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THE ORIGIN OF SPECIES
Volume II
1^
V
ART-TYPE EDITION
THE ORIGIN
OF SPECIES
VOLUME II
By
CHARLES DARWIN
THE WORLD'S
POPULAR CLASSICS
BOOKS, INC.
PUBLISHERS
NEW YORK BOSTON
2)G5
02.
PRINTED IN THE UNITED STATES OF AMERICA
CONTENTS
Volume II
PAGX
CHAPTER IX
HYBRIDISM
Distinction between the sterility of first crosses and of hybrids —
Sterility various in degree, not universal, affected by close inter-
breeding, removed by domestication — Laws governing the sterility
of hybrids — Sterility not a special endowment, but incidental on
other differences, not accumulated by natural selection — Causes of
the steriUty of first crosses and of hybrids — Parallelism between
the effects of changed conditions of life and of crossing — Dimor-
phism and Trimorphism — Fertility of varieties when crossed and
of their mongrel offspring not universal — Hybrids and mongrels
compared independently of their fertility — Summary .... 239
CHAPTER X
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD
On the absence of intermediate varieties at the present day— On the
nature of extinct intermediate varieties; on their number — On the
lapse of time, as inferred from the rate of denudation and of
deposition — On the lapse of time as estimated by years — On the
poorness of our 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 fossiUferous strata — ^Antiquity of
the habitable earth 269
CHAPTER XI
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
On the slow and successive appearance of new species — On their dif-
ferent rates of change — Species once lost do not reappear — Groups
of species follow the same general rules in their appearance and
disappearance as do single species — On extinction— On simulta-
neous changes in the forms of life throughout the world — On the
X CONTENTS
PAGE
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 296
CHAPTER XII
GEOGRAPHICAL DISTRIBUTION
Present distribution cannot be accounted for by differences in physical
conditions — Importance of barriers — Affinity of the productions
of the same continent — Centres of creation — Means of dispersal
by changes of climate and of the level of the land, and by occa-
sional means — Dispersal during the Glacial period — Alternate
Glacial periods in the north and south 323
CHAPTER XIII
GEOGRAPHICAL DISTRIBUTION — Continued
Distribution of fresh-water productions — On the inhabitants of oceanic
islands — Absence of Batrachians and of terrestrial Mammals — On
the relation of the inhabitants of islands to those of the nearest
mainland — On colonization from the nearest source with subse-
quent modification — Summary of the last and present chapter . 3S1
CHAPTER XIV
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY
EMBRYOLOGY RUDIMENTARY ORGANS
Classification, groups subordinate to groups — ^Natural system — Rules
and difiiculties in classification, explained on the theory of descent
with modification — Classification of varieties — Descent always used
in classification — Analogical or adaptive characters — Affinities, gen-
eral, complex and radiating — Extinction separates and defines
groups — Morphology, between members of the same class, between
parts of the same individual — Embryology, laws of, explained by
variations not supervening at an early age, and being inherited
at a corresponding age — Rudimentary organs, their origin ex-
plained— Summary 371
CHAPTER XV
RECAPITULATION AND CONCLUSION
Recapitulation of the objections to the theory of Natural Selection —
Recapitulation of the general and special circumstances in its favor
— Causes of the general belief in the immutability of species —
How far the theory of Natural Selection may be extended —
Effects of its adoption on the study of Natural History — Con-
cluding remarks 413
Glossary of Scientific Terms 441
Index 459
THE ORIGIN OF SPECIES
Volume II
CHAPTER IX
Hybridism
Distinction between the Sterility of First Crosses and of Hybrids — SteriOlity
Various in Degree, not Universal, affected by Close Interbreeding, re-
moved by Domestication — Laws governing the Sterility of Hybrids —
Sterility not a Special Endowment, but Incidental on Other Differences,
not accumulated by Natural Selection — Causes of the Sterility of First
Crosses and of Hybrids — ^Parallelism between the Effects of Changed
Conditions of Life and of Crossing — Dimorphism and Trimorphism —
Fertility of Varieties when crossed and of their Mongrel Offspring not
Universal — Hybrids and Mongrels compared independently of their
Fertihty — Summary.
The view commonly entertained by naturalists is that species,
when intercrossed, have been specially endowed with sterility, in
order to prevent their confusion. This view certainly seems at first
highly probable, for species living together could hardly have been
kept distinct had they been capable of freely crossing. The sub-
ject is in many ways important for us, more especially as the ster-
ility of species when first crossed, and that of their hybrid off-
spring, cannot have been acquired, as I shall show, by the preser-
vation of successive profitable degrees of sterility. It is an inci-
dental result of differences in the reproductive systems of the
parent-species.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely,
the sterility of species when first crossed, and the sterility of the
hybrids produced from them.
Pure species have of course their organs of reproduction in a
perfect condition, yet when intercrossed they produce either few
or no offspring. Hybrids, on the other hand, have their reproduc-
tive organs functionally impotent, as may be clearly seen in the
state of the male element in both plants and animals; though the
formative organs themselves are perfect in structure, as far as the
microscope reveals. In the first case the two sexual elements which
go to form the embryo are perfect; in the second case they are
either not at all developed, or are imperfectly developed. This dis-
239
240 THE ORIGIN OF SPECIES
tinction is important, when the cause of the sterility, which is
common to the two cases, has to be considered. The distinction
probably has been slurred over, owing to the sterility in both cases
being looked on as a special endowment, beyond the province of
our reasoning powers.
The fertility of varieties, that is of the forms known or believed
to be descended from common parents, when crossed, and likewise
the fertility of their mongrel offspring, is, with reference to my
theory, of equal importance with the sterility of species; for it
seems to make a broad and clear distinction between varieties and
species.
DEGREES OF STERILITY
First, for the sterility of species when crossed and of their hy-
brid offspring. It is impossible to study the several memoirs and
works of those two conscientious and admirable observers, Kolreu-
ter 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. Gartner, also,
makes the rule equally universal; and he disputes the entire fer-
tility of Kolreuter's ten cases. But in these and in many other
cases, Gartner is obliged carefully to count the seeds, in order to
show that there is any degree of sterility. He always compares the
maximum number of seeds produced by two species when first
crossed, and the maximum produced by their hybrid offspring,
with the average number produced by both pure parent-species in
a state of nature. But causes of serious error here intervene; a
plant, to be hybridized, must be castrated, and, what is often more
important, must be secluded in order to prevent pollen being
brought to it by insects from other plants. Nearly all the plants
experimented on by Gartner were potted, and were kept in a
chamber in his house. That these processes are often injurious to
the fertility of a plant, cannot be doubted; for Gartner gives in
his table about a score of cases of plants which he castrated, and
artificially fertilized with their own pollen, and (excluding all
cases such as the Leguminosse, in which there is an acknowledged
difficulty in the manipulation) half of these twenty plants had
their fertility in some degree impaired. Moreover, as Gartner re-
peatedly crossed some forms, such as the common red and blue
pimpernels (Anagallis arvensis and coerulea), which the best
HYBRIDISM 241
botanists rank as varieties, and found them absolutely sterile, we
may doubt whether many species are really so sterile, when inter-
crossed, as he believed.
It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all prac-
tical purposes it is most difficult to say where perfect fertility ends
and sterility begins. I think no better evidence of this can be re-
quired than that the two most experienced observers who have
ever lived, namely, Kolreuter and Gartner, arrived at diametrically
opposite conclusions in regard to some of the very same forms. It
is also most instructive to compare — but I have not space here
to enter on details — the evidence advanced by our best botanists
on the question whether certain doubtful forms should be ranked
as species or varieties, with the evidence from fertility adduced
by different hybridizers, or by the same observer from experiments
made during different years. It can thus be shown that neither
sterility nor fertility affords any certain distinction between species
and varieties. The evidence from this source graduates away, and
is doubtful in the same degree as is the evidence derived from
other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations;
though Gartner was enabled to rear some hybrids, carefully guard-
ing them from a cross with either pure parent, for six or seven,
and in one case for ten generations, yet he asserts positively that
their fertility never increases, but generally decreases greatly and
suddenly. With respect to this decrease, it may first be noticed
that when any deviation in structure or constitution is common
to both parents, this is often transmitted in an augmented degree
to the offspring; and both sexual elements in hybrid plants are
already affected in some degree. But I believe that their fertility
has been diminished in nearly all these cases by an independent
cause, namely, by too close interbreeding. I have made so many
experiments and collected so many facts, showing on the one hand
that an occasional cross with a distinct individual or variety in-
creases the vigor and fertility of the offspring, and on the other
hand that very close interbreeding lessens their vigor 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
242 THE ORIGIN OF SPECIES
generally be fertilized during each generation by pollen from the
same flower; and this would probably be injurious to their fertihty,
already lessened by their hybrid origin. I am strengthened in
this conviction by a remarkable statement repeatedly made by
Gartner, namely, that if even the less fertile hybrids be artificially
fertilized with hybrid pollen of the same kind, their fertility, not-
withstanding the frequent ill effects from manipulation, some-
times decidedly increases, and goes on increasing. Now, in the
process of artificial fertilization, pollen is as often taken by chance
(as I know from my own experience) from the anthers of another
flower, as from the anthers of the flower itself which is to be
fertilized; so that a cross between two flowers, though probably
often on the same plant, would be thus effected. Moreover, when-
ever complicated experiments are in progress, so careful an ob-
server as Gartner would have castrated his hybrids, and this
would have insured 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 in-
crease of fertility in the successive generations of artificially fer-
tilized hybrids, in contrast with those spontaneously self-fertilized,
may, as I believe, be accounted for by too close interbreeding
having been avoided.
Now let us turn to the results arrived at by a third most ex-
perienced hybridizer, namely, the Hon. and Rev. W. Herbert. He
is as emphatic in his conclusion that some hybrids are perfectly
fertile — as fertile as the pure parent-species — as are Kolreuter
and Gartner that some degree of sterility between distinct species
is a universal law of nature. He experimented on some of the very
same species as did Gartner. The difference in their results may, I
think, be in part accounted for by Herbert's great horticultural
skill, and by his having hot-houses at his command. Of his many
important statements I will here give only a single one as an
example, namely, that "every ovule in a pod of Crinum capense
fertilized by C. revolutum produced a plant, which I never saw
to occur in a case of its natural fecundation." So that here we
have perfect, or even more than commonly perfect, fertility, in
a first cross between two distinct species.
This case of the Crinum leads me to refer to a singular fact,
namely, that individual plants of certain species of Lobelia, Ver-
bascum, and Passiflora, can easily be fertilized by the pollen from
a distinct species, but not by pollen from the same plant, though
this pollen can be proved to be perfectly sound by fertilizing other
plants or species. In the genus Hippeastrum, in Corydalis, as
HYBRIDISM 243
shown by Professor Hildebrand, in various orchids as shown by
Mr. Scott and Fritz Miiller, all the individuals are in this peculiar
condition. So that with some species certain abnormal individuals,
and in other species all the individuals, can actually be hybridized
much more readily than they can be fertilized by pollen from the
same individual plant! To give oi^e instance, a bulb of Hippeas-
trum aulicum produced four flowers; three were fertilized by
Herbert with their own pollen, and the fourth was subsequently
fertilized by the pollen of a compound hybrid descended from
three distinct species; the result was that "tie ovaries of the three
first flowers soon ceased to grow, and after a few days perished
entirely, whereas the pod impregnated by the pollen of the hybrid
made vigorous growth and rapid progress to maturity, and bore
good seed, which vegetated freely." Mr. Herbert tried similar
experiments during many years, and always with the same result.
These cases serve to show on what slight and mysterious causes
the lesser or greater fertility of a species sometimes depends.
The practical experiments of horticulturists, though not made
with scientific precision, deserve some notice. It is notorious in
how complicated a manner the species of Pelargonium, Fuchsia,
Calceolaria, Petunia, Rhododendron, etc., have been crossed, yet
many of these hybrids seed freely. For instance, Herbert asserts
that a hybrid from Calceolaria integrifolia and plantaginea,
species most widely dissimilar in general habit, "reproduces itself
as perfectly as if it had been a natural species from the mountains
of Chili." I have taken some pains to ascertain the degree of
fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them are perfectly fertile. Mr. C. Noble,
for instance, informs me that he raises stocks for grafting from a
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 nurserymen. Horticulturists raise
large beds of the same hybrid, and such alone are fairly treated,
for by insect agency the several individuals are allowed to cross
freely with each other, and the injurious influence of close inter-
breeding is thus prevented. Any one may readily convince himself
of the efficiency of insect agency by examining the flowers of the
most sterile kinds of hybrid Rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen brought
from other flowers.
In regard to animals, much fewer experiments have been care-
244 THE ORIGIN OF SPECIES
fully 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 distinct species of finches, but,
as not one of these breeds freely in confinement, we have no right
to expect that the first crosses between them and the canary, or
that their hybrids, should be perfectly fertile. Again, with respect
to the fertility in successive generations of the more fertile hybrid
animals, I hardly know of an instance in which two families of the
same hybrid have been raised at the same time from different
parents, so as to avoid the ill effects of close interbreeding. On
the contrary, brothers and sisters have usually been crossed in
each successive generation, in opposition to the constantly re-
peated admonition of every breeder. And in this case, it is not
at all surprising that the inherent sterility in the hybrids should
have gone on increasing.
Although I know of hardly any thoroughly well-authenticated
cases of perfectly fertile hybrid animals, I have reason to believe
that the hybrids from Cervulus vaginalis and 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 to-
gether, produce offspring which are highly fertile when crossed
with one of the parent-species. The hybrids from the common and
Chinese geese (A. cygnoides), species, which are so different that
they are generally ranked in distinct genera, have often bred in
this country with either pure parent, and in one single instance
they have bred inter se. This was effected by Mr. Eyton, who raised
two hybrids from the same parents, but from different hatches ; and
from these two birds he raised no less than eight hybrids (grand-
children of the pure geese) from one nest. In India, however,
these cross-bred geese must be far more fertile ; for I am assured
by two eminently capable judges, namely, Mr. Blyth and Captain
Hutton, that whole flocks of these crossed geese are kept in
various parts of the country; and as they are kept for profit,
HYBRIDISM 245
where neither pure parent-species exists, they must certainly be
highly or perfectly fertile.
With our domesticated animals, the various races when crossed
together are quite fertile; yet in many cases they are descended
from two or more wild species. From this fact we must conclude
either that the aboriginal patent-species at first produced per-
fectly fertile hybrids, or that the hybrids subsequently reared
under domestication became quite fertile. This latter alternative,
which was first propounded by Pallas, seems by far the most
probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild
stock; yet, with perhaps the exception of certain indigenous
domestic dogs of South America, all are quite fertile together;
but analogy makes me greatly doubt, whether the several aborig-
inal species would at first have freely bred together and have
produced quite fertile hybrids. So again I have lately acquired
decisive evidence that the crossed offspring from the Indian
humped and common cattle are inter se perfectly fertile ; and from
the observations by Riitimeyer on their important osteological
differences, as well as from those by Mr. Blyth on their differences
in habits, voice, constitution, etc., these two forms must be re-
garded 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 steriHty 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 intercross-
ing of plants and animals, it may be concluded that some degree
of sterility, both in first crosses and in hybrids, is an extremely
general result; but that it cannot, under our present state of
knowledge, be considered as absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND
OF HYBRIDS
We will now consider a little more in detail the laws governing
the sterility of first crosses and of hybrids. Our chief object will
be to see whether or not these laws indicate that species have been
specially endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following
conclusions are drawn up chiefly from Gartner's admirable work
on the hybridization of plants. I have taken much pains to as-
246 THE ORIGIN OF SPECIES
certain how far they apply to animals, and, considering how
scanty our knowledge is in regard to hybrid animals, I have been
surprised to find how generally the same rules apply to both
kingdoms.
It has been already remarked, that the degree of fertility, both
of first crosses and of hybrids, graduates from zero to perfect
fertility. It is surprising in how many curious ways this gradation
can be shown; but only the barest outline of the facts can here
be given. When pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no more influence
than so much inorganic dust. From this absolute zero of fertility,
the pollen of different species applied to the stigma of some one
species of the same genus, yields a perfect gradation in the num-
ber of seeds produced, up to nearly complete or even quite com-
plete fertility; and, as we have seen, in certain abnormal cases,
even to an excess of fertility, beyond that which the plant's own
pollen produces. So in hybrids themselves, there are some which
never have produced, and probably never would produce, even
with the pollen of the pure parents, a single fertile seed: but in
some of these cases a first trace of fertility may be detected, by
the pollen of one of the pure parent-species causing the flower of
the hybrid to wither earlier than it otherwise would have done;
and the early withering of the flower is well known to be a sign
of incipient fertilization. From this extreme degree of sterility
we have self-fertilized hybrids producing a greater and greater
number of seeds up to perfect fertility.
The hybrids raised from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very
sterile; but the parallelism between the difficulty of making a
first cross, and the sterility of the hybrids thus produced — two
classes of facts which are generally confounded together — is by
no means strict. There are many cases, in which two pure species,
as in the genus Verbascum, can be united with unusual facility,
and produce numerous hybrid offspring, yet these hybrids are
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 fertihty, both of first crosses and of hybrids, is more easily
affected by unfavorable conditions, than is that of pure species.
But the fertility of first crosses is likewise innately variable; for
it is not always the same in degree when the same two species are
HYBRIDISM 247
crossed under the same circumstances; it depends in part upon
the constitution of the individuals which happen to have been
chosen for the experiment. So it is with hybrids, for their degree
of fertility is often found to differ greatly in the several individuals
raised from seed out of the same capsule and exposed to the same
conditions.
By the term systematic affinity is meant, the general resem-
blance between species in structure and constitution. Now the
fertility of first crosses, and of the hybrids produced from them,
is largely governed by their systematic affinity. This is clearly
shown by hybrids never having been raised between species
ranked by systematists in distinct families; and on the other hand,
by very closely allied species generally uniting with facility. But
the correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be
given of very closely allied species which will not unite, or only
with extreme difficulty; and on the other hand of very distinct
species which unite with the utmost facility. In the same family
there may be a genus, as Dianthus, in which very many species
can most readily be crossed; and another genus, as Silene, in
which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits
of the same genus, we meet with this same difference; for in-
stance, the many species of Nicotiana have been more largely
crossed than the species of almost any other genus; but Gartner
found that N. acuminata, which is not a particularly distinct
species, obstinately failed to fertilize, or to be fertilized by, no
less than eight other species of Nicotiana. Many analogous facts
could be given.
No one has been able to point out what kind or what amount
of difference, in any recognizable character, is sufficient to prevent
two species crossing. It can be shown that plants most widely
different in habit and general appearance, and having strongly
marked differences in every part of the flower, even in the pollen,
in the fruit, and in the cotyledons, can be crossed. Annual and
perennial plants, deciduous and evergreen trees, plants inhabiting
different stations, and fitted for extremely different climates, can
often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for
instance, of a female ass being first crossed by a stallion, and then
a mare by a male ass; these two species may then be said to have
been reciprocally crossed. There is often the widest possible differ-
ence in the facility of making reciprocal crosses. Such cases are
248 THE ORIGIN OF SPECIES
highly important, for they prove that the capacity in any two
species to cross is often completely independent of their system-
atic affinity, that is, of any difference in their structure or consti-
tution, 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 fertilized by the pollen of M. longiflora, and
the hybrids thus produced are sufficiently fertile; but Kolreuter
tried more than two hundred times, during eight following years,
to fertilize reciprocally M. longiflora with the pollen of M. jalapa,
and utterly failed. Several other equally striking cases could be
given. Thuret has observed the same fact with certain sea-weeds
or Fuci. Gartner, moreover, found that this difference of facility
in making reciprocal crosses is extremely common in a lesser
degree. He has observed it even between closely related forms (as
Matthiola annua and glabra) which many botanists rank only as
varieties. It is also a remarkable fact, that hybrids raised from
reciprocal crosses, though of course compounded of the very same
two species, the one species having first been used as the father
and then as the mother, though they rarely differ in external
characters, yet generally differ in fertility in a small, and 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 inter-
mediate character between their two parents, always closely re-
semble one of them; and such hybrids, though externally so like
one of their pure parent-species, are with rare exceptions ex-
tremely sterile. So again among hybrids which are usually inter-
mediate in structure between their parents, exceptional and ab-
normal 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 fertihty. These
facts show how completely the fertility of a hybrid may be in-
dependent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the
fertility of first crosses and of hybrids, we see that when forms,
which must be considered as good and distinct species, are united,
their fertility graduates from zero to perfect fertility, or even to
HYBRIDISM 249
fertility under certain conditions in excess; that their fertility,
besides being eminently susceptible to favorable and unfavorable
conditions, is innately variable; that it is by no means always the
same in degree in the first cross and in the hybrids produced from
this cross; that the fertility of hybrids is not related to the degree
in which they resemble in external appearance either parent; and
lastly, that the facility of making a first cross between any two
species is not always governed by their systematic affinity or
degree of resemblance to each other. This latter statement is
clearly proved by the difference in the result of reciprocal crosses
between the same two species, for, according as the one species
or the other is used as the father or the mother, there is generally
some difference, and occasionally the widest possible difference,
in the facility of effecting an union. The hybrids, moreover, pro-
duced from reciprocal crosses often differ in fertility.
Now, do these complex and singular rules indicate that species
have been endowed with sterility simply to prevent their becom-
ing 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 individuals of the same
species? Why should some species cross with facility, and yet
produce very sterile hybrids; and other species cross with ex-
treme 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 limited a nature, that, in reciprocal crosses between the same
two species, the male sexual element of the one will often freely
act on the female sexual element of the other, but not in a re-
versed direction. It will be advisable to explain a little more fully,
by an example, what I mean by sterility being incidental on other
differences, and not a specially endowed quality. As the capacity
of one plant to be grafted or budded on another is unimportant
250 THE ORIGIN OF SPECIES
for their welfare in a state of nature, I presume that no one will
suppose that this capacity is a specially endowed quality, but will
admit that it is incidental on differences in the laws of growth of
the two plants. We can sometimes see the reason why one tree will
not take on another, from differences in their rate of growth, in
the hardness of their wood, in the period of the flow or nature
of their sap, etc.; but in a multitude of cases we can assign no
reason whatever. Great diversity in the size of two plants, one
being woody and the other herbaceous, one being evergreen and
the other deciduous, and adaptation to widely different climates,
do not always prevent the two grafting together. As in hybridiza-
tion, so with grafting, the capacity is limited by systematic affinity,
for no one has been able to graft together trees belonging to quite
distinct families; and, on the other hand, closely allied species and
varieties of the same species can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridization, is by no
means absolutely governed by systematic affinity. Although many
distinct genera within the same family have been grafted together,
in other cases species of the same genus will not take on each
other. The pear can be grafted far more readily on the quince,
which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear
take with different degrees of facility on the quince; so do differ-
ent varieties of the apricot and peach on certain varieties of the
plum.
As Gartner found that there was sometimes an innate difference
in different individuals of the same two species in crossing; so
Sageret believes this to be the case with different individuals of
the same two species in being grafted together. As in reciprocal
crosses, the facility of effecting an union is often very far from
equal, so it sometimes is in grafting. The common gooseberry, for
instance, cannot be grafted on the current, whereas the current
will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids which have their re-
productive organs in an imperfect condition, 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 Robinia, which
seeded freely on their own roots, and which could be grafted with
no great difficulty on a fourth species, when thus grafted were
rendered barren. On the other hand, certain species of Sorbus,
when grafted on other species, yielded twice as much fruit as
HYBRIDISM 251
when on their own roots. We are reminded by this latter fact of
the extraordinary cases of hippeastrum, passiflora, etc., which
seed much more freely when fertilized with the pollen of a distinct
species than when fertilized with pollen from the same plant.
We thus see, that, although there is a clear and great difference
between the mere adhesion of grafted stocks and the union of the
male and female elements in the act of reproduction, yet that
there is a rude degree of parallelism in the results of grafting
and of crossing distinct species. And as we must look at the
curious and complex laws governing the facility with which trees
can be grafted on each other as incidental on unknown differences
in their vegetative systems, so I believe that the still more com-
plex laws governing the facility of first crosses are incidental on
unknown differences in their reproductive systems. These differ-
ences in both cases follow, to a certain extent, as might have been
expected, systematic affinity, by which term every kind of re-
semblance and dissimilarity between organic beings is attempted
to be expressed. The facts by no means seem to indicate that the
greater or lesser difficulty of either grafting or crossing various
species has been a special endowment; although in the case of
crossing, the difficulty is as important for the endurance and
stabihty of specific forms as in the case of grafting it is unim-
portant for their welfare.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES
AND OF HYBRIDS
At one time it appeared to me probable, as it has to others,
that the sterility of first crosses and of hybrids might have been
slowly acquired through the natural selection of slightly lessened
degrees of fertility, which, like any other variation, spontaneously
appeared in certain individuals of one variety when crossed with
those of another variety. For it would clearly be advantageous to
two varieties or incipient species if they could be kept from blend-
ing, on the same principle that, when man is selecting at the same
time two varieties, it is necessary that he should keep them sepa-
rate. In the first place, it may be remarked that species inhabiting
distinct regions are often sterile when crossed ; now it could clearly
have been of no advantage to such separated species to have been
rendered mutually sterile, and consequently this could not have
been effected through natural selection; but it may perhaps be
argued, that, if a species was rendered sterile with some one com-
patriot, sterility with other species would follow as a necessary
contingency. In the second place, it is almost as much opposed
252 THE ORIGIN OF SPECIES
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, while at
the same time the male element of this second form is enabled
freely to fertilize the first form; for this peculiar state of the re-
productive system could hardly have been advantageous to either
species.
In considering the probability of natural selection having come
into action, in rendering species mutually sterile, the greatest
difficulty will be found to lie in the existence of many graduated
steps, from slightly lessened fertility to absolute sterility. It may
be admitted that it would profit an incipient species, if it were
rendered in some slight degree sterile when crossed with its parent
form or with some other variety; for thus fewer bastardized and
deteriorated offspring would be produced to commingle their
blood with the new species in process of formation. But he who
will take the trouble to reflect on the steps by which this first
degree of sterility could be increased through natural 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 com-
plex. 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 favor the survival of
those individuals which happened to be endowed in a slightly
higher degree with mutual infertility, and which thus approached
by one small step toward absolute sterihty? 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 modifications in their structure and fertility
have been slowly accumulated by natural selection, from an ad-
vantage having been thus indirectly given to the community to
which they belonged over other communities of the same species;
but an individual animal not belonging to a social community, if
rendered slightly sterile when crossed with some other variety,
would not thus itself gain any advantage or indirectly give any
advantage to the other individuals of the same variety, thus lead-
ing to their preservation.
But it Vv'ould 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
HYBRIDISM . 253
of natural selection. Both Gartner and Kolreuter have proved
that in genera including numerous species, a series can be formed
from species which when crossed yield fewer and fewer seeds, to
species which never produce a single seed, but yet are affected
by the pollen of certain other species, for the germen swells. It is
here manifestly impossible to select the more sterile individuals,
which have already ceased to yield seeds; so that this acme of
sterility, when the germen alone is affected, cannot have been
gained through selection ; and from the laws governing the various
grades of sterility being so uniform throughout the animal and
vegetable kingdoms, we may infer that the cause, whatever it may
be, is the same or nearly the same in all cases.
We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses
and in hybrids. In the case of first crosses, the greater or less
difficulty in effecting an union and in obtaining offspring ap-
parently depends on several distinct causes. There must some-
times 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 ob-
served that when the pollen of one species is placed on the stigma
of a distantly allied species, though the pollen-tubes protrude,
they do not penetrate the stigmatic surface. Again, the male
element may reach the female element, but be incapable of caus-
ing 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 com-
municated to me by Mr. Hewitt, who has had great experience in
hybridizing pheasants and fowls, that the early death of the
embryo is a very frequent cause of sterility in first crosses. Mr.
Salter has recently given the results of an examination of about
500 eggs produced from various crosses between three species of
Gallus and their hybrids; the majority of these eggs had been
fertilized; and in the majority of the fertilized eggs, the embryos
had either been partially developed and had then perished, or had
become nearly mature, but the young chickens had been unable
to break through the shell. Of the chickens which were born, more
than four-fifths died within the first few days, or at latest weeks,
^'without any obvious cause, apparently from mere inability to
254 THE ORIGIN OF SPECIES
live;" so that from the 500 eggs only twelve chickens were reared.
With plants, hybridized embryos probably often perish in a like
manner; at least it is known that hybrids raised from very dis-
tinct species are sometimes weak and dwarfed, and perish at an
early age; of which fact Max Wichura has recently given some
striking cases with hybrid willows. It may be here worth noticing
that in some cases of parthenogenesis, the embryos within the
eggs of silk moths which had not been fertilized, pass through their
early stages of development and then perish like the embryos
produced by a cross between distinct species. Until becoming ac-
quainted with these facts, I was unwilling to believe in the fre-
quent early death of hybrid embryos; for hybrids, when once
born, are generally healthy and long-lived, as we see in the case
of the common mule. Hybrids, however, are differently circum-
stanced before and after birth: when born and living in a country
where their two parents live, they are generally placed under
suitable conditions of life. But a hybrid partakes of only half
of the nature and constitution of its mother; it may therefore,
before birth, as long as it is nourished within its mother's womb,
or within the egg or seed produced by the mother, be exposed to
conditions in some degree unsuitable, and consequently be liable
to perish at an early period; more especially as all very young
beings are eminently sensitive to injurious or unnatural condi-
tions of life. But after all, the cause more probably lies in some
imperfection in the original act of impregnation, causing the em-
bryo to be imperfectly developed, rather than in the conditions to
which it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual ele-
ments are imperfectly developed, the case is somewhat different.
I have more than once alluded to a large body of facts showing
that, when animals and plants are removed from their natural
conditions, they are extremely liable to have their reproductive
systems seriously affected. This, in fact, is the great bar to the
domestication of animals. Between the sterility thus superinduced
and that of hybrids, there are many points of similarity. In both
cases the sterility is independent of general health, and is often
accompanied by excess of size or great luxuriance. In both cases
the sterility occurs in various degrees; in both, the male element
is the most liable to be affected; but sometimes the female more
than the male. In both, the tendency goes to a certain extent with
systematic affinity, for whole groups of animals and plants are
rendered impotent by the same unnatural conditions; and whole
groups of species tend to produce sterile hybrids. On the other
HYBRIDISM 2SS
hand, one species in a group will sometimes resist great changes
of conditions with unimpaired fertility; and certain species in a
group will produce unusually fertile hybrids. No one can tell till
he tries, whether any particular animal will breed under 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
disturbed, though often in so slight a degree as to be inappreciable
by us; in the other case, or that of hybrids, the external condi-
tions have remained the same, but the organization has been dis-
turbed by two distinct structures and constitutions, including of
course the reproductive systems, having been blended into one.
For it is scarcely possible that two organizations should be com-
pounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relations of the differ-
ent parts and organs one to another or to the conditions of life.
When hybrids are able to breed inter se, they transmit to their
offspring from generation to generation the same compounded
organization, and hence we need not be surprised that their steril-
ity, though in some degree variable, does not diminish; it is even
apt to increase, this being generally the result, as before explained,
of too close interbreeding. The above view of the sterility of
hybrids being caused by two constitutions being compounded into
one has been strongly maintained by Max Wichura.
It must, however, be owned that we cannot understand, on the
above or any other view, several facts with respect to the sterility
of hybrids; for instance, the unequal fertihty of hybrids produced
from reciprocal crosses; or the increased sterility in those hybrids
which occasionally and exceptionally resemble closely either pure
parent. Nor do I pretend that the foregoing remarks go to the
root of the matter; no explanation is offered why an organism,
when placed under unnatural conditions, is rendered sterile. All
256 THE ORIGIN OF SPECIES
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 hfe having been disturbed, in the other
case from the organization having been disturbed by two organiza-
tions being compounded into one.
A similar parallelism holds good with an allied yet very differ-
ent class of facts. It is an old and almost universal belief, founded
on a considerable body of evidence, which I have elsewhere given,
that slight changes in the conditions of life are beneficial to all
living things. We see this acted on by farmers and gardeners in
their frequent exchanges of seed, tubers, etc., from one soil or
climate to another, and back again. During the convalescence of
animals, great benefit is derived from almost any change in their
habits of life. Again, both with plants and animals, there is the
clearest evidence that a cross between individuals of the same
species, which differ to a certain extent, gives vigor and fertility
to the offspring; and that close interbreeding continued during
several generations between the nearest relations, if these be
kept under the same conditions of life, almost always leads to
decreased size, weakness, or steriHty.
Hence it seems, that, on the one hand, slight changes in the
conditions of life benefit all organic beings, and on the other hand,
that slight crosses, that is, crosses between the males and females
of the same species, which have been subjected to slightly differ-
ent conditions, or which have slightly varied, give vigor and
fertility to the offspring. But, as we have seen, organic beings
long habituated to certain uniform conditions under a state of
nature, when subjected, as under confinement, to a considerable
change in their conditions, very frequently are rendered more or
less sterile; and we know that a cross between two forms that
have become widely or specifically different, produce hybrids
which are almost always in some degree sterile. I am fully per-
suaded that this double parallelism is by no means an accident
or an illusion. He who is able to explain why the elephant, and
a multitude of other animals, are incapable of breeding when kept
under only partial confinement in their native country, will be
able to explain the primary cause of hybrids being so generally
sterile. He will at the same time be able to explain how it is that
the races of some of our domesticated animals, which have often
been subjected to new and not uniform conditions, are quite
fertile together, although they are descended from distinct species,
which would probably have been sterile if aboriginally crossed.
The above two parallel series of facts seem to be connected to-
HYBRIDISM 2S7
gether by some common but unknown bond, which is essentially
related to the principle of life; this principle, according to Mr.
Herbert Spencer, being that life depends on, or consists in, the
incessant action and reaction of various forces which, as through-
out nature, are always tending toward 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 dis-
tinct orders present two forms, which exist in about equal numbers
and which differ in no respect except in their reproductive organs;
one form having a long pistil with short stamens, the other a short
pistil with long stamens; the two having differently sized pollen-
grains. With trimorphic plants there are three forms likewise
differing in the lengths of their pistils and stamens, in the size
and color of the pollen-grains, and in some other respects; and
as in each of the three forms there are two sets of stamens, the
three forms possess altogether six sets of stamens and three kinds
of pistils. These organs are so proportioned in length to each other
that half the stamens in two of the forms stand on a level with
the stigma of the third form. Now I have shown, and the result
has been confirmed by other observers, that in order to obtain
full fertility with these plants, it is necessary that the stigma of
the one form should be fertilized by pollen taken from the stamens
of corresponding height in another form. So that with dimorphic
species two unions, which may be called legitimate, are fully
fertile; and two, which may be called illegitimate, are more or
less infertile. With trimorphic species six unions are legitimate, or
fully fertile, and twelve are illegitimate, or more or less infertile.
The infertility which may be observed in various dimorphic
and trimorphic plants, when they are illegitimately fertilized, that
is, by pollen taken from stamens not corresponding in height with
the pistil, differs much in degree, up to absolute and utter sterility;
just in the same manner as occurs in crossing distinct species. As
the degree of sterility in the latter case depends in an eminent
degree on the conditions of life being more or less favorable, so I
have found it with illegitimate unions. It is well known that if
pollen of a distinct species be placed on the stigma of a flower,
and its own pollen be afterward, even after a considerable interval
of time, placed on the same stigma, its action is so strongly pre-
potent that it generally annihilates the effect of the foreign pollen;
258 THE ORIGIN OF SPECIES
SO it is with the pollen of the several forms of the same species,
for legitimate pollen is strongly prepotent over illegitimate pollen,
when both are placed on the same stigma. I ascertained this by
fertilizing several flowers, first illegitimately and twenty-four
hours afterward legitimately, with pollen taken from a peculiarly
colored variety, and all the seedlings were similarly colored; tljis
shows that the legitimate pollen, though applied twenty-four hours
subsequently, had wholly destroyed or prevented the action of the
previously applied illegitimate pollen. Again, as in making re-
ciprocal crosses between the same two species, there is occasionally
a great difference in the result, so the same thing occurs with
trimorphic plants; for instance, the mid-styled form of Lythrum
salicaria was illegitimately fertilized 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 fertilized by the longer stamens of the mid-styled form.
In all these respects, and in others which might be added, the
forms of the same undoubted species, when illegitimately united,
behave in exactly the same manner as do two distinct species
when crossed. This led me carefully to observe during four years
many seedlings, raised from several illegitimate unions. The chief
result is that these illegitimate plants, as they may be called, are
not fully fertile. It is possible to raise from dimorphic species,
both long-styled and short-styled illegitimate plants, and from
trimorphic plants all three illegitimate forms. These can then be
properly united in a legitimate manner. When this is done, there
is no apparent reason why they should not yield as many seeds as
did their parents when legitimately fertilized. But such is not
the case. They are all infertile, in various degrees; some being
so utterly and incurably sterile that they did not yield during
four seasons a single seed or even seed-capsule. The sterility of
these illegitimate plants, when united with each other in a legiti-
mate manner, may be strictly compared with that of hybrids
when crossed inter se. If, on the other hand, a hybrid is crossed
with either pure parent-species, the sterility is usually much
lessened and so it is when an illegitimate plant is fertilized by a
legitimate plant. In the same manner as the sterility of hybrids
does not always run parallel with the difficulty of making the first
cross between the two parent-species, so that sterility of certain
illegitimate plants was unusually great, while the sterility of the
union from which they were derived was by no means great. With
hybrids raised from the same seed-capsule the degree of sterility
is innately variable, so it is in a marked manner with illegitimate
HYBRIDISM 259
plants. Lastly, many hybrids are profuse and persistent flowerers,
while other and more sterile hybrids produce few flowers, and are
weak, miserable dwarfs; exactly similar cases occur with the
illegitimate offspring of various dimorphic and trimorphic plants.
Altogether there is the closest identity in character and behavior
between illegitimate plants and hybrids. It is hardly an exaggera-
tion to maintain that illegitimate plants are hybrids, produced
within the limits of the same species by the improper union of
certain forms, while ordinary hybrids are produced from an im-
proper union between so-called distinct species. We have also al-
ready seen that there is the closest similarity in all respects be-
tween first illegitimate unions and first crosses between distinct
species. This will perhaps be made more fully apparent by an
illustration; we may suppose that a botanist found two well-
marked varieties (and such occur) of the long-styled form of
the trimorphic Lythrum salicaria, and that he determined to
try by crossing whether they were specifically distinct. He would
find that they yielded only about one-fifth of the proper number
of seed, and that they behaved in all the other above specified
respects as if they had been two distinct species. But to make the
case sure, he would raise plants from his supposed hybridized seed,
and he would find that the seedlings were miserably dwarfed and
utterly sterile, and that they behaved in all other respects like
ordinary hybrids. He might then maintain that he had actually
proved, in accordance with the common view, that his two vari-
eties 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 con-
clude that there is some unknown bond which connects the in-
fertility of illegitimate unions with that of their illegitimate off-
spring, and we are led to extend the same view to first crosses and
hybrids; thirdly, because we find, and this seems to me of es-
pecial importance, that two or three forms of the same species
may exist and may differ in no respect whatever, either in struc-
ture 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; while it is the union of the sexual elements proper
to two distinct forms which is fertile. Hence the case appears at
260 THE ORIGIN OF SPECIB.S
first sight exactly the reverse of what occurs, in the ordinary
unions of the individuals of the same species and with crosses
between distinct species. It is, however, doubtful whether this is
really so; but I will not enlarge on this obscure subject.
We may, however, infer as probable from the consideration of
dimorphic and trimorphic plants, that the sterility of distinct
species when crossed and of their hybrid progeny, depends ex-
clusively on the nature of their sexual elements, and not on any
difference in their structure or general constitution. We are also
led to this same conclusion by considering reciprocal crosses, in
which the male of one species cannot be united, or can be united
with great difficulty, with the female of a second species, while
the converse cross can be effected with perfect facility. That
excellent observer, Gartner, likewise concluded that species when
crossed are sterile owing to differences confined to their reproduc-
tive systems.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL
OFFSPRING, NOT UNIVERSAL
It may be urged as an overwhelming argument that there
must be some essential distinction between species and varieties,
inasmuch as the latter, however much they may differ from each
other in external appearance, cross with perfect facility, and
yield perfectly fertile offspring. With some exceptions, presently
to be given, I fully admit that this is the rule. But the subject
is surrounded by difficulties, for, looking to varieties produced
under nature, if two forms hitherto reputed to be varieties be
found in any degree sterile together, they are at once ranked
by most naturalists as species. For instance, the blue and red
pimpernel, which are considered by most botanists as varieties,
are said by Gartner to be quite sterile when crossed, and he
consequently ranks them as undoubted species. If we thus argue
in a circle, the fertility of all varieties produced under nature
will assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been
produced, under domestication, we are still involved in some
doubt. For when it is stated, for instance, that certain South
American indigenous domestic dogs do not readily unite with
European dogs, the explanation which will occur to every one,
and probably the true one, is that they are descended from
aboriginally distinct species. Nevertheless the perfect fertility
of so many domestic races, differing widely from each other in
HYBRIDISM 261
appearance, for instance, those of the pigeon, or of the cabbage,
is a remarkable fact; more especially when we reflect how
many species there are, which, though resembling each other
most closely, are utterly sterile when intercrossed. Several con-
siderations, however, render the fertility of domestic varieties
less remarkable. In the first place, it may be observed that the
amount of external difference between two species is no sure
guide to their degree of mutual sterility, so that similar differ-
ences in the case of varieties would be no sure guide. It is cer-
tain that with species the cause lies exclusively in differences in
their sexual constitution. Now the varying conditions to whicfi
domesticated animals and cultivated plants have been subjected,
have had so little tendency toward modifying the reproductive
system in a manner leading to mutual sterility, that we have
good grounds for admitting the directly opposite doctrine of
Pallas, namely, that such conditions generally eliminate this
tendency; so that the domesticated descendants of species, which
in their natural state probably would have been in some degree
sterile when crossed, become perfectly fertile together. With
plants, so far is cultivation from giving a tendency toward sterility
between distinct species, that in several well-authenticated cases
already alluded to,' certain plants have been affected in an
opposite manner, for they have become self-impotent, while still
retaining the capacity of fertilizing, and being fertilized by, other
species. If the Pallasian doctrine of the elimination of sterility
through long-continued domestication be admitted, and it can
hardly be rejected, it becomes in the highest degree improbable
that similar conditions long-continued should likewise induce
this tendency; though in certain cases, with species having a
peculiar constitution, sterility might occasionally be thus caused.
Thus, as I believe, we can understand why, with domesticated
animals, varieties have not been produced which are mutually
sterile; and why with plants only a few such cases, immediately
to be given, have been observed.
The real difficulty in our present subject is not, as it appears
to me, why domestic varieties have not become mutually in-
fertile when crossed, but why this has so generally occurred with
natural varieties, as soon as they have been permanently modi-
fied 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
262 THE ORIGIN OF SPECIES
species, owing to their struggle for existence with numerous
competitors, will have been exposed during long periods of time
to more uniform conditions, than have domestic varieties; and
this may well make a wide difference in the result. For we know
how commonly wild animals and plants, when taken from their
natural conditions and subjected to captivity, are rendered
sterile; and the reproductive functions of organic beings whicli
have always lived under natural conditions would probably in
like manner 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 domestication, were
not originally highly sensitive to changes in their conditions of
life, and which can now generally resist with undiminished
fertihty repeated changes of conditions, might be expected to
produce varieties, which would be little hable to have their
reproductive powers injuriously affected by the act of cross-
ing 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 sepa-
rated sexes, they never naturally crossed. He then fertilized
thirteen flowers of the one kind with pollen of the other; but
only a single head produced any seed, and this one head pro-
duced only five grains. Manipulation in this case could not have
been injurious, as the plants have separated sexes. No one, I
believe, has suspected that these varieties of maize are distinct
species; and it is important to notice that the hybrid plants thus
raised were themselves perfectly fertile; so that even Gartner
did not venture to consider the two varieties as specifically dis-
tinct.
Girou de Buzareingues crossed three varieties of gourd, which
like the maize has separate sexes, and he asserts that their mu-
tual fertilization is by so much the less easy as their differences
are greater. How far these experiments may be trusted, I know
not; but the forms experimented on are ranked by Sageret, who
HYBRIDISM 263
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 Ver-
bascum, by so good an observer and so hostile a witness as Gart-
ner: namely, that the yellow and white varieties when crossed
produce less seed than the similarly colored varieties of the same
species. Moreover, he asserts that, when yellow and white vari-
eties of one species are crossed with yellow and white varieties
of a distinct species, more seed is produced by the crosses between
the similarly colored flowers, than between those which are
differently colored. Mr. Scott also has experimented on the
species and varieties of Verbascum; and although unable to con-
firm Gartner's results on the crossing of the distinct species, he
finds that the dissimilarly colored varieties of the same species
yield fewer seeds, in the proportion of eighty-six to one hundred,
than the similarly colored varieties. Yet these varieties differ
in no respect, except in the color of their flowers; and one
variety can sometimes be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every sub-
sequent observer, has proved the remarkable fact that one
particular variety of the common tobacco was more fertile than
the other varieties, when crossed with a widely distinct species.
He experimented on five forms which are commonly reputed to
be varieties, and which he tested by the severest trial, namely,
by reciprocal crosses, and he found their mongrel offspring
perfectly fertile. But one of these five varieties, when used
either as the father or mother, and crossed with the Nicotiana
glutinosa, always yielded hybrids not so sterile as those which
were 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 ot varieties in a state of nature, for
a supposed variety, if proved to be infertile in any degree, would
almost universally be ranked as a species; from man attending
only to external characters in his domestic varieties, and from
such varieties not having been exposed for very long periods to
uniform conditions of life; from these several considerations we
may conclude that fertility does not constitute a fundamental
distinction between varieties and species when crossed. The
264 THE ORIGIN OF SPECIES
general sterility of crossed species may safely be looked at, not
as a special acquirement or endowment, but as incidental on
changes of an unknown nature in their sexual elements.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF
THEIR FERTILITY
Independently of the question of fertility, the offspring of
species and of varieties when crossed may be compared in
several other respects. Gartner, whose strong wish it was to draw
a distinct line between species and varieties, could find very few,
and, as it seems to me, quite unimportant differences between
the so-called hybrid offspring of species, and the so-called mon-
grel 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 mon-
grels are more variable than hybrids; but Gartner admits that
hybrids from species which have long been cultivated are often
variable in the first generation; and I have myself seen striking
instances of this fact. Gartner further admits that hybrids be-
tween very closely allied species are more variable than those
from very distinct species; and this shows that the difference in
degree of variability graduates away. When mongrels and the
more fertile hybrids are propagated for several generations, an
extreme amount of variability in the offspring in both cases is
notorious; but some few instances of both hybrids and mongrels
long retaining a uniform character could be given. The varia-
bility, 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 aug-
ment that arising from the act of crossing. The slight variability
of hybrids in the first generation, in contrast with that in the
succeeding generations, is a curious fact and deserves attention.
For it bears on the view which I have taken of one of the causes
of ordinary variability, namely, that the reproductive system,
from being eminently sensitive to changed conditions of life,
fails under these circumstances to perform its proper function of
producing offspring closely similar in all respects to the parent
form. Now, hybrids in the first generation are descended from
HYBRIDISM 265
species (excluding those long cultivated) which have not had
their reproductive systems in any way affected, and they are not
variable; but hybrids themselves have the reproductive systems
seriously affected and their descendants are highly variable.
But to return to our comparison of mongrels and hybrids:
Gartner states that mongrels are more liable than hybrids to re-
vert to either parent form; but this, if it be true, is certainly only
a difference in degree. Moreover, Gartner expressly states that
the hybrids from long cultivated plants are more subject to re-
version than hybrids from species in their natural state; and thi§
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, while Naudin, on the other
hand, insists in the strongest terms on the almost universal tend-
ency to reversion in hybrids, and he experimented chiefly on
cultivated plants. Gartner further states that when any two spe-
cies, 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 ex-
periment, and seems directly opposed to the results of several
experiments made by Kolreuter.
Such alone are the unimportant differences which Gartner is
able to point out between hybrid and mongrel plants. On the
other hand, the degrees and kinds of resemblance in mongrels
and in hybrids to their respective parents, more especially in
hybrids produced from nearly related species, follow, according
to Gartner, the same laws. When two species are crossed, one
has sometimes a prepotent power of impressing its likeness on the
hybrid. So I believe it to be with varieties of plants; and with
animals, one variety certainly often has this prepotent power
over another variety. Hybrid plants produced from a reciprocal
cross generally resemble each other closely, and so it is with mon-
grel plants from a reciprocal cross. Both hybrids and mongrels
can be reduced to either pure parent form by repeated crosses in
successive generations with either parent.
These several remarks are apparently applicable to animals,
but the subject is here much complicated, partly owing to the
existence of secondary sexual characters, but more especially
owing to prepotency in transmitting likeness running more
strongly in one sex than in the other, both when one species is
266 THE ORIGIN OF SPECIES
crossed with another and when one variety is crossed with an-
other variety. For instance, I think those authors are right who
maintain that the ass has a prepotent power over the horse, so
that both the mule and the hinny resemble more closely the ass
than the horse; but that the prepotency runs more strongly in
the male than in the female ass, so that the mule, which is an off-
spring of the male ass and mare, is more like an ass than is the
hinny, which is the offspring of the female ass and stallion.
Much stress has been laid by some authors on the supposed
fact, that it is only with mongrels that the offspring are not in-
termediate 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 al-
most monstrous in their nature, and which have suddenly ap-
peared— such as albinism, melanism, deficiency of tail or horns,
or additional fingers and toes; and do not relate to characters
which have been slowly acquired through selection. A tendency
to sudden reversions to the perfect character of either parent
would, also, be much more likely to occur with mongrels, which
are descended from varieties often suddenly produced and semi-
monstrous in character, than with hybrids, which are descended
from species slowly and naturally produced. On the whole, I
entirely agree with Dr. Prosper Lucas, who, after arranging an
enormous body of facts with respect to animals, comes to the
conclusion that the laws of resemblance of the child to its parents
are the same, whether the two parents differ little or much from
each other, namely, in the union of individuals of the same vari-
ety, or of different varieties, or of distinct species.
Independently of the question of fertility and sterility, in all
other respects there seems to be a general and close similarity in
the offspring of crossed species, and of crossed varieties. If we
look at species as having been specially created, and at varieties
as having been produced by secondary laws, this similarity would
be an astonishing fact. But it harmonizes perfectly with the view
that there is no essential distinction between species and varieties.
SUMMARY OF CHAPTER
First crosses between forms, sufficiently distinct to be ranked
as species, and their hybrids, are very generally, but not univer-
sally, sterile. The sterility is of all degrees, and is often so sHght
that the most careful experimentaUsts have arrived at diamet-
HYBRIDISM 267
rically opposite conclusions in ranking forms by this test. The
sterility is innately variable in individuals of the same species,
and is eminently susceptible to action of favorable and unfavor-
able conditions. The degree of sterility does not strictly follow
systematic affinity, but is governed by several curious and com-
plex laws. It is generally different, and sometimes widely differ-
ent, in reciprocal crosses between the same two species. It is not
always equal in degree in a first cross and in the hybrids pro-
duced from this cross.
In the same manner as in grafting trees, the capacity in one
species or variety to take on another, is incidental on differences,
generally of an unknown nature, in their vegetative systems, so
in crossing, the greater or less facility of one species to unite
with another is incidental on unknown differences in their repro-
ductive systems. There is no more reason to think that species
have been specially endowed with various degrees of sterility to
prevent their crossing and blending in nature, than to think that
trees have been specially endowed with various and somewhat
analogous degrees of difficulty in being grafted together in order
to prevent their inarching in our forests.
The sterility of first crosses and of their hybrid progeny has
not been acquired through natural selection. In the case of first
crosses it seems to depend on several circumstances; in some in-
stances, in chief part on the early death of the embryo. In the
case of hybrids, it apparently depends on their whole organization
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 ex-
plain 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 vigor 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 var-
ied, favors the size, vigor, and fertility of their offspring. The
facts given on the sterility of the illegitimate unions of dimorphic
and trimorphic plants and of their illegitimate progeny, perhaps
render it probable that some unknown bond in all cases connects
the degree of fertility of first unions with that of their offspring.
The consideration of these facts on dimorphism, as well as of the
results of reciprocal crosses, clearly leads to the conclusion that the
primal cause of the sterility of crossed species is confined to
differences in their sexual elements. But why, in the case of dis-
268 THE ORIGIN OF SPECIES
tinct species, the sexual elements should so generally have become
more or less modified, leading to their mutual infertility, we do not
know; but it seems to stand in some close relation to species having
been exposed for long periods of time to nearly uniform conditions
of life.
It is not surprising that the difficulty in crossing any two spe-
cies, and the sterility of their hybrid offspring, should in most
cases correspond, even if due to distinct causes; for both depend
on the amount of difference between the species which are
crossed. Nor is it surprising that the facility of effecting a first
cross, and the fertility of the hybrids thus produced, and the ca-
pacity of being grafted together — though this latter capacity evi-
dently depends on widely different circumstances — should all
run, to a certain extent, parallel with the systematic affinity of
the forms subjected to experiment; for systematic affinity in-
cludes resemblances of all kinds.
First crosses between forms known to be varieties, or suffi-
ciently alike to be considered as varieties, and their mongrel off-
spring, are very generally — ^but not, as is so often stated, invar-
iably— 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 pro-
duced under domestication by the selection of mere external dif-
ferences, and that they have not been long exposed to uniform
conditions of life. It should also be especially kept in mind, that
long-continued domestication tends to eliminate sterility, and is
therefore little likely to induce this same quality. Independently
of the question of fertility, in all other respects there is the
closest general resemblance between hybrids and mongrels, in
their variability, in their power of absorbing each other by re-
peated 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 condi-
tions become sterile, yet the facts given in this chapter do not
seem to me opposed to the belief that species aboriginally existed
as varieties.
CHAPTER X
On the Imperfection of the Geological Record
On the Absence cf Intermediate Varieties at the Present Day — On the Na-
ture 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 Forma-
tions— On the Denudation of Granitic Areas — On the Absence of Inter-
mediate Varieties in any one Formation — On the Sudden Appearance of
Groups of Species — On their Sudden Appearance in the lowest known
Fossiliferous Strata — Antiquity of the Habitable Earth.
In the sixth chapter I enumerated the chief objections which
might be justly urged against the views maintained in this vol-
ume. Most of them have now been discussed. One, namely, the
distinctness of specific forms and their not being blended to-
gether by innumerable transitional links, is a very obvious diffi-
culty. I assigned reasons why such links do not commonly occur
at the present day under the circumstances apparently most fa-
vorable for their presence, namely, on an extensive and continu-
ous area with graduated physical conditions. I endeavored to
show that the life of each species depends in a more important
manner on the presence of other already defined organic forms,
than on climate, and, therefore, that the really governing condi-
tions of life do not graduate away quite insensibly like heat or
moisture. I endeavored, also, to show that intermediate varieties,
from existing in lesser numbers than the forms which they con-
nect, will generally be beaten out and exterminated during the
course of further modification and improvement. The main cause,
however, of innumerable intermediate links not now occurring
everywhere throughout nature, depends on the very process of
natural selection, through which new varieties continually take
the places of and supplant their parent-forms. But just in pro-
portion as this process of extermination has acted on an enor-
mous scale, so much the number of intermediate varieties, which
have formerly existed, be truly enormous. Why then is not every
geological formation and every stratum full of such intermediate
269
270 THE ORIGIN OF SPECIES
links? Geology assuredly does not reveal any such finely-gradu-
ated organic chain; and this, perhaps, is the most obvious and
serious objection which can be urged against the theory. The ex-
planation lies, as I believe, in the extreme imperfection of the
geological record.
In the first place, it should always be borne in mind what sort
of intermediate forms must, on the theory, have formerly existed.
I have found it difficult, when looking at any two species, to
avoid picturing to myself forms directly intermediate between
them. But this is a wholly false view; we should always look for
forms intermediate between each species and a common but un-
known progenitor; and the progenitor will generally have dif-
fered in some respects from all its modified descendants. To give
a simple illustration: the fantail and pouter pigeons are both
descended from the rock-pigeon; if we possessed all the inter-
mediate varieties which have ever existed, we should have an ex-
tremely close series between both and the rock-pigeon; but we
should have no varieties directly intermediate between the fan-
tail and pouter; none, for instance, combining a tail somewhat
expanded with a crop somewhat enlarged, the characteristic fea-
tures of these two breeds. These two breeds, moreover, have be-
come so much modified, that, if we had i^o 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 organization much general re-
semblance to the tapir and to the horse; but in some points of
structure may have differed considerably from both, even per-
haps more than they differ from each other. Hence, in all such
cases, we should be unable to recognize the parent form of any
two or more species, even if we closely compared the structure
of the parent with that of its modified descendants, unless at the
same time we had a nearly perfect chain of the intermediate
links.
It is just possible, by the theory, that one of two living forms
might have descended from the other; for instance, a horse from
a tapir; and in this case direct intermediate links will have
existed between them. But such a case would imply that one
THE IMPERFECTION OF THE GEOLOGICAL RECORD 271
form had remained for a very long period unaltered, while its de-
scendants had undergone a vast amount of change; and the prin-
ciple of competition between organism and organism, between
child and parent, will render this a very rare event; for in all
cases the new and improved forms of life tend to supplant the
old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences
not greater than we see between the natural and domestic varie-
ties of the same species at the present day; and these parent spe-
cies, now generally extinct, have in their turn been similarly con-
nected with more ancient forms; and so on backward, always
converging to the common ancestor of each great class. So that
the number of intermediate and transitional links, between all
Hving and extinct species, must have been inconceivably great.
But assuredly, if this theory be true, such have lived upon the
earth.
ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF
DEPOSITION AND EXTENT OF DENUDATION
Independently of our not finding fossil remains of such in-
finitely numerous connecting links, it may be objected that time
cannot have sufficed for so great an amount of organic change,
all changes having been effected slowly. It is hardly possible for
me to recall to the reader who is not a practical geologist, the
facts leading the mind feebly to comprehend the lapse of time.
He who can read Sir Charles Ly ell's grand work on the Princi-
ples of Geology, which the future historian will recognize as hav-
ing produced a revolution in natural science, and yet does not
admit how vast have been the past periods of time, may at once
close this volume. Not that it suffices to study the Principles of
Geology, or to read special treatises by different observers on
separate formations, and to mark how each author attempts to
give an inadequate idea of the duration of each formation, or
even of each stratum. We can best gain some idea of past time by
knowing the agencies at work, and learning how deeply the sur-
face 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 superimposed strata, and watch the rivulets bring-
ing down mud, and the waves wearing away the sea-cliffs, in order
272 THE ORIGIN OF SPECIES
to comprehend something about the duration of past time, the
monuments of which we see all around us.
It is good to wander along the coast, when formed of moder-
ately hard rocks, and mark the process of degradation. The tides
in most cases reach the cliffs only for a short time twice a day,
and the waves eat into them only when they are charged with
sand or pebbles ; for there is good evidence that pure water effects
nothing in wearing away rock. At last the base of the cliff is un-
dermined, 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 bowlders,
all thickly clothed by marine productions, showing how little
they are abraded, and how seldom they are rolled about! More-
over, if we follow for a few miles any line of rocky cliff, which is
undergoing degradation, we find that it is only here and there,
along a short length or 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 v/ashed their base.
We have, however, recently learned from the observations of
Ramsay, in the van of many excellent observers — of Jukes,
Geikie, Croll, and others, that subaerial degradation is a much
more important agency than coast-action, or the power of the
waves. The whole surface of the land is exposed to the chemical
action of the air and of the rain-water, with its dissolved carbonic
acid, and in colder countries to frost; the disintegrated matter is
carried down even gentle slopes during heavy rain, and to a
greater extent than might be supposed, especially in arid districts,
by the wind; it is then transported by the streams and rivers,
which, when rapid, deepen their channels, and triturate the frag-
ments. On a rainy day, even in a gently undulating country, we
see the effects of subaerial degradation in the muddy rills which
flow down every slope. Messrs. Ramsay and Whitaker have
shown, and the observation is a most striking one, that the great
lines of escarpment in the Wealden district and those ranging
across England, which formerly were looked at as ancient sea-
coasts, cannot have been thus formed, for each Hne is composed
of ^one and the same formation, while our sea cliffs are every-
where formed by the intersection of various formations. This
being the case, we are compelled to admit that the escarpments
owe their origin in chief part to the rocks of which they are com-
THE IMPERFECTION OF THE GEOLOGICAL RECORD 273
posed, having resisted suba^rial denudation better than the sur-
rounding surface; this surface consequently has been gradually
lowered, with the lines of harder rock left projecting. Nothing
impresses the mind with the vast duration of time, according to
our ideas of time, more forcibly than the conviction thus gained
that subaerial agencies, which apparently have so little power,
and which seem to work so slowly, have produced great results.
When thus impressed with the slow rate at which the land is
worn away through subaerial and littoral action, it is good, in
order to appreciate the past duration of time, to consider, on the
one hand, the masses of rock which have been removed over
many extensive areas, and on the other hand the thickness of our
sedimentary formations. I remember having been much struck
when viewing volcanic islands, which have been worn by the
waves and pared all round into perpendicular cliffs of one or two
thousand feet in height; for the gentle slope of the lava streams,
due to their formerly liquid state, showed at a glance how far the
hard, rocky beds had once extended into the open ocean. The
same story is told still more plainly by faults — those great cracks
along which the strata have been upheaved on one side, or thrown
down on the other, to the height or depth of thousands of feet;
for since the crust cracked, and it makes no great difference
whether the upheaval was sudden, or, as most geologists now
believe, was slow and effected by many starts, the surface of the
land has been so completely planed down that no trace of these
vast dislocations is externally visible. The Craven fault, for in-
stance, extends for upward of thirty miles, and along this line
the vertical displacement of the strata varies from 600 to 3,000
feet. Professor Ramsay has published an account of a down-
throw in Anglesea of 2,300 feet; and he informs me that he fully
believes that there is one in Merionethshire of 12,000 feet;
yet in these cases there is nothing on the surface of the land to
show such prodigious movements; the pile of rocks on either side
of the crack having been smoothly swept away.
On the other hand, in all parts of the world the piles of sedi-
mentary strata are of wonderful thickness. In the Cordillera, I
estimated one mass of conglomerate at 10,000 feet; and although
conglomerates have probably been accumulated at a quicker
rate than finer sediments, yet from being formed of worn and
rounded pebbles, each of which bears the stamp of time, they
are good to show how slowly the mass must have been heaped
together. Professor Ramsay has given me the maximum thick-
ness, from actual measurement in most cases, of the successive
274 THE ORIGIN OF SPECIES
formations in different parts of Great Britain; and this is the
result:
Feet.
Palaeozoic strata (not including igneous beds) .... 57, 154
Secondary strata 13,190
Tertiary strata 2,240
— making altogether 72,584 feet; that is, very nearly thirteen
and three-quarters British miles. Some of the formations, which
are represented in England by thin beds, are thousands of feet
in thickness on the Continent. Moreover, between each succes-
sive 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 idea of the time which
has elapsed during their accumulation. The consideration of these
various facts impresses the mind almost in the same manner as
does the vain endeavor to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in an
interesting paper, remarks that we do not err "in forming too
great a conception of the length of geological periods," but in
estimating them by years. When geologists look at large and
complicated phenomena, and then at the figures representing
several million years, the two produce a totally different effect on
the mind, and the figures are at once pronounced too small. In
regard to subaerial denudation, Mr. Croll shows, by calculating
the known amount of sediment annually brought down by cer-
tain rivers, relatively to their areas of drainage, that 1,000 feet
of solid rock, as it became gradually disintegrated, would thus
be removed from the mean level of the whole area in the course
of six million years. This seems an astonishing result, and some
considerations lead to the suspicion that it may be too large, but
if halved or quartered it is still very surprising. Few of us, how-
ever, know what a million really means. Mr. Croll gives the fol-
lowing illustration: Take a narrow strip of paper, eighty-three
feet four inches in length, and stretch it along the wall of a
large hall; then mark off at one end the tenth of an inch. This
tenth of an inch will represent one hundred years, and the entire
strip a million years. But let it be borne in mind, in relation to
the subject of this work, what a hundred years implies, repre-
sented as it is by a measure utterly insignificant in a hall of the
above dimensions. Several eminent breeders, during a single life-
time, have so largely modified some of the higher animals, which
propagate their kind much more slowly than most of the lower
THE IMPERFECTION OF THE GEOLOGICAL RECORD 275
animals, that they have formed what well deserves to be called
a new sub-breed. Few men have attended with due care to any
one strain for more than half a century, so that a hundred years
represents the work of two breeders in succession. It is not to be
supposed that species in a state of nature ever change so quickly
as domestic animals under the guidance of methodical selection.
The comparison would be in every way fairer with the effects
which follow from unconscious selection, that is, the preservation
of the most useful or beautiful animals, with no intention of
modifying the breed; but by this process of unconscious selec-
tion, 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 coun-
try 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 cir-
cumstances, 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 TKE POORNESS OF PAL^ONTOLOGICAL COLLECTIONS
Now let us turn to our richest geological museums, and what
a paltry display we behold! That our collections are imperfect,
is admitted by every one. The remark of that admirable palaeon-
tologist, Edward Forbes, should never be forgotten, namely,
that very many fossil species are known and named from single
and often broken specimens, or from a few specimens collected
on some one spot. Only a small portion of the surface of the earth
has been geologically explored, and no part with sufficient care,
as the important discoveries made every year in Europe prove.
No organism wholly soft can be preserved. Shells and bones decay
and disappear when left on the bottom of the sea, where sedi-
ment is not accumulating. We probably take a quite erroneous
view, when we assume that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to
embed and preserve fossil remains. Throughout an enormously
large proportion of the ocean, the bright blue tint of the water
276 THE ORIGIN OF SPECIES
bespeaks its purity. The many cases on record of a formation
conformably covered, after an immense interval of time, by an-
other and later formation, without the underlying bed having
suffered in the interval any wear and tear, seem explicable only
on the view of the bottom of the sea not rarely lying for ages in
an unaltered condition. The remains which do become embedded,
if in sand or gravel, will, when the beds are upraised, generally
be dissolved by the percolation of rain water charged with car-
bolic acid. Some of the many kinds of animals which live on the
beach between high and low water mark seem to be rarely pre-
served. 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 excep-
tion of a single Mediterranean species, which inhabits deep
water, and this has been found fossil in Sicily, whereas not one
other species has hitherto been found in any tertiary formation:
yet it is known that the genus Chthamalus existed during the
Chalk period. Lastly, many great deposits, requiring a vast
length of time for their accumulation, are entirely destitute of
organic remains, without our being able to assign any reason:
one of the most striking instances is that of the Flysch forma-
tion, which consists of shale and sandstone, several thousand,
occasionally even six thousand, feet in thickness, and extending
for at least 300 miles from Vienna to Switzerland; and although
this great mass has been most carefully searched, no fossils,
except a few vegetable remains, have been found.
With respect to the terrestrial productions which lived during
the Secondary and Palaeozoic periods, it is superfluous to state
that our evidence is fragmentary in an extreme degree. For in-
stance, until recently not a land-shell was known belonging to
either of these vast periods, with the exception of one species dis-
covered by Sir C. Lyell and Dr. Dawson in the carboniferous
strata of North America; but now land-shells have been found
in the lias. In regard to mammiferous remains, a glance at the
historical table published in Lyell's Manual will bring home the
truth, how accidental and rare is their preservation, far better
than pages of detail. Nor is their rarity surprising, when we re-
member how large a proportion of the bones of tertiary mammals
have been discovered either in caves or in lacustrine deposits;
and that not a cave or true lacustrine bed is known belonging to
the age of our secondary or palaeozoic formations.
But the imperfection in the geological record largely results
from another and more important cause than any of the fore-
THE IMPERFECTION OF THE GEOLOGICAL RECORD 277
going; namely, from the several formations being separated
from each other by wide intervals of time. This doctrine has been
emphatically admitted by many geologists and palaeontologists,
who, like E. Forbes, entirely disbelieve in the change of species.
When we see the formations tabulated in written works, or when
we follow them in nature, it is difficult to avoid believing that
they are closely consecutive. But we know, for instance, from
Sir R. Murchison's great work on Russia, what wide gaps there
are in that country between the superimposed formations; so it
is in North America, and in many other parts of the world. The
most skilful geologist, if his attention had been confined exclu-
sively to these large territories, would never have suspected that
during the periods which were blank and barren in his own coun-
try, great piles of sediment, charged with new and peculiar forms
of life, had elsewhere been accumulated. And if, in every separate
territory, hardly any idea can be formed of the length of time
which has elapsed between the consecutive formations, we may
infer that this could nowhere be ascertained. The frequent and
great changes in the mineralogical composition of consecutive
formations, generally implying great changes in the geography
of the surrounding lands, whence the sediment was derived, ac-
cord with the belief of vast intervals of time having elapsed be-
tween each formation.
We can, I think, see why the geological formations of each re-
gion are almost invariably intermittent; that is, have not fol-
lowed each other in close sequence. Scarcely any fact struck me
more when examining many hundred miles of the South American
coasts, which have been upraised several hundred feet within the
recent period, than the absence of any recent deposits sufficiently
extensive to last for even a short geological period. Along the
whole west coast, which is inhabited by a peculiar marine fauna,
tertiary beds are so poorly developed that no record of several
successive and peculiar marine faunas will probably be pre-
served to a distant age. A little reflection will explain why, along
the rising coast of the western side of South America, no ex-
tensive formations with recent or tertiary remains can anywhere
be found, though the supply of sediment must for ages have been
great, from the enormous degradation of the coast rocks and
from the muddy streams entering the sea. The explanation, no
doubt, is that the littoral and sub-littoral deposits are continu-
ally 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
278 THE ORIGIN OF SPECIES
We may, I think, conclude that sediment must be accumu-
lated 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 subse-
quent subaerial degradation. Such thick and extensive accumu-
lations of sediment may be formed in two ways; either in pro-
found depths of the sea, in which case the bottom will not be
inhabited by so many and such varied forms of life as the more
shallow seas; and the mass when upraised will give an imperfect
record of the organisms which existed in the neighborhood dur-
ing 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 sub-
sidence and the supply of sediment nearly balance each other,
the sea will remain shallow and favorable for many and varied
forms, and thus a rich fossiliferous formation, thick enough,
when upraised, to resist a large amount of denudation, may be
formed.
I am convinced that nearly all our ancient formations, which
are throughout the greater part of their thickness rich in fossils,
have thus been formed during subsidence. Since publishing my
views on this subject in 1845, I have watched the progress of
geology, and have been surprised to note how author after au-
thor, in treating of this or that great formation, has come to the
conclusion that it was accumulated during subsidence. I may
add, that the only ancient tertiary formation on the west coast
of South America, which has been bulky enough to resist such
degradation as it has as yet suffered, but which will hardly last
to a distant geological age, was deposited during a downward
oscillation of level, and thus gained considerable thickness.
All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscilla-
tions have affected wide spaces. Consequently formations rich
in fossils and sufficiently thick and extensive to resist subsequent
degradation will have been formed over wide spaces during pe-
riods of subsidence, but only where the supply of sediment was
sufficient to keep the sea shallow and to embed and preserve
the remains before they had time to decay. On the other hand,
as long as the bed of the sea remains stationary, thick deposits
cannot have been accumulated in the shallow parts, which are
the most favorable to life. Still less can this have happened dur-
ing the alternate periods of elevation; or, to speak more accu-
rately, the beds which were then accumulated will generally have
THE IMPERFECTION OF THE GEOLOGICAL RECORD 279
been destroyed by being upraised and brought within the limits
of the coast-action.
These remarks apply chiefly to littoral and sub-littoral deposits.
In the case of an extensive and shallow sea, such as that within
a large part of the Malay Archipelago, where the depth varies
from thirty or forty to sixty fathoms, a widely extended forma-
tion might be formed during a period of elevation, and yet not
suffer excessively from denudation during its slow upheaval; but
the thickness of the formation could not be great, for owing to
the elevatory movement it would be less than the depth in which
it was formed; nor would the deposit be much consolidated, nor
be capped by overlying formations, so that it would run a good
chance of being worn away by atmospheric degradation and by
the action of the sea during subsequent oscillations of level. It
has, however, been suggested by Mr. Hopkins^ that if one part of
the area, after rising and before being denuded, subsided, the
deposit formed during the rising movement, though not thick,
might afterward become protected by fresh accumulations, and
thus be preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds
of considerable horizontal extent have rarely been completely
destroyed. But all geologists, excepting 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 stripped of their covering to an enormous extent.
For it is scarcely possible that such rocks could have been solidi-
fied and crystallized while uncovered; but if the metamorphic
action occurred at profound depths of the ocean, the former pro-
tecting mantle of rock may not have been very thick. Admitting
then that gneiss, mica-schist, granite, diorite, etc., were once
necessarily covered up, how can we account for the naked and
extensive areas of such rocks in any parts of the world, except
on the belief that they have subsequently been completely de-
nuded 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 Switzer-
land. South of the Amazon, Boue colors 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 travelers, the granitic area is very large: thus Von Esch-
wege gives a detailed section of these rocks, stretching from Rio
de Janeiro for 260 geographical miles inland in a straight line;
280 THE ORIGIN OF SPECIES
and I travelled for 150 miles in another direction, and saw noth-
ing but granitic rocks. Numerous specimens, collected along the
whole coast, from near Rio Janeiro to the mouth of the Plata, a
distance of 1,100 geographical miles, were examined by me, and
they all belonged to this class. Inland, along the whole northern
bank of the Plata, I saw, besides modern tertiary beds, only one
small patch of slightly metamorphosed rock, which alone could
have formed a part of the original capping of the granitic series.
Turning to a well-known region, namely, to the United States
and Canada, as shown in Professor H. D. Rogers's beautiful
map, I have estimated the areas by cutting out and weighing the
paper, and I find that the metamorphic (excluding the "semi-
metamorphic") and granite rocks exceed, in the proportion of
19 to 12.5, the whole of the newer Palaeozoic formations. In many
regions the metamorphic and granite rocks would be found much
more widely extended than they appear to be, if all the sedi-
mentary beds were removed which rest unconformably on them,
and which could not have formed part of the original mantle
under which they were crystallized. Hence, it is probable that
in some parts of the world whole formations have been com-
pletely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods of
elevation, the area of the land and of the adjoining shoal parts of
the sea will be increased, and new stations will often be formed
— all circumstances favorable, as previously explained, for the
formation of new varieties and species; but during such periods
there will generally be a blank in the geological record. On the
other hand, during subsidence, the inhabited area and number of
inhabitants will decrease (excepting on the shores of a continent
when first broken up into an archipelago), and consequently,
during subsidence, though there will be much extinction, few
new varieties or species will be formed; and it is during these
very periods of subsidence that the deposits which are richest in
fossils have been accumulated.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN
ANY SINGLE FORMATION
From these several considerations it cannot be doubted that
the geological record, viewed as a whole, is extremely imperfect;
but if we confine our attention to any one formation, it becomes
much more difficult to understand why we do not therein find
closely graduated varieties between the allied species which lived
at its commencement and at its close. Several cases are on record
THE IMPERFECTION OF THE GEOLOGICAL RECORD 281
of the same species presenting varieties in the upper and lower
parts of the same formation. Thus Trautschold gives a number
of instances with Ammonites, and Hilgendorf has described a
most curious case of ten graduated forms of Planorbis multi-
formis in the successive beds of a fresh-water formation in
Switzerland. Although each formation has indisputably required
a vast number of years for its deposition, several reasons can be
given why each should not commonly include a graduated series
of links between the species which lived at its commencement
and close, but I cannot assign due proportional weight to the
following considerations.
Although each formation may mark a very long lapse of years,
each probably is short compared with the period requisite to
change one species into another. I am aware that two palaeon-
tologists, whose opinions are worthy of much deference, namely
Bronn and Woodward, have concluded that the average duration
of each formation is twice or thrice as long as the average 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 forma-
tion, it would be rash in the extreme to infer that it had not else-
where previously existed. So again, when we find a species dis-
appearing before the last layers have been deposited, it would
be equally rash to suppose that it then became extinct. We forget
how small the area of Europe is, compared with the rest of the
world ; nor have the several stages of the same formation through-
out Europe been correlated with perfect accuracy.
We may safely infer that with marine animals of all kinds
there has been a large amount of migration due to climatal and
other changes; and when we see a species first appearing in any
formation, the probability is that it only then first immigrated
into that area. It is well known, for instance, that several species
appear somewhat earlier in the palaeozoic beds of North America
than in those of Europe; time having apparently been required
for their migration from the American to the European seas. In
examining the latest deposits, in various quarters of the world,
it has everywhere been noted, that some few still existing species
are common in the deposit, but have become extinct in the im-
mediately surrounding sea; or, conversely, that some are now
abundant in the neighboring sea, but are rare or absent in this
particular deposit. It is an excellent lesson to reflect on the ascer-
tained amount of migration of the inhabitants of Europe during
the glacial epoch, which forms only a part of one whole geological
282 THE ORIGIN OF SPECIES
period; and likewise to reflect on the changes of level, on the ex-
treme change of climate, and on the great lapse of time, all in-
cluded within this same glacial period. Yet it may be doubted
whether, in any quarter of the world, sedimentary deposits, in-
cluding fossil remains, have gone on accumulating within the
same area during the whole of this period. It is not, for instance,
probable that sediment was deposited during the whole of the
glacial period near the mouth of the Mississippi, within that
limit of depth at which marine animals can best flourish: for we
know that great geographical changes occurred in other parts of
America during this space of time. When such beds as were de-
posited in shallow water near the mouth of the Mississippi during
some part of the glacial period shall have been upraised, organic
remains will probably first appear and disappear at different
levels, owing to the migrations of species and to geographical
changes. And in the distant future, a geologist, examining these
beds, would be tempted to conclude that the average duration of
life of the embedded fossils had been less than that of the glacial
period, instead of having been really far greater, that is, extend-
ing from before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the
upper and lower parts of the same formation, the deposit must
have gone on continuously accumulating during a long period,
sufficient for the slow process of modification; hence, the deposit
must be a very thick one; and the species undergoing change
must have lived in the same district throughout the whole time.
But we have seen that a thick formation, fossiliferous throughout
its entire thickness, can accumulate only during a period of sub-
sidence; and to keep the depth approximately the same, which is
necessary that the same marine species may live on the same
space, the supply of sediment must nearly counterbalance the
amount of subsidence. But this same movement of subsidence
will tend to submerge the area whence the sediment is derived,
and thus diminish the supply, while the downward movement
continues. In fact, this nearly exact balancing between the supply
of sediment and the amount of subsidence is probably a rare con-
tingency; for it has been observed by more than one palaeontolo-
gist that very thick deposits are usually barren of organic re-
mains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile
of formation in any country, has generally been intermittent in
its accumulation. When we see, as is so often the case, a forma-
tion composed of beds of widely different mineralogical compo-
THE IMPERFECTION OF THE GEOLOGICAL RECORD 283
sition, we may reasonably suspect that the process of deposition
has been more or less interrupted. Nor will the closest inspection
of a formation give us any idea of the length of time which its
deposition may have consumed. Many instances could be given
of beds, only a few feet in thickness, representing formations
which are elsewhere thousands of feet in thickness, and which
must have required an enormous period for their accumulation;
yet no one ignorant of this fact would have even suspected the
vast lapse of time represented by the thinner formation. Many
cases could be given of the lower beds of a formation having
been upraised, denuded, submerged, and then re-covered by the
upper beds of the same formation — facts, showing what wide,
yet easily overlooked, intervals have occurred in its accumula-
tion. In other cases we have the plainest evidence in great fossil-
ized trees, still standing upright as they grew, of many long in-
tervals of time and changes of level during the process of depo-
sition, which would not have been suspected, had not the trees
been preserved: thus Sir C. Lyell and Dr. Dawson found car-
boniferous beds 1,400 feet thick in Nova Scotia, with ancient
root-bearing strata, one above the other, at no less than eighty-
six 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 depo-
sition, 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 depo-
sition of any one geological formation, a section would not in-
clude all the fine intermediate gradations which must, on our
theory, have existed, but abrupt, though perhaps slight, changes
of form.
It is all-important to remember that naturalists have no golden
rule by which to distinguish species and varieties; they grant
some little variability to each species, but when they meet with a
somewhat greater amount of difference between any two forms,
they rank both as species, unless they are enabled to connect
them together by the closest intermediate gradations; and this,
from the reasons just assigned, we can seldom hope to effect in
any one geological section. Supposing B and C to be two species,
and a third. A, to be found in an older and underlying bed; even
if A were strictly intermediate between B and C, it would simply
be ranked as a third and distinct species, unless at the same time
it could be closely connected by intermediate varieties with either
one or both forms. Nor should it be forgotten, as before explained.
284 THE ORIGIN OF SPECIES
that A might be the actual progenitor of B and C, and yet would
not necessarily be strictly intermediate between them in all re-
spects. So that we might obtain the parent-species and its several
modified descendants from the lower and upper beds of the same
formation, and unless we obtained numerous transitional grada-
tions, we should not recognize their blood-relationship, and
should consequently rank them as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this
the more readily if the specimens come from different sub-stages
of the same formation. Some experienced conchologists are now
sinking many of the very fine species of D'Orbigny and others
into the rank of varieties; and on this view we do find the kind
of evidence of change which on the theory we ought to find.
Look again at the later tertiary deposits, which include many
shells believed by the majority of naturalists to be identical with
existing species; but some excellent naturalists, as Agassiz and
Pictet, maintain that all these tertiary species are specifically
distinct, though the distinction is admitted to be very slight; so
that here, unless we believe that these eminent naturalists have
been misled by their imaginations, and that these late tertiary
species really present no difference whatever from their living
representatives, or unless we admit, in opposition to the judgment
of most naturalists, that these tertiary species are all truly dis-
tinct from the recent, we have evidence of the frequent occur-
rence 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-
form until they have been modified and perfected in some con-
siderable degree. According to this view, the chance of discover-
ing 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
THE IMPERFECTION OF THE GEOLOGICAI. RECORD 285
spot. Most marine animals have a wide range; and we have seen
that with plants it is those which have the widest range, that
oftenest present varieties; so that, with shells and other marine
animals, it is probable that those which had the widest range,
far exceeding the limits of the known geological formations in
Europe, have oftenest given rise, first to local varieties and ulti-
mately to new species; and this again would greatly lessen the
chance of our being able to trace the stages of transition in any
one geological formation.
It is a more important consideration, leading to the same re-
sult, as lately insisted on by Dr. Falconer, namely, that the pe-
riod during which each species underwent modification, though
long as measured by years, was probably short in comparison
with that during which it remained without undergoing any
change.
It should not be forgotten, that at the present day, with per-
fect specimens for examination, two forms can seldom be con-
nected by intermediate varieties, and thus proved to be the same
species, until many specimens are collected from many places;
and with fossil species this can rarely be done. We shall, perhaps,
best perceive the improbability of our being enabled to connect
species by numerous, fine, intermediate, fossil links, by asking
ourselves whether, for instance, geologists at some future period
will be able to prove that our different breeds of cattle, sheep,
horses, and dogs are descended from a single stock or from sev-
eral aboriginal 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 rep-
resentatives, and by other conchologists as only varieties, are
really varieties, or are, as it is called, specifically distinct. This
could be effected by the future geologist only by his discovering
in a fossil state numerous intermediate gradations; and such
success is improbable in the highest degree.
It has been asserted over and over again, by writers who be-
lieve in the immutability of species, that geology yields no link-
ing forms. This assertion, as we shall see in the next chapter, is
certainly erroneous. As Sir J. Lubbock has remarked, ''Every
species is a link between other allied forms." If we take a genus
having a score of species, recent and extinct, and destroy four-
fifths of them, no one doubts that the remainder will stand much
more distinct from each other. If the extreme forms in the genus
happen to have been thus destroyed, the genus itself will stand
more distinct from other allied genera. What geological research
286 THE ORIGIN OF SPECIES
has not revealed, is the former existence of infinitely numerous
gradations, as fine as existing varieties, connecting together
nearly all existing and extinct species. But this ought not to be
expected ; yet this has been repeatedly advanced as a most serious
objection against my views.
It may be worth while to sum up the foregoing remarks on the
causes of the imperfection of the geological record under an
imaginary illustration. The Malay Archipelago is about the size
of Europe from the North Cape to the Mediterranean, and from
Britain to Russia, and therefore equals all the geological forma-
tions which have been examined with any accuracy, excepting
those of the United States of America. I fully agree with Mr.
Godwin-Austen, that the present condition of the Malay Archi-
pelago, with its numerous large islands separated by wide and
shallow seas, probably represents the former state of Europe,
while most of our formations were accumulating. The Malay
Archipelago is one of the richest regions in organic beings; yet
if all the species were to be collected which have ever lived there,
how imperfectly would they represent the natural history of the
world!
But we have every reason to believe that the terrestrial pro-
ductions of the archipelago would be preserved in an extremely
Imperfect manner in the formations which we suppose to be
there accumulating. Not many of the strictly littoral animals,
or of those which lived on naked submarine rocks, would be em-
bedded; and those embedded in gravel or sand would not endure
to a distant epoch. Wherever sediment did not accumulate on the
bed of the sea, or where it did not accumulate at a sufficient rate to
protect organic bodies from decay, no remains could be preserved.
Formations rich in fossils of many kinds, and of thickness
sufficient to last to an age as distant in futurity as the secondary
formations lie in the past, would generally be formed in the archi-
pelago only during periods of subsidence. These periods of sub-
sidence would be separated from each other by immense inter-
vals of time, during which the area would be either stationary or
rising; while rising, the fossiliferous formations on the steeper
shores would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South
America. Even throughout the extensive and shallow seas within
the archipelago, sedimentary beds could hardly be accumulated
of great thickness during the periods of elevation, or become
capped and protected by subsequent deposits, so as to have a
good chance of enduring to a very distant future. During the
THE IMPERFECTION OF THE GEOLOGICAL RECORD 287
periods of subsidence, there would probably be much extinction
of life; during the periods of elevation, there would be much var-
iation, but the geological record would then be less perfect.
It may be doubted whether the duration of any one great pe-
riod of subsidence over the whole or part of the archipelago, to-
gether with a contemporaneous accumulation of sediment, would
exceed the average duration of the same specific forms; and these
contingencies are indispensable for the preservation of all the
transitional gradations between any two or more species. If such
gradations were not all fully preserved, transitional varieties
would merely appear as so many new, though closely allied
species. It is also probable that each great period of subsidence
would be interrupted by oscillations of level, and that slight
cHmatical changes would intervene during such lengthy periods;
and in these cases the inhabitants of the archipelago would mi-
grate, and no closely consecutive record of their modifications
could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now
range thousands of miles beyond its confines; and analogy plainly
leads to the belief that it would be chiefly these far-ranging
species, though only some of them, which v/ould oftenest produce
new varieties; and the varieties would at first be local or confined
to one place, but if possessed of any decided advantage, or when
further modified and improved, they would slowly spread and
supplant their parent forms. When such varieties returned to
their ancient homes, as they would differ from their former state
in a nearly uniform, though perhaps extremely slight degree, and
as they would be found embedded in slightly different substages
of the same formation, they would, according to the principles
followed by many palaeontologists, be ranked as new and distinct
species.
If then there be some degree of truth in these remarks, we
have no right to expect to find, in our geological formations, an
infinite number of those fine transitional forms which, on our
theory, have connected all the past and present species of the
same group into one long and branching chain of life. We ought
only to look for a few links, and such assuredly we do find —
some more distantly, some more closely, related to each other;
and these links, let them be ever so close, if found in different
stages of the same formation, would, by many palaeontologists,
be ranked as distinct species. But I do not pretend that I should
ever have suspected how poor was the record in the best pre-
served geological sections, had not the absence of innumerable
288 THE ORIGIN OF SPECIES
transitional links between the species which lived at the com-
mencement and close of each fonnation, pressed so hardly on my
theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF
ALLIED SPECIES
The abrupt manner in which whole groups of species suddenly
appear in certain formations, has been urged by several palaeon-
tologists— for instance, by Agassiz, Pictet, and Sedgwick — ^as a
fatal objection to the belief in the transmutation of species. If
numerous species, belonging to the same genera or families, have
really started into life at once, the fact would be fatal to the
theory of evolution through natural selection. For the develop-
ment by this means of a group of forms, all of which are de-
scended from some one progenitor, must have been an extremely
slow process; and the progenitors must have lived long before
their modified descendants. But we continually overrate the per-
fection of the geological record, and falsely infer, because certain
genera or families have not been found beneath a certain stage,
that they did not exist before that stage. In all cases positive
palaeontological evidence may be implicitly trusted; negative evi-
dence is worthless, as experience has so often shown. We con-
tinually forget how large the world is, compared with the area
over which our geological formations have been carefully exam-
ined; we forget that groups of species may elsewhere have long
existed, and have slowly multiplied, before they invaded the
ancient archipelagoes of Europe and the 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 multipli-
cation of species from some one parent-form: and in the succeed-
ing formation, such groups or species will appear as if suddenly
created.
I may here recall a remark formerly made, namely, that it
might require a long succession of ages to adapt an organism to
some new and peculiar line of life, for instance, to fly through the
air; and consequently that the transitional forms would often
long remain confined to some one region; but that, when this
adaptation had once been effected, and a few species had thus
acquired a great advantage over other organisms, a comparatively
short time would be necessary to produce many divergent forms,
which would spread rapidly and widely throughout the world.
THE IMPERFECTION OF THE GEOLOGICAL RECORD 289
Professor Pictet, in his excellent review of this work, in com-
menting on early transitional forms, and taking birds as an illus-
tration, carmot 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 suppose that we
here see the real transitional grades through which the wings of
birds have passed; but what special difficulty is there in believ-
ing 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 re-
marks, 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 dis-
appearance of several groups of animals have been considerably
modified; and a third edition would require still further changes.
I may recall the well-known fact that in geological treatises, pub-
lished not many years ago, mammals were always spoken of as
having abruptly come in at the commencement of the tertiary
series. And now one of the richest known accumulations of fossil
mammals belongs to the middle of the secondary series; and true
mammals have been discovered in the new red sandstone at
nearly the commencement of this great series. Cuvier used to
urge that no monkey occurred in any tertiary stratum; but now
extinct species have been discovered in India, South America,
and in Europe, as far back as the miocene stage. Had it not been
for the rare accident of the preservation of footsteps in the new
red sandstone of the United States, who would have ventured to
suppose that no less than at least thirty different bird-like ani-
mals, 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 green sand; and still
more recently, that strange bird, the Archeopteryx, with a long
290 THE ORIGIN OF SPECIES
Mzard-like tail, bearing a pair of feathers on each joint, and with
its wings furnished with two free claws, has been discovered in
the oolitic slates of Solenhofen. Hardly any recent discovery
shows more forcibly than this how little we as yet know of the
former inhabitants of the world.
I may give another instance, which, from having passed under
my own eyes, has much struck me. In a memoir on Fossil Sessile
Cirripedes, I stated that, from the large number of existing and
extinct tertiary species; from the extraordinary abundance of the
individuals of many species all over the world, from the arctic
regions to the equator, inhabiting various zones of depths, from
the upper tidal limits to fifty fathoms; from the perfect manner
in which specimens are preserved in the oldest tertiary beds;
from the ease with which even a fragment of a valve can be rec-
ognized; from all these circumstances, I inferred that, had sessile
cirripedes existed during the secondary periods, they would cer-
tainly 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 commence-
ment of the tertiary series. This was a sore trouble to me, adding,
as I then thought, one more instance of the abrupt appearance of
a great group of species. But my work had hardly been published,
when a skilful palaeontologist, M. Bosquet, sent me a drawing of
a perfect specimen of an unmistakable sessile cirripede, which he
had himself extracted from the chalk of Belgium. And, as if to
make the case as striking as possible, this cirripede was a Chtham-
alus, a very common, large, and ubiquitous genus, of which not
one species has as yet been found even in any tertiary stratum.
Still more recently, a Pyrgoma, a member of a distinct sub-family
of sessile cirripedes, has been discovered by Mr. Woodward in the
upper chalk; so that we now have abundant evidence of the exist-
ence of this group of animals during the secondary period.
The case most frequently insisted on by palaeontologists, of the
apparently sudden appearance of a whole group of species, is that
of the teleostean fishes, low down, according to Agassiz, in the
Chalk period. This group includes the large majority of existing
species. But certain Jurassic and Triassic forms are now commonly
admitted to be teleostean; and even some palaeozoic forms have
thus been classed by one high authority. If the teleosteans had
really appeared suddenly in the northern hemisphere at the com-
mencement of the chalk formation, the fact would have been
highly remarkable; but it would not have formed an insuperable
difficulty, unless it could likewise have been shown that at the same
THE IMPERFECTION OF THE GEOLOGICAL RECORD 291
period the species were suddenly and simultaneously developed in
other quarters of the world. It is almost superfluous to remark
that hardly any fossil fish are known from south of the equator;
and by running through Pictet's Palaeontology it will be seen that
very few species are known from several formations in Europe.
Some few families of fish now have a confined range; the teleostean
fishes might formerly have had a similarly confined range, and
after having been largely developed in some one sea, have spread
widely. Nor have we any right to suppose that the seas of the
world have always been so freely open from south to north as they
are at present. Even at this day, if the Malay Archipelago were
converted into land, the tropical parts of the Indian Ocean would
form a large and perfectly enclosed basin, in which any great
group of marine animals might be multiplied ; and here they would
remain confined, until some of the species became adapted to a
cooler climate, and were enabled to double the southern capes of
Africa or Australia and thus reach other and distant seas.
From these considerations, from our ignorance of the geology
of other countries beyond the confines of Europe and the United
States, and from the revolution in our palaeontological knowledge
effected by the discoveries of the last dozen years, it seems to me
to be about as rash to dogmatize on the succession of organic forms
throughout the world, as it would be for a naturalist to land for
five minutes on a barren point in Australia, and then to discuss the
number and range of its productions.
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN
THE LOWEST KNOV^N FOSSILIFEROUS STRATA
There is another and allied difficulty, which is much more seri-
ous. I allude to the manner in which species belonging to several
of the main divisions of the animal kingdom suddenly appear in
the lowest known fossiliferous rocks. Most of the arguments which
have convinced me that all the existing species of the same group
are descended from a single progenitor, apply with equal force to
the earliest known species. For instance, it cannot be doubted that
all the Cambrian and Silurian trilobites are descended from some
one crustacean, which must have lived long before the Cambrian
age, and which probably differed greatly from any known animal.
Some of the most ancient animals, as the Nautilus, Lingula, etc.,
do not differ much from living species; and it cannot on our theory
be supposed, that these old species were the progenitors of all the
species belonging to the same groups which have subsequently ap-
peared, for they are not in any degree intermediate in character.
292 THE ORIGIN OF SPECIES
Consequently, if the theory be true, it is indisputable that before
the lowest Cambrian stratum was deposited long periods elapsed,
as long as, or probably far longer than, the whole interval from
the Cambrian age to the present day; and that during these vast
periods the world swarmed with living creatures. Here we en-
counter a formidable objection; for it seems doubtful whether
the earth, in a fit state for the habitation of living creatures, has
lasted long enough. Sir W. Thompson concludes that the consoli-
dation of the crust can hardly have occurred less than twenty or
more than four hundred million years ago, but probably not less
than ninety-eight or more than two hundred million years. These
very wide limits show how doubtful the data are; and other ele-
ments may have hereafter to be introduced into the problem. Mr.
CroU estimates that about sixty million years have elapsed since
the Cambrian period, but this, judging from the small amount of
organic change since the commencement of the Glacial epoch,
appears a very short time for the many and great mutations of
life, which have certainly occurred since the Cambrian formation ;
and the previous one hundred and forty million years can hardly
be considered as sufficient for the development of the varied forms
of life which already existed during the Cambrian period. It is
however probable, as Sir William Thompson insists, that the world
at a very early period was subjected to more rapid and violent
changes in its physical conditions than those now occurring; and
such changes would have tended to induce changes at a corre-
sponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous deposits
belonging to tJiese assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer. Several eminent geol-
ogists, with Sir R. Murchison at their head, were until recently
convinced that we beheld in the organic remains of the lowest
Silurian stratum the first dawn of life. Other highly competent
judges, as Lyell and E. Forbes, have disputed this conclusion. We
should not forget that only a small portion of the world is known
with accuracy. Not very long ago M. Barrande added another and
lower stage, abounding with new and pecuHar species, beneath
the then known Silurian system; and now, still lower down in
the Lower Cambrian formation, Mr. Hicks has found South
Wales beds rich in trilobites, and containing various mollusks and
annelids. The presence of phosphatic nodules and bituminous mat-
ter, even in some of the lowest azotic rocks, probably indicates life
at these periods; and the existence of the Eozoon in the Lauren-
THE IMPERFECTION OF THE GEOLOGICAL RECORD 293
tian formation of Canada is generally admitted. There are three
great series of strata beneath the Silurian system in Canada, in
the lowest of which the Eozoon is found. Sir W. Logan states that
their "united thickness may possibly far surpass that of all the
succeeding rocks, from the base of the palaeozoic series to the
present time. We are thus carried back to a period so remote, that
the appearance of the so-called primordial fauna (of Barrande)
may by some be considered as a comparatively modern event."
The Eozoon belongs to the most lowly organized of all classes of
animals, but is highly organized for its class; it existed in count-
less 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 exist-
ence of living beings long before the Cambrian period, and which
are almost the same with those since used by Sir W. Logan, have
proved true. Nevertheless, the difficulty of assigning any good
reason for the absence of vast piles of strata rich in fossils be-
neath the Cambrian system is very great. It does not seem prob-
able that the most ancient beds have been quite worn away by
denudation, or that their fossils have been wholly obliterated by
metamorphic action, for if this had been the case we should have
found only small remnants of the formations next succeeding
them in age, and these would always have existed in a partially
metamorphosed condition. But the descriptions which we possess
of the Silurian deposits over immense territories in Russia and in
North America, do not support the view that the older a forma-
tion is, the more invariably it has suffered extreme denudation
and metamorphism.
The case at present must remain inexplicable, and may be truly
urged as a valid argument against the views here entertained. To
show that it may hereafter receive some explanation, I will give
the following hypothesis. From the nature of the organic remains
which do not appear to have inhabited profound depths, in the
several formations of Europe and of the United States; and from
the amount of sediment, miles in thickness, of which the forma-
tions are composed, we may infer that from first to last large
islands or tracts of land, whence the sediment was derived, oc-
curred in the neighborhood of the now existing continents of
Europe and North America. This same view has since been main-
tained by Agassiz and others. But we do not know what was the
state of things in the intervals between the several successive
formations; whether Europe and the United States during these
294 THE ORIGIN OF SPECIES
intervals existed as dry land, or as a submarine surface near land,
on which sediment was not deposited, or as the bed of an open
and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive
as the land, we see them studded with many islands; but hardly
one truly oceanic island (with the exception of New Zealand, if
this can be called a truly oceanic island) is as yet known to afford
even a remnant of any palaeozoic or secondary formation. Hence,
we may perhaps infer, that during the palaeozoic and secondary
periods, neither continents nor continental islands existed where
our oceans now extend; for had they existed, palaeozoic and sec-
ondary 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 enormously long periods.
If, then, we may infer anything from these facts, we may infer
that, where our oceans now extend, oceans have extended from
the remotest period of which we have any record; and on the
other hand; that where continents now exist, large tracts of land
have existed, subjected, no doubt, to great oscillations of level,
since the Cambrian period. The colored map appended to my
volume on Coral Reefs led me to conclude that the great oceans
are still mainly areas of subsidence, the great archipelagoes still
areas of oscillations of level, and the continents areas of elevation.
But we have no reason to assume that things have thus remained
from the beginning of the world. Our continents seem to have
been formed by a preponderance, during many oscillations of level,
of the force of elevation. But may not the areas of preponderant
movement have changed in the lapse of ages? At a period long
antecedent to the Cambrian epoch, continents may have existed
where oceans are now spread out, and clear and open oceans may
have existed where our continents now stand. Nor should we be
justified in assuming that if, for instance, the bed of the Pacific
Ocean were now converted into a continent, we should there find
sedimentary formations, in recognizable condition, older than the
Cambrian strata, supposing such to have been formerly deposited;
for it might well happen that strata which had subsided some
miles nearer to the 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 always remained nearer to the surface. The immense
areas in some parts of the world, for instance in South America,
of naked metamorphic rocks, which must have been heated under
THE IMPERFECTION OF THE GEOLOGICAL RECORD 295
great pressure, have always seemed to me to require some special
explanation; and we may perhaps believe that we see in these
large areas the many formations long anterior to the Cambrian
epoch in a completely metamorphosed and denuded condition.
The several difficulties here discussed, namely, that, though we
find in our geological formations many links between the species
which now exist and which formerly existed, we do not find
infinitely numerous fine transitional forms closely joining them
all together, the sudden manner in which several groups of species
first appear in our European formations, the almost entire ab-
sence, as at present known, of formations rich in fossils beneath
the Cambrian strata, are all undoubtedly of the most serious na-
ture. We see this in the fact that the most eminent palaeontologists,
namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E. Forbes,
etc., and all our greatest geologists, as Lyell, Murchison, Sedg-
wick, etc., have unanimously, often vehemently, maintained the
immutability of species. But Sir Charles Lyell now gives the sup-
port of his high authority to the opposite side, and most geologists
and palaeontologists are much shaken in their former belief. Those
who believe that the geological record is in any degree perfect,
will undoubtedly at once reject the theory. For my part, follow-
ing out Lyell's metaphor, I look at the geological record as a his-
tory of the world imperfectly kept and written in a changing
dialect. Of this history we possess the last volume alone, relating
only to two or three countries. Of this volume, only here and there
a short chapter has been preserved, and of each page, only here
and there a few lines. Each word of the slowly-changing language,
more or less different in the successive chapters, may represent
the forms of life, which are entombed in our consecutive forma-
tions, and which falsely appear to have been abruptly introduced.
On this view the difficulties above discussed are greatly diminished
or even disappear.
CHAPTER XI
On The Geological Succession of Organic Beings
On the Slow and Successive Appearance of New Species — On their Different
Rates of Change — Species once lost do not reappear — Groups of Spedes
follow the Same General Rules in Their Appearance and Disappearance
as do Single Species — On Extinction — On Simultaneous Changes in the
Forms of Life throughout the World — On the Affinities of Extinct Spe-
des 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 com-
mon view of the immutability of species, or with that of their
slow and gradual modification through variation and natural
selection.
New species have appeared very slowly, one after another, both
on the land and in the waters. Lyell has shown that it is hardly
possible to resist the evidence on this head in the case of the
several tertiary stages; and every year tends to fill up the blanks
between the stages, and to make the proportion between the lost
and existing forms more gradual. In some of the most recent beds,
though undoubtedly of high antiquity if measured by years, only
one or two species are extinct, and only one or two are new, hav-
ing 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 forma-
tion 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 ter-
tiary beds a few living shells may still be found in the midst of
a multitude of extinct forms. Falconer has given a striking in-
stance of a similar fact, for an existing crocodile is associated with
many lost mammals and reptiles in the sub-Himalayan deposits.
The Silurian Lingula differs but little from the living species of
this genus; whereas most of the other Silurian Molluscs and all
296
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 297
the Crustaceans have changed greatly. The productions of the
land seem to have changed at a quicker rate than those of the
sea, of which a striking instance has been observed in Switzerland.
There is some reason to believe that organisms high in the scale,
change more quickly than those that are low: though there are
exceptions to this rule. The amount of organic change, as Pictet
has remarked, is not the same in each successive so-called forma-
tion. Yet if we compare any but the most closely related forma-
tions, all the species will be found to have undergone some
change. When a species has once disappeared from the face of
the earth, we have no reason to believe that the same identical
form ever reappears. The strongest apparent exception to this
latter rule is that of the so-called "colonies" of M. Barrande,
which intrude for a period in the midst of an older formation, and
then allow the pre-existing fauna to reappear; but Lyell's ex-
planation, 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 in-
cludes no fixed law of development, causing all the inhabitants of
an area to change abruptly, or simultaneously, or to an equal
degree. The process of modification must be slow, and will gen- ^'
erally affect only a few species at the same time; for the variability
of each species is independent of that of all others. Whether such
variations or individual differences as may arise will be accu-
mulated 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 immi-
gration of new colonists, and on the nature of the other inhabitants
with which the varying species come into competition. Hence it is
by no means surprising that one species should retain the same
identical form much longer than others; or, if changing, should
change in a less degree. We find similar relations between the
existing inhabitants of distinct countries; for instance, the land-
shells and coleopterous insects of Madeira have come to differ
considerably from their nearest allies on the continent of Europe,
whereas the marine shells and birds have remained unaltered. We
can perhaps understand the apparently quicker rate of change in
terrestrial and in more highly organized productions compared
with marine and lower productions, by the more complex relations
of the higher beings to their organic and inorganic conditions of
life, as explained in a former chapter. When many of the in-
298 THE ORIGIN OF SPECIES
habitants of any area have become modified and improved, we
' can understand, on the principle of competition, and from the all-
I important relations of organism to organism in the struggle for
\ Hie, that any form which did not become in some degree modified
Vand improved, would be liable to extermination. Hence, we see
I why all the species in the same region do at last, if we look to
I 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 ac-
cumulated at wide and irregularly intermittent intervals of time;
consequently the amount of organic change exhibited by the fos-
sils embedded in consecutive formations is not equal. Each forma-
tion, on this view, does not mark a new and complete act of
creation, but only an occasional scene, taken almost at hazard, in
an ever slowly changing drama.
We can clearly understand why a species when once lost should
never reappear, even if the very same conditions of life, organic
and inorganic, should recur. For though the offsprmg of one species
might be adapted (and no doubt this has occurred in innumerable
instances) to fill the place of another species in the economy of
nature, and thus supplant it; yet the two forms — the old and the
new — would not be identically the same; for both would almost
certainly inherit different characters from their distinct progeni-
tors; and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail pigeons were
destroyed, that fanciers might make a new breed hardly dis-
tinguishable 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 exter-
minated by their improved offspring, it is incredible that a fantail,
identical with the existing breed, could be raised from any other
species of pigeon, or even from any other well-established race of
the domestic pigeon, for the successive variations would almost
certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some character-
istic differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 299
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 ex-
ceptions are surprisingly few, so few that E. Forbes, Pictet, and
Woodward (though all strongly opposed to such views as I main-
tain) admit its truth; and the rule strictly accords with the theory.
For all the species of the same group, however long it may have
lasted, are the modified descendants one from the other, and all
from a common progenitor. In the genus Lingula, for instance,
the species which have successively appeared at all ages must have
been connected by an unbroken series of generations, from the
lowest Silurian stratum to the present day.
We have seen in the last chapter that whole groups of species
sometimes falsely appear to have been abruptly developed; and
I have attempted to give an explanation of this fact, which if
true would be fatal to my views. But such cases are certainly ex-
ceptional; 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 repre-
sented by a vertical line of varying thickness, ascending through
the successive geological formations in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upward, often keeping of equal thickness for a space, and ulti-
mately thins out in the upper beds, marking the decrease and
final extinction of the species. This gradual increase in number
of the species of a group is strictly conformable with the theory,
for the species of the same genus, and the genera of the same
family, can increase only slowly and progressively; the process
of modification and the production of a number of allied forms
necessarily being a slow and gradual process, one species first
giving rise to two or three varieties, these being slowly converted
into species, which in their turn 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 disappearance
of species and of groups of species. On the theory of natural se-
lection, 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
300 THE ORIGIN OF SPECIES
by catastrophes at successive periods is very generally given up,
even by those geologists, as Elie de Beaumont, Murchison, Bar-
rande, etc., whose general views would naturally lead them to
this conclusion. On the contrary, we have every reason to believe,
from the study of the tertiary formations, that species and groups
of species gradually disappear, one after another, first from one
spot, then from another, and finally from the world. In some few
cases, however, as by the breaking of an isthmus and the conse-
quent irruption of a multitude of new inhabitants into an ad-
joining sea, or by the final subsidence of an island, the process of
extinction may have been rapid. Both single species and whole
groups of species last for very unequal periods; some groups, as
we have seen, have endured from the earliest known dawn of life
to the present day; some have disappeared before the close of
the palaeozoic period. No fixed law seems to determine the length
of time during which any single species or any single genus en-
dures. There is reason to believe that the extinction of a whole
group of species is generally a slower process than their produc-
tion: if their appearance and disappearance be represented, as
before, by a vertical line of varying thickness, the line is found to
taper more gradually at its upper end, which marks the progress
of extermination, than at its lower end, which marks the first ap-
pearance and the early increase in number of the species. In some
cases, however, the extermination of whole groups, as of am-
monites, toward the close of the secondary period, has been won-
derfully sudden.
The extinction of species has been involved in the most gra-
tuitous mystery. Some authors have even supposed that, as the
individual has a definite length of life, so have species a definite
duration. No one can have marvelled more than I have done at
the extinction of species. When I found in La Plata the tooth of a
horse embedded with the remains of Mastodon, Megatherium,
Toxodon, and other extinct monsters, which all co-existed with
still living shells of a very late geological period, I was filled with
astonishment; for, seeing that the horse, since its introduction by
the Spaniards into South America, has run wild over the whole
country and has increased in numbers at an unparalleled rate, I
asked myself what could so recently have exterminated the former
horse under conditions of life apparently so favorable. But my
astonishment was groundless. Professor Owen soon perceived that
the tooth, though so like that of the existing horse, belonged to an
extinct species. Had this horse been still living, but in some degree
rare, no naturalist would have felt the least surprise at its rarity;
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 301
for rarity is the attribute of a vast number of species of all classes,
in all countries. If we ask ourselves why this or that species is
rare, we answer that something is unfavorable in its conditions of
life; but what that something is, we can hardly ever tell. On the
supposition of the fossil horse still existing as a rare species, we
might have felt certain, from the analogy of all other mammals,
even of the slow-breeding elephant, and from the history of the
naturalization of the domestic horse in South America, that under
more favorable conditions it would in a very few years have
stocked the whole continent. But we could not have told what the
unfavorable conditions were which checked its increase, whether
some one or several contingencies, and at what period of the
horse's life, and in what degree, they severally acted. If the con-
ditions had gone on, however slowly, becoming less and less favor-
able, we assuredly should not have perceived the fact, yet the fos-
sil horse would certainly have become rarer and rarer, and finally
extinct — its place being seized on by some more successful com-
petitor.
It is most difficult always to remember that the increase of
every creature is constantly being checked by unperceived hostile
agencies; and that these same unperceived agencies are amply
sufficient to cause rarity, and finally extinction. So little is this
subject understood, that I have heard surprise repeatedly ex-
pressed at such great monsters as the Mastodon and the more
ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the con-
trary, would in some cases determine, as has been remarked by
Owen, quicker extermination, from the greater amount of requi-
site 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 weaking the ele-
phant in India, check its increase; and this was Bruce's conclu-
sion with respect to the African elephant in Abyssinia. It is cer-
tain that insects and bloodsucking bats determine the existence
of the larger naturalized quadrupeds in several parts of South
America.
We see in many cases in the more recent tertiary formations,
that rarity precedes extinction; and we know that this has been
the progress of events with those animals which have been ex-
terminated, 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
302 THE ORIGIN OF SPECIES
no surprise at the rarity of a species, and yet to marvel greatly
when the species ceases to exist, is much the same as to admit
that sickness in the individual is the forerunner of death — to feel
no surprise at sickness, but, when the sick man dies, to wonder,
and to suspect that he died by some deed of violence.
The theory of natural selection is grounded on the belief that
each new variety, and ultimately each new species, is produced
and maintained by having some advantage over those with which
it comes into competition; and the consequent extinction of the
less-favored forms almost inevitably follows. It is the same with
our domestic productions; when a new and slightly improved
variety has been raised, it at first supplants the less improved
varieties in the same neighborhood; when much improved it is
transported far and near, like our short-horn cattle, and takes
the place of other breeds in other countries. Thus the appearance
of new forms and the disappearance of old forms, both those
naturally and those artificially produced, are bound together. In
flourishing groups, the number of new specific forms which have
been produced within a given time has at some periods probably
been greater than the number of the old specific forms which
have been exterminated; but we know that species have not gone
on indefinitely increasing, at least during the later geological
epochs, so that, looking to later times, we may believe that the
production of new forms has caused the extinction of about the
same number of old forms.
The competition will generally be most severe, as formerly ex-
plained and illustrated by examples, between the forms which
are most like each other in all respects. Hence the improved and
modified descendants of a species will generally cause the exter-
mination of the parent-species; and if many new forms have been
developed from any one species, the nearest allies of that species,
i. e., the species of the same genus, will be the most liable to exter-
mination. 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 hap-
pened that a new species belonging to some one group has seized
on the place occupied by a species belonging to a distinct group,
and thus have caused its extermination. If many allied forms be
developed from the successful intruder, many will have to yield
their places; and it will generally be the allied forms, which will
suffer from some inherited inferiority in common. But whether it
be species belonging to the same or to a distinct class, which have
yielded their places to other modified and improved species, a few
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 303
of the sufferers may often be preserved for a long time, from be-
ing fitted to some peculiar line of life, or from inhabiting some
distant and isolated station, where they will have escaped severe
competition. For instance, some species of Trigonia, a great genus
of shells in the secondary formations, survive in the Australian
seas; and a few members of the great and almost extinct group
of Ganoid fishes still inhabit our fresh waters. Therefore, the utter
extinction of a group is generally, as we have seen, a slower proc-
ess than its production.
With respect to the apparently sudden extermination of whole
families or orders, as of Trilobites at the close of the palaeozoic
period, and of Ammonites at the close of the secondary period, we
must remember what has been already said on the probable wide
intervals of time between our consecutive formations; and in these
intervals there may have been much slow extermination. More-
over, when, by sudden immigration or by unusually rapid develop-
ment, many species of a new group have taken possession of an
area, many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus yield
their places will commonly be allied, for they will partake of the
same inferiority in common.
Thus, as it seems to me, the manner in which single species and
whole groups of species become extinct accords well with the
theory of natural selection. We need not marvel at extinction; if
we must marvel, let it be at our own presumption in imagining
for a moment that we understand the many complex contingencies
on which the existence of each species depends. If we forget for
an instant that each species tends to increase inordinately, and
that some check is always in action, yet seldom perceived by us,
the whole economy of nature will be utterly obscured. Whenever
we can precisely say why this species is more abundant in in-
dividuals than that; why this species and not another can be
naturalized in a given country; then, and not until then, we may
justly feel surprise why we cannot account for the extinction of
any particular species or group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY
THROUGHOUT THE WORLD
Scarcely any 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 recognized in many distant regions, under the most different
climates, where not a fragment of the mineral chalk itself can
304 THE ORIGIN OF SPECIES
be found ; namely in North America, in equatorial South America,
in Tierra del Fuego, at the Cape of Good Hope, and in the penin-
sula of India. For at these distant points, the organic remains in
certain beds present an unmistakable resemblance to those of the
Chalk. It is not that the same species are met with; for in some
cases not one species is identically the same; but they belong to
the same families, genera, and sections of genera, and sometimes
are similarly characterized in such trifling points as mere super-
ficial sculpture. Moreover, other forms, which are not found in
the Chalk of Europe, but which occur in the formations either
above or below, occur in the same order at these distant points
of the world. In the several successive palaeozoic formations of
Russia, Western Europe, and North America, a similar parallelism
in the forms of life has been observed by several authors; so it
is, according to Lyell, with the European and North American
tertiary deposits. Even if the few fossil species which are 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 correlated.
These observations, however, relate to the marine inhabitants
of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water at distant points change
in the same parallel manner. We may doubt whether they have
thus changed: if the Megatherium, Mylodon, Macrauchenia, and
Toxodon had been brought to Europe from La Plata, without any
information in regard to their geological position, no one would
have suspected that they had co-existed with sea-shells all still
living ; but a.s these anomalous monsters co-existed with the Masto-
don 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 cen-
tury, or even that it has a very strict geological sense; for if all
the marine animals now living in Europe, and all those that lived
in Europe during the pleistocene period (a very remote period as
measured by years, including the whole glacial epoch) were com-
pared with those now existing in South America or in Australia,
the most skilful naturalist would hardly be able to say whether
the present or the pleistocene inhabitants of Europe resembled
most closely those of the southern hemisphere. So, again, several
highly competent observers maintain that the existing produc-
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS »S
tions of the United States are more closely related to those which
lived in Europe during certain late tertiary stages, than to the
present inhabitants of Europe; and if this be so, it is evident that
fossiliferous beds now deposited on the shores of North America
would hereafter be liable to be classed with somewhat older Eu-
ropean beds. Nevertheless, looking to a remotely future epoch,
there can be little doubt that all the more modem marine forma-
tions, namely, the upper pliocene, the pleistocene, and strictlj
modern beds of Europe, North and South America, and Australia,
from containing fossil remains in some degree allied, and from
not including those forms which are found only in the older under-
lying deposits, would be correctly ranked as simultaneous in a
geological sense.
The fact of the forms of life changing simultaneously in the
above large sense, at distant parts of the world, has greatly struck
those admirable observers, MM. de Verneuil and d'Archiac. After
referring to the parallelism of the palaeozoic forms of life in various
parts of Europe, they add: "If, struck by this strange sequence,
we turn our attention to North America, and there discover a
series of analogous phenomena, it will appear certain that all
these modifications of species, their extinction, and the introduc-
tion of new ones, cannot be owing to mere changes in marine cur-
rents 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 conditions, as the cause of these great mutations
in the forms of life throughout the world, under the most differ-
ent climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries and
the nature of their inhabitants.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selec-
tion. New species are formed by having some advantage over older
forms; and the forms which are already dominant, or have some
advantage over the other forms in their own country, give birth to
the greatest number of new varieties or incipient species. We have
distinct evidence on this head, in the plants which are dominant,
that is, which are commonest and most widely diffused, producing
the greatest number of new varieties. It is also natural that the
dominant, varying and far-spreading species, which have already
306 THE ORIGIN OF SPECIES
invaded, to a certain extent, the territories of other species, should
be those which would have the best chance of spreading still far-
ther, 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 ac-
cidents, and on the gradual acclimatization of new species to the
various climates through which they might have to pass, but in
the course of time the dominant forms would generally succeed
in spreading and would ultimately prevail. The diffusion would,
it is probable, be slower with the terrestrial inhabitants of distinct
continents than with the marine inhabitants of the continuous sea.
We might therefore expect to find, as we do find, a less strict de-
gree of parallelism in the succession of the productions of the
land than with those of the sea.
Thus, as it seems to me, the parallel, and, taken in a large
sense, simultaneous, succession of the same forms of life through-
out the world, accords well with the principle of new species hav-
ing been formed by dominant species spreading widely and vary-
ing; the new species thus produced being themselves dominant,
owing to their having had some advantage over their already
dominant parents, as well as over other species, and again spread-
ing, 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 ever3^where tends to cor-
respond both in their first appearance and final disappearance.
There is one other remark connected with this subject worth
making. I have given my reasons for believing that most of our
great formations, rich in fossils, were deposited during periods of
subsidence; and that blank intervals of vast duration, as far as
fossils are concerned, occurred during the periods when the bed
of the sea was either stationary or rising, and likewise when sedi-
ment was not thrown down quickly enough to embed and preserve
organic remains. During these long and blank intervals I suppose
that the inhabitants of each region underwent a considerable
amount of modification and extinction, and that there was much
migration from other parts of the world. As we have reason to
believe that large areas are affected by the same movement, it is
probable that strictly contemporaneous formations have often
been accumulated over very wide spaces in the same quarter of
the world ; but we are very far from having any right to conclude
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 307
that this has invariably been the case, and that large areas have
invariably been affected by the same movements. When two for-
mations have been deposited in two regions during nearly, but
not exactly, the same period, we should find in both, from the
causes explained in the foregoing paragraphs, the same general
succession in the forms of life; but the species would not exactly
correspond; for there will have been little more time in the one
region than in the other for modification, extinction, and immi-
gration.
I suspect that cases of this nature occur in Europe. Mr. Prest-
wich, in his admirable Memoirs on the eocene deposits of Eng-
land and France, is able to draw a close general parallelism be-
tween the successive stages in the two countries; but when he
compares certain stages in England with those in France, al-
though 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 contem-
poraneous faunas. Lyell has made similar observations on some
of the later tertiary formations. Barrande, also, shows that there
is a striking general parallelism in the successive Silurian deposits
of Bohemia and Scandinavia; nevertheless he finds a surprising
amount of difference in the species. If the several formations in
these regions have not been deposited during the same exact pe-
riods— 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 accumulation of the several
formations and during the long intervals of time between them;
in this case the several formations in the two regions could be
arranged in the same order, in accordance with the general suc-
cession of the forms of life, and the order would falsely appear to
be strictly parallel ; nevertheless the species would not be all the
same in the apparently corresponding stages in the two regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER,
AND TO LIVING FORMS
Let us now look to the mutual affinities of extinct and living
species. All fall into a few grand classes; and this fact is at once
explained on the principle of descent. The more ancient any form
is, the more, as a general rule, it differs from living forms. But,
as Buckland long ago remarked, extinct species can all be classed
either in still existing groups, or between them. That the extinct
306 THE ORIGIN OF SPECIES
forms of life help to fill up the intervals between existing genera,
families, and orders, is certainly true; but as this statement has
often been ignored or even denied, it may be well to make some
remarks on this subject, and to give some instances. If we con-
fine our attention either to the living or to the extinct species of
the same class, the series is far less perfect than if we combine
both into one general system. In the writings of Professor Owen
we continually meet with the expression of generalized 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 dis-
tinguished palaeontologist, M. Gaudry, has shown in the most
striking manner that many of the fossil mammals discovered by
him in Attica serve to break down the intervals between existing
genera. Cuvier ranked the Ruminants and Pachyderms as two
of the most distinct orders of mammals; but so many fossil links
have been disentombed that Owen has had to alter the whole
classification, and has placed certain Pachyderms in the same
sub-order with ruminants; for example, he dissolves by gradations
the apparently wide interval between the pig and the camel. The
Ungulata or hoofed quadrupeds are now divided into the even-
toed or odd-toed divisions; but the Macrauchenia of South Amer-
ica connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing
horse and certain other ungulate forms. What a wonderful con-
necting link in the chain of mammals is the Typotherium from
South America, as the name given to it by Professor Gervais ex-
presses, and which cannot be placed in any existing order. The
Sirenia form a very distinct group of the mammals, and one of
the most remarkable peculiarities in existing dugong and lamen-
tin is the entire absence of hind limbs, without even a rudiment
being left; but the extinct Halitherium had, according to Profes-
sor Flower, an ossified thigh-bone "articulated to a well-defined
acetabulum in the pelvis," and it thus makes some approach to
ordinary hoofed quadrupeds, to which the Sirenia are in other
respects allied. The cetaceans or whales are widely different from
all other mammals, but the tertiary Zeuglodon and Squalodon,
which have been placed by some naturalists in an order by them-
selves, are considered by Professor Huxley to be undoubtedly
cetaceans, "and to constitute connecting links with the aquatic
carnivora."
Even the wide interval between birds and reptiles has been
shown by the naturalist just quoted to be partially bridged over
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 309
in the most unexpected manner, on the one hand, by the ostrich
and extinct Archeopteryx, and on the other hand by the Comp-
sognathus, one of the Dinosaurians — that group which includes
the most gigantic of all terrestrial reptiles. Turning to the Inverte-
brata, Barrande asserts (a higher authority could not be named)
that he is every day taught that, although palaeozoic animals can
certainly be classed under existing groups, yet that at this an-
cient period the groups were not so distinctly separated from
each other as they now are.
Some writers have objected to any extinct species, or group of
species, being considered as intermediate between any two living
species, or groups of species. If by this term it is meant that an
extinct form is directly intermediate in all its characters between
two living forms or groups, the objection is probably valid. But
in a natural classification many fossil species certainly stand be-
tween living species, and some extinct genera between living
genera, even between genera belonging to distinct families. The
most common case, especially with respect to very distinct groups,
such as fish and reptiles, seems to be that, supposing them to be
distinguished at the present day by a score of characters, the an-
cient members are separated by a somewhat lesser number of
characters, so that the two groups formerly made a somewhat
nearer approach to each other than they now do.
It is a common belief, that the more ancient a form is, by so
much the more it tends to connect by some of its characters groups
now widely separated from each other. This remark no doubt
must be restricted to those groups which have undergone much
change in the course of geological ages; and it would be difficult
to prove the truth of the proposition, for every now and then even
a living animal, as the Lepidosiren, is discovered having affinities
directed toward very distinct groups. Yet if we compare the older
reptiles and Batrachians, the older fish, the older cephalopods,
and the eocene mammals, with the 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 num-
bered letters in Italics represent genera, and the dotted lines
diverging from them the species in each genus. The diagram is
much too simple, too few genera and too few species being given,
but this is unimportant for us. The horizontal lines may represent
successive geological formations, and all the forms beneath the
310 THE ORIGIN OF SPECIES
uppermost line may be considered as extinct. The three existing
genera a^'*, ^^*, p^"^, will form a small family; b'^* and /^^, a closely
allied family or sub-family; and o^*, e^*, w^^, 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 com-
mon from their ancient progenitor. On the principle of the con-
tinued tendency to divergence of character, which was formerly
illustrated by this diagram, the more recent any form is, the more
it will generally differ from its ancient progenitor. Hence, we can
understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the de-
scendants 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 ex-
tinct forms supposed to be embedded 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^^,
P, m^, m^, m^j were disinterred, these three families would be so
closely linked together that they probably would have to be
united into one great family, in nearly the same manner as has
occurred with ruminants and certain pachyderms. Yet he who
objected to consider as intermediate the extinct genera, which
thus link together the living genera of three families, would be
partly justified, for they are intermediate, not directly, but only
by a long and circuitous course through many widely different
forms. If many extinct forms were to be discovered above one of
the middle horizontal lines or geological formations — for instance,
above No. VI. — but none from beneath this line, then only two
of the families (those on the left hand, a}^, etc., and b^^, etc.)
GEOLCXJICAL SUCCESSION OF ORGANIC BEINGS 311
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 (fl^* to w^*), on the uppermost
line, be supposed to differ from each other by half-a-dozen im-
portant characters, then the families which existed at a period
marked VI. would certainly have differed from each other by a
less number of characters; for they would at this early stage of
descent have diverged in a less degree from their common pro-
genitor. Thus it comes that ancient and extinct genera are often
in a greater or less degree intermediate in character between their
modified descendants, or between their collateral relations.
Under nature the process will be far more complicated than is
represented in the diagram; for the groups will have been more
numerous; they will have endured for extremely unequal lengths
of time, and will have been modified in various degrees. As we
possess only the last volume of the geological record, and that in
a very broken condition, we have no right to expect, except in
rare cases, to fill up the wide intervals in the natural system, and
thus to unite distinct families or orders. All that we have a right
to expect is, that those groups which have, within known geological
periods, undergone much modification, should in the older forma-
tions make some slight approach to each other; so that the older
members should differ less from each other in some of their
characters than do the existing members of the same groups; and
this by the concurrent evidence of our best palaeontologists is
frequently the case.
Thus, on the theory of descent with modification, the main
facts with respect to the mutual affinities of the extinct forms of
life to each other and to living forms, are explained in a satisfac-
tory manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna during any
one great period in the earth's history will be intermediate in
general character between that which preceded and that which
succeeded it. Thus the species which lived at the sixth great stage
of descent in the diagram are the modified offspring of those which
lived at the fifth stage, and are the parents of those which be-
came still more modified at the seventh stage; hence they could
hardly fail to be nearly intermediate in character between the
forms of life above and below. We must, however, allow for the
entire extinction of some preceding forms, and in any one region
for the immigration of new forms from other regions, and for a
large amount of modification during the long and blank intervals
312 THE ORIGIN OF SPECIES
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 recognized by palaeontologists as intermediate in
character between those of tie overlying carboniferous and under-
lying Silurian systems. But each fauna is not necessarily exactly
intermediate, as unequal intervals of time have elapsed between
consecutive formations.
It is no real objection to the truth of the statement that the
fauna of each period as a whole is nearly intermediate in char-
acter between the preceding and succeeding faunas, that certain
genera offer exceptions to the rule. For instance, the species of
mastodons and elephants, when arranged by Dr. Falconer in two
series — in the first place according to their mutual affinities, and
in the second place according to their periods of existence — do
not accord in arrangement. The species extreme in character are
not the oldest or the most recent; nor are those which are inter-
mediate in character, 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 complete, which
is far from the case, we have no reason to believe that forms
successively produced necessarily endure for corresponding lengths
of time. A very ancient form may occasionally have lasted much
longer than a form elsewhere subsequently produced, especially
in the case of terrestrial productions inhabiting separated districts.
To compare small things with great; if the principal living and
extinct races of the domestic pigeon were arranged in serial affinity,
this arrangement would not closely accord with the order in time
of their production, and even less with the order of their disap-
pearance; for the parent rock-pigeon still lives; and many vari-
eties between the rock-pigeon and the carrier have become extinct;
and carriers which are extreme in the important character of
length of beak originated earlier than short-beaked tumblers,
which are at the opposite end of the series in this respect.
Closely connected with the statement, that the organic remains
from an intermediate formation are in some degree intermediate
in character, is the fact, insisted on by all palaeontologists, that
fossils from two consecutive formations are far more closely re-
lated to each other, than are the fossils from two remote forma-
tions. Pictet gives as a well-known instance, the general resem-
blance of the organic remains from the several stages of the
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 313
Chalk formation, though the species are distinct in each stage.
This fact alone, from its generality, seems to have shaken Pro-
fessor Pictet in his belief in the immutability of species. He who
is acquainted with the distribution of existing species over the
globe, will not attempt to account for the close resemblance of
distinct species in closely consecutive formations, by the physical
conditions of the ancient areas having remained nearly the same.
Let it be remembered that the forms of life, at least those in-
habiting 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 being closely related, though
ranked as distinct species, is obvious. As the accumulation of each
formation has often been interrupted, and as long blank intervals
have intervened between successive formations, we ought not to
expect to find, as I attempted to show in the last chapter, in any
one or in any two formations, all the intermediate varieties be-
tween the species which appeared at the commencement and close
of these periods: but we ought to find after intervals, very long
as measured by years, but only moderately long as measured
geologically, closely allied forms, or, as they have been called
by some authors, representative species; and these assuredly we
do find. We find, in short, such evidence of the slow and scarcely
sensible mutations of specific forms, as we have the right to ex-
pect.
ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED
WITH LIVING FORMS
We have seen in the fourth chapter that the degree of differ-
entiation and specialization of the parts in organic beings, when
arrived at maturity, is the best standard, as yet suggested, of their
degree of perfection or highness. We have also seen, that, as the
specialization of parts is an advantage to each being, so natural
selection will tend to render the organization of each being more
specialized and perfect, and in this sense higher; not but that it
may leave many creatures with simple and unimproved structures
fitted for simple conditions of life, and in some cases will even
degrade or simplify the organization, yet leaving such degraded
beings better fitted for their new walks of life. In another and
314 THE ORIGIN OF SPECIES
more general manner, new species become superior to their pred-
ecessors ; for they have to beat in the struggle for life all the older
forms, with which they come into close competition. We may
therefore conclude that if under 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 ex-
terminated by the latter, as would the secondary by the eocene,
and the palaeozic by the secondary forms. So that by this funda-
mental test of victory in the battle for life, as well as by the
standard of the specialization of organs, modern forms ought, on
the theory of natural selection, to stand higher than ancient forms.
Is this the case? A large majority of palaeontologists would an-
swer in the affirmative; and it seems that this answer must be
admitted as true, though difficult of proof.
It is no valid objection to this conclusion, that certain Brachio-
pods have been but shghtly 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 organization since even the Laurentian epoch; for some or-
ganisms would have to remain fitted for simple conditions of life,
and what could be better fitted for this end than these lowly or-
ganized Protozoa? Such objections as the above would be fatal
to my view, if it included advance in organization as a necessary
contingent. They would likewise be fatal, if the above Foramini-
fera, for instance, could be proved to have first come into existence
during the Laurentian epoch, or the above Brachiopods during
the Cambrian formation; for in this case, there would not have
been time sufficient for the development of these organisms up to
the standard which they had then reached. When advanced up
to any given point, there is no necessity, on the theory of natural
selection, for their further continued process; though they will,
during each successive age, have to be slightly modified, so as to
hold their places in relation to slight changes in their conditions.
The foregoing objections hinge on the question whether we really
know how old the world is, and at what period the various forms
of life first appeared; and this may well be disputed.
The problem whether organization on the whole has advanced
is in many ways excessively intricate. The geological record, at
all times imperfect, does not extend far enough back to show with
unmistakable clearness that within the known history of the
world organization has largely advanced. Even at the present day.
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 315
looking to members of the same class, naturalists are not unani-
mous which forms ought to be ranked as highest: thus, some look
at the selaceans or sharks, from their approach in some important
points of structure to reptiles, as the highest fish; others look at
the teleosteans as the highest. The ganoids stand intermediate
between the selaceans and teleosteans; the latter at the present
day are largely preponderant in number; but formerly selaceans
and ganoids alone existed; and in this case, according to the
standard of highness chosen, so will it be said that fishes have
advanced or retrograded in organization. To attempt to compare
members of distinct types in the scale of highness seems hopeless;
who will decide whether a cuttle-fish be higher than a bee — that
insect which the great Von Baer believed to be "in fact more
highly organized than a fish, although upon another type"? In
the complex struggle for life it is quite credible that crustaceans,
not very high in their own class, might beat cephalopods, the high-
est mollusks; 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. Be-
sides these inherent difficulties in deciding which forms are the
most advanced in organization, we ought not solely to compare
the highest members of a class at any two periods — though un-
doubtedly 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 two periods. At an ancient epoch the highest and
lowest moUuscoidal animals, namely, cephalopods and brachio-
pods, swarmed in numbers; at the present time both groups are
greatly reduced, while others, intermediate in organization, have
largely increased; consequently some naturalists maintain that
mollusks were formerly more highly developed than at present;
but a stronger case can be made out on the opposite side, by
considering the vast reduction of brachiopods, and the fact that
our existing cephalopods, though few in number, are more highly
organized than their ancient representatives. We ought also to
compare the relative proportional numbers, at any two periods,
of the high and low classes throughout the world: if, for instance,
at the present day fifty thousand kinds of vertebrate animals
exist, and if we knew that at some former period only ten thou-
sand kinds existed, we ought to look at this increase in number
in the highest class, which implies a great displacement of lower
forms, as a decided advance in the organization of the world. We
thus see how hopelessly difficult it is to compare with perfect
fairness, under such extremely complex relations, the standard
316 THE ORIGIN OF SPECIES
of organization 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 man-
ner in which European productions have recently spread over
New Zealand, and have seized on places which must have been
previously occupied by the indigenes, we must believe, that if all
the animals and plants of Great Britain were set free in New
Zealand, a multitude of British forms would in the course of time
become thoroughly naturalized there, and would 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 pro-
ductions of New Zealand were set free in Great Britain, any
considerable number would be enabled to seize on places now
occupied by our native plants and animals. Under this point of
view, the productions of Great Britain stand much higher in the
scale than those of New Zealand. Yet the most skilful naturalists,
from an examination of the species of the two countries, could not
have foreseen this result.
Agassiz and several other highly competent judges insist that
ancient animals resemble to a certain extent the embryos of recent
animals belonging to the same classes; and that the geological
succession of extinct forms is nearly parallel with the embryo-
logical development of existing forms. This view accords 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, while it leaves the embryo
almost unaltered, continually adds, in the course of successive
generations, more and more difference to the adult. Thus the em-
bryo comes to be left as a sort of picture, preserved by nature, of
the former and less modified condition of the species. This view
may be true, and yet may never be capable of proof. Seeing, for
instance, that the oldest known mammals, reptiles, and fishes
strictly belong to their proper classes, though some of these old
forms are in a slight degree less distinct from each other than are
the typical members of the same groups at the present day, it
would be vain to look for animals having the common embryo-
logical character of the vertebrata, until beds rich in fossils are
discovered far beneath the lowest Cambrian strata — a discovery
of which the chance is small.
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 317
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 mar-
supials of that continent. In South America a similar relationship
is manifest, even to an uneducated eye, in the gigantic pieces of
armor, like those of the armadillo, found in several parts of La
Plata ; and Professor Owen has shown in the most striking manner
that most of the fossil mammals, buried there in such numbers, are
related to South American types. This relationship is even more
clearly seen in the wonderful collection of fossil bones made by
MM. Lund and Clausen in the caves of Brazil. I was so much
impressed with these facts that I strongly insisted, in 1839 and
1845, on this "law of the succession of types," — on "this wonder-
ful relationship in the same continent between the dead and the
living.'^ Professor Owen has subsequently extended the same
generalization to the mammals of the Old World. We see the same
law in this author's restorations of the extinct and gigantic birds
of New Zealand. We see it also in the birds of the caves of Brazil.
Mr. Woodward has shown that the same law holds good with
sea-shells, but, from the wide distribution of most mollusks, 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 continents;
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
318 THE ORIGIN OF SPECIES
partook strongly of the present character of the southern half of
the continent; and the southern half was formerly more closely
allied, than it is at present, to the northern half. In a similar man-
ner we know, from Falconer and Cautley's discoveries, that North-
ern India was formerly more closely related in its mammals to
Africa than it is at the present time. Analogous facts could be
given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of
the long-enduring, but not immutable, succession of the same
types within the same areas, is at once explained; for the in-
habitants oi 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 inter-
vals of time, and after great geographical changes, permitting
much inter-migration, the feebler will yield to the more dominant
forms, and there will be nothing immutable in the distribution
of organic beings.
It may be asked in ridicule whether I suppose that the mega-
therium and other allied huge monsters, which formerly lived in
South America, have left behind them the sloth, armadillo, and
ant-eater, as their degenerate descendants. This cannot for an
instant be admitted. These huge animals have become wholly
extinct, and have left no progeny. But in the caves of Brazil there
are many extinct species which are closely allied in size and in all
other characters to the species still living in South America; and
some of these fossils may have been the actual progenitors of the
living species. It must not be forgotten, that, on our theory, all
the species of the same genus are the descendants of some one
species; so that, if six genera, each having eight species, be found
in one geological formation, and in a succeeding formation there
be six other allied or representative genera, each with the same
number of species, then we may conclude that generally only
one species of each of the older genera has left modified descend-
ants, 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,
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 319
with the genera and species decreasing in numbers as is the case
with the Edentata of South America, still fewer genera and species
will leave modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS
I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geo-
logically explored with care; that only certain classes of organic
beings have been largely preserved in a fossil state; that the num-
ber both of specimens and of species, preserved in our museums,
is absolutely as nothing compared with the number of generations
which must have passed away even during a single formation;
that, owing to subsidence being almost necessary for the accumu-
lation 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 least perfectly
kept; that each single formation has not been continuously de-
posited; that the duration of each formation is probably short
compared with the average duration of specific forms ; that migra-
tion has played an important part in the first appearance of new
forms in any one af ea 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
numerous transitional stages, it is probable that the periods, dur-
ing which each underwent modification, though -many and long
as measured by years, have been short in comparison with the
periods during which each remained in an unchanged condition.
These causes, taken conjointly, will to a large extent explain why
— though we do find many links — ^we do not find interminable
varieties, connecting together all extinct and existing forms by
the finest graduated steps. It should also be constantly borne in
mind that any linking variety between two forms, which might
be found, would be ranked, unless the whole chain could be per-
fectly 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
320 THE ORIGIN OF SPECIES
vain where are the numberless transitional links which must
formerly have connected the closely allied or representative
species, found in the successive stages of the same great forma-
tion? He may disbelieve in the immense intervals of time which
must have elapsed between our consecutive formations; he may
overlook how important a part migration has played, when the
formations of any one great region, as those of Europe, are con-
sidered; he may urge the apparent, but often falsely apparent,
sudden coming in of whole groups of species. He may ask where
are the remains of those infinitely numerous organisms which must
have existed long before the Cambrian system was 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 be-
fore that epoch, the world presented a widely different aspect;
and that the older continents, formed of formations older than
any known to us, exist now only as remnants in a metamorphosed
condition, or lie still buried under the ocean.
Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with
modification through variation and natural selection. We can
thus understand how it is that new species come in slowly and
successively; how species of different classes do not necessarily
change together, or at the same rate, or in the same degree; yet
in the long run that all undergo modification to some extent. The
extinction of old forms is the almost inevitable consequence of
the production of new forms. We can understand why, when a
species has once disappeared, it never reappears. Groups of species
increase in numbers slowly, and endure for unequal periods of
time; for the process of modification is necessarily slow, and de-
pends on many complex contingencies. The dominant species be-
longing to large and dominant groups tend to leave many modi-
fied descendants, which form new sub-groups and groups. As
these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to be-
come extinct together, and to leave no modified offspring on the
face of the earth. But the utter extinction of a whole group of
species has sometimes been a slow process, from the survival of
a few descendants, lingering in protected and isolated situations.
When a group has once wholly disappeared, it does not reappear;
for the link of generation has been broken.
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS i2\
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 aUied, but modified, descendants; and these wil
generally succeed in displacing the groups which are their in-
feriors in the struggle for existence. Hence, after long intervals
of time, the productions of the world appear to have changed
simultaneously.
We can understand how it is that all the forms of life, ancient
and recent, make together a few grand classes. We can under-
stand, from the continued tendency to divergence of character,
why, the more ancient a form is, the more it generally differs from
those now living; why ancient and extinct forms often tend to
fill up gaps between existing forms, sometimes blending two
groups, previously classed as distinct, into one; but more com-
monly bringing them only a little closer together. The more
ancient a form is, the more often it stands in some degree inter-
mediate between groups now distinct; for the more ancient a
form is, the more nearly it will be related to, and consequently
resemble, the common progenitor of groups, since become widely
divergent. Extinct forms are seldom directly intermediate be-
tween existing forms; but are intermediate only by a long and
circuitous course through other extinct and different forms. We
can clearly see why the organic remains of closely consecutive
formations are closely allied; for they are closely linked together
by generation. We can clearly see why the remains of an inter-
mediate formation are intermediate in character.
The inhabitants of the world at each successive period in its
history have beaten their predecessors in the race for life, and
are, in so far, higher in the scale, and their structure has generally
become more speciaHzed; and this may account for the common
belief held by so many palaeontologists, that organization on the
whole has progressed. Extinct and ancient animals resemble to
a certain extent the embryos of the more recent animals belonging
to the same classes, and this wonderful fact receives a simple
explanation according to our views. The succession of the same
types of structures within the same areas during the later geo-
logical periods ceases to be mysterious, and is intelligible on the
principle of inheritance.
If, then, the geological record be as imperfect as many believe,
and it may at least be asserted that the record cannot be proved
to be much more perfect, the main objections to the theory of
natural selection are greatly diminished or disappear. On the
other hand, all the chief laws of palaeontology plainly proclaim,
322 THE ORIGIN OF SPECIES
as it seems to me, that species have been produced by ordinary
generation: old forms having been supplanted by new and im-
proved forms of life, the products of Variation and the Survival of
the Fittest.
CHAPTER XII
GEOGRAPHICAL DISTRIBUTION
Present Distribution cannot be acx:ounted for by Differences in Physical
Conditions — Importance of Barriers — ^Affinity of the Productions of the
Same Continent — Centres of Creation — Means of Dispersal by Changes
of Climate and of the Level of the Land, and by Occasional Means —
Dispersal during the Glacial Period — ^Alternate Glacial Periods in the
North and South.
In considering the distribution of organic beings over the face of
the globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various re-
gions can be wholly accounted for by climatal and other physical
conditions. Of late, almost every author who has studied the sub-
ject has come to this conclusion. The case of America alone would
almost suffice to prove its truth; for if we exclude the arctic and
northern temperate parts, all authors agree that one of the most
fundamental divisions in geographical distribution is that be-
tween the New and the Old Worlds; yet if we travel over the
vast American continent, from the central parts of the United
States to its extreme southern point, we meet with the most
diversified conditions; humid districts, arid deserts, lofty moun-
tains, grassy plains, forests, marshes, lakes and great rivers,
under almost every temperature. There is hardly a climate or con-
dition in the Old World which cannot be paralleled in the New —
at least so closely as the same species generally require. No doubt
small areas can be pointed out in the Old World, hotter than any
in the New World ; but these are not inhabited by a fauna differ-
ent from that of the surrounding districts; for it is rare to find a
group of organisms confined to a small area, of which the condi-
tions are peculiar in only a slight degree. Notwithstanding this
general parallelism in the conditions of 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 degrees, we shall find parts extremely
similar in all their conditions, yet it would not be possible to point
323
324 THE ORIGIN OF SPECIES
out three faunas and floras more utterly dissimilar. Or, again,
we may compare the productions of South America south of lati-
tude 35 degrees with those north of 25 degrees, which conse-
quently are separated by a space of ten degrees of latitude, and
are exposed to considerably different conditions; yet they are
incomparably more closely related to each other than they are
to the productions of Australia or Africa under nearly the same
climate. Analogous facts could be given with respect to the in-
habitants 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 re-
lated in a close and important manner to the differences between
the productions of various regions. We see this in the great differ-
ence 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 cHmate, there might
have been free migration for the northern temperate forms, as
there now is for the strictly arctic productions. We see the same
fact in the great difference between the inhabitants of Australia,
Africa, and South America, under the same latitude; for these
countries are almost as much isolated from each other as is pos-
sible. On each continent, also, we see the same fact; for on the
opposite sides of lofty and continuous mountain-ranges, of great
deserts and even of large rivers, we find different productions;
though as mountain-chains, deserts, etc., are not as impassable,
or likely to have endured so long, as the oceans separating con-
tinents, the differences are very inferior in degree to those char-
acteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabit-
ants of the eastern and western shores of South America are very
distinct, with extremely few shells, crustacea, or echinodermata
in common; but Dr. Gunther has recently shown that about
thirty per cent of the fishes are the same on the opposite sides
of the isthmus of Panama; and this fact has led naturalists to
believe that the isthmus was formerly open. Westward of the
shores of America, a wide space of open ocean extends, with not
an island as a halting-place for emigrants; here we have a barrier
of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific with another and totally distinct
fauna. So that three marine faunas range northward and southward
in parallel lines not far from each other, under corresponding cli-
mate; but from being separated from each other by impassable
GEOGRAPHICAL DISTRIBUTION 325
barriers, either of land or open sea, they are almost wholly distinct.
On the other hand, proceeding still farther westward from the east-
ern islands of the tropical parts of the Pacific, we encounter no im-
passable barriers, and we have innumerable islands as halting-
places, or continuous coasts, until, after travelHng over a hemis-
phere, we come to the shores of Africa; and over this vast space
we meet with no well-defined and distinct marine faunas. Al-
though so few marine animals are common to the above-named
three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes range from the Pacific
into the Indian Ocean, and many shells are common to the eastern
islands of the Pacific and the eastern shores of Africa on almost
exactly opposite meridians of longitude.
A third great fact, partly included in the foregoing statement,
is the affinity of the productions of the same continent or of the
same sea, though the species themselves are distinct at different
points and stations. It is a law of the widest generality, and every
continent offers innumerable instances. Nevertheless, the nat-
uralist, in travelling, for instance, from north to south, never
fails to be struck by the manner in which successive groups of
beings, specifically distinct, though nearly related, replace each
other. He hears from closely allied yet distinct kinds of birds,
notes nearly similar, and sees their nests similarly constructed,
but not quite alike, with eggs colored in nearly the same manner.
The plains near the Straits of Magellan are inhabited by one
species of Rhea (American ostrich), and northward the plains of
La Plata by another species of the same genus ; and not by a true
ostrich or emu, like those inhabiting Africa and Australia under
the same latitude. On these same plains of La Plata we see the
agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits, and belonging to the same order of rodents,
but they plainly display an American type of structure. We as-
cend the lofty peaks of the Cordillera, and we find an alpine
species of bizcacha; we look to the waters, and we do not find the
beaver or muskrat, but the coypu and capybara, rodents of the
South American type. Innumerable other instances could be given.
If we look to the islands off the American shore, however much
they may differ in geological structure, the inhabitants are essen-
tially American, though they may be all peculiar species. We may
look back to past ages, as shown in the last chapter, and we find
American types then prevailing on the American continent and
in the American seas. We see in these facts some deep organic
326 THE ORIGIN OF SPECIES
bond, throughout space and time, over the same areas of land
and water, independently of physical conditions. The naturalist
must be dull who is not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone, as far
as we positively know, produces organisms quite like each other,
or, as we see in the case of varieties, nearly alike. The dissimilarity
of the inhabitants of different regions may be attributed to modi-
fication through variation and natural selection, and probably in
a subordinate degree to the definite influence of different physical
conditions. The degrees of dissimilarity will depend on the migra-
tion of the more dominant forms of life from one region into an-
other 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 rela-
tion of organism to organism in the struggle for life being, as I
have already often remarked, the most important of all relations.
Thus the high importance of barriers comes into play by checking
migration; as does time for the slow process of modification
through natural selection. Widely ranging species, abounding in
individuals, which have already triumphed over many competitors
in their own widely extended homes, will have the best chance of
seizing on new places, when they spread out into new countries.
In their new homes they will be exposed to new conditions, and
will frequently undergo further modification and improvement;
and thus they will become still further victorious, and will produce
groups of modified descendants. On this principle of inheritance
with modification we can understand how it is that sections of
genera, whole genera, and even families, are confined to the same
areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of
the existence of any law of necessary development. As the vari-
ability of each species is an independent property, and will be
taken advantage of by natural selection, only so far as it profits
each individual in its complex struggle for life, so the amount of
modification in different species will be no uniform quantity. If
a number of species, after having long competed with each other
in their old home, were to migrate in a body into a new and after-
ward isolated country, they would be little liable to modification ;
for neither migration nor isolation in themselves effect any thing.
These principles come into play only by bringing organisms into
new relations with each other and in a lesser degree with the sur-
rounding physical conditions. As we have seen in the last chapter
GEOGRAPHICAL DISTRIBUTION 327
that some forms have retained nearly the same character from an
enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly or at all
modified.
According to these views, it is obvious that the several species
of the same genus, though inhabiting the most distant quarters of
the world, must originally have proceeded from the same source,
as they are descended from the same progenitor. In the case of
those species which have undergone, during whole geological pe-
riods, little modification, there is not much difficulty in believing
that they have migrated from the same region; for during the
vast geographical and climatical 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 beHeve that
the species of a genus have been produced within comparatively
recent times, there is great difficulty on this head. It is also ob-
vious that the individuals of the same species, though now in-
habiting 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 distinct.
SINGLE CENTRES OF SUPPOSED CREATION
We are thus brought to the question which has been largely
discussed by naturalists, namely, whether species have been
created at one or more points of the earth's surface. Undoubtedly
there are many cases of extreme difficulty in understanding how
the same species could possibly have migrated from some one
point to the several distant and isolated points where now found.
Nevertheless the simplicity of the view that each species was first
produced within a single region captivates the mind. He who re-
jects it, rejects the vera causa of ordinary generation with subse-
quent migration, and calls in the agency of a miracle. It is univer-
sally admitted, that in most cases the area inhabited by a species is
continuous ; and that when a plant or animal inhabits two points so
distant from each other, or with an interval of such a nature, that
the space could not have been easily passed over by migration, the
fact is given as something remarkable and exceptional. The in-
capacity of migrating across a wide sea is more clear in the case
of terrestrial mammals than perhaps with any other organic be-
ings; and, accordingly, we find no inexplicable instances of the
same mammals inhabiting distant points of the world. No geol-
ogist feels any difficulty in Great Britain possessing the same
328 THE ORIGIN OF SPECIES
quadrupeds with the rest of Europe, for they were no doubt once
united. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe
and Australia or South America? The conditions of life are nearly
the same, so that a multitude of European animals and plants
have become naturalized in America and Australia; and some of
the aboriginal plants are identically the same at these distant
points of the northern and southern hemispheres. The answer, as
I believe, is, that mammals have not been able to migrate,
whereas some plants, from their varied means of dispersal, have
migrated across the wide and broken interspaces. The great and
striking influence of barriers of all kinds, is intelligible only on
the view that the great majority of species have been produced on
one side, and have not been able to migrate to the opposite side.
Some few families, many sub-famihes, very many genera, a still
greater number of sections of genera, are confined to a single re-
gion; 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 con-
tinuous. What a strange anomaly it would be if a directly opposite
rule were to prevail when we go down one step lower in the series,
namely, to the individuals of the same species, and these had not
been, at least at first, confined to some one region!
Hence, it seems to me, as it has to many other naturalists,
that the view of each species having been produced in one area
alone, and having subsequently migrated from that area as far
as its powers of migration and subsistence under past and present
conditions permitted, is the most probable. Undoubtedly many
cases occur in which we cannot explain how the same species
could have passed from one point to the other. But the geograph-
ical and climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the
formerly continuous range of many species. So that we are re-
duced to consider whether the exceptions to continuity of range
are so numerous, and of so grave a nature, that we ought to give
up the belief, rendered probable by general considerations, that
each species has been produced within one area, and has migrated
thence as far as it could. It would be hopelessly tedious to discuss
all the exceptional cases of the same species, now living at distant
and separated points, nor do I for a moment pretend that any
explanation could be offered of many instances. But, after some
preliminary remarks, I will discuss a few of the most striking
GEOGRAPHICAL DISTRIBUTION 329
classes of facts, namely, the existence of the same species on the
summits of distant mountain ranges, and at distant points in the
Arctic and Antarctic regions; and secondly (in the following
chapter), the wide distribution of fresh-water productions; and
thirdly, the occurrence of the same terrestrial species on islands
and on the nearest mainland, though separated by hundreds of
miles of open sea. If the existence of the same species at distant
and isolated points of the earth's surface can in many instances
be explained on the view of each species having migrated from a
single birthplace, then, considering our ignorance with respect to
former climatical and geographical changes, and to the various
occasional means of transport, the belief that a single birthplace
is the law seems to me incomparably the safest.
In discussing this subject we shall be enabled at the same time
to consider a point equally important for us, namely, whether the
several species of a genus which must on our theory all be de-
scended from a common progenitor, can have migrated, undergoing
modification during their migration from some one area. If, when
most of the species inhabiting one region are different from those
of another region, though closely allied to them, it can be shown
that migration from the one region to the other has probably oc-
curred at some former period, our general view will be much
strengthened; for the explanation is obvious on the principle of
descent with modification. A volcanic island, for instance, up-
heaved and formed at the distance of a few hundreds of miles from
a continent, would probably receive from it in the course of time
a few colonists, and their descendants, though modified, would
still be related by inheritance to the inhabitants of that con-
tinent. Cases of this nature are common, and are, as we shall
hereafter see, inexplicable on the theory of independent creation.
This view of the relation of the species of one region to those of
another, does not differ much from that advanced by Mr. Wal-
lace, who concludes that "every species has come into existence
coincident both in space and time with a pre-existing closely allied
species." And it is now well known that he attributes this coinci-
dence to descent with modification.
The question of single or multiple centres of creation differs
from another though allied question; namely, whether all indi-
viduals of the same species are descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from
many individuals simultaneously created. With organic beings
which never intercross, if such exist, each species must be de-
scended from a succession of modified varieties, that have sup-
330 THE ORIGIN OF SPECIES
planted 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 de-
scended from a single parent. But in the great majority of cases,
namely, with all organisms which habitually unite for each birth,
or which occasionally intercross, the individuals of the same
species inhabiting the same area will be kept nearly uniform by
intercrossing; so that many individuals will go on simultaneously
changing, and the whole amount 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 select-
ing and training of many individuals during each generation.
Before discussing the three classes of facts, which I have se-
lected as presenting the greatest amount of difficulty on the theory
of "single centres of creation," I must say a few words on the
means of dispersal.
MEANS OF DISPERSAL
Sir C. Lyell and other authors have ably treated this subject.
I can give here only the briefest abstract of the more important
facts. Change of climate must have had a powerful influence on
migration. A region nov/ impassible to certain organisms from
the nature of its climate, might have been a high road for migra-
tion, when the climate was different. I shall, however, presently
have to discuss this branch of the subject in some detail. Changes
of level in the land must also have been highly influential: a nar-
row isthmus now separates two marine faunas; submerge it, or
let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea
now extends, land may at a former period have connected islands
or possibly even continents together, and thus have allowed ter-
restrial 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 connected with
Europe or Africa, and Europe likewise with America. Other au-
thors have thus hypothetically bridged over every ocean, and
united almost every island with some mainland. If, indeed, the
arguments used by Forbes are to be trusted, it must be admitted
that scarcely a single island exists which has not recently been
united to some continent. This view cuts the Gordian knot of the
GEOGRAPHICAL DISTRIBUTION 331
dispersal of the same species to the most distant points, and re-
moves many a difficulty; but to the best of my judgment we are
not authorized 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 pe-
riod to each other and to the several intervening oceanic islands.
I freely admit the former existence of many islands, now buried
beneath the sea, which may have served as halting-places for
plants and for many animals during their migration. In the coral-
producing oceans such sunken islands are now marked by rings of
coral or atolls standing over them. Whenever it is fully admitted,
as it will some day be, that each species has proceeded from a
single birthplace, and when in the course of time we know some-
thing definite about the means of distribution, we shall be anabled
to speculate with security on the former extension of the land.
But I do not believe that it will ever be proved that within the
recent period most of our continents which now stand quite sepa-
rate, 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 inhabitants of several lands and even seas
to their present inhabitants — the degree of affinity between the
mammals inhabiting islands with those of the nearest continent,
being in part determined (as we shall hereafter see) by the depth
of the intervening ocean — these and other such facts are opposed
to the admission of such prodigious geographical revolutions
within the recent period, as are necessary on the view advanced by
Forbes and admitted by his followers. The nature and relative
proportions of the inhabitants of oceanic islands are likewise op-
posed to the belief of their former continuity of continents. Nor
does the almost universally volcanic composition of such islands
favor the admission that they are the wrecks of sunken continents;
if they had originally existed as continental mountain ranges,
some at least of the islands would have been formed, like other
mountain summits, of granite, metamorphic schists, old fossilifer-
ous and other rocks, instead of consisting of mere piles of vol-
canic 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
332 THE ORIGIN OF SPECIES
works, this or that plant is often stated to be ill adapted for wide
dissemination; but the greater or less facilities for transport
across the sea may be said to be almost wholly unknown. Until
I tried, with Mr, Berkeley's aid, a few experiments, it was not even
known how far seeds could resist the injurious action of sea-
water. To my surprise I found that out of eighty-seven kinds,
sixty-four germinated after an immersion of twenty-eight days,
and a few survived an immersion of 137 days. It deserves notice
that certain orders were far more injured than others; nine
Leguminosae were tried, and, with one exception, they resisted
the salt water badly ; seven species of the allied orders, Hydrophyl-
laceae and Polemoniaceae, were all killed by a month's immersion.
For convenience sake I chiefly tried small seeds without the cap-
sules or fruit; and as all of these sunk in a few days, they could
not have been floated across wide spaces of the sea, whether or
not they were injured by salt water. Afterward I tried some larger
fruits, capsules, etc., and some of these floated for a long time.
It is well known what a difference there is in the buoyancy of
green and seasoned timber; and it occurred to me that floods
would often wash into the sea dried plants or branches with seed-
capsules or fruit attached to them. Hence I was led to dry the
stems and branches of ninety-four plants with ripe fruit, and to
place them on sea-water. The majority sunk quickly, but some
which, while green, floated, for a very short time, when dried,
floated much longer; for instance, ripe hazel-nuts sunk imme-
diately, but when dried they floated for ninety days, and after-
ward when planted germinated; an asparagus plant with ripe
berries floated for twenty-three days, when dried it floated for
eighty-five days, and the seeds afterward germinated; the ripe
seeds of Helosciadium sunk in two days, when dried they floated
for above ninety days, and afterward germinated. Altogether, out
of the ninety-four dried plants, eighteen floated for above twenty-
eight days; and some of the eighteen floated for a very much
longer period. So that as ff kinds of seeds germinated after an
immersion of twenty-eight 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 twenty-eight
days, we may conclude, as far as anything can be inferred from
these scanty facts, that the seeds of -^ kinds of plants of any
country might be floated by sea-currents during twenty-eight
days, and would retain their power of germination. In Johnston's
Physical Atlas, the average rate of the several Atlantic currents
is thirty- three miles per diem (some currents running at the rate
GEOGRAPHICAL DISTRIBUTION 333
of sixty miles per diem) ; on this average, the seeds of ^^ plants
belonging to one country might be floated across 924 miles of
sea to another country, and when stranded, if blown by an in-
land gale to a favorable 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 floating plants. He tried ninety-eight 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 favored both the average length of their flotation and their
resistance to the injurious action of the salt water. On the other
hand, he did not previously dry the plants or branches with the
fruit; and this, as we have seen, would have caused some of them
to have floated much longer. The result was that -Jf of his seeds
of different kinds floated for forty-two days, and were then ca-
pable 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 per-
haps 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 facts of
the larger fruits often floating longer than the small, is interest-
ing; as plants with large seeds or fruit which, as Alph. de Can-
dolle has shown, generally have restricted ranges, could hardly
be transported by any other means.
Seeds may be occasionally transported in another manner.
Drift timber is thrown up on most islands, even on those in the
midst of the widest oceans; and the natives of the coral islands
in the Pacific procure stones for their tools, solely from the roots
of drifted trees, these stones being a valuable royal tax. I find
that when irregularly shaped stones are embedded in the roots of
trees, small parcels of earth are frequently enclosed in their in-
terstices and behind them, so perfectly that not a particle could
be washed away during the longest transport: out of one small
portion of earth thus completely inclosed by the roots of an oak
about fifty years old, three dicotyledonous plants germinated: I
am certain of the accuracy of this observation. Again, I can show
that the carcasses of birds, when floating on the sea sometimes
escape being immediately devoured: and many kinds of seeds in
the crops of floating birds long retain their vitality: pease 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
334 THE ORIGIN OF SPECIES
floated on artificial sea-water for thirty days, to my surprise nearly
all germinated.
Living birds can hardly fail to be highly effective agents in
the transportation of seeds. I could give many facts showing how
frequently birds of many kinds are blown by gales to vast dis-
tances across the ocean. We may safely assume that under such
circumstances their rate of flight would often be thirty-five miles
an hour; and some authors have given a far higher estimate. I
have never seen an instance of nutritious seeds passing through the
intestines of a bird; but hard seeds of fruit pass uninjured through
even the digestive organs of a turkey. In the course of two
months, I picked up in my garden twelve kinds of seeds, out of
the excrement of small birds, and these seemed perfect, and some
of them, which were tried, germinated. But the following fact is
more imE>ortant: the crops of birds do not secrete gastric juice,
and do not, as I know by trial, injure in the least the germination
of seeds; now, after a bird has found and devoured a large supply
of food, it is positively asserted that all the grains do not pass
into the gizzard for twelve or even eighteen hours. A bird in this
interval might easily be blown to the distance of five hundred
miles, and hawks are known to look out for tired birds, and the
contents of their torn crops might thus readily get scattered.
Some hawks and owls bolt their prey whole, and, after an interval
of from twelve to twenty hours, disgorge pellets, which, as I know
from experiment made in the Zoological Gardens, include seeds
capable of germination. Some seeds of the oat, wheat; millet,
canary, hemp, clover, and beet germinated after having been
from twelve to twenty-one hours in the stomachs of different
birds of prey; and two seeds of beet grew after having been thus
retained for two days and fourteen hours. Fresh-water fish, I find,
eat seeds of many land and water plants; fish are frequently de-
voured by birds, and thus the seeds might be transported from
place to place. I forced many kinds of seeds into the stomachs of
dead fish, and then gave their bodies to fishing-eagles, storks and
pelicans; these birds, after an interval of many hours, either re-
jected the seeds in pellets or passed them in their excrement; and
several of these seeds retained the power of germination. Certain
seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from the land.
I myself caught one 370 miles from the coast of Africa, and have
heard of others caught at greater distances. The Rev. R. T. Lowe
informed Sir C. Lyell that in November, 1844, swarms of locusts
visited the island of Madeira. They were in countless numbers.
GEOGRAPHICAL DISTRIBUTION 335
as thick as the flakes of snow in the heaviest snowstorm, and ex-
tended upward as far as could be seen with a telescope. During
two or three days they slowly careered round and round in an
immense ellipse, at least five or six miles in diameter, and at
night alighted on the taller trees, which were 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 in-
sufficient evidence, that injurious seeds are introduced into their
grass-land in the dung left by the great flights of locusts which
often visit that country. In consequence of this belief Mr. Weale
sent me in a letter a small packet of the dried pellets, out of which
I extracted under the microscope several seeds, and raised from
them seven grass plants, belonging to two species, of two genera.
Hence a swarm of locusts, such as that which visited Madeira,
might readily be the means of introducing several kinds of plants
into an island lying far from the mainland.
Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one
grains, and in another case twenty-two grains of dry argillaceous
earth from the foot of a partridge, and in the earth there was a
pebble as large as the seed of a vetch. Here is a better case: the
leg of a woodcock was sent to me by a friend, with a little cake of
dry earth attached to the shank, weighing only nine grains; and
this contained a seed of the toad-rush (Juncus bufonius) which
germinated and flowered. Mr. Swaysland, of Brighton, who dur-
ing the last forty years has paid close attention to our migratory
birds, informs me that he has often shot wagtails (Motacillae),
wheatears, and whinchats (Saxicolae), 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 in-
stance. Professor Newton sent me the leg of a red-legged partridge
(Caccabis rufa) which had been wounded and could not fly, with
a ball of hard earth adhering to it, and weighing six and a half
ounces. The earth had been kept for three years, but when
broken, watered and placed under a bell glass, no less than eighty-
two plants sprung from it: these consisted of twelve monocotyle-
dons, including the common oat, and at least one kind of grass,
and of seventy 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
336 THE ORIGIN OF SPECIES
migrate — for instance, the millions of quails across the Mediter-
ranean— must occasionally transport a few seeds embedded in
dirt adhering to their feet or beaks? But I shall have to recur to
this subject.
As icebergs are known to be sometimes loaded with earth and
stones, and have even carried brushwood, bones, and the nest of
a land-bird, it can hardly be doubted that they must occasionally,
as suggested by Lyell, have transported seeds from one part to
another of the arctic and antarctic regions; and during the Gla-
cial period from one part of the now temperate regions to another.
In the Azores, from the large number of plants common to Eu-
rope, in comparison with the species on the other island 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-born seeds during the Glacial
epoch. At my request Sir C. Lyell wrote to M. Hartung to in-
quire whether he had observed erratic bowlders on these islands,
and he answered that he had found large fragments of granite and
other rocks, which do not occur in the archipelago. Hence we may
safely infer that icebergs formerly landed their rocky burdens on
the shores of these mid-ocean islands, and it is at least possible
that they may have brought thither some few seeds of Northern
plants.
Considering that these several means of transport, and that
other means, which without doubt remain to be discovered, have
been in action year after year for tens of thousands of years, it
would, I think, be a marvellous fact if many plants had not thus
become widely transported. These means of transport are some-
times called accidental; but this is not strictly correct: the cur-
rents of the sea are not accidental, nor is the direction of prevalent
gales of wind. It should be observed that scarcely any means of
transport would carry seeds for very great distances: for seeds
do not retain their vitality when exposed for a great length of
time to the action of sea-water; nor could they be long carried in
the crops or intestines of birds. These means, however, would
suffice for occasional transport across tracts of sea some hundred
miles in breadth, or from island to island, or from a continent to
a neighboring island, but not from one distant continent to an-
other. 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
GEOGRAPHICAL DISTRIBUTION 337
from the West Indies to our western shores, where, if not killed
by their very long immersion in salt water, they could not en-
dure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to
the western shores of Ireland and England; but seeds could be
transported by these rare wanderers only by one means, namely,
by dirt adhering to their feet or beaks, which is in itself a rare
accident. Even in this case, how small would be the chance of a
seed falling on favorable 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 cen-
turies, through occasional means of transport, immigrants from
Europe or any other continent, that a poorly stocked island, though
standing more remote from the mainland, would not receive col-
onists by similar means. Out of a hundred kinds of seeds or
animals transported to an island, even if far less well stocked than
Britain, perhaps not more than one would be so well fitted to its
new home, as to become naturalized. But this is no valid argu-
ment against what would be effected by occasional means of
transport, during the long lapse of geological time, while the is-
land was being upheaved, and before it had become fully stocked
with inhabitants. On almost bare land, with few or no destructive
insects or birds living there, nearly every seed which chanced to
arrive, if fitted for the climate, would germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD
The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where
alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points, without
the apparent possibility of their having migrated from one point
to the other. It is indeed a remarkable fact to see so many plants
of the same species living on the snowy regions of the Alps or
Pyrenees, and in the extreme northern parts of Europe; but it
is far more remarkable, that the plants on the White Mountains,
in the United States of America, are all the same with those of
Labrador, and neaily all the same, as we hear from Asa Gray,
with those on the loftiest mountains of Europe. Even as long ago
as 1747, such facts led Gmelin to conclude that the same species
must have been independently created at many distinct points;
and we might have remained in this same belief, had not Agassiz
and others called vivid attention to the Glacial period, which, as
338 THE ORIGIN OF SPECIES
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 burned by fire do not tell their tale more plainly
than do the mountains of Scotland and Wales, with their scored
flanks, polished surfaces, and perched bowlders, of the icy streams
with which their valleys were lately filled. So greatly has the
climate of Europe changed, that in 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
bowlders 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 follov/ the changes more
readily, by supposing a new glacial period slowly to come on, and
then pass away, as formerly occurred. As the cold came on, and
as each more southern zone became fitted for the inhabitants of
the north, these would take the places of the former inhabitants
of the temperate regions. The latter, at the same time, would
travel farther and farther southward, unless they were stopped
by barriers, in which case they would perish. The mountains
would become covered with snow and ice, and their former alpine
inhabitants would descend to the plains. By the time that the
cold had reached its maximum, we should have an arctic fauna
and flora, covering the central parts of Europe, as far south as
the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered
by arctic plants and animals, and these woulc be nearly the same
with those of Europe; for the present circumpolar inhabitants,
which we suppose to have everywhere travelled southward, are
remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat north-
ward, closely followed up in their retreat by the productions of
the more temperate regions. And as the snow melted from the
bases of the mountains, the arctic forms would seize on the cleared
and thawed ground, always ascending, as the warmth increased
and the snow still further disappeared, higher and higher, while
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
GEOGRAPHICAL DISTRIBUTION 339
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-migra-
tion on the returning warmth, would generally have been due
south and north. The alpine plants, for example, of Scotland, as
remarked by Mr. H. C. Watson, and those of the Pyrenees, as
remarked by Ramond, are more especially allied to the plants of
Northern Scandinavia; those of the United States, to Labrador;
those of the mountains of Siberia, to the arctic regions of that
country. These views, grounded as they are on th^ 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 mounS;ain
summits, we may almost conclude, without other evidence, that
a colder climate formerly permitted their migration across the
intervening lowlands, now become too warm for their existence.
As the arctic forms moved first southward and afterward back-
ward to the north, in unison with the changing climate, they will
not have been exposed during their long migrations to any great
diversity of temperature; and as they all migrated in a body to-
gether, their mutual relations will not have been much disturbed.
Hence, in accordance with the principles inculcated in this volume,
these forms will not have been liable to much modification. But
with the alpine productions, left isolated from the moment of
the returning warmth, first at the bases and ultimately on the
summits of the mountains, the case will have been somewhat
different; for it is not likely that all the same arctic species will
have been left on mountain ranges far distant from each other,
and have survived there ever since; they will also, in all prob-
ability, have become mingled with ancient Alpine species, which
must have existed on the mountains before the commencement of
the Glacial epoch, and which during the coldest period will have
been temporarily driven down to the plains; they will, also, have
been subsequently exposed to somewhat different climatical in-
fluences. Their mutual relations will thus have been in some de-
gree disturbed; consequently they will have been liable to modi-
340 THE ORIGIN OF SPECIES
fication; and they have been modified, for if we compare the
present alpine plants and animals of the several great European
mountain ranges, one with another, though many of the species
remain identically the same, some exist as varieties, some as
doubtful forms of sub-species, and some as distinct yet closely
allied species representing each other on the several ranges.
In the foregoing illustration I have assumed that at the com-
mencement of our imaginary Glacial period, the arctic productions
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 temporate forms were the same round the world, for
some of the species which now exist on the lower mountain slopes
and on the plains of North America and Europe are the same;
and it may be asked how I account for this degree of uniformity
in the sub-arctic and temperate forms round the world, at the
commencement of the real Glacial period. At the present day,
the sub-arctic and northern temperate productions of the Old
and New Worlds are separated from each other by the whole
Atlantic Ocean and by the northern part of the Pacific. During
the Glacial period, when the inhabitants of the Old and New
Worlds lived further southward 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 con-
tinents. The explanation, I believe, lies in the nature of the
climate before the commencement of the Glacial period. At this,
the newer Pliocene period, the majority of the inhabitants of the
world were specifically the same as now, and we have good reason
to believe that the climate was warmer than at the present day.
Hence we may suppose that the organisms which now live under
latitude 60 degrees, lived during the Pliocene period further north,
under the Polar Circle, in latitude 66-67 degrees; and that the
present arctic productions then lived on the broken land still
nearer to the pole. Now, if we look at a terrestrial globe, we see
under the Polar Circle that there is almost continuous land from
Western Europe through Siberia, to Eastern America. And this
continuity of the circumpolar land, with the consequent freedom
under a more favorable climate for intermigration, will account for
the supposed uniformity of the sub-arctic and temperate produc-
tions 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
GEOGRAPHICAL DISTRIBUTION 341
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 PHocene period, a large
number of the same plants and animals inhabited the almost
continuous circumpolar land; and that these plants and animals,
both in the Old and New Worlds, began slowly to migrate south-
ward as the climate became less warm, long before the commence-
ment of the Glacial period. We now see, as I believe, their de-
scendants mostly in a modified condition, in the central parts
of Europe and the United States. On this view we can under-
stand the relationship, with very little identity, between the
productions of North America and Europe, — a relationship which
is highly remarkable, considering the distance of the two areas,
and their separation by the whole Atlantic Ocean. We can further
understand the singular fact remarked on by several observers,
that the productions of Europe and America during the later
tertiary stages were more closely related to each other than they
are at the present time; for during these warmer periods the
northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered
impassable by cold, for inter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period,
as soon as the species in common, which inhabited the New and
Old Worlds, migrated south of the Polar Circle, they will have
been completely cut off from each other. This separation, as far
as the more temperate productions are concerned, must have taken
place long ages ago. As the plants and animals migrated south-
ward, they will have become mingled in the one great region with
the native American productions, and would have had to compete
with them; and in the other great region, with those of the Old
World. Consequently we have here everything favorable for mucb
modification — for tar 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 North America. Hence, it has come, that when we compare
the now living productions of the temperate regions of the New
and Old Worlds, we find very few identical species (though Asa
Gray has lately shown that more plants are identical than was
formerly supposed), but we find in every great class many forms,
which some naturalists rank as geographical races, and others as
distinct species; and a host of closely allied or representative
forms which are ranked by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern
342 THE ORIGIN OF SPECIES
migration of a marine fauna, which, during the Pliocene or even
a somewhat earher period, was nearly uniform along the con-
tinuous 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 understand the
presence of some closely allied, still existing and extinct tertiary
forms, on the eastern and western shores of temperate North
America; and the still more striking fact of many closely allied
crustaceans (as described in Dana's admirable work), some fish
and other marine animals, inhabiting the Mediterranean and the
seas of Japan — these two areas being now completely separated
by the breath 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 con-
vinced that Forbes' view may be largely extended. In Europe we
meet with the plainest evidence of the Glacial period, from the
western shores of Britain to the Ural range, and southward to
the Pyrenees. We may infer from the frozen mammals and nature
of the mountain vegetation, that Siberia was similarly affected.
In the Lebanon, according to Dr. Hooker, perpetual snow formerly
covered the central axis, and fed glaciers which rolled 4,000 feet
down the valleys. The same observer has recently found great
moraines at a low level on the Atlas range in North Africa. Along
the Himalaj''a, 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 continent, on the opposite side of the equator, we
know, from the excellent researches of Dr. J. Haast and Dr. Hec-
tor, that in New Zealand immense glaciers formerly descended to
a low level; and the same plants found by Dr. Hooker on widely
separated m.ountains in this island tell the same story of a former
cold period. From facts communicated to me by the Rev. W. B.
GEOGRAPHICAL DISTRIBUTION 343
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 latitude thirty-six and thirty-seven degrees, and
on the shores of the Pacific, where the climate is now so different,
as far south as latitude forty-six degrees. Erratic bowlders have,
also, been noticed on the Rocky Mountains. In the Cordillera of
South America, nearly under the equator, glaciers once extended
far below their present level. In Central Chili I examined a vast
mound of detritus with great bowlders, crossing the Portillo
Valley, which, there can hardly be a doubt, once formed a huge
moraine; and Mr. D. Forbes informs me that he found in various
parts of the Cordillera, from latitude thirteen to thirty degrees
south, at about the height of 12,000 feet, deeply-furrowed rocks,
resembhng those with which he was familiar in Norway, and like-
wise great masses of detritus, including grooved pebbles. Along
this whole space of the Cordillera true glaciers do not now exist
even at much more considerable heights. Further south, on both
sides of the continent, from latitude forty-one degrees to the
southern -most extremity, we have the clearest evidence of former
glacial action, in numerous immense bowlders transported far from
their parent source.
From these several facts, namely, from the glacial action hav-
ing 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 mem-
oirs, has attempted to show that a glacial condition of climate is
the result of various physical causes, brought into operation by
an increase in the eccentricity of the earth's orbit. All these
causes tend toward the same end ; but the most powerful appears
to be the indirect 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 in-
tervals are extremely severe, owing to certain contingencies, of
which the most important, as Sir C. Lyell has shown, is the relative
position of the land and water. Mr. Croll believes that the last
344 THE ORIGIN OF SPECIES
great glacial period occurred about 240,000 years ago, and en-
dured, with slight alterations of climate, for about 160,000 years.
With respect to more ancient glacial periods, several geologists
are convinced, from direct evidence, that such occurred during
the miocene and eocene formations, not to mention still more
ancient formations. But the most important result for us, arrived
at by Mr. Croll, is that whenever the northern hemisphere passes
through a cold period the temperature of the southern hemisphere
is actually raised, with the winters rendered much milder, chiefly
through changes in the direction of the ocean currents. So con-
versely it will be with the northern hemisphere, while the southern
passes through a glacial period. This conclusion throws so much
light on geographical distribution, that I am strongly inclined to
trust in it: but I will first give the facts which demand an ex-
planation.
In South America, Dr. Hooker has shown that besides many
closely allied species, between forty and fifty of the flowering
plants of Tierra del Fuego, forming no inconsiderable part of its
scanty flora, are common to North America and Europe, enor-
mously 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 belong-
ing to genera characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on
the mountains of Abyssinia. At the Cape of Good Hope a very few
European species, believed not to have been introduced by man,
and on the mountains several representative European forms, are
found which have not been discovered in the 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 neighboring Cameroon Mountains, in the Gulf of
Guinea, are closely related to those on the mountains of Abyssinia,
and likewise to those of temperate Europe. It now also appears,
as I hear from Dr. Hooker, that some of these same temperate
plants have been discovered by the Rev. R. T. Lowe on the moun-
tains of the Cape Verde Islands. This extension of the same tem-
perate forms, almost under the equator, across the whole continent
of Africa and to the mountains of the Cape Verde archipelago, is
GEOGRAPHICAL DISTRIBUTION 345
one of the most astonishing facts ever recorded in the distribution
of plants.
On the Himalaya, and on the isolated mountain ranges of the
peninsula of India, on the heights of Ceylon and on the volcanic
cones of Java, many plants occur either identically the same or
representing each other, and at the same time representing plants
of Europe not found in the intervening hot lowlands. A list of the
genera of plants collected on the loftier peaks of Java, raises a
picture of a collection made on a hillock in Europe. Still more
striking is the fact that peculiar Australian forms are represented
by certain plants growing on the summits of the mountains of
Borneo. Some of these Australian forms, as I hear from Dr.
Hooker, extend along the heights of the peninsula of Malacca, and
are thinly scattered on the one hand over India, and on the
other hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Miiller has dis-
covered 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 Aus-
tralia, but not in the intermediate torrid regions. In the admirable
"Introduction to the Flora of New Zealand," by Dr. Hooker,
analogous and striking facts are given in regard to the plants of
that large island. Hence, we see that certain plants growing on
the more lofty mountains of the tropics in all parts of the world,
and on the temperate plains of the north and south, are either the
same species or varieties of the same species. It should, however,
be observed that these plants are not strictly arctic forms ; for, as
Mr. H. C. Watson has remarked, "in receding from polar toward
equatorial latitudes, the alpine or mountain flora really become
less and less arctic." Besides these identical and closely allied
forms, many species inhabiting the same widely sundered areas,
belong to genera not now found in the intermediate tropical low-
lands.
These brief remarks apply to plants alone; but some few anal-
ogous facts could be given in regard to terrestrial animals. In
marine productions, similar cases likewise occur; as an example,
I may quote a statement by the highest authority. Professor Dana,
that "it is certainly a wonderful fact that New Zealand should
have a closer resemblance in its Crustacea to Great Britain, its
antipode, than to any other part of the world." Sir J. Richardson,
also, speaks of the reappearance on the shores of New Zealand,
Tasmania, etc., of northern forms of fish. Dr. Hooker informs me
that twenty-five species of algae are common to New Zealand and
346 THE ORIGIN OF SPECIES
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 Archipel-
ago, 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 considerable number of temperate
forms. At this period the equatorial climate at the level of the sea
was probably about the same with that now experienced at the
height of from five to six thousand feet under the same latitude_,
or perhaps even rather cooler. During this, the coldest period, the
lowlands under the equator must have been clothed with a min-
gled 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 per-
haps a still greater preponderance of temperate forms. So again in
the mountainous island of Fernando Po, in the Gulf of Quinea,
Mr. Mann found temperate European forms beginning to appear
at the height of about five thousand feet. On the mountains of
Panama, at the height of only two thousand feet. Dr. Seemann
found the vegetation like that of Mexico, "with forms of the torrid
zone harmoniously blended with those of the temperate."
Now let us see whether Mr. CroU's conclusion that when the
northern hemisphere suffered from the extreme cold of the great
Glacial period, the southern hemisphere was actually warmer,
throws any clear light on the present apparently inexplicable dis-
tribution of various organisms in the temperate parts of both hemi-
spheres, and on the mountains of the tropics. The Glacial period,
as m.easured by years, must have been very long; and when we
remember over what vast spaces some naturalized plants and ani-
mals have spread within a few centuries, this period will have
been ample for any amount of migration. As the cold became more
and more intense, we know that arctic forms invaded the temper-
ate regions; and, from the facts just given, there can hardly be
a doubt that some of the more vigorous, dominant, and widest-
spreading temperate forms invaded the equatorial lowlands. The
inhabitants of these hot lowlands would at the same time have
migrated to the tropical and subtropical regions of the south, for
the southern hemisphere was at this period warmer. On the de-
cline of the Glacial period, as both hemispheres gradually recov-
GEOGRAPHICAL DISTRIBUTION 347
ered their former temperature, the northen temperate forms liv-
ing on the lowlands under the equator, would have been driven to
their former homes or have been destroyed, being replaced by the
equatorial forms returning from the south. Some, however, of the
northern temperate forms would almost certainly have ascended
any adjoining high land, where, if sufficiently lofty, they would
have long survived like the arctic forms on the mountains of
Europe. They might have survived, even if the climate was not
perfectly fitted for them, for the change of temperature must
have been very slow, and plants undoubtedly possess a certain
capacity for acclimatization, as shown by their transmitting to
their offspring different constitutional powers of resisting heat
and cold.
In the regular course of events the southern hemisphere would
in its turn be subjected to a severe Glacial period, with the north-
ern hemisphere rendered warmer; and then the southern temper-
ate form? would invade the equatorial lowlands. The northern
forms which had before been left on the mountains would now
descend and mingle with the southern forms. These latter, when
the warmth returned, would return to their former homes, leaving
some few species on the mountains, and carrying southward with
them some of the northern temperate forms which had descended
from their mountain fastnesses. Thus, we should have some few
species identically the same in the northern and southern temper-
ate zones and on the mountains of the intermediate tropical re-
gions. But the species left during a long time on these mountains,
or in opposite hemispheres, would have to compete with many
new forms, and would be exposed to somewhat different physical
conditions: hence, they would be eminently liable to modification,
and would generally now exist as varieties or as representative
species; and this is the case. We must, also, bear in mind the oc-
currence 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 CandoUe in regard to Aus-
tralia, that many more identical or slightly modified species have
migrated from the north to the south, than in a reversed direc-
tion. We see, however, a few southern forms on the mountains of
Borneo and Abyssinia. I suspect that this preponderant migration
from the north to the south is due to the greater extent of land
348 THE ORIGIN OF SPECIES
in the north, and to the northern forms having existed in their own
homes in greater numbers, and having consequently been ad-
vanced 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 alternations of the Glacial periods, the north-
ern forms were the more powerful and were able to hold their
places on the mountains, and afterward to migrate southward
with the southern forms; but not so the southern in regard to the
northern forms. In the same manner, at the present day, we see
that very many European productions cover the ground in La
Plata, New Zealand, and to a lesser degree in Australia, and have
beaten the natives; whereas extremely few southern forms have
become naturalized in any part of the northern hemisphere, though
hides, wool, and other objects likely to carry seeds have been
largely imported into Europe during the last two or three cen-
turies from La Plata, and during the last forty or fifty ^^ears from
Australia. The Neilgherrie Mountains in India, howei^er, offer a
partial exception; for here, as I hear from Dr. Hooker. Australian
forms are rapidly sowing themselves, and becomir-i naturalized.
Before the last great Glacial period, no doubt the inter-tropical
mountains were stocked with endemic alpine forms; but these
have almost everywhere yielded to the more dominant forms gen-
erated in the larger areas and more efficient workshops of the
north. In many islands the native productions are nearly equalled,
or even outnumbered, by those which have become naturalized;
and this is the first stage toward their extinction. Mountains are
islands on the land, and their inhabitants have yielded to those
produced within the larger areas of the north, just in the same
way as the inhabitants of real islands have everywhere yielded
and are still yielding to continental forms naturalized through
man's agency.
The same principles apply to the distribution of terrestrial ani-
mals and of marine productions, in the northern and southern
temperate zones, and on the inter-tropical mountains. When, dur-
ing the height of the Glacial period, the ocean-currents were
widely different to what they now are, some of the inhabitants of
the temperate seas might have reached the equator; of these a
few would perhaps at once be able to migrate southward, by
keeping to the cooler currents, while others might remain and sur-
vive in the colder depths until the southern hemisphere was in
its turn subjected to a glacial climate and permitted their further
progress; in nearly the same manner as, according to Forbes,
GEOGRAPHICAL DISTRIBUTION 349
isolated spaces inhabited by arctic productions exist to the present
day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and some-
times on the intermediate mountain-ranges, are removed on the
views above given. The exact lines of migration cannot be in-
dicated. We cannot say why certain species and not others have
migrated; why certain species have been modified and have given
rise to new forms, while others have remained unaltered. We can-
not hope to explain such facts, until we can say why one species
and not another becomes naturalized by man's agency in a foreign
land ; why one species ranges twice or thrice as far, and is twice or
thrice as common, as another species within their own homes.
Various special difficulties also remain to be solved; for in-
stance, the occurrence, as shown by Dr. Hooker, of the same
plants at points so enormously remote as Kerguelen Land, New
Zealand, and Fuegia; but icebergs, as suggested by Lyell, may
have been concerned in their dispersal. The existence at these and
other distant points of the southern hemisphere, of species, which,
though distinct, belong to genera exclusively confined to the south,
is a more remarkable case. Some of these species are so distinct,
that we cannot suppose that there has been time since the com-
mencement of the last Glacial period for their migration and sub-
sequent modification to the necessary degree. The facts seem to
indicate that distinct species belonging to the same genera have
migrated in radiating lines from a common centre; anr I am in-
clined to look in the southern, as in the northern hemisphere, to
a former and warmer period, before the commencement of the last
Glacial period, when the antarctic lands, now covered with ice,
supported a highly peculiar and isolated flora. It may be suspected
that before this flora was exterminated during the last Glacial
epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of trans-
port, 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 Gla-
cial periods in the one hemisphere coincide with warmer periods
in the opposite hemisphere, together with the admission of the
350 THE ORIGIN OF SPECIES
slow modification of species, explains a multitude of facts in the
distribution of the same and of the allied forms of life in all parts
of the globe. The living waters have flowed during one period from
the north and during another from the south, and in both cases
have reached the equator; but the stream of life has flowed with
greater force from the north than in the opposite direction, and
has consequently more freely inundated the south. As the tide
leaves its drift in horizontal lines, rising higher on the shores
where the tide rises highest, so have the living waters left their
living drift on our mountain summits, in a line gently rising from
the arctic lowlands to a great altitude under the equator. The
various beings thus left stranded may be compared with savage
races of man, driven up and surviving in the mountain fastnesses
of almost every land, which serves as a record, full of interest to
us, of the former inhabitants of the surrounding lowlands.
I
CHAPTER XIII
Geographical Distribution — continued
Distribution of Fresh-water Productions — On the Inhabitants of Oceanic
Islands — ^Absence of Batrachians and of Terrestrial Mammals — On the
Relation of the Inhabitants of Islands to those of the Nearest Mainland
— On Colonization from the Nearest Source with Subsequent Modifica-
tion— Summary of the Last and Present Chapters.
FRESH-WATER PRODUCTIONS
As lakes and river systems are separated from each other by bar-
riers of land, it might have been thought that fresh-water produc-
tions would not have ranged widely within the same country, and
as the sea is apparently a still more formidable barrier, that they
would never have extended to distant countries. But the case is
exactly the reverse. Not only have many fresh-water species be-
longing to different classes, an enormous range, but allied species
prevail in a remarkable manner throughout the world. When first
collecting in the fresh waters of Brazil, I well remember feeling
much surprise at the similarity of the fresh-water insects, shells,
etc., and at the dissimilarity of the surrounding terrestrial beings,
compared with those of Britain.
But the wide ranging power of fresh-water productions can, I
think, in most cases be explained by their having become fitted,
in a manner highly useful to them, for short and frequent migra-
tions from pond to pond, or from stream to stream, within their
own countries; and liability to wide dispersal would follow from
this capacity as an almost necessary consequence. We can here
consider only a few cases; of these, some of the most difficult to
explain are presented by fish. It was formerly believed that the
same fresh-water species never existed on two continents distant
from each other. But Dr. Giinther has lately shown that the
Galaxias attenuatus inhabits Tasmania, New Zealand, the Falk-
land Islands, and the mainland of South America. This is a won-
derful case, and probably indicates dispersal from an antarctic
centre during a former warm period. This case, however, is ren-
dered in some degree less surprising by the species of this genus
351
352 THE ORIGIN OF SPECIES
having the power of crossing by some unknown means consid-
erable 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 re-
moval 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 differences of the fish on the opposite
sides of most mountain-ranges, which are continuous and conse-
quently must, from an early period, have completely prevented
the inosculation of the river-systems on the two sides, leads to the
same conclusion. Some fresh-water fish belong to very ancient
forms, and in such cases there will have been ample time for great
geographical changes, and consequently time and means for much
migration. Moreover, Dr. Gunther has recently been led by several
considerations to infer that with fishes the same forms have a 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 confined to fresh
water, so that a marine species belonging to a fresh-water group
might travel far along the shores of the sea, and could, it is prob-
able, become adapted without much difficulty to the fresh waters
of a distant land.
Some species of fresh-water shells have very wide ranges, and
allied species which, on our theory, are descended from a common
parent, and must have proceeded from a single source, prevail
throughout the world. Their distribution at first perplexed me
much, as their ova are not likely to be transported by birds; and
the ova, as well as the adults, are immediately killed by sea-
water. I could not even understand how some naturalized species
have spread rapidly throughout the same country. But two facts,
which I have observed — and many others no doubt will be discov-
ered— throw some light on this subject. When ducks suddenly
emerge from a pond covered with duck-weed, I have twice seen
these little plants adhering to their backs ; and it has happened to
GEOGRAPHICAL DISTRIBUTION 353
me, in removing a little duck-weed from one aquarium to an-
other, that I have unintentionally stocked the one with fresh-
water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where
many ova of fresh-water shells were hatching; and I found that
numbers of the extremely minute and just-hatched shells crawled
on the feet, and clung to them so firmly that when taken out of
the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily. drop off. These just-hatched
mollusks, though aquatic in their nature, survived on the duck's,
feet, in damp air, from twelve to twenty hours; and in this length
of time a duck or heron might fly at least six or seven hundred
miles, and if blown across the sea to an oceanic island, or to any
other distant point, would be sure to alight on a pool or rivulet.
Sir Charles Lyell informs me that a dytiscus has been caught with
an ancylus (a fresh-water shell like a limpet) firmly adhering to
it; and a water-beetle of the same family, a colymbetes, once
flew on board the Beagle, when forty-five miles distant from the
nearest land: how much farther it might have been blown by a
favoring gale, no one can tell.
With respect to plants, it has long been known what enormous
ranges many fresh-water and even marsh species have, both over
continents and to the most remote oceanic islands. This is strik-
ingly illustrated, according to Alph. de CandoUe, in those large
groups of terrestrial plants, which have very few aquatic members;
for the latter seem immediately to acquire, as if in consequence,
a wide range. I think favorable 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 likely to have muddy feet. Birds of this order wander
more than those of any other; and they are occasionally found
on the most remote and barren islands of the open ocean; they
would not be likely to alight on the surface of the sea, so that any
dirt on their feet would not be washed off; and when gaining the
land, they would be sure to fly to their natural fresh-water haunts.
I do not believe that botanists are aware how charged the mud
of ponds is with seeds; I have tried several little experiments, but
will here give only the most striking case : I took in February three
tablespoonfuls of mud from three different points, beneath water,
on the edge of a little pond; this mud when dried weighed only
six and three-fourths ounces; I kept it covered up in my study
for six months, pulling up and counting each plant as it grew; the
354 THE ORIGIN OF SPECIES
plants were of many kinds, and were altogether 537 in number;
and yet the viscid mud was all contained in a breakfast cup I
Considering these facts, I think it would be an inexplicable cir-
cumstance if water birds did not transport the seeds of fresh-water
plants to unstocked ponds and streams, situated at very distant
points. The same agency may have come into play with the eggs
of some of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part.
I have stated that fresh- water fish eat some kinds of seeds, though
they reject many other kinds after having swallowed them; even
small fish swallow seeds of moderate size, as of the yellow water-
lily and Potamogeton. Herons and other birds, century after cen-
tury, 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
fiiat seeds retain their power of germination, when rejected many
hours afterward in pellets or in the excrement. When I saw the
great size of the seeds of that fine water-lily, the Nelumbium, and
remembered Alph. de Candolle'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
Nelumbium luteum) in a heron's stomach. Now this bird must
often have flown with its stomach thus well stocked to distant
ponds, and then, getting a hearty meal of fish, analogy makes me
believe that it would have rejected the seeds in the pellet in a fit
state for germination.
In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for in-
stance, on a rising islet, it will be unoccupied; and a single seed
or egg will have a good chance of succeeding. Although there will
always be a struggle for life between the inhabitants of the same
pond, however few in kind, yet as the number even in a well-
stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them
will probably be less severe than between terrestrial species; con-
sequently an intruder from the waters of a foreign country would
have a better chance of seizing on a new place, than in the case
of terrestrial colonists. We should also remember that many fresh-
water productions are low in the scale of nature, and we have
reason to believe that such beings become modified more slowly
than the high ; and this will give time for the migration of aquatic
species. We should not forget the probability of many fresh-water
forms having formerly ranged continuously over immense areas.
GEOGRAPHICAL DISTRIBUTION 3SS
and then having become extinct at intermediate points. But the
wide distribution of fresh-water plants, and of the lower animals,
whether retaining the same identical form, or in some degree
modified, apparently depends in main part on the wide dispersal
of their seeds and eggs by animals, more especially by fresh-
water birds, which have great powers of flight, and naturally
travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS
We now come to the last of the three classes of- facts, whick
I have selected as presenting the greatest amount of difficulty
with respect to distribution, on the view that not only all the in-
dividuals of the same species have migrated from some one area, ^
but that allied species, although now inhabiting the most distant
points, have proceeded from a single area, the birthplace of their
early progenitors, I have already given my reasons for disbelicT-
ing in continental extensions within the period of existing species
on so enormous a scale that all the many islands of the several
oceans were thus stocked with their present terrestrial inhabitants.
This view removes many difficulties, but it does not accord witJi
all the facts in regard to the productions of islands. In the fol-
lowing 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 islands are few
in number compared with those on equal continental areas: Alph.
de Candolle admits this for plants, and Wollaston for insects.
New Zealand, for instance, with its lofty mountains and diversi-
fied stations, extending over 780 miles of latitude, together with
the outlying islands of Auckland, Campbell, and Chatham, con-
tain altogether only 960 kinds of flowering plants; if we compare
this moderate number with the species which swarm over equal
areas in Southwestern Australia or at the Cape of Good Hopcj,
we must admit that some cause, independently of different physi-
cal conditions, has given rise to so great a difference in number.
Even the uniform county of Cambridge has 847 plants, and the
little island of Anglesea 764, but a few ferns and a few intro-
duced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that
the barren island of Ascension aboriginally possessed less than
half a dozen flowering plants; yet many species have now be-
come naturalized on it, as they have in New Zealand and on
356 THE ORIGIN OF SPECIES
every other oceanic island which can be named. In St. Helena
there is reason to believe that the naturalized plants and animals
have nearly or quite exterminated many native productions. He
who admits the doctrine of the creation of each separate species,
will have to admit that a sufficient number of the best-adapted
plants and animals were not created for oceanic islands; for man
has unintentionally stocked them far more fully and perfectly
than did nature.
Although in oceanic islands the species are few in number the
proportion of endemic kinds (i. e., those found nowhere else in
the world) is often extremely large. If we compare, for instance,
the number of 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 occa-
sionally 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 difference seems to depend partly on
the species 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 unmodified immigrants from the
mother-country, with which the insular forms have intercrossed.
It should be borne in mind that the offspring of such crosses would
certainly gain in vigor; so that even an occasional cross would
produce more effect than might have been anticipated. I will give
a few illustrations of the foregoing remarks: in the Galapagos
Islands there are twenty-six land birds; of these, twenty-one
(or perhaps twenty- three) are peculiar, whereas of the eleven
marine birds only two are peculiar; and it is obvious that marine
birds could arrive at those islands much more easily and fre-
quently than land birds. Bermuda, on the other hand, which lies
at about the same distance from North America as the Gala-
pagos 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' 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.
Harcourt, many European and African birds are blown to Ma-
GEOGRAPHICAL DISTRIBUTION 357
deira; this island is inhabited by ninety-nine 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 neighboring continents with birds, which
for long ages have there struggled together, and have become
mutually co-adapted. Hence, when settled in their new homes,
each kind will have been kept by the others to its proper place
and habits, and will consequently have been but little liable to
modification. Any tendency to modification will also have been
checked by intercrossing with the unmodified immigrants, often
arriving from the mother-country. Madeira again is inhabited by
a wonderful number of peculiar land-shells, whereas not one
species of sea-shell is peculiar to its shores: now, though we do
not know how sea-shells are dispersed, yet we can see that their
eggs or larvae, perhaps attached to sea-weed or floating timber,
or to the feet of wading birds, might be transported across three
or four hundred miles of open sea far more easily than land-
shells. The different orders of insects inhabiting Madeira present
nearly parallel 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. Al-
though 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 character and the direction of its
mountain ranges, the Rev. W. B. Clarke has lately maintained
that this island, as well as New Caledonia, should be considered
as appurtenances of Australia. Turning to plants. Dr. Hooker has
shown that in the Galapagos Islands the proportional numbers
of the different orders are very different from what they are else-
where. All such differences in number, and the absence of certain
whole groups of animals and plants, are generally accounted for
by supposed differences in the physical conditions of the islands;
but this explanation is not a little doubtful. Facility of immigra-
tion seems to have been fully as important as the nature of the
conditions.
Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands
not tenanted by a single mammal, some of the endemic plants
have beautifully hooked seeds; yet few relations are more mani-
358 THE ORIGIN OF SPECIES
fest 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 appendage, like the shrivelled wings under
the soldered wing-covers of many insular beetles. Again, islands
often possess trees or bushes belonging to orders which elsewhere
include only herbaceous species; now trees, as Alph. de CandoUe
has shown, generally have, whatever 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 over-topping them. In this case, natural selection would
tend to add to the stature of the plant, to whatever order it be-
longed, and thus first convert it into a bush and then into a tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON
OCEANIC ISLANDS
With respect to the absence of whole orders of animals on
oceanic islands, Bory St. Vincent long ago remarked that Ba-
trachians (frogs, toads, newts) are never found on any of the
many islands with which the great oceans are studded. I have
taken pains to verify this assertion, and have found it true, with
the exception of New Zealand, New Caledonia, the Andaman
Islands, and perhaps the Solomon Islands and the Seychelles. But
I have already remarked that it is doubtful whether New Zealan4
and New Caledonia ought to be classed as oceanic islands; and
this is still more doubtful with respect to the Andaman and
Solomon groups and the Seychelles. This general absence of frogs,
toads, and newts on so many true oceanic islands cannot be ac-
counted for by their physical conditions: indeed, it seems that
islands are peculiarly fitted for these animals; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and
their spawn are immediately killed (with the exception, as far
as known, of one Indian species) by sea-water, there would be
great difficulty in their transportal across the sea, and 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 explain.
Mammals offer another and similar case. I have carefully
GEOGRAPHICAL DISTRIBUTION 359
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 about
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
a distance of about 280 miles; moreover, icebergs formerly brought
bowlders to its western shores, and they may have formerly
transported foxes, as now frequently happens in the arctic re-
gions. 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 naturalized 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 ap-
pear and disappear at a quicker rate than other and lower ani-
mals. 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: Nor-
folk Island, the Viti Archipelago, the Bonin Islands, the Caroline
and Marianne Archipelagoes, and Mauritius, all possess their
peculiar bats. Why, it may be asked, has the supposed creative
force produced bats and no other mammals on remote islands?
On my view this question can easily be answered; for no ter-
restrial 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 islands.
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 mammals.
Another interesting relation exists, namely, between the depth
360 THE ORIGIN OF SPECIES
of the sea separating islands from each other, or from the nearest
continent, and the degree of affinity of their mammahan inhabit-
ants. 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 is-
lands stand on a moderately shallow submarine bank, and these
islands are inhabited by the same or by closely allied quadrupeds.
I have not as yet had time to follow up this subject in all quarters
of the world; but as far as I have gone, the relation holds good.
For instance, Britain is separated by a shallow channel from
Europe, and the mammals are the same on both sides; and so
it is with all the islands near the shores of Australia. The West
Indian Islands, on the other hand, stand on a deeply submerged
bank, nearly one thousand fathoms in depth, and here we find
American forms, but the species and even the genera are quite
distinct. As the amount of modification which animals of all
kinds undergo partly depends on the lapse of time, and as the
islands which are separated from each other, or from the main-
land, by shallow channels, are more likely to have been con-
tinuously united within a recent period than the islands separated
by deeper channels, we can understand how it is that a relation
exists between the depth of the sea separating two mammalian
faunas, and the degree of their affinity, a relation which is quite
inexplicable on the theory of independent acts of creation.
The foregoing statements in regard to the inhabitants of
oceanic islands, namely, the fewness of the species, with a large
proportion consisting of endemic forms — the members of certain
groups, but not those of other groups in the same class, having
been modified — the absence of certain whole orders, as of batra-
chians and of terrestrial mammals, notwithstanding the presence
of aerial bats, the singular proportions of certain orders of plants,
herbaceous forms having been developed into trees, etc., seem
to me to accord better with the belief in the efficiency of occa-
sional means of transport, carried on during a long course of time,
than with the belief in the former connection of all oceanic
islands with the nearest continent; for on this latter view it is
probable that the various classes would have immigrated more
uniformly, and from the species having entered in a body, their
mutual relations would not have been much disturbed, and, con-
sequently, they would either have not been modified, or all the
species in a more equable manner.
I
GEOGRAPHICAL DISTRIBUTION 361
I do not deny that there are many and serious difficulties in
understanding how many of the inhabitants of the more remote
islands, whether still retaining the same specific form or subse-
quently 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 difficult case. Almost all oceanic islands, even the
most isolated and smallest, are inhabited 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 efficient, means for their transportal. Would the
just-hatched young sometimes adhere to the feet of birds roost-
ing on the ground and thus get transported? It occurred to me
that land-shells, when hibernating and having a membraneous
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 hibernating,
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 membraneous one, I
again immersed it for fourteen days in sea-water, and again it
recovered and crawled away. Baron Aucapitaine has since tried
similar experiments. He placed 100 land-shells, belonging to ten
species, in a box pierced with holes, and im.mersed it for a fort-
night in the sea. Out of the hundred shells twenty-seven recovered.
The presence of an operculum seems to have been of importance,
as out of twelve specimens of Cyclostoma elegans, which is thus
furnished, eleven revived. It is remarkable, seeing how well the
Helix pomatia resisted with me the salt water, that not one of
fifty-four specimens belonging to four other species of Helix tried
by Aucapitaine recovered. It is, however, not at all probable that
land-shells have often been thus transported; the feet of birds
offer a more probable method.
362 THE ORIGIN OF SPECIES
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO
THOSE OF THE NEAREST MAINLAND
The most striking and important fact for us is the affinity of
the species which inhabit islands to those of the nearest main-
land, 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 con-
tinent. 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 naturalist, looking at the inhabitants of these
volcanic islands in the Pacific, distant several hundred miles from
the continent, feels that he is standing on American land. Why
should this be so? Why should the species which are supposed
to have been created in the Galapagos 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 geo-
logical nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together,
which closely resembles the conditions of the South American
coast. In fact, there is a considerable dissimilarity in all these re-
spects. On the other hand, there is a considerable degree of re-
semblance 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 inhabitants 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 or-
dinary view of independent creation; whereas, on the view here
maintained, it is obvious that the Galapagos Islands would be
likely to receive colonists from America, whether by occasional
means of transport or (though I do not believe in this doctrine)
by formerly continuous land, and the Cape Verde Islands from
Africa; such colonists would be liable to modification — the prin-
ciple of inheritance still betraying their original birthplace.
Many analogous facts could be given: indeed, it is an almost
GEOGRAPHICAL DISTRIBUTION 363
universal rule that the endemic productions of islands are re-
lated to those of the nearest continent, or of the nearest large
island. The exceptions are few, and most of them can be ex-
plained. Thus, although Kerguelen Land stands nearer to Africa
than to America, the plants are related, and that very closely,
as we know from Dr. Hooker's account, to those of America: but
on the view that this island has been mainly stocked by seeds
brought with earth and stones on icebergs, drifted by the pre-
vailing currents, this anomaly disappears. New Zealand in its
endemic plants is much more closely related to Australia, the
nearest mainland, than to any other region: and this is what
might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is so enor-
mously remote, that the fact becomes an anomaly. But this diffi-
culty 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 Antarctic Islands, when they were clothed with vegetation,
during a warmer tertiary period, before the commencement of
the last Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of the south-
western comer of Australia and of the Cape of Good Hope, is
a far more remarkable case; but this affinity is confined to the
plants, and will, no doubt, some day be explained.
The same law which has determined the relationship between
the inhabitants of islands and the nearest mainland, is sometimes
displayed on a small scale, but in a most interesting manner,
within the limits of the same archipelago. Thus each separate is-
land of the Galapagos Archipelago is tenanted, and the fact is a
marvellous one, by many distinct species; but these species are
related to each other in a very much closer manner than to the
inhabitants of the American continent, 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, the islands situated
within sight of each other, having the same geological nature, the
same height, climate, etc.? This long appeared to me a great
difficulty: 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 im-
364 THE ORIGIN OF SPECIES
portant, and generally a far more important element of success.
Now, if we look to the species which inhabit the Galapagos
Archipelago, and are likewise found in other parts of the world,
we find that they differ considerably in the several islands. This
difference might indeed have been expected if the islands have
been stocked by occasional means of transport — a seed, for in-
stance, of one plant having been brought to one island, and that
of another plant to another island, though all proceeding from
the same general source. Hence, when in former times an im-
migrant first settled on one of the islands, or when it subsequently
spread from one to another, it would undoubtedly be exposed to
different conditions in the different islands, for it would have to
compete with a different set of organisms; a plant, for instance,
would find the ground best fitted for it occupied by somewhat
different species in the different islands, and would be exposed to
the attacks of somewhat different enemies. If, then, it varied,
natural selection would probably favor different varieties in the
different islands. Some species, however, might spread and yef
retain the same character throughout the group, just as we see
some species spreading widely throughout a continent and re-
maining the same.
The really surprising fact in this case of the Galapagos Archi-
pelago, and in a lesser degree in some analogous cases, is that
each new species, after being formed in any one island, did not
spread quickly to the other islands. But the islands, though in
sight of each other, are separated by deep arms of the sea, in
most cases wider 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 deep between the
islands, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than
they appear on a map. Nevertheless, some of the species, both of
those found in other parts of the world and of those confined to
the archipelago, are common to the several islands; and we may
infer from the 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 in-
vading each other's territory, when put into free intercommunica-
tion. Undoubtedly, if one species has any advantage over another,
it will in a very brief time wholly or in part supplant it; but if
both are equally well fitted for their own places, both will prob-
ably hold their separate places for almost any length of tim.e.
Being familiar with the fact that many species, naturahzed
GEOGRAPHICAL DISTRIBUTION 36S
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
naturalized in new countries are not generally closely allied to
the aboriginal inhabitants, but are very distinct forms, belong-
ing 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 an-
nually more eggs are laid and young birds hatched than can pos-
sibly be reared; and we may infer that the mocking-thrush
peculiar to Charles Island is at least as well fitted for its home
as is the species peculiar to Chatham Island. Sir C. Lyell and
Mr. WoUaston have communicated to me a remarkable fact bear-
ing 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 al-
though large quantities of stone are annually transported from
Porto Santo to Madeira, yet this latter island has not become
colonized by the Porto Santo species; nevertheless, both islands
have been colonized by European land-shells, which no doubt had
some advantage over the indigenous species. From these considera-
tions 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 con-
tinent, 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 conditions. Thus,
the south-east and south-west corners of Australia have nearly
the same physical conditions, and are united by continuous land,
yet they are inhabited by a vast number of distinct mammals,
birds, and plants; so it is, according to Mr. Bates, with the butter-
flies and other animals inhabiting the great, open, and continuous
valley of the Amazons.
The same principle which governs the general character 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
366 THE ORIGIN OF SPECIES
throughout nature. We see this on every mountain-summit, in
every lake and marsh. For alpine species, excepting in as far as
the same species have become widely spread during the Glacial
epoch, are related to those of the surrounding lowlands; thus we
have in South America, alpine humming-birds, alpine rodents,
alpine plants, etc., all strictly belonging to American forms; and
it is obvious that a mountain, as it became slowly upheaved,
would be colonized from the surrounding 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 the same principle in
the character of most of the blind animals inhabiting the caves
of America and of Europe. Other analogous facts could be given.
It will, I believe, be found universally true, that wherever in two
regions, let them be ever so distant, many closely allied or repre-
sentative species occur, there will likewise be found some identical
species ; and wherever many closely allied species occur, there will
be found many forms which some naturalists rank as distinct
species, and others as mere varieties; these doubtful forms show-
ing us the steps in the progress of modification.
The relation between the power and extent of migration in
certain species, either at the present or at some former period,
and the existence at remote points of the world of closely allied
species, is shown in another 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 generally true, though difficult
of proof. Among mammals, we see it strikingly displayed in bats,
and in a lesser degree in the Felidae and Canidae. We see the same
rule in the distribution of butterflies and beetles. So it is with
most of the inhabitants of fresh water, for many of the genera
in the most distinct classes range over the world, and many of
the species have enormous ranges. It is not meant that all, but
that some of the species, have very wide ranges in the genera
which range very widely. Nor is it meant that the species in such
genera have, on an average, a very wide range; for this will
largely depend on how far the process of modification has gone;
for instance, two varieties of the same species inhabit America
and Europe, and thus the species has an immense range; but, if
variation were to be carried a little further, the two varieties
would be ranked as distinct species, and their range would be
greatly reduced. Still less is it meant, that species which have
the capacity of crossing barriers and ranging widely, as in the
GEOGRAPHICAL DISTRIBUTION 367
case of certain powerfully winged birds, will necessarily range
widely; for we should never forget that to range widely implies
not only the power of crossing barriers, but the more important
power of being victorious in distant lands in the struggle for life
with 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 widely.
We should bear in mind that many genera in all classes are of
ancient origin, and the species in this case will have had ample
time for dispersal and subsequent 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 rang-
ing widely and of still retaining the same specific character. This
fact, together with that of the seeds and eggs of most lowly or-
ganized forms being very minute and better fitted for distant
transportal, probably accounts for a law which has long been ob-
served, and which has lately been discussed by Alph. de CandoUe,
in regard to plants; namely, that the lower any group of organ-
isms stands, the more widely it ranges.
The relations just discussed — namely, lower organisms rang-
ing more widely than the higher — ^some of the species of widely
ranging genera themselves ranging widely — such facts, as alpine,
lacustrine, and marsh productions being generally related to those
which live on the surrounding low lands and dry lands — the
striking relationship between the inhabitants of islands and those
of the nearest mainland — the still closer relationship of the dis-
tinct inhabitants of the islands in the same archipelago — are in-
explicable on the ordinary view of the independent creation of
each species, but are explicable if we admit colonization from the
nearest or readiest source, together with the subsequent adapta-
tion of the colonists to their new homes.
SUMMARY OF THE LAST AND PRESENT CHAPTERS
In these chapters I have endeavored 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 oc-
curred within the recent period, and of other changes which have
probably occurred — if we remember how ignorant we are with
respect to the many curious means of occasional transport — if
we bear in mind, and this is a very important consideration, how
368 THE ORIGIN OF SPECIES
often a species may have ranged continuously over a wide area,
and then have become extinct in the intermediate tracts — the
difficulty is not insuperable in believing that all the individuals
of the same species, wherever found, are descended from common
parents. And we are led to this conclusion, which has been ar-
rived at by many naturalists under the designation of single
centres of creation, by various general considerations, more es-
pecially from the importance of barriers of all kinds, and from
the analogical distribution of subgenera, genera, and famihes.
With respect to distinct species belonging to the same genus,
which on our theory have spread from one parent-source; if we
make the same allowances as before for our ignorance, and re-
member that some forms of life have changed very slowly, enor-
mous periods of time having been thus granted for their migra-
tion, 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 climatical changes on distribu-
tion, 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 alternations of the cold in the
north and the south, allowed the productions of opposite hemi-
spheres to mingle, and left some of them stranded on the moun-
tain-summits in all parts of the world. As showing how diversified
are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the
long course of time all the individuals of the same species, and
likewise of the several species belonging to the same genus, have
proceeded from some one source; then all the grand leading facts
of geographical distribution are explicable on the theory of migra-
tion, together with subsequent modification and the multiplica-
tion of new forms. We can thus understand the high importance
of barriers, whether of land or water, in not only separating but
in apparently forming the several zoological and botanical prov-
inces. We can thus understand the concentration of related
species within the same areas; and how it is that under different
latitudes, for instance, in South America, the inhabitants of the
plains and mountains, of the forests, marshes and deserts, are
linked together in so mysterious a manner, and are likewise linked
to the extinct beings which formerly inhabited the same continent.
Bearing in mind that the mutual relation of organism to organism
is of the highest importance, we can see why two areas, having
GEOGRAPHICAL DISTRIBUTION 369
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 al-
lowed 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 ca-
pable of varying more or less rapidly, there would ensue in the
two or more regions, independently of their physical conditions,
infinitely diversified conditions of life; there would be an al-
most 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 en-
deavored to show, why oceanic islands should have few inhabit-
ants, but that, of these, a large proportion should be endemic or
peculiar; and why, in relation to the means of migration, one
group of beings should have all its species peculiar, and another
group, even within the same class, should have all its species the
same with those in an adjoining quarter of the world. We can
see why whole groups of organisms, as batrachians and terrestrial
mammals, should be absent from oceanic islands, while the most
isolated islands should possess their own peculiar species of aerial
mammals or bats. We can see why, in islands, there should be
some relation between the presence of mammals, in a more or
less modified condition, and the depth of the sea between such
islands and the mainland. We can clearly see why all the in-
habitants 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 de-
rived. We can see why, if there exist very closely allied or repre-
sentative 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 governing at the present time the differences
in different areas. We see this in many facts. The endurance of
each species and group of species is continuous in time; for the
370 THE ORIGIN OF SPECIES
apparent exceptions to the rule are so few that they may fairly
be attributed to our not having as yet discovered in an inter-
mediate deposit certain forms which are absent in it, but which
occur both above and below: so in space, it certainly is the gen-
eral 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 characterized by trifling
features in common, as of sculpture or color. 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, while 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 organized members of each
class generally change less than the highly organized; but there
are in both cases marked exceptions to the rule. According to our
theory, these several relations throughout time and space are
intelligible; for whether we look to the allied forms of life whic'h
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 genera-
tion; in both cases the laws of variation have been the same, and
modifications have been accumulated by the same means of nat-
ural selection.
I
' CHAPTER XIV
Mutual Affinities of Organic Beings: Morphology — Embryology
— Rudimentary Organs
I
Classification, Groups Subordinate to Groups — ^Natural System — Rules and
Difficulties in Classification, explained on the Theory of Descent with
Modification — Classification of Varieties — Descent always used in Classi-
fication— ^Analogical or Adaptive Characters — ^Affinities, General, Com-
plex, and Radiating — Extinction separates and defines Groups — ^Mor-
phology, between Members of the Same Class, between Parts of the
Same Individual — Embryology, Laws of, explained by Variations not
supervening at an Early Age, and being inherited at a Corresponding
Age — Rudimentary Organs, their Origin explained — Summary.
CLASSIFICATION
From the most remote period in the history of the world, organic
beings have been found to resemble each other in descending de-
grees, so that they can be classed in groups under groups. This
classification is not arbitrary Hke the grouping of the stars in
constellations. The existence of groups would have been of sim-
ple significance, if one group had been exclusively fitted to in-
habit the land, and another the water; one to feed on flesh, an-
other on vegetable matter, and so on; but the case is widely
different, for it is notorious how commonly members of even the
same sub-group have different habits. In the second and fourth
chapters, on Variation and on Natural Selection, I have attempted
to show that within each country it is the widely ranging, the
much diffused and common, that is the dominant species, belong-
ing to the larger genera in each class, which vary most. The
varieties, or incipient species, thus produced, ultimately become
converted into new and distinct species; and these, on the prin-
ciple of inheritance, tend to produce other new and dominant
species. Consequently the groups which are now large, and which
generally include many dominant species, tend to go on increas-
ing in size. I further attempted to show that from the varying
descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they con-
stantly tend to diverge in character. This latter conclusion is sup-
371
372 THE ORIGIN OF SPECIES
ported by observing the great diversity of forms, which, in any
small area, come into the closest competition, and by certain
facts in naturalization.
I attempted also to show that there is a steady tendency in
the forms which are increasing in number and diverging in char-
acter, 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 principles; and he will see that the inevitable result is,
that the modified descendants proceeding from one progenitor
become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus
including several species; and the whole of the genera along this
upper line form together one class, for all are descended from
one ancient parent, and, consequently, have inherited something
in common. But the three genera on the left hand have, on this
same principle, much in common, and form a sub-family distinct
from that containing the next two genera on the right hand, which
diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than
when grouped in sub-families; and they form a family distinct
from that containing the three genera still farther to the right
hand, which diverged at an earlier period. And all these genera,
descended from (A), form an order distinct from the genera
descended from (I). So that we here have many species descended
from a single progenitor, grouped into genera; and the genera
into sub-families, families, and orders, all under one great class.
The grand fact of the natural subordination of organic beings
in groups under groups, which, from its familiarity, does not al-
ways sufficiently strike us, is in my judgment thus explained. No
doubt organic beings, like all other objects, can be classed in many
ways, either artificially by single characters, or more naturally
by a number of characters. We know, for instance, that minerals
and the elemental substances can be thus arranged. In this case
there is of course no relation to genealogical succession, and no
cause can at present be assigned for their falling into groups.
But with organic beings the case is different, and the view above
given accords with their natural arrangement in group under
group; and no other explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera,
and families in each class, on what is called the Natural System.
But what is meant by this system? Some authors look at it merely
as a scheme for arranging together those living objects which are
MUTUAL AFFINITIES OF ORGANIC BEINGS 373
most alike, and for separating those which are most unlike; or
as an artificial method of enunciating, as briefly as possible, gen-
eral propositions — that is, by one sentence to give the characters
common, for instance, to all mammals, by another those common
to all carnivora, by another those common to the dog-genus, and
then, by adding a single sentence, a full description is given of
each kind of dog. The ingenuity and utility of this system are in-
disputable. But many naturalists think that something more is
meant by the Natural System; they beheve 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 Linnaeus, which we
often meet with in a more or less concealed form, namely, that
the characters do not make the genus, but that the genus gives
the characters, seem to imply that some deeper bond is included
in our classifications than mere resemblance. I believe that this
is the case, and that community of descent — the one known
cause of close similarity in organic beings — is the bond, , which,
though observed by various degrees of modification, is partially
revealed to us by our classifications.
Let us now consider the rules followed in classification, and
the difficulties which are encountered on the view that classi-
fication either gives some unknown plan of creation, or is simply
a scheme for enunciating general propositions and of placing to-
gether the forms most like each other. It might have been thought
(and was in ancient times thought) that those parts of the struc-
ture which determined the habits of life, and the general place
of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one
regards the external similarity of a mouse to a shrew, of a dugong
to a whale, of a whale to a fish, as of any importance. These re-
semblances, though so intimately connected with the whole life
of the being, are ranked as merely "adaptive or analogical char-
acters:" but to the consideration of these resemblances we shall
recur. It may even be given as a general rule, that the less any
part of the organization is concerned with special habits, the
more important it becomes for classification. As an instance:
Owen, in speaking of the dugong, says, "The generative organs,
being those which are most remotely related to the habits and
food of an animal, I have always regarded as affording very clear
indications of its true affinities. We are least likely in the modi-
fications of these organs to mistake a merelv adaptive for an
374 THE ORIGIN OF SPECIES
essential character." With plants how remarkable it is that the
organs of vegetation, on which their nutrition and life depend,
are of little signification; whereas the organs of reproduction,
with their product the seed and embryo, are of paramount im-
portance! 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 throughout many allied groups; and
their constancy chiefly depends on any slight deviations not hav-
ing been preserved and accumulated by natural selection, which
acts only on serviceable characters.
That the mere physiological importance of an organ does not
determine its classificatory value, is almost proved by the fact,
that in allied groups, in which the 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, Robert
Brown, who, in speaking of certain organs in the Proteaceae, says
their generic importance, "like that of all their parts, not only in
this, but, as I apprehend, in every natural family, is very un-
equal, and in some cases seems to be entirely lost." Again, in
another work he says, the genera of the Connaracese "differ in
having one or more ovaria, in the existence or absence of albumen,
in the imbricate or valvular aestivation. Any one of these char-
acters singly is frequently of more than generic importance,
though here, even when all taken together, they appear insuffi-
cient to separate Cnestis from Connarus." To give an example
among insects: in one great division of the Hymenoptera, the
antennse, as Westwood has remarked, are most constant in struc-
ture; 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 organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs
are of high physiological or vital importance; yet, undoubtedly,
organs in this condition are often of much value in classification.
No one will dispute that the rudimentary teeth in the upper jaws
of young ruminants, and certain rudimentary bones of the leg,
MUTUAL AFFINITIES OF ORGANIC BEINGS 37S
are highly serviceable in exhibiting the close affinity between
ruminants and pachyderms. Robert Brown has strongly insisted
on the fact that the position of the rudimentary florets is of the
highest importance in the classification of the grasses.
Numerous instances could be given of characters derived from
parts which must be considered of very trifling physiological im-
portance, but which are universally 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 distinguishes
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 color in certain 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 external and trifling
character would have been considered by naturalists as an im-
portant aid in determining the degree of affinity of this strange
creature to birds.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of
more or less importance. The value indeed of an 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 organization 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 character; for this seems founded on
the appreciation of many trifling points of resemblance, too
slight to be defined. Certain plants belonging to the Malpighiaceae
bear perfect and degraded flowers; in the latter, as A. de Jussieu
has remarked, "The greater number of the characters proper to
the species, to the genus, to the family, to the class, disappear,
and thus laugh at our classification." When Aspicarpa produced
in France, during several years, only these degraded flowers, de-
parting so wonderfully in a number of the most important points
of structure from the proper type of the order, yet M. Richard
376 THE ORIGIN OF SPECIES
sagaciously saw, as Jussieu observes, that this genus should still
be retained among the Malpighiaceae. This case well illustrates
the spirit of our classifications.
Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which
they use in defining a group or in allocating any particular species.
If they find a character nearly uniform, and common to a great
number of forms, and not common to others, they use it as one
of high value; if common to some lesser number, they use it as
of subordinate value. This principle has been broadly confessed
by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. Saint-Hilaire. If several
trifling 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, im-
portant organs, such as those for propelling the blood, or for
aerating it, or those for propagating the race, are found nearly
uniform, they are considered as highly serviceable in classifica-
tion; but in some groups all these, the most important vital or-
gans, are found to offer characters of quite subordinate value.
Thus, as Fritz MUller has lately remarked, in the same group of
crustaceans, Cypridina is furnished with a heart, while in two
closely allied genera, namely Cypris and Cytherea, there is no
such organ ; one species of Cypridina has well-developed branchiae,
while 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 ordinary view, why the structure of
the embryo should be more important for this purpose than that
of the adult, which alone plays its full part in the economy of
nature. Yet it has been strongly urged by those great naturalists,
Milne Edwards and Agassiz, that embryological characters are
the most important of all; and this doctrine has very generally
been admitted as true. Nevertheless, their importance has some
times been exaggerated, 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. But there can be no doubt that embryonic, excluding larval,
characters, are of the highest value for classification, not only
with animals but with plants. Thus the main divisions of flower-
ing plants are founded on differences in the embryo — on the
MUTUAL AFFINITIES OF ORGANIC BEINGS 377
number and position of the cotyledons, and on the mode of devel-
opment 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 ar-
rangement.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of char-
acters common to all birds ; but with crustaceans, any such defini-
tion has hitherto been found impossible. There are crustaceans
at the opposite ends of the series, which have hardly a character
in common ; yet the species at both ends, from being plainly allied
to others, and these to others, and so onward, can be recognized
as unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps
not quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and
it has been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various
groups of species, such as orders, sub-orders, families, sub-fam-
ilies, and genera, they seem to be, at least at present, almost
arbitrary. Several of the best botanists, such as Mr. Bentham,
and others, have strongly insisted on their arbitrary value. In-
stances could be given among 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 spe-
cies, with slightly different grades of difference, have been sub-
sequently discovered.
All the foregoing rules and aids and difficulties in classification
may be explained, if I do not greatly deceive myself, on the view
that the Natural System is founded on descent with modification
— that the characters which naturalists consider as showing true
affinity between any two or more species, are those which have
been inherited from a common parent, all true classification be-
ing genealogical — that community of descent is the hidden bond
which naturalists have been unconsciously seeking, and not some
unknown plan of creation, or the enunciation of general proposi-
tions, and the mere putting together and separating objects more
or less alike.
But I must explain my meaning more fully. I believe that the
378 THE ORIGIN OF SPECIES
arrangement of the groups within each class, in due subordina-
tion 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 aUied in the same degree in
blood to their common progenitor, may differ greatly, being due
to the different degrees of modification which they have under-
gone; 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
and 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 transmitted modified
descendants to the present day, represented by the fifteen genera
(a^^ to z^^) on the uppermost horizontal line. Now, all these
modified descendants from a single species are related in blood
or descent in the same degree. They may metaphorically be
called cousins to the same millionth degree, yet they differ widely
and in different degrees from each other. The forms descended
from A, now broken up into two or three families, constitute a
distinct order from those descended from I, also broken up into
two families. Nor can the existing species descended from A be
ranked in the same genus with the parent A, or those from I with
the parent I. But the existing genus f^* may be supposed to have
been but slightly modified, and it will then rank with the parent
genus F, just as some few still living organisms belong to Silurian
genera. So that the comparative value of the differences between
these organic beings, which are all related to each other in the
same degree in blood, has come to be widely different. Never-
theless, their genealogical arrangement remains strictly true, not
only at the present time, but at each successive period of de-
scent. All the modified descendants from A will have inherited
something in common from their common parents, 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
MUTUAL AFFINITIES OF ORGANIC BEINGS 379
diagram on a flat surface, is much too simple. The branches
ought to have diverged in all directions. It the names of the
groups had been simply written down in a linear series, the repre-
sentation would have been still less natural; and it is notoriously
not possible to represent in a series, on a flat surface, the affinities
which we discover in nature among 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 undergone has to be expressed by rankmg them
under different so-called genera, sub-families, families, sections,
orders, and classes.
It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would
afford the best classification of the various languages now spoken
throughout the world; and if all extinct languages, and all inter-
mediate and slowly changing dialects, were to be included, such an
arrangement would be the only possible one. Yet it might be that
some ancient languages had altered very little and had given rise
to few new languages, while others had altered much, owing to the
spreading, isolation, and state of civilization 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 subordi-
nate to groups ; but the proper or even the only possible arrange-
ment would still be genealogical; and this would be strictly nat-
ural, as it would connect together all languages, extinct and recent,
by the closest affinities, and would give the filiation and origin of
each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a
single species. These are grouped under the species, with the sub-
varieties under the varieties; and in some cases, as with the do-
mestic pigeon, with several other grades of difference. Nearly the
same rules are followed as in classifying species. Authors have in-
sisted 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 pineapple 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. WTiatever part is
found to be most constant, is used in classing varieties; thus the
great agriculturist Marshall says the horns are very useful for this
I
380 THE ORIGIN OF SPECIES
purpose with cattle, because they are less variable than the shape
or color of the body, etc.; whereas with sheep the horns are much
less serviceable, because less constant. In classing varieties, I ap-
prehend 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 this habit ; neverthe-
less, 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 low-
est grade, that of species, the two sexes ; and how enormously these
sometimes differ in the most important characters is known to
every naturalist: scarcely a single fact can be predicated in 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 sometimes produced on the same plant, they were
immediately considered as varieties; and now I have been able
to show that they are the male, female, and hermaphrodite forms
of the same species. The naturalist includes as one species the
various larval stages of the same individual, however much they
may differ from each other and from the adult, as well as the so-
called alternate generations of Steenstrup, which can only in a
technical sense be considered as the same individual. He includes
monsters and varieties, not from their partial resemblance to the
parent-form, but because they are descended from it.
As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larvae are sometimes extremely different; and as it has been used
in classing varieties which have undergone a certain, and some-
times a considerable, amount of modification; may not this same
element of descent have been unconsciously used in grouping
species under genera, and genera under higher groups, all under
the so-called natural system? I believe it has been unconsciously
used; and thus only can I understand the several rules and guides
MUTUAL AFFINITIES OF ORGANIC BEINGS 381
which have been followed by our best systematists. As we have no
written pedigrees, we are forced to trace community of descent by
resemblances of any kind. Therefore, we choose those characters
which are the least likely to have been modified, in relation to the
conditions of life to which each species has been recently exposed.
Rudimentary structures on this view are as good, or even some-
times better than other parts of the organization. We care not how
trifling a character may be — let it be the mere inflection of the
angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers — if it prevail
throughout many and different species, especially those having
very different habits of life, it assumes high value ; for we can ac-
count for its presence in so many forms with such different habits,
only by inheritance from a common parent. We may err in this
respect in regard to single points of structure, but when several
characters, let them be ever so trifling, concur throughout 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 char-
acters have especial value in classification.
We can understand why a species or a group of species may de-
part 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, betray the hidden bond of community of
descent. Let two forms have not a single character in common, yet,
if these extreme forms are connected together by a chain of inter-
mediate groups, we may at once infer their community of descent,
and we put them all into the same class. As we find organs of high
physiological importance — those which serve to preserve life
under the most diverse conditions of existence — are generally the
most constant, we attach especial value to them; but if these same
organs, in another group or section of a group, are found to differ
much, we at once value them less in our classification. We shall
presently see why embryological characters are of such high classi-
ficatory importance. Geographical distribution may sometimes be
brought usefully into play in classing large genera, because all the
species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
ANALOGICAL RESEMBLANCES
We can understand, on the above views, the very important dis-
tinction between real affinities and analogical or adaptive resem-
382 THE ORIGIN OF SPECIES
blances. Lamarck first called attention to this subject, and he has
been ably followed by Macleay and others. The resemblances in
the shape of the body and in the fin-like anterior limbs between
dugongs and whales, and between these two orders of mammals
and fishes, are analogical. So is the resemblance between a mouse
and a shrew-mouse (Sorex), which belong to different orders;
and the still closer resemblance, insisted on by Mr. Mivart, be-
tween 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.
Among insects there are innumerable similar instances; thus
Linnaeus, 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 simi-
larly thickened stems of the common and specifically distinct
Swedish turnip. The resemblance between the greyhound and the
race-horse 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 classi-
fication, only in so far as they reveal descent, we can clearly under-
stand why analogical or adaptive characters, although of the ut-
most importance to the welfare of the being, are almost valueless
to the systematist. For animals, belonging to two most distinct
lines of descent, may have become adapted to similar conditions,
and thus have assumed a close external resemblance; but such
resemblances will not reveal — will rather tend to conceal their
blood-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 mem-
bers of the whale family, the shape of the body and the fin-like
limbs offer characters exhibiting true affinity; for as these parts
are so nearly similar throughout the whole family, we cannot
doubt that they have been inherited from a common ancestor. So
it is with fishes.
Numerous cases could be given of striking resemblances in
quite distinct beings between single parts of organs, which have
MUTUAL AFFINITIES OF ORGANIC BEINGS 383
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 appear-
ance, as in the prominence of the canines, and in the cutting shape
of the molar teeth. For the teeth really differ much: thus the dog
has on each side of the upper jaw four pre-molars and only two
molars; while 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 ad-
mitted in the one case, it is unintelligible to me that it should be
denied in the other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of widely
different fishes possessing electric organs — of widely different in-
sects possessing luminous organs — and of orchids and asclepiads
having pollen-masses with viscid disks, 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 de-
velopment of the parts, and generally in their matured structure,
can be detected. The end gained is the same, but the means,
though appearing superficially to be the same, are. essentially dif-
ferent. The principle formerly alluded to under the term of ana-
logical 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 pro-
genitor.
As species belonging to distinct classes have often been adapted
by successive slight modifications to Hve under nearly similar cir-
cumstances— to inhabit, for instance, the three elements of land,
air, and water — we can perhaps understand how it is that a nu-
merical parallelism has sometimes been observed between the sub-
groups of distinct classes. A naturalist, struck with a parallelism of
this nature, by arbitrarily raising or sinking the value of the groups
in several classes (and all our experience shows that their valua-
b
384 THE ORIGIN OF SPECIES
tion is as yet arbitrary), could easily extend the parallelism over
a wide range; and thus the septenary, quinary, quaternary, and
ternary classifications have probably arisen.
There is another and curious class of cases in which close ex-
ternal resemblance does not depend on adaptation to similar habits
of life, but has been gained for the sake of protection. I allude to
the wonderful manner in which certain butterflies imitate, as first
described by Mr. Bates, other and quite distinct species. This
excellent observer has shown that in some districts of South
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 color, and even in the shape
of its wings, that Mr. Bates, with his eyes sharpened by collecting
during eleven years, was, though always on his guard, continually
deceived. When the mockers and the mocked are caught and com-
pared, they are found to be very different in essential structure,
and to belong not only to distinct genera, but often to distinct
families. Had this mimicry occurred in only one or two instances,
it might have been passed over as a strange coincidence. But, if we
proceed from a district where one Leptalis imitates an Ithomia,
another mocking and mocked species, belonging to the same two
genera, equally close in their resemblance, may be found. Alto-
gether no less than ten genera are enumerated, which include
species that imitate other butterflies. The mockers and mocked
always inhabit the same region; we never find an imitator Hving
remote from the form which it imitates. The mockers are almost
invariably rare insects; the mocked in almost every case abounds
in swarms. In the same district in which a species of Leptalis
closely imitates an Ithomia, there are sometimes other Lepidoptera
mimicking the same Ithomia: so that in the same place, species of
three genera of butterflies and even a moth are found all closely
resembling a butterfly belonging to a fourth genus. It deserves
especial notice, that many of the mimicking forms of the Leptalis,
as well as of the mimicked forms, can be shown by a graduated
series to be merely varieties of the same species; while others are
undoubtedly distinct species. But why, it may be asked, are cer-
tain 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, while the counterfeiters have changed their dress and
do not resemble their nearest allies.
We are next led to inquire what reason can be assigned for cer-
MUTUAL AFFINITIES OF ORGANIC BEINGS 385
tain butterflies and moths so often assuming the dress of another
and quite distinct form ; why, to the perplexity of naturalists, has
nature condescended to the tricks of the stage? Mr. Bates has, no
doubt, hit on the true explanation. The mocked forms, which al-
ways abound in numbers, must habitually escape destruction to a
large extent, otherwise they could not exist in such swarms ; and a
large amount of evidence has now been collected, showing that
they are distasteful to birds and other insect-devouring animals.
The mocking forms, on the other hand, that inhabit the same
district, are comparatively rare, and belong to rare groups; hence,
they must suffer habitually from some danger, for otherwise, from
the number of eggs laid by all butterflies, they would in three or
four generations swarm over the whole country. Now if a member
of one of these persecuted and rare groups were to assume a dress
so like that of a well-protected species that it continually deceived
the practised eyes of an entomologist, it would often deceive pre-
daceous 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 butterflies, varied in an extreme degree. In
one district several varieties occurred, and of these one alone re-
sembled, to a certain extent, the common Ithomia of the same dis-
trict. 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 in-
habiting the same district, this variety, from its resemblance to a
flourishing and little persecuted kind, has a better chance of
escaping destruction from predaceous birds and insects, and is
consequently oftener preserved; "the less perfect degrees of re-
semblance being generation after generation eliminated, and only
the others left to propagate their kind." So that here we have an
excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several
equally striking cases of imitation in the Lepidoptera of the Malay
Archipelago and Africa, and with some other insects. Mr. Wallace
has also 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 in-
deed the kinds furnished with a sting, and I have never heard of
386 THE ORIGIN OF SPECIES
an instance of such kinds mocking other insects, though they are
mocked; insects cannot easily escape by flight from the larger
animals which prey on them; therefore, speaking metaphorically,
they are reduced, like most weak creatures, to trickery and dis-
simulation.
It should be observed that the process of imitation probably
never commenced between forms widely dissimilar in color. 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 gradually
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 coloring wholly
unlike that of the other members of the family to which it be-
longed. There is, however, some difficulty on this head, for it is
necessary to suppose in some cases that ancient members belong-
ing to several distinct groups, before they had diverged to their
present extent, accidentally resembled a member of another and
protected group in a sufficient degree to afford some slight protec-
tion, this having given the basis for the subsequent acquisition of
the most perfect resemblance.
ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS
As the modified descendants of dominant species, belonging to
the larger genera, tend to inherit the advantages which made the
groups to which they belong large and their parents dominant,
they are almost sure to spread widely, and to seize on more and
more places in the economy of nature. The larger and more domi-
nant groups within each class thus tend to go on increasing in size,
and they consequently supplant many smaller and feebler groups.
Thus, we can account for the fact that all organisms, recent and
extinct, are included under a few great orders and under still
fewer classes. As showing how few the higher groups are in num-
ber, and how widely they are spread throughout the world, the
fact is striking that the discovery 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 fami-
lies 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
MUTUAL AFFINITIES OF ORGANIC BEINGS 387
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
exterminated and utterly lost. And we have evidence of aberrant
groups having suffering severely from extinction, for they are al-
most always represented by extremely few species, and such
species as do occur are generally very distinct from each other,
which again implies extinction. The genera Ornithorhynchus and
Lepidosiren, for example, would not have been less aberrant had
each been represented by a dozen species, instead of as at present
by a single one, or by two or three. We can, I think, account for
this fact only by looking at aberrant groups as forms which have
been conquered by more successful competitors, with a few mem-
bers still preserved under unusually favorable conditions.
Mr. Waterhouse has remarked that when a member belonging
to one group of animals exhibits an affinity to a quite distinct
group, this affinity in most cases is general and not special; thus,
according to Mr. Waterhouse, of all Rodents, the bizcacha is most
nearly related to Marsupials; but in the points in which it ap-
proaches this order, its relations are general, that is, not to any
one Marsupial species more than to another. As these points of
affinity are believed to be real and not merely adaptive, they must
be due, in accordance with our view, to inheritance from a com-
mon progenitor. Therefore, we must suppose either that all Ro-
dents, including the bizcacha, branched off from some ancient
Marsupial, which will naturally have been more or less inter-
mediate 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 sup-
pose that the bizcacha has retained, by inheritance, more of the
characters of its ancient progenitor than have other Rodents;
and therefore it will not be specially related to any one existing
Marsupial, but indirectly to all or nearly all Marsupials, from hav-
ing partially retained the character of their common progenitor, or
of some early member of the group. On the other hand, of all Mar-
supials, as Mr. Waterhouse has remarked, the Phascolomys re-
sembles 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 Rodent. The elder
388 THE ORIGIN OF SPECIES
De Candolle has made nearly similar observations on the general
nature of the affinities of distinct families of plants.
On the principle of the multiplication and gradual divergence
in character of the species descended from a common progenitor,
together with their retention by inheritance of some characters in
common, we can understand the excessively complex and radiat-
ing affinities 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 extinction into distinct groups
and sub-groups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they
will consequently be related to each other by circuitous lines of
affinity of various lengths (as may be seen in the diagram so often
referred to), mounting up through many predecessors. As it is
difficult to show the blood relationship between the numerous
kindred of any ancient and noble family even by the aid of a
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 perceive between the many living and
extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account for the distinct-
ness 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 pro-
genitors of birds were formerly connected with the early progeni-
tors of the other and at that time less differentiated vertebrate
classes. There has been much less extinction of the forms of life
which once connected fishes with Batrachians. There has been
still less within some whole classes, for instance the Crustacea, for
here the most wonderfully diverse forms are still linked together
by a long and only partially broken chain of affinities. Extinction
has only defined the groups: it has by no means made them; for
if every form which has ever lived on this earth were suddenly to
reappear, though it would be quite impossible to give definitions
by which, each group could be distinguished, still a natural classi-
fication, or at least a natural arrangement, would be possible. We
shall see this by turning to the diagram; the letters, A to L, may
represent eleven Silurian genera, some of which have produced
large groups of modified descendants, with every link in each
branch and sub-branch still alive; and the links not greater than
MUTUAL AFFINITIES OF ORGANIC BEINGS 389
those between existing varieties. In this case it would be quite im-
possible to give definitions by which the several members of the
several groups could be distinguished from their more immediate
parents and descendants. Yet the arrangement in the diagram
would still hold good and would be natural; for, on the principle
of inheritance, all the forms descended, for instance, from A,
would have something in common. In a tree we can distinguish
this or that branch, though at the actual fork the two unite and
blend together. We could not, as I have said, define the several
groups; but we could pick out types, or forms, representing most
of the characters of each group, whether large or small, and thus
give a general idea of the value of the differences between them.
This is what we should be driven to, if we were ever to succeed in
collecting all the forms in any one class which have lived through-
out all time and space. Assuredly we shall never succeed in making
so perfect a collection: nevertheless, in certain classes, we are
tending toward this end; and Milne Edwards has lately insisted,
in an able paper, on the high importance of looking to types,
whether or not we can separate and define the groups to which
such types belong.
Finally, we have seen that natural selection, which follows
from the struggle for existence, and which almost inevitably leads
to extinction and divergence of character in the descendants from
any one parent species, explains that great and universal feature
in the affinities of all organic beings, namely, their subordination
in group under group. We use the element of descent in classing
the individuals of both sexes and of all ages under one species,
although they may have but few characters in common; we use
descent in classing acknowledged varieties, however different
they may be from their parents; and I believe that this element of
descent is the hidden bond of connection which naturalists 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, genea-
logical in its arrangement, with the grades of difference expressed
by the terms genera, families, orders, etc., 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 be-
tween one group and another, we summarily reject analogical or
adaptive characters, and yet use these same characters within the
limits of the same group. We can clearly see how it is that all living
and extinct forms can be grouped together within a few great
390 THE ORIGIN OF SPECIES
classes; and how the several members of each class are connected
together by the most complex and radiating lines of affinities. We
shall never, probably, disentangle the inextricable web of the
affinities between the members of any one class ; but when we have
a distinct object in view, and do not look to some unknown plan
of creation, we may hope to make sure but slow progress.
Professor Hackel in his "Generelle Morphologic,'' and in other
works, has recently brought his great knowledge and abilities to
bear on what he calls phylogeny, or the lines of descent of all
organic beings. In drawing up the several series he trusts chiefly to
embryological characters, but receives aid from homologous and
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.
MORPHOLOGY
We have seen that the members of the same class, independently
of their habits of life, resemble each other in the general plan of
their organization. This resemblance 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 subject is included under the general term of Morphology.
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 digging, the leg of a horse, the paddle of the porpoise,
and the wing of the bat, should all be constructed on the same pat-
tern, and should include similar bones, in the same relative posi-
tions? How curious it is, to give a subordinate though striking
instance, that the hind feet of the kangaroo, which are so well
fitted for bounding over the open plains — those of the climbing,
leaf-eating koala, equally well fitted for grasping the branches of
trees — those of the ground-dwelling, insect or root eating, bandi-
coots— and those of some other Australian marsupials — should
all be constructed on the same extraordinary type, namely with
the bones of the second and third digits extremely slender and
enveloped within the same skin, so that they appear like a single
toe furnished with two claws. Notwithstanding the similarity of
pattern, it is obvious that the hind feet of these several animals
are used for as widely different purposes as it is possible to con-
ceive. The case is rendered all the more striking by the American
MUTUAL AFFINITIES OF ORGANIC BEINGS 391
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 phenome-
non;" and he then adds, "but is it not powerfully suggestive of
true relationship, of inheritance from a common ancestor?"
Geoffroy Saint-Hilaire has strongly insisted on the high im-
portance of relative position or connection in homologous parts;
they may differ to almost any extent in form and size, and yet
remain connected together in the same invariable order. We never
find, for instance, the bones of the arm and forearm, 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 dif-
ferent purposes, are formed by infinitely numerous modifications
of an upper lip, mandibles, and two pairs of maxillae. The same
law governs the construction of the mouths and limbs of crusta-
ceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this
similarity of pattern in members of the same class, by utility or by
the doctrine of final causes. The hopelessness of the attempt has
been expressly admitted by Owen in his most interesting work on
the "Nature of Limbs." On the ordinary view of the independent
creation of each being, we can only say that so it is; that it has
pleased the Creator to construct all the animals and plants in each
great class on a uniform plan; but this is not a scientific explana-
tion.
The explanation is to a large extent simple, on the theory of
the selection of successive slight modifications, each being profit-
able in some way to the modified form, but often affecting by
correlation other parts of the organization. In changes of this
nature, there will be little or no tendency to alter the original
pattern, or to transpose the parts. The bones of a limb might be
shortened and flattened to any extent, becoming at the same time
enveloped in thick membrane, so as to serve as a fin ; or a webbed
hand 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 relative connection of the
392 THE ORIGIN OF SPECIES
parts. If we suppose that an early progenitor — the archetype, as
it may be called — of all mammals, birds, and reptiles, had its
limbs constructed on the existing general pattern, for whatever
purpose they served, we can at once perceive the plain significa-
tion 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 definite diversity
in the structure and functions of the mouths of insects. Neverthe-
less, it is conceivable that the general pattern of an organ might
become so much obscured as to be finally lost, by the reduction
and ultimately by the complete abortion of certain parts, by the
fusion of other parts, and by the doubling or multiplication of
others, variations which we know to be within the limits of possi-
bility. 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 of the different parts
or organs in the same individual, and not of the same parts or
organs in different members of the same class. Most physiologists
believe that the bones of the skull are homologous — that is, cor-
respond in number and relative connection — with the elemental
parts of a certain number of vertebrae. The anterior and posterior
limbs in all the higher vertebrate classes are plainly homologous.
So it is with the wonderfully complex jaws and legs of crustaceans.
It is familiar to almost every one, that in a flower the relative posi-
tion of the sepals, petals, stamens, and pistils, as well as their inti-
mate 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, during the
early or embryonic stages of development in flowers, as well as in
crustaceans and many other animals, that organs which when
mature become extremely different are at first exactly alike.
How inexplicable are the cases of serial homologies on the ordi-
nary view of creation ! Why should the brain be enclosed in a box
composed of such numerous and such extraordinarily shaped
pieces of bone, apparently 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.
MUTUAL AFFINITIES OF ORGANIC BEINGS 393
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 pur-
poses, namely, flying and walking? Why should one crustacean,
which has an extremely complex mouth formed of many parts,
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 dis-
tinct purposes, be all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent,
answer these questions. We need not here consider how the bodies
of some animals first became divided into a series of segments, or
how they became divided into right and left sides, with corre-
sponding organs, for such questions are almost beyond investiga-
tion. It is, however, probable that some serial structures are the
result of cells multiplying by division, entailing the multiplication
of the parts developed from such cells. It must suffice for our pur-
pose to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has remarked, of
all low or httle specialized forms; therefore the unknown pro-
genitor of the Vertebrata probably possessed many vertebrae; the
unknown progenitor of the Articulata, many segments; and the
unknown progenitor of flowering plants, many leaves arranged in
one or more spires. We have also formerly seen that parts many
times repeated are eminently liable to vary, not only in number,
but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally
afford the materials for adaptation to the most different purposes ;
yet they would generally retain, through the 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, would tend from the first to be similar; the parts
being at an early stage of growth alike, and being subjected to
nearly the same conditions. Such parts, whether more or less modi-
fied, unless their common origin became wholly obscured, would
be serially homologous.
In the great class of mollusks, though the parts in distinct
species can be shown to be homologous, only a few serial homolo-
gies, such as the valves of Chitons, can be indicated; that is, we
are seldom enabled to say that one part is homologous with an-
other part in the same individual. And we can understand this
fact; for in mollusks, even in the lowest members of the class,
we do not find nearly so much indefinite repetition of any one
394 THE ORIGIN OF SPECIES
part as we find in the other great classes of the animal and vege-
table 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. Ray Lankester, who has drawn an important distinction
between certain classes of cases which have all been equally
ranked by naturalists as homologous. He proposes to call the
structures which resemble each other in distinct animals, owing to
their descent from a common progenitor with subsequent modifi-
cation, homogenous ; and the resemblances which cannot thus be
accounted for, he proposes to call homoplastic. For instance, he
believes that the hearts of birds and mammals are as a whole
homogenous — that is, have been derived from a common pro-
genitor; but that the four cavities of the heart in the two classes
are homoplastic — that is, have been independently developed.
Mr. Lankester also adduces the close resemblance of the parts on
the right and left sides of the body, and in the successive seg-
ments of the same individual animal; and here we have parts com-
monly called homologous which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic struc-
tures are the same with those which I have classed, though in a
very imperfect manner, as analogous modifications or resem-
blances. Their formation may be attributed in part to distinct
organisms, or to distinct parts of the same organism, having varied
in an analogous manner; and in part to similar modifications,
having been preserved for the same general purpose or function,
of which many instances have been given.
Naturalists frequently speak of the skull as formed of meta-
morphosed vertebrae; the jaws of crabs as metamorphosed legs; the
stamens and pistils in flowers as metamorphosed leaves; but it
would in most cases be more correct, as Professor Huxley has re-
marked, to speak of both skull and vertebrae, jaws and legs, etc.,
as having been metamorphosed, not one from the other, as they
now exist, but from some common and simpler element. Most
naturalists, however, use such language only in a metaphorical
sense; they are far from meaning that during a long course of
descent, primordial organs of any kind — vertebrae in the one case
and legs in the other — have actually been converted into skulls or
jaws. 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,
MUTUAL AFFINITIES OF ORGANIC BEINGS 395
which they probably would have retained through inheritance, if
they had really been metamorphosed from true though extremely
simple legs, is in part explained.
DEVELOPMENT AND EMBRYOLOGY
This is one of the most important subjects in the whole round
of natural history. The metamorphoses of insects, with which
every one is familiar, are generally effected abruptly by a few
stages; but the transformations are in reality numerous and
gradual, though concealed. A certain ephemerous insect (Chloeon)
during its development moults, as shown by Sir J. Lubbock, above
twenty times, and each time undergoes a certain amount of
change; and in this case we see the act of metamorphosis per-
formed in a primary and gradual manner. Many insects, and espe-
cially certain crustaceans, show us what wonderful changes of
structure can be effected during development. Such changes, how-
ever, reach their acme in the so-called alternate generations of
some of the lower animals. It is, for instance, an astonishing fact
that a delicate branching coralline, studded with polypi, and at-
tached to a submarine rock, should produce, first by budding and
then by transverse division, a host of huge floating jelly-fishes;
and that these should produce eggs, from which are hatched
swimming animalcules, which attach themselves to rocks and be-
come developed into branching corallines ; and so on in an endless
cycle. The belief in the essential identity of the process of alternate
generation and of ordinary metamorphosis has been greatly
strengthened by Wagner's discovery of the larva or maggot of a
fly, namely the Cecidomyia, producing asexually other larvae, and
these others which finally are developed into mature males and
females, propagating their kind in the ordinary manner by eggs.
It may be worth notice, that when Wagner's remarkable dis-
covery was first announced, I was asked how was it possible to
account for the larvae of this fly having acquired the power of
asexual reproduction. As long as the case remained unique, no an-
swer could be given. But already Grimm has shown that another
fly, a Chironomus, reproduces itself in nearly the same manner,
and he believes that this occurs frequently in the order. It is the
pupa, and not the larva, of the Chironomus which has this power;
and Grimm further shows that this case, to a certain extent,
^'unites that of the Cecidomyia, with the parthenogenesis of the
Coccidae;" the term parthenogenesis implying that the mature
females of the Coccidae are capable of producing fertile eggs with-
out the concourse of the male. Certain animals belonging to sev-
396 THE ORIGIN OF SPECIES
eral classes are now known to have the power of ordinary repro-
duction at an unusually early age ; and we have only to accelerate
parthenogenetic reproduction by gradual steps to an earlier and
earlier age — Chironomus showing us an almost exactly inter-
mediate stage, viz., that of the pupa — and we can perhaps account
for the marvellous case of the Cecidomyia.
It has already been stated that various parts in the Same indi-
vidual, 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 em-
bryos of the most distinct species belonging to the same class are
closely similar, but become, when fully developed, widely dis-
similar. 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 the 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 pos-
session 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, however,
are still absent in these embryos. But even if they had existed in
the earhest stage of their development we should learn nothing,
for the feet of lizards and mammals, the wings and feet of birds,
no less than the hands and feet of man, all arise from the same
fundamental form." The larvae of most crustaceans, at correspond-
ing stages of development, closely resemble each other, however
different the adults may become; and so it is with very many other
animals. A trace of the law of embryonic resemblance occasion-
ally 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 occa-
sionally, 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 leguminosae.
The points of structure, in which the embryos of widely differ-
ent animals within the same class resemble each other, often have
MUTUAL AFFINITIES OF ORGANIC BEINGS 397
no direct relation to their conditions of existence. We cannot, for
instance, suppose that in the embryos of the vertebrata the pecu-
liar, loop-like courses of the arteries near the branchial slits are
related to similar conditions — in the young mammal which is
nourished in the womb of its mother, in the egg of the bird which
is hatched in a nest, and in the spawn of a frog under water. We
have no more reason to believe in such a relation than we have to
beheve that the similar bones in the hand of a man, wing of a bat,
and fin of a porpoise, are related to similar conditions of life. 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, during any part
of its embryonic career, is active, and has to provide for itself.
The period of activity may come on earher 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 labor during the
different stages of development, as when the same larva has dur-
ing one stage to search for food, and during another stage has to
search for a place of attachment. Cases can even be given of the
larvae of allied species, or groups of species, differing more from
each other than do the adults. In most cases, however, the larvae,
though active, still obey, more or less closely, the law of common
embryonic resemblance. Cirripedes afford a good instance of this;
even the illustrious Cuvier did not perceive than 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 pedun-
culated and sessile, though differing widely in external appear-
ance, have larvae in all their stages barely distinguishable.
The embryo in the course of development generally rises in or-
ganization. I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by organization
being higher or lower. But no one probably will dispute that the
butterfly is higher than the caterpillar. In some cases, however,
the mature animal must be considered as lower in the scale than
the larva, as with certain parasitic crustaceans. To refer once again
to cirripedes: the larvae in the first stage have three pairs of loco-
398 THE ORIGIN OF SPECIES
motive organs, a simple single eye, and a probosciformed mouth,
with which they feed largely, for they increase much in size. In
the second stage, answering to the chrysalis stage of butterflies,
they have six pairs of beautifully constructed natatory legs, a
pair of magnificent compound eyes, and extremely complex an-
tennae; but they have a closed and imperfect mouth and cannot
feed: their function at this stage is, to search out by their well-
developed organs of sense, and to reach by their active powers of
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,
cirripedes may be considered as either more highly or more lowly
organized 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 retro-
grade, for the male is a mere sac, which lives for a short time and
is destitute of mouth, stomach, and every other organ of impor-
tance, 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 porpoise, should not have been sketched out with all their
parts in proper proportion, as soon as any part became visible. In
some whole groups of animals and in certain members of other
groups this is the case, and the embryo does not at any period
differ widely from the adult: thus Owen has remarked, in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic char-
acter is manifested long before the parts of the embryo are com-
pleted." Land-shells and fresh-water crustaceans are born having
their proper forms, while 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
I
MUTUAL AFFINITIES OF ORGANIC BEINGS 399
the development of this insect, by Professor Huxley, we see hardly
any trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which
fail. Thus, Fritz Miiller has made the remarkable discovery that
certain shrimp-like crustaceans (allied 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 mala-
costracan order, to which these crustaceans belong, no other mem-
ber is as yet known to be first developed under the nauplius-form,
though many appear as zoeas; nevertheless Miiller assigns reasons
for his belief, that if there had been no suppression of develop-
ment, all these crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embryology —
namely, the very general, though not universal, difference in struc-
ture 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 species in the same class;
the embryo often retaining, while within the egg or womb, struc-
tures which are of no service to it, either at that or at a later
period of Hfe; on the other hand, larvae which have to provide for
their own wants, being perfectly adapted to the surrounding con-
ditions; and lastly, the fact of certain larvae standing higher in
the scale of organization 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 indi-
vidual 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 and demerits 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 dis-
played. 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 importance to a very young animal, as long
400 THE ORIGIN OF SPECIES
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 char-
acters are acquired a little earlier or later in life. It would not
signify, for instance, to a bird which obtained its food by having
a much-curved beak, whether or not while 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 varia-
tion first appears in the parent, it tends to reappear at a corre-
sponding age in the offspring. Certain variations can only appear
at corresponding ages ; for instance, peculiarities in the caterpillar,
cocoon, or imago states of the silk-moth; or, again, in the full-
grown horns of cattle. But variations which, for all that we can
see, might have first appeared either earlier or later in life, like-
wise tend to reappear at a corresponding age in the offspring and
parent. I am far from meaning that this is invariably the case, and
I could give several exceptional cases of variations (taking the
word in the largest sense) which have supervened at an earlier age
in the child than in the parent.
These two principles, namely, that slight variations generally
appear at a not very early period of life, and are inherited at a
corresponding not early period, explain, as I believe, all the above
specified leading facts in embryology. But first let us look to a few
analogous cases in our domestic varieties. Some authors who have
written on dogs maintain that the greyhound and bull-dog, though
so different, are really closely allied varieties, descended from the
same wild stock, hence I was curious to see how far their puppies
differed from each other. I was told by breeders that they differed
just as much as their parents, and this, judging by the eye, seemed
almost to be the case ; but on actually measuring the old dogs and
their six-days-old puppies, I found that the puppies had not ac-
quired nearly their full amount of proportional difference. So,
again, I was told that the foals of cart and race horses — breeds
which have been almost wholly formed by selection under domesti-
cation— 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 case.
As we have conclusive evidence that the breeds of the pigeon
are descended from a single wild species, I compared the young
within twelve hours after being hatched. I carefully measured the
proportions (but will not here give the details) of the beak, width
of mouth, length of nostril and of eyelid, size of feet and length of
leg, in the wild parent species, in pouters, fantails, runts, barbs,
i
MUTUAL AFFINITIES OF ORGANIC BEINGS 401
dragons, carriers, and tumblers. Now, some of these birds, when
mature, differ in so extraordinary a manner in the length and form
of beak, and in other characters, that they would certainly have
been ranked as distinct genera if found in a state of nature. But
when the nestling birds of these several breeds were placed in a
row, though most of them could just be distinguished, the propor-
tional differences in the above specified points were incomparably
less than in the full-grown birds. Some characteristic points of
difference — for instance, that of the width of mouth — could hardly
be detected in the young. But there was one remarkable exception
to this rule, for the young of the short-faced tumbler differed from
the young of the wild rock-pigeon, and of the other breeds, in al-
most exactly the same proportions as in the adult stage.
These facts are explained by the above two principles. Fanciers
lect their dogs, horses, pigeons, etc., for breeding, when nearly
own up. They are indifferent whether the desired qualities are
quired earlier or later in life, if the full-grown animal possesses
em. And the cases just given, more especially that of the pigeons,
show that the characteristic differences which have been accumu-
lated 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 super-
vened in the several species at a not early age, and having been
inherited at a corresponding age, the young will have been but
little modified, and they will still resemble each other much more
closely than do the adults, just as we have seen with the breeds of
the pigeon. We may extend this view to widely distinct structures
and to whole classes. The fore limbs, for instance, which once
served as legs to a remote progenitor, may have become, through a
long course of modification, adapted in one descendant to act as
hands, in another as paddles, in another as wings; but on the
above two principles the fore limbs will not have been much
modified in the embryos of these several forms; although in each
form the fore limb will differ greatly in the adult state. Whatever
402 THE ORIGIN OF SPECIES
influence long continued use or disuse may have had in modifying
the limbs or other parts of any species, this will chiefly or solely
have affected it when nearly mature, when it was compelled to use
its full powers to gain its own living; and the effects thus produced
will have been transmitted to the offspring at a corresponding
nearly mature age. Thus the young will not be modified, or will be
modified only in a slight degree, through the effects of the in-
creased use or disuse of parts.
With some animals the successive variations may have super-
vened at a very early period of life, or the steps may have been
inherited at an earlier age than that at which they first occurred. In
either of these cases, the young or embryo will closely resemble
the mature parent-form, as we have seen with the short-faced
tumbler. And this is the rule of development in certain whole
groups, or in certain sub-groups alone, as with cuttle-fish, land-
shells, fresh-water crustaceans, spiders, and some member 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 undergo any meta-
morphosis, while marine members of the same groups pass through
various transformations, Fritz Miiller has suggested that the
process of slowly modifying and adapting an animal to live on the
land or in fresh water, instead of in the sea, would be greatly 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 unoccupied or ill-occupied by other organisms.
In this case the gradual acquirement at an earlier and earlier age
of the adult structure would be favored by natural selection; and
all traces of former metamorphoses would finally be lost.
If, on the other hand, it profited the young of an animal to fol-
low habits of life slightly different from those of the parent-form,
and consequently to be constructed 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 corresponding
ages, the young or the larvae might be rendered by natural selec-
tion more and more different from their parents to any conceivable
MUTUAL AFFINITIES OF ORGANIC BEINGS 403
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 be-
come fitted for sites or habits, in which organs of locomotion or of
the senses, etc., would be useless; and in this case the metamor-
phosis 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 corresponding ages, animals might
come to pass through stages of development, perfectly distinct
from the primordial condition of their adult progenitors. Most of
our best authorities are now convinced that the various larval
and pupal stages of insects have thus been acquired through adap-
tation, and not through inheritance from some ancient form. The
curious case of Sitaris — a beetle which passes through certain un-
usual stages of development — ^will illustrate how this might occur.
The first larval form is described by M. Fabre, as an active, minute
insect, furnished with six legs, two long antennae, and four eyes.
These larvae are hatched in the nests of bees; and when the male
bees emerge from their burrows, in the spring, which they do before
the females, the larvae spring on them, and afterward crawl on to
the females while paired with the males. As soon as the female bee
deposits her eggs on the surface of the honey stored in the cells,
the larvae of the Sitaris leap on the eggs and devour them. After-
ward they undergo a complete change ; their eyes disappear ; their
legs and antennae become rudimentary, and they feed on honey;
so that they now more closely resemble the ordinary larvae of in-
sects; 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 pro-
genitor of a whole new class of insects, the course of development
of the new class would be widely different from that of our exist-
ing insects; and the first larval stage certainly would not repre-
sent 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, entomostraca,
and even the malacostraca, appear at first as larvae under the nau-
pliusform ; and as these larvae live and feed in the open sea, and are
not adapted for any peculiar habits of life, and from other rea-
404 THE ORIGIN OF SPECIES
sons assigned by Fritz Miiller, it is probable that at some very
remote period an independent adult animal, resembling the Nau-
plius, existed, and subsequently produced, along several divergent
lines of descent, the above-named great Crustacean groups. So
again, it is probable, from what we know of the embryos of mam-
mals, birds, fishes, and reptiles, that these animals are the modified
descendants of some ancient progenitor, which was furnished in its
adult state with branchiae, a swim-bladder, four fin-like limbs, and
a long tail, all fitted for an aquatic life.
As all the organic beings, extinct and recent, which have ever
lived, can be arranged within a few great classes; and as all within
each class have, according to our theory, been connected together
by fine gradations, the best, and, if our collections were nearly
perfect, the only possible arrangement would be genealogical;
descent being the hidden bond of connection which naturalists
have been seeking under the term of the Natural System. On this
view we can understand how it is that, in the eyes of most nat-
uralists, the structure of the embryo is even more important for
classification than that of the adult. In two or more groups of
animals, however much they may differ from each other in struc-
ture 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 re-
lated. Thus, community in embryonic structure reveals com-
munity 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, cr may have
been so greatly modified through adaptation to new habits of life
as to be no longer recognizable. Even in groups in which the adults
have been modified to an extreme degree, community of origin is
often revealed by the structure of the larvse; we have seen, for
instance, that cirripedes, though externally so like shell-fish, are
at once known by their larvae to belong to the great class of crusta-
ceans. As the embryo often shows us more or less plainly the struc-
ture of the less modified and ancient progenitor of the group, we
can see why ancient and extinct forms so often resemble in their
adult state the embryos of existing species of the same class.
Agassiz believes this to be a universal law of nature; and we may
hope hereafter to see the law proved true. It can, however, be
proved true only in those cases in which the ancient state of the
progenitor of the group has not been wholly obliterated, either
by successive variations having supervened at a very early period
of growth, or by such variations having been inherited at an earlier
MUTUAL AFFINITIES OF ORGANIC BEINGS 405
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 forever, incapable of demonstration. The law will
not strictly hold good in those cases in which an ancient form be-
came adapted in its larvae state to some special line of life, and
transmitted the same larval state to a whole group of descendants ;
for such larval will not resemble any still more ancient form in its
adult state.
Thus, as it seems to me, the leading facts in embryology, which
are second to none in importance, are explained on the principle
of variations in the many descendants from some one ancient pro-
genitor, having appeared at a not very early period of life, and
having been inherited at a corresponding period. Embryology
rises greatly in interest, when we look at the embryo as a picture,
more or less obscured, of the progenitor, either in its adult or larval
state, of all the members of the same great class.
RUDIMENTARY, ATROPHIED AND ABORTED ORGANS
Organs or parts in this strange condition, bearing the plain
stamp of inutility, are extremely common, or even general,
throughout nature. It would be impossible to name one of the
higher animals in which some part or other is not in a rudimen-
tary condition. In the mammalia, for instance, the males possess
rudimentary mammae ; in snakes one lobe of the lungs is rudimen-
tary; in birds the "bastard-wing" may safely be considered as a
mdimentary digit, and in some species the whole wing is so far
rudimentary that it cannot be used for flight. What can be more
curious than the presence of teeth in foetal whales, which when
grown up have not a tooth in their heads; or the teeth, which
never cut through the gums, in the upper jaws of unborn calves?
Rudimentary organs plainly declare their origin and meaning
in various ways. There are beetles belonging to closely allied
species, or even to the same identical species, which have either
full-sized and perfect wings, or mere rudiments of membrane,
which not rarely lie under wing-covers firmly soldered together;
and in these cases it is impossible to doubt that the rudiments
represent wings. Rudimentary organs sometimes retain their po-
tentiality: this occasionally occurs with the mammae of male
mammals, which have been known to become 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
406 THE ORIGIN OF SPECIES
and yield milk. In regard to plants, the petals are sometimes rudi-
mentary, and sometimes well developed in the individuals of the
same species. In certain plants having separated sexes, Kolreuter
found that by crossing a species, in which the male flowers in-
cluded a rudiment of a pistil, with an hermaphrodite species, hav-
ing of course a well-developed pistil, the rudiment in the hybrid
offspring was much increased in size; and this clearly shows that
the rudimentary and perfect pistils are essentially alike in nature.
An animal may possess various parts in a perfect state, and yet
they may in one sense be rudimentary, for they are useless: thus
the tadpole of the common salamander or water-newt, as Mr.
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 full-formed. This animal never
lives in the water. Yet if we open a gravid female, we find tad-
poles inside her with exquisitely feathered gills; and when placed
in water they swim about like the tadpoles of the water-newt.
Obviously this aquatic organization 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 re-
peats a phase in the development of its progenitors."
An organ, serving for two purposes, may become rudimentary
or utterly aborted for one, even the more important purpose, and
remain perfectly efficient for the other. Thus, in plants, the office
of the pistil is to allow the pollen tubes to reach the ovules within
the ovarium. The pistil consists of a stigma supported on a style;
but in some Compositae, the male florets, which of course cannot
be 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 manner with hairs, which serve to brush the pollen
out of the surrounding and conjoined anthers. Again, an organ
may become rudimentary for its proper purpose, and be used for
a distinct one: in certain fishes the swim-bladder seems to be
rudimentary for its proper function of giving buoyancy, but has
become converted into a nascent breathing organ or lung. Many
similar instances could be given.
Useful organs, however little they may be developed, unless we
have reason to suppose that they were formerly more highly de-
veloped, ought not to be considered as rudimentary. They may
be in a nascent condition, and In progress toward further develop-
ment. Rudimentary 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
MUTUAL AFFINITIES OF ORGANIC BEINGS 407
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 se-
lection, which acts solely by the preservation of useful modifica-
tions. They have been partially retained by the power of inherit-
ance, and relate to a former state of things. It is, however, often
difficult to distinguish between rudimentary and nascent organs;
for we can judge only by analogy whether a part is capable of
further development, in which case alone it deserves to be called
nascent. Organs in this condition will always be somewhat rare;
for beings thus provided will commonly have been supplanted by
their successors with the same organ in a more perfect state, and
consequently will have become long ago extinct. The wing of the
penguin is of high service, acting as a fin ; it may, therefore, repre-
sent the nascent state of the wing; not that I believe this to be
the case; it is more probably a reduced organ, modified for a new
function; the wing of the Apteryx, on the other hand, is quite
useless, and is truly rudimentary. Owen considers the simple
filamentary limbs of the Lepidosiren as the "beginnings of organs
which attain full functional development in higher vertebrates;"
but, according to the view lately advocated by Dr. GUnther, they
are probably remnants, consisting of the persistent axis of a fin,
with the lateral rays or branches aborted. The mammary glands
of the Omithorhynchus may be considered, in comparison with
the udders of a cow, as in a nascent condition. The ovigerous
frena of certain cirripedes, which have ceased to give attachment
to the ova and are feebly developed, are nascent branchiae.
Rudimentary organs in the individuals of the same species are
very liable to vary in the degree of their development and in other
respects. In closely allied species, also, the extent to which the
same organ has been reduced occasionally differs much. This
latter fact is well exemplified in the state of the wings of female
moths belonging to the same family. Rudimentary organs may be
utterly aborted ; and this implies, that in certain animals or plants,
parts are entirely absent which analogy would lead us to expect
to find in them, and which are occasionally found in monstrous
individuals. Thus in most of the Scrophulariacese the fifth stamen
is utterly aborted; yet we may conclude that a fifth stamen once
existed, for a rudiment of it is found in many species of the family,
and this rudiment occasionally becomes perfectly developed, as
may 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 understand
408 THE ORIGIN OF SPECIES
the relations of the parts, more useful than the discovery of rudi-
ments. 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 afterward wholly disappear. It is also, I believe,
a universal rule, that a rudimentary part is of greater size in the
embryo relativdy 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 or-
gans in the adult are often said to have retained their embryonic
condition.
I have now given the leading facts with respect to rudimentary
organs. In reflecting on them, every one must be struck with
astonishment; for the same reasoning power which tells us that
most parts and organs are exquisitely adapted for certain pur-
poses, 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 cre-
ated "for the sake of symmetry," 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 thought of an astron-
omer 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 ac-
counts 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 rap-
idly growing embryonic calf by removing matter so precious as
phosphate of lime? When a man's fingers have been amputated,
imperfect nails have been known to appear on the stumps, and I
could as soon believe that these vestiges of nails are developed in
order to excrete horny matter, as that the rudimentary nails on
the fin of the manatee have been developed for this same purpose.
On the view of descent with modification, the origin of nidi-
MUTUAL AFFINITIES OF ORGANIC BEINGS 409
mentary organs is comparatively simple; and we can understand
to a large extent the laws governing their imperfect development.
We have plenty of cases of rudimentary organs in our domestic
productions, as the stump of a tail in tailless breeds, the vestige of
an ear in earless breeds of sheep — the re-appearance of minute
dangling horns in hornless breeds of cattle, more especially, ac-
cording to Youatt, in young animals — and the state of the whole
flower in the cauliflower. We often see rudiments of various parts
in monsters; but I doubt whether any of these cases throw light
on the origin of rudimentary organs in a state of nature, further
than by showing that rudiments can be produced; for the balance
of evidence clearly indicates that species under nature do not
undergo great and abrupt changes. But we learn from the study
of our domestic productions that the disuse of parts leads to their
reduced size; and that the result is inherited.
It appears probable that disuse has been the main agent in ren-
dering 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 in-
habiting dark caverns, and of the wings of birds inhabiting oceanic
islands, which have seldom been forced by beasts of pray to take
flight, and have ultimately lost the power of flying. Again, an
organ, useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small and ex-
posed islands; and in this case natural selection will have aided
in reducing the organ, until it was rendered harmless and rudi-
mentary.
Any change in structure and function, which can be effected by
small stages, is within the power of natural selection; so that an
organ rendered, through changed habits of life, useless or injurious
for one purpose, might be modified and used for another purpose.
An organ, might, also, be retained for one alone of its former
functions. Organs, originally formed by the aid of natural selec-
tion, when rendered useless may well be variable, for their varia-
tions can no longer be checked by natural selection. All this
agrees well with what we see under nature. Moreover, at whatever
period of life either disuse or selection reduces an organ, and this
will generally be when the being has come to maturity and has to
exert its full powers of action, the principle of inheritance at cor-
responding ages will tend to reproduce the organ in its reduced
state at the same mature age, but will seldom affect it in the em-
bryo. Thus we can understand the greater size of rudimentary
organs in the embryo relatively to the adjoining parts, and their
410 THE ORIGIN OF SPECIES
lesser relative size in the adult. If, for instance, the digit of an
adult animal was used less and less during many generations,
owing to some change of habits, or if an organ or gland was less
and less functionally exercised, we may infer that it would be-
come 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 consequence much reduced,
how can it be still further reduced in size until the merest vestige
is left; and how can it be finally quite obliterated? It is scarcely
possible that disuse can go on producing any further effect after
the organ has once been rendered functionless. Some additional
explanation is here requisite which I cannot give. If, for instance,
it could be proved that every part of the organization tends to
vary in a greater degree toward diminution than toward augmenta-
tion of size, then we should be able to understand how an organ
which has become useless would be rendered, indep)endently of
the effects of disuse, rudimentary, and would at last be wholly
suppressed; for the variations toward diminished size would no
longer be checked by natural selection. The principle of the econ-
omy of growth, explained in a former chapter, by which the ma-
terials forming any part, if not useful to the possessor, are saved
as far as is possible, will perhaps come into play in rendering a
useless part rudimentary. But this principle will almost necessarily
be confined to the earlier stages of the process of reduction; for we
cannot suppose that a minute papilla, for instance, representing in
a male flower the pistil of the female flower, and formed merely
of cellular tissue, would be further reduced or absorbed for the
sake of economizing 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 inheritance — 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 some-
times more useful than, parts of high physiological importance.
Rudimentary organs may be compared with the letters in a word,
still retained in the spelling, but become useless in the pronuncia-
tion, but which serve as a clew for its derivation. On the view of
descent with modification, we may conclude that the existence of
organs in a rudimentary, imperfect, and useless condition, or quite
MUTUAL AFFINITIES OF ORGANIC BEINGS 411
aborted, far from presenting a strange difficulty, as they assuredly
do on the old doctrine of creation, might even have been antici-
pated in accordance with the views here explained.
SUMMARY
In this chapter I have attempted to show that the arrangement
of all organic beings throughout all time in groups under groups —
that the nature of the relationships by which all living and extinct
organisms are united by complex, radiating, and circuitous lines
of affinities into a few grand classes — the rules followed and the
difficulties encountered by naturalists in their classifications — the
value set upon characters, if constant and prevalent, whether of
high or of the most trifling importance, or, as with rudimentary
organs, of no importance — the wide opposition in value between
analogical or adaptive characters, and characters of true affinity;
and other such rules; — all naturally follow if we admit the com-
mon parentage of allied forms, together 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 element of de-
scent has been universally used in ranking together the sexes, ages,
dimorphic forms, and acknowledged varieties of the same species,
however much they may differ from each other in structure. If we
extend the use of this element of descent^ — the one certainly known
cause of similarity in organic beings — we shall understand what
is meant by the Natural System: it is genealogical in its attempted
arrangement, with the grades of acquired difference marked by the
terms, varieties, species, genera, families, orders, and classes.
On this same view of descent with modification, most of the
great facts in Morphology become intelligible — whether we look
to the same pattern displayed by the different species of the same
class in their homologous organs, to whatever purpose applied;
or to the serial and lateral homologies in each individual animal
and plant.
On the principle of successive slight variations, not necessarily
or generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the leading
facts in embryology; namely, the close resemblance in the indi-
vidual 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
412 THE ORIGIN OF SPECIES
been specially modified in a greater or less degree in relation to
their habits of life, with their modifications inherited at a cor-
responding 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 im-
portance of embryological characters and of rudimentary organs
in classification is intelligible, on the view that a natural arrange-
ment 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 in-
numerable species, genera, and families, with which this world is
peopled, are all descended, each within its own class or group,
from common parents, and have all been modified in the course
of descent, that I should without hesitation adopt this view, even
if it were unsupported by other facts or arguments.
CHAPTER XV
Recapitulation and Conclusion
Recapitulation of the Objections to the Theory of Natural Selection — ^Re-
capitulation of the General and Special Circumstances in its Favor —
Causes of the General Belief in the Immutability of Species — How far
the Theory of Natural Selection may be extended — Effects of its Adop-
tion 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 re-
capitulated.
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 endeavored to give to them their
full force. Nothing at first can appear more difficult to believe
than that the more complex organs and instincts have been per-
fected, not by means superior to, though analogous with, human
reason, but by the accumulation of innumerable slight variations,
each good for the individual possessor. Nevertheless, this diffi-
culty, though appearing to our imagination insuperably great, can-
not be considered real if we admit the following propositions,
namely, that all parts of the organization and instincts offer, at
least, individual differences — that there is a struggle for existence
leading to the preservation of profitable deviations of structure or
instinct — and, lastly, that gradations 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
among 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 ar-
rived 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
413
414 THE ORIGIN OF SPECIES
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 recapitulation 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 reproductive systems of
the inter-crossed 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 di-
morphic and trimorphic plants clearly leads to the same conclu-
sion, 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 respect except in their reproductive organs and func-
tions.
Although the fertility of varieties when intercrossed, and of their
mongrel offspring, has been asserted by so many authors to be
universal, this cannot be considered as quite correct after the facts
given on the high authority of Gartner and Kolreuter. Most of
the varieties which have been experimented on have been produced
under domestication; and as domestication (I do not mean mere
confinement) almost certainly tends to 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 gradually 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 conditions of life give vigor 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 vigor. This is chiefly owing to the forms which
RECAPITULATION AND CONCLUSION 415
are crossed having been exposed to somewhat different conditions
of life; for I have ascertained by a laborious series of experiments
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 con-
ditions, either perish, or if they survive, are rendered sterile,
though retaining perfect health. This does not occur, or only in a
very slight degree, with our domesticated productions, which
have long been exposed to fluctuating conditions. Hence when we
find that hybrids produced by a cross between two distinct species
are few in number, owing to their perishing soon after conception
or at a very early age, or if surviving that they are rendered more
or less sterile, it seems highly probable that this result is due to
their having been in fact subjected to a great change in their con-
ditions of life, from being compounded of two distinct organiza-
tions4 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, while two domesticated varieties when crossed
and their mongrel offspring are perfectly fertile.'
Turning to geographical distribution, the difficulties encoun-
tered 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 com-
mon 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 aU
the others. We are often wholly unable even to conjecture how
this could have been effected. Yet, as we have reason to believe
that some species have retained the same specific form for very
long periods of time, immensely long as measured by years, too
much stress ought not to be laid on the occasional wide diffusion
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
416 THE ORIGIN OF SPECIES
the various climatical and geographical changes which have af-
fected 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 distribution of the same and of allied species throughout
the world. We are as yet profoundly ignorant of the many occa-
sional 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 migra-
tion 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 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 con-
tinuous, and of which the climatic and other conditions of life
change insensibly in proceeding from a district occupied by one
species into another district occupied by a closely allied species,
we have no just right to expect often to find intermediate varieties
in the intermediate zones. For we have reason to believe that only
a few species of a genus ever undergo change; the other species
becoming utterly extinct and leaving no modified progeny. Of the
species which do change, only a few within the same country
change at the same time; and all modifications are slowly effected.
I have also shown that the intermediate varieties which probably
at first existed in the intermediate zones, would be liable to be
supplanted by the allied forms on either hand; for the latter,
from existing in greater numbers, would generally be modified and
improved at a quicker rate than the intermediate varieties, which
existed in lesser numbers; so that the intermediate varieties
would, in the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of con-
necting 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
RECAPITULATION AND CONCLUSION 417
plain evidence of the gradation and mutation of the forms of life?
Although geological research has undoubtedly revealed the former
existence of many links, bringing numerous forms of life much
closer together, it does not yield the infinitely m.any fine grada-
tions between past and present species required on the theory, and
this is the most obvious of the many objections which may be
urged against it. Why, again, do whole groups of allied species
appear, though this appearance is often false, to have come in
suddenly on the successive geological stages? Although we now
know that organic beings appeared on this globe, at a period in-
calculably remote, long before the lowest bed of the Cambrian
system was deposited, why do we not find beneath this system
great piles of strata stored with the remains of the progenitors of
the Cambrian fossils? For on the theory, such strata must some-
where have been deposited at these ancient and utterly unknown
epochs of the world's history.
I can answer these questions and objections only on the sup- r
position that the geological record is far more imperfect 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 characters di-
rectly intermediate between its modified offspring, any more than
the rock-pigeon is directly intermediate in crop and tail between
its descendants, the pouter and fantail pigeons. We should not
be able to recognize a species as the parent of another and modified
species, if we were to examine the two ever so closely, unless we
possessed most of the intermediate links; and owing to the im-
perfection of the geological record, we have no just right to expect
to find so many links. If two or three, or even more linking forms
were discovered, they would simply be ranked by many naturalists
as so many new species, more especially if found in different
geological sub-stages, let their differences be ever so slight. Nu-
merous existing doubtful forms could be named which are prob-
ably varieties; but who will pretend that in future ages so many
fossil links will be discovered, that naturalists will be able to de-
cide whether or not these doubtful forms ought to be called
varieties? Only a small portion of the world has bfn 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
I
418 THE ORIGIN OF SPECIES
measured by years, have probably been short in comparison with
the periods during which they retained the same form. It is the
dominant and widely ranging species which vary most frequently
and vary most, and varieties are often at first local — both causes
rendering the discovery of intermediate links in any one formation
less likely. Local varieties will not spread into other and distant
regions until they are considerably modified and improved; and
when they have spread, and are discovered in a geological 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 duration has probably been shorter
than the average duration of specific forms. Successive forma-
tions are in most cases separated from each other by blank in-
tervals of time of great length, for fossiliferous formations thick
enough to resist future degradation can, as a general rule, be ac-
cumulated 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 rela-
tive positions, we have no reason to assume that this has always
been the case; consequently formations much older than any now
known may lie buried beneath the great oceans. With respect to the
lapse of time not having been sufficient since our planet was con-
solidated 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 constitution of the universe and of
the interior of our globe to speculate with safety on its past dura-
tion.
That the geological record is imperfect, all will admit; but
that it is imp '^rf ect 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
RECAPITULATION AND CONCLUSION 419
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 Dis-
tribution during the long lapse of years, or that we know how im-
perfect is the Geological Record. Serious as these several objections
are, in my judgment they are by no means sufficient to overthrow
the theory of descent with subsequent modification.
Now let us turn to the other side of the argument. Under do-
mestication 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. Varia-
bility is governed by many complex laws, by correlated growth,
compensation, the increased use and disuse of parts, and the
definite action of the surrounding conditions. There is much diffi-
culty in ascertaining how largely our domestic productions have
been modified ; but we may safely infer that the amount has been
large, and that modifications 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 evi-
dence 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 uninten-
tionally exposes organic beings to new conditions of life, and then
nature acts on the organization and causes it to vary. But man C2in
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 me-
thodically, or he may do it unconsciously by preserving the indi-
viduals most useful or pleasing to him without any intention of
420 THE ORIGIN OF SPECIES
altering the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive generation,
individual differences so slight as to be inappreciable except by
an educated eye. This unconscious process of selection 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 in-
extricable doubts whether many of them are varieties or aborigi-
nally distinct species.
There is no reason why the principles which have acted so
efficiently under domestication should not have acted under na-
ture. In the survival of favored individuals and races, during the
constantly recurrent Struggle for Existence, we see a powerful and
ever-acting form of Selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is com-
mon 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 naturalized in
new countries. More individuab are born than can possibly sur-
vive. 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 individuals of the same species come in all respects into
the closest competition with each other, the struggle will generally
be most severe between them; it will be almost equally severe
between the varieties of the same species, and next in severity be-
tween 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 compe-
tition, or better adaptation in however slight a degree to the
surrounding physical conditions, will, in the long-run, turn the
balance.
With animals having separated sexes, there will be in most
cases a struggle between the males for the possession of the
females. The most vigorous males, or those which have most suc-
cessfully struggled with their conditions of life, will generally
leave most progeny. But success will often depend on the males
having special weapons or means of defence or charms; and a
slight advantage will lead to victory.
As geology plainly proclaims that each land has undergone
great physical changes, we might have expected to find that or-
ganic beings have varied under nature, in the same way as they
RECAPITULATION AND CONCLUSION 421
have varied under domestication. And if there has been any vari-
ability under nature, it would be an unaccountable fact if natural
selection had not come into play. It has often been asserted, but
the assertion is incapable of proof, that the amount of variation
under nature is a strictly limited quantity. Man, though acting
on external characters alone and often capriciously, can produce
within a short period a great result by adding up mere individual
differences in his domestic productions; and every one admits that
species present individual differences. But, beside such differences,
all naturalists admit that natural varieties exist, which are con-
sidered sufficiently distinct 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 separate continents, and
on different parts of the same continent, 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 individual 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 chang-
ing 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 scrutinizing the whole constitution, structure, and habits of
each creature, favoring 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 further than this, seems to be in the
highest degree probable. 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 favor of the theory.
On the view that species are only strongly marked and perma-
nent varieties, and that each species first existed as a variety, we
can see why it is that no line of demarcation can be drawn be-
tween species, commonly supposed to have been produced by
special acts of creation, 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
422 THE ORIGIN OF SPECIES
genus have been produced, and where they now flourish, these
same species should present many varieties; for where the manu-
factory of species has been active, we might expect, as a general
rule, to fmd it still in action; and this is the case if varieties be
incipient species. Moreover, the species of the larger genera, which
afford the greater number of varieties or incipient species, retain
to a certain degree the character of varieties; for they differ from
each other by a less amount of difference than do the species of
smaller genera. The closely allied species also of a 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 reproduction
to increase inordinately in number; and as the modified descend-
ants of each species will be enabled to increase by as much as they
become more diversified in habits and structure, so as to be able
to seize on many and widely different places in the economy of
nature, there will be a constant tendency in natural selection to
preserve the most divergent offspring of any one species. Hence,
during a long-continued course of modification, the slight differ-
ences characteristic of varieties of the same species, tend to be
augmented into the greater differences characteristic of the species
of the same genus. New and improved varieties will inevitably
supplant and exterminate the older, less improved, and intermedi-
ate varieties; and thus species are rendered to a large exten;
defined and distinct objects. Dominant species belonging to th;
larger groups within each class tend to give birth to new and domi •
nant 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 character, 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 inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, succes-
sive, favorable variations, it can produce no great or sudden
modifications; it can act only by short and slow steps. Hence, the
RECAPITULATION AND CONCLUSION 423
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 modi-
fied 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 I and so in endless other cases. But on the view of each
species constantly trying to increase in number, with natural se-
lection always ready to adapt the slowly varying descendants of
each to any unoccupied 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 at-
tributed to the agency of selection. That beauty, according to our
sense of it, is not universal, must be admitted by every one who
will look at some venomous snakes, at some fishes, and at certain
hideous bats with a distorted resemblance to the human face.
Sexual selection has given the most brilliant colors, elegant pat-
terns, 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 colors in contrast with the green foliage, in
order that the flowers may be easily seen, visited and fertilized
by insects, and the seeds disseminated by birds. How it comesN
that certain colors, 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 odors and flavors 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
424 THE ORIGIN OF SPECIES
created and specially adapted for that country, being beaten and
supplanted by the naturalized productions from another land. Nor
ought we to marvel if all the contrivances in nature be not, as far
as we can judge, absolutely perfect, as in the case even of the
human eye; or if some of them be abhorrent to our ideas of fitness.
We need not marvel at the sting of the bee, when used against an
enemy, causing the bee's own death; at drones being produced in
such great numbers for one single act, and being then slaughtered
by their sterile sisters; at the astonishing waste of pollen by our
fir-trees; at the instinctive hatred of the queen bee for her own
fertile daughters ; at ichneumonidae feeding within the living bodies
of caterpillars; or at other such cases. The wonder, indeed, is, on
the theory of natural selection, that more cases of the want of
absolute perfection have not been detected.
The complex and little known laws governing the production of
varieties are the same, as far as we can judge, with the laws which
have governed the production of distinct species. In both cases
physical conditions seem to have produced some direct and definite
effect, but how much we cannot say. Thus, when varieties enter
any new station, they occasionally assume some of the characters
proper to the species of that station. With both varieties and
species, use and disuse seem to have produced a considerable
effect; for it is impossible to resist this conclusion when we look,
for instance, at the logger-headed duck, which has wings in-
capable of flight, in nearly the same condition as in the domestic
duck; or when we look at the borrowing tucu-tucu, which is oc-
casionally 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 inhabiting 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 modified. With both
varieties and species, reversions to long-lost characters occasion-
ally occur. How inexplicable on the theory of creation is the oc-
casional appearance of stripes on the shoulders and legs of the
several species of the horse-genus and of their hybrids! How
simply is this fact explained if we believe that these species are
all descended from a striped progenitor, in the same manner as
the several domestic breeds of the pigeon are descended from the
blue and barred rock-pigeon !
On the ordinary view of each species having been independently
created, why should specific characters, or those by which the
RECAPITULATION AND CONCLUSION 425
species of the same genus differ from each other, be more variable
than generic characters in which they all agree? Why, for instance,
should the color of a flower be more likely to vary in any one
species of a genus, if the other species possess differently colored
flowers, than if all possessed the same colored flowers? If species
are only well-marked varieties, of which the characters have be-
come in a high degree permanent, we can imderstand this fact;
for they have already varied since they branched off from a com-
mon 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 im-
mense 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 impor-
tance to that species, should be eminently liable to variation ; but,
on our view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount of
variabihty 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
constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no
greater difficulty than do corporeal structures on the theory of the
natural selection of successive, slight, but profitable modifications.
We can thus understand why nature moves by graduated steps in
endowing different animals of the same class with their several in-
stincts. I have attempted to show how much light the principle
of gradation throws on the admirable architectural powers of the
hive-bee. Habit no doubt often comes into play in modifying in-
stincts ; but it certainly is not indispensable, as we see in the case
of neuter insects, which leave no progeny to inherit the effects of
long-continued habit. On the view of all the species of the same
genus having descended from a common parent, and having in-
herited much in common, we can understand 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
temperate South America, for instance, line their nests with mud
like our British species. On the view of instincts having been
426 THE ORIGIN OF SPECIES
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 varieties, 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 acknowl-
edged 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 ex-
treme degree, then the facts, which the record does give, strongly
support the theory of descent with modification. New species have
come on the stage slowly and at successive intervals; and the
amount of change, after equal intervals of time, is widely different
in different 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 prin-
ciple 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 modifica-
tion of their descendants, causes the forms of life, after long in-
tervals of time, to appear as if they had changed simultaneously
throughout the world. The fact of the fossil remains of each forma-
tion 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, natu-
rally follows from the living and the extinct being the offspring
of common parents. As species have generally diverged in char-
acter during their long course of descents and jnodification , we
can understand 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 organization 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 specialized for different func-
tions. This fact is perfectly compatible with numerous beings still
retaining simple and but little improved structures, fitted for
}
RECAPITULATION AND CONCLUSION 427
simple conditions of life; it is likewise compatible with some forms
having retrograded in organization, 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 climatical and
geographical changes and to the many occasional and unknown
means of dispersal, then we can understand, on the theory of
descent with modification, most cf the great leading facts in dis-
tribution. We can see why there should be so striking a parallel-
ism 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 modification have been the same. We see the
full meaning of the wonderful fact, which has struck every travel-
ler, 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 inhabitants 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
understand, by the aid of the Glacial period, the identity of some
few plants, and the close alliance of many others, on the most dis-
tant mountains, and in the northern and southern temperate
zones; and likewise the close alliance of some of the inhabitants
of the sea in the northern and southern temperate latitudes,
though separated by the whole intertropical ocean. Although two
countries may present physical conditions as closely similar as
the same species ever require, we need feel no surprise at their
inhabitants being widely different, if they have been for a long
period completely sundered from each other; for as the relation
of organism to organism is the most important of all relations, and
as the two countries will have received colonists at various periods
and in different proportions, from some other country or from
each other, the course of modification in the two areas will in-
evitably have been different.
On this view of migration, with subsequent modification, we see
why oceanic islands are inhabited by only few species, but of
428 THE ORIGIN OF SPECIES
these, why many are peculiar or endemic forms. We clearly see
why species belonging to those groups of animals which cannot
cross wide spaces of the ocean, as frogs and terrestrial mammals,
do not inhabit oceanic islands; and why, on the other hand, new
and peculiar species of bats, animals which can traverse the ocean,
are often found on islands far distant from any continent. Such
cases as the presence of peculiar species of bats on oceanic islands
and the absence of all other terrestrial mammals, are facts utterly
inexplicable on the theory of independent acts of creation.
The existence of closely allied representative species 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 ^ecies
inhabit two areas, some identical species are still common to
both. Wherever many closely allied yet distinct species occur,
doubtful forms and varieties belonging to the same groups like-
wise occur. It is a rule of high generality that the 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 Gala-
pagos Archipelago, of Juan Fernandez, and of the other American
islands, to the plants and animals of those of the mainland; and
of those of the Cape de Verde Archipelago, 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 inteUigible on the theory of
natural selection with its contingencies of extinction and diver-
gence of character. On these same principles we see how it is that
the mutual 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 importance to the beings, are of hardly any
importance in classification; why characters derived from rudi-
mentary parts, though of no service to the beings, are often of
high classificatory value; and why embryological characters are
often the most valuable of all. The real affinities of all organic
beings, in contra-distinction to their adaptive resemblances, are
due to inheritance or community of descent. The Natural System
is a genealogical arrangement with the acquired grades of differ-
ence, marked by the terms, varieties, species, genera, families,
RECAPITULATION AND CONCLUSION 429
etc.; and we have to discover the lines of descent by the most
permanent characters, whatever they may be, and of however
slight vital importance.
The similar framework of bones m the hand of a man, wing
of a bat, fin of the porpoise, and leg of the horse — the same num-
ber of vertebrae forming the neck of the giraffe and of the ele-
phant— and innumerable other such facts, at once explain them-
selves 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 pistils of a flower,
is likewise, to a large extent, intelligible on the view of the grad-
ual modification of parts or organs, which were aboriginally
alike in an early progenitor in each of these classes. On the prin-
ciple of successive variations not always supervening at an early
age, and being inherited at a corresponding not early period of
life, we clearly see why the embryos of mammals, birds, reptiles,
and fishes should be so closely similar and so unlike the adult
forms. We may cease marvelling at the embryo of an air-breath-
ing mammal or bird having branchial slits and arteries running
in loops, like those of a fish which has to breathe the air dis-
solved 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 understand on this view the mean-
ing of rudimentary organs. But disuse and selection will generally
act on each creature, when it has come to maturity and has to
play its full part in the struggle for existence, and will thus have
little power on an organ during early life; hence the organ will
not be reduced or rendered rudimentary at this early age. The
calf, for instance, has inherited teeth, which never cut through
the gums of the upper jaw, from an early progenitor having well-
developed teeth ; and we may beheve, 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 unaffected, and on the principle of inheritance
at corresponding 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 inex-
plicable 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
430 THE ORIGIN OF SPECIES
occur. Nature may be said to have taken pains to reveal her
scheme of modification, by means of rudimentary organs, of em-
bryological and homologous structures, but we are too blind to
understand her meaning.
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, favor-
able variations ; aided in an important manner by the inherited ef-
fects of the use and disuse of parts ; and in an unimportant manner,
that is, in relation to adaptive structures, whether past or present,
by the direct action of external conditions, and by variations
which seem to us in our ignorance to arise spontaneously. It ap-
pears 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 con-
clusions have lately been much misrepresented, and it has been
stated that I attribute the rnodification of species exclusively to
natural selection, rinay^i~pefmltted to remark that in the first
edition of this work, and subsequently, I placed in a most con-
spicuous position — 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 modification." 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 selec-
tion, the several large classes of facts above specified. It has re-
cently been objected that this is an unsafe method of arguing;
but it is a method used in judging of the common events of life,
and has often been used by the greatest natural philosophers. The
undulatory theory of light has thus been arrived at ; and 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;
notwithstanding that Leibnitz formerly accused Newton of in-
troducing "occult qualities and miracles into philosophy."
I see no good reasons why the views given in this volume
should shock the religious feelings of any one. It is satisfactory,
RECAPITULATION AND CONCLUSION 431
as showing how transient such impressions are, to remember that
the greatest discovery ever made by man, namely, the law of the
attraction of gravity, was also attacked by Leibnitz, "as sub-
versive of natural, and inferentially of revealed, religion." A
celebrated author and divine has written to me that "he has
gradually learned to see that it is just as noble a conception of
the Deity to believe that He created a few original forms capable
of self-development into other and 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 disbelieve in the mu-
tability of species? It cannot be asserted that organic beings in a
state of nature are subject to no variation; it cannot be proved
that the amount of variation in the course of long ages is a
limited quantity; no clear distinction has been, or can be, drawn
between species and well-marked varieties. It cannot be main-
tained that species when intercrossed are invariably sterile and
varieties invariably fertile; or that sterility is a special endow-
ment and sign of creation. The belief that species were immu-
table productions was almost unavoidable as long as the history
of the world was thought to be of short duration; and now that
we have acquired some idea of the lapse of time, we are too apt
to assume, without proof, that the geological record is so perfect
that it would have afforded us plain evidence of the mutation of
species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that
one species has given birth to other and distinct species, is that
we are always slow in admitting great changes of which we do
not see the steps. The difficulty is the same as that felt by so
m-any geologists, when Lyell first insisted that long lines of in-
land cliffs had been formed, and great valleys excavated, by the
agencies which we still see 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 varia-
tions, accumulated during an almost infinite number of genera-
tions.
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 ex-
pect 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 crea-
432 THE ORIGIN OF SPECIES
tion,'^ "unity of design," etc., and to think that we give an ex-
planation when we only restate a fact. Any one whose disposition
leads him to attach more weight to unexplained difficulties than
to the explanation of a certain number of facts will certainly
reject the theory. A few naturalists, endowed with much flexibil-
ity 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 im-
partiality. Whoever is led to believe 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 in-
dependently 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 con-
sequently have all the external characteristic features of true spe-
cies— they admit that these have been produced by variation, but
they refuse to extend the same view to other and slightly differ-
ent 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
blindness 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 innumerable periods in the
earth's history certain elemental atoms have been commanded
suddenly to flash into living tissues? Do they believe that at each
supposed act of creation one individual or many were produced?
Were all the infinitely numerous kinds of animals and plants
created as eggs or seed, or as full grown? and in the case of mam-
mals, were they created bearing the false marks of nourishment
from the mother's womb? Undoubtedly some of these same ques-
tions 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 Mauper-
RECAPITULATION AND CONCLUSION 433
tuis' philosophical axiom of "least action" leads the mind more
willingly to admit the smaller number; and certainly we ought
not to believe that innumerable beings within each great class
have been created with plain, but deceptive, marks of descent
from a single parent.
As a record of a former state of things, I have retained in the
foregoing paragraphs, and elsewhere, several sentences which
imply that naturalists believe in the separate creation of each
species ; and I have been much censured for having thus expressed
myself. But undoubtedly this was the general belief when the first
edition of the present work appeared. I formerly spoke to very
many naturalists on the subject of evolution, and never once met
with any sjnupathetic 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. There are,
however, some who still think that species have suddenly given
birth, through quite unexplained means, to new and totally differ-
ent forms. But, as I have attempted to show, weighty evidence
can be opposed to the admission of great and abrupt modifications.
Under a scientific point of view, and as leading to further in-
vestigation, but little advantage is gained by believing that new
forms are suddenly developed in an inexplicable 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 modifica-
tion of species. The question is difficult to answer, because the
more distinct the forms are which we consider, by so much the
arguments in favor of community of descent become fewer in
number and less in force. But some arguments of the greatest
weight extend very far. All the members of whole classes are
connected together by a chain of affinities, and all can be classed
on the same principle, in groups subordinate to groups. Fossil
remains sometimes tend to fill up very wide intervals between
existing orders.
Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed condition, and this
in some cases implies an enormous amount of modification in the
descendants. Throughout whole classes various structures are
formed on the same pattern, and at a very early age the embryos
closely resemble each other. Therefore I cannot doubt that the
theory of descent with modification embraces all the members of
434 THE ORIGIN OF SPECIES
the same great class or kingdom. I believe that animals are de-
scended from at most only four or five progenitors, and plants
from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief
that all animals and plants are descended from some one proto-
type. But analogy may be a deceitful guide. Nevertheless all liv-
ing things have much in common, in their chemical composition,
their cellular structure, their laws of growth, and their liability
to injurious 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, ex-
cepting perhaps some of the very lowest, sexual reproduction
seems to be essentially similar. With all, as far as is at present
known, the germinal vesicle is the same; so that all organisms
start from a common origin. If we look even to the two main
divisions — namely, to the animal and vegetable kingdoms — cer-
tain low forms are so far intermediate in character that naturalists
have disputed to which kingdom they should be referred. As
Professor Asa Gray has remarked, "the spores and other reproduc-
tive 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 character, 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 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 Vertebrata, Articulata, etc., we have dis-
tinct evidence in their embryological, homologous, and rudimen-
tary structures, 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 labors as at present; but they will not be
RECAPITULATION AND CONCLUSION 435
incessantly haunted by the shadowy doubt whether this or that
form be a true species. This, I feel sure, and I speak after ex-
perience, will be no slight relief. The endless disputes whether
or not some fifty species of British brambles are good species will
cease. Systematists will have only to decide (not that this will be
easy) whether any form be sufficiently constant, and distinct
from other forms, to be capable of definition; and if definable,
whether the differences be sufficiently important to deserve a
specific name. This latter point will become a far more essential
consideration that it is at present; for differences, however slight,
between any two forms, if not blended by intermediate gradations,
are looked at by most naturalists as sufficient to raise both forms
to the rank of species.
Hereafter, we shall be compelled to acknowledge that the only
distinction between species and well-marked 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 existence of intermediate gradations between any two
forms, we shall be led to weigh more carefully and to value higher
the acturl amount of difference between them. It is quite possible
that forms now generally acknowledged to be merely varieties
may hereafter be thought worthy of specific names; and in this
case scientific and common language will come into accordance.
In short, we shall have to treat species in the same manner as
those naturalists treat genera, who admit that genera are merely
artificial combinations made for convenience. This may not be a
cheering prospect; but we shall at least be freed from the vain
search for the undiscovered and undiscoverable essence of the term
species.
The other and more general departments of natural history
will rise greatly in interest. The terms used by naturalists, of
affinity, relationship, community of type, paternity, morphology,
adaptive characters, rudimentary and aborted organs, etc., will
cease to be metaphorical, 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 labor, the experience, the reason, and even the
blunders of numerous workmen; when we thus view each organic
436 THE ORIGIN OF SPECIES
being, how far more interesting — I speak from experience — does
the study of natural history become!
A grand and almost untrodden field of inquiry will be opened,
on the causes and laws of variation, on correlation, 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 classifying will no doubt become simpler when we have
a definite object in view. We possess no pedigree or armorial bear-
ings; 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. Spe-
cies and groups of species which are called aberrant, and which
may fancifully be called living fossils, will aid us in forming a
picture of the ancient forms of life. Embryology will often reveal
to us the structure, in some degree obscured, of the prototypes
of each great class.
When we can feel assured that all the individuals of the same
species, and all the closely aUied species of most genera, have,
within a not very remote period, descended from one parent, and
have migrated from some one birthplace; 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, we shall surely
be enabled to trace in an admirable manner the former migra-
tions of the inhabitants of the whole world. Even at present, by
comparing the differences between the inhabitants of the sea on
the opposite sides of a continent, and the nature of the various 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 im-
perfection of the record. The crust of the earth, with its em-
bedded remains, must not be looked at as a well-filled museum,
but as a poor collection made at hazard and at rare intervals.
The accumulation of each great fossiliferous formation will be
recognized as having depended on an unusual occurrence of favor-
able circumstances, and the blank intervals between the successive
stages as having been of vast duration. But we shall be able to
RECAPITULATION AND CONCLUSION 437
gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We
must be cautious in attempting to correlate as strictly contem-
poraneous two formations, which do not include many identical
species, by the general succession of the forms of life. As species
are produced and exterminated by slowly acting and still existing
causes, and not by miraculous acts of creation; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism
— the improvement of one organism entailing the improvement
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 relative, though not actual lapse
of time. A number of species, however, keeping in a body might
remain for a long period unchanged, while within the same pe-
riod, several of these species, by migrating into new countries
and coming into competition with foreign associates, might be-
come modified; so that we must not overrate the accuracy of
organic change as a measure of time.
In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well
laid by Mr. Herbert Spencer, that of the necessary acquirement
of each mental power and capacity by gradation. Much light will
be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with
the view that each species has been independently created. To my
mind it accords better with what we know of the laws impressed
on matter by the Creator, that the production and extinction of
the past and present inhabitants of the world should have been
due to secondary causes, like those determining the birth and
death of the individual. When I view all beings not as special
creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Cambrian system was de-
posited, they seem to me to become ennobled. Judging from the
past, we may safely infer that not one living species will trans-
mit its unaltered likeness to a distinct futurity. And of the species
now living, very few will transmit progeny of any kind to a far
distant futurity; for the manner in which all organic beings are
grouped shows that the greater number of species in each genus,
and all the species in many genera, have left no descendants, but
have become utterly extinct. We can so far take a prophetic
glance into futurity as to foretell that it will be the common and
438 THE ORIGIN OF SPECIES
widely spread species, belonging to the larger and dominant
groups within each class, which will ultimately prevail and pro-
create new and dominant species. As all the living forms of life
are the lineal descendants of those which lived long before the
Cambrian epoch, we may feel certain that the ordinary succession
by generation has never once been broken, and that no cataclysm
has desolated the whole world. Hence, we may look with some
confidence to a secure future of great length. And as natural se-
lection works solely by and for the good of each being, all corpo-
real and mental endowments will tend to progress toward per-
fection.
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 con-
structed 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; Inheritance which is almost
implied by reproduction; Variability from the indirect and direct
action of the conditions of life, and from use and disuse: a Ratio
of Increase so high as to lead to a Struggle for Life, and as a
consequence to Natural Selection, entailing Divergence of Char-
acter 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 conceiving, namely, the production of
the higher animals, directly follows. There is grandeur in this
view of life with its several powers, having been originally breathed
by the Creator into a few forms or into one; and that, while
this planet has gone circling 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.
APPENDIX
APPENDIX
AN HISTORICAL SKETCH
OF THE PROGRESS OF OPINION ON THE ORIGIN
OF SPECIES,
PREVIOUSLY TO THE PUBLICATION OF THE
FIRST EDITION OF THIS WORK
I WILL here give a brief sketch of the progress of opinion on
the Origin of Species. Until recently the great majority of nat-
uralists believed that species were immutable productions, and had
been separately created. This view has been ably maintained by
many authors. Some few naturalists, on the other hand, have be-
lieved that species undergo modification, and that the existing
forms of life are the descendants by true generation of pre-exist-
ing forms. Passing over allusions to the subject in the classical
writers,* the first author who in modern times has treated it in
a scientific spirit was Buff on. But as his opinions fluctuated greatly
at different periods, and as he does not enter on the causes or
means of the transformation of species^ I need not here enter on
details.
Lamarck was the first man whose conclusions on the subject
♦Aristotle, in his "Physicae Auscultationes" (lib. 2, cap. 8, s. 2), after re-
marking that rain does not fall in order to make the corn grow, any more
than it falls to spoil the farmer's com when threshed out of doors, applies
the same argument to organization; and adds (as translated by Mr. Clair
Grece, who first pointed out the passage to me), "So what hinders the dif-
ferent parts [of the body] from having this merely accidental relation in
nature? as the teeth, for example, grow by necessity, the front ones sharp,
adapted for dividing, and the grinders flat, and serviceable for masticating
the food ; since they were not made for the sake of this, but it was the result
of accident. And in like manner as to other parts in which there appears to
exist an adaptation to an end. Wheresoever, therefore, all things together
(that is, all the parts of one whole) happened like as if they were made for
the sake of something, these were preserved, having been appropriately con-
stituted by an internal spontaneity; and whatsoever things were not thus
constituted, perished and still perish." We here see the principle of natural
selection shadowed forth, but how little Aristotle fully comprehended the
principle, is shown by his remarks on the formation of the teeth.
441
442 THE ORIGIN OF SPECIES
excited much attention. This justly celebrated naturalist first pub-
lished his views in 1801; he much enlarged them in 1809 in his
"Philosophie Zoologique," and subsequently, 1815, in the Intro-
duction to his "Hist. Nat. des Animaux sans Vertebres." In these
works he upholds the doctrine that all species, including man, are
descended from other species. He first did the eminent service of
arousing attention to the probability of all change in the organic,
as well as in the inorganic world, being the result of law, and not
of miraculous interposition. Lamarck seems to have been chiefly
led to his conclusion on the gradual change of species, by the
difficulty of distinguishing species and varieties, by the almost per-
fect gradations of forms in certain groups, and by the analogy of
domestic productions. With respect to the means of modification,
he attributed something to the direct action of the physical con-
ditions of life, something to the crossing of already existing forms,
and much to use and disuse, that is, to the effects of habit. To this
latter agency he seems to attribute all the beautiful adaptations
in nature; such as the long neck of the giraffe for browsing on the
branches of trees. But he likewise believed in a law of progressive
development; and as all the forms of life thus tend to progress, in
order to account for the existence at the present day of simple
productions, he maintains that such forms are now spontaneously
generated.*
Geoffroy Saint-Hilaire, as is stated in his ''Life," written by
his son, suspected, "s early as 1795, that what we call species are
various degenerations of the same type. It was not until 1828
that he published his conviction that the same forms have not
been perpetuated since the origin of all things. Geoffroy seems to
have relied chiefly on the conditions of life, or the "monde am-
* I have taken the date of the j&rst publication of Lamarck from Isidore
Geoffroy Saint-Hilaire's ("Hist. Nat. Generale," torn. ii. p. 405, 1859) ex-
cellent history of opinion on this subject. In this work a full account is given
of Buffon's conclusions on the same subject. It is curious how largely my
grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous
grounds of opinion of Lamarck in his "Zoonomia" (vol. i. pp. 509-510), pub-
lished in 1794. According to Isid. Geoffroy there is no doubt that Goethe was
an extreme partisan of similar views, as shown in the introduction to a work
written in 1794 and 1795, but not published till long afterward: he has
pointedly remarked ("Goethe als Naturforscher," von Dr. Karl Meding, s.
34) that the future question for naturalists will be how, for instance, cattle
got their horns, and not for what they are used. It is rather a singular in-
stance of the manner in which .similar views arise at about the same time,
that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint-Hilaire
(as we shall immediately see) in France, came to the same conclusion on the
origin of species, in the years 1794-95.
HISTORICAL SKETCH 443
biant" as the cause of change. He was cautious in drawing con-
clusions, and did not believe that existing species are now under-
going modification; and, as his son adds, "C'est done un prob-
leme a reserver entierement a I'avenir, suppose meme que I'avenir
doive avoir prise sur lui."
In 1813 Dr. W. C. Wells read before the Royal Society "An
Account of a White Female, part of whose skin resembles that of
a Negro;" but his paper was not published until his famous "Two
Essays upon Dew and Single Vision" appeared in 1818. In this
paper he distinctly recognizes the principle of natural selection,
and this is the first recognition which has been indicated; but he
applies it only to the races of man, and to certain characters alone.
After remarking that negroes and mulattoes enjoy an immunity
from certain tropical diseases, he observes, firstly, that all animals
tend to vary in some degree, and, secondly, that agriculturists
improve their domesticated animals by selection; and then, he
adds, but what is done in this latter case "by art, seems to be done
with equal efficacy, though more slowly, by nature, in the forma-
tion of varieties of mankind, fitted for the country which they
inhabit. Of the accidental varieties of man, which would occur
among the first few and scattered inhabitants of the middle re-
gions of Africa, some one would be better fitted than others to
bear the diseases of the country. This race wculd consequently
multiply, while the others would decrease; not only from their
inability to sustain the attacks of disease, but from their incapacity
of contending with their more vigorous neighbors. The color of
this vigorous race I take for granted, from what has been already
said, would be dark. But the same disposition to form varieties
still existing, a darker and a darker race would in the course of
time occur, and as the darkest would be the best fitted for the
climate, this would at length become the most prevalent, if not
the only race, in the particular country in which it had originated."
He then extends these same views to the white inhabitants of
colder climates. I am indebted to Mr. Rowley, of the United States,
for having called my attention, through Mr. Brace, to the above
passage of Dr. Wells's work.
The Hon. and Rev. W. Herbert, afterward Dean of Manchester,
in the fourth volume of the "Horticultural Transactions," 1822,
and in his work on the "Amaryllidaceae" (1837, pp. 19, 339), de-
clares that "horticultural experiments have established, beyond
the possibility of refutation, that botanical species are only a
higher and more permanent class of varieties." He extends the
same view to animals. The dean believes that single species of
444 THE ORIGIN OF SPECIES
each genus were created in an originally highly plastic condition,
and that these have produced, chiefly by intercrossing, but like-
wise by variation, all our existing species.
In 1826 Professor Grant, in the concluding paragraph in his
well-known paper ("Edinburgh Philosophical Journal," vol. xiv.
p. 283) on the Spongilla, clearly declares his belief that species
are descended from other species, and that they become improved
in the course of modification. This same view was given in his
Fifty-fifth Lecture, published in the "Lancet" in 1834.
In 1831 Mr. Patrick Matthew published his work on "Naval
Timber and Arboriculture," in which he gives precisely the same
view on the origin of species as that (presently to be alluded to)
propounded by Mr. Wallace and myself in the "Linnean Journal,"
and as that enlarged in the present volume. Unfortunately the
view was given by Mr. Matthew very briefly in scattered passages,
in an appendix to a work on a different subject, so that it re-
mained unnoticed until Mr. Matthew himself drew attention to it
in the "Gardeners' Chronicle," on April 7, 1860. The differences
of Mr. Matthew's views from mine are not of much importance:
he seems to consider that the world was nearly depopulated at
successive periods, and then restocked ; and he gives as an alterna-
tive, that new forms may be generated "without the presence of
any mould or germ of former aggregates." I am not sure that I
understand some passages; but it seems that he attributes much
influence to the direct action of the conditions of life. He clearly
saw, however, the full force of the principle of natural selection.
The celebrated geologist and naturalist. Von Buch, in his ex-
cellent "Description Physique des Isles Canaries" (1836, p. 147),
clearly expresses his belief that varieties slowly become changed
into permanent species, which are no longer capable of intercross-
ing.
Rafinesque, in his "New Flora of North America," published
in 1836, wrote (p. 6) as follows: "All species might have been
varieties once, and many varieties are gradually becoming species
by assuming constant and peculiar characters;" but further on
(p. 18), he adds, "except the original types or ancestors of the
genus."
In 1843-44 Professor Haldeman ("Boston Journal of Nat. Hist.
U. States," vol. iv. p. 468) has ably given the arguments for and
against the hypothesis of the development and modification of
species: he seems to lean toward the side of change.
The "Vestiges of Creation" appeared in 1844. In the tenth and
much improved edition (1853) the anonymous author says (p.
HISTORICAL SKETCH 44S
155): "The proposition determined on after much consideration
is, that the several series of animated beings, from the simplest
and oldest up to the highest and most recent, are, under the provi-
dence of God, the results, first, of an impulse which has been im-
parted to the forms of life, advancing them in definite times, by
generation, through grades of organization terminating in the
highest dicotyledons and vertebrata, these grades being few in
number, and generally marked by intervals of organic character,
which we find to be a practical difficulty in ascertaining affinities;
second, of another impulse connected with the vital forces, tend-
ing, in the course of generations, to modify organic structures in
accordance with external circumstances, as food, the nature of
the habitat, and the meteoric agencies, these being the 'adapta-
tions' of the natural theologian." The author apparently believes
that organization progresses by sudden leaps, but that the effects
produced by the conditions of life are gradual. He argues with
much force on general grounds that species are not immutable
productions. But I cannot see how the two supposed "impulses"
account in a scientific sense for the numerous and beautiful co-
adaptations which we see throughout nature; I cannot see that
we thus gain any insight how, for instance, a woodpecker has be-
come adapted to its peculiar habits of life. The work, from its
powerful and brilliant style, though displaying in the early edi-
tions little accurate knowledge and a great want of scientific cau-
tion, immediately had a very wide circulation. In my opinion it
has done excellent service in this country in calling attention to
the subject, in removing prejudice, and in thus preparing the
ground for the reception of analogous views.
In 1846 the veteran geologist M. J. d'Omalius d'Halloy pub-
lished in an excellent though short paper ("Bulletins de I'Acad.
Roy. Bruxelles," tom. xiii. p. 581) his opinion that it is more
probable that new species have been produced by descent with
modification than that they have been separately created: the
author first promulgated this opinion in 1831.
Professor Owen, in 1849 ("Nature of Limbs," p. 86), wrote
as follows: "The archetypal idea was manifested in the flesh under
diverse such modifications, upon this planet, long prior to the
existence of those animal species that actually exemplify it. To
what natural laws or secondary causes the orderly succession and
progression of such organic phenomena may have been committed,
we, as yet, are ignorant." In his address to the British Association,
in 1858, he speaks (p. li.) of "the axiom of the continuous opera-
tion of creative power, or of the ordained becoming of living
446 THE ORIGIN OF SPECIES
things." Further on (p. xc), after referring to geographical dis-
tribution, he adds, "These phenomena shake our confidence in
the conclusion that the Apteryx of New Zealand and the Red
Grouse of England were distinct creations in and for those islands
respectively. Always, also, it may be well to bear in mind that
by the word 'creation' the zoologist means 'a process he knows
not what.' " He amplifies this idea by adding that when such
cases as that of the Red Grouse are "enumerated by the zoologist
as evidence of distinct creation of the bird in and for such islands,
he chiefly expresses that he knows not how the Red Grouse came
to be there, and there exclusively; signifying also, by this mode
of expressing such ignorance, his belief that both the bird and the
islands owed their origin to a great first Creative Cause." If we
interpret these sentences given in the same address, one by the
other, it appears that this eminent philosopher felt in 1858 his
confidence shaken that the Apteryx and the Red Grouse first ap-
peared in their respective homes "he knew not how," or by some
process "he knew not what."
This address was delivered after the papers by Mr. Wallace
and myself on the Origin of Species, presently to be referred to,
had been read before the Linnean Society. When the first edition
of this work was published, I was so completely deceived, as
were many others, by such expressions as "the continuous opera-
tion of creative power," that I included Professor Owen with
other palaeontologists as being firmly convinced of the immutabil-
ity of species; but it appears ("Anat. of Vertebrates," vol. iii. p.
796) that this was on my part a preposterous error. In the last
edition of this work I inferred, and the inference still seems to
me perfectly just, from a passage beginning with the words "no
doubt the type-form," etc. (Ibid., vol. i. p. xxxv.), that Professor
Owen admitted that natural selection may have done something
in the formation of a new species; but this it appears (Ibid., vol.
iii. p. 798) is inaccurate and without evidence. I also gave some
extracts from a correspondence between Professor Owen and the
editor of the "London Review," from which it appeared manifest
to the editor as well as to myself, that Professor Owen claimed
to have promulgated the theory of natural selection before I had
done so; and I expressed my surprise and satisfaction at this
announcement; but as far as it is possible to understand certain
recently published passages (Ibid., vol. iii. p. 798) I have either
partially or wholly again fallen into error. It is consolatory to me
that others find Professor Owen's controversial writings as diffi-
cult to understand and to reconcile with each other, as I do. As
HISTORICAL SKETCH 447
far as the mere enunciation of the principle of natural selection
is concerned, it is quite immaterial whether or not Professor Owen
preceded me, for both of us, as shown in this historical sketch,
were long ago preceded by Dr. Wells and Mr. Matthews.
M. Isidore Geoff roy Saint-Hilaire, in his lectures delivered in
1850 (of which a resume appeared in the ''Revue et Mag. de
Zoolog.," Jan., 1851), briefly gives his reason for believing that
specific characters "sont fixes, pour chaque espece, tant qu'elle
se perpetue au milieu des memes circonstances: ils se modifient,
si les circonstances ambiantes viennent a changer." "En resum6,
Vohservation des animaux sauvages demontre deja la variabilite
limitce des especes. Les experiences sur les animaux sauvages
devenus domestiques, et sur les animaux domestiques redevenus
sauvages, la demontrent plus clairement encore. Ces memes ex-
periences prouvent, de plus, que les differences produites peuvent
etre de valeur gen^rique*' In his "Hist. Nat. Generale" (torn. ii.
p. 430, 1859) he amplifies analogous conclusions.
From a circular lately issued it appears that Dr. Freke, in
1851 ("Dublin Medical Press," p. 322), propounded the doctrine
that all organic beings have descended from one primordial form.
His grounds of belief and treatment of the subject are wholly
different from mine; but as Dr. Freke has now (1861) published
his Essay on the "Origin of Species by means of Organic Affinity,"
the difficult attempt to give any idea of his views would be super-
fluous on my part.
Mr. Herbert Spencer, in an essay (originally published in the
"Leader," March, 1852, and republished in his "Essays," in
1858), has contrasted the theories of the Creation and the Devel-
opment of organic beings with remarkable skill and force. He
argues from the analogy of domestic productions, from the changes
which the embryos of many species undergo, from the difficulty
of distinguishing species and varieties, and from the principle of
general graduation, that species have been modified; and he
attributes the modification to the change of circumstances. The
author (1855) has also treated Psychology on the principle of
the necessary acquirement of each mental power and capacity by
gradation.
In 1852 M. Naudin, a distinguished botanist, expressly stated,
in an admirable paper on the Origin of Species ("Revue Horti-
cole," p. 102; since partly republished in the "Nouvelles Archives
du Museum," tom. i. p. 171), his belief that species are formed
in an analogous manner as varieties are under cultivation; and
the latter process he attributes to man's power of selection. But
448 THE ORIGIN OF SPECIES
he does not show how selection acts under nature. He believes,
like Dean Herbert, that species, when nascent, were more plastic
than at present. He lays weight on what he calls the principle of
finality, "puissance mysterieuse, indeterminee ; fatalite pour les
uns; pour les autres volont6 providentielle, dont Taction inces-
sante sur les etres vivantes determine, a toutes les epoques de
I'existence du monde, la forme, le volume, et la duree de chacun
d'eux, en raison de sa destinee dans I'ordre de choses dont il fait
partie. C'est cette puissance qui harmonise chaque membre a
['ensemble, en Fappropriant a la fonction qu'il doit remplir dans
i'organisme generale de la nature, fonction qui est pour lui sa
raison d'etre."*
In 1853 a celebrated geologist, Count Keyserling ("Bulletin
de la Soc. Geolog.," 2d ser., tom. x. p. 357) suggested that as new
diseases, supposed to have been caused by some miasma, have
arisen and spread over the world, so at certain periods the germs
of existing species may have been chemically affected by circum-
ambient molecules of a particular nature, and thus have given
rise to new forms.
In this same year, 1853, Dr. Schaaffhausen published an ex-
cellent pamphlet ("Verhand. des Naturhist. Vereins der Preuss.
Rheinlands," etc.), in which he maintains the development of
organic forms on the earth. He infers that many species have kept
true for long periods, whereas a few have become modified. The
distinction of species he explains by the destruction of interme-
diate graduated forms. "Thus living plants and animals are not
separated from the extinct by new creations, but are to be re-
garded as their descendants through continued reproduction."
A well-known French botanist, M. Lecoq, writes in 1854
("Etudes sur Geograph. Bot.," tom. i. p. 250): "On voit que nos
recherches sur la fixite ou la variation de I'espece, nous conduisent
directement aux idees emises par deux hommes justement cele-
bres, Geoffroy Saint-Hilaire et Goethe." Some other passages scat-
* From references in Bronn's "Untersuchungen iiber die Entwickelungs-
Gesetze," it appears that the celebrated botanist and palaeontologist Unger
published, in 1852, his belief that species undergo development and modifi-
cation. Dalton, likewise, in Pander and Dalton's work on Fossil Sloths, ex-
pressed, in 1821, a similar belief. Similar views have, as is well known, been
maintained by Oken in his mystical "Natur-Philosophie." From other refer-
ences in Godron's work "Sur I'Espece," it seems that Bory St. Vincent, Bur-
dach, Poiret, and Fries have all admitted that new species are continually
being produced. I may add, that of the thirty-four authors named in this
Historical Sketch who believe in the modification of species, or at least dis-
believe in separate acts of creation, twenty-seven have written on special
branches of natural history or geology.
HISTORICAL SKETCH 449
tered through M. Lecoq's large work make it a little doubtful how
far he extends his views on the modification of species.
The "Philosophy of Creation" has been treated in a masterly
manner by the Rev. Baden Powell, in his "Essays on the Unity of
Worlds," 1855. Nothing can be more striking than the manner in
which he shows that the introduction of new species is "a regular,
not a casual phenomenon," or, as Sir John Herschel expresses it,
"a natural in contradistinction to a miraculous process."
The third volume of the "Journal of the Linnean Society" con-
tains papers, read July 1, 1858, by Mr. Wallace and myself, in
which, as stated in the introductory remarks to this volume, the
theory of Natural Selection is promulgated by Mr. Wallace with
admirable force and clearness.
Von Baer, toward whom all zoologists feels so profound a re-
spect, expressed about the year 1859 (see Prof. Rudolph Wagner,
"Zoologisch-Anthrppologische Untersuchungen," 1861, s. 51) his
conviction, chiefly grounded on the laws of geographical distribu-
tion, that forms now perfectly distinct have descended from a
single parent-form.
In June, 1859, Professor Huxley gave a lecture before the Royal
Institution on the "Persistent Types of Animal Life." Referring to
such cases, he remarks, "It is difficult to comprehend the mean-
ing of such facts as these, if we suppose that each species of ani-
mal and plant, or each great type of organization, was formed
and placed upon the surface of the globe at long intervals by a
distinct act of creative power; and it is well to recollect that such
an assumption is as unsupported by tradition or revelation as it
is opposed to the general analogy of nature. If, on the other hand,
we view 'Persistent Types' in relation to that hypothesis which
supposes the species living at any time to be the result of the
gradual modification of pre-existing species, a hypothesis which,
though unproven, and sadly damaged by some of its supporters,
is yet the only one to which physiology lends any countenance;
their existence would seem to show that the amount of modifica-
tion which living beings have undergone during geological time is
but very small in relation to the whole series of changes which
they have suffered."
In December, 1859, Dr. Hooker published his "Introduction
to the Austrahan Flora." In the first part of this great work he
admits the truth of the descent and modification of species, and
supports this doctrine by many original observations.
The first edition of this work was published on November 24,
1859, and the second edition on January 7, 1860.
GLOSSARY
i
GLOSSARY
OF THE
PRINCIPAL SCIENTIFIC TERMS USED IN THE
PRESENT VOLUME 1
Aberrant. — Forms or groups of animals or plants which deviate in im-
portant characters from their nearest alUes, so as not to be easily
included in the same group with them, are said to be aberrant.
Aberration (in Optics). — In the refraction of light by a convex lens the
rays passing through different parts of the lens are brought to a
focus at slightly different distances — this is called spherical aber-
ration; at the same time the colored rays are separated by the
prismatic action of the lens and likewise brought to a focus at dif-
ferent 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 coloring matters
characteristic of the species have not been produced in the skin and
its appendages. Albinism is the state of being an Albino.
Alg^. — ^A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.
Alternation of Generations. — ^This term is applied to a peculiar
mode of reproduction which prevails among many of the lower
animals, in which the egg produces a living form quite different
from its parent, but from which the parent-form is reproduced by
a process of budding, or by the division of the substance of the first
product of the egg.
*I am indebted to the kindness of Mr. W. S. Dallas for this Glossary,
which has been given because several readers have complained to me that
some of the terms used were unintelligible to them. Mr. Dallas has endeav-
ored to give the explanations of the terms in as popular a form as possible.
453
4S4 THE ORIGIN OF SPECIES
Ammonites. — ^A group of fossil, spiral, chambered shells, allied to the
existing pearly Nautilus, but having the partitions between the
chambers waved in complicated patterns at their junction with the
outer wall of the shell.
Analogy. — The resemblance of structures which depends upon simi-
larity of function, as in the wings of insects and birds. Such struc-
tures are said to be analogous, and to be analogues of each other.
Animalcule. — A minute animal : generally applied to those visible only
by the microscope.
Annelids. — A class of worms in which the surface of the body exhibits
a more or less distinct division into rings or segments, generally
provided with appendages for locomotion and with gills. It includes
the ordinary marine worms, the earth-worms and the leeches.
Antennae. — ^Jointed organs appended to the head in Insects. Crustacea
and Centipedes, and not belonging to the mouth.
Anthers. — ^The summits of the stamens of flowers, in which the pollen
or fertilizing dust is produced.
Aplacentalia, Aplacentata or Aplacental Mammals. See Mammalia.
Archetypal. — Of or belonging to the Archetype, or ideal primitive
form upon which all the beings of a group seem to be organized.
Articulata. — A great division of the Animal Kingdom characterized
generally by having the surface of the body divided into rings
called segments, a greater or less number of which are furnished
with jointed legs (such as Insects, Crustaceans and Centipedes).
Asymmetrical. — Having the two sides unlike.
Atrophied. — Arrested in development at a very early stage.
Balanus. — ^The genus including the common Acom-shells which live in
abundance on the rocks of the seacoast.
Batrachians. — A class of animals allied to the Reptiles, but undergoing
a peculiar metamorphosis, in which the young animal is generally
aquatic and breathes by gills. (Examples, Frogs, Toads, and
Newts.)
Bowlders. — ^Large transported blocks of stone generally embedded in
clays or gravels.
Brachiopoda. — A class of marine MoUusca, or soft-bodied animals,
furnished with a bivalve shell, attached to submarine objects by a
stalk which passes through an aperture in one of the valves, and
furnished with fringed arms, by the action of which food is carried
to the mouth.
Branchle. — Gills or organs for respiration in water.
Branchial. — Pertaining to gills or branchiae.
GLOSSARY 4S5
Cambrian System. — A series of very ancient Palaeozoic rocks, between
the Laurentian and the Silurian. Until recently these were re-
garded as the oldest fossiliferous rocks.
CANiDiE. — The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.
Carapace. — The shell enveloping the anterior part of the body in
Crustaceans generally; applied also to the hard shelly pieces of
the Cirripedes.
Carboniferous. — This term is applied to the great formation which
includes, among other rocks, the coal-measures. It belongs to the
, oldest, or Palaeozoic, system of formations.
Caudal. — Of or belonging to the tail.
Cephalopods. — ^The highest class of the MoUusca, or soft-bodied ani-
mals, characterized 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. (Examples, Cuttle-fish,
Nautilus.)
Cetacea. — An order of Mammalia, including the Whales, Dolphins,
etc., 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, etc.
Cirripedes. — An order of Crustaceans including the Barnacles and
Acorn-shells. Their young resemble those of many other Crusta-
ceans in form; but when mature they are always attached to other
objects, either directly or by means of a stalk, and their bodies are
enclosed by a calcareous shell composed of several pieces, two of
which can open to give issue to a bunch of curled, jointed tentacles,
which represent the limbs.
Coccus. — The genus of Insects including the Cochineal. In these the
male is a minute, winged fly, and the female generally a motion-
less, berry-like mass.
Cocoon. — A case usually of silky material, in which insects are fre-
quently enveloped during the second or resting-stage (pupa) of
their existence. The term "cocoon-stage" is here used as equivalent
to "pupa-stage."
Ccelgspermous. — A term applied to those fruits of the Umbelliferae
which have the seed hollowed on the inner face.
CoLEOPTERA. — 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 Une down the middle
of the back.
Column. — A peculiar organ in the flowers of Orchids, in which the
stamens, style and stigma (or the reproductive parts) are united.
456 THE ORIGIN OF SPECIES
CoMPOSiTiE or CoMPOSiTOUs Plants. — Plants in which the inflores-
cence consists of numerous small flowers (florets) brought together
into a dense head, the base of which is enclosed by a common
envelope. {Examples, the Daisy, Dandelions, etc.)
CoNFERViE. — The filamentous weeds of fresh water.
Conglomerate. — A rock made up of fragments of rock or pebbles,
cemented together by some other material.
Corolla. — The second envelope of a flower usually composed of col-
ored, leaf-like organs (petals), which may be united by their edges
either in the basal part or throughout.
Correlation. — The normal coincidence of one phenomenon, character,
etc., with another.
Corymb. — A bunch of flowers in which those springing from the lower
part of the flower-stalks 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 cal-
cereous matter, breathing by means of gills. (Examples, Crab, Lob-
ster, Shrimp, etc.)
CuRCULio. — The old generic term for the Beetles known as Weevils,
characterized by their four jointed feet, and by the head being
produced into a sort of beak, upon the sides of which the antennse
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 water.
Devonian System or Formation. — ^A series of Palaeozoic rocks, in-
cluding the Old Red Sandstone.
Dicotyledons or Dicotyledonous Plants. — A class of plants charac-
terized by having two seed-leaves, by the formation of new wood
between the bark and the old wood (exogenous growth) and by
the reticulation of the veins of the leaves. The parts of the flowers
are generally in multiples of five.
Differentiation. — The separation or discrimination of parts or organs
which in simpler forms of life are more or less united.
Dimorphic. — ^Having two distinct forms — Dimorphism is the condition
of the appearance of the same species under two dissimilar forms.
DicECious. — Having the organs of the sexes upon distinct individuals.
Diorite. — A peculiar form of Greenstone.
Dorsal. — Of or belonging to the back.
GLOSSARY 457
Edentata. — ^A peculiar order of Quadrupeds, characterized by the
absence of at least the middle incisor (front) teeth in both jaws.
(Examples, the Sloths and Armadillos.)
Elytra. — The hardened fore-wings of Beetles, serving as sheaths for
the membraneous 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 seg-
ments of the body usually distinct, gills attached to the feet or
organs of the mouth, and the feet fringed with fine hairs. They are
generally of small size.
Eocene. — The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells
identical with species now living.
Ephemerous Insects. — Insects alHed to the May-fly.
Fauna. — ^The totality of the animals naturally inhabiting a certain
country or region, or which have lived during a given geological
period.
Felid^. — The Cat-family.
Feral. — Having become wild from a state of cultivation or domesti-
cation.
Flora. — The totality of the plants growing naturally in a country, or
during a given geological period.
Florets. — Flowers imperfectly developed in some respects, and col-
lected into a dense spike or head, as in the Grasses, the Dande-
lion, etc.
Fgetal. — Of or belonging to the foetus, or embryo in course of devel-
opment.
Foraminifera. — ^A class of animals of very low organization and gen-
erally of small size, having a jelly-Hke body, from the surface of
which delicate filaments can be given off and retracted for the
prehension of external objects, and having a calcareous oi sandy
shell, usually divided into chambers and perforated with small
apertures.
FossiLiFEROUS. — Containing fossils.
FossoRiAL. — ^Having a faculty of digging. The Fossorial Hymenoptera
are a group of Wasp-like Insects, which burrow in sandy soil to
make nests for their young.
Frenum (pi. Frena). — A small band or fold of skin.
458 THE ORIGIN OF SPECIES
Fungi (sing. Fungus). — A class of cellular plants, of which Mush-
rooms, Toadstools, and Moulds are familiar examples.
FuRCULA. — The forked bone formed by the union of the collar-bones
in many birds, such as the common Fowl.
Gallinaceous Birds. — An order of Birds of which the common
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 as
from a centre.
Ganoid Fishes. — Fishes covered with peculiar enamelled bony scales.
Most of them are extinct.
Germinal Vesicle. — A minute vesicle in the eggs of animals, from
which the development of the embryo proceeds.
Glacial Period. — A period of great cold and of enormous extension
of ice upon the surface of the earth. It is believed that glacial
periods have occurred repeatedly during the geological history of
the earth, but the term is generally applied to the close of the Ter-
tiary epoch, when nearly the whole of Europe was subjected to an
arctic climate.
Gland. — An organ which secretes or separates some peculiar product
from the blood or sap of animals or plants.
Glottis. — The opening of the windpipe into the oesophagus or gullet.
Gneiss. — A rock approaching granite in composition, but more or less
laminated, and really produced by the alteration of a sedimentary
deposit after its consolidation.
Grallatores. — The so-called Wading-birds (Storks, Cranes, Snipes,
etc.), which are generally furnished with long legs, bare of feathers
above the heel, and have no membranes between the toes.
Granite. — ^A rock consisting essentially of crystals of felspar and mica
in a mass of quartz.
Habitat. — The locality in which a plant or animal naturally lives.
Hemiptera. — An order or sub-order of Insects, characterized by the
possession of a jointed beak or rostrum, and by having the fore-
wings horny in the basal portion and membraneous at the ex-
tremity, where they cross each other. This group includes the
various species of Bugs.
Hermaphrodite. — Possessing the organ of both sexes.
Homology. — The relation between parts which results from their de-
velopment 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 individual, as
GLOSSARY 459
in the case of the fore and hind legs in quadrupeds, and the seg-
ments or rings and their appendages of which the body of a worm,
a centipede, etc., is composed. The latter is called serial homology.
The parts which stand in such a relation to each other are said to
be homologous, and one such part or organ is called the homologue
of the other. In different plants the parts of the flower are homol-
ogous, and in general these parts are regarded as homologous with
leaves.
HoMOPTERA. — ^An order or sub-order of Insects having (like tlie
Hemiptera) a jointed beak, but in which the fore-wings are either
wholly membranous or wholly leathery. The Cicada, Frog-hop-
pers, and Aphides, are well-known examples.
Hybrid. — The offspring of the union or two distinct species.
Hymenoptera. — An order of Insects possessing biting jaws and usu-
ally four membranous wings in which there are a few veins. Bees
and Wasps are familiar examples of this group.
Hypertrophied. — Excessively developed.
IcHNEUMONiD^. — ^A family of Hymenopterous insects, the members of
which lay their eggs in the bodies or eggs of other insects.
Imago. — ^The perfect (generally winged) reproductive state of an in-
sect.
Indigens. — The aboriginal animal or vegetable inhabitants of a coun-
try 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 mem-
brane, the whole or part of which is furnished with short vibrating
hairs (called ciha), by means of which the animalcules swim
through the water or convey the minute particles of their food to
the orifice of the mouth.
Insectivorous. — Feeding on Insects.
Invertebrata or Invertebrate Animals. — Those animals which do
not possess a backbone or spinal column.
Lacunae. — Spaces left among the tissues in some of the lower animals,
and serving in place of vessels for the circulation of the fluids of
the body.
Lamellated. — Furnished with lemellae or little plates.
Larva (pi. Larvae). — ^The first condition of an insect at its issuing from
the egg, when it is usually in the form of a grub, caterpillar or
maggot.
460 THE ORIGIN OF SPECIES
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. Lawrence, whence
the name. It is in these that the earliest known traces of organic
bodies have been found.
Leguminos^e. — An order of plants represented by the common Pease
and Beans, having an irregular flower in which one petal stands
up like a wing, and the stamens and pistil are enclosed in a sheath
formed by two other petals. The fruit is a pod (or legume).
Lemurid^. — A group of four-handed animals, distinct from the Mon-
keys, 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, characterized by the possession of
a spiral proboscis, and of four large more or less scaly wings. It
includes the well-known Butterflies and Moths.
Littoral. — Inhabiting the seashore.
Loess. — A marly deposit of recent (Post-Tertiary) date, which occu-
pies a great part of the valley of the Rhine.
Malacostraca. — The higher division of the Crustacea, including the
ordinary Crabs, Lobsters, Shrimps, etc., together with the Wood-
lice and Sand-hoppers.
Mammalia. — The highest class of animals, including the ordinary hairy
quadrupeds, the Whales and Man, and characterized by the produc-
tion of living young which are nourished after birth by milk from
the teats (MammcB, Mammary glands) of the mother. A striking
difference in embryonic development has led to the division of this
class into two great groups; in one of these, when the embryo has
attained a certain stage, a vascular connection, called the placenta,
is formed between the embryo and the mother; in the other this
is wanting, and the young are produced in a very incomplete state.
The former, including the greater part of the class, are called
Placental mamm,als; the latter, or Aplacental mammals, include the
Marsupials and Monotremes (Ornithorhynchus) .
Mammiferous. — Having mammae or teats (see Mammalia).
Mandibles in Insects. — The first or uppermost pair of jaws, which
are generally solid, horny, biting organs. In Birds the term is
applied to both jaws with their homy coverings. In quadrupeds
the mandible is properly the lower jaw.
Marsupials. — ^An order of Mammalia in which the young are born in
GLOSSARY 461
a very incomplete state of development and carried by the mother,
while sucking, in a ventral pouch (marsupium), such as the Kan-
garoos, Opossums, etc. (see Mammalia).
Maxilla in Insects. — The second or lower pair of jaws, which are
composed of several joints and furnished with peculiar jointed
appendages called palpi or feelers.
Melanism. — The opposite of albinism; an undue development of color-
ing material in the skin and its appendages.
Metamorphic Rocks. — Sedimentary rocks which have undergone al-
teration, generally by the action of heat, subsequently to their
deposition and consoHdation.
MOLLUSCA. — One of the great divisions of the Animal Kingdom, in-
cluding 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 Monocotyledonous Plants. — Plants in which
the seed sends up only a single seed-leaf (or cotyledon) ; charac-
terized by the absence of consecutive layers of wood in the stem
(endogenous growth), by the veins of the leaves being generally
straight, and by the parts of the flowers being generally in mul-
tiples of three. (Examples, Grasses, LiHes, Orchids, Palms, etc.)
Moraines. — The accumulations of fragments of rock brought down by
glaciers.
Morphology. — ^The law of form or structure independent of function.
Mysis-stage. — A stage in the development of certain Crustaceans
(Prawns), in which they closely resemble the adults of a genus
(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 divi-
sion 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.
Nictitating Membrane. — A semi-transparent membrane, which can
462 THE ORIGIN OF SPECIES
be drawn across the eye in Birds and Reptiles, either to mod-
erate the effects of a strong light or to sweep particles of dust, etc.,
from the surface of the eye.
Neuters. — Imperfectly developed females of certain social insects
(such as Ants and Bees), which perform all the labors of the com-
munity. Hence they are also called workers.
Ocelli. — ^The simple eyes or stemmata of Insects, usually situated on
the crown of the head between the great compound eyes.
CEsoPHAGUS. — 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 MoUusca to close
the aperture of their shell. The opercular valves of Cirripedes are
those which close the aperture of the shell.
Orbit. — The bony cavity for the reception of the eye.
Organism. — ^An organized being, whether plant or animal.
Orthospermous. — ^A term applied to those fruits of the Umbelliferae
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, etc.
Palaeozoic. — The oldest system of fossiliferous rocks.
Palpi. — ^Jointed appendages to some of the organs of the mouth in
Insects and Crustacea.
pAPiLiONACEiE. — An Order of Plants (see LEGUMiNOSiE). — ^The flowers
of these plants are called papilio'naceous, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings
of a butterfly.
Parasite. — ^An animal or plant living upon or in, and at the expense of,
another organism.
Parthenogenesis.— The production of living organisms from unim-
pregnated eggs or seeds.
GLOSSARY 463
Pedunculated. — Supported upon a stem or stalk. The pedunculated
oak has its acorns borne upon a footstool.
Peloria or Pelorism. — The appearance of regularity of structure in
the flowers of plants which normally bear irregular flowers.
Pelvis. — The bony arch to which the hind limbs of vertebrate animals
are articulated.
Petals. — ^The leaves of the corolla, or second circle of organs in a
flower. They are usually of delicate texture and brightly colored.
Phyllodineous. — Having flattened, leaf-like twigs or leaf-stalks in-
stead of true leaves.
Pigment. — The coloring material produced generally in the superficial
parts of animals. The cells secreting it are called pigment-cells.
Pinnate. — Bearing leaflets on each side of a central stalk.
Pistils. — ^The female organs of a flower, which occupy a position in
the centre of the other floral organs. The pistil is generally divisi-
ble into the ovary or germen, the style and the stigma.
Placentalia, Placentata, or Placental Mammals. — See Mammalia.
Plantigrades. — Quadrupeds which walk upon the whole sole of the
foot, hke 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 igneous
action in the depths of the earth.
Pollen. — ^The male element in flowering plants; usually a fine dust
produced by the anthers, which, by contact with the stigma effects
the fecundation of the seeds. This impregnation is brought about
by means of tubes {pollen-tubes) which issue from the pollen-
grains adhering to the stigma, and penetrate through the tissues
until they reach the ovary.
PoLYANDROus (flowers). — Flowers having many stamens.
Polygamous Plants. — Plants in which some flowers are unisexual and
others hermaphrodite. The unisexual (male and female) flowers
may be on the same or on different plants.
Polymorphic. — Presenting many forms.
PoLYZOARY. — The common structure formed by the cells of the 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 man.
464 THE ORIGIN OF SPECIES
Processes. — Projecting portions of bones, usually for the attachment
of muscles, ligaments, etc.
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^e). — The second stage in the development of an Insect,
from which it emerges in the perfect (winged) reproductive form.
In most insects the pupal stage is passed in perfect repose. The
chrysalis is the pupal state of Butterflies.
Ramus. — One half of the lower jaw in the Mammalia. The portion
which rises to articulate with the skull is called the ascending
ramus.
Radicle. — The minute root of an embryo plant.
Range. — The extent of coimtry over which a plant or animal is natu-
rally spread. Range in time expresses the distribution of a species
or group through the fossiliferous beds of the earth's crust.
Retina. — The delicate inner coat of the eye, formed by nervous fila-
ments spreading from the optic nerve and serving for the percep-
tion of the impressions produced by light.
Retrogression. — Backward development. When an animal, as it ap-
proaches maturity, becomes less perfectly organized than might
be expected from its early stages and known relationships, it is said
to undergo a retrograde development or metamorphosis.
Rhizopods. — A class of lowly organized 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 characterized by the possession of a
single pair of chisel-like cutting teeth in each jaw, between which
and the grinding teeth there is a great gap.
RuBUS. — The Bramble Genus.
Rudimentary. — Very imperfectly developed.
Ruminants. — The group of Quadrupeds which ruminate or chew the
cud, such as oxen, sheep and deer. They have divided hoofs, and
are destitute of front teeth in the upper jaw.
GLOSSARY 465
Sacral. — 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^. — 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 water.
Segments. — The transverse rings of which the body of an articulate
animal or Annelid is composed.
Sepals. — The leaves or segments of the calyx, or outermost envelope
of an ordinary flower. They are usually green, but sometimes
brightly colored.
Serratures. — Teeth Hke those of a saw.
Sessile. — ^Not supported on a stem or footstalk.
Silurian System. — A very ancient system of fossiliferous rocks be-
longing to the earlier part of the Palaeozoic series.
Specialization. — The setting apart of a particular organ for the per-
formance of a particular function.
.,^. Spinal Cord. — The central portion of the nervous system in the Verte-
I^K brata, which descends from the brain through the arches of the
^Ir vertebrae, and gives off nearly all the nerves to the various organs
of the body.
Stamens. — ^The male organs of flowering plants, standing in a circle
within the petals. They usually consist of a filament and an anther,
the anther being the essential part in which the pollen, or fecun-
dating dust, is formed.
Sternum. — The breast-bone.
Stigma. — The apical portion of the pistil in flowering plants.
Stipules. — Small leafy organs placed at the base of the footstalks of
the leaves in many plants.
Style. — The middle portion of the perfect pistil, which rises like a
column from the ovary and supports the stigma at its summit.
Subcutaneous. — Situated beneath the skin.
Suctorial. — Adapted for sucking.
Sutures (in the skull). — The lines of junction of the bones of which
the skull is composed.
Tarsus (pi. Tarsi). — The jointed feet of articulate animals, such as
insects.
Teleostean Fishes. — Fishes of the kind familiar to us in the present
day, having the skeleton usually completely ossified and the scales
homy.
466 THE ORIGIN OF SPECIES
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 at-
tached to a common base.
Trilobites. — A peculiar group of extinct Crustaceans, somewhat re-
sembling the Wood-lice 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.
UMBELLiFERiE. — ^Ah Order of plants in which the flowers, which contain
five stamens and a pistil with two styles, are supported upon foot-
stalks 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.
Vertebrata; 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 vertebrcB, which con-
stitutes the centre of the skeleton and at the same time supports
and protects the central parts of the nervous system.
Whorls. — ^The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.
Workers. — See Neuters.
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,
etc.) reproduction takes place in two ways, namely, by means of
eggs and by a process of budding with or without separation from
GLOSSARY 467
the parent of the product of the latter, which is often very differ-
ent from that of the egg. The individuality of the species is repre-
sented by the whole of the form produced between two sexual
reproductions; and these forms, which are apparently individual
animals, have been called zooids.
'^
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