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



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Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close in- 
terbreeding, 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 uni- 
versal — Hybrids and mongrels compared independently of their 
fertility — Summary . Page 1 



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 palaeontological collections — On the 
intermittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden appearance of groups of species 
— On their sudden appearance in the lowest known f ossiliferous 
strata — Antiquity of the habitable earth .... 48 




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 . , Page 89 



Present distribution cannot be accounted for by differences ii» 
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 ocoasional means — Dispersal during the Glacial period 
—Alternate Glacial periods in the North and South . . 12S 



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 171 




Classification, groups subordinate to groups — Natural system — 
Rules and difficulties in classification, explained on the theory 
of descent with modification — Classification of varieties — De- 
scent always used in classification — Analogical or adaptive char- 
acters—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 super- 
vening at an early age, and being inherited at a correspond- 
ing age — Rudimentary organs : their origin explained — Sum- 
mary Page 203 


recapitulation and conclusion. 

Recapitulation of the objections to the theory of Natural Selection 
— Recapitulation of the general and special circumstances in its 
favour — Causes of the general belief in the immutability of 
species — How far the theory of Natural Selection may be ex- 
tended — Effects of its adoption on the study of Natural History 
— Concluding remarks 267 

Glossary of Scientific Terms 307 

Index • 323 




Distinction between the sterility of first crosses and of hybrids- 
Sterility various in degree, not universal, affected by close in- 
terbreeding, 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 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 en- 
dowed 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 dis- 
tinct had they been capable of freely crossing. The 
subject is in many ways important for us, more especial- 
ly 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 dif- 
ferences in the reproductive systems of the parent- 

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

Pure species have of course their organs of 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 per- 
fect; in the second case they are either not at all de- 
veloped, 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 en- 
dowment, beyond the province of our reasoning 

The fertility of varieties, that is of the forms known 
or believed to be descended from common parents, when 
crossed, and likewise the fertility of their mongrel off- 
spring, is, with reference to my theory, of equal im- 
portance 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 spe- 


cies when crossed and of their hybrid offspring. It is 
impossible to study the several memoirs and works of 
those two conscientious and admirable observers, Kol- 
reuter 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 to- 
gether, he unhesitatingly ranks them as varieties. Gart- 
ner, also, makes the rule equally universal; and he dis- 
putes 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 alwa5's 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 (ex- 
cluding all cases such as the Leguminosae, in which there 
is an acknowledged dijB&culty in the manipulation) 
half of these twenty plants had their fertility in some 
degree impaired. Moreover, as Gartner repeatedly 

4 HYBRIDISM. [Chap. IX. 

crossed some forms, such as the common red and 
blue pimpernels (Anagallis arvensis and ccerulea), 
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 be- 

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 ex- 
perienced observers who have ever lived, namely Kol- 
reuter 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 con- 
stitutional 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 for 


ten generations, yet he asserts positively that their fer- 
tility never increases, but generally decreases greatly and 
suddenly. With respect to this decrease, it may first be 
noticed that when any de^dation in structure or constitu- 
tion 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 di- 
minished in nearly all these cases by an independent 
cause, namely, by too close interbreeding. I have made 
so many experiments and collected so many facts, show- 
ing on the one hand that an occasional cross with a dis- 
tinct individual or variety increases the vigour and fer- 
tility 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 strengthened in this conviction 
by a remarkable statement repeatedly made by Gartner, 
namely, that if even the less fertile hybrids be artificially 
fertilised with hybrid pollen of the same kind, their 
fertility, notwithstanding the frequent ill effects from 
manipulation, sometimes decidedly increases, and goes 
on increasing. Now, in the process of artificial fertilisa- 
tion, pollen is as often taken by chance (as I know from 
my own experience) from the anthers of another flower. 


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

Now let us turn to the results arrived at by a third 
most experienced hybridiser, namely, the Hon. and Rev, 
W. Herbert. He is as emphatic in his conclusion that 
some hybrids are perfectly fertile — as fertile as the pure 
parent-species — as are 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 ex- 
ample, namely, that " every ovule in a pod of Crinum 
capense fertilised by C. revolutum produced a plant, 
which I never saw to occur in a case of its natural fecun- 
dation." 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 


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 soiind by fertilising other 
plants or species. In the genus Hippeastrum, in Cory- 
dalis as shown by Professor Hildebrand, in various or- 
chids as shown by IVIr. 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 vege- 
tated freely." Mr. Herbert tried similar experiments 
during many years, and always with the same result. 
These cases serve to show on what slight and mysterious 
causes the lesser or greater fertility of a species some- 
times 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, Khodo- 
dendron, &c., have been crossed, yet many of these 
hybrids seed freely. For instance, Herbert asserts that 

8 HYBRIDISM. [Chap. IX. 

a hybrid from Calceolaria integrifolia and plantaginea, 
species most widely dissimilar in general habit, " re- 
produces 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 Ehododendrons, and I am 
assured that many of them are perfectly fertile. Mr. 
C. Noble, for instance, informs me that he raises stocks 
for grafting from a hybrid between Rhod. ponticum and 
catawbiense, and that this hybrid " seeds as freely as it 
is possible to imagine." Had hybrids when fairly 
treated, always gone on decreasing in fertility in each 
successive generation, as Gartner believed to be the 
case, the fact would have been notorious to nursery- 
men. Horticulturists raise large beds of the same hy- 
brid, 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 hy- 
brid Ehododendrons, 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 breeding freely under 


confinement, few experiments have been fairly tried: for 
instance, the canary-bird has been crossed with nine dis- 
tinct 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 in- 
stance 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 con- 
stantly 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 increas- 

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 Reevesii, and from Phasianus colchicus 
with P. torquatus, are perfectly fertile. M. Quatrefages 
states that the hybrids from two moths (Bombyx cyn- 
thia 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 off- 
spring, which are highly fertile when crossed with one 
of the parent-species. The hybrids from the common 
and Chinese geese (A. cygnoides), species which are 
so different that they are generally ranked in distinct 
genera, have often bred in this country with either pure 
parent, and in one single instance they have bred inter 

10 HYBRIDISM. [Chap. IX. 

se. This was effected by Mr. Eyton, who raised two 
hybrids from the same parents, but from different 
hatches; and from these two birds he raised no less than 
eight hybrids (grandchildren of the pure geese) from one 
nest. In India, however, these cross-bred geese must 
be far more fertile; for I am assured by two eminently 
capable judges, namely Mr. Blyth and Capt. Hutton, 
that whole flocks of these crossed geese are kept in 
various parts of the country; and as they are kept for 
profit, where neither pure parent-species, exists, they 
must certainly be highly or perfectly fertile. 

With our domesticated animals, the various races 
when crossed together are quite fertile; yet in many 
cases they are descended from two or more wild species. 
From this fact we must conclude either that the abo- 
riginal parent-species at first produced perfectly fertile 
hybrids, or that the hybrids subsequently reared under 
domestication became quite fertile. This latter alter- 
native, 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 sev- 
eral 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 ob- 
servations by Eiitimeyer on their important osteological 
differences, as well as from those by Mr. Blyth on their 
differences in habits, voice, constitution, &c., these two 


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

Finally, considering all the ascertained facts on the 
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 

"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 king- 

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


many curious ways this gradation can be shown; but 
only the barest outline of the facts can here be given. 
When pollen from a plant of one family is placed on 
the stigma of a plant of a distinct family, it exerts no 
more influence than so much inorganic dust. From 
this absolute zero of fertility, the pollen of different spe- 
cies 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 with- 
ering of the flower is well known to be a sign of in- 
cipient fertilisation. From this extreme degree of 
sterility we have self-fertilised hybrids producing a 
greater and greater number of seeds up to perfect fer- 

The hybrids raised from two species which are very 
difiicult 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- 


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 condi- 

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 hy- 
brids 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 syste- 
matic affinity and the facihty 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 dis- 
tinct species which unite with the utmost facility. In 
the same family there may be a genus, as Dianthus, in 


which very many species can most readily be crossed; 
and another genus, as Silene, in which the most 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 Nicotiana have been 
more largely crossed than the species of almost any other 
genus; but Gartner found that ]S1. acuminata, which 
is not a particularly distinct species, obstinately failed 
to fertilise, or to be fertilised by no less than eight other 
species of Nicotiana. Many analogous facts could be 

No one has been able to point out what kind or what 
amount of difference, in any recognisable character, is 
sufficient to prevent two species crossing. It can be 
shown that plants most widely different in habit and 
general appearance, and having strongly marked differ- 
ences in every part of the flower, even in the pollen, in 
the fruit, and in the cotyledons, can be crossed. An- 
nual and perennial plants, deciduous and evergreen trees, 
plants inhabiting different stations and fitted for ex- 
tremely different climates, can often be crossed with 

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


their reproductive systems. The diversity of the result 
in reciprocal crosses between the same two species was 
long ago observed by Kolreuter. To give an instance: 
Mirabilis jalapa can easily be fertilised by the pollen of 
M. longiflora, and the hybrids thus produced are suffi- 
ciently fertile; but Kolreuter tried more than two hun- 
dred 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 recipro- 
cal crosses is extremely common in a lesser degree. He 
has observed it even between closely related forms (as 
Matthiola annua and gilabra) 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 spe- 
cies having first been used as the father and then as 
the mother, though they rarely differ in external char- 
acters, yet generally differ in f ertihty in a small, and oc- 
casionally 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 ol them; and such hybrids, 
though externally so like cue cf their pure parent- 
epecies, are with rare exception; sstremely sterile. So 


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

Considering the several rules now given, whicli 
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 unfa- 
vourable 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 syste- 
matic affinity or degree of resemblance to each other. 
This latter statement is clearly proved by the differ- 
ence 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 gen- 
erally some dijfference, and occasionally the widest pos- 
sible difference, in the facility of effecting an union. 
The hybrids, moreover, produced from reciprocal crosses 
often differ in fertility. 

Now do these complex and singular rules indicate 


that species have been endor7ed 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 im- 
portant to keep from blending together? Why should 
the degree of sterility be innately variable in the in- 
dividuals of the same species? Why should some spe- 
cies 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 re- 
ciprocal 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 lim- 
ited 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 ad- 
visable 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 un- 
important for their welfare in a state of nature, I pre- 


Bume that no one will suppose that this capacity is a 
specially endowed quality, but will admit that it is inci- 
dental 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 di- 
versity 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 ap- 
ple, which is a member of the same genus. Even dif- 
ferent varieties of the pear take with different de- 
grees of facility on the quince; so do different varie- 
ties of the apricot and peach on certain varieties of the 

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 spe- 


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

We have seen that the sterility of hybrids, which 
have their reproductive organs in an imperfect 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 fertil- 
ised 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 dis- 
tinct 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 differ- 
ences in their vegetative systems, so I believe that the 
still more complex laws governing the facility of first 


crosses are incidental on unknown differences in their 
reproductive systems. These differences in both cases, 
follow to a certain extent, as might have been expected, 
systematic affinity, by which term every kind of resem- 
blance and dissimilarity between organic beings is at- 
tempted 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 diffi- 
culty is as important for the endurance and stability of 
specific forms, as in the case of grafting it is unimpor- 
tant 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 cer- 
tain individuals of one variety when crossed with those 
of another variety. For it would clearly be advantage- 
ous 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 sepa- 
rated species to have been rendered mutually sterile, 
and consequently this could not have been effected 
through natural selection; but it may perhaps be argued. 



that, if a species was rendered sterile with some one com- 
patriot, sterility with other species would follow as a 
necessary contingency. In the second place, it is almost 
as much opposed to the theory of natural selection as to 
that of special creation, that in reciprocal crosses the 
male element of one form should have been rendered 
utterly impotent on a second form, whilst at the same 
time the male element of this second form is enabled 
freely to fertilise the first form; for this peculiar state 
of the reproductive system could hardly have been ad- 
vantageous 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 
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? Yet 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 modifica- 
tions in their structure and fertility have been slowly 
accumulated by natural selection, from an advantage 
having been thus indirectly given to the community to 
which they belonged over other communities of the same 
species; but an individual animal not belonging to a so- 
cial community, if rendered slightly sterile when crossed 
with some other variety, would not thus itself gain any 
advantage or indirectly give any advantage to the other 
individuals of the same variety, thus leading to their 

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 in- 
cluding 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- 
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 na- 
ture 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 spe- 
cies 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 communi- 
cated to me by Mr. Hewitt, who has had great experience 
in hybridising pheasants and fowls, that the early death 
of the embryo is a very frequent cause of sterility in 
first crosses. Mr. Salter has recently given the results 
of an examination of about 500 eggs produced from 
Various crosses between three species of Gallus and their 
hybrids; the majority of these eggs had been fertilised; 
and in the majority of the fertilised eggs, the embryos 
had either been partially developed and had then per- 


ished, or had become nearl}- 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 tlie first few days, or at latest weeks, " with- 
out any obvious cause, apparently from mere inability 
to Uve; " so that from the 500 eggs only twelve chick- 
ens were reared. With plants, hybridised embryos prob- 
ably often perish in a like manner; at least it is known 
that hybrids raised from very distinct species are some- 
times 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 eases 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 pro- 
duced by a cross between distinct species. Until becom- 
ing acquainted with these facts, I was unwilling to be- 
lieve 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. Hy- 
brids, 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 par- 
takes of only half of the nature and constitution of ita 
mother; it may therefore before birth, as long as it is 
nourished within its mother's womb, or within the egg 
or seed produced by the mother, be exposed to condi- 
tions in some degree unsuitable, and consequently bf 
liable to perish at an early period; more especially as all 
very young beings are eminently sensitive to injuriou.s 
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 imper- 
fectly 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 inde- 
pendent 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 some- 
times the female more than the male. In both, the 
tendency goes to a certain extent with systematic affin- 
ity, 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 fer- 
tility; and certain species in a group will produce un- 
usually fertile hybrids. No one can tell, till he tries, 
whether any particular animal will breed under confine- 
ment, or any exotic plant seed freely under culture; 
nor can he tell till he tries, whether any two species 
of a genus will produce more or less sterile hybrids. 
Lastly, when organic beings are placed during several 
generations under conditions not natural to them. 


they are extremely liable to vary, which seems to be 
partly due to their reproductive systems having been 
specially affected, though in a lesser degree than 
when sterility ensues. So it is with hybrids, for their 
offspring in successive generations are eminently liable 
to vary, as every experimentalist has observed. 

Thus we see that when organic beings are placed 
under new and unnatural conditions, and when hybrids 
are produced by the 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 varia- 
ble, does not diminish; it is even apt to increase, this 
being generally the result, as before explained, of too 
close interbreeding. The above view of the sterility 
of hybrids being caused by two constitutions being com- 
pounded into one has been strongly maintained by Max 


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 un- 
natural conditions, is rendered sterile. All that I have 
attempted to show is, that in two cases, in some respects 
allied, sterility is the common result, — in the one case 
from the conditions of life having been disturbed, in 
the other case from the organisation having been dis- 
turbed 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 evi- 
dence, 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 con- 
valescence of animals, great benefit is derived from al- 
most 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 fer- 
tility to the offspring; and that close interbreeding 
continued during several generations between the near- 
est 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 be- 
ings, and on the other hand, that slight crosses, that is 
crosses between the males and females of the same spe- 
cies, which have been subjected to slightly different 
conditions, or which have slightly varied, give vigour 
and fertility to the offspring. But, as we have seen, or- 
ganic beings long habituated to certain uniform condi- 
tions under a state of nature, when subjected, as under 
confinement, to a considerable change in their condi- 
tions, 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 gen- 
erally sterile. He will at the same time be able to ex- 
plain how it is that the races of some of our domesticated 
animals, which have often been subjected to new and 
not uniform conditions, are quite fertile together, al- 
though 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 con- 
nected together by some common but unknown bond, 
which is essentially related to the principle of life; this 
principle, according to Mr. Herbert Spencer, being that 
life depends on, or consists in, the incessant action and 
reaction of various forces, which, as throughout nature, 
are always tending towards an equilibrium; and when 


this tendency is slightly disturbed by any change, the 
vital forces gain in power. 

Reciprocal Dimorphism and 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 dif- 
ferently 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 sta- 
mens 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 cor- 
responding height in another form. So that with di- 
morphic species two unions, which may be called legiti- 
mate, are fully fertile; and two, which may be called 
illegitimate, are more or less infertile. With trimor- 
phic species six unions are legitimate, or fully fer- 
tile, — and twelve are illegitimate, or more or less infer- 


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, up to absolute and utter steril- 
ity; just in the same manner as occurs in crossing dis- 
tinct species. As the degree of sterility in the latter 
ease depends in an eminent degree on the conditions 
of life being more or less favourable, so I have found 
it with illegitimate unions. It is well known that if 
pollen of a distinct species be placed on 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 anni- 
hilates 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 il- 
legitimately, and twenty-four hours afterwards legiti- 
mately with the 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 il- 
legitimate pollen. Again, as in making reciprocal 
crosses between the same two species, there is occasion- 
ally 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 fer- 
tilised with the greatest ease by pollen from the longer 
stamens of the short-styled form, and yielded many 
seeds; but the latter form did not yield a single seed 


when fertilised by the longer stamens of the mid-styled 

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 re- 
sult is that these illegitimate plants, as they may be 
called, are not fully fertile. It is possible to raise from 
dimorphic species, both long-styled and short-styled 
illegitimate plants, and from trimorphic plants all three 
illegitimate forms. These can then be properly united 
in a legitimate manner. When this is done, there is no 
apparent reason why they should not yield as many 
seeds as did their parents when legitimately fertilised. 
But such is not the case. They are all infertile, in 
various degrees; some being so utterly and incurably 
sterile that they did not yield during four seasons a 
single seed or even seed-capsule. The sterility of these 
illegitimate plants, when united with each other in ? 
legitimate manner, may be strictly compared with that 
of hybrids when crossed inter se. If, on the other hand, 
a hj'brid is crossed with either pure parent-species, the 
sterility is usually much lessened: and so it is when 
an illegitimate plant is fertilised by a legitimate plant. 
In the same manner as the sterility of hybrids does not 
always run parallel with the difficulty of making the 
first cross between the two parent-species, so the sterility 
of certain illegitimate plants was unusually great, whilst 
the sterility of the union from which they were derived 
was by no means great. With hybrids raised from the 
same seed-capsule the degree of sterility i« 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 pro- 
duce few flowers, and are weak, miserable dwarfs; 
exactly similar cases occur with the illegitimate off- 
spring 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 re- 
spects 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 seeds, and that thej 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 
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, doubt- 
ful whether this is really so; but I will not enlarge 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 
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, Gart- 
ner, likewise concluded that species when crossed are 
sterile owing to differences confined to their reproduc- 
tive svstems. 

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

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

If we turn to varieties, produced, or supposed to have 
been produced, under domestication, we are still in- 
volved in some doubt. For when it is stated, for in- 
stance, that certain South American indigenous domes- 
tic dogs do not readily unite with European dogs, the 
explanation which will occur to every one, and probably 

Chap. IX.] WHEN CROSSED. 35 

the true one, is that they are descended from aborigi- 
nally distinct species. Nevertheless the perfect fertil- 
ity 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 es- 
pecially 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 amoiant of external difference between two 
species is no sure guide to their degree of mutual steril- 
ity, 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 modify- 
ing the reproductive system in a manner leading to 
mutual sterility, that we have good grounds for admit- 
ting 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 fer- 
tile 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 op- 
posite manner, for they have become self-impotent 
whilst still retaining the capacity of fertilising, and 
being fertilised by, other species. If the Pallasian doc- 
trine of the elimination of sterility through long-con- 


tinued 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 our present subject is not, as it 
appears to me, why domestic varieties have not become 
mutually infertile when crossed, but why this has so 
generally occurred with natural varieties, as soon as they 
have been permanently modified in a sufficient degree 
to take rank as species. We are far from 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 ex- 
istence with numerous competitors, will have been ex- 
posed 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 man- 
ner be eminently sensitive to the influence of an un- 
natural 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 

Chap. iX.] WHEN CROSSED, 37 

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 sepa- 
rated sexes. No one, I believe, has suspected that these 
varieties of maize are distinct species; and it is impor- 
tant 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 

Girou de Buzareingues crossed three varieties of 
gourd, which like the maize has separated sexes, and he 
asserts that their mutual fertilisation is by so much the 


less easy as their differences are greater. How far these 
experiments may be trusted, I know not; but the forms 
experimented on are ranked by Sageret, who mainly 
founds his classification by the test of infertility, as 
varieties, and Naudin has come to the same conclusion. 

The following case is far more remarkable, and seems 
at first incredible; but it is the result of an astonishing 
number of experiments made during many years on 
nine species of 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 varie- 
ties 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 Gart- 
ner'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 
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 

Chap. IX.] WHEN CROSSED. 39 

reciprocal crosses, and he found their mongrel offspring 
perfectly fertile. But one of these five varieties, when 
used either as the father or mother, and crossed with 
the Nicotiana glutinosa, always yielded hybrids not so 
sterile as those which were 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 fer- 
tility does not constitute a fundamental distinction be- 
tween varieties and species when crossed. The gen- 
eral sterility of crossed species may safely be looked at, 
not as a special acquirement or endowment, but as in- 
cidental on changes of an unknown nature in their sex- 
ual elements. 

Hyhrids and Mongrels compared, independently of their 

Independently of the question of fertility, the off- 
spring of species and of varieties when crossed may be 
compared in several other respects. Gartner, whose 
strong wish it was to draw a distinct line between spe- 
cies 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 variabil- 
ity 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 charac- 
ter 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 
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 con- 
ditions of life, fails under these circumstances to per- 
form its proper function of producing offspring closely 
similar in all respects to the parent-form. Now hy- 
brids in the first generation are descended from spe- 
cies (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 Uable 
than hybrids to revert to either parent-form; but this, 
if it be true, is certainly only a difference in degree. 
Moreover, Gartner expressly states that hybrids from 
long cultivated plants are more subject to reversion 
than hybrids from species in their natural state; and 
this probably explains the singular difference in the 
results arrived at by different observers: thus Max 
Wichura doubts whether hybrids ever revert to their 
parent-forms, and he experimented on uncultivated 
species of willows; whilst Naudin, on the other hand, 
insists in the strongest terms on the almost universal 
tendency to reversion in hybrids, and he experimented 
chiefly on cultivated plants. Gartner further states 
that when any two species, although most closely allied 
to each other, are crossed with a third species, the 
hybrids are widely different from each other; whereas 
if two very distinct varieties of one species are crossed 
with another species, the hybrids do not differ much. 
But this conclusion, as far as I can make out, is 
founded on a single experiment; and seems directly 


opposed to the results of several experiments made by 

Such alone are the unimportant differences which 
Oartner is able to point out between hybrid and mon- 
grel plants. On the other hand, the degrees and kinds 
of resemblance in mongrels and in hybrids to their 
respective parents, more especially in hybrids pro- 
duced 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 cer- 
tainly 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 char- 
acters; 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- 
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 

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 sud- 
den reversions to the perfect character of either parent 
would, also, be much more likely to occur with mon- 
grels, which are descended from varieties often sud- 
denly 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 steril- 
ity, in all other respects there seems to be a general and 
close similarity in the offspring of crossed species, and 
of crossed varieties. If we look at species as having 
been specially created, and at varieties as having been 

44 SUMMARY. [Chap. IX 

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 gen- 
erally, 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 spe- 
cies, and is eminently susceptible to the action of fa- 
vourable 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 re- 
ciprocal 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 capac- 
ity in one species or variety to take on another, is inci- 
dental on differences, generally of an unknown nature, 
in their vegetative sjfstems, so in crossing, the greater 
or less facility of one species to unite with another is in^ 
cidental 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 na- 
ture, than to think that trees have been specially en- 
dowed with various and somewhat analogous degrees of 

Chap. IX.] SUMMARY. 45 

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 cliief 
part on the early death of the embryo. In the case of 
hybrids, it apparently depends on their whole organi- 
sation 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 fer- 
tility of their offspring. The facts given on the steril- 
ity of the illegitimate unions of dimorphic and trimor- 
phic plants and of their illegitimate progeny, perhaps 
render it probable that some unknown bond in all cases 
connects the degree of fertility of first unions with that 
of their offspring. The consideration of these facts on 
dimorphism, as well as of the results of reciprocal 
crosses, clearly leads to the 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 should 
so generally have become more or less modified, leading 
to their mutual infertility, we do not know; but it 

46 SUMMARY. [Chap. IX. 

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 sur- 
prising 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 ic there- 
fore little likely to induce this same quality. Inde- 
pendently of the question of fertility, in all other re- 
spects there is the closest general resemblance between 
hybrids and mongrels, — in their variability, in their 

Chap. IX.] SUMMARY. 47 

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 varie- 




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 palasontological collections — On the 
intermittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden appearance of groups of species 
— On their sudden appearance in the lowest known fossiliferous 
strata — Antiquity of the habitable earth. 

In the sixth chapter I enumerated the chief objec- 
tions which might be justly urged against the views 
maintained in this volume. Most of them have now 
been discussed. One, namely the distinctness of spe- 
cific forms, and their not being blended together by in- 
numerable transitional links, is a very obvious difl&culty. 
I assigned reasons why such links do not commonly oc- 
cur at the present day under the circumstances ap- 
parently most favourable for their presence, namely, on 
an extensive and continuous area with graduated phys- 
ical 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 govern- 
ing conditions of life do not graduate away quite insen- 
sibly like heat or moisture. I endeavoured, also, to 


show that intermediate varieties, from existing in lesser 
numbers than the forms which they connect, will gen- 
erally be beaten out and exterminated during the course 
of further modification and improvement. The main 
cause, however, of innumerable intermediate links not 
now occurring everj'where throughout nature, depends 
on the very process of natural selection, through which 
new varieties continually take the places of and sup- 
plant 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 stra- 
tum full of such intermediate links? Geology assured- 
ly does not reveal any such finely-graduated organic 
chain; and this, perhaps, is the most obvious and seri- 
ous objection which can be urged against the theory. 
The explanation lies, as I believe, in the extreme imper- 
fection of the geological record. 

In the first place, it should always be borne in mind 
what sort of intermediate forms must, on the theory, 
have formerly existed. I have found it diflQcult, when 
looking at any two species, to avoid picturing to my- 
self 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 modi- 
fied descendants. To give a simple illustration: the 
fantail and pouter pigeons are both descended from 
the rock-pigeon; if we possessed all the intermediate 
varieties which have ever existed, we should have an 
extremely close series between both and the rock- 


pigeon; but we should have no varieties directly in- 
termediate 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. cenas. 

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 spe- 
cies, 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 in- 
termediate links. 

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


and the principle of competition between organism and 
organism, between child and parent, will render this a 
very rare event; for in all cases the new and improved 
forms of life tend to supplant the old and unimproved 

By the theory' of natural selection all li^ang 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 gen- 
erally extinct, have in their turn been similarly con- 
nected with more ancient forms; and so on backwards, 
always converging to the common ancestor of each 
great class. So that the number of intermediate and 
transitional links, between all living and extinct spe- 
cies, must have been inconceivably great. But as- 
suredly, if this theory be true, such have lived upon the 

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

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

52 THE LAPSE OF TIME. [Chap, X. 

been the past periods of time, may at once close this 
volume. Not that it suffices to study the Principles of 
Geology, or to read special treatises by different ob- 
servers on separate formations, and to mark how each 
author attempts to give an inadequate idea of the dura- 
tion 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-clifEs, in 
order to comprehend something about the duration of 
past time, the monuments of which we see all around 

It is good to wander along the coast, when formed of 
moderately hard rocks, and mark the process of degra- 
dation. 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 noth- 
ing 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 productions, 

Chap. X.] THE LAPSE OP TIME. 63 

showing how little they are abraded and how seldom 
they are rolled about! Moreover, if we follow for a few 
miles any line of rocky clift', which is undergoing deg- 
radation, 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 Ramsay, in the van of many excellent ob- 
servers — of Jukes, Geikie, CroU, and others, that sub- 
aerial 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 car- 
bonic acid, and in colder countries to frost; the disin- 
tegi-ated 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 undulat- 
ing country, we see the effects of subaerial degradation 
in the muddy rills which flow down every slope. Messrs. 
Ramsay and Whitaker have shown, and the observation 
is a most striking one, that the great lines of escarp- 
ment 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 intersection 
of various formations. This being the case, we are 
compelled to admit that the escarpments owe their 

54 THE LAPSE OF TIME. ^Chap. X. 

origin in chief part to the rocks of which they are com- 
posed having resisted subaerial denudation better than 
the surrounding surface; this surface consequently has 
been gradually lowered, wdth the lines of harder rock 
left projecting. Nothing impresses the mind with the 
vast duration of time, according to our ideas of time, 
more forcibly than the conviction thus gained that sub- 
aerial agencies which apparently have so Uttle power, 
and which seem to w^ork so slowdy, have produced great 

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 dura- 
tion of time, to consider, on the one hand, the masses 
of rock which have been removed over many extensive 
areas, and on the other hand the thickness of our sedi- 
mentary formations. I remember having been much 
struck when viewdng volcanic islands, which have been 
worn by the waves and pared all round into perpen- 
dicular 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 up- 
heaved 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 sur- 
face 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 tliis line the vertical 
displacement of the strata varies from 600 to 3000 feet. 
Professor Eamsay 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 13,000 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: — 


Palseozoic 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 

56 THE LAPSE OP TIME. [Chap. X. 

idea of the time which has elapsed during their accumu- 
lation. The consideration of these various facts im- 
presses 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. CroU 
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 haired or quartered 
it is still very surprising. Few of us, however, know 
what a million really means: Mr. Croll gives the fol- 
lowing 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, 
wnat 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 OP 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 repre- 
sents 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 guid- 
ance 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 uncon- 
scious selection, various breeds have been sensibly 
changed in the course of two or three centuries. 

Species, however, probably change much more 
slowly, and within the same country only a few change 
at the same time. This slowness follows from all the 
inhabitants of the same country being already so well 
adapted to each other, that new places in the polity of 
nature do not occur until after long intervals, due to the 
occurrence of physical changes of some kind, or through 
the immigration of new forms. Moreover variations or 
individual differences of the right nature, by which 
some of the inhabitants might be better fitted to their 
new places under the altered circumstances, would not 
always occur at once. 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. 


On the Poorness of Pakeontological 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 dis- 
coveries made every year in Europe prove. No organ 
ism wholly soft can be preserved. Shells and bones de- 
cay 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 ia 
being deposited over nearly the whole bed of the sea, at 
a rate sufficiently quick to embed and preserve fossil re- 
mains. 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 con- 
formably 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 o? 
gravel, will, when the beds are upraised, generally be 
dissolved by the percolation of rain-water charged with 
carbonic acid. Some of the many kinds of animals 
which live on the beach between high and low water 
mark seem to be rarely preserved. For instance, the 


several species of the Chthamalinae (a sub-family of 
sessile cirripedes) coat the rocks all over the world in 
infinite numbers: they are all strictly littoral, with the 
exception of a single Mediterranean species, which in- 
habits 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 Palseozoic periods, it is 
superfluous to state that our evidence is fragmentary in 
an extreme degree. For instance, until recently not a 
land-shell was known belonging to either of these vast 
periods, with the exception of one species discovered by 
Sir C. Lyell and Dr. Dawson in the carboniferous strata 
of N"orth 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. 
"Not is their rarity surprising, when we remember how 
large a proportion of the bones of tertiary mammals 
have been discovered either in caves or in lacustrine 


deposits; and that not a cave or true lacustrine bed is 
known belonging to the age of our secondary or palaeo- 
zoic 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 disbeheve in the change of species. 
When we see the formations tabulated in written works, 
or when we follow them in nature, it is difficult to 
avoid believing that they are closely consecutive. But 
we know, for instance, from Sir E. Murchison's great 
work on Eussia, what wide gaps there are in that coun- 
try between the superimposed formations; so it is in 
North America, and in many other parts of the world. 
The most skilful geologist, if his attention had been 
confined 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 consecu- 
tive formations, we may infer that this could nowhere 
be ascertained. The frequent and great changes in 
the mineralogical composition of consecutive forma- 
tions, generally impl}'ing great changes in the geography 
of the surrounding lands, whence the sediment was 
derived, accord with the belief of vast intervals of time 
having elapsed between each formation. 

We can, I think, see why the geological formations 


of each region are almost invariably intermittent; that 
is, have not followed each other in close sequence. 
Scarcely any fact struck me more when examining 
many hundred miles of the South American coasts, 
which have been upraised several hundred feet within 
the recent period, than the absence of any recent de- 
posits sutiiciently 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 up- 
raised will give an imperfect record of the organisms 


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

I am convinced that nearly all our ancient forma- 
tions, 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, an<J 
have been surprised to note how author after author, 
in treating of this or that great formation, has come to 
the conclusion that it was accumulated during subsi- 
dence. I may add, that the only ancient tertiary forma- 
tion 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 dis- 
tant geological age, was deposited during a downward 
oscillation of level, and thus gained considerable thick- 

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 shallow and to embed 

chap.x.] pal^ontological collections. 63 

and preserve the remains before they had time to de- 
cay. On the other hand, as long as the bed of the sea 
remains stationary, thich deposits cannot have been ac- 
cumulated 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 accumu- 
lated will generally have been destroyed by being 
upraised and brought within the limits of the coast- 

These remarks apply chiefly to littoral and sub-lit- 
toral 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 de- 
posit be much consolidated, nor be capped by overlying 
formations, so that it would run a good chance of being 
worn away by atmospheric degradation and by the ac- 
tion 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 ris- 
ing movement, though not thick, might afterwards be- 
come protected by fresh accumulations, and thus be 
preserved for a long period. 

Mr. Hopkins also expresses his belief that sedimen- 
tary beds of considerable horizontal extent have rarely 
Vleen completely destroyed. But all geologists, except- 


ing the few who believe that our present metamorphic 
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 ex- 
tent. For it is scarcely possible that such rocks could 
have been solidified and crystalUzed whilst uncovered; 
but if the metamorphic action occurred at profound 
depths of the ocean, the former protecting mantle of 
rock may not have been very thick. Admitting then 
that gneiss, mica-schist, granite, diorite, &c., were 
once necessarily covered up, how can we account for 
the naked and extensive areas of such rocks in many 
parts of the world, except on the belief that they have 
subsequently been completely denuded of all 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 testi- 
mony of travellers, the granitic area is very large: thus, 
Von Eschwege gives a detailed section of these rocks, 
stretching from Eio de Janeiro for 260 geographical 
miles inland in a straight Une; and I travelled for 150 
miles in another direction, and saw nothing but granitic 
rocks. Numerous specimens, collected along the 
whole coast from near Eio Janeiro to the mouth of the 
Plata, a distance of 1100 geographical miles, were ex- 
amined by me, and they all belonged to this class. In- 
land, along the whole northern bank of the Plata I 
saw, besides modem tertiary beds, only one small patch 


of slightly metamorphosed rock, which alone could have 
formed a part of the original capping of the granitic 
series. Turning to a well-known region, namely, to 
the United States and Canada, as shown in Professor 
H. D. 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 propor- 
tion of 19 to 13.5, the whole of the newer Palaeozoic 
formations. In many regions the metamorphic and 
granitic rocks would be found much more widely ex- 
tended 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 forma- 
tions 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 ad- 
joining shoal parts of the sea will be increased, and new 
stations will often be formed: — all circumstances favour- 
able, 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 varie- 
ties 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. 


Oil, tlie 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 gradu- 
ated 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, 
'irautschold 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 Swit- 
zerland. Although each formation has indisputably 
required a vast number of years for its deposition, sev- 
eral reasons can be given why each should not commonly 
include a graduated series of links between the species 
which lived at its commencement and close; but I 
cannot assign due proportional weight to the following 

Although each formation may mark a very long 
lapse of years, each probably is short compared with the 
period requisite to change one species into another. I 
am aware that two 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 dura- 
tion of specific forms. But insuperable difficulties, 
as it seems to me, prevent us from coming to any just 
conclusion on this head. When we see a species first 


appearing in the middle of any formation, it would be 
rash in the extreme to infer that it had not elsewhere 
previously existed. So again when we find a species 
disappearing before the last layers have been deposited, 
it would be equally rash to suppose that it then became 
extinct. We forget how small the area of Europe is 
compared with the rest of the world; nor have the sev- 
eral 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 probabil- 
ity is that it only then first immigrated into that area. 
It is well-known, for instance, that several species ap- 
pear 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 neighbour- 
ing sea, but are rare or absent in this particular deposit. 
It is an excellent lesson to reflect on the ascertained 
amount of migration of the inhabitants of Europe dur- 
ing 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 anv quarter of the world, sedimentarv deposits, in- 


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 de- 
posited during the whole of the glacial period near 
the mouth of the Mississippi, within that limit of depth 
at which marine animals can best flourish: for we know 
that great geographical changes occurred in other parts 
of America during this space of time. When such 
beds as were deposited in shallow water near the mouth 
of the Mississippi during some part of the glacial period 
shall have been upraised, organic remains will prob- 
ably first appear and disappear at different levels, ow- 
ing to the migrations of species and to geographical 
changes. And in the distant future, a geologist, ex- 
amining 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 hav- 
ing been really far greater, that is, extending from be- 
fore 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 forma- 
tion, the deposit must have gone on continuously ac- 
cumulating 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 ac- 
cumulate only during a period of subsidence; and to 
keep the depth approximately the same, which is neces- 
sary that the same marine species may live on the same 
space, the supply of sediment must nearly counterbal- 
ance the amount of subsidence. But this same move- 



meat of subsidence will tend to submerge the area 
whence the sediment is derived, and thus diminish the 
supply, wliilst 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 paleontologist, that very thick deposits are usually 
barren of organic remains, except near their upper or 
lower limits. 

It would seem that each separate formation, like the 
whole pile of formations in any country, has generally 
been 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 accumula- 
tion; 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, de- 
nuded, 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 ac- 
cumulation. In other cases we have the plainest evi- 
dence in great fossilised trees, still standing upright 
as they grew, of many long intervals of time and changes 
of level during the process of deposition, which would 


not have been suspected, had not the trees been pre- 
served: thus Sir C. Lyell and Dr. Dawson found carbon- 
iferous beds 1400 feet thick in Nova Scotia, with an- 
cient 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 species, unless they are enabled to connect them 
together by the closest intermediate gradations; and 
this, from the reasons just assigned, we can seldom 
hope to effect in any one geological section. Supposing 
B and C to be two species, and a third, A, to be found in 
an older and underlying bed; even if A were strictly 
intermediate between B and C, it would simply be 
ranked as a third and distinct species, unless at the 
same time it could be closely connected by 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 


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 un- 
less we obtained numerous transitional gradations, we 
should not recognise their bloou-relationship, and 
should consequently rank them as distinct species. 

It is notorious on what excessively slight diiferences 
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 natu- 
ralists have been misled by their imaginations, and 
that these late tertiary species really present no dif- 
ference whatever from their living representatives, or 
unless we admit, in opposition to the judgment of most 
naturalists, that these tertiary species are all truly dis- 
tinct from the recent, we have evidence of the frequent 
occurrence of slight modifications of the kind required. 
If we look to rather wider intervals of time, namely, 
to distinct but consecutive stages of the same great 
formation, we find that the embedded fossils, though 
universally ranked as specifically different, yet are far 
more closely related to each other than are the species 


found in more widely separated formations; so that 
here again we have undoubted evidence of change in 
the direction required by the theory; but to this latter 
subject I shall return in the following 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 de- 
gree. According to this view, the chance of discov- 
ering 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 ani- 
mals 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 to local varieties and ultimately to new spe- 
cies; and this again would greatly lessen the chance of 
our being able to trace the stages of transition in any 
one geological formation. 

It is a more important consideration, leading to the 
same result, as lately insisted on by Dr. Falconer, name- 
ly, that the period during which each species under- 
went 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 can 

Chap.X.] in any single FORMATION. 73 

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 per- 
ceive the improbability of our being enabled to con- 
nect species by numerous, fine, intermediate, fossil links, 
by asking ourselves whether, for instance, geologists 
at some future period will be able to prove that our 
different breeds of cattle, sheep, horses, and dogs are 
descended from a single stock or from several abori- 
ginal stocks; or, again, whether certain sea-shells in- 
habiting the shores of North America, which are 
ranked by some conchologists as distinct species from 
their European representatives, and by other con- 
chologists as only varieties, are really varieties, or 
are, as it is called, specifically distinct. This could 
be effected by the future geologist only by his discover- 
ing in a fossil state numerous intermediate grada- 
tions; and such success is improbable in the highest de- 

It has been asserted over and over again, by writers 
who believe in the immutability of species, that geology 
yields no hnking 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 genu3 
having a score of species, recent and extinct, and de- 
stroy four-fifths of them, no one doubts that the re- 
mainder will stand much more distinct from each other. 
If the extreme forms in the genus happen to have been 
thus destroyed, the genus itself will stand more distinct 
from other allied genera. What geological research 
has not revealed, is the former existence of infinitely 


numerous gradations^ as fine as existing varieties, con- 
necting 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 geo- 
logical record under an imaginary illustration. The 
Malay Archipelago is about the size of Europe from the 
North Cape to the Mediterranean, and from Britain to 
Eussia; 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, prob- 
ably 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 ter- 
restrial productions of the archipelago would be pre- 
served in an extremely imperfect manner in the forma- 
tions which we suppose to be there accumulating. Not 
many of the strictly littoral animals, or of those which 
lived on naked submarine rocks, would be embedded; 
and those embedded in gravel or sand would not en- 
dure to a distant epoch. Wherever sediment did not 
accumulate on the bed of the sea, or where it did not 
accumulate at a sufficient rate to protect organic bodies 
from decay, no remains could be preserved. 


Formations rich in fossils of many kinds, and of 
thickness sufficient to last to an age as distant in futu- 
rity as the secondary formations he in the past, would 
generally be formed in the archipelago only during 
periods of subsidence. These periods of subsidence 
would be separated from each other by immense in- 
tervals of time, during which the area would be either 
stationary or rising; whilst rising, the fossiliferous for- 
mations on the steeper shores would be destroyed, al- 
most 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 with- 
in the archipelago, sedimentary beds could hardly be 
accumulated of great thickness during the periods of 
elevation, or become capped and protected by subse- 
qu'int deposits, so as to have a good chance of enduring 
to a very distant future. During the periods of sub- 
sidence, there would probably be much extinction of life; 
during the periods of elevation, there would be much 
variation, but the geological record would then be less 

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 accumu- 
lation of sediment, would exceed the average duration of 
the same specific forms; and these contingencies are in- 
dispensable 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 os- 
cillations of level, and that slight cHmatal changes 


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

Very many of the marine inhabitants of the archi- 
pelago now range thousands of miles beyond its con- 
fines; and analogy plainly leads to the behef that it 
would be chiefly these far-ranging species, though only 
some of them, which would oftenest produce new varie- 
ties; and the varieties would at first be local or con- 
fined to one place, but if possessed of any decided ad- 
vantage, 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, accord- 
ing 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 geo- 
logical 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 
©ne long and branching chain of life. We ought only 
to look for a few links, and such assuredly we do find — 
some more distantly, some more closely, related to 
each other; and these links, let them be ever so close, 
if found in different stages of the same formation, 
would, by many palasontologists, 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 FORMATION. 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 
60 hardly on my theory. 

On the sudden Appearance of whole Groups of allied 

The abrupt manner in which whole groups of spe- 
cies suddenly appear in certain formations, has been 
urged by several palaeontologists — for instance, by Agas- 
siz, Pictet, and Sedgwick — as a fatal objection to the be- 
lief in the transmutation of species. If numerous spe- 
cies, 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 cer- 
tain 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 


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 adapta- 
tion had once been effected, and a few species had thus 
acquired a great advantage over other organisms, a 
comparatively short time would be necessary to produce 
many divergent forms, which would spread rapidly and 
widely, throughout the world. Professor Pictet, in his 
excellent Eeview of this work, in commenting on early 
transitional forms, and taking birds as an illustration, 
cannot see how the successive modifications of the an- 
terior limbs of a supposed prototype could possibly 
have been of any advantage. But look at the penguins 
of the Southern Ocean; have not these birds their front 
limbs in this precise intermediate state of " neither true 
"arms nor true wings"? Yet these birds hold their 
place victoriously in the battle for life; for they exist 
in infinite numbers and of many kinds. I do not sup- 
pose that we here see the real transitional grades 
through which the wings of birds have passed; but 
what special difficulty is there in believing that it 
might profit the modified descendants of the penguin, 
first to become enabled to flap along the surface of the 


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

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 re- 
quire 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 dis- 
covered 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 pres- 
ervation 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 fragment 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 during 


the deposition of the upper greensand; and still more 
recently, that strange bird, the Archeopteryx, with a 
long hzard-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 httle 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 spe- 
cies; from the extraordinary abundance of the indi- 
viduals of many species all over the world, from the 
Arctic regions to the equator, inhabiting various zones 
of depths from the upper tidal limits to 50 fathoms; 
from the perfect manner in which specimens are pre- 
served 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 cirri- 
pedes existed during the secondary periods, they would 
certainly have been preserved and discovered; and as 
not one species had then been discovered in beds of 
this age, I concluded that this great group had been 
suddenly developed at the commencement of the ter- 
tiary series. This was a sore trouble to me, adding as 
I then thought one more 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 unmistakable sessile cirripede, which he had him- 
self 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 Chthamalus, a very common, large, and ubiqui- 
tous genus, of which not one species lias as yet been 
found even in any tertiary stratum. Still more re- 
cently, 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 palaeonto- 
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 adnaitted to be teleostean; and even some palaBO- 
zoic forms have thus been classed by one high authority*'. 
If the teleosteans had really appeared suddenly in the 
northern hemisphere at the commencement of the chalk 
formation the fact would have been highly remarkable; 
but it would not have formed an insuperable difiiculty, 
unless it could likewise have been shown that at the 
same period the species were suddenly and simultane- 
ously 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 run- 
ning through Pictet's Palaeontology it will be seen that 
very few species are known from several formations 
in Europe. Some few families of fish now have a con- 
fined range; the teleostean fishes might formerly have 
had a similarly confined range, and after having been 
largely developed in some one sea, have spread widely. 
Nor have we any right to suppose that the seas of the 
•world have always been so freely open from south to 


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

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 palaeontological 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 Aus- 
tralia, and then to discuss the number and range of its 

On the sudden Appearance of Groups of allied Species 
in the lowest known Fossiliferous Strata. 

There is another and allied difficulty, which is much 
more serious. I allude to the manner in which species 
belonging to several of the main divisions of the animal 
kingdom suddenly appear in the lowest known fossili- 
ferous rocks. Most of the arguments which have con- 
vinced 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 in- 
stance, it cannot be doubted that all the Cambrian and 


Silurian trilobites are descended from some one crusta- 
cean, 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 sub- 
sequently appeared, for they are not in any degree in- 
termediate 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 Kving 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 for- 
mation; and the previous 140 million years can hardly 
be considered as sufficient for the development of the 


varied forms of life which already existed during the 
Cambrian period. It is, however, probable, as Sir Wil- 
liam 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 

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. Mur- 
chison 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 dis- 
puted this conclusion. We should not forget that only 
a small portion of the world is known with accuracy. 
Not very long ago M. Barrande added another and 
lower stage, abounding with new and peculiar species, 
beneath the then known Silurian system; and now, 
still lower down in the Lower Cambrian formation, Mr. 
Hicks has found in South Wales beds rich in trilobites, 
and containing various molluscs and annelids. The 
presence of phosphatic nodules and bituminous matter, 
even in some of the lowest azoic rocks, probably indi- 
cates life at these periods; and the existence of the 
Eozoon in the Laurentian formation of Canada is gener- 
ally admitted. There are three great series of strata be- 
neath 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 palaeo- 


" zoic series to the present time. We are thus carried 
" back to a period so remote, that the appearance of the 
" so-called Primordial fauna (of Barraude) 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 diffi- 
culty 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 ob- 
literated by metamorphic action, for if this had heen 
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 hy- 
pothesis. From the nature of the organic remains 
which do not appear to have inhabited profound depths, 

86 GROUPS OF ALLffit) SPECIES [Chap. X. 

in the several formations of Europe and of the United 
States; and from the amount of sediment, miles in 
thickness, of which the formations are composed, we 
may infer that from first to last large islands or tracts 
of land, whence the sediment was derived, occurred in 
the neighbourhood of the now existing 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 be- 
tween the several successive formations; whether Europe 
and the United States during these intervals existed as 
dry land, or as a submarine surface near land, on which 
sediment was not deposited, or as the bed of an open and 
unfathomable sea. 

Looking to the existing oceans, which are thrice as 
extensive as the land, we see them studded with many 
islands; but hardly one truly oceanic island (with the 
exception of New Zealand, if this can be called a truly 
oceanic island) is as yet known to afford even a remnant 
of any palaeozoic or secondary formation. Hence we 
may perhaps infer, that during the palaeozoic and sec- 
ondary periods, neither continents nor continental is- 
lands existed where our oceans now extend; for had 
they existed, palasozoic 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 


land have existed, subjected no doubt to great oscilla- 
tions of level, since the Cambrian period. The col- 
oured map appended to my volume on Coral Eeefs, led 
me to conclude that the great oceans are still mainly 
areas of subsidence, the great archipelagoes still areas of 
oscillations of level, and the continents areas of eleva- 
tion. But we have no reason to assume that things have 
thus remained from the beginning of the world. Our 
continents seem to have been formed by a preponder- 
ance, during many oscillations of level, of the force of 
elevation; but may not the areas of preponderant move- 
ment 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. Xor should we be justified in assuming 
that if, for instance, the bed of the Pacific Ocean were 
now converted into a continent we should there find 
sedimentary formations in a recognisable condition older 
than the Cambrian strata, supposing such to have been 
formerly deposited; for it might well happen that strata 
which had subsided some miles nearer to the centre of the 
earth, and which had been pressed on by an enormous 
weight of superincumbent water, might have undergone 
far more metamorphic action than strata which have al- 
ways remained nearer to the surface. The immense 
areas in some parts of the world, for instance in South 
America, of naked metamorphic rocks, which must have 
been heated under great pressure, have always seemed to 
me to require some special explanation; and we may per- 
haps believe that we see in these large areas, the many 
formations long anterior to the Cambrian epoch in a 
completely metamorphosed and denuded condition. 


The several difficulties here discussed, namely — tbat, 
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 fos- 
sils 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,Murchison, Sedgwick, 
&c., have unanimously, often vehemently, maintained 
the immutability of species. But Sir Charles Lyell 
now gives the suj.port of his high authority to the op- 
posite side; and most geologists and palseontologists are 
much shaken in their former belief. Those who believe 
that the geological record is in any degree perfect, will 
undoubtedly at once reject the theory. For my part, 
following out Lyell's metaphor, I look at the geological 
record as a history of the world imperfectly kept, and 
written in a changing dialect; of this history we pos- 
sess the last volume alone, relating only to two or three 
countries. Of this volume, only here and there a short 
chapter has been preserved; and of each page, only here 
and there a few lines. Each word of the slowly-changing 
language, more or less different in the successive chap- 
ters, may represent the forms of life, which are en- 
tombed in our consecutive formations, and which falsely 
appear to have been abruptly introduced. On this 
view, the difficulties above discussed are greatly dimin 
ished, or even disappear. 




On the slow and successiye 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 afl&nities of extinct species to each other and to 
living species — On the state of development of ancient forms — 
On the succession of the same types within the same areas — 
Summary of preceding and present chapter. 

Let us now see whether the several facts and laws 
relating to the geological succession of organic beings 
accord best with the common view of the immutability 
of species, or with that of their slow and gradual 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 exist- 
ing forms more gradual. In some of the most recent 
beds, though undoubtedly of high antiquity if measured 
by years, only one or two species are extinct, and only 
one or two are new, having appeared there for the first 
time, either locally, or, as far as we know, on the face of 
the earth. The secondary formations are more broken; 


but, as Bronn has remarked, neither the 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 or- 
ganic 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 spe- 
cies will be found to have undergone some change. 
When a species has once disappeared from the face of 
the earth, we have no reason to believe that the same 
identical form ever reappears. The strongest apparent 
exception to this latter rule is that of the so-called 
" colonies " of M. Barrande, which intrude for a period 
in the midst of an older formation, and then allow the 
pre-existing fauna to reappear; but Lyell's explanation, 
namely, that it is a case of temporary migration from a 
distinct geographical province, seems satisfactory. 

These several facts accord well with our theory. 


which includes no fixed law of development, causing all 
the inhabitants of an area to change abruptly, or simul- 
taneously, or to an equal degree. The process of modi- 
fication must be slow, and will generally efEect 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 sur- 
prising that one species should retain the same identi- 
cal form much longer than others; or, if changing, 
should change in a less degree. We find similar rela- 
tions between the existing inhabitants of distinct coun- 
tries; for instance, the land-shells and coleopterous in- 
sects 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 understand the apparently quicker rate 
of change in terrestrial and in more highly organised 
productions compared with marine and lower produc- 
tions, by the more complex relations of the higher 
beings to their organic and inorganic conditions of life, 
as explained in a former chapter. When many of the 
inhabitants of any area have become modified and im- 
proved, we can understand, on the principle of compe- 
tition, and from the all-important relations of organ- 


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

In members of the same class the average amount of 
change, during long and equal periods of time, may, 
perhaps, be nearly the same; but as the accumulation 
of enduring formations, rich in fossils, depends on great 
masses of sediment being deposited on subsiding areas, 
our formations have been almost necessarily accumu- 
lated at wide and irregularly intermittent intervals of 
time; consequently the amount of organic change ex- 
hibited by the fossils embedded in consecutive forma- 
tions 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 
instances) to fill the place of another species in the econ- 
omy 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 organ- 
isms already differing would vary in a different manner. 
For instance, it is possible, if all our fantail pigeons 
were destroyed, that fanciers might make a new breed 
hardly distinguishable from the present breed; but if 


the parent rock-pigeon were likewise destroyed, and 
under nature we have every reason to believe that 
parent-forms are generally supplanted and exterminated 
by their improved offspring, it is incredible that a fantail, 
identical with the existing breed, could be raised from 
any other species of pigeon, or even from any other well- 
established race of the domestic pigeon, for the successive 
variations would almost certainly be in some degree dif- 
ferent, and the newly-formed variety would probably in- 
herit from its progenitor some characteristic differences. 

Groups of species, that is, genera and families, fol- 
low 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 abrupt- 
ly 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 
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 geologi- 
cal formations, in which the species are found, the line 
will sometimes falsely appear to begin at its lower end, 
not in a sharp point, but abruptly; it then gradually 
thickens upwards, often keeping of equal thickness for 
a space, and ultimately thins out in the upper beds, 
marking the decrease and final extinction of the species. 
This gradual increase in number of the species of a 
group is strictly conformable with the theory, for the 
species of the same genus, and the genera of the same 
family, can increase only slowly and progressively; the 
process of modification and the production of a number 
of allied forms necessarily being a slow and gradual pro- 
cess, — one species first giving rise to two or three varie- 
ties, these being slowly converted into species, which in 
their turn produce by equally slow steps other varieties 
and species, and so on, like the branching of a great tree 
from a single stem, till the group becomes large. 

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, Mur- 


chison, Barrande, &c., whose general views would natu- 
rally 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 isth- 
mus 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 paleozoic 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 rep- 
resented, as before, by a vertical line of varying thick- 
ness the line is found to taper more gradually at its up- 
per 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 m5'stery. 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 
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, see- 
ing that the horse, since its introduction by the Span- 
iards into South America, has run wild over the whole 
country and has increased in numbers at an unparal- 
leled rate, I asked myself what could so recently have 
exterminated the former horse under conditions of life 
apparently so favourable. But my astonishment was 
groundless. Professor Owen soon perceived that the 
tooth, though so like that of the existing horse, be- 
longed 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 cer- 
tain, 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 condi- 
tions were which checked its increase, whether some one 
or several contingencies, and at what period of the 
horse's life, and in what degree they severally acted. If 
the conditions had gone on, however slowly, becoming 
less and less favourable, we assuredly should not have 


perceived the fact, yet the fossil horse would certainly 
have become rarer and rarer, and finally extinct; — its 
place being seized on by some more successful competitor. 

It is most difficult always to remember that the 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 inhabited India or Africa, some cause must have 
checked the continued increase of the existing elephant. 
A highly capable judge, Dr. Falconer, believes that it is 
chiefly insects which, from incessantly harassing and 
weakening the elephant in India, check its increase; and 
this was Bruce's conclusion with respect to the African 
elephant in Abyssinia. It is certain that insects and 
blood-sucking bats determine the existence of the larger 
naturalised 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 sxterminated, 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 mar- 
vel greatly when the species ceases to exist, is much the 


same as to admit that sickness in the individual is the 
forerunner of death — to feel no surprise at sickness, 
but, when the sick man dies, to wonder and to suspect 
that he died by some deed of violence. 

The theory of natural selection is grounded on the 
belief that each new variety and ultimately each new 
species, is produced and maintained by having some 
advantage over those with which it comes into competi- 
tion; and the consequent extinction of the less-fa- 
voured 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 to- 
gether. In flourishing groups, the number of new spe- 
cific forms which have been produced within a given 
time has at some periods probably been greater than the 
number of the old specific forms which have been ex- 
terminated; but we know that species have not gone on 
indefinitely increasing, at least during the later geo- 
logical 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 spe- 
cies will generally cause the extermination of the parent- 
species; and if many new forms have been developed 


from any one species, the nearest allies of that species, 
i.e. the species of the same genus, will be the most liable 
to extermination. Thus, as I believe, a number of new 
species descended from one species, that is a new genus, 
comes to supplant an old genus, belonging to the same 
family. But it must often have happened that a new 
species belonging to some one group has seized on the 
place occupied by a species belonging to a distinct group, 
and thus have caused its extermination. If many allied 
forms be developed from the successful intruder, many 
will have to }ield their places; and it will generally be 
the alHed forms, which will suffer from some inherited 
inferiority in common. But whether it be species be- 
longing to the same or to a distinct class, which have 
yielded their places to other modified and improved spe- 
cies, a few of the sufferers may often be preserved for 
a long time, from being fitted to some peculiar line of 
Kfe, or from inhabiting some distant and isolated station, 
where they will have escaped severe competition. For 
instance, some species of Trigonia, a great genus of shells 
in the secondary formations, survive in the Australian 
seas; and a few members of the great and almost extinct 
group of Ganoid fishes still inhabit our fresh waters. 
Therefore the utter extinction of a group is generally, as 
we have seen, a slower process than its production. 

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


ally 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 correspond- 
ingly rapid manner; and the forms which thus yield 
their places will commonly be allied, for they will par- 
take of the same inferiority in common. 

Thus, as it seems to me, the manner m 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 mo- 
ment that we understand the many complex contin- 
gencies 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 econ- 
omy 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 an- 
other can be naturalised in a given country; then, and 
not until then, we may justly feel surprise why we can- 
not account for the extinction of any particular spe- 
cies or group of species. 

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

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


in Tierra del Fuego, at the Cape of Good Hope, and in 
the peninsula of India. For at these distant points, 
the organic remains in certain beds present an unmis- 
takeahle 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. Moreover, other 
forms, which are not found in the Chalk of Europe, 
but which occur in the formations either above or be- 
low, occur in the same order at these distant points 
of the world. In the several successive palaeozoic for- 
mations of Eussia, Western Europe, and North America, 
a similar parallelism in the forms of life has been ob- 
served by several authors; so it is, according to Lyell, 
with the European and North American tertiary de- 
posits. Even if the few fossil species which are com- 
mon to the Old and New Worlds were kept wholly out 
of view, the general parallelism in the successive forms 
of life, in the palaeozoic and tertiary stages, would still 
be manifest, and the several formations could be easily 

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 geological 
position, no one would have suspected that they had co- 
existed with sea-shells all still living; but as these 


anomalous monsters co-existed with the Mastodon and 
Horse, it might at least have been 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 gla- 
cial epoch) were compared with those now existing in 
South America or in Australia, the most skilful natu- 
ralist would hardly be able to say whether the present 
or the pleistocene inhabitants of Europe resembled most 
closely those of the southern hemisphere. So, again, 
several highly competent observers maintain that the 
existing productions of the United States are more 
closely related to those which lived in Europe during 
certain late tertiary stages, than to the present inhabi- 
tants of Europe; and if this be so, it is evident that fos- 
siliferous beds now deposited on the shores of North 
America would hereafter be liable to be classed with 
somewhat older European beds. Nevertheless, looking 
to a remotely future epoch, there can be little doubt 
that all the more modern marine formations, namely, 
the upper pliocene, the pleistocene and strictly modern 
beds of Europe, North and South America, and Aus- 
tralia, from containing fossil remains in sonie 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 simultaneous- 


ly, in the above large sense, at distant parts of the world, 
has greatly struck these 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 greai mutations in the 
forms of life throughout the world, under the most dif- 
ferent climates. We must, as Barrande has remarked, 
look to some special law. We shall see this more clearly 
when we treat of the present distribution of organic 
beings, and find how slight is the relation between the 
physical conditions of various countries and the nature 
of their inhabitants. 

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


new varieties. It is also natural that the dominant, 
varying, and far-spreading species, which have already 
invaded to a certain extent the territories of other 
species, should be those which would have the best 
chance of spreading still further, and of giving rise in 
new countries to other new varieties and species. The 
process of diffusion would often be very slow, 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 ulti- 
mately prevail. The diffusion would, it is probable, be 
slower with the terrestrial inhabitants of distinct conti- 
nents than with the marine inhabitants of the continu- 
ous sea. We might therefore expect to find, as we do 
find, a less strict degree of parallelism in the succesion of 
the productions of the land than with those of the sea. 

Thus, as it seems to me, the parallel, and, taken in a 
large sense, simultaneous, succession of the same forms 
of life throughout the world, accords well with the 
principle of new species having been formed by domi- 
nant species spreading widely and varying; the new 
species thus produced being themselves dominant, ow- 
ing to their having had some advantage over their al- 
ready 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 


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 be- 
lieve that large areas are affected by the same move- 
ment, it is probable that strictly contemporaneous for- 
mations 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 dur- 
ing nearly, but not exactly, the same period, we should 
find in both, from the causes explained in the fore- 
going paragraphs, the same general succession in the 
forms of life; but the species would not exactly cor- 
respond; for there will have been a httle 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 


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 long 
intervals of time between them; in this case the several 
formations in the two regions could be arranged in the 
same order, in accordance with the general succession 
of the forms of life, and the order would falsely appear 
to be strictly parallel; nevertheless the species would 
not be all the same in the apparently corresponding 
stages in the two regions. 

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

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


descent. The more ancient any form is, the more, as 
a general rule, it differs from living forms. But, as 
Buckland long ago remarked, extinct species can all be 
classed either in still existing groups, or between them. 
That the extinct forms of life help to fill up the in- 
tervals 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 sys- 
tem. In the writings of Professor Owen we continu- 
ally 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 palaeontolo- 
gist, 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 exist- 
ing genera. Cuvier ranked the Euminants and Pachy- 
derms as two of the most distinct orders of mammals: 
but so many fossil links have been disentombed that 
Owen has had to alter the whole classification, and has 
placed certain pachyderms in the same sub-order with 
ruminants; for example, he dissolves by gradations the 
apparently wide interval between the pig and the camel. 
The Ungulata or hoofed quadrupeds are now divided 
into the even-toed or odd-toed divisions; but the Mac- 
rauchenia of S. America connects to a certain extent 
these two grand divisions. No one will deny that the 
Hipparion is intermediate between the existing horse 


and certain older ungulate forms. What a wonderful 
connecting link in the chain of mammals is the Typo- 
therium 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 dis- 
tinct group of mammals, and one of the most remark- 
able peculiarities in the existing dugong and lamentin 
is the entire absence of hind limbs without even a rudi- 
ment being left; but the extinct Halitherium had, ac- 
cording to Professor Flower, an ossified thigh-bone 
" articulated to a well-defined acetabulum in the pelvis," 
and it thus makes some approach to ordinary hoofed 
quadrupeds, to which the Sirenia are in other respects 
allied. The cetaceans or whales are widely different 
from all other mammals, but the tertiary Zeuglodon 
and Squalodon, which have been placed by some natu- 
ralists in an order by themselves, are considered by Pro- 
fessor 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, and a higher author- 
ity could not be named, that he is every day taught that, 
although palaeozoic animals can certainly be classed 
under existing groups, yet that at this ancient period 
the groups were not so distinctly separated from each 
other as they now are. 

Some writers have objected to any extinct species, 


or group of species, being considered as intermediate 
between any two living species, or groups of species. 
If by this term it is meant that an extinct form is di- 
rectly intermediate in all its characters between two 
living forms or groups, the objection is probably valid. 
But in a natural classification many fossil species cer- 
tainly stand between living species, and some extinct 
genera between living genera, even between genera be- 
longing 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 charac- 
ters, the ancient members are separated by a somewhat 
lesser number of characters; so that the two groups 
formerly made a somewhat nearer approach to each 
other than they now do. 

It is a common belief that the more ancient a form 
is, by so much the more it tends to connect by some of 
its characters groups now widely separated frO'" 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 


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 be^ 
ing given, but this is unimportant for us. The hori- 
zontal lines may represent successive geological forma^ 
tions, and all the forms beneath the uppermost line may 
be considered as extinct. The three existing genera a^*, 
g", p^*, will form a small family; b^* and /^* a closely 
allied family or sub-family; and o^*, e^*, m^*, a third 
family. These three families, together with the many 
extinct genera on the several lines of descent diverging 
from the parent-form (A) will form an order, for all 
will have inherited something in common from their 
ancient progenitor. On the principle of the continued 
tendency to divergence of character, which was formerly 
illustrated by this diagram, the more recent any form 
is, the more it will generally differ from its ancient 
progenitor. Hence we can understand the rule that 
the most ancient fossils differ most from existing forms. 
We must not, however, assume that divergence of char- 
acter 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^*. 

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


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

By looking at the diagram we can see that if many 
of the extinct forms supposed to be imbedded in the 
successive 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^, a^, a^°, 
/^, m^, m®, m®, were disinterred, these three families 
would be so closely linked together that they probably 
would have to be united into one great family, in near- 
ly the same manner as has occurred with ruminants 
and certain, pachyderms. Yet he who objected to con- 
sider as intermediate the extinct genera, which thus 
link together the living genera of three families, would 
be partly justified, for they are intermediate, not direct- 
ly, but only by a long and circuitous course through 
many widely different forms. If many extinct forms 
were to be discovered above one of the middle hori- 
zontal lines or geological formations — for instance, 
above No. VI. — ^but none from beneath this line, then 
only two of the families (those on the left hand, a^*, &c., 
and ¥*, &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^* to m^*), on the uppermost line, be sup- 
posed to differ from each other by half-a-dozen impor- 
tant characters, then the families which existed at the 
period marked VI. would certainly have differed from 


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

Under nature the process will be far more compli- 
cated than is represented in the diagram; for the groups 
vvill have been more numerous; they will have en- 
dured 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 for- 
mations 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 palasontologists is frequently the case. 

Thus, on the theory of descent with modification, 
the main facts with respect to the mutual afl&nities 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 spe- 


cies 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 forme of life above and below. 
We must;, however, allow for the entire extinction of some 
preceding forms, and in any one region for the immi- 
gration 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 inter- 
mediate in character between the preceding and suc- 
ceeding 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 ex- 
istence, — 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 charac- 


ter, intermediate in age. But supposing for an instant, 
in this and other such cases, that the record of the first 
appearance and disappearance of the species was com- 
plete, which is far from the case, we have no reason to 
beheve that forms successively produced necessarily en- 
dure for corresponding lengths of time. A very an- 
cient form may occasionally have lasted much longer 
than a form elsewhere subsequently produced, especially 
in the case of terrestrial productions inhabiting sepa- 
rated districts. To compare small things with great; 
if the principal living and extinct races of the domestic 
pigeon were arranged in serial affinity, this arrange- 
ment would not closely accord with the order in time 
of their production, and even less with the order of 
their disappearance; for the parent rock-pigeon still 
lives; and many varieties between the rock-pigeon and 
the carrier have become extinct; and carriers which are 
extreme in the important character of length of beak 
originated earlier than short-beaked tumblers, which 
are at the opposite end of the series in this respect. 

Closely connected with the statement, that the or- 
ganic remains from an intermediate formation are in 
some degree intermediate in character, is the fact, in- 
sisted 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 forma- 
tions. Pictet gives as a well-known instance, the gen- 
eral resemblance of the organic remains from the sev- 
eral stages of the Chalk formation, though the species 
are distinct in each stage. This fact alone, from its 
generality, seems to have shaken Professor Pictet in his 
belief in the immutability of species. He who is ac- 
quainted with the distribution of existing species over 


the globe, will not attempt to account for the close re- 
semblance of distinct species in closely consecutive for- 
mations, by the physical conditions of the ancient areas 
having remained nearly the same. Let it be remembered 
that the forms of life, at least those inhabiting the sea, 
have changed almost simultaneously throughout the 
world, and therefore under the most different climates 
and conditions. Consider the prodigious vicissitudes of 
climate during the pleistocene period, which includes 
the whole glacial 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 be- 
ing 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 in- 
termediate 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 geologi- 
cally, closely allied forms, or, as they have been called 
by some authors, representative species; and these as- 
suredly 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. 


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 or- 
ganic 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 hfe 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 natu- 
ral selection, to stand higher than ancient forms. Is 
this the case? A large majority of palaeontologists 
would answer in the affirmative; and it seems that this 
answer must be admitted as true, though difficult of 


It is no valid objection to this conclusion, that cer- 
tain Brachiopods have been but slightly modified from 
an extremely remote geological epoch; and that certain 
land and fresh-water shells have remained nearly the 
same, from the time when, as far as is known, they 
first appeared. It is not an insuperable difficulty that 
Foraminifera have not, as insisted on by Dr. 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 Brachiopods 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 suc- 
cessive age, have to be slightly modified, so as to hold 
their places in relation to slight changes in their con- 
ditions. The foregoing objections hinge on the ques- 
tion 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 ex- 
tend far enough back, to shew with unmistakeable clear- 


ness that within the known history of the world or- 
ganisation 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 inter- 
mediate 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 Von Baer be- 
lieved to be " in fact more highly organised than a fish, 
although upon another type " ? In the complex strug- 
gle 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. Besides these inherent 
difficulties in deciding which forms are the most ad- 
vanced in organisation, we ought not solely to compare 
the highest members of a class at any two periods — 
though undoubtedly this is one and perhaps the most 
important element in striking a balance — ^but we ought 
to compare all the members, high and low, at the two 
periods. At an ancient epoch the highest and lowest 
molluscoidal animals, namely, cephalopods and brachio- 


pods, swarmed in numbers; at the 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 representa- 
tives. 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 ccmplex relations, the stand- 
ard 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 Zea- 
land, 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 of the 


southern hemisphere has become wild in any part of 
Europe, we may well doubt whether, if all the produc- 
tions of Xew Zealand were set free in Great Britain, any 
considerable number would be enabled to seize on places 
now occupied by our native plants and animals. Under 
this point of view, the productions of Great Britain 
stand much higher in the scale than those of New Zea- 
land. Yet the most skilful naturalist, from an exami- 
nation 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 
yet may never be capable of proof. Seeing, for instance, 
that the oldest kno\vn mammals, reptiles, and fishes 
strictly belong to their proper classes, though some of 
these old forms are in a slight degree less distinct from 
each other than are the typical members of the same 
groups at the present day, it would be vain to look for 
animals having the common embryological character of 


the Vertebrata, until beds rich in fossils are discovered 
far beneath 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 strik- 
ing manner that most of the fossil mammals, buried 
there in such numbers, are related to South American 
types. This relationship is even more clearly seen in 
the wonderful collection of fossil bones made by MM. 
Lund and Clausen in the caves of Brazil. I was so 
much impressed with these facts that I strongly insisted, 
in 1839 and 1845, on this " law of the succession of 
types," — on " this wonderful relationship in the same 
continent between the dead and the living." Professor 
Owen has subsequently extended the same generalisation 
to the mammals of the Old World. "We see the same 
law in this author's restorations of the extinct and 
gigantic birds of New Zealand. We see it also in the 
birds of the caves of Brazil. Mr. Woodward has shown 
that the same law holds good with sea-shells, but, from 
the wide distribution of most molluscs, it is not well 
displayed by them. Other cases could be added, as the 
relation between the extinct and living land-shells of 
Madeira; and between the extinct and living brackish 
water-shells of the Aralo-Caspian Sea. 


Now what does this remarkable law of the succession 
of the same types within the same areas mean? He 
would be a bold man who, after comparing the present 
climate of Australia and of parts of South America, 
under the same latitude, would attempt to account, on 
the one hand through dissimilar physical 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. North 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 Northern India was formerly more 
closely related in its mammals to Africa than it is at the 
present time. Analogous facts could be given in rela- 
tion to the distribution of marine animals. 

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


though in some degree modified descendants. If the 
inhabitants of one continent formerly differed greatly 
from those of another continent, so will their modified 
descendants still differ in nearly the same manner and 
degree. But after very long intervals of time, and 
after great geographical changes, permitting much in- 
termigration, the feebler will yield to the more domi- 
nant 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 alhed 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 forma- 
tion there be six other alHed or representative genera 
each with the same number of species, then we may con- 
clude 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 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 spe- 
cies 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 al- 
most 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 less perfectly kept; that each single for- 
mation has not been continuously deposited; that the 
duration of each formation is probably short compared 
with the average duration of specific forms; that mi- 
gration has played an important part in the first ap- 
pearance 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 must have passed through numer- 
ous 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 re- 
mained 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 inter- 
minable varieties, connecting together all extinct and 
existing forms by the finest graduated steps. It should 
also be constantly borne in mind that any linking va- 
riety between two forms, which might be found, would 
be ranked, unless the whole chain could be perfectly 
restored, as a new and distinct species; for it is not pre- 
tended that we have any sure criterion by which species 
and varieties can be discriminated. 

He who rejects this view of the imperfection of the 
geological record, will rightly reject the whole theory. 
For he may ask in vain where are the numberless tran- 
sitional hnks which must formerly have connected the 
closely alhed 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 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 lead- 
ing 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 in- 
crease in numbers slowly, and endure for unequal 
periods of time; for the process of modification is neces- 
sarily slow, and depends on many complex contingen- 
cies. The dominant species belonging to large and 
dominant groups tend to leave many modified descend- 
ants, which form new sub-groups and groups. As these 
are formed, the species of the less vigorous groups, from 
their inferiority inherited from a common progenitor, 
tend to become extinct together, and to leave no modi- 
fied offspring on the face of the earth. But the utter 
extinction of a whole group of species has sometimes 
been a slow process, from the survival of a few descend- 


ants, lingering in protected and isolated situations. 
When a group has once wholly disappeared, it does not 
reappear; for the link of generation has been broken. 

We can understand how it is that dominant forms 
which spread widely and yield the greatest number of 
varieties tend to people the world with allied, but modi- 
fied, 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 to- 
gether. 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 ex- 
tinct 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 
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 special- 
ised; and this may account for the common belief held 
by so many palaeontologists, that organisation on the 
whole has progressed. Extinct and ancient animals re- 
semble to a certain extent the embryos of the more re- 
cent 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 geologi- 
cal 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 gen- 
eration: old forms having been supplanted by new and 
improved forms of life, the products of Variation and 
the Survival of the Fitteet. 




Present distribution cannot be accounted for by dififerences 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. 

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 dissimi- 
larity 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 divi- 
sions 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 least as closely as the same species generally re- 
quire. No doubt small areas can be pointed out in the 
Old World hotter than any in the Xew 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. Notwith- 
standing this general parallelism in the conditions of 
the Old and New Worlds, how widely different are their 
living productions! 

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

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


northern temperate forms, as there now is for the strict- 
ly 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 con- 

Turning to the sea, we find the same law. The 
marine inhabitants of the eastern and western shores 
of South America are very distinct, with extremely few 
shells, Crustacea, or echinodermata in common; but Dr. 
Giinther has recently shown that about thirty per cent, 
of the fishes are the same on the opposite sides of the 
isthmus of Panama; and this fact has led naturalists 
to beheve that the isthmus was formerly open. West- 
ward 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 sepa- 
rated from each other by impassable barriers, either of 
land or open sea, they are almost wholly distinct. On 
the other hand, proceeding still farther westward from 


the eastern islands of the tropical parts of the Pacific, 
we encounter no impassable barriers, and we have in- 
numerable islands as halting-places, or continuous 
coasts, until, after travelling over a hemisphere, we 
come to the shores of Africa; and over this vast space 
we meet with no well-defined and distinct marine fau- 
nas. Although so few marine animals are common 
to the above-named three approximate faunas of East- 
ern and Western America and the Eastern Pacific is- 
lands, yet many fishes range from the Pacific into the 
Indian Ocean, and many shells are common to the east- 
ern islands of the Pacific and the eastern shores of 
Africa on almost exactly opposite meridians of longi- 

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 close- 
ly 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 Ehea (American ostrich) and north- 
ward 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 lati- 
tude. On these same plains of La Plata we see the 


agouti and bizcacha, animals having nearly the same 
habits as our hares and rabbits, and belonging to the 
same order of 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 bizca- 
cha; we look to the waters, and we do not find the 
beaver or musk-rat, but the coypu and capybara, ro- 
dents of the S. American type. Innumerable other 
Instances could be given. If we look to the islands ofE 
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 natu- 
ralist must be dull who is not led to enquire 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 varie- 
ties, nearly alike. The dissimilarity of the inhabitants 
of different regions may be attributed to modification 
through variation and natural selection, and probably 
in a subordinate degree to the definite influence of dif- 
ferent physical conditions. The degrees of dissimilar- 
ity will depend on the migration of the more dominant 
forms of life from one region into another having been 
more or less effectually prevented, at periods more or 
less remote; — on the nature and number of the former 
immigrants; — and on the action of the inhabitants on 


each other in leading to the preservation of different 
modifications; the relation of organism to organism in 
the struggle for life being, as I have already often re- 
marked, the most important of all relations. Thus the 
high importance of barriers comes into play by check- 
ing migration; as does time for the slow process of 
modification through natural selection. Widely-rang- 
ing species, abounding in individuals, which have al- 
ready triumphed over many competitors in their own 
widely-extended homes, will have the best chance of 
seizing on new places, when they spread into new coun- 
tries. In their new homes they will be exposed to new 
conditions, and will frequently undergo further modi- 
fication and improvement; and thus they will become still 
further victorious, and will produce groups of modified 
descendants. On this principle of inheritance with 
modification we can understand how it is that sec- 
tions of genera, whole genera, and even families, are 
confined to the same areas, as is so commonly and notori- 
ously the case. 

There is no evidence, as was remarked in the last 
chapter, of the existence of any law of necessary de- 
velopment. 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 any- 
thing. These principles come into play only by bring- 


ing organisms into new relations with each other and in 
a lesser degree with the surrounding physical condi- 
tions. 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 hecome 
greatly or at all modified. 

According to these views, it is obvious that the sev- 
eral species of the same genus, though inhabiting the 
most distant quarters of the world, must originally have 
proceeded from the same source, as they are descended 
from the same progenitor. In the case of those species 
which have undergone during whole geological periods 
httle modification, there is not much difficulty in be- 
lieving 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 spe- 
cies of a genus have been produced within comparative- 
ly recent times, there is great difficulty on this head. 
It is also obvious that the individuals of the same spe- 
cies, though now inhabiting distant and isolated regions, 
must have proceeded from one spot, where their parents 
were first produced: for, as has been explained, it is 
incredible that individuals identically the same should 
have been produced from parents specifically dis- 

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 


difficulty in understanding how the same species could 
possibly have migrated from some one point to the sev- 
eral distant and isolated points, where now found. 
Nevertheless the simplicity of the view that each spe- 
cies was first produced within a single region captivates 
the mind. He who rejects it, rejects the vera causa of 
ordinary generation with subsequent migration, and 
calls in the agency of a miracle. It is universally ad- 
mitted, that in most cases the area inhabited by a spe- 
cies is continuous; and that when a plant or animal 
inhabits two points so distant from each other, or with 
an interval of such a nature, that the space could not 
have been easily passed over by migration, the fact is 
given as something remarkable and exceptional. The 
incapacity of migrating across a wide sea is more clear 
in the case of terrestrial mammals than perhaps with 
any other organic beings; and, accordingly, we find no 
inexplicable instances of the same mammals inhabit- 
ing distant points of the world. No geologist feels any 
difficulty in Great Britain possessing the same quad- 
rupeds with the rest of Europe, for they were no doubt 
once united. But if the same species can be produced 
at two separate points, why do we not find a single mam- 
mal common to Europe and Australia or South Amer- 
ica? The conditions of life are nearly the same, so that 
a multitude of European animals and plants have be- 
come 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 mam- 
mals have not been able to migrate, whereas some plants, 
from their varied means of dispersal, have migrated 
across the wide and broken interspaces. The great 


and striking influence of barriers of all kinds, is intelli- 
gible 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 natu- 
ralists, that the view of each species having been pro- 
duced in one area alone, and having subsequently mi- 
grated from that area as far as its powers of migration 
and subsistence under past and present conditions per- 
mitted, 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 cer- 
tainly occurred within recent geological times, must 
have rendered discontinuous the formerly continuous 
range of many species. So that we are reduced to con- 
sider 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 con- 
siderations, that each species has been produced within 
one area and has migrated thence as far as it could. 


It would be hopelessly tedious to discuss all the excep- 
tional eases of the same species, now living at distant 
and separated points, nor do I for a moment pretend that 
any explanation could be offered of many instances. 
But, after some preliminary remarks, I will discuss a 
few of the most striking classes of facts; namely, the 
existence of the same species on the summits of distant 
mountain ranges, and at distant points in the arctic and 
antarctic regions; and secondly (in the following chap- 
ter), the wide distribution of freshwater productions; 
and thirdly, the occurrence of the same terrestrial species 
on islands and on the nearest mainland, though sepa- 
rated by hundreds of miles of open sea. If the exist- 
ence 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 incom- 
parably 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 modifica- 
tion during their migration, from some one area. If, 
when most of the species inhabiting one region are dif- 
ferent 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 strength- 
ened; for the explanation is obvious on the principle of 


descent with modification. A volcanic island, for in- 
stance, upheaved and formed at the distance of a few 
hiindreds of miles from a continent, would probably re- 
ceive from it in the course of time a few colonists, and 
their descendants, though modified, would still be re- 
lated by inheritance to the inhabitants of that continent. 
Cases of this nature are common, and are, as we shall 
hereafter see, inexplicable on the theory of independ- 
ent 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 spe- 
cies." And it is now well known that he attributes this 
coincidence to descent with modification. 

The question of single or multiple centres of crea- 
tion diff'ers from another though allied question, — 
nameh', 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 individ- 
uals 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, name- 
ly, 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 individ- 
uals will go on simultaneously changing, and the whole 


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

Before discussing the three classes of facts, which I 
have selected as presenting the greatest amount of diffi- 
culty 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, pres- 
ently 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 sepa- 
rates two marine faunas; submerge it, or let it formerly 
have been submerged, and the two faunas will now 
blend together, or may formerly have blended. Where 
the sea now extends, land may at a former period have 
connected islands or possibly even continents together, 
and thus have allowed terrestrial productions to pass 
from one to the other. No geologist disputes that 
great mutations of level have occurred within the period 
of existing organisms. Edward Forbes insisted that all 
the islands in the Atlantic must have been recently 

Chap. Xn.] MEANS OF DISPERSAL. 14 ^ 

connected with Europe or Africa, and Europe likewise 
with America, Other authors have thus hypotheticallj 
bridged over every ocean, and united almost every is- 
land 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 re- 
cently been united to some continent. This view cuts 
the Gordian knot of the dispersal of the same species to 
the more 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 ad- 
mit 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 for- 
mer extension of the land. But I do not believe that it 
will ever be proved that within the recent period most 
of our continents which now stand quite separate have 
been continuously, or almost continuously united with 
each other, and with the many existing oceanic islands. 
Several facts in distribution, — such as the great difference 


in the marine faunas on the opposite sides of almost 
every continent, — the close relation of the tertiary in- 
habitants of several lands and even seas to their present 
inhabitants, — the degree of affinity between the mam- 
mals inhabiting islands with those of the nearest conti- 
nent, 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 rela- 
tive proportions of the inhabitants of oceanic islands are 
likewise opposed to the belief of their former continu- 
ity with continents. Nor does the almost universally 
volcanic composition of such islands favour the admis- 
sion 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, meta- 
morphic schists, old fossiliferous and other rocks, instead 
of consisting of mere piles of volcanic matter. 

I must now say a few words on what are called 
accidental means, but which more properly should be 
called occasional means of distribution. I shall here 
confine myself to plants. In botanical works, this or 
that plant is often stated to be ill adapted for wide dis- 
semination; but the greater or less facilities for trans- 
port 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 sur- 
prise I found that out of 87 kinds, 64 germinated after 
an immersion of 28 davs, and a few survived an immer- 


sion of 137 days. It deserves notice that certain orders 
were far more injured than others: nine Leguminosae 
were tried, and, with one exception, they resisted the 
salt-water badly; seven species of the allied orders, Hy- 
drophyllaceffi and Polemoniacese, were all killed by a 
month's immersion. 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 rapidly, but 
some which, whilst green, floated for a 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 after- 
wards 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 ^ 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 as anything can be inferred from these 
scanty facts, t^^at the seeds of yVV kinds of plants of any 
country might oe floated by sea-currents during 28 days, 
and would retain their power of germination. In 
Johnston's Physical Atlas, the average rate of the sev- 
eral Atlantic currents is 33 miles per diem (some cur- 
rents running at the rate of 60 miles per diem); on this 
average, the seeds of ^^ plants belonging to one coun- 
try 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 flota- 
tion and their resistance to the injurious action of the 
salt-water. On the other hand, he did not previously 
dry the plants or branches with the fruit; and this, as 
we have seen, would have caused some of them to have 
floated much longer. The result was that -^f of his 
seeds of different kinds floated for 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 -^^ plants of a flora, 
after having been dried, could be floated across a space 
of sea 900 miles in width, and would then germinate. 
The fact of the larger fruits often floating longer than 


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

Seeds may he 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 per- 
fectly 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 observation. Again, I 
can show that the carcases of birds, when floating on the 
sea, sometimes escape being immediately devoured: and 
many kinds of seeds in the crops of floating birds long 
retain their vitality: peas and vetches, for instance, are 
killed by even a few days' immersion in sea- water; but 
some taken out of the crop of a pigeon, which had floated 
on artificial sea-water for 30 days, to my surprise nearly 
all germinated. 

Living birds can hardly fail to be highly effective 
agents in the transportation of seeds. I could give 
many facts showing how frequently birds of many kinds 
are blown by gales to vast 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. T 


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 13 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 ia 
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 germi- 
nated 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 fre- 
quently 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 intprval of many hours, either re- 
jected the seeds in pellets or passed them in their excre- 


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

Locusts are sometimes blown to great distances from 
the land; I myself caught one 370 miles from the coast 
of Africa, and have heard of others caught at greater 
distances. The 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 
telescope. During two or three days they slowly ca- 
reered round and round in an immense eUipse, 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 insufl&cient evidence, that injurious 
seeds are introduced into their grass-land in the dung 
left by the great flights of locusts which often visit that 
country. In consequence of this belief Mr. Weale sent 
me in a letter a small packet of the dried pellets, out of 
which I extracted under the microscope several seeds, 
and raised from them seven grass plants, belonging to 
two species, of two genera. Hence a swarm of locusts, 
such as that which visited Madeira, might readily be the 
means of introducing several kinds of plants into an 
island lying far from the mainland. 

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


partridge, and in the earth there was a pebble as large as 
the seed of a vetch. Here is a better case: the leg of a 
woodcock was sent to me by a friend, with a little cake 
of dry earth attached to the shank, weighing only nine 
grains; and this contained a seed of the toad-rush (Jun- 
eus 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 

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


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

Considering that these several means of transport, 
and that other means, which without doubt remain to 
be discovered, have been in action year after year for 
tens of thousands of years, it would, I think, be a mar- 
vellous 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 direc- 
tion of prevalent gales of wind. It should be observed 
that scarcely any means of transport would carry seeds 
for very great distances: for seeds do not retain their 


vitality when exposed for a great length of time to the 
action of sea-water; nor could they be long carried in 
the crops or intestines of birds. These means, how- 
ever, would suffice for occasional transport across tracts 
of sea some hundred miles in breadth, or from island 
to island, or from a continent to a neighbouring island, 
but not from one distant continent to another. The 
floras of distant continents would not by such means 
become mingled; but would remain as distinct as they 
now are. The currents, from their course, would never 
bring seeds from North America to Britain, though 
they might and do bring seeds from the West Indies 
to our western shores, where, if not killed by their very 
long immersion in salt water, they could not endure our 
climate. Almost every year, one or two land-birds are 
blown across the whole Atlantic Ocean, from North 
America to the western shores of Ireland and England; 
but seeds could be transported by these rare wanderers 
only by one means, namely, by dirt adhering to their 
feet or beaks, which is in itself a rare accident. Even in 
this case, how small would be the chance of a seed fall- 
ing 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 occa- 
sional means of transport, immigrants from Europe or 
any other continent, that a poorly-stocked island, 
though standing more remote from the mainland, would 
not receive colonists by similar means. Out of a hun- 
dred kinds of seeds or animals transported to an island, 
even if far less well-stocked than Britain, perhaps not 
more than one would be so well fitted to its new home, 


as to become naturalised. But this is no valid argu- 
ment against what would be effected by occasional 
means of transport, during the long lapse of geologi- 
cal 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 in- 
sects or birds living there, nearly every seed which 
chanced to arrive, if fitted for the climate, would ger- 
minate 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 ex- 
treme northern parts of Europe; but it is far more re- 
markable, 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 
GmeHn to conclude that the same species must have 
been independently created at many distinct points; 
and we might have remained in this same belief, had 
not Agassiz and others called vivid attention to the 
Glacial period, which, as we shall immediately see, 
affords a simple explanation of these facts. We have 
evidence of almost every conceivable kind, organic and 


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

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


present circumpolar inhabitants, which we suppose to 
have everywhere travelled southward, are remarkably 
uniform round the world. 

As the warmth returned, the arctic forms would 
retreat northward, closely followed up in their retreat 
by the productions of the more temperate regions. And 
as the snow melted from the bases of the mountains, 
the arctic forms would seize on the cleared and thawed 
ground, always ascending, as the warmth increased and 
the snow still further disappeared, higher and higher, 
whilst their brethren were pursuing their northern 
journey. Hence, when the warmth had fully returned, 
the same species, which had lately lived together on the 
European and North American lowlands, would again 
be found in the arctic regions of the Old and New 
Worlds, and on many isolated mountain-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 


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 chang- 
ing climate, they will not have been exposed during 
their long migrations to any great diversity of tem- 
perature; and as they all migrated in a body together, 
their mutual relations will not have been much dis- 
turbed. Hence, in accordance with the principles in- 
culcated in this volume, these forms will not have been 
liable to much modification. But with the Alpine pro- 
ductions, left isolated from the moment of the return- 
ing 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 some- 
what different climatal influences. Their mutual rela- 
tions will thus have been in some degree disturbed; con- 
sequently they will have been liable to modification; and 
they have been modified; for if we compare the present 


Alpine plants and animals of the several great Euro- 
pean mountain-ranges one with another, though many 
of the species remain identically the same, some exist 
as varieties, somt as doubtful forms or sub-species, and 
some as distinct yet closely alKed 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 moun- 
tain-slopes and on the plains of North America and 
Europe are the same; and it may be asked how I ac- 
count 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 in- 
habitants 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 


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°-G7°; 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 num- 
ber of the same plants and animals inhabited the al- 
most continuous circumpolar land; and that these plants 
and animals, both in the Old and New Worlds, began 
slowly to migrate southwards as the climate became 
less warm, long before the commencement of the Glacial 
period. We now see, as I believe, their descendants, 
mostly in a modified condition, in the central parts of 
Europe and the United States. On this view we can 
understand the relationship with very little identity, 
between the productions of North America and Europe, 
— a relationship which is highly remarkable, consider- 
ing the distance of the two areas, and their separation 
by the whole Atlantic Ocean. We can further under- 
stand the singular fact remarked on by several observers 


that the productions of Europe and America during 
the later tertiary stages were more closely related to 
each other than they are at the present time; for dur- 
ing 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 in- 

During the slowly decreasing warmth of the Plio- 
cene 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 geo- 
graphical races, and others as distinct species; and a 
host of closely allied or representative forms which are 
ranked by all naturalists as specifically distinct. 


As on the land, so in the waters of the sea, a slow 
southern migration of a marine fauna, which, during 
the Pliocene or even a somewhat earlier period, was 
nearly uniform along the continuous shores of the Polar 
Circle, will account, on the theory of modification, for 
many closely allied forms now living in 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 North America; and the still more 
striking fact of many closely allied crustaceans (as de- 
scribed in Dana's admirable work), some fish and other 
marine animals, inhabiting the Mediterranean and the 
seas of Japan, — these two areas being now completely 
separated by the breadth of a whole continent and by 
wide spaces of ocean. 

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

Alternate Glacial Periods in the North and South. 

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


tended. In Europe we meet with the plainest evi- 
dence of the Glacial period, from the western shores 
of Britain to the Oural range, and southward to the 
Pyrenees. We ma}- 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 400 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 Rev. W. B. 
Clarke, it appears also that there are traces of former 
glacial action on the mountains of the south-eastern 
corner of Australia. 

Looking to America; in the northern half, ice-borne 
fragments of rock have been observed on the eastern 
side of the continent, as far south as lat. 36°-37°, and 
on the shores of the Pacific, where the climate is now 
so different, as far south as lat. -i6°. Erratic boulders 
have, also, been noticed on the Rocky Mountains. In 
the Cordillera of South America, nearly under the 
equator, glaciers once extended far below their present 
level. In Central Chile I examined a vast mound of 


detritus with great boulders, crossing the Portillo valley, 
which there can hardly be a doubt once formed a huge 
moraine; and Mr. D. Forbes informs me that he found 
in various parts of the Cordillera, from lat. 13° to 30° S., 
at about the height of 12,000 feet, deeply-furrowed 
rocks, resembling those with which he was familiar in 
Norway, and likewise great masses of detritus, including 
grooved pebbles. Along this whole space of the 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 ex- 
tremity, we have the clearest evidence of former glacial 
action, in numerous immense boulders transported far 
from their parent source. 

From these several facts, namely from the glacial 
action having extended all round the northern and 
southern hemispheres — from the period having been in 
a geological sense recent in both 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 in- 
direct influence of the eccentricity of the orbit upon 
oceanic currents. According to Mr. Croll, cold periods 



regularly recur every ten or fifteen thousand years; and 
these at long intervals are extremely severe, owing to cer- 
tain contingencies, of which the most important, as Sir 
C. Lyell has shown, is the relative position of the land 
and water. Mr. CroU 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 hemi- 
sphere 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 north- 
ern hemisphere, whilst the southern passes through 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 be- 
sides many closely allied species, between forty and 
fifty of the flowering plants of Tierra del Fuego, form- 
ing no inconsiderable part of its scanty flora, are com- 
mon to North America and Europe, enormously remote 
as these areas in opposite hemispheres are from each 
other. On the lofty mountains of equatorial America 
a host of peculiar species belonging to European genera 
occur. On the Organ mountains of Brazil, some few 
temperate European, some Antarctic, and some Andean 
genera were found by Gardner, which do not exist 


in the low intervening hot countries. On the Silla of 
Caraccas, the illustrious Humboldt long ago found 
species belonging to genera characteristic of the Cordil- 

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, be- 
lieved not to have been introduced by man, and on the 
mountains several representative European forms are 
found, which have not been discovered in the inter- 
tropical 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 neigh- 
bouring Cameroon mountains, in the Gulf of Guinea, 
are closely related to those on the mountains of Abys- 
sinia, 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 temper- 
ate forms, almost under the equator, across the whole 
continent of Africa and to the mountains of the Cape 
Verde archipelago, is one of the most astonishing facts 
ever recorded in the distribution of plants. 

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


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

On the southern mountains of Australia, Dr. F. 
Miiller has discovered several European species; other 
species, not introduced by man, occur on the lowlands; 
and a long list can be given, as I am informed by Dr. 
Hooker, of European genera, found in Australia, but 
not in the intermediate torrid regions. In the admir- 
able ' Introduction to the Flora of New Zealand,' by Dr. 
Hooker, analogous and striking facts are given in re- 
gard to the plants of that large island. Hence we see 
that certain plants growing on the more lofty moun- 
tains 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 equatorial latitudes, 
the Alpine or mountain floras really become less and 
less Arctic." Besides these identical and closely allied 
forms, many species inhabiting the same widely sun- 
dered areas, belong to genera not now found in the inter- 
mediate 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 


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." Sir 
J. Eichardson, 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 prob- 
ably about the same with that now experienced at 
the height of from five to six thousand feet under the 
same latitude, or perhaps even rather cooler. During 
this, the coldest period, the lowlands under the equator 
must have been clothed with a mingled tropical and 
temperate vegetation, like that described by Hooker as 
growing luxuriantly at the height of from four to five 
thousand feet on the lower slopes of the Himalaya, but 
with perhaps a still greater preponderance of temperate 
forms. So again in the mountainous island of Fer- 
nando Po, in the Gulf of Guinea, Mr. Mann found tem- 
perate European forms beginning to appear at the height 


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

Now let us see whether Mr. Croll's conclusion that 
when the northern hemisphere suffered from the ex- 
treme 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 hemi- 
spheres, 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 vigor- 
ous, dominant, and widest-spreading temperate forms in- 
vaded the equatorial lowlands. The inhabitants of 
these hot lowlands would at the same time have migrated 
to the tropical and subtropical regions of the south, for 
the southern hemisphere was at this period warmer. 
On the decline of the Glacial period, as both hemi- 
spheres gradually recovered their former temperatures, 
the northern temperate forms living on the lowlands 
under the equator, would have been driven to their 
former homes or have been destroyed, being replaced 
by the equatorial forms returning from the south. 
Some, however, of the northern temperate forms would 
almost certainly have ascended any adjoining high land, 


where, if sufficiently lofty, they would have long sur- 
vived like the Arctic forms on the mountains of Europe. 
They might have survived, even if the chmate was not 
perfectly fitted for them, for the change of temperature 
must have been very slow, and plants undoubtedly pos- 
sess a certain capacity for acclimatisation, as shown by 
their transmitting to their offspring different consti- 
tutional 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 de- 
scend 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 
their mountain fastnesses. Thus, we should have some 
few species identically the same in the northern and 
southern temperate zones and on the mountains of the 
intermediate tropical regions. But the species left dur- 
ing a long time on these mountains, or in opposite 
hemispheres, would have to compete with many new 
forms and would be exposed to somewhat different 
physical conditions; hence they would be eminently 
liable to modification, and would generally now exist 
as varieties or as representative species; and this is the 
case. "We must, also, bear in mind the occurrence in 
both hemispheres of former Glacial periods; for these 
will account, in accordance with the same principles, for 
the many quite distinct species inhabiting the same 


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

It is a remarkable fact strongly insisted on by 
Hooker in regard to America, and by Alph. de Can- 
dolle 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 preponder- 
ant 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 pro- 
ductions 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 hemi- 
sphere, 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, 


Australian forms are rapidly sowing themselves and be- 
coming naturalised. Before the last great Glacial 
period, no doubt the intertropical mountains were 
stocked with endemic Alpine forms; but these have al- 
most 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 pro- 
ductions 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 con- 
tinental forms naturalised through man's agency. 

The same principles apply to the distribution of ter- 
restrial animals and of marine productions, in the north- 
ern and southern temperate zones, and on the inter- 
tropical mountains. When, during the height of the 
Glacial period, the ocean-currents were widely differ- 
ent 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, iso- 
lated 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 


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 
ahove 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 Ker- 
guelen Land, New Zealand, and Fuegia; but icebergs, 
as suggested by Lyell, may have been concerned in their 
dispersal. The existence at these and other distant 
points of the southern hemisphere, of species, which, 
though distinct, belong to genera exclusively confined 
to the south, is a more remarkable case. Some of these 
species are so distinct, that we cannot suppose that there 
has been time since the commencement of the last Gla- 
cial period for their migration and subsequent modi- 
fication 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 that before this flora was exterminated 
during the last Glacial epoch, a few forms had been al- 
ready 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 New Zealand 
may have become slightly tinted by the same peculiar 
forms of life. 

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




distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial 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 

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 ap- 
parently 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 enor- 
mous 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 in- 
sects, shells, &c., and at the dissimilarity of the sur- 
rounding terrestrial beings, compared with those of 

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


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


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 geo- 
graphical changes, and consequently time and means for 
much migration. Moreover, Dr. Giinther has recently 
been led by several considerations to infer that with 
fishes the same forms have a long endurance. Salt- 
water fish can with care be slowly accustomed to live in 
fresh water; and, according to Valenciennes, there is 
hardly a single group of which all the members are con- 
fined to fresh water, so that a marine species belonging 
to a fresh-water group might travel far along the shores 
of the sea, and could, it is probable, become adapted 
without much difficulty to the fresh waters of a distant 

Some species of fresh-water shells have very wide 
ranges, and allied species which, on our theory, are de- 
scended 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 ob- 
served — and many others no doubt will be discovered — 
throw some light on this subject. When ducks sudden- 
ly emerge from a pond covered with duck-weed, I have 
twice seen these little plants adhering to their backs; 
and it has happened to me, in removing a little duck- 


weed from one aquarium to another, that I have unin- 
tentionally stocked the one with fresh-water shells from 
the other. But another agency is perhaps more effec- 
tual: 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 ad- 
vanced age they would voluntarily drop off. These 
just-hatched molluscs, though aquatic in their nature, 
survived on the duck's feet, in damp air, from twelve to 
twenty hours; and in this length of time a duck or 
heron might fly at least six or seven hundred miles, and 
if blown across the sea to an oceanic island, or to any 
other distant point, would be sure to alight on a pool 
or rivulet. Sir Charles Lyell informs me that a Dytis- 
cus has been caught with an Ancylus (a fresh-water 
shell like a limpet) firmly adhering to it; and a water- 
beetle of the same family, a Colymbetes, once flew on 
board the ' Beagle,' when forty-five miles distant from 
the nearest land: how much farther it might have been 
blown by a 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 re- 
mote oceanic islands. This is strikingly illustrated, ac- 
cording 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 con- 
sequence, 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 


feet and beaks of birds. Wading birds, which frequent 
the muddy edges of ponds, if suddenly flushed, would be 
the most hkely 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 ofE; 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, be- 
neath water, on the edge of a little pond: this mud 
when dried weighed only 6f 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, cen- 
tury after century, have gone on daily devouring fish; 


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

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 num- 
ber 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 would have 
a better chance of seizing on a new place, than in the 
case of terrestrial colonists. We should also remember 
that many fresh-water productions are low in the scale 
of nature, and we have reason to believe that such be- 
ings become modified more slowly than the high; and 


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

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 produc- 
tions of islands. In the following remarks I shall not 
confine myself to the mere question of dispersal, but 
shall consider some other cases bearing on the truth of 
the two theories of independent creation and of descent 
with modification. 


The species of all kinds which inhabit oceanic is- 
lands are few in number compared with those on equal 
continental areas: Alph. de Candolle admits this for 
plants, and Wollaston for insects. New Zealand, for 
instance, with its lofty mountains and diversified sta- 
tions, extending over 780 miles of latitude, together 
with the outlying islands of Auckland, Campbell and 
Chatham, contain altogether only 960 kinds of flower- 
ing plants; if Ave 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 differ- 
ence in number. Even the uniform county of Cam- 
bridge has 847 plants, and the little island of Angle- 
sea 764, but a few ferns and a few introduced plants 
are included in these numbers, and the comparison in 
some other respects is not quite fair. We have evi- 
dence 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 New Zealand and on every other oceanic 
island which can be named. In St. Helena there is 
reason to believe that the naturalised plants and ani- 
mals 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 suffi- 
cient number of the best adapted plants and animals 
were not created for oceanic islands; for man has unin- 
tentionally stocked them far more fully and perfectly 
than did nature. 

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


found nowhere else in the world) is often extremely 
large. If we compare, for instance, the number of 
endemic land-shells in Madeira, or of endemic birds in 
the Galapagos Archipelago, with the number found on 
any 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 pro- 
duce 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 difEerence seems to depend partly on the spe- 
cies which are not modified having immigrated in a 
body, so that their mutual relations have not been much 
disturbed; and partly on the frequent arrival of un- 
modified 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 fore- 
going 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 is- 
lands 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 South America, and which has 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 North American 
birds occasionally or even frequently visit this island. 
Almost every year, as I am informed by Mr. E. V. 
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 to- 
gether, 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 
modification. Any tendency to modification will also 
have been checked by intercrossing with the un- 
modified immigrants, often arriving from the mother- 
country. Madeira again is inhabited by a wonder- 
ful 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 transported across three or four hun- 
dred miles of open sea far more easily than land-shells. 
The different orders of insects inhabiting Madeira pre- 
sent nearly parallel cases. 

Oceanic islands are sometimes deficient in animals of 
certain whole classes, and their places are occupied by 
other classes; thus in the Galapagos Islands reptiles. 


and in New Zealand gigantic wingless birds, take, or 
recently took, the 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 ap- 
purtenances 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 dif- 
ferences 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 condi- 

Many remarkable little facts could be given with 
respect to the inhabitants of oceanic islands. For in- 
stance, in certain islands not tenanted by a single mam- 
mal, 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 might 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 append- 
age like the shrivelled wings under the soldered wing- 
covers of many insular beetles. Again, islands often 
possess trees or bushes belonging to orders which else- 


where 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 suc- 
cessfully 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 

Absence of Batrachians and Terrestrial Mammals on 
Oceanic Islands. 

With respect to the absence of whole orders of ani- 
mals on oceanic islands, Bory St. Vincent long ago 
remarked that Batrachians (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 Sey- 
chelles. But I have already remarked that it is doubt- 
ful whether New Zealand and New 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 introduced into Ma- 


deira, 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 excep- 
tion, as far as known, of one Indian species) by sea- 
water, there would be great difficulty in their trans- 
portal across the sea, and therefore 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 ex- 

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 380 miles; moreover, icebergs formerly brought 
boulders to its western shores, and they may have for- 
merly 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 quadrupeds 
have not become naturalised and greatly multiplied. 
It cannot be said, on the ordinary view of creation, that 
there has not been time for the creation of mammals; 
many volcanic islands are sufficiently ancient, as shown 


by the stupendous degradation which they have suffered, 
and by their tertiary strata: there has also been time 
for the production of endemic species belonging to 
other classes; and on continents it is 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. 
New Zealand possesses two bats found nowhere else 
in the world: Norfolk Island, the Viti Archipelago, the 
Bonin Islands, the Caroline and Marianne Archipela- 
goes, 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 is- 
lands? On my view this question can easily be an- 
swered; for no terrestrial mammal can be transported 
across a wide space of sea, but bats can fly across. 
Bats have been seen wandering by day far over the 
Atlantic Ocean; and two North American species 
either regularly or occasionally visit Bermuda, at 
the distance of 600 miles from the mainland. I hear 
from Mr. Tomes, who has specially studied this 
family, that many species have enormous ranges, 
and are found on continents and on far distant is- 
lands. Hence we have only to suppose that such 
wandering species have been modified in their new 
homes 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- 

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 


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 shal- 
low 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 near the 
shores of Australia. The West Indian Islands, on the 
other hand, stand on a deeply submerged bank, nearly 
1000 fathoms in depth, and haee 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 chan- 
nels, 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 afl&nity, — a relation 
which is quite inexplicable on the theory of independ- 
ent acts of creation. 

The foregoing statements in regard to the inhabi- 
tants 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 groups in the same class, having been modified — 
the absence of certain whole orders, as of batrachians 
and of terrestrial mammals, notwithstanding the pres- 
ence of aerial bats, — the singular proportions of certain 
orders of plants, — herbaceous forms having been de- 
veloped into trees, &c., — seem to me to accord better 
with the belief in the efl&ciency of 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 hav- 
ing 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 diflBi- 
culties in understanding how many of the inhabitants 
of the more remote islands, whether still retaining the 
same specific form or subsequently modified, 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 difl&cult case. Almost all oceanic 
islands, even the most isolated and smallest, are in- 
habited by land-shells, generally by endemic species, 
but sometimes by species found elsewhere, — 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-water; their eggs, 
at least such as I have tried, sink in it and are killed. 
Yet there must be some unknown, but occasionally effi- 
cient means for their transportal. 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 hyhernating and 
having a membranous diaphragm over the mouth of 
the shell, might be floated in chinks of drifted timber 
across moderately wide arms of the sea. And I 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 hyhernating 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 geo- 
graphical miles. As this Helix has a thick calcareous 
operculum, I removed it, and when it had formed a 
new membranous one, I again immersed it for four- 
teen days in sea-water, and again it recovered and 
crawled away. Baron Aucapitaine has since tried simi- 
lar experiments: he placed 100 land-shells, belonging 
to ten species, in a box pierced with holes, and im- 
mersed it for a fortnight in the sea. Out of the hun- 
dred 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 be- 
longing to four other species of Helix tried by Aucapi- 
taine, 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. 


On ike 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 being actually the same. 
Numerous 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 is- 
lands 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 asso- 
ciated together, which closely resembles the conditions 
of the South American coast: in fact, there is a con- 
siderable dissimilarity in all these respects. On the 


other hand, there is a considerable degree of resem- 
blance in the volcanic nature of the soil, in the climate, 
height, and size of the islands, between the Galapagos 
and Cape Verde Archipelagoes: but what an entire and 
absolute difference in their inhabitants! The inhabi- 
tants of the Cape Verde Islands are related to those of 
Africa, like those of the Galapagos to America. Facts 
such as these, admit of no sort of explanation on the 
ordinary view of independent creation; whereas on the 
view here maintained, it is obvious that the Galapagos 
Islands would be likely to receive colonists from Amer- 
ica, whether by occasional means of transport or (though 
I do not believe in this doctrine) by formerly continu- 
ous land, and the Cape Verde Islands from Africa; 
such colonists would be Hable to modification, — the 
principle of inheritance still betraying their original 

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 Amer- 
ica, 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 Austraha, 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 


80 enormously remote, that the fact becomes an anom- 
aly. 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 
intermediate though distant point, namely from the ant- 
arctic islands, when they were clothed with vegetation, 
during a warmer tertiary period, before the commence- 
ment of the last Glacial period. The affinity, which 
though feeble, I am assured by Dr. Hooker is real, be- 
tween 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 relation- 
ship 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 & 
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 witliin 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 


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 had been stocked by occasional means of trans- 
port — a seed, for instance, of one plant having been 
brought to one island, and that of another plant to 
another island, though all proceeding from the same 
general source. Hence, when in former times an im- 
migrant first settled on one of the islands, or when it 
eubsequently spread from one to another, it would un- 
doubtedly 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 fa- 
vour 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 Gala- 
pagos Archipelago, and in a lesser degree in some an- 
alogous cases, is that each new species after being formed 
in any one island, did not spread quickly to the other is- 
lands. But the islands, though in sight of each other, 
are separated by deep arms of the sea, in most cases 


wider than the British Channel, and there is no reason 
to suppose that they have at any former period been 
continuously united. The currents of the sea are rapid 
and sweep 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 tlie several 
islands; and we may infer from their present manner 
of distribution, that they have 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 intercom- 
munication. Undoubtedly, if one species has any ad- 
vantage 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 sepa- 
rate places for almost any length of time. Being famil- 
iar 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 inhabi- 
tants, but are very distinct forms, belonging in a large 
proportion of cases, as shown by Alph. de Candolle, to 
distinct genera. In the Galapagos Archipelago, many 
even of the birds, though so well adapted for flying 
from island to island, differ on the different islands; 
thus there are three closely-allied species of mocking- 
thrush, each confined to its own island. Now let us 
suppose the mocking-thrush of Chatham Island to be 


blown to Charles Island, which has its own mocking- 
thrush; why should it succeed in establishing itself 
there? We may safely infer that Charles Island is 
well stocked with its own species, for annually more 
eggs are laid and young birds hatched, than can possibly 
be reared; and we may infer that the mocking-thrush 
pecuhar to Charles's 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 butterflies and 
other animals inhabiting the great, open, and continu- 
ous vallev of the Amazons. 


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 
most 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, Al- 
pine 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 surround- 
ing lowlands. So it is with the inhabitants of lakes 
and marshes, excepting in so far as great facility of 
transport has allowed the same forms to prevail through- 
out large portions of the world. We see this same prin- 
ciple in the character of most of the blind animals in- 
habiting 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-al- 
lied 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 mi- 
gration in certain species, either at the present or at 
some former period, and the existence at remote points 
of the world of closely-allied species, is shown in an- 


other and more general way. Mr. Gould remarked 
to me long ago, that in those genera of birds which 
range over the world, many of the species have very 
wide ranges. I can hardly doubt that this rule is gen- 
erally true, though difficult of proof. Amongst mam- 
mals, we see it strikingly displayed in Bats, and in a 
lesser degree in the Fehdae 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 
epecies have very wide ranges in the genera which range 
very widely. Nor is it meant that the species in 
guch genera have on an average a very wide range; for 
this will largely depend on how far the process of modi- 
fication 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 pow^erfully- 
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 im- 
portant power of being victorious in distant lands 
in the struggle for life with foreign 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 rule we 


do find, that some at least of the species range very 

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 spe- 
cific character. This fact, together with that of the 
seeds and eggs of most lowly organised forms being very 
minute and better fitted for distant transportal, prob- 
ably accounts for a law which has long been observed, 
and which has lately been discussed by Alph. de Can- 
dolle in regard to plants, namely, that the lower 
any group of organisms stands the more widely it 

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 re- 
lationship 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. 


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 witliin the recent 
period, and of other changes which have probably oc- 
curred, — 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 com- 
mon parents. And we are led to this conclusion, which 
has been arrived at by many naturalists under the desig- 
nation of single centres of creation, by various gen- 
eral considerations, more especially from the impor- 
tance 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 be- 
fore 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 


even the equatorial regions^, and which, during the al- 
ternations 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 geo- 
graphical 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 form- 
ing 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 dif- 
ferent latitudes, for instance in South America, the in- 
habitants 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. Bear- 
ing in mind that the mutual relation of organism to 
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 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 con- 
ditions 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 propor- 
tion should be endemic or peculiar; and why, in rela- 
tion 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 
batrachians and terrestrial mammals, should be absent 
from oceanic islands, whilst the most isolated islands 
should possess their own peculiar species of aerial mam- 
mals 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 
be closely related to each other; and should likewise 


be related, but less closely, to those of the nearest con- 
tinent, or other source whence immigrants might 
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 spe- 
cies 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 liv- 
ing 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 
find that species in certain classes differ little from each 
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 sev- 
eral 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 modifications 
have been accumulated by the same means of natural 



mutual affinities of oeganic beings: mob- 
phology: embeyology: eudimentary organs. 

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


From the 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 signifi- 
cance, if one group had been exclusively fitted to in- 
habit 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 com- 
monly members of even the same sub-group have dif- 
ferent habits. In the second and fourth chapters, on 
Variation and on Natural Selection, I have attempted 


to show that within each country it is the widely rang- 
ing, the much diffused and common, that is the domi- 
nant species, belonging to the larger genera in each 
class, which vary most. The varieties, or incipient spe- 
cies, thus produced, ultimately become converted into 
new and distinct species; and these, on the principle 
of inheritance, tend to produce other new and domi- 
nant species. Consequently the groups which are now 
large, and which generally include many dominant spe- 
cies, tend to go on increasing in size. I further at- 
tempted to show that from the varying descendants 
of each species trying to occupy as many and as differ- 
ent 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 tend- 
ency 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 progeni- 
tor 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 com- 
mon. But the three genera on the left hand have, 
on this same principle, much in common, and form a 


sub-family, distinct from that containing the next two 
genera on the right hand, which diverged from a com- 
mon 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 far- 
ther 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 subordina- 
tion 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 or- 
ganic 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 spe- 
cies, genera, and families in each class, on what is called 
the Natural System. But what is meant by this sys- 
tem? Some authors look at it merely as a scheme for 
arranging together those living objects which are most 
alike, and for separating those which are most unlike; 


or as an artificial method of enunciating, as briefly as 
possible, general propositions, — that is, by one sentence 
to give the characters common, for instance, to all mam- 
mals, 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 Katural 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 knowl- 
edge. Expressions such as that famous one by Lin- 
nasus, which we often meet with in a more or less con- 
cealed 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 classi- 
fications 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 classifica- 

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 gen- 
eral propositions and of placing together the forms most 
like each other. It might have been thought (and 
was in ancient times thought) that those parts of the 
structure which determined the habits of life, and the 
general place of each being in the economy of nature, 


would be of very high importance in classification. 
Nothing can be more false. No one regards the ex- 
ternal 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 con- 
sideration 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 indica- 
tions of its true affinities. We are least likely in the 
modifications of these organs to mistake a merely adap- 
tive 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 significa- 
tion; whereas the organs of reproduction, with their prod- 
uct 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 
classification. This depends on their constancy through- 
out many allied groups; and their constancy chiefly 
depends on any slight deviations not having been pre- 
served 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 


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 Connaraceae " differ in having one or more ovaria, 
in the existence or absence of albumen, in the imbricate 
or valvular aestivation. Any one of these characters 
singly is frequently of more than generic importance, 
though here even when all taken together they appear 
insufficient to separate Cnestis from Connarus." To 
give an example amongst insects: in one great division 
of the Hymenoptera, the antennae, as Westwood has re- 
marked, 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 varying 
importance for classification of the same important or- 
gan 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 rumi- 


nants, and certain rudimentary bones of the leg, are 
highly serviceable in exhibiting the close affinity be- 
tween ruminants and pachyderms. Kobert 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 wings of insects are folded — mere colour in cer- 
tain Algae — mere pubescence on parts of the flower in 
grasses — ^the nature of the dermal covering, as hair or 
feathers, in the Vertebrata. If the Ornithorhynchus 
had been covered with feathers instead of hair, this ex- 
ternal and trifling character would have been consid- 
ered by naturalists as an important aid in determin- 
ing the degree of afl&nity of this strange creature to 

The importance, for classification, of trifling charac- 
ters, mainly depends on their being correlated with many 
other characters of more or less importance. The value 
indeed of an aggregate of characters is very evident in 
natural history. Hence, as has often been remarked, a 
species may depart from its allies in several characters, 
both of high physiological importance, and of almost 
universal prevalence, and yet leave us in no doubt where 
it should be ranked. Hence, also, it has been found 


that a classification founded on any single character, 
however important that may be, has always failed; for 
no part of the organisation is 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 appreciation of many 
trifling points of resemblance, too slight to be defined. 
Certain plants, belonging to the Malpighiacese, bear 
perfect and degraded flowers; in the latter, as A. de 
Jussieu has remarked, " the greater number of the char- 
acters proper to the species, to the genus, to the family, 
to the class, disappear, and thus laugh at our classifi- 
cation." 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 num- 
ber, 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 ex- 
cellent botanist, Aug. St. Hilaire. If several trifling 

210 CLASSIFICATION. [Chap. Xlv. 

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 prop- 
agating 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 Miiller 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 
C3'pridina has well-developed branchise, 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 doc- 
trine has very generally been admitted as true. Never- 
theless, their importance has sometimes been exagger- 
ated, owing to the adaptive characters of larvae not 
having been excluded; in order to show this, Fritz 
Miiller arranged by the aid of such characters alone the 
great class of crustaceans, and the arrangement did not 
prove a natural one. T?nt there can be no doubt that 
embryonic, excluding larval characters, are of the high- 


est 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 de- 
fine a number of characters common to all birds; but 
with crustaceans, any such definition has hitherto been 
found impossible. There are crustaceans at the oppo- 
site ends of the series, which have hardly a character in 
common; yet the species at both ends, from being plain- 
ly 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 x\rticulata. 

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 neces- 
sity 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 strong- 
ly 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 this has 


been done, not because further research has detected 
important structural differences, at first overlooked, but 
because numerous allied species with slightly differ- 
ent grades of difference, have been subsequently dis- 

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 Natural System is 
founded on descent with modification; — that the char- 
acters which naturalists consider as showing true affin- 
ity 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 propositions, 
and the mere putting together and separating objects 
more or less alike. 

But I must explain my meaning more fully. I be- 
lieve that the arrangement of the groups within each 
class, 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 
or groups, though allied in the same degree in blood to 
their common progenitor, may differ greatly, being due 
to the different degrees of modification which they have 
undergone; and this is expressed by the forms being 
ranked under different genera, families, sections, or 
orders. The reader will best understand what is meant, 
if he will take the trouble to 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 


three of these genera (A, F, and I), a species has transmit- 
ted modified descendants to the present day, represented 
by the fifteen genera (a^* to 2^*) on the uppermost hori- 
zontal 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 de- 
scended 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^* 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 differ- 
ences between these organic beings, which are all re- 
lated 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 pres- 
ent 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 


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 dia- 
gram 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 afl&nities 
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 
modification which the different groups have under- 
gone has to be expressed by ranking them under dif- 
ferent 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 lan- 
guages 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 to 


groups; but the proper or even the only possible ar- 
rangement would still be genealogical; and this would 
be strictly natural, as it would connect together all lan- 
guages, 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 difllerence. 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 agri- 
culturist 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 


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 his 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 natu- 
ralist: scarcely a single fact can be predicated in com- 
mon of the adult males and hermaphrodites of certain 
cirripedes, and yet no one dreams of separating them. 
As soon as the three Orchidean forms, Monachanthus, 
Myanthus, and Catasetum, which had previously been 
ranked as three distinct genera, were known to be some- 
times produced on the same plant, they were imme- 
diately considered as varieties; and now I have been 
able to show that they are the male, female, and herma- 
phrodite forms of the same species. The naturaUst in- 
cludes 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 to- 
gether the individuals of the same species, though the 
males and females and larvag are sometimes extremely 
different; and as it has been used in classing varieties 


which have undergone a certain, and sometimes a con- 
siderable 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 be- 
lieve 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 com- 
munity of descent by resemblances of any kind. There- 
fore we chose those characters which are the least likely 
to have been modified, in relation to the conditions 
of life to which each species has been recently exposed. 
Rudimentary structures on this view are as good as, or 
even sometimes better than, other parts of the organisa- 
tion. We care not how trifling a character may be — ^let it 
be the mere inflection of the angle of the jaw, the man- 
ner 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 sevei-al 
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 
gpecies may depart from its allies, in several of its most 


important characteristics, and yet be safely classed with 
them. This may be safely done, and is often done, as 
long as a sufficient number of characters, let them be 
ever so unimportant, betrays the hidden bond of com- 
munity 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 exist- 
ence — 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 classificatory importance. Geographical dis- 
tribution 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 be- 
tween these two orders of mammals and fishes, are ana- 
logical. 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. Mi- 
vart, between the mouse and a small marsupial animal 


(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 Linnasus, misled by external appearances, 
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 adaptive 
characters, although of the utmost importance to the 
welfare of the being, are almost valueless to the system- 
atist. For animals, belonging to two most distinct lines 
of descent, may have become adapted to similar condi- 
tions, and thus have assumed a close external resem- 
blance; but such resemblances will not reveal — will 
rather tend to conceal their blood-relationship. 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 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 

Numerous cases could be given of striking resem- 
blances 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 
system. But this resemblance is confined to general ap- 
pearance, 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 
jaw 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 imintelligible 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 

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 with 


viscid discs, come under this same head of analogical 
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 gen- 
erally in their matured structure, can be detected. The 
end gained is the same, but the means, though appear- 
ing superj&cially 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 in- 
stance, 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 sink- 
ing the value of the groups in several classes (and all 
our experience shows that their valuation is as yet arbi- 
trary), could easily extend the parallelism over a wide 
range; and thus the septenary, quinary, quarternary and 
ternary classifications have probably arisen. 

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


tion to similar habits of life, but has been gained for 
the sake of protection. I allude to the wonderful man- 
ner in which certain butterflies imitate, as first de- 
scribed by Mr. Bates, other and quite distinct species 
Ihis 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, 
e even years, was, though always on his guard, continut 
ally deceived. When the mockers and the mocked are 
caught and compared, they are found to be very differ- 
ent m essential structure, and to belong not only to dis- 
tinct genera, but often to distinct families. Had this 
mimicry occurred in only one or two instances, it might 
have been passed over as a strange coincidence But if 
we proceed from a district where one Leptalis imitates 
an Ithomia, another mocking and mocked species be- 
longing 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 imi- 
tsLtoT living remote from the form which it imitates 
The mockers are almost invariably rare insects; the 
mocked in almost every case abound in swarms. In 
the same district in which a species of Leptalis closely 
imitates an Ithomia, there are sometimes other Lepi- 
doptera mimicking the same Ithomia: so that in the 
same place, species of three genera of butterflies and 
even a moth are found all closely resembling a butter- 


fly belonging to a fourth genus. It deserves especial 
notice that many of the mimicking forms of the Lep- 
talis, 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 wdiy, 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 counterfeit- 
ers have changed their dress and do not resemble their 
nearest allies. 

We are next led to inquire what reason can be as- 
signed for certain butterflies and moths so often assum- 
ing the dress of another and quite distinct form; why, 
to the perplexity of naturalists, has nature condescended 
to the tricks of the stage? Mr. Bates has, no doubt, 
hit on the true explanation. The mocked forms, which 
always abound in numbers, must habitually escape de- 
struction 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 distaste- 
ful 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 gen- 
erations 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 


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 butter- 
flies, varied in an extreme degree. In one district sev- 
eral 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 inhabit- 
ing the same district, this variety, from its resem- 
blance 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 resem- 
blance being generation cfter generation eliminated, 
and only the others left to propagate their kind." So 
that here we have an excellent illustration of natural 

Messrs. Wallace and Trimen have likewise described 
several equally striking cases of imitation in the Lepi- 
doptera of the Malay Archipelago and Africa, and with 
some other insects. Mr. Wallace has also detected one 
such case with birds, but we have none with the larger 
quadrupeds. The much greater frequency 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 string, and I have 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; there- 
fore, speaking metaphorically, they are reduced, 
like most weak creatures, to trickery and dissimula- 

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 difiiculty on this head, for it 
is necessary to suppose in some cases that ancient mem- 
bers belonging to several distinct groups, before they 
had diverged to their present extent, accidentally re- 
sembled a member of another and protected group in 
a sufiicient 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 spe- 
cies, 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 al- 
most 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 tbus tend to 


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 ex- 
tinct, 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 dis- 
covery of Australia has not added an insect belonging 
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 long-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 lost. 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 Omithorhynchus and 
Lepidosiren, for example, would not have been less aber- 
rant had each been represented by a dozen species, in- 
stead 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 con- 


quered by more successful competitors, with a few mem- 
bers still preserved under unusually favourable con- 

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


to the Phascolomys having become adapted to habits 
like those of a Kodent. 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 un- 
derstand the excessively complex and radiating affini- 
ties by which all the members of the same family or 
higher group are connected together. For the common 
progenitor of a whole family, now broken up by ex- 
tinction 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 con- 
sequently be related to each other by circuitous lines 
of affinity of various lengths (as may be seen in the dia- 
gram so often referred to), mounting up through many 
predecessors. As it is difficult to show the blood-re- 
lationship between the numerous kindred of any an- 
cient and noble family even by the aid of a genealogical 
tree, and almost impossible to do so without this aid, 
we can understand the extraordinary difficulty which 
naturalists have experienced in describing, without the 
aid of a diagram, the various affinities which they per- 
ceive 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 


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 with- 
in 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 natu- 
ral 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 Si- 
lurian 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 dis- 
tinguished from their more immediate parents and de- 
scendants. Yet the arrangement in the diagram would 
still hold good and would be natural; for, on the prin- 
ciple of inheritance, all the forms descended, for in- 
stance, 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, whether large or small, 


and thus give a general idea of the value of the differ- 
ences 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 mak- 
ing 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 be- 

Finally, we have seen that natural selection, which 
follows from the struggle for existence, and which al- 
most inevitably leads to extinction and divergence of 
character in the descendants from anyone 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 char- 
acters in common; we use descent in classing acknowl- 
edged varieties, however different they may be from 
their parents; and I believe that this element of de- 
scent is the hidden bond of connection 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., we can understand the rules which 
we are compelled to follow in our classification. "We 
can understand why we value certain resemblances far 
more than others; why we use rudimentary and useless 
organs, or others of trifling 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 Bee 
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 to- 
gether by the most complex and radiating lines of 
afl&nities. We shall never, probably, disentangle the 
inextricable web of the affinities between the members 
of any one class; but when we have a distinct object 
in view, and do not look to some unknown plan of crea- 
tion, we may hope to make sure but slow pro- 

Professor Hackel in his ' Generelle Morphologic ' 
and in other works, has recently brought his great 
knowledge and abilities to bear on what he calls phylo- 
geny, or the lines of descent of all organic beings. In 
drawing up the several series he trusts chiefly to em- 
bryological characters, but receives aid from homologous 
and rudimentary 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. 


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 whole 

232 MORPHOLOGY. [Chap. XIV. 

subject is included under the general term of Morphol- 
ogy. This is one of the most interesting departments 
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 dig- 
ging, the leg of the horse, the paddle of the porpoise, 
and the wing of the bat, should all be constructed on the 
same pattern, and should include 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 eat- 
ing, bandicoots, — and those of some other Australian 
marsupials, — should all be constructed on the same ex- 
traordinary 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 fur- 
nished with two claws. Notwithstanding this similar- 
ity of pattern, it is obvious that the hind feet of these 
several animals are used for as widely different pur- 
poses as it is possible to conceive. The case is ren- 
dered all the more striking by the American opossums, 
which follow nearly the same habits of life as some of 
their Australian relatives, having feet constructed 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 an- 

Chap. XIV.] MORPHOLOGY. 233 

Geoffrey St. Hilaire has strongly insisted on the high 
importance of relative position or connexion in homo- 
logous parts; they may differ to almost any extent in 
form and size, and yet remain connected together in the 
same invariahle 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 modifica- 
tions of an upper lip, mandibles, and two pairs of max- 
illae. 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 ad- 
mitted by Owen in his most interesting work on the 
' Nature of Limbs.' On the ordinary view of the in- 
dependent creation of each being, we can only say that 
so it is; — that it has pleased the Creator to construct 
all the animals and plants in each great class on a uni- 
form 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 modifications, 
— 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 nature. 

234 MORPHOLOGY. [Chap. XIV. 

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, becom- 
ing at the same time enveloped in thick membrane, so 
as to serve as a fin; or a webbed hand might 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 rela- 
tive connection 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 con- 
structed 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 di- 
versity in the structure and functions of the mouths 
of insects. Nevertheless, it is conceivable that the gen- 
eral 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 multi- 
plication 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 
subject; namely, serial homologies, or the comparison 

Chap. XIV.] MORPHOLOGY. 235 

of the different parts or organs in the same individual, 
and not of the same parts or organs in different mem- 
bers of the same class. Most physiologists believe that 
the bones of the skull are homologous — that is, cor- 
respond 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 verte- 
brate classes are plainly homologous. So it is with 
the wonderfully complex jaws and legs of crustaceans. 
It is familiar to almost every one, that in a flower the 
relative position of the sepals, petals, stamens, and pis- 
tils, as well as their intimate structure, are intelligible 
on the view that they consist of metamorphosed leaves, 
arranged in a spire. In monstrous plants, we often get 
direct evidence of the possibility of one organ being 
transformed into another; and we can actually see, dur- 
ing the early or embryonic stages of development in 
flowers, as well as in crustaceans and many other ani- 
mals, 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 vertebrae? 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 crustacean, which 
has an extremely complex mouth formed of many parts, 

236 MORPHOLOGY. [Chap. XIV. 

consequently always have fewer legs; or conversely, those 
with many legs have simpler mouths? Why should 
the sepals, petals, stamens, and pistils, in each flower, 
though fitted for such distinct purposes, be all con- 
structed on the same pattern? 

On the theory of natural selection, we can, to a cer- 
tain 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 corresponding 
organs, for such questions are almost beyond investiga- 
tion. It is, however, probable that some serial struc- 
tures 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 re- 
marked, of all low or little specialised forms; therefore 
the unknown progenitor of the Vertebrata probably pos- 
sessed many vertebrae; the unknown progenitor of the 
Articulata, many segments; and the unknown progeni- 
tor 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, not 
only in number, but in form. Consequently such parts, 
being already present in considerable numbers, and 
being highly variable, would naturally afford the ma- 
terials for adaptation to the most different purposes; 
yet they would generally retain, through the force 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 
their subsequent modification through natural selection, 

Chap. XIV.] MORPHOLOGY. 237 

woiild tend from the first to be similar; the parts be- 
ing at an early stage of growth alike, and being sub- 
jected to nearly the same conditions. Such parts, 
whether more or less modified, unless their common 
origin became wholly obscured, would be serially homo- 

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 much 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 ani- 
mals, owing to their descent from a common progenitor 
with subsequent modification, homogenous; and the re- 
semblances which cannot thus be accounted for, he pro- 
poses to call Jiomoplastic. For instance, he believes that 
the hearts of birds and mammals are as a whole homo- 
genous, — that is, have been derived from a common pro- 
genitor; but that the four cavities of the heart in the 
two classes are homoplastic, — that is, have been inde- 
pendently developed. Mr. Lankester also adduces the 
close resemblance of the parts on the right and left sides 

238 MORPHOLOGY. [Chap. XIV, 

of the body, and in the successive segments of the same 
individual animal; and here we have parts commonly 
called homologous, which bear no relation to the descent 
of distinct species from a common progenitor. Homo- 
plastic structures are the same with those which I have 
classed, though in a very imperfect manner, as analo- 
gous modifications or resemblances. 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 modifica- 
tions, having been preserved for the same general pur- 
pose or function, — of which many instances have been 

Naturalists frequently speak of the skull as formed 
of metamorphosed vertebrae; the jaws of crabs as meta- 
morphosed legs; the stamens and pistils in flowers as 
metamorposed 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 hav- 
ing 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. Yet 
so 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 
would have retained through inheritance, if they had 


really been metamorphosed from true through extremely 
simple legs, is in part explained. 

Development and Embryology. 

This is one of the most important subjects in the 
whole round of history. The metamorphoses of insects, 
with which every one is familiar, are generally effected 
abruptly by a few stages; but the transformations are 
in reality numerous and gradual, though concealed. 
A certain ephemerous insect (Chloeon) during its devel- 
opment, 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 metamor- 
phosis 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 as- 
tonishing fact that a delicate branching coralline, stud- 
ded 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 swim- 
ming animalcules, which attach themselves to rocks 
and become developed into branching corallines; and so 
on in an endless cycle. The belief in the essential iden- 
tity of the process of alternate generation and of ordi- 
nary metamorphosis has been greatly strengthened by 
Wagner's discovery of the larva or maggot of a fly, name- 
ly the Cecidomyia, producing asexually other larvae, 
and these others, which finally are developed into ma- 


ture 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 hav- 
ing 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 man- 
ner, and he believes that this occurs frequently in the 
Order. It is the pupa, and not the larva, of the Chiro- 
nomus which has this power; and Grimm further shows 
that this case, to a certain extent, " unites that of the 
Cecidom}da with the parthenogenesis of the Coccidae; " 
— the term parthenogenesis implying that the mature 
females of the Coccidas are capable of producing fertile 
eggs without the concourse of the males. Certain ani- 
mals belonging to several classes are now known to have 
the power of ordinary reproduction at an unusually 
early age; and we have only to accelerate parthenoge- 
netic production by gradual steps to an earlier and ear- 
lier age, — Chironomus showing us an almost exactly in- 
termediate stage, viz., that of the pupa — and we can 
perhaps account for the marvellous case of the Cecido- 

It has already been stated that various parts in the 
same individual which are exactly alike during an early 
embryonic period, become widely different 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 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 mammalia, so complete is 
*' the similarity in the mode of formation of the head 
" and trunk in these animals. The extremities, how- 
" ever, are still absent in these embryos. But even 
"if they had existed in the earliest stage of their de- 
" velopment 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 ani- 
mals. 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. 


are pinnate or divided like the ordinary leaves of the 

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 con- 
ditions of existence. We cannot, for instance, sup- 
pose 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 mam- 
mal 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 be- 
lieve that the similar bones in the hand of a man, wing 
of a bat, and fin of a porpoise, are related to similar con- 
ditions of life. No 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 dur- 
ing 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 


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 he 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 ohey, more or less closely, 
the law of common embryonic resemblance. Cirripedes 
afford a good instance of this; even the illustrious Cu- 
vier 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 

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 ma- 
ture 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 magniflcent 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 or- 
gans of sense, and to reach by their active powers of 


swimming, a proper place on which to become attached 
and to undergo their final metamorphosis. When this 
is completed they are fixed for life: their legs are now 
converted into prehensile organs; they again obtain a 
well-constructed mouth; but they have no antennae, and 
their two eyes are now reconverted into a minute, single, 
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 cer- 
tain 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 char- 
acter is manifested long before the parts of the embryo 
are completed." Land-shells and fresh-water crusta- 
ceans are bom having their proper forms, whilst the 
marine members of the same two great classes pass 
through considerable and often great changes during 


their development. Spiders, again, barely undergo any 
metamorphosis. The larvae 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 develop- 
ment 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 Miiller has made the remark- 
able discovery that certain shrimp-like crustaceans (al- 
lied to Penoeus) 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 mem- 
ber is as yet known to be first developed under the nau- 
plius-form, though many appear as zoeas; nevertheless 
Miiller assigns reasons for his belief, that if there had 
been no suppression of development, all these crusta- 
ceans would have appeared as nauplii. 

How, then, can we explain these several facts in 
embryology, — namely, the very general, though not uni- 
versal, difference in structure between the embryo and 
the adult; — the various parts in the same individual 
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 larvae of the most distinct spe- 
cies 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 period of 


life; on the other hand larvas, which have to provide 
for their own wants, being perfectly adapted to the sur- 
rounding conditions; — and lastly the fact of certain 
larvag 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 de- 
merits of their young animals. We 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 has 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 ob- 
tained its food 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. Cer- 


tain 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 corre- 
sponding 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 su- 
pervened at an earlier age in the child than in the 

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, ex- 
plain, as I believe, all the above specified leading facts 
in embryology. But first let us look to a few analo- 
gous cases in our domestic varieties. Some authors 
who have written on Dogs, maintain that the greyhound 
and bulldog, though so different, are really closely al- 
lied 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 meas- 
uring the old dogs and their six-days-old puppies, I 
found that the puppies had not acquired nearly their 
ixdl 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 measurements made 
of the dams and of three-days-old colts of race and 


heavy cart-horses^, I find that this is by no means the 

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 eyelid, size of feet and length 
of leg, in the wild parent-species, in pouters, fantails, 
runts, barbs, dragons, carriers, and tumblers. Now 
some of these birds, when mature, differ in so extraordi- 
nary 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 dis- 
tinguished, 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 remark- 
able 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 facts are explained by the above two principles. 
Fanciers select their dogs, horses, pigeons, &c., for breed- 
ing, 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 give value to 


his breeds, do not generally appear at a very early period 
of life, and are inherited at a corresponding 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 in- 
lerited 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 ex- 
tend 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 ma- 
ture, when it was compelled to use its full powers to 
gain its own living; and the effects thus produced will 


have been transmitted to the offspring at a correspond- 
ing 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 

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 ma- 
ture 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 contingencies; 
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 under- 
go any metamorphosis, whilst marine members of the 
same groups pass through various transformations, Fritz 
Miiller has suggested that the process of slowly modify- 
ing and adapting an animal to live on the land or in 
fresh water, instead of in the sea, would be greatly sim- 
plified 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 greatly 


changed habits of life, would commonly be found unoc- 
cupied 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 selec- 
tion; 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 larvae might be ren- 
dered 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 cor- 
responding ages, animals might come to pass through 
stages of development, perfectly distinct from the pri- 
mordial 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 


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 anten- 
nae, 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 Sitarif 
leap on the eggs and devour them. Afterwards the}! 
undergo a complete change; their eyes disappear; theil 
legs and antennae become rudimentary, and they feed on 
honey; so that they now more closely resemble the ordi- 
nary larvse 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 development 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- 
Btraca, 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 


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 sev- 
eral 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 

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; de- 
scent being the hidden bond of connexion which natural- 
ists 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 em- 
bryo is even more important for classification than that 
of the adult. In two or more groups of animals, how- 
ever 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 em- 
bryonic 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 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, community of origin is often re- 
vealed by the structure of the larva?; we have seen, for 
instance, that cirripedes, though externally so Hke 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 modi- 
fied 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 supervened at a very early 
period of growth, or by such variations having been in- 
herited 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 
which an ancient form became adapted in its larval state 
to some special line of life, and transmitted the same 
larval state to a whole group of descendants; for such 
larva3 will not resemble any still more ancient form in 
its adult state. 

Thus, as it seems to me, the leading facts in embry- 
olog)', which are second to none in importance, are ex- 
plained on the principle of variations in the many de- 


scendants from some one ancient progenitor, having ap- 
peared at a not very early period of life, and having been 
inherited at a corresponding period. Embryology rises 
greatly in interest, when ve 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 mam- 
mae; in snakes one lobe of the lungs is rudimentarv^ 
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 

Budimentary organs plainly declare their origin and 
meaning in various ways. There are beetles belonging 
to closely alHed 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 in these 
cases it is impossible to doubt, that the rudiments repre- 
eent wings. Eudimentary organs sometimes retain their 
potentiality: this occasionally occurs with the mam- 


mae of male mammals, which have been known to be- 
come well developed and to secrete milk. So again in 
the udders in the genus Bos, there are normally four 
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 some- 
times rudimentary, and sometimes well-developed in the 
individuals of the same species. In certain plants hav- 
ing 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 hy- 
brid offspring was much increased in size; and this clear- 
ly shows that the rudimentary and perfect pistils are es- 
sentially 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. G. H. 
Lewes remarks, " has gills, and passes its existence in 
" the water; but the Salamandra atra, which lives high 
" up among the mountains, brings forth its young f uU- 
" formed. This animal never lives in the water. Yet 
" if we open a gravid female, we 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 ancestral adaptations, it 
" repeats a phase in the development of its progeni- 
" tors." 

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


important purpose, and remain perfectly efficient for 
the other. Thus in plants, the office of the pistil is to 
allow the pollen-tuhes to reach the ovules within the 
ovarium. The pistil consists of a stigma supported on 
a style; but in some Compositae, the male florets, which 
of course cannot he fecundated, have a rudimentary 
pistil, for it is not crowned with a stigma; but the style 
remains well developed and is clothed in the usual man- 
ner with hairs, which serve to brush the pollen out of 
the surrounding and conjoined anthers. Again, an or- 
gan 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 for- 
merly more highly developed, ought not to be consid- 
ered as rudimentary. Thsy may be in a nascent condi- 
tion, and in progress towards further development. 
Kudimentary 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 condition 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 in- 
heritance, and relate to a former state of things. It is, 
however, often difficult to distinguish between rudimen- 
taryand nascent organs; for we can judge only byanalogy 


whether a part is capable of further development, in 
which case alone it deserves to be called nascent. Or- 
gans in this condition will always be somewhat rare; 
for beings thus provided will commonly have been sup- 
planted by their successors with the same organ in a 
more perfect state, and consequently 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 probably a reduced organ, modi- 
fied 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 Lepi- 
dosiren as the " beginnings of organs which attain full 
functional development in higher vertebrates; " but, ac- 
cording to the view lately advocated by Dr. Giinther, 
they are probably remnants, consisting of the persist- 
ent axis of a fin, with the lateral rays or branches abort- 
ed. The mammary glands of the Ornithorhynchus may 
be considered, in comparison with the udders of a cow, 
as in a nascent condition. The ovigerous frena of cer- 
tain cirripedes, which have ceased to give attachment 
to the ova and are feebly developed, are nascent bran- 

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 or- 
gans may be utterly aborted; and this implies, that in 
certain animals or planti, parts are entirely absent which 


analogy would lead us to expect to find in them, and 
which are occasionally found in monstrous individuals. 
Thus in most of the Scrophulariaceae 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 occasion- 
ally becomes perfectly developed, as may sometimes 
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 un- 
derstand 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 rela- 
tively 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 reason- 
ing 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 svmmetry," or in order 


" to complete the scheme of nature." But this is not an 
explanation, merely a re-statement of the fact. Nor 
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 the 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 rudimentary 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 embr}^onic 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 de- 
veloped 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 eases 
of rudimentary organs in our domestic productions, — as 
the stump of a tail in tailless breeds, — the vestige of an 


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 tliese casts tiirow 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 re- 
duced 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 condi- 
tions, 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 
the organ, until it was rendered harmless and rudi- 

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- 


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 dis- 
use 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 effect it in the embryo. Thus we can un- 
derstand 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 i1 
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 eon- 
sequence much reduced, how can it be still further re- 
duced in size until the merest vestige is left; and how 
can it be finally quite obliterated? It is scarcely pos- 
sible 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 
degree 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- 


dependently 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 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 re- 
duced 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 physiologi- 
cal 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 condi- 
tion, 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 SUMMARY. [Chap. XIV. 


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 con- 
stant and prevalent, whether of high or of the most tri- 
fling importance, or, as with rudimentary organs, of no 
importance, — the wide opposition in value between an- 
alogical or adaptive characters, and characters of true 
affinity; and other such rules; — all naturally follow if 
we admit the common parentage of allied forms, to- 
gether with their modification through variation and 
natural selection, with the contingencies of extinction 
and divergence of character. In considering this view 
of classification, it should be borne in mind that the ele- 
ment of descent has been universally used in ranking to- 
gether the sexes, ages, dimorphic forms, and acknowl- 
edged 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 

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 

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. 
Larvae 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 fami- 
lies, with which this world is peopled, are all descended, 

266 SUMMARY. [Chap. XIV. 

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




Recapitulation of the objections to the theory of Natural Selection 
— Recapitulation of the general and special circumstances in its 
favour — Causes of the general belief in the immutability of 
species — How far the theory of Natural Selection may be ex- 
tended — 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 diflficult 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- 
tisms of Jtructure or instinct — and, lastly, that grada- 


tions 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 


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 re- 
spect 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 varie- 
ties which have been experimented on have been pro- 
duced under domestication; and as domestication (I do 
not mean mere confinement) almost certainly tends to 
eliminate 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; 


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 conditions, 
when they are subjected under confinement to new and 
greatly changed conditions, either perish, or if they sur- 
vive, are rendered sterile, though retaining perfect 
health. This does not occur, or only in a very slight de- 
gree, 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 dis- 
tinct 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, 
are generally rendered more or less sterile, whilst two 
domesticated 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 


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 re- 
tained 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 con- 
sequently 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 


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? Why are not all organic beings blended together 
in an inextricable chaos? With respect to existing forms, 
we should remember that we have no right to expect 
(excepting in rare cases) to discover directly connecting 
links between them, but only between each and some 
extinct and supplanted form. Even on a wide area, 
which has during a long period remained continuous, 
and of which the climatic and other conditions of life 
change insensibly in proceeding from a district occupied 
by one species into another district occupied by a closely 
alliel 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 be- 
coming 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 modi- 
fications 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 sup- 
planted by the allied forms on either hand; for the lat- 
ter, 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 infini- 
tude of connecting links, between the living and extinct 
inhabitants of the world, and at each successive period 
between the extinct and still older species, why is not 


every geological formation charged with such links? 
Why does not every collection of fossil remains afford 
plain evidence of the gradation and mutation of the 
forms of life? Although geological research has uji- 
doubtedly revealed the former existence of many links, 
bringing numerous forms of life much closer together, it 
does not yield the infinitely many fine gradations be- 
tween 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 geologi- 
cal stages? Although we now know that organic be- 
ings appeared on this globe, at a period incalculably re- 
mote, 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, 
yuch strata must somewhere have been deposited at 
these ancient and utterly unknown epochs of the world's 

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 modi- 


fied species, if we were to examine the two ever so close- 
ly, 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 dis- 
covered, they wouM simply be ranked by many natu- 
ralists as so many new species, more especially if found 
in different geological sub-stages, let their differences 
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 modification, 
though long as measured by years, have probably been 
short in comparison with the periods during which 
they retain 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 forma- 
tion, 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 dura- 


tion has probably been shorter than the average dura- 
tion 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 posi- 
tions, 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 



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 ques- 
tions on which we are confessedly ignorant; nor do 
we know how ignorant we are. We do not know all 
the possible transitional gradations between the simplest 
and the most perfect organs; it cannot be pretended 
that we know all the varied means of Distribution dur- 
ing the long lapse of years, or that we know how im- 
perfect is the Geological Record. Serious as these sev- 
eral objections are, in my judgment they are by no means 
sufficient to overthrow the theory of descent with sub- 
sequent 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 


productions have been modified; but we may safely in^ 
fer 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 


acted so efficiently under domestication should not havo 
aoted under nature. In the survival of favoured indi- 
viduals 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 al- 
most 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 conditiocs, 
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 pos- 
session of the females. The most vigorous males, or 
those which have most successfully struggled with their 
conditions of life, will generally leave most progeny. 
But success will often depend on the males having spe- 

Chaf. xv.j recapitulation. 279 

cial weapons, or means of defence, or charms; and a 
flight 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 do- 
mestication. And if there has been any variability 
under nature, it would be an unaccountable fact if natu- 
ral 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 dif- 
ferences in his domestic productions; and every one ad- 
mits that species present individual differences. But, 
besides such differences, all naturalists admit that natu- 
ral varieties exist, which are considered sufficiently dis- 
tinct to be worthy of record in systematic works. No 
one has drawn any clear distinction between individual 
differences and slight varieties; or between more plainly 
marked varieties and sub-species, and species. On sepa- 
rate continents, and on different parts of the same conti- 
nent when divided by barriers of any kind, and on out- 
lying 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 


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 far- 
ther than this, seems to be in the highest degree prob- 
able. I have already recapitulated, as fairly as I could, 
the opposed difficulties and objections: now let us turn 
to the special facts and arguments in favour of the theory. 

On the view that species are only strongly marked 
and permanent varieties, and that each species first 
existed as a variety, we can see why it is that no line 
of demarcation can be drawn between species, commonly 
supposed to have been produced by special acts of crea- 
tion, and varieties which are acknowledged to have been 
produced 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 varie- 
ties; for where the manufactory of species has been ac- 
tive, we might expect, as a general rule, to find it still in 
action; and this is the case if varieties be incipient spe- 
cies. Moreover, the species of the larger genera, which 
afford the greater number of varieties or incipient spe- 
cies, retain to a certain degree the character of varieties; 
for they differ from each other by a less amount of differ- 
ence than do the species of smaller genera. The closely 


allied species also of the larger genera apparently have 
restricted 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 spe- 
cies. Hence, during a long-continued course of modi- 
fication, the slight differences characteristic of varie- 
ties 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 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 in- 
explicable 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 " Natura non facit 
saltum," 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. We 
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 unoccu- 
pied 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 


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 univer- 
sal, 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 readily seen, 
visited and fertilised by insects, and the seeds dissem- 
inated 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 per- 
fect, 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 against 


an enemy, causing the bee's own death; at drones be^ 
mg 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 the ichneumonidae feeding within the liv- 
ing 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 produc- 
tion of distinct species. In both cases physical condi- 
tions seem to have produced some direct and definite 
effect, but how much we cannot say. Thus, when varie- 
ties 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 
played an important part, so that when one part has 
been modified other parts have been necessarily modi- 
fied. "With both varieties and species, reversions to long- 
lost characters occasionally occur. How inexplicable on 


the 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 man- 
ner as the several domestic breeds of the pigeon are de- 
scended 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 char- 
acters 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 


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 diflficulty than do corporeal structures 
on the theory of the natural selection of successive, 
slight, but profitable modifications. We can thus under- 
stand 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 in- 
sects, 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 tropical and tem- 
perate 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 and kinds 


of resemblance to their parents, — in being absorbed 
into each other by successive crosses, and in other such 
points, — as do the crossed offspring of acknowledged 
varieties. 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 slow- 
ly 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 


progenitors of each group, so often occupy a position 
in some degree intermediate between existing groups. 
Recent 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 
migration from one part of the world to another, owing 
to former climatal and geographical changes and to the 
many occasional and unknown means of dispersal, then 
we can understand, on the theory of descent with 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. We see the full meaning of 
the wonderful fact, which has struck every traveller 


namely, that on the same continent, under the most 
diverse conditions, under heat and cold, on mountain 
and lowland, on deserts and marshes, most of the 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 alli- 
ance of some of the inhabitants of the sea in the north- 
ern and southern temperate latitudes, though separated 
by the whole intertropical ocean. Although two coun- 
tries 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 dis- 


taut from any continent. Sucli cases as the presence of 
peculiar species of bats on oceanic islands and the ab- 
sence of all other terrestrial mammals, are facts utterly 
inexplicable on the theory of independent acts of crea- 

The existence of closely allied or representative spe- 
cies in any two areas, implies, on the theory of descent 
with modification, that the same parent-forms formerly 
inhabited both areas; and we almost invariably find that 
wherever many closely allied species inhabit two areas, 
some identical species are still common to both. Where- 
ever many closely allied yet distinct species occur, doubt- 
ful forms and varieties belonging to the same groups 
likewise occur. It is a rule of high generality that the 
inhabitants of each area are related to the inhabitants of 
the nearest source whence immigrants might have been 
derived. We see this in 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 mu- 
tual affinities of the forms within each class are so com- 
plex and circuitous. We see why certain characters 


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 
afl&nities 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 de- 
scent 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 porpoise, and leg of the 
horse, — the same number of vertebrae forming the neck 
of the giraffe and of the elephant, — and innumerable 
other such facts, at once explain themselves on the theory 
of descent with slow and slight successive modifications. 
The similarity of pattern in the wing and 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 pis- 
tils of a flower, is likewise, to a large extent, intelligible 
on the view of the gradual modification of parts or or- 
gans, which were aboriginally alike in an early progeni- 
tor in each of these classes. On the principle of succes- 
sive 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 


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 crea- 
ture, 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 wings under the soldered wing- 
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 man- 
ner, 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 
Tindulatory theory of light has thus been arrived at; and 

294 CONCLUSION. [Chap. XV. 

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? No one now objects to following 
out the results consequent on this unknown element 
of attraction; nowithstanding that Leibnitz formerly ac- 
cused Newton of introducing " occult qualities and 
miracles into philosophy." 

I see no good reason why the views given in this vol- 
ume should shock the religious feelings of any one. It 
is satisfactory, as showing how transient such impres- 
sions are, to remember that the greatest discovery ever 
made by man, namely, the law of the attraction of grav- 
ity, was also attacked by Leibnitz, " as subversive of 
natural, and inferentially of revealed, religion." A cele- 
brated author and divine has written to me that " he has 
" gradually learnt to see that it is Just as noble a concep- 
" tion of the Deity to believe that He created a few ori- 
" ginal forms capable of self-development into other and 
" needful forms, as to believe that He required a fresh act 
" of 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 as- 
serted that organic beings in a state of nature are sub- 
ject to no variation; it cannot be proved that the 
amount of variation in the course of long ages is a lim- 
ited 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 

Chap. XV.] CONCLUSION. 295 

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

But the chief cause of our natural unwillingness to 
admit that one species has given birth to clear and dis- 
tinct 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 cliff's 
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 

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 be- 
lieve that species are mutable will do good service by 
conscientiously 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 liv- 
ing 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 re- 
tained in the foregoing paragraphs, and elsewhere, sev- 
eral 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 edi- 
tion 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 fur- 
ther investigation, but little advantage is gained by be- 
lieving that new forms are suddenly developed in an in- 
explicable 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 sub- 
ordinate to groups. Fossil remains sometimes tend 
to fill up very wide intervals between existing 

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. Through- 
out 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 only four or 

Chap. XV.] CONCLUSION. 299 

five progenitors, and plants from an equal or lesser 

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 production seems to be es- 
sentially similar. With all, as far as is at present known, 
the germinal vesicle is the same; so that all organ- 
isms 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- 
" duetive bodies of many of the lower algae 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 Ver- 
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 la- 
bours 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 sujfficiently 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 sujBBcient 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 acknowl- 
edged 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 free 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 sum- 
ming up of the labour, the experience, the reason, and 
even the blunders of numerous workmen; when we thus 
view each organic being, how far more interesting — I 

302 CONCLUSION. [Chap. XV. 

speak from experience — does the study of natural history 

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. We possess no pedigrees or armorial 
bearings; and we have to discover and trace the many 
diverging lines of descent in our natural genealogies, by 
characters of any kind which have long been inherited. 
Rudimentary organs will speak infallibly with respect 
to the nature of long-lost structures. Species and groups 
of species which are called aberrant, and which may 
fancifully be called living fossils, will aid us in form- 
ing a picture of the ancient forms of life. Embryology 
will often reveal to us the structure, in some degree ob- 
scured, of the prototypes of each great class. 

When we can feel assured that all the individuals 
of the same species, and all the closely allied species 
of most genera, have within a not very remote period 
descended from one parent, and have migrated from 
some one 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 differ- 
ences between the inhabitants of the sea on the opposite 
sides of a continent, and the nature of the various in- 
habitants 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 f ossilif erous 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 at- 
tempting to correlate as strictly contemporaneous two 
formations, which do not include many identical species, 
by the general succession of the forms of life. As spe- 
cies 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. [Chap. 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 in- 
dependently 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 utterlv 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 suc- 
cession 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 per- 

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 Reproduction; Inherit- 
ance which is almost implied by reproduction; Varia- 
bility from the indirect and direct action of the condi- 
tions of life, and from use and disuse: a Ratio of In- 
crease 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, having been originally breathed 
by the Creator into a few forms or into one; and that, 
whilst this planet has gone cycling on according to the 
fixed law of gravity, from so simple a beginning endless 
forms most beautiful and most wonderful have been, and 
are being evolved. 



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 

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 spJierical 
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. 

Alg^e. — 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. 


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 differ- 
ent from its parent, but from which the parent-form is repro- 
duced 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 be- 
tween 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 ex- 
hibits 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, Crus- 
tacea 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- 

Archetypal. — Of or belonging to the Archetype, or ideal primi- 
tive form upon which all the beings of a group seem to be 

Articulata. — A great 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 

Asymmetrical. — Having the two sides unlike. 

Atrophied. — Arrested in development at a very early stage. 

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

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


Tiindergoing 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. 

Branchls;. — Gills or organs for respiration in water. 

Beanchial. — Pertaining to gills or branchiae. 

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

Canid^.— 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 be- 
longs to the oldest, or Palaeozoic, system of formations. 

Caudal. — Of or belonging to the tail. 

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

Getacea. — 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, 

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 at- 
tached to other objects, either directly or by means of a stalk, 
and their bodies are enclosed by a calcareous shell composed of 
several pieces, two of which can open to give issue to a bunch 
of curled, jointed tentacles, which represent the limbs. 


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

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

CffiLOSPEEMOus. — A term applied to those fruits of the Umbellif- 
erae 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 homy, 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 

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

CoNFEEV^. — The filamentous weeds of fresh water. 

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

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

CoERELATioN. — The normal coincidence of one phenomenon, char- 
acter, &c., with another. 

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

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

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

CuECULio. — The old generic term for the Beetles known as Wee- 
vils, characterised 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. 


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 

Devonian System or formation. — A series of Palaeozoic rocks, in- 
cluding 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 

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

DicEcious. — Having the organs of the sexes upon distinct indi- 

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-YPings 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, giUs 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 cer- 
tain country or region, or which have lived during a given 
geological period. 

Fei,idm. — The Cat-family. 

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

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

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

F(ETAL. — Of or belonging to the foetus, or embryo in course of de- 

FoEAMiNiFEEA. — A class of auimals of very low organisation, and 
generally of small size, having a jelly-like body, from the sur- 
face 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. 

FossiLi FEEOUS. — Con taini ng fossils. 

FossoEiAL,— 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. 

Feenum (pi. Feena).— 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. 

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

Gallinaceous Bieds. — An order of Birds of which the common 
Fowl, Turkey, and Pheasant, are well-known examples. 

Gallus. — The genus of birds which includes the common FowL 

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

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

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

Glacial Peeiod. — A period of great cold and of enormous exten- 
sion of ice upon the surface of the earth. It is believed that 
glacial periods have occurred repeatedly during the geological 


history of the earth, but the term is generally applied to the 
close of the Tertiary epoch, when nearly the whole of Europe 
was subjected to an arctic climate. 

Gland. — An organ which secretes or separates some peculiar prod- 
uct from the blood or sap of animals or plants. 

Glottis. — The opening of the windpipe into the oesophagus or 

Gneiss. — A rock approaching granite in composition, but more or 
less laminated, and really produced by the alteration of a sedi- 
mentary deposit after its consolidation. 

Grallatores. — The so-called Wading-birds (Storks, Cranes, Snipes, 
&c.), which are generally furnished with long legs, bare of 
feathers above the heel, and have no membranes between the 

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 rela- 
tion 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 Cicadce, 
Frog-hoppers, and Aphides, are well-known examples. 

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


Htmenopteba. — An order of insects possessing biting jaws and 
usuiiilj" ^-^ur membranous wings in which there are a few veins. 
Bees and Wasps are familiar examples of this group. 

Hypeeteophied. — Excessively developed. 

IcHNEUMONiD^. — A family of Hymenopterous insects, the mem- 
bers of which lay their eggs in the bodies or eggs of other 

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

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

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

Infusoria. — A class of microscopic Animalcules, so called from 
their having originally been observed in infusions of vegetable 
matters. They consist of a gelatinous material enclosed in a 
delicate membrane, the whole or part of which is fumishet^ 
with short vibrating hairs (called cilia), by means of which th« 
animalcules swim through the water or convey the minute par- 
ticles of their food to the orifice of the mouth. 

XNSECTITOEOUS. — Feeding on Insects. 

Invertebeata, or Invertebeate Animals. — Those animals which 
do not possess a backbone or spinal column. 

LACXTNiE. — Spaces left among the tissues in some of the lower ani' 
mals, and serving in place of vessels for the circulation of the 
fluids of the body. 

Lamellated. — Furnished with lamellae or little plates. 

Laeva (pi. Larv^). — The first condition of an insect at its issuing 
from the egg, when it is usually in the form of a grub, cater- 
pillar, 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. Lau- 
rence, whence the name. It is in these that the earliest known 
traces of organic bodies have been found. 

Leguminos^. — 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 


Lemttrib^. — A group of four-handed animals, distinct from the 
Monkeys and approaching the Insectivorous Quadrupeds in 
some of their characters and habits. Its members have the 
nostrils curved or twisted, and a claw instead of a nail upon 
the first finger of the hind hands. 

Lepidoptera. — An order of Insects, characterised by the posses- 
sion of a spiral proboscis, and of four large more or less 
scaly wings. It includes the well-known Butterflies and 

LiTTOEAL. — Inhabiting the seashore. 

Loess.— A marly deposit of recent (Post-Tertiary) date, which 
occupies a great part of the valley of the Rhine. 

Malacostraca. — The higher division of the Crustacea, including 
the ordinary Crabs, Lobsters, Shrimps, &c., together with the 
Woodlice and Sand-hoppers. 

Mammalia. — The highest class of animals, including the ordinary 
hairy quadrupeds, the Whales, and Man, and characterised by 
the production of living young which are nourished after birth 
by milk from the teats {MamnuB, 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 vas- 
cular connection, caUed 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 (OmitJwrhynchus). 

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 Quadru- 
peds the mandible is properly the lower jaw. 

Marsupials. — An order of Mammalia in which the young are bom 
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 Masimalia). 

MAXiLLiE, in Insects. — The second or lower pair of jaws, which are 
composed of several joints and furnished with peculiar jointed 
appendages called palpi, or feelers. 


Melanism. — The opposite of albinism ; an undue development of 
colouring material in the skin and its appendages. 

Metamorphic Rocks. — Sedimentary rocks which have undergone 
alteration, generally by the action of heat, subsequently to 
their deposition and consolidation. 

MoLLUSCA — One of the great divisions of the Animal Kingdom, 
including those animals which have a soft body, usually 
furnished with a shell, and in which the nervous ganglia, or 
centres, present no definite general arrangement. They are 
generally known under the denomination of " shell-fish ; " the 
cuttle-fish, and the common snails, whelks, oysters, mussels, 
and cockles, may serve as examples of them. 

Monocotyledons, or Monocottledonous 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 beinji 
generally straight, and by the parts of the flowers being gener- 
ally in multiples of three. {Examples, Grasses, LUies, Orchids, 
Palms, &c.) 

Moraines. — The accumulations of fragments of rock brought down 
by glaciers. 

Morphology. — The law of form or structure independent of 

Mysis-staqe. — A stage in the development of certain Crustaceans 
(Prawns), in which they closely resemble the adults of a genu* 
(Mysis) belonging to a slightly lower group. 

Nascent. — Commencing development. 

Natatory. — Adapted for the purpose of swimming. 

Nauplius-form. — The earliest stage in the development of many 
Crustacea, especially belonging to the lower groups. In this 
stage the animal has a short body, with indistinct indications 
of a division into segments, and three pairs of fringed limbs. 
This form of the common fresh-water Cyclops was described 
as a distinct genus under the name of Nauplius. 

Neuration. — The arrangement of the veins or nervures in the 
wings of Insects. 

Neuters. — Imperfectly developed females of certain social insects 
(such as Ants and Bees), which perform all the labours of the 
community. Hence they are also called workers. 

Nictitating Membrane. — A semi-transparent membrane, which 


can be drawn across the eye in Birds and Reptiles, either to 
moderate the effects of a strong light or to sweep particles of 
dust, &c., from the surface of the eye. 

Ocelli. — The simple eyes or stemmata of Insects, usually situated 
on the crown of the head between the great compound eyes. 

(Esophagus. — The gullet. 

Oolitic. — A great series of secondary rocks, so called from the 
texture of some of its members, which appear to be made up 
of a mass of small egg-like calcareous bodies. 

Operculum. — A calcareous plate employed by many Mollusca to 
close the aperture of their shell. The opercular valves of Cir- 
ripedes are those which close the aperture of the shell. 

Orbit. — The bony cavity for the reception of the eye. 

Organism. — An organised being, whether plant or animal. 

Orthospermous. — A term applied to those fruits of the Umbel- 
liferae 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, 

Palaeozoic — 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^). — The 
flowers of these plants are c&WedL 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 ex- 
pense of, another organism. 

Parthenogenesis.— The production of living organisms from un- 
impregnated eggs or seeds. 


Pedunculated. — Supported upon a stem or stalk. The peduncu- 
lated oak has its acorns borne upon a footstalk. 

Peloria or Peloeism. — The appearance of regularity of structure 
in the flowers of plants which normally bear irregular flowers. 

Pelvis. — The bony arch to which the hind limbs of vertebrate 
animals are articulated. 

Petals. — The leaves of the corolla, or second circle of organs in 
a flower. They are usually of delicate texture and brightly 

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 pig- 

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- 

Plantigrades. — Quadrupeds which walk upon the whole sole of 
the foot, like the Bears. 

Plastic. — Readily capable of change. 

Pleistocene Period. — The latest portion of the Tertiary epoch. 

Plumule (in plants). — The minute bud between the seed-leaves of 
newly-germinated plants. 

Plutonic Rocks. — Rocks supposed to have been produced by igne- 
ous action in the depths of the earth. 

Pollen. — The male element in flowering plants; usually a fine 
dust produced by the anthers, which, by contact with the 
stigma effects the fecundation of the seeds. This impregnation 
is brought about by means of tubes (pollen-tubes) which issue 
from the pollen-grains adhering to the stigma, and penetrate 
through the tissues until they reach the ovary. 

PoLYANDROUS (flowers). — Flowers having many stamens. 

Polygamous Plants.— Plants in which some flowers are tinisexual 
and others hermaphrodite. The unisexual (male and female) 
flowers, may be on the same or on different plants. 

Polymorphic. — Presenting many forms. 

PoLYZOARY. — The common structure formed by the cells of the 
Polyzoa, such as the well-known Sea-mats. 


•PREHENSILE. — Capable of grasping. 

Prepotent. — Having a superiority of power. 

'Primaries. — The feathers forming the tip of the wing of a bird, 
and inserted upon that part which represents the hand of 

Processes.— Projecting portions of bones, usually for the attach- 
ment of muscles, ligaments, &c. 

Propolis. — A resinous material collected by the Hive-Bees from 
the opening buds of various trees. 

Protean. — Exceedingly variable. 

Protozoa. — The lowest great division of the Animal Kingdom. 
These animals are composed of a gelatinous material, and 
show scarcely any trace of distinct organs. The Infusoria, 
Foraminifera, and Sponges, with some other forms, belong to 
this division. 

Pupa (pi. Pup^). — The second stage in the development of an 
Insect, from which it emerges in the perfect (winged) repro- 
ductive 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 

Range. — The extent of country over which a plant or animal is 
naturally spread. Range in time expresses the distribution of 
a species or group through the fossilif erous beds of the earth's 

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- 

Rhizopods. — A class of lowly organised animals (Protozoa), having 
a gelatinous body, the surface of which can be protruded in 
the form of root-like processes or filaments, which serve for 
locomotion and the prehension of food. The most important 
order is that of the Foraminifera. 


Rodents. — The gnawing Mammalia, such as the Rats, Rabbits, 
and Squirrels. They are especially characterised by the pos- 
session of a single pair of chisel-like cutting teeth in each 
jaw, between which and the grinding teeth there is a great 


RuBUS. — The Bramble Genus. 

Rudimentary. — Very imperfectly developed. 

Ruminants. — The group of Quadrupeds which ruminate or chew 
the cud, such as oxen, sheep, and deer. They have divided 
hoofs, and are destitute of front teeth in the upper jaw. 

Saceax. — Belonging to the sacrum, or the bone composed usually 
of two or more united vertebrae 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-aE. — The horny plates with which the feet of birds are gen- 
erally more or less covered, especially in front. 

Sedimentary Formations. — Rocks deposited as sediments from 

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 enve- 
lope of an ordinary flower. They are usually green, but some- 
times brightly coloured. 

Serratures. — Teeth like those of a saw. 

Sessile. — Not supported on a stem or footstalk. 

Silurian System. — A very ancient system of fossiliferous rocks 
belonging to the earlier part of the Palaeozoic series. 

Specialisation. — The setting apart of a particular organ for the 
performance of a particular function. 

Spinal Chord. — The central portion of the nervous system in the 
Vertebrata, which descends from the brain through the arches 
of the vertebrae, and gives off nearly all the nerves to the va- 
rious organs of the body. 

Stamens. — The male organs of flowering plants, standing in a circle 
within the petals. They usually consist of a filament and an 
anther, the anther being the essential part in which the pollen, 
or fecundating dust, is formed. 

Sternum. — The breast-bone. 

Stigma. — The apical portion of the pistil in flowering plants. 


Stipules. — Small leafy organs placed at the base of the footstalks 

of the leaves in many plants. 
Style. — The middle portion of the perfect pistil, which rises like 

a column from the ovary and supports the stigma at its 

Subcutaneous. — Situated beneath the skin. 
Suctorial. — Adapted for sucking. 
Sutures (in the skull). — The lines of junction of the bones of which 

the skull is composed. 

Tarsus (pi. Tarsi). — The jointed feet of articulate animals, such 

as Insects. 
Tkleostean Fishes. — Fishes of the kind familiar to us in the 

present day, having the skeleton usually completely ossified 

and the scales homy. 
Tentacula or Tentacles. — Delicate fleshy organs of prehension 

or touch possessed by many of the lower animals. 
Tertiary. — The latest geological epoch, immediately preceding 

the establishment of the present order of things. 
Trachea. — The windpipe or passage for the admission of air to 

the lungs. 
Tridactyle. — Three-fingered, or composed of three movable parts 

attached to a common base. 
Trilobites. — A peculiar group of extinct Crustaceans, somewhat 

resembling the Woodlice in external form, and, like some of 

them, capable of rolling themselves up into a ball. Their 

remains are found only in the Palaeozoic rocks, and most 

abundantly in those of Silurian age. 
Trimorphic. — Presenting three distinct forms. 

TJmbellifer^. — An order of plants in which the flowers, which 
contain five stamens and a pistil with two styles, are supported 
upon footstalks which spring from the top of the flower stem 
and spread out like the wires of an umbrella, so as to bring all 
the flowers in the same head (umbel) nearly to the same leveL 
{Examples, Parsley and Carrot). 

Ungulata. — Hoofed quadrupeds. 

Unicellular. — Consisting of a single cell. 

Vascular. — Containing blood-vessels. 
Vermiform. — Like a worm. 


Vektebeata : 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 vertebree, 
which constitutes the centre of the skeleton and at the same 
time supports and protects the central parts of the nervous 

Whorls. — The circles or spiral lines in which the parts of plants 

are arranged upon the axis of growth. 
Workers. — See neuters. 

ZoEA-STAGE. — The earliest stage in the development of many of 
the higher Crustacea, so called from the name of Zoea applied 
to these young animals when they were supposed to constitute 
a peculiar genus. 

ZooiDS. — In many of the lower animals (such as the Corals, Medusae, 
&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 be- 
tween two sexual reproductions ; and these forms, which ar" 
apparently individual animals, have been called zooida. 



Abkrrakt groups, ii. 227. 

Abysainia, plants of, ii. 167. 

Acclimatisation, i. 173. 

Adoxa, i. 270. 

AflBinities of extinct species, ii. 106. 

of organic beings, ii. 225. 

Agassiz, on Amblyopsis, i. 173. 
, on groups of species suddenly 

appearing, li. 88. 

, on prophetic forms, ii. 107. 

, on embryological succession, 


, on the Glacial period, ii. 151. 

, on embryological characters, 

iL 210. 
, on the latest tertiary forms, ii. 

, on parallelism of embryologi- 
cal development and geological 
succession, ii. 254. 

, Alex., on pedicellariae, L 298. 

Algae of New Zealand, ii. 164. 

Alfigators, 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, 

u. 159. 

, South, no modem formations 

on west coast, ii. 61. 

Ammonites, sudden extinction of, 
ii. 99. 

Anagallis, sterility of, ii. 4. 

Analogy of variations, i. 197. 

AncyluB, 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 of, i. 175. 

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. 

Antechinu.s, ii. 219. 

Ants attending aphides, i. 323. 

, slave-making instinct, i. 336. 

, neuters, structure of, i. 359. 

Apes, not ha^•ing acquired intel- 
lectual powers, i. 282. 

Aphides, attended by ants, i. 323. 

Aphis, development of, iL 245. 

Apteryx, i. 218. 

Arab horses, L 40. 

Aralo-Caspian Sea, ii. 121. 

Archeopteryx, ii. 80. 

Archiac, M.' de, on the succession of 
species, ii. 103. 

Artichoke, Jerusalem, i. 176. 

Ascension, plants of, ii. 178. 

Asclepias, pollen of, L 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- 

AuduDon, on habits of frigate-bird, 
i. 222. 

, on variation in birds' nests, i. 


, on heron eating seeds, iL 176. 

Australia, animals o^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. 

Azores, flora of, ii. 149. 


Babington, Mr., on British plants, 

i. 58. 
Baer, Von, standard of Highness, i. 

, comparison of bee and fish, ii. 

, embryonic similarity of the 

Vertebrata, ii. 241. 
Baker, Sir S., on the giraffe. L 278. 
Balancement of growth, i. 182. 



Baleen, i. 285. 

Barberry, tlowers of, i. 121. 

Barrande, M., on Silurian colonies, 

ii. 90. 
•^— , on the succession of species, 

, on parallelism of palaeozoic 

formations, ii. 106. 
< , on affinities of ancient species, 

ii. 108. 
Barriers, importance of, ii. 130. 
Bates, Mr., on mimetic butterflies, 

ii. 222, 223, 224. 
Batrachians on islands, ii. 182. 
Bats, how structure acquired, L 


. , distribution of, ii. 184. 

Bear, catching water-insects, L 220. 
Beauty, how acquired, i. 249; ii. 

Bee, sting of, i. 255. 

, queen, killing rivals, i. 256. 

. , Australian, extermination of, 

i. 93. 
Bees fertilising flowers, i. 90. 
, hive, not sucking the red 

clover, i. 117. 

, Ligurian, i. 117. 

, hive, cell-making instinct, i. 


, variation in habits, i. 324. 

, parasitic, i. 336. 

, humble, cells of, i. 343. 

Beetles, wingless, in Madeira, i. 169. 

with deficient tarsi, i. 168. 

Bentham, Mr., on British plants, i. 


, on classification, ii. 211. 

Berkeley, Mr., on seeds in salt 

water, ii. 142. 
Bermuda, birds of, ii. 180. 
Birds acquiring fear, i. 325. 

, beautv of, i. 252. 

annually cross the Atlantic, ii. 

, colour of, on continents, i. 

, footsteps and remains of, in 

secondary rocks, iL 79. 
, fossil, in caves of Brazil, ii. 

, of Madeira. Bermuda, and 

Galapagos, ii. 179, 180. 

, song of males, i. 109. 

transporting seeds, ii. 148. 

, waders, ii. 175. 

, wingless, i. 167, 218. 

Bizeacha, ii. 133. 

, affinities of, ii. 227. 

Bladder for swimming, in fish, i. 230. 
Blindness of cave animals, i. 170. 
Blyth, Mr., on distinctness of Indian 

cattle, i. 21. 

, on striped hemionus, i. 199. 

, on crossed geese, ii. 10. 

Borrow, Mr., on the Spanish pointer, 

i. 40. 
Bory St. Vincent, on Batrachians, 

u. 182. 
Bosquet, M., on fossil Chthamalusi 

ii. 80. 
Boulders, erratic, on the Azores, ii. 

Branchiae, i. 231, 232. 

of crustaceans, i. 238. 

Braun, Prof., on the seeds of Fuma' 

riaceae, i. 271. 
Brent, Mr., on house-tumblers, L 

Britain, mammals of, ii. 185. 
Broca, Prof., on Natural Selection, 

i. 265. 
Bronn, Prof, on duration of specific 

forms, ii. 66. 

, various objections by, i. 266. 

Brown, Eobert, on classification, iL 

, Sequard, on inherited muti- 
lations, i. 168. 
Busk, Mr., on the Polyzoa, i. 301 . 
Butterflies, mimetic, ii. 222, 223, 

Buzarcingues, on sterility of varie- 
ties, ii. 38. 


Cabbage, varieties of, crossed, i, 12a 

Calceolaria, ii. 7, 8. 

Canary-birds, sterility of hybrids! 

iL 9. 
Cape de Verde islands, productions 

of, iL 189. 
, plants of, on mountains, iL 

Cape of Good Hope, plants of, i, 

158; iL 178. 
Carpenter, Dr., on foraminifera, iL 

Carthamus, i. 271. 
Catasetum, i. 243; iL 216. 
Cat8, with blue eyes, deaf, i. J3. 

, variation in habits of, i. 325. 

curling tail when going to 

spring, i. 254. 



Cattle destroying fir-trees, i. 88. 

. destroyed by flies in Paraguay, 

i. 89. 

, breeds of. locally extinct, i. 134. 

. , fertility of Indian and Euro- 
pean breeils, ii. 10. 

, Indian, i. 21 ; ii. 10. 

Cave, inhabitants of, blind, i. 170. 

Cecidomyia, ii. 239. 

Celts, proving antiquity of man, 
i. 21. 

Centres of Creation, ii. 135. 

Cephalopodae, structures of eyes, L 

, development of, ii. 244. 

Cercopithecus, tail of, i. 294. 

Ceroxylus laceratus, i. 284. 

Cervulus, ii. 9. 

Cetacea, teeth and hair, 1. 179. 

, development of the whale- 
bone, i. 285. 

Cetaceans, i. 285. 

Ceylon, plants of. ii. 164. 

Chalk formation, ii. 100. 

Characters, divergence of, i. 134. 

, sexual, variable, i. 185, 191. 

, adaptive or analogical, ii. 218. 

Charlock, i. 94. 

Checks, to increase, i. 83. 

, mutual, i. 86. 

Chelae of Crustaceans, i. 300. 

Chickens, instinctive taraeness of, 
i. 329. 

Chironomus, its asexual reproduc- 
tion, ii. 240. 

Chthamalinae, ii. 59. 

Chthamalus, cretacean species of, 
ii. 81. 

Circumstances favourable to selec- 
tion of domestic products, i. 46. 

to natural selection, i. 124. 

Cirripedes capable of crossing, i. 124. 

, carapace aborted, i. 184. 

, their ovigerous frena, i. 232. 

, fossil, ii. 80. 

, larvae of, ii. 243. 

Claparede, Prof., on the hair-clasp- 
ers of the Acaridae, i. 239. 

Clarke, Rev. W. B., on old glaciers 
in Australia, ii. 159. 

Classification, ii. 202. 

Clift, Mr., on the succession of types, 
ii. 121. 

Climate, eflfects of. in checking in- 
crease of beings, i. 84. 

— , adaptation of, to organisms, i. 

Climbing plants, i. 230. 

, development of, i. 305. 

Clover visited by bees, i. 117. 

Cobites, intestine of, i. 229. 

Cockroach, i. 93. 

Collections, palaeontological, poor, 
ii. 58. 

Colour, influenced by climate,!. 165. 

, in relation to attack by flies, 

i. 248. 

Columba livia, parent of domestic 
pigeons, i. 26. 

Colymbetes, ii. 174. 

Compensation of growth, i. 182. 

Compositae, flowers and seeds of, i. 

, outer and inner florets of, i. 


, male flowers of, ii. 257. 

Conclusion, general, ii. 293. 

Conditions, slight changes in, fa- 
vourable to fertility, ii. 27. 

Convergence of genera, i. 156. 

Coot, i. 222. 

Cope, Prof., on the acceleration or 
retardation of the period of repro- 
duction, i. 232. 

Coral-islands, seeds drifted to, ii. 

reefs, indicating movements of 

earth, ii. 145. 

Corn-crake, i. 223. 

Correlated variation in domestic 
productions, i. 13. 

Coryanthes, i. 241. 

Creation, single centres of, ii. 135. 

Crinum, iL 6. 

CroU, Mr., on subaerial denudation, 
iL 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, L 


Cruger, Dr., on Coryanthes, i. 241. 

Crustacea of New Zealand, ii. 164. 

Crustacean, blind, i. 171. 

air-breathers, i. 238. 

Crustaceans, their chelae, i. 300. 

Cryptocerus, i. 359. 

Ctenomys, blind, i. 170. 

Cuckoo, instinct of, i. 319, 330. 




Cunningham, Mr., on the flight of 
the logger-headed duck, i. 167. 

Currants, grafts of, ii. 19. 

Currents of sea, rate of, ii. 144. 

Cuvier, on conditions of existence, 
i. 320. 

Cuvier. on fossil monkeys, ii. 79. 

, Fred., on instinct, i. 320. 

Cyclostoma, resisting salt water, ii. 

Dana, Prof., on blind cave-animals, 
i. 172. 

, on relations of crustaceans of 

Japan, ii. 158. 

, on crustaceans of New Zea- 
land, ii. 164. 

Dawson, Dr., on eozoon, ii. 85. 

De Candolle, Aug. Pyr., on struggle 
for existence, i. 77. 

, on umbelliferae, i. 181. 

, on general atfinities, ii. 228. 

, Alph., on the variability of 

oaks, i. 62. 

, on low plants, widely dis- 
persed, ii. 196. 

, on widely-ranging plants be- 
ing 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. 


, 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. 

Devonian system, iL 113. 

Dianthus, ftrtUity 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, geographical, ii. 129. 

' , means of, ii. 140. 

Disuse, effect of, under nature, i. 

Divergence of character, L 134. 

Diversification of means for same 

general purpose, i. 240. 
Division, physiological, of labour, 

i. 139. 
Dog, resemblance of jaw to that of 

the Thykcinus, ii. 220. 
Dogs, hairless, 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, iL 


, of crosses, ii. 35. 

, proportions of body in differ^ 

ent breeds, when young, ii. 247. 
Domestication, variation under, i. 7. 
Double flowers, i. 358. 
Downing, Mr., on fruit-trees in 

America, i. 104 
Dragon flies, intestines of, i. 229. 
Drift-timber, ii. 145. 
Driver-ant, i. 361. 
Drones killed by other bees, i. 256. 
Duck, domestic, wings of, reduced, 

i. 12. 

, beak of. i. 285. 

, logger-headed, i. 218. 

Duckweed, ii. 173. 
Dugong, affinities of, ii. 206. 
Dung-beetles with deficient tansi, i. 

Dytiscus, ii. 174. 


Eari, Mr. W., on the Malay Archi- 

pelago, ii. 185. 
Ears, drooping, in domestic ani- 
mals, i. 13. 

, rudimentary, ii. 261. 

Earth, seeds in roots of trees, ii. 


charged with seeds, ii. 148. 

Echinodermata, their pedicel arise, 

i. 297. 
Eciton, i. 359. 

Economy of organisation, L 182. 
Edentata, teeth and hair, i. 179. 

, fossil species of, ii. 288. 

Edwards, Milne, on physiological 

division of labour, i. 139. 
, on gradations of structure, i. 

, on embryological characters, 

ii. 210. 




Eggs, young birds escaping from, i. 

Egypt, productions of, not modified, 

1. 263. 
Electric organs, i. 234. 
Elephant, rate of increase, i. 80. 

, of Glacial period, i. 176. 

Embryology, ii. 239. 
Eozoon Canadense, ii. 84. 
Epilepsy inherited, i. 167. 
Existence, struggle for, i. 75. 

, condition of, i. 261. 

Extinction, as bearing on natural 

selection, i. 150. 

of domestic varieties, i. 145. 

, ii. 94. 

Eye, structure of, i. 225. 

, correction for aberration, i. 255. 

Eyes, reduction in moles, i. 170. 

Fabre, M., on hymenoptera fight- 
ing, i. 108. 

, on parasitic sphex, i. 336. 

, on Sitaris, ii. 252. 

Falconer, Dr., on naturalisation of 
plants in India, i. 80. 

, on elephants and maatodons, 

u. 113. 

and CauHey, on mammals of 

sub-Himalayan beds, ii. 122. 

Falkland Islands, wolf of, ii 183. 

Faults, ii. 54. 

Faunas, marine, ii. 131. 

Fear, instinctive, in birds, i. 329. 

Feet of birds, young molluscs ad- 
hering to, ii. 174. 

Fertilisation variously effected, i. 
241, 252. 

Fertility of hybrids, ii. 6. 

, from slight changes in condi- 
tions, ii. 28. 

of crossed varieties, ii. 34. 

S'ir-trees destroyed by cattle, i. 88. 

, pollen of, 1. 257. 

Fish, flying, i. 218. 

, teleostean, sudden appearance 

of, ii. 81. 

, eating seeds, ii. 146, 175. 

, fresh-water, distribution of, 

ii. 172. 

Fishes, ganoid, now confined to 
fresh water, i. 130. 

> , electric organs of, i. 234. 

• , ganoid, living in fresh water, 

Fishes, of southern hemisphere, ii. 

Flat-fish, their structure, i. 290. 
Flight, powers of, how acquired, i. 

Flint-tools, proving antiquity of 

man, i. 21. 
Flower, Prof., on the Larynx, i. 297. 

, on Halitherium, ii. 108. 

, on the resemblance between 

the jaws of the dog and Thyla- 

cinus, ii. 220. 
, on the homology of the feet of 

certain marsupials, ii. 232. 
Flowers, structure of, in relation to 

crossing, i. 114. 
, of compositse and umbelli- 

ferae, i. 179, 270. 

, beauty of, i. 252. 

, double, i. 358. 

Flysch formation, destitute of or- 
ganic remains, ii. 59. 
Forbes, Mr. D., on glacial action in 

the Andes, ii. 160. 

, E., on colours of shells, i. 165. 

, on abrupt range of shells in 

depth, i. 210. 
, on poorness of palseontological 

collections, ii. 58. 
, on continuous succession of 

genera, ii. 93. 
, on continental extensions, ii. 

140, 141. 
, on distribution during Glacial 

period, ii. 152. 
, on parallelism in time and 

space, ii. 200. 
Forests, changes in, in America, i. 

Formation, Devonian, ii. 113. 

, Cambrian, ii. 84. 

Formations, thickness of, in Britain, 

u. 55. 

, intermittent, ii. 69. 

Formica, rufescens, i. 336. 

, sanguinea, i. 338. 

, flava, neuter of, i. 360. 

Forms, lowly organised, long en- 
during, i. 154. 
Frena, ovigerous, of cirripedes, i. 

Fresh-water productions, dispersal 

of, ii. 171. 
Fries, on species in large genera 

being closely allied to other spe- 
cies, 1. 71. 
Frigate-bird, i. 222. 


Frogs on islands, ii. 1 82. 
Fruit-trees, gradual improvement 

of, i. 42. 

in United States, i. 104 

, varieties of, acclimatised in 

United States, i. 176. 
Fuci, crossed, ii. 15, 23. 
Fur, thicker in cold climates, i. 166. 
Furze, iL 241. 


Galapagos Archipelago, birds of, 
ii. 179. 

, productions of, ii. 188, 190. 

Galaxias, its wide range, iL 172. 

Galeopithecus, i. 217. 

Game, increase of, checked by ver- 
min, i. 86. 

Gartner, on sterility of hybrids, ii. 
3, 4, 11. 

, on reciprocal crosses, iL 15. 

, on crossed maize and verbas- 

cum, ii. 37. 

, on comparison of hybrids and 

mongrels, li. 40, 41, 42. 

Gaudry, Prof., on intermediate ge- 
nera of fossil mammals in Attica, 
ii. 107. 

Geese, fertility when crossed, ii. 9, 

, upland, i. 222. 

Geikie, Mr., on subaerial denuda- 
tion, ii. 53. 

Genealogy, important in classifica- 
tion, ii. 212. 

Generations, alternate, iL 239. 

GeoflFroy St. Hilaire, on balance- 
ment, L 182. 

, on homologous organs, ii. 233. 

, Isidore, on variability of re- 
peated parts, i. 184. 

, or correlation, in monstrosi- 
ties, i. 13. 

, on correlation, i. 179. 

, on variable parts being often 

monstrous, L 190. 

Geographical distribution, ii. 129. 

Geography, ancient, ii. 303. 

Geology, future progress of, ii. 302. 

, imperfection of the record, ii. 


Gervais, Prof., on Typotherium, ii. 

Giraffe, tail of, L 245. 

, structure of, i. 276. 

Glacial period, ii. 161. 

Glacial period, affecting the ?^orth 

and South, ii. 158. 
Glands, mammary, L 295. 
Gmelin, on distribution, ii. 151. 
Godwin- Austen, Mr., on the Malay 

Archipelago, ii. 74. 
Goethe, on compensation of growth, 

L 182. 
Gomphia, i. 272. 
Gooseberry, grafts of, ii. 19. 
Gould, Dr. Aug. A., on land-shells, 

iL 186. 

, Mr., on colours of birds, L 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. &4. 
Grasses, varieties of, i. 137. 
Gray, Dr. Asa, on the variability of 

oaks, i. 62. 
, on man not causing variabil- 
ity, i. 98. 

, on sexes of the holly, L 116. 

, on trees of the United Stat««, 

L 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, L 

Grebe, i. 221. 
Grimm, on asexual reproduction, ii 

Groups, aberrant, iL 227. 
Grouse, colours of, i. 104. 

, red, a doubtful species, L 59. 

Growth, compensation of, i. 182. 
Glinther, Dr., on flat-fish, i. 292. 

, on prehensile tails, i. 295. 

, on the fishes of Panama, ii. 

, on the range of fresh-water 

fishes, ii. 172. 
, OQ the limbs of LepidoBiren, 

ii. 258. 


Haast, Dr., on glsciers of New Zea- 
land, ii. 159. 

Habit, effect of, under domestic*, 
tion, i. 12. 



Habit, effect of, under nature, i. 168. 

, diversified, of same species, 

i. 219. 

Hackel, Prof., on classification and 
the lines of descent, ii. 231. 

Hair and teeth, correlated, i. 179. 

Halitherium, ii. 108. 

Harcourt, Mr. E. V., on the birds of 
Madeira, ii. 180. 

Hartung, M., on boulders in the 
Azores, ii. 149. 

Hazel-nuts, ii. 143. 

Hearne, on habits of bears, i. 220. 

Heath, changes in vegetation, i. 

Hector, Dr., on glaciers of New Zea- 
land, ii. 159. 

Heer, Oswald, 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. 

Hemsen, 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 the self-ste- 
rility of Corydalis, ii. 7. 

Hilgendorf, 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, L 115. 

Hooker, Dr., on trees of New Zea- 
land, i. 123. 

- — , on acclimatisation of Hima- 
layan trees, i. 174. 

,on flowers of umbelliferse, i. 


■ , on the position of ovules, L 268. 

Hooker, Dr., on glaciers of Himala- 
ya, ii. 159. 

, on algae of New Zealand, ii. 


, on vegetation at the base of 

the Himalaya, ii. 164. 

, on plants of Tierra del Fuego, 

ii. 161. 

, on Australian plants, iL 163, 


, on relations of flora of Amer- 
ica, iL 167. 

, on flora of the Antarctic lands, 

iL 169, 189. 

, on the plants of the Gala- 
pagos, iL 181, 188. 

, on glaciers of the Lebanon, 

ii. 159. 

, on man not causing variabil- 
ity, L 97. 

, on plants of mountains of 

Fernando Po, ii. 162. 

Hooks on palms, i. 247. 

on seeds, on islands, ii. 181. 

Hopkins, Mr., on denudation, iL 63. 

Horn bill, remarkable instinct of, L 

Horns, rudimentary, ii. 261. 

Horse, fossil, in La Plata, ii. 96. 

, proportions of, when young, 

iL 247. 

Horses destroyed by flies in Para- 
guay, L 89. 

, striped, L 199. 

Horticulturists, selection applied by, 

Huber, on cells of bees, i. 349. 

, P., on reason blended with 

instinct, i. 320. 

, on habitual nature of instincts, 

L 320. 

, on slave-making ants, i. 336. 

, on Melipona domestica, i. 343. 

Hudson, Mr., on the Ground- Wood- 
pecker of La Plata, i. 221. 

, on the Molothrus, L 334. 

Humble-bees, cells of, i. 343. 

Himter, J., on secondary sexual 
characters, i. 185. 

Hutton, Captain, on crossed geese, 
iL 10. 

Huxley, Prof, on structure of her- 
maphrodites, i. 124. 

, on the atfinities of the Sirenia, 

ii. 108. 

, on forms connecting birda and 

reptiles, iL 108. 



Huxley, Prof., on homologous or- 
gans, ii. 238. 

, on the development of aphis, 

ii. 245. 

Hybrids and mongrels compared, ii. 

Hybridism, ii. 1. 

Hydra, structure of, i. 229. 

Hymenoptera, fighting, i. 108. 

Hymenopterous insect, diving, i. 222. 

Hyoseris, i. 271. 

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, 

Insects, colour of, fitted for their 

stations, i. 103. 

, sea-side, colours of, i. 165. 

, blind, in caves, i. 171. 

, luminous, i. 236. 

, their resemblance to certain 

objects, i. 283. 

, neuter, i. 359. 

Instinct, i. 319. 

, not varying simultaneously 

with structure, i. 357. 
Instincts, domestic, i. 325. 
Intercrossing, advantages of, i. 149, 

ii. 27. 
Islands, oceanic, ii. 177. 
Isolation favourable to selection, i. 



Japan, productions of, ii. 158. 

Java, plants of. ii. 162. 

Jones, Mr. J. M., on the birds of 
Bermuda, ii. 180. 

Jourdain, M., on the eye-spots of 
star-fishes, i. 225. 

Jukes, Prof., on subaerial denuda- 
tion, ii. 53. 

Juseieu, on classification, ii. 209. 


Kentucky, caves of, i. 172. 
Kerguelen-land, flora of, ii. 169, 189. 

Kidney-bean, acclimatisation of, L 


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 barberrv, i. 121. 

, on sterility of hybridSj 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, iL 

Lancelot, 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. 


, resisting salt water, ii. 187. 

Languages, classification of, ii. 214. 
Lankester, Mr. E. Ray, on Longe- 
vity, i. 263. 

, on homologies, ii. 237. 

Lapse, great, of time, ii. 51. 

Larvae, u. 241, 242, 243. 

Laurel, nectar secreted by the leaves, 

i. 114. 
Laurentian formation, ii. 84. 
Laws of variation, i. 164. 
Leech, varieties of, i. 93. 
Leguminosae, nectar secreted by 

glands, i. 114. 
Leibnitz' attack on Newton, ii. 294. 
Lepidosiren, i. 130 ; ii. 109. 
, limbs in a nascent conditio^ 

ii. 258. 
Lewes, Mr. G. H., on species not 

having changed in Egypt, i. 263. 

, on the Salamandra atra, ii. 256. 

, on many forms of life having 

been at first evolved, ii. 300. 
Life, struggle for, i. 77. 
Lingula, Silurian, ii. 83. 
Linnaeus, aphorism of, ii. 205. 
Lion, mane of, i. 109. 

, young of, striped, ii. 241. 

Lobelia fulgens, i. 90, 121. 
, sterility of crosses, ii. 7. 



Lockwood, Mr., on the ova of the 

Hippocampus, i. 295. 

Locusts transporting seeds, ii. 147. 

Logan, Sir W., on Laurentian for- 
mation, ii. 84. 

Lowe, Kev. R. T., on locusts visiting 
Madeira, ii. 147. 

Lowness of structure connected with 
variability, i. 184. 

, related to wide distribution, ii. 


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, ofl 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 hav- 
ing been developed on islands, i. 

, on a carboniferous land-shell. 

ii. 59. 

, on strata beneath Silurian sys- 
tem, ii. 84. 

, on the imperfection of the geo- 
logical 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. 

, on parallelism of tertiary for- 
mations, ii. 106. 

, on transport of seeds by ice- 
bergs, ii. 148. 

, on great alterations of climate, 

ii. 170. 

, on the distribution of fresh- 
water shells, ii. 174. 

, on land-shells of Madeira, ii. 


Lyell and Dawson, on fossilized trees 
in Nova Scotia, ii. 70. 

Lythrum salicaria, trimorphic, ii. 

Macleay, on analogical characters, 

ii. 218. 
Macrauchenia, ii. 107. 
M'Donnell, Dr., on electric organs, 

i. 234, 
Madeira, plants of. i. 130. 

, beetles of, wingless, i. 169. 

, fossil land-shells of, ii. 121, 

, birds of, ii. 180. 

Magpie tame in Norway, i. 325, 

Males iightinff, i. 108, 

Maize, crossed, ii. 37. 

Malay Archipelago compared with 

Europe, ii. 74, 

, mammals of, ii. 185, 

Malm, on flat-fish, i. 291. 
Malpighiacese, small imperfect flow- 
ers of, i. 269, 

, ii. 209. 

Mammae, their development, i. 295. 

, rudimentaryj ii. 255. 

Mammals, fossil, m secondary for- 
mation, ii. 79. 

, insular, ii. 183. 

Man, origin of, ii. 304. 

Manatee, rudimentary nails of, ii. 

Marsupials of Australia, i. 140, 

, structure of their feet, ii, 232. 

, fossil species of, ii. 121. 

Martens, M., experiment on seeds, 

ii. 144. 
Martin, Mr. W. 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. 

Maurandia, i. 307. 
Means 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, L 

344, 350. 
Mirabilis, crosses of, ii. 15. 
Missel-thrush, i. 93. 
Mistletoe, complex relations of, i. 3, 
Mivart, Mr., on the relation of hail 

and teeth, i, 179, 



Mivart, Mr., on the eyes of cephalo- 
poda, L 237. 

, various objections to Natural 

Selection, i. 275. 

, on abrupt modifications, i. 313. 

, on the resemblance of the 

mouse and antechinus, iL 218. 

Mocking-thrush of the Galapagos, 
iL 193. 

Modification of species not abrupt, 
iL 298. 

Moles, blind, L 170. 

Molothrus, habits of, i. 334. 

Mongrels, fertility and sterility of, 

and hybrids compared, ii. 39. 

Monkeys, tossil, iL 79. 

Monachanthus, li. 216. 

Mons, Van, on the origin of fruit- 
trees, L 33. 

Monstrosities, i. 51. 

Moquin-Tandon, on sea-side plants, 

Morphology, ii. 231. 

Morren, on the leaves of Oxalis, i. 

Moths, hybrid, ii. 9. 

Mozart, musical powers of, L 321. 

Mud, seeds in, iL 175. 

Mules, striped, i. 201. 

Muller, Adolf, on the instincts of the 
cuckoo, i. 331. 

Muller, Dr. Ferdinand, on Alpine 
Australian plants, ii. 163. 

Muller, Fritz, on dimorphic crusta- 
ceans, i. 55, 362. 

, on the lancelet, i. 154. 

, on air-breathing crustaceans, 

L 238. 

, on climbing plants, L 307. 

, on the self-sterility of orchids, 


, on embryology in relation to 

classification, ii. 210. 

, on the metamorphoses of crus- 

Uceans, ii. 245, 253. 

, on terrestrial and fresh-water 

organisms not undergoing any 
metamorphosis, ii. 250. 

Multiplication of species not indefi- 
nite, i. 157. 

Murchison, Sir R., on the forma- 
tions of Russia, iL 60. 

, on azoic formations, ii. 84 

^j on extinction, ii. 94. 

Marie. Dr., on the modification of 
the skull in ok age, L 233. 

Murray, Mr. A., on cave-insects, 

L 173. 
Mustela vision, L 216. 
Myanthus, iL 216. 
Myrmecocystus, L 359. 
Myrmica, eyes of, i. 361. 


Nageli, on morphological charaotera, 

i. 266. 
Nails, rudimentary, ii. 260. 
Nathusius, Von, on pigs, i. 249. 
Natural historv, future progress of, 
iL 301. 

selection, L 97. 

system, iL 204. 

Naturalisation of forms distinct 

from the indigenous species, L 138. 

Naturalisation in New Zealand, i. 

Naudin, on analogous variations in 

gourds, L 195. 

, on hybrid gourds, ii. 38. 

, on reversion, ii. 4i. 

NautUus, Silurian. iL 83. 
Nectar of plants, L 114. 
Nectaries, now formed, L 114. 
Nelumbium luteum, iL 176. 
Nests, variations in, L 324, 355, 364. 
Neuter insects, i. 359, 360. 
Newman, Col., on humble-bees, L 90. 
New Zealand, productions of, not 

perfect, i. 255. 

, naturalised products of, iL 119. 

, fossU birds of, iL 121. 

, glaciers of, ii. 159. 

, crustaceans of, iL 164. 

, algae of, ii. 164. 

, number of plants of, ii. 178. 

, flora of, iL 189. 

Newton, Sir I., attacked for irre- 

ligion, iL 294. 
, Prof., on earth attached to a 

partridge's foot, ii. 148. 
Nicotiana, crossed varieties of, ii. 39. 
, certain species very sterile, iL 

Nitsche, Dr., on the Polyzoa, i. 301. 
Noble, Mr., on fertility of Rhodo- 
dendron, ii. 8. 
Nodules, phosphatic, in azoic rocka, 

ii. 84. 

Oaks, variability of, L 62. 
Unites, 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. 

Ornithorhynchus, i. 130 ; ii. 208. 

, mammae of, i. 296. 

Ostrich not capable of flight, i. 281. 
, habit of laying eggs together, 

i. 335. 
, American, two species of, ii. 

Otter, habits of, how acquired, i. 

Ouzel, water, i. 222. 
Owen, Prof., on birds not flying, i. 

, on vegetative repetition, L 

, on variability of unusually 

developed parts, i. 185. 

, on the eyes of fishes, i. 227. 

, on the swim-bladder of fishes, 

i. 231. 
, on fossil horse of La Plata, ii. 


, on generalized form, ii. 107. 

— — , on relation of ruminants and 

pachyderms, ii. 107. 
, on fossil birds of New Zea- 
land, ii. 121. 

, on succession of types, ii. 121. 

, on affinities of the dugong, ii. 


, on homologous organs, ii. 233. 

, on the metamorphosis of ce- 

phalopods, ii. 244. 

Pacific Ocean, faunas of, ii. 131. 
Pacini, on electric organs, i 235. 
Paley, on no organ formed to give 

Sain, i. 254. 
las, on the fertility of the domes- 
ticated descendants of wild stocks, 
ii. 10. 

Palm with hooks, i. 247. 
Papaper bracteatum, i. 272. 
Paraguay, cattle destroyed by flies, 
i. 89. ^ .^ — I 

Parasites, i. 334. 

Partridge, with ball of earth at- 
tached to foot, ii. 148. 
Parts greatly developed, variable, 

i. 185. 
Parus major, L 220. 
Passiflora, ii. 7. 

Peaches in United States, i. 104. 
Pear, grafts of, ii. 18. 
Pedicellariae, i. 298. 
Pelagornium, flowers of, i. 180. 

, sterility of, ii. 7. 

Pelvis of women, i. 178. 
Peloria, i. 180. 
Period, glacial, ii. 151. 
Petrels, habits of, i. 221. 
Phasianus, fertility of hybrids, ii. 9. 
Pheasant, young, wild, i. 329. 
Pictet, Prof, on groups of species 

suddenly appearing, ii. 77. 

, on rate of organic change, ii. 90. 

, on continuous succession of 

genera, ii. 93. 
, on change in latest tertiary 

forms, ii. 71. 
, on close alliance of fossils in 

consecutive formations, ii. 114. 
, on early transitional links, ii. 

Pierce, Mr., on varieties of wolves, 

i. 111. 
Pigeons with feathered feet and skin 

between toes, i. 14. 
, breeds described, and origin 

of, i. 23. 
, breeds of, how produced, i. 44, 

, tumbler, not being able to get 

out of egg, i. 106. 

, reverting to blue colour, i. 197. 

, instinct of tumbling, i. 327. 

, young of, ii. 248. 

Pigs, black, not aff'ected by the 

paint-root, i. 13. 
, modified by want of exercise, 

i. 249. 
Pistil, rudimentary, ii. 256. 
Plants, poisonous, not aflfecting cer- 
tain coloured animals, i. 13. 

, selection, applied to, i. 41. 

, gradual improvement of, i. 42. 

. not improved in barbaroui 

coimtries, i. 43. 




Plants, dimorphic, i. 55 ; ii. 29. 

, destroyed by insects, i. 83. 

, in midst of range, have to 

struggle with other plants, L 95. 

, nectar of, i. 114. 

, tleshy, on sea-shores, i. 166. 

, climbing, i. 230, 305. 

, fresh-water, distribution of, ii. 

, low in scale, widely distri- 
buted, ii. 196. 
Fleuronectidae, their structure, i. 

Plumage, laws of change in sexes 

of birds, i. 109. 
Plums in the United States, i. 104. 
Pointer dog, origin of, L 40. 

, habits of, i. 327. 

Poison not affecting certain coloured 

animals, i. 13. 
, similar effect of, on animals 

and plants, iL 299. 
Pollen of fir-trees, i. 257. 
transported by various means, 

i. 241, 252. 
PoUinia, their development, i. 304. 
Polyzoa, their avicularia, i. 301. 
Poole, Col., on striped hemionus, i. 

Potemogeton, ii. 175. 
Pouchet, on the colours of flat-fish, 

i. 293. 
Prestwich, Mr., on English and 

French eocene formations, iL 105. 
Proctotrupes, i. 222. 
Proteolepas, i. 183. 
Proteus, i. 173. 
Psychology, future progress of, ii. 

Pyrgoma, found in the chalk, iL 81. 


Quagga, striped, i. 201. 
Quatrefages, M., on hybrid moths, 

ii. 9. 
Quercus, variabilitjy of, L 62. 
Quince, grafts of, ii. 18. 

Babbits, disposition of young, i. 328- 
Baces, domestic, characters of, L 18. 
Bace-horses, Arab. i. 40. 

, English, ii. 140. 

Eadcliffe, Dr., the electrical organs 
of the torpedo, L 234. 

Eamond, on plants of Pyrenees, iL 

Kamsay, Prof., on subaerial denu- 
dation, ii. 53. 

, on thickness of the British 

formations, ii. 55, 56. 

, on faults, iL 55. 

Kamsay, Mr., on instincts of cuckoo, 
i. 333. 

Katio of increase, L 79. 

Rats supplanting each other, i. 93. 

, acclimatisation of, i. 175. 

, blind, in cave, i. 171. 

Battle-snake, i. 254. 

Reason and instinct, L 319. 

Eecapitulation, general, iL 267. 

Reciprocity of crosses, ii. 14. 

Record, geological, imperfect, iL 48. 

Rengger, on flies destroying cattle, 

Reproduction, rate of, L 79. 

Resemblance, protective, of 

to parents in mongrels and 

hybrids, ii. 41. 

Reversion, law of inheritance, L 

, in pigeons, to blue colour, 1. 


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, Dlind, 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, L 21 ; 
ii. 10. 


Salamandra atra, ii. 256. 
Saliva used in nests, i. 855. 
Salvin, Mr., on the beaks of ducks, 

L 287. 
Sageret, on grafts, ii. 18. 
Salmons, males fighting, and hooked 

jaws of, i. 108. 
Salt water, how far injurious to 

seeds, ii. 142. 
not destructive to land-shellB, 

IL 187. 




Salter, Mr., on early death of hybrid 
embryos, ii. 23. 

Saurophagus sulphuratus, i. 220. 

Schacht, Prof., on Phyllotaxy, i. 

Schiodte, on blind insects, i. 172. 

, on flat-fish, i. 290. 

Schlegel, on snakes, i. 178. 

Schobl, Dr., on the ears of mice, i. 

Scott, J., Mr., on the self-sterility of 
orchids, ii. 7. 

, on the crossing of varieties of 

verbascum, ii. 38. 

Sea-water, how far injurious to 
seeds, ii. 142. 

not destructive to land-shells, 

ii. 187. 

Sebright, Sir J., on crossed animals, 
i. 23. 

Sedgwick, Prof., on groups of spe- 
cies suddenly appearing, ii. 77. 

Seedlings destroyed by insects, i. 

Seeds, nutriment in, i. 94. 

, winged, i. 181. 

, means of dissemination, i. 

240, 252 ; ii. 146. 

• , power of resisting salt water, 

ii. 143. 

, in crops and intestines ot 

birds, ii. 146. 

, eaten by lish, ii. 146, 176. 

, in mud, ii. 175. 

, hooked, on islands, ii. 181. 

Selection of domestic products, i. 34. 

, principle not of recent origin, 

i. 39. 

, unconscious, i. 39. 

, natural, i. 97. 

, sexual, i. 107. 

, objections to term, i. 99. 

natural, has not induced steri- 
lity, ii. 20. 

Sexes, relations of, i. 108. 

Sexual characters variable, i. 191. 

selection, i. 107. 

Sheep, Merino, their selection, i. 36. 

, two sub-breeds, unintention- 
ally produced, i. 41. 

, mountain varieties of, i. 93. 

Shells, colours of, i. 165. 

, hinges of, i. 240. 

, littoral, seldom embedded, ii. 


, fresh -water, long retain the 

same forms, IL 117. 

Shells, fresh-water, dispersal of, ii. 


, of Madeira, ii. 180. 

, land, distribution of, ii. 180. 

, land, resisting salt water, ii. 

Shrew-mouse, ii. 218. 
Silene, infertility of crosses, ii. 14. 
SiUiman, Prof, on blind rat, i. 171. 
Sirenia, th «ir athnities, ii. 108. 
Sitaris, metamorphosis of, ii. 252. 
Skulls of young mammals, i. 248; 

ii. 235. 
Slave-making instinct, i. 336. 
Smith, Col. Hamilton, on striped 

horses, i. 200. 
, Mr. Fred., on slave-making 

ants, i. 337. 

, on neuter ants, i. 360. 

Smitt, Dr., on the Polyzoa, i. 301. 
Snake with tooth for cutting through 

egg-shell, i. 334. 
Soraerville, Lord, on selection of 

sheep, i. 35. 
Sorb us, grafts of, ii. 19. 
Sorex, ii. 218. 

Spaniel, King Charles's breed, i. 40. 
Specialisation of organs, i. 152. 
Species, polymorphic, i. 54. 

, dominant, i. 67. 

, common, variable, i. 66. 

in large genera variable, i. 69. 

, groups of, suddenly appear- 
ing, ii. 77, 82. 

beneath Silurian formations, 

ii. 84. 

successively appearing, ii. 89. 

changing simultaneously 

throughout the world, ii. 100. 

Spencer, Lord, on increase in size of 
cattle, i. 40. 

, Herbert, Mr., on the first steps 

in ditferentiation, i. 155. 

, on the tendency to an equili- 
brium in all forces, ii. 29. 

Sphex, parasitic, i. 336. 

Spiders, development of, ii. 245. 

Sports in plants, i. 11. 

Sprengel, C. C, on crossing, i. 119. 

, on ray-fiorets, i. 180. 

Squalodon, ii. 108. 

Squirrels, gradations in structure, L 

Staffordshire, heath, changes in, L 

Stag-beetles, fighting, i. 108. 

Star-fishes, eyes of, i. 225. 




Star-Fishes, their pedicellariae, i. 299. 
Sterility from changed conditions 

of life, L 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 classilication, ii. 209. 

St John, Mr., on habits of cats, L 

Sting of bee, i. 256. 
Stocks, aboriginal, of domestic ani- 
mals, L 22. 
Strata, thickness of, in Britain, ii. 55. 
Stripes on horses, i. 199. 
Structure, degrees of utility of, L 249. 
Struggle for existence, i. 75. 
Succession, geological, ii. 89. 

of types in same areas, ii 121. 

Swallow, one species supplanting 

another, i. 93. 
Swaysland, Mr., on earth adhering 

to the feet of migratory birds, ii. 

Swifts, nests of, i. 355. 
Swim-bladder, i. 230. 
Switzerland, lake habitations of, L 

System, natural, ii. 204 


Tail of giraffe, i. 245. 

• of aquatic animals, i. 246. 

, prehensile, i. 294. 

, rudimentary, il. 260. 

Tanais, dimorphic, i. 55. 

Tarsi, deficient, i. 168. 

Tausch, Dr., on umbelliferae, i. 271. 

Teeth and hair correlated, i. 179. 

, rudimentary, in embryonic, 

calf, ii. 255, 292. 
Tegetmeier, Mr., on cells of bees, i. 

346, 352. 
Temminck, on distribution aiding 

classitication, ii. 211. 
Tendrils, their development, i. 305. 
Thompson, Sir W., on the age of 

the nabitable world, ii. 83. 
• , on the consolidation of the 

cnist of the earth, ii. 275. 

Thouin, on grafts, ii. 19. 

Thrush, aquatic species of, i. 222. 

, mocking, of the Galapagos, iL 


, young of, spotted, iL 241. 

, nest of, i. 364. 

Thuret, M., on crossed fuci, ii. 15. 

Thwaites, Mr. on acclimatisation, 
i. 174. 

Thylacinus, iL 220. 

Tierra del Fuego, dogs of, L 328. 

, plants of, iL 169. 

Timber-drift, li. 145. 

Time, lapse of, ii. 51. 

by itself not causing modifica- 
tion, L 126. 

Titmouse, i. 220. 

Toads on islands, ii. 182. 

Tobacco, crossed varieties of, ii. 38- 

Tomes, Mr., on the distribution ot 
bats, iL 184. 

Transitions in varieties rare, L 208. 

Traquair, Dr., on flat-fish, i. 293. 

Trautschold, on intermediate varie- 
ties, ii. 66. 

Trees on islands belong to peculiar 
orders, ii. 182. 

with separated sexes, i. 123. 

Trifolium pratenae, i. 90, 117. 

incamatum, L 117. 

Trigonia, iL 99. 

Trilobites, ii. 83. 

, sudden extinction of, iL 99. 

Trimen, Mr., on imitating-insects, 
ii. 224. 

Trimorphism in plants, i. 55 ; ii. 29. 

Troglodytes, L 364. 

Tuco-tuco, blind, L 170. 

Tumbler pigeons, habits of, heredi- 
tary, L 327. 

, yoimg of, ii. 248. 

Turkey-cock, tuft of hair on breast, 
L 110. 

, naked skin on head, L 248. 

. young of, instinctively wild, 

i. 329. 

Turnip and cabbage, analogous 
variations of, L 195. 

Type, unity of, L 260, 261. 

Types, succession of, in same areaa 
il. 121. 

Typotherium, iL 108. 

Udders enlarged by use, L 18. 
, rudimentary, ii. 256. 



Dlex, young leaves of, ii. 241. 
Umbelliferae, flowers and seeds of, 

i. 180. 

. , outer and inner florets of, i. 270. 

Unity of type, i. 260, 261. 

Uria lacrymans, i. 113. 

Use, efl'ects of, under domestication, 

i. 12. 
, effects of, in a state of nature, 

Utility, how far important in the 

construction of each part, i. 249. 

Valenciennes, on fresh-water fish, 

ii. 173. 
Variability of mongrels and hy- 
brids, ii. 39. 
Variation under domestication, i. 8. 
caused by reproductive system 

being affected by conditions of 

Ufe, 1. 10. 

under nature, i. 51. 

, laws of, i. 164. 

, correlated, i. 13, 177, 248. 

Variations appear at corresponding 

ages, i. 16, 105. 
analogous in distinct species, 

i. 193. 
Varieties, natural, i. 50. 

, struggle between, i. 93. 

, domestic, extinction of, i. 134. 

, transitional, rarity of, i. 208. 

, 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. 
Verneuil, M. de, on the succession 

of species, ii. 103. 
Vibracula of the Polyzoa, i. 301. 
Viola, small imperfect flowers of, 

1. 269. 

, tricolor, i. 90. 

Virchow, on the structure of the 

crystalline lens, i. 227. 

Virginia, pigs of, i. 104. 
Volcanic islands, 

denudation of, ii. 


Vulture, naked skin on head, i. 247. 


"Wading-birds, ii. 175. 

Wagner, Dr., on Cecidomyia, ii. 239. 

Wagner, Moritz, on the importance 

of isolation, L 127. 
Wallace, Mr., on origin of species, 

, on the limit of variation under 

domestication, i. 48. 

, on dimorphic lepidoptera, i. 

55, 362. 

, on races in the Malay Archi- 
pelago, i. 58. 

, on the improvement of the 

eye, i. 227. 

. on the walking-stick insect, i. 


, on laws of geographical dis- 
tribution, ii. 139. 

, on the Malay Archipelago, ii. 


, on mimetic animals, ii. 224. 

Walsh, Mr. B. D., on phytophagic 
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. 

, on greatly developed parts 

being variable, i. 185. 

, on the cells of bees, i. 343. 

, on general affinities, ii. 227. 

Water-ouzel, i. 222. 

Watson, Mr. H. C, on range of 
varieties of British plants, L 57, 

, 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 the 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 the causes of 
variability, i. 8. 

, on rudimentary organs, ii. 260. 

West Indian Islands, mammals of, 
ii. 185. 

Westwood, on species in large gen- 
era being closely allied to others, 
i. 71. 

, on the tarsi of Engidae, i. 192. 

, on the antennae 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. 

Wichura, Max, on hybrids, ii. 24, 
27, 41. 

Wings, reduction of size, i. 169. 

of insects homologous with 

branchiae, i. 231. 

, rudimentary, in insects, ii. 


"vVolf crossed with dog, i. 327. 

of Falkland Isles, ii. 183. 

Wollaston, Mr., on varieties of in- 
sects, i. 59. 

, on fossil varieties of shells in 

Madeira, i. 65. 

, on colours of insects on sea- 
shore, 1. 165. 

, on wingless beetles, i. 169. 

, on rarity of intermediate va- 
rieties, i. 212. 

, on insular insects, ii. 178. 

, on land-shells of Madeira nat- 
uralised, 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, iL 


World, species changing simultane- 
ously throughout, iL 100. 

Wrens, nest of, i. 364. 

Wright, Mr. Chauncey, on the gi- 
raffe, i. 278. 

, on abrupt modifications, L 


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 ill 

young cattle, iL 261. 


Zanthoxylon, i. 272. 
Zebra, stripes on, L 1 
Zeuglodon, ii. 108.