Lyell and Dr. Dawson in the carboniferous 
strata of North America ; but now land-shells have been found 
in. the lias. In regard to mammiferous remains, a glance at the 
historical table published in 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 largely results 
from another and more important cause than any of the fore- 
going ; namely, from the several formations being separated from 
each other by wide intervals of time. This doctrine has been 
emphatically admitted by many geologists and palaeontologists, 
who, like E. Forbes, entirely disbelieve in the change of species. 
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 E. 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 confined ex- 
clusively 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 was 


derived, accord with the belief of vast intervals of time having 
elapsed between each formation. 

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 
within the recent period, than the absence of any recent deposits 
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 poorly 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 con- 
tinually worn away, as soon as they are brought up by the slow 
and gradual rising of the land within the grinding action of the 

We may, I think, conclude that sediment must be accumulated 
in extremely thick, solid, or extensive masses, in order to with- 
stand the incessant action of the waves, when first upraised and 
during successive oscillations of level, as well as the subsequent 
subaerial degradation. Such thick and extensive accumulations 
of sediment may be formed in two ways ; either in profound 
depths of the sea, in which case the bottom will not be inhabited 
by so many and such varied forms of life, as the more shallow 
seas ; and the mass when upraised will give an imperfect record 
of the organisms which existed in the neighbourhood during the 
period of its accumulation. Or, sediment may be deposited 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 the supply of sediment nearly balance each other, 
the sea will remain shallow and favourable for many and varied 
forms, and thus a rich fossiliferous formation, thick enough, when 
upraised, to resist a large amount of denudation, may be formed. 

I am convinced that nearly all our ancient formations, which 
are throughout the greater part of their thickness 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 con- 
clusion 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 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 oscilla- 
tions have affected wide spaces. Consequently, formations rich 
in fossils and sufficiently thick and extensive to resist subsequent 
degradation, will 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 remains stationary, thick deposits 
cannot have been accumulated in the shallow parts, which are 
the most favourable to life. Still less can this have happened 
during the alternate periods of elevation ; or, to speak more 
accurately, the beds which were then accumulated will generally 
have been destroyed by being upraised and brought within the 
limits of the coast-action. 

These remarks apply chiefly to littoral and sub-littoral deposits. 
In the case of an extensive and shallow sea, such as that within 
a large part of the Malay Archipelago, where the depth varies 
from 30 or 40 to 60 fathoms, a widely extended formation might 
be formed during a period of elevation, and yet not suffer exces- 
sively from denudation during its slow upheaval ; but the thick- 
ness of the formation could not be great, for owing to the elevatory 
movement it would be less than the depth in which it was formed ; 
nor would the deposit be much consolidated, nor be capped by 
overlying formations, so that it would run a good chance of being 
worn away by atmospheric degradation and by the action of the 
sea during subsequent oscillations of level. It has, however, been 
suggested by Mr. Hopkins, that if one part of the area, after 
rising and before being denuded, subsided, the deposit formed 
during the rising movement, though not thick, might afterwards 
become protected by fresh accumulations, and thus be preserved 
for a long period. 

Mr. Hopkins also expresses his belief that sedimentary beds of 
considerable horizontal extent have rarely been completely de- 
stroyed. But all geologists, excepting the few who believe that 
our present inetamorphic schists and plutonic rocks once formed 
the primordial nucleus of the globe, will admit that these latter 
rocks have been stript of their covering to an enormous extent. 
For it is scarcely possible that such rocks could have been solidified 
and crystallized whilst uncovered ; but if the metamorphic action 
occurred at profound depths of the ocean, the former protecting 


mantle of rock may not have been very thick. Admitting then 
that gneiss, mica-schist, granite, diorite, &c., were once necessarily 
covered up, how can we account for the naked and extensive areas 
of such rocks in many parts of the world, except on the belief 
that they have subsequently been completely denuded of all over- 
lying strata? That such extensive areas do exist cannot be 
doubted : the granitic region of Parime is described by Humboldt 
as being at least nineteen times as large as Switzerland. South 
of the Amazon, Boue colours an area composed of rocks of this 
nature as equal to that of Spain, France, Italy, part of Germany, 
and the British Islands, all conjoined. This region has not been 
carefully explored, but from the concurrent testimony of travellers, 
the granitic area is very large: thus, Von Eschwege gives a 
detailed section of these rocks, stretching from Rio de Janeiro for 
260 geographical miles inland in a straight line ; and I travelled 
for 150 miles in another direction, and saw nothing but granitic 
rocks. Numerous specimens, collected along the whole coast from 
near Rio Janeiro to the mouth of the Plata, a distance of 1100 
geographical miles, were examined by me, and they all belonged 
to this class. Inland, along the whole northern bank of the Plata 
I saw, besides modern tertiary beds, only one small patch of 
slightly metamorphosed reck, which alone could have formed a 
part of the original capping of the granitic series. Turning to a 
well-known region, namely, to the United States and Canada, as 
shown in Professor H. D. Rogers's beautiful map, I have esti- 
mated the areas by cutting out and weighing the paper, and I 
find that the metamorphic (excluding " the semi-metariiorphic ") 
and granitic rocks exceed, in the proportion of 19 to 12'5, the 
whole of the newer Palaeozoic formations. In many regions the 
metamorphic and granitic rocks would be found much more widely 
extended than they appear to be, if all the sedimentary beds were 
removed which rest unconformably on them, and which could not 
have formed part of the original mantle under which they were 
crystallized. Hence it is probable that in some parts of the world 
whole formations have been completely denuded, with not a 
wreck left behind. 

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 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 on the shores of a continent 
when first broken up into an archipelago), and consequently 
during subsidence, though there will be much extinction, few 


new varieties or species will be formed; and it is during these 
very periods of subsidence, that the deposits which are richest in 
fossils have been accumulated. 

On the Absence of Numerous Intermediate Varieties in any 
Single Formation. 

From these several 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 
much 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. Several cases are on record 
of the same species presenting varieties in the upper and lower 
parts of the same formation; thus, Trautschold gives a number 
of instances with Ammonites; and Hilgendorf has described a 
most curious case of ten graduated forms of Planorbis multiformis 
in the successive beds of a fresh-water formation in Switzerland. 
Although each formation has indisputably required a vast number 
of years for its deposition, several reasons can be given why each 
should not commonly include a graduated series of links between 
the species which lived at its commencement and close ; but I 
cannot assign due proportional weight to the following con- 

Although each formation may mark a very long lapse of years, 
each probably is short compared with the period requisite to 
change one species into another. I am aware that two palaeonto- 
logists, 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 from 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 last layers have been deposited, 
it would be equally rash to suppose that it then became 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 forma- 
tion throughout Europe been correlated with perfect accuracy. 

We may safely infer that with marine animals of all kinds 
there has been a large amount of migration due to 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 
appear 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 in 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 imme- 
diately surrounding sea ; or, conversely, that some are now abun- 
dant 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 epoch, which forms only a part of one whole 
geological period ; and likewise to reflect on the changes of level, 
on the extreme change of climate, and on the great 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 best 
flourish : for we know that great 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 migrations of species and to 
geographical changes. And in the distant future, a geologist, 
examining these beds, would 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 continuously accumulating during a long period, 
sufficient for the slow process of modification ; hence the deposit 
must be a very thick one; and the species undergoing change 
must have lived in the same district throughout the whole time. 
But we have seen that a thick formation, fossiliferous throughout 
its entire thickness, can accumulate only during a period of sub- 
sidence; and to keep the depth approximately the same, which 
is necessary that the same marine species may live on the same 
space, the supply of sediment must nearly counterbalance the 
amount of subsidence. But this same movement of subsidence 
will tend to submerge 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 forma- 
tion composed of beds of widely different mineralogical com- 
position, we may reasonably suspect that the process of deposition 
has been more or less interrupted. Nor will the closest inspection 
of a formation give us any idea of the length of time which its 
deposition may have consumed. Many instances could be given 
of beds only a few feet in thickness, representing formations, 
which are 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 even 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 not have been suspected, had not the trees been preserved : 
thus Sir C. Lyell and Dr. 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 occurs a,t the bottom, middle, and top of a 
formation, the probability is that it has not lived on the same 
spot during the whole period of deposition, but has disappeared 
and reappeared, perhaps many times, during the same geological 
period. Consequently if it were to undergo a considerable amount 
of modification during the deposition of any one geological for- 
mation, a section would not include all the fine intermediate 
gradations which must on our theory have existed, but abrupt, 
though perhaps 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 the closest 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 older and 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 closely connected by intermediate varieties with either 
one or both forms. Nor should it be forgotten, as before explained, 
that A might be the actual progenitor of B and C, and yet would 
not necessarily be strictly intermediate between them in all 
respects. So that we might obtain the parent-species and its 
several modified descendants from the lower and upper beds of 
the same formation, and unless we obtained numerous transitional 
gradations, we should not recognise their blood-relationship, and 
should consequently rank them 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 the theory we ought to find. 
Look again at the later tertiary deposits, which include many 
shells believed by the majority of naturalists to be identical with 
existing species ; but some excellent naturalists, as Agassiz and 
Pictet, maintain that all these tertiary species are specifically 
distinct, though the distinction is admitted to be very slight ; so 
that here, unless we believe that these eminent naturalists have 
been misled by their imaginations, and that these late tertiary 
species really present no difference whatever from their living 
representatives, or unless we admit, in opposition to the judgment 
of most naturalists, that these tertiary species are all truly distinct 
from the recent, we have evidence of the frequent occurrence of 
slight modifications of the kind required. If we look to rather 
wider intervals of time, namely, to distinct but consecutive stages 
of the same great formation, we find that the embedded fossils, 
though universally ranked as specifically different, yet are far 
more closely related to each other than are the species found in 
more widely separated formations ; so that here again we have 
undoubted evidence of change in the direction required by the 
theory ; but to this latter subject I shall return in the following 

With animals and plants that propagate rapidly and do not 
wander much, 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 dis- 
covering 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 vide range ; and we hare 
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 probable that those which had the widest 
range, far exceeding the limits of the known geological formations 
in Europe, 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 is a more important consideration, leading to the same 
result, as lately insisted on by Dr. Falconer, namely, that the 
period during which each species underwent modification, though 
long as measured by years, was probably short in comparison 
with that during which it remained without undergoing any 

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 are collected from many places ; and with 
fossil species this can rarely be done. 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 are 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 con- 
chologists 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 
by the future geologist only by his discovering in a fossil state 
numerous intermediate gradations ; and such success is im- 
probable in the highest degree. 

It has been asserted over and over again, by writers who 
believe in the immutability of species, that geology yields no 
linking forms. This assertion, as we shall see in the next chapter, 
is certainly erroneous. As Sir J. Lubbock has remarked, " Every 
"species is a link between other allied forms." If we take a genus 
having a score of species, recent and extinct, and destroy four- 
fifths of them, no one doubts that the remainder will stand much 
more distinct from each other. If the extreme forms in the genus 
happen to have been thus destroyed, the genus itself will stand 
more distinct from other allied genera. What geological research 
has not revealed, is the former existence of infinitely numerous 
gradations, as fine as existing varieties, connecting together nearly 
all existing and extinct species.. But this ought not to be expected ; 


yet this has been repeatedly advanced as a most serious objection 
against my views. 

It may be worth while to sum up the foregoing remarks on the 
causes of the imperfection of the geological record under an 
imaginary illustration. The Malay Archipelago is about the size 
of Europe from the North Cape to the Mediterranean, and from 
Britain to Russia ; and therefore equals all the geological forma- 
tions 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 Archi- 
pelago, with its numerous large islands separated by wide and 
shallow seas, probably represents the former state of Europe, 
whilst most of our formations were accumulating. The Malay 
Archipelago is one of the richest regions 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 produc- 
tions of the archipelago would be preserved in an extremely 
imperfect manner in the formations which we suppose to be there 
accumulating. 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. 

Formations rich in fossils of many kinds, and of thickness suffi- 
cient to last to an age as distant in futurity as the secondary forma- 
tions lie in the past, would generally be formed in the archipelago 
only during periods of subsidence. These periods of subsidence 
would be separated from each other by immense intervals of time, 
during which the area would be either stationary or rising ; whilst 
rising, the fossiliferous formations on the steeper shores would be 
destroyed, almost as soon as accumulated, by the incessant coast- 
action, as we now see on the shores of South America. Even 
throughout the extensive and shallow seas within the archipelago, 
sedimentary beds could hardly be accumulated of great thick- 
ness during the periods of elevation, or become capped and 
protected by subsequent deposits, so as to have a good chance of 
enduring to a very distant future. During the periods of sub- 
sidence, 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 less 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 con- 
tingencies are indispensable for the preservation of all the transi- 
tional gradations between any two or more species. If such 
gradations were not all fully preserved, transitional varieties 
would merely appear as so many new, though closely allied 
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 
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 plainly 
leads to the belief that it would be chiefly these far-ranging 
species, though only some of them, which would oftenest produce 
new varieties ; and the varieties would at first 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, and 
as they would be found embedded in slightly different sub-stages 
of the same formation, they would, according to the principles 
followed by many palaeontologists, be ranked as new and distinct 

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 our theory, 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, and such assuredly we do find some more distantly, 
some more closely, related to each other ; and these links, let them 
be ever so close, if found in different stages of the same formation, 
would, by many palaeontologists, be ranked as distinct species. 
But I do not pretend that I should ever have suspected how poor 
was the record in the best preserved geological sections, had not 
the absence of innumerable transitional links between the species 
which lived 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 palaeonto- 
logists for instance, by Agassiz, Pictet, and 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 at once, the fact would be fatal to the theory of evolution, 
through natural selection. For the development by this means 
of a group of forms, all of which are descended from some one 
progenitor, must have been an extremely slow process ; and the 
progenitors must have lived long before their modified descendants. 
But we continually overrate the perfection of the geological 
record, and falsely infer, because certair genera or families have 
not been found beneath a certain stage, that they did not exist 
before that stage. In all cases positive palseontological evidence 
may be implicitly trusted ; negative evidence is worthless, as 
experience has so often shown. 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 multi- 
plied, before they invaded the ancient archipelagoes of Europe and 
the United States. We do not make due allowance for the 
intervals of time which have elapsed between our consecutive 
formations, longer perhaps in many 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 parent- 
form: and in the succeeding formation, such groups or 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 ; 
and consequently that the transitional forms would often long 
remain confined to some one region ; but that, when this adapta- 
tion had once 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 
spread rapidly and widely, throughout the world. Professor Pictet, 
in his excellent Review of this work, in commenting on early 
transitional forms, and taking birds as an illustration, cannot see 
hoAv the successive modifications of the anterior limbs of a sup- 
posed prototype could possibly have been of any advantage. But 
look at the penguins of the Southern Ocean ; have not these birds 
their front limbs in this precise intermediate state of " neither true 
" arms nor true wings " ? Yet these birds hold their place victori- 
ously in the battle for life ; for they exist in infinite numbers and 
of many kinds. I do not suppose that we here see the real 
transitional grades through which the wings of birds have passed ; 
but what special difficulty is there in believing that it might profit 
the modified descendants of the penguin, first to become enabled 
to flap along the surface of the sea like the logger-headed duck, 
and ultimately to rise from its surface and glide through the air 1 

I will now give a few examples to illustrate the foregoing, 


remarks, and to show how liable we are to error in supposing that 
whole groups of species have suddenly been produced. Even in 
so short an interval as that between the first and second editions 
of Pictet's great work on Palaeontology, published in 1844-46 and 
in 1853-57, the conclusions on the first appearance and disappear- 
ance of several groups of animals have been considerably modified ; 
and a third edition would require still further changes. I may 
recall the well-known fact that in geological treatises, published 
not many years ago, mammals were 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 true 
mammals have 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, as far back as the miocene stage. Had it not been for the 
rare accident of the preservation of footsteps in the new red sand- 
stone of the United States, who would have ventured to suppose 
that no less than at least thirty different bird-liKe animals, some 
of gigantic size, existed during that period ? Not a fragment of 
bone has been discovered in these beds. Not long ago, palaeonto- 
logists maintained that the whole class of birds came suddenly 
into existence during the eocene period ; but now we know, on the 
authority of Professor Owen, that a bird certainly lived during the 
deposition of the upper greensand ; and still more recently, that 
strange bird, the Archeopteryx, with a long lizard-like tail, bearing 
a pair of feathers on each joint, and with its wings furnished with 
two free claws, has been discovered in the oolitic slates of Solen- 
hofen. Hardly any recent discovery shows more forcibly than 
this, how little we as yet know of the former inhabitants of the 

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 stated that, from the large 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 then 
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 then 
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 per- 
fect 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 cirripede was a Chthamalus, a 
very common, large, and ubiquitous genus, of which not one species 
has as yet been found even in any tertiary stratum. Still more 
recently, a Pyrgoma, a member of a distinct sub-family of sessile 
cirripedes, has been discovered by Mr. Woodward in the upper 
chalk ; so that we now have abundant evidence of the existence 
of this group of animals during the secondary period. 

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, according to Agassiz, in the 
Chalk period. This group includes the large majority of existing 
species. But certain Jurassic and Triassic forms are now commonly 
admitted to be teleostean ; and even some palaeozoic forms have 
thus been classed by one high authority. If the teleosteans had 
really appeared suddenly in the northern hemisphere at the com- 
mencement of the chalk formation, the fact would have been 
highly remarkable ; but it would not have formed an insuperable 
difficulty, unless it could likewise have been shown that at the 
same period the species were suddenly and simultaneously 
developed in other quarters of the world. 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 fishes might formerly have had a similarly confined 
range, and after having been largely developed in some one sea, 
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 considerations, from our ignorance of the geology of 
other countries beyond the confines of Europe and the United 
States, and from the revolution in our palaaontological knowledge 
effected by the discoveries of the last dozen years, it seems to me 



to be about as rash to dogmatize on the succession of organic 
forms throughout the world, as it would be for a naturalist to land 
for five minutes on a 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 the lowest 
known Fossil if erous Strata. 

There is another and allied difficulty, which is much more 
serious. I allude to the manner in which species belonging to 
several of the main divisions of the animal kingdom suddenly 
appear in the lowest known fossiliferous rocks. Most of the argu- 
ments which have convinced me that all the existing species of the 
same group are descended from a single progenitor, apply with 
equal force to the earliest known species. For instance, it cannot 
be doubted that all the Cambrian and Silurian trilobites are 
descended from some one crustacean, which must have lived long 
before the Cambrian age, and which probably differed greatly from 
any known animal. Some of the most ancient animals, as the 
Nautilus, Lingula, &c., do not differ much from living species; 
and it cannot on our theory be supposed, that these old species 
were the progenitors of all the species belonging to the same 
groups which have subsequently appeared, for they are not in any 
degree intermediate in character. 

Consequently, if the theory be true, it is indisputable that 
before the lowest Cambrian stratum was deposited long periods 
elapsed, as long as, or probably far longer than, the whole interval 
from the Cambrian age to the present day ; and that during these 
vast periods the world swarmed with living creatures. Here we 
encounter a formidable objection; for it seems doubtful whether 
the earth, in a fit state for the habitation of living creatures, has 
lasted long enough. Sir TV. Thompson concludes that the con- 
solidation of the crust can hardly have occurred less than 20 or 
more than 400 million years ago, but probably not less than 98 or 
more than 200 million years. These very wide limits show how 
doubtful the data are ; and other elements may have hereafter to 
be introduced into the problem. Mr. Croll estimates that about 
60 million years have elapsed since the Cambrian period, but this, 
judging from the small amount of organic change since the 
commencement of the Glacial epoch, appears a very short time 
for the many and great mutations of life, which have certainly 
occurred since the Cambrian formation; and the previous 140 
million years can hardly be considered as sufficient for the 
development of the varied forms of life which already existed 
during the Cambrian period. It is, however, probable, as Sir 
William Thompson insists, that the world at a very early period 
was subjected to more rapid and violent changes in its physical 


conditions than those now occurring; and such changes would 
have tended to induce changes at a corresponding rate in the 
organisms which then existed. 

To the question why we do not find rich fossiliferous deposits 
belonging to these assumed earliest periods prior to the Cambrian 
system, I can give no satisfactory answer. Several eminent 
geologists, with Sir R. Murchison at their head, were until 
recently convinced that we beheld in the organic remains of the 
lowest Silurian stratum the first dawn of life. Other highly 
competent judges, as Lyell and E. Forbes, have disputed this 
conclusion. We should not forget that only a small portion of 
the world is known with accuracy. Not very long ago M. 
Barrande added another and lower stage, abounding with new 
and peculiar species, beneath the then known Silurian system ; 
and now, still lower down in the Lower Cambrian formation, Mr. 
Hicks has found in South Wales beds rich in trilobites, and 
containing various molluscs and annelids. The presence of 
phosphatic nodules and bituminous matter, even in some of the 
lowest azoic rocks, probably indicates life at these periods ; and 
the existence of the Eozoon in the Laurentian formation of 
Canada is generally admitted. There are three great series of 
strata beneath the Silurian system in Canada, in the lowest of 
which the Eozoon is found. Sir W. Logan states that their 
'united thickness may possibly far surpass that of all the 
'succeeding rocks, from the base of the palaeozoic series to the 
' present time. We are thus carried back to a period so remote, 
'that the appearance of the so-called Primordial fauna (of 
'Barrande) may by some be considered as a comparatively 
' modern event." The Eozoon belongs to the most lowly organ- 
ised of all classes of animals, but is highly organised for its class ; 
it existed in countless numbers, and, as Dr. Dawson has remarked, 
certainly preyed on other minute organic beings, which must have 
lived in great numbers. Thus the words, which I wrote in 1859, 
about the existence of living beings long before the Cambrian 
period, and which are almost the same with those since used by 
Sir W. Logan, have proved true. Nevertheless, the difficulty of 
assigning any good reason for the absence of vast piles of strata 
rich in fossils beneath the Cambrian system is very great. It 
does not seem probable that the most ancient beds have been 
quite worn away by denudation, or that their fossils have been 
wholly obliterated by metamorphic action, for if this had been 
the case we should have found only small remnants of the for- 
mations next succeeding them in age, and these would always 
have existed in a partially metamorphosed condition. But the 
descriptions which we 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 in- 
variably it has suffered extreme denudation and metamorphism. 

The case at present must remain inexplicable; and may be 
truly urged as a valid argument against the views here enter- 
tained. 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 now existing con- 
tinents of Europe ai d North America. The same view has since 
been maintained by Agassiz and others. But we do not know 
what was the state of things in the intervals between the several 
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 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 hardly 
one truly oceanic island (with the exception of New Zealand, if 
this can be called a truly 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, palaeozoic and 
secondary formations would in all probability have been accumu- 
lated from sediment derived from their wear and tear ; and these 
would have been at least partially upheaved by the oscillations of 
level, which 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 were continents now exist, large tracts of 
land have existed, subjected no doubt to great oscillations of 
level, since the Cambrian 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 we have no reason to assume that things have 
thus remained from the beginning of the world. 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 long antecedent to the Cambrian 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 sedimentary formations in a 
recognisable condition older than the Cambrian 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 meta- 
morphic 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 naked 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 Cambrian epoch in a completely metamorphosed 
and denuded condition. 

The several difficulties here discussed, namely that, though we 
find in our geological formations many links between the species 
which now exist and which formerly existed, we do not find 
infinitely numerous fine transitional forms closely joining them all 
together ; the sudden manner in which several groups of species 
first appear in our European formations ; the almost entire 
absence, as at present known, of formations rich in fossils beneath 
the Cambrian strata, are all undoubtedly of the most serious 
nature. We see this in the fact that the most eminent palaeon- 
tologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E. 
Forbes, <fec., and all our greatest geologists, as Lyell, Murchison, 
Sedgwick, &c., have unanimously, often vehemently, maintained 
the immutability of species. But Sir Charles Lyell now gives 
the support of his high authority to the opposite side ; and most 
geologists and palaeontologists are much shaken in their former 
belief. Those who believe that the geological record is in any 
degree perfect, will undoubtedly at once reject the theory. For 
my part, following out Lyell's metaphor, I look at the 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, more or less different in the successive 
chapters, may represent the forms of life, which are entombed in 
our consecutive formations, and which falsely appear to have 
been abruptly introduced. 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 pre- 
ceding and present chapter. 

LET us now see whether the several facts and laws relating to 
the geological succession of organic beings accord best with the 
common view of the immutability of species, or with that of their 
slow and gradual modification, through variation and natural 

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 the stages, and to make the proportion between the lost 
and existing 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 extinct, and only one or two are new, 
having appeared there for the first time, either locally, or, as far 
as we know, on the face of the earth. The secondary formations 
are more broken ; but, as Bronn has remarked, neither the appear- 
ance nor disappearance of the many species embedded in each 
formation has been simultaneous. 

Species belonging to different genera and classes have not 
changed at the same rate, or in the same degree. In the older 
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, for an existing crocodile is associated 
with many 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 have changed at a quicker rate than those of 
the sea, of which a striking instance has been observed in Switzer- 
land. There is some reason to believe that organisms high in the 
scale, change more quickly than those that are low : though there 
are exceptions to this rule. The amount of organic change, as 


Pictet has remarked, is not the same in each successive so-called 
formation. Yet if we compare any but the most closely related 
format ions, 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 no reason to believe that the same identical form 
ever 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 satisfactory. 

These several facts accord well with our theory, which includes 
no fixed law of development, causing all the inhabitants of an 
area to change abruptly, or simultaneously, or to an equal degree. 
The process of modification must be slow, and will generally affect 
only a few species at the same time ; for the variability of each 
species is independent of that of all others. Whether such 
variations or individual differences as may arise will be accumu- 
lated through natural selection in a greater or less degree, thus 
causing a greater or less amount of permanent modification, will 
depend on many complex contingencies on the variations being 
of a beneficial nature, on the freedom of intercrossing, on the 
slowly changing physical conditions of the country, on the 
immigration of new colonists, and on the nature of the other 
inhabitants with which the varying species come into competi- 
tion. Hence it is by no means surprising that one species should 
retain the same identical form much longer than others; or, if 
changing, should change in a less degree. We find similar relations 
between the existing inhabitants of distinct countries ; for in- 
stance, the land-shells and coleopterous insects of Madeira have 
come to differ considerably from their nearest allies on the con- 
tinent 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 any area have become modified 
and improved, we can understand, on the principle of competition, 
and from the all -important relations of organism to organism in 
the struggle for life, that any form which did not become in some 
degree modified and improved, would be liable to extermination. 
Hence we see why all the species in the same region do at last, if 
we look to long enough intervals of time, become modified, for 
otherwise they would 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 enduring formations, rich in 
fossils, depends on great masses of sediment being deposited on 
subsiding areas, our formations have been almost necessarily accu- 
mulated at wide and irregularly intermittent intervals of time ; con- 
sequently 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 an ever 
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 in- 
numerable instances) to fill the 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 ; and organisms already differing would vary 
in a different manner. For instance, it is possible, if all our fan- 
tail pigeons were destroyed, that fanciers might make a new breed 
hardly distinguishable from the present breed ; but if the parent 
rock-pigeon were likewise destroyed, and under nature we have 
every reason to believe that parent-forms are generally supplanted 
and exterminated by their improved offspring, it is incredible that 
a fantail, identical with the existing breed, could be raised from 
any other species of pigeon, or even from any other well-established 
race of the domestic pigeon, for the successive variations would 
almost certainly be in some degree different, and the newly-formed 
variety would probably inherit from its progenitor some character- 
istic 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, when it has once disappeared, never reappears ; 
that is, 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 main- 
tain) admit its truth ; and the rule strictly accords with the theory. 
For all the species of the same group, however long it may have 
lasted, are the modified descendants one from the other, and all 
from a common progenitor. In the genus Lingula, Tor instance, 
the species which have successively appeared at all ages must have 
been connected by an unbroken series of generations, from the 
lowest Silurian stratum to the present day. 


We have seen in the last chapter that whole groups of species 
sometimes falsely appear to have been abruptly developed ; and I 
have attempted to give an explanation of this fact, which if true 
would be fatal to my views. But such cases are certainly excep- 
tional ; the general rule being a gradual increase in number, until 
the group reaches its maximum, and then, sooner or later, a gradual 
decrease. If the number of the species included within a genus, 
or the number of the genera within a family, be represented by a 
vertical line of varying thickness, ascending through 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, often 
keeping of equal thickness for a space, 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 the theory, for the species of the same 
genus, and the genera of the same family, can increase only slowly 
and progressively ; the process of modification and the production 
of a number of allied forms necessarily being a slow and gradual 
process, one species first giving rise to two or three varieties, 
these being slowly converted into species, which in their turn pro- 
duce by equally slow steps other varieties and 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 only spoken incidentally of the disappearance of 
species and of groups of species. On the theory of natural selec- 
tion, 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 by 
catastrophes at successive periods is very generally given up, even 
by those geologists, as Elie de Beaumont, Murchison, Barrande, 
Ac., whose general views would naturally lead them to this con- 
clusion. 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. In some few cases 
however, as by the breaking of an isthmus and the consequent 
irruption of a multitude of new inhabitants into an adjoining sea, 
or by the final subsidence of an island, the process of extinction 
may have been rapid. Both single species and whole groups of 
species last for very unequal periods; some groups, as we have 
seen, have endured from the earliest known dawn of life to the 
present day; some have 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 extinction of a whole group of 
species is generally a slower process than their production : if their 
appearance and disappearance 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 extermina- 
tion, than at its lower end, which marks the first appearance and 
the early increase in number of the species. In some cases, how- 
ever, the extermination of whole groups, as of ammonites, towards 
the close of the secondary period, has been wonderfully sudden. 

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 can have marvelled more than I have 
done at the extinction of species. 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 my astonishment was groundless. 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 attri- 
bute 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 


It is most difficult always to remember that the increase of 
every creature is constantly being checked by unperceived hostile 
agencies; and that these same unperceived agencies are amply 
sufficient to cause rarity, and finally extinction. So little is this 
subject understood, that I have heard surprise repeatedly ex- 
pressed at such great monsters as the Mastodon and the more 
ancient Dinosaurians having become extinct; as if mere bodily 
strength gave victory in the battle of life. Mere size, on the 
contrary, would in some cases determine, as has been remarked 
by Owen, quicker extermination from the greater amount of 
requisite food. Before man inhabited India or Africa, some 
cause must have checked the continued increase of the existing 
elephant. A highly capable judge, Dr. Falconer, believes that it 
is chiefly insects which, from incessantly harassing and weakening 
the elephant in India, check its increase; and this was Brace's 
conclusion with respect to the African elephant in Abyssinia. It 
is certain that insects and blood-sucking bats determine the 
existence of the larger naturalized quadrupeds in several parts 
of S. America. 

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 exter- 
minated, 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 the species 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 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 the 
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 take? 
the place of other breeds in other countries. Thus the appearance 
o new forms and the disappearance of old forms, both those 
naturally and those artificially produced, are bound together. 
In flourishing groups, the number of new specific forms which 
have been produced within a given time has at some periods 
probably been greater than the number of the old specific forms 
which have been exterminated; but we know that species 


not gone on indefinitely increasing, at least during the later geo- 
logical epochs, so that, looking to later times, we may beliave 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 exter^ 
mination 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 has seized on the place occupied by a species belonging 
to a distinct group, and thus have caused its extermination. If 
many allied forms be developed from the successful intruder, 
many will have to yield their places ; and it will generally be the 
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 have yielded their places to other modified 
and improved species, a few of the sufferers may often be preserved 
for a long time, from being fitted to some peculiar line of life, or 
from inhabiting some distant and isolated station, where they will 
have escaped severe competition. For instance, some species of 
Trigonia, a great genus of shells in the secondary formations, 
survive 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 an area, many of the older species will have been exterminated 
in a correspondingly rapid manner; and the forms which thus 
yield their places will commonly be allied, for they will partake 
of the same 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 own 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. When- 
ever 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 until then, we may 
justly feel surprise why we cannot account for the extinction of 
any particular species or group of species. 

Qn the Forms of Life changing almost simultaneously throughout 
tfo World. 

Scarcely any pal aeon tological 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 regions, 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 resem- 
blance 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, occur in the same 
order 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 European and North American tertiary deposits. 
Even if the few fossil species which are common to the Old 
and New Worlds were kept wholly out of view, the general 
parallelism in the successive forms of life, in the palaeozoic and 
tertiary stages, would still be manifest, and the several formations 
could be easily correlated. 

These observations, however, relate to the marine inhabitants of 
the world : we have not sufficient data to judge whether the pro- 
ductions of the land and of fresh water at distant points change in 
the same parallel manner. We may doubt whether they have thus 
changed : if the Megatherium, Mylodon, Macrauchenia. and Toxo- 
don had been brought to Europe from La Plata, without anv 


information in regard to their geological position, no one would 
have suspected that they had co-existed with sea-shells all still 
living; but as these anomalous monsters co-existed with the Mas- 
todon and Horse, it might at least have been inferred that they 
had lived during one of the later 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 year, or to the same century, 
or even that it has a very strict geological sense ; for if all the 
marine animals now living in Europe, and all those that lived in 
Europe during the pleistocene period (a very remote period as 
measured by years, including the whole glacial epoch) were com- 
pared with those now existing in South America or in Australia, 
the most skilful naturalist would hardly be able to say whether the 
present or the pleistocene inhabitants of Europe resembled most 
closely those of the southern hemisphere. So, again, several highly 
competent observers maintain that the existing productions of the 
United States are more closely related to those which lived in 
Europe during certain late tertiary stages, than to the present 
inhabitants of Europe ; and if this be so, it is evident that fossili- 
ferous beds now deposited 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 
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 in- 
cluding those forms which are found only in the older underlying 
deposits, would be correctly ranked as simultaneous in a geological 

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. Bar- 
rande 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 selec- 
tion. New species are formed by having some advantage over 
older forms ; and the forms, which are already dominant, or have 
some advantage over the other forms in their own country, give 
birth to the greatest number of new varieties or incipient species. 
We have distinct evidence on this head, in the plants which are 
dominant, that is, which are commonest and most widely diffused, 
producing the greatest number of new varieties. It is also natural 
that the dominant, varying, and far-spreading species, which have 
already 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 other new varieties 
and species. The process of diffusion would often be very slow, de- 
pending on climatal and geographical changes, on strange accidents, 
and on the gradual acclimatisation of new species to the various 
climates through which they might have to pass, but in the course 
of time the dominant forms would generally succeed in spreading 
and would ultimately prevail. The diffusion would, it is probable, 
be slower with the terrestrial inhabitants of the distinct continents 
than with the marine inhabitants of the continuous sea. We might 
therefore expect to find, as we do find, a less strict degree of paral- 
lelism in the succession of the productions of the land than with 
those of the sea. 

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 
their having had some advantage over their already dominant 
parents, as well as over other species, and again spreading, vary- 
ing, and producing new forms. The old forms which are beaten 
and which yield their places to the new and victorious forms, wiU 
generally be allied in groups, from inheriting some inferiority in 
common; and therefore, as new and improved groups spread 
throughout the world, old groups disappear from the world ; and 
the succession of forms everywhere tends to correspond both in 
their first appearance and final disappearance. 

There is one other remark connected with this subject worth 
making. I have given my reasons for believing that most of our 
great formations, rich in fossils, were deposited during periods of 
subsidence; and that blank intervals of vast duration, as far as 


fossils are concerned, 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 I 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 very far from having any right to conclude that 
this has invariably been the case, and that large areas have in- 
variably been affected by the same movements. When two forma- 
tions 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 occur in Europe. Mr. Prest- 
wich, 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 he compares 
certain stages in England with those in France, although he finds 
in both a curious accordance in the numbers of the species belong- 
ing 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 forma- 
tion 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 forms 
of life, and the order would falsely appear to be strictly parallel ; 
nevertheless the species would not be all 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. All fall into a few grand classes; 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, extinct species can all be classed 
either in still existing groups, or between them. That the extinct 
forms of life help to till up the intervals between existing genera, 
families, and orders, is certainly true ; but as this statement has 
often been ignored or even denied, it may be well to make some 
remarks on this subject, and to give some instances. If we confine 
our attention either to the living or to the extinct species of the 
same class, the series is far less perfect than if we combine both 
into one general system. In the writings of Professor Owen we 
continually meet with the expression of generalised forms, as 
applied to extinct animals; and in the writings of Agassiz, of 
prophetic or synthetic types; and these terms imply that such 
forms are in fact intermediate or connecting links. Another 
distinguished palaeontologist, M. Gaudry, has shown in the most 
striking manner that many of the fossil mammals discovered by 
him in Attica serve to break down the intervals between existing 
genera. Cuvier ranked the lluminants and Pachyderms, as two 
of the most distinct orders of mammals : but so many fossil links 
have been disentombed that Owen has had to alter the whole 
classification, and has placed certain pachyderms in the same sub- 
order with ruminants ; for example, he dissolves by gradations the 
apparently wide interval between the pig and the camel. The 
Ungulata or hoofed quadrupeds are now divided into the even- 
toed or odd-toed divisions ; but the Macrauchenia of S. America 
connects to a certain extent these two grand divisions. No one 
will deny that the Hipparion is intermediate between the existing 
horse and certain older ungulate forms. What a wonderful con- 
necting link in the chain of mammals is the Typotherium from 
S. America, as the name given to it by Professor Gervais expresses, 
and which cannot be placed in any existing order. The Sirenia 
form a very distinct group of mammals, and one of the most 
remarkable peculiarities in the existing dugong and lamentin is 
the entire absence of hind limbs without even a rudiment being 
left ; but the extinct Halitherium had, according to Professor 
Flower, an ossified thigh-bone "articulated to a well-defined 
acetabulum in the pelvis," and it thus makes some approach to 
ordinary hoofed quadrupeds, to which the Sirenia are in other 
respects allied. The cetaceans or whales are widely different from 
all other mammals, but the tertiary Zeuglodon and Squalodon, 


which have been placed by some naturalists in an order by them- 
selves, are considered by Professor Huxley to be undoubtedly 
cetaceans, "and to constitute connecting links with the aquatic 

Even the wide interval between birds and reptiles has been 
shown by the naturalist just quoted to be partially bridged over 
in the most unexpected manner, on the one hand, by the ostrich 
and extinct Archeopteryx, and on the other hand, by the Compsog- 
nathus, one of the Dinosaurians that group which includes the 
most gigantic of all terrestrial reptiles. Turning to the Inverte- 
brata, Barrande asserts, a higher authority could not be named, 
that he is every day taught that, although palaeozoic animals can 
certainly be classed under existing groups, yet that at this ancient 
period the groups were not so distinctly separated from each other 
as they now are. 

Some writers have objected to any extinct species, or group of 
species, being considered as intermediate between any two living 
species, or groups of species. If by this term it is meant that an 
extinct form is directly intermediate in all its characters between 
two living forms or groups, the objection is probably valid. But 
in a natural classification many fossil species certainly 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 by a score of characters, the 
ancient members are separated by a somewhat lesser number 
of characters ; so that the two groups formerly made a somewhat 
nearer approach to each other than they now do. 

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 com- 
pare 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 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 in italics 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 14 , q 14 , p u , will form a small family ; b u and f lt a closely 
allied family or sub-family ; and o 14 , e 14 , m u , 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 progenitor. On the principle of the continued 
tendency to divergence of character, which was formerly illus- 
trated 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 diver- 
gence 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 M . 

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 imbedded in the successive forma- 
tions, 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 a 1 , a 5 , a 10 , /*, m 3 , m e , m 9 , 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 certain pachyderms. 
Yet he who objected to consider as intermediate the extinct 
genera, which thus link together the living genera of three 
families, would be partly justified, for 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 dis- 
covered above one of the middle horizontal lines or geological 
formations for instance, above No. VI. but none from beneath 


this line, then only two of the families (those on the left hand, 
14 , &c., and b l \ &c.) would have to be united into one : and there 
would remain two families, which would be less distinct from 
each other than they were before the discovery of the fossils. 
So again if the three families formed of eight genera (a 14 to 
m 14 ), on the uppermost line, be supposed to differ from each other 
by half-a-dozen important characters, then the families which 
existed at the period marked VI. would certainly have differed 
from each other by a less number of characters ; for they would 
at this early stage of descent have diverged in a less degree from 
their common progenitor. Thus it comes that ancient and extinct 
genera are often in a greater or less degree intermediate in cha- 
racter between their modified descendants, or between their 
collateral relations. 

Under nature the process 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 
rare cases, to fill up the wide intervals in the natural system, and 
thus to unite distinct families or orders. All that we have a right 
to expect is, that those groups which have, within known geolo- 
gical 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 palaeon- 
tologists is frequently 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, are explained in a 
satisfactory manner. And they are wholly inexplicable on any 
other view. 

On this same theory, it is evident that the fauna during any one 
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 in any one region for the 
immigration of new forms from other regions, 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 dis- 
covered, were at once recognised by palaeontologists as intermediate 
in character between those of the overlying carboniferous, and 
underlying Silurian systems. 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, the species of masto- 
dons and elephants, when arranged by Dr. Falconer in two series, 
in the first place according to their mutual affinities, and in the 
second place 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 appear- 
ance and disappearance of the species was complete, which is far 
from the case, we have no reason to believe that forms successively 
produced necessarily endure for corresponding lengths of time. A 
very ancient form may occasionally have lasted 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 in serial 
affinity, this arrangement would not closely accord with the 
order in time of their production, and even less with the order 
of their disappearance ; for the parent rock -pigeon still 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 back originated earlier than short-beaked 
tumblers, which are at the opposite end of the series in this 

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 Pro- 


fessor Pictet in his 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 reseaiblance of 
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 inhabit- 
ing the sea, have changed almost simultaneously throughout the 
world, and therefore under the most different climates and con- 
ditions. Consider the prodigious vicissitudes of climate during 
the pleistocene period, which includes the whole glacial epoch, 
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 fossil remains 
from closely consecutive formations being closely related, though 
ranked as distinct species, 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 in any 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 assuredly we do 
find. We find, in short, such evidence of the slow and scarcely 
sensible mutations of specific forms, as we have the right to 

On the State of Development of Ancient compared with Living 

We have seen in the fourth chapter that the degree of differen- 
tiation and specialisation of the parts in organic beings, when 
arrived at maturity, is the best standard, as yet suggested, of 
their degree of perfection or highness. We have also seen that, 
as the specialisation of parts is an advantage to each being, so 
natural selection will tend to render the organisation of each 
\)eing more specialised and perfect, and in this sense higher ; not 
but that it may leave many creatures with simple and unimproved 
structures fitted for simple conditions of life, and in some cases 
will even degrade or simplify the organisation, yet leaving such 
degraded beings better fitted for their new walks of life. In 
another and more general manner, new species become superior 
to their predecessors ; for they have to beat in the struggle for 
life all the older forms, with which they come into close compe- 
tition. We may therefore conclude that if under a nearly similar 
climate the eocene inhabitants of the world could be put into 


competition with the existing inhabitants, the former would be 
beaten and exterminated by the latter, as would the secondary 
by the eocene, and the paheozoic by the secondary forms. So that 
by this fundamental test of victory in the battle for life, as well 
as by the standard of the specialisation of organs, modern forms 
ought, on the theory of natural selection, to stand higher than 
ancient forms. Is this the case? A large majority of palaeon- 
tologists would answer in the affimative ; and it seems that this 
answer must be admitted as true, though difficult of proof. 

It is no valid objection to this conclusion, that certain Brachio- 
pods have been but slightly modified from an extremely remote 
geological epoch ; and that certain land and fresh- water shells 
have remained nearly the same, from the time when, as far as is 
known, they first appeared. It is not an insuperable difficulty 
that Foraminifera have not, as insisted on by Dr. Carpenter, pro- 
gressed in organisation since even the Lauren tian epoch ; for some 
organisms would have to remain fitted for simple conditions of 
life, and what could be better fitted for this end than these lowly 
organised Protozoa ? Such objections as the above would be fatal 
to my view, if it included advance in organisation as a necessary 
contingent. They would likewise be fatal, if the above Fora- 
minifera, for instance, could be proved to have first come into 
existence during the Laurentian epoch, or the above Brachiopods 
during the Cambrian formation ; for in this case, there would not 
have been time sufficient for the development of these organisms 
up to the standard which they had then reached. When advanced 
up to any given point, there is no necessity, on the theory of 
natural selection, for their further continued progress; though 
they will, during each successive age, have to be slightly modified, 
so as to hold their places in relation to slight changes in their 
conditions. The foregoing objections hinge on the question 
whether we really know how old the world is, and at what period 
the various forms of life first appeared; and this may well be 

The problem whether organisation on the whole has advanced 
is in many ways excessively intricate. The geological record, at 
all times imperfect, does not extend far enough back, to shew 
with unmistakeable clearness that within the known history of 
the world organisation has largely advanced. Even at the present 
day, looking to members of the same class, naturalists are not 
unanimous which forms ought to be ranked as highest : thus, 
some look at the selaceans or sharks, from their approach in 
some important points of structure to reptiles, as the highest fish ; 
others look at the teleosteans as the highest. The ganoids stand 
intermediate between the selaceans and teleosteans ; the latter at 
the present day are largely preponderant in number; but fromerly 


selaceans and ganoids alone existed ; and in this case, according 
to the standard of highness chosen, so will it be said that fishes 
have advanced or retrograded in organisation. To attempt to 
compare members of distinct types in the scale of highness seems 
hopeless ; who will decide whether a cuttle-fish be higher than a 
bee that insect which the great Von Baer believed to be " in fact 
more highly organised than a fish, although upon another type " ? 
In the complex struggle for life it is quite credible that crusta- 
ceans, not very high in their own class, might beat cephalopods, 
the highest molluscs; and such crustaceans, though not highly 
developed, would stand very high in the scale of invertebrate 
animals, if judged by the most decisive of all trials the law of 
battle. Beside these inherent difficulties in deciding which forms 
are the most advanced in organisation, we ought not solely to 
compare the highest members of a class at any two periods 
though undoubtedly this is one and perhaps the most important 
element in striking a balance but we ought to compare all the 
members, high and low, at the two periods. At an ancient epoch 
the highest and lowest molluscoidal animals, namely, cephalopoda 
and brachiopods, swarmed in numbers ; at the present time both 
groups are greatly reduced, whilst others, intermediate in organi- 
sation, have largely increased; consequently some naturalists 
maintain that molluscs were formerly more highly developed than 
at present ; but a stronger case can be made out on the opposite 
side, by considering the vast reduction of the brachiopods, and 
the fact that our existing cephalopods, though few in number, 
are more highly organised than their ancient representatives. 
We ought also to compare the relative proportional numbers at 
any two periods of the high and low classes throughout the world : 
if, for instance, at the present day fifty thousand kinds of verte- 
brate animals exist, and if we knew that at some former period 
only ten thousand kinds existed, we ought to look at this increase 
in number in the highest class, which implies a great displacement 
of lower forms, as a decided advance in the organisation of the 
world. We thus see how hopelessly difficult it is to compare with 
perfect fairness under such extremely complex relations, the stan- 
dard of organisation of the imperfectly-known faunas of successive 

We shall appreciate this difficulty more clearly, by looking 
to certain existing faunas and floras. 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 by the indigenes, we must believe, that 
if all the animals and plants of Great Britain were set free in 
New Zealand, a multitude of British forms would in the course 
of time become thoroughly naturalised there, and would exter- 


ruinate many of the natives. On the other hand, from the fact 
that hardly a single inhabitant of the southern hemisphere has 
become wild in any part of Europe, we may well doubt whether, 
if all the productions of New Zealand were set free in Great 
Britain, 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 stand much 
higher in the scale 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 and several other highly competent judges insist that 
ancient animals resemble to a certain extent the embryos of recent 
animals belonging to the same classes ; and that the geological 
succession of extinct forms is nearly parallel with the embryo- 
logical development of existing forms. This view accords admir- 
ably well with our theory. In a future chapter I shall attempt 
to show that the adult differs from its embryo, owing to variations 
having supervened at a not early age, and having been 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 former and less modified condition of the species. This 
view may be true, and yet may never be capable of proof. Seeing 
for instance, that the oldest known mammals, reptiles, and fishes 
strictly belong to their 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 embryo- 
logical character of the Vertebrata, until beds rich in fossils are 
discovered far beneath the lowest Cambrian strata a discovery 
of which the chance is small. 

On the Succession of the same Types within the same Areas, 
during t/ie 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 ol 
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 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 through dissimilar physical condi- 
tions, for the dissimilarity of the inhabitants of these two con- 
tinents ; and, on the other hand through similarity of conditions, 
for the uniformity of the same types in each continent 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 

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 inhabi- 
tants 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 inhabi- 
tants 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, per- 


mitting much intermigration, the feebler will yield to the more 
dominant forms, and there will be nothing immutable in the 
distribution of organic beings. 

It may be asked in ridicule, whether I suppose that the mega- 
therium and other allied huge monsters, which formerly lived in 
South America, have left behind them 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 all other characters to the species still living in South 
America ; and some of these fossils may have been the actual 
progenitors of the living species. It must not be forgotten that, 
on our theory, all the species of the same genus are the descend- 
ants of some one species ; so that, if six genera, each having eight 
species, be found in one geological formation, and in a succeeding 
formation there be six other allied or representative genera each 
with the same number of species, then we may conclude that 
generally only one species of each of the older genera has left 
modified descendants, which constitute the new genera con- 
taining the several species ; the other seven species of each old 
genus having died out and left no progeny. Or, and this will be 
a far commoner case, two or three species in two or three alone 
of the six older gene'iu will be the parents of the new genera : the 
other species and the other old genera having become utterly 
extinct. In failing orders, with the genera and species decreasing 
in numbers as is the case with the Edentata of South America, 
etill fewer genera and species will leave modified blood-descendants. 

Summary of tlie 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 geo- 
logically 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 number 
of generations which must have passed away even during a 
single formation ; that, owing to subsidence being almost neces- 
sary for the accumulation of deposits rich in fossil species of 
many kinds, and thick enough to outlast future degradation, 
great intervals of time must have elapsed between most of our 
successive formations ; that there has probably been more extinc- 
tion 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 deoosited that the duration of each formation 


is probably 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 fre- 
quently, and have oftenest given rise to new species ; that 
varieties have at first been local ; and lastly, although each 
species must have passed through numerous transitional stages, 
it is probable that the periods, during which each underwent 
modification, though many and long as measured by years, have 
been short in comparison with the periods during which each 
remained in an unchanged condition. These causes, taken con- 
jointly, will to a large extent explain why though we do find 
many links we do not find interminable varieties, connecting 
together all extinct and existing forms by the finest graduated 
steps. It should also be constantly borne in mind that any 
linking variety between two forms, which might be found, would 
be ranked, unless the whole chain could be perfectly restored, as 
a new and distinct species ; for it is not pretended that we have 
any sure criterion by which species and varieties can be dis- 

He who rejects this view of the imperfection of the geological 
record, will rightly reject the 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 successive stages of the same great forma- 
tion ? He may disbelieve in the immense intervals of time which 
must have elapsed between our consecutive formations ; he may 
overlook how important a part migration has played, when the 
formations of any one great region, as those of Europe, are con- 
sidered ; 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 Cambrian system was de- 
posited ? We now know that at least one animal did then exist ; 
but I can answer this last question only by supposing that where 
our oceans now extend they have extended for an enormous 
period, and where our oscillating continents now stand they have 
stood since the commencement of the Cambrian system ; but 
that, long before that epoch, the world presented a widely 
different aspect ; and that the older continents, formed of forma- 
tions older than any known to us, exist now only as remnants in 
a metamorphosed condition, or lie still buried under the ocean. 

Passing from these difficulties, the other great leading facts in 
palaeontology agree admirably with the theory of descent with 
.modification through variation and 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 
belonging to large and dominant groups tend to leave many 
modified descendants, which form new sub-groups and groups. 
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 has sometimes been a 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 it is that dominant forms which spread 
widely and yield the greatest number of varieties tend to people 
the world with allied, but modified, descendants ; and these will 
generally succeed in displacing the groups which are their in- 
feriors in the struggle for existence. Hence, after long intervals 
of time, the productions of the world appear to have changed 

We can understand how it is that all the forms of life, ancient 
and recent, make together a few grand classes. We can under- 
stand, 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 com- 
monly bringing them only a little closer together. The more 
ancient a form is, the more often it stands in some degree inter- 
mediate 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 be- 
tween existing forms ; but are intermediate only by a long and 
circuitous course through other extinct and different forms. We 
can clearly see why the organic remains of closely consecutive 
formations are closely allied ; for they are closely linked together 
by generation. We can clearly see why the remains of an inter- 
mediate formation are intermediate in character. 

The inhabitants of the world at each successive period in its 


history have beaten their predecessors in the race for life, and 
are, in so far, higher in the scale, and their structure has generally 
become more specialised ; and this may account for the common 
belief held by so many palaeontologists, that organisation on the 
whole has progressed. Extinct and ancient animals resemble to 
a certain extent the embryos of the more recent animals belonging 
to the same classes, and this wonderful fact receives a simple 
explanation according to our views. The succession of the same 
types of structure within the same areas during the later geo- 
logical periods ceases to be mysterious, and is intelligible on the 
principle of inheritance. 

If then the geological record be as imperfect as many believe, 
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, the products of Variation and the Survival 
of the Fittest. 


Present distribution cannot be accounted for by differences in physical con- 
ditions Importance of barriers Affinity of the productions o'f 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 Alternate Glacial periods in the North and South. 

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 wholly accounted for by 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 arctic and northern temperate parts, all authors agree 
that one of the most fundamental divisions in geographical distri- 
bution is that between the New and Old Worlds ; yet if we travel 
over the vast American continent, from the central parts of the 
Unit 3d States to its extreme southern point, we meet with the 
most diversified conditions; 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. 
No doubt small areas can be pointed out in the Old World hotter 
than any in the New World ; but these are not inhabited by a 
fauna different from that of the surrounding districts; for it is 
rare to find a group of organisms confined to a small area, of 
which the conditions are peculiar in only a slight degree. Not- 
withstanding this general 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 35, 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 are separated by a space 
of ten degrees of latitude, and are exposed to considerably different 
conditions ; yet they are 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 in 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, of great deserts and 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. The marine inhabi- 
tants of the eastern and western shores of South America are 
very distinct, with extremely few shells, Crustacea, or echino-- 


clermata in common ; but Dr. Giinther has recently shown that 
about thirty per cent, of the fishes are the same on the opposite 
sides of the isthmus of Panama ; and this fact has led naturalists 
to believe that the isthmus was formerly open. 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 three marine faunas range far northward and 
southward in parallel lines not far from each other, under corre- 
sponding climates ; but from being separated from each other by 
impassable barriers, either of land or open sea, they are almost 
wholly distinct. On the other hand, proceeding still farther 
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, or continuous coasts, 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 so few marine animals are 
common to the above-named three approximate faunas of Eastern 
and Western America and the eastern Pacific islands, yet many 
fishes 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 

A third great fact, partly included in the foregoing statement, 
is the affinity of the productions of the same continent or of the 
same 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 
naturalists, in travelling, for instance, from north to south, never 
fails to be struck by the manner in which successive groups of 
beings, specifically distinct, though nearly 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 emu, like those inhabiting 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 nd an alpine 
epecies 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 
S. 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 are essen- 
tially American, though they may be all peculiar species. We 
may look back to past ages, as shown in the last chaper, and we 
find American types then prevailing on the American continent 
and in the American seas. We see in these facts some deep 
organic bond, throughout space and time, over the same areas 
of land and water, independently of physical conditions. The 
naturalist must be dull who is not led to inquire what this 
bond is. 

The bond is simply inheritance, that cause which alone, as far 
as we positively know, produces organisms quite like each other, 
or, as we see in the case of varieties, nearly alike. The dis- 
similarity of the inhabitants of different regions may be attributed 
to modification through variation and natural selection, and 
probably in a subordinate degree to the definite influence of 
different physical conditions. The degrees of dissimilarity will 
depend on the migration of the more dominant forms of life from 
one region into another having been more or less effectually 
prevented, at periods more or less remote ; on the nature and 
number of the former immigrants ; and on the action of the 
inhabitants on each other in leading to the preservation of 
different modifications ; the relation of organism to organism in 
the struggle for life 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. 

There is no evidence, as was remarked in the last chapter, of 
the existence of any 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 each individual in its complex struggle for life, so the 
amount of modification in different species will be no uniform 


quantity. If a number of species, after having long competed 
with each other in their old home, were to migrate in a body into 
a new and afterwards isolated country, they would be little liable 
to modification ; for neither migration nor isolation in themselves 
effect 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 or at all modified. 

According to 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 are descended from the same progenitor. In the case of 
those species which have undergone during whole geological 
periods little modification, there is not much difficulty in believing 
that they have migrated from the same region ; for during the 
vast geographical and climatal changes which 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 
has been explained, it is incredible that individuals identically 
the same should have been produced nx/m parents specifically 

Single Centres of supposed Creation. 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 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, w-here 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 
that 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 have been easily passed over by migration, the fact is 
given as something remarkable and exceptional. The incapacity 
of migrating across a wide sea is more clear in the case of terres- 


trial mammals than perhaps with any other organic beings ; and, 
accordingly, we find no inexplicable instances of the same 
mammals inhabiting distant points of the world. No geologist 
feels any difliculty in Great Britain possessing the same quad- 
rupeds with the rest of Europe, for they were no doubt once 
united. 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 dis- 
persal, have migrated across the wide and broken interspaces. 
The great and striking influence of barriers of all kinds, is 
intelligible only on the view that the greac majority of species 
have been produced on one side, and have not been able to 
migrate to the opposite 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 confined to the same country, or if they have a wide 
range that their range is continuous. What a strange anomaly 
it would be, if a directly opposite rule were to prevail, when we 
go down one step lower in the series, namely, to the individuals 
of the same species, and these had not been, at least at first, con- 
fined to some one region ! 

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 con- 
ditions 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 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 instances. 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 nearest 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 to the various 
occasional means of transport, the belief that a single birthplace 
is the 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 species of a genus which must on our theory all be 
descended from a common progenitor, can have migrated, under- 
going modification during their migration, from some one area. 
If, when most of the species inhabiting one region are different 
from those of another region, though closely allied to them, it can 
be shown that migration from the one region to the other has 
probably occurred at some former period, our general view will 
be much strengthened; for the explanation is obvious on the 
principle of descent with modification. 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 related by inheritance to the inhabitants 
of that continent. Cases of this nature are common, and are, as 
we shall hereafter see, inexplicable on the theory of independent 
creation. This view of the relation of the species of one region 
to those of another, does not differ much from that advanced by 
Mr. Wallace, who concludes that " every species has come into 
existence coincident both in space and time with a pre-existing 
closely allied species." And it is now well known that he attri- 
butes this coincidence to descent with modification. 

The question of single or multiple centres of creation differs 
from another though allied question, namely, whether all the 
individuals of the same species are descended from a single pair, 
or single hermaphrodite, or whether, as some authors suppose, 
from many individuals simultaneously created. With organic 
beings which never intercross, if such exist, each species must 
be descended from a succession of modified varieties, that have 
supplanted each other, but have never blended with other indi- 


viduals or varieties of the same species ; so that, at each successive 
stage of modification, all the individuals of the same form will be 
descended from a single parent. But in the great majority of 
cases, namely, with all organisms which habitually unite for each 
birth, or which occasionally intercross, the individuals of the 
same species inhabiting the same area will be kept nearly uniform 
by intercrossing; so that many individuals will go on simul- 
taneously changing, and the whole amount of modification at 
each stage will not be due to descent from a single parent. To 
illustrate what I mean : our English race-horses differ 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 the selecting and training of many individuals during 
each generation. 

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 now impassable to certain organisms from 
the nature of its climate, might have been a high road for migra- 
tion, when the climate was different. 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 together, 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 disputes that great mutations of level have occurred 
within the period of existing organisms. Edward Forbes insisted 
that all the islands in the Atlantic must have been recently con- 
nected with Europe or Africa, and Europe likewise with America. 
Other authors have thus hypothetically bridged over every ocean, 
and united almost every island with some mainland. If indeed 
the arguments used by Forbes are to be trusted, it must be ad- 
mitted 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 my judgment 
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 in the level 
of the land or sea ; but not of such vast changes in the position 
and extension of our continents, 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 by rings of coral or atolls standing over them. 
Whenever it is fully admitted, as 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 most of our 
continents which now stand quite separate, have been continu- 
ously, 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 oppo- 
site sides of almost every continent, the close relation of the 
tertiary inhabitants of several lands and even seas to their pre- 
sent inhabitants, the degree of affinity between the mammals 
inhabiting islands with those of the nearest continent, being in 
part determined (as we shall hereafter see) by the depth of the 
intervening ocean, these and other such facts are opposed to the 
admission of such prodigious geographical revolutions within 
the recent period, as are necessary on the view advanced by 
Forbes and admitted by his followers. The nature and relative 
proportions of the inhabitants of oceanic islands are likewise 
opposed to the belief of their former continuity with continents. 
Nor does the almost universally volcanic composition of such 
islands favour the admission that they are the wrecks of sunken 
continents ; if they had originally existed as continental moun- 
tain ranges, some at least of the islands would have been formed, 
like other mountain summits, of granite, metamorphic schists, old 
fossiliferous and other 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 should be called occasional 
means of distribution. I shall here confine myself to plants. 
In botanical works, this or that plant is often stated to be ill 
adapted for wide dissemination ; but the greater or less facilities 
for transport across the sea may be said to be almost wholly 
unknown. Until I tried, with Mr. Berkeley's aid, a few experi- 
ments, 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. It deserves notice that 
certain orders were far more injured than others: nine Legu- 
minosae were tried, and, with one exception, they resisted the 
salt-water badly; seven species of the allied orders, Hydro- 
phyllaceae and Polemoniacese, were all killed by a month's im- 
mersion. 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 have been floated across wide spaces of the 
sea, whether or not they were injured by the salt-water. After- 
wards 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 would often wash into the sea dried plants or 
branches with seed-capsules or fruit attached to them. Hence I 
was led to dry the 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 after- 
wards when planted 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 {j-f kinds of seeds germinated 
after an immersion of 28 days; and as ^f distinct species with 
ripe fruit (but not all the same species as in the foregoing experi- 
ment) floated, after being dried, for above 28 days, we may con- 
clude, as far as anything can be inferred from these scanty facts, 
that the seeds of ^ kinds of 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 Y\J^ plants belonging to one country might 
be floated across 924 miles of sea to another country, and when 
stranded, if blown by an inland gale to a favourable spot, would 

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 both the average length of their flotation and 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 |,| of his seeds 
of different kinds 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 ffo 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 inter- 
esting ; as plants with large seeds or fruit which, as Alph. de 
Candolle has shown, generally have restricted ranges, could 
hardly be transported by any other means. 

Seeds may be occasionally transported in another manner. 
Drift timber is threwn 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 
that when irregularly shaped stones are embedded in the roots of 
trees, small parcels of earth are frequently enclosed in their inter- 
stices and behind them, so perfectly that not a particle could be 
washed away during the longest transport: out of one small 
portion of earth thus cvnipletely enclosed by the roots of 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 carcases of birds, when floating on the sea, sometimes 
escape being immediately devoured : and many kinds of seeds 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 sea- 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 dis- 
tances across the ocean. We may 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 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 
were tried, germinated. But the following fact is more impor- 
tant : the crops of birds do not secrete gastric juice, and do not, 
as I know by trial, injure in the least 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 twelve or even eighteen 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. 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 germina- 
tion. 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. i'resh-water 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 the power of germination. Certain seeds, however, 
were always killed by this process. 

Locusts are sometimes blown to great distances from the land ; 
I myself caught one 370 miles from the coast of Africa, and have 
heard of others caught at greater distances. The Rev. E. T. Lowe 
informed Sir C. Lyell that in November 1844 swarms of locusts 
visited the island of Madeira. They were in countless numbers, 
as thick as the flakes of snow in the heaviest snowstorm, and ex- 
tended upwards as far as could be seen with a telescope. During 
two or three days they slowly careered round and round in an 
immense ellipse, at least five or six miles in diameter, and at 
night alighted on the taller trees, which were completely coated 
with them. They then disappeared over the sea, as suddenly as 
they had appeared, and have not since visited the island. Now, 
in parts of Natal it is believed by some farmers, though on insuffi- 
cient evidence, that injurious seeds are introduced into their grass- 
land in the dung left by the great flights of locusts which often 
visit that country. In consequence of this belief Mr. Weale sent 
me in a letter a small packet of the dried pellets, out of which I 
extracted under the microscope several seeds, and raised from 
them seven grass plants, belonging to two species, of two genera. 
Hence a swarm of locusts, such as that which visited Madeira. 



might readily be the means of introducing several kinds of plants 
into an island lying far from the mainland. 

Although the beaks and feet of birds are generally clean, earth 
sometimes adheres to them : in one case I removed sixty-one 
grains, and in another case twenty -two grains of dry argillaceous 
earth from the foot of a partridge, and in the earth there was a 
pebble as large as the seed of a vetch. Here is a better case : the 
leg of a woodcock was sent to me by a friend, with a little cake 
of dry earth attached to the shank, weighing only nine grains ; 
and this contained a seed of the toad-rush (Juncus bufonius) 
which germinated and flowered. Mr. Swayslaiid, of Brighton, 
who during the last forty years has paid close attention to our 
migratory birds, informs me that he has often shot wagtails 
(Motacillae), wheatears, and whinchats (Saxicolse), on their first 
arrival on our shores, before they had alighted ; and he has 
several times noticed little cakes of earth attached to their feet. 
Many facts could be given showing how generally soil is charged 
with seeds. For instance, Prof. Newton sent me the leg of a red- 
legged partridge (Caccabis rafa) which had been wounded and 
could not fly, with a ball of hard earth adhering to it, and Aveigh- 
ing six and a half ounces. The earth had been kept for three 
years, but when broken, watered and placed under a bell-glass, no 
less than 82 plants sprung from it : these consisted of 12 mono- 
cotyledons, including the common oat, and at least one kind of 
grass, and of 70 dicotyledons, which consisted, judgirg from the 
young leaves, of at least three distinct species. With such facts 
before us, can we doubt that the many birds which are annually 
blown by gales across great spaces of ocean, and which annually 
migrate for instance, the millions of quails across the Mediter- 
ranean must occasionally transport a few seeds embedded in dirt 
adhering to their feet or beaks ? But I shall have to recur to this 

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, it can hardly be doubted that they must occasionally, 
as suggested by Lyell, have transported seeds from one part to 
another of the arctic and antarctic regions ; and during the Glacial 
period from one part of the now temperate regions to another. 
In the Azores, from the large number of plants common to 
Europe, in comparison with the species on the other islands of the 
Atlantic, which stand nearer to the mainland, and (as remarked 
by Mr. H. C. Watson) from their somewhat northern character 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 some few seeds of northern 

Considering that these several means of transport, and that 
other means, which without doubt remain to be discovered, have 
been in action year after year for 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 some- 
times 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 sea-water ; 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 ; but would remain as distinct as 
they now are. 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 their very long 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 rare wanderers only by one means, namely, 
by dirt adhering to their feet or beaks, which is in itself a rare 
accident. Even in this case, how small would be the chance 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 stand- 
ing more remote from the mainland, would not receive colonists 
by similar means. Out of a hundred kinds of seeds or animals 
transported to an island, even if far less well-stocked than Britain, 
perhaps not more than one would be so well fitted to its new 
home, as to become naturalised. But this is no valid argument 
against what would be effected by occasional means of transport, 
during the long lapse of geological time, whilst the island was 


being upheaved, 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, if fitted for the climate, would 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 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 point to the other. It is indeed a remarkable fact to see so 
many plants of the same species 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 many 
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 tell 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 scored rocks plainly reveal a 
former cold period. 

The former influence of the glacial climate on the distribution 
of the inhabitants of Europe, as explained by Edward Forbes, is 
substantially as follows. But we shall follow the changes more 
readily, by supposing a new glacial period slowly to come on, and 
then pass away, as formerly occurred. As the cold came on, and 
as each more southern zone became fitted for the inhabitants of 
the north, these would take the places of the former inhabitants 
of the temperate regions. The latter, at the same time, would 
travel further and further 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 an 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. 

As the warmth returned, the arctic forms would retreat north- 
ward, 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, as the warmth 
increased and the snow still further disappeared, higher and 
higher, whilst their brethren were pursuing their northern journey. 
Hence, when the warmth had fully returned, the same species, 
which had lately lived together on the European and North 
American lowlands, would again be found in the arctic regions of 
the Old and New Worlds, and on many isolated mountain-sum- 
mits far distant from each other. 

Thus we can understand the identity of many plants at points 
so immensely remote as the mountains of the United States and 
those 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 first migration when the cold came on, and the re-migra- 
tion on the returning warmth, would 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 moun- 
tain-summits, we may almost conclude, without other evidence, that 
a colder climate formerly permitted their migration across the 
intervening lowlands, now become too warm for their existence. 

As the arctic forms moved first southward and afterwards back- 
wards to the north, in unison with the changing climate, they 
will not have been exposed during their long migrations to any 
great diversity of temperature ; and as they all migrated in a body 
together, their mutual relations will not have been much disturbed. 


Hence, in accordance with the principles inculcated in this volume, 
these forms will not have been liable to much modification. But 
with the 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 far 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 the coldest period will have been temporarily 
driven down to the plains ; they will, also, have been subsequently 
exposed to somewhat different climatal influences. Their mutual 
relations will thus have been in some degree disturbed; conse- 
quently they will have been liable to modification ; and they have 
been modified ; for if we compare the present Alpine plants and 
animals of the several great European mountain-ranges one with 
another, though many of the species remain identically the same, 
some exist as varieties, some as doubtful forms or sub-species, and 
some as distinct yet closely allied species representing each other 
on the several ranges. 

In the foregoing illustration I have assumed that at the com- 
mencement of our imaginary Glacial period, the arctic productions 
Avere as uniform round the polar regions as they are at the present 
day. But it is also necessary to assume that many sub-arctic and 
some few temperate forms were the same round the world, for 
some of the species which now exist on the lower mountain-slopes 
and on the plains of North America and Europe are the same ; 
and it may be asked how I account for this degree of uniformity 
in the sub-arctic and temperate forms round the world, at the 
commencement of the real 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 whole Atlantic 
Ocean and by the northern part of the Pacific. During the 
Glacial period, when the inhabitants of the Old and New Worlds 
lived farther southwards than they do at present, they must have 
been still more completely separated from each other by wider 
spaces of ocean ; so that it may well be asked, how the same 
species could then or previously have entered the two continents. 
The explanation, I believe, lies in the nature of the climate 
before the commencement of the Glacial period. At this, the 
newer Pliocene period, the majority of the inhabitants of the 
world were specifically the same as now, and we have good reason 
to believe that the climate was warmer than at the present day. 
Hence we may suppose that the organisms which now live under 
latitude 60, lived during tLe Pliocene period farther north under 


the Polar Circle, in latitude 66-67 ; and that the present arctic 
productions then lived on the broken land still nearer to the pole. 
Now, if we look at a terrestrial globe, we see under the Polar 
Circle that there is almost continuous land from western Europe, 
through Siberia, to eastern America. And this continuity of the 
circumpolar land, with the consequent freedom under a more 
favourable climate for intermigration, will account for the sup- 
posed uniformity of the sub-arctic and temperate productions of 
the Old and New Worlds, at a period anterior to the Glacial 

Believing, from reasons before alluded to, that our continents 
have long remained in nearly the same relative position, though 
subjected to great oscillations of level, I am strongly inclined to 
extend the above view, and to infer that during some still earlier 
and still warmer period, such as the older Pliocene period, a large 
number of the same plants and animals inhabited the almost con- 
tinuous circumpolar land; and that these plants and animals, 
both in the Old and New Worlds, began slowly to migrate south- 
wards as the climate became less warm, long before the commence- 
ment 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 under- 
stand the relationship with very little identity, between the pro- 
ductions of North America and Europe, a relationship which is 
highly remarkable, considering the distance of the two areas, and 
their separation by the whole 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 intermigration 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 will have been 
completely cut oft' from each other. This separation, as far as the 
more temperate productions are concerned, must have taken place 
long ages ago. As the plants and animals migrated southward, 
they will have become mingled in the one great region with the 
native American productions, and would 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 


Europe and N. America. 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 modifica- 
tion, for many closely allied forms now living in marine areas 
completely sundered. Thus, I think, we can understand the 
presence of some closely allied, still existing and extinct tertiary 
forms, on the eastern and western shores of temperate North 
America; and the still more striking fact of many closely allied 
crustaceans (as described in Dana's admirable work), some fish 
and other marine animals, inhabiting the Mediterranean and the 
seas of Japan, these two areas being now completely separated 
by the breadth of a whole continent and by wide spaces of ocean. 

These cases of close relationship in species either now or 
formerly inhabiting the seas on the eastern and western shores of 
North America, the Mediterranean and Japan, and the temperate 
lands of North America and Europe, are inexplicable on the 
theory of creation. We cannot maintain that such species 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 parts of South Africa or 
Australia, we see countries closely similar in all their physical 
conditions, with their inhabitants utterly dissimilar. 

Alternate Glacial Periods in the North and South. 

But we must return to our more immediate subject. I am 
convinced that Forbes's view may be largely extended. In 
Europe we meet with the plainest evidence of the Glacial 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. In the Lebanon, according to Dr. Hooker, 
perpetual snow formerly covered the central axis, and fed glaciers 
which rolled 4000 feet down the valleys. The same observer has 
recently found great moraines at a low level on the Atlas range 
in N. Africa. Along the Himalaya, at points 900 miles apart, 
glaciers have left the marks of their former low descent ; and IB 


Sikkim, Dr. Hooker saw maize growing on ancient and gigantic 
moraines. Southward of the Asiatic continent, on the opposite 
side of the equator, we know, from the excellent researches of 
Dr. J. Haast and Dr. Hector, that in New Zealand immense 
glaciers formerly descended to a low level ; and the same plants 
found by Dr. Hooker on widely separated mountains in this 
island tell the same story of a fonner cold period. From facts 
communicated to me by the Rev. W. B. Clarke, it appears also 
that there are traces of former glacial action on the mountains of 
the south-eastern corner of Australia. 

Looking to America ; in the northern half, ice-borne fragments 
of rock have been observed on the eastern side of the continent, 
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 South America, nearly under the equator, glaciers 
once extended far below their present level. In Central Chile I 
examined a vast mound of detritus with great boulders, crossing 
the Portillo valley, which there can hardly be a doubt once 
formed a huge moraine ; and Mr. D. Forbes informs me that he 
found in various parts of the Cordillera, from lat. 13 to 30 S., at 
about the height of 12,000 feet, deeply-furrowed rocks, resembling 
those with which he was familiar in Norway, and likewise great 
masses of detritus, including grooved pebbles. Along this whole 
space of the Cordillera true glaciers do not now exist even at 
much more considerable heights. Farther south on both sides of 
the continent, from lat. 41 to the southernmost extremity, we 
have the clearest evidence of former glacial action, in numerous 
immense boulders transported far from their parent source. 

From these several facts, namely from the glacial action having 
extended all round the northern and southern hemispheres from 
the period having been in a geological sense recent in both hemi- 
spheres from its having lasted in both during a great length of 
time, as may be inferred from the amount of work effected and 
lastly from glaciers having recently descended to a low level 
along the whole line of the Cordillera, it at one time appeared to 
me that we could not avoid the conclusion that the temperature 
of the whole world had been simultaneously lowered during the 
Glacial period. But now Mr. Croll, in a series of admirable 
memoirs, has attempted to show that a glacial condition of 
climate is the result of various physical causes, brought into 
operation by an increase in the eccentricity of the earth's orbit. 
All these causes tend towards the same end; but the most 
powerful appears to be the indirect influence of the eccentricity 
of the orbit upon oceanic currents. According to Mr. Croll, cold 
periods regularly recur every ten or fifteen thousand years ; and 


these at long intervals are extremely severe, owing to certain 
contingencies, of which the most important, as Sir C. Lyell has 
shown, is the relative position of the land and water. Mr. Croll 
believes that the last great Glacial period occurred about 240,000 
years ago, and endured with slight alterations of climate for about 
160,000 years. With respect to more ancient Glacial periods, 
several geologists are convinced from direct evidence that such 
occurred during the Miocene and Eocene formations, not to 
mention still more ancient formations. But the most important 
result for us, arrived at by Mr. Croll, is that whenever the 
northern hemisphere passes through a cold period the tempera- 
ture of the southern hemisphere is actually raised, with the 
winters rendered much milder, chiefly through changes in the 
direction of the ocean-currents. So conversely it will be with 
the northern hemisphere, whilst the southern passes through a 
Glacial period. This conclusion throws so much light on geo- 
graphical distribution that I am strongly inclined to trust in it; 
but I will first give the facts, which demand an explanation. 

In South America, Dr. Hooker has shown that besides many 
closely allied species, between forty and fifty of the flowering 
plants of Tierra del Fuego, forming no inconsiderable part of 
its scanty flora, are common to North America and Europe, 
enormously remote as these areas in opposite hemispheres are 
from each other. On the lofty mountains of equatorial America 
a host of peculiar species belonging to European genera occur. 
On the Organ mountains of Brazil, some few temperate European, 
some Antarctic, and some Andean genera were found by Gardner, 
which do not exist in the low intervening hot countries. On the 
Silla of Caraccas, the illustrious Humboldt long ago found species 
belonging to genera characteristic of the Cordillera. 

In Africa, several forms characteristic of Europe and some few 
representatives of the flora of the Cape of Good Hope occur on 
the mountains of Abyssinia. At the Cape of Good Hope a very 
few European species, believed not to have been introduced by 
man, and on the mountains several representative European forms 
are found, which have not been discovered in the intertropical 
parts of Africa. Dr. Hooker has also lately shown that several 
of the plants living on the upper parts of the lofty island oi 
Fernando Po and on the neighbouring Cameroon mountains, in 
the Gulf of Guinea, are closely related to those on the mountains 
of Abyssinia, and likewise to those of temperate Europe. It now 
also appears, as I hear from Dr. Hooker, that some of these same 
temperate plants have been discovered by the Rev. R. T. Lowe on 
the mountains of the Cape Verde islands. This extension of the 
same temperate forms, almost under the equator, across the whole 
continent of Africa and to the mountains of the Cape Verde 



archipelago, is one of the most astonishing facts ever recorded in 
the distribution of plants. 

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 of plants collected on the loftier peaks of Java, raises 
a picture of a collection made on a hillock in Europe ! Still more 
striking is the fact that peculiar Australian forms are represented 
by certain 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 hand as far north as Japan. 

On the southern mountains of Australia, Dr. F. Muller 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 certain plants 
growing on the more lofty mountains of the tropics in all parts 
of the world, and on the temperate plains of the north and south, 
are either the same species or varieties of the same species. It 
should, however, be observed that these plants are not strictly 
arctic forms; for, as Mr. H. C. Watson has remarked, "in re- 
ceding from polar towards equatorial latitudes, the Alpine or 
mountain floras really become less and less Arctic." Besides 
these identical and closely allied forms, many species inhabiting 
the same widely sundered areas, belong to genera not now found 
in the intermediate tropical lowlands. 

These brief remarks apply to plants alone; but some few 
analogous facts could be given in regard to terrestrial animals. 
In marine productions, similar cases likewise occur; as an 
example, I may quote a statement 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. Iiichardson, also, speaks of the reappearance on the shores 
of New Zealand, Tasmania, ttc., 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. 

From the foregoing facts, namely, the presence of temperate 


forms on the highlands across the whole of equatorial Africa, 
and along the Peninsula of India, to Coy Ion and the Malay 
Archipelago, and in a less well-marked manner across the wide 
expanse of tropical South America, it appears almost certain that 
at some former period, no doubt during the most severe part of a 
Glacial period, the lowlands of these great continents were every- 
where tenanted under the equator by a considerable number of 
temperate forms. At this period the equatorial climate at the 
level of the sea was probably about the same with that now 
experienced at the height of from five to six thousand feet under 
the same latitude, or perhaps even rather cooler. During this, 
the coldest period, the lowlands under the equator must have 
been clothed with a mingled tropical and temperate vegetation, 
like that described by Hooker as growing luxuriantly at the 
height of from four to five thousand feet on the lower slopes of 
the Himalaya, but with perhaps a still greater preponderance of 
temperate forms. So again in the mountainous island of Fernando 
Po, in the Gulf of Guinea, Mr. Mann found temperate European 
forms beginning to appear at the height of about five thousand 
feet. On the mountains of Panama, at the height of only two 
thousand feet, Dr. Seemann found the vegetation like that of 
Mexico, "with forms of the torrid zone harmoniously blended 
with those of the temperate." 

Now let us see whether Mr. Croll's conclusion that when the 
northern hemisphere suffered from the extreme cold of the great 
Glacial period, the southern hemisphere was actually warmer, 
throws any clear light on the present apparently inexplicable 
distribution of various organisms in the temparate parts of both 
hemispheres, and on the mountains of the tropics. 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 
became more and more intense, we know that Arctic forms 
invaded the temperate regions; and, from the facts just given, 
there can hardly be a doubt that some of the more vigorous, 
dominant and widest-spreading temperate forms invaded the 
equatorial lowlands. The inhabitants of these hot lowlands 
would at the same time have migrated to the tropical and sub- 
tropical regions of the south, for the southern hemisphere was at 
this period warmer. On the decline of the Glacial period, as both 
hemispheres gradually recovered their former temperatures, the 
northern temperate forms living on the lowlands under the 
equator, would have been driven to their former homes or have 
been destroyed, being replaced by the equatorial forms returning 
from the south. Some, however, of the northern temperate forms 


would almost certainly have ascended any adjoining high land, 
where, if sufficiently lofty, they would have long survived like the 
Arctic forms on the mountains of Europe. They might have 
survived, even if the climate was not perfectly fitted for them, 
for the change of temperature must have been very slow, and 
plants undoubtedly possess a certain capacity for acclimatisation, 
as shown by their transmitting to their offspring different con- 
stitutional powers of resisting heat and cold. 

In the regular course of events the southern hemisphere would 
in its turn be subjected to a severe Glacial period, with the 
northera hemisphere rendered warmer; and then the southern 
temperate forms would invade the equatorial lowlands. The 
northern forms which had before been left on the mountains 
would now descend and mingle with the southern forms. These 
latter, when the warmth returned, would return to their former 
homes, leaving some few species on the mountains, and carrying 
southward with them some of the northern temperate forms 
which had descended from their mountain fastnesses. Thus, 
we should have some few species identically the same in the 
northern and southern temperate zones and on the mountains of 
the intermediate tropical regions. But the species left during a 
long time on these mountains, or in opposite hemispheres, would 
have to compete with many new forms and would be exposed to 
somewhat different physical conditions; hence they would be 
eminently liable to modification, and would generally now exist 
as varieties or as representative species ; and this is the case. We 
must, also, bear in mind the occurrence in both hemispheres of 
former Glacial periods ; for these will account, in accordance with 
the same principles, for the many quite distinct species inhabiting 
the same widely separated areas, and belonging to genera not now 
found in the intermediate torrid zones. 

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 or slightly modified species have migrated 
from the north to the south, than in a reversed direction. We 
see, however, a few southern forms on the mountains of Borneo 
and Abyssinia. I suspect that this preponderant migration from 
the north to the 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 the two sets became com- 
mingled in the equatorial regions, during the alternations of 
the Glacial periods, the northern forms were the more powerful 
and were able to hold their places on the mountains, and after- 


wards to migrate southward with the southern forms; but not so 
the southern in regard to the northern forms. In the same 
manner at the present day, we see that very many European 
productions cover the ground in La Plata, New Zealand, and 
to a lesser degree in Australia, and have beaten the natives ; 
whereas extremely few southern forms have become naturalised 
in any part of the northern hemisphere, 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 forty or fifty years from Australia. The 
Neilgherrie mountains in India, however, offer a partial excep- 
tion; for here, as I hear from Dr. Hooker, Australian forms are 
rapidly sowing themselves and becoming naturalised. Before the 
last great Glacial period, no doubt the intertropical mountains 
were stocked with endemic Alpine forms ; but these have almost 
everywhere yielded to the more dominant forms generated in the 
larger areas and more efficient workshops of the north. In many 
islands the native productions are nearly equalled, or even out- 
numbered, by those which have become naturalised ; and this is 
the first stage towards their extinction. Mountains are islands 
on the land, and their inhabitants have yielded to those produced 
within the larger areas of the north, just in the same way as the 
inhabitants of real islands have everywhere yielded and are still 
yielding to continental forms naturalised through man's agency. 

The same principles apply to the distribution of terrestrial 
animals and of marine productions, in the northern and southern 
temperate zones, and on the intertropical mountains. When, 
during the height of the Glacial period, the ocean-currents were 
widely different to what they now are, some of the inhabitants of 
the temperate seas might have reached the equator; of these a 
few would perhaps at once be able to migrate southward, by 
keeping to the cooler currents, whilst others might remain and 
survive in the colder depths until the southern hemisphere was 
in its turn subjected to a glacial climate and permitted their 
further progress ; in nearly the same manner as, according to 
Forbes, isolated spaces inhabited by Arctic productions exist 
to the present day in the deeper parts of the northern temperate 

I am far from supposing that all the difficulties in regard to the 
distribution and affinities of the identical and allies species, which 
now live so widely separated in the north and south, and some- 
times on the intermediate mountain-ranges, are removed on the 
views above given. The exact lines of migration cannot be 
indicated. We cannot say why certain species and not others 
have migrated ; why certain species have been modified and have 
given rise to new forms, whilst 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 species ranges twice or thrice as far, and is 
twice or thrice as common, as another species within their own 

Various special difficulties also remain to be solved ; for instance, 
the occurrence, as shown by Dr. Hooker, of the same plants at 
points so enormously remote as Kerguelen Land, New Zealand, 
and Fuegia ; but icebergs, as suggested by Lyell, may have been 
concerned in their dispersal. The existence at these and other 
distant points of the southern hemisphere, of species, which, 
though distinct, belong to genera exclusively confined to the 
south, is a more remarkable case. Some of these species are so 
distinct, that we cannot suppose that there has been time since 
the commencement of the last Glacial period for their migration 
and subsequent modification to the necessary degree. The facts 
seem to indicate that distinct species belonging to the same 
genera have migrated in radiating lines from a common genera ; 
and I am inclined to look in the southern, as in the northern 
hemisphere, to a former and warmer period, before the commence- 
ment of the last Glacial period, when the Antarctic lands, now 
covered with ice, supported a highly peculiar and isolated flora. 
It may be suspected that before this flora was exterminated during 
the last Glacial epoch, a few forms had been already widely dis- 
persed to various points of the southern hemisphere by occasional 
means of transport, and by the aid as halting-places, of now 
sunken islands. Thus the southern shores of America, Australia, 
and New Zealand may have become slightly tinted by the same 
peculiar forms of life. 

Sir C. Lyell in a striking passage has speculated, in language 
almost identical with mine, on the effects of great alterations of 
climate throughout the world on geographical distribution. And 
we have now seen that Mr. Croll's conclusion that successive 
Glacial periods in the one hemisphere coincide with warmer 
periods in the opposite hemisphere, together with the admission 
of the slow modification of species, explains a multitude of facts 
in the distribution of the same and of the allied forms of life in 
all parts of the globe. The living waters have flowed during one 
period from the north and during another from the south, and in 
both cases have reached the equator ; but the stream of life has 
flowed with greater force from the north than in the opposite 
direction, and has consequently more freely inundated the south. 
As the tide leaves its drift in horizontal lines, 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 altitude under the 


equator. The various beings thus left stranded may be compared 
with savage races of man, driven up and surviving in the moun- 
tain fastnesses of almost every land, which serves as a record, 
full of interest to us, of the former inhabitants of the surrounding 


Distribution of fresh-water productions On the inhabitants of oceanic islands 
Absence of Batrachians and of terrestrial Mammals On the relation 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 chapter. 

Fresh-water Productions. 

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 formidable 
barrier, that they would never have extended to distant countries. 
But the case is exactly the reverse. Not only have many fresh- 
water species, belonging to different classes, an enormous range, 
but allied species prevail in a remarkable manner throughout the 
world. When first collecting in the fresh waters of Brazil, I well 
remember feeling much surprise at the similarity of the fresh- 
water insects, shells, &c., and at the dissimilarity of the surround- 
ing terrestrial beings, compared with those of Britain. 

But the wide ranging power of fresh-water productions can, 1 
think, in most cases be explained by their having become fitted, 
in a manner highly useful to them, for short and frequent migra- 
tions from pond to pond, or from stream to stream, within their 
own countries ; and liability to wide dispersal would follow from 
this capacity as an almost necessary consequence. We can here 
consider only a few cases; of these, some of the most difficult to 
explain are presented by fish. It was formerly believed that the 
same fresh-water species never existed on two continents distant 
from each other. But Dr. Giinther has lately shown that the 
Galaxias attenuatus inhabits Tasmania, New Zealand, the Falk- 
land Islands, and the mainland of South America. This is a 
wonderful case, and probably indicates dispersal from an Ant- 
arctic centre during a former warm period. This case, however, 
is rendered in some degree less surprising by the species of this 
genus having the power of crossing by some unknown means 


considerable spaces of open ocean : thus there is one species 
common to New Zealand and to the Auckland Islands, though 
separated by a distance of about 230 miles. On the same continent 
fresh-water fish often range widely, and as if capriciously; for in 
two adjoining river-systems some of the species may be the same, 
and some wholly different. 

It is probable that they are occasionally transported by what 
may be called accidental means. Thus fishes still alive are not 
very rarely dropped at distant points by whirlwinds; and it is 
known that the ova retain their vitality for a considerable time 
after removal from the water. Their dispersal may, however, be 
mainly attributed to changes in the level of the land within the 
recent period, causing rivers to flow into each other. Instances, 
also, could be given of this having occurred during floods, without 
any change of level. The wide difference of the fish on the oppo- 
site sides of most mountain-ranges, which are continuous, and 
which consequently must from an early period have completely 
prevented the inosculation of the river-systems on the two sides, 
leads to the same conclusion. 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. Moreover Dr. Giinthor has recently 
been led by several considerations to infer that with fishes the 
same forms have a long endurance. Salt-water fish can with care 
be slowly accustomed to live in fresh water; and, according *~o 
Valenciennes, there is hardly a single group of which all the 
members are confined to fresh water, so that a marine species 
belonging to a fresh-water group might travel far along the shores 
of the sea, and could, it is probable, become adapted without 
much difficulty to the fresh waters of a distant land. 

Some species of fresh-water shells have very wide ranges, and 
allied species which, on our 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 
the ova, as well as the adults, are immediately killed by sea- water. 
I could not even understand how some naturalised species have 
spread rapidly throughout the same country. But two facts, 
which I have observed and many others no doubt will be dis- 
covered throw some light on this subject. When ducks suddenly 
emerge from a pond covered with duck -weed, I have twice seen 
these little plants adhering to their backs ; and it has happened 
to me, in removing a little duck-weed from one aquarium to 
another, that I have unintentionally stocked the one with fresh- 
water shells from the other. But another agency is perhaps more 
effectual : I suspended the feet of a duck 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 if blown across the sea to an oceanic island, 
or to any other distant point, would be sure to alight on a pool or 
rivulet. Sir Charles Lyell informs me that a Dytiscus 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 Colym- 
betes, once flew on board the 'Beagle,' when forty-five miles 
distant from the nearest land : how much farther it might have 
been blown by 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 illustrated, according to Alph. de Candolle, in those 
large groups of terrestrial plants, which have very few aquatic 
members; for the latter seem immediately to acquire, as if in 
consequence, a wide range. I think favourable means of dis- 
persal explain this fact. I have before mentioned that earth 
occasionally 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 wander more than those of any other; and 
they are occasionally found on the most remote and barren islands 
of the open ocean ; they would not be likely to alight on the surface 
of the sea, so that any dirt on their feet would not be washed off; 
and when gaining the land, they would be sure to fly to their 
natural fresh-water haunts. I do not believe that botantists 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 dried weighed only 6| 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 break- 
fast cup ! Considering these facts, I think it would be an inex- 
plicable circumstance if water-birds did not transport the seeds 
of fresh-water plants to unstocked ponds and streams, situated at 
very distant points. 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 
many hours afterwards in pellets or in the excrement. When I 
saw the great size of the seeds of that fine water-lily, the Nelum- 
bium, and remembered Alph. de Candolle's remarks on the distri- 
bution of this plant, I thought that the means of its dispersal must 
remain 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. Now this bird 
must often have flown with its stomach thus well stocked to distant 
ponds, and then getting a hearty meal of fish, analogy makes me 
believe that it would have rejected the seeds in a pellet in a fit 
state for germination. 

In considering these several means of distribution, it should be 
remembered that when a pond or stream is first formed, for in- 
stance, 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 inhabitants of the same 
pond, however few in kind, yet as the number even in a well- 
stocked pond is small in comparison with the number of species 
inhabiting an equal area of land, the competition between them 
will probably be less severe than between terrestrial species ; con- 
sequently an intruder from the waters of a foreign country would 
have a better chance of seizing on a new place, than in the case of 
terrestrial colonists. We should also remember that many fresh- 
water productions are low in the scale of nature, and we have 
reason to believe that such beings become modified more slowly 
than the high ; and this will give time for the migration of aquatic 
species. We should not forget the probability of many fresh- water 
forms having formerly ranged continuously over immense areas, 
and then having become extinct at intermediate points. But the 
wide distribution of fresh-water plants and of the lower animals, 
whether retaining the same identical form or in some degree 
modified, apparently depends in main part on the wide dispersal 
of their seeds and eggs by animals, more especially by fresh- water 
birds, which have great powers of flight, and naturally travel from 
one piece of water to another. 


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 with 
respect to distribution, on the view that not only all the indi- 
viduals of the same species have migrated from some one area, but 
that allied species, although now inhabiting the most distant 
points, have proceeded from a single area, the birthplace of their 
early progenitors. I have already given my reasons for disbeliev- 
ing in continental extensions within the period of existing species, 
on so enormous a scale that all the many islands of the several 
oceans were thus stocked with their present terrestrial inhabitants. 
This view removes many difficulties, but it does not accord with 
all the facts in regard to the productions of islands. In the follow- 
ing remarks I shall not confine myself to the mere question of 
dispersal, but shall consider some other cases bearing on the truth 
of the two theories of independent creation and of descent with 

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. New 
Zealand, for instance, with its lofty mountains and diversified 
stations, extending over 780 miles of latitude, together with the 
outlying islands of Auckland, Campbell and Chatham, contain 
altogether only 960 kinds of flowering plants ; if we compare this 
moderate number with the species which swarm over equal areas 
in South- Western Australia or at the Cape of Good Hope, we 
must admit that some cause, independently of different physical 
conditions, has given rise to 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 less than half-a-dozen flower- 
ing plants ; yet many species have now become naturalised on it, 
as they have in 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 extermi- 
nated 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 were not 
created for oceanic islands ; for man has unintentionally stocked 
them far more fully and perfectly than did nature. 

Although in oceanic islands the species are few in number, the 
proportion of endemic kinds (i.e. those found nowhere else in the 
world) is often extremely large. If we compare, for instance, the 


number of endemic land-shells in Madeira, or of endemic birds in 
the Galapagos Archipelago, with the number found on any conti- 
nent, and then compare the area of the island with that of the 
continent, we shall see that this is true. This fact might have 
been theoretically expected, for, as already explained, species 
occasionally arriving after long intervals of time in the new and 
isolated district, and having to compete with new associates, would 
be eminently liable to modification, and would 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 partly on the species 
which are not modified having immigated in a body, so that their 
mutual relations have not been much disturbed ; and partly on 
the frequent arrival of unmodified immigrants from the mother- 
country, with which the insular forms have intercrossed. It 
should be borne in mind that the offspring of such crosses would 
certainly gain in vigour ; so that even an occasional cr