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Full text of "The origin of species by means of natural selection; or, The preservation of favored races in the struggle for life"



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THE 

OBIG-IN OF SPECIES 



BY MEAN'S OF NATURAL SELECTION 



OR THE PRESERVATION OF FAVORED RACES IN 
THE STRUGGLE FOR LIFE 






BY 



AMERICAN ETI 

6015 

is©e 



CHARLES DARWIN, M. A., LL. D., F. R. S. 



WITH ADDITIONS AND CORRECTIONS 
FROM SIXTH AND LAST ENGLISH EDITION 



IN TWO VOLUMES 
VOLUME n 



NEW YORK 

D. APPLETON AND COMPANY 

1889 



Authorized Edition. 



CONTENTS OF VOL. II. 



CHAPTER IX. 

Hybridism. 

Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close 
interbreeding, removed by domestication — Laws governing the 
sterility of hybrids — Sterility not a special endowment, but 
incidental on other differences, 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 » .. ., Page 1 



CHAPTER X. 

On the Imperfection of the G-eological Record. 

On the absence of intermediate varieties at the present day — On 
the nature of extinct intermediate varieties ; on their number 
— On the lapse of time, as inferred from the rate of denudation 
and of deposition — On the lapse of time as estimated by years 
— On the poorness of our palseontological collections — On the 
intermittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden appearance of groups of species 
— On their sudden appearance in the lowest known fossiliferous 
strata — Antiquity of the habitable earth .. 48 



IT CONTEXTS. 

CHAPTEE XI. 

On the Geological Succession of Organic Beings. 

On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their appear- 
ance and disappearance as do single species — On Extinction — 
On simultaneous changes in the forms of life throughout the 
world — On the affinities of extinct species to each other and to 
living species — On the state of development of ancient forms — 
On the succession of the same types within the same areas — 
Summary of preceding and present chapter •* .. Page 89 



CHAPTER XII. 

Geographical Distribution. 

Present distribution cannot be accounted for by differences in 
physical conditions — Importance of barriers — Affinity of the 
productions of the same continent — Centres of creation — Means 
of dispersal, by changes of climate and of the level of the land, 
and by occasional means — Dispersal during the Glacial period 
— Alternate Glacial periods in the north and south .. 129 



CHAPTER XIII. 

Geographical Distribution — continued. 

Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial 
Mammals— On the relation of the inhabitants of islands to 
those of the nearest mainland — On colonisation from the nearest 
source with subsequent modification — Summary of the last and 
present chapter 171 



CONTENTS. 



CHAPTEE XIV. 

Mutual Affinities of Organic Beings : Morphology : 
Embryology : Eudimentary Oegans. 

Classification, groups subordinate to groups — Natural system — 
Eules and difficulties in classification, explained on the theory 
of descent with modification — Classification of varieties — 
Descent always used in classification — Analogical or adaptive 
characters — Affinities, general, complex, and radiating — Ex- 
tinction separates and defines groups — Morphology, between 
members of the same class, between parts of the same 
individual — Embryology, laws of, explained by variations not 
supervening at any early age, and being inherited at a 
corresponding age — Eudimentary organs; their origin ex- 
plained — Summary ., ,. .. Page 202 



CHAPTEE XY. 

Eecapitulation and Conclusion. 

Eecapitulation of the objections to the theory of Natural Selection 
— Eecapitulation of the general and special circumstances in 
its favour — Causes of the general belief in the immutability of 
species — How far the theory of Natural Selection may be 
extended — Effects of its adoption on the study of Natural 
History — Concluding remarks .. , 267 

Glossary of Scientific Teems « 307 

Index .. .. , .. 323 



OKXGIN OF SPECIES. 



CHAPTEE IX. 

Hybeidism. 

Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close inter- 
breeding, removed by domestication — Laws governing the 
sterility of hybrids — Sterility not a special endowment, but 
inc ; dental on other differences, not accumulated by natural 
selection — Causes of the sterility of first crosses and of hybrids 
— Parallelism between the effects of changed conditions of life 
and of crossing — Dimorphism and trimorphism — Fertility of 
varieties when crossed and of their mongrel offspring not uni- 
versal — Hybrids and mongrels compared independently of their 
fertility — Summary. 

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



2 HYBRIDISM. [Chap. IX. 

degrees of sterility. It is an incidental result of 
differences in the reproductive systems of the parent- 
species. 

In treating this subject, two classes of facts, to a large 
extent fundamentally different, have generally been 
confounded ; namely, the sterility of species yrhen first 
crossed, and the sterility of the hybrids produced from 
them. 

Pure species have of course their organs of reproduc- 
tion in a perfect condition, yet when intercrossed they 
produce either few or no offspring. Hybrids, on the 
other hand, have their reproductive organs functionally 
impotent, as may be clearly seen in the state of the 
male element in both plants and animals ; though the 
formative organs themselves are perfect in structure, as 
far as the microscope reveals. In the first case the two 
sexual elements which go to form the embryo are 
perfect ; in the second case they are either not at all 
developed, or are imperfectly developed. This distinc- 
tion is important, when the cause of the sterility, which 
is common to the two cases, has to be considered. The 
distinction probably has been slurred over, owing to the 
sterility in both cases being looked on as a special 
endowment, bevond the province of our reasoning 
powers. 

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

Degrees of Sterility. — First, for the sterility of species 



Chap. IX.] DEGREES OF STERILITY. 3 

when crossed and of their hybrid offspring. It is 
impossible to study the several memoirs and works 
of those two conscientious and admirable observers, 
Kolreuter and Gartner, who almost devoted their lives 
to this subject, without being deeply impressed with the 
high generality of some degree of sterility. Kolreuter 
makes the rule universal ; but then he cuts the knot, 
for in ten cases in which he found two forms, considered 
by most authors as distinct species, quite fertile 
together, he unhesitatingly ranks them as varieties. 
Gartner, also, makes the rule equally universal ; and he 
disputes the entire fertility of Kolreuter' s ten cases. 
But in these and in many other cases, Gartner is 
obliged carefully to count the seeds, in order to show 
that there is any degree of sterility. He always 
compares the maximum number of seeds produced by 
two species when first crossed, and the maximum 
produced by their hybrid offspring, with the average 
number produced by both pure parent- species in a state 
of nature. But causes of serious error here intervene : 
a plant, to be hybridised, must be castrated, and, what 
is often more important, must be secluded in order to 
prevent pollen being brought to it by insects from other 
plants. Nearly all the plants experimented on by 
Gartner were potted, and were kept in a chamber in his 
house. That these processes are often injurious to the 
fertility of a plant cannot be doubted ; for Gartner gives 
in his table about a score of cases of plants which he 
castrated, and artificially fertilised with their own 
pollen, and (excluding all cases such as the Leguminosse, 
in which there is an acknowledged difficulty in the 
manipulation) half of these twenty plants had their 
fertility in some degree impaired. Moreover, as 



4 HYBRIDISM. [Chap. IX. 

Gartner repeatedly crossed some forms, such as the 
common red and blue pimpernels (Anagallis arvensis 
and coerulea), which the best botanists rank as varieties, 
and found them absolutely sterile, we may doubt 
whether many species are really so sterile, when 
intercrossed, as he believed. 

It is certain, on the one hand, that the sterility of 
various species when crossed is so different in degree 
and graduates away so insensibly, and, on the other 
hand, that the fertility of pure species is so easily 
affected by various- circumstances, that for all practical 
purposes it is most difficult to say where perfect fertility 
ends and sterility begins. I think no better evidence 
of this can be required than that the two most 
experienced observers who have ever lived, namely 
Kolreuter and Gartner, arrived at diametrically opposite 
conclusions in regard to some of the very same forms. 
It is also most instructive to compare — but I have not 
space here to enter on details — the evidence advanced 
by our best botanists on the question whether certain 
doubtful forms should be ranked as species or varieties, 
with the evidence from fertility adduced by different 
hybridisers, or by the same observer from experiments 
made during different years. It can thus be shown 
that neither sterility nor fertility affords any certain 
distinction between species and varieties. The evidence 
from this source graduates away, and is doubtful in the 
same degree as is the evidence derived from other 
constitutional and structural differences. 

In regard to the sterility of hybrids in successive 
generations ; though Gartner was enabled to rear some 
hybrids, carefully guarding them from a cross with 
either pure parent, for six or seven, and in one case foi 



Chap. IX.] DEGREES OF STERILITY. 5 

ten generations, yet he asserts positively that their 
fertility never increases, but generally decreases 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 offspring; 
and both sexual elements in hybrid plants are already 
affected in some degree. But I believe that their 
fertility has been diminished in nearly all these cases 
by an independent cause, namely, by too close inter- 
breeding. I have made so many experiments and 
collected so many facts, showing on the one hand that 
an occasional cross with a distinct individual or variety 
increases the vigour and fertility of the offspring, and on 
the other hand that very close interbreeding lessens 
their vigour and fertility, that I cannot doubt the 
correctness of this conclusion. Hybrids are seldom 
raised by experimentalists in great numbers ; and as the 
parent-species, or other allied hybrids, generally grow 
in the same garden, the visits of insects must be 
carefully prevented during the flowering season : hence 
hybrids, if left to themselves, will generally be fertilised 
during each generation by pollen from the same flower ; 
and this would probably be injurious to their fertility, 
already lessened by their hybrid origin. I am strength- 
ened in this conviction by a remarkable statement 
repeatedly made by Gartner, namely, that if even the 
less fertile hybrids be artificially fertilised with hybrid 
pollen of the same kind, their fertility, notwithstanding 
the frequent ill effects from manipulation, sometimes 
decidedly increases, and goes on increasing. Now, in 
the process of artificial fertilisation, pollen is as often 
taken by chance (as I know from my own experience) 



6 HYBRIDISM. [Chap. IX. 

from the anthers of another flower, as from the anthers 
of the flower itself which is to be fertilised ; so that a 
cross between two flowers, though probably often on the 
same plant, would be thus effected. Moreover, when- 
ever complicated experiments are in progress, so careful 
an observer as Gartner would have castrated his 
hybrids, and this would have ensured in each generation 
a cross with pollen from a distinct flower, either from 
the same plant or from another plant of the same hybrid 
nature. And thus, the strange fact of an increase of fer- 
tility in the successive generations of artificially fertilised 
hybrids, in contrast with those spontaneously self- 
fertilised, may, as I believe, be accounted for by too close 
interbreeding having been avoided. 

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

This case of the Crinum leads me to refer to a 



Chap. IX.] DEGEEES OF STERILITY. 7 

singular fact, namely, that individual plants of certain 
species of Lobelia, Verbascum and Passiflora, can easily 
be fertilised by pollen from a distinct species, but not 
by pollen from the same plant, though this pollen can 
be proved to be perfectly sound by fertilising other 
plants or species. In the genus Hippeastrum, in 
Corydalis as shown by Professor Hildebrand, in various 
orchids as shown by Mr. Scott and Fritz Miiller, all the 
individuals are in this peculiar condition. So that with 
some species, certain abnormal individuals, and in other 
species all the individuals, can actually be hybridised 
much more readily than they can be fertilised by pollen 
from the same individual plant ! To give one instance, 
a bulb of Hippeastrum aulicum produced four flowers ; 
three were fertilised by Herbert with their own pollen, 
and the fourth was subsequently fertilised by the pollen 
of a compound hybrid descended from three distinct 
species : the result was that " the ovaries of the three 
first flowers soon ceased to grow, and after a few days 
perished entirely, whereas the pod impregnated by the 
pollen of the hybrid made vigorous growth and rapid 
progress to maturity, and bore good seed, which 
vegetated freely." Mr. Herbert tried similar experi- 
ments during many years, and always with the same 
result. These cases serve to show on what slight and 
mysterious causes the lesser or greater fertility of a 
species sometimes depends. 

The practical experiments of horticulturists, though 
not made with scientific precision, deserve some notice. 
It is notorious in how complicated a manner the 
species of Pelargonium, Fuchsia, Calceolaria, Petunia, 
Pthododendron, &c, have been crossed, yet many of 
these hybrids seed freely. For instance, Herbert 
19 



8 HYBRIDISM. [Chap. IX. 

asserts that a hybrid from Calceolaria integrifolia and 
plantaginea, species most widely dissimilar in general 
habit, " reproduces itself as perfectly as if it had been a 
natural species from the mountains of Chili." I have 
taken some pains to ascertain the degree of fertility of 
some of the complex crosses of Bhododendrons, and I 
am assured that many of them are perfectly fertile. 
Mr. C. Xoble, for instance, informs me that he raises 
stocks for grafting from a hybrid between Ehod. 
ponticum and catawbiense, and that this hybrid " seeds 
as freely as it is possible to imagine." Had hybrids 
when fairly treated, always gone on decreasing in fertility 
in each successive generation, as Gartner believed to be 
the case, the fact would have been notorious to nursery- 
men. Horticulturists raise large beds of the same 
hybrid, and such alone are fairly treated, for by insect 
agency the several individuals are allowed to cross 
freely with each other, and the injurious influence of 
close interbreeding is thus prevented. Any one may 
readily convince himself of the efficiency of insect- 
agency by examining the flowers of the more sterile 
kinds of hybrid Rhododendrons, which produce no 
pollen, for he will find on their stigmas plenty of pollen 
brought from other flowers. 

In regard to animals, much fewer experiments have 
been carefully tried than with plants. If our systematic 
arrangements can be trusted, that is, if the genera of 
animals are as distinct from each other as are the genera 
of plants, then we may infer that animals more widely 
distinct in the scale of nature can be crossed more 
easily than in the case of plants; but the hybrids 
themselves are, I think, more sterile. It should, 
however, be borne in mind that, owing to few animals 



Chap. IX.] DEGEEES OF STEKILITY. 9 

breeding freely under confinement, few experiments 
have been fairly tried : for instance, the canary-bird has 
been crossed with nine distinct species of finches, but, 
as not one of these breeds freely in confinement, we 
have no right to expect that the first crosses between 
them and the canary, or that their hybrids, should be 
perfectly fertile. Again, with respect to the fertility 
in successive generations of the more fertile hybrid 
animals, I hardly know of an instance in which two 
families of the same hybrid have been raised at the 
same time from different parents, so as to avoid the ill 
effects of close interbreeding. On the contrary, brothers 
and sisters have usually been crossed in each successive 
generation, in opposition to the constantly repeated 
admonition of every breeder. And in this case, it is 
not at all surprising that the inherent sterility in the 
hybrids should have gone on increasing. 

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



10 HYBRIDISM [Chap. IX. 

bred inter se. This was effected by Mr. Eyton, who 
raised two hybrids from the same parents, but from 
different hatches ; and from these two birds he raised 
no less than eight hybrids (grandchildren of the pnre 
geese) from one nest. In India, however, these cross- 
bred geese must be far more fertile ; for I am assured 
by two eminently capable judges, namely Mr. Blyth and 
Capt. Hutton, that whole flocks of these crossed geese 
are kept in various parts of the country ; and as they 
are kept for profit, where neither pure parent-species 
exists, they must certainly be highly or perfectly fertile. 
With our domesticated animals, the various races 
when crossed together are quite fertile; yet in many 
cases they are descended from two or more wild species. 
From this fact we must conclude either that the 
aboriginal parent-species at first produced perfectly 
fertile hybrids, or that the hybrids subsequently reared 
under domestication became quite fertile. This latter 
alternative, which was first propounded by Pallas, seems 
by far the most probable, and can, indeed, hardly be 
doubted. It is, for instance, almost certain that our 
dogs are descended from several wild stocks ; yet. with 
perhaps the exception of certain indigenous domestic 
dogs of South America, all are quite fertile together; 
but analogy makes me greatly doubt, whether the 
several aboriginal species would at first have freely bred 
together and have produced quite fertile hybrids. So 
again I have lately acquired decisive evidence that the 
crossed offspring from the Indian humped and common 
cattle are inter se perfectly fertile ; and from the 
observations by Eutimeyer on their important osteo- 
logical differences, as well as from those by Mr. Blyth 
on their differences in habits, voice, constitution, &c, 



Chap. IX.] DEGEEES OF STERILITY. 11 

these two forms must be regarded as good and distinct 
species. The same remarks may be extended to the two 
chief races of the pig. We must, therefore, either give 
up the belief of the universal sterility of species when 
crossed ; or we must look at this sterility in animals, 
not as an indelible characteristic, but as one capable of 
being removed by domestication. 

Finally, considering all the ascertained facts on the 
intercrossing of plants and animals, it may be concluded 
that some degree of sterility, both in first crosses and in 
hybrids, is an extremely general result ; but that it 
cannot, under our present state of knowledge, be con- 
sidered as absolutely universal. 

Laws governing the Sterility of first Crosses and of 
Hybrids. 

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

It has been already remarked, that the degree of 
fertility, both of first crosses and of hybrids, graduates 
from zero to perfect fertility. It is surprising in how 



12 LAWS GOVERNING THE STERILITY [Chap. IX. 

many curious ways this gradation can be shown ; but 
only the barest outline of the facts can here be given. 
When pollen from a plant of one family is placed on 
the stigma of a plant of a distinct family, it exerts no 
more influence than so much inorganic dust. From 
this absolute zero of fertility, the pollen of different 
species applied to the stigma of some one species of the 
same genus, yields a perfect gradation in the number 
of seeds produced, up to nearly complete or even quite 
complete fertility ; and, as we have seen, in certain 
abnormal cases, even to an excess of fertility, beyond 
that which the plant's own pollen produces. So in 
hybrids themselves, there are some which never have 
produced, and probably never would produce, even 
with the pollen of the pure parents, a single fertile 
seed : but in some of these cases a first trace of fertility 
may be detected, by the pollen of one of the pure 
parent-species causing the flower of the hybrid to 
wither earlier than it otherwise would have done; 
and the early withering of the flower is well known 
to be a sign of incipient fertilisation. From this 
extreme degree of sterility we have self- fertilised 
hybrids producing a greater and greater number of 
seeds up to perfect fertility. 

The hybrids raised from two species which are very 
difficult to cross, and which rarely produce any off- 
spring, are generally very sterile ; but the parallelism 
between the difficulty of making a first cross, and the 
sterility of the hybrids thus produced — two classes of 
facts which are generally confounded together — is by 
no means strict. There are many cases, in which two 
pure species, as in the genus Verbascum, can be united 
with unusual facility, and produce numerous hybrid- 



Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 13 

offspring, yet these hybrids are remarkably sterile. 
On the other hand, there are species which can be 
crossed very rarely, or with extreme difficulty, but 
the hybrids, when at last produced, are very fertile. 
Even within the limits of the same genus, for instance 
in Dianthus, these two opposite cases occur. 

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

By the term systematic affinity is meant, the general 
resemblance between species in structure and constitu- 
tion. Now the fertility of first crosses, and of the 
hybrids produced from them, is largely governed by 
their systematic affinity. This is clearly shown by 
hybrids never having been raised between species 
ranked by systematists in distinct families ; and on 
the other hand, by very closely allied species generally 
uniting with facility. But the correspondence between 
systematic affinity and the facility of crossing is by no 
means strict. A multitude of cases could be given of 
very closely allied species which will not unite, or only 
with extreme difficulty ; and on the other hand of very 
distinct species which unite with the utmost facility. 
In the same family there may be a genus, as Dianthus, 



14 LAWS GOVERNTSTG THE STEEILITY [Chap. IX. 

in which very many species can most readily be crossed ; 
and another genus, as Silene. in which the most perse- 
vering efforts have failed to produce between extremely 
close species a single hybrid. Even within the limits 
of the same genus, we meet with this same difference; 
for instance, the many species of Xicotiana have been 
more largely crossed than the species of almost any 
other genus ; but Gartner found that X. acuminata, 
which is not a particularly distinct species, obstinately 
failed to fertilise, or to be fertilised by no less than 
eight other species of Xicotiana. Many analogous facts 
could be given 

Xo one has been able to point out what kind or what 
amount of difference, in any recognisable character, is 
sufficient to prevent two species crossing. It can be 
shown that plants most widely different in habit and 
general appearance, and having stronglv marked differ- 
ences in every part of the flower, even in the pollen, in 
the fruit, and in the cotyledons, can be crossed. Annual 
and perennial plants, deciduous and evergreen trees, 
plants inhabiting different stations and fitted for ex- 
tremely different climates, can often be crossed with 
ease. 

By a reciprocal cross between two species, I mean 
the case, for instance, of a female-ass being first crossed 
by a stallion, and then a mare by a male-ass ; these two 
species may *hen be said to have been reciprocally 
crossed. There is often the widest possible difference 
in the facility of making reciprocal crosses. Such 
cases are highly important, for they prove that the 
capacity in any two species to cross is often completely 
independent of their systematic affinity, that is of any 
difference in their structure or constitution, excepting 



Chap. IX.] OF FIEST CROSSES AND OF HYBRIDS. 15 

in their reproductive systems. The diversity of the 
result in reciprocal crosses between the same two 
species was long ago observed by Kolreuter. To give 
an instance: Mirabilis jalapa can easily be fertilised 
by the pollen of M. longiflora, and the hybrids thus 
produced are sufficiently fertile; but Kolreuter tried 
more than two hundred times, during eight following 
years, to fertilise reciprocally M. longiflora with the 
pollen of M. jalapa, and utterly failed. Several other 
equally striking cases could be given. Thuret has 
observed the same fact with certain sea-weeds or Fuci. 
Gartner, moreover, found that this difference of facility 
in making reciprocal crosses is extremely common in a 
lesser degree. He has observed it even between closely 
related forms (as Matthiola annua and glabra) which 
many botanists rank only as varieties. It is also a 
remarkable fact, that hybrids raised from reciprocal 
crosses, though of course compounded of the very same 
two species, the one species having first been used as 
the father and then as the mother, though they rarely 
differ in external characters, yet generally differ in 
fertility in a small, and occasionally in a high degree. 

Several other singular rules could be given from 
Gartner : for instance, some species have a remarkable 
power of crossing with other species ; other species of 
the same genus have a remarkable power of impressing 
their likeness on their hybrid offspring ; but these two 
powers do not at all necessarily go together. There are 
certain hybrids which, instead of having, as is usual, 
an intermediate character between their two parents, 
always closely resemble one of them ; and such hybrids, 
though externally so like one of their pure parent- 
species, are with rare exceptions extremely sterile. So 



16 LAWS GOVERNING THE STEEILITY [Chap. IX. 

again amongst hybrids which are usually intermediate 
in structure between their parents, exceptional and 
abnormal individuals sometimes are born, which closely 
resemble one of their pure parents ; and these hybrids 
are almost always utterly sterile, even when the other 
hybrids raised from seed from the same capsule have a 
considerable degree of fertility. These facte show how 
completely the fertility of a hybrid may be independent 
of its external resemblance to either pure parent. 

Considering the several rules now given, which 
govern the fertility of first crosses and of hybrids, we 
see that when forms, which must be considered as good 
and distinct species, are united, their fertility graduates 
from zero to perfect fertility, or even to fertility under 
certain conditions in excess -that their fertility, besides 
being eminently susceptible to favourable and un- 
favourable conditions, is innately variable ; that it is by 
no means always the same in degree in the first cross 
and in the hybrids produced from this cross ; that the 
fertility of hybrids is not related to the degree in which 
they resemble in external appearance either parent ; and 
lastly, that the facility of making a first cross between 
any two species is not always governed by their 
systematic affinity or degree of resemblance to each 
other. This latter statement is clearly proved by the 
difference in the result of reciprocal crosses between 
the same two species, for, according as the one species 
or the other is used as the father or the mother, there is 
generally some difference, and occasionally the widest 
possible difference, in the facility of effecting an union. 
The hybrids, moreover, produced from reciprocal crosses 
often differ in fertility. 

]\ow do these complex and singular rules indicate 



Chap. IX.] OF FIRST GROSSES AND OF HYBRIDS. 17 

that species have been endowed with sterility simply 
to prevent their becoming confounded in nature? I 
think not. For why should the sterility be so 
extremely different in degree, when various species 
are crossed, all of which we must suppose it would 
be equally important to keep from blending together ? 
"Why should the degree of sterility be innately variable 
in the individuals of the same species ? Why should 
some species cross with facility, and yet produce very 
sterile hybrids; and other species cross with extreme 
difficulty, and yet produce fairly fertile hybrids ? Why 
should there often be so great a difference in the result 
of a reciprocal . cross between the same two species ? 
Why, it may even be asked, has the production of 
hybrids been permitted ? To grant to species the 
special power of producing hybrids, and then to stop 
their further propagation by different degrees of sterility, 
not strictly related to the facility of the first union 
between their parents, seems a strange arrangement. 

The foregoing rules and facts, on the other hand, 
appear to me clearly to indicate that the sterility both 
of first crosses and of hybrids is simply incidental or 
dependent on unknown differences in their reproductive 
systems ; the differences being of so peculiar and 
limited a nature, that, in reciprocal crosses between the 
same two species, the male sexual element of the one 
will often freely act on the female sexual element of the 
other, but not in a reversed direction. It will be 
advisable to explain a little more fully by an example 
what I mean by sterility being incidental on other 
differences, and not a specially endowed quality. As 
the capacity of one plant to be grafted or budded on 
another is unimportant for their welfare in a state of 



18 LAWS GOVERNING THE STERILITY [Chap. IX. 

nature, I presume that no one will suppose that this 
capacity is a specially endowed quality, but will admit 
that it is incidental on differences in the laws of growth 
of the two plants. We can sometimes see the reason 
why one tree will not take on another, from differences 
in their rate of growth, in the hardness of their wood, 
in the period of the flow or nature of their sap, &c. ; 
but in a multitude of cases we can assign no reason 
whatever. Great diversity in the size of two plants, 
one being woody and the other herbaceous, one being 
evergreen and the other deciduous, and adaptation to 
widely different climates, do not always prevent the 
two grafting together. As in hybridisation, so with 
grafting, the capacity is limited by systematic affinity, 
for no one has been able to graft together trees 
belonging to quite distinct families ; and, on the other 
hand, closely allied species, and varieties of the same 
species, can usually, but not invariably, be grafted with 
ease. But this capacity, as in hybridisation, is by 
no means absolutely governed by systematic affinity. 
Although many distinct genera within the same family 
have been grafted together, in other cases species of the 
same genus will not take on each other. The pear can 
be grafted far more readily on the quince, which is 
ranked as a distinct genus, than on the apple, which is 
a member of the same genus. Even different varieties 
of the pear take with different degrees of facility on the 
quince ; so do different varieties of the apricot and 
peach on certain varieties of the plum. 

As Gartner found that there was sometimes an 
innate difference in different individuals of the same 
two species in crossing ; so Sageret believes this to be 
the case with different individuals of the same two 



Chap. IX.] OF FIKST CROSSES AND OF HYBKIDS. 19 

species in being grafted together. As in reciprocal 
crosses, the facility of effecting an union is often very 
far from equal, so it sometimes is in grafting ; the 
common gooseberry, for instance, cannot be grafted on 
the currant, whereas the currant will take, though with 
difficulty, on the gooseberry. 

We have seen that the sterility of hybrids, which 
have their reproductive organs in an imperfect con- 
dition, is a different case from the difficulty of uniting 
two pure species, which have their reproductive organs 
perfect ; yet these two distinct classes of cases run to 
a large extent parallel. Something analogous occurs 
in grafting ; for Thouin found that three species of 
Eobinia, which seeded freely on their own roots, and 
which could be grafted with no great difficulty on a 
fourth species, when thus grafted were rendered barren. 
On the other hand, certain species of Sorbus, when 
grafted on other species yielded twice as much fruit as 
when on their own roots. We are reminded by this 
latter fact of the extraordinary cases of Hippeastrum, 
Passiflora, &c, which seed much more freely when 
fertilised with the pollen of a distinct species, than 
when fertilised with pollen from the same plant. 

We thus see, that, although there is a clear and great 
difference between the mere adhesion of grafted stocks, 
and the union of the male and female elements in the 
act of reproduction, yet that there is a rude degree of 
parallelism in the results of grafting and of crossing 
distinct species. And as we must look at the curious 
and complex laws governing the facility with which 
trees can be grafted on each other as incidental on 
unknown differences in their vegetative systems, so I 
believe that the still more complex laws governing the 

20 



20 ■ CAUSES OF THE STEEILITY [Chap. IX. 

facility of first crosses are incidental on unknown 
differences in their reproductive systems. These 
differences in both cases, follow to a certain extent; as 
might have been expected, systematic affinity, by which 
term every kind of resemblance and dissimilarity 
between organic beings is attempted to be expressed. 
The facts by no means seem to indicate that the greater 
or lesser difficulty of either grafting or crossing various 
species has been a special endowment ; although in the 
case of crossing, the difficulty is as important for the 
endurance and stability of specific forms, as in the case 
of grafting it is unimportant for their welfare. 



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

At one time it appeared to me probable, as it has to 
others, that the sterility of first crosses and of hybrids 
might have been slowly acquired through the natural 
selection of slightly lessened degrees of fertility, which, 
like any other variation, spontaneously appeared in 
certain individuals of one variety when crossed with 
those of another variety. For it would clearly be 
advantageous to two varieties or incipient species, if 
they could be kept from blending, on the same 
principle that, when man is selecting at the same time 
two varieties, it is necessary that he should keep them 
separate. In the first place, it may be remarked that 
species inhabiting distinct regions are often sterile 
when crossed; now it could clearly have been of no 
advantage to such separated species to have been 
rendered mutually sterile, and consequently this could 
not have been effected through natural selection ; but 



Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 21 

it may perhaps be argued, that, if a species was 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 have been rendered utterly impotent on a 
second form, whilst at the same time the male element 
of this second form is enabled freely to fertilise the 
first form ; for this peculiar state of the reproductive 
system could hardly have been advantageous to either 
species. 

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



22 CAUSES OF THE STEEILITY [Chap. IX. 

the survival of those individuals which happened to 
be endowed in a slightly higher degree with mutual 
infertility, and which thus approached by one small 
step towards absolute sterility ? Tet an advance of 
this kind, if the theory of natural selection be brought to 
bear, must have incessantly occurred with many species, 
for a multitude are mutually quite barren. With 
sterile neuter insects we have reason to believe that 
modifications in their structure ctnd fertility have been 
slowly accumulated by natural selection, from an 
advantage having been thus indirectly given to the 
community to which they belonged over other com- 
munities of the same species ; but an individual animal 
not belonging to a social community, if rendered slightly 
sterile when crossed with some other variety, would not 
#hus itself gain any advantage or indirectly give any 
advantage to the other individuals of the same variety, 
thus leading to their preservation. 

But it would be superfluous to discuss this question in 
detail; for with plants we have conclusive evidence 
that the sterility of crossed species must be due to 
some principle, quite independent of natural selection. 
Both Gartner and Kolreuter have proved that in genera 
including numerous species, a series can be formed from 
species which when crossed yield fewer and fewer seeds, 
to species which never produce a single seed, but yet 
are affected by the pollen of certain other species, for 
the germen swells. It is here manifestly impossible to 
select the more sterile individuals, which have already 
ceased to yield seeds ; so that this acme of sterility, 
when the germen alone is affected, cannot have been 
gained through selection ; and from the laws governing 
the various grades of sterility being so uniform through- 



Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 23 

out the animal and vegetable kingdoms, we may infer 
that the cause, whatever it may be, is the same or 
nearly the same in all cases. 

"We will now look a little closer at the probable 
nature of the differences between species which induce 
sterility in first crosses and in hybrids. In the case of 
first crosses, the greater or less difficulty in effecting an 
union and in obtaining offspring apparently depends on 
several distinct causes. There must sometimes be a 
physical impossibility in the male element reaching 
the ovule, as would be the case with a plant having a 
pistil too long for the pollen-tubes to reach the ovarium. 
It has also been observed that when the pollen of one 
species is placed on the stigma of a distantly allied 
species, though the pollen-tubes protrude, they do not 
penetrate the stigmatic surface. Again, the male 
element may reach the female element but be incapable 
of causing an embryo to be developed, as seems to have 
been the case with some of Thuret's experiments on 
Fuci. No explanation can be given of these facts, any 
more than why certain trees cannot be grafted on others. 
Lastly, an embryo may be developed, and then perish 
at an early period. This latter alternative has not been 
sufficiently attended to ; but I believe, from observations 
communicated to me by Mr. Hewitt, who has had great 
experience in hybridising pheasants and fowls, that the 
early death of the embryo is a very frequent cause of 
sterility in first crosses. Mr. Salter has recently given 
the results of an examination of about 500 eggs 
produced from various crosses between three species 
of Gallus and their hybrids ; the majority of these eggs 
had been fertilised ; and in the majority of the fertilised 



24 CAUSES OF THE STERILITY [Chap. IX. 

eggs, the erabrvos had either been partially developed 
and had then perished, or had become nearly mature, 
but the young chickens had been unable to break 
through the shell. Of the chickens which were born, 
more than four-fifths died within the first few days, or 
at latest weeks, " without any obvious cause, apparently 
from mere inability to live ; " so that from the 500 
eggs only twelve chickens were reared. With plants, 
hybridised embryos probably often perish in a like 
manner ; at least it is known that hybrids raised from 
very distinct species are sometimes weak and dwarfed, 
and perish at an early age ; of which fact Max Wichura 
has recently given some striking cases with hybrid 
willows. It may be here worth noticing that in some 
cases of parthenogenesis, the embryos within the eggs 
of silk moths which had not been fertilised, pass 
through their early stages of development and then 
perish like the embryos produced by a cross between 
distinct species. Until becoming acquainted with these 
facts, I was unwilling to believe in the frequent early 
death of hybrid embryos ; for hybrids, when once born, 
are generally healthy and long-lived, as we see in the 
case of the common mule. Hybrids, however, are 
differently circumstanced before and after birth : when 
born and living in a country where their two parents 
live, they are generally placed under suitable conditions 
of life. But a hybrid partakes of only half of the 
nature and constitution of its mother; it may therefore 
before birth, as long as it is nourished vathin its 
mother's womb, or within the egg or seed produced by 
the mother, be exposed to conditions in some degree 
unsuitable, and consequently be liable to perish at an 
early period ; more especially as all very young beings 



Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 25 

are eminently sensitive to injurious or unnatural 
conditions of life. But after all, the cause more 
probably lies in some imperfection in the original act of 
impregnation, causing the embryo to be imperfectly 
developed, rather than in the conditions to which it is 
subsequently exposed. 

In regard to the sterility of hybrids, in which the 
sexual elements are imperfectly developed, the case is 
somewhat different. I have more than once alluded to 
a large body of facts showing that, when animals and 
plants are removed from their natural conditions, they 
are extremely liable to have their reproductive systems 
seriously affected. This, in fact, is the great bar to 
the domestication of animals. Between the sterility 
thus superinduced and that of hybrids, there are many 
points of similarity. In both cases the sterility is 
independent of general health, and is often accompanied 
by excess of size or great luxuriance. In both cases 
the sterility occurs in various degrees ; in both, the 
male element is the most liable to be affected; but 
sometimes the female more than the male. In both, 
the tendency goes to a certain extent with systematic 
affinity, for whole groups of animals and plants are 
rendered impotent by the same unnatural conditions ; 
and whole groups of species tend to produce sterile 
hybrids. On the other hand, one species in a group 
will sometimes resist great changes of conditions with 
unimpaired fertility; and certain species in a group 
will produce unusually fertile hybrids. No one can 
tell, till he tries, whether any 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 



26 CAUSES OF THE STERILITY [Chap. IX 

or less sterile hybrids. Lastly, when organic beings 
are placed during several generations under conditions 
not natural to them, they are extremely liable to vary, 
which seems to be partly due to their reproductive 
systems having been specially affected, though in a 
lesser degree than when sterility ensues. So it is with 
hybrids, for their offspring in successive generations 
are eminently liable to vary, as every experimentalist 
has observed. 

Thus we see that when organic beings are placed 
under new and unnatural conditions, and when hybrids 
are produced by the unnatural crossing of two species, 
the reproductive system, independently of the general 
state of health, is affected in a very similar manner. 
In the one case, the conditions of life have been dis- 
turbed, though often in so slight a degree as to be in- 
appreciable by us ; in the other case, or that of hybrids, 
the external conditions have remained the same, but 
the organisation has been disturbed by two distinct 
structures and constitutions, including of course the 
reproductive systems, having been blended into one. 
For it is scarcely possible that two organisations should 
be compounded into one, without some disturbance 
occurring in the development, or periodical action, or 
mutual relations of the different parts and organs one 
to another or to the conditions of life. "When hybrids 
are able to breed inter se, they transmit to their off- 
spring from generation to generation the same com- 
pounded organisation, and hence we need not be sur- 
prised that their sterility, though in some degree 
variable, does not diminish ; it is even apt to increase, 
this being generally the result, as before explained, 
of too close interbreeding. The above view of the 



Chap. IX.] OF FIEST CEOSSES AND OF HYBKIDS. 27 

sterility of hybrids being caused by two constitutions 
being compounded into one has been strongly main- 
tained by Max Wichura. 

It must, however, be owned that we cannot under- 
stand, on the above or any other view, several facts 
with respect to the sterility of hybrids; for instance, 
the unequal fertility of hybrids produced from recipro- 
cal crosses ; or the increased sterility in those hybrids 
which occasionally and exceptionally resemble closely 
either pure parent. Nor do I pretend that the fore- 
going remarks go to the root of the matter ; no explana- 
tion is offered why an organism, when placed under 
unnatural conditions, is rendered sterile. All that I 
have attempted to show is, that in two cases, in some 
respects allied, sterility is the common result, — in the 
one case from the conditions of life having been dis- 
turbed, in the other case from the organisation having 
been disturbed by two organisations being compounded 
into one. 

A similar parallelism holds good with an allied yet 
very different class of facts. It is an old and almost 
universal belief founded on a considerable body of 
evidence, which I have elsewhere given, that slight 
changes in the conditions of life are beneficial to all 
living things. We see this acted on by farmers and 
gardeners in their frequent exchanges of seed, tubers, 
&c, from one soil or climate to another, and back again. 
During the convalescence of animals, great benefit is 
derived from almost any change in their habits of life. 
Again, both with plants and animals, 'there is the 
clearest evidence that a cross between individuals of the 
same species, which differ to a certain extent, gives 
vigour and fertility to the offspring; and that close 

AMERICAN ETHNC 



189G 
LIBRARY 



28 STERILITY OF HYBEIDS. [Chap. IX. 

interbreeding continued during several generations 
between the nearest relations, if these be kept under 
the same conditions of life, almost always leads to 
decreased size, weakness, or sterility. 

Hence it seems that, on the one hand, slight changes 
in the conditions of life benefit all organic beings, and 
on the other hand, that slight crosses, that is crosses 
between the males and females of the same species, 
which have been subjected to slightly different con- 
ditions, or which have slightly varied, give vigour and 
fertility to the offspring. But, as we have seen, organic 
beings long habituated to certain uniform conditions 
under a state of nature, when subjected, as under con- 
finement, to a considerable change in their conditions, 
very frequently are rendered more or less sterile ; and 
we know that a cross between two forms, that have 
become widely or specifically different, produce hybrids 
which are almost always in some degree sterile. I am 
fully persuaded that this double parallelism is by no 
means an accident or an illusion. He who is able to 
explain why the elephant and a multitude of other 
animals are incapable of breeding when kept under 
only partial confinement in their native country, will 
be able to explain the primary cause of hybrids being 
so generally sterile. He will at the same time be able 
to explain how it is that the races of some of our do- 
mesticated animals, which have often been subjected 
to new and not uniform conditions, are quite fertile 
together, although they are descended from distinct 
species, which would probably have been sterile if 
aboriginally crossed. The above two parallel series of 
facts seem to be connected together by some common 
but unknown bond, which is essentially related to the 



Chap. IX.] DIMORPHISM AND TEIMORPHISM. 29 

principle of life ; this principle, according to Mr. Herbert 
Spencer, being that life depends on, or consists in, the 
incessant action and reaction of various forces, which, 
as throughout nature, are always tending towards an 
equilibrium ; and when this tendency is slightly dis- 
turbed by any change, the vital forces gain in power. 

Reciprocal Dimorphism and Trimorphism. 

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



30 RECIPEOCAL DBIOEPHISM [Chap. IX. 

legitimate, or fully fertile, — and twelve are illegitimate, 
or more or less infertile. 

The infertility which may be observed in various 
dimorphic and trimorphic plants, when they are il- 
legitimately fertilised, that is by pollen taken from 
stamens not corresponding in height with the pistil, 
differs much in degree, np to absolute and utter 
sterility ; just in the same manner as occurs in crossing 
distinct species. As the degree of sterility in the 
latter case depends in an eminent degree on the con- 
ditions of life being more or less favourable, so I have 
found it with illegitimate unions. It is well known 
that if pollen of a distinct species be placed on the stigma 
of a flower, and its own pollen be afterwards, even after 
a considerable interval of time, placed on the same 
stigma, its action is so strongly prepotent that it 
generally annihilates the effect of the foreign pollen ; 
so it is with the pollen of the several forms of the same 
species, for legitimate pollen is strongly prepotent over 
illegitimate pollen, when both are placed on the same 
stigma. I ascertained this by fertilising several flowers, 
first illegitimately, and twenty-four hours afterwards 
legitimately, with pollen taken from a peculiarly 
coloured variety, and all the seedlings were similarly 
coloured ; this shows that the legitimate pollen, though 
applied twenty-four hours subsequently, had wholly 
destroyed or prevented the action of the previously 
applied illegitimate pollen. Again, as in making re- 
ciprocal crosses between the same two species, there is 
occasionally a great difference in the result, so the same 
thing occurs with trimorphic plants ; for instance, the 
mid-styled form of Lythrum salicaria was illegitimately 
fertilised with the greatest ease by pollen from the 



Chap. IX.] AND TKIMOKPHISM. 31 

longer stamens of the short-styled form, and yielded 
many seeds ; but the latter form did not yield a single 
seed when fertilised by the longer stamens of the mid- 
styled form. 

In all these respects, and in others which might be 
added, the forms of the same undoubted species when 
illegitimately united behave in exactly the same 
manner as do two distinct species when crossed. This 
led me carefully to observe during four years many 
seedlings, raised from several illegitimate unions. The 
chief result is that these illegitimate plants, as they 
may be called, are not fully fertile. It is possible to 
raise from dimorphic species, both long-styled and 
short-styled illegitimate plants, and from trimorphic 
plants all three illegitimate forms. These can then be 
properly united in a legitimate manner. When this is 
done, there is no apparent reason why they should not 
yield as many seeds m as did their parents when legiti- 
mately fertilised. But such is not the case. They are 
all infertile, in various degrees ; some being so utterly 
and incurably sterile that they did not yield during four 
seasons a single seed or even seed-capsule. The 
sterility of these illegitimate plants, when united with 
each other in a legitimate manner, may be strictly 
compared with that of hybrids when crossed inter se. 
If, on the other hand, a hybrid is crossed with either 
pure parent-species, the sterility is usually much 
lessened: and so it is when an illegitimate plant is 
fertilised by a legitimate plant. In the same manner 
as the sterility of hybrids does not always run parallel 
with the difficulty of making the first cross between the 
two parent-species, so the sterility of certain illegiti- 
mate plants was unusually great, whilst the sterility of 
21 



32 RECIPROCAL DIMORPHISM [Chap. IX. 

the union from which they were derived was by no 
means great. With hybrids raised from the same seed- 
capsule the degree of sterility is innately variable, so it 
is in a marked manner with illegitimate plants. Lastly, 
many hybrids are profuse and persistent flowerers, 
whilst other and more sterile hybrids produce few 
flowers, and are weak, miserable dwarfs ; exactly 
similar cases occur with the illegitimate offspring of 
various dimorphic and trimorphic plants. 

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



Chap. IX.] AND TEIMOEPHISM. 33 

the common view, that his two varieties were as good 
and as distinct species as any in the world; but he 
would be completely mistaken. 

The facts now given on dimorphic and trimorphic 
plants are important, because they show us, first, that 
the physiological test of lessened fertility, both in first 
crosses and in hybrids, is no safe criterion of specific 
distinction ; secondly, because we may conclude that 
there is some unknown bond which connects the in- 
fertility of illegitimate unions with that of their illegiti- 
mate offspring, and we are led to extend the same view 
to first crosses and hybrids ; thirdly, because we find, 
and this seems to me of especial importance, that two 
or three forms of the same species may exist and may 
differ in no respect whatever, either in structure or in 
constitution, relatively to external conditions, and yet 
be sterile when united in certain ways. For we must 
remember that it is the union of the sexual elements of 
individuals of the same form, for instance, of two long- 
styled forms, which results in sterility ; whilst it is the 
union of the sexual elements proper to two distinct 
forms which is fertile. Hence the case appears at first 
sight exactly the reverse of what occurs, in the ordinary 
unions of the individuals of the same species and 
with crosses between distinct species. It is, however, 
doubtful whether this is really so ; but I will not en- 
large on this obscure subject. 

"We may, however, infer as probable from the con- 
sideration of dimorphic and trimorphic plants, that the 
sterility of distinct species when crossed and of their 
hybrid progeny, depends exclusively on the nature of 
their sexual elements, and not on any difference in their 
structure or general constitution. We are also led to 



34 FEETILITY OF VARIETIES [Chap. IX- 

this same conclusion by considering reciprocal crosses, 
in which the male of one species cannot be united, 
or can be united with great difficulty, with the female 
of a second species, whilst the converse cross can be 
effected with perfect facility. That excellent observer, 
Gartner, likewise concluded that species when crossed 
are sterile owing to differences confined to their repro- 
ductive systems. 



Fertility of Varieties ivhen Crossed, and of their 
Mongrel Ojfsjrring, not universal. 

It may be urged, as an overwhelming argument, that 
there must be some essential distinction between species 
and varieties, inasmuch as the latter, however much 
they may differ from each other in external appearance, 
cross with perfect facility, and yield perfectly fertile 
offspring. With some exceptions, presently to be 
given, I fully admit that tins is the rule. But the 
subject is surrounded by difficulties, for, looking to 
varieties produced under nature, if two forms hitherto 
reputed to be varieties be found in any degree sterile 
ther, they are at once ranked by most naturalists 
as species. F r instance, the blue and red pimpernel, 
which are considered by most botanists as varieties, are 
said by Gartner to be quite sterile when crossed, and 
he consequently ranks them as undoubted species. If 
we thus argue in a circle, the fertility of all varieties 
produced under nature will assuredly have to be 
granted. 

If we turn to varieties, produced, or supposed to have 
been produced, under domestication, we are still in- 
volved in some doubt. For when it is stated, for 



Chap. IX.] WHEN CROSSED. 35 

instance, that certain South American indigenous do- 
mestic dogs do not readily unite with European dogs, 
the explanation which will occur to every one, and 
probably the true one, is that they are descended from 
aboriginally distinct species. Nevertheless the perfect 
fertility of so many domestic races, differing widely 
from each other in appearance, for instance those of the 
pigeon, or of the cabbage, is a remarkable fact ; more 
especially when we reflect how many species there are, 
which, though resembling each other most closely, are 
utterly sterile when intercrossed. Several considera- 
tions, however, render the fertility of domestic varieties 
less remarkable. In the first place, it may be observed 
that the amount of external difference between two 
species is no sure guide to their degree of mutual 
sterility, so that similar differences in the case of 
varieties would be no sure guide. It is certain that with 
species the cause lies exclusively in differences in their 
sexual constitution. Now the varying conditions to 
which domesticated animals and cultivated plants have 
been subjected, have had so little tendency towards 
modifying the reproductive system in a manner leading 
to mutual sterility, that we have good grounds for ad- 
mitting the directly opposite doctrine of Pallas, namely, 
that such conditions generally eliminate this tendency ; 
so that the domesticated descendants of species, which 
in their natural state probably would have been in 
some degree sterile when crossed, become perfectly 
fertile together. With plants, so far is cultivation from 
giving a tendency towards sterility between distinct 
species, that in several well-authenticated cases already 
alluded to, certain plants have been affected in an 
opposite manner, for they have become self-impotent 



36 FERTILITY OF VARIETIES [C^ap. IX 

whilst still retaining the capacity of fertilising, and 
being fertilised by, other species. If the Pallasian 
doctrine of the elimination of sterility through long- 
continued domestication be admitted, and it can hardly 
be rejected, it becomes in the highest degree improbable 
that similar conditions long-continued should likewise 
induce this tendency; though in certain cases, with 
species having a peculiar constitution, sterility might 
occasionally be thus caused. Thus, as I believe, we 
can understand why with domesticated animals varieties 
have not been produced which are mutually sterile; 
and why with plants only a few such cases, immediately 
to be given, have been observed. 

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



Chap. IX.] WHEN CEOSSED. 37 

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

I have as yet spoken as if the varieties of the same 
species were invariably fertile when intercrossed. But 
it is impossible to resist the evidence of the existence 
of a certain amount of sterility in the few following- 
cases, which I will briefly abstract. The evidence is 
at least as good as that from which we believe in the 
sterility of a multitude of species. The evidence is, 
also, derived from hostile witnesses, who in all other 
cases consider fertility and sterility as safe criterions of 
specific distinction. Gartner kept during several years 
a dwarf kind of maize with yellow seeds, and a tall 
variety with red seeds growing near each other in his 
garden ; and although these plants have separated 
sexes, they never naturally crossed. He then fertilised 
thirteen flowers of the one kind with pollen of the 
other ; but only a single head produced any seed, and 
this one head produced only five grains. Manipulation 
in this case could not have been injurious, as the plants 
have separated sexes. No one, I believe, has suspected 
that these varieties of maize are distinct species ; and it 
is important to notice that the hybrid plants thus 
raised were themselves 'perfectly fertile; so that even 
Gartner did not venture to consider the two varieties as 
specifically distinct. 



38 FERTILITY OF VARIETIES [Chap. IX. 

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

The following case is far more remarkable, and seems 
at first incredible ; but it is the result of an astonishing 
number of experiments made during many years on 
nine species of Verbascum, by so good an observer and 
so hostile a witness as Gartner : namely that the yellow 
and white varieties when crossed produce less seed than 
the similarly coloured varieties of the same species. 
Moreover, he asserts that, when yellow and white 
varieties of one species are crossed with yellow and 
white varieties of a distinct species, more seed is 
produced by the crosses between the similarly coloured 
flowers, than between those which are differently 
coloured. Mr. Scott also has experimented on the 
species and varieties of Verbascum; and although 
unable to confirm Gartner's results on the crossing of 
the distinct species, he finds that the dissimilarly 
coloured varieties of the same species yield fewer seeds, 
in the proportion of 86 to 100, than the similarly 
coloured varieties. Yet these varieties differ in no 
respect except in the colour of their flowers ; and one 
variety can sometimes be raised from the seed of another. 

Kolreuter, whose accuracy has been confirmed by 
every subsequent observer, has proved the remarkable 
fact, that one particular variety of the common tobacco 
was more fertile than the other varieties, when crossed 



Chap. IX.] WHEN CROSSED. 89 

with a widely distinct species. He experimented on 
five forms which are commonly reputed to be varieties, 
and which he tested by the severest trial, namely, by 
reciprocal crosses, and he found their mongrel offspring 
perfectly fertile. But one of these five varieties, when 
used either as the father or mother, and crossed with 
the Mcotiana glutinosa, always yielded hybrids not so 
sterile as those which were produced from the four 
other varieties when crossed with N". glutinosa. Hence 
the reproductive system of this one variety must have 
been in some manner and in some degree modified. 

From these facts it can no longer be maintained that 
varieties when crossed are invariably quite fertile. 
From the great difficulty of ascertaining the infertility 
of 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 attend- 
ing only to external characters in his domestic varieties, 
and from such varieties not having been exposed for 
very long periods to uniform conditions of life ; — from 
these several considerations we may conclude that 
fertility does not constitute a fundamental distinction 
between varieties and species when crossed. The 
general sterility of crossed species may safely be looked 
at, not as a special acquirement or endowment, but as 
incidental on changes of an unknown nature in their 
sexual elements. 



Hybrids and Mongrels compared, independently of their 

fertility. 

Independently of the question of fertility, the off- 
spring of species and of varieties when crossed may be 



40 HYBEIDS AXD aIONGEELS COMPAEED. [Chap. IX. 

compared in several other respects. Gartner, whose 
strong wish it was to draw a distinct line between 
species and varieties, could find very few, and, as it 
seems to me, quite unimportant differences between the 
so-called hybrid offspring of species, and the so-called 
mongrel offspring of varieties. And, on the other hand, 
they agree most closely in many important respects. 

I shall here discuss this subject with extreme brevity. 
The most important distinction is, that in the first 
generation mongrels are more variable than hybrids ; 
but Gartner admits that hybrids from species which 
have long been cultivated are often variable in the 
first generation ; and I have myself seen striking 
instances of this fact. Gartner further admits that 
hybrids between very closely allied species are more 
variable than those from very distinct species ; and 
this shows that the difference in the degree of variability 
graduates away. When mongrels and the more fertile 
hybrids are propagated for several generations, an extreme 
amount of variability in the offspring in both cases is 
notorious ; but some few instances of both hybrids and 
mongrels long retaining a uniform character could be 
given. The variability, however, in the successive 
generations of mongrels is, perhaps, greater than in 
hybrids. 

This greater variability in mongrels than in hybrids 
does not seem at all surprising. For the parents of 
mongrels are varieties, and mostly domestic varieties 
(very few experiments having been tried on natural 
varieties), and this implies that there has been recent 
variability, which would often continue and would 
augment that arising from the act of crossing. The 
slight variability of hybrids in the first generation, in 



Chap. IX.] HYBEIDS AND MONGEELS COMPARED. 41 

contrast with that in the succeeding generations, is a 
curious fact and deserves attention. For it bears on 
the view which I have taken of one of the causes of 
ordinary variability; namely, that the reproductive 
system from being eminently sensitive to changed 
conditions of life, fails under these circumstances to 
perform its proper function of producing offspring 
closely similar in all respects to the parent-form. "Now 
hybrids in the first generation are descended from 
species (excluding those long- cultivated) which have 
not had their reproductive systems in any way affected, 
and they are not variable ; but hybrids themselves have 
their reproductive systems seriously affected, and their 
descendants are highly variable. 

But to return to our comparison of mongrels and 
hybrids : Gartner states that mongrels are more liable 
than hybrids to revert to either parent-form ; but this, 
if it be true, is certainly only a difference in degree. 
Moreover, Gartner expressly states that hybrids from 
long cultivated plants are more subject to reversion 
than hybrids from species in their natural state; and 
this probably explains the singular difference in the 
results arrived at by different observers : thus Max 
Wichura doubts whether hybrids ever revert to their 
parent-forms, and he experimented on uncultivated 
species of willows ; whilst Naudin, on the other hand, 
insists in the strongest terms on the almost universal 
tendency to reversion in hybrids, and he experimented 
chiefly on cultivated plants. Gartner further states 
that when any two species, although most closely allied 
to each other, are crossed with a third species, the 
hybrids are widely different from each other ; whereas 
if two very distinct varieties of one species are crossed 



42 HYBRIDS AND MONGRELS COMPARED. [Chap. IX. 

with another species, the hybrids do not differ mnch. 
But this conclusion, as far as I can make out, is 
founded on a single experiment; and seems directly 
opposed to the results of several experiments made by 
Kolreuter. 

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

These several remarks are apparently applicable to 
animals ; but the subject is here much complicated, 
partly owing to the existence of secondary sexual 
characters ; but more especially owing to prepotency in 
transmitting likeness running more strongly in one sex 
than in the other, both when one species is crossed with 
another, and when one variety is crossed with another 
variety. For instance, I think those authors are right 
who maintain that the ass has a prepotent power over 
the horse, so that both the mule and the hinny resemble 
more closely the ass than the horse ; but that the pre- 



Chap. IX.] HYBRIDS AND MONGRELS COMPARED. 43 

potency runs more strongly in the male than in the 
female ass, so that the mule, which is the offspring of 
the male ass and mare, is more like an ass, than is 
the hinny, which is the offspring of the female ass and 
stallion. 

Much stress has been laid by some authors on the 
supposed fact, that it is only with mongrels that the 
offspring are not intermediate in character, but closely 
resemble one of their parents ; but this does sometimes 
occur with hybrids, yet I grant much less frequently 
than with mongrels. Looking to the cases which I 
have collected of cross-bred animals closely resembling 
one parent, the resemblances seem chiefly confined to 
characters almost monstrous in their nature, and which 
have suddenly appeared — such as albinism, melanism, 
deficiency of tail or horns, or additional fingers and 
toes ; and do not relate to characters which have been 
slowly acquired through selection. A tendency to 
sudden reversions to the perfect character of either 
parent would, also, be much more likely to occur with 
mongrels, which are descended from varieties often 
suddenly produced and semi-monstrous in character, 
than with hybrids, which are descended from species 
slowly and naturally produced. On the whole, I 
entirely agree with Dr. Prosper Lucas, who, after 
arranging an enormous body of facts with respect to 
animals, comes to the conclusion that the laws of 
resemblance of the child to its parents are the same, 
whether the two parents differ little or much from each 
other, namely, in the union of individuals of the same 
variety, or of different varieties, or of distinct species. 

Independently of the question of fertility and sterility, 

in all other respects there seems to be a general and 
22 



44 SUMMAEY, [Chap. IX 

close similarity in the offspring of crossed species, and 
of crossed varieties. If we look at species as having 
been specially created, and at varieties as having been 
produced by secondary laws, this similarity would be 
an astonishing fact. But it harmonises perfectly with 
the view that there is no essential distinction between 
species and varieties. 

Summary of Chapter. 

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

In the same manner as in grafting trees, the capacity 
in one species or variety to take on another, is incidental 
on differences, generally of an unknown nature, in their 
vegetative systems, so in crossing, the greater or less 
facility of one species to unite with another is incidental 
on unknown differences in their reproductive systems. 
There is no more reason to think that species have been 
specially endowed with various degrees of sterility to 
prevent their crossing and blending in nature, than to 



Chap. IX.] SUMMARY. 45 

think that trees have been specially endowed with 
various and somewhat analogous degrees of difficulty 
in being grafted together in order to prevent their 
inarching in our forests. 

The sterility of first crosses and of their hybrid 
progeny has not been acquired through natural 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 
hybrids, it apparently depends on their whole or- 
ganisation having been disturbed by being compounded 
from two distinct forms ; the sterility being closely 
allied to that which so frequently affects pure species, 
when exposed to new and unnatural conditions of 
life. He who will explain these latter cases will 
be able to explain the sterility of hybrids. This 
view is strongly supported by a parallelism of another 
kind : namely, that, firstly, slight changes in the 
conditions of life add to the vigour and fertility of all 
organic beings; and secondly* that the crossing of 
forms, which have been exposed to slightly different 
conditions of life or which have varied, favours the size, 
vigour, and fertility of their offspring, The facts given 
on the sterility of the illegitimate unions of dimorphic 
and trimorphic plants and of their illegitimate progeny, 
perhaps render it probable that some unknown bond in 
all cases connects the degree of fertility of first unions 
with that of their offspring. The consideration of these 
facts on dimorphism, as well as of the results of reci- 
procal crosses, clearly leads to the conclusion that the 
primary cause of the sterility of crossed species is 
confined to differences in their sexual elements. But 
why, in the case of distinct species, the sexual elements 



46 SUMMARY. [Chap. IX. 

should so generally have become more or less modified, 
leading to their mutual infertility, we do not know ; but 
it seems to stand in some close relation to species having 
been exposed for long periods of time to nearly uniform 
conditions of life. 

It is not surprising that the difficulty in crossing any 
two species, and the sterility of their hybrid offspring, 
should in most cases correspond, even if due to distinct 
causes : for both depend on the amount of difference 
between the species which are crossed. Nor is it 
surprising that the facility of effecting a first cross, and 
the fertility of the hybrids thus produced, and the 
capacity of being grafted together — though this latter 
capacity evidently depends on widely different circum- 
stances — should all run, to a certain extent, parallel 
with the systematic affinity of the forms subjected to 
experiment ; for systematic affinity includes resem- 
blances of all kinds. 

First crosses between forms known to be varieties, or 
sufficiently alike to be considered as varieties, and their 
mongrel offspring, are very generally, but not, as is so 
often stated, invariably fertile. Nor is this almost 
universal and perfect fertility surprising, when it is 
remembered how liable we are to argue in a circle with 
respect to varieties in a state of nature ; and when we 
remember that the greater number of varieties have 
been produced under domestication by the selection of 
mere external differences, and that they have not been 
long exposed to uniform conditions of life. It should 
also be especially kept in mind, that long-continued 
domestication tends to eliminate sterility, and is there- 
fore little likely to induce this same quality. Indepen- 
dently of the question of fertility, in all other respects 



Chap. IX.] SUMMARY. 47 

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



48 IMPERFECTION OF THE [Chap. X. 



CHAPTEE X. 
On the Imperfection of the Geological Record. 

On the absence of intermediate varieties at the present day — On 
the nature of extinct intermediate varieties ; on their number — 
On the lapse of time, as inferred from the rate of denudation 
and of deposition — On the lapse of time as estimated by years 
— On the poorness of our palceontological collections — On the 
intermittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden aj3pearance of groups of species — - 
On their sudden appearance in the lowest known fossiliferous 
strata — Antiquity of the habitable earth. 

In the sixth chapter I enumerated the chief objections 
which might be justly urged against the views main- 
tained in this volume. Most of them have now been 
discussed. One, namely the distinctness of specific 
forms, and their not being blended together by innumer- 
able transitional links, is a very obvious difficulty. I 
assigned reasons why such links do not commonly occur 
at the present day under the circumstances apparently 
most favourable for their presence, namely on an 
extensive and continuous area with graduated physical 
conditions. I endeavoured to show, that the life of 
each species depends in a more important manner on 
the presence of other already defined organic forms, 
than on climate, and, therefore, that the really governing 
conditions of life do not graduate away quite insensibly 
like heat or moisture. I endeavoured, also, to show 



Chap. X.] GEOLOGICAL KECOED. 49 

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 
supplant their parent-forms. But just in proportion as 
this process of extermination has acted on an enormous 
scale, so must the number of intermediate varieties, 
which have formerly existed, be truly enormous. Why 
then is not every geological formation and every stratum 
full of such intermediate links ? Geology assuredly does 
not reveal any such finely-graduated organic chain ; and 
this, perhaps, is the most obvious and serious objection 
which can be urged against the theory. The explanation 
lies, as I believe, in the extreme imperfection of the 
geological record. 

In the first place, it should always be borne in mind 
what sort of intermediate forms must, on the theory, 
have formerly existed. I have found it difficult, when 
looking at any two species, to avoid picturing to 
myself forms directly intermediate between them. But 
this is a wholly false view ; we should always look for 
forms intermediate between each species and a common 
but unknown progenitor ; and the progenitor will gen- 
erally have differed in some respects from all its 
modified descendants. To give a simple illustration: 
the fantail and pouter pigeons are both descended from 
the rock-pigeon ; if we possessed all the intermediate 
varieties which have ever existed, we should have an 
extremely close series between both and the rock- 



50 IMPERFECTION OF THE [Chap. X 

pigeon ; but we should have no varieties directly 
intermediate between the fantail and pouter; none, 
for instance, combining a tail somewhat expanded with 
a crop somewhat enlarged, the characteristic features of 
these two breeds. These two breeds, moreover, have 
become so much modified, that, if we had no historical 
or indirect evidence regarding their origin, it would not 
have been possible to have determined, from a mere 
comparison of their structure with that of the rock- 
pigeon, C. livia, whether they had descended from this 
species or from some other allied form, such as C. oenas. 

So, with natural species, if we look to forms very 
distinct, for instance to the horse and tapir, we have 
no reason to suppose that links directly intermediate 
between them ever existed, but between each and an 
unknown common parent. The common parent will 
have had in its whole organisation much general re- 
semblance to the tapir and to the horse ; but in some 
points of structure may have differed considerably from 
both, even perhaps more than they differ from each 
other. Hence, in all such cases, we should be unable 
to recognise the parent-form of any two or more species, 
even if we closely compared the structure of the parent 
with that of its modified descendants, unless at the 
same time we had a nearly perfect chain of the inter- 
mediate links. 

It is just possible by the theory, that one of two 
living forms misfht have descended from the other ; for 
instance, a horse from a tapir ; and in this case direct 
intermediate links will have existed between them. 
But such a case would imply that one form had 
remained for a very long period unaltered, whilst its 
descendants had undergone avast amount of change; 



Chap. X.] GEOLOGICAL KECOED. 51 

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 differences not greater than we see between 
the natural and domestic varieties of the same species 
at the present day; and these parent-species, now 
generally extinct, have in their turn been similarly 
connected with more ancient forms ; and so on back- 
wards, always converging to the common ancestor of 
each great class. So that the number of intermediate 
and transitional links, between all living and extinct 
species, must have been inconceivably great. But 
assuredly, if this theory be true, such have lived upon 
the earth. 



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

Independently of our not rinding fossil remains of 
such infinitely numerous connecting links, it may be 
objected that time cannot have sufficed for so great an 
amount of organic change, all changes having been 
effected slowly. It is hardly possible for me to recall 
to the reader who is not a practical geologist, the facts 
leading the mind feebly to comprehend the lapse of time. 
He who can read Sir Charles Ly ell's grand work on the 
Principles of Geology, which the future historian will 
recognise as having produced a revolution in natural 
science, and yet does not admit how vast have been the 



52 THE LAPSE OF TIME. [Chap. X- 

past periods of time, may at once close this volume. 
Not that it suffices to study the Principles of Geology, 
or to read special treatises by different observers on 
separate formations, and to mark how each author 
attempts to give an inadequate idea of the duration of 
each formation, or even of each stratum. We can best 
gain some idea of past time by knowing the agencies at 
work, and learning how deeply the surface of the land 
has been denuded, and how much sediment has been 
deposited. As Lyell has well remarked, the extent and 
thickness of our sedimentary formations are the result 
and the measure of the denudation which the earth's 
crust has elsewhere undergone. Therefore a man 
should examine for himself the great piles of super- 
imposed strata, and watch the rivulets bringing down 
mud, and the waves wearing away the sea-cliffs, in 
order to comprehend something about the duration of 
past time, the monuments of which we see all around 
us. 

It is good to wander along the coast, when formed of 
moderately hard rocks, and mark the process of degrad- 
ation. The tides in most cases reach the cliffs only for 
a short time twice a day, and the waves eat into them 
only when they are charged with sand or pebbles; 
for there is good evidence that pure water effects 
nothing in wearing away rock. At last the base of the 
cliff is undermined, huge fragments fall down, and 
these, remaining fixed, have to be worn away atom by 
atom, until after being reduced in size they can be 
rolled about by the waves, and then they are more 
quickly ground into pebbles, sand, or mud. But how 
often do we see along the bases of retreating cliffs 
rounded boulders, all thickly clothed by marine pro- 



Chap. X.] THE LAPSE OF TIME. 53 

ductions, showing how little they are abraded and how 
seldom they are rolled about ! Moreover, if we follow 
for a few rniles any line of rocky cliff, which is under- 
going degradation, we find that it is only here and there, 
along a short length or round a promontory, that the 
cliffs are at the present time suffering. » The appearance 
of the surface and the vegetation show that elsewhere 
years have elapsed since the waters washed their base. 

We have, however, recently learnt from the obser- 
vations of Eamsay, in the van of many excellent 
observers — of Jukes, Geikie, Croll, and others, that 
subaerial degradation is a much more important agency 
than coast-action, or the power of the waves. The 
whole surface of the land is exposed to the chemical 
action of the air and of the rain-water with its dissolved 
carbonic acid, and in colder countries to frost; the 
disintegrated matter is carried down even gentle slopes 
during heavy rain, and to a greater extent than might 
be supposed, especially in arid districts, by the wind ; it 
is then transported by the streams and rivers, which 
when rapid deepen their channels, and triturate the 
fragments. On a rainy day, even in a gently undula- 
ting country, we see the effects of subaerial degradation 
in the muddy rills which flow down every slope. 
Messrs. Eamsay and Whitaker have shown, and the 
observation is a most striking one, that the great lines 
of escarpment in the Wealden district and those ranging 
across England, which formerly were looked at as 
ancient sea-coasts, cannot have been thus formed, for 
each line is composed of one and the same formation, 
whilst our sea-cliffs are everywhere formed by the 
mtersection of various formations. This being the case, 
we are compelled to admit that the escarpments owe 



54 THE LAPSE OF TDIE. [Chap. X. 

their origin in chief part to the rocks of which they are 
composed having resisted subaerial denudation better 
than the surrounding surface ; this surface consequently 
has been gradually lowered, with the lines of harder 
rock left projecting. jSTothing impresses the mind with 
the vast duration of time, according to our ideas of time, 
more forcibly than the conviction thus gained that 
subaerial agencies which apparently have so little 
power, and which seem to work so slowly, have pro- 
duced great results. 

When thus impressed with the slow rate at which 

the land is worn away through subaerial and littoral 

action, it is good,' in order to appreciate the past 

duration of time, to consider, on the one hand, the 

masses of rock which have been removed over many 

extensive areas, and on the other hand the thickness of 

our sedimentary formations. I remember having been 

much struck when viewing volcanic islands, which have 

been worn by the waves and pared all round into 

perpendicular cliffs of one or two thousand feet in 

height ; for the gentle slope of the lava-streams, due to 

their formerly liquid state, showed at a glance how far 

the hard, rocky beds had once extended into the open 

ocean. The same story is told still more plainly by 

faults, — those great cracks along which the strata have 

been upheaved on one side, or thrown down on the 

other, to the height or depth of thousands of feet ; for 

since the crust cracked, and it makes no great difference 

whether the upheaval was sudden, or, as most geologists 

now believe, was slow and effected by many starts, the 

surface of the land has been so completely planed down 

that no trace of these vast dislocations is externally 

visible. The Craven fault, for instance extends for 



Chap. X.] THE LAPSE OF TIME. 55 

upwards of 30 miles, and along this line the vertical 
displacement of the strata varies from 600 to 3000 feet. 
Professor Bamsay has published an account of a down- 
throw in Anglesea of 2300 feet; and he informs me 
that he fully believes that there is one in Merioneth- 
shire of 12,000 feet ; yet in these cases there is nothing 
on the surface of the land to show such prodigious 
movements ; the pile of rocks on either side of the 
crack having been smoothly swept away. 

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

Feet. 

Paleozoic strata (not including igneous beds) .. .. 57,154 

Secondary strata .. .. „ 13,190 

Tertiary strata 2,240 

— making altogether 72,584 feet; that is, very nearly 
thirteen and three-quarters British miles. Some of the 
formations, which are represented in England by thin 
beds, are thousands of feet in thickness on the Con- 
tinent. Moreover, between each successive formation, 
we have, in the opinion of most geologists, blank 
periods of enormous length. So that the lofty pile of 

sedimentary rocks in Britain gives but an inadequate 
23 



56 THE LAPSE OF TD1E. [Chap. X. 

idea of the time which has elapsed during their accumula- 
tion. The consideration of these various facts impresses 
the mind almost in the same manner as does the vain 
endeavour to grapple with the idea of eternity. 

Nevertheless this impression is partly false. Mr. 
Croll, in an interesting paper, remarks that we do not 
err " in forming too great a conception of the length of 
" geological periods," but in estimating them by years. 
When geologists look at large and complicated phe- 
nomena, and then at the figures representing several 
million years, the two produce a totally different effect 
on the mind, and the figures are at once pronounced too 
small. In regard to subaerial denudation, Mr. Croll 
shows, by calculating the known amount of sediment 
annually brought down by certain rivers, relatively to 
their areas of drainage, that 1000 feet of solid rock, as 
it became gradually disintegrated, would thus be re- 
moved from the mean level of the whole area in the 
course of six million years. This seems an astonishing 
result, and some considerations lead to the suspicion 
that it may be too large, but even if halved or quartered 
it is still very surprising. Few of us, however, know 
what a million really means : Mr. Croll gives the 
following illustration : take a narrow strip of paper, 83 
feet 4 inches in length, and stretch it along the wall of 
a large hall ; then mark off at one end the tenth of an 
inch. This tenth of an inch will represent one hundred 
years, and the entire strip a million years. But let it 
be borne in mind, in relation to the subject of this work, 
what a hundred years implies, represented as it is by a 
measure utterly insignificant in a hall of the above 
dimensions. Several eminent breeders, during a single 
lifetime, have so largely modified some of the higher 



Chap. X.] THE LAPSE OF TIME. 57 

animals, which propagate their kind much more slowly 
than most of the lower animals, that they have formed 
what well deserves to be called a new sub-breed. Few men 
have attended with due care to any one strain for more 
than half a century, so that' a hundred years represents 
the work of two breeders in succession. It is not to be 
supposed that species in a state of nature ever change 
so quickly as domestic animals under the guidance of 
methodical selection. The comparison would be in 
every way fairer with the effects which follow from 
unconscious selection, that is the preservation of the 
most useful or beautiful animals, with no intention of 
modifying the breed ; but by this process of unconscious 
selection, various breeds have been sensibly changed in 
the course of two or three centuries. 

Species, however, probably change much more slowly, 
and within the same country only a few change at the 
same time. This slowness follows from all the inhabit- 
ants of the same country being already so well adapted 
to each other, that new places in the polity of nature do 
not occur until after long intervals, due to the occur- 
rence of physical changes of some kind, or through the 
immigration of new forms. Moreover variations or 
individual differences of the right nature, by which 
some of the inhabitants might be better fitted to their 
new places under the altered circumstances, would not 
always occur at once. Unfortunately we have no means 
of determining, according to the standard of years, how 
long a period it takes to modify a species ; but to the 
subject of time we must return. 



58 THE POORNESS OF OUR [Chap. X 

On the Poorness of Palccontological Collections. 

Now let us turn to our richest geological museums, 
and what a paltry display we behold ! That our col- 
lections are imperfect is admitted by every one. The 
remark of that admirable palaeontologist, Edward Forbes, 
should never be forgotten, namely, that very many fossil 
species are known and named from single and often 
broken specimens, or from a few specimens collected on 
some one spot. Only a small portion of the surface of 
the earth has been geologically explored, and no part 
with sufficient care, as the important discoveries made 
every year in Europe prove. Xo organism wholly soft 
can be preserved. Shells and bones decay and disappear 
when left on the bottom of the sea, where sediment is 
not accumulating. We probably take a quite erroneous 
view, when we assume that sediment is being deposited 
over nearly the whole bed of the sea, at a rate 
sufficiently quick to embed and preserve fossil remains. 
Throughout an enormously large 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, without the underlying bed having 
suffered in the interval any wear and tear, seem 
explicable only on the view of the bottom of the sea not 
rarely lying for ages in an unaltered condition. The 
remains which do become embedded, if in sand or 
gravel, will, when the beds are upraised, generally be 
dissolved by the percolation of rain-water charged with 
carbolic acid. Some of the many kinds of animals 
which live on the beach between high and low water 
mark seem to be rarely preserved. For instance, the 



Chap. X.] PAL^ONTOLOGICAL COLLECTIONS. 59 

several species of the Chthamalinse (a sub-family of 
sessile cirripedes) coat the rocks all over the world in 
infinite numbers : they are all strictly littoral, with the 
exception of a single Mediterranean species, which 
inhabits deep water, and this has been found fossil in 
Sicily, whereas not one other species has hitherto been 
found in any tertiary formation : yet it is known that 
the genus Chthamalus existed during the Chalk period. 
Lastly, many great deposits requiring a vast length of 
time for their accumulation, are entirely destitute of 
organic remains, without our being able to assign any 
reason : one of the most striking instances is that of the 
Flysch formation, which consists of shale and sandstone, 
several thousand, occasionally even six thousand feet in 
thickness, and extending for at least 300 miles from 
Vienna to Switzerland; and although this great mass 
has been most carefully searched, no fossils, except a 
few vegetable remains, have been found. 

With respect to the terrestrial productions which lived 
during the Secondary and Palaeozoic periods, it is super- 
fluous to state that our evidence is fragmentary in an 
extreme degree. For instance, until recently not a land- 
shell was known belonging to either of these vast 
periods, with the exception of one species discovered by 
Sir C. Lyell and Dr. Dawson in the carboniferous strata 
of North America ; but now land-shells have been found 
in the lias. In regard to mammiferous remains, a 
glance at the historical table published in Lyell's 
Manual will bring home the truth, how accidental and 
rare is their preservation, far better than pages of detail. 
Nor is their rarity surprising, when we remember how 
large a proportion of the bones of tertiary mammals 
have been discovered either in caves or in lacustrine 



60 THE POOEXESS OF OUE [Chap. X. 

deposits ; and that not a cave or true lacustrine bed is 
known belonging to the age of our secondary or 
palaeozoic formations. 

But the imperfection in the geological record largely 
results from another and more important cause than 
any of the foregoing ; namely, from the several forma- 
tions being separated from each other by wide intervals 
of time. This doctrine has been emphatically admitted 
by many geologists and palaeontologists, who, like E. 
Forbes, entirely disbelieve in the change of species. 
When we see the formations tabulated in written 
works, or when we follow them in nature, it is difficult 
to avoid believing that they are closely consecutive. 
But we know, for instance, from Sir R. Murchison's 
great work on Russia, what wide gaps there are in that 
country between the superimposed formations ; so it is 
in Xortk America, and in many other parts of the 
world. The most skilful geologist, if his attention had 
been confined exclusively to these large territories, 
would never have suspected that, during the periods 
which were blank and barren in his own country, great 
piles of sediment, charged with new and peculiar forms 
of life, had elsewhere been accumulated. And if, in 
each separate territory, hardly any idea can be formed 
of the length of time which has elapsed between the 
consecutive formations, we may infer that this could 
nowhere be ascertained. The frequent and great 
changes in the mineralogical composition of consecutive 
formations, generally implying great changes in the 
geography of the surrounding lands, whence the sedi- 
ment was derived, accord with the belief of vast inter- 
vals of time having elapsed between each formation. 

We can, I think, see why the geological formations 



Chap. X.] PALaEONTOLOGICAL COLLECTIONS. 61 

of each region are almost invariably intermittent ; that 
is, have not followed each other in close sequence. 
Scarcely any fact struck me more when examining 
many hundred miles of the South American coasts, 
which have been upraised several hundred feet within 
the recent period, than the absence of any recent de- 
posits sufficiently extensive to last for even a short 
geological period. Along the whole west coast, which is 
inhabited by a peculiar marine fauna, tertiary beds are 
so poorly developed, that no record of several successive 
and peculiar marine faunas will probably be preserved 
to a distant age. A little reflection will explain why, 
along the rising coast of the western side of South 
America, no extensive formations with recent or ter- 
tiary remains can anywhere be found, though the supply 
of sediment must for ages have been great, from the 
enormous degradation of the coast-rocks and from 
muddy streams entering the sea. The explanation, no 
doubt, is, that the littoral and sub-littoral deposits are 
continually worn away, as soon as they are brought up 
by the slow and gradual rising of the land within the 
grinding action of the coast- waves. 

We may, I think, conclude that sediment must be 
accumulated in extremely thick, solid, or extensive 
masses, in order to withstand the incessant action of 
the waves, when first upraised and during successive 
oscillations of level, as well as the subsequent subaerial 
degradation. Such thick and extensive accumulations 
of sediment may be formed in two ways ; either in pro- 
found depths of the sea, in which case the bottom will 
not be inhabited by so many and such varied forms of 
life, as the more shallow seas ; and the mass when 
upraised will give an imperfect record of the organisms 



62 THE POOEXESS OF OUR [Chap. X. 

which existed in the neighbourhood during the period 
of its accumulation. Or, sediment may be deposited to 
any thickness and extent over a shallow bottom, if it 
continue slowly to subside. In this latter case, as long 
as the rate of subsidence and the supply of sediment 
nearly balance each other, the sea will remain shallow 
and favourable for many and varied forms, and thus 
a rich fossiliferous formation, thick enough, when up- 
raised, to resist a large amount of denudation, may be 
formed. 

I am convinced that nearly all our ancient formations, 
which are throughout the greater part of their thickness 
rich in fossils, have thus been formed during subsidence. 
Since publishing my views on this subject in 1845, I 
have watched the progress of Geology, and have been 
surprised to note how author after author, in treating 
of this or that great formation, has come to the con- 
clusion that it was accumulated during subsidence. I 
may add, that the only ancient tertiary formation on 
the west coast of South America, which has been bulky 
enough to resist such degradation as it has as yet 
suffered, but which will hardly last to a distant 
geological age, was deposited during a downward 
oscillation of level, and thus gained considerable 
thickness. 

All geological facts tell us plainly that each area 
has undergone numerous slow oscillations of level, 
and apparently these oscillations have affected wide 
spaces. Consequently, formations rich in fossils and 
sufficiently thick and extensive to resist subsequent 
degradation, will have been formed over wide spaces 
during periods of subsidence, but only where the 
supply of sediment was sufficient to keep the sea 



Chap. X.] PAL^ONTOLOGICAL COLLECTIONS. 63 

shallow and to embed and preserve the remains before 
they had time to decay. On the other hand, as long as 
the bed of the sea remains stationary, thick deposits 
cannot have been accumulated in the shallow parts, 
which are the most favourable to life. Still less can 
this have happened during the alternate periods of 
elevation ; or, to speak more accurately, the beds which 
were then accumulated will generally have been 
destroyed by being upraised and brought within the 
limits of the coast-action. 

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

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



64 THE POORNESS OF OUR [Chap. X. 

the few who believe that our present met amorphic 
schists and plutonic rocks once formed the primordial 
nucleus of the globe, will admit that these latter rocks 
have been stript of their covering to an enormous 
extent. For it is scarcely possible that such rocks 
could have been solidified and crystallized whilst 
uncovered ; but if the metamorphic action occurred at 
profound depths of the ocean, the former protecting 
mantle of rock may not have been very thick. Ad- 
mitting then that gneiss, mica-schist, granite, diorite, 
&c, were once necessarily covered up, how can we 
account for the naked and extensive areas of such rocks 
in many parts of the world, except on the belief that 
they have subsequently been completely denuded of all 
overlying strata ? That such extensive areas do exist 
cannot be doubted: the granitic region of Parime is 
described by Humboldt as being at least nineteen times 
as large as Switzerland. South of the Amazon, Boue 
colours an area composed of rocks of this nature as 
equal to that of Spain, France, Italy, part of Germany, 
and the British Islands, all conjoined. This region has 
not been carefully explored, but from the concurrent 
testimony of travellers, the granitic area is very large : 
thus, Von Eschwege gives a detailed section of these 
rocks, stretching from liio de Janeiro for 260 geo- 
graphical miles inland in a straight line; and I 
travelled for 150 miles in another direction, and saw 
nothing but granitic rocks. jSTumerous specimens, 
collected along the whole coast from near Eio Janeiro to 
the mouth of the Plata, a distance of 1100 geographical 
miles, were examined by me, and they all belonged to 
this class. Inland, along the whole northern bank of 
the Plata I saw, besides modern tertiary beds, only one 



Chap. X.] PAL^ONTOLOGICAL COLLECTIONS. 65 

small patch of slightly metamorphosed rock, which 
alone could have formed a part of the original capping 
of the granitic series. Turning to a well-known region, 
namely, to the United States and Canada, as shown in 
Professor H. D. Eogers's beautiful map, I have estimated 
the areas by cutting out and weighing the paper, and I 
find that the metamorphic (excluding " the semi-meta- 
" morphic ") and granitic rocks exceed, in the proportion 
of 19 to 12 -5, the whole of the newer Palaeozoic 
formations. In many regions the metamorphic and 
granitic rocks would be found much more widely 
extended than they appear to be, if all the sedimentary 
beds were removed which rest unconformably on them, 
and which could not have formed part of the original 
mantle under which they were crystallized. Hence it 
is probable that in some parts of the world whole 
formations have been completely denuded, with not a 
wreck left behind. 

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



66 ABSENCE OF IXTEEMEDIATE VAEIETIES [Chap. X. 

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

From these several considerations, it cannot be 
doubted that the geological record, viewed as a whole, 
is extremely imperfect ; but if we confine our attention 
to any one formation, it becomes much more difficult to 
understand why we do not therein find closely graduated 
varieties between the allied species which lived at its 
commencement and at its close. Several cases are on 
record of the same species presenting varieties in the 
upper and lower parts of the same formation ; thus, 
Trautschold gives a number of instances with Am- 
monites; 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 indispu- 
tably required a vast number of years for its deposition, 
several reasons can be given why each should not 
commonly include a graduated series of links between 
the species winch lived at its commencement and close ; 
but I cannot assign due proportional weight to the 
following considerations. 

Although each formation may mark a very long lapse 
of years, each probably is short compared with the 
period requisite to change one species into another. I 
am aware that two palaeontologists, whose opinions are 
worthy of much deference, namely Bronn and Wood- 
ward, have concluded that the average duration of each 
formation is twice or thrice as long as the average 
duration of specific forms. But insuperable difficulties, 
as it seems to me, prevent us from coming to any just 
conclusion on this head. Wnen we see a species first 



Chap. X.] IN ANY SINGLE FOEMATION. 67 

appearing in the middle of any formation, it would be 
rash in the extreme to infer that it had not elsewhere 
previously existed. So again when we find a species 
disappearing before the last layers have been deposited, 
it would be equally rash to suppose that it then became 
extinct. We forget how small the area of Europe is 
compared with the rest of the world ; nor have the 
several stages of the same formation throughout Europe 
been correlated with perfect accuracy. 

We may safely infer that with marine animals of all 
kinds there has been a large amount of migration due 
to climatal and other changes ; and when we see a 
species first appearing in any formation, the probability 
is that it only then first immigrated into that area. It 
is well-known, for instance, that several species appear 
somewhat earlier in the palaeozoic beds of North 
America than in those of Europe ; time having appa- 
rently been required for their migration from the 
American to the European seas. In examining the 
latest deposits in various quarters of the world, it has 
everywhere been noted, that some few still existing 
species are common in the deposit, but have become 
extinct in the immediately surrounding sea; or, con- 
versely, that some are now abundant in the neighbouring 
sea, but are rare or absent in this particular deposit. 
It is an excellent lesson to reflect on the ascertained 
amount of migration of the inhabitants of Europe 
during the glacial epoch, which forms only a part of 
one whole geological period ; and likewise to reflect on. 
the changes of level, on the extreme change of climate, 
and on the great lapse of time, all included within this 
same glacial period. Yet it may be doubted whether, 
in any quarter of the world, sedimentary deposits, 
24 



68 ABSENCE OF INTERMEDIATE VARIETIES [Chap. X. 

including fossil remains, have gone on accumulating 
within the same area during the whole of this period. 
It is not, for instance, probable that sediment was 
deposited during the whole of the glacial period near 
the mouth of the Mississippi, within that limit of depth 
at winch marine animals can best nourish : for we 
know that great geographical changes occurred in 
other parts of America during this space of time. 
"When such beds as were deposited in shallow water 
near the mouth of the Mississippi during some part of 
the glacial period shall have been upraised, organic 
remains will probably first appear and disappear at 
different levels, owing to the migrations of species and 
to geographical changes. And in the distant future, a 
geologist, examining these beds, would be tempted to 
conclude that the average duration of life of the 
embedded fossils had been less than that of the glacial 
period, instead of having been really far greater, that is, 
extending from before the glacial epoch to the present day. 
In order to get a perfect gradation between two forms 
in the upper and lower parts of the same formation, the 
deposit must have gone on continuously accumulating 
during a long period, sufficient for the slow process of 
modification ; hence the deposit must be a very thick 
one; and the species undergoing change must have 
lived in the same district throughout the whole time. 
But we have seen that a thick formation, fossiliferous 
throughout its entire thickness, can accumulate only 
dming a period of subsidence; and to keep the depth 
approximately the same, which is necessary that the 
same marine species may live on the same space, the 
supply of sediment must nearly counterbalance the 
amount of subsidence. But this same movement of 



Chap. X.] IN ANY SINGLE FORMATION. 69 

subsidence will tend to submerge the area whence the 
sediment is derived, and thus diminish the supply, 
whilst the downward movement continues. In fact, 
this nearly exact balancing between the supply of 
sediment and the amount of subsidence is probably a 
rare contingency ; for it has been observed by more than 
one palaeontologist, that very thick deposits are usually 
barren of organic remains, except near their upper or 
lower limits. 

It would seem that each separate formation, like the 
whole pile of formations in any country, has generally 
been intermittent in its accumulation. When we see, 
as is so often the case, a formation composed of beds 
of widely different mineralogical composition, we may 
reasonably suspect that the process of deposition has 
been more or less interrupted. Nor will the closest 
inspection of a formation give us any idea of the length 
of time which its deposition may have consumed. 
Many instances could be given of beds only a few feet 
in thickness, representing formations, which are else- 
where thousands of feet in thickness, and which must 
have required an enormous period for their accumu- 
lation ; yet no one ignorant of this fact would have 
even suspected the vast lapse of time represented by 
the thinner formation. Many cases could be given of 
the lower beds of a formation having been upraised, 
denuded, submerged, and then re-covered by the upper 
beds of the same formation, — facts, showing what wide, 
yet easily overlooked, intervals have occurred in its 
accumulation. In other cases we have the plainest 
evidence in great fossilised trees, still standing uprio-ht 
as they grew, of many long intervals of time and 
changes of level during the process of deposition, which 



70 ABSENCE OF EnTEEVEEDIATE VARIETIES [Chap. X. 

would not have been suspected, had not the trees been 
preserved: thus Sir C. Lyell and Dr. Dawson found 
carboniferous beds 1400 feet thick in ISTova Scotia, with 
ancient root-bearing strata, one above the other at no 
less than sixty-eight different levels. Hence, when the 
same species occurs at the bottom, middle, and top of a 
formation, the probability is that it has not lived on the 
same spot during the whole period of deposition, but 
has disappeared and reappeared, perhaps many times, 
during the same geological period. Consequently if it 
were to undergo a considerable amount of modification 
during the deposition of any one geological formation, 
a section would not include all the fine intermediate 
gradations which must on our theory have existed, but 
abrupt, though perhaps slight, changes of form. 

It is all-important to remember that naturalists have 
no golden rule by which to distinguish species and 
varieties ; they grant some little variability to each 
species, but when they meet with a somewhat greater 
amount of difference between any two forms, they rank 
both as unless they are enabled to connect 

them together by the closest intermediate gradations; 
and this, from th ngned, we can seldom 

hope to effect in any one geological section. Supposing 
B and C to be two species, and a third, A, to be found 
in an older and underlying bed ; even if A were strictly 
intermediate between B and C, it would simply be 
ranked as a third and distinct species, unless at the 
same time it could be closely connected by interme- 
diate varieties with either one or both forms. Nor 
should it be forgotten, as before explained, that A 
might be the actual progenitor of B and C, and yet 
would not necessarily be strictly intermediate between 



Chap. X.] IN ANY SINGLE FOEMATION. 71 

them in all respects. So that we might obtain the 
parent-species . and its several modified descendants 
from the lower and upper beds of the same formation, 
and unless we obtained numerous transitional grada- 
tions, we should not recognise their blood-relationship, 
and should consequently rank them as distinct species. 
It is notorious on what excessively slight differences 
many palaeontologists have founded their species ; and 
they do this the more readily if the specimens come 
from different sub-stages of the same formation. Some 
experienced conchologists are now sinking many of the 
very fine species of D'Orbigny and others into the rank 
of varieties ; and on this view we do find the kind of 
evidence of change which on the theory we ought to 
find. Look again at the later tertiary deposits, which 
include many shells believed by the majority of natu- 
ralists to be identical with existing species ; but some 
excellent naturalists, as Agassiz and Pictet, maintain 
that all these tertiary species are specifically distinct, 
though the distinction is admitted to be very slight ;' 
so that here, unless we believe that these eminent 
naturalists have been misled by their imaginations, 
and that these late tertiary species really present no 
difference whatever from their living representatives, or 
unless we admit, in opposition to the judgment of 
most naturalists, that these tertiary species are all 
truly distinct from the recent, we have evidence of the 
frequent occurrence of slight modifications of the kind 
required. If we look to rather wider intervals of time, 
namely, to distinct but consecutive stages of the same 
great formation, we find that the embedded fossils, 
though universally ranked as specifically different, yet 
are far more closely related to each other than are the 



72 ABSENCE OF INTERMEDIATE VARIETIES [Chap. X. 

species found in more widely separated formations ; so 
that here again we have undoubted evidence of change 
in the direction required by the theory ; but to this 
latter subject I shall return in the following chapter 

With animals and plants that propagate rapidly and 
do not wander much, there is reason to suspect, as we 
have formerly seen, that their varieties are generally at 
first local ; and that such local varieties do not spread 
widely and supplant their parent-forms until they 
have been modified and perfected in some considerable 
degree. According to this view, the chance of dis- 
covering in a formation in any one country all the 
early stages of transition between any two forms, is 
small, for the successive changes are supposed to have 
been local or confined to some one spot. Most marine 
animals have a wide range; and we have seen that 
with plants it is those which have the widest range, 
that oftenest present varieties ; so that, with shells and 
other marine animals, it is probable that those which 
had the widest range, far exceeding the limits of the 
known geological formations in Europe, have oftenest 
given rise, first bo 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 g 1 formation. 

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

It should not be forgotten, that at the present day, 
with perfect specimens for examination, two forms 



Chap. X.] IN ANY SINGLE FOKMATION. 73 

can seldom be connected by intermediate varieties, 
and thus proved to be the same species, until many 
specimens are collected from many places ; and with 
fossil species this can rarely be done. We shall, 
perhaps, best perceive the improbability of our being 
enabled to connect species by numerous, fine, inter- 
mediate, fossil links, by asking ourselves whether, for 
instance, geologists at some future period will be able 
to prove that our different breeds of cattle, sheep, 
horses, and dogs are descended from a single stock or 
from several aboriginal stocks ; or, again, whether 
certain sea -shells inhabiting the shores of North 
America, which are ranked by some conchologists as 
distinct species from their European representatives, 
and by other conchologists as only varieties, are really 
varieties, or are, as it is called, specifically distinct. 
This could be effected by the future geologist only by 
his discovering in a fossil state numerous intermediate 
gradations; and such success is improbable in the 
highest degree. 

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



74 ABSEXCE OF INTERMEDIATE VARIETIES [Chap. X. 

of infinitely numerous gradations, as fine as existing 
varieties, connecting together nearly all existing and 
extinct species. But this ought not to be expected ; yet 
this has been repeatedly advanced as a most serious 
objection against my views. 

It may.be worth while to sum up the foregoing 
remarks on the causes of the imperfection of the 
geological record under an imaginary illustration. The 
Malay Archipelago is about the size of Europe from the 
Xorth Cape to the Mediterranean, and from Britain to 
Bussia ; and therefore equals all the geological forma- 
tions which have been examined with any accuracy, 
excepting those of the United States of America. I 
fully agree with Mr. Godwin-Austen, that the present 
condition of the Malay Archipelago, with its numerous 
large islands separated by wide and shallow seas, pro- 
bably represents the former state of Europe, whilst 
most of our formations were accumulating. The Malay 
Archipelago is one of the richest regions in organic 
beings ; yet if all the species were to be collected which 
have ever lived there, how imperfectly would they 
represent the natural history of the world \ 

But we have every reason to believe that the 
terrestrial productions of the archipelago would be 
preserved in an extremely imperfect manner in the 
formations which we suppose to be there accumulating. 
Not many of the strictly littoral animals, or of those 
which lived on naked submarine rocks, would be em- 
bedded ; and those embedded in gravel or sand would 
not endure to a distant epoch. Wherever sediment did 
not accumulate on the bed of the sea, or where it did 
not accumulate at a sufficient rate to protect organic 
bodies from decay, no remains could be preserved. 



Chap. X.] IN ANY SINGLE FORMATION. 75 

Formations rich in fossils of many kinds, and of 
thickness sufficient to last to an age as distant in 
futurity as the secondary formations lie in the past, 
would generally be formed in the archipelago only 
during periods of subsidence. These periods of subsi- 
dence would be separated from each other by immense 
intervals of time, during which the area would be 
either stationary or rising; whilst rising, the fossili- 
ferous formations on the steeper shores would be de- 
stroyed, almost as soon as accumulated, by the incessant 
coast-action, as we now see on the shores of South 
America. Even throughout the extensive and shallow 
seas within the archipelago, sedimentary beds could 
hardly be accumulated of great thickness during the 
periods of elevation, or become capped and protected 
by subsequent deposits, so as to have a good chance of 
enduring to a very distant future. During the periods 
of subsidence, there would probably be much extinction 
of life ; during the periods of elevation, there would be 
much variation, but the geological record would then be 
less perfect. 

It may be doubted whether the duration of any one 
great period of subsidence over the whole or part of the 
archipelago, together with a contemporaneous accumula- 
tion 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, though 
closely allied species. It is also probable that each 
great period of subsidence would be interrupted by 
oscillations of level, and that slight climatal changes 



76 ABSENCE OE INTERMEDIATE YAKIETIES [Chap. X. 

would intervene during such lengthy periods; and in 
these cases the inhabitants of the archipelago would 
migrate, and no closely consecutive record of their 
modifications could be preserved in any one formation. 

Very many of the marine inhabitants of the archi- 
pelago now range thousands of miles beyond its con- 
fines ; and analogy plainly leads to the belief that it 
would be chiefly these far-ranging species, though only 
some of them, which would oftenest produce new 
varieties ; and the varieties would at first be local or 
confined to one place, but if possessed of any decided 
advantage, or when further modified and improved, they 
would slowly spread and supplant their parent-forms. 
When, such varieties returned to their ancient homes, 
as they would differ from their former state in a nearly 
uniform, though perhaps extremely slight degree, and 
as they would be found embedded in slightly different 
sub-stages of the same formation, they would, according 
to the principles followed by many palaeontologists, be 
ranked as new and distinct species. 

If then there be some degree of truth in these re- 
marks, we have no right to expect to find, in our 
geological formations, an infinite number of those fine 
transitional forms which, on our theory, have connected 
all the past and present species of the same group into 
one long and branching chain of life. AVe ought only 
to look for a few links, and such assuredly we do find 
— some more distantly, some more closely, related to 
each other ; and these links, let them be ever so close, 
if found in different stages of the same formation, 
would, by many palaeontologists, be ranked as distinct 
species. But I do not pretend that I should ever have 
suspected how poor was the record in the best preserved 



Chap. X.] IN ANY SINGLE FOEMATION. 77 

geological sections, had not the absence of innumerable 
transitional links between the species which lived at 
the commencement and close of each formation, pressed 
so hardly on my theory. 



On the sudden Appearance of whole Groups of 
allied Species. 

The abrupt manner in which whole groups of species 
suddenly appear in certain formations, has been urged 
by several palaeontologists — for instance, by Agassiz, 
Pictet, and Sedgwick — as a fatal objection to the belief 
in the transmutation of species. If numerous species, 
belonging to the same genera or families, have really 
started into life at once, the fact would be fatal to the 
theory of evolution through natural selection. For the 
development by this means of a group of forms, all of 
which are descended from some one progenitor, must 
have been an extremely slow process ; and the progeni- 
tors must have lived long before their modified descen- 
dants. But we continually overrate the perfection of 
the geological record, and falsely infer, because certain 
genera or families have not been found beneath a 
certain stage, that they did not exist before that stage. 
In all cases positive palseontological evidence may be 
implicitly trusted; negative evidence is worthless, as 
experience has so often shown. We continually forget 
how large the world is, compared with the area over 
which our geological formations have been carefully 
examined; we forget that groups of species may else- 
where have long existed, and have slowly multiplied, 
before they invaded the ancient archipelagoes of Europe 
and the United States. We do not make due allowance 



78 SUDDEN APPEARANCE OF [Chap. X. 

for the intervals of time which have elapsed between our 
consecutive formations, — longer perhaps in many cases 
than the time required for the accumulation of each 
formation. These intervals will have given time for 
the multiplication of species from some one parent- 
form : and in the succeeding formation, such groups or 
species will appear as if suddenly created. 

I may here recall a remark formerly made, namely, 
that it might require a long succession of ages to adapt 
an organism to some new and peculiar line of life, for 
instance, to fly through the air ; and consequently that 
the transitional forms would often long remain con- 
fined 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 be necessary to produce 
many divergent forms, which would spread rapidly and 
widely, throughout the world. Professor Pictet, in his 
excellent Review of this work, in commenting on early 
transitional forms, and taking birds as an illustration, 
cannot see how the successive modifications of the 
anterior limbs of a supposed prototype could possibly 
have been of any advantage. Bnt 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 victors rasly in the battle for life ; for they exist 
in infinite numbers and of many kinds. I do not 
suppose that we here see the real transitional grades 
through which the wings of birds have passed; but 
what special difficulty is there in believing that it 
might profit the modified descendants of the penguin, 
first to become enabled to flap along the surface of the 



Chap. X.] GEOUPS OF ALLIED SPECIES. 79 

sea like the logger-headed duck, and ultimately to rise 
from its surface and glide through the air ? 

I will now give a few examples to illustrate the 
foregoing remarks, and to show how liable we are to 
error in supposing that whole groups of species have 
suddenly been produced. Even in so short an interval 
as that between the first and second editions of Pictet's 
great work on Palaeontology, published in 1844-46 and 
in 1853-57, the conclusions on the first appearance and 
disappearance of several groups of animals have been 
considerably modified; and a third edition would 
require still further changes. I may recall the well- 
known fact that in geological treatises, published not 
many years ago, mammals were always spoken of as 
having abruptly come in at the commencement of the 
tertiary series. And now one of the richest known ac- 
cumulations of fossil mammals belongs to the middle of 
the secondary series ; and true mammals have been 
discovered in the new red sandstone at nearly the com- 
mencement of this great series. Cuvier used to urge that 
no monkey occurred in any tertiary stratum ; but now 
extinct species have been discovered in India, South 
America and in Europe, as far back as the miocene stage. 
Had it not been for the rare accident of the preservation 
of footsteps in the new red sandstone of the United States, 
who would have ventured to suppose that no less than 
at least thirty different bird-like animals, some of 
gigantic size, existed during that period ? Not a frag- 
ment of bone has been discovered in these beds. Not 
long ago, palaeontologists maintained that the whole 
class of birds came suddenly into existence during the 
eocene period ; but now we know, on the authority of 

Professor Owen, that a bird certainly lived durino- the 
25 



80 SUDDEN APPEAKANCE OF [Chap. X. 

deposition of the upper greensand; and still more 
recently, that strange bird, the Archeopteryx, with a 
long lizard-like tail, bearing a pair of feathers on each 
joint, and with its wings furnished with two free claws, 
has been discovered in the oolitic slates of Solenhofen. 
Hardly any recent discovery shows more forcibly than 
this, how little we as yet know of the former inhabitants 
of the world. 

I may give another instance, which, from having 
passed under my own eyes, has much struck me. In a 
memoir on Fossil Sessile Cirripedes, I stated that, from 
the large number of existing and extinct tertiary 
species ; from the extraordinary abundance of the in- 
dividuals of many species all over the world, from the 
Arctic regions to the equator, inhabiting various zones of 
depths from the upper tidal limits to 50 fathoms ; from 
the perfect manner in winch specimens are preserved 
in the oldest tertiary beds ; from the ease with which 
even a fragment of a valve can be recognised ; from all 
these circumstances, I inferred that, had sessile cirripedes 
existed during the secondary periods, they would 
duly have been preserved and discovered; and as 
not one had then been discovered in beds of 

this age, I concluded that this great group had been 
suddenly developed at the commencement of the ter- 
tiary series. This was a sore trouble to me, adding as 
I then thought one mora instance of the abrupt ap- 
pearance of a great group of species. But my work had 
hardly been published, when a skilful palaeontologist, 
M. Bosquet, sent me a drawing of a perfect specimen 
of an unmistakeable sessile cirripede, which he had 
himself extracted from the chalk of Belgium. And, as 
if to make the case as striking as possible, this cirripede 



Chap. X.] GROUPS OF ALLIED SPECIES. 81 

was a Chthainalus, a very common, large, and ubi- 
quitous genus, of which not one species has as yet 
been found even in any tertiary stratum. Still more 
recently, a Pyrgoma, a member of a distinct sub-family 
of sessile cirripedes, has been discovered by Mr. "Wood- 
ward in the upper chalk ; so that we now have abun- 
dant evidence of the existence of this group of animals 
during the secondary period. 

The case most frequently insisted on by paleonto- 
logists of the apparently sudden appearance of a whole 
group of species, is that of the teleostean fishes, low 
down, according to Agassiz, in the Chalk period. This 
group includes the large majority of existing species. 
But certain Jurassic and Triassic forms are now com- 
monly admitted to be teleostean ; and even some 
palaeozoic forms have thus been classed by one high 
authority. If the teleosteans had really appeared 
suddenly in the northern hemisphere at the commence- 
ment of the chalk formation, the fact would have been 
highly remarkable ; but it would not have formed an 
insuperable difficulty, unless it could likewise have 
been shown that at the same period the species were 
suddenly and simultaneously developed in other 
quarters of the world. It is almost superfluous to 
remark that hardly any fossil-fish are known from south 
of the equator ; and by running through Pictet's Palaeon- 
tology it will be seen that very few species are known 
from several formations in Europe. Some few families 
of fish now have a confined range ; the teleostean fishes 
might formerly have had a similarly confined range, 
and after having been largely developed in some one 
sea, have spread widely. Nor have we any right to 
suppose that the seas of the world have always been 



82 GROUPS OF ALLIED SPECIES [Chap. X. 

so freely open from south to north as they are at 
present. Even at this day, if the Malay Archipelago 
were converted into land, the tropical parts of the 
Indian Ocean would form a large and perfectly enclosed 
basin, in which any great group of marine animals 
might be multiplied ; and here they would remain 
confined, until some of the species became adapted to 
a cooler climate, and were enabled to double the 
Southern capes of Africa or Australia, and thus reach 
other and distant seas. 

From these considerations, from our ignorance of the 
geology of other countries beyond the confines of 
Europe and the United States, and from the revolution 
in our pakeontological knowledge effected by the dis- 
coveries of the last dozen years, it seems to me to be 
about as rash to dogmatize on the succession of organic 
forms throughout the world, as it would be for a natura- 
list to land for five minutes on a barren point in 
Australia, and then to discuss the number and range cf 
its productions. 

On iht Appearance of Groups of allied Species 

FossUift rous Strata. 

There is another and allied difficulty, which is much 
more serious. I allude to the manner in which species 
aging to several of the main divisions of the animal 
kingdom suddenly appear in the lowest known fossili- 
ferous rocks. Most of the arguments which have 
convinced me that all the existing species of the same 
group are descended from a single progenitor, apply 
with equal force to the earliest known species. For 
instance, it cannot be doubted that all the Cambrian 



Chap. X.J IN LOWEST FOSSILIFEEOUS STEATA. 83 

and Silurian trilobites are descended from some one 
crustacean, which must have lived long before the 
Cambrian age, and which probably differed greatly from 
any known animal. Some of the most ancient animals, 
as the Nautilus, Lingula, &c, do not differ much from 
living species ; and it cannot on our theory be supposed, 
that these old species were the progenitors of all the 
species belonging to the same groups which have 
subsequently appeared, for they are not in any degree 
intermediate in character. 

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



84 GROUPS OF ALLIED SPECIES [Chap. X 

forms of life which already existed during the Cam- 
brian period. It is, however, probable, as Sir William 
Thompson insists, that the world at a very early period 
was subjected to more rapid and violent changes in 
its physical conditions than those now occurring ; and 
such changes would have tended to induce changes 
at a corresponding rate in the organisms which then 
existed. 

To the question why we do not find rich fossiliferous 
deposits belonging to these assumed earliest periods 
prior to the Cambrian system, I can give no satisfactory 
answer. Several eminent geologists, with Sir E. 
Murchison at their head, were until recently convinced 
that we beheld in the organic remains of the lowest 
Silurian stratum the first dawn of life. Other highly 
competent judges, as Lyell and E. Forbes, have disputed 
this conclusion. "We should not forget that only a small 
portion of the world is known with accuracy. Not 
very long ago M. Barrande added another and lower 
stage, abounding with new and peculiar species, beneath 
the then known Silurian system ; and now, still lower 
down in the Lower Cambrian formation, Mr. Hicks has 
found in South Wales beds rich in trilobites, and con- 
taining various molluscs and annelids. The presence of 
phosphatic nodules and bituminous matter, even in 
some of the lowest azoic rocks, probably indicates life 
at these periods ; and the existence of the Eozoon in the 
Laurentian formation of Canada is generally admitted. 
There are three great series of strata beneath the Silurian 
system in Canada, in the lowest of which the Eozoon is 
found. Sir W. Logan states that their " united thickness 
" may possibly far surpass that of all the succeeding 
" rocks, from the base of the palaeozoic series to the 



Chap. X.] IN LOWEST FOSSILIFEROUS STRATA. 85 

" present time. We are thus carried back to a period 
" so remote, that the appearance of the so-called 
" Primordial fauna (of Barrande) may by some be 
"considered as a comparatively modern event." The 
Eozoon belongs to the most lowly organised of all 
classes of animals, but is highly organised for its class ; 
it existed in countless numbers, and, as Dr. Dawson has 
remarked, certainly preyed on other minute organic 
beings, which must have lived in great numbers. Thus 
the words, which I wrote in 1859, about the existence 
of living beings long before the Cambrian period, and 
which are almost the same with those since used by Sir 
W. Logan, have proved true. Nevertheless, the difficulty 
of assigning any good reason for the absence of vast 
piles of strata rich in fossils beneath the Cambrian 
system is very great. It does not seem probable that 
the most ancient beds have been quite worn away by 
denudation, or that their fossils have been wholly 
obliterated by metamorphic action, for if this had been 
the case we should have found only small remnants of 
the formations next succeeding them in age, and these 
would always have existed in a partially metamorphosed 
condition. But the descriptions which we possess of 
the Silurian deposits over immense territories in Eussia 
and in North America, do not support the view, that 
the older a formation is, the more invariably it has 
suffered extreme denudation and metamorphism. 

The case at present must remain inexplicable ; and 
may be truly urged as a valid argument against the 
views here entertained. To show that it may hereafter 
receive some explanation, I will give the following 
hypothesis. From the nature of the organic remains 
which do not appear to have inhabited profound depths, 



86 GROUPS OF ALLIED SPECIES [Chap. X. 

in the several formations of Europe and of the United 
States ; and from the arnonnt of sediment, miles in 
thickness, of which the formations are composed, we 
may infer that from first to last large islands or tracts 
of land, whence the sediment was derived, occurred in 
the neighbourhood of the now existing continents of 
Europe and North America. This same view has since 
been maintained by Agassiz and others. But we do not 
know what was the state of things in the intervals 
between the several successive formations ; whether 
Europe and the United States during these intervals 
existed as dry land, or as a submarine surface near land, 
on which sediment was not deposited, or as the bed of 
an open and unfathomable sea. 

Looking to the existing oceans, which are thrice as 
extensive as the land, we see them studded with many 
islands ; but hardly one truly oceanic island (with the 
exception of New Zealand, if this can be called a truly 
oceanic island) is as yet known to afford even a remnant 
of any palaeozoic or secondary formation. Hence we 
may perhaps infer, that during the pakeozoic and 
secondary periods, neither continents nor continental 
islands existed where our oceans now extend ; for had 
they existed, palaeozoic and secondary formations would 
in all probability have been accumulated from sediment 
derived from their wear and tear ; and these would have 
been at least partially upheaved by the oscillations of 
level, which must have intervened during these enor- 
mously long periods. If then we may infer anything 
from these facts, we may infer that, where our oceans 
now extend, oceans have extended from the remotest 
period of which we have any record ; and on the other 
hand, that where continents now exist, large tracts of 



Chap. X.] IN LOWEST FOSSILIFEROUS STRATA. 87 

land have existed, subjected no doubt to great oscilla- 
tions of level, since the Cambrian period. The coloured 
map appended to my volume on Coral Beefs, led me to 
conclude that the great oceans are still mainly areas of 
subsidence, the great archipelagoes still areas of oscilla- 
tions of level, and the continents areas of elevation. 
But we have no reason to assume that things have thus 
remained from the beginning of the world. Our contin- 
ents seem to have been formed by a preponderance, during 
many oscillations of level, of the force of elevation ; but 
may not the areas of preponderant movement have 
changed in the lapse of ages ? At a period long antecedent 
to the Cambrian epoch, continents may have existed where 
oceans are now spread out ; and clear and open oceans 
may have existed where our continents now stand. Nor 
should we be justified in assuming that if, for instance, 
the bed of the Pacific Ocean were now converted into a 
continent we should there find sedimentary formations in 
a recognisable condition older than the Cambrian strata, 
supposing such to have been formerly deposited ; for it 
might well happen that strata which had subsided some 
miles nearer to the centre of the earth, and which had 
been pressed on by an enormous weight of superincum- 
bent water, might have undergone far more metamorphic 
action than strata which have always remained nearer 
to the surface. The immense areas in some parts of the 
world, for instance in South America, of naked meta- 
morphic rocks, which must have been heated under 
great pressure, have always seemed to me to require 
some special explanation ; and we may perhaps believe 
that we see in these large areas, the many formations 
long anterior to the Cambrian epoch in a completely 
metamorphosed and denuded condition. 



88 IMPERFECTION OF GEOLOGICAL EECOKD. [Chap. X. 

The several difficulties here discussed, namely — that, 
though we find in our geological formations many links 
between the species which now exist and which formerly 
existed, we do not find infinitely numerous fine transi- 
tional forms closely joining them all together ; — the 
sudden manner in which several groups of species first 
appear in our European formations ; — the almost entire 
absence, as at present known, of formations rich in fossils 
beneath the Cambrian strata, — are all undoubtedly of the 
most serious nature. We see this in the fact that the 
most eminent palaeontologists, namely, Cuvier, Agassiz 
Barrande, Pictet, Falconer, E. Forbes, &c, and all our 
greatest geologists, as Lyell, Murehison, Sedgwick, &c, 
have unanimously, often vehemently, maiutained the 
immutability of species. But Sir Charles Lyell now 
gives the support of his high authority to the opposite 
side ; and most geologists and palaeontologists are much 
shaken in their former belief. Those who believe that 
the geological record is in any degree perfect, will un- 
doubtedly at once reject the theory. For my part, 
following out Lyell's metaphor, I look at the geological 
record as a history of the world imperfectly kept, and 
written in a changing dialect ; of this history we possess 
the last volume alone, relating only to two or three 
countries. Of this volume, only here and there a short 
chapter has been preserved ; and of each page, only here 
and there a few lines. Each word of the slowly-changing 
language, more or less different in the successive chapters, 
may represent the forms of life, which are entombed in 
our consecutive formations, and which falsely appear 
to have been abruptly introduced. On this view, the 
difficulties above discussed are greatly diminished, or 
even disappear. 



Chap. XL] SUCCESSION OF OBGANIC BEINGS. 89 



CHAPTEE XL 
On the Geological Succession of Organic Beings. 

On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their ap- 
pearance and disappearance as do single species — On extinction 
— On simultaneous changes in the forms of life throughout the 
world — On the affinities of extinct species to each other and to 
living species — On the state of development of ancient forms — 
On the succession of the same types within the same areas — 
Summary of preceding and present chapter. 

Let tis now see whether tho several facts and laws 
relating to the geological succession of organic beings 
accord best with the common view of the immutability 
of species, or with that of their slow and gradual modi- 
fication, through variation and natural selection. 

New species have appeared very slowly, one after 
another, both on the land and in the waters. Lyell has 
shown that it is hardly possible to resist the evidence on 
this head in the case of the several tertiary stages ; and 
every year tends to fill up the blanks between the stages, 
and to make the proportion between the lost and existing 
forms more gradual. In some of the most recent beds, 
though undoubtedly of high antiquity if measured by 
years, only one or two species are extinct, and only one 
or two are new, having appeared there for the first time, 
either locally, or, as far as we know, on the face of 
the earth. The secondary formations are more broken 3 



90 THE GEOLOGICAL SUCCESSION [Chap. XI. 

but, as Bronn has remarked, neither the appearance 
nor disappearance of the many species embedded in 
each formation has been simultaneous. 

Species belonging to different genera and classes have 
not changed at the same rate, or in the same degree. 
In the older tertiary beds a few living shells may still 
be found in the midst of a multitude of extinct forms. 
Falconer has given a striking instance of a similar fact, 
for an existing crocodile is associated with many lost 
mammals and reptiles in the sub-Himalayan deposits. 
The Silurian Lingula differs but little from the living 
species of this genus ; whereas most of the other Silurian 
Molluscs and all the Crustaceans have changed greatly. 
The productions of the land seem to have changed at a 
quicker rate than those of the sea, of which a striking 
instance has been observed in 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 hitter rule is that of the so-called "colonies " of 
\L Barrande, which intrude for a period in the midst of 
an older formation, and then allow the pre-existing 
fauna to reappear ; but LyelTs explanation, namely, that 
it is a case of temporary migration from a distinct geo- 
graphical province, seems satisfactory. 

These several facts accord well with our theory, which 



Chap. XI.] OF ORGANIC BEINGS. 91 

includes no fixed law of development, causing all the 
inhabitants of an area to change abruptly, or simul- 
taneously, or to an equal degree. The process of modifi- 
cation must be slow, and will generally affect only a 
few species at the same time ; for the variability of each 
species is independent of that of all others. Whether 
such variations or individual differences as may arise will 
be 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 
contingencies — on the variations being of a beneficial 
nature, on the freedom of intercrossing, on the slowly 
changing physical conditions of the country, on the 
immigration of new colonists, and on the nature of the 
other inhabitants with which the varying species come 
into competition. Hence it is by no means surprising 
that one species should retain the same identical form 
much longer than others ; or, if changing, should change 
in a less degree. We find similar relations between the 
existing inhabitants of distinct countries ; for instance, 
the land-shells and coleopterous insects of Madeira have 
come to differ considerably from their nearest allies on 
the continent of Europe, whereas :the marine shells and 
birds have remained unaltered. We can perhaps under- 
stand the apparently quicker rate of change in terrestrial 
and in more highly organised productions compared 
with marine and lower productions, by the more complex 
relations of the higher beings to their organic and in- 
organic conditions of life, as explained in a former 
chapter. When many of the inhabitants of any area 
have become modified and improved, we can understand, 
on the principle of competition, and from the all-import- 
ant relations of organism to organism in the struggle for 
26 



92 THE GEOLOGICAL SUCCESSION [Chap. XL 

life, that any form which did not become in some degree 
modified and improved, would be liable to extermination. 
Hence we see why all the species in the same region do 
at last, if we look to long enongh intervals of time, be- 
come modified, for otherwise they would become extinct. 

In members of the same class the average amount of 
change, during long and equal periods of time, may, 
perhaps, be nearly the same ; but as the accumulation 
of enduring formations, rich in fossils, depends on great 
masses of sediment being deposited on subsiding areas, 
our formations have been almost necessarily accumulated 
at wide and irregularly intermittent intervals of time ; 
consequently the amount of organic change exhibited by 
the fossils embedded in consecutive formations is not 
equal. Each formation, on this view, does not mark a new 
and complete act of creation, but only an occasional scene, 
taken almost at hazard, in an ever slowly changing drama. 

We can clearly understand why a species when once 
lost should never reappear, even if the very same con- 
ditions of life, organic and inorganic, should recur. For 
though the offspring of one species might be adapted 
(and no doubt this has occurred in innumerable in- 
stances) to fill the place of another species in the economy 
of nature, and thus supplant it ; yet the two forms — 
the old and the new — would not be identically the 
same ; for both would almost certainly inherit different 
characters from their distinct progenitors ; and organisms 
already differing would vary in a different manner. 
For instance, it is possible, if all our fantail pigeons 
were destroyed, that fanciers might make a new breed 
hardly distinguishable from the present breed ; but if 
the parent rock-pigeon were likewise destroyed, and 
under nature we have every reason to believe that 



Chap. XL] OF ORGANIC BEINGS. 93 

parent-forms are generally supplanted and exterminated 
by their improved offspring, it is incredible that a fantail, 
identical with the existing breed, could be raised from 
any other species of pigeon, or even from any other well- 
established race of the domestic pigeon, for the successive 
variations would almost certainly be in some degree 
different, and the newly-formed variety would probably 
inherit from its progenitor some characteristic differences. 

Groups of species, that is, genera and families, follow 
the same general rules in their appearance and dis- 
appearance as do single species, changing more or less 
quickly, and in a greater or lesser degree. A group, 
when it has once disappeared, never reappears ; that is, 
its existence, as long as it lasts, is continuous. I am 
aware that there are some apparent exceptions to this 
rule, but the exceptions are surprisingly few, so few that 
E. Forbes, Pictet, and Woodward (though all strongly 
opposed to such views as I maintain) admit its truth ; 
and the rule strictly accords with the theory. For all 
the species of the same group, however long it may 
have lasted, are the modified descendants one from the 
other, and all from a common progenitor. In the genus 
Lingula, for instance, the species which have successively 
appeared at all ages must have been connected by an 
unbroken series of generations, from the lowest Silurian 
stratum to the present day. 

We have seen in the last chapter that whole groups 
of species sometimes falsely appear to have been abruptly 
developed ; and I have attempted to give an explana- 
tion of this fact, which if true would be fatal to my 
views. But such cases are certainly exceptional ; the 
general rule being a gradual increase in number, until 
the group reaches its maximum, and then, sooner or 



94 EXTINCTION. [Chap. XI. 

later, a gradual decrease. If the number of the species 
included within a genus, or the number of the genera 
within a family, be represented by a vertical line of vary- 
ing thickness, ascending through the successive geological 
formations, in which the species are found, the line will 
sometimes falsely appear to begin at its lower end, not 
in a sharp point, but abruptly ; it then gradually thickens 
upwards, often keeping of equal thickness for a space, 
and ultimately thins out in the upper beds, marking the 
decrease and final extinction of the species. This gradual 
increase in number of the species of a group is strictly 
conformable with the theory, for the species of the same 
genus, and the genera of the same family, can increase 
only slowly and progressively ; the process of 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 into species, which in their turn 
produce by equally slow steps other varieties and species, 
and so on, like the brandling of a great tree from a single 
stem, till the group becomes large. 



On Extinction. 

We have as yet only spoken incidentally of the dis- 
appearance of species and of groups of species. On the 
theory of natural selection, the extinction of old forms 
and the production of new and improved forms are 
intimately connected together. The old notion of all 
the inhabitants of the earth having been swept away by 
catastrophes at successive periods is very generally 
given up, even by those geologists, as Elie de Beaumont, 
Murchison, Barrande, &c, whose general views would 



Ciap. XI.] EXTINCTION. 95 

naturally lead them to this conclusion. On the contrary, 
we have every reason to believe, from the study of the 
tertiary formations, that species and groups of species 
gradually disappear, one after another, first from one spot, 
then from another, and finally from the world. In some 
few cases however, as by the breaking of an isthmus 
and the consequent irruption of a multitude of new 
inhabitants into an adjoining sea, or by the final subsi- 
dence of an island, the process of extinction may have 
been rapid. Both single species and whole groups of 
species last for very unequal periods ; some groups, 
as we have seen, have endured from the earliest known 
dawn of life to the present day ; some have disappeared 
before the close of the palaeozoic period. No fixed law 
seems to determine the length of time during which 
any single species or any single genus endures. There 
is reason to believe that the extinction of a whole group 
of species is generally a slower process than their pro- 
duction : if their appearance and disappearance be re- 
presented, as before, by a vertical line of varying thick- 
ness the line is found to taper more gradually at its upper 
end, which marks the progress of extermination, than 
at its lower end, which marks the first appearance and 
the early increase in number of the species. In some 
cases, however, the extermination of whole groups, as 
of ammonites, towards the close of the secondary period, 
has been wonderfully sudden. 

The extinction of species has been involved in the 
most gratuitous mystery. Some authors have even sup- 
posed that, as the individual has a definite length of 
life, so have species a definite duration. No one can 
have marvelled more than I have done at the extinction 
of species. When I found in La Plata the tooth of a 



96 EXTINCTION. [Chap. XI. 

horse embedded with the remains of Mastodon, Mega- 
therium, Toxodon, and other extinct monsters, which 
all co-existed with still living shells at a very late geo- 
logical period, I was filled with astonishment ; for, seeing 
that the horse, since its introduction by the Spaniards 
into South America, has run wild over the whole 
country and has increased in numbers at an unparalleled 
rate, I asked myself what could so recently have exter- 
minated the former horse under conditions of life ap- 
parently so favourable. But my astonishment was 
groundless. Professor Owen soon perceived that the 
tooth, though so like that of the existing horse, belonged 
to an extinct species. Had this horse been still living, 
but in some degree rare, no naturalist would have felt 
the least surprise at its rarity ; for rarity is the attribute 
of a vast number of species of all classes, in all countries. 
If we ask ourselves why this or that species is rare, we 
answer that something is unfavourable in its conditions 
of life ; but what that something is we can hardly ever 
tell. On the supposition of the fossil horse still existing 
as a rare species, we might have felt certain, from the 
analogy of all other mammals, even of the slow-breeding 
elephant, and from the history of the naturalisation of 
the domestic horse in South America, that under more 
favourable conditions it would in a very few years have 
stocked the whole continent. But we could not have 
told what the unfavourable conditions were which 
cheeked its increase, whether some one or several con- 
tingencies, 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 favour- 
able, we assuredly should not have perceived the fact, 
yet the fossil horse would certainly have become rarer 



Chap. XI.] EXTINCTION. 97 

and rarer, and finally extinct ; — its place being seized 
on by some more successful competitor. 

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

We see in many cases in the more recent tertiary for- 
mations, that rarity precedes extinction ; and we know 
that this has been the progress of events with those 
animals which have been exterminated, either locally or 
wholly, through man's agency. I may repeat what I 
published in 1845, namely, that to admit that species 
generally become rare before they become extinct — to feel 
no surprise at the rarity of a species, and yet to marvel 
greatly when the species ceases to exist, is much the same 
as to admit that sickness in the individual is the fore- 



98 EXTINCTION. [Chap. XI. 

runner of death — to feel no surprise at sickness, but, 
when the sick man dies, to wonder and to suspect that 
he died by some deed of violence. 

The theory of natural selection is grounded on the 
belief that each new variety and ultimately each new 
species, is produced and maintained by having some 
advantage over those with which it comes into competi- 
tion ; and the consequent extinction of the less-favoured 
forms almost inevitably follows. It is the same with 
our domestic productions ; when a new and slightly 
improved variety has been raised, it at first supplants 
the less improved varieties in the same neighbourhood ; 
when much improved it is transported far and near, like 
our short-horn cattle, and takes the place of other breeds 
in other countries. Thus the appearance of new forms 
and the disappearance of old forms, both those naturally 
and those artificially produced, are bound together. In 
flourishing groups, the number of new specific forms 
which have been produced within a given time has at 
some periods probably been greater than the number of 
the old specific forms which have been exterminated ; 
but we know that species have not gone on indefinitely 
increasing, at least during the later geological epochs, so 
that, looking to later times, we may believe that the 
production of new forms has caused the extinction of 
about the same number of old forms. 

The competition will generally be most severe, as 
formerly explained and illustrated by examples, between 
the forms which are most like each other in all respects. 
Hence the improved and modified descendants of a species 
will generally cause the extermination of the parent- 
species ; and if many new forms have been developed 
from any one species, the nearest allies of that species, 



Chap. XI.] EXTINCTION. 99 

i.e. the species of the same genus, will be the most liable 
to extermination. Thus, as I believe, a number of new 
species descended from one species, that is a new genus, 
comes to supplant an old genus, belonging to the same 
family. But it must often have happened that a new 
species belonging to some one group has seized on the 
place occupied by a species belonging to a distinct group, 
and thus have caused its extermination. If many allied 
forms be developed from the successful intruder, many 
will have to yield their places ; and it will generally be 
the allied forms, which will suffer from some inherited 
inferiority in common. But whether it be species 
beloncnnsf 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 competi- 
tion. Tor instance, some species of Trigonia, a great 
genus of shells in the secondary formations, survive in 
the Australian seas ; and a few members of the great and 
almost extinct group of Ganoid fishes still inhabit our 
fresh waters. Therefore the utter extinction of a group 
is generally, as we have seen, a slower process than its 
production. 

With respect to the apparently sudden extermination 
of whole families or orders, as of Trilobites at the close 
of the palaeozoic period and of Ammonites at the close of 
the secondary period, we must remember what has been 
already said on the probable wide intervals of time 
between our consecutive formations ; and in these 
intervals there may have been much slow extermina- 
tion. Moreover, when, by sudden immigration or by 



100 POEMS OF LIFE CHANGING [Chap. XI. 

unusually rapid development, many species of a new 
group have taken possession of an area, many of the 
older species will have been exterminated in a corre- 
spondingly rapid manner; and the forms which thus 
yield their places will commonly be allied, for they will 
partake of the same inferiority in common. 

Thus, as it seems to me, the manner in which single 
species and whole groups of species become extinct 
accords well with the theory of natural selection. We 
need not marvel at extinction ; if we must marvel, let 
it be at our own presumption in imagining for a 
moment that we understand the many complex con- 
tingencies on which the existence of each species 
depends. If we forget for an instant that each species 
tends to increase inordinately, and that some check is 
always in action, yet seldom perceived by us, the whole 
economy of nature will be utterly obscured. Whenever 
we can precisely say why this species is more abundant 
in individuals than that; why this species and not 
another can be naturalised in a given country ; then, 
and not until then, we may justly feel surprise why we 
cannot account for the extinction of any particular 
species or group of species. 

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

Scarcely any paheontological discovery is more 
striking than the fact that the forms of life change 
almost simultaneously throughout the world. Thus our 
European Chalk formation can be recognised in many 
distant regions, under the most different climates, where 
not a fragment of the mineral chalk itself can be found ; 
namely in Xorth America, in equatorial South America, 



Chap. XL] THROUGHOUT THE WORLD. 101 

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 unmis- 
takeable 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 be- 
long to the same families, genera, and sections of 
genera, and sometimes are similarly characterised in 
such trifling points as mere superficial sculpture. More- 
over, 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 palaeo- 
zoic formations of Eussia, Western Europe, and North 
America, a similar parallelism in the forms of life has 
been observed by several authors; so it is, according 
to Lyell, with the European and North American 
tertiary deposits. Even if the few fossil species which 
are common to the Old and New Worlds were kept 
wholly out of view, the general parallelism in the 
successive forms of life, in the palaeozoic and tertiary 
stages, would still be manifest, and the several forma- 
tions could be easily correlated. 

These observations, however, relate to the marine 
inhabitants of the world : we have not sufficient data 
to judge whether the productions of the land and of 
fresh water at distant points change in the same parallel 
manner. We may doubt whether they have thus 
changed : if the Megatherium, Mylodon, Macrauchenia, 
and Toxodon had been brought to Europe from La 
Plata, without any information in regard to their geo- 
logical position, no one would have suspected that they 
had co-existed with sea-shells all still living ; ..but as 



102 FOEMS OF LIFE CHANGING [Chap. XI 

these anomalous monsters co-existed with the Mastodon 
and Horse, it might at least have been inferred that 
they had lived during one of the later tertiary stages. 

"When the marine forms of life are spoken of as 
having changed simultaneously throughout the world, 
it must not be supposed that this expression relates to 
the same year, or to the same century, or even that it 
has a very strict geological sense ; for if all the marine 
animals now living in Europe, and all those that lived 
in Europe during the pleistocene period (a very remote 
period as measured by years, including the whole 
glacial epoch) were compared with those now existing 
in South America or in Australia, the most skilful 
naturalist would hardly be able to say whether the 
present or the pleistocene inhabitants of Europe re- 
sembled most closely those of the southern hemisphere. 
So, again, several highly competent observers maintain 
that the existing productions of the United States are 
more closely related to those which lived in Europe 
during certain late tertiary stages, than to the present 
inhabitants of Europe ; and if this be so, it is evident 
that fossiliferous beds now deposited on the shores of 
North America would hereafter be liable to be classed 
with somewhat older European beds. Xevertheless, 
Looking to a remotely future epoch, there can be little 
doubt that all the more modern marine formations, 
namely, the upper pliocene, the pleistocene and strictly 
modern beds of Europe, Xorth and Suuth America, and 
Australia, from containing fossil remains in some 
degree allied, and from not including those forms which 
are found only in the older underlying deposits, would 
be correctly ranked as simultaneous in a geological sense. 

The fact of the forms of life changing simultaneously, 



Chap. XI.] THROUGHOUT THE WORLD. 103 

in the above large sense, at distant parts of the world, 
has greatly struck those admirable observers, MM. de 
Verneuil and d'Archiac. After referring to the parallel- 
ism of the palaeozoic forms of life in various parts of 
Europe, they add, " If, struck by this strange sequence, 
we turn our attention to North America, and there 
discover a series of analogous phenomena, it will appear 
certain that all these modifications of species, their 
extinction, and the introduction of new ones, cannot 
be owing to mere changes in marine currents or other 
causes more or less local and temporary, but depend on 
general laws which govern the whole animal kingdom." 
M. Barrande has made forcible remarks to precisely 
the same effect. It is, indeed, quite futile to look to 
changes of currents, climate, or other physical con- 
ditions, as the cause of these great mutations in the 
forms of life throughout the world, under the most 
different climates. We must, as Barrande has re- 
marked, look to some special law. We shall see this 
more clearly when we treat of the present distribution 
of organic beings, and find how slight is the relation 
between the physical conditions of various countries 
and the nature of their inhabitants. 

This great fact of the parallel succession of the forms 
of life throughout the world, is explicable on the theory 
of natural selection. New species are formed by having 
some advantage over older forms ; and the forms, which 
are already dominant, or have some advantage over 
the other forms in their own country, give birth to the 
greatest number of new varieties or incipient species. 
We have distinct evidence on this head, in the plants 
which are dominant, that is, which are commonest and 
most widely diffused, producing the greatest number of 
27 



104 FOEMS OF LIFE CHANGING [Chap. XL 

new varieties. It is also natural that the dominant, 
varying, and far-spreading species, which have already 
invaded to a certain extent the territories of other 
species, should be those which would have the best 
chance of spreading still further, and of giving rise in 
new countries to other new varieties and species. The 
process of diffusion would often be very slow, depending 
on climatal and geographical changes, on strange acci- 
dents, and on the gradual acclimatisation of new species 
to the various climates through which they might have to 
pass, but in the course of time the dominant forms would 
generally succeed in spreading and would ultimately 
prevail. The diffusion would, it is probable, be slower 
with the terrestrial inhabitants of distinct continents 
than with the marine inhabitants of the continuous sea. 
"We might therefore expect to find, as we do find, a less 
strict degree of parallelism in the succession of the pro- 
ductions of the land than with those of the sea. 

Thus, as it seems to me, the parallel, and, taken in a 
large sense, simultaneous, succession of the same forms 
of life throughout the world, accords well with the 
principle of new species having been formed by domi- 
nant species spreading widely and varying ; the new 
species thus produced being themselves dominant, 
owing to their having had some advantage over their 
already dominant parents, as well as over other species, 
and again spreading, varying, and producing new forms. 
The old forms which are beaten and which yield their 
places to the new and victorious forms, will generally 
be allied in groups, from inheriting some inferiority in 
common; and therefore, as new and improved groups 
spread throughout the world, old groups disappear from 
the world; and the succession of forms everywhere 



Chap. XL] THROUGHOUT THE WORLD. 105 

tends to correspond both in their first appearance and 
final disappearance. 

There is one other remark connected with this subject 
worth making. I have given my reasons for believing 
that most of our great formations, rich in fossils, were 
deposited during periods of subsidence ; and that blank 
intervals of vast duration, as far as fossils are concerned, 
occurred during the periods when the bed of the sea 
was either stationary or rising, and likewise when sedi- 
ment was not thrown down quickly enough to embed 
and preserve organic remains. During these long and 
blank intervals I suppose that the inhabitants of each 
region underwent a considerable amount of, modification 
and extinction, and that there was much migration 
from other parts of the world. As we have reason to 
believe that large areas are affected by the same move- 
ment, it is probable that strictly contemporaneous 
formations have often been accumulated over very wide 
spaces in the same quarter of the world; but we are 
very far from having any right to conclude that this 
has invariably been the case, and that large areas have 
invariably been affected by the same movements. When 
two formations have been deposited in two regions 
during nearly, but not exactly, the same period, we 
should find in ' both, from the causes explained in the 
foregoing paragraphs, the same general succession in 
the forms of life ; but the species would not exactly 
correspond ; for there will have been a little more time 
in the one region than in the other for modification, 
extinction, and immigration. 

I suspect that cases of this nature occur in Europe. 
Mr. Prestwich, in his admirable Memoirs on the eocene 
deposits of England and France, is able to draw a close 



106 AFFINITIES OF EXTINCT SPECIES. [Chap. XL 

general parallelism between the successive stages in 
the two countries ; but when he compares certain stages 
in England with those in France, although he finds in 
both a curious accordance in the numbers of the species 
belonging to the same genera, yet the species them- 
selves differ in a manner very difficult to account for 
considering the proximity of the two areas, — unless, 
indeed, it be assumed that an isthmus separated two 
seas inhabited by distinct, but contemporaneous, faunas. 
Lyell has made similar observations on some of the 
later tertiary formations. Barrande, also, shows that 
there is a striking general parallelism in the successive 
Silurian deposits of Bohemia and Scandinavia ; never- 
theless he finds a surprising amount of difference in 
the species. If the several formations in these regions 
have not been deposited during the same exact periods, 
— a formation in one region often corresponding with a 
blank interval in the other, — and if in both regions the 
species have gone on slowly changing during the accu- 
mulation of the several formations and during the lon^ 
intervals of time between them ; in this case the several 
formations in the two regions could be arranged in the 
same order, in accordance with the general succession 
of the forms of life, and the order would falsely appear 
to be strictly parallel ; nevertheless the species would 
not be all the same in the apparently corresponding 
stages in the two regions. 

On the Affinities of Extinct Species to each other, and 
to Living Forms. 

Let us now look to the mutual affinities of extinct 
and living species. All fall into a few grand classes ; 
and this fact is at once explained on the principle of 



Chap. XL] AFFINITIES OF EXTINCT SPECIES. 107 

descent. The more ancient any form is, the more, as 
a general rule, it differs from living forms. But, as 
Buckland long ago remarked, extinct species can all be 
classed either in still existing groups, or between them. 
That the extinct forms of life help to fill up the 
intervals between existing genera, families, and orders, 
is certainly true ; but as this statement has often been 
ignored or even denied, it may be well to make some 
remarks on this subject, and to give some instances. 
If we confine our attention either to the living or to the 
extinct species of the same class, the series is far less 
perfect than if we combine both into one general 
system. In the writings of Professor Owen we con- 
tinually meet with the expression of generalised forms, 
as applied to extinct animals; and in the writings of 
Agassiz, of prophetic or synthetic types ; and these 
terms imply that such forms are in fact intermediate or 
connecting links. Another distinguished paleontologist, 
M. Gaudry, has shown in the most striking manner that 
many of the fossil mammals discovered by him in Attica 
serve to break down the intervals between existing 
genera. Cuvier ranked the Euminants and Pachyderms, 
as two of the most distinct orders of mammals : but so 
many fossil links have been disentombed that Owen 
has had to alter the whole classification, and has placed 
certain pachyderms in the same sub-order with rumi- 
nants ; for example, he dissolves, by gradations the 
apparently wide interval between the pig and the 
camel. The Ungulata or hoofed quadrupeds are now 
divided into the even-toed or odd-toed divisions ; but 
the Macrauchenia of S. America connects to a certain 
extent these two grand divisions. No one will deny 
that the Hipparion is intermediate between the existing 



108 AFFINITIES OF EXTINCT SPECIES. [Chap. XI. 

horse and certain older ungulate forms. What a won- 
derful connecting link in the chain of mammals is 
the Typotherium from S. America, as the name given 
to it by Professor Gervais expresses, and which cannot 
be placed in any existing order. The Sirenia form a 
very distinct group of mammals, and one of the most 
remarkable peculiarities in the existing dugong and 
lamentin is the entire absence of hind limbs without 
even a rudiment being left ; but the extinct Halitherium 
had, according to Professor Flower, an ossified thigh- 
bone " articulated to a well-defined acetabulum in the 
pelvis," and it thus makes some approach to ordinary 
hoofed quadrupeds, to which the Sirenia are in other 
respects allied. The cetaceans or whales are widely 
different from all other mammals, but the tertiary Zeug- 
lodon and Squalodon, which have been placed by some 
naturalists in an order by themselves, are considered by 
Professor Huxley to be undoubtedly cetaceans, " and to 
constitute connecting links with the aquatic carnivora." 

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

Some writers have objected to any extinct species, 



Chap. XI.] AFFINITIES OF EXTINCT SPECIES. 109 

or group of species, being considered as intermediate 
between any two living species, or groups of species. 
If by this term it is meant that an extinct form is 
directly intermediate in all its characters between two 
living forms or groups, the objection is probably valid. 
But in a natural classification many fossil species 
certainly stand between living species, and some extinct 
genera between living genera, even between genera 
belonging to distinct families. The most common case, 
especially with respect to very distinct groups, such as 
fish and reptiles, seems to be, that, supposing them to 
be distinguished at the present day by a score of cha- 
racters, the ancient members are separated by a some- 
what lesser number of characters ; so that the two groups 
formerly made a somewhat nearer approach to each 
other than they now do. 

It is a common belief that the more ancient a form 
is, by so much the more it tends to connect by some of 
its characters groups now widely separated from each 
other. This remark no doubt must be restricted to 
those groups which have undergone much change in 
the course of geological ages ; and it would be difficult 
to prove the truth of the proposition, for every now and 
then even a living animal, as the Lepidosiren, is dis- 
covered having affinities directed towards very distinct 
groups. Yet if we compare the older Eeptiles and 
Batrachians, the older Fish, the older Cephalopods, and 
the eocene Mammals, with the more recent members of 
the same classes, we must admit that there is truth in 
the remark. 

Let us see how far these several facts and inferences 
accord with the theory of descent with modification. 
As the subject is somewhat complex, I must request 



110 AFFINITIES OF EXTINCT SPECIES. [Chap. XI. 

the reader to turn to the diagram in the fourth chapter. 
We may suppose that the numbered letters in italics 
represent genera, and the dotted lines diverging from 
them the species in each genus. The diagram is much 
too simple, too few genera and too few species being 
given, but this is unimportant for us. The horizontal 
lines may represent successive geological formations, 
and all the forms beneath the uppermost line may be 
considered as extinct. The three existing genera a 14 . 
£ 14 ) p li > vvill form a small family ; 6 14 and Z 14 a closely 
allied family or sub-family ; and o 14 , e 14 , m 14 , a third 
family. These three families, together with the many 
extinct genera on the several lines of descent diverging 
from the parent-form (A) will form an order , for all 
will have inherited something in common from theii 
ancient progenitor. On the principle of the continued 
tendency to divergence of character, which was formerly 
illustrated by this diagram, the more recent any form 
is, the more it will generally differ from its ancient 
progenitor. Hence we can understand the rule that 
the most ancient fossils differ most from existing forms. 
We must not, however, assume that divergence of 
character is a necessary contingency ; it depends solely 
on the descendants from a species being thus enabled 
to seize on many and different places in the economy of 
nature. Therefore it is quite possible, as we have seen 
in the case of some Silurian forms, that a species might 
go on being slightly modified in relation to its slightly 
altered conditions of life, and yet retain throughout a 
vast period the same general characteristics. This is 
represented in the diagram by the letter F 14 . 

All the many forms, extinct and recent, descended 
from (A), make, as before remarked, one order; and 



Chap. XL] AFFINITIES OF EXTINCT SPECIES. Ill 

this order, from the continued effects of extinction and 
divergence of character, has become divided into several 
sub-families and families, some of which are supposed 
to have perished at different periods, and some to have 
endured to the present day. 

By looking at the diagram we can see that if many 
of the extinct forms supposed to be imbedded in the 
successive formations, were discovered at several points 
low down in the series, the three existing families on 
the uppermost line would be rendered less distinct 
from each other. If, for instance, the genera a 1 , a 5 , a 10 , 
f 8 , m 3 , m 6 , m 9 , were disinterred, these three families 
would be so closely linked together that they probably 
would have to be united into one great family, in 
nearly the same manner as has occurred with rumi- 
nants and certain pachyderms. Yet he who objected 
to consider as intermediate the extinct genera, which 
thus link together the living genera of three families, 
would be partly justified, for they are intermediate, not 
directly, but only by a long and circuitous course 
through many widely different forms. If many ex- 
tinct forms were to be discovered above one of the 
middle horizontal lines or geological formations — for 
instance, above No. VI. — but none from beneath this 
line, then only two of the families (those on the left 
hand, a u , &c, and b u , &c.) would have to be united 
into one ; and there would remain two families, which 
would be less distinct from each other than they were 
before the discovery of the fossils. So again if the three 
families formed of eight genera (a 14 to m 14 ), on the 
uppermost line, be supposed to differ from each other 
by half-a-dozen important characters, then the families 
wliich existed at the period marked VI. would certainly 



112 AFFINITIES OF EXTINCT SPECIES. [Chap. XI. 

have differed from each other by a less number of cha- 
racters ; for they would at this early stage of descent 
have diverged in a less degree from their common 
progenitor. Thus it comes that ancient and extinct 
genera are often in a greater or less degree intermediate 
in character between their modified descendants, or 
between their collateral relations. 

Under nature the process will be far more complicated 
than is represented in the diagram ; for the groups will 
have been more numerous ; they will have endured for 
extremely unequal lengths of time, and will have been 
modified in various degrees. As we possess only the 
last volume of the geological record, and that in a very 
broken condition, we have no right to expect, except in 
rare cases, to fill up the wide intervals in the natural 
system, and thus to unite distinct families or orders. 
All that we have a right to expect is, that those groups 
which have, within known geological periods, undergone 
much modification, should in the older formations make 
some slight approach to each other ; so that the older 
members should differ less from each other in some of 
their characters than do the existing members of the 
same groups ; and this by the concurrent evidence of 
our best palaeontologists is frequently the case. 

Thus, on the theory of descent with modification, the 
main facts with respect to the mutual affinities of the 
extinct forms of life to each other and to living forms, 
are explained in a satisfactory manner. And they are 
wholly inexplicable on any other view. 

On this same theory, it is evident that the fauna 
during any one great period in the earth's history will be 
intermediate in general character between that which 
preceded and that which succeeded it. Thus the species 



Chap. XL] AFFINITIES OF EXTINCT SPECIES. 113 

which lived at the sixth great stage of descent in the 
diagram are the modified offspring of those which lived 
at the fifth stage, and are the parents of those which 
became still more modified at the seventh stage ; hence 
they could hardly fail to be nearly intermediate in 
character between the forms of life above and below. 
We must, however, allow for the entire extinction of 
some preceding forms, and in any one region for the 
immigration of new forms from other regions, and for a 
large amount of modification during the long and blank 
intervals between the successive formations. Subject 
to these allowances, the fauna of each geological period 
undoubtedly is intermediate in character, between the 
preceding and succeeding faunas. I need give only one 
instance, namely, the manner in which the fossils of the 
Devonian system, when this system was first discovered, 
were at once recognised by palaeontologists as inter- 
mediate in character between those of the overlying 
carboniferous, and underlying Silurian systems. But 
each fauna is not necessarily exactly intermediate, as 
unequal intervals of time have elapsed between conse- 
cutive formations. 

It is no real objection to the truth of the statement that 
the fauna of each period as a whole is nearly intermediate 
in character between the preceding and succeeding 
faunas, that certain genera offer exceptions to the rule. 
For instance, the species of mastodons and elephants, 
when arranged by Dr. Falconer in two series, — in the 
first place according to their mutual affinities, and in 
the second place according to their periods of existence, 
— do not accord in arrangement. The species extreme 
in character are not the oldest or the most recent ; nor 
are those which are intermediate in character, inter- 



114 AFFINITIES OF EXTINCT SPECIES. [Chap. XL 

mediate in age. But supposing for an instant, in this 
and other such cases, that the record of the first appear- 
ance and disappearance of the species was complete, 
which is far from the case, we have no reason to believe 
that forms successively produced necessarily endure for 
corresponding lengths of time. A very ancient form 
may occasionally have lasted much longer than a form 
elsewhere subsequently produced, especially in the case 
of terrestrial productions inhabiting separated districts. 
To compare small things with great ; if the principal 
living and extinct races of the domestic pigeon were 
arranged in serial affinity, this arrangement would not 
closely accord with the order in time of their produc- 
tion, and even less with the order of their disappearance ; 
for the parent rock-pigeon still lives ; and many varieties 
between the rock-pigeon and the carrier have become 
extinct ; and carriers which are extreme in the impor- 
tant character of length of beak originated earlier than 
short-beaked tumblers, which are at the opposite end of 
the series in this respect. 

Closely connected with the statement, that the organic 
remains from an intermediate formation are in some 
degree intermediate in character, is the fact, insisted on 
by all palaeontologists, that fossils from two consecutive 
formations are far more closely related to each other, 
than are the fossils from two remote formations. Pictet 
gives as a well-known instance, the general resem- 
blance of the organic remains from the several stages 
of the Chalk formation, though the species are dis- 
tinct in each stage. Tins fact alone, from its generality, 
seems to have shaken Professor Pictet in his belief 
in the immutability of species. He who is acquainted 
with the distribution of existing species over the globe, 



Chap. XI.] AFFINITIES OF EXTINCT SPECIES. 115 

will not attempt to account for the close resemblance of 
distinct species in closely consecutive formations, by 
the physical conditions of the ancient areas having 
remained nearly the same. Let it be remembered that 
the forms of life, at least those inhabiting the sea, have 
changed almost simultaneously throughout the world, 
and therefore under the most different climates and con- 
ditions. Consider the prodigious vicissitudes of climate 
during the pleistocene period, which includes the whole 
glacial epoch, and note how little the specific forms of 
the inhabitants of the sea have been affected. 

On the theory of descent, the full meaning of the 
fossil remains from closely consecutive formations being 
closely related, though ranked as distinct species, is 
obvious. As the accumulation of each formation has 
often been interrupted, and as long blank intervals have 
intervened between successive formations, we ought not 
to expect to find, as I attempted to show in the last 
chapter, in any one or in any two formations, all the 
intermediate varieties between the species which 
appeared at the commencement and close of these 
periods : but we ought to find after intervals, very long 
as measured by years, but only moderately long as 
measured geologically, closely allied forms, or, as they 
have been called by some authors, representative species; 
and these assuredly we do find. We find, in short, such 
evidence of the slow and scarcely sensible mutations of 
specific forms, as we have the right to expect 



28 



116 STATE OF DEVELOPMENT OF [Chap. XI. 

On the State of Development of Ancient compared with 

Living Forms. 

We have seen in the fourth chapter that the degree 
of differentiation and specialisation of the parts in 
organic beings, when arrived at maturity, is the best 
standard, as yet suggested, of their degree of perfection 
or highness. We have also seen that, as the speciali- 
sation of parts is an advantage to each being, so natural 
selection will tend to render the organisation of each 
being more specialised and perfect, and in this sense 
higher ; not but that it may leave many creatures with 
simple and unimproved structures fitted for simple con- 
ditions of life, and in some cases will even degrade or 
simplify the organisation, yet leaving such degraded 
beings better fitted for their new walks of life. In 
another and more general manner, new species become 
superior to their predecessors ; for they have to beat in 
the struggle for life all the older forms, with which they 
come into close competition. We may therefore con- 
clude that if under a nearly similar climate the eocene 
inhabitants of the world could be put into competition 
with the existing inhabitants, the former would be beaten 
and exterminated by the latter, as would the secondary 
by the eocene, and the palaeozoic by the secondary 
forms. So that by this fundamental test of victory in 
the battle for life, as well as by the standard of the 
specialisation of organs, modern forms ought, on the 
theory of natural selection, to stand higher than ancient 
forms. Is this the case ? A large majority of palaeon- 
tologists would answer in the affirmative ; and it seems 
that tins answer must be admitted as true, though 
difficult of proof. 



Chap. XL] ANCIENT AND LIVING FORMS. 117 

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

The problem whether organisation on the whole has 
advanced is in many ways excessively intricate. The 
geological record, at all times imperfect, does not 
extend far enough back, to shew with unmistakeable 



118 STATE OF DEVELOPMENT OF [Chap. XL 

clearness that within the known history of the world 
organisation has largely advanced. Even at the present 
day, looking to members of the same class, naturalists 
are not unanimous which forms ought to be ranked as 
highest : thus, some look at the selaceans or sharks, 
from their approach in some important points of struc- 
ture to reptiles, as the highest fish; others look at 
the teleosteans as the highest. The ganoids stand 
intermediate between the selaceans and teleosteans ; 
the latter at the present day are largely preponderant 
in number ; but formerly selaceans and ganoids alone 
existed ; and in this case, according to the standard of 
highness chosen, so will it be said that fishes have 
advanced or retrograded in organisation. To attempt 
to compare members of distinct types in the scale of 
highness seems hopeless; who will decide whether a 
cuttle-fish be higher than a bee — that insect which the 
great Yon Baer believed to be " in fact more highly 
organised than a fish, although upon another type " ? 
In the complex struggle for life it is quite credible that 
crustaceans, not very high in their own class, might beat 
cephalopods, the highest molluscs ; and such crustaceans, 
though not highly developed, would stand very high in 
the scale of invertebrate animals, if judged by the most 
decisive of all trials — the law of battle. Beside these 
inherent difficulties in deciding which forms are the 
most advanced in organisation, we ought not solely to 
compare the highest members of a class at any two 
periods — though undoubtedly this is one and perhaps 
the most important element in striking a balance — but 
we ought to cumpare all the members, high and low, at the 
two periods. At an ancient epoch the highest and lowest 
raolluscoidal animals, namely, cephalopods and brachio 



Chap. XL] ANCIENT AND LIVING FOKMS. 119 

pods, swarmed in numbers ; at trie present time both 
groups are greatly reduced, whilst others, intermediate 
in organisation, have largely increased; consequently 
some naturalists maintain that molluscs were formerly 
more highly developed than at present ; but a stronger 
case can be made out on the opposite side, by consider- 
ing the vast reduction of brachiopods, and the fact 
that our existing cephalopods, though few in number, 
are more highly organised than their ancient represen- 
tatives. We ought also to compare the relative propor- 
tional numbers at any two periods of the high and low 
classes throughout the world : if, for instance, at the 
present day fifty thousand kinds of vertebrate animals 
exist, and if we knew that at some former period only 
ten thousand kinds existed, we ought to look at this in- 
crease in number in the highest class, which implies a 
great displacement of lower forms, as a decided advance 
in the organisation of the world. We thus see how 
hopelessly difficult it is to compare with perfect fair- 
ness under such extremely complex relations, the stan- 
dard of organisation of the imperfectly-known faunas of 
successive periods. 

We shall appreciate this difficulty more clearly, by 
looking to certain existing faunas and floras. From the 
extraordinary manner in which European productions 
have recently spread over New Zealand, and have seized 
on places which must have been previously occupied by 
the indigenes, we must believe, that if all the animals 
and plants of Great Britain were set free in New 
Zealand, a multitude of British forms would in the 
course of time become thoroughly naturalised there, 
and would exterminate many of the natives. On the 
other hand, from the fact that hardly a single inhabitant 



120 STATE OF DEVELOPMENT OF [Chap. XI. 

of the southern hemisphere has become wild in any part 
of Europe, we may well doubt whether, if all the 
productions of New Zealand were set free in Great 
Britain, any considerable number would be enabled to 
seize on places now occupied by our native . plants and 
animals. Under this point of view, the productions of 
Great Britain stand much higher in the scale than those 
of Xew Zealand. Yet the most skilful naturalist, from 
an examination of the species of the two countries, 
could not have foreseen this result. 

Agassiz and several other highly competent judges 
insist that ancient animals resemble to a certain extent 
the embryos of recent animals belonging to the same 
classes ; and that the geological succession of extinct 
forms is nearly parallel with the embryological de- 
velopment of existing forms. This view accords ad- 
mirably well with our theory. In a future chapter I 
shall attempt to show that the adult differs from its 
embryo, owing to variations having supervened at a not 
early age, and having been inherited at a corresponding 
age. This process, whilst it leaves the embryo almost 
unaltered, continually adds, in the course of successive 
generations, more and more difference to the adult. 
Thus the embryo comes to be left as a sort of picture, 
preserved by nature, of the former and less modified 
condition of the species. This view may be true, and 
vet may never be capable of proof. Seeing, for instance, 
that the oldest known mammals, reptiles, and fishes 
strictly belong to their proper classes, though some of 
these old forms are in a slight degree less distinct from 
each other than are the typical members of the same 
groups at the present day, it would be vain to look for 
animals having the common embryological character of 



Chap. XL] ANCIENT AND LIVING FORMS. 121 

the Vertebrata, until beds rich in fossils are discovered 
far beneath the lowest Cambrian strata — a discovery of 
which the chance is small. 

On the Succession of the same Types within the same 
Areas, during the later Tertiary periods. 

Mr. Clift many years ago showed that the fossil 
mammals from the Australian caves were closely allied 
to the living marsupials of that continent. In South 
America, a similar relationship is manifest, even to an 
uneducated eye, in the gigantic pieces of armour, like 
those of the armadillo, found in several parts of La 
Plata ; and Professor Owen has shown in the most 
striking manner that most of the fossil mammals, buried 
there in such numbers, are related to South American 
types. This relationship is even more clearly seen in 
the wonderful collection of fossil bones made by MM. 
Lund and Clausen in the caves of Brazil. I was so 
much impressed with these facts; that I strongly insisted, 
in 1839 and 1845, on this "law of the succession of 
types/' — on "this wonderful relationship in the same 
continent between the dead and the living." Professor 
Owen has subsequently extended the same generalisation 
to the mammals of the Old World. We see the same 
law in this author's restorations of the extinct and 
gigantic birds of New Zealand. We see it also in the 
birds of the caves of Brazil. Mr. Woodward has shown 
that the same law holds good with sea-shells, but, from 
the wide distribution of most molluscs, it is not well 
displayed by them. Other cases could be added, as the 
relation between the extinct and living land-shells of 
Madeira ; and between the extinct and living brackish 
water-shells of the Aralo-Caspian Sea. 



122 SUCCESSION OF THE [Chap. XI. 

Now what does this remarkable law of the succession 
of the same types within the same areas mean ? He 
would be a bold man who, after comparing the present 
climate of Australia and of parts of South America, 
under the same latitude, would attempt to account, on 
the one hand through dissimilar physical conditions, for 
the dissimilarity of the inhabitants of these two con- 
tinents; and, on the other hand through similarity of 
conditions, for the uniformity of the same types in each 
continent during the later tertiary periods. Nor can it 
be pretended that it is an immutable law that marsupials 
should have been chiefly or solely produced in Australia ; 
or that Edentata and other American types should have 
been solely produced in South America. For we know 
that Europe in ancient times was peopled by numerous 
marsupials ; and I have shown in the publications above 
alluded to, that in America the law of distribution of 
terrestrial mammals was formerly different from what it 
now is. Xorth America formerly partook strongly of 
the present character of the southern half of the con- 
tinent ; 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 Xorthern India was formerly more 
closely related in its mammals to Africa than it is at 
the present time. Analogous facts could be given in 
relation to the distribution of marine animals. 

On the theory of descent with modification, the great 
law of the long enduring, but not immutable, succession 
of the same types within the same areas, is at once ex- 
plained ; for the inhabitants of each quarter of the world 
will obviously tend to leave in that quarter, during the 
next succeeding period of time, closely allied though in 



Chap. XL] SAME TYPES IN THE SAME AREAS. 123 

some decree modified descendants. If the inhabitants 
of one continent formerly differed greatly from those of 
another continent, so will their modified descendants still 
differ in nearly the same manner and degree. But after 
very long intervals of time, and after great geographical 
changes, permitting much intermigration, the feebler 
will yield to the more dominant forms, and there will 
be nothing immutable in the distribution of organic 
beings. 

It may be asked in ridicule, whether I suppose that 
the megatherium and other allied huge monsters, which 
formerly lived in South America, have left behind them 
the sloth, armadillo, and anteater, as their degenerate 
descendants. This cannot for an instant be admitted. 
These huge animals have become wholly extinct, and 
have left no progeny. But in the caves of Brazil, there 
are many extinct species which are closely allied in size 
and in all other characters to the species still living in 
South America ; and some of these fossils may have been 
the actual progenitors of the living species. It must not 
be forgotten that, on our theory, all the species of the 
same genus are the 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 representative genera each 
with the same number of species, then we may conclude 
that generally only one species of each of the older 
genera has left modified descendants, which constitute 
the new genera containing the several species ; the other 
seven species of each old genus having died out and left 
no progeny. Or, and this will be a far commoner case, 
two or three species in two or three alone of the six older 
genera will be the parents of the new genera : the other 



124 SUMMAEY OF THE [Chap. XL 

species and the other old genera having become utterly 
extinct. In failing orders, with the genera and species 
decreasing in numbers as is the case with the Edentata 
of South America, still fewer genera and species will 
leave modified blood-descendants. 



Summary of the preceding and present Chapters. 

I have attempted to show that the geological record is 
extremely imperfect ; that only a small portion of the 
globe has been geologically explored with care ; that 
only certain classes of organic beings have been largely 
preserved in a fossil state ; that the number both of 
specimens and of species, preserved in our museums, is 
absolutely as nothing compared with the number of 
generations which must have passed away even during a 
single formation ; that, owing to subsidence being almost 
necessary for the accumulation of deposits rich in fossil 
species of many kinds, and thick enough to outlast future 
degradation, great intervals of time must have elapsed 
between most of our successive formations ; that there 
has probably been more extinction during the periods of 
subsidence, and more variation during the periods of 
elevation, and during the latter the record will have been 
perfectly kept ; that each single formation has not 
been continuously deposited ; that the duration of each 
formation is probably short compared with the average 
duration of specific forms ; that migration has played an 
important part in the first appearance of new forms in 
any one area and formation ; that widely ranging species 
are those which have varied most frequently, and have 
oftenest given rise to new species ; that varieties have 
at first been local; and lastly, although each species 



Chap. XL] PEECEDING AND PEESENT CHAPTEES. 125 

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

He who rejects this view of the imperfection of the 
geological record, will rightly reject the whole theory. 
For he may ask in vain where are the numberless 
transitional links which must formerly have connected 
the closely allied or representative species, found in 
the successive stages of the same great formation ? He 
may disbelieve in the immense intervals of time which 
must have elapsed between our consecutive formations ; 
he may overlook how important a part migration has 
played, when the formations of any one great region, 
as those of Europe, are considered; he may urge the 
apparent, but often falsely apparent, sudden coming in 
of whole groups of species. He may ask where are the 
remains of those infinitely numerous organisms which 
must have existed long before the Cambrian system 
was deposited ? We now know that at least one animal 
did then exist ; but I can answer this last question 
only by supposing that where our oceans now extend 



126 SUMMAEY OF THE [Chap. XL 

they have extended for an enormous period, and where 
our oscillating continents now stand they have stood 
since the commencement of the Cambrian system ; but 
that, long before that epoch, the world presented a 
widely different aspect ; and that the older continents, 
formed of formations older than any known to us, exist 
now only as remnants in a metamorphosed condition, 
or lie still buried under the ocean. 

Passing from these difficulties, the other great leading 
facts in palaeontology agree admirably with the theory 
of descent with modification through variation and 
natural selection. We can thus understand how it is 
that new species come in slowly and successively ; how 
species of different classes do not necessarily change 
together, or at the same rate, or in the same degree ; 
yet in the long run that all undergo modification to 
some extent. The extinction of old forms is the almost 
inevitable consequence of the production of new forms. 
"We can understand why, when a species has once dis- 
appeared, it never reappears. Groups of species increase 
in numbers slowly, and endure for unequal periods of 
time; for the process of modification is necessarily 
slow, and depends on many complex contingencies. 
The dominant species belonging to large and dominant 
groups tend to leave many modified descendants, which 
form new sub-groups and groups. As these are formed, 
the species of the less vigorous groups, from their in- 
feriority inherited from a common progenitor, tend to 
become extinct together, and to leave no modified off- 
spring on the face of the earth. But the utter extinc- 
tion of a whole group of species has sometimes been a 
slow process, from the survival of a few descendants, 
lingering in protected and isolated situations. When a 



Chap XI.] PRECEDING AND PRESENT CHAPTEES. 127 

group has once wholly disappeared, it does not reappear ; 
for the link of generation has been broken. 

We can understand how it is that dominant forms 
which spread widely and yield the greatest number of 
varieties tend to people the world with allied, but 
modified, descendants ; and these will generally succeed 
in displacing the groups which are their inferiors in 
the struggle for existence. Hence, after long intervals 
of time, the productions of the world appear to have 
changed simultaneously. 

We can understand how it is that all the forms of 
life, ancient and recent, make together a few grand 
classes. We can understand, from the continued ten- 
dency to divergence of character, why the more ancient 
a form is, the more it generally differs from those now 
living ; why ancient and extinct forms often tend to 
fill up gaps between existing forms, sometimes blending 
two groups, previously classed as distinct, into one ; but 
more commonly bringing them only a little closer 
together. The more ancient a form is, the more often 
it stands in some degree intermediate between groups 
now distinct ; for the more ancient a form is, the more 
nearly it will be related to, and consequently resemble, 
the common progenitor of groups, since become widely 
divergent. Extinct forms are seldom directly inter- 
mediate between existing forms ; but are intermediate 
only by a long and circuitous course through other 
extinct and different forms. We can clearly see why 
the organic remains of closely consecutive formations 
are closely allied ; for they are closely linked together 
by generation. We can clearly see why the remains of 
an intermediate formation are intermediate in character. 

The inhabitants of the world at each successive 
29 



128 SUMMARY OF CHAPTEES. [Chap. XL 

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

If then the geological record be as imperfect as many 
believe, and it may at least be asserted that the record 
cannot be proved to be much more perfect, the main 
objections to the theory of natural selection are greatly 
diminished or disappear. On the other hand, all the 
chief laws of palaeontology plainly proclaim, as it seems 
to me, that species have been produced by ordinary 
generation : old forms having been supplanted by new 
and improved forms of life, the products of Variation 
and the Survival of the Fittest 



Chap. XII.] GEOGKAPHICAL DISTRIBUTION. 129 



CHAPTEE XII. 

Geographical Distribution. 

Present distribution cannot be accounted for by differences in 
physical conditions — Importance of barriers — Affinity of the pro- 
ductions 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 wholly 
accounted for by climatal and other physical conditions. 
Of late, almost every author who has studied the subject 
has come to this conclusion. The case of America 
alone would almost suffice to prove its truth ; for if we 
exclude the arctic and northern temperate parts, all 
authors agree that one of the most fundamental divisions 
in geographical distribution is that between the New and 
Old Worlds ; yet if we travel over the vast American 
continent, from the central parts of the United States 
to its extreme southern point, we meet with the most 
diversified conditions; humid districts, arid deserts, 
lofty mountains, grassy plains, forests, marshes, lakes, 
and great rivers, under almost every temperature. 
There is hardly a climate or condition in the Old 
World which cannot be paralleled in the New — at 



130 GEOGRAPHICAL DISTRIBUTION. [Chap. XII 

least as closely as the same species generally require. 
ISTo doubt small areas can be pointed out in the Old 
World hotter than any in the New World ; but these 
are not inhabited by a fauna different from that of the 
surrounding districts ; for it is rare to find a group of 
organisms confined to a small area, of which the con- 
ditions are peculiar in only a slight degree. Xotwith- 
standing this general parallelism in the conditions of 
the Old and ]STew "Worlds, how widely different are 
their living productions ! 

In the southern hemisphere, if we compare large 
tracts of land in Australia, South Africa, and western 
South America, between latitudes 25° and 35°, we shall 
find parts extremely similar in all their conditions, yet 
it would not be possible to point out three faunas and 
floras more utterly dissimilar. Or, again, we may com- 
pare the productions of South America south of lat. 
35° with those north of 25°, which consequently are 
separated by a space of ten degrees of latitude, and are 
exposed to considerably different conditions ; yet they 
are incomparably more closely related to each other 
than they are to the productions of Australia or Africa 
under nearly the same climate. Analogous facts could 
be given with respect to the inhabitants of the sea. 

A second great fact which strikes us in our general 
review is, that barriers of any kind, or obstacles to 
free migration, are related in a close and important 
manner to the differences between the productions of 
various regions. We see this in the great difference 
in nearly all the terrestrial productions of the Xew 
and Old Worlds, excepting in the northern parts, where 
the land almost joins, and where, under a slightly 
different climate, there might have been free migration 



Chap. XII.] GEOGRAPHICAL DISTRIBUTION. 131 

for the northern temperate forms, as there now is for 
the strictly arctic productions. We see the same fact 
in the great difference between the inhabitants of 
Australia, Africa, and South America under the same 
latitude ; for these countries are almost as much isolated 
from each other as is possible. On each continent, 
also, we see the same fact ; for on the opposite sides of 
lofty and continuous mountain-ranges, of great deserts 
and even of large rivers, we find different productions ; 
though as mountain-chains, deserts, &c, are not as 
impassable, or likely to have endured so long, as the 
oceans separating continents, the differences are very 
inferior in degree to those characteristic of distinct 
continents. 

Turning to the sea, we find the same law. The 
marine inhabitants of the eastern and western shores 
of South America are very distinct, with extremely 
few shells, Crustacea, or echinodermata in common ; 
but Dr. Gunther has recently shown that about thirty 
per cent, of the fishes are the same on the opposite 
sides of the isthmus of Panama ; and this fact has led 
naturalists to believe that the isthmus was formerly 
open. Westward of the shores of America, a wide 
space of open ocean extends, with not an island as a 
halting-place for emigrants ; here we have a barrier of 
another kind, and as soon as this is passed we meet in 
the eastern islands of the Pacific with another and 
totally distinct fauna. So that three marine faunas 
range far northward and southward in parallel lines 
not far from each other, under corresponding climates ; 
but from being separated from each other by impass- 
able barriers, either of land or open sea, they are almost 
wholly distinct. On the other hand, proceeding still 



132 GEOGEAPHICAL DISTRIBUTION. [Chap. XII. 

farther -westward from the eastern islands of the tropical 
parts of the Pacific, we enconnter no impassable barriers, 
and we have innumerable islands as halting-places, or 
continuous coasts, until, after travelling over a hemi- 
sphere, we come to the shores of Africa ; and over this 
vast space we meet with no well-defined and distinct 
marine faunas. Although so few marine animals are 
common to the above-named three approximate faunas 
of Eastern and Western America and the eastern 
Pacific islands, yet many fishes range from the Pacific 
into the Indian Ocean, and many shells are common 
to the eastern islands of the Pacific and the eastern 
shores of Africa on almost exactly opposite meridians 
of longitude. 

A third great fact, partly included in the foregoing 
statement, is the affinity of the productions of the same 
continent or of the same sea, though the species them- 
selves are distinct at different points and stations. It 
is a law of the widest generality, and every continent 
offers innumerable instances. Nevertheless the natural- 
ist, in travelling, for instance, from north to south, 
never fails to be struck by the manner in which suc- 
cessive groups of beings, specifically distinct, though 
nearly related, replace each other. He hears from 
closely allied, yet distinct kinds of birds, notes nearly 
similar, and sees their nests similarly constructed, but 
not quite alike, with eggs coloured in nearly the same 
manner. The plains near the Straits of Magellan are 
inhabited by one species of Pihea (American ostrich), 
and northward the plains of La Plata by another species 
of the same genus ; and not by a true ostrich or emu, 
like those inhabiting Africa and Australia under the 
same latitude. On these same plains of La Plata we 



Chap. XII.] GEOGRAPHICAL DISTRIBUTION. 133 

see the agouti and bizcacha, animals having nearly 
the same habits as our hares and rabbits, and belonging 
to the same order of Eodents, but they plainly display 
an American type of structure. "We ascend the lofty 
peaks of the Cordillera, and we find an alpine species 
of bizcacha ; we look to the waters, and we do not find 
the beaver or musk-rat, but the coypu and capybara, 
rodents of the S. American type. Innumerable other 
instances could be given. If we look to the islands off 
the American shore, however much they may differ in 
geological structure, the inhabitants are essentially 
American, though they may be all peculiar species. 
We may look back to past ages, as shown in the last 
chapter, and we find American types then prevailing 
on the American continent and in the American seas. 
We see in these facts some deep organic bond, through- 
out space and time, over the same areas of land and 
water, independently of physical conditions. The 
naturalist must be dull who is not led to inquire what 
this bond is. 

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



134 GEOGRAPHICAL DISTRIBUTION. [Chap. XIL 

of the inhabitants on each other in leading to the 
preservation of different modifications ; the relation of 
organism to organism in the struggle for life being, as 
I have already often remarked, the most important of 
all relations. Thus the high importance of barriers 
comes into play by checking migration ; as does time 
for the slow process of modification through natural 
selection. Widely-ranging species, abounding in in- 
dividuals, which have already triumphed over many 
competitors in their own widely-extended homes, will 
have the best chanee 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- 
quently undergo further modification and improvement ; 
and thus they will become still further victorious, and 
will produce groups of modified descendants. On this 
principle of inheritance with modification we can under- 
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. 

There is no evidence, as was remarked in the last 
chapter, of the existence of any law of necessary 
development. As the variability of each species is an 
independent property, and will be taken advantage of 
by natural selection, only so far as it profits each in- 
dividual in its complex struggle for life, so the amount 
of modification in different species will be no uniform 
quantity. If a number of species, after having long 
competed with each other in their old home, were to 
migrate in a body into a new and afterwards isolated 
country, they would be little liable to modification ; for 
neither migration nor isolation in themselves effect 
anything. These principles come into play only by 



Chap. XII.] SINGLE CENTRES OF CREATION. 135 

bringing organisms into new relations with each other 
and in a lesser degree with the surrounding physical 
conditions. As we have seen in the last chapter that 
some forms have retained nearly the same character 
from an enormously remote geological period, so certain 
species have migrated over vast spaces, and have not 
become greatly or at all modified. 

According to these views, it is obvious that the 
several species of the same genus, though inhabiting 
the most distant quarters of the world, must originally 
have proceeded from the same source, as they are 
descended from the same progenitor. In the case of 
those species which have undergone during whole geo- 
logical periods little modification, there is not much 
difficulty in believing that they have migrated from 
the same region ; for during the vast geographical and 
climatal changes which have supervened since ancient 
times, almost any amount of migration is possible. But 
in many other cases, in which we have reason to believe 
that the species of a genus have been produced within 
comparatively recent times, there is great difficulty on 
this head. It is also obvious that the individuals of 
the same species, though now inhabiting distant and 
isolated regions, must have proceeded from one spot, 
where their parents were first produced: for, as has 
been explained, it is incredible that individuals iden- 
tically 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 dis- 
cussed by naturalists, namely, whether species have 
been created at one or more points of the earth's sur- 
face. Undoubtedly there are many cases of extreme 



138 SINGLE CENTRES OF CREATION. [Chap. XII. 

difficulty in understanding how the same species could 
possibly have migrated from some one point to the 
several distant and isolated points, where now found. 
Nevertheless the simplicity of the view that each 
species was first produced within a single region cap- 
tivates the mind. He who rejects it, rejects the vera 
causa of ordinary generation with subsequent migra- 
tion, and calls in the agency of a miracle. It is 
universally admitted, that in most cases the area in- 
habited by a species is continuous ; and that when a 
plant or animal inhabits two points so distant from 
each other, or with an interval of such a nature, that 
the space could not have been easily passed over by 
migration, the fact is given as something remarkable 
and exceptional. The incapacity of migrating across a 
wide sea is more clear in the case of terrestrial mam- 
mals than perhaps with any other organic beings ; and, 
accordingly, we find no inexplicable instances of the 
same mammals inhabiting distant points of the world. 
No geologist feels any difficulty in Great Britain pos- 
sessing the same quadrupeds with the rest of Europe, 
for they were no doubt once united. But if the same 
species can be produced at two- separate points, why do 
we not find a single mammal common to Europe and 
Australia or South America ? The conditions of life 
are nearly the same, so that a multitude of European 
animals and plants have become naturalised in America 
and Australia; and some of the aboriginal plants are 
identically the same at these distant points of the 
northern and southern hemispheres ? The answer, as 
I believe, is, that mammals have not been able to 
migrate, whereas some plants, from their varied means 
of dispersal, have migrated across the wide and broken 



Chap. XII.] SINGLE CENTEES OF CKEATION. 137 

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 confined to a single region ; and it has been observed 
by several naturalists that the most natural genera, or 
those genera in which the species are most closely 
related to each other, are generally confined to the 
same country, or if they have a wide range that their 
range is continuous. What a strange anomaly it would 
be, if a directly opposite rule were to prevail, when we 
go down one step lower in the series, namely, to the 
individuals of the same species, and these had not 
been, at least at first, 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 present conditions 
permitted, is the most probable. Undoubtedly many 
cases occur, in which we cannot explain how the same 
species could have passed from one point to the other. 
But the geographical and climatal changes which have 
certainly occurred within recent geological times, must 
have rendered discontinuous the formerly continuous 
range of many species. So that we are reduced to consider 
whether the exceptions to continuity of range are so 
numerous and of so grave a nature, that we ought to give 
up the belief, rendered probable by general considerations, 
that each species has been produced within one area, and 
has migrated thence as far as it could. It would be 



138 SINGLE CENTRES OF CEEATION. [Chap. XII. 

hopelessly tedious to discuss all the exceptional cases 
of the same species, now living at distant and separated 
points, nor do I for a moment pretend that any 
explanation could be offered of many instances. But, 
after some preliminary remarks, I will discuss a few of 
the most striking classes of facts ; namely, the existence 
of the same species on the summits of distant mountain 
ranges, arid at distant points in the arctic and antarctic 
regions; and secondly (in the following chapter), the 
wide distribution of freshwater productions ; and thirdly, 
the occurrence of the same terrestrial species on islands 
and on the nearest mainland, though separated by 
hundreds of miles of open sea. If the existence of the 
same species at distant and isolated points of the earth's 
surface, can in many instances be explained on the view 
of each species having migrated from a single birthplace ; 
then, considering our ignorance with respect to former 
climatal and geographical changes and to the various 
occasional means of transport, the belief that a single 
birthplace is the law, seems to me incomparably the 
safest. 

In discussing this subject, we shall be enabled at the 
same time to consider a point equally important for us, 
namely, whether the several species of a genus which 
must on our theory all be descended from a common 
progenitor, can have migrated, undergoing modification 
during their migration, from some one area. If, when 
most of the species inhabiting one region are different 
from those of another region, though closely allied to 
them, it can be shown that migration from the one region 
to the other has probably occurred at some former period, 
our general view will be much strengthened ; for the 
explanation is obvious on the principle of descent with 



Chap. XII.] SINGLE CENTRES OF CREATION. 139 

modification. A volcanic island, for instance, upheaved 
and formed at the distance of a few hundreds of miles 
from a continent, would probably receive from it in the 
course of time a few colonists, and their descendants, 
though modified, would still be related by inheritance to 
the inhabitants of that continent. Cases of this nature 
are common, and are, as we shall hereafter see, inexplic- 
able on the theory of independent creation. This view 
of the relation of the species of one region to those of 
another, does not differ much from that advanced by Mr. 
Wallace, who concludes that " every species has come 
into existence coincident both in space and time with a 
pre-existing closely allied species." And it is now well 
known that he attributes this coincidence to descent with 
modification. 

The question of single or multiple centres of creation 
differs from another though allied question, — namely, 
whether all the individuals of the same species are 
descended from a single pair, or single hermaphrodite, or 
whether, as some authors suppose, from many individuals 
simultaneously created. With organic beings which 
never intercross, if such exist, each species must be de- 
scended from a succession of modified varieties, that 
have supplanted each other, but have never blended with 
other individuals or varieties of the same species ; so that, 
at each successive stage of modification, all the individuals 
of the same form will be descended from a single parent. 
But in the great majority of cases, namely, with all 
organisms which habitually unite for each birth, or which 
occasionally intercross, the individuals of the same 
species inhabiting the same area will be kept nearly 
uniform by intercrossing ; so that many individuals will 
go on simultaneously changing, and the whole amount 
30 



140 MEANS OF DISPERSAL. [Chap. XJX 

of modification at each stage will not be due to descent 
from a single parent. To illustrate what I mean : our 
English race-horses differ from the horses of every 
other breed ; but they do not owe their difference and 
superiority to descent from any single pair, but to 
continued care in the selecting and training of many 
individuals during each generation. 

Before discussing the three classes of facts, which I 
have selected as presenting the greatest amount of 
difficulty on the theory of " single centres of creation," 
I must say a few words on the means of dispersal. 

Means of Dispersal. 

Sir C. Lyell and other authors have ably treated 
this subject. I can give here only the briefest abstract 
of the more important facts. Change of climate must 
have had a powerful influence on migration. A region 
now impassable to certain organisms from the nature of 
its climate, might have been a high road for migration, 
when the climate was different. I shall, however, 
presently have to discuss this branch of the subject in 
some detail. Changes of level in the land must also 
have been highly influential: a narrow isthmus now 
separates two marine faunas ; submerge it, or let it 
formerly have been submerged, and the two faunas will 
now blend together, or may formerly have blended. 
"Where the sea now extends, land may at a former period 
have connected islands or possibly even continents to- 
gether, and thus have allowed terrestrial productions to 
pass from one to the other. No geologist disputes that 
great mutations of level have occurred within the period 
of existing organisms. Edward Eorbes insisted that all 
the islands in the Atlantic must have been recently 



Chap. XII.] MEANS OF DISPERSAL. 141 

connected with Europe or Africa, and Europe likewise 
with America. Other authors have thus hypothetically 
bridged over every ocean, and united almost every island 
with some mainland. If indeed the arguments used by 
Forbes are to be trusted, it must be admitted that 
scarcely a single island exists which has not recently 
been united to some continent. This view cuts the 
Gorclian knot of the dispersal of the same species to the 
most distant points, and removes many a difficulty ; but 
to the best of my judgment we are not authorised in 
admitting such enormous geographical changes within 
the period of existing species. It seems to me that we 
have abundant evidence of great oscillations in the level 
of the land or sea ; but not of such vast changes in the 
position and extension of our continents, as to have united 
them within the recent period to each other and to the 
several intervening oceanic islands. I freely admit the 
former existence of many islands, now buried beneath 
the sea, which may have served as halting-places for 
plants and for many animals during their migration. In 
the coral-producing oceans such sunken islands are now 
marked by rings of coral or atolls standing over them. 
Whenever it is fully admitted, as it will some day be, that 
each species has proceeded from a single birthplace, and 
when in the course of time we know something definite 
about the means of distribution, we shall be enabled 
to speculate with security on the former extension of the 
land. But I do not believe that it will ever be proved 
that within the recent period most of our continents 
which now stand quite separate, have been continuously, 
or almost continuously united with each other, and with 
the many existing oceanic islands. Several facts in 
distribution, — such as the great difference in the marine 



142 MEANS OF DISPEESAL. [Chap. XII. 

faunas on the opposite sides of almost every continent, 
— the close relation of the tertiary inhabitants of several 
lands and even seas to their present inhabitants, — the 
degree of affinitv between the mammals inhabiting 
islands with those of the nearest continent, being in 
part determined (as we shall hereafter see) by the depth 
of the intervening ocean, — these and other such facts are 
opposed to the admission of such prodigious geographical 
revolutions within the recent period, as are necessary 
on the view advanced by Forbes and admitted by his 
followers. The nature and relative proportions of the 
inhabitants of oceanic islands are likewise opposed to the 
belief of their former continuity with continents. !N"or 
does the almost universally volcanic composition of such 
islands favour the admission that they are the wrecks of 
sunken continents ; — if they had originally existed as 
continental mountain ranges, some at least of the islands 
would have been formed, like other mountain summits, 
of granite, metamorphic schists, old fossiliferous and 
other rocks, instead of consisting of mere piles of volcanic 
matter. 

I must now say a few words on what are called 
accidental means, but which more properly should be 
called occasional means of distribution. I shall here 
confine myseK to plants. In botanical works, this or 
that plant is often stated to be ill adapted for wide 
dissemination ; but the greater or less facilities for 
transport across the sea may be said to be almost wholly 
unknown. Until I tried, with Mr. Berkeley's aid, a few 
experiments, it was not even known how far seeds could 
resist the injurious action of sea- water. To my surprise 
I found that out of 87 kinds, 64 germinated after an 
immersion of 28 davs, and a few survived an immersion 



Chap. XII.] MEANS OF DISPEKSAL. 143 

of 137 days. It deserves notice that certain orders were 
far more injured than others : nine Leguminosse were 
tried, and, with one exception, they resisted the salt-water 
badly ; seven species of the allied orders, Hydrophyllacese 
and Polemoniacese, were all killed by a month's im- 
mersion. For convenience' sake I chiefly tried small 
seeds without the capsule or fruit ; and as all of these 
sank in a few days they could not have been floated 
across wide spaces of the sea, whether or not they were 
injured by the salt-water. Afterwards I tried some larger 
fruits, capsules, &c, and some of these floated for a long 
time. It is well known what a difference there is in the 
buoyancy of green and seasoned timber ; and it occurred 
to me that floods would often wash into the sea dried 
plants or branches with seed- capsules or fruit attached 
to them. Hence I was led to dry the stems and branches 
of 94 plants with ripe fruit, and to place them on sea- 
water. The majority sank quickly, but some which, 
whilst green, floated for a very short time, when dried 
floated much longer ; for instance, ripe hazel-nuts sank 
immediately, but when dried they floated for 90 days, 
and afterwards when planted germinated ; an asparagus- 
plant with ripe berries floated for 23 days, when dried it 
floated for 85 days, and the seeds afterwards germinated ; 
the ripe seeds of Helosciadium sank in two days, when 
dried they floated for above 90 days, and afterwards 
germinated. Altogether, out of the 94 dried plants, 18 
floated for above 28 days ; and some of the 18 floated 
for a very much longer period. So that as § f kinds of 
seeds germinated after an immersion of 28 days ; and as 
if distinct species with ripe fruit (but not all the same 
species as in the foregoing experiment) floated, after 
being dried, for above 28 days, we may conclude, as far 



144 MEANS OF DISPEKSAL. [Chap. XII. 

as anything can be inferred from these scanty facts, that 
the seeds of ^5 kinds of plants of any country might be 
floated by sea-currents during 28 days, and would re- 
tain their power of germination. In Johnston's Physical 
Atlas, the average rate of the several Atlantic currents 
is 33 miles per diem (some currents running at the rate 
of 60 miles per diem) ; on this average, the seeds of yj^ 
plants belonging to one country might be floated across 
924 miles of sea to another country, and when stranded, 
if blown by an inland gale to a favourable spot, would 
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 float- 
ing plants. He tried 98 seeds, mostly different from 
mine; but he chose many large fruits and likewise 
seeds from plants which live near the sea; and this 
would have favoured both the average length of their 
flotation and their resistance to the injurious action of 
the salt-water. On the other hand, he did not pre- 
viously 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 -J| of 
his seeds of different kinds floated for 42 days, and were 
then capable of germination. But I do not doubt that 
plants exposed to the waves would float for a less time 
than those protected from violent movement as in our 
experiments. Therefore it would perhaps be safer to 
assume that the seeds of about jffa plants of a flora, 
after having been dried, could be floated across a space 
of sea 900 miles in width, and would then germinate. 
The fact of the larger fruits often floating longer than 



Chap. XH.] MEANS OF DISPEKSAL. 145 

the small, is interesting ; as plants with large seeds or 
fruit which, as Alph. de Candolle has shown, generally 
have restricted ranges, could hardly be transported by 
any other means. 

Seeds may be occasionally transported in another 
manner. Drift timber is thrown up on most islands, 
even on those in the midst of the widest oceans ; and 
the natives of the coral-islands in the Pacific procure 
stones for their tools, solely from the roots of drifted 
trees, these stones being a valuable royal tax. I find 
that when irregularly shaped stones are embedded in 
the roots of trees, small parcels of earth are frequently 
enclosed in their interstices and behind them, — so 
perfectly that not a particle could be washed away 
during the longest transport : out of one small portion 
of earth thus completely enclosed by the roots of an 
oak about 50 years old, three dicotyledonous plants 
germinated : I am certain of the accuracy of this obser- 
vation. Again, I can show that the carcases of birds, 
when floating on the sea, sometimes escape being im- 
mediately devoured: and many kinds of seeds in the 
crops of floating birds long retain their vitality : peas 
and vetches, for instance, are killed by even xa few days' 
immersion in sea- water ; but some taken out of the 
crop of a pigeon, which had floated on artificial sea- 
water for 30 clays, to my surprise nearly all germinated. 
Living birds can hardly fail to be highly effective 
agents in the transportation of seeds. I could give 
many facts showing how frequently birds of many kinds 
are blown by gales to vast distances across the ocean. 
We may safely assume that under such circumstances 
their rate of flight would often be 35 miles an hour; 
and some authors have given a far higher estimate. I 



146 MEANS OF DISPERSAL. [Chap. XII. 

have never seen an instance of nutritious seeds passing 
through the intestines of a bird ; but hard seeds of fruit 
pass uninjured through even the digestive organs of a 
turkey. In the course of two months, I picked up in 
my garden 12 kinds of seeds, out of the excrement of 
small birds, and these seemed perfect, and some of 
them, which were tried, germinated. But the following 
fact is more important : the crops of birds do not secrete 
gastric juice, and do not, as I know by trial, injure in 
the least the germination of seeds ; now, after a bird 
has found and devoured a large supply of food, it is 
positively asserted that all the grains do not pass into 
the gizzard for twelve or even eighteen hours. A bird 
in this interval might easily be blown to the distance 
of 500 miles, and hawks are known to look out for tired 
birds, and the contents of their torn crops might thus 
readily get scattered. Some hawks and owls bolt their 
prey whole, and, after an interval of from twelve to 
twenty hours, disgorge pellets, which, as I know from 
experiments made in the Zoological Gardens, include 
seeds capable of germination. Some seeds of the oat 
wheat, millet, canary, hemp, clover, and beet germinated 
after having been from twelve to twenty-one hours 
in the stomachs of different birds of prey; and two 
seeds of beet grew after having been thus retained 
for two days and fourteen hours. Fresh- water fish, I 
find, eat seeds of many land and water plants ; fish are 
frequently devoured by birds, and thus the seeds might 
be transported from place to place. I forced many 
kinds of seeds into the stomachs of dead fish, and then 
gave their bodies to fishing-eagles, storks, and pelicans ; 
these birds, after an interval of many hours, either 
rejected the seeds in pellets or passed them in their 



Chap. XII.] MEANS OF DISPEESAL. 147 

excrement; and several of these seeds retained the 
power of germination. Certain seeds, however, were 
always killed by this process. 

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

Although the beaks and feet of birds are generally 
clean, earth sometimes adheres to them : in one case I 
removed sixty-one grains, and in another case twenty-two 
grains of dry argillaceous earth from the foot of a part- 



148 MEANS OF DISPEKSAL. [Chap. XII. 

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

As icebergs are known to be sometimes loaded with 
earth and stones, and have even carried brushwood, 



Chap. XII.] MEANS OF D1SPEESAL. 149 

bones, and the nest of a land-bird, it can hardly be 
doubted that they must occasionally, as suggested by 
Lyell, have transported seeds from one part to another 
of the arctic and antarctic regions ; and during the 
Glacial period from one part of the now temperate 
regions to another. In the Azores, from the large 
number of plants common to Europe, in comparison 
with, the species on the other islands of the Atlantic, 
which stand nearer to the mainland, and (as remarked 
by Mr. H. C. Watson) from their somewhat northern 
character in comparison with the latitude, I suspected 
that these islands had been partly stocked by ice-borne 
seeds, during the Glacial epoch. At my request Sir C. 
Lyell wrote to M. Hartung to inquire whether he had 
observed erratic boulders on these islands, and he 
answered that he had found large fragments of granite 
and other rocks, which do not occur in the archipelago. 
Hence we may safely infer that icebergs formerly 
landed their rocky burthens on the shores of these 
mid-ocean islands, and it is at least possible that they 
may have brought thither some few seeds of northern 
plants. 

Considering that these several means of transport, 
and that other means, which without doubt remain to 
be discovered, have been in action year after year for 
tens of thousands of years, it would, I think, be a 
marvellous fact if many plants had not thus become 
widely transported. These means of transport are 
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 



150 MEANS OF DISPEESAL. [Chap. XII. 

retain their vitality when exposed for a great length of 
time to the action of sea-water ; nor could they be long 
carried in the crops or intestines of birds. These 
means, however, would suffice for occasional transport 
across tracts of sea some hundred miles in breadth, or 
from island to island, or from a continent to a neigh- 
bouring island, but not from one distant continent to 
another. The floras of distant continents would not by 
such means become mingled ; but would remain as 
distinct as they now are. The currents, from their 
course, would never bring seeds from North America to 
Britain, though they might and do bring seeds from 
the West Indies to our western shores, where, if not 
killed by their very long immersion in salt water, they 
could not endure our climate. Almost every year, one 
or two land-birds are blown across the whole Atlantic 
Ocean, from North America to the western shores of 
Ireland and England ; but seeds could be transported 
by these rare wanderers only by one means, namely, by 
dirt adhering to their feet or beaks, which is in itself a 
rare accident. Even in this case, how small ^ould be 
the chance of a seed falling on favourable soil, and 
coming to maturity ! But it would be a great error to 
argue that because a well-stocked island, like Great 
Britain, has not, as far as is known (and it would be 
very difficult to prove this), received within the last 
few centuries, through occasional means of transport, 
immigrants from Europe or any other continent, that a 
poorly-stocked island, though standing more remote 
from the mainland, would not receive colonists by simi- 
lar means. Out of a hundred kinds of seeds or animals 
transported to an island, even if far less well-stocked 
than Britain, perhaps not more than one would be so 



Chap. XII.] THE GLACIAL PERIOD. 151 

well fitted to its new home, as to become naturalised 
But this is no valid argument against what would be 
effected by occasional means of transport, during the 
long lapse of geological time, whilst the island was 
being upheaved, and before it had become fully stocked 
with inhabitants. On almost bare land, with few or no 
destructive insects or birds living there, nearly every 
seed which chanced to arrive, if fitted for the climate, 
would germinate and survive. 

Dispersal during the Glacial Period. 

The identity of many plants and animals, on moun- 
tain-summits, separated from each other by hundreds 
of miles of lowlands, where Alpine species could not 
possibly exist, is one of the most striking cases known 
of the same species living at distant points, without 
the apparent possibility of their having migrated from 
one point to the other. It is indeed a remarkable fact 
to see so many plants of the same species living on the 
snowy regions of the Alps or Pyrenees, and in the 
extreme northern parts of Europe ; but it is far more 
remarkable, that the plants on the White Mountains, in 
the United States of America, are all the same with 
those of Labrador, and nearly all the same, as we hear 
from Asa Gray, with those on the loftiest mountains 
of Europe. Even as long ago as 1747, such facts led 
Gmelin to conclude that the same species must have 
been independently created at many distinct points ; 
and we might have remained in this same belief, had 
not Agassiz and others called vivid attention to the 
Glacial period, which, as we shall immediately see, 
affords a simple explanation of these facts. We have 
evidence of almost every conceivable kind, organic and 
31 



152 DISPERSAL DUEING [Chap. XII. 

inorganic, that, within a very recent geological period, 
central Europe and North America suffered under an 
arctic climate. The ruins of a house burnt by fire do 
not tell their tale more plainly than do the mountains 
of Scotland and Wales, -with their scored flanks, polished 
surfaces, and perched boulders, of the icy streams with 
which their valleys were lately filled. So greatly has 
the climate of Europe changed, that in Northern Italy, 
gigantic moraines, left by old glaciers, are now clothed 
by the vine and maize. Throughout a large part of the 
United States, erratic boulders and scored rocks plainly 
reveal a former cold period. 

The former influence of the glacial climate on the dis- 
tribution of the inhabitants of Europe, as explained by 
Edward Eorbes, is substantially as follows. But we shall 
follow the changes more readily, by supposing a new 
glacial period slowly to come on, and then pass away, 
as formerly occurred. As the cold came on, and as each 
more southern zone became fitted for the inhabitants of 
the north, these would take the places of the former 
inhabitants of the temperate regions. The latter, at 
the same time, would travel further and further south- 
ward, unless they were stopped by barriers, in which 
case they would perish. The mountains would become 
covered with snow and ice, and their former Alpine 
inhabitants would descend to the plains. By the time 
that the cold had reached its maximum, we should have 
an arctic fauna and flora, covering the central parts of 
Europe, as far south as the Alps and Pyrenees, and even 
stretching into Spain. The now temperate regions of 
the United States would likewise be covered by arctic 
plants and animals and these would be nearly the same 
with those of Europe ; for the present circumpolar 



Chap. XII.] THE GLACIAL PEKIOD. 153 

inhabitants, which we suppose to have everywhere tra- 
velled 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 productions of the more temperate regions. 
And as the snow melted from the bases of the moun- 
tains, the arctic forms would seize on the cleared and 
thawed ground, always ascending, as the warmth in- 
creased and the snow still further disappeared, higher 
and higher, whilst their brethren were pursuing their 
northern journey. Hence, when the warmth had fully 
returned, the same species, which had lately lived 
together on the European and North American low- 
lands, would again be found in the arctic regions of the 
Old and New Worlds, and on many isolated mountain- 
summits far distant from each other. 

Thus we can understand the identity of many plants 
at points so immensely remote as the mountains of the 
United States and those of Europe. We can thus also 
understand the fact that the Alpine plants of each 
mountain-range are more especially related to the arctic 
forms living due north or nearly due north of them : 
for the first migration when the cold came on, and the 
re-migration on the returning warmth, would generally 
have been due south and north. The Alpine plants, 
for example, of Scotland, as remarked by Mr. H. C. 
Watson, and those of the Pyrenees, as remarked by 
Eamond, are more especially allied to the plants of 
northern Scandinavia; those of the United States to 
Labrador ; those of the mountains of Siberia to the 
arctic regions of that country. These views, grounded 
as they are on the perfectly well-ascertained occurrence 



154 DISPEESAE DURING [Chap. XII, 

of a former Glacial period, seem to me to explain in so 
satisfactory a manner the present distribution of the 
Alpine and Arctic productions of Europe and America, 
that when in other regions we find the same species 
on distant mountain-summits, we may almost conclude, 
without other evidence, that a colder climate formerly 
permitted their migration across the intervening low- 
lands, now become too warm for their existence. 

As the arctic forms moved first southward and after- 
wards backwards to the north, in unison with the 
changing climate, they will not have been exposed 
during their long migrations to any great diversity of 
temperature; and as they all migrated in a body to- 
gether, their mutual relations will not have been much 
disturbed. Hence, in accordance with the principles 
inculcated in this volume, these forms will not have 
been liable to much modification. But with the Alpine 
productions, left isolated from the moment of the re- 
turning warmth, first at the bases and ultimately on 
the summits of the mountains, the case will have been 
somewhat different ; for it is not likely that all the same 
arctic species will have been left on mountain -ranges far 
distant from each other, and have survived there ever 
since ; they will also in all probability, have become 
mingled with ancient Alpine species, which must have 
existed on the mountains before the commencement of the 
Glacial epoch, and which during the coldest period will 
have been temporarily driven down to the plains ; they 
will, also, have been subsequently exposed to somewhat 
different climatal influences. Their mutual relations 
will thus have been in some degree disturbed ; conse- 
quently they will have been liable to modification ; and 
they have been modified ; for if we compare the present 



Chap. XIL] THE GLACIAL PERIOD. 155 

Alpine plants and animals of the several great Euro- 
pean mountain-ranges one with another, though many 
of the species remain identically the same, some exist 
as varieties, some as doubtful forms or sub-species, and 
some as distinct yet closely allied species representing 
each other on the several ranges. 

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



156 DISPERSAL DURING [Chap. XII. 

day. Hence we may suppose that the organisms which 
now live under latitude 60°, lived during the Pliocene 
period farther north under the Polar Circle, in latitude 
66°- 6 7°; and that the present arctic productions then 
lived on the broken land still nearer to the pole. Now, 
if we look at a terrestrial globe, we see under the Polar 
Circle that there is almost continuous land from wes- 
tern Europe, through Siberia, to eastern America. And 
this continuity of the circumpolar land, with the con- 
sequent freedom under a more favourable climate for in- 
termigration, will account for the supposed uniformity of 
the sub-arctic and temperate productions of the Old and 
New Worlds, at a period anterior to the Glacial epoch. 
Believing, from reasons before alluded to, that our 
continents have long remained in nearly the same 
relative position, though subjected to great oscillations 
of level, I am strongly inclined to extend the above 
view, and to infer that during some still earlier and 
still warmer period, such as the older Pliocene ^period, 
a large number of the same plants and animals in- 
habited the almost continuous circumpolar land; and 
that these plants and animals, both in the Old and 
New "Worlds, began slowly to migrate southwards as 
the climate became less warm, long before the com- 
mencement of the Glacial period. We now see, as 
I believe, their descendants, mostly in a modified con- 
dition, in the central parts of Europe and the United 
States. On this view we can understand the relation- 
ship with very little identity, between the productions 
of North America and Europe, — a relationship which is 
highly remarkable, considering the distance of the two 
areas, and their separation by the whole Atlantic Ocean. 
We can further understand the singular fact remarked 



Chap. XII.] THE GLACIAL PERIOD. 157 

on by several observers that the productions of Europe 
and America during the later tertiary stages were more 
closely related to each other than they are at the present 
time ; for during these warmer periods the northern 
parts of the Old and New Worlds will have been 
almost continuously united by land, serving as a bridge, 
since rendered impassable by cold, for the intermigration 
of their inhabitants. 

During the slowly decreasing warmth of the Pliocene 
period, as soon as the species in common, which in- 
habited the New and Old Worlds, migrated south of 
the Polar Circle, they will have been completely cut off 
from each other. This separation, as far as the more 
temperate productions are concerned, must have taken 
place long ages ago. As the plants and animals mi- 
grated southward, they will have become mingled in the 
one great region with the native American productions, 
and would have had to compete with them ; and in the 
other great region, with those of the Old World. Con- 
sequently we have here everything favourable for much 
modification, — for far more modification than with the 
Alpine productions, left isolated, within a much more 
recent period, on the several mountain-ranges and on 
the arctic lands of Europe and N. America. Hence it 
has come, that when we compare the now living pro- 
ductions of the temperate regions of the New and Old 
Worlds, we find very few identical species (though Asa 
Gray has lately shown that more plants are identical 
than was formerly supposed), but we find in every 
great class many forms, which some naturalists rank as 
geographical races, and others as distinct species ; and 
a host of closely allied or representative forms which 
are ranked by all naturalists as specifically distinct. 



158 ALTERNATE GLACIAL PERIODS [Chap. XIL 

As on the land, so in the waters of the sea, a slow 
southern migration of a marine fauna, which, during 
the Pliocene or even a somewhat earlier period, was 
nearly uniform along the continuous shores of the Polar 
Circle, will account, on the theory of modification, for 
many closely allied forms now living in marine areas 
completely sundered. Thus, I think, we can under- 
stand the presence of some closely allied, still existing 
and extinct tertiary forms, on the eastern and western 
shores of temperate Xorth America ; and the still more 
striking fact of many closely allied crustaceans (as 
described in Dana's admirable work), some fish and 
other marine animals, inhabiting the Mediterranean 
and the seas of Japan, — these two areas being now 
completely separated by the breadth of a whole con- 
tinent and by wide spaces of ocean. 

These cases of close relationship in species either 
now or formerly inhabiting the seas on the eastern and 
western shores of Xorth America, the Mediterranean 
and Japan, and the temperate lands of ISTorth America 
and Europe, are inexplicable on the theory of creation. 
"We cannot maintain that such species have been created 
alike, in correspondence with the nearly similar physical 
conditions of the areas ; for if we compare, for instance, 
certain parts of South America with parts of South 
Africa or Australia, we see countries closely similar in 
all their physical conditions, with their inhabitants 
utterly dissimilar. 

Alternate Glacial Periods in the North and South. 

But we must return to our more immediate subject. 
I am convinced that Forbes's view may be largely 



Chap. XII.] IN THE NOKTH AND SOUTH. 159 

extended. In Europe we meet with the plainest 
evidence of the Glacial period, from the western shores 
of Britain to the Onral range, and southward to the 
Pyrenees. We may infer from the frozen mammals 
and nature of the mountain vegetation, that Siberia was 
similarly affected. In the Lebanon, according to Dr. 
Hooker, perpetual snow formerly covered the central 
axis, and fed glaciers which rolled 4000 feet down the 
valleys. The same observer has recently found great 
moraines at a low level on the Atlas range in N. Africa. 
Along the Himalaya, at points 900 miles apart, glaciers 
have left the marks of their former low descent ; and 
in Sikkim, Dr. Hooker saw maize growing on ancient 
and gigantic moraines. Southward of the Asiatic con- 
tinent, on the opposite side of the equator, we know, 
from the excellent researches of Dr. J. Haast and Dr. 
Hector, that in New Zealand immense glaciers formerly 
descended to a low level ; and the same plants found 
by Dr. Hooker on widely separated mountains in this 
island tell the same story of a former cold period. 
From facts communicated to me by the Eev. W. B. 
Clarke, it appears also that there are traces of former 
glacial action on the mountains of the south-eastern 
corner of Australia. 

Looking to America ; in the northern half, ice-borne 
fragments of rock have been observed on the eastern 
side of the continent, as far south as lat. 36°-37°, and 
on the shores of the Pacific, where the climate is now 
so different, as far south as lat. 46°. Erratic boulders 
have, also, been noticed on the Eocky Mountains. In 
the Cordillera of South America, nearly under the 
equator, glaciers once extended far below their present 
]eveL In Central Chile I examined a vast mound of 



160 ALTERNATE GLACIAL PEEIODS [Chap. XII. 

detritus with great boulders, crossing the Portillo valley, 
which there can hardly be a doubt once formed a huge 
moraine ; and Mr. D. Forbes informs me that he found 
in various parts of the Cordillera, from lat. 13° to 30° S., 
at about the height of 12,000 feet, deeply-furrowed 
rocks, resembling those with which he was familiar in 
Norway, and likewise great masses of detritus, including 
grooved pebbles. Along this whole space of the Cor- 
dillera true glaciers do not now exist even at much 
more considerable heights. Farther south on both sides 
of the continent, from lat. 41° to the southernmost 
extremity, we have the clearest evidence of former gla- 
cial action, in numerous immense boulders transported 
far from their parent source. 

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



Chap. XE] IN THE NOKTH AND SOUTH. 161 

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

In South America, Dr. Hooker has shown that 
besides many closely allied species, between forty and 
fifty of the flowering plants of Tierra del Fuego, forming 
no inconsiderable part of its scanty flora, are common 
to North America and Europe, enormously remote as 
these areas in opposite hemispheres are from each 
other. On the lofty mountains of equatorial America 
a host of peculiar species belonging to European 
genera occur. On the Organ mountains of Brazil, 
some few temperate European, some Antarctic, and 
some Andean genera were found by Gardner, which do 



162 ALTERNATE GLACIAL PERIODS [Chap. XII. 

not exist in the low intervening hot countries. On the 
Silla of Caraccas, the illustrious Humboldt long ago 
found species belonging to genera characteristic of the 
Cordillera, 

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

On the Himalaya, and on the isolated mountain- 
ranges of the peninsula of India, on the heights of 
Ceylon, and on the volcanic cones of Java, many plants 
or, either identically the same or representing each 
other, and at the same time representing plants of 
Europe, not found in the intervening hot lowlands. 
A list of the genera of plants collected on the loftier 
peaks of Java, raises a picture of a collection made on 



Chap. XII.] IN THE NORTH AND SOUTH. 163 

a hillock in Europe ! Still more striking is the fact 
that peculiar Australian forms are represented by 
certain plants growing on the summits of the moun- 
tains of Borneo. Some of these Australian forms, as I 
hear from Dr. Hooker, extend along the heights of the 
peninsula of Malacca, and are thinly scattered on the 
one hand over India, and on the other hand as far 
north as Japan. 

On the southern mountains of Australia, Dr. 
F. Muller has discovered several European species ; 
other species, not introduced by man, occur on the 
lowlands; and a long list can be given, as I am 
informed by Dr. Hooker, of European genera, found in 
Australia, but not in the intermediate torrid regions. 
In the admirable 'Introduction to the Flora of New 
Zealand,' by Dr. Hooker, analogous and striking facts 
are given in regard to the plants of that large island. 
Hence we see that certain plants growing on the more 
lofty mountains of the tropics in all parts of the world, 
and on the temperate plains of the north and south, are 
either the same species or varieties of the same species. 
It should, however, be observed that these plants are 
not strictly arctic forms ; for, as Mr. H. C. Watson 
has remarked, " in receding from polar towards equa- 
torial latitudes, the Alpine or mountain floras really 
become less and less Arctic." Besides these identical 
and closely allied forms, many species inhabiting the 
same widely sundered areas, belong to genera not now 
found in the intermediate tropical lowlands. 

These brief remarks apply to plants alone ; but some 
few analogous facts could be given in regard to terres- 
trial animals. In marine productions, similar cases 

likewise occur ; as an example, I may quote a statement 
32 



164 ALTERNATE GLACIAL PERIODS [Chap. XII. 

by the highest .authority, Prof. Dana, that "it is cer- 
tainly a wonderful fact that New Zealand should have 
a closer resemblance in its Crustacea to Great Britain, 
its antipode, than to any other part of the world/ 1 ' 
Sir J. Bichardson, also, speaks of the reappearance on 
the shores of New Zealand, Tasmania, &c, of northern 
forms of fish. Dr. Hooker informs me that twenty -five 
species of Algae are common to New Zealand and to 
Europe, but have not been found in the intermediate 
tropical seas. 

From the foregoing facts, namely, the presence of 
temperate forms on the highlands across the whole of 
equatorial Africa, and along the Peninsula of India, to 
Ceylon and the Malay Archipelago, and in a less well- 
marked manner across the wide expanse of tropical 
South America, it appears almost certain that at some 
former period, no doubt during the most severe part of 
a Glacial period, the lowlands of these great continents 
were everywhere tenanted under the equator by a con 
siderable number of temperate forms. At this period 
the equatorial climate at the level of the sea was pro- 
bably about the same with that now experienced at the 
height of from five to six thousand feet under the same 
latitude, or perhaps even rather cooler. During this, the 
coldest period, the lowlands under the equator must 
have been clothed with a mingled tropical and temper- 
ate vegetation, like that described by Hooker as growing 
luxuriantly at the height of from four to five thousand 
feet on the lower slopes of the Himalaya, but with 
perhaps a still greater preponderance of temperate 
forms. So again in the mountainous island of Fernando 
Po, in the Gulf of Guinea, Mr. Mann found temperate 
European forms beginning to appear at the height of 



Chap. XII.] IN THE NORTH AND SOUTH. 165 

about five thousand feet. On the mountains of Panama, 
at the height of only two thousand feet, Dr. Seemann 
found the vegetation like that of Mexico, " with forms 
of the torrid zone harmoniously blended with those of 
the temperate." 

Now let us see whether Mr. Croll's conclusion that 
when the northern hemisphere suffered from the extreme 
cold of the great Glacial period, the southern hemisphere 
was actually warmer, throws any clear light on the 
present apparently inexplicable distribution of various 
organisms in the temperate parts of both hemispheres, 
and on the mountains of the tropics. The Glacial 
period, as measured by years, must have been very 
long ; and when we remember over what vast spaces 
some naturalised plants and animals have spread within 
a few centuries, this period will have been ample for 
any amount of migration. As the cold became more 
and more intense, we know that Arctic forms invaded 
the temperate regions ; and, from the facts just given, 
there can hardly be a doubt that some of the more 
vigorous, dominant and widest-spreading temperate 
forms invaded the equatorial lowlands. The inha- 
bitants of these hot lowlands would at the same time 
have migrated to the tropical and subtropical regions 
of the south, for the southern hemisphere was at this 
period warmer. On the decline of the Glacial period, 
as both hemispheres gradually recovered their former 
temperatures, the northern temperate forms living on 
the lowlands under the equator, would have been driven 
to their former homes or have been destroyed, being re- 
placed by the equatorial forms returning from the south. 
Some, however, of the northern temperate forms would 
almost certainly have ascended any adjoining high land, 



166 ALTERNATE GLACLAX PERIODS [Chap. XU. 

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

In the regular course of events the southern hemi- 
sphere would in its turn be subjected to a severe Glacial 
period, with the northern hemisphere rendered warmer ; 
and then the southern temperate forms would invade 
the equatorial lowlands. The northern forms which 
had before been left on the mountains would now 
descend and mingle with the southern forms. These 
latter, when the warmth returned, would return to their 
former homes, leaving some few species on the moun- 
tains, and carrying southward with them some of the 
northern temperate forms which had descended from 
tin ir mountain fastnesses. Thus, we should have some 
few species identically the same in the northern and 
southern temperate zones and on the mountains of the 
intermediate tropical regions. But the species left 
during a long time on these mountains, or in opposite 
hemispheres, would have to compete with many new 
forms and would be exposed to somewhat different 
physical conditions ; hence they would be eminently 
liable to modification, and would generally now exist 
as varieties or as representative species ; and this is the 
case. We must, also, bear in mind the occurrence in 
both hemispheres of former Glacial periods ; for these 
will account, in accordance with the same principles, for 
the many quite distinct species inhabiting the same 



Chap. XII.] IN THE NOETH AND SOUTH. 167 

widely separated areas, and belonging to genera not now 
found in the intermediate torrid zones. 

It is a remarkable fact strongly insisted on by Hooker 
in regard to America, and by Alph. de Canclolle in 
regard to Australia, that many more identical or 
slightly modified species have migrated from the north 
to the south, than in a reversed direction, We see, 
however, a few southern forms on the mountains of 
Borneo and Abyssinia. I suspect that this preponderant 
migration from the north to the south is due to the 
greater extent of land in the north, and to the northern 
forms having existed in their own homes in greater 
numbers, and having consequently been advanced 
through natural selection and competition to a higher 
stage of perfection, or dominating power, than the 
southern forms. And thus, when the two sets became 
commingled in the equatorial regions, during the alter- 
nations of the Glacial periods, the northern forms were 
the more powerful and were able to hold their places 
on the mountains, and afterwards to migrate southward 
with the southern forms ; but not so the southern in 
regard to the northern forms. In the same manner at 
the present day, we see that very many European 
productions cover the ground in La Plata, New Zealand, 
and to a lesser degree in Australia, and have beaten 
the natives; whereas extremely few southern forms 
have become naturalised in any part of the northern 
hemisphere, though hides, wool, and other objects likely 
to carry seeds have been largely imported into Europe 
during the last two or three centuries from La Plata 
and during the last forty or fifty years from Australia. 
The Neilgherrie mountains in India, however, offer a 
partial exception ; for here, as I hear from Dr. Hooker, 



168 ALTERNATE GLACIAL PEEIODS [Chap. XII. 

Australian forms are rapidly sowing themselves and 
becoming naturalised. Before the last great Glacial 
period, no doubt the intertropical mountains were 
stocked with endemic Alpine forms ; but these have 
almost everywhere yielded to the more dominant forms 
generated in the larger areas and more efficient work- 
shops of the north. In many islands the native 
productions are nearly equalled, or even outnumbered, 
by those which have become naturalised; and this is 
the first stage towards their extinction. Mountains are 
islands on the land, and their inhabitants have yielded 
to those produced within the larger areas of the north, 
just in the same way as the inhabitants of real islands 
have everywhere yielded and are still yielding to 
continental forms naturalised through man's agency. 

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

I am far from supposing that all the difficulties in 
regard to the distribution and affinities of the identical 



Chap. XII.] IN THE NORTH AND SOUTH. 169 

and allied species, which now live so widely separated 
in the north and south, and sometimes on the inter- 
mediate mountain-ranges, are removed on the views 
above given. The exact lines of migration cannot be 
indicated. We cannot say why certain species and not 
others have migrated ; why certain species have .been 
modified and have given rise to new forms, whilst others 
have remained unaltered. We cannot hope to explain 
such facts, until we can say why one species and not 
another becomes naturalised by man's agency in a 
foreign land ; why one species ranges twice or thrice as 
far, and is twice or thrice as common, as another species 
within their own homes. 

Various special difficulties also remain to be solved ; 
for instance, the occurrence, as shown by Dr. Hooker, of 
the same plants at points so enormously remote as 
Kerguelen Land, jSTew Zealand, and Fuegia ; but icebergs, 
as suggested by Lyell, may have been concerned in their 
dispersal. The existence at these and other distant 
points of the southern hemisphere, of species, which, 
though distinct, belong to genera exclusively confined 
to the south, is a more remarkable case. Some of these 
species are so distinct, that we cannot suppose that there 
has been time since the commencement of the last 
Glacial period for their migration and subsequent 
modification to the necessary degree. The facts seem to 
indicate that distinct species belonging to the same 
genera have migrated in radiating lines from a common 
centre ; and I am inclined to look in the southern, as in 
the northern hemisphere, to a former and warmer period, 
before the commencement of the last Glacial period, 
when the Antarctic lands, now covered with ice, supported 
a highly peculiar and isolated flora. It may be suspected 



170 ALTERNATE GLACIAL PERIODS. [Chap. XII. 

that before this flora was exterminated during the last 
.Glacial epoch, a few forms had been already widely 
dispersed to various points of the southern hemisphere 
by occasional means of transport, and by the aid 
as halting-places, of now sunken islands. Thus the 
southern shores of America, Australia, and Xew Zealand 
may have become slightly tinted by the same peculiar 
forms of life. 

Sir C. Lyell in a striking passage has speculated, in 
language almost identical with mine, on the effects of 
great alterations of climate throughout the world on 
geographical distribution. And we have now seen that 
Mr. Croll's conclusion that successive Glacial periods in 
the one hemisphere coincide with warmer periods in the 
opposite hemisphere, together with the admission of the 
slow modification of species, explains a multitude of facts 
in the distribution of the same and of the allied forms 
of life in all parts of the globe. The living waters have 
flowed during one period from the north and during 
another from the south, and in both cases have reached 
the equator ; but the stream of life has flowed with 
greater force from the north than in the opposite direction, 
and has consequently more freely inundated the south, 
As the tide leaves its drift in horizontal lines, rising 
higher on the shores where the tide rises highest, so haA^e 
the living waters left their livinsr drift on our mountain 
summits, in a line gently rising from the Arctic lowlands 
to a great altitude under the equator. The various 
beings thus left stranded may be compared with savage 
races of man, driven up and surviving in the mountain 
fastnesses of almost every land, which serves as a record 
full of interest to us, of the former inhabitants of the 
surrounding lowlands. 



Chap. XIIL] FRESH- WATER PRODUCTIONS 171 



CHAPTER XIIL 

Geographical Distribution — continued. 

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

Fresh-water Productions. 

As lakes and river-systems are separated from each 
other by barriers of land, it might have been thought 
that fresh-water productions would not have ranged 
widely within the same country, and as the sea is 
apparently a still more formidable barrier, that they 
would never have extended to distant countries. But 
the case is exactly the reverse. Not only have many 
fresh-water species, belonging to different classes, an 
enormous range, but allied species prevail in a 
remarkable manner throughout the world. When first 
collecting in the fresh waters of Brazil, I well remember 
feeling much surprise at the similarity of the fresh- 
water insects, shells, &c, and at the dissimilarity of the 
surrounding terrestrial beings, compared with those of 
Britain. 

But the wide ranging power of fresh- water productions 
can, I think, in most cases be explained by their having 
become fitted, in a manner highly useful to them, for 



172 FEESH-WATEU PEODTJCTIONS. [Chap. XUI. 

short and frequent migrations from pond to pond, or 
from stream to stream, within their own countries ; 
and liability to wide dispersal would follow from this 
capacity as an almost necessary consequence. We can 
here consider only a few cases ; of these, some of the 
most difficult to explain are presented by fish. It was 
formerly believed that the same fresh-water species 
never existed on two continents distant from each other. 
But Dr. Giinther has lately shown that the Galaxias 
attenuates inhabits Tasmania, jSTew Zealand, the Falk- 
land Islands, and the mainland of South America. This 
is a wonderful case, and probably indicates dispersal 
from an Antarctic centre during a former warm period. 
This case, however, is rendered in some degree less 
surprising by the species of this genus having the 
power of crossing by some unknown means considerable 
spaces of open ocean : thus there is one species common 
to Xew Zealand and to the Auckland Islands, though 
separated by a distance of about 230 miles. On the same 
continent fresh-water fish often range widely, and as if 
capriciously ; for in two adjoining river-systems some of 
the species may be the same, and some wholly different. 
It is probable that they are occasionally transported 
by what may be called accidental means. Thus fishes 
still alive are not very rarely dropped at distant points 
by whirlwinds ; and it is known that the ova retain 
their vitality for a considerable time after removal from 
the water. Their dispersal may, however, be mainly 
attributed to changes in the level of the land within the 
recent period, causing rivers to flow into each other. 
Instances, also, could be given of this having occurred 
during floods, without any change of level. The wide 
difference of the fish on the opposite sides of most 



Chap. XIII.] FEESH- WATER PRODUCTIONS. 173 

mountain-ranges, which are continuous, and which con- 
sequently must from an early period have completely 
prevented the inosculation of the river-systems on the 
two sides, leads to the same conclusion. Some fresh- 
water fish belong to very ancient forms, and in such cases 
there will have been ample time for great geographical 
changes, and consequently time and means for much 
migration. Moreover Dr. Gunther has recently been 
led by several considerations to infer that with fishes 
the same forms have a loDg endurance. Salt-water fish 
can with care be slowly accustomed to live in fresh 
water ; and, according to Valenciennes, there is hardly a 
single group of which all the members are confined to 
fresh water, so that a marine species belonging to a 
fresh- water group might travel far along the shores of 
the sea, and could, it is probable, become adapted 
without much difficulty to the fresh waters of a distant 
land. 

Some species of fresh-water shells have very wide 
ranges, and allied species which, on our theory, are 
descended from a common parent, and must have pro- 
ceeded from a single source, prevail throughout the 
world. Their distribution at first perplexed me much* 
as their ova are not likely to be transported by birds ; 
and the ova, as well as the adults, are immediately 
killed by sea-water. I could not even understand how 
some naturalised species have spread rapidly through- 
out the same country. But two facts, which I have 
observed — and many others no doubt will be discovered 
— throw some light on this subject. When ducks 
suddenly emerge from a pond covered with duck- weed, 
I have twice seen these little plants adhering to their 
backs ; and it has happened to me, in removing a little 



174 FEESH-WATEE PRODUCTIONS. [Chap. XIH. 

duck-weed from one aquarium to another, that I have 
unintentionally stocked the one with fresh-water shells 
from the other. But another agency is perhaps more 
effectual : I suspended the feet of a duck in an 
aquarium, where many ova of fresh-water shells were 
hatching ; and I found that numbers of the extremely 
minute and just-hatched shells crawled on the feet, and 
clung to them so firmly that when taken out of the 
water they could not be jarred off, though at a somewhat 
more advanced age they would voluntarily drop off. 
These just-hatched molluscs, though aquatic in their 
nature, survived on the duck's feet, in damp air, from 
twelve to twentv hours ; and in this length of time a 
duck or heron might fly at least six or seven hundred 
miles, and if blown across the sea to an oceanic island, 
or to any other distant point, would be sure to alight on 
a pool or rivulet. Sir Charles Lyell informs me that a 
Dytiscus has been caught with an Ancylus (a fresh- 
water shell like a limpet) firmly adhering to it ; and a 
water-beetle of the same family, a Colvmbetes, once flew 
on board the ' Beagle,' when forty-five miles distant 
from the nearest land : how much farther it might have 
been blown by a favouring gale no one can tell 

With respect to plants, it has long been known what 
enormous ranges many fresh-water, and even marsh 
species, have, both over continents and to the most 
remote oceanic islands. This is strikingly illustrated, 
according to Alph. de Candolle, in those large groups of 
terrestrial plants, which have very few aquatic members ; 
for the latter seem immediately to acquire, as if in 
consequence, a wide range. I think favourable means 
of dispersal explain this fact. I have before mentioned 
that earth occasionally adheres in some quantity to the 



Chap. XIII] FEESH-WATEE PEODUCTIONS. 175 

feet and beaks of birds. Wading birds, which frequent 
the muddy edges of ponds, if suddenly flushed, would be 
the most likely to have muddy feet. Birds of this order 
wander more than those of any other; and they are 
occasionally found on the most remote and barren 
islands of the open ocean ; they would not be likely to 
alight on the surface of the sea, so that any dirt on their 
feet would not be washed off; and when gaining the 
land, they would be sure to fly to their natural fresh- 
water haunts. I do not believe that botanists are 
aware how charged the mud of ponds is with seeds ; I 
have tried several little experiments, but will here give 
only the most striking case : I took in February three 
table-spoonfuls of mud from three different points, 
beneath water, on the edge of a little pond : this mud 
when dried weighed only 6| ounces ; I kept it covered 
up in my study for six months, pulling up and counting 
each plant as it grew ; the plants were of many kinds, 
and were altogether 537 in number ; and yet the viscid 
mud was all contained in a breakfast cup ! Considering 
these facts, I think it would be an inexplicable cir- 
cumstance if water-birds did not transport the seeds 
of fresh- water plants to unstocked ponds and streams, 
situated at very distant points. The same agency may 
have come into play with the eggs of some of the 
smaller fresh- water animals. 

Other and unknown agencies probably have also 
played a part. I have stated that fresh-water fish eat 
some kinds of seeds, though they reject many other 
kinds after having swallowed them; even small fish 
swallow seeds of moderate size, as of the yellow water- 
lily and Potamogeton. Herons and other birds, century 
after century, have gone on daily devouring fish ; they 
33 



176 FRESH- WATEE PRODUCTIONS. [Chap. XIII. 

then take flight and go to other waters, or are blown 
across the sea ; and we have seen that seeds retain their 
power of germination, when rejected many hours after- 
wards in pellets or in the excrement. When I saw 
the great size of the seeds of that fine water-lily. 
the Xelumbium, and remembered Alph. de Candolle's 
remarks on the distribution of this plant, I thought that 
the means of its dispersal mnst remain inexplicable; 
but Audubon states that he found the seeds of the great 
southern water-lily (probably, according to Dr. Hooker, 
the Xelumbium luteum) in a heron's stomach. Xow 
this bird must often haye flown with its stomach thus 
well stocked to distant ponds, and then getting a hearty 
meal of fish, analogy makes me believe that it would 
have rejected the seeds in a pellet in a fit state for 
germination. 

In considering these several means of distribution, it 
should be remembered that when a pond or stream is 
first formed, for instance, on a rising islet, it will be 
unoccupied ; and a single seed or egg will have a good 
chance of succeeding. Although there will always be a 
struggle for life between the inhabitants of the same 
pond, however few in kind, yet as the number even in a 
well-stocked pond is small in comparison with the 
number of species inhabiting an equal area of land, the 
competition between them will probably be less severe 
than between terrestrial species ; consequently an in- 
truder from the waters of a foreign country woidd have 
a better chance of seizing on a new place, than in the 
case of terrestrial colonists. We should also remember 
that many fresh-water productions are low in the scale 
of nature, and we have reason to believe that such 
beings become modified more slowly than the high,* 



Chap. XIII.] INHABITANTS OF OCEANIC ISLANDS. 177 

and this will give time for the migration of aquatic 
species. We should not forget the probability of many 
fresh-water forms having formerly ranged continuously 
over immense areas, and then having become extinct 
at intermediate points. But the wide distribution of 
fresh-water plants and of the lower animals, whether 
retaining the same identical form or in some degree 
modified, apparently depends in main part on the wide 
dispersal of their seeds and eggs by animals, more 
especially by fresh- water birds, which have great powers 
of flight, and naturally travel from one piece of water to 
another. 

On the Inhabitants of Oceanic Islands. 

We now come to the last of the three classes of facts, 
which I have selected as presenting the greatest amount 
of difficulty with respect to distribution, on the view 
that not only all the individuals of the same species 
have migrated from some one area, but that allied 
species, although now inhabiting the most distant 
points, have proceeded from a single area, — the birth- 
place of their early progenitors. I have already given 
my reasons for disbelieving in continental extensions 
within the period of existing species, on so enormous a 
scale that all the many islands of the several oceans 
were thus stocked with their present terrestrial inhabi- 
tants. This view removes many difficulties, but it 
does not accord with all the facts in regard to the 
productions of islands. In the following remarks T 
shall not confine myself to the mere question of 
dispersal, but shall consider some other cases bearing 
on the truth of the two theories of independent creation 
and of descent with modification. 



178 INHABITANTS OF OCEANIC ISLANDS. [Chap. XIII. 

The species of all kinds which inhabit oceanic islands 
are few in number compared with those on equal con- 
tinental areas : Alph. de Candolle admits this for 
plants, and Wollaston for insects. Xew Zealand, for 
instance, with its lofty mountains and diversified 
stations, extending over 7S0 miles of latitude, together 
with the outlying islands of Auckland, Campbell and 
Chatham, contain altogether only 960 kinds of flower- 
ing plants ; if we compare this moderate number with 
the species which swarm over equal areas in South- 
western Australia or at the Cape of Good Hope, 
we must admit that some cause, independently of 
different physical conditions, has given rise to so great 
a difference in number. Even the uniform county of 
Cambridge has 847 plants, and the little island of 
Anglesea 764, but a few ferns and a few introduced 
plants are included in these numbers, and the com- 
parison in some other respects is not quite fair. "We 
have evidence that the barren island of Ascension 
aboriginally possessed less than half-a-dozen flowering 
plants ; yet many species have now become naturalised 
on it, as they have in Xew Zealand and on every other 
oceanic island which can be named. In St. Helena 
there is reason to believe that the naturalised plants 
and animals have nearly or quite exterminated many 
native productions. He who admits the doctrine of 
the creation of each separate species, will have to admit 
that a sufficient number of the best adapted plants and 
animals were not created for oceanic islands ; for man 
has unintentionally stocked them far more fully and 
ctly than did nature. 

Although in oceanic islands the species are few in 
number, the proportion of endemic kinds (i. c. those 



Chap. XIIL] INHABITANTS OF OCEANIC ISLANDS. 179 

found nowhere else in the world) is often extremely 
large. If we compare, for instance, the number of 
endemic land-shells in Madeira, or of endemic birds in 
the Galapagos Archipelago, with the number found on 
any continent, and then compare the area of the island 
with that of the continent, we shall see that this is 
true. This fact might have been theoretically expected, 
for, as already explained, species occasionally arriving 
after long intervals of time in the new and isolated 
district, and having to compete with new associates, 
would be eminently liable to modification, and would 
often produce groups of modified descendants. But it 
by no means follows that, because in an island nearly 
all the species of one class are peculiar, those of 
another class, or of another section of the same class, 
are peculiar ; and this difference seems to depend 
partly on the species which are not modified having 
immigated in a body, so that their mutual relations 
have not been much disturbed; and partly on the 
frequent arrival of unmodified immigrants from the 
mother-country, with which the insular forms have 
intercrossed. It should be borne in mind that the 
offspring of such crosses would certainly gain in 
vigour ; so that even an occasional cross would produce 
more effect than might have been anticipated. I will 
give a few illustrations of the foregoing remarks : in 
the Galapagos Islands there are 26 land-birds ; of these 
21 (or perhaps 23) are peculiar, whereas of the 11 
marine birds only 2 are peculiar ; and it is obvious that 
marine birds could arrive at these islands much more 
easily and frequently than land-birds. Bermuda, on 
the other hand, which lies at about the same distance 
from North America as the Galapagos Islands do from 



180 INHABITANTS OF OCEANIC ISLANDS. [Chap. XIII. 

South America, and which ha3 a very peculiar soil, 
does not possess a single endemic land-bird; and we 
know from Mr. J. M. Jones's admirable account of 
Bermuda, that very many Xorth American birds occa- 
sionally or even frequently visit this island. Almost 
every year, as I am informed by Mr. E. Y. Harcourt, 
many European and African birds are blown to 
Madeira ; this island is inhabited 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 Bermuda and Madeira have been stocked 
from the neighbouring continents with birds, which for 
long ages have there struggled together, and have 
become mutually co-adapted. Hence when settled in 
their new homes, each kind will have been kept by 
the others to its proper place and habits, and will 
consequently have been but little liable to modifi- 
cation. Any tendency to modification will also have 
been checked by intercrossing with the unmodified 
inmigrants, often arriving from the mother-country. 
Madeira again is inhabited by a wonderful number of 
peculiar land-shells, whereas not one species of sea- 
shell is peculiar to its shores : now, though we do not 
know how sea-shells are dispersed, yet we can see that 
their eggs or larvae, perhaps attached to seaweed or 
floating timber, or to the feet of wading-birds, might be 
trans] orted across three or four hundred miles of open 
sea far more easily than land-shells. The different 
orders of insects inhabiting Madeira present nearly 
parallel cas 5. 

Oceanic islands are sometimes deficient in animals of 
certain whole classes, and their places are occupied by 



Chap. XIII.] INHABITANTS OF OCEANIC ISLANDS. 181 

other classes ; thus in the Galapagos Islands reptiles, 
and in New Zealand gigantic wingless birds, take, or 
recently took, the place of mammals. Although New 
Zealand is here spoken of as an oceanic island, it is in 
some degree doubtful whether it should be so ranked ; 
it is of large size, and is not separated from Australia 
by a profoundly deep sea ; from its geological charac- 
ter and the direction of its mountain-ranges, the Eev. 
W. B. 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 the 
proportional numbers of the different orders are very 
different from what they are elsewhere. All such 
differences in number, and the absence of certain whole 
groups of animals and plants, are generally accounted 
for by supposed differences in the physical conditions 
of the islands ; but this explanation is not a little 
doubtful. Facility of immigration seems to have been 
fully as important as the nature of the conditions. 

Many remarkable little facts could be given with 
respect to the inhabitants of oceanic islands. For 
instance, in certain islands not tenanted by a single 
mammal, some of the endemic plants have beautifully 
hooked seeds ; yet few relations are more manifest than 
that hooks serve for the transportal of seeds in the 
wool or fur of quadrupeds. But a hooked seed mio-ht 
be carried to an island by other means ; and the plant 
then becoming modified would form an endemic species, 
still retaining its hooks, which would form a useless 
appendage like the shrivelled wings under the soldered 
wing-covers of many insular beetles. Again, islands 
often possess trees or bushes belonging to orders which 



182 ABSENCE OF TERRESTRIAL [Chap. XHL 

elsewhere include only herbaceous species ; now trees, 
as Alph. de Candolle has shown, generally have, what- 
ever the cause may be, confined ranges Hence trees 
would be little likely to reach distant oceanic islands ; 
and an herbaceous plant, which had no chance of 
successfully competing with the many fully developed 
trees growing on a continent, might, when established 
on an island, gain an advantage over other herbaceous 
plants by growing taller and taller and overtopping 
them. In this case, natural selection would tend to 
add to the stature of the plant, to whatever order it 
belonged, and thus first convert it into a bush and then 
into a tree. 

Absence of Batracliians and Terrestrial Mammals on 
Oceanic Islands. 

With respect to the absence of whole orders of 
animals on oceanic islands, Bqry St. Vincent long ago 
remarked that Batracliians (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 true, with the ex- 
ception of New Zealand, New Caledonia, the Andaman 
Islands, and perhaps the Salomon Islands and the 
■belles. But I have already remarked that it is 
doubtful whether New Zealand and Xew Caledonia 
ought to be classed as oceanic islands ; and this is still 
more doubtful with respect to the Andaman and 
Salomon groups and the Seychelles. This general 
absence of frogs, toads, and newts on so many true 
oceanic islands cannot be accounted for by their 
physical conditions : indeed it seems that islands are 
peculiarly fitted for these animals ; for frogs have been 



Chap. XIIL] MAMMALS ON OCEANIC ISLANDS. 183 

introduced into Madeira, the Azores, and Mauritius, 
and have multiplied so as to become a nuisance. But 
as these animals and their spawn are immediately 
killed (with the exception, as far as known, of one 
Indian species) by sea-water, there would be great 
difficulty in their transportal across the sea, and there- 
fore we can see why they do not exist on strictly 
oceanic islands. But why, on the theory of creation, 
they should not have been created there, it would be 
very difficult to explain. 

Mammals offer another and similar case. I have 
carefully searched the oldest voyages, and have not 
found a single instance, free from doubt, of a terrestrial 
mammal (excluding domesticated animals kept by the 
natives) inhabiting an island situated above 300 miles 
from a continent or great continental island ; and many 
islands situated at a much less distance are equally 
barren. The Falkland Islands, which are inhabited by 
a wolf-like fox, come nearest to an exception ; but this 
group cannot be considered as oceanic, as it lies on a 
bank in connection with the mainland at the distance 
of about 280 miles ; moreover, icebergs formerly brought 
boulders 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 mammals, 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 our smaller quad- 
rupeds have not become naturalised and greatly 
multiplied. It cannot be said, on the ordinary view of 
creation, that there has not been time for the creation 
of mammals; many volcanic islands are sufficiently 



184 ABSENCE OF TERRESTRIAL [Chap. XHI. 

ancient, as shown by the stupendous degradation which 
they have suffered, and by their tertiary strata : there 
has also been time for the production of endemic species 
belonging to other classes ; and on continents it is 
known that new species of mammals appear and 
disappear at a quicker rate than other and lower 
animals. " Although terrestrial mammals do not occur 
on oceanic islands, aerial mammals do occur on almost 
every island. Xew Zealand possesses two bats found 
nowhere else in the world : Norfolk Island, the Viti 
Archipelago, the Bonin Islands, the Caroline and 
Marianne Archipelagoes, and Mauritius, all possess 
their peculiar bats. Why, it may be asked, has the 
supposed creative force produced bats and no other 
mammals on remote islands ? On my view this 
question can easily be answered; for no terrestrial 
mammal can be transported across a wide space of sea, 
but bats can fly across. Bats have been seen wandering 
by day far over the Atlantic Ocean; and two North 
American species either regularly or occasionally visit 
Bermuda, at the distance of GOO miles from the main- 
land. I hear from Mr. Tomes, who has specially 
studied this family, that many species have enormous 
ranges, and are found on continents and on far distant 
ads. Hence we have only to suppose that such 
wandering species have been modified in their new 
In imes in relation to their new position, and we can 
understand the presence of endemic bats on oceanic 
islands, with the absence of all other terrestrial mam- 
mals. 

Another interesting relation exists, namely between 
the depth of the sea separating islands from each other 
or from the nearest continent, and the degree of affinity 



Chap. XIII.] MAMMALS ON OCEANIC ISLANDS. 185 

of their mammalian inhabitants. Mr. Windsor Earl 
has made some striking observations on this head, since 
greatly extended by Mr. Wallace's admirable researches, 
in regard to the great Malay Archipelago, which is 
traversed near Celebes by a space of deep ocean, and 
this separates two widely distinct mammalian faunas. 
On either side the islands stand on a moderately shallow 
submarine bank, and these islands are inhabited by the 
same or by closely allied quadrupeds. I have not as 
yet had time to follow up this subject in all quarters of 
the world ; but as far as I have gone, the relation holds 
good. For instance, 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 hear the 
shores of Australia. The West Indian Islands, on the 
other hand, stand on a deeply submerged bank, nearly 
1000 fathoms in depth, and here we find American 
forms, but the species and even the genera are quite 
distinct. As the amount of modification which animals 
of all kinds undergo partly depends on the lapse of 
time, and as the islands which are separated from each 
other or from the mainland by shallow channels, are 
more likely to have been continuously united within a 
recent period than the islands separated by deeper 
channels, we can understand how it is that a relation 
exists between the depth of the sea separating two 
mammalian faunas, and the degree 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, — namely, the fewness of the species, 

with a large proportion consisting of endemic forms 

the members of certain groups, but not those of other 



186 ABSENCE OF TEEEESTEIAL [Chap. XIII. 

groups in the same class, haying been 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 developed 
into trees, &c, — seem to me to accord better with the 
belief in the efficiency o: occasional means of transport, 
carried on during a long course of time, than with the 
belief in the former connection 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 disturbed, and consequently they would either 
have not been modified, or all the species in a more 
equable manner. 

I do not deny that there are many and serious 
difficulties in understanding how many of the in- 
habitants of the more remote islands, whether still 
retaining the same specific form or subsequently modi- 
have reached their present homes. But the 
probability of other islands having once existed as 
halting-places, of which not a wreck now remains, 
must not be overlooked. I will specify one difficult 
case. Almost all oceanic islands, even the most isolated 
and smallest, are inhabited by land-shells, generally by 
endemic but sometimes by species found else- 

where, — striking instances of which have been given 
by Dr. A. A. Gould in relation to the Pacific. Now it 
is notorious that land-shells are easily killed by sea- 
watei ; their eggs, at least such as I have tried, sink in 
it and are killed. Yet there must be some unknown, 
but occasionally efficient means for their transports!. 



Chap. XIII.] MAMMALS ON OCEANIC ISLANDS. 187 

"Would the just-hatched young sometimes adhere to the 
feet of birds roosting on the ground, and thus get 
transported ? It occurred to me that land-shells, when 
hybernating and having a membranous diaphragm over 
the mouth of the shell, mio-ht be floated in chinks of 
drifted timber across moderately wide arms of the sea. 
And I find that several species in this state withstand 
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 days, and perfectly recovered. During 
this length of time the shell might have been carried 
by a marine current of average swiftness, to a distance 
of 660 geographical miles. As this Helix has a thick 
calcareous operculum, I removed it, and when it had 
formed a new membranous one, I a^ain immersed it for 
fourteen days in sea-water, and again it recovered and 
crawled away. Baron Aucapitaine has since tried 
similar experiments : he placed 100 land-shells, be- 
longing to ten species, in a box pierced with holes, and 
immersed it for a fortnight in the sea. Out of the 
hundred shells, twenty-seven recovered. The presence 
of an operculum seems to have been of importance, as 
out of twelve specimens of Cyclostoma elegans, which 
is thus furnished, eleven revived. It is remarkable, 
seeing how well the Helix pomatia resisted with me the 
salt-water, that not one of fifty-four specimens belonging 
to four other species of Helix tried by Aucapitaine, 
recovered. It is, however, not at all probable that 
land-shells have often been thus transported ; the feet 
of birds offer a more probable method. 



34 



188 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 

On the Relations of the Inhabitants of Islands to those of 
the nearest Mainland. 

The most striking and important fact for us is the 
affinity of the species which inhabit islands to those of 
the nearest mainland, without bein^ actually the same. 
Xumerous instances could be given. The Galapagos 
Archipelago, situated under the equator, lies at the 
distance of between 500 and 600 miles from the shores 
of South America. Here almost every product of the 
land and of the water bears the unmistakable stamp of 
the American continent. There are twenty-six land- 
birds; of these, twenty-one, or perhaps twenty-three 
are ranked as distinct species, and would commonly be 
assumed to have been here created ; yet the close affinity 
of most of these birds to American species is manifest 
in every character, in their habits, gestures, and tones 
of voice. So it is with the other animals, and with a 
large proportion of the plants, as shown by Dr. Hooker 
in his admirable Flora of this archipelago. The natu- 
ralist, looking at the inhabitants of these volcanic 
islands in the Pacific, distant several hundred miles 
from the continent, 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 Gala- 
pagos Archipelago, and nowhere else, bear so plainly 
the stamp of affinity to those created in America? 
There is nothing in the conditions of life, in the 
geological nature of the islands, in their height or 
climate, or in the proportions in which the several 
classes are associated together, which closely resembles 
the conditions of the South American coast : in fact, 
there is a considerable dissimilarity in all these respects. 



Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 189 

On the other hand, there is a considerable degree of 
resemblance in £he volcanic nature of the soil, in the 
climate, height, and size of the islands, between the 
Galapagos and Cape Yerde Archipelagoes : but what an 
entire and absolute difference in their inhabitants ! 
The inhabitants of the Cape Yerde 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 from America, whether by occasional 
means of transport or (though I do not believe in this 
doctrine) by formerly continuous land, and the Cape 
Yerde Islands from Africa; such colonists would be 
liable to modification, — the principle of inheritance still 
betraying their original birthplace. 

Many analogous facts could be given : indeed it is an 
almost universal rule that the endemic productions of 
islands are related to those of the nearest continent, or 
of the nearest large 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 account, to those of America : but 
on the view that this island has been mainly stocked 
by seeds brought with earth and stones on icebergs, 
drifted by the prevailing currents, this anomaly dis- 
appears. New Zealand in its endemic planes is much 
more closely related to Australia, the nearest mainland, 
than to any other region: and this is what might have 
been expected ; but it is also plainly related to South 
America, which, although the next nearest continent, is 



190 RELATIONS OF THE INHABITANTS OF [Chap. XttL 

so enormously remote, that the fact becomes an anomaly. 
But this difficulty partially disappears on the view that 
New Zealand, South America, and the other southern 
lands have been stocked in part from a nearly inter- 
mediate though distant point, namely from the antarctic 
islands, "when they were clothed with vegetation, during 
a warmer tertiary period, before the commencement of 
the last Glacial period. The affinity, which though 
feeble, I am assured by Dr. Hooker is real, between the 
flora of the south-western corner of Australia and of the 
Cape of Good Hope, is a far more remarkable case ; but 
this affinity is confined to the plants, and will, no doubt, 
some day be explained. 

The same law which has determined the relationship 
between the inhabitants of islands and the nearest 
mainland, is sometimes displayed on a small scale, but 
in a most interesting manner, within the limits of the 
same archipelago. Thus each separate island of the 
Galapagos Archipelago is tenanted, and the fact is a 
marvellous one, by many distinct species ; but these 
species are related to each other in a very much closer 
manner than to the inhabitants of the American con- 
tinent, or of any other quarter of the world. This is 
what might have been expected, for islands situated so 
near to each other would almost necessarily receive 
immigrants from the same original source, and from 
each other. But how is it that many of the immigrants 
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, &c. ? This long appeared to me a great diffi- 
culty : but it arises in chief part from the deeply-seated 
error of considering the physical conditions of a country 



Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 191 

as the most important ; whereas it cannot be disputed 
that the nature of the other species with which each has 
to compete, is at least as important, and generally a far 
more important element of success. Now if we look to 
the species which inhabit the Galapagos Archipelago, 
and are likewise found in other parts of the world, we 
find that they differ considerably in the several islands. 
This difference might indeed have been expected if the 
islands have been stocked by occasional means of 
transport — a seed, for instance, of one plant having 
been brought to one island, and that of another plant to 
another island, though all proceeding from the same 
general source. Hence, when in former times an im- 
migrant 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 organisms ; a plant, for instance, would 
find the ground best fitted for it occupied by somewhat 
different species in the different islands, and would be 
exposed to the attacks of somewhat different enemies. 
If then it varied, natural selection would probably 
favour different varieties in the different islands. Some 
species, however, might spread and yet retain the same 
character throughout the group, just as we see some 
species spreading widely throughout a continent and 
remaining the same. 

The really surprising fact in this case of the Galapagos 
Archipelago, and in a lesser degree in some analogous 
cases, is that each new species after being formed in any 
one island, did not spread quickly to the other islands. 
But the islands, though in sight of each other, are 
separated by deep arms of the sea, in most cases wider 



192 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 

than the British Channel, and there is no reason to 
suppose that they have at any former period been 
continuously united. The currents of the sea are rapid 
and sweep between the islands, and gales of wind are 
extraordinarily rare ; so that the islands are far more 
effectually separated from each other than they appear 
on a map. Nevertheless some of the species, both of 
those found in other parts of the world and of those 
confined to the archipelago, are common to the several 
islands ; and we may infer from their present manner 
of distribution, that they haye spread from one island 
to the others. But we often take, I think, an erroneous 
view of the probability of closely-allied species invading 
each other's territory, when put into free intercommuni- 
cation. Undoubtedly, 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 will probably hold their separate 
places for almost any length of time. Being familiar 
with the fact that many species, naturalised through 
man's agency, have spread with astonishing rapidity 
over wide areas, we are apt to infer that most species 
would thus spread; but we should remember that the 
species which become naturalised in new countries are 
not generally closely allied to the aboriginal inhabitants, 
but are very distinct forms, belonging in a large pro- 
portion of cases, as shown by Alph. de Candolle, to 
distinct genera. In the Galapagos Archipelago, many 
even of the birds, though so well adapted for flying 
from island to island, differ on the different islands ; 
thus there are three closely-allied species of moeking- 
thrush, each confined to its own island. Now let us 
suppose the mocking-thrush of Chatham Island to be 



Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 193 

blown to Charles Island, which has its own mocking- 
thrash; why should it succeed in establishing itself 
there ? We may safely infer that Charles Island is 
well stocked with its own species, for annually more 
eggs are laid and young birds hatched, than can possibly 
be reared ; and we may infer that the mocking-thrush 
peculiar to Charles Island is at least as well fitted for 
its home as is the species peculiar to Chatham Island. 
Sir C. Lyell and Mr. Wollaston have communicated to 
me a remarkable fact bearing on this subject ; namely, 
that Madeira and the adjoining islet of Porto Santo 
possess many distinct but representative species of land- 
shells, some of which live in crevices of stone ; and 
although large quantities of stone are annually trans- 
ported from Porto Santo to Madeira, yet this latter 
island has not become colonised by the Porto Santo 
species ; nevertheless both islands have been colonised 
by European land-shells, which no doubt had some 
advantage over the indigenous species. From these 
considerations I think we need not greatly marvel at 
the endemic species which inhabit the several islands of 
the Galapagos Archipelago, not having all spread from 
island to island. On the same continent, also, pre- 
occupation has probably played an important part in 
checking the commingling of the species which inhabit 
different districts with nearly the same physical condi- 
tions. Thus, the south-east and south-west corners of 
Australia have nearly the same physical conditions, and 
are united by continuous land, yet they are inhabited 
by a vast number of distinct mammals, birds, and 
plants ; so it is, according to Mr. Bates, with the butter- 
flies and other animals inhabiting the great, open, and 
continuous valley of the Amazons. 



194 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 

The same principle which governs the general char- 
acter of the inhabitants of oceanic islands, namely, the 
relation to the source whence colonists could have been 
7nostly easily derived, together with their subsequent 
modification, is of the widest application throughout 
nature. We see this on every mountain-summit, in 
every lake and marsh. For Alpine species, excepting 
in as far as the same species have become widely 
spread during the Glacial epoch, are related to those 
of the surrounding lowlands ; thus we have in South 
America, Alpine humming-birds, Alpine rodents, Alpine 
plants, &c, all strictly belonging to American forms; 
and it is obvious that a mountain, as it became slowly 
upheaved, would be colonised from the surrounding 
lowlands. So it is with the inhabitants of lakes and 
marshes, excepting in so far as great facility of trans- 
port has allowed the same forms to prevail throughout 
large portions of the world. We 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 
found universally true, that wherever in two regions, 
let them be ever so distant, many closely allied or 
representative species occur, there will likewise be 
found some identical species ; and wherever many 
closely-allied species occur, there will be found many 
forms which 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 between the power and extent of 
migration in certain species, either at the present or 
at some former period, and the existence at remote 
points of the world of clusely -allied species, is shown 



Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 195 

in another and more general way. Mr. Gould re- 
marked to me long ago, that in those genera of birds 
which range over the world, many of the species have 
very wide ranges. I can hardly doubt that this rule 
is generally true, though difficult of proof. Amongst 
mammals, we see it strikingly displayed in Bats, and 
in a lesser degree in the Felidae and Canidae. We see 
the same rule in the distribution of butterflies and 
beetles. So it is with most of the inhabitants of fresh 
water, for many of the genera in the most distinct 
classes range over the world, and many of the species 
have enormous ranges. It is not meant that all, but 
that some of the species have very wide ranges in the 
genera which range very widely. 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 modification has gone ; for instance, two 
varieties of the same species inhabit America and 
Europe, and thus the species has an immense range ; 
but, if variation were to be carried a little further, 
the two varieties would be ranked as distinct species, 
and their range would 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 powerfully-winged birds, will necessarily 
range widely ; for we should never forget that to range 
widely implies not only the power of crossing barriers, 
but the more important power of being victorious in 
distant lands in the struggle for life with foreigu 
associates. But according to the view that all the 
species of a genus, though distributed to the most 
remote points of the world, are descended from a single 
progenitor, we ought to find, and I believe as a general 



196 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 

rule we do find, that some at least of the species range 
very widely. 

"We should bear in mind that many genera in all 
classes are of ancient origin, and the species in this 
case will have had ample time for dispersal and sub- 
sequent modification. There is also reason to believe 
from geological evidence, that within each great class 
the lower organisms change at a slower rate than the 
higher ; consequently they will have had a better 
chance of ranging widely and of still retaining the 
same specific character. This fact, together with that 
of the seeds and eggs of most lowly organised forms 
being very minute and better fitted for distant trans- 
portal, probably accounts for a law winch has long 
been observed, and which has lately been discussed by 
Alph. de Candolle in regard to plants, namely, that 
the lower any group of organisms stands the more 
widely it ranges. 

The relations just discussed, — namely, lower organ- 
isms ranging more widely than the higher, — some of 
the species of widely-ranging genera themselves ranging 
widely, — such facts, as alpine, lacustrine, and marsh 
productions being generally related to those which live 
on the surrounding low lands and dry lands, — the 
striking relationship between the inhabitants of islands 
and those of the nearest mainland — 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, but 
are explicable if we admit colonisation from the nearest 
or readiest source, together with the subsequent adap- 
tation of the colonists to their new homes. 



Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 197 

Summary of the last and present Chapters. 

In these chapters I have endeavoured to show, that 
if we make due allowance for our ignorance of the full 
effects of changes of climate and of the level of the 
land, which have certainly occurred within the recent 
period, and of other changes which have probably 
occurred, — if we remember how ignorant we are with 
respect to the many curious means of occasional trans- 
port, — if we bear in mind, and this is a very important 
consideration, 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 
insuperable 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 im- 
portance of barriers of all kinds, and from the analogical 
distribution of sub-genera, genera, and families. 

With respect to distinct species belonging to the 
same genus, which on our theory have spread from one 
parent-source; if we make the same allowances as 
before for our ignorance, and remember that some 
forms of life have changed very slowly, enormous 
periods of time having been thus granted for their 
migration, the difficulties are far from insuperable ; 
though in this case, as in that of the individuals of the 
same species, they are often great. 

As exemplifying the effects of climatal changes on 
distribution, I have attempted to show how important 
a part the last Glacial period has played, which affected 



198 SUMMARY OF THE [Chap. XILL 

even the equatorial regions, and which, during the 
alternations of the cold in the north and south, allowed 
the productions of opposite hemispheres to mingle, and 
left some of them stranded on the mountain-summits 
in all parts of the world. As showing how diversified 
are the means of occasional transport, I have discussed 
at some little length the means of dispersal of fresh- 
water productions. 

If the difficulties be not insuperable in admitting 
that in the long course of time all the individuals of 
the same species, and likewise of the several species 
belonging to the same genus, have proceeded from 
some one source ; then all the grand leading facts of 
geographical distribution are explicable on the theory 
of migration, together with subsequent modification and 
the multiplication of new forms. We can thus under- 
stand the high importance of barriers, whether of land 
or water, in not only separating, but in apparently 
forming the several zoological 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 inhabitants of the plains and mountains, of the 
forests, marshes, and deserts, are linked together in so 
mysterious a manner, and are likewise linked to the 
extinct beings which formerly inhabited the same 
continent Bearing in mind that the mutual relation 
of organism t>» organism is of the highest importance, 
we can see why two areas having nearly the same 
physical conditions should often be inhabited by very 
different forms of life ; for according to the length of 
time which has elapsed since the colonists entered one 
of the regions, or both ; according to the nature of the 



Chap. XIII.] LAST AND PRESENT CHAPTEES. 199 

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 competition with each 

other and with the aborigines; and according as the 

immigrants were capable of varying more or less 

rapidly, there would ensue in the two or more regions, 

independently of their physical conditions, infinitely 

diversified conditions of life, — there would be an almost 

endless amount of organic action and reaction, — and 

we should find some groups of beings greatly, and some 

only slightly modified, — some developed in great force, 

some existing in scanty numbers — and this we do find 

in the several great geographical provinces of the 

world. 

On these same principles we can understand, as I have 

endeavoured to show, why oceanic islands should have 

few inhabitants, but that of these, a large proportion 

should be endemic or peculiar ; and why, in relation to 

the means of migration, one group of beings should 

have all its species peculiar, and another group, even 

within the same class, should have all its species the 

same with those in an adjoining quarter of the world. 

We can see why whole groups of organisms, as batra- 

chians and terrestrial mammals, should be absent from 

oceanic islands, whilst the most isolated islands should 

possess their own peculiar species of aerial mammals or 

bats. We can see why, in islands, there should be 

some relation between the presence of mammals, in a 

more or less modified condition, and the depth of the 

sea between such islands and the mainland. We can 

clearly see why all the inhabitants of an archipelago, 

though specifically distinct on the several islets, should 
35 



200 SUMMAKY OF THE [Chap. XIII 

be closely related to each other; and should likewise 
be related, but less closely, to those of the nearest 
continent, or other source whence immigrants nrioht 
have been derived. We can see why, if there exist 
very closely allied or representative species in two 
areas, however distant from each other, some identical 
species will almost always there be found. 

As the late Edward Forbes often insisted, there is a 
striking parallelism in the laws of life throughout time 
and space ; the laws governing the succession of forms 
in past times being nearly the same with those govern- 
ing at the present time the differences in different 
areas. "We see this in many facts. The endurance of 
each species and group of species is continuous in time ; 
for the apparent exceptions to the rule are so few, that 
they may fairly be attributed to our not having as yet 
discovered in an intermediate deposit 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 group of 
species, is continuous, and the exceptions, which are not 
rare, may. as I have attempted to show, be accounted for 
by former migrations under different 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 maximum development. Groups 
of species, living during the same period of time, or 
living within the same area, are often characterised by 
trifling features in common, as of sculpture or colour- 
In looking to the long succession of past ages; as in 
looking to distant provinces throughout the world, we 
rind that species in certain classes differ little from each 



Chap. XIIL] LAST AND PKESENT CHAPTEKS. 201 

other, whilst those in another class, or only in a different 
section of the same order, differ greatly from each other. 
In both time and space the lowly organised members 
of each class generally change less than the highly 
organised ; but there are in both cases marked excep- 
tions to the rule. According to our theory, these 
several relations throughout time and space are intelli- 
gible ; for whether we look to the allied forms of life 
which have changed during successive ages, or to 
those which have changed after having migrated into 
distant quarters, in both cases they are connected by 
the same bond of ordinary generation ; in both cases 
the laws of variation have been the same, and modifi- 
cations have been accumulated by the same means of 
natural selection. 



202 CLASSIFICATION. [Chap. XIV. 



CHAPTER XIV. 

Mutual Affinities of Oeganic Beings : Mor- 
phology : Embryology : Rudimentary Organs. 

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

Classification. 

From the most remote period in the history of the world 
organic beings have been found to resemble each other 
in descending degrees, so that they can be classed in 
groups under groups. This classification is not arbi- 
trary like the grouping of the stars in constellations. 
The existence of groups would have been of simple 
significance, if one group had been exclusively fitted to 
inhabit the land, and another the water; one to feed 
on flesh, another on vegetable matter, and so on ; but 
the case is widely different, for it is notorious how 
commonly members of even the same sub-group have 
different habits. In the second and fourth chapters, on 
Variation and on Natural Selection, I have attempted 



Chap. XIV.] CLASSIFICATION. 203 

to show that within each country it is the widely 
ranging, the much diffused and common, that is the 
dominant species, belonging to the larger genera in each 
class, which vary most. The varieties, or incipient 
species, thus produced, ultimately become converted 
into new and distinct species ; and these, on the prin- 
ciple of inheritance, tend to produce other new and 
dominant species. Consequently the groups which are 
now large, and which generally include many dominant 
species, tend to go on increasing in size. I further 
attempted to show that from the varying descendants 
of each species trying to occupy as many and as different 
places as possible in the economy of nature, they 
constantly tend to diverge in character. This latter 
conclusion is supported by observing the great diversity 
of forms which, in any small area, come into the closest 
competition, and by certain facts in naturalisation. 

I attempted also to show that there is a steady 
tendency in the forms which are 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 explained, of these several prin- 
ciples ; and he will see that the inevitable result is, 
that the modified descendants proceeding from one 
progenitor become broken up into groups subordinate 
to groups. In the diagram each letter on the uppermost 
line may represent a genus including several species ; 
and the whole of the genera along this upper line form 
together one class, for all are descended from one ancient 
parent, and, consequently, have inherited something in 
common. But the three genera on the left hand have, 
on this same principle, much in common, and form a 



204 CLASSIFICATION. [Chap. XIV. 

sub-family, distinct from that containing the next two 
genera on the right hand, which diverged from a common 
parent at the fifth stage of descent. These five genera 
have also much in common, though less than when 
grouped in sub-families ; and they form a family distinct 
from that containing the three genera still farther to 
the right hand, which diverged at an earlier period. 
And all these genera, descended from (A), form an 
order distinct from the genera descended from (I). So 
that we here have many species descended from a single 
progenitor grouped into genera; and the genera into 
sub-families, families, and orders, all under one great 
class. The grand fact of the natural subordination of 
organic beings in groups under groups, which, from its 
familiarity, does not always sufficiently strike us, is in 
my judgment thus explained. No doubt organic beings, 
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 
arranged. In this case there is of course no relation to 
genealogical succession, and no cause can at present be 
assigned for their falling into groups. But with organic 
beings the case is different, and the view above given 
accords with their natural arrangement in group under 
group ; and no other explanation has ever been 
attempted. 

Naturalists, as we have seen, try to arrange the 
species, genera, and families in each class, on what is 
called the Natural System. But what is meant by this 
•m ? Some authors look at it merely as a scheme 
for arranging together those living objects which are 
most alike, and for separating those which are most 



Chap. XIV.] CLASSIFICATION. 205 

unlike ; or as an artificial method of enunciating, as 
briefly as possible, general propositions, — that is, by one 
sentence to give the characters common, for instance, to 
all mammals, by another those common to all carnivora, 
by another those common to the dog- genus, and then, 
by adding a single sentence, a full description is given 
of each kind of dog. The ingenuity and utility of this 
system are indisputable. But many naturalists think 
that something more is meant by the Natural System ; 
they believe that it reveals the plan of the Creator ; 
but unless it be specified whether order in time or 
space, or both, or what else is meant by the plan of the 
Creator, it seems to me that nothing is thus added to 
our knowledge. Expressions such as that famous one 
by Linnaeus, which we often meet with in a more or 
less concealed form, namely, that the characters do not 
make the genus, but that the genus gives the characters, 
seem to imply that some deeper bond is included in our 
classifications than mere resemblance. I believe that 
this is the case, and that community of descent — the 
one known cause of close similarity in organic beings — 
is the bond, which though observed by various degrees 
of modification, is partially revealed to us by our 
classifications. 

Let us now consider the rules followed in classifica- 
tion, and the difficulties which are encountered on the 
view that classification either gives some unknown 
plan of creation, or is simply a scheme for enunciating 
general propositions and of placing together the forms 
most like each other. It might have been thought 
(and was in ancient times thought) that those parts of 
the structure which determined the habits of life, and 
the general place of each being in the economy of 



206 CLASSIFICATION. [Chap. XTV. 

nature, would be of very high importance in classifica- 
tion. Nothing can be more false. Xo one regards the 
external similarity of a mouse to a shrew, of a dugong 
to a whale, of a whale to a fish, as of any importance. 
These resemblances, though so intimately connected 
with the whole life of the being, are ranked as merely 
" adaptive or analogical characters ; " but to the 
consideration of these resemblances we shall recur. It 
may even be given as a general rule, that the less any 
part of the organisation is concerned with special 
habits, the more important it becomes for classification. 
As an instance : Owen, in speaking of the dugong, 
says, " The generative organs, being those which are 
most remotely related to the habits and food of an 
animal, I have always regarded as affording very clear 
indications of its true affinities. We are least likely in 
the modifications of these organs to mistake a merely 
adaptive for an essential character." With plants how 
remarkable it is that the organs of vegetation, on which 
their nutrition and life depend, are of little signification ; 
whereas the organs of reproduction, with their product 
the seed and embryo, are of paramount importance ! 
So again in formerly discussing certain morphological 
characters which are not functionally important, we 
have seen that they are often of the highest service in 
ihcation. This depends on their constancy through- 
out many allied groups ; and their constancy chiefly 
depends on any slight deviations not having been 
preserved and accumulated by natural selection, which 
acts only on serviceable characters. 

That the mere physiological importance of an organ 
does not determine its classificatory value, is almost 
proved by the fact, that in allied groups, in which the 



Chap. XIV.] CLASSIFICATION. 207 

same organ, as we have every reason to suppose, has 
nearly the same physiological value, its classificatory 
value is widely different. No naturalist can have 
worked long at any 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 authority, Eobert Brown, who, in 
speaking of certain organs in the Proteacese, says their 
generic importance, "like that of all their parts, not 
only in this, but, as I apprehend, in every natural 
family, is very unequal, and in some cases seems to be 
entirely lost." Again, in another work he says, the 
genera of the Connaracese " differ in having one or more 
ovaria, in the existence or absence of albumen, in the 
imbricate or valvular aestivation. Any one of these 
characters singly is frequently of more than generic 
importance, though here even when all taken together 
they appear insufficient to separate Cnestis from 
Connarus." To give an example amongst insects : in 
one great division of the Hymenoptera, the antennae, as 
Westwood has remarked, are most constant in structure ; 
in another division they differ much, and the differences 
are of quite subordinate value in classification ; yet no 
one will say that the antennae in these two divisions of 
the same order are of unequal physiological importance. 
Any number of instances could be given of the vary- 
ing importance for classification of the same important 
organ within the same group of beings. 

Again, no one will say that rudimentary or atrophied 
organs are of high physiological or vital importance ; 
yet, undoubtedly, organs in this condition are often of 
much value in classification. No one will dispute that 
the rudimentary teeth in the upper jaws of young 



208 CLASSIFICATION. [Chap. XTV. 

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 
rudimentary florets is of the highest importance in the 
classification of the grasses. 

Numerous instances could be given of characters 
derived from parts which must be considered of very 
trifling physiological importance, but which are univer- 
sally admitted as highly serviceable in the definition of 
whole groups. For instance, whether or not there is an 
open passage from the nostrils to the mouth, the only 
character, according to Owen, which absolutely dis- 
tinguishes fishes and reptiles — the inflection of the 
angle of the lower jaw in Marsupials — the manner in 
which the win^s of insects are folded — mere colour in 
certain Algae — mere pubescence on parts of the flower 
in grasses — the nature of the dermal covering, as hair or 
feathers, in the Vertebrate. If the Ornithorhynchus 
had been covered with feathers instead of hair, this 
external and trilling character would have been con- 
sidered by naturalists as an important aid in deter- 
mining the degree of affinity of this strange creature to 
birds. 

The importance, for classification, of trifling characters, 
mainly depends on their being correlated with many 
other characters of more or less importance. The value 
indeed of an be of characters is very evident in 

natural history. Hence, as has often been remarked, a 
species may depart from its allies in several characters, 
both of high physiological importance, and of almost 
universal prevalence, and yet leave us in no doubt 
where it should be ranked. Hence, also, it has been 



Chap. XIV.] CLASSIFICATION. 209 

found that a classification founded on any single 
character, however important that may be, has always 
failed; for no part of the organisation is invariably 
constant. The importance of an aggregate of characters, 
even when none are important, alone explains the 
aphorism enunciated by Linnaeus, namely, that the 
characters do not give the genus, but the genus gives 
the characters ; for this seems founded on the apprecia- 
tion of many trifling points of resemblance, too slight to 
be defined. Certain plants, belonging to the Malpighi- 
aceae, bear perfect and degraded flowers ; in the latter, 
as A. de Jussieu has remarked, " the greater number of 
the characters proper to the species, to the genus, to the 
family, to the class, disappear, and thus laugh at our 
classification." When Aspicarpa produced in France, 
during several years, only these degraded flowers, 
departing so wonderfully in a number of the most 
important points of structure from the proper type of 
the order, yet M. Eichard sagaciously saw, as Jussieu 
observes, that this genus should still be retained 
amongst the Malpighiacese. This case well illustrates 
the spirit of our classifications. 

Practically, when naturalists are at work, they do 
not trouble themselves about the physiological value 
of the characters which they use in defining a group 
or in allocating any particular species. If they find 
a character nearly uniform, and common to a great 
number of forms, and not common to others, they use it 
as one of high value ; if common to some lesser number, 
they use it as of subordinate value. This principle has 
been broadly confessed by some naturalists to be the 
true one; and by none more clearly than by that 
excellent botanist, Aug. St. Hilaire. If several trifling 



210 CLASSIFICATION. [Chap. XIV. 

characters are always found in combination, though no 
apparent bond of connection can be discovered between 
them, especial value is set on them. As in most groups 
of animals, important organs, such as those for pro- 
pelling the blood, or for aerating it, or those for pro- 
pagating the race, are found nearly uniform, they are 
considered as highly serviceable in classification ; but in 
some groups all these, the most important vital organs, 
are found to offer characters of quite subordinate value. 
Thus, as Fritz Muller has lately remarked, in the same 
group of crustaceans, Cypridina is furnished with a 
heart, whilst in two closely allied genera, namely 
Cypris and Cytherea, there is no such organ ; one 
species of C)^ridina has well-developed branchiae, 
whilst another species is destitute of them. 

We can see why characters derived from the embryo 
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 obvious, on the ordi- 
nary view, why the structure of the embryo should be 
more important for this purpose than that of the adult, 
which alone plays its full part in the economy of nature. 
Yet it has been strongly urged by those great natural- 
ists, Milne Edwards and Agassiz, that embryological 
characters are the most important of all ; and this 
doctrine has very generally been admitted as true. 
Nevertheless, their importance has sometimes been 
exaggerated, owing to the adaptive characters of larvae 
not having been excluded ; in order to show this, Fritz 
Muller arranged by the aid of such characters alone the 
great class of crustaceans, and the arrangement did not 
prove a natural one. But there can be no doubt that 
embryonic, excluding larval characters, are of the 



Chap. XIY.] CLASSIFICATION. 211 

highest value for classification, not only with animals 
but with plants. Thus the main divisions of flowering 
plants are founded on differences in the embryo, — on 
the number and position of the cotyledons, and on the 
mode of development of the plumule and radicle. We 
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 affinities. Nothing can be easier than to 
define a number of characters common to all birds ; but 
with crustaceans, any such definition has hitherto been 
found impossible. There are crustaceans at the opposite 
ends of the series, which have hardly a character in 
common ; yet the species at both ends, from being 
plainly allied to others, and these to others, and so 
onwards, can be recognised as unequivocally belonging 
to this, and to no other class of the Articulata. 

Geographical distribution has often been used, though 
perhaps not quite logically, in classification, more 
especially in very large groups of closely allied forms. 
Temminck insists on the utility or even necessity of 
this practice in certain groups of birds ; and it has been 
followed by several entomologists and botanists. 

Finally, with respect to the comparative value of the 
various groups of species, such as orders, sub-orders, 
families, sub-families, and genera, they seem to be, at 
least at present, almost arbitrary. Several of the best 
botanists, such as Mr. Bentham and others, have strongly 
insisted on their arbitrary value. Instances could be 
given amongst plants and insects, of a group first 
ranked by practised naturalists as only a genus, and 
then raised to the rank of a sub-family or family ; and 
38 



212 CLASSIFICATION. [Chap. XIV. 

this has been done, not because further research has 
detected important structural differences, at first over- 
looked, but because numerous allied species with slightly 
different grades of difference, have been subsequently 
discovered. 

All the foregoing rules and aids and difficulties in 
classification may be explained, if I do not greatly 
deceive myself, on the view that the Xatural System is 
founded on descent with modification ; — that the cha- 
racters which naturalists consider as showing true 
affinity between any two or more species, are those 
which have been inherited from a common parent, all 
true classification being genealogical ; — that community 
of descent is the hidden bond which naturalists have 
been unconsciously seeking, and not some unknown 
plan of creation, or the enunciation of general proposi- 
tions, and the mere putting together and separating 
objects more or less alike. 

But I must explain my meaning more fully. I 
believe that the arrai of the groups within each 

js, in due subordination and relation to each other, 
must be strictly genealogical in order to be natural; 
but that the amount of difference in the several branches 
roups, though allied in the same degree in blood to 
their common progenitor, may differ greatly, being due 
to the different degrees of modification which they have 
undergone ; and this is expressed by the forms being 
ranked under different genera, families, sections, or 
orders. The reader will best understand what is meant, 
if he will take the trouble to refer to the diagram in the 
fourth chapter. We will suppose the letters A to L to 
represent allied genera existing during the Silurian 
epoch, and descended from some still earlier form. In 



Chap. XIV.] CLASSIFICATION. 213 

three of these genera (A, F, and I), a species has trans- 
mitted modified descendants to the present day, repre- 
sented by the fifteen genera (a 14 to 2 14 ) on the uppermost 
horizontal line. Now all these modified descendants 
from a single species, are related in blood or descent in 
the same degree; they may metaphorically be called 
cousins to the same millionth degree ; yet they differ 
widely and in different degrees from each other. The 
forms descended from A, now broken up into two or 
three families, constitute a distinct order from those 
descended from I, also broken up into two families. 
Nor can the existing species, descended from A, be 
ranked in the same genus with the parent A ; or those 
from I, with the parent I. But the existing genus F 14 
may be supposed to have been but slightly modified ; 
and it will then rank with the parent-genus F; just 
as some few still living organisms belong to Silurian 
genera. So that the comparative value of the differences 
between these organic beings, which are all related to 
each other in the same degree in blood, has come to 
be widely different. Nevertheless their genealogical 
arrangement remains strictly true, not only at the 
present time, but at each successive period of descent. 
All the modified descendants from A will have inherited 
something in common from their common parent, as 
will all the descendants from I ; so will it be with each 
subordinate branch of descendants, at each successive 
stage. If, however, we suppose any descendant of A, 
or of I, to have become so much modified as to have 
lost all traces of its parentage, in this case, its place in 
the natural system will be lost, as seems to have occurred 
with some few existing organisms. All the descendants 
of the genus F, along its whole line of descent, are 



214 CLASSIFICATION. [Chap. XIV. 

supposed to have been but little modified, and they form 
a single genus. But this genus, though much isolated, 
will still occupy its proper intermediate position. The 
representation of the groups, as here given in the diagram 
on a flat surface, is much too simple. The branches 
ought to have diverged in all directions. If the names 
of the groups had been simply written down in a linear 
series, the representation would have been still less 
natural ; and it is notoriously not possible to represent 
in a series, on a flat surface, the affinities which we 
discover in nature amongst the beings of the same 
group. Thus, the natural system is genealogical in its 
arrangement, like a pedigree : but the amount of modifi- 
cation which the different groups have undergone has to 
be expressed by ranking them under different so-called 
genera, sub-families, families, sections, orders, and 
classes. 

It may be worth while to illustrate this view of 
classification, by taking the case of languages. If we 
possessed a perfect pedigree of mankind, a genealogical 
arrangement of the races of man would afford the best 
classification of the various languages now spoken 
throughout the world ; and if all extinct languages, and 
all intermediate and slowly changing dialects, were to 
be included, such an arrangement would be the only 
possible one. Yet it might be that some ancient 
languages had altered very little and had given rise to 
few new languages, whilst others had altered much 
owing to the spreading, isolation, and state of civilisation 
of the several co-descended races, and had thus given 
rise to many new dialects and languages. The various 
degrees of difference between the languages of the same 
stock, would have to be expressed by groups subordinate 



Chap. XIV.] CLASSIFICATION. 215 

to groups ; but the proper or even the only possible 
arrangement would still be genealogical ; and this would 
be strictly natural, as it would connect together all 
languages, extinct and recent, by the closest affinities, 
and would give the filiation and origin of each tongue. 

In confirmation of this view, let us glance at the 
classification of varieties, which are known or believed 
to be descended from a single species. These are 
grouped under the species, with the sub-varieties under 
the varieties ; and in some cases, as with the domestic 
pigeon, with several other grades of difference. Nearly 
the same rules are followed as in classifying 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 the Swedish and common turnip 
together, though the esculent and thickened stems are 
so similar. Whatever part is found to be most con- 
stant, is used in classing varieties : thus the great 
agriculturist Marshall says the horns are very useful for 
this purpose with cattle, because they are less variable 
than the shape or colour of the body, &c. ; whereas with 
sheep the horns are much less serviceable, because less 
constant. In classing varieties, I apprehend 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, whether there had 
been more or less modification, that the principle of 
inheritance would keep the forms together which were 
allied in the greatest number of points. In tumbler 
pigeons, though some of the sub-varieties differ in the 



216 CLASSIFICATION. [Chap. XIV. 

important character of the length of the beak, yet all 
are kept together from having the common habit of 
tumbling; but the short-faced breed has nearly or quite 
lost this habit : nevertheless, without any thought on 
the subject, these tumblers are 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 he 
includes in his lowest grade, that of species, the two 
sexes; and how enormously these sometimes differ in 
the most important characters, is known to every 
naturalist : scarcely a single fact can be predicated in 
common of the adult males and hermaphrodites of 
certain cirripedes, and yet no one dreams of separating 
them. As soon as the three Orchidean forms, Mo- 
nachanthus, Myanthus, and Catasetum, which had 
previously been ranked as three distinct genera, were 
known to be sometimes produced on the same plant, 
they were immediately considered as varieties; and 
now I have been able to show that they are the male, 
female, and hermaphrodite forms of the same species. 
The naturalist includes as one species the various larval 
stages of the same individual, however much they may 
differ from each other and from the adult, as well as the 
so-called alternate generations of Steenstrup, 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 

her the individuals of the same species, though the 

males and females and larvae are sometimes extremely 

different ; and as it has been used in classing varieties 



Chap. XIV.] CLASSIFICATION. 217 

which have undergone a certain, and sometimes a 
considerable amount of modification, may not this same 
element of descent have been unconsciously used in 
grouping species under genera, and genera under higher 
groups, all under the so-called natural system ? I 
believe it has been unconsciously used ; and thus only 
can I understand the several rules and guides which 
have been followed by our best systematists. As we 
have no written pedigrees, we are forced to trace 
community of descent by resemblances of any kind. 
Therefore we choose those characters which are the least 
likely to have been modified, • in relation to the con- 
ditions of life to which each species has been recently 
exposed. Kudimentary structures on this view are as 
good as, or even sometimes better than, other parts of 
the organisation. We care not how trifling a character 
may be — let it be the mere inflection of the angle of the 
jaw, the manner in which an insect's wing is folded, 
whether the skin be covered by hair or feathers — if it 
prevail throughout many and different species, especially 
those having very different habits of life, it assumes high 
value ; for we can account for its presence in so many 
forms with such different habits, only by inheritance 
from a common parent. We may err in this respect in 
regard to single points of structure, but when several 
characters, let them be ever so trifling, concur through- 
out a large group of beings having different habits, we 
may feel almost sure, on the theory of descent, that 
these characters have been inherited from a common 
ancestor ; and we know that such aggregated characters 
have especial value in classification. 

We can understand why a species or a group of 
species may depart from its allies, in several of its most 



218 CLASSIFICATION. [Chap. XIV. 

important characteristics, and jet be safely classed with 
them. This may be safely done, and is often done, as 
long as a sufficient number of characters, let them be eyer 
so unimportant, betrays the hidden bond of community 
of descent. Let two forms have not a single character 
in common, yet, if these extreme forms are connected 
together by a chain of intermediate groups, we may at 
once infer their community of descent, and we put them 
all into the same class. As we find organs of high 
physiological importance — those which serve to preserve 
life under the most diverse conditions of existence — are 
generally the most constant, we attach especial value to 
them; but if these same organs, in another group or 
section of a group, are found to differ much, we at once 
value them less in our classification. "We shall presently 
see why embryological characters are of such high classi- 
ficatory importance. Geographical distribution may 
sometimes be brought usefully into play in classing 
large genera, because all the species of the same genus, 
inhabiting any distinct and isolated region, are in all 
probability descended from the same parents. 

Analogical Resemblances. — We can understand, on 
the above views, the very important distinction between 
real affinities and analogical or adaptive resemblances. 
Lamarck first called attention to this subject, 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 dugongs and whales, and between 
these two orders of mammals and fishes, are analogical. 
So is the resemblance between a mouse and a shrew- 
mouse (Sorex), which belong to different orders; and 
the still closer resemblance, insisted on by Mr. Mivart, 
between the mouse and a small marsupial animal 



Chap. XIY.] ANALOGICAL KESEMBLANCE8. 219 

(Antechinus) of Australia. These latter resemblances 
may be accounted for, as it seems to me, by adaptation 
for similarly active movements through thickets and 
herbage, together with concealment from enemies. 

Amongst insects there are innumerable similar in- 
stances ; thus Linnaeus, misled by external appear- 
ances, actually classed an homopterous insect as a moth. 
We see something of the same kind even with our 
domestic varieties, as in the strikingly similar shape 
of the body in the improved breeds of the Chinese and 
common pig, which are descended from distinct species ; 
and in the similarly thickened stems of the common and 
specifically distinct Swedish turnip. The resemblance 
between the greyhound and the racehorse is hardly more 
fanciful than the analogies which have been drawn by 
some authors between widely different animals. 

On the view of characters being of real importance 
for classification, only in so far as they reveal descent, 
we can clearly understand why analogical or adap- 
tive characters, although of the utmost importance to 
the welfare of the being, are almost valueless to the 
systematist. For animals, belonging to two most 
distinct lines of descent, may have become adapted to 
similar conditions, and thus have assumed a close 
external resemblance; but such resemblances will not 
reveal — will rather tend to conceal their blood-relation- 
ship. We can thus also understand the apparent 
paradox, that the very same characters are analogical 
when one group is compared with another, but give true 
affinities when the members of the same group are 
compared together: thus, the shape of the body and 
fin-like limbs are only analogical when whales are 
compared with fishes, being adaptations in both classes 



220 CLASSIFICATION. [Chap. XIV. 

for swimming through the water; but between the 
several members of the whale family, the shape of the 
body and the fin-like limbs offer characters exhibiting 
true affinity; for as these parts are so nearly similar 
throughout the whole family, we cannot doubt that they 
have been inherited from a common ancestor. So it is 
with fishes. 

Numerous cases could be given of striking resemblances 
in quite distinct beings between single parts or organs. 
which have been adapted for the same functions. A 
good instance is afforded by the close resemblance of the 
jaws of the dog and Tasmanian wolf or Thylacinus, — 
animals which are widely sundered in the natural sys- 
tem. But this resemblance is confined to general appear- 
ance, as in the prominence of the canines, and in the 
cutting shape of the molar teeth. For the teeth really 
differ much : thus the dog has on each side of the upper 
j aw four pre-molars and only two molars ; whilst the 
Thylacinus has three pre-molars and four molars. The 
molars also differ much in the two animals in relative 
size and structure. The adult dentition is preceded by 
a widely different milk dentition. Any one may of 
course deny that the teeth in either case have been 
adapted for tearing flesh, through the natural selection 
of successive variations ; but if this be admitted in the 
one case, it is unintelligible to me that it should be 
denied in the other. I am glad to find that so high an 
authority as Professor Flower has come to this same 
conclusion. 

The extraordinary cases given in a former chapter, of 
widely different fishes possessing electric organs, — of 
widely different insects possessing luminous organs, — 
and of orchids and asclepiads having pollen-masses 



Chap XIV.] ANALOGICAL RESEMBLANCES. 221 

with viscid discs, come under this same head of ana- 
logical resemblances. But these cases are so wonderful 
that they were introduced as difficulties or objections 
to our theory. In all such cases some fundamental 
difference in the growth or development of the parts, 
and generally in their matured structure, can be 
detected. The end gained is the same, but the means, 
though appearing superficially to be the same, are 
essentially different. The principle formerly alluded to 
under the term of analogical variation has probably in 
these cases often come into play; that is, the members 
of the same class, although only distantly allied, have 
inherited so much in common in their constitution, 
that they are apt to vary under similar exciting causes 
in a similar manner ; and this would obviously aid in 
the acquirement through natural selection of parts or 
organs, strikingly like each other, independently of 
their direct inheritance from a common progenitor. 

As species belonging to distinct classes have often been 
adapted by successive slight modifications to live under 
nearly similar circumstances, — to inhabit, for instance, 
the three elements of land, air, and water, — we can 
perhaps understand how it is that a numerical paral- 
lelism has sometimes been observed between the sub- 
groups of distinct classes. A naturalist, struck with a 
parallelism of this nature, by arbitrarily raising or 
sinking the value of the groups in several classes (and 
all our experience shows that their valuation is as yet 
arbitrary), could easily extend the parallelism over a 
wide range ; and thus the septenary, quinary, quater- 
nary and ternary classifications have probably arisen. 

There is another and curious class of cases in which. 
close external resemblance does not depend on adapta- 



222 CLASSIFICATION. [Chap. XIV. 

tion to similar habits of life, but has been gained for 
the sake of protection. I allude to the wonderful 
manner in which certain butterflies imitate, as first 
described by Mr. Bates, other and quite distinct species. 
This excellent observer has shown that in some districts 
of S. America, where, for instance, an Ithomia abounds 
in gaudy swarms, another butterfly, namely, a Leptalis, 
is often found mingled in the same flock ; and the 
latter so closely resembles the Ithomia in every shade 
and stripe of colour and even in the shape of its wings, 
that Mr. Bates, with his eyes sharpened by collecting 
during eleven years, was, though always on his guard, 
continually deceived. When the mockers and the 
mocked are caught and compared, they are found to be 
very different in essential structure, and to belong not 
only to distinct genera, but often to distinct families. 
Had this mimicry occurred in only one or two in- 
stances, 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 two genera, 
equally close in their resemblance, may be found. 
Altogether no less than ten genera are enumerated, 
which include species that imitate other butterflies. 
The mockers and mocked always inhabit the same 
region ; we never find an imitator living remote from 
the form which it imitates. The mockers are almost 
invariably rare u the mocked in almost every 

abound in swarms. In the same district in which 
a specie- of Leptalis closely imitates an Ithomia, there 
are sometimes other Lepidoptera mimicking the same 
Ithomia : so that in the same place, species of three 
genera of butterflies and even a moth are found all 



Chap. XIV.] ANALOGICAL EESEMBLANCES. 223 

closely resembling a butterfly belonging to a fourth 
genus. It deserves especial notice that many of the 
mimicking forms of the Leptalis, as well as of the 
mimicked forms, can be shown by a graduated series to 
be merely varieties of the same species ; whilst others 
are undoubtedly distinct species. But why, it may be 
asked, are certain forms treated as the mimicked and 
others as the mimickers ? Mr. Bates satisfactorily 
answers this question, by showing that the form which 
is imitated keeps the usual dress of the group to which 
it belongs, whilst the counterfeiters have changed, their 
dress and do not resemble their nearest allies. 

We are next led to inquire what reason can be 
assigned for certain butterflies and moths so often 
assuming the dress of another and quite distinct form ; 
why, to the perplexity of naturalists, has nature con- 
descended to the tricks of the stage ? Mr. Bates has, 
no doubt, hit on the true explanation. The mocked 
forms, which always abound in numbers, must habitu- 
ally escape destruction to a large extent, otherwise they 
could not exist in such swarms ; and a large amount of 
evidence has now been collected, showing that they are 
distasteful to birds and other insect-devouring animals. 
The mocking forms, on the other hand, that inhabit the 
same district, are comparatively rare, and belong to 
rare groups ; hence they must suffer habitually from 
some danger, for otherwise, from the number of eggs 
laid by all butterflies, they would in three or four 
generations swarm over the whole country. Now if a 
member of one of these persecuted and rare groups were 
to assume a dress so like that of a well-protected species 
that it continually deceived the practised eyes of an 
entomologist, it would often deceive predaceous birds 
37 



224 CLASSIFICATION. [Chap. XIV. 

and insects, and thus often escape 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 Leptalis 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 common 
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 Leptalis first varies ; and 
when a variety happens to resemble in some degree 
any common butterfly inhabiting the same district, 
this variety, from its resemblance to a flourishing and 
little-persecuted kind, has a better chance of escaping 
destruction from predaceous birds and insects, and is 
consequently oftener preserved; — "the less perfect 
degrees of resemblance being generation after generation 
eliminated, and only the others left to propagate their 
kind." So that here we have an excellent illustration 
of natural selection. 

Messrs. Wallace and Trimen have likewise described 
several equally striking cases of imitation in the 
Lepidoptera of the Malay Archipelago and Africa, and 
with some other insects. Mr. Wallace has also de- 
tected one such case with birds, but we have none 
with the larger quadrupeds. The much greater fre- 
quency of imitation with insects than with other 
animals, is probably the consequence of their small 
size ; insects cannot defend themselves, excepting 
indeed the kinds furnished with a sting, and I have 



Chap. XIV.] ANALOGICAL RESEMBLANCES. 225 

never heard of an instance of such kinds mocking other 
insects, though they are mocked ; insects cannot easily 
escape by flight from the larger animals which prey 
on them; therefore, speaking metaphorically, they are 
reduced, like most weak creatures, to trickery and 
dissimulation. 

It should be observed that the process of imitation 
probably never commenced between forms widely dis- 
similar in colour. But starting with species already 
somewhat like each other, the closest resemblance, if 
beneficial, could readily be gained by the above means ; 
and if the imitated form was subsequently and gradu- 
ally modified through any agency, the imitating form 
would be led along the same track, and thus be altered 
to almost any extent, so that it might ultimately 
assume an appearance or colouring wholly unlike that 
of the other members of the family to which it belonged. 
There is, however, some difficulty on this head, for it 
is necessary to suppose in some cases that ancient 
members belonging to several distinct groups, before 
they had diverged to their present extent, accidentally 
resembled a member of another and protected group in 
a sufficient degree to afford some slight protection ; this 
having given the basis for the subsequent acquisition of 
the most perfect resemblance. 

On the Nature of the Affinities connecting Organic 
Beings. — As the modified descendants of dominant 
species, belonging to the larger genera, tend to inherit 
the advantages which made the groups to which they 
belong large and their parents dominant, they are 
almost sure to spread widely, and to seize on more and 
more places in the economy of nature. The larger and 
more dominant groups within each class thus tend to 



226 AFFINITIES CONNECTING [Chap. XIV. 

go on increasing in size ; and they consequently sup- 
plant many smaller and feebler groups. Thus we can 
account for the fact that all organisms, recent and 
extinct; are included under a few great orders, and 
'under still fewer classes. As showing how few the 
higher groups are in number, and how widely they are 
spread throughout the world, the fact is striking that 
the discovery of Australia has not added an insect 
belon£inCT to a new class ; and that in the vegetable 
kingdom, as I learn from Dr. Hooker, it has added only 
two or three families of small size. 

In the chapter on Geological Succession I attempted 
to show, on the principle of each group having generally 
diverged much in character during; the lons;-continued 
process of modification, how it is that the more ancient 
forms of life often present characters in some degree 
intermediate between existing groups. As some few of 
the old and intermediate forms have transmitted to 
the present day descendants but little modified, these 
constitute our so-called osculant or aberrant species. 
The more aberrant any form is, the greater must be the 
number of connecting forms which have been exter- 
minated and utterly tost. And we have some evidence 
of aberrant groups having suffered severely from ex- 
tinction, for they are almost always represented by 
extremely few species ; and such species as do occur 
are generally very distinct from each other, which 
again implies extinction. The genera Ornithorhynchus 
and Lepidusiren, for example, would not have been less 
aberrant had each been represented by a dozen species, 
instead of as at present by a single one, or by two or 
three. We can, I think, account for this fact only by 
looking at aberrant groups as forms which have been 



Chap. XIV.] OEGANIC BEINGS. 227 

conquered by more successful competitors, with a few 
members still preserved under unusually favourable 
conditions. 

Mr. Waterhouse has remarked that, when a member 
belonging to one group of animals exhibits an affinity to 
a quite distinct group, this affinity in most cases is 
general and not special ; thus, according to Mr. Water- 
house, of all Eodents, the bizcacha is most nearly related 
to Marsupials ; but in the points in which it approaches 
this order, its relations are general, that is, not to any 
one marsupial species more than to another. As these 
points of affinity are believed to be real and not merely 
adaptive, they must be due in accordance with our view 
to inheritance from a common progenitor. Therefore 
we must suppose either that all Eodents, including the 
bizcacha, branched off from some ancient Marsupial, 
which will naturally have been more or less intermediate 
in character with respect to all existing Marsupials ; or 
that both Eodents and Marsupials branched off from a 
common progenitor, and that both groups have since 
undergone much modification in divergent directions. 
On either view we must suppose that the bizcacha has 
retained, by inheritance, more of the characters of its 
ancient progenitor than have other Eodents ; and there- 
fore it will not be specially related to any one existing 
Marsupial, but indirectly to all or nearly all Marsupials, 
irom having partially, retained the character of their 
common progenitor, or of some early member of the 
group. On the other hand, of all Marsupials, as Mr. 
Waterhouse has remarked, the Phascolomys resembles 
most nearly, not any one species, but the general order 
of Eodents. In this case, however, it may be strongly 
suspected that the resemblance is only analogical, owing 



228 AFFINITIES CONNECTING [Chap. XIV. 

to the Phascolomys having become adapted to habits 
like those of a Eodent. The elder De Candolle has 
made nearly similar observations on the general nature 
of the affinities of distinct families of plants. 

On the principle of the multiplication and gradual 
divergence in character of the species descended from a 
common progenitor, together with their retention by 
inheritance of some characters in common, we can 
understand the excessively complex and radiating 
affinities by which all the members of the same family 
or higher group are connected together. For the com- 
mon progenitor of a whole family, now broken up by 
extinction into distinct groups and sub-groups, will have 
transmitted some of its characters, modified in various 
ways and degrees, to all the species ; and they will 
consequently be related to each other by circuitous lines 
of affinity of various lengths (as may be seen in the 
diagram so often referred to), mounting up through 
many predecessors. As it is difficult to show the blood- 
relationship between the numerous kindred of any 
ancient and noble family even by the aid of a genea- 
logical tree, and almost impossible to do so without this 
aid, we can understand the extraordinary difficulty 
which naturalists have experienced in describing, with- 
out the aid of a diagram, the various affinities which 
they perceive between the many living and extinct 
members of the same great natural class. 

Extinction, as we have seen in the fourth chapter, has 
played an important part in defining and widening the 
intervals between the several groups in each class. "We 
may thus account for the distinctness of whole classes 
from each other — for instance, of birds from all other 
vertebrate animals — by the belief that many ancient 



Chap. XIV.] OEGANIC BEINGS, 229 

forms of life have been utterly lost, through which the 
early progenitors of birds were formerly connected with 
the early progenitors of the other and at that time less 
differentiated vertebrate classes. There has been much 
less extinction of the forms of life which once connected 
fishes with batrachians. There has been still less 
within some whole classes, for instance the Crustacea, 
for here the most wonderfully diverse forms are still 
linked together by a long and only partially broken 
chain of affinities. Extinction has only defined the 
groups : it has by no means made them ; for if every 
form which has ever lived on this earth were suddenly 
to reappear, though it would be quite impossible to give 
definitions by which each group could be distinguished, 
still a natural classification, or at least a natural 
arrangement, would be possible. We shall see this by 
turning to the diagram; the letters, A to L,may represent 
eleven Silurian genera, some of which have produced 
large groups of modified descendants, with every link in 
each branch and sub-branch still alive ; and the links 
not greater than those between existing varieties. In 
this case it would be quite impossible to give definitions 
by which the several members of the several groups 
could be distinguished from their more immediate 
parents and descendants. Yet the arrangement in the 
diagram would still hold good and would be natural ; 
for, on the principle of inheritance, all the forms 
descended, for instance, from A, would have something 
in common. In a tree we can distinguish this or that 
branch, though at the actual fork the two unite and 
blend together. We could not, as I have said, define 
the several groups ; but we could pick out types, or 
forms, representing most of the characters of each group, 



230 AFFINITIES CONNECTING [Chap. XIV. 

whether large or small, and thus give a general idea 
of the value of the differences between them. This is 
what we should be driven to, if we were ever to succeed 
in collecting all the forms in any one class which have 
lived throughout all time and space. Assuredly we 
shall never succeed in making so perfect a collection : 
nevertheless, in certain classes, we are tending towards 
this end : and Milne Edwards has lately insisted, in an 
able paper, on the high importance of looking to types, 
whether or not we can separate and define the groups 
to which such types belong. 

Finally, we have seen that natural selection, which 
follows from the struggle for existence, and which almost 
inevitably leads to extinction and divergence of charac- 
ter in the descendants from any one parent-species, 
explains that great and universal feature in the affinities 
of all organic beings, namely, their subordination in 
group under group. We use the element of descent in 
classing the individuals of both sexes and of all ages 
under one species, although they may have but few 
characters in common : we use descent in classing 
acknowledged varieties, however different they may be 
from their parents ; and I believe that this element of 
descent is the hidden bond of connexion which natur- 
alists have sought under the term of the Natural System. 
On this idea of the natural system being, in so far as it 
has been perfected, genealogical in its arrangement, 
with the grades of difference expressed by the terms 
genera, families, orders, &c. 3 we can understand the 
rules which we are compelled to follow in our classifi- 
cation. We can understand why we value certain 
resemblances far more than others ; why we use 
rudimentary and useless organs, or others of trifling 



Chap. XIV.] OKGANIC BEINGS. 231 

physiological importance ; why, in finding the relations 
between one group and another, we summarily reject 
analogical or adaptive characters, and yet use these 
same characters within the limits of the same group. 
We can clearly see how it is that all living and extinct 
forms can be grouped together within a few great 
classes ; and how the several members of each class are 
connected together by the most complex and radiating 
lines of affinities. We shall never, probably, disentangle 
the inextricable web of the affinities between the mem- 
bers of any one class ; but when we have a distinct 
object in view, and do not look to some unknown plan 
of creation, we may hope to make sure but slow 
progress. 

Professor Hackel in his ' G-enerelle Morphologie ' and 
in other works, has recently brought his great knowledge 
and abilities to bear on what he calls phylogeny, or the 
lines of descent of all organic beings. In drawing up 
the several series he trusts chiefly to embryological 
characters, but receives aid from homologous and rudi- 
mentary organs, as well as from the successive periods 
at which the various forms of life are believed to have 
first appeared in our geological formations. He has thus 
boldly made a great beginning, and shows us how 
classification will in the future be treated. 

Morphology, 

We have seen that the members of the same class, 
independently of their habits of life, resemble each other 
in the general plan of their organisation. This resem- 
blance is often expressed by the term " unity of type ; " 
or by saying that the several parts and organs in the 
different species of the class are homologous. The 



232 MOEPHOLOGY [Chap. XIV. 

whole subject is included under tbe general term of 
Morphology. This is one of the most interesting de- 
partments of natural history, and may almost be said to 
be its very soul. What can be more curious than that 
the hand of a man, formed for grasping, that of a mole 
for digging, the leg of the horse, the paddle of the por- 
poise, and the wing of the bat, should all be constructed 
on the same pattern, and should include similar bones, 
in the same relative positions ? How curious it is, to 
give a subordinate though striking instance, that the 
hind-feet of the kangaroo, which are so well fitted for 
bounding over the open plains, — those of the climbing, 
leaf-eating koala, equally well fitted for grasping the 
branches of trees, — those of the ground-dwelling, insect 
or root eating, bandicoots, — and those of some other 
Australian marsupials, — should all be constructed on 
the same extraordinary type, namely with the bones of 
the second and third digits extremely slender and 
enveloped within the same skin, so that they appear like 
a single toe furnished with two claws. Notwithstand- 
ing this similarity of pattern, it is obvious that the 
hind feet of these several animals are used for as widely 
different purposes as it is possible to conceive. The 
case is rendered all the more striking by the American 
opossums, which follow nearly the same habits of life as 
some of their Australian relatives, having feet con- 
structed on the ordinary plan. Professor Flower, from 
whom these statements are taken, remarks in conclusion : 
" We may call this conformity to type, without getting 
much nearer to an explanation of the phenomenon ; " 
and he then adds " but is it not powerfully suggestive 
of true relationship, of inheritance from a common 
ancestor ? "' 



Chap. XIV.] MORPHOLOGY. 233 

Geoffroy St. Hilaire has strongly insisted on the high 
importance of relative position or connexion in homo- 
logons parts ; they may differ to almost any extent in 
form and size, and yet remain connected together in the 
same invariable order. We never find, for instance, the 
bones of the arm and fore-arm, or of the thigh and leg, 
transposed. Hence the same names can be given to the 
homologous bones in widely different animals. We see 
the same great law in the construction of the mouths of 
insects : what can be more different than the immensely 
long spiral proboscis of a sphinx-moth, the curious 
folded one of a bee or bug, and the great jaws of a 
beetle ? — yet all these organs, serving for such widely 
different purposes, are formed by infinitely numerous 
modifications of an upper lip, mandibles, and two pairs 
of maxillae. The same law governs the construction of 
the mouths and limbs of crustaceans. So it is with the 
flowers of plants. 

Nothing can be more hopeless than to attempt to 
explain this similarity of pattern in members of the 
same class, by utility or by the doctrine of final causes. 
The hopelessness of the attempt has been expressly 
admitted by Owen in his most interesting work on the 
'Nature of Limbs.' On the ordinary view of the 
independent creation of each being, we can only say 
that so it is ; — that it has pleased the Creator to con- 
struct all the animals and plants in each great class on 
a uniform plan ; but this is not a scientific explanation. 

The explanation is to a large extent simple on the 
theory of the selection of successive slight modifica- 
tions, — each modification being profitable in some way 
to the modified form, but often affecting by correlation 
other parts of the organisation, In changes of this 



234 MOEPHOLOGY. [Chap. XIV. 

nature, there will be little or no tendency to alter the 
original pattern, or to transpose the parts. The bones 
of a limb might be shortened and flattened to any 
extent, becoming at the same time enveloped in thick 
membrane, so as to serve as a fin ; or a webbed hand 
miqht have all its bones, or certain bones, lengthened 
to any extent, with the membrane connecting them 
increased, so as to serve as a wing ; yet all these 
modifications would not tend to alter the framework of 
the bones or the relative connexion of the parts. If 
we suppose that an early progenitor — the archetype as 
it may be called — of all mammals, birds, and reptiles, 
had its limbs constructed on the existing general 
pattern, for whatever purpose they served, we can at 
once perceive the plain signification of the homologous 
construction of the limbs throughout the class. So 
with the mouths of insects, we have only to suppose 
that their common progenitor had an upper lip, 
mandibles, and two pairs of maxillae, these parts 
being perhaps very simple in form ; and then natural 
selection will account for the infinite diversity in the 
structure and functions of the mouths of insects. 
Nevertheless, it is conceivable that the general pattern 
of an organ might become so much obscured as to be 
finally lost, by the reduction and ultimately by the 
complete abortion of certain parts, by the fusion of 
other parts, and by the doubling or multiplication of 
others, — variations which we know to be within the 
limits of possibility. In the paddles of the gigantic 
extinct sea-lizards, and in the mouths of certain 
suctorial crustaceans, the general pattern seems thus 
to have become partially obscured. 

There is another and equally curious branch of our 



Chap. XIV.] MOEPHOLOGY. 235 

subject ; namely, serial homologies, or the comparison 
of the different parts or organs in the same individual, 
and not of the same parts or organs in different 
members of the same class. Most physiologists believe 
that the bones of the skull are homologous — that is, 
correspond in number and in relative connexion — with 
the elemental parts of a certain number of vertebrae. 
The anterior and posterior limbs in all the higher 
vertebrate classes are plainly homologous. So it is 
with the wonderfully complex jaws and legs of crus- 
taceans. It is familiar to almost every one, that in 
a flower the relative position of the sepals, petals, 
stamens, and pistils, as well as their intimate structure, 
are intelligible on the view that they consist of meta- 
morphosed leaves, arranged in a spire. In monstrous 
plants, we often get direct evidence of the possibility of 
one organ being transformed into another ; and we can 
actually see, during the early or embryonic stages of 
development in flowers, as well as in crustaceans and 
many other animals, that organs, which when mature 
become extremely different are at first exactly alike. 

How inexplicable are the cases of serial homologies 
on the ordinary view of creation ! Why should the 
brain be enclosed in a box composed of such numerous 
and such extraordinarily shaped pieces of bone, appa- 
rently representing vertebras ? As Owen has remarked, 
the benefit derived from the yielding of the separate 
pieces in the act of parturition by mammals, will by 
no means explain the same construction in the skulls 
of birds and reptiles. Why should similar bones have 
been created to form the wing and the leg of a bat, 
used as they are for such totally different purposes, 
namely flying and walking? Why should one crus- 
38 



236 MOKPHOLOGY. [Chap. XIY. 

tacean, which has an extremely complex month formed 
of many parts, consequently always have fewer legs; 
or conversely, those with many legs have simpler 
months ? Why should the sepals, petals, stamens, and 
pistils, in each flower, though fitted for such distinct 
purposes, be all constructed on the same pattern ? 

On the theory of natural selection, we can, to a 
certain extent, answer these questions. We need not 
here consider how the bodies of some animals first 
became divided into a series of segments, or how they 
became divided into right and left sides, with corre- 
sponding organs, for such questions are almost beyond 
investigation. It is, however, probable that some serial 
structures are the result of cells multiplying by division, 
entailing the multiplication of the parts developed from 
such cells. It must suffice for our purpose to bear in 
mind that an indefinite repetition of the same part or 
organ is the common characteristic, as Owen has 
remarked, of all low or little specialised forms ; there- 
fore the unknown progenitor of the Vertebrata probably 
possessed many vertebrae ; the unknown progenitor of 
the Articulate, many segments ; and the unknown 
progenitor of flowering plants, many leaves arranged in 
one or more spires. We have also formerly seen that 
parts many times repeated are eminently liable to vary, 
Ik »t only in number, but in form. Consequently such 
parts, being already present in considerable numbers, 
and being highly variable, would naturally afford the 
materials for adaptation to the most different purposes; 
yet they would generally retain, through the furce of 
inheritance, plain traces of their original or fundamental 
resemblance. They would retain this resemblance all 
the more, as the variations, which afforded the basis for 



Chap. XIY.] MOEPHOLOGY. 237 

their subsequent modification through natural selection, 
would tend from the first to be similar ; the parts 
being at an early stage of growth alike, and being 
subjected to nearly the same conditions. Such parts, 
whether more or less modified, unless their common 
origin became wholly obscured, would be serially 
homologous. 

In the great class of molluscs, though the parts in 
distinct species can be shown to be homologous, only a 
few serial homologies, such as the valves of Chitons, 
can be indicated ; that is, we are seldom enabled to say 
that one part is homologous with another part in the 
same individual. And we can understand this fact; 
for in molluscs, even in the lowest members of the 
class, we do not find nearly so milch indefinite repetition 
of any one part as we find in the other great classes of 
the animal and vegetable kingdoms. 

But morphology is a much more complex subject 
than it at first appears, as has lately been well shown 
in a remarkable paper by Mr. E. Eay Lankester, who 
has drawn an important distinction between certain 
classes of cases which have all been equally ranked by 
naturalists as homologous. He proposes to call the 
structures which resemble each other in distinct 
animals, owing to their descent from a common 
progenitor with subsequent modification, homogenous; 
and the resemblances which cannot thus be accounted 
for, he proposes to call homoplastic. For instance, he 
believes that the hearts of birds and mammals are as a 
whole homogenous, — that is, have been derived from a 
common progenitor; but that the four cavities of the 
heart in the two classes are homoplastic, — that is, have 
been independently developed. Mr. Lankester also 



238 MOEPHOLOGY. [Chap. XIV. 

adduces the close resemblance of the parts on the right 
and left sides of the body, and in the successive 
segments of the same mdividual animal; and here we 
have parts commonly called homologous, which bear 
no relation to the descent of distinct species from a 
common progenitor. Homoplastic structures are the 
same with those which I have classed, though in a very 
imperfect manner, as analogous modifications or re- 
semblances. Their formation may be attributed in part 
to distinct organisms, or to distinct parts of the same 
organism, having varied in an analogous manner ; and 
in part to similar modifications, having been preserved 
for the same general purpose or function, — of which 
many instances have been given. 

Naturalists frequently speak of the skull as formed 
of metamorphosed vertebrae ; the jaws of crabs as 
metamorphosed legs ; the stamens and pistils in flowers 
as metamorphosed leaves ; but it would in most cases 
be more correct, as Professor Huxley has remarked, to 
speak of both skull and vertebrae, jaws and legs, &c, as 
having been metamorphosed, not one from the other, as 
they now exist, but from some common and simpler 
element. Most naturalists, however, use such language 
only in a metaphorical sense ; they are far from meaning 
that during a long course of descent, primordial organs 
of any kind — vertebrae in the one case and legs in the 
other — have actually been converted into skulls or jaws. 
5 30 strong is the appearance of this having occurred, 
that naturalists can hardly avoid employing language 
having this plain signification. According to the views 
here maintained, such language may be used literally ; 
and the wonderful fact of the jaws, for instance, of a 
crab retaining numerous characters which they probably 



Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 239 

would have retained through inheritance, if they had 
really been metamorphosed from true though extremely 
simple legs, is in part explained. 

Development and Embryology. 

This is one of the most important subjects in the 
whole round of natural history. The metamorphoses of 
insects, with which every one is familiar, are generally 
effected abruptly by a few stages ; but the transforma- 
tions are in reality numerous and gradual, though 
concealed. A certain ephemerous insect (Chloeon) 
during its development, moults, as shown by Sir J. 
Lubbock, above twenty times, and each time undergoes 
a certain amount of change ; and in this case we see 
the act of metamorphosis performed in a primary and 
gradual manner. Many insects, and especially certain 
crustaceans, show us what wonderful changes of 
structure can be effected during development. Such 
changes, however, reach their acme in the so-called 
alternate generations of some of the lower animals. It 
is, for instance, an astonishing fact that a delicate 
branching coralline, studded with polypi and attached 
to a submarine rock, should produce, first by budding 
and then by transverse division, a host of huge floating 
jelly-fishes ; and that these should produce eggs, from 
which are hatched swimming animalcules, which attach 
themselves to rocks and become developed into branch- 
ing corallines; and so on in an endless cycle. The 
belief in the essential identity of the process of 
alternate generation and of ordinary metamorphosis 
has been greatly strengthened by Wagner's discovery 
of the larva or maggot of a fly, namely the Cecidomyia, 
producing asexually other larvae, and these others, which 



240 DEVELOPMENT AND EMBEYOLOGY. [Chap. XIY. 

finally are developed into mature males and females, 
propagating their kind in the ordinary manner by eggs. 

It may be worth notice that when Wagner's remark- 
able discovery was first announced, I was asked how 
was it possible to account for the larvae of this fly 
having acquired the power of asexual reproduction. As 
long as the case remained "unique no answer could be 
given. But already Grimm has shown that another fly, 
a Chironomus, reproduces itself in nearly the same 
manner, and he believes that this occurs frequently in 
the Order. It is the pupa, and not the larva, of the 
Chironomus which has this power ; and Grimm further 
shows that this case, to a certain extent, " unites that 
of the Cecidomyia with the parthenogenesis of the 
Coccidee;" — the term parthenogenesis implying that 
the mature females of the Coceidse are capable of 
producing fertile eggs without the concourse of the 
male. Certain animals belonging to several classes are 
now known to have the power of ordinary reproduction 
at an unusually early age ; and we have only to acceler- 
ate parthenogenetic reproduction by gradual steps to an 
earlier and earlier age, — Chironomus showing us an 
almost exactly intermediate stage, viz., that of the pupa 
— and we can perhaps account for the marvellous case 
of the Cecidomyia. 

It has already been stated that various parts in 
the same individual which are exactly alike dur- 
ing an early embryonic period, become widely dif- 
ferent and serve for widely different purposes in 
the adult state. So again it has been shown that 
generally the embryos of the most distinct species 
belonging to the same class are closely similar, but 
become, when fully developed, widely dissimilar. A 



Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 241 

better proof of this latter fact cannot be given than the 
statement by Von Baer that " the embryos of mammalia, 
" of birds, lizards, and snakes, probably also of chelonia, 
" are in their earliest states exceedingly like one another. 
" both as a whole and in the mode of development of 
"their parts; so much so, in fact, that we can often 
"distinguish the embryos only by their size. In my 
"possession are two little embryos in spirit, whose 
" names I have omitted to attach, and at present I am 
" quite unable to say to what class they belong. They 
" may be lizards or small birds, or very young mam- 
"malia, so complete is the similarity in the mode 
" of formation of the head and trunk in these animals. 
"The extremities, however, are still absent in these 
" embryos. But even if they had existed in the earliest 
" stage of their development we should learn nothing, for 
" the feet of lizards and mammals, the wings and feet of 
" birds, no less than the hands and feet of man, all arise 
" from the same fundamental form." The larvae of most 
crustaceans, at corresponding stages of development, 
closely resemble each other, however different the 
adults may become ; and so it is with very many other 
animals. A trace of the law of embryonic resemblance 
occasionally lasts till a rather late age : thus birds of 
the same genus, and of allied genera, often resemble 
each other in their immature plumage ; as we see in the 
spotted feathers in the young of the thrush group. In 
the cat tribe, most of the species when adult are striped 
or spotted in lines ; and stripes or spots can be plainly 
distinguished in the whelp of the lion and the puma. 
"We occasionally though rarely see something of the 
same kind in plants ; thus the first leaves of the ulex or 
furze, and the first leaves of the phyllodineous acacias, 



242 DEVELOPMENT AXD EMBRYOLOGY. [Chap. XIV. 

are pinnate or divided like the ordinary leaves of the 
legiiroinosae. 

The points of structure, in which the embryos of 
widely different animals within the same class resemble 
each other, often have no direct relation to their 
conditions of existence. We cannot, for instance, 
suppose that in the embryos of the vertebrata the 
peculiar loop-like courses of the arteries near the 
branchial slits are related to similar conditions, — in the 
young mammal which is nourished in the womb of its 
mother, in the egg of the bird which is hatched in a 
nest, and in the spawn of a frog under water. "We have 
no more reason to believe in such a relation, than we 
have to believe that the similar bones in the hand of a 
man, wing of a bat, and fin of a porpoise, are related to 
similar conditions of life. ISTo one supposes that the 
stripes on the whelp of a lion, or the spots on the 
young blackbird, are of any use to these animals. 

The case, however, is different when an animal during 
any part of its embryonic career is active, and has to 
provide for itself. The period of activity may come on 
earlier or later in life ; but whenever it comes on, the 
adaptation of the larva to its conditions of life is just as 
perfect and as beautiful as in the adult animal. In how 
important a manner this has acted, has recently been 
well shown by Sir J. Lubbock in his remarks on the 
close similarity of the larvae of some insects belonging to 
very different orders, and on the dissimilarity of the 
larvae of other insects within the same order, according 
to their habits of life. Owing to such adaptations, the 
similarity of the larvae of allied animals is sometimes 
greatly obscured ; especially when there is a division of 
labour during the different stages of development, as 



Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 243 

when the same larva has during one stage to search for 
food, and during another stage has to search for a place 
of attachment. Cases can even be given of the larvae of 
allied species, or groups of species, differing more from 
each other than do the adults. In most cases, however, 
the larvae, though active, still obey, more or less closely, 
the law of common embryonic resemblance. Cirripedes 
afford a good instance of this; even the illustrious 
Cuvier did not perceive that a barnacle was a crustacean : 
but a glance at the larva shows this in an unmistakable 
manner. So again the two main divisions of cirripedes, 
the pedunculated and sessile, though differing widely in 
external appearance, have larvae in all their stages 
barely distinguishable. 

The embryo in the course of development generally 
rises in organisation ; I use this expression, though I am 
aware that it is hardly possible to define clearly what is 
meant by the organisation being higher or lower. But 
no one probably will dispute that the butterfly is 
higher than the caterpillar. In some cases, however, the 
mature animal must be considered as lower in the scale 
than the larva, as with certain parasitic crustaceans. 
To refer once again to cirripedes : the larvae in the first 
stage have three pairs of locomotive organs, a simple 
single eye, and a probosciformed mouth, with which 
they feed largely, for they increase much in size. In 
the second stage, answering to the chrysalis stage of 
butterflies, they have six pairs of beautifully constructed 
natatory legs, a pair of magnificent compound eyes, and 
extremely complex antennae; but they have a closed 
and imperfect mouth, and cannot feed : their function at 
this stage is, to search out by their well-developed 
organs of sense, and to reach by their active powers of 



244 DEVELOPMENT AXD EMBEYOLOGY. [Chap. XIV 

swimming, a proper place on which to become attached 
and to undergo their final metamorphosis. When this 
is completed they are fixed for life : their legs are now 
converted into prehensile organs ; they again obtain a 
well-constructed mouth ; but they have no antennae, and 
their two eyes are now reconverted into a minute, single, 
simple eye-spot. In this last and complete state, cirri- 
pedes may be considered as either more highly or more 
lowly organised than they were in the larval condition. 
But in some genera the larvae become developed into 
hermaphrodites having the ordinary structure, and into 
what I have called complemental males ; and in the 
latter the development has assuredly been retrograde, for 
the male is a mere sack, which lives for a short time 
and is destitute of mouth, stomach, and every other 
organ of importance, excepting those for reproduction. 

We are so much accustomed to see a difference in 
structure between the embryo and the adult, that we are 
tempted to look at this difference as in some necessary 
manner contingent on growth. But there is no reason 
why, for instance, the wing of a bat, or the fin of a por- 
poise, should not have been sketched out with all their 
parts in proper proportion, as soon as any part became 
visible. In some whole groups of animals and in 
certain members of other groups this is the case, and 
the embryo does not at any period differ widely from 
the adult : thus Owen has remarked in regard to cuttle- 
fish, " there is no metamorphosis ; the cephalopodic 
character is manifested long before the parts of the 
embryo are completed." Land-shells and fresh-water 
crustaceans are born having then proper forms, w T hilst 
the marine members of the same two great classes pass 
through considerable and often great changes during 



Chap. XIV.] DEVELOPMENT AND EMBKYOLOGY. 245 

their development. Spiders, again, barely undergo any 
metamorphosis. The larvse of most insects pass through 
a worm-like stage, whether they are active and adapted 
to diversified habits, or are inactive from being placed 
in the midst of proper nutriment or from being fed by 
their parents; but in some few cases, as in that of 
Aphis, if we look to the admirable drawings of the 
development of this insect, by Professor Huxley, we see 
hardly any trace of the vermiform stage. 

Sometimes it is only the earlier developmental stages 
which fail. Thus Fritz Muller has made the remarkable 
discovery that certain shrimp-like crustaceans (allied to 
Penceus) first appear under the simple nauplius-form, 
and after passing through two or more zoea-stages, and 
then through the mysis-stage, finally acquire their 
mature structure : now in the whole great malacostracan 
order, to which these crustaceans belong, no other 
member is as yet known to be first developed under the 
nauplius-form, though many appear as zoeas ; neverthe- 
less Muller assigns reasons for his belief, that if there 
had been no suppression of development, all these 
crustaceans would have appeared as nauplii. 

How, then, can we explain these several facts in 
embryology, — namely, the very general, though not 
universal, difference in structure between the embryo 
and the adult ; — the various parts in the same in- 
dividual embryo, which ultimately become very unlike 
and serve for diverse purposes, being at an early period 
of growth alike; — the common, but not invariable, 
resemblance between the embryos or larvse of the most 
distinct species in the same class ; — the embryo often 
retaining whilst within the egg or womb, structures 
which are of no service to it, either at that or at a later 



246 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 

period of life ; on the other hand larvae, which have to 
provide for their own wants, being perfectly adapted 
to the surrounding conditions ; — and lastly the fact of 
certain larvae standing higher in the scale of organisation 
than the mature animal into which they are developed ? 
I believe that all these facts can be explained, as follows. 

It is commonly assumed, perhaps from monstrosities 
affecting the embryo at a very early period, that slight 
variations or individual differences necessarily appear 
at an equally early period. We have little evidence on 
this head, but what we have certainly points the other 
way ; for it is notorious that breeders of cattle, horses, 
and various fancy animals, cannot positively tell, until 
some time after birth, what will be the merits or 
demerits of their young animals. "VVe see this plainly 
in our own children ; we cannot tell whether a child 
will be tall or short, or what its precise features will be. 
The question is not, at what period of life each variation 
may have been caused, but at what period the effects 
are displayed. The cause may have acted, and I believe 
often lias acted, on one or both parents before the act of 
generation. It deserves notice that it is of no import- 
ance to a very young animal, as long as it remains in 
its mother's womb or in the egg, or as long as it is 
nourished and protected by its parent, whether most of 
its characters are acquired a little earlier or later in life. 
It would not signify, for instance, to a bird which 
obtained its fond by having a much-curved beak 
whether or not whilst young it possessed a beak of this 
shape, as long as it was fed by its parents. 

I have stated in the first chapter, that at whatever 
age a variation first appears in the parent, it tends to 
re-appear at a corresponding age in the offspring. 



Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 247 

Certain variations can only appear at corresponding 
ages; for instance, peculiarities in the caterpillar, 
cocoon, or imago states of the silk-moth : or, again, in 
the full-grown horns of cattle. But variations, which, 
for all that we can see might have first appeared either 
earlier or later in life, likewise tend to reappear at a 
corresponding age in the offspring and parent. I am 
far from meaning that this is invariably the case, and 
I could give several exceptional cases of variations 
(taking the word in the largest sense) which have 
supervened at an earlier age in the child than in the 
parent. 

These two principles, namely, that slight variations 
generally appear at a not very early period of life, and 
are inherited at a corresponding not early period, 
explain, as I believe, all the above specified leading 
facts in embryology. But first let us look to a few 
analogous cases in our domestic varieties. Some 
authors who have written on Dogs, maintain that the 
greyhound and bulldog, though so different, are really 
closely allied varieties, descended from the same wild 
stock ; hence I was curious to see how far their puppies 
differed from each other : I was told by breeders that 
they differed just as much as their parents, and this, 
judging by the eye, seemed almost to be the case ; but 
on actually measuring the old dogs and their six-days- 
old puppies, I found that the puppies had not acquired 
nearly their full amount of proportional difference. So, 
again, I was told that the foals of cart and race-horses 
— breeds which have been almost wholly formed by 
selection under domestication — differed as much as the 
full-grown animals; but having had careful measure- 
ments made of the dams and of three-days-old colts of 
39 



248 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 

race and heavy cart-horses, I find that this is by no 
means the case. 

As we have conclusive evidence that the breeds of 
the Pigeon are descended from a single wild species, I 
compared the young within twelve hours after being 
hatched; I carefully measured the proportions (but 
will not here give the details) of the beak, width of 
mouth, length of nostril and of evelid, size of feet and 
length of leg, in the wild parent-species, in pouters, 
fantails, runts, barbs, dragons, carriers, and tumblers. 
Xow some of these birds, when mature, differ in so 
extraordinary a manner in the length and form of beak, 
and in other characters, that they would certainly have 
been ranked as distinct genera if found in a state of 
nature. But when the nestling birds of these several 
breeds were placed in a row, though most of them could 
just be distinguished, the proportional differences in 
the above specified points were incomparably less than 
in the full-grown birds. Some characteristic points of 
difference — for instance, that of the width of mouth — 
could hardly be detected in the young. But there was 
one remarkable exception to this rule, for the young of 
the short-faced tumbler differed from the young of the 
wild rock-pigeon and of the other breeds, in almost 
exactly the same proportions as in the adult state. 

These tacts are explained by the above two principles. 
Fanciers select their dogs, horses, pigeons, &c, for 
breeding, when nearly grown up : they are indifferent 
whether the desired qualities are acquired earlier or 
later in life, if the full-grown animal possesses them. 
And the cases just given, more especially that of the 
pigeons, show that the characteristic differences which 
have been accumulated by man's selection, and which 



Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 249 

give value to his breeds, do not generally appear at a 
very early period of life, and are inherited at a corre- 
sponding not early period. But the case of the short- 
faced tumbler, which when twelve hours old possessed 
its proper characters, proves that this is not the 
universal rule; for here the characteristic differences 
must either have appeared at an earlier period than 
usual, or, if not so, the differences must have been 
inherited, not at a corresponding, but at an earlier age. 

Now let us apply these two principles to species in a 
state of nature. Let us take a group of birds, descended 
from some ancient form and modified through natural 
selection for different habits. Then, from the many 
slight successive variations having supervened in the 
several species at a not early age, and having been 
inherited at a corresponding age, the young will have 
been but little modified, and they will still resemble 
each other much more closely than do the adults, — -just 
as we have seen with the breeds of the pigeon. We 
may extend this view to widely distinct structures and 
to whole classes. The fore-limbs, for instance, which 
once served as legs to a remote progenitor, may have 
become, through a long course of modification, adapted 
in one descendant to act as hands, in another as paddles, 
in another as wings ; but on the above two principles 
the fore-limbs will not have been much modified in the 
embryos of these several forms ; although in each form 
the fore-limb will differ greatly in the adult state. 
Whatever influence long-continued use or disuse may 
have had in modifying the limbs or other parts of any 
species, this will chiefly or solely have affected it when 
nearly mature, when it was compelled to use its full 
powers to gain its own living; and the effects thus 



250 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 

produced will have been transmitted to the offspring at 
a corresponding nearly mature age. Thus the young 
will not be modified, or will be modified only in a slight 
degree, through the effects of the increased use or 
disuse of parts. 

With some animals the successive variations may 
have supervened at a very early period of life, or the 
steps may have been inherited at an earlier age than 
that at which they first occurred. In either of these 
cases, the young or embryo will closely resemble the 
mature parent-form, as we have seen with the short- 
faced tumbler. And this is the rule of development in 
certain whole groups, or in certain sub-groups alone, 
as with cuttle-fish, land-shells, fresh-water crustaceans, 
spiders, and some members of the great class of insects. 
With respect to the final cause of the young in such 
groups not passing through any metamorphosis, we can 
see that this would follow from the following contin- 
ences ; namely, from the young having to provide at a 
very early age for their own wants, and from their 
following the same habits of life with their parents ; for 
in this case, it would be indispensable for their existence 
that they should be modified in the same manner as 
their parents. Again, with respect to the singular fact 
that many terrestrial and fresh-water animals do not 
undergo any metamorphosis, whilst marine members of 
the same groups pass through various transformations, 
Fritz Mallei has ~ igg ssted that the process of slowly 
modifying and adapting an animal to live on the land 
or in fresh water, instead of in the sea, would be greatly 
simplified by its not passing through any larval stage ; 
for it is not probable that places well adapted for both 
the larval and mature stages, under such new and 



Chap. XIV.] DEVELOPMENT AND EMBEYOLOGY. 251 

greatly changed habits of life, would commonly be found 
unoccupied or ill-occupied by other organisms. In this 
case the gradual acquirement at an earlier and earlier 
age of the adult structure would be favoured by natural 
selection ; and all traces of former metamorphoses would 
finally be lost. 

If, on the other hand, it profited the young of an 
animal to follow habits of life slightly different from 
those of the parent-form, and consequently to be con- 
structed on a slightly different plan, or if it profited a 
larva already different from its parent to change still 
further, then, on the principle of inheritance at corre- 
sponding ages, the young or the larvse might be rendered 
by natural selection more and more different from their 
parents to any conceivable extent. Differences in the 
larva might, also, become correlated with successive 
stages of its development ; so that the larva, in the first 
stage, might come to differ greatly from the larva in the 
second stage, as is the case with many animals. The 
adult might also become fitted for sites or habits, in 
which organs of locomotion or of the senses, &c, would 
be useless ; and in this case the metamorphosis would 
be retrograde. 

From the remarks just made we can see how by 
changes of structure in the young, in conformity with 
changed habits of life, together with inheritance at corres- 
ponding ages, animals might come to pass through stages 
of development, perfectly distinct from the primordial 
condition of their adult progenitors. Most of our best 
authorities are now convinced that the various larval 
and pupal stages of insects have thus been acquired 
through adaptation, and not through inheritance from 
some ancient form. The curious case of Sitaris — a 



252 DEVELOPMENT AXD EMBEYOLOGY. [Chap. XIV. 

beetle which passes through, certain unusual stages of 
development — will illustrate how this might occur. 
The first larval form is described by M. Fabre, as an 
active, minute insect, furnished with six legs, two long 
antennae, and four eyes. These larvae are hatched in 
the nests of bees ; and when the male-bees emerge from 
their burrows, in the spring, which they do before the 
females, the larvae spring on them, and afterwards crawl 
on to the females whilst paired with the males. As 
soon as the female bee deposits her eggs on the surface 
of the honey stored in the cells, the larvae of the Sitaris 
leap on the eggs and devour them. Afterwards they 
undergo a complete change ; their eyes disappear ; their 
legs and antennae become rudimentary, and they feed on 
honey; so that they now more closely resemble the 
ordinary larvae of insects ; ultimately they undergo a 
further transformation, and finally emerge as the perfect 
beetle. Now, if an insect, undergoing transformations 
like those of the Sitaris, were to become the progenitor 
of a whole new class of insects, the course of develop- 
ment of the new class would be widely different from 
that of our existing insects ; and the first larval stage 
certainly would not represent the former condition of 
any adult and ancient form. 

On the other hand it is highly probable that with 
many animals the embryonic or larval stages show us, 
more or less completely, the condition of the progenitor 
of the whole group in its adult state. In the great class 
of the Crustacea, forms wonderfully distinct from each 
other, namely, suctorial parasites, cirripedes, entomo- 
straca, and even the malacostraca, appear at first as 
larvae under the nauplius-form ; and as these larvae live 
and feed in the open sea, and are not adapted for any 



Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 253 

peculiar habits of life, and from other reasons assigned 
by Fritz Miiller, it is probable that at some very remote 
period an independent adult animal, resembling the 
Nauplius, existed, and subsequently produced, along 
several divergent lines of descent, the above-named 
great Crustacean groups. , So again it is probable, from 
what we know of the embryos of mammals, birds, fishes, 
and reptiles, that these animals are the modified descen- 
dants of some ancient progenitor, which was furnished 
in its adult state with branchiae, a swim-bladder, four 
fin-like limbs, and a long tail, all fitted for an aquatic 
life. 

As all the organic beings, extinct and recent, which 
have ever lived, can be arranged within a few great 
classes ; and as all within each class have, according to 
our theory, been connected together by fine gradations, 
the best, and, if our collections were nearly perfect, 
the only possible arrangement, would be genealogical ; 
descent being the hidden bond of connexion which 
naturalists have been seeking under the term of the 
Natural System. On this view we can understand how 
it is that, in the eyes of most naturalists, the structure 
of the embryo is even more important for classification 
than that of the adult. In two or more groups of 
animals, however much they may differ from each other 
in structure and habits in their adult condition, if they 
pass through closely similar embryonic stages, we may 
feel assured that they all are descended from one 
parent- form, and are therefore closely related. Thus, 
community in embryonic structure reveals community 
of descent; but dissimilarity in embryonic development 
does not prove discommunity of descent, for in one of 
two groups the developmental stages may have been 



254 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 

suppressed, or may have been so greatly modified 
through adaptation to new habits of life, as to be no 
longer recognisable. Even in groups, in which the 
adults have been modified to an extreme degree, com- 
munity of origin is often revealed by the structure of 
the larvae ; we have seen, for instance, that cirripedes, 
though externally so like shell-fish, are at once known 
by their larvae to belong to the great class of crustaceans. 
As the embryo often shows us more or less plainly the 
structure of the less modified and ancient progenitor of 
the group, we can see why ancient and extinct forms so 
often resemble in their adult state the embryos of 
existing species of the same class. Agassiz believes this 
to be a universal law of nature ; and we may hope 
hereafter to see the law proved true. It can, however, 
be proved true only in those cases in which the ancient 
state of the progenitor of the group has not been wholly 
obliterated, either by successive variations having super- 
vened at a very early period of growth, or by such 
variations having been inherited at an earlier age than 
that at which they first appeared. It should also be 
borne in mind, that the law may be true, but yet, owing 
to the geological record not extending far enough back 
in time, may remain for a long period, or for ever, 
incapable of demonstration. The law will not strictly 
hold good in those cases in winch an ancient form 
became adapted in its larvae state to some special line 
of life, and transmitted the same larval state to a whole 
group of descendants ; for such larval will not resemble 
any still more ancient form in its adult state. 

Thus, as it seems to me, the leading fact3 in 
embryology, which are second to none in importance, 
are explained on the principle of variations in the 



Chap. XIV.] EUDBIENTAEY OKGANS. 255 

many descendants from some one ancient progenitor, 
having appeared at a not very early period" of life, and 
having been inherited at a corresponding period. Em- 
bryology rises greatly in interest, when we look at the 
embryo as a picture, more or less obscured, of the 
progenitor, either in its adult or larval state, of all the 
members of the same great class. 

Rudimentary, Atrophied, and Aborted Organs. 

Organs or parts in this strange condition, bearing the 
plain stamp of inutility, are extremely common, or even 
general, throughout nature. It would be impossible to 
name one of the higher animals in which some part or 
other is not in a rudimentary condition. In the mam- 
malia, for instance, the males possess rudimentary 
mammas ; in snakes one lobe of the lungs is rudimen- 
tary ; in birds the " bastard- wing " may safely be 
considered as a rudimentary digit, and in some species 
the whole wing is so far rudimentary that it cannot be 
used for flight. What can be more curious than the 
presence of teeth in foetal whales, which when grown 
up have not a tooth in their heads ; or the teeth, which 
never cut through the gums, in the upper jaws of 
unborn calves ? 

Eudimentary organs plainly declare their origin and 
meaning in various ways. There are beetles belonging 
to closely allied species, or even to the same identical 
species, which have either full-sized and perfect wings, 
or mere rudiments of membrane, which not rarely lie 
under wing-covers firmly soldered together; and hi 
these cases it is impossible to doubt, that the rudiments 
represent wings. Eudimentary organs sometimes retain 
their potentiality: this occasionally occurs with the 



256 PwUDtMEXTAEY, ATEOPHIED, [Chap. XIV 

mammae of male mararnals, which have been known to 
become well ^developed and to secrete milk. So again 
in the ndders in the genus Bos, there are normally fonr 
developed and two rudimentary teats ; but the latter in 
our domestic cows sometimes become well developed 
and yield milk. In regard to plants the petals are 
sometimes rudimentary, and sometimes well-developed 
in the individuals of the same species. In certain 
plants having separated sexes Kolreuter found that by 
crossing a species, in which the male flowers included a 
rudiment of a pistil, with an hermaphrodite species, 
having of course a well-developed pistil, the rudiment 
in the hybrid offspring was much increased in size ; 
and this clearly shows that the rudiment ary and perfect 
pistils are essentially alike in nature. An animal may 
possess various parts in a perfect state, and yet they 
may in one sense be rudimentary, for they are useless : 
thus the tadpole of the common Salamander or Water- 
newt, as Mr. Gr. H. Lewes remarks, "has gills, and 
"passes it- existence in the water; but the Salamandra 
"atra, which lives high up among the mountains, brings 
" forth its young full-formed. This animal never lives 
" in the water. Yet if we open a gravid female, we 
u . find tadpoles inside her with exquisitely feathered 
" gills ; and when placed in water they swim about 
" like the tadpoles of the water-newt. Obviously this 
" aquatic organisation has no reference to the future 
'• life of the animal, nor has it any adaptation to its 
"embryonic condition; it has solely reference to ances- 
'• tral adaptations, it repeats a phase in the development 
" of its progenitors." 

An organ, serving for two purposes, may become 
rudimentary or utterly aborted for one, even the more 



Chap. XIY.] AND ABORTED OEGANS. 257 

important purpose, and remain perfectly efficient for 
the other. Thus in plants, the office of the pistil is to 
allow the pollen-tubes to reach the ovules within the 
ovarium. The pistil consists' of a stigma supported 
on a style ; but in some Composite, the male florets, 
which of course cannot be fecundated, have a rudimen- 
tary pistil, for it is not crowned with a stigma ; but the 
style remains well developed and is clothed in the usual 
manner with hairs, which serve to brush the pollen out 
of the surrounding and conjoined anthers. Again, an 
organ may become rudimentary for its proper purpose, 
and be used for a distinct one: in certain fishes the 
swim-bladder seems to be rudimentary for its proper 
function of giving buoyancy, but has become converted 
into a nascent breathing organ or lung. Many similar 
instances could be given. 

Useful organs, however little they may be developed, 
unless we have reason to suppose that they were 
formerly more highly developed, ought not to be con- 
sidered as rudimentary. They may be in a nascent 
condition, and in progress towards further development. 
Eudimentary organs, on the other hand, are either quite 
useless, such as teeth which never cut through the 
gums, or almost useless, such as the wings of an ostrich, 
which serve merely as sails. As organs in this con- 
dition would formerly, when still less developed, have 
been of even less use than at present, they cannot 
formerly have been produced through variation and 
natural selection, which acts solely by the preservation 
of useful modifications. They have been partially 
retained by the power of inheritance, and relate to a 
former state of things. It is, however, often difficult to 
distinguish between rudimentary and nascent organs ; 



258 KUDIMENTABY, ATKOPHIED, [Chap. XIV. 

for we can judge only by analogy whether a part is 
capable of further development, in which case alone it 
deserves to be called nascent. Organs in this condition 
will always be somewhat rare ; for beings thus provided 
will commonly have been supplanted by their successors 
with the same organ in a more perfect state, and conse- 
quently will have become long ago extinct. The wing 
of the penguin is of high service, acting as a fin; it 
may, therefore, represent the nascent state of the wing : 
not that I believe this to be the case ; it is more pro- 
bably a reduced organ, modified for a new function : the 
wing of the Apteryx, on the other hand, is quite useless, 
and is truly rudimentary. Owen considers the simple 
filamentary limbs of the Lepidosiren as the " beginnings 
of organs which attain full functional development in 
higher vertebrates ; " but, according to the view lately 
advocated by Dr. Giinther, they are probably remnants, 
consisting of the persistent axis of a fin, with the lateral 
rays or branches aborted. The mammary glands of the 
Ornithorhynchus may be considered, in comparison 
with the adders of a cow, as in a nascent condition. 
The ovigerous frena of certain cirripedes, which have 
ceased to give attachment to the ova and are feebly 
developed, are nascent branchia}. 

Rudimentary organs in the individuals of the same 
species are very liable to vary in the degree of their 
development and in other respects. In closely allied 
species, also, the extent to which the same organ has 
been reduced occasionally differs much. This latter fact 
is well exemplified in the state of the wings of female 
moths belonging to the same family. Rudimentary 
organs may be utterly aborted ; and this implies, that 
in certain animals or plants, parts are entirely absent 



Chap. XIV.] AND ABOKTED OKGANS. 259 

which analogy would lead us to expect to find in them, 
and which are occasionally found in monstrous indivi- 
duals. Thus in most of the Scrophulariacese the fifth 
stamen is utterly aborted ; yet we may conclude that a 
fifth stamen once existed, for a rudiment of it is found 
in many species of the family, and this rudiment 
occasionally becomes perfectly developed, as may some- 
times be seen in the common snap-dragon. In tracing 
the homologies of any part in different members of the 
same class, nothing is more common, or, in order fully 
to understand the relations of the parts, more useful 
than the discovery of rudiments. This is well shown 
in the drawings given by Owen of the leg-bones of the 
horse, ox, and rhinoceros. 

It is an important fact that rudimentary organs, such 
as teeth in the upper jaws of whales and ruminants, can 
often be detected in the embryo, but afterwards wholly 
disappear. It is also, I believe, a universal rule, that 
a rudimentary part is of greater size in the embryo 
relatively to the adjoining parts, than in the adult ; so 
that the organ at this early age is less rudimentary, or 
even cannot be said to be in any degree rudimentary. 
Hence rudimentary organs in the adult are often said 
to have retained their embryonic condition. 

I have now given the leading facts with respect 
to rudimentary organs. In reflecting on them, every 
one must be struck with astonishment ; for the same 
reasoning power which tells us that most parts and 
organs are exquisitely adapted for certain purposes, tells 
us with equal plainness that these rudimentary or 
atrophied organs are imperfect and useless. In works 
on natural history, rudimentary organs are generally 
said to have been created " for the sake of symmetry," 
40 



260 SEDIMENTARY, ATROPHIED, [Chap. XW 

or in order " to complete the scheme of nature." But 
this is not an explanation, merely a re-statement of the 
fact. Xor is it consistent with itself: thus the boa- 
constrictor has rudiments of hind-limbs and of a pelvis, 
and if it be said that these bones have been retained 
" to complete the scheme of nature," why, as Professor 
Weismann asks, have they not been retained by other 
snakes, which do not possess even a vestige of these 
same bones ? What would be thought of an astronomer 
who maintained that the satellites revolve in elliptic 
courses round their planets " for the sake of symmetry," 
because the planets thus revolve round the sun ? An 
eminent physiologist accounts for the presence of rudi- 
mentary organs, by supposing that they serve to excrete 
matter in excess, or matter injurious to the system ; but 
can we suppose that the minute papilla, which often 
represents the pistil in male flowers, and which is 
formed of mere cellular tissue, can thus act ? Can we 
suppose that rudimentary teeth, which are subsequently 
absorbed, are beneficial to the rapidly growing embryonic 
calf by removing matter so precious as phosphate of 
lime ? When a man's fingers have been amputated, 
imperfect nails have been known to appear on the 
stumps, and I could as soon believe that these vestiges 
of nails are developed in order to excrete horny matter, 
as that the rudimentary nails on the fin of the manatee 
have been developed for this same purpose. 

On the view of descent with modification, the origin 
of rudimentary organs is comparatively simple ; and we 
can understand to a large extent the laws governing 
their imperfect development. We have plenty of cases 
of rudimentary organs in our domestic productions, — as 
the stump of a tail in tailless breeds, — the vestige of an 



Chap. XIV.] AND ABOETED OKGANS. 261 

ear in earless breeds of sheep, — the reappearance of 
minute dangling horns in hornless breeds of cattle, 
more especially, according to Youatt, in young animals, 
— and the state of the whole flower in the cauliflower. 
We often see rudiments of various parts in monsters ; 
but I doubt whether any of these cases throw light on 
the origin of rudimentary organs in a state of nature, 
further than by showing that rudiments can be pro- 
duced ; for the balance of evidence clearly indicates that 
species under nature do not undergo great and abrupt 
changes. But we learn from the study of our domestic 
productions that the disuse of parts leads to their 
reduced size ; and that the result is inherited. 

It appears probable that disuse has been the main 
agent in rendering organs rudimentary. It would at 
first lead by slow steps to the more and more complete 
reduction of a part, until at last it became rudimentary, 
— as in the case of the eyes of animals inhabiting dark 
caverns, and of the wings of birds inhabiting oceanic 
islands, which have seldom been forced by beasts of prey 
to take flight, and have ultimately lost the power of 
flying. Again, an organ, useful under certain conditions, 
might become injurious under others, as with the wings 
of beetles living on small and exposed islands ; and in 
this case natural selection will have aided in reducing 

o 

the organ, until it was rendered harmless and rudi- 
mentary. 

Any change in structure and function, which can be 
effected by small stages, is within the power of natural 
selection ; so that an organ rendered, through changed 
habits of life, useless or injurious for one purpose, might 
be modified and used for another purpose. An organ 
might, also, be retained for one alone of its former func- 



262 KUDIMEXTAEY, ATEOPHIED, [Chap. XIV. 

tions. Organs, originally formed by the aid of natural 
selection, when rendered useless may well be variable, 
for their variations can no longer be checked by natural 
selection. All this agrees well with what we see under 
nature. Moreover, at whatever period of life either disuse 
or selection reduces an organ, and this will generally 
be when the being has come to maturity and has to 
exert its full powers of action, the principle of inherit- 
ance at corresponding ages will tend to reproduce the 
organ in its reduced state at the same mature age, but 
will seldom affect it in the embryo. Thus we can 
understand the greater size of rudimentary organs in 
the embryo relatively to the adjoining parts, and their 
lesser relative size in the adult. If, for instance, the 
digit of an adult animal was used less and less during 
many generations, owing to some change of habits, or if 
an organ or gland was less and less functionally exercised, 
we may infer that it would become reduced in size in the 
adult descendants of this animal, but would retain nearly 
its original standard of development in the embryo. 

There remains, however, this difficulty. After an 
organ has ceased being used, and has become in con- 
sequence much reduced, how can it be still further 
reduced in size until the merest vestige is left; and 
how can it be finally quite obliterated ? It is scarcely 
possible that disuse can go on producing any further 
effect after the organ has once been rendered functionless. 
Some additional explanation is here requisite which I 
cannot give. If, for instance, it could be proved that 
every part of the organisation tends to vary in a greater 
decree towards diminution than towards augmentation 
of size, then we should be able to understand how an 
organ which has become useless would be rendered, in- 



Chap. XIV.] AND ABORTED ORGANS. 263 

dependency of the effects of disuse, rudimentary and 
would at last be wholly suppressed ; for the variations 
towards diminished size would no longer be checked by 
natural selection. The principle of the economy of 
growth, explained in a former chapter, by which the 
materials forming any part, if not useful to the pos- 
sessor, are saved as far as is possible, will perhaps come 
into play in rendering a useless part rudimentary. But 
this principle will almost necessarily be confined to the 
earlier stages of the process of reduction ; for we cannot 
suppose that a minute papilla, for instance, representing 
in a male flower the pistil of the female flower, and 
formed merely of cellular tissue, could be further reduced 
or absorbed for the sake of economising nutriment. 

Finally, as rudimentary organs, by whatever steps 
they may have been degraded into their present useless 
condition, are the record of a former state of things, and 
have been retained solely through the power of inherit- 
ance, — we can understand, on the genealogical view of 
classification, how it is that systematists, in placing 
organisms in their proper places in the natural system, 
have often found rudimentary parts as useful as, or even 
sometimes more useful than, parts of high physiological 
importance. Eudimentary organs may be compared 
with the letters in a word, still retained in the spelling, 
but become useless in the pronunciation, but which serve 
as a clue for its derivation. On the view of descent 
with modification, we may conclude that the existence 
of organs in a rudimentary, imperfect, and useless con- 
dition, or quite aborted, far from presenting a strange 
difficulty, as they assuredly do on the old doctrine of 
creation, might even have been anticipated in accordance 
with the views here explained. 



264 SUMMAEY. [Chap. XIY. 



Summary, 

In this chapter I have attempted to show, that the 
arrangement of all organic beings throughout all time 
in groups under groups — that the nature of the relation- 
ships by which all living and extinct organisms are 
united by complex, radiating, and circuitous lines of 
affinities into a few grand classes, — the rules followed 
and the difficulties encountered by naturalists in their 
classifications, — the value set upon characters, if constant 
and prevalent, whether of high or of the most trifling 
importance, or, as with rudimentary organs, of no 
importance, — the wide opposition in value between 
analogical or adaptive characters, and characters of true 
affinity ; and other such rules ; — all naturally follow if 
we admit the common parentage of allied forms, together 
with their modification through variation and natural 
selection, with the contingencies of extinction and di- 
vergence of character. In considering this view of classifi- 
cation, it should be borne in mind that the element of 
descent has been universally used in ranking together 
the sexes, ages, dimorphic forms, and acknowledged 
varieties of the same species, however much they may 
differ from each other in structure. If we extend the 
use of this element of descent, — the one certainly 
known cause of similarity in organic beings, — we shall 
understand what is meant by the Natural System : 
it is genealogical in its attempted arrangement, 
with the grades of acquired difference marked by 
the terms, varieties, species, genera, families, orders, 
and classes. 

On this same view of descent with modification, most 



Chap. XIV.] SUMMARY. 265 

of the great facts in Morphology become intelligible, — 
whether we look to the same pattern displayed by the 
different species of the same class in their homologous 
organs, to whatever purpose applied; or to the serial 
and lateral homologies in each individual animal and 
plant. 

On the principle of successive slight variations, not 
necessarily or generally supervening at a very early 
period of life, and being inherited at a corresponding 
period, we can understand the leading facts in Embry- 
ology ; namely, the close resemblance in the individual 
embryo of the parts which are homologous, and which 
when matured become widely different in structure and 
function ; and the resemblance of the homologous parts 
or organs in allied though distinct species, though fitted 
in the adult state for habits as different as is possible. 
Larva? are active embryos, which have been specially 
modified in a greater or less degree in relation to their 
habits of life, with their modifications inherited at a 
corresponding early age. On these same principles, — 
and bearing in mind that when organs are reduced in 
size, either from disuse or through natural selection, it 
will generally be at that period of life when the being 
has to provide for its own wants, and bearing in mind 
how strong is the force of inheritance — the occurrence 
of rudimentary organs might even have been anticipated. 
The importance of embryological characters and of rudi- 
mentary organs in classification is intelligible, on the 
view that a natural arrangement must be genealogical. 

Finally, the several classes of facts which have been 
considered in this chapter, seem to me to proclaim so 
plainly, that the innumerable species, genera and 
families, with which this world is peopled, are all 



266 SUMMAEY. [Chap. XIV. 

descended, each within its own class or group, from 
common parents, and have all been modified in the 
course of descent, that I should without hesitation adopt 
this view, even if it were unsupported by other facts or 
arguments. 



Chap. XV J RECAPITULATION. 267 



CHAPTEE XV. 

Eecapitulation and Conclusion. 

Recapitulation of the objections to the theory of Natural Selection 
— Eecapitulation of the general and special circumstances in its 
favour — Causes of the general belief in the immutability of 
species — How far the theory of Natural Selection may be 
extended — Effects of its adoption on the study of Natural 
History — Concluding remarks. 

As this whole volume is one long argument, it may 
be convenient to the reader to have the leading facts 
and inferences briefly recapitulated. 

That many and serious objections may be advanced 
against the theory of descent with modification through 
variation and natural selection, I do not deny. I have 
endeavoured to give to them their full force. Nothing 
at first can appear more difficult to believe than that 
the more complex organs and instincts have been per- 
fected, not by means superior to, though analogous with, 
human reason, but by the accumulation of innumerable 
slight variations, each good for the individual possessor. 
Nevertheless, this difficulty, though appearing to our 
imagination insuperably great, cannot be considered 
real if we admit the following propositions, namely, 
that all parts of the organisation and instincts offer, at 
least, individual differences — that there is a struggle for 
existence leading to the preservation of profitable devia- 
tions of structure or instinct — and, lastly, that gradations 



268 RECAPITULATION. [Chap. XV. 

in the state of perfection of each organ may have existed, 
each good of its kind. The truth of these propositions 
cannot, I think, be disputed. 

It is, no doubt, extremely difficult even to conjecture 
by what gradations many structures have been perfected, 
more especially amongst broken and failing groups of 
organic beings, which have suffered much extinction; 
but we see so many strange gradations in nature, that 
we ought to be extremely cautious in saying that any 
organ or instinct, or any whole structure, could not have 
arrived at its present state by many graduated steps. 
There are, it must be admitted, cases of special difficulty 
opposed to the theory of natural selection ; and one of 
the most curious of these is the existence in the same 
community of two or three defined castes of workers or 
sterile female ants ; but I have attempted to show how 
these difficulties can be mastered. 

"With respect to the almost universal sterility of 
species when first crossed, which forms so remarkable a 
contrast with the almost universal fertility of varieties 
when crossed, I must refer the reader to the recapitula- 
tion of the facts given at the end of the ninth chapter, 
which seem to me conclusively to show that this sterility 
is no more a special endowment than is the incapacity of 
two distinct kinds of trees to be grafted together ; but 
that it is incidental on differences confined to the repro- 
ductive systems of the intercrossed species. We see the 
truth of this conclusion in the vast difference in the 
results of crossing the same two species reciprocally, — 
that is, when one species is first used as the father and 
then as the mother. Analogy from the consideration of 
dimorphic and trimorphic plants clearly leads to the 
same conclusion, for when the forms are illegitimately 



Chap. XV.] EE CAPITULATION. 269 

united, they yield few or no seed, and their offspring are 
more or less sterile ; and these forms belong to the same 
■undoubted species, and differ from each other in no respect 
except in their reproductive organs and functions. 

Although the fertility of varieties when intercrossed 
and of their mongrel offspring has been asserted by so 
many authors to be universal, this cannot be considered 
as quite correct after the facts given on the high 
authority of Gartner and Kolreuter. Most of the varieties 
which have been experimented on have been produced 
under domestication ; and as domestication (I do not 
mean mere confinement) almost certainly tends to elimi- 
nate that sterility which, judging from analogy, would 
have affected the parent-species if intercrossed, we ought 
not to expect that domestication would likewise induce 
sterility in their modified descendants when crossed. 
This elimination of sterility apparently follows from 
the same cause which allows our domestic animals to 
breed freely under diversified circumstances ; and this 
again apparently follows from their having been gradu- 
ally accustomed to frequent changes in their conditions 
of life. 

A double and parallel series of facts seems to throw 
much light on the sterility of species, when first crossed, 
and of their hybrid offspring. On the one side, there is 
good reason to believe that slight changes in the con- 
ditions of life give vigour and fertility to all organic 
beings. We know also that a cross between the distinct 
individuals of the same variety, and between distinct 
varieties, increases the number of their offspring, and 
certainly gives to them increased size and vigour. This 
is chiefly owing to the forms which are crossed having 
been exposed to somewhat different conditions of life ; 



270 RECAPITULATION. [Chap. XV. 

for I have ascertained by a laborious series of experi- 
ments that if all the individuals of the same variety 
be subjected during several generations to the same 
conditions, the good derived from crossing is often much 
diminished or wholly disappears. This is one side of 
the case. On the other side, we know that species 
which have long been exposed to nearly uniform condi- 
tions, when they are subjected under confinement to 
new and greatly changed conditions, either perish, or 
if they survive, are rendered sterile, though retaining 
perfect health. This does not occur, or only in a very 
slight degree, with our domesticated productions, which 
have long been exposed to fluctuating conditions. Hence 
when we find that hybrids produced by a cross between 
two distinct species are few in number, owing to their 
perishing soon after conception or at a very early age, 
or if surviving that they are rendered more or less sterile, 
it seems highly probable that this result is due to their 
having been in fact subjected to a great change in their 
conditions of life, from being compounded of two distinct 
organisations. He who will explain in a definite manner 
why, for instance, an elephant or a fox will not breed 
under confinement in its native country, whilst the 
domestic pig or dog will breed freely under the most 
diversified conditions, will at the same time be able to 
give a definite answer to the question why two distinct 
species, when crossed, as well as their hybrid offspring, 
arc generally rendered more or less sterile, whilst two 
dom I varieties when crossed and their mongrel 

offspring are perfectly fertile. 

Turning to geographical distribution, the difficulties 
encountered on the theory of descent with modification 
are serious enough. All the individuals of the same 



Chap. XV.] RECAPITULATION. 271 

species, and all the species of the same genus, or even 
higher group, are descended from common parents ; and 
therefore, in however distant and isolated parts of the 
world they may now be found, they must in the course 
of successive generations have travelled from some one 
point to all the others. We are often wholly unable even 
to conjecture how this could have been effected. Yet, 
as we have reason to believe that some species have 
retained the same specific form for very long periods of 
time, immensely long as measured by years, too much 
stress ought not to be laid on the occasional wide diffu- 
sion of the same species ; for during very long periods 
there will always have been a good chance for wide 
migration by many means. A broken or interrupted 
range may often be accounted for by the extinction of 
the species in the intermediate regions. It cannot be 
denied that we are as yet very ignorant as to the full 
extent of the various climatal and geographical changes 
which have affected the earth during modern periods ; 
and such changes will often have facilitated migration. 
As an example, I have attempted to show how potent 
has been the influence of the Glacial period on the dis- 
tribution of the same and of allied species throughout 
the world. We are as yet profoundly ignorant of the 
many occasional means of transport. With respect to 
distinct species of the same genus inhabiting distant 
and isolated regions, as the process of modification has 
necessarily been slow, all the means of migration will 
have been possible during a very long period; and 
consequently the difficulty of the wide diffusion of the 
species of the same genus is in some degree lessened. 

As according to the theory of natural selection an 
interminable number of intermediate forms must have 
41 



272 RECAPITULATION. [Chap. XV. 

existed, linking together all the species in each group by 
gradations as fine as are our existing varieties, it may be 
asked, Why do we not see these linking forms all around 
us ? Whv are not all organic beings blended together 
in an inextricable chaos ? With respect to existing forms, 
we should remember that we have no right to expect 
(excepting in rare cases) to discover directly connecting 
links between them, but only between each and some 
extinct and supplanted form. Even on a wide area, 
which has during a long period remained continuous, 
and of which the climatic and other conditions of life 
change insensibly in proceeding from a district occupied 
by one species into another district occupied by a closely 
allied species, we have no just right to expect often to 
find intermediate varieties in the intermediate zones. 
For we have reason to believe that only a few species 
of a genus ever undergo change ; the other species 
becoming utterly extinct and leaving no modified 
progeny. Of the species which do change, only a few 
within the same country change at the same time ; and 
all modifications are slowly effected. I have also shown 
that the intermediate varieties which probably at first 
existed in the intermediate zones, would be liable to be 
supplanted by the allied forms on either hand ; for the 
latter, from existing in greater numbers, would generally 
be modified and improved at a quicker rate than the 
intermediate varieties, which existed in lesser numbers ; 
so that the intermediate varieties would, in the long 
run, be supplanted and exterminated. 

On this doctrine of the extermination of an infinitude 
of connecting links, between the living and extinct in- 
habitants of the world, and at each successive period 
between the extinct and still older species, why is not 



Chap. XV.] RECAPITULATION. 273 

every geological formation charged with such links ? 
Why does not every collection of fossil remains afford 
plain evidence of the gradation and mutation of the forms 
of life ? Although geological research has undoubtedly 
revealed the former existence of many links, bringing 
numerous forms of life much closer together, it does not 
yield the infinitely many fine gradations between past 
and present species required on the theory ; and this is 
the most obvious of the many objections which may be 
urged against it. Why, again, do whole groups of allied 
species appear, though this appearance is often false, to 
have come in suddenly on the successive geological 
stages ? Although we now know that organic beings 
appeared on this globe, at a period incalculably remote, 
long before the lowest bed of the Cambrian system 
was deposited, why do we not find beneath this system 
great piles of strata stored with the remains of the pro- 
genitors of the Cambrian fossils ? For on the theory, 
such strata must somewhere have been deposited at 
these ancient and utterly unknown epochs of the world's 
history. 

I can answer these questions and objections only on 
the supposition that the geological record is far more im- 
perfect than most geologists believe. The number of 
specimens in all our museums is absolutely as nothing 
compared with the countless generations of countless 
species which have certainly existed. The parent-form 
of any two or more species would not be in all its char- 
acters directly intermediate between its modified off- 
spring, any more than the rock-pigeon is directly inter- 
mediate in crop and tail between its descendants, the 
pouter and fantail pigeons. We should not be able 
to recognise a species as the parent of another and 



274 RECAPITULATION. [Chap. XV. 

modified species, if we were to examine the two ever so 
closely, unless we possessed most of the intermediate 
links ; and owing to the imperfection of the geological 
record, we have no just right to expect to find so many- 
links. If two or three, or even more linking forms were 
discovered, they would simply be ranked by many 
naturalists as so many new species, more especially if 
found in different geological sub-stages, let their dif- 
ferences be ever so slight. Xumerous existing doubtful 
forms could be named which are probably varieties ; 
but who will pretend that in future ages so many fossil 
links will be discovered, that naturalists will be able to 
decide whether or not these doubtful forms ought to be 
called varieties ? Only a small portion of the world 
has been geologically explored. Only organic beings of 
certain classes can be preserved in a fossil condition, at 
least in any great number. Many species when once 
formed never undergo any further change but become 
extinct without leaving modified descendants ; and the 
periods, during which species have undergone modifica- 
tion, though long as measured by years, have probably 
been short in comparison with the periods during winch 
they retained the same form. It is the dominant and 
widely ranging species which vary most frequently and 
vary most, and varieties are often at first local — both 
causes rendering the discovery of intermediate links in 
any one formation less likely. Local varieties will 
not spread into other and distant regions until they are 
considerably modified and improved; and when they 
have spread, and are discovered in a geological formation, 
they appear as if suddenly created there, and will be 
simply classed as new species. Most formations have 
been intermittent in their accumulation; and their 



Chap. XY.] RECAPITULATION. 275 

duration has probably been shorter than the average 
duration of specific forms. Successive formations are in 
most cases separated from each other by blank intervals 
of time of great length; for fossiliferous formations thick 
enough to resist future degradation can as a general rule 
be accumulated only where much sediment is deposited 
on the subsiding bed of the sea. During the alternate 
periods of elevation and of stationary level the record 
will generally be blank. During these latter periods 
there will probably be more variability in the forms of 
life ; during periods of subsidence, more extinction. 

With respect to the absence of strata rich in fossils 
beneath the Cambrian formation, I can recur only to 
the hypothesis given in the tenth chapter ; namely, that 
though our continents and oceans have endured for an 
enormous period in nearly their present relative positions, 
we have no reason to assume that this has always been 
the case ; consequently formations much older than any 
now known may lie buried beneath the great oceans. 
With respect to the lapse of time not having been suffi- 
cient since our planet was consolidated for the assumed 
amount of organic change, and this objection, as urged 
by Sir William Thompson, is probably one of the gravest 
as yet advanced, I can only say, firstly, that we do not 
know at what rate species change as measured by years, 
and secondly, that many philosophers are not as yet 
willing to admit that we know enough of the constitu- 
tion of the universe and of the interior of our globe to 
speculate with safety on its past duration. 

That the geological record is imperfect all will admit ; 
but that it is imperfect to the degree required by our 
theory, few will be inclined to admit. If we look to 
long enough intervals of time, geology plainly declares 



276 RECAPITULATION. [Chap. XV 

that species have all changed ; and they have changed 
in the manner required by the theory, for they have 
changed slowly and in a graduated manner. We clearly 
see this in the fossil remains from consecutive formations 
invariably being much more closely related to each other, 
than are the fossils from widely separated formations. 

Such is the sum of the several chief objections and 
difficulties which may be justly urged against the 
theory; and I have now briefly recapitulated the 
answers and explanations which, as far as I can see, 
may be given. I have felt these difficulties far too 
heavily during many years to doubt their weight. But 
it deserves especial notice that the more important 
objections relate to questions on winch we are con- 
fessedly ignorant ; nor do we know how ignorant we 
are. We do not know all the possible transitional 
gradations between the simplest and the most perfect 
organs ; it cannot be pretended that we know all the 
varied means of Distribution during the long lapse of 
years, or that we know how imperfect is the Geological 
Record. Serious as these several objections are, in my 
judgment they are by no means sufficient to overthrow 
the theory of descent with subsequent modification. 

Now let us turn to the other side of the argument. 
Under domestication we see much variability, caused, or 
at least excited, by changed conditions of life ; but often 
in so obscure a manner, that we are tempted to consider 
the variations as spontaneous. Variability is governed 
by many complex laws, — by correlated growth, compen- 
sation, the increased use and disuse of parts, and the 
definite action of the surrounding conditions. There is 
much difficulty in ascertaining how largely our domestic 



Chap. XV.] RECAPITULATION. 277 

productions have been modified; but we may safely 
infer that the amount has been large, and that modifica- 
tions can be inherited for long periods. As long as the 
conditions of life remain the same, we have reason to 
believe that a modification, which has already been 
inherited for many generations, may continue to be 
inherited for an almost infinite number of generations. 
On the other hand, we have evidence that variability 
when it has once come into play, does not cease under 
domestication for a very long period ; nor do we know 
that it ever ceases, for new varieties are still occasionally 
produced by our oldest domesticated productions. 

Variability is not actually caused by man ; he only 
unintentionally exposes organic beings to new condi- 
tions of life, and then nature acts on the organisation 
and causes it to vary. But man can and does select the 
variations given to him by nature, and thus accumulates 
them in any desired manner. He thus adapts animals 
and plants for his own benefit or pleasure. He may 
do this methodically, or he may do it unconsciously by 
preserving the individuals most useful or pleasing to 
him without any intention of altering the breed. It is 
certain that he can largely influence the character of a 
breed by selecting, in each successive generation, indi- 
vidual differences so slight as to be inappreciable except 
by an educated eye. This unconscious process of selec- 
tion has been the great agency in the formation of the 
most distinct and useful domestic breeds. That many 
breeds produced by man have to a large extent the 
character of natural species, is shown by the inextric- 
able doubts whether many of them are varieties or 
aboriginally distinct species. 

There is no reason why the principles which have 



278 KECAPITULATION. [Chap. XV. 

acted so efficiently under domestication should not have 
acted under nature. In the survival of favoured 
individuals and races, during the constantly -recurrent 
Struggle for Existence, we see a powerful and ever- 
acting form of Selection. The struggle for existence 
inevitably follows from the high geometrical ratio of 
increase which is common to all organic beings. This 
high rate of increase is proved by calculation, — by the 
rapid increase of many animals and plants during a 
succession of peculiar seasons, and when naturalised in 
new countries. More individuals are born than can 
possibly survive. A grain in the balance may determine 
which individuals shall live and which shall die, — which 
variety or species shall increase in number, and which 
shall decrease, or finally become extinct. As the indi- 
viduals of the same species come in all respects into the 
closest competition with each other, the struggle will 
generally be most severe between them ; it will be 
almost equally severe between the varieties of the same 
species, and next in severity between the species of the 
same genus. On the other hand the struggle will often 
be severe between beings remote in the scale of nature. 
The slightest advantage in certain individuals, at any 
age or during any season, over those with which they 
come into competition, or better adaptation in however 
slight a degree to the surrounding physical conditions, 
will, in the long run, turn the balance. 

With animals having separated sexes, there will be 
in most cases a struggle between the males for the posses- 
sion of the females. The most vigorous males, or those 
which have most successfully struggled with their con- 
ditions of life, will generally leave most progeny. But 
success will often depend on the males having special 



Chap. XV.] RECAPITULATION. 279 

weapons, or means of defence, or charms ; and a slight 
advantage will lead to victory. 

As geology plainly proclaims that each land has 
undergone great physical changes, we might have ex- 
pected to find that organic beings have varied under 
nature, in the same way as they have varied under 
domestication. And if there has been any variability 
under nature, it would be an unaccountable fact if natural 
selection had not come into play. It has often been 
asserted, but the assertion is incapable of proof, that the 
amount of variation under nature is a strictly limited 
quantity. Man, though acting on external characters 
alone and often capriciously, can produce within a short 
period a great result by adding up mere individual 
differences in his domestic productions ; and every one 
admits that species present individual differences. But, 
besides such differences, all naturalists admit that natural 
varieties exist, which are considered sufficiently distinct 
to be worthy of record in systematic works. JSTo one 
has drawn any clear distinction between individual 
differences and slight varieties ; or between more plainly 
marked varieties and sub-species, and species. On 
separate continents, and on different parts of the same 
continent when divided by barriers of any kind, and on 
outlying islands, what a multitude of forms exist, which 
some experienced naturalists rank as varieties, others as 
geographical races or sub-species, and others as distinct, 
though closely allied species ! 

If then, animals and plants do vary, let it be ever so 
slightly or slowly, why should not variations or indi- 
vidual differences, which are in any way beneficial, be 
preserved and accumulated through natural selection, or 
the survival of the fittest ? If man can by patience 



280 RECAPITULATION. [Chap. XV. 

select variations useful to him, why, under changing 
and complex conditions of life, should not variations 
useful to nature's living products often arise, and be 
preserved or selected ? What limit can be put to this 
power, acting during long ages and rigidly scrutinising 
the whole constitution, structure, and habits of each 
creature, — favouring the good and rejecting the bad ? I 
can see no limit to this power, in slowly and beautifully 
adapting each form to the most complex relations of life. 
The theory of natural selection, even if we look no farther 
than this, seems to be in the highest degree probable. 
I have already recapitulated, as fairly as I could, the 
opposed dithculties and objections : now let us turn to 
the special facts and arguments in favour of the theory. 

On the view that species are only strongly marked 
and permanent varieties, and that each species first 
existed as a variety, we can see why it is that no line 
of demarcation can be drawn between species, commonly 
supposed to have been produced by special acts of crea- 
tion, and varieties which are acknowledged to have been 
pr« kduced by secondary laws. On this same view we can 
understand how it is that in a region where many species 
of a genus have been produced, and where they now 
flourish, these same species should present many varieties; 
for where the manufactory of species has been active, we 
might expect, as a general rule, to find it still in action ; 
and this is the case if varieties be incipient species 
Moreover, the species of the larger genera, which afford 
the greater number of varieties or incipient species, 
retain to a certain degree the character of varieties ; for 
they differ from each other by a less amount of difference 
than do the species of smaller genera. The closely allied 



Chap. XV.] RECAPITULATION. 281 

species also of the larger genera apparently have re- 
stricted ranges, and in their affinities they are clustered 
in little groups round other species — in both respects 
resembling varieties. These are strange relations on 
the view that each species was independently created, 
but are intelligible if each existed first as a variety. 

As each species tends by its geometrical rate of repro- 
duction to increase inordinately in number ; and as the 
modified descendants of each species will be enabled to 
increase by as much as they become more diversified in 
habits and structure, so as to be able to seize on many 
and widely different places in the economy of nature, 
there will be a constant tendency in natural selection 
to preserve the most divergent offspring of any one 
species. Hence, during a long-continued course of 
modification, the slight differences characteristic of 
varieties of the same species, tend to be augmented into 
the greater differences characteristic of the species of the 
same genus. New and improved varieties will inevitably 
supplant and exterminate the older, less improved, and 
intermediate varieties ; and thus species are rendered to 
a large extent defined and distinct objects. Dominant 
species belonging to the larger groups within each class 
tend to give birth to new and dominant forms ; so that 
each large group tends to become still larger, and at the 
same time more divergent in character. But as all 
groups cannot thus go on increasing in size, for the 
world would not hold them, the more dominant groups 
beat the less dominant. This tendency in the large 
groups to go on increasing in size and diverging in char- 
acter, together with the inevitable contingency of much 
extinction, explains the arrangement of all the forms of 
life in groups subordinate to groups, all within a few great 



282 EECAPITULATION. [Chap. XV. 

classes, which has prevailed throughout all time. This 
grand fact of the grouping of all organic beings under 
what is called the Natural System, is utterly inexplic- 
able on the theory of creation. 

As natural selection acts solely by accumulating slight, 
successive, favourable variations, it can produce no great 
or sudden modifications ; it can act only by short and 
slow steps. Hence, the canon of "datura non facit 
sakum," which every fresh addition to our knowledge 
tends to confirm, is on this theory intelligible. "We can 
see why throughout nature the same general end is 
gained by an almost infinite diversity of means, for every 
peculiarity when once acquired is long inherited, and 
structures already modified in many different ways have 
to be adapted for the same general purpose. Wq can, 
in short, see why nature is prodigal in variety, though 
niggard in innovation. But why this should be a law 
of nature if each species has been independently created 
no man can explain. 

Many other facts are, as it seems to me, explicable on 
this theory. How strange it is that a bird, under the 
form of a woodpecker, should prey on insects on the 
ground ; that upland geese which rarely or never swim, 
should possess webbed feet; that a thrush-like bird 
should dive and feed on sub-aquatic insects ; and that a 
petrel should have the habits and structure fitting it for 
the life of an auk ! and so in endless other cases. But 
on the view of each species constantly trying to increase 
in number, with natural selection always ready to adapt 
the slowly varying descendants of each to any un- 
occupied or ill-occupied place in nature, these facts cease 
to be strange, or might even have been anticipated. 

We can to a certain extent understand how it is that 



Chap. XV.] RECAPITULATION. 283 

there is so much beauty throughout nature ; for this 
may be largely attributed to the agency of selection. 
That beauty, according to our sense of it, is not universal, 
must be admitted by every one who will look at some 
venomous snakes, at some fishes, and at certain hideous 
bats with a distorted resemblance to the human face. 
Sexual selection has given the most brilliant colours, 
elegant patterns, and other ornaments to the males, and 
sometimes to both sexes of many birds, butterflies, and 
other animals. With birds it has often rendered the 
voice of the male musical to the female, as well as to 
our ears. Flowers and fruit have been rendered con- 
spicuous by brilliant colours in contrast with the green 
foliage, in order that the flowers may be easily seen, 
visited, and fertilised by insects, and the seeds dis- 
seminated by birds. How it comes that certain colours, 
sounds, and forms should give pleasure to man and the 
lower animals, — that is, how the sense of beauty in its 
simplest form was first acquired, — we do not know any 
more than how certain odours and flavours were first 
rendered agreeable. 

As natural selection acts by competition, it adapts 
and improves the inhabitants of each country only in 
relation to their co-inhabitants ; so that we need feel no 
surprise at the species of any one country, although on 
the ordinary view supposed to have been created and 
specially adapted for that country, being beaten and 
supplanted by the naturalised productions from another 
land. Nor ought we to marvel if all the contrivances 
in nature be not, as far as we can judge, absolutely 
perfect, as in the case even of the human eye ; or if 
some of them be abhorrent to our ideas of fitness. We 

need not marvel at the sting of the bee, when used 
42 



284 RECAPITULATION. [Chap. XV. 

against an enemy, causing the bee's own death; at 
drones being produced in such great numbers for one 
single act, and being then slaughtered by their sterile 
sisters ; at the astonishing waste of pollen by our fir- 
trees ; at the instinctive hatred of the queen-bee for her 
own fertile daughters ; at ichneumonidse feeding within 
the living bodies of caterpillars ; or at other such cases. 
The wonder indeed is, on the theory of natural selection, 
that more cases of the want of absolute perfection have 
not been detected. 

The complex and little known laws governing the 
production of varieties are the same, as far as we can 
judge, with the laws which have governed the production 
of distinct species. In both cases physical conditions 
seem to have produced some direct and definite effect, 
but how much we cannot say. Thus, when varieties 
enter any new station, they occasionally assume some 
of the characters proper to the species of that station. 
With both varieties and species, use and disuse seem to 
have produced a considerable effect ; for it is impossible 
to resist this conclusion when we look, for instance, at 
the logger-headed duck, which has wings incapable of 
flight, in nearly the same condition as in the domestic 
duck ; or when we look at the burrowing tucu-tucu, 
which is occasionally blind, and then at certain moles, 
which are habitually blind and have their eyes covered 
with skin ; or when we look at the blind animals in- 
habiting the dark caves of America and Europe. "With 
varieties and species, correlated variation seems to have 
plaved an important part, so that when one part has 
been modified other parts have been necessarily modified. 
With both varieties and species, reversions to long-lost 
characters occasionally occur. How inexplicable on the 



Chap. XV.] RECAPITULATION. 285 

theory of creation is the occasional appearance of stripes 
on the shoulders and legs of the several species of the 
horse-genus and of their hybrids ! How simply is this 
fact explained if we believe that these species are all 
descended from a striped progenitor, in the same .manner 
as the several domestic breeds of the pigeon are descended 
from the blue and barred rock-pigeon ! 

On the ordinary view of each species having been 
independently created, why should specific characters, 
or those by which the species of the same genus 
differ from each other, be more variable than generic 
characters in which they all agree ? Why, for instance, 
should the colour of a flower be more likely to vary in 
any one species of a genus, if the other species possess 
differently coloured flowers, than if all possessed the 
same coloured flowers ? If species are only well-marked 
varieties, of which the characters have become in a high 
degree permanent, we can understand this fact ; for they 
have already varied since they branched off from a 
common progenitor in certain characters, by which they 
have come to be specifically distinct from each other ; 
therefore these same characters would be more likely 
again to vary than the generic characters which have 
been inherited without change for an immense period. 
It is inexplicable on the theory of creation why a part 
developed in a very unusual manner in one species 
alone of a genus, and therefore, as we may naturally infer, 
of great importance to that species, should be eminently 
liable to variation ; but, on our view, this part has under- 
gone, since the several species branched off from a 
common progenitor, an unusual amount of variability 
and modification, and therefore we might expect the 
part generally to be still variable. But a part may be 



286 RECAPITULATION. [Chap. XT. 

developed in the most unusual manner, like the wing 
of a bat, and yet not be more variable than any other 
structure, if the part be common to many subordinate 
forms, that is, if it has been inherited for a very long 
period ; for in this case it will have been rendered con- 
stant by long-continued natural selection. 

Glancing at instincts, marvellous as some are, they 
offer no greater difficulty than do corporeal structures 
on the theory of the natural selection of successive, slight, 
but profitable modifications. We can thus understand 
why nature moves by graduated steps in endowing 
different animals of the same class with their several 
instincts. I have attempted to show how much light 
the principle of gradation throws on the admirable archi- 
tectural powers of the hive- bee. Habit no doubt often 
comes into play in modifying instincts ; but it certainly 
is not indispensable, as we see in the case of neuter 
insects, which leave no progeny to inherit the effects of 
long-continued habit. On the view of all the species of 
the same genus having descended from a common parent, 
and having inherited much in common, we can under- 
stand how it is that allied species, when placed under 
widely different conditions of life, yet follow nearly the 
same instincts: why the thrushes of tropiealand temperate 
South America, for instance, line their nests with mud 
like our British species. On the view of instincts having 
been slowly acquired through natural selection, we need 
not marvel at some instincts being not perfect and liable 
to mistakes, and at many instincts causing other animals 
to suffer. 

If species be only well-marked and permanent varie- 
ties, we can at once see why their crossed offspring 
should follow the same complex laws in their degrees 



Chap. XV.] KECAPITULATION. 287 

and kinds of resemblance to their parents, — in being 
absorbed into each other by successive crosses, and in 
other such points, — as do the crossed offspring of ac- 
knowledged varieties. This similarity would be a strange 
fact, if species had been independently created and 
varieties had been produced through secondary laws. 

If we admit that the geological record is imperfect 
to an extreme degree, then the facts, which the record 
does give, strongly support the theory of descent with 
modification. New species have come on the stage slowly 
and at successive intervals ; and the amount of change, 
after equal intervals of time, is widely different in dif- 
ferent groups. The extinction of species and of whole 
groups of species, which has played so conspicuous a 
part in the history of the organic world, almost inevitably 
follows from the principle of natural selection ; for old 
forms are supplanted by new and improved forms. 
Neither single species nor groups of species reappear 
when the chain of ordinary generation is once broken. 
The gradual diffusion of dominant forms, with the slow 
modification of their descendants, causes the forms of 
life, after long intervals of time, to appear as if they had 
changed simultaneously throughout the world. The fact 
of the fossil remains of each formation being in some 
degree intermediate in character between the fossils in 
the formations above and below, is simply explained by 
their intermediate position in the chain of descent. The 
grand fact that all extinct beings can be classed with 
all recent beings, naturally follows from the living and 
the extinct being the offspring of common parents. As 
species have generally diverged in character during their 
long course of descent and modification, we can under^ 
stand why it is that the more ancient forms, or early 



288 EECAPITULATIOff. [Chap. XV. 

progenitors of each group, so often occupy a position 
in some degree intermediate between existing groups. 
Eecent forms are generally looked upon as being, on the 
-whole, higher in the scale of organisation than ancient 
forms ; and they must be higher, in so far as the later 
and more improved forms have conquered the older and 
less improved forms in the struggle for life ; they have 
also generally had their organs more specialised for dif- 
ferent functions. This fact is perfectly compatible with 
numerous beings still retaining simple and but little 
improved structures, fitted for simple conditions of life ; 
it is likewise compatible with some forms having retro- 
graded in organisation, by having become at each stage 
of descent better fitted for new and degraded habits of 
life. Lastly, the wonderful law of the long endurance 
of allied forms on the same continent, — of marsupials 
in Australia, of edentata in America, and other such 
cases, — is intelligible, for within the same country the 
existing and the extinct will be closely allied by descent. 
Looking to geographical distribution, if we admit that 
there has been during the long course of ages much migra- 
tion from one part of the world to another, owing to 
former climatal and geographical changes and to the 
many occasional and unknown means of dispersal, then 
we can understand, on the theory of descent with modi- 
fication, most of the great leading facts in Distribution. 
We can see why there should be so striking a parallelism 
in the distribution of organic beings throughout space, 
and in their geological succession throughout time ; for 
in both cases the beings have been connected by the 
bond of ordinary generation, and the means of modifica- 
tion have been the same. AYe see the full meaning of 
the wonderful fact, which has struck every traveller 



Chap. XV.] RECAPITULATION. 289 

namely, that on the same continent, tinder the most 
diverse conditions, nnder heat and cold, on mountain 
and lowland, on deserts and marshes, most of the inhabit- 
ants within each great class are plainly related ; for they 
are the descendants of the same progenitors and early 
colonists. On this same principle of former migration, 
combined in most cases with modification, we can under- 
stand, by the aid of the Glacial period, the identity of 
some few plants, and the close alliance of many others, 
on the most distant mountains, and in the northern and 
southern temperate zones ; and likewise the close alliance 
of some of the inhabitants of the sea in the northern and 
southern temperate latitudes, though separated by the 
whole intertropical ocean. Although two countries may 
present physical conditions as closely similar as the 
same species ever require, we need feel no surprise at 
their inhabitants being widely different, if they have 
been for a long period completely sundered from each 
other ; for as the relation of organism to organism is the 
most important of all relations, and as the two countries 
will have received colonists at various periods and in 
different proportions, from some other country or from 
each other, the course of modification in the two areas 
will inevitably have been different. 

On this view of migration, with subsequent modifica- 
tion, we see why oceanic islands are inhabited by only 
few species, but of these, why many are peculiar or 
endemic forms. We clearly see why species belonging 
to those groups of animals which cannot cross wide 
spaces of the ocean, as frogs and terrestrial mammals, do 
not inhabit oceanic islands ; and why, on the other hand, 
new and peculiar species of bats, animals which can 
traverse the ocean, are often found on islands far distant 



290 RECAPITULATION. [Chap. XV. 

from any continent. Such cases as the presence of 
peculiar species of bats on oceanic islands and the 
absence of all other terrestrial mammals, are facts utterly 
inexplicable on the theory of independent acts of 
creation. 

The existence of closely allied or representative species 
in any two areas, implies, on the theory of descent with 
modification, that the same parent-forms formerly in- 
habited both areas : and we almost invariably find that 
wherever many closely allied species inhabit two areas, 
some identical species are still common to both. Wherever 
many closely allied yet distinct species occur, doubtful 
forms and varieties belonging to the same groups like- 
wise occur. It is a rule of high generality that the in- 
habitants of each area are related to the inhabitants of 
the nearest source whence immigrants might have been 
derived. We see this in the striking relation of nearly 
all the plants and animals of the Galapagos archipelago, 
of Juan Fernandez, and of the other American islands, 
to the plants and animals of the neighbouring American 
mainland; and of those of the Cape de Verde archi- 
pelago, and of the other African islands to the African 
mainland. It must be admitted that these facts receive 
no explanation on the theory of creation. 

The fact, as we have seen, that all past and present 
organic beings can be arranged within a few great classes, 
in groups subordinate to groups, and with the extinct 
groups often falling in between the recent groups, is 
intelligible on the theory of natural selection with its 
contingencies of extinction and divergence of character. 
On these same principles we see how it is, that the 
mutual affinities of the forms within each class are so 
complex and circuitous. ~\Ye see why certain characters 



Chap. XV.] RECAPITULATION. 291 

are far more serviceable than others for classification ; — 
why adaptive characters, though of paramount import- 
ance to the beings, are of hardly any importance in 
classification ; why characters derived from rudimentary 
parts, though of no service to the beings, are often of 
high classificatory value ; and why embryological char- 
acters are often the most valuable of all. The real 
affinities of all organic beings, in contradistinction to 
their adaptive resemblances, are due to inheritance or 
community of descent. The Natural System is a gene- 
alogical arrangement, with the acquired grades of dif- 
ference, marked by the terms, varieties, species, genera, 
families, &c. ; and we have to discover the lines of 
descent by the most permanent characters ■ whatever 
they may be and of however slight vital importance. 

The similar framework of bones in the hand of a man, 
wing of a bat, fin of the poipoise, and leg of the horse, 
— the same number of vertebras forming the neck of the 
giraffe and of the elephant, — and innumerable other such 
facts, at once explain themselves on the theory of descent 
with slow and slight successive modifications. The simi- 
larity of pattern in the wing and in the leg of a bat, 
though used for such different purpose, — in the jaws 
and legs of a crab, — in the petals, stamens, and pistils 
of a flower, is likewise; to a large extent, intelligible on 
the view of the gradual modification of parts or organs, 
which were aboriginally alike in an early progenitor in 
each of these classes. On the principle of successive 
variations not always supervening at an early age, and 
being inherited at a corresponding not early period of 
life, we clearly see why the embryos of mammals, birds, 
reptiles, and fishes should be so closely similar, and so 
unlike the adult forms. We may cease marvelling at 



292 KECAPITULATION. [Chap. XV. 

the embryo of an air-breathing mammal or bird having 
branchial slits and arteries running in loops, like those 
of a fish which has to breathe the air dissolved in water 
by the aid of well-developed branchiae. 

Disuse, aided sometimes by natural selection, will 
often have reduced organs when rendered useless under 
changed habits or conditions of life ; and we can under- 
stand on this view the meaning of rudimentary organs. 
But disuse and selection will generally act on each 
creature, when it has come to maturity and has to play 
its full part in the struggle for existence, and will thus 
have little power on an organ during early life ; hence 
the organ will not be reduced or rendered rudimentary 
at this early age. The calf, for instance, has inherited 
teeth, which never cut through the gums of the upper 
jaw, from an early progenitor having well-developed 
teeth ; and we may believe, that the teeth in the mature 
animal were formerly reduced by disuse, owing to the 
tongue and palate, or lips, having become excellently 
fitted through natural selection to browse without their 
aid ; whereas in the calf, the teeth have been left un- 
affected, and on the principle of inheritance at corre- 
sponding ages have been inherited from a remote period 
to the present day. On the view of each organism with 
all its separate parts having been specially created, how 
utterly inexplicable is it that organs bearing the plain 
stamp of inutility, such as the teeth in the embryonic 
calf or the shrivelled win^s under the soldered wim?- 
covers of many beetles, should so frequently occur. 
Nature may be said to have taken pains to reveal her 
scheme of modification, by means of rudimentary organs, 
of embryological and homologous structures, but we are 
too blind to understand her meaning. 



Chap. XV.] CONCLUSION. 293 

I have now recapitulated the facts and considerations 
which have thoroughly convinced me that species have 
been modified, during a long course of descent. This 
has been effected chiefly through the natural selection 
of numerous successive, slight, favourable variations ; 
aided in an important manner by the inherited effects 
of the use and disuse of parts ; and in an unimportant 
manner, that is in relation to adaptive structures, whether 
past or present, by the direct action of external condi- 
tions, and by variations which seem to us in our ignor- 
ance to arise spontaneously. It appears that I formerly 
underrated the frequency and value of these latter forms 
of variation, as leading to permanent modifications of 
structure independently of natural selection. But as my 
conclusions have lately been much misrepresented, and 
it has been stated that I attribute the modification of 
species exclusively to natural selection, I may be per- 
mitted to remark that in the first edition of this work, 
and subsequently, I placed in a most conspicuous posi- 
tion — namely, at the close of the Introduction — the 
following words : " I am convinced that natural selection 
has been the main but not the exclusive means of modi- 
fication." This has been of no avail. Great is the power 
of steady misrepresentation ; but the history of science 
shows that fortunately this power does not long endure. 

It can hardly be supposed that a false theory would 
explain, in so satisfactory a manner as does the theory 
of natural selection, the several large classes of facts 
above specified. It has recently been objected that this 
is an unsafe method of arguing ; but it is a method used 
in judging of the common events of life, and has often 
been used by the greatest natural philosophers. The 
undulatory theory of light has thus been arrived at ; and 



294 CONCLUSION. [Chap. XY. 

the belief in the revolution of the earth on its own axis 
was until lately supported by hardly any direct evidence. 
It is no valid objection that science as yet throws no 
light on the far higher problem of the essence or origin 
of life. Who can explain what is the essence of the 
attraction of gravity ? jSTo one now objects to following 
out the results consequent on this unknown element 
of attraction; notwithstanding that Leibnitz formerly 
accused Newton of introducing f( occult qualities and 
miracles into philosophy." 

I see no good reason why the views given in this volume 
should shock the religious feelings of any one. It is 
satisfactory, as showing how transient such impressions 
are, to remember that the greatest discovery ever made 
by man, namely, the law of the attraction of gravity, 
was also attacked by Leibnitz, " as subversive of natural, 
and inferentially of revealed, religion." A celebrated 
author and divine has written to me that " he has gradu- 
" ally learnt to see that it is just as noble a conception 
" of the Deity to believe that He created a few original 
" forms capable of self-development into other and need- 
" fal forms, as to believe that He required a fresh act of 
t( creation to supply the voids caused by the action of 
« His laws." 

Why, it may be asked, until recently did nearly all 
the most eminent living naturalists and geologists dis- 
believe in the mutability of species. It cannot be 
asserted that organic beings in a state of nature are 
subject to no variation ; it cannot be proved that the 
amount of variation in the course of long ages is a limited 
quantity ; no clear distinction has been, or can be, drawn 
between species and well-marked varieties. It cannot 
be maintained that species when intercrossed are invari* 



Chap. XV.] CONCLUSION. 295 

ably sterile, and varieties invariably fertile; or that 
sterility is a special endowment and sign of creation. 
The belief that species were immutable productions was 
almost unavoidable as long as the history of the world 
was thought to be of short duration ; and now that we 
have acquired some idea of the lapse of time, we are too 
apt to assume, without proof, that the geological record 
is so perfect that it would have afforded us plain evidence 
of the mutation of species, if they had undergone 
mutation. 

But the chief cause of our natural unwillingness to 
admit that one species has given birth to other and 
distinct species, is that we are always slow in admitting 
great changes of which we do not see the steps. The 
difficulty is the same as that felt by so many geologists, 
when Lyell first insisted that long lines of inland cliffs 
had been formed, and great valleys excavated, by the 
agencies which we see still at work. The mind cannot 
possibly grasp the full meaning of the term of even a 
million years ; it cannot add up and perceive the full 
effects of many slight variations, accumulated during an 
almost infinite number of generations. 

Although I am fully convinced of the truth of the 

views given in this volume under the form of an abstract, 

I by no means expect to convince experienced naturalists 

whose minds are stocked with a multitude of facts all 

viewed, during a long course of years, from a point of 

view directly opposite to mine. It is so easy to hide 

our ignorance under such expressions as the "plan of 

creation," "unity of design," &c, and to think that we 

give an explanation when we only re-state a fact. Any 

one whose disposition leads him to attach more weight 

to unexplained difficulties than to the explanation of a 
43 



296 CONCLUSION. [Chap. XV 

certain number of facts will certainly reject the theory. 
A few naturalists, endowed with much flexibility of 
mind, and who have already begun to doubt the immu- 
tability of species, may be influenced by this volume ; 
but I look with confidence to the future, — to young and 
rising naturalists, who will be able to view both sides of 
the question with impartiality. Whoever is led to believe 
that species are mutable will do good service by con- 
scientiously expressing his conviction ; for thus only can 
the load of prejudice by which this subject is over- 
whelmed be removed. 

Several eminent naturalists have of late published 
their belief that a multitude of reputed species in each 
genus are not real species ; but that other species are 
real, that is, have been independently created. This 
seems to me a strange conclusion to arrive at. They 
admit that a multitude of forms, which till lately they 
themselves thought were special creations, and which 
are still thus looked at by the majority of naturalists, 
and which consequently have all the external character 
istic features of true species, — they admit that these 
have been produced by variation, but they refuse to 
extend the same view to other and slightly different 
forms. Nevertheless they do not pretend that they can 
define, or even conjecture, which are the created forms 
of life, and which are those produced by secondary laws. 
They admit variation as a vera causa in one case, they 
arbitrarily reject it in another, without assigning any 
distinction in the two cases. The day will come when 
this will be given as a curious illustration of the blind- 
ness of preconceived opinion. These authors seem no 
more startled at a miraculous act of creation than at an 
ordinary birth. But do they really believe that at innu- 



Chap. XV.] CONCLUSION. 297 

merable periods in the earth's history certain elemental 
atoms have been commanded suddenly to flash into 
living tissues ? Do they believe that at each supposed 
act of creation one individual or many were produced ? 
Were all the infinitely numerous kinds of animals and 
plants created as eggs or seed, or as full grown ? and in 
the case of mammals, were they created bearing the 
false marks of nourishment from the mother's womb ? 
Undoubtedly some of these same questions cannot be 
answered by those who believe in the appearance or 
creation of only a few forms of life, or of some one form 
alone. It has been maintained by several authors that 
it is as easy to believe in the creation of a million beings 
as of one ; but Maupertuis' philosophical axiom " of least 
action" leads the mind more willingly to admit the 
smaller number ; and certainly we ought not to believe 
that innumerable beings within each great class have 
been created with plain, but deceptive, marks of descent 
from a single parent. 

As a record of a former state of things, I have retained 
in the foregoing paragraphs, and elsewhere, several 
sentences which imply that naturalists believe in the 
separate creation of each species ; and I have been much 
censured for having thus expressed myself. But un- 
doubtedly this was the general belief when the first 
edition of the present work appeared. I formerly spoke 
to very many naturalists on the subject of evolution, 
and never once met with any sympathetic agreement. 
It is probable that some did then believe in evolution, 
but they were either silent, or expressed themselves so 
ambiguously that it was not easy to understand their 
meaning. Now things are wholly changed, and almost 
every naturalist admits the great principle of evolution. 



298 CONCLUSION. [Chap. XV. 

There are, however, some who still think that species 
have suddenly given birth, through quite unexplained 
means, to new and totally different forms : but, as I 
have attempted to show, weighty evidence can be op- 
posed to the admission of great and abrupt modifications. 
Under a scientific point of view, and as leading to further 
investigation, but little advantage is gained by believing 
that new forms are suddenly developed in an inexplic- 
able manner from old and widely different forms, over 
the old belief in the creation of species from the dust of 
the earth. 

It may be asked how far I extend the doctrine of the 
modification of species. The question is difficult to 
answer, because the more distinct the forms are which 
we consider, by so much the arguments in favour of 
community of descent become fewer in number and 
less in force. But some arguments of the greatest 
weight extend very far. All the members of whole 
classes are connected together by a chain of affinities, 
and all can be classed on the same principle, in 
groups subordinate to groups. Fossil remains some- 
times tend to fill up very wide intervals between 
existing orders. 

Organs in a rudimentary condition plainly show that 
an early progenitor had the organ in a fully developed 
condition ; and this in some cases implies an enormous 
amount of modification in the descendants. Throughout 
whole classes various structures are formed on the same 
pattern, and at a very early age the embryos closely 
resemble each other. Therefore I cannot doubt that the 
theory of descent with modification embraces all the 
members of the same great class or kingdom. I believe 
that animals are descended from at most onlv four or 



Chap. XV.] CONCLUSION. 299 

five progenitors, and plants from an equal or lesser 
number. 

Analogy would lead me one step farther, namely, to 
the belief that all animals and plants are descended from 
some one prototype. But analogy may be a deceitful 
guide. Nevertheless all living things have much in 
common, in their chemical composition, their cellular 
structure, their laws of growth, and their liability to in- 
jurious influences. We see this even in so trifling- a 
fact as that the same poison often similarly affects 
plants and animals ; or that the poison secreted by the 
gall-fly produces monstrous growths on the wild rose or 
oak-tree. With all organic beings, excepting perhaps 
some of the very lowest, sexual reproduction seems to 
be essentially similar. With all, as far as is at present 
known, the germinal vesicle is the same; so that all 
organisms start from a common origin. If we look even 
to the two main divisions — namely, to the animal and 
vegetable kingdoms — certain low forms are so far inter- 
mediate in character that naturalists have disputed to 
which kingdom they should be referred. As Professor 
Asa Gray has remarked, " the spores and other repro- 
" ductive bodies of many of the lower algse may claim 
" to have first a characteristically animal, and then an 
" unequivocally vegetable existence." Therefore, on the 
principle of natural selection with divergence of char- 
acter, it does not seem incredible that, from some such 
low and intermediate form, both animals and plants 
may have been developed; and, if we admit this, we 
must likewise admit that all the organic beings which 
have ever lived on this earth may be descended from 
some one primordial form. But this inference is chiefly 
grounded on analogy, and it is immaterial whether or 



300 CONCLUSION. [Chap. XV. 

not it be accepted. No doubt it is possible, as Mr. G. 
H. Lewes has urged, that at the first commencement of 
life many different forms were evolved ; but if so, we 
may conclude that only a very few have left modified 
descendants. For, as I have recently remarked in regard 
to the members of each great kingdom, such as the Yer- 
tebrata, Articulata, &c, we have distinct evidence in 
their embryological, homologous, and rudimentary struc- 
tures, that within each kingdom all the members are 
descended from a single progenitor. 

When the views advanced by me in this volume, and 
by Mr. "Wallace, or when analogous views on the origin 
of species are generally admitted, we can dimly foresee 
that there will be a considerable revolution in natural 
history. Systematists will be able to pursue their labours 
as at present ; but they will not be incessantly haunted 
by the shadowy doubt whether this or that form be a 
true species. This, I feel sure and I speak after ex- 
perience, will be no slight relief. The endless disputes 
whether or not some fifty species of British brambles 
are good species will cease. Systematists will have only 
to decide (not that this will be easy) whether any form 
be sufficiently constant and distinct from other forms, 
to be capable of definition; and if definable, whether 
the differences be sufficiently important to deserve a 
specific name. This latter point will become a far more 
essential consideration than it is at present ; for differ- 
ences, however slight, between any two forms, if not 
blended by intermediate gradations, are looked at by 
most naturalists as sufficient to raise both forms to the 
rank of species. 

Hereafter we shall be compelled to acknowledge that 
the only distinction between species and well-marked 



Chap. XV.] CONCLUSION. 301 

varieties is, that the latter are known, or believed, to be 
connected at the present day by intermediate gradations 
whereas species were formerly thus connected. Hence, 
without rejecting the consideration of the present exist- 
ence of intermediate gradations between any two forms, 
we shall be led to weigh more carefully and to value 
higher the actual amount of difference between them. 
It is quite possible that forms now generally acknow- 
ledged to be merely varieties may hereafter be thought 
worthy of specific names ; and in this case scientific and 
common language will come into accordance. In short, 
we shall have to treat species in the same manner as 
those naturalists treat genera, who admit that genera 
are merely artificial combinations made for convenience. 
This may not be a cheering prospect ; but we shall at 
least be freed from the vain search for the undiscovered 
and undiscoverable essence of the term species. 

The other and more general departments of natural 
history will rise greatly in interest. The terms used 
by naturalists, of affinity, relationship, community of 
type, paternity, morphology, adaptive characters, rudi- 
mentary and aborted organs, &c, will cease to be meta- 
phorical, and will have a plain signification. When we 
no longer look at an organic being as a savage looks at 
a ship, as something wholly beyond his comprehension ; 
when we regard every production of nature as one 
which has had a long history; when we contemplate 
every complex structure and instinct as the summing 
up of many contrivances, each useful to the possessor, 
in the same way as any great mechanical invention is 
the summing up of the labour, the experience, the reason, 
and even the blunders of numerous workmen ; when we 
thus view each organic being, how far more interesting 



302 CONCLUSION. [Chat. XV 

— I speak from experience — does the study of natural 
history become ! 

A grand and almost untrodden field of inquiry will 
be opened, on the causes and laws of variation, on cor- 
relation, on the effects of use and disuse, on the direct 
action of external conditions, and so forth. The study 
of domestic productions will rise immensely in value. 
A new variety raised by man will be a more important 
and interesting subject for study than one more species 
added to the infinitude of already recorded species. 
Our classifications will come to be, as far as they can 
be so made, genealogies ; and will then truly give what 
may be called the plan of creation. The rules for classi- 
fying will no doubt become simpler when we have a defi- 
nite object in view. TTe possess no pedigrees or armorial 
bearings ; and we have to discover and trace the many 
diverging lines of descent in our natural genealogies, by 
characters of anv kind which have lon^ been inherited. 
Eudimentary organs will speak infallibly with respect 
to the nature of long-lost structures. Species and 
groups of species which are called aberrant, and which 
may fancifully be called living fossils, will aid us in 
forming a picture of the ancient forms of life. Embry- 
ology will often reveal to us the structure, in some 
decree obscured, of the prototypes of each great class. 

THien we can feel assured that all the individuals 
of the same species, and all the closely allied species 
of most genera, have within a not very remote period 
descended from one parent, and have migrated from 
some one birth-place ; and when we better know the 
many means of migration, then, by the light which 
geology now throws, and will continue to throw, on 
former changes of climate and of the level of the land, 



Chap. XV.] CONCLUSION. 303 

we shall surely be enabled to trace in an admirable 
manner the former migrations of the inhabitants of the 
whole world. Even at present, by comparing the 
differences between the inhabitants of the sea on the 
opposite sides of a continent, and the nature of the 
various inhabitants on that continent in relation to their 
apparent means of immigration, some light can be thrown 
on ancient geography. 

The noble science of Geology loses glory from the 
extreme imperfection of the record. The crust of the 
earth with its imbedded remains must not be looked at 
as a well-filled museum, but as a poor collection made at 
hazard and at rare intervals. The accumulation of each 
great fossiliferous formation will be recognised as having 
depended on an unusual concurrence of favourable cir- 
cumstances, and the blank intervals between the suc- 
cessive stages as having been of vast duration. But we 
shall be able to gauge with some security the duration 
of these intervals by a comparison of the preceding 
and succeeding organic forms. We must be cautious in 
attempting to correlate as strictly contemporaneous two 
formations, which do not include many identical species, 
by the general succession of the forms of life. As 
species are produced and exterminated by slowly acting 
and still existing causes, and not by miraculous acts of 
creation; and as the most important of all causes of 
organic change is one which is almost independent of 
altered and perhaps suddenly altered physical conditions, 
namely, the mutual relation of organism to organism, — 
the improvement of one organism entailing the improve- 
ment or the extermination of others ; it follows, that the 
amount of organic change in the fossils of consecutive 
formations probably serves as a fair measure of the 



304 CONCLUSION. [CSap. XV. 

relative, though not actual lapse of time. A number 
of species, however, keeping in a body might remain for 
a long period unchanged, whilst within the same period 
several of these species by migrating into new countries 
and coming into competition with foreign associates, 
might become modified ; so that we must not overrate 
the accuracy of organic change as a measure of time. 

In the future I see open fields for far more important 
researches. Psychology will be securely based on the 
foundation already well laid by Mr. Herbert Spencer, 
that of the necessary acquirement of each mental power 
and capacity by gradation. Much light will be thrown 
on the origin of man and his history. 

Authors of the highest eminence seem to be fully 
satisfied with the view that each species has been 
independently created. To my mind it accords better 
with what we know of the laws impressed on matter by 
the Creator, that the production and extinction of the 
past and present inhabitants of the world should have 
been due to secondary causes, like those determining the 
birth and death of the individual. When I view all 
beings not as special creations, but as the lineal descend- 
ants of some few beings which lived long before the first 
bed of the Cambrian system was deposited, they seem 
to me to become ennobled. Judging from the past, we 
may safely infer that not one living species will transmit 
its unaltered likeness to a distant futurity. And of the 
species now living very few will transmit progeny of 
any kind to a far distant futurity; for the manner in 
which all organic beings are grouped, shows that the 
greater number of species in each genus, and all the 
species in many genera, have left no descendants, but 
have become utterly extinct. We can so far take a 



Chap. XV.] CONCLUSION. 305 

prophetic glance into futurity as to foretell that it will 
be the common and widely-spread species, belonging to 
the larger and dominant groups within each class, which 
will ultimately prevail and procreate new and dominant 
species. As all the living forms of life are the lineal 
descendants of those which lived long before the Cam- 
brian epoch, we may feel certain that the ordinary 
succession by generation has never once been broken, 
and that no cataclysm has desolated the whole world. 
Hence we may look with some confidence to a secure 
future of great length. And as natural selection works 
solely by and for the good of each being, all corporeal and 
mental endowments will tend to progress towards 
perfection. 

It is interesting to contemplate a tangled bank, 
clothed with many plants of many kinds, with birds 
singing on the bushes, with, various insects flitting about, 
and with worms crawling through the damp earth, and 
to reflect that these elaborately constructed forms, so 
different from each other, and dependent upon each 
other in so complex a manner, have all been produced 
by laws acting around us. These laws, taken in the 
largest sense, being Growth with Eeproduction ; In- 
heritance which is almost implied by reproduction; 
Variability from the indirect and direct action of the 
conditions of life, and from use and disuse : a Eatio of 
Increase so high as to lead to a Struggle for Life, and as 
a consequence to Natural Selection, entailing Divergence 
of Character and the Extinction of less-improved forms. 
Thus, from the war of- nature, from famine and death, 
the most exalted object which we are capable of con- 
ceiving, namely, the production of the higher animals, 
directly follows. There is grandeur in this view of life, 



306 CONCLUSION. [Chap. XV. 

with its several powers, haying been originally breathed 
by the Creator into a few forms or into one ; and that, 
whilst tnis planet has gone cycling on according to the 
fixed law of gravity, from so simple a beginning endless 
forms most beautiful and most wonderful have been, and 
are being evolved. 



GLOSSARY 

OF THE 

PRINCIPAL SCIENTIFIC TERMS USED IN THE 
PRESENT VOLUME.* 



Aberrant. — Forms or groups of animals or plants which deviate 
in important characters from their nearest allies, so as not to be 
easily included in the same group with them, are said to be 
aberrant. 

Aberration (in Optics). — In the refraction of light by a convex lens 
the rays passing through different parts of the lens are brought 
to a focus at slightly different distances, — this is called spherical 
aberration ; at the same time the coloured rays are separated by 
the prismatic action of the lens and likewise brought to a focus 
at different distances, — this is chromatic aberration. 

Abnormal. — Contrary to the general rule. 

Aborted. — An organ is said to be aborted, when its development 
has been arrested at a very early stage. 

Albinism. — Albinos are animals in which the usual colouring 
matters characteristic of the species have not been produced in 
the skin and its appendages. Albinism is the state of being an 
albino. 

Algm. — A class of plants including the ordinary sea-weeds and the 
filamentous fresh-water weeds. 



* I am indebted to the kindness of Mr. W. S. Dallas for this 
Glossary, which has been given because several readers have com- 
plained to me that some of the terms used were unintelligible to them. 
Mr. Dallas has endeavoured to give the explanations of the terms in 
as popular a form as possible, 

4A 



308 GLOSSAEY. 

Alternation of Generations. — This term is applied to a peculiar 

mode of reproduction which prevails among many of the lower 

animals, in which the egg produces a living form quite different 

from its parent, but from which the parent-form is reproduced 

by a process of budding, or by the division of the substance 

of the first product of the egg. 

Ammonites. — A group of fossil, spiral, chambered shells, allied to 

the existing pearly Nautilus, but having the partitions between 

the chambers waved in complicated patterns at their junction 

with the outer wall of the shell. 

Analogy. — That resemblance of structures which depends upon 

similarity of function, as in the wings of insects and birds. 

Such structures are said to be analogous, and to be analogues of 

each other. 

Animalcule. — A minute animal : generally applied to those visible 

only by the microscope. 
Annelids. — A class of worms in which the surface of the body 
exhibits a more or less distinct division into rings or segments, 
generally provided with appendages for locomotion and with 
gills. It includes the ordinary marine worms, the earthworms, 
and the leeches. 
Antenna. — Jointed organs appended to the head in Insects, 

Crustacea and Centipedes, and not belonging to the mouth. 
Anthers. — The summits of the stamens of flowers, in which the 

pollen or fertilising dust is produced. 
Aplacentalia, Aplacentata or Aplacental Mammals. See Mam- 

malia. 
Archetypal. — Of or belonging to the Archetype, or ideal primi- 
tive form upon which all the beings of a group seem to be 
organised, 
Articulata. — A groat division of the Animal Kingdom character- 
ised generally by having the surface of the body divided into 
rings called segments, a greater or less number of which are 
furnished with jointed legs (such as Insects, Crustaceans and 
Centipedes). 
Asymmetrical. — Having the two sides unlike. 
Atrophied. — Arrested in development at a very early stage. 

Balanus. — The genus including the common Acorn-shells which 

live in abundance on the rocks of the sea-coast. 
Batrachiaxs. — A class of animals allied to the Keptiles, but 



GLOSSARY. 309 

undergoing a peculiar metamorphosis, in which the young 
animal is generally aquatic and breathes by gills. (Examples, 
Frogs, Toads, and Newts.) 

Boulders. — Large transported blocks of stone generally imbedded 
in clays or gravels. 

Beachiopoda. — A class of marine Mollusca, or soft-bodied animals, 
furnished with a bivalve shell, attached to submarine objects 
by a stalk which passes through an aperture in one of the 
valves, and furnished with fringed arms, by the action of which 
food is carried to the mouth. 

Branchle. — Gills or organs for respiration in water. 

Branchial. — Pertaining to gills or branchias. 

Cambrian System. — A Series of very ancient Palaeozoic rocks, 
between the Laurentian and the Silurian. Until recently these 
were regarded as the oldest fossil iferous rocks. 

Canid^e. — The Dog-family, including the Dog, Wolf, Fox, Jackal, 
&c. 

Carapace. — The shell enveloping the anterior part of the body in 
Crustaceans generally ; applied also to the hard shelly pieces of 
the Cirripedes. 

Carboniferous. — This term is applied to the great formation which 
includes, among other rocks, the coal-measures. It belongs to 
the oldest, or Palaeozoic, sj-stem of formations^ 

Caudal. — Of or belonging to the tail. 

Cephalopods. — The highest class of the Mollusca, or soft-bodied 
animals, characterised by having the mouth surrounded by a 
greater or less number of fleshy arms or tentacles, which, in 
most living species, are furnished with sucking-cups. {Ex- 
amples, Cuttle-fish, Nautilus.) 

Cetacea. — An order of Mammalia, including the Whales* Dolphins, 
&c, having the form of the body fish-like, the skin naked, and 
only the fore-limbs developed. 

Chelonia. — An order of Reptiles including the Turtles, Tortoises, &c. 

Cirripedes. — An order of Crustaceans including the Barnacles and 
Acorn-shells. Their young resemble those of many other 
Crustaceans in form ; but when mature they are always attached 
to other objects, either directly or by means of a stalk, and their 
bodies are enclosed by a calcareous shell composed of several 
pieces, two of which can open to give issue to a bunch of curled, 
jointed tentacles, which represent the limbs. 



310 . GLOSSAEY. 

Coccus. — The genus of Insects including the Cochineal. In these 
the male is a minute, winged fly, and the female generally a 
motionless, berry-like mass. 

Cocoox. — A case usually of silky material, in which insects are 
frequently enveloped during the second or resting-stage (pupa) 
of their existence. The term "cocoon-stage" is here used as 
equivalent to " pupa-stage." 

CffiLosPERiious. — A term applied to those fruits of the Umbelliferse 
which have the seed hollowed on the inner face. 

Coleopteea. — Beetles, an order of Insects, having a biting mouth 
and the first pair of wings more or less horny, forming sheaths 
for the second pair, and usually meeting in a straight line down 
the middle of the back. 

Column. — A peculiar organ in the flowers of Orchids, in which 
the stamens, style and stigma (or the reproductive parts) are 
united. 

Composite or Compqsitous Plants. — Plants in which the inflores- 
cence consists of numerous small flowers (florets) brought 
together into a dense head, the base of which is enclosed by a 
common envelope. (Examples, the Daisy, Dandelion, &c.) 

Conferva. — The filamentous weeds of fresh water. 

Conglomerate. — A rock made up of fragments of rock or pebbles, 
cemented together by some other material. 

Corolla. — The second envelope of a flower usually composed of 
coloured, leaf-like organs (petals), which may be united by their 
edges either in tbe basal part or throughout. 

Correlation. — The normal coincidence of one phenomenon, 
character, &c, with another. 

Corymb. — A bunch of flowers in which those springing from the 
lower part of the flower stalk are supported on long stalks so as 
to be nearly on a level with the upper ones. 

Cotyledons. — The first or seed-leaves of plants. 

Crustaceans. — A class of articulated animals, having the skin of 
the body generally more or less hardened by the deposition of 
calcareous mutter, breathing by means of gills. (Examples, 
Crab, Lobster, Shrimp, Arc.) 

Curculio. — The old generic term for the Beetles known as "Weevils, 
charactei ised by their four -jointed feet, and by the head being 
produced into a sort of beak, upon the sides of which the 
antennae are inserted. 

Cutaneous. — Of or belonging to the skin. 



GLOSSARY- 311 

Degradation. — The wearing down of land by the action of the sea 
or of meteoric agencies. 

Denudation. — The wearing away of the surface of the land by water. 

Devonian System or formation. — A series of PalEeozoic rocks, 
including the Old Red Sandstone. 

Dicotyledons or Dicotyledonous Plants.— A class of plants 
characterised by having two seed-leaves, by the formation of 
new wood between the bark and the old wood (exogenous 
growth) and by the reticulation of the veins of the leaves. The 
parts of the flowers are generally in multiples of five. 

Differentiation. — The separation or discrimination of parts or 
organs which in simpler forms of life are more or less united. 

Dimorphic — Having two distinct forms. — Dimorphism is the con- 
dition of the appearance of the same species under two dissimilar 
forms. 

Dicecious. — Having the organs of the sexes upon distinct indi- 
viduals. 

Diorite. — A peculiar form of Greenstone, 

Dorsal. — Of or belonging to the back. 

Edentata. — A peculiar order of Quadrupeds, characterised by the 

absence of at least the middle incisor (front) teeth in both jaws. 

(Examples, the Sloths and Armadillos.) 
Elytra. — The hardened fore-wings of Beetles, serving as sheaths 

for the membranous hind-wings, which constitute the true 

organs of flight. 
Embryo. — The young animal undergoing development within the 

egg or womb. 
Embryology. — The study of the development of the embryo. 
Endemic — Peculiar to a given locality. 
Entomostraca. — A division of the class Crustacea, having all the 

segments of the body usually distinct, gills attached to the feet 

or organs of the mouth, and the feet fringed with fine hairs. 

They are generally of small size. 
Eocene. — The earliest of the three divisions of the Tertiary epoch 

of geologists. Rocks of this age contain a small proportion of 

shells identical with species now living. 
Ephemerous Insects. — Insects allied to the May-fly. 

Fauna. — The totality of the animals naturally inhabiting a certain 
country or region, or which have lived during a given geological 
period. 



312 GLOSSARY. 

Feluxe. — The Cat-family. 

Feral. — Having become wild from a state of cultivation or domes- 
tication. 

Flora. — The totality of the plants growing naturally in a country, 
or during a given geological period. 

Florets. — Flowers imperfectly developed in some respects, and 
collected into a dense spike or head, as in the Grasses, the 
Dandelion, &c. 

Fcetal. — Of or belonging to the foetus, or embryo in course of 
development. 

Foraminifera. — A class of animals of very low organisation, and 
generally of small size, having a jelly-like body, from the surface 
of which delicate filaments can be given off and retracted for the 
prehension of external objects, and having a calcareous or sandy 
shell, usually divided into chambers, and perforated with small 
apertures. 

Fossiliferous. — Containing fossils. 

Fossorial. — Having a faculty of digging. - The Fossorial Hymen- 
optera are a group of Wasp-like Insects, which burrow in sandy 
soil to make nests for their young. 

Frenum (pi. Fbena). — A small band or fold of skin. 

Fungi (sing. Fungus). — A class of cellular plants, of which Mush- 
rooms, Toadstools, and Moulds, are familiar examples. 

Furcula. — The furked bone formed by the union of the collar-bones 
in many birds, such as the common Fowl. 

Gallinaceous Birds. — An order of Birds of which the common 
F( wl, Turkey, and Pheasant, are well-known examples. 

Gal us. — The genus of birds wh ch includes the common Fowl. 

Ganglion. — A swelling or knot from which nerves are given off as 
from a centre. 

Ganoid Fishes. — Fishes covered with peculiar enamelled bony 
scales. Most of them are extinct. 

Germinal Vesicle. — A minute vesicle in the eggs of animals, from 
which the development of the embryo proceeds. 

Glacial Period. — A period of great cold and of enormous extension 
of ice upon the surface of the earth. It is believed that glacial 
periods have occurred repeatedly during the geological history of 
the earrh, but the term is gen-rally applied to the close of the 
Tertiary e; och, when nearly the whole of Europe was subjected 
to an arctic climate. 



GLOSSAEY. 313 

Gland. — An organ which secretes or separates some peculiar 

product from the blood or sap of animals or plants. 
Glottis. — The opening of the windpipe into the oesophagus or gullet. 
Gneiss. — A rock approaching granite in composition, but more or 

less laminated, and really produced by the alteration of a 

sedimentary deposit after its consolidation. 
Geallatores. — The so-called Wading-birds (Storks, Cranes, Snipes, 

&c), which are generally furnished with long legs, bare of 

feathers above the heel, and have no membranes between the 

toes. 
Granite. — A rock consisting essentially of crystals of felspar and 

mica in a mass of quartz. 

Habitat. — The locality in which a plant or animal naturally lives. 

Hemiptera. — An order or sub-order of Insects, characterised by the 
possession of a jointed beak or rostrum, and by having the fore- 
wings horny in the basal portion and membranous at the 
extremity, where they cross each other. This group includes 
the various species of Bugs. 

Hermaphrodite. — Possessing the organs of both sexes. 

Homology. — That relation between parts which results from their 
development from corresponding embryonic parts, either in 
different animals, as in the case of the arm of man, the fore-leg 
of a quadruped, and the wing of a bird; or in the same in- 
dividual, as in the case of the fore and hind legs in quadrupeds, 
and the segments or rings and their appendages of which the 
body of a worm, a centipede, &c, is composed. The latter is 
called serial homology. The parts which stand in such a relation 
to each other are said to be homologous, and one such part or 
organ is called the homologue of the other. In different plants 
the parts of the flower are homologous, and in general these 
parts are regarded as homologous with leaves. 

Homoptera. — An order or sub-order of Insects having (like the 
Hemiptera) a jointed beak, but in which the fore-wings are 
either wholly membranous or wholly leathery. The Cicadss, 
Frog-hoppers, and Aphides, are well-known examples. 

Hybrid. — The offspring of the union of two distinct species. 

Hymenoptera. — An order of Insects possessing biting jaws and 
usually four membranous wings in which there are a few veins. 
Bees and Wasps are familiar examples of this group. 

Hypertrophied. — Excessively developed. 



314 GLOSSARY. 

IcHNEUHGNms. — A family of Hymenopterous insects, the members 
of which lay their eggs in the bodies or eggs of other insects. 

Imago.— The perfect (generally winged) reproductive state of an 
insect. 

Lndigens. — The aboriginal animal or vegetable inhabitants of a 
country or region. 

Inflorescence. — The mode of arrangement of the flowers of plants. 

Infusoria. — A class of microscopic Animalcules, so called from 
their having originally been observed in infusions of vegetable 
matters. They consist of a gelatinous material enclosed in a 
delicate membrane, the whole or part of which is furnished with 
short vibrating hairs (called cilia), by means of which the 
animalcules swim through the water or convey the minute 
particles of their food to the orifice of the mouth. 

Insectivorous. — Feeding on Insects. 

Invertebrata, or Invertebrate Animals. — Those animals which 
do not possess a backbone or spinal column. 

Lacuna. — Spaces left among the tissues in some of the lower 
animals, and serving in place of vessels for the circulation of the 
fluids of the body. 

Lamellated. — Furnished with lamella or little plates. 

Larva (pi. Larv.e). — The first condition of an insect at its issuing 
from the egg, when it is usually in the form of a grub, caterpillar, 
or maggot. 

Larynx. — The upper part of the windpipe opening into the gullet. 

Laurentian. — A group of greatly altered and very ancient rocks, 
which is greatly developed along the course of the St. Laurence, 
whence the name. It is in these that the earliest known traces 
of organic bodies have been found. 

Leguminosje. — An order of plants represented by the common Peas 
and Beans, having an irregular flower in which one petal stands 
up like a wing, and the stamens and pistil are enclosed in a 
sheath formed by two other petals. The fruit is a pod (or 
legume). 

Lemueid-£. — A group of four-handed animals, distinct from the 
Monkeys and approaching the Insectivorous Quadrupeds in some 
of their characters and habits. Its members have the nostrils 
curved or twisted, and a claw instead of a nail upon the first 
finger of the hind hands. 

Lepldoptera. — An order of Insects, characterised by the possession 



GLOSSAEY. 315 

of a spiral proboscis, and of four large more or less scaly wings. 

It includes the well-known Butterflies and Moths. 
Littoral. — Inhabiting the seashore. 
Loess. — A marly deposit of recent (Post-Tertiary) date, which 

occupies a great part of the valley of the Ehine. 

Malacqstraca. — The higher division of the Crustacea, including 
the ordinary Crabs, Lobsters, Shrimps, &c, together with the 
Woodlice and Sand-hoppers. 

Mammalia. — The highest class of animals, including the ordinary 
hairy quadrupeds, the Whales, and Man, and characterised by 
the production of living young which are nourished after birth 
by milk from the teats {Mammas, Mammary glands) of the 
mother. A striking difference in embryonic development has 
led to the division of this class into two great groups ; in one of 
these, when the embryo has attained a certain stage, a vascular 
connection, called the placenta, is formed between the embryo 
and the mother ; in the other this is wanting, and the young are 
produced in a very incomplete state. The former, including the 
greater part of the class, are called Placental mammals; the 
latter, or Aplacental mammals, include the Marsupials and 
Monotremes ( Ornithorhynchus). 

Mammiferous. Having mammae or teats (see Mammalia). 

Mandibles, in Insects. — The first or uppermost pair of jaws, which 
are generally solid, horny, biting organs. In Birds the term is 
applied to both jaws with their horny coverings. In Quadrupeds 
the mandible is properly the lower jaw. 

Marsupials. — An order of Mammalia in which the young are born 
in a very incomplete state of development, and carried by the 
mother, while sucking, in a ventral pouch (marsupium), such as 
the Kangaroos, Opossums, &c. (see Mammalia). 

Maxilla, in Insects. — The second or lower pair of jaws, which are 
composed of several joints and furnished with peculiar jointed 
appendages called palpi, or feelers. 

Melanism. — The opposite of albinism ; an undue development of 
colouring material in the skin and its appendages. 

Metamorphic Kocks. — Sedimentary rocks which have undergone 
alteration, generally by the action of heat, subsequently to their 
deposition and consolidation. 

Mollusca. — One of the great divisions of the Animal Kingdom, 
including those animals which have a soft body, usually 



316 GLOSSARY. 

furnished with a shell, and in which the nervous ganglia, or 
centres, present no definite general arrangement. They are 
generally known under the denomination of " shell-fish ; " the 
cuttle-fish, and the common snails, whelks, oysters, mussels, and 
cockles, may serve as examples of them. 

Monocotyledons, or Monocotyledonous Plants.— Plants in 
which the seed sends up only a single seed-leaf (or cotyledon) ; 
characterised by the absence of consecutive layers of wood in 
the stem (endogenous growth), by the veins of the leaves being 
generally straight, and by the parts of the flowers being generally 
in multiples of three. {Examples, Grasses, Lilies, Orchids, 
Palms, &c.) 

Mobaines. — The accumulations of fragments of rock brought down 
by glaciers. 

Morphology. — The law of form or structure independent of 
function. 

Mysis-stage.^A stage in the development of certain Crustaceans 
(Prawns), in which they closely resemble the adults of a genus 
{My sis) belonging to a slightly lower group. 

Nascent. — -Commencing development* 

Natatory. — Adaped for the purpose of swimming. 

Nauplius-form. — The earliest stage in the development of many 
Crustacea, especially belonging to the lower groups. In this 
stage the animal has a short body, with indistinct indications of 
a division into segments, and three pairs of iringed limbs. This 
form of the common fresh-water Cyclops was described as a 
distinct genus under the name of KaupJius. 

Neuration. — The arrangement of the veins or nervures in the 
wings of Insects. 

Neuters. — Imperfectly developed females of certain social insects 
(such as Ants and Bees), which perform all the labours of the 
community. Hence they are also called workers. 

Nictitating Membrane. — A semi-transparent membrane, which 
can be drawn across the eye in Birds and Eeptiles, either to 
moderate the effects of a strong light or to sweep particles of 
dust, &c, from the surface of the eye. 

Ocelli. — The simple eyes or stemmata of Insects, usually situated 

on the crown of the head between the great compound eyes. 
(Esophagus. — The gullet. 



GLOSSAEY. 317 

Oolitic. — A great series of secondary rocks, so called from the 
texture of some of its members, which appear to be made up of 
a mass of small egg-like calcareous bodies. 

Operculum. — A calcareous plate employed by many Mollusca to 
close the aperture of their shell. The opercular valves of 
Cirripedes are those which close the aperture of the shell. 

Orbit. — The bony cavity for the reception of the eye. 

Organism. — An organised being, whether plant or animal. 

Orthospermous. — A term applied to those fruits of the Umbelliferse 
which have the seed straight. 

Osculant. — Forms or groups apparently intermediate between and 
connecting other groups are said to be osculant. 

Ova. — Eggs. 

Ovarium or Ovary (in plants).— The lower part of the pistil or 
female organ of the flower, containing the ovules or incipient 
seeds; by growth after the other organs of the flower have 
fallen, it usually becomes converted into the fruit. 

Ovigerous. — Egg-bearing. 

Ovules (of plants). — The seeds in the earliest condition. 

Pachyderms. — A group of Mammalia, so called from their thick 
skins, and including the Elephant, Rhinoceros, Hippopotamus, 
&c. 

Paleozoic. — The oldest system of fossiliferous rocks. 

Palpi. — Jointed appendages to some of the organs of the mouth in 
Insects and Crustacea. 

Papilionace^:. — An order of Plants (see Leguminos<e). — The 
flowers of these plants are called papilionaceous, or butterfly- 
like, from the fancied resemblance of the expanded superior 
petals to the wings of a butterfly. 

Parasite. — An animal or plant living upon or in, and at the 
expense of, another organism. 

Parthenogenesis. — The production of living organisms from 
unimpreguated eggs or seeds. 

Pedunculated. — Supported upon a stem or stalk. The peduncu- 
lated oak has its acorns borne upon a footstool. 

Peloria or Pelorism. — The appearance of regularity of structure in 
the flowers of plants which normally bear irregular flowers. 

Pelvis. — The bony arch to which the hind limbs of vertebrate 
animals are articulated. 

Petals. — The leaves of the corolla, or second circle of organs in a 



318 GLOSSARY. 

flower. They are usually of delicate texture and brightly 
coloured. 

Phyllodineous. — Having flattened, leaf-like twigs or leafstalks 
instead of true leaves. 

Pigment. — The colouring material produced generally in the super- 
ficial parts of animals. The cells secreting it are called pigment- 
cells. 

Pinnate. — Bearing leaflets on each side of a central stalk. 

Pistils. — The female organs of a flower, which occupy a position 
in the centre of the other floral organs. The pistil is generally 
divisible into the ovary or germen, the style and the 
stigma. 

Placentalia, Placentata, or Placental Mammals. — See Mam- 
malia. 

Plantigrades. — Quadrupeds which walk upon the whole sole of 
the foot, like the Bears. 

Plastic. — Eeadily capable of change. 

Pleistocene Period. — The latest portion of the Tertiary epoch. 

Plumule (in plants). — The minute bud between the seed-leaves of 
newly-germinated plants. 

Plutonic Rocks. — Rocks supposed to have been produced by 
igneous action in the depths of the earth. 

Pollen. — The male element in flowering plants ; usually a fine dust 
produced by the anthers, which, by contact with the stigma 
effects the fecundation of the seeds. This impregnation is 
brought about by means of tubes {pollen-tubes) which issue 
from the pollen-grains adhering to the stigma, and penetrate 
through the tissues until they reach the ovary. 

Polyandrous (flowers). — Flowers having many stamens. 

Polygamous Plants. — Plants in w T hich some flowers are unisexual 
and others hermaphrodite. The unisexual (male and female) 
flowers, may be on the same or on different plants. 

Polymorphic. — Presenting many forms. 

Polyzoary. — The common structure formed by the cells of the 
Polvzoa, such as the well-known Sea-mats. 

Prehensile. — Capable of grasping. 

Prepotent. — Having a superiority of power. 

Primaries. — The feathers forming the tip of the wing of a bird, and 
inserted upon that part which represents the hand of man. 

Processes. — Projecting portions of bones, usually for the attach- 
ment of muscles, ligaments, &c. 



GLOSSARY. 319 

Propolis, — A resinous material collected by the Hive-Bees from the 
opening buds of various trees. 

Protean. — Exceedingly variable. 

Protozoa. — The lowest great division of the Animal Kingdom. 
These animals are composed of a gelatinous material, and show 
scarcely any trace of distinct organs. The Infusoria, Foramini- 
fera, and Sponges, with some other forms, belong to this 
division. 

Pupa (pi. Pup^e). — The second stage in the development of an 
Insect, from which it emerges in the perfect (winged) reproduc- 
tive form. In most insects the pupal stage is passed in perfect 
repose. The chrysalis is the pupal state of butterflies. 

Radicle. — The minute root of an embryo plant. 

Ramus. — One half of the lower jaw in the Mammalia. The portion 
which rises to articulate with the skull is called the ascending 
ramus. 

Range. — The extent of country over which a plant or animal is 
naturally spread. Bange in time expresses the distribution of a 
species or group through the fossiliferous beds of the earth's 
crust. 

Retina. — The delicate inner coat of the eye, formed by nervous 
filaments spreading from the optic nerve, and serving for the 
perception of the impressions produced by light. 

Retrogression. — Backward development. When an animal, as it 
approaches maturity, becomes less perfectly organised than 
might be expected from its early stages and known relation- 
ships, it is said to undergo a retrograde development or meta- 
morpjJwsis. 

Rhlzopods. — A class of lowly organised animals (Protozoa), having a 
gelatinous body, the surface of which can be protruded in the 
form of root-like processes or filaments, which serve for loco- 
motion and the prehension of food. The most important order 
is that of the Foraminifera. 

Rodents. — The gnawing Mammalia, such as the Rats, Rabbits, and 
Squirrels. They are especially characterised by the possession 
of a single pair of chisel-like cutting teeth in each jaw, between 
which and the grinding teeth there is a great gap. 

Rubus. — The Bramble Genus. 

Rudimentary. — Very imperfectly developed. 

Ruminants. — The group of Quadrupeds which ruminate or chew 
45 



320 GLOSSAEY. 

the cud, such as oxen, sheep, and deer. They have divided 
hoofs, and are destitute of front teeth in the upper jaw. 

Saceal. — Belonging to the sacrum, or the bone composed usually 

of two or more united vertebras to which the sides of the pelvis 

in vertebrate animals are attached. 
Sarcode. — The gelatinous material of which the bodies of the 

lowest animals (Protozoa) are composed. 
Scutell^e. — The horny plates with which the feet of birds are 

generally more or less covered, especial: y in front. 
Sedimentary Formations. — Hocks deposited as sediments from 

water. 
Segments. — The transverse rings of which the body of an articulate 

animal or Annelid is composed. 
Sepals. — The leaves or segments of the calyx, or outermost envelope 

of an ordinary flower. They are usually green, but sometimes 

brightly coloured. 
Serratures. — Teeth like those of a saw. 
Sessile. — Xot supported on a stem or footstalk. 
Silurian System. — A very ancient system of fossiliferous rocks 

belonging to the earlier part of the Palaeozoic series. 
Specialisation. — The setting apart of a particular organ for the 

] erformance of a particular function. 
Spinal Chord. — The central portion of the nervous system in the 

Vertebrate, which descends frum the brain through the arches 

of the vertebrae, and gives off nearly all the nerves to the various 

organs of the body. 
STAMENS. — The male organs of flowering plants, standing in a circle 

within the petals. They usually c insist of a filament and an 

anther, the anther being the essential part in which the pollen, 

or fecundating dust, is formed. 
Sternum. — The breast-bone. 

Stigma. — The apical portion of the pistil in flowering plants. 
Stipules. — Small leafy organs placed at the base of the footstalks 

of the leaves in many plants. 
Style. — The middle portion of the perfect pistil, which rises like a 

column from the ovary and supports the stigma at its summit. 
Subcutaneous. — Situated beneath the skin. 
Suctorial. — Adapted for sucking. 
Sutures (in the skull). — The lines of junction of the bones of which 

the skull is composed. 



GLOSSARY. 321 

Tarsus (pi. Tarsi). — The jointed feet of articulate animals, such as 
Insects. 

Teleostean Fishes. — Fishes of the kind familiar to us in the 
present day, having the skeleton usually completely ossified and 
the scales horny. 

Tentacula or Tentaci.es. — Delicate fleshy organs of prehension or 
touch possessed by many of the lower animals. 

Tertiary. — The latest geological epoch, immediately preceding the 
establishment of the present order of things. 

Trachea. — The windpipe or passage for the admission of air to the 
lungs. 

Tridacttle. — Three-fingered, or composed of three movable parts 
attached to a common base. 

Trilobites. — A peculiar group of extinct Crustaceans, somewhat 
resembling the Woodlice in external form, and, like some of 
them, capable of rolling themselves up into a ball. Their 
remains are found only in the Palasozoic rocks, and most abun- 
dantly in those of Silurian age. 

Trimorphic. — Presenting three distinct forms. 

Umbellifer^:. — An order of plants in which the flowers, which 
contain five stamens and a pistil with two styles, are supported 
upon footstalks which spring from the top of the flower stem 
and spread out like the wires of an umbrella, so as to bring all 
the flowers in the same head (umbel) nearly to the same level. 
(Examples, Parsley and Carrot.) 

Ungulata. — Hoofed quadrupeds. 

Unicellular. — Consisting of a single cell. 

Y a scular. — Containing blood-vessels. 

Vermiform. — Like a worm. 

Yertebrata : or Vertebrate Animals. — The highest division of 
the animal kingdom, so called from the presence in most 
cases of a backbone composed of numerous joints or vertebrae, 
which constitutes the centre of the skeleton and at the same 
time supports and protects the central parts of the nervous 
system. 

Whorls. — The circles or spiral lines in which the parts of plants are 

arranged upon the axis of growth. 
Workers. — See Neuters. 



322 GLOSSAEY. 

Zoea-stage. — The earliest stage in the development of many of 
the higher Crustacea, so called from the name of Zoea applied to 
these young animals when they were supposed to constitute a 
peculiar genus. 

Zooms. — In many of the lower animals (such as the Corals, Medusa?, 
&c.) reproduction takes place in two ways, namely, by means of 
eggs and by a process of budding with or without separation 
from the parent of the product of the latter, which is often very 
different from that of the egg. The individuality of the species 
is represented by the whole of the form produced between two 
sexual reproductions ; and these forms, which are apparently 
individual animals, have been called zoo ids. 



INDEX. 



ABERRANT. 



AZORES. 



Aberrant groups, ii. 227. 

Abyssinia, plants of, ii. 167. 

Acclimatisation, i. 173. 

Adoxa, i. 270 

Affinities of extinct species, ii. 106. 

of organic beiugs, ii. 225. 

Agassiz, on Amblyopsis, i. 173. 
, on groups of species suddenly 

appearing, ii. 88. 

, on propbetic forms, ii. 107. 

, on embryological succession, 

ii. 120. 

, on tbe Glacial period, ii. 151. 

, on embryological characters, 

ii. 210. 
, on the latest tertiary forms, 

ii. 71. 
, on parallelism of embryologi- 
cal development and geological 

succession, ii. 254. 

, Alex., on pedicellarise, i. 297. 

Algse of New Zealand, ii. 164. 
Alligators, males, fighting, i. 108. 
Alternate generations, ii. 239. 
Amblyopsis, blind fish, i. 173. 
America, North, productions allied 

to those of Europe, ii. 156. 
, , boulders and glaciers of, 

ii. 159. 
, South, no modern formations 

on west coast, ii. 61. 
Ammonites, sudden extinction of, 

ii. 99. 
Anagallis, sterility of, ii. 4. 
Analogy of variations, i. 197. 
Ancylus, ii. 174. 
Andaman Islands inhabited by a 

toad, ii. 182. 
Animals, not domesticated from 

being variable, i. 19. 
, domestic, descended from seve- 
ral stocks, i, 21. 
■ , , acclimatisation o£ i. 

175. 



Animals of Australia, i. 140. 
■ with thicker fur in cold cli- 
mates, i. 166. 

, blind, in caves, i. 172. 

extinct, of Australia, ii. 121. 

Anomma, i. 361. 

Antarctic islands, ancient flora of, 
ii. 190. 

Antechinus, ii. 219. 

Ants attending aphides, i. 323. 

, slave-making instinct, i. 

336. 

, neuters, structure of, i. 359. 

Apes, not having acquired intel- 
lectual powers, i. 282. 

Aphides, attended by ants, i. 323 

Aphis, development of, ii. 245. 

Apteryx, i. 218. 

Arab horses, i. 40. 

Aralo-Caspian Sea, ii. 121. 

Archeoptervx, ii. 80. 

Archiac, M. de, on the succession of 
species, ii. 103. 

Artichoke, Jerusalem, i. 176. 

Ascension, plants of, ii. 178. • 

Asclepias, pollen of, i. 236. 

Asparagus, ii. 143. 

Aspicarpa, ii. 209. 

Asses, striped, i. 198. 

, improved by selection, i. 48. 

Ateuchus, i. 168. 

Aucapitaine, on land-shells, ii. 
187. 

Audubon, on habits of frigate-bird, 
i. 222. 

, on variation in birds' nests, i. 

324. 

■ , on heron eating seeds, ii. 176. 

Australia, animals of, i. 140. 

, dogs of, i. 328. 

■ , extinct animals of, ii. 121. 

, European plants in, ii. 163. 

, glaciers of, ii. 159. 

Azara, on flies destroying cattle, i 
89. 

Azores, flora of, ii. 149. 



524 



BABINGTON. 



INDEX. 



BUZABETNGUES. 



B. 


Birds acquiring fear, i. 325. 




, beauty of, i. 252. 


Babington, Mr., on British plants, 


annually cross the Atlantic, 


i. 53. 


ii. 150. 


Ba t r. Yon, standard of Highness, i. 


, colour of, on continents, i. 


151. 


165. 


, comparison of bee and fish, 


, footsteps, and remains of, in 


ii. 118. 


secondary rocks, ii. 79. 


, embryonic similarity of the 


. fossil, in caves of Brazil, ii. 


Yertebrata, ii. 241. , 


121. 


Baker, Sir S., on the giraffe, i. 278. 


. of Madeira. Bermuda, and 


Balancement of growth, i. 182. 


Galapagos, ii. 179, 180. 


Baleen, i. 285. 


, song of males, i. 109. 


Barb.rry, flowers of, i. 121. 


transporting >eeds, ii. 148. 


Barrande, M., on Silurian colonies, 


, waders, ii. 175. 


ii. 90. 


, wingless, i. 167, 218. 


, on the succession of species, 


Bizeacha, ii. 133. 


ii. 103. 


. affinities of, ii. 227. 


, on parallelism of palaeozoic 


Bladder for swimming, in fish, i 


formations, ii. 106. 


230. 


, on affinities of ancient species, 


Biindness of cave animals, i. 170. 


ii. 108. 


Blyth, Mr., on distinctness of Indian 


Barriers, importance of. ii. 130. 


cattle, i. 21. 


Bate?. Mr., on mimetic butterflies, 


, on striped hemionus, i. 199. 


ii. 222, 223. 224. 


, on crossed geese, ii. 10. 


B atraebiaus on islands, ii. 182. 


Borrow, Mr., on the Spanisb pointer, 


Bats, how structure acquired, i. 


i. 40. 


218. 


Borv St. Vincent, on Batrachians, 


, distribution of, ii. 184. 


ii. 182. 


Bear, catching water-in.^e ts. i. 220. 


t, M., on fossil Chthamalus, 


Beauty, how acquired, i. 249; ii. 


ii. 80. 




B<>uldcrs, erratic, on the Azores, ii. 


Bee, stilly of, i. 255. 


14V*. 


, queen, killing rivals, i. 256. 


Branchia, i. 231, 232. 


, Australian, extermination of, 


of crustaceans, i. 238. 


i. '.'3. 


Braun, Prof., on the seeds of Fuma- 


Bees fertilising flowers, i. 90. 


riac. SB, i. 271. 


, hive, not sucking the red 


Mr., on bouse-tumblers, i. 326. 


clov. r, i. 117. 


Britain, mammals of, ii. 185. 


, Ligurian, i. 117. 


Broca, Prof., on Natural Selection, 


. hive, Cell-making instinct, L 


i. 265. 


342. 


Bronu, Prof., on duration of specific 


, variation in habits, i. 324. 


forms, ii. 6Q. 


, parasiti •. i. 330. 


, \aiious objections by, i. 265. 


, bumble, cells of, i. 343. 


Brown, Kobert, on classification, ii 


in M ideira, i. 169. 




with deficient tarsi, i. 16S. 


, Se'quard. on inberited muti- 


B_ ntbam, Mr., on British plants, 


laion.-, i. 168. 


i. 58. 


Butk, Mr., on tie Polyzoa, i. 301. 


, on classification, ii. 211. 


Butterflies, mimetic, ii. 222, 223, 


Berkeley, Mr., on seeds in salt 


--4. 


water, ii. 112. 


eingu< s, on sterility of varie- 


Bermuda, buds of, ii. 180. 


ties, ii. 66. 



INDEX. 



325 



c. 


Chickens, instinctive tameness of, 

i. 329. 
Chironomus, its asexual reproduc- 


Cabbage, varieties of, crossed, L 


122. 


tion, ii. 240. 


Calceolaria, ii. 7, 8. 


Chthamalinse, ii. 59. 


Canary-birds, sterility of hybrids, 


Chthamalus, cretacean species of, 


ii. 9. 


ii. 81. 


Cape de Verde islands, productions 


Circumstances favourable to selec- 


of, ii. 189. 


tion of domestic products, i. 46. 


, plants of, on mountains, ii. 


to natural selection, i. 124. 


162. 


Cirripedes capable of crossing, i. 124. 


Cape of Good Hope, plants of, i. 


, carapace aborted, i. 184. 


158 ; ii. 178. 


, their ovigerous frena, i. 232. 


Carpenter, Dr., on foraminifera, ii. 


, fossil, ii. 80. 


117. 


, larvse of, ii. 243. 


Carthamus, i. 271. 


Claparede, Prof., on the hair-clas- 


Catasetum, i. 243 ; ii. 216. 


pers of the Aearidae, i. 239. 


Cats, with blue eyes, deaf, i. 13. 


Clarke, Eev. W. B., on old glaciers 


, variation in habits of, i. 325. 


in Australia, ii. 159. 


curling tail when going to 


Classification, ii. 202. 


spring, i. 251. 


Clift, Mr., on the succession of 


Cattle destroying fir-trees, i. 88. 


types, ii. 121. 


destroyed by flies in Paraguay, 


Climate, effects of, in checking in- 


i. 89. 


crease of beings, i. 84. 


breeds of, locally extinct, i. 


, adaptation of, to organisms, 


134. 


i. 174. 


, fertility of Indian and Euro- 


Climbing plants, i. 230. 


pe, m breeds, ii. 10. 


, development of, i. 305. 


, Indian, i. 21 ; ii. 10. 


Clover visited by bees, i. 117. 


Cave, inhabitants of, blind, i. 170. 


Co bites, intestine of, i. 229. 


Cecidornyia, ii. 239. 


Cockroach, i. 93. 


Celts, proving antiquity of man, 


Collections, palseontological, poor, 


i. 21. 


ii. 58. 


Centres of Creation, ii. 135. 


Colour, influenced by climate, i. 165. 


Cephalopoda, structures of eyes, 


, in relation to attack by flies, 


i. 236. 


i. 218. 


, development of, ii. 244. 


Col urn ba livia, parent of domestic 


Cercopithecus, tail of, i. 294. 


pigeons, i. 26. 


Ceroxyins laceratus, i. 284. 


Colymbetes, ii. 174. 


Cervulus, ii. 9. 


Compensation of growth, i. 182. 


Cetacea, ti eth and hair, i. 179. 


Compositae, flowers and seeds of, 


, development of the whale- 


i. 179. 


bone, i. 285. 


, outer and inner florets of, 


Cetaceans, i. 285. 


i. 270. 


Ceylon, plants of, ii. 164. 


, male flowers of, ii. 257. 


Cnrilk formation, ii. 100. 


Conclusion, general, ii. 2 '3. 


Characters, divergence of, i. 134. 


Condif ions, slight changes in, 


■ , sexual, variable, i. 185, 191. 


favourable to fertility, ii. 27. 


, adaptive or analogical, ii. 218. 


Convergence of genera, i. 156. 


Charlock, i. 94. 


Coot, i. 222. 


Checks to increase, i. 83. 


Cope, Prof., on the acceleration or 


— — , mutual, i. 86. 


retardation of the period of re- 


Chelee of Crustaceans, i. 300. 


production, i. 232. 



326 



COEAL-ISLAXDS. 



INDEX. 



Coral-islands, seeds drifted to, 
ii. 145. 

reefs, indicating movements 

of earth, ii. 115. 

Corn-crake, i. 223. 

Correlated variation in domestic 
productions, i. 13. 

Coryanthes, i. 241. 

Creation, single centres of, ii. 135. 

Crinum, ii. 6. 

Croll, Mr., on subaerial denuda- 
tion, ii. 53, 56. 

, on the age of our oldest for- 
mations, ii. 83. 

, on alternate Glacial periods 

in the North and South, ii. 160. 

Crosses, reciprocal, ii. 14. 

Crossing of domestic animals, im- 
portance in altering breeds, i. 23. 

, advantages of, i. 119, 120. 

, unfavourable to selection, 

i. 125. 

Criiger, Dr., on Coryanthes, i. 211. 

Crustacea of New Zealand, ii. 164. 

Crustacean, blind, i. 171. 

air-breathers, i. 238. 

Crustaceans, their chela, i. 300. 

Cry ptocerus, L 359. 

Cten 'in vs. blind, i. 170. 

Cuckoo.* instinct of, i. 319, 330. 

Cunningham, Mr., on the flight of 
the Logger-beaded duck, i. 167. 

Cam - f, ii. 19. 

Currents of sea. rate of, ii. 144. 

Cuvier, on conditions of existence, 
i. 320. 

Cuvier. on fre-sil monkeys, ii. 79. 

, Fred., on instinct, L 320. 

Cyclostoma, renting salt water, 
"ii. 187. 



Dana. Prof., on blind cave-animals, 
i. 172. 

, on relations of crustaceans of 

■i. ii 158. 

, on crustaceans of New Zea- 
land, ii. 161. 

Dawson, Dr., on eozoon, ii. 85. 

1> C rndolle, Aug. Pyr., on struggle 
for existe: oe, i. 77. 

• , on umb llif MB, i. 181. 

, on general affinities, ii. 228. 



De Candolle, Alph., on the varia- 
bility of oaks, i. 62. 
, on low plants, widely dis- 
persed, ii. 196. 
, on widely-ranging plants 

being variable, i. 67. 

, on naturalisation, i. 139. 

, on winged seeds, i. 181. 

, on Alpine species suddenly 

becoming rare, i. 210. 
, on distribution of plants with 

large seeds, ii. 145. 
, on vegetation of Australia, 

ii. 167. 

, on fresh-water plants, ii. 174. 

, on insular plants, ii. 178. 

Degradation of rocks, ii. 52. 
Denudation, rate of, ii. 54. 

of oldest rocks, ii. 85. 

of granitic areas, ii. 64. 

Development of ancient forms, 

ii. 116. 
Devonian system, ii. 113. 
Dianthus, fertility of crosses, ii. 

13. 
Dimorphism in plants, i. 55 ; ii. 29. 
Dirt on feet of birds, ii. 148. 
Dispersal, means of, ii. 140. 

during Glacial period, ii. 151. 

Distribution, geograpical, ii. 129. 

, means of, ii. 140. 

Disuse, effect of, under Dature, 

i. 167. 
Divergence of character, i. 134. 
Diversification of means for same 

general purpose, i. 240. 
Division, physiological, of labour, 

i. 139. 
Dog, resemblance of jaw to that of 

the Tliylacinus, ii. 220. 
Dogs, hairkss, with imperfect teeth, 

i. 14. 
descended from several wild 

stocks, i. 22. 

, domestic instincts of, i. 327. 

, inherited civilisation of, i. 327. 

, fertility of breeds together, 

ii. 10. 

, of crosses, ii. 35. 

, proportions of body in diff. r- 

ent breed-, when young, ii. 247. 
Domestication, variation under, i.7. 
Double flowers, i. 358. 



INDEX. 



327 



1 )owning, Mr., on fruit-trees in Ame- 


Eve, correction for aberration, i. 


rica, i. 104. 


*255. 


Dragon flies, intestines of, i. 229. 


Eyes, reduced in moles, i. 170. 


Drift-timber, ii. 145. 




Driver-ant, i. 361. 


F. 


Drones killed by other bees, i. 256. 




Duck, domestic, wings of, reduced, 


Fabre, M., on hymenoptera fight- 


i. 12. 


ing, i. 108. 


, beak of, i. 285. 


, on parasitic sphex, i. 336. 


, logger-headed, i. 218. 


, on Sitaris, ii. 252. 


Duckweed, ii. 173. 


Falconer, Dr., on naturalisation of 


Dugong, affinities of, ii. 206. 


plants in India, i. 80. 


Dung-beetles with deficient tarsi, 


on elephants and mastodons, 


i. 168. 


ii. 113. 


Dytiscus, ii. 174. 


and Cautley, on mammals of 




sub-Himalayan beds, ii. 122. 


E. 


Falkland Islands, wolf of, ii. 183. 


Earl, Mr. W., on the Malay Archi- 


Faults, ii. 54. 


pelago, ii. 185. 


Faunas, marine, ii. 131. 


Ears, drooping, in domestic animals, 


Fear, instinctive, in birds, i. 329. 


i. 13. 


Feet of birds, young molluscs ad- 


, rudimentary, ii. 261. 


hering to, ii. 174. 


Earth, seeds in roots of trees, ii. 145. 


Fertilisation variously effected, i. 


charged with seeds, ii. 148. 


241, 252. 


Echinodermata, their pedicell arise, 


Fertility of hybrids, ii. 6. 


i. 297. 


, from slight changes in con- 


Eciton, i. 359. 


ditions, ii. 28. 


Economy of organisation, i. 1S2. 


of crossed varieties, ii. 34. 


Edentata, teeth and hair, i. 179. 


Fir-trees destroyed by cattle, i. 88. 


, fossil species of, ii. 288. 


, pollen of, i. 257. 


Edwards, Milne, on physiological 


Fish, flying, i. 218. 


division of labour, i. 139. 


, teleostean, sudden appear- 


, on • gradations of structure, 


ance of, ii. 81. 


i. 244. 


, eating seeds, ii. 146, 175. 


, on embryological characters, 


, fresh-water, distribution of, 


ii. 210. 


ii. 172. 


Eg?s, young birds escaping from, 


Fishes, ganoid, now confined to fresh 


L106. 


water, i. 130. 


Egypt, productions of, iaot modified. 


, electric organs of, i. 234. 


i. 263. 


, ganoid, living in fresh water, 


Electric organs, i. 234. 


ii. 99. 


Elephant, rate of increase, i. 80. 


, of southern hemisphere, ii. 


• , of Glacial period, i. 176. 


164. 


Embryology, ii. 239. 


Flat-fish, their structure, i. 290. 


Eozoon Canadense, ii. 84. 


Flight, powers of, how acquired, 


Epilepsy inherited, i. 167. 


i. 218. 


Existence, struggle for, i. 75. 


Flint-tools, proving antiquity of 


, condition of, i. 261. 


man, i. 21. 


Extinction, as bearing on natural 


Flower, Prof, on the Larnyx, i. 297. 


selection, i. 150. 


, on Halitherium, ii. 108. 


. of domestic varieties, i. 145. 


, on the resemblance between 


, ii. 94. 


the jaws of the dog and Thyla- 


Eye, structure of, i. 225. 


oinus, ii. 220. 



328 



INDEX. 



Flower, Prof., on the homology of the 


Fur. thicker in cold climates, i. 166. 


feet of certain marsupials, ii. 232. 


Furze, ii. 241. 


Flowers, structure of, in relation to 


G. 


crossing, i. 114. 


, of compositse and umbelli- 


Galapasros Archipelago, birds of, 


ferae, i. 179, 270. 


ii. 179. 


, beauty of, i. 252. 


productions of, ii. 188, 190. 


, double, i. 358. 


Ga'axias, its wide range, ii. 172. 


Flysch formation, drstitute of or- 


Galeopitkeeus, i. 217. 


ganic remains, ii. 59. 


Game, increase of, checked by ver- 


Forbes, Mr. D., on glacial action in 


min, i. 86. 


the Andes, ii. 160. 


Gartner, on sterility of hvbrids, ii. 


, E., on colours of shells, i. 


3. 4. 11. 


165. 


, on reciprocal crosses, ii. 15. 


. on abrupt range of shells in 


, on crossed maize and verbas- 


depth, i. 210. 


cum. ii. 37. 


, on poorness r,f palseontological 


, on comparison of hybrids and 


collections, ii. 58. 


mongrels, ii. 40, 41, 42. 


, on continuous succession of 


Gaudry, Prof., on intermediate ge- 


genera, ii. 93. 


nera of fossil mammals in Attica, 


, on continental extensions, ii. 


ii. 107. 


140, 141. 


Geese, fertility when crossed, ii. 


, on distribution during Glacial 


9, 10. 


period, ii. 152. 


. upland, i. 222. 


, on parallelism in time and 


Geikie, Mr., on subaerial denuda- 


space, ii. 200. 


tion, ii. 53. 


Forests, changes in, in America, 


Genealogv. important in classifica- 


i. 91. 


tion, if.*212. 


Formation, Devonian, ii. 113. 


Generations, alternate, ii. 239. 


. Cambrian, ii. 84. 


Geoffroy St. Hilaire, on balance- 


Formations, tliickn<-ss of, in Britain, 


ment, i. 182. 


ii. 5o. 


, ou homologous organs, ii. 233. 


. intermittent, ii. 69. 


, Isi .ore, on variability of re- 


Formic . rufi Bcens, i i 


ted parts, i. 184. 


. sangouH a, i 


, or correlation, in monstroci- 


, flava, ni ater of, i. 300. 


ties, i. 13. 


! sganised, long en- 


, on correlation, i. 179. 


dnring, i. 154. 


, on variable parts being often 


Fn na, ovigerous, of cirripedcs, i. 


monstrous, i. 19u. 


i - 


iphieal distribution, ii. 129. 


Fresh-wat r productions, dispersal 


< ■ srraphy, ancient, ii. 303. 


of, ii. 171. 


Geology, future progress of, ii. 302. 


B in large genera 


, imperfection of the record, 


_ < 1 » 1\ allied to other 


ii. 303. 


Bpecies, i. 71. 


Gervais, Prof., on Typotherium, ii. 


Frigate-bird, i. 222. 


108. 


»a in islands, ii. 182. 


Giraffe, tail of, i. 245. 


Fruit-trees, gradual improvement 


, structure of, i. 276. 


of, i. 42. 


Glacial T'<rri<>d. ii. 151. 


in Unit- d States, i. 104. 


, atf.cting the Xorthand South, 


. varieties of, a climatised in 


ii. 158. 


United Stiles, i. 176. 


Glands, mammary, i. 295. 


Fuci, crossed, ii. 15, 23. 


C'._._lin, on distribution, ii. 151. 



GODWIN-AUSTEN. 



INDEX. 



329 



Godwin-Austen, Mr., on the Malay 

Archipelago, ii. 74. 
Goethe, on compensation of growth, 

i. 182. 
Gomphia, i. 272. 
Gooseberry, grafts of, ii. 19. 
Gould. Dr. Aug. A. on land-shells, 

ii. 186. 

, Mr., on colours of birds, i. 165. 

, on instincts of cuckoo, i. 333. 

, on distribution of genera of 

birds, ii. 195. 
Gourds, crossed, ii. 38. 
Graba, on the Uria lacrymas, i. 113. 
Grafting, capacity of, ii. 18, 19, 20. 
Granite, areas of denuded, ii. 61. 
Grasses, varieties of, i. 137. 
Grav, Dr. Asa, on the variability of 

oak-, i. 62, 
■ , on roan not causing varia- 
bility, i. 98. 

, on sexes of the holly, i. 116. 

■ , on trees of the United States, 

i. 123. 
■ , on naturalised plants in the 

United States, i. 139. 

, on aestivation, i. 272. 

, on Alpine plants, ii. 151. 

, on rarity of intermediate va- 
rieties, i. 212. 
, Dr. J. E., on striped mule, 

i. 199. 
Grebe, i. 221. 
Grimm, on asexual reproduction, 

ii. 240. 
Groups, aberrant, ii. 227. 
Grouse, colours of, i. 104. 

, red, a doubtful species,!. 59. 

Growth, compensation of, i. 182. 
Giinther, Dr., on flat-fish, i. 292. 

> , on preV ensile tails, i. 294. 

, on the fishes of Panama, ii. 131. 

, on the range of fresh-water 

fishes, ii. 172. 
, on the limb3 of Lepidosiren, 

ii. 258. 

E. 

Haast, Dr., on glaciers of New Zea- 
land, ii. 159. 

Habit, effect of, under domestica- 
tion, i. 12. 

, effect of, under nature, i. 168. 



Habit, diversified, of same species, 
i. 219. 

H'aekel, Prof., on classification and 
the lines of descent, ii. 231. 

Hair and teeth, correlated, i. 179. 

Haiitheiium, ii. 108. 

Harcourt, Mr. E. V., on the birds of 
Madeira, ii. 180. 

Hartuog, M., on boulders in the 
Azores, ii. 149. 

Hazel-nuts, ii. 143. 

Hearne, on habits of bears, i. 220. 

Heath, changes in vegetation, i. 
87. 

Hector, Dr., on glaciers of New Zea- 
land, ii. 159. 

Heer, OswaM, on ancient cultivated 
plants, i. 20. 

, on plants of Madeira, i. 130. 

Helianthemum, i. 272. 

Helix pomatia, ii. 187. 

, resisting salt water, ii. 187. 

Helmholtz, M., on the imperfection 
of the human eye, i. 255. 

Helosciadium, ii. 143. 

Hemionus. striped, i. 202. 

Hensen, Dr., on the eyes of Cepha- 
lopods, i. 237. 

Herbert, W., on struggle for exist- 
ence, i. 77. 

, on sterility of hybrids, ii. 6. 

Hermaphrodites crossing, i. 119. 

Heron eating seed, ii. 176. 

Heron, Sir K., on peacocks, i. 109. 

Heusinger, on white animals poi- 
soned by certain plants, i. 13. 

Hewitt, Mr., on sterility of first 
crosses, ii. 23. 

Hildebrand, Prof., on +he self-ste- 
rility of Corylalis, ii. 7. 

Hilg-eridorf, on intermediate varie- 
ties, ii. 66. 

Himalaya, glaciers of, ii. 159. 

, plants of, ii. 162. 

Hippeastrum, ii. 7. 

Hippocampus, i. 295. 

Hofmeister, Prof., on the move- 
ments of plants, i. 308. 

Holly-trees, sexes of, i. 115. 

Hooker, Dr., on trees of New Zea- 
land, i. 123. 

, on acclimatisation of Hima- 
layan trees, i. 174. 



330 



HOOKER 



INDEX. 



Hooker, Dr.. on flowers of uinbel- 
liferse, i. 180. 

, on the position of ovules, i. 

268. 

, on glaciers of Himalaya, ii. 

159. 

, on algse of New Zealand, ii. 

164. 

, on vegetation at the base of 

the Himalaya, ii. 164. 

, <m plants of Tierra del Fuego, 

ii. 161. 

, on Australian plants, ii. 163, 

190. 

, on relations of flora of Ame- 
rica,- ii. 167. 

, on flora of the Antarctic lands, 

ii. 169, 189. 

, on the plants of the Gala- 
pagos, ii. 181, 188. 

, on glaciers of the Lebanon, 

ii. 159. 

, on man not causing varia- 
bility, i. 97. 

, on plants of mountains of 

Fernando Po, ii. 162. 

Hooks on palms, i. 2 17. 

om Beads, on islands, ii. 181. 

Hopkins, Mr., on denudation, ii. 63. 

lluinliill, remarkable instinct of, i. 
364. 

. rudimentary, ii. 261. 

. in La Plata, ii. 96. 

. proportions of, when young, 

ii. 247. 

Horses destr yed by flies in Para- 
guay, i. ! 

striped, i. 199. 

Hortioultui i:-ts, selection applied by, 
i. 37. 

Huber, on cells of bees, i. 349. 

. P., i n r aeon bknded with 

instinct, i 

,on habitual nature of instincts, 

i. 32 

, on slave-making ants, i. 336. 

, on Melipona domestics, i. 343. 

Hudson, Mr., on the Ground- Wood- 
pecker of La Plata, i. 221. 

, on the Molothrus. i. 

Humble-bees, Cells, uf, i. 343. 

Hunter, J., on secondary sexual 
characters, i. 185. 



Hutton, Captain, on crossed geese, 

ii. 10. 
Huxley, Prof., on structure of her- 
maphrodites, i. 124. 
, on the affinities of the Sirenia, 

ii. 108. 
, on forms connecting birds and 

reptiles, ii. 108. 

, on homologous organs, ii. 238. 

. on the development of aphis, 

ii. 245. 
Hybrids and mongrels compared, 

ii. 39. 
Hybridism, ii. 1. 
Hydra, structure of, i. 229. 
Hymenoptera, fighting, i. 108. 
Hvmenopterous insect, diving, i. 

222. 
Hyoseris, i. 271. 

I. 

Ibla. i. 183. 

Icebergs transporting; seeds, ii. 148. 
Increase, rate of, i. 79. 
Individuals, numbers favourable to 

selection, i. 124. 
. many, whether simultaneously 

created, ii. 139. 
Inheritance, laws of, i. 15. 
. at corresponding ages, i. 15, 

105. 
1 sects, colour of, fitted for their 

stations, i. 103. 

. sea-si le, colours of, i. 165. 

, blind, in caves, i. 171. 

. luminous, i. 236. 

, their resemblance to certain 

objects, i. 283. 

, neuter, i, 359. 

Instinct, i. 310. 

, not varying simultaneously • 

with structure, i. 307. 
Instincts, domestic, i. 325. 
Int. rcrossing, advantages of, i. 119, 

ii. 27. 
Islands, oceanic, ii. 177. 
Isolation favourable to selection, L 

127. 

J. 

Japan, productions of, ii. 158. 
Java, plants of, ii. 162. 



INDEX. 



331 



Jones, Mr. J. M., on the birds of 
Bermuda, ii. 180. 

Jour- lain, M., on the eye-spots of 
star-fishes, i. 225. 

Jukes, Prnf., on subaerial denuda- 
tion, ii. 53. 

Jussieu, on classification, ii. 209. 



Kentucky, caves of, i. 172. 

Kerguelen-land, flora of,ii. 169, 189. 

Kidney-bean, acclimatisation of, i. 
177. 

Kidneys of birds, i. 178. 

Kirby, on tarsi deficient in beetles, 
i. 168. 

Knight, Andrew, on cause of varia- 
tion, i. 8. 

Kolreuter, on Intercrossing, i. 119. 

, on the barberry, i. 121. 

, on sterility of hybrids, ii. 3, 4. 

, on reciprocal crosses, ii. 15. 

, on crossed varieties of nico- 

tiana, ii. 38. 

, on crossing male and herma- 
phrodite flowers, ii. 256. 



Lamarck, on adaptive characters, ii. 
218 

Lancelet, i. 154. 

, eyes of, i. 227. 

Landois, on the development of the 
wings of insects, i. 231. 

Land-shells, distribution of, ii. 186. 

, of Madeira, naturalised, ii. 

193. 

, resisting salt water, ii. 187. 

Languages, classification of, ii. 214. 

Lankester, Mr. E. Kay, on Longe- 
vity, i. 263. 

, on homologies, ii. 237. 

Lapse, great, of time, ii. 51. 

Larvae, ii. 241, 242, 243. 

Laurel, nectar secreted by the leaves, 
i. 114. 

Laurc-ntian formation, ii. 84, 

Laws of variation, i. 164. 

Leech, varieties of, i. 93. 

Leguminosse, nectar secreted by 
glands, i. 114. 
46 



Leibnitz' attack on Newton, ii. 294. 

Lepidosiren, i. 130 ; ii. 109. 

, limbs in a nascent condition, 

ii. 258. 
Lewes, Mr. G-. H., on species not 

having changed in Egypt, i. 263. 
■ , on the Salaniandra atra, ii. 

256. 
, on many forms of life having 

been at first evolved, ii. 300. 
Life, struggle for, i. 77. 
Lingula, Silurian, ii. 83. 
Linneeus, aphorism of, ii. 205. 
Lion, mane of, i. 109. 

, young of, striped, ii. 241. 

Lobelia fulgens, i. 90, 121. 

, s-terility of crosses, ii. 7. 

Lockwood, Mr., on the ova of the 

Hippocampus, i. 295. 
Locusts transporting seeds, ii. 147. 
Logan, Sir W.,on Lauren tian forma- 
tion, Ji. 84. 
Lowe, Rev. R. T., on locusts visiting 

Madeira, ii. 147. 
Lowness of structure connected 

with variability, i. 184. 
, related to wide distribution, 

ii. 196. 
Lubbock, Sir J., on the nerves of 

coccus, i. 54. 
, on secondary sexual charac- 
ters, i. 193. 
, on a diving hymenopterous 

insect, i. 222. 

, on affinities, ii. 73. 

, on metamorphoses, ii. 239, 

242. 
Lucas, Dr. P., on inheritance, i. 14. 
, on resemblance of child to 

parent, ii. 43. 
Lund and Clausen, on fossils of 

Brazil, ii. 121. 
Lyell, Sir C, on the struggle for 

existence, i. 77. 
, on modern changes of the 

earth, i. 118. 
, on terrestrial animals not 

having been developed on islands, 

i. 281. 
, on a carboniferous land-shell, 

ii. 59. 
, on strata beneath Siluria-B 

system, ii. 84. 



332 



LTELL. 



INDEX. 



Lyell, Sir C, on the imperfection of 
the geological record, ii. 88. 

, on the appearance of species, 

ii. 88. 

, on Barrande's colonies, ii. 90. 

, on tertiary formations of 

Europe and North America, ii. 
101. 

, on parallelism of tertiary for- 
mations, ii. 106. 

, on transport of seeds by ice- 
bergs, ii. 148. 

, on great alterations of cli- 
mate, ii. 170. 

, on the distribution of fresh- 
water shells, ii. 171. 

, on land-shells of Madeira, 

ii. 193. 

Lyell and Dawson, on fossilized trees 
in Nova Scotia, ii. 70. 

Lvthruru salicaria, trimorphic, ii. 
'32. 

M. 

Macleay, on analogical characters, 

ii. 218. 

uchenia, ii. 107. 
M'Donuell, Dr., on electric organs, 

i. 234. 
Madeira, plants of. i. 130. 

, beetles of, windless, i. 169. 

, fossil land-shells of, ii. 121. 

, birds of, ii 

Magpie t note in X" way, i. 325. 
L08. 

I > compared with 
pe, ii. 71. 

, mamma] - of, ii. 1 85. 

Malm, on flat-fish, i. 291. 
Bialpig • .. imperfect 

flowi rsof, i. _ 

, ii 

Ma urn. a, their development, i. 295. 

, rudimentary, ii. 255. 

Mammals, fossil, in secondary for- 
mation, ii. 

, insular, ii. 1S3. 

Mau, origin of, ii. 304. 

Manatee, rudimentary nails of, ii. 
260. 

Marsupials of Australia, i. 140. 

, structure of their feet, ii. 232 



Marsupials, fossil species of, ii. 121. 
Martens, M., experiment on seeds, 

ii. 144. 
Martin, Mr. TV". C, on striped mules, 

i. 201. 
Masters, Dr., on Saponaria, i. 272. 
Matteucci, on the electric organs of 

rays, i. 234. 
Matthiola, reciprocal crosses of, ii. 

15. 
Maurandia, i. 307. 
Menus of dispersal, ii. 140. 
Melipona domestica, i. 343. 
Merrell, Dr., on the American 

cuckoo, i. 330. 
Metamorphism of oldest rocks, ii. 85. 
. Mice destroying bees, i. 90. 

, acclimatisation of, i. 175. 

, tails of, i. 294. 

Miller, Prof., on the cells of bees, 

i. 344, 850. 
Mirabilis, crosses of, ii. 15. 
Missel-thrush, i. 93. 
Mistletoe, complex relations of, i. 3. 
Mivart. Mr., on the relation of hair 

and teeth, i. 179. 
■ . on the eyes of cephalopods, 

i. 237. 

, various objections to Natural 

tion, i. 275. 

, on abrupt modifications, i. 313. 

, on the resemblance of the 

mouse and antechinus, ii. 218. 
MocMng-throsh of the Galapagos 

ii. 193. 
Modiiic.ition of species not abrupt, 

ii. 2 
Moles, blind, i. 170. 
Molothrus, habits of, i. 334. 
Mongrel?, fertility and sterility of, 

ii. 34. 

and hybrids compared, ii. 39. 

Monkeys, fossil, ii. 79. 
Monaciiauthus, ii. 216. 
Mons, Van, on the origin of fruit- 
trees, i. 33. 
M< nstn si ties, i. 51. 
Moquin-Tandon, on sea-aide plants, 

i. 166. 
Morphology, ii. 231. 
Morren, on the leaves of Oxalis, 

i. 308. 
Moths, hybrid, ii. 9. 



MOZART. 



INDEX. 



333 



Mozart, musical powers of, i. 321. 

Mud, seeds in, ii. 175. 

Mules, striped, i. 201. 

Miiller, Adolf, on the instincts of the 
cuckoo, i. 331. 

Miiller, Dr. Ferdinand, on Alpine 
Australian plants, ii. 163. 

Miiller, Fritz, on dimorphic crus- 
taceans, i. 55, 362. 

, on the lance! et, i. 154. 

, on air-breathing crustaceans^ 

i. 238. 

, on climbing plants, i. 307. 

, on the self-sterility of orchids, 

ii. 7. 

, on embryology in relation to 

classification, ii. 210. 

, on the metamorphoses of crus- 
taceans, ii. 245, 253. 

, on terrestrial and fresh- water 

organisms not undergoing any 
metamorphosis, ii. 250. 

Multiplication of species not indefi- 
nite, i. 157. 

Muichison, Sir K.* on the forma- 
tions of Russia, ii. 60. 

■ , on azoic formations, ii. 84. 

, on extinction, ii. 94. 

Murie, Dr., on the modification of 
the sknll in old age, i. 233. 

Murray, Mr. A., on cave-insects, 
i. 173. 

Mustela vison, i. 216. 

Myanthus, ii. 216. 

M\ rmecocj stus, i. 359. 

Myrmica, eyes of, i. 361. 

N. 

N'ageli, on morphological characters, 

i. 266. 
Nails, rudimentary, ii. 260. 
Nathu>ius, Von, on pigs, i. 249. 
Natural history, luture progress of, 

ii. 301. 

selection, i. 97. 

system, ii. 204. 

Naturalisation of forms distinct from 

the indigenous species, i. 138. 
Naturalisation in New Zealand, i. 

255. 
Naudin, on analogous variations in 

gourds, i. 195. 
, on hybrid gourds, ii. 38. 



Naudin, on reversion, ii. 41. 
Nautilus, Silurian, ii. 83. 
Nectar of plants, i. 114. 
Nectaries, how formed, i. 114. 
Nelumbium luteum, ii. 176. 
Nests, variations in, i. 324, 355, 364. 
Neuter insects, i. 359, 360. 
Newman, Col., on humble-bees, i. 90. 
New Zealand, productions of, not 

perfect, i. 255. 

i naturalised products of,ii. 119. 

-, fossil birds of, ii. 121. 

, glaciers of, ii. 159. 

, crustaceans of, ii. 164. 

— — , algae of, ii. 164. 

, number of plants of, ii. 178. 

— -, flora of, ii. 189. 

Newton, Sir I., attacked for irre- 

ligion, ii. 294. 
— — , Prof., on earth attached to a 

partridge's foot, ii. 148. 
Nicotiaua, crossed varieties of, ii. 39. 
, certain species very sterile, 

ii. 14. 
Nitsche, Dr., on the Polyzoa, i. 301. 
Noble, Mr., on fertility of Khodo- 

dendron, ii. 8. 
Nodules, phosphatic, in azoic rocks, 

ii. 84. 

0. 

Oaks, variability of, i. 62. 

Onites, appelles, i. 168. 

Ononis, small imperfect flowers of, 

i. 269. 
Orchids, fertilisation of, i. 241. 
■ , the development of their 

flowers, i. 303. 

, forms of, ii. 216. 

Orchis, pollen of, i. 236. 
Organisation, tendency to advance, 

i. 151. 
Organs of extreme perfection, i. 223. 

, electric, of fishes, i. 234. 

— — of little importance, i. 245. 

, homologous, ii. 233. 

, rudiments of, and nascent, 

ii. 255. 
Ornithorhynchus, i. 130 ; ii. 208. 

, mammae of, i. 296. 

Ostrich not capable of flight, i. 281. 
— T-, habit of laying eggs together, 

i. 335. 



334 



INDEX. 



Ostrich, American, two species of, 


Pelvis of women, i. 178. 


ii. 132. 


Peloria, i. 180. 


Ot r er. habits of, how acquired, i. 


Period, glacial, ii. 151. 


216. 


Petrels, habits of, i. 221. 


Ouzel, water, i. 222. 


Phasianus, fertility of hybrids, ii. 9. 


.ven, Prof., on birds not flying, i. 


Pheasant, young, wild, i. 329. 


167. 


Pictet. Prof., on groups of species 


, on vegetative repetition, i. 


suddenly appearing, ii. 77. 


181. 


, on rate of organic change, ii 


— — , on variability of unusually 


90. 


developed parts, i. 185. 


, on continuous succession of 


, on the eyes of fishes, i. 227; 


genera, ii. 93. 


, on the swim-bladder of fishes, 


, on change in latest tertiary 


i. 231. 


forms, ii. 71. 


, on fossil horse of La Plata, ii. 


-, on close alliance of fossils in 


96. 


consecutive formations, ii. 114. 


. , on generalized form, ii. 107. 


, on early transitional links, ii. 


, on relation of ruminants and 


78. 


pachyderms, ii. 107. 


Pierce, Mr., on varieties of wolves, 


, on fossil birds of Xew Zea- 


i. 111. 


land, ii. 121. 


Pigeons with feathered feet and skin 


, on succession of types, ii. 121. 


between toes. i. 14. 


, on affinities of the dugong, 


, breeds described, and origin 


ii. 206. 


of, i. 23. 


, on homologous organs, ii. 233. 


, breeds of, how produced, i. 44, 


, on the metamorphosis of ce- 


47. 


phalopoda, ii. 244. 


, tumbler, not being able to get 




out of egir, i. 106. 


P. 


, reverting to blue colour, i. 197. 




, instinct of tumbling, i. 327. 


Pacific Ocean, faunas of, ii. 131. 


, young of, ii. 248. 


Pacini, on electric organs, i. 235. 


Pigs, black, not affected by the 


Pal« y. vn no organ formed to give 


paint-root, i. 13. 


paiu, i. 251. 


. modified by want of exercise. 


Pallas, on the fertility of the domes- 


i. 249. 


ticated descendants of wild stocks, 


Pisil, rudimentary, ii. 256. 


ii. 10. 


Plants, poisonous, not affecting cer- 


Palm with hooks, i. 247. 


tain coloured animals, i. 13. 


Papaper bracteatum, i. 272. 


, selection, applied to, i. 41 


Paraguay, cattle destroyed by flies, 


, gradual improvement of, i 42. 


i. 89 


, not improved in barbarous 


Parasites, i. 334. 


countries, i. 43. 


Partridge, with ball of earth at- 


, dimorphic, i. 55 ; ii. 29. 


tached to foot, ii. 148. 


, destroyed by insects, i. 83. 


Parts greatly developed, variable, 


, in midsr of range, have to 


i. 1-5. 


struggle with other p. ants, i. 95. 


Parns major, i. 220. 


, pectar of, i. 114. 


Pa^irlora, ii. 7. 


, flesh v, on sea-.- Lores, i. 166. 


Peaches in United States, i. 104. 


, climbing, i. 230, 305. 


Pear, srafts of, ii. 18. 


, fresh-water, distribution of, ii. 


Pedicellariae, i. 29S. 


174. 


Pelagornium, flowers of, i. 180. 


, l'-w in fcale, widely distri- 


, sterility of, ii. 7. 


buted, ii. Vjo. 



PLEURONEcrnm 



INDEX. 



EArEOPHAGUS. 



335 



Pleuronectidse, their structure, i. 

290. 
Plumage, laws of change in sexes 

of birds, i. 109. 
Plums in the United States, i. 104. 
Pointer dog, origin of, i. 40. 

, habits of, i. 327. 

Poison not affecting certain coloured 

animals, i. 13. 
, similar effect of, on animals 

and plants, ii. 299. 
Pollen of fir-trees, i. 257. 
transported by various means, 

i. 241, 252. 
Po'ilinia, their development, i. 304. 
Polyzoa, their avicularia, i. 301. 
Poole, Col., on striped hemionus, 

i. 202. 
Potemogeton, ii. 175. 
Pouchet, on the colours of flat-fish, 

i. 293. 
Presrwich, Mr., on English and 

Frenci: eocene formations, ii. 105. 
Pro totrupeo, i. 222. 
Pioteolepas, i. 183. 
Proteus, i. 173. 
Psychology, future progress of, ii. 

304. 
Pyrgoma, found in the chalk, ii. 81. 

Q. 

Quagga, striped, i. 201. 
Quatrefages, M., on hybrid moths, 

ii. 9. 
Qnercus, variability of, i. 62. 
Quince, grafts of, ii. 18. 

R. 

Rabbits, disposition of young, i 
328. 

Ra<-es, domestic, characters of, i. 18. 

Kuce-lmrses, Arab. i. 40. 

, English, ii. 140. 

Radcliffe, Dr., the electrical organs 
of the torpedo, i. 234. 

Ramond, on plants of Pyrenees, ii. 
153. 

Ramsay, Prof., on subacrial denu- 
dation, ii. 53. 

■ , on thickness of the British 

formations, ii. 55, 56. 

. on faults, ii. 55. 



Ramsay, Mr., on instincts of cuckoo, 
i. 333. 

Ratio of increase, i. 79. 

Rats supplanting each other, i. 93. 

, acclimatisation of, i. 175. 

— — , blind, in cave, i. 171. 

Rattle-snake, i. 254. 

Reason and instinct, i. 319. 

Recapitulation, general, ii. 267. 

Reciprocity of crosses, ii. 14. 

Record, geological, imperfect, ii. 48. 

Rengger, on flies destroying cattle, 
i. 89. 

Reproduction, rate of, i. 79. 

Resemblance, protective, of insects, 
i. 283. 

to parents in mongrels and 

hybrids, ii. 41. 

Reversion, law of inheritance, i. 
16. 

, in pigeons, to blue colour, i. 

198. 

Rhododendron, sterility of, ii. 7, 8. 

Richard, Prof., on Aspicarps, ii. 209. 

Richardson, Sir J., on structure of 
squirrels, i. 216. 

, on fishes of the southern hemi- 
sphere, ii. 164. 

Robinia, grafts of, ii. 19. 

Rodents, blind, i. 170. 

Rogers, Prof., Map of N. America, 
ii. 65. 

Rudimentary organs, ii. 255. 

Rudiments important for classifica- 
tion, ii. 207. 

Riitimeyer, on Indian cattle, i. 21 ; 
ii. 10. 

S. 

Salamandra atra, ii. 256. 
Saliva used in nests, i. 355. 
Salvin, Mr., on the beaks of ducks, 

i. 287. 
Sageret, on grafts, ii. 18. 
Salmons, males fighting, and hooked 

jaws of, i. 108. 
Salt water, how far injurious te 

seeds, ii. 142. 
not destructive to land-shells, 

ii. 187. 
Salter, Mr., on early death of hybrid 

embryos, ii. 23. 
Saurophagus sulphuratus, i. 220. 



336 



SCHACHT. 



INDEX. 



BQUAliODON. 



Schacht, Prof., on Phyllotaxy, i. 


Shells, fresh-water, long retain the 


270. 


same forms, ii. 117. 


Sehiodte, on blind insects^ i. 172. 


, fresh-water, dispersal of, ii. 


, on flat-fish, i. 290. 


173. 


Schlegel, on snakes, i. 178. 


, of Madeira, ii. 180. 


School, Dr., on the ears of mice, i. 


, land, distribution of, ii. 180. 


268. 


, land, resisting salt water, ii. 


Scott, J.. Mr., on the self-sterility 


187. 


of orchids, ii. 7. 


Shrew-mouse, ii. 218. 


— — , on the crossing of varieties of 


Silene, infertility of crosses, ii. 11. 


verbascuni, ii. 38. 


Silliman, Prof., on blind rat, i. 171. 


Sea-watt r, how far injurious to 


Sirenici, then- affinities, ii. 108. 


seeds, ii. 142. 


Sitaris, metamorphosis of, ii. 252. 


not destructive to land-shells, 


Skulls of young mammals, i. 218 ; 


ii. 187. 


ii. 235. 


Sebright, Sir J., on crossed animals, 


Slave-making instinct, i. 336. 


i. 23. 


Smith, Col. Hamilton, on striped 


Sedgwick, Prof., on groups of spe- 


h rses, i. 200. 


cies suddenly appearing, ii. 77. 


, Mr. Fred., on slave-making 


Seedlings destroyed by insects, i. 


ants, i. 337. 


83. 


, on neuter ants, i. 360. 


Seeds, nutriment in, i. 91. 


Smitt, Dr.. on the Polyzoa, i. 301. 


, winged, i. 181. 


Snake with tooth for cutting through 


, means of dissemination, i. 


egg-shell, i. 331. 


210, 252; ii. 116. 


Somerville, Lord, on selection of 


, power of resisting salt water, 


sheep, i. 35. 


ii. 113. 


Sorbus, grafts of, ii. 19. 


, in crops and intestines of 


Sorex, ii. 218. 


birds, ii. 116. 


Spaniel. King Charles'sbreed,i. 40. 


, eaten by fish. ii. 116, 176. 


Specialisation of organs, i. 152. 


, in mud, ii. 1 75. 


bpeeies, polymorp, ic, i. 51. 


, hooked, on islands, ii. 181. 


, dominant, i. 67. 


Selection of domestic products, i. 


, commou, variable, i. 66. 


31. 


in large genera variable, i. 69. 


, principle not of recent origin, 


, groups of, suddenly appear- 


i. 3.'. 


in g, ii. 77, 82. 


, unconscious, i. 39. 


beneath Silurian formations, 


, natural, i. y7. 


ii. 84. 


, sexu d, i. 107. 


successively appearing, ii. 89. 


, objections to term, i. 99. 


ehauging ,-imultaneously 


natural, has not induced steri- 


throughout the world, ii. Io0. 


lity, ii. 20. 


Spencer, L >rd, on increase in size of 


Sexes, relations of, i. 108. 


cattle, i. 10. 


Sexual characters vaiable, i. 191. 


. Herbert, Mr., on the first steps 


Selection, i. 1U7. 


in differentiation,! 155. 


Sheep. Merino, their selection, i. 36. 


, on the tendency to an equili- 


, two sub-breeds, unintention- 


brium in all forces, ii. 29. 


ally produced, i. 11. 


Sphex. parasitic, i. 336. 


, mountain varieties of, i. 93. 


Spiders, development of, ii. 245. 


Shells, colours of, i. 165. 


Sports in plants, i. 11. 


, hinges of, i. 210. 


fcjprengel, C. C, on crossing, i. 119. 


, littoral, seldom embedded, ii. 


. on ray-florets, i. ISO. 


5b. 


Squ.iLdon, ii. 108. 



SQUIRRELS. 



INDEX. 



TUMBLER. 



337 



Squirrels, gradations in structure, i. 

216. 
Staffordshire, heath, changes in, i. 

87. 
Stag-beetles, fighting, i. 108. 
Star-fishes, eyes of, i. 225. 

, their pedieellarise, i. 299. 

Sterility from changed conditions of 

life, i. 10. 

of hybrids, ii. 3. 

, laws of, ii. 11. 

, causes of, ii. 20. 

, from unfavourable conditions, 

ii. 26. 
not induced through natural 

selection, ii. 21. 
St. Helena, productions of, ii. 178. 
St. Hilaire, Aug., on variability of 

certain plants, i. 272. 

, on classification, ii. 209. 

St. John, Mr., on habits of cats, i. 

325. 
Sting of bee, i. 256. 
Stocks, aboriginal, of domestic ani- 

mals, i. 22. 
Strata, thickness of, in Britain, ii. 

55. 
Stripes on horses, i. 199. 
Structure, degrees of utility of, i. 

249. 
Struggle for existence, i. 75. 
Succession, geological, ii. 89. 

of types in same areas, ii. 121. 

Swallow, one spedes supplanting 

another, i. 93. 
Swaysland, Mr., on earth adhering 

to the feet of migratory birds, ii. 

148. 
Swifts, nests of, i. 355. 
Swim-bladder, i. 230. 
Switzerland, lake habitations of, i. 

20. 
Sy=ttm, natural, ii. 204. 



T. 



Tail of giraffe, i. 245. 

of aquatic animals, i. 246. 

, prehensile, i. 294. 

, rudimentary, ii. 260. 

Tanais, dimorphic, i. 55. 

Tarsi, deficient, i. 168. 

Tausch, Dr., on umbelliferaB, i. 271. 



Teeth and hair correlated, i. 179. 

, rudimentary, in embryonic, 

calf, ii. 255, 292. 

Tegetmeier, Mr., on cells of bees,i. 
316, 352. 

Temminck, on distribution aiding 
classification, ii. 211. 

Tendrils, their development, i. 
305. 

Thompson, Sir W., on the age of 
the habitable world, ii. 83. 

, on the consolidation of the 

crust of the earth, ii. 275. 

Thouin, on grafts, ii. 19. 

Thrush, aquatic species of, i. 222. 

, mocking, of the Galapagos, ii. 

192. 

, young of, spotted, ii. 241. 

, nest of, i. 364. 

Thuret, M., on crossed fnci, ii. 15. 

Thwaites, Mr., on acclimatisation, 
i. 174. 

Thylacinus, ii. 220. 

Tierra del Fuego, dogs of, i. 328. 

, plants of, ii. 169. 

Timber-drift, ii. 145. 

Time, lapse of, ii. 51. 

by itself not causing modifica- 
tion, i. 126. 

Titmouse, i. 220. 

Toads on islands, ii. 182. 

Tobacco, crossed varieties of, ii. 38. 

Tomes, Mr., on the distribution of 
bats, ii. 184. 

Transitions in varieties rare, i. 208. 

Traquair, Dr., on flat-fish, i. 293. 

Traulschold, on intermediate varie- 
ties, ii. 66. 

Trees on islands belong to peculiar 
orders, ii. 182. 

with separated sexes, i. 123, 

Trifolium pratense, i. 90, 117. 

incarnatum, i. 117. 

Trigonia, ii. 99. 

Trilobites, ii. 83. 

, sudden extinction of, 99. 

Trimen, Mr., on imitating-iusects, 
ii. 224. 

Trimorphism in plants, i. 55; ii. 29. 

Troglodytes, i. 364. 

Tuco-tuco, blind, i. 170. 

Tumbler pigeons, habits of, heredi- 
tary, i. 327. 



338 



TOIBLEB. 



INDEX. 



WATERH0T7SE. 



Tumbler, young of, ii. 248. 
Turkev-cock, tuft of hair on breast, 

i. lib. 

, naked skin on head, i. 24S. 

. voun? of, instinctively wild, 

i. 329. 
Turnip and cabbage, analogous 

variations of, i. 195. 
Type, unity of, i. 260, 261. 
Tvpes. succession of, in same areas, 

"ii. 121. 
Typotherium, ii. 108. 



U. 



Udders enlarged by use, i. 12. 

, rudimentary, ii. 256. 

Ulex, young leaves of, ii. 241. 
Umbelliftrje, flowers and seeds of, 

i. 180. 
, outer aud inner florets of, i. 

270. 
Unity of type, i. 260, 261. 
Uria lacrymans, i. 113. 
Use, effects of, under domestication, 

i. 12. 
, effects of. in a state of nature, 

i. 167. 
Utility, lmw far important in the 

construction of each part, i. 249. 



V. 



Valenciennes, on fresh-water fish, 

ii. 173. 
Varia bility of mongrels and hybrids, 

ii. 39. 
Variation under domestication, i. 8. 
caused by reproductive system 

being . by conditions of 

life, i. 10. 

under nature, i. 51. 

, la\\> o\ i. 164. 

. correlated, i. 13, 177, 248. 

Variations appear at corresponding 

. i. 16, 1 5. 
• analogous in distinct species, 

i. 19a 
Varieties, natural, i. 50. 

, struggle b tween, i. 93. 

, domestic, extinction of, i. 131. 

, transitional, rarity of. i. 208. 

i , when crossed, fertile, ii. 34. 



Varieties, when crossed, sterile, ii. 37 

, classification of, ii, 215. 

Verbascum, sterility of, ii. 7. 

. varieties of crossed, ii. 38. 

Verlot, M., on double stocks, i. 358. 
Vemeuil. M. de, on the succession 

of species, ii. 103. 
Vibracula of the Polyzoa, i. 301. 
Viola, small imperfect flowers of, i. 

269. 

-, tricolor, i. 90. 

Viichow, on the structure of the 

crystalline lens, i. 227. 
Virginia, pigs of, i. 104. 
Volcanic islands, denudation of, ii. 

54. 
Vulture, naked skin on head, i. 247. 



W 



Wading-birds, ii. 175. 

Wagner, Dr., on Cecidomyia, ii 

239. 
Wagner, Moritz, on the importance 

of isolation, i. 127. 
Wallace, Mr., on origin of species, 

i. 2. 
, on the limit of variation under 

domestication, i. 48. 
, on dimorphic lepidoptera, i. 

55, 362. 
, i n races in the Malay Archi- 

pelagi », i. 58. 
, on the improvement of the 

eye, i. 227. 
, on the walking-stick insect, i 

284. 
, on laws of geographical dis- 
tribution, ii. 139. 
, on the Malay Archipelago, ii 

185. 

, on mimetic animals, ii. 224. 

Walsh, Mr. B. D., on phytuphagic 

forms, i. 60. 

. on equal variability, i. 195. 

Water, fresh, productions of, ii. 171. 

Water-hen. i. 222. 

Waterhouse, Mr., on Australian 

marsupials, i. 140. 
. en greatly developed parts 

being variable, i. 185. 

, on the cells of bees, i. 313. 

, on general affinitk-s, ii. 227. 



■WATEE-OrZEL. 



INDEX. 



ZErGLODON. 



339 



Water-ouzel, i. 222. 

Watson, Mr. H. C, on range of 
varieties of British plants, i. 57, 
73. 

, on acclimatisation, i. 134. 

, on flora of Azores, ii. 149. 

, on Alpine plants, ii. 153. 

, on rarity of intermediate va- 
rieties, i. 212. 

, on convergence, i. 156. 

, on tbe indefinite multiplica- 
tion of species, i. 157. 

Weale, Mr., on locusts transporting 
seeds, ii. 147. 

Web of feet in water-birds, i. 223. 

Weismann, Prof., on tbe causes of 
variability, i. 8. 

, on rudimentary organs, ii. 260. 

"West Indian Islands, mammals of, 
ii. 185. 

Westwood, on species in large genera 
being closely allied to others, i. 
71. 

, on tbe tarsi of Engidae, i. 192. 

, on the antenna? of hymeno- 

pterous insects, ii. 207. 

Whales, i. 285. 

Wheat, varieties of, i. 137. 

White Mountains, flora of, ii. 151. 

Whittaker, Mr., on lines of escarp- 
ment, ii. 53. 

Wiehura, Max, on hybrids, ii. 24, 
27, 41. 

Wings, reduction of size, i. 169. 

of insects homologous with 

branchia3, i. 231. 

, rudimentary, in insects, ii. 

255. 

Wolf crossed with dosr, i. 327. 

of Falkland Ides, ii. 183. 

Wollaston, Mr., on varieties of in- 
sects, i. 59. 

, on fossil varieties of shells in 

Madeira, i. 65. 



Wollaston, Mr., on colours of insects 
on sea-shore, i. 165. 

, on wingless beetles, i. 169. 

, on rarity of intermediate va- 
rieties, i. 212. 

, on insular insects, ii. 178. 

, on land-shells of Madeira 

naturalised, ii. 193. 

Wolves, varieties of, i. 111. 

Woodcock with earth attached to 
leg, ii. 148. 

Woodpecker, habits of, i. 220. 

, green colour of, i. 247. 

Woodward. Mr., on the duration of 
specific forms, ii. 66. 

, on Pyrgoma, ii. 81. 

, on the continuous succession 

of genera, ii. 93. 

, on the succession of types, ii. 

121. 

World, species changing simultane- 
ously throughout, ii. 100. 

Wrens, nest of, i. 364. 

Wright, Mr. Chauncey, on tbe 
giraffe, i. 278. 

, on abrupt modifications, i. 

316. 

Wyman, Prof., on correlation of 
colour and effects of poison, i. 
13. 

, on the cells of the bee, i. 345 



Youatt, Mr., on selection, i. 35. 

, on sub-breeds of sheep, i. 41. 

, on rudimentary horns in 

young cattle, ii. 261. 

Z. 

Zanthoxylon, i. 272. 
Zebra, stripes on, i. 199, 
Zeuglodon, ii. 108. 



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