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" But with regard to tlic luatcrial world, vfc can at least go so far as tliia 
— wc can perceive that events are brought about not by insulated interpo- 
sitions of Divine power, exerted in each particular case, but by the estab- 
lishment of general laws." 

Whewkll: Bridjcwatcr Treatise. 

" The only distinct moaning of the word ' natural ' is slalnl, Jixcel, or 
tcllled ; since what is natural as much requires and presupposes an intelli- 
gent agent to render it so, i. c., to effect it continually or at stated times, 
as what is supernatural or miraculous does to effect it for once." 

Butler : An(do<ji/ of Revealed Religion, 

" To conclude, therefore, let no man out of a weak conceit of sobriety, 
or an ill-applied moderation, think or maintain, that a man can search too 
far or be too well studied in the book of God's word, or in the book of 
God's works ; divinity or philosophy ; but rather let men endeavor an end- 
less progress or proCcicncc in both." 

Bacon : Advancement of Learning. 

ON , . ^ 









5 4 9 & 5 5 1 BROADWAY. 


nisTORiCAL Sketch ........ paoe 9 

I.VTBOnUCTION ......... 17 



Causes of Variability— EfTocts of Habit— Correlated Variation— Inheritance— Char- 
acter of Domestic Varieties — Diflftculty 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 23 



Vailability- Individual Differences— Doubtful Species— Wide-ranging, much dif- 
fused, 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 . 52 



its bearing on Natural Selection— The Term used in a wide Sense— Geometrical rjutio 
of Increase — Rai)id Increase of Naturalized Animals and Plants — Nature of tlio 
Checks to Increase— Competition universal— Effects of Climate— Protection fri)ni 
the Number of Individuals— Complex Relations of all Animals and Plants through 


out Nature— Strugiclo for Life most severe hctwccn Individuals and Varieties of 
the same Species : often severe between Species of the same Genns— The Rela- 
tion of Organism to Organism the most important of all Relations faoe 69 



Natural Selection— its Power compared with Man's Selection — its Power on Char- 
acters of trifling Importance— its Power at all Ages and on both Sexes — Sexual 
Selection— On the Generality of Intercrosses bi^tween Individuals of the same 
Species — Circumstances fiivorable and unfavorable to the Uesults of Natural Se- 
lection, 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 Nat- 
ural Selection, through Divergence of Character and Extinction, on the Descend- 
ants from a Common Parent — Explains the Grouping of all Organic Beings— Ad- 
vance in Organization— Low Forms preserved — Objections considered— Uniform- 
ity of certain Characters due to their Unimportance and to their not having been 
acted on by Natural Selection— Indefinite Multiplication of Species— Summary. 



Effects of changed Conditions — Use and Disuse, combined with Natural Selection ; 
Organs of Flight and of Vision — Acclimatization — Correlated Variation — Com- 
pensation and Economy of Growtli — False Correlations — Multiple, Rudimentary, 
and Lowly-organized Structures variable — Parts developed in an Unusual Man- 
ner are highly variable: Specific Characters more variable than Generic: Second- 
ary Sexual Characters variable — Species of the same Genus vary in an analogous 
Manner- Reversions to long-lost Characters — Summary . . . 137 



Difficulties of the Theory of Descent with Modification— Transitions— Absence or 
Rarity of Transitional Varieties — Transitions in Habits of Life — Diversified Elal)- 
its in the same Species— Species with Habits widely diO'erent from those of their 
Allies— Organs of Extreme Perfection — Modes of Traifsition — Cases of Difficulty 
— Natura non facit saltum— Organs of small Importance— Organs not in all Cases 
absolutely perfect — The Law of Unity of Type and of the Conditions of Existence 
embraced by the Theory of Natural Selection .... 167 



Instincts comjiarable with Ilabils, but different in tlu-ir Origin— Instincts graduated 
— Aphides and Ants— Instincts variable— Domestic Instincts, their Origin— Nat- 


nral lOBtincls of the Cuckoo, Ostrich, and Parasitic Bees— Slave-making Ants — 
Hive-bee, its cell-making instinct— ChanKCs of In!<linct and Structure not neces- 
earlly simultaneous— Difllcultics of the Theory of the Natural Selection of In- 
stincts— Xeuter or Sterile Insects— Summary . . . . PAGE 201 



Distinction between the Sterility of First Crospoe and of Hybrids— Sterility various 
in Degree, not universal, affected by close Interbreeding, removed by Domesti- 
cation-Laws goveiTiing the Sterility of Hybrids- Sterility not a special Endow- 
ment, but incidental on other DilTerenccB, not accumulated by Natural Selection 
— Causes of the Sterility of First Crosses and of Hybrids— Parallelism between 
the Effects of Changed Conditions of Life and of Crossing— Dimorphism and 
Trimorphism — Fertility of Varieties when crossed and of their Mongrel Offspring 
not universal— Hybrids and Mongrels compared independently of their Fertility 
—Summary ......... 233 



On the Absence oflntcrmediatc Varieties at the Present Day — On the Nature of Ex- 
tinct Intermediate Varieties ; on their Number— On the Lapse of Time, as in- 
ferred from the Kjite of Denudation and of Deposition— On the Lapse of Time as 
estimated by Years- On the Poorness of our Palcontological Collections— on the 
Denudation of Granitic Areas— On the Interaiittence of Geological Formations — 
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 Fossilifcrous Strata— Antiquity of the Habitable Earth . . 2CC 



Ou the Slow and Successive Appearance of New Species— On their Different Rates 
of Cliange— Species once lost do not reappear — Groups of Species follow the 
same General Kules 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 E.\tinct Species to each other and to Living Spe- 
cles- On the State of Development of Ancient Forms— On the Succession of the 
eamc Types within the same Areas— Summary of preceding and present Chapter 




PrcBont Distribntlon cannot bo accounted for by Differences In Physical Conditions 
— Importance of Carriers — Aflliiity of the Productions of the same Continent— 


Centres of Creation— Means of Dispersal by Changes of Climate and of the Level 
of the Land, and by Occasional Means— Dispersal during the Glacial Period- 
Alternate Glacial Periods in the North and South . . . page 322 



Distribution of Fresh-water Productions— On the Inhabitants of Oceanic Islands- 
Absence of Batrachians and of Terrestrial Mammals— On the Relation of the In- 
habitants of Islands to those of the nearest Main-land— On Colonization from the 
nearest Source with subsequent Modification— Summary of the last and present 
Chapter 351 



Classiflcation, Groups subordinate to Groups— Natural System— Eules and Difficulties 
in Classification, explained on the Theory of Descent with Modification— Classi- 
fication of Varieties- Descent always used in Classification— Analogical or Adap- 
tive Characters— Affinities, General, Complex, and Radiating— Extinction sepa- 
rates and defines Groups— Morphology, between members of the same Class, be- 
tween 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 . . 372 



Recapitulation of the Objections to the Theory of Natural Selection— Recapitulation 
of the General and Special Circumstances in its favor — Causes of the General 
Belief in the Immutability of Species— IIow far the Theory of Natural Selection 
may be extended- Efl"ects of its Adoption on the Study of Natural History— Con- 
cluding Remarks ......... 412 


Instruction to Binder.— The Diagram to front page 114. 

An Historical Sketcu of the Recent Progress of 
Opinion on tue Origin of Species. 

I WILL here give a brief slcetch of the progress of opinion on the Origin 
of Species. Until recently, the great majority of naturalists believed that 
species were immutable productions, and had been separately created. This 
view has been ably maintained by many authors. Some few naturalists, on 
the other hand, have believed that species undergo Tiiodification, and that 
the existing forms of life arc the descendants by true generation of pre- 
existing forms. Passing over allusions to the subject in the classical 
writers,* the first author who in modern times has treated it in a scientific 
spirit was Buffon. But, as his opinions fluctuated greatly at different 
periods, and as he does not enter on the causes or means of the 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 enlarged them in 1809 in his " Philosophic Zoologique," 
and subsequently, in 1815, in the introduction to his "Hist. Nat. des Ani- 
maux sans Vcrtcbres." 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 the 
organic as well as in the inorganic world being the result of law, and not of 
miraculous interposition. Lamarck seems to have been chiefly led to his 

* Aristotle, In his " Phygictc Anecultationes " (I'l'- 2, cap. 8, s. 2), after remarking 
that rain docs not fall in order to make the com grow, auy more than it falls to gpoil 
the farmer's com when threshed out of doors, applies the same arf::umcnt to orfrani- 
zaliou, and adds (as translated by Mr. Clair Grecc, who first pointed out the passaio 
to me): "-So, what hinders the different parts [of the body] from haviuii 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 masticat- 
ing the food ; since they were not made for the sake of tliiH, but it was the result of 
accident. And in like manner as to the other parts in which there appears to exist 
nn adaptation to an end. Wheresoever, therefore, all things together ^tllat is, all tho 
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 spon- 
taneity ; and whatsoever things were not thus constituted perished, and still perish." 
Wo hero sec the principle of natural selection shadowed forth ; but how little 
Aristotle fully comprehended the principle is shown byhia remarks on the formation 
of tho teeth. 


conclusion on the graJiuil cliangc of species by the dilTicully of distinguish- 
ing species and varieties, bj- the almost perfect gradation of forms in certain 
groups, and by the analogy of domestic productions. With respect to the 
means of modification, he attributed something to the direct action of the 
physical conditions of life, something to the crossing of already existing 
forms, and much to use and disuse — that is, to the efTccts of habit. To this 
latter agency he seems to attribute all the beautiful adaptations in Nature — 
such as the long neck of the giraffe for browsing on the branches of trees. 
But he likewise believed in a law of progressive development ; and, as all 
the forms of life thus tend to progress, in order to account for the existence 
at the present day of simple productions, he maintains that such forms arc 
now spontaneously generated.* 

Geoffroy Saint-IIilaire, as is stated in his "Life," written by his son, sus- 
pected, as early as IvOo, that what we call species are various degenerations 
of the same type. It was not until 1828 that he puljjishcd his conviction 
that the same forms have not been perpetuated since the origin of all things. 
Geoffroy seems to have relied chiefly on the conditions of life, or the " monde 
ambiant,''^ as the cause of change. lie was cautious in drawing conclusions, 
and did not believe that existing species are now undergoing modification ; 
and, as his son adds, " C'cst done un probleme h, reserver cntierement h 
I'avenir, suppos6 meme que I'avenir doive avoir prise sur lui." 

In 1813 Dr. "W. C. AVells read before the Eoyal Society "An Account 
of a White Female, Part of whose Skin resembles that of a Negro;" but 
his paper was not published until his famous " Two Essays upon Dew and 
Single Vision " appeared, in 1818. In this paper he distinctly 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- 
nniuity from certain tropical di.-cases, he observes, firstly, that all animals 

* I have taken the date of tlic first publication of Lamarck from Isid. Qeofl'roy 
Saint-Ililaire'8 ("Hist. Nat. Guncralc," torn. 11., p. 405, 1S50) cxrcllent history of 
opinion on this subject. In this work a full account is pivcn of Buftbn's conclusious 
on the pame subject. It is curious how largely my grandfather, Dr. Erasmus Dar. 
win, anticipated the views and erroneous grounds of opinion of Lamarck in Iiis 
"Zoononiia" (vol. i., pp. 500-510), published in ITUt. According to Isid. Geoffroy, 
there is no doubt that Goethe was an extreme partisan of similar views, ns shown 
in llic introduction to a work written in 1794 and 1795, but not published <\\\ long 
afterward: he has pointedly remarked ("Goethe als Naturforsclicr," von Dr. Karl 
Meding, s. 31) tbat the future question for naturalists will be how, for Instance, cattle 
got tlielr horns, and not for wbat they are used. It is rather a singular instance of 
the manner in wliieh similar views arise at about the same time, that Goethe in Ger- 
many, Dr. Darwin in England, and (icoffroy Saint-Ililai re (as we shall immediately 
eee) in France, came to the same conclusion on the origin of species, in the years 


tend to varj- iu some deforce, and, secondly, that agriculturists improve their 
domesticated animals by selection ; and then, he adds, but what is done in 
this latter case " by art seems to be done with equal cdicacy, though more 
slowly, by Nature, in the formation of varieties of mankind, fitted for the 
country which they inhabit. Of the accidental varieties of man, which 
would occur among the first few and scattered inhabitants of the middle 
regions of Africa, some one would be better fitted than the others to bear 
the diseases of the country. This race would consequently multiply, while 
the others would decrease — not only from their inability to sustain the at- 
tacks 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 already said, would be dark. But, tlie 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." lie then extends 
these same views to the white inhabitants of colder climates. I am in- 
debted to Mr. Rowley, of the Uniteil States, for having called my attention, 
through Mr. Brace, to the above passage in Dr. Wells's work. 

The lion, and Kcv. W. Herbert, afterward Dean of Manclicster, in the 
fourth volume of the " Horticultural Transactions," 1822, and in his work on 
the " Amaryllidacere " (1S37, pp. 10, 339), declares that " horticultural exper- 
iments have established, beyond the possibility of refutation, that botanical 
species are only a higher and more permanent class of varieties." He ex- 
tends the same view to animals. The dean believes that single species of 
each genus were created in an originally highly plastic condition, and that 
these have produced, chiefly by intercrossing, but likewise by variation, all 
ovir existing species. 

In 182G Professor Grant, in the concluding paragraph in his well-known 
paper {Edinhtirgh Philosoplikal Journal, vol. xiv., p. 283) on the Spongilla, 
clearly declares his belief that species are descended from other species, and 
that they become improved in the course of modification. This same view 
was given in his 55th Lecture, published in the Lancet in 1834. 

In 1831 Mr. Patrick Matthew published his work on "Xaval Timl)er and 
Arboriculture," in which he gives precisely the same view on the origin of 
species as that (presently to be alluded to) propounded by ^Ir. Wallace and 
myself in the Lluncan 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 adiflcrcnt sul)jeet, so that it remained 
unnoticed until Mr. Matthew himself drew attention to it in the Gardcncr^a 
Chronicle, on April Y, 1860. The difTcrcnecs of Mr. JIatthew's view from 
mine arc 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 new forms may be generated " without the presence 
of any mould or germ of former aggregates." I am not sure that I under- 
stand some passages ; but it seems tliat he attributes much influence to the 
direct action of the conditions of life. He clearly saw, however, the full 
force of tlie principle of natural selection. 

The" celebrated geologist and naturalist. Yon Buch, in his excellent "De- 
scription Physique dcs lies Canaries" (1836, p. 147), clearly expresses his 
belief that varieties slowly become changed into permanent species, which 
arc no longer capable of intercrossing. 

Rafinesquc, in his "New Flora of North America," published in 1836, 
wrote (p. C) as follows : " All species might have been varieties once, and 
many varieties are gradually becoming species by assuming constant and pe- 
culiar characters:" but further on (p. 18) he adds, "except the original 
types or ancestors of the genus." 

In 1813-41 Prof. Ilaldcman {Boston Journal of Nat. ///.s/., U. States, vol. 
iv., p. 463) has ably given the arguments for and against the hypothesis of 
the development and modification of species ; he seems to lean toward the 
side of change. 

The " Ycstigcs of Creation" appeared in 1844. In the tenth and much 
improved edition (1853) the anonymous author says (p. 155): "The propo- 
sition determined on after much consideration is, that the several series of 
animated beings, from the simplest and oldest up to the highest and most 
recent, are, under the providence of God, the results, first, of an impulse 
which has been imparted to the forms of life, advancing them, in definite 
times, by generation, through grades of organization terminating in the high- 
est dicotyledons and vertebrata, these grades being few in number, and gen- 
erally marked by intervals of organic character, which we find to be a prac- 
tical difficulty in ascertaining affinities; second, of another impulse connected 
with the vital forces, tending, in the course of generations, to modify organic 
structures in accordance with external circumstances, as food, the nature of 
the habitat, and the meteoric agencies, these being the ' adaptations ' of the 
natural theologian." The author apparently believes thr.t organization pro- 
gresses by sudden leaps, but that the eflccts produced by the conditions of 
life arc gradual. lie argues with much force on general grounds that species 
arc not immutable productions. But I cannot see how the two supposed 
"impulses" account in a scientific sense for the numerous and beautiful co- 
adaptations which wc sec throughout Nature ; I cannot see that we thus gain 
any insight how, for instance, a woodpecker has become adapted to its pecu- 
liar habits of life. " The work, from its powerful and brilliant style, though 
di:<playing in the earlier editions little accurate knowledge and a gi-eat want 
of scientific caution, immedi.itcly had a very wide circulation. In my opiu- 


ion it has done excellent service in this country in callinj; attention to the 
subject, in removing prejudice, and in thus preparing the ground lor the re- 
ception of analogous views. 

In 1816 the veteran geologist, M. J. d'Omalius d'llalioy, published in au 
excellent, though short paper {Bulletins dc PAcad. Jioy. JSi-uxcllcs, torn, 
xiii., p. 581), his oinuion tliut it is more probable that new species have been 
produced by descent with modification, than that they have been separately 
created. The author first promulgated this opinion in 1831. 

Trof. Owen, in 1819 ("Nature of Limbs," p. SO), wrote as follows: 
" The archetypal idea was mauirested in the flesh under divers such modifi- 
cations, upon this planet, long prior to the existence of those animal species 
that actually exemplify it. To what natural laws or secondary causes the or- 
derly succession and progression of such organic phenomena may have been 
committed, we, as yet, are ignorant." In his Address to the British Asso- 
ciation, in 1858, he speaks (p. li.) of "the axiom of the continuous operation 
of creative pow'er; or of the ordained becoming of living things." Further on 
(p. xc), after referring to geograpliical distribution, he adds : " These phe- 
nomena shake our confidence in the conclusion that the Apteryx of New 
Zealand and the Red Grouse of England were distinct creations, in and for 
tho.^e islands respectively. Always, also, it may be well to bear in mind 
that by the word 'creation' the zoologist means 'a process he knows not 
what.' " lie amplifies this idea by adding that, when such eases as that of 
the IJed Grouse arc " 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 lied Grouse came to be there, and there exclusively, sig- 
nifying also, by this mode of expressing such ignorance, his belief that both 
the bird and" the islands owed their origin to a great first Creative Cause." 
If we interpret these sentences given in the same Address, one by the other, 
it appears that this eminent philosopher felt, in 1858, his confidence shaken 
that the Apteryx and the Red Grouse first appeared in their respective 
homes, "he knew not how," or by some process, "he knew not what." 

This Address was delivered after the papers, by Mr. "Wallace and myself, 
on the Origin of .'Species, presently to 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 Prof. 
Owen with other paleontologists, as being firmly convinced of the immuta- 
bility 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 bo perfectly just — from a passage 
beginning with the words " no doubt the type-form," etc. (ibid., vol. i., p. 
XXXV.), that Prof. Owen admitted that natural selection may have done 


Bometliinp; in tlio foriuation of new ppccics ; but tliir:, it appears (ibiil., vol. 
ii., p. 708), is inaccurate and witliout evidence. I also gave some extracts 
from a correspondence between Prof. Owen and the editor of the Lon- 
don licvicw, from which it appeared manifest to the editor as well as to 
myself, that Prof. Owen claimed to have promulgated the theory of nat- 
ural selection before I had done so ; and I expressed my surprise and satis- 
faction at this announcement ; but, as far as it is possible to understand cer- 
tain reccntl}--published passages (ibid., vol. iii., p. "JOS), I have, cither par- 
tially or wholly, again fallen into error. It is consolatory to me that others 
find Prof. 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 Prof. Owen preceded me, for both of us, as shown in tliis historical 
sketch, were long ago preceded by Dr. "Wells and Mr. Matthew. 

M. Isidore GcoflVoy Saint-IIilaire, in his Lectures delivered in 1850 (of 
which a resume appeared iu the Hevue ctMn(/. dc ZooJog.^ January, 1851), 
briefly gives his reason for believing that specific characters " sont fixes, 
pour chaque esijuce, tant qu'elle se perpetue au milieu des m6mes circon- 
stanccs: lis se modifient, si Icscirconstanccsambiantcs vienncnti changer." 
"En r6sume, Vohservation des animaux sauvages dcmontre dejiHavariabihte 
limiiie des cspeces. Les experiences sur Ics animaux sauvages dcvcnus do- 
mcstiqucs, ct sur les animaux domestiques redcvenus sauvages, la demon- 
trent plus claircment encore. Ccs memcs experiences prouvcnt, deplus, que 
les diirercnces produites peuvent ctre de valcur gcniriqne.''^ In his " Hist. 
Nat. Generalc" (tom. ii., p. 430, 1859) he amplifies analogous conclusions. 

From a circular lately issued it appears that Dr. Frekc, in 1851 {Dublin 
Medical Frcsa, p. 322), propounded the doctrine that all organic beings have 
descended from one primordial form. His grounds of belief and treatment 
of the subject are wholly diflVrent from mine; but, as Dr. Freke has now 
(1801) ptiblishcd his Essay on the "Origin of Species by means of Organic 
Affinity," the difficult attempt to give any idea of his views would be super- 
fluous on my part. 

Mr. Herbert Spencer, in an Essay (originally published in the Leader, 
March, 1852, and republished in his " Essays " in 1858), has contrasted the 
theories of the Creation and the Development of organic beings with re- 
markable skill and force. He argues from the analogy of domestic pro- 
ductions, from the changes which the embryos of many species undergo, 
from the difficulty of distinguishing species and A-arictics, and from the prin- 
ciple of general gradation, that species have been modified ; and he attributes 
the modification to the change of circumstances. The author (1855) has 
also treated Psychology on the principle of tliencce.-sary acquirement of each 
mental power and capacity by gradation. 


In 1852 M. Naudin, a distinguished botanist, expressly stated, in an ad- 
mirable paper on the Origin of Species {Revue Horlkolc^ p. 102 ; sir.ce 
partlj' republished in the "Nouvelles Archives du Museum," torn, i., p. 
171), his belief that species are formed in an analogous manner as varie- 
ties arc under cultivation ; and the latter process he attributes to man's 
power of selection. But he does not show how selection acts under Nature. 
lie believes, like Dean Herbert, that species, when nascent, were more 
plastic than at present. lie lays weight on what he calls the principle of 
linality, "puissance mysturieusc, indeterminee; fatalite pour les uns ; pour 
les autres, volonto providentielle, dont Taction inecssante sur les etres vi- 
vants determine, ii toutcs les epoqucs de Texistcncc du mondc, la forme, Ic 
volume, et la durcc do chacun d'cux, en raison dc sa destiuC-e dans I'ordre 
de choses dont il fait partie. C'est cette puissance qui harmonise chaquc 
membrc i rensemble en I'appropriant h. la fonction qu'il doit rcmplir dans 
Torganisme general de la nature, fonction qui est pour lui sa raison 

In 1853 a celebrated geologist. Count Keyscrling {BuV.ei'ni ilc la Soc. 
Gcohg., 2d Scr., tom. x., p. SSV), suggested that as new diseases, supposed 
to have been caused by some miasma, have arisen and spread over the 
world, so at certain periods the germs of existing species may have been 
cliemically affected by circumambient molecules of a particular nature, and 
thus have given rise to new forms. 

In this same year, 1853, Dr. SchaalTliauscn published an excellent pam- 
phlet (" Yerhand. des Naturhist. Yereins dcr Prcus.-!. Ilhcinlands," etc.), in 
which he maintains the progressive development of organic forms on the 
earth. lie infers that many species have kept true for long periods, 
whereas a few become modified. The distinction of species he explains 
by the destruction of intermediate graduated forms. " Thus living plants 
and animals are not separated from the extinct by new creations, but are to 
be regaiilod as their descendants through continued reproduction." 

A well-known French botanist, 51. Lccoq, writes in 185-1 ("Etudes sur 

♦ From rofcrcnccs in Bronn's " Untcrsucluui^xon iibcr die Entwifkchins-'-Oe- 
pptze " it nppca^^^ that the celebrated botanist and paleontologifit Unser published in 
]S5'2 his belief that Bpcclcs undergo development and modillcntion. D'Alton, likc- 
wi!»c, in Pander and Dalton's? work on Fosfil Slothp, cxprcc-scd, in 1821, a similar 
belief. Similar views have, as is well known, been uinintained by Okcn in his mys- 
tical " Niitur-PhiloKophic." From other refcrencci? in Godron's work "Siir VEi- 
pc^co," it seems that Bory St. Vinecut, Burdacli, Poiret, and Fries, have all admitted 
that new species are continually bciii:; produced. 

I may add that, of the thirfy-foiir authors named in this lliKtnrical Skcicli, who 
believe In the modification of species, or nt leasit disbelieve in sepanite acts of 
creation, twenty-seven have written on special branches of natural history or 


Geograph. Bot.," torn, i., p. 250), " Oa voit que nos rcchcrchcs sur la fixate 
ou la variation ilc Tespece, nous conduiscnt dircctcment aux idecs emisea 
par deux honiincs justcmcnt celebrcs, Geofifroy Saint-llilairc ct Goethe." 
Some other passages, scattered through M. Lccoq's large work, make it a 
little doubtful how far he extends his views on the modification of species. 

The " Philosophy of Creation " has been treated in a masterly manner 
by the Rev. Baden Powell, in his "Essays on the Unity of Worlds," 1855. 
Nothing can be more striking than the manner in which he shows that the 
introduction of a new species is " a regular, not a casual phenomenon," or, 
as Sir John llerschel expresses it, " a natural in contradistinction to a mi- 
raculous process." 

The third volume of the Journal of the Linnean Sociehj 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 clearness. 

Yon Baer, toward whom all zoologists feel so profound a respect, ex- 
pressed, about the year 1859 (see Prof. Rudolph Wagner, " Zoologisch-An- 
thropologische Untcrsuchungen," 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, Prof. Huxley gave a lecture before the Royal Institution 
on the "Persistent Types of Animal Life." Referring to such cases, he re- 
marks : " 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 or- 
ganization, 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 op- 
posed to the general analogy of Nature. If, on the other hand, we view 
' Persistent Types' in relation to that hypothesis which supposes the species 
living at any time to be the result of the gradual modification of pre- 
existing species — a hypothesis which, though unproven, and sadly damaged 
by some of its supporters, is yet the only one to which physiology lends any 
countenance ; their existence would seem to show that the amount of modi- 
fication which living beings have undergone during geological time is but 
very small in relation to the whole scries of changes which they have suf- 

In December, 1859, Dr. Hooker published his " Introduction to the Aus- 
tralian Flora." In the first part of this great work he admits the truth of 
the descent and modiCcatiou of species, and supports this doctrine by many 
original observations. 

The first edition of this work was published on November 24, 1850, and 
the second edition on January 7, 18G0. 



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 Amdrica, and in the geological rela- 
tions 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 bcei\ called by one of our great- 
est pliiloso^ihcrs. On my return home, it occurred to me, in 
1837, that something might perhaps be made out on this ques- 
tion by patiently accumulating and reflecting on all sorts of 
facts which could possibly have any bearing on it. After five 
years' work I allowed myself to speculate on the subject, and 
drew up some short notes ; these I enlarged in 1844 into a 
sketch of the conclusions, which then seemed to me probable : 
fiom that period to the present day I have steachly pursued 
tlie same object. I hope that I may be excused from entering 
on these personal details, as I give them to show that I have 
not been hasty in coming to a decision. 

My work is now nearly finished ; but as it will take me two 
or three more years to complete it, and as my health is far 
from strong, I have been urged to publish this Abstract. I 
have more especially been induced to do this, as Mr. Wallace, 
who is now studying the natural history of the Malay archi- 
pelago, has arrived at almost exactly the same general conclu- 
sions 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 Lycll, who sent it to the Linnean Society, and 


it is published in the third vohime of the Journal of tliat So- 
ciety, Sir C Lycll :uid Dr. Hooker, who both Icnew of my 
work — tlic hitter having read my sketch of 1844 — honored me 
by thinking it advisable to publish, with Mr. Wallace's excel- 
lent memoir, some brief extracts from my manuscripts. 

This Abstract, -which I now publish, must necessarily be 
imperfect. I cannot here give references and authorities for 
my several statements ; and I must trust to the reader reposing 
some confidence in my accurac}'. No doubt errors Avill have 
crept in, tliough I hope I have always been cautious in trust- 
ing to good authorities alone. I can here give only the general 
conclusions at which I have arrived, with a few facts in illustra- 
tion, but which, I hope, in most cases will suffice. No one can 
feel more sensible than I do of the necessity of hereafter pul> 
lishing in detail all the facts, Avith references, on wliich 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 Avjiich facts cannot be adduced, 
often apparently leading to conclusions directly opposite to 
those at Avhich I have arrived. A fair result can be obtained 
only by fully stating and balancing tlie facts and arguments 
on both sides of each question ; and this is here imj^ossible. 

I much regret that Avant of space prevents my having the 
satisfaction of acknowledging the generous assistance Avhich I 
have received from A^ery many naturalists, some of them per- 
sonally unknoAvn to me. I cannot, hoAvevcr, let this 02')por- 
tunity pass Avithout expressing my deep obligations to Dr. 
Hooker, Avho for tlie last fifteen years has aided me in CA'ery 
[wssible Avay by his large stores of knoAAdedge and liis excellent 

In considering the Origin of Species, it is quite conceivable 
that a naturalist, reflecting on the mutual affinities of organic 
beings, on their embryological relations, their geographical 
distribution, geological succession, and other such facts, might 
come to the conclusion that species had not been independently 
created, l)ut had descended, like varieties, from other species. 
Nevertlielcss, such a conclusion, even if Avell founded, Avould be 
unsatisfactory, imtil it could be shoAvn how the innumerable 
species inhabiting this Avorld haA'e been modified, so as to ac- 
quin? that perfection of structure and coadaptation Avhich justly 
excites our admiration. Naturalists continually refer to exter- 
nal conditions, such as climate, food, etc., as the only possible 
cause of variation. In one limited sense, as Ave shall hereafter 


see, this may bo true ; but it is preposterous to attribute 
to mere exteriuil conditions, the structure, for instance, of 
the woodpecker, with its feet, tail, beak, and tonjTue, so ad- 
mirably adapted to catch insects under the bark of trees. In 
the case of the mistletoe, which draws its nourishment from 
certain trees, M'hich has seeds that must be transported by cer- 
tain birds, and which has flowers with separate sexes absolutely 
requiring the ag^ency of certain insects to brin<^ pollen from one 
flower to the other, it is equally preposterous to account for 
the structure of this parasite, with its relations to several dis- 
tinct organic beings, by the efl'ects 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 coramencem(mt of my observations it seemed to me prob- 
able that a carcfyl study of domesticated animals and of culti- 
vated plants would oiler the best chance of making out this 
obscure problem. Nor have I been disappointed; in this and 
in all other perplexing cases I have invariably found that our 
knowledge, imperfect though it be, of variation under domesti- 
cation, aflbrded the best and safest clew. I may venture to 
express my conviction of the high value of such studies, al- 
though they have been very commonly neglected by natural- 

From these considerations, I shall devote the first chapter 
of this Aljstract to Variation under Domestication. We shall 
thus see that a large amount of hereditary modification is at 
least possible ; and, what is equally or more important, we 
shall see how great is the jiower of man in accumulating by 
his Selection successive slight variations. I will then pass on 
to the variability of species in a state of nature ; but I shall, 
unfortunately, be compelled to treat this subject far too briefly, 
as it can be treated properly only by giving long catalogues 
of facts. We shall, however, be enabled to discuss what cir- 
cumstances are most favorable to variation. In the next chap- 
ter the Struggle for Existence among all organic beings 
througliout the world, which inevitably follows from the high 
geometrical ratio of their increase, Avill be treated of. This is 
the doctrine of Malthus, applied to the whole animal and vege- 
table kingdoms. As many more individuals of each species 
are born than can possil)ly survive ; and as, consequently, there 
is a frequently-recurring struggle for existence, it follows that 
any being, if it vary however slightly in any manner profitable 


to itself, under tlie complex and sometimes varying conditions 
of life, uill have a better chance of survivinq-, and thus be nat- 
la'cdhj sdectcd. From the strong principle of inheritance, any 
selected variety will tend to propagate its new and modified 

This fundamental subject of Natural Selection will be 
treated at some length in tlie 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 com2:)lcx and little-known laws of variation. 
In the four succeeding chajitcrs, the most apparent and gravest 
difficulties in accepting the theory will be given : namely, first, 
the difficulties of transitions, or how a simple being or a simple 
organ can be changed and perfected into a highly-developed 
being or into an elaborately-constructed organ ; secondly, the 
subject of Instinct, or the mental powers of animals ; thirdly, 
Hybridism, or the infertility of species, and the fertility of va- 
rieties when intercrossed ; and fourthly, the imperfection of 
the Geological Record. In the next chapter I shall consider 
the geological succession of organic beings throughout time ; 
in the eleventh and twelfth, their geographical distribution 
throughout space ; in the thirteenth, their classification or mu- 
tual atlinities, both when mature and in an embrj-onic condi- 
tion. 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. AVho can explain why one species ranges Avidely and is 
very numerous, and why another allied species has a narrow 
range and is rare ? Yet these relations are of the highest im- 
]')ortance, for they determine the present welfare, and, as I be- 
lieve, the future success and modification of every inhabitant 
of tins world. Still less do we know of tlic mutual relations 
of the innumerable inhabitants of the world during tlie man}"- 
past geological epochs in its history. Although much remains 
obscure, and Avill long remain obscure, I can entertain no 
doubt, after the most deliberate study and dispassionate judg- 
ment of which I am cajiable, that the view which most natu- 
ralists entertain, and which I fonnerly entertained — namely, 
that each species has been independently created — is erio- 


neons. I am fully convinced that species arc not immutable , 
but that those belonging to what are called the same genera 
are hncal descendants of some other and generally extinct spe- 
cies, in the same manner as the acknowledged varieties of any 
one sjiecies arc the descendants of that species. Further- 
more, I am convinced that Natural Selection has been the 
most important but not the exclusive means of modification. 

22 VARIATION Chap. I. 



C!aii8PS of Variability— Effects of Habit— Correlated Variation— Inheritance— Char- 
acter of Domestic Varieties — Ditliculty of (liHtintruiehinj,' between Varieties and 
Species — Orii^in of Dome.stic Varieties from oiie or inorc (species — Domestic 
Pigeons, their Differences and Origin— Principles of Selection, anciently followed, 
their Effects — Methodical and Uuconscious Selection— Unknown Orisin of our 
Domestic Productions— Circumstances favorable to Man's Power of Selection. 

Causes of Yariahility. 

WnEN 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 generallj' differ 
from each other more than do the indi^aduals of any one spe- 
cies or variety in a state of nature. And if we reflect on the 
vast diversity of the plants and animals which have been cul- 
tivated, and which have varied during all ages under the most 
different climates and treatment, Ave are driven to conclude 
that this great variability is due to our domestic productions 
having been raised under conditions of life not so uniform as, 
and somewhat different from, those to which the parent-species 
had been exposed luider nature. There is also, I think, some 
probability in the view propounded by Andrew Knight, that 
this variability may be partly connected with excess of food. 
It seems clear that organic beings must be exposed during 
several generations to new conditions to cause any appreciable 
amount of variation ; and that, when the organization has once 
l)cgun to vary, it generally continues varying for many gencr- 
atitms. No case is on record of a variable organism ceasing 
to vary imder cultivation. Our oldest cultivated plants, siicli 
as wheat, still yield new varieties : our oldest domesticated 
animals arc still capable of rapid improvement or modifica- 

As far as I am able to judge, after long attending to the 
subject, the conditions of life ajipcar to act in two ways — di- 


rcctly ou the -wliole orn-anizalion or on certain parts alone, and 
indirectly by affecting the reproductive system. With respect 
to the direct action, Ave must bear in mind that in every case, 
as Prof. Weismann has lately insisted, and as I have incident- 
ally shown in my -work on " Variation under Domestication," 
there are two factors : namely, the nature of the organism, and 
the nature of the conditions. The former seems to be much 
the more important; for nearly similar variations sometimes 
arise under, as far as we can judge, dissimilar conditions ; and, 
on the other hand, dissimilar variations under conditions which 
appear to be nearly uniform. The effects on the offspring are 
cither defmite or indefmitc. They may be considered as defi- 
nite 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 rcgjird 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 
amoimt of food, color from the nature of the food, thickness of 
the skin and hair from climate, etc. Each of the endless vari- 
ations which we sec in the plumage of our fowls must hare 
had some efficient cause ; and if the same cause were to act 
uniformly during a long scries of generations on many indi- 
viduals, all probably would be modified in the same manner. 
Such facts as the complex and extraordinary outgrowths 
which invariably follow from the insertion of a minute drop of 
])oison by a gall-producing insect, show us what singular mod- 
ifications might result in the case of plants from a chemical 
change in the nature of the sap. 

Indefinite variability is a much more common result of 
changed conditions than definite variability, and has probably 
played a more important part in the formation of our domestic 
races. We see indefinite Aariability in the endless slight pe- 
culiarities which distinguish the individuals of the same spe- 
cies, and which cannot be accounted for by inheritance from 
cither 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-capsules. 
At long intervals of time, out of millions of individuals rc^ared 
in the same country and fed on nearly the same fooJ, dona- 
tions of structure so strongly pronounced as to deserve to be 
called monstrosities arise; but monstrosities cannot be sepa- 
rated by any distinct Une from slighter variations. All such 

24 VARIATION Chap. I. 

changes of structure, ■whether extremely slight or strongly 
marked, which appear among many individuals living together, 
may be considered as the indefinite elTects of the conditions of 
life on each individual organism, in nearly the' same manner as 
a chill allxicts diflerent men in an indefinite manner, according 
to their state of body or constitution, causing coughs or colds, 
rheumatism, or inflammations of various organs. 

With respect to what I have called the indirect action of 
changed conditions, namely, through the reproductive system 
being afTected, Ave 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 Kolreutcr and others have remarked, between the variabil- 
ity which follows from the crossing of distinct species, and 
that which may be observed with all plants and animals 
Avhen reared under new or unnatural conclitions. Many facts 
clearly show how eminently susceptible the reproductive sys- 
tem 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, 
oven Avhen the male and female unite. How many animals 
there are Avhich Avill not breed, though kept in an almost free 
state in their native country ! This is generally, but errone- 
ously, 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 liave 
collected and elscAvhere published on this curious subject ; but, 
to show how singular the laws are Avhich determine the repro- 
duction of animals under confinement, I may mention that car- 
nivorous animals, even from the tropics, breed in this country 
pretty freely under confinement, with the exception of the 
jilantigrades or bear family, Avhich seldom produce young; 
whereas carnivorous birds, with the rarest exceptions, hardly 
ever lay fertile eggs. Many exotic plants have jwllen utterly 
worthless, in the same condition as in the most sterile hybrids. 
When, on the one hand, we sec domesticated animals and 
jilants, though often weak and sickly, yet breechng freely un- 
der confinement ; and when, on the other hand, we see indi- 
viduals, though taken young from a state of nature, ]>erfectly 
tamed, long-lived, and healthy (of which I could give numerous 


instances), yet having their reproductive system so seriously 
anccted by uiiperceived causes as to fail to act, Ave need not 
he surprised at this system, when it does act under confine- 
UKMit, acting irregularly, and jiroducing offspring' somewhat 
imlike their parents. I may adil, that as some organisms breed 
fn^ely under the most unnatural conditions (for instance, rali- 
bits and ferrets kept in hutches), showing that their reproduc- 
tive organs arc not affected ; so will some animals and plants 
withstand domestication or cultivation, and vary very slightly 
— perhaps hardly more than in a state of nature. 

Some naturalists have maintained that all variations are 
connected with the act of sexual reproduction ; but this is cer- 
tainly an eiTor; for I have given in another woi'k 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-diilerent character from that of the 
other buds on the same plant. These bud-variations, as they 
may be named, can be propagated by grafts, offsets, etc., and 
sometimes by seed. They occur rarely under nature, but far 
from rarely under culture. As a single bud out of the manj^ 
thousands produced year after year under uniform conditions 
on the same tree, has been known suddenly to assmne a new 
character ; and as buds on distinct trees growing und(^r differ- 
ent conditions, have sometimes yielded nearly the same variety 
— for instance, buds on peach-trees producing nectarines, and 
buds on common roses producing moss-roses — we clearly see 
tliat the nature of the conditions is of quite subordinate impor- 
tance in comparison with the nature of the organism in deter- 
mining each particular form of variation — of not more impor- 
tance than the nature of the spark by wliich a mass of combus- 
t \h\o matter is ignited, has in determining the nature of the 

Effects of Habit ; Correlated Variation / Inheritance. 

Habits are inherited and have a decided influence ; as in 
the period of the flowering of plants when transported from 
one climate to another. In animals they have a more marktnl 
effect ; for instance, I llnd in the domestic duck that the bones 
of the wing weigh less and the bones of the leg more, in pro- 
portion to the whole skeleton, than do the same bones in the 
wild-duck ; and this change may be safely attributed to the do- 
mestic duck flying mucli less, and walking more, than its Mild 
]):irents. The great and inherite<l development of the udders 

26 VAKIATION Chap. I. 

in cows and goats in countries where they arc habitually milked, 
in comparison with the state of these organs in other coimtries, 
is probably another instance of the cfl'ects of use. Not one of 
our domestic animals can be named which has not in some 
country drooping ears ; and the view which has been suggested 
that the drooping is due to the disuse of the muscles of the 
ear, from the animals being seldom alarmed by danger, seems 

^lany laws regulate variation, some few of which can be 
dimly seen, and will hereafter be briefly discussed. I will here 
only allude to what may be called correlated variation. Im- 
portant changes in the embryo or larva will probably entail 
changes in the mature animal. In monstrosities, the correla- 
tions between quite distinct parts are very curiovis ; and many 
instances are given in Isidore Geofl'roy 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. Color and coii- 
stitutional peculiarities go together, of which many remarkable 
cases could be given among animals and plants. From facts 
collected by Heusinger, it appears that white sheep and pigs 
are injured by certain plants, while dark-colored mdii-iduals es- 
cape : Prof. Wyman has recently communicated to me a good 
illustration of this fact ; on asking some farmers in Florida 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 off; and one of the "crackers" (i. e., Florida 
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 ani- 
mals are apt to have, as is asserted, long or many horns ; 
pigeons with feathered feet have skin between their outer 
toes; pigeons with short beaks have small feet, and those 
with long beaks large feet. Hence if man goes on selecting, 
and thus augmenting, any peculiarity, he will almost certainly 
modify imintentionally other parts of the structure, owing to 
tlie mysterigus laws of correlation. 

The results of the various, unknown, or but dimly under- 
stood laws of variation are infinitely complex and diversified. 
It is well worth Avhile can-fiilly to study the several treatises 
on some of our ol<l cultivated jilants, as on tlie ]iyaci:ith, potato. 


even the dahlia, etc. ; and it is really surprising to note the 
endless points in structure and constitution in which the varie- 
ties and sub-varieties differ slig'litly from each other. The 
whole organization seems to have become plastic, and tends to 
depart 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 struc- 
ture, both those of slight and those of considerable physiological 
importance, are endless. Dr. Prosper Lucas's treatise, in two 
large volumes, is the fullest and the best on this subject. No 
breeder doubts how strong is the tendency to inheritance : that 
like produces like is his fundamental belief : doubts have been 
tlirown on this principle only by theoretical writers. When 
any deviation of structure often appears, and we see it in the 
father and child, we cannot tell Avhether it may not be due to 
the same cause having acted on both ; but when among indir 
viduals, apparently exposed to the same conditions, any very 
rare deviation, due to some extraordinary combination of cir- 
cumstances, appears in the parent — say, once among several 
million individuals — and it reappears in the child, the mere 
doctrine of chances almost compels us to attribute its re- 
appearance to inheritance. Every one must have heard of 
cases of albinism, prickly sldn, hairy bodies, etc., appearing in 
several members of the same family. If strange and rare devia- 
tions of structure are really inherited, less strange and com- 
moner deviations may be freely admitted to be inheritable. 
Perhaps the correct way of viemng the whole subject, Avould 
be, to look at the inheritance of every character whatever as 
tlie rule, and non-inheritance as the anomaly. 

The hiAvs governing inheritance are for tlie most part un- 
Icnown. No one can say why the same peculiarity in tlifferent 
individuals of the same species, or in different species, is some- 
tiiucs inherited and sometimes not so; why the child often re- 
verts in certain characters to its grandfather or grandmother or 
more remote ancestor ; why a peculiarity is often transmitted 
fi'om 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 impor- 
tance to us, that jK'culiarities appearing in the males of our 
domestic breeds are ofLen transmitted, either exclusively or in 
a nmch greater degree, to tlie males alone. A much more im- 
portant rule, which I think may be trusted, is that, at Avhatevrr 
period of life a peculiarity first appears, it tends to reappear in 
the ollspring at a corresponding age, though sometimes earlier. 

23 VARIATION Chap. I. 

In many cases this could not be otherwise : thus the inherited 
pocuharities in tlie horns of cattle could appear only in the 
oflsprini^ when nearly mature ; peculiarities in the silkworm 
are known to appear at the corrcsjoondinf^ caterpillar or cocoon 
staple. But hereditary diseases and some other facts make mc 
believe that the rule has a Avider extension, and that, when 
there is no apparent reason why a peculiarity should appear at 
any particular ai^e, yet that it does tend to appear in the off- 
spring at the same period at which it first appeared in the par- 
ent. I believe this rule to be of the hig-hest importance in 
explaining- the laws of embryology. These remarks are of 
course confined to the first appearance of the peculiarity, and 
not to its primary cause, which may have acted on the ovules 
or on the male element ; in nearly the same manner as in the 
ofi'spring- from a short-horned cow by a long-horned bull, the 
greater length of horn, though appearing late in life, is clearly 
due to the male element. 

Having alluded to the sul)ject of reversion, I may here refer 
to a statement often made by naturalists — namely, that our 
domestic varieties, "when rim Avild, gradually but invariably re- 
vert 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 Avould be great difli- 
cidiy 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 
Avhat the aboriginal stock was, and so could not tell whether 
or not nearly perfect reversion had ensued. It Avould be ne- 
cessary, 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 
i-evert in some of their characters to ancestral forms, it seems 
to mc not improbable, that, if we could succeed in naturalizing, 
or were to cultivate, duringmany generations, the several races, 
for instance, of the cabbage, in very poor soil (in which case, 
however, some effect would have to 1)C attributed to the definite 
action of the poor soil), 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 imjiortance 
for our line of argument ; for by the experiment itself the con- 
ditions of life are chanixed. If it couUl be sliown that our do 


incstic 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 considerable body, so that 
free intercrossin2^ mig-ht 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 we could not breed our cart and race horses, long 
and short horned cattle, and poultry of various breeds, and 
esculent vegetables, for an unlimited number of generations, 
would be opposed to all experience. 

Character of Domestic Varieties ; Difficult}/ of distinguish- 
inff heticeen Varieties and Species y Origin of Domestic 
Varieties from one or more /Sjjecies. 

When we look to the hereditary varieties or races of our 
domestic animals and plants, and compare them with closely- 
alUed species, wc generally perceive in each domestic race, as 
already remarked, less uniformity of character than in true 
species. Domestic races often have a somewhat monstrous 
character ; by which I mean that, although differing from each 
other, and from f)thcr species of the same genus, in several 
trifling res]iects, 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 
witli 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, only in most cases in a less de- 
gree, in the same manner as do closely-allied species of the 
same genus in a state of natiu-e. This must be admitted as 
true, for the domestic races of many animals and plants have 
•been rank(!d by some competent judges as the descendants of 
aboriginally distinct species, and by other competent judges as 
mere varieties. If any well-marked distinction existed be- 
tween a domestic race and a species, this source of doubt 
would not so perpetually recur. It has often been stated that 
domestic races do not diffcT from each other in characters of 
generic value. It can ])e shown that this statement is not cor- 
rect ; but naturalists differ nuich in determining what char- 
acters are of gimerio value ; all such valuations being at 
present empirical. When it is explained how genera origi- 
nate under nature, it will bo seen that we have no right to 

so CIIAEACTEK OF Ciiap. 1. 

expect often to find a generic amount of difference in our do- 
mesticated races. 

In attempting to estimate tlie amount of structural difler- 
encc between the domestic races of the same species, we are 
soon involved in doubt, from not knowing whether they have 
descended from one or several parent-species. This point, if 
it could be cleared up, would be interesting ; if, for instance, 
it could be shown that the grevhound, bloodhound, terrier, 
spaniel, and bull-dog, which we all know propagate their kind 
so truly, were the offspring of any single species, then such 
facts would have great weight in making us doubt about the 
immutability of the many very closely-allied natural species — 
for instance, of the many foxes — inhabiting different quarters 
of the Avorld. I do not believe, as we shall presently see, that 
the whole amount of difference between the several breeds of 
the dog has been produced under domestication ; I believe 
that some small part of the difference is due to their having 
descended from distinct species. In the case of strongly- 
marked races in 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 domes- 
tication animals and plants having an extraordinary inherent 
tendency to var}^, and likewise to withstand diverse climates. 
I do not dispute that these capacities have added largely to 
the value of most of our domesticated productions ; but how 
could a savage possibly know, when he first tamed an animal, 
M'hether it would vary in succeeding 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 domestication ? I cannot doubt that if other animals and 
plants, equal in number to our domesticated productions, and 
belonging to equally diverse classes and countries, were taken 
from a state of nature, and could be made to breed for an equal 
number of generations under domestication, they would on an 
average vary as largely as the parent-species of our existing 
domesticated productions have varied. 

In the case of most of our anciently-domesticated animals 
and jilants, it is not possible to come to any definite conclusion, 
whether they are descended from one or several wild species. 
The argiunent mainly relied on by those who believe in the 
multiple origin of our domestic animals is, that we find in the 


most ancient times, on tlie 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 ani- 
mals Avere domesticated at a much earlier period than has hith- 
erto l>een supposed. The lake-inhabitants of Switzerland cul- 
tivated several kinds of wheat and barley, the pea, the poppy 
t\)v oil, and llax ; and they possessed several domesticatcnl ani- 
mals ; they had also commerce with other nations. All this 
clearly shows, as Heer has remarked, that they had at this 
early ag-c progressed considerably in civilization ; and this 
again implies a long-continued previous period of less advanced 
civilization, during which the domesticated animals, kept by 
the different tribes in different districts, might have varied and 
given rise to distinct races. Since the discovery of flint tools 
or celts in the superficial formations in many parts of the world, 
all geologists believe that barbax-ian man existed at an enor- 
mously remote period ; and we know that at the present day 
there is hardly a tribe so barbarous as not to have domesti- 
cated at least the dog. 

The origin of most of our domestic animals will probably 
forever remain vague. But I may here state that, looking 
to the domestic dogs of the Avholc world, I have, after a la- 
borious collection of all known facts, come to the conclusion 
that several wild species of Canidre have been tamed, and that 
their blood, in some cases mingled together, llows in the A-eins 
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 they are de- 
scended 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 those 
deserve to be called species or races. This conclusion, as Avell 
as the specific distinction between the humped and common 
cattle, may, indeed, be looked at as established by the recent 
admirable researches of Prof. Kutimeyer. 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 saine species. Having kept nearly all the 
English breeds of the fowl alive, having bred and crossed them, 
and examined their skeletons, it appears to me almost certain 

32 CU^AKACTER OF Chap.*. 

that all are the descendants of the wild Indian fowl, Gallus 
bankiva; and Ihis is tlie conclusion of Mr. Bljth, and of others 
who have studied this bird in India. In reg'ard to ducks and 
rabbits, some breeds of which difler much from each otiier, the 
evidence is clear that they arc all descended from the common 
wild duck and rabbit. 

The doctrine of the origin of our several domestic races from 
several aborig'inal stocks, has been carried to an absurd ex- 
treme by some authors. They believe that every race which 
breeds true, let the distinctive characters be ever so slight, 
has had its wild protot^qDe. At this rate, there must have ex- 
isted at least a score of species of wild cattle, as many sheep, 
and several goats, in Europe alone, and several even within 
Great Britain. One author believes that there formerly existed 
in Great Britain eleven wild species of sheej? peculiar to it ! 
When v,e bear in mind that Britain has now hardly one pecu- 
liar mammal, and France but few distinct from those of Ger- 
many, and conversely, and so with Hungary, Spain, etc., but 
that each of these kingdoms possesses several peculiar breeds 
of cattle, sheep, etc., we must admit that many domestic breeds 
have originated in Europe ; for whence have they been derived, 
as these several countries could not possess so large a number 
of peculiar species for parent-stocks ? So it is in India. Even 
in the case of the domestic dogs of the whole world, which I 
admit to have descended from several wild species, it cannot be 
doubted that there has been an immense amount of inherited 
variation ; for who will believe tliat animals closely resembling 
the Italian greyhound, the bloodhound, the bull-dog, pug-dog, 
or Blenheim spaniel, etc. — so xmlike all Avild Canida? — ever ex- 
isted freely in a state of nature ? It has often been loosely 
said that all our races of dogs have been produced by the 
crossing of a few aboriginal species ; but by crossing Ave can 
only get forms in some degree intermediate between their par- 
ents ; and if we account for our several domestic races by this 
process, we must admit the former existence of the most ex- 
treme forms, as the Italian greyhound, bloodhound, bull-dog, 
etc., in the wild state. Moreover, the possibility of making 
distinct races by crossing has been greatly exaggerated. Many 
cases are on record, showing that a race may be modified by 
occasional crosses, if aided by the careful selection of the in- 
dividuals which present the desired character; but to obtaui a 
race nearly intermediate between two extronely dilTerent races 
or species, would be very dillicult. Sir J. Sebright expressly 


experimented for this object, and failed. The oflspriii^v from 
the first cross between two pure breeds is tolerably and some- 
times (as I have found with pip^eons) extremely uniform, and 
every thing seems simple enouji-h ; but when these mongrels 
are crossed one "with another for several generations, hardly 
two of them are alike ; and then the extreme dilEculty of the 
task becomes apparent. Certainly, a breed intermediate be- 
tween tico very distinct breeds could not be got without ex- 
treme care and long-continued selection ; nor can I find a case 
on record of a permanent race having been thus formed. 

Breeds of the Domestic Pigeon, their Differences and Origin. 

Belie\ing that it is alwa^'S best to study some special group, 
I have, after deliberation, taken up domestic pigeons. I have 
kept every breed whieli I could purchase or obtain, and have 
lieen most kindly fav(^red with skins from several quarters of 
the world, more especially by the Hon. W. Elliot from India, 
and by the Hon. C. Murray from Persia. Many treatises in 
different languages have been published on pigeons, and some 
of them are very important, as being of considerable antiquity. 
I have associated witli several eminent fanciers, and have been 
permitted to join two of the London Pigeon Clubs. The diver- 
sity of the breeds is something astonishing. Compare the Eng- 
lish carrier and the short-faced tumbler, and see the Avonderful 
difference in tlieir beaks, entailing corresponding differences in 
tlieir skulls. The carrier, more especially the male bird, is also 
remarlvable from the wonderful development of the caruncu- 
lated skin about the head; and this is accompanied by greatly- 
elongated eyelids, very large external orifices to the nostrils, 
and a wide gape of mouth. The short-faced tumbler has a 
b(>ak in outline almost like that of a finch; and the common 
tuin])ler 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 tlie sub-ljreeds of runts have very long 
necks, others very long Avings and tails, others singularly short 
tails. The barb is allied to the carrier, but, instead of a long 
beak, has a very short and broad one. The ])outer has a much- 
elongated body, wings, and legs ; and its enormously devel- 
oped crop, which it glories in inllating, may well excite aston- 
ishment and even laughter. The turbit has a very short and 
conical beak, with a line of reversed feathers down the breast; 


and it has the hal)it of continually expanding slightly the up- 
per part of tlie a?so2:)hagus. The Jacobin has the feathers so 
much reversed along the back of the neck that the}' form a 
hood ; and it has, proportionally to its size, much-elongated 
wing and tail feathers. The trumpeter and laugher, as their 
names express, utter a very different coo from the otlier breeds. 
The fantail has thirty or even forty tail-feathers, instead of 
twelve or fourteen — the normal number in all members of the 
great pigeon family ; and these feathers are kept expanded, 
and are carried so erect, that in good birds the head and tail 
touch : the oil-gland is quite aborted. Several other less dis- 
tinct breeds might be specified. 

In the skeletons of the several breeds, the development of 
the bones of the face in length and breadth and curvature dif- 
fers enormously. The shape, as Avell 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 mouth, the proportional 
length of the eyelids, of the orifice of the nostrils, of the tongue 
(not always in strict correlation with the length of beak), the 
size of the crop and of the upper part of the ccso})liagus ; the 
development and abortion of the oil-gland ; the number of the 
primary wing and caudal feathers ; the relative length of wing 
and tail to each other and to the body ; the relative length of leg 
and of the feet ; the number of scutcHa^ on the toes, the develop- 
ment of skin between the toes, arc all points of structure -which 
are variable. The period at which the perfect plumage is ac- 
quired, varies, as does the state of the do^^^l with which the nest- 
ling birds are clothed when hatched. The shape and size of the 
eggs vary. The manner of flight, and in some breeds the voice 
and disposition, differ remarkably. Lastly, in certain breeds, the 
males and females have come to dilVer 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-de- 
fined species. Moreover, I do not believe that any ornitliolo- 
gist would place the English canicr, the short-faced tumbler, 
the runt, tlic barb, pouter and fantail in the same genus; 


more especially as in each of these breeds several truly-inher- 
ited sub-breeds, or s2Dccies as he would have called them, could 
be shown him. 

Great as the differences are between the breeds of pigeons, 
I am fully convinced that the common opinion of naturalists 
is correct, namely, that all are descended from the rock-pigeon 
(Columba livia), including under this term several geograplii- 
cal races or sul>specics, Avhich 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 Ijricfly give them. If the several breeds are 
not varieties, and have not proceeded from the rock-pigeon, 
they must have descended from at least seven or eight abori- 
ginal 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 enonnous 
crop? The supposed aboriginal stocks must all have been 
rock-pigeons, that is, not breeding or willingly perching on 
trees. But besides Columba livia, Avith its geographical sub- 
species, only two or three other species of rock-pigeons are 
known ; and these have not any of the characters of the 
domestic breeds. Hence the supposed aboriginal stocks must 
either still exist in the countries where they were originally 
domesticated, and yet be unknown to ornithologists ; and this, 
considering their size, habits, and remarkable characters, seems 
improbable ; or they must have become extinct in the wild 
state. But birds breeding on precijiices, and good fliers, are 
unlikely to be exterminated ; and the common rock-pigeon, 
which has the same habits with the domestic breeds, has not 
been exterminated even on several of the smaller British isiecs, 
or on the shores of the Mediterranean. Hence the supposed 
extermination of so many species having similar habits with 
the rock-pigeon seems a very rash assumption. Moreover, 
the several above-named domesticated breeds have been trans- 
ported to all parts of tlie world, and, therefore, some of them 
must have been carried back again into their native country; 
but not one has ever become wild or feral, though the dovecot- 
pigeon, which is the rock-pigeon in a very slightly altered 
state, has become feral in several places. Again, all recent 
experience shows that it is difficult to get any wild animal to 
breed freely imder domestication : yet, on the hypothesis of tlie 
multiple oritrin 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 un- 
der confinement. 

An argument of grcat_ weight, and applicable in several 
other cases, is, that the above-specified breeds, though agree- 
ing generally with the wild rock-pigeon in constitution, habits, 
voice, coloring, and in most parts of their structure, yet are 
certainly highly abnormal in other parts ; we may look in vain 
throughout the whole great family of Columbida? for a beak 
liUe that of the English carrier, or that of the short-faced tum- 
bler, or barb; for reversed feathers like those of the Jacobin; 
for a crop like that of the pouter; for tail-feathers like those 
of the fantail. Hence it must be assumed, not only that half- 
civilized man succeeded in thoroughly domesticating several 
species, but that he intentionally or by chance picked out ex- 
traordinarily abnormal species ; and further, that these very 
species have since all become extinct or unknown. So many 
strange contingencies are improbable in the highest degree. 

Some facts in regard to the coloring of pigeons Avell de- 
serve consideration. The rock-pigeon is of a slaty-blue, with 
white loins ; the Indian sub-species, C intermedia of Strick- 
land, having this part bluish ; the tail has a terminal 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 Avhole 
family. Now, in every one of the domestic breeds, taking 
thoroughly well-bred birds, all the above marks, even to the 
white edging of the outer tail-feathers, sometimes concur per- 
fectly developed. Moreover, when birds belonging to two or 
more distinct breeds are crossed, none of Avhich 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 Avhich notoriously breeds very 
true ; the mongrels were dusky and mottled. I then crossed 
one of the mongrel barb-fanlails with a mongrel barb-spot, and 


tlicy produced a bird of as beautiful a bhie color, with the 
white loius, double black Aviiif^-bar, and barred and Avliite- 
ed^cd tail-feathers, as any wild rock-pi2;'eon ! We can under- 
stand these facts, on the well-known principle of reversion to 
ancestral characters, if all the domestic breeds are descended 
from the rock-])igeoii. ]}ut if we deny this, we must make 
one of the two following; highly improbable suppositions : 
cither, 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 with- 
in a score, of generations, been crossed by the rock-pigeon : 
I say within a dozen or twenty generations, for no instance 
is known of crossed descendants reverting to an ancestor 
of foreign' blood, removed by a greater nmnbcr of genera- 
tions. In a breed which has been crossed only once, the 
tendency to revert to any character derived from such a cross 
will naturall}'^ become less and less, as in each succeeding 
generation there will be less of the foreign blood ; but when 
there has been no cross, and there is a tendency in the breed 
to revert to a character which was lost during some former 
generation, this tendency, for all that we can see to the contrary, 
may be transmitted imdiminishcd for an indefinite nunil)er of 
generations. These two distinct cases of reversion are often 
confounded together by those who have Avritten on inheritance. 

Lastly, the hybrids or mongrels from between all the 
domestic breeds of pigeons are perfectly fertile. I can state 
this from my own oljservationR, purposely made, on the most 
distinct breeds. Now, it is diflicult, perhaps impossible, to 
bring forward one case of the hybrid offspring of two animals 
clenrbj distbict being themselves perfectly fertile. Some 
authors believe that long-continu(?d domestication eliminates 
this strong tendency to sterilily: from the histor}' of tlie dog, 
and of some other domestic animals, there is great probability 
in this hypothesis, if applied to species closely related to each 
other, thougli it is imsupported by a single experiment. But 
A.O extend the h^11otIlesis so far as to suppose that sjx'cies, 
aboriginally as distinct as carriers, tumblers, pouters, and fan- 
tails now are, should yield offspring perfectly fertile inter se^ 
seems to me rash in the extreme. 

From these several reasons, namely, the improbability of 


man having formerly got seven or eight supposed species of 
pigeons to breed freely under domestication ; these supposed 
species being quite unknown in a ■wild state, and their be- 
coming nowhere feral ; these species having very al^normal 
characters in certain respects, as conijoared with all other Co- 
lumbida?, thougli so like in most other respects to the rock- 
pigeon ; the bkic color and various black marks occasionally 
apjiearing in all the breeds, both when kept pure and when 
crossed ; the mongrel offspring being perfectly fertile — from 
these several reasons, taken together, we may safely conclude 
that all our domestic breeds have descended from the Columba 
livia Avith its geographical sub-species. 

In favor of this view, I may add, first, that C. livia, or the 
rock-pigeon, has been found capable of domestication in Europe 
and in India ; and that it agrees in habits and in a great num- 
ber of points of structure with all the domestic breeds. Second- 
ly, although an English carrier or a short-faced tumbler differs 
immensely in certain characters from the rock-pigeon, yet by 
comparing the several sub-breeds of these varieties, more 
especially those brought from distant countries, we can make 
in these two cases, and in some but not in all other cases, 
an almost perfect series between the extremes of stlncture. 
Thirdly, those characters which are mainly distinctive of each 
breed, for instance the wattle and length of beak of the car- 
rier, the shortness of that of the tumbler, and the number of 
tail-feathers in the fantail, ai'C in each breed eminently variable; 
and the explanation of this fact Avill be obvious Avhcn we come 
to treat of Selection. Fourthly, pigeons have been Avatched 
and tended Avith the utmost care, and loved by many people. 
They liaA'C been domesticated for thousands of years in several 
quarters of the Avorld ; the earliest knoAvn record of pigeons 
is in the fifth Eg^-ptian dynasty, about 3000 B. c, as Avas 
pointed out to me by Prol; Lcpsius ; 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 AA'e hear from Pliny, 
immense prices Averc giAxn for pigeons ; " nay, they are come 
to this pass, that they can reckon up their pedigree and race." 
Pigeons Avere much A'alued by Akbar Ivlian in India, about 
the j-ear IGOO; never less than 20,000 pigeons Avere taken 
Avith the tourt. " The monarchs of Iran and Turan sent him 
some very rare birds;" and, continues the courtly historian, 
" his Majesty by crossing the breeds, Avhich method Avas ncA'cr 
practised before, has improved them astonishingly." About 


this saino period the Dutch were as easier about pigeons as 
were the old Romans. The paramount importance of these 
considerations in explainini^ the immense amount of variation 
Avhich pigeons have undcr2;one, will be obvious when Ave treat 
of Selection. We shall then, also, see how it is that the 
several breeds so often have a somewhat monstrous character. 
It is also a most 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 tlie probable origin of domestic pigeons 
at some, yet quite insufiieient, length ; because when I first 
kept pigeons and watched the several kinds, avcU knowing how 
truly they breed, I felt fully as mucli dilliculty 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, v/ith 
whom I have conversed, or Avhose treatises I have read, are 
firmly convinced that the several breeds to Avhich each has 
attended, are descended from so many aboriginally distinct 
species. Ask, as I have asked, a celebrated raiser of Here- 
ford cattle, whether his cattle might not have descended from 
Long-horns, or both from a common parent-stock, and he Avill 
laugh you to scorn, I have never met a pigeon, or })oultry, 
or duck, or rabljit fancier, Avho 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 tlie several sorts, for instance a Ribston- 
pippin or Codlin-apple, could ever have proceeded from the 
seeds of the same tree. Innumerable other examples could 
be given. The explanation, I think, is simple : from long- 
continued study they are strongly impressed with the differ- 
ences 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 u[) in their minds slight differ- 
ences accumulated during many successive generations. May 
not those naturalists who, knowing far less of the laws of 
inheritance than does the breeder, and knowing no more than 
be docs 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 tlicy deride the idea of species in a state of nature being 
lineal descendants of other species ? 

Princi2>les of Selection anciently foUoiccd, and their Effects. 

r^et us now briefly consider the steps by which domestic 
races have been produced, either from one or from several 
allied species. Some elTect may be attributed to the direct 
and definite action of the external conditions of life, and some 
little to habit ; but he would be a bold man who would ac- 
count by such agencies for the differences between a dray and 
race horse, a greyhound and bloodhound, a carrier and tumbler 
jiigeon. One of the most remarkable features in our domesti- 
cated 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 suddenl}-, or by 
one step ; many botanists, for instance, ])elieve that the fuller's 
teasel, with its hooks, which cannot be rivalled by any mechan- 
ical 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 tlie turnspit-tlog ; and tliis is 
known to have been the case with the ancon sheep. But when 
we compare the dray-horse and race-horse, the dromedary and 
camel, the various breeds of sheep fitted either for cultivated 
land or mountain-pasture, with the wool of one breed good for 
one purpose, and that of another breed for another purpose ; 
when we compare the many breeds of dogs, each good for man 
in very different ways ; when we compare the game-cock, so 
pertinacious in battle, with other breeds so little quarrelsome, 
with "everlasting layers" which never desire to sit, and Avith 
the bantam so small and elegant ; w^hen we compare the host 
of agricultural, culinary, orchard, and flower-garden races of 
plants, most useful to man at dillerent seasons and for differ- 
ent purposes, or so beautiful in his eyes, we must, I think, 
look fi'rther than to mere variability. "We cannot sup]")ose 
that all the breeds were suddenly produced as perfect and as 
useful as we now see them ; indeed, in many cases, we know 
that this has not been their history. The key is man's ])ower 
of accumulative selection: Nature gives successive variations; 
man a<lds them up in certain directions useful to him. In tliig 
sense he may be said to have made for himself useful breeds. 


Tlie great poAVcr of tliis j^rinciple of selection is not hj- 
potlietical. It is certain that several of our eminent breeders 
nave, even within a single lifetime, modilied to a large extent 
the breeds of cattle and sheep. In order fully to realize -what 
they have done, it is almost necessary to read several of the 
many treatises devoted to this subject, and to inspect the ani- 
mals. Breeders habitually speak of an animal's organization 
as something quite plastic, which they can model almost as 
thcv 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 agricultu- 
rists than almost any other individual, and who was himself a 
very good judge of an animal, speaks of the principle of selec- 
tion as " that Avliich enables the agriculturist, not only to mod- 
ify the character of his flock, but to change it altogether. It 
is the magician's wand, by means of which he may summon 
into life whatever form and mould he pleases." Lord Somer- 
ville, speaking of what breeders have done for sheep, says : 
" It would seem as if they had chalked out upon a -wall a form 
perfect in itself, and then had given it existence." 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 arc studied, like a picture by 
a connoisseur ; this is done three times at intervals of months, 
and the sheep are each time marked and classed, so that the 
very best may ultimately be selected for breeding. 

What English breeders have actually eifectecl is proved by 
the enormous prices given for animals with a good pedigree ; 
and these have now been exported to almost every quarter of 
the world. The imjirovement is by no means generally due 
to crossing diilerent breeds ; all the best breeders are strongly 
opposed to this practice, except sometimes among closely-al- 
lied sub-breeds. And when a cross has been made, the closest 
selection is far more indispensable even than in ordinary cases. 
If selection consisted merely in separating some very distinct 
variety, and breeding from it, the princi^Dlc would be so obvi- 
ous as liardly to be worth notice ; but its imjiortance consists 
in the great edec't produced by tlie accumulation in one direc- 
tion, during successive generations, of diiferences absolutely 
inappreciable by an imeducated e^-c — diflerences which I for 
one have vainly attempted to appreciate. Not one man in a 
thousand has accuracy of eye and judgment sufficient to be- 
come an eminent breeder. If gifted with these qualities, and 

io SELECTION BY MAN. Ca.vr. 1, 

lie studies his subject for years, and devotes his lifetime to it 
witli indomitable ])erscverance, he "sviU succeed, and may make 
great improvements ; if he wants any of these qualities, he aa'III 
assuredly fail. Few -would readily believe in the natural ca- 
pacity and years of practice requisite to become even a skilful 

The same principles are followed by horticulturists ; but the 
variations are here often more abrupt. No one supposes that 
our choicest productions have been produced by a single vari- 
ation from the aboriginal stock. "W^e have proofs that this is 
not so in some cases in Avhich exact records have been kept : 
thus, to give a very trifling instance, the steadily-increasing 
size of the common gooseberry may be quoted. We see an 
astonishing improvement in many florists' flowers, when the 
flowei'S of the present day are compared with drawings made 
only twenty or thirty years ago. When a race of plants is 
once pretty Avell 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 
]iroper standard. With animals this kind of selection is, in 
fact, also followed ; for hardly any one is so careless as to 
allow his Avorst animals to breed. 

In regard to plants, there is another means of observing the 
accmnulated 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 compari- 
son with the flowers of the same varieties ; and the diversity 
of fruit of the same species in the orchard, in comparison with 
the leaves and flowers of the same set of varieties. See how 
different the leaves of the cabbage are, and how extremely 
alike the flowers ; how unlike the flowers of the heart's-ease 
are, and how alike the leaves ; how much the fruit of the dif- 
ferent kinds of gooseberries differ in size, color, shape, and 
hairiness, and yet the flowers present very slight differences. 
It is not that tlie 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. 
Tlie law of correlated variation, the importance of which should 
never be overlooked, will insure some diflerences ; but, as a 
general rule, I cannot doul)t that the continued selection of 
slight v:iriations, either in the leaves, the flowers, or the fruit, 
will produce races dillering from each other chiefly in these 


It may be objected that the principle of selection has been 
reduced to methodical practice ibr scarcely more than three- 
quarters of a century ; it has certainly been more attended to 
of late years, and many treatises have been published on the 
subjc^ct ; and the result has been, in a correspondinjr degree, rapid 
and important. But it is very far from true that the jjrinciple is 
a modern discovery. I could give several references to the full 
acknowledgment of the importance of the principle in woiks 
of higli anti(iuity. In rude and barbarous periods of English 
history, choice animals -were often imported, and laws were 
passed to prevent their exportation : the destruction of horses 
imdcr a certain size was ordered, and this may be comjiared 
to tlic " roguing " of plants by nurserymen. Tlic principle of 
selection 1 lind distinctly given in an ancient Chinese encyclo- 
pajdia. Exjilicit 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 draught^cattle by color, as do some of the PJsqui- 
maux their teams of dogs. Livingstone states that good do- 
mestic breeds arc highly valued by the negroes in the interior 
of Africa who have not associated with Euroj^eans. Some of 
these facts do not show actual selection, but they show that 
the breeding of domestic animals was carefully attended to in 
ancient times, and is now attended to by the lowest savages. 
It would, indeed, have been a strange fact, had attention not 
been paid to breeding, for the inheritance of good and bad 
qualities is so obvious. 

Unconscious Selection. 

At the present time, eminent breeders try by methodical 
selection, with a distinct object in view, to make a new strain 
or sul>breed, superior to any thing existing in the country. 
But, for our purpose, a kind of Selection, which may be called 
Unconscious, and which results frojn every one trying to pos- 
sess and breed from the best individual animals, is more im- 
portant. Thus, a man who intends keeping pointere naturally 
tries to get as good dogs as he can, and afterward breeds from 
his own best dogs, but he has no wish or exjiectation of per- 
manently altering the breed. Nevertheless, we may infer that 


Iliis process, continued during centuries, Avould improve and 
modify any I)recd, in the same "way as Bakeivell, Collins, etc., 
by this very same process, only carried on more metliodically, 
did greatly modify, even during their own lifetimes, the forms 
and qualities of their cattle. Slow and insensible changes of 
this kind could never be recognized unless actual measure- 
ments or careful drawings of the breeds in cjuestion had been 
made long ago, which might serve for comparison. In some 
cases, however, imchanged or but little changed individuals 
of the same breed may be found in less civilized districts, 
Avhere the breed has been less improved. There is reason to 
believe that King Charles's spaniel has been unconsciously 
modified to a large extent since the time of that monarch. 
Some highly-competent authorities are convinced that the 
setter is directly derived from the spaniel, and has probably 
been slowly altered from it. It is known that the English 
pointer has been greatly changed within the last century, and 
in this case the change has, it is believed, been chiefly elfected 
by crosses Avith the fox-hound ; but what concerns us is, that 
the change has ]:)ccn elTccted unconsciously and gradually, and 
yet so eifectually, that, thougli the old Spanish pointer cer- 
tainly came from Spain, Mr. Boitow has not seen, as I am 
informed by him, any native dog in Spain like our pointer. 

By a similar process of selection, and by careful training, 
the whole body of English race-horses have come to surpass 
in fleetness and size the parent Arab stock, so that the latter, 
1)}' the regulations for the Goodwood Races, are favored in the 
Aveights tlicy carry. Lord Sjicncer and others have shown 
how the cattle of England have increased in weight and in 
early maturity, compared with the stock formerly kept in this 
country. By comparing the accounts given in old pigeon 
treatises of carriers ami tumblers with these breeds as now 
existing in Britain, India, and Persia, Ave can, I think, clearly 
trace the stages through Avhich they have insensibly passed, 
and come to dilfer so greatly from the rock-pigeon. 

Youatt gives an excellent illustration of the effects of a 
course of selection, AA-hich may be considered as unconsciously 
followed, in so far that the breeders could never have expect- 
ed, or even have A\ished, to produce the result Avliich ensueil — 
namely, the production of tAvo distinct strains. The two Hocks 
of Leicester sheep ke])t by Mr. Buckley and INIr. Burgess, as 
Mr. Youatt remarks, " have been purely bred from the original 
stock of Mr. Bakewell for upAvard of lifty years. There is not 


a suspicion existing in the mind of any one at all acquainted 
with the subject, that the owner of cither of them has deviated 
in any one instance from tlie pure blood of Mr. BakewcU's 
flock, and yet the difference between tlie sheep possessed by 
these two f^entlemen is so f^reat that they have the appear- 
ance of being quite difl'erent varieties." 

If there exist savages so barbarous as never to think of 
the inherited character of the offspring of their domestic ani- 
mals, yet any one animal particularly useful to them, for any 
special purpose, would be carefully preserved during famines 
and other accidents, to Avhich savages are so liable, and such 
choice animals would thus generally leave more offspring than 
the inferior ones ; so that in this case there would be a kind * 
of unconscious selection going on. We see the value set on 
animals even by the barbarians of Tierra del Fuego, by their 
killing and devouring their old women, in times of dearth, as 
of less value than their dogs. 

In plants the same gradual process of improvement, through 
the occasional preservation of the best indi\4duals, whether or 
not sufBciently distinct to be ranked at their first appearance 
as distinct varieties, and whether or not two or more species 
or races have become blended together by crossing, may plainly 
be recognized in the increased size and beauty which we now 
see in the varieties of the heart's-easo, rose, pelargonium, 
dahlia, and other plants, when compared \\-ith the older varie- 
ties or with their parent-stocks. No one would ever expect 
to get a first-rate heart's-ease or dahlia from the seed of a wild 
plant. No one would expect to raise a first-rate melting ]jear 
from the seed of the wild pear, though he might succeed from 
a poor seedling growing wild, if it had come from a garden- 
stock. The pear, though cultivated in classical times, appears, 
from Pliny's description, to have been a fruit of very inferior 
quality. I have seen great surprise expressed in horticultm-al 
works at the wonderful skill of gardeners, in having produced 
such splendid results from such poor materials ; but the art 
has been simple, and, as far as tlie final result is concerned, 
lias been followed almost unconsciously. It has consisted in 
always cultivating the best-known variety, sowing its seeds, 
antl, when a sliglitly-better variety has chanced to appear, 
selecting it, and so onward. But the gardeners of the classi- 
cal period, who cultivated the best pear they could procure, 
never thought Avhat splendid fruit we should eat; though wo. 
owe our excellent fruit, in some small degree, to their having 


naturally chosen and preserved the best varieties they could 
anywhere inid. 

A laro-e amount of chang'c in our cultivated plants, thus 
slowly and unconsciously accumulated, explains, as I 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. If it has taken centuries or thousands 
of 3'^ears 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 aiforded us 
a single plant worth culture. It is not that these countries, so 
rich in species, do not by a strange chance possess the abori- 
ginal stocks of any useful plants, but that the native plants 
have not been improved by continued selection up to a stand- 
ard of perfection comparable with that given to tiie plants in 
countries anciently civilized. 

In regard to the 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 diifcrently circumstanced, individ- 
\ials of the same species, having slightly-different constitutions 
or structure, would often succeed better in the one country 
than in the other; and thus by a process of "natural selec- 
tion," as will hereafter be more fully explained, two sub-breeds 
might be formed. This, perhaps, partly explains Avhat has 
been remarked by soiiic authors, namely, that the varieties 
kept by savages have more of the character of species than 
the varieties kept in civilized countries. 

On the view here given of the all-important part which 
selection by man has played, it becomes at once ob\-ious how 
it is that our domestic races show adaptation in their stnicture 
or in their habits to man's wants or fancies. We can, I think, 
further understand the frequently-abnormal character of our 
domestic races, and likewise their differences being so great 
in external characters, and relatively so slight in internal parts 
or organs. Man can hardly select, or only with much dilhculty, 
any deviation of structure, excepting such as is externally vis- 
ible ; and indeed he rarely cares for what is internal. He can 
never act l)y 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 de- 


veloped in some slis^lit degree in an unusual manner, or a 
pouter till lie saw a pi<j^eon -with a crop of somewhat unusual 
size ; and the more abnormal or unusual any character was 
when it first appeared, the more likely it would be to catch his 
attention. But to use such an expression as trying to make 
a fantail, is, I have no doubt, in most cases, utterly incorrect. 
The man Avho first selected a pigeon with a slightly-larger 
tail, never dreamed what the descendants of that pigeon would 
ht'come through long-continued, partly-unconscious and parti y- 
nicthodical selection. Perhaps the parent-bird of all fantails 
had only fourteen tail-feathers somewhat expanded, like the 
jircsent Java fantail, or like individuals of other and distinct 
breeds, in which as many as seventeen tail-feathers have been 
counted. Perhaps tlie first pouter-pigeon did not inflate its 
crop much more than the turbit now does the upper part of its 
ccsojihagus-^a habit which is disregarded by aU fanciers, as it 
is not one of the points of the breed. 

Nor let it be thought that some great de\'iation 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 be set on any slight 
differences in the individuals of the same species, be judged of 
by the value which would now be set on them, after several 
])reeds have once fairly been established. Many slight differ- 
ences might, and indeed do now, arise among pigeons, which 
are rejected as faults or deviations from the standard of per- 
fection of each breed. The common goose has not given rise 
to any marked varieties; hence the Toulouse and the common 
breed, Avhich differ only in color, that most fleeting of char- 
acters, have lately been exhibited as distinct at our poultry- 

I think those views explain what has sometimes been 
noticed — namely, that we know nothing about the origin or 
history of any of our domestic breeds. But, in fact, a breed, 
like a dialect of a language, can hardly be said to have a dis- 
tinct 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 imjiroves them, 
and the improved animals slowly spread in the immediate 
neighborhood. But as yet they will hardly have a distinct 
name, and, from being only slightly valued, their history will 
be disregardt>d, ^^''hen further improved by the same slow 


and gradual process, tliey will spread more widely, and will get 
recognized as sometliing 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 in a new strain are once acknowledged, the principle, 
as I have called it, of unconscious selection will always tend — per- 
haps 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 inhabitants — slowly 
to add to the characteristic features of the breed, whatever 
they may be. But the chance may be infinitely small of any 
record having been jireserved of such slow, varying, and insen- 
sible changes. 

Circumstances favorable to Mail's Power of Selection. 

I must now say a few words on the circumstances, favor- 
able, or the reverse, to man's power of selection. A high de- 
gree of variability is obviously favorable, as freely giving the 
materials for selection to work on ; not that mere individual 
differences are not amply sufficient, with extreme care, to allow 
of the accumulation of a large amount of modification in almost 
any desired direction. But, as variations manifestly useful or 
pleasing to man appear only occasionally, the chance of their 
appearance will be much increased by a large number of indi- 
viduals being kept ; and hence tliis comes to be of the highest 
importance to success. On this principle Marshall formerly 
remarked, with respect to the sheep of parts of Yorkshu-e, that, 
" as they generally belong to poor people, and are mostly in 
small lots, they never can l)e improved." On the other hand, 
nurservmen, from raising large stocks of the same plants, arc 
generally far more successful than amateurs in getting new 
and valuable varieties. The keejiing of a large number of in- 
dividuals of a species in any country requires that the species 
should be placed under favorable conditions of life, so as to 
breed freely in that countr>', A\^hcn the indi\'iduals of any 
species are scanty, all the individuals, whatever their quality 
may be, will generally be allowed to l)reed, and this will effect- 
ually prevent selection. But probably the most important 
point of all is, that the animal or plant should be so highly 
useful to man, or so much valued hy him, that tlie closest at- 
Irntion is paid to even the slightest dmiation in the qualities 


or structure of each individual. Unless such attention be paid 
nothinf^ can be effected. I have seen it gravely remarked, that 
it was most fortimate that tlic strawberry began to vary just 
when gardeners began to attend closely to this plant. No doubt 
the strawberry had always varied since it was cultivated, but 
the slight varieties had been neglected. As soon, however, 
as gardeners picked out individual plants with slightly larger, 
earlier, or better fruit, and raised seedlings from them, and 
again })ickcd out the best seedlings and bred from them, then, 
there appeared (aided by some crossing with distinct species) 
those many admirable varieties of the strawberry which have 
been raised during the last thirty or forty years. 

In the case of animals with separate sexes, facility in pre- 
venting crosses is an important element of success in the for- 
mation of new races — at least, in a country which is already 
stocked with -other races. In this respect, enclosure of the 
land plays a part. Wandering savages or the inhabitants of 
open plains rarely possess more than one breed of the same 
species. Pigeons can be mated for life, and this is a great 
convenience to the fancier, for thus many races may be im- 
proved 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 tliey serve for food. On the 
other hand, cats, from their nocturnal rambling habits, cannot 
be easily matched, and, although so much valued by women 
and children, we hardly ever see a distinct breed kept up ; such 
breeds as we do sometimes sec are iilmost always imported 
from some other country. Although I do not doubt that some 
domestic animals vary less than others, yet the rarity or ab- 
sence of distinct l)reeds of the cat, the donkey, peacock, goose, 
etc., may be attril)uted in main part to selection not having 
been l)rought into play : in cats, from tlie difhculty in pairing 
them : in donkeys, from onl}'^ a few ])oing kept by poor jieople, 
and little attention paid to their breeding; for recently, in cer- 
tain parts of Spain and of the United States, tliis animal has 
been surprisingly modified and improved by careful selection : 
in peacocks, from not being very easily reared and a large 
stock not kejit : in geese, from being valuable only for two 
puqioses, food and featliers, and more especially from no 
plccisure having lieen felt in the display of distinct breeds ; but 
the goose seems to have a singularly inflexible organization. 


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 g'reatly improved 
in many ways within a recent period ; and this imjilies varia- 
tion. It would be equally rash to assert that characters now 
increased to their vitmost limit, could not, after remaining fixed 
for many centuries, again vary under new conditions of hfe. 
No doul)t, as Mr. Wallace has remarked with much truth, a 
limit will be at last reached. For instance, there must be a 
limit to the fleetness of any terrestrial animal, as this will be 
determined by the friction to be overcome, the weight of body 
to be carried, and the power of contraction in the muscular 
fibres. But what concerns us is that the domestic varieties 
of the same species differ from each other in almost every 
character, Mhich man has attended to and selected, more than 
do the distinct species of the same genera. Isidore Geoffroy 
St.-Hilaire has proved this in regard to size, and so it is with 
color, and probably Muth the length of hair. With respect to 
fleetness, which depends on many bodily characteristics. Eclipse 
was far fleeter, and a drajMiorse is incomparably stronger, than 
any two equine species. So with plants, the seeds of the dif- 
ferent varieties of the bean or maize difl'cr more in size than 
do the seeds of the distinct species in any one genus of the 
same two families. The same remark holds good in regard 
to the fruit of the several varieties c»f the plum, and still more 
so with the melon, as well as in endless other analogous cases. 

To sum up on the origin of our domestic races of animals 
and plants. Changed conditions of life are of the highest im- 
portance in causing variability, both directly l)y acting on the 
organization, and indirectly by aft'ccting the reproductive sj's- 
tem. It is not probable that variability is an inherent and 
necessary contingent, under all circumstances. The greater 
or less force of inheritance and reversion determine Avhether 
variations shall endure. Yariability is governed by many un- 
known laws, more especially by that of correlation. Some- 
Ihing may be attributed to the definite action of the conditions 
of life, but how much, we do not know. Something must be 
:iltiil)uted to use and disuse. The final result is thus rendered 
inllnilely comjilex. In some cases the intercrossing of abori- 
ginally distinct species, has probably played an important part 
in the orijxin of oin* domestic breeds. When several breeds 


have once been formed in any country, their occasional inter- 
crossing-, -with the aid of selection, has, no doubt, largely aided 
in forming new sub-breeds ; but the importance of crossing 
has been much exaggerated, both in regard to animals and 
to those plants which are propagated by seed. With plants 
which are temporarily propagated by cuttings, buds, etc, the 
importance of crossing is immense ; for the cultivator may 
here disregard the extreme variability both of hybrids and of 
mongrels, and the frequent sterility of hybrids ; but plants 
not propagated by seed are of little importance to us, for their 
endurance is only temporary. Over all these causes of Change 
the accumulative action of Selection, whether applied method- 
ically and quickly, or unconsciously and slowly, but more efB- 
ciently, is by far the predominant Power. 




Variability— Individual Differences— Doubtful Species— Wide-ranging, much dif- 
fused, and Common Species, vary meet- Species of the Larger Genera in each 
Country vai"y more frequently than the Species of the Smaller Genera— Many of 
the Species of the Larger Genera resemble Varieties in being very closely, bat 
unequally, related to each other, and in having Restricted Ranges. 

Before applying the principles arrived at in the last cha|> 
ter to organic beings in a state of nature, we must briefly dis- 
cuss whether these latter are subject to any variation. To 
treat this subject at all properlj^ a long catalogue of dry facts 
ought to be given; but these I shall reserve for my future 
work. Nor shall I here discuss the various definitions which 
have been given of the term species. No one definition has 
as yet satisfied all naturalists ; yet every naturalist knows 
vaguely what he means when he speaks of a species. Gen- 
erally the term includes the unknown element of a distinct 
act of creation. The term "variety" is almost equally difficult 
to define ; but here community of descent is almost universally 
implied, though it can rarely be proved. We have also what 
are called monstrosities ; but they graduate into varieties. 
By a monstrosity I presume is meant some considerable devia- 
tion of structure, generally injurious to or not useful to the 
species. Some authors use the term "variation" in a techni- 
cal sense, as implying a modification directly due to the physi- 
cal conditions of life ; and " variations " in this sense are sup- 
posed 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 be inher- 
ited for at least some few generations ? and in this c;ise I 
presume that the form would be called a variety. 

It may be doubted whether sudden and great deviations of 
structure such as we occasionally see in our domestic produc- 


tions, more especially with plants, are ever permanently prop- 
agated in a state of nature. Almost every part of every 
organic being is so beautifully related in 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 invente4 by man in a perfect state. Under 
domestication monstrosities often occur which resemble noi 
mal structures in widely-different animals. Thus pigs have 
often been born with a sort of proboscis like that of tlie tapir 
or elephant. Now, if any wild species of the pig-genus had 
naturally possessed a proboscis, it might have been argued that 
tliis in like manner had suddenly 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 would bear on the question. If mon- 
strous forms of this kind ever do appear in a state of nature 
and are capable of propagation (which is not always the case), 
as they occur rarely and singly, their preservation would de- 
pend on unusually favorable circumstances. They would, also, 
during the first and succeeding generations cross with the or- 
dinary form, and thus they would almost inevitably lose their 
abnormal character. But I shall have to retvu^n in a future 
cliapter to the preservation and perpetuation of occasional va- 

Individual Differences. 

Tlie many slight differences Avhich frequently appear in the 
offspring from the same parents, or which may be presumed to 
have thus arisen, from being frequently observed in the indi- 
viduals of the same species inhabiting the same confined local- 
ity, may be called indiA-idual differences. No one supposes 
that all the indi\'iduals of the same species are cast in the same 
actual mould. These individual differences are of the highest 
importance for us, for they are often inherited, as must be fa- 
miliar to every one ; and they thus afford materials for natural 
selection to act on and accumulate, in the same manner as man 
accumulates in any given direction individual differences in 
his domesticated production.s. These individual differences 
generally affect what naturalists consider unimportant parts ; 
but I could show, by a long catalogue of facts, that parts which 
must be called important, whether viewed under a physiolo- 
gical or classificatory point of view, sometimes vary in the indi- 
viduals of the same species. I am convinced that the most ex- 


perienced naturalist would be surprised at the number of the 
oases of variability, even in important parts of structure, which 
lie could collect on ^ood authority, as I have collected, during 
a course of years. It should be remembered that systematists 
are far from bcing^ pleased at finding variability in important 
characters, and that there are not many men who wiU labori- 
ously examine internal and important ort^ans, and compare 
tliem in many specimens of the same species. It Avould 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, recently, Sir J. Lubbock has shown a degree of . 
variability in these main nerves in Coccus, which ma}' almost 
be compared to the irregular branching of the stem of a tree. 
This philosophical naturalist, I may add, has also recently 
sliowu that the muscles in the larva? of certain insects are far 
from luiiform. 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 natu- 
ralists have honestly confessed), which do not vary ; and, under 
this point of view, no instance will ever be found of an impor- 
tant 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 the 
species present an inordinate amount of variation ; and about 
which hardly two naturalists agree whether to rank them as 
species or as varieties. We niay instance Rubus, Rosa, and 
Hieracium among plants, several genera of insects, several gen- 
era of Brachiopod shells, and the Ruff (Machetes pugnax) 
among birds. In most polpnorphic genera some of the species 
have fixed and definite characters. Genera which are poly- 
morpliic in one country seem to be, with some few exceptions, 
])olymorphic in other countries, and likewise, judging from 
Brachiopod shells, at former periods of time. These facts are 
very perplexing, for they seem to sliow that this kind of varia- 
bility is independent of the conditions of life. I am inclined 
to suspect that we have, at least in some of these polymorphic 
gen(»ra, variations which are of no service or disservice to the 
Hpccics, and which, consequently, have not been seized on and 
reiiden^d delinite by natural selection, as hereafter to be ex- 


Individuals of the same species often present ^cat difTcr- 
eiices of structure, as in the two sexes of various animals, iuthe 
two or three castes of sterile females or workers amon^^ insects, 
and in the immature and larval states of many of the lower ani- 
mals. Tlierc are, however, other cases, namely, of dimori)hism 
and trimorphism, which might easily be, and have frecjucntly 
l)een, confounded with varial^ility, but which are quite distinct. 
I refer to the two or three dillerent forms which certain animals 
of either sex, and certain hermaphrodite plants, habitually pre- 
sent. Thus, Mr. Wallace, who has lately called special atten- 
tion to the subject, has shown that the females of certain spe- 
cies of butterflies, in the Malayan archipelaj^o, regularly ap- 
pear under two or even three consjoicuously distinct forms, not 
connected together by intermediate varieties. The winged and 
frequently wingless states of so many Hemipterous insects may 
probably be included as a case of dimorphism, and not of mere 
variability. Fritz Muller, also, has recently described analo- 
gous but more extraordinary cases with the males of certain 
Brazilian Crustaceans : thus, the male of a Tanais regularly 
occurs under two widely-dillerent forms, not connected by any 
intermediate links ; one of these forms has much stronger and 
differently-shaped pincers for seizing the female, and the other, 
as if for compensation, has anlennaj much more abundantly 
furnished with smelhng-hairs, so as to have a better chance of 
finding the female. Again, the males of another Crustacean, 
an Orchestia, occur under two distinct forms, with pincers dif- 
fering much more from each other in structure, than do the 
pincers of most species of the same genus. With respect to 
plants, I have recently shown that in several -widely-distinct 
orders, the species present two or even three forms, which arc 
abruptly distinguished from each other in several important 
]ioints, as in the size and color of the pollen-grains ; and these 
forms, though all hermaphrotlites, differ from each other in 
their reproductive power, so that for full fertility, or indeed in 
some cases for any fertility, they must reciprocally impregnate 
each other. Although the forms of the few dimorphic and tri- 
morphic animals and plants which have been studied, are not 
now connected together by intermi^diate links, it is ]irol)able 
that thi^ will be found to occur in other cases ; for Mr. \\''allacc 
observed a certain Ijutterlly which jiresentcd in the same island 
a great range of varieties connected by intermediate links, and 
the extreme links of the chain closely resembled the two forms 
of an allied dimorphic species inhabiting another part of the 


Malay archipelago. Tlius also with ants, the several worker- 
castes are generally cjuite distinct ; but in some cases, as we 
shall hereafter see, the castes are connected together by gradu- 
ated varieties. It certainly at first appears a highly-remark- 
able fact that the same female butterfly should have the power 
of producing at the same time three distinct female forms and 
a male ; that a male Crustacean should generate two male 
forms and a female form, all widely different from each other ; 
and that an hermaphrodite plant should produce from the same 
seed-capsule three distinct hermaplu-odite forms, bearing three 
different kinds of females and three or even six different kinds 
of males. Nevertheless these cases are only exaggerations of 
the universal fact that every female produces males and females, 
Avhich in some instances differ in a wonderful manner fj-omeach 

Doubtful jSpecies. 

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 grada- 
tions, that naturalists do not like to rank them as distinct 
species, are in several respects the most important for us. We 
have every reason to believe that many of these doubtful and 
closely-allied forms have permanently retained their characters 
in their own country for a long time ; for as long, as far as we 
knoAV, as have good and true species. Practically, when a 
naturalist can unite two forms together by othei*s having inter- 
mediate characters, he treats the one as a variety of the other, 
ranking the most common, but sometimes the one first de- 
scribed, as the species, and the other as the variety. But 
cases of great difliculty, which I will not here enumerate, 
sometimes occur in deciding whether or not to rank one form 
as a variety of another, even when they are closely connected 
by intermediate links ; nor will the commonly-assumed hybrid 
nature of the intermediate links always remove the difficulty. 
In very many cases, however, one form is ranked as a variety 
of another, not because the intermediate links have actually 
been found, but because analogy leads the observer to suppose 
either that they do now somewhere exist, or may formerly 
have existed ; and here a wide door for tlie 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 


judgment and wide experience seems the only guide to follow. 
itVe must, however, in many cases, decide by a majority of 
naturalists, for few well-marked and well-known varieties can 
be named which have not been ranked as species by at least 
some competent judges. 

That varieties of this doubtful nature are far from uncom- 
mon cannot be disputed. Compare the several floras of Great 
Britain, of France, or of the United States, drawn up by dif- 
ferent botanists, and see Avhat a surprising number of forms 
have been ranked by one botanist as good species, and by an- 
other as mere varieties. ^Ir. H. C. Watson, to whom I lie 
inider deep obligation for assistance of all kinds, has marked 
fur me 18"v British plants, which are generally considered as 
varieties, but which have all been ranked by botanists as spe- 
cies ; and in making this list he has omitted many trifling varie- 
ties, but which nevertheless have been ranked by some bota- 
nists as specie's, and he has entirely omitted several bighly-poly- 
morj^hic genera. Under genera, including the most polymor- 
{)hic forms, Mr. Babington gives 251 species, whereas Mr. 
Benthara gives only 112 — a difference of 139 doubtful forms ! 
Among animals which unite for each birth, and which are 
highly locomotive, doubtful forms, ranked by one zoologist as 
a species and by another as a variety, can rarely be found 
within the same country, but are common in separated areas. 
How many of those birds and insects in North America and 
Europe, which differ very slightly from each other, have been 
ranked by one eminent naturalist as undoubted species, and by 
another as varieties, or, as they are often called, as geographical 
races ! 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 sul)-species, and as true representative 
species. The first or variable forms vary much within the 
limits of the same island. The local forms are moderately 
constant and distinct in eqch separate island; but when all 
the forms from the several islands arc compared, the differ- 
ences are seen to be so slight and graduated, that it is impos- 
sible to define or describe them, though at the same time the 
extreme forms are sufficiently distinct. The geographical 
races or sul>speoies are local forms completely fixed and iso- 
lated; 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 consid- 
ered as species and -which as varieties." Lastly, representa- 
tive species fill the same place in the natural economy of each 
island as do the local forms and sub-species ; but, as they are 
distinp^uished from each other by a greater amount of differ- 
ence than that between the local forms and sub-species, they 
are almost imiversally ranked by naturalists as true species. 
Nevertheless, no certain criterion can possibly be given by 
which variable forms, local forms, sub-species, and representa- 
tive species can be recognized. 

Many years ago, when comparing, and seeing others com- 
pare, the birds from the closely-neighboring islands of the 
Galapagos archipelago, both one with another, and with those 
from the American main-land, I was much struck how entirely 
vague and arbitrary is the distinction between species and 
varieties. On the islets of the little Madeira group there are 
many insects Avhich are characterized as varieties in Mr. Wol- 
laston's admirable work, but which would certainly be ranked 
as distinct species by many entomologists. Even Ireland has 
a few animals, now generally regarded as varieties, but which 
have been ranked as species by some zoologists. Several 
experienced ornithologists consider our British red grouse as 
only a strongly-marked race of a Norwegian species, whereas 
the greater number rank it as an undoubted species peculiar 
to Great Britain. A wide distance between the homes of two 
doubtful forms leads many naturalists to rank them as distinct 
species ; but what distance, it has been Avell asked, Avill suf- 
fice ; if that between America and Europe is ample, will that 
lietween Europe and the Azores, or Madeira, or tlie Canaries, 
or between the several islets of these small archipelagos, be 
sufficient ? 

Mr. B. D. Walsh, a distinguished entomologist of the United 
States, has lately 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 in- 
discriminately 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, either ex- 
el usivel}' 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 l)()th males and females, liave been olisen'ed to be 
thus alTected in a shght degree. When the diflerencos are 


rather more strongly marked, and when both sexes and all aj^os 
are afTectcd, the forms -would be ranked by all entomolo<^ists as 
species. But no observer can determine for others, 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 together, 
as varieties ; and those Avhich appear to have lost this jjower, 
as species. As the ditfcrences depend on the insects having 
long fed on distinct plants, it cannot be expected that inter- 
mediate links connecting tlie several forms should now be 
found. The naturalist thus loses his best guide in determining 
whether to rank such doubtful forms as varieties or sjiecies. 
This likewise necessarily occurs with closely-allied organisms, 
which inhabit distinct continents or islands. When, on the 
other hand, an animal or plant ranges over the same continent, 
or inhabits many islands in the same archipelago, and presents 
different forms in the different areas, there is always a good 
chance that intermediate forms may be discovered which shall 
link together the extreme states ; and these arc then degraded 
to the rank of varieties. 

Some few naturalists maintain that animals never present 
varieties ; but then these same; naturalists rank the slightest 
differences as of specific value; and when the same identical 
form is met with in two distant countries, or in tM'o geological 
formations, they believe that two distinct species are hidden 
under the same dress. Tlie term species thus comes to be a 
mere useless mental abstraction, implying and assuming a sepa 
rate act of creation. It is certain that many forms, considered 
by highly-competent judges as varieties, so completely resemble 
species in character, that they have been thus ranked by other 
highly-competent judges. But to discuss whether they ought 
to lie called species or varieties, before any definition of these 
terms has been generally accepted, is A'ainly to beat the air. 

Many of the cases of strongly-marked varieties or doul)tful 
species well deserve consideration ; for several interesting lines 
of argument, from geographical distribution, analogical varia- 
tion, hybridism, etc., have been brought to l)ear on tlie attempt 
to determine their rank ; but space does not here permit me 
to discuss them. Close investigation, in many cases, will bring 
naturalists to an agreement how to rank doubtfid forms. Yet 
it must be confessed that it is in the best-known countries that 
we find the greatest number of forms of doubtful value. I 
have been struck Avith the fact that, if any animal or plant in a 


state of nature be liigbly useful to man, or from any cause 
closely attract his attention, varieties of it will almost univer- 
sally 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 
imiversally considered as varieties ; and in this country the 
highest botanical authorities and practical men can Ix) quoted 
to show that the sessile and pedunculated oaks are either good 
and distinct species or mere varieties. 

I may here allude to a remarkable memoir lately published 
by A. de Candolle, on the oaks of the whole world. No one 
ever had more ample materials for the discrimination of the 
species, or could have worked on them with more zeal and 
sagacity. He first gives in detail all the many points of struc- 
ture which vary in the species, and estimates numerically the 
relative frequency of the variations. He specifics above a 
dozen characters Avliich niay be found varying even on the same 
branch, sometimes according to age or development, sometimes 
without any assignable reason. Such characters of course are 
not 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 saj^- that he 
gives the rank of species to the forms that differ by characters 
never varying on the same tree, and never found connected by 
intermediate states. After this discussion, the result of so 
much labor, he emphatically remarks : " They are mistaken, 
who repeat that the greater part of our species arc 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 Avere fovuidcd 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 spe- 
cific 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. peduuculata, sessilifiora, and 
pubescens. The forms which connect these three sulvspecies 
are comparatively rare ; and, as Asa Gray remarks, if these 
connecting forms, whi(;h are now rare, were to become wholly 
extinct, the three sub-species Avould hold exactly the same 
relation to each other, as do the four or five provisionally- 


admitted species which closely surround the typical Qucnuis 
rol)ur. Finally, De Candolle admits that, out of the three hun- 
dred species, Avhicli will l^e enumerated in his Prodromus as 
belonging to the oak family, at least two-thirds are provisional 
species, that is, are not known strictly to fulfil the definition 
above given of a true species. For it should be added that De 
Candolle no longer believes that species are immutable crea- 
tions, but concludes that the derivative theory of the succession 
of forms is the most natural one, " and the most accordant with 
tlie known facts in paleontology, geographical botany and zo- 
ology, of anatomical structure and classification ; " but, he adds, 
direct proof is still wanting. 

When a young naturalist commences the study of a group 
of organisms quite unknown to him, he is at first much per- 
plexed to determine what difiercnces 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 tendency will be to make many species, for he will 
become impressed, just like the pigeon or poultry fancier l)eforc 
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 corrett his first impressions. As he extends the 
range of his oljservations, he will meet Avith more cases of diffi- 
culty ; for ho will encounter a greater number of closely-allied 
forms. But if his observations be widely extended, he will in 
the end generally l)e enaljled to make up his own mind which 
to call varieties and wliich species ; but he will succeed in this 
at the expense of admitting much variation — and the truth of 
this admission will often be disputed by other naturalists. 
AV'hen, moreover, he comes to study allied forms brought from 
countries not now continuous, in which case he cannot hope to 
find tlie intermediate links between his doubtful forms, he will 
have to trust almost entirely to analog}-, and his difliculties rise 
to a climax. 

Certainl}^ no clear line of demarcation has as yet been drawn 
between species and sub-sju^cies — that is, the forms which in 
the ojjinion of some naturalists come very near to, but do not 
quite arrive at, the rank of species: or, again, between sub- 
species and well-marked varieties, or between lesser varieties and 


individual differences. These differences blend into each other 
in an insensible scries ; and a seiies impresses the mind with 
the idea of an actual passage. 

Hence, I look at individual differences, though of small 
interest to the systematist, as of the highest importance for 
us, as being the first steps 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 latter, as leading to sub- 
species or species. The passages from one stage of difference 
to another may, in some cases, be the simple result of the 
long-continued action of different physical conditions ; but in 
most cases they may be attributed to the gradual accumulative 
'action of natural selection, as hereafter to be more fullj" ex- 
plained, on fluctuating variability. • Hence a well-marked 
variety may be called an incipient species ; but whether this 
belief is justifiable must be judged of by the general weight 
of the facts and considerations given throughout this Avork. 

It need not be supposed that all varieties or incipient spe- 
cies necessarily attain the rank of species. They may become 
extinct, or they may endure as varieties for very long periods, 
as has been shown to be the case by Mr. Wollaston with the 
varieties of certain fossil land-shells in Madeira, and with plants 
by Gaston de Saporta, If a variety were to flourish so as to 
exceed in immbers the parent-species, it would then rank as 
the species, and the species as the variety ; or it might come 
1o supplant and exterminate the parent-species ; or both might 
coexist, and both rank as independent species. But we shall 
hereafter return to this subject. 

From these remarks it will be seen that I look at the term 
species as one arbitrarily given, for tlie sake of convenience, 
to a set of individuals closely resembling each other, and that 
it does not essentially differ from the term variety, which is 
given to less distinct and more fluctuating forms. The term 
variety, again, in comparison with mere individual differences, 
is also applied arbitrarily, and for mere convenience' sake. 

Wide-ranging^ much-d {fused, and Commo7i Siyecics, vary 


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 tabulatinj^ 
all the varieties in several Avell-workcd floras. At first this 
seemed a simple task ; but Mr. H. C. W^atson, to whom I am 
much indebted for valuable advice and assistance on this sub- 
ject, soon convinced me that there were many difficulties, as 
did subsequently Dr. Hooker, even in strono^er terms. I shall 
reserve • for my future work the discussion of these difficulties, 
and the tables themselves of the proportional numbers of the 
varying species. Dr. Hooker permits me to add that, after 
liaviufT' carefully read my manuscript, and examined the tables, 
he tliinks tliat the follo\\ing statements are fairly well estab- 
lished. The whole subject, however, treated as it necessarily 
here is with much brevity, is rather pcrplexin;T, and allusions 
cannot be avoided to the "struggle for existence," "divergence 
of character," and other cjuestions, hereafter to be discussed. 

Alphonse de Candollc and others have shown that plants 
which have very wide ranges generally present varieties ; and 
this might have been expected, as they become exposed to 
diverse physical conditions, and as they come into competition 
(which, as we shall hereafter see, is an equally or more im- 
portant circumstance, with different sets of organic beings. But 
my ta])les further show that, in any limited country, the species 
winch arc most common, that is, abound most in individuals, 
and the species Avhich are most widely diifused within their 
own country (and this is a different 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 flourishing, or, as 
they may be called the dominant species — those which range 
widely, are the most diffused in their own country, and arc the 
most numerous in individuals — wln'ch oftenest produce well- 
marked varieties, or, as I consider them, incipient species. 
And this, perhaps, might have been anticipated; for, as varie- 
ties, in order to become in any degree permanent, necessarily 
have to struggle with the other inhabitants of the coimtry, 
the species which are already dominant will be the most likely 
to yield olTspring, which, though in some slight degree modi- 
fied, still inlu^rit those advantages tliat enabled their parents to 
become dominant over their compatriots. In these remai'ks 
on predominance, it should l)e understood that reference is 
marie only to those forms which come into competition Avith 
each other, and more especially to the members of the same 
genus or class having nearly similar habits of life. With re- 


spect to cominoiinoss or the number of individuals of any spe- 
cies, the comparison, of course, relates only to the members of 
the same group. A plant 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, not livini^^ under widely- 
different conditions of Hfe. Such a plant is not the less domi- 
nant in the sense here used, because some conferva inhabiting 
the water or some parasitic fungus is infinitely more numerous 
in individuals, and more widely diffused ; if one kind of conferva 
or parasitic fungus exceeded its allies in the above respects, 
it would be a dominant form within its own class. 

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

If the plants inhabiting a country and descrilicd in any 
Flora be divided into two ecjual masses, all those in the larger 
genera (i. e., those including many species) being placed on 
one side, and all those in the smaller genera on the other side, 
a somewhat larger number of the very common and much-dif- 
fused or dominant species will be found on the side of the 
larger genera. This, again, might have been anticipated ; for 
the mere fact of many species of the same genus inhabiting any 
country, shows that there is something in the organic or inor- 
ganic conditions of that country favorable to the genus ; and, 
consequently, we might have expected to have found in the 
larger genera, or those including many species, a large propor- 
tional number of dominant species. But so many causes tend 
to obscure this result, that I am surprised that my tables show 
even a small majority on the side of the larger genera. I will 
here allude to only two causes of obscurity. Fresh-water and 
salt-loving plants have generally very wide ranges and are 
much diffused, but this seems to be connected with the nature 
of the stations inhabited by them, and has little or no relation 
to the size of the genera to which the species belong. Again, 
plants low in the scale of 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. Tlie 
cause of lowly-organized plants ranging widely will be discussed 
in our chapter on Geographical Distribution. 

From looking at species as only strongly-marked and well- 
defined varieties, I was led to anticipate tliat the species of the 
larger genera in each country would oftener present varieties, 


than the species of the smaller g'enera ; for wherever many 
closely-related species (i. e., species of the same genus) have 
been fornietl, many varieties or incipient species ought, as a 
general rule, to be now forming. Where many large trees 
grow, we expect to find saplings. Where many species of a 
genus have been formed tkrough variation, circumstances have 
been favorable for variation ; and hence we might expect thai 
the circumstances would generally be still favorable to varia- 
tion. On the other hand, if we look at each species as a special 
act of creation, there is no apparent reason why more varieties 
should occur in a group having many species, than in one 
having few. 

To test the truth of this anticipation, I have arranged the 
j^lants 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 gCnera on 
the other sid(?, and it has invariably proved to be the case that 
a larger proportion of ±he 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 pre- 
sent any varieties, invariably present a larger average number 
of varieties than do the species of the small genera. Both these 
results follow when another diWsion is made, and when all the 
least genera, with from only one to four species, are absolutely 
excluded from the tables. These facts are of plain significa- 
tion 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 expres- 
sion, the manufactory of species has been active, we ought 
generally to find the manufactory still in action, more espe- 
cially as we have every reason to believe the process of manu- 
facluring new species to be a slow one. And this certainly 
is the case, if varieties be looked at as incipient species ; for 
my tables clearly show as a general rule that, wherever many 
speci(^s of a genus have been formed, the species of that genus 
present a nvunber of varieties, that is, of incipient species, be- 
yond the average. It is not that all large genera are now 
varying nuich, and are thus increasing in the lunnber of their 
species, or that no small genera are now varying and increas- 
ing; for if this had l)een so, it would have been fatal to my 
theory ; inasmuch as geology plainly tells us that small genera 
have in the lapse of time often increased greatly in size; and 
that large genera have often come to their maxima, declined, 


aiul disappeared. All that we -want to show is, that, where 
many species of a genus have been formed, on an average 
many arc still forming ; and this certainly holds good. 

Many of tJic Sjiccies included iclthln the Larger Genera re- 
semble Varieties in being very closely, but unequally, related 
to each other, and in having liestricted Manges. 

There are other relations between the species of large gen- 
era and their recorded varieties which deserve notice. We 
have seen that there is no infallible criterion by which to dis- 
tinguish species and well-marked varieties ; and in those cases 
in which intermediate links have not been found between 
doubtful forms, naturalists are compelled to come to a deter- 
mination by the amount of difference between them, judging by 
analogy whether or not the amoimt suffices to raise one or both 
to the rank of species. Hence the amount of difference is one 
very important criterion in settling whether two forms should be 
ranked as species or varieties. Now Fries has remarked in 
regard to plants, and Westwood in regard to insects, that in 
large genera the amount of difference between the sjiecies is 
often exceedingly small. I have endeavored to test this nu- 
merically 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, tlie 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 num- 
ber of varieties or incipient species greater than the average 
are now manufacturing, many of the species already manufac- 
tured 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 large genera are related to 
each other, in the same manner as the varieties of any one 
species are related to each other. No naturalist pretends that 
all the species of a genus arc equally distinct from each other ; 
they may generally be divided into sub-genera, or sections, or 
lesser groups. As Fries has well remarked, little groups of 
species are generally clustered like satellites around certain 
other species. And what are varieties but groups of forms, 
imequally related to each other, and clustered round certain 
forms — that is, round their parent-species ? Undoubtedly there 


is one most important point of dilFerencc between varieties and 
species ; namely, that the amount of diflerencc between varieties, 
M'hen compared with each other or with their parent-species, is 
much less than that betw'een the species of the same genus. But 
■when WG come to discuss the principle, as I call it, of Diver- 
gence of Character, we shall see how this may be explained, 
and how the lesser diiferences between varieties tend to in- 
crease into the greater differences between species. 

There is one other point which is worth notice. Varieties 
generally have much-restricted ranges : this statement is in- 
deed scarcely more than a truism, for, if a variety were found 
to have a wider range thaii that of its sujiposed parent-species, 
their denominations would be reversed. But there is also 
reason to believe that those species which are very closely 
allied to other species, and in so far resemble varieties, often 
have much-restricted ranges. For instance, Mr. II. C. Watson 
has marked for me, in the well-sifted London Catalogue of 
plants (fourth edition), G3 plants which are therein ranked as 
species, but which he considers as so closely allied to other 
species as to be of doubtful value : these 63 reputed species 
range on an average over G.9 of the provinces into which Mr. 
Watson has divided Great Britain. Now, in this same cata- 
logue, 53 acknowledged varieties are recorded, and these range 
over 7.7 jirovinces ; whereas, the species to Avhich these vari- 
eties belong range over 14.3 provinces. So that the acknowl- 
edged varieties have very nearly the same restricted average 
range as have those very closely allied forms, marked for me 
liy Mr. Watson as doubtful species, but which are almost uni- 
versally ranked by British botanists as good and true species. 


Finally, then, varieties caimot be distinguished from species 
— except, first, by the discovery of intermediate linking forms; 
and, secondly, by a certain indefinite amount of difference be- 
tween them ; for two forms, if differing very little, are gener- 
ally ranked as varieties, notwithstanding that they cannot be 
closely connected ; but the amount of difference considered 
necessary to give to any two forms the rank of species cannot 
be defined. In genera having more than the average number 
of species in any country, the species of these genera have 
more than the average number of varieties. In large genera 
the species are apt to be closely, but uncfiually, allied together. 

68 SUMMARY. Chap. II. 

forming little clusters round certain other species. Species 
very closoly allied to other species apparently have restricted 
rang'cs. In all these several respects the species of large genera 
present a strong analogy with varieties. And we can clearly 
understand these analogies, if species once existed as varieties, 
and thus originated ; whereas, these analogies are utterly in- 
explicable if species are independent creations. 

We have, also, seen that it is the most nourishing or dom- 
inant species of the larger genera within each class which on 
an average 3'ield the greatest number of varieties ; and varie- 
ties, as we shall hereafter see, tend to become converted into 
new and distinct species. Thus the larger genera tend to be- 
come larger ; and throughout Nature the forms of life which 
are now dominant tend to become still more dominant by leav- 
ing many modified and dominant descendants. But by steps 
hereafter to be explained, the larger genera also tend to break 
up into smaller genera. And thus, the forms of life through- 
out the universe become divided into groups subordhiate to 




Its bearing on Natural Selection— The Term used In a wide Sense— Geometrical Ratio 
oriucroase — Rapid Iiicreaao of Nuturalized Animals and Plants— Nature of tho 
Checks to Incroase— Competition universal— EftectR of Climate — Protection from 
the Number of Individuals — Complex Relations of all Animals and Plants throush- 
out Nature— St ru;;srlo for Life most severe between Individuals and Varieties of 
the same *Specics: often severe between Species of the same Genus — The Rela- 
tion of Organism to Or^^anism the most important of all Relations. 

Before entering on the subject of this chapter, I must 
make a few prehminary 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 immaterial for us 
whether a multitude of doubtful forms be called species or 
sub-species or varieties ; what rank, for instance, the two or 
three hundred doubtful forms of British plants are entitled to 
hold, if the existence of any well-marked varieties be admitted. 
But the mere existence of individual variability and of some 
few well-marked varieties, though necessary as the foundation 
for the work, helps us but little in understanding how species 
arise in Nature. How have all those exquisite adaptations of 
one part of the organization to another part, and to the condi- 
tions of life, and of one organic being to another being, been 
perfected ? We see these beautiful coadaptations most plain- 
ly in the woodpecker and the mistletoe ; and only a little less 
])lainly in the hinnblest parasite which clings to the hairs of a 
quadruped or feathers of a bird; in the structure of the beetle 
wliich dives through the water ; in the ])lumed seed which is 
wafted by the gentlest breeze ; in short, we see beautiful 
adaptations everywhere and in every part of the organic 

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 oilier far more than do the varieties of the same 
species? How do those groups of species, wliich 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. 0\\4ng to this struggle, 
variations, however slight, and from whatever cause proceed- 
ing, 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, wUl thus have a better 
chance of surviving, for, of the many individuals of an}^ species 
Avhich 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 expres- 
sion often used by Mr. Herbert Spencer of the Survival of the 
Fittest is more accurate, and is sometimes equally convenient. 
'SVe have seen that man by selection can certainly produce 
great results, and can adapt organic beings to his own uses, 
through the accumulation of slight but useful variations, given 
to him by the hand of Nature. But Natural Selection, as we 
shall hereafter see, is a power incessantly ready for action, and 
is as immeasurably superior to man's feeble efforts as the 
works of Nature are to those of Art. 

We will now discuss in a little more detail the struggle for 
existence. In my future work this subject will be treated, as 
it well deserves, at greater length. The elder De Candolle 
and Ijyell have largely and philosophically shown that all 
organic beings are exposed to severe competition. In regard 
to plants, no one has treated this subject Avith more spirit and 
ability than AV. Herbert, Dean of Manchester, evidently the 
result of his great horticultural knowledge. Nothing is easier 
than to admit in words the truth of the universal struggle for 
life, or more difficult — at least I have found it so — than con- 
stantly to bear this conclusion in mind. Yet unless it be 
thoroughly engrained in the mind, the whole economy of 
Nature, with every fact on distriljution, rarity, abundance, ex- 
tinction, and variation, will be dimly seen or quite misunder- 
stood. We behold the face of Nature bright Avith gladness, 
Ave often sec superabundance of food ; Ave do not see, or Ave 


forget, tliat the birds which arc idly singing round us mostly 
live on insects or seeds, and are thus constantly destroying 
life ; or we forget how largely these songsters, or their eggs, 
or their nestlings, are destroyed by birds and beasts of prey ; 
we do not always bear in mind, that, though food may be now 
superabundant, it is not so at all seasons of each recurring 

Tlie 2h'm, Stu-uggle for Existence^ used in a Large Sense. 

I should premise that I use this term in a large and meta- 
phorical sense, including dependence of one being on another, 
and including (which is more important) not only the life of 
the indi\-idual, but success in lea\'ing progeny. Two canine 
animals, in a time of dearth, may be truly said to struggle with 
each -other which shall get food and live. But a plant on the 
edge of a desert is said to struggle for life against the drought, 
though more properly it should be said to be dependent on the 
moisture. A plant which annually produces a thousand seeds, 
of which on an average only one comes to maturity, may be 
more truly said to struggle with the plants of the same and 
other kinds which already clothe the ground. The 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 will 
languish and die. But several seedling mistletoes, growing 
close together on the same branch, may more truly be said to 
struggle with each other. As the mistletoe is disseminated by 
birds, its existence depends on birds ; and it may metaphori- 
cally be said to struggle with other fruit-bearing plants, in 
order to tempt birds to devour and thus disseminate its seeds 
rather than those of other plants. In these several senses, 
which pass into each other, I use for convenience' sake the 
general term of struggle for existence. 

Geometrical Ratio of Increase. 

A struggle for existence inevitably follows from the high 
rate at which all organic beings tend to increase. Every being 
which during its natural lifetime produces several eggs or 
seeds, must suffer destruction during some period of its life, 
and during some season or occasional year, otherwise, on the 
principle of geometrical increase, its numbers would quickly 


become so inordinately great that no country could support the 
product. Hence, as more individuals are produced than can 
possibly survive, there must in every case be a struggle for 
existence, cither one individual with another of the same spe- 
cies, or -with the individuals of distinct species, or with the phys- 
ical conditions 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 for marriage. Although some 
species may be now increasing, more or less rapidly, in num- 
bers, all cannot do so, for the world would not hold them. 

There is no exception to the rule that every organic being 
naturally increases at so high a rate, that, if not destroyed, the 
earth would soon be covered by the progeny of a single pair. 
Even slow-breeding man has doubled in twenty-five years, and 
at this rate, in a few thousand years, there would literally not 
be standing-room for his progeny. Linnasus has calculated 
that if an annual plant produced only two seeds — and there is 
no plant nearly so unproductive as this — and their seedlings 
next year produced two, and so on, then in twenty years thei'C 
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 go'es 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 seven hundred 
and forty to seven hundi'ed and fifty years, there would be alive 
nearly nineteen million elephants descended from the first pair. 

But we have better evidence on this subject than mere 
theoretical calculations, namely, the numerous recorded cases 
of the astonishingly ra\nd increase of various animals in a state 
of nature, "when circumstances have been favorable to them 
during two or three following seasons. Still more striking is 
the evidence from our domestic animals of many kinds which 
liave 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 Avould have been incredible. So it is with 
plants : cases could be given of introduced plants which have 
b(>come common throughout whole islands in a period of less 
than ten years. Several of the plants, such as the caitloon, 
and a tall thistle, now most numerous over the wide plains of 


La Plata, clothing square leagues of surface almost to the ex- 
clusion of all other plants, have been introduced from Europe ; 
and there are plants which now range in India, as I hear from 
Dr. Falconer, from Cape Comorin to tlie Himalaya, which have 
been imported from xVmerica since its discovery. In such 
cases, and endless instances could be given, no one supposes 
that the fertility of these animals or phuits has been suddenly 
and temporarily increased in any sensil)le degree. The obvious 
explanation is that the conditions of lifeliave been very favor- 
able, and that there has consequently been less destruction of 
the old and 3'oung, and that nearly all the young have been 
enabled to breed. In such cases the geometrical ratio of in- 
crease, the result of which never fails to be surprising, simply 
explains the extraordinarily rapid increase and wide diffusion 
of naturalized productions in their new homes. 

In a state of nature almost every plant produces seed, and 
among animals there are very few which do not annually pair. 
Hence we may confidently assert, that all plants and animals 
are tending to increase at a geometrical ratio — that all would 
most rapidly stock every station in which they could anjdiow 
exist — and that the geometrical tendency to increase must be 
checked by destruction at some period of life. Our familiarity 
with the larger domestic animals tends, I think, to mislead us : 
Ave see no great destruction falling on them, and we forget that 
thousands are annually slaughtered for food, and that in a 
state of nature an equal number would have somehow to be 
disposed of. 

The only difference between organisms which annually pro- 
duce eggs or seeds by the thousand, and those which produce 
extremely few, is, that the slow-breeders would require a few 
more years to people, under favorable conditions, a Avliole dis- 
trict, let it be ever so large. The condor lays a couple of eggs 
and tlic 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 o<i;g, yet it is believed to be the most numerous bird in 
the world. One fly deposits hundreds of eggs, and another, 
like the hippolwsca, a single one; but this dillerence does not 
det(^nnine how many individuals of the two species can be sup- 
ported in a district. A large nmnber of eggs is of some im- 
j)ortance to those species which depend on a rapidly-fluctuating 
amount of food, for it allows them rapidly to increase in num- 
ber. 15ut the real importance of a large number of eggs or 
seeds is to make up for nuich 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 i)roduccd, and yet the average stock be 
fully kept up; ])ut if many eggs or young are dcstroj'ed, many 
must be produced, or the species will become extinct. It 
would suilicc to keep up the full number of a tree, which lived 
on an average for a thousand years, if a single seed were pro- 
duced once in a thousand years, supposing that this seed were 
never destroyed, and Could lie insured to germinate in a fitting 
place. So that, in all cases, the average number of any animal 
or plant depends only indirectly on the number of its eggs or 

In looking at Nature, it is most necessary to keep the fore- 
going considerations always in mind — never to forget that 
everj- single organic being around us may be said to be striving 
to the utmost to increase in numbers ; that each lives by a 
struggle at some period of its life ; that hea\'y destruction in- 
evitably falls either on the young or old, during each genera- 
tion or at recurrent intervals. Lighten any check, mitigate 
the destruction ever so little, and the number of the species 
will almost instantaneously increase to any amount. 

Nature of the Checks to Increase. 

The causes which check the natural tendency of each spe- 
cies to increase are most obscure. Look at the most vigorous 
species ; by as much as it swarms in numbers, by so much will 
its tendency to increase be still further increased. We know 
not exactly what the checks are in even one single instance. 
Nor will this surprise any one who reflects hov/ ignorant we 
are on this head, even in regard to mankind, so incomparably 
better known than any other animal. This subject has been 
alily treated by several authors, and I shall, in my future work, 
discuss some of the checks at considerable length, more es- 
pecially 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 genei-ally to sufler most, but this is not invariably the 
case. "With plants there is a vast destruction of seeds, but, 
from some observations which I have made, I believe that it 
is the seedlings which sufier most from germinating in ground 
iilrcvady thickly stocked with othcn- jilants. Seedlings, also, are 
destroyed in vast numbers by various enemies ; for instance. 


on a jiicco of ground three feet long and two wide, dug and 
cleared, and wliere there could be no choking from other plants, 
I marked all the seedlings of our native weeds as they came 
up, and out of the 357 no less than 295 were destroyed, chielly 
by slugs and insects. If turf which has long been mown, and 
the case would be the same with turf closely browsed by quad- 
rupeds, be let to grow, the more vigorous plants gradually 
kill the less vigorous though fully-grown plants ; thus out of 
twenty species growing on a little plot of turf (three feet by 
four) nine species perished from the other species being allowed 
to grow up freely. 

The amount of food for each species of course gives the 
extreme limit to which each can increase ; but very frequently 
it is not the obtaining food, but the serving as prey to other 
animals, which determines the average numbers of a species. 
Thus', there seems to be little doubt that the stock of par- 
tridges, grouse, and hares, on any large estate depends chiefly 
on the destruction of vermin. If not one head of game were 
shot during the next twenty years in England, and, at the 
same time, if no vermin were destroyed, tliere would, in all 
prnba])ility, be less game tlian at present, although hundreds 
of thousands of game animals are now annually killed. On the 
other hand, in some eases, as with the elephant, none are de- 
stroyed by beasts of prey; for even the tiger in India most 
larely dares to attack a young elephant protected by its dam. 

Climate plays an important part in determining the aver- 
ige numbers of a species, and periodical seasons of extreme 
cold or drought seem to be the most effective of all checks. I 
estimated (chiefly from the greatly reduced numbers of nests 
in the sjiring) that the winter of 1854-55 destroyed four-fifths 
of the birds in my own grounds ; and this is a tremendous 
destruction, when we remember that ten per cent, is an ex- 
traordinarily 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 cliiefly 
acts in reducing food, it brings on the most severe struggle 
between the individuals, whether of the same or of distinct 
species, which subsist on the same kind of food. Evon when 
climate, for instance extreme cold, acts directly, it will be the 
least vigorous, or thost; whicli have got least i'ood through the 
advancing winter, which will suffer most. "When we travel 
from south to north, or from a damp region to a dry, we in- 
variidily sec some species gradually getting rarer and rarer, 


and finally disappearing- ; and the change of climate being con- 
spicuous, we are tempted to attribute the whole effect to its 
direct action. But tliis is a false view : we forget that each 
species, even where it most abounds, is constantly suffering 
enormous destruction at some period of its life, from enemies 
or from competitors for the same place and food;. and if these 
enemies or competitors be in the least degree favored by any 
slight change of climate, they will increase in numbers, and, 
as each area is aheady fully stocked with inhabitants, the other 
species will decrease. "\\Tien we travel southward and see a 
species decreasing in numbei"s, Ave may feel sure that the cause 
lies quite as much in other species being favored, as in tliis 
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; hence, in 
going northward, or in ascending a mountain, we far oftener 
meet with stvuited forms, due to the directly injurious action 
of climate, than we do in proceeding southward or in descend- 
ing a mountain. When we reach the Arctic regions, or snow- 
capped summits,- or absolute deserts, the struggle for life is 
almost exclusively with the elements. 

That climate acts in main part indirectly by favoring other 
species, Ave may clearly see in the prodigious number of plants 
in our gardens which can perfectly Avell endure our chmate, 
l)ut which never become naturalized, for they cannot compete 
with om- native plants, nor resist destmction by our native 

When a species, owing to highly favorable circumstances, 
increases inordinately in numbers in a small tract, epidemics — 
at least this seems generally to occur with our game animals — 
often ensue : and here we have a limiting check independent 
of the struggle for life. But even some of these so-called 
epidemics appear to be due to parasitic worms, which have 
from some cause, possibly in jiart through facihty of diffusion 
among the crowded animals, l)ecn disproportionally favored ; 
and here comes in a sort of sti-uggle between the parasite and 
its prey. 

On the other hand, in many cases, a large stock of individ- 
uals of the same species, relatively to the numbers of its ene- 
mies, is absolutely necessary for its preservation. Thus we can 
easily raise plenty of corn and rape-seed, etc., in our fields, be- 
cause the seeds are in great excess compared with the mnnber 
of birds which feed on them ; nor can the birds, though liavinq 


a superabundance of food at tliis one season, increase in num- 
ber proportionally to the supply of seed, as their numbers are 
checked during -winter ; but any one who has tried, knows 
how troublesome it is to get seed from a few wheat or other 
such plants in a garden : I have in this case lost every single 
seed. This view of the necessity of a large stock of the same 
species for its preservation, explains, I believe, some singular 
facts in Nature, such as that of very rare plants being some- 
times extremely abundant in the few spots where they do 
occur; and that of some social plants being social, that is, 
abounding in individuals, even on the extreme confines of their 
range. For in such cases, we may believe, that a plant could 
exist only where the conditions of its life were so favorable 
that many could exist together, and thus save the species from 
utter destruction. I should add that the good effects of fre- 
quent intercrossing, and the ill eff'ects of close interbreeding, 
probably come into play in some of these cases ; but on tliis 
intricate subject I will not here enlarge. 

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

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, has 
interested me. In Staffordshire, on the estate of a relation, 
where I had ample means of investigation, there was a large 
and extremely barren heath, which had never been touched by 
the hand of man ; but several hundred acres of exactly the 
same nature had been enclosed twenty-five years previously 
and planted with Scotch fir. The change in the native vegeta- 
tion of the ])lanted part of the heath was most remarkable, 
more than is generally seen in passing from one quite diiTerent 
soil to another; not only the proportional numbers of the heath- 
plants were wholly changed, but twelve species of j)lants (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 conmion 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 cff"cct of the introduction of a single tree, nothing 


wliatever Cise liavinji^ been clone, "vvith the cxce}7tion of the 
huid having 1)ecn enclosed, so that cattle could not enter. But 
how important an clement enclosure is, I plainly saw near Farn- 
ham, in Surrc}'. Plcrc there are extensive heaths, with a few 
clumps of old Scotch firs, on the distant hill-tops : within the 
last ten j'ears larg'e spaces have been enclosed, and self-sown 
firs are now sprini!:iii2^ up in multitudes, so close together that 
all cannot live. When I ascertained that these young trees 
had not been sown or planted, I was so much surprised at their 
numbcre that I went to several points of view, whence I could 
examine hundreds of acres of the unenclosed heath, and liter- 
ally I could not see a single Scotch fir, except the old planted 
clumps. But on looking closely between the st^ms of the 
heath, I found a multitude of seedlings and little trees, which 
had been perpetually browsed down by the cattle. In one 
square yfu'd, at a point some himdred 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 laud was enclosed, 
it became thickly clothed with vigorously-growing young firs. 
Yet the heath was so extremely barren and so extensive that 
no one would ever have imagined that cattle would have so 
closely and effectually searched it for food. 

Here we see that cattle absolutely determine the existence 
of the Scotch fir ; but in several parts of the world insects 
determine the existence of cattle. Perhaps Paraguay offers 
the most curious instance of this ; for here neither cattle nor 
horses nor dogs have ever run wild, though they swarm south- 
ward and northwai'd in a feral state ; and ^Vzara and Pengger 
have shown that this is caused by the greater number in Para- 
guay of a certain fly, which lays its eggs in the navels of these 
animals when first born. The increase of these flics, numerous 
as they are, must be habitually checked by some mean?, proba- 
bly l)y other parasitic insects. Hence, if certain insectivorous 
birds were to decrease in Paraguay, the parasitic insects would 
probably increase ; and this Avould lesson the num])er of the 
navel-frequenting flies — then cattle and horses would become 
f(>ral, and this would certainly greatly alter (as indeed I have 
obseiTod in parts of South America) the vegetation : this again 
would largely aflect the insects; and this, as Ave have just seen 
in Staffordshire, the insectivorous birds, and so onward in 
ever-increasing circles of complexity. We began this scries 


by insectivorous birds, and -we have ended with tlieni. Not 
tliat in nature the rehitions can ever be as simple as this. Bat- 
tle within battle must ever be recurring' with varying' success ; 
and yet in the long-run the forces are so nicely balanced, that 
the lace of Nature remains uniform for long periods of time, 
tliough assuredly the merest trifle would often give the victory 
to one organic being over another. Nevertheless, so profound 
is our ignorance, and so high our presumption, that we marvel 
when we hear of the extinction of an organic being ; and, as 
we do not see the cause, we invoke cataclysms to desolate the 
world, or invent laws on the duration of the forms of life ! 

I am tempted to give one more instance showing how 
plants and animals, most remote in the scale of Nature, are 
bound together by a web of complex relations. I shall here- 
after have occasion to show that the exotic Lobelia fulgens, in 
this part of England, is never visited by insects, and conse- 
fiucntly, from its peculiar structure, never sets a seed. Near- 
ly 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 indispen- 
sable to the fertilization of the heart's-ease (Viola 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, 20 heads of Dutch clover (Trifolium 
repens) yielded 2,290 seeds, but 20 other heads protected from 
bees produced not one. Again, 100 heads of red clover (T. 
pratense) produced 2,700 seeds, but the same number of pro- 
tected heads produced not a single seed. Humble-bees alone 
visit red clover, as other bees cannot reach the nectar. It has 
been suggested that moths may 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 humljle-bces became extinct or very rare in 
England, ihe heart's-ease and red clover would become very 
rare, or wholly disappear. The number of humble-bees in any 
district depends in a great degree on the number of field-mice, 
which destroy their combs and nests ; and Colonel Newman, 
who has long attended lo 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 hirgely depend- 
ent, as every one knows, on tlic niunber of cats : and Colonel 
Newman says, " Near villages and small towns I have found 


the nests of humble-bees more numerous than elsewhere, Mhich 
I attribute to the number of cats that destroy the mice." Hence 
it is quite credible that the presence of a fefine animal in large 
numbers in a district might determine, through the interven- 
tion first of mice and then of bees, the frequency of certain 
flowers in that district ! 

In the case of every species, many different checks, acting 
at diflerent 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 concur in determining 
the average number or even the existence of the species. In 
some cases it can be shown that widely-different checks act on 
the same species in different districts. When we look at the 
jilants and bushes clothing an entangled bank, Ave are tempted 
to attribute their proportional numbers and kinds to what we 
call chance. But how false a view is this ! Every one has 
heard that when an American forest is cut down, a very differ- 
ent vegetation springs up ; but it has been observed that an- 
cient Indian ruins in the Southern United States, which must 
formerly haA'e been cleared of trees, now display the same 
beavitiful diversity and proportion of kinds as in the surround- 
ing virgin forest. What a struggle must have gone on during 
long centuries betAveen the scA'cral kinds of trees, each annual- 
ly scattering its seeds by the thousand ; Avhat Avar betAvcen 
insect and insect — between insects, snails, and other animals, 
Avith birds and beasts of pre}^ — all striving to increase, ail 
fcx'ding on each other, or on the trees, their seeds and seed- 
lings, or on the other plants Avhich first clothed the groimd 
and thus checked the groAvth of the trees ! ThroAV vip a handful 
of feathers, and all must fall to the ground according to defi- 
nite laAvs ; but Iioav simple is the jiroblem Avhere each shall fall 
compared to that of the action and reaction of the innumerable 
plants and animals which haA'C determined, in the course of 
centuries, the proportional numbers and kinds of trees noAV 
ffroAving on the old Indian ruins ! 

The dependency of one organic being on another, as of a 
parasite on its prey, lies generally betAveen beings remote in 
the scale of nature. This is likcAvise sometimes the case Avith 
those Avhich may strictly be said to struggle Avith each other 
for existence, as in the case of locusts and grass-feeding quad- 
rupeds. But the struggle Avill almost invariably be most 
severe betAveen the individuals of the same species, for they 
frequent the same districts, require the same food, and are 


exposed to the same dangers. lu the case of vai-ietics 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 Avhich best 
suit the soil or climate, or are naturally the most fertile, Avill 
beat the others and so yield more seed, and will consequently 
in a few years quite su^iplant the other varieties. To keep vip 
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, other- 
wise the weaker kinds Avill steadily decrease in number and 
disappear. So, again, with the varieties of sheep : it has licen 
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 varie- 
ties 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 could be kept up for half 
a dozen generations, if they were allowed to struggle together, 
like beings in a stUte of nature, and if the seed or young were 
not annually sorted. 

' Struggle for Life most severe between Individuals and Vari- 
eties of the same Sjyecles. 

As species of the same genus have usually, though by no 
means invariably, much similarity in habits and constitution, 
and always in structure, the struggle will generally be more 
severe between species of the same genus, when they come 
into competition Avith each other, than between species of dis- 
tinct genera. We see this in the recent extension over parts 
of the United States of one species of swallow having caused 
the decrease of another species. The recent increase of the 
missel-thrush in parts of Scotland has caused the decrease of 
the song-thrush. How frequently we hear of one. species of 
rat taking the place of another species under the most differ- 
ent climates ! In Russia the small Asiatic cockroach has every- 
where driven before it its great congener. In Australia the 
imported hivG^bce is rapidly exterminating the small, .stingless 
native bee. One species of charlock has been known to sup- 
plant another species ; and so in other cases. We can dimly 


see why the competition should be most severe between allied 
forms, which fill nearly the same place in the economy of Na- 
ture ; but probably in no one case could "vve precisely say why 
one species has been victorious over another in the great bat- 
tle of life. 

A corollary of the highest importance may be deduced 
from the foregoing remarks, namely, that the structure of 
every organic being is related, in the most essential yet often 
hidden manner, to that of all 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 dandehon, and in the flattened and fringed legs of the 
water-beetle, the relation seems at first confined to the ele- 
ments of air and Avater. Yet the advantage of plumed seeds 
no doubt stands in the closest relation to the land being 
already thickly clothed Avith other plants ; so that the seeds 
may be widely distributed and fall on iinoccupied ground. In 
the water-beetle, the structm-e of its legs, so well adapted for 
diving, allows it to compete with other a'quatic insects, to 
hunt for its own prey, and to escape ser\dng as prey to other 

The store of nutriment laid up witliin the seeds of many 
plants seems at first sight to have no sort of relation to other 
jilants. 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 
luitriment in the seed is to favor the growth of the young seed- 
ling, 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 qviadruplc its nimibers ? We know that it can per- 
fectly 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 sec that 
if we wished in imagination to give the plant the power of in- 
creasing in number, we should have to give it some advantage 
over its competitors, or over the aTiimals which jireyed on it. 
( )n the confines of its geographical range, a change of consti- 
tution Avith respect to climate would clearly be an advan- 
tage to our plant ; but we have reason to believe that only a 


few plants or aniniiils rani]fc so far, that tbcy are destroyed by 
the rigor of the climate alone. Not until we reach the extreme 
confines of life, in the Arctic regions or on the borders of an 
utter desert, will competition cease. The land may be ex- 
tremely cold or dry, yet there will be competition between 
some few species, or between the individuals of the same spe- 
cies, for the Avarmest or dampest spots. 

Hence, also, we can see that when a plant or animal is 
placed in a new country among new competitors, tliough the 
climate may be exactly the same as in its former home, yet 
the conditions of its life will generally be changed in an essen- 
tial manner. If we wished to increase its average numbers in 
its new home, we should have to modify it in a diflerent way 
to what we should have to do in its native country; for we 
should have to give it some advantage over a different set of 
convpetitors or enemies. 

It is good thus to try in imagination to give any form 
some advantage over another. Probably in no single instance 
should we know what to do, so as to succeed. It will con- 
A-ince us of our ignorance on the mutual relations of all organic 
beings ; a conviction as necessar}', as it seems difficult to ac- 
quire. All that we can do is, to keep steadily in mind that 
each organic being is striving to increase in a geometrical 
ratio ; that each at some period of its life, during some season 
of the year, during 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 jirompt, and that the vigorous, the 
healthy, and the haj^py, survive and multiply. 




Natural Selection— its Power compared with jMan's Selection— its Power on Char- 
acters of trifling Importance— its Power at all A<;e8 and on both Soxes— Sexual 
Selection— On the CJenenility of Intercrosses between Individuals of the same 
Species — Circumstances favorable and unfavorable to the Results of Natural Se- 
lection, namely. Intercrossing:, Isolation, Nnnibor 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 Nat- 
ural Selection, through Divergence of Character and Extinction, on the Descend- 
ants from a C(<mmou Parent— Explains the Grouping of all Organic Heings— Ad- 
vance in Organization — Low Forms preserved — Objections considered— Uniform- 
ity of certain Characters due to their Unimportance and to their not having been 
acted on by Natural Selection— Indefinite Multiplication of Species — Summary. 

How will the struggle for existence, briefly discussed in 
tlie last chapter, act in regard to variation ? Can the principle 
of selection, which we have seen is so potent in the hands of 
man, apply in Nature ? I think we shall see that it can act 
most effectually. Let the endless number of peculiar varia- 
tions in our domestic productions, and, in a lesser degree, in 
those under Nature, be borne in ^ind ; as well as the strength 
of the hereditary tendency. Under domestication, it may be 
tridy said that the Avliole organization becomes in some degree 
plastic. But the variability, which we almost universally meet 
with in our domestic productions, is not directly produced, as 
Hooker and Asa Gray have well remarked, by man ; he can 
neither originate varieties, nor prevent their occurrence ; he 
can onlv preserve and acciunulate such as do occur; uninten- 
tionally he exjioses organic beings to new and changing con- 
ditions of life, and variability ensues; but similar changes of 
conditions might and do occur under Nature. Let it als«> be 
borne in mind how inlinitely complex and close-fitting are the 
mutual relations of all organic beings to each other and to 
tlieir physical conditions of life ; and consequently what in- 
finitely-varied diversities of structure may be of use to each 
l)cing under changing conditions of life. Can it, then, be 
thought improbable, seeing that variations usefid to man have 
undoubtedly occurred, that other variations useful in some way 


to each beinp^ in the frrcat and complex battle of life, sliouUl 
sometimes occur in the course of thousands of generations? 
If such do occur, can we doubt (remembering' that many more 
individuals are born than can possibly survive) that individuals 
having any advantage, however slight, over others, would have 
the best chance of surviving and of procreating their kind ? 
On the other hand, we may feel sure that any variation in the 
least degree injurious would be rigidly destroyed. This pres- 
ervation of favorable variations, and the destruction of injuri- 
ous variations, I call Natural Selection, or the Survival of the 
Fittest. Variations neither useful nor injurious would not be 
affected by natural selection, and would be left either a fluc- 
tuating 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 
tenn Natural Selection. Some have even imagined that nat- 
ural selection induces variability, whereas it imphes only the 
preservation of such variations as occur and are beneficial to 
the being under its conditions of life. No one objects to agri- 
culturists speaking of the potent effects of man's selection ; and 
in this case the individual differences given by Nature, which 
man for some object selects, must of necessity first occur. 
Others have objected that the term selection implies conscious 
choice in the animals Avhich become modified; and it has even 
been urged that, as plants have no volition, natural selection is 
not applicable to them ! In the literal sense of the word, no 
doubt, natural selection is a false term ; but who ever objected 
to chemists speaking of the elective affinities of the various 
elements ? — and yet an acid cannot strictl}^ 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 ex- 
])ressions ; and they are almost necessary for brevity. So, 
again, it is difficult to avoid personifying the word Nature ; but 
I mean by Nature, only the aggregate action and product of 
many natural laws, and by laws the sequence of events as as- 
certained by us. With a little familiarity such superficial ob- 
jections Avill be forgotten. 

We shall best understand the probable course of natural 
selection by taking the case of a country undergoing some 


slii^ht iihysical chanire, for instance, of climate. The propor- 
tional numbers of its inhabitants -would almost immediately 
undergo a change, and some species might become extinct. 
AVe may conclude, from what avc have seen of the intimate 
and complex manner in which the inhabitants of each country 
arc boinid together, that any change in the numerical jiropor- 
tions of some of the inhabitants, independently of the change 
of climate itself, would seriously affect the others. If the 
country were open on its borders, new fonns would certainly 
innnigrate, and this also would often seriously disturb the re- 
lations of some of the former inhabitants. Let it be remem- 
bered how powerful the influence of a single introduced tree 
or mammal has been sliOAvn 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 Natvire Avhich would 
assuredly be better fdled up, if some of the original inhabitants 
were in some manner modified ; for, had the area been ojien to 
immigration, these same places would have been seized by in- 
truders. In such cases, slight modifications, which in any way 
favored the individuals of any species, by better adajiting them 
to their altered conditions, would tend to be prcsen-ed ; and 
natural selection would have free scope for the Avork of im- 

We have reason to believe, as stated in the first chapter, 
that changes in the conditions of life cause or excite a ten- 
dency to vary ; and in the foregoing case the conditions are 
supposed to have changed, and this would manifestly be favor- 
able to natural selection, by giving a better chance of profit- 
able variations occurring; and unless such do occur, natural 
selection can do nothing. Under the term of "variations," it 
must never be forgotten that mere individual diflerences are 
always included. As man can certainly ]noducc a great result 
with his domestic animals and plants by adding up in any 
given direction individual differences, so could natural selection, 
but far more easily, from having incomparably longer time for 
action. Nor do I believe that any great physical change, as 
of climate, or any imusual degree of isolation to check immi- 
gration, is actually necessary to ])roduce new and unoccujiied 
places for natural selection to fill up by modifying and im])rf)v- 
ing some of the A-arying inhabitants. For as all the inhabit- 
ants of each country are struggling together Avith nic(^ly-bal- 
anccd forces, extremely-slight modifications in the structure oi 


habits of one si)ecics would often give it an advantugc over 
others ; and still further modifications of the same liind Avould 
often still further increase the advantage, as long as the species 
continued under the same conditions of life and profited by 
similar means of sul)sistence and defence. No country can be 
named, in which all the native inhabitants are now so perfectly 
adapted to each other and to the physical conditions under 
which they live, that none of them could be still better adapted 
or improved ; for, in all countries, the natives have been so far 
conquered by naturalized productions, tliat they have allowed 
foreigners to take iirin ])ossession of the land. And, as for- 
eigners have thus in every country beaten some of the natives, 
Ave may safely conclude that the natives might have been mod- 
ified with advantage, so as to have better resisted the in- 

As man can produce and certainly has produced a great 
result by his methodical and unconscious means of selection, 
what may not natural selection effect ? Man can act only on 
external and visible characters : Nature, if I may be allowed 
to j)ersonify the natural preservation or survival of the fittest, 
cares nothing for appearances, except in so far as they are use- 
ful 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 peculiac 
and fitting manner ; he feeds a long and a short beaked pigeon 
on the same food ; he does not exercise a long-backed or long- 
legged quadrup(^(l in any peculiar manner ; he exposes sheep 
with long and short wool to the same climate. lie 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 produc- 
tions. He often begins his selection by some half-monstrous 
form; or at least by some modification prominent enough to 
catch the eye or to be plainly useful to him. Under Nature, 
the slightest differences of structure or constitution may well 
turn the nicely-balanceil scale in the struggle for life, and so be 
preserved. How fleet uig are the wishes and eflbrts of man! 
how short his time ! and consequently how poor will be his 
results, comjiared with those accumulated l)y Nature during 


whole geological periods ! Can we wonder, then, that Nature's 
productions should be far " truer " in character than man's pro- 
ductions ; tliat they should be infinitely better adapted to the 
most complex conditions of life, and should plainly bear the 
stamp of far higher workmanship? 

It may metaphorically l)e said that natural selection ig daily 
and hourly scrutinizing, throughout the world, the slightest vari- 
ations ; rejecting those that are bad, preserving and adding up 
all that are good ; silently and insensibly working, whenever 
and wherever opportunity oft'ers, 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 tlie lapse of ages, and then 
so imperfect is our view into long-past geological ages, that 
we see only that the forms of life are now different from what 
they formerly were. 

In order that any great amount of modification in any part 
should be eflectcd, a variety when once formed must again, 
perliaps after a long interval of time, vary or present individ- 
ual difl"erences of the same favorable nature, and these must be 
again preserved, and so onward step by step. Seeing that 
individual differences of all kinds perpetually recur, this can 
hardly be considered as an unwarrantable assumption. But 
whether all this has actually taken place must be judged by 
how far the h}q5othesis accords with and explains tlie general 
phenomena of Nature. On the other liand, the ordinary belief 
that the amount of possible variation is a strictly-limited quan- 
tity is 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, avc must believe that these 
tints are of service to these birds and insects in preserving them 
from danger. Grouse, if not destroyed at some period of their 
lives, would increase in countless numbers ; they are known to 
suffer largely from bii'ds of prey ; and hawks are guided by eye- 
sight to their prey — so much so, that on parts of the Continent 
persons are warned not to keep white pigeons, as being the 
most liable to destruction. Hence natural selection miglit be 
most eflective in giving the proper color to each kind of grouse, 
and in keeping that color, when once acquired, true and con 


stant. Nor ougbt we to think that the occasional destruction 
of an animal of any particular color would produce little effect : 
we should remember how essential it is in a flock of white 
slicep to destroy every lamb with the faintest trace of black. 
A\'e have seen how the color of the ho<^s, when fcedino: on the 
" paint-root " in Florida, 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, Down- 
ing, that in the United States smooth-skinned fruits sullcr far 
more from a beetle, a curculio, than those Avith down ; that pur- 
ple plums suffer far more from a certain disease than 3X'llow 
plums ; whereas another disease attacks yellow-fleshed peaches 
far more than those with other colored flesh. If, with all the 
aids of art, these slight differences make a great difl'crence 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 
whicli variety, whether a smooth or downy, a yellow or purple 
fleshed fruit, should succeed. 

In looking at many small points of difference between spe- 
cies, which, as far as our ignorance permits us to judge, seem 
quite unimportant, we must not forget that climate, food, etc., 
may have produced some direct eflect. It is also necessary to 
bear in mind that, owing to the law of correlation, Avhen one 
part varies, and the A\ariations are accumulated through natural 
selection, other modifications, often of the most unexpected 
nature, avUI ensue. 

As we see that those variations Avhicli under domestication 
appear at any particular })eriod of life, tend to rcapjwar 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 tlieir chickens; in the horns of our 
sheep and cattle when nearly adult — so, in a state of nature, 
natural selection Avill be enabled to act on and modify organic 
beings at any age, by the accumulation of variations profitable 
j-t that age, and by their inheritance at a corresponding age. 
If it profit a plant to have its seeds more and more Avidely dis- 
seminated by the wind, I can sec no greater difliculty in this 
being effected tlirough natural selection than in the cotton- 
planter increasing and improving by selection the down in the 


pods on his cotton-trces. Natural selection may modify and 
adapt the larva of an insect to a score of continovncics, wholly 
tlillcrent from tliose which concern the mature insect; and 
these modilicalions may affect, throug'h correlation, the stnio 
ture of the adult. So, conversely, modifications in the adult 
may affect tlie 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 in 
relation to the parent, and of the parent in relation to the 
young. In social animals it will adapt the structure of each 
indivitlual for the benefit of the Avhole community ; if this in 
consequence profits by the selected change. What natural 
selection cannot do, is to modify the structure of one species, 
without giving it any advantage, for the good of another spe- 
cies ; and, though statements to this effect may be found in 
works of natural history, I cannot find one case which will 
bear investigation. A structure used only once in an animal's 
life, if of high importance to it, might be modified to any ex- 
tent by natural selection ; for instance, the great jaws pos- 
sessed by certain insects, used exclusively for opening the 
cocoon — or the hard tip to the beak of nestling birds, used for 
breaking th(^ eir'^. It has been asserted that, of the best short- 
lieaked tumbler-pigeons, a greater number 2:)erish in the egg 
than are al^le 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- 
j^rown pigeon very short for the bird's own advantage, the pro- 
cess of modification Avould be very slow, and there would be 
simultaneously the most rigorous selection of all the young 
birds within the egg, which had the most powerful and hardest 
beaks, for all with weak beaks would inevitably perish ; or, 
more delicate and more easily-l)rokcn shells might be se- 
lected, the thickness of the shell being knoAvn to vary like 
every other structure. 

Sexual Selection. 

Inasmuch as peculiarities often appear under domestication 
in one sex and become hereditarily attached to that sex, the 
same fact no doubt occurs under Nature, and if so, natural se- 
lection will be able to modify one sex in its functional n'lations 
to the otlier sex, or in relation to Avholly-ditrerent liabits of life 
in the two sexes, as is sometimes the case with insects. And 


this leads ine to saj a few words on what I call Sexual Selec- 
tion. This depends, not on a sti'ugf^le for existence, l)ut on a 
struo^g'lc between the males for possession of tlie females; the 
result is not death to the inisuccessful competitor, but few or 
HO oifsprino-. Sexual selection is, therefore, less rit^orous than 
natural selection. Generally, the most vigorous males, those 
\vhich are best fitted for their places in Nature, will leave most 
progeny. But, in many cases, victory depends not on general 
vigor, but on having special weapons, confined to the male 
sex. A hornless stag or spurless cock would have a poor 
chance of leaving numerous ollspring. Sexual selection, by 
always allowing the victor to breed, might surely give indom- 
itable courage, length to the spur, and strength to the wing 
to strike in the spurred leg, as in the case of the brutal cock- 
fighter, who knows well how to improve his breed hy the care- 
ful selection of the best cocks. How low in the scale of Nature 
the law of battle descends, I know not ; male alligators have 
been described as fighting, bellowing, and Avhirling round, like 
Indians in a war-dance, for the possession of the females ; male 
salmons have been seen fighting all day long ; male stag-])eetles 
sometimes bear wounds from the huge mandibles of other 
mules ; tlie males of certain hymcnopterous insects have been 
fivcjuently seen by that inimitable observer M. Fabre, fighting 
for a particular female, Avho sits by, an apparently imcon- 
cerned beholder of the struggle, and then retires with the con- 
queror. The war is, perhaps, severest between the males of 
l)olygamous animals, and these seem oftenest provided with 
special weapons. The males of carnivorous animals arc already 
well armed ; though to them and to others, special means of de- 
fence may be given through means of sexual selection, as the 
mane to 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 char- 
acter. All those who have attended to the subject, believe 
that there is the severest rivalry between the males of many 
species to attract l)y singing the females. The rock-thrush of 
Guiana, birds of Paradise, and some others, congregate; and 
successive males display tlieir gorgeous plumage and perform 
strange antics l)efore the females, whieli, standing bv as spec- 
tators, at last choose the most attractive partner. 1'hose who 
have closely attended to birds in confinement well know that 
they often take individual preferences and dislikes: thus Sir 


Iv. Iloroii has described how one pied peacock was eminently 
attractive to all his hen-birds. I cannot here enter on the ne- 
cessary details ; but if man can in a short time jjive elegant 
carriage and beauty to his bantams according to liis standard 
of beauty, I can see no good reason to doubt that female 
birds, by selecting, during thousands of generations, the most 
melodious or beautiful males, according to their standard of 
beauty, might jiroduce a marked effect. Some well-known 
laws, with respect to the plumage of male and female birds, in 
comparison with the plumage of the young, can be explained 
through the action of sexual selection on variations occurring 
at diiierent ages, and being transmitted to the males alone or 
to both sexes at a corresponding age ; 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 diller in 
structure, color, or ornament, such differences have been main- 
ly caused by sexual selection ; that is, by individual males hav- 
ing had, in successive generations, some slight advantage over 
other males, in their weapons, means of defence, or charms ; 
and having transmitted these advantages to their male off- 
spring. Yet, I would not Avish to attribute all such sexual dif- 
ferences to this agency : for we see peculiarities arising and 
becoming attached to the male sex in our domestic animals (as 
the greater development of the Avattle in male carrier-pigeons, 
horn-like protuberances in certain fowls, etc.), Avhich are in no 
way useful. We see analogous cases under Nature — for in- 
stance, the tuft of hair on the breast of the turkej'-cock, Avliich 
cannot be useful, and can hardly be ornamental ; indeed, had 
the tuft appeared under domestication, it would have been 
called a monstrosity. 

Illustratio7is of the Action of Natural Selection, or the Sur- 
vival of the Fittest. 

In order to make it clear how, as I believe, natural selec- 
tion acts, I must beg permission to give one or two imaginary 
illustrations. Let us take the case of a wolf, which preys on 
various animals, securing some by craft, some by strength, and 
some by (leetness ; and let us suppose that tlie fleetest prey, 
a deer for instance, had from any change in the country in- 
creased in numbers, or that other prey had decreased in num- 
bers, during that season of the year when the wolf was hardest 


pressed for food. Under such cLrcumstances the swiftest and 
slimmest wolves would have the best chance of survdving-, and 
so be preserved or selected — provided always that they retained 
strength to master their prey at this or at some other period 
of the year, when they might bo compelled to prey on other 
animals. I can see no more reason to doubt tliis, than that 
man can improve the fleetness of his greyhounds by careful and 
methodical selection, or by unconscious selection which results 
from each man trying to keep the best dogs without any 
thought of modifying the breed. I may add, that, according 
to Mr. Pierce, there are two varieties of the wolf inhabiting 
the Catskill Mountains in the United States, one with a light, 
greyhound-like form, which pursues deer, and the other more 
bulky, with shorter legs, which more frequently attacks the 
shepherd's flocks. 

It should be observed that, in the above illustration, I speak 
of the slimmest individual wolves, and not of any single strong- 
Ij'-marked variation having been preserved. In former editions 
of this work I sometimes spoke as if this latter alternative had 
frequently occurred. I saw the great importance of individual 
differences, and this led me fully to discuss the results of un- 
conscious selection by man, which depends on the preservation 
of the better-adapted or more 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 preserved, 
it would generally be lost by subsequent intercrossing with 
ordinary individuals. Nevertheless, until reading an able and 
valuable article in the N'orth Uritish lievieio (18G7), I did not 
appreciate how rarely single variations, whether slight or 
strongly-marked, could be perpetuated. The author takes the 
case of a pair of animals, whicli produce during their lifetime 
two hundred offspring, of which, from various causes of de- 
struction, only two on an average survive to procreate their 
kind. Tliis is rather an extreme estimate for most of the 
higher animals, but by no means so for many of the lower 
organisms. He then shows that if a single individual were 
born, whicli varied in souk* maimer, giving it twice as g'ood a 
chance of life as that of the other individuals, yet the chances 
would be strongly against its survival. Supposing it to sur- 
vive and to breed, and that half its young inherited the favor- 
able variation ; still, as the Reviewer goes on to show, the 
young would liavo only a slightly-better chance of survaving 


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 
couhl procure its food more easily by having its beak curved, 
and if one Avere born with its beak strongly carved, and 
wliich consequently flourished, nevertheless there would be 
a very poor chance of this one individual perpetuating its kind 
to the exclusion of the common form ; but there can hardly be 
a doubt, judging by what we see taking place imder domesti- 
cation, that this result Avould follow from the preservation dur- 
ing many generations of a large number of indiWduals with 
more or less curved beaks, and from the destruction of a still 
larger numlier Avith the straightest beaks. 

It should not, however, be overlooked that certain varia- 
tions, which no one would rank as mere individual differences, 
frequently recur, owing to a similar organization being simi- 
larly acted on — of which fact numerous instances could be 
given with our domestic productions. In such cases, if a vary- 
ing individual did not actually transmit to its offspring its 
newly-acquired character, it would imdoubtedly transmit, as 
long as the existing conditions remained the same, a still 
stronger tendency to vary in the same manner. The condi- 
tions might indeed act in so energetic and definite a manner as 
to lead to the same modification in all the individuals of the 
species without the aid of selection. But we may suppose 
that the conditions sufficed to effect only a third, or fourth, or 
tenth part of the individuals ; and several such cases could be 
given ; for instance, it has been estimated by Graba that in the 
Faroe Islands about one-fiftli of the guillemots, Avhich all breed 
together, consist of a well-marked variety ; and this Avas for- 
meily ranked as a distinct species under the name of Uria 
lacrymans. Now, in such cases, if the variation were of a ben- 
eficial nature, the original form would soon be sup})lantcd by 
the modified form, through the survival of the fittest. 

With reference to the eftects of intercrossing and of com- 
petition, it should be borne in mind that most animals antl plants 
keep to their proper homes, and do not needlessly Avander 
aljout ; Ave see this CA'cn Avith migratory birds, Avhich almost 
always return to the same district. Consequently each newly- 
formed variety Avould generally be at lirst local, as seems to 
1)0 the common rule Avith A'arieties in a state of nature ; so that 
siinilarlA'-modified individuals Avould soon exist in a small l)ody 
together, and would often breed together. If the v.vw variety 


was successful in its battle for life, it would slowly spread from 
a central spot, competinp^ Avitli and conf[U(>rinof the uncluuifred 
individuals on the niar<>-ins of an ever-increasing circle. But 
to the subject of intercrossing we shall have to return. It may 
be objected by those who have not attended to natural history, 
that tlie long-continued accumulation of individual differences 
could not give rise to parts or organs which seem to us, and 
arc often called, new. But, as we shall hereafter find, it is dif- 
ficult to advance any good instance of a really new organ ; 
even so complex and perfect an organ as the eye can be showa t(< 
graduate downward into mere tissue sensitive to diffused light 
It may be worth Avhile to give another and more complex 
illustration of tlie action of natural selection. Certain plants 
excrete sweet juice, apparently for the sake of eliminating 
something injurious from their sap: this is effected, for in- 
stance, by glands at the base of the stipules in some Legunii- 
nosa^, and at the backs of the leaves of the common laurel. 
This juice, though small in quantity, is greedily sought by in- 
sects ; but their visits do not in any way benefit the plant. 
Now, let us suppose that the juice or nectar was excreted 
from the inside of the flowers of a certain number of plants of 
anv species. Insects in seeking the nectar would get dusted 
with pollen, and would certainly 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 cross- 
ing, as Ave have good reason to believe, would produce vigor- 
ous seedlings, which consequently would have the best chance 
of flourishing and surviving. The plants which produced flow- 
ers with the largest glands or nectaries, excreting most nectar, 
would oftencst be visited by insects, and would oftenest be 
crossed ; and so in the long-run would gain the upper hand and 
form a local variet}'. The flowers, also, wliich had their sta- 
mens and pistils placed, in relation to the size and habits of the 
particular insect which visited them, so as to favor in any 
dcgr(>e the transportal of the pollen, would likewise be favored. 
AVe might have taken the case of insects visiting flowers for 
Ihc sake of collecting pollen instead of nectar; and as pollen is 
formed for the sole purpose of fertilization, its destruction ap- 
pears to be a simple loss to the plant ; 3-et if a little pollen 
"were canied, at first occasionally and then habitually, by the 
pollen-devouring insects from flower to flower, and a cross thus 
ellected, although nine-tenths of the pollen were destroyed, it 
might still be a great gain to the plant; 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, unin- 
tentionally on their part, regularly carry pollen from flower to 
flower; and that they do this cfl'ectually, I could easily show 
l)y many striking facts. I will give only one, as likewise illus- 
trating one step in the separation of the sexes of plants. Some 
liolly-trees bear. only male flowers, which have four stamens 
])roducing a rather small quantity of pollen, and a rudimentary 
l)istil ; other holly-trees bear only female flowers ; these have 
a full-sized pistil, and four stamens with shrivelled anthers, in 
which not a grain of pollen can be detected. Having found 
a female tree exactly sixty yards from a male tree, I put the 
stigmas of twenty flowers, taken from difierent 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, neverthe- 
less 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 rendered so highly attractive to 
insects that pollen was regularly carried from flower to flower, 
another process might commence. No naturalist doubts the 
advantage of what has been called the " physiological division 
of labor ; " hence we may believe that it would be advanta- 
geous 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, 
tlien, 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 di\'ision of 
labor, individuals with this tendency more and more increased, 
would be continually favored or selected, until at last a eoin- 
phite separation of the sexes might be effected. It would 
take up too much space to show the various steps, through 
diinorpliism and other means, by which the separation of the 
sexes in plants of various kinds is aiiparently now in progress ; 


but I may add that .some of the species of holly in North 
America are, accordiiifj to Asa Gray, in an intermediate con- 
dition, or, as he cxpixvsses it, arc more or less diceciously po- 

Let us now turn to the nectar-feeding insects. We may 
suppose the plant, of which we have been slowly increasing 
the nectar hy continued selection, to be a common plant ; and 
that certain insects depended in main part on its nectar for 
fooil. I could give many facts, showing how anxious bees are 
to save time: for instance, their habit of cutting holes and 
sucking the nectar at the bases of certain flowers, which they 
can, with a very little more troul)le, enter by the mouth. 
Bearing such facts in mind, it may be believed that, under cer- 
tain circumstances, individual differences in the curvature or 
length of the proboscis, etc., too slight to be appreciated by 
us, might profit a bee or other insect, so that certain individ- 
uals would be able to obtain their food more quickly than 
others ; and thus the communities to which they belonged 
would ilourish and throw off many swarms inheriting the same 
peculiarities. The tulles of the corolla of the common red and 
incarnate clovers (Trifolium pratense and incarnatum) do not 
on a hasty glance appear to differ in length ; yet the hive-bee 
ran easily suck the nectar out of the incarnate clover, but not 
out of the common red clover, Avhich is visited by humble-bees 
alone ; so that whole fields of the red clover in vain offer an 
abundant supply of precious nectar to the hive-bee. Tliat this 
nectar is much liked by the hive-bee is certain ; for I have re- 
])eatedly seen, but only in the autumn, many hive-bees sucking 
the (lowei-s 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 
nve 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 and which freely crosses with the common hive-bee, is 
able to. reach and suck the nectar of the connnon red clover. 
Thus, in a country where this kind of clover abounded, it might 
be a great advantage to the hive-bee to have a slightly-longer 
or differently-constructed proboscis.. On the other hand, as 
the fertility of this clover absolutely depends on bees visiting 


tlio llowiTri, if humble-bees were to become rare in any country, 
it miglit be a great advantage to the plant to have a shorter 
or more deeply-divided corolhi, so that the hive-bees should be 
induced to suck its flowers. Thus I can understand how a 
flower and a bee might slowly become, either simultaneously 
or one after the other, modified and adapted to each other in 
the most perfect manner, by the continued preservation of all 
the individuals M'hich presented slight deviations of structure 
mutually favorable to each other. 

I am Avcll aware that this doctrine of natural selection, 
exemplified in the above imaginary instances, is open to the 
same objections which were at first urged against Sir Charles 
Lycll's noble \aews on " the modern changes of the earth, as 
illustrative of geology ; " but Ave now seldom hear the agencies, 
still at work, spoken of as trifling or insignificant, when applied 
to the excavation of the deepest valleys or to the formation of 
long lines of inland cliffs. Natural selection acts only by the 
preservation and accumulation of small inherited modifications, 
each profitable to the preserved being ; and as modern geology- 
has almost banished such views as the excavation of a great 
valley by a single diluvial wave, so will natural selection, if 
it be a true principle, banish the belief of the continued crea- 
tion of new organic beings, or of any great and sudden modifi- 
cation in their structure. 

On the Infererossmff of Individuals. 

I must here introduce a short digression. In the case of 
animals and plants with separated sexes, it is of course obvi- 
ous that two individuals must always (with the exception of 
the curious and not well-understood cases of parthenogenesis) 
unite for each birth ; but in the case of hermaphrodites this is 
far from obvious. Nevertheless tliere is reason to believe that 
with all hermaphrodites two individuals, either occasionally or 
habitually, concur for the reproduction of their kind. This 
vicAV was first suggested by Andrew Knight. "SVe shall pres- 
enth' see its importance : but I must here treat the subject 
with extreme 1)revity, thougli I have the materials prepared 
for an anijile discussion. All vertebrate animals, all insects, 
and some other large groups of animals, pair for each birth. 
Modern research has nuich diminished the number of supjiosed 
hermajihrodites, and of real hermaphrodites a large number 
pair; that is, two individuals regularly unite for reproduction, 


Aviiieh is all that concerns us. But still there arc many lier- 
maphrodito animals wliicli certainly do not habitually pair, and 
a \-ast majority of plants are hermaphrodites. What reason, it 
may be asked, is there for supposing in these cases that two 
individuals ever concur in reproduction? As it is impossible 
here to enter on details, I nuist trust to some general consid- 
erations alone. 

In the first place, I have collected so large a body of facts, 
showing, in accordance with the almost universal belief of 
breeders, that Avith animals and plants a cross between differ- 
ent 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 dose interbreeding diminishes vig- 
or and fertility ; tliat these facts alone incline me to believe 
that it is a general law of Nature that no organic being fertil- 
izes itself for a perpetuity of generations ; but that a cross 
with another individual is occasionally — perhaps at long inter- 
vals of time — indispensable. 

On the belief that this is a law of Nature, we can, I think, 
miderstand several large classes of facts, such as the follow- 
ing, which on any other view are inexplicable. Every hybrid- 
izer knows how unfavorable exposure to wet is to the fertili- 
zation of a flower, yet Avhat a multitude of flowers have their 
anthers and stigmas fully exposed to the weather ! If an 
occasional cross be indispensable, notwithstanding that the 
plant's own anthers and pistil stand so near each other as 
almost to insure self-fertilization, the fullest freedom for the 
entrance of pollen from another individual Avill explain the 
above state of exposure of the organs. Many flowers, on the 
other hand, have their organs of fructification closely enclosed, 
as in the great papilionaceous or pea-fainily ; but in most of 
these flowers there is a curious adaptation between their struc- 
ture and the manner in which bees suck the nectar; for, in 
doing this, they either push the flower's own pollen on the 
stigma, or bring pollen from another flower. So necessary are 
the visits of bees to many ]iapiIionaceous flowers, that I have 
found, by experiments published elsewhere, that their fertility 
is greatly diminished if these visits be prevented. Now, it is 
scarcely possible that bees should fly from flower to flower, 
and not carry pollen from one to the other, to the great good, 
as I believe, of the plant. IJees will act lik(^ a camel-hair pen- 
cil, and it is quite sufficient just to touch the anthers of one 
flower and then the stigma of another with the same brush to 


insure fertilization ; but it must not be supposed that bees 
would thus produce a multitude of hybrids between distinct 
species ; for if you brinf^ on the same brush a plant's own pol- 
len and pollen from another species, the former will have such 
a prepotent effect, that it will invariably and completely de- 
stroy, as has been shown by Gartner, any influence from the 
foreig'n pollen, 

^V'hen the stamens of a flower suddenly spring toward the 
pistil, or slowly move one after the other toward it, the con- 
trivance seems adapted solely to insure 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 Kol- 
reutcr 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, so largely do they naturally cross. In 
many other cases, far from there being any aids for self-fertili- 
zation, there are special contrivances, as I could show from the 
writings of C C. Sprengel and from my own observations, 
which effectually prevent the stigma receiving pollen from its 
own flower : for instance, in Lobelia fulgens, there is a really 
beautiful and elaborate contrivance by which all the infinitely 
numerous pollen-granules are swept out of the conjoined an- 
thers of each flower, before the stigma of that individual flower 
is ready to receive them; and as this flower is never visited, at 
least in my garden, b}^ insects, it never sets a seed, though by 
placing pollen from one flower on the stigma of another, I 
raised plenty of seedlings ; and while another species of Lobe- 
lia growing close by, which is visited by bees, seeds freely. 
In very many other cases, though there be no special mechani- 
cal contrivance to prevent the stigma of a flower receiving its 
own pollen, yet, as C. C. Sprengel has shown, and as I can 
confirm, either the anthers burst before the stigma is ready for 
fertilization, or the stigma is ready before the pollen of that 
flower is ready, so tliat these plants have in fact separated 
sexes, and nnist habitually be crossed. So it is with the recip- 
rocally dimorphic and trimorphic plants pre\dously alluded to. 
How strange are these facts ! How strange that the pollen 
and stigmatic surface of the same flower, though placed so 
close together, as if for the very purpose of self-fertilization, 
should in so many cases be nuitually useless to each other! 
Hov>' simjily are these facts explained on the view of an oeca- 


sional cross Avilli a distinct individual being advantageous or 
indispensable ! 

If several varieties of tlie cabbage, radish, onion, and of 
some otlicr plants, be allowed to seed near eacli other, a large 
majority, as I have found, of the seedlings thus raised will 
turn out mongrels : for instance, I raised 233 seedling cab- 
Ixigcs from some jilants 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 l)y its own six sta- 
mens, but by those of the many other flowers on the same plant; 
and the pollen of each flower readily gets on its own stigma 
without insect-agency ; for I have found that a plant carefully 
protected produced the full number of pods. How, then, 
comes it that sucli a vast number of the seedlings are mongrel- 
ized ? • I suspect that it must arise from the pollen of a distinct 
varii'tj/ having a prepotent effect over a flower's own pollen ; 
and that this is part of the general law of good being derived 
from the intercrossing of distinct individuals of the same spe- 
cies. When distinct species are crossed the case is directly the 
reverse, for a jilant's own pollen is almost always prepotent 
over foreign pollen ; but to this subject Ave shall return in a 
future chapter. 

In the case of a large tree covered with innumerable flow- 
ers, 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 tre*, and that flowers on the same tree can be con- 
sidered as distinct individuals only in a limited sense. I be- 
lieve this objection to be valid, but that Nature has largely pro- 
vided against it by giving to trees a strong tendency to bear 
flowers with separated sexes. When the sexes are separated, 
although the male and female flowers may be produced on the 
same tree, we can see that pollen must be regularly carried 
from flower to flower; and this will give a better chance of 
]i()llen being occasionally carried from tree to tree. That trees 
belonging to all Ord<»rs have their sexes more often separated 
than other plants, I And to be the case in this country ; and at 
my request Dr. Hooker tabulated the trees of New Zealand, 
and Dr. Asa Gray those of the United States, and the result 
was as I anticipated. On the other hand. Dr. Hooker has re- 
cently informc(l me that he flnds that the rule does not hold in 
Australia ; and I have made these few remarks on the sexes of 
trees simply to call attention to the subject. 


Turning- for a very brief space to animals : on ilic land 
there are some hcrmaplirodites, as land-moUusca and eartli- 
worms ; but these all pair. As yet I have not found a single 
case of a terrestrial animal Avhich fertilizes itself. Wc can un- 
derstand this remarkable fact, Avhich offers so strong a contrast 
Avith terrestrial plants, on the view of an occasional cross being 
indispensable, by considering the medium in which terrestrial 
animals live, and the nature of the fertilizing element ; for we 
know of no means, analogous to the action of insects and of 
the wind in the case of plants, by which an occasional cross 
could be effected "with terrestrial animals, "without the concur- 
rence of two individuals. Of aquatic animals, there are many 
self-fertilizing hermaphrodites ; but here currents in the water 
offer an obvious means for an occasional cross. And, as in the 
case of flowers, I have as yet failed, after consultation Avith 
one of the highest authorities, namely, Prof. Huxley, to dis- 
cover a single case of an hermaphrodite anim:il with the organs 
of reproduction so perfectly enclosed within the body, that ac- 
cess from without and the occasional influence of a distinct in- 
dividual can be shown to be physically impossible. Cirripedes 
long appeared to me to present a case of very great difficulty 
under this point of view ; but I have been enabled, by a 
fortunate chance, elsewhere to prove that two individuals, 
though both are self-fertilizing hermaphrodites, do sonietimcs 

It must have struck most naturalists as a strange anomaly 
that, in the case of both animals and plants, species of the same 
family and even of the same genus, though agreeing closely 
with each other in almost their whole organization, yet are not 
rarely, some of them hermaphrodites, and some of them uni- 
sexual. But if, in fact, all hermaphrodites do occasionally in- 
tercross with other individuals, the difference between hermaph- 
rodites and unisexual species, as far as function is concerned, 
becomes very small. 

From these several considerations and from the many spe- 
cial facts which I have collected, but which I am not here able 
to give, I am strongly inclined to suspect that, both in the 
vegetable and animal kingdoms, an occasional intercross with 
a distinct individual is a law of Nature. I am well aware that 
there are, on this view, many cases of difficulty, some of which 
I am trying to investigate. Finally, then, we may conclude 
that, in many organic beings, a cross between two individuals 
is an ()l)vious necessity for each l)irth ; in many others it occurs 


perhaps only at long' intervals ; but in none, as I suspect, can 
self-fertilization go on for perpetuity. 

Clrcnmstanccs favorable for the Production of Neio Forms 
through Natural Selection. 

Tliis is an extremely intricate subject. A great amount of 
variability, under which term individual differences are always 
included, will evidently be favorable. A large number of in- 
dividuals, by giving a better chance for the appearance of 
profitable variations within any given period, will compensate 
for a lesser amount of variability in each individual, and is, I 
believe, an extremely important element of success. Though 
Nature grants long periods of time for the work of natural se- 
lection, she does not grant an indefinite period ; for, as all or- 
ganic -beings arc striving to seize on each place in the economy 
of Nature, if any one species docs not become modified and im- 
proved 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 nat- 
ural sclecticjn. Tlie tendency to reversion may. often check or 
prevent the work ; but as tliis tendency has not prevented 
man from forming by selection nimierous domestic races, why 
.sliould it prevail against natural selection ? 

In the case of methodical selection, a breeder selects for 
some definite object, and free intercrossing Avill wholly stop 
his work. But when many men, without intending to alter the 
breed, have a nearly common standard of perfection, and all 
try to procure and breed from the best animals, much improve- 
ment surely but slowly follows from this unconscious process 
of selection, notwithstanding a large amount of crossing with 
inferior animals. Thus it Avill be in Nature; for within a con- 
Ihied area, with some place in its polity not perfectly occu]iicd, 
natural selection will always tend to preserve all the individ- 
uals varying in the right direction, though in different degrees, 
so as better to fill up the imoccupied place. But if the area be 
very large, its several districts will almost certainly present 
different conditions of life ; and then, if the same species under- 
g()(^s modification in different parts, the newly-formed varieties 
will intercross on the confines of each district. But we shall 
see in the seventh chapter that intermediate varieties, inhabit- 
ing an intermediate district, whether the result of the crossing 
of other varieties, or originally formed with an intermediate 


character, will in llie lonrr-run generally be supplanted by one 
of the varieties on either hand. Intercrossing will aflect those 
animals most which unite for each birth and Avauder 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 hennaphrodite organisms which cross only occasionally, 
and likewise with animals which unite for each l)irth,but which 
wander little and can increase at a very rapid rate, a new and 
iinpro\'ed variety might be quickly formed on any one spot, 
and might there maintain itself in a body and afterward 
spread, so that the crossing would be chiefly between the indi- 
viduals of the new variety living together in the same place. 
On this principle, nurserymen always prefer saving seed from 
a large body of plants, as the chance of intercrossing is thus 

Even in the case of animals which breed slowly and xmite 
for each birtli, Ave must not assume that the effects of natural 
selection Avill always be immediately oveq:)OAvered by free inter- 
crossing ; for I can bring a considerable body of facts, showing 
that, Avitljin the- same area, varieties of the same animal may 
long remain distinct, from haunting different stations, from 
breeding at slightly-tlifferent seasons, or from varieties of the 
same kind preferring to pair together. 

Intercrossing plays a very important part in Nature in keep- 
ing the individuals of the same species, or of the same \'ariety, 
true and xmifonn in character. It will obviously thus act far 
more elViciently with those animals Avhich unite for each birth ; 
but as already stated we have reason to believe that occasional 
intercrosses take place with all animals and with all plants. 
Even if these take ]ilace only at long intcr\-als of time, the 
young thus produced Avill gain so much in vigor and fertihty 
over the offspring from long-continued self-fertilization, that 
the}' Avill have a better chance of surviving and propagat- 
ing their kind ; and thus, in the long-run, the influence of inter- 
crosses, even at rare intervals, will be great. If there exist 
organic beings which never intercross, uniformity of character 
ran be retained among them, as long as their conditions of 
life remain tho same, only through the principle of inheritance 
and through natural selection destroying any which depart from 
tlic proper type ; but if their conditions of life change and they 
undergo modification, xmiformity of character can be given to 
their modified offspring, solely b^' natural selection prescn-ing 
similar favorable variations. 


Isolation, also, is an important clement in the changes 
effected tlirout^h natural selection. In a confined or isolated 
area, if not very large, the organic and inorganic conditions of 
life will generally be almost uniform ; so that natural selection 
will tend to modify all the varying individuals of the same spe- 
cies in the same manner. Intercrossing with the inhal)itants 
of the surrounding districts ^vill, also, be prevented. Moritz 
Wagner has lately published an interesting essay on this sub- 
ject, and has shown that the service rendered by isolation in 
preventing crosses between newly-formed varieties is probably 
greater even than I have supposed. But from reasons already 
assigned I can by no means agree with this naturalist, that 
migration and isolation are necessary for the formation of new 
species. The importance of isolation is likewise great in pre- 
venting, 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 econ- 
cmy of the district are left open for the old inhabitants to strug- 
gle for and become ada])ted to. Lastly, isolation will give time 
for a new variety to be slowly improved ; and this may some- 
times be of importance in the productioTi of new species. If, 
however, an isolated area be very small, either from being sur- 
rounded by jjarriers, or from having very peculiar physical con- 
ditions, the total number of the inlialiitants will be small ; and 
this will retard the production of new species through natural 
selection, by decreasing the chances of the appearance of favor- 
able individual diflerences. 

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-imjiortant part in modifying species, as if 
all ^vere necessarily undergoing change through the action of 
some iimate law. Lapse of time is only so fur injportant, and 
its importance in this respect is great, that it gives a better 
chaiKMj of beneficial variations arising, being selected, increased, 
and fixed, in relation to the; slowly-clianging organic and in- 
organic (•f)iiditions of life. It likewise favors the definite action 
of the conditions of life. 

If \\c turn to Nature to test the truth of these remarks, and 
look at any small, isolated area, such as an oceanic island, al- 
though the number of the dilTt^rent species inhabiting it is small, 
as we shall see in our chapter on Geographical Distribution; 
yet of the species a very large proportion are endemic — that is, 


have l.ccn produced llicre, and nowhere else in the world. 
Hencn^ :m oceanic ishxnd at first sie^ht seems to have been 
hig'hly 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 production of new organic forms, we 
ought to make the comparison -within equal times ; and this 
wc are incajiahlo of doing. 

Although isolation is of great importance in the production 
of new species, on the Avhole I am inclined to believe that large- 
ness of area is still more important, especially for the produc- 
tion of species which shall prove capable of enduring for a long 
period, and of spreading Avidcly. Throughout a great and open 
area, not only Avill there be a better chance of favorable varia- 
tions arising from the large number of individuals of the same 
species there supported, but the conditions of life are much 
more complex from the large number of already existing spe- 
cies ; 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 compe- 
tition with many others. Moreover, great areas, though now 
continuous, owing to former oscillations of level, will often 
have existed in a broken condition, so that the good effects of 
isolation will generally, to a certain extent, have concuiTed. 
Finally, I conclude that, although small isolated areas probably 
have beeii in some respects highly favorable for the production 
of new species, yet that the course of modification will gener- 
ally have been more rapid on large areas ; and, what is more 
important, that the new forms j)roduced on large areas, which 
already have been victorious over many competitors, will be 
tliose that will s])rea(l most widely, will give rise to most new 
varit^ties and species, and will thus play the most important 
part in the changing history of the organic Avorld. 

We can, perhaps, on these Aiews, imderstand some facts 
wliich will be again alluded to in our chapter on Geographical 
Distribution ; for instance, that the ]iroductions of the smaller 
continent of Australia are now yielding before those of the 
larger Europa'o- Asiatic area. Thus, also, it is that continental 
pniductions 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 ex- 


termination. Ilcncc, perhaps, it is liiat the flora of Madeira, 
accordin<^ to Oswald Heer, resembles the extinct tertiary flora 
of Europe. ^Vll fresh-water basins, taken together, make a 
small area compared with that of the sea or of the land ; and, 
consequently, the competition between fresh-water productions 
will have been less severe than elsewhere ; new forms will have 
been more slowly formed, and old forms more slowly extermi- 
nated. And it is in fresh water that Ave find seven genera of 
Ganoid fishes, remnants of a once preponderant order : and in 
ii?sh water we find some of the most anomalous forms now 
known in the world, as the Ornithorhynchus and Lepidosiren, 
which, like fossils, connect to a certain extent orders at present 
widely separated 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 having been 
exposed to less varied and therefore less severe competition. 

To sum up the circumstances favorable and unfavorable for 
the production of new species through natural selection, as far 
as the extreme intricacy of the svibject permits. I conclude 
that fur terrestrial productions a large continental area, which 
has imdergone many oscillations of level, will have been the 
most favorable for the production of many ncAV forms of life, 
fitted to endure for a long time and to spread widely. While 
the area existed as a continent, the inhabitants will have been 
numerous in individuals and kinds, and Avill have been subjected 
to severe competition. When converted by subsidence 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, immigration will 
have been prevented, so that new places in the polity of each 
island will have had to be filled up by modifications of the old 
inhabitants; and time will have been allowed for the varieties 
in each to become well modified and perfected. When, by 
renewed elevation, the islands were reconverted into a conti- 
nental area, there will again have been severe competition : 
the most faxorcd or improved varieties will have been enabled 
to spread : there will have been much extinction of the less 
improved forms, and the relative proportional ninnbcrs of the 
various inhabitants of tin; reunited continent will again have 
been changed ; and, again, there will have been a fair lield for 
natural selection to improve still further the inhabitants, and 
thus to produce new species. 


That natural sc^lcction acts "vvith extreme slowness I fully 
iidniit. The result depends on there beings places in the polity 
of Nature, wliich can be better filled through some of the in- 
habitants of the country undergoinf^ modifications of some kind. 
The existence of such places will often depend on physical 
changes, which are generally very slow, and on the immigra- 
tion of better-adapted forms being checked. But the effects 
of natural selection will probably still oftener depend on some 
few of the inhabitants becoming slowly modified ; the mutual 
relations of the other inhabitants being thus disturbed. Al- 
though all the individuals of the same species differ more or 
less from each other, differences of the right nature, better 
adapted to the then existing conditions, may not soon occur. 
The results will often be greatly retarded by free intercrossing. 
Many will exclaim that these several causes are amply suffi- 
cient to neutralize the power of natural selection. I do not 
believe so. But I do believe that natural selection generally 
acts very slowly in effecting changes, at long intervals of time, 
and only on a few of the inhabitants of the same region. I 
further believe that these slow, intermittent results of natural 
selection accord perfectly with what geology tells us of the 
rate and manner at which the inhabitants of the world have 

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 infinite complexity of the 
coadaptations between all organic beings, one with another and 
with their physical conditions of life, "which may be effected in 
the long course of time by Nature's power of selection, or the 
survival of the fittest. 

Extinction caused by Natural Selection, 

This subject will be more fully discussed in our chapter on 
Geology ; but it must be here alluded to from being inti- 
mately connected with natural selection. Natural selection 
acts solely through the preservation of variations in some way 
advantageous, which consequently endure. Owing to the high 
geometrical ratio of increase of all organic beings, each area is 
already stocked with the full number of its existing inhabit- 
ants, and as most areas are already stocked with a great diver 
sity of forms, it follows that, as each selected and favored form 
increases in number, so generally will the less favored forms 


decrease and l^eooinc rare. Rarity, as geolo<T:y tells us, is the 
precursor to e.vtinclion. We can, also, see that any form rep- 
resented l>y few individuals will, during fluctuations in the sea- 
sons or in the numlxir of its enemies, run a good chance of 
utter extinction. But we may go further than this ; for, as 
new forms are continually and slowly being produced, unless 
we believe that tlie mnnber of specific forms goes on perpet- 
ually and almost indetinitely increasing, many inevitably must 
become extinct. That the number of speciiic forms has not 
indcHnitely increased, geology tells us plainly ; and we shall 
presently attempt to show why it is that the number of species 
throughout the world has not become immeasurably great. 

AW; have seen tliat the species which are most numerous in 
individuals have the best chance of producing favorable varia- 
tions within any given period. We have evidence of this, in 
the facts stated in the second chapter, showing that it is the 
common species which offer the greatest number of recorded 
varieties, or incipitnit species. Hence, rare species will be less 
quickly moditieil or improved within any given period, and 
they will consequently be beaten in the race for life by the 
modified descendants of the commoner species. 

From the sesevcral considerations I think it inevitably fol- 
lows, that, as new species in the course of time are formed 
through natural selection, others will become rarer and rarer, 
and finally extinct. The forms Avhich stand in closest compe- 
tition with those imderg^oing modification and improvement, 
will naturally suffer most. And we have seen in the chapter 
nn the Struggle for Existence that it is the most closely-allied 
forms — varieties of the same species, and species of the same 
genus or of related genera — which, from having nearly the 
same structure, constitution, and habits, generally come into 
tlie severest competition with each other. Consequently, eacli 
new variety or species, during the progress of its formation, 
will generally press hardest on its nearest kindred, and tcTid to 
exterminate them. We see tlie same process of extermination 
among our domesticated productions, through the selection of 
improved forms by man. Many curious instances could l)e given 
showing how quickly new breeds of cattle, sheep, and other 
animals, and varieties of flowers, take the place of older and 
inferior kinds. In Yorkshire, it is historically known that the 
ancient black cattle were displaced by the long-horns, and that 
these " were swept away by the short-horns " (I quote the 
words of an agricultural Avriter) "as if by some murderous pes- 


Divergence of Character, 

The principle, which I liavc dcsip^nated by this term, is of 
high importance, and cxphiins, as I believe, several important 
facts, in tlie first place, varieties, even strongly-marked ones, 
though having somewhat of the character of species — as is 
shown by the hopeless doubts in many cases how to rank 
them — yet certainly differ from each other far less than do 
good and distinct species. Nevertheless, according to my 
view, varieties are species in the process of formation, or are, 
as I have called them, incipient species. How, then, does the 
lesser difference between varieties become augmented into the 
greater difference between species ? That this does habitually 
happen, we must infer from most of the innumerable species 
throughout Nature presenting well-marked differences ; where- 
as varieties, the supposed protot}-pes and parents of future 
well-marked species, jiresent 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 produc- 
tion of races so different as short-horn and Hereford cattle, race 
and cart horses, the several breeds of pigeons, etc., could never 
have been effected by the mere chance accumulation of varia- 
tions of a similar character during many successive generations. 
In ])ractice, a fancier is, for instance, struck by a pigeon having 
a sliglitly shorter beak ; another fancier is struck by a pigeon 
having a rather longer beak ; and, on the acknowledged prin- 
ciple that " fanciers do not and will not admire a medium stand- 
ard, but like extremes," they both go on (as has actually oc- 
curred with the sub-breeds of the tumbler-pigeon) choosing and 
l)rceding from birds with longer and longer beaks, or with 
shorter and shorter beaks. Again, we may suppose that at an 
early period one man preferred swifter horses ; another stronger 
and more bulky horses. The early differences would be very 
Blight; in the course of time, from the continued selection of 
swifter horses by some breeders, and of stronger ones by others, 
the differences would become greater, and would be noted as 


Ibnnin;^ two sub-breeds; finally, after the lapse of centuries 
the sub-breods would become converted into two ■well-estab- 
lished and distinct breeds. As the dilVerenccs slowly became 
,e:reater, the inferior animals with intermediate characters, bein;i^ 
neither very swift nor very strong, would have been neglected, 
and will have disappeared. Here, then, we see in man's pro- 
ductions 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 eacli other and from their common pax-ent. 

But 1»()W, it may be asked, can any analogous principle 
apply in Nature ? t believe it can and does apply most effi- 
ciently (though it Avas a long time before I saw how), from tlie 
simple circumstance that the more diversified the descendants 
from any one species become in structure, constitution, and 
habrts, 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 numbers. 

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 an}?- country has 
long ago arrived at its full average. If its natural powers of 
increase be allowed to act, it can succeed in increasing (the 
country not imdergoing any change in its conditions) only by 
its varying descendants seizing on places at present occupied 
by other animals: some of them, for instance, being enabled to 
feed on new kinds of prey, either dead or alive ; some inhabit- 
ing new stations, climbing trees, frequenting water, and some 
[lerliaps becoming less carnivorous. The more diversified in 
lialjits and structure the descendants of our carnivorous animal 
lir'came, the more places they would bo enabled to occupy. 
AViiat applies to one animal will apply throughout all time to 
all animals — that is, if they vary — for otherwise natural selec- 
tion can effect nothing. So it will be witli j^lants. It has been 
experimentally proved that, if a plot of ground be sown witli 
one species of grass, and a similar plot be sown with several 
distinct genera of grasses, a greater number of phmts and a 
greater weight of dry herbage can be raised by the latter jjro- 
ccss. Tlie same has been found to hold good when one va- 
riety and several mixed varieties of wlieat liave been sown on 
equal spaces of ground. Hence, if any one species of grass 
were to go on varying, and those varieties were continually 
selected which differed from each other in at all the same man- 


lUT as distiiKl species and gwiora of grasses differ from each 
other, a greater miinber of individual phmts of this species of 
grass, inchiding its modified descendants, ■would succeed in 
living on the same piece of ground. And we well know that 
each species and each variety of grass is aimually sowing al- 
most countless seeds : and thus, as it may be said, is striving 
its utmost to increase its numbers. Consequently, in the course 
of many thousand generations, the most distinct varieties of 
any one species of gTass would always have the best chance of 
succeeding and of increasing in numbers, and thus of supplant- 
ing the less distinct varieties ; and varieties, when rendered 
very distinct from each other, take the rank of species. 

The truth of the principle that the greatest amount of life 
can be supported by great diversification of structure, is seen 
iinder many natural circumstances. In an extremely small 
area, especially if freely open to immigration, and where the 
contest between indivitkial and individual must be severe, we 
always find groat diversity in its inhabitants. For instance, I 
found that a piece of turf, three feet by four in size, which had 
been exposed for many years to exactly the same conditions, 
supported twenty species of plants, and these belonged to 
eighteen genera and to eight orders, which shoAvs how much 
these plants differ from each other. So it is with tlie plants 
and insects on small and imiform islets ; also in small ponds 
of fresh water. Farmers find that they can raise most food by 
a rotation of plants belonging to the most different orders : 
Nature follows what may be called a simultaneous rotation. 
Most of the animals and plants which live close round any 
small piece of ground, could live on it (supposing it not to be 
in any way peculiar in its nature), and may be said to be 
striving to the utmost to live there ; but, it is seen, that where 
tliey come into the closest competition with each other, the 
advantages of diversification of structure, with the accom])any- 
ing difVerenccs 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 

The same principle is seen in the naturalization of plants 
through man's agency in foreign lands. It might have been 
expected that the plants which would succeed in becoming 
naturalized in any land would generally have been closely 
aUied to tlie indigenes ; for these are commonly looked at as 
specially created and adapted for their own country. It might 


also, pcrliaps, liave been expected that naturalized plants would 
have beloufred to a few groups more especially adapted to 
certain stations in their new homes. But the case is very dif- 
ferent ; and Alpli. dc Candolle has well remarked, in his great 
and admirable work, that floras gain by naturalization, propor- 
tionally with the number of the native genera and species, far 
more in new genera than in new species. To give a single 
instance : in tiie last edition of Dr. Asa Gray's " Manual of 
the Flora of the Northern United States," 260 naturalized 
plants are enumerated, and these belong to 162 genera. Wc 
thus see that these naturalized plants are of a highly-diversified 
nature. The}'" difler, moreover, to a large extent, from the in- 
digenes, for, out of the 162 naturalized genera, no less than 100 
genera arc not there indigenous, and thus a large proportional 
addition is made to the genera now living in the United States. 

By considering tlie nature of the plants or animals which 
have struggled successfully with the indigenes of any country, 
and have there become naturalized, we may gain some crude 
idea in what manner some of the natives would have to be 
modilied, in order to gain an advantage over the other natives ; 
and we may at least safely infer that diversification of struc- 
ture, amounting to new generic differences, would be profit- 
able to them. 

The advantage of diversification in the inhabitants of the 
same region is, in fact, the same as that of the physiological 
division of labor in the organs of the same individual body — 
a subject so well elucidated by Milne Edwards. Xo physiolo- 
gist doubts that a stomach adapted to digest vegetable matter 
alone, or llesh alone, draws most nutriment from these sub- 
stances. So in the general economy of an}'' land, the more 
widely and perfectly the animals and plants are diversified for 
dilferent haljits of life, so will a greater number of inchviduals 
be capable of there sujiporting themselves. A set of animals, 
with their organization but little diversified, coulti hardly com- 
pete with a set more perfectly diversified in structure. It may 
be doubt(M], for instance, whether the Australian marsupials, 
which are divided into groups differing but little from each 
other, and feebly representing, as Mr, AVaterhouse antl others 
have remarked, our carnivorous, ruminant, and rodent mam- 
mals, could successfully compete with these well-pronounced 
orders. In the Australian manmials, we see the process of 
diversification in an early and incomplete stage of develop- 


Tlie Pi'ohdhle Results of the Action of Katural Selection 
through Diverrjence of Character and JSxtinctiony vi the 
Descendants of a Common Ancestor. 

After tlie forcg'oing discussion, which has been much com- 
pressed, we may assume that the modified descendants of any 
one species Avill succeed by so much the better as they become 
more diversified in structure, and are thus enabled to encroach 
on places occupied by other beings. Now let us see how this 
principle of benefit being derived from divergence of character, 
combined with the principles of i>atural selection and of ex- 
tinction, tends to act. 

The accompanying diagram will aid us in understanding 
this rather perplexing subject. Let A to L represent the spe- 
cies 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 
cliagram by the letters standing at unequal distances. I have 
said a large genus, because Ave have seen in the second chap- 
ter that, on an average, more of the species of large genera 
vary than of small genera ; and the varying species of the 
large genera present a greater number of varieties. We have, 
also, seen that the species, which are the commonest and the 
most widely-diilused, 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 pro- 
ceeding 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 simul- 
taneously, but often after long intervals of time ; nor are they 
all supposed to endure for equal periods. Only those varia- 
tions which are in some Avay profitable will be preserved or 
naturally selected. And here the importance of the principle 
of benefit being derived from divergence of character comes 
in ; for this will generally lead to the most different or diver- 
gent variations (represented by the outer dotted lines) being 
preserved and accumulated by natural selection. When a dot- 
ted line reaches one of the horizontal lines, and is there marked 
by a small numbered letter, a sufficient ainount of variation is 
supjiosed to have been accunuilated to have formed a fairly 
well-marked variety, such as would l)e thought worthy of rec- 
ord in a systematic work. 

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'. XII 


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The intervals between the horizontal lines in the dia^ara, 
may ix'prescnt each a thousand generations, or ten thousand. 
After a thousand generations, species (A) is supposed to have 
produced two fairly well-marked varieties, namely a} and rn). 
These two varieties will generally continue to be exposed to 
the same conditions Avhich made their parents variable, and 
the tendency to variability is in itself hereditary, consequently 
tliey will tend to vary, and generally to vary in nearl}' the 
same manner as their j)arents varied. Moreover, these two 
varieties, being only slightl^'-niodilied forms, will tend to in- 
herit those advantages which made their parent (A) more 
numerous than most of the other inhabitants of the same coun- 
try ; they will likewise partake of those more general advan- 
tages v/hich made the genus to which the parent-species be- 
longed a large genus in its ov,n country. And these circinn- 
stanccs we know to bo favoral)le to tiic production of new 

If, then, these two varieties be variable, the most divergent 
of their variations will generally be ])reserved during the next 
thousand generations. And after this interval, variety 0} is 
supposed in the diagram to have pi'oduccd variety a^, which 
will, owing to the principle of divergence, difler more from (A) 
than did variety a*. Variety «^' is supposed to have produced 
two varieties, namely Wi" and s", diflering from each other, and 
more considerably from their common parent (A). Wc may 
continue the process by similar steps for any length of time ; 
some of the varieties, after each thousand generations, produ- 
cing only a single variety, but in a more and more modi lied con- 
dition, some producing two or three varieties, and some failing 
to produce any. Thus the varieties, or modified descendants, 
proceeding from the common ])arent (A), will generally go on 
increasing in numljcr, and diverging in character. In the dia- 
gram tlie process is represented up to the ten-thousandth gen- 
eration, and under a condensed and siniplilled form up to the 
fourteen-thousandth generation, 

IJut I must hen; remark that I do not suppose that the 
process ever goes on so regularly as is represented in the dia- 
gram, though in itself made somewhat irregular, nor that it 
goes on continuously ; it is far more ])robable that each form 
remains for long periods unaltered, and tlien again imdcr- 
gocs modification. Nor do I suppose that the most di- 
vergent varieties are invariably preserved: a medium form 
may often long endure, and may or may not produce more 


than one modified descendant; for nalural selection Avill al- 
ways act according to the nature of the places which arc either 
unoccupied or not perfectly occupied by other beings ; and 
this will depend on infinitely-complex relations. But, as a 
general ruh>, the more diversified in structure tlic descendants 
from any one species can be rendered, the more places they 
will be enabled to seize on, and the more their modified pro- 
geny will increase. In our diagram the line of succession is 
liroken at regular intervals by small numbered letters marking 
the succ(>ssivc forms which have become sufficiently distinct to 
be recorded as varieties. But these breaks are imaginary, and 
might have been inserted an^^vhere, after intervals long 
enough to have allowed the accumulation of a considerable 
amount of divergent variation. 

As all the modified descendants from a common and wide- 
ly-diffused species, belonging to a large genus, Avill tend to 
partake of the same advantages which made their parent suc- 
cessful in life, they will generally go on multiplying in number 
as well as diverging in character: this is represented in the 
diagram by the several divergent branches proceeding from 
(A). The modified offspring from the latter 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 im- 
j)i(ived branches : this is represented in the diagram by some 
of the lower branches not reaching to the upper horizontal 
lines. In some cases I do not doubt that the jirocess of modi- 
fication will be confined to a single line of descent, and the 
ntunber of the descendants Mill not be increased; althouglithe 
amount of divergent modification may have been increased in 
the successive generations. This case would be represented in 
tlic diagram, if all the lines proceeding from (A) Avere removed, 
excepting that from «* to rt'°. In the same way, for instance, the 
English race-horse and English pointer have apparently both 
gone on slowly diverging in character from their original stocks, 
Mithout either having given oft' any fresh branches or races. 

After ten thousand generations, species (A) is sujiposed to 
liave produced three forms, «",/'", and wi", which, from hav- 
ing diverged in character during the successive generations, 
will have come to differ largely, but jicrhaps unequally, from 
each other and from their common jvarcnt. If we suppose the 
amount of change between each horizontid line in our diagram 
to be excessively small, these three forms may still be only 
w('ll-mark,Ml varieties ; but we have only to suppose the steps 


ill tiie process of modification to be more numerous or j^rcatcr 
in amount, to convert these three forms into doubtful or at 
last wcU-denncd species : thus the diao^ram iUustratcs the 
steps by wliich the small diflerences distinguishing varieties 
are increased into the larger differences distinguishing species. 
By continuing the same process for a greater number of gener- 
ations (as shown in the diagram in a condensed and simplified 
manner), we get eight species, marked by the letters between 
«" and m^\ all descended from (A). Thus, as I believe, spe- 
cies are multiplied and genera are formed. 

In a large genus it is probable that more than one species 
would vary. In the diagram I have assumed that a second 
species (I) has produced, by analogous steps, after ten thou- 
sand generations, either two well-marked varieties (w'° and z'") 
or two species, according to the amount of change supposed to 
be'rcjircsented between the horizontal lines. After fourteen 
thousand generations, six new species, marked by the letters 
w'* to z'*, are supposed to have been produced. In any genus, 
the species which are already very different in character from 
each other will generally tend to produce the greatest num- 
ber of modified descendants ; for they will have the best 
chance of filling new and widely-different places in the polity 
of Nature : hence in the diagram I have chosen the extreme 
species (A), and the nearly extreme species (I), as those which 
have largely varied, and have given rise to new varieties and 
species. The other nine species (marked by capital letters) 
of our original genus, may for long but unequal periods con- 
tinue to transmit unaltered descendants ; -and this is shown in 
tlu; 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 extinction, 
will have played an important part. As in each fully-stocked 
country natural selecti(jn necessarily acts by the selected form 
having some advantage in the struggle for life over other forms, 
there will be a constant tondcMicy in the improved descendants 
of any one species to supplant and exterminate in each stage 
of descent their pred(X"essors and their original progenitor. 
For it should be remembered that the competition will gen- 
erally be most severe between those forms which are most 
nearly related to each other in habits, constitution, and struc- 
ture. Hence all the intermediate forms between the earlier 
and later states, that is, between the less and more improved 
states of a species, as well as the original parent-species itself. 


will generally tend to l^ecome extinct. So it probably Avill 
be ^vith many -whole collateral lines of descent, Avhich ^vill be 
conquered by later and improved lines of descent. If, liow- 
ever, the modified offspring of a species get into some distinct 
country, or become quickly adapted to some cjuite new station, 
in whic-h offspring and progenitor do not come into compe- 
tition, both may continue to exist. 

If, then, our diagram be assumed to represent a consider- 
able amount of modification, species (A) and all the earlier va- 
rieties Avill have become extinct, having" been replaced by eight 
new species (a" to wi") ; and (I) will have been replaced 
by six (?i" to 2'*) new species. 

But we may go further than this. The original species 
of our genus were supposed to resemble eacli other in unequal 
degrees, as is so generally the case in Nature ; species (A) 
being more nearly related to B, C, and D, than to the other 
species ; and species (I) more to G, H, K, L, than to the 
others. These two species (A) and (I) were also supposed to 
be very common and widely-dilfuscd species, so that they must 
originally have had some advantage over most of the other 
species of the genus. Their modified descendants, fourteen in 
number at the fourteen-thousandth generation, will probably 
have inherited some of the same advantages : they have also 
been modified and improved in a diversified manner at each 
stage of descent, so as to have become adapted to many related 
places .in the natural economy of their country. It seems, 
therefore, extremely probable that they will have taken the 
places of, and thus exterminatod, not only their parents (A) 
and (I), but likewise some of the original species which were 
most nearly related to their parents. Hence very few of the 
original species will have transmitted offspring to the fourteen- 
tliousandth generation. 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 origi- 
nal eleven species, will now be lifteen in number. Owing to 
the divergent tendency of natural selection, the extreme 
amoimt of difference in character betAveen species «'* and 2'* 
will be much greater than that between the most distinct of 
the original eleven species. The new species, moreover, wiK 
be allied to each other in a Avidely-differcnt manner. Of the 
eight descendants from (A) the three marked a", <j^\ p^\ will 


bo noavly related from liaving^ recently branched off from a'°/ 
J/*, and /'*, from liavin": diverged at an earlier period from a'', 
\\\\\ be in some deo^ree distinct from the three first-named spe- 
ties ; and lastly, o'\ e'*, and »i'*, will be nearly related one to 
the other, but, from havinfj; diverf^ed at the first commence- 
ment of tlie process of modification, will be widely different 
from the other five species, and may constitute a sub-genus or 
a distinct genus. 

The six descendants from (I) will form two sub-genera or 
genera. But as the original species (I) differed largely from 
(A), standing nearly at the extreme end of the original genus, 
llie six descendants from (I) will, owing to inheritance alone, 
differ considerably from the eight descendants from (A) ; the 
two groups, moreover, are supposed to have gone on diverging 
in different directions. The intermediate species, also (and 
this is a very important consideration), which connected the 
original species (A) and (I), have all become, excepting (F), 
extinct, and have left no descendants. Hence the six new spe- 
cies descended from (I), and the eight descended from (A), 
will have to be ranked as very distinct genera, or even as dis- 
tinct sub- families. 

Thus it is, as I believe, that two or more genera are pro- 
duced, by descent Avith modification, from two or more species 
of the same genus. And the two or more parent-species are 
supposed to have descended from some one species of an ear- 
lier 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 representing a 
single species, the supposed progenitor of our several new sub- 
genera and genera. 

It is worth while to reflect for a moment on the character 
of the new species f'*, which is supposed not to have diverged 
much in character, but to have retained the fomi of (F), either 
unaltered or altered only in a slight degree. In this case, its 
allinitics to the other fourteen new species Avill be of a curious 
and circuitous nature. Having descended from a form which 
stood l)etwe(Mi the two parent-species (A) and (I), now sup- 
posed to 1)0 extinct and imknown, it will be in some degree in- 
l«-rmediate in character between the two groups descended 
from these species. I5ut as these two groups have gone on 
diverging in character from the type of their parents, the new 
species (f'*) will not be directly intermediate between them, 
but ratlier l^etween t}^es of the two groups; and every 


naturalist will be able to bring some such case before his 

In the diagram, each liorizonal line has hitherto been sup- 
posed to represent a tliousand generations, but each may rep- 
resent a million or several million generations ; it may also 
represent a section of the successive strata of the earth's crust 
including extinct remains. ' We shall, when we come to our 
chapter on Geology, have to refer again to this subject, and I 
think we shall then sec that the diagram throws light on the 
affinities of extinct beings, Avhich, though generally belonging 
to the same orders, or families, or genera, ■with those now liv- 
ing, yet are often, in some degree, intermediate in character 
between existing groups ; and we can understand this fact, for 
the extinct species lived at very ancient epochs when the 
branching lines of descent had diverged less. 

I see no reason to limit the process of modification, as now 
explained, to the formation of genera alone. If, in the diagram, 
we suppose the amount of change represented by each succes- 
sive group of diverging dotted lines to be great, the forms 
marked a'* to^J>", those marked i'* and/"'\ and those marked 
o'* to ni^*, will form three very distinct genera. We shall also 
have two very distinct genera descended from (1), differing 
widely from the descendcnts of (A). These two groups of 
genera will thus form two distinct families, or orders, accord- 
ing to the amount of divergent modification supposed to be 
represented in the diagram. And the two new families, or 
orders, are descended from two species of the original genus, 
and these are suj^posed to be descended from some still more 
ancient and unknown form. 

We have seen that in each countrj^ it is the species belong- 
ing to the larger genera which oftcnest present varieties or in- 
cipient species. This, indeed, might have been expected ; for 
as natural selection acts throvigh one form having some advan- 
tage over other forms in the struggle for existence, it will 
chiefly act on those which already have some advantage ; and 
the largeness of any grouji shows that its species have inherited 
from a connnon ancestor some advantage in common. Hence, 
the struggle for the production of new and modified descend- 
ants will mainly lie between the larger groups which are all 
trying to increase in number. One large grouj) will slowl}' 
ct)nfjuer another large group, reduce its numbers, and thus les- 
sen its chance of further variation and improveuKMit. Within 
the same large group, the later and more highly -perfected su]> 


j^roups, from hraneliiiig" out and seizinc^ on many new plar-cs in 
the polity of Nature, will constantly tend to sujiplant and de- 
stroy the earlier and Icss-iinp.roved svib-<^oups. Small and 
broken ""roups and sul>irroups will hnally disappear, Lookinnf 
to the future, Ave can ])redict that the groups of organic beings 
which are now large and triumphant, and Avbich.are least 
broken u]i, that is, which as yet have sulTered least extinction, 
will for a long period continue to increase. But which groups 
will ultimately prevail, no man can predict ; for we well know 
that many groups formerly most extensively develojied, have 
now become extinct. Looking still more remotely to the 
future, we may predict that, owing to the continued and steady 
increase of the larger groups, a multitude of smaller groups will 
become utterly extinct, and leave no modified descendants; 
and consequently that, of the species living at any one period, 
extremely (gw will transmit descendants to a remote futurity. 
I shall have to return to this subject in the chapter on Classifi- 
cation, but I may add that as according to this view extremely 
few of the more ancient species have transmitted descendants, 
and as :ill the descendants of the same species form a class, we 
can imdcrstand 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 motlified descendants, 
yet, at remote geological periods, the earth may have been 
almost as well jjcopled with species of many genera, families, 
oiders, and classes, as at the; present day. 

O71 the Degree to which Organization tends to advance. 

Natural Selection exclusively acts by the preservation and 
accunmlation of variations, which are beneficial under the or- 
ganic and inorganic conditions of life to which each creature is 
exposed at each successive period 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 bi'ings throughout the world. But 
here we enter on a very intricate subject, for naturalists have 
not defined to each other's satisfaction what is meant by an 
advance in org-anization. Among the vertcbrata the degree of 
intellect and approac-h in structure to man clearly come into 
play. It might be thought that the amount of change Avhich 
the various ])arts and organs pass through in their develop- 


incnt from the cnil)ryo to maturity would suffice as a standard 
of comparison ; but there are cases, as with certain parasitic 
crustaceans, in Avhich several parts of the structure become 
less perfect, so that the mature animal cannot be called higher 
than its larva. Von Baers standard seems the most widely 
applica])le and the Ix'st, namely, the amount of diflerentiation 
of the diiferent parts of the same organic being, in the adult 
state as I should be inclined to add, and their specialization 
for difl'ercnt functions ; or, -as Milne Edwards would express it, 
the completeness of the division of physiological labor. But we 
shall sec how obscure this subject is if Ave look, for instance, to 
fish, among which some naturalists rank those as highest which, 
like the sliarks, 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. Still more plainly we 
see 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 sepals, jietals, stamens, and pistils, fully devel- 
oped in each flower ; whereas other botanists, probably with 
more truth, look at the plants which have their several organs 
much modified and reduced in number as the highest. 

If we take, as the standard of high organization, the 
amount of differentiation and speciahzation of the several or- 
gans in each being when adult (and this will include the ad- 
vancement of the brain for intellectual purposes), natural selec- 
tion clearly leads toward highness ; for all physiologists admit 
that the specialization of organs, inasmuch as they perform in 
this state their functions better, is an advantage to each being; 
and hence the accumulation of variations tending toward spe- 
cialization is within the scope of natural selection. On the 
other hand, Ave can see, bearing iu mind that all organic beings 
are striving to increase at a high ratio and to seize on every 
ill-occupied place in the economy of Nature, that it is quite pos- 
sible for natural selection gradually to fit an organic being to 
a situation in AA'hich scA^eral organs would be superfluous or 
useless : in such cases there might be retrogression in the scale 
of organization, AN'hcther organization on the Avhole has 
actually adA'anced from the remotest geological jieriods to the 
present day Avill be more conveniently discussed in om- chapter 
on Geological Succession. 

But it may be objected that, if all organic beings thus tend 



to rise iu llie scale, liow is it that throughout tlie Avorkl a mul- 
titude of the lowest forms still exist ; and how is it that in 
each great class some forms are far more hiofhly developed than 
otliers ? Why have not the more highly-developed forms 
everywhere supplanted and exterminated the lower? Lamarck, 
who lielieved iu an innate and ineWtable tendency toward per- 
fection in all organic beings, seems to have felt this difficulty 
so strongly, that he was led to suppose that new and simple 
forms are continually being produced by spontaneous genera- 
tion. Science, however, under her present aspect does not 
countenance tlie belief, whatever the futiu-e may reveal, that 
living creatures are now being generated. On our theory the 
continued existence of lowly organisms oflcrs no difficulty ; for 
natural selection, or the survival of the fittest, does not neces- 
sarily include progressive development — it only takes advan- 
tage of "such variations as arise and are bcnetlcial to each crea- 
ture imder its complex relations of life. And it may be asked 
what advantage, as far as we can see, would it be to an infu- 
sorian animalcule — to an intestinal worm — or even to an earth- 
worm, to be highly organized? If it were no advantage, tlu^se 
forms would be left by natural selection miimproved or but 
little improved; and might remain for indefmite ages in their 
present little advanced condition. And geology tells us that 
some of the lowest forms, as the infusoria and rizopods, have 
remained for an enormous period in nearly their present state. 
But to suppose that most of the many now existing low forms 
have not in the least advanced since the first dawn of life 
would be 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 organ- 

Nearly the same remarks are applicable if we look to the 
dilVt -rent grades of organization within each great group; for 
instance, in the vertebrata, to the coexistence of manunals and 
fish — among manunalia, to the coexistence of man and tlu; 
ornitliorliynchus — among fishes, to the coexistence of the shark 
and lancelet (Brancliiostoma), which latter fish in the extreme 
8imi)licily of its structiue approaches the invertebrate classes. 
But mammals and fisli hardly come into competition with each 
other ; tiie advancement of tlie whole class of mammals, or of 
certain memb(^rs in this class, to the highest grade would not 
lead to their taking the place of, and thus exterminating, fishes. 
Physiologists believe that the brain must be bathed by warm 


blood to be highly active, and tliis I'cquires auiial respiration ; 
so that Avarm-bloodod maininals when inhabiting the water live 
under some disadvantages in comparison with iishes. In this 
latter class, members of the shark family would not tend to 
supplant the lancelet ; for the lancelet, as I hear from Fritz 
Miiller, has, as sole companion and competitor on the barren, 
sandy shore of South Brazil, an anomalous annelid. The three 
lowest orders of mammals, namely, marsupials, edentata, and 
rodents, coexist in South America in the same region with 
numerous monkeys, and probably interfere little with each 
other. Altliougli oi'ganization, on the whole, may have ad- 
vanced and be still advancing throughout the world, yet the 
scale will always present many degrees of perfection; for the 
high advancement of certain whole classes, or of certain mem- 
bers of each class, does not at all necessarily lead to the ex- 
tinction of those groups with Avhich they do not enter into 
close competition. In some cases, as we shall hereafter see, 
lowly-organized forms appear to have been preserved to the 
present day from inhabiting confmed or peculiar stations, where 
they have been subjected to less severe competition, and where 
their scant}- numbers have retarded the chance of favorable 
variations arising. 

Finally, I believe that many lowly-organized forms now 
exist throughout the Avorld, 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 sufficed for the 
utmost possible amount of development. In some few cases 
there has been Avhat we must call retrogression of organization. 
But the main cause lies in the fact that under very simple con- 
ditions of life a high organization would be of no service — 
possibly would be of actual disservice, as being of a more 
delicate nature, and more liable to be put out of order and 

Looking to the first dawn of life, when all organic beings, 
as w^e may believe, presented the simplest structun^, how, it 
has been asked, couhl the first steps in the advancement or 
differentiation of parts have arisen'? Mr. Herbert S])encer 
would probably answer that as soon as simple unicellular or- 
ganism came by growth or division to be coinj^ounded of sev- 
eral cells, or became attached to any supporting surface, his 
law " that homologous imits of any order become ilifferentiated 
in proportion as their relations to incident forces become differ- 


eiit," would come into action. But as we have no facts to 
guide us, all speculation on the subject is useless. It is, how- 
ever, an en-or to suppose that there would be no struggle for 
existence, and, conseciuently, no natural selection, until many 
forms had been produced : variations in a single species in- 
habiting an isolated station might be beneficial, and thus the 
Avhole 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 ignorance on the mutual relations 
of the inhabitants of the world during the past epochs in its 

Various Objectiofis considered. 

I will here notice a few miscellaneous objections Avhich 
nave been advanced against my views, as some of the previous 
discussions may perhaps thus be made clearer; but it would 
be useless to discuss all of them, as many have been made by 
writers Avho have not taken the trouble to understand my views. 
Thus a distinguished German naturalist has recently asserted 
that the weakest part of my theory is, that I consider all or- 
ganic beings as imperfect : what I have really said is, that all 
are not as ]ierfect in relation to their conditions as they might 
be ; and this is shown to be the case by so many native forms 
in many quarters of the world yielding their places to intrud- 
ing foreigners which have become naturalized. Nor can all 
organic beings, even if they were at any one time perfectly 
adapted to their conditions of life, remain so, when the condi- 
tions slowly change, unless they likewise change; and no one 
will dispute that the physical conditions of each country, as 
well as the numbers and kinds of its inhabitants, are liable to 

It has been argued that, as none of the animals and plants 
of Egypt, of which we know any thing, have clianged dining 
the last 3,000 years, so pro1)ably none have been modified in 
any other part of the world. The many animals which have 
remained unehanged since the commencement of the glacial 
jieriod would have been an incomparably stronger case, for 
tliese have been exposed to great changes of climate and have 
migrated over great distances ; whereas, in Egypt, during the 
last 3,000 years, the conditions of life, as far as we know, have 
remained abs(jlutely uniform. The fact of little or no modifi- 


cation liuviu^ bjcn cficctcd sinco the f^lacial iicriocl would ha 
of some avail ag'ainst those who believe iu an innate and 
necessary law of development, l:)ut is powerless against the 
doctrine of natural selection or the survival of the fittest, which 
implies only that variations or individual differences of a favor- 
able nature occasionally arise in a few s])ecies, and are then 

It has been objected, if natural selection be so powerful 
an agent, why has not this or that organ been modified and 
improved? Why has not the proboscis of the hive-bee been 
lengthened so as to reach the nectar of tlie red-clover ? Why 
has not the ostrich acquired the power of llight ? But grant- 
ing that these parts and organs have varied in the right direc- 
tion — granting that there has been time sufficient for the slow 
work of natural selection, the effects being often checked, as 
they will be by intercrossing and the tendency to reversion, 
who will pretend that he knows the life-history of any one 
organic being sufficiently well to say that any particular change 
would be, on the whole, to its advantage ? Can we feel sure 
that a long proboscis would not be a disadvantage to the bive- 
bee in sucking the innumerable small flowers which it fre- 
quents ? Can we feel sure that a long proboscis would not, 
by correlation, almost necessarily give increased size to other 
parts of the mouth, perhaps interfering with the delicate cell- 
constructing Avork? In the case of the ostrich, a moment's 
reflection will show what an enormous supply of food would 
be necessary to supply force for this bird of the desert to move 
its huge body through the air. But such ill-considered objec- 
tions are hardly worth notice. 

The celebrated paleontologist, Bronn, at the close of his 
German translation of this work, asks, how, on the principle 
of natural selection, can a variety live side by side with the 
parent-species ? If both have become fitted for slightly-differ- 
ent habits of life or conditions, they might live together ; 
though, in the case of animals which freely cross and wander 
much about, varieties seem to be almost always confined to 
distinct localities. But if we put on one side polymorphic 
species, in which the variability seems to be of a peculiar na- 
ture, and all mere temporary variations, such as size, albinism, 
etc., the more permanent varieties are generally fmmd, as far 
as I can judge, inhabiting distinct stations, high land or low 
land, dry or moist districts, or distinct regions. Bronn also 
insists that distinct species never differ from each other only 


ill single cliaractcrs, ])ut in man}- parts; and he asks, how it 
comes that natural selection should invariably have afrectcd 
simultaneously many parts of the org'anization i' But there is 
not the loast necessity for believing that all the parts have been 
simultaneously modilied ; they may have been gained one after 
the other, and from being transmitted together, they appeal 
to us as if simultaneously formed. Correlation, however, will 
account for various parts changing, when any one part changes. 
We have e\'idence of this in our domestic races, which, though 
they may difl'er greatly in some one selected character, always 
diller to a certain extent in other characters. 

Bronn, again, asks how natural selection can account for 
dilTerences between species, which appear to be of no service 
to these species, such as the length of the ears or tail, or the 
folds of the enamel in the teeth, of the several species of hares 
and mire ? With respect to plants, this subject has been 
recently discussed by Niigeli in an admirable essay. He admits 
that natural selection has effected much, but he urges that the 
families of plants diil'er chiefly from each other in morphologi- 
cal characters, which seem quite imimportant for the welfare 
of the species. He consequently believes in an innate ten- 
dency toward perfection or progressive development. He 
specifies the arrangement of the cells in the tissues, and of the 
leaves on the axis, as cases in which natural selection Avould 
fail to act. To these may be added the numerical divisions 
in the parts of the flower, the position of the ovniles, the shape 
of the seed, when not of any use for dissemination, etc Prof. 
Weismann, in discussing Niigeli's essay, accounts for such dif- 
f(!rences by the nature of the varying organism under the action 
of certain conditions ; and this is the same with what I have 
called the direct and definite action of the conditions of lifo, 
causing all or nearly all the individuals of the same species to 
vary in the same manner. When we remember such cases as 
the formation of the more complex galls, and certain monstros- 
ities, which cannot be accounted for l)y reversion, cohesion, 
etc., and sudden strongly-marked deviations of structure, such 
as the ajipearance of a moss-roscj on a connnon rose, we must 
admit that the organization of the individual is capable through 
its own laws of growth, under certain conditions, of undergoing 
great modifications, independently of the gradual accumulation 
of slight inherited modifications. \'arious morphological differ- 
ences probably come under this head, to which we sliall recur; 
but many differences may at the present time be of high ser- 


vice, or may formerly have been so, although >vc arc not able 
to perceive their use ; and these will have been acted on by 
natural selection, A still larger number of mor}:)hological dif- 
ferences may certainly be looked at as the necessary result — 
through pressure, the withdrawal or excess of nutriment, an 
early-formed jiart affecting a part subsequently developed, cor- 
relation, etc. — of other adaptive changes, through wliich all 
species must have j)assed during their long course of descent 
and modilication. 

No one will maintain that we as yet know the uses of all 
the pai-ts of any one plant, or the functions of each cell in any 
one organ. Five or six years ago, endless peculiarities of 
structure in the flowers of orchids, great ridges and crests, and 
the relative positions of the various parts AVould have been con 
sidered as useless moiphological difl'erences ; but now we know 
that they are of great service, and must have been under the 
dominion of natural selection. No one at present can explain 
Avhy the leaves in a spire diverge from each other at certain 
angles ; but we can see that their arrangement is related to 
their standing at equal distances from the leaves on all sides ; 
and we may reasonably expect that the angles will hereafter 
be shown to follow from some such cause, as the addition of 
new leaves to the crowded spire in the bud, as inevitably as 
the angles of a bee"'s cell follow from the manner in which the 
insects work together. 

In certain Avhole groups of ])lants the ovules stand erect, 
and in others they are suspended ; and in some few plants 
within the same ovarium one ovule holds the former and a sec- 
ond ovule the latter position. These positions seem at first 
purely moi-jihological and of no physiological signification ; 
but Dr. Hooker informs me that, of the ovules within the same 
ovarium, in some cases the upper ones alone and in other cases 
the lower ones alone are fertilized ; and he suggests that this 
probably depcn^ls on the direction in which the pollen-tubes 
enter. If so, the position of the ovules, even when one is erect 
and the other suspended, would follow from the selection of any 
slight deviation in position which might favor their fertilization 
and the production of seed. 

Several plants Ix'longing to distinct orders habitually pro- 
duce flowers of two kinds — the one open and of the orthnary 
structure, the other closed and impM'fect. In the latter the 
l)etals are almost alwaj-s reductxl to the merest rudiments ; the 
pollen-grains are reduced in diameter ; five of the alternate 


stamens are ruilimcntury in Ononis columnas ; and in some spe- 
(•i(^s of Viola three stamens arc in this state, two retaining their 
proper fimction, but being' of very small size. In six out of 
lliirtv of tlie closed flowers in an Indian violet (name unknown, 
for the plants have not as yet jiroduced perfect flowers), the 
sepals were reduced from the normal lunnber of live to three. 
In one section of the Malpighiacea3 the closed flowers, according 
to A. de Jussieu, are still further modified, for the five stamens 
which stand opposite to tlie sepals are all aborted, a sixth 
stamen standhig opposite to a j)etal being alone developed; 
and this stamen is not present in the ordinary flowers of these 
species ; the style is aborted ; and the ovaria are reduced froni 
three to two. In all the foregoing plants the minute closed 
flowers are of high service, for they yield with perfect seciu-ity, 
!Uid with the expenditure of extremely little pollen, or other 
organized matter, a large supply of seed ; while the perfect 
flowers permit occasional crosses with distinct individuals. 
Therefore, these changes may have been, and no doubt have 
been, efTi.'cted through natural selection ; and I may add that 
nearly all the gradations between the perfect and imperfect 
flowers may sometimes be observed on the same plant. 

With respect to modifications which necessarily follow from 
other cljangcs — through the withdrawal or excess of nutriment 
— througli pressiu-e and other unknown influences — there is 
space here only for a few brief illustrations. 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, 
ojicns first, and has five sepals and petals, and five divisions to 
the ovarium ; while all the other flowers on the plant are te- 
tramerous. In the British Adoxa the uppermost flower gener- 
ally has two cjilyx-lobes with the other organs tetramerous, 
while the surrounding flowers generally have three cal^'x-lobes 
with the other organs pentamerous ; and this difference appears 
to follow from the manner in which the flowers are closely 
]> icked together. In many Compositic and Umbellifera?, and 
in some other plants, the circumferential flowers have their 
corollas much more developed tlian those of the centre; and 
this is probably the result of natural selection, for all the 
flowers are thus rendered nuich more conspicuous to those in- 
sects which are useful or even neccs.sary for their fertilization. 
In connection with the greater development of the corolla, the 


reproductive organs are frequently more or less aborted. It is 
a more curious fact that the achenes or seeds of the circum- 
ference and of the centre sometimes differ greatly in form, 
color, and other characters. In Carthamus and some othoi 
Composita^ the central achenes alone are furnished with a pap- 
pus ; and in llyoseris the same head yields achenes of three 
different forms. In certain Umbcllifeni:; the exterior seeds, ac- 
cording to Tausch, are orthospermous, and the central one 
coelospermous, and this difference has been considered by De 
Candolle as of the highest systematic importance in the family. 
If in such cases as the foregoing all the leaves, flowers, fruits, 
etc., on the same plant had been subjected to precisely the 
same external and internal conditions, all no doubt would have 
presented the same morjohological characters ; and there clearly 
woiild have been no need to call in the aid of the principle of 
progressive development. With the minute closed flowers, as 
well as ^^^tll many degraded parasitic animals, if it be assumed 
that any such aid is requisite, we sliould have to call in an in- 
nate tendency to retrogi-essive development. 

]\Iany instances could be given of morphological characters 
varying greatly in plants of the same species growing close 
together, or even on the same individual plant ; and some of 
these characters are considered as systematically important. 
I will specify only a few cases Avhich have first occurred to me. 
It is not necessary to give instances of flowers on the same 
plant being indifferently tctramcrous, pentamcrous, etc. ; but 
as when the parts are few, numerical variations are in all cases 
comparatively rare, I may mention that, according to De Can- 
dolle, the flowers of Papavcr bracteatum offer two sepals with 
four petals (and this is the common type with jioj^pies), or 
three se}>als with six petals. The manner in which the petals 
are folded in the bud is in most groups a constant morpho- 
logical character ; but Prof. Asa Gray states that with some 
species of Mimulus, the aestivation is almost as frequently that 
of the Rhinanthidea3 as c>f the Antirrhinidea?, to which tribe 
the genus belongs. Aug. St.-Hilaire gives the following cases ; 
the genus Zanthoxylon belongs to a di\-ision of the Kutacea3 
with a single ovary, but in some of the species flowers may bo 
found on the same plant, and even in the same panicle, with 
cither one or two ovaries. In Helianthennnn the capsule has 
been described as unilocular or 3-locular ; and in H. nuitabih^ 
" Une lame, plus ou moins large, sY'tend entre le pericarpe et 
Ic placenta." In the flowers of Saponaria ollicinalis, Dr. Mas- 


tcrs also observed instances of botli mar<^inal and free central 
placentation. Lastl}', St.-Hilaire foinid toward the southern 
extreme of the ranfjc of Goniphia oletvforniis two forms which 
he did not at first doubt were distinct species, but he subse- 
quently saw them g-rowinp^ on the same bush ; and he then 
adds, " A'oihl done dans uu memo individu des lof^es et un 
style qui sc rattachent tantOt il un axe verticale et tantot ;l un 

In the case of these plants, will it be said that they have 
been detected in the act of progTessin<T toward a hijjher state 
of development ? On the contrary', I should infer, from such 
characters varying;' so f^reatly, that tliey Avere of extremely 
small importance to the plants themselves, of whatever impoi-- 
tance they may be to us in our classifications. Although we 
are quite ipiorant of the excitin<^ cause of each modification, 
vet it st'cms probaljle from what we know of the relations of 
varialjility to changed conditions, that under certain conditions 
the one structure Avould have prevailed over the other, and 
thus might have been rendered almost or quite constant. 
l'>om th<? very fact of such diflerences being unimportant for 
the welfare of the species, any slight dcAiations which did 
occur Avould not be augmented or accumulated through natural 
selection ; and they would be liable to obliteration through the 
occasional intercrossing of distinct individuals. A structure 
Avhich has hccn developed through long-continued selection, 
when it ceases to be of service to the species, will generally 
become variable, as we see with rudimentary organs ; for it 
will no longer be regulated by this same power of selection ; 
but on the other hand, when from the nature of the organism 
and from a change in the conditions definite modifications have 
been produced which are imimportant for the welfare of the 
species, they may be, and apparently often have been, trans- 
mitted in nearly the same state to numerous, otherwise modi- 
fied descendants. Hair has been transmitted to almost all 
mammals, feathers to all birds, and scales to all true reptiles. 
A structure, whatever it may be, wiiich is common to many 
alli(Ml forms, is ranked by us as of high systematic importance, 
and consequently is often assimied by us in be of high vital 
importance to tin* species. Thus, as I am inclined to believe, 
morphological diilerenccs, which we consider as important — 
such as the arrangement of the leaves, the divisions of the 
ovarium, the position of the ovules, etc. — first appeared in 
many cases as fluctuating variations, which sooner or later 


became almost constant tln-ough the nature of tlic organism 
and of the surrounding' conditions, as Avell as through inter- 
crossing ; for as these morphological charactei'S do not aflfect 
the welfare of the species, any slight deviations in them would 
not be acted on or accumulated through natural selection. It 
is a strange result which we thus arrive at, namely, that char- 
acters of slight vital importance to the species, are the most 
important to the systematist; but, as we shall hereafter see 
when we treat of the genetic principle of classification, this is 
by no means so paradoxical as it at first appears. Finally, 
whatever may be thought of this view, in none of the forego- 
ing cases do the facts, as far as I can judge, afford any evi- 
dence of the existence of an innate tendency toward perfecti- 
bility or progressive development, 

I need allude only to two other objections : a distinguished 
botanist, Mr. H. C. ^\'atson, behoves that I have overrated the 
importance of divergence of character (in which, however, he 
apparently believes), and that convergence of character, as it 
may be called, has likewise played a part, Tliis is an intricate 
subject which need not be fully discussed. I will only remark 
that if two species of two allied genera, both produced a num- 
ber of new and divergent species, I can believe that they 
might sometimes approach each other so closely that they 
would for convenience' sake be classed in the same new genus, 
and thus two genera would converge into one ; but from the 
strength of the principle of inheritance, and from the two par- 
ent-species already differing and consequently tending to vary 
in a somewhat different manner, it seems hardly credible that 
the two new groups would not at least form distinct sections 
in the genus. 

Mr. AVatson has also objected that the continued action of 
natui'al selection with divergence of character will tend to 
make an indefinite number of specific forms. As far as mere 
inorganic conditions are concerned, it seems probable that a 
sufficient mmiber 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 increas- 
ing, the organic conditions of life become more and more com- 
plex. Conscquentl}' there seems at first sight no limit to the 
amount of prolitaljle diversification of structure, and therefore 
no limit to the number of species wliich might be })roduced. 
A\'e 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 astonishinnf number of spe- 
cies, many European plants have become naturalized. But 
fj^coloiry shows us, that from an early part of the long' tertiary 
jicriod the numljcr of species of shells, and that from the mid- 
dle part of this same period the numlxT of mammals, has not 
greatly or at all increased. AVhat, tluMi, checks an indefinite 
increase in the number of species? The amount of life (I do 
ncjt mean the lunnbcr of specific forms) supported on any area 
nuist have a limit, depending;' so larj^ely as it does on physical 
conditions ; therefore, if an area be inhabited by very many 
species, each or neaily each s})ecies "will be represented by few 
individuals; and such species will be liable to extermination 
fiom accidental lluctuations in the nature of the seasons or in 
the number of their enemies. The process of extermination in 
these <;!ases will be ra]>id, whereas the production of new spe- 
cies must alwa3'S be slow. Imac^inc the extreme case of as 
many species as individuals in England, and the first severe 
winter or A-ery dry summer would exterminate thousands on 
thousands of species. Kare species, and each species will be- 
come rare if the number of species in any country becomes 
indefinitely increased, will, on the principle often explained, 
j)resent within a given period few favorable variations; conse- 
<|uently, the process of giving birth to new specific forms will 
thus be retarded. When any species becomes very rare, close 
interbreeding will help to exterminate it ; authors have thought 
that this comes into play in accounting for the deterioration of 
the Aurochs in Lithuania, of Ked Deer in Scotland, and of 
liears in Norway, etc. Lastly, and this I am inclined to think 
is the most important element, a dominant species, Avhich has 
nlnMdy beaten many competitors in its own home, Avill tend to 
spread and supplant many others. Alph. de Candolle has 
shown that those species which spread widely tend generally to 
spread vcnj widely ; and, consequently, they will tend to sup- 
|)lMnt and exterminate 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 south- 
east corner of Australia, where, apparently, there are manv in- 
vaders from difTerent quarters of the world, the endemic Aus- 
tralian species have been greatly reduced in number. ITow 
nuich weight to attribute to these several considerations I do 
not jiretend to assign ; but ronjointly they must limit in each 
country the tendency to an indefinite augmentation of specific 

134 SUMMARY. Chap. IV. 

Sum)) tart/ of Chcfpter. 

If iiiulcr changing conditions of life organic beings present 
individual difiercnces in all parts of their structure, and this 
cannot be disputed ; if there bo, owing to the high geometrical 
ratio of increase of each species, a severe struggle for life at 
some age, season, or year, and tliis certainly cannot 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 extraor- 
dinary fact if no variations ever occurred useful to each be- 
ing's OAvn welfare, in the same inanner as so many variations 
have occurred useful to man. /But if variations useful to any 
organic being do ever occur, assuredly individuals thus charac- 
terized will have the best chance of being preserved in the 
struggle for life ; and from the strong principle of inheritance, 
they will tend to produce ofi'spring similarly characterized. 
This principle of preservation, or the survival of the fittest, I 
have called Natural Selection^ It leads to the improvement 
of each creature in relation to its organic and in organic condi- 
tions of life ; and consequent]}', in most cases, to what must 
be regarded as an advance in organization. Nevertheless, low 
and simple 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 modify the egg, seed, or 
young, as easily as the adult. Among many animals, sexual 
selection Avill give its aid to ordinary selection, by assuring to 
the most vigorous and best adapted males the greatest num- 
ber of offspring. Sexual selection will also give characters useful 
only to the males, in their struggles with other males ; and 
these characters Avill 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 conditions and stations, 
must be judged of by the general tenor and balance of the evi- 
dence given in the following chapters. But we already see 
how it entails extinction ; and how largely extinction has acted 
in the world's history, geology plainly tleclarcs. Natural selec- 
tion, also, leads to divergence of character ; for the more organic 
beings diverge in structure, habits, and constitution, by so much 
can a greater number be supported on the same area — of which 


we see proof by looking to the inhabitants of any small spot 
or (o naturalized productions. Therefore, durin> the modifica- 
tion of the descendants of anj- one species, and dm'in<>* the in- 
cessant stru^f^le of all species to increase in numbers, the more 
diversified these descendants become, tlic better will be their 
chance of succccdinrr in the battle for life. Thus, the small 
dill'erences distin<]^uishint^ varieties of the same species, steadily 
tend to increase, till they come to equal the greater diflfereuces 
between species of tlie same genus, or even of distinct genera. 

We have seen that it is the common, the widely-dilfused, 
and widely-ranging species, belonging to the larger genera 
within each class, which vary most; and these tend to transmit 
((> their modified offspring that superiority which now makes 
them dominant in their own countries. Natural selection, as 
has just been remarked, leads to divergence of character and to 
much Extinction of the less improved and intermediate forms of 
life. On these principles, the nature of the affinities, and the 
generally AvoU-dellned distinctions of the innumerable organic 
beings in each class througliout the Avorld, may be explained. 
It is a truly wonderful fact — the wonder of M'hich we are apt 
to overlook from familiarity — that all animals and all plants 
tliroughout all time and space should be related to each other 
in natural groups subordinate to groups, in the manner 
which Ave everywhere behold — namely, varieties of the same 
species most closely related together ; species of the same genus 
less closely and unequally related together, forming sections 
and sub-genera ; species of distinct genera much less closely 
related, and genera related m difierent degrees, forming sub- 
families, families, orders, sub-classes, and classes. The several 
subordinate groups in any class cannot be ranked in a single 
lile, but seem rather to be clustered round points, and these 
round other points, and so on in almost endless cycles. If 
each species has been independently created, no explanation 
rm be given of this great fact in the classification of all organic 
beings; but it is explained through inheritance and tlie com- 
plex action of natural selection, entailing extinction and diver- 
gence of character, as we have seen illustrated in the diagram. 

The affinities of all the beings of the same class have some- 
limes been repres(Mited by a great tree. I believe this simile 
largely speaks (lie trulli. The green and budding twigs may 
represent existing species ; and those produced during former 
years may represent the long succession of extinct species. At 
each period of growth all the growing twigs have tried to 

133 SUMMARY. CuAr. IV. 

brancli out on all siili^s, and to overtop and kill the surrounding 
t\vii>s and l)ranchcs, in the same manner as species and groups 
of speeies have at all times overmastered other species in the 
great battle for life. The limbs, di\'ided into great branches, 
and those into lesser and lesser branches, were themselves once, 
when the tree Avas small, budding twigs ; and this connec- 
tioTi of the former and present Inids by ramifying branches may 
well represent the classification of all extinct and living species 
in groups subordinate to groups. Of the many twigs which 
nourished when the tree was a mere bush, only two or three, 
now grown into great branches, yet survive and bear all the 
other branches ; so with the species which lived during long- 
])ast geological periods, very few now have living and modified 
descendants. From the first growth of the tree, many a limb 
and branch has decayed and dropped off; and these lost 
branches of various sizes may represent those whole orders, 
families, and genera, which have now no living representatives, 
and whicli are known to us only from being found in a fossil 
state. As we here and there see a thin, straggling branch 
sj)ringing 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 Ornithorh^mchus or Lepi- 
dosiren, which in some small degree connects by its affinities 
two large branches of life, and which has apparently been 
saved from fatal competition by having inhabited a protected 
station. As buds give rise by growth to fresh buds, and these, 
if vigorous, branch out and overtop on all sides many a feebler 
branch, so by generation I believe it has been Avith the great 
Tree of Life, which fills with its dead and broken branches the 
crust of the earth, and covers the surface with its ever-branch- 
inc: and beautiful ramifications. 


ciiArxER y. 


Effects of clmntrofl Coiuiillont'—T'iJo and Dis>nso, combined with Natural Selection; 
Orirtiiii (if Fliirlit and of Vision— Acclimatization— Correlated Variatiou— Com- 
pensation and Keonoiny of (imwtli— False C:orrclation» — Mnlti|)le, Uudinicntary, 
and l.iiwly-or^'anized Stnietuies< variable— Parts developed in an Unusual Man- 
ner are hij;ldy variable : ?>poeific Characters more variable ttwn Generic: Second- 
ary Sexual characters variable— Species of the t^anic Genus vary in au analoguua 
llauucr— Ueverbioua to long-lost Charactera — Sumaiary. 

. I HAVE hitherto sometimes spoken as if the variations — so 
comn'ion and multilorin with org'anic beings nnder domestication, 
and in a lesser degree with those in a state of nature — had 
l)een due to cliance. This, of course, is a wholly incorrect ex- 
pression, but it serves to acknowledge plainly our ignorance 
of the cause of each particular variation. Some authors be- 
lieve it to be as much the function of the reproductive system 
to produce individual difTerences, or slight deviations of struc- 
tuie, 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 varia- 
l>ility of species having wide ranges than of those having re- 
stricted ranges, lead to the conclusion that variability is directly 
related to the conditions of life to Avhich each species has been 
exposed during several successive generations. In the first 
cliapter I attempt(>d to show that changed conditions act in 
two ways, directly on the whole organization or on certain 
])ar(s alone, jind indirectly through the reproductive sj'stcm. 
Jn all cases th(M-e ate two factors, the nature of the organism, 
which is nuich the most important of the two, and the nature 
of the conditions. The direct acticm of changed conditions, 
leads to (h'Tmite or indefinite results. In the latter case the 
organization seems to become plastic, and we have nnich fluc- 
tuating variability. In the former case the nature of the or- 
ganism is such that it yields readily, when subjected to cer- 
lain conditions, and all, or nearly all the individuals become 
modified in the same way. 


It is very difficult to decide how far clianii^cd conditions, 
such as of climate, food, etc., have acted in a definite manner. 
There is some reason to believe that in the course of time the 
effects have been greater than can be proved to be the case by 
any clear evidence. But we may safely conclude that the in- 
niunerable complex coadaptations of structure which we see 
throughout Nature between various organic beings, cannot be 
attriljuted simply to such action. In the following cases the 
conditions seem to have produced some slight definite effect : 
E. Forbes asserts that shells at their southern limit, and when 
living in shallow water, are more brightly colored than those 
of the same species from farther north or from a greater depth; 
but these statements have lately been disputed, Mr. Gould 
believes that birds of the same species are more brightly col- 
ored under a clear atmosjihere, than when living on islands or 
near the coast ; and WoUaston is convinced that residence near 
tlie sea affects the colors of insects. Moquin-Tandon gives a of plants which when growing near the sea-shore have their 
leaves in some degree ileshy, though not elsewhere fleshj'. 
Other similar facts could be given. 

The fact of varieties of one species, when they range into 
the habitations of other species, often acquiring in a slight 
degree some of their characters, accords Avith the view that 
species arc only well-marked and permanent varieties. Thus 
the species of shells Avhich are confined to tropical and shal- 
low seas are generally brighter-colored than those confined to 
cold and deeper seas. The birds Avhich are confined to conti- 
nents are, according to Mr. Gould, brighter-colored than those 
of islands. The insect-species confined to sea-coasts, as every 
collector knows, are often brassy or lurid. Plants which live 
exclusively on the sea-side are very apt to have Ileshy leaves. 
He who believes in the creation of each species, will have to 
say that this insect, for instance, was created of a brassy color, 
because it was intended to live near the sea, but that this other 
insect became brassy through variation as soon as it reached 
the sea-coast. 

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. Tlius, it is avcU known to furriers that ani- 
mals of the same species have thicker and better fur tlie far- 
tlier north they live ; but who can tell liow much of this dif- 
ference may be due to the warmest-clad individuals having 


1)ftM» favored and preserved during many generations, and how 
much lo the action of tlie severe climate ? for it would appear 
that climate has some direct action on the hair of our domestic 

Instances could be given of similar varieties being produced 
from the same species under external conditions of life as dif- 
ferent as can well be conceived ; and, on the other hand, of 
dissimilar varieties being produced imder apparently the same 
external conditions. Again, innumerable instances are known 
to ev(;ry naturalist, of species keeping true, or not varying at 
all, although living und<'r the most opposite climates. Such 
considerations as these incline me not to lay much weight on 
the direct and definite action of the conditions of life; but I 
fully admit that strong arguments of a general nature may be 
advanced on the other side. 

In one sense the conditions of life may be said, not only to 
cause variability, but likewise to include natural selection ; for 
the conditions determine whether this or that variety shall sur- 
yi\i.\ ]3ut when man is the selecting agent, we clearly see 
that the two elements of change are distinct ; the conditions 
cause the variability; the will of man, acting either consciously 
or unconsciously, accumulates the variations in certain direc- 
tions, and tills answers to the survival of the fittest under 

J^Jfects of Use and Disrise, as controlled hy Natural Selection. 

From the facts alluded to in the first chapter, I think there 
can be no doubt that use in our domestic animals strengthens 
and enlarges certain parts, and disuse diminishes them ; and 
that such modifications are inherited. Under free Nature, we 
liave no standard of comparison, by which to judge of the 
elfecls of long-continued use or disuse, for "we know not the 
parent-fonns; but many animals have structures which- can be 
(•xi)lain<'d by the efl'ects of disuse. As Prof. Owen has remarked, 
llierc is no greater anomaly in Nature than a bird that cannot 
l!y ; yet there are several in this state. The logger-headed duck 
of South America can only llap along the surface of the water, 
and has its wings in nearly the same condition as the domestic 
Aylesbury duck. As the larger ground-feeding birds seldom 
tak(> fiight except to escap<> danger, I believe that the nearly 
wingless condition of several birds, which now inhabit or have 
lately inhabited several oceanic islands, tenanted by no beast 


of prey, lias been caused by disuse. The ostricli, indeed, in- 
Imbils continents and is exposed to dann^er from -wliicli it cannot 
escape by llii^^lit, but by kickinp^ it can defend itself from ene- 
mies, as well as any of the smaller ciuadrupeds, "We may be- 
lieve that the progenitor of the ostrich fj^enns had habits like 
those of a bustard, and that, as natural selection increased in 
successive g-enerations the size and weight of its body, its legs 
were used more, and its wings less, until they became incapa- 
ble of flight. 

Kirby has remarked (and I have observed the same fact) 
that the anterior tarsi, or feet, of many male dung-feeding 
beetles are very often broken off ; he examined seventeen spe- 
cimens in his own collection, and not one had even a relic left. 
In the Onites apclles the tarsi are so habitually lost, that the 
insect has been described as not having them. In some other 
genera they are present, but in a rudimentary condition. In 
the Ateuchus or sacred beetle of the Egyptians, they are totally 
deficient. The evidence that accidental mutilations can be in- 
herited is at present not quite decisive ; but the remarkable 
case observed by Brown-Sequard of inherited epilepsy in guinea- 
pigs, caused by an operation performed on the spinal cord, 
should make us cautious in denying such power. Hence it 
Avill ]>ei]iaps be safest to look at the entire absence of the an- 
terior tarsi in Ateuchus, and their rudimentary condition in 
some other genera, as due to the effects of long-continued 
disuse; for, as many dung-feeding beetles are generally found 
with their tarsi lost, this nnist happen earlv in life; therefore 
tlie tarsi cannot be of much impoi'tance or be nnich used by 
these insects. 

In some cases we might easily put do"^^'n to disuse modifi- 
cations of structure which are wholly, or mainly, due to natural 
selection. Mr. AYollaston has discovered the remarkable fact 
that ::300 beetles, out of the 550 species (but more are now 
known) which inhabit Madeira, are so far deficient in wings 
that they cannot fly; and that, of the twent^'-nine endemic 
genera, no less than twenty-three genera have all their species 
in this condition ! Several facts — namely, that beetles in many 
parts of the world are frequently blown to sea and perish ; 
that the beetles in Madeira, as observed by Mr. AVollaston, lie 
much concealed, until the wind lulls and the sun shines ; that 
the ])roportion of wingless beetles is larger on the exposed 
Desertas than in Madeira itself; and especially the extraor- 
dinary fact, so strongly insisted on by Mr. AVoUaston, of the 

CiiAP. V. EFFECTS OF USE AND DISUSE. 141 entire abs'jnco of certain lar^jo groups of beetles, else- 
where excessively numerous, and which groups have ha])its of 
life almost necessitating frequent flight — tliese several con- 
siderations have made me believe that the wingless condition 
of so many Madeira beetles is mainly due to the action of 
natural selection, but combined probably with disuse. For 
during many successive generations each individual beetle 
which Hew least, cither from its wings having been ever so 
little less perfectly developed or from indolent habit, will have 
had the best chance of surviving from not being blown out to 
sea ; and, on the other hand, those beetles whicli most readily 
took to flight would oftenest have been blown to sea and thus 
have been destroyed. 

The insects in Madeira which arc not ground-feeders, and 
which, as the flower-feeding coleoptera and lepidoptera, must 
habilually use their wings to gain their subsistence, have, as 
Mr. Wuilaslon suspects, their wings not at all reduced, but 
even elilarged. This is quite compatible with the action of 
natural selection. For when a new insect first arrived on the 
ishind, the tendency of natural selection to enlarge or to reduce 
the wings, would depend on whether a greater number of in- 
dividuals were saved by successfully battling with the winds, 
or by giving up the attempt and rarely or never flying. As 
wilh mariners shipwrecked near a coast, it would liave been 
bcttiT for the good swimmers if they had been able to swim 
still farther, whereas it would have been better for the bad 
swinnners 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 arc 
rudimentary in size, and in some cases are quite covered up ])y 
skin and fur. Tliis state of the eyes is probably due to gradual 
reduction from disuse, but aided perhaps by natural selection. 
In Soutli America, a burrowing rodent, the tuco-tuco, or Ctcn- 
omys, is even more subt(«rranean in its habits than the mole; 
.and 1 was assured by a Spaniard, who had often caught them, 
that th(»y were frequently blind; one which I kept alive was 
certainly in this condition, the cause, as appeared on dissection, 
having been inllammation of the nictitating membrane. As 
frefjuent inflammation of the eyes nnist be injurious to any 
animal, and as eyes are certaiidy not necessary to aiiimals 
having su])terranean 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 
conslanlly aid the efl'ects of disuse. 

It is well known that several animals, belonging to the 
most different classes, which inhabit the caves of Carniola and 
of Kentucky, are blind. In some of the crabs the foot-stalk for 
tlie eye remains, though the eye is gone ; the stand for the 
telescope is there, though the telescope with its glasses has 
been lost. As it is difficult to imagine that eyes, though use- 
less, could be in any way injurious to animals living in dark- 
ness, I attribute their loss wholly to disuse. In one of the 
bUnd animals, namely, the cave-rat (Ncotoma), two of which 
were cajotured by Prof. Silliman at above half a mile dis- 
tance fj-om the moutli 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 Prof. Silliman, after 
having been exposed for about a month to a graduated light, 
acquired a dim perception of objects. 

It is difficult to imagine conditions of life more similar tlian 
deep limestone caverns imdcr a nearly similar climate ; so that, 
on the common view of the blind animals haWng been sepa- 
rately created for the American and European caverns, very 
close similarity in their organization and affinities might have 
been expected. This is certainly not the case if we look at the 
two whole faunas ; and, with respect to the insects alone, 
Schiodte has remarked : " We are accordingly prevented from 
considering the entire phenomenon in any other light than 
something purely local, and the similarity which is exhibited 
in a few forms between the INIannnoth Cave (in Kentucky) and 
the caves in Carniola, otherwise than as a very plain expression 
of that analogy which subsists generally between the fauna of 
Europe and of North America." On my view we must suppose 
that American animals, having in most cases ordinary powers of 
vision, slowly migrated by successive generations from the outer 
world into the deeper and deeper recesses of the Kentucky 
caves, as did European animals into the caves of Europe. We 
have some evidence of this gradation of habit ; for, as Schiodte 
remarks, " We accordingly look upon the subterranean faunas 
as small ramifications which have penetrated into the earth from 
the geograjihically limited faunas of the adjacent tracts, and 
which, as they extended themselves into darkness, have been 
accommodated to surrounding circumstances. Animals not far 
remote from ordinary forms, proiwre the transition from light 
to darkness. Next follow those that are constructed for twi- 


li^lit ; and, last of all, tlioso destined for total darkness, and 
Aviiose formation is (juitc peculiar." These remarks of Schiiidte's, 
it should be understood, apply not to the same, but to distinct 
species. ]Jy the time that an animal had reached, after num- 
berless generations, the deepest recesses, disuse will on this 
view have more or less perfectly obliterated its eyes, and 
natural selection will often have ell'ected other changes, such 
as an increase in the length of the antenmu or palpi, as a com- 
pensation for l)lindness. Notwithstanding such modifications, 
we might expect still to sec in the cave-animals of America, 
allinities to tlie 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 
hi^ar from Prof. Dana ; and some of the European cave-insects 
arc very closely allied to those of the surroimding countrv. It 
woulil be most dillicult to give any rational explanation of the 
allinities of the blind cave-animals to the other inhabitants of 
the two continents on the ordinary view of their independent 
creation. That several of the inhabitants of the caves of the 
Old and New Worlds should l)e closely related, we might ex- 
pect from the Avell-knowu relationship of most of their other 
jiroductions. As a blind species of Bathyscia is found in abun- 
dance on shady rocks far from caves, the loss of vision in the 
cave-species of this one genus has probably had no relation to 
its dark habitation ; and it is very natural that an insect al- 
ready deprived of vision should readily become adapted to 
dark caverns. Another blind genus (Anophthalmus) offers 
this remarkable peculiarity: the several distinct species, as Mr. 
MuiTay has remarked, iidiabit several distinct European caves 
and likewise those of Kentucky, and the genus is found no- except in caves ; but it is possible that the progenitor 
or progenitors of these several spec-ies, while furnished with 
eyes, formerly may have ranged widely over both continents, 
and then have become extinct, excepting in their ])resent se- 
cluded abodes. Far from feeling surprise that some of the 
rave-animals should be very anomalous, as Agassis has remarked 
in regard to flu' blind fisli, the Amblyopsis, and as is the case 
with the blind I'rotcus with refertMice to the reptiles of Europe, 
1 am only surprised that more wrecks of ancient HA; have not 
]>ccn preserved, owing to tin; less severe competition to which 
the inhabitants of these daik abodes must have been exposed. 



Habit is hereditary with plants, as in the period of flower- 
iii<T, in the amount of raui requisite for seeds to germinate, in 
the time of sleep, etc., and tliis leads me to say a few words on 
acclimatization. As it is extremely common for species of the 
same genus to inhaljit hot and cold countries, if it be true that 
all the species of the sanic g-cnus are descended from a single 
parent-form, acclimatization must be readily effected during a 
long course of descent. It is notorious that each species 
is adapted to the climate of its own home : species from an 
arctic or even from a temperate region cannot endure a tropical 
climate, or conversely. So, again, many succulent plants can- 
not endure a damp climate. But the degree of adaptation of 
species to the climates under which they live is often overrated. 
We may infer this from our frequent inability to predict 
whether or not an im})ortcd 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 cli- 
mates. But whether or not the adaptation be generally very 
close, we have evidence, in the case of some few plants, of 
their becoming, to a certain extent, naturally habituated to 
diff(>rent temperatures ; that is, they become acclimatized: thus 
the pines and rhododendrons, raised from seed collected by Dr. 
Hooker from the same species gro^^^ng at different heights on 
the Himalaya, were found in this country to possess different 
constitutional powers of resisting cold. ^Ir. Thwaites informs 
me that he has observed similar facts in Ceylon ; analogous 
observations have been made by Mr. H. C. Watson on Euro- 
pean species of plants brought from the Azores to England ; 
and I could give other cases. In regard to animals, several 
autlientic instances could be adduced of species within histori- 
cal times having largely extended their range from warmer to 
cooler latitudes, and conversely ; but we do not positively know 
tliat these animals were strictly adapted to their native climate, 
though in all ordinary cases we assume such to be the case ; 
nor do we know that they have subsequently become specially 
acclimatized to their new homes, so as to be better fitted for 
lliiMU than they Avere at first. 

As we may infer that our domestic animals were originally 


chosen by uncivilized nuin because they ■were useful^ and be- 
cause they bred readily inider confinement, and not because 
they were subsequently found cai)ablc of far-extended trans- 
portation, the common and extraordinary capacity in our do- 
mestic animals of not only withstandinf^ the most dillcrent 
climates, but of beiii<T perfectly fertile (a far severer test) under 
tliem, may be used as an argument that a large proportion of 
other animals now in a state of nature could easily be brought 
to bear widely-different climates. We must not, however, 
push the foregoing argument too far, on account of the prob- 
able origin of some of our domestic animals from several wild 
stocks ; the blood, for instance, of a tropical and arctic wolf or 
wild dog may perhaps be mingled in our domestic breeds. 
The rat and mouse cannot be considered as domestic animals, 
but they have been transported by man to many parts of the 
world, and now have a far wider range than any other rodent ; 
for they, live under the cold climate of Faroe in the north and 
of the Falklands in the south, and on many islands in the tor- 
rid zones. Hence adaptation to an}' special climate may be 
looked at as a quality readily grafted on an innate wide ilexi- 
bility of constitution, common to most animals. On this view, 
the ca]iacity of enduring the most different climates by man 
himself and by his domestic animals, and the fact of the extinct 
elephant and rhinoceros ha\nng formerly endured a glacial cli- 
mate, whereas the living species are now all trojiical or sub- 
tropical in their habits, ought not to be looked at as anomalies, 
but as examples of a very common flexibility of constitution, 
brought, under peculiar 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 se- 
lection of varieties having diilerent innate constitutions, and 
how much to both means combined, is an obscure question. 
Tliat habit or custom has some influence I nuist believe, both 
from analogy and from the incessant advice given in agricul- 
tural works, even in the ancient Encyclopicdias of China, to be 
very cautious in trans])orting animals from one district to 
another; for as it is not likely that man should have succeeded 
in selecting so many breeds and sub-breeds with constitutions 
specially fitted for their own districts, the result nuist, I think-, 
be due to habit. On the other hand, natural si^lection would 
inevitably tend to preserve those individuals wliich were born 
with constitutions l)est adapted io any country which they in- 
li ibih'd. In treatises on many kinds of cultivated plants, ccr- 


t:\in varieties are said to withstand certain climates better than 
others ; this is verv strikingly shown in works on fruit-trees 
published in the United States, in which certain varieties are 
liabituallj recommended for the Northern and others for the 
Southern States ; and, as most of these vari(>tics arc of recent 
origin, they cannot owe their constitutional differences to hab- 
it. The case of the Jerusalem artichoke, which is never propa- 
gated in England by seed, and of which consequently new va- 
rieties have not been produced, has even been advanced — for 
it is now as tender as ever it was — as proving that acclimatiza- 
tion cannot be effected ! Tlie case, also, of the kidney-bean 
has been often cited for a similar purpose, and with much 
greater weight; but until some one will sow, during a score of 
generations, his kidney-beans so early that a very large pro- 
portion 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 precau- 
tions, the experiment cannot be said to have been tried. Nor 
let it be supposed that differences in the constitution of seed- 
ling kidney-beans never appear, for an account has been pub- 
lished how much more hardy some seedlings were than others; 
and of this fact I have myself observed striking instances. 

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

Correlated 'Variation. 

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 accumulated 
through natural selection, other parts become modified. This 
is a very important subject, most imperfectly understood, and 
no doubt totally different classes of facts may be here easily 
confounded together : we shall presently see that simple in- 
heritance often gives the false appearance of correlation. The 
most obvious instance of real correlation is, that variations of 
structure arising in the young or in the larvae naturally tend 
to affect the structin-e of the mature animal ; in the same man- 
ner as ;my malconfonnation in the early embryo is knoAvn 


seriously to .ifFect the whole org'anization of the adult. The 
several parts of the body which are homolof^ous, and which, 
at an early embryonic period, are identical in structure, and 
which are necessarily exposed to similar conditions, seem emi- 
nently 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 
bs homologous with the limbs. These tendencies, I do not 
doubt, may be mastered more or less completely by natural 
selection : thus a family of stags once existed with an antler 
only on one side ; and if this had been of any great use to the 
breed, it might probably have been rendered permanent by 

Homologous parts, as has been remarked by some authors, 
tend to cohere ; this is often seen in monstrous plants : and 
nothing is more common than the union of homologous parts 
in normal structures, as the imion of the petals of the corolla 
into a tube. Hard parts seem to affect the forms of adjoining 
soft parts ; it is believed by some authors that the diversity in 
the shape of the pelvis in birds causes the remarkable diver- 
sity in the shape of their kidneys. Others believe that the 
shape of the pelvis in the human mother influences by pressure 
the shape of the head of the child. In snakes, according to 
Schlegol, the shape of the body and the manner of swallowing 
determine the position and form of several of the most impor- 
tant viscera. 

The nature of the bond of correlation is frequently quite 
obscure. M. Isidore Geoffroy St.-Hilaire has forcibly remarked 
that certain malconformations very frequently, and that others 
rarely coexist, without our being able to assign any reason. 
^\''hat can be more singular than in cats the relation between 
complete whiteness with blue eyes and deafness, or between 
the tortoise-shell color and the female sex? or in pigeons be- 
tween their feathered feet and skin betwixt the outer toes, or 
b;^tween the jiresence of more or less down on the young birtl 
when first hatched, with the future color of its plumage ; or, 
again, the relation between the hair and teeth in the naked 
Turkish dog, though here no doubt homology comes into play? 
With respect to tliis latter case of correlation, I think it can 
hardly be accidental, that, if we pick out the two orders of" 
mammals which are most abnormal in their dermal covering, 
viz., Cetacca (whales) and Edentata (amindilloes, scaly ant- 


eaters, etc.), these are likewise the most abnormal in tlieir 

I know of no case better adapted to show the importance 
of the laws of variation and correlation, independently of util- 
ity and therefore of natural selection, than that before referred 
to, of the difference between the outer and inner flowers in 
some Conipositous and Umbelliferous plants. Every one knows 
the dilTercnce in the ray and central florets of, for instiuice, th{^ 
daisy, and this difference is often accompanied with the partial 
or complete abortion of the rejoroductive organs. But in some 
of these plants, the seeds also differ in shape and sculpture. 
These differences have been attributed by some authors to the 
I^rcssure of the involucra on the florets, or to their mutual press- 
in-e, and the shape of the seeds in the ray-florets of some Com- 
positie countenances this idea; but with the Umbellifene, it 
is by no means, as Dr. Hooker informs me, the species with 
the densest heads Avhich most frequently differ in their inner 
and outer flowers. It might have been tliought that the de- 
velopment of the ray-pc^tals by drawing- nourishment from the 
r(>productivc organs had caused their abortion ; but this can 
hardly be the S(jle cause, for in some Compositir; the seeds of 
the outer and inner florets differ without any difference in the 
corolla. Possibly these several differences may be connected 
Avith the diflcrent flow of nutriment toward tlie central and 
external flowers : we know, at least, that, with normally iireg- 
ular flowers, those nearest to the axis are most subject to 
jK'loria, that is, they become S3'mmetrical. I may add, as an 
instance of this, and of a striking case of correlation, that I 
have recently observed in many pelargoniums, that in the cen- 
tral flower of the truss the two upjier petals often lose their 
])atches of darker color; and when this occurs, the adherent 
nectary is qviite aborted. 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 developmoit of the corolla in the cen- 
tral and exterior flowers, Sprengel's idea that the ray-florets 
serve to attract insects, whose agency is highly advantageous 
or necessary for the fertilteation of these plants, is highly jirob- 
able ; and if so, natural selection may have come into i^lay. 
]>ut, witli respect to the seeds, it seems impossible that their 
diiVcrences in shape, which ai-e not ahvaj's correlated with any 
difference in the corolla, can be in any way beneficial : yet in 
the Umbellifere these differences arc of such apparent impor- 


tiincc — the seeds bcinj^ sometimes orlliospermons in tlic exte- 
rior llowers and ca-lospennoiis in the central ilowors — that the 
elder iJe Caiidollo founded his main divisions in the order on 
such char;- -ters. Hence, as before remarked, wc see that modi- 
fications of structure, viewed by systematists as of hi^h value, 
may be AvlK)lly due to the laws of variation and correlation, 
without beiuf^, as far as we can judf^e, of the slightest service 
t(^ the species. 

We may often falsely attribute fo correlated variation 
structures which arc common to whole groups of species, and 
which in truth arc simply due to inheritance : for an ancient 
progenitor may have acquired through natural selection some 
one modification in structure, and, after thousands of genera- 
tions, some other and independent modilication ; and these 
two modiiications, liaving been transmitted to a whole group 
of descendants witli diverse habits, would naturally l^e tlumght 
_to be correlated in some necessary manner. Some correlations 
are apparently due to the manner in wliich natural selection 
acts. For instance, Al]ih. de Candollc has remarked that 
winged seeds are never found in fruits wliich do not open : I 
should ex})lainthis rule by the impossiljility of seeds gradually 
becoming winged through natural selection, unless the ca])- 
sules first opened themselves ; for in this case alone could the 
seeds, which were a little better adapted to be Avafted by the 
wind, gain an advantage over those less well fitted for wide 

Compensation and Economy of Groxcth. 

llie elder GeoiTroy and Goethe propounded, at about the 
same period, their law of compensation or balanccment 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 domestic produc- 
tions: if nourishment flows to one part or organ in excess, it 
rarely flows, at least in excess, to another part; thus it is dif- 
ficult to get a cow to give nuicli milk and to fatten readily. 
The same varieties of tlic cabbage do not yield abundant and 
luitritious foliage and a copious supply of oil-bearing seeds. 
When the seeils in our fruits become atrophied, tlie fruit itself 
gains largely in size and quality. In our poultry, a large tuft 
of feathers on the head is generally accompanied by a dimin- 
ished comb, and a large beard by diminished wattles. With 


species in a state of nature it can hardly be maintained that 
the law is of universal application ; but many jrood observers, 
more especially botanists, believe in its truth. I will not, how- 
ever, here give any instances, for I see hardly any way of dis- 
tinguishing between the effects, on the one hand, of a part 
being largel}' developed through natural selection and another 
and adjoining part being reduced by this same process or by 
disuse, and, on the other hand, the actual willidrawal of nutri- 
ment from one part owing to the excess of growth in another 
and adjoining part. 

I suspect, also, that some cases of compensation which have 
been advanced, and likewise some other facts, may be merged 
inider a more general principle, namel}', that natural selection 
is continually trying to economize in every part of the organ- 
ization. If under changed conditions of life a structure before 
useful becomes less useful, any diminution, however slight, in 
its development, Avill be seized on by natural selection, for it 
will ]iroiit 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 cirri- 
pedes, and of which many other instances could be given : 
namely, that when a cirripede is parasitic within another and 
is thus protected, it loses more or less completely its own shell 
or carapace. This is the case with the male Ibla, and in a 
truly extraordinary manner with the Proteolepas ; for the car- 
apace in all other cirripedes consists of the three highly-impor- 
tant anterior segments of the head enormously developed, and 
furnished with great nerves and muscles ; but in the parasitic 
and protected Proteolepas, the whole anterior part of the head 
i.s reduced to the merest rudiment attached to the bases of the 
prehensile antenna?. Now the saving of a large and complex 
structure, when rendered superfluous by the parasitic habits of 
the Proteolepas, though effected by slow steps, would be a 
decided advantage to each successive individual of the species ; 
for in the struggle for life to which every animal is exposed, 
each individual Proteolepas would have a better chance of 
supporting itself, by less nutriment being wasted in develop- 
ing a structure now become useless. 

Thus, as I believe, natural selection will always succeed in 
the long-run in reducing and saving every part of the organi- 
zation, as soon as it is rendered by changed habits of life super- 
fluous, without by any means causing some other part to be 
largely develojicd in a corresponding degree; and, converse- 


ly, that natural selection may perfectly well succeed in largely 
developing any organ, without requiring as a necessary com- 
pensation the reduction of some adjoinhig part. 

Mullij)le, liudimentaryy and Loichj-organized Structures are 


It seems to be a rule, as remarked by Is. Geoffroy St.-IIi- 
laire, both in varieties and in species, that, when any part or 
organ is repeated many times in the structure of the same 
individual (as the vertebnu in snakes, and the stamens in poly- 
androus ilowers), tlie number is variable; whereas the number 
of the same part or organ, when it occurs in lesser num])ers, is 
constant. The same author and some botanists have further 
remarked that multiple parts are also very liable to variation 
in structure. Inasmuch as this " vegetative repetition," to use 
Prof. Owen's expression, seems to be a sign of low organiza- 
tion, the foregoing remark seems connected with the very gen- 
eral opinion of Jiaturalists that beings low in the scale of Na- 
ture arc more variable than those which are liigher. I pre- 
sume that lowncss in this case 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 diver- 
sified Avork, wo can perhaps see Avhy it should remain variable, 
that is, why natural selection should not have preserved or 
rejected eacli little deviation of form so carefully as when the 
part had to serve for one special purpose alone — in the same 
way that a knife which has to cut all sorts of things may be of 
almost any shape, while a tool for some particular j^urpose 
had better be of some particular shape. Natural selection, it 
should never be forgotten, can act on each part of each being, 
solely through and for its advantage. 

Kudimentary parts, it has been stated by some authors, and 
I believe with truth, are apt to be highly varialjle. AVe shall 
have to recur to tlie general subject of rudimentary and aborted 
organs; and I will here only add tliat tlieir variability seems 
to be owing to their uselessness, and therefore to natiu-al selec- 
tion having no power to check deviations in their structure. 
Thus rudimentary parts are left to the free play of the various 
laws of growth, to the effects of long-continued disuse, and to 
tlic tendency to reversion. 


A Part developed in any F^pccks in an extraordinary Degree 
or Manner, in comparison with the same Part in allied 
JS^)ccic^, tends to he highly variable. 

Several years ago I was much struck by a rcmaik, to the 
above effect, made by Mr, Waterliouse ; Prof. Owen, also, 
seems to have come to a nearly similar conclusion. It is hope- 
less to attempt to convince any one of the truth of the aljovc 
j)roposition witliout giving the long array of facts which I have 
collected, and which cannot possibly be here introduced. I can 
only state my conviction that it is a rule of high generality. 
I am aware of several causes of error, but I hope that I have 
made due allowance for them. It should be understood that 
the rule by no means applies to any part, however unusually 
developed, unless it be unusually developed in comparison with 
the same part in closely-allied species. Thus, the wing of the 
bat is a most abnormal structure in the class of mammalia ; 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 develojoed in a remarkable manner in comparison with 
other species of the same genus. The rule applies very strongly 
in the case of secondary sexual characters, when displayed in 
any unusual manner. The term, secondary sexual characters, 
used by Hunter, applies to characters which are attached to 
one sex, but are not directly connected Avith the act of repro- 
duction. The nde applies to males and females ; but as females 
more rarely offer remarkable secondary sexual characters, it ap- 
plies more rarely to them. The rule being so plainly applicable 
in the case of secondary sexual characters, may be due to the 
great variability of these characters, whether or not displayed 
in any unusual manner — of Avhich fact I think there can be 
little doubt. But that our rule is not confined to secondary 
sexual characters is clearly shown in the case of hermaphrodite 
cirripedes ; I particularly attended to ^Ir. \Vaterhouse's remark, 
while investigating this order, and I am fully convinced that 
the rule almost invariably holds good. I shall, in a future; 
work, give a list of all the more remarkable cases ; I will lierc 
only give one, as it illustrates the rule in its largest application. 
The opercular valves of sessile cirripedes (rock barnacles) are, 
in every sense of the word, very important structures, and they 
diller extremely little even in different genera ; but in the several 
species of one genus, Pyrgoma, these valves present a marvel- 
lous amount of diversification ; the homologous valves in the 


different species beinp^ sometimes wlioUy unlike in shape; and 
the amount of variation in the individuals of the same speciey 
is so great, that it is no exaf^goration to state that the varieties 
differ more from each other in the characters derived from these 
important valves than do other species belonging to distinct 

As birds within the same country vary in a remarkably 
small degree, I have jiarticuhu-ly attended to them, and the 
rule seems to me certainly to hold good in this class. I cannot 
make out that it applies to plants, and this would seriously 
have shaken my belief in its truth, had not the great variability 
in plants mad(; it jiarticularly dillicult to compare their relative 
degrees of variabilit}'. 

When we see any part or organ developed in a remarkable 
degree or maimer in any species, the fair presumption is, that 
it is of high importance to that species ; nevertheless it is in 
this case eminently liable to variation. Why should this be 
so ? On the view that each species has been independently 
created, with all its ])arts as we now see them, I can see no 
explanation. But on the view that groups of species have 
descended from other species, and have been modified through 
natural selection, T think we can obtain some light. First, let 
me remark that if any part in our domestic animals, or the 
whole animal, be neglected and no selection be applied, that 
I)art (for instance, the comb in the Dorking fowl) or the whole 
bref'd will cease to have a imiforin character. The breed will 
then be said to have degenerated. In rudimentary organs, and 
in tijose which have been but little specialized for an}'' partic- 
ular purpose, and perhaps in poljmiorphic groups, we see a 
nearly parallel natural 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 
concerns us is, that those points in our domestic animals, whicli 
at the present time are undergoing rapid change by continued 
selection, are also eminently liable to variation. Look at the 
breeds of the pigeon ; see what a prodigious amount of differ- 
ence there is in tht; beaks of tumblers, in the beaks and wattle 
of carriers, in the carriage and tail of fantails, etc., these being 
the points now mainly attiMided to by Englisli fanciers. Even 
in sub-breeds, as in that of the short-faced tumbler, it is notori- 
ously dillicult to breed nearly perfect birds, some fre(|uently 
departing widely from the standard. There may truly be said 
to be a constant struggle going on between, on the one hand, 


the tendency to reversion to a less perfect state, as well as an 
innate tendency to fnrtlier variability, 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 faU 
so far as to breed a bird as cf)arse as a common tumbler from a 
good short-faced strain. Bnt as long as selection is rapidly 
going on, nnich variability in the parts \mdergoing modification 
may always he, expected. It further deserves notice tliat char- 
acters, modified through selection by man, are sometimes trans- 
mitted, from causes quite unknown to us, more to one sex than 
to the other, generally to the male sex, as with the wattle of 
carriers and the enlarged crop of pouters. 

Now let us turn to Nature. AYlien a part has been devel- 
oped 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 modifi- 
cation since the period when the species branchetl off from the 
connnon progenitor of the genus. This period will seldom be 
remote in any extreme degree, as species rarely endure for more 
than one geological period. An extraordinary amovmt 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 
■];)een so great and long-continued within a period not exces- 
sively remote, Ave might, as a general rule, still expect to find 
more A-ariability in such parts than in otlier parts of the organi- 
zation which have remained for a much longer period nearly 
constant. And this, I am convinced, is the case. Tliat the 
struggle between natural selection on the one hand, and the 
tendency to reversion and variability on the other hand, will in 
the course of time cease ; and that the most abnormally devel- 
oped organs may be made constant, I see no reason to doubt. 
Ilence, when an organ, however abnormal it may be, has been 
transmitted in approximately the same condition to many modi- 
fied descendants, as in the case of the wing of the bat, it must 
have existed, according to my theory, for an immense period 
in nearly the same state; and thus it comes to be no more vari- 
al)le than any other structure. It is only in those cases in 
wliich the modification has been comparatively recent and 
extraordinarily great that we ought to find i\\o generative vari- 
(ihllitij^ as it may be called, still present in a high degree. For 
in this case tlie variability will seldom as yet have been fixed 


by the continued selection of the individuals varj-ing in the 
•required manner and degree, and by the continued rejection 
of those tending to revert to a former and less-modified con- 

Si'iecific Characters more variable than Generic Characters^ 

The principle included in the above remarks may be ex- 
tended. It is notorious that specific characters are more va- 
riable than generic. To exjilain by a simple example -what is 
meant : If in a large genus of jilants some species had blue 
flowers and some had red, the color Avould be only a specific 
character, and no one "would be surprised at one of the blue 
species varying into red, or conversely ; but if all the species 
had blue ilowers, the color would become a generic character, 
and its variation Avould be a more unusual circumstance. I 
liave chosen this example because an explanation is not in this 
case applicable, which most naturalists would advance, namely, 
that specific characters are more variable than generic, because 
they are taken from parts of less physiological importance than 
those commonly used for classing genera. I believe this ex- 
planation is partly, yet only indirectly, true ; I shall, however, 
have to return to this subject in the cliapter on Classification. 
It would be almost superfluous to adduce evidence in support 
of the a])ove statement, that specific characters are more va- 
riable than generic ; but I have repeatedly noticed in Avorks on 
natural history, that, when an author has remarked with sur- 
prise that some imjiortant organ or part, which is generally 
very constant throughout large groups of species, has differed 
considerably in closelj'-allied species, it has also been vari- 
able in the individuals of some of the species. And this fact 
shows that a character, Avhicli is generally of generic value, 
when it sinks in value and becomes only of specific value, often 
becomes variable, though its physiological importance may re- 
main the same. Something of the same kind applies to mon- 
strosities : at least Is. GeoflVoy St.-Hilaire seems to entertain 
no doubt that the more an organ normally differs in the differ- 
ent species of the same group, the more subject it is to indi- 
vidual anomalies. 

On the ordinary view of each species having been inde- 
pendently created, why should that part of the structure, 
which differs from the same part in other independently-created 
species of the same genus, be more variable than those parts 


which are closely alike in the several species? I do not sec 
that any explanation can be given. But on the vie'.v that spe- 
cies are only stron2;ly marked and fixed varieties, Ave might ex- 
pect to find them still oil en continuing to vary in those parts 
of their structure which had varied witliin a moderately-recent 
period, and which had thus come to diCfer, 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 charac- 
ters in common I attribute to inheritance from a common pro- 
genitor, for it can rarely have happened that natural selection 
Avill have modified several species, fitted to more or less wide- 
ly-diflerent habits, in exactly the same manner : and as these 
so-called generic characters have been inherited from before 
tlie period when the different species first branched off from 
their common progenitor, and subsequently have not varied or 
come to differ in any degree, or only in a slight degree, it is 
not probaljle that tliey should vary at the present day. On the 
other hand, the points in which species differ from other spe- 
cies of the same genus are called specific characters ; and as 
these specific characters have varied and come to differ since 
tlie period when the species branched off from a common pro- 
genitor, it is probable that they should still often be in some 
degree variable — at least more variable than those parts of the 
organization which have for a very long period remained con- 

Secondary Sexual Characters variable. 

In coimection with the present subject, I Avill make only 
two other remarks. I think it will be admitted, without my 
entering on details, that secondary sexual characters are very 
variable; I think it also will be admitted that species of the 
same group diller from each other more widely in their second- 
ary sexual characters tlian in other parts of their organization; 
(^ompare, for instance, tlie amount of difference between the 
males of gallinaceous birds, in which secondary sexual charac- 
ters are strongly displayed, with the amount of difference be- 
tween the females; and the tnith of tliis jiroposition will be 
granted. Tiie cause of the original variability of secondary 
S(\xual characters is not manifest ; but we can see why these 
characters should not have been rendered as constant and imi- 
form as other parts of the organization; for secondary sexual 
characters have been accumulated by sexual selection, which is 


less rigud in its action tlian ordinary selection, as it does not 
entail death, bnt only gives fewer offspring to the less favored 
males. Whatever tlie cause may be of the variability of sec- 
ondary sexual characters, as they are highly variable, sexual se- 
lection, will have had a wide scope for action, and may thus 
readily have succeeded in giving to the species of the same 
group a greater amount of difference in their sexual characters 
tlian in other parts of their structure. 

It is a remarkable fact that the secosdary sexual differ- 
ences between the two sexes of the same sjiecies arc generally 
displayed in the very same parts of the organization in whicli 
the different species of the same genus dilfer from each other. 
Of this fact I will give two instances in illustration, the first 
which hap])en to stand on my list; and, as the differences in 
these cases are of a very unusual nature, the relation can hard- 
ly be accidental. The same number of joints in the tarsi is a 
character generally common to very large gi-oups of beetles, 
but in the Engid:e, as Westwood has remarked, the number 
varies greatly; and the number likewise differs in the two 
sexes of the same species : again, in fossorial liymenoptera, the 
manner of neuration of the wings is a character of the highest 
importance, because common to large groups ; but in certain 
genera the ncin-ation differs in the different species, and like- 
wise in the two sexes of the same species. Sir J. Lubbock 
has recently remarked that several minute crustaceans offer ex- 
cellent illustrations of this law. " In Pontella, for instance, 
the sexual characters arc afforded mainly l)y the anterior an- 
tenna) 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 of the subject : I look at all th(! 
species of the same genus as ha^nng as certainly descended 
from the same progenitor as have the two sexes of any one of 
the species. Conseijuently, whatever part of the structure of 
the common progenitor, or of its early descendants, became 
A-ariable, variations of this part Avould, it is highly jiroljable, be 
taken advantage of by natural and sexual selection, in order to 
fit the several species to their several j)lac(\s in the economy of 
Nature, and likewise to fit the two sexes of the same S]iecies to 
each other, or to fit the males and females to different habits of 
life, or the males to struggle with other males for the jiosses- 
sion of the females. 

Finally, then, I conclude that the greater variability of 
specific characteis, or those which distinguish species from 


species, ihan of generic characters, or those which the species 
possess in roinmon ; that the frequent extreme variability of 
any part which is developed in a species in an extraordinary 
manner in comparison with the same part in its congeners ; 
and the slight degree of variability in a part, however extraor- 
dinarily it may be developed, if it be common to a whole group 
of species ; that the great variability of secondarj' sexual char- 
acters, and the great amount of difference in these same char- 
acters between closely-allied species ; that secondary sexual 
and ordinary specific differences are generally displayed in the 
same parts of the organization — are all principles closely con- 
nected together. All being mainly due to the species of the 
same group having descended from a common progenitor, from 
whom they have inherited much in common — to parts whicli 
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 com- 
pletely, according to the lapse of time, overmastered the ten- 
dency to reversion and to further variability — to sexual selec- 
tion being less rigid than orchnary selection — and to variations 
in the same parts having been accumulated by natural and 
sexual selection, and having been thus adapted for secondary 
sexual, and for ordinary purposes. 

Distinct Species present analogous Variations ; and a Variety 
of one Species often assumes some of the Characters of an 
allied Species, or reverts to soyne of the Characters of an 
early Progenitor. 

These propositions will be most readily understood by look- 
ing to our domestic races. The most distinct breeds of pigeons, 
in countries most widely apart, present sub-varieties with re- 
versed feathers on the head and feathers on the feet — charac- 
ters not possessed by the aboriginal rock-pigeon ; these, then, 
are analogous variations in two or more distinct races. The 
frequent presence of fourteen or even sixteen tail-feathers in the 
pouter may be considered as a variation representing the nor- 
mal 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 
]>arent the same constitution and tendency to variation, when 
acted on bv similar unknown influences. In the vegetable 
kingdom we have a case of analogous variation in the enlarged 


stems, or roots as commonly called, of the Swedish turnip and 
Ruta-ba^'a, })hmts wliich several botanists rank as varieties 
produced by cultivation from a common parent; if this be not 
so, the case will then be one of analogous variation in two so- 
called distinct species ; and to these a third may be added, 
namely, the common turnip. AccoriUng to the ordinary view 
of each species having be(?n independently created, we should 
have to attribute this similarity in the enlarged stems of these 
three plants, not to the vera causa of community of descent, 
and a consequent tendency to vary in a like manner, but to 
three separate yet closely-related acts of creation. Many simi- 
lar cases of analogous variation have been observed by Naudin 
in the great gourd-family, and by various authors in oiu* cereals. 
Similar cases occurring with insects under their natural con- 
ditions have lately been discussed with much al)ility by Mr. 
Walsh, who has grouped theni under his law of Equable Va- 

With pigeons, however, we have another case, namely, the 
occasional appearance in all the breeds of slaty-blue birds with 
two black bars on the v/ings, white loins, a bar at the end of 
the tail, with the outer feathers externally edged near their 
bases with white. As all these marks are characteristic of the 
parent rock-pigeon, I presume that no one will doubt that this 
is a case of reversion, and not of a new yet analogous variation 
appearing in the several breeds. ^Ve may, I think, confidently 
come to tliis conclusion, because, as we have seen, these colored 
marks are eminently liable to appear in tlie crossed oflsining 
of two distinct and differently-colored breeds; and in this case 
there is nothing in the external conditions of life to cause tlie 
reappearance of the slaty-blue, with the several marks, beyond 
the influence of the mere act of crossing on the laws of inher- 

No doubt it is a very surprising fact that characters should 
reappear after having lieen lost for many, probably for hun- 
dreds of generations. But when a ])reed has been crossed only 
once by some otlier breed, the offspring occasionally show a 
tendency to revert in character to the foreign breed for many 
generations — some say for a dozen or even a score of genera- 
tions. After twelve general i(ms, the proportion of blood, to 
use a common expression, of any one ancestor, is only 1 in 
3,048 ; and yet, as we see, it is generally believed that a ten- 
dency to reversion is retained by tliis very small projiortion of 
foreign blood. In a breed which has not been crossed, but 


in which both parents liave lost some character which ilieir 
progenitor possessed, the tendency, Avliether stronf^ or -weak, 
to reproduce the lost character might be, as was formerly re- 
marked, for all that wc can sec to the contrary, transmitted 
for almost any uuml:)cr of generations. When a character which 
h;is been lost in a lu'ced, reappears after a gTcat immber of 
generations, the most probable hypothesis is, not that the ofl- 
spring suddenly takes after an ancestor removed by some hun- 
dred generations, but that in each successive generation the 
character in question has been lying latent, and at last, luidei 
imknown favorable conditions, is developed. With the barb- 
pigeon, for instance, which very rarely produces a blue bird, 
it is probable that a latent tendency exists in each generation 
to produce blue plumage. The possibility of chai'acters long 
lying latent can be imderstood according to the hypothesis of 
})angenesis, which I have given in another work. The abstract 
improbability of a latent tendency being transmitted through 
a vast number of generations, is not greater tlian that of quite 
useless or rudimentary organs being thus transmitted. A mere 
tendency to produce a nuHment is indeed sometimes inherited. 
As all the species of the same genus are supposed, on our 
theory, to be descended from a connnon progenitor, it might 
be expected that they would occasionally vary in an analogous 
manner; so that the varieties of two or more species would re- 
semble each other, or that a variety of some 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 thus gained 
would probably be of an unimportant nature, for the presence 
of all important characters will be governed by natural selec- 
tion, in accordance with tlic tlifTerent habits of the species, and 
Avill not be left to the mutual action of the nature of tlie organ- 
ism and of the conditions of life. It might further be expected 
that the species of the same genus Avould occasionally exhil^it 
reversions to long-lost ancestral characters. As, however, we 
never know tlic exact character of the common ancestor of a 
natural group, we could not distinguish these two cases : if, 
iV)r instance, we did not know that the rock-pigeon was not 
feather-footed or turn-crowned, we could not have told whether 
these characters in our domestic breeds were reversions or only 
analogous variations ; but wc might have inferred that the blue 
color was a case of reversion from llie number of the markings, 
whicli arc correlated with this tint, and which it docs not ap- 


pear probable would all appear top^cthcr from simple ^•arialion. 
More especially we might have infen-ed this, from the blue 
color and the several marks so often appearinp^ when distinct 
breeds of distinct colors are crossed. Hence, although under 
Nature it must generally be left doubtful, what cases are re- 
versions to a formerly-existing character, and what are ncAV 
l)ut analogous variations, yet we ought, on oiu* theory, some- 
times to iind the varying offspring of a species assuming 
characters ((^ither from nivcrsion or from analogous variation) 
which already are jirescnt in other members of the same group 
and tliis undoubteclly is the case. 

A considcrabl(! jjart of the difficulty in recognizing in our 
systematic works a variable species, is due to its varieties 
mocking, as it were, other species of the same genus. A con- 
siderable catalogue, also, could be given of forms intermediate 
between two other forms, which themselves can only doubt- 
fully be ranked as species ; and this shows, unless all these 
forms be considered as independently-created species, that the 
one in varjing has assumed some of the characters of the other, 
so as to produce the intermediate forms. But the best evi- 
dence is afforded by parts or organs of an important and gen- 
erally uniform nature occasionally varying so as to acquire, in 
some degree, the character of the same part or organ in an 
allied species. I have collected a long list of such cases ; but 
here, as before, I lie mider the great disadvantage of not being 
able to give them. I can only repeat that such cases certainly 
do occur, and seem to me very remarkable. 

I will, however, give one curious and complex case, not in- 
deed as affecting any important character, but from occurring 
in several species of the same genus, partly under domestica- 
tion and partly under Nature. It is a case almost certainly of 
reversion. The ass sometimes has very distinct transverse 
bars on its legs, like those on the legs of the zebra : it has 
been asserted that these are jilainest in the foal, and, from in- 
qiiirics which I have made, I believe this to be true. The 
stripe on the shoulder is sometimes double and is very variable 
in length and outline. A white ass, but not an albino, has 
been described without either spinal or shoulder stripe : and 
these stripes are sometimes verv obscure, or actually (luitelost, 
in dark-colored asses. The koulan or Pallas is said to have 
been seen with a double shoulder-stripe. Mr. I31yth has seen 
a specimen of the hemionus with a distinct shoulder-stripe, 
tliough it properly has none ; and I have been informed by 


Colonel Poole tliat the foals of this species arc generally striped 
on the legs, and faiiitly on the shoulder. The quagga, though 
so plainly barred like a zebra over the body, is without bars 
on the legs ; but ]3r. Gray has figured one specimen with very 
distinct zebra-like bars on the hocks. 

"With respect to the horse, I have collected cases in Eng- 
land 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 chestnut: a faint 
shoulder-stripe may sometimes be seen in duns, and I have seen 
a trace in a bay horse. JNIy son made a careful examination and 
sketch for me of a dun Belgian cart-horse Avith a double stripe 
on each shoulder and with leg-stripes ; I have myself seen a 
dun Devonshire ponj^, and a small dun Welsh pony has been 
carefully described to me, both with three parallel stripes on 
each shoulder. 

In the northwest part of India the Kattywar breed of horses 
is so generally striped, that, as I hear from Colonel Poole, who 
examined the breed for the Indian Government, a horse with- 
out stripes is not considered as purely-bred. The spine is 
always striped; the legs are generally barred; and the shoul- 
der-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 Kattywar horses striped Avhen first foaled. I have 
also reason to suspect, from information given me by Mr. AV. 
W. Edwards, that Avith the English race-horse the spinal strijie 
is much commoner in the foal than in the full-grown animal. 
I have myself recently bred a foal from a bay mare (offspring 
of a Turcoman horse and a Flemish mare) by a bay English 
race-horse ; this foal when a week old Avas marked on its hinder 
quarters and on its forehead Avith numerous, A'ery narroAV, dark, 
zebra-like bars, and its legs were feebly striped : all the stripes 
soon disappeared completely. Without here entering on fur- 
ther details, I may state that I have collected cases of leg and 
shoulder stripes in horses of A'ery different breeds, in A'arious 
countries from Britain to Eastern China ; and from NorAvay in 
the north to the ISIalay Archipelago in the south. In all jiarts 
of the Avorld these stripes occur far oftenest in duns and mouse- 
duns ; by the term dun a large range of color is included, from 
one between broAvn and black to a close approach to cream- 


I am uwaro tliat Colonel Hamilton Smith, who lias written 
on this subject, believes that the several breeds of the horse 
are descended from several abori^^inal species — one of which, 
the dun, was striped ; and that the above-described appear- 
ances are all due to ancient crosses with the dun stock. But 
this view may be safely rejected ; for it is highly improbable 
that the heavy Belgian cart-horse, Welsh ponies, cobs, the lanky 
Kattywar race, etc., inhabiting the most distant parts of the 
world, should all have been crossed with one supposed aborigi- 
nal stock. 

Now let us turn to the effects of crossing the several species 
of the horse-genus. lloUin asserts that the common mule 
from the ass and horse is particularly apt to have bars on its 
legs ; according to Mr. Gosse, in certain parts of the United 
States about nine out of ten mules have striped legs. I once 
saw a mule with its legs so much striped that any one might 
have thought that it was a hybrid-zebra ; and Mr. W. C. Mar- 
tin, in his excellent treatise on the horse, has given a figure of 
a similar mule. In four colored draAvings, which I have seen, 
of hybrids between the ass and zebra, the legs were much more 
plainly barred tlian 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 mare 
l)y a black Araljian sire, were much more plainly barred across 
the legs than is even the pure quagga. Lastly, and this is an- 
other 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 throe short shoulder-stripes, like those 
on tlie dun Devonshire and Welsh ponies, and even had some 
zebra-like stripes on the sides of its face. With respect to 
this last fact, I was so convinced that not even a stripe of 
color appears from what is commonly called chance, that I was 
led solel}' 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 
l)reed of horses, and was, as we have seen, answered in the 

AVhat now arc we to say to these several facts ? We sec 
several very distinct species of the horse-genus becoming, by 


siin])le vaiiiition, slripod on tlio legs like a zebra, or stri})cd on 
the sliouUler.s like an ass. In the liorsc we see this tendency 
strong^ whenever a dun tint appears — a tint Avhidi approaches 
to that of the general coloring' of the other species of the ge- 
nus. The appearance of the stripes is not accompanied by any 
cliange of form or by any other new character. AVe see this 
tendency to become striped most strongly displayed in hybrids 
from between several of the most distinct species. Now ob- 
serve the case of the several breeds of pigeons : they are de- 
scended from a pigeon (including two or three sub-species or 
geographical races) of a bluish color, with certain bars and 
other marlcs ; and Avhcn any breed assmnes by simple variation 
a l)luish tint, these bars and other marks invariably reappear ; 
but without any other change of form or character. When the 
oldest and truest breeds of various colors are crossed, wc sec a 
strong tendency for the blue tint and bars and marks to reap- 
pear in the mongrels. I have stated that the most probable 
hypothesis to account for the reappearance of xery ancient 
characters, is — that there is a tendency in the young of each 
successive generation to produce the long-lost character, and 
that this tendency, from unknown causes, sometimes prevails. 
And we have just seen that in several species of the horse- 
genus the stripes arc either plainer or appear more commonly 
in tlie young than in the old. Call the breeds of pigeons, some 
of which have bred true for centuries, species ; and how exact- 
ly parallel is the case with that of the species of the horse- 
genus ! For myself, I venture confidently to look back thou- 
sands on thousands of generations, and I see an animal striped 
like a zebra, but perhaps other^vise very differently constructed, 
the common parent of our domestic lu)rse (whether or not it be 
descended from one or more wild stocks), of the ass, the hemi- 
onus, quagga, and zebra. 

He who believes tliat each equine sjiccies was indepen- 
dently created, will, I presume, assert that each species has 
been created with a tendency t(i vary, botli under Nature and 
under domestication, in this particular manner, so as often to 
become striped like other species of tlie genus ; and that each 
has been created with a strong tendency, when crossed with 
species inhabiting distant quarters of tlie world, to produce 
hybrids resembling in tlieir stri})es, 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 imreal, or at least for an unknown, 
cause. It makes the works of God a mere mockery and decep 


tioii; I would almost as soon believe ■with the old and if^no- 
rant cosinof>;'oiusts, that fossil shells had never lived, but had 
been created in stone so as to mock the shells living on the 


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, liut whenever we 
have the means of instituting a comparison, the same laws 
ajipear to have acted in jiroducing the lesser dillerences be- 
tween varieties of the same species, and the greater differences 
between species of the same genus. Changed conditions gen- 
erally induce mere fluctuating variability, but sometimes they 
cause direct and definite effects ; and these may become 
strpngly marked in the coiu'se of time, though we have not 
sulhcient evidence on this head. Habit in producing constitu- 
tional 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 way, and homologous parts tend to cohere, 
ilodilications in hard parts and in external parts sometimes 
affect softer and internal parts. When one part is largely 
developed, perhaps it tends to draw nourishment from the ad- 
joining parts ; and every part of the structure which can be 
saved without detriment will be saved. Changes of structure 
at an early age may aflect parts subsefjuently developed ; and 
many cases of correlated variation, the nature of which we are 
unable to understand, undoubtedly occur. Multiple parts are 
variable in number and in structure, perhaps arising from such 
])arts 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 vari- 
able than those standing higher in the scale, and which have 
llieir whole organization more specialized. Kudiinentary or- 
gans, from being useless, are not regulated by natural selec- 
tion, and hence arc varial)le. Specific characters — that is, the 
characters which have come to differ since the several species 
of (he same genus branched off from a common parent — are 
more varia1)le than generics characters, or those which have 
long becii inherited, and have not differed within this same 
perioil. In these remarks we have referred to special parts or 

1G6 SUMMARY. Chap. V. 

org-ans bcin,;^ still variable, because they have recently viiried 
and thus come lo differ; but vrc have also seen in the second 
chapter that the same principle applies to the whole individual ; 
for in a district Mhcre many species of any genus are found — 
that is, where there has been much former variation and differ- 
entiation, or where the manufactory of new specific forms has 
been actively at work — in that district and among these spe- 
cies, 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 ad- 
vantage of in giving secondary sexual differences to the sexes 
of the same species, and specific differences to the several spe- 
cies of the same genus. Any part or organ developed to an 
extraordinary size or in an extraordinary manner, in compari- 
son with the same part or organ in the allied species, must 
have gone through an extraordinary amount of modification 
since the genus arose ; and thus Ave can understand why it 
should often still be variable in a much higher degree than 
other parts ; for variation is a long-continued and slow process, 
and natiu"al selection will in such cases not as yet have had 
time to overcome the tendency to further variability and to 
reversion to a less modified state. But when a species with 
any extraordinarily-developed organ has become the parent of 
many modified descendants — which on our Wew 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 constitu- 
tion from a common parent and exposed to similar influences, 
natui-ally tend to present analogous variations, or these same 
species may occasionally revert to some of the characters of 
their ancient progenitors. Although new and important modi- 
fications may not arise from reversion and analogous variation, 
such modifications will add to the beautiful and harmonious 
diversity of Nature. 

Whatever the cause may be of each slight difference be- 
tween the offspring and their parents — and a cause for each 
must exist — it is the steady accumulation, through natural se- 
lection, of beneficial differences that has given rise to all those 
modifications of structure which are the most important for the 
welfare of each species. 




Difflcnilies of the Theory of Descent with Modification— Transitions— Ahsence or 
Karlty of Transitional Varieties— Transitions in Habits of Life— Diversified Hab- 
its iu the same Specios— Species with Habits widely different from those of their 
Allies — Orj^iiis of Extreme Perfection— Modes of Tranh-ition— Cases of Ditflcully 
— Natura nou fucit saltnm— Ort'ans of small Importance— Organs not in all Cases 
absolutely perfect- The Law of XJniiy of Type aud of the Conditions of Existence 
embraced by the Theory of Natural Selection. 

Loxr, before havins; anivcd at this part of my ^-ork, a 
crowd of dilFiculties will have occurred to the reader. Some 
of them are so serious that to this day I can hardly reflect on 
them without bein<i; stapffrered ; but, to the best of my judf^- 
ment, the prreater number arc only «pparcnt, and those that are 
real are not, I think, fatal to my theory. 

These dilliculties and objections may be classed under the 
followiuf^ heads : First, Avhy, if species have descended from 
other species by insensibly line p;radations, do Ave not every- 
where see ir.numcrable transitional fonns? 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 liavino", for instance, 
the structure and habits of a bat, could have been formed by 
the modification of some animal Avith Avidely-difl'erent structine 
and habits ? Can Ave believe that natural selection could pro- 
duce, on the one hand, organs of trifling importance, such as 
the tail of a giraffe, Avhich serA'cs as a tly-fiapper, and, on the 
other hand, organs of sucli wonderful structure, as the eye, of 
Avhich we hardly as yet fiillv imderstand the inimitable perfec- 
tion ? 

Thirdly, can instincts be acquired and modified through 
natural selection ? What shall Ave say to so marvellous an 
instinct as that Avhich leads the bee to make cells, Avhich has 
jiracticall}^ anticipated i\m discoveries of profound mathema- 
ticians ? 


Fourthl}', how can -wc account for species, Vv'hen crossed, 
being sterile and jiroducins;^ sterile offsprinp-, "vvhcreas, when 
varieties are crossed, their fertility is unimpaired? 

Tiie tlrst two heads shall be here discussed — Instinct and 
Hybridism in separate chapters. 

On the Absence or Harlty of Transltio7ial Varieties. 

As natural selection acts solely by the preservation of 
profitable modifications, each new form will tend in a fully- 
stocked country to take the place of, and finally to exterminate, 
its own less improved parent-form and other less-favored forms 
with Avhich it comes into competition. Thus extinction and 
natural selection go hand in hand. Hence, if we look at each 
species as descended from some other iniknown form, both the 
parent and all the transitional varieties will generally have 
been exterminated by the very process of the formation and 
perfection of the new form. 

But, as by this theory innumerable transitional forms must 
have existed, why do we not find them embedded in countless 
numbers in the crust of the earth. It will be more convenient 
to discuss tliis question in the chapter on the Imperfection of 
the Geological liccord ; and I will here only st:ite that I be- 
lieve the answer 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 imper- 
fectly made, and only at long intervals of time. 

But it may be lu-ged that when several closely-allied species 
inhabit the same teixitory, we surely ought to find at the pres- 
ent time many transitional forms. Let us take a simple case : 
in travelling from north to south over a continent, we generally 
meet at successive intervals with closely-allied or representa- 
tive species, (Tvidently tilling neaily the same place in the nat- 
ural economy of the land. These reiM-esentativc species often 
meet and interlock ; and as the one l)ecomcs rarer and rarer, 
the other becomes more and more fret(uent, till the one re- 
places the other. But if we compare these species Avhere they 
intermingle, they are generally as absolutel}" distinct from each 
other in every detail of structiu'c as are specimens taken from 
the metropolis inhalntcd by each. By my tlieory these allied 
species are descended from a common parent ; and, during the 
jHoccss of modilication, each has become adapted to the con- 
ditions of lift^ of its own region, and has suj^planted and ex- 


IcriniiKited its ori^i'inal parent-form and all the transitional va- 
rieties between its past and present states. Hence we ought 
not to expect at the present time to mccft with numerous tran- 
sitional varieties in each rei^ion, though they must have ex- 
isted there, and may be embedded there in a fossil condition. 
But in the intermediate region, having intermediate conditions 
of life, why do we not now find closely-linking intermediatt^ 
varieties? This dilliculty for a long time quite confounded 
nie. But I think it can be in large part explained. 

In the lii-st jjlace, we should be extremely cautious in infer- 
ring, be(\iuse an area is now continuous, that it has been con- 
tinuous during a long period. Geology would lead us to be- 
lieve that most continents have been broken up into islands 
even during the later tertiary periods ; and in such islands 
distinct species might have been separately formed without 
the possibilit}' of intermediate varieties existing in the inter- 
mediate zones. By changes in the form of the land and of 
climate, marine areas now continuous must often have existed 
within recent times in a far less continuous and uniform condi- 
tion than at present. But I will pass over this way of escaj> 
ing from the difficulty ; for I believe that many perfcctlj^Ie- 
fmed sjiecies have been formed on strictly continuous areas ; 
though I do not doubt that the formerly broken condition of 
areas now continuous has played an important part in the for- 
mation of new species, more especially ^vith frcelj'-crossing and 
wandering animals. 

In looking at species as they are now distributed over a 
^vide 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 gener- 
ally narrow in comparison with the territory projier to each. 
^^'e see the same fact in ascending mountains, and sometimes 
it is quite remarkable how abruptly, as Alph. de Candollc has 
(il)served, a common alpine species disappears. The same fact 
has been noticed by E. Forbes in sounding the depths of the 
s(>a with the dredge. To those who look at climate and the 
])]iysical conditions of life as the all-important elements of dis- 
tribution, these facts ought to cause surprise, as climate and 
height or depth graduate away insensibly. But when we bear 
in mind that almost every species, even in its metropolis, would 
increase immensely in numbeis, were it not for other com- 
peting species; that nearly aH either prey on or serve as prey 


for otlicrs ; in short, that cadi organic beinf^ is cither directly 
or indirectly related in the most important manner to other 
organic beings, \vc must see that the range of the inhabitants 
of any country by no means exclusively depends on insensibly 
changing pliysical conditions, but in large part on the presence 
of other species, on which it lives, or by ■which it is destroyed, 
or with which it comes into comjietition ; and as these species 
arc already defmed objects, not blending one into another by 
insensil)le gradations, the range of any one species, depending 
as it does on the range of others, will tend to be sharply de- 
fined. Moreover, each species on the confines of its range, 
wherc it exists in lessened numbers, will, during fluctuations 
in the number of its enemies or of its prey, or in the seasons, 
be extremely liable to utter extermination ; and thus its geo- 
graphical range Avill come to be still more sharply defined. 

If I am right in believing that allied or representative spe- 
cies, when inhabiting a continuous area, are generally so dis- 
tributed that each has a wide range, with a comparatively 
narrow neutral territory between tliem, in which they become 
rather suddenly rarer and rarer ; then, as varieties do not essen- 
tially differ from species, the same rule will probably apply to 
both ; and if we take a varying species inhabiting a very large 
area, we shall have to adapt two varieties to two large areas, 
and a third variety to a narrow intermediate zone. The inter- 
mediate variety, consequently, will exist in lesser numbers 
from inhabiting a narrow and lesser area ; and practical!}', as 
fiir as I can make out, this rule holds good with varieties in a 
state of Tjature. I have met with striking instances of the 
rule in the case of varieties intermediate between well-marked 
varieties in the genus Balanus. And it would appear from 
information given me by ]\Ir. "Watson, Dr. Asa Gray, and Mr. 
Wollaston, that generally, when varieties intermediate between 
two other forms occur, they arc much rarer numerically than 
the forms which they connect. Now, if we may trust these 
facts and inferences, and therefore conclude that varieties link- 
ing two other varieties together hare generally existed in 
lesser numbers than the forms which they connect, then, I think, 
we can understand wh}' intermediate varieties should not en- 
dure for very long periods ; Avhy, as a general rule, they should 
be exterminated and disaiii)ear, sooner than the forms v>hich 
they originally linked together. 

"For any form existing in lesser numbers would, as already 
remarked, ran a greater chance of being exterminated tlian 


one exist inn" in laro-c numbers; and in this particular case the 
interniedi:it(! form would be cminentl}"^ liable to the inroads 
of closely-allied forms existing on both sides of it. But a far 
more important consideration, as I believe, is that, during the 
process of further modification, by Avhich two varieties are 
supposed to be converted and perfected into two distinct spe- 
cies, the two which exist in larger numbers, from inhabiting 
larger areas, will have a great advantage over the intermediate 
variet}', which exists in smaller numbers in a narrow and inter- 
mediate zone. For forms existing in larger numbers will al- 
ways have a better chance, within any given period, of present- 
ing further favorable variations for natural selection to seize 
on, than will the rarer fcjrms which exist in lesser numbers. 
Hence, the more common forms, in the race for life, will tend 
to beat and supplant the less common forms, for these will be 
more slowly modified and improved. It is the same principle 
which, as I believe, accounts for the common species in each 
country, as shown in the second chapter, presenting on an 
average a greater number of well-marked varieties than do the 
rarer species. I may illustrate what I mean by supposing three 
varieties of sheep to be kept: one adapted to an extensive 
mountainous region ; a second to a comparatively narrow, hilly 
tract ; and a third to wide plains at the base ; and that the 
inhabitants are all trying with equal steadiness and skill to 
improve their stocks by selection ; the chances in this case will 
be strongly in favor of the great holders on the mountains or 
on the plains improving th.eir breeds more quicldy than the 
small holders on the intermediate narrow, hilly tract; and con- 
sequently the improved mountain or j^lain breed Avill soon 
take the place of the less improved hill-breed ; and thus the 
two breeds, which originally existed in greater numbers, will 
come into close contact with each other, without the interpo- 
sition of the supplanted, intermediate hill-variety. 

To sum up, I believe that species come to be tolerabh' well- 
dcfini^d objects, and do not at any one period present an inex- 
tricable chaos of varying and intermediate links : first, because 
new varieties are very slowly formed, for variation is a slow 
])roccss, and natural selection can do nothing until favorable 
individual diflerences or variations occur, and until a place in 
the natural polity of the country can be better filled by some 
modification of some one or more of its inhabitants. And such 
«ncw places will depend on slow changer> of climate, or on the 
occasional innnigratinn of new inhabitants, and, ]m)bably, in a 


still more important doffrco, on some of the old inhabitants be- 
coming slowly moditied, with the new forms thus produced 
and the old ones acting and reacting on each other. So that, 
in any one region and at any one time, we ought only to sec 
a few species j)resenting slight modiiications of structure in 
some degree permanent ; and this assuredly we do see. 

Secondly, areas now continuous must often have existed 
within the recent period as isolated portions, in which many 
forms, more especially among the classes which unite for each 
birth and wander much, may have separately been rendered 
suiKciently distinct to rank as representative species. In this 
case, intermediate varieties betw^een the several representative 
species and their common parent, must have existed formerly 
within each isolated portion of the land, but these links during 
the process of natural selection will have been supplanted and 
exterminated, so that they will no longer be found in a living 

Thirdly, wlien two or more varieties have been formed in 
diflerent j)ortions of a strictly continuous area, intermediate 
varieties Avill, it is probable, at first have been formed in the 
intermediate zones, but they Avill generally have had a short 
duration. For these intermediate varieties will, from reasons 
already assigned (namely, from what we know of the actual 
distribution of closely-allied or representative species, and like- 
Avise of acknowledged varieties), exist in the intermediate zones 
in lesser numbers than the varieties which they tend to con- 
nect. From this cause alone the intennediate varieties will 
be hable to accidental extemiination ; and during the process 
of further modification through natural selection, they will al- 
most certainly lie beaten and supplanted by the forms which 
they connect ; for these from existing in greater numbers will, 
in the aggregate, present more variation, and thus be further 
improved through natural selection and gain further advan- 

Lastly, looking not to any one time, but to all time, if 
my theory be true, numberless intermediate varieties, linking 
closely together all the species of the same grouji, must assur- 
edly have existed; but the very process of natural si'leetion 
constantly tends, as has been so often remarked, to exterminate 
the parent-forms and the intermediate links. Consequently 
evidence of their former existence could be found only among 
fossil remains, Avhich are preserved, as we shall in a future* 
chapter attempt to show, in an extremely imperfect and inter- 
mittent record. 


On the Origin and IW'nsifions of Orr/anlc Heings wifhjjecH' 
liar Habits and /Structure. 

It has been asked by the opponents of such views as I hold, 
how, for instance, a land carnivorous animal could have been 
converted into one with aquatic habits ; for how could tlu; ani- 
mal in its transitional state have subsisted ? It would be easy 
to show that within the same group carnivorous animals exist, 
having' every intermediate grade between truly aquatic and 
strictly terrestrial habits ; and as each exists by a struggle for 
life, it is clear that each is well adapted in its habits to its 
place in Nature. Look at the Mustela vison of North Amei-ica, 
which has webbed feet, and which resembles an otter in its fur, 
short legs, and form of tail; during summer this animal dives 
for and preys on fish, but during the long winter it leaves the 
frpzen waters, and iM'e3'S, like other polecats, on mice and land 
animals. If a different case had been taken, and it had 1)een 
asked how an insectivorous quadruped could possibly have 
been converted into a flying bat, the question would have been 
far more difficult to answer. Yet I think such difficulties have 
little weight. 

Here, as on other occasions, I lie imder a heavy disadvan- 
tage, for, out of the many striking cases which I have collected, 
I can give only one or two instances of transitional hal)its and 
structures in closely-allied species of the same genus; and of 
diversified habits, either constant or occasional, in the satiKj 
species. And it seems to me that nothing less than a long 
list of such cases is sufficient to lessen the difficulty in an}- ])ar- 
ticidar case like that of the bat. 

Ix)ok at the family of squirrels : here we have the finest gra- 
dation from animals with their tails only slightly flattened, and 
from others, as Hir J. IJichardson has remarked, with the pos- 
terior part of their bodies rather wide and with the skin on 
their flanks rather full, to the so-called flying-squirrels ; and fly- 
ing-squirrels have their limbs and even the base of the tail united 
l)y a broad expanse of skin, which serves as a parachute and 
allows them to glide through the air to an astonishing distance 
from tree to tree. We cannot doubt that each structure is of 
use to each kind of squirrel in its own country, by enabling it 
to escape birds or beasts of prey, or to coUect food moie 
quickly, or, as there is reason to believe, by lessming the dan- 
ger from occasional falls, lint it does not follow from this fact 
that the structure of each sfjuirrel is the best that it is possible 


lo conceive under all natural conditions. Lot the climate and 
vegetation change, let other coTnpcting rodents or new beasts 
of prey immigrate, or old ones become modified, and all anal- 
ogy would lead us to believe that some at least of the squirrels 
would decrease in numbers or become exterminated, unless 
they also became modified and improved in structure in a cor- 
responding manner. Therefore, I can see no difficulty, more 
especially under changing conditions of life, in the continued 
preservation of individuals with fuller and fuller flunk-mem- 
branes, each modification being useful, each being propagated, 
until, by the accumulated efl'ects of this process of natural selec- 
tion, a perfect so-called flying-squirrel was produced. 

Now look at the Galeopithecus, or flying lemur, which for- 
merly was ranked among bats. It has an extremely wide 
flank-membrane, stretching from the corners of the jaw to the 
tail, and including the limbs and the elongated fingers : the 
flank-membrane is, also, furnished with an extensor muscle. 
Although no graduated links of stracture, fitted for gliding 
through the air, now connect the Galeopithecus with the other 
LemuridcTJ, yet there is no difficulty in supposing that such links 
formerly existed, and that each had been formed by the same 
steps as in the case of the less perfectly gliding squurels ; and 
that each grade of structure was useful to its possessor. Nor 
can I see any insuperable difficulty in further belie\-ing it possi- 
ble that the membrane-connected fingers .and forearm of the 
Galeopithecus might be greatly lengthened by natural selec- 
tion ; and this, as far as the organs of flight 'are concerned, 
would convert it into a bat. In certain bats which have the 
wing-mem])rane extended from the top of the shoulder to the 
tail, including the hind legs, we perhaps yet see actual traces 
of an apparatus originally fitted for gliding through the air 
rather than for flight. 

If about a dozen genera of birds had become extinct or 
were unknown, who would have ventured to surmise that birds 
might have existed which used their wings solely as flappers, 
like the logger-headed duck (Micropterus of Eyton) ; as tins in 
the water and front legs on the land, like the penguin; as 
sails, like the ostrich ; and functionallj' for no purpose, like the 
Apteryx? Yet the structure of each of these birds is good for 
it, under the conditions of life to which it is exposed, for each 
has to live by a struggle ; but it is not necessarily the best 
possible under all possible conditions. It must not be inferred 
from these remarks tliat any of the grades of wing-structure 


here alluded to, ■\vliicli j)c*rliiips may all have resulted from dis- 
use, indicate the natural stops by whicli birds have acquired 
their perfect jK)wer of ilifj^ht ; but they serve, at least, to show 
what diversified means of transition are possible. 

Seeing that a few members of such water-breathing classes 
as tlie Crustacea and Mollusca are adapted to live on the land; 
and seeing that we have flying birds and mammals, lining in- 
sects of the most divei-sified types, and formerly had Hying rej)- 
tiles, it is conceivable that liying-flsh, Mhich now glide far 
through the air, slightly rising and turning by the aid of their 
fluttering llns, might liave been modified into perfectly-winged 
animals. K this had Ix^en effected, who would have ever im- 
agined that in an early ti-ansitional state they had been the in- 
habitants of the open ocean, and had used their incipientorgans 
of flight exclusively, as far as wc know, to escape being de- 
voured by other fish ? 

AVhen m'C see any structure liighlj' perfected for an}- par- 
ticular habit, as the wings of a bird for llight, we should bear 
in mind that animals displaying early transitional grades of the 
structure will seldom exist at the present day, lor they will 
have been supplanted by their successors, which Avere gradu- 
ally rendered more perfect through natural selection. Further- 
more, we may conclude that transitional states between struct- 
ures fitted for very difl'erent habits of life Avilkrarely have been 
developed at an early period in great numbers and under many 
subordinate forms. Thus, to return to our imaginary illustra- 
tion of the llying-fish, it does not seem probable that fishes 
capable of true flight would have been develoi^ed under many 
subordinate forms, for taking prey of many kinds in many 
ways, on the land and in the water, imtil their organs of flight 
had come to a high stage of perfection, so as to have given 
them a decided advantage over other animals in the battle for 
life. Hence the chance of discovering species with transition- 
al 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 ihstances of diversified and of 
changed habits in the individuals of the same species. In 
either case it Avould be easy for natural selection to adapt the 
structure of the animal to its changed habits, or exclusively to 
one of its several habits. It is, however, dilhcult to decide, and 
immaterial for us, whether habits generally change first and 
structure afterward ; or Avhether slight modifications of struo 


turc lead to clian^ed habits ; botli probably often occurring- al- 
most sinuiltaneouslj. Of cases of champed habits it will sullice 
merely to allude to that of the many British insects which now- 
feed on exotic plants, or exclusively on artificial substances. 
Of diversified habits innumerable instances could Ixi given : I 
have often watched a tyrant flycatcher (Sauropha<^us sulphu- 
ratus) in South America, hovering over one spot and then 
proceeding to another, like a kestrel, and at other times stand- 
ing 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 break- 
ing them like a nuthatch. In North xVmerica 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 of a species following 
habits widely different from those of their own species and of 
the other species of the same genus, "we might expect that 
such individuals Avould occasionally give rise to new sjiccies, 
having anomalous habits, and with their structure either 
slightly or considerably modified from that of their proper 
type. And such*nstanc<?s do occur in Nature. Can a more 
striking instance of adaptation be given than that of a wood- 
pecker for climbing trees and seizing insects in the chinks of the 
bark ? Yet in North America there are Avoodpcckci-s which 
feed largely on fruit, and others with elongated wings which 
chase insects on the wing. On the plains of La Plata, where 
not a tree grows, there is a woodpecker (Colaptcs campestris) 
which has two toes before and two behind, a long pointed 
tongue, pointed tail-feathers, sufficiently stiff to support the 
bird in a vertical position on a post, but not so stiff as in the 
typical woodpeckers, and a straight, strong beak. The beak, 
however, is not so straight or so strong as in the typical wood- 
peckers, but it is strong enough to bore into wood. Hence this 
Colaptes in all the essential parts of its structure is a Avoodptxk- 
er. Even in such trifling characters as the coloring, the harsh 
tone of the voice, and inidulatory flight, all plainly dcclareil its 
close blood-relationship to ouj common Avoodpecker ; yet, as I 
can assert, not only from my own observation, but from that 
of the accurate Azara, it never climbs a tree ! I may mention, 
as another illustration of the varied haliits of the tril)C, that a 



Mexican Colaptes has been described by Do Saussurc as boring 
holes into hard wood in order to hiy up a store of acorns, but 
for what use is not yet known. 

Petrels are the most ar-rial and oceanic of l)inls, l)ut in the 
quiet sounds of Tierra del Fucf^o, the Pulhnuria berardi, in its 
l^eneral habits, in its astouisliinii^ power of divinjr, its manner 
of swiinminij;-, and of flying when made to take lliglit, would be 
mistaken by anyone for an auk or a grebe; nevertheless it is es- 
sentially a petrel, but with many parts of its organization pro- 
foundly modified in relation to its new haliits of life ; Avhereas 
the woodpecker of L:i Plata has had its structure only slightly 
modified. In the case of the Avater-ouzcl, the acutest observer 
by examining its dead body would never have suspected its 
sub-aquatic hal)its; yet this binl, which is allied to the thrusli 
familv, wholly subsists by diving — using its wings under 
\yater, and grasjiing stones with its feet. AH the members of 
the great order of Hymenopterous insects are terrestrial, ex- 
cepting the genus Proctotrupes, which Sir J(jhn Lubbock has 
recently discovered to be aquatic in its habits ; it often enters 
the water and dives about by the use not of its legs but of its 
wings, and remains as long as four hours beneath the surface ; 
yet not the least modification in its structure can be detected 
in accordance Anth such abnormal habits. 

He who believes that eacli being has been created as we 
now see it, must occasionally have felt surprise when he has 
met with an animal having habits and structure not at all in 
agreement. What can be jilaincr than that the webbed feet of 
ducks and geese are formed for swimming ? Yet, there are up- 
land geese with webbed feet which rarely or never go near the 
water; and no one except Audubon has seen the frigate-bird, 
which has all its four toes webbed, alight on the surface of the 
ocean. On the other hand, grebes and coots are eminently 
aquatic, although their toes are only bordered by membrane. 
What .seems jilaincr than that the long toes, not furnished with 
membrane, of the Grallatores are formed for walking over 
swamps and lloating plants? The water-hen and landrail are 
members of this order, yet the first is nearly as acjuatic as the 
coot, and the second nearly as terrestrial as the quail or ])ar- 
tridge. In such eases, and many others could be given, habits 
have changc^d Avithout a corresponding change of structure. 
The webbed feet of the upland goose may be said to have be- 
come rudimentary in function, though not in structure. In the 


frigate-bird, the dee])ly-scooped membrane between the toes 
shows that structure has begun to change. 

He Avho believes in separate and innumerable acts of crea- 
tion 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 t^-pe ; but this seems to me only restating the fact in 
ditrnified lano-uaffe. He Avho believes in the strug"-le for ex- 
istcnce and in the principle of natural selection, will acknowl- 
edge that every organic being is constantly endeavoring to 
increase in numbers ; and that if any one varies ever so little, 
either in habits or structure, and thus gains an advantage over 
some other inhabitant of the country, it will seize on the j)lace 
of that inhabitant, however different it may be from its own 
place. Hence it will cause him no surprise that there should 
be geese and frigate-birds with webbed feet, living on the dry 
land, or most rai'ely alighting on the "water; that there should 
be long-toed corncrakes, living in meadows instead of in 
swamps ; that there should be woodpeckers Mhere not a tree 
grows ; that there should be diving thnishes and diving Hymen- 
optera, and petrels with the habits of auks. 

Organs of Extreme Perfection and Complication. 

To suppose that the eye with all its inimitable contrivances 
for adjusting the focus to different distances, for admittmg 
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. 
"SVlien it was first said that the sun stood still and the world 
turned round, the common-sense of mankind declared the doc- 
trine false ; Imt the old saying of ] 'ox popxdi^ vox Dei, as every 
j)hilosopher knows, cannot be trusted in science, lieason tells 
me, that if numerous gradations from an imperfect and simple 
eye to one jierfect and complex, each grade being useful to its 
possessor, can be shown to exist, as is certainly the case; if 
further, the eye ever slightly varies, and the variations be inher- 
ited, as is likewise certainly the case ; and if such variations 
should ever be useful to any animal under changing conditions of 
life, then the dilliculty of believing that a jierfect and complex 
eye could be formed l>y natural selection, though insuperable 
by our imagination, cannot be considered real. How a nerve 
comes to be S(Misitive to light, hardly concerns us more than 
how life itself first originated ; but I may remark that, as some 


of the lowest organisms, in Avbich nerves cannot be detected, 
are known to be sensitive to light, it does not seem impossible 
that certain elements in the sarcode, of which they are mainly 
composed, 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 
j)arent-form, in order to see what gradations arc possible, and 
for the cliance of some gradations liaving been transmitted in 
an unaltered or little altered condition. But the state of the 
organ even in distinct classes may incidentally throw light on 
the steps hy which it has been perfected in any one species. 

The simplest organ which can be called an eye consists of 
an optic nerve, surrounded by pigment-cells covered by trans- 
lucent skin, but without any lens or other refractive body. AVe 
may, however, according to M. Jourdain, descend even a step 
lower and find aggregates of pigment-cells, apparently serving 
as an organ of vision, but without any nerve, 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 starfishes, small depressions in the 
layer of pigment which surrounds tlie nerve are filled, as de- 
scribed by the author just quoted, Avith transparent gelatinous 
matter, projecting with a convex surface, like the cornea in the 
higher animals. He suggests that this serves not to form an 
image, but only to concentrate the luminous raj's and render 
their perception more easy. In this concentration of the rays 
we gain the first and by 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 concentrat- 
ing ap]iaratus, and an image will be formed on it. 

In the great class of tlie Articulata, we may start from an 
optic nerve simply coatccl with pigment, the latter sometimes 
forming a sort of pupil, but destitute of a lens or other optical 
contrivance. "NVitii insects it is now known that the numer- 
ous facets on the cornea of the great comjiound eyes form 
true lenses, and that the cones include curiously-modified 
nervous filaments. But these organs in the Articulata are so 


mucli diversified that Milller formerly made three main chxsses 
of compound eyes -with seven subdivisions, l^esidcs a fourth 
main chiss of ag-j^regated simple eyes. 

AVhcn we reflect on these facts, here f^ivcn too briefly, with 
respect to the wide, diversified, and graduated range of struct- 
in-e in the eyes of the lower animals ; and when we bear in 
mind how small the number of all the forms now living must 
he in comparison with those which have become extinct, the 
ditliculty ceases to be very great in believing that natural se- 
lection may have converted the simple apparatus of an optic 
nerve, coated with pigment and invested by transparent mem- 
brane, into an optical instrument as perfect as is possessed by 
an}' member of the great Articulate Class. 

He who will go thus far, ought not to hesitate to go one 
step further, if he finds on finishing this volume that large bod- 
ies of facts, otherwise inexplicable, can be explained by the 
theory of descent with modification ; he ought to admit that a 
structure even as perfect as an eagle's eye might be formed by 
natural selection, although in tliis case he does not know the 
transitional states. It has been objected that, in order to mod- 
ify 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 do- 
mestic animals, it is not necessary to suppose that all the modi- 
fications were simultaneous, if tliey were extremely slight and 
gradual. Even in the most highly-organized division of the 
animal kingdom, namely, the Vcrtebrata, 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 both fishes 
and reptiles, as Owen has remarked, " the range of gradations 
of dioptric structures is very great." It is a significant fiict 
that even in man, according to tlie high authority of Virchow, 
tlic beautiful crj^stalline lens is formed in tlie embryo by an 
accumulation of epidermic cells, lying in a sack-like fold of the 
skin ; and tlie vitreous body is formed from embryonic subcu- 
taneous tissue?. It is indeed indispensable, in order to arrive at 
a just conclusion regarding the formation of the eye, with all 
its marvellously perfect characters, that the reason should con- 
quer the imagination ; but I have felt this difficulty far too 
keenly to be surprised at any degree of hesitation in extend 
ing the principle of natural selection to so startling a length. 


It is scarcely possible to avoid comparing tlic eye with a 
telescope. We know tliat tliis instrument lias been peri'eeted 
by the long-continued ell'orts of the highest human intellects ; 
and we natm-ally infer that the eye has been formed by a some- 
what analogous process. But may not this inference be pre- 
sumptuous ? Have we any right to assume that the Creator 
works by intellectual powers like those of man? If we must 
compare the e^'e to an ojjtical instrument, we ought in imagi- 
nation to take a thick layer of transparent tissue, with spaces 
tilled with fluid, and with a nerve sensitive to light beneatlt, 
and then suppose every ])art of this layer to be continually 
changing slowly in d(Misity, so as to separate into layers of 
dillerent densities and thicknesses, placed at different distances 
from each other, and with the surfaces of each layer slowly 
changing in form. Further, Ave must suppose that there is a 
power, r(>presented by natural selection or the survival of the 
Mttest, always intently watching each sliglit alteration in the 
transparent la^'ers ; and carefully preserving eaoli which, undc^' 
varied circumstances, in any way or in any degree, tends to 
jiroduce 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 produced, and then the old ones 
to be all destroj-ed. In living bodies, variation will cause the 
slight alterations, generation will multiply them almost infi- 
nitely, and natural selection Avill pick out Avith unerring skill 
each improvement. Let this process go on for millions of 
years ; and during each year on millions of individuals of many 
kinds; and may we not believe that a living optical instrument 
might tluis be formed as superior to one of glass, as the works 
of the Creator are to those of man ? 

Modes of Transition. 

If it could be demonstrated that any com])lex organ existed, 
which could not possibly have been formed by numerous, suc- 
cessive, slight modifications, my theor}- would absolutely break 
down. But I can find out no such case. No doubt manv or- 
gans exist of which we do not know the transitional grades, 
more especially if we look to much-isolated spt^cies, roimd 
which, according to the theory, there has been nujch extinction. 
Or again, if we take an organ common to all the members of 
a large class, for in this latter case the organ must have been 
originally fonnod at an extremely remote period, since which 


all the mail)'- members of the class have been developed ; and, 
iu order to discover the early transitional grades through 
which the organ has passed, Ave should have to look to very 
ancient ancestral forms, long since become extinct. 

We should be extremely cautious in concluding that an or- 
gan could not have been formed by transitional gradations 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 the alimentary canal respires, digests, 
and excretes, in the larva of the dragon-fly and in the fish Co- 
bites. 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 advan- 
tage were thus gained, the whole or part of an organ, Avhich 
had previously performed two functions, for one function alone, 
and thus by insensible steps greatly change its nature. Many 
plants are known which regularly produce at the same time 
differently-constructed flowers ; and if such plants were to pro- 
duce one kind alone, a great change would in some cases be 
effected in the character of the species. It can also be shown 
that the pi'oduction of the two sorts of flowers by the same 
plant has been effected by finely-graduated steps. Again, two 
distinct organs in the same individual may simultaneously per- 
form the same function, and this is a highly-imj^ortant means 
of transition : to give one instance — there are fish with gills 
or branchiae that breathe the air dissolved in the water, at the 
same time that they breathe free air in their swim-bladders, 
this latter organ being divided by highly-vascular partitions, 
and having a ductus pneumaticus for the supply of air. To 
give another instance from the vegetable kingdom: plants 
climb l^y three distinct means, by spirally twining, by clasping 
a support with their sensitive tendrils, and by the emission of 
aerial rootlets ; tliese three means are usually found in distinct 
groups, but some few plants exhibit two of the means, or even 
all three, combined in ttie same individual. In all such cases 
one of the two organs for performing the same function might 
be modified and perfected so as to ])erf()rm all the work, being 
aided during the progress of modification by the other organ ; 
and then this other organ might be modified for some other 
and quite distinct purpose, or be wholly obliterated. 

The illustration of the swim-l)ladder in fishes is a good one, 
becausi? it shows us clearly the higher important fact that an 
organ originally constructed for one purpose, namely flotation, 


may be converted into one for a widcly-tliffercnt purpose, 
namely, respiration. Tlie swim-bladder has, also, been worked 
in as an accessory to the auditory org'ans of certain hsh. All 
physiolog^ists 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 vertebrate 
animals with true lungs have descended by ordinary generation 
from an ancient and unknown prototype, which was furnished 
witii a floating apparatus or swim-bladder. We can thus, as I 
infer from Owen's interesting description of these parts, under- 
sta,nd the strange fact that every particle of food and drink 
which we swallow has to pass over the orifice of the trachi^a, 
with some risk of falling into the lungs, notwithstanding the 
beautiful contrivance by which the glottis is closed. In the 
higlier Vertebrata the branchiaj 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 tlunr former position. But 
it is conceivable that the now utterly lost branchiae might have 
been gradually worked in by natural selection for some distinct 
purpose : for instance, the branchiir; and dorsal scales of Anne- 
lids are believed to be homologous with the wings and wing- 
covcrs of insects, and it is not improbable that with our existing 
insects organs, which at an ancient period served for respiration, 
have actually been converted into organs of flight. 

In considering transitions of organs, it is so important to 
bear in mind the probability of conversion from one function to 
another, that I will give another instance. Pedunculated cir- 
ripcdes have two minute folds of skin, called by me the ovigerous 
frena, Avhidi serve, through the means of a sticky secretion, to 
retain the eggs until they are hatched within the; sack. These 
cirri|)edes have no branclu;ii, the whole surface of the body and 
of the sack, together with the small frena, serving for respira- 
tion. Tiic 13alanid;\3 or sessih; cirripedes, on the other hand, 
have no ovigerous frena, the eggs lying loose at the bottom of 
the sack, within the well-enclosed shell ; but they iiave, in the 
same relative position with the frena, large, much-fokU'd mem- 
branes, which freely communicate with the circulatory lacunixj 
of the sack and body, and which have been considered to be 
branchiaj by Prof. Owen and by all other naturalists who have 
treated on the sul)ject. Now I think no one will dispute that 


the ovigcrous frcna in the one family arc strictly homologous 
■with the braiicliiiu of tlic other family; indeed, they graduate 
into each other. Therefore it need not be doubted that the 
two little folds of skin, -which originally served as ovigerous 
frena, but -which, likewise, very slightly aid in the act of res- 
piration, have been gradually con\erte(l by natural selection 
into branchije, simply through an increase in their size and the 
obliteration of their adhesive glands. If all pedunculated cir- 
rijiedes had become extinct, and they have already suffered far 
more extinction than have sessile cirripedes, who Avonld ever 
have imagined that the branchioi in this latter family had 
originally existed as organs for pre\'cnting the ova from being 
washed out of the sack ? 

iSj^ecial Difficulties of the, Theory of Natural Selection. 

Although -we must be extremely cautious in concluding 
that any organ could not have been produced by successive 
transitional gradations, yet vmdoubtedly serious cases of difli- 
culty occur, some of which will be discussed in my fiiture work. 

One of the most serious is that of neuter insects, which are 
often diiFcrently constructed from either the males or fertile 
females; but this case will be treated of in the next chapter. 
Tlie electric organs of fishes offer another case of special diffi- 
culty ; for it is impossible to conceive by what steps these 
Avondrous organs have been produced. As Owen has remarked, 
there is much analogy between them and ordinary muscles, in 
their manner of action, in the influence on them of the nervous 
power and of stimulants svich as strychnine, and as some be- 
lieve in their intimate structure. M^e do not even know of 
-what use these organs are ; though in the Gjinnotus and Tor- 
pedo they no doubt serve as powerful means of defence and 
perhaps for securing prey; 3'et in the Kay an analogous organ 
in the tail, even when greatly irritated, manifests, as lately 
observed by Matteuccu, but little electricity — so little that it 
can hardl}^ be of use for such purposes. ^Moreover, in the Kay, 
besides the organ just referred to, there is, as Dr. K. ^McDonnell 
has shown, another organ near the head, not known to be elec- 
trical, but which appears to be the real homologue of the elec- 
tric batter}' in the torpedo. Lastly, as we know nothing about 
the lineal progenitors of these fishes, it must be admitted that 
we are too ignorant to be enabled to aflirm that no transitions 
arc possible, through which the electric organs might have 
been developed. 


Tliesc same organs at first appear to offer anotncr and far 
more serious difficulty ; for they occur in about a dozen kinds 
of fisli, of Avliich. several arc Avidely remote in their affinities. 
Generally when the same organ is found in several niemhers 
of the same class, especially if in mc^mbers having very differ- 
ent habits of life, we may attribute its presence to inheritance 
from a common ancestor; and its absence in some of the mem- 
bers 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 Ijclief 
that most fishes formerly possessed electric organs, which their 
modified descendants have now lost. But when we look at 
the subject more closely, wc find in the several fishes provided 
with electric organs that these are situated in diiferent parts 
of the body — that they differ in construction, as in the arrange- 
ment of the plates, and according to Pacini, in the process or 
means by which the electricity is excited — and lastly, in the 
requisite nervous power being supplied through different nerves 
from widely-diffi;rent sources, and this is perhaps the most im- 
])ortant of all the diflerences. Hence in the several remotely- 
atlied fishes furnished with electric organs, these cannot be 
ccnisidered as homologous, but only as analogous in function. 
Consequently there is no reason to suppose that they have 
been inherited from a common progenitor ; for, had this been 
the case, they would have closely resembled each other in all 
respects. Thus tlic greater difficulty disappears, leaving only 
the lesser yet still great difliculty ; namely, by what graduated 
steps these organs have arisen and been develojied in each sepa- 
rate grouji of fishes. 

Tiic luminous organs which occur only in a few insects, 
belonging to widely-different fiunilies and orders, and v.hich 
arc situated in different jiarts of the body, offer a difficidty 
almost exactly jiaralhd witli tiiat of the electric organs. Other 
cases could be given; for instance, in plants, the very curious 
contrivance of a mass of pollen-grains, borne on a foot-stalk 
with an adhesive gland, is apparently the same in Orchis and 
Asclcpias — genera almost as remote as is possible among 
flowering ])lants. In all such cases of two species, fiir removed 
from each other in the scale of organization, being furnislied 
with similar anomalous organs, it should be observed that, al- 
though the general appearance and function of the organ may 


1)0 iilcMitically the saiiie, yet some fundamental difference be- 
tween them can always, or almost always, be detected. I am 
inclined to believe that, in the same manner as two inen have 
sometimes independently hit on the same invention, so natural 
selection, working for tlie good of each being and taking ad- 
vantage of analogous variations, has sometimes modified in 
nearly the same way two organs in two distinct organic beings, 
which owe but little of tlieir structure in common to inheritance 
from a common progenitor. 

Fritz MllUcr, in a remarkable work recently published, has 
investigated a nearly parallel case, in order to test the views 
advanced in this volume. Several families of crustaceans in- 
clude a few species which possess an air-breathing apparatus, 
and are fitted to live out of the water. In two of these fam- 
ilies, 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-or- 
gans, circulating system, in the position of the tufts of hair 
with whicli their complex stomachs arc lined, and lastly in the 
v/hole structure of the Avater-breatliing Ijranchia^, even to the 
microscopical hooks by Avhich they arc cleansed. Hence it 
might have been expected that the equally important air- 
breathing apparatus would have been the same in the fevv^ 
species in both families Avhich live on the land ; and this might 
Iiavc been tlie more confidently expected by those wlio believe 
in distinct creations ; for why sliould this one apparatus, given 
for the same special piu^ose to these species, have been made 
to differ, Avhile all the other important organs are closely simi- 
lar or rather identical ? 

Fritz Mllller argues that this close similarity in so many 
points of structure must, in accordance with the views ad- 
vanced by me, be accounted for by inheritance from a common 
progenitor. But as the vast majority of the species in tlie 
above two families, as well as most crustaceans of all orders, 
are aquatic in their habits, it is improbable in the highest degree, 
that their common progenitor should have been adapted for 
breathing air. Miiller was thus led carefully to examine the 
apparatus in the air-breathing species; and in each he found 
it to diffor 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 intelli- 
gible, and might even have been anticipated, on the siip]iositi<-)n 
that species belonging to distinct families had slowly become 


adapted to live more and more out of "water, and to breathe 
the air. For these species, from beloiif^ing to distinct families, 
■would difler to a certain extent, and in accordance with the 
principle that the nature of each variation depends on two fac- 
tors, viz., the nature of the orp^anism and that of the condi- 
tions, the variability of these crustaceans assuredly would not 
have been exactly the same. Consequently natural selection 
would have had diiTerent materials or variations to work on, in 
order tu arrive at the same functional result; and the structures 
thus acquired would almost necessarily have dillered. On the 
hypotliesis of 6C]>arate acts of creation the whole case remains 
unintelli,Lrible. The above line of arn^mcnt, as advanced by 
Fritz Miiller, seems to have had j^reat weiglit in leadino^ this 
distin<^uislicd naturalist to accept the views maintained by me 
in this volume. 

• In the several cases just discussed, wc have seen that in 
beings more or less remotely allied, the same end is gained 
and the same function performed by organs in appearance, 
though not in truth, closely similar. But the common rule 
throughout Nature is that the same end is gained, even some- 
times in the case of beings closely related to each other, by the 
most diversified means. How differently constructed is the 
feathered wing of a bird and the membrane-covered yying of a 
l)at with all the digits largely developed ; and still more so the 
four Avings of a butterfly, the two wings of a fly, and the two 
wings of a beetle, together with the elytra ! 13ivalve shells 
are made to open and shut, but on what a immber of ])atterns 
is the hinge constructed, from the long row of neatly inter- 
locking teetli in a Nucula to the simple ligament of a Mussel ! 
tSeeds arc disseminated by their minuteness — by their capsule 
being converted into a light balloon-like envelope — by being- 
embedded in pulp or flcsli, formed of tlie most diverse parts, 
and rendered nutritious, as well as conspicuously colored, so as 
to attract and be devoured by birds — ])y having hooks and 
grapnels of many kinds and serrated awns, so as to adhere to 
the fur of quadmpeds — and by being furnish(>d Avith wings and 
plumes, as different in shape as elegant in structure, so as to 
be wafted by every breez(\ I will give one other instance ; 
for this subject of the same end being gained by the niost di- 
versified 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 incrediljlo. Witli plants having' scjiarated 
Bcxcs, and witli those in whicli, tlious-li herniaplnodites, the 
pollen docs not spontaneously fall on tlie stig'ma, some aid is 
necessary for their fertilization. AVith several kinds this is 
cHected by the polli'U-yrains, which are lig'ht and incoherent, 
Ix'ino; blown 1)y the wind through mere chance on to the stigma ; 
and this is the simj)lest plan which can well be conceived. 
An almost equally simple, though very different, plan occurs 
in many plants in which a symmetrical flower secretes a few 
drops of nectar, and is consequently visited by insects ; and 
these carry the pollen from the anthers to the stigma. 

From this simple stage we may pass through an inexhaust- 
ible number of contrivances, all for the same purpose and 
cflected in essentially the same manner, but entailing changes 
in every part of the llower. The nectar may be stored in vari- 
ously-shaped receptacles, with the stamens and pistils modi- 
fied in many ways, sometimes forming traj)-like contrivances, 
and sometimes capable of neatly-adapted movements through 
irritability or elasticity. From such structures we may ad- 
vance till Ave come to such a case of extraordinary adaptation 
as that lately described by Dr. Crugcr in the Coryanthcs. Tliis 
orchid has jiart of its labellum or lower lip hollowed out into 
a great bucket, into which drops of almost pure water contin- 
ually fall from two secreting horns which stand above it ; and 
v.hen the bucket is half full, the water overflows by a spout 
on one side. The basal part of the labellum stands over the 
bucket, and is itself hollowed out into a sort of chamber with 
two lateral entrances ; Avithin tliis chamber there are curious 
fleshy ridges. The most ingenious man, if he had not Avit- 
nessed Avhat takes ])lace, could never liaA^e imagined Avhat pur- 
pose all these parts serve. But Dr. Crtiger saAV croAvds of 
large humble-bees A'isiting the gigantic flowers of this orchid, 
not in order to suck nectar, but to gnaAV ofi" the ridges Avithin 
the chamber aboA^e the bucket ; in doing this they frequently 
I)ushe(l each other into the bucket, and their wings being thus 
Avetted they could not fly away, liut had to craAvl out through 
the passage formed by tlie spout or OA-erflow. Dr. Crllger saw 
a "continual procession" of bees thus crawling out of their in- 
A'oluntary bath. The passage is narroAver, antl is roofed over 
by the column, so that a bee, in forcing its Avay out, first rubs 
its back against the viscid stigma and then against the viscid 
glands of the pollen-masses. The jiollen-masses are thus glued 
to the back of that bee Avhich first happens to craAvl out through 


the passage of a litely-cxpandcd flower, and arc thus carried 
away. Dr. Cril<^('r sent ine a (lower in spirits of wine, with a 
bee which he liad killed before it liad ([uite crawled ont with a 
pollen-mass still fastened to its back. When the bee, thus 
provided, (lies to another (lower, or to the same flower a sec- 
ond time, and is jnished by its comrades into the bucket and 
then crawls out by the iiassag-e, the pollen-mass necessarily 
comes first into contact with the viscid stig'ma, and adheres to 
it, and the (lower 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, wliicli prevents the bees from 
flving away and forces them to crawl out through the spout, 
and lub against the properly-placed viscid pollen-masses and 
viscid stigma. 

The construction of tlie (lower in another closely-allied or- 
chid, namely, the Catasetum, is widely dilVerent, though serving 
the same end; and is equally curious. Bees visit these flow- 
ers, like those of the Coryanthes, in order to gnaw the label- 
lum ; in doing this they inevitably touch a long, tapering, 
sensitive projection, or, as I have called it, the antenna. This 
antenna, when touched, transmits a sensation or vibration to a 
certain membrane which is instantly ruptured ; this sets free 
a spring by which the pollen-mass is shot forth, like an arrow, 
in the right direction, and adheres by its viscid extremity'' to 
the back of the bee. The pollen-mass of a male plant is thus 
carried to the flower of a female plant, where it is brought into 
contact with the stigma, which is A'iscid enough to break cer- 
tain elastic threads, and, retaining the pollen, fertilization is 

How, it may be asked, in the foregoing and in innumerable 
other instances, can we imderstand the graduated scale of 
complexity and the multifarious means for gaining the same 
end V The answer no doubt is, as already remarked, that when 
two forms vary, which already diller from each other even in a 
slight degree, tht; variability will not be of the same exact na- 
ture, and conseciuently the results obtained through natural 
selection for the same general ])urpose will not be the same. 
We should also bear in mind that every highly-developed or- 
ganism has passed through a long course of modilication; and 
that each moditied structure tends to be inherited, so that it 
will not readily be wholly lost, but may be modified again and 
again. Hence the structure of each part of each species, for 
whatever purpose used, is the sum of the many inherited 


changes, through which that species has passed during its suc- 
cessive adaptations to changed habits and conditions of life. 

Finally, then, although in many cases it is most difficult 
even to conjecture by what transitions many organs have ar- 
rived at their present state; yet, considering how small the 
proportion of living and known forms is to the extinct and un- 
known, 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 specially 
created for some purjDOse, rarely or never appear suddenly in 
any class ; as indeed is sho^\'n by that old, but somewhat ex- 
aggerated, canon in natural history of " Natura non facit sal- 
tum." Vio 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 inno- " 
vation. Why, on the theory of Creation, should there be so 
much variety and so little novelty ? "Why should all the parts 
and organs of many independent beings, each supposed to 
have been separately created for its proper place in Nature, 
be so commonly linked together by graduated steps ? Why 
should not Nature take a sudden leap from structm-e to struct- 
ure ? On the theory of natural selection, we can clearly un- 
derstand why she should not; for natural selection acts only 
by taking advantage of slight successive variations ; she can 
never take a sudden leap, but must advance by short and sure 
though slow steps. 

Organs of little apparent Importance^ as affected by Natural 

As natural selection acts by life and death — by the survival 
of the fittest, and by the destruction of the less wcll-litted indi- 
viduals — I have sometimes felt great dilllculty in understand- 
ing the origin or formation of parts of little importance ; al- 
most as great, though of a very different kind, as in the case 
of the most perfect and complex organs. 

In the fii-st 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 for- 
mer chapter I have given instances of very trifling characters, 
such as tlie down on fruit and the color of its ilesh, the color 
of the skin and hair of quailrupeds, which, from being corre- 
lated with constitutional differences or from determining the 


attacks of insects, mif^lit assuredly he acted on by natural se- 
lection. Tlie tail of tiie g^iraffc looks like an artificially-con- 
structed fly-flapper; and it seems at first incredible that tliis 
could have been adapteil for its present purpose by successive 
slight niodiiications, each better and better lifted, for so triliino; 
an object as to drive away flies ; yet avc should pause before 
being too positive even in this case, for we know that the dis- 
tribution and existence of cattle and other animals in South 
America absolutely depend on their power of resisting the at- 
tacks 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 (juadrupeds are actually destroyed (exceiDt 
in some rare cases) by flies, but they are incessantly harassed 
and their strength reduced, so that they are more subject to 
disease, or not so well enabled in a coming dearth to search 
for food, or to escape from beasts of pre}'. 

Organs now of trifling importance have probably in some 
cases been of high imjiortance 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 ver}' slight use ; but any actually injurious 
deviations in their structure will of course have been checked 
by natiu'al selection. Seeing how important an organ of loco- 
motion the tail is in most aquatic animals, its general prcsenco 
and use for many purposes in so many land animals, which in 
their lungs or modified swim-bladders betray their aquatic ori- 
gin, 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 Ily- 
flapper, an organ of prehension, or as an aid in turning, as with 
the dog, though the aid in this latter respect nuist be slight, 
for the hare, with hardly any tail, can double cjuickly eTiough. 

In the second ])lace, ^ve may sometimes wrongly attribute 
im]X)rtance to characters which have originated from quiti; sec- 
ondary causes, independently of natural selection. We should 
remember that climate, food, etc., probably have had some, 
])erhaps a considerable, direct influence on the organization ; 
that characters reajjpear from the law of reversion ; that ct>r- 
relation is an important element of change; and finallv, that 
sexual selection has often largely modified the external i-Iiarac- 
ters of the higher animals, so as to give one male an advantage 
in fighting witli other males, or in charming the female ; and 


characters gained through sexual selection may be transmitted 
to both sexes. Moreover, a modification, caused in any of tlic 
above specified ways, may at first have been of no direct ad- 
vantage to a species, ])ut may subsequently have been taken 
advantage of by its descendants under new conditions of life 
and newly-acquired luibits. 

If, for instance, green woodpeckers alone had existed, and 
"\ve did not know that there Avere many black and pied kinds, 
I dare say that we should have thought that the green color 
was a beautiful adaptation to hide 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 selec- 
tion. A trailing palm in the Malay Archipelago climbs tlie 
loftiest trees by the aid of exquisitely-constructed hooks clus- 
tered 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 there is reason to l:)elieve, from tlie distribution of 
the thorn-bearing species in Africa and South America, serves 
as a defence against browsing quadrupeds, so tlie hooks on the 
palm may first have been developed for this object, and subse- 
quently been taken advantage of by the plant as it imderwent 
further modification and became a climber. The naked skin 
on the head of a vulture is generally considered as a direct 
adaptation for wallowing in putridity ; and so it may be, or it 
may possibly be due to tlie 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 adapta- 
tion for aiding parturition, and no doubt they facilitate, or may 
be indispensable for this act ; but as sutures occur in the skulls 
of young birds and reptiles, Avliich have only to escape from 
a broken C(i;ji:, 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 va- 
riation or individual diflbrence ; and we are immediately made 
conscious of this by reflecting on the diiferences in the breeds 
of our domesticated animals in different countries — more espc- 
ially in the less civilized countries where there has been but 
littie methodical selection. Animals kept by savages in difler- 


ent countries often have to struggle for their own subsistence, 
and are exposed to a certain extent to natural selection, and 
individuals witli sliglitly-dillerent constitutions would succeed 
best under dilTerent climates. A good observer states tliat in 
cattle susceptibility to the attacks of flies is correlated with 
color, as is the liability to be poisoned by certain plants ; so 
that even color would be thus subjected to the action of nat- 
ural selection. Other observers are convinced that a damp 
climate affects the growth of the hair, and tliat with the hair 
the horns are correlated. Mountain-breeds alwaj-s differ from 
lowland-breeds ; and a mountainous country would probably 
affect the hind-limbs fmm exercising them more, and possibly 
even the form of the jielvis ; and then, by the law of homolo- 
gous variation, the fiont-liml)s and the head would probably be 
affected. Tlie shape, also, of the pelvis might affect by press- 
ure the shape of certain parts of the young in the womb. 
The laborious breathing necessary in high regions would, we 
have reason to believe, increase the size of the chest ; and 
again- correlation would come into play. The effects on the 
whole organization of lessened exercise, together with abun- 
dant food, is probably still more important ; and this, as H. von 
Nathusius has lately shown in his excellent Treatise, is appar- 
ently one chief cause of the great modilication which the 
breeds of swine have undergone. But Ave are far too ignorant 
to speculate on the relative importance of the several known 
and unknown causes of variation ; and I have made these 
remarks only to show that, if Ave are unable to account for the 
characteristic differences of our domestic breeds, which never- 
theless are generally admitted to have arisen through ordinary 
generation from one or a few parent-stocks, we ought not to 
lay too much stress on our ignorance of the precise cause of 
the sliglit analogous differences between species. I might 
have adduced for this same purpose the dilferences between 
the races of man, which are so strongly marked ; I may add 
that some light can apparently be thrown on these differences, 
through sexual selection of a particular kind, but without 
entering on full details my reasoning Avould appear frivolous. 

Uillliarian Doctrine howfac true: JJcauti/ /tow acquired. 

The foregoing remarks lead me to say a few Avords on the 
protest, lately made by some naturalists, against th«> utilitarian 
doctrine that CA'cry detail of structure has been produced for 


the good of its possessor. Tliey believe that many structures 
have been created for l)eauty in the eyes of man, or, as already 
discussed, for the sake of mere variety. Such doctrines, if 
true, Avould be aljsolulely fatal to my theory. Yet I fully 
admit that many structures are now of no direct use to their 
possessors, and may never have been of an}- use to their pro- 
genitors. No doubt, as recently remarked, the definite action 
of changed conditions, correlated variation, and reversion, have 
all produced their efTects. But the most important consider- 
ation is that the chief part of the organization of every liv- 
ing creature is simply due to inheritance; and consequently, 
though each assuredly is well fitted for its place in Nature, 
many structures now have no direct relation to existing habits 
of life. Thus, we can hardly believe that the webbed feet of 
the upland goose or of the frigate-bird are of special use to 
these birds ; we cannot believe that the similar bones in the 
arm of the monkey, in the fore-leg of the horse, in the wing of 
the bat, and in the flipper of the seal, are of special use to 
these animals. We may safely attribute these structures to 
inheritance. But to the progenitor of the upland goose and 
of the frigate-bird, webbed feet no doubt were as useful as 
they now are to the most aquatic of living birds. So we ma^' 
believe that the progenitor of the seal did not possess a flipper, 
but a foot with live toes fitted for walking or grasping; and 
we may further venture to believe that the several bones in 
the liml)s of the monkey, horse, and bat, which have been in- 
herited from some ancient progenitor, were formerly of more 
special use than they now are to these animals M'ith their 
Avidely-diversificd habits, and might consequently have been 
modilied through natural selection, M:>king due allowance 
for the definite action of changed conditions, correlation, rever- 
sion, etc., we may conclude that every detail of structure in 
every living creature is either now or was formerly of use — 
directly or indirectly through the comj^lex laws of growth. 

With respect to the belief that organic beings have been 
created beautiful for the delight of man — a view which it has 
been pronounced may safely be accepted as true, and as sub- 
versive of my whole theory — I may first remark that the idea 
of the beauty of any object obviously d(^pends on the mind of 
man, irrespective of any real quality in the a(hnircd object ; and 
that the idea is not an innate and imalterable element in the 
mind. We see this in men of ditlerent races admiring an en- 
tirely diirerent standard of beauty in their women ; neither the 


Negro nor the Chinese admires the Caucasian bcau-idcal. The 
idea also of picturesque beauty in scenery has arisen only within 
modern times. On the view of beautiful objects havinj^ been 
created for man's gratification, it ought to be shown that there 
was less beauty on tlie face of the earth before man appeared 
than since he canae on the stage. Were the l^cautiful volute 
and cone shells of the Eocene epoch, and the gracefully-sculi> 
tured ammonites of the Secondary period, created that man 
might ages afterward admire them in his cabinet ? Few ob- 
jects are more beautiful than the minute siliceous cases of the 
diatomacea? : were these created that they might be examined 
and admired xmder 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 ; and they have become 
through natural selection beautiful, or rather conspicuous in 
'contrast with the green leaves, that they might easily be ob- 
served and visited by insects, so that their fertilization miglit 
b'3 favored. 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 gayly-colored corolla. Again, several plants habitu- 
ally produce two kinds of flowers : one kind open and colored 
so as to attract insects; the other closed and iiot colored, desti- 
tute of nectar, and never \-isited by insects. Hence we may 
conclude that, if insects had never existed on the face of the 
earth, the vegetation would not have been decked Avith beauti- 
ful flowers, but Avould have produced only such poor flowers as 
we now see on our firs, oaks, nut and ash trees, on grasses, 
spinach, docks, and nettles. A similar line of argument holds 
good with the many kinds of beautiful fruits ; that a ripe straw- 
berry or cherry is as pleasing to the eye as to the palat<% that the 
gayly-colored fruit of the spindle-wood-tree and the scarlet 
berries of the holly are beautiful objects, will be admitted by 
every one. But this beauty serves merely as a guide to birds 
and beasts, that the fruit may be devoured and the manured 
seeds thus disscminatod : I infer that this is the case from havhig 
as yet found in every instance that seeds, Avhich are embedded 
within a fruit of any king, tliat is witliin a fleshy or pulpy envel- 
ope, if it be colored of any brilliant tint, or merely rendered 
conspicuous by being white or black, are always disseminated 
by being first devoured. 

On the other hand, I willinglv admit that a great nuinlKT 
of male animals, as all our ir.< st jrorgcous birds, some f\^he3 


some maimnals, and a host of majTnificently-colored butterflies 
and some other insects, have been rendered beautiful for 
beauty's sake ; but this has Ijcen cfTected not for the dehf^ht 
of man, but throup^'h sexual selection, that is, from the more 
beautiful males liaving been continually preferred by their less 
ornamented females. So it is Avith the music of birds. We 
may infer from all this that a similar taste for lieautiful colors 
ami for musical sounds runs throug-h a large part of the animal 
kingilom. AVhen tlie female is as beautifully colored as the 
male, Avhich is not rarely the case with birds and butterflies, 
the cause simply lies in the colors acquired throuo-h sexual se- 
lection having been transmitted to both sexes, instead of to 
the males alone. In some instances, however, the acquirement 
of conspicuous colors by the female may have been checked 
through natural selection, on account of the danger to which 
she would thus have been exposed during incubation. 

Natural selection cannot possibly produce any modification 
in any one species exclusively for the good of another species ; 
though throughout Nature one species incessantly takes advan- 
tage of, and profits by, the structure of others. But natural 
selection can and does often produce structures for the direct 
injury of otlicr animals, as we see in the fang of the adder, and 
in the ovipositor of the ichneumon, by which its eggs are depos- 
ited in the living bodies of other insects. If it could be proved 
that any part of the structure of any one species had been 
formed for the exclusive good of another species, it would 
annihilate my theory, for such could not have been produced 
through natural selection. Although many statements may 
be found in Avorks on natural history to tliis eflcct, I cannot 
find even one which seems to me of any weight. It is admitted 
that the rattlesnake has a poison-fang for its own defence and 
for the destruction of its prey ; but some authors supjiose that 
at the same time this snake is furnished with a rattle for its 
own injury, namely, to warn its prey to escape. I would 
almost as soon believe that the cat curls the end of its tail 
wlicn ])reparing to spring, in order to warn the doomed mouse. 
But I have not space here to enter on this and other such cases. 

Natural selection will never produce in a being any thing 
injurious to itself, for natural selection acts solely by and for 
the good of each. No organ will be formed, as Palcy has 
remarked, for the purpose of causing pain or fordoing an injury 
to its possessor. If a fivir balance be struck between the good evil caused by each part, each will 1w found on the whole 


advantagccous. After the lapse of time, under changing con- 
ditions of life, if any part comes to be injurious, it will be modi- 
fied; or, if it be not so, the being will become extinct, as 
myriads have become extinct. 

Natural selection tends only to make each organic being as 
perfect as, or sliglitly more perfect than, the other inliabitants 
of the same country with which it has to struggle for existence. 
And we see that this is the degree of perfection attained under 
nature. Tlie endemic productions of New Zealand, for instance, 
are perfect one compared with another; but tlicy are now 
rapidly yielding before the advancing legions of ])lants and 
animals introduced from Europe. Natural selection will nc»t 
produce absolute perfection, nor do we always meet, as far as 
we can judge, with tliis high standard under nature. The cor- 
rection for the al)erration of light is said by !Mliller not to be 
perfect even in that most perfect organ, the human eye. If our 
•reason leads us to admire with enthusiasm a multitude of in- 
imitable contrivances in nature, this same reason tells us, though 
we may easily err on both sides, that some other contrivances 
are less perfect. Can we consider the sting of the bee as per- 
fect, which, when used against many attacking animals, cannot 
be withdrawn, owing to the backward scrratures, and so inevi 
tably causes the death of the insect by tearing out its viscera ? 

if we look at the sting of the bee, as having originally ex- 
isted in a remote progenitor as a boring and serrated instru- 
ment, like that in so many members of the same great order, 
and wliich has been modified but not perfected for its present 
purjiose, with the poison originally adapted for some other 
pur[)ose, such as to produce galls, subsequently intensified, mo 
can ]ierhaps 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 conununity, it will 
fulfil all the requirements of natural selection, though it may 
cause the death of some few members. If we admire the truly 
wonderful power of scent by which the males of many insects 
find their females, can we admire the production for this single 
purpose of thousands of drones, which are utterly useless to the 
community for any other purpose, and which are ultimately 
slaughtered by their industrious and sterile sisters ? It may be 
ditVicult, Init 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 l)orn, or to perish herself in the combat ; 
for undoubtedly this is for the good of the community ; and 

108 SUMMARY. Cuap. VI. 

inatcriKil love or maternal hatred, though the latter fortunately 
is most rare, is all the same to the inexr)rablc principle of natu- 
ral selection. If we admire the several ini^enious contrivances, 
by which orchids and many other plants are fertilized through 
insect agency, can we consider as equally perfect the elabora- 
tion of dense clouds of pollen by our fir-trees, so that a few 
granules may be wafted by a chance breeze on to the ovules ? 

Summary : tJie Law of Unity of Type and oftJie Conditions 
of Existence embraced by the Theory of JVcitural Se- 

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 indepen- 
dent acts of creation are uttercly obscure. We have seen that 
species at any one period are not indefinitely variable, and are 
not linked together by a multitude of intermediate gradations, 
partly because the process of natural selection will always be 
very slow, and will act, at any one time, only on a few forms ; 
and partly because the very process of natural selection implies 
the continual supplanting and extinction of preceding and in- 
termediate gradations. Closely-allied species, now living on a 
continuous area, must often have been formed when the area 
was not continuous, and when the conditions of life did not in- 
sensibly graduate away from one part to another. When two 
varieties are formed in two districts of a continuous area, an 
intermediate variety will often be formed, fitted for an interme- 
diate zone ; but, from reasons assigned, the intermediate variety 
will usually exist in lesser numbers than the two forms which it 
connects ; consequently the two latter, during the course of 
further modification, from existing in greater numbers, Avill have 
a great advantage over the less numerous intermediate variety, 
and Avill thus generally svicceed in sujiplanting and exterminat- 
ing it. 

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

We have seen that a species may under new conditions of 
life change its habits, or have diversified habits, Avith some very 

Chap. VI. SUMMARY. 199 

unlike those of its ne:u-est conveners. Hence, we can under- 
stand, bearing in mind that each organic being is trying to Hvc 
wliercver it can live, how it has arisen that there arc upland 
geese with Avebbcd feet, ground woodpeckers, diving thrushes, 
and jx'trels with the hal)its of auks. 

Although the belief that an organ so perfect as the eye 
could have been formed by natural selection, is more than 
enouiih to sta^rger any one ; yet in the case of any organ, if we 
know of a long series of gradations m complexity, each good 
for its possessor, then, under changing conditions of life, there 
is no logical impossibilit}'^ in the acquirement of any conceivable 
degree of perfection through natural selection. In the cases 
in which we know of no intermediate or transitional states, we 
sliouhl be very cautious in concluding that none have existed, 
for the homologies of man}' organs and their intermediate states 
show what wonderful metamorphoses in function are at least 
'possible. For instance, a swim-bladder has apparently been 
converted into an air-breathing lung. The same organ having 
performed simultaneously very diflerent functions, and then 
liaving Ixicn in part or in whole specialized for one function ; 
and two distinct organs having performed at the same time the 
same function, the one having been perfected while aided by 
the other, must often have largely-facilitated transitions. 

We have seen in two beings, widely remote from each other 
in the natural scale, that an organ serving in both for the same 
purpose, and appearing closely similar, may have been sepa- 
rately and independently formed ; but when such organs are 
closely examined, essential dilTerences in their structure can 
almost always be detected; and this naturally follows from the 
principle of natural selection. On the other hand, the conunon 
rule throughout Nature is infinite diversity of structure for 
gaining the same end; and this again naturally, follows on the 
same great principle. 

In almost every case we are far too ignorant to be enaljled 
lo assert that any part or organ is so unimportant for the wel- 
fare of a species, that modifications in its structure could not 
have been slowly accumulated by means of natural selection. 
But Ave may confidently believe that many modifications, 
wholly due to the laws of growth, and at first in no way advanta- 
geous to a species, have been subsef|uently taken advantage of 
by the still furtlier modified descendants of this species. We 
may also believe tliat a part formerly of high importance has 
often been retained (as tlie tail of an aquatic animal by its ter- 
restrial descendants), though it has become of such small im- 

200 SUMMAEY. CiiAr. VI. 

portancc that it could not, in its present state, have been ac- 
quired by natural selection — a power which acts solely through 
tlic sur^-ival of tlie best-fitted individuals in the strufrgle for life. 

Natural selection will produce nothing in one sjiecies foi 
the exclusive good or injury of anotlier; though it may well 
produce parts, organs, and excretions, highly useful or even in- 
dispensable, or highly injurious to another species, but in all 
cases at the same time useful to the owner, Natural selection 
in each well-stocked country must act chiefly through the com- 
pc^tition of the inhabitants one with another, and consequently 
will produce perfection, or strength in the battle for life, only 
according to the standard of that countrj^ Hence the inhab- 
itants of one countr}', generally the smaller one, will often yield 
to the inhabitants of another and generally larger country. For 
in the larger country there will have existed more individuals, 
and more diversified forms, and the competition will have been 
severer, and thus the standard of perfection will have been 
rendered higher. Natural selection will not nccessarilj' pro- 
duce absolute perfection ; nor, as far as Ave can judge by our 
limited faculties, can absolute perfection be evei-ywhcrc found. 

On the theory of natural selection we can clearly inider- 
stand the full meaning of that old canon in natural history, 
" Natura non facit saltum." This canon, if we look only to 
the jiresent inhabitants of the world, is not strictly correct ; 
but if we include all those of past times, whether known or 
not yet known, it must by my theory be strictly true. 

It is generally acknowledged that all organic beings have 
been formed on two groat laws — Unity of Type, and the Con- 
ditions of Existence. B}^ imity of type is meant that funda- 
mental agTcement in structure which we see in organic beings 
of the same class, and which is quite independent of their 
halnts of life. . On my theory, unity of type is explained by 
unity of descent. The expression of conditions of existence, so 
often insisted on by the illustrious Cuvier, is fully embraced by 
tlie principle of natural selection. For natural selection acts 
by either now adapting the varying parts of each being to its 
organic and inorganic conditions of life ; or by having adapted 
i\unn during long-past periods of time : the adaptations being 
aided in some cases by use and disuse, being affected by the 
direct action of the external conditions of life, and being in all 
crises subjected to the several laws of growth. Hence, in fact, 
the law of the Conditions of Existence is the higher law ; as it 
iiuhidcs, thrnugli the inheritance of former adaptations, that 
of Unity of Type, 

Cii.i.Vll. INSTINCT. 201 



Instincts coniparnl)lo with Ilabiis, but difTcront in their Orii^in— Inptincls £;radu.ited 
— Aphides and Ants— Instincts vnriable— DomcBtic Instincts, tlicir Origin— Nat- 
ural luHtincta of tlic Cuckoo. Ostrich, and Parasitic Bees— Slave-niakin:,' Ants— 
Hive-bee, its cell-inakini; instinct— Changes of Instinct and Structure not neces- 
sarily simultaneous- DilDculties of the Theory of the Natural tjelectiou of lu- 
btincta— Neuter or Sterile Insects — Summary. 

Instincts might have been worked into the previous chap- 
ters ; but I thought that it would be more convenient to treat 
the subject separately, especially as an instinct so wonderful as 
that of the construction of the comb by the hive-bee will prob- 
ably have occurred to many readers, :is a difficulty sufficit-nt to 
overthrow the whole theory. I must premise that I have 
notliing to do with the origin of tlic primary 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 qualities of animals Avithin the same class. 

I will not attempt any definition of instinct. It would be 
easy to show that several distinct mental actions are commoidy 
embraced by this term ; but every one underst;uids what is 
meant, when it is said that instinct impels the cuckoo to mi- 
grate and to lay her eggs in other birds' nests. An action, 
which we ourselves should require experience to enable us to 
perform, when performed by an animal, more especially by a 
very young one, without any experience, and when performed 
by many individuals in the same way, without their knowing 
for what purpose it is performed, is usutilly said to he instinc- 
tive. But I could show that none of these cliaracters of in- 
stinct are universal. A little dose, as Pierre Hubcr expresses 
it, of judgmtMit or rea.son, often comes into play, even in ani- 
mals low in the scale of nature. 

Frederick Cuvier and several of the older m('tai)hysicians 
have compared instinct with habit. This comparLsou gives, I 
think, an accurate notion of the frame of mind under which 
an instinctive action is performed, but not necessarily of its 

202 INSTINCT. Chap. VII. 

origin. How unconsciously many habitual actions are per* 
fornieil, indeed not rarely in direct opposition to our conscious 
Avill ! yet tlicy may be modified by the ■will or reason. Habits 
easily become associated with other habits, and witli certain 
periods of time and states of the body. A\'hcn 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 ])erson be in- 
terrupted in a song, or in repeating any thing by rote, he is 
generally forced to go back to recover the habitual train of 
thought : so P. Huber found it Avas with a caterpillar, which 
makes a very complicated hammock ; for if he took a cater- 
pillar 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 reperformed 
the fourth, fifth, and sixth stages of construction. If, however, 
a cater])illar were taken out of a hammock made up, for in- 
stance, to the third stage, and were put into one finished up to 
the sixth stage, so that much of its work was already done for 
it, far from feeling the benefit of this, it was much embarrassed, 
and, in order to complete its hammock, seemed forced to start 
from the third stage, where it had left off, and thus tried to 
complete the already-finished work. 

If we suppose any habitual action to become inherited — and 
I think it can be shown that tliis does sometimes happen — 
then the resemblance between Avhat originally was a habit and 
an instinct becomes so close as not to be distinguished. If 
Mozart, instead of playing the jiiano-forte at three years old 
with wonderfully little jiracticc, had played a tune with no 
l)ractice at all, he might truly be said to have done so instinc- 
tivclv. But it would be a serious error to suppose that the 
greater number of instincts have been acquired by habit in one 
generation, and then transmitted by inheritance to succeeding 
generations. It can be clearly shown that the most wonder- 
ful instincts with which we arc acquainted, namely, those of 
the hive-bee and of many ants, could not possibly have been 
acqtiired by habit. 

It will be universally admitted that instincts are as imjior- 
tant as corporeal structure for the welf:ire of each species, un- 
der its present conditions of life. lender changed conditions 
of life, it is at least possible that slight modifications of in- 
stinct might be profitable to a species ; and if it can be shown 

Chap. VII. INSTINCT. 203 

that instincts do vary ever so little, then I can see no dilTiculty 
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 in- 
stiijcts have originated. As modifications of corporeal struct- 
ure arise from, and arc increased b}'", 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 arc of 
quite subordinate importance to the effects of the natural selec- 
tion of what may be called spontaneous variations of instincts; 
tliat is, of variations produced by tlie same unknown causes 
which produce slight deviations of bodily structure. 

No comj^lex instinct can possibly be produced througli nat- 
ural selection, except by the slow and gradual accumulation 
of numerous, slight, yet profitable, variations. Hence, as in 
the case of corporeal structures, we ought to find in nature, not 
the actual transitional gradations by which each complex in- 
stinct 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 shoAV 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- kno^vn among extinct 
species, how very generall^'^ gradations, leading to the most 
complex instincts, can be discovered. Changes of instinct may 
sometimes be facilitated by the same species having different 
instincts at different periods of life, or at different seasons of the 
year, or when placed imder difi'erent circumstances, etc. ; in 
which case cither the one or the other instinct might be pre- 
served by natural selection. And such instances of diversity 
of instinct in the same species can be shown to occur in nature. 

Again, as in the case of corporeal structure, and conform- 
ably to my theorj-, the instinct of each species is good for it- 
self, but has never, as far as we can judge, been produced for 
the exclusive good of others. One of the strongest instances 
of an animal apparently performing an action for the sole good 
of another, witli which I am acquainted, is that of ajihides vol- 
untarily yielding, as was first observed l)y Hu})er, their sweet 
excretion to ants ; that they do so voluntarily, the following 
facts show : I removed all the ants from a group of about a 
dozen aphides on a dock-plant, and prevented their attendance 

204 INSTINCT. CiiAr. VII. 

during several hours. After this interval, I felt sure that the 
aphides would Avant to excrete. I watched thcni 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 tlie ants do with their antennre; 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 Hock it had discovered ; it then began to play witli 
its anteun;i3 on the abdomen first of one aphis and then of an- 
other; and each, as soon as it felt the antenna?, immediately 
lifted up its abdomen and excreted a limpid drop of sweet 
juice, which was eagerly devoured by the ant. Even the 
quite young aphides behaved in this manner, showing that the 
action Avas instinctive, and not the result of experience. It is 
certain, from the observations of Huber, that the aphides show 
no dislike to the ants : if the latter be not present they are at 
last compelled to eject their excretion. But as the excretion 
is extremely viscid, it is no doubt a convenience to the aphides 
to have it removed ; tlierefore, 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 in- 
stincts of others, as each takes advantage of the weaker bodily 
structure of other species. So, again, certain instincts cannot 
be considered as absolutely perfect ; but as details on this and 
other such points are not indispensable, they may be here 
passed over. 

As some degree of variation in instincts under a state of 
nature, and the inheritance of such variations, arc indispensable 
for the action of natural selection, as many instances as pos- 
sible ought to be here given ; but want of space prevents me. 
I can only assert that instincts certainly do vary — for instance, 
the migratory instinct — both in extent and direction, and in its 
total loss. So it is with the nests of birds, which vary partly 
in dependence on the situations chosen, and on the nature and 
temperature of the country inhabited, but often from causes 
Avliolly unknown to us : Audubon has given several remarkable 
cases of differences in the nests of the same species in the 
Northern and Southern United States. A\''hy, it has been asked, 
if instinct be variable, has it not granted to the bee " the ability 
to use some other material Avhen wax was deficient ? " But 
what other material could bees use? They Avill work with 
and use, as I have seen, wax hardened with vermilion and 


softened" with lard. Andrew Knight observed that his bees, 
instead of laboriously collecting propolis, used a cement of 
wax and turpentine, with which he had covered decorticated 
trees. It has lately been shown that bees, instead of search- 
ing flowers for their pollen, will gladly use a very different 
substance, namely, oatmeal. Fear of any particular enemy is 
certainly an instinctive quality, as may be seen in nestling 
birds, though it is strengthened by experience, and by the 
sight of fear of the same enemy in other animals. The fear of 
man is slowly acquired, as I have elsewhere shown, by the 
various animals which inhabit desert islands ; and we see an 
instance of this even in England, in the greater wildness of 
all our large birds in comparison with our small birds; for 
llie large birds have been most persecuted by man. '\\'e may 
safely attribute the greater Avildness of our large birds to this 
cause ; for in iminhabited islands large birds are not more fear- 
'ful than small ; and the magjDie, so wary in England, is tame 
in Norwa}', as is the hooded crow in Egypt. 

That the mental qualities of animals of the same kind, 
bom in a state of nature, vary much, could be shown by 
many facts. Several cases could also be given of occasional 
and strange habits in wild animals, which, if advantageous to 
the species, might give rise, through natural selection, to 
new instincts. But 1 am well aware that these general state- 
ments, without the facts in detail, Avill produce but a feeble 
effect on the reader's mind. I can only repeat my assurance, 
that I do not speak without good evidence. 

Inherited Chanr/cs of Ilah'it or Instinct in Domesticated Animals. 

The possibilit}', or even probability, of inherited varial ions 
of instinct in a state of nature Avill be strengthened In' briefly 
considering a few cases under domestication. Wc shall thus 
b(; enabled to see the part Avhich habit and the selection of 
so-called accidental or spontaneous variations have plaj-ed in 
modifying the mentiil ([ualilies of our domestic animals. It is 
notorious how much domestic animals vary in their mental 
qualities. With cats, for instance, one naturally takes to catch- 
ing rats, and another mice, and these tendencies are known 
to be inherited. One cat, according to Mr. St. John, alwavs 
brought home game-birds, another hares or rabbits, and another 
Inmtcd on marshy ground and almost nightly caught wood- 
cocks or snipes. A number of curious and authentic instancca 


could be g'iven of various shades ol' disjKjsitiou and of taste, 
• and likcAvisc of the oddest tricks, associated with certain frames 
of mind or periods of time, being inherited. But let us 
look to the familiar case of the breeds of the dof^ : it cannot 
be doubted that younj^ pointers (I have myself seen a striking 
instance) "will sometimes point and even back other dogs 
the very first time that they are taken out ; retrie\-ing is 
certainly in some degree inherited by retrievers ; and a ten- 
dency to run round, instead of at, a flock of sheep, by shep- 
herd-dogs. I cannot see that these actions, performed with- 
out experience by the young, and in nearly the same manner 
by each indi\'idual, performed with eager delight by each 
breed, and "without the end being known — for the young 
pointer can no more know that he points to aid his master, 
than the white butterfly knows why she lays her eggs on 
the leaf of the cabbage — I cannot see that these actions 
differ essentially from true instincts. If we were to behold 
one kind of wolf, when young and without any training, as 
soon as it scented its prey, stand motionless like a statue, 
and 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 assur- 
edly call these actions instinctive. Domestic instincts, as they 
may be called, are certainly far less fixed than natural in- 
stincts; but they have been acted on by far less rigorous selec- 
lion, and have been transmitted for an incomparably shorter 
period, imder less fixed conditions of life. 

HoAV strongly these domestic instincts, habits, and dis- 
positions, are inherited, and how curiously they become 
mingled, is well shown when diflferent breeds of dogs are 
crossed. Thus it is known that a cross with a bull-dog has 
afiFected for many generations the courage and obstinacy of 
greyhounds ; and a cross Avith a greyhound has given to a 
whole famih^ of shepherd-dogs a tendency to hunt hares. 
These domestic instincts, when thus tested by crossing, resem- 
ble 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 Koy describes a dog, 
whose great-grandfather was a wolf, and this dog showed a 
trace of its wild parentage only in one way, by not com- 
ing in a straight line to his master, when called. 

Domestic instmcts are sometimes spoken of as actions 
which have bec^.me inherited solelv fi"om 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 light tendency to this strange 
habit, and tlie 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 cannot fly eighteen inches high without 
going head over heels. It may be doubted whether any 
one would have thought of training a dog to point, had not 
some one dog naturally shown a tendency in this line ; and 
this is known occasionally to happen, as I once saw, in 
a pure terrier : the act of pointing is probably, as many 
have thought, only the exaggerated pause of an animal 
preparing to sjiring on its prey. When the first tendency 
to point was once displayed, methodical selection and the 
inherited effects of compulsory training in each successive 
generation would soon complete the work ; and unconscious 
selection is still in progress, as each man tries to procure, 
without intending to improve the breed, dogs which will 
stand and hunt best. On the other hand, habit alone in some 
cases has sufficed ; hardly any animal is more diflicult, in most 
cases, to tame than the young of the Avild rabbit ; scarcely any 
animal is tamer than tlic young of the tame rabbit; bull 
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 remark- 
able instance of this is seen in those breeds of fowls which 
very rarely or never become " broo<ly," that is, never wish 
to sit on tlieir eggs. Familiarity alone prevents our seeing 
how largely and how permanently tlie minds of our domestic 
animals have been modified. It is scarcely possible to doubt 
that the love of man has become instinctive in tlie dog. 
All wolves, foxe.s, jackals, and species of the cat genus, when 
kept tame, an; most eager to attack poultry, sheep, and pigs; 
and this tendency has been found incural)le in dogs which 
liave been l)rought home as jiuppies from coimtries, such 
as Tierra del Fuego and Australia, where the savages do 


not keep lliese 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, with some de- 
gree of selection, has probably concmred in civilizing by in- 
heritance our dogs. On the other hand, young chickens 
have lost, wholly by habit, that fear of the dog and cat 
which no doubt was originally instinctive in them •, for I 
am informed by Captain Hutton that the young chickens of 
the parent-stock, the Gallus bankiva, when reared in India 
imder a hen, are at first excessively wild. So it is with 
young pheasants reared in England under a hen. It is not 
that chickens have lost all fear, but fear only of dogs and 
cats, for if the hen gives the danger-chuckle, they will nm 
(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 
tliis instinct retained by our chickens has become useless 
imder domestication, 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 accumu- 
lating, during successive generations, peculiar mental habits 
and actions, which at first appeared from what we must in our 
ignorance call an accident. In some cases compulsory habit 
alone has sufficed to produce inherited mental changes ; in 
othf^r cases compulsory habit has done nothing, and all has 
been the result of selection, pursued both methodically and 
imconsciously : but in most cases habit and selection have 
probably acted together. 

Special J/isti?ii't^'i. 

We shall, perhaps, best understand how instincts in a slate 
of nature have become modified by selection, by considering a 
few cases. I will select only three, out of tliose Avhich I sliall 
have to discuss in my future work — namely, the instinct which 
leads the cuckoo to lay her eggs in other birds' nests : the 
slave-making instinct of certain ants ; and the cell-making 
power of the hive-bee : these two latter instincts have gcner- 


ally and justly been ranked by naturalists as the most wonder- 
ful of all known instincts. 

Instincts of the Cuckoo. — It is supposed by some natu- 
ralists that the more immediate cause of the instinct of the 
cuckoo is that she lays her ep^gs, 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 eg'p;s, those first laid would have to be left 
for some time unincubatcd, or there would be eggs and young 
birds of different ages in tlie same nest. If this Avere the case, 
tlie process of laying and hatching might be inconveniently 
long, more especially as she migrates at a very early period ; 
and the first hatched young would prol:)abl3' have to be fed by 
the male alone. But the American cuckoo is in this predica- 
ment ; for she makes her own nest, and has eggs and young 
successively hatched, all at the same time. It has been both 
asserted and denied that the American cuckoo occasionally 
lays her eggs in other birds' nests ; but I have lately heard 
from Dr. Merrcll, of Iowa, that he once found in Illinois a 
young cuckoo together with a young jay in the nest of a Blue 
jay (Garrulus cristatus) ; and as both were nearly fully feath- 
ered, there could be no mistake in their identification. I could 
also give several instances of various birds which have been 
known occasionally to lay their eggs in other birds' nests. 
Now let us suppose that the ancient progenitor of our Euro- 
pean cuckoo had the habits of the American cuckoo, and that 
she occasionally laid an e^^ in another bird's nest. If the old 
bird profited by this occasional habit through being enabled to 
migrate earlier or through any other cause ; or if the yoimg 
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 eggs and young of different ages at the same time ; 
then the old birds or the fostered young would gain an advan- 
tage. And analogy would lead us to believe that the young 
thus roared would be apt to follow by inheritance the occasion- 
al and aberrant habit of their mother, and in their turn would 
be apt to lay their eggs in other birds' nests, and thus be more 
successful in rearing their young. By a continued process of 
this nature, I believe that the strange instinct of our cuckoo 
has been generated. It has, also, recently been ascertained 
that the cuckoo occasionally lays her eggs on the bare ground, 
sits on them and feeds her young ; this rare and strange event 
evidently is a case of reversion to the long-lost aboriginal in- 
stinct of nidification. 


It has been objected that I liave not noticed other related 
instincts and adaptations in the cuckoo, which are falsely 
spoken of as necessarily coordinated. But in all cases, specu- 
lation on an instinct known only in a single species is useless, 
for we have no facts to guide us. Until quite recently the in- 
stincts of the European and of the non-parasitic American 
cuckoo alone Avere known ; now, owing to Mr. E. Ramsaj^'s 
observations, we know something about three Australian spe- 
cies, which lay their eggs in other birds' nests. The chief 
points are three : first, that the cuckoo, with rare exceptions, 
lays only one egg in a nest, so that the large and voracious 
young l)ird may receive ample food. Secondly, that the eggs 
are of remarkably small size, not exceeding those of the sky- 
lark — a bird about one-fourth as large as the cuckoo. That 
the small size of the egg is a real case of adaptation we may 
infer from the fact of the non-parasitic American cuckoo laying 
full-sized eggs. Thirdl}"-, that the young cuckoo, soon after 
birth, has the instinct, the strength, and a properly-shaped 
back, for ejecting its foster-brothers, which then perish from 
cold and hunger. This, it has been boldly maintained, is a 
beneficent arrangement, in order that the young cuckoo may 
get sufficient food, and that its foster-brothers may perish be- 
fore they have acquired much feeling. 

Turning now to the Australian species ; though these birds 
generally lay only one egg in a nest, it is not rare to find two 
and even three eggs in the same nest. In the Bronze cuckoo 
the eggs vary greatly in size, from eight to ten lines in length. 
Now if it had been of any 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 
tliere is a relation between size and the period of incubation), 
then there is no difficulty in believing that a race or species 
might have been formed which would have laid smaller and 
smaller eggs ; for these would have been more safely hatched 
and reared. ^Ir. Ramsay remarks that two of the Australian 
cuckoos, Avhen they lay their eggs in an open nest, manifest a 
decided preference for nests containing eggs similar in color to 
their own. Tlie Eurojiean sjiecies certainly 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-Avarbler with bright greenish-blue eggs. 
Had our cuckoo invariably displaj-ed the above instinct, it 


would assuredly have been added to those Mhich it is assumed 
must all have been acquired together. The eg-gs of the Aus- 
tralian ]3ronze cuckoo vary, according to Mr. Ixanisay, to an 
extraordinary degree in color; so that in this respect, as well 
as in size, natural selection assuredly might have secured and 
iixed any advantageous variation. 

With reference to the young European cuckoo ejecting its 
foster-brothers — it must first be remarked that Mr. Gould, who 
has paid particular attention to this subject, is convinced that 
the belief is an error : he asserts that the young foster-birds 
are generally ejected during the first three days, when the 
yoimg cuckoo is quite powerless ; he maintains that the young 
cuckoo exerts, l)y its hunger-cries, or by some other means, 
such a fascination over its foster-parents, that it alone receives 
food, so that the others are starved to death, and are then 
tljrown out, like the egg-shells or the excrement, by the old 
birds. He admits, however, that the 3'^oung cuckoo Avheu 
giown older and stronger may have the power, and perhaps the 
instinct, of ejecting its foster-brothers, if they hapj)en to escape 
starvation during the first few days after birth. Mr. Ramsay 
has arrived at the same conclusion regarding the Australian 
species : he states that the young cuckoo is at first a little 
helpless fat creature, but, " as it grows rapidly, it soon fills up 
the greater part of the nest, and its unfortunate companions, 
either smothered by its weight, or starved to death through its 
greediness, are thrown out by their parents." Nevertlieless 
there is so much evidence, both ancient and recent, that the 
young Eurojican cuckoo does eject its foster-brothers, that this 
can hardly be doubted. Now, if it were of great importance 
to the young cuckoo to receive as much food as possible soon 
after birth, I can sec no special difficulty in its gradually 
acrjuiring, during successive generations, the habit (perhaps 
through mere unintentional restlessness), the strength, and the 
structure, best fitted for ejecthig its foster-brothers; for those 
young cuckoos which had such habits and structure would be 
the best fed and most securely reared. I can sec no more 
difficulty in this, than in young birds acquiring the instinct and 
the temporary hard tips to their beaks for breaking through 
their own shells — or than in young snakes having 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 any age, and the variations 
tend to be inherited at a corresponding age — propositions 


which caiiiioL be disputed — then the instincts and structure of 
the youns-;' could be slowly modified as well as those of the 
adult; antl both cases must stand or fidl with the whole theory 
of natural selection. 

The occasional habit of birds laying their eggs in other 
birds' nests, either of the same or of distinct species, is not 
very uncommon with the Gallinaceas ; and this perhaps explains 
the origin of a singular instinct in the nearest allied group, 
that of ostriches. For several hen-ostriches unite and lay first 
a few eggs in one nest and then in another; and these arc 
hatched by the males. This instinct may probably be accounted 
for by the ftict of the hens lajang a large number of eggs, but, 
as in the case of the cuckoo, at intervals of two or three days. 
The instinct, however, of the American ostrich has not as yet 
been perfected ; for a surprising number of eggs lie strewed 
over the plains, so that in one day's hunting I picked up no 
less than twenty lost and wasted eggs. 

T\Iany bees are parasitic, and regularly lay their eggs in the 
nests of other kinds of bees. This case is more remarkable 
than that of the cuckoo ; for these bees have not only had their 
instincts but their structure modified in accordance with their 
parasitic habits ; for they do not possess the pollen-collecting 
apparatus which Avould have been indispensable if they had 
stored food for tlieir own yoimg. Some species, likewise, of 
Sphegida) (wasp-like insects) are parasitic in the same manner 
on other species ; and M. Fabre has lately shown good reason 
for believing that, although the Tachytes nigra generally makes 
its OAvn burrow and stores it with paralyzed prey for its own 
larv;e, yet that, when this insect finds a burrow already made 
and stored by another sphex, it takes advantage of the prize, 
and becomes for the occasion parasitic. In this case, as Avith 
the supposed case of the cuckoo, I can see no difficulty in nat- 
ural selection making an occasional habit permanent, if of ad- 
vantage to the species, and if the insect, whose nest and stored 
food are thus feloniously appropriated, be not thus extermi- 

Slave-male hip Instinct. — This remarkable instinct Avas first 
discovered in the Formica (Polyerges) rufesccns by Pien-o 
Huber, a better oliserv-er even than his celebrated father. This 
ant is absolutely dependent on its slaves ; Avithout their aid, 
the species Avould certainly become extinct in a single year. 
The males and fertile females do no Avork of any kind, and the 
Avorkers or sterile females, though most energetic and coura- 


{2:eous in capturinp^ slaves, do no otlier work. They are inca- 
paljle of makiiii^ their own nests, or of feeding their own larv'a3. 
When the old nest is found inconvenient, 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 Hubcr shut up thirty of them with- 
out a slave, but with plenty of the food Avhich they like best, 
and with tlieir own larvie and pupa3 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 larv.T, and put all to 
rights. What can be more extraordinary than these avcII- 
ascertaincd facts ? If we had not known of any other slave- 
making ant, it would have been hopeless to speculate how So 
wonderful an instinct could have been perfected. 

Another species, Formica sanguinea, Avas likewise first dis- 
covered by P. Huber to be a slave-making ant. This species 
is found in the southern parts of England, and its habits have 
been attended to by Mr. F. Smith, of the British Museum, to 
whom I am nnich indebted for information on this and other 
subjects. Although fully trusting to the statements of Hulxu' 
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 truth of so extraordinary and odious an instinct as that (jf 
making slaves. Hence I will give the observations wliicli I 
made, in some little detail. I oj^ened fourteen nests of F. san- 
guinea, and found a few slaves in all. Males and fertile fe- 
males of the slave sj)ecics (F. fusca) are found only in their 
own proper communities, and have never been observed in the 
nests of F. sanguinea. The slaves are black and not above 
half the size of their red masters, so that the contrast in their 
appearance is great. W^hen 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 larva^ and pup:c 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 nujuths of June and July, on 
three successive years, I have 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 numljcr, I thought that they might behave differently wlien 


more numerous ; but Mr. Smith informs me that he has watched 
the nests at various liours during' May, June, and Aui^ust, both 
in Surrey and Hamjishire, and lias never seen tlie slaves, 
thougli present in large nundjers in August, either leave or 
enter the nest. Hence he considers them as strictly house- 
hold slaves. The masters, on the other hand, maybe constant- 
ly seen bringing' in materials for the nest, and food of all kinds. 
During the ^-ear 18G0, however, in the month of July, I came 
across a community with an unusually large stock of slaves, 
and I observed a few slaves mingled with their masters leaving 
the nest, and marching along the same road to a tall Scotch-fir- 
tree, twenty-five yards distant, which they ascended together, 
prol)ably in search of aphides or cocci. According to Huber, 
who had ample opportimitics for observation, the slaves in 
Switzerland habitually Avork with their masters in making the 
nest, and they alone open and close the doors in the morning 
and evening; and, as Pluber expressly states, their jmncipal 
office is to search for aphides. This difference in the usual 
habits of the masters and slaves in the two coinitries probably 
depends merely on the slaves being captured in greater num- 
bers in Switzerland than in England. 

One day I fortunately witnessed a migration of F. sanguinea 
from one nest to another, and it Avas a most interesting spec- 
tacle 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 b}^ an independent community of the slave-species 
(F. fusca) ; sometimes as many as tliree of these ants cUnging 
to the legs of the slave-making F. sanguinea. The latter ruth- 
lessly 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 pupre 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 perhaps fancied that, after all, tliC}' had been vic- 
torious in their late combat. 

At the same time I laid on the same place a small parcel 
of the ]nipre of another species, F. flava, with a few of these 
little yellow ants still clinging to the fragments of their nest. 
Tliis species is sometimes, though rarely, made into slaves, as 


has been described by Mr. Smith. Although so small a species, 
it is very com'ageoiis, and I have seen it ferociously attack other 
ants. In one instance I found to my surprise an independent 
connnunity of F. llava under a stone beneath a nest of the 
slave-making F. sanguinea ; and when I had accidentally dis- 
turbed both n«'sts, tlie little ants attacked their big neighbors 
with surprising courage. Now I was curious to ascertain wheth- 
er F. sanguinea could distinguish the pupae of F. fusca, which 
they habitually make into slaves, from those of the little and 
furious F, llava, 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 pupre of F. fusca, whereas 
they were much terrified when they came across the pupa?, or 
even the earth from the nest, of F. flava, and cjuickly 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 pupaj. 

One evening I visited another community of F. sanguinea, 
and found a number of these ants returning home and entering 
their nests, carrying the dead bodies of F. fusca (showing that 
it was not a migration) and numerous pupa?. I traced a long 
file of ants burdened Avith 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, h^wnver, 
must have been close at hand, for two or three individuals of 
F. fusca were rushing al^out 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 tliey did not need confirmation 
by me, in regard to the wonderful instinct of making slaves. 
Let it be observed what a contrast the instinctive habits of F. 
sanguinea jiresent with those of the Continental F. rufescens. 
The latter does not build its own nest, does not determine its 
own migrations, does not collect food for itself or its young, 
and cannot even feed itself: it is absolutely dependent on its 
numerous slaves. Formica sanguinea, on the other hand, pos- 
sesses much fewer slaves, a!id in the early part of the summer 
extremely few : the mastcTS 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 lurviu, and the masters alono 


go on slave-making expeditions. In Switzerland the slaves 
and masters Avork together, making and bringing materials for 
the nest; both, l)iit cliiefly 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 Ijuilding-materials and food for themselves, their 
slaves and larva^. So that the masters in this country receive 
much less service from their slaves than they do in Switzerland. 

By what steps the instinct of F. sanguinea originated I 
will not pretend to conjecture. But as ants, which are not 
slave-makers, Avill, as I have seen, carry off pupoe of other spe- 
cies, if scattered near their nests, it is possible that such pupaj 
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 pres- 
ence proved useful to the species which had seized them — if it 
Avere more advantageous to tliis species to capture workers 
than to procreate them — the habit of collecting pupa? originally 
for food might by natural selection be strengthened and ren- 
dered i^ermanent for the very different purpose of raising slaves. 
When the instinct was once acquired, if carried out to a much 
less extent even than in our British F. sanguinea, which, as 
we have seen, is less aided by its slaves tlian the same species 
in Switzerland, natural selection might increase and modify the 
instinf"*^ — always supposing each modification to be of use to 
th(; species — until an ant Avas fcn-med as abjectly dependent on 
its slaves as is the Formica rufcscens. 

Cell-making Instinct of the IIivc-J3ee. — I will not here 
enter on minute details on this subject, but Avill merely give an 
outline of the conclusions at Avhich I have arrived. He must 
be a dull man Avho can examine the exquisite structure of a 
comb, so beautifully adapted to its end, without enthusiastic 
admiration. We hear from mathematicians that bees have 
jiractically solved a recondite ]irobk'm, and have made their 
cells of the proper shape to hold the greatest possible amount 
of hone}', with the least possible consumption of precious Avax 
in their construction. It has been remarked that a skilful Avork- 
man Avith fitting tools and measures, Avould find it very difficult 
to make cells of Avax of the true form, though this is jierfectly 
elTected by a croAvd of bees Avorking in a dark hive. Granting 
whatever instincts you please, it seems at first quite inconceiv- 
able how they can make all the necessary angles and planes, 
or even perceive Avhen they are correctly made. But the dilli- 


culty is not nearly so great as it at first appears : all this beau- 
tiful work can be shown, I think, to follow from a few simple 

I was led to investigate this subject by INIr. Waterhoiisc, 
who hasshown that the form of the cell stands in close relation 
to the presence of adjoining cells ; and the following view may, 
perhaps, be considered only as a modilicalion of his theory : 
let us look to the great priucijile of gradation, and see Avhether 
Nature does not reveal to us licr method of woik. At one end 
of a .short series we have huiuble-l)ees, which use their old 
cocoons to hold honey, soinctiTucs adding to them short tubes 
of wax, and liUewi.s<; making separate and very iiTCgular rounded 
cells of wax. At the other end (if the series Ave have the cells 
of the hive-bee, jilaced in a doiil)le la^-er: each cell, as is well 
known, is an hexagonal prism, with the basal edges of its six 
sides bevelled so as to lit on to a pj-ramid, formed 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 simpli- 
city of those of the humble-bee, we have the cells of the Mexi- 
can ^lelipona domestica, carefully described and figured by 
Pierre Iluber. llie I\Ielipona itself is intermediate in structure 
between the hive and humble bee, but more nearly related to 
the latter; it forms a nearly regular waxen comb of cylindrical 
cells, in which the young are hatched, and, in addition, some 
large cells of wax for holding honey. These latter cells ai'e 
nearly spherical and of nearly equal sizes, and are aggregated 
into an irregular mass. Ikit the important point to notice is, 
that these cells are always made at that degree of nearness to 
each other, that they v>ould have intersected or broken into 
each other, if the spheres had been completed ; but this is never 
permitted, the bees building pel-fectly Hat walls of wax between 
the spheres which thus tend to intersect. Hence each cell con- 
sists of an outer spherical portion, and of two, three, or more 
perfectly flat surfaces, according as the cell adjoins two, three, 
or more other cells. When one cell 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 arc 
united into a p}Tamid ; and this pyramid, as Huber has re- 
marked, is manifestlv a gross imitation of the three-sid(Ml jiyram- 
'dal baseji of th(> cell of the liivc'liee. As in the fells of the 


hive-bee, so licre, the three plane surfaces in anyone cell neces- 
sarily enter into the construction of three adjoining cells. It 
is obvious that the Meli])ona saves wax, and, what is more im- 
portant, lal)or, by this manner of buildinjr; for the Hat walls 
between the adjoining cells are not double, but are of the Siune 
thickness as the outer spherical portions, and yet each flat por- 
tion forms a part of two cells. 

Rc^flecting 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 erjnal sizes, and had arranged 
them s^-nnnetrically in a double layer, the resulting structure 
would probably have been as perfect as the comb of the hive- 
bee. Accordingly, I wrote to Prof. Miller, of Cambridge, and 
this geometer has kindly read over the following statement, 
drawn up from his information, and tells me that it is strictly 
correct : 

If a number of equal spheres be described with their centres 
placed in two parallel layers, with the centre of each sphere at 
the distance of radius X 1^ 2, or radius X 1.414:'21 (or at some 
lesser distance), from the centres of the six surrounding spheres 
in the same layer; and at the same distance from the centres 
of the adjoining spheres in the other and parallel layer ; then, 
if planes of intersection between the several spheres in both 
layers be formed, there will result a double layer of hexagonal 
prisms united together by pyramidal bases formed of three 
rhombs ; and the rhombs and the sides of the hexagonal prisms 
will have every angle identically the same with the best meas- 
urements Avhich have been made of the cells of the hive-bee. 
But I hear from Prof. Wyman, who has made numerous care- 
ful measurements, that the accuracy of the workmanship of the 
bee has been greatly exaggerated ; so much so, that, as he adds, 
whatever the typical form of the cell may be, it is rarely, if 
ever, realized. 

Hence, we may safely conclude that, if Ave could slightly 
modify the instincts already possessed by the Melipona, and in 
themselves not very wonderful, this bee would make a structure 
as wonderfully perfect as that of the hive-bee. "We must sup- 
pose the Melipona to have the power of forming her cells truly 
spherical, and of equal sizes ; and this would not be very sur- 
prising, seeing that she already does so to a certain extent, and 
seeing what perfectly cvlindrical burrows in wood many insects 
can make, apparently by turning round on a fixed jioint. AVc 
must suppose the Melipona (o arrange her cells in level layers, 


as she already does her cylindrical cells ; and wc uiiist further 
suppose, and this is the greatest difliculty, that she can some- 
how 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 alwa3-s 
describes her spheres so as to intersect largely ; and then she 
unites tlie points of intersection by perfectly flat surfaces. 
We have further to suppose, but this is no difficulty, that after 
hexagonal ])risnis have been formed by the intersection of ad- 
joining spheres in the same layer, she can jirolong the hexagon 
to any length requisite to hold the stock of hone}*; in the same 
way as the rude humble-bee adds cylinders of wax to the circu- 
lar mouths of her old cocoons, ]5y such modifications of in- 
stincts, in themselves not very wonderful — hardly more wonder- 
ful than those which guide a bird to make its nest — I believe 
tluit 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 jierfectly true or parts of a sphere, and of about the 
diameter of a cell. It was most interesting to me to observe 
that, •wherever several bees had begun to excavate these basins 
near together, they had begun their work at such a distance from 
each other, that by tlie time the basins had acquired the above- 
stated width (i. e., about the width of an ordinary cell), and 
were in d<^pth about one-sixth of the diameter of the sphere of 
M'hiih they formed a part, the rims of the Ixisins intersected or 
broke into each other. As soon as this occurred, the bees 
ceased to excavate, and began to build up flat walls of wax on 
the lines of intersection between the basins, so that each hex- 
agonal prism was built upon the scalloped edge of a smooth 
l)asin, instead of on the straight edges of a three-sided pyramid 
as in the case of ordinary cells. 

I then put into the hive, instead of a thick, rectangular 
\)\cce of wax, a thin and narrow, knife-edged ridge, colored 
with vermilion. The bees instantlv began on both sides to ex- 
cavate little basins near to each other, in the same way as be- 
fore; 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 tlio 


former experiment, would liavc broken into eacli otlier from 
the opposite sides. The bees, liowcver, did not sufl'er 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 bottoms with flat sides ; and these flat sides, 
formed by little thin plates of the vermilion wax left un- 
p^nawed, were situated, as far as the eye could judge, exactly 
along the planes of imaginary intersection between the basins 
on tiie opposite sides of the ridge of wax. In some parts, only 
small portions, in other parts, large portions of a rhombic jilate 
had been left between the opposed basins ; but the work, from 
the mmatural 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 vennilion wax, in order to have thus suc- 
ceeded in leaving flat plates between the basins, by stopping 
work at the jilanes of intersection. 

Considering how flexible thin wax is, I do not see that 
there is any dilficulty 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 then stopping their work. 
In ordinarv combs it has appeared to me that the bees do not 
always succeed in working at exactly the same rate from the 
opposite sides ; for I have noticed half-completed rhombs at 
the base of a just-commenced cell, which were slightly concave 
on one side, where I suppose that the bees had excavated too 
quickly, and convex on the opposed side where the bees had 
■worked less quickly. In one well-marked instance, I jiut the 
coml) back into the hive, and allowed the bees to go on work- 
ing for a sliort time, and again examined the cell, and I found 
that the rhombic plate had been completed, and had become 
pcrfectli/ JJat : i^ was absolutely impossil)le, from the extreme 
thinness of the little plate, that they could have effected this 
by gnawing away the convex side ; and I suspect that the bees 
in such cases stand on opposite sides and push and bend the 
ductile and warm -wax (which, as I have tried, is easily done) 
into its ])roper intermediate plane, and thus flatten it. 

From the experiment of the ridge of vermilion wax we can 
Bee that, if the bees were to build for themselves a thin wall 
of wax, they could make their cells of the proper shape, by 
standing at the proper distance from each other, by excavat- 
ing at the same rate, and by endeavoring to make equal spheri- 
cal hollows, but never allowing the s])hcres to break into each 


other. Now bees, as may be clearly seen hy examining' the 
od^e of a ^rowinp^ comb, do make a roug'li, circumferential 
^vall or rim all round the com)) ; and they g'liaw this away from 
the op])()sitc sides, always workinj]^ 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 tliat one rhom- 
bic plate Avhich stands on the extreme growing marg-in, or the 
two plates, as the case may be ; and th(>y never complete the 
upper edges of the rhoml)ic plates, until the hexagonal walls 
are comnifmced. Some of these statements dilTer from those 
made by the justly-celebrated elder Uuber, but I am convinced 
of their accuracy ; and, if I had space, I could show that they 
are conformable with my theory. 

Huber's statement, that the very first cell is excavated ouJ 
of a little parallel-sided wall of wax, is not, as far as I liavo 
seen, strictly correct ; the first commencement ha^'ing always 
bben a little hood of wax ; but I will not here enter on these 
details. We see how important a part excavation plays in the 
construction of the cells ; but it would be a gn^at error to sup- 
pose that the bees cannot l)uild up a rough Avail of M'ax in the 
proper position — that is, along the plane of intersection be- 
tween two adjoining si)heres. I have several specimens show- 
ing clearly that they can do this. Even in the rude circum- 
ferential rim or wall of wax round a growing comb, flexures 
may sometimes be observed, corresponding in position to the 
planes of the rhombic basal plates of future cells. But the 
rough wall of wax has in every case to be finished ofl", l)y 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. 
\\^o shall understand how they work, by supposing masons first 
to pile up a broad ridge of cement, and then to Ix^gin cutting 
it away erjually on l)()th 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 sum- 
mit of tilt; ridge. "We shall thus have a thin wall steadily 
growing upward but always crowned by a gigantic coping. 
From all tlu; cells, both those just commenced and those com- 
pleted, Ijcing tluis crowned by a strong coping of wax, the 
bees can cluster and crawl over the comb without injuring the 
delicate hexagonal walls. These walls, as Prof. Miller has 
kindly ascertained for me, vary greatly in thickness ; being, 


on an average of twelve measurements made near tlie border 
of tlic comb, Tp^ of an inch in thickness ; whereas the basal 
rhomboidal plates are thicker nearly in the proportion of three 
to two, having a mean tliickness, from twenty-one measure 
mcnts, of -rAs of an inch, liy the above singular maimer of 
building, strength is continually given to the coml), with the 
utmost ultimate economy of wax. 

It seems at first to add to the difficulty of understanding 
how the cells are made, that a multitude of bees all work to- 
gether ; one bee after Avorking a sliort time at one cell going 
to another, so that, as Hubcr has stated, a score of individuals 
work even at the commencement of the first cell. I was able 
practicall}'" to show this fact, by covering the edges of tlie hex- 
agonal walls of a single cell, or the extreme margin of the cir- 
cumferential rim of a growing comb, with an extremely thin 
layer of melted vermilion wax ; and I invariably found that the 
color was most delicately difiused by tlie bees — as delicately 
as a painter could have done with his brush — by atoms of the 
colored wax having been taken from the spot on which it had 
been placed, and Avorked into the growing edges of the cells 
all round. The Avork of construction seems to be a sort of 
balance struck between many bees, all instinctively standing 
at the same relative distance from each other, all trying to 
SAveep equal spheres, and tlien building up, or leaving un- 
gnaAved, the planes of intersection betAveen these spheres. It 
was really curious to note in cases of difficulty, as Avhen two 
pieces of comb met at an angle, hoAV often the bees Avould pull 
down and rebuild in different ways the same cell, sometimes 
recurring to a shaj^e which they had at first rejected. 

When bees haA'e a place on Avhich they can starid in their 
proper positions for Avorking — for instance, on a slip of Avood, 
placed directly under the middle of a comb groAving doAvnAvard 
so that the comb has to be built OA'er one face of the slip — in 
this case the bees can lay the foundations of one Avail of a ncAV 
hexagon, in its strictly proper place, projecting beyond the 
other com])leted cells. It suffices that the bees should be en- 
al)led to stand at their proper relative distances from each 
other and from the A\-alls of the last completed cells, and then, 
by striking imaginary spheres, they can build up a Avail inter- 
mediate betAveen tAvo adjoining s])heres ; but, as far as I have 
seen, they ncAxr gnaw aA\-ay 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 doAvn under ccr- 


tain circumsluuccs a rough wall in its proper place l>elwccn two 
just-commenced cells, is important, as it bears on a fact, which 
seems at first quite subversive of the foregoing theory ; namely, 
that the cells on the extreme margin of Avasp-combs are some- 
times strictly hexagonal ; but I have not space here to enter 
on this subject Nor does there seem to me any great difii- 
culty in a single insect (as in the case of a queen-wasp) making 
hexagonal cells, if she were to work alternately on the inside 
and outside of two or three cells commenced at the same time, 
always standing at the proper relative distance from the parts 
of the cells just Ix^guji, sweeping spheres or cylinders, and 
building up intermediate planes. 

As natural selection acts only by the accumulation of slight 
modificiitions of structure or instinct, each profitable to the 
individual under its conditions of life, it may reasonably be 
asked, how a long and graduated succession of motlified archi- 
tectural instincts, all tending toward the prcsent perfect plan 
of construction, could have profited the progenitoi-s of the hive- 
bee ? I think the answer is not diflicult: cells 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 experi- 
mentally proved that from twelve to fifteen poimds of dry 
sugar arc consumed by a liive of bees for the secretion of each 
pound of wax; so that a prodigious quantity of fluid nectar 
nmst be collected and consumed by the bees in a hive for the 
secretion of the wax necessary for the construction of their 
combs. Moreover, many bees have to remain idle for many 
days during the jirocess of secretion. A large store of honey 
is indispensable to support a largt; 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 col- 
lecting the honey must be an important element of success to 
any family of bees. Of course, the success of the species ma\- 
be dependent on the number of its enemies, or parasites, or on 
(juite distinct caiuses, and so be altogether independent of the 
quantity of honey which the bees could collect. But let us 
suppose that this latter circumstance determined, as it proba- 
bly often has determined, whether a bee allied to our liumble- 
bces could exist in large nuinbere in any country ; and let us 


further suppose lliat tlie community lived tlirou;^h 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 imagi- 
nary humble-bee, if a slight modification in her instincts led 
her to make her waxen cells near together, so as to intci*sect ;•- 
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 to- 
gether, and aggregated into a mass, like the cells of the Jleli- 
])ona ; 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 Avould bo saved. Again, from the same cause, 
it Avould 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 sjiherical sm- 
faces would wholly disappear and be replaced by plane sur- 
faces ; and the Melipona Avould make a comb as perfect as that 
of the hive-bee. Beyond this stage of perfection in architect- 
ure, 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 v.'onderful of all known in- 
stincts, that of the hive-bee, can be explained by natural selec- 
tion having taken advantsige of numerous, successive, sUght 
modifications of simpler instincts; natural selection having, by 
slow degrees, more and more perfectly led the bees to sweej) 
etjual spheres at a given distance from each other in a double 
layer, and to build up and excavate the wax along the planes 
of intersection ; the bees, of course, no more knowing that 
they swept their spheres at one particular distance from each 
other, than they know what are the several angles of the 
hexagonal prisms and of tlie basal rhombic plates ; the motive 
jiower of tlie process of natural selection having been the con- 
struction of cells of due strength and of the proper size and 
shape for tlie larv.o?, this being etlected Avith the greatest pos- 
sible economy of labor and Avax ; that individual swarm which 
thus made tlie best cells with least lalior, and least waste of 
honey in tlie secretion of wax, having succeeded best, and liav- 
ing transmitted by inheritance their newly-acquired economical 
instincts to new swarms, which in their turn will have had tho 
best chance of succeeding in the struggle for existence. 


Objectiojis to the Tlicory of Natural Selection as cq^plied to 
Instincts y' Axuter and Sterile Insects. 

It has been objected to the foregoing view of the origin of 
instincts that " the variations of structure and of instinct must 
have been simultaneous and accurately adjusted to each other, 
as a modification in the one without an immediate corre- 
sponding change in the other Avould have been fatal." The 
force of this olijection rests entirely on the assumption that the 
changes in both instinct and structure are abrupt. To take as 
an illustration the case of the larger titmouse (Parus major) al- 
luded to in the last chapter : this l)ird often holds the seeds of 
the yew between its feet on a branch, and hannners away till 
it gets into the kernel. Now what special diiliculty would 
there be in natural selection preserving all the slight individ- 
ual variations in the shape of the beak, Avhich were better and 
better adapted to break ojien 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 sponta- 
neous variations of taste, led the bird to become more and more 
of a seed-eater ? In this case the beak is supposed to be slowly 
modified by natural selection, subsequently to, but in accordance 
with, slowly-changing habits or taste ; but let the feet of the 
titmouse vary and grow larger from correlation with tlie beak, 
or from any other imknown 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 
jwwer of the nuthatch. In this case a gradual change of struct- 
ure 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. iSoine birds build their nests 
of nnid, 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 selec- 
tion of individual swifts, wliich secreted more and more saliva, 
should at last produce a species with instincts leading it to 
neglect other matc^rials^ and to make its nest exclusively of 
inspissated saliva? And so in other cases. It must, however, 
be admitted tliat in many instances we cannot conjecture 
whetlier it was instinct or structure which first varied. 

No doubt many instincts of very dillicult explanation could 


be 0])poscJ to tlic theory of natunil selection — cases, in -which 
Ave cannot sec how an instinct could ])ossibly liave originated; 
cases, in ■which no intermediate gradations arc known to exist ; 
cases of instinct of such trifling importance, that they could 
hardly have been acted on by natural selection ; cases of in- 
stincts 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 independently acciuircd through natural 
selection. I will not here enter on these several cases, but 
will confine myself to one special difficulty, which at first ap- 
jieared to me insuperable, and actuall}' 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 Avill here take only a single case, that of working or 
sterile ants. Hoav the workers have been rendered sterile is a 
difriculty : but not much greater than that of any other strik- 
ing modification of structure ; for it can be shown that some 
insects and other articulate animals in a state of nature occa- 
sionally become sterile ; and if such insects had been social, 
and it had been profitable to the community that a numl^er 
should have been annually born capable of work, but incapable 
of procreation, I can see no especial difficulty in this having 
been efT(M'ted through natural selection. But I must pass over 
this preliminary difficulty. The great difiiculty lies in the 
working ants differing widely from both the males and the 
fertile females in structure, as in the shape of the thorax, and 
in being destitute of wings and sometimes of eyes, and in in- 
stinct. As far as instinct alone is concerned, the Avonderful 
difference in this respect between the Avorkers and the perfect 
females, would have been better exemplified by the hive-bee. 
If a working ant or other neuter insect had been an ordinary 
animal, 1 should have mihesitatingly assumed that all its char- 
acters had been slowly acquired through natural selection ; 
namely, by individuals having been born with slight profitable 
modifications, which were inherited by -the offspring; and that 
these again varied and again were selected, and so onward. 
But with the Avorking ant we have an insect differing greatly 
from its parents, yet. absolutely sterile ; so that it could never 
have transmitted successively-acquired modifications of struct- 


lire or instinct to its proc^ony. It may well be asked how is 
it possible to reconcile this case with the theory of natural 
selection ? 

First, let it be remembered that wc have innumeral)le in- 
stances, both in our domestic productions and in those in a 
state of nature, of all sorts of differences of inherited structure 
which are correlated with certain ages, and with either sex. 
We have differences correlated not only Avith one sex, but 
with that short period when the reproductive system is active, 
as in the nuptial plumage of many birds, and in the hooked 
jaws of the male salmon. We have even slight differences in 
the horns of diQ'erent breeds of cattle in relation to an artifi- 
cially imperfect state of the male sex ; for oxen of certain 
breeds have longer horns than tlie oxen of other breeds, rela- 
tively to the h'ugth of the horns in both the bulls and cows of 
these same breeds. Hence I can see no great difficulty in any 
character becoming correlated with the sterile condition of 
certain members of insect communities : the difficulty lies in 
understanding how such correlated modifications of structure 
could have been slowly accumulated by natural selection. 

Tliis dilficulty, though ajipearing insuperable, is lessened, 
' or, as I believe, disajjpears, when it is remembered that selec- 
tion may l)e applied to the family, as well as to the individual, 
and may thus gain the desired end. Thus, breeders of cattle 
wish the flesh and fat to be well marbled together ; the animal 
has been slaughtered, but the breeder has gone witli confidence 
to the same stock and has succeeded. Such faith may be 
placed in the power of selection, that probably a breed of 
cattle, always yielding oxen Avith extraordinarily long horns, 
could be slowly formed by carefully Avatching Avhich individual 
bulls and cows, Avhen matched, produced oxen Avith the longest 
horns ; and yet no one ox Avould ever have propagated its 
kind. Here is a better and real illustration : according to M. 
Vcrlot, some varieties of the double annual stock of various 
colors, from having been long carefully selected to the right 
degree, always produce by seed a large projinrtion of plants 
l)earing double andf|nite sterile floAvers ; so that, if the A'ariety 
had not likewise yielded others, it Avould at once have become 
extinct; but it always yields some single and fertile plants, 
Avhich differ from ordinary single A'arieties only in their power 
of producing the two forms. Thus the fertile plants producing 
single flowers may be compared Avith the males and females of 
an ant-comnumity, and the sterile double-flowered plants, Aviiich 


arc rcg'ularly produced in l:ir^e numbers, with tlu; iiKiny st4?rilo 
neuters of the same coininunity. Thus I l>elieve it has been 
"vvith social insects : a slij^ht modification of structure, or of in- 
stinct, correlated with tlic sterile condition of certain members 
of the conmiunity, has been advantageous to the community: 
consequently the fertile males and females of tlie same cona- 
munity flourished, and transmitted to their fertile ofl'sprinfr^ a 
tendency to produce sterile members having the same modifi- 
cation. And I believe that this process has been repeated, 
luitil that prodigious amount of difference between the fertile 
and sterile females of the same species has been produced, 
which wo see m so many social insects. 

But we have not as yet touched on the climax of the difE- 
culty : 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. Tiie castes, moreover, 
do not generally graduate into each other, but are perfectly 
well defined ; being as distinct from each other as are any two 
species of the same genus, or rather as any two genera of the 
same family. Thus in Eciton, there are working and soldier 
neuters, wnth jaws and instincts extraordinarily difi'ercnt : in 
Cryptocerus, the workers of one caste alone carry a wonderful 
sort of shield on their heads, the use of which is quite imknown : 
in the Mexican JMyrmecocystus, 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 se- 
cretes 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 con- 
fidence in the principle of natural selection, when I do not ad- 
mit that such wonderful and Avell-established facts at once an- 
jiiliilate the theory. In the simpler case of neuter insects all 
of one caste, which, as I believe, have been rendered different 
from the fertile males and females through natural selection, 
we may conclude from the analogy of ordinary variations, that 
the successive, slight, profitable modifications did not first 
arise in all the neuters in the same nest, but in some few alone ; 
and that by the survival of the communities with females which 
])r()(lure most neuters having the advantageous modification, all 
the neuters ultimately come to be thus characterized. Accord- 
ing to this view, Ave ought occasionally to find in the same nest 


neuter insects, presenting gradations of structure ; and this we 
do find, even frcquenth'-, considering liow few neuter insects 
out of Europe have been carefully examined, ^[r. F. JSinith 
lias shown that the neuters of several British ants differ sur- 
])risingly from each other in size, and sometimes in color ; and 
that the extreme forms can be linked together by individuals 
taken out of the sanu; nest ; I have myself compared j^erfect 
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 interme- 
diate 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 workere 
have simj)lc eyes (ocelli), which though small can be plainly 
distinguished, whereas the smaller workers have their ocelli 
rudimentary. IlaWng carefully dissected several specimens ot 
these workers, I can affirm that the eyes are far more rudimen- 
tary in the smaller workers than can be accounted for merely 
by their proportionally lesser size ; and I fully believe, though 
I dare not assert so positively, that the workers of intermediate 
size have their ocelli in an exactly intermediate condition. So 
that here we have two bodies of sterile workers in the same 
nest, differing not only in size, but in their organs of vision, yet 
conn(^cted by some few members in an intermediate condition. 
I may digress by adding, that if the smaller Avorkers had been 
the most useful to the community, and those males and females 
had been continually selected, which produced more and more 
of the smaller workers, until all the workers were in this condi- 
tion, we should then have had a species of ant with neuters 
nearly in the same condition with those of Mynnica. For the 
workers of Mynnica have not even rudiments of ocelli, though 
the male and female ants of this genus have Avcll-develoi)ed 

I may give one other case : so confidently did I expect to find 
gradations in important points of structure between the differ- 
ent castes of neuters in the same species, that I glaiUy availed 
myself of Mr. F. Smith's offer of numerous specimens from the 
same? nest of tlie driver-ant (Aiiomma) of West Africa. The 
reader will peihajis best a]i])reciate the amount of difference in 
these workers, by my giving not the actual measurements, but 
a strictly aecurate illustration : the difference was the same as 
if we were to sec a set of workmen building a house, of whom 
many were five feet four inches high, and many sixteen feet 


high ; but wc must suppose that the larger workmen had heads 
four instead of three times as l:)ig 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 f(^r us is, that, though the workers can be grouped into castes 
of different sizes, yet they graduate insensibly into each other, 
as does the widely-different structure of their jaws. I speak 
confidently on this latter point, as Sir J. Lubbock made draw- 
ings for me, with the camera lucida, of the jaws which I dis- 
sected from the workers of the several sizes. Mr. Bates, in his 
most interesting "Naturalist on the Amazons," has described 
analogous cases. 

With these facts before me, I believe that natural selection, 
by acting on the fertile ants or parents, could form a species 
which should regularly produce neuters, either all of large size 
with one form of jaw, or all of small size M-ith jaws having a 
Avidely-different structure; or lastl}^, and this is the clima.x of 
dilliculty, one set of workers of one size and structure, and at 
the same time 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 com[)lcx case, of certain Malayan butterflies 
regularly appearing under two or even three distinct female 
forms ; and by Fritz Miiller, of certain Brazilian crustaceans 
likewise appearing under two widely-distinct male forms. But 
this subject need not here be discussed. 

I have now explained how, as I believe, the wonderful fact 
of two distinctly-defined castes of sterile workers existing in 
the same nest, both widely different from each other and from 
their parents, has originated. We can see how useful their 
production niay have been to a social comnnmity of ants, on 
the same, principle that the division of labor is useful to civil- 
ized man. Ants, however, work by inherited instincts and by 
inherited organs or tools, while man works by acquired knowl- 
edge and manufactured instruments. But I must confess, that, 
Avith all my faith in natural selection, I should never have an- 
ticipated tiiat this principle could have been efficient in so high 
a degree, had not the case of these neuter insects convinced me 


of the fact. I have, therefore, discussed this case, at some little 
but wholly insuniciout length, in order to show tlio power of nat- 
ural selection, and likewise because this is by far the most serious 
special dillieulty which my theory has encountered. The case, 
also, is very intercstinir, as it proves that with animals, as with 
plants, any amount of modification may be cUc'cted ])y the ac- 
cumulation o( numerous, slight, spontaneous variations, which 
are in any way jirolitable, without exercise or hal)it havini^ 
been brouf^ht into play. For peculiar hal^its confined to the 
workers or sterile females, however long they mif^ht be fol- 
lowed, could not possibly affect the males and fertUe female, 
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 inherited 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 vari- 
ations are inlierited. Still more briefly I have attempted to show 
that instincts A'ary slightly in a state of nature. No one will 
dispute that instincts are of the highest importance to each 
animal. Therefore there is no real diiliculty, under changing 
conditions of life, in natural selection accumulating to any ex- 
tent slight modifications of instinct which are in any way use- 
ful. In some cases habit or use, and disuse, have proliably 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 diiliculty, to the best of my judgment, annihilate it. 
On the other hand, the fact that instincts are not always ab- 
solutely perfect, and are liable to'mistakes ; that no instinct 
can be shown to have been produced for the good of other 
animals, tliough animals take advantage of the instincts of 
others; that tlie canon in natural history of " Natura non facit 
saltuin,"isap]ili(able to instincts as well as to corporeal struct- 
ure, and is plainly explicable on the foregoing views, but is 
otherwise inexplicable — all tend to corroborate the theory of 
natural selection. 

This theory is, also, strengthened by some few other facts in 
regard to instincts ; as by that common case of closely-allied, 
but distinct, species, when inhabiting distant parts of the 
world and living under considerably diffcM-ent conditions of 
lifCj yet often retaining nearly the same instincts. For instance, 

232 SUMMARy. Chap. VII. 

we can understand, on the principle of inheritance, how it is 
that the thrush of tropical South America hues its nest -with 
mud, in the same peculiar manner as docs our British thrush : 
how it is that the liornbills of Africa and India have the same 
extraordinary instinct of plastering' up and imprisoning the 
females in a hole in a tree, with only a small hole left in the 
plaster throug-h Avhich the males feed them and the young 
Avhen 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 lnrva3 of ichneumonidas feeding within the 
live bodies of caterj:)illars — not as specially endowed or created 
instincts, but as small consequences of one general laAV, lead- 
ing to the advancement of all organic beings — namely, multi- 
ply, vary, let the strongest live and the weakest die. 

Ci:\v. Vlil. lIYBiaDISM. 23J 



Distinction Ijctwpon the Stcrilifir of First CropfCF and of Hybrids— Slorilil.v various 
in JX'iXTCf, nut uiiivtM>jil. all'i'ctcd l)y cIojji! Interbrccdincr, removed by Domesti- 
cation— Lawf LTovi'miiiL,' llie Sterility of Hybrids— Sterility not a special Endow- 
ment, bat incidental on oilier DilTerences, not accnmiilated by Natural Selection 
— Causes of the Sterility f)f First Crosses and of Hybrids— i'urallilisrn between 
llie Ell'ectrt of Clianjred Conditions of Life and of Crossin;,'— Diinnrnldsm and 

, Triuiorpliism — Fertility of Varieties when crossed and of their Monjirel Ollsprinij 
not universal— Hybrids and Mongrels compared independently of tlicir Fertility 

The view generally entertained by naturalists is that 
species, when intercrossed, have been specially endowed witli 
sterility, in order to prevent their confusion. This view cer- 
tainly seems at first hifi;hly pro])able, for s])ecies within the 
same country 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 tlieir hybrid offspring, cannot 
have been acquired by the continued preservation of successive, 
]irolit:d)lc degrees of sterility. It is, as I hope to show, inci- 
dental on differences in tlie reproductive system of the jiarciit- 
spceics, and is not cither a specially acquired or endoAVcd 

In treating this subject, two classes of facts, to a large ex- 
tent fundamentally different, have generally been confounded 
together ; namely, the sterility of two species when first crossed, 
antl 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 offs])ring. Hybrids, on the other hand, have their 
reproductive organs functionally impotent, as may ])e dearly 
seen in the; statt; of the malt; element in both ])hints and ani- 
mals ; though the formative; organs themselves are perfect in 
structure, as far as the microscope reveals. In the first case 
the two sexual elements which go to form the embryo are per- 


feet ; in the second case they are either not at all developed, 
or are imperfectly developed. This distinction is important, 
■when the cause of the sterility, -which is common to the two 
cases, has to be considered. The distinction probably has been 
slurred over, owiniii; to the sterility in botli cases being' looked 
on as. a special endowment, Ijoyond the province of our reason- 
ing powers. 

The fertility of vaiietics, that is of the forms known or be- 
lieved to have descended from common parents, when inter- 
crossed, and likewise the fertility of their mongrel offspring, is, 
with reference to my theory, of equal importance with the 
sterility of species ; for it seems to make a broad and clear dis- 
tinction between varieties and species. 

Defjrees of Sterility. — First, for the sterility of species when 
crossed and of their hybrid offspring. It is impossible to study 
the several memoirs and works of those two conscientious and 
admirable observers, Kolrcuter and Gartner, who almost de- 
voted their lives to this subject, without being deeply im- 
pressed with the high generality of some degree of steriUty. 
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 un- 
hesitatingly ranks them as varieties. Gartner, also, makes the 
rule equally universal ; and he disputes the entire fertility of 
Kolrcuter'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 off- 
spring, with the average number produced by both pure par- 
ent-species in a state of nature, lint a serious cause of error 
seems to me to be here introduced : a plant, to be hybridized, 
nuist be castrated, and Avhat is often more imjiortant, must be 
secluded in order to prevent pollen being brought to it by in- 
sects 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 Giirtner gives in his table 
about a score of cases of plants Avhich he castrated, and arti- 
ficially fertilized with their own pollen, and (excluding all cases 
such as the Leguminosa', in which there is an acknowledged 
difliculty in the manipidation) half of these twenty plants had 
their fertility in some degree impaired. Moreover, as Giirlncr 


ropcalcdly cnissed some forms, such as the common red and 
blue pimpernels (Anagallis arvensis and ccerulea), wliicli the 
best botanists rank as varieties, and found them absolutely 
sterile, -sve may doubt whether many species are really so ster- 
ile, when intercrossed, as he believed. 

It is certain, on the one hand, that the sterility of various 
species "when crossed is so dilferent in deg'ree and graduates 
away so insensibly, and, on the other hand, that the fertility 
of pure sjiecies is so easily aflected 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 ex- 
perienced observers who have ever lived, namely, Kolrcuter 
and Gartner, should luivc arrived at diametrically opposite con- 
clusions in regard to the very same species. It is also most 
instructive to compare — but I have not space here to enter on 
details — the evidence advanced by our best botanists on the 
question whether certain doubtful forms should be ranked as 
species or varieties, with the evidence from fertility adduced 
by different hybridizers, or by the same author, from ex])eri- 
ments made during different years. It can thus be shown that 
neither sterility nor fertility alfords any clear distinction ];c- 
tween species and varieties; but tliat the evidence from this 
source graduates away, and is doubtful in the same degree as 
is the evidence derived from other constitutional and structural 

In regard to the sterility of hybrids in successive generations; 
though Giirtner was enabled to rear some hybrids, carefully 
guarding them from a cross with either pure parent, for six or 
seven, and in one case for ten generations, yet he asserts posi- 
tively that their fertility never increased, but generally de- 
creased greatly and suddenly. With respect to this decrease, 
it may first be noticed that, when any deviation in structure or 
constitution is common to both parents, this is often transmitted 
in an augmented degree to the oiTspring; and both sexual ele- 
ments in hyljritl ])lants are already allected in some degree. 
15ut I believe in nearly all these cases, that the fertility has 
been diminished by an independent cause, namely, by too close 
interbreeding. I have collected so large a body of facts, show- 
ing on the one liand 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 clc>8e interbreeding 
lessons their vigor and fcrtihty, that I must admit the correct- 


ness of this almost universal belief among breeders. Hybrids 
are seldom raised b}-- experimentalists in great numl)ers ; and 
as the parent-species, or other allied hybrids, generally grow 
in the same garden, the visits of insects must be carefully pre- 
vented during the ilowering season: hence hyljrids will gener- 
ally have to be fertilized during each generation by their own 
individual pollen ; and this would probably be injurious to their 
fertility, already lessened by their hybrid origin. I am strength- 
ened in this conviction by a remarkable statement repeatedly 
made by Giirtner, namely, that if even the less fertile hybrids 
be artilicially fertilized with hybrid pollen of the same kind, 
tlieir fertility, notwithstanding the frequent ill elFects from 
manipulation, sometimes decidedly increases, and goes on in- 
creasing. Now, in the process of ai'tilicial fertilization, pollen 
is as often taken by chance (as I know from my own experi- 
vncc) from the anthers of another tlower, as from the anthers 
of the tlower itself which is to be fertilized ; so that a cross be- 
tween two flowers, though probably often on the same plant, 
would be thus etfected. Moreover, whenever complicated ex- 
jieriments are in jirogress, so careful an observer as Gartner 
would have castrated his hybrids, and this wovdd have insured 
in each generation a cross with pollen from a distinct tlower, 
either from the same plant or from another plant of the same 
hj'brid nature. And thus the strange fact of an increase of 
fertility in the successive generations of art tficicdhf -fertilized 
hylirids, in contrast Avith those spontaneously 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 l)y a third most 
experienced hybridizer, namely, the Hon. and Rev. W. Herbert. 
He is as emphatic in his conclusion that some hybrids are per- 
fectly fertile — as fertile as the pure parent-species — as are Kiil- 
rcuter and Gartner that some degree of sterility between dis- 
tinct species is a imiversal law of Nature. He experimented 
on some of the very same species as did Gartner, The diflVr- 
ence in their results may, I think, be in part accounted for by 
Herliert's great horticultural skill, and by his having hot-houses 
at his command. Of his many important statements I will here 
give only a single one as an examjile, namcdy, that " every 
ovule in a pod of Crinum capense fertilized by C. revolutuni 
])roduced a plant, which I never saw to occur in a case of its 
natural fecundation." So that here wc have perfect, or even 
more than commonly perfect, fertility in a first cross between 
two distinct species. 


This case of tlie Crinum leads me to refer to a singular fact, 
namely, that individual plants of certain species of Lobelia, 
Vcrbascuni, and Passillora, can easily be fertilized by pollen 
from a distinct species, but not by pollen from the same; plant, 
though this pollen can be proved to l)e perfectly sound by fer- 
tilizing other plants or species. In the genus llippeastrum, in 
Corydalis as shown by Prof. Hildebrand, in various orchids as 
shown by Mr. Scott and Fritz Mliller, all the individuals are in 
this })eculiar 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 jiollen from the same individual plant ! To 
give one instance, a bulb of llippeastrum aulicum produced four 
flowers ; three were ferlilizod by Herbert with their own pollen, 
and the fourth was subsequently fertilized by the pollen of a 
compound hybrid descended from three distinct species: the 
result was, that " the ovaries of the first three flowers soon 
ceased to grow, and after a few days perished entirely, whereas 
the pod impregnated by the pollen of the hybrid made vigorous 
growth and rapid progress to maturity, and bore good seed, 
which vegetated freely." Mr. Herbert tried similar experi- 
ments during many y(iars, and always with the same result. 
With those plants in which certain individuals alone fail to be 
fertilized by their own pollen, though thej^ appear cjuite healthy 
and although both ovules and pollen are perfectly good with 
reference to other species, yet they must be in some way in an 
unnatural condition. Tliese cases serve to show on what slight 
and mysterious causes the lesser or greater fertility of a species 
sometimes depends. 

The practical experiments of horticulturists, though not 
made with scientific ])recision, deserve some notice. It is no- 
torious in how complicated a manner the species of Pelargo- 
nium, Fuchsia, Calceolaria, Petunia, Rhododendron, etc., have 
been crossed, yet many of these hybrids seed freely. For in- 
stance, Herbert asserts that a hybrid from Calceolaria integri- 
folia and jilanlaginea, species most widely dissimilar in general 
habit, "reproduced itself as perfectly as if it had been a natu- 
ral species from the mountains of Chili." I have taken some 
pains to ascertain the degree of fertility of some of the complex 
crosses of Khod(jdendrons, and I am assured that many of thcrn 
are perfectly fertile. Mr. C. Noble, for instance, informs mc 
that he raises stocks for grafting from a hvbrid between Rhod. 
Poiifieiiin and Catawbiense, and that this hybrid "seeds as 


freely as it is possible to imac^ine." Had liybrids, when fairly 
treated, always p^ono on decreasing in fertility in each succes- 
sive generation, as Gartner believed to be the case, the fact 
would have been notorious to nursery-men. Horticulturists 
raise large beds of the same hybrids, and such alone are fairly 
treated, for by insect-agency the several individuals of the 
same hybrid variety are allowed to freely cross with each other, 
and the injurious influence of close interbreeding is thus ]irc- 
vented. Any one may readily convince himself of the eih- 
ciency of insect-agency by examining the lloAvers of the more 
sterile kinds of hybrid Rhododendrons, which produce no pol- 
len, 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 jilants. If our systematic arrange- 
ments can be trusted, that is, if the genera of animals are as 
distinct from each other as are the genera of plants, then we 
may infer that animals more widely separated in the scale of 
Nature can be more easily crossed than in the case of plants ; 
l)ut the hybrids themselves are, I think, more sterile. 1 doubt 
whether any case of a perfectly-fertile hybrid animal can be 
considered as thoroughly well authenticated. It should, how- 
ever, 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 Avith nine 
other finches, but, as not one of these nine species breeds 
freely in confinement, we have no right to expect that the first 
crosses between them and the canary, or that their Inbrids, 
should be perfectly fertile. Again, with resjiect to the fertility 
in successive generations of the more fertile hybrid animals, 
1 hardly know of an instance in which two fiimilies of the same 
hyl>rid have been raised at the same time from difierent par- 
ents, so as to avoid the ill-effects of close interbreeding. On 
the contrary, brothers and sisters have usually been crossed 
in each STiccessive generation, in opposition to the constantly- 
repeated admonition of every breeder. And in this case, it is 
not at all surprising that the inherent sterility in the hybrids 
should have gone on increasing. If we were to act thus, and 
pair brothers and sisters in the case of any pure animal, which 
from any cause had the least tendency to sterility, the breed 
would assuredly be lost in a few generations. 

Although I do not know of any thoroughly well-autlienli- 
catcd cases of perfectly-fertile liybrid animals, I have reason to 


believe that tlio liyl)ri(ls from Cervulus vaginalis and Rccvesii, 
and from Pliasianus colchicus with P. toniuatus, arc perfectly 
fertile. It has lately been asserted in France, that two such 
distinct species as the hare and the raljbit, Avhen they can be 
|Tot to breed together, produce offspring, which are highly fer- 
tile when crossed with one of the parent-species. The hybrids 
from the common and Chinese geese (A. cygnoides), species 
which are so dillerent that they are generally ranked in dis- 
tinct genera, have often bred in this country with either pure 
parent, and, in one single instance, they have bred inter se. This 
was eifected by Mr. Eyton, who raised two hybrids from the 
same parents, but from difVerent 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 emi- 
nently-capable judges, namely, Mr. Blyth and Captain Hutton, 
that whole flocks of these crossed geese are kept in various 
parts of the country ; and as they are kept for profit, where 
neither pure parent-species exists, they must certainly be high- 
ly or perfectly fertile. 

Tlie various races of each kind of domesticated animals are 
quite fertile when crossed together ; yet in many cases they 
are descended from two or more wild species. From this fact 
wc must conclude either that the aboriginal parent-species 
produced at first perfectly fertile hybrids, or that the hybrids 
subsequently reared under domestication became quite fertile. 
This latter alternative, which was first propounded by Pallas, 
seems tlie most probable, and can, indeed, hardly be doubted. 
It is, for instance, almost certain that our dogs are descended 
from several wild stocks ; yet, witli perhaps the exception of 
certain indigenous domestic dogs of South America, all are 
quite fertile together; and anahigy makes me greatly doubt 
whether the several aljoriginal species would at first have 
freely bred together, and have produced quite fertile hybrids. 
So, again, I have lately acquired decisive evidence tliat the 
crossed offspring from the Indian humped and common cattle 
are inter se perfectly fcrtih^; and from the observations by 
Ililtimeyer on their important osteijlogical differences, as well 
as from tliose by Mr. IJlytli on their differences in habits, voice, 
constitution, etc., these two forms must be regarded as good 
and distinct species as any in the world. According to this 
view of the origin of many domestic animals, we must either 
give up tlie belief of the almost universal sterility of distinct 


species of animals when crossed ; or we must look at sterility, 
not as an indelible characteristic, but as one capable of being 
removed by domestication. 

Finally, 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 univ'crsal. 

Laics f/overnlng the Sterility of First Grosses and of Hybrids. 

We will now consider a little more in detail the circum- 
stances and rules governing the sterility of first crosses and of 
hybrids. Our chief object will be to see whether or not the 
rules indicate that species have specially been endowed with 
this quality, in order to prevent their crossing and blending to- 
gether in utter confusion. The following rules and conclusions 
are chiefly drawn \\\) from Gartner's admirable work on the 
hybridization of plants. I have taken much pains to ascertain 
how far the rules apply to animals, and, considering how scanty 
our knowledge is in regard to hybrid animals, I have been sur- 
prised to find how generally the same rules apply to both king- 

It has been already remarked, that the degree of fertilitj'', 
both of first crosses and of hybrids, graduates from zero to per- 
fect 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 ])lant 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 dilferent species of 
the same genus applied to the stigma of some one of the 
species, 3-ields a jierfect gradation in the number of seeds pro- 
duced, up to nearly comj)lete or even quite comi)lete fertility ; 
and, as we have seen, in certain abnormal cases, even to an 
excess of fertility, beyond tiiat which the plant's own pollen 
j^roduces. So in hybrids themselves, there are some which 
never have produced, and probably never would produce, even 
with the pollen of the pure parents, a single fertile seed : but 
in some of these cases a first trace of fertility mvcy be detected, 
by the jiollen of one of the pure parent-sjiecies 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 wc have self-fertilized hybrids jiroduting a greater 
and greater number of seeds up to perfect fertility, 

Hyljrids from two species which are very difiicult to cross, 
and which rarely produce any offspring, are generally very 
sterile ; but the parallelism between the dilliculty of making a 
first cross, and the sterility of the hybrids thus jiroduced — two 
classes of facts which are generall}- confounded together — is by 
no means strict. There are many cases, in Avhich two pure 
species, :is in the genus Yerbascum, can be united with un- 
usual facility, and produce numerous hybrid-offspring, yet these 
hybrids are remarkabh' sterile. On the other hand, there are 
sp(M-ies which can be crossed very rarelv, or with extreme diflli- 
culty, but the hybrids, ■when at last produced, are very fertile. 
Even within the limits of the same genus, for instance in Dian- 
thus, these two opj^osite cases occur. 

• The fertility, both of first crosses and of hybrids, is more 
easily affected by unfavorable conditions, than is the fertility 
of pure species. But the degree of fertility is likewise innately 
variable ; for it is not always the same when the same two 
species arc crossed under the same circumstances, but depends 
in part upon the constitution of the individuals which happen 
to have been chosen for the experiment. So it is with hybrids, 
for their degree of fertility is often found to differ greatly in 
the several individuals raised from seed out of the same ca})sule 
and exposed to the same conditions. 

By the term systematic affinity is meant, the resemblance 
between species in structure and in constitution, more espe- 
cially in the structure of parts which are of high physiological 
importance and which differ little in the allied sjiecies. Now 
the fertility of fust crosses between species, and of the hybrids 
protluced from them, is largely governed by their systematic 
affmity. This is clearly shown by hybrids never having lieen 
raised l)etween species ranked by systematists in distinct fami- 
lies ; and, on the other hand, by very closely-allied species gener- 
ally uniting Mith facility. But the correspondence between 
systematic affmit}- and the facility of crossing is by no means 
strict, A nudtitude of cases could be given of very closely- 
allied species which Avill not imite, or only with extreme diffi- 
culty ; and, on the other hand, of very distinct species which 
unite with the utmost facility. In the same family there may 
be a genus, as Dianthus, in which very many species can most 
readilv be crossed ; and another genus, as ISilcne, in which the 


most persevering efforts have failed to produce between ex- 
tremely-close species a single hybrid. Even within the limits 
of the same genus, wc 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 Giirt- 
ner found that N. acuminata, which is not a particularly dis- 
tinct species, obstinately failed to fertilize, or to be fertilized 
by, no less than eight other species of Nicotiana. Many analo- 
gous facts could be given. 

No one has been able to point out what kind, or what 
amount, of difference in any recognizalile character is suflicient 
to prevent two species crossing. It can be shown that plants 
most Avidely different in habit and general appearance, and 
having strongly-marked differences in every part of the flower, 
even in the pollen, in the fruit, and in the cotyledons, can be 
crossed. Annual and perennial plants, deciduous and ever- 
green trees, plants inhabiting difl'erent stations and fitted for 
extremely different climates, can often be crossed with ease. 

By a reciprocal cross between two species, I mean the case, 
for instance, of a stallion-horse being first crossed with a fe- 
male-ass, and then a male-ass with a mare : these two species 
may then be said to have been reciprocally crossed. There is 
often the widest possible difference in the facility of making 
reciprocal crosses. Such cases are highly important, for they 
prove tliat the capacity in any two species to cross is often 
completely independent of their systematic affinity, or of any 
difference in their whole organization, except in their rej^ro- 
ductive systems. The diversity of result in reciprocal crosses 
between the same two species was long ago observed by Kiil- 
reuter. To give an instance : Mirabilis jalapa can easily be 
fertilized by the pollen of M. longiflora, and the hybrids thus 
25roduced are suiliciontly fertile ; but Kolreuter tried more than 
two hundred times, during eight following j-ears, to fertilize 
reciprocally M, longiflora v»itli the pollen of ^I. 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 l)etween closely-re- 
lated forms (as Matthiola annua and glabra) which many bota- 
nists rank only as varieties. It is also a remarkable fact, that 
hybrids raised from reciprocal crosses, though of course com- 
pounded of the very same two species, fhe one species having 


first been used as the ftithcr and tlien as the mot her, though 
thev rarely ditFer in external characters, yet generally difier in 
fertility in a small, and oeeasionally in a high degree. 

Several other singular rules could be given iVom Gartner ; 
for instance, some species have a remarkable power of crossing 
with other species ; other species of tlie same genus have a re- 
markablt; power of impressing their likeness on their hybrid 
ollspring ; 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 par- 
(mts, always closely resemble one of them ; and such hybrids, 
tliougli externally so like one of their pure parent-species, are 
witli rare exceptions extremely sterile. So again among hy- 
brids which arc usuall}' intermediate in structure between their 
jiarents, exceptional and abnormal individuals sometimes are 
l)orn, Avhich closely resemble one of their pure parents; and 
these hybrids are almost always utterly sterile, even Avhen the 
other hybrids raised from seed from tlie same capsule have a 
considerable degree of fertility. These facts .show how com- 
pletely fertility in the hybrid is independent of its external 
lesemblance to either pure parent. 

Considering the several rules now given, Avhich govern the 
fertility of first crosses and of hybrids, we see that when forms, 
wliich must be considered as good and distinct species, are 
united, tlieir fertility graduates from zero to perfect fertility. 
<ir even to fertility iinder certain conditions in excess ; that 
their fertility, besides being eminently susceptible to favorable 
and imfavorable conditions is innately variable ; that it is by 
no means always the same in degree in the first cross and in 
th(> hybrids produced from this cross ; that the fertility of hy- 
brids is not related to the degree in which the}' resemljle in 
external appearance either parent ; and, lastly, that the facili- 
ty of making a first cross between any two species is not al- 
ways governed by their systematic aiHnity or degree of re- 
semblance to each other. This latter statement is clearly 
proved by the dillerencc in the result of reciprocal crosses be- 
tween the same two species, for, according as the one species 
or the other is used as the father or the mother, there is gen- 
erally some difieriMice, and occasionally the widest possible dif- 
ference, in the facility of effecting a union. Tlie hybrids, more- 
over, produ(<'d from reciprocal crosses often dilTer in fertility. 

Now do these complex and singular rules indicate that .spe- 
cies have b(>en endowed with sterility simply to prevent their 


becoming confounded in nature ? I think not. For why 
should the sterility be so extremely difTerent in degree, Avhen 
various species are crossed, all of Avhicli we must suppose it 
would l)e equally important to keej") from blending together? 
Why should the degree of sterility lie innately variable in the 
individuals of the same species ? Why should some species 
cross with facility, and yet produce very sterile hybrids ; and 
odier species cross with extreme dilliculty, and yet produce 
fairly fertile hybrids ? Why should there often be so great a 
diflerence in the result of a reciprocal cross between the same 
two species ? Why, it may even be asked, has the production 
of hybrids been permitted ? To grant to species the special 
power of producing hybrids, and then to stop their further 
l)ro]iagation by dilferent 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 simplv 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 ster- 
ility being incidental on other differences, and not a specially- 
endowed quality. As the capacity of one plant to be grafted 
or budded on another is so unimportant for its Avelfare in a 
state of nature, I prcsmne that no one will suppc^se 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 na- 
ture of their sap, etc. ; but in a multitj^de of cases we can as- 
sign no reason whatever. Great diAersity in the size of two 
plants, one being Avoody and the other herbaceous, one being 
evergreen and the other deciduous, and adaptation to widely- 
different climates, do not alwaA'S prevent the two grafting to- 
gether. As in hybridization, so with grafting, the capacity is 
limited by systematic alhniU', for no one has been able to graft 
trees together belonging to ((uite distinct families: and, on the 
other hand, closely-allied species, and varieties of the same 


species, can usually, but not iuvariabl}', be grafted with case, 
liut this capacity, as in hybridization, is by no means absolute- 
ly jToverned by systematic allinity. Althoup^'h many distinct 
genera within the same family have been grafted together, in 
otiier cases species of the same genus will not take on each 
other. The j)ear can be grafted far more readily on the 
rpiince, which is ranked as a distinct genus, than on the apple, 
wliich is a member of the same genus. Even dilferent varieties 
of the pear take with different degrees of facility on the 
quince ; so do dilTerent varieties of the apricot and peach on 
certain varieties of the plum. 

As Gartner found that there was sometimes an innate dif- 
ference in dilferent individuals of the same two species in 
crossing ; so Sageret behoves this to be the case with different 
individuals of the same two species in being grafted together. 
As in recipi-ocal crosses, the facility of effecting a union is 
often very far from equal, so it sometimes is in grafting; the 
couimon gooseberry, for instance, cannot be grafted on tlie cur- 
rant, Avhereas the currant will take, though with difficulty on 
the gooseberry. 

AVe have seen that the sterility of hybrids, which have 
their reproductive organs in an imperfect condition, is a differ- 
ent case from the dilHculty of uniting two pure species, which 
have their reproductive organs perfect ; yet these two distinct 
cases run to a certain extent parallel. Something analogous 
occurs in grafting ; for Thouin found that three species of llo- 
bini;i, which seeded freely on their own roots, and which could 
bo grafted with no great difficulty on another species, when 
thus grafted were rendered barren. On the otiier hand, cer- 
tain species of Sorbus, when grafted on other species, yielded 
twice as much fruit as Avhen on their own roots. AVe are re- 
minded by this latter fact of the extraordinary cases of Ilip- 
jieastrum, Passillora, etc., which seed much more freely wIkmi 
fertilized with the ])ollen of a distinct species, than wIk.mi fer- 
tilized with }K)llen from tlie same plant. 

Wc thus see, that, althougli 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 reproduc- 
tion, yet that there is a rude degree of parallelism in the re- 
sults of grafting and of crossing distinct species. And as wc 
must look at the curious and complex laws governing the 
facility with which tre(\s can be grafted on each other as inci- 
dental on unknown differences in their vegetative SA'stcms, so 


I believe that the still more complex laws governing!: the facili- 
ty of first crosses are incidental on unknown dillerences in 
their reproductive systems. These dilTcrences, in both cases, 
follow to a certain extent, as might have been expected, sys- 
tematic allinity, by Avhieh every kind of resemblance and dis- 
similarity between organic beings is attempted to be ex- 
jiressed. The facts by no means seem to me to indicate that 
the greater or lesser dilFiculty of either grafting or crossing va- 
rious species has been a special endowment ; although in the 
Ciise of crossing, the difficulty is as important for the endur- 
ance and stabiUty of specific forms, as in the case of grafting it 
is unimportant for their Avelfare. 

Origin and Causes of the Ster'tUty of First Crosses and of 

At one time it appeared to me probable, as it has to others, 
that the sterility of iirst crosses and of hybrids might have 
been slowly acquired through the natural selection of slightly- 
lessened degrees of fertility, which s]iontaneously appeared, 
like any other variation, in certain indivitluals of one variety 
"when crossed with 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 Iirst place, it may be 
remarked that distinct regions are ofteii inhabited by groups 
of species and by single species, which, when brought together 
and crossed, are found to be more or less sterile ; now it could 
clearly have been of no advantage to such sejiarated species to 
have been rendered mutually sterile, and consequently this 
could not have been effected through natiu-al selection ; but it 
may perhaps be argued that, if a species were rendered sterile 
with some one compatriot, sterility with other species would 
follow as a necessary contingency. In the second place it is 
almost as much opposed to the theory of natural selection as 
to that of special creation, that in reciprocal crosses the male 
element of one form should be rendered utterly impotent on a 
second form, Avhile at the same time the male element of this 
second form is enabled freely to fertilize the Iirst form ; for this 
peculiar state of the reproductive system could not possibly be 
advantageous to either species. 

In considering the probability of natiual selection having 


come into action, in rendering species mutually sterile, one 
frrcat clifTiculty will he found to lie in the existence of many 
graduated steps from slin^litly-lesscned fertility to absolute 
sterility. It may be admitted, on the principle above ex- 
plained, that it would \n-oi\t an incipient species if it were 
rendered in some slight degree sterile when crossed with its 
j)ar(>nt-form or with some other variety ; for thus fewer bas- 
tartUzed and deteriorated odspiing would be produced to com- 
mingle their blood with the new species in jirocess of forma- 
tion. But he who will take the trouble to rellcct on the steps 
by which this first degree of steriUty could be increased through 
natural selection to that high degree which is common with so 
many species, and which is iniiversol with species which have 
been diflerentiated to a generic or family rank, will find the 
subject extraordinarily complex. After mature rellection it 
seems to me that this could not have been effected through 
natural selection ; for it could have been of no direct advantiigc 
to an individual animal to breed badly with another indi\-idual 
of a dilVcrent variety, and thus to leave few offspring ; conse- 
quently such individuals could not have been preserved or se- 
lected. Or take the case of two species which, in their present 
state when crossed, produce few and sterile oflspring ; now, 
what is there which coidd favor the survival of those individuals 
which ha})pe!ied to be endowed in slightly-higher degree Avith 
nnitual infertility, and which thus approached by one small step 
toward absolute sterility ? Yet an advance of this kind, if the 
theory of natural selection be brought to bear, must have in- 
cessantly occurred with many species, for a multitude are mutu- 
ally quite barren. With sterile neuter insects we have reason 
to believe that modifications in their structure and fertility have 
been slowly accumulated by natural selection, from an advan- 
tage having been thus indirectly given to the conununity to 
which they belonged over other communities of the same spe- 
cies ; but an individual animal not belonging to a social com- 
numity, if rendered slightly sterile when crossed with some 
other variety, would not thus itself gain any advantage or in- 
directly give any advantage to the other individnals of the 
same variety, thus leading to their preservation. From these 
considerations I infer, as far as animals are concerncil, that the 
various degrees of lessened fertility which occur with species 
when crossed cannot have been slowly accumulated by means 
of natural s(>l(>ction. 

With plants, it is possible that the case may be somewhat 


(lifTcront. With many kinds, insects constantly ciirry pollen 
from neigliboriniT plants to the stif^mas of each flower ; and 
"with some species this is effected by the •wind. Now, if the 
pollen of a variet}', when deposited on the stij]^ia of the same va- 
riety, should become by spontaneous variation in ever so slig'ht a 
degree prepotent over the pollen of other varieties, this would 
certainly be an advantage to the variety ; for its own pollen 
would thus obliterate the efl\3cts of the pollen of other varieties, 
and prevent deterioration of character. And the more pre- 
])(^tcnt the variety's own pollen could be rendered through nat- 
ural selection, the greater the advantage would be. We know 
from the researches of Giirtncr that, with s^xjcies which are 
mutually sterile, the pollen of each is always prepotent on its 
own stigma over that of the other species ; but we do not 
know whether this prepotency is a consequence of the mutual 
sterility, or the sterility a consequence of the prepotency. If 
the latter view be correct, as the prepotency became stronger 
through natural selection, from being advantageous to a spe- 
cies in process of formation, so the sterility consequent on pre- 
potency would at the same time be augmented; and the final 
result would be various degrees of sterility, such as occurs with 
existing species. This view might be extended to animals, if 
the female before each birth received several males, so that the 
sexual element of the prepotent male of her own variety ob- 
literated the effects of the access of previous males belonging 
to other varieties, but we have no reason to believe, at least 
with terrestrial animals, that this is the case ; as most males 
and females pair for each birth, and some few for life. 

On the whole we may conclude, that with animals the ster- 
ility of crossed species has not been slowly augmented, through 
natural selection ; and as this sterility follows the same general 
laws in the vegetable as in the animal kingdom, it is improba- 
ble, though apparently possible, that with plants crossed spe- 
cies should have been rendered sterile by a different process. 
From this consideration, and remembering that species Avhich 
have never coexisted in the same countiy, and Avhich therefore 
could not have received any advantage from having been ren- 
dei'ed mutually infertile, yet are generally sterile when cix)ssed ; 
and bearing in mind that in reciprocal crosses between the 
same two species there is sometimes the widest difference in 
their sterility, we must give up the belief that natural selection 
has come into play. We are thus driven to our former prop- 
osition, namely, that the sterility of first crosses, and indi- 


rcctly of hybrids, is simply incidental on unknow n ditFcrcnccs 
in tli(^ reproductive sx'slems of the parent-species. 

We may now try and look a little closer at tlic probable 
nature of these dilVerenies, Avhicli induce sterility in first crosses 
and in hybrids. I*iire species and hybrids difl'er, as already re- 
marked, in the state of their reproductive or<z;ans ; but from 
what Avill presently follow on reciprocally dimorphic and tri- 
inorphic plants, it would ajipear as if some unknown bond or 
law existed, which causes the young from a union not fully 
fertile to be themselves more or less infertile. 

In the case of first crosses between pure species, the greater 
or less difiiculty in eflecting a union antl in obtaining offspring 
aj^parently depends on several distinct causes. There must 
sometimes be a ])hysical impossibilit}^ in the male element reach- 
ing the ovule, as would be the case with a plant having a pistil 
too long for tlie pollen-tubes to reach the ovarium. It has also 
been observed that when pollen of one species is placed on the 
stigma of a distantly-allied species, though the pollen-tubes 
protrude, they do not penetrate the stigmatic surface. Again, 
the male element may reach the female element, but be 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 caimot be grafted on others. Lastly, an embryo 
may be developed, and then perish at an early jieriod. This 
latter alternative has not been sufHciently attended to ; but I 
believe, fj-om observations communicated to me by Mr. Hewitt, 
who has had great exjierience in hybridizing pheasants and 
fowls, that the early death of the emliryo 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 b(itween three species of Gallus and their hy- 
brids; the majority of these eggs had been fertilized; and in 
the majority of the fertilized eggs, the embryos either had been 
partially develojK'd and had then aborted, or had become nearly 
mature, but the young chickens had been imable 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, " with- 
out any obvious cause, apparently from mere inability to live;" 
so that from the oOO eggs only twelve chickens were reared. 
The early death of hybrid embryos jiroliably occurs in like 
manner Avith plants; at least it is known that hybrids raised 
from very distinct species arc sometimes weak and dwarfed, 


and perish at an early aa;o ; of ■vvliicli fact Max Wirliura has 
recently given some striking' cases with hybrid -willows. It 
may be here worth noticing that in some cases of partheno- 
genesis, embryos jiroduccd from the eggs of silk-motlis, which 
liad not been fertilized, passed through their early stages of 
development and then perished like the embryos produced by 
a cross between two distinct species. Until becoming ac- 
quainted with these facts, I was unwilling to believe in the fre- 
fjuent early death of hybrid embryos ; for hj-brids, wlien once 
born, are generally healthy and long-lived, as we see in the 
case of the common mule. Hybrids, liowever, are differently 
circumstanced before and after birth : when born and living in 
a country where their two parents live, they are generally 
placed under suitable conditions of life. But a hj-brid partakes 
of only half of the nature and constitution of its mother, and 
therefore before birth, as long as it is nourished within its 
mother's womb, or within the egg or seed produced by the 
mother, it may be exposed to conditions in some degree unsuit- 
able, and consequently be liable to perish at an early period; 
more especially as all very 3-oung beings are eminently sensitive 
to injurious or uTuiatural conditions of life. But, after all, the 
cause more jirobal)!}' lies in some imperfection in the original 
act of impregnation, causing the embryo to be imperfectly 
developed, rather than in the conditions to which it is subse- 
quently exposed. 

In regard to the sterility of hyl)rids, in whicli the sexual ele- 
ments are imperfectly dcveloi)cd, the case is dilferent. I have 
more than once alluded to a large body of facts, which I have 
collected, showing that, when animals and plants are removed 
from tlieir natin-al conditions, they are extremely liable to have 
their reproductive svstems seriously affected. This, in fact, is 
the great bar to the domestication of animals. Between the 
sterility thus sujicrinduced 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 vari- 
ous degrees ; in both, the male element is the most liable to 
be affected ; but sometimes the female more than the male. In 
both, the tendency goes to a certain extent with S3-stematic 
affinity, for whole groups of animals and jilants are rendered 
impotent by the same unnatural conditions; and whole groups 
of species lend to produce sterik^ hvbrids. On the other hand, 
one species in a grouji will sometimes resist great changes of 


conditions with unimpaired fertility; and certain species in a 
p^roiip will produce unusually fertile hybrids. No one can tell, 
till he tries, whether any particular animal will breed under 
confinement, or any exotic plant seed freely under culture ; nor 
can he tell, till he tries, whether any two species of a genus 
will produce more or less sterile hybrids. Lastl}^ 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 arc eminently liable to vary, as every 
experimentalist has observed. 

Thus we s(>e that when organic beings are placed under new 
and unnatural conditions, and when hybrids are produced by 
the unnatural crossing of two species, the reproductive system, 
independently of the general state of health, is affected by 
sterility in a very similar manner. In the one case, tlie condi- 
tions of life have been disturbed, though ofteit in so slight a 
degree as to be inappreciable by vis ; in the other case, or that 
of hybrids, the external conditions have remained the same, 
but the organization has been disturbed by two different struct- 
ures and constitutions having been blended into one. For it 
is scarcely possible that two organizations should be com- 
pounded into one, without some disturbance occurring in the de- 
velopment, or periodical action, or mutual relations of the differ- 
ent parts and organs one to another or to the conditions of life. 
When hyl)rids are able to breed inter se, they transmit to their 
offsj)ring from generation to generation the same compounded 
organization, and hence we need not be surprised that their 
sterility, thougli in some degree variable, does not diminish; it 
is even a]it to increase, this being generally the result, as l)eforc 
explained, of too close interbreeding. The above view of the 
sterility of hybrids being caused by two different constitutions 
being conlounded into one, has lately been strongly maintained 
l)y Max Wichura ; but it must be owned that the sterility (as 
will be immediately ex])lained) which affects the offspring of 
dimorphic and trimorpliic plants, when individuals l)elonging 
to the same form are united, makes this view rather doulitful. 
It should, however, be borne in mind that the sterilitv of these 
plants has been acfpiired for a special purpose, and may differ 
in origin from that of hybrids. 

It must be owned that we cannot understand, on the above 


or any other view, scvenil facts with respect to the sterility of 
hybrids ; for instance, the unequal fertility of hybrids produced 
from reciprocal crosses; or the increased sterility in those hy- 
brids Avhich occasionally and exceptionally reseniljle closely 
cither pure parent. Nor do I pretend that the foroi^oing re- 
marks go to the root of the matter ; no explanation is oflered 
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 
disturljed, in the other case from the organization or constitu- 
tion having been disturbed by two organizations being com- 
pounded into one. 

A similar parallelism apparently extends to an allied yet 
very different class of facts. It is an old and almost universal 
belief, founded on a considerable body of evidence, that slight 
changes in the conditions of life are beneficial to all living 
things. We sec this acted on by fiirmcrs 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 cliange in 
the habits of life. Again, both with plants and animals, there 
is abundant evidence that a cross between indiWduals of the 
same species, which difltr to a certain extent, gives vigor and 
fertility to the oflspring; and that close interbreeding con- 
tinued during several generations between the nearest rela- 
tions, especially if these be kept under the same conditions of 
life, almost always induces weakness and sterility. 

Hence it seems that, on the one hand, slight clianges in the 
conditions of life benefit all organic beings, and, on the other 
hand, that slight crosses, that is, crosses between the males and 
females of the same species, which have varied and become 
slightly different, give vigor and fertility to the offspring. But 
we have seen that greater changes, or changes of a jiarticular 
nature, often render organic beings in some degree sterile ; 
and that greater crosses, tliat is, crosses between males and 
females which have become widely or specifically different, 
produce hybrids which are generally sterile in some degree. 
I cannot persuade myself that this parallelism is an accident 
or an illusion. Both series of facts seem to be connected to- 
gether by some common but unknown bond, which is essen- 
tially related to the principle of life ; this principle apparently 
being that life, as Mr. Herbert Spencer has remarked, depends 


on, or consists in, the incessant action and reaction of various 
forces, which, as throui^liout Nature, are always tending toward 
an e(iuilihriuiii ; and when this tendency is slip^htly disturbed 
by any change, tlie vital forces apparently gain in power. 

Meciprocal Dimorphism and Trimorpliism. 

This subject may he here briefly discussed, and will l)e 
found to throw some light on hybridism. Several plants belong- 
ing to distinct orders present two forms, which exist in al)out 
equal numbers, and whicli differ in no respect except in tlieir 
re])roductive organs ; one form having a long pistil with short 
stamens, tlie other a short ])istil with long stamens; botli with 
dilVerently-sized pollen-grains. With trimorphic plants there 
are three forms likewise difl'ering in the lengths of their pistils 
and stamens, in the size and color of the pollen-grains, and in 
some otlier respects ; and as in each of the three forms there 
are two sets of stamens, there arc altogether six sets of stamens, 
and three kinds of pistils. These organs are so proportioned 
in length to each other, that, in any two of the forms, half the 
stamens in each stand on a level with the stigma of the third 
fonn. Now I have shown, and the result has been confirmed 
Ijy other observers, that, in order to obtain full fertility Avith 
these ])lants, it is necessary that the stigma of the one form 
should be fertilized by pollen taken from the stamens of corre- 
sponding height in the other form. So that with dimorphic 
species two unions, wliich may be called legitimate, arc fully 
fertile ; and two, which may be called illegitimate, are more 
or less infertile. AVith tnmorj>hic species six imions are legiti- 
mate or fully fertile, and twelve are illegitimate or more or 
less infertile. 

The infertility which may be observed in various dimorphic 
and trimorphic ])lants, when they are illegitimately fertilized, 
that is, by ]io]Um) taken from stamens not coi'responding in 
height villi the pistil, dilTers much in degree, up to absolute 
and utter sttM-ility ; just in the same manner as occurs in cross- 
ing 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 witli iUegitimate 
imions. It is well known that if pollen of a distinct species 
be placed on the stigma of a flower, and its own ]iollen be 
afterward, even after a considerable interval of time, placed on 
the same stigma, its action is so strongly prepotent that it 


generally amiiliilatcs 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 botii 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 wholly destroyed or pre- 
vented the action of the previously-applied illegitimate pollen. 
Again, as in making reciprocal crosses between the same two 
species, there is occasionally a great difference in the result, so 
the same thing occurs Avith trimorphic plants ; for instance, 
the mid-styled form of Lythrum salicaria Avas illegitimately 
fertilized with the greatest case by pollen from the longer sta- 
mens of the short-styled form, and yielded many seeds ; but 
the latter form did not yield a single seed when fertilized by 
the longer stamens of the mid-styled form. 

In all these respects and in others which might have been 
adduced, the forms of the same imdoubted species when ille- 
gitimately united behave in exactly the same manner as do two 
distinct S[)ecics wheii crossed. This led me carefully to ob- 
serve during four years many seedlings, raised from several 
illegitimate unions. The chief result is that these illegitimate 
])lants, as they may be called, are not fully fertile. It is possi- 
ble to raise from dimorphic species both long-styled and short- 
styled illegitimate plants, and from trimorphic plants all three 
illegitimate forms ; these can then be properly united in a le- 
gitimate maimer. When this is done, there is no apparent 
reason why they should not yield as many seeds as did their 
parents when legitimately fertilized. But such is not the case ; 
they are all infertile, but in various degrees; some being so 
utterly and incurably sterile that they did not yield during 
four seasons a single seed or even seed-capsule. The sterility 
of these illegitimate plants, when united with each other in a 
legitimate manner, may be strictly compared with that of hy- 
brids when crossed inter sc. When on the other* hand a hybrid 
is crossed with either pure parent-species, the sterility is usu- 
ally much lessened : and so it is when an illegitimate plant is 
fertilized by a legitimate plant. In the same manner as the 
sterility of hybrids does not always run parallel with the difii- 
culty of making the first cross between the two j^arent-species, 
so the sterility of certain illegitimate plants was usually great, 


while tlic sterility of the union from which they were derived 
was hy no means f^n\it. ^Vith liybrids raised from the same 
seed-capsule the deforce of sterility is innately variable, so it 
is in a marked manner with illefritimate plants. Lastly, many 
hybrids arc profuse and persistent flowcrers, while other and 
more sterile hybrids produce few flowers, and arc weak, miser- 
able dwarfs ; exactly similar cases occur with the illegitimate 
oflspring of various dimordhic and trimorphic plants. 

Altogether there is the closest identity in character and be- 
havior between illegitimate plants and hybrids. It is hardly 
an exaggeration to maintain that the former are hybrids, but 
jjroduced within the limits of the same species by the imjiroper 
imion 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 
l)e+wcen distinct species. This will perhaps be made more 
fully ajiparcnt by an illustration : we may suppose that a bot- 
anist found two well-marked varieties (and such occur) of the 
loiig-stjled form of the trimorphic Lythrum salicaria, and that 
he determined to try by crossing whether they Mere specifi- 
cally 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 dis- 
tinct 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 common view, that his two varieties were as good 
and as distinct species as any in the world ; but he would be 
completely mistaken. 

The facts now given on dimorphic and trimorphic plants 
an» important, because they show us, first, that the ])hysi()logi- 
cal test of lessened fertility, both in first crosses and in hyl)ri(ls, 
is no safe critericm of specific distinction ; secondly, because we 
may conclude that there is some unknown bond which connects 
(li(> infertility of illegitimate unions with that of their illegiti- 
mate oflspring, and we an* led to extend the same view to lirst 
crosses and hybrids ; thirdly, because we find, and this seems 
to me of especial importance, that two or three forms of the 
same species may exist and may differ in no respect, except in 
their reproductive organs, and yet be sterile when united in 


certain -ways. With dimorpliic plants, tlie unions between the 
two distinct forms arc alone quite fertile, and jmxluce quite 
fertile ofl'spring-, while unions between individuals belonging 
to the same form are more or less sterile ; so that the result is 
exactly the reverse of what occurs with distinct sj)ecies. With 
dimorphic plants the resultant sterility is quite indejiendent of 
any difference in general structure or constitution, for it arises 
from the union of individuals belonging not only to the same 
species, but to the same form. It must, therefore, depend on 
the nature of the sexual elements, which are so adapted to each 
other, that the male and female elements occurring in the same 
form do not suit each other, Avhile those occurring in the two 
distinct forms are mutually suited to each other. From these 
considerations, it seems proljable that the sterility of distinct 
species when crossed, and of their hybrid progeny, dejiends ex- 
clusively on the nature of their sexual elements, and not on 
any general difference in structure or constitution. We are, 
indeed, led to this same conclusion by considering reciprocal 
crosses, in which the male of one species cannot be imited, or 
can be imited with great dilhculty, with the female of a sec- 
ond species, while the converse cross can be effected with per- 
fect facility ; for this difference in the facility of making recip- 
rocal crosses and in the fertility of their offspring must be at- 
tributed cither to the male or to the female element in the first 
species having been differentiated, with reference to the sexual 
elements of the second species in a higher degree than in the 
converse case. That excellent observer, Gartner, likewise 
came to this same conclusion, namely, that species when 
crossed are sterile owing to differences confined to their repro- 
ductive systems. 

Fertility of Varieties tchen crossed, and of their Mongrel 


It may be urged, as an overwhelming argument, that there 
must be some essential distinction between species and varie- 
ties, inasmuch as the latter, however nuich tliey may differ 
from each other in external ajipearance, cross with perfect facil- 
it>-, and yield perfectly fertile offspring. Witli some excep- 
tions, presently to be given, I fully admit that this is the rule. 
But the subject is surrounded by difliculties, for, looking to va- 
rieties, produced under nature, if two forms hitherto reputed to 
be varieties be found in any degree sterile together, they are 


.it onre ranked l)v naturalists as species. For instance, 
the l)lue and red ijiiiipi'rnel, wliich are considered by most bot- 
anists as varieties, are said l)y Giirtncr not to be quite fertile 
when crossed, and he consequently ranks them as undoubted 
species. If we thus art»-uo in a circle, the fertility of all varie- 
ties produced under nature will assuredly have to be granted. 

If we turn to varieties, produced, or supposed to have been 
j)roduced, under domestication, we arc still involved in doubt. 
For when it is stated, for instance, that the German Spitz dog 
crosses more easily with the fox than do other dogs, or that 
(v-rtain South American indigenous domestic dogs do not 
rcadilv unite with European dogs, the explanation which will 
o^'cur to every one, and jirobably the true one, is, that these 
dogs are descended from aboriginally distinct species. Never- 
theless the perfect fertility of so many domestic varieties, dif- 
fering widely from each other in appearance, for instance those 
of -the pigeon, or of the cabbage, is a remarkable fact; more 
especially when we reflect how many species there arc, which, 
though resembling each other most closely, are utterly sterile 
wiien intercrossed. Several considerations, how'ever, render 
the fertility of domestic varieties less remarkable. In the first 
])hice, it may be observed that the amount of external dill'er- 
cnce between two species is no suni guide to their degree of 
mutual sterility, so that similar differences in the case of vari- 
eties would be no sure guide. It is almost certain that with 
species the cause lies exclusively in differences in their sexual 
constitution. Now the conditions to which domesticated ani- 
mals and cultivated plants have been subjected, have had so 
little tendency tow^ard modifying the reproductive system in a 
manner leading to mutual sterility, that we have good grounds 
for admitting the directly opposite doctrine of Pallas, namely, 
that such conditions generally eliminate this tendency; so that 
the domesticated descendants of species, which in their natural 
state would have becMi in some degree sterile when crossed, 
become ])(;rfectly fertile together. With plants, so far is cul- 
tivation from giving a tendency toward sterility between dis- 
tinct species, that in several well-authenticated cases already 
alluded to, certain ])lants have been affected in an opposite 
mann(^r, for they^ have become self-impotent, while still retain- 
ing the ea|)a(itv of fertilizing and being fertilized by, other 
species. If the Pallasian doctrine of the elimination of sterility 
through long-continued domestication be admitted, and it can 
hardly be rejected, it becomes in the highest degree improl> 


tiblc tliat similar circumstances should both induce and elimi- 
nate the same tendency ; though in certain cases, with species 
liaving a peculiar constitution, sterility might occasionally be 
thus induced. Tims, as I believe, we can understand why with 
domesticated animals varieties have not been produced which 
arc mutually sterile ; and why with plants only a few such 
cases, immediately to l)e given, have been observed. 

The real difficulty in our present subject is not, as it ap- 
pears to me, why domestic varieties have not become mutually 
infertile when crossed, but why this has so generally occun-ed 
Avith natural varieties as soon as they have been modified in a 
sufficient and permanent degree to take rank as species. "We 
are far from precisely knowing the cause ; nor is this surpris- 
ing, seeing hovv' profoundly ignorant we are in regard to the 
normal action of the reproductive system. But we can see that 
species, owing to their struggle for existence with numerous 
competitors, must have been exposed to more uniform condi- 
tions during long periods of time, than have been domestic 
varieties ; and this may well make a wide difference in the re- 
sult. For we know how commonly wild animals and plants, 
when taken from their natural conditions and subjected to cap- 
tivity, are rendered sterile ; and the reproductive functions of 
organic beings, which have always lived and been slowly mod- 
ilied under natural conditions, would probably in like manner 
be eminently sensitive to the influence of an unnatural cross. 
Domesticated productions, on the other hand, which, as shown 
by the mere fact of their domestication, were not originally 
highly sensitive to changes in their conditions of life, and 
winch can now generally resist with undiminished fertility 
repeated changes of conditions, might be expected to produce 
varieties, Avhich would be little liable to have their reproduc- 
tive powers injuriously affected by the act of crossing with 
other varieties which had originated in a like manner. 

I have as yet spoken as if the varieties of the same species 
were invariably fertile when intercrossed. But it is impossible 
to resist the evidence of the existence of a certain amount of 
sterility in the few following cases, which I will briefly abstract. 
Tiie evidence is at least as good as that from which we believe 
in the sterility of a multitude of s]iecies. The evidence is, also, 
derived from hostile Avitncsses, who in all other cases consider 
fertility and sterility as safe critej-ions of specific distinction. 
Giirtner kept during several years a dwarf kind of maize with 
yellow seeds, and a tall variety with red seeds growing ncai 


each oilier in his r.arclcn ; and although these j)lants liavc 
separated sexes, they never naturally crossed. lie then fertil- 
ized thirteen flowers of the one with the pollen of the other; 
but only a sing-le head jiroduced any seed, and this one head 
produced onl}' live j^rains. Manipulation in this case could not 
have l)een injurious, as the plants have separated sexes. No 
one, I believe, has suspected that these varieties of maize arc 
distinct species; and it is important to notice that the hybrid 
plants thus raised were themselves j^erfecthj fertile ; so that 
even Gartner did not venture to consider the two varieties as 
specilically distinct. 

Girou do Buzareingues crossed three varieties of gourd, 
which, like the maize, has separated sexes, and he asserts that 
their mutual fertilization is by so much the less easy as their 
differences are greater. How far these experiments may be 
trusted, I know not ; but the forms experimented on are 
rapked by Sageret, who mainly founds his classification by the 
test of infertility, as varieties; and Naudin has come to the 
same conclusion. 

The following case is far more remarkable, and seems at 
fh-st quite incredible ; but it is the result of an astonishing 
number of experiments made during many years on nine spe- 
cies of Verbascmn, by so good an observer and so hostile a 
witness as Gartner; namely, that the yellow and white varie- 
ties when crossed i)roduce less seed than the similarly-colored 
varic;ties of the same species. Moreover, he asserts that, when 
yellow and white varieties of one species are crossed with yel- 
\o\v and white varieties of a distinct species, more seed is pro- 
duced by the crosses between the similarly-colored flowers 
than between those which are diflerently colored. ]\lr. Scott, 
also, has experimented on the species and varieties of Verbas- 
cum ; and, although unable to ctrnfinn 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 8G to 100, than the similarly-colored varieties, 
^'et these varieties difl'er in no respect, except in the color of 
their flowers; and one variety can sometimes be raised from 
the seed of another. 

Kolrcuter, whose accuracy has been confirmed by every 
subscijucnt observer, has proved th(» remarkal)le fact that one 
particular variety of the connnon tobacco was more fertile than 
the other variiMies, when cro.ssed with a widely-distinct species. 
He experimented on live fonns, which are commonly reputed 


to be varieties, and which he tested by the severest trial, 
namel}', by reciprocal crosses, and he found their mongrel ofl- 
sprinf^ perfectly fertile. But one of these five varieties, when 
used either as the father or mother, and crossed with the Nico- 
tiana glutinosa, always yielded hybrids not so sterile as those 
which were produced from the four other varieties when crossed 
with Nicotiana glutinosa. Hence the reproductive system of 
this one variety must have been in some manner and in some 
degree modified. 

From these facts it cannot be maintained that varieties 
when crossed are invariably (luitc fertile : from the great diffi- 
culty of ascei'taining the infertility of varieties in a state of 
nature, for a supposed variety, if proved to be infertile in any 
degree, would almost universally be ranked as a species ; from 
man attending only to external characters in liis domestic va- 
rieties, and from such varieties not having been exposed for a 
very long period to uniform conditions of life. From these sev- 
eral considerations we may conclude that fertility does not con- 
stitute a fundamental distinction between varieties and species 
when crossed. The general sterility of crossed species may 
safely be looked at not as a special acquirement or endowment, 
but as incidental on changes of an vmknown nature in their 
sexual elements. 

Hybrids and 3fonr/rels compared^ independently of their 

Independently of the quest i(m of fertility, the offspring of 
species when crossed, and of varieties when crossed, may be 
compared in several other respects. Giirtuer, whose strong 
wish it was to draw a distinct line between species and varie- 
ties, 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 im2:)ortant 

I shall here discuss this subject with extreme brevity. The 
most important chstinction is, that in the first generation mon- 
grels are more variable than hybrids ; but Gartner admits that 
hybrids from species which have long been cultivated are often 
variable in the first generation; and 1 have myself seen striking 
instances of this fact. Gartner further admits that liybrida 
between very closely-allied species are more variable than 


those from very distinct species ; and this shows that the dif- 
ference in the degree of variability graduates away. When 
mongrels and the more fertile liybrids are propagated for sev- 
eral generations, an extreme amount of variability in the off- 
spring in both cases is notorious ; but some few instances of 
botli hybrids and mongrels long retaining a imifonn character 
could be given. The variability, however, in the successive 
generations of mongrels is, perhaps, p^eater than in hybrids. 

This greater variability in mongrels than in hybrids docs 
not seem at all surprising. For the parents of mongrels are 
varieties, and mostly domestic varieties (very few ex])eriments 
having been tried on natural varieties), and this imjjlies that 
there lias been recent variability, Avhich would often continue 
and be added to that arising from the act of crossing. The 
slight variability of hybrids in the first generation, in contrast 
with the succeeding generations, is a curious fixct, and deserves 
attention. For it bears on the view which I have taken of one 
of the causes of ordinary variability ; namely, that the repro- 
ductive systeni from being eminently sensitive to changed con- 
ditions of life, fails under these circumstances to perform its 
proper function of producing offspring identical in all respects 
with the parent-form. Now, hybrids in the first generation 
are descended from sjiecies (excluding those long cultivated) 
which have not had their reproductive systems in any way 
allected, and they are not variable ; but hybrids themselves 
have their reproductive systems seriously affected, and their 
descendants are highly variable. 

Biit to return to our comparison of mongrels and hybrids : 
Gfirtner states that mongrels are more liable than hybrids to 
revert to cither parent-form ; but this, if it be true, is certainly 
only a difference in degree. Mort>over, Gartner expressly states 
that hylirids from long-cultivated plants are more subject to 
reversion than hybrids from species in their natural state; and 
this probably explains the singular difference in the results 
arrived at by dilferent obser\'ers : thus, Max Wichura doubts 
whether hybrids ever revert to their parent-forms, and he ex- 
perimented on uncultivated species of willows ; while Naudin, 
on the other hand, insists in the strongest terms on the almost 
universal tendencv to reversion in hybrids, and he experiment- 
ed chiedy on cultivated plants. Gartner further stiites that 
when any two species, although most closely allied to each 
other, are crossed with a third species, the hybrids arc widely 
different from each other; Avhereas, if two vtM-y distinct varie- 


ties of one species are crossed with another species, the hybrids 
do not diiler much. But this conclusion, as far as I can make 
out, is founded on a sinp^le experiment; and seems directly 
opposed to tlie results of several experiments made by Kol- 

Such alone arc the imimportant dilTercnces which Gartner 
is able to point out between hybrid and moiig'rel plants. On 
the other hand, the dco-recs and kinds of resemblance in mon- 
g^rels and in hybrids to their respective parents, more especially 
in hybrids produced from nearly related species, follow accord- 
ing to Giirtncr the same laws. When two species are crossed, 
one has sometimes a prepotent power of impressing its likeness 
on the hybrid ; and so I believe it to be with varieties of plants. 
With animals one variety certainly often has this prepotent 
power over another variety. Hybrid plants produced from a 
reciprocal cross, generally resemble each other closely ; and so 
it is with mongrel plants from a reciprocal cross. Both hybrids 
and mongrels can be reduced to either pure parent-form, by 
repeated crosses in successive generations Avith cither parent. 

These several remarks are apparently applicable to animals ; 
but the subject is here much complicated, partly owing to the 
existence of secondary sexual characters ; but more especially 
owing to prepotency in transmitting likeness running more 
strongly in one sex than in the other, both when one species 
is crossed with another, and when one variety is crossed with 
another variety. For instance, I think those authors are right, 
Avho maintain that the ass has a prepotent power over the horse, 
so that botli the mule and the hinny more resemble the ass than 
tlie horse ; but that the prepotency runs more strongly in the 
male-ass than in the female, so that the mule, which is the off- 
spring of the male-ass and mare, is more like an ass, than is the 
hinny, Avhich is the offspring of the female-ass and stallion. 

Much stress has been laid by some authors on the supposed 
fact, that it is only Avith mongrels that the offspring are not 
intermediate in character, but closely resemble one of their 
parents ; but this does sometimes occur Avitli hybrids, yet I 
grant much less frequently Avith them than Avitli mongrels. 
Looking to the cases Avliich I have collected of cross-bred ani- 
mals closely resembling one parent, the resemblances seem 
cliiefly confined to characters almost monstrous in their nature, 
and Avhich have suddcidy appeared — such as albinism, melanism, 
deficiency of tail or horns, or additional fingers and toes; and 
do not relate to characters Avhich have been sloAvly ac(|uired 

Chap. VIU. SUMMARY. 263 

through selection. Consequently, sudden reversions to the 
perfect character of either parent would be much more likely 
to occur with mono-rels, Avhicli are descended from varieties 
often sudd(Mily produced and semi-monstrous in character, than 
with hvbrids, which are descended from species slowly and 
naturally produced. On the whole, I entirely agree witli Dr. 
Prosper Lucas, who, after arranging an enormous body of facts 
with respect to animals, comes to the conclusion that the laws 
of resemblance of the child to its parents are the same, whether 
the two parents dilTer little or nmch from each other, namely 
in the imion of individuals of the same variety, or of different 
varieties, or of distinct species. 

Independently of the question of fertility and sterility, in 
all other respects there seems to be a general and close simi- 
larity in the offspring of crossed species, and of crossed vari- 
eties. If we look at species as having been specially created, 
an'd at varieties as having been produced by secondary laws, 
this similarity would be an astonishing fact. But it harmo- 
nizes perfectly with the view that there is no essential dis- 
tinction l^etwcen species and varieties. 

Summary of Chapter. 

First crosses between forms sufficiently distinct to be ranked 
as species, and their hybrids, are very generally but not univer- 
sally sterile. The sterility is of all degrees, and is often so 
slight that the most careful experimentalists have arrived at 
diametrically ojiposite conclusions in ranking forms by this 
test. The sterility is innately variable in individuals of the 
same species, and is eminently susceptible to the action of 
favorable and imfavorable conditions. The degree of sterility 
does not strictly follow systematic affinity, but is governed by 
several curious and complex laws. It is generally different, 
and sometimes widely different, in reciprocal crosses between 
tlie same two species. It is not always equal in degree in a 
first cross and in the hybrids produced from this cross. 

In the same manner as in grafting trees, the capacity of one 
species or variety to take on another, is incidental on differ- 
ences, generally of an unknown nature, in their vegetative sys- 
tems, so in crossing, the greater or less facility of one species 
to unite with another is incidental on imknown differences in 
their reproductive systems. There is no more reaiion to think 
that species have been specially endowed with various degrees 

2G4 6UMMAEY. Chap. VIH. 

of sterility to prevent their crossing and blending in nature, 
than to think that trees have been specially endowed with vari- 
ous and somewhat analogous degrees of ditliculty in being 
grafted together in order to prevent their inarching in our 

Tlie sterility of first crosses and of their hylirid progeny lias 
not, as far as we can judge, been acquired through natural se- 
lection. In the case of first crosses it seems to depend on 
several circumstances ; in some instances in chief part on the 
early death of the embryo. In the case of liybrids, it perhajis 
depends on tlieir whole organization having been disturbed by 
lieing compounded from two distinct forms ; tlie sterility being 
closely allied to that Avhich so frequently aii'ects pure species, 
when exposed to unnatural conditions of life. This view is 
supported by a parallelism of another kind : namely, that, first, 
the crossing of forms only slightly diiferentiated favors the 
vigor and fertility of their offspring, while close interbreeding 
is injurious ; and secondly, that slight changes in the conditions 
of life apparently add to the vigor and fertility of all organic 
beings, while greater changes are often injurious. But the 
facts given on the sterility of the illegitimate unions of dimor- 
phic and trimorphic plants and of their illegitimate progeny, 
render it probable that some unknown bond in all cases con- 
nects the degree of fertility of first unions with that of their 
offspring. The consideration of these facts on dimorphism, as 
well as the results of reciprocal crosses, clearly leads to the 
conclusion that the primary cause of tlie sterility is confined to 
differences in the sexual elements. But why, in the case of 
species, the sexual elements should so generally liave become 
more or less modified, leading to their mutual infertility, we do 
not know% 

It is not surprising that the diniculty in crossing any two 
species, and the sterility of their hybrid-offspring, should in 
most cases correspond, even if due to distinct causes ; for both 
depend on the amount of difference between the species which 
are crossed. Nor is it surprising that the facility of effecting 
a first cross, and the fertility of tlie 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 system- 
atic afiinity of the forms subjected to experiment ; for system- 
atic airiiiity includes resemblances of all kinds. 

First crosses between forms known to be varieties, or snffi- 


cicntly alike to be considered as varieties, and their mongrel 
ofl'spring, are very generally, but not, as is so often stated, 
invariably fertile. Nor is this almost universal and perfect 
fertility surprising, Avhen we remember liow 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 jiroduced under domestication by the selection of 
mere external difl'crences, and that they have not been long 
exposed to uniform conditions of life. It should also be espe- 
cially kept in mind, that long-continued domestication tends to 
eliminate sterility, and is therefore little likely to induce this 
same cjuality. Independently of the question of fertility, in all 
other respects there is the closest general resemblance between 
hybrids and mongrels, in their variability, in their power of 
absorbing each other by repeated crosses, and in their inheri- 
tance of characters from both parent-forms. Finally, then, 
although we are profoundly ignorant of the precise cause of 
the sterility of first crosses and of h^'brids, the facts given in 
this chapter do not seem to me opposed to the belief that vari- 
eties and species are not fundamentally different. 





On the Absence of Intermediate Varieties at the Present Day— On the Nature of Ex- 
tinct Intermediate Variotii's : on their Number— On the Lapse of Time, as in- 
ferred from the Ilate of Denudation and of Deposition — On the Lapse of Time as 
estimated V)y Years — On the Poorness of our Paleontologieal Collections — on the 
Denudation of Granitic Areas— On the Intermittence of Geolo'jrical Formations — 
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 Uubitablc Earth. 

In the sixth chapter I enumerated tlie chief objections 
which iTiio;lit be justly urged against the views maintained in 
this A-olume. Most of them have now been discussed. One, 
namely, the distinctness of specific forms, and their not being 
blended together by innumerable transitional links, is a very 
obvious dilliculty. I assigned reasons why such links do not 
commonly occur at the present day, under the circumstances 
apparently most favorable for their presence, namely, on an 
extensive and continuous area with graduated physical condi- 
tions. I endeavored to show that the life of each species 
depends in a more important manner on the presence of other 
alrcady-deiined organic forms, than on climate ; and, therefore, 
that the really governing conditions of life do not graduate 
away quite insensibly like heat or moisture. I endeavored, 
also, to show that intermediate varieties, from existing in lesser 
numbers than the forms which they connect, will generally be 
beaten out and exterminated during the course of further 
modification and improvement. The main cause, however, of 
innumerable intermediate links not now occurring everywhere 
throughout Nature depends on the very process of natural 
selection, through which new varieties continually take the 
places of and exterminate their parent-forms. But just in pro- 
portion as this process of extermination has acted on an enor- 
mous scale, so must tlie number of intermediate varieties, 
which have formerly existed, be truly enormotis. ^V^ly, then, 


is not every pfColor;ical formation and every stratum full of 
such iiitermcdiate links V GeolojTy assuredly does not reveal 
any such tinclv-^radiiated organic; chain; and this, perhaps, is 
the most obvious and serious objection which can be urf^ed 
ar^ainst the theor3^ The explanation lies, as I believe, in the 
extreme imperfection of the geological record. 

In the first place, it should always be borne in mind what 
sort of intermediate forms must, on the tiieory, have formerly 
existed. I have found it diilicult, when looking at any two 
species, to avoid picturing to myself forms directbj intermedi- 
ate between them. But this is a wholly false view ; we should 
alwa3's look for forms intermediate between each species and 
a common but unknown progenitor ; and the progenitor will 
generally have ditlered in some respects from all its modified 
descendants. To give a simple illustration: the fantail and 
pouter pigeons have both descended from the rock-pigeon; 
if we possessed all the intermediate varieties which have ever 
existed, we should have an extremely close series between 
both and the rock-pigeon ; but we should have no varieties 
directly intermediate between the fantail and jiouter ; 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 re- 
garding their origin, it would not have been possible to have 
determined, from a mere comparison of their structure with 
tliat of the rock-pigeon, C. livia, whether they had descended 
irom this species or from some other allied species, such as C. 

So with natural species, if we look to forms very distinct, 
for instance to the horse and tapir, we have no reason to sup>- 
pose that links ever existed directly intermediate between 
them, but between each and an unknown connnon parent. The 
common parent will have had in its whole; organization much 
general resemblance to the tapir and to the horse ; but in some 
])oints of structure may have difTcMcd considerably from both, 
even perhaps more than they differ from each other. Hence, 
in all such cases, we should be unable to recognize the parent- 
form of any two or more s])ecies, 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 jierfect chain of 
the intermediate links. 

It is just possible by the theory that one of two living 


forms might have ilescended from the other; lor instance, a 
horse from a tapir ; and in this case direct intermediate links 
will have existed between them. But such a case "would im- 
ply that one form had remained for a very long period unal- 
tered, while its descendants had undergone a vast amount of 
change ; and the principle of competition between organism 
and organism, between child and parent, will render this a very 
rare event ; for in all cases the new and improved forms of life 
tend to supplant 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 dif- 
ferences not greater than we see between the varieties of the 
same species at the present day ; and these parent-species, 
now generally extinct, have in their turn been similarly con- 
nected with more ancient species ; and so on backward, always 
converging to the common ancestor of each great class. So 
that the number of intermediate and transitional links, be- 
tween all living and extinct species, must have been incon- 
ceivably great. But assuredly, if this theory be true, such 
have lived upon the earth. 

On the Lapse of Time, as inferred from the Mate of Deposi- 
tion and Extent of Denudation. 

Ind(^pendently of our not hnding fossil remains of such in- 
finitely numerous connecting links, it may be objected that 
time cannot have sufficed for so great an amount of organic 
change, all changes having been etlected very sIomIv. It is 
hardh' possible for me even to recall to the reader, who is not 
a practical geologist, the facts leading the mind feebly to com- 
prehend the lapse of time. He who can read Sir Cliarles 
Ly ell's grand Avork on the Principles of Geology, which the 
future historian will recognize as having produced a revolu- 
tion in natural science, 3'et does not admit how vast have been 
the past periods of time, may at once close this volume. Not 
that it suffices to study the Princij^les of Geology, or to read 
special treatises by d liferent observers on separate formations, 
and to mark how each author attempts to give an inadequate 
idea of the duration of each formation or even of each stratum. 
We can best gain some idea of jiast time by knowing the agen- 
cies at work, and learning how nuich of the surface of the land 
has l)cen denuded, and how nnich sediment has been deposited. 
As I.yell has well remarked, the extent and thickness of our 

CiiAP. IX. THE LAl'SK OF T1MI<:. 2G9 

sciliincntiiry formations arc the result and the measure of the 
rlrnudation which the eartli's crust lias elsewhere under<roue. 
Therefore a man should examine for himself the great piles of 
superimposed strata, and watch the rivulets brini^ini^ down 
niuil, and the waves wearing away the sca-clills, in order to 
comprehend something about the duration of past time, the 
monuments of which we sec all around us. 

It is good to wander along the coast, when formed of raod- 
eratcly-hard rocks, and mark the process of degradation. The 
tides in most cases reach the cliffs only for a short time twice a 
day, and the waves cat into them, only when they are charged 
with sand or pel)bles ; for there is good evidence that pure 
water elfects 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 iii size they can be rolled about by the 
waves, and then they are more quickly ground into pebbles, 
sand, or mud. But how often do we see along the bases of 
retreating cliffs rounded bowlders, all thickly clothed by marine; 
productions, showing how little they are abraded, and how 
seldom they are rolled about ! Moreover, if we follow for a 
few miles any line (jf rocky cliff, which is undergoing degrada- 
tion, we find that it is only here and there, along a sliort length 
or round a promontory, that the clifl's arc at the present time 
suffering. The appearance of the surface and the vegetation, 
show that elsewhere yeai'S have elapsed since the Avaters 
washed their base. 

We have, however, recently learned from the observations 
of Ramsay, in the van of" excellent observers, of Jukes, Geikie, 
Croll, and others, that subaerial degradation is a much more 
imi)ortant agency than coast-action, or the power of the waves. 
Tiie whole surface of the land is exposed to the chemical ac- 
tion of the air and of the rain-water with its dissolved carbonic 
acid, and in colder countries to frost; the disintegrated matter 
is carried down even gentle slopes during heavy rain, and to a 
greater extent than might be supposed, especially in arid dis- 
tricts, by the wind ; it is then transported by the streams and 
rivers, which when rapid deepen their chainicls, and triturate 
the fragments. On a rainy day, even in a gently-undulating 
country, we see the effects of subaerial degradation in the mud- 
dy rills Avhich flow down each slope. Messrs. Kamsay and 
Whitaker have shown, and the observation is a most striking 
one, that the great lines of escarpment in the Wcaldon district 

270 '^^iE LAPSE OF TIME. CiiAr. IX. 

and those ranging across England, which formerly were looked 
at by every one Jis ancient sea-coasts, cannot have been thus 
formed, for each line is composed of one and the s;une forma- 
tion, while our j^resent sea-clifls are eveiywhere formed by the 
intersection of various formations. This being tlic case, we 
are compcllcil to admit that the escaq^ments owe their origin 
in chief part to the rocks of Avhich they are composed having 
resisted subacrial denudation better than the surrounding sur- 
face ; this surface consequently has l^een gradually lowered, 
with the lines of harder rock left projecting. Nothing im- 
presses the mind with the vast duration of time, according to 
our ideas of time, more forcibly tlian the conviction thus gained 
that subaiirial agencies, which apparently have so little power, 
and which seem to work so slowly, have produced such great 

When thus impressed with the slow rate at Vv'liich tlie 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 mass of rock which has been removed over 
many extensive areas, and on the other hand the tliickness of 
our sedimentary formations. I remember haAnng been much 
struck M'hen viewing volcanic islands, which have been worn 
by the waves and pared all around 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 very slow and 
eflected 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 dis- 
placement of tlie strata varies from 600 to 3,000 feet. Prof, 
liamsay has published an account of a downtlirow in Anglesea 
of 2,300 feet; and lie informs me tliat he fully believes that 
tliere is one in ]M(>rionethshire of 12,000 feet; yet in these 
cas(>s there is nothing on the sm-face of the land to show such 
prodigious movements ; tlie jiile of rocks on cither side of the 
crack having been smoothly SAvej)t away. 


Oil the other Imnd, in all parts of the world the piles of 
Kcdimentary strata arc of wonderful thickness. In the Cordil- 
lera I estimated one mass of cono:lomcrate at ten thousand 
feet ; and althou_Q;h conf2;lomerates have probably been accu- 
nuilated at a quicker rate than liner sediments, yet from being 
formed of worn and rounded pebbles, each of which bears the 
stamp of time, they are f:^ood to show how slowly tlic mass 
must have been heaped to<j;'ether. Prof. Ramsay has <;iven me 
the thickness, from actual measurement in most 
cases, of the successive formations in dijf'ercnt parts of Great 
Britain; and this is the result: 


Paleozoic BtratA (not including? igneous beds) 57,154 

Secondary strata 13.100 

Tertiary strata 2,240 

— makinfi;' altogether 72,584 feet ; that is, very nearly thirteen 
and three-(iuarters British miles. Some of the formations, 
Avhich are represented in England by thin beds, are thousands 
of feet in thickness on the Continent. Moreover, between each 
successive formation, we have, in the o])inion of most geolo- 
gists, enormously long blank periods. So that the lofty pile 
of sedimentary rocks in Britain gives but an inadequate idea 
of the time which has elapsed during their accumulation. 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 eteniity. 

Nevertheless this impression is partly false. Mr. Croll, in 
a most interesting paper, remarks that we do not err " in form- 
ing too great a conception of the length of geological periods," 
but in estimating them by years. When geologists look at 
large and complicated phenomena, and then at the figures rep- 
resenting several million years, the two produce a totally dif- 
ferent effect on the mind, and the figures are at once pro- 
nounced to be too small. But in regard to denudation, Mr. 
Croll shows, by calculating the known amount of sediment an- 
nually brought down by certain rivers, relatively to the areas 
of drainage, that 1,000 feet of rock, disintegrated through sub- 
ai'rial agenci(>s, would thus be removed from the mean level of 
the whoh* area in the course of six million vears. This seems 
an astonishing result, and some considerations lead to the sus- 
picion that it may be much too large, but even if halved or 
(juartered it is still very surprising. Few of us, however, know 
what a million really means: Mr. Croll gives the following 

272 Tilt: LAPSE OF TIME. Cuap. IX. 

illustration : take a narrow strip of paper, 83 feet 4 inclies in 
length, and stretch it along the Avail 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 dimen- 
sions. Several eminent breeders, during a single lifetime, have 
so largely modified some of the higher animals, Avhich propa- 
gate their kind much more slowly than most of the lower ani- 
mals, that they have formed Avliat Avell deserve to be called 
new sub-breeds. Few men have attended AA-ith due care to 
any one strain for more than half a century, so that a hundred 
years represent the Avork of two breeders in succession. It is 
not to be supposed that species in a state of nature ever 
change so quickly as domestic animals under the guidance of 
methodical selection. The comparison Avould be in every AA'ay 
fairer Avitli the results Avhich follow from unconscious selection, 
that is, the preservation of the most useful or beautiful animals, 
Avith no intention of modifying the breed ; but by this process 
of unconscious selection, various breeds haA"e been sensibly 
changed in the course of two or three centuries. 

Species, however, probably change much more slowly, and 
within the same country only a few change at the same time. 
This slowness follows from all the inhabitants of the same 
country being already so avcU adapted to each other that ncAV 
places in the polity of Nature do not occiir until after long in- 
tervals, Avhen changes of some kind in the physical conditions 
or through immigration haA^e occurred; and indiA^idual differ- 
ences or variations of the right nature, by Avliich some of the 
inhabitants might be better fitted to their new places under 
the altered circumstances, might not at once occur. According 
to the standard of years Ave have no means of determining 
how long a period it takes to modify a species. Mr. Croll, 
judging from the amount of heat-energy in the sun and from 
the date Avhich he assigns to the last glacial epoch, estimates 
that only sixty million years lia\'e elapsed since the deposi- 
tion of the first Cambrian formation. This appears a A'ery 
short period for so many and such great mutations in the forms 
of life as have certainly since occurred. It is admitted that 
many of the elements in the calculation are more or less doubt- 
ful, and Sir "W. Thomson giA'Cs a Avide margin to the possible 
age of the habitable Avorld. But, as Ave have seen, Ave cannot 


comprehend what tlio firrures 00,000,000 really imply ; and 
during this, or perhaps a lonfrcr roll of years, the land and the 
waters have everywhere teemed with livinj^ creatures, all ex- 
posed to the struggle for life and undergoing change. 

On the Poorness of oitr Paleontological Collections. 

Now let us turn to our richest geological museums, and 
what a paltry display we behold ! That our collections arc 
very imperfect is admitted by every one. The remark of that 
admira1:)lc paleontologist, Edward Forbes, should not be for- 
gotten, namely, that numbers of our fossil species are known 
and named from single and often broken specimens, or from a 
few specimens collected on some one spot. Only a small portion 
of the surface of the earth has been geologically explored, and 
no part with sufficient care, as the important discoveries made 
eVery year in Europe prove. No organism wholly soft can be 
preserved. Shells and bones Avill decay and disappear when 
left on the bottom of the sea, where sediment is not accumu- 
lating. I believe we often take an erroneous view, when we 
tacitly admit to ourselves that sediment is being deposited 
over nearly the whole bed of the sea, at a rate sufficiently 
quick to embed and preserve fossil remains. Throughout an 
enormously large proportion of the ocean, the bright blue tint 
of the water bespeaks its purity. The many cases on record 
of a formation conformably covered, after an immense interval 
of time, by another and later formation, Avithout tlic underly- 
ing bed having suffered in the interval any wear and tear, seem 
explicable only on the view of the l)ottom of the sea not rarely 
lying for ages in an unaltered condition. The remains Avhich 
do become embedded, if in sand or gravel, will when the beds 
are upraised generally be dissolved by the percolation of rain- 
water chargetl with carbonic acid. Some of the many kinds 
of animals which live on the beach between high and low 
water mark seem to be rarely jireserved. For instance, the 
several species of the Chthamalin;!:? (a sub-family of sessile 
cirripcdes) coat the rocks all over the world in infinite 
numbers: they are all slridly littoral, with the exception of a 
single Medit(;rranean species, which inhabits deep water, and 
this has been f(jund fossil in Sicily, whereas not one other 
species has hitherto been found in any tertiary formation : yet 
it is not known that the genus Chthamalus existeil during the 
T^halk period. Lastly, many great deposits requiring a vast 


length of time for llicir Jiccuinuliilion, are entirely destitute of 
organic remains, witliout our being able to assign any reason : 
one of tlic most striking instances is that of the Flysch forma- 
tion, -wliich 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, al- 
though this great mass has been most carefully searched, no 
fossils, except a fcAv vegetable remains, have been found. 

"With respect to the terrestrial productions which lived 
during the Secondary and Paleozoic periods, it is superfluous 
to state that our evidence from fossil-remahis is fragmentary 
in an extreme degree. For instance, not a land-shell until quite 
recently was known belonging to cither of these vast periods, 
with the exception of one species discovered by Sir C. Lyell 
and Dr. DaAvsou in the carboniferous strata of North Amei'ica, 
of which shell above a hundred specimens have now been col- 
lected. In regard to mammiferous remains, a single glance at the 
historical table published inLyell's Manual will bring home the 
Iruth, how acciilental and rare is tlicir preservation, far better 
than pages of detail. Nor is their rarity surprising, when we 
remember how large a proportion of the bones of tertiary mam- 
mals have been discovered either in caves or in lacustrine 
deposits; and that not a cave or true lacustrine bed is known 
belonging to the age of oiu- secondary or paleozoic formations. 

But the imperfection in the geological record largely results 
from another and more important cause than any of the forego- 
ing ; namely, from the several formations being separated from 
each other l^y wide intervals of time. This doctrine has been 
most emphatically admitted by many geologists and paleon- 
tologists, who, like E. Forbes, entirely disbelieve in the change 
of species. When we see tiie formations tabulated in written 
works, or when we follow them in Nature, it is diihcult to 
avoid believing that they arc closely consecutive. But we 
know, for instance, from Sir R. Murchison's great work on Rus- 
sia, what wide gaps there are in that country between the 
superimposed formations ; Sf) it is in North America, and in 
many otlier parts of the world. The most skilful geologist, if 
his attention had been confined exclusively to tliese large ter- 
ritories, Avould never have suspected that, during the periods 
which were blank and barren in his own country, great piles 
of sediment, charged Avith new and peculiar forms of life, had 
elsewhere been accumulated. And if, in each separate territory, 
hardly any idea can be formed of the length of time which has- 


elapsed lictwoL'u Iho consecutive formations, we may infer that 
this could nowhere be ascertained. The frequent and great 
changes in the miiieralogieal composition of consecutive forma- 
tions, generall}^ implying great changes in the geography of 
the surronnding lands, whence the sediment has been derived, 
accords with tlie belief of vast intervals of time having elapsed 
between each formation. 

But we can, I thinlc, see wliy the geological formations of 
each region are almost invariably intermittent; that is, have 
not followed each other in close sequence. Scarcely any fact 
struck me more when examining many hundred miles of the 
South American coasts, which have been uj)raised several hun- 
dred feet Avithin the recent period, than the absence of any 
recent deposits sufliciently extensive to last for even a short 
geological period. Along the whole west coast, Avhich is in- 
liabited by a peculiar marine fauna, tertiary beds are so poorly 
cTeveloped, that no 'record of several successive and peculiar 
marine faunas will probably be preserved to a distant age. A 
little refl(^ction will explain Avhy, along the rising coast of the 
western side of South America, no extensive formations with 
recent or tertiary remains can anywhere be found, though the 
supply of sediment must for ages have been great, from the 
enormous degradation of the coast-rocks and from muddy 
streams entering the sea. The exjilanation, no doubt, is, that 
the littoral and sub-littoral deposits are continually worn away 
as soon as they nro, lirought up by the slow and gradual rising 
of the land witliin the grinding action of the coast-waves. 

We niay, I think, conclude that sediment must be accumu- 
lated in extremely thick, solid, or extensive masses, in order 
to withstand the incessant action of tlie waves, wlien lirst up- 
raised and during successive oscillations of level, as well as 
the subsequent subaerial degradation. Such thick and exten- 
sive accumulations of sediment may be formed in two waj-s; 
either in profound depths of the sea, in which case the bottom 
will not be inhabited by so many and such varied f(jrms of life 
as the more shallow seas; and the* mass Avhen upraised will 
give an imperfect record of the organisms which existed 
tliroughout the world during the period of its accunuilation; 
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 hmg as the rate of subsidence and the suj> 
ply of sediment nearly balance each other, the sea will remain 
shallow and favorable for many and varied fonns, and thus a 


rich fossiliferous formation, thick cnougli, \\hvn 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 rlcJi in fos- 
sils, have thus been formed during subsidence. Since publish- 
ing my views on this subject in 1845, 1 have watched the prog- 
ress 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. sub- 
sidence. I may add that the only ancient tertiary formation 
on the west coast of South America, which has been bulky 
enough to resist such degradation as it has as yet suffered, but 
which will hardly last to a distant geological age, was deposit- 
ed during a downward oscillation of level, and thus gained 
considerable thickness. 

All geological facts toll us plainly that each area has under- 
gone numerous slow oscillations of level, atid apparently these 
oscillations have affected Avide spaces. Consequently forma- 
tions rich in fossils and sufficiently thick and extensive to re- 
sist subsequent degradation, may have been formed over wide 
spaces during periods of subsidence, but only where the sup- 
ply of sechment was sufficient to keep the sea shallow and to 
embed and preserve the remains before they had time to deca3% 
On the other hand, as long as the bed of the sea remained sta- 
tionary, thich deposits could not hare been accumulated in the 
shallow parts, which are the most favorable to life. Still less 
could this have happened during the alternate periods of ele- 
vation ; or, to speak more accurately, the beds which were 
then accumulated Avill generally have been destroyed by being 
upraised and brought within the limits of the coast-action. 

These remarks appl}'" chiefly to littoral and sub-littoral de- 
jiosits. In the case of an extensive and shallow sea, such as 
that within a large part of the Malay Archipelago, Avhere the 
depth varies from 30 or 40 to 60 fathoms, widely-extended for- 
mation might be formed during a jicriod of elevation, and yet 
not suffer excessively fr©m denudation during its slow up- 
heaval ; but the thickness of the formation could not be great, 
for owing to the elevatory movement it would be less than the 
depth in whidi it was formed ; nor would the deposits be much 
consolidated, nor be cajiped by ovcrlj-ing formations, so that 
it would run a good chance of being worn away by atmos- 
])heric dc^gradation and by the action of the sea during subse- 
quent oscillations of level. It has, however, been suggested 


by Mr. Hopkins, lliat if one part of the area, after rising and 
before being denuded, subsided, the deposit formed during the 
rising movement, though not thick, might afterward become 
protected by fresh accumuhitions, and thus be jireserved for a 
long period. 

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 plutonic rocks once 
formed the primordial nucleus of the globe, Avill admit that 
tlicse latter rocks have been denuded on an enormous scale. 
For it is scarcely possible that such rocks could have been 
solidilied and crystallized while imcovered ; but if the meta- 
morphic 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., 
w-ere once necessarily covered up, how can we account for the 
naked and extensive areas of such rocks in many jiarts of the 
world, except on the belief that they have subsequently been 
completely denuded of all overl^ang strata ? lliat such ex- 
tensive areas do exist cannot be doubted : the granitic region 
of Parime is described by Humboldt as being at least nineteen 
times as large as Switzerland. South of the Amazon, Boue 
colors an area comjiosed of rocks of this nature as equal to tliat 
of Spain, France, Italy, part of German}-, and the British Isl- 
ands, all conjoined. This region has not been carefully ex- 
plored, but, from tlie concurrent testimony of travellers, the 
granitic area is very large : thus. Von Eschwege gives a de- 
tailed section of these rocks, stretching from Rio dc Janeiro 
for 260 geograjihical miles inland in a straight line ; and I trav- 
elled for 150 miles in another direction, and saw nothing but 
granitic rocks. Numerous specimens, collected along the whole 
c(jast from near Rio Janeiro to tlie mouth of the Plata, a dis- 
tance of 1,100 geographical miles, were examined l)y me, and 
they all belonged to this class. Inland, along the whole north- 
ern bank of the Plata, I saw, besides modern tertiary beds, only 
one small patch of slightly-metamorphosed rock, ^\■hich alone 
could have formed a part of the original capping of the granitic 
scries. Tuining to a well-known region, namely, to the United 
States and Canada, as shown in Prof. II. I). Rogers's beauti- 
ful map, I have estimated the areas l)y (Hitting out and weigh- 
ing the paper, and I find that liie metamorphic (including "the 
Henii-metamorphic") and granitic rocks exceed, in the i)n)p()rtion 


of 19 to 1:2.5, the Avholc of the newer Paleozoic formations. 
In many rcg'ions the metamorphic and granitic rocks would be 
seen to be much more widely extended, if all the sedimentary 
beds Averc removed which rest unconformably on them, and 
whicli could not have formed part of the original mantle under 
which they were crystallized. Hence it is probable that in some 
parts of the world whole formations have been completely de- 
nuded, with not a wreck left behind. 

One remark is here worth a passing notice. During peri- 
ods of elevation the area of the land and of the adjoining shoal 
parts of the sea will be increased, and new stations will often 
be formed — all circumstances favorable, as previously explained, 
for the formation of new varieties and species ; but during such 
periods there will generally be a blank in the geological record. 
On the other hand, during subsidence, the inhabited area and 
number of inhabitants Avill decrease (excepting on the shores 
of a continent when fii-st broken up into an archipelago), and 
consequently during subsidence, though there will bo much ex- 
tinction, few new varieties or species will be formed ; and it 
is during these very periods of subsidence that the deposits 
which are richest in fossUs have been accumulated. 

On the Absence of JVumcrous Intermediate Varieties in any 
one Single Formatio7i, 

From these several considerations, it cannot be doubted 
that the geological record, viewed as a whole, is extremely im- 
perfect ; but if we conline our attention to any one formation, 
it becomes much more diilicult to understand why we do not 
therein find closely-graduated varieties between the allied 
species which lived at its commencement and at its close. 
Several cases arc on record of the same species presenting va- 
rieties in the upper and lower parts of the same formation : 
thus, Trautschold gives a number of instances with Ammon- 
ites ; and Hilgendorf has described a most curious case of ten 
graduated forms of Planorbis multiformis in the successive 
beds of a fresh-water formation in Switzerland. Although 
each formation has indisputably required a vast number of 
years for its deposition, several reasons can be given why each 
should not connnonly include a graduated series of links be- 
tween 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 requi- 
site to change one sjieoies into another. I am aware that two 
paleontologists, whose opinions are worthy of much deference, 
namely, Broim and AVoodward, have concluded that the aver- 
age duration of each fcjrmation is twice or thrice as long as the 
average duration of specific forms. But insuperable difficul- 
ties, as it seems to me, prevent us from coming to an}"^ just 
conclusion on this head. W^hen we see a species first appear- 
ing in the middle of any formation, it would be rash in the ex- 
treme to infer that it had not elsewhere previously existed. 
So, again, when we find a species disappearing before the last 
layers have been deposited, it would be equally rash to sup- 
pose that it then became extinct. We forget how small the 
area of Europe is compared with the rest of the world ; nor 
have the several stages of the same formation throughout Eu- 
rope been correlated Avith perfect accuracy. 

With marine animals of all kinds, we may safelv infer a 
large amount of migration during climatal and other changes ; 
and when we see a species first appearing in any formation, the 
probability is, that it only then first iimnigrated into that area. 
It is well known, for instance, that several species ajipeared 
somewhat earlier in the paleozoic beds of North America than 
in those of Europe; time having apparently been required for 
their Tnigration from the American to the European seas. In 
examining the latest deposits in various quai'ters of the world, 
it has everywhere been noted that some few still existing spe- 
cies are common in the deposit, but have become extinct in the 
immediately surrounding sea ; or, conversely, that some arc 
now abundant in the neighboring sea, but are rare or absent in 
tliis particular deposit. It is an excellent lesson to reflect on 
the ascertained amount of migration of the inhabitants of Eu- 
rope during the glacial epoch, which forms only a part of one 
whole geolf)gical 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, sedi- 
mentary deposits, indiidlitf/ fossil rcmahis, have gone on ac- 
cumulating within the same area during the whole of this 
period. It is not, for instance, probable that S(Mliment was de- 
posited during the whole of the glacial jieriod near the mouth 
of the Mississippi, within that limit of (ie])th at which marine 
animals can best flourish ; for we know that great geographical 


changes oocurrcd in other parts of America duriiifr this space 
of time. "\\'h(ui such beds as were deposited in shallow water 
near the mouth of the Mississippi during some part of the gla- 
cial period shall have been upraised, organic remains will prob- 
ably lirst appear and disappear at different levels, owing to 
the migrations of species and to geographical changes. And 
in the distant future, a geologist, examining these beds, would 
be tempted to conclude that the average duration of life of the 
embedded fossils had been less than that of the glacial period, 
instead of having been really far greater, that is, extending 
from before the glacial ejioch 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 
will have to go on continuously accumulating during a very 
long period, so that there may be time sufficient for the slow 
procT^ss of modification ; hence the deposit will have to be a 
very thick one ; and the species undergoing change will have 
to live in the same district throughout this whole time. But 
we have seen that a thick formation, fossiliferous throughout 
its entire thickness, can accumulate only during a period of 
subsidence ; and to keep the depth approximately the same, 
which is necessary that the same marine species may hve on 
the same space, the supply of sediment must nearly counter- 
balance the amount of subsidence. But this same movement 
of subsidence will tend to submerge the area whence the sedi- 
ment is derived, and thus diminish the supply while the down- 
ward movement continues. In fact, this nearly exact balancing 
between the supply of sediment and the amount of subsidence 
is probably a rare contingency ; for it has been observed, by 
more than one paleontologist, that very thick deposits are 
usually barren of organic remains, except near their upper or 
lower limits. 

It would seem that each separate formation, like the whole 
pile of formations in any country, has generally been intermit- 
tent in its accumulation. When we see, as is so often the case, 
a formation composed of beds of dilVercnt mineralogical com- 
position, we may reasonably suspect that the jirocess of deposi- 
tion has been much intcrnij)t(Ml, as a change in the currents of 
the sea, and a supply of sediment of a different nature will 
generally have been due to geographical changes requiring 
much time. Nor will the closest ins]iection of a formation give 
any idea of the time which its deposition has consumed. Many 
instances could be given of beds only a few feet in thickness, 


roiM(\scntIiig formations, elsewhere thousands of feet in thick- 
ness, and which must have required an enormous period for 
(heir accumuhition ; yet no one ignorant of this fact woukl have 
snspecled the vast lapse of time represented by the thinner for- 
mation. Many cases could be f]fiven of the lower beds of a 
formation havin*^ been upraised, denuded, submcrj^ed, and then 
i('-covercd by the upper beds of the same formation — facts, 
shcnving what wide, yet easily-overlooked, intervals have oc- 
curred in its accumulation. In other cases we have the plainest 
evidence in great fossilized trees, still standing upright as they 
grew, of many long intervals of time and changes of level 
(luring the process of deposition, which would never even have 
been suspected, had not the trees chanced to have been pre- 
served : tluis Sir C Lyell and Dr. Dawson found carboniferous 
beds 1,400 feet tliick in Nova Scotia, with ancient root-bearing 
strata, one above the other, at no less than sixty-eight diiferent 
levels. Hence, when the same species occur at the bottom, 
middle, and top of a formation, the probability is that they have 
not lived on the same spot during the whole period of depo- 
sition, but have disappeared and reapj)eared, perhaps many 
times, during the same geological period. So that, if such spe- 
cies were to undergo a consid(n-al)le amount of modification 
during any one geological period, a section would not include 
all the fin(^ intermediate gradations which must on our theory 
have existed between them, but aljrupt, though perhaps slight, 
changes of form. 

It is all-important to remember that natin-alists have no 
golden rule by which to distinguish species and varieties ; they 
grant some little variability to each species, but when they 
meet with a somewhat greater amount of dilference between 
any two forms, they rank both as species, uidess tliey are en- 
abled to coimect them together by the closest intermediate 
gradations. And this from the reasons just assigned we can 
seldom hope to effect in any one geological section. Supposing 
B and C to b(; two sjiecies, and a third. A, to l)e found in an 
older and underlying bed; even if A were strietly intermediate 
l)(!tween B and C, it would simply be ranked as a tliird and 
distinct sjiecies, unl(\ss at the sam«; time it could be most 
closely coimected with either one or l)oth forms 1)>' intermediate 
varieties. Nor should it be forgotten, as before explaitied, that 
A might ])(; the actual ]irogenitor of B and C, and yet might 
not necessarily be strictly intermediate between them in all re- 
opccts. So that we might obtain the parcnt>specics and its 


several modified descendants from tlu^ lower and upper beds of 
the same formation, and unless "\ve obtained numerous transi- 
tional gradations, we should not reco^ize their blood-relation- 
ship, and should consequently be compelled to rank them as 
distinct species. 

It is notorious on what excessively slight differences many 
paleontologists liave founded their species; and they do this 
the more readily if the specimens come from different sub- 
stages of the same formation. Some experienced conchologists 
are now sinking many of the very fine species of D'Orbigny 
and others into the rank of varieties ; and on this view we do 
find the kind of evidence of change which on tlie theory we 
ought to find. Look, again, at the later tertiary deposits, 
which include many shells believed by the majority of natural- 
ists to be identical with existing species ; but some excellent 
naturalists, as Agassiz and Pictet, maintain that all these ter- 
tiary species are specifically distinct, though the distinction is 
admitted to be very slight ; so that here, unless we believe that 
these eminent naturalists have been misled by their imaginations, 
and that these late tertiary species really present no difference 
whatever from their living representatives, or unless we believe 
that the great majority of naturalists are wrong, and that the 
tertiar}' sjiecies are all truly distinct from the recent, we have 
evidence of the frequent occurrence of sliglit modifications of 
the kind required. If we look to rather wider intervals of time, 
namely, to distinct but consecutive stages of the same great 
formation, we find that the embedded fossils, though almost 
universally ranked as specifically different, yet are far more 
closely related to each other than are the species found in more 
widely-separated formations ; so that here again we have un- 
doubted evidence of change in the direction required by the 
theory ; but to this latter subject I shall have to return in the 
following chapter. 

With animals and plants that propagate rapidly and do 
not wander much, there is reason to suspect, as we have for- 
merly seen, that their varieties are generally at first local ; and 
that such local varieties do not spread widely and supplant 
their parent-forms imtil they have been modified and perfected 
in some considerable degree. According to this A'icw, the 
chance of discovering in a fonnation in any one country all the 
early stages of transition between any two such forms is small, 
for the successive changes are supposed to liave been local, or 
confined to some one spot. Most marine animals have a wide 


range ; and wo have soon tliat with plants it is those wliicli 
have the widest rani>-o, that oftonest present varieties; so that, 
witli shells and other marine animals, it is pro])able that those 
which liavc had the widest range, far exceeding the limits of 
the known geological formations of Europe, have oftcnest given 
rise, ilrst 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 for- 

It is a more important consideration, leading to the same 
result, as lately insisted on by Dr. Falconer, namely, that the 
period during which each species underwent modification, 
though long as measured by years, was, from the reasons 
lately assigned, probably short in comparison with that during 
which it remained without undergoing any ch.ange. 

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, initil many specimens are collected from many 
]ilaces ; and, in tlie case of fossil species, this can rarely be 
effected by paleontologists. "We shall, perhaps, best perceive 
the improbability of our being enabled to connect species by 
numerous fine, intermediate fossil links, by asking ourselves 
^^•hether, for instance, geologists at some future period will be 
able to prove that our different breeds of cattle, sheep, horses, 
and dogs, are descended from a single stock or from several 
alwriginal stocks; or, again, Avhether certain sea-shells inliabit- 
ing the shores of North America, which are ranked by some 
conchologists as distinct species from their European represent- 
atives, and by other conchologists as only varieties, are really 
varieties, or are, as it is called, specifically distinct. This could 
be effected only by the future geologist discovering in a fossil 
state numerous intermediate gradations ; and such success is 
imj>r(>bable in tlu; iiighest degree. 

It has been assorted over and over again, l)y Avritors who 
believe in the immutability of species, that geology yields no 
linking forms. This assertion is entirely erroneous. As Sir 
J. Lubbock has nMuarkod, "Every species is a link between 
other allied forms." We clearly see this if we take a genus 
having a score of recent and extinct species and destroy four- 
lifths of thorn ; for in tliis case no one doubts that the ri'inain- 
dor will stand much more distinct from each other. If the ex- 
treme forn)s in the genus happen to have boon thus destroyed, 


the genus itself in most cases will stand more distinct from 
ntlicr allied genera. The camel and the pig, or the horse and 
the tapir, are now obviously very distinct forms ; but if we add 
the several fossil quadrupeds which have already been discov- 
ered to the families including the camel and pig, these forma 
become joined by links not extremely wide apart. The chain 
of linking forms does not, hovrever, in these cases, or in any 
case, run straight from the one living form to the other, but 
takes a circuitous sweep through the forms which lived during 
long-past ages. What geological research has not revealed, 
is the former existence of infinitely numerous gradations, as 
fine as existing varieties, connecting nearly all our existing 
species w'itli extinct species. But this ought not to be ex- 
pected ; 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 liluvopc from the North Cape to the Mediterranean, 
and fi'om Britain to Russia ; and therefore equals all the geo- 
logical formations which have been examined with any accu- 
racy, excepting those of the United States of America. I fully 
agree with Mr. Godwin-x^usten, that the present condition of 
the Malay Archipelago, with its numerous large islands sep- 
arated by wide and shallow seas, probably represents the for- 
mer state of Europe, while most of our formations were accu- 
mulating. The Malay Archipelago is one of the richest re- 
gions of the "whole world in organic beings ; yet, if all the 
species were to be collected which have ever lived there, how 
imperfectly would they represent the natural history of the 
world ! 

But Ave have every reason to believe that the terrestrial 
productions of the archipelago would be preserved in an ex- 
tremely imperfect manner in the formations which we suppose 
to be there accumulating. Not many of the strictly littoral 
animals, or of those Avhich 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 accu- 
mulate at a sufficient rate to protect organic bodies from decay, 
no remains could be preserved. 

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, -wonkl f^cncrally 
be formed in the archipehigo only during poriods 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 sleeper shores would be 
destroyed, almost as soon as accumulated, by the incessant 
coast-action, as we now see on the shores of South America ; 
even throughout the extensive and shallow seas witiiin 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 ex- 
tinction of life ; during the periods of elevation, there would 
be much variation, but the geological record would then be 
least perfect. 

It may be doubted whether the duration of any one 
great period of subsidence over the whole or part of the 
archipelago, together with a contemporaneous accumulation 
of sediment, would exceed the average duration of the same 
specific forms; and these contingencies are indispensable for 
the preservation of all the transitional gradations between 
any two or more species. If such gradations were not all 
fully preserved, transitional varieties would merely appear 
as so many new and distinct species. It is also probable 
that each great period of subsidence would be interrupted 
by oscillations of level, and that slight climatal changes 
would intervene during such lengthy periods ; and in these 
cases the inhabitants of the archipehigo would migrate, and 
no closely-consecutive record of their modifications could be 
preserved in any one formation. 

Very many of the marine inhabitants of the archipelago 
now range: thousands of miles beyond its confines ; and 
analogy plainly leads to the belief that it would be chiefly 
these far-ranging species, though only some of them, which 
would oftenest produce new varieties ; and the varieties 
would at first generally be local or confined to one place ; 
but if jiossessed of any decided advantage, or when further 
modified and improve<l, they would slowly spread and supplant 
their parent-forms. ^V^hen such varieties returned to their 
ancient homes, as they would difTcr from their former state, 
in a nearlv-unifonn, though ]>orhaj)s extremely-slight degree. 


and as they would Ijc found embedded in sliglilly-different 
sub-stages of the same formation, they would, according to 
the principles followed l)y many paleontologists, be ranked 
as new and distinct species. 

If, then, there be some degree of truth in these remarks, 
we have no right to expect to find, in our geological for- 
mations, an infinite number of those fine transitional forma 
which, on our theor}', have connected all the past and present 
species of the same gi-oup into one long and branching 
chain of life. We ought only to look for a few links, and 
such assuredly Ave do find — some more distantly, some more 
closely, related to each other ; and these links, let them be 
ever so close, if found in different stages of the same forma- 
tion, would, by many paleontologists, be ranked as distinct 
species. But I do not pretend that I should ever have suspect- 
ed how poor was the record in the best preserved geologi- 
cal sections, had not the absence of innumerable transitional 
links between the species which lived at the commencement 
and close of each formation, pressed so hardly on my theory. 

On the sudden Apjjearance of ichole Groups of allied 

The abrupt manner in which Avhole groups of species 
suddenly appear in certain formations, has been urged by 
several paleontologists — for instance, by Agassiz, Pictet, and 
Sedgwick — as a fatal objection to the belief in the trans- 
mutation of species. If numerous species, belonging to the 
same genera or families, have really started into life at 
once, the fact would be fatal to the theory of descent with 
slow modification through natural selection. For the devel- 
opment of a group of forms, all of which have descended 
from some one progenitor, nuist have been an extremely slow 
process; and the progenitors must have lived long ages 
before their mothfied descendants. But we continually over- 
rate the perfection of the geological record and falsely infer, 
because certain genera or families have not been foimd 
beneath a certain stage, that they did not exist before that 
stage. In all cases positive paleontological evidence may 
be implicitly trusted ; negative evidence is worthless, as 
experience has so often sho"wn. We continually forget how 
large the world is, compared with the area over which our 
geological formations have been carefully examined ; we 


forf^ot that g-ioups of species may elsewhere have long: 
existed, and liavc slowly multiplied, before they invaded 
the ancient archipela^^oes of Europe and the United States. 
We do not make due allowance for the enormous intervals 
of time which have elapsed between our consecutive forma- 
tions — loiiocr perhaps in many cases than the time required 
for the accumulation of each formation. These intervals 
will have g-iven time for the multiplication of species from 
some one or some few parent-forms ; and in the succeeding^ 
formation such groups of species will appear as if suddenly 

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, 
lo liy through the air; and consequently that the transition- 
al forms would often remain confined to some one region ; 
but that, when this adaptation had once been effected, and 
a few species had thus acquired a great advantage over 
other organisms, a comparatively short time would 1)0 neces- 
sary to produce many divergent forms, ■which would spread 
rapidly and widely throughout the world. Prof. Pictet, 
in his excellent review of this work, in commenting on 
early transitional forms, and taking birds as an illustration, 
cannot see how the successive modifications of the ante- 
rior limbs of a supposed protot\'pe could possibly have been 
of any advantage. But look at the penguins of the Southern 
Ocean; have not these birds their front limbs in this precise 
intermediate state of "neither true arms nor true wings?" 
Yet these birds hold their place victoriously in the battle 
for life; for they exist in infinite numbers and of many kinds. 
I do not suppose that we here see the real transitional 
grades through which the wings of birds have passed ; but 
what special dilliculty is there in believing that it might 
profit the modified descendants of the ]ienguin, first to become 
enabled to flap along the surface of the sea like the logger- 
headed duck, and ultimately to rise from its surface and glide 
through the air? 

I will now give a fmv examples to illustrate the fore- 
going remarks, and to show how liable we are to error in 
supposing that whole groups of sj^ecies have suddenly been 
produced. Even in so short an interval as that between the 
iirst and second editions of Pictet's great work on Paleon- 
tologv, pu])lished in 18-i4-'4G and in ISSS-^o?, the conclusions 


on the first appearance and disappearance of several groups 
of animals have l)ocn considerably modified ; and a third 
edition •would rociuire still further changes. I may recall 
the ■well-known fact that in geological treatises, published 
not many years ago, mammals were always spoken of as 
liaving abruptly come in at the commencement of the tertiary 
series. And now one of the richest known accumulations of 
fossil mammals, for its thickness, belongs to the middle of 
the secondary series ; and true mammals have been discov- 
ered in the new red sandstone at nearly the commencement of 
tliis great series. Cuvier used to urge that no monkey oc- 
curred 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 acci- 
dent of the preservation of footsteps in the new red sandstone 
of the United States, who would have A^entured to suppose 
that, besides reptiles, no less than at least thirty kinds of birds, 
some of gigantic size, existed during that period ? Not a frag- 
ment of bone has been discovered in these beds. Notwith- 
standing that the number of joints shown in the fossil impres- 
sions corresponds with the number in the several toes of living 
birds' feet, some authors doubt whether the animals which left 
these impressions were really birds. Until quite recently these 
authors might have maintained, and some have maintained, that 
the whole class of birds came suddenly into existence during 
the eocene period; but now we know, on the authority of 
Prof. Owen, that a bird certainly lived during the deposition 
of the upper greensand ; and still more recently, that strange 
bird, the ArchcopterA'x, with a long, lizard-like tail, bearing a 
pair of feathers on each joint, and with its wings furnished with 
two free claAvs, has been discovered in the oolitic slates of So- 
Icnhofcn. Hardly any recent discovery shows more forcibly 
than this, how little we as yet know of the former inhabitants 
of the Avorld. 

I may give another instance, which, from having passed im- 
der my own eyes, has much struck me. In a memoir on Fossil 
Sessile Cirripedes, I have stated that, from the large number 
of existing and extinct tertiary species ; from the extraordinary 
abundance of the individuals of many sj:)ecies all over the world, 
from the Arctic regions to the equator, inhabiting A-arious 
zones of depths from the upper tidal limits to 50 fathoms ; from 
the perfect manner in which specimens are preserved in the 
oldest tertiary beds ; from the case with which even a frag- 


nicnt of a valve can be recorrnizetl ; from all these circum- 
stances, I infer that, had sessile cirripcdes existed durinn^ the 
secondary periods, they would certainly have Ijcen preserved and 
discovered ; and as not one species had then been discovered in 
oeds of this age, I concluded that this grcjit jrroup had been 
suddenly develo])ed at the commencement of the tertiary series. 
This was a sore trouble to me, adding as I thought one more 
instance of the abrupt appearance of a great group of species. 
But my work had hardly been published, when a skilful pale- 
ontologist, M. Bosquet, sent me a drawing of a perfect speci- 
men of an unmistakable sessile cirripede, Avhich he had him- 
self extracted from the chalk of Belgium. And, as if to make 
tlio case as striking as possil)le, this sessile ciiripede was a 
C'hthamalus, a very common, large, and ubiquitous genus, of 
which not one specimen has as yet been foimd even in any ter- 
tiary stratum. Hence we now positively know that sessile 
cirripedes existed during the secondary period ; and these cir- 
ripcdes might have been the progenitors of our many tertiary 
and existhig species. Still more recently Tyrgoma has been 
discovered by Mr, AA'oodward in the upper chalk. 

The case most frecjucntly insisted on by paleontologists of 
the apparently sudden appearance of a whole group of species, 
is that of the teieostean iishes, low down in the Chalk period. 
This group includes the large majority of existing species. 
Lately, Prof. Pictet has carried their existence one sub-stage 
further back ; and some paleontologists believe that certain 
nmch older fishes, of which the affinities are as yet imperfectly 
known, are really teieostean. Assuming, however, that the 
whole of them did appear, as Agassiz maintains, at the com- 
mencement of the chalk formation, the fact would certainly be 
highly remarkal)lc ; but I cannot see that it would be an in- 
superalile objection to these views, unless it could likewise be 
shown that tlie species of this group appeared suddenly and 
simultaneously throughout the world at tliis same period. It 
is almost superfluous to remark that hardly an}' fossil-lish are 
known from south of the equator; and by running tlu-ougli 
Pictet's Paleontology it will be seen that very few species are 
Ivtiown from several formations in Europe. Some few families 
of (ish now have a confined range ; the teieostean fish miglit 
formerly have had a simihirly confined range, and after having 
been largely developed in some one sea, might have spread 
widely. Nor have we any right to suppose that the seas of 
the world have always been so freely open from eouth to north 


as they are at present. Even at tliis day, if the Malay Archi 
pehigo "svcre converted into land, the tropical parts of the In- 
dian Ocean would form a large and perfectly-enclosed basin, 
in Avliich 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 geol- 
ogy of other countries beyond the confines of Europe and the 
United States, and fi'om the revolution in our paleontological 
knowledge eflected by the discoveries of the last dozen years, 
it seems to me to be about as rash to dogmatize on the succes- 
sion of organic forms throughout the world, as it would be for 
a naturalist to land for five minutes on a barren point in Aus- 
tralia, and then to discuss the number and range of its jjroduc- 

On the sudden A2)2)earance of Groups of allied Species in 
the loicest Jcnoicn Fossil if eroiis Strata. 

There is another and allied difficulty, which is much more 
serious. I allude to the manner in Avhich many species in sev- 
eral of the main divisions of the animal kingdom suddenly 
appear in the lowest known fossiliferous rocks. Most of the 
arguments which have convinced me that all the existing 
species of the same group are descended from a single pro- 
genitor, apply with nearly equal force to the earliest known 
species. For instance, it cannot be doubted that all the Silu- 
rian trilobites are descended from some one crustacean, which 
must have lived long before the Silurian age, and which prob- 
ably difl'ered greatly from an>' known animal. Some of the 
most ancient Silurian animals, as the Nautilus, Lingula, etc., 
do not difter much from living sjiccies; and it cannot on our 
theory be sujiposed that these old species were the progenitors 
of all the species belonging to the same groups whicli have 
subseciucntly appeared, for they are not in any degree inter- 
mediate in character. 

Consequently, if the tlicory be true, it is indis]nitable 
that, before the lowest Silurian or Cambrian stratum was do- 
posited long periods elapsed, as long as, or probably far longer 
than, tlie whole interval from the Cambrian age to the present 
day; I'.nd that during these vast periods the world swarmed 


■\vitli livinp; creatures. Here we encounter a formidable objec- 
tion ; for it sceins dou])tful whether the eartli in a lit state for 
the habitation of livin"^ creatures has lasted lono; cnouo-h. Sir 
W. Tiionipson concludes that the consolidation of the crust can 
hardly have occurred less than 20 or more than 400 million 
years ago, Ijut probably not less than 98 or more than 200 mill- 
ion years. Tliese very wide limits show how doubtful the 
data are ; and other elements may have to be introduced into 
tlie problem. Mr. C'roll estimates that about 00 million years 
have elapsed since the Cambrian period, but this, judging from 
the small amoiuit of organic change since the commencement 
of the Glacial epoch, seems a very short time for the many and 
great mutations of life, which have certainlv occurred since the 
Cambrian formation; and the previous 140 million years can 
hardly be considered as sufficient for the development of the 
varied forms of life which certainly existed toward the close of 
the Cambrian period. It is, however, probable, as Sir ^\^ 
Thompson insists, that the world at a verj- early period was 
subjected to more rapid and violent changes in its physical con- 
ditions than those now occurring; and such changes would 
have led to corresponding rapid changes in the organic being-s 
which inhabited the world at this remote period. 

To the question why we do not hnd rich fossiliferous de- 
posits belonging to these assumed earliest periods, I can give 
no satisfactory answer. Several eminent geologists, with Sir 
li. Murehison at their head, were until recently convinced that 
we beheld in the organic remains of the lowest Silurian stratum 
the first dawn of life. Other highly-competent judges, as Ja-qM 
and E. Forbes, have disputed this conclusion. We should not 
forget that only a small jiortion of the world is known with ac- 
curacy. Not long ago, M. Barrande added another and lower 
stage, abounding with new and peculiar species beneath the 
old Silurian system. Henmants of several forms have also 
been detected beneath BaiTande's so-called primordial zone in 
the Longmynd group, now divided into two stages, and con- 
stituting the Lower Cambrian system. The presence also of 
])hosphatie nodules and bitunnnous matter in some of the low- 
est azoic rocks, jirobably indicates life at these periods. Now 
the great discovery of the Eozoon in the Laurent ian f(jrmation 
of Canada has been made, for, after reading Dr. Carpenter's 
description of tliis fossil, it is scarcely possible to doubt regard- 
ing its organic nature. There are tiiree great series of strata 
beneath tiie Siluriiin svslein in Canad:i, in the lowest of which 


the Eozoon was found ; and Sir W. Logan states that their 
" united thickness may jiossibly far surpass that of all the suc- 
ceeding rocks, fiom llie base of the paleozoic series to the pres- 
ent time. We arc thus carried back to a period so remote, 
that the appearance of the so-called Primordial fauna (of 
Barraudc) may by some be considered as a comparatiyely 
modern eyent." The Eozoon belongs to the most lowly or- 
ganized of all classes of animals, but for its class is highly organ- 
ized ; it existed in countless numbers, and, as Dr, Dawson has re- 
marked, certainly preyed on other minute organic beings, which 
must have lived in great numbers. Thus the words, which I 
wrote in 1859, about the vast periods which had probably 
elapsed before the Cambrian system, are almost the same with 
those since used by Sir W. Logan. Nevertheless, the difliculty 
of assigning any good reason for the absence beneath the up- 
per Cambrian formations of vast piles of strata rich in fossils, 
is very great. It does not seem probable that the most an- 
cient beds have been quite worn away by denudation, or that 
their fossils have been wholly obliterated liy raetamorphic ac- 
tion, for if this had been the case we should have found only 
small remnants of the formations next succeeding them in age, 
and these -would always have existed in a partially-metamor- 
])hoscd condition. But the descriptions which we possess of 
the Silurian deposits over immense territories in Russia and in 
North America, do not support the view, that the older a for- 
mation is, the more it has invariably suffered extreme denuda- 
tion and metamorphism. 

The case at present must remain inexplicable ; and may be 
truly urged as a valid argument against the views here enter- 
tained. To show that it may hereafter receive some explana- 
tion, I will give the following hypothesis : From the nature ot 
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 Avhich the formations arc composed, we may 
infer that from first to last large islands or tracts of land, 
whence the sediment was derived, occurred in the neighbor- 
hood of the now existing continents of Europe and North 
America. But we do not know what Avas the state of tilings 
in the intervals between the several successive fonnations ; 
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 
a lid unfathomable sea. 


Lookinp^ lo the existing oceans, whicli are thrice as exten- 
sive as the hind, we see them studded with many ishmds ; but 
not one truly oceanic ishind (with the exception of New Zea- 
hind, if this can be called a truly oceanic island) is as ^-^et known 
to afford even a renmant of any jialeozoic or secondary forma- 
tion. Hence we may perhaps infer that, duriufr the paleozoic 
and secondary periods, neither continents nor continental islands 
existed where our oceans now extend ; for had they existed, 
])aleozoicand secondary formations would in all j)roi)ability have 
been accumulated from sediment derived from their wear and 
tear ; and these would have been at least partiall}' upheaved 
by the oscillations of level, which must have intervened during 
these enormousl3'-long' periods. If, then, we may infer any 
thing from these facts, we may infer that, where our oceans 
now extend, oceans have extended from the remotest period of 
which we have any record ; and, on the other hand, that where 
cofitinents now exist, large tracts of hind have existc.'d, subjected 
no doubt to great oscillations of level, since the earliest Silurian 
j)criod. The colored map appended to my volume on Coral 
lleefs led me to conclude that the great oceans are still mainly 
areas of subsidence, the great archipelagoes still areas of oscil- 
lations of level, and the continents areas of elevation. IJut we 
have no reason to assume that things have thus remained from 
the beginning of the world. Our continents seem to have been 
formed by a jireponderance, during many oscillations of level, 
of the force of elevation ; but may not the areas of preponder- 
ant movement have changed in the lapse of ages ? At a period 
long antecedent to the Silurian epoch, continents may have ex- 
isted where oceans are now spread out ; and clear and open 
oceans may have existed where our continents now stand. Nor 
should we be justilied in assuming that if, for instance, the bed 
of the Pacific Ocean were now converted into a continent, we 
shouUl there find sedimentary formations in a recognizable con- 
dition older than the Silurian strata, suj^posing such to have 
been formerly deposited; for it might well happen that strata 
which had subsided some miles nearer to the centre of the 
earth, and which had been pressed on by an enormous weight 
of superincumbent water, might have undergcme far more met- 
amorphic action than strata Avhich have always remained 
nearer to the surface. The immense areas in some parts of the 
world, for instance in South America, of naked metamorphic 
rocks, which must have been heated under great pressure, have 
always seemed to me to require some special explanation; and 


we ma}' pcrliaps believe that avc sec, in these large areas, the 
many formations long anterior to the Silurian epoch in a com- 
pletely metamorphosed and denuded condition. 

The several difficulties here discussed — namely, that, though 
•\vc find in our geological formations many links between the 
species which now exist and which formerly existed, we do not 
find infiiutely numerous fine transitional forms closely jouiing 
them all together; the sudden manner in which several whole 
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, undoubted- 
ly, of the most serious nature. A\'e see this in the fact that 
tlie most eminent paleontologists — namely, CuA-ier, Agassiz, 
Barrande, Pictet, Falconer, E, Forbes, etc., and all our great- 
est geologists, as Lyell, IMurchison, Sedgwick, etc., have unan- 
imously, 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 other geologists 
and paleontologists are much shaken in their former belief. 
Those who believe that the geological record is in any d(>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 possess the last vol- 
ume alone, relating only to two or three countries. Of this 
volume, only here and there a short chapter has been pre- 
served; and of each page, only here and there a few lines. 
Each word of the slowly-changing language, more or less dif- 
ferent in the successive chapters, may represent the forms of 
life, which arc entombed in our consecutive formations, and 
which falsely appear to us to have been abruptl}' introduced. 
On this vicAV, the difficulties above discussed are greatly dimin- 
ished, or even disappear. 




On the Slow nnd Snccci>9ivc Appearance of New Species— On their Diflfcrcnt Rates 
of t'haii;,'C— Species once lost do not reappear — Groui)8 of Species follow the 
game (Jc-ncnil Rules in their Appearance and Uisappearunce as do Sinfrlc Species 
— On Extinction— On Sininltaneoiis C'hnn;,'es in the Forms of Life throu^rhout 
the World— On the Affinities of Extinct Species to cacli other and to Li\ in;; Spe- 
cies—On the State of Develo[)nKiit of Ancient Forms— On the Succession of tho 
same Typea within the same Areas- Summary of preccdinjj and present Chapter. 

Let us now see whether the several facts and laws relatint^ 
to the geological succession of organic beings better accord 
with the common view of the immutability of species, or with 
that of their slow and gradual modification, through descent 
and natural selection. 

New species have appeared very slowly, one after another, 
both on the land and in the waters. Lyell has shown that it 
is hardly possible to resist the evidence on this head in the 
case of the several tertiary stages ; and every year tends to fill 
up the blanks between the stages, and to make the percentage 
system of lost and new forms more gradual. In some of the 
most recent beds, though undoubtedly of high antitjuity if 
measured by years, only one or two species arc extinct, and 
only one or two arc new, having appeared there for the first 
time, either locally, or, as far as we know, on the face of 
the cartli. The secondary formations are more broken ; but, 
as Bronn has remarked, neither the appearance nor. disappear- 
ance of the many extinct species embedded in each formation 
has been simultaneous. 

Species of different genera iind 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 foimd in the midst of a 
multitude of extinct fonns. Falconer has given a striking in- 
stance of a similar fact, for an existing crocodile is associated 
with many lost mammals and reptiles in the sub-IIimalayan 
deposits. The Silurian Liiigula dilTcrs but little from the 


livinfj: spocics of this p^'onus ; whereas most of the other Silu- 
rian Molhiscs and all the; Crustaceans have changed greatly. 
The productions of the land seem to change at a quicker rate 
than those of the sea, of whicli a striking instance has lately 
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 JM. Barrande, which 
intrude for a period in the midst of an older formation, and 
then allow the preexisting fauna to reappear ; but Lyell's ex- 
jjlanation, namely, that it is a case of temporary migration 
from a distinct geographical province, seems to me satisfac- 

These several facts accord well Avith our theory, which in- 
cludes no fixed law of development, causing all the inhabitants 
of an area to change abruptl}^, or simultaneously, or to an equal 
degree. Tlic 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 quite independent of that of all 
others. Whether such variations or individual differences as 
may arise will be accumulated through natural selection in a 
greater or less degree, thus causing a greater or less amount 
of permanent modification, will depend on many complex con- 
tingencies — 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 
means surprising that one species should retain the' same iden- 
tical form much longer than others ; or, if changing, that it 
should chang(! in a less degree. We find similar relations be- 
tween the inhaljitants of distinct countries ; for instance, the 
land-sliclls and coleopterous insects of jNIadeira have come to 
diller considerably from their nearest allies oh the continent 
of l^^urope, whereas the marine shells and birtls have remained 
unaltered. We can perhaps understand the apparently quicker 


rate of change in terrestrial and in more higlily-org'anized pro- 
ductions compared with marine and lower productions, by the 
more complex relations of tlie hi<^her beings to tlieir organic 
and inor^^nic conditions of life, as explained in a former chap- 
ter. When many of the inhabitants of any area have become 
modified and imjiroved, we can imderstand, on the principle of 
competition, and from tlie all-important relations of organism 
to organism in the struggle for life, that any form which does 
not become in some deg'ree modified and improved, will be 
liable to extermination. Ilencc we see why all the species in 
the same region do at last, if we look to long-enougli intervals 
of time, become niodilicd, for otherwise they would become 

In members of the same class the average amount of 
change, during long and equal periods of time, may, perhaps, be 
nearly the same ; but as the accumulation of long'-enduring' for- 
mations, 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 inter- 
mittent intervals of time ; consequently the amount of organic 
change exhibited by the fossils embedded in consecutive for- 
mations is not equal. Each formation, on this view, does not 
mark a new and complete act of creation, but only an occa- 
sional scene, taken almost at hazard, in an ever slowly-chan- 
ging drama. 

We can clearly understand why a species when once lost 
should never reappear, even if the very same conditions of life, 
organic and inorganic, should recur. For thougli the offspring 
of one species might be ada])t{Hl (and no doubt this has occurred 
in innunieral)le instances) to 1111 the place of another species' in 
tlie economy of Nature, and thus supplant it; yet the two 
forms — the- old and the new — would not be identically th.e 
same ; for both would almost certainly inherit different char- 
acters from their distinct progenitors, and organisms already 
differing would vary in a different manner. For instance, it is 
pist possible, if all our fantail jiigeons were destroyed, that 
fanciers might make a new breed hardly distinguishable from 
tlie present breed ; but if the parent rock-pigeon were likewise 
destroyed, and imdcr Nature we have every reason to believe 
that j»arent-fonns are generallv supplanted and exterminated 
bv tlieir improved offspring, it is incredible that a fantail, iden- 
tical with th(! existing breed, could he raised from any other 
species of pigeon, or even from any other well-established race 


of the domestic pigeon, for the successive variations would al- 
most certainly be in some degree different, and the newly- 
formed variety would probably inherit from its progenitor some 
characteristic differences. 

Groups of species, that is, genera and families, follow the 
same general rules in their appearance and disappearance as 
do single s))ecies, changing more or less qviickly, and in a 
greater or lesser degree. A group, when it has once disap- 
peared, never reappears ; that is, its existence, as long as it 
lasts, is contiruious. I am aware that there are some apparent 
excei:)tions to this rule, but the exceptions are surprisingly few, 
so few that E. Forbes, Pictet, and Woodward (though all 
strongly opposed to such views as I maintain), admit its truth ; 
and the rule strictly accords with the theor}'. For all the 
species of the same group, however long it may have lasted, 
are the modified descendants of each other, and of some com- 
mon progenitor. In the genus Lingula, for instance, the 
species which have successively appeared at all ages nmst have 
been connected by an unbroken series of generations, from the 
loAvest Silurian stratum to the present day. 

We have seen in the last chapter that many species of a 
group sometimes falsely appear to have come in abruptly in a 
body ; and I liavc attempted to give an explanation of tliis 
fact, which if true would be fatal to my views. But such cases 
are certainly exceptional ; the general rule being a gradual in- 
crease 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 gen- 
era Avithin a family, be represented by a vertical line of vary- 
ing thickness, ascending through the successive geological for- 
mations in which the species are found, the line will sometimes 
falsely appear to begin at its lower end, not in a sharp point, 
but al^ruptly ; it then gradually thickens upward, often keep- 
ing for a space of equal thickness, and xdtimately thins out in 
the upper beds, marking the decrease and final extinction of the 
species. This gradual increase in number of the species of a 
group is strictly conformable Avith the theory, for the species 
of the same genus, and the genera of the same family, can in- 
crease only slowly and progressively ; the process of modifica- 
tion and the production of a number of allied forms necessarily 
being a slow and gradual process — one species first giving 
rise to two or three varieties, these being slowly converted in- 
to specie?, Avhich in their turn produce by equally slow steps 

Chap. X. EXTINCTION. 099 

other varieties and species, and so on, like tlie branching of a 
great tree from a single stem, till the group becomes large. 

On Extinction. 

We have as yet spoken only incidentally of the disappear- 
ance of species and of groups of species. On the theory of 
natural selection the extinction of old forms and the produc- 
tion of ne\v and improved forms are intimately connected to- 
gether. The old notion of all tlie inhabitants of the earth hav- 
ing been swept away by catastrophes at successive periods is 
very generally given up, even by tliose geologists, as Elie de 
Beaumont, ]\Iurcliison, Barrande, 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 disap- 
pear, one after another, first from one sj^t, then from another, 
and finally from tlie world. In some few cases, however, as by 
the breaking of an isthmus and the conseciuent irrujition of a 
multitude of new inhabitants into an adjoining sea, or by the 
final subsidence of an island, the process of extinction may have 
been rapid. Both single species and whole groups of species 
last for very unequal periods ; some groups, as we have seen, 
have endured from the earliest known dawn of life to the pres- 
ent day ; some have disappeared before the close of the paleo- 
zoic period. No fixed law seems to determine the length of 
time during which any single species or any single genus en- 
dures. There is reason to believe that tlic extinction of a 
whole group of species is generally a slower process than their 
production : if their appearance and disappearance be repre- 
sented, as before, by a vertical line of varying tliickness, tlie 
line is found to taper more gradually at its ujiper end, wliich 
marks tlie 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 extermina- 
tion of whole groups, as of ammonites toward the close of the 
secondary periofl, 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 liav<; species 
a definite duration. No one can have marvelled more than I 
have done at the extinction of species. "When I found in La 
Plata the tooth of a horse embedded witli the remains of Mas- 

300 EXTINCTION, Chap. X. 

iodon, Mcgaihcrium, Toxodon, and other extinct monsters, 
^vllich all coexisted with still liviiii? sliells at a very late geolo- 
gical period, I Avas filled Avilh astonishment; for, seeing that 
the horse, since its introduction by the Spaniards into South 
America, has run wild over the Avholc country, and has in- 
creased in numbers at an unparalleled rate, I asked myself 
what could so recently have exterminated the former horse un- 
der conditions of life apparently so favorable. But my aston- 
ishment was groundless. Prof. Owea 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 Avould have felt the least sur- 
prise 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 what that something 
is, we can hardly ever tell. On the supposition of the fossil- 
horse still existing as a rare species, we might have felt certain, 
from the analogy of all other mammals, even of the slow-breed- 
ing ele])hant, and from the history of the naturalization of the 
domestic horse in South Amei'ica, that, xuider more favorable 
conditions, it would, in a very few years, have stocked the 
whole continent. 13ut wc could not have told what the un- 
favorable conditions Avere which checked its increase, Avhether 
some one or several contingencies, and at what period of the 
horse'^s life, and in what degree, they severally acted. If the 
conditions had gone on, however slowly, becoming less and 
less favorable, we assuredly should not have perceived the fact, 
yet the fossil-horse w^ould certainly have become rarer and 
rarer, and finally extinct — its place being seized on by some 
more successful competitor. 

It is most diilicult alwa3's to renaembcr that the increase 
of every creature is constantly being checked by imperceived 
hostile agencies ; and that these same unperceived agencies 
are amply sufficient to cause rarity, and finally extinction. So 
little is this subject understood, that I have heard surprise re- 
peatedly expressed at such great monsters as the Mastodon 
and the more ancient Dinosaurians having become extinct; as 
if mere bodily strength ga\e victory in the battle of life. Mere 
size, on the contrary, would in some cases determine, as has 
been remarked by Owen, quicker extermination from the greater 
amount of requisite food. Before man inhabited India or Africa, 
some cause must have checked the continued increase of the 


existing; elephant. A lii;;lily-capablc judge, Dr. Falconer, be- 
lieves that it is chielly insects which, from incessantly harass- 
ing and weakening the elephant in India, check its increase ; 
and this was Bruce's conclusion with respect to the African 
elephant in Abj'ssinia. It is certain that insects and blood- 
sucking bats determine the existence of the larger naturalizec 
quadrupeds in several parts of South America. 

We sec 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 animals which have 
been exterminated, eillier locally or wholly, through man's 
agency. I may repeat what I published in 1815, uamel}', that 
to admit that sjiecies generally become rare before they be- 
come extinct — to feel no surprise at the rarity of a species, 
and yet to marvel greatly when the species ceases to exist, is 
much the same as to admit that sickness in the individual is 
t)ie forerunner of death — to feel no surprise at sickness, but, 
when the sick man dies, to wonder and to suspect that he died 
by some deed of violence. 

The theory of natural selection is grounded on the belief 
that each new variety, and ultimately each new species, is pro- 
duced and maintained by having some advantage over those 
with which it comes into competition ; and the consequent 
oxtinctioii of the less-favored forms almost inevitably follows. 
It is the same with our domestic productions ; when a new 
and slightly-improved variety has been raised, it at first sup- 
plants the less improved varieties in the same neighborhood; 
Avhen 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 a})pcarance of new forms, and the disap- 
pearance of old forms, both those naturally and those artifi- 
cially produced, are bound together. In nourishing groups, 
the number of new specific f(jrms which have been produced 
within a given time, has at some periods probably been greater 
than the number of the old specific forms which have been ex- 
terminated; but we know tliat species have not gone on indefi- 
nitely increasing, 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. 

llie competition will generally be most severe, as formerly 
explained and illustrated by examples, between the forms 
which are most like each other in all respects. Hence the 

302 EXTINCTION. Chap. X. 

improved and modified dcsrcudants of a species will generally 
cause the externiiiiatiou 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 
he the most liable to extermination. Thus, as I believe, a 
number of new speci(3s descended from one species, that is, a 
new genus, comes to sujiplant an old geiuis, belonging to the 
same family. But it must often have happened that a new 
species belonging to some one group has seized on the placfa 
occupied by a species belonging to a distinct group, and thus 
have caused its extermination. If many allied forms be de- 
veloped from the successful intruder, many will have to yield 
tlunr places ; and it Avill generally be the allied forms which 
will suffer from some inherited inferiority in common. But 
whetlicr it be species belonging to the same or to a distinct 
class, which have yielded their places to other modified and 
improved species, a few of the sufferers may often be preserved 
for a long time, from being fitted to some peculiar line of life, 
or from inhabiting some distant and isolated station, where 
they will have escaped severe competition. For instance, 
some species of Trigonia, a great genus of shells in the second- 
ary formations, survive in the Australian seas; and a few mem- 
bers of the great and almost extinct group of Ganoid fishes 
still inhabit our fresh waters. Therefore the utter extinction 
of a group is generalh', as Ave have seen, a slower process than 
its production. 

With res^DCct to the apparently svidden extermination of 
whole families or orders, as of Trilobites at the close of the 
paleozoic period and of Ammonites at 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 exter- 
mination. I\Ioreover, when, by sudden immigration or by un- 
usually 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 
fonns Avhich thus yi(^^ld their places vrill commonly be allied, 
for they Avill partake of the same inferiority in common. 

Thus, as it seems to me, the manner in which single species 
and Avliole groups of species become extinct accords well with 
the theory of natural selection. We need not marvel at ex- 
tinction ; if we must marvel, let it be at our own presumption 
in imagining for a moment tliat wc understand the many com- 


plex cuntinoencies, on which ilic existence of e;ich species de- 
pends. If "SVC for<ret, for an instant, that each species tends to 
-Ticreasc inordinately, and that some check is always in action, 
yet seldom perceived l)y us, the whole economy of Nature will 
be utterly obscured. Whenever we can precisely say why this 
species is more alnmdant in individuals than that ; why this 
species and not another can be naturalized in a given country; 
then, and not until then, we may justly feel surprised why we 
cannot account for the extinction of any j)articular species or 
any grouj) of species. 

On the Forms of Life changlnff almost simultaneously 
throughout the World. 

Scarcely any jialeontological discovery is more striking 
than the fact that the forms of life change almost simultane- 
ously throughout the world. Thus our European Chalk for- 
jnation can be recognized in many distant parts of the world, 
imder the most different climates, where not a fragment of the 
mineral chalk itself can be found ; namely, in North America, 
in equatorial South America, in Tierra del Fuego, at the Cape 
of Good Hope, and in the peninsula of India. For, at these 
distant points, the organic remains in certain beds present an 
unmistakable resemblance to those of the Chalk. It is not that 
the same species are met with ; for in some cases not one 
species is identically the same, but they belong to the same 
families, genera, and sections of genera, and sometimes arc 
Kimilarly characterized in such trilling points as mere superficial 
sculpture. Moreover, other forms, which are not found in the 
Chalk of Europe, but which occur in the formations either above 
or below, occur in the same order at these distant points of the 
world. In the several successive paleozoic formations of Rus- 
sia, Western Europe, and North America, a similar parallelism 
in the forms of life has been observed by several authors : so it 
is, according to Lycll, with the several European and North 
American tertiary deposits. Even if the few fossil species 
which are common to the Old and New Worlds were kept 
wholly out of view, the general ])arallelism in the successive 
forms of life, in the paleozoic and tertiary stages, would still 
be manifest, and the several formations could be easily corre- 

These observations, however, relate to the marine inhabit- 
ants of the Avorld: we have not sufTirient data to judge whelhcr 


the productions of llie land and of fresh water at distant points 
clianfj^e in the same jiarallcl manner. We may doubt wliethcr 
thoy have tluis changed: if the Meg-atherium, Milodon, Macra- 
iiclicnia, and Toxodon had been brought to Europe from La 
Plata, without any information in regard to their geological 
position, no one would have suspected that they had coexisted 
with sea-shells all still living; but as these anomalous monsters 
coexisted with the Mastodon and Horse, it might at least have 
been inferred that they had lived during one of the later ter- 
tiary stages. 

When the marine forms of life are spoken of as haWng 
changed simultaneously throughout the world, it must not be 
supposed that tliis expression relates to the same thousandth 
or ten-thousandth year, or even that it has a very strict geologi- 
cal sense ; for if all the marine animals now living in Europe, 
and all those that lived in Europe during the pleistocene period 
(a very remote period as measured by years, including the 
Avhole glacial epoch) Avcre compared Avith those now existing 
in South America or in Australia, the most skilful naturalist 
would hardly be able to say whetlicr the present or the pleis- 
tocene inhabitants of Europe resembled most closely those of 
the southern hemisphere. So, again, several highly-competent 
obs(>rvers 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 inhabit- 
ants of Europe ; and if this be so, it is e\ddent that fossilifer- 
ous beds now being deposited on the shores of North America 
would hereafter be liable to be classed with somewhat older 
European beds. Nevertheless, looking to a remotely future 
epoch, there can be little doubt that all the more modern ma- 
rine formations, namely, the upper pliocene, the pleistocene 
and strictly modern beds, of Europe, North and South Amer- 
ica, and Australia, from containing fossil remains in some de- 
gree allied, and from not including those forms which are found 
only in the older underlying deposits, would be correctly 
ranked as simultaneous in a geological sense. 

The fact of the forms of life changing simultaneously, in the 
above large sense, at distant parts of the Avorld, has greatly 
struck those admirable observers, MM. do Verneuil and d'Arcli- 
iac. After referring to the parallelism of the paleozoic forms 
of life in various pails of Europe, they 'add : " If. struck by tliis 
strange sequence, Ave turn our attention to North AmcricM, 
and there discover a series of analogous iihenomena, it wi!.' 


appear certain that all these modifications of species, tlieir ex- 
tinction, and the introduction of new ones, cannot be owin^r to 
move chani^es in marine currents or other causes more or less 
local and temporarv, but de])cnd on general laws which govern 
the whole animal kingdom." M. Barrande has made forcible 
remarks to j)recisely tin; same eflect. It is, indeed, quite futile 
to look to changes of currents, climate, or other physical con- 
ditions, as the cause of these great mutations in the forms of 
lift; throughout the world, under the most different climates. 
Wc must, as Barrande has remarked, look to some special law. 
AVe shall see this more clearly when Ave treat of the present 
distribution of organic l)cings, and find how slight is the rela- 
tion between the ])hysical conditions of various countries, and 
the nature of their inhabitants. 

This great fact of the parallel succession of the forms of life 
througlu^ut the world, is explicable on the theory of natural 
selection. New species arc formed by having some advantage 
over older forms ; and the forms, whicli are already dominant, 
or have some advantage over the other forms in their own 
country, would be the most likely to give birth to the greatest 
number of new varieties or incipient sjiccies. 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 numljcr of new varieties. It is also natural that 
the dominant, varying, and far-spreading species, which already 
'lave invaded to a certain extent the territories of other species, 
should be those which would have the best chance of spreading 
still further, and of giving rise in new countries to other new 
varieties and species. The process of diffusion would often be 
very slow, 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 jxiss, but in the course of time the dominant forms would 
generally succeed in spreading, and would ultimately prevail. 
Th(' diffusion would, it is probable, be slower with the terres- 
trial inhabitants of distinct continents than with the marine 
inhabitants of llie 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 

Thus, as it seems to me, the parallel, and,- taken in a large 
sense, simultaneous, succession of the same forms of life through- 
out the world, accords well witli tlic j>rinciple of new species 


havinfi^ been formed by dominant species spreading widely and 
varying; the new species thus produced being themselves dom- 
inant, owing to their having had some advantage over their 
already dominant parents, as well as over other species, ana 
again spreading, varying, and producing new forms. The old 
forms which are beaten and Avhicli yield their places to the new 
and victorious forms, will generally be allied in groups, from 
inheriting some inferiority in common ; and therefore, as new 
and imjjroved groups spread throughout the world, old groups 
disappear from the world ; and the succession of forms every- 
where tends to corres])ond both in their first appearance and 
final disappearance. 

There is one other remark connected with this subject worth 
making. I have given my reasons for believing that most of 
our great formations, rich in fossils, were deposited during 
periods of subsidence ; and that blank intervals of vast dura- 
tion, as far as fossils are concerned, occurred during the periods 
when the bed of the sea was cither stationary or rising, and 
likewise when sediment was not thrown down quiclcly enough 
to embed and preserve organic remains. During these long 
and blank intervals I suppose that the inhabitants of each re- 
gion underwent a considerable amount of modification and ex- 
tinction, and that there was much migration from other parts 
of the world. As we have reason to believe that large areas 
are affected by the same movement, it is probable that strictly 
contemporaneous formations have often been accumulated over 
very wide spaces in the same quarter of the world ; but we are 
very far from having any right to conclude that this has invari- 
ably been the ease, and that large areas have invariably been 
affected by the same movements. Wlien two formations have 
been deposited in two regions during nearly, but not exactly, 
the same period, we should find in both, from the causes ex- 
plained in the foregoing paragraphs, the same general succes- 
sion in the forms of life ; but the species would not exactly 
correspond ; for there will have been a little jnore time in the 
one region than in the other for modification, extinction, and 

I suspect that cases of this nature occur in Europe. Mr. 
Prestwich, in his admirable memoirs on the eocene dejiosits 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 tlie species bclongiiijr to the same genera, yet the species 
thetiisehes diircr in a manner very difTicult to account for, con- 
sidcrin<x the proximit}- of tlic tuo areas, unless indeed it be 
assumed that an isthmus separated two seas inhabited by dis- 
tinct but contemporaneous faunas. Lyell has made similar 
observations on some of the later tertiary formations. Barrande, 
also, shows that there is a striking general parallelism in the 
successive Silurian deposits of Bohemia and Scandinavia ; never- 
theless he finds a surprising amount of dillcrence in the species. 
If the several formations in these regions have not been de- 
jiosited during the same exact periods — a formation in one re- 
gion often corresponding with a blank interval in the other — 
and if in both regions the species have gone on slowly chan- 
ging during tlie accumulation of tlie several formations, and 
(hning the long intervals of time between tliem ; 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 aiiparcntly corresponding stages in the two regions. 

On the Affinities of J^xtinct Species to each other, and to 
Livinf/ J/ornis. 

Let us now look to the mutual affinities of extinct and liv- 
ing species. They all fall into a few grand classes ; and this 
fact is at once explain(Ml on the princij)le of descent. The more 
ancient any form is, the more, as a general rule, it differs from 
living forms. But, as Buckland long ago remarked, all extinct 
species can be classed eitlier in still existing grou})s, or between 
tliem. That the extinct forms of life help to fill up the inter- 
vals between existing genera, families, and orders, cannot be 
disputed. For if we confine our attention cither to the living 
or to the extinct alone, the series is far less perfect than if we 
combine both into one general system. With respect to the 
vertebrata, whole pages could be filled with illustrations from 
Owen, showing how extinct animals fall in between existing 
groups. Cuvier ranked the Ruminants and Pachyderms, as the 
two most distinct orders of mammals ; but Owen has discovered 
so many fossil links, that he has had to alter th(; whole classifi- 
cation, and has placed certain j)achyderms in the same sub-order 
with nnuinants : for example, he dissolves by fine gradations 
the apparently wide difference between the pig and the camel. 


Another distinguished paleontologist, M. Gaudry, shows, in 
the most striking manner, that very many of the fossil mam- 
mals discDvered by him in Attica, connect in the plainest man- 
ner existing genera. AMiat a -wonderful intermediate form is 
the Tyiiotherium from South America, as the name given to it 
by Prof. Gervais expresses, and Avliich cannot be placed in any 
existing order of mammals ! Even the wide interval between 
lairds and reptiles has been shown by Prof. Huxley to be par- 
tially bridged over in the most unexpected manner, by, on the 
one hand, the ostrich and extinct Archeopteryx ; and, on the 
other hand, the Compsognathus, one of the Dinosaurians — that 
group which includes the most gigantic of all terrestrial rep- 
tiles. Turning to the Invertebrata, Barrande asserts, and a 
higher authority could not be named, that he is every day 
taught that, although paleozoic animals can certainly be classed 
under existing groups, yet that at this ancient period the 
groups were not so distinctly separated from each other as they 
now are. 

Some writers have objected to an}' extinct species or group 
of species being considered as intermediate between living 
species or groups. If by this term it is meant that an extinct 
form is directly intermediate in all its characters between two 
living forms, the objection is valid. But in a natural classifi- 
cation many fossil species certainly stand between living spe- 
cies, and some extinct genera between living genera, even 
between genera belonging to distinct families. The most 
common case, especially with respect to very distinct groups, 
such as fish and reptiles, seems to be, that, supposing them to 
be distinguished at the present day by a dozen characters, the 
ancient members are separated by a somewhat lesser number 
of characters, so that the two groups, though formerly quite 
distinct, made at that period a somewhat nearer approach to 
each other. 

It is a common belief that the more ancient a form is, by 
so much the more it tends to connect by some of its characters 
groups now widely separated from each other. This remark 
no doubt must be restricted to those groups Avhich have mider- 
gone 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 Batra- 
chians, the older Fish, the older Cephalopods, and the eocene 


Mammals, witli llio more recent members of the same classes, 
we must admit that there is truth in tlie i-emark. 

Let us see how far these several facts and inferences accord 
with tlic theory of descent with modification. As the subject 
is somewhat complex, I must request the reader to turn to the 
diafi^ram in the fourth chapter. We may suppose that the 
numbered letters represent genera, and the dotted lines di- 
verging from them the species in each genus. The diagram 
is much too simple, too few genera and too few species being 
given, but this is unimportant for us. The horizontal lines 
may represent successive geological formations, and all the 
forms beneath the uppermost line may be considered as ex- 
tinct. The three existing genera, «'*, Q^*,])'*, will form a small 
family ; i'* and _/"'*, a closely-allied family or sub-family ; and 
o'*, e'*, Wi", a third lamily. These three families, together vvitli 
the many extinct genera on the several lines of descent di- 
verging irom the parent-form (A), will form an order; for all 
Avill have inherited something in common from their ancient 
and common ])rogenitor. On the principle of the continued 
tendency to divergence of character, which was formerly illus- 
trated by this diagram, the more recent any form is, the more 
it will generally differ from its ancient progenitor. Hence we 
can understand the rule that the most ancient fossils ditfer most 
from existing forms. We must not, hoAvever, assume that di- 
vergence of character is a necessary contingency; it depends 
solely on the descendants from a species being thus enabled 
to seize on many and different places in the economy of Natme. 
Therefore it is quite possible, as we have seen in the case of 
some Silurian forms, that a species might go on being slightly 
modified in relation to its slightly-altered conditions of life, 
and yet retain throughout a vast period the same general 
characteristics. This is represented in the diagram by the let- 
ter f'\ 

All the many forms, extinct and recent, descended from 
(A), mak(% as Ix-fore remarked, one order;' and this order, from 
the continued eflects of extinction and divergence of character, 
has l)ecoine divided into several sul>families and families, some 
of which are supposed to have perished at dilVercnt periods, 
and some to have endured to the present da}'. 

By looking at the diagram we can sec that if many of the 
extinct forms, supposed to be embedded in the successive for- 
mations, were discovered at several jioints low down in the 
series, the three existing families on the uppermost line would 


be rendered less distinct from each otlior. If, for instance, the 
g'enera «', «V ^'"5 /*» ''^*» ^''^S ''^'^ Averc disinterred, these three 
families would be so closely linked together that they proba- 
bly M'ould have to be xniited into one great family, in nearly 
the same manner as has occurred -with ruminants and certain 
pach^'dtM-ms. Yet he who objected to call the extinct genera 
which thus linked the living genera of three families together, 
intermediate in character, would be justified, as they are inter- 
mediate, not directly, but only by a long and circuitous course 
through many "vndely-different forms. If many extinct forms 
were to be discovered above one of the middle horizontal lines 
or geological formations — for instance, above No. VI. — but 
none from beneatli this line, then only two of the families 
(those on the left hand, «'*, etc., and b^*, etc.) would have to be 
imited into one ; and there wovild 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 («" to ni^*), on the uppermost line, 
be supposed to differ from each other by half a dozen impor- 
tant characters, then the families which existed at the peiiod 
marked VI. would certainly have differed from each other by 
a less numljer 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 some slight degree intermediate in character be- 
tween their modilled descendants, or between their collateral 

In Nature the case will be far more complicates! 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 Avill have been modified in various degrees. 
As we possess only the last volume of the geological record, 
and that in a very broken condition, Ave have no right to expect, 
except in rare cases, to fill up the wide intervals in the natural 
system, and thus imife distinct fiimilies or orders. All that we 
have a right to expect is, that those groups which have within 
knoA\ni geological periods undergone much modification, should 
in the older formations make some slight approach to each 
other ; so that the older members should differ less from each 
other in some of their characters than do the existing members 
of the same groups; and this, by the concurrent evidence of 
our best paleontologists, is frequently the case. 

Thus, on the thcorv of dcsrcnt witli modification, the main 


facts with respect to the mutual affinities of the extinct forms 
of life to each other and to livinf^ 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 of any 
one great ])eriod in the earth's history will be intermediate 
in g'eneral character between that which preceded and that 
which succeeded it. Thus the species which lived at the sixth 
p^reat stage of descent in the diagram are the modified ofTsjjring 
of those which lived at the fifth stage, and are the parents of 
1 hose which became still more modified at the seventh stage; 
hence they could hardly fail to be nearly intermediate in char- 
acter between the forms <jf life above and below. We must, hoAV- 
ever, allow ior the entire extinction of some preceding forms, 
and in any one region for the immigration of new fijrms from 
other regions, and for a large amount of modification during 
the long and blank intervals between the successive forma- 
tions. Subject to these allowances, the fauna of each geolo- 
gical 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 Devo- 
nian system, when this system was first discovered, were at 
once recognized by paleontologists as intermediate in charac- 
ter between those of the overlying carboniferous, and under- 
lying Silurian system. But each fauna is not necessarily ex- 
actly intermediate, as unequal intervals of time have elapsed 
between consecutive formations. 

It is no real objection to the truth of the statement that 
the fauna of each period as a whole is nearly intermediate in 
character between the preceding and succeeding faunas, that 
certain genera offer exceptions to the rule. For instance, 
mastodons and elephants, when arranged by Dr. Falconer in 
two scries, first according to their mutual affinities and then 
according to their periods of existence, do not accord in ar- 
rangement. The species extreme in character are not the old- 
est or the most recent ; nor are those which are intermediate 
in character, intermediate in age. But supposing for an 
instant, in this and other sucli cases, "that the recoril of the 
lir^t appearance and disappearance of the species was perfect, 
we have no reason to believe that forms successively produced 
necessarily endure for corresponding lengths of time : a very 
ancient form might occasionally last much longer than a form 
elsewhere subsequently produced, csjiecially in the case of 


ten-estrial productions inliabiting separated districts. To com- 
pare small tliiii;^s with p^reat : if the principal living and ex- 
tinct races of the domestic pigeon were arranged as well as 
they could be in serial affinity, this arrangement would not 
accord closely with the order in time of their production, and 
even less with the order of their disappearance ; for the par- 
cnt-rock-pigeon still lives ; and many varieties between the 
rock-pigeon and the carrier have become extinct ; and carriers 
Avhich are extreme in the important character of length of 
beak originated earlier than short-beaked tumblcrs,which are 
at the opposite end of the series in this respect. 

Closely connected with tlie statement that the organic re- 
mains from an intermediate formation are in some degree in- 
termediate in character, is the fact, insisted on by all paleon- 
tologists, that fossils from two consecutive formations are far 
more closclj' related to each other than are the fossils from 
two remote formations. Pictet gives us a well-known in- 
stance, 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 Prof. Pictet in his firm belief in the im- 
mutability of species. He who is acquainted with the distribu- 
tion of existing species over the globe, will not attempt to 
account for the close resemblance of distinct species in close- 
ly consecutive formations, by the ph^-sical conditions of the 
ancient areas having remained nearly the same. Let it be re- 
membered that the forms of life, at least those inhabiting the 
sea, have changed almost simultaneously throughout the Avorld, 
and therefore under the most different climates and conditions. 
Consider the prodigious vicissitudes of climate during the pleis- 
tocene period, which includes the whole glacial epoch, and note 
how little the specific forms of the inhabitants of the sea have 
been affected. 

On the theory of descent, the full meaning of the fossil re- 
mains from closely-consecutive formations being closely related, 
though ranked as distinct species, is obvious. As the accumu- 
lation of each formation has often been interrupted, and as 
long blank intervals have intervened between successive forma- 
tions, we ought not to expect to find, as I attempted to show 
in the last chapter, in any one or in any two formations all the 
intermediate vaiietics between the species which appeared at 
the commencement and close of these periods : but we ought 
to find after intervals, very long as measureil by years, but 


only moderately lonp;' as measured f^coloo-ically, closely-allied 
Forms, or, as they have been called by some authors, represent- 
ative species ; and these assuredly we do find. We find, in 
short, such evidence of the slow and scarcely-sensible muta- 
tion of specific forms as we have the right to expect. 

On the State of DevelojJmoit of AncieJit comjtared with Liv- 
ing Forms. 

We have seen in the fourth chapter that the degree of 
differentiation and specialization of the parts in all organic 
beings, when arrived at maturity, is the best standard, as yet 
suggested, of their degree of perfection or highness. We 
have also seen that, as the specialization of parts and organs 
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 that it may and 
will leave many creatures with siin])li' and unimproved struct- 
ures 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 will become 
superior to their predecessors; for they will have to beat in the 
struggle for life all the older forms with which they come into 
close competition. We may therefore conclude that, if under 
n nearly similar climate the eocene inhabitants of the world 
could be put into competition with the existing inhobitants, 
(he former would be beaten and exterminated by the latter, as 
would the secondary by the eocene, and the paleozoic by the 
secondary forms. So that by this fundamental test of victory 
in the battle for life, as well as by the standard of the spociali- 
zaticjn of organs, modern forms ought on the theory of natu- 
ral selection to stand higher than ancient fonns. Is this the 
case ? A large majority of paleontologists wcnikl answer in 
the affirmative ; and I suppose that the answer must he ad- 
mitted as true, though dillicult of full proof. 

It is no valid objection to this conclusion, that certain 
BrachiopcKls have been but slightly modilied from an extremely 
remote geological epoch. It is not an insuperable diniculty 
that Foramiiiifera have not, as insisted on b}* Dr. ('arp<'nter, 
])rogress*ed in organization since even the Laurentian epoch : 
for some organisms would have to remain iitted for simple con- 
ditions of life, and what could be better fitted for this end than 


these lowly-organized Protozoa ? It is no great difficulty that 
fresh-water shells, as Prof, Piiillips has urged, have remained 
almost unaltered from the time when they first ajipeared to 
the present day ; for these shells will have been subjected to 
less severe competition than the moUusks inhabiting the more 
extensive area of the sea with its innumerable inhabitants. 
Such objections as the above v/ould be fatal to any view which 
included advance in organization as a necessary contingent. 
They would likewise be fatal to my view if Foraminifera, for 
instance, could be proved to have first come into existence, 
during the Laurentian epoch, or Brachiopods during the Cam- 
brian formation ; for, in this case, there Avould not have been 
time sufficient for the development of these organisms up to 
the standard which they then reached. When once advanced 
up to any given point, there is no necessity on the theory of 
natvu'al selection for their further continued progress ; though 
they will, during each successive age, have to be slightly 
modified, so as to hold their places in relation to the changing 
conditions of life. All such objections hinge on the question 
whether w^e I'eally know how old the world is, and at what 
periods the various forms of life first appeared ; and this may 
be disputed. 

The problem whether organization on the whole has ad- 
vanced is in many ways excessively intricate. The geological 
record, at all times imperfect, does not extend far enough back, 
as I believe, to show Avith unmistakable clearness that within 
the known history of the world organization has largely ad- 
vanced. Even at the present day, looking to members of the 
same class, naturalists are not unanimous which forms are to be 
ranked as highest: thus, some look at the selaceans or sharks, 
from their approach in some important points of structure to rep- 
tiles, as the highest fish ; others look at the teleosteans as the 
highest. The ganoids stand intermediate between the selaceans 
and teleosteans ; the latter at the present day are largely 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 retro- 
graded in organization. To attempt to compare in the scale 
of highness members of distinct types seemed hopeless : who 
will decide whether a cuttle-fish be higher tlian a bee — that in- 
sect which the great Von Baer believed to be " in fact more 
highly organized than a fish, although upon another type?" 
In the comjilex struggle for life it is quite credible that cms- 


taceaus, not very high in tlrcir own class, might beat cephalo- 
pods, tlic highest nioUusks ; and such crustaceans, though not 
highlv devek)pccl, wouhl stand very high in the scale of inverte- 
brate animals if judged by the most decisive of all trials — the 
law of battle. Besides these inherent difficulties in deciding 
which forms are the most advanced in organization, we ouglit 
not solely to compare the highest members of a class at any 
two periods — though undoul)tcdly this is one and perhaps the 
most important element in striking a balance — but we ought 
to compare all the members, high and low, at the two periods: 
At an ancient epoch the highest and lowest mollusks, namely, 
cephalopods an(l brachiopods, swarmed in numbers ; at the pres- 
sent tim(^ both ordei*s are greatly reduced, while other orders, 
intermediate in organization, have largely increased; conse- 
quently some naturalists maintain that mollusks were formerly 
more highly developed than at present ; but a stronger case 
Clin be made out on the opposite side, by considering the vast 
reduction of the lowest mollusks, and the fact that our existing 
cephalopods, though few in nimibcr, arc more highh'' organized 
than their ancient representatives. We ought also to compare 
the relative proportional numbers of the high and low classes 
throughout the world at any two periods : if, for instance, at 
the present day fifty thousand kinds of A-ertebrate animals ex- 
ist, and if we knew that at some former period only ten thou- 
sand kinds existed, we ought to look at this increase in num- 
ber in the highest class, which implies a great displacement of 
lower forms, as a decided advance in the org-anization of the 
world. AVe thus see how hopelessly dillicnlt it is to compare 
with perfect fairness, under such extremely complex relations, 
tlw; standard of organization of the imperfectly-known faunas 
of successive j^eriods. 

We shall apjireciatc this difficnlly the more cleaily, by 
looking to certain existing faimas and iioras. From the extraor- 
dinary manner in which European productions have recently 
spread over New Zealand, and have seized on places whicli 
must have lieen previously occupied, we must believe that, if 
nil the animals and plants of Great Britain were set free in 
New Zeahuul, in the course of time a multitude of ]3ritish forms 
would become thoroughly naturaliz("d there, and would exter- 
minate many of the natives. On the other hand, from hardly a 
single inhabitant of the southern hemisphere having become 
Anld in any jiart of Emope, we may well doubt whether, if all 
the productions of New Zealand vicrc set free in Great Britain, 


any considcriiblc number -would be enabled to seize on jilaces 
now occupied by our native plants and animals. Under this 
point of view, the productions of Great Britain stand much 
hioher in the scale than those of New Zealand. Yet the most 
skilful naturalist, from an examination of the species of the two 
countries could not have foreseen this result. 

Agassiz and several other hif^hly-competent judges insist 
that ancient animals resemble to a certain extent the embryos 
of recent animals belonging to the same classes ; and that the 
geological succession of extinct forms is nearly parallel Avith 
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, 
OM'ing to variations supervening at a not early age, and being 
inherited at a corresponding age. This process, while it leaves 
the embryo almost unaltered, continually adds, in the course 
of successive generations, more and more difference to the adult. 
Thus the embryo comes to be left as a sort of picture, preserved 
by Nature, of the ancient and less modified condition of the 
animal. This view may be true, and yet may never be capable 
of full proof. Seeing, for instance, that the oldest known mam- 
mals, reptiles, and fishes, strictly belong to their proper classes, 
though some of these old forms are in a slight degree less dis- 
tinct 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 Yertebrata, 
until beds rich in fossils are discovered far beneath the lowest 
Silurian stratum — a discovery of which the chance is small. 

On the Succession of the same Types xcitJtin the same Areas, 
during the later Tertiary Periods. 

Mr. Clift many years ago showed that the fossil mammals 
from the Australian caves were closely allied to the living mar- 
supials of that continent. In South America, a similar relation- 
ship is manifest, even to an uneducated eye, in the gigantic 
pieces of armor, like those of tlie armadillo, found in several 
parts of I^a Plata ; and Prof. Owen has shown in the most 
striking manner that most of the fossil manmials, buried there 
in such lunnbers, are related to South American t^-pes. This 
relationship is even more clearl}' seen in the wonderful collection 
of fossil bones made by MM. Lund and Clausen in the caves of 
Urazil. I was so much injprcssed with these facts that I 


stroni^ly insisted, in 1839 and 1845, on tliis " law of the suc- 
cession of types," on " tliis -wonderful relationship in the same 
continent between the dead and the livinrr." Prof. Owen has 
subsecjuentlv extended the same ^"eneralization to tlie mammals 
of the Old World. We see the same law in this author's res- 
torations of the extinct and ^ijj^antic birds of New Zealand. 
We see it also in the birds of the caves of Brazil. Mr. ^V^)^)d- 
ward has shown that the same law holds good with sea-shells, 
but, from the wide distribution of most genera of mollusks, it 
is not well dis})layed by them. Other cases could be added, as 
the relation between the extinct and living land-shells of Ma- 
deira ; and between the extinct and living brackish-water shells 
of the Aralo-Casjiian Sea. 

Now what does this remarkable law of the succession of 
the same types within the same areas mean ? He would be a 
bold man, who, after comparing the present climate of Aus- 
tralia and of parts of South America under the same lati- 
tude, would attempt to account, on the one hand,bv dissimilar 
physical conditions, for the dissimilarity of the inhabitants of 
these two continents, and, on the other hand, by similarity of 
conditions, for the uniformity of the same types in each during 
the later tertiary periods. Nor can it be pretended that it is 
an immutable law that marsupials should have been chiefly or 
solely produced in Australia; or that Edentata and otln^r 
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 
])ublications above alluded to, that in America the law of dis- 
tribution of terrestrial matnmals was foniierly dillV-rent from 
what it now is. North America formerly partook strongly of 
the present character of the southern half of the continent ; 
and the southern half was formerly more closely allied, than it 
is at present, to the northern half. In a similar manner we 
know, from Falconer and Cautley's discoveries, that Northern 
India was formerly more closely related in its mammals to 
Africa than it is at the present time. Analogous facts could 
be given in relation to the distribution of marine animals. 

On the theory of descent with modillcation, the great law 
of the long-enduring, but not inmiutable, succession of the 
same t^'pes within the same areas, is at onct; explained; for the 
inhabitants of each <|uarter of tin; world will obviously tend to 
leave in that quarter, during the next succeeding period of 
time closely-allied though in some degree modified descend 

318 SUMMAKY OF THE Cuap. X. 

ants. If llio inhabitants of one continent formerly differed 
greatly from those of another continent, so will their modified 
descendants still dillcr in nearly the same manner and degree, 
lint after very long intervals of time, and after great geo- 
graphical changes, permitting much inter-migration, the fee- 
bler will yield to the more dominant forms, and there will be 
nothing innnutable in the laws of past and present distribu- 

It may be asked in ridicule, whctlier I suppose tliat the 
megatherium and other allied huge monsters, which formerly 
lived in South America, have left behind them the sloth, arma- 
dillo, and anteater, as their degenerate descendants. This can- 
not for an instant be admitted. These huge animals have 
become wlioll^'^ extinct, and have left no progeny. But in the 
caves of Brazil, there are many extinct species Avhich are close- 
ly allied in size and in all other characters to the species still 
liv-ing in South America ; and some of these fossils may be the 
actual progenitors of living species. It must not be forgotten 
that, on our 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 formation, and 
in a succeeding formation there be six other allied or repre- 
sentative genera each Avith the same number of species, then we 
may conclude that generally only one species of each of the 
older genera has left modified descendants, which constitute 
the several species of the new genera ; the other seven species 
of each old genus ha^ang died out and left no progeny. Or, 
and this probably 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 whole genera having become utterly extinct. In failing 
ordcre, 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. 

Summary of the preceding andprese7it Chapter. 

I have attempted to show that the geological record is ex- 
tremely imperfect ; that only a small portion of the globe has 
been geologically explored with care ; that only certain class- 
es of organic beings have been largely preserved in a fossil 
state; that the number both of specimens and of species, pre- 
served in our museums, is absolutely as notliing, compared 


with the number of p^encralions which must have iiasscd away 
even during a siii<:;le formation; lliat, owinfj;' to subsidence 
being ahuost necessary for the accumulation of deposits rich in 
fossil species of very many kinds and thick enough to resist fu- 
ture degradation, great intervals of time must have elapsed 
between most of our successive formations; that there has 
probably been more extinction during the periods of subsi- 
dence, and more variation during tlie 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 tlH> duration of each formation is, probably, 
short compared with the average duration of specidc forms ; 
that migration has played an important part in the first ap- 
pearance of now forms in any one area and fonnation ; that 
widely-ranging species are those which have varied most fre- 
quently, and have oftenest given rise to new species ; that va- 
rieties have at first been local ; and lastly, although each spe- 
cies must have j^asscd through numerous transitional stages, it 
is probable that the periods, during which each underwent 
modification, though many and long as measured l)y years, 
have been short in comparison with the periods during which 
each remained in an unchanged condition. These causes, taken 
conjointly, will, to a large extent, explain why — though we do 
find many links — we do not find interminable varieties, con- 
necting together all extinct and existing forms liy the finest 
graduated steps. It should also be constantly borne in mind 
that any linking varieties between two or more forms, which 
might be found, would be ranked, unless the whole chain could 
be perfectly restored, as so many new and disthict species ; for 
it is not pretended that we have any sure criterion by which 
species and varieties can be discriminated. 

H(! who rejt;cts these views on the imperfection of tlie 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 rep- 
resentative species found in the successive stages of the same 
great formation ? lie may disbelieve in the immense intervals 
of time which have elapsed between our consecutivt; forma- 
tions ; he may overlook how important a part migration has 
tjlaycd, when the fonnations of any one gieat region alone, as 
that of Europe, are considen-d ; he may urge the apparent, but 
often falsely appanMit, sudden coming in of whole groups of 
species. He may ask where are the remains of those infinitely- 


numerous or<Tanisnis wliicli must have existed long before the 
Cambrian system ^vas deposited ? We now know that at least 
one animal did then exist ; but I can answer the above ques- 
tion only l)y supjiosin<^ that where our oceans now extend tliey 
have extended for an enormous .period, and wliere our oscillat- 
ing continents now stand, they have stood since the coinmence- 
mcnt of the Cambrian system ; but that, long before that 
epoch, the world presented a Avidcly-diirerent aspect ; and that 
the older continents, formed of formations older than any 
known to us, exist now only as remnants in a metamorphosed 
condition, or he still buried under the ocean. 

Passing from these difficulties, the other great leading 
facts in paleontology seem to me simply to follow on the the- 
ory of descent with modification through natural selection. 
We can thus understand how it is that new species come in 
slowly and successively ; how species of different classes do 
not necessarily change together, or at the same rate, or in the 
same degree ; yet in the long-run that all undergo modification 
to some extent. The extinction of old forms is the almost 
inevitable consequence of the production of new fonns. We 
can imderstand Avhy Avhcn a species has once disappeared it nev- 
er reappears. Groups of species increase in numbers slowly, 
and endure for unequal periods of time ; for the process of 
modification is necessarily slow, and depends on many com- 
plex 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 infe- 
riority inherited from a common progenitor, tend to become 
extinct together, and to leave no modified oQ'spring on the face 
of the earth. But the utter extinction of a whole group of 
species has sometimes been a slow process, from the surnval 
of a few descendants, lingering in protected and isolated situ- 
ations. When a group has once wholly disappeared, it does 
not reappear; for the link of generation has been broken. - 

AVe can understand how the dominant fonns which spread 
widel}' and yield the ;;Teatest number of varieties Avill tend to 
people the world with allied, but modified, descendants ; and 
these will generally succeed in displacing the groups which are 
their inferiors in the struggle for existence. Hence, after long 
intervals of time, the jiroductions of the world apjicar to have 
changed sinmltaneously. 

We can understand how it is that all the forms of life 


ancient and recent, make tog-ctlier a (c\v f^rand classes ; for all 
arc at least thus far connected by generation. We can undci'- 
stand, from the continued tendency to divergence of character, 
why the more ancient a form is, the more it gcnc^rally differs 
from those now living; why ancient and extinct forms often 
tend to fill up gaps between existing forms, sometimes blend- 
ing two groups, previously classed as • distinct, into one ; but 
more connnonly 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 direct- 
ly intermediate between existing forms ; but are intermcfhate 
onl}' by a long and circuitous course through other extinct and 
difl'erent forms. We can clearly sec why the organic remains of 
closely-consecutive formations are closely allied ; for they are 
closely linked togetlier by generation. ^Ve can clearly see why 
the remains of an intermediate formation are intermediate in 

The inhabitants of each successive period in the world's 
history must have beaten their predecessors in the race for life, 
and are, in so far, higher in the scale of Nature, and tlieir 
structure has generally become more specialized ; and this may 
account for the common belief held by so many paleontolo- 
gists, 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 explanation according to 
our views. The succession of the same types of structure with- 
in the same areas during the later geological periods ceases to 
be mysterious, and is intelligible on the principle of inlierit- 

If, then, the geological record be as imperfect as many be- 
lieve, and it may at least be asserted that the record cannot be 
jiroved to be much more perfect, the main ol)jections to the 
theory of natural selection are greatly diminished or disappear. 
On the other hand, all the chief laws of paleontology plainly 
jiroflaim, as it seems to me, that species have been produced 
by ordinary generation: old forms having been supjilanted by 
new and imjjroved forms of life, the products of Variation and 
the Survival of the Fittest. 




f resent Distribution cnnnot be accounted for by Differences in Phj-sical Conditions 
— Importance of Barriers — Afllniiy of the Prodnctiotis of the same Continent — 
Centres of Creation — Means of Dispersal by Changes of Climate and of the Level 
of the Land, and by Occasional Means — Dispersal during the Glacial Period — 
Alternate Glacial Periods in the North and South. 

In' considering the distribution of organic beings over the 
face of the globe, the first great fact which strikes us is, that 
neither the similarity nor the dissimilarity of the inhabitants 
of various regions can be accoimted for by their climatal and 
other jihysical conditions. Of late, almost every author who 
has studied the subject has come to this conclusion. The case 
of America alone would almost sulllce to prove its truth : for, if 
we exclude th.e northern parts where the circumpolar land is 
almost continuous, all authors agree that one of the most fun- 
damental divisions in geographical distribution is that between 
the New and Old Worlds ; yet, if Ave travel over the vast 
American Continent, from the central parts of the United 
States to its extreme southern point, we meet with tin; 
most diversified conditions ; liinnid districts, arid deserts, lofty 
moimtains, grassy plains, forests, marshes, lakes, and great 
rivers, under almost every temperature. Tlierc is hardly a 
climate or condition iii the Old World which cannot be paral- 
leled in the New — at least as closely as the same species gen- 
erally require. No doubt small areas can be jiointed out in 
the Old A\''orld hotter than any in the New A^'orld, but these 
are not inhabited by a fauna different from that of the sur- 
rounding districts ; for it is very rare to find a group of organ- 
isms conilned to a small area, having conditions peculiar in 
only a slight degree. Notwithstanding this general parallel- 
ism in the conditions of the Old and New Worlds, how widely 
different are their living productions! 

In tho southern hemisphere, if wc compare large tracts of 


land in Australia, South Africa, and Western South America, 
between latitudes 20"^ and 35°, we shall find parts extremely 
similar in all their conditions, yet it would not he possible to 
point out three faunas and floras more utterly dissimilar. Or 
again, we may compare the productions of South America 
south of hit, 35° with those north of 25°, which consequently 
are separated by a space of ten deg'rces of latitude and are ex- 
posed to considerably different conditions, yet they are incom- 
I)arably more closely related to each other than they arc to the 
productions of Australia or Africa under nearly the same cli- 
mate. Analog'ous facts could be given with respect to the 
inhabitants of the sea. 

A second great fact which strikes us in our general review 
is, that barriers of any kind, or obstacles to free migration, are 
related in a close and important manner to the differences be- 
tween the productions of various regions. We sec this in the 
great difference of nearly all the terrestrial productions of the 
New and Old Worlds, excepting in the northern parts, Avhere 
the land almost joins, and where, under a slightly-different cli- 
mate, there might have been free mij^Tation for the northern 
temperate forms, as there now is for the strictly arctic produc- 
tions. We see the same fact in the great difference betwefMi 
the inhabitants of Australia, Africa, and South America, under 
the same latitude : for these countries are almost as much iso- 
lated from each other as is possible. On each continent, also, 
we see the same fact ; for, on the opposite sides of lofty and 
continuous mountain-ranges, of great deserts, and even of large 
rivers, we find different productions ; though as mountain- 
chains, deserts, etc., are not as impassable, or likely to have 
endured so long, as the oceans separating continents, the differ- 
ences are very inferior in degree to those characteristic of dis- 
tinct continents. 

Turning to the sea, we find the same law. The marine in- 
habitants of the eastern and western shores of South America 
arc very distinct, with extremely few fishes, shell, or crabs, in 
common ; but Dr. GUnther has recently shown that on opjio- 
site sides of the Isthmus of Panama, about thirty jier cent, of 
the fishes are the same ; and this fact has led naturalists to be- 
lieve that the istlnnus was formerly open. Westward of the 
shores of America, a wide space of open ocean extends, with 
not an island as a hahing-place for emigrants ; here we have a 
barrier of another kind, and as soon as this is passed we meet 
in the eastern islands of the Pacilic with another and totally 


distinct fauna. So that three marine faunas range far north- 
ward and southward in parallel lines not far from each other, 
imder corrcspondini^ climates ; but from beinf^ separated from 
each other by impassable barriers, either of land or open sea, 
they are almost wholly distinct. On the other hand, proceed- 
ing still farther westward from the eastern islands of the tropi- 
cal parts of the Pacific, we encounter no impassable barriers, 
and we have innumerable islands as halting--places, or continu- 
ous coasts, imtil after travelling over a hemisphere we come to 
the shores of Africa ; and over this vast space we meet witli 
no well-defined and distinct marine faunas. Although so few 
shells, crabs, or fishes, arc common to the above-named three 
approximate faunas of Eastern and Western America and the 
eastern Pacific islands, yet many fish range from the Pacific 
into the Indian Ocean, and many shells are common to the 
eastern islands of the Pacific and the eastern shores of Africa, 
on almost exactly opposite meridians of longitude. 

A third great fact, partly included in the foregoing state- 
ment, is the ailinity of the productions of the same continent, 
or of the same sea, though the species themselves are distinct 
at different points and stations. It is a law of the widest 
generality, and every continent offers innumerable instances. 
Nevertheless the naturalist, in travelling, for instance, from 
north to south, never fails to be struck by the manner in which 
successive groups of beings, specifically distinct, nearly related, 
replace each other. He hears from closely-allied, yet distinct 
kinds of birds, notes nearly similar, and sees their nests simi- 
larly constructed, but not quite alike, with eggs colored in 
nearly the same manner. The plains near the Straits of Magel- 
lan 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 in- 
habiting Africa and Australia imdcr the same latitude. On 
these same plains of La Plata, we see the agouti and bizcacha, 
animals having nearly the same habits as our hares and rabbits 
and belonging to the same order of Rodents, but they plainly 
display an American type of structure. We ascend the lofty 
peaks of the Cordillera, and we find an alpine species of biz- 
cacha ; we look to the waters, and we do not find the beaver 
or nuisk-rat, but the coypu and capybara, rodents of the South 
Americaii ty\)C. Inmunerable 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 


(>sscnlially Aniorlcan, though they may be all peculiar species. 
We may look back to past ages, as shown in tli(j last chapter, 
and we lind American types then prevailing on the American 
Continent and in the American seas. AVe see in these fact.s 
some deep organic bond, throughout space and time, over the 
same areas of land and water, and indej)endent of physical con- 
ditions. 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 positive!}' know, produces organisms quite Hke each 
other, or, as we see in the case of varieties, nearly alike. The 
dissimilarity of the inhabitants of different regions may be at- 
trilnited to modification througli natural selection, and in a 
subordinate degree to the definite inlluence of different phys- 
ical conditions. The degree of dissimilarity will depend on 
the migration of the more dominant forms of life from one re- 
gion into another having been prevented more or less effectually, 
at periods more or less remote — oii the nature and number of 
the former inmiigrants — and on the action of the inhabitants 
on each other in leading to the preservation of different modifi- 
cations; the relation of organism to organisin in the struggle 
for life being, as I have already often remarked, the most im- 
portant of all relations. Thus the high importance of barriers 
comes into play by checking migration ; as does time for the 
slow process of modification througli natural selection. Widely- 
ranging species, abounding in individuals, which have already 
triumphed over many competitors in their own widelj'-extendcd 
homes, will have the best chance of seizing on new places, when 
they spread into new countries. In their new homes they will 
be exposed to new conditions, and will fre(iuently undergo 
further modification and improvement ; and thus they will be- 
come still further victorious, and will produce grou[is of modi- 
fied descendants. On this principle of inheritance with modili- 
eation, we can uiidci-stand how it is that sections of genera, 
whole genera, and even families, are confined to the same areas, 
as is so commonly and notoriously the case. 

I believe, as was remarked in the last chapter, in no law of 
necessary d(?velopment. As the variability of each species is 
an independent property, and will Ix; taken advantage of by 
natural selection, only so far as it jirolits each individual in its 
complex struggle for life, so the amount of modification in 
difierent species Avill be no uniform (juantity. If a number of 
species, after having long conipeteil witli each otlier in their 


old home, were to mi^^atc in a body into a new and afterward 
isolated country, they Avould be little liable to modification ; 
for n('itli(>r niig-ration nor isolation in themselves can effect any 
tiling. These principles come into play only by bringin*:^ or- 
ganisms into new relations with each other, and in a lesser de- 
gree Avith the surrounding physical conditions. As we have 
seen in the last chai)tcr that some forms have retained nearly 
the same character from an enormously remote geological 
period, so certain s})ecies have migrated over vast spaces, and 
have not become greatly or at all modified. 

According to these views, it is obvious that the several 
species of the same genus, though inhabiting the most distant 
quarters of the world, must originally have proceeded from the 
same source, as they are descended from the same progenitor. 
In the case of those species, which have undergone during 
whole geological periods but little modification, there is not 
much difficulty in believing that they may have migrated from 
the same region ; for, during the vast geographical and climatid 
changes which have supervened since ancient times, almost 
any amount of migration is possible. But in many other cases, 
in Avhich we have reason to believe that the species of a genus 
have been produced within comparatively recent times, there is 
great difficulty on this head. It is also obvious that the individ- 
uals of the same species, though now inhabiting distant and 
isolated regions, must have proceeded from one spot, where 
their parents were lirst produced: for, as explained in the last 
chajiler, it is incredible that individuals identically the same 
should have been produced from parents specifically distinct. 

Single Centres of supposed Creation. — We are thus brought 
to the question which has been largely discussed by naturalists, 
namely, whether species have beeia created at one or more 
points of the earth's surface. Undoubtedly there are many 
cases of extreme difficulty in understanding how the same species 
could possibly have migrated from some one point to the several 
distant and isolated points where noAV found. Nevertheless the 
simplicity of the view that each species was first produced 
within a single region captivates the mind. lie Avho rejects it, 
rejects the vera causa of ordinary generation with subsequent 
migration, and calls in the agency of a miracle. It is univer- 
sally admitted that in most cases the area inhabited by a spe- 
cies 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 be easily passed over 


by mip^raliun, the fact is g'iv^cn as something njiiarkablc and 
exceptional. The incapacity of migrating across a wide sea is 
more clear in the case of terrestrial mannnals than perhaps with 
any other organic beings; and, accordingly, Ave iind no inex- 
plicable instances of the same mammals inhabiting distant 
points of the world. No geologist feels any dilliculty in Great 
Britain possessing tlu^ same (juadrupeds 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 Soutlx 
America ? The conditions of life are nearly the same, so that 
a multitude of European animals and plants have become nat- 
uralized 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 
]>lants, from their varied means of disp(>rsal, have migrated 
across the wide and broken interspaces. The great and striking 
influence of barriers of all kinds is intelligible only on the view 
that the great majority of species have been produced on one 
side, and have not been able to migrate to the opposite side. 
Some few families, many sub-families, very many genera, and a 
still greater number of sections of genera, are conlined to a sin- 
gle region ; and it has been oljserved by several naturalists that 
the most natural genera or those genera in whicii the species are 
most closely related to each other, are generally confined to the 
same countr}', or if they have a wide range that their range 
is continuous. What a strange anomaly it woidd be, if a di- 
rectly opposite rule were to prevail, when we go down one step 
lower in the series, namely, to the individuals of the same spe- 
cies, and these had not been, at least at first, confined to some 
one region ! 

Hence it seems to me, as it has to many other naturalists, 
that the view of each species having been produced in one area 
alone, and having subsequently migrated from that area as far 
as its powers of migration and subsistence under past and pres- 
ent conditions permitted, is the most jirobable. Undoubtedly 
many cases occur, in which we cannot explain how the same 
species could have jiassed from one point to the other. But 
tlic geographical and dimatal dianges, which have certainly 
occurred within recent geological times, nuist have rendered 
discontinuous the formerly conliinious range of many species. 
So that we are reduced to consider whether the exceptions to 


contiiuiity of rani^c arc 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 
^vould be hopelessly tedious to discuss all the exceptional cases 
of the same species, now living- at distant and separated points; 
nor do I for a moment i)retend that any explanation could l)c 
olVercd of many instances. But after some jireliminary remarks, 
I will discuss a few of the most striking classes of facts ; namely, 
the existence of the same species on the summits of distant; 
mountain-ranges, and at distant points in the arctic and ant- 
arctic regions; and secondly (in the following chapter), tlic 
wide distribution of fresh-water productions ; and thirdly, the 
occurrence of the same terrestrial species on islands and on the 
main-land, 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 ex- 
plained on the view of each species having migrated from a 
single birthplace; then, considering our ignorance Avith respect 
to former climatal and geographical changes, and to the various 
occasional means of transport, the belief that a single birthplace 
is the law, seems to me incomparably the safest. 

In discussing this subject, we shall be enabled at the same 
time to consider a point equally important for us, namely, 
whether the several species of a genus, which must on the 
iheorj- all be descended from a common progenitor, can have 
migrated, undergoing modification during their migration, from 
some one area. When most of the species inhabiting one re- 
gion are diflcrcnt from those of another region, but are closely 
allied or belong to the same genera, if in all such cases it can 
be shown that there probably has been at some former period 
migration from the one region to the other, our general view 
will be much strengthened ; for the explanation is obvious on 
the principle of descent with modification. A volcanic island, 
for instance, upheaved and fomied at the distance of a few 
hundreds of miles from a continent, would prol^ably 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 tlicory of 
indi'pendent 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 spc- 


cics has come into existence coincident both in sjKice and time 
with a preexist ino- closely-allied species." And I now know that 
he attributes this coincidence to descent with modification. 

The discussion on " single and multiple centres of crea- 
tion " does not directly l)ear on another allied question — 
namely, whether all the individuals of the same species are 
descended from a single pair, or single hermaphrodite, or 
whether, as some authors suppose, from many individuals si- 
uuiltan<'()usly created. With organic beings which never inter- 
cross, if such exist, each species must be descended from a suc- 
cession of modified varieties, which have sujiplanted each 
other, but which have never blended with other individuals 
or varieties of the same species ; so that, at each successive 
stage of modification and improvement, all the individuals 
of the same variety will be descended from a single parent. 
IJut in the great majority of cases, namely, with all organisms 
which habitually unite for each birth, or which occasionally inter- 
cross, 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 chfler 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 difliculty on the 
theory of " single centres of creation," I must say a few words 
on the means of dispersal. 

Means of Dlq:)crsaL 

Sir C Lyell and other authors have ably treated this sub- 
ject. I can give here only the l^riefest abstract of the more 
important facts. Change of climate must have had a p(jwcr- 
ful influence on migration ; an impassable region when its 
climate was different from what it now is, may have been a 
high-road for migration; I shall, however, presently have to 
discuss this branch of the subject in some detail. Changes of 
level in the land must also have been highly influential : a 
narrow isthnuis now separates two marine faunas ; submerge 
it, or let it fonncrly have been submerged, and the two faunas 


\vill now Ijlend or may fonnerly have blended : where the sea 
now extends, land may at former pciiods have connected 
islands or possibly even continents together, and thus have 
allowed terrestrial productions to pass from one to the other. 
No geologist will dispute that great mutations of level have 
occurred within the period of existing organisms. Edward 
Forbes insisted that all the islands in the Atlantic must have 
been recently connected with Europe or Africa, and Europe 
likewise with America. Other authors have thus hj'potheti- 
cally bridged over every ocean, and united almost every island 
to some main-land. 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 conti- 
nent. This view cuts the Gordian knot of the dispersal of the 
same species to the most distant points, and removes many a 
dilhculty : but, to the best of my judgment, we are not author- 
ized in admitting such enormous geograplucal 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 exten- 
sion of our continents, as to have imited them within the 
recent period to each other and to the several intervening 
oceanic islands. I freely admit the former existence of many 
islands, now buried beneath the sea, which may have served 
as halting-places for plants and for many animals during their 
migration. In the coral-pi'oducing oceans such sunken islands 
are now marked by rings of coral or atolls standing over them. 
AVhcneverit is fully admitted, as no doubt it Avill some day be, 
that each species has proceeded from a single birthplace, and 
Avlien 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 be- 
lieve that it Avill 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 oth- 
er, and Avith the many existing oceanic islands. Several facts in 
distribution — such as the great difference in the marine faunas 
on the opposite sides of almost every continent — the close re- 
lation 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 of the nearest con- 
tinent, being in ])art determined (as we shall hereafter see) by 
the depth of the intervening ocean — these and other such facts 


sceni to me oj^poscd to tlic julinissioii of such jirodigious geo- 
graphical revolutions -within the recent period, as are necessary 
on the view advanced b}^ Forbes and admitted by his many- 
followers. The nature and relative proportions of the inhab- 
itants of oceanic islands likewise seem to me opposed to the 
belief of their former continuity with continents. Nor does 
the almost universall}' volcanic composition of such islands 
iavor the admission that they are the wrecks of sunken conti- 
nents; if they had originally existed as continental mountain- 
ranges, some at least of the islands would ha^-e been formed, 
like other mountain-sinnmits, of granite, metamorphic schists, 
old fossiliferous and other rocks, instead of consisting of mere 
pih.'S of volcanic matter. 

I nmst now say a few words on -what are called accidental 
means, but which more properly should be called occasional 
means of distribution. 1 shall here coniine myself to plants. In 
botanical works, this or tliat plant is stated to be ill adapted for 
-wide dissemination ; but for transport across the sea the greater 
or less facilities may be said to be almost wholly unknown. 
Until I tried, -with JMr. ]5erkeley's aid, a few experiments, it 
-was not even known how far seeds could resist the injurious 
action of sea-water. To my surprise, I found that, out of 87 
kinds, 6-4 germinated after an immersion of 28 days, and a few 
survived an inmiersion of 137 days. It deserves notice that 
certain orders Avere far more injured than others : nine Legu- 
minoste were tried, and, with one exception, they resisted the 
salt-water badly ; seven species of the allied orders, Hydro- 
phjdlaccw and Polemoniace:e were all killed by a month's im- 
mersion. For convenience' sake I chiefly tried small seeds, 
without the cajisule or fruit ; and, as all of these sank in a few 
(laj-s, they could not have been floated across wide spaces of 
th(^ sea, whether or not they were injured by the salt--water. 
Afterward I tried some larger fruits, capsules, etc., and some 
of these floated for a long time. It is well kncnvn what a dif- 
ference there is in the buoyancy of green aiid seasoned tim- 
ber; and it occurred to me that floods might wash down plants 
or branches, and that these might be dried on the banks, and 
then liy a fresh rise in the stream be washed into the sea. 
Hence I was led to dry stems and branches of 9-i ])lants with 
ripe fruit, and to plac(; them on sea-water. The majority sank 
quickly, but some, which while green floated for a very short 
time, when drietl floated nmch longer ; for instance, ripe hazel- 
nuts sank immediately, but when dried they floated for 90 


days, and afterward -wlioii planted tliey cremunatcd ; an aspar- 
a2:u.s-j)lant with ripe berries floated for ^3 days, when dried it 
floated for 85 (lays, and the seeds afterward germinated; the 
ripe seeds of llelosciadimn sank in two days, when dried they 
floated for above 90 days, and afterward germinated. Alto- 
gether, out of the 94 clried plants, 18 floated for above '28 
days, and some of the 18 floated for a very much longer period. 
So that as -J;^- seeds germinated after an immersion of 28 days ; 
and as 1} ])lants with ripe fruit (but not all the same species 
as in the foregoing experiment) floated, after being dried, for 
above 28 days, as far as we may infer any thing from these 
scanty fticts, we may conclude that the seeds of ^^ plants of 
any country might be floated by sea-currents during 28 days, 
and would retain their power of germination. In Johnston's 
Physical Atlas, the average rate of the several Atlantic cur- 
rents is 33 miles per diem (some currents running at the rate 
of GO miles per diem) ; on this average, the seeds of -jJ^g- plants 
belonging to one country might be floated across 924 miles 
of sea to another country ; and when stranded, if blown to a 
favorable spot by an inland gale, they Avould germinate. 

Subsequently to my experiments, M. Martens tried similar 
ones, but in a much better manner, for he placed the seeds in 
a box in the actual sea, so that they were alternately wet and 
exposed to the air like really floating plants. He tried 98 
seeds, mostly different from mine ; but he chose many large 
fruits and likewise seeds from plants which live near the sea; 
and this would have fiivored the average length of their flota- 
tion and of their resistance to the injurious action of the salt- 
water. On the other hand, he did not pre^^iously dry the 
plants or branches with the fruit ; and this, as we have seen, 
would have caused some of them to have floated much longer. 
The result was, that ^ of his seeds floated for 42 days, and 
were then capable of germination. But I do not doubt that 
plants exposed to the waves would float for a less time than 
tliose protected from violent movement as in our experiments. 
Therefore it would perhaps be safer to assume that the seeds 
of about -^/g- plants of a flora, after having been dried, could 
be floated across a space of sea 900 miles in Avidth, and would 
then germinate. The fact of the larger fruits often floating 
longer than the small, is interesting ; as plants with large 
seeds or fruit could hardly be transported by any other means ; 
and Alph. do Candollo has shown that such plants generally 
have restricted ranges. 


But seeds 111:13' l)c occasionally transported in another man- 
ner. 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 valualjle 
royal tax. I find, on examination, that, when irregularly- 
shaped stones are embedded in the roots of trees, small parcels 
of earth are very frequently enclosed in their interstices and 
behind them — so perfectly that not a particle could be washed 
away in the longest transport : out of one small portion of. 
earth thus completely enclosed by wood in an oak about 50 
years old, three dicotyledonous plants germinated : I am cer- 
tain of the accuracy of this observation. Again, I can show 
that the carcasses of birds, when floating on the sea, sometimes 
escape being immediately devoured ; and seeds of many kinds 
in the crops of floating birds long retain their vitality : peas 
andvetches, for instance, are killed by even a few days' immer- 
sion in sea-water ; but some taken out of the crop of a pigeon, 
which had floated on artificial salt-water for 30 days, to my 
surprise nearly all germinated. 

LiWng 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 bloAvn by gales to vast 
distances across the ocean. We may safely assume that undei" 
such circumstances their rate of flight would often be 35 miles 
an hour ; and some authors have given a far higher estimat'C. 
1 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 ctHirse of two months, I picked up in my garden 12 kinds 
of seeds, out of the excrement of small birds, and these seemed 
jierfect, and some of them, which were tried, germinated. But 
the following fact is more important : the crops of birds do iif)t 
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 tli<? distance of 500 miles, and hawks are known to 
look out for tired birds, and the contents of their torn crops 
might thus readily get scattered. Some hawks and owls bolt 
their prey whole, and, after an interval of from twelve to 
twenty hours, disgorge pellets, which, as I know from cxperi- 


iiicnls iimdc in the Zoological Gardens, include seeds capable of 
genniiiiit ion. Some seeds of the oat, -wheat, millet, canary, hemp, 
clover, and beet, germinated after ha\nng 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 d;iys and fourteen hours. Fresh-water fisli, I find, eat 
seeds of many land and water plants : fish are frequently de- 
voured by birds, and thus the seeds might Ije transported fi-om 
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 excre- 
ment ; and several of these seeds retained the power of ger- 
mination. Certain seeds, however, were always killed by this 

Locusts are sometimes blown to great distances from the 
land; I myself caught one 370 miles from the coast of Africa, 
and have heard of others caught at greater distances. The 
Kev. R. T. Lowe infonns Sir C. Lyell that, in November, 1844, 
swarms of locusts visited the island of Madeira. They were in 
countless numbers, as thick as the fiakes of snow in the heavi- 
est snow-storm, and extended upward as far as could be seen 
with a telescope. During two or "three days they slowly ca- 
reered round in the air in an immense ellipse, at least five or 
six miles in diameter, and at night alighted on the taller trees, 
which were completely coated with them. They then disap- 
peared over the sea, as suddenly as they had appeared, and 
have not since visited the island. Now, in parts of Natal it is 
believed by some of the farmers, thougli on quite insufficient 
evidence, that injurious seeds are introduced into their grass- 
land in the dung left by the great fiights of locusts which often 
visit that country. In consequence of this belief, Mr. Weale 
sent me in a letter a small jiacket of the dried pellets, out of 
which I extracted imder the microscope several seeds, and 
raised from them seven gi-ass-i)lants, belonging to two spe- 
cies, in two genera. Hence a swarm of locusts, such as that 
which visited Madeira, might readily be the means of introdu- 
cing several kinds of plants into an island lying far from the 

Although the beaks and fix-t of birds are generally clean, 
earth sometimes adheres to them : in one easel removed sixty- 
one grains, and in another case twenty-two grains of dry argil- 
laceous earth from one foot of a partridge, and in the earth 


there was a pebble as larf^e as the seed of a vetch. Here is a 
better case : the h\2: <if a woodcock was sent to mo by a friend, 
with a little cake of dry earth attached to the sliaiik, weighini^ 
only nine 2,rains ; and this contained a seed of the toad- 
rush (Juncus bufonius) which germinated and flowered. Mr. 
Swaysland, of Brighton, who has paid such close attention to 
our migratory birds during the last forty years, informs me that 
he has often shot wagtails (motacilla?)', wheatears, and whin- 
chats (saxicoloe), on their first arrival, before they had alight- 
ed on our shores, and has several times noticed little cakes of 
earth on their feet. Many facts could be given showing how 
the soil is almost everywhere charged with seeds. For in- 
stance. Prof. 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 jilaced under a bell glass, no less 
than 82 plants sprung from it: these consisted of 13 monoco- 
tyledons, including the common oat, and at least one kind of 
grass, and of 70 dicotyledons, which consisted, judging from 
tlie young leaves, of at least three distinct species. With such 
facts before us, can we doubt that the many birds which are 
annually blown l)y gales across great spaces of ocean, and which 
annually migrate — for instance, tlie millions of quails across 
the ^Mediterranean — nuist occasionally transport a few seeds 
embedded in dirt adiiering 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 transported seeds 
from one part to another of the arctic and antarctic regions ; 
and, during the Glacial period, from one part of the now tem- 
Ijcrate rcgiims to another. In the Azores, from the large 
number of plants common to Europe, in comparis(m with the 
species in the otlier islands of thi^ Atlantic, which stand nearer 
to the main-land, and (as remarked by Mr. H. C Watson) from 
their somewhat northern character in comj^arison with the lati- 
tude, I suspected that th(>S(^ islands had been partly stocked 
by ice-borne seeds, during the Glacial epoch. At my request 
Sir C. Lyell wrote to M. llartung to incjuire whether he had 
observed erratic bowlders on these islands, and he answered 
that lie had found large fragments of granite and other rocks, 


■wliicli do not occur in the archipelago. Hence we may safely 
infer that icebergs formerly landed their rocky burdens on the 
shores of these niid-occau islands, and it is at least possible 
that they may have brought thither the seeds of northern 

Considering that tliese several means of transport, and that 
other means, which without doubt remain to be discovered, 
have been in action year after year, for tens of thousands of 
years, it would, I think, be a marvellous fact if many plants had 
not thus become widely transported. These means of trans- 
port 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 
Avhen exposed for a great length of time to the action of sea- 
water; nor could they be long carried in the crops or intes-« 
tines of birds. These means, however, would suffice for occa- 
sional 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 by such 
means become mingled ; but would remain as distinct as tliey 
now are. The currents, from their course, would never bring 
seeds from North America to Britain, though they might and 
do bring seeds from the West Indies to our western shores, 
where, if not killed by their very long immersion in salt-water, 
they could not endure our climate. Almost every year, one or 
two land-birds are blown across the whole Atlantic Ocean, 
from North America to the western shores of Ireland and 
England ; but seeds could be transported by these rare wan- 
derers only by one means, namely, by dirt adhering to their 
feet or beaks, which is in itself a rare accident. Even in this 
case, how small the chance would be of a seed falling on fa- 
vorable soil, and coming to matuiity ! But it would be a great 
error to argue that because a well-stocked island, like Great 
Britain, has not, as far as is known (and it would be very diffi- 
cult to prove this), received within the last few centuries, 
through occasional means of transport, immigrants from Eu- 
lope or any other continent, that a poorly-storked island, 
though standing more remote from the main-land, would not 
receive colonists by similar means. Out of a hundred seeds 
or animals transported to an island, even if far less well 


stocked than Britain, perhaps not more than one would be so 
well litted to its new home as to become naturalized. But 
this, as it seems to me, is no valid arf^ument against "what 
would be effected by occasional means of transport, during tlu; 
long lapse of geological time, while the island was being up- 
heaved, and before it had become fully stocked with inhabit- 
ants. 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, 

Disjyersal durbuj the Glacial Period. 

The identity of many plants and animals, on mountain- 
gunnnits, separated from each other by hundreds of miles of 
lowlands, where Alpine species could not possibly exist, is one 
of llie 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 remark- 
able fact to see so many plants of the same species living on 
the snowy regions of the Alps or Pyrenees, and in the extreme 
jiorthern parts of Europe ; but it is far more remarkable that 
the plants on the White Mountains, in the United States of 
America, arc all the same with those of Labrador, and nearly 
all the sanie, as we hear from Asa Gray, with those on the 
loftiest mountains of Europe. Even as long ago as 1747, such 
facts led Gmelin to conclude that the same species must have 
Ijcen independently created at several distinct points ; and we 
might have remained in this same belief, had not Agassiz and 
others called vivid attention to the Glacial period, which, as 
we shall immediately see, affords a simple exfjlanation of these 
facts. We have evidence of almost every conceivable kind, 
organic and inorganic, that, within a very recent geological 
jx'riod, central Em-ope and North America suffered under an 
arctic climate. The ruins of a house burnt by lire do not tell 
their tale moni plainly than do the mountains of Scotland and 
Wales, with their scored flanks, polishi^d surfaces, and perched 
bowlders, of the icy streams with which their valleys W(M-e late- 
Iv filled. So grcatlv has the climate of Europe clianged, thai, 
in Northern Jtalv, gigantic moraines, left bv old glaciers, are 
now clothed by tlie vine and maize, 'J'hroughout a larg(^ part 
of the United States, erratic bowlders and scored rocks ])jainly 
reveal a former cold j)eriod, 

Tlie former influence of the glacial climate on the distribu- 

338 DISPERSAL Cuat, Xi. 

lion of the inliabitiints of Europe, as cxj)lained hy Edward 
Forbes, is substantially as follows. But we shall follow the 
changes more readil}', by supposing a new glacial period slowly 
to come on, and then pass away, as formerly occurred. As the 
cold came on, and as each more southern zone became fitted 
for the inhabitants of the north, they would take the places of 
the former inhabitants of the temperate regions. The latter, 
at the same time, would travel farther and farther southward, 
unless thoy were stopped by bairiers, in which case they would 
perish. The mountains would become covered with snow and 
ice, and their former Alpine inhabitants would descend to the 
plains. By the time that the cold had reached its maximum, 
we should have an arctic fauna and flora, covering the central 
parts of Europe, as far south as the Alps and Pyrenees, and 
even stretching into Spain. The now temperate regions of the 
United States would likewise be covered by arctic plants and 
animals, and these would be nearly the same with those of 
Europe ; for the present cireumpolar inhabitants, which we sup- 
pose to have everywhere travelled 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 produc- 
tions of the more temperate regions. And as the snow melted 
from the bases of the mountains, the arctic forms Avould seize 
on the cleared and thawed ground, always ascending, as the 
warmth increased and the snow still further disappeared, higher 
and higher, while their brethren were pursuing their northern 
journey. Hence, when the warmth had fully returned, the 
same species, which had lately lived together in a body on the 
European and North American lowlands, Avould 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 understalid the identity of many plants at 
points so immensely remote as on the mountains of the United 
States and of Europe. We can thus also understand the fact 
that the Alpine plants of each mountain-range are more espe- 
cially 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 remigration on the returning warmth, would generally 
have been due south and north. The Alpine plants, for exam- 
ple, of Scotland, as remarked by Mr. II. C. Watson, and those 
of the Pyrenees, as remarked by Ramond, are more especially 
allied to the plants of northern Scandinavia ; those of the 


United States to Labrador ; those of the mountains of Siberia 
to the arctic regions of that country. These views, f^rounded 
as they are 011 the perfectly well-ascertained occurrence of a 
former Glacial period, seem to me to explain in so satisfactory 
a manner the present distribution of the Alpine and arctic pro- 
ductions of Europe and .Vmcrica, that, when in other regions 
we lind the same species on distant mountain-summits, Ave may 
almost conclude, without other evidence, that a colder climate 
lV)rnierly permitted their migration across the intervening low- 
lands, now become too warm for their existence. 

As the arctic forms moved first southward and afterward 
backward to the north, in unison with the changing climate, 
they w ill not have been exposed during their long migrations 
to any great diversity of temperature ; and, as they all migrated 
in a body tog-ether, their mutual relations will not have been 
much disturlicd. Hence, in accordance with the principles in- 
culcated in tliis volume, these forms will not have been liabli> 
to much modification. But with the Alpine productions, left 
isolated fi-om 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, antl have survived there ever since; 
they will, also, in all probability, have become mingled Avitli 
ancient Alpine species, which must have existed on the moun- 
tains before tlie commencement of the Glacial epoch, and which 
during its coldest ])eriod will have been temporarily driven 
down to the plains ; they will, also, have been exposed to some- 
what different climatal influences. Their mutual relations will 
thus have been in some degree disturbed ; consequiMitlv they 
will have been liable to modification ; and this we find has been 
the case : for if we compare the present Alpine j)lants and ani- 
mals of the several great European mountain-ranges one with 
another, though many of the species remain idmitically the same, 
some exist as varieties, some as doubtful forms or sub-species, 
and some as certainly distinct yet closely-allied species repre- 
senting each other on the several ranges. 

In the foregoing illustration T have assumed that at the 
commencement of our imagin.ary Glacial period, the arctic pro- 
ductions were as uniform round the polar regions as they arc 
at the present day. But it is necessary also to include many 
sulvarctic and some few temperate forms, for some of these are 
the same on the lower mountain-slopes and on the plains of 

340 DISPERSAL Chaj-. XI. 

North America and Europe ; and it may be asked how I ac- 
count for tliis degree of uniformity in the sub-arctic and temper- 
ate forms round tlie world, at the commencement of the real 
Glacial period. At the present day, the sub-arctic and northern 
temperate productions of the Old and New Worlds are sepa- 
rated from each other by the whole Atlantic Ocean and by the 
nortliern part of the Pacific. During the Glacial period, when 
the inhabitants of the Old and New Worlds lived farther south- 
ward than they do at present, they must have been still more 
completely separated from each other by wider spaces of ocean ; 
so that it may well be asked how the same species could have 
entered the two continents then so widely separated. The ex- 
planation, I believe, lies in the nature of the climate before the 
commencement of the Glacial period. At this, the newer Plio- 
cene period, the majority of the inhabitants of the world were 
specifically the same as now, and we have good reason to 
believe that the climate was warmer than at the present day. 
Hence we may suppose that the organisms which now live un- 
der latitude 60°, lived during the Pliocene jieriod farther north 
under the Polar Circle, in latitude CG°-G(° ; and that the pres- 
ent arctic productions then lived on the broken land still nearer 
to the pole. Now, if we look at a terrestrial globe, we see un- 
der the Polar Circle that there is almost continuous land from 
western Europe, through Siberia, to eastern America. And 
this ccHitinuity of the circumpolar land, with the consequent 
freedom imder a more favorable climate for intermigration, will 
account for the supposed uniformity of the sub-arctic and tem- 
perate productions of the Old and New Worlds, at a period 
anterior to the Glacial epoch. 

Believing, from reasons before alluded to, that our conti- 
nents have long remained in nearly the same relative position, 
though sul)jected to large but partial oscillations of level, I 
am strongly inclined to extend the above view, and to infer 
that during some still earlier and still vrarmer period, such as 
the older Pliocene period, a large number of the same plants 
and animals inhabited the almost continuous circumpolar land ; 
and that these plants and animals, both in the Old and New 
Worlds, began slowly to migrate southward as the climate be- 
came less warm, long before the commencement of the Glacial 
period. We now see, as I believe, their descendants, mostly 
in a modified condition, in the central parts of Europe and the 
United States. ( )n this A-iew we can understand tlie relation- 
shi]), ^^itll very little identity, l)etween the productions of 


North America and Europe — a relationship whicli is highly 
romarkai)lc, considering the distance of the two areas, and 
tlieir separation hy tlie whole Atlantic Ocean. We can fur- 
ther understand the singular fact remarked on by several 
observers, that the productions of Europe and America during 
the later tertiary stages -were more closely related to each 
other than they are at the present time ; for during tliese 
warmer periods the northern parts of the Old and New Worlds 
will have been almost conthiuously united by land, serving as 
a bridge, since rendered impassable by cold, for the intermi- 
gration of their inhabitants. 

During the slowly-decreasing warmth of the Pliocene pe- 
riod, as soon as the species in common, which inliabitcd the 
New and 0!d Worlds, migrated south of the Polar Circle, they 
would be 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 would become mingled in the one 
great region with the native American productions, and would 
have had to compete with them ; and, in the other great region, 
with those of the Old World. Consequently we have here 
every tiling favorable for much modification — for far more 
modification than with the Alpine productions, left isolated, 
within a much more recent period, on tlie several mountain- 
ranges and on the arctic lands of Europe and North America. 
Hence it has come that, when we compare the now living pro- 
ductions of the temperate regions of the New and Old Worlds, 
we find very few identical species (though Asa Gray has late- 
ly shown that more plants are identical than was formerly sup- 
j)Osed), but we find in every great class many forms, which 
some naturalists rank as geographical races, and others as dis- 
tinct species ; and a host of closely-allied or representative 
forms which are ranked by all naturalists as specifically dis- 

As on the land, so in the waters of the sea, a slow southern 
migration of a marine fauna, which, during the Pliocene or 
even a somewhat earlier pcried, was nearly imiform along the 
continuous shores of tlie Pohir Circle, will account, on tlie the- 
ory of modification, for many closely-allied forms now living 
in marine areas completely sundered. Thus, I tliink, we can 
imderstand the ])resence of some still existing and of some ter- 
tiary closely-allied fijrms on the eastern anti western shores of 
temperate North America ; and the still more striking fact of 

342 alti:i:nate glacial tliuods chap. xl 

many clost'ly-allicd crustaceans (as described in Dana's admir- 
able work), of some fish and other marine animals, in the Med- 
iterranean and in the seas of Japan — these two areas being 
now completely separated by the breadth of a whole continent 
and by a wide space 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 tem- 
perate lands of North America and Europe, are inexplicable 
on the theory of creation. AVc cannot maintain that such spe- 
cies have been created alike, in correspondence with the neai'- 
ly similar physical conditions of the areas ; for if we compare, 
for instance, certain parts of South America with j^arts of 
South Africa or Australia, Ave see countries closely similar in 
all their j^hysical conditions, hut with inhabitants utterly dis- 

Alternate Glacial Periods of the Korth and South. 

But we must return to our more immediate subject. I am 
convinced that Forbes's view may be largely extended. In 
Europe we meet with the plainest evidence of the Glacial pe- 
riod, from the Avcstcrn 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 aflected. In the Lebanon, according to Dr. 
Hooker, perpetual snow formerly covered the central axis, and 
fed glaciers which rolled 4,000 feet down its valleys. Along 
the Himalaya, at points 900 miles apart, glaciers have left the 
marks of their former low descent ; and, in Sikkim, Dr. Hooker 
saw maize growing on gigantic ancient moraines. Southward 
of the Asiatic Continent, on the opposite side of the equator, 
we now know, from the excellent researches of Dr. J. Haast 
and Dr. Hector, that immense glaciers formerly descended to 
a low level in New Zealand ; and the same plants foimd by 
Dr. Hooker on wideh'-separated mountains in this island tell 
the same story of a former cold period. From facts commimi- 
catcd to me by tlie Rev. W. B. Clarke, it appears also that 
there are traces of former glacial action on the mountains of 
the southeastern corner of Australia. 

Looking to America: in the northern half, ice-born frag- 
ments of rock have been observed on the eastern side of the 
oontinent, as far south as kit. 3G°-37°, and on the shores of the 


Pncific, where tlic climate is now so difiercnt, as far south as 
lat 4G''. Erratic bowlders have, also, been noticed on the 
Rocky Mountains. In the Cordillera of South America, nearly 
under the equator, glaciers once extended far Ix'low their pres- 
ent level. In central Chili I examined a vast mound of detri- 
tus with grciit bowlders, crossing the Portillo valle}', which 
there can hardly be a doubt once formed a huge moraine ; and 
Mr. D. Forbes informs me that he found in various parts of the 
Cordillera, from lat, 13° to 30° S., at about the height of 
12,000 feet, deeply-furrowed rocks, resembling those with 
which he was familiar in Norway, and likewise great masses 
of detritus, including grooved pebbles. Along this whole 
space of the Cordillera true glaciers do not now exist even at 
nmcli more considerable heights. Farther south on both sides 
of the continent, from lat. 41° to the southernmost extremity, 
we have the clearest evidence of former glacial action, in nu- 
merous immense bowlders transported far from their parent 

From these several facts, namely, from the glacial action 
having extended all round the northern and southern hemis- 
pheres — from tlic period having been in a geological sense re- 
cent in both hemispheres — from its having lasted in both dur- 
ing a great length of time, as may be inferred from the amount 
of work effected — and lastly from glaciers having recently de- 
scended to a low level along the whole line of the Cordillera, it 
formerly appeared to me that we could not avoid the conclusion 
that the temperature of the whole world had been simultane- 
ously lowered during the Glacial period. But now Mr. CroU, 
in a series of admirable memoirs, lias 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 orlnt. All these causes tend toward the same 
end ; but the most powerful appears to be the influence of the 
oocentricity of the orbit upon oceanic ciuTcnts. It follows, from 
Mr. Croll's researches, that cold periods regularly recur every 
ten or fifteen thousand years; but that at much longer inter- 
vals the cold, owing to certain contingencies, is extremely se- 
vere, and lasts for a great length of time. Mr. Cn)ll believes 
that the last great Glacial period occurred about 2-40,000 years 
ago, and endured with slight alterations of climate for about 
160,000 years. With respect to more ancient Glacial periods, 
several geologists arc convinced from direct evidence that such 
occurred during the Miocene and Eocene fonnations, not to 


mention still more ancient formations. But in relation to our 
present subject, the most important result arrived at by Mr, 
Croll is, that whenever the northern hemisphere passes through 
a cold period, the temperature of the southern hemisphere is 
actually raisctl, with the winters rendered much milder, chiefly 
through changes in the direction of the ocean-currents. So, 
conversely, it is with the northern hemisphere, when the south- 
ern passes through a glacial period. These conclusions have, 
as we shall immediately see, a most important bearing on geo- 
graphical distribution ; but I will first give the facts which de- 
mand an explanation. 

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

In Africa, several forms characteristic of Europe and some 
few representatives of the flora of the Cape of Good Hope oc- 
cur on the Mountains of Abj'ssinia. At the Cape of Good 
Hope a very few European species, believed not to have been 
introduced by man, and on the mountains several representa- 
tive European forms are found, which have not been discovered 
in the intertrojDical 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 Cam- 
eroon Mountains, in the Gulf of Guinea, are closely related to 
those on the Mountains of Abyssinia, and likewise to those of 
Icinperate P^urope. It now also appears, as I hear froni Dr. 
Hooker, tliat some of these same temperate plants have been 
discovered by the R;'v. R. T. Lowe on the mountains of the 
Cape de V^erde Islands. This extension of the same temperate 
forms, almost under the equator, across the whole continent of 
Africa and to the mountains of the Cape de Verde archipelago, 
is one of the most astonishing facts ever recorded in the dis' 
tribution of plants. 


On the Himalaya, and on the isolated mountain-ranges of 
the Peninsula of India, on the heights of Cejlon, and on the 
volcanic cones of Java, many plants occur, cither identicall}' the 
same or representing each other, and at the same time repre- 
senting plants of Europe, not found in the intervening hot low- 
lands. A list of the genera of plants collected on the loftier 
peaks of Java raises a picture of a collection made on a hillock 
in Europe ! Still more striking is the fact that pecidiar south- 
ern Australian forms are represented by certain j)lants grow- 
ing 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 nortli as 

On the southern mountains of Australia, Dr. F. Muller has 
discovered several European species ; other species, not intro- 
duced by man, occur on the lowlands; and a long list can be 
given, as I am informed by Dr. Hooker, of European genera, 
found in Australia, but not in the intermediate torrid regions. 
In the admirable " Introduction to the Flora of New Zealand," 
by Dr. Hooker, analogous and strilving facts are given in re- 
gard to the ])lants 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 identical species or 
varieties of the same species. It should, however, be observed 
that these plants are not strictly Arctic forms ; for, as Mr. H. 
V. Watson has remarked, " in receding from polar toward 
equatorial latitudes, the Alpine or mountain floras really be- 
come less and less Arctic." Besides these identical and closely- 
allied forms, many species inhabiting the same widely-sundered 
areas behjng to genera not now found in the intermediate trop- 
ical lowlands. 

These brief remarks a]iply to plants alone ; but some few 
analogous facts could be givi'U in regard to terrestrial animals. 
In marine productions, similar cases likewise occur; as an ex- 
ample, I may quote a statement by the highest authority, Prof. 
Dana, that ''it is certainly a wonderful fact that New Zealand 
should have a closer reseml)lanc«; in its Crustacea to Great 
Ijiitain, its anlijiode, than to anv otiier part of the world." 
Sir J. Kiclianlson, also, speaks of the reappearance on the 
shores of New Zealand, Tasnianiii, etc., of northern forms of 
fish. Dr. Hooker informs me that twenty-five species of Algaj 


arc common to Ncn' Zealand and to Europe, Inti Iiave not beer 
found in the inlennediate tropical seas. 

From the foregoing^ facts, namely, the presence of temperate 
forms on the highlands across the whole of equatorial iVfrica, 
and along the Peninsula of India to Ceylon and the Malay archi- 
pelago, and in a less ■\vell-raarked mann(T across the wide ex- 
panse of tropical South America, it appears almost certain that 
at some former period, no doubt during the most severe part 
of the Glacial period, the lowlands of these great continents 
were everyAvhere tenanted under the equator by a considerable 
number of temperate forms. At tliis ])criod the equatorial cli- 
mate at the level of the sea was probably about the same with 
that now^ experienced at the height of from five to six thousand 
feet vmder the same latitudes, or perhaps even rather cooler. 
During this, the coldest period, the lowlands under the equator 
must have been clothed with a mingled tropical and temperate 
vegetation, like that described by Hooker as growing luxuri- 
antly 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, on the moun- 
tainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann 
found temperate European forms beginning to appear at the 
height of about five thousand feet. On the mountains of 
Panama, at the height of only two thousand feet. Dr. Seemann 
found the vegetation like that of Mexico, " Avith forms of the 
torrid zone harmoniously blended with those of the temperate." 

Now let us see Avhether Mr. Croll's conclusion, that Avhen 
the northern hemisphere sufTcred fi-om the extreme cold of the 
great Glacial period, the southern hemisphere Avas actually 
Avarmer, throAvs any clear light on the present apparently inex- 
])licable distribution of A^arious organisms in the temperate 
jiarts of both hemispheres, and on mountains of the tropics. 
The Glacial period as measured by ^-ears, 7uust have been very 
long; and, Avhcn avc remember OA'cr Avhat vast spaces some 
naturalized plants and animals have spread Avithin a fcAV cen- 
turies, this period Avill haAC been ample for any amoimt of mi- 
gration. As the cold became more and more intense, Ave knoAV 
that Arctic forms iuA'aded the temperate regions ; and, from the 
facts just given, there can hardly be a doubt that some of the 
more A'igorous, dominant, and widest-spreading temperate forms 
actually then iuA'aded the equatorial loAvlands. The inhabitants 
of these lowlands Avould at the same time migrate to the tropi- 
Ciil and sub-tropical regions of the south, for the southern hemi- 


splicre was at this period warmer. On the decline of the Ghi- 
cial ]>eriod, as both liemis])heres gradually recovered their 
former temperatures, the northern temperate forms, livins; on 
the lowlands under tlie equator, would be driven to their former 
homes or be destroyed, being replaced by the equatorial forms 
returning from the south. Some, howeveK, of the northern 
temperate forms would almost certainly ascend any adjoining 
highland, where, if sufficiently lofty, they would long survive, 
like the Arctic forms on the mountains of Europe. They might 
survive, even if the climate was not perfectly fitted for them, 
for the change of temperature must have been very slow, and 
plants undoubtedly possess a certain capacity for acclimatiza- 
tion, as slio\\Ti by their transmitting to their offspring different 
constitutional powers of resisting heat and cold. 

In the regular course of events the southern hemisphere 
would be subjected to a severe Glacial period, with the northern 
hemisphere rendered warmer ; and then the southern temperate 
forms would in their turn invade the equatorial lowlands. The 
northern forms which had before been left on the mountains 
would now descend and mingle Avith the southern forms. 
These latter, Avhen the warmth returned, Avould return to their 
former homes, leaving some few species on the mountains, and 
carrying southward with them some of the northern temper- 
ate 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 moun- 
tains of the intermediate tropical regions. But the species left 
during a long time on these mountains, or in opposite hemi- 
spheres, would have to compete Avith many new forms, and 
would be exposed to somewhat clifTerent physical conditions ; 
hence tlicy would be eminently lialjle to modification, and 
would generally now exist as varieties or as representative s]>c- 
cies ; and this is the case. We must, also, bear in mind the 
occurrence in both hemispheres of former Glacial jieriods ; for 
these will account, in accordance with the same principles, for 
the many quite distinct species inhabiting the same widely- 
separated areas, and belonging to genera not now found in the 
intermediate torrid zones. 

It is a remarkaljlc fact, strongly insisted on by Hooker in 
regard to America, and by Alph. de Candolle in regard to Aus- 
tralia, that inany more identical or now slightly-modified spe- 
cies have migrated from the north to the south, than in a re- 
versed direction. We see, however, a few southern forms on 


tlie mountains of Borneo and Abyssinia. I suspect that this 
preponderant niij^ration from the north to t!ie south is due to 
the f^rcatcr extent of hmd in the north, and to tlie noi'tbeni 
forms liaviuii;' existed in their own homes in g-reater numbers, 
and havini^ eonsccfucntly been advanced throu<>;h natural selec- 
tion and comjM'tition to a liigher stage of perfection, or domi- 
nating jKnver, tliiui the soutliern forms. And thus, when the 
two sets became commingled in the equatorial regions, during 
the alternations of the Glacial periods, the northern forms were 
the more powerful and were able to hold their places on the 
mountains, and afterward to migrate southward with the south- 
ern forms ; but not so the southern in regard to the northern 
forms. In the same manner we see, at the present day, that 
very many European productions cover the groimd in La Plata, 
New Zealand, and to a lesser degree in Australia, and have 
beaten the natives ; whereas extremely few southern 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 
ccnturietj from La Plata, and during the last thirty or forty 
years from Australia. The Ncilgherric Mountains in India, 
however, offer a part ial exception ; for here, as I hear from Dr. 
Hooker, Australian forms are rapidly sowing themselves and 
becoming naturalized. Before the last great Glacial period, 
no doubt the intertropical mountains Avere stocked with en- 
demic 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 pi'oductions are nearly equalled, or even outnumbered, 
by those which have become naturalized ; and this is the llrst 
stage toward their extinction. Mountains are islands on the 
land, and their inhabitants have yielded to those produced with- 
in 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 naturalizetl through man's 

The same principles apply to the distribution of terrestrial 
animals and of marine productions, in the northern and south- 
ern temperate zones, and on the intertropical mountains. 
When during the height of the Glacial period the ocean-cur- 
rents Avere Avidely diiferent to Avhat thev noAV are, some of the 
inhabitants of the temperate seas might have reached the equa- 
tor ; of these a fcAV would perhaps at once be able to migrate 


southward, by kpcpiiiG^ to the cooler currents, Avhile others 
might remain and survive in the cooler depths, until the south- 
ern hemisphere was in its turn subjected to a glacial climate 
and permitted of their lurtlier progress ; in nearly the same 
manner as, accortling to Forbes, isolated spaces inhabited by 
Arctic productions exist to the present day in the deeper parts 
of the temperate seas. 

I am far from supposing that all dilhculties in regard to 
the distribution and ailinities of the identical and allied spe- 
cies, which now live so widely separated in the north and 
south, and sometimes on the intermediate mountain-ranges, 
are removed on the views above given. The exact lines of mi- 
gration cannot be indicated. We cannot say why certain spe- 
cies and not others have migrated ; why certain species have 
been modified and have given rise to new forms, while others 
liave remained unaltered. We cannot hope to explain such 
facts, until we can say why one species and not another be- 
comes naturalized l^y man's agency in a foreign land ; Avhy 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 in- 
stance, the occurrence, as shown by Dr. Hooker, of tiie same 
plants at points so enormously remote as Kerguelen Land, 
New Zealand, and Fuegia; but icebergs, as suggested by Ly- 
cll, may have l)een concerned in tlieir dispersal. The existence, 
at these and other distant points of the southern hemisphere, 
of species, which, though distinct, belong to genera exclusively 
confined to the south, is a more remarkable case. Some of 
these species are so distinct, that we cannot suppose that there 
has been time since the commencement of the last Glacial 
period for their migration and subsequent modification to the 
necessary degree. The facts seem to me to indicate that dis- 
tinct species belonging to the same genera have migrated in 
radiating lines from a common centre ; and I am inclined to 
look in tiie southern, as in the northern heniisphere, to a former 
and warmer ])erio(l, before the commencement of the Ghicial 
period, when the Antarctic lands, now covere^l with ice, sup- 
ported a liighly pecuhar 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 dispersed to vari- 
ous points of the southern hemisphere l)y occasional means of 
transport, and by the aid, as halting-places, of now sunken 
islands. Thus tlie southern shores of America, Australia, and 


New Zealand, niio^'ht have become slightly tinted by the same 
peculiar forms of life. 

Sir C. Lycll in a striking' passage has speculated, in lan- 
guage almost identical Avith mine, on the effects of great alter- 
nations of climate throughout the world on geographical dis- 
tribution. And we have uow seen that Mr. Oroll's conclusion, 
that successive Glacial periods in the one hemisphere coin- 
cided with warmer periods in the opposite hemisphere, to- 
gether with the admission of the slow modification of species, 
explains a multitude of facts in the distribution of the same 
and of the allied forms of life in all parts of the globe. The 
living waters have flowed during certain periods from the 
north and afterward 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 Hving 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 com- 
pared with savage races of man, driven up and surviving in 
the mountain-fastnesses of almost every land, Avhich serve as a 
record, full of interest to us, of the former inhabitants of the 
surroundinjr lowlands. 



GEOGnAPiiicAL DisTKiBUTiox — Continued. 

Distribution of Prcsh-wnfcr Productions— On the Inhabitants of Ocranic IrJlands— 
Absence of 15atrucl)ian8 and of Terrestrial Mammals— On the Relation of tlie In- 
tuibitants of I^landx to those of the nearest Main-land— On Colouizaticm from tho 
nearest Source with subsequent Modification— Summary of the last and present 

Fresh-water Productions. 

As lakes and river-sj'stenis arc 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 for- 
midable barrier, that they never would have extended to dis- 
tant countries. But the case is exactly the reverse. Not only 
have many fresh-water species, belonging to quite different 
classes, an enormous range, but allied species prevail in a re- 
markable manner throughout the world. I well remember, 
when first collecting in the fresh waters of Brazil, feeling much 
surprise at the similarity of the fresh-water insects, shells, etc., 
and at the dissimilarity of the surrounding terrestrial beings, 
compared with those of Britain. 

But this power in fresh- water productions of ranging wide- 
ly, though so imcxpected, can, I think, in most cases be ex- 
plaiiiod by their having become fitted, in a manner highly 
useful to them, for short and frequent migrations from pond to 
pond, or from stream to stream ; and liability to wide disper- 
sal would follow from this capacity as an almost necessary con- 
seipience. We can here consider only a few cases. In regard 
to fish, I believe that the same species never occur in the 
fresh waters of distant continents. But on the same continent 
the species often range widely and almost capriciously ; for 
two river-systems will have some fish in common and some 
different. A few facts seem to favor the possibility of their 
occasi(mal transport by accidental means ; like that of the live 


fish not rarely dropped hy Avhirlwinds in India, and the vitality 
t)f their ova when removed from the Avater. ]3ut I am inclined 
to attribute the dispersal of frcsh-Avater iish mainly to changes 
in the level of the land within the recent period having caused 
rivers to flow into each other. Instances, also, coidd be given 
of this having occniTcd during floods, without any change of 
level. The wide dilTerence of the fish on the opposite sides 
of continuous mountain-ranges, -which from an early period 
must have completely prevented the inosculation of the river- 
systems, seems to lead to the same conclusion. "With respect 
to allied fresh-water fish occurring at very distant points of the 
world, no doubt there arc many cases which cannot at present 
be explained : but some fresh- Avater fish belong to very ancient 
fonns, and in such cases there will have been ample time for 
great geograpliical changes, and consequently time and means 
for such migration. In the second place, salt-water fish can 
Avith care be sloAvly accustomed to Ha'c in fresh Avater; and, 
according to Valenciennes, there is hardly a single group of 
Avhich all the members are confined exclusively to fresh Avatcr, 
so that a marine species of a frcsh-Avater group might travel 
far along the shores of the sea, and subsequently liecome modi- 
fied and adapted to the fresh Avaters of a distant land. 

Some species of fresh-Avater shells haA'e A'crA' Avide ranges, 
and allied species Avhich, on our theory, are descended from a 
common parent, and must haA'C proceeded from a single source, 
prevail throughout the Avorld. Their distribution at first per- 
plexed me much, as their OA'a are not likely to be transported 
by birds, and are immediately killed by sea-Avater, as are the 
adults. I could not even understand hoAV some naturalized 
species have spread rapidly throughout the same country. But 
tAvo facts, Avhich I have observed — and no doubt many others 
remain to be observed — throAV some light on this subject. 
^^'hen a duck suddenly emerges from a pond coA'cred Avith 
duck-Avced, I have tAvicc seen these little plants adhering to 
its back ; and it has happened to me, in remoAing a little 
duck-AVoed from one aquarium to another, that I have quite un- 
intentionally stocked the one Avith fresh-water shells from the 
other. 13ut another agency is perhaps more eftectual : I sus- 
pended a duck's feet in an aquarium, Avhere many ova of fresh- 
Avater shells Avere hatching ; and I found that numbers of the 
extremely minute and just-hatched shells craAvled on the feet, 
and clung to them so firmly that, Avhen taken out of the Avater, 
they could not be jarred off, though at a someA\hat more ad- 


vanced a;^c ihoy would voluntarily drop olT. These just- 
hatched moUusks, thoui^h aquatic in their nature, survived on 
the duck's feet, in damp air, from twelve to twenty hours ; and 
in this lenp^tli of time a duck or heron mij^ht (ly 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 Dj'tiscus 
has been cau2;ht with an Ancylus (a fresh-water shell like a 
limpet) firmly adhering- to it ; and a Avater-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 with a favoring gale no one 
can tell. 

With respect to plants, it has long been known what 
enormous ranges many fresh-water, and even marsh-species, 
have, both over continents and to the most remote oceanic 
isFands, This is strikingly shown, as remarked by Alph. de 
Candolle, in large groups of terrestrial plants, ^vluc]l have only 
a very few aquatic members ; for these latter seem immediate- 
ly to acquire, as if in consequence, a very wide range. I 
think favorable means of dispersal explain this fact. I have be- 
fore mentioned that earth occasionally, though rarely, adheres 
in some quantity to the feet and beaks of birds. Wading- 
birds, which frequent the muddy edges of ponds, if suddenly 
flushed, would be the most likely to have muddy feet. Birds 
of this order, I can show, arc the greatest wanderers, and are 
occasionally found on the most remote and barren islands in 
the open ocean ; they would not be likely to alight on the sur- 
face of the sea, so that the dirt would not be washed off their 
feet ; and, when making land, they would be sure to fly to their 
natural fresh-water haunts. I do not believe that botanists 
an; aware how charged the mud of ponds is with seeds : I 
have tried several little experiments, l)ut will here give only 
the most striking case : I took in February three table-spoon- 
fuls of mud from three different points, beneath water, on the 
edge of a little pond: this mud when dry weighed only GJ 
ounces ; I kcjit it covered up in my study for six months, pull- 
ing up and CHjunting each plant as it grew ; the plants were 
f)f many kinds, and were altogether 537 in number; and yet 
the viscid mud was all contiiined in a breakfast cup ! Consid- 
ering these facts, I think it would be an inexplic;d)l(> circum- 
stance if water-birds did not transport the seeds of the same 
fivsh-water plants to unstocked ponds and streams, situated at 


very distant points. The same ag'ency may lia\'C come into 
play with tlic e<xgs of some of the smaller fresh-water animals. 

Other and unknown agencies probably have also played a 
part. I have stated that fresh-water fish eat some kinds of 
seeds, though they reject man}' other lands after having swal- 
lowed them ; even small fish swallow seeds of moderate size, 
as of tlie yellow water-lily and Potamogeton. Herons and 
other birds, century after century, have gone on daily devour- 
ing fish ; they then take flight and go to other waters, or are 
blown across the sea ; and Ave have seen that seeds retain their 
power of germination, when rejected in pellets or in excre- 
ment, many hours afterward. AVhen I saw the great size of 
the seeds of that fine water-lily, the Nelumbium, and remem- 
bered Alph. de Candolle's remarks on this plant, I thought 
that its distribution must remain quite inexplicable ; but 
Audubon states that he found the seeds of the great southern 
water-lily (probably, according to Dr. Hooker, the Nelumbium 
luteum) in a heron's stomach ; although I do not know the 
fact, yet analogy makes me believe that a heron, fl^'ing to an- 
other pond and getting a heai-ty meal of fish, Avould probably 
reject from its stomach a pellet containing the seeds of the 
Nelumbium undigested ; or the seeds might be dropped by the 
bird Avhile feeding its young, in the same way as fish are known 
sometimes to be dropped. 

In considering these several means of distribution, it shoukl 
be remembered that Avhen a pond or stream is first formed, for 
instance, on a rising islet, it Avill be unoccupied ; and a single 
seed or e^^g will have a good chance of succeeding. Although 
there Anil always be a struggle for life between the inhabitants 
of the same pond, hoAvever few in Idnd, yet, as the number even 
in a Avell-stocked pond is small in comparison Avitli the number 
of species inhabiting an equal area of land, the competition 
Avill probably be less severe between aquatic than betAveen ter- 
restrial species ; consequently an intruder from the waters of 
a foreign country Avillhave a better chance of seizing on a ncAV 
place, than in the case of terrestrial colonists. We should also 
remember that many frcsh-AAMter productions are low in the 
scale of nature, and Ave have reason to believe that Ioav beings 
change or become modified less quickl}^ than the high ; and this 
Avill giA'e a longer time than the aA'erage for the rj;igration of 
the same aquatic species. We should not forget the probability 
of many species having formerly ranged as continuously as 
fresh-Avater productions ever can range, over immense areas 


and having subseqiienlly become extinct in intermediate re- 
gions. But the wide distribution of fresli-wuter pkxnts and of 
the lower animals, whether retaining the same identical form 
or in some degree modifieJ, I believe mainly depends on the 
wide disjiersal of their seeds and eggs by animals, more es- 
pecially by fresh-water birds, which have great poAvers of 
flight, and naturally travel from one piece of water to another. 

On the InJuihltayds of Oceanic Islcmds. 

We now come to the last of the three classes of facts, 
which I have selected as presenting the greatest amount of 
dilliculty, if we accept the view that not only all the individ- 
uals of the same species, wherever found, have migrated from 
some one area, but that allied species, although now inhabiting 
the most distant points, have proceeded from a single area — 
tlie birthplace of their early prog(Miitor. I have already stated 
that I cannot honestly admit For])es's view en continental ex- 
tensions, which, if legitimately followed out, would lead to the 
belief that all existing islands have been continuously or 
almost continuously joined to some continent within the recent 
period. This view Avould remove many dilliculties, but it 
would not explain all the facts in regard to insular produc- 
tions. In the following remarks I shall not confine myself to 
the mere question of dispersal ; but shall consider some other 
facts, which bear on the truth of the two theories of indepen- 
dent creation and of descent with modification. 

The species of all kinds which inhabit oceanic islands are 
few in number compared Avith those on equal continental 
areas : Alph. de Candolle admits this for plants, and AVollas- 
ton for insects. New Zealand, for instance, with its lofty 
moimlains and diversified stations, extending over 780 miles 
of latitufle, together with the outlying islands of Auckland, 
Campbell, and Chatham, contain altogether only DCO lands 
of flowering plants; if we compare this moderate number with 
the species which swann over equal areas in Southwestern 
Australia or at the Cape of Good Ilojie, we nnist admit that 
somclliing, quite indcpenilently of a difVerence in tlie ])hvsical 
conditions, has caused so great a difVerence in number. Even 
the uniform county of Cambridge has 847 plants, and the little 
island of Anglesea 704, but a few ferns and a few introduced 
plants are included in these nundiers, and the comparison in 
some other respects is not quite fair, "\\'e have evidence that 


the barren isluiul of Ascension aboriginally possessed less than 
half a dozen lowering plants ; yet many have now become 
naturalized on it, as they have on New Zealand and on every 
other oceanic island Avhich 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 Avorld) is often extremely large. If Ave compare, 
for instance, the number of endemic land-shells in Madeira, or 
of endemic birds in the Galapagos Archipelago, Avith the num- 
ber found on any continent, and then compare the area of the 
island Avith that of the continent, Ave shall see that this is true. 
This fact might have been theoretically expected, for, as al- 
ready explained, species occasionally arriving after long inter- 
A'als of time in a ncAV and isolated district, and having to com- 
pete AA'ith ncAV associates, Avould be eminently liable to modifi- 
cation, and Avould often produce groups of modified descend- 
ants. But it by no means folloAvs that, because in an island 
nearly all the species of one class are peculiar, those of an- 
other class, or of another section of the same class, are pecu- 
liar ; and this difference seems to depend partly on the species 
Avhich are not modified having immigrated in a body, so that 
their mutual relations haA^e not been much disturbed; and 
partly on the frequent arrival of immodified immigrants from 
the mother-country, Avith Avhich the insular forms have inter- 
crossed. It should be borne in mind that the offspring of such 
crosses AA'ould almost certainly gain in vigor ; so that even an 
occasional cross AA'ould produce more effect than might have 
been anticipated. I Avill give a fcAV illustrations of the forego- 
ing remarks : in the Galapagos Islands there are 20 land-birds; 
of these, 21 (or perhaps 23) are peculiar, AA'hereas of the 11 
marine birds only 2 are peculiar ; and it is obvious that marine 
lairds could arrive at these islands much more easily and fre- 
quently than land-birds. Bermuda, on the other hand, Avhich 
lies at about the same distance from North America as the 
Galapagos Islands do from South America, and Avhich has a 
A'cry peculiar soil, does not possess a single endemic land-bird ; 


and we know, from Mr. J. M. Jones's admirable account of Ber- 
muda, that very many North American birds occasional!}' or 
even frequently visit this island. Almost every year, as I am 
informed l)y !Mr. E. V. Harcourt, many EuropeaTi and African 
birds arc blown to Madeira ; this island is inhaljited by 99 
kinds, of which one alone is peculiar, though very closely 
related to a European form ; and three or four other species 
are confined to this island and to the Canaries. So that the 
islands of 13ermuda and Madeira have been stocked from the 
neighboring continents with birds, whicli for long ages have 
struggled together, and become mutually adapted ; hence, 
when settled in their new homes, each kind wduld be kept by 
the others to its proper place and habits, and would conse- 
quently be but little liable to modification. Any tendency to 
modification would also be checked by intercrossing with the 
unmodified immigrants from the mother-country. Madeira, 
again, is inhabited by a wonderful lumiber of peculiar land- 
sliells, whereas not one species of sea-shell is peculiar to its 
shores : now, though we do not know how sea-shells are dis- 
persed, yet we can sec that their eggs or larva?, perhaps at- 
tached to seaweed or floating timber, or to the feet of wading- 
birds, might be transported across three or four hundi'cd miles 
of open sea far more easily than land-shells. Tlie dilTerent 
orders of insects inhabiting Madeira present nearly similar 

Oceanic islands are sometimes deficient in animals of cer- 
tain Avhole classes, and their places are occupied by other 
classes: thus in the Galapagos Islands reptiles, and in New 
Zealand gigantic wingless birds, take, or recently took, the 
j)lace of mammals. Altliough New Zealand is here spoken of 
as an oceanic island, it is in some degree doubtful whether it 
should 1)0 so ranked ; it is of large size, and is not separated 
from Australia by a profoundly deep sea; from its geological 
character and tlie direction of its mountain-ranges, the Rev. 
W. 15. Clarke has lately maintained that this island, as well 
as New Caledonia, should be considered as appurtenances of 
Australia. Turning to plants. Dr. Hooker has shown that in 
the Galapagos Islands tlie ])roportional numbers of the difl'er- 
ent orders are very different from what they are elsewhere. 
All such diflerences in luimber, and the absence of certain 
whole grou]\s of animals and jilants on islands, are gtMierally 
account(>d for by su]iposed ditVerences in their ])hysical condi- 
tions; but this explanation is not a little doubtful. Facility 


of inimioTalion seoins to liavc been fully as impoptant as the 
nature of the conditions. 

JIany remarkable little facts could bo given with respect to 
the inhabit:iuts of oceanic islands. For instance, in certain 
islands not tenanted Iiy a sing-le mammal, some of the endemic 
plants have beautifully hooked seeds; and few relations arc 
more manifest than that hooked seeds are adapted for trans- 
portal in the wool or fin- of quadi-upeds. But a hooked seed 
might be carried to an island by other means ; and the plant 
then becoming modified Avould form an endemic species, Avhich 
might still retain the hooks, Avhich would not form a more use- 
less appendage than the shrivelled wings imder the soldered 
elytra of many insular beetles. Again, islands often possess 
trees or bushes belonging to orders elsewhere including only 
herbaceous species ; now trees, as Alph. de Candolle has shown, 
generally have, whatever the cause may be, confined ranges. 
Hence trees Avould be little likely to reach distant oceanic 
islands ; and an herbaceous plant, which had no chance of suc- 
cessfully competing Avitli the many fully-developed trees grow- 
ing on a continent, might, when established on an island, gain 
an advantage by growing taller and taller and overtopping the 
other herbaceous plants. In this case, natural selection would 
tend to add to the stature of the plant, to whatever order it 
belonged, and thus convert it first into a bush and then into a 

Absence of Batrachians and Terrestrial j[Tammals on Oce- 
anic Islands. 

"With resjiect to the absence of whole orders of animals on 
oceanic islands, I5ory St. Vincent long ago remarked that Ba- 
trachians (frogs, toads, newts) are never found on any of the 
many islands with which the great oceans are studded. I have 
taken pains to verify this assertion, and have found it strictly 
true, with the exception of New Zealand, of the Andaman 
Islands, and perhaps of the Salomon Islands. But I have al- 
ready remarked that it is doubtful whether New Zealand ought 
to be classed as an oceanic island ; and this is still more doubt- 
ful with respect to the Andaman and Salomon groups. This 
general absence of frogs, toads, and newts, on so many oceanic 
islands cannot be accounted for by their physical conditions; 
indeed, it seems that islands are peculiarly well fitted for these 
animals: for frogs have been introduced into Madeira, the 


Azores, and Mauritius, and liavo multiplied so as to become a 
nuisance. But, as these animals ami their spawn are known to 
be immediately killed hy sea-wat<^r, there wovdd be great dilfi- 
culty in their transportal across the sea, and therefore we can 
sec why they do not exist on any oceanic island. But why, on 
tlie theory of creation, they should not have been created there, 
it would be very dillicult to explain. 

Mammals ofler another and similar case. I have carefully 
searched the oldest voyao-es, and as yet I have not found a 
single instance, free frt)m doubt, of a terrestrial mammal (ex- 
cluding domesticated animals kept by the natives) inhabiting' 
an island situated above 300 miles from a continent or great 
continental island ; and many islands situated at a much less 
distance are equally l)arren. The Falkland Islands, which are 
inhabited by a wolf-like fox, come nearest to an exception ; but 
tlws group cannot be considered as oceanic, as it lies on a bank 
in connection with the main-land at the distance of about 280 
miles ; moreover, icebergs formerly brought bowlders to its 
western shores, and they may have formerly transported foxes, 
as now frequently happens in the arctic regions. Yet it cannot 
be said that small islands will not support at least small mam- 
mals, for they occur in many parts of the world on very small 
islands, when lying close to a continent; and hardly an island 
can be named on which oin- smaller quadrupeds have not be- 
come naturalized and greatly nuiltiplicd. It capnot be said, 
on the ordinary view of creation, tliat there has not been time 
for the creation of mammals; many volcanic islands are sufTi- 
ciently ancient, as shown l)y the stupendous degradation which 
they have suffered, and by their tertiary strata : there has also 
bcHMi time for the production of endemic species belonging to 
other classes ; and on continents it is known that mammals ap- 
pear and disappear at a quicker rate than any other and lower 
animals. Although terrestrial mammals do not occur on oce- 
anic islands, aerial mammals do occur on almost every island. 
New Zealand possesses two bats found nowhere else in the 
■world : Norfolk Island, the Viti Archipelago, the Bonin Islands, 
the Caroline and Mariannt" Archipelagoes, and Mauritius, 
all possess their jieculiar bats. N\ hy, it may be asked, has 
the supposed creative force ])roduced bats and no other mam- 
mals on remote islands ? On my view this (juestion can easily 
be answered; for no terrestrial mammal can be transported 
across a wide space of sea, but bats can fly across. Bats have 
been seen wandering by d:iy far over the Atlantic Ocean; and 


two North American species either ref^ularly or occasionally 
visit Bermuda, at the distance of GOO miles from the main-land. 
I hear from Mr. Tomes, who has specially studied this family, 
that many species have enormous ranj^es, and are found on con- 
tinents and on far-distant islands. Hence we have only to sup- 
pose that such wandering species have been modified in their 
new homes in relation to their new position, and we can under- 
stand the presence of endemic bats on oceanic islands, with the 
absence of all other terrestrial mammals. 

Another interesting relation exists, namely, between the 
depth of the sea separating islands from each other or from the 
nearest continents, and the degree of affinity of their mamma- 
lian inhabitants. Mr. Windsor Earl has made some striking 
observations on this head, since greatly extended by Mr. Wal- 
lace's admirable researches, in regard to the great Malay Ar- 
chipelago, which is traversed near Celebes by a space of deep 
ocean, and tliis separates two widely-distinct mammalian fau- 
nas. On either side the islands stand on a moderately shallow 
submarine bank, and these are inhabited by the same or by 
very closely-allied quadrupeds. I have not as yet had time to 
follow uj") this subject in all quarters of the world ; but, as far 
as I have gone, the relation generally holds good. For in- 
stance, Britain is separated by a shallow channel from Europe, 
and the mammals are the same on both sides ; and so it is with 
all the islands near the shores of Australia. The West-Indian 
Islands, on the other hand, stand on a deeply-submerged bank, 
nearly 1,000 fathoms in depth, and here we find American 
forms, but the species and even the genera arc quite distinct. 
As the amount of modification which animals of all kinds un- 
dergo partly depends on the lapse of time, and as islands sep- 
arated from each other or from the main-land by shallow chan- 
nels are more likely to have been continuously united within a 
recent period than islands separated by deeper channels, we 
can imdcrstand how it is that a relation exists l)etween the 
depth of the sea separating two mammalian fiiimas, and the de- 
gree of their affinity — a relation which is quite inexplicable on 
the theory of independent acts of creation. 

The foregoing statements in regard to the inhabitants of 
oceanic islands — namelv, the fewness of the species, with a 
large proportion consisting of endemic forms — the members 
of certain groups, and not of other groups in the same class, 
having hoen modified — the absence of certain whole orders, as 
of batrachians and of terrestrial mammals, notwithstanding the 


presence of aerial bats — the singular proportions of certain 
orders of plants — herbaceous forms having been develoi)cd into 
trees, etc., seem to me to accord better with the belief in the 
cfiiciency of occasional means of transport, carried on during a 
long course of time, than Avith the belief in the former connec- 
tion of all oceanic islands with the nearest continent ; for, on 
this latter view, it is probable that the various classes would 
have immigrated more uniformly, and, from the species having 
entered in a body, their mutual relations would not have been 
much distiu-bed, and consequently tliey would have been modi- 
fied citlier not at all or in a more equal manner. 

I do not deny that there arc many and serious difficulties 
in understanding how many of the inhabitants of the more 
remote ishxnds, whether still retaining the same specific form 
or subsequently modified, have reached their present homes. 
Hut the probability of islands having existed as halting-places, 
of which not a wreck now remains, must not he overlooked. 
T will specify one such dillicult case. Almost all oceanic 
islands, even the most isolated and smallest, are inhabited l)y 
land-shells, generally by endemic species, but sometimes l)y 
species found elsewhere — striking instances of which have 
been given by Dr. A. A. Gould in relation to the Pacific. 
Kow, it is notorious that land-shells are easily killed by sea- 
water; their eggs, at least such as I have tried, sink in it and 
are killed. Yet there must l)e, according to our view, some 
luiknown, but occasionally eflicient, means for their trauspor- 
tal. ^Vould the just-hatched young sometimes adhere to the 
fe(>t of l)irds roosting on the ground, and thus get transported? 
It occurred to me that land-shells, when hj'bcrnating, and hav- 
ing a nuMnljranous dia]ihragm over the mouth of the shell, 
might be floated in chinks of drifted tunl)er across moderately 
wide arms of the sea. And I found that several species in this 
state withstood uninjured an immersion in sea-water during 
seven days : one shell, the Helix pomatia, after having been 
thus treated and again hybernating, was put into sea-water for 
twenty da\s, and ])erfectly recovered. During tliis length of 
time tlie shell might have been carried, by a marine current of 
average sv.iftness, to a distance of GGO geogra])liical miles. 
As this Helix has a thick calcareous operculum, I removed it, 
and, when it had formed a new membranous one, I again im- 
mersed it for fourteen days in se:irwater, and again it recov- 
ered and crawled away. Baron Aucapitaine has recently tried 
similar experiments: he placed 100 land-shells, belonging to 


ten species, in :i ])ox pierced with holes, and immersed it for a 
fortnight in the sea. Out of the hundred shells, twenty-seven 
recovered. The presence of an ojierculum seems to have been 
of importance, as out of twelve sj^ecimcns of Cydostoma cle- 
gans, which is thus furnished, eleven revived. It is remarkable, 
seeing how well with me the Helix pomatia resisted the salt- 
water, that not one of fifty-four specimens, l^elonging to four 
species of Helix tried by Aueapitaine, recovered. It is, how- 
ever, not at all probable that land-shells have often been thus 
transported ; the feet of birds is a more probable method. 

On the Relations of the Inhabitants of Islaiids to those oftJie 
nearest Main-land. 

The most striking and important fact for us is the affinity 
of the species which inhabit islands to those of the nearest 
main-land, without being actually the same. Numerous in- 
stances could be given. The Galapagos Archipelago, situated 
under the equator, lies at the distance of between 500 and GOO 
miles from the shores of Soutli America. Here almost every 
jiroduct of the land and the water l)ears the unmistakable 
stamp of the American Continent. There are twenty-six land- 
birds, and twenty-one, or, perhaps, twenty-three, of these arc 
)"anked as distinct species, and are supposed to have been 
created here ; yet the close affinity of most of these birds to 
American species in every character, in their habits, gestures, 
and tones of A^oice, was manifest. So it is with the other ani- 
mals, and with a large proportion of the plants, as shown by 
Dr. Hooker in his admirable Flora of this archipelago. The 
naturalist, looking at the inhabitants of these volcanic islands 
in the Pacific, distant several hundred miles from the continent, 
yet feels that he is standing on American land. Why should 
this be so ? why should the species which are supposed to 
have been created in the Galapagos Archipelago, and nowhere 
else, bear so plain a stamp of affinity to those created in Amer- 
ica ? There is nothing in the conditions of life, in the geologi- 
cal nature of the islands, in their height or climate, or in the 
proportions in which the several classes are associated to- 
gether, which closely resembles the conditions of the South 
American coast : in fact, there is a considerable dissimilarity 
in all the foregoing respects. On the other hand, there is a 
considerable degree of resemblance in the volcanic nature of 
the soil, in the climate, height, and size of the islands, be- 


twccn the Galap.'ifjos aiul Capo dc Verde Arcliipclaii-oes : but 
Avhat an entire and absolute difTorence in their inlialjitants ! 
The inliabitants of the Cape de Verde Islands are related to 
those of Africa, like those of the Galapagos to America. 
Facts such as these admit of no sort of explanation on the 
ordinary view of independent creation ; whereas, on the view, 
here maintained, it is obvious that the Galapagos Islands 
would be likely to receive colonists, whether by occasional 
means of transport or by formerly continuous land, fi'om Amer- 
ica ; the Cape de Verde Islands from Africa : and that such 
colonists would be liable to modification — the principle of in- 
heritance still betraving their original birthplace. 

Many analogous facts could be given : indeed, it is an al- 
most universal rule that the endemic productions of islands 
are related to those of the nearest continent, or of the nearest' 
island. The exceptions are few, and most of them can be 
explained. Thus, although Kerguelen Land stands nearer to 
Africa than to America, the plants are related, and that very 
closely, as we know from Dr. Hooker's accoimt, to those of 
America : but, on the view that this island has been mainly 
stocked by seeds brought with earth and stones on icel)ergs, 
drifted by the prevailing currents, this anomaly disappears. 
New Zealand in its endemic plants is much more closely re- 
lated to Australia, tlie nearest main-land, than to any other 
region: and this is what might have been ex])ccted ; but it is 
also plainly related to South America, which, although the 
next nearest continent, is so enormously remote, that the fact 
becomes an ancjinaly. But this difFiculty almost disajipears on 
the view that New Zealand, South America, and tlie other 
southern lands, have been partially stocked from a nearly in- 
termediate though distant point, namely, from the antaictic 
islands, when they were clothed with vegetation, during a 
warmer tertiary period, before the commencement of the last 
Glacial period. The allinity, which, though feeble, I am 
assured by Dr. Hooker is real, between the flora of the S(juth- 
western corner of Australia and of the Ca]ie of Good Ilojie, is 
a far more remarkable case ; but this ailinity is ccmlined to the 
])lan(s, and will, no doubt, be some day explained. 

The same law which has determined the relati(mshi|) be- 
tween the inhabitants of islands and the nearest main-land i.s 
sometimes displayed on a small scale, but in a most interesting 
manner, within tlie limits of the same archipelago. Thus each 
separate island of the Galapagos Archipelago is ti-nanted, and 


the fact is a iiiarvcllous ono, by distinct species; but these 
species arc related in a very much closer manner to each other 
than to the inhabitants of any other quarter of the world. Tliis 
is what mii^ht have been expected, for islands situated so near 
each other would almost necessarily receive immi<Ti"ants from 
the same orig'inal source, and from each other. But how is it 
that many of the immif^rants have been differently modified, 
though only in a small degree, in islands situated within sight 
of each other, having the same geological nature, the same 
height, climate, etc. ? This long appeared to me a great diffi- 
culty: but it ai-ises in chief part from the deeply-seated error 
of considering the physical conditions of a country as the most 
important ; whereas it cannot be disputed that the nature of 
the other inhabitants with Avhich each has to compete is at 
least as important, and generally a far more important element 
of success. Now, if we look to the species Avliich inhabit the 
Galapagos Archipelago and are likewise found in other parts 
of the world, Ave find that they differ considerably in the sev- 
eral islands. This difference might indeed have been expected 
if the islands have been stocked by occasional means of trans- 
port — a seed, for instance, of one plant having been brought to 
one island, and that of another plant to another island, though 
all proceeding from the same general source. Hence, when in 
former times an immigrant first settled on one of the islands, 
or when it subsequently spread from one to another, it would 
undoubtedly be exposed to different conditions in the different 
islands, for it would have to compete with a different set of or- 
ganisms : a plant, for instance, would find the ground best 
fitted for it occujiied by somewhat different species in the dif- 
ferent islands, and would be exposed to the attacks of some- 
what dillerent enemies. If, then, it varied, natural selection 
would probably ffivor different varieties in the different islands. 
Some species, however, might spread and yet retain the same 
character throughout the group, just as we see some species 
spreading widely throughout a continent and remaining the 

The really surprising fact in this case of the Galapagos 
Archipelago, and in a lesser degrcx; in some analogous cases, 
is, that each new species, after being formed in any one island, 
did not quickly spread to the other islands. But the islands, 
though in siglit of each other, are se])arated by deep arms of 
the sea, in most cases wider than the Britisli Channel, and 
there is no reason to suppose that they have at any former 


period boon continuously uuitccL The currcuts of the sea arc 
rapid and sweep across the archipela;^o, and gales of wind arc 
extraordinarily rare ; so that the islands are far more ciTcct- 
iially separated from each other than they appear on a map. 
Nevertlieless, some of tlie species, both those foimd in other 
parts of tlic world and tliose confined to the archii)elag-o, are 
common to the several islands; and we may infer from their 
present manner of distribution that they have spread frona one 
island to the othei-s. But we often take, I think, au erroneous 
view of the probability of closely-allied species invading each 
other's territory, when put into free intercommunication. Un- 
doubtedly, if one species has any advantage over another, it 
will in a very brief time wholly or in part supplant it ; but, if 
both are equally well fitted for their own places, both proba- 
bly will hold their jilaces and keep separate for almost any 
length of time. Being familiar with the fact that many spe- 
cies, naturalized through man's agency, have spread with as- 
tonishing raj)idity over wide areas, we are apt to infer that 
most species would thus spread ; but we should remember that 
the speci(;s which become naturalized in new countries are not 
generally closely allied to the aboriginal inhabitants, but are 
very distinct forms, belonging in a large proportion of cases, 
as shown by Alph. de CandoUe, to distinct genera. In the 
(jrulapagos Archipelago, many even of the birds, though so 
well adapted for Hying from island to island, are distinct on 
each ; thus there are three closely-allied species of mocking- 
thrush, each confined to its own island. Now let us suppose 
the mocking-thrush of Cliatham Island to be blown to Charles 
Island, which has its own mocking-thrush ; why should it suc- 
ceed in establishing itself there ? We may safely infer that 
Charles Island is well stocked with its own species, for aiuiu- 
ally more eggs are laid and yv>ung birds hatched, than can 
possibly be reared ; and we may infer that the mocking-tlirush 
l)eculiar to Charles Island is at least as well fitted for its home 
as is the species pecuhar to Chatham Island. Sir C. Lyell and 
Mr. Wollaston have communicated to me a remarkable fact 
bearing on this subject ; namely, that Madeira and the adjoining 
islet of Porto Santo possess many distinct but representative 
species of land-shells, some of which live in crevices of stone; 
and, although large quantities of stone are annuallv transported 
from Porto Santo to Madeira, yet this latter island has not be- 
come coloniz(vl by the Porto Santo species : nevertheless, both 
islands have been colunized by European land-shells, which no 


doubt liad some advanta<Te over tlie indig-cnous species. From 
these considerations I think wc need not g^reatl}- marvel at the 
endemic and representative species, Avliicli inhabit the several 
islands of tlic Galapag-os Archipelago, not Laving universally 
spread from island to island, Preoccu])ation has also probably 
played an important part in checking the connningling of the 
species Avhich inhabit different districts with nearly the same 
physical conditions on the same continent. Thus, the south- 
cast and southwest corners of Australia have nearly the same 
physical conditions, and are united by continuous land, yet 
they are inhabited by a vast number of distinct mammals, 
birds, and plants. 

The same principle which governs the general character of 
the inhal)itants of oceanic islands, namely, their relation to tho 
source whence colonists could have been most easily derived, 
together with their subsequent modification, is of the widest 
apjilication throughout Nature. We see this on every moun- 
tain-summit, in every lake and marsh. For Alpine species, 
cxcci)ting in as far as the same species have become widely 
spread during the recent Glacial epoch, are related to those of 
the surrounding lowlands : thus we have, in South America, 
Aljiine humming-birds, Alpine rodents, Alpine plants, etc., all 
strictly belonging to American forms ; and it is obvious that 
a mountain, as it liccamo slowly upheaved, Avould he colonized 
from the surrounding lowlands. So it is with the inhabitants 
of lakes and marshes, excepting in so far that great facility of 
transport has allowed many of the same species to prevail 
through large portions of the world. AVe see this same prin- 
ciple in the character of most of the blind animals inhabiting 
the caves of America and of Europe. Other analogous facts 
could be given. It will, I believe, be universally found true, 
that wherever in two regions, let them be ever so distant, 
many closely-allied or representative species occur, there will 
likewise be found some identical sjiecies ; and wherever many 
closely-allied species occur, there Avill be found many fonns 
wliich some naturalists rank as distinct species, and others as 
mere varieties ; these doubtful forms showing us the steps in 
the progress of modification. 

The relation l)etweeu the power and extent of migration in 
certain species, either at the present or at some fomier jieriod, 
and the existence at remote jioints of the world of allied spe- 
cies, is shown in another and more general way. Mr. Gould 
remarked to me long ago, that, in those genera of birds which 


range over tlic world, many of the species have very wide 
rana^es. I can hardly doubt that this rule is generally true, 
though it would be difficult to prove it. Among mammals, we 
see it strikingly displayed in bats, and in a lesser degree in 
the Felid;e and CanidiV. We see the same rule in the distribu- 
tion of butterflies and beetles. So it is with most of the in- 
habitants of fresh water, for many of the genera in the most 
distinct classes range over the world, and many of the species 
have enonnous ranges. It is not meant that all^ but that some 
of the species in the genera which range very widely, have 
themselves very wide ranges. Nor is it meant that the species 
in such genera have on an average a very wide range ; for this 
will largely depend on how far the process of modilicatiou has 
gone; for instance, two varieties of the same species inhabit 
America and Europe, and thus the species has an immense 
rjtnge ; but, if variation were to be carried a little further, the 
two varieties would be ranked as distinct species, and the 
range Avould be greatly reduced. Still less is it meant, that 
species which have the capacity of crossing barriers and ranging 
widely, as in the case of certain powerfullj'-winged birds, will 
necessarily range widely ; for we should never forget that to 
range widely implies not only the power of crossing barriers, 
but the more important power of being ^'ictorious in distant 
lands in the struggle for life with foreign associates. But, ac- 
cording to the vi(.'w that all the species of the same genus, 
though now distributed at the most remote points of the worhl, 
are descended from a single progenitor, we ought to find, and 
I believe as a general rule we do find, that some at least of the 
species range very widely. 

We should bear in mind, in relation to all organic beings, 
that many genera arc of very ancient origin, and the species in 
this case will have had ample time for dispersal and subsequent 
modificati(.)n. There is also reason to believe, fiom geological 
evidence, that Avithin each great class the lower organisms 
change at a slower rate than the higher; consequently they 
will liave had a better chance of ranging widely and of still re- 
taining the same sjjeeific character. This fact, together with 
the seeds and eggs of almost all lowly-organized forms l)eing 
very minute and better fitted for distant transportal, probably 
accounts for a law which has long been observed, and which 
has lately been discussed by Alpli. de CandoUe in regard to 
plants, namely, that the lower any group of organisms stands, 
the more widely it ranges. 


The relations just discussed — namely, lower oriranisms ran- 
ging' more "widely than the higher — some of the species of widely- 
ranging genera themselves ranging widely — such facts, as al- 
l)ine, lacustrine, and marsh productions being generally related 
to those which live on the surrounding low-lands and drylands 
— the striking relationship between the inhabitants of islands 
and tliose of the nearest main-land — the still closer relationship 
of the distinct inhabitants of the islands in the same archipelago 
— are inexplicable on the ordinary view of the independent 
creation of each species, l)ut arc explicable if we admit coloni- 
zation from the nearest or readiest source, together with the 
subsequent adaptation of the colonists to their new homes. 

Summary of the last and 2)^'c^C}it Chapter. 

In these chapters I have endeavored to show that, if we 
make due allowance for our ignorance of the full effects of the 
changes of climate and of the level of the land, which have cer- 
tainly occurred within the recent period, and of other changes 
which have probably occurred — if Ave remember how ignorant 
M'e are with respect to the many curious means of occasional 
transport — if we bear in mind how often a species may have 
ranged continuously over a wide area, and then have become 
extinct in the intermediate tracts — the difficulty is not insuper- 
able in believing that all the individuals of the same species, 
wherever found, are descended from common parents. And 
we are led to this conclusion, which has been arrived at by 
many naturalists under the designation of single centres of 
creation, by various general considerations, more especially 
from the importance of barriers of all kinds, and from the ana- 
logical distribution of sub-genera, genera, and families. 

With respect to distinct species belonging to the same 
genus, Avhich on our theory must have spread from one parent- 
source ; if Ave make the same allowance as before for our igno- 
rance, and remember that some forms of life have changed A'cry 
slowly, enormous periods of time having been thus granted for 
tlieir migration, the difficulties are far from insuperable; though 
in this case, and in that of the individuals of the same species, 
they are often great. 

As exemplifying the effects of climatal changes on distribu- 
tion, I liaA'c attempted to shoAV how important a part the Gla- 
cial period has played, Avhich affected even the equatorial 
regions, and Avhich, during the alternations of the cold in the 


north and soutli, allowed the productions of ojipositc hemi- 
spheres to mingle, and left some of them stranded in all parts 
of the world on the mountain-summits. As showing how diver- 
sified are the means of occasional transport, I have discussed 
at some little length the means of dispersal of fresh-water pro- 

If the difliculties be not insuperable in admitting that in 
the long course of time all the individuals of the same species, 
and likewise of the several species belonging to the same ge- 
nus, have proceeded from some one source ; then all the grand 
leading facts of geographical distribution are explicable on the 
theory of mioTation, together with subsetiuent modification and 
the multiplication of new forms. We can thus understand the 
high imixtrtancc of barriers, whether of land or water, in not 
only s{'j)arating, but in apparently forming the several zoologi- 
cal and botanical provinces. We can thus understand the 
concentration of related species within the same areas; and 
how it is that under different latitudes, for instance in South 
America, the inliabitants of the i>lains and mountains, of the 
forests, marshes, and deserts, are linked together in so mysteri- 
ous a manner, and are likewise linked to the extinct beings 
which fonnerly inhabited the same continent. Bearing in mind 
that the mutual relation of organism to organism is of the high- 
est importance, we can see why two areas liaving nearly the 
same phj'sical conditions sliould often be inhabited by very 
different forms of life ; for, according to the length of time 
which has elapsed since the colonists entered one of the regions, 
or both; according to the nature of the communication which 
allowed certain forms and not others to enter, either in greater 
or lesser numbers; according or not as those which entered 
happened to come into more or less direct coinpetiti(m with 
each other and with the aborigines ; and according as the im- 
migrants were capable of varying more or less rapidly, there 
would ensue in the two or more regions, indejiendently of their 
phj-sical conditions, infinitcly-diver.sified conditions of life — 
there would be an almost endless amount of organic action and 
reaction — and we should lind, as we. do find, some groups of 
beings greatly, and some only slightly, modified — some devel- 
oped in great force, some existing in scanty numbers — in the 
several great geographical provinces of the world. 

On these same principles we can undc^rstand, as I have en- 
deavored to show, why oceanic islands should have few in- 
habitants, but of these a great number should be endemic or 


peculiar; aii<l 'wliy, in rclalion to tlie means of iiii<>ration, one 
group of beings, even Avithin the same class, should have all its 
species peculiar, and another group should have all its species 
the same Avith those in other quarters of the world. We can 
see why "whole groups of organisms, as batrachians and terres- 
trial mammals, should be absent from oceanic islands, Avhilc the 
most isolated islands should possess their own peculiar species 
of aerial mammals or bats. We can see why there should be 
some relation between the presence, in islands, of mammals, in 
a more or less modified condition, and the depth of the sea be- 
tween such islands and the main-land. We can clearly see why 
all the inhabitants of an archipelag-o, though specifically distinct 
on the several islets, should be closely related to each other, 
and likewise be related, but less closely, to those of the nearest 
ccnitinent or other source whence immigrants might have been 
derived. AVe can sec why, in two areas, however distant from 
each other, where very closely-allied or representative species 
exist, there should almost always exist some identical species. 
As the late Edward Forbes often insisted, there is a striking 
parallelism in the laws of life throughout time and space: the 
laws governing the successioni of fonns in past times being 
nearly the same with those governing at the present time the 
differences in difTerent areas. We see this in many facts, llie 
endurance of each species and group of species is continuous in 
time ; for the exceptions to the rule are so few, that they may 
be fairly attributed to our not having as yet discovered in an 
intermediate dei")Osit certain forms which are absent in it, but 
which occur both above and below : so in space, it certainly is 
the general rule that the area inhabited by a single species, or 
by a grouji of species, is continuous, and the exceptions, which 
are not rare, may, as I have attempted to show, be accounted 
for by former migrations mider difTerent circumstances, or 
through occasional means of transport, or by the species having 
become extinct in the intermediate tracts. Both in time and 
space, species and groups of species have their points of maxi- 
mum development. Groups of species, living during the same 
j)eri(jd of time, or living jvitliin the same area, are often char- 
acterized by trifling features in common, as of sculpture or 
color. In looking to the long succession of past ages, as in 
now looking to distant provinces throughout the world, we 
find that species in certain classes differ little from each other, 
while others in a different class, or only in a different family of 
the same order, differ greatly from each other. In both time 


and space the lowly-orn^auized members of each class generally 
change less than the highly-organized ; but there are in botli 
cases marked exceptions to the rule. According to our theory 
these several relations throughout time and space arc intelli- 
gible ; for, whether we look to the forms of life which have 
changed during successive ages, or to those which have changed 
after having migrated into distant quarters, in both cases the 
forms witliin each class arc connected by the same bond of or- 
dinary generation ; and in both cases the laws of variation have 
been the same, and modifications have been accumulated by 
the same means of natural selection. 




Classifiriition. Groups subordinate fn Group? — Natural System— Rnlcs ;ind Difficiiltica 
in Chissilio.'UiDii. explained on tlio Theory of Descent with Moditicalion — Classi- 
fication of Vnrieliew— Descent alwavH used in Classification — Analofrical or Adap- 
tive Characters — Affinities, General. Complex, and liadiatincj— Extinction sepa- 
rates and defines Groups — Morpliolou'V- between members of the same Class, be- 
tween parts of the same Individual — Embryoloiry. Laws of. explained by Vari.i- 
tions not supervening at an early Age, and being inherited at a corresponding 
Age — Kudimeutary Organs ; their Origin explainuU — Suinniar3'. 


From a very remote period in the history of the world or- 
ganic beings have resembled each other iu descending degrees, 
so that they can be classed in groups under groups, litis 
classiftcation is not arbitrary, like the grouping of the stars in 
constellations. The existence of groups would have been of 
simple signification, if one group had been exclusively fitted to 
inhtibit the land, and another tlie water; one to feed on flesh, 
another on vegetable matter, and so on ; but the case is widely 
different in Nature ; for it is notorious how commonly members 
of even the same sub-group have dilfercnt habits. In the second 
:ind fourth chapters, on Variation and on Natural Selection, I 
have attempted to show that within each country it is the 
Avidel^'-ranging, the much-dilfused and common, that is, the 
dominant sj^ccies belonging to the larger genera in each class, 
which vary most. The varieties, or incipient species, thus pro- 
duced, ultimately become converted into new and distinct spe- 
cies ; and these, on the principle of inheritance, tend to pro 
duce other new and dominant species. Consequently, the 
groups which are now large, and which generally include many 
dominant sjiecies, tend to go on increasing in size. I further 
attempted to show that, from the varying descendants of each 
species t tying to occupy as many and as different places as pos- 


siblc in the economy of Nature, there is a constant tendency in 
their characters to diverge. This latter conclusion is supported 
by observing tlio great diversity of forms which in any small 
area come into the closest competition, and by certain facts in 

I attempted also to show that there is a constant tendency 
in the forms -which arc increasing in number and diverging in 
character, to supplant and exterminate the preceding, less 
divergent and less improved forms. I request the reader to 
turn to the diagram illustrating the action, as formerly ex- 
])lained, of these several jmnciples ; and he will see that the 
inevitable result is, that tlie modilied descendants proceeding 
from one progenitor become broken up into groups subordi- 
nate to groups. In the diagram each letter on the uppermost 
line may represent a genus including sevei'al species ; and the 
whole of tlie genera along this upper line form together one 
class, for all arc descended from one ancient parent, and, con- 
sequentl}', have inherited something in common. But the 
three genera on the left hand have, on this same principle, 
nnich in common, and form a sul>family, distinct from that in- 
cluding the next two genera on the right hand, w^hich diverged 
from a common parent at the fifth stage of descent. These 
five genera have also much, though less, in common ; and they 
form a faniilv distinct from that including the three genera still 
fartlier to tlie right liand, which diverged at a still earlier 
period. And all these genera, descended from (A), form an 
order distinct from the genera descended from (1). So tliat 
we here have many species descended from a single progenitor 
groujied into genera ; and the genera in sub-families, families, 
and orders, all in one great class. Thus, the grand fact of the 
natural suliordination of all organic beings in groups under 
groups, which, from its familiarity^, does not always sulliciently 
strike us, is in my judgment explained. No doubt organic 
lieings, like all other objects, can be classed in many ways, 
either artificially by single characters or more naturally by a 
number of characters. We know, for instance, that minerals 
and the elemental substances can be thus aiTanged ; in this 
case there is of course no relation to genealogical succession, 
and no cause can at present be assigned for tlicir falling into 
group.s. But with organic beings the case is different, and the 
view above given explains their natural an-angement in group 
under gr(jup ; and no other explanation has ever hccn at- 


Naturalists, as we have seen, try to arraiioc the species, 
genera, and families in each class, on what is called the Natu- 
ral System. But -what is meant by this system ? Some 
authors look at it merely as a scheme for arranginj^ tog'ethcr 
those llvina^ objects which are most alike, and for separating 
those which are most inilike ; or as an artificial means for 
enunciating, as briefly as possible, general propositions — that 
is, by one sentence to give the characters common, for instance, 
to all mammals, by another those common to all carnivora, by 
another those conunon to the dog-genus, and then, by adding 
a single sentence, a full description is given of each kind of 
dog. The ingenuity and utility of this system are indispu- 
table. But many naturalists think that something more is 
meant ])y the Natural System ; they believe that it reveals the 
plan of the Creator ; but, unless it be specified Avhether order 
in time or space, or both, or what else is meant by the plan 
of the Creator, it seems to me that nothing is thus added to 
our knowledge. Such expressions as that fiimous one by Lin- 
nieus, and which wc often meet with in a more or less con- 
cealed form, that the characters do not make the genus, but 
that the genus gives the characters, seem to imply that some- 
thing more is included in our classification than mere resem- 
blance. I believe that something more is included, and that 
propin(juity of descent — the only known cause of the similarity 
of organic beings — is the bond, hidden as it is by various de- 
grees of modification, which is partially revealed to us by our 

Let us now consider the rules followed in classification, 
and the difficulties which are encountered, on the view that 
classification either gives some imknown plan of creation, or is 
simply a scheme for enunciating general propositions and of 
jilacing together the forms most like each other. It might 
have been thought (and Avas in ancient times thought) that 
those parts of the structure which determined the habits of life, 
and the general place of each being in the economy of Nature, 
would be of very high importance in classification. Nothing 
can be more false. No one regards the external similarity of a 
mouse to a shrew, of a dugong to a whale, of a whale to a fish, 
as of any importance. These reseml)lances, though so intimate- 
ly connected with the whole life of the being, are ranked as 
merely " adaptive or analogical characters ; " but to the con- 
sideration of these resemblances we shall recur. It may even 
be given as a general rul(>, that the less nny part of the organi- 


zation is concerned with special habits, the more important it 
becomes for chissilication. As an instance — Owen, in speak- 
inf];' of the dufjonp:, says : " The generative or<^ans, Ijein^r those 
wiiich are most remotely related to the habits and food of an 
animal, I have always regarded as alTordinti; very clear indica- 
tions of its true airmities. We are least likely in the modifi- 
cations of these organs to mistake a merely adaptive for an 
essential character." With plants, how remarkable it is that 
the organs of vegetation, on which tlieir nutrition and life de- 
]icnd, are of little signification ; whereas the organs of repro- 
duction, with their product tlie seed and embr^'o, are of ])ara- 
mount importance ! So, again, in formerly discussing morpho- 
logical dilleronces M'hich are not jihysiologically important, we 
have seen that they are often of the highest service in classifi- 
cation. Tliis depends on their constancy througliout many 
allied groups ; and the constancy depends chiefly on any shght 
deviations of structure in such parts not having been pre- 
served and accumulated by natural selection, wliich acts only 
on useful characters. 

That tlie mere jihysiological importance of an organ does 
not determine its classificatory value, is almost proved by the 
fact that in allied groups, in which the same organ, as w^e have 
every reason to suppose, has nearly the same physiologi- 
cal value, its classificatory value is widely difltM-ent. No 
naturalist can have worked at aily group without being struck 
with this fact ; and it has been fully acknowledged in the 
writings of almost every author. It will suffice to quote the 
highest aulhorily, Kobert ]}ro'\\Ti, who, in speaking of certain 
organs in the Proteacere, says, their generic importance, " like 
that of all their parts, not only in this, Init, as I apprehend, in 
every natural familv, is verv unequal, and in some cases seems 
to be entirely lost." Again, in another work, he says, the 
genera of the Connaracea? " differ in having one or more 
ovaria, in the existence or absence of albumen, in the imbri- 
cate or valvular rcstivation. Any one of these characters 
singly is fref|uently of more than generic importance, though 
here even when all taken together they a])pear insufficient 
to separate Cnestis from Connarus." To give an example 
among insects: in one great division of the Ilymenoptera, 
the antenna?, as Westwood has remarked, arc most constant 
in structure ; in another division they differ nuich, and the 
differences are of (juitc subordinate vahie in classification ; yet 
no one will say that the antennif in these two divisions of the 


same order are of unequal jilijsiological importance. Any 
number of instances could be given of the varying importance 
for classification of the same important organ williin the same 
group of beings. 

Agahi, no one Avill say that rudimentary or atrophied or- 
gans are of high physiological or ^ital importance ; ^-et, un- 
doubtedly, organs in this condition are often of high value in 
classification. No one will dispute that the rudimentary teeth 
in the upper jaws of young ruminants, and certain rudimentary 
bones of the leg, are highly serviceable in exhibiting the close 
affinity between Ruminants and Pachyderms. Robert Brown 
has strongly insisted on the fact that the position of the rudi- 
mentary florets is of the highest importance in the classification 
of the Grasses. 

Numerous instances could be given of characters derived 
Ij-om parts which must be considered of Acry trifling physiolo- 
gical importance, but which are universally admitted as highly 
serviceable in the definition of Avhole groups. For instance, 
whether or not there is an open passage from the nostrils to 
the mouth, the only character, according to Owen, which abso- 
lutely disting-uishcs fishes and reptiles — the inflection of the 
angle of the jaws in ^larsupials — the manner in which the 
wings of insects are folded — mere color in certain Alga; — mere 
pubescence on parts of the tlower in grasses — the nature of 
the dermal covering, as hair or feathers, in the Vertebrata. 
If the Ornithorhyncluis had been covered with feathers instead 
of hair, this external and trifling character would have been 
considered by naturalists as an important aid in determining 
the degree of affinity of this strange creature to birds. 

The importance, for classilication, of trifling characters, 
mainly depends on their being correlated Avith several other 
characters of more or less importance. The value indeed of an 
aggregate of characters is very evident in natural history. 
Hence, as has often been remarked, a species may depart from 
its allies in several characters, both of high physiological im- 
portance and of almost universal prevalence, and A'ct leave us 
in no doubt where it sliould be ranked. Hence, also, it has 
been found that a classification founded on any single charac- 
ter, however important that may be, has always failed ; for no 
part of the organization is invariably constant. The impor- 
tance of an aggregate of characters, even when none are im- 
portant, alone explains the aphorism by Linuiens, naTuely, that 
the characters do not give the genus, but the genus gives the 


characters ; for this seems foun Jed on an .appreciation of many 
trillinp;' points of resemblance, too slif^ht to be delinetl. Cer- 
tain plants, belongiiii^ to the Mali)igliiacene, bear perfect and 
degraded fio\vcrs; in the hitter, as A. do Jussicuhas remarked, 
" the grc'iter nuinl)er of the characters proper to the species, 
to the genus, to tlie family, to the class, disappear, and thus 
laugh at our classification." But when Aspicarpa produced in 
France, during several years, only degraded flowers, departing 
so wonderfully in a number of the most important points of 
structure from the proper type of the order, yet M. Richard 
sagaciously saw, as Jussieu observes, that this genus should 
still be retained among the MalpighiacefO. This case seems to 
me well to illustrate the spirit of our classification. 

Practically, Avhen naturalists are at work, they do not 
trouble themselves about the physiological value of the charac- 
ters which they use in defining a group or in allocating any 
particular species. If they find a character nearly uniform, and 
common to a great number of forms, and not common to 
others, they use it as one of high value ; if common to some 
lesser number, they use it as of subordinate value. This prin- 
ciple has been broadly confessed by some naturalists to be the 
true one; and by none more clearly than by that excellent 
botanist, Aug. St. Hilaire. If certain characters are always 
found correlated with others, though no apparent bond of con- 
nection can be discovered between them, especial value is set 
on them. As in most groups of animals, important organs, 
such as those fc^r propelling the blood, or for aerating it, or 
those for propagating the race, arc foimd nearly uniform, they 
are considered as highly sernceable in classification; but in 
some groups of anim:ds all these, the most important vital 
organs, are found to ofler characters of quite subordinate value. 
Thus, as Fritz MllUer has lately remarked, in the same group 
of crustaceans, ('ypridina is furnished Avith a heart, while in 
two closely-allied genera, namely, C^-pris and Cytherea, there 
is no such organ : one species of Cypridina has well-developed 
branchi.'C, while another species is destitute of them. 

Wo can see why characters derived from the embrj^o should 
be of equal importance with those derived from the adult, for 
a natural classification of course includes all ages. But it is 
by no means ol)vious, on the ordinary view, why the structure 
of the embryo should be more important for this purpose than 
that of the adult, which alone plays its full part in the econo- 
my of Nature. Yet it has been strongly urged by those great 


naturalists, iMiliic Edwards and Ag'assiz, tliat embryoloijifal 
characters ai o the most important of all ; and this doctrhie has 
very g'encrally been admitted as true. Nevertheless, their im- 
portaTicc has sometimes been exaggerated; in order to show 
tlii.s, Fritz Miillcr arranged by the aid of such characters the 
great class of crustaceans, and the arrangement did not prove 
a natural one. But there can be no douljt that characters de- 
rived from tlie embryo are generally of the highest value, not 
only Avith animals but Avith plants. Thus the two main divis- 
ions of flowering plants are founded on differences in the em- 
bryo — on the number and position of the cotyledons, and on 
the mode of development of the plumule and radicle. Wc 
shall immediately see why these characters possess so high a 
value in classification, namely, from the natural system being 
genealogical in its arrangement. 

Our classifications are often plainly influenced by chains of 
aflinities. Nothing can be easier than to define a number of 
chanicters common to all birds ; but in the case of crustaceans, 
such definition has hitherto been found impossible. There are 
crustaceans at the opposite ends of the series, which have 
hardly a character in common ; yet the species at both ends, 
from being plainly allied to others, and these to others, and 
so onward, can be recognized as unequivocally belonging to 
this, and to no other class of the Articulata. 

Geographical distribution has often been used, though per- 
haps not quite logically, in classification, more especially in 
very largo groups of closely-allied forms. Temminck insists 
on the utility or even necessity of this practice in certain 
groups of birds ; and it has been followed by several entomol- 
ogists and botanists. 

Finally, wutli respect to the comparative value of the vari- 
ous groups of species, such as orders, sub-orders, families, sub- 
families, and genera, they seem to be, at least at present, 
almost arbitrary. Several of the best botanists, such as Mr. 
IJontham and others, have strongly insisted on their arbitrary 
value. Instances could be given among plants and insects, of 
:i group of forms, first ranked by ])ractised naturalists as only 
a genus, and then raised to the rank of a sub-family or family ; 
and this has been done, not because further research has de- 
tected important structural differences, at first overlooked, but 
because nmnerous allied species, with slightly-different grades 
of difference, have been subsequently discovered. 

All the foregoing rules and aids and difBcultics in classifi- 


cation arc explained, if I do not greatly deceive myself, on the 
view that the Natural System is founded on descent Avitli 
modification; that the characters which naturalists consider as 
showing true allinity between any two or more species, are 
those which have been inherited from a connnon parent, all true 
classification being genealogical ; that community of descent 
is the hidden bond Avhich naturalists have been unconsciously 
seeking, and not some unknown plan of creation, or the enun- 
ciation of general propositions, and the mere putting together 
and separating objects more or less alike. 

But I must explain my meaning more fully. I believe that 
the arret)} rjcment of the groujis within each class, in due sub- 
ordination and relation to each other, must be strictly genea- 
logical in order to be natural ; but that the amount of diflerence 
in the several branches or groups, though allied in the same 
(h'grce in blood to their common progenitor, may diller greatly, 
In'ing duo to the different degrees of modification Avhich they 
have luidergone ; and this is expressed by the forms being 
ranked under different genera, families, sections, or orders. 
The reader will best understand what is meant, if he will take 
the trouble to refer to the diagram in the fourth chapter. We 
will sup]iose the letters A to L to rcpi'csent during the Silurian 
epoch allied genera, descended from some still earlier forms. 
In three of these genera (A, F, and I) the species have trans- 
mitted modified descendants to the present day, represented l)y 
the fifteen genera («'* to s'*) on the uppermost horizontal line. 
Now all these modified descendants from a single species, are 
related in blood or descent to the same degree ; they may 
metaphorically be called cousins to the same millionth degTce ; 
y(}t tiiey differ widely and in diU'ercnt degrees from each other. 
The forms descended from A, now broken up into two or three 
families, constitute a distinct order from those descended from 
I, also broken up into two families. Nor can the existing sjie- 
cies, desecnded from A, be ranked in the same genus with the 
parent A; or those from I, with the parent I. But the exist- 
ing geiuis f" may be supposed to have been but slightly modi- 
tied ; and it will then rank with tlie parent-genus F ; just as 
some few still living organisms belong to Silurian genera. So 
Ihat the amount or value of the differences between these or- 
ganic beings which arc all related to each other in the same 
degree in blood, has come to be widely different. Nevertheless, 
their genealogi(Ml arranffcment remains strictly true, not only 
at tlie present time, but at each successive period of descent. 


All the modified descendants from A will have inherited some- 
thing' in common from their common parent, as will all the de- 
scendants from I ; so will it be with each subordinate branch of 
descendants, at each successive stage. If, however, we suppose 
any descendant of A or of I to have been so much modified as 
to have lost all traces of its parentage, in this case, its place in 
the natural system will likev>dse be lost — as seems to have oc- 
curred with some few existing organisms. All the descendants 
of the genus F, along its whole line of descent,' are supposed 
to have been but little modified, and they form a single genus 
But this genus, tliough much isolated, will stiil occupy its projv 
er intermediate position. This natural arrangement is shown 
in the diagram as far as is possible on paper, but in much too 
simple a manner. If a branching diagram had not been used, 
and only the names of the groups had been written in a linear 
series, it would have been still less possible to have given a 
natural arrangement ; and it is notoriously not possible to rep- 
resent in a series, on a flat surface, tlic aflinities which we dis- 
cover in Nature among the beings of the same group. Thus, 
on the view which I hold, the natural system is genealogical 
in its arrangement, like a i^edigrcc ; but the amount of modifi- 
cation which the different groups have undergone has to be 
expressed by ranking them und(?r different so-called genera, 
sub-families, families, sections, orders, and classes. 

It may be worth while to illustrate this view of classification, 
by taking the case of languages. If we possessed a perfect 
jiedigree of mankind, a genealogical arrangement of the races 
of man would afford the best classification of the various lan- 
guages now spoken throughout the world ; and if all extinct lan- 
guages, and all intermediate and slowly-changing dialects, had 
to be included, such an arrangement would be the only pos- 
sible one. Yet it might be that some ancient languages had 
altered very little and had given rise to few new languages, 
while others had altered much, o\\'ing to the spreading, isola- 
tion, and state of civilization of the several co-descended races, 
and liad thus given rise to many new dialects and languages. 
The various degrees of difference between the languages of 
tlie same stock would have to be expressed by groups subordi- 
nate to groups ; but the proper or even only possible arrange- 
ment would still be genealogical ; and this would be strictly 
natimal, as it would connect together all languages, extinct and 
recent, by the closest affmitics, and would give the filiation and 
origin of each tongue. 


In confirmation of this view, let us glance at the classifica- 
tion of varieties, which are believed or known to be descended 
i'roin a sinu:le sjiecies. These are grouped under the species, 
witli the sub-variclies under the varieties ; and in some cases, 
as with domestic pigeons, several otlier grades of dilVerence are 
requisite. Nearly the same rules are followed as in classifj-ing 
species. Authors have insisted on the necessity of arranging 
varieties on a natural instead of an artificial system ; we are 
cautioned, for instance, not to class two varieties of the pine- 
apple together, merely because their fruit, though the most 
important part, happens to be nearly identical; no one puts 
llie Swedish and conunon turnijjs together, though the escu- 
h'nt and thickened stems are so similar. Whatever part is 
found to be most constant, is used in classing varieties : thus 
the great agriculturist Marsiiall says the liorns are very useful 
loE this purpose with cattle, because they are less variable than 
tlie shape or color of the body, etc. ; whereas with sheep the 
horns are nuicli less serviceable, because less constant. In 
classing varieties, I apprehend that if we had a real pedigree, 
a genealogical classification would be universally preferred ; 
and it has been attempted in some cases. For we might feel 
sure, wlicther there had been more or less modification, the 
])rinciple of inheritance would keep the forms together which 
were alHed in the greatest number of points. In tundjler 
])igeons, tliougli some of the sul>varieties difler in the impor- 
tant character of having a longer beak, yet all are kept togeth- 
er, from having the common habit of tumbling; but the shoit- 
faccd breed has nearly or quite lost this haljit; nevertheless, 
without any thought on the sul)ject, these tumblers arc kept 
in the same group, because allied in blood and alike in some 
other respects. 

With species in a state of nature, every naturalist has in 
fact brought descent into his classification ; for lie includes in 
his lowest grade, that of the species, the two sexes ; and how 
enormously these sometimes dilh^r in the most important char- 
acters, is known to every naturalist : scarcely a single fact can 
be predicated in common of the adult males and hermajihro- 
• lites of certain cirripedes, and yet no one dreams of sejiarating 
ihem. As soon as the three Orchidean forms, Monachanthus, 
-Myanthus, and Catasetum, which had jireviously been ranked 
as three distinct genera, were known to bo sometimes pro- 
duced on the same plant, they were inunediately considered as 
varieties; and now 1 have been able to show that they are the 


male, female, and hermaphrodite forms of the same species. 
The naturalist ineludes as one species the various larval stages 
of the same individual, however much they may differ from 
each other and from the adult, as Avell as the so-called alter- 
nate generations of Stcenslrup, which can only in a technical 
sense be considered as the same individual. He includes 
monsters and varieties, not from their partial resemblance to 
the parent-form, but because they are descended from it. 

As descent has universally been used in classing together 
the individuals of the same species, though the males and fe- 
males and larvfc are sometimes extremely different ; and as it 
has been nscd in classing varieties which have undergone a 
certain, and sometimes a considerable amount of moditication, 
may not this same element of descent have been unconsciously 
used in grouping species under genera, and genera imder 
higher groujis, all under the so-called natural system? I be- 
lieve it has been unconsciously used; and thus only can I im- 
derstand the several rules and guides which have been fol- 
lowed by our best systematists. We have no written pedi- 
grees ; Ave have to make out community of descent by resem- 
blances of any kind. Therefore Ave choose those characters 
Avhich, as far as Ave can jucige, are the least likely to Ikia'C been 
modified in relation to the conditions of life to Avhich each spe- 
cies has been recently exposed. Rudimentary structures on 
this vicAV are as good as, or CA'en sometimes better than, other 
]")arts of the organization. We care not hoAV trilling a charac- 
tcn- may be — let it be the mere inflection of the angle of the 
jaAv, the manner in Avhich an insect's Aving is folded, Avhet'licr 
the skin be covered b}' hair or feathers— if it prevail thn^ugh- 
out many and different species, especially those haA'ing very 
different habits of life, it assumes high A'alue ; for Ave can ac- 
coimt for its presence in so many forms Avith such different 
habits, only by inheritance from a common ]iareut. A\^e may 
err in this resjiect in regard to single points of structure, but 
Avhen scA'eral characters, let them be eA'cr so trifling, concur 
throughout a large group of beings haA'ing different habits, aa'C 
may feel almost sure, on tlieory of descent, that these charac- 
ters have been inlierited from a common ancestor. And aa-g 
knoAV that such correlated or aggregated charactei's haA'e es 
jiecial value in classification. 

We can understand Avhy a sjiecies or a group of sjiecies 
may depart, in several of its most important characteristics, 
from its allies, and yet be safely classed Avith them. This may 


be safely doiip, aiul is often done, as long as a sufficient num- 
ber of characters, let them be ever so unimportant, betrays the 
hidden bond of community of descent. Let two forms have 
not a single character in conunon, yet if these extreme forms 
are connected together by a chain of intermediate groups, we 
may at once infer their community of descent, and we put 
them all into the same class. As we find organs of high 
physiologiortl importance — those which serv'C to preserve life 
under the most diverse conditions of existence — are generally 
the most constant, we attach especial value to them ; but if 
these same organs, in another group or section of a group, are 
found to differ much, we at once value them less in our classi- 
fication. We sliall presently see why embryological charac- 
ters are of such high classiticatpry importance. Geographical 
distribution may sometimes be brought usefully into play in 
classing large genera, because all the species of the same 
genus, inlial)iting any distinct and isolated region, are in all 
probability descended from the same ]>arents. 

jinalof/lcal Resemhlanccs. — We can understand, on the 
above views, the very important distinction between real af- 
finities and analogical or adaptive resemblances. Lamarck 
first called attention to this distinction, and he has been ably 
followed by Macleay and others. The resemblance in the 
shape of the body and in the fin-like anterior limbs, between 
the dugong, which is a pachydermatous animal, and the 
whale, and between both these mammals and fishes, is analo- 
gical. Among insects there are innumerable instances: tlujs 
Linnicus, misled by external appearances, actually classed an 
ho;noi)terous insect as a moth. We see something of the same 
kind even in our domestic varieties, as in the thickened stems 
of the common and Swedish turnips. The resemblance of the 
greyhound and race-horse is hardly more fanciful than the 
analogies whicli have been drawn by some authors between 
widely-distinct animals. On my view of characters being of 
real importance for classification, only in so far as they reveal 
descent, we can clearlv understand why analogical or adaj^tive 
characters, although of the utmost importance to the welfare 
of tlie being, are almost valueless to the systematists. For 
aniiMiils belonging to two most distinc-t lines of descent may 
readily have become adapted to similar conditions, and thus 
have assumed a close external resemblance ; but such resem- 
blances will not reveal — will rather tend to conceal their 
blood-rclalionsliip. We can thus also understand the appar- 


cut paradox, that the very same characters are analogical 
when one class or one order is compared -with another, but 
give true aflniities Avhen the members of the same class or or- 
der are compared together: thus, the shape of the body and 
iin-likc Hmbs are only analogical "when whales are compared 
AviLli fishes, being adaptations in both classes for swimming 
through the water; but the shape of the body and fin-like 
limbs serve as characters exhibiting true aflinity between the 
several members of the whale family ; for these cetaceans 
agree in so many characters, great and small, that we cannot 
doubt that they have Hiherited their general shape of body and 
structure of limbs from a common ancestor. So it is with 

The most remarkable case of analogical resemblance ever 
recorded, though not dependent on adaptation to similar con- 
ditions of life, is that given by Mr. Bates with respect to cer- 
tain butterflies in the Amazonian region closely mimicking 
other kinds. This excellent observer shows that in a district 
where, for instance, an Ithomia abounds in gaudy swarms, 
another butterfly, namely, a Leptalis, is often found mingled 
in the same flock, and so closely resembles the Ithomia in 
every shade and stripe of color, and even in the shape of its 
wings, that jNIr. Bates, with his eyes sharpened by collecting 
during eleven years, was, though always on his guard, con- 
tinually deceived. When the mockers and the mocked are caught 
and comjiared, they are found to be totally different in essen- 
tial structure, and to belong, not only to distinct genera, but 
often to distinct families. Had this mimicry occurred in only 
one or two instances, it might have been passed over as a 
strange coincidence. But, if we proceed from a district where 
one Leptalis imitates an Ithomia, another mocking and mocked 
species belonging to the same genera, equally close in their re- 
semblance, will be found. Altogether no less than ten genera 
are enumerated, which include species that iinitate other but- 
terflies. The mockers and mocked alwaj's inhabit the same 
region ; we never find an imitator living remote from the 
form which it imitates. The mockers are almost invariably 
rare insects ; the mocked in almost every case aboimd in 
swanns. In the same district in which a species of Lcjitalis 
closely imitates an Ithomia, there are sometimes other Lejn- 
doptera mimicking the same Ithomia ; so that in the same 
place, species of three genera of butterflies and even a moth 
are found nil closely resembling a butterfly belonging to a fourth 


p^emis. It deserves especial notice that many of the mimick- 
inn^ forms of the Lcptalis, as ■well as of the mimicked forms, 
can be shown hy a g^raduated series to be merely varieties of 
the same species ; while others are undoubtedly distinct spe- 
cies. But why, it may be asked, are certain Jbrms treated as 
tlie mimicked and othei's as tlic mimickers ? Mr. Bates satis- 
factorily answers this question, by showing);' that the form 
Avliicli is imitated keeps the usual dress of the f^roup to which 
it lielongs, while the counterfeiters have changed their dress 
and do not resemble their nearest allies. 

^\'e are next led to inquire what reason can possibly be as- 
signed for certain butterflies and moths so often assuming the 
dress of another and quite distinct form ; why, to the perplexity 
of naturalists, has Nature condescended to the tricks of the 
stage? ]Mr. Bates has, no doubt, hit on the true explanation. 
The mocked forms, wliich always aboimd in numbers, nuist 
habitually escape destruction to a large extent, otherwise they 
could not exist in such swarms ; and Mr. Bates never saw 
them preyed on l)y birds and certain large insects which at- 
tack other butterllies. He has good reason to believe that 
this innnunity is owing to a peculiar and offensive odor which 
they emit. The mocking forms, on the other hand, that in- 
habit the same district, are comparatively rare, and belong to 
lare groups ; hence they must suffer habitually from some 
danger, A^r otherwise, from the number of eggs laid bv all but- 
terflies, they would in three or four generations swarm over the 
wliole country. Now if a member of one of these persecuted 
and rare groups were to assume a dress so like that of a well- 
])rotected species that it continually deceived the practised 
eyes of an entom(ilogist, it would often deceive predacious birds 
and insects, and thus escape much destruction. Mr. Bates 
may almost be said to have actually witnessed the process 
by which the mimickers have come so closely to resemble the 
mimicked ; for he found that some of the forms of Lcptalis 
which mimic so many other butterflies, varied in an extreme 
degree. In one district several varieties occurred, and of these 
one alone resembled, to a certain extent, the conunon Ithomia 
of the same district. In another district there were two or 
three varieties, one of which was much commoner than the 
others, and this closely mocked another form of Ithomia. 
From facts^of this nature, Mr. Bates concludes that the I^op- 
talis first varies ; and when a variety hajipens to resemble in 
some degree any common butterfly inhabiting the same district, 


tliis variety, from its rcseinl)lancc to a flourishing aiul little-per- 
secuted kind, has a better chance of escaping' destruction from 
predacious birds and insects, and is consequently oftener pre- 
served ; " the less ];)erfect degrees of resemblance being gener- 
ation after generation eliminated, and only the others left to 
]')ropagate their kind." So that here we have an excellent il- 
lustration of natural selection. 

Mr, Wallace has recently described several equally striking 
cases of mimicry in the Lepidoptera of the Malay Archipelago, 
and other instances could be given with other orders of insects. 
Mr. Wallace has also described one case of mimicry among 
birds, but we have no such cases Avith the larger quadrupeds. 
Tlie much greater fi-equency of mimicry with insects than with 
other animals, is probably the consequence of their small size ; 
insects cannot defend themselves, excepting indeed the kinds 
that sting, and I have never heard of an ins