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SIl|P E B. Ml Slibrarg 

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This book was presented by 

Dr. Ernest J. Schreiner 
Forest Geneticist 


- V, 

S004 18824 R 


WC 1 5 1993 



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Charles Darwin. 


The Origin of Species 

By Means of Natural Selection 

Or the Preservation of Favored 
Races in the Struggle for Life 

Charles Darwin 



New York and London 


''But with regard to the material world, we can at least 
go so far as this — we can perceive tliat events are brought 
about not by insulated interpositions of Divine power, 
exerted in each particular case, but by the establishment 
of general laws." — Whewell: Bridgeivater Treatise. 

''The only distinct meaning of the word * natural' is 
stated, fixed or settled; since what is natural as much re- 
quires and presu Piloses an intelligent agent to render it so, 
i.e., ^o effect it continually or at stated times, as what is 
supernatural or miraculous does to effect it for once." — 
Butler: Analogy of Revealed Religion. 

*' To conclude, therefore, let no man out of a weak con- 
ceit of sobriety, or an ill-applied moderation, think or 
maintain, that a man can search too far or be too welt 
studied in the book of God's word, or in the book of God'*% 
works; divinity or philosophy; but rather let me^i 
endeavor an endless progress or proficience in both/^—- 
Bacoj^: Advancement of Learning, 








I WILL here give a brief sketch of the progress of opin- 
ion on the Origin of Species. Until recently the great 
majority of naturalists believed that species were immut- 
able 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 ])re exist- 
ing forms. Passing over allusions to the subject in the 
classical writers,* the first author who in modern times 

*Aristotle, in liis " Pliysicae Auscultationes " (lib. 2, cap. 8, s. 2), 
after remarking that rain does not fall in order to make tlie corn 
grow, any more than it falls to spoil the farmer's corn when threshed 
out of doors, applies the same argument to organization; and adds (as 
translated by Mr. Clair Grece, who first pointed out the passage to 
me), '' So what hinders the different parts [of the body] from having 
this merely accidental relation in nature ? as the teeth, for example, 
grow by necessity, the front ones sharp, adapted for dividing, and the 
grinders flat, and serviceable for masticating the food; since they 
were not made for the sake of this, but it was the result of accident, 
Afi f] in like manner as to other parts in which there appears to exist 
an adaptation to an end. Wheresoever, therefore, all things 
together (that is all the parts of one whole) happened like as if they 
were made for the sake of something, these were preserved, having 
been appropriately constituted by an internal spontaneity, and what- 
soever things were not thus constituted, perished and still peri.'=;h." 
We here see the principle of natural selection shadowed forth, but 
how little Aristotle fully comprehended the principle, is shown by 
his remarks on the formation of the teeth. 


has treated it in a scientific spirit was Buffon. Bn' as his 
opinions fluctuated greatly at different periods, and as he 
does not enter on the causes or means of the transforma- 
tion of species, I need not here enter on details. 

Lamarck was the first man whose conclusions on the 
subject excited much attention. This justly celebrated 
naturalist first published his views in 1801; he much en- 
larged them in 1809 in his " Philosophic Zoologique/^ and 
subsequently, 1815, in the Introduction to his *'Hist. Kat. 
des Animaux sans Vertebres." In these works he upholds 
the doctrine that all species, including man, are descended 
from other species. He first did the eminent service of 
arousing attention to the probability of all change in tlie 
organic, as well as in the inorganic world, being the result 
of law, and not of miraculous interposition. Lamarck 
seems to have been chiefly led to his conclusion on the 
gradual change of species, by the difficulty of distinguish- 
ing 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 ph3'S- 
ieal conditions of life, something to the crossing of 
already existing forms, and much to use and disuse, that 
is, to the etiects of habit. To this latter agency he seems 
to attribute all the beautiful adaj^tations in nature; such 
as the long ne'3k of the girafl'e 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 spontaneous!}' generated.* 

Geoffroy Saint-Hilaire, as is stated in his ^^Life," writ- 
ten by his son, suspected, as early as 1795, that what we 
call species are various degenerations of the same type. It 

* I have taken the date of the first publication of Lamarck from 
Isidore Geoifroy 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 DarAvin, 
anticipated the views and erroneous grounds of opinion of Lamarck 
in his "Zoonomia" (vol. i. pp. 500-510), published in 1704. Accord- 
ing to Isid. Geoft'roy there is no doubt that Goethe was an extreme 
pOFiasan ci amilax views^ as shown in the iartroduclaon fco a woa-k 


was not until 1828 that he published his conviction that 
the same forms have not been perpetuated since the origin 
of all things. Geoffroy seems to have relied chiefly on the 
conditions of life, or the ^'moncle ambiant" as the cause of 
change. He was cautious in drawing conclusions, aud did 
not believe that existing species are now undergoing modi- 
fication; and, as his son adds, "C'est done un probleme a 
reserver enti^rement a Favenir, suppose meme que Tavenir 
doive avoir prise sur lui/' 

In 1813 Dr. W. C. Wells read before the Royal Society 
"An Account of a White Female, part of whose skin 
resembles that of a ISTegro ; " but his paper was not pub- 
lished until his famous " Two Essays upon Dew and Single 
Vision^' appeared in 1818. In this pai:)er he distinctly 
recognizes 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 im- 
munity from certain tropical diseases, he observes, firstly, 
that all animals tend to vary in some degree, and, secondly, 
that agriculturists improve their domesticated amimals 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 varie- 
ties 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 
others to bear the diseases of the country. This race 
would consequently multiply, while the others would 
decrease; not only from their inability to sustain the 
attacks of disease, but from their incapacity of contending 
with their more vigorous neighbors. The color of this 
vigorous race I take for granted, from what has been 

written in 1794 and 1795, but not pubUslied till long afterward: lie 
lias pointedly remarked ("Goethe als Naturforscher," von Dr. Karl 
Meding, 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 simihir views arise 
at about the same time, that Goethe in Germany, Dr. Darwin in Eng- 
land, and GeofEroy Saint- Hilaire (as we shall immediately see) in 
France, came to the same conclusion on the origin of species, in the 
years 1794-5. 


already said, would be dark. But the same disposition to 
form varieties still existing, a darker and a darker race 
would in the course of time occur; and as the darkest 
would be the best fitted for the climate, this would at 
length become the most prevalent, if not the only race, in 
the particular country in which it had originated." He 
then extends these samxC 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 of Dr. Wells' work. 

The Hon. and Rev. W. Herbert, afterward Dean of 
Manchester, in the fourth volume of the '^ Horticultural 
Transactions," 1822, and in his work on the *' Amarylli- 
dacese*' (1837, pp. 19, 339), declares that ^'horticultural 
experiments have established, beyond the possibility of 
refutation, that botanical species are only a higher and 
more permanent class of varieties." He extends the same 
view to animals. The dean believes that single species of 
each genus were created in an originally highly plastic con- 
dition, and that these have produced, chiefly by inter- 
crossing, but likewise by variation, all our existing species. 

In 1826 Professor Grant, in the concluding paragraph 
in his well-known paper (*'* Edinburgh Philosophical 
Journal," vol. xiv, p. 283) on the Spongilla, clearly de- 
clares 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 Fifty-fifth 
Lecture, published in the *' Lancet" in 1834. 

In 1831 Mr. Patrick Matthew published his work on 
"Naval Timber and Arboriculture," in which he gives 
precisely the same view on the origin of species as that 
(presently to be alluded to) propounded by Mr. Wallace 
and myself in the "Linnean Journal," and as that enlarged 
in the present volume. Unfortunately the view was given 
by Mr. Matthew very briefly in scattered passages in an 
appendix to a work on a different subject, so that it re- 
mained unnoticed until Mr. Matthew himself drew atten- 
tion to it in the '* Gardeners' Chronicle," on April 7, 
1860. The differences of Mr. Matthew's views from mine 
are not of much importance: he seems to consider that the 
world was nearly depopulated at successive periods, and 
then restocked; and he gives as an alternative, that uew 


forms may be generated '' without the presence of any 
mold 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 prin- 
ciple of natural selection. 

The celebrated geologist and naturalist. Von Buch, in 
his excellent '^ Description Physique des Isles Canaries'' 
(1836, p. 147), clearly expresses his belief that varieties 
slowly become changed into permanent species, which are 
no longer capable of intercrossing. 

Eafinesque, in his '^ New Flora of North America," pub- 
lished 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 char- 
acters;" but further on (p. 18) he adds, ''except the 
original types or ancestors of the genus." 

In 1843-44 Professor Haldeman f " Boston Journal of 
Nat. Hist. U. States," vol. iv, p. 468) has ably given the 
arguments for and against the hypothesis of the develop- 
ment and modification of species: he seems to lean toward 
the side of change. 

The " Vestiges of Creation" appeared in 1844. In the 
tenth and much improved edition (1853) the anony- 
mous author says (p. 155): "The proposition determined 
on after much consideration is, that the several series 
of animated beings, from the simplest and oldest up to 
the highest and most recent, are, under the providence 
of God, the results, first, of an impulse which has been im- 
parted to the forms of life, advancing them, in definite 
times, by generation, through grades of organization ter- 
minating in the highest dicotyledons and vertebrata, these 
grades being few in number, and generally marked by in- 
tervals of organic character, which we find to be a practi 
cal difficulty in ascertaining affinities; second, oi another 
impulse connected with the vital forces, tending, in the 
course of generations, to modify organic structures in ac- 
cordance Avith external circumstances, as food, the nature 
of the habitat, and the meteoric agencies, these being the 
' adaptations ' of the natural theologian." The author ap- 
parently believes that organization progresses by sudden 
leaps, but that the effects produced by the conditions of 


life are gradual. He argues witli 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 coadaptations 
which we see tliroughout nature; I cannot see that we 
thus gain an 3^ 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 early editions little accurate knowledge and a great 
want of scientific caution, immediately had a very wide 
circulation. In my opinion it has done excellent service 
in this country in calling attention to the subject, in re- 
moving prejudice, and in thus preparing the ground for 
the reception of analogous views. 

In 1846 the veteran geologist M. J. d'Omalins d^Halloy 
published in an excellent though short paper (" Bulletins 
de FAcad. Roy. Bruxelles," torn, xiii, p. 581) his opinion 
that it is more probable that new species have been pro- 
duced by descent with modification than that they have 
been separately created: the author first promulgated this 
opinion in 1831. 

Professor Owen, in 1819 (^'ISTature 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 secon- 
dary 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 xA.ssociation, 
in 1858, he speaks (p. li) of ''the axiom of the continuous 
operation of creative power, or of the ordained becoming 
of living things.''^ Further on (p. xc), after referring to 
geographical distribution, he adds, "These phenomena 
shake our confidence in the conclusion that the Apteryx of 
New Zealand and the Red Grouse of England were distinct 
creations in and for those islands respectively. Always, 
also, it may be well to bear in mind that by the word 
'creation' tlie 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 exclu- 
sively; signifying also, by this mode of expressing such 
ignorance, his belief that both the bird and the islands 
owed their origin to a great first Creative Cause." If we 
interpret these sentences given in the same address, one by 
the other, it appears that this eminent philosopher felt in 
1858 his confidence shaken that the Apteryx and the Red 
Grouse first appeared in their respective homes *' he knew 
not how," or by some process *'he knew not what." 

Tliis 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 palgeon- 
tologists as being firmly convinced of the immutability of 
species; but it appears (^'Anat. of Vertebrates," vol. iii, 
p. 796) that this was on my part a preposterous error. In 
the last edition of this work I inferred, and the inference 
still seems to me perfectly just, from a passage beginning 
with the words "no doubt the type-form," etc. (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 sur- 
prise and satisfaction at this announcement ; but as far as 
it is possible to understand certain recently published pas- 
sages (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 pre- 
ceded by Dr. Wells and Mr. Matthews. 


M. Isidore Geoffroy Saint-Hilaire, in his lectures deliv- 
ered 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 circonstauces: ils se moditient, si les circonstances 
ambiantes viennent a changer." '^En resume, Vobserva- 
tion des animaux sauvages demontre deja la variabilite 
limit ee des especes. Les experiences sur les animaux 
sauvages devenus domestiques, et sur les animaux domes- 
tiques redevenus sauvages, la demontrent plus clairement 
encore. Ces memes experiences prouvent, de plus, que les 
differences produites peuvent etre de valeu7' generique.'* 
In his ^aiist. Nat. Generale'' (tom. ii, p. 430, 1859) he 
amplifies analogous conclusions. 

From a circular lately issued it appears that Dr. Freke, 
m 1851 {'' Dublin Medical Press," p. 332), propounded 
the doctrine that all organic beings have descended from 
one primordial form. His grounds of belief and treat- 
ment of the subject are wholly different from mine ; 
but as Dr. Freke has now (1861) published his Essay on 
^he ^' Origin of Species by means of Organic Affinity,'^ 
the difficult attempt to give any idea of his views would 
be superfluous on my part. 

Mr. Herbert Spencer, in an Essay (originally pub- 
lished 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 em- 
bryos of many species undergo, from the difficulty of dis- 
tinguishing species and varieties, and from the principle of 
general gradation, that species have been modified ; and 
he attributes the modification to the change of circum- 
Btances. The author (1855) has also treated Psychology 
on the principle of the necessary acquirement of each 
mental power and capacity by gradation. 

In 1852 M. Naudin, a distinguished botanist, expressly 
stated, in an admirable paper on the Origin of Species 
(•'Revue Horticole," p. 1k)Z \ since partly republished in 
the *']SIouvelles Archives du Museum," tom. i, p. 171), his 
belief that species are formed in an analogous manner as 


varieties are under cultivation ; and the latter process lie 
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 tlian at present. He lays weight on wliat he calls 
the principle of finality, " puissance mysterieuse, ind6- 
terminee ; fatality pour les uns ; pour les autres volonte 
providentielle, dont Taction incessante sur les ^tres vi- 
vantes determine, a toutes les epoques de Fexistence du 
monde, la forme, le volume, et la duree de chacun d'eux, 
en raison de sa destinee dans Tordre de choses dont il fait 
partie. C'est cette puissance qui harmonise chaque 
membre a Fensemble, en Tappropriant a la fonction qu'il 
doit remplir dans Torganisme general de la nature, fonc- 
tion qui est pour lui sa raison d'etre." * 

In 1853 a celebrated geologist, Count Keyserling (*^ Bul- 
letin de la Soc. Geolog.," 2d Ser., tom. x, p. 357), sug- 
gested 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 alfected by circumambient molecules of a 
particular nature, and thus have given rise to new forms. 

In this same year, 1853, Dr. Schaaffhausen published an 
excellent pamphlet (^^Verhand. des Naturhist. Vereins der 
Preuss. Rheinlands," etc.), in which he maintains the de- 
velopment of organic forms on the earth. He infers that 
many species have kept true for long periods, whereas a 
fcAV have become modified. The distinction of species he 
explains by the destruction of intermediate graduated 

*Fiom references in Bronn's " Untersuchungen ilber die Ent- 
wickeluiigs-Gesetze," it appears that the celebrated bot-anist and 
palaeontologist Unger published, in 1852, his belief that species 
undergo development and modification. Balton, likewise, in Pander 
and Dalton's work on Fossil Sloths, expressed, in 1821, a similar 
belief. Similar views have, as is well known, been maintained by 
Oken in his my?;tical "Natur-Philosophie." From other references 
in Godron's work "Sur I'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-fonr 
authors named in this Historical Sketch, who believe in the modi- 
cation of species, or at least disbelieve in separate acts of crea- 
tion, twenty-seven have written on ^)ecial branches of natural 
kistory or geology. 


forms. ** Thus living plants and animals are not sepa- 
rated from the extinct by new creations, but are to be 
regarded as their descendants through continued repro- 

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 
Tespece, nous conduisent directement aux idees emises 
par deux hommes justement celebres, Geoffroy Saint- 
Hilaire et Goethe." Some other passages scattered through 
M. TiCcoq^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 
m-: ^terly manner by the Rev. Baden Powell, in his "Essays 
on the Unity of Worlds," 1855. Nothing can be more 
striking than the manner in which he shovv^s that the intro- 
duction of new species is "a regular, not a casual phenom- 
enon," or, as Sir John Herschel expresses it, "a natural 
in contradistinction to a miraculous process." 

The third volume of the "Journal of the Linnean 
Society " contains papers, read July 1, 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 

Von Baer, toward whom all zoologists feel so profound a 
respect, expressed about the year 1859 (see Prof. Rudolph 
Wagner, " Zoologisch-Anthropologische Untersuchungen," 
1861, s. 51) his conviction, chiefly grounded on the laws of 
geographical distribution, that forms now perfectly distinct 
have descended from a single parent-form. 

In June, 1859, Professor Huxley gave a lecture before 
the Royal Institution on the " Persistent Types of Animal 
Life." Referring to such cases, he remarks, " It is difficult 
to comprehend the meaning of such facts as these, if we 
suppose that each species of animal and plant, or each 
great type of organization, 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 r^ation to 


that hypothesis which supposes the species living at any 
time to be the result of the gradual modification of pre- 
existing species, a hypothesis which, though unproven, and 
sadly damaged by some of its supporters, is yet the only 
one to which physiology lends any countenance; their 
existence would seem to show that the amount of modifica- 
tion 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 ^' Intro- 
duction to the Australian Flora/' In the first part of this 
great work he admits the truth of the descent and modifica- 
tion of species, and supports this doctrine by many original 

The first edition of this work was published on Novem- 
ber 24, 1859, and the second edition on January 7, 1860. 




Introduction 1 



Causes of Variability — EfEects of Habit and the use or disuse 
of Parts — Correlated Variation— Inberitance — Character of 
Domestic Varieties — Difficulty of distinguishing between 
Varieties and Species — Origin of Domestic Varieties from 
one or more Species — Domestic Pigeons, their Diiferences 
and Origin — Principles of Selection, anciently followed, 
their Elfects — Methodical and Unconscious Selection — 
Unknown Origin of our Domestic Productions — Circum- 
stances favorable to Man's power of Selection 6 



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 89 



Its bearing on natural selection — The term used in a wide 
sense — Geometrical ratio of increase — Rapid increase of 
naturalized animals and plants — Nature of the checks to 
increase — Competition universal — Effects of climate — Pro- 
tection from the number of individuals— Complex relations 
of all animals and plants throughout nature— Struggle for 

xviii CONTENTS. 

life most severe between individuals and varieties of tlie 
same species: often severe between species of the same 
genus — The relation of organism to organism the most 
important of all relations 57 



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 favorable and unfavorable 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 naturaliza- 
tion — 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 organization — Low forms preserved — 
Convergence of character — Indefinite multiplication of 
species — Summary 73 



Effects of changed conditions — Use and disuse, combined 
with natural selection; organs of flight and of vision — 
Acclimatization — Correlated variation — Compensation and 
economy of growth — False correlations — Multiple, rudi- 
mentary and lowly organized structures variable — Parts 
developed in an unusual manner ai'e highly variable; 
specific characters more variable than generic; secondaiy 
sexual characters variable — Species of the same genus vary 
in an analogous manner — Reversions to long-lost charac- 
ters — Summary 126 



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 diflBculty — Natura non facit saltum — Organs of 


small importance — Organs not in all cases absolutely per- 
fect — The law of Unity of Type and of the Conditions of 
Existence embraced by the theory of Natural Selection 108 




Longevity — Modifications not necessarily simultaneous — Modi- 
fications apparently of no direct service — Progressive 
development — Characters of small functional importance, 
the most constant — Supposed incompetence of natural selec 
tion to account for the incipient stages of useful structures 
— Causes which interfere with the acquisition through 
natural selection of useful structures — Gradations of struct- 
ure 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 199 



Instincts comparable with habits, but different in their 
origin — Instincts graduated — Aphides and ants — Instincts 
variable — Domestic instincts, their origin — Natural instincts 
of the cuckoo, 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 243 



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 accumu- 
lated 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 univereal— Hybrids and mon- 
grels compared independently of their fertility— Summary... 277 




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 rale of 
denudation and of deposition — On the lapse of time as esti- 
mated by years — On the poorness of our palseontological col- 
lections — On the intermittence of geological formations — On 
the denudation of granitic areas — On the absence of inter- 
mediate varieties in any one formation — On the sudden 
appearance of groups of species — On their sudden appear- 
ance in the lowest known fossiliferous strata — Antiquity of 
the habitable earth 313 



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 843 



Present distribution cannot be accounted for by differences 
in physical conditions — Importance of barriers — Affinity 
of the productions of the same continent — Centers of crea- 
tion — 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 „ 375 



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 



tliose of the nearest mainland — On colonization from the 
nearest source with subsequent modification— Summary of 
the last and present chapter 405 




Classification, groups subordinate to groups — Natural system — 
Rules and difficulties in classification, explained on the 
theory of descent with modification — Classification of varie- 
ties — Descent always used in classification — Analogical or 
adaptive characters — Affinities, general, complex and radiat- 
ing — Extinction separates and defines groups — Morphology, 
between members of the same class, between parts of the 
same individual — Embryology, laws of, explained by varia- 
tions not supervening at an early age, and being inherited 
at a corresponding age — Rudimentary organs, their origin 
explained — Summary 428 



Recapitulation of the objections to the theory of Natural Selec- 
' tion — Recapitulation of the general and special circum- 
stances in its favor — 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 476 

Glossary op Scientific Terms 507 

Index ^21 



When on board H. M. S. Beagle, as naturalist, I was 
much struck with certain facts in the distribution of 
the organic beings inhabiting 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 accumu- 
lating and reflecting on all sorts of facts which could possi- 
bly have any bearing on it. After five years' work I 
allowed myself to speculate on the subject, and drew up 
some short notes; these I enlarged in 1844 into a sketch of 
the conclusions, which then seemed to me probable: from 
that period to the present day I have steadily pursued the 
same object. I hope that I may be excused for entering 
on these personal details, as I give them to show that I 
have not been hasty in coming to a decision. 

My work is now (1859) nearly finished; but as it will 
take me many more years to com23lete it, and as my health 
is far from strong, I have been urged to publish this ab- 
stract. 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 
ihird volume of the Journal of that Society. Sir C. Lyell 


and Dr. Hooker, who both knew of my work — the latter 
having read my sketch of 18-4-i — lionored me by thinking 
it advisable to publish, with Mr. Wallace's excellent memoir, 
gome brief extracts from my manuscripts. 

This abstract, which I now publish, must necessarily be 
imperfect. I cannot here give references and authorities 
for my several statements; and I must trust to the reader 
reposing some confidence in my accuracy. No doubt errors 
may have crept in, though I hope I have always been cau- 
tious 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 ap- 
parently leading to conclusions directly opposite to those 
at which I have arrived. A fair result can be obtained 
only by fully stating and balancing the facts and arguments 
on both sides of each question; and this is here impossible. 

I much regret that want of space prevents my having 
the satisfaction of acknowledging the generous assistance 
which I have received from very many naturalists, some 
of them personally unknov/n to me. I cannot, however, 
let this opportunity i3ass 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 conceiv- 
able that a naturalist, reflecting on the mutual affinities of 
organic beings, on their embryological relations, their 
geographical distribution, geological succession, and other 
such facts, might come to the conclusion that species had 
not been independently created, but had descended, like 
varieties, from other species. Nevertheless, such a con- 
clusion, even if well founded, would be unsatisfactory, until 
it could be shown how the innumerable s]">ecies, inhabit- 
ing 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, each 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 admirably 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 cer- 
tain 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 commencement of my observations it seemed to me 
probable that a careful study of domesticated animals and 
of cultivated plants would offer the best chance of making 
out this obscure problem. 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 vari- 
ation 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 com- 
monly neglected by naturalists. 

From these considerations, I shall devote the first chap- 
ter of this abstract to variation under domestication. "We 
shall thus see that a large amount of hereditary modifica- 
tion is at least possible; and, what is equally or more im- 
portant, we shall see how great is the power of man in 
accumulating by his selection successive slight variations. 
I will then pass on to the variability of species in a state of 
nature; but I shall, unfortunately, be compelled to treat 
this subject far too briefly, as it can be treated properly 
only by giving long catalogues of facts. We shall, how- 
ever, be enabled to discuss what circumstances are most 
favorable to variation. In the next chapter the strug- 
gle for existence among all organic beings throught)ut 
the world, which inevitably follows from the high geomet- 
rical ratio of their increase, will be considered. This i^s 


the doctrine of Malthns, 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 recurring struggle for exist- 
ence, it follows that any being, if it vary however slightly 
in any manner profitable to itself, under the complex and 
sometimes varying conditions of life, will have a better 
chance of surviving, and thus be naturally selected. From 
the strong principle of inheritance, any selected variety 
will tend to propagate its new and modified form. 

This fundamental subject of natural selection will be 
treated at some length in the fourth chapter; and we shall 
then see how natural selection almost inevitably causes 
much extinction of the less improved forms of life, and 
leads to what I have called divergence of character. In the 
next chapter I shall discuss the complex and little known 
laws of variation. In the five succeeding chapters, the 
most apparent and gravest difficulties in accepting the 
theory will be given: namely, first, the difficulties of tran- 
sitions, or how a simple being or a simple organ can be 
changed and perfected into a highly developed being or 
into an elaborately constructed organ; secondly, the sub- 
ject of instinct, or the mental powers of animals; thirdly, 
hybridism, or the infertility of species and the fertility of 
varieties when intercrossed; and fourthly, the imperfection 
of the geological record. In the next chapter I shall con- 
sider the geological succession of organic beings through- 
out time; in the twelfth and thirteenth, their geogi'aphical 
distribution throughout 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 work, and a few 
concluding remarks. 

No one ought to feel surprise at much remaining as yet 
unexplained in regard to the origin of species and varieties, 
if he 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 ransre 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 modifica- 


tion of every inhabitant of this world. Still less do we 
know of the mutual relations of the innumerable inhab- 
itants of the world during the many past geological epochs 
in :.ts history. Although much remains obscnre^ and will 
long remain obscure, I can entertain no doubt, after the 
most deliberate study and dispassionate 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 erroDeous. 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 descend- 
ants of that species. Furthermore, I am convinced that 
natural selection has been the most important, but not tho 
exclusive, means of modification. 




Causes of Varialjility — Effects of Habit and tlie use or disuse of 
Parts — Correlated Variation — Inheritance — Character of Domes- 
tic Varieties — Dilficulty 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 favorable to Man's 
power of Selection. 


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 gen- 
erally 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 treatment, 
we are driven to conclude that this great variability is due 
to our domestic productions having been raised under con- 
ditions 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 gener- 
ations to new conditions to cause any great amount of 
variation; and that, when tlie organization has once begun 
to vary, it generally continues varying for many gener- 
ations. No case is on record of a variable organism ceas- 
ing to vary under cultivation. Our oldest cultivated 
plants, such as wheat, still yield new varieties: our oldest 

vauiation under domestication. 7 

domesticated animals are still capable of rapid improve- 
ment or modificationc 

As far as I am able to judge, after long attending to the 
subject, the conditions of life appear to act in two wavs — 
directly on the wliole organization or on certain parts 
alone^ and indirectly by aifecting 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 incidently shown in my work on 
'^Variation under Domestication,^^ there are two factors: 
namely; the nature of the organism and the nature of the 
conditionSc The former seeuis to be much the more 
important, for nearly similar variations sometimes arise 
under, as far as we can judge, dissimihir conditions; aud, 
on the other hand, dissimilar variations arise under condi- 
tions which appear to be nearly 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 
generations 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, 
color 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 
outgrowths which variably follow from the insei'tiou of a 
minute drop of poison by a gall-producing insect, shows us 
what singular modifications might result in the case ot 
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 prob- 
ably played a more important part in the formation of our 
domestic races. We see indefinite variability in the end- 
less 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 occasionally 
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 monstros- 
ities arise; but monstrosities cannot be separated by any 
distinct line from slighter variations. All such changes of 
itructure, whether extremely slight or strongly marked, 
which appear among many individuals living together, 
m-dy be considered as the indefinite effects of the condi- 
tions of life on each individual organism, in nearly the 
same manner as the chill effects different men in an indefi- 
nite manner, according to their state of body or constitu- 
tion, causing coughs or colds, rheumatism, 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 unnat- 
ural conditions. 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 confinement, 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 I This is generally, but erroneously 
attributed to vitiated instincts. Many cultivated plants 
display the utmost vigor, 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 thiit carnivorous 
animals, even from the tropics, breed in tliis country 
pretty freely under confinement, with the exception of the 
plantigrades or bear family, which seldom produce young; 
whereas carnivorous birds, with the rarest exception, 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 domesti- 
cated 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 a state 
of nature perfectly tamed, long-lived and healthy ( of 
which I could give numerous instances), yet having their 
reproductive system so seriously affected by unperceived 
causes as to fail to act, we need not be surprised at this 
system, when it does act under confinement, acting irregu- 
larly, 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 reproduct- 
ive 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 

Some naturalists have maintained that all variations are 
connected with the act of sexual reproduction; but this is 
certainly an error; for I have given in anotlier 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 propagated 
by grafts, off'sets, etc., and sometimes by seed. They 
©ocur rarely under nature, but are far from rare under 
culture. As a single bud out of many thousands produced 
year after year on the same tree under uniform conditions, 
nas been known suddenly to assume a new cha7*acter; and 
as buds on distinct trees, growing under different con- 
ditions, have sometimes yielded nearly the same variety — 
for instance, buds on peach-trees producing nectarines, 
and buds on common roses producing moss-roses — we 
cl«arly see that the nature of the conditions is of subordi- 


nate importance in comparison with the nature of the 
organism in determining each particular form of varia- 
tion; 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. 


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; 
thas 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 prob- 
ably 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 sug- 
gested that the drooping is due to disuse of the muscles of 
the ear, from t!.e animals being seldom much alarmed, 
seems probable. 

Many laws regulate variation, some few of which can bo 
dimly seen, and will hereafter be briefly discussed. I will 
here only allude to what may he called correlated variation. 
Important changes in the embryo or larva will probablv 
entail changes in the mature animal. In monstrosities, the 
correlations between quite distinct parts are very curious; 
and many instances are given in Isidore G-eoffroy 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 conflned to the males. Color and constitu- 
tional pecularities go together, of which many remarkable 


cases could be given among animals and plants. From 
facts collected by Heusinger, it appears that wliite sheep 
and pigs are injured by certain pUints, while dark-colored 
individuals escape: Professor Wyman has recently com- 
municated to me a good illustration of this fact; on asking 
some farmers in Virginia how it was that all their pigs were 
black, they informed him that the pigs ate the paint-root 
(Lachnanthes), which colored their bones pink, and which 
caused the hoofs of all but the black varieties to drop otf; 
and one of the '^ crackers" (/. 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; pigeon.s 
with feathered feet have skin between their outer toes; 
pigeons with short beaks have small feet, and those with 
long beaks large feet. Hence if man goes on selecting, 
and thus augmenting, any peculiarity, he will almost cer- 
tainly 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 structure and con- 
stitution in which the varieties and sub-varieties differ 
slightly from each other. The whole organization 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 consider- 
able physiological importance, are endless. Dr. Prosper 
Lucas^ treatise, in two large volumes, is the fullest 
and the best on this subject. No breeder doubts liow 
strong is the tendency to inheritance; that like pi'o 
duces like is his fundamental belief : doubts have 
been thrown on this principle only by theoretical 
writers. AYhen any deviation of structure often appears, 
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 among individuals, apparently exposed 
to the same conditions, any very rare deviation, due to 
some extraordinary combination of circumstances, appeals 
in the parent — say, once among several million individu- 
als — and it reappears in the child, the mere doctrine of 
ciiances almost compels us to attribute its reappearance to 
inheritance. Every one must have heard of crises of al- 
binism, prickly skin, hairy bodies, etc., appearing in 
several members of the same family. If strange and rare de- 
viations of structure are really inherited, less strange and 
commoner deviations may be freely admitted to be inherit- 
able. Perha^DS 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 

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 sOo why 
the child often reverts in certain characteristics to its grand- 
father or grandmother or more remote ancestor; why a 
peculiarity is often transmitted from one sex to both sexes, 
or to one sex alone, more commonly but not exclusively to 
the like sex. It is a fact of some importance to us, that 
peculiarities appearing in the males of our domestic breeds 
are often transmitted, either exclusively or in a much 
greater degree, to the males alone. A much more im- 
portant rule, which I think may be trusted, is that, at 
whatever period of life a peculiarity first appears, it tends 
to reappear in the offspring at a corresponding age, though 
sometimes earlier. In many cjises this could not be other- 
wise; thus the inherited peculiarities in the horns of cattle 
could appear only in the offspring when nearly mature; 
peculiarities in the silk-worm 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 ap- 
parent reason why a peculiarity should appear at any 
particular age, yet that it does tend to appear in the off- 
spring at the same period at Avhich 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 oifspring froa 
a short-horned cow by a long-horned bull, though appear, 
ing 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 character to their aboriginal stocks. 
Hence it has been argued that no deductions can be drawn 
from domestic races to species in a state of nature. I have 
in vain endeavored 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 per- 
fect reversion had ensued. It would be necessary, in order 
to prevent the effects of intercrossing, that only a single 
variety should have 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 improbable that if we could succeed in 
naturalizirg, or were to cultivate, during many gener- 
ations, the several races, for instance, of the cabbage, in 
very poor soil — in which case, however, some effect would 
have to be attributed to the 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, while 
kept under the same conditions and while kept in a con- 
siderable 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 favor of this view: to assert that wo 


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. 


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 domes- 
tic race, as already remarked, less uniformity of character 
than in true species. Domestic races often have a some- 
what monstrous character ; by which I mean, that, 
although dillering 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 of 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 differences 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 com- 
petent 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 per- 
petually recur. It has often been stated that domestic races 
do not differ fi'om each other in characters of generic value. 
It can be shown that this statetnent is not correct; but 
naturalists differ much in determing what characters are 
of generic value; all such valuations being at present 
empirical. When it is explained how genera originate 
under nature, it will be seen that we have no right to 
expect often to find a generic amount of difference in our 
domesticated races. 


In attempting to estimate the amount of structural dif- 
ference between allied domestic races, we are soon involved 
in doubt, from not knowing whether they are descended 
from one or several parent species. This point, if it could 
be cleared up, would be interesting; if, for instance, it 
could be shown that the greyhound, bloodhound, terrier, 
spaniel and bull-dog, which we all know propagate their kind 
truly, were the offspring of any single species, then such 
facts would have great weight in making us doubt about 
the immutability of the many closely allied natural species 
— for instance, of the many foxes — inhabiting the different 
quarters of the world. I do not believe, as we shall pres- 
ently see, that the whole amount of difference between the 
several breeds of the dog has been produced under domes- 
tication; I believe that a small part of the difference is due 
to their being descended from distinct species. In the case 
of strongly marked races of some other domesticated 
species, there is presumptive or even strong evidence that 
all are descended from a single wild stock. 

It has often been assumed that man has chosen for do- 
mestication 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, wdien 
he first tamed an animal, whether it would varv in sue- 
ceeding generations, and whether it would endure other 
climates? Has the little variability of the ass and goose, or 
the small power of endurance of warmth by the reindeer, 
or of cold by the common camel, prevented their domesti- 
cation? I cannot doubt that if other animals and plants, 
equal in number to our domesticated productions, and 
belonging to equally diverse classes and countries, were 
taken from a state of nature, and could be made to breed 
for an equal number of generations under domestication, 
they would on an average vary as largely as the jiarent 
species of our existing domesticated productions have 

In the case of most of our anciently domesticated animals 
and plants, it is not possible to come to any definite con- 
clusion, whether they are descended fi'om one or several 
-ivild s^'eoi^s. The argument mainly relied on by those wlio 


believe in the multiple origin of onr domestic animals is, 
that we find in the most ancient times, on the monuments 
of Egypt, and in tlie lake-habitations of Switzerland, 
much diversity in the breeds ; and that some of these 
ancient breeds closely resemble, or are even identical with, 
those still existing. But this only throws far backward 
the history of civilization, and shows that animals were 
domesticated at a much earlier period than has hitherto 
been supposed. The lake-inhabitants of Switzerland cul- 
tivated several kinds of wheat and barley, the pea, the 
poppy for oil and flax ; and they possessed several domesti- 
cated 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 
civilization ; and this again implies a long continued pre- 
vious period of less advanced civilization, 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 forma- 
tions of many parts of the world, all geologists believe that 
barbarian men exsisted 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 prob- 
ably forever 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 Canidse 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, constitution and structure of the humped 
Indian cattle, it is almost certain that thev 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 establislied 
by the admirable researches of Professor Kiitimeyer. 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 skele- 
tons, it appears to me almost certain that all are the 
descendents of the wild Indian fowl, Gallus bankiva; and 
this is the conclusion 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 otlier, tlie 
evidence is clear that they are all descended from the 
common duck and wild rabbit. 

The doctrine of the origin of our several domestic races 
from several aboriginal stocks, has been carried to an 
absurd extreme by some authors. They believe that every 
race which breeds true, let the distinctive characters be 
ever so slight, has had its wild prototype. At this rate 
there must have existed at least a score of species of Avild 
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 sev- 
eral peculiar breeds of cattle, sheep, etc., Ave must admit 
that many domestic breeds must have originated in Europe; 
for 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 S23aniel, etc. — so unlike 
all wild Canidae — ever existed in a state of nature? It 
has often been loosely said that all our races of dogs have 
been produced by the crossing of a few aboriginal species; 
but by crossing we can only get forms in some degree inter- 
mediate between their parents; and if we account for our 
several domestic races by this i)rocess, 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 niakiug distinct races by cross- 
ing has been greatly exaggerated. Many cases are on record 
showing that a race may be modified by occasional crosses 
if aided by the careful selection of the individuals which 
present the desired character; but to obtain a race inter- 
mediate 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 
every thing 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. 



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 favored with skins 
from several quarters of the world, more especially by the 
Hon. V/, Elliot, from India, and by the Hon. C. Murray, 
from Persia. Many treatises iii 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 per- 
mitted 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 corres- 
ponding differences in their skulls. The carrier, more 
especially the male bird, is also remarkable from the won- 
derful development of the carunculated skin about the 
head; and tins is accompanied by greatly elongated eyelids, 
very large external oriiices 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 tery 
long necks, others very long wings and tails, othcs sinmj^ 
larl}^ short tails. The barb is allied to the carreer, 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 
continually expanding, slightly, the upper part of the 
esophagus. The Jacobin has the feathers so much reversed 
along the back of the neck that they forma 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 
f of the bones of the face, in length and breadth and curva- 
ture, 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 vertebra 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 m.outh, the proportional length of the eye- 
lids, 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 esophagus; 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 scutellie 
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 disposi- 
tion, differ remarkabl}'. Lastly, in certain breeds, the 
males and females have come to differ in a slight degree 
from each other. ' 

Altogether at least a score of pigeons might be chosen 
which, "if shown to an ornithologist, and he were told that 
they were wild birds, would certainly 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 (Columbia 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 de- 
scended from at least seven or eight aboriginal stocks; for 
it is impossible to make the present domestic breeds by the 
crossing of any lesser number ; how, for instance, could 
a pouter be produced by crossing two breeds unless 
one of the parent-stocks possessed the characteristic 
enormous crop? The supposed aboriginal stocks must 
all have been rock-pigeons, that is, 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 sup- 
posed aboriginal stocks must either still exist in the coun- 
tries 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 flyers, 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 sup- 
posed extermination of so many species having similar 
habits with the rock-pigeon seems a very rash assumption. 
Moreover, the seveial above-named domesticated breeds 
have been transported to all parts of the world, and, there- 
fore, 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 diffi- 
cult to get wild animals to breed freely under domestica- 
tion; yet on the hypothesis of the multiple origin of our 
pigeons, it must be assumed that at least seven or eight 
species were so thoroughly domesticated in ancient times 
by half-civilized man as to be quite prolific under confine- 

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 constitu- 
tion, habits, voice, coloring, and in most parts of their 
structure, yet are certainly highly abnormal in other parts; 
we may look in vain through the whole great family of Col- 
umbidae for a beak like that of the English carrier, or that 
of the short-faced tumbler, or barb; for reversed feathers 
like those of the Jacobin; for a crop like that of the pouter; 
for tail-feathers like those of the fantail. Hence it must 
be assumed, not only that half-civilized man succeeded in 
thoroughly domesticating several species, but that he in- 
tentionally or by chance picked out extraordinarily abnor- 
mal species; and further, that these very species have since 
all become extinct or unknown. So many strange contin- 
gencies are improbable in the highest degree. 

Some facts in regard to the coloring of pigeons well de- 
serve consideration. The rook-pigeon is of a slaty-blue, 
with white loins; but the Indian sub-species, C. interme- 
dia of Strickland, has this part bluish. The tail lias a ter- 
minal dark bar, with the outer feathers externally 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 checkered with 
black. These several marks do not occur together in any 
other species of the whole family. Xow, 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. More- 
over, 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 tlie mongrel 
barb-fantails with a mongrel barb-spot, and they produced 
a bird of as beautiful a blue color, 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 de- 
scended 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 colored and marked like the rock- 
pigeon, although no other existing species is thus colored 
and marked, so that in each separate breed there might be 
a tendency to revert to the very same colors 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 descend- 
ants reverting to an ancestor of foreign blood, removed by 
a greater number of generations. In a : reed 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 hreed to revert to a character 
which was lost during some former generation, this 
tendency, for all that we can see to the contrary, mav be 
transmitted undiminished for an indefinite number of gen- 
erations. These two distinct cases of reversion are often 
confounded together by those who have written on in- 

Lastly, the hybrids or mongrels from between all the 
breeds of the pigeon are perfectly fertile, as I can state 
from my own observations, purposely made, on tlie most 
distinct breeds. Now, hardly any cases have been ascer- 
tained 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 con- 
clusion 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 off- 
spring 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 Columbidae, though so like the rock-pigeon 
in most res|)ects — the occasional reappearance of the blue 
color 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 
Columbfe livia with its geographical sub-species. 

In favor of this view, I may add, firsth\ 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 tliese 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 w^ere 
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 breed?, which method was 
never practiced before, has improved them astonishingly."' 
About this same period the Dutch were as eager about 
pigeons as were the old Eomans. The paramount impor- 
tance of these considerations in explaining the immense 
amount of variation which pigeons have undergone, will 
likewise be obvious when we treat of selection. We shall 
then, also, sec how it is that the several breeds so often 
have a somewhaj . lonstrous character. It is also a most 
favorable 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 wa+ched 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 dis- 
tinct species. Ask, as I have asked, a celebrated raiser ol 
Hereford cattle, whether his cattle might not have de- 
scended 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 Kibston-pippin or Codlin-apple, could 
ever have proceeded from the seeds of the same tree. In- 
numerable other examples could be given. The explana- 
tion, 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 know- 
ing no more than he does of the intermediate 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 



Let US now briefly consider the steps by which domestic 
races have been produced, either from one or from seveial 


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 ac- 
count by such agencies for the differences between a di-ay 
and race-horse, a greyhound and bloodhound, a carrier and 
tumbler pigeon. One of the most remarkable features in 
our domesticated races is that we see in them adaptation, 
not indeed to the animal's or plant's own good, but to 
man's use or fancy. Some variations useful to him have 
probably arisen suddenly, or by one step; many botauists, 
for instance, believe that the fuller's teasel, with its hooks, 
which can not be rivaled by any mechanical contrivance, 
is only a variety of the wild Dipsacus; and this amount of 
change may have suddenly arisen in a seedling. So it has 
probably been with the turnspit dog; and this is known to 
have been the case with the ancon sheep. But when we 
compare the dray-horse and race-horse, the dromedary and 
camel, the various breeds of sheep fitted either for culti- 
vated 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, culi- 
nary, orchard and flower-garden races of plants, most useful 
to man at different seasons and for different purposes, or so 
beautiful in his eves, we must, I think, look further than 
to mere variability. We can not suppose that all the 
breeds were suddenly j^roduced 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 successive variations; 
man adds them up in certain directions useful to him. In 
this sense he may be said to have made for himself useful 

The great power of this principle of selection is not 
hypothetical. It is certain that several of our eminent 
breed ei's have, even within a single lifetime, modified to a 
large extent their breeds of cattle and sheep. In order 
fully to realize what they iiave done it is almost necessary 


to read several of the many treatises devoted to this sub- 
ject, and to inspect the animals. Breeders habitually 
speak of an animal's organization as something plastic, 
which they can model almost as they please. If I had 
space I could quote numerous passages to this effect from 
highly competent authorities. ^Youatt, who was probably 
better acquainted with the works of 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 mold he K 
pleases,"" Lord Somerville, speakins^ of what breeders ^ 
nave 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 
recognized that men follow it as a trade: the sheep are 
placed on a table and are studied, like a picture by a con- 
noisseur; this is done three times at intervals of months, 
and the sheep are each time marked and classed, so that 
the very best may ultimately be selected for breeding. 
^What English breeders have actually effected is proved 
by the enormous prices given for animals with a good ped- 
igree; and these have been exported to almost every 
quarter of the world. The improvement is by no means 
generally due to crossing different breeds; all the best 
breeders are strongly opposed to this practice, except some- 
times among closely allied sub-breeds. And when a cross 
has been made, the closest selection is far more indispen- 
sable even than in ordinary cases. If selection consisted 
merely in separating some very distinct variety and breed- 
ing from it, the principle would be so obvious as hardly to 
be worth notice; but its importance consists in the great 
effect produced by the accumulation in one direction, 
during successive generations, of differences absolutely 
inappreciable by an uneducated eye — differences which I 
for one have vainly attempted to appreciate. Not one man 
in a thousand has accuracy of eye and judgment sufficient 
to become an eminent breeder. If gifted with these qual- 
ities, and he studies his subject for years, and devotes his 


lifetime to it witli indomitable perseverance, he will suc- 
ceed, and may make great improvements; if he wants any 
of these qualities, he will assuredly fail. Few would read- 
ily believe in the natural capacity and years of practice 
requisite to become even a skillful pigeon-fancier,>^ 

The same principles are followed by horticulturi:;ts; but 
the variations are here often more abrupt. No one sup- 
poses 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 goose- 
berry may be quoted. We see an astonishing improvement 
in many florists' flowers, when the flowers of the present 
day are compared with drawings made only twenty or 
thirty years ago. -^ W^hen 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, 
m 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 diversity of flowers in the different varieties of the same 
species in the flower garden; the diversity of leaves, pods, 
or tubers, or whatever part is valued, in the kitchen-garden, 
in comparison with the flowers of the same varieties; and 
the diversity of fruit of the same species in the orchard, in 
comparison with the leaves and flowers of the same set of 
varieties. See how different the leaves of the cabbage are, 
and how extremely alike the flowers; how unlike the flow- 
ers of the heartsease are, and how alike the leaves; how 
much the fruit of the different kinds of gooseberries differ 
in size, color, shape and hairyness, and yet the flowers pre- 
sent 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 corre- 
lated variation, the importance cf which should never be 
overlooked, will insure: some dif erences; but, as a general 
rule, it cannot be doubted that the continued selection of 

VJsrcoj^ serous selection. 29 

slight variations, either in the leaves, the flowers, or tlie 
fruit, will produce races ditrering from eacli 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 at- 
tended to of late years, and many treatises have been pub- 
lished on the subject; and the result has been, in a corre- 
sponding degree, rapid and important. But it is very far 
from true that the principle is a modern discovery. I could 
give several references to works of high antiquity, in wliich 
the full importance of the principle is acknowledged. In 
rude and barbarous periods of English history clioice ani- 
mals 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 ency- 
clopedia. Explicit rules are laid down by some of the 
Roman classical writers. From passages in Genesis, it is 
clear that the color of domestic animals was at that early 
period attended to. Savages now sometimes cross their 
dogs with wild canine animals, to improve the breed, and 
they formerly did so, as is attested by passages in Pliny. 
The savages in South Africa match their draft cattle by 
color, as do some of the Esquimaux their team 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 vra-s carefully attended to in ancient 
times, and is now attended to by the lowes-t savages. It 
would, indeed, have been a strange fact, had attention not 
been paid to breeding, for the inheritance of good and bud 
qualities is so obvious. 


At the present time, eminent breeders try by methodical 
selection, with a distinct object in view, to make ;i new 
strain or sub-breed, sujierior 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 tiying to possess and breed from the best individual 
animals, is more important. Tims, a man who intends 
keeping pointers naturally tries to get as good dogs as he can, 
and afterward breeds from his own best dogs, but he has 
no wish or expectation of permanently altering the breed. 
Nevertheless we may infer that this process, continued 
during centuries, would improve and modify any breed, in 
tlie same way as Bakewell, 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 recognized unless actual measurements or care- 
ful drawings of the breeds in question have been made long 
ago, which may serve for comparison. In some cases, how- 
ever, unchanged, or but little changed, individuals of the 
same breed exist in less civilized districts, where the breed 
has been less improved. There is reason to believe that 
King Charles^ spaniel has been unconsciously modified to a 
large extent since the time of that monarch. Some highly 
competent authorities are convinced that the setter is 
directly derived from the spaniel, and has probably been 
slowly altered from it. It is known that the English 
pointer has been greatly changed within the last century, 
and in this case the change has, it is believed, been chiefly 
effected by crosses with the foxhound; but what concerns 
us is, that the change has been effected unconsciously and 
gradually, and yet so effectually that, though the old 
Spanish j)ointer certainly came from Spain, Sir. Borrow 
has not seen, as I am informed by him, any native dog in 
Spain like our pointer. 

By a similar process of selection, and by careful training, 
English race-horses have come to surpass in fleetness and size 
the parent Arabs, so that the latter, by the regulations for 
the Goodwood Races, are favored in the weights which 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. 


Yonatt gives an excellent illustration of the effects of a 
course of selection v/hich maybe 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 the 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 upward 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 pur230se, would be carefully preserved 
during famines and other accidents, to which savages are 
so liable, and such choice animals would thus generally 
leave more oft'spring 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 bv the bar- 
barians of Tierra del Fuego, by their killing and devouring 
their old women, in times of dearth, as of less value than 
their dogs. 

In plants the same gradual process of improvement 
through the occasional preservation of the best individuals, 
whether or not sufficiently distinct to be ranked at their 
first appearance as distinct varieties, and whether or not 
two or more species or races have become blended togetlier 
by crossing, may plainly be recognized in the increased size 
and beauty which we now see in the varieties of the hearts- 
ease, 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 qudity. I have seen great surprise expressed 
in horticultural works at the wonderful skill of gardeners 
in having produced sucli splendid results from such poor 
materials; but the art has been simple, and, as far as the 
final result is concerned, has been followed almost uncon- 
sciously. 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 onward. 
But the gardeners of the classical period, who cultivated 
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 any- 
where find. 

/a large amount of change, thus slowly and uncon- 
sciously accumulated, explains, as 1 believe, the well- 
known fact, that in a number of cases we cannot 
recognize, and therefore do not know, the wild parent- 
stocks of the plants which have been longest cultivated 
in our flower and kitchen gardens. It 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 uncivilized 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 per- 
fection comparable with that acquired by the plants in 
countries anciently civilized. 

In regard to tlie domestic animals kept by uncivilized 
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 circum- 
stanced, individuals of the same species, having slightly 
different constitutions or structure, would often suc- 
ceed 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 
tnie species than the varieties kept in civilized countries..^-- 
/On the view here given of the important part whicli 
selection by man has played, it becomes at once obvious, 
how it is that our domestic races show adaptation in their 
structure or in their habits to man's wants or fancies. We 
can, I think, further understand the frequently abnormal 
character of our domestic races, and likewise their differ- 
ences 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 ex- 
pression as trying to make a fantail 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, selec- 
tion. Perhaps the parent-bird of all fantails had only 
fourteen tail-feathers somewhat expanded, like the present 
Java fantail, or like individuals of other and distinct 
breeds, in which as many as seventeen tail-feathers have 
been counted. Perhaps the first pouter-pigeon did not in- 
flate its crop much more than the turbit now does 
the upper part of its esophagus — a habit which is disre- 
garded by all fanciers, as it is not one of the points of the^ 

/Nor let it be thought that some great deviation of struc- 
ture would be necessary to catch the fancier's eye: he per- 
ceives extremely small differences, and it is in human 
nature to value any novelty, however slight, in 
one's own possession. Nor must the value which would 
formerly 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 re- 
jected as faults or deviations from the standard of per- 
fection in each breed. The common goose has not given 
rise to any marked varieties; hence the Toulouse and the 
common breed, which dilfer only in color, that most fleet- 
ing 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 
fa^'t, a breed, like a dialect of a language, can hardly be 
sai 1 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 neighborhood. But they will as 
yet hardly have a distinct name, and from being only 
slightly valued, their history will have been disregarded. 
When farther improved by the same slow and gradual 
process, they will spread more widely, and will be recog- 
nized as something distinct and valuable, and will then 
probably first receive a provincial name. In semi-civilized 
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 civilization of the in- 
habitants — slowly to add to the characteristic features of 
the breed, whatever thev mav be. But the chance v.-iil be 
infinitely small of any record having been preserved of such 
slow, varying and insensible changes. .^-^ 


I will now say a few words on the circumstances, favor- 
able or the reverse, to man's power of selection. A high 
degree of variability is obviously favorable, as freely giving 


the materials for selection to work on; not that mere in- 
dividual differences are not amply sufficient, with extreme 
care, to allow of the accumulation of a large amount of 
modification in almost any desired direction. But as 
variations manifestly useful or pleasing to man appear only 
occasionally, the chance of their appearance will be much 
increased by a large number of individuals being kept. 
Hence, number is of the highest importance for success. 
On this principle Marshall formerly remarked, with respect 
to the sheep of part of Yorkshire, ^'As they generally 
belong to poor people, and are mostly iyi 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 v,diere the conditions for its 
propagation are favorable. 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. Ko 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-century. 

With animals, facility in preventing crosses is an im- 
portant element in the formation of new races — at least, 
in a country which is already stocked with other races. In 
this respect inclosure of the land plays a part. Wander- 
ing 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 favored 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 noctural rambling habits, can not 
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 
alv,ays 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, etc., 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 atten- 
tion paid to their breeding; for recently in certain parts of 
Spain and of the United States this animal has been sur- 
prisingly 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 
organization, though it has varied to a slight extent, as I 
have elsewhere described. 

Some authors have maintained that the amount of varia- 
tion in our domestic productions is soon reached, and can 
never afterward 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 im- 
proved in many ways within a recent period; and this im- 
plies variation. It would be equally rash to assert that 
characters now increased to their utmost limit, could not, 
after remaining fixed for many centuries, again vary under 
new conditions of life. ]No doubt, as Mr. Wallace has re- 
marked with much truth, a limit will be at last reached. 
For instance, tiiere must be a limit to the fleetness of any 
terrestrial animal, as this will be determined by the fric- 
tion to be overcome, the weight of the body to be carried, 
and the power of contraction in the muscular fibers. But 


what concerns us is that the domestic varieties of the same 
species diifer from each other in almost every character, 
which man lias attended to and selected, more than do tlie 
distinct species of the same genera. Isidore Geolfroy St. 
Hilaire has proved this in regard to size, and so it is with 
color, and probahly with the length of hair. With respect 
to fleetness, which depends on many bodily character's, 
Eclipse was far fleeter, and a dray-horse is comparably 
stronger, than any two natural species belonging to the same 
genus. So with plants, the seeds of the different varieties 
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 ani- 
mals and plants. Changed conditions of life are of tho 
highest importance in causing variability, both by acting 
directly on the organization, and indirectly by affecting the 
reproductive system. It is not probable that variability is 
an inherent and necessary contingent, under all circum- 
stances. The greater or less force of inheritance and 
reversion determine whether variations shall endure. 
Variability is governed by many unknown laws, of which 
correlated growth is probably the most important. Some- 
thing, but how much we do not know, may be attributed 
to the definite action of the conditions of life. Some, per- 
haps a great, effect may be attributed 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 occa- 
sional 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 wliich are 
propagated by seed. With plants which are temporarily 
propagated by cuttings, buds, etc., the importance of cross- 
ing is immense; for the cultivator may here disregard the 
extreme variability both of hybrids and of mongrels, and 


the sterility of hybirds; but plants not propagated by seed 
are of little importance to ns, for their endurance is only 
temporary. Over all these causes of change, the accumu- 
lative 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 rang- 
ing, 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 th© 
larger genera resemble varieties in being very closely, but un- 
equally, related to each other, and in having restricted ranges. 

Before applying the principles arrived at in the last 
chapter to organic beings in a state of nature, we must 
briefly discuss whether these latter are subject to any 
variation. To treat this subject 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 defini- 
tions 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. 
Generallv the term includes the unknown element of r 
distinct act of creation. The term *' variety^"* is almost 
equally difficult to define; but here community of descent 
is almost universally implied, though it can rarely be 
proved. We have also what are called monstrosities; but 
they graduate into varieties. By a monstrosity I presume 
is meant some considerable deviation of structure, gener- 
ally injurious, or not useful to the species. Some authors 
use the term "variation'^ in a technical sense, as implying 
a modification directly due to the physical conditions of 
life; and '* variations '^ in this sense are supposed not to be 
inherited; but who can say that the dwarfed condition of 
shells in the brackish waters of the Baltic, or dwarfed 
plants on Alpine summits, or the thicker fur of an animal 
from far northward, would not in some cases bo inherited 
for at least a few generations? 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 productions, 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 monstrosi- 
ties sometimes occur which resemble normal structures in 
widely diiferent 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 favorable circum- 
stances. 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 preserva- 
tion and perpetuation of single or occasional variations. 


The many slight differences which appear in the offspring 
from the same jiarents, or which it may be presumed have 
thus arisen, from being observed in the individuals of the 
same species inhabiting 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 mold. 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 mate- 
rials 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 imimportant 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 natu- 
ralist would be surprised at the number of the cases of 
variability, even in important parts of structure, which he 
could collect on good authority, as I have collected, during 
a course of years. It should be remembered that systema- 
tists 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 been 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 
the irregular branching of the stem of a tree. This philo- 
sophical naturalist, I may add, has also shown that the 
muscles in tiie larvae of certain insects are far from uni- 
form. 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 varying; 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 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 among 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 Briichiopod 


shells, at former periods of time. These facts are very 
perplexing, for they seem to show that this kind of varia- 
bility is independent of the conditions of life. I am in- 
clined to suspect that we see, at least in some of these 
polymoq:)hic genera, variations which are of no service or 
disservice to the species, and which consequently have not 
been seized on and rendered definite by natural selection, 
as hereafter to be explained. 

Individuals of the same species often present, as is 
known to every one, great ditferences of structure, in- 
dependently of variation, as in the two sexes of various 
animals, in the two or three castes of sterile females or 
workers among 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, reg- 
ularly appear under two or even three conspicuously dis- 
tinct forms, not connected bv intermediate varieties. Fritz 
Muller 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 smelling-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 in- 
termediate links, and the extreme links of the chain closely 
resemble the two forms of an allied dimorphic species in- 
habiting another part of the Malay Archipelago. Thus 
also with ants, the several worker-castes are generally quite 
distinct; but in some cases, as we shpJl 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 plaait sliouM pro- 


dnce from the same seed-capsule three distinct herma- 
phrodite forms, bearing three different kinds of females and 
three or even six different kinds of males. Nevertheless 
these cases are only exao^gerations of the common fact that 
the female produces offspring of two sexes which some- 
times differ from each other in a wonderful manner. 


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 inter- 
mediate links; nor will the commonly assumed hybrid 
nature of the intermediate forms alwavs remove the difti- 
culty. In very man}^ cases, however, one form is ranked 
as a vaiiety of another, not because the intermediate links 
have actually been found, but because analogy leads the 
observer to sujopose 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 judgm.ent and wide experience seems the only guide 
to follow. We must, however, in many cases, decide by a 
majority of naturalists, for few well-mai-ked and well- 
known varieties can be named which have not been ranked 
as species by at least some competent judges. 

That varieties of this doubtful nature are far from 


uncommon cannot be disputed. Compare the several floras 
of Great Britain, of France, or of the United States, drawn 
up by different botanists, and see what a surprising num- 
ber of forms have been ranked by one botanist as good 
Bpecies, and by another as mere varieties. Mr. H. C. 
Watson, to whom I lie under deep obligation for assistance 
of all kinds, has marked for me 182 British plants, which 
are generally considered as varieties, but which have all 
been ranked by botanists as species; and in making this 
list he has omitted many trifling varieties, but which never- 
theless have been ranked by some botanists as species, and 
he has entirely omitted several highly polymorphic genera. 
Under genera, including the most poiymorijhic forms, Mr. 
Babington gives 251 species, whereas Mr. Bentham gives 
only 112 — a difference of 139 doubtful forms! Among 
animals w^hich 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 un- 
doubted species, and by another as varieties, or, as they 
are often called, geogi-aphical races ! Mr. Wallace, in 
several valuable papers on the various animals, especially 
on the Lepidoptera, inhabiting the islands of the great 
Malayan Archipelago, 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 representa- 
tive species. The first or variable forms vary much within 
the limits of the same island. The local forms are moder- 
ately 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 bo 

Many years ago, when comparing, and seeing others com- 
pare, the birds from the closely neighboring islands of the 
Galapagos Archipelago, one with another, and with those 
from the American mainland, I was much struck how en- 
tirely vague and arbitrary is the distinction between species 
and varieties. On the islets of the little Maderia group 
there are many insects which are characterized as 
varieties in Mr. Wollaston's admirable work, but which 
would certainly be ranked as distinct species by many en- 
tomologists. Even Ireland has a few animals, now gen- 
erally regarded as varieties, but which have been ranked as 
species by some zoologists. Several experienced ornitholo- 
gists 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 pecuhar to Great 
Britain. A wide distance between the homes of two doubt- 
ful forms leads many naturalists to rank them as distinct 
species; but what distance, it has been well asked, will suf- 
fice if that between America and Europe is ample, will that 
between Europe and the Azores, or Maderia, or the Cana- 
ries, or between the several islets of these small archipelagos, 
be sufficient? 

Mr. B. D. "Walsh, a distinguished entomologist of tlie 
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 indiscriminately 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 color, 
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 ranl<:ed 
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 varie- 
ties; 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 can not 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 inter- 
mediate 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 pre- 
sent varieties; but then these same naturalists rank the 
slightest difference as of specific value; and when the same 
identical form is met with in two distant countries, or in 
two geological formations, they believe that two distinct 
species are hidden under the same dress. The term 
species thus comes to be a mere useless abstraction, imply- 
ing 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 compe- 
tent 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 doubt- 
ful species well deserve consideration; for several interest- 
ing lines of argument, from geographical distribution, 
analogical variation, hybridism, etc., have been brought to 
bear in the attempt to determine their rank; but space 


does not here permit me to discuss them. Close investiga- 
tion, 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 ^vill 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 vari- 
eties; 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 pub- 
lished by A. de Candolle, on the oaks of the whole world. 
No one ever had more ample materials for the discrimina- 
tion 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 vari- 
ations. 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 assignable 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, aud 
never found connected by intermediate states. After 
this discussion, the result of so much labor, 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. Thus 
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 con- 
necting forms which are now rare were to become totally 
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 Quer- 
cus 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 provis- 
ional 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 classific- 
ation. '^ 

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 variation to which the group is 
subject; and this shows, at least, how very generally there 
is some variation. But if he confine his attention to one 
class within one country he will soon make up his mind 
how to rank most of the doubtful forms. His general ten- 
dency will be to make many species, for he will become 
impressed, just like the pigeon or poultry fancier before 
alluded to, with the amount of difference in the forms 
which he is continually studying; and he has little general 
knowledge of analogical variation in other groups and in 
other countries by which to correct his first impressions. 
As he extends the range of his observations he will meet 
with more cases of difficulty; for he will encounter a 
greater number of closely allied forms. But if his obser- 
vations 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 natural- 
ists. 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 compelled to 
trust almost entirely to analogy, and his difficulties Avill 
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 insensible series; 
and a series impresses the mind with the idea of an actual 

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 toward 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 toward more 
strongly marked and permanent varieties; and at the lat- 
ter, as leading to sub-species, and then to sjoecies. 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 exposed; but with respect to the more im- 
portant 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 ex- 
plained, 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 considera- 
tions 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 ex- 
tinct, or they may endure as varieties for very long periods, 
as ha^ been shown to be the case by Ish. 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 

From these remarks it will be seen that I look at the 
term species as one arbitrarily given, for the sake of con- 
venience, 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 ajoplied arbitrarily, for con- 
venience sake. 



Guided by theoretical considerations, I thought that 
some interesting results might be obtained in regard to the 
nature and relations of the species which vary most, by 
tabulating all the varieties in several well- worked floras. 
At first this seemed a simple task; but Mr. H. 0. 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 
proportional numbers of the varying species. Dr. Hooker 
permits me to add that after having carefully read my 
manuscript, and examined the tables, he thinks that the 
following statements are fairly well established. The whole 
subject, however, treated as it necessarily here is with much 
brevity, is rather perplexing, and allusions cannot be 
avoided to the *^ struggle for existence," '^divergence of 
character," and other questions, hereafter to be discussed. 

Alphonso 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 widely diffused within their own 
country (and this is a dili'erent consideration from wide 
range, and to a certain extent from commonness), oftenest 
give rise to varieties sufficiently well-marked to have been 
recorded in botanical works. Hence it is the most flourish- 
ing, 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 
oftenest produce well-marked varieties, or, as I consider 
them, incipient species. And this, perhaps, might have 
been anticipated; for, as varieties, in order to become in 
any degree permanent, necessarily have to struggle with the 
other inliabitants of the country, the species which are 
already dominant will be the most likely to yield offspring, 
which, though in some slight degree modified, still inherit 
those advantages that enabled their parents to become 
dominant over their compatriots. In these remarks on 
predominence, 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 live under nearly the same 
conditions. A plant of this kind is not the less dominant 
because some conferva inhabiting the water or some para- 
sitic 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. 


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 iucludiug 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 some- 
what 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 in- 
habiting any country, shows that there is something in 
the organic or inorganic conditions of that country favor- 
able to the genus; and, consequently, we might have ex- 
pected to have found in the larger genera, or those includ- 
ing 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 dif- 
fused, 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 organization 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 organized plants ranging 
widely will be discussed in our chapter on Geographical 

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 incip- 
ient 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 favorable for variation; 
and hence we might expect that the circumstances would 
generally still be favorable to variation. On tlie 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 

To test the truth of this anticipation I have arranged 


che 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, and 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. 


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 com^oelled to come to a determination 
by the amount of difference between them, judging by anal- 
ogy 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 whetlier 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 
endeavored 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 manu- 
facturing, 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. 
Undoubtedly there is one most important point of differ- 
ence between varieties and sj)ecies, namely, that the 
amount of difference between varieties, when compared 
with each other or with their parent species, is much less 
than that between the species of the same genus. But when 
we come to discuss the principle, as I call it, of divergence 
of character, we shall see how this may be explained, and 


how the lesser differences between varieties tend to increase 
into the greater differences between species. 

There is one other point which is worth notice. Varie- 
ties generally have much restricted ranges. This state- 
ment is indeed scarceh^ 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. 0. Watson has 
marked for me in the well-sifted London catalogue of 
Plants (4th edition) sixty-three 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 
sixty-three 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, fifty- 
three acknowledged varieties are recorded, and these 
range over 7.7 provinces; whereas, the species to which 
these varieties belong range over 14.3 provinces. So 
that the acknowledged varieties have 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 species, 
— except, first, by the discovery of intermediate Imking 
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 countrv, 
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 
plusters round other species. Species very closely allied to 


other species apparently have restricted rauges. 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 inex- 
plicable if species are independent creations. 

We have also seen that it is the most flourishing or domi- 
nant 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 con- 
verted 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, tho 
larger genera also tend to break up into smaller genera. 
And thus, the forms of life tb^'oughout the universe be- 
come divided into groups subordinate to groups. 




Its bearing on natural selection — The term used in a wide sense — 
Geometrical ratio of increase — Eapid increase of naturalized 
animals and plants — Nature of the checks to increase — Com- 
petition 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 among organic beings in a state of 
nature there is some individual variability: indeed I am 
not aware that this has ever been disputed. It is imma- 
terial 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 tlie mere exist- 
ence 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 organization to another part, and to tlie condi- 
tions of life and of one organic being to another being, 
been perfected? 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 iu 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 sj^ecies? How do those groups of 
species, which constitute what are called distinct genera 
and which differ from each other more than do the species 
of the same genus, arise? All these results, as we shall 
more fully see in the next chapter, follow from the struggle 
for life. Owing to this struggle, variations, hoAvever 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 survive. 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 Sur- 
vival 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 accumulation of slight 
but useful variations, given to him by the hand of Nature. 
But Natural Selection, 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. 

AVe 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 philosophicajly 
shown that all organic beings are exposed to severe compe- 
tition. In regard to plants, no one has treated this sub- 
ject with more spirit and ability than W. Herbert, Dean 
of Manchester, evidently the result of his great horticul- 


tural knowledge. Nothing is easier than to admit iu 
words the truth of the universal struggle for life, or more 
difficult — at least I found it so — than constantly to bear 
this conclusion in mind. Yet unless it be thoroughly en- 
grained 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 gladness, we 
often see superabundance of food; we do not see or we 
forget that the birds which are idly singing round ns 
mostly live on insects or seeds, and are thus constantly 
destroying life; or we forget how largely these songsters, 
or their eggs, or their nestlings, are destroyed by birds and 
beasts of prey; we do not always bear in mind, that, 
though food may be now superabundant, it is not so at all 
seasons of each recurring year. 



I should premise that I use this term in a large and 
metaphorical sense, including dependence of one being on 
another, and including (which is more important) not 
only the life of the individual, but success in leaving prog- 
eny. 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 jorop- 
erly 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 mistle- 
toes, growing close together on the same branch, may more 
truly be said to struggle with each other. As the mistle- 
toe is disseminated by birds, its existence depends on them; 
and it may metaphorically be said to struggle with other 
fruit-bearing plants, in tempting the birds to devour and 


thus disseminate its seeds. In these several senses, which 
pass into each other, I use for convenience sake the gen- 
eral term of Struggle for Existence. 


A struggle for existence inevitably follows from the high 
rate at which all organic beings tend to increase. Every 
being, which during its natural lifetime produces several 
eggs or seeds, must suffer destruction during some period 
of its life, and during some season or occasional year, 
otherwise, on the principle of geometrical increase, its 
numbers would quickly become so inordinately great that 
no country could support the product. Hence, as more 
individuals are produced than can possibly survive, there 
must in every case be a struggle for existence, either one in- 
dividual with another of the same species, or with the 
individuals of disMnct species, or with the physical con- 
ditions of life. It is the doctrine of Malthus applied with 
manifold force to the whole animal and vegetable king- 
doms; 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 can not 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. Linngeus has calculated that if an annual plant 
produced only two seeds — and there is no plant so unpro- 
ductive 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. 

Bat we have better evidence on this subject than mere 
theoretical calculations, namely, the numerous recorded 
cases of the astonishingly rapid increase of various animals 
in a state of nature, when circumstances have been favor- 
able 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 introduced 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 ex- 
clusion 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 inported 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 temporarily increased in any 
sensible degree. The obvious explanation is that the con- 
ditions of life have been highly favorable, 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 and rapid increase and wide diffusion in their 
new homes. 

In a state of nature almost every full-grown plant an- 
nually produces seed, and among animals there are very 
few which do not annually pair. Hence we may confi- 
dently assert that all plants and animals are tending to in- 
crease 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 cheolcod 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 re- 
quire a few more years to people, under favorable condi- 
tions, 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 Qgg, yet it is be- 
lieved to be the most numerous bird in the world. One 
fly deposits hundreds of eggs, and another, like the hippo- 
bosca, 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 im- 
portance 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 produced 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 insured to germinate in a fitting 
place; so that, in all cases, the average number of any an- 
imal 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 gen- 
eration or at recurrent intervals. Lighten any check, mit- 
igate the destruction ever so little, and the number of the 
species will almost instantaneously increase to any amount 



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 mankind, 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 seed- 
lings 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 de- 
stroyed, 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 grown on a 
little plot of mown turf (three feet by four) nine species 
perished, from the other species being allowed to grow up 

The amount of food for each species, of course, gives 
the extreme limit to which each can increase; but very fre- 
quently it is not the obtaining food, but the serving as prey 
to other animals, which determines the average number of 
a species. Thus, there seems to be little doubt that the 
stock of partridges, grouse and hares on any large estate 
depends chiefly on the destruction of vermin. If not one 
head of game were shot during the next twenty years in 


England, and, at the same time, if no vermin were de- 
stroyed, 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 ex- 
treme cold or drought seem to be the most effective of all 
checks. I estimated (chiefly from the 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 extraordinarially 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 indi- 
viduals, 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 the 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 
cliange of climate being conspicious, 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 enormous destruc- 
tion at some period of its life, from enemies or from com- 
petitors for the same place and food; and if these enemies 
or competitors be in the least degree favored 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 
favored, 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 northward, or in ascending a mount- 
ain, we far oftener meet with stunted forms, due to the 
directly injurious action of climate, than we do in proceed- 
ing southward or in descending a mountain. When we 
reach the Arctic regions, or snow-capped summits, or abso- 
lute deserts, the struggle for life is almost exclusively with 
the elements. 

That climate acts in main part indirectly by favoring 
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 naturalized, for they 
cannot compete with our native plants nor resist destruc- 
tion by our native animals. 

When a species, owing to highly favorable circumstances, 
increases inordinately in numbers in a small tract, epidem- 
ics — 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 among the crowded animals, 
but disproportionally favored: 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 indi- 
viduals 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, etc., 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 the 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 preser- 
vation, 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 conld exist only 


where the conditions of its life were so favorable that maay 
could exist together, and thus save the species from utter 
destruction. I should add that the good effects of inter- 
crossing, 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 subject. 


Many cases are on record showing how complex and unex- 
pected 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 Staffordshire, on the estate of a 
relation, where I had ample means of investigation, there 
was a large and extremely barren heath, which had never 
been touched by the hand of man; but several hundred 
acres of exactly the same nature had been inclosed twenty- 
five years previously and planted with Scotch fir. The change 
in the native vegetation of the planted part of the heath 
was most remarkable, more than is generally seen in pass- 
ing from one quite different soil to another: not only the 
proportional numbers of the heath-plants were wholly 
changed, but twelve species of plants (not counting grasses 
and carices) flourished in the plantations, which could not 
be found on the heath. The effect on the insects must 
have been still greater, for six insectivorous birds were very 
common in the plantations, which were not to be seen on 
the heath; and the heath was frequented by two or three 
distinct insectivorous birds. Here we see how potent has 
been the effect of the introduction of a single tree, nothing 
whatever else having been done, with the exception of the 
land having been inclosed, so that cattle could not enter. 
But how important an element inclosure 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 
inclosed, and self-sown firs are now springing up in multi- 
tudes, so close together that all cannot live. When I 
ascertained that these young trees had not been sown or 
pljiijted I was m much surprised R-t their numbers that I 


went to several points of view, whence I could exuniine 
hundreds of acres of the uninclosed heath, and literally J 
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 sqiiare 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 inclosed, it became thickly clothed 
with vigorously growing young firs. Yet the heath was so 
extremelv barren and so extensive that no one would ever 
have imagined that cattle would have so closely and effec- 
tually searched it for food. 

Here we see that cattle absolutely determine the 
existence of the Scotch fir; but in several parts of the 
world insects determine the existence of cattle. Perhaps 
Paraguay offers the most curious instance of this; for here 
neither cattle nor horses nor dogs have ever run wild, 
though they swarm southward and northward in a feral 
state; and Azara and Rengger have shown that this is 
caused by the greater number in Paraguay of a certain fly, 
which lays its eggs in the navels of these animals when 
first born. The increase of these flies, numerous as they 
are, must be habitually checked by some means, probably 
by other parasitic insects. Hence, if certain insectivorous 
birds were to decrease in Paraguay, the parasitic insects 
would probably increase; and this would lessen the num- 
ber 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 
vegetation: this again would largely affect the insects; and 
this, as we have just seen in Staffordshire, the insectivor- 
ous birds, and so onward in ever-increasing circles of com- 
plexity. 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 deso- 
late 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 
folgens is never visited in my garden by insects, and con- 
sequently, 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 
fertilize them. I find from experiments that humble-bees 
are almost indispensable to the fertilization of the hearts- 
ease (Violo tricolor), for other bees do not visit this flower. 
I have also found that the visits of bees are necessary for the 
fertilization of some kinds of clover; for instance twenty 
heads of Dutch clover (Trifolium repens) yielded 2,290 
seeds, but twenty other heads, protected from bees, produced 
not one. Again, 100 heads of red clover (T. pratense) 

Eroduced 2,700 seeds, but the same number of protected 
eads 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 fertilize 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 Colonel 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 Colonel 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, act- 
ing 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 dis- 
tricts. When we look at the plants and busbes clothing 
an entangled bank, we are tempted to attribute their pro- 
portional 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 forests. 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 tlie 
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 invari- 
ably be most severe between the individuals of the same 


species, for they frequent the same districts, require the 
same food, and are exposed to the same dangers. In the 
case of varieties of the same species, the struggle will 
generally be almost equally severe, and we sometimes see 
the contest soon decided: for instance, if several varieties 
of wheat be sown together and the mixed seed be resown, 
some of the varieties which best suit the soil or climate, 
or are naturally the most fertile, will beat the others 
and so yield more seed, and will consequently in a 
few years supplant the other varieties. To keep up a 
mixed stock of even such extremely close varieties as 
the variously colored 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 man- 
ner as beings in a state of nature, and if the seed or young 
were not annually preserved in due proportion. 


As the species of the same genus usually have, though 
by no means invariabl}^ 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 ther recent extension 
over parts of the United States of one species of swallow 
having caused the decrease of another species. The recent 
increase of the missel-thrush in parts of Scotland has caused 
the decrease of the song-thrush. How frequently we hear 
of onQ 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 com- 
petition 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 Uie most essential yet 
often hidden manner, to that of all the other organic 
beings, with which it comes into competition for food or 
residence, or from which it has to escape, or on which it 
preys. This is obvious in the structure of the teeth and 
talons of the tiger; and in that of the legs and claws of the 
parasite which clings to the hair on the tiger's body. But 
in the beautifully plumed seed of the dandelion, and in 
the flattened and fringed legs of the water-beetle, the re- 
lation seems at first confined to the elements of air and 
water. Yet the advantage of the plumed seeds no doubt 
stands in the closest relition 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 favor the growth 
of the seedlings, while struggling with other plants growing 
vigorously all around. 

Look at a plant in the midst of its range! Why does it 
not double or quadruple its numbers? We know that it 
can perfectly well withstand a little more heat or cold, 


dampness or dryness, for elsewhere it ranges into slightly 
hotter or colder, damper or drier districts. In this case we 
can clearly see that if we wish in imagination to give the 
plant the power of increasing in numbers, 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 
rigor of the climate. Not until we reach the extreme con- 
fines of life, in the Arctic regions or on the borders of an 
utter desert, will competition cease. The land may be ex- 
tremely 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, among 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 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 conviction as necessary, as it is difficult to acquire. All 
that we can do is to keep steadily in mind that each or- 
ganic being is striving to increase in a geometrical ratio; 
that each, at some period of its life, during some season of 
the year, during each generation, or at intervals, has to 
struggle for life and to suffer great destruction. When we 
reflect on this struggle we may console ourselves with the 
full belief that the war of nature is not incessant, that no 
fear is felt, that death is generally prompt, and that the 
vigorous, the healthy and the happy survive and multiply. 




Natural Selection — its power compared with man's selection — its 
power on characters of trifling importance — its power at all ages 
and on both sexes — Sexual Selection — On the generality of inter- 
crosses between individuals of the same species — Circumstances 
favorable and unfavorable 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 naturalization — 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 organization — Low fonns preserved — Con- 
vergence of character — Indefinite multiplication of species — 

How will the struggle for existence, briefly discussed in 
the last chapter, act in regard to variation? Can the prin- 
ciple of selection, which we have seen is so potent in the 
hands of man, apply under nature? I think we shall 
see that it can act most efi&ciently. 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 domesti- 
cation, it may truly be said that the whole organization 
becomes in some degree plastic. But the variability, which 
we almost universally meet with in our domestic produc- 
tions 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 pre- 
serve and accumulate such as do occur. Unintentionally 
he exposes organic beings to new and changiug conditions 
of life, and variability ensues; but similar changes of con- 
ditions might and do occur under nature. Let it also be 


borne in mind how infinitely complex and close-fitting are 
the mutual relations of all organic beings to each other 
and to their physical conditions of life; and consequently 
what infinitely varied diversities of structure mis'ht be of 
use to each being under changing conditions of life. Can 
it then be thousfht im^Drobable, seeing that variations 
useful to man have undoubtedl}^ 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? If such do occur, can we 
doubt (remembering that many more individuals are born 
than can possibly survive) that individuals having any ad- 
vantage, liowever slight, over others, would have the best 
chance of surviving and procreating their kind? On the 
other hand, we may feel sure that any variation in the least 
degree injurious would be rigidh^ destroyed. This preser- 
vation of favorable individual differences and variations, 
and the destruction of those which are injurious, I have 
called Katural Selection, or the Survival of the Fittest. 
Variations neither useful nor injurious would not be affected 
by natural selection, and w^ould 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 conditions. 

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 preservation 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 differ- 
ences 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 applica- 
ble 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 aftinities 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 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 per- 
sonifying 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 

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 pro- 
portional numbers of its inhabitants will almost immedi- 
ately undergo a change, and some species will probably be- 
come extinct. We may conclude, from what we have seen 
of the intimate and complex manner in which the inhabi- 
tants of each country are bound together, that any change 
in the numerical proportions of the inhabitants, independ- 
ently 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 inhab- 
itants. 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 man- 
ner 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 
favored the individuals of any species, by better adapting 
them to their altered conditions, would tend to be pre- 
served; and natural selection would have free scope for the 
work of improvement. 

We have' good reason to believe, as shown in the first 
chapter, that changes in the conditions of life give a ten- 
dency to increased variability; and in the forgoing cases 
the conditions have changed, and this would manifestly be 
favorable to natural selection; by affording a better chance 


of the occurrence of i^rofitable variations. TJnless sucli 
occur, natural selection can do nothing. Under the 
term of ^^ variations," it must never be forgotten 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 dif- 
ferences, so could natural selection, but far more easily 
from having incomparably 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 unoccupied 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 inhab- 
itants 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 
naturalized productions that they have allowed some for- 
eigners to take firm possession of the land. And as for- 
eigners 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 selec- 
tion, what may not natural selection effect? 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 constitutional difference 
on the whole machinery of life. Man selects only for his 
own good; Nature only for that of the being which she 
tends. Every selected character is fully exercised by her, 


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 fit- 
ting 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; 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 selec- 
tion by some half -monstrous form, or at least by some mod- 
ification prominent enough to catch the eye or to be 
plainly useful to him. Under nature, the slightest differ- 
ences 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 productions should be far ''truer" in char- 
acter 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 workman- 

It may metaphorically be said that natural selection 
is daily and hourly scrutinizing, throughout the world, the 
slightest varifitionri; rejecting those that are bad, preserv- 
ing and adding up all that are good; silently and insen- 
sibly working, tvhenever and ivherever 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 tliat 
the forms of life are now different from what they formerly 

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 favorable 
nature as before; and these must again be preserved, and 


so onward, step by step. Seeing that individual differences 
of the same kind perpetually recur, this can hardly be con- 
sidered as an unwarrantable assumption. But whether it 
is true, we can judge only by seeing how far the hypothe- 
sis 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-gray; the alpine ptarmigan 
white in winter, the red grouse the color of heather, we 
must believe that these tints are of service to these birds 
and insectr 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 e3^esight to 
their prey — so much so that on j^arts of the continent per- 
sons are warned not to keep white pigeons, as being the 
most liable to destruction. Hence natural selection might 
be effective in giving the j)roper color to each kind of 
grouse, and in keeping that color, when once acquired, 
true and constant. Nor ought we to think that the oc- 
casional destruction of an animal of any particular color 
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 
color of 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 color of the flesh are considered 
by botanists as characters of the most trifling importance; 
yet we hear from an excellent horticulturist. Downing, 
that in the United States that smooth-skinned fruits suffer 
far more from a beetle, a Curcuho, than those with down; 
that purple jolums suffer far more from a certain disease 
than yellow plums; whereas another disease attacks yellow- 
fleshed peaches far more those with other colored 
flesh. If, with all the aids of art, these slight differ- 
ences 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 a 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, etc., 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 tlie variations are 
accumulated through natural selection, other modifica- 
tions, often of the most unexpected nature, will ensue. 

As we see that those variations which, under domesti- 
cation, appear at any particular period of life, tend to 
reappear in the offspring at the same period; for instance, 
in the shape, size and flavor of the seeds of the many 
varieties of our culinary and agricultural plants; in the 
caterpillar and cocoon stages of the varieties of the silk- 
worm; in the eggs of poultry, and in the color 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 dis- 
seminated 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 eft'ect, 
through correlation, the structure of the adult. So, con- 
versely, modifications in the adult may affect the structure 
of the larva; but in all cases natural selection will insure 
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 
m 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. V, hat 


natural selection cannot do, is to modify the structure of 
one species, without giving it any advantage, for the good 
of another sj^ecies; and though statements to this effect 
may be found in works of natural history, I cannot find 
one case whicii will bear investigation. A structure used 
only once in an animaFs 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 eggs. 
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 simul- 
taneously 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 dis- 
trict 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, sup- 
posiug that there is any variability in a favorable 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 ben- 
eficial directions; but this is no valid objection to its effi- 
ciency 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 area. 


Inasmuch as peculiarities often appear under domestica- 
tion in one sex and become hereditarily attached to that 
sex, so no doubt it will be under nature. Thus it is ren- 
dered 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 rela- 
tion 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 exist- 
tence in relation to other organic beings or to external 
conditions, 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 competi- 
tor, but few or no offspring. Sexual selection is, there- 
fore, 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 vigor, a son 
having special weapons, confined to the male sex. A horn- 
less stag or spurless cock would have a poor chance of 
leaving numerous off'spring. Sexual selection, by always 
allowing the victor to breed, might surely give indomitable 
courage, length of 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 fight- 
ing, bellowing and whirling round, like Indians in a war- 
dance, for the possession of the females; male salmons have 

82 Sexual selection. 

been observed fighting all day long; male stag-beetles 
sometimes bear wounds from tlie 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 ap- 
parently unconcerned beholder of the struggle, and then 
retires with the conqueror. The war is, perhaps, severest 
between the males of pol3'gamous 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 

Among 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 attact, by singing, the females. The 
rock-thrush of Guiana, birds of paradise, and some others^ 
congregate, and successive males display 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 individual preferences and dislikes: thus 
Sir E. Heron has described how a pied peacock was emi- 
nently 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, accord- 
ing to his standard of beauty, I can see no good reason to 
doubt that female birds, by selecting, during thousands of 
generations, the most melodious or beautiful males, accord- 
to their standard of beauty, might produce a marked 
effect. Some well-known laws, with respect to the pumage 
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, color, 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 defense, 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 
augmented 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. 


In order to make it clear how, as I believe, natural selec- 
tion acts, I must beg permission to give one or two imagi- 
nary illustrations. Let us take the case of a wolf, which 
preys on various animals, securing some by craft, some by 
strength, and some by fleetuess; and let us suppose that 
the fleetest prey, a deer for instance, had from any change 
in the country increased in numbers, or tliat 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 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 compelled 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 fleetnessof 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 tliought of mod- 
ifying 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 


greyliouud-like form, which pursues deer, and the other 
more bulk}^, 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 ^Dreserved. 
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 unconscious 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 inter- 
crossing with ordinary individuals. 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 j^air of ani- 
mals, 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 organ- 
isms. 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. SupjDOsing 
it to survive and to breed, and that half its young 
inherited the favorable variation; still, as the Reviewer 
goes on to show, the youug would have only a slightly 
better chance of surviving and breeding; 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 individual perpet- 
uating its kind to the exclusion of the common form; but 


there can hardly be a doubt, judging by wluit we see taking 
place under domestication, that this result would follow 
from the preservation during many generations of a large 
number of individuals 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 organization being similarly acted on — 
of which fact numerous instances could be given with our 
domestic productions. In such cases, if the varying indi- 
vidual 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 individ- 
uals of the same species have been similarly modified with- 
out the aid of any form of selection. 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 scon 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 maybe 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 gen- 
erally be at first local, as seems to be the common rule with 
varieties in a state of nature; so that similarly modified indi- 
viduals would soon exist in a small body together, and 
would often breed together. If the new variety were suc- 
cessful in its battle for life, it would slowly spread from a 
central district, competing with and conquering the ini- 
changed individuals ou the margins of an ever-increasing 


It maybe worth while to give another and more complex 
illustration of the action of natural selection. Certain 
plants excrete sweet juice, apparently for the sak« of elim- 
inating something injurious from the sap: this is effected, 
for instance, by glands at the base of the stipules in some 
Leguminos^e, 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 ben- 
efit 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 individuals 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, Avhich had their stamens and 
pistils placed, in relation to the size and habits of the par- 
ticular insect which visited them, so as to favor in any 
degree the transportal of the pollen, would likewise be 
favored. 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 fertilization, 
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 destro3^ed 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 

OF NATURAL selection: 37 

sexes of planes. Some holly-trees bear only male flowers, 
which have four stamens producing a rather small quan- 
tity 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 polen 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 profusion. 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 favorable 
to bees, nevertheless every female flower which I examined 
had been effectually fertilized 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 ren- 
dered so highly attractive to insects that pollen was regu- 
larly carried from flower to flower, another process might 
commence. No naturalist doubts the advantage of what 
has been called the '*^ physiological division of labor;" 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 labor, individuals with this tendency 
more and more increased, would be continually favored 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, though dimorphism_ and other 
means, by which the separation 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 intermediate 
condition, or, as he expresses it, are more or less dia^ciously 


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 probocis, etc., 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 or incarnate clovers (Trifo- 
lium pratense and incarnatum) do not on a hasty glance 
appear to differ in length; yet the hive-bee can easily suck 
the nectar out of the incarnate clover, but not out of the 
common red clover, which is visited by humble-bees alone, 
so that whole fields of the red clover offer in vain an abun- 
dant supply of precious nectar to the hive-bee. That this 
nectar is much liked bv 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 somewhat 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 com- 
mon hive-bee, and which freel}^ 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 vis- 
iting the flowers, if humble-bees were to become rare in any 


country, it might be a great advantage to the nlant to have 
a shorter or more deeply diviied corolla, so that tlie hive- 
bees should be enabled to suck its flowers. Thus I can un- 
derstand how a flower and a bee might slowly become, 
either simal'^aneously or one after the other, modified and 
adapted to each other in the most perfect manner, by 
the continued preservation of all the" individuals which 
presented flight deviations of structure mutually favorable 
to each other. 

I am well aware that this doctrine of natural selection, 
exemplified :n the above 'maginary instances, is open to 
the same objections which were first urged against Sir 
Charles LyelFs noble views on " the modorn changes of 
the earth, as Illustrative of geology;-*' but we now seldom 
hear the agencies which we see still a: Tork, spoken of as 
trifling or insignificant, when used In explaining the excava- 
tion of the deepest valleys or the formation of long Lines of 
inland oiiffs. Natural selection acts only by the preserva- 
tion and accumulation of small inherited modifications, 
each profitable to the preserved being; and as modern geol- 
ogy has almost banished such views as the excavation of a 
great valley by a single diluvial wave, so will natural selec- 
tion banish the belief of the continued creation of new or- 
ganic beings, or of any great and sudden modification in 
their structure. 


I must here introduce a short digression. In the case of 
a,nimals and plants with separated sexes, it is of course ob- 
vious that two individuals must always (with the ex- 
ception 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 hermaphrodites 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 hero 
treat the subject with extreme brevity, though I have the 
materials prepared for an ample discussion. All vertebrate 
animals, all insects and some other large groups of animals. 


pair for each birth. Modern research has much diminished 
the number of supposed hermaphrodites and of real herma- 
phrodites a large number pair; that is, two individuals regu- 
larly unite for reproduction, which is all that concerns us. 
But still there are many hermaphrodite animals which cer- 
tainly do not habitually pair, and a vast majority of plants 
are hermaphrodites. What reason, it may be asked, is 
there for sujDposingin 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 

In the first place, I have collected so large a body of 
facts, and made so many experiments, showing, in accord- 
ance 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 vigor and fertility to the offspring; and on 
the other hand, that close interbreeding diminishes vigor 
and fertility; that these facts alone incline me to believe 
that it is a general law of nature that no organic being 
fertilizes 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 unfavorable exposure to wet 
is to the fertilization 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, notwith- 
standing that the plant's own anthers and pistil stand so 
near each other as almost to insure self-fertilization, the 
fullest freedom for the entrance of pollen from another in- 
dividual will explain the above state of exposure of the 
organs. Many flowers, on the other hand, have their 
organs of fructification closely inclosed, as in the great 
papilionaceous or pea-family; but these almost invariably 
present beautiful and curious adajDtations 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. JMow, 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 suffi- 
cient, to insure fertilization, 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 specie* 
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 toward 
the pistil, or slowly move one after the other toward 
it, the contrivance seems adapted solely to ensure self- 
fertilization; 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-fertilization, it is well 
known that, if closely-allied forms or varieties are planted 
near each other, it is hardly possible to raise pure seedlings, 
60 largely do they naturally cross. In numerous other 
cases, far from self-fertilization being favored, 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 
fhat 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 liave 
shown, and as I can confirm, either the anthers burst 
before the stigma is ready for fertilization, or the stigma is 
ready before the pollen of that flower is ready, so that these 
^o-named dichoganiotis plants have in fact separated sexes^ 


and must habitually be crossed. So it is with the recipro- 
cally dimorphic and trimorDhic plants previously alluded 
to. How strange are these facts! How strange that the 
pollen and stigmatic surface of the flower, though 
placed so close together, as if for the very purpose of self- 
fertilization, should be in so many cases mutually useless 
to each otlier? How simply are these facts explained on 
the view of an occasional cross with a distinct individual 
being advantageous or indispensable! 

If several varieties of the cabbage, radish, onion and of 
some other plants, be allowed to seed near each other, a 
large majority of the seedling thus raised turn out, as I 
founds mongrels: for instance, I raised 233 seedling cab- 
bages from some plants of different varieties growing near 
each other, and of these only 78 were true to their kind, 
and some even of these were not perfectly true. Yet the 
pistil of each cabbage-flower is surrounded not only by its 
own six stamens but by those of the many other flowers on 
the same plant; and the pollen of each flower readily gets 
on its 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 intercross- 
ing of distinct individuals of the same species. When dis- 
tinct 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 individuals only in a limited 
sense. I believe this objection to be valid, but that nature 
has largely provided against it by giving to trees a strong 
tendency to bear flowers with separated sexes. When the 
sexes are separated, although the male and female flowers 
may be produced on the same tree, pollen must be regu- 
larly carried from flower to flower; and this will give 
a better chance of pollen being occasionally carried from 

OP mniviDUALs, 03 

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 antici- 
pated. 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 trees 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 1 have not found 
a single terrestrial animal which can fertilize itself. This 
remarkable fact, which offers so strong a contrast with ter- 
restrial plants, is intelligible on the view of an occasional 
cross being indispensible; for owing to the nature of the 
fertilizing element there are no means, analogous to the 
action of insects and of the wind with plants, by which an 
occasional cnoss could be effected with terrestrial animals 
without the concurrence of two individuals. Of aquatic 
animals, there are many self-fertilizing 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 consultation with one of the highest 
authorities, namely. Professor Huxley, to discover a single 
hermaphrodite animal with the organs of rej^ reduction so 
perfectly enclosed that access from without, and the occa- 
sional 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 diflB- 
culty; but I have been enabled, by a fortunate chance, to 
prove that two individuals, though both of self-fertilizing 
hermaphrodites, 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 organization, are 
hermaphrodities, and some unisexual. But if, in fact, all 
hermaphrodities do occasionally intercross, the difference 
between them and unisexual species is, as far as fuuction 
is concerned, ver/ 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. 


This is an extremely intricate subject. A great amount 
of variability, under which term individual differences are 
always included, will evidently be favorable. A large num- 
ber of individuals, by giving a better chance within any 
giveJi 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 indefi- 
nite 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 corresponding 
degree with its competitors it will be exterminated. Unless 
favorable 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 intercross, 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 pro- 
cure and breed from the best animals, improvement surely 
but slowly follows from this unconscious process of selec- 
tion, notwithstanding that there is no separation of selected 
individuals. Thus it v/ill be under nature; 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 pre- 
served. But if the area be large, its several districts will 
almost certainly present different conditians of life; and 


then, if the same species undergoes modification in differ- 
ent districts, the newly formed varieties will intercross on 
the confines of each. But we shall see in the sixth chap- 
ter that intermediate varieties, inhabiting intermediate dis- 
tricts, 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 hermaphrodite or- 
ganisms which cross only occasionally, and likewise Avitli 
animals which unite for each birth, but which wander lit- 
tle and can increase at a rapid rate, a new and improved 
variety might be quickly formed on any one spot, and 
might there maintain itself in a body and afterward 
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 in- 
tercrossing 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 distinct, 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 occesional intercrosses take 
place with all animals and plants. Even if these take 
place only at long intervals of time, the young thus pro- 
duced will gain so much in vigor and fertility over the 
offspring from long-continued self-fertilization, 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 or- 


gallic beings extremely low in the scale, which do not 
propagate sexually, nor conjugate, and which cannot pos- 
sibly intercross, uniformity of character can be retained 
by them under the same conditions of life, only through 
the principle of inheritance, 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 favorable variations. 

Isolation also is an important element in the modifica- 
tion 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 indi- 
viduals of the same species in the same manner. Inter- 
crossing with the inhabitants of the surrounding districts, 
will also be thus prevented. Moritz Wagner has lately pub- 
lished an interesting essay on this subject, and has sho\7n 
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 isola- 
tion are necessary elements for the formation of new species. 
The importance of isolation is likewise great in prevent- 
ing, after any physical change in the conditions, such as 
of climate, elevation of the land, etc., 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 import- 
ance. If, however, an isolated area be very small, either 
from being surrounded by barriers, or from having very 
peculiar physical conditions, the total number of the in- 
habitants will be small; and this will retard the production 
of new species through natural selection, by decreasing the 
chances of favorable variations arising. 

The mere lapse of time by itself does nothing, either for 
or against natural selection. I state this because it has 
been erroneously asserted that the element of time has 
been assumed by me to play an all-important part in modi- 


f jing species, as if all tlie forms of life were necessarily 
undergoing change through some innate law. Lai)8e of 
fime 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, accumulated, 
and fixed. It likewise tends to increase the direct action 
of the physical conditions of life, in relation to the consti- 
tution 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 
smal], as we shall see in our chapter on Geographical Dis- 
tribution; 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 favorable 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 favorable for the pro- 
duction of new organic forms, we ought to make the com- 
parison within equal times; and this we are incapable of 

Although isolation is of gi^eat importance in the produc- 
tion 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 cajDable 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 favorable variations, arising from the 
large number of individuals of the same species there sup- 
ported, 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 exterminated. Each new form, also, as soon as 
it has been much improved, will be able to spread over the 
open and continuous area, and will thus come into competi- 
tion 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 favorable 
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 victor- 
ious 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, under- 
stand some facts which will be again alluded to in our 
chapter on Geographical Distribution; for instance, the 
fact of the productions of the smaller continent of Austra- 
lia now yielding before those of the larger Europaeo- Asiatic 
area. Thus, also, it is that continental productions have 
everywhere become so largely naturalized 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 Maderia, 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. Con- 
sequently, 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, con- 
nect 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, com- 

To sum up, as far as the extreme intricacy of the subject 
permits, the circumstances favorable and unfavorable for 
the production of new species through natural selection. 
I conclude that for terrestrial productions a large conti- 


nental area, whicli has undergone many oscillations o^ 
level, will have been the most favorable for the prod net iou 
of many new forms of life, fitted to endure for a long time 
and to spread widely. While the area existed as a conti- 
nent the inhabitants will have been numerous in individu- 
als and kinds, and will have been subjected to severe com- 
petition. ^ When converted by subsistance into large 
separate islands there will still have existed many indi- 
viduals 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 immigra- 
tion 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 modi- 
fied and perfected. "When, by renewed elevation, the 
islands were reconverted into a continental area, there will 
again have been very severe competition; the most favored 
or improved varieties will have been enabled to spread; 
there will have been much extinction of the less improved 
forms, and the relative proportional 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 slow- 
ness I fully admit. It can act only when there are places 
in the natural polity of a district which can be better occu- 
pied by the modification of some of its existing inhabitants. 
The occurrence of such places will often depend on physi- 
cal changes, which generally take place very slowly, and 
on the immigration of better adapted forms being pre- 
vented. As some few of the old inhabitants become modi- 
fied 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 organization might occur. The result would 
often be greatly retarded by free intercrossing. Many will 
exclaim that these several causes are amply sufficient to 


neutralize 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, intermittant 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 


This subject will be more fully discussed in our chap- 
ter on Geology; but it must here be alluded to from 
being intimately connected with natural selection. Nat- 
ural selection acts solely through the preservation of vari- 
ations in some way advantageous, which consequently en- 
dure. 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 favored 
forms increase in number, so, generally, will the less fa- 
vored decrease and become rare. Earity, 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 or the seasons, or from a temporary increase 
ir. 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 increasing 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 presently attempt 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 numer- 
ous in individuals have the best chance of producing favor- 


able variations within any given period. We have evi- 
dence 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 vari- 
eties. Hence, rare species will be less quickly modified or 
improved within any given period; they will consequently 
be beaten in the race for life by the modified and improved 
descendants of the commoner species. 

From these several considerations I think it inevitably 
follows, that as new species in the course of time are 
foniied 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 suifer 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 related 
genera — which, from having nearly the same structure, 
constitution and habits, generally come into the severest 
competition with each other consequently, each new vari- 
ety or species, during the progress of its formation, will 
generally press hardest on its nearest kindred, and tend to 
exterminate them. We see the same process of extermina- 
tion among our domesticated productions, through the 
selection of improved forms by man. Many curious in- 
stances 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 agricul- 
tural writer) '^as if by some murderous pestilence.^' 


The principle, which I have designated by this tenn, ia 
of high importance, and explains, as I believe, several im- 
portant facts. In the first place, varieties, even strongly 
marked ones, though having somewhat of the chanic- 
ter 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 differ- 
ence between varieties become augmented into the greater 
difference between species? That this does habitually 
happen, we must infer from most of the innumerable 
species throughout nature presenting well-marked differ- 
ences; 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 something analogous. It will be admitted that the 
production of races so different as short-horn and Here- 
ford cattle, race and cart horses, the several breeds of 
pigeons, etc., could never have been effected by the mere 
chance accumulation of similar variations during many 
successive generations. In practice, a fancier is, for in- 
stance, 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 breed- 
ing 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, while those of another re- 
quired 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 dis- 
tinct 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? I believe it can and does apply most 
etficiently (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 in- 
creasing (the country not undergoing any change in con- 
ditions) 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, climb- 
ing trees, frequenting water, and some perhaps becoming 
less carnivorous. The more diversified in habits and 
structure the descendants 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 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 countless 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 dis- 
tinct 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 trutli of the principle that the greatest amount of 
life can be supported by great diversification of structure, 
is seen under many natural circumstances. In an extremely 
small area, especially if freely open to immigration, and 
where the contest between individual and individual must 
be very severe, we always find great diversity in its inhabi- 
tants. 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 j^lants 
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 more food 
by a rotation of plants belonging to the m^ost 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 (suppos- 
ing 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 accom- 
panying 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 naturalization of 


plants through man's agency in foreign lands. It miglit 
have been expected that the plants which would suc- 
ceed in becoming naturalized 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 county. It might also, perhaps, have been ex- 
pected that naturalized 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 naturalization, pro- 
portionally 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 Xorthern United States," 260 
naturalized plants are enumerated, and these belong to 1G2 
genera. We thus see that these naturalized plants are of 
a highly diversified nature. They differ, moreover, to a 
large extent, from the indigenes, for out of the 1G2 
naturalized genera, no less than 100 genera are not there 
indigenous, and thus a large proportional addition is made 
to the genera now living in the United States. 

By considering the nature of the plants or animals which 
have in any country struggled successfully with the indi- 
genes, and have there become naturalized, 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 dif- 
ferences, 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 labor 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 alone, 
draws most nutriment from these substances. So in the 
general economy of any land, the more widely and per- 
fectly 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 organization but little diversified, could hardly 


compete with a set more perfectly diversified in structure. 
It may be doubted, for instance, whether the Austrahan 
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 com- 
pete with these well-developed orders. In the Australian 
mammals, we see the process of diversification in an early 
and incomplete stage of development. 


After the foregoing discussion, which has been much 
compressed, 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. ]N^ow let 
us see how this principle of benefit being derived from 
divergence of character, combined with the principles of 
natural selection and of extinction, tends to act. 

The accompanying diagram will aid us in understanding 
this rather perplexing subject. Let A to L represent the 
species of a genus large in its own country; these species are 
supposed to resemble each other in unequal degrees, as is 
so generally the case in nature, and as is represented 
in the diagram by the letters standing at unequal distances. 
I have said a large genus, because as we saw in the second 
chapter, on an average more sj^ecies 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 
most widely diffused, vary more than do the rare and 
restricted species. Let (A) be a common, widely-diffused, 
and varying species, belonging 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 tliey all supposed to 
endure for equal periods. Only those variations wliich arc 
in some way profitable will be preserved or naturally 
selected. And here the 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. AVhen 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 tlie dia- 
gram, 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^ and m^. 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 hered- 
itary; 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 numerous 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 favorable to the 
production of new varieties. 

If, then, these two varieties be variable, the most 
divergent of their variations will generally be preserved 
during the next thousand generations. And after this 
interval, variety cc^ is supposed in the diagram to have pro- 
duced variety a^, which will, owing to the principle of 
divergence, differ more from (A) than did variety a^. 
Variety m^ is supposed to have produced two varieties, 
namely m^ and s^ 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 genera- 
tions, producing only a single variety, but in a more and 




o- p-:. 




^^* .t,m 



1 / 







i / 

1 > 







' / 









% ' 

% 1 
























_W v^^ 



••. \ 







G H 





more modified condition, some producing two or three 
varieties, and some failing to produce any. Thus the 
Tttrieties or modified descendants of the common parent 
(A), will generally go on increasing in number and diverg- 
ing in character. In the diagram the process is repre- 
sented up to the ten-thousandth generation, and under a 
condensed and sim23lified form up to the fourteen-thous- 
andth generation. 

But I must here remark that I do not suppose that the 
process aver goes on so regularly as is represented in the 
diagj'am, 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 produce 
more than one modified descendant; for natural selection 
will always act according to the nature of the places which 
are either unoccupied or not perfectly occupied by other 
beings; and this will depend on infinitely complex rela- 
tions. But as a general rule, the more diversified in struc- 
ture the descendants from any one species can be rendered, 
the more places they will be enabled to seize on, and the 
more their modified progeny will 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 
anywhere, after intervals long enough to allow the accumu- 
lation of a considerable amount of divergent variation. 

As all the modified descendents from a common and 
widely-dilfused 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 multi- 
plying in number as well as diverging in character: this 
is represented in the diagram by the several divergent 
branches proceeding from (A). The modified offspring 
from the later and more highly improved branches in the 
lines of descent, will, it is probable, often take the place 
of, and so destroy, the earlier and less improved branches: 
this is represented in the diagram by some of the lower 
branches not reaching to the upper horizontal lines, lu 


some cases no doubt the process of modification will be 
confined to a single line of descent, and the number of 
modified descendants will not be increased; although tbe 
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 «! to «!". In the same way the English racehorse 
and English pointer have apparently both gone on slowly 
diverging in character from their original stocks, without 
either having given off any fresh branches or races. 

After ten thousand generations, species (A) is supposed 
to have produced three forms, a^^,f^^, and m'^^, 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 hori- 
zontal 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 modi- 
fication 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 con- 
densed and simplified manner), we get eight species, marked 
by the letters between a^^ and m^*, 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 sj^e- 
cies 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'^^ and z'^^) or two species, according to the amount of 
change supposed to be represented between the horizontal 
lines. After fourteen thousand generations, six new spe- 
cies, marked by the letters n'^'^ to 2;i*, are supnosed to 
have been produced. In any genus, the species which are 
already very ditt'erent 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 different places in the polity of nature; 
hence in the diagram I have chosen the extreme species 
(A), and the nearly extreme species (I), as tliose 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 upward. 

But during the process of modification, represented in 
the diagram, another of our principles, namely that of ex- 
tinction, 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 anyone species to supplant 
and exterminate in each stage of descent their predecessors 
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 proba- 
bly 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 offspring and progen- 
itor do not come into competition, both may continue to 

If, then, our diagram be assumed to represent a consid- 
erable amount of modification, species (A) and all the 
earlier varieties will have become extinct, being replaced by 
eight new species (a^* to m^*), and species (I) will be re- 
placed by six {n^^ to z^^) new species. 

But we may go further than this. The original species of 
our genus were supposed to resemble each other in unequal 
degrees, as is so generally the case in nature; species (A) being 
more nearly related to B, and D than to the other spe- 
cies; 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, four- 
teen 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 tliat 
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 fourteenth thousandth genera- 
tion. AVe 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^* and z^^ 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", q^*, 7.?^*, will be nearly related from 
having recently branched off from a}^; b^^ and/^*, from hav- 
ing diverged at an earlier period from a^, will be in some 
degree distinct from the three first-named species; and 
lastly, o^S e" and m}^, will be nearly related one to the 
other, but, from having diverged at the first commence- 
ment of the process of modification, will be widely differ- 
ent 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 de- 
scendants from (A); the two groups, moreover, are sup- 


posed to have gone on diverging in different directions. 
The intermediate species, also (and this is a very import- 
ant consideration), which connected the original species 
(A) and (I), have all become, except (F), extinct, and 
have left no descendants. Hence the six new species de- 
scended from fl), and the eight descendants from (A), 
will have to be ranked as very distinct genera, or even as 
distinct sob-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 desceuded 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 downward toward a single point; this point rep- 
resents a species, the supposed progenitor of our several 
new sub-genera and genera. 

It is worth while to reflect for a moment on the charac- 
ter of the new species f^*, wdiich 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 de- 
scended from a form that stood between the parent- 
species (A) and (I), now supposed to be extinct and un- 
known, it vrill be in some degree iiitermediate in character 
between the two groups descended from these two species. 
But as these two groups have gone on diverging in charac- 
ter from the type of their parents, the new species (f^*) 
will not be 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 diao-ram 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 sub- 
ject, and I think we shall then see that the diagram throws 
light on the afUnities of extinct beings, which, though 
generally belonging to the same orders, families or genera, 
with those now living, yet are often, in some degree, inter- 


mediate in character between existing groups; and we can 
understand this fact, for the extinct species lived at various 
remote epochs when the branching Unes 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}^ to p^^, those marked h^^ and 
/^S and those marked o^* to ?;i^*, will form three very dis- 
tinct 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 dis- 
tinct families, or orders, according to the amount of diver- 
gent modification supposed to be represented in the dia- 
gram. And the two new families, or orders, are descended 
from two species of the original genus, and these are sup- 
posed to be descended from some still more ancient and 
unknown form. 

We have seen that in each country it is the species be- 
longing to the larger genera which oftenest present varie- 
ties 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 pro- 
duction of new and modified descendants will mainly lie 
between the larger groups which are all trying to incroase 
in number. One large group will slowly conquer another 
large group, reduce its number, 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 sup- 
})iant and destroy the earlier and less improved sub-gi-oups. 
Small and broken groups and sub-groups will finally disap- 
pear. Looking to the future, we can predict that the 
groups of organic beings which are now hirge and trium- 
phant, and which are least broken up, that is, which have 
as yet suffered least extinction, will, for a long period, con- 


tinue to increase. But which groups will ultimately pre* 
vail, no man can predict; for we know that many groups, 
formerly most extensively developed, have now become ex- 
tinct. Looking still more remotely to the future, we may 
predict that, owing to the continued and steady increase of 
the larger groups, a multitude of smaller groups will be- 
come 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 more 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 exist so few classes in 
each main division of the animal and vegetable kingdoms. 
Although few of the most ancient species have left modi- 
fied 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 



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 organization of 
the greater number of living beings throughout the world. 
But here we enter on a very intricate subject, for natural- 
ists have not defined to each other's satisfaction what is 
meant by an advance in organization. Among the verte- 
brata 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 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 liiglier tliau its 
larva. Von Baer's standard seems the most widely appli- 
cable 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 specialization for 
different functions; or, as Milne Edwards would express it, 
the completeness of the division of physiological labor. 
But we shall see how obscure this subject is if we look, for 
instance, to fishes, among which some naturalists rank 
those as highest which, like the sharks, approach nearest 
to amphibians; while 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, among which the 
standard of intellect is of course quite excluded; and here 
some botanists rank those plants as highest which have 
every organ, as sepels, petals, stamens and pistils, fully 
developed in each flower; whereas other botanists, probably 
with more truth, look at the plants v/hich have their 
several organs much modified and reduced in number as 
the highest. 

If we take as the standard of high organization, the 
amount of differentiation and specialization 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 toward this standard: for 
all physiologists admit that the specialization 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 toward specialization 
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 organiza- 
tion. Whether organization on the whole has actually ad- 
vanced 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 every where supplanted and exter- 
minated the lower? Lamarck, who believed in an innate 
and inevitable tendency toward 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 continu- 
ally 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 selec- 
tion, or the survival of the fittest, does not necessarily in- 
clude 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 in- 
fusorian animalcule — to an intestinal worm — or even to au 
earth-worm, to be highly organized. If it were no advan- 
tage, these forms would be left, by natural selection, un- 
improved or but little improved, and might remain for in- 
definite ages in their present lowly condition. And geol- 
ogy 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 ad- 
vanced since the first dawn of life would be extremely 
rash; for every naturalist who has dissected some of the 
beings now ranked as very low in the scale, must have been 
struck with their really wondrous and beautiful organiza- 

Nearly the same remarks are applicable, if we look to the 
different grades of organization within the same great 
group; for instance, in the vertebrata, to the co-existence 
of mammals and fish — among mammalia, to the co-exist- 
ence of man and the ornithorhynchus — among fishes, to 
the co-existence of the shark and thelancelet (Amphioxus), 
which latter fish in the extreme simplicity of its structure 


approaches the invertebrate classes. But mammals and 
fish hardly come into competition with each other; the ad- 
vancement of the whole class of mammals, or of certain 
members in this class, to the highest grjide 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 companiou and 
competitor on the barren sandy shore of South Brazil, an 
anomalous annelid. The three lowest orders of nuun- 
mals, namely, marsupials, edentata, and rodents, co-exist 
in South America in the same region with nnmeroua 
monkeys, and probably interfere little with each other. 
Although organization, on the whole, may have advanced 
and be still advanciug 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 here- 
after see, lowly organized forms appear to have been pre- 
served to the jDresent day, from inhabiting confined or 
peculiar stations, where they have been subjected to less 
severe competion, and where their scanty numbers have re- 
tarded the chance of favorable variations arising. 

Finally, I believe that many lowly organized forms nov 
exist throughout the world, from various causes. In some 
cases variations or individual differences of a favorable 
nature may never have arisen for natural selection to act 
on and accumulate. In no case, probably, has time suf- 
ficed for the utmost possible amount of development. In 
some few cases there has been what we must call retro- 
gression or organization. But the main cause lies in the 
fact that under very simple conditions of life a hi;: h organi- 
zation would be of no service, — possibly would bo of actual 
disservice, as being of a more delicate nature, and more 
liable to be put out of order and injured. 

Looking to the firgt dawu of life,' when all orgauio beiugd^ 


as we may believe, presented the simplest structure, how, 
it has been asked, could the first step 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 rela- 
"tions to incident forces become different'' would come 
into action. But as we have no facts to guide us, specula- 
tion on the subject is almost useless. It is, however, an 
error to suppose that there would be no struggle for exist- 
ence, 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 toward 
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 ignor- 
ance on the mutual relations of the inhabitants of the 
world at the present time, and still more so during past 


Mr. H. C. Watson thinks that I have overrated the im- 
portance 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 at- 
tribute to convergence a close and general similarity of struct- 
ure 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 de- 
pends on an infinitude of complex relations, 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 de- 
gree 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 eoually com- 
plex relations. It is incredible that the descendants of two 
organisms, which had originally differed in a marked man- 
ner, should ever afterward converge so closely as to lead 
to a near approach to identity throughout their whole 
organization. If this had occurred, we should meet with 
the same form, independently of genetic connection, re- 
curring 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, etc.; 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 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 naturalized. 
But geology shows us, that from an early part of the ter- 
tiary period the number of species of shells, and tliat from 
the middle pa/t of this same period, the number of mam- 
mals has not greatly or at all increased. What then^checks 
an indefinite increase in the number of species? Tlio 
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 represented 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. Imao^ine the extreme case of as manv 
Species as individuals in England, and the first severe winter 
or very dry summer would exterminate thousands on thou- 
sands of species. Eare species, and each species will become 
rare if the number of species in any country becomes in- 
definitely increased, will, on the principle often explained, 
present within a given period few favorable variations; con- 
sequently, the process of giving birth to new specific forms 
would thus be retarded. AYhen 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, etc. Lastly, and this 
I am inclined to think is the most important element, a 
dominant species, which has already beaten many compet- 
itors 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 veri/ 
widely, consequently they will tend to supplant and exter- 
minate several species in several areas, and thus check the 
inordinate increase of specific forms throughout the world. 
Dr. Hooker has recently shown that in the southeast cor- 
ner of Australia, where, apparently, there are many in- 
vaders from different quarters of the globe, the endemic 
Australian species have been greath^ reduced in number. 
How much weight to attribute to these several considera- 
tions I will not pretend to say; but conjointly they must 
limit in each country the tendency to an indefinite aug- 
mentation of specific forms. 


If under clianging conditions of life organic beings pre- 
sent individual 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 cannoi 

SUMMARY. 1*^1 

be disputed; then, considering the infinite 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 variations useful to any organic being ever 
do occur, assuredly individuals thus characterized 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 ofl:'spring similarly characterized. This 
principle of preservation, or the survival of the fit- 
test, I have called natural selection. It leads to the im- 
provement of each creature in relation to its organic and in- 
organic conditions of life; and consequently, in most cases, 
to what must be regarded as an advance in organization. 
Nevertheless, low and sim])le forms will long endure if well 
fitted for their simple conditions of life. 

Natural selection, on the principle of qualities being in- 
herited at corresponding ages, can miodify the egg, seed or 
young as easily as the adult. Among many animals sexual 
selection will have given its aid to ordinary selection by 
assuring to the most vigorous and best adapted males tlie 
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, according to the 
form of inheritance which prevails. 

Whether natural selection has really thus acted in 
adapting the various forms of life to their several condi- 
tions 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, leads 
lo divergence of character; for the more organic beings 
diverge in structure, habits and constitution, by so mucli 
the more can a large number be supported on the area, of 
which we see proof by looking to tlie inhabitants of any 
small spot, and to the productions naturalized in foreign 
l^uds. Therefore^ during the modification of the descend- 


ants 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 suc- 
cess 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 modified offspring that superiority which 
now makes them dominant in their own countries. Nat- 
ural selection, as has just been remarked, leads to diverg- 
ence of character and to much extinction of the less 
improved and intermediate forms of life. On these prin- 
ciples, 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 seen clustered round 
points, and these round other points, and so on in almost 
endless cycles. If species had been independently created, 
no explanation would have been possible of this kind of class- 
ification; 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 grow- 
ing twigs have tried to branch out on all sides, and to over- 
top and kill the surrounding twigs and branches, in tlie 
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 bnds by ramifying branches may well 
represent the classification of all extinct and living species 
in groups subordinate to gi'oups. 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 modified descendants. From the first growth 
of the tree, many a limb and branch has decayed and 
dropped of!;; 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 w'e here and 
there see a thin, straggling branch springing from a fork 
low down in a tree, and which by some chance has been 
favored and is still alive on its summit, so we occasionally 
see an animal like the Ornithorhynchus or Lepidosiren, 
which in some small degree connects by its aflinities two 
large branches of life, and which has apparently been 
saved from fatal competition by having inhabited a pro- 
tected station. As buds give rise by growth to fresh buds, 
and these, if vigorous, branch out and overtop on all sides 
many a feebler branch, so by generation I believe it has 
been with the great Tree of Life, which fills witli its dead 
and broken branches the crust of the earth, and covers tlie 
Hurfacewith its ever-branching and beautiful ramifications. 




Effects of changed conditions — Use and disuse, combined •with 
natural selection; organs of flight and of vision — Acclimatisa- 
tion — Correlated variation — Compensation and economy of 
growth — False correlations — Multiple, rudimentary and lowly 
organized 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 vary in an analogous manner — Reversions to 
long-lost characters — Summary. 

I HAVE hitherto sometimes spoken as if the variations — 
BO common and multiform with organic beins's under do- 
niestication, and inalesser degree with those under nature 
< — were due to chance. Tliis, of course is a wholly incorrect 
expression, but it serves to acknowledge plainly our igno- 
rance of the cause of each particular variation. Some 
authors believe it to be as much the function of the repro- 
ductive S3'stem 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 frequently under domestication than under 
nature, and the greater variability of species having wide 
ranges than of those with restricted ranges, lead to the con- 
clusion that variability is generally related to the conditions 
of life to which each species has been exposed during sev- 
eral successive generations. In the first chapter I at- 
tempted to show that changed conditions act in two ways, 
directly on the whole organization or on certain parts alone, 
and indirectly through the reproductive system. In all cases 
there are tw^o 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 
organization 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 subjected 
to certain conditions, and all, or nearly all, the individuaU 
become modified in the same way. 

It is very difficult to decide how far changed conditions, 
such as of climate, food, etc., have acted in a definite 
manner. There is reason to believe that in the course of 
time the effects have been greater tlian can be j^roved by 
clear evidence. But we may safely conclude that the 
innumerable complex co-adaptations of structure, wliich 
we see throughout nature between various organic beings, 
cannot be attributed simply to such action. In tlie follow- 
ing 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 colored 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 colored under a clear 
atmosphere, than when living near the coast or on islands; 
and Wollaston is convinced that residence near the sea 
affects the colors 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 analogous to those pos- 
sessed by the species which are confined to similar condi- 

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 further nortli they live; but who can 
tell how much of this difference may be due to the warmest; 
clad individuals having been favored 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 pro- 
duced from the same species under external conditions 
ot life as different as can well be conceived; and, on th« 


other hand, of dissimilar varieties being produced under 
apparently the same external conditions. Again, innumer- 
able instances are known to every naturalist, of species 
keeping true, or not varying at all, although living under 
the most opposite climates. Such considerations as these 
incline me to lay 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 like- 
wise 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 accumu- 
lates the variations in certain direction; and it is this latter 
agency which answers to the survival of the fittest under 


From the facts alluded to in the first chapter, I think 
there can be no doubt that use in our domestic animals has 
strengthened and enlarged certain parts, and disuse dimin- 
ished 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 inliabits con- 
tinents, and is exposed to danger from which it cannot 
escape by flight, but it can defend itself, by kicking its 
enemies, as efticiently as many quadrupeds. We may 
believe that the progenitor of the ostrich genus liad 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 be- 
came incapable of flight. 

Kirby has remarked (and I have observed the same fact) 
that the anterior tarsi, or feet, of many male dung-feeding 
beetles are often broken off; he examined seventeen speci- 
mens 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 Egyp- 
tians, they are totally deficient. The evidence that acci- 
dental mutilations can be inherited is at present not de- 
cisive; 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 con- 
dition in some other genera, not as cases of inherited mu- 
tilations, 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 duo 
to natural selection. Mr. Wollaston has discovered the 
remarkable fact that 200 beetles, out of the 550 species (but 
more are now known) inhabiting Maderia, are so far defi- 
cient 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 fre- 
quently blown to sea and perish; that the beetles in Maderia, 
as observed by Mr. Wollaston, lie much concealed, until 
the wind lulls and the sun shines; that the proportion of 


wingless beetles is larger on the exposed Desertas than in 
Maderia 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 considerations make me be- 
lieve that the wingless condition of so many Maderia 
beetles is mainly due to the action of natural selection, 
combined probably with disuse. For during many succes- 
sive generations each individual beetle which flew least, 
either from its wings having been ever so little less per- 
fectly developed or from indolent habit, will have had the 
best chance of surviving from not being blown out to sea; 
and, on the other hand, those beetles which most readily 
took to flight would oftenest have been blown to sea, and 
thus destroyed. 

The insects in Maderia 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 com- 
patable 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 shipwrecked 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 
swimmers if they had not been able to swim at all and had 
stuck to the wreck. 

The eyes of moles and of some burrowing rodents are 
rudimentary in size, and in some cases are quite covered 
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 con- 
clition^ the cause, as appeared on dissection, having been 


inflammation of the nictitating membrane. As frequent 
inflammation of the eyes must be injurious to any animal, 
and as eyes are certainly not 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 Kentucky, are blind. In some of the crabs the 
foot-stalk for the eyes 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 imag- 
ine 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, tlie 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 ex- 
posed for about a month to a graduated light, acquired a 
dim perception of objects. 

It is difficult to imasfine 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 ani- 
mals having been separately created for the American and 
European caverns, very close similarity in their organiza- 
tion and affinities might have been expected. This is cer- 
tainly 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 ordi- 
nary powers of vision, slowly migrated by successive gener- 
ations from the outer world into the deeper and deeper re- 
cesses of the Kentucky caves, as did European animals into 


the caves of Europe. "We have some evidence of this gra- 
dation of habit; for, as Schiodte remarks: " We accord- 
ingly look upon the subterranean faunas as small ramifica- 
tions which have penetrated into the earth from the geo- 
graphically limited faunas of the adjacent tracts, and 
which, as they extended themselves into darkness, have 
been accommodated to surrounding circumstances. Ani- 
mals not far remote from ordinary forms, prepare the tran- 
sition 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 elfected other 
changes, such as an increase in the length of the 
antennae or palpi, as a compensation for blindness. 
Notwithstanding such modifications, we might expect still 
to see in the cave-animals of America, affinities to the other 
inhabitants of that continent, and in those of Europe to 
the inhabitants of the European continent. And this is the 
case with some of the American cave-animals, as I hear from 
Professor Dana; and some of the European cave-insects are 
very closely allied to those of the surrounding country. It 
would be difficult to give any rational explanation of the 
affinities of the blind cave-animals to the other inhabit- 
ants 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 sliould be closely related, 
we might expect from the well-known relationship 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 prob- 
ably had no relation to its dark habitation; for it is natu- 
ral that an insect already deprived of vision should readily 
become adapted to dark caverns. Anotlier blind genus 
(Anophthalmus) offers his 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, while 
they were furnished with eyes, may formerly have ranged 
over both continents, and then have become extinct, ex- 
cepting 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 bliird 
fish, the Amblyopsis, and as is the case with the blind 
Proteus, with reference to the reptiles of Europe, I am only 
surprised that more wrecks of ancient life have not been 
preserved, owing to the less severe competition to which 
the scanty inhabitants of these dark abodes will have been 


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, etc., and this leads me to 
say a few words on acclimatization. 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, acclimatization must be readily effected dur- 
ing 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 
thev live is often overrated. We mav infer this from our 
frequent inability to predict whether or not an imported 
plant will endure our climate, and from the number of 
plants and animals brought from 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 not this adaptation is in most cases very 
close, we have evidence with some few plants, of their be- 
coming, to a certain extent, naturally habituated to differ- 
ent temperatures; that is, they become acclimatized: thus 
the pines and rhododendrons, raised from seed collected 


by Dr. Hooker from the same species growing at different 
heights on the Ilimahiyas, 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. 0, Watson on European species of flants 
brought from the Azores to England; and I could give 
other cases. In regard to animals, several autbeiitic 
instances could be adduced of species having h.rgely 
extended, within historical times, their range f rom wiiinier 
to colder latitudes, and conversely; but w^e do not posi- 
tively know that these animals were strictly adapted to 
their native climate, though in all ordinary casos we 
assume such to be the case; nor do we know that they 
have subsequently become specially acclimated 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 origin- 
ally chosen by uncivilized 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 perfectl} fertile 
(a far severer test) under them, may be used as an argument 
that a large proportion of other animals liow in a state of 
nature could easily be brought to bear widely dilferent 
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 per- 
haps 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 quality readily grafted on an innate 
wide flexibility of constitution, common to most animals. 
On this view, the capacity of enduring the most dilferent 
climates by man himself and by his domestic animals, and 


the fact of the extinct elephant and rhinoceros liavin^ for- 
merly endured a glacial climate, whereas the living species 
are now all tropical or sub-tropical in their habits, ought 
not to be looked at as anomalies, but as examples of a vtrv 
common flexibility of constitution^ brought, under pecuhur 
circumstances, into action. 

How much of the acclimatization of species to any 
peculiar climate is due to mere habit, and how much to the 
natural selection of varieties having different innate con- 
stitutions, and how much to both means combined, is an 
obscure question. That habit or custom has some influ- 
ence, I must believe, both from analogy and from tlie in- 
cessant advice given in agricultural works, even in the 
ancient Encyclopedias of China, to be very cautious in 
transporting animals from one district to another. And 
as it is not likely that man should have succeeded in select- 
ing 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 selec- 
tion would inevitably tend to preserve those individuals 
which were born with constitutions best adapted to any 
country whicli they inhabited. In treatises on many kinds 
of cultivated plants, certain varieties are said to with- 
stand certain climates better than others; this is strik- 
ingly shown in works on fruit-trees published in the United 
States, in which certain varieties are habitually recom- 
mended for the northern and others for the southern states; 
and as most of these varieties are of recent origin, they can 
not 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 prov- 
ing that acclimatization 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 some one will sow, during a score 
of generations, his kidney-beans so |early tliat a very large 
proportion are destroyed by frost, and then collect seed 
from the few survivors, with care to prevent accidental 
crosses, and then again get seed from these seedlings, with 
the same precautions, the experiment cannot be said to 
have been tried. Nor let it be supposed that differences ia 


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, Ave 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 


I mean by this expression that the whole organization is 
so tied together, during its growth and development, that 
when slight variations in any one part occur and are accu- 
mulated through natural selection, other parts become 
modified. This is a very important subject, most 
imperfectly understood, and no doubt wholly different 
classes of facts may be here easily confounded together. 
We 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 larvae naturally tend to affect the structure of 
the mature animal. The several parts which are homo- 
logous, and which, at an early embryonic period, are 
identical in structure, and which are necessarily 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 per- 
manent 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 homolo- 
gous parts in normal structures, 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 tlieir 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. 
I The nature of the bond is frequently quite obscure. M. Is. 
I Geoffroy St. Hilaire has forcibly remarked that certain mal- 
'conformations frequently, and that others rarely, coexist 
without our being able to assign any reason. What can 
be more singular than the relation in cats between com- 
plete whiteness and blue eyes with deafness, or between 
the tortoise-shell color 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 color 
of its plumage; or again, the relation between the hair and 
the 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, etc.), 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 umbelliferous plants. Everyone 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 in- 
volucra on the florets, or to their mutual pressure, and the 
shape of the seeds in the ray florets of some composite 


lC3 l,> \ 

countenances this idea; but with the umbellifera3 it u 
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 composita? 
the seeds of the outer and inner florets difl'er, without any 
difference in the corolla. Possibly these several differ- 
ences may be connected with the different flow of nutri- 
ment toward 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 upx)er petals in the central flower of the truss often 
lose their patches of darker color; and when this occurb, 
the adherent nectary is quite aborted, the central flower 
thus becoming peloric or regular. When the color 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, Spren- 
geFs idea that the ray-florets serve to attract insects, whose 
agency is highly advantageous, or necessary for the fertili- 
zation of tliese plants, is highly probable; and if so, nat- 
ural 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 um- 
belliferge these differences are of such apparent importance 
— the seeds being sometimes orthospermous in the exterior 
flowers and co^lospermous 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 systematists 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 

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 inhericance; 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, huvin^^ been 
transmitted to a whole group of descendants witli 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 impossibihty 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. 


The elder Geoff roy 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 economize on the other side.'" 
I think this holds true to a certain extent with our domes- 
tic productions: if nourishment flows to one part or organ 
in excess, it rarely flows, at least in excess, to another part; 
thus it is difficult to get a cow to give much milk and to 
fatten readily. The same varieties of the cabbage do not 
yield abundant and nutritious foliage and a copious supply 
of oil-bearing seeds. When the seeds in our fruits become 
atrophied, the fruit itself gains largely in size and quality. 
In our poultry, a large tuft of feathers on the head is gen- 
erally accompanied by a diminished comb, and a large 
beard by diminished wattles. With species in a state of 
nature it can hardlv be maintained that the law is of uni- 
versal application; but many good observers, more espe- 
cially botanists, believe in its truth. I will not, however, 
here give any instances, for I see hardly any way of distin- 
guishing between the effects, on the one hand, of a part 
being largely developed through natural selection and 
another and adjoining part being reduced by the same pro- 
cess or by disuse, and, on the other hand, the actual with- 
drawal of nutriment from one part owing tc the excess of 
growth in another and adjoining part. 


I suspect, also, that some of the cases of compensation 
which have been advanced, and likewise some other facts, 
may be merged under a more general principle, namely, 
that natural selection is continually trying to econo- 
mize every part of the organization. If under changed 
conditions of life a structure, before useful, becomes less 
useful, its diminution will be favored, for it will profit the 
individual not to have its nutriment wasted in building up 
a useless structure. I can thus only understand a fact with 
which I 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 Proteo- 
lepas: 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 Proteole- 
pas, the whole anterior part of the head is reduced to the 
merest rudiment attached to the bases of the prehensile 
antennae. Now the saving of a large and complex struc- 
ture, when rendered superfluous, would be a decided ad- 
vantage 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 organization, as soon as it 
becomes, through changed habits, superfluous, without by 
any means causing some other part to be largely devel- 
oped in a corresponding degree. And conversely, that 
natural selection may perfectly well succeed in largely de- 
veloping an organ without requiring as a necessary com- 
pensation the reduction of some adjoining part. 



It seems to be a rule, as remarked by Is. Geoffrey 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 pol3'androu8 
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 re- 
marked that multiple parts are extremely liable to vary 
in structure. As ''vegetable repetition/' to use Professor 
Owen's expression, is a sign of low organization, the fore- 
going statements accord with the common opinion of natu- 
ralists, that beings which stand low in the scale of nature 
are more variable than those which are higher. I presume 
that lowness here means that the several parts of the 
organization have been but little specialized for particular 
functions; and as long as the same part has to perform 
diversified work, we can perhaps see why it should remain 
variable, that is, why natural selection should not have 
preserved or rejected each little deviation of form so care- 
fully as when the part has to serve for some one special 
purpose. In the same way that a knife which has to cut 
all sorts of things may be of almost any shape; while 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 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 re- 
sult from their uselessness, and consequently from natural 
selection having had no power to check deviations in their 


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 bo 
here introduced. I can only state my conviction that it is 
a rule of high generality. I am aware of several causes of 


error, but I hope that I have made due allowances fot 
them. It should be understood that the rule by no means 
applies to any part, hov/ever unusually developed, unless 
it be unusually developed in one species or in a lew species 
in comparison with the same part in many closely allied 
species. Thus, the wing of the 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 con- 
nected 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 char- 
acters is clearly shown in the case of hermaphrodite cirri- 
pedes; I particularly attended to Mr. Waterhouse^s remark, 
while investigating this order, and I am fully convinced 
that the rule almost always holds good. I shall, in a 
future work, give a list of all the more remarkable cases. 
I will hero give only one, as it illustrates the rule in its 
largest application. Tiie opercular valves of sessile cirri- 
pedes (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 belonging to other distinct genera. 
As with birds the individuals of the same species, in- 


lial3iting the same country, vary extremely little, I h:iv© 
particularly attended to them; and the rule certainly seems 
to hold good in this class. I cannot make out that it 
appHes to plants, and this would have seriously sliaken mr 
belief in its truth, had not the great variability in plants 
made it particularly difficult to compare their relative de- 
grees of variability. 

When we see any part or organ developed in a remark- 
able degree or manner in a species, the fair presumption ia 
that it is of high importance to that species: nevertheless 
it is in this case eminently liable to variation. AVhy 
should this be so? On the view that each species has been 
independently created, with all its parts as we now see 
them, I can see no explanation. But on tlie view that 
groups of species are descended from some other species, 
and have been modified through natural splection, I think 
we can obtain some light. First let me make some pre- 
liminary remarks. If, in our domestic animals, any part 
or the whole animal be neglected, and no selection be ap- 
plied, 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 degeneraung. 
In rudimentary organs, and in those which have been but 
little specialized for any particular purpose, and perhaps 
in polymorphic groups, we see a nearly parallel case; for in 
such cases natural selection either has not or cannot have 
come into full play, and thus the organization 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 difference there is in 
the beaks of tumblers, in the beaks and wattle of carriers, 
in the carriage and tail of fantails, etc., these being the 
points now mainly attended to by Englisli fanciers. Even 
in the same sub-breed, as in that of the short-faced tumbler, 
it is notoriously 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 tho 
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 power of steady selection to keep the 
breed true. In the long run selection gains the day, 
and we do not expect to fail so 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 ra^oidly 
going on, much variability in the parts undergoing modi- 
fication 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 geolog- 
ical period. An extraordinary amount of modification 
implies an unusually large and long-continued amount of 
variability, which has continually been accumulated by 
natural selection for the benefit of the species. But as the 
variability of the extraordinarily developed part or organ 
has been so great and long-continued^ within a period 
not excessively remote, we might, as a general rule, still 
expect to find more variability in such parts than in 
other parts of the organization which have remained for 
a much longer period nearly constant. And this, I am 
convinced, is the case. That the struggle between nat- 
ural selection on the one hand, and the tendency to rever- 
sion 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 tlie 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 extraordinarily great that we 
ought to find the generative variability, as it may be 
called, still present in a high degree. For in this case the 
variability will seldom as yet have been fixed by the con- 
tiuued selection of the individuals varying in the required 


manner and degree, and by the continued rejection of tlioHe 
tending to revert to a former and less modified condition. 



The principle discussed under the last heading may be 
applied to our present subject. It is notorious that spe- 
cific characters are more variable than generic. 'Vo ex- 
plain by a simple example what is meant: if in a large 
genus of plants some species had blue flowers and some 
had red, the color would be only a specific character, and 
no one would be surprised at one of the blue species vary- 
ing into red, or conversely; but if all the species had blue 
flowers, the color would become a generic character, and 
its variation would be a more unusual circumstance. I 
have chosen this example because the explanation which 
most naturalists would advance is not here applicable, 
namely, that specific characters are more variable than gen- 
eric, 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 ordi- 
nary specific characters are more variable than generic; 
but with respect to important characters, I have repeatedly 
noticed in works on natural history, that when an author 
remarks with surprise that some im.portant organ or part, 
which is generally very constant throughout a large group 
of species, differs considerably 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 gen- 
eric value, when it sinks in value and becomes only of spe- 
cific value, often becomes variable, though its i)hysiol()gical 
importance may remain the same. Something of the same 
kind applies to monstrosities: at least Is. Gcoft'roy 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 individ- 

On the ordinary view of each species having been inde- 


pendently created, why should that part of the struc- 
ture, 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 struc- 
ture 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 char- 
acters may be attributed to inheritance from a common 
progenitor, for it can rarely have happened that natural selec- 
tion will have modified several distinct species, fitted to 
more or less widely different habits, in exactly the same 
manner: and as those so-called generic characters have been 
inherited from before the period when the several species 
first branched off from their common progenitor, and sub- 
sequently have not varied or come to differ in any degree, 
or only in a slight degi'ee, 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 stili often be in 
some degree variable — at least more variable than those 
parts of the organization which have for a very long period 
remained constant. 


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 
organization: compare, for instance, the amount of differ- 
ence between the males of gallinaceous birds, in which 
secondary sexual characters are strongly displayed, with 


the tamount of difference between the fetnales. Tlie c;ins«^ 
of the original variability of these cliaracters is not mani- 
fest; but we can see why they should not liave been ren- 
dered as coQstant and uniform as otliers, for tliev are ac- 
cumulated by sexual selection, which is less rigid in ita 
action than ordinary selection, as it does not entail death, 
but only gives fewer offspring to the less favored males. 
Whatever the cause may be of the variability of secondary 
sexual characters, as they are highly variable, sexual selec- 
tion 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 

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 organization in 
which the species of the same genus differ from each other. 
Of this fact I will give in illustration the two first in- 
stances which happen to stand on my list; and as the dif- 
ferences 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 Engid^e, as West- 
wood 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 neu ra- 
tion of the wings is a character of the highest importance, 
because common to large groups; but in certain genera the 
neuration dift'ers in the different species, and likewise in 
the two sexes of the same species. Sir J. Lubbock has 
recently remarked, that several mi ante crustaceans offer 
excellent illustrations of this law. *' In Pontella, for 
instance, the sexual characters are afforded mainly by the 
anterior antenna3 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 aud 


sexual selection, in order to fit the 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 females. 

Finally, then, I conclude that the greater variability of 
specific characters, or those which distinguish species from 
species, than of generic characters, or those which are pos- 
sessed by all the species; that the frequent extreme varia- 
bility of any part which is developed in a species in an 
extraordinary manner in comparison vv^ith 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 varia- 
bility 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 of the organization, 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. 


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-varie- 
ties 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 con- 
sidered 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 commonly called roots, of the 
Swedish turnip and ruta-baga, plants which several bot- 
anists 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 maybe 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 conse- 
quent 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 wath insects under nat- 
ural conditions have lately been discussed with much abil- 
ity by Mr. Walsh, w^ho has grouped them under his law of 
equal3le variability. 

With pigeons, however, we have another case, namely, 
the occasional appearance in all the breeds, of slaty-bhie 
birds with two black bars on the wings, white loins, a bar 
at the end of the tail, with the outer feathers externally 
edsred near their basis with white. As all these marks are 
characteristic of the parent rock-pigeon, I presume tiiat 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 con- 
clusion, because, as we have seen, these colored marks are 
eminently liable to appear in tlic crossed oilspring of two 
distinct and differently colored breeds; and in this case 
there is nothing in the external conditions of life to cause 


the reappearance of the slaty-bhie, 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 occa- 
sionally 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 pro- 
portion of blood, to use a common expression, from one 
ancestoi", is only one in 2048; and yet, as we see, it is gen- 
erally 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 hoth 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 gen- 
erations. 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 genera- 
tions, but that in each successive generation the character 
iu question has been lying latent, and at last, under 
unknown favorable conditions, is developed. With the 
barb-i:)igeon, 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 trans- 
mitted. 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 \TOuId probably be of an unim 
portant nature, for the preservation of all fiuictionallv im- 
portant characters will have been determined through 
natural selection, in accordance with the different hiibits 
of the species. It might further be expected thut the 
species of the same genus would occasionally exhibit rever- 
sions to long-lost characters. As, however, we do not 
know the common ancestor of any natural group, we 
cannot distinguish between reversionary and analogous 
characters. If, for instance, we did not know that tlie 
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 varia- 
tions; but we might have inferred that the blue color was 
a case of reversion from the number of the markings, 
which are correlated with this tint, and whicli would not 
probaby have all appeared together from simple variation. 
More especially we might have inferred this from the blue 
color and the several marks so often appearing wlien dif- 
ferently colored 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 assum- 
ing 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 forma, 
which themselves can only doubtfully be ranked as species; 
and this shows, unless all these closely allied forms be con- 
sidered as independently created species, that they have in 
varing assumed some 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 disadvantage 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 tranverse 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 some- 
times very obscure, or actually quite lost, in dark-colored 
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 colors; transverse bars on the legs are not 
rare in duns, mouse duns, and in one instance in a chest- 
nut; 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 

In the northwest part of India the Kattywar breed of 
horses is so generally striped, that, as I hear from Colonel 
Poole, w^ho examined this iDreed for the Indian Govern- 
ment, a horse without stripes is not considered as purely 
bred. The spine is always striped, the legs are generally 
barred, and the shoulder-stripe, which is sometimes double 
and sometimes treble, is common; the side of the face, 
moreover, is sometimes striped. The stripes are often 


plainest in the foal, and sometimes quite disappear in old 
horses. Colonel Poole has seen both gray and bay Katty- 
war horses striped when first foaled. I have also reason to 
suspect, from information given me by Mr. W. \\ . Ed- 
wards, 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 (off- 
spring 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 com- 
pletely. Without here entering on further details I may 
state that I have collected cases of leg and shoulder-stripes 
in horses of very different breeds in various countries from 
Britain to Eastern China, and from Norway in the north 
to the Malay Archipelago in the south. In all parts of the 
world these stripes occur far oftenest in duns and mouse- 
duns. By the term dun a large range of color is included, 
from one between brown and black to a close approach to 
cream color. 

I am aware that Colonel Hamilton Smith, who has writ- 
ten 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 
higlily improbable that the heavy Belgian cart-horse, 
Welsh ponies, Norwegian cobs, the lanky Kattywar race, 
etc., inhabiting the most distant parts of the world, 
should all have been crossed with one supposed aboriginal 

Now let us turn to the effects of crossing the several 
species of the horse genus. Eollin asserts that the com- 
mon 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 colored drawings, which I have seen, of liybridu 


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 produced from 
the same mare by a black Arabian Bire, 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 tliat 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 AVelsh 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 color 
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 
Been, answered in the affirmative. 

What now are we to say to these several facts? 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 coloring 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 niost 
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 (in- 
cluding two or three sub-species or geographical races) of 
a bluish color, with certain bars and other marks; and 
when any breed assumes by simple variation a bluish tint, 
these bars and other marks invariably reappear; but with- 
out any other change of form or character. When the 
oldest and truest breed of various colors are crossed, we see 
a strong tendency for the blue tint and bars and marks 


to reappear in the nK)ngrels. I have stated tliat the 
most probable hypothesis to account for the reappear- 
ance of very ancient characters, is — that there is a 
tendency in the young of each successive generation 
to produce the long-lost character, and that this ten- 
dency, 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 com- 
monly 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 genera- 
tions, 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 independ- 
ently created, will, I presume, assert that each species has 
been created with a tendency to vary, both under nature 
and under domestication, in this particular manner, so as 
often to become striped like 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 cosmo- 
gonists, that fossil shells had never lived, but had been 
created in stone so as to mock the shells living on the sea- 


Our ignorance of the laws of variation is profound. 
Not in one case out of a hundred can we pretend to assign 
any reason why this or that part has varied. But when- 
ever we have the means of instituting a comparison, the 
same laws appear to have acted in producing the lesser dif- 



ferences between varieties of the same species, and the 
greater diffences between species of the same genus. Changed 
conditions generally induce mere fluctuating 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 man- 
ner, and homologous parts tend to cohere. Modifications 
in hard parts and in external parts sometimes affect softer 
and internal parts. AVhen one part is largely developed, per- 
haps it tends to draw nourishment from the adjoining parts; 
and every part of the structure which can be saved without 
detriment 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, undoubtedly occur. Mutiple parts 
are variable in number and in structure, perhaps arising 
from such parts not having been closely specialized 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 organization more specialized. 
Rudimentary organs, from bfjing useless, are not regulated by 
natural selection, and hence ^u'e 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 char- 
acters, 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 
among 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 organization 
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 sizu or in 
an extraordinary manner, in comparison with the same 
part or organ in the allied species, must have gone tlirough 
an extraordinary amount of modilication since the geims 
arose; and thus we can understand why it slioukl 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 an extraordinarily developed organ has become the 
parent of many modified descendants — wliich 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 progeni- 
tors. Although new^ and important modifications may not 
arise from reversion and analogous variation, such modi- 
fications will add to the beautiful and harmonious diver- 
sity of nature. 

Whatever the cause may be of each slight difference be- 
tween 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 difi'crences which has 
given rise to all the more important modifications of struc- 
ture in relation to the habits of each species. 




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 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 

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 ejections 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 
confusion, instead of the species being, as we see them, 
well defined? 

Secondly, is it possible that an animal having, for 
instance, the structure and habits of a bat, could have been 
formed by the modification of some other animal with 
widely different habits and structure? Can we believe that 
natural selection could produce, 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? 

Thirdly, can instincts be acquired and modified through 
natural selection? AVhat shall we say to the instinct which 


leads the bee to make cells, and which has practically 
anticipated the discoveries of profound mathematicians? 

Fourthly, how can we account for species, when crossed, 
"being sterile and producing sterile offspring, wherejvs, 
when varieties are crossed, their fertility is uniin})aired? 

The two first heads will here be discussed; some miscel- 
laneous objections in the following chapter; Instinct and 
Hybridism in the two succeeding chapters. 


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-favored 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 

But, as by this theory innumerable transitional forms 
must have existed, why do we not find them imbedded 
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 onlv state that I believe the anr^wer mainly 
lies in the record being incomparably less perfect than is 
generally supposed. The crust of the earth is a vast 
museum; but 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 tenitory, we surely ougiit to find 
at the present time many transitional forms. Let us take 
a simple case: in traveling from north to south over a con- 
tinent, we generally meet at successive intervals with 
closely allied or representative species, evidently filling 
nearly the same place in the natural economy of the land. 
These representative species often meet and interlock; and 
as the one becomes rarer and rarer, the other becomes more 
and more frequent, till the one replaces the other. But 


if we compare these species where they intermingle, they 
are generally as absolutely distinct from each other in every 
detail of structure as are specimens taken from the metrop- 
olis 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 supplanted and 
exterminated its original parent-form and all the transi- 
tional varieties between its past and present states. Hence 
we ought not to expect at the present time to meet with 
numerous transitional varieties in each region^ though they 
must have existed there, and may be imbedded there in a 
fossil condition. But in the intermediate region, having 
intermediate conditions of life, why do we not now find 
closely linking intermediate varieties? This difficulty for 
a long time quite confounded me. But I think it can be 
in large part explained. 

In the first place we should be extremely cautious in in- 
ferring, 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 liave 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 continu- 
ous 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 
formerlv 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 comparison with the territory proper 
to each. We see the same fact in ascending mountains. 


and sometimes it is quite remarkable how abruptly, a- 
Alph. de CandoUe has observed, a common alpine species 
disappears. The same fact has been noticed by E. Forbes 
in sounding the depths of the st;a \vitii 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 iii 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 inhab- 
itants 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 com- 
petition; and as these species are already dehned 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 sharj^ly defined. Moreover, each 
species on the confines of its range, where it exists in les- 
sened 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 

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 es- 
sentially 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 adai)t two varieties to two 
large areas, and a third variety to a narrow intermediate zone. 
The intermediate variety, consequently, will exist in lesser 
numbers from inhabiting a narrow and lesser area; and 
practically, as far as I can make out, this rule holds good 
with varieties in a state of nature. I have met with strik- 
ing 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 con- 
nect. Now, if we may trust these facts and inferences, 
and conclude that varieties linking two other varieties to- 
gether 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 exter- 
minated and disappear, sooner than the forms which they 
originally linked together. 

For any form existing in lesser numbers would, as already 
remarked, run a greater chance of being exterminated 
than one existing in large numbers; and in this particular 
case the intermediate form would be eminently liable 
to the inroads of closely allied forms existing on both 
sides of it. But it is a far more important considera- 
tion, that during the process of further modification, 
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 pre- 
senting further favorable variations for natural selection to 
seize on, than will the rarer forms which exist in lesser 
numbers. Hence, the more common forms, in the race for 
life, will tend to beat and supplant the less common forms, 
for these will be more slowly modified and improved. It 
is the same principle which, as I believe, accounts for the 
common species in each country, as shown in the second 
chapter, presenting on an average a greater number of well- 
marked varieties than do the rarer species. I may illus- 
trate 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 inhabi- 
tants are all trying with equal steadiness and skill to im- 
prove their stocks by selection; the chances in this case 


will be strongly in favor of the great liolderson the mount- 
ains or on the plains, improving their breeds nioro 
quickly than the small holders on the intermediate narrow, 
hilly tract; and consequently the improved moimiain or 
plain breed will soon take the place of the less improved 
hill breed; and thus the two breeds, which originallv ex- 
isted in greater numbers, will come into close contact' with 
each other, without the interposition of the supplanted, in- 
termediate 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 inextricable chaos of varying and intermediate links: 
first, because new varieties are veiy slowly formed, for vari- 
ation is a slow process, and natural selection can do noth- 
ing nntil favorable individual differences or variations 
occur, and until a place in the natural polity of the coun- 
try can be better filled by some modification of some one 
or more of its inhabitants. And such new places will de- 
pend on slow changes of climate, or on the occasional im- 
migration 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 struc- 
ture in some degree permanent; and this assuredly we do 

Secondly, areas now continuous must often have existed 
within the recent period as isolated portions, in wliich 
many forms, more especially among the classes which 
unite for each birth and wander much, may have sepa- 
rately been rendered sufficiently distinct to rank as repi-e- 
sentative species. In this case, intermediate varieties be- 
tween 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 extern) in- 
ated, so that they will no longer be found in a living state. 

Thirdly, when two or more vai-ieties have been formed 
in different portions of a strictly continuous area, interme- 
diate varieties will, it is probable, at first bave been formed 
in the intermediate zones, but they will generally have liad 


a short duration. For these intermediate varieties will, 
from reasons already assigned (namely from what 
we know of the actual distribution of closely allied 
or representative species, and likewise of acknowl- 
edged 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 at all time, if 
my theory be true, numberless intermediate varieties, link- 
ing 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 among fossil remains, which are preserved, as we 
shall attempt to show in a future chapter, in an extremely 
imperfect and intermittent record. 


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 sub- 
sisted? 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 
vision 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 hiwl been taken, and it 
had been asked how an insectivorous quadruped could pos- 
sibly have been converted into a flying bat, tlie question 
would have been far more difficult to answer. Yet I think 
such difficulties luive little weight. 

Here, as on other occasions, 1 he under a heavy disad- 
vantage, for, out of the many striking cases wliich I liave 
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 difficulty in any par- 
ticular case like that of the bat. 

Look at the family of squirrels; here we have the finest 
gradation from animals with their tails only slightly flat- 
tened, and from others, as Sir J. Richardson lias 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 can- 
not doubt that each structure is of use to each kiiul of 
squirrel in its own country, by enabling it to e^scape birds 
or beasts of pre}^, 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 moditied, 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 become modified and improved 
in structure in a correspondirg 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. 

How look at the Galeopithecus or so-called flying lemur, 


which was formerly ranked among 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 inckides 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 Galeopi- 
thecus with the other Insectivora, yet there is no dif^ 
ficiilty in supposing that such links formerly existed, 
and that each was developed in the same manner 
as with the less perfectly gUding squirrels; each grade 
of structure having been useful to its possessor. Nor 
can I see any insuperable difficulty in further believ- 
ing that the membrane connected fingers and forearm 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 apparatus originally 
fitted for gliding through the air rather than for flight. 

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 hand, 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 necessarially the best possible under all possible con- 
ditions. 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 reptilesj it is conceivable that flying- 


fish, which now glide far through the air, slightly rising 
and turning by the aid of their fluttering lins, might liave 
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, so far as we know, to escape being de- 
voured by other fish? 

When we see any structure highly perfected for any par- 
ticular habit, as the wings of a bird for flight, we should 
bear in mind that animals displaying early transitional 
grades of the structure will selclom 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 con- 
clude 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 sub- 
ordinate forms. Thus, to return to our imaginary illus- 
tration 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 perfec- 
tion, 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 selec- 
tion 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 afterward; or 
whether slight modifications of structure lead to changed 
habits; both probably often occurring almost simulUne- 
ouslv. Of cases of changed habits it will suffice mei-eiy to 
allude to that of the many British insects which now teed 
on exotic plants, or exclusively on artificial hubbtancee. 


Of diversified habits innumerable instances could be given: 
I have often watclied a tyrant flycatcher (Saurophagus sul- 
phuratus) 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 nut- 
hatch. 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 differ- 
ent from those proper to their species and to the other 
s.pecies of the same genus, we might expect that such in- 
dividuals 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 two 
toes before and two behind, a long-pointed tongue, pointed 
tail-feathers, sufficiently stiff to support the bird in a verti- 
cal position on a post, but not so stiff as in the typical wood- 
peckers, 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 wood- 
pecker. Even in such trifling characters as the coloring, 
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 ob- 
servations, 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 boles in tbe trunk for its nest. 1 may 
mention as anotber illustration of tbe varied babits of tbis 
genus, tbat a Mexican Colaptes lias been described by De 
Saussure as boring boles into bard wood in order to lay up a 
store of acorns. 

Petrels are tbe most aerial and oceanic of birds, but, 
m tbe quiet sounds of Tierra del Fuego, tbe Puffinuria 
berardi, in its general babits, in its astonisbing power of 
diving, in its manner of swimming and of flying wben 
made to take fligbt, would be mistaken by any one 
for an auk or a grebe; nevertbeless it is essentially a 
petrel, but witb many parts of its organization pro- 
foundly modified in relation to its new babits of life; 
fs^bereas tbe woodpecker of La Plata bas bad its structure 
only sligbtly modified. In tbe case of tbe water-ouzel, 
tbe acutest observer, by examining its dead body, would 
never bave suspected its sub-aquatic babits; yet tbis bird, 
wbicb is allied to tbe tbrusb family, subsists by diving — 
using its wings under water, and grasping stones witb its 
feet. All tbe members of tbe great order of Hymenopter- 
ous insects are terrestrial, excepting tbe genus Procto- 
trupes, wbicb Sir Jobn Lubbock bas discovered to be 
acquatic in its babits; it often enters tbe water and dives 
about by tbe use not of its legs but of its wings, and re- 
mains as long as four bours beneatb tbe surface; yet it 
exbibits no modification in structure in accordance witb 
its abnormal babits. 

He wbo believes tbat eacb being bas been created as we 
now see it, must occasionally bave felt surprise wben be 
has met witb an animal having babits and structure not in 
asfreement. What can be plainer than that tbe webbed 
feet of ducks and geese are formed for swimming? Yet 
tbere are upland geese witb webbed feet wbicb rarely go 
near tbe water; and no one, except Audubon, bas seen 
tbe frigate-bird, wbicb bas all its four toes webbed, aligbt 
on tbe surface of tbe ocean. On tbe otber band, grebes 
and coots are eminently aquatic, although tbeir toes are 
only bordered by membrane. Wliat seems plainer tban 
tbat the long toes, not furnished witb membrane of tbo 
Grallatores, are formed for walking over swamps and float- 
ing plants? The water-hen and landrail are members of 


this order, yet the first is nearly as aquatic, as the coot, and 
the second is nearly as terrestrial as the quail or partridge. 
In such cases, and many others could be given, habits have 
changed without a corresponding change of structure. The 
webbed feet of the upland goose may be said to have be- 
come almost rudiuientary in function, though not in struc- 
ture. In the frigate-bird, the deeply scooped membrane 
between the toes shows that structure has begun to change. 
He who believes in separate and innumerable acts of 
creation 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 the fact in dignified language. He who believes in 
the struggle for existence and in the principle of natural 
selection, will acknowledge that every organic being is con- 
stantly endeavoring to increase in numbers; and that if 
any one being varies ever so little, either in habits or struc- 
ture, and thus gains an advantage over some other inhab- 
itant 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 thrushea 
and diving Hymenoptera, and petrels with the habits of 


To suppose that the eye wath all its inimitable contri- 
vances 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 com- 
mon sense of mankind declared the doctrine false; but the 
old saying of Vox 2^opuli, vox Dei, as every philosopher 
knows, can not be trusted in science. Reason tells me, 
that if numerous gradations from a simple and imperfect 

OHQANS of extreme perfection. 171 

eye to one complex and perfect can be shown to exist, each 
grade being useful to its possessor, as is certaitily the case; 
if further, the eye ever varies and the variations be in- 
herited, as is likewise certainly the case; and if such varia- 
tions should be useful to any animal under changing con- 
ditions of life, then the difficulty of believing that a per- 
fect and complex eye could be formed by natural selection, 
though insuperable by our imagination, should not be con- 
sidered as subversive of the theorv. 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 low- 
est organisms in which nerves can not 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 in- 
cidentally throw light on the steps by which it has been 

The simplest organ which can be called an eye consists 
of an optic nerve, surrounded by pigment-cells and covered 
by translucent skin, but without any lens or other refract- 
ive 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 pig- 
ment which surrounds the nerve are filled, as described by 
the author just quoted, with transparent gelatinous matter, 
projecting with a convex surface, like tiie cornea in the 
higher animals. He suggests that this serves not to form 
an image, but only to concentrate the luminous rays and 


render their perception more easy. In this concentration 
of the rays we gain the first and hy far the most important 
step toward the formation of a true, picture-forming eye; 
for we have only to place the naked extremity of the optic 
nerve, which in some 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 some- 
times forming a sort of pupil, but destitute of lens or 
other optical contrivance. With insects it is now known 
that the numerous facets on the cornea of their great com- 
pound 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 comparison 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 
Articulata 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 len 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 curva- 
ture will 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 re- 
marked, " The range of gradation 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 accumula- 
tion 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 marvellous 
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 selec- 
tion to so startling a length. 

It is scarcely possible to avoid comparing the eye with 
a telescope. We know that this instrument has been per- 
fected by the long-continued efforts of the highest human 
intellects; and we naturally infer that the eye has been 
formed by a somewhat analogous process. But may not 
this inference be presumptuous? Have we any right to 
assume that the Creator works by intellectual powers like 
those of man? If we must compare the eye to an optical 
instrument, we ought in imagination to take a thick layer 
of transparent tissue, with spaces filled with fluid, and 
with a nerve sensitive to light beneath^ and then suppose 


every part of this layer to be continually changing slowly 
in density, so as to separate into layers of different densi- 
ties 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, 
represented 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 degree, 
tends to produce a distincter image. We must suppose 
each new state of the instrument to be multiplied by the 
million; each to be preserved until a better one is pro- 
duced, and then the old ones to be all destroyed. In 
living bodies, variation will cause the slight alteration, 
generation will multiply them almost infinitely, and natural 
selection will pick out with unerring skill each improve- 
ment. Let this process go on for millions of years; and 
during each 5^ear on millions of individuals of many kinds; 
and may we not believe that a living optical instrument 
might thus be formed as superior to one of glass, as the 
works of the Creator are to those of man? 


If it could be demonstrated that any complex organ ex- 
isted, which could not possibly have been formed by nu- 
merous, successive, slight modifications, my theory would 
absolutely break down. But I can find out no such case. 
No doubt many organs exist of which we do not know the 
transitional grades, more especially if we look to much- 
isolated species, around which, according to the theory, 
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 developed; and in order to discover the 
early transitional grades through which the organ has 
passed, we should have to look to very ancient ancestral 
forms, long since become extinct. 

We should be extremely cautious in concluding that an 
organ could not have been formed by transitional grada- 
tions of some kind. Numerous cases could be given 


among the lower animals of the same organ performing at 
the same time wholly distinct functions; thus in the hirva 
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 selection might specialize, 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 differentlv constructed flowers: and if 
such plants were to produce one kind alone, a great change 
would be effected with comparative suddenness in the 
character of the species. It 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 branchiae that breathe the air dis- 
solved in the water, at the same time that they breathe 
free air in their swim-bladders, this latter organ being 
divided by highly vascular partitions and having a ductus 
pneumaticus for the supply of air. To give another in- 
stance 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 oUiterated. 

The illustration of the swim-bladder in tishes 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 swim-bladder has, also, been worked in as an accessory 
to the auditory organs of certain fishes. All physiologists 
admit that the swim-bladder 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 swim-bladder has actually been converted 
into lungs, or an organ used exclusively for respiration. 

According to this view it may be inferred that all verte- 
brate animals with true lungs are descended by ordinary 
generation from an ancient and unknown prototype, which 
was furnished with a floating apparatus or swim-bladder. 
We can thus, as I infer from Owen's interesting descrip- 
tion of these parts, understand the strange fact that every 
particle of food and drink which we swallow has to pass 
over the orifice of the trachea, with some risk of falling 
intc the lungs, notwithstanding the beautiful contrivance 
by which tho glottis is closed. In the higher Vertebrata 
the branchi^ 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 dis- 
tinct purpose: for instance, Laudois has shown that tho 
wings of insects are developed from the trachea; it is there- 
fore 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 func- 
tion to another, that I will give another instance. Pedun- 
culated 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 Balanid^ or 
sessile cirripedes, on the other hand, have no ovigerous 
frena, the eggs lying loose at the bottom of the sack, 
within the well-inclosed shell; but they have, in the same 
relative position with the frena, large, much-folded mem- 
branes, which freely communicate with the circulatory 


lacunae of the sack and body, and which have been consid- 
ered by all naturalists to act as branchiae. Now 1 tliink 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. Tlierefore 
it need not be doubled that the two little folds of skin, 
which originally served as ovigerous frena, but which, like- 
wise, 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 obliteratii^n 
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 im- 
agined that the branchiae in this latter family had origin- 
ally existed as organs for preventing the ova from being 
washed out of the sack? 

There is anotlier possible mode of transition, namely, 
through the acceleration or retardation of the period of re- 
production. This has lately been insisted on by Professor 
Cope and others in the United States. It is now known 
that some animals are capable of reproduction at a very 
early age, before they have acquired their perfect charac- 
ters; and if this power became thoroughly well developed 
in a species, it seems probable that the adult stage of devel- 
opment 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 aiul moi-e 
branched, and the plumes of some birds become more finely 
developed, as they grow older. Professor Cope states that 
the teeth of certain lizards change much in shajie with ad- 
vancing years. With crustaceans not only many trivial, 
but some important parts assume a new character, as re- 
corded by Fritz Miiller, 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 com- 
paratively 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 be- 
tween the mature and the old, were primordially acquired 
by graduated steps. 



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 occur. 

One of the most series is that of neuter insects, which 
are often differently constructed from either the males or 
fertile females; but tliis case will be treated of in the next 
chapter. The electric organs of fishes offer another case 
of special difficulty, 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 defense, and perhaps for secur- 
ing prey; yet in the ra,y, 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. E. McDonnell 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 tor- 
pedo. It is generally admitted that there exists between 
these organs and ordinary muscle a close analogy, in inti- 
mate 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 con- 
traction is accompanied by an electrical discharge; and, as 
Dr. Eadcliffe 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 diinng the rest, and the discharge of the torpedo, 
instead of being peculiar, may be only another form of the 
discharge which attends npon the action of muscle and 
motor nerve." Beyond this we cannot at present go in tlie 
way of explanation; but as we know so little about tlie uses 
of these organs, and as we know notliing 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 mem- 
bers 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 the belief that most fishes for- 
merly possessed electric organs, which their modified 
descendants have now lost. But when we look at the sub- 
ject more closely, we find in the several fishes provided with 
electric organs, that these are situated in different parts of 
the body, that they differ in construction, as in the 
arrangement of the plates, and, according to Pacini, in the 
process or means by which the electricity is excited — and 
lastly, in being supplied with nerves proceeding from dif- 
ferent sources, and this is j^erhaps the most important of 
all the differences. Hence in the several fishes furnished 
with electric organs, these cannot be considered as hom- 
ologous, 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, aris- 
ing 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 widely different families, and which are sit- 
uated in diiferent parts of the body, offer, under our pres- 
ent 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 curious contriv- 
ance of a mass of pollen-grains, borne on a foot-stalk with 
an adhesive gland, is apparently the same in Orchis and 
Asciepias, genera almost as remote as is possible among 
flowering plants; but here again the parts are not 
homologous. In all cases of beings, far removed from 
each other in the scale of organization, which are fur- 
nished 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 Gephalopods or cuttle-fish and of vertebrate ani- 
mals appear wonderfully alike; and in such widely sun- 
dered groups no part of this resemblance can be due 
to inheritance from a common progenitor, Mr. Mivart 
has advanced this case as one of special difficult}^, 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 re- 
semblance, there is hardly any real similarity between the 
eyes of cuttle-fish and vertebrates, as may be seen by con- 
sulting 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 
wnat occurs in the vertebrata. The retina is wholly dif- 
ferent, with an actual inversion of the elemental parts, 
and with a large nervous ganglion included within the 
membranes of the eye. Tlie relations of the muscles 
are as different as it is possible to conceive, and so in 
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 Cephalopoda 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 ad- 
mitted 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 ac- 
cordance with this view of their manner of formation. 
As two men have sometimes independently hit on tlie 
same invention, so in the several foregoing cases it appears 
that natural selection, working for the good of eacli being, 
and taking advantage of all favorable variations, has pro- 
duced 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 fev/ species, possessing an air-breathing appara- 
tus 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 systems, in the position of 
the tufts of hair within their complex stomachs, and lastly in 
the whole structure of the water-breathing branchia3, even to 
the microscopical hooks by which they are cleansed. Hence 
it might have been expected that in the few species belong- 
ing to both families which live on the land, the equally 
important air-breathing apparatus would have been the 
same; for why should this one apparatus, given for the 
same purpose, have been made to differ, while 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 improb- 
able 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 intelligi- 
ble, and might even have been expected, on the supposition 
that species belonging to distinct families had slowly be- 
come adapted to live more and more out of water, and to 
breathe the air. For these species, from belonging to dis- 
tinct 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 surrounding 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 hypothesis 
of separate acts of creation the whole case remains unintel- 
ligible. This line of argument seems to have had great 
weiglit in leading Fritz Mliller to accept the views main- 
tained by me in this volume. 

Another distinguished zoologist, the late Professor Cla- 
parede, has argued in the same manner, and has arrived at 
the same result. He shows that there are parasitic mites 
(Acarid^), belonging to distinct sub-families and families, 
which are furnishes 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 grou23S they are formed by the modification of the 
fore legs, of the hind legs, of the maxillae or lips, and 
of appendages on the under side of the hind part of the 

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 throughout nature that the same end should 
be gained, even sometimes in the case of closely related be- 
ings, by the most diversified means. How differently con- 
structed 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 con- 
Btructed, from the long row of neatly interlocking teeth 
in a Nucula to the simple ligament of a Mussel! Seeds 
are disseminated by their minuteness, by tlieir 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 conspicu- 
ously colored, 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 pluoes, as different in 
shape as they are elegant in structure, so as to be wafted 
by every breeze. I will gi^e one other Instance: for this 
subject of the same end being gained by the most diversi- 
fied means well deserves attention. Some authors main- 
tain 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 fertilization. 
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 sim- 
plest 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 droi:)s of nectar, 
and is consequently visited by insects; and these carry the 
pollen from the anthers to the stigma. 

From this simple stage we may pass through an inex- 
haustible number of contrivances, all for the same pur- 
pose and effected in essentially the same manner, but en- 
tailing changes in every part of the flower. The nectar 
may be stored in variously shaped receiDtacles, with the 
stamens and pistils modified in many ways, sometimes 
forming trap-like contrivances, and sometimes capable of 
neatly adapted movements through irritability or ehisticity. 
From such structures we may advance till we come to 
such a case of extraordinary adaptation as that lately de- 
scribed by Dr. Criiger in the Coryanthes. This orchid lias 
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 cham- 
ber 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 gigan- 
tic 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 con- 
tact with the viscid stigma, and adheres to it, and the 
flower is fertilized. 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 l3ees from flyins: 
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 Cor3^anthes, in order to gnaw 
the labelhim; in doing this they inevitably touch a long, 
tapering, sensitive projection, or, as 1 have called it, the 


antenna. ^ This antenna, when touched, transmits a sensa- 
tion or vibration to a certain membrane which is instantly 
ruptured; this sets free a spring by which tlie pollen- mass 
is shot .^orth, 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 retain the pollen, thus effecting fertilization. 

How, it may be asked, in the foregoing and in innumer- 
able other instances, can we understand the graduated 
scale of complexity and the multifarious means for gaining 
the same end. The answer no doubt is, as already re- 
marked, 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 difticult 
even to conjecture by what transitions organs have ariivcd 
at their present state; yet, considei'ing how small the pro- 
portion of living and known forms is to the extinct and 
unknown, I have been astonished how rarely an organ can 
be named, toward 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 expressed it, ^* Nature is prodigal in variety, but 
niggard in innovation.'' 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 own proper place in nature, be so commonly linked to- 
gether 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 ad- 
vantage of slight successive variations; she can never take 
a great and sudden leap, but must advance by short and 
sure, though slow steps. 



As natural selection acts by life and death, by the sur- 
vival of the fittest, and by the destruction of the less well- 
fitted individuals, 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 

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 color of its 
flesh, the color 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 distribu- 
tion 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 


advantage. It is not that the larger quadrupeds are 
actually destroyed (except in some rare cases) hy flics, but 
they are incessantly harassed and their strength reduced, 
so that they are more subject to disease, or not so well 
enabled in a corning dearth to search for food, or to escape 
from beasts of prey. 

Organs now of trifling importance liave 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 state, 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 subse- 
quently come to be worked in for all sorts of purposes, as a 
fly-flapper, an organ of prehension, or as an aid in turn- 
ing, 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. AVe must by 
no means overlook the effects of the definite action of 
changed conditions of life, of so-called spontaneous varia- 
tions, 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, comprehension, 
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 the sex. But structures 
thus indirectly gained, altliough at first of no advantage to 
a species, may subsequently have been taken advantage of 
by its modified descendants, under new condiiions 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 color was a 
beautiful adaptation 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 color is probably in chief part due to 
sexual selection. A trailing palm in the Malay Archipel- 
ago 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 distribution of the thorn-bearing species 
in Africa and South America, serve as a defense 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 advantage of by the plant, 
as it underwent further modification and became a climber. 
The naked skin on the head of a vulture is generally con- 
sidered 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 indis- 
23ensable for this act: but as sutures occur in the skulls of 
young birds and reptiles, which have only to escape from a 
broken ^gg, 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 of this by reflecting on the differences 
between the breeds of our domesticated animals in differ- 
ent countries, more especially in the less civilized coun- 
tries, where there has been but little methodical selection. 
Animals kept by savages in different countries often have 
to sti'uggle 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 color, as is the liability to be pois- 
oned by certain plants; so that even color would be thus 
subjected to the action of natural selection. Some observ- 
ers are convinced that a damp climate affects the growth 
of the hair, and that with the hair the horns are corre- 
lated. 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 organization 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 unknow^n 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 ignor- 
ance of the precise cause of the slight analogous differences 
between true species. 



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 beautv, 
to delight man or the Creator (but this latter point is be- 
yond 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 an effect, 
probably a great effect, independently of any advantage 
thus gained. But a still more important consideration is 
that the chief part of the organization 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 rela- 
tion 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 can not 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 variations, 
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 de- 


pends 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 en- 
tirely different standard of beauty in their women. If beau- 
tiful 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 siiells of 
the Eocene epoch, and the gracefully sculptured ammonites 
of the Secondary period, created that man might ages 
afterward admire them in his cabinet? Few objects are 
more beautiful than the minute siliceous cases of the 
diatomacese : were these created that they might be ex- 
amined 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 among the most beautiful productions of 
nature; but they have been rendered conspicuous in con- 
trast 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 fertilized by the 
wind it never has a gaily-colored corolla. Several plants 
habitually produce two kinds of flowers; one kind open and 
colored so as to attract insects; the other closed, not 
colored, destitute of nectar, and never visited by insects. 
Hence, we may conclude that, if insects had not been de- 
veloped on the face of the earth, our plants would not have 
been decked with beautiful flowers, but would have pro- 
duced only such poor flowers as we see on our tir, oak, nut 
and ash trees, on grasses, spinach, docks and nettles, 
which are all fertilized 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 tlie 
palate— that the gaily-colored fruit of the spindle-^yood 
tree and the scarlet hemes of the holly are beautifui objects 
—will be admitted by every one. -But this beauty serves 
merely as a guide to birds and beasts, in order that the 
fruit may be devoured and the matured seeds dissem- 
inated. I infer that this is the case from having as yet 
found no exception to the rule that seeds are always thus 


disseminated when imbedded within a fruit of any kind 
(that is witliin a fleshy or pulpy envelope), if it be colored 
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 colored 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 colors and for musical sounds runs 
through a large part of the animal kingdom. When the 
female is as beautifully colored as the male, which is not 
rarely the case with birds and butterflies, the cause ap- 
parently lies in the colors acquired through sexual selec- 
tion 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 colors, forms and sounds — -nvas first developed 
in the mind of man and of the lower animals, is a very ob- 
scure subject. The same sort of difficulty is presented if 
we inquire how it is that certain flavors and odors 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 the nervous system in each species. 

Natural selection cannot possibly produce any modifica- 
tion in a species exclusively for the good of another 
species, though throughout nature one species inces- 
santly 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 
p3,rt of the structure of any one species had been formed 
for the exclusive good of another species, it would anni- 
hilate my theory, for such could not have been produced 


through natural selection. Although many statements 
may be found in works on natural history to this effect, I 
cannot find even one which seems to me of any weiglit. '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 while 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 endeavor 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 pos- 
sessor. 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 ex- 
tinct as myriads have become extinct. 

Natural selection tends only to make each organic being 
^s perfect as, or slightly more perfect than the other in- 
habitants of the same countrv with which it comes into com- 
petition. And we see that this is the standard of perfec- 
tion attained under nature. The endemic productions of 
New Zealand, for instance, are perfect, one compai-ed with 
another; but they are now rapidly yielding before the ad- 
vancing legions of plants and animals introduced from 
Europe. Natural selection will not produce absolute per- 
fection, nor do we always meet, as far as we can judge, 
with this high standard under nature. The correction for 
the aberration of light is said by Miiller not to be perfect 



even in that most perfect organ, the human eye. Hehn- 
holtz, whose judgment no one will dispute, after describ- 
ing in the strongest terms the wonderful powers 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 sen- 
sations. One might say that nature has taken delight in 
accumulating contradictions in order to remove all founda- 
tion 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 con- 
trivances in nature, this same reason tells us, though we 
may easily err on both sides, that some other contrivances 
are less perfect. Can we consider the sting of the bee as 
perfect, which, when used against many kinds of enemies, 
can not be withdrawn, owing to the backward serratures, 
and thus inevitably causes the death of the insect by tear- 
ing 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 inten- 
sified, 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 mem- 
bers. If we admire the truly wonderful jiower 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 utterlv useless to the communitv foj 
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 principles of natural selection. If we admire 
the several ingenious contrivances by which orchids and 
many other plants are fertilized 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? 


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 obsure. 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 pro- 
cess of natural selection implies the continual supplanting 
and extinction of preceding and intermediate gradations. 
Closely allied sj^ecies, now living on a continuous area, 
must often have been formed when the area was not con- 
tinuous, and when the conditions of life did not insensibly 
graduate away from one part to another. When two vari- 
eties 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 inter- 
mediate 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 ex- 
isting in greater numbers, will have a great advantage over 
the less numerous intermediate variety, and will thus gen- 
erally succeed in supplanting and exterminating it. 

We have seen in this chapter how cautious we should be 
in concluding that the most different habits of life could 
not graduate into each other; that a bat, for instance, 
could not have been formed by natural selection from an 
animal which at first only glided through the air. 

1-^a BVMMAItT, 

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 understand, bearing in mind that each or- 
ganic 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 anyone; 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 con- 
ceivable degree of perfection through natural selection. 
In the cases in which we know of no intermediate or transi- 
tional 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 swim-bladder has appar- 
ently 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 special- 
ized for one function; and two distinct organs having per- 
formed at the same time the same function, the one having 
been perfected while aided by the other, must often have 
largely facilitated transitions. 

We liave seen that in two beings widely remote from each 
other in the natural scale, organs serving for the same pur- 
pose 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 throughout nature is infinite 
diversity of structure for gaining the same end; and this 
again naturally follows from the same great principle. 

In many cases we are far too ignorant to be enabled to 
assert that a part or organ is so unimportant for the wel- 
fare of a species, that modifications in its striicture could 
not have been slowly accum';iated by means of natural 
selection. In many other cases, modifications are probably 



the direct result of the laws of variation or of growth, in- 
dependently 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 hi^h 
importance has frequently been retained (as the tail of an 
aquatic animal by its terrestrial descendants), tliough it 
has become of such small importance that it could not, in 
its present state, have been acquired by means of natural 

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 inhabi- 
tants 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 com- 
petition will have been severer, and thus the standard of 
perfection will have been rendered higher. Xatural selec- 
tion 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 under- 
stand the full meaning of that old canon in natural history, 
*' Natura non facit saltum." This canon, if we look to the 
present inhabitants alone of the world, is not strictly cor- 
rect; 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 agreement in structure which we see in 
organic beings of the same class, and which is quite inde- 
pendent 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 illus- 
trious Cuvier, is full}^ embraced by the principle of natural 
selection. For natural selection acts by either now adapt- 
ing the varying parts of each being to its organic and in- 
organic 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 
inheritance of former variations and adaptations, that of 
Unity of Type. 





Longevity — Modifications not necessarily simultaneous — Modifica- 
tions apparently of no direct service — Progressive develop- 
ment — Characters of small functional importance, the most con- 
stant — 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 struct- 
ures — 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 miscellaneous 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 dis- 
guished 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 condi- 
tions; and this is shown to be the case by so many native 
forms in many quarters of the world having yielded tiieir 
places to intruding foreigners. Nor can organic beings, 
even if they were at any one time perfectly adajHed to 
their conditions of life, have remained so, when their con- 
ditions changed, unless they themselves likewise changed; 
and no one will dispute that the physical conditions of 
each country, as well as the number and kinds of its inhab- 
itants, have undergone many mutations. 

A critic has lately insisted, with some parade of mathe- 
matical accuracy, that longevity is a great advantage to all 
species, so that he who believes in natural selection " must 


arrange his genealogical tree '^ in such a manner that all 
the descendants have longer lives than their progenitors! 
Cannot our critics 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 advantages 
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 judg- 
ment, that longevity is generally related to the standard of 
each species in the scale of organization, as well as to the 
amount of expenditure in reproduction 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 
modification 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 con- 
ditions 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 pre- 
served ; but this will be effected only under certain favor- 
able circumstances. 

The celebrated palaeontologist, Bronn, at the close of his 
German translation of this work, asks how, on the prin- 
ciple of natural selection, can a variety live side by side 


with th© parent species? If both have become fitted for 
slightly different habits of life or conditions, tbuy might 
live together; and if we lay on one side pol^morpliic 
species, in which the variability seems to be of a peculiar 
nature, and all mere temporary variations, such as size, 
albinism, etc., the more permanent varieties are generally 
found, as far as lean discover, inhabiting distinct stations, 
such as high land or low laud, 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 
organization should have been modified at the same time 
through 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 vari- 
ations, if slight, first in one part and then in another; and 
as they would be transmitted all together, they would ap- 
pear 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 greyhound and 
mastiff. Their whole frames, and even their mental char- 
acteristics, have been modified; but if we could trace eauli 
step in the history of their transformation — and the hitter 
steps can be traced — we should not see great and simulta- 
neous changes, but first one part and then another slightly 
modified and improved. Even when selection has been ap- 
plied 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 tlie 
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 ciiaracters 


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 multi- 
tude of analogous cases. With respect to plants, this sub- 
ject 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 morphological characters, which aj^pear to be quite 
nnimportant for the welfare of the species. He conse- 
quently believes in an innate tendency toward progressive 
and more perfect development. He specifies the arrange- 
ment 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 dissemina- 
tion, etc. 

There is much force in the above objection. Neverthe- 
less, 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 diminished flow of nutri- 
ment to a part, mutual pressure, an early developed 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 under- 
stand. 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 conditions of life, and for so-called spontaneous 
variations, in which the nature of the conditions appar- 
ently 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 spon- 
taneous 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 indi- 
vidual 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 moditied. 

In the earlier editions of this work I underrated, as it 
now seems probable, the frequency and importance of 
modifications due to spontaneous variability. But it is im- 
possible to attribute to this cause the innumerable struct- 
ures 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 understood, ex- 
cited so much surprise in the minds of the older natural- 
ists, 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 struct- 
ures exist, which are so highly developed 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. Schobl, 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 unim})or- 
tant. 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 Niigeli's 
essay I shall confine myself in the following remarks, it 
will be admitted that the flowers of the orchids present a 
multitude of curious structures, which a few years ago 
would have been considered as mere morphological differ- 
ences without any special function; but they are now 
known to be of the highest importance for the fertilization 
of the species through the aid of insects, and have prob- 


ably been gained through natural selection. No one until 
lately would have imagined that in dimorphic and tri- 
morphic 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 others the lower ones alone are fertilized; and he sug- 
gests 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 the selection of any 
slight deviations in position which favored their fertiliza- 
tion, and the production of seed. 

Several plants belonging to distinct orders habitually 
produce flowers of two kinds — the one open, of the ordi- 
nary 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 pro- 
cess 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 ex- 
penditure 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 columnae five of the alternate sta- 
mens are rudimentary; and in some species of Viola three 
stamens are in this state, two retaining their proper func- 
tion, 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 modi- 


fied, for the five stamens whicli stand opposite to the sepals 
are all aborted, a sixth stamen standing opposite to a petal 
being alone developed; and tiiis stamen is not present in 
the ordinary flowers of this species; the style is aborted; 
and the ovaria are reduced from three to two. Now al- 
though natural selection may well have had the power to 
prevent some of the flowers from expanding, and to reduce 
the amount of pollen, wdien 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 func- 
tional inactivity of parts, during the progress of the reduc- 
tion 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 flrst, and 
has five sepals and petals, and five divisions to the ovarium; 
while all the other flowers on the plant are tetramerous. 
In the British Adoxa the uppermost flower generally bas 
two calyx-lobes with the other organs tetramerous, while 
the surrounding flowers generally have three calyx-lobes 
with the other organs pentamerous. In many CompositaB 
and Umbellifera? (and in some other plants) the circum- 
ferential flowers have their corollas much more developed 
than those of the center; 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 circumference and center sometimes differ 
greatly in form, color and other characters. In Cartha- 
mus and some other Compositae the central achenes alone 
are furnished with a pappus; and in Hyoseris the same 
head yields achenes of three different forms. In certain 
irmbellifer,3e the exterior seeds, according to Tausch, are 
orthospermous, and the central onecrelospermous, and this 
is a character which was considered by De CandoUu to be iu 
other species of the highest systematic importance. Pro- 


fessor Braun mentions a Fumariaceous 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-devel- 
oped 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 sub- 
ordinate 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 

In numerous other cases we find modifications of struct- 
ure, 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 indiffer- 
ently tetramerous, pentamerous, etc., that I need not give 
examples; but as numerical variations are comparatively 
rare when the parts are few, I may mention that, accord- 
ing 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 char- 
acter; but Professor Asa Gray states that with some species 
of Mimulus, the aestivation is almost as frequently that of 
the Rhinanthidege as of the Antirrhinideae, to which latter 
tribe the genus belongs. Aug. St. Hilaire gives the fol- 
lowing cases: the genus Zanthoxylon belongs to a division 
of the Eutaceae 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 


Helianthemum the capsule has been described as uiiilocu- 
lar or tri-locular; and in H. mutabile, " Une hime plus on 
moins large, s'etend entre le pericarpe et le placenta." In 
the flowers of Saponaria officinalis Dr. Masters lias 
observed instances of both marginal and free central plac- 
entation. Lastly, St. Hilaire found toward the southern 
extreme of the range of Gomphia oleseformis two forms 
which he did not at first doubt were distinct species, but 
he subsequently saw them growing on tlie same bush; and 
he then adds, " Voila done dans un rneme indivielu des 
loges et un style qui se rattaclient tantot a un axe verticale 
et tantot a un gynobase/' 

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 tend- 
ency toward perfection or progressive development, can it 
be said in the case of these strongly pronounced variations, 
that the plants have been caught in the act of progressiug 
toward a higher state of development? On the contrary, 
I should infer from the mere fact of the parts in questioi 
differing or varying greatly on the same plant, that such 
modifications were of extremely small importance to the 
i:)lants themselves, of whatever importance they may gener- 
ally 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 tlian 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 unimpor- 
tant for the welfare of the species, any slight vai-iations 
which occurred in them would not have been accumulated 
and augmented through natural selection. A structure 
which has been developed througli long-continued selec- 
tion, when it ceases to be of service to a species, generally 


becomes variable, as we see with rndimentar}^ organs; for 
it will no longer be regulated by this same power of selec- 
tion. But when, from the nature of the organism and of 
the conditions, modifications have been induced which are 
unimportant 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 v/ere 
clothed with ha,ir, feathers or scales; yet hair has been trans- 
mitted 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, etc., 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 characters do 
not ailect 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 S3'steniatist; but, 
as we shall hereafter see when we treat of the genetic 
principle of classiflcation, this is by no means so paradoxi- 
cal as it may at first appear. 

Although we have no good evidence of the existence 
in organic beings of an innate tendency toward progressive 
development, yet this necessarily follows, as I have attempt- 
ed 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 organization, is 
the degree to which the parts have been specialized or dif- 
ferentiated; and natural selection tends toward this end, 
inasmuch as the parts are thus enabled to perform their 
functions more efi&ciently. 


A distinguished zoologist, Mr. St. George Mivart, has 
recently collected all the objections wliicli luive ever been 
advanced by myself and others against the theory of natui-al 
selection, as propounded by Mr. Wallace and myself, and 
has illustrated them with admirable art and force. When 
thus marshaled, 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 conclusions, no 
slight effort of reason and memory is left to the reader, 
who may wish to weigh the evidence on both sides. Wlu-n 
discussing special cases, Mr. Mivart passes over the effects 
of the increased use and disuse of parts, which 1 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 work 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 con- 
clusions here arrived at, subject, of course, in so intricate 
a subject, to much partial error. 

All Mr. Miva^rt^s objections will be, or have been, con- 
sidered in the present volume. The one new point wliich 
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 the 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 otlier Uugulata 


or hoofed animals inhabiting the same country; and this 
mnst be a great advantage to it during dearths. The Niata 
cattle in South x\merica show us how small a difference in 
structure may make, during such periods, a great differ- 
ence 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 of ten recurrent droughts, 
browse on the twigs of trees, reeds, etc., 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 Avell to explain once again how natural 
selection will act in all ordinary cases. Man has 
modified some of his animals, without necessarily haying 
attended to special points of structure, by simply pre- 
serving 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 underniature 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 pre- 
served; 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, con- 
sidering its probable habits of life; for those individuals' 
which had some one part or several parts 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 ; while the 
individuals less favored 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: natu-- 
ral selection will preserve and thus separate all the superior 
individuals, allowing them freely to intercross, and will 
destroy all the inferior individuals. By this process long- 


continued, which exactly corresponds with wlmi I luive 
called unconscious selection by man, combined, no doubt, 
in a most important manner with the inherited elTects 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. ^[ivart brings forward two ob- 
jections, One is that the increased size of the body woultl 
obviously require an increased supply of food, and lie con- 
siders it as '' very problematical wiiether the disadvantages 
thence arising would not, in times of scarcity, more than 
counterbalance the advantages." But as tlie giraffe does 
actually exist in large numbers in Africa, and as some of the 
largest antelopes in the world, taller than aji ox, abound 
there, why should we doubt that, as far as size is concerned, 
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 in- 
creased bulk wonld 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 more difficult to stalk than the giraffe. This 
animal also uses its long neck as a means of offence or de- 
fence, 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 quad- 
ruped 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 
South 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 in- 
stance, to sheep, if kept there, to acquire slightly longer 
necks? 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 
South Africa the competition for browsing on the higher 
branches of the acacias and other trees must be between 
giraife and giraffe, and not with the other ungulate 

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 unreasonable to expect a distinct 
answer to such a question as why some event in the history 
of mankind did not occur in one country while 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 favorable 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 at a consid- 
erable height (without climbing, for which hoofed animals 
are singularly ill-constructed) implies greatly increased 
bulk of body; and we know that some areas support singu- 
larly few large quadrupeds, for instance South America, 
though it is so luxuriant, while South 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 favorable 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 favorable 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 coadapted. 
Although every part of the body varies slightly, it does not 


follow that the necessary parts should always vary in thu 
right direction and to the right degree. With the differ- 
ent 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 spe- 
cies. For instance, if the number of individuals 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, nat- 
ural selection is a slow process, and the same favorable 
conditions 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 brows- 
ing 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 in- 
terfered with the acquisition through natural selection of 
structures, which it is thought would be beneficial to cer- 
tain species. One writer asks, why has not the ostrich 
acquired the power of tiiglit? But a moment's reflection 
will show what an enormous supply of food would be nec- 
essai-y 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 in- 
habited 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 ]n-eyed 
Qn by the reptiles or birds, which first colonize and abound 


on most oceanic islands. Gradations of structure, with each, 
stage beneficial to a changing species, will be favored 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 acquatic as to brave the open 
ocean. But seals would not find on oceanic islands the 
conditions favorable to their gradual reconversion into a 
terrestrial form. Bats, as formerly shown, probably ac- 
quired 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 iu 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 can not 
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 
civilization than the other; and this apparently implies in- 
creased 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. Tlie resein- 
blauce is often Avonderfully close, and is not confined to 
color, but extends to form, and even to the manner in 
which the insects hold themselves. Tlie caterpillars which 
project motionless like dead twigs from the bushes on 
which they feed, olfer an excellent instance of a resem- 
blance 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 diffi- 
cult, if not impossible, to see how such indefinite oscilla- 
tions of infinitesimal beginnings can ever buikl up a suffi- 
ciently appreciable resemblance to a leaf, bamboo, or other 
object, for natural selection to seize upon and perpetuate." 

But in all the forgoing cases the insects in their original 
state no doubt presented some rude and accidental resem- 
blance to an object commonly found in the stations fre- 
quented by them. Nor is this at all improbable, consider- 
ing the almost infinite number of surrounding objects and 
the diversity in form and color 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 sur- 
rounds them, and this chiefly in color. 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 in- 
sect at all more like any such object, and thus favored its 
escape, would be preserved, while other variations would 
be neglected and ultimately lost; or, if they rendered tlie 
insect at all less like the imitated object, they would be 
eliminated. There would indeed be force in Mr. ^livart'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 

Nor can I see any force in Mr. Mivart's difficulty wuth 


respect to '' the last touches of perfection in the mimicry;^' 
as in the case given by Mr. Wallace, of a walking-stick in- 
sect (Ceroxylus laceratus), which resembles '^a stick grown 
over oy 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 ether enemies whose sight is probably 
sharper than ours, and every grade in resemblance which 
aided an insect to escape notice or detection, would tend 
toward its preservation; and the more perfect the resem- 
blance 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 im- 
probable in this insect having varied in the irregularities 
on its surface, and in these having become more or less 
green-colored; for in every group the characters which 
differ in the sevei-al species are the most apt to vary, whiltf 
the generic characters, or those common to all the species, 
are the most constant. 

The Greenland whale is one of the most wonderful ani- 
mals in the world, and the baleen, or whalebone, 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 subsid- 
iary 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 ani- 
mals subsist. The middle and longest lamina in ^he Green- 
land whale is ten, twelve, or even fifteen feet in length; 
but in the different species of Cetaceans there are grada- 
tions in length; the middle lamina being in one species, 
according to Scoresby, four feet, in another three, in 
another eighteen inches, and in the Balasnoptera rostrata 
only about nine inches in lengtli. The quality of the 
whalebone 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?'^ 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 laniel- 
lated beak of a duck? Ducks, like whales, subsist by sift- 
ing the mud and water; and the family has sometimes 
been called Onblatores, 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, elastio lamellae, obliquely bevelled so as to be 
pointed, and placed transversely 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 toward the middle are the longest, being about 
one-third of an inch in length, and they project fourteen 
one-hundreths 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 v/hale. But toward the ex- 
tremity of the beak they differ much, as they project in- 
ward, instead of straight downward. 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 lam- 
ellae 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 these above, but liner; 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 lamella3 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 furnislied with lamellae, which are well 
developed and project beneath the margin; so that the 
beak of this bird resembles in this respect the mouth of a 

From the highly developed structure of the shoveller's 
beak we may proceed (as I have learned 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 coarsei 
than in the shoveller, and are firmlv attached to the sides 
of the mandible; they are only about fifty in number oi 
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 inferio 
as a sifter to that of a 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 
lamellse 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 sg 
numerous, nor so distinct from each other, nor do they 
project so much inward; yet this goose, as I am informed 
by Mr. E. Bartlett, ''uses its bill like a duck by throwing 
the water out at the corners/' Its chief food, however, is 
grass, which it crops like the common goose. In this latter 
bird the lamellse of the upper mandible are much coarser 
than in the common duck, almost confluent, about twenty- 
seven in number on each side, and terminating upward 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 cut- 


ting herbage, for which purpose it is so well fitted that it 
can crop grass closer than almost any ether animal. There 
are other species of geese, as I hear from Mr. Bartlett, in 
which the lamellae are less developed than in the common 

We thus see that a member of the duck family, with a 
beak constructed like that of a common goose and adapted 
Bolely for grazing, or even a member with a beak having 
less well-developed lamellae, might be converted by small 
changes into a species like the Egyptian goose — this into 
one like the common duck — and, lastly, into one like the 
shoveller, provided with a beak almost exclusively adapted 
for sifting the water; for this bird could hardly use any 
part of its beak, except the hooked tip, for seizing or tear- 
ing solid food. The beak of a goose, as I may add, might 
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 differ- 
ent purpose of securing live fish. 

Keturning 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, noth- 
ing 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 tear- 
ing 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 on- 
ward, until they became as well constructed as those of the 
ehoveller, 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 Balaenoptera rostrata, gradations, which may 
be observed in still-existing Cetaceans, lead us onward 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 clmnging 
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 Pleuronectid^e, or Flat-fish, are remarkable for 
their asymmetrical bodies. They rest on one side — in the 
greater number of species on the left, but in some on the 
right side; and occasionally 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 
color, 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, 
however, they stand opposite to each other, and the whole 
body is then symmetrical, with both sides equally colored. 
Soon the eye proper to the lower side begins to glide 
elovvly 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 while 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 Pleuronectidae are admirably adapted by their 
flattened and asymmetrical structure for their habits of 
life, is manifest from several species, such as soles, flound- 
ers, etc., being extremely common. The chief advantages 
thus gained seem to be protection from their enemies, and 
facility for feeding on the ground. The different mem- 
bers, 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 consid^ 
erable 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 toward 


the other side of the head could benefit the iiidividudl i,-, 
indeed, far from clear. It seems, even, that sucli an in- 
cipient transformation must rather have been iiijurious." 
But he might have found an answer to this objection in 
the excellent observations published in 1867 by Malm. 
The Pleuronectidffi, while very young and still symmetri- 
cal, with their eyes standing on opposite sides of the head, 
cannot long retain a vertical position, owing to the exces- 
sive depth of their bodies, the small size of their lateral 
fins, and to their being destitute of a swim-bladder. 
Hence, soon growing tired, they fall to the bottom on one 
side. While thus at rest they often twist, as Malm ob- 
served, the lower eye upward, 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, tem- 
porarily contracted in breadth. On one occasion Malm 
saw a young fish raise and depress the lower eye through 
an angular distance of about seventy degrees. 

We should remember that the skull at this early age is 
cartilaginous and flexible, so that it readily yields to mus- 
cular action. It is also known with the higher animals, 
even after early vouth, that the skull vields and is altered 
in shape, if the skin or muscles be permanently contracted 
through disease or some accident. With long-eared rab- 
bits, if one ear flops forward and downward, its weight 
drags forward all the bones of the skull on the same side, 
of which I have given a figure. Malm states that tlie 
newly-hatched young of perches, salmon, and several other 
symmetrical fishes, have the hal3it 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 upward; 
and their skulls are thus rendered rather crooked. These 
fishes, however, are soon able to hold themselves in a ver- 
tical position, and no permanent effect is thus produced. 
With the Pleuronectidse, 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, tlie tendency 
to distortion would no doubt be increased through the 
principle of inheritance. Schiodte believes, in opposition 


to some other naturalists, that the Pleuronecticlae are not 
quite symmetrical even in the embryo; and if this be so, 
we could understand how it is that certain species, while 
young, habitually fall over and rest on the left side, and 
other species on the right side. Malm adds, in confirma- 
tion of the above view, that the adult Trachypterus arcti- 
cus, which is not a member of the Pleuronectid^, 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 Pleu- 

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 endeavoring to look upward with both 
eyes, while 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 toward the 
lower surface, with the jaw bones stronger and more effect- 
ive 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 condition of the whole inferior half 
of the body, including the lateral fins; though Yarrel 
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 colorless state of the ventral sur- 
face of most fishes and of many other animals, we may 
reasonably suppose that the absence of color in flat-fish on 
the side, whether it be the right or left, which is under- 
most, is due to the exclusion of light. But it cannot 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 color in accordance with tlie surround- 
ing surface, or the presence of bony tubercles on the u})per 
side of the tnrbot, are due to the action of the liglit. 
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 per- 
haps of their disuse, will be strengthened by natural selec- 
tion. For all spontaneous variations in the right direc- 
tion 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 appar- 
ently owes its origin exclusively to use or habit. The 
extremity of the tail in some American monkeys has been 
converted into a wonderfully perfect prehensile organ, and 
serves as a fifth hand. A reviewer, who agrees with Mr. 
Mivart in everv detail, remarks on this structure: "It is 
impossible to believe that in any number of ages the first 
slight incipieut tendency to grasp could preserve the lives 
of the individuals possessing it, or favor their chance of 
having and of rearing offspring. ^^ But there is no neces- 
sity 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 (Cercopithecus) 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 pos- 
sess a structurally prehensive 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 analo- 
gous 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 


Cercopitheciis, considering its habits while 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 ita 
prodigious leaps, than as a prehensile organ. 

The mammary glands are common to the whole class of 
mammals, and are indispensable for their existence; they 
must, therefore, have been developed at an extremely 
remote period, and v/e 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 scarcel}'' 
nutritious fluid from an accidentally hypertrophied cuta- 
neous 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 
evolutionists that mammals are descended, from a marsu- 
pial 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 desig- 
nated, is it not at least possible that the young might have 
been similiarly nourished? And in this case, the individu- 
als which secreted a fluid, in some degree or manner the 
most nutritious, so as to partake of the nature of milk, 
Avould 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 liomologues of the mammary glands, would 
have been improved or rendered more effective. It accords 
with the widely extended principle of specialization, 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 Ornithorhyncus, at the base of 
the mammalian series. Through what agency the glands 


over a certain space became more highly specialized than 
the others, I will not pretend to decide, wlietlier 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 atfected 
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 learned to suck the breast, than in under- 
standing how unhatched chickens have learned 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 
learned 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 after- 
ward 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 oif spring. On 
this head Mr. Mivart remarks: **Did no special provision 
exist, the young one must infallibly be choked by the in- 
trusion of the milk into the wind-pipe. 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 assump- 
tion 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 cer- 
tainly of high importance to many animals, could hardly 
have been used with full force as long as the larynx en- 
tered the nasal passage; and Prof essor 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 Echinoderniata (star-fishes, 
sea-urchins, etc.) are furnished with remarkable organs, 
called pedicellarias, which consist, when well developed, of 


a tridactyle forceps — that is, of one formed of three ser« 
rated arms, neatly fitting together and placed on the 
summit of a flexible stem, moved by muscles. These for- 
ceps can seize firmly hold of any object; and Alexander 
Agassiz has seen an Echinus or sea-urchin rapidly passing 
particles 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 hegimiings of such structures, and 
how could such insipient buddings have ever preserved the 
life of a single Echinus?" He adds, ^'^not even the sudden 
development of the snapping action could have been bene- 
ficial without the freely movable stalk, nor could the 
latter have been efficient without the snapping jaws, yet 
no minute, merely indefinite variations could simultane- 
ously 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 capa- 
ble 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, in- 
forms 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 de- 
scribed by M. Perrier as bearing two kinds of pedicellariae, 
one resembling those of Echinus, and the other those of 
Spatangus; and such cases are always interesting as afford- 
ing the means of ajoparently 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 re- 
searches and those of Mr. Miiller, that both in star-fishes 
and sea-urchins the pedicellariBe 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 pedicellariae. The gradation 
extends even to the manner in whicli 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 pedicel- 
lariae are only modified branching spines'' may be found. 
Thus we have fixed spines, with three equi-distant, serrated, 
movable branches, articulated to near their bases; and 
higher up, on the same spine, three other movable 
branches. Now when the latter arise from the summit of 
a spine they form, in fact, a rude tridactyle pedicellariae, 
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 pedicellariae and the movable 
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 fur- 
nished with serrated and movable 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 
prehensile or snapping apparatus. Thus every gradation, 
from an ordinary fixed spine to a fixed pedicellariae, 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 movable. I wish I had 
space here to give a fuller abstract of Mr. Agassiz's inter- 
esting observations on the development of the pedicellarite. 
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 ob- 
served by me, all the avicularia on the same branch often 
moved simultaneously backward and forward, with the 
lower jaw widely open, through an angle of about 90 
degrees, 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 shaken. 

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 lie considers as ^^ essentially similar,^' having been 
developed through natural selection in widely distinct 
divisions of the animal kingdom. But, as far as struct- 
ure is concerned, I can see no similarity between tridac- 
tyle pedicellariae and avicularia. The latter resembles 
somewhat more closely the chelae or pincers of Crusta- 
ceans; 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 movable mandible of 
the avicularium. Mr. Busk, however, does not know of 
any gradations now existing between a zooid and an avicu- 
larium. 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 loco- 
motion. We next find one corner of the broad penulti- 
mate segment sliglitly prominent, sometimes furnished 
with irregular teeth, and against these the terminal seg- 
ment shuts down. By an increase in the size of this pro- 
jection, with its sluipe, as well as that of the terminal 
segment, slightly moclitied and improved, the pincers are 
rendered more and more perfect, until we have at last an 
instrument as efficient as the chelsB of a lobster. And all 
these gradations can be actually traced. 

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 ofteu 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 vio- 
lent 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 poly- 
zoary, removing what m.ight be noxious to the delicate 
inhabitants of the cells when their tentaciila are pro- 
truded.^^ The avicularia, like the vibracula, probably 
serve for defence, but they also catch and kill small living 
animals, which, it is believed, are afterward 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 dif- 
ferent in appearance than a bristle or vibraculum, and an 
avicularium" like the head of a bird: vet thev 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 movable mandible is so much produced ami 
is so like a bristle that the presence of the upper or lixod 
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 trans- 
formation, 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 sup- 
port in some species is quite absent. This view of the de- 
velopment 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 imagina- 
tion would ever have thought that the vibracula had orig- 
inally 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 movable lip of the cell serves as a protec- 
tion 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 
thoroughly unsatisfactory — utterly insufficient to explain 
the incipient, infinitesimal beginnings 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 strik- 
ing peculiarities of the flowers of orchids, namely, their pol- 
linia. 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 in- 
sects 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 consid- 
ered; yet I may mention that at the base of the orchida- 


ceous 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 sometimes be detected em- 
bedded within the central and solid parts. 

Vv'ith respect to the second chief peculiarity, namely, the 
little mass of viscid matter attached to the end of the cau- 
dicle, 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 matter. 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 consequence 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 cau- 
dicle — to others in which the caudicle becomes firmly at- 
tached to the viscid matter, with the sterile stigma itself 
much modified. In this latter case we have a polliuium 
in its most highly developed and perfect condition. He 
who will carefully examine the flowers of orchids for 
himself will not clenv 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 the ordinary flowers, to a highly comjilex 
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 fertilization by different 
insects. In this, and in almost every other case, the in- 
quiry may be pushed further backward ; and it may be 
asked how did the stigma of an ordinary flower become 
viscid, but as we do not biow 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 ar- 
ranged in a long series, from those which 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 revolv- 
ing, which the tendrils likewise possess. The gradations 
from leaf-climbers to tendril bearers are wonderfully close, 
and certain plants may be differently placed in either 
class. But in ascending the series from simple twiners to 
leaf-climbers, an important quality is added, namely sen- 
sitiveness to a touch, by which means the foot-stalks of the 
leaves or flowers, or these modified and converted into ten- 
drils, 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 sen- 
sitiveness 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, 
afterward to be improved and increased through natural 
selection. The power of twining depends, firstly, on the 
stems while young being extremely 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 revolv- 
ing, 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 
qf revolving, and have thus become twiners, they must have 


in'lependently acquired it, and cannot have inherited it from 
a common progenitor. Hence, I was led 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 pre- 
diction, I knew of only one imperfect case, namelv, 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 afterward 
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 ques- 
tion; anyhow, they are not of the least use in the way of 
climbing, which is the point that concerns us. Neverthe- 
less we can see that if the stems of these plants had been 
flexible, and if under the conditions to which they are ex- 
posed it had profited them to ascend to a height, then the 
habit of slightly and irregularly revolving might have been 
increased and utilized through natural selection, until they 
had become converted into well-developed twining species. 
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 revolv- 
ing 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 be- 
come climbers. This is the case: I observed that the 
young flower-peduncles of the above Maurandia curved 
themselves a little toward the side which was touched. 
Morren found in several species of Oxaiis that the leaves 
and their foot-stalks moved, especially after exposure to a 
hot sun, when they were gently and repeatedly touciied, or 
when the plant was shaken. I repeated these observations 
on some other species of Oxaiis with the same result; in 
some of them the movement wjis 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 Ilofmeister, 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 tliat the foot-stalks and tendrils are sensitive. 

It is scarcely possible that the above slight movements, 
due to a touch or shake, in the 3'Oung and growing organs 
of plants, can be of any functional importance to them. 
But plants i^ossess, in obedience to various stimuli, powers 
of movement^ which are of manifest importance to them; 
for instance, toward 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 
a?iimal are excited by galvanism or by the absorption 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 in- 
cidental 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 adrantage of and increased through natural 
selection. It is, however, probable, from reasons which I 
have assigned in my memoir, that this wdll have occurred 
only with plants which had already acquired the power 
of revolving, and had thus become twiners. 

I have already endeavored 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, nat- 
ural 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 skillful natu- 
ralist, to prove that natural selection is incompetent to ac- 
count for the incipient stages of useful structures; and I 


have shown, as I hope, that there is no great difliculty on 
this head. A good opportunity has thus been alTorded for 
enlarging a little on gradations of structure, often associ- 
ated with strange functions — an important subject, which 
was not treated at sufificient length in the former editions 
of this work. I will now briefly recapitulate tlie foregoing 

With the giraffe, the continued preservation of tlie indi- 
viduals of some extinct high-reaching ruminant, whicli had 
the longest necks, legs, etc., 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-ordina- 
tion. With the many insects which imitate various ob- 
jecii^, there is no improbability in the belief that an acci- 
dental resemblance to some common object was in each case 
the foundation for the work of natural selection, since per- 
fected 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 favor- 
able 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 teetli, 
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 lamella3 of horn or 
whalebone, habit or use can have done little or nothing, as 
far as we can judge, toward their development. On the 
other hand^ the transportal 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 con- 


tinned use, together with inheritance. With respect to 
the mammae of tlie higher animals, the most probable con- 
jcctui-e 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, Tn 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 pedicellarice, than in understand- 
ing tlie development of the pincers of crustaceans, through 
slight, serviceable modifications in the ultimate and pe- 
nultimate 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 grada- 
tions 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 advan- 
tageous? But it is unreasonable to expect a precise answer 
to such questions, considering 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, many 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 struct- 
nres, they could not have been acquired through natural 
selection. In many cases complex and long-enduring con- 
ditions, 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, v^ould have been gained under 
all circumstances through natural selection, is opposed to 
fvhat we can understand of its manner of action. Mr. 
Mivart does not deny that natural selection has effected 
something; but he considers it as " demonstrably insuf- 
ficient " to account for the phenomena which I explain by 
its agency. His chief arguments have now been con- 
sidered, and the others will hereafter be considered. They 
seem to me to partake little of the character of demonstra- 
tion, and to have little weight in comparison with those in 
favor 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 ad- 
vanced for the same purpose in an able article lately pub- 
lished in the ''Medico-Chirurgical Review." 

At the present day almost all naturalists admit evolution 
under some form. 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 evolu- 
tionists; but there is no need, as it seems to me, to invoke 
anv internal force beyond the tendency to ordinary varia- 
bility, 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 ex- 
plained, an advance, but in some few cases a retrogression, 
in organization. 

Mr. Mivart is further inclined to believe, and some 
naturalists agree with him, that new species manifest 
themselves '* with suddenness and by modifications appear- 


ing at once." For instance, he supposes that the differ- 
ences 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 cul- 
tivated than under their natural conditions, it is not prob- 
able 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, etc.; 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 hare occasionally domestic races, 
and for entirely disbelieving that xhey 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 subse- 
quent intercrossing; and so it is known to be under do- 
mestication, unless abrupt variations of this kind are spec- 
ially preserved and separated by the care of man. Hence, 
in order that a new species should suddenly a^^pear in the 
manner supposed by Mr. Mivart, it is almost necessary to 


believe, in opposition to all analogy, that several wonder- 
fully changed individuals appeared simultaneously within 
the same district. This difficulty, as in the case of uncon- 
scious selection by man, is avoided on the theory of gradual 
evolution, through the preservation of a large number of 
individuals, which varied more of less in any favorable 
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, etc., 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 out- 
lying 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 
imbedded in the sub-stages of the same geological forma- 
tion. 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 main- 
tained 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 dis- 
tinct species, that numerous and wonderfully fine grada- 
tions can be traced, connecting together widely different 

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 pi-esent 
other analogies with varieties, as was shown in our second 


chapter. On this same principle we can understand liow 
it is that specific characters are more variable than generic 
characters; and how the parts which are developed in an 
extraordinary 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 favor of 
this view, such as the sudden crystallization 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 sup- 
ports at first sight the belief in abrupt development. But 
the value of this evidence depends entirely on the perfec- 
tion of the geological record, in relation to periods remote 
in the history of the world. If the record is as frag- 
mentary 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 Ilipparion into a horse, hardly any light is thrown by the 
belief in abrupt modifications on the deficiency of connect- 
ing 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 undis- 
tinguishable at an eariy embryonic period, and that they 
become differentiated by insensibly fine steps. Embryo- 
logical resem Glances 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 trans- 
mitted their newly-acquired characters to their offspring, 
at a corresponding age. The embryo is thus left almost 



unailiocted, 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 erabryological resemblances, and 
indeed on any view, it is incredible that an animal 
should have undergone such momentous and abrupt trans- 
formations 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 com- 
pelled to assume, in opposition to all analogy, that many 
individuals varied simultaneouslv. 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 — 
Instincts graduated — Aphides and ants — Instincts variable — 
Domestic instincts, their origin — 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 — 

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 con- 
cerned only with the diversities of instinct and of the 
other mental faculties in animals of the same class. 

I will not attempt any definition of instinct. It would 
be easy to show that several distinct mental actions are 
commonly embraced by this term; but every one under- 
stands 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 judg- 
ment or reason, as Pierre Ruber 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 oppo- 
sition 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 
throughout life. Several other points of resemblance 
between instincts and habits could be pointed out. As in 
repeating a well-known song, so in instincts, one action 
follows another by a sort of rhythm; if a person be inter- 
rupted 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 con- 
struction. 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 embaiTassed, 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 resemblance between what originally was a habit 
and an instinct becomes so close as not to be distinguished. 
If Mozart, instead of playing the piano-forte 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 hj 
habit in one generation, and then transmitted by inherit- 
ance to succeeding generations. It can be clearly shown 
that the most wonderful instincts with whicli 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 instiucts are as im- 


portant 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 sub- 
ordinate 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 accumu- 
lation 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 among extinct species, how 
very generally gradations, leading to the most complex in- 
stincts, can be discovered. Changes of instinct may some- 
times be facilitated by the same species having different in- 
stincts at different periods of life, or at different seasons of 
the year, or when placed under different circumstances, 
etc. ; in which case either the one or the other instinct 
might be preserved by natural selection. And such in- 
stances of diversity of instinct in the same species can be 
shown to occur in nature. 

Again, as in the case of corporeal structure, and con- 
formably to my theory, the instinct of each species is good 


for itself; but has never, as far as we can judge, been jiro- 
diiced for the exclusive good of others. One of tlie strou"-- 
est instances of an animal apparently perform in or an 
action for the sole good of another, with which 1 uui 
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 re- 
moved 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. Afterward, 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 begun to play with its antennas on the abdomen 
first of one aphis and then of another; and each, as soon us 
it felt the antennae, immediately lifted up its abdomen and 
excreted a limped drop of sweet juice, which was eagerly 
devoured by the ant. Even the quite young aphides be- 
haved in this manner, showing that the action was instinc- 
tive, and not the result of experience. It is certain, from 
the observations of Huber, that the aphides show no dis- 
like to the ants: if the latter be not present they are at 
lavSt compelled to eject their excretion. But as the excre- 
tion 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 can not be considered as 
absolutely perfect; but as details on this and other such 
points are not indispensable, they may be here passed 

As some degree of variation in instincts under a state of 
nature, and the inheritance of sucli variations, are indis- 
pensable 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 tlie same species in the northern and south- 
ern 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 w^as deficient?" But what 
other natural material could bees use? They will work, as 
I have seen, with wax hardened with vermilion or soft- 
ened 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 decor- 
ticated trees. It has lately been shown that bees, instead 
of searching for pollen, will gladly use a very different 
Bubstance, namely, oatmeal. Fear of any particular enemy 
is certainly an instinctive quality, as may be seen in nest- 
ling 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. 

Tliat 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 occa- 
sional and strange habits in wild animals, which, if advan- 
tageous 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. 




^ The possibility, or even probability, of inherited varia- 
tions of instinct in a state of nature will be strengtliened 
by briefly considering a few cases under domestication. 
We shall thus be enabled to see the part wliich 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 tlioir 
mental qualities. With cats, for instance, one naturally 
takes to catching rats, and anotlier mice, and these ten- 
dencies are known to be inherited. One cat, according to 
Mr. St. John, always brought home game birds, anotlier 
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 minds 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 striking instances) 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 experi- 
ence 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 dilTep 
essentially from true instincts. If we were to beliold one 
kind of wolf, when young and without any training, aa 
soon as it scented its prey, stand motionless like a statue, 
and then slowly crawl forward with a peculiar gait; and 
another kind of wolf rushing round, instead of at, a herd 
of deer, and driving them to a distant point, we should 
assuredly call these actions instinctive. Domestic instincts, 
as they may be called, are certainly far less fixed than 
natural instincts; but they have been acted on by far lesa 


rigorous selections, and have been transmitted for an 
incomparably shorter period, under less fixed conditions of 


IIow strongly these domestic instincts, habits, and 
dispositions are inherited, and how curiously they 
become mingled, is well shown when different breeds of 
dogs are crossed. Thus it is known that a cross with a 
bull-dog has affected for many generations the courage and 
obstinacy of greyhounds; and a cross with a greyhound 
has given to a whole family of shepherd-dogs a tendency 
to hunt hares. These domestic instincts, when thus tested 
by crossing, resemble natural instincts, which in a like 
manner become curiously blended together, and for a long 
period exhibit traces of the instincts of either parent: for 
example, Le Eoy describes a dog, whose great-grandfather 
was a wolf, and this dog showed a trace of its wild parent- 
age 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 individ- 
uals in successive generations made tumblers what they 
now are; and near Glasgow there are house- tumblers, as I 
hear from Mr. Brent, which can not 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 prob- 
ably, as many have thought, only the exaggerated 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 comj^ulsory 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 liund, 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 re- 
markable instance of this is seen in those breeds of fowls 
which A^ery rarely or never become "broody," that is, never 
wish to sit on their eggs. Familiarity alone prevents our 
seeing how largely and how permanently the minds of our 
domestic animals have been modified. It is scarcely pos- 
sible 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 incur- 
able 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 civilized 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 proba- 
bly concurred in civilizing 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 
ibankiva, when reared in India under a hen, are at first ex- 
cessively 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 domes- 


ticatiou, for the mother hen has almost lost by disuse the 
power of flight. 

Hence, we may conclude that under domestication in- 
stincts 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 in- 
herited 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 


We shall, perhaps, best understand how instincts in a 
state of nature have become modified by selection by con- 
sidering 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 natural- 
ists as the most wonderful of all known instincts. 


It is supposed by some naturalists that the more imme- 
diate 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 inconveni- 
ently long, more especially as she migrates at a very 
early period, and the first hatched young would probably 
have to be fed by the male alone. But the American 
cuckoo is in this predicament, for she makes her own nest 
and has eggs and young successively hatched, all at the 
same time. It has been both asserted and denied that 


the American cuckoo occasionally lays her eggs in other 
birds'* nests; but 1 have lately heard from Dr. Merrill, 
of Iowa, that he once found in Illinois a young cuckoo, 
together with a young jay in the nest of a blue juy ((Jar- 
rulus cristatus); and as both were nearly full fuathered, 
there could be no mistake in their identification. 1 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 emigrate 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, 
encumbered as she could hardly fail to be by having eg^s 
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' nest, and thus be more 
(Successful in rearing their young. By a continued jDrocess 
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 prob- 
ably a case of reversion to the long-lost, aboriginal instinct 
of nidification. 

It has been objected that I have not noticed other re- 
lated 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 observations; we 
have learned something about three Australian species, 
which lay their eggs in other birds' nests. The cliii'f 
points to be referred to are throe: first, that the comnioy 


cuckoo, with rare exceptions, lays only one Qgg in a nest, 
60 that the large and voracious 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 Qgg is a 
real case of adaptation we may infer from the fact of the 
uou-parasitic American cuckoo laying full-sized eggs. 
Thirdly, that the young cuckoo, soon after birth, has the 
instiuct, the strength and a properly shaped back for eject- 
ing its foster-brothers, which then perish from cold and 
hunger. This has been boldly called a beneficent arrange- 
ment, in order that the young cuckoo may get sufficient 
food, and that its foster-brothers may perish before they 
nad acquired much feeling! 

Turning now to the Australian species: though these 
birds generally lay only one Qgg 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 diffi.culty 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. Eamsav remarks that two of the Australian 
cuckoos, when they lay their eggs in an open nest, mani- 
fest a decided preference for nests containing eggs similar 
in color to their own. The European species apparently 
manifests some tendency toward a similar instinct, but not 
rarely departs from it, as is shown by her laying her dull 
and pale-colored eggs in the nest of the hedge-warbler 
with bright greenish-blue eggs. Had our cuckoo invari- 
ably displayed 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 color; so that in this respect, as well as in size, 
natural selection might have secured and fixed any advan- 
tageous variation. 


In the case of the European cuckoo, the offspriug of the 
foster-parents are commonly ejected from the nest within 
three daj's 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, while 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 ac- 
quired, 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 cuckoos which had such habits and 
structure best developed would be the most securely reared. 
The first step toward the acquisition of the proper instinct 
might have been mere unintentional restlessness on the 
part of the young bird, wdien somewhat advanced in age 
and strength; the habit having been afterward improved, 
and transmitted to an earlier age. I uan see no more diffi- 
culty 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 — proposi- 
tions 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 wdth 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 fiocks, and sometimes to pair. They either 


build a nest of their own or seize on one belonging to some 
other bird, occtisionally throwing out the nestlings of the 
stranger. They either lay their eggs in the nest thus appro- 
priated, 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 occa- 
sionally parasitic, for he has seen the young of this species 
following old birds of a distinct kind and clamoring to be 
fed by them. The parasitic habits of another species of 
Molothrus, the M. bonariensis, are much more highly de- 
veloped than those of the last, but are still far from per- 
fect. 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 singular 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 extra- 
ordinary 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 e>gg 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 
habits, not as especially endowed or created instincts, but 
as small consequences of one general law, namely, 

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 Gallinacese, and throws some 
light on the singular instinct of the ostrich. In this 
family several hen birds unite and lay first a few eggfs 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 tliecnckoo, at intervals of two or three clays. Tlie 
instinct, however, of the American ostrich, as in the case 
of the Molothrus bonariensis, has not as yet been per- 
fected; 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 re- 
markable 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 
possessess 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 gen- 
erally makes it own burrow and stores it with paralyzed 
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 par- 
asitic. 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. 


This remarkable instinct was first discovered in the For- 
mica (Polyerges) rufescens by Pierre Iluber, a better 
observer even than his celebrated father. This ant is ab- 
solutely 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 pleuty of food which they like best, and with their 
own larvse and pupae 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 
lliese well-ascertained facts ? If we had not known of any 
other slave-making ant, it would have been hopeless to 
.^peculate how so wonderful an instinct could have beer*, 

Another species, Formica 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 Brit- 
ish 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 skeptical frame of mind, as any one may 
well be excused for doubting the existence of so extraordi- 
nary an instinct as that of making slaves. Hence, I will 
give the observations 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 communi- 
ties, and have never been observed in the nests of F. san- 
guinea. Tlie slaves are black and not above half the size 
of tlieir 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 dis- 
turbed, 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 numerous; but 
Mr.^ Smith informs me that he has watched the nests at 
various hours during May, June and August, both in Sur- 


rey 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 con- 
stantly seen bringing in materials for the nest, and food of 
all kinds. During the year 18G0, however, in the montli 
of July, I came across a community with an unusual! v 
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 
opportunities 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 an' 1 
evening; and, as Huber expressly states, their principle 
office is to search for aphides. This difference in the usual 
habits of the masters and slaves in the two countries, prob- 
ably depends merely on the slaves being captured in greater 
numbers in Switzerland than in England. 

One day I fortunately witnessed a migration of F. san- 
guina from one nest to another, and it was a most interest- 
ing 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 independ- 
ent 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 pupse to rear as slaves. I then 
dug up a small parcel of the pupa? 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 i^erhaps 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 pupse of another species, F. flav;i, with a few of 
these little yellow ants still clinging to the fragments 0^ 


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. 
sauguinea; and when I had accidentally disturbed both 
nests, the little ants attacked their big neighbors with sur- 
prising courage. JSTow I was curious to ascertain whether 
F. sanguinea could distinguish the pupae of F. fusca, 
wliich 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 pups 
of F. fusca, wiiereas they were much terrified when they 
came across the pupse, 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 1 visited another community of F. san- 
guinea, 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 burdened 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 

Such are the facts, though they did not need confirma- 
tion 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 conti- 
nental F. rufescens. The latter does not build its own 
nest, does not determine its own migrations, does not col- 
lect food for itself or its young, and cannot even feed 
itself:^ it is absolutely dependent on its numerous slaves. 
Formica sanguinea, on the other hand, possesaea much 


fewer slaves, and in the early part of the summer extremely 
lew: the masters determine when and where a new nest 
shall be formed, and when they migrate, the masters carry 
the slaves. Both in Switzerland and England the slaves 
seem to have the exclusive care of the larvae, and the mas- 
ters alone go on slave-making expeditions. In Switzerhmd 
the slaves and masters work together, making and bringing 
materials for the nest; both, but chiefly the slaves, tend 
and milk, as it may be called, their aphides; and thus both 
collect food for the community. In England the masters 
alone usually leave the nest to collect building materials 
and food for themselves, their slaves and larvae. So that 
the masters in this countrv receive much less service from 
their slaves than they do in Switzerland. 

By what steps the instinct of F. sanguinea originated I 
will not pretend to conjecture. But as ants which are not 
slave- makers will, as I have seen, carry oif the pupae of 
other species, if scattered near their nests, it is possible 
that such pupge originally stored as food might become 
developed; and the foreign ants thus unintentionally reared 
would then follow their proper instincts, and do what work 
they could. If their presence proved useful to the species 
which had seized them — if it were more advantageous to 
this species, to capture workers than to procreate them — 
the habit of collecting pupae, originally for food, might by 
natural selection be strengthened and rendered permanent 
for the very different purpose of raising slaves. When the 
instinct was once acquired, if carried out to a much less 
extent even than in our British F. sanguinea, wliich, 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 
nse to the species — until an ant was formed as abjectly 
dependent on its slaves as is the Formica rufescens. 


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 recon- 
dite problem, and have made their cells of the proper 
shape to hold tlie greatest possible amount of honey, with 
the least possible consumption of precious wax in their 
construction. It has been remarked that a sliillful work- 
man 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 at first 
appears: all this beautiful work can be shown^ I think, to 
follow from a few simple instincts. 

I was led to investigate this subject by Mr. Waterhouse, 
who has shown that the form of the cell stands in close re- 
lation 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 
beveled 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 ad- 
joining 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 interme- 
diate 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 hold- 
ing honey. These latter cells are nearly spherical and of 
nearly equal sizes, and are aggregated into an irregular 


mass. But tlie 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 fiat walls of wax be- 
tween 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 imi- 
tation 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, 
labor, by this manner of building; for the flat walls be- 
tween 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. 

Eeflecting on this case, it occurred to me that if the Meli- 
pona 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 WTote to Professor Miller, of Cam- 
bridge, and this geometer has kindly read over the follow- 
ing statement, drawn up from his information, and tells 
nie that it is strictly correct : 

If a number of equal spheres be described with their 
centers placed in two parallel layers; with the center of 
each sphere at the distance of radius X ^/ '^, or radius 
X 1.41421 (or at some lesser distance), from the centers of 
the six surrounding spheres in the same layer; and at tho 
same distance from the centers of the adjoining spheres in 
the other and parallel layer; then, if ])lane6 of intersec- 
tion 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 tliree rhombs; and 
the rhombs and the sides of the hexagonal prisms will have 


every angle identically the same with the best measure- 
ments which have been made of the cells of the hive-bee. 
But I hear from Professor Wyman, who has made numer- 
ous careful measurements, that the accuracy of the work- 
manship of the bee has been greatly exaggerated; so much 
80, that whatever the typical form of the cell may be, it is 
rarely, if ever, realized. 

Hence we may safely conclude that, if we could slightly 
modify the instincts already possessed by the Melipona, 
iind 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 form- 
ing her cells truly spherical, and of equal sizes; and this 
would not be very surprising, seeing that she already does 
60 to a certain extent, and seeing what perfectly cylindri- 
cal burrows many insects make in wood, apparently by 
turning round on a fixed point. We must sujopose 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- 
laborers 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 
fiat 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 as they deepened these little pits, they made 
them wider and wider until they were converted into 
shallow basins, appearing to the eye perfectly true or parts 
of a spliere, 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 evcavate, and began to build up flat walls of wax on the 
lines of intersection between the basins, so that each hexa- 
gonal prism was built upon the scalloped edge of a smooth 
basin, instead of on the straight edges of a three-sided 
pyrauiid 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, colored 
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 ver- 
milion wax left ungnawed, were situated, as far as the eye 
could Judge, exactly along the planes of imaginary inter- 
section between the basins on the opposite side of the ridge 
of wax. In some parts, only small portions, in otlier 
parts, large portions of a rhombic plate were thus left be- 
tween the opposed basins, but the work, from the unnat- 
ural 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 of intersection. 

Considering how flexible thin wax is, I do not see that 
there is any difficulty in the bees, while at work on the 
two sides of a strip of wax, perceiving when they have 
gnawed the wax away to the proper thinness, and tlien 
stopping their work. In ordinary combs it has apj)earcd 
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 com- 
menced cell, wdiich were sHghtly concave on one side, 


•where I suppose that the bees had excavated too quickly, 
and convex on the opposed side wliere 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 affected 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 
liave 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 proj)er distance from each other, 
by excavating at the same rate, and by endeavoring 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 jdates, 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. 

Ruber'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 
alwavs 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 
waxin the proper position — that is, along the plane of inter- 
section 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, cor- 
responding 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 under- 
stand how they work, by supposing masons first to pile up 
a broad ridge of cement, and then to begin cutting it away 
equally on both sides near the ground, till a smooth, very 
thin wall is left in the middle; the masons always piling 
up the cut away cement, and adding fresh cement on the 
summit of the ridge. We shall thus have a thin wall 
steadily growing upward but always crowned by a gigantic 
coping. From all the cells, both those just commenced 
and those completed, being thus crowned by a strong coping 
of wax, the bees can cluster and crawl over the comb with- 
out 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 measure- 
ments made near the border of the comb, -g^ of an 
inch in thickness; whereas the basal rhomboiclal j^lates are 
thicker, nearly in the proportion of three to two, having a 
mean thickness, from twenty-one measurements, of -^Ij ^^ 
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 woik 
together; one bee after working a short time at one cell 
going to another, so that, as Huber has stated, a score of 
individuals work even at the commencement of the fir.^t 
cell. I was able practically to show this fact, by covering 
the edges of the hexagonal walls of a sing-le cell, or the ex- 
treme margin of the circumferential rim of a growing 
comb, with an extremely thin layer of melted vermilion 
wax; and I invariably found that the color was most deli- 
cately diffused by the bees — as delicately as a painter could 
have done it with his brush— by atoms of the colored 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 stand- 
ing at the same relative distance from each other, all trying 
to sweep equal spheres, and then building up, or leaving 
nno"nawed, the planes of intej'section between these spheres. 
It was really curious to note in cases of difficulty, as Avhen 
two pieces of comb met at an angle, how often the bees 
would i^ull down and rebuild in different ways the same 
cell, sometimes recurring to a shape which they had at first 

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 
downward, so that the comb has to be built over one face 
of the slip — in this case the bees can lay the foundations of 
one wall of a new hexagon, in its strictly proper place, 
projecting beyond the other completed cells. It suffices 
that the bees should be enabled to stand at their proper 
relative distances from each other and from the walls of 
the last completed cells, and then, by striking imaginary 
spheres, they can build up a wall intermediate between two 
adjoining spheres; but as far as 1 have seen, they never 
gnaw away and finish off the angles of a cell till a large part 
both of that cell and of the adjoining cells has been built. 
This capacity in bees of laying down under certain circum- 
stances 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 alter- 
nately on the inside and outside of two or three cells com- 
menced 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 interme- 
diate planes. 

As natural selection acts only by the accumulation of 
slight modifications of structure or instinct, each profitable 


to the individual under its conditions of life, it may reason- 
ably be asked, how along and graduated 8ucces.->ion of 
modified architectural instincts, all tending toward the pres- 
ent perfect plan of construction, could have profited the 
progenitors of the hive-bee? I think the answer is not 
difficult: ceils constructed like those of the bee or the wasp 
gain in strength, and save much in labor 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. Tegetmeier that it has been experimentally proved 
that from twelve to fifteen pounds of dry sugar are con- 
sumed 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 days daring the process of secretion. A large 
store of honey is indispensable to support a large stock of 
bees during the winter; and the security of the hive is 
known mainly to depend on a large number of bees being 
supported. Hence the saving of wax by largely saving 
honey, 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 de- 
pendent on the number of its enemies, or parasites, or on 
quite distinct causes, and so be altogether independent 
of the quantity oi* honey which the bees can collect. But 
let us suppose that this latter circumstance 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 tliat 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 
labor 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 ]\[eiiponaj 


for in this case a large part of the bounding surface of each 
cell would serve to bound the adjoining cells, and much 
labor 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 re- 
placed 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 economizing labor and wax. 

Thus, as I believe, the most wonderful of all known in- 
stincts, 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 larvse, 
this being effected with the greatest possible economy of 
labor and wax; that individual swarm which thus made 
the best cells with least labor, 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 succeeding in the struggle for existence. 


It has been objected to the foregoing view of the origin 
of instincts that ^Hhe 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 assuniptiou 
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 iu natu- 
ral 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, subse- 
quently 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 WAV?., 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, wnich secreted 
more and more saliva, should at last produce a species with 
instincts leading it to neglect other materials and to make 
its nest exclusively of inspissated saliva? And so in other 
cases. It must, however, be admitted that in^ many 
instances we Ccinnot 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 origin- 
ated; 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 progenitor, 
and consequently must believe that they were independ- 
ently acquired through natural selection. I will not here 
enteV 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 procrea- 
tion, 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 some- 
times of eyes, and in instinct. As far as instinct 
alone is concerned, the wonderful difference in this respect 
between the workers and the j^erfect 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 profit- 
able modifications, which were inherited by the offspring, 
and that these again varied and again were selected, and so 
onward. 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 


moditications of structure or instinct to its progeny. It 
may well be asked how it is possible to reconcile this case 
with the theory of natural selection? 

First, let it be remembered that we have innumerable in- 
stances, both in our domestic productions and in those in 
a state of nature, of all sorts of diiferences of inherited 
structure which are correlated with certain ages and with 
either sex. We have differences correlated not only with 
one sex, but Avith 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. AVe 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 cor- 
related 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 remem- 
bered 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 characterized has been slaugh- 
tered, 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 yield- 
ing oxen with extraordinarily long horns, could, it is prob- 
able, 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 propa- 
gated 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 cai'efully 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. Tliese 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 com- 
munity. 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 ma}' conclude that slight modifications of 
structure or of instinct, correlated with the sterile condi- 
tion 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 modifi- 
cations. 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 dif- 
ficulty; 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 extraordinaril}^ 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 bv tlie 
survival of the communities with females wliich produced 
most neuters having the advantageous modification, all 
the neuters ultimately came to be thus characterized. Ac- 
cording to this view we ought occasionally to find in tlie 
same nest neuter insects, presenting gradations of struct- 
ure; and this we do find, even not rarely, considering how 
few neuter insects out of Europe have been carefully ex- 
amined. Mr. F. Smith has shown that the neuters of 
several British ants diller surprisingly from each other in 
size and sometimes in color; 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 sometimes happens that the larger or the smaller sized 
workers are the most numerous; or that both large and 
small are numerous, while 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 dissected 
several specimens of these workers, I can affirm that the 
eyes are far more rudimentary in the smaller workers than 
can be accounted for merely by their proportionately lesser 
size; and I fully believe, though I dare not assert so posi- 
tively, that the workers of intermediate size have their 
ocelli in an exactlv intermediate condition. So that here 
we have two bodies of sterile workers in the same nest, dif- 
fering 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 continuallv 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 
Myrmicahave not even rudiments of ocelli, though the male 
and female ants of this genus have well-developed ocelli. 

I may give one other case: so confidently did I expect 
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. Smithes offer of 
numerous specimens from the same nest of the driver ant 
(Anomma) of West Africa. The reader will perhaps best 
appreciate the amount of difference in these workers by 
my giving, not the actual measurements, but a strictly 
accurate illustration: the difference was the same as if we 
were to see a set of workmen building a house, of whom 
many were five feet four inches high, and many sixteen feet 
high; but we must 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 dif- 
ferent 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, vrith 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 selec- 
tion, by acting on the fertile ants or parents, could form a 
species which should regularly produce neuters, all of large 
size w^ith one form of jaw, or all of small size with widely 
different jaws; or lastly, and this is the greatest difficult)^ 
one set of workers of one size and structure, and simulta- 
neously 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, 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 ha"e been to a social commu- 
nity of ants, on the same principle that the division of 
labor is useful to civilized man. Ants, however, work by 
inherited instincts and by inherited organs or tools, 
while 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 con- 
clusion. 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 amouut 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 of sterile females, however long they might be fol- 
lowed, could not possibly affect the males and fertile 
females, which alone leave descendants. I am surprised 
that no one has hitherto advanced this demonstrative case 
of neuter insects, against the well-known doctrine of in- 
herited habit, as advanced by Lamarck. 


I have endeavored 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 at- 
tempted to show that instincts vary slightly in a state of 
nature. No one will dispute that instincts are of the high- 
est 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 considerable different con- 
ditions of life, yet often retaining nearly the same instincts. 
For instance, we can understand, on the principle of in- 
heritance, how it is that the thrash of tropical South Amer- 
ica 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 i^laster 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 spe- 
cially endowed or created instincts, but as small conse- 
quences of one general law leading to the advancement of 
all organic beings — namely, multiply, vary, let the strong- 
est live and the weakest die. 




Distinction "beW-^pn the sterility of first crosses and of hybrids— . 
Sterility various in degree, not universal, affected by close inter- 
breeding, removed by domestication — Laws governing the 
sterility of hybrids — Sterility not a special endowment, but inci- 
dental on other differences, not accumulated by natural selec- 
tion — 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 sterlility, 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 off- 
spring, cannot have been acquired, as I shall show, by the 
preservation of successive profitable degrees of sterility. 
It is an incidental result of differences in the reproductive 
systems of the parent-species. 

In treating this subject, two classes of facts, to a large 
extent fundamentally different, have generally been con- 
founded; 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 reproductive 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 niicro- 
scope 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 2)robably has been slurred 
over, owing to the sterility in both cases being looked on 
as a special endowment, beyond the province of our reason- 
ing 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 off- 
spring, 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. 


First, for the sterility of species when crossed and of 
their hybrid offspring. It is impossible to study the sev- 
eral memoirs and works of those two conscientious and ad- 
mirable observers, Kolreuter and Gartner, who almost 
devoted their lives to this subject, without being deeply 
impressed with the high generality of some degree of ster- 
ility. 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. Gart- 
ner, also, makes the rule equally universal; and he disputes 
the entire fertility of Kolreuter'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 produced by both pure parent-species 
in a state of nature. But causes of serious error here in- 
tervene: a plant, to be hybridized, 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 fertilized with their own pollen, and (excluding 
all cases such as the Leguminosae, in which there is an 
acknowledged difficulty in the manipulation) half of these 
twenty plants had their fertility in some degree impaired. 
Moreover, as Gartner repeatedly crossed some forms, such 
as the common red and blue pimpernels (Anagalhs 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 

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 on details — the evidence 
advanced by our best botanists on the question whether 
certain doubtful forms should be ranked as species or 
varieties, with the evidence from fertility adduced by dif- 
ferent hybridizers, or by the same observer from experi- 
ments 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 consti- 
tutional and structural differences. 

In regard to the sterility of hybrids in successive genera- 
tions; though Gartner was enabled to rear some iiybrids, 
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 and suddonlv. With re^ 


spect 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 augmented 
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 inter- 
breeding. 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 
vigor and fertility of the offspring, and on the other hand 
that very close interbreeding lessens their vigor and fertil- 
ity, that I can not doubt the correctness of this conclusion. 
Hybrids are seldom raised by experimentalists in great 
numbers; and as the parent-species, or other allied hybrids, 
generally grow in the same garden, the visits of insects 
must be carefully prevented during the flowering season: 
hence hybrids, if left to themselves, will generally be fer- 
tilized during each generation by pollen from the same 
flower; and this would probably be injurious to their fertil- 
ity, 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 fertilized with hybrid pollen 
of the same kind, their fertility, notwithstanding the fre- 
quent ill effects from manipulation, sometimes decidedly 
increases, and goes on increasing. Now, in the process of 
artificial fertilization, 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 fertilized; so that a cross between two flow- 
ers, though probably often on the same plant, would be 
thus effected. Moreover, whenever complicated experi- 
ments are in progress, so careful an observer as Gartner 
would have castrated his hybrids, and this would have in- 
sured in each generation a cross with pollen from a dis- 
tinct flower, either from the same jDlant or from another 
plant of the same hybrid nature. And thus, the strange 
fact of an increase of fertility in the successive generations 
of artificAciUy fertilized hybrids, in contrast with those spon- 
taneously self-fertilized, may, as I believe, be accounted 
for by too close interbreeding having been avoided. 



Now let us turn to the results arrived at by a third most 
experienced hybridizer, namely, the Hon. and Rev. W. 
Herbert. He is as emphatic in his conclusion that some 
hybrids are perfectly fertile — as fertile as the pure parent- 
species — as are Kolreuter 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 horticul- 
tural 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 Orinum capense fertilized by C. revolutum pro- 
duced 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 case of the Orinum leads me to refer to a singular 
fact, namely, that individual plants of certain species 
of Lobelia, Verbascum and Passiflora, can easily be fer- 
tilized by the pollen from a distinct species, but not by 
pollen from the same plant, though this pollen can be 
proved to be perfectly sound by fertilizing other plants or 
species. In the genus Hippeastrum, in Oorydalis as 
shown by Professor Hildebrand, in various orchids as 
shown by Mr. Scott and Fritz Miiller, all the individuals 
are in this peculiar condition. So that with some species, 
certain abnormal individuals, and in other species all the 
individuals, can actually be hybridized much more readily 
than they can be fertilized by pollen from the same indi- 
vidual plant! To give one instance, a bulb of Hippeas- 
trum aulicum produced four flowers; three were fertilized 
by Herbert with their own pollen, and the fourth was sub- 
sequently fertilized by the pollen of a compound hybrid 
descended from three distinct species: the result was that 
"the ovaries of the three first flowers soon ceased to grow, 
and after a few days perished entirely, whereas the pod 
impregnated by thepollen of the hybrid made vigorous 
growth and rapid progress to maturity, and bore good seed, 
which vegetated freely.'' Mr. Herbert tried simihir experi- 
ments 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 

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, Rhododen- 
dron, etc., have been crossed, yet many of these hybrids 
seed freely. For instance, Herbert asserts that a hybrid 
from Calceolaria integrifolia and plantaginea, species most 
widely dissimilar in general habit, *' reproduces itself as 
perfectly as if it had been a natural species from the 
mountains of Chili. ^' I have taken some pains to ascer- 
tain the degree of fertility of some of the complex crosses 
of Rhododendrons, and I am assured that many of them 
are perfectly fertile. Mr. C. Noble, for instance, informs 
me that he raises stocks for grafting from a hybrid between 
Rhod. ponticum and catawbiense, and that this hybrid 
" seeds as freely as it is possible to imagine.'^ Had hybrids, 
when fairly treated, always gone on decreasing in fertility 
in each successive generation, as Gartner believed to be 
the case, the fact would have been notorious to nursery- 
men. Horticulturists 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 hybrid Rhododendrons, which 
produce no pollen, for he will find on their stigmas plenty 
of pollen brought from other flowers. 

In regard to animals, much fewer experiments have been 
carefully tried than with plants. If our systematic arrange- 
ments can be trusted, that is, if the genera of animals are 
as distinct from each others as are the genera of plants, 
then we may infer that animals more widely distinct in 
the scale of nature can be crossed more easily than in 
the case of plants; but the hybrids themselves are, I 
think, more sterile. It should, however, be borne in 
mind that, owing to few animals breeding freely 
under confinement, few experiments have been fairly 
tried: for instance, the canary-bird has been crossed with 
nine distinct species of finches, but, as not one of these 


breeds freely in confinement, we have no right to eipect 
that the first crosses between them and the canary, or that 
their hybrids, should be perfectly fertile. Again, with 
respect to the fertility in successive generations of the nioro 
fertile hybrid animals, I hardly know of an instance in 
which two families of the same hybrid have been raised at 
the same time from different parents, so as to avoid the ill 
effects of close interbreeding. On the contrary, brothers 
and sisters have usually been crossed in each successive 
generation, in opposition to the constantly repeated admon- 
ition of every breeder. And in this case, it is not at all 
surprising that the inherent sterility in the hybrids should 
have gone on increasing. 

Although I know of hardly any thoroughly well-authen- 
ticated cases of perfectly fertile hybrid animals, I have 
reason to believe that the hybrids from Cervulus vaginalis 
and Reevesii, and from Phasianus colchicus with P. tor- 
quatus, 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 genera- 
tions. 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 genera, have often bred in this country with either 
pure parent, and in one single instance they have bred 
inter se. This was effected by Mr. Eyton, who raised two 
hybrids from the same parents, but from different hatches; 
and from these two birds he raised no less than eight 
hybrids (grandchildren of the pure geese) from one nest. 
In India, however, these cross-bred geese must be far more 
fertile; for I am assured by two eminently capable judges, 
namely Mr. Blyth and Captain Hutton, that whole flocks 
of these crossed geese are kept in various parts of the coun- 
try; and as they are kept for profit, where neither pure 
parent-species exists, they must certainly be highly or per- 
fectly fertile. 

With our domesticated animals, the various races when 
crossed together are quite fertile; yet in many cases thej 
^re descended from two or more wild species. From thia 


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 moso 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 in- 
digenous domestic dogs of South America, all are quite 
fertile together; but analogy makes me greatly doubt, 
whether the several aboriginal species would at first have 
freely bred together and have produced quite fertile hybrids. 
So again I have lately acquired decisive evidence that the 
crossed offspring from the Indian humped and common 
cattle are intei' se perfectly fertile; and from the observa- 
tions by Riitimeyer on their important osteological differ- 
ences, as well as from those by Mr. Bly th on their differences 
in habits, voice, constitution, etc., 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 



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 admirabb work on the hybridization 
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 aur 
prised 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 cer- 
tain 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 pro- 
duced, 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 fertilization. From 
this extreme degree of sterility we have self-fertilized 
hybrids producing a greater and greater number of seeds 
up to perfect fertility. 

The hybrids raised from two species which are very diffi- 
cult 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 
remarkably sterile. On the other liand, there are species 
which can be crossed very rarely, or with extreme ditliculty, 
but the hybrids, when at last produced, are very fertile. 
Even within the limits of the same genus, for instance ii> 
pianthus, these two opposite cases occur. 


The fertility, both of first crosses and of hybrids, is more 
easily affected by unfavorable 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 cir- 
cumstances; 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 fer- 
tility is often found to differ greatly in the several indi- 
viduals raised from seed out of the same capsule and ex- 
posed to the same conditions. 

By the term systematic affinity is meant, the general re- 
semblance between species in structure and constitution. 
Now the fertility of first crosses, and of the hybrids pro- 
duced from them, if largely governed by their systematic 
affinity. This is clearly shown by hybrids never having 
been raised between species ranked by systematists in dis- 
tinct families; and on the other hand, by very closely 
allied species generally uniting with facility. But the cor- 
respondence 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 siDecies 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 in- 
stance, 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 par- 
ticularly distinct species, obstinately failed to fertilize, or 
to be fertilized, by no less than eight other species of Nico- 
tiana. Many analogous facts could be given. 

No one has been able to point out what kind or what 
amount of difference, in any recognizable character, is suf- 
ficient to prevent two species crossing. It can be shown 
that plants most widely different in habit and general ap- 
pearance, and having strongly marked differences in every 
part of the flower, even in the pollen, in the fruit, and iii 


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 sytematic affinity, that is of any difference 
in their structure or constitution, excepting in their 
reproductive systems. The diversity of the result in recip- 
rocal crosses between the same two species was long ago 
observed by Kolreuter. To give an instance: Mirabilis 
jalapa can easily be fertilized by the pollen of M. long- 
iflora, and the hybrids thus produced are sufficiently fer- 
tile; but Kolreuter tried more than two hundred times, 
during eight following years, to fertilize reciprocally M. long- 
iflora with the pollen of M. jalapa, and utterly failed. 
Several other equally 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 dift'er in fertility in a small, and occasionally 
in a high degree. 

Several other singular rules could be given from Giirt- 
ner: 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 like- 
ness on their hybrid offspring; but these two powers do not 
at all necessarily go together. There are certain hybrids 
which, instead of having, as is usual, an intermediate 


character between their two parents, always closely resem- 
ble one of them; and such hybrids, though externally so 
like one of their pure parent-species, are with rare excep- 
tions extremely sterile. So again among 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 parent. 

Considering the several rules now given, which govern 
the fertility of first crosses and of hybrids, we see that 
when forms, which must be considered as good and distinct 
sjoecies, are united, their fertility graduates from zero to 
perfect fertility, or even to fertility under certain con- 
ditions in excess; that their fertility, besides being emi- 
nently susceptible to favorable and unfavorable 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 dif- 
ference 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 some 
dift'erence, and occasionally the widest possible dif- 
ference, in the facility of effecting an union. The 
hybrids, moreover, produced from reciprocal crosses often 
differ in fertility. 

Now, do these complex and singular rules indicate 
that species have been endowed with sterility simply to 
prevent their becoming confounded in nature? I think 
not.^ For why should the sterility be so extremely differ- 
ent in degree, when various species are crossed, all of which 
we must sup|)ose it would be equally important to keep 
frorn^ blending together? Why should the degree of 
sterility be innately variable in the individuals of the same 


species? Why slioiild some species cross with facility und 
yet produce very sterile hybrids; and other species cross 
with extreme difficulty, aud yet produce fairly fertile 
hybrids? Why should there often be so groat a difTer- 
ence in the result of a reciprocal cross between the i^ame 
two species? Why, it may even be asked, has the pro- 
duction of hybrids been permitted? To grant to species 
the special power of producing hybrids, and then to stop 
their further propagation by different degrees of sterility, 
not strictly related to the facility of the first union between 
their parents, seems 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 in 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 inci- 
dental on other differences, 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 specialli/ 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, etc.; 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 hybridization, 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 ca})acity, 
as in hybridization, 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 facihty 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 

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 spe- 
cies, which have their reproductive 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 Eobinia, 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 grafted on other species, yielded twice 
as much fruit as when on their own roots. We are re- 
minded by this latter fact of the extraordinary cases of 
hippeastrum, passiflora, etc., which seed much more freely 
when fertilized with the pollen of a distinct species than 
when fertilized with pollen from the same plant. 
. We thus see that, although there is a clear and great dif- 
ference 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 gov- 
erning the facility with which trees can be grafted on each 


other as incidental on unknown differences in their vege- 
tative systems, so I believe that the still more complex 
laws governing the facility of first crosses are inciaental on 
unknown differences in their reproductive systems. These 
differences in both cases follow, to a certain extent, as 
might have been expected, systematic affinity, bv 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 diffi- 
culty 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. 



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 re- 
marked 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 compatriot, sterility with other spe- 
cies would follow as a necessary contingency. In tlie 
second place, it is almost as much opposed to the theory of 
natural selection as to that of special creation, that in re- 
ciprocal crosses the male element of one form should have 
been rendered utterly impotent on a second form, while at 
the same time the male element of this second form is en- 


abled freely to fertilize the first form; for this peculiar 
state of the reproductive system could hardly have been 
advantageous to either species. 

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 fertiUtyto ab- 
solute 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 bastardized 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 selec- 
tion 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 reflec- 
tion, it seems to me that this could not have been effected 
through natural selection. Take the case of any two spe- 
cies which, when crossed, produced few and sterile off- 
spring; now, what is there which could favor the survival 
of those individuals which happened to be endowed in a 
slightly higher degree with mutual infertility, and which 
thus approached by one small step toward absolute sterility? 
Yet an advance of this kind, if the theory of natural se- 
lection be brought to bear, must have incessantly occurred 
with many species, for a multitude are mutually quite 
barren. With sterile neuter insects we have reason to be- 
lieve that modifications in their structure and fertility 
have been slowly accumulated by natural selection, from 
an advantage having been thus indirectly given to the com- 
munity 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 preser- 

But it would be superfluous to discuss this question in 
detail: for with plants we have conclusive evidence that 



the sterility of crossed species mast be due to some princi- 
ple, quite iiideijendeiit of natural selection. Botli Giirtner 
and Kolreuter have proved that in genera includin;,Miuiner- 
ous species, a series can be formed from species which wheu 
crossed yield fewer and fewer seeds, to species wliich never 
produce a single seed, but yet are affected by the pollen of 
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 effected, cannot have 
been gained through selection; and from the laws govern- 
ing the various grades of sterility being so uniform through- 
out 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 apparently depends on several distinct 
causes. There must sometimes be a physical impossibility 
in the male element reaching the ovule, as would be the 
case with a plant having a pistil too long for the pollen- 
tubes to reach the ovarium. It has also been observed tliat 
when the pollen of one species is placed on the stigma of a 
distantly allied species, though the pollen-tubes protrude, 
they do not penetrate the stigmatic surface. Again, the 
male element may reach the female element, but be inca- 
pable 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 can not be grafted on others. 
Lastly, an embryo may be developed, and then perish at 
an early period. This latter alternative has not been suf- 
ficiently attended to; but I believe, from observations com- 
municated to me by Mr. Hewitt, who has had great ex- 
perience in hybridizing 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 Callus and their hybrids; 



the majority of these eggs had been fertilized; and iix the 
majority of the fertilized eggs, the embryos had either been 
partially developed and had then perished, or had become 
neai'ly 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, hybridized em- 
bryos 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 strik- 
ing cases with hybrid willows. It may be here worth 
noticing that in some cases of parthenogenesis, the em- 
bryos within the eggs of silk moths which had not been 
fertilized, pass through their early stages of development 
and then perish like the embryos produced by a cross be- 
tween 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 
generally healthy and long-lived, as we see in the case of 
the common mule. Hybrids, however, are differently cir- 
cumstanced before and after birth: when born and living 
in a country where their two parents live, they are gener- 
ally placed under suitable conditions of life. But a hybrid 
partakes of only half of the nature and constitution of its 
mother; it may therefore, before birth, as long as it is 
nourished within its mother^s womb, or within the Qgg 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 in some imperfection in the original act of impregna- 
tion, causing the embryo to be imperfectly developed, 
rather than in the conditions to which it is subsequently 

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 animals and plants are 


removed from their natural conditions, they are extremely 
liable to have their reproductive systems seriously affected. 
This, in fact, is the great bar to the domestication of ani- 
mals. Between the sterility thus superinduced and that of 
hybrids, there are many points of similarity. In both 
cases the sterility is independent of general health, and is 
often accompanied by excess of size or great luxuriance. 
In both cases the sterility occurs in various degrees; in 
both, the m.ale element is the most liable to be affected; 
but sometimes the female more than the male. In both, 
the tondency goes to a certain extent with systematic afii- 
nity, for whole groups of animals and plants are rendered 
impotent by the same unnatural conditions; and whole 
groups of species tend to produce sterile hybrids. On the 
other hand, one species in a group will sometimes resist 
great changes of conditions with unimpaired fertility; and 
certain species in a group will produce unusually fertile 
hybrids. No one can tell till he tries, whether any partic- 
ular 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 pro- 
duced by the unnatural crossing of two species, the repro- 
ductive system, independently of the general state of 
health, is affected in a very similar manner. In the one 
case, the conditions of hfe have been disturbed, tliough 
often in so slight a degree as to be inappreciable by us; in 
the other case, or that of hybrids, the external conditions 
have remained the same, but the organization has been dis- 
turbed by two distinct structures and constitutions, includ- 
ing of course the reproductive systems, having been 
blended into one. For it is scarcely ^ possible that two 
organizations should be compounded into one, without 


some disturbance occurring in the development, or periodi- 
cal 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 com- 
pounded organization, 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 gener- 
ally the result, as before explained, of too close interbreed- 
ing. The above view of the sterility of hybrids being 
caused by two constitutions being compounded into one 
has been strongly maintained by Max Wichura. 

It must, however, be owned that we cannot understand, 
on the above or any other view, several facts with respect 
to the sterility of hybrids; for instance, the unequal fer- 
tility of hybrids produced from reciprocal crosses; or the 
increased sterility in those hybrids which occasionally and 
exceptionally resemble closely either pure parent. Nor do 
I pretend that the foregoing remarks go to the root of the 
matter; no explanation is offered why an organism, when 
placed under unnatural conditions, is rendered sterile. All 
that I have attempted to show is, that in two cases, in 
some respects allied, sterility is the common result — in the 
one case from the conditions of life having been disturbed, 
in the other case from the organization having been dis- 
turbed by two organizations 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 elsewhere 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, etc., 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 individuals of 
the same species, which differ to a certain extent, gives 
vigor and fertility to the offspring; and that close inter- 
breeding 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 


Hence it seems that, on the one hand, slight changes in 
the conditions of hfe benefit all organic beings, and on tlie 
other hand, that slight crosses, that is, crosses between the 
males and females of the same species, whicli have been 
subjected to slightly different conditions, or which have 
slightly varied, give vigor and fertility to the offspring. 
But, as we have seen, organic beings long habituated to 
certain uniform conditions under a state of nature, wiien 
subjected, as under confinement, to a considerable 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 persuaded that this double parallelism is by no 
means an accident or an illusion. He who is able to ex- 
plain why the elephant, and a multidude 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 seiies of facts 
seem to be connected together by some common but un- 
known 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 throughout 
nature, are always tending toward an equilibrium; and 
when this tendency is slightly disturbed by any change, 
the vital forces gain in power. 

RECiPEOCAL dimorphism: a:n"d trimorphism. 

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 reproductive organs; one form having a 
long pistil with short stamens, the other a short jnstil 



with long stamens; the two having differently sized pollen- 
grains. With triniorphic plants there are three forma 
likewise differing in the lengths of their pistils and stamens, 
in the size and color 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 proportioned 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 neces- 
sary that the stigma of the one form should be fertilized 
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 legiti- 
mate, or fully fertile, and twelve are illegitimate, or more 
or less infertile. 

The infertility which may be observed in various dimor- 
phic and trimorphic plants, when they are illegitimately 
fertilized, that is by pollen taken from stamens not cor- 
responding 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 favorable, so I 
have found it with illegitimate unions. It is well known 
that if pollen of a distinct species be placed on the stigma 
of a flower, and its own pollen be afterward, even after 
a considerable interval of time, placed on the same stigma, 
its action is so strongly prepotent that it generally annihi- 
lates 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 prepotent over illegitimate pollen, when 
both are placed on the same stigma. I ascertained this 
by fertilizing several flowers, first illegitimately, and twenty- 
four hours afterward legitimately, with pollen taken 
from a peculiarly colored variety, and all the seedlings 
were similarly colored; this shows that the legitimate pollen, 
though applied twenty-four hours subsequently, had 


vvliolly destroyed or prevented the action of the previously 
applied illegitimate pollen. Again, as in making recipro^ 
cal crosses between the same two species, there is occa- 
sionally a great difference in the result, so the same thing 
occurs with trimorphic plants; for instance, the mid- 
styled form of Ly thrum salicaria was illegitimately fer- 
tilized 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 fer- 
tilized by the longer stamens of the mid-styled 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 species 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-syled illegitimate plants, and 
from trimorphic plants all three illegitimate forms. These 
can then be properly united in a legitimate manner. AVhen 
this is done, there is no apparent reason why they should 
not yield as many seeds as did their parents when legiti- 
mately fertilized. But such is not the case. They are all 
infertile, in various degrees; some being so utterly and in- 
curably 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 legiti- 
mate manner, may be strictly compared with that of 
hybrids when crossed i7iter se. If, on the other hand, a 
hybrid is crossed with either pure parent-species, the ster- 
ility is usually much lessened: and so it is when an illegiti- 
mate plant is fertilized 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 that sterility of certain illegiti- 
mate plants was unusually great, while the sterility of the 
union from which they were derived was by no moans 
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, mnny 
hybrids are profuse and persistent flowerers, while ot^:-r 


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 

Altogether there is the closest indentity in character and 
behavior 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, while 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 illus* 
tration; 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 deter- 
mined 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 hybridized 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 com- 
mon view, that his two varieties were as good and as dis- 
tinct species as any in the world; but he would be com- 
pletely mistaken. 

The facts now given ondimorj^hic and trimorphic plants 
are important, because they show us, first, that the physio- 
logical 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 impor- 
tance, 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 con' 


ditions, and yet be sterile when united in certain wiivs. 
For we must remember that it is the union of the sexuai 
elements of individuals of the same form, for instance, of 
two long-styled forms, which results in sterility; ^vhile it 
is the union of the sexual elements proper to two disiinct 
forms which is fertile. Hence the case appears at iirst sic^ht 
exactly the reverse of what occurs, in the ordinary unions 
of the individuals of the same species and witli crosses 
between distinct species. It is, however, doubtful whether 
this is really so; but I will not enlarge on this obscure 

We may, however, infer as probable from the considera- 
tion of dimorphic and trimorphic plants, that the sterility 
of distinct species when crossed and of their hybrid pro- 
geny, depends exclusively on the nature of their sexual 
elements, and not on any difference in their structure or 
general constitution. AVe are also led to this same con- 
clusion 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, 
while the converse cross can be effected with perfect 
facility. That excellent observer, Gartner, likewise con- 
cluded that species when crossed are sterile owing to dif- 
ferences confined to their reproductive systems. 


It may be urged as an overwhelming argument tliat 
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 rod 
pimpernel, which are considered by most botanists a5 
varieties, are said by Gartner to be quite sterile wlien 
crossed, and he consequentlv 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 

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 witli European dogs, the explanation 
which will occur to every one, and probably the true one, 
is tliat they are descended from aboriginally distinct 
species. Nevertheless the perfect fertility of so many do- 
mestic races, dillering widely from each other in appear- 
ance, 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 considerations, however, render the fertility of do- 
mestic varieties less 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 differences in the case of rarieties 
would be no sure guide. It is certain that with species the 
cause lies exclusively in differences in their sexual consti- 
tution. Now the varying conditions to which domesticated 
animals and cultivated plants have been subjected, have 
had so little tendency toward modifying the reproductive 
system in a manner leading to mutual sterility, that we 
have good grounds for admitting the directly opposite doc- 
trine of Pallas, namely, that such conditions generally 
eliminate this tendency; so that the domesticated descend- 
ants of species, which in their natural state probably would 
have been in some degree sterile when crossed, become per- 
fectly fertile together. With plants, so far is cultivation 
from giving a tendency toward sterility between distinct 
species, that in several well-authenticated cases already 
alluded to, certain plants have been affected in an opposite 
manner, for they have become self-impotent, while still 
retaining the capacity of fertilizing, and being fertilized 
by, other species. If the Pallasian doctrine of the elimi- 
nation of sterility through long continued domestication be 
admitted, and it can hardly be rejected, it becomes in the 
highest degree improbable that similar conditions long-con- 


tinned should likewise induce this tendency; though in 
certain cases, with species liaving a peculiar constitution, 
sterility niiglit occasionally be thus caused. Thus, as I 
believe, we can understand why, with domesticated ani- 
mals, varieties have not been produced which are mutually 
sterile; and why with plants only a few such cases, imme- 
diately to be given, have been observed. 

The real difficulty in our present subject is not, as it 
appears to me, why domestic varieties have iu:)t 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 de- 
gree to take rank as species. We are far from precisely 
knowing the cause; nor is this surprising, seeing how 
profoundly ignorant we are in I'egard to the uornud 
and abnormal action of the reproductive system. But 
we can see that species, owing to their struggle for exist- 
ence with numerous competitors, will have been exposed 
during long periods of time to more uniform 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 conditions and subjected to captivity, 
are rendered sterile ; and the reproductive functions 
of ora:anic beins^s vv^hich have alwavs lived under natural 
conditions would probably in like manner be emi- 
nently 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 undimin- 
ished fertility repeated changes of conditions, might be 
expected to produce varieties, which would be little liable 
to have their reproductive powers injuriously alTected by 
the act of crossing with other varieties which had origi- 
nated in a like manner. 

I have as yet spoken as if the varieties of the saine species 
were invariably fertile when intercrossed. But it is impos- 
sible to resist the evidence of the existence of a certain 
amount of sterility in the few following cases, which 1 will 
briefly abstract. The evidence is at least as good as that 
from/which we believe in the sterility of a multitude oi 


species. The evidence is also derived from hostile wit- 
nesses, who in all other ca-^es consider fertility and sterility 
as safe criterions of sioecific 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 sepa- 
ated sexes, they never naturally crossed. He then fertil- 
ized 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 sepa- 
rated sexes. No one, I believe, has suspected that these 
varieties of maize are distinct species; and it is important 
to notice that the hybrid plants thus raised were them- 
selves ^?e?/ec% fertile; so that even Gartner did not ven- 
ture to consider the two varieties as specifically distinct. 

Girou de Buzareiugues crossed three varieties of gourd, 
which like the maize has separate sexes, and he asserts that 
their mutual fertilization is by so much the less easy as 
their dilferences 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 classifi- 
cation bv the test of infertilitv, 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 num- 
ber of experiments made during many years on nine species 
of Verbascum, by so good an observer and so hostile a wit- 
ness as Gartner: namely, that the yellow and white varie- 
ties when crossed produce less seed than the similarly 
colored 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 colored flowers, than between those whicli are dif- 
ferently colored. 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 colored varieties of 
the same species yield fewer seeds, in the proportion of 
eighty-six to 100, than the similarly colored varieties. Yet 
these varieties differ in no respect, except in the color of 

WHEN CROStiED. 3(^5 

their flowers; and one variety can sometimes be raised 
from the seed of another. 

Kolreuter, whose accuracy has been confirmed bv 
every subsequent observer, lias proved the remurkablo 
fact that one particuhir variety of the common tobacco 
v/as more fertile than the other varieties, when crossed 
mi\\ a widely distinct species, lie experimented on five 
forms which are commonly reputed to be varieties, and 
which he tested by the severest trial, namely, by reci})ro- 
cal 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 Xico- 
tiana glutinosa, always yielded hybrids not so sterile as 
those which were produced from the four other varieties 
when crossed with N. glutinosa. Hence, the reproductive 
system of this one variety must have been in some manner 
and in some degree modified. 

From these facts it can no longer be maintained that 
varieties when crossed are invariably quite fertile. From 
the great difficulty of ascertaining the infertility of varie- 
ties 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 considera- 
tions 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 endow- 
ment, but as incidental on changes of an unknown nature 
in their sexual elements. 



Independentlv of the question of fertility, the offspring 
of species and of varieties when crossed may be comj)areci 
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 unim- 
portant 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 genera- 
tion mongrels are more variable than hybrids; but Giirtner 
admits that hybrids from species which have long been cul- 
tivated 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 distinct 
species; and this shows that the difference in the degree of 
variability graduates away. When mongrels and the more 
fertile h3'brids 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 gener- 
ations of mongrels is, perhaps, greater than in hybrids. 

This greater variability in mongrels than in hybrids 
does not seem at all surprising. For the parents of 
mongrels are varieties, and mostly domestic varieties 
(very few experiments having been tried on natural varie- 
ties), and this implies that there has been recent variabil- 
ity, which would often continue and would augment 
that arising from the act of crossing. The slight 
variability of hybrids in the first generation, in coatrast 
with that in the succeeding generations, is a carious 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 producing 
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 reproductive systems in any way affected, and 
thev are not variable; but hvbrids themselves have the re- 
productive systems seriously affected and their descendants 
are highly variable. 

But to return to our comparison of mongrels and 
hybrids: Gartner states that mongrels are more liable 


than liybritis to revert to either parent form; but tliis, if it 
be true, is certainly only a difference iu dci^ree. More- 
over, Gartner expressly states tliat tlie hybri'ls from long 
cultivated plants are more subject to reversion tluin 
hybrids from species in their natural state; and this prob- 
aijly explains the singular difference in the results arrived 
at by different observers. Thus Max Wichura doubts 
vv^hether hybrids ever revert to their parent forms, and lie 
experimented on uncultivated species of willows, while 
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; whei-eas 
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 re- 
semblance 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 ani- 
mals, 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 ^nongrels can be reduced to either pure parent 
form by repeated crosses in successive generations with 
either parent. 

These several remarks are apparently api)licable to ani- 
mals, but the subject is here much comi)licated, partly 
owing to the existence of secondary sexual characters, but 
more especially owing to prepotency in transmitting like- 
ness 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 anotlier 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 hinuy 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 an 
offspring of the male ass and mare, is more like an ass than 
is the hinny, which is the offspring of the female ass and 

Much stress has been laid by some anthors on the sup- 
posed fact, that it is only with mongrels that the offspring 
are not intermediate in character, but closely resemble one 
of their parents; but this does sometimes occur with 
hybrids, yet I grant much less frequently than with mon- 
grels. Looking to the cases which I have collected of 
cross-bred animals closely resembling one parent, the re- 
semblances seem chiefly confined to characters almost mon- 
strous in their nature, and v/hich have suddenly appeared 
— such as albinism, melanism, deficiency of tail or horns, 
or additional fingers and toes; and do not relate to char- 
acters which have been slowly acquired through selection. 
A tendency to sudden reversions to the perfect character 
of either parent v/ould, 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 otlier, namely, in the union of individ- 
uals 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 vaiieties as having been produced by sec- 
ondary laws, this similarity would be an astonishing fact. 
But it harmonizes perfectly with the view that there is no 
essential distinction between species and varieties. 



first crosses between forms,, sufficiently distinct to be 
ranked as species, and their hybrids, are veiy general Iv, 
but not universally, sterile. The sterility is of all degrees', 
imd is often so slight that the most careful experimental- 
ists have arrived at diametrically opposite conclusions in 
ranking forms bj this test. The sterility is innately vari- 
able in individuals of the same species, and is eminentlv 
susceptible to action of favorable and unfavorable condi- 
tions. The degree of sterility does not strictly fallow 
systematic affinity, but is governed by several curious and 
complex laws. It is generally different, and sometimes 
widely different in reciprocal crosses between the same two 
species. It is not always equal in degree in a lirst 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 incidentaf on 
differences, generally of an unknown nature, in their vege- 
tative 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 en- 
dowed 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 circum- 
stances; in some instances in chief part on the early death 
of the embryo. In the case of hybrids, it apparently de- 
pends on their whole organization having been disturbed 
by being compounded from two distinct forms; the ster- 
ility being closely allied to that which so frequently affects 
pure species, when exposed to new and unnatural con- 
ditions "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, tliat, firstly, slight changes in the conditions of 
life add to the vigor and fertility of all organic beings; and 


secondly, that the crossing of forms, which have been ex- 
posed to slightly different conditions of life, or which have 
varied, favors the size, vigor and fertility of their offspring. 
Tiie 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 re- 
ciprocal crosses, clearly leads to the conclusion that the 
primary cause of the sterility of crossed species is confined 
to differences in their sexual elements. But wh}^, in the 
case of distinct species, the sexual elements should so gen- 
erally 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 be- 
tween the species which are crossed. Nor 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 resemblances 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 uni- 
versal and perfect fertility surprisiug, when it is remem- 
bered how liable we are to argue in a circle with respect to 
varieties in a state of nature; and when we remember that 
the greater number of varieties have been produced under 
domestication by the selection of mere external differences, 
and that they have not been long exposed to uniform con- 
ditions of life. It should also be especially kept in mind, 
that long-continued domestication tends to eliminate ster- 
ility, 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 resenil)lance 
between hybrids and mongrels, in their variability, in tlieir 
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 tlie 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 tha*. 
species aboriginally existed as varieties. 





On tlie 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 palaeontological 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. 

In the sixth chapter I enumerated the chief objections 
which might be justly urged against the views maintained 
in this volume. Most of them have now been discussed. 
One, namely, the distinctness of si^ecific 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 favorable for their pres- 
ence, namely, on an extensive and continuous area with 
graduated physical conditions. I endeavored to show, that 
the life of each species depends in a more important manner 
on the presence of other already defined organic forms, than 
on climate, and, therefore, that the really governing condi- 
tions of life do not graduate away quite insensibly like 
heat or moisture. I endeavored, also, to shov/ that inter- 
mediate varieties, from existing in lesser numbers than 
the forms which they connect, will generally be beaten out 
and exterminated during the course of further modifica- 
tion and improvement. The main cause, however, of 
innumerable intermediate links not now occurring every- 
where throughout nature, depends on the very process of 
natural selection, through which new varieties continually 
take the places of and supplant their parent-forms. But 


just in proportion as this process of extermination lias 
acted on an enormous scale, so must the number of inter- 
mediate varieties, which have formerly existed, be truly 
enormous. Why then is not every geological formation 
and every stratum full of such intermediate links? Geo- 
logy assuredly does not reveal any such tlnelv-graduated 
organic chain; and this, perhaps, is the most obvious and 
serious objection Avhich can be urged against the theory. 
The explanation lies, as I believe, in the extreme imper- 
fection 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 wlu^lly 
false view; we should always look for forms intermediate 
between each species and a common but unknown pro- 
genitor; and the progenitor will generally have dilTei-ed 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 intermediate varieties which have ever existed, we 
should have an extremely close series between both and 
the rock-pigeon; but we should have no varieties directly 
intermediate between the fantail and pouter; none, for 
instance, combining a tail somewhat expanded with a crop 
somewhat enlarged, the characteristic features of these two 
breeds. These two breeds, moreover, have become so 
much modified, that, if we had no historical or indirect 
evidence regarding their origin, it would not have been 
possible to have determined, from a mere comparison of 
their structure with that of the rock-pigeon, C. livia, 
whether they had descended from this species or from 
some other allied form, such as 0. oenas. 

So, with natural species, if we look to forms very dis- 
tinct, 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 organization much general resemblance to the tapir 
and to the horse; but in some points of structure may 
have differed considerably from both, even perhaps mora 


than they differ from each other. Hence, in all such 
cases, we should be unable to recognize the j^arent-form of 
any two or more species, even if we closely compared the 
structure of the parent with that of its modified descend- 
ants, 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, while its descend- 
ants had undergone a vast amount of change; and the 
principle of competition between organism and organism, 
between child and parent, will render this a very rare 
event; for in all cases the new and improved forms of life 
tend to snpplant the old and unimproved 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 backward, always converging to the 
common ancestor of each great class. So that the number 
of intermediate and transitional links, between all living 
and extinct species, must have been inconceivably great. 
But assuredly, if this theory be true, such have lived upon 
the earth. 


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 liardly possible for me to recall to the reader who is 
not a practical geologist, the facts leading the mind feebly 
to comprehend the (apse of time. He who can read Sir 
Charles LyelTs grand work on tlie Principles of Geology, 
which the future historian will recognize as having pro- 


diiced a revolution in natunil science, and yet does not 
admit how vast have been the past periods of lime, may at 
once close this volume. Not tliat it suilices to study tlie 
Principles of Geolog-y, or to read special treatises by dltler- 
ent observers on separate formations, and to mark liow 
each author attempts to give an inadequate idea of tlie 
duration of each foj-mation, or even of each stratum. Wc 
can best gain some idea of past time by knowin.i? tlie agen- 
cies at work; and learning how deeply the surface of^tiie 
land has been denuded, and how much sediment lias been 
deposited. As Lyell has well remarked, 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 examine 
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 some- 
thing 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 degrada- 
tion. The tides in most cases reach the cliffs only fur a 
short time twice a day, and the waves eat into them only 
when they are charged with sand or 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 be 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 re- 
treating cliffs rounded boulders, all thickly clothed 
by marine productions, showing how little they are 
abraded, and how seldom they are rolled about I More- 
over, 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 rouiul a 
promontory, that the cliffs are at the present time suffering. 
The appearance of the surface and the vegetation show tliat 
elsewhere years have elapsed since the waters washed iheir 

We have, however^ recently learned from the observa- 



tious of Rainsay, in the van of many excellent observers — 
of Jukes, Geikie, Croll and others, that subaerial degrada- 
tion is a mucli more important agency than 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, witli its dissolved carbonic acid, and in colder coun- 
tries 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 wdiich flow down every slope. Messrs. Eamsay 
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, can not have 
been thus formed, for each line is composed of one and the 
same formation, while our sea-cliffs are everywhere formed 
by the intersection of various formations. This being the 
case, we are compelled to admit that the escarpments 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 worn away through subaerial and littoral action, it 
is good, in 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 sedimentary forma- 
tions. I remember having been much struck when view- 
ing 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 upward of thirty miles, and 

along this line the vertical displacement of the strata varies 

from 600 to 3,0U0 feet. Professor Eamsay has published 

an account of a downthrow in Anglesea of 2,300 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 

sedimentary 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 Eamsay has given me the maximum thickness, 

from actual measurement in most cases, of the successive 

formations in dirfferent parts of Great Britain; and this is 

the result: 


Palaeozoic strata (not including igneous beds) 57,154 

Secondary strata "^o'o m 

Tertiary strata w,L40 

—making altogether 72,584 feet; that is, very nearly thir- 
teen and three-quarters British miles. Some of the for- 
mations, which are represented in England by thm beds, 
are thousands of feet in thickness on the Continent. More- 
over, betv/een each successive formation we have, in tlie 
opinion of most geologists, blank periods of enormous 
length. So that the lofty pile of sedimentary 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 endeavor to grapple with the idea 
of eternity. 

Nevertheless this impression is partly false. Mr. Croll, 
in an interesting paper, remarks that we do not err *Mn 
forming too great a conception of the length of " geological 
periods,'^ but in estimating them by years. When geolo- 
gists look at large and complicated phenomena, and then 
at the figures representing several million years, the two 
produce a totally dilferent 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 
1,000 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 if halved or quartered 
it is still very surprising. Few of us, however, know what 
a million really means: Mr. Croll gives the following illus- 
tration: Take a narrow strip of paper, eighty-three feet 
four 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 sup- 
posed that species in a state of nature ever change so 
quickly as domestic animals under the guidance of method- 
ical 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 modifyino- 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 slowlv, 
and within the same country only a few change at tlie sanie 
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 tlie 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 
circumstance, would not alwavs occur at once. Un- 
fortunately we have no means 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 


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 palaeontologist, 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 eartli has been 
geologically explored, and no part with sufficient care, as 
the important discoveries made every year in Europe 
prove. No organism wholly soft can be preserved. Shells 
and bones decay and disappear when left on the bottom of 
the sea, where sediment is not accumulating. AVe prob- 
ably take a quite erroneous view, when we assume tluit 
sediment is being deposited over nearly the whole bed of 
the sea, at a rate sufficiently quick to imbed and preserve 
fossil remains. Throughout an enormously large propor- 
tion of the ocean, the bright blue tint of the water be- 


speaks its purity. The many cases on record of a forma- 
tion conformably covered, after an immense interval of 
time, by another and later formation, without the under- 
lying 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 /emains which do become imbedded, if in sand 
or gravel, will, when the beds are upraised, generally 
be dissolved by the percolation of rain water charged 
with caibolic acid. Some of the many kinds of animals 
which live on the beach between high and low water 
mark scbni to be rarely preserved. For instance, the 
several species of the Chthamalina3 (a sub-family of sessile 
cirripedco) coat the rocks all over the world in intinite 
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 for- 
mation: yet it is known that the genus Olithamalus 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 in- 
stances 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 resj^ect to the terrestrial productions which lived 
during the Secondary and Palaeozoic periods, it is super- 
fluous to state that our evidence is fragmentary in an 
extreme degree. For instance, untill 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 marnmiferous remains, a glance at the histor- 
ical table published in Lyell's Manual will bring home tlie 
truth, how accidenral and rare is their preservation, f;i!- 
better than pages of detail. Nor is their rarity surprisi ::,;•, 
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 hicustrino 
bed is known belonging to the age of our secondary or 
palaeozoic formations. 

But the imperfection in the geological record largely re- 
sults from another and more important cause than any of 
the foregoing; namely, from the several formations being 
separated from each other by wide intervals of time. This 
doctrine has been emphatically admitted by many geologists 
and palaeontologists, who, like E. Forbes, entirely disbe- 
lieve in the change of species. When we see the forma- 
tions 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 
K. 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 skillful geologist, if his attention had 
been confined exclusively to these large territories, would 
never have suspected that during the periods which were 
blank and barren in his own country, great piles of sedi- 
ment, charged with new and peculiar forms of life, had 
elsewliere been accumulated. And if, in every separate 
territory, hardly any idea can be formed of the length of 
time which has elapsed between the consecutive forma- 
tions, we may infer that this could nowhere be ascertained. 
The frequent and great changes in the mineralogical com- 
position 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 for- 

We can, I think, see why the geological formations 
of each region are almost invariable 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 w^- 
raised several hundred feet within the recent period, than 
the absence of anv 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 sev- 



eral 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 sedi- 
ment must for ages have been great, from the enormous 
degradation of the coast rocks and from the muddy streams 
entering the sea. The explanation, no doubt, is that the 
littoral and sublittoral deposits are continually worn away, 
as soon as they are brought up by the slow and gradual 
rising of the land within the grinding action of the coast 


"We may, I think, conclude that sediment must be ac- 
cumulated in extremely thick, solid, or extensive masses, 
in order to withstand 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 neighborhood 
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 sedi- 
ment nearly balance each other, the sea will remain shallow 
and favorable 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 conclusion that it was 
accumulated during subsidence. I may add, that the only 
ancient tertiary formation on the west coast of South 
America, which has been bulky enough to resist such de- 
gradation as it has as yet suffered, but which will hardly 


last to a distant geological age, was deposited duriijfr a 
downward oscillation of level, and thus gained considerable 

All geological facts tell us plainly tluit each area has 
"undergone numerous slow oscillations of level, and appar- 
ently these oscillations have affected wide spaces. Conse- 
quently, formations rich in fossils and sumciently tliick 
and extensive to resist subsequent degradation, will have 
been formed over wide spaces during periods of sub- 
sidence, 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 sta- 
tionary, thick deposits cannot have been accumulated in 
the shallow parts, which are the most favorable to life. 
Still less can this have happened during the alternate 
periods of elevation; or, to speak more accurately, tiie beds 
which were then accumulated will generally have been de- 
stroyed by being upraised and brought within the limits of 
the coast-action. 

These remarks apply chiefly to littoral and sublittoral 
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 thirty or forty to sixty fath- 
oms, a widely extended formation migiit be formed during 
a period of elevation, and yet not suffer excessively from 
denudation during its slow upheaval; but the tiiickness 
of the formation could not be great, for owing to the ele- 
vatory movement it would be less than the depth in which 
it was formed; nor would the deposit be much consoli- 
dated, nor be capped by overlying formations, so that it 
would run a good chance of being worn away by atmos- 
pheric 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, tlie 
deposit formed during the rising movement, thougii not 
thick, might afterward become protected by fresh accumu- 
lations, and thus be preserved for a long pei'iixl. 

Mr, Hopkins also expresses his belief that sedimentary 
beds of considerable horizontal extent have rarely been 
completely destroyed. But all geologists, excepting the 


few who believe that our present metamorphic schists and 
phitonic rocks once formed the primordial nucleus of the 
globe, will admit that these latter rocks have been stripped 
of their coverina: to an enormous extent. For it is 
scarcely possible that such rocks could have been solidified 
and crystallized while 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, etc., were once necessarially 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 Switzerlandc South of the Amazon, Boue colors 
an area composed of rocks of this nature as equal to that 
of Spain. Prance^ -^taly. part of Germany, and the British 
Islands^ all conjoined. This region has not been carefully 
explored, but from the concurrent testimony of travelers, 
the granitic area is very large: thus Von Eschwege gives a 
detailed section of these rocks, stretching from Eio de 
Janeiro for 260 geographical miles inland in a straight 
line; and I traveled for 150 miles in another direction, and 
saw nothing but granitic rocks. Numerous specimens, 
collected along the whole coast, from near Eio Janeiro to 
the mouth of the Plata, a distance of 1,100 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 rock, 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' beautiful map, I have estimated tlie areas by 
cutting out and weighing the paper, and I find that the 
metamorphic (excluding the " semi-metamorphic^') and 
granite rocks exceed, in the proportion of 19 to 12.5, 
the whole of the newer Palaeozoic formations. In many 
regions tlie metamorphic and granite rocks would be found 
much more widely extended than they appear to be, if all 


the sedimentary beds were removed whicli rest uiicoii- 
formably on them, and which could not have formed part 
of the original mantle nnder which they were crystallized. 
Hence, it is probable that in some parts of the world wh(>le 
formations have been completely denuded, witli not a wreck 
left behind. 

One remark is here worth a passing notice. During 
periods of elevation the area of the laud and of the adjoin- 
ing shoal parts of the sea will be increased and now t^ta- 
tions will often be formed — all circumstances favorable, as 
previously explained, for the formation of new varieties 
and species; but during such periods there will general Iv 
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 np into an archipelago), and 
consequently during subsidence, though there will bo 
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 

ON" THE abse:n'ce of numerous intermediate varie- 

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 commence- 
ment 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 Trautsehold gives a 
number of instances with Ammonites, and Ililgendorf 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 

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 palasontolo^ists, whose opinions are worthy of 
much deference, namely Bronn and Woodward, have con- 
cluded 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 forma- 
tion, it would be rash in the extreme to infer that it had 
not elsewhere previously existed. 80 again, when we find 
a species disappearing before the last layers have been de- 
posited, 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 sev- 
eral stages of the same formation throughout Europe been 
correlated with perfect accuracy. 

We may safeW 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 
apiDcaring 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 lias everywhere been noted, that some few still 
existing species are common in the deposit, but have 
become extinct in the immediately surrounding sea; or, 
conversely, that some are now abundant in the neighbor- 
ing 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, inchidincj fossil rcmaini^y 
have gone on accumulating within tlie same ?\rea during 
the whole of this period. It is not, for instance, pr()])ablo 
that sediment was deposited during tlie whole of tlie 
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 geograpliical 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 diiferent levels, 
owing to the migrations of species and to geographical 
changes. And in the distant future, a geologist, examin- 
ing these beds, would be tempted to conclude that tlio 
average duration of life of the 3mbedded fossils had been 
less than that of tlie 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 oidy 
during a period of subsidence; and to keej) Hie deptli 
approximately the same, which is necessary that tiic 
same marine species may live on the same space, the sup- 
ply of sediment must nearly counterbalance the amount 
of subsidence. But this same movement of subsi- 
dence will tend to submerge tlie area whence the sediment 
is derived, and thus diminish the supply, while tlie down- 
ward movement continues. In fact, this neai-ly exact bal- 
ancing 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 tlu* 


whole pile of formation in any country, has generally been 
intermittent in its accumulation. When we see, as is so 
often the case^ a formations composed of beds of widely 
different mineralogical composition^ we may reasonably 
suspect that the process of deposition has been more or less 
interrupted, ."N'or will the closest inspection of a forma- 
tion give us any idea of the length of time which its depo- 
sition 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 thick- 
ness, and which must have required an enormous period 
for their accumulation; 3^et no one ignorant of this fact 
would have even suspected the vast lapse of time repre- 
sented by the thinner formatioUo Many cases could be 
given of the lower beds of a formation having been up- 
raised, denuded, submerged, and then recovered 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 evi- 
dence in great fossilized 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 
1,400 feet thick in Nova Scotia, with ancient root bearing 
strata, one above the other, at no less than sixty-eight dif- 
ferent levels Hence, when the same species occurs at tlie 
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 
formation, a section would not include all the fine inter- 
mediiite 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 wliich 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 geolog- 
ical 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, uidess at 
the same time it could be closely connected by intermedi- 
ate 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. 80 
that we might obtain the parent-species and its several , 
modified descendants from the low^er^and upper beds of the 
same formation, and unless we obtained numerous transi- 
tional gradations, we should not recognize their blood-rela- 
tionship, and should consequently rank them as distinct 

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 differ- 
ent 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 chanije 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 aiul 
Pictet, maintain that all these tertiary species are specific- 
ally distinct, though the distinction is admitted to be very 
slight; so that here, unless we believe that tliese 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 ratlior 
wider intervals of time, namely, to distinct but consecu- 
tive stages of the same great formation, we find that the 
embedded fossils, though universally ranked as specific- 
ally different, yet are far more closely related to each other 


than are the species found in more widely separated forma- 
tions; so that here again we have undoubted evidence of 
change in the direction ri>quired by the theory; but to this 
latter subject I shall return in the following chapter. 

AVith 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 
widel}^ and supplant their parent-form until they have 
been modified and perfected in some considerable degree. 
According to this view, the chance of discovering in a 
formation in any one country all the early stages of transi- 
tion between any two forms, is small, for the successive 
changes are sujjposed to have been local or confined to 
some one spot. Most marine animals have a wide range; 
and we have seen that with plants it is those which have 
the widest range, that oftenest present varieties; so that, 
with shells and other marine animals, it is j^robable that 
those which had the widest range, far exceeding the limits 
of the known geological formations in Euroj^e, have often- 
est 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 modifica- 
tion, though long as measured by years, was probably short 
in comparison with that during which it remained without 
undergoing any change. 

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 our- 
selves 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 
Btock or from several aboriginal stocks; or again, whether 


certain sea-shells inhabiting the shores of North America, 
which are ranked by some conchologists as distinct species 
from their European representatives, and by other con- 
chologists 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 improbable 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-tifths 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 geo- 
logical 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 Kussia ; and therefore 
equals all the geological formations which have been 
examined with any accuracy, excepting those of the United 
States of America. I fully agree with Mr. God win- Austen, 
that the present condition of the Malay Archipehigo, with 
its numerous large islands separated by wide and shallow 
seas, probably represents the former state of Europe, 
while most of our formations were accumulatiusx. 'I'he 
Malav 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 repre- 
sent the natural history of the world ! 

But we have every reason to believe that the terrestrial 


productions of the archipelago would be preserved in an 
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 

Formations rich in fossils of many kinds, and of thick- 
ness sufficient to last to an age as distant in futurity as the 
secondary formations lie in the past, would generally be 
formed in the archipelago onl}^ during periods of subsi- 
dence. 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; while 
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 
Beas within the archipelago, sedimentary beds could hardly 
be accumulated of great thickness 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 subsidence, there 
would probably be much extinction of life; during the 
periods of elevation, there would be much variation, but 
the geological record would then be less perfect. 

It may be doubted whether the duration of anyone great 
period of subsidence over the whole or part of the archi- 
pelago, together with a contemporaneous accumulation of 
sediment, would exceed the average duration of the same 
specific forms; and these contingencies are indispensable 
for the pi'eservation of all the transitional gradations be- 
tween 9.ny 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 probo,ble tha^ each great period of subsidence would 
be interrupted by oscillations of level, and that slight cli- 
matical phanges would intervene during such lengthy 
periods^ and in these cases the inhabitants of the archi- 


pelago would migrate, and no closely consecutive record of 
their modifications could bo preserved in any one forma- 

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 ciiielly 
these far-ranging species, though only some of thenf. 
which wo'ald oftenest produce new varieties; and the vari- 
eties would at first be local or confined to one })lace, but if 
possessed of any decided advantage, or when further modi- 
fied and improved, they would slowly spread and suj)plant 
their parent-forms. When such varieties returned to their 
ancient homes, as they would diifer from their former state 
in a nearly uniform, though perhaps extremely slight degree, 
and as they would be found imbedded in slightly ditter- 
ent sub-stages of the same formation, they would, accord- 
ing to the principles followed by many palaeontologists, bo 
ranked as new and distinct species. 

If then there be some degree of truth in these remarks, 
we have no right to expect to find, in our geological for- 
mations, an infinite number of those fine transitional forms 
which, on our theory, have connected all the jiast and 
present species of the same group into one long and branch- 
ing 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 
tliem be ever so close, if found in difi'erent stages of tho 
same formation, would, by many palaeontologists, bo 
ranked as distinct species. But I do not pretend that I 
should ev3r have suspected how poor was the record in the 
best preserved geological sections, had not the absence of 
innumerable transitional links between the species wliicii 
lived at the commencement and close of each formation, 
pressed so hardly on my theory. 



The abrupt manner in which whole groups of species 
suddenly appear in certain formations, has been urged by 
several palaeontologists— for instance, by Agassiz. Pictet, 
and Sedgwick— as a fatal objection to tho belief in tho 


transmutation of species. If numerous species, belonging 
to the same genera or families, have reall}^ started into life 
at once, the fact would be fatal to the tlieory of evolution 
tlirough 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 certain genera or families have not been found be- 
neath a certain stage, that they did not exist before that 
stage. In all cases positive palaeontological 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 multiplied, before they invaded 
the ancient archipelagoes of Europe and the Unites 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 apjoear 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 or- 
ganism to some new and peculiar line of life, for instance, 
to fly through the air; and consequently that the transi- 
tional forms would often long remain confined to some one 
region; but that, when this adaptation had once been 
effected, and a few species had thus acquired a great ad- 
vantage 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 how the succes- 
sive modifications of the anterior limbs of a supposed pro- 
totype 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 intermediuto 
state of "neither true arms nor true wings?" Yet 
birds hold their place victoriously in the battle for life; fur 
they exist in infinite numbers and of many kinds. 1 do 
not suppose that we here see the real transitional grades 
through which the wings of birds have passed; but wliat 
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 loggi*r- 
headed duck, and ultimately to rise from its surface and 
glide through the air? 

I will now give a few examples to illustrate the fore- 
going 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 1844-4G and in 1853-57, the 
conclusions on the first appearance and disappearance of 
several grouj^s 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, pub- 
lished not many years ago, mammals were always sj^oken 
of as having abruptly come in at the commencement of the 
tertiary series. And now one of the richest known ac- 
cumulations 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 footstens 
in the new red sandstone of the United States, who would 
have ventured to suppose tliat 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 dis- 
covered in these beds. JS^'ot long ap^o, pahTontoh-)gi.st3 
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, tlio 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 Solenho- 
fen. Hardly any recent discovery shows more forcibly 
than this how little we as yet know of the former inhab- 
itants of the world. 

I may give another instance, which, from having passed 
under ray own eyes, has much struck me. In a memoir on 
Fossil Sessile Cirripedes, I stated that, from the large num- 
ber of existing and extinct tertiary species; from the ex- 
traordinary abundance of tlie individuals of many species 
all over the world, from the Arctic regions to tiie equator, 
inhabiting various zones of depths, from the upper tidal 
limits to fifty fatiioms; from the perfect manner in v^diicli 
specimens are preserved in the oldest tertiary beds; from 
the ease with v>^hich even a fragment of a valve can be 
recognized; from all these circumstances, I inferred that, 
had sessile cirripedes existed during the secondary periods, 
they would certainly have been preserved and dis- 
covered; and as not one species had then been discov- 
ered in beds of this age, I concluded that this great 
group had been suddenly developed at the commencement 
of the tertiar}^ series. This ^vas a sore trouble to me, 
adding, as 1 then thought, one more instance of the 
abrupt appearance of a gi'eat group of species. But my 
work had hardly been published, wdien a skillful pal^eon- 
togist, M. Bosquut, sent me a drawing of a perfect speci- 
men of an unmistakable 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 subfamily of sessile cirrii^edes, 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 thet eleostean 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 
teieostean; and even some palseozoic forms have thus been 
classed by one high authority. If the teleosteans liud 
really appeared suddenly in the northern hemisphere at 
the commencement 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 ])een 
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 teieostean 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 inclosed basin, in 
which any great group of marine animals might be multi- 
plied; 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 
palseontological 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 foims 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. 


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 fossiliferoiis 
rocks. Most of the arguments 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, etc., do not differ much 
from living species; and it cannot on our theory be sup- 
posed, that these old species were tlie progenitors of all the 
species belonging to the same groups which have subse- 
quently appeared, for they are not in any degree inter- 
mediate 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 W. Thompson concludes that 
the consolidation of the crust can hardly have occurred less 
than twenty or more than four hundred million years ago, 
but probably not less than ninety-eight or more than two 
hundred million years. These very wide limits show how 
doubtful the data are; and other elements mav have here- 
after to be introduced into the problem. Mr. Ci'oll esti- 
mates that about sixty 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 certainlv occurred since the 
Cambrian formation; and the previous one hundred and 
forty million years can hardly be considered as sufficient 
for the development of the varied forms of life which 
already existed during tlie Cambrian period. It is, how- 
ever, 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 tlioso 
now occurring; and such changes would have tended to 
induce changes at a corresponding rate in the or^'anisma 
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 tiiat we beheld in tlie 
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 shoiild not 
forget than only a small portion of the world is known with 
accuracy. Not very lon^ ago M. Barrande added anotiier 
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, ^Ir. Hicks 
has found South Wales beds rich in trilobites, and con- 
taining various molluscs and annelids. The presence of 
phosphatic nodules and bituminous matter, even in some 
of the lowest azotic rocks, probablv indicates life at tliese 
periods; and the existence of the Eozoon in the Lauren tiaii 
formation of Canada is generally adinitted. There are three 
great series of strata beneath the Silurian system in Can- 
ada, 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 tho 
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 organized 
of all classes of animals, but is highly organized 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. Tims 
the words, which I wrote in 1S59, about the existence of 
living beings long before the Cambrian perio<l, 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 oy metamorphic action, for 
if this had been the case we should have found only small 
remnants of the formations next succeeding them in age, 
aud these would always have existed in a partially meta- 
morpliosed condition. But the descriptions which we 
possess of the silurian deposits over immense territories in 
Russia and in Xorth America, do not support the view 
that the older a formation is the more invariably it has 
sutfored extreme denudation and metamorphism. 

The case at present must remain inexplicable, and may 
be truly urged as a valid argument against the views here 
entertained. To show that it may hereafter receive some 
explanation, I will give the following hypothesis. From 
the nature of the organic remains which do not appear to 
have inhabited profound depths, in the several formations 
of Europe and of the United States; and from the amount 
of sediment, miles in thickness, of which the formations 
are composed, we may infer that from first to last large 
islands or tracts of land, whence the sediment was derived, 
occurred in the neighborhood of the now existing conti- 
nents of Europe and North America. This 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 ex- 
tensive 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 pala3ozoic and secondary periods, neither 
continents nor continental islands existed where our oceans 
now extend; for had they existed, palaeozoic and second- 
ary 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, wo may 
infer anything from these facts, we may infer that, whoro 
our oceans now extend, oceans have extended from the 
remotest perioc] of which we have any record; and on 
the other h^md, that where continents now exist, 
large tracts of land have existed, subjected, no doubt, to 
great oscillations of level, since the Cambrian period. The 
colored map appended to my volume on Coral Keefs, led 
me to conclude that the great oceans are still mainlv areas 
of subsidence, the great archipelagoes still areas of oscilla- 
tions of level, and the continents areas of elevation. i^>nt 
we have no reason to assume that things have thus re- 
mained 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. Bu't 
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 nov/ spread out, and clear and open 0(,'eans 
may have existed where our continents now stand. Xor 
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 
recognizable 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 center of the earth, and which had 
been pressed on by an enormous weight of superincumbent 
water, might have undergone far more metamorphic action 
than strata which have always remained nearer to the sur- 
face. The immense areas in some parts of the world, for 
instance in South America, of naked metamorphic rorks, 
which must have been heated under great pressure, have 
always seemed to me to require some special explana- 
tion; and we may perhaps believe that we see in these 
large areas the many formations long {interior to the Cam- 
■^rian epoch in a completely metamorphosed and denuded 


The several difficulties here discussed, nanu'ly, that, 
though we find in our geological formations niany links 
^fitween the species which n^w exist and which formerly 


existed, we do not find infinitely nnmerons fine transitional 
forms closely joining them ail together. The sudden man- 
ner 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 em- 
inent paleontologists, namely, Cuvier, Agassiz, Barrande, 
Pictet, Falconer, E. Forbes, etc., and all our greatest geol- 
ogists, as Lyell, Murchison, Sedgwick, etc., have unani- 
mously, 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 de- 
gree 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 pos- 
sess 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 




On the slow and successive appearance of new species — On tlieir 
different rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their appear- 
ance 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. 

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 modifica- 
tion, through variation and natural selection. 

New species have appeared very slowly, one after another, 
both on the land and in the waters. Lyell has shown tliat 
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 second- 
ary formations are more broken; but, as Bronn has 
remarked, neither the appearance nor disappearance of the 
many species embedded in each formation nas been simul- 

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 
fo^ind 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 snb-Himalayaii 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 greatty. 
The productions of the land soem to have changed at a 
quicker rate than those of the sea, of which a striking 
instance has been observed in Switzerland. 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 formations, all the species will be found to 
have undergone some change. When a species has once 
disappeared from the face of the earth, we have 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 expla- 
nation, 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 simultane- 
ously, 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 varia- 
tions or individual differences as may arise will be aocu- 
mulated through natural selection in a greater or less 
degree, thus causing a greater or less amount of perma- 
nent modification, will depend on many complex contin- 
gencies — 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 competition. 
Hence it is by no menus surprising tliat one species should 
retain the same identical form much longer than others; 


or, if changing, should change in a less degree. We lind 
similar relations between the existing inhubitunts of dis- 
tinct_ countries; for instance, the land-shells and coU-opter- 
ous insects of Madeira have come to ditler considerably 
from their nearest allies on the continent of Europe, 
whereas the marine shells and birds have remained 
unaltered. We can perhaps understand the a])parently 
quicker rate of change in terrestrial and in more higlilv 
organized productions compared with marine and lowe'r 
productions, by the more complex relations of tlie liigher 
beings to their organic and inorganic conditions of life, as 
explained in a former chapter. When many of the inhab- 
itants of any area have become modified and imj)roved, 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 

In members of the same class the average amount of 
change, during long and equal periods of time, may, per- 
haps, 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 accumulated at 
wide and irregularly intei'mittent intervals of time; conse- 
quently the amount of organic change exhibited by the fos- 
sils 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 onre lost 
should never reappear, even if the very same conditions of 
life, organic and inorganic, should recur. For though the 
•offspring of one species might be adapted (and no doubt 
this has occurred in innumerable instances) to fill the 
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 dib- 


tinct progenitors; and organisms already differing would 
vary in a different manner. For instance, it is possible, if 
all our fantail 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 estab- 
lished race of the domestic pigeon, for the successive varia- 
tions would almost certainly be in some degree different, 
and the newly-formed variety would probably inherit from 
its progenitor some characteristic differences. 

Groups of species, that this, genera and families, follow 
the same general rules in their appearance and disappear- 
ance 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. 1 am aware that there are 
some apparent exceptions to this rule, but the exceptions 
are surprisinglv few, so few that E. Forbes, Pictet, and 
Woodward (though all strongly opposed to such views as I 
maintain) admit its truth; and the rule strictly accords 
with the theory. For all the species of the same group, 
however long it may have lasted, are the modified descend- 
ants one from the other, and all from a common progeni- 
tor. In the genus Lingula, for 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 exceptional; 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 bei-in 
at Its lower end, not in a sharp point, but abruptly it 
then gradually thickens upward, often keepiug of equal 
thickness for a space, and ultimately tliins out in the 
upper beds, marking the decrease and finiil 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 p>roduction 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 
produce 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. 


"We have as yet only spoken incidentally of the disap- 
pearance of species and of groups of species. On the 
theory of natural selection, the extinction of old forms 
and the production of new and improved forms are inti- 
mately connected together. The old notion of all the 
inhabitants of the earth having been swept away by catas- 
trophes at successive periods is very generally given up, even 
by those geologists, as Elie de Beaumont, Murchison, Bar- 
rande, etc., whose general views would naturally lead 
them to this conclusion. On the contrary, we have every 
reason to believe, from the study of the tertiary formations, 
that species and groups of species gradually disappear, one 
after another, first from one spot, then from another, and 
finally from the world. 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, tlie process of ex- 
tinction may have been rapid. Both single species and 
whole groups of species last for very unequal ])eriods; some 
groups, as we have seen, have endured from the earliest 
known dawn of life to the present day; some have disap- 
peared before the close of the palaeozoic period. Xo fixed 
taw 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 t'ne line is 
found to taper more gradually at its upper end, which 
marks the progress of extermination, than at its lower end, 
which marks the first appearance and the early increase in 
number of the species. In some cases, however, the extermi- 
nation of whole groups, as of ammonites, toward 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. Xo 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 v/ith 
the remains of Mastodon, Megatherium, Toxodon and 
other extinct monsters, Avhich 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 in- 
troduction by the Spaniards into South America, has run 
Avild over the whole country and has increased in numbers 
at an unparalleled rate, I asked myself what could so re- 
cently have exterminated the former horse under conditions 
of life apparently so favorable. 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 attribute of a vast 
number of species of all classes, in all countries. If we 
ask ourselves why this or that species is rare, we answer 
that something is unfavorable in its conditions of life; but 
whai 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 naturalization of the domestic horse in 
South America, that under more favorable conditions it 
would in a very few years have stocked the wdiole con- 
tinent. But we could not have told what the unfavorable 


conditions were Avhicli checked its incre<ase, whether Roine 
one or several contingencies, and at what pericxl of tlje 
horse's life, and in what degree tliey severally acLod. If 
the conditions had gone on, however slowly, becomii\g less 
and less favorable, we assuredly should not have perceived 
the fact, yet the fossil horse would certaiiily have become 
rarer aiul rarer, and finally extinct — its place being seizx'd 
on by some more successful competitor. 

It is most difficult always to remember that the increase 
of every creature is constantly being checked by unper- 
ceived hostile agencies; and tliat these same unper- 
ceived agencies are amply sufficient to cause rarity, and 
fi.ually extinction. So little is this subject understood, 
that I have heard surprise repeatedly expressed at such 
great monsters as the Mastodon and the more ancient I)in- 
osaurians having become extinct; as if mere bodily strength 
gave victory in the battle of life. Mere size, on the con- 
trary, 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 highlj capable judge. Dr. Fal- 
coner, believes that it is chiefly insects which, from inces- 
santly harassing and weakening the elepliant in India, 
check its increase; and this was Bruce'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 
South America. 

We see in many cases in the more recent tertiary forma- 
tions that rarity precedes extinction; and we know that 
this has been the progress of events with those animah 
which have been exterminated, either locally or wliollv, 
tliroudi man's agency. I may repeat what I i)ul)lished in 
1845,^namelv, that to admit that species generally become 
rare before they become extinct— to feel no surpn.-e at the 
rarity of a species, and yet to marvel greatly when the spe- 
cies ceases to exist, is much the same as to adnnt that sick- 
ness in the individual is the forerunner of deatii— to feel 
no surprise at sickness, but, when the sick man dies, to 
wonder and to suspect that he died by some deed of 


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 con- 
sequent extinction of the less-favored forms almost inevita- 
bly follows. It is the same with our domestic productions; 
wlien a new and slightly improved variety has been raised, 
it at first supplants the less improved varieties in the same 
neighborhood; when much improved it is transported far 
and near, like our short-horn cattle, and takes the place of 
other breeds in other countries. Thus the appearance of 
new forms and the disappearance of old forms, both those 
naturally and those artificially produced, are bound 
together. In flourishing groups, the number of new spe- 
cific forms which have been produced within a given time 
has at some periods probably been greater than the number 
of the old sj^ecific forms which have been exterminated; 
but we know that species have not gone on indefinitely in- 
creasing, at least during the later geological epochs, so 
that, looking to later times, we may believe that the pro- 
duction of new forms has caused the extinction of about 
the same number of old forms. 

The competition will generally be most severe, as for- 
merly explained and illustrated by examples, between the 
forms which are most like each other in all respects. Hence 
the improved and modified descendants of a species will 
generally cause the extermination of the parent-species; 
and if many new forms have been developed from any 
one species, the nearest allies of that species, i. e. 
the species of the same genus, will be the most liable 
to extermination. Thus, as I believe, a number of new 
species descended from one species, that is a new genus, 
comes to supplant an old genus, belonging to the same 
family. But it must often have happened that a new 
species belonging to some one group 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 'n 
common. But whether it be species belonging to the 
same or to a distinct class, wlaich have yielded their places 



to other modified and improved species, a few of the suf- 
ferers may often be preserved for a long time, from bein^' 
fitted to some peculiar line of life, or from inhabiting some 
distant and isolated station, where they will have escai)L'd 
severe competition. For iiistance, some species of Tri-^o- 
nia, 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 un- 
derstand 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 per- 
ceived by us, the whole economy of nature will be utterly 
obscured. Whenever we can precisely say why this species 
is more abundant in individuals than that; why this 
species and not another can be naturalized in a given coun- 
try; then, and not until then, we may justly feel surprise 
why we cannot account for the extintion of any particular 
species or group of species. 





Scarcely any pal^eontological discovery is more striking 
than the fact tliat the forms of life change almost simul- 
taneously throughout the world. Thus our European 
Chalk formation can be recognized in many distant regions, 
under the most different climates, where not a frag- 
ment 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 unmistakable resemblance to 
those of ihe 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 sectioiis of genera, and sometimes are similarly char- 
acterized in such trifling points as mere superficial sculp- 
ture. Moreover, other forms, which are not found in the 
Chalk of Europe, but whicli 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 
formatioiis 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 Lvell, with the 
Fiuropean and North American tertiary deposits. Even if 
the few fossil species which are common to the Did and 
New Worlds were kept wholly out of view, the general 
parallelism in the successive forms of life, in the palceozoic 
and tertiary stages, would still be manifest, and the several 
formations could be easily correlated. 

These observations, however, relate to the marine inhabi- 
tants of the world: we have not sufficient data to judge 
whether the productions 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 Mega- 
therium, Mylodon, Macrauchenia, and Toxodon had been 
brought to Europe from La Plata, without any information 
in regard to their geological position, no one would have 
suspected that they had co-existed with sea-shells all still 
living; but as these anomalous monsters co-existed with 
the Mastodon and Horse, it might at least have been in- 


ferred that they had lived during one of the later tertiary 


AVlien the marine forms of life are spoken of as having 
changed simultaneously throughout the world, it must nol 
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 tlie 
pleistocene period (a very remote period as measured by 
years, including the whole glacial epoch) were compjired 
with those now existing in South America or in Australia, 
the most skillful naturalist would hardly be able to Bay 
whether the present or the pleistocene inhabitants of 
Europe resembled most closely those of the southern hemi- 
sphere. So, agai]i, 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 in- 
habitants of Europe; and if this be so, it is evident that 
fossiliferous beds now deposited on the sliorcs of Korth 
America would hereafter be liable to be classed with some- 
what older European beds. Nevertheless, looking to a 
remotely future epoch, there can be little doubt that all 
the more modern marine formations, namely, the upjjcr 
pliocene, the pleistocene and strictly modern beds of 
Europe, North and South America, and Australia, from 
containing fossil remains in some degree allied, and from 
not including those forms which are found only in the 
older underlying deiDosits, would be correctly ranked as 
simultaneous in a geological sense. 

The fact of the forms of life changing simultaneously in 
the above large sense, at distant parts of the world, has 
greatly struck those admirable observers, ^iM. de Verneiiil 
and d^Archiac. After referring to the j)ai-allelism of flic 
paleozoic forms of life in various j)arts of Europe, they add, 
"11, struck by this strange sequence, we turn our atten- 
tion to North America, and there discover a series of ana- 
logous phenomena, it will appear certain that all these 
modifications of species, their extinction, and the intro- 
duction of new ones, can not be owing to mere changes in 
marine currents or other causes more or less local and 
temporaiy, but depend on general laws which govern the 


whole animal kingdom." M. Barrande has made forcible 
remarks to precisely the same effect. It is, indeed, quite 
futile to look to changes of cicrrents, climate, or other phy- 
sical 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 
wlien we treat of the present distribution of organic beings, 
and find how slight is the relation between the johysical 
conditions of various countries and the nature of their 

This great fact of the parallel succession of the forms of 
lif? throughout the world, is explicable on the theory of 
nainral selection. 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 giviug rise in new countries to other new varieties 
and species. The process of diffusion would often be very 
elow, depending on climatal and geographical changes, on 
strange accidents, and on the gradual acclimatization of 
new species to the various climates through which they 
might have to pass, but in the course of time the domi- 
nant forms would generally succeed in spreading and would 
ultimately prevail. The diffusion would, it is probable, be 
slower with the terrestrial inhabitants of distinct conti- 
nents than with the marine inhabitants of the continuous 
sea. We might therefore expect to find, as we do find, a 
less strict degree of parallelism 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 pro- 
duced being themselves dominant, owing to their having 
had some advantage over their already dominant pan-rits, 
as well as over other species, and again spreading, varying! 
and producing new forms. The old forms wliich are 
beaten and which yield their places to the new and victori- 
'^us forms, will generally be allied in groups, from- inherit- 
ing some inferiority in common; and, therefore, as new 
and improved groups spread throughout the world, oKl 
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 tliat blank 
intervals of vast duration, as far as fossils are concerned, 
occurred during the periods wlien the bed of the sea was 
either stationary or rising, and likewise wlien sediment 
was not thrown down quickly enough to imbed and pre- 
serve organic remains. During these long and blank inter- 
vals 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 tliat 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 invariably been affected by the 
same movements. When two formations have been depos- 
ited 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 Eurojie. 
Mr. Prestwich, in his admirable .Memoirs on the eocene 
deposits of England and France, is able to draw a close 


general parallelism between the successive stages in the 
two countries ; but when he compares certain stages in 
England with those in France, although he finds in both a 
curious accordance in the numbers of the species belonging 
to the same genera, yet the species tJiemselves differ in a 
manner very difficult to account for considering the prox- 
imity 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 obser- 
vations 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 suprising amount of difference 
in the species. If the several formations in these regions 
have not been deposited during the same exact periods 
— a formation in one region often corresponding v*dth a 
blank interval in the other — and if in both regions the 
species have gone on slowly changing during the accumu- 
lation of the several formations and during the long inter- 
vals 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. 



Let ns 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 fill up the intervals between existing 
genera, families and orders, is certainly true; but as this 
statement has often been is^nored or even denied, it mav 
be well to make some remarks on this subject, and to give 
Bome instances. If we confine our attention either to the 


living or to the extinct species of the same class, tlie Buries 
is far less perfect than if we combine both into one geiiorai 
system. In the writings of Professor Owen we contimialiv 
meet with the expression of generalized forms, as apj.lifd 
to extinct animals; and in the writings of Agassiz, of 
prophetic or synthetic types; and these terms irnply that 
such forms are, in fact, intermediate or connecting "links. 
Another distinguished paleontologist, M. Gaud7v, has 
shown in the most striking manner Uiat many of the fossil 
mammals discovered by him in iittica serve to break down 
the intervals between existing genera. Cuvier ranked the 
Ruminants and Pachyderms as two of the most distinct 
orders of mammals; but so many fossil links have been dis- 
entombed that Owen has had to alter the whole classifica- 
tion, and has placed certain Pachyderms in the same 
sub-order Avith 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 South America connects to a certain 
extent these two grand divisions. Ko one will deny that 
the Hipparion is intermediate between the existing horse 
and certain other ungulate forms. What a wonderful con- 
necting link in the chain of mammals is the Typotherium 
from South America, as the name given to it by Professor 
Gervais expresses, and which cannot be placed in any ex- 
isting order. The Sirenia form a very distinct group of tlie 
mammals, and one of the most remarkable peculiarities in 
existing dugong and lamentin is the entire absence of hind 
limbs, without even a ludiment being left; but the extinct 
Halitherium had, according to Professor Flower, an ossitiod 
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 otht-r re- 
spects allied. The cetaceans or whales are widely ditTerent 
from all other mammals, but the tertiary Zouglodon and 
Squalodon. which have been placed by some naturalists in 
an order by themselves, are considered by Professor Huxley 
to be undoubtedly cetaceans, *'aud to constitute connect- 
ing links with the aquatic carnivora.'^ 

Even the wide interval between birds and reptiles has 
been shown by the naturalist j'ust quoted to be ])ariiully 


bridged over in the most unexpected manner, on the one 
hand, by the ostrich and extinct Archeopter3^x, and on the 
other hand by tlie Conipsognathus, one of the Dinosaurians 
— that group which inchides the most gigantic of all ter- 
restrial reptiles. Turning to the Invertebrata, Barrande 
asserts, a higher authority could not be named, that he is 
every day taught that, although palaeozoic animals can cer- 
tainly be classed nnder 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 interme- 
diate in all its characters between two livinsr 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 dis- 
tinct 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 char- 
acters 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 
toward very distinct groups. Yet if we compare the older 
reptiles and Batrachians, the older fish, the older cephalo- 
pods, and the eocene mammals, with the recent members 
of tlie 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 itaHcs represent 
genera, and the dotted lines diverging from them the spe- 
cies in each genus. The diagram is much too siinple, too 
few genera and too few species being given, but tliis is un- 
important for us. The horizontal lines may represent suc- 
cessive geological formations, and all the forms beneath 
the uppermost line may be considered as extinct. The 
three existing genera «i*, q^^, jj^^, will form a small 
family; b^^ and/^*, a closely allied family or subfamily, 
and ci*, e^*, m^*, 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 illustrated by this diagram, the more recent any 
form is the more it will generally differ from its ancient 
progenitor. Hence, we can understand the rule that the 
most ancient fossils differ most from existing forms. We 
must not, however, assume that divergence of character is 
a necessary contingency; it depends solely on the descend- 
ants 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 slio^htlv altered conditions of 
life, and yet retain throughout a vast period tne same gen- 
eral characteristics. This is represented in the diagram by 
the letter F^*. 

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 diverg- 
ence of character, has become divided into several siih- 
families and families, some of which are supi)osed to have 
perished at different periods, and some to have endured to 
the present day. 

By looking at the diagram we can see tliat if many of 
the extinct forms supposed to be imbedded in the successive 
formations, were discovered at several points low down in 
the series, the three existing fa?nilies on the unpcrmost 
line would be rendered less distinct from each other. If, 


for instance, the genera a}, a^, a}'^, /', m', m", m^, were dis- 
interred, 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 fami- 
lies, 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 discovered above one of the middle hori- 
zontal 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, a}^, etc., and Z>^*, 
etc.) 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^* to wi^*), on the uppermost line, be supposed to 
differ from each other by half-a-dozen iuiportaut char- 
acters, then the families which existed at a 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 
charactei' 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 geological periods, undergone much modification, 
should in the older formations make some slight approach 
to each other; so that the older members should differ less 
from each other in some of their characters than do thQ 


existing members of the same groups; and tliis by tlio 
concurrent evidence of our best palseontologists is fre- 
quently the case. 

Thus, on the theory of descent witb modification, tlio 
main facts with respect to the mutual aOiiiiiies of tbo 
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 tiie 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 fiftli stage, 
and are the parents of those which became still more 
modihed at the seventh stao^e; hence thev could hardlv fail 
to be nearly intermediate in character between the forma 
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 otlier 
regions, and for a large amount of modification during the 
long and blank intervals between the successive formationd. 
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 svstem, when this svstem was first discovered, 
were at once recognized by palaeontologists as intermodiato 
in character between those of the overlying carboniferous 
and underlying Silurian systems. But each fauna is not 
necessarily exactly intermedi ite, as unequal intervale 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 intennediato 
in character between the preceding and succeeding fauna?, 
that certain genera offer exceptions to tbe rule. For instance, 
the species of mastodons 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 
moat recent; nor are those which are intermediate m ciiar- 



acter, intermediate iu 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 
successivel}^ 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 principle living and extinct races of the 
domestic pigeon were arranged in serial affinity, this ar- 
rangement 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 beak originated earlier 
than short-beaked tumblers, which are at the opposite end. 
of the series in this respect. 

Closely connected with the statement, that the organic 
remains from an intermediate formation are in some degree 
intermediate in character, is the fact, insisted on by all 
pala3ontologists, that fossils from two consecutive forma- 
tions are far more closely related to each other, than are 
the fossils from two remote formations. Pictet gives as 
a well-known instance, the general resemblance of the 
organic remains from the several stages of the Chalk 
formation, though the species are distinct in each stage. 
This fact alone, from its generality, seems to have 
shaken Professor Pictet in his belief in the immutability 
of species. He who is acquainted with the distribution 
of existing species over the globe, will not attempt to 
account for the close resemblance of distinct species in 
closely consecutive formations, by the physical conditions 
of the ancient areas having remained nearly the same. 
Let it be remembered that the forms of life, at least those 
inhabiting the sea, have changed almost simultaneously 
throughout the world, and therefore under the most 
different climates and conditions. Consider the prodig- 
ious viscisitudes 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, tlie full of the fosHil 
remains from closely consecutive fonnutions being- closelv 
related, though ranked as distinct species, is obvious. As 
the accumulation of each formation has often bfeii inter- 
rupted, 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 anv 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 lind after 
intervals, very long as measured by years, but only moder- 
ately long as measured geologically, closely allied forms, 
or, as they have been called by some authors, representa- 
tive 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 



We have seen in the fourth chapter that the degree of 
differentiation and specialization of the parts in organic 
beings, when arrived at maturity, is the best standard, as 
vet suggested, of their degree of perfection or highness. 
We have also seen that, as the specialization of parts is an 
advantage to each being, so natural selection will tend to 
render the organization of each being more specialized and 
perfect, and in this sense higher; not but tliat 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 organization, 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 liave to 
beat in the struggle for life all the older forms, with which 
they come into close competition. We may therefore con- 
clude that if under a nearly similar climate tiie eocene 
inhabitants of the world could be put into competition 
with the existing iidiabitants, the former would be beaten 
and exterminated by the latter, as would the secondary by 
the eocene, and the palaeozoic by the secondary forms. iJo 


that by this fundamental test of victory in the battle for 
life, as well as by the standard of the specialization 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 palaeontologists would 
answer in the affirmative; and it seems that this answer 
must be admitted as true, though difficult of pi-oof. 

It is no valid objection to this conclusion, that certain 
Brachiopods 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. Carpenrer, progressed in 
organization siuce even the Laurentian epoch; for some 
organisms would have to remain fitted for simple condi- 
tions of life, and what could be better fitted for this end 
than these lowly organized Protozoa? Such objections as 
the above would be fatal to my view, if it included advance 
in organization as a necessary contingent. They would 
likewise be fatal, if the above Foraminifera, 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 organ- 
isms up to the standard which they had tlien reached. 
When advanced up to any given point, there is no neces- 
sity, on the theory of natural selection, for their further 
continued process; though they will, during each succes- 
sive 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 
disputed. ? 

The problem whether organization on the whole has 
advanced is in many ways excessively intricate. The geo- 
logical record, at all times imperfect, does not extend far 
enough back to show with unmistakable clearness that 
within the known historv of the world ora,-anization 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 liigh- 
est 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 pre- 
ponderant in number; but formerly 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 organization. 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 organized than a fish, 
although upon another type?'^ In the complex struggle 
for life it is quite credible that crustaceans, not very high 
in their own class, might beat cephalopods, the highest 
mollucs; and such crustaceans, though not highly devel- 
oped, would stand very high in the scale of invertebrate 
animals, if iuda*ed bv the most decisive of all trials — the 
law of battle. Beside these inherent difficulties in decid- 
ing which forms are the most advanced in organization, 
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 bal- 
ance — but we ought to compare all the members, high and 
low, at two periods. At an ancient epoch the highest and low- 
est molluscoidal animals, namely, cephalopods and brachio- 
pods, swarmed in numbers; at the present time both groups 
are greatly reduced, while others, intermediate in orgiini- 
zation, have largely increased ; consequently some natu- 
ralists 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 brachiopods, and the fact that our existing cephalopods, 
though few in number, are more highly oj'gauized than 
their ancient representatives. AVe 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 im- 
plies a great displacement of lower forms, as a decided 
advance in the organization of the world. We thus see 
how hopelessly difficult it is to compare with perfect fair- 
ness, under such extremely complex relations, the standard 
of organization of the imperfectly- known faunas of succes- 
sive periods. 

We shall appreciate this difficulty more clearly by look- 
ing to certain existing faunas and floras. From the extra- 
ordinary 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 naturalized there, and would exter- 
minate many of the natives. On the other hand, from the 
fact that hardly a single inhabitant of the southern hemi- 
sphere 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 skillful 
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 em- 
bryos of recent animals belonging to the same classes; and 
that the geological succession of extinct forms is nearly 
parallel with the embryological development of existing 
forms. This view accords admirably well with our theory. 
In a future chapter I shall attempt to show that the adult 
differs from its embryo, owing to variations having super- 
vened at a not early age, and having been inherited at a 
corresponding age. This process, while it leaves the em- 
bryo 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 modi- 
fied 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 embryological character of the vertebrata^ 
until beds rich in fossils are discovered far beneath tlie 
lowest Cambrian strata — a discovery of which the chance 
is small. 


Mr. Clift many years ago showed that the fossil mam- 
mals from the Australian caves were closely allied to the 
living marsupials of that continent. In South America, a 
similar relationship is manifest, even to an uneducated 
eye, in the gigantic pieces of armor, like those of the arma- 
dillo, 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 re- 
lated 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 in- 
sisted, in 1839 and 1845, on this "law of the succession of 
types, ^^ — on "this wonderful relationship in the same con- 
tinent between the dead and the living." Professor Owen 
has subsequently extended the same generalization 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 showm 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 sncoession of 
the same types within the same areas mean? He would be a 


bold mau 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 conditions, for the dissimilarity 
of the inhabitants of these two continents ; and, on the 
other hand through similarity of conditions, for the uni- 
formity 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 Amer- 
ica the law of distribution of terrestrial mammals was for- 
merly different from what it now is. North America for- 
merly 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 Fal- 
coner and Cautley's discoveries, that Northern India was 
formerly more closely related in its mammals to Africa 
than it is at the present time. Analogous facts could be 
given in relation to the distribution of marine animals. 

On the theory of descent with modification, the great law 
of the long enduring, but not immutable, succession of the 
same types within the same areas, is at once explained ; for 
the inhabitants of each quarter of the world will obviously 
tend to leave in that quarter, during the next succeeding 
period of time, closely allied though in some degree modi- 
fied descendants. If the inhabitants of one continent for- 
merly differed greatly from those of another continent, so 
will their modified descendants still differ in nearly the 
same manner and degree. But after very long intervals of 
time, and after great geographical changes, permitting 
much 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 
megatherium and other allied huge monsters, which for- 
merely lived in South America, have left behind them the 
sloth, armadillo, and ant-eater, as their degenerate descend. 


ants. This cannot for an instant be admitted. 'J'hese 
huge animals have become wholly extinct, and have left no 
progeny. Bnt 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 progeni- 
tors of the living species. It must not be forgotten that, 
on onr theory, all the species of the same genus are the 
descendants of some one species ; so that, if six genera, 
each having eight species, be found in one geological for- 
mation, 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 mo(iified 
descendants, which constitute the new genera containing 
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 genera 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, still fewer genera 
and species will leave modified blood-descendants. 


I have attempted to show that the geological record is 
extremely imperfect ; that only a small portion of the 
globe has been geologically explored with care ; that only 
certain classes of organic beings have been largely pi-eserved 
iu 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 hav« 
passed away even during a single formation ; that, owing 
to subsidence being almost necessary for the accumulation 
of deposits rich in fossil species of many kinds, and tliick 
enough to outlast future degradation, great intervals of 
time must have elapsed between most of our successive for- 
mations ; that there has probably been more extinction 
during the periods of subsidence, and more variati-.n 
during the periods of elevation, and during the latter 


the record will have been least perfectly kept ; that each 
single formation has not been continuously deposited; 
that the duration of each formation is probably short com- 
pared with the average duration of specific forms ; that 
migration has played an important part in the first appear- 
ance of new forms in any one area and formation ; that 
widely ranging sj)ecies are those which have varied most 
frequently, 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 meas- 
ured by years, have been short in comparison with the 
periods during which each remained in an unchanged con- 
dition. These causes, taken conjointly, 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 pre- 
tended that we have any sure criterion by which species 
and varieties can be discriminated. 

He who rejects this view of the imperfection of the geo- 
logical 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 formation? 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 forma- 
tions 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 nu- 
merous organisms which must have existed long before the 
Cambrian system was deposited? We now know that at 
least one animal did then exist; but I can answer this last 
question only by supposing that where our oceaus 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 w'idely 
different aspect; and that the older continents, formed o'f 
formations older than any known to iis, 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 ex- 
tinction 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 cf modifica- 
tion is necessarily slow, and depends on many complex 
contingencies. The dominant species belonging to large 
and dominant groups tend to leave many modified descend- 
ants, 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 off- 
spring on the face of the earth. But the utter extinction 
of a whole grou]) of species has sometimes been a slow pro- 
cess, 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, de- 
scendants; and these will generally succeed in dis})hicing 
the groups which are their inferiors in the struggle for 
existence." Hence, after long intervals of time, the pro- 
ductions of the world appear to have changed simultane- 

We can understand how it is that all the forms of hfe, 


ancient and recent, make together a few grand classes. 
We can understand, from the continued tendency to di- 
vergence of character, why the more ancient a form is, the 
more it genei'ally differs from those now living; why 
ancient and extinct forms often tend to fill up gaps be- 
tween existing forms, sometimes blending two groups, pre- 
viously classed as distinct, into one; but more commonly 
bringing them only a little closer together. The more 
ancient a form is, the more often it stands in some degree 
intermediate between groups now distinct; for the more 
ancient a form is, the more nearly it will be related to, and 
consequently resemble, the common progenitor of groups, 
since become widely divergent. Extinct forms are seldom 
directly intermediate between existing forms; but are in- 
termediate 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 gen- 
eration. 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 specialized; and this 
may account for the common belief held by so many palae- 
ontologists, that organization 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 explana- 
tion according to our views. The succession of the same 
types of structure within the same areas during the later 
geological 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 




Present distribution cannot be accounted for by differences in 
physical conditions — Importance of barriers— Affinity of the 
productions of the same continent — Centers of creation— Means 
of dispersal by changes of climate and of the level of the lan.l, 
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 whicli 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 
distribution is that between the New and the Old Worlds; 
yet if we travel over the vast American continent, from 
the central parts of the United States to its extreme south- 
ern point, we meet with the most diversified conditions; 
humid districts, arid deserts, lofty mountains, grassy 
plains, forests, m.arshes, 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 so closely as tlie 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 fauiia different from that of tlie sur- 
rounding districts; for it is rare to find a group of organ- 
isms confined to a small area, of which the conditions are 
peculiar in only a slight degree. Notwithstanding thia 


general parallelism in the conditions of Old and New 
Worlds, liow 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 degrees, 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 latitude 35 degrees 
with those north of 25 degrees, 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 ns 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 pro- 
ductions. 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 op- 
posite sides of lofty and continuous mountain-ranges, of 
great deserts and even of large rivers, we find different 
productions; though as mountain-chains, deserts, etc., are 
not as impassable, or likely to have endured so long, as the 
oceans separating continents, the differences are very in- 
ferior in degree to those characteristic of distinct con- 

Turning to the sea, we find the same law. The marine 
inhabitants of the eastern and western shores of South 
America are very distinct, v»"ith extremely few shells, Crus- 
tacea, or echinodermata in common; but Dr. Giinther has 


recently shown that about thirty per cent, of tlie fislies are 
the same on the opposite sides of the isthmus of Panama; 
and this fact has led naturalists to believe tliat the istlimua 
was formerly open. Westward of the shores of America, 
a wide space of open ocean extends, with not an ishind as 
a halting-place for emigi-ants; 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 dis- 
tinct fauna. So that three marine faunas range northward 
and southward in parallel lines not far from each otlier, 
under corresponding climate; but from being separated 
from each other by impassable barriers, either of laud or. 
open sea, they are almost wholly distinct. On the other 
hand, proceeding still further westward from the eastern 
islands of the tropical parts of the Pacific, we encounter 
no impassable barriers, and we have innumerable islands as 
halting-places, or continuous coasts, until, after traveling 
over a hemisphere, we come to the shores of Africa; and 
over this vast space we meet with no well-defined and dis- 
tinct 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 longitude. 

A third great fact, partly included in the foregoing 
statement, is the affinity of the productions of tlie 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 innum- 
erable instances. Nevertheless, the naturalist, in travel- 
ing, for instance, from north to south, never fails to be 
struck by the manner in which successive groups of beings, 
specificallv distinct, though nearly related, replace each 
other. He hears from closely allied, yet distinct knids of 
birds, notes nearly similar, and sees their nests simihirly con- 
structed, but not quite alike, with eggs colored in nearly the 
same manner. The plains near the Sti'aits 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 find an alpine species ol bizcacha; we look to the 
waters, and we do not find the beaver or muskrat, but the 
coypu and capybara, rodents of the South 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 
chapter, and we find American types then prevailing on the 
American continent and in the American seas. AVe 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 dissimilarity of the inhabitants of different 
regions maybe 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 inhab- 
itants on each other in leading to the preservation of differ- 
ent modifications; the relation of organism to organism in 
the struggle for life being, as I have already often re- 
marked, 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 oi modifica- 
tion 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 out into new conntries. In tlieir new liotnes 
they will be exposed to new conditions, and will froqucntlv 
undergo further modification and improvement; and thus 
they will become still further victorious, and will })roduce 
groups of modified descendants. On this i)rimji})le of in- 
heritance with modification we can understand how it is 
that sections of genera, whole genera, and even famihes, 
are confined to the same areas, as is so commonly and noto- 
riously the case. 

There is no evidence, as was remarked in the la^t 
chapter, of the existence of any law of necessary develop- 
ment. As the variability of each species is an in(le})endent 
property, and will be taken advantage of by natural selec- 
tion, only so far as it profits each iiulividual in its complex 
struggle for life, so the amount of modificatioii in dilTerent 
species will be r.o 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 afterward 
isolated country, they would be little liable to modification; 
for neither migration nor isolation in themselves effect any 
thing. 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 tiie several 
species of the same genus, though inhabiting the most 
distant quarters of the world, must originally have jiro- 
ceeded from the same source, as they are descended 
from the same progenitor. In the case of those 
species which have undergone, during whole cfeological 
periods, little modification, there is not much ditliculty in 
believing that thev have migrated from the same region; 
for during the vast geographical and clin«atieal chunge8 
which have supervened since ancient times, almost any 
amount of migration is possible. Bnt in many other o.ises. 
in which we have reason to believe that the species of a 
genus have been produced within comparatively recent 
times, there is great dimcultv on this head. It is hIro 
obvious that the individuals of the same species. thongU 


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 indentically the same should have been pro- 
duced from parents specifically distinct. 


We are thus brought to the question which has been 
largely discussed by naturalists, namely, whether species 
have been created at one or more points of the earth^s sur- 
face. Undoubtedly there are many cases of extreme diffi- 
culty in understanding how the same species could possi- 
bly have migrated from some one point to the several dis- 
tant and isolated points, where now found. Nevertheless 
the simplicity of the view that each species was first pro- 
duced within a single region captivates the mind. He who 
rejects it, rejects the vera causa of ordinary generation 
v»'ith 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 terrestrial mammals than perhaps with any 
other organic beings; and, accordingly, we find no inex- 
plicable instances of the same mammals inhabiting distant 
points of the world. No geologist feels any difficulty in ,| 

Great Britain possessing the same quadrupeds 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 I 

are nearly the same, so that a multitude of European 
animals and plants have become naturalized in America '••. 

and Australia; and some of the aboriginal plants are | 

identically the same at these distant points of the ^ 

northern and southern hemispheres? The answer, as 
I believe, is, that mammals have not been able to 
migrate, whereas some plants, from their varied means 


of dispersal, have migrated across tlie wide and broken 
interspaces. The great and striking influence of barriera 
of all kinds, is intelligible oidy on the view that tlie <nvut 
majority of species have been produced on one siderand 
have not been able to migrate to the opposite side. Some 
few families, many subfamilies, very many genera, a still 
greater number of sections of genera, are confined to a 
single region; and it has been observed by several natural- 
ists that the most natural genera, or those genera in which 
the species are most closely related to each other, are gen- 
erally confined to the same country, or if they have a wide 
range that their range is continuous. AVhat 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, confined to some one 

Hence, it seems to me, as it has to many other natu- 
ralists, that the view of each species having been produced 
in one area alone, and having subsequently migrated from 
that area as far as its powers of migration and subsistence 
under past and present conditions permitted, is the most 
probable. Undoubtedly many cases occur in which wo 
cannot explain how the same species could have passed from 
one point to the'other. But the geographical and climatical 
changes which have certainly occurred within recent geo- 
logical times, must have rendered discontinuous the for- 
merly 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 tlie exceptional 
cases of the same species, now living at distant and sep- 
arated points, nor do I for a moment protend that any 
explanation could be offered of many instances, l^iit, 
after some preliminary remarks, I will discuss a fi3W of the 
most striking classes of facts, namely, the existence of the 
same species on the summits of distant mountain rangos, 
and at distant points in the Arctic and Antarctic regions; 
and secondly (in the following chapter), the wide Jistri- 


bution of fresh water productions; and tiiirdl}^ 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, consider- 
ing our ignorance with respect to former climatical and geo- 
graphical 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, undergoing modiiication during their 
mis^ration from some one area. If, when most of the spe- 
cies 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 in- 
stance, 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 attributes this coincidence to descent with modi- 

The question of single or multiple centres of creation 
differs from another though allied question^ namely, 
whether all individuals of the same species are descended 
from a single pair, or single hermaphrodite, or whether, as 
some authors suppose, from many individuals simultane- 


ously created. Witli organic beings which never intercross*, 
if such exist, each species must be descended from u suc- 
cession of modified varieties, that liave supplanted eacii 
other, but have never blended with other individuals or 
varieties of the same species; so that, at each snocessivo 
stage of modification, all the individuals of the same form 
will be descended from a single parent. But in the groat 
majority of cases, namely, with all organisms which habit- 
ually 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 simultaneously 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 centers of creation/' I must say a 
few words on the 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 impass- 
able to certain organisms from the nature of its climate, 
might have been a high road for migration, when the 
climate was different. I shall, however, j)resently 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; sub- 
merge 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 ^a^- 
sibly even continents together, and thus have allowed terres- 
trial productions to pass fi'om one to the other. No geolo-isi 


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 
connected with Euroj^e or Africa, and Europe likewise 
with America. Other authors have thus hypotlietically 
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 admitted that 
scarcely a single island exists which has not recently 
been united to some continent. This view cuts the 
Gordian knot of the disjiersal of the same species to the 
most distant points, and removes many a difficulty; but to 
the best of my judgment we are not authorized in admit- 
ting 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 ex- 
tension of our continents, as to have united them within 
the recent period to each other and to the several interven- 
ing 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 continuously, or almost continuously united 
with each other, and with the many existing oceanic 
islands. Several facts in distribution — such as the great 
difference in the marine faunas on the oi^posite sides of 
almost every continent — the close relation of the tertiary 
inhabitants of several lands and even seas to their present 
inhabitants — the degree of affinity between the mammals 
inhabiting islands with those af 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 sncli prodigious geo- 
grapiiical revolutious 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 of continents. Nor does 
the almost universally volcanic composition of such islands 
favor the admission that they are the wrecks of sunken 
continents; if they had originally existed as continental 
mountain ranges, some at least of the islands would havo 
been formed, like other mountain summits, of granite, 
metamorphic schists, old fossiliferous and other rocks, in- 
stead of consisting of mere piles of volcanic matter. 

I must now say a few words on what are called acci- 
dental 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 experiments, it 
was not even known how far seeds could resist the in- 
jurious action of sea-water. To my surprise I found 
that out of eighty-seven kinds, sixty-four germinated 
after an immersion of twenty-eight days, and a few 
survived an immersion of 137 days. It deserves notice 
that certain orders were far more injured than others: nine 
LeguminosaB were tried, and, wdth one exception, they 
resisted the salt-water badly; seven species of the allied 
orders, Hydrophyllaceae and Polemoniacea;, were all killed 
by a month's immersion. For convenience sake I chiefly 
tried small seeds without the capsules or fruit; and as all 
of these sunk in a few days, they could not have been 
floated across wide spaces of the sea, whether or not they 
were injured by salt water. Afterward I tried some larger 
fruits, capsules, etc., 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 
ninety-four plants with ripe fruit, and to place them on 


sea-Tvater. The majority sunk quickly, but some whicli, 
while green, floated, for a very short time, when dried, 
floated much longer; for instance, ripe hazel-nuts sunk 
immediately, but when dried they floated for ninety days, 
and afterward when planted germinated; an asparagus- 
plant with ripe berries floated for twenty-three days, when 
dried it floated for eighty-five days, and the seeds after- 
ward germinated; the ripe seeds of Helosciadium sunk in 
two days, when dried they floated for above ninety days, 
and afterward germinated. Altogether, out of the ninety- 
four dried plants, eighteen floated for above twenty-eight 
days; and some of the eighteen floated for a very much longer :| 

period. So that as |4 kinds of seeds germinated after an 
immersion of twenty-eight days; and as -^f distinct species 
with ripe fruit (but not all the same species as in the forego- 
ing experiment) floated, after being dried, for above twenty- 
eight days, we may conclude, 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 twenty-eight days, and would retain their 
power of germination. In Johnston^s Physical Atlas, the 
average rate of the several Atlantic currents is thirty-three 
miles per diem (some currents running at the rate of sixty 
miles per diem); on this average^ the seeds of -^ 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 favorable spot, would germinate. 

Subsequently to my experiments, M. Martens tried sim- 
ilar ones, but in a much better manner, for he placed the 
seeds in a box in the actual sea, so that they were alter- 
nately wet and exposed to the air like really floating 
plants. He tried ninety-eight seeds, mostly difl'erent from 
mine, but he chose many large fruits, and likewise seeds, 
from plants which live near the sea; and this would have 
favored 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 -^f of his seeds of different kinds 
floated for forty-two days, and were then capable of ger- 
mination. 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 
-^%- plants of a flora, after having been dried, could bo 
floated across a space of sea 900 miles in width, and would 
then germinate. The facts of the larger fruits often float- 
ing longer than the small, is interesting; as plants with 
large seeds or fruit which, as Alph. de Candolle has shown, 
generally have restricted ranges, could hardly bo trans- 
ported by any other means. 

Seeds may be occasionally transported in another 
manner. Drift timber is thrown up on most islands, 
even on those in the midst of the widest oceans; and the 
natives of the coral islands in the Pacific procure stones 
for their tools, solely from the roots of drifted trees, these 
stones being a valuable royal tax. I find that when irregu- 
larly shaped stones are embedded in the roots of trees, 
small parcelsof earth are frequently inclosed 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 completely inclosed by the 
roots of an oak about fifty years old, three dicotyledonous 
plants germinated: I am certain of the accuracy of this 
observation. Again, I can show that the carcasses of birds, 
when floating on the sea, sometimes escape being immedi- 
ately 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 thirty days, to 
my surprise nearly all germinated. 

Living birds can hardly fail to be highly effective agents 
in the transportation of seeds. I could give many facts 
showing how frequently birds of many kinds are blown by 
gales to vast distances across the ocean. We may safely 
assume that under such circumstances their rate of flight 
would often be thirty-five 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 twelve kinds of 


seeds, out of tlie excrement of small birds, and these 
seemed perfect, and some of them, which were tried, ger- 
minated. But the following fact is more important: 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 five hundred 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 ex- 
periments made in the Zoological Gardens, include seeds 
capable of germination. Some seeds of the oat, wheat, 
millet, canary, hemp, clover, and beet germinated after 
having been from twelve to twenty-one hours in the 
stomachs of different birds of prey; and two seeds of beet 
grew after having been thus retained for two days and 
fourteen hours. Fresh-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 m3^self caught one 370 miles from the coast of 
Africa, and have heard of others caught at greater distances. 
The Rev. R. T. Lowe informed Sir C. Lyell that in No- 
vember, 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 extended 
upward 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 com- 
pletely coated with them. They then disappeared over the 


sea, as suddenly as they had appeared, and luive not since 
visited the island. Now, in parts of Natul it is believed by 
some farmers, though on insufficient evidence, thai injuri- 
ous 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 wliich 
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 in- 
troducing 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 arg:illaceous 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 germin- 
ated and flowered. Mr. Swaysland, of Brighton, who dur- 
ing the last forty years has paid close attention to our 
migratory birds, informs me that he has often shot wag- 
tails (Motacillae), wheatears, and whinchats (Saxicoht), 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 cliarged with seeds. For instance, 
Prof essor Newton sent me the leg of a red-legged partridge 
(Caccabis rufa) which had been wounded and could not 
fly, with a ball of hard earth adhering to it, and weighing 
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 eighty-two plants sprung from it: those 
consisted of twelve monocotyledons, including the common 
oat, and at least one kind of grass, and of seventy dicotyle- 
dons, which consisted, judging 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 an- 


nually migrate — for instance, the millions of quails across 
the Mediterranean — must occasionally transport a few 
seeds imbedded in dirt adhering to their feet or beaks? 
But I shall have to recur to this subject. 

As icebergs are known to be sometimes loaded with earth 
and stones, and have even carried brushwood, bones, 
and the nest of a land-bird, it can hardly be doubted that 
they must occasionally, as suggested by Lyell, have trans- 
ported 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 main- 
land, and (as remarked by Mr. H. 0. AYatson) from their 
somewhat northern character, in comparison with the lati- 
tude, I suspected that these islands had been partly stocked 
by ice-born seeds during the Glacial epoch. At my request 
Sir C. Lyell wrote to Si. Hartung to inquire whether he 
had observed erratic bowlders 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 burdens 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 plants. 

Considering that these several means of transport, and 
that other means, which without doubt remain to be dis- 
covered, have been in action year after year for tens of 
thousands of years, it would, I think, be a marvelous fact 
if many plants had not thus become widely transported. 
These means of transport are sometimes called accidental, 
but this is not strictly correct : the currents of the sea are 
not accidental, nor is the direction of prevalent gales of 
wind. It should be observed that scarcely any means of 
transport would carry seeds for very great distances: for 
seeds do not retain their vitality when exposed for a great 
length of time to the action of 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 neighboring 


island, but not from one distant continent to another. 
The floras of distant continents would not bv sucli means 
become mingled; but would remain as distinct as they now 
are. The currents, from their course, would never \v\uit 
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 thy 
whole Atlantic Ocean, from Kortli America to the western 
shores of Ireland and England; but_ seeds could be tnins- 
ported by these rare wanderers only by one means, luimelv, 
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 favorable soil, and coming to 
maturity! But it would be a great error to argue tliat 
because a well-stocked island, like Great Britain, has not, 
as far as is known (and it would be very dithcult to prove 
this), received within the last few centuries, through occa- 
sional means of transport, immigrants from Europe or any 
other continent, that a poorly-stocked island, though 
standing more remote from the mainland, would not 
receive colonists by similar means. 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 
naturalized. But this is no valid argument against what 
would be effected by occasional means of transpoit, 
during the long lapse of geological time, while 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. 


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 i)ossibly 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 snow^ 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 tliose of Labrador, and nearly all the same, as 
we hear from Asa Gray, with those on the loftiest mount- 
ains 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 expla- 
nation 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 burned 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 bowlders, of the icy 
streams with which their valleys were lately filled. So 
greatly has the climate of Europe changed, that in North- 
ern Italy, gigantic moraines, left by old glaciers, are now 
clothed by the vine and maize. Throughout a large part 
of the United States, erratic bowlders and scored rocks 
plainly reveal a former cold jDcriod. 

The former influence of the glacial climate on the dis- 
tribution 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 south- 
ern zone became fitted for the inhabitants of the north, 
these would take the places of the former inhabitants of 
the temperate regions. Tlie 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 maxi- 
mum, 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. 'IMie now 
temperate regions of the United States would likewise bo 
covered by arctic plants aud animals and these would be 
nearly the same with those of Europe; for the present 
eircumpolar ijihabitants, which we suppose to have every- 
where traveled southward, are remarkably uniform round 
the world. 

As the warmth returned, the arctic forms would retreat 
northward, closely followed up in their retreat by the pro- 
ductions 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, while their breth- 
ren 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 summits 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-migration on the return- 
ing warmth, would generally have been due south and 
north. The Alpine plants, for example, of Scotland, as 
remarked by Mr. H. 0. Watson, and those of the Pyrenees, 
as remarked by Eamond, are more especially allied t^o tiio 
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 satis- 
factory a manner the present distribution of the Alpine 
and Arctic productions of Europe and America, tluit wiien 
in other regions we find the same species on distant mount- 
ain-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 after- 
ward backward 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 rela- 
tions 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 2^1'obability, 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 climatical influences. Their mutual relations will 
thus have been in some degree disturbed; consequently 
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 m.any of the species 
remain identically the same, some exist as varieties, some 
as doubtful forms or sub-species and some as distinct 3^et 
closely allied species representing each other on the several 

In the foregoing illustration I have assumed that at the 
commencement of our imaginary Glacial period, tlie arctic 
productions were 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 worlds at 


the commencement of the real Glacial period. At the 
present day, the sub-arctic and northern temperate pro- 
ductions of the Old and New AVorlds are separated from 
each other by the whole Atlantic Ocean and by the norih- 
ern part of the Pacific. During the Glacial })eriod, when 
the inhabitants of the Old and New Worlds lived further 
southward then 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 con- 
tinents. 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 in- 
habitants 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 tliC 
organisms which now live under latitude 60 degrees, lived 
during the Pliocene period further north, under the Polar 
Circle, in latitude 66-67 degrees; and that the present arctic 
productions then lived on the broken land still nearer to the 
23ole. 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 conse- 
quent freedom under a more favorable climate for inter- 
migration, will account for the supposed uniformity of tlie 
sub-arctic and temperate productions of the Old and New 
Worlds, at a period anterior to the Glacial epoch. 

Believing, from reasons before alluded to, that our con- 
tinents have long remained in nearly the same relative 
position, though subjected to great oscillations of level. 1 
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 continuous cir- 
cumpolar land; and that these plants and animals, both in 
the Old and New Worlds, begun slowly to migrate south- 
ward as the climate became less warm, long before the 
commencement of the Glacial period. We now see, as 1 
believe, their descendants, mostly in a modified condition, 
in the central parts of Europe and the United States. On 
this view we can understand the relationship with very 


little identity, between the productions of North America 
and Europe — a relationship which is highly remarkable, 
considering the distance of the two areas, and their sepa^ 
ration by the whole Atlantic Ocean. We can further under- 
stand tiie 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 impassible by cold, for intermigra- 
tion 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 off 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 com- 
pete with them; and in the other great region, with those 
of the Old World. Consequently we have here everything 
favorable for much modification — for far more modifica- 
tion 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 North America. Hence, 
it has come, that when we compare the now living produc- 
tions 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 for- 
merly 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 south- 
ern migration of a marine fauna, which, daring the Pliocene 
or even a somewhat earlier period, was nearly uniform along 
the continuous shores of the Polar Circle, v/ill account, on 
the theory of modification, 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 crus- 
taceans (as described in Dana's admirafjle work), some fish 
and other marine animals, inhabiting the Mediterranean 
and the seas of Jajmn — these two areas being now com- 
pletely 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 cor- 
respondence 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. 


But we must return to our more immediate subject. I 
am convinced that Forbes' 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 Ural 
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 Leb- 
anon, according to Dr. Hooker, perpetual snow formerly 
covered the central axis, and fed glaciers which rolled 
4,000 feet down the valleys. The same observer has 
recently found great moraines at a low level on the Atlas 
range in North Africa. Along the Himalaya, at jioints 
900 miles apart, glaciers have left the marks of tht'ir 
former low descent; and in 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. Ilaast 
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 m 
this island tell the same story of a former cold period. 
From facts communicated to me by the Eev. W. B. 
Clarke, it appears also that there are traces of former gla- 
cial 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 latitude thirty-six and 
thirty-seven degrees, and on the shores of the Pacific, 
Avhere the climate is now so different, as far south as lati- 
tude forty-six degrees. Erratic bowlders 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 Chili I 
examined a vast mound of detritus with great bowlders, 
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 latitude thirteen to thirty degrees south, at about the 
height of 12,000 feet, deeply-furrowed rocks, resembling 
those with which he was familiar in l^orway, and likewise 
great masses of detritus, inchiding grooved pebbles. Along 
this whole space of the Cordillera true glaciers do not now 
exist even at much more considerable heights. Further 
south, on both sides of the continent, from latitude forty- 
one degrees to the southernmost extremity, we have the 
clearest evidence of former glacial action, in numerous 
immense bowlders transported far from their parent 

From these several facts, namely, from the glacial action 
having extended all round tlie northern and southern 
hemispheres — from the period having been in a geological 
sense recent in both hemispheres — 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 gla- 
ciers 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 tempera- 
ture 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 toward the same 
end; but the most powerful appears to be the indirect in- 
fluence of the eccentricity of the orbit upon oceanic cur- 
rents. According to Mr. Croll, cold periods reguhirly recur 
every ten or fifteen thousand years; and tliese at long in- 
tervals are extremely severe, owing to certain contingen- 
cies, 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 altera- 
tions of climate, for about 160,000 years. With respect to 
more ancient glacial periods, several geologists arc con- 
vinced, 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 temperature 
of the southern hemisphere is actually raised, with the 
winters rendered much milder, chiefly through changes in 
the direction of the ocean currents. So converselv it will 
be with the northern hemisphere, while the southern passes 
through a glacial period. This conclusion throws so much 
light on geographical distribution that I am strongly in- 
clined to trust in it; but I will first give the facts which 
demand an explanation. 

In South America, Dr. Hooker has showm that besides 
many closely allied species, between forty and fifty of the 
flowering plants of Tierra del Fuego, forming no inconsid- 
erable part of its scanty flora, are common to North 
America and Europe, enormously remote as these areas in 
opposite liemispheres 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 (iardner 
which do not exist in the low intervcTiing hot countries. 
On the Silla of Caraccas the illustrious Humboldt long ago 
found species belonging to genera characteristic of the 

In Africa, several forms characteristic of Europe, and 


some few representatives of the flora of the Cape of 
Grood 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 
of Fernando Po, and on the neighboring Cameroou Mount- 
ains, in the Gulf of Guinea, are closely related to those on 
the mountains of Abvssinia, and likewise to those of tem- 
perate Europe. It now also appears, as I hear from Dr. 
Hooker, that some of these same temperate plants have 
been discovered by the Rev. E. T. Lowe on the mountains 
of the Cape Verde Islands. This extension of the same tem- 
perate 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 re- 
corded 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 identi- 
cally the same or representing each other, and at the same 
time representing plants of Europe not found in the inter- 
vening hot lowlands. A list of the genera of plants col- 
lected on the loftier peaks of Java, raises a picture of a 
collection made on a hillock in Europe. Still more strik- 
ing is the fact that peculiar Australian forms are repre- 
sented 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 

On the southern mountains of Australia, Dr. F. Miiller 
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 inter- 
mediate 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 re- 
marked, " in receding from polar toward equatorial lati- 
tudes, the Alpine or mountain flora 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. Professor Dana, that '' it is certainly a 
wonderful fact that New Zealand should have a closer re- 
semblance in its Crustacea to Great Britain, its antipode, 
than to any other part of the world." Sir J. Richardson, 
also, speaks of the reappearance on the shores of New 
Zealand, Tasmania, etc., of northern forms of fish. Dr. 
Hooker informs me that twenty-five species of Algse 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 tem- 
perate forms on the highlands across the whole of equatorial 
Africa, and along the Peninsula of India, to Ceylon and 
the Malay Archipelago, and in a less well-marked manner 
across the wide expanse of tropical South America, it ap- 
pears almost certain that at some former period, no doubt 
during the most severe part of a Glacial period, the low- 
lands of these great continents were everywhere 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 ex- 
perienced 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 tlie 
equator must have been clothed with a mingled tropical 
and temperate vegetation, like that described by Ilojkor 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 begiuing to appear ab 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 temper- 

Now let us see whether Mr. CrolFs 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 ap- 
parently inexplicable distribution of various organisms in 
the temperate 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 remem- 
ber over what vast spaces some naturalized plants and ani- 
mals 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 subtropical regions of the south, for the 
southern hemisphere was at this period warmer. On the 
decline of the Glacial period, as both hemispheres gradu- 
ally recovered their former temperature, the northern tem- 
perate 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 return- 
ing 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 tem- 
perature must have been very slow, and plants undoubtedly 
possess a cerfeain capacity for acclimatization, as shown by 


their transmitting to their offspring different constitutional 
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 northern 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 mount- 
ains of the intermediate tropical regions. But the species 
left during a long time on these mountains, or in opi^osite 
hemispheres, would have to compete with many new forms 
and would be exposed to somewhat different ph3"sical con- 
ditions; hence, they would be eminently liable to modifica- 
tion, and would generally now exist as varieties or as rep- 
resentative 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 in- 
habiting 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 sus- 
pect 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 
commingled in the equatorial regions, during the alterna- 
tions of the Glacial periods, the northern forms were tho 


more powerful and were able to hold their places on the 
mountains, and afterward 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 soathern forms have become naturalized 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 cen- 
turies from La Plata and during the last forty or fifty years 
from Australia. The K"eilgherrie Mountains in India, 
however, ofler a partial exception; for here, as I hear from 
Dr. Hooker, Australian forms are rapidly sowing them- 
selves and becoming naturalized. 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 outnumbered, by those which have 
become naturalized; and this is the first stage toward 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 naturalized through 
man^s agency. 

The same principles apply to the distribution of terres- 
trial 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 per- 
haps at once be able to migrate southward, by keeping to 
the cooler currents, while others might remain and sur- 
vive 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 Arctio 


productions exist to the present day in the deeper parts of 
the northern temperate seas. 

I am far from siipposinoj that all the difficulties in 
regard to the distribution and affinities of the identical and 
allied species, which now live so widely separated in the 
north and south, and sometimes on the intermediate mount- 
ain-ranges, are removed on the views above given. The 
exact lines of migration cannot be indicated. We cannot 
say why certain species and not otliers have migrated; why 
certain species have been modified and have given rise to 
new forms, while others have remained unaltered. We 
cannot hope to explain such facts, until we can say why 
one species and not another becomes naturalized 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 homes. 

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, ISTew 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 remark- 
able case. Some of these species are so distinct, that we 
cannot suppose that there has been time since the com- 
mencement 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 center; and I am inclined to look in the southern, 
as in the northern hemisphere, to a former and warmer 
period, before the commencement of the 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 hemisiDhere by oc- 
casional means of transport, and by the aid, as lial ting- 
places, of now sunken islands. Thus the soutliern 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 lias speculated, in lan- 
guage almost identical with mine, on the effects of great 
alterations of climate throughout the world on geograph- 
ical distribution. And we have now seen that Mr. CrolFs 
conclusion that successive Glacial periods in the one hemi- 
sphere coincide with warmer periods in the opposite hemi- 
sphere, together with the admission of the slow modifica- 
tion of species, explains a multitude of facts in the distri- 
bution 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 surviv- 
ing in the mountain fastnesses of almost every land, which 
serves as a record, full of interest to us, of tlae former in- 
habitants of the surrounding lowlands. 




Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial Mam- 
mals — On the relation of the inhabitants of islands to those of 
the nearest mainlaind — On colonization from the nearest source 
with subsequent modification— Summary of the last and present 


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, belong- 
ing 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 w^ell remember feeling much surprise at the similarity of 
the fresh-water insects, shells, etc., and at the dissimilarity 
of the surrounding terrestrial beings, compared with those 

of Britain. 

But the wade ranging power of fresh-water productions 
can, I think, in most cases be explained by their having 
become fitted, in a manner highly useful to them, for short 
and frequent migrations from pond to pond, or from 
stream to stream, 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 Falkland Islands and the mainland of 
South America. This is a wonderful case, and probably 
indicates dispersal from an Antarctic center 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 con- 
siderable 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 

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 attribu- 
ted 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 differences of the 
fish on the opposite sides of most mountain-ranges, which 
are continuous and consequently must, from an early 
period, have completely prevented the inosculation of the 
river, systems on the two sides, leads to the same conclu- 
sion. 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. Giinther 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 to Valenciennes, there is 
^ hardly a single group of which all the members are con- 
fined 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 naturalized species have spread 
rapidly throughout the same country. But two facts, 
which I have observed — and many others no doubt will be 
discovered — 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 colymbetes, 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 favoring gale no one can 

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 

408 :fbesh-water PEdDUCTiomr 

immediately to acquire, as if in consequence, a wide 
range. I think favorable means of dispersal explain 
this fact. I have before mentioned that earth occa- 
tionally adheres in some quantity to the feet and beaks 
of birds. Wading birds, wiiich 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 botanists are aware how charged the mud of ponds is 
with seeds; 1 have tried several little experiments, but will 
here give only the most striking case: I took in February 
three tablespoonfuls of mud from three different points, 
beneath water, on the edge of a little pond; this mud when 
dried weighed only six and three-fourth ounces; I kept it 
covered up in my study for six months, pulling up and 
counting each plant as it grew; the plants were of many 
kinds, and were altogether 537 in number; and yet the 
viscid mud was all contained in a breakfast cup! Consider- 
ing these facts, I think it v/ould be an inexplicable cir- 
cumstance 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 

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 afterward in pellets or in the excrement. 
"When I saw the great size of the seeds of that fine water- 
lily, the Nelumbium, and remembered Oandolle's 
remarks on the distribution 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 Nelumt 
bium luteum) in a heron^s stomach. Now this bird mus- 
often have flown with its stomach thus well stocked to dis- 
tant ponds, and, then getting a hearty meal of fish, 
analogy makes me believe that it would have rejected the 
seeds in the 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 instance on a rising islet, it will be unoccupied; 
and a single seed or e,gg will have a good chance of succeed- 
ing. Although there will always be a struggle for life be- 
tween 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 tlie number of species inhabiting an 
equal area of land, the competition between them will proba- 
bly be less severe than between terrestrial species; conse- 
quently 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 re- 
member 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. 


We now come to the last of the three classes of facts, 
which 1 have selected as presenting the greatest amount 
of difficulty with respect to distribution, on the view that 
not only all the individuals of the »am© speci«s have 


migrated from some one area, but that allied species, 
although now inhabiting the most distant points, have pro- 
ceeded from a single area, the birthplace of their early pro- 
genitors. 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 j^resent 
terrestrial inhabitants. This view removes many difficul- 
ties, but it does not accord with all the facts in regard to 
the productions of islands. In the following remarks I 
shall not confine myself to the mere question of dispersal, 
bat shall consider some other cases bearing on the truth of 
the two theories of independent creation and of descent 
with modification. 

The species of all kinds which inhabit oceanic islands 
are few in number compared with those on equal continen- 
tal areas: Alph. de Candolle admits this for plants, and 
WoUaston for insects, ^""ew 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 moder- 
ate number with the species which swarm over equal areas 
in Southwestern 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. AYe have evidence that the barren island 
of Ascension aboriginally possessed less than half a dozen 
flowering plants; yet many species have now become 
naturalized 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 naturalized plants 
and animals have nearly or quite exterminated many native 
productions. He who admits the doctrine of the creation 
of each separate species, will have to admit that a sufficient 
number of the best adapted plants and animals 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