On the Origin of Species By Means of Natural Selection Charles Darwin 22351 downloads.pdf

CHAPTER IX.
ON THE IMPERFECTION OF THE GEOLOGICAL
RECORD.

On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their
number. On the vast lapse of time, as inferred from the rate
of deposition and of denudation. On the poorness of our
palæontological 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.

On the lapse of Time.—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 result:—
Feet

Palæozoic 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 slow.
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 visible.
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 palæozoic 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 period.
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!

On the poorness of our Palæontological collections.—That our
palæontological collections are very imperfect, is admitted by every one.
The remark of that admirable palæontologist, 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 Chthamalinæ (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 Palæozoic 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 palæozoic 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 coast-
action.
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 conclusion.
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 palæontologists, 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 palæozoic 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
palæontologist, 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
palæontologists 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 formation.
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 palæontologists. 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 palæontologists, 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
palæontologists, 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.

On the sudden appearance of whole groups of Allied Species.—The
abrupt manner in which whole groups of species suddenly appear in certain
formations, has been urged by several palæontologists, 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
palæontologist, 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 palæontologists 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 palæontologists 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 Palæontology 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 palæontological 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.

On the sudden appearance of groups of Allied Species in the lowest
known fossiliferous strata.—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 descendants.
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 palæozoic or secondary formation.
Hence we may perhaps infer, that during the palæozoic and secondary
periods, neither continents nor continental islands existed where our oceans
now extend; for had they existed there, palæozoic 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 condition.

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 palæontologists,
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.

CHAPTER X.
ON THE GEOLOGICAL SUCCESSION OF
ORGANIC BEINGS.

On the slow and successive appearance of new species. On
their different rates of change. Species once lost do not
reappear. Groups of species follow the same general rules in
their 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 species. On the state of development
of ancient forms. On the succession of the same types within
the same areas. Summary of preceding and present 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 large.

On Extinction.—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 palæozoic 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 palæozoic 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.

On the Forms of Life changing almost simultaneously throughout the
World.—Scarcely any palæontological 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 palæozoic 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 palæozoic 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 sense.
The fact of the forms of life changing simultaneously, in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d’Archiac. After referring to the
parallelism of the palæozoic 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.

On the Affinities of extinct Species to each other, and to living forms.—
Let us now look to the mutual affinities of extinct and living species. 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 palæontologist, 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 palæozoic 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 species being given, but this is

unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a14, q14, p14, will form a
small family; b14 and f14 a closely allied family or sub-family; and o14, e14,
m14, a third family. These three families, together with the many extinct
genera on the several lines of descent diverging from the parent-form A,
will form an order; for all will have inherited something in common from
their ancient and common progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by this
diagram, the more recent any form is, the more it will generally differ from
its ancient progenitor. Hence we can understand the rule that the most
ancient fossils differ most from existing forms. We must not, however,
assume that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize on
many and different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a species
might go on being slightly modified in relation to its slightly altered
conditions of life, and yet retain throughout a vast period the same general
characteristics. This is represented in the diagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, as
before remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred, these three
families would be so closely linked together that they probably would have
to be united into one great family, in nearly the same manner as has
occurred with ruminants and pachyderms. Yet he who objected to call the
extinct genera, which thus linked the living genera of three families
together, intermediate in character, would be justified, as they are
intermediate, not directly, but only by a long and circuitous course through
many widely different forms. If many extinct forms were to be discovered
above one of the middle horizontal lines or geological formations—for

instance, above Number VI.—but none from beneath this line, then only the
two families on the left hand (namely, a14, etc., and b14, etc.) would have to
be united into one family; and the two other families (namely, a14 to f14 now
including five genera, and o14 to m14) would yet remain distinct. These two
families, however, would be less distinct from each other than they were
before the discovery of the fossils. If, for instance, we suppose the existing
genera of the two families to differ from each other by a dozen characters,
in this case the genera, at the early period marked VI., would differ by a
lesser number of characters; for at this early stage of descent they have not
diverged in character from the common progenitor of the order, nearly so
much as they subsequently diverged. Thus it comes that ancient and extinct
genera are often in some slight degree intermediate in character between
their modified descendants, or between their collateral relations.
In nature the case will be far more complicated than is represented in the
diagram; for the groups will have been more numerous, they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in very rare cases, to fill up wide intervals in the natural system, and
thus unite distinct families or orders. All that we have a right to expect, is
that those groups, which have within known geological periods undergone
much modification, should in the older formations make some slight
approach to each other; so that the older members should differ less from
each other in some of their characters than do the existing members of the
same groups; and this by the concurrent evidence of our best
palæontologists seems frequently to be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each other and
to living forms, seem to me explained in a satisfactory manner. And they are
wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period in the
earth’s history will be intermediate in general character between that which
preceded and that which succeeded it. Thus, the species which lived at the
sixth great stage of descent in the diagram are the modified offspring of
those which lived at the fifth stage, and are the parents of those which
became still more modified at the seventh stage; hence they could hardly

fail to be nearly intermediate in character between the forms of life above
and below. We must, however, allow for the entire extinction of some
preceding forms, and for the coming in of quite new forms by immigration,
and for a large amount of modification, during the long and blank intervals
between the successive formations. Subject to these allowances, the fauna
of each geological period undoubtedly is intermediate in character, between
the preceding and succeeding faunas. I need give only one instance, namely,
the manner in which the fossils of the Devonian system, when this system
was first discovered, were at once recognised by palæontologists as
intermediate in character between those of the overlying carboniferous, and
underlying Silurian system. But each fauna is not necessarily exactly
intermediate, as unequal intervals of time have elapsed between consecutive
formations.
It is no real objection to the truth of the statement, that the fauna of each
period as a whole is nearly intermediate in character between the preceding
and succeeding faunas, that certain genera offer exceptions to the rule. For
instance, mastodons and elephants, when arranged by Dr. Falconer in two
series, first according to their mutual affinities and then according to their
periods of existence, do not accord in arrangement. The species extreme in
character are not the oldest, or the most recent; nor are those which are
intermediate in character, intermediate in age. But supposing for an instant,
in this and other such cases, that the record of the first appearance and
disappearance of the species was perfect, we have no reason to believe that
forms successively produced necessarily endure for corresponding lengths
of time: a very ancient form might occasionally last much longer than a
form elsewhere subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things with
great: if the principal living and extinct races of the domestic pigeon were
arranged as well as they could be in serial affinity, this arrangement would
not closely accord with the order in time of their production, and still less
with the order of their disappearance; for the parent rock-pigeon now lives;
and many varieties between the rock-pigeon and the carrier have become
extinct; and carriers which are extreme in the important character of length
of beak originated earlier than short-beaked tumblers, which are at the
opposite end of the series in this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the

fact, insisted on by all palæontologists, that fossils from two consecutive
formations are far more closely related to each other, than are the fossils
from two remote formations. Pictet gives as a well-known instance, the
general resemblance of the organic remains from the several stages of the
chalk formation, though the species are distinct in each stage. This fact
alone, from its generality, seems to have shaken Professor Pictet in his firm
belief in the immutability of species. He who is acquainted with the
distribution of existing species over the globe, will not attempt to account
for the close resemblance of the distinct species in closely consecutive
formations, by the physical conditions of the ancient areas having remained
nearly the same. Let it be remembered that the forms of life, at least those
inhabiting the sea, have changed almost simultaneously throughout the
world, and therefore under the most different climates and conditions.
Consider the prodigious vicissitudes of climate during the pleistocene
period, which includes the whole glacial period, and note how little the
specific forms of the inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fact of fossil remains
from closely consecutive formations, though ranked as distinct species,
being closely related, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened between
successive formations, we ought not to expect to find, as I attempted to
show in the last chapter, in any one or two formations all the intermediate
varieties between the species which appeared at the commencement and
close of these periods; but we ought to find after intervals, very long as
measured by years, but only moderately long as measured geologically,
closely allied forms, or, as they have been called by some authors,
representative species; and these we assuredly do find. We find, in short,
such evidence of the slow and scarcely sensible mutation of specific forms,
as we have a just right to expect to find.

On the state of Development of Ancient Forms.—There has been much
discussion whether recent forms are more highly developed than ancient. I
will not here enter on this subject, for naturalists have not as yet defined to
each other’s satisfaction what is meant by high and low forms. But in one
particular sense the more recent forms must, on my theory, be higher than
the more ancient; for each new species is formed by having had some

advantage in the struggle for life over other and preceding forms. If under a
nearly similar climate, the eocene inhabitants of one quarter of the world
were put into competition with the existing inhabitants of the same or some
other quarter, the eocene fauna or flora would certainly be beaten and
exterminated; as would a secondary fauna by an eocene, and a palæozoic
fauna by a secondary fauna. I do not doubt that this process of improvement
has affected in a marked and sensible manner the organisation of the more
recent and victorious forms of life, in comparison with the ancient and
beaten forms; but I can see no way of testing this sort of progress.
Crustaceans, for instance, not the highest in their own class, may have
beaten the highest molluscs. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have
seized on places which must have been previously occupied, we may
believe, if all the animals and plants of Great Britain were set free in New
Zealand, that in the course of time a multitude of British forms would
become thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from what we see now occurring in New
Zealand, and from hardly a single inhabitant of the southern hemisphere
having become wild in any part of Europe, we may doubt, if all the
productions of New Zealand were set free in Great Britain, whether any
considerable number would be enabled to seize on places now occupied by
our native plants and animals. Under this point of view, the productions of
Great Britain may be said to be higher than those of New Zealand. Yet the
most skilful naturalist from an examination of the species of the two
countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the embryological
development of recent forms. I must follow Pictet and Huxley in thinking
that the truth of this doctrine is very far from proved. Yet I fully expect to
see it hereafter confirmed, at least in regard to subordinate groups, which
have branched off from each other within comparatively recent times. For
this doctrine of Agassiz accords well with the theory of natural selection. In
a future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations supervening at a not early age, and being
inherited at a corresponding age. This process, whilst it leaves the embryo

almost unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal. This view
may be true, and yet it may never be capable of full proof. Seeing, for
instance, that the oldest known mammals, reptiles, and fish strictly belong
to their own proper classes, though some of these old forms are in a slight
degree less distinct from each other than are the typical members of the
same groups at the present day, it would be vain to look for animals having
the common embryological character of the Vertebrata, until beds far
beneath the lowest Silurian strata are discovered—a discovery of which the
chance is very small.

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

Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing
the present climate of Australia and of parts of South America under the
same latitude, would attempt to account, on the one hand, by dissimilar
physical conditions for the dissimilarity of the inhabitants of these two
continents, and, on the other hand, by similarity of conditions, for the
uniformity of the same types in each during the later tertiary periods. Nor
can it be pretended that it is an immutable law that marsupials should have
been chiefly or solely produced in Australia; or that Edentata and other
American types should have been solely produced in South America. For
we know that Europe in ancient times was peopled by numerous
marsupials; and I have shown in the publications above alluded to, that in
America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly of
the present character of the southern half of the continent; and the southern
half was formerly more closely allied, than it is at present, to the northern
half. In a similar manner we know from Falconer and Cautley’s discoveries,
that northern India was formerly more closely related in its mammals to
Africa than it is at the present time. Analogous facts could be given in
relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the world
will obviously tend to leave in that quarter, during the next succeeding
period of time, closely allied though in some degree modified descendants.
If the inhabitants of one continent formerly differed greatly from those of
another continent, so will their modified descendants still differ in nearly
the same manner and degree. But after very long intervals of time and after
great geographical changes, permitting much inter-migration, the feebler
will yield to the more dominant forms, and there will be nothing immutable
in the laws of past and present distribution.
It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This cannot
for an instant be admitted. These huge animals have become wholly extinct,
and have left no progeny. But in the caves of Brazil, there are many extinct
species which are closely allied in size and in other characters to the species

still living in South America; and some of these fossils may be the actual
progenitors of living species. It must not be forgotten that, on my theory, all
the species of the same genus have descended from some one species; so
that if six genera, each having eight species, be found in one geological
formation, and in the next succeeding formation there be six other allied or
representative genera with the same number of species, then we may
conclude that only one species of each of the six older genera has left
modified descendants, constituting the six new genera. The other seven
species of the old genera have all died out and have left no progeny. Or,
which would probably be a far commoner case, two or three species of two
or three alone of the six older genera will have been the parents of the six
new genera; the other old species and the other whole genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of South
America, still fewer genera and species will have left modified blood-
descendants.

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

must have tended to make the geological record extremely imperfect, and
will to a large extent explain why we do not find interminable varieties,
connecting together all the extinct and existing forms of life by the finest
graduated steps.
He who rejects these views on the nature of the geological record, will
rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the several stages of the
same great formation. He may disbelieve in the enormous intervals of time
which have elapsed between our consecutive formations; he may overlook
how important a part migration must have played, when the formations of
any one great region alone, as that of Europe, are considered; he may urge
the apparent, but often falsely apparent, sudden coming in of whole groups
of species. He may ask where are the remains of those infinitely numerous
organisms which must have existed long before the first bed of the Silurian
system was deposited: I can answer this latter question only hypothetically,
by saying that as far as we can see, where our oceans now extend they have
for an enormous period extended, and where our oscillating continents now
stand they have stood ever since the Silurian epoch; but that long before
that period, the world may have presented a wholly different aspect; and
that the older continents, formed of formations older than any known to us,
may now all be in a metamorphosed condition, or may lie buried under the
ocean.
Passing from these difficulties, all the other great leading facts in
palæontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence of
the production of new forms. We can understand why when a species has
once disappeared it never reappears. Groups of species increase in numbers
slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species of the larger dominant groups tend to
leave many modified descendants, and thus new sub-groups and groups are
formed. As these are formed, the species of the less vigorous groups, from

their inferiority inherited from a common progenitor, tend to become extinct
together, and to leave no modified offspring on the face of the earth. But the
utter extinction of a whole group of species may often be a very slow
process, from the survival of a few descendants, lingering in protected and
isolated situations. When a group has once wholly disappeared, it does not
reappear; for the link of generation has been broken.
We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to people the
world with allied, but modified, descendants; and these will generally
succeed in taking the places of those groups of species which are their
inferiors in the struggle for existence. Hence, after long intervals of time,
the productions of the world will appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient and recent,
make together one grand system; for all are connected by generation. We
can understand, from the continued tendency to divergence of character,
why the more ancient a form is, the more it generally differs from those
now living. Why ancient and extinct forms often tend to fill up gaps
between existing forms, sometimes blending two groups previously classed
as distinct into one; but more commonly only bringing them a little closer
together. The more ancient a form is, the more often, apparently, it displays
characters in some degree intermediate between groups now distinct; for the
more ancient a form is, the more nearly it will be related to, and
consequently resemble, the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate between
existing forms; but are intermediate only by a long and circuitous course
through many extinct and very different forms. We can clearly see why the
organic remains of closely consecutive formations are more closely allied to
each other, than are those of remote formations; for the forms are more
closely linked together by generation: we can clearly see why the remains
of an intermediate formation are intermediate in character.
The inhabitants of each successive period in the world’s history have
beaten their predecessors in the race for life, and are, in so far, higher in the
scale of nature; and this may account for that vague yet ill-defined
sentiment, felt by many palæontologists, that organisation on the whole has
progressed. If it should hereafter be proved that ancient animals resemble to
a certain extent the embryos of more recent animals of the same class, the

fact will be intelligible. The succession of the same types of structure within
the same areas during the later geological periods ceases to be mysterious,
and is simply explained by inheritance.
If then the geological record be as imperfect as I believe it to be, and it
may at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of
palæontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new
and improved forms of life, produced by the laws of variation still acting
round us, and preserved by Natural Selection.

CHAPTER XI.
GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences
in physical conditions. Importance of barriers. Affinity of the
productions of the same continent. Centres of creation.
Means of dispersal, by changes of climate and of the level of
the land, and by occasional means. Dispersal during the
Glacial period co-extensive with the world.

In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the similarity nor
the dissimilarity of the inhabitants of various regions can be accounted for
by their climatal and other physical conditions. Of late, almost every author
who has studied the subject has come to this conclusion. The case of
America alone would almost suffice to prove its truth: for if we exclude the
northern parts where the circumpolar land is almost continuous, all authors
agree that one of the most fundamental divisions in geographical
distribution is that between the New and Old Worlds; yet if we travel over
the vast American continent, from the central parts of the United States to
its extreme southern point, we meet with the most diversified conditions;
the most humid districts, arid deserts, lofty mountains, grassy plains,
forests, marshes, lakes, and great rivers, under almost every temperature.
There is hardly a climate or condition in the Old World which cannot be
paralleled in the New—at least as closely as the same species generally
require; for it is a most rare case to find a group of organisms confined to
any small spot, having conditions peculiar in only a slight degree; for
instance, small areas in the Old World could be pointed out hotter than any
in the New World, yet these are not inhabited by a peculiar fauna or flora.
Notwithstanding this parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25°
and 35°, we shall find parts extremely similar in all their conditions, yet it
would not be possible to point out three faunas and floras more utterly
dissimilar. Or again we may compare the productions of South America
south of lat. 35° with those north of 25°, which consequently inhabit a
considerably different climate, and they will be found incomparably more
closely related to each other, than they are to the productions of Australia or
Africa under nearly the same climate. Analogous facts could be given with
respect to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that barriers
of any kind, or obstacles to free migration, are related in a close and
important manner to the differences between the productions of various
regions. We see this in the great difference of nearly all the terrestrial
productions of the New and Old Worlds, excepting in the northern parts,
where the land almost joins, and where, under a slightly different climate,
there might have been free migration for the northern temperate forms, as
there now is for the strictly arctic productions. We see the same fact in the
great difference between the inhabitants of Australia, Africa, and South
America under the same latitude: for these countries are almost as much
isolated from each other as is possible. On each continent, also, we see the
same fact; for on the opposite sides of lofty and continuous mountain-
ranges, and of great deserts, and sometimes even of large rivers, we find
different productions; though as mountain chains, deserts, etc., are not as
impassable, or likely to have endured so long as the oceans separating
continents, the differences are very inferior in degree to those characteristic
of distinct continents.
Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of the
eastern and western shores of South and Central America; yet these great
faunas are separated only by the narrow, but impassable, isthmus of
Panama. Westward of the shores of America, a wide space of open ocean
extends, with not an island as a halting-place for emigrants; here we have a
barrier of another kind, and as soon as this is passed we meet in the eastern
islands of the Pacific, with another and totally distinct fauna. So that here
three marine faunas range far northward and southward, in parallel lines not
far from each other, under corresponding climates; but from being separated

from each other by impassable barriers, either of land or open sea, they are
wholly distinct. On the other hand, proceeding still further westward from
the eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places,
until after travelling over a hemisphere we come to the shores of Africa; and
over this vast space we meet with no well-defined and distinct marine
faunas. Although hardly one shell, crab or fish is common to the above-
named three approximate faunas of Eastern and Western America and the
eastern Pacific islands, yet many fish range from the Pacific into the Indian
Ocean, and many shells are common to the eastern islands of the Pacific
and the eastern shores of Africa, on almost exactly opposite meridians of
longitude.
A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the species
themselves are distinct at different points and stations. It is a law of the
widest generality, and every continent offers innumerable instances.
Nevertheless the naturalist in travelling, for instance, from north to south
never fails to be struck by the manner in which successive groups of beings,
specifically distinct, yet clearly related, replace each other. He hears from
closely allied, yet distinct kinds of birds, notes nearly similar, and sees their
nests similarly constructed, but not quite alike, with eggs coloured in nearly
the same manner. The plains near the Straits of Magellan are inhabited by
one species of Rhea (American ostrich), and northward the plains of La
Plata by another species of the same genus; and not by a true ostrich or
emeu, like those found in Africa and Australia under the same latitude. On
these same plains of La Plata, we see the agouti and bizcacha, animals
having nearly the same habits as our hares and rabbits and belonging to the
same order of Rodents, but they plainly display an American type of
structure. We ascend the lofty peaks of the Cordillera and we find an alpine
species of bizcacha; we look to the waters, and we do not find the beaver or
musk-rat, but the coypu and capybara, rodents of the American type.
Innumerable other instances could be given. If we look to the islands off the
American shore, however much they may differ in geological structure, the
inhabitants, though they may be all peculiar species, are essentially
American. We may look back to past ages, as shown in the last chapter, and
we find American types then prevalent on the American continent and in the
American seas. We see in these facts some deep organic bond, prevailing

throughout space and time, over the same areas of land and water, and
independent of their physical conditions. The naturalist must feel little
curiosity, who is not led to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone,
as far as we positively know, produces organisms quite like, or, as we see in
the case of varieties nearly like each other. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
natural selection, and in a quite subordinate degree to the direct influence of
different physical conditions. The degree of dissimilarity will depend on the
migration of the more dominant forms of life from one region into another
having been effected with more or less ease, at periods more or less remote;
—on the nature and number of the former immigrants;—and on their action
and reaction, in their mutual struggles for life;—the relation of organism to
organism being, as I have already often remarked, the most important of all
relations. Thus the high importance of barriers comes into play by checking
migration; as does time for the slow process of modification through natural
selection. Widely-ranging species, abounding in individuals, which have
already triumphed over many competitors in their own widely-extended
homes will have the best chance of seizing on new places, when they spread
into new countries. In their new homes they will be exposed to new
conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and will
produce groups of modified descendants. On this principle of inheritance
with modification, we can understand how it is that sections of genera,
whole genera, and even families are confined to the same areas, as is so
commonly and notoriously the case.
I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent property,
and will be taken advantage of by natural selection, only so far as it profits
the individual in its complex struggle for life, so the degree of modification
in different species will be no uniform quantity. If, for instance, a number of
species, which stand in direct competition with each other, migrate in a
body into a new and afterwards isolated country, they will be little liable to
modification; for neither migration nor isolation in themselves can do
anything. These principles come into play only by bringing organisms into
new relations with each other, and in a lesser degree with the surrounding
physical conditions. As we have seen in the last chapter that some forms

have retained nearly the same character from an enormously remote
geological period, so certain species have migrated over vast spaces, and
have not become greatly modified.
On these views, it is obvious, that the several species of the same genus,
though inhabiting the most distant quarters of the world, must originally
have proceeded from the same source, as they have descended from the
same progenitor. In the case of those species, which have undergone during
whole geological periods but little modification, there is not much difficulty
in believing that they may have migrated from the same region; for during
the vast geographical and climatal changes which will have supervened
since ancient times, almost any amount of migration is possible. But in
many other cases, in which we have reason to believe that the species of a
genus have been produced within comparatively recent times, there is great
difficulty on this head. It is also obvious that the individuals of the same
species, though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for, as
explained in the last chapter, it is incredible that individuals identically the
same should ever have been produced through natural selection from
parents specifically distinct.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more points
of the earth’s surface. Undoubtedly there are very many cases of extreme
difficulty, in understanding how the same species could possibly have
migrated from some one point to the several distant and isolated points,
where now found. Nevertheless the simplicity of the view that each species
was first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with subsequent
migration, and calls in the agency of a miracle. It is universally admitted,
that in most cases the area inhabited by a species is continuous; and when a
plant or animal inhabits two points so distant from each other, or with an
interval of such a nature, that the space could not be easily passed over by
migration, the fact is given as something remarkable and exceptional. The
capacity of migrating across the sea is more distinctly limited in terrestrial
mammals, than perhaps in any other organic beings; and, accordingly, we
find no inexplicable cases of the same mammal inhabiting distant points of
the world. No geologist will feel any difficulty in such cases as Great
Britain having been formerly united to Europe, and consequently

possessing the same quadrupeds. But if the same species can be produced at
two separate points, why do we not find a single mammal common to
Europe and Australia or South America? The conditions of life are nearly
the same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal plants are
identically the same at these distant points of the northern and southern
hemispheres? The answer, as I believe, is, that mammals have not been able
to migrate, whereas some plants, from their varied means of dispersal, have
migrated across the vast and broken interspace. The great and striking
influence which barriers of every kind have had on distribution, is
intelligible only on the view that the great majority of species have been
produced on one side alone, and have not been able to migrate to the other
side. Some few families, many sub-families, very many genera, and a still
greater number of sections of genera are confined to a single region; and it
has been observed by several naturalists, that the most natural genera, or
those genera in which the species are most closely related to each other, are
generally local, or confined to one area. What a strange anomaly it would
be, if, when coming one step lower in the series, to the individuals of the
same species, a directly opposite rule prevailed; and species were not local,
but had been produced in two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the view of
each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur, in which we cannot explain how
the same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous and
of so grave a nature, that we ought to give up the belief, rendered probable
by general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now living
at distant and separated points; nor do I for a moment pretend that any
explanation could be offered of many such cases. But after some
preliminary remarks, I will discuss a few of the most striking classes of

facts; namely, the existence of the same species on the summits of distant
mountain-ranges, and at distant points in the arctic and antarctic regions;
and secondly (in the following chapter), the wide distribution of freshwater
productions; and thirdly, the occurrence of the same terrestrial species on
islands and on the mainland, though separated by hundreds of miles of open
sea. If the existence of the same species at distant and isolated points of the
earth’s surface, can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatal and geographical changes and
various occasional means of transport, the belief that this has been the
universal law, seems to me incomparably the safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from a
common progenitor, can have migrated (undergoing modification during
some part of their migration) from the area inhabited by their progenitor. If
it can be shown to be almost invariably the case, that a region, of which
most of its inhabitants are closely related to, or belong to the same genera
with the species of a second region, has probably received at some former
period immigrants from this other region, my theory will be strengthened;
for we can clearly understand, on the principle of modification, why the
inhabitants of a region should be related to those of another region, whence
it has been stocked. A volcanic island, for instance, upheaved and formed at
the distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their descendants,
though modified, would still be plainly related by inheritance to the
inhabitants of the continent. Cases of this nature are common, and are, as
we shall hereafter more fully see, inexplicable on the theory of independent
creation. This view of the relation of species in one region to those in
another, does not differ much (by substituting the word variety for species)
from that lately advanced in an ingenious paper by Mr. Wallace, in which he
concludes, that “every species has come into existence coincident both in
space and time with a pre-existing closely allied species.” And I now know
from correspondence, that this coincidence he attributes to generation with
modification.
The previous remarks on “single and multiple centres of creation” do not
directly bear on another allied question,—namely whether all the

individuals of the same species have descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have descended
from a succession of improved varieties, which will never have blended
with other individuals or varieties, but will have supplanted each other; so
that, at each successive stage of modification and improvement, all the
individuals of each variety will have descended from a single parent. But in
the majority of cases, namely, with all organisms which habitually unite for
each birth, or which often intercross, I believe that during the slow process
of modification the individuals of the species will have been kept nearly
uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not
have been due, at each stage, to descent from a single parent. To illustrate
what I mean: our English racehorses differ slightly from the horses of every
other breed; but they do not owe their difference and superiority to descent
from any single pair, but to continued care in selecting and training many
individuals during many generations.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of “single centres
of creation,” I must say a few words on the means of dispersal.

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

recently have been connected with Europe or Africa, and Europe likewise
with America. Other authors have thus hypothetically bridged over every
ocean, and have united almost every island to some mainland. If indeed the
arguments used by Forbes are to be trusted, it must be admitted that
scarcely a single island exists which has not recently been united to some
continent. This view cuts the Gordian knot of the dispersal of the same
species to the most distant points, and removes many a difficulty: but to the
best of my judgment we are not authorized in admitting such enormous
geographical changes within the period of existing species. It seems to me
that we have abundant evidence of great oscillations of level in our
continents; but not of such vast changes in their position and extension, as
to have united them within the recent period to each other and to the several
intervening oceanic islands. I freely admit the former existence of many
islands, now buried beneath the sea, which may have served as halting
places for plants and for many animals during their migration. In the coral-
producing oceans such sunken islands are now marked, as I believe, by
rings of coral or atolls standing over them. Whenever it is fully admitted, as
I believe it will some day be, that each species has proceeded from a single
birthplace, and when in the course of time we know something definite
about the means of distribution, we shall be enabled to speculate with
security on the former extension of the land. But I do not believe that it will
ever be proved that within the recent period continents which are now quite
separate, have been continuously, or almost continuously, united with each
other, and with the many existing oceanic islands. Several facts in
distribution,—such as the great difference in the marine faunas on the
opposite sides of almost every continent,—the close relation of the tertiary
inhabitants of several lands and even seas to their present inhabitants,—a
certain degree of relation (as we shall hereafter see) between the
distribution of mammals and the depth of the sea,—these and other such
facts seem to me opposed to the admission of such prodigious geographical
revolutions within the recent period, as are necessitated on the view
advanced by Forbes and admitted by his many followers. The nature and
relative proportions of the inhabitants of oceanic islands likewise seem to
me opposed to the belief of their former continuity with continents. Nor
does their almost universally volcanic composition favour the admission
that they are the wrecks of sunken continents;—if they had originally
existed as mountain-ranges on the land, some at least of the islands would

have been formed, like other mountain-summits, of granite, metamorphic
schists, old fossiliferous or other such rocks, instead of consisting of mere
piles of volcanic matter.
I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution. I
shall here confine myself to plants. In botanical works, this or that plant is
stated to be ill adapted for wide dissemination; but for transport across the
sea, the greater or less facilities may be said to be almost wholly unknown.
Until I tried, with Mr. Berkeley’s aid, a few experiments, it was not even
known how far seeds could resist the injurious action of sea-water. To my
surprise I found that out of 87 kinds, 64 germinated after an immersion of
28 days, and a few survived an immersion of 137 days. For convenience
sake I chiefly tried small seeds, without the capsule or fruit; and as all of
these sank in a few days, they could not be floated across wide spaces of the
sea, whether or not they were injured by the salt-water. Afterwards I tried
some larger fruits, capsules, etc., and some of these floated for a long time.
It is well known what a difference there is in the buoyancy of green and
seasoned timber; and it occurred to me that floods might wash down plants
or branches, and that these might be dried on the banks, and then by a fresh
rise in the stream be washed into the sea. Hence I was led to dry stems and
branches of 94 plants with ripe fruit, and to place them on sea water. The
majority sank quickly, but some which whilst green floated for a very short
time, when dried floated much longer; for instance, ripe hazel-nuts sank
immediately, but when dried, they floated for 90 days and afterwards when
planted they germinated; an asparagus plant with ripe berries floated for 23
days, when dried it floated for 85 days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when dried
they floated for above 90 days, and afterwards germinated. Altogether out
of the 94 dried plants, 18 floated for above 28 days, and some of the 18
floated for a very much longer period. So that as 64/87 seeds germinated
after an immersion of 28 days; and as 18/94 plants with ripe fruit (but not
all the same species as in the foregoing experiment) floated, after being
dried, for above 28 days, as far as we may infer anything from these scanty
facts, we may conclude that the seeds of 14/100 plants of any country might
be floated by sea-currents during 28 days, and would retain their power of
germination. In Johnston’s Physical Atlas, the average rate of the several
Atlantic currents is 33 miles per diem (some currents running at the rate of

60 miles per diem); on this average, the seeds of 14/100 plants belonging to
one country might be floated across 924 miles of sea to another country;
and when stranded, if blown to a favourable spot by an inland gale, they
would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried 98 seeds, mostly different from mine; but he chose many
large fruits and likewise seeds from plants which live near the sea; and this
would have favoured the average length of their flotation and of their
resistance to the injurious action of the salt-water. On the other hand he did
not previously dry the plants or branches with the fruit; and this, as we have
seen, would have caused some of them to have floated much longer. The
result was that 18/98 of his seeds floated for 42 days, and were then capable
of germination. But I do not doubt that plants exposed to the waves would
float for a less time than those protected from violent movement as in our
experiments. Therefore it would perhaps be safer to assume that the seeds
of about 10/100 plants of a flora, after having been dried, could be floated
across a space of sea 900 miles in width, and would then germinate. The
fact of the larger fruits often floating longer than the small, is interesting; as
plants with large seeds or fruit could hardly be transported by any other
means; and Alph. de Candolle has shown that such plants generally have
restricted ranges.
But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral-islands in the Pacific, procure
stones for their tools, solely from the roots of drifted trees, these stones
being a valuable royal tax. I find on examination, that when irregularly
shaped stones are embedded in the roots of trees, small parcels of earth are
very frequently enclosed in their interstices and behind them,—so perfectly
that not a particle could be washed away in the longest transport: out of one
small portion of earth thus completely enclosed by wood in an oak about 50
years old, three dicotyledonous plants germinated: I am certain of the
accuracy of this observation. Again, I can show that the carcasses of birds,
when floating on the sea, sometimes escape being immediately devoured;
and seeds of many kinds in the crops of floating birds long retain their
vitality: peas and vetches, for instance, are killed by even a few days’

immersion in sea-water; but some taken out of the crop of a pigeon, which
had floated on artificial salt-water for 30 days, to my surprise nearly all
germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may I think safely assume that under such circumstances their rate of
flight would often be 35 miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit will pass uninjured
through even the digestive organs of a turkey. In the course of two months, I
picked up in my garden 12 kinds of seeds, out of the excrement of small
birds, and these seemed perfect, and some of them, which I tried,
germinated. But the following fact is more important: the crops of birds do
not secrete gastric juice, and do not in the least injure, as I know by trial, the
germination of seeds; now after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains do not pass into
the gizzard for 12 or even 18 hours. A bird in this interval might easily be
blown to the distance of 500 miles, and hawks are known to look out for
tired birds, and the contents of their torn crops might thus readily get
scattered. Mr. Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English coast
destroyed so many on their arrival. Some hawks and owls bolt their prey
whole, and after an interval of from twelve to twenty hours, disgorge
pellets, which, as I know from experiments made in the Zoological
Gardens, include seeds capable of germination. Some seeds of the oat,
wheat, millet, canary, hemp, clover, and beet germinated after having been
from twelve to twenty-one hours in the stomachs of different birds of prey;
and two seeds of beet grew after having been thus retained for two days and
fourteen hours. Freshwater fish, I find, eat seeds of many land and water
plants: fish are frequently devoured by birds, and thus the seeds might be
transported from place to place. I forced many kinds of seeds into the
stomachs of dead fish, and then gave their bodies to fishing-eagles, storks,
and pelicans; these birds after an interval of many hours, either rejected the
seeds in pellets or passed them in their excrement; and several of these
seeds retained their power of germination. Certain seeds, however, were
always killed by this process.

Although the beaks and feet of birds are generally quite clean, I can show
that earth sometimes adheres to them: in one instance I removed twenty-two
grains of dry argillaceous earth from one foot of a partridge, and in this
earth there was a pebble quite as large as the seed of a vetch. Thus seeds
might occasionally be transported to great distances; for many facts could
be given showing that soil almost everywhere is charged with seeds. Reflect
for a moment on the millions of quails which annually cross the
Mediterranean; and can we doubt that the earth adhering to their feet would
sometimes include a few minute seeds? But I shall presently have to recur
to this subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, I can
hardly doubt that they must occasionally have transported seeds from one
part to another of the arctic and antarctic regions, as suggested by Lyell; and
during the Glacial period from one part of the now temperate regions to
another. In the Azores, from the large number of the species of plants
common to Europe, in comparison with the plants of other oceanic islands
nearer to the mainland, and (as remarked by Mr. H. C. Watson) from the
somewhat northern character of the flora in comparison with the latitude, I
suspected that these islands had been partly stocked by ice-borne seeds,
during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung to
inquire whether he had observed erratic boulders on these islands, and he
answered that he had found large fragments of granite and other rocks,
which do not occur in the archipelago. Hence we may safely infer that
icebergs formerly landed their rocky burthens on the shores of these mid-
ocean islands, and it is at least possible that they may have brought thither
the seeds of northern plants.
Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in
action year after year, for centuries and tens of thousands of years, it would
I think be a marvellous fact if many plants had not thus become widely
transported. These means of transport are sometimes called accidental, but
this is not strictly correct: the currents of the sea are not accidental, nor is
the direction of prevalent gales of wind. It should be observed that scarcely
any means of transport would carry seeds for very great distances; for seeds
do not retain their vitality when exposed for a great length of time to the
action of seawater; nor could they be long carried in the crops or intestines

of birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to island,
or from a continent to a neighbouring island, but not from one distant
continent to another. The floras of distant continents would not by such
means become mingled in any great degree; but would remain as distinct as
we now see them to be. The currents, from their course, would never bring
seeds from North America to Britain, though they might and do bring seeds
from the West Indies to our western shores, where, if not killed by so long
an immersion in salt-water, they could not endure our climate. Almost every
year, one or two land-birds are blown across the whole Atlantic Ocean,
from North America to the western shores of Ireland and England; but seeds
could be transported by these wanderers only by one means, namely, in dirt
sticking to their feet, which is in itself a rare accident. Even in this case,
how small would the chance be of a seed falling on favourable soil, and
coming to maturity! But it would be a great error to argue that because a
well-stocked island, like Great Britain, has not, as far as is known (and it
would be very difficult to prove this), received within the last few centuries,
through occasional means of transport, immigrants from Europe or any
other continent, that a poorly-stocked island, though standing more remote
from the mainland, would not receive colonists by similar means. I do not
doubt that out of twenty seeds or animals transported to an island, even if
far less well-stocked than Britain, scarcely more than one would be so well
fitted to its new home, as to become naturalised. But this, as it seems to me,
is no valid argument against what would be effected by occasional means of
transport, during the long lapse of geological time, whilst an island was
being upheaved and formed, and before it had become fully stocked with
inhabitants. On almost bare land, with few or no destructive insects or birds
living there, nearly every seed, which chanced to arrive, would be sure to
germinate and survive.

Dispersal during the Glacial period.—The identity of many plants and
animals, on mountain-summits, separated from each other by hundreds of
miles of lowlands, where the Alpine species could not possibly exist, is one
of the most striking cases known of the same species living at distant
points, without the apparent possibility of their having migrated from one to
the other. It is indeed a remarkable fact to see so many of the same plants

living on the snowy regions of the Alps or Pyrenees, and in the extreme
northern parts of Europe; but it is far more remarkable, that the plants on
the White Mountains, in the United States of America, are all the same with
those of Labrador, and nearly all the same, as we hear from Asa Gray, with
those on the loftiest mountains of Europe. Even as long ago as 1747, such
facts led Gmelin to conclude that the same species must have been
independently created at several distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which, as we shall immediately see, affords
a simple explanation of these facts. We have evidence of almost every
conceivable kind, organic and inorganic, that within a very recent
geological period, central Europe and North America suffered under an
Arctic climate. The ruins of a house burnt by fire do not tell their tale more
plainly, than do the mountains of Scotland and Wales, with their scored
flanks, polished surfaces, and perched boulders, of the icy streams with
which their valleys were lately filled. So greatly has the climate of Europe
changed, that in Northern Italy, gigantic moraines, left by old glaciers, are
now clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders, and rocks scored by drifted icebergs and coast-ice,
plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period to come slowly on, and then pass
away, as formerly occurred. As the cold came on, and as each more
southern zone became fitted for arctic beings and ill-fitted for their former
more temperate inhabitants, the latter would be supplanted and arctic
productions would take their places. The inhabitants of the more temperate
regions would at the same time travel southward, unless they were stopped
by barriers, in which case they would perish. The mountains would become
covered with snow and ice, and their former Alpine inhabitants would
descend to the plains. By the time that the cold had reached its maximum,
we should have a uniform arctic fauna and flora, covering the central parts
of Europe, as far south as the Alps and Pyrenees, and even stretching into
Spain. The now temperate regions of the United States would likewise be
covered by arctic plants and animals, and these would be nearly the same
with those of Europe; for the present circumpolar inhabitants, which we

suppose to have everywhere travelled southward, are remarkably uniform
round the world. We may suppose that the Glacial period came on a little
earlier or later in North America than in Europe, so will the southern
migration there have been a little earlier or later; but this will make no
difference in the final result.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the arctic
forms would seize on the cleared and thawed ground, always ascending
higher and higher, as the warmth increased, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same arctic species, which had lately lived in a body together
on the lowlands of the Old and New Worlds, would be left isolated on
distant mountain-summits (having been exterminated on all lesser heights)
and in the arctic regions of both hemispheres.
Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of Europe.
We can thus also understand the fact that the Alpine plants of each
mountain-range are more especially related to the arctic forms living due
north or nearly due north of them: for the migration as the cold came on,
and the re-migration on the returning warmth, will generally have been due
south and north. The Alpine plants, for example, of Scotland, as remarked
by Mr. H. C. Watson, and those of the Pyrenees, as remarked by Ramond,
are more especially allied to the plants of northern Scandinavia; those of the
United States to Labrador; those of the mountains of Siberia to the arctic
regions of that country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the Alpine
and Arctic productions of Europe and America, that when in other regions
we find the same species on distant mountain-summits, we may almost
conclude without other evidence, that a colder climate permitted their
former migration across the low intervening tracts, since become too warm
for their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe to
have been the case, chiefly from the distribution of the fossil Gnathodon),

then the arctic and temperate productions will at a very late period have
marched a little further north, and subsequently have retreated to their
present homes; but I have met with no satisfactory evidence with respect to
this intercalated slightly warmer period, since the Glacial period.
The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as is
especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed, and,
in accordance with the principles inculcated in this volume, they will not
have been liable to much modification. But with our Alpine productions,
left isolated from the moment of the returning warmth, first at the bases and
ultimately on the summits of the mountains, the case will have been
somewhat different; for it is not likely that all the same arctic species will
have been left on mountain ranges distant from each other, and have
survived there ever since; they will, also, in all probability have become
mingled with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which
during its coldest period will have been temporarily driven down to the
plains; they will, also, have been exposed to somewhat different climatal
influences. Their mutual relations will thus have been in some degree
disturbed; consequently they will have been liable to modification; and this
we find has been the case; for if we compare the present Alpine plants and
animals of the several great European mountain-ranges, though very many
of the species are identically the same, some present varieties, some are
ranked as doubtful forms, and some few are distinct yet closely allied or
representative species.
In illustrating what, as I believe, actually took place during the Glacial
period, I assumed that at its commencement the arctic productions were as
uniform round the polar regions as they are at the present day. But the
foregoing remarks on distribution apply not only to strictly arctic forms, but
also to many sub-arctic and to some few northern temperate forms, for
some of these are the same on the lower mountains and on the plains of
North America and Europe; and it may be reasonably asked how I account
for the necessary degree of uniformity of the sub-arctic and northern
temperate forms round the world, at the commencement of the Glacial
period. At the present day, the sub-arctic and northern temperate
productions of the Old and New Worlds are separated from each other by

the Atlantic Ocean and by the extreme northern part of the Pacific. During
the Glacial period, when the inhabitants of the Old and New Worlds lived
further southwards than at present, they must have been still more
completely separated by wider spaces of ocean. I believe the above
difficulty may be surmounted by looking to still earlier changes of climate
of an opposite nature. We have good reason to believe that during the newer
Pliocene period, before the Glacial epoch, and whilst the majority of the
inhabitants of the world were specifically the same as now, the climate was
warmer than at the present day. Hence we may suppose that the organisms
now living under the climate of latitude 60°, during the Pliocene period
lived further north under the Polar Circle, in latitude 66°-67°; and that the
strictly arctic productions then lived on the broken land still nearer to the
pole. Now if we look at a globe, we shall see that under the Polar Circle
there is almost continuous land from western Europe, through Siberia, to
eastern America. And to this continuity of the circumpolar land, and to the
consequent freedom for intermigration under a more favourable climate, I
attribute the necessary amount of uniformity in the sub-arctic and northern
temperate productions of the Old and New Worlds, at a period anterior to
the Glacial epoch.
Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to large, but
partial oscillations of level, I am strongly inclined to extend the above view,
and to infer that during some earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly to migrate
southwards as the climate became less warm, long before the
commencement of the Glacial period. We now see, as I believe, their
descendants, mostly in a modified condition, in the central parts of Europe
and the United States. On this view we can understand the relationship,
with very little identity, between the productions of North America and
Europe,—a relationship which is most remarkable, considering the distance
of the two areas, and their separation by the Atlantic Ocean. We can further
understand the singular fact remarked on by several observers, that the
productions of Europe and America during the later tertiary stages were
more closely related to each other than they are at the present time; for
during these warmer periods the northern parts of the Old and New Worlds

will have been almost continuously united by land, serving as a bridge,
since rendered impassable by cold, for the inter-migration of their
inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they must have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and have had to compete with them;
and in the other great region, with those of the Old World. Consequently we
have here everything favourable for much modification,—for far more
modification than with the Alpine productions, left isolated, within a much
more recent period, on the several mountain-ranges and on the arctic lands
of the two Worlds. Hence it has come, that when we compare the now
living productions of the temperate regions of the New and Old Worlds, we
find very few identical species (though Asa Gray has lately shown that
more plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical races,
and others as distinct species; and a host of closely allied or representative
forms which are ranked by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which during the Pliocene or even a somewhat earlier period,
was nearly uniform along the continuous shores of the Polar Circle, will
account, on the theory of modification, for many closely allied forms now
living in areas completely sundered. Thus, I think, we can understand the
presence of many existing and tertiary representative forms on the eastern
and western shores of temperate North America; and the still more striking
case of many closely allied crustaceans (as described in Dana’s admirable
work), of some fish and other marine animals, in the Mediterranean and in
the seas of Japan,—areas now separated by a continent and by nearly a
hemisphere of equatorial ocean.
These cases of relationship, without identity, of the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the
temperate lands of North America and Europe, are inexplicable on the
theory of creation. We cannot say that they have been created alike, in

correspondence with the nearly similar physical conditions of the areas; for
if we compare, for instance, certain parts of South America with the
southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their inhabitants
utterly dissimilar.
But we must return to our more immediate subject, the Glacial period. I
am convinced that Forbes’s view may be largely extended. In Europe we
have the plainest evidence of the cold period, from the western shores of
Britain to the Oural range, and southward to the Pyrenees. We may infer,
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. Along the Himalaya, at points 900 miles
apart, glaciers have left the marks of their former low descent; and in
Sikkim, Dr. Hooker saw maize growing on gigantic ancient moraines. South
of the equator, we have some direct evidence of former glacial action in
New Zealand; and the same plants, found on widely separated mountains in
this island, tell the same story. If one account which has been published can
be trusted, we have direct evidence of glacial action in the south-eastern
corner of Australia.
Looking to America; in the northern half, ice-borne fragments of rock
have been observed on the eastern side as far south as lat. 36°-37°, and on
the shores of the Pacific, where the climate is now so different, as far south
as lat. 46 deg; erratic boulders have, also, been noticed on the Rocky
Mountains. In the Cordillera of Equatorial South America, glaciers once
extended far below their present level. In central Chile I was astonished at
the structure of a vast mound of detritus, about 800 feet in height, crossing a
valley of the Andes; and this I now feel convinced was a gigantic moraine,
left far below any existing glacier. Further south on both sides of the
continent, from lat. 41° to the southernmost extremity, we have the clearest
evidence of former glacial action, in huge boulders transported far from
their parent source.
We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have good
evidence in almost every case, that the epoch was included within the latest
geological period. We have, also, excellent evidence, that it endured for an
enormous time, as measured by years, at each point. The cold may have
come on, or have ceased, earlier at one point of the globe than at another,

but seeing that it endured for long at each, and that it was contemporaneous
in a geological sense, it seems to me probable that it was, during a part at
least of the period, actually simultaneous throughout the world. Without
some distinct evidence to the contrary, we may at least admit as probable
that the glacial action was simultaneous on the eastern and western sides of
North America, in the Cordillera under the equator and under the warmer
temperate zones, and on both sides of the southern extremity of the
continent. If this be admitted, it is difficult to avoid believing that the
temperature of the whole world was at this period simultaneously cooler.
But it would suffice for my purpose, if the temperature was at the same time
lower along certain broad belts of longitude.
On this view of the whole world, or at least of broad longitudinal belts,
having been simultaneously colder from pole to pole, much light can be
thrown on the present distribution of identical and allied species. In
America, Dr. Hooker has shown that between forty and fifty of the
flowering plants of Tierra del Fuego, forming no inconsiderable part of its
scanty flora, are common to Europe, enormously remote as these two points
are; and there are many closely allied species. On the lofty mountains of
equatorial America a host of peculiar species belonging to European genera
occur. On the highest mountains of Brazil, some few European genera were
found by Gardner, which do not exist in the wide intervening hot countries.
So on the Silla of Caraccas the illustrious Humboldt long ago found species
belonging to genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the
peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a
very few European species, believed not to have been introduced by man,
and on the mountains, some few representative European forms are found,
which have not been discovered in the intertropical parts of Africa. On the
Himalaya, and on the isolated mountain-ranges of the peninsula of India, on
the heights of Ceylon, and on the volcanic cones of Java, many plants
occur, either identically the same or representing each other, and at the same
time representing plants of Europe, not found in the intervening hot
lowlands. A list of the genera collected on the loftier peaks of Java raises a
picture of a collection made on a hill in Europe! Still more striking is the
fact that southern Australian forms are clearly represented by plants
growing on the summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend along the heights of the

peninsula of Malacca, and are thinly scattered, on the one hand over India
and on the other as far north as Japan.
On the southern mountains of Australia, Dr. F. Müller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr. Hooker,
of European genera, found in Australia, but not in the intermediate torrid
regions. In the admirable ‘Introduction to the Flora of New Zealand,’ by Dr.
Hooker, analogous and striking facts are given in regard to the plants of that
large island. Hence we see that throughout the world, the plants growing on
the more lofty mountains, and on the temperate lowlands of the northern
and southern hemispheres, are sometimes identically the same; but they are
much oftener specifically distinct, though related to each other in a most
remarkable manner.
This brief abstract applies to plants alone: some strictly analogous facts
could be given on the distribution of terrestrial animals. In marine
productions, similar cases occur; as an example, I may quote a remark by
the highest authority, Professor Dana, that “it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its crustacea to Great
Britain, its antipode, than to any other part of the world.” Sir J. Richardson,
also, speaks of the reappearance on the shores of New Zealand, Tasmania,
etc., of northern forms of fish. Dr. Hooker informs me that twenty-five
species of Algæ are common to New Zealand and to Europe, but have not
been found in the intermediate tropical seas.
It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges of
the intertropical regions, are not arctic, but belong to the northern temperate
zones. As Mr. H. C. Watson has recently remarked, “In receding from polar
towards equatorial latitudes, the Alpine or mountain floras really become
less and less arctic.” Many of the forms living on the mountains of the
warmer regions of the earth and in the southern hemisphere are of doubtful
value, being ranked by some naturalists as specifically distinct, by others as
varieties; but some are certainly identical, and many, though closely related
to northern forms, must be ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that the
whole world, or a large part of it, was during the Glacial period

simultaneously much colder than at present. The Glacial period, as
measured by years, must have been very long; and when we remember over
what vast spaces some naturalised plants and animals have spread within a
few centuries, this period will have been ample for any amount of
migration. As the cold came slowly on, all the tropical plants and other
productions will have retreated from both sides towards the equator,
followed in the rear by the temperate productions, and these by the arctic;
but with the latter we are not now concerned. The tropical plants probably
suffered much extinction; how much no one can say; perhaps formerly the
tropics supported as many species as we see at the present day crowded
together at the Cape of Good Hope, and in parts of temperate Australia. As
we know that many tropical plants and animals can withstand a
considerable amount of cold, many might have escaped extermination
during a moderate fall of temperature, more especially by escaping into the
warmest spots. But the great fact to bear in mind is, that all tropical
productions will have suffered to a certain extent. On the other hand, the
temperate productions, after migrating nearer to the equator, though they
will have been placed under somewhat new conditions, will have suffered
less. And it is certain that many temperate plants, if protected from the
inroads of competitors, can withstand a much warmer climate than their
own. Hence, it seems to me possible, bearing in mind that the tropical
productions were in a suffering state and could not have presented a firm
front against intruders, that a certain number of the more vigorous and
dominant temperate forms might have penetrated the native ranks and have
reached or even crossed the equator. The invasion would, of course, have
been greatly favoured by high land, and perhaps by a dry climate; for Dr.
Falconer informs me that it is the damp with the heat of the tropics which is
so destructive to perennial plants from a temperate climate. On the other
hand, the most humid and hottest districts will have afforded an asylum to
the tropical natives. The mountain-ranges north-west of the Himalaya, and
the long line of the Cordillera, seem to have afforded two great lines of
invasion: and it is a striking fact, lately communicated to me by Dr. Hooker,
that all the flowering plants, about forty-six in number, common to Tierra
del Fuego and to Europe still exist in North America, which must have lain
on the line of march. But I do not doubt that some temperate productions
entered and crossed even the lowlands of the tropics at the period when the
cold was most intense,—when arctic forms had migrated some twenty-five

degrees of latitude from their native country and covered the land at the foot
of the Pyrenees. At this period of extreme cold, I believe that the climate
under the equator at the level of the sea was about the same with that now
felt there at the height of six or seven thousand feet. During this the coldest
period, I suppose that large spaces of the tropical lowlands were clothed
with a mingled tropical and temperate vegetation, like that now growing
with strange luxuriance at the base of the Himalaya, as graphically
described by Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the lowlands; those which had not reached the equator,
would re-migrate northward or southward towards their former homes; but
the forms, chiefly northern, which had crossed the equator, would travel
still further from their homes into the more temperate latitudes of the
opposite hemisphere. Although we have reason to believe from geological
evidence that the whole body of arctic shells underwent scarcely any
modification during their long southern migration and re-migration
northward, the case may have been wholly different with those intruding
forms which settled themselves on the intertropical mountains, and in the
southern hemisphere. These being surrounded by strangers will have had to
compete with many new forms of life; and it is probable that selected
modifications in their structure, habits, and constitutions will have profited
them. Thus many of these wanderers, though still plainly related by
inheritance to their brethren of the northern or southern hemispheres, now
exist in their new homes as well-marked varieties or as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical plants and allied forms have apparently migrated from the north to
the south, than in a reversed direction. We see, however, a few southern
vegetable forms on the mountains of Borneo and Abyssinia. I suspect that
this preponderant migration from north to south is due to the greater extent
of land in the north, and to the northern forms having existed in their own
homes in greater numbers, and having consequently been advanced through
natural selection and competition to a higher stage of perfection or

dominating power, than the southern forms. And thus, when they became
commingled during the Glacial period, the northern forms were enabled to
beat the less powerful southern forms. Just in the same manner as we see at
the present day, that very many European productions cover the ground in
La Plata, and in a lesser degree in Australia, and have to a certain extent
beaten the natives; whereas extremely few southern forms have become
naturalised in any part of Europe, though hides, wool, and other objects
likely to carry seeds have been largely imported into Europe during the last
two or three centuries from La Plata, and during the last thirty or forty years
from Australia. Something of the same kind must have occurred on the
intertropical mountains: no doubt before the Glacial period they were
stocked with endemic Alpine forms; but these have almost everywhere
largely yielded to the more dominant forms, generated in the larger areas
and more efficient workshops of the north. In many islands the native
productions are nearly equalled or even outnumbered by the naturalised;
and if the natives have not been actually exterminated, their numbers have
been greatly reduced, and this is the first stage towards extinction. A
mountain is an island on the land; and the intertropical mountains before the
Glacial period must have been completely isolated; and I believe that the
productions of these islands on the land yielded to those produced within
the larger areas of the north, just in the same way as the productions of real
islands have everywhere lately yielded to continental forms, naturalised by
man’s agency.
I am far from supposing that all difficulties are removed on the view here
given in regard to the range and affinities of the allied species which live in
the northern and southern temperate zones and on the mountains of the
intertropical regions. Very many difficulties remain to be solved. I do not
pretend to indicate the exact lines and means of migration, or the reason
why certain species and not others have migrated; why certain species have
been modified and have given rise to new groups of forms, and others have
remained unaltered. We cannot hope to explain such facts, until we can say
why one species and not another becomes naturalised by man’s agency in a
foreign land; why one ranges twice or thrice as far, and is twice or thrice as
common, as another species within their own homes.
I have said that many difficulties remain to be solved: some of the most
remarkable are stated with admirable clearness by Dr. Hooker in his
botanical works on the antarctic regions. These cannot be here discussed. I

will only say that as far as regards the occurrence of identical species at
points so enormously remote as Kerguelen Land, New Zealand, and Fuegia,
I believe that towards the close of the Glacial period, icebergs, as suggested
by Lyell, have been largely concerned in their dispersal. But the existence
of several quite distinct species, belonging to genera exclusively confined to
the south, at these and other distant points of the southern hemisphere, is,
on my theory of descent with modification, a far more remarkable case of
difficulty. For some of these species are so distinct, that we cannot suppose
that there has been time since the commencement of the Glacial period for
their migration, and for their subsequent modification to the necessary
degree. The facts seem to me to indicate that peculiar and very distinct
species have migrated in radiating lines from some common centre; and I
am inclined to look in the southern, as in the northern hemisphere, to a
former and warmer period, before the commencement of the Glacial period,
when the antarctic lands, now covered with ice, supported a highly peculiar
and isolated flora. I suspect that before this flora was exterminated by the
Glacial epoch, a few forms were widely dispersed to various points of the
southern hemisphere by occasional means of transport, and by the aid, as
halting-places, of existing and now sunken islands, and perhaps at the
commencement of the Glacial period, by icebergs. By these means, as I
believe, the southern shores of America, Australia, New Zealand have
become slightly tinted by the same peculiar forms of vegetable life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt one of
his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the present
distribution both of the same and of allied forms of life can be explained.
The living waters may be said to have flowed during one short period from
the north and from the south, and to have crossed at the equator; but to have
flowed with greater force from the north so as to have freely inundated the
south. As the tide leaves its drift in horizontal lines, though rising higher on
the shores where the tide rises highest, so have the living waters left their
living drift on our mountain-summits, in a line gently rising from the arctic
lowlands to a great height under the equator. The various beings thus left
stranded may be compared with savage races of man, driven up and
surviving in the mountain-fastnesses of almost every land, which serve as a

record, full of interest to us, of the former inhabitants of the surrounding
lowlands.

CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION—continued.

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

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more impassable barrier, that they never would have extended to distant
countries. But the case is exactly the reverse. Not only have many fresh-
water species, belonging to quite different classes, an enormous range, but
allied species prevail in a remarkable manner throughout the world. I well
remember, when first collecting in the fresh waters of Brazil, feeling much
surprise at the similarity of the fresh-water insects, shells, etc., and at the
dissimilarity of the surrounding terrestrial beings, compared with those of
Britain.
But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having become
fitted, in a manner highly useful to them, for short and frequent migrations
from pond to pond, or from stream to stream; and liability to wide dispersal
would follow from this capacity as an almost necessary consequence. We
can here consider only a few cases. In regard to fish, I believe that the same
species never occur in the fresh waters of distant continents. But on the
same continent the species often range widely and almost capriciously; for
two river-systems will have some fish in common and some different. A

few facts seem to favour the possibility of their occasional transport by
accidental means; like that of the live fish not rarely dropped by whirlwinds
in India, and the vitality of their ova when removed from the water. But I
am inclined to attribute the dispersal of fresh-water fish mainly to slight
changes within the recent period in the level of the land, having caused
rivers to flow into each other. Instances, also, could be given of this having
occurred during floods, without any change of level. We have evidence in
the loess of the Rhine of considerable changes of level in the land within a
very recent geological period, and when the surface was peopled by
existing land and fresh-water shells. The wide difference of the fish on
opposite sides of continuous mountain-ranges, which from an early period
must have parted river-systems and completely prevented their inosculation,
seems to lead to this same conclusion. With respect to allied fresh-water
fish occurring at very distant points of the world, no doubt there are many
cases which cannot at present be explained: but some fresh-water fish
belong to very ancient forms, and in such cases there will have been ample
time for great geographical changes, and consequently time and means for
much migration. In the second place, salt-water fish can with care be slowly
accustomed to live in fresh water; and, according to Valenciennes, there is
hardly a single group of fishes confined exclusively to fresh water, so that
we may imagine that a marine member of a fresh-water group might travel
far along the shores of the sea, and subsequently become modified and
adapted to the fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not likely to
be transported by birds, and they are immediately killed by sea water, as are
the adults. I could not even understand how some naturalised species have
rapidly spread throughout the same country. But two facts, which I have
observed—and no doubt many others remain to be observed—throw some
light on this subject. When a duck suddenly emerges from a pond covered
with duck-weed, I have twice seen these little plants adhering to its back;
and it has happened to me, in removing a little duck-weed from one
aquarium to another, that I have quite unintentionally stocked the one with
fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended a duck’s feet, which might represent those of a bird

sleeping in a natural pond, in an aquarium, where many ova of fresh-water
shells were hatching; and I found that numbers of the extremely minute and
just hatched shells crawled on the feet, and clung to them so firmly that
when taken out of the water they could not be jarred off, though at a
somewhat more advanced age they would voluntarily drop off. These just
hatched molluscs, though aquatic in their nature, survived on the duck’s
feet, in damp air, from twelve to twenty hours; and in this length of time a
duck or heron might fly at least six or seven hundred miles, and would be
sure to alight on a pool or rivulet, if blown across sea to an oceanic island or
to any other distant point. Sir Charles Lyell also informs me that a Dyticus
has been caught with an Ancylus (a fresh-water shell like a limpet) firmly
adhering to it; and a water-beetle of the same family, a Colymbetes, once
flew on board the ‘Beagle,’ when forty-five miles distant from the nearest
land: how much farther it might have flown with a favouring gale no one
can tell.
With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and to
the most remote oceanic islands. This is strikingly shown, as remarked by
Alph. de Candolle, in large groups of terrestrial plants, which have only a
very few aquatic members; for these latter seem immediately to acquire, as
if in consequence, a very wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally, though
rarely, adheres in some quantity to the feet and beaks of birds. Wading
birds, which frequent the muddy edges of ponds, if suddenly flushed, would
be the most likely to have muddy feet. Birds of this order I can show are the
greatest wanderers, and are occasionally found on the most remote and
barren islands in the open ocean; they would not be likely to alight on the
surface of the sea, so that the dirt would not be washed off their feet; when
making land, they would be sure to fly to their natural fresh-water haunts. I
do not believe that botanists are aware how charged the mud of ponds is
with seeds: I have tried several little experiments, but will here give only
the most striking case: I took in February three table-spoonfuls of mud from
three different points, beneath water, on the edge of a little pond; this mud
when dry weighed only 6 3/4 ounces; I kept it covered up in my study for
six months, pulling up and counting each plant as it grew; the plants were of
many kinds, and were altogether 537 in number; and yet the viscid mud was
all contained in a breakfast cup! Considering these facts, I think it would be

an inexplicable circumstance if water-birds did not transport the seeds of
fresh-water plants to vast distances, and if consequently the range of these
plants was not very great. The same agency may have come into play with
the eggs of some of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and we
have seen that seeds retain their power of germination, when rejected in
pellets or in excrement, many hours afterwards. When I saw the great size
of the seeds of that fine water-lily, the Nelumbium, and remembered Alph.
de Candolle’s remarks on this plant, I thought that its distribution must
remain quite inexplicable; but Audubon states that he found the seeds of the
great southern water-lily (probably, according to Dr. Hooker, the
Nelumbium luteum) in a heron’s stomach; although I do not know the fact,
yet analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach a
pellet containing the seeds of the Nelumbium undigested; or the seeds
might be dropped by the bird whilst feeding its young, in the same way as
fish are known sometimes to be dropped.
In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance, on a
rising islet, it will be unoccupied; and a single seed or egg will have a good
chance of succeeding. Although there will always be a struggle for life
between the individuals of the species, however few, already occupying any
pond, yet as the number of kinds is small, compared with those on the land,
the competition will probably be less severe between aquatic than between
terrestrial species; consequently an intruder from the waters of a foreign
country, would have a better chance of seizing on a place, than in the case
of terrestrial colonists. We should, also, remember that some, perhaps many,
fresh-water productions are low in the scale of nature, and that we have
reason to believe that such low beings change or become modified less
quickly than the high; and this will give longer time than the average for the
migration of the same aquatic species. We should not forget the probability
of many species having formerly ranged as continuously as fresh-water

productions ever can range, over immense areas, and having subsequently
become extinct in intermediate regions. But the wide distribution of fresh-
water plants and of the lower animals, whether retaining the same identical
form or in some degree modified, I believe mainly depends on the wide
dispersal of their seeds and eggs by animals, more especially by fresh-water
birds, which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful gardener,
thus takes her seeds from a bed of a particular nature, and drops them in
another equally well fitted for them.

On the Inhabitants of Oceanic Islands.—We now come to the last of the
three classes of facts, which I have selected as presenting the greatest
amount of difficulty, on the view that all the individuals both of the same
and of allied species have descended from a single parent; and therefore
have all proceeded from a common birthplace, notwithstanding that in the
course of time they have come to inhabit distant points of the globe. I have
already stated that I cannot honestly admit Forbes’s view on continental
extensions, which, if legitimately followed out, would lead to the belief that
within the recent period all existing islands have been nearly or quite joined
to some continent. This view would remove many difficulties, but it would
not, I think, explain all the facts in regard to insular productions. In the
following remarks I shall not confine myself to the mere question of
dispersal; but shall consider some other facts, which bear on the truth of the
two theories of independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large size and
varied stations of New Zealand, extending over 780 miles of latitude, and
compare its flowering plants, only 750 in number, with those on an equal
area at the Cape of Good Hope or in Australia, we must, I think, admit that
something quite independently of any difference in physical conditions has
caused so great a difference in number. Even the uniform county of
Cambridge has 847 plants, and the little island of Anglesea 764, but a few
ferns and a few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence that
the barren island of Ascension aboriginally possessed under half-a-dozen

flowering plants; yet many have become naturalised on it, as they have on
New Zealand and on every other oceanic island which can be named. In St.
Helena there is reason to believe that the naturalised plants and animals
have nearly or quite exterminated many native productions. He who admits
the doctrine of the creation of each separate species, will have to admit, that
a sufficient number of the best adapted plants and animals have not been
created on oceanic islands; for man has unintentionally stocked them from
various sources far more fully and perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is scanty,
the proportion of endemic species (i.e. those found nowhere else in the
world) is often extremely large. If we compare, for instance, the number of
the endemic land-shells in Madeira, or of the endemic birds in the
Galapagos Archipelago, with the number found on any continent, and then
compare the area of the islands with that of the continent, we shall see that
this is true. This fact might have been expected on my theory, for, as already
explained, species occasionally arriving after long intervals in a new and
isolated district, and having to compete with new associates, will be
eminently liable to modification, and will often produce groups of modified
descendants. But it by no means follows, that, because in an island nearly
all the species of one class are peculiar, those of another class, or of another
section of the same class, are peculiar; and this difference seems to depend
on the species which do not become modified having immigrated with
facility and in a body, so that their mutual relations have not been much
disturbed. Thus in the Galapagos Islands nearly every land-bird, but only
two out of the eleven marine birds, are peculiar; and it is obvious that
marine birds could arrive at these islands more easily than land-birds.
Bermuda, on the other hand, which lies at about the same distance from
North America as the Galapagos Islands do from South America, and which
has a very peculiar soil, does not possess one endemic land bird; and we
know from Mr. J. M. Jones’s admirable account of Bermuda, that very
many North American birds, during their great annual migrations, visit
either periodically or occasionally this island. Madeira does not possess one
peculiar bird, and many European and African birds are almost every year
blown there, as I am informed by Mr. E. V. Harcourt. So that these two
islands of Bermuda and Madeira have been stocked by birds, which for long
ages have struggled together in their former homes, and have become
mutually adapted to each other; and when settled in their new homes, each

kind will have been kept by the others to their proper places and habits, and
will consequently have been little liable to modification. Madeira, again, is
inhabited by a wonderful number of peculiar land-shells, whereas not one
species of sea-shell is confined to its shores: now, though we do not know
how seashells are dispersed, yet we can see that their eggs or larvæ, perhaps
attached to seaweed or floating timber, or to the feet of wading-birds, might
be transported far more easily than land-shells, across three or four hundred
miles of open sea. The different orders of insects in Madeira apparently
present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the Galapagos
Islands reptiles, and in New Zealand gigantic wingless birds, take the place
of mammals. In the plants of the Galapagos Islands, Dr. Hooker has shown
that the proportional numbers of the different orders are very different from
what they are elsewhere. Such cases are generally accounted for by the
physical conditions of the islands; but this explanation seems to me not a
little doubtful. Facility of immigration, I believe, has been at least as
important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not tenanted
by mammals, some of the endemic plants have beautifully hooked seeds;
yet few relations are more striking than the adaptation of hooked seeds for
transportal by the wool and fur of quadrupeds. This case presents no
difficulty on my view, for a hooked seed might be transported to an island
by some other means; and the plant then becoming slightly modified, but
still retaining its hooked seeds, would form an endemic species, having as
useless an appendage as any rudimentary organ,—for instance, as the
shrivelled wings under the soldered elytra of many insular beetles. Again,
islands often possess trees or bushes belonging to orders which elsewhere
include only herbaceous species; now trees, as Alph. de Candolle has
shown, generally have, whatever the cause may be, confined ranges. Hence
trees would be little likely to reach distant oceanic islands; and an
herbaceous plant, though it would have no chance of successfully
competing in stature with a fully developed tree, when established on an
island and having to compete with herbaceous plants alone, might readily
gain an advantage by growing taller and taller and overtopping the other
plants. If so, natural selection would often tend to add to the stature of

herbaceous plants when growing on an island, to whatever order they
belonged, and thus convert them first into bushes and ultimately into trees.
With respect to the absence of whole orders on oceanic islands, Bory St.
Vincent long ago remarked that Batrachians (frogs, toads, newts) have
never been found on any of the many islands with which the great oceans
are studded. I have taken pains to verify this assertion, and I have found it
strictly true. I have, however, been assured that a frog exists on the
mountains of the great island of New Zealand; but I suspect that this
exception (if the information be correct) may be explained through glacial
agency. This general absence of frogs, toads, and newts on so many oceanic
islands cannot be accounted for by their physical conditions; indeed it
seems that islands are peculiarly well fitted for these animals; for frogs have
been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their spawn
are known to be immediately killed by sea-water, on my view we can see
that there would be great difficulty in their transportal across the sea, and
therefore why they do not exist on any oceanic island. But why, on the
theory of creation, they should not have been created there, it would be very
difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not found a
single instance, free from doubt, of a terrestrial mammal (excluding
domesticated animals kept by the natives) inhabiting an island situated
above 300 miles from a continent or great continental island; and many
islands situated at a much less distance are equally barren. The Falkland
Islands, which are inhabited by a wolf-like fox, come nearest to an
exception; but this group cannot be considered as oceanic, as it lies on a
bank connected with the mainland; moreover, icebergs formerly brought
boulders to its western shores, and they may have formerly transported
foxes, as so frequently now happens in the arctic regions. Yet it cannot be
said that small islands will not support small mammals, for they occur in
many parts of the world on very small islands, if close to a continent; and
hardly an island can be named on which our smaller quadrupeds have not
become naturalised and greatly multiplied. It cannot be said, on the ordinary
view of creation, that there has not been time for the creation of mammals;
many volcanic islands are sufficiently ancient, as shown by the stupendous
degradation which they have suffered and by their tertiary strata: there has

also been time for the production of endemic species belonging to other
classes; and on continents it is thought that mammals appear and disappear
at a quicker rate than other and lower animals. Though terrestrial mammals
do not occur on oceanic islands, ærial mammals do occur on almost every
island. New Zealand possesses two bats found nowhere else in the world:
Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline and
Marianne Archipelagoes, and Mauritius, all possess their peculiar bats.
Why, it may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily be
answered; for no terrestrial mammal can be transported across a wide space
of sea, but bats can fly across. Bats have been seen wandering by day far
over the Atlantic Ocean; and two North American species either regularly or
occasionally visit Bermuda, at the distance of 600 miles from the mainland.
I hear from Mr. Tomes, who has specially studied this family, that many of
the same species have enormous ranges, and are found on continents and on
far distant islands. Hence we have only to suppose that such wandering
species have been modified through natural selection in their new homes in
relation to their new position, and we can understand the presence of
endemic bats on islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the remoteness
of islands from continents, there is also a relation, to a certain extent
independent of distance, between the depth of the sea separating an island
from the neighbouring mainland, and the presence in both of the same
mammiferous species or of allied species in a more or less modified
condition. Mr. Windsor Earl has made some striking observations on this
head in regard to the great Malay Archipelago, which is traversed near
Celebes by a space of deep ocean; and this space separates two widely
distinct mammalian faunas. On either side the islands are situated on
moderately deep submarine banks, and they are inhabited by closely allied
or identical quadrupeds. No doubt some few anomalies occur in this great
archipelago, and there is much difficulty in forming a judgment in some
cases owing to the probable naturalisation of certain mammals through
man’s agency; but we shall soon have much light thrown on the natural
history of this archipelago by the admirable zeal and researches of Mr.
Wallace. I have not as yet had time to follow up this subject in all other
quarters of the world; but as far as I have gone, the relation generally holds
good. We see Britain separated by a shallow channel from Europe, and the

mammals are the same on both sides; we meet with analogous facts on
many islands separated by similar channels from Australia. The West Indian
Islands stand on a deeply submerged bank, nearly 1000 fathoms in depth,
and here we find American forms, but the species and even the genera are
distinct. As the amount of modification in all cases depends to a certain
degree on the lapse of time, and as during changes of level it is obvious that
islands separated by shallow channels are more likely to have been
continuously united within a recent period to the mainland than islands
separated by deeper channels, we can understand the frequent relation
between the depth of the sea and the degree of affinity of the mammalian
inhabitants of islands with those of a neighbouring continent,—an
inexplicable relation on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic islands,—namely,
the scarcity of kinds—the richness in endemic forms in particular classes or
sections of classes,—the absence of whole groups, as of batrachians, and of
terrestrial mammals notwithstanding the presence of ærial bats,—the
singular proportions of certain orders of plants,—herbaceous forms having
been developed into trees, etc.,—seem to me to accord better with the view
of occasional means of transport having been largely efficient in the long
course of time, than with the view of all our oceanic islands having been
formerly connected by continuous land with the nearest continent; for on
this latter view the migration would probably have been more complete;
and if modification be admitted, all the forms of life would have been more
equally modified, in accordance with the paramount importance of the
relation of organism to organism.
I do not deny that there are many and grave difficulties in understanding
how several of the inhabitants of the more remote islands, whether still
retaining the same specific form or modified since their arrival, could have
reached their present homes. But the probability of many islands having
existed as halting-places, of which not a wreck now remains, must not be
overlooked. I will here give a single instance of one of the cases of
difficulty. Almost all oceanic islands, even the most isolated and smallest,
are inhabited by land-shells, generally by endemic species, but sometimes
by species found elsewhere. Dr. Aug. A. Gould has given several interesting
cases in regard to the land-shells of the islands of the Pacific. Now it is
notorious that land-shells are very easily killed by salt; their eggs, at least
such as I have tried, sink in sea-water and are killed by it. Yet there must be,

on my view, some unknown, but highly efficient means for their transportal.
Would the just-hatched young occasionally crawl on and adhere to the feet
of birds roosting on the ground, and thus get transported? It occurred to me
that land-shells, when hybernating and having a membranous diaphragm
over the mouth of the shell, might be floated in chinks of drifted timber
across moderately wide arms of the sea. And I found that several species
did in this state withstand uninjured an immersion in sea-water during seven
days: one of these shells was the Helix pomatia, and after it had again
hybernated I put it in sea-water for twenty days, and it perfectly recovered.
As this species has a thick calcareous operculum, I removed it, and when it
had formed a new membranous one, I immersed it for fourteen days in sea-
water, and it recovered and crawled away: but more experiments are wanted
on this head.
The most striking and important fact for us in regard to the inhabitants of
islands, is their affinity to those of the nearest mainland, without being
actually the same species. Numerous instances could be given of this fact. I
will give only one, that of the Galapagos Archipelago, situated under the
equator, between 500 and 600 miles from the shores of South America.
Here almost every product of the land and water bears the unmistakeable
stamp of the American continent. There are twenty-six land birds, and
twenty-five of these are ranked by Mr. Gould as distinct species, supposed
to have been created here; yet the close affinity of most of these birds to
American species in every character, in their habits, gestures, and tones of
voice, was manifest. So it is with the other animals, and with nearly all the
plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this
archipelago. The naturalist, looking at the inhabitants of these volcanic
islands in the Pacific, distant several hundred miles from the continent, yet
feels that he is standing on American land. Why should this be so? why
should the species which are supposed to have been created in the
Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity
to those created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South American coast: in fact there
is a considerable dissimilarity in all these respects. On the other hand, there
is a considerable degree of resemblance in the volcanic nature of the soil, in
climate, height, and size of the islands, between the Galapagos and Cape de

Verde Archipelagos: but what an entire and absolute difference in their
inhabitants! The inhabitants of the Cape de Verde Islands are related to
those of Africa, like those of the Galapagos to America. I believe this grand
fact can receive no sort of explanation on the ordinary view of independent
creation; whereas on the view here maintained, it is obvious that the
Galapagos Islands would be likely to receive colonists, whether by
occasional means of transport or by formerly continuous land, from
America; and the Cape de Verde Islands from Africa; and that such
colonists would be liable to modification;—the principle of inheritance still
betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of the
nearest continent, or of other near islands. The exceptions are few, and most
of them can be explained. Thus the plants of Kerguelen Land, though
standing nearer to Africa than to America, are related, and that very closely,
as we know from Dr. Hooker’s account, to those of America: but on the
view that this island has been mainly stocked by seeds brought with earth
and stones on icebergs, drifted by the prevailing currents, this anomaly
disappears. New Zealand in its endemic plants is much more closely related
to Australia, the nearest mainland, than to any other region: and this is what
might have been expected; but it is also plainly related to South America,
which, although the next nearest continent, is so enormously remote, that
the fact becomes an anomaly. But this difficulty almost disappears on the
view that both New Zealand, South America, and other southern lands were
long ago partially stocked from a nearly intermediate though distant point,
namely from the antarctic islands, when they were clothed with vegetation,
before the commencement of the Glacial period. The affinity, which, though
feeble, I am assured by Dr. Hooker is real, between the flora of the south-
western corner of Australia and of the Cape of Good Hope, is a far more
remarkable case, and is at present inexplicable: but this affinity is confined
to the plants, and will, I do not doubt, be some day explained.
The law which causes the inhabitants of an archipelago, though
specifically distinct, to be closely allied to those of the nearest continent, we
sometimes see displayed on a small scale, yet in a most interesting manner,
within the limits of the same archipelago. Thus the several islands of the
Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite
marvellous manner, by very closely related species; so that the inhabitants

of each separate island, though mostly distinct, are related in an
incomparably closer degree to each other than to the inhabitants of any
other part of the world. And this is just what might have been expected on
my view, for the islands are situated so near each other that they would
almost certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of the
islands may be used as an argument against my views; for it may be asked,
how has it happened in the several islands situated within sight of each
other, having the same geological nature, the same height, climate, etc., that
many of the immigrants should have been differently modified, though only
in a small degree. This long appeared to me a great difficulty: but it arises in
chief part from the deeply-seated error of considering the physical
conditions of a country as the most important for its inhabitants; whereas it
cannot, I think, be disputed that the nature of the other inhabitants, with
which each has to compete, is at least as important, and generally a far more
important element of success. Now if we look to those inhabitants of the
Galapagos Archipelago which are found in other parts of the world (laying
on one side for the moment the endemic species, which cannot be here
fairly included, as we are considering how they have come to be modified
since their arrival), we find a considerable amount of difference in the
several islands. This difference might indeed have been expected on the
view of the islands having been stocked by occasional means of transport—
a seed, for instance, of one plant having been brought to one island, and that
of another plant to another island. Hence when in former times an
immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly be
exposed to different conditions of life in the different islands, for it would
have to compete with different sets of organisms: a plant, for instance,
would find the best-fitted ground more perfectly occupied by distinct plants
in one island than in another, and it would be exposed to the attacks of
somewhat different enemies. If then it varied, natural selection would
probably favour different varieties in the different islands. Some species,
however, might spread and yet retain the same character throughout the
group, just as we see on continents some species spreading widely and
remaining the same.
The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous instances, is that the new species

formed in the separate islands have not quickly spread to the other islands.
But the islands, though in sight of each other, are separated by deep arms of
the sea, in most cases wider than the British Channel, and there is no reason
to suppose that they have at any former period been continuously united.
The currents of the sea are rapid and sweep across the archipelago, and
gales of wind are extraordinarily rare; so that the islands are far more
effectually separated from each other than they appear to be on a map.
Nevertheless a good many species, both those found in other parts of the
world and those confined to the archipelago, are common to the several
islands, and we may infer from certain facts that these have probably spread
from some one island to the others. But we often take, I think, an erroneous
view of the probability of closely allied species invading each other’s
territory, when put into free intercommunication. Undoubtedly if one
species has any advantage whatever over another, it will in a very brief time
wholly or in part supplant it; but if both are equally well fitted for their own
places in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man’s agency, have spread with astonishing
rapidity over new countries, we are apt to infer that most species would thus
spread; but we should remember that the forms which become naturalised
in new countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct species, belonging in a large proportion of
cases, as shown by Alph. de Candolle, to distinct genera. In the Galapagos
Archipelago, many even of the birds, though so well adapted for flying
from island to island, are distinct on each; thus there are three closely-allied
species of mocking-thrush, each confined to its own island. Now let us
suppose the mocking-thrush of Chatham Island to be blown to Charles
Island, which has its own mocking-thrush: why should it succeed in
establishing itself there? We may safely infer that Charles Island is well
stocked with its own species, for annually more eggs are laid there than can
possibly be reared; and we may infer that the mocking-thrush peculiar to
Charles Island is at least as well fitted for its home as is the species peculiar
to Chatham Island. Sir C. Lyell and Mr. Wollaston have communicated to
me a remarkable fact bearing on this subject; namely, that Madeira and the
adjoining islet of Porto Santo possess many distinct but representative land-
shells, some of which live in crevices of stone; and although large quantities
of stone are annually transported from Porto Santo to Madeira, yet this

latter island has not become colonised by the Porto Santo species:
nevertheless both islands have been colonised by some European land-
shells, which no doubt had some advantage over the indigenous species.
From these considerations I think we need not greatly marvel at the
endemic and representative species, which inhabit the several islands of the
Galapagos Archipelago, not having universally spread from island to island.
In many other instances, as in the several districts of the same continent,
pre-occupation has probably played an important part in checking the
commingling of species under the same conditions of life. Thus, the south-
east and south-west corners of Australia have nearly the same physical
conditions, and are united by continuous land, yet they are inhabited by a
vast number of distinct mammals, birds, and plants.
The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not identically
the same, yet are plainly related to the inhabitants of that region whence
colonists could most readily have been derived,—the colonists having been
subsequently modified and better fitted to their new homes,—is of the
widest application throughout nature. We see this on every mountain, in
every lake and marsh. For Alpine species, excepting in so far as the same
forms, chiefly of plants, have spread widely throughout the world during
the recent Glacial epoch, are related to those of the surrounding lowlands;
—thus we have in South America, Alpine humming-birds, Alpine rodents,
Alpine plants, etc., all of strictly American forms, and it is obvious that a
mountain, as it became slowly upheaved, would naturally be colonised from
the surrounding lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has given the same general
forms to the whole world. We see this same principle in the blind animals
inhabiting the caves of America and of Europe. Other analogous facts could
be given. And it will, I believe, be universally found to be true, that
wherever in two regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some identical
species, showing, in accordance with the foregoing view, that at some
former period there has been intercommunication or migration between the
two regions. And wherever many closely-allied species occur, there will be
found many forms which some naturalists rank as distinct species, and
some as varieties; these doubtful forms showing us the steps in the process
of modification.

This relation between the power and extent of migration of a species,
either at the present time or at some former period under different physical
conditions, and the existence at remote points of the world of other species
allied to it, is shown in another and more general way. Mr. Gould remarked
to me long ago, that in those genera of birds which range over the world,
many of the species have very wide ranges. I can hardly doubt that this rule
is generally true, though it would be difficult to prove it. Amongst
mammals, we see it strikingly displayed in Bats, and in a lesser degree in
the Felidæ and Canidæ. We see it, if we compare the distribution of
butterflies and beetles. So it is with most fresh-water productions, in which
so many genera range over the world, and many individual species have
enormous ranges. It is not meant that in world-ranging genera all the
species have a wide range, or even that they have on an average a wide
range; but only that some of the species range very widely; for the facility
with which widely-ranging species vary and give rise to new forms will
largely determine their average range. For instance, two varieties of the
same species inhabit America and Europe, and the species thus has an
immense range; but, if the variation had been a little greater, the two
varieties would have been ranked as distinct species, and the common range
would have been greatly reduced. Still less is it meant, that a species which
apparently has the capacity of crossing barriers and ranging widely, as in
the case of certain powerfully-winged birds, will necessarily range widely;
for we should never forget that to range widely implies not only the power
of crossing barriers, but the more important power of being victorious in
distant lands in the struggle for life with foreign associates. But on the view
of all the species of a genus having descended from a single parent, though
now distributed to the most remote points of the world, we ought to find,
and I believe as a general rule we do find, that some at least of the species
range very widely; for it is necessary that the unmodified parent should
range widely, undergoing modification during its diffusion, and should
place itself under diverse conditions favourable for the conversion of its
offspring, firstly into new varieties and ultimately into new species.
In considering the wide distribution of certain genera, we should bear in
mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have
been ample time for great climatal and geographical changes and for
accidents of transport; and consequently for the migration of some of the

species into all quarters of the world, where they may have become slightly
modified in relation to their new conditions. There is, also, some reason to
believe from geological evidence that organisms low in the scale within
each great class, generally change at a slower rate than the higher forms;
and consequently the lower forms will have had a better chance of ranging
widely and of still retaining the same specific character. This fact, together
with the seeds and eggs of many low forms being very minute and better
fitted for distant transportation, probably accounts for a law which has long
been observed, and which has lately been admirably discussed by Alph. de
Candolle in regard to plants, namely, that the lower any group of organisms
is, the more widely it is apt to range.
The relations just discussed,—namely, low and slowly-changing
organisms ranging more widely than the high,—some of the species of
widely-ranging genera themselves ranging widely,—such facts, as alpine,
lacustrine, and marsh productions being related (with the exceptions before
specified) to those on the surrounding low lands and dry lands, though these
stations are so different—the very close relation of the distinct species
which inhabit the islets of the same archipelago,—and especially the
striking relation of the inhabitants of each whole archipelago or island to
those of the nearest mainland,—are, I think, utterly inexplicable on the
ordinary view of the independent creation of each species, but are
explicable on the view of colonisation from the nearest and readiest source,
together with the subsequent modification and better adaptation of the
colonists to their new homes.

Summary of last and present Chapters.—In these chapters I have
endeavoured to show, that if we make due allowance for our ignorance of
the full effects of all the changes of climate and of the level of the land,
which have certainly occurred within the recent period, and of other similar
changes which may have occurred within the same period; if we remember
how profoundly ignorant we are with respect to the many and curious
means of occasional transport,—a subject which has hardly ever been
properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that all the
individuals of the same species, wherever located, have descended from the

same parents, are not insuperable. And we are led to this conclusion, which
has been arrived at by many naturalists under the designation of single
centres of creation, by some general considerations, more especially from
the importance of barriers and from the analogical distribution of sub-
genera, genera, and families.
With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life change most slowly, enormous periods of time being thus granted for
their migration, I do not think that the difficulties are insuperable; though
they often are in this case, and in that of the individuals of the same species,
extremely grave.
As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how diversified
are the means of occasional transport, I have discussed at some little length
the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long course
of time the individuals of the same species, and likewise of allied species,
have proceeded from some one source; then I think all the grand leading
facts of geographical distribution are explicable on the theory of migration
(generally of the more dominant forms of life), together with subsequent
modification and the multiplication of new forms. We can thus understand
the high importance of barriers, whether of land or water, which separate
our several zoological and botanical provinces. We can thus understand the
localisation of sub-genera, genera, and families; and how it is that under
different latitudes, for instance in South America, the inhabitants of the
plains and mountains, of the forests, marshes, and deserts, are in so
mysterious a manner linked together by affinity, and are likewise linked to
the extinct beings which formerly inhabited the same continent. Bearing in
mind that the mutual relations of organism to organism are of the highest
importance, we can see why two areas having nearly the same physical
conditions should often be inhabited by very different forms of life; for
according to the length of time which has elapsed since new inhabitants
entered one region; according to the nature of the communication which

allowed certain forms and not others to enter, either in greater or lesser
numbers; according or not, as those which entered happened to come in
more or less direct competition with each other and with the aborigines; and
according as the immigrants were capable of varying more or less rapidly,
there would ensue in different regions, independently of their physical
conditions, infinitely diversified conditions of life,—there would be an
almost endless amount of organic action and reaction,—and we should find,
as we do find, some groups of beings greatly, and some only slightly
modified,—some developed in great force, some existing in scanty numbers
—in the different great geographical provinces of the world.
On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a great
number should be endemic or peculiar; and why, in relation to the means of
migration, one group of beings, even within the same class, should have all
its species endemic, and another group should have all its species common
to other quarters of the world. We can see why whole groups of organisms,
as batrachians and terrestrial mammals, should be absent from oceanic
islands, whilst the most isolated islands possess their own peculiar species
of ærial mammals or bats. We can see why there should be some relation
between the presence of mammals, in a more or less modified condition,
and the depth of the sea between an island and the mainland. We can clearly
see why all the inhabitants of an archipelago, though specifically distinct on
the several islets, should be closely related to each other, and likewise be
related, but less closely, to those of the nearest continent or other source
whence immigrants were probably derived. We can see why in two areas,
however distant from each other, there should be a correlation, in the
presence of identical species, of varieties, of doubtful species, and of
distinct but representative species.
As the late Edward Forbes often insisted, there is a striking parallelism in
the laws of life throughout time and space: the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see this
in many facts. The endurance of each species and group of species is
continuous in time; for the exceptions to the rule are so few, that they may
fairly be attributed to our not having as yet discovered in an intermediate
deposit the forms which are therein absent, but which occur above and
below: so in space, it certainly is the general rule that the area inhabited by

a single species, or by a group of species, is continuous; and the exceptions,
which are not rare, may, as I have attempted to show, be accounted for by
migration at some former period under different conditions or by occasional
means of transport, and by the species having become extinct in the
intermediate tracts. Both in time and space, species and groups of species
have their points of maximum development. Groups of species, belonging
either to a certain period of time, or to a certain area, are often characterised
by trifling characters in common, as of sculpture or colour. In looking to the
long succession of ages, as in now looking to distant provinces throughout
the world, we find that some organisms differ little, whilst others belonging
to a different class, or to a different order, or even only to a different family
of the same order, differ greatly. In both time and space the lower members
of each class generally change less than the higher; but there are in both
cases marked exceptions to the rule. On my theory these several relations
throughout time and space are intelligible; for whether we look to the forms
of life which have changed during successive ages within the same quarter
of the world, or to those which have changed after having migrated into
distant quarters, in both cases the forms within each class have been
connected by the same bond of ordinary generation; and the more nearly
any two forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have been
the same, and modifications have been accumulated by the same power of
natural selection.

CHAPTER XIII.
MUTUAL AFFINITIES OF ORGANIC BEINGS:
MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS.

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

From the first dawn of life, all organic beings are found to resemble each
other in descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the grouping of the
stars in constellations. The existence of groups would have been of simple
signification, if one group had been exclusively fitted to inhabit the land,
and another the water; one to feed on flesh, another on vegetable matter,
and so on; but the case is widely different in nature; for it is notorious how
commonly members of even the same subgroup have different habits. In our
second and fourth chapters, on Variation and on Natural Selection, I have
attempted to show that it is the widely ranging, the much diffused and
common, that is the dominant species belonging to the larger genera, which
vary most. The varieties, or incipient species, thus produced ultimately

become converted, as I believe, into new and distinct species; and these, on
the principle of inheritance, tend to produce other new and dominant
species. Consequently the groups which are now large, and which generally
include many dominant species, tend to go on increasing indefinitely in
size. I further attempted to show that from the varying descendants of each
species trying to occupy as many and as different places as possible in the
economy of nature, there is a constant tendency in their characters to
diverge. This conclusion was supported by looking at the great diversity of
the forms of life which, in any small area, come into the closest
competition, and by looking to certain facts in naturalisation.
I attempted also to show that there is a constant tendency in the forms
which are increasing in number and diverging in character, to supplant and
exterminate the less divergent, the less improved, and preceding forms. I
request the reader to turn to the diagram illustrating the action, as formerly
explained, of these several principles; and he will see that the inevitable
result is that the modified descendants proceeding from one progenitor
become broken up into groups subordinate to groups. In the diagram each
letter on the uppermost line may represent a genus including several
species; and all the genera on this line form together one class, for all have
descended from one ancient but unseen parent, and, consequently, have
inherited something in common. But the three genera on the left hand have,
on this same principle, much in common, and form a sub-family, distinct
from that including the next two genera on the right hand, which diverged
from a common parent at the fifth stage of descent. These five genera have
also much, though less, in common; and they form a family distinct from
that including the three genera still further to the right hand, which diverged
at a still earlier period. And all these genera, descended from (A), form an
order distinct from the genera descended from (I). So that we here have
many species descended from a single progenitor grouped into genera; and
the genera are included in, or subordinate to, sub-families, families, and
orders, all united into one class. Thus, the grand fact in natural history of
the subordination of group under group, which, from its familiarity, does
not always sufficiently strike us, is in my judgment fully explained.
Naturalists try to arrange the species, genera, and families in each class,
on what is called the Natural System. But what is meant by this system?
Some authors look at it merely as a scheme for arranging together those
living objects which are most alike, and for separating those which are most

unlike; or as an artificial means for enunciating, as briefly as possible,
general propositions,—that is, by one sentence to give the characters
common, for instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and then by adding a
single sentence, a full description is given of each kind of dog. The
ingenuity and utility of this system are indisputable. But many naturalists
think that something more is meant by the Natural System; they believe that
it reveals the plan of the Creator; but unless it be specified whether order in
time or space, or what else is meant by the plan of the Creator, it seems to
me that nothing is thus added to our knowledge. Such expressions as that
famous one of Linnæus, and which we often meet with in a more or less
concealed form, that the characters do not make the genus, but that the
genus gives the characters, seem to imply that something more is included
in our classification, than mere resemblance. I believe that something more
is included; and that propinquity of descent,—the only known cause of the
similarity of organic beings,—is the bond, hidden as it is by various degrees
of modification, which is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each other.
It might have been thought (and was in ancient times thought) that those
parts of the structure which determined the habits of life, and the general
place of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one regards the
external similarity of a mouse to a shrew, of a dugong to a whale, of a
whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
“adaptive or analogical characters;” but to the consideration of these
resemblances we shall have to recur. It may even be given as a general rule,
that the less any part of the organisation is concerned with special habits,
the more important it becomes for classification. As an instance: Owen, in
speaking of the dugong, says, “The generative organs being those which are
most remotely related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true affinities. We are
least likely in the modifications of these organs to mistake a merely
adaptive for an essential character.” So with plants, how remarkable it is

that the organs of vegetation, on which their whole life depends, are of little
signification, excepting in the first main divisions; whereas the organs of
reproduction, with their product the seed, are of paramount importance!
We must not, therefore, in classifying, trust to resemblances in parts of
the organisation, however important they may be for the welfare of the
being in relation to the outer world. Perhaps from this cause it has partly
arisen, that almost all naturalists lay the greatest stress on resemblances in
organs of high vital or physiological importance. No doubt this view of the
classificatory importance of organs which are important is generally, but by
no means always, true. But their importance for classification, I believe,
depends on their greater constancy throughout large groups of species; and
this constancy depends on such organs having generally been subjected to
less change in the adaptation of the species to their conditions of life. That
the mere physiological importance of an organ does not determine its
classificatory value, is almost shown by the one fact, that in allied groups,
in which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No
naturalist can have worked at any group without being struck with this fact;
and it has been most fully acknowledged in the writings of almost every
author. It will suffice to quote the highest authority, Robert Brown, who in
speaking of certain organs in the Proteaceæ, says their generic importance,
“like that of all their parts, not only in this but, as I apprehend, in every
natural family, is very unequal, and in some cases seems to be entirely lost.”
Again in another work he says, the genera of the Connaraceæ “differ in
having one or more ovaria, in the existence or absence of albumen, in the
imbricate or valvular æstivation. Any one of these characters singly is
frequently of more than generic importance, though here even when all
taken together they appear insufficient to separate Cnestis from Connarus.”
To give an example amongst insects, in one great division of the
Hymenoptera, the antennæ, as Westwood has remarked, are most constant
in structure; in another division they differ much, and the differences are of
quite subordinate value in classification; yet no one probably will say that
the antennæ in these two divisions of the same order are of unequal
physiological importance. Any number of instances could be given of the
varying importance for classification of the same important organ within the
same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this condition
are often of high value in classification. No one will dispute that the
rudimentary teeth in the upper jaws of young ruminants, and certain
rudimentary bones of the leg, are highly serviceable in exhibiting the close
affinity between Ruminants and Pachyderms. Robert Brown has strongly
insisted on the fact that the rudimentary florets are of the highest
importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance, but
which are universally admitted as highly serviceable in the definition of
whole groups. For instance, whether or not there is an open passage from
the nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles—the inflection of the angle of
the jaws in Marsupials—the manner in which the wings of insects are
folded—mere colour in certain Algæ—mere pubescence on parts of the
flower in grasses—the nature of the dermal covering, as hair or feathers, in
the Vertebrata. If the Ornithorhynchus had been covered with feathers
instead of hair, this external and trifling character would, I think, have been
considered by naturalists as important an aid in determining the degree of
affinity of this strange creature to birds and reptiles, as an approach in
structure in any one internal and important organ.
The importance, for classification, of trifling characters, mainly depends
on their being correlated with several other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiological importance
and of almost universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found, that a classification
founded on any single character, however important that may be, has
always failed; for no part of the organisation is universally constant. The
importance of an aggregate of characters, even when none are important,
alone explains, I think, that saying of Linnæus, that the characters do not
give the genus, but the genus gives the characters; for this saying seems
founded on an appreciation of many trifling points of resemblance, too
slight to be defined. Certain plants, belonging to the Malpighiaceæ, bear
perfect and degraded flowers; in the latter, as A. de Jussieu has remarked,

“the greater number of the characters proper to the species, to the genus, to
the family, to the class, disappear, and thus laugh at our classification.” But
when Aspicarpa produced in France, during several years, only degraded
flowers, departing so wonderfully in a number of the most important points
of structure from the proper type of the order, yet M. Richard sagaciously
saw, as Jussieu observes, that this genus should still be retained amongst the
Malpighiaceæ. This case seems to me well to illustrate the spirit with which
our classifications are sometimes necessarily founded.
Practically when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining a
group, or in allocating any particular species. If they find a character nearly
uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If certain characters are
always found correlated with others, though no apparent bond of connexion
can be discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for propelling the blood,
or for ærating it, or those for propagating the race, are found nearly
uniform, they are considered as highly serviceable in classification; but in
some groups of animals all these, the most important vital organs, are found
to offer characters of quite subordinate value.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications of
course include all ages of each species. But it is by no means obvious, on
the ordinary view, why the structure of the embryo should be more
important for this purpose than that of the adult, which alone plays its full
part in the economy of nature. Yet it has been strongly urged by those great
naturalists, Milne Edwards and Agassiz, that embryonic characters are the
most important of any in the classification of animals; and this doctrine has
very generally been admitted as true. The same fact holds good with
flowering plants, of which the two main divisions have been founded on
characters derived from the embryo,—on the number and position of the
embryonic leaves or cotyledons, and on the mode of development of the
plumule and radicle. In our discussion on embryology, we shall see why

such characters are so valuable, on the view of classification tacitly
including the idea of descent.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but in the case of crustaceans, such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series, which
have hardly a character in common; yet the species at both ends, from being
plainly allied to others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of closely
allied forms. Temminck insists on the utility or even necessity of this
practice in certain groups of birds; and it has been followed by several
entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and genera, they
seem to be, at least at present, almost arbitrary. Several of the best botanists,
such as Mr. Bentham and others, have strongly insisted on their arbitrary
value. Instances could be given amongst plants and insects, of a group of
forms, first ranked by practised naturalists as only a genus, and then raised
to the rank of a sub-family or family; and this has been done, not because
further research has detected important structural differences, at first
overlooked, but because numerous allied species, with slightly different
grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification; that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, and, in
so far, all true classification is genealogical; that community of descent is
the hidden bond which naturalists have been unconsciously seeking, and not
some unknown plan of creation, or the enunciation of general propositions,
and the mere putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination and

relation to the other groups, must be strictly genealogical in order to be
natural; but that the amount of difference in the several branches or groups,
though allied in the same degree in blood to their common progenitor, may
differ greatly, being due to the different degrees of modification which they
have undergone; and this is expressed by the forms being ranked under
different genera, families, sections, or orders. The reader will best
understand what is meant, if he will take the trouble of referring to the
diagram in the fourth chapter. We will suppose the letters A to L to represent
allied genera, which lived during the Silurian epoch, and these have
descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted modified
descendants to the present day, represented by the fifteen genera (a14 to z14)
on the uppermost horizontal line. Now all these modified descendants from
a single species, are represented as related in blood or descent to the same
degree; they may metaphorically be called cousins to the same millionth
degree; yet they differ widely and in different degrees from each other. The
forms descended from A, now broken up into two or three families,
constitute a distinct order from those descended from I, also broken up into
two families. Nor can the existing species, descended from A, be ranked in
the same genus with the parent A; or those from I, with the parent I. But the
existing genus F14 may be supposed to have been but slightly modified; and
it will then rank with the parent-genus F; just as some few still living
organic beings belong to Silurian genera. So that the amount or value of the
differences between organic beings all related to each other in the same
degree in blood, has come to be widely different. Nevertheless their
genealogical arrangement remains strictly true, not only at the present time,
but at each successive period of descent. All the modified descendants from
A will have inherited something in common from their common parent, as
will all the descendants from I; so will it be with each subordinate branch of
descendants, at each successive period. If, however, we choose to suppose
that any of the descendants of A or of I have been so much modified as to
have more or less completely lost traces of their parentage, in this case,
their places in a natural classification will have been more or less
completely lost,—as sometimes seems to have occurred with existing
organisms. All the descendants of the genus F, along its whole line of
descent, are supposed to have been but little modified, and they yet form a
single genus. But this genus, though much isolated, will still occupy its

proper intermediate position; for F originally was intermediate in character
between A and I, and the several genera descended from these two genera
will have inherited to a certain extent their characters. This natural
arrangement is shown, as far as is possible on paper, in the diagram, but in
much too simple a manner. If a branching diagram had not been used, and
only the names of the groups had been written in a linear series, it would
have been still less possible to have given a natural arrangement; and it is
notoriously not possible to represent in a series, on a flat surface, the
affinities which we discover in nature amongst the beings of the same
group. Thus, on the view which I hold, the natural system is genealogical in
its arrangement, like a pedigree; but the degrees of modification which the
different groups have undergone, have to be expressed by ranking them
under different so-called genera, sub-families, families, sections, orders,
and classes.
It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, had to be included, such an arrangement would, I think, be the only
possible one. Yet it might be that some very ancient language had altered
little, and had given rise to few new languages, whilst others (owing to the
spreading and subsequent isolation and states of civilisation of the several
races, descended from a common race) had altered much, and had given
rise to many new languages and dialects. The various degrees of difference
in the languages from the same stock, would have to be expressed by
groups subordinate to groups; but the proper or even only possible
arrangement would still be genealogical; and this would be strictly natural,
as it would connect together all languages, extinct and modern, by the
closest affinities, and would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one species.
These are grouped under species, with sub-varieties under varieties; and
with our domestic productions, several other grades of difference are
requisite, as we have seen with pigeons. The origin of the existence of
groups subordinate to groups, is the same with varieties as with species,
namely, closeness of descent with various degrees of modification. Nearly

the same rules are followed in classifying varieties, as with species. Authors
have insisted on the necessity of classing varieties on a natural instead of an
artificial system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the swedish and
common turnips together, though the esculent and thickened stems are so
similar. Whatever part is found to be most constant, is used in classing
varieties: thus the great agriculturist Marshall says the horns are very useful
for this purpose with cattle, because they are less variable than the shape or
colour of the body, etc.; whereas with sheep the horns are much less
serviceable, because less constant. In classing varieties, I apprehend if we
had a real pedigree, a genealogical classification would be universally
preferred; and it has been attempted by some authors. For we might feel
sure, whether there had been more or less modification, the principle of
inheritance would keep the forms together which were allied in the greatest
number of points. In tumbler pigeons, though some sub-varieties differ from
the others in the important character of having a longer beak, yet all are
kept together from having the common habit of tumbling; but the short-
faced breed has nearly or quite lost this habit; nevertheless, without any
reasoning or thinking on the subject, these tumblers are kept in the same
group, because allied in blood and alike in some other respects. If it could
be proved that the Hottentot had descended from the Negro, I think he
would be classed under the Negro group, however much he might differ in
colour and other important characters from negroes.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, or that of
a species, the two sexes; and how enormously these sometimes differ in the
most important characters, is known to every naturalist: scarcely a single
fact can be predicated in common of the males and hermaphrodites of
certain cirripedes, when adult, and yet no one dreams of separating them.
The naturalist includes as one species the several larval stages of the same
individual, however much they may differ from each other and from the
adult; as he likewise includes the so-called alternate generations of
Steenstrup, which can only in a technical sense be considered as the same
individual. He includes monsters; he includes varieties, not solely because
they closely resemble the parent-form, but because they are descended from
it. He who believes that the cowslip is descended from the primrose, or

conversely, ranks them together as a single species, and gives a single
definition. As soon as three Orchidean forms (Monochanthus, Myanthus,
and Catasetum), which had previously been ranked as three distinct genera,
were known to be sometimes produced on the same spike, they were
immediately included as a single species. But it may be asked, what ought
we to do, if it could be proved that one species of kangaroo had been
produced, by a long course of modification, from a bear? Ought we to rank
this one species with bears, and what should we do with the other species?
The supposition is of course preposterous; and I might answer by the
argumentum ad hominem, and ask what should be done if a perfect
kangaroo were seen to come out of the womb of a bear? According to all
analogy, it would be ranked with bears; but then assuredly all the other
species of the kangaroo family would have to be classed under the bear
genus. The whole case is preposterous; for where there has been close
descent in common, there will certainly be close resemblance or affinity.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvæ are sometimes
extremely different; and as it has been used in classing varieties which have
undergone a certain, and sometimes a considerable amount of modification,
may not this same element of descent have been unconsciously used in
grouping species under genera, and genera under higher groups, though in
these cases the modification has been greater in degree, and has taken a
longer time to complete? I believe it has thus been unconsciously used; and
only thus can I understand the several rules and guides which have been
followed by our best systematists. We have no written pedigrees; we have to
make out community of descent by resemblances of any kind. Therefore we
choose those characters which, as far as we can judge, are the least likely to
have been modified in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view are
as good as, or even sometimes better than, other parts of the organisation.
We care not how trifling a character may be—let it be the mere inflection of
the angle of the jaw, the manner in which an insect’s wing is folded,
whether the skin be covered by hair or feathers—if it prevail throughout
many and different species, especially those having very different habits of
life, it assumes high value; for we can account for its presence in so many
forms with such different habits, only by its inheritance from a common
parent. We may err in this respect in regard to single points of structure, but

when several characters, let them be ever so trifling, occur together
throughout a large group of beings having different habits, we may feel
almost sure, on the theory of descent, that these characters have been
inherited from a common ancestor. And we know that such correlated or
aggregated characters have especial value in classification.
We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, and yet be
safely classed with them. This may be safely done, and is often done, as
long as a sufficient number of characters, let them be ever so unimportant,
betrays the hidden bond of community of descent. Let two forms have not a
single character in common, yet if these extreme forms are connected
together by a chain of intermediate groups, we may at once infer their
community of descent, and we put them all into the same class. As we find
organs of high physiological importance—those which serve to preserve
life under the most diverse conditions of existence—are generally the most
constant, we attach especial value to them; but if these same organs, in
another group or section of a group, are found to differ much, we at once
value them less in our classification. We shall hereafter, I think, clearly see
why embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play in
classing large and widely-distributed genera, because all the species of the
same genus, inhabiting any distinct and isolated region, have in all
probability descended from the same parents.
We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this distinction, and he has been ably followed by
Macleay and others. The resemblance, in the shape of the body and in the
fin-like anterior limbs, between the dugong, which is a pachydermatous
animal, and the whale, and between both these mammals and fishes, is
analogical. Amongst insects there are innumerable instances: thus Linnæus,
misled by external appearances, actually classed an homopterous insect as a
moth. We see something of the same kind even in our domestic varieties, as
in the thickened stems of the common and swedish turnip. The resemblance
of the greyhound and racehorse is hardly more fanciful than the analogies
which have been drawn by some authors between very distinct animals. On
my view of characters being of real importance for classification, only in so
far as they reveal descent, we can clearly understand why analogical or

adaptive character, although of the utmost importance to the welfare of the
being, are almost valueless to the systematist. For animals, belonging to two
most distinct lines of descent, may readily become adapted to similar
conditions, and thus assume a close external resemblance; but such
resemblances will not reveal—will rather tend to conceal their blood-
relationship to their proper lines of descent. We can also understand the
apparent paradox, that the very same characters are analogical when one
class or order is compared with another, but give true affinities when the
members of the same class or order are compared one with another: thus the
shape of the body and fin-like limbs are only analogical when whales are
compared with fishes, being adaptations in both classes for swimming
through the water; but the shape of the body and fin-like limbs serve as
characters exhibiting true affinity between the several members of the
whale family; for these cetaceans agree in so many characters, great and
small, that we cannot doubt that they have inherited their general shape of
body and structure of limbs from a common ancestor. So it is with fishes.
As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,—to inhabit
for instance the three elements of land, air, and water,—we can perhaps
understand how it is that a numerical parallelism has sometimes been
observed between the sub-groups in distinct classes. A naturalist, struck by
a parallelism of this nature in any one class, by arbitrarily raising or sinking
the value of the groups in other classes (and all our experience shows that
this valuation has hitherto been arbitrary), could easily extend the
parallelism over a wide range; and thus the septenary, quinary, quaternary,
and ternary classifications have probably arisen.
As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages, which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups thus tend to go on increasing in size; and
they consequently supplant many smaller and feebler groups. Thus we can
account for the fact that all organisms, recent and extinct, are included
under a few great orders, under still fewer classes, and all in one great
natural system. As showing how few the higher groups are in number, and
how widely spread they are throughout the world, the fact is striking, that
the discovery of Australia has not added a single insect belonging to a new

order; and that in the vegetable kingdom, as I learn from Dr. Hooker, it has
added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some slight degree intermediate
between existing groups. A few old and intermediate parent-forms having
occasionally transmitted to the present day descendants but little modified,
will give to us our so-called osculant or aberrant groups. The more aberrant
any form is, the greater must be the number of connecting forms which on
my theory have been exterminated and utterly lost. And we have some
evidence of aberrant forms having suffered severely from extinction, for
they are generally represented by extremely few species; and such species
as do occur are generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been represented by a dozen
species instead of by a single one; but such richness in species, as I find
after some investigation, does not commonly fall to the lot of aberrant
genera. We can, I think, account for this fact only by looking at aberrant
forms as failing groups conquered by more successful competitors, with a
few members preserved by some unusual coincidence of favourable
circumstances.
Mr. Waterhouse has remarked that, when a member belonging to one
group of animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse, of
all Rodents, the bizcacha is most nearly related to Marsupials; but in the
points in which it approaches this order, its relations are general, and not to
any one marsupial species more than to another. As the points of affinity of
the bizcacha to Marsupials are believed to be real and not merely adaptive,
they are due on my theory to inheritance in common. Therefore we must
suppose either that all Rodents, including the bizcacha, branched off from
some very ancient Marsupial, which will have had a character in some
degree intermediate with respect to all existing Marsupials; or that both
Rodents and Marsupials branched off from a common progenitor, and that
both groups have since undergone much modification in divergent
directions. On either view we may suppose that the bizcacha has retained,
by inheritance, more of the character of its ancient progenitor than have

other Rodents; and therefore it will not be specially related to any one
existing Marsupial, but indirectly to all or nearly all Marsupials, from
having partially retained the character of their common progenitor, or of an
early member of the group. On the other hand, of all Marsupials, as Mr.
Waterhouse has remarked, the phascolomys resembles most nearly, not any
one species, but the general order of Rodents. In this case, however, it may
be strongly suspected that the resemblance is only analogical, owing to the
phascolomys having become adapted to habits like those of a Rodent. The
elder De Candolle has made nearly similar observations on the general
nature of the affinities of distinct orders of plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common parent, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
parent of a whole family of species, now broken up by extinction into
distinct groups and sub-groups, will have transmitted some of its characters,
modified in various ways and degrees, to all; and the several species will
consequently be related to each other by circuitous lines of affinity of
various lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the
blood-relationship between the numerous kindred of any ancient and noble
family, even by the aid of a genealogical tree, and almost impossible to do
this without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram, the
various affinities which they perceive between the many living and extinct
members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account even for the distinctness of whole classes
from each other—for instance, of birds from all other vertebrate animals—
by the belief that many ancient forms of life have been utterly lost, through
which the early progenitors of birds were formerly connected with the early
progenitors of the other vertebrate classes. There has been less entire
extinction of the forms of life which once connected fishes with
batrachians. There has been still less in some other classes, as in that of the
Crustacea, for here the most wonderfully diverse forms are still tied

together by a long, but broken, chain of affinities. Extinction has only
separated groups: it has by no means made them; for if every form which
has ever lived on this earth were suddenly to reappear, though it would be
quite impossible to give definitions by which each group could be
distinguished from other groups, as all would blend together by steps as fine
as those between the finest existing varieties, nevertheless a natural
classification, or at least a natural arrangement, would be possible. We shall
see this by turning to the diagram: the letters, A to L, may represent eleven
Silurian genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and their
primordial parent, and every intermediate link in each branch and sub-
branch of their descendants, may be supposed to be still alive; and the links
to be as fine as those between the finest varieties. In this case it would be
quite impossible to give any definition by which the several members of the
several groups could be distinguished from their more immediate parents;
or these parents from their ancient and unknown progenitor. Yet the natural
arrangement in the diagram would still hold good; and, on the principle of
inheritance, all the forms descended from A, or from I, would have
something in common. In a tree we can specify this or that branch, though
at the actual fork the two unite and blend together. We could not, as I have
said, define the several groups; but we could pick out types, or forms,
representing most of the characters of each group, whether large or small,
and thus give a general idea of the value of the differences between them.
This is what we should be driven to, if we were ever to succeed in
collecting all the forms in any class which have lived throughout all time
and space. We shall certainly never succeed in making so perfect a
collection: nevertheless, in certain classes, we are tending in this direction;
and Milne Edwards has lately insisted, in an able paper, on the high
importance of looking to types, whether or not we can separate and define
the groups to which such types belong.
Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction and
divergence of character in the many descendants from one dominant parent-
species, explains that great and universal feature in the affinities of all
organic beings, namely, their subordination in group under group. We use
the element of descent in classing the individuals of both sexes and of all
ages, although having few characters in common, under one species; we use

descent in classing acknowledged varieties, however different they may be
from their parent; and I believe this element of descent is the hidden bond
of connexion which naturalists have sought under the term of the Natural
System. On this idea of the natural system being, in so far as it has been
perfected, genealogical in its arrangement, with the grades of difference
between the descendants from a common parent, expressed by the terms
genera, families, orders, etc., we can understand the rules which we are
compelled to follow in our classification. We can understand why we value
certain resemblances far more than others; why we are permitted to use
rudimentary and useless organs, or others of trifling physiological
importance; why, in comparing one group with a distinct group, we
summarily reject analogical or adaptive characters, and yet use these same
characters within the limits of the same group. We can clearly see how it is
that all living and extinct forms can be grouped together in one great
system; and how the several members of each class are connected together
by the most complex and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of affinities between the
members of any one class; but when we have a distinct object in view, and
do not look to some unknown plan of creation, we may hope to make sure
but slow progress.

Morphology.—We have seen that the members of the same class,
independently of their habits of life, resemble each other in the general plan
of their organisation. This resemblance is often expressed by the term
“unity of type;” or by saying that the several parts and organs in the
different species of the class are homologous. The whole subject is included
under the general name of Morphology. This is the most interesting
department of natural history, and may be said to be its very soul. What can
be more curious than that the hand of a man, formed for grasping, that of a
mole for digging, the leg of the horse, the paddle of the porpoise, and the
wing of the bat, should all be constructed on the same pattern, and should
include the same bones, in the same relative positions? Geoffroy St. Hilaire
has insisted strongly on the high importance of relative connexion in
homologous organs: the parts may change to almost any extent in form and
size, and yet they always remain connected together in the same order. We
never find, for instance, the bones of the arm and forearm, or of the thigh

and leg, transposed. Hence the same names can be given to the homologous
bones in widely different animals. We see the same great law in the
construction of the mouths of insects: what can be more different than the
immensely long spiral proboscis of a sphinx-moth, the curious folded one
of a bee or bug, and the great jaws of a beetle?—yet all these organs,
serving for such different purposes, are formed by infinitely numerous
modifications of an upper lip, mandibles, and two pairs of maxillæ.
Analogous laws govern the construction of the mouths and limbs of
crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of final
causes. The hopelessness of the attempt has been expressly admitted by
Owen in his most interesting work on the ‘Nature of Limbs.’ On the
ordinary view of the independent creation of each being, we can only say
that so it is;—that it has so pleased the Creator to construct each animal and
plant.
The explanation is manifest on the theory of the natural selection of
successive slight modifications,—each modification being profitable in
some way to the modified form, but often affecting by correlation of growth
other parts of the organisation. In changes of this nature, there will be little
or no tendency to modify the original pattern, or to transpose parts. The
bones of a limb might be shortened and widened to any extent, and become
gradually enveloped in thick membrane, so as to serve as a fin; or a webbed
foot might have all its bones, or certain bones, lengthened to any extent, and
the membrane connecting them increased to any extent, so as to serve as a
wing: yet in all this great amount of modification there will be no tendency
to alter the framework of bones or the relative connexion of the several
parts. If we suppose that the ancient progenitor, the archetype as it may be
called, of all mammals, had its limbs constructed on the existing general
pattern, for whatever purpose they served, we can at once perceive the plain
signification of the homologous construction of the limbs throughout the
whole class. So with the mouths of insects, we have only to suppose that
their common progenitor had an upper lip, mandibles, and two pair of
maxillæ, these parts being perhaps very simple in form; and then natural
selection will account for the infinite diversity in structure and function of
the mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally lost, by

the atrophy and ultimately by the complete abortion of certain parts, by the
soldering together of other parts, and by the doubling or multiplication of
others,—variations which we know to be within the limits of possibility. In
the paddles of the extinct gigantic sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems to have been thus to a
certain extent obscured.
There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual. Most
physiologists believe that the bones of the skull are homologous with—that
is correspond in number and in relative connexion with—the elemental
parts of a certain number of vertebræ. The anterior and posterior limbs in
each member of the vertebrate and articulate classes are plainly
homologous. We see the same law in comparing the wonderfully complex
jaws and legs in crustaceans. It is familiar to almost every one, that in a
flower the relative position of the sepals, petals, stamens, and pistils, as well
as their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we often
get direct evidence of the possibility of one organ being transformed into
another; and we can actually see in embryonic crustaceans and in many
other animals, and in flowers, that organs, which when mature become
extremely different, are at an early stage of growth exactly alike.
How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and such
extraordinarily shaped pieces of bone? As Owen has remarked, the benefit
derived from the yielding of the separate pieces in the act of parturition of
mammals, will by no means explain the same construction in the skulls of
birds. Why should similar bones have been created in the formation of the
wing and leg of a bat, used as they are for such totally different purposes?
Why should one crustacean, which has an extremely complex mouth
formed of many parts, consequently always have fewer legs; or conversely,
those with many legs have simpler mouths? Why should the sepals, petals,
stamens, and pistils in any individual flower, though fitted for such widely
different purposes, be all constructed on the same pattern?
On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebræ bearing

certain processes and appendages; in the articulata, we see the body divided
into a series of segments, bearing external appendages; and in flowering
plants, we see a series of successive spiral whorls of leaves. An indefinite
repetition of the same part or organ is the common characteristic (as Owen
has observed) of all low or little-modified forms; therefore we may readily
believe that the unknown progenitor of the vertebrata possessed many
vertebræ; the unknown progenitor of the articulata, many segments; and the
unknown progenitor of flowering plants, many spiral whorls of leaves. We
have formerly seen that parts many times repeated are eminently liable to
vary in number and structure; consequently it is quite probable that natural
selection, during a long-continued course of modification, should have
seized on a certain number of the primordially similar elements, many times
repeated, and have adapted them to the most diverse purposes. And as the
whole amount of modification will have been effected by slight successive
steps, we need not wonder at discovering in such parts or organs, a certain
degree of fundamental resemblance, retained by the strong principle of
inheritance.
In the great class of molluscs, though we can homologise the parts of one
species with those of another and distinct species, we can indicate but few
serial homologies; that is, we are seldom enabled to say that one part or
organ is homologous with another in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one part, as
we find in the other great classes of the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ: the jaws of crabs as metamorphosed legs; the stamens and pistils
of flowers as metamorphosed leaves; but it would in these cases probably
be more correct, as Professor Huxley has remarked, to speak of both skull
and vertebræ, both jaws and legs, etc.,—as having been metamorphosed,
not one from the other, but from some common element. Naturalists,
however, use such language only in a metaphorical sense: they are far from
meaning that during a long course of descent, primordial organs of any kind
—vertebræ in the one case and legs in the other—have actually been
modified into skulls or jaws. Yet so strong is the appearance of a
modification of this nature having occurred, that naturalists can hardly
avoid employing language having this plain signification. On my view these
terms may be used literally; and the wonderful fact of the jaws, for instance,

of a crab retaining numerous characters, which they would probably have
retained through inheritance, if they had really been metamorphosed during
a long course of descent from true legs, or from some simple appendage, is
explained.

Embryology.—It has already been casually remarked that certain organs
in the individual, which when mature become widely different and serve for
different purposes, are in the embryo exactly alike. The embryos, also, of
distinct animals within the same class are often strikingly similar: a better
proof of this cannot be given, than a circumstance mentioned by Agassiz,
namely, that having forgotten to ticket the embryo of some vertebrate
animal, he cannot now tell whether it be that of a mammal, bird, or reptile.
The vermiform larvæ of moths, flies, beetles, etc., resemble each other
much more closely than do the mature insects; but in the case of larvæ, the
embryos are active, and have been adapted for special lines of life. A trace
of the law of embryonic resemblance, sometimes lasts till a rather late age:
thus birds of the same genus, and of closely allied genera, often resemble
each other in their first and second plumage; as we see in the spotted
feathers in the thrush group. In the cat tribe, most of the species are striped
or spotted in lines; and stripes can be plainly distinguished in the whelp of
the lion. We occasionally though rarely see something of this kind in plants:
thus the embryonic leaves of the ulex or furze, and the first leaves of the
phyllodineous acaceas, are pinnate or divided like the ordinary leaves of the
leguminosæ.
The points of structure, in which the embryos of widely different animals
of the same class resemble each other, often have no direct relation to their
conditions of existence. We cannot, for instance, suppose that in the
embryos of the vertebrata the peculiar loop-like course of the arteries near
the branchial slits are related to similar conditions,—in the young mammal
which is nourished in the womb of its mother, in the egg of the bird which
is hatched in a nest, and in the spawn of a frog under water. We have no
more reason to believe in such a relation, than we have to believe that the
same bones in the hand of a man, wing of a bat, and fin of a porpoise, are
related to similar conditions of life. No one will suppose that the stripes on
the whelp of a lion, or the spots on the young blackbird, are of any use to
these animals, or are related to the conditions to which they are exposed.

The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just as perfect and as
beautiful as in the adult animal. From such special adaptations, the
similarity of the larvæ or active embryos of allied animals is sometimes
much obscured; and cases could be given of the larvæ of two species, or of
two groups of species, differing quite as much, or even more, from each
other than do their adult parents. In most cases, however, the larvæ, though
active, still obey more or less closely the law of common embryonic
resemblance. Cirripedes afford a good instance of this: even the illustrious
Cuvier did not perceive that a barnacle was, as it certainly is, a crustacean;
but a glance at the larva shows this to be the case in an unmistakeable
manner. So again the two main divisions of cirripedes, the pedunculated and
sessile, which differ widely in external appearance, have larvæ in all their
several stages barely distinguishable.
The embryo in the course of development generally rises in organisation:
I use this expression, though I am aware that it is hardly possible to define
clearly what is meant by the organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar. In some
cases, however, the mature animal is generally considered as lower in the
scale than the larva, as with certain parasitic crustaceans. To refer once
again to cirripedes: the larvæ in the first stage have three pairs of legs, a
very simple single eye, and a probosciformed mouth, with which they feed
largely, for they increase much in size. In the second stage, answering to the
chrysalis stage of butterflies, they have six pairs of beautifully constructed
natatory legs, a pair of magnificent compound eyes, and extremely complex
antennæ; but they have a closed and imperfect mouth, and cannot feed: their
function at this stage is, to search by their well-developed organs of sense,
and to reach by their active powers of swimming, a proper place on which
to become attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennæ, and their two eyes are now reconverted into a minute,
single, and very simple eye-spot. In this last and complete state, cirripedes
may be considered as either more highly or more lowly organised than they
were in the larval condition. But in some genera the larvæ become

developed either into hermaphrodites having the ordinary structure, or into
what I have called complemental males: and in the latter, the development
has assuredly been retrograde; for the male is a mere sack, which lives for a
short time, and is destitute of mouth, stomach, or other organ of importance,
excepting for reproduction.
We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to look
at these facts as necessarily contingent in some manner on growth. But
there is no obvious reason why, for instance, the wing of a bat, or the fin of
a porpoise, should not have been sketched out with all the parts in proper
proportion, as soon as any structure became visible in the embryo. And in
some whole groups of animals and in certain members of other groups, the
embryo does not at any period differ widely from the adult: thus Owen has
remarked in regard to cuttle-fish, “there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the embryo
are completed;” and again in spiders, “there is nothing worthy to be called a
metamorphosis.” The larvæ of insects, whether adapted to the most diverse
and active habits, or quite inactive, being fed by their parents or placed in
the midst of proper nutriment, yet nearly all pass through a similar worm-
like stage of development; but in some few cases, as in that of Aphis, if we
look to the admirable drawings by Professor Huxley of the development of
this insect, we see no trace of the vermiform stage.
How, then, can we explain these several facts in embryology,—namely
the very general, but not universal difference in structure between the
embryo and the adult;—of parts in the same individual embryo, which
ultimately become very unlike and serve for diverse purposes, being at this
early period of growth alike;—of embryos of different species within the
same class, generally, but not universally, resembling each other;—of the
structure of the embryo not being closely related to its conditions of
existence, except when the embryo becomes at any period of life active and
has to provide for itself;—of the embryo apparently having sometimes a
higher organisation than the mature animal, into which it is developed. I
believe that all these facts can be explained, as follows, on the view of
descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily appear at an
equally early period. But we have little evidence on this head—indeed the
evidence rather points the other way; for it is notorious that breeders of
cattle, horses, and various fancy animals, cannot positively tell, until some
time after the animal has been born, what its merits or form will ultimately
turn out. We see this plainly in our own children; we cannot always tell
whether the child will be tall or short, or what its precise features will be.
The question is not, at what period of life any variation has been caused, but
at what period it is fully displayed. The cause may have acted, and I believe
generally has acted, even before the embryo is formed; and the variation
may be due to the male and female sexual elements having been affected by
the conditions to which either parent, or their ancestors, have been exposed.
Nevertheless an effect thus caused at a very early period, even before the
formation of the embryo, may appear late in life; as when an hereditary
disease, which appears in old age alone, has been communicated to the
offspring from the reproductive element of one parent. Or again, as when
the horns of cross-bred cattle have been affected by the shape of the horns
of either parent. For the welfare of a very young animal, as long as it
remains in its mother’s womb, or in the egg, or as long as it is nourished
and protected by its parent, it must be quite unimportant whether most of its
characters are fully acquired a little earlier or later in life. It would not
signify, for instance, to a bird which obtained its food best by having a long
beak, whether or not it assumed a beak of this particular length, as long as it
was fed by its parents. Hence, I conclude, that it is quite possible, that each
of the many successive modifications, by which each species has acquired
its present structure, may have supervened at a not very early period of life;
and some direct evidence from our domestic animals supports this view.
But in other cases it is quite possible that each successive modification, or
most of them, may have appeared at an extremely early period.
I have stated in the first chapter, that there is some evidence to render it
probable, that at whatever age any variation first appears in the parent, it
tends to reappear at a corresponding age in the offspring. Certain variations
can only appear at corresponding ages, for instance, peculiarities in the
caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns
of almost full-grown cattle. But further than this, variations which, for all
that we can see, might have appeared earlier or later in life, tend to appear

at a corresponding age in the offspring and parent. I am far from meaning
that this is invariably the case; and I could give a good many cases of
variations (taking the word in the largest sense) which have supervened at
an earlier age in the child than in the parent.
These two principles, if their truth be admitted, will, I believe, explain all
the above specified leading facts in embryology. But first let us look at a
few analogous cases in domestic varieties. Some authors who have written
on Dogs, maintain that the greyhound and bulldog, though appearing so
different, are really varieties most closely allied, and have probably
descended from the same wild stock; hence I was curious to see how far
their puppies differed from each other: I was told by breeders that they
differed just as much as their parents, and this, judging by the eye, seemed
almost to be the case; but on actually measuring the old dogs and their six-
days old puppies, I found that the puppies had not nearly acquired their full
amount of proportional difference. So, again, I was told that the foals of cart
and race-horses differed as much as the full-grown animals; and this
surprised me greatly, as I think it probable that the difference between these
two breeds has been wholly caused by selection under domestication; but
having had careful measurements made of the dam and of a three-days old
colt of a race and heavy cart-horse, I find that the colts have by no means
acquired their full amount of proportional difference.
As the evidence appears to me conclusive, that the several domestic
breeds of Pigeon have descended from one wild species, I compared young
pigeons of various breeds, within twelve hours after being hatched; I
carefully measured the proportions (but will not here give details) of the
beak, width of mouth, length of nostril and of eyelid, size of feet and length
of leg, in the wild stock, in pouters, fantails, runts, barbs, dragons, carriers,
and tumblers. Now some of these birds, when mature, differ so
extraordinarily in length and form of beak, that they would, I cannot doubt,
be ranked in distinct genera, had they been natural productions. But when
the nestling birds of these several breeds were placed in a row, though most
of them could be distinguished from each other, yet their proportional
differences in the above specified several points were incomparably less
than in the full-grown birds. Some characteristic points of difference—for
instance, that of the width of mouth—could hardly be detected in the
young. But there was one remarkable exception to this rule, for the young
of the short-faced tumbler differed from the young of the wild rock-pigeon

and of the other breeds, in all its proportions, almost exactly as much as in
the adult state.
The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties. Fanciers
select their horses, dogs, and pigeons, for breeding, when they are nearly
grown up: they are indifferent whether the desired qualities and structures
have been acquired earlier or later in life, if the full-grown animal possesses
them. And the cases just given, more especially that of pigeons, seem to
show that the characteristic differences which give value to each breed, and
which have been accumulated by man’s selection, have not generally first
appeared at an early period of life, and have been inherited by the offspring
at a corresponding not early period. But the case of the short-faced tumbler,
which when twelve hours old had acquired its proper proportions, proves
that this is not the universal rule; for here the characteristic differences must
either have appeared at an earlier period than usual, or, if not so, the
differences must have been inherited, not at the corresponding, but at an
earlier age.
Now let us apply these facts and the above two principles—which latter,
though not proved true, can be shown to be in some degree probable—to
species in a state of nature. Let us take a genus of birds, descended on my
theory from some one parent-species, and of which the several new species
have become modified through natural selection in accordance with their
diverse habits. Then, from the many slight successive steps of variation
having supervened at a rather late age, and having been inherited at a
corresponding age, the young of the new species of our supposed genus will
manifestly tend to resemble each other much more closely than do the
adults, just as we have seen in the case of pigeons. We may extend this view
to whole families or even classes. The fore-limbs, for instance, which
served as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles—namely of
each successive modification supervening at a rather late age, and being
inherited at a corresponding late age—the fore-limbs in the embryos of the
several descendants of the parent-species will still resemble each other
closely, for they will not have been modified. But in each individual new
species, the embryonic fore-limbs will differ greatly from the fore-limbs in
the mature animal; the limbs in the latter having undergone much

modification at a rather late period of life, and having thus been converted
into hands, or paddles, or wings. Whatever influence long-continued
exercise or use on the one hand, and disuse on the other, may have in
modifying an organ, such influence will mainly affect the mature animal,
which has come to its full powers of activity and has to gain its own living;
and the effects thus produced will be inherited at a corresponding mature
age. Whereas the young will remain unmodified, or be modified in a lesser
degree, by the effects of use and disuse.
In certain cases the successive steps of variation might supervene, from
causes of which we are wholly ignorant, at a very early period of life, or
each step might be inherited at an earlier period than that at which it first
appeared. In either case (as with the short-faced tumbler) the young or
embryo would closely resemble the mature parent-form. We have seen that
this is the rule of development in certain whole groups of animals, as with
cuttle-fish and spiders, and with a few members of the great class of insects,
as with Aphis. With respect to the final cause of the young in these cases
not undergoing any metamorphosis, or closely resembling their parents
from their earliest age, we can see that this would result from the two
following contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for their
own wants at a very early stage of development, and secondly, from their
following exactly the same habits of life with their parents; for in this case,
it would be indispensable for the existence of the species, that the child
should be modified at a very early age in the same manner with its parents,
in accordance with their similar habits. Some further explanation, however,
of the embryo not undergoing any metamorphosis is perhaps requisite. If,
on the other hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to be constructed in a
slightly different manner, then, on the principle of inheritance at
corresponding ages, the active young or larvæ might easily be rendered by
natural selection different to any conceivable extent from their parents.
Such differences might, also, become correlated with successive stages of
development; so that the larvæ, in the first stage, might differ greatly from
the larvæ in the second stage, as we have seen to be the case with
cirripedes. The adult might become fitted for sites or habits, in which
organs of locomotion or of the senses, etc., would be useless; and in this
case the final metamorphosis would be said to be retrograde.

As all the organic beings, extinct and recent, which have ever lived on
this earth have to be classed together, and as all have been connected by the
finest gradations, the best, or indeed, if our collections were nearly perfect,
the only possible arrangement, would be genealogical. Descent being on my
view the hidden bond of connexion which naturalists have been seeking
under the term of the natural system. On this view we can understand how it
is that, in the eyes of most naturalists, the structure of the embryo is even
more important for classification than that of the adult. For the embryo is
the animal in its less modified state; and in so far it reveals the structure of
its progenitor. In two groups of animal, however much they may at present
differ from each other in structure and habits, if they pass through the same
or similar embryonic stages, we may feel assured that they have both
descended from the same or nearly similar parents, and are therefore in that
degree closely related. Thus, community in embryonic structure reveals
community of descent. It will reveal this community of descent, however
much the structure of the adult may have been modified and obscured; we
have seen, for instance, that cirripedes can at once be recognised by their
larvæ as belonging to the great class of crustaceans. As the embryonic state
of each species and group of species partially shows us the structure of their
less modified ancient progenitors, we can clearly see why ancient and
extinct forms of life should resemble the embryos of their descendants,—
our existing species. Agassiz believes this to be a law of nature; but I am
bound to confess that I only hope to see the law hereafter proved true. It can
be proved true in those cases alone in which the ancient state, now
supposed to be represented in many embryos, has not been obliterated,
either by the successive variations in a long course of modification having
supervened at a very early age, or by the variations having been inherited at
an earlier period than that at which they first appeared. It should also be
borne in mind, that the supposed law of resemblance of ancient forms of
life to the embryonic stages of recent forms, may be true, but yet, owing to
the geological record not extending far enough back in time, may remain
for a long period, or for ever, incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many descendants
from some one ancient progenitor, at a very early period in the life of each,
though perhaps caused at the earliest, and being inherited at a

corresponding not early period. Embryology rises greatly in interest, when
we thus look at the embryo as a picture, more or less obscured, of the
common parent-form of each great class of animals.

Rudimentary, atrophied, or aborted organs.—Organs or parts in this
strange condition, bearing the stamp of inutility, are extremely common
throughout nature. For instance, rudimentary mammæ are very general in
the males of mammals: I presume that the “bastard-wing” in birds may be
safely considered as a digit in a rudimentary state: in very many snakes one
lobe of the lungs is rudimentary; in other snakes there are rudiments of the
pelvis and hind limbs. Some of the cases of rudimentary organs are
extremely curious; for instance, the presence of teeth in foetal whales,
which when grown up have not a tooth in their heads; and the presence of
teeth, which never cut through the gums, in the upper jaws of our unborn
calves. It has even been stated on good authority that rudiments of teeth can
be detected in the beaks of certain embryonic birds. Nothing can be plainer
than that wings are formed for flight, yet in how many insects do we see
wings so reduced in size as to be utterly incapable of flight, and not rarely
lying under wing-cases, firmly soldered together!
The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same species)
resembling each other most closely in all respects, one of which will have
full-sized wings, and another mere rudiments of membrane; and here it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality, and are merely not developed:
this seems to be the case with the mammæ of male mammals, for many
instances are on record of these organs having become well developed in
full-grown males, and having secreted milk. So again there are normally
four developed and two rudimentary teats in the udders of the genus Bos,
but in our domestic cows the two sometimes become developed and give
milk. In individual plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In plants with
separated sexes, the male flowers often have a rudiment of a pistil; and
Kölreuter found that by crossing such male plants with an hermaphrodite
species, the rudiment of the pistil in the hybrid offspring was much

increased in size; and this shows that the rudiment and the perfect pistil are
essentially alike in nature.
An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is to allow the
pollen-tubes to reach the ovules protected in the ovarium at its base. The
pistil consists of a stigma supported on the style; but in some Compositæ,
the male florets, which of course cannot be fecundated, have a pistil, which
is in a rudimentary state, for it is not crowned with a stigma; but the style
remains well developed, and is clothed with hairs as in other compositæ, for
the purpose of brushing the pollen out of the surrounding anthers. Again, an
organ may become rudimentary for its proper purpose, and be used for a
distinct object: in certain fish the swim-bladder seems to be rudimentary for
its proper function of giving buoyancy, but has become converted into a
nascent breathing organ or lung. Other similar instances could be given.
Rudimentary organs in the individuals of the same species are very liable
to vary in degree of development and in other respects. Moreover, in closely
allied species, the degree to which the same organ has been rendered
rudimentary occasionally differs much. This latter fact is well exemplified
in the state of the wings of the female moths in certain groups. Rudimentary
organs may be utterly aborted; and this implies, that we find in an animal or
plant no trace of an organ, which analogy would lead us to expect to find,
and which is occasionally found in monstrous individuals of the species.
Thus in the snapdragon (antirrhinum) we generally do not find a rudiment
of a fifth stamen; but this may sometimes be seen. In tracing the
homologies of the same part in different members of a class, nothing is
more common, or more necessary, than the use and discovery of rudiments.
This is well shown in the drawings given by Owen of the bones of the leg
of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterwards wholly disappear. It is also, I believe, a universal rule, that a
rudimentary part or organ is of greater size relatively to the adjoining parts
in the embryo, than in the adult; so that the organ at this early age is less
rudimentary, or even cannot be said to be in any degree rudimentary. Hence,

also, a rudimentary organ in the adult, is often said to have retained its
embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment: for the
same reasoning power which tells us plainly that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness that
these rudimentary or atrophied organs, are imperfect and useless. In works
on natural history rudimentary organs are generally said to have been
created “for the sake of symmetry,” or in order “to complete the scheme of
nature;” but this seems to me no explanation, merely a restatement of the
fact. Would it be thought sufficient to say that because planets revolve in
elliptic courses round the sun, satellites follow the same course round the
planets, for the sake of symmetry, and to complete the scheme of nature?
An eminent physiologist accounts for the presence of rudimentary organs,
by supposing that they serve to excrete matter in excess, or injurious to the
system; but can we suppose that the minute papilla, which often represents
the pistil in male flowers, and which is formed merely of cellular tissue, can
thus act? Can we suppose that the formation of rudimentary teeth which are
subsequently absorbed, can be of any service to the rapidly growing
embryonic calf by the excretion of precious phosphate of lime? When a
man’s fingers have been amputated, imperfect nails sometimes appear on
the stumps: I could as soon believe that these vestiges of nails have
appeared, not from unknown laws of growth, but in order to excrete horny
matter, as that the rudimentary nails on the fin of the manatee were formed
for this purpose.
On my view of descent with modification, the origin of rudimentary
organs is simple. We have plenty of cases of rudimentary organs in our
domestic productions,—as the stump of a tail in tailless breeds,—the
vestige of an ear in earless breeds,—the reappearance of minute dangling
horns in hornless breeds of cattle, more especially, according to Youatt, in
young animals,—and the state of the whole flower in the cauliflower. We
often see rudiments of various parts in monsters. But I doubt whether any of
these cases throw light on the origin of rudimentary organs in a state of
nature, further than by showing that rudiments can be produced; for I doubt
whether species under nature ever undergo abrupt changes. I believe that
disuse has been the main agency; that it has led in successive generations to
the gradual reduction of various organs, until they have become

rudimentary,—as in the case of the eyes of animals inhabiting dark caverns,
and of the wings of birds inhabiting oceanic islands, which have seldom
been forced to take flight, and have ultimately lost the power of flying.
Again, an organ useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small and exposed
islands; and in this case natural selection would continue slowly to reduce
the organ, until it was rendered harmless and rudimentary.
Any change in function, which can be effected by insensibly small steps,
is within the power of natural selection; so that an organ rendered, during
changed habits of life, useless or injurious for one purpose, might easily be
modified and used for another purpose. Or an organ might be retained for
one alone of its former functions. An organ, when rendered useless, may
well be variable, for its variations cannot be checked by natural selection.
At whatever period of life disuse or selection reduces an organ, and this will
generally be when the being has come to maturity and to its full powers of
action, the principle of inheritance at corresponding ages will reproduce the
organ in its reduced state at the same age, and consequently will seldom
affect or reduce it in the embryo. Thus we can understand the greater
relative size of rudimentary organs in the embryo, and their lesser relative
size in the adult. But if each step of the process of reduction were to be
inherited, not at the corresponding age, but at an extremely early period of
life (as we have good reason to believe to be possible) the rudimentary part
would tend to be wholly lost, and we should have a case of complete
abortion. The principle, also, of economy, explained in a former chapter, by
which the materials forming any part or structure, if not useful to the
possessor, will be saved as far as is possible, will probably often come into
play; and this will tend to cause the entire obliteration of a rudimentary
organ.
As the presence of rudimentary organs is thus due to the tendency in
every part of the organisation, which has long existed, to be inherited—we
can understand, on the genealogical view of classification, how it is that
systematists have found rudimentary parts as useful as, or even sometimes
more useful than, parts of high physiological importance. Rudimentary
organs may be compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which serve as a clue
in seeking for its derivation. On the view of descent with modification, we
may conclude that the existence of organs in a rudimentary, imperfect, and

useless condition, or quite aborted, far from presenting a strange difficulty,
as they assuredly do on the ordinary doctrine of creation, might even have
been anticipated, and can be accounted for by the laws of inheritance.

Summary.—In this chapter I have attempted to show, that the
subordination of group to group in all organisms throughout all time; that
the nature of the relationship, by which all living and extinct beings are
united by complex, radiating, and circuitous lines of affinities into one
grand system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters, if constant
and prevalent, whether of high vital importance, or of the most trifling
importance, or, as in rudimentary organs, of no importance; the wide
opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules;—all naturally follow on the
view of the common parentage of those forms which are considered by
naturalists as allied, together with their modification through natural
selection, with its contingencies of extinction and divergence of character.
In considering this view of classification, it should be borne in mind that the
element of descent has been universally used in ranking together the sexes,
ages, and acknowledged varieties of the same species, however different
they may be in structure. If we extend the use of this element of descent,—
the only certainly known cause of similarity in organic beings,—we shall
understand what is meant by the natural system: it is genealogical in its
attempted arrangement, with the grades of acquired difference marked by
the terms varieties, species, genera, families, orders, and classes.
On this same view of descent with modification, all the great facts in
Morphology become intelligible,—whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on the
same pattern in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited at a
corresponding period, we can understand the great leading facts in
Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from
each other in structure and function; and the resemblance in different

species of a class of the homologous parts or organs, though fitted in the
adult members for purposes as different as possible. Larvæ are active
embryos, which have become specially modified in relation to their habits
of life, through the principle of modifications being inherited at
corresponding ages. On this same principle—and bearing in mind, that
when organs are reduced in size, either from disuse or selection, it will
generally be at that period of life when the being has to provide for its own
wants, and bearing in mind how strong is the principle of inheritance—the
occurrence of rudimentary organs and their final abortion, present to us no
inexplicable difficulties; on the contrary, their presence might have been
even anticipated. The importance of embryological characters and of
rudimentary organs in classification is intelligible, on the view that an
arrangement is only so far natural as it is genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families of organic beings, with which this world is peopled,
have all descended, each within its own class or group, from common
parents, and have all been modified in the course of descent, that I should
without hesitation adopt this view, even if it were unsupported by other
facts or arguments.

CHAPTER XIV.
RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural
Selection. Recapitulation of the general and special
circumstances in its favour. Causes of the general belief in
the immutability of species. How far the theory of natural
selection may be extended. Effects of its adoption on the
study of Natural history. Concluding remarks.

As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I have
endeavoured to give to them their full force. Nothing at first can appear
more difficult to believe than that the more complex organs and instincts
should have been perfected, not by means superior to, though analogous
with, human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot be
considered real if we admit the following propositions, namely,—that
gradations in the perfection of any organ or instinct, which we may
consider, either do now exist or could have existed, each good of its kind,—
that all organs and instincts are, in ever so slight a degree, variable,—and,
lastly, that there is a struggle for existence leading to the preservation of
each profitable deviation of structure or instinct. The truth of these
propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially amongst broken and
failing groups of organic beings; but we see so many strange gradations in

nature, as is proclaimed by the canon, “Natura non facit saltum,” that we
ought to be extremely cautious in saying that any organ or instinct, or any
whole being, could not have arrived at its present state by many graduated
steps. There are, it must be admitted, cases of special difficulty on the
theory of natural selection; and one of the most curious of these is the
existence of two or three defined castes of workers or sterile females in the
same community of ants; but I have attempted to show how this difficulty
can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter, which seem
to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two trees to be grafted together, but
that it is incidental on constitutional differences in the reproductive systems
of the intercrossed species. We see the truth of this conclusion in the vast
difference in the result, when the same two species are crossed reciprocally;
that is, when one species is first used as the father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel offspring
cannot be considered as universal; nor is their very general fertility
surprising when we remember that it is not likely that either their
constitutions or their reproductive systems should have been profoundly
modified. Moreover, most of the varieties which have been experimentised
on have been produced under domestication; and as domestication
apparently tends to eliminate sterility, we ought not to expect it also to
produce sterility.
The sterility of hybrids is a very different case from that of first crosses,
for their reproductive organs are more or less functionally impotent;
whereas in first crosses the organs on both sides are in a perfect condition.
As we continually see that organisms of all kinds are rendered in some
degree sterile from their constitutions having been disturbed by slightly
different and new conditions of life, we need not feel surprise at hybrids
being in some degree sterile, for their constitutions can hardly fail to have
been disturbed from being compounded of two distinct organisations. This
parallelism is supported by another parallel, but directly opposite, class of
facts; namely, that the vigour and fertility of all organic beings are increased

by slight changes in their conditions of life, and that the offspring of slightly
modified forms or varieties acquire from being crossed increased vigour
and fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of life and
crosses between less modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are grave enough. All the individuals of
the same species, and all the species of the same genus, or even higher
group, must have descended from common parents; and therefore, in
however distant and isolated parts of the world they are now found, they
must in the course of successive generations have passed from some one
part to the others. We are often wholly unable even to conjecture how this
could have been effected. Yet, as we have reason to believe that some
species have retained the same specific form for very long periods,
enormously long as measured by years, too much stress ought not to be laid
on the occasional wide diffusion of the same species; for during very long
periods of time there will always be a good chance for wide migration by
many means. A broken or interrupted range may often be accounted for by
the extinction of the species in the intermediate regions. It cannot be denied
that we are as yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern periods;
and such changes will obviously have greatly facilitated migration. As an
example, I have attempted to show how potent has been the influence of the
Glacial period on the distribution both of the same and of representative
species throughout the world. We are as yet profoundly ignorant of the
many occasional means of transport. With respect to distinct species of the
same genus inhabiting very distant and isolated regions, as the process of
modification has necessarily been slow, all the means of migration will
have been possible during a very long period; and consequently the
difficulty of the wide diffusion of species of the same genus is in some
degree lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our present varieties, it may be asked,
Why do we not see these linking forms all around us? Why are not all
organic beings blended together in an inextricable chaos? With respect to

existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover directly connecting links between
them, but only between each and some extinct and supplanted form. Even
on a wide area, which has during a long period remained continuous, and of
which the climate and other conditions of life change insensibly in going
from a district occupied by one species into another district occupied by a
closely allied species, we have no just right to expect often to find
intermediate varieties in the intermediate zone. For we have reason to
believe that only a few species are undergoing change at any one period;
and all changes are slowly effected. I have also shown that the intermediate
varieties which will at first probably exist in the intermediate zones, will be
liable to be supplanted by the allied forms on either hand; and the latter,
from existing in greater numbers, will generally be modified and improved
at a quicker rate than the intermediate varieties, which exist in lesser
numbers; so that the intermediate varieties will, in the long run, be
supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? We meet with no such evidence, and this is
the most obvious and forcible of the many objections which may be urged
against my theory. Why, again, do whole groups of allied species appear,
though certainly they often falsely appear, to have come in suddenly on the
several geological stages? Why do we not find great piles of strata beneath
the Silurian system, stored with the remains of the progenitors of the
Silurian groups of fossils? For certainly on my theory such strata must
somewhere have been deposited at these ancient and utterly unknown
epochs in the world’s history.
I can answer these questions and grave objections only on the supposition
that the geological record is far more imperfect than most geologists
believe. It cannot be objected that there has not been time sufficient for any
amount of organic change; for the lapse of time has been so great as to be
utterly inappreciable by the human intellect. The number of specimens in all
our museums is absolutely as nothing compared with the countless
generations of countless species which certainly have existed. We should

not be able to recognise a species as the parent of any one or more species if
we were to examine them ever so closely, unless we likewise possessed
many of the intermediate links between their past or parent and present
states; and these many links we could hardly ever expect to discover, owing
to the imperfection of the geological record. Numerous existing doubtful
forms could be named which are probably varieties; but who will pretend
that in future ages so many fossil links will be discovered, that naturalists
will be able to decide, on the common view, whether or not these doubtful
forms are varieties? As long as most of the links between any two species
are unknown, if any one link or intermediate variety be discovered, it will
simply be classed as another and distinct species. Only a small portion of
the world has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great number.
Widely ranging species vary most, and varieties are often at first local,—
both causes rendering the discovery of intermediate links less likely. Local
varieties will not spread into other and distant regions until they are
considerably modified and improved; and when they do spread, if
discovered in a geological formation, they will appear as if suddenly
created there, and will be simply classed as new species. Most formations
have been intermittent in their accumulation; and their duration, I am
inclined to believe, has been shorter than the average duration of specific
forms. Successive formations are separated from each other by enormous
blank intervals of time; for fossiliferous formations, thick enough to resist
future degradation, can be accumulated only where much sediment is
deposited on the subsiding bed of the sea. During the alternate periods of
elevation and of stationary level the record will be blank. During these latter
periods there will probably be more variability in the forms of life; during
periods of subsidence, more extinction.
With respect to the absence of fossiliferous formations beneath the lowest
Silurian strata, I can only recur to the hypothesis given in the ninth chapter.
That the geological record is imperfect all will admit; but that it is imperfect
to the degree which I require, few will be inclined to admit. If we look to
long enough intervals of time, geology plainly declares that all species have
changed; and they have changed in the manner which my theory requires,
for they have changed slowly and in a graduated manner. We clearly see
this in the fossil remains from consecutive formations invariably being

much more closely related to each other, than are the fossils from
formations distant from each other in time.
Such is the sum of the several chief objections and difficulties which may
justly be urged against my theory; and I have now briefly recapitulated the
answers and explanations which can be given to them. I have felt these
difficulties far too heavily during many years to doubt their weight. But it
deserves especial notice that the more important objections relate to
questions on which we are confessedly ignorant; nor do we know how
ignorant we are. We do not know all the possible transitional gradations
between the simplest and the most perfect organs; it cannot be pretended
that we know all the varied means of Distribution during the long lapse of
years, or that we know how imperfect the Geological Record is. Grave as
these several difficulties are, in my judgment they do not overthrow the
theory of descent with modification.

Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the reproductive
system being eminently susceptible to changes in the conditions of life; so
that this system, when not rendered impotent, fails to reproduce offspring
exactly like the parent-form. Variability is governed by many complex laws,
—by correlation of growth, by use and disuse, and by the direct action of
the physical conditions of life. There is much difficulty in ascertaining how
much modification our domestic productions have undergone; but we may
safely infer that the amount has been large, and that modifications can be
inherited for long periods. As long as the conditions of life remain the same,
we have reason to believe that a modification, which has already been
inherited for many generations, may continue to be inherited for an almost
infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently domesticated
productions.
Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts on the
organisation, and causes variability. But man can and does select the
variations given to him by nature, and thus accumulate them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.

He may do this methodically, or he may do it unconsciously by preserving
the individuals most useful to him at the time, without any thought of
altering the breed. It is certain that he can largely influence the character of
a breed by selecting, in each successive generation, individual differences
so slight as to be quite inappreciable by an uneducated eye. This process of
selection has been the great agency in the production of the most distinct
and useful domestic breeds. That many of the breeds produced by man have
to a large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or aboriginal
species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In the
preservation of favoured individuals and races, during the constantly-
recurrent Struggle for Existence, we see the most powerful and ever-acting
means of selection. The struggle for existence inevitably follows from the
high geometrical ratio of increase which is common to all organic beings.
This high rate of increase is proved by calculation, by the effects of a
succession of peculiar seasons, and by the results of naturalisation, as
explained in the third chapter. More individuals are born than can possibly
survive. A grain in the balance will determine which individual shall live
and which shall die,—which variety or species shall increase in number,
and which shall decrease, or finally become extinct. As the individuals of
the same species come in all respects into the closest competition with each
other, the struggle will generally be most severe between them; it will be
almost equally severe between the varieties of the same species, and next in
severity between the species of the same genus. But the struggle will often
be very severe between beings most remote in the scale of nature. The
slightest advantage in one being, at any age or during any season, over
those with which it comes into competition, or better adaptation in however
slight a degree to the surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled with
their conditions of life, will generally leave most progeny. But success will
often depend on having special weapons or means of defence, or on the
charms of the males; and the slightest advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected that organic beings would have varied
under nature, in the same way as they generally have varied under the
changed conditions of domestication. And if there be any variability under
nature, it would be an unaccountable fact if natural selection had not come
into play. It has often been asserted, but the assertion is quite incapable of
proof, that the amount of variation under nature is a strictly limited quantity.
Man, though acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere individual
differences in his domestic productions; and every one admits that there are
at least individual differences in species under nature. But, besides such
differences, all naturalists have admitted the existence of varieties, which
they think sufficiently distinct to be worthy of record in systematic works.
No one can draw any clear distinction between individual differences and
slight varieties; or between more plainly marked varieties and sub-species,
and species. Let it be observed how naturalists differ in the rank which they
assign to the many representative forms in Europe and North America.
If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any way
useful to beings, under their excessively complex relations of life, would be
preserved, accumulated, and inherited? Why, if man can by patience select
variations most useful to himself, should nature fail in selecting variations
useful, under changing conditions of life, to her living products? What limit
can be put to this power, acting during long ages and rigidly scrutinising the
whole constitution, structure, and habits of each creature,—favouring the
good and rejecting the bad? I can see no limit to this power, in slowly and
beautifully adapting each form to the most complex relations of life. The
theory of natural selection, even if we looked no further than this, seems to
me to be in itself probable. I have already recapitulated, as fairly as I could,
the opposed difficulties and objections: now let us turn to the special facts
and arguments in favour of the theory.
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see why it is
that no line of demarcation can be drawn between species, commonly
supposed to have been produced by special acts of creation, and varieties
which are acknowledged to have been produced by secondary laws. On this
same view we can understand how it is that in each region where many

species of a genus have been produced, and where they now flourish, these
same species should present many varieties; for where the manufactory of
species has been active, we might expect, as a general rule, to find it still in
action; and this is the case if varieties be incipient species. Moreover, the
species of the larger genera, which afford the greater number of varieties or
incipient species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of the larger
genera apparently have restricted ranges, and they are clustered in little
groups round other species—in which respects they resemble varieties.
These are strange relations on the view of each species having been
independently created, but are intelligible if all species first existed as
varieties.
As each species tends by its geometrical ratio of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by so much the more as they become more
diversified in habits and structure, so as to be enabled to seize on many and
widely different places in the economy of nature, there will be a constant
tendency in natural selection to preserve the most divergent offspring of any
one species. Hence during a long-continued course of modification, the
slight differences, characteristic of varieties of the same species, tend to be
augmented into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and exterminate
the older, less improved and intermediate varieties; and thus species are
rendered to a large extent defined and distinct objects. Dominant species
belonging to the larger groups tend to give birth to new and dominant
forms; so that each large group tends to become still larger, and at the same
time more divergent in character. But as all groups cannot thus succeed in
increasing in size, for the world would not hold them, the more dominant
groups beat the less dominant. This tendency in the large groups to go on
increasing in size and diverging in character, together with the almost
inevitable contingency of much extinction, explains the arrangement of all
the forms of life, in groups subordinate to groups, all within a few great
classes, which we now see everywhere around us, and which has prevailed
throughout all time. This grand fact of the grouping of all organic beings
seems to me utterly inexplicable on the theory of creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification; it can
act only by very short and slow steps. Hence the canon of “Natura non facit
saltum,” which every fresh addition to our knowledge tends to make more
strictly correct, is on this theory simply intelligible. We can plainly see why
nature is prodigal in variety, though niggard in innovation. But why this
should be a law of nature if each species has been independently created, no
man can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have been
created to prey on insects on the ground; that upland geese, which never or
rarely swim, should have been created with webbed feet; that a thrush
should have been created to dive and feed on sub-aquatic insects; and that a
petrel should have been created with habits and structure fitting it for the
life of an auk or grebe! and so on in endless other cases. But on the view of
each species constantly trying to increase in number, with natural selection
always ready to adapt the slowly varying descendants of each to any
unoccupied or ill-occupied place in nature, these facts cease to be strange,
or perhaps might even have been anticipated.
As natural selection acts by competition, it adapts the inhabitants of each
country only in relation to the degree of perfection of their associates; so
that we need feel no surprise at the inhabitants of any one country, although
on the ordinary view supposed to have been specially created and adapted
for that country, being beaten and supplanted by the naturalised productions
from another land. Nor ought we to marvel if all the contrivances in nature
be not, as far as we can judge, absolutely perfect; and if some of them be
abhorrent to our ideas of fitness. We need not marvel at the sting of the bee
causing the bee’s own death; at drones being produced in such vast numbers
for one single act, and being then slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fir-trees; at the instinctive hatred of the
queen bee for her own fertile daughters; at ichneumonidæ feeding within
the live bodies of caterpillars; and at other such cases. The wonder indeed
is, on the theory of natural selection, that more cases of the want of absolute
perfection have not been observed.
The complex and little known laws governing variation are the same, as
far as we can see, with the laws which have governed the production of so-

called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone, they
occasionally assume some of the characters of the species proper to that
zone. In both varieties and species, use and disuse seem to have produced
some effect; for it is difficult to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings incapable of flight, in
nearly the same condition as in the domestic duck; or when we look at the
burrowing tucutucu, which is occasionally blind, and then at certain moles,
which are habitually blind and have their eyes covered with skin; or when
we look at the blind animals inhabiting the dark caves of America and
Europe. In both varieties and species correlation of growth seems to have
played a most important part, so that when one part has been modified other
parts are necessarily modified. In both varieties and species reversions to
long-lost characters occur. How inexplicable on the theory of creation is the
occasional appearance of stripes on the shoulder and legs of the several
species of the horse-genus and in their hybrids! How simply is this fact
explained if we believe that these species have descended from a striped
progenitor, in the same manner as the several domestic breeds of pigeon
have descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently created,
why should the specific characters, or those by which the species of the
same genus differ from each other, be more variable than the generic
characters in which they all agree? Why, for instance, should the colour of a
flower be more likely to vary in any one species of a genus, if the other
species, supposed to have been created independently, have differently
coloured flowers, than if all the species of the genus have the same coloured
flowers? If species are only well-marked varieties, of which the characters
have become in a high degree permanent, we can understand this fact; for
they have already varied since they branched off from a common progenitor
in certain characters, by which they have come to be specifically distinct
from each other; and therefore these same characters would be more likely
still to be variable than the generic characters which have been inherited
without change for an enormous period. It is inexplicable on the theory of
creation why a part developed in a very unusual manner in any one species
of a genus, and therefore, as we may naturally infer, of great importance to
the species, should be eminently liable to variation; but, on my view, this
part has undergone, since the several species branched off from a common

progenitor, an unusual amount of variability and modification, and therefore
we might expect this part generally to be still variable. But a part may be
developed in the most unusual manner, like the wing of a bat, and yet not be
more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long period; for
in this case it will have been rendered constant by long-continued natural
selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural selection
of successive, slight, but profitable modifications. We can thus understand
why nature moves by graduated steps in endowing different animals of the
same class with their several instincts. I have attempted to show how much
light the principle of gradation throws on the admirable architectural
powers of the hive-bee. Habit no doubt sometimes comes into play in
modifying instincts; but it certainly is not indispensable, as we see, in the
case of neuter insects, which leave no progeny to inherit the effects of long-
continued habit. On the view of all the species of the same genus having
descended from a common parent, and having inherited much in common,
we can understand how it is that allied species, when placed under
considerably different conditions of life, yet should follow nearly the same
instincts; why the thrush of South America, for instance, lines her nest with
mud like our British species. On the view of instincts having been slowly
acquired through natural selection we need not marvel at some instincts
being apparently not perfect and liable to mistakes, and at many instincts
causing other animals to suffer.
If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents,—in being absorbed
into each other by successive crosses, and in other such points,—as do the
crossed offspring of acknowledged varieties. On the other hand, these
would be strange facts if species have been independently created, and
varieties have been produced by secondary laws.
If we admit that the geological record is imperfect in an extreme degree,
then such facts as the record gives, support the theory of descent with
modification. New species have come on the stage slowly and at successive
intervals; and the amount of change, after equal intervals of time, is widely

different in different groups. The extinction of species and of whole groups
of species, which has played so conspicuous a part in the history of the
organic world, almost inevitably follows on the principle of natural
selection; for old forms will be supplanted by new and improved forms.
Neither single species nor groups of species reappear when the chain of
ordinary generation has once been broken. The gradual diffusion of
dominant forms, with the slow modification of their descendants, causes the
forms of life, after long intervals of time, to appear as if they had changed
simultaneously throughout the world. The fact of the fossil remains of each
formation being in some degree intermediate in character between the
fossils in the formations above and below, is simply explained by their
intermediate position in the chain of descent. The grand fact that all extinct
organic beings belong to the same system with recent beings, falling either
into the same or into intermediate groups, follows from the living and the
extinct being the offspring of common parents. As the groups which have
descended from an ancient progenitor have generally diverged in character,
the progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can see why
the more ancient a fossil is, the oftener it stands in some degree
intermediate between existing and allied groups. Recent forms are generally
looked at as being, in some vague sense, higher than ancient and extinct
forms; and they are in so far higher as the later and more improved forms
have conquered the older and less improved organic beings in the struggle
for life. Lastly, the law of the long endurance of allied forms on the same
continent,—of marsupials in Australia, of edentata in America, and other
such cases,—is intelligible, for within a confined country, the recent and the
extinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading facts
in Distribution. We can see why there should be so striking a parallelism in
the distribution of organic beings throughout space, and in their geological
succession throughout time; for in both cases the beings have been
connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful

fact, which must have struck every traveller, namely, that on the same
continent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on deserts and marshes, most of the inhabitants
within each great class are plainly related; for they will generally be
descendants of the same progenitors and early colonists. On this same
principle of former migration, combined in most cases with modification,
we can understand, by the aid of the Glacial period, the identity of some
few plants, and the close alliance of many others, on the most distant
mountains, under the most different climates; and likewise the close
alliance of some of the inhabitants of the sea in the northern and southern
temperate zones, though separated by the whole intertropical ocean.
Although two areas may present the same physical conditions of life, we
need feel no surprise at their inhabitants being widely different, if they have
been for a long period completely separated from each other; for as the
relation of organism to organism is the most important of all relations, and
as the two areas will have received colonists from some third source or
from each other, at various periods and in different proportions, the course
of modification in the two areas will inevitably be different.
On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that many
should be peculiar. We can clearly see why those animals which cannot
cross wide spaces of ocean, as frogs and terrestrial mammals, should not
inhabit oceanic islands; and why, on the other hand, new and peculiar
species of bats, which can traverse the ocean, should so often be found on
islands far distant from any continent. Such facts as the presence of peculiar
species of bats, and the absence of all other mammals, on oceanic islands,
are utterly inexplicable on the theory of independent acts of creation.
The existence of closely allied or representative species in any two areas,
implies, on the theory of descent with modification, that the same parents
formerly inhabited both areas; and we almost invariably find that wherever
many closely allied species inhabit two areas, some identical species
common to both still exist. Wherever many closely allied yet distinct
species occur, many doubtful forms and varieties of the same species
likewise occur. It is a rule of high generality that the inhabitants of each
area are related to the inhabitants of the nearest source whence immigrants
might have been derived. We see this in nearly all the plants and animals of
the Galapagos archipelago, of Juan Fernandez, and of the other American

islands being related in the most striking manner to the plants and animals
of the neighbouring American mainland; and those of the Cape de Verde
archipelago and other African islands to the African mainland. It must be
admitted that these facts receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group, and
with extinct groups often falling in between recent groups, is intelligible on
the theory of natural selection with its contingencies of extinction and
divergence of character. On these same principles we see how it is, that the
mutual affinities of the species and genera within each class are so complex
and circuitous. We see why certain characters are far more serviceable than
others for classification;—why adaptive characters, though of paramount
importance to the being, are of hardly any importance in classification; why
characters derived from rudimentary parts, though of no service to the
being, are often of high classificatory value; and why embryological
characters are the most valuable of all. The real affinities of all organic
beings are due to inheritance or community of descent. The natural system
is a genealogical arrangement, in which we have to discover the lines of
descent by the most permanent characters, however slight their vital
importance may be.
The framework of bones being the same in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse,—the same number of vertebræ
forming the neck of the giraffe and of the elephant,—and innumerable other
such facts, at once explain themselves on the theory of descent with slow
and slight successive modifications. The similarity of pattern in the wing
and leg of a bat, though used for such different purpose,—in the jaws and
legs of a crab,—in the petals, stamens, and pistils of a flower, is likewise
intelligible on the view of the gradual modification of parts or organs,
which were alike in the early progenitor of each class. On the principle of
successive variations not always supervening at an early age, and being
inherited at a corresponding not early period of life, we can clearly see why
the embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling at
the embryo of an air-breathing mammal or bird having branchial slits and
arteries running in loops, like those in a fish which has to breathe the air
dissolved in water, by the aid of well-developed branchiæ.

Disuse, aided sometimes by natural selection, will often tend to reduce an
organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the meaning
of rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in the
struggle for existence, and will thus have little power of acting on an organ
during early life; hence the organ will not be much reduced or rendered
rudimentary at this early age. The calf, for instance, has inherited teeth,
which never cut through the gums of the upper jaw, from an early
progenitor having well-developed teeth; and we may believe, that the teeth
in the mature animal were reduced, during successive generations, by
disuse or by the tongue and palate having been fitted by natural selection to
browse without their aid; whereas in the calf, the teeth have been left
untouched by selection or disuse, and on the principle of inheritance at
corresponding ages have been inherited from a remote period to the present
day. On the view of each organic being and each separate organ having
been specially created, how utterly inexplicable it is that parts, like the teeth
in the embryonic calf or like the shrivelled wings under the soldered wing-
covers of some beetles, should thus so frequently bear the plain stamp of
inutility! Nature may be said to have taken pains to reveal, by rudimentary
organs and by homologous structures, her scheme of modification, which it
seems that we wilfully will not understand.

I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability of
species? It cannot be asserted that organic beings in a state of nature are
subject to no variation; it cannot be proved that the amount of variation in
the course of long ages is a limited quantity; no clear distinction has been,
or can be, drawn between species and well-marked varieties. It cannot be
maintained that species when intercrossed are invariably sterile, and
varieties invariably fertile; or that sterility is a special endowment and sign
of creation. The belief that species were immutable productions was almost
unavoidable as long as the history of the world was thought to be of short

duration; and now that we have acquired some idea of the lapse of time, we
are too apt to assume, without proof, that the geological record is so perfect
that it would have afforded us plain evidence of the mutation of species, if
they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow in
admitting any great change of which we do not see the intermediate steps.
The difficulty is the same as that felt by so many geologists, when Lyell
first insisted that long lines of inland cliffs had been formed, and great
valleys excavated, by the slow action of the coast-waves. The mind cannot
possibly grasp the full meaning of the term of a hundred million years; it
cannot add up and perceive the full effects of many slight variations,
accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of facts
all viewed, during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under such expressions
as the “plan of creation,” “unity of design,” etc., and to think that we give
an explanation when we only restate a fact. Any one whose disposition
leads him to attach more weight to unexplained difficulties than to the
explanation of a certain number of facts will certainly reject my theory. A
few naturalists, endowed with much flexibility of mind, and who have
already begun to doubt on the immutability of species, may be influenced
by this volume; but I look with confidence to the future, to young and rising
naturalists, who will be able to view both sides of the question with
impartiality. Whoever is led to believe that species are mutable will do good
service by conscientiously expressing his conviction; for only thus can the
load of prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This seems
to me a strange conclusion to arrive at. They admit that a multitude of
forms, which till lately they themselves thought were special creations, and
which are still thus looked at by the majority of naturalists, and which
consequently have every external characteristic feature of true species,—

they admit that these have been produced by variation, but they refuse to
extend the same view to other and very slightly different forms.
Nevertheless they do not pretend that they can define, or even conjecture,
which are the created forms of life, and which are those produced by
secondary laws. They admit variation as a vera causa in one case, they
arbitrarily reject it in another, without assigning any distinction in the two
cases. The day will come when this will be given as a curious illustration of
the blindness of preconceived opinion. These authors seem no more startled
at a miraculous act of creation than at an ordinary birth. But do they really
believe that at innumerable periods in the earth’s history certain elemental
atoms have been commanded suddenly to flash into living tissues? Do they
believe that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants
created as eggs or seed, or as full grown? and in the case of mammals, were
they created bearing the false marks of nourishment from the mother’s
womb? Although naturalists very properly demand a full explanation of
every difficulty from those who believe in the mutability of species, on their
own side they ignore the whole subject of the first appearance of species in
what they consider reverent silence.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct the
forms are which we may consider, by so much the arguments fall away in
force. But some arguments of the greatest weight extend very far. All the
members of whole classes can be connected together by chains of affinities,
and all can be classified on the same principle, in groups subordinate to
groups. Fossil remains sometimes tend to fill up very wide intervals
between existing orders. Organs in a rudimentary condition plainly show
that an early progenitor had the organ in a fully developed state; and this in
some instances necessarily implies an enormous amount of modification in
the descendants. Throughout whole classes various structures are formed on
the same pattern, and at an embryonic age the species closely resemble each
other. Therefore I cannot doubt that the theory of descent with modification
embraces all the members of the same class. I believe that animals have
descended from at most only four or five progenitors, and plants from an
equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy

may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see this
even in so trifling a circumstance as that the same poison often similarly
affects plants and animals; or that the poison secreted by the gall-fly
produces monstrous growths on the wild rose or oak-tree. Therefore I
should infer from analogy that probably all the organic beings which have
ever lived on this earth have descended from some one primordial form,
into which life was first breathed.

When the views entertained in this volume on the origin of species, or
when analogous views are generally admitted, we can dimly foresee that
there will be a considerable revolution in natural history. Systematists will
be able to pursue their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be in essence a
species. This I feel sure, and I speak after experience, will be no slight
relief. The endless disputes whether or not some fifty species of British
brambles are true species will cease. Systematists will have only to decide
(not that this will be easy) whether any form be sufficiently constant and
distinct from other forms, to be capable of definition; and if definable,
whether the differences be sufficiently important to deserve a specific name.
This latter point will become a far more essential consideration than it is at
present; for differences, however slight, between any two forms, if not
blended by intermediate gradations, are looked at by most naturalists as
sufficient to raise both forms to the rank of species. Hereafter we shall be
compelled to acknowledge that the only distinction between species and
well-marked varieties is, that the latter are known, or believed, to be
connected at the present day by intermediate gradations, whereas species
were formerly thus connected. Hence, without quite rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that forms
now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names, as with the primrose and cowslip; and in
this case scientific and common language will come into accordance. In
short, we shall have to treat species in the same manner as those naturalists

treat genera, who admit that genera are merely artificial combinations made
for convenience. This may not be a cheering prospect; but we shall at least
be freed from the vain search for the undiscovered and undiscoverable
essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity, relationship,
community of type, paternity, morphology, adaptive characters, rudimentary
and aborted organs, etc., will cease to be metaphorical, and will have a plain
signification. When we no longer look at an organic being as a savage looks
at a ship, as at something wholly beyond his comprehension; when we
regard every production of nature as one which has had a history; when we
contemplate every complex structure and instinct as the summing up of
many contrivances, each useful to the possessor, nearly in the same way as
when we look at any great mechanical invention as the summing up of the
labour, the experience, the reason, and even the blunders of numerous
workmen; when we thus view each organic being, how far more interesting,
I speak from experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects of use
and disuse, on the direct action of external conditions, and so forth. The
study of domestic productions will rise immensely in value. A new variety
raised by man will be a far more important and interesting subject for study
than one more species added to the infinitude of already recorded species.
Our classifications will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of
creation. The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigrees or armorial
bearings; and we have to discover and trace the many diverging lines of
descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect
to the nature of long-lost structures. Species and groups of species, which
are called aberrant, and which may fancifully be called living fossils, will
aid us in forming a picture of the ancient forms of life. Embryology will
reveal to us the structure, in some degree obscured, of the prototypes of
each great class.

When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not very
remote period descended from one parent, and have migrated from some
one birthplace; and when we better know the many means of migration,
then, by the light which geology now throws, and will continue to throw, on
former changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the
inhabitants of the whole world. Even at present, by comparing the
differences of the inhabitants of the sea on the opposite sides of a continent,
and the nature of the various inhabitants of that continent in relation to their
apparent means of immigration, some light can be thrown on ancient
geography.
The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous formation
will be recognised as having depended on an unusual concurrence of
circumstances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with some
security the duration of these intervals by a comparison of the preceding
and succeeding organic forms. We must be cautious in attempting to
correlate as strictly contemporaneous two formations, which include few
identical species, by the general succession of their forms of life. As species
are produced and exterminated by slowly acting and still existing causes,
and not by miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism,—the improvement of
one being entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the lapse of actual time. A
number of species, however, keeping in a body might remain for a long
period unchanged, whilst within this same period, several of these species,
by migrating into new countries and coming into competition with foreign
associates, might become modified; so that we must not overrate the
accuracy of organic change as a measure of time. During early periods of
the earth’s history, when the forms of life were probably fewer and simpler,

the rate of change was probably slower; and at the first dawn of life, when
very few forms of the simplest structure existed, the rate of change may
have been slow in an extreme degree. The whole history of the world, as at
present known, although of a length quite incomprehensible by us, will
hereafter be recognised as a mere fragment of time, compared with the ages
which have elapsed since the first creature, the progenitor of innumerable
extinct and living descendants, was created.
In the distant future I see open fields for far more important researches.
Psychology will be based on a new foundation, that of the necessary
acquirement of each mental power and capacity by gradation. Light will be
thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of the
world should have been due to secondary causes, like those determining the
birth and death of the individual. When I view all beings not as special
creations, but as the lineal descendants of some few beings which lived long
before the first bed of the Silurian system was deposited, they seem to me to
become ennobled. Judging from the past, we may safely infer that not one
living species will transmit its unaltered likeness to a distant futurity. And of
the species now living very few will transmit progeny of any kind to a far
distant futurity; for the manner in which all organic beings are grouped,
shows that the greater number of species of each genus, and all the species
of many genera, have left no descendants, but have become utterly extinct.
We can so far take a prophetic glance into futurity as to foretel that it will be
the common and widely-spread species, belonging to the larger and
dominant groups, which will ultimately prevail and procreate new and
dominant species. As all the living forms of life are the lineal descendants
of those which lived long before the Silurian epoch, we may feel certain
that the ordinary succession by generation has never once been broken, and
that no cataclysm has desolated the whole world. Hence we may look with
some confidence to a secure future of equally inappreciable length. And as
natural selection works solely by and for the good of each being, all
corporeal and mental endowments will tend to progress towards perfection.

It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent on each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with Reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the external
conditions of life, and from use and disuse; a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less-improved
forms. Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur in this view of
life, with its several powers, having been originally breathed into a few
forms or into one; and that, whilst this planet has gone cycling on according
to the fixed law of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being, evolved.

INDEX.

Aberrant groups, 429.

Abyssinia, plants of, 375.

Acclimatisation, 139.

Affinities:
of extinct species, 329.
of organic beings, 411.

Agassiz:
on Amblyopsis, 139.
on groups of species suddenly appearing, 302, 305.
on embryological succession, 338.
on the glacial period, 366.
on embryological characters, 418.
on the embryos of vertebrata, 439.
on parallelism of embryological development and geological succession,
449.

Algæ of New Zealand, 376.

Alligators, males, fighting, 88.

Amblyopsis, blind fish, 139.

America, North:
productions allied to those of Europe, 371.
boulders and glaciers of, 373.

South, no modern formations on west coast, 290.

Ammonites, sudden extinction of, 321.

Anagallis, sterility of, 247.

Analogy of variations, 159.

Ancylus, 386.

Animals:
not domesticated from being variable, 17.
domestic, descended from several stocks, 19.
acclimatisation of, 141.
of Australia, 116.
with thicker fur in cold climates, 133.
blind, in caves, 137.
extinct, of Australia, 339.

Anomma, 240.

Antarctic islands, ancient flora of, 399.

Antirrhinum, 161.

Ants:
attending aphides, 211.
slave-making instinct, 219.

Ants, neuter, structure of, 236.

Aphides attended by ants, 211.

Aphis, development of, 442.

Apteryx, 182.

Arab horses, 35.

Aralo-Caspian Sea, 339.

Archiac, M. de, on the succession of species, 325.

Artichoke, Jerusalem, 142.

Ascension, plants of, 389.

Asclepias, pollen of, 193.

Asparagus, 359.

Aspicarpa, 417.

Asses, striped, 163.

Ateuchus, 135.

Audubon:
on habits of frigate-bird, 185.
on variation in birds’-nests, 212.
on heron eating seeds, 387.

Australia:
animals of, 116.
dogs of, 215.
extinct animals of, 339.
European plants in, 375.

Azara on flies destroying cattle, 72.

Azores, flora of, 363.

Babington, Mr., on British plants, 48.

Balancement of growth, 147.

Bamboo with hooks, 197.

Barberry, flowers of, 98.

Barrande, M.:
on Silurian colonies, 313.
on the succession of species, 325.
on parallelism of palæozoic formations, 328.
on affinities of ancient species, 330.

Barriers, importance of, 347.

Batrachians on islands, 393.

Bats:
how structure acquired, 180.
distribution of, 394.

Bear, catching water-insects, 184.

Bee:
sting of, 202.
queen, killing rivals, 202.

Bees fertilising flowers, 73.

Bees:
hive, not sucking the red clover, 95.
cell-making instinct, 224.
humble, cells of, 225.
parasitic, 218.

Beetles:
wingless, in Madeira, 135.
with deficient tarsi, 135.

Bentham, Mr.:
on British plants, 48.
on classification, 419.

Berkeley, Mr., on seeds in salt-water, 358.

Bermuda, birds of, 391.

Birds:
acquiring fear, 212.
annually cross the Atlantic, 364.
colour of, on continents, 132.
fossil, in caves of Brazil, 339.
of Madeira, Bermuda, and Galapagos, 390.
song of males, 89.
transporting seeds, 361.
waders, 386.
wingless, 134, 182.
with traces of embryonic teeth, 451.

Bizcacha, 349.
affinities of, 429.

Bladder for swimming in fish, 190.

Blindness of cave animals, 137,

Blyth, Mr.:
on distinctness of Indian cattle, 18.
on striped Hemionus, 163.
on crossed geese, 253.

Boar, shoulder-pad of, 88.

Borrow, Mr., on the Spanish pointer, 35.

Bory St. Vincent on Batrachians, 393.

Bosquet, M., on fossil Chthamalus, 304.

Boulders, erratic, on the Azores, 363.

Branchiæ, 190.

Brent, Mr.:
on house-tumblers, 214.
on hawks killing pigeons, 362.

Brewer, Dr., on American cuckoo, 217.

Britain, mammals of, 395.

Bronn on duration of specific forms, 293.

Brown, Robert, on classification, 414.

Buckman on variation in plants, 10.

Buzareingues on sterility of varieties, 270.

Cabbage, varieties of, crossed, 99.

Calceolaria, 251.

Canary-birds, sterility of hybrids, 252.

Cape de Verde islands, 398.

Cape of Good Hope, plants of, 110, 375.

Carrier-pigeons killed by hawks, 362.

Cassini on flowers of compositæ, 145.

Catasetum, 424.

Cats:
with blue eyes, deaf, 12.
variation in habits of, 91.
curling tail when going to spring, 201.

Cattle:
destroying fir-trees, 71.
destroyed by flies in La Plata, 72.
breeds of, locally extinct, 111.
fertility of Indian and European breeds, 254.

Cave, inhabitants of, blind, 137.

Centres of creation, 352.

Cephalopodæ, development of, 442.

Cervulus, 253.

Cetacea, teeth and hair, 144.

Ceylon, plants of, 375.

Chalk formation, 322.

Characters:
divergence of, 111.
sexual, variable, 156.
adaptive or analogical, 427.

Charlock, 76.

Checks:
to increase, 67.

mutual, 71.

Chickens, instinctive tameness of, 216.

Chthamalinæ, 288.

Chthamalus, cretacean species of, 304.

Circumstances favourable:
to selection of domestic products, 40.
to natural selection, 101.

Cirripedes:
capable of crossing, 101.
carapace aborted, 148.
their ovigerous frena, 192.
fossil, 304.
larvæ of, 440.

Classification, 413.

Clift, Mr., on the succession of types, 339.

Climate:
effects of, in checking increase of beings, 68.
adaptation of, to organisms, 139.

Cobites, intestine of, 190.

Cockroach, 76.

Collections, palæontological, poor, 287.

Colour:
influenced by climate, 132.
in relation to attacks by flies, 198.

Columba livia, parent of domestic pigeons, 23.

Colymbetes, 386.

Compensation of growth, 147.

Compositæ:
outer and inner florets of, 144.
male flowers of, 451.

Conclusion, general, 480.

Conditions, slight changes in, favourable to fertility, 267.

Coot, 185.

Coral:
islands, seeds drifted to, 360.
reefs, indicating movements of earth, 309.

Corn-crake, 185.

Correlation:
of growth in domestic productions, 11.
of growth, 143, 198.

Cowslip, 49.

Creation, single centres of, 352.

Crinum, 250.

Crosses, reciprocal, 258.

Crossing:
of domestic animals, importance in altering breeds, 20.
advantages of, 96.

unfavourable to selection, 102.

Crustacea of New Zealand, 376.

Crustacean, blind, 137.

Cryptocerus, 238.

Ctenomys, blind, 137.

Cuckoo, instinct of, 216.

Currants, grafts of, 262.

Currents of sea, rate of, 359.

Cuvier:
on conditions of existence, 206.
on fossil monkeys, 303.

Cuvier, Fred., on instinct, 208.

Dana, Professor:
on blind cave-animals, 139.
on relations of crustaceans of Japan, 372.
on crustaceans of New Zealand, 376.

De Candolle:
on struggle for existence, 62.
on umbelliferæ, 146.
on general affinities, 430.

De Candolle, Alph.:
on low plants, widely dispersed, 406.
on widely-ranging plants being variable, 53.
on naturalisation, 115.
on winged seeds, 146.

on Alpine species suddenly becoming rare, 175.
on distribution of plants with large seeds, 360.
on vegetation of Australia, 379.
on fresh-water plants, 386.
on insular plants, 389.

Degradation of coast-rocks, 282.

Denudation:
rate of, 285.
of oldest rocks, 308.

Development of ancient forms, 336.

Devonian system, 334.

Dianthus, fertility of crosses, 256.

Dirt on feet of birds, 362.

Dispersal:
means of, 356.
during glacial period, 365.

Distribution:
geographical, 346.
means of, 356.

Disuse, effects of, under nature, 134.

Divergence of character, 111.

Division, physiological, of labour, 115.

Dogs:
hairless, with imperfect teeth, 12.
descended from several wild stocks, 18.

domestic instincts of, 213.
inherited civilisation of, 215.
fertility of breeds together, 254.
of crosses, 268.
proportions of, when young, 444.

Domestication, variation under, 7.

Downing, Mr., on fruit-trees in America, 85.

Downs, North and South, 285.

Dragon-flies, intestines of, 190.

Drift-timber, 360.

Driver-ant, 240.

Drones killed by other bees, 202.

Duck:
domestic, wings of, reduced, 11.
logger-headed, 182.

Duckweed, 385.

Dugong, affinities of, 414.

Dung-beetles with deficient tarsi, 135.

Dyticus, 386.

Earl, Mr. W., on the Malay Archipelago, 395.

Ears:
drooping, in domestic animals, 11.
rudimentary, 454.

Earth, seeds in roots of trees, 361.

Eciton, 238.

Economy of organisation, 147.

Edentata:
teeth and hair, 144.
fossil species of, 339.

Edwards, Milne:
on physiological divisions of labour, 115.
on gradations of structure, 194.
on embryological characters, 418.

Eggs, young birds escaping from, 87.

Electric organs, 192.

Elephant:
rate of increase, 64.
of glacial period, 141.

Embryology, 439.

Existence:
struggle for, 60.
conditions of, 206.

Extinction:
as bearing on natural selection, 109.
of domestic varieties, 111.
317.

Eye:
structure of, 187.

correction for aberration, 202.

Eyes reduced in moles, 137.

Fabre, M., on parasitic sphex, 218.

Falconer, Dr.:
on naturalization of plants in India, 65.
on fossil crocodile, 313.
on elephants and mastodons, 334.
and Cautley on mammals of sub-Himalayan beds, 340.

Falkland Island, wolf of, 393.

Faults, 285.

Faunas, marine, 348.

Fear, instinctive, in birds, 212.

Feet of birds, young molluscs adhering to, 385.

Fertility:
of hybrids, 249.
from slight changes in conditions, 267.
of crossed varieties, 267.

Fir-trees:
destroyed by cattle, 71.
pollen of, 203.

Fish:
flying, 182.
teleostean, sudden appearance of, 305.
eating seeds, 362, 387.
fresh-water, distribution of, 384.

Fishes:
ganoid, now confined to fresh water, 107.
electric organs of, 192.
ganoid, living in fresh water, 321.
of southern hemisphere, 376.

Flight, powers of, how acquired, 182.

Flowers:
structure of, in relation to crossing, 97.
of compositæ and umbelliferæ, 144.

Forbes, E.:
on colours of shells, 132.
on abrupt range of shells in depth, 175.
on poorness of palæontological collections, 287.
on continuous succession of genera, 316.
on continental extensions, 357.
on distribution during glacial period, 366
on parallelism in time and space, 409.

Forests, changes in, in America, 74.

Formation, Devonian, 334.

Formations:
thickness of, in Britain, 284.
intermittent, 290.

Formica rufescens, 219.

Formica sanguinea, 219.

Formica flava, neuter of, 239.

Frena, ovigerous, of cirripedes, 192.

Fresh-water productions, dispersal of, 383.

Fries on species in large genera being closely allied to other species, 57.

Frigate-bird, 185.

Frogs on islands, 393.

Fruit-trees:
gradual improvement of, 37.
in United States, 85.
varieties of, acclimatised in United States, 142.

Fuci, crossed, 258.

Fur, thicker in cold climates, 133.

Furze, 439.

Galapagos Archipelago:
birds of, 390.
productions of, 398, 400.

Galeopithecus, 181.

Game, increase of, checked by vermin, 68.

Gärtner:
on sterility of hybrids, 247, 255.
on reciprocal crosses, 258.
on crossed maize and verbascum, 270.
on comparison of hybrids and mongrels, 272.

Geese:
fertility when crossed, 253.
upland, 185.

Genealogy important in classification, 425.

Geoffrey St. Hilaire:
on balancement, 147.
on homologous organs, 434.

Geoffrey St. Hilaire, Isidore:
on variability of repeated parts, 149.
on correlation in monstrosities, 11.
on correlation, 144.
on variable parts being often monstrous, 155.

Geographical distribution, 346.

Geography, ancient, 487.

Geology:
future progress of, 487.
imperfection of the record, 279.

Giraffe, tail of, 195.

Glacial period, 365.

Gmelin on distribution, 365.

Gnathodon, fossil, 368.

Godwin-Austen, Mr., on the Malay Archipelago, 299.

Goethe on compensation of growth, 147.

Gooseberry, grafts of, 262.

Gould, Dr. A., on land-shells, 397.

Gould, Mr.:

on colours of birds, 132.
on birds of the Galapagos, 398.
on distribution of genera of birds, 404.

Gourds, crossed, 270.

Grafts, capacity of, 261.

Grasses, varieties of, 113.

Gray, Dr. Asa:
on trees of United States, 100.
on naturalised plants in the United States, 115.
on rarity of intermediate varieties, 176.
on Alpine plants, 365.

Gray, Dr. J. E., on striped mule, 165.

Grebe, 185.

Groups, aberrant, 429.

Grouse:
colours of, 84.
red, a doubtful species, 49.

Growth:
compensation of, 147.
correlation of, in domestic products, 11.
correlation of, 143.

Habit:
effect of, under domestication, 11.
effect of, under nature, 134.
diversified, of same species, 183.

Hair and teeth, correlated, 144.

Harcourt, Mr. E. V., on the birds of Madeira, 391.

Hartung, M., on boulders in the Azores, 363.

Hazel-nuts, 359.

Hearne on habits of bears, 184.

Heath, changes in vegetation, 72,

Heer, O., on plants of Madeira, 107.

Helix pomatia, 397.

Helosciadium, 359.

Hemionus, striped, 163.

Herbert, W.:
on struggle for existence, 62.
on sterility of hybrids, 249.

Hermaphrodites crossing, 96.

Heron eating seed, 387.

Heron, Sir R., on peacocks, 89.

Heusinger on white animals not poisoned by certain plants, 12.

Hewitt, Mr., on sterility of first crosses, 264.

Himalaya:
glaciers of, 373.
plants of, 375.

Hippeastrum, 250.

Holly-trees, sexes of, 93.

Hollyhock, varieties of, crossed, 271.

Hooker, Dr., on trees of New Zealand, 100.

Hooker, Dr.:
on acclimatisation of Himalayan trees, 140.
on flowers of umbelliferæ, 145.
on glaciers of Himalaya, 373.
on algæ of New Zealand, 376.
on vegetation at the base of the Himalaya, 378.
on plants of Tierra del Fuego, 374, 378.
on Australian plants, 375, 399.
on relations of flora of South America, 379.
on flora of the Antarctic lands, 381, 399.
on the plants of the Galapagos, 391, 398.

Hooks:
on bamboos, 197.
to seeds on islands, 392.

Horner, Mr., on the antiquity of Egyptians, 18.

Horns, rudimentary, 454.

Horse, fossil, in La Plata, 318.

Horses:
destroyed by flies in La Plata, 72.
striped, 163.
proportions of, when young, 445.

Horticulturists, selection applied by, 32.

Huber on cells of bees, 230.

Huber, P.:
on reason blended with instinct, 208.
on habitual nature of instincts, 208.
on slave making ants, 219.
on Melipona domestica, 225.

Humble-bees, cells of, 225.

Hunter, J., on secondary sexual characters, 150.

Hutton, Captain, on crossed geese, 253.

Huxley, Professor:
on structure of hermaphrodites, 101.
on embryological succession, 338.
on homologous organs, 438.
on the development of aphis, 442.

Hybrids and mongrels compared, 272.

Hybridism, 245.

Hydra, structure of, 190.

Ibla, 148.

Icebergs transporting seeds, 363.

Increase, rate of, 63.

Individuals:
numbers favourable to selection, 102.
many, whether simultaneously created, 356.

Inheritance:

laws of, 12.
at corresponding ages, 14, 86.

Insects:
colour of, fitted for habitations, 84.
sea-side, colours of, 132.
blind, in caves, 138.
luminous, 193.
neuter, 236.

Instinct, 207.

Instincts, domestic, 213.

Intercrossing, advantages of, 96.

Islands, oceanic, 388.

Isolation favourable to selection, 104.

Japan, productions of, 372.

Java, plants of, 375.

Jones, Mr. J. M., on the birds of Bermuda, 391.

Jussieu on classification, 417.

Kentucky, caves of, 137.

Kerguelen-land, flora of, 381, 399.

Kidney-bean, acclimatisation of, 142.

Kidneys of birds, 144.

Kirby on tarsi deficient in beetles, 135.

Knight, Andrew, on cause of variation, 7.

Kölreuter:
on the barberry, 98.
on sterility of hybrids, 247.
on reciprocal crosses, 258.
on crossed varieties of nicotiana, 271.
on crossing male and hermaphrodite flowers, 451.

Lamarck on adaptive characters, 427.

Land-shells:
distribution of, 397.
of Madeira, naturalised, 402.

Languages, classification of, 422.

Lapse, great, of time, 282.

Larvæ, 440.

Laurel, nectar secreted by the leaves, 92.

Laws of variation, 131.

Leech, varieties of, 76.

Leguminosæ, nectar secreted by glands, 92.

Lepidosiren, 107, 330.

Life, struggle for, 60.

Lingula, Silurian, 306.

Linnæus, aphorism of, 413.

Lion:
mane of, 88.
young of, striped, 439.

Lobelia fulgens, 73, 98.

Lobelia, sterility of crosses, 250.

Loess of the Rhine, 384.

Lowness of structure connected with variability, 149.

Lowness, related to wide distribution, 406.

Lubbock, Mr., on the nerves of coccus, 46.

Lucas, Dr. P.:
on inheritance, 12.
on resemblance of child to parent, 275.

Lund and Clausen on fossils of Brazil, 339.

Lyell, Sir C.:
on the struggle for existence, 62.
on modern changes of the earth, 95.
on measure of denudation, 283.
on a carboniferous land-shell, 289.
on fossil whales, 303.
on strata beneath Silurian system, 307.
on the imperfection of the geological record, 310.
on the appearance of species, 312.
on Barrande’s colonies, 313.
on tertiary formations of Europe and North America, 323.
on parallelism of tertiary formations, 328.
on transport of seeds by icebergs, 363.
on great alternations of climate, 382.

on the distribution of fresh-water shells, 385.
on land-shells of Madeira, 402.

Lyell and Dawson on fossilized trees in Nova Scotia, 296.

Macleay on analogical characters, 427.

Madeira:
plants of, 107.
beetles of, wingless, 135.
fossil land-shells of, 339.
birds of, 390.

Magpie tame in Norway, 212.

Maize, crossed, 270.

Malay Archipelago:
compared with Europe, 299.
mammals of, 395.

Malpighiaceæ, 417.

Mammæ, rudimentary, 451.

Mammals:
fossil, in secondary formation, 303.
insular, 393.

Man, origin of races of, 199.

Manatee, rudimentary nails of, 454.

Marsupials:
of Australia, 116.
fossil species of, 339.

Martens, M., experiment on seeds, 360.

Martin, Mr. W. C., on striped mules, 165.

Matteuchi on the electric organs of rays, 193.

Matthiola, reciprocal crosses of, 258.

Means of dispersal, 356.

Melipona domestica, 225.

Metamorphism of oldest rocks, 308.

Mice:
destroying bees, 74.
acclimatisation of, 141.

Migration, bears on first appearance of fossils, 296.

Miller, Professor, on the cells of bees, 226.

Mirabilis, crosses of, 258.

Missel-thrush, 76.

Misseltoe, complex relations of, 3.

Mississippi, rate of deposition at mouth, 284.

Mocking-thrush of the Galapagos, 402.

Modification of species, how far applicable, 483.

Moles, blind, 137.

Mongrels:

fertility and sterility of, 267.
and hybrids compared, 272.

Monkeys, fossil, 303,

Monocanthus, 424.

Mons, Van, on the origin of fruit-trees, 29, 39.

Moquin-Tandon on sea-side plants, 132.

Morphology, 434.

Mozart, musical powers of, 209.

Mud, seeds in, 386.

Mules, striped, 165.

Müller, Dr. F., on Alpine Australian plants, 375.

Murchison, Sir R.:
on the formations of Russia, 289.
on azoic formations, 307.
on extinction, 317.

Mustela vison, 179.

Myanthus, 424.

Myrmecocystus, 238.

Myrmica, eyes of, 240.

Nails, rudimentary, 453.

Natural history:

future progress of, 484.
selection, 80.
system, 413.

Naturalisation:
of forms distinct from the indigenous species, 115.
in New Zealand, 201.

Nautilus, Silurian, 306.

Nectar of plants, 92.

Nectaries, how formed, 92.

Nelumbium luteum, 387.

Nests, variation in, 212.

Neuter insects, 236.

Newman, Mr., on humble-bees, 74.

New Zealand:
productions of, not perfect, 201.
naturalised products of, 337.
fossil birds of, 339.
glacial action in, 373.
crustaceans of, 376.
algæ of, 376.
number of plants of, 389.
flora of, 399.

Nicotiana:
crossed varieties of, 271.
certain species very sterile, 257.

Noble, Mr., on fertility of Rhododendron, 251.

Nodules, phosphatic, in azoic rocks, 307.

Oak, varieties of, 50.

Onites apelles, 135.

Orchis, pollen of, 193.

Organs:
of extreme perfection, 186.
electric, of fishes, 192.
of little importance, 194.
homologous, 434.
rudiments of, 450.

Ornithorhynchus, 107, 416.

Ostrich:
not capable of flight, 134.
habit of laying eggs together, 218.
American, two species of, 349.

Otter, habits of, how acquired, 179.

Ouzel, water, 185.

Owen, Professor:
on birds not flying, 134.
on vegetative repetition, 149.
on variable length of arms in ourang-outang, 150.
on the swim-bladder of fishes, 191.
on electric organs, 192.
on fossil horse of La Plata, 319.
on relations of ruminants and pachyderms, 329.
on fossil birds of New Zealand, 339.
on succession of types, 339.

on affinities of the dugong, 414.
on homologous organs, 435.
on the metamorphosis of cephalopods and spiders, 442.

Pacific Ocean, faunas of, 348.

Paley on no organ formed to give pain, 201.

Pallas on the fertility of the wild stocks of domestic animals, 253.

Paraguay, cattle destroyed by flies, 72.

Parasites, 217.

Partridge, dirt on feet, 362.

Parts:
greatly developed, variable, 150.
degrees of utility of, 201.

Parus major, 183.

Passiflora, 251.

Peaches in United States, 85.

Pear, grafts of, 261.

Pelargonium:
flowers of, 145.
sterility of, 251.

Pelvis of women, 144.

Peloria, 145.

Period, glacial, 365.

Petrels, habits of, 184.

Phasianus, fertility of hybrids, 253.

Pheasant, young, wild, 216.

Philippi on tertiary species in Sicily, 312.

Pictet, Professor:
on groups of species suddenly appearing, 302, 305.
on rate of organic change, 313.
on continuous succession of genera, 316.
on close alliance of fossils in consecutive formations, 335.
on embryological succession, 338.

Pierce, Mr., on varieties of wolves, 91.

Pigeons:
with feathered feet and skin between toes, 12.
breeds described, and origin of, 20.
breeds of, how produced, 39, 42.
tumbler, not being able to get out of egg, 87.
reverting to blue colour, 160.
instinct of tumbling, 214.
carriers, killed by hawks, 362.
young of, 445.

Pistil, rudimentary, 451.

Plants:
poisonous, not affecting certain coloured animals, 12.
selection applied to, 32.
gradual improvement of, 37.
not improved in barbarous countries, 38.
destroyed by insects, 67.
in midst of range, have to struggle with other plants, 77.

nectar of, 92.
fleshy, on sea-shores, 132.
fresh-water, distribution of, 386.
low in scale, widely distributed, 406.

Plumage, laws of change in sexes of birds, 89.

Plums in the United States, 85.

Pointer dog:
origin of, 35.
habits of, 213.

Poison not affecting certain coloured animals, 12.

Poison, similar effect of, on animals and plants, 484.

Pollen of fir-trees, 203,

Poole, Col., on striped hemionus, 163.

Potamogeton, 387.

Prestwich, Mr., on English and French eocene formations, 328.

Primrose, 49.
sterility of, 247.

Primula, varieties of, 49.

Proteolepas, 148.

Proteus, 139.

Psychology, future progress of, 488.

Quagga, striped, 165.

Quince, grafts of, 261.

Rabbit, disposition of young, 215.

Races, domestic, characters of, 16.

Race-horses:
Arab, 35.
English, 356.

Ramond on plants of Pyrenees, 368.

Ramsay, Professor:
on thickness of the British formations, 284.
on faults, 285.

Ratio of increase, 63.

Rats:
supplanting each other, 76.
acclimatisation of, 141.
blind in cave, 137.

Rattle-snake, 201.

Reason and instinct, 208.

Recapitulation, general, 459.

Reciprocity of crosses, 258.

Record, geological, imperfect, 279.

Rengger on flies destroying cattle, 72.

Reproduction, rate of, 63.

Resemblance to parents in mongrels and hybrids, 273.

Reversion:
law of inheritance, 14.
in pigeons to blue colour, 160.

Rhododendron, sterility of, 251.

Richard, Professor, on Aspicarpa, 417.

Richardson, Sir J.:
on structure of squirrels, 180.
on fishes of the southern hemisphere, 376.

Robinia, grafts of, 262.

Rodents, blind, 137.

Rudimentary organs, 450.

Rudiments important for classification, 416.

Sageret on grafts, 262.

Salmons, males fighting, and hooked jaws of, 88.

Salt-water, how far injurious to seeds, 358.

Saurophagus sulphuratus, 183.

Schiödte on blind insects, 138.

Schlegel on snakes, 144.

Sea-water, how far injurious to seeds, 358.

Sebright, Sir J.:
on crossed animals, 20.
on selection of pigeons, 31.

Sedgwick, Professor, on groups of species suddenly appearing, 302.

Seedlings destroyed by insects, 67.

Seeds:
nutriment in, 77.
winged, 146.
power of resisting salt-water, 358.
in crops and intestines of birds, 361.
eaten by fish, 362, 387.
in mud, 386.
hooked, on islands, 392.

Selection:
of domestic products, 29.
principle not of recent origin, 33.
unconscious, 34.
natural, 80.
sexual, 87.
natural, circumstances favourable to, 101.

Sexes, relations of, 87.

Sexual:
characters variable, 156.
selection, 87.

Sheep:
Merino, their selection, 31.
two sub-breeds unintentionally produced, 36.
mountain, varieties of, 76.

Shells:

colours of, 132.
littoral, seldom embedded, 288.
fresh-water, dispersal of, 385.
of Madeira, 391.
land, distribution of, 397.

Silene, fertility of crosses, 257.

Silliman, Professor, on blind rat, 137.

Skulls of young mammals, 197, 437.

Slave-making instinct, 219.

Smith, Col. Hamilton, on striped horses, 164.

Smith, Mr. Fred.:
on slave-making ants, 219.
on neuter ants, 239.

Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.

Snap-dragon, 161.

Somerville, Lord, on selection of sheep, 31.

Sorbus, grafts of, 262.

Spaniel, King Charles’s breed, 35.

Species:
polymorphic, 46.
common, variable, 53.
in large genera variable, 54.
groups of, suddenly appearing, 302, 306.
beneath Silurian formations, 306.
successively appearing, 312.

changing simultaneously throughout the world, 322.

Spencer, Lord, on increase in size of cattle, 35.

Sphex, parasitic, 218.

Spiders, development of, 442.

Spitz-dog crossed with fox, 268.

Sports in plants, 9.

Sprengel, C. C.:
on crossing, 98.
on ray-florets, 145.

Squirrels, gradations in structure, 180.

Staffordshire, heath, changes in, 72.

Stag-beetles, fighting, 88.

Sterility:
from changed conditions of life, 9.
of hybrids, 246.
laws of, 254.
causes of, 263.
from unfavourable conditions, 265.
of certain varieties, 269.

St. Helena, productions of, 389.

St. Hilaire, Aug., on classification, 418.

St. John, Mr., on habits of cats, 91.

Sting of bee, 202.

Stocks, aboriginal, of domestic animals, 18,

Strata, thickness of, in Britain, 284.

Stripes on horses, 163.

Structure, degrees of utility of, 201.

Struggle for existence, 60.

Succession, geological, 312.

Succession of types in same areas, 338.

Swallow, one species supplanting another, 76.

Swim-bladder, 190.

System, natural, 413.

Tail:
of giraffe, 195.
of aquatic animals, 196.
rudimentary, 454.

Tarsi deficient, 135.

Tausch on umbelliferous flowers, 146.

Teeth and hair:
correlated, 144.
embryonic, traces of, in birds, 451.
rudimentary, in embryonic calf, 450, 480.

Tegetmeier, Mr., on cells of bees, 228, 233.

Temminck on distribution aiding classification, 419.

Thouin on grafts, 262.

Thrush:
aquatic species of, 185.
mocking, of the Galapagos, 402.
young of, spotted, 439.
nest of, 243.

Thuret, >M., on crossed fuci, 258.

Thwaites, Mr., on acclimatisation, 140.

Tierra del Fuego:
dogs of, 215.
plants of, 374, 378.

Timber-drift, 360.

Time, lapse of, 282.

Titmouse, 183.

Toads on islands, 393.

Tobacco, crossed varieties of, 271.

Tomes, Mr., on the distribution of bats, 394.

Transitions in varieties rare, 172.

Trees:
on islands belong to peculiar orders, 392.
with separated sexes, 99.

Trifolium pratense, 73, 94.

Trifolium incarnatum, 94.

Trigonia, 321.

Trilobites, 306.
sudden extinction of, 321.

Troglodytes, 243.

Tucutucu, blind, 137.

Tumbler pigeons:
habits of, hereditary, 214.
young of, 446.

Turkey-cock, brush of hair on breast, 90.

Turkey:
naked skin on head, 197.
young, wild, 216.

Turnip and cabbage, analogous variations of, 159.

Type, unity of, 206.

Types, succession of, in same areas, 338.

Udders:
enlarged by use, 11.
rudimentary, 451.

Ulex, young leaves of, 439.

Umbelliferæ, outer and inner florets of, 144.

Unity of type, 206.

Use:
effects of, under domestication, 11.
effects of, in a state of nature, 134.

Utility, how far important in the construction of each part, 199.

Valenciennes on fresh-water fish, 384.

Variability of mongrels and hybrids, 274.

Variation:
under domestication, 7.
caused by reproductive system being affected by conditions of life, 8.
under nature, 44.
laws of, 131.

Variations:
appear at corresponding ages, 14, 86.
analogous in distinct species, 159.

Varieties:
natural, 44.
struggle between, 75.
domestic, extinction of, 111.
transitional, rarity of, 172.
when crossed, fertile, 267.
when crossed, sterile, 269.
classification of, 423.

Verbascum:
sterility of, 251.
varieties of, crossed, 270.

Verneuil, M. de, on the succession of species, 325.

Viola tricolor, 73.

Volcanic islands, denudation of, 284.

Vulture, naked skin on head, 197.

Wading-birds, 386.

Wallace, Mr.:
on origin of species, 2.
on law of geographical distribution, 355.
on the Malay Archipelago, 395.

Wasp, sting of, 202.

Water, fresh, productions of, 383.

Water-hen, 185.

Waterhouse, Mr.:
on Australian marsupials, 116.
on greatly developed parts being variable, 150.
on the cells of bees, 225.
on general affinities, 429.

Water-ouzel, 185.

Watson, Mr. H. C.:
on range of varieties of British plants, 58.
on acclimatisation, 140.
on flora of Azores, 363.
on Alpine plants, 367, 376.
on rarity of intermediate varieties, 176.

Weald, denudation of, 285.

Web of feet in water-birds, 185.

West Indian islands, mammals of, 395.

Westwood:
on species in large genera being closely allied to others, 57.
on the tarsi of Engidæ, 157.
on the antennæ of hymenopterous insects, 416.

Whales, fossil, 303.

Wheat, varieties of, 113.

White Mountains, flora of, 365.

Wings, reduction of size, 134.

Wings:
of insects homologous with branchiæ, 191.
rudimentary, in insects, 451.

Wolf:
crossed with dog, 214.
of Falkland Isles, 393.

Wollaston, Mr.:
on varieties of insects, 48.
on fossil varieties of land-shells in Madeira, 52.
on colours of insects on sea-shore, 132.
on wingless beetles, 135.
on rarity of intermediate varieties, 176.
on insular insects, 389.
on land-shells of Madeira, naturalised, 402.

Wolves, varieties of, 90.

Woodpecker:
habits of, 184.
green colour of, 197.

Woodward, Mr.:
on the duration of specific forms, 293.
on the continuous succession of genera, 316.
on the succession of types, 339.

World, species changing simultaneously throughout, 322.

Wrens, nest of, 243.

Youatt, Mr.:
on selection, 31.
on sub-breeds of sheep, 36.
on rudimentary horns in young cattle, 454.

Zebra, stripes on, 163.

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