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On the Origin of Species by Charles Darwin

Part 5 out of 9

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

We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and of crossing
distinct species. And as we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other
as incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of
first crosses, are incidental on unknown differences, chiefly in their
reproductive systems. These differences, in both cases, follow to a
certain extent, as might have been expected, systematic affinity, by
which every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed. The facts by no means seem to me
to indicate that the greater or lesser difficulty of either grafting
or crossing together various species has been a special endowment;
although in the case of crossing, the difficulty is as important for
the endurance and stability of specific forms, as in the case of
grafting it is unimportant for their welfare.


We may now look a little closer at the probable causes of the
sterility of first crosses and of hybrids. These two cases are
fundamentally different, for, as just remarked, in the union of two
pure species the male and female sexual elements are perfect, whereas
in hybrids they are imperfect. Even in first crosses, the greater or
lesser difficulty in effecting a union apparently depends on several
distinct causes. There must sometimes be a physical impossibility in
the male element reaching the ovule, as would be the case with a plant
having a pistil too long for the pollen-tubes to reach the ovarium. It
has also been observed that when pollen of one species is placed on
the stigma of a distantly allied species, though the pollen-tubes
protrude, they do not penetrate the stigmatic surface. Again, the male
element may reach the female element, but be incapable of causing an
embryo to be developed, as seems to have been the case with some of
Thuret's experiments on Fuci. No explanation can be given of these
facts, any more than why certain trees cannot be grafted on others.
Lastly, an embryo may be developed, and then perish at an early
period. This latter alternative has not been sufficiently attended to;
but I believe, from observations communicated to me by Mr. Hewitt, who
has had great experience in hybridising gallinaceous birds, that the
early death of the embryo is a very frequent cause of sterility in
first crosses. I was at first very unwilling to believe in this view;
as hybrids, when once born, are generally healthy and long-lived, as
we see in the case of the common mule. Hybrids, however, are
differently circumstanced before and after birth: when born and living
in a country where their two parents can 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, and therefore
before birth, as long as it is nourished within its mother's womb or
within the egg or seed produced by the mother, it may be exposed to
conditions in some degree unsuitable, and consequently be liable to
perish at an early period; more especially as all very young beings
seem eminently sensitive to injurious or unnatural conditions of life.

In regard to the sterility of hybrids, in which the sexual elements
are imperfectly developed, the case is very different. I have more
than once alluded to a large body of facts, which I have collected,
showing that when animals and plants are removed from their natural
conditions, they are extremely liable to have their reproductive
systems seriously affected. This, in fact, is the great bar to the
domestication of animals. Between the sterility thus superinduced and
that of hybrids, there are many points of similarity. In both cases
the sterility is independent of general health, and is often
accompanied by excess of size or great luxuriance. In both cases, the
sterility occurs in various degrees; in both, the male element is the
most liable to be affected; but sometimes the female more than the
male. In both, the tendency goes to a certain extent with systematic
affinity, for whole groups of animals and plants are rendered impotent
by the same unnatural conditions; and whole groups of species tend to
produce sterile hybrids. On the other hand, one species in a group
will sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce unusually
fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement or any 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 is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids 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 by sterility 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 conditions have
remained the same, but the organisation has been disturbed by two
different structures and constitutions having been blended into one.
For it is scarcely possible that two organisations should be
compounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relation of the different
parts and organs one to another, or to the conditions of life. When
hybrids are able to breed inter se, they transmit to their offspring
from generation to generation the same compounded organisation, and
hence we need not be surprised that their sterility, though in some
degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent.
Nor do I pretend that the foregoing remarks go to the root of the
matter: no explanation is offered why an organism, when placed under
unnatural conditions, is rendered sterile. All that I have attempted
to show, is that in two cases, in some respects allied, sterility is
the common result,--in the one case from the conditions of life having
been disturbed, in the other case from the organisation having been
disturbed by two organisations having been compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends
to an allied yet very different class of facts. It is an old and
almost universal belief, founded, I think, on a considerable body of
evidence, 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, we plainly see that great benefit is derived from almost any
change in the habits of life. Again, both with plants and animals,
there is abundant evidence, that a cross between very distinct
individuals of the same species, that is between members of different
strains or sub-breeds, gives vigour and fertility to the offspring. I
believe, indeed, from the facts alluded to in our fourth chapter, that
a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and
sterility in the progeny.

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 varied and become slightly different, give vigour
and fertility to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between
males and females which have become widely or specifically different,
produce hybrids which are generally sterile in some degree. I cannot
persuade myself that this parallelism is an accident or an illusion.
Both series of facts seem to be connected together by some common but
unknown bond, which is essentially related to the principle of life.


It may be urged, as a most forcible argument, that there must be some
essential distinction between species and varieties, and that there
must be some error in all the foregoing remarks, inasmuch as
varieties, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably the case. But
if we look to varieties produced under nature, we are immediately
involved in hopeless difficulties; for if two hitherto reputed
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, the primrose and cowslip, which are considered by many of
our best botanists as varieties, are said by Gartner not to be quite
fertile 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 doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily
than other dogs with foxes, or that certain South American indigenous
domestic dogs do not readily cross with European dogs, the explanation
which will occur to everyone, and probably the true one, is that these
dogs have descended from several aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic varieties,
differing widely from each other in appearance, for instance of the
pigeon or of the cabbage, is a remarkable fact; more especially when
we reflect how many species there are, which, though resembling each
other most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic varieties
less remarkable than at first appears. It can, in the first place, be
clearly shown that mere external dissimilarity between two species
does not determine their greater or lesser degree of sterility when
crossed; and we may apply the same rule to domestic varieties. In the
second place, some eminent naturalists believe that a long course of
domestication tends to eliminate sterility in the successive
generations of hybrids, which were at first only slightly sterile; and
if this be so, we surely ought not to expect to find sterility both
appearing and disappearing under nearly the same conditions of life.
Lastly, and this seems to me by far the most important consideration,
new races of animals and plants are produced under domestication by
man's methodical and unconscious power of selection, for his own use
and pleasure: he neither wishes to select, nor could select, slight
differences in the reproductive system, or other constitutional
differences correlated with the reproductive system. He supplies his
several varieties with the same food; treats them in nearly the same
manner, and does not wish to alter their general habits of life.
Nature acts uniformly and slowly during vast periods of time on the
whole organisation, in any way which may be for each creature's own
good; and thus she may, either directly, or more probably indirectly,
through correlation, modify the reproductive system in the several
descendants from any one species. Seeing this difference in the
process of selection, as carried on by man and nature, we need not be
surprised at some difference in the result.

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

Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.

The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness, as Gartner: namely, that
yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilised with pollen from their own coloured flowers. Moreover, he
asserts that when yellow and white varieties of one species are
crossed with yellow and white varieties of a DISTINCT species, more
seed is produced by the crosses between the same coloured flowers,
than between those which are differently coloured. Yet these varieties
of Verbascum present no other difference besides the mere colour of
the flower; and one variety can sometimes be raised from the seed of
the other.

From observations which I have made on certain varieties of hollyhock,
I am inclined to suspect that they present analogous facts.

Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one variety of the
common tobacco is more fertile, when crossed with a widely distinct
species, than are the other varieties. He experimentised 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 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; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety
if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most
distinct domestic varieties, and from not wishing or being able to
produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think
that the very general fertility of varieties can be proved to be of
universal occurrence, or to form a fundamental distinction between
varieties and species. The general fertility of varieties does not
seem to me sufficient to overthrow the view which I have taken with
respect to the very general, but not invariable, sterility of first
crosses and of hybrids, namely, that it is not a special endowment,
but is incidental on slowly acquired modifications, more especially in
the reproductive systems of the forms which are crossed.


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

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

This greater variability of mongrels than of hybrids does not seem to
me 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 in most cases that there has been
recent variability; and therefore we might expect that such
variability would often continue and be super-added to that arising
from the mere act of crossing. The slight degree of variability in
hybrids from the first cross or in the first generation, in contrast
with their extreme variability in the succeeding generations, is a
curious fact and deserves attention. For it bears on and corroborates
the view which I have taken on the cause of ordinary variability;
namely, that it is due to the reproductive system being eminently
sensitive to any change in the conditions of life, being thus often
rendered either impotent or at least incapable of its proper function
of producing offspring identical with the parent-form. Now hybrids in
the first generation are descended from species (excluding those long
cultivated) which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves have their
reproductive systems seriously affected, and their descendants are
highly variable.

But to return to our comparison of mongrels and hybrids: Gartner
states that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference
in degree. Gartner further insists that when any two species, although
most closely allied to each other, are crossed with a third species,
the hybrids are widely different from each other; whereas if two very
distinct varieties of one species are crossed with another species,
the hybrids do not differ much. But this conclusion, as far as I can
make out, is founded on a single experiment; and seems directly
opposed to the results of several experiments made by Kolreuter.

These alone are the unimportant differences, which Gartner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species,
follows according to Gartner the same laws. When two species are
crossed, one has sometimes a prepotent power of impressing its
likeness on the hybrid; and so I believe it to be with varieties of
plants. With animals one variety certainly often has this prepotent
power over another variety. Hybrid plants produced from a reciprocal
cross, generally resemble each other closely; and so it is with
mongrels 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 excessively complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to
prepotency in transmitting likeness running more strongly in one sex
than in the other, both when one species is crossed with another, and
when one variety is crossed with another variety. For instance, I
think those authors are right, who maintain that the ass has a
prepotent power over the horse, so that both the mule and the hinny
more resemble the ass than the horse; but that the prepotency runs
more strongly in the male-ass than in the female, so that the mule,
which is the offspring of the male-ass and mare, is more like an ass,
than is the hinny, which is the offspring of the female-ass and

Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but
it can be shown that this does sometimes occur with hybrids; yet I
grant much less frequently with hybrids than with mongrels. Looking to
the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly
appeared--such as albinism, melanism, deficiency of tail or horns, or
additional fingers and toes; and do not relate to characters which
have been slowly acquired by selection. Consequently, sudden
reversions to the perfect character of either parent would be 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 much or
little from each other, namely in the union of individuals of the same
variety, or of different varieties, or of distinct species.

Laying aside 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 harmonises perfectly with the view that there
is no essential distinction between species and varieties.


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

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

The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several
circumstances; in some cases largely on the early death of the embryo.
The sterility of hybrids, which have their reproductive systems
imperfect, and which have had this system and their whole organisation
disturbed by being compounded of two distinct species, seems closely
allied to that sterility which so frequently affects pure species,
when their natural conditions of life have been disturbed. This view
is supported by a parallelism of another kind;--namely, that the
crossing of forms only slightly different is favourable to the vigour
and fertility of their offspring; and that slight changes in the
conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of
their hybrid-offspring should generally correspond, though due to
distinct causes; for both depend on the amount of difference of some
kind between the species which are crossed. Nor is it surprising that
the facility of effecting a first cross, the fertility of the hybrids
produced, and the capacity of being grafted together--though this
latter capacity evidently depends on widely different
circumstances--should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment;
for systematic affinity attempts to express all kinds of resemblance
between all species.

First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable
we are to argue in a circle with respect to varieties in a state of
nature; and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In all
other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels. Finally, then, the facts
briefly given in this chapter do not seem to me opposed to, but even
rather to support the view, that there is no fundamental distinction
between species and varieties.


On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
On the vast lapse of time, as inferred from the rate of deposition and
of denudation.
On the poorness of our palaeontological collections.
On the intermittence of geological formations.
On the absence of intermediate varieties in any one formation.
On the sudden appearance of groups of species.
On their sudden appearance in the lowest known fossiliferous strata.

In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific
forms, and their not being blended together by innumerable
transitional links, is a very obvious difficulty. I assigned reasons
why such links do not commonly occur at the present day, under the
circumstances apparently most favourable for their presence, namely on
an extensive and continuous area with graduated physical conditions. I
endeavoured to show, that the life of each species depends in a more
important manner on the presence of other already defined organic
forms, than on climate; and, therefore, that the really governing
conditions of life do not graduate away quite insensibly like heat or
moisture. I endeavoured, also, to show that intermediate varieties,
from existing in lesser numbers than the forms which they connect,
will generally be beaten out and exterminated during the course of
further modification and improvement. The main cause, however, of
innumerable intermediate links not now occurring everywhere throughout
nature depends on the very process of natural selection, through which
new varieties continually take the places of and exterminate their
parent-forms. But just in proportion as this process of extermination
has acted on an enormous scale, so must the number of intermediate
varieties, which have formerly existed on the earth, be truly
enormous. Why then is not every geological formation and every stratum
full of such intermediate links? Geology assuredly does not reveal any
such finely graduated organic chain; and this, perhaps, is the most
obvious and gravest objection which can be urged against my theory.
The explanation lies, as I believe, in the extreme imperfection of the
geological record.

In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid
picturing to myself, forms DIRECTLY intermediate between them. But
this is a wholly false view; we should always look for forms
intermediate between each species and a common but unknown progenitor;
and the progenitor will generally have differed in some respects from
all its modified descendants. To give a simple illustration: the
fantail and pouter pigeons have both descended from the rock-pigeon;
if we possessed all the intermediate varieties which have ever
existed, we should have an extremely close series between both and the
rock-pigeon; but we should have no varieties directly intermediate
between the fantail and pouter; none, for instance, combining a tail
somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have become
so much modified, that if we had no historical or indirect evidence
regarding their origin, it would not have been possible to have
determined from a mere comparison of their structure with that of the
rock-pigeon, whether they had descended from this species or from some
other allied species, such as C. oenas.

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

It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case DIRECT intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount
of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare
event; for in all cases the new and improved forms of life will 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 varieties of the same species at the
present day; and these parent-species, now generally extinct, have in
their turn been similarly connected with more ancient species; and so
on backwards, always converging to the common ancestor of each great
class. So that the number of intermediate and transitional links,
between all living and extinct species, must have been inconceivably
great. But assuredly, if this theory be true, such have lived upon
this earth.


Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected, that time will not have
sufficed for so great an amount of organic change, all changes having
been effected very slowly through natural selection. It is hardly
possible for me even to recall to the reader, who may not be a
practical geologist, the facts leading the mind feebly to comprehend
the lapse of time. He who can read Sir Charles Lyell's grand work on
the Principles of Geology, which the future historian will recognise
as having produced a revolution in natural science, yet does not admit
how incomprehensibly 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 each
stratum. A man must for years examine for himself great piles of
superimposed strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend anything
of the lapse of time, the monuments of which we see around us.

It is good to wander along lines of sea-coast, when formed of
moderately 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 reason to believe that pure water can effect
little or nothing in wearing away rock. At last the base of the cliff
is undermined, huge fragments fall down, and these remaining fixed,
have to be worn away, atom by atom, until reduced in size they can be
rolled about by the waves, and then are more quickly ground into
pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine
productions, showing how little they are abraded and how seldom they
are rolled about! Moreover, if we follow for a few miles any line of
rocky 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 washed their base.

He who most closely studies the action of the sea on our shores, will,
I believe, be most deeply impressed with the slowness with which rocky
coasts are worn away. The observations on this head by Hugh Miller,
and by that excellent observer Mr. Smith of Jordan Hill, are most
impressive. With the mind thus impressed, let any one examine beds of
conglomerate many thousand feet in thickness, which, though probably
formed at a quicker rate than many other deposits, yet, from being
formed of worn and rounded pebbles, each of which bears the stamp of
time, are good to show how slowly the mass has been accumulated. Let
him remember Lyell's profound remark, that the thickness and extent of
sedimentary formations are the result and measure of the degradation
which the earth's crust has elsewhere suffered. And what an amount of
degradation is implied by the sedimentary deposits of many countries!
Professor Ramsay has given me the maximum thickness, in most cases
from actual measurement, in a few cases from estimate, of each
formation in different parts of Great Britain; and this is the

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 these formations, which are
represented in England by thin beds, are thousands of feet in
thickness on the Continent. Moreover, between each successive
formation, we have, in the opinion of most geologists, enormously long
blank periods. So that the lofty pile of sedimentary rocks in Britain,
gives but an inadequate idea of the time which has elapsed during
their accumulation; yet what time this must have consumed! Good
observers have estimated that sediment is deposited by the great
Mississippi river at the rate of only 600 feet in a hundred thousand
years. This estimate may be quite erroneous; yet, considering over
what wide spaces very fine sediment is transported by the currents of
the sea, the process of accumulation in any one area must be extremely

But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember having been much struck with the evidence of denudation, 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 still more
plainly told 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, the
surface of the land has been so completely planed down by the action
of the sea, that no trace of these vast dislocations is externally

The Craven fault, for instance, extends for upwards of 30 miles, and
along this line the vertical displacement of the strata has varied
from 600 to 3000 feet. Professor Ramsay has published an account of a
downthrow in Anglesea of 2300 feet; and he informs me that he fully
believes there is one in Merionethshire of 12,000 feet; yet in these
cases there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having been
smoothly swept away. The consideration of these facts impresses my
mind almost in the same manner as does the vain endeavour to grapple
with the idea of eternity.

I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the
denudation of the Weald has been a mere trifle, in comparison with
that which has removed masses of our palaeozoic strata, in parts ten
thousand feet in thickness, as shown in Professor Ramsay's masterly
memoir on this subject. Yet it is an admirable lesson to stand on the
North Downs and to look at the distant South Downs; for, remembering
that at no great distance to the west the northern and southern
escarpments meet and close, one can safely picture to oneself the
great dome of rocks which must have covered up the Weald within so
limited a period as since the latter part of the Chalk formation. The
distance from the northern to the southern Downs is about 22 miles,
and the thickness of the several formations is on an average about
1100 feet, as I am informed by Professor Ramsay. But if, as some
geologists suppose, a range of older rocks underlies the Weald, on the
flanks of which the overlying sedimentary deposits might have
accumulated in thinner masses than elsewhere, the above estimate would
be erroneous; but this source of doubt probably would not greatly
affect the estimate as applied to the western extremity of the
district. If, then, we knew the rate at which the sea commonly wears
away a line of cliff of any given height, we could measure the time
requisite to have denuded the Weald. This, of course, cannot be done;
but we may, in order to form some crude notion on the subject, assume
that the sea would eat into cliffs 500 feet in height at the rate of
one inch in a century. This will at first appear much too small an
allowance; but it is the same as if we were to assume a cliff one yard
in height to be eaten back along a whole line of coast at the rate of
one yard in nearly every twenty-two years. I doubt whether any rock,
even as soft as chalk, would yield at this rate excepting on the most
exposed coasts; though no doubt the degradation of a lofty cliff would
be more rapid from the breakage of the fallen fragments. On the other
hand, I do not believe that any line of coast, ten or twenty miles in
length, ever suffers degradation at the same time along its whole
indented length; and we must remember that almost all strata contain
harder layers or nodules, which from long resisting attrition form a
breakwater at the base. Hence, under ordinary circumstances, I
conclude that for a cliff 500 feet in height, a denudation of one inch
per century for the whole length would be an ample allowance. At this
rate, on the above data, the denudation of the Weald must have
required 306,662,400 years; or say three hundred million years.

The action of fresh water on the gently inclined Wealden district,
when upraised, could hardly have been great, but it would somewhat
reduce the above estimate. On the other hand, during oscillations of
level, which we know this area has undergone, the surface may have
existed for millions of years as land, and thus have escaped the
action of the sea: when deeply submerged for perhaps equally long
periods, it would, likewise, have escaped the action of the
coast-waves. So that in all probability a far longer period than 300
million years has elapsed since the latter part of the Secondary

I have made these few remarks because it is highly important for us to
gain some notion, however imperfect, of the lapse of years. During
each of these years, over the whole world, the land and the water has
been peopled by hosts of living forms. What an infinite number of
generations, which the mind cannot grasp, must have succeeded each
other in the long roll of years! Now turn to our richest geological
museums, and what a paltry display we behold!


That our palaeontological collections are very imperfect, is admitted
by every one. The remark of that admirable palaeontologist, the late
Edward Forbes, should not be forgotten, namely, that numbers of our
fossil species are known and named from single and often broken
specimens, or from a few specimens collected on some one spot. Only a
small portion of the surface of the earth has been geologically
explored, and no part with sufficient care, as the important
discoveries made every year in Europe prove. No organism wholly soft
can be preserved. Shells and bones will decay and disappear when left
on the bottom of the sea, where sediment is not accumulating. I
believe we are continually taking a most erroneous view, when we
tacitly admit to ourselves that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed
and preserve fossil remains. Throughout an enormously large proportion
of the ocean, the bright blue tint of the water bespeaks its purity.
The many cases on record of a formation conformably covered, after an
enormous interval of time, by another and later formation, without the
underlying bed having suffered in the interval any wear and tear, seem
explicable only on the view of the bottom of the sea not rarely lying
for ages in an unaltered condition. The remains which do become
embedded, if in sand or gravel, will when the beds are upraised
generally be dissolved by the percolation of rain-water. I suspect
that but few of the very many animals which live on the beach between
high and low watermark are preserved. For instance, the several
species of the Chthamalinae (a sub-family of sessile cirripedes) coat
the rocks all over the world in infinite numbers: they are all
strictly littoral, with the exception of a single Mediterranean
species, which inhabits deep water and has been found fossil in
Sicily, whereas not one other species has hitherto been found in any
tertiary formation: yet it is now known that the genus Chthamalus
existed during the chalk period. The molluscan genus Chiton offers a
partially analogous case.

With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with one exception discovered by Sir C. Lyell in the
carboniferous strata of North America. In regard to mammiferous
remains, a single glance at the historical table published in the
Supplement to Lyell's Manual, will bring home the truth, how
accidental and rare is their preservation, far better than pages of
detail. Nor is their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been discovered
either in caves or in lacustrine 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 mainly results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time. 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 exclusively confined to
these large territories, would never have suspected that during the
periods which were blank and barren in his own country, great piles of
sediment, charged with new and peculiar forms of life, had elsewhere
been accumulated. And if in each separate territory, hardly any idea
can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great
changes in the geography of the surrounding lands, whence the sediment
has been derived, accords with the belief of vast intervals of time
having elapsed between each formation.

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

We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during
subsequent oscillations of level. Such thick and extensive
accumulations of sediment may be formed in two ways; either, in
profound depths of the sea, in which case, judging from the researches
of E. Forbes, we may conclude that the bottom will be inhabited by
extremely few animals, and the mass when upraised will give a most
imperfect record of the forms of life which then existed; or, sediment
may be accumulated to any thickness and extent over a shallow bottom,
if it continue slowly to subside. In this latter case, as long as the
rate of subsidence and supply of sediment nearly balance each other,
the sea will remain shallow and favourable for life, and thus a
fossiliferous formation thick enough, when upraised, to resist any
amount of degradation, may be formed.

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

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, may
have been formed over wide spaces during periods of subsidence, but
only where the supply of sediment was sufficient to keep the sea
shallow and to embed and preserve the remains before they had time to
decay. On the other hand, as long as the bed of the sea remained
stationary, THICK deposits could not have been accumulated in the
shallow parts, which are the most favourable to life. Still less could
this have happened during the alternate periods of elevation; or, to
speak more accurately, the beds which were then accumulated will have
been destroyed by being upraised and brought within the limits of the

Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for
they are in strict accordance with the general principles inculcated
by Sir C. Lyell; and E. Forbes independently arrived at a similar

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 most favourable, as previously explained, for the
formation of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other hand,
during subsidence, the inhabited area and number of inhabitants will
decrease (excepting the productions on the shores of a continent when
first broken up into an archipelago), and consequently during
subsidence, though there will be much extinction, fewer new varieties
or species will be formed; and it is during these very periods of
subsidence, that our great deposits rich in fossils have been
accumulated. Nature may almost be said to have guarded against the
frequent discovery of her transitional or linking forms.

From the foregoing 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 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. Some cases are on record of the same species
presenting distinct varieties in the upper and lower parts of the same
formation, but, as they are rare, they may be here passed over.
Although each formation has indisputably required a vast number of
years for its deposition, I can see several reasons why each should
not include a graduated series of links between the species which then
lived; but I can by no means pretend to assign due proportional weight
to the following considerations.

Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite to change one
species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us coming to any
just conclusion on this head. When we see a species first appearing in
the middle of any formation, it would be rash in the extreme to infer
that it had not elsewhere previously existed. So again when we find a
species disappearing before the uppermost layers have been deposited,
it would be equally rash to suppose that it then became wholly
extinct. We forget how small the area of Europe is compared with the
rest of the world; nor have the several stages of the same formation
throughout Europe been correlated with perfect accuracy.

With marine animals of all kinds, we may safely infer a large amount
of migration during climatal and other changes; and when we see a
species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for
instance, that several species appeared 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 of various quarters of
the world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now
abundant in the neighbouring sea, but are rare or absent in this
particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during
the Glacial period, which forms only a part of one whole geological
period; and likewise to reflect on the great changes of level, on the
inordinately great change of climate, on the prodigious lapse of time,
all included within this same glacial period. Yet it may be doubted
whether in any quarter of the world, sedimentary deposits, INCLUDING
FOSSIL REMAINS, have gone on accumulating within the same area during
the whole of this period. It is not, for instance, probable that
sediment was deposited during the whole of the glacial period near the
mouth of the Mississippi, within that limit of depth at which marine
animals can flourish; for we know what vast geographical changes
occurred in other parts of America during this space of time. When
such beds as were deposited in shallow water near the mouth of the
Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to
geographical changes. And in the distant future, a geologist examining
these beds, might be tempted to conclude that the average duration of
life of the embedded fossils had been less than that of the glacial
period, instead of having been really far greater, that is extending
from before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will
generally have to be a very thick one; and the species undergoing
modification will have had to live on the same area throughout this
whole time. But we have seen that a thick fossiliferous formation can
only be accumulated during a period of subsidence; and to keep the
depth approximately the same, which is necessary in order to enable
the same species to live on the same space, the supply of sediment
must nearly have counterbalanced the amount of subsidence. But this
same movement of subsidence will often tend to sink the area whence
the sediment is derived, and thus diminish the supply whilst the
downward movement continues. In fact, this nearly exact balancing
between the supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of different mineralogical composition, we may
reasonably suspect that the process of deposition has been much
interrupted, as a change in the currents of the sea and a supply of
sediment of a different nature will generally have been due to
geographical changes requiring much time. Nor will the closest
inspection of a formation give any idea of the time which its
deposition has consumed. Many instances could be given of beds only a
few feet in thickness, representing formations, 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
suspected the vast lapse of time represented by the thinner formation.
Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation,--facts, showing what wide, yet easily overlooked,
intervals have occurred in its accumulation. In other cases we have
the plainest evidence in great fossilised trees, still standing
upright as they grew, of many long intervals of time and changes of
level during the process of deposition, which would never even have
been suspected, had not the trees chanced to have been preserved:
thus, Messrs. Lyell and Dawson found carboniferous beds 1400 feet
thick in Nova Scotia, with ancient root-bearing strata, one above the
other, at no less than sixty-eight different levels. Hence, when the
same species occur at the bottom, middle, and top of a formation, the
probability is that they have not lived on the same spot during the
whole period of deposition, but have disappeared and reappeared,
perhaps many times, during the same geological period. So that if such
species were to undergo a considerable amount of modification during
any one geological period, a section would not probably include all
the fine intermediate gradations which must on my theory have existed
between them, but abrupt, though perhaps very 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 close
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 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 most closely connected with either one or both forms
by intermediate varieties. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet
might not at all necessarily be strictly intermediate between them in
all points of structure. So that we might obtain the parent-species
and its several modified descendants from the lower and upper beds of
a formation, and unless we obtained numerous transitional gradations,
we should not recognise their relationship, and should consequently be
compelled to rank them all 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 my
theory we ought to find. Moreover, if we look to rather wider
intervals, namely, to distinct but consecutive stages of the same
great formation, we find that the embedded fossils, though almost
universally ranked as specifically different, yet are far more closely
allied to each other than are the species found in more widely
separated formations; but to this subject I shall have to return in
the following chapter.

One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, there is reason
to suspect, as we have formerly seen, that their varieties are
generally at first local; and that such local varieties do not spread
widely and supplant their parent-forms until they have been modified
and perfected in some considerable degree. According to this view, the
chance of discovering in a formation in any one country all the early
stages of transition between any two forms, is small, for the
successive changes are supposed to have been local or confined to some
one spot. Most marine animals have a wide range; and we have seen that
with plants it is those which have the widest range, that oftenest
present varieties; so that with shells and other marine animals, it is
probably those which have had the widest range, far exceeding the
limits of the known geological formations of Europe, which have
oftenest given rise, first to local varieties and ultimately to new
species; and this again would greatly lessen the chance of our being
able to trace the stages of transition in any one geological

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until
many specimens have been collected from many places; and in the case
of fossil species this could rarely be effected by palaeontologists.
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 have descended from a single stock or
from several aboriginal stocks; or, again, whether certain sea-shells
inhabiting the shores of North America, which are ranked by some
conchologists as distinct species from their European representatives,
and by other conchologists as only varieties, are really varieties or
are, as it is called, specifically distinct. This could be effected
only by the future geologist discovering in a fossil state numerous
intermediate gradations; and such success seems to me improbable in
the highest degree.

Geological research, though it has added numerous species to existing
and extinct genera, and has made the intervals between some few groups
less wide than they otherwise would have been, yet has done scarcely
anything in breaking down the distinction between species, by
connecting them together by numerous, fine, intermediate varieties;
and this not having been effected, is probably the gravest and most
obvious of all the many objections which may be urged against my
views. Hence it will be worth while to sum up the foregoing remarks,
under an imaginary illustration. The Malay Archipelago is of about the
size of Europe from the North Cape to the Mediterranean, and from
Britain to Russia; and therefore equals all the geological formations
which have been examined with any accuracy, excepting those of the
United States of America. I fully agree with Mr. Godwin-Austen, that
the present condition of the Malay Archipelago, with its numerous
large islands separated by wide and shallow seas, probably represents
the former state of Europe, when most of our formations were
accumulating. The Malay Archipelago is one of the richest regions of
the whole world in organic beings; yet if all the species were to be
collected which have ever lived there, how imperfectly would they
represent the natural history of the world!

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

In our archipelago, I believe that fossiliferous formations could be
formed of sufficient thickness to last to an age, as distant in
futurity as the secondary formations lie in the past, only during
periods of subsidence. These periods of subsidence would be separated
from each other by enormous intervals, during which the area would be
either stationary or rising; whilst rising, each fossiliferous
formation would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America.
During the periods of subsidence there would probably be much
extinction of life; during the periods of elevation, there would be
much variation, but the geological record would then be least perfect.

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

Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to
believe that it would be chiefly these far-ranging species which would
oftenest produce new varieties; and the varieties would at first
generally 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, 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 my theory assuredly
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, some more closely, some more distantly related to each other;
and these links, let them be ever so close, if found in different
stages of the same formation, would, by most palaeontologists, be
ranked as distinct species. But I do not pretend that I should ever
have suspected how poor a record of the mutations of life, the best
preserved geological section presented, had not the difficulty of our
not discovering innumerable transitional links between the species
which appeared at the commencement and close of each formation,
pressed so hardly on my theory.


The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists, for
instance, by Agassiz, Pictet, and by none more forcibly than by
Professor 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 all at once, the
fact would be fatal to the theory of descent with slow modification
through natural selection. For the development of a group of forms,
all of which have descended from some one progenitor, must have been
an extremely slow process; and the progenitors must have lived long
ages before their modified descendants. But we continually over-rate
the perfection of the geological record, and falsely infer, because
certain genera or families have not been found beneath a certain
stage, that they did not exist before that stage. We continually
forget how large the world is, compared with the area over which our
geological formations have been carefully examined; we forget that
groups of species may elsewhere have long existed and have slowly
multiplied before they invaded the ancient archipelagoes of Europe and
of the United States. We do not make due allowance for the enormous
intervals of time, which have probably elapsed between our consecutive
formations,--longer perhaps in some cases than the time required for
the accumulation of each formation. These intervals will have given
time for the multiplication of species from some one or some few
parent-forms; and in the succeeding formation such 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; but that
when this had 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 be able to
spread rapidly and widely throughout the world.

I will now give a few examples to illustrate these remarks; and to
show how liable we are to error in supposing that whole groups of
species have suddenly been produced. I may recall the well-known fact
that in geological treatises, published not many years ago, the great
class of mammals was 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 one true mammal has 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 even as far back as the eocene stage. The most striking case,
however, is that of the Whale family; as these animals have huge
bones, are marine, and range over the world, the fact of not a single
bone of a whale having been discovered in any secondary formation,
seemed fully to justify the belief that this great and distinct order
had been suddenly produced in the interval between the latest
secondary and earliest tertiary formation. But now we may read in the
Supplement to Lyell's 'Manual,' published in 1858, clear evidence of
the existence of whales in the upper greensand, some time before the
close of the secondary period.

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
have stated that, from the 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 50
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 recognised; from all these circumstances, I inferred
that had sessile cirripedes existed during the secondary periods, they
would certainly have been preserved and discovered; and as not one
species had been discovered in beds of this age, I concluded that this
great group had been suddenly developed at the commencement of the
tertiary series. This was a sore trouble to me, adding as I thought
one more instance of the abrupt appearance of a great group of
species. But my work had hardly been published, when a skilful
palaeontologist, M. Bosquet, sent me a drawing of a perfect specimen
of an unmistakeable sessile cirripede, which he had himself extracted
from the chalk of Belgium. And, as if to make the case as striking as
possible, this sessile cirripede was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one specimen has as yet been
found even in any tertiary stratum. Hence we now positively know that
sessile cirripedes existed during the secondary period; and these
cirripedes might have been the progenitors of our many tertiary and
existing species.

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 in the Chalk period. This group
includes the large majority of existing species. Lately, Professor
Pictet has carried their existence one sub-stage further back; and
some palaeontologists believe that certain much older fishes, of which
the affinities are as yet imperfectly known, are really teleostean.
Assuming, however, that the whole of them did appear, as Agassiz
believes, at the commencement of the chalk formation, the fact would
certainly be highly remarkable; but I cannot see that it would be an
insuperable difficulty on my theory, unless it could likewise be shown
that the species of this group appeared suddenly and simultaneously
throughout the world at this same period. 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
fish might formerly have had a similarly confined range, and after
having been largely developed in some one sea, might have spread
widely. Nor have we any right to suppose that the seas of the world
have always been so freely open from 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 and similar considerations, but chiefly 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
ideas on many points, which the discoveries of even the last dozen
years have effected, it seems to me to be about as rash in us to
dogmatize on the succession of organic beings throughout the world, as
it would be for a naturalist to land for five minutes on some one
barren point in Australia, and then to discuss the number and range of
its productions.


There is another and allied difficulty, which is much graver. I allude
to the manner in which numbers of species of the same group, 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 have descended from one progenitor, apply with nearly equal
force to the earliest known species. For instance, I cannot doubt that
all the Silurian trilobites have descended from some one crustacean,
which must have lived long before the Silurian age, and which probably
differed greatly from any known animal. Some of the most ancient
Silurian animals, as the Nautilus, Lingula, etc., do not differ much
from living species; and it cannot on my theory be supposed, that
these old species were the progenitors of all the species of the
orders to which they belong, for they do not present characters in any
degree intermediate between them. If, moreover, they had been the
progenitors of these orders, they would almost certainly have been
long ago supplanted and exterminated by their numerous and improved

Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long
as, or probably far longer than, the whole interval from the Silurian
age to the present day; and that during these vast, yet quite unknown,
periods of time, the world swarmed with living creatures.

To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most
eminent geologists, with Sir R. Murchison at their head, are convinced
that we see in the organic remains of the lowest Silurian stratum the
dawn of life on this planet. Other highly competent judges, as Lyell
and the late E. Forbes, dispute this conclusion. We should not forget
that only a small portion of the world is known with accuracy. M.
Barrande has lately added another and lower stage to the Silurian
system, abounding with new and peculiar species. Traces of life have
been detected in the Longmynd beds beneath Barrande's so-called
primordial zone. The presence of phosphatic nodules and bituminous
matter in some of the lowest azoic rocks, probably indicates the
former existence of life at these periods. But the difficulty of
understanding the absence of vast piles of fossiliferous strata, which
on my theory no doubt were somewhere accumulated before the Silurian
epoch, is very great. If these most ancient beds had been wholly worn
away by denudation, or obliterated by metamorphic action, we ought to
find only small remnants of the formations next succeeding them in
age, and these ought to be very generally in a metamorphosed
condition. But the descriptions which we now possess of the Silurian
deposits over immense territories in Russia and in North America, do
not support the view, that the older a formation is, the more it has
suffered the extremity of denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged
as a valid argument against the views here entertained. To show that
it may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains, which do not
appear to have inhabited profound depths, in the several formations of
Europe and of the United States; and from the amount of sediment,
miles in thickness, of which the formations are composed, we may infer
that from first to last large islands or tracts of land, whence the
sediment was derived, occurred in the neighbourhood of the existing
continents of Europe and North America. But we do not know what was
the state of things in the intervals between the successive
formations; whether Europe and the United States during these
intervals existed as dry land, or as a submarine surface near land, on
which sediment was not deposited, or again 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 not one 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
there, palaeozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which we may fairly conclude 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 earliest silurian period. The coloured 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 have
we any right to assume that things have thus remained from eternity?
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 immeasurably antecedent to the silurian 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
formations older than the silurian 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 bare metamorphic
rocks, which must have been heated under great pressure, have always
seemed to me to require some special explanation; and we may perhaps
believe that we see in these large areas, the many formations long
anterior to the silurian epoch in a completely metamorphosed

The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between
the many species which now exist or have existed; the sudden manner in
which whole groups of species appear in our European formations; the
almost entire absence, as at present known, of fossiliferous
formations beneath the Silurian strata, are all undoubtedly of the
gravest nature. We see this in the plainest manner by the fact that
all the most eminent palaeontologists, namely Cuvier, Owen, Agassiz,
Barrande, Falconer, E. Forbes, etc., and all our greatest geologists,
as Lyell, Murchison, Sedgwick, etc., have unanimously, often
vehemently, maintained the immutability of species. But I have reason
to believe that one great authority, Sir Charles Lyell, from further
reflexion entertains grave doubts on this subject. I feel how rash it
is to differ from these great authorities, to whom, with others, we
owe all our knowledge. Those who think the natural geological record
in any degree perfect, and who do not attach much weight to the facts
and arguments of other kinds given in this volume, will undoubtedly at
once reject my theory. For my part, following out Lyell's metaphor, I
look at the natural geological record, as a history of the world
imperfectly kept, and written in a changing dialect; of this history
we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has
been preserved; and of each page, only here and there a few lines.
Each word of the slowly-changing language, in which the history is
supposed to be written, being more or less different in the
interrupted succession of chapters, may represent the apparently
abruptly changed forms of life, entombed in our consecutive, but
widely separated formations. On this view, the difficulties above
discussed are greatly diminished, or even disappear.


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

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

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between them, and
to make the percentage system of lost and new 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 lost forms, and only
one or two are new forms, having here appeared for the first time,
either locally, or, as far as we know, on the face of the earth. If we
may trust the observations of Philippi in Sicily, the successive
changes in the marine inhabitants of that island have been many and
most gradual. The secondary formations are more broken; but, as Bronn
has remarked, neither the appearance nor disappearance of their many
now extinct species has been simultaneous in each separate formation.

Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest tertiary beds a few living
shells may still be found in the midst of a multitude of extinct
forms. Falconer has given a striking instance of a similar fact, in an
existing crocodile associated with many strange and lost mammals and
reptiles in the sub-Himalayan deposits. The Silurian Lingula differs
but little from the living species of this genus; whereas most of the
other Silurian Molluscs and all the Crustaceans have changed greatly.
The productions of the land seem to change at a quicker rate than
those of the sea, of which a striking instance has lately been
observed in Switzerland. There is some reason to believe that
organisms, considered high in the scale of nature, change more quickly
than those that are low: though there are exceptions to this rule. The
amount of organic change, as Pictet has remarked, does not strictly
correspond with the succession of our geological formations; so that
between each two consecutive formations, the forms of life have seldom
changed in exactly the same degree. Yet if we compare any but the most
closely related formations, all the species will be found to have
undergone some change. When a species has once disappeared from the
face of the earth, we have reason to believe that the same identical
form never reappears. The strongest apparent exception to this latter
rule, is that of the so-called "colonies" of M. Barrande, which
intrude for a period in the midst of an older formation, and then
allow the pre-existing fauna to reappear; but Lyell's explanation,
namely, that it is a case of temporary migration from a distinct
geographical province, seems to me satisfactory.

These several facts accord well with my theory. I believe in no fixed
law of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species
is quite independent of that of all others. Whether such variability
be taken advantage of by natural selection, and whether the variations
be accumulated to a greater or lesser amount, thus causing a greater
or lesser amount of modification in the varying species, depends on
many complex contingencies,--on the variability being of a beneficial
nature, on the power of intercrossing, on the rate of breeding, on the
slowly changing physical conditions of the country, and more
especially on the nature of the other inhabitants with which the
varying species comes 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, that it should change less. We
see the same fact in geographical distribution; for instance, in the
land-shells and coleopterous insects of Madeira having come to differ
considerably from their nearest allies on the continent of Europe,
whereas the marine shells and birds have remained unaltered. We can
perhaps understand the apparently quicker rate of change in
terrestrial and in more highly organised productions compared with
marine and lower 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 inhabitants of a
country have become modified and improved, we can understand, on the
principle of competition, and on that of the many all-important
relations of organism to organism, that any form which does not become
in some degree modified and improved, will be liable to be
exterminated. Hence we can see why all the species in the same region
do at last, if we look to wide enough intervals of time, become
modified; for those which do not change will 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 long-enduring fossiliferous formations depends on
great masses of sediment having been deposited on areas whilst
subsiding, our formations have been almost necessarily accumulated at
wide and irregularly intermittent intervals; consequently the amount
of organic change exhibited by the fossils embedded in consecutive
formations is not equal. Each formation, on this view, does not mark a
new and complete act of creation, but only an occasional scene, taken
almost at hazard, in a 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 offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances)
to fill the exact place of another species in the economy of nature,
and thus supplant it; yet the two forms--the old and the new--would
not be identically the same; for both would almost certainly inherit
different characters from their distinct progenitors. For instance, it
is just possible, if our fantail-pigeons were all destroyed, that
fanciers, by striving during long ages for the same object, might make
a new breed hardly distinguishable from our present fantail; but if
the parent rock-pigeon were also destroyed, and in nature we have
every reason to believe that the parent-form will generally be
supplanted and exterminated by its improved offspring, it is quite
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from the other
well-established races of the domestic pigeon, for the newly-formed
fantail would be almost sure to inherit from its new progenitor some
slight characteristic 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
degree. A group does not reappear after it has once disappeared; or
its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions
are surprisingly few, so few, that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain) admit its
truth; and the rule strictly accords with my theory. For as all the
species of the same group have descended from some one species, it is
clear that as long as any species of the group have appeared in the
long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the
same old and unmodified forms. Species of the genus Lingula, for
instance, must have continuously existed by an unbroken succession of
generations, from the lowest Silurian stratum to the present day.

We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule
being a gradual increase in number, till the group reaches its
maximum, and then, sooner or later, it gradually decreases. If the
number of the species of a genus, or the number of the genera of a
family, be represented by a vertical line of varying thickness,
crossing the successive geological formations in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the
species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of
modification and the production of a number of allied forms must be
slow and gradual,--one species giving rise first to two or three
varieties, these being slowly converted into species, which in their
turn produce by equally slow steps other species, and so on, like the
branching of a great tree from a single stem, till the group becomes


We have as yet spoken only incidentally of the disappearance of
species and of groups of species. On the theory of natural selection
the extinction of old forms and the production of new and improved
forms are intimately connected together. The old notion of all the
inhabitants of the earth having been swept away at successive periods
by catastrophes, is very generally given up, even by those geologists,
as Elie de Beaumont, Murchison, Barrande, etc., whose general views
would naturally lead them to this conclusion. On the contrary, we have
every reason to believe, from the study of the tertiary formations,
that species and groups of species gradually disappear, one after
another, first from one spot, then from another, and finally from the
world. Both single species and whole groups of species last for very
unequal periods; some groups, as we have seen, having endured from the
earliest known dawn of life to the present day; some having
disappeared before the close of the palaeozoic period. No fixed law
seems to determine the length of time during which any single species
or any single genus endures. There is reason to believe that the
complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of
a group of species 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 appearance and increase in numbers of
the species. In some cases, however, the extermination of whole groups
of beings, as of ammonites towards the close of the secondary period,
has been wonderfully sudden.

The whole subject of the extinction of species has been involved in
the most gratuitous mystery. Some authors have even supposed that as
the individual has a definite length of life, so have species a
definite duration. No one I think can have marvelled more at the
extinction of species, than I have done. 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 at 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 favourable. But how utterly groundless was my
astonishment! Professor Owen soon perceived that the tooth, though so
like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity is the
attribute of a vast number of species of all classes, in all
countries. If we ask ourselves why this or that species is rare, we
answer that something is unfavourable in its conditions of life; but
what that something is, we can hardly ever tell. On the supposition of
the fossil horse still existing as a rare species, we might have felt
certain from the analogy of all other mammals, even of the
slow-breeding elephant, and from the history of the naturalisation of
the domestic horse in South America, that under more favourable
conditions it would in a very few years have stocked the whole
continent. But we could not have told what the unfavourable conditions
were which checked its increase, whether some one or several
contingencies, and at what period of the horse's life, and in what
degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not
have perceived the fact, yet the fossil horse would certainly have
become rarer and rarer, and finally extinct;--its place being seized
on by some more successful competitor.

It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply
sufficient to cause rarity, and finally extinction. 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 exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely,
that to admit that species generally become rare before they become
extinct--to feel no surprise at the rarity of a species, and yet to
marvel greatly when it 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 unknown 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 less-favoured forms
almost inevitably follows. It is the same with our domestic
productions: when a new and slightly improved variety has been raised,
it at first supplants the less improved varieties in the same
neighbourhood; when much improved it is transported far and near, like
our short-horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance of
old forms, both natural and artificial, are bound together. In certain
flourishing groups, the number of new specific forms which have been
produced within a given time is probably greater than that of the old
forms which have been exterminated; but we know that the number of
species has not gone on indefinitely increasing, at least during the
later geological periods, so that looking to later times we may
believe that the production of new forms has caused the extinction of
about the same number of old forms.

The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like
each other in all respects. Hence the improved and modified
descendants of a species will generally cause the extermination of the
parent-species; and if many new forms have been developed from any one
species, the nearest allies of that species, i.e. the species of the
same genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species, that is a
new genus, comes to supplant an old genus, belonging to the same
family. But it must often have happened that a new species belonging
to some one group will have seized on the place occupied by a species
belonging to a distinct group, and thus caused its extermination; and
if many allied forms be developed from the successful intruder, many
will have to yield their places; and it will generally be 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 yield their places to other species which have been
modified and improved, a few of the sufferers may often long be
preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a
great genus of shells in the secondary formations, survives in the
Australian seas; and a few members of the great and almost extinct
group of Ganoid fishes still inhabit our fresh waters. Therefore the
utter extinction of a group is generally, as we have seen, a slower
process than its production.

With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palaeozoic period and
of Ammonites at the close of the secondary period, we must remember
what has been already said on the probable wide intervals of time
between our consecutive formations; and in these intervals there may
have been much slow extermination. Moreover, when by sudden
immigration or by unusually rapid development, many species of a new
group have taken possession of a new area, they will have exterminated
in a correspondingly rapid manner many of the old inhabitants; and the
forms which thus yield their places will commonly be allied, for they
will partake of some 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 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 individuals than that; why this species and not
another can be naturalised in a given country; then, and not till
then, we may justly feel surprise why we cannot account for the
extinction of this particular species or group of species.


Scarcely any palaeontological discovery is more striking than the
fact, that the forms of life change almost simultaneously throughout
the world. Thus our European Chalk formation can be recognised in many
distant parts of the world, under the most different climates, where
not a fragment of the mineral chalk itself can be found; namely, in
North America, in equatorial South America, in Tierra del Fuego, at
the Cape of Good Hope, and in the peninsula of India. For at these
distant points, the organic remains in certain beds present an
unmistakeable degree of 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 characterised in
such trifling points as mere superficial sculpture. Moreover other
forms, which are not found in the Chalk of Europe, but which occur in
the formations either above or below, are similarly absent 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 several European and North
American tertiary deposits. Even if the few fossil species which are
common to the Old and New Worlds be kept wholly out of view, the
general parallelism in the successive forms of life, in the stages of
the widely separated palaeozoic and tertiary periods, would still be
manifest, and the several formations could be easily correlated.

These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge
whether the productions of the land and of fresh water change at
distant points 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 coexisted with still living sea-shells; but as
these anomalous monsters coexisted with the Mastodon and Horse, it
might at least have been inferred that they had lived during one of
the latter 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 thousandth or hundred-thousandth year,
or even that it has a very strict geological sense; for if all the
marine animals which live at the present day in Europe, and all those
that lived in Europe during the pleistocene period (an enormously
remote period as measured by years, including the whole glacial
epoch), were to be compared with those now living in South America or
in Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers believe that the existing
productions of the United States are more closely related to those
which lived in Europe during certain later tertiary stages, than to
those which now live here; and if this be so, it is evident that
fossiliferous beds deposited at the present day on the shores of North
America would hereafter be liable to be classed with somewhat older
European beds. Nevertheless, looking to a remotely future epoch, there
can, I think, be little doubt that all the more modern MARINE
formations, namely, the upper pliocene, the pleistocene and strictly
modern beds, of Europe, North and South America, and Australia, from
containing fossil remains in some degree allied, and from not
including those forms which are only found in the older underlying
deposits, would be correctly ranked as simultaneous in a geological

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 introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom." M. Barrande has made forcible remarks to precisely
the same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these
great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to
some special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries, and the
nature of their inhabitants.

This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural
selection. New species are formed by new varieties arising, which have
some advantage over older forms; and those forms, which are already
dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or
incipient species; for these latter must be victorious in a still
higher degree in order to be preserved and to survive. We have
distinct evidence on this head, in the plants which are dominant, that
is, which are commonest in their own homes, and are most widely
diffused, having produced the greatest number of new varieties. It is
also natural that the dominant, varying, and far-spreading species,
which already have invaded to a certain extent the territories of
other species, should be those which would have the best chance of
spreading still further, and of giving rise in new countries to new
varieties and species. The process of diffusion may often be very
slow, being dependent on climatal and geographical changes, or on
strange accidents, but in the long run the dominant forms will
generally succeed in spreading. 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 apparently do find, a less strict degree of
parallel succession in the productions of the land than of the sea.

Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of
individuals, from giving a better chance of the appearance of
favourable variations, and that severe competition with many already
existing forms, would be highly favourable, as would be the power of
spreading into new territories. A certain amount of isolation,
recurring at long intervals of time, would probably be also
favourable, as before explained. One quarter of the world may have
been most favourable for the production of new and dominant species on
the land, and another for those in the waters of the sea. If two great
regions had been for a long period favourably circumstanced in an
equal degree, whenever their inhabitants met, the battle would be
prolonged and severe; and some from one birthplace and some from the
other might be victorious. But in the course of time, the forms
dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these inferior forms
would be allied in groups by inheritance, whole groups would tend
slowly to disappear; though here and there a single member might long
be enabled to survive.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new species having been
formed by dominant species spreading widely and varying; the new
species thus produced being themselves dominant owing to inheritance,
and to having already had some advantage over their parents or over
other species; these again spreading, varying, and producing new
species. The 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 will disappear
from the world; and the succession of forms in both ways will
everywhere tend to correspond.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of
the sea was either stationary or rising, and likewise when sediment
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 far from having
any right to conclude that this has invariably been the case, and that
large areas have invariably been affected by the same movements. When
two formations have been deposited in two regions during nearly, but
not exactly the same period, we should find in both, from the causes
explained in the foregoing paragraphs, the same general succession in
the forms of life; but the species would not exactly correspond; for
there will have been a little more time in the one region than in the
other for modification, extinction, and immigration.

I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England
and France, is able to draw a close general parallelism between the
successive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult
to account for, considering the proximity of the two areas,--unless,
indeed, it be assumed that an isthmus separated two seas inhabited by
distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; 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 periods,--a formation in one region often corresponding with a
blank interval in the other,--and if in both regions the species have
gone on slowly changing during the 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 succession of the form
of life, and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.

Let us now look to the mutual affinities of extinct and living
species. They all fall into one grand natural system; 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, all fossils can be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up the wide intervals between existing genera,
families, and orders, cannot be disputed. For if we confine our
attention either to the living or to the extinct alone, the series is
far less perfect than if we combine both into one general system. With
respect to the Vertebrata, whole pages could be filled with striking
illustrations from our great palaeontologist, Owen, showing how
extinct animals fall in between existing groups. Cuvier ranked the
Ruminants and Pachyderms, as the two most distinct orders of mammals;
but Owen has discovered so many fossil links, that he has had to alter
the whole classification of these two orders; and has placed certain
pachyderms in the same sub-order with ruminants: for example, he
dissolves by fine gradations the apparently wide difference between
the pig and the camel. In regard to the Invertebrata, Barrande, and a
higher authority could not be named, asserts that he is every day
taught that palaeozoic animals, though belonging to the same orders,
families, or genera with those living at the present day, were not at
this early epoch limited in such distinct groups as they now are.

Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If
by this term it is meant that an extinct form is directly intermediate
in all its characters between two living forms, the objection is
probably valid. But I apprehend that in a perfectly natural
classification many fossil species would have to stand between living
species, and some extinct genera between living genera, even between
genera belonging to distinct families. The most common case,
especially with respect to very distinct groups, such as fish and
reptiles, seems to be, that supposing them to be distinguished at the
present day from each other by a dozen characters, the ancient members
of the same two groups would be distinguished by a somewhat lesser
number of characters, so that the two groups, though formerly quite
distinct, at that period made some small approach to each other.

It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted to
those groups 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 towards very
distinct groups. Yet if we compare the older Reptiles and Batrachians,
the older Fish, the older Cephalopods, and the eocene Mammals, with
the more recent members of the same classes, we must admit that there
is some truth in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the
fourth chapter. We may suppose that the numbered letters 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

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