Charles Darwin.

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which became still more modified at the seventh stage; hence they could
hardly fail to be nearly intermediate in character between the forms of life
above and below. We must, however, allow for the entire extinction of some
preceding forms, and in any one region for the immigration of new forms
from other regions, and for a large amount of modification during the long
and blank intervals between the successive formations. Subject to these allow-
ances, 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 sys-
tem, when this system was first discovered, were at once recognized by palaeon-
tologists as intermediate in character between those of the overlying car-
boniferous and underlying Silurian systems. But each fauna is not neces-


sarily exactly intermediatej as unequal intervals of time have elapsed be-
tween 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, the species of mastodons and elephants, when arranged by Dr. Fal-
coner in two series — in the first place according to their mutual affinities, and
in the second place according to their periods of existence — do not accord
in arrangement. The species extreme in character are not the oldest or the
most recent; nor are those which are intermediate in character, intermediate
in age. But supposing for an instant, in this and other such cases, that the
record of the first appearance and disappearance of the species was complete,
which is far from the case, we have no reason to believe that forms suc-
cessively produced necessarily endure for corresponding lengths of time. A
very ancient form may occasionally have lasted much longer than a form
elsewhere subsequently produced, especially in the case of terrestrial pro-
ductions inhabiting separated districts. To compare small things with great;
if the principal living and extinct races of the domestic pigeon were arranged
in serial affinity, this arrangement would not closely accord with the order in
time of their production, and even less with the order of their disappear-
ance; for the parent rock-pigeon still lives; and many varieties between the
rock-pigeon and the carrier have become extinct; and carriers which are
extreme in the important character of length of beak originated earlier than
short-beaked tumblers, which are at the opposite end of the series in this

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two consecutive for-
mations 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 re-
semblance of the organic remains from the several stages of the Chalk for-
mation, though the species are distinct in each stage. This fact alone, from
its generality, seems to have shaken Professor Pictet in his belief in the im-
mutability of species. He who is acquainted with the distribution of existing
species over the globe, will not attempt to account for the close resemblance
of distinct species in closely consecutive formations, by the physical conditions
of the ancient areas having remained nearly the same. Let it be remembered
that the forms of life, at least those 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 dur-
ing the pleistocene period, which includes the whole glacial epoch, and note
how little the specific forms of the inhabitants of the sea have been affected.

On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations being closely related, though ranked as dis-
tinct species, is obvious. As the accumulation of each formation has often
been interrupted, and as long blank intervals have intervened between sue-


cessive formations, we ought not to expect to find, as I attempted to show in
the last chapter, in any one or in any two formations, all the intermediate
varieties between the species which appeared at the commencement and
close of these periods : but we ought to find after intervals, very long as meas-
ured by years, but only moderately long as measured geologically, closely
allied forms, or, as they have been called by some authors, representative
species; and these assuredly we do find. We find, in short, such evidence of
the slow and scarcely sensible mutations of specific forms, as we have the
right to expect.



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

It is no valid objection to this conclusion, that certain Brachiopods have
been but slightly modified from an extremely remote geological epoch; and
that certain land and fresh-water shells have remained nearly the same, from
the time when, as far as is known, they first appeared. It is not an insuperable
difficulty that Foraminifera have not, as insisted on by Dr. Carpenter, pro-
gressed in organization since even the Laurentian epoch ; for some organisms
would have to remain fitted for simple conditions of life, and what could be
better fitted for this end than these lowly organized Protozoa? Such objec-
tions as the above would be fatal to my view, if it included advance in organ-
ization as a necessary contingent. They would likewise be fatal, if the above
Foraminifera, for instance, could be proved to have first come into existence
during the Laurentian epoch, or the above Brachiopods during the Cambrian
formation; for in this case, there would not have been time sufficient for the


development of these organisms up to the standard which they had then
reached. When advanced up to any given point, there is no necessity, on the
theory of natural selection, for their further continued process; though they
will, during each successive age, have to be slightly modified, so as to hold
their places in relation to slight changes in their conditions. The foregoing
objections hinge on the question whether we really know how old the world
is, and at what period the various forms of life first appeared; and this may
well be disputed.

The problem whether organization on the whole has advanced is in many
ways excessively intricate. The geological record, at all times imperfect, does
not extend far enough back to show with unmistakable clearness that within
the known history of the world organization has largely advanced. Even at
the present day, looking to members of the same class, naturalists are not
unanimous which forms ought to be ranked as highest: thus, some look at
the selaceans or sharks, from their approach in some important points of
structure to reptiles, as the highest fish; others look at the teleosteans as the
highest. The ganoids stand intermediate between the selaceans and teleos-
teans; the latter at the present day are largely preponderant in number; but
formerly selaceans and gamoids alone existed; and in this case, according to
the standard of highness chosen, so will it be said that fishes have advanced
or retrograded in organization. To attempt to compare members of distinct
types in the scale of highness seems hopeless; who will decide whether a
cuttle-fish be higher than a bee — that insect which the great Von Baer be-
Heved to be "in fact more highly organized than a fish, although upon an-
other type"? In the complex struggle for life it is quite credible that crusta-
ceans, not very high in their own class, might beat cephalopods, the highest
mollusks; and such crustaceans, though not highly developed, would stand
very high in the scale of invertebrate animals, if judged by the most decisive
of all trials — the law of battle. Besides these inherent difficulties in deciding
which forms are the most advanced in organization, we ought not solely to
compare the highest members of a class at any two periods — though un-
doubtedly this is one and perhaps the most important element in striking a
balance — but we ought to compare all the members, high and low, at two
periods. At an ancient epoch the highest and lowest molluscoidal animals,
namely, cephalopods and brachiopods, swarmed in numbers; at the present
time both groups are greatly reduced, while others, intermediate in organ-
ization, have largely increased; consequently some naturalists maintain that
mollusks were formerly more highly developed than at present; but a stronger
case can be made out on the opposite side, by considering the vast reduction
of brachiopods, and the fact that our existing cephalopods, though few in
number, are more highly organized than their ancient representatives. We
ought also to compare the relative proportional numbers, at any two periods,
of the high and low classes throughout the world: if, for instance, at the
present day fifty thousand kinds of vertebrate animals exist, and if we knew
that at some former period only ten thousand kinds existed, we ought to look
at this increase in number in the highest class, which implies a great dis-


placement of lower forms, as a decided advance in the organization of the
world. We thus see how hopelessly difficult it is to compare with perfect fair-
ness, under such extremely complex relations, the standard of organization
of the imperfectly known faunas of successive periods.

We shall appreciate this difficulty more clearly by looking to certain ex-
isting faunas and floras. From the extraordinary manner in which European
productions have recently spread over New Zealand, and have seized on
places which must have been previously occupied by the indigenes, we must
believe, that if all the animals and plants of Great Britain were set free in
New Zealand, a multitude of British forms would in the course of time be-
come thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from the fact that hardly a single inhabitant of
the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in
Great Britain, any considerable number would be enabled to seize on places
now occupied by our native plants and animals. Under this point of view,
the productions of Great Britain stand much higher in the scale than those of
New Zealand. Yet the most skilful naturalists, from an examination of the
species of the two countries, could not have foreseen this result.

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



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 armor, like those of the armadillo, found in sev-


eral 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 generalization
to the mammals of the Old World. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see it also
in the birds of the caves of Brazil. Mr. Woodward has shown that the same
law holds good with sea-shells, but, from the wide distribution of most mol-
lusks, 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 compar-
ing the present climate of Australia and of parts of South America, under
the same latitude, would attempt to account, on the one hand through dis-
similar physical conditions, for the dissimilarity of the inhabitants of these
two continents; and, on the other hand through similarity of conditions, for
the uniformity of the same types in each continent during the later tertiary
periods. Nor can it be pretended that it is an immutable law that marsupials
should have been chiefly or solely produced in Australia; or that Edentata
and other American types should have been solely produced in South
America. For we know that Europe in ancient times was peopled by numer-
ous marsupials ; and I have shown, in the publications above alluded to, that
in America the law of distribution of terrestrial mammals was formerly dif-
ferent 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 rela-
tion to the distribution of marine animals.

On the theory of descent with modification, the great law of the long-
enduring, but not immutable, 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
distribution of organic beings.


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


I have attempted to show that the geological record is extremely imper-
fect; that only a small portion of the globe has been geologically explored
with care; that only certain classes of organic beings have been largely pre-
served in a fossil state; that the number both of specimens and of species,
preserved in our museums, is absolutely as nothing compared with the num-
ber of generations which must have passed away even during a single forma-
tion; that, owing to subsidence being almost necessary for the accumulation
of deposits rich in fossil species of many kinds, and thick enough to outlast
future degradation, great intervals of time must have elapsed between most
of our successive formations; that there has probably been more extinction
during the periods of subsidence, and more variation during the periods of
elevation, and during the latter the record will have been least perfectly
kept; that each single formation has not been continuously deposited; that
the duration of each formation is probably short compared with the average J
duration of specific forms; that migration has played an important part in
the first appearance of new forms in any one area and formation ; that widely
ranging species are those which have varied most frequently, and have often-
est given rise to new species ; that varieties have at first been local ; and lastly,
although each species must have passed through numerous transitional stages, i
it is probable that the periods, during which each underwent modification,
though many and long as measured by years, have been short in comparison
with the periods during which each remained in an unchanged condition.]


These causes, taken conjointly, will to a large extent explain why — though
we do find many links — we do not find interminable varieties, connecting to-
gether all extinct and existing forms by the finest graduated steps. It should
also be constantly borne in mind that any linking variety between two forms,
which might be found, would be ranked, unless the whole chain could be per-
fectly restored, as a new and distinct species; for it is not pretended that we
have any sure criterion by which species and varieties can be discriminated.

He who rejects this view of the imperfection of the geological record, will
rightly reject the whole theory. For he may ask in vain where are the num-
berless transitional links which must formerly have connected the closely
allied or representative species, found in the successive stages of the same
great formation? He may disbelieve in the immense intervals of time which
must have elapsed between our consecutive formations; he may overlook
how important a part migration has played, when the formations of any one
great region, as those of Europe, are considered; he may urge the apparent,
but often falsely apparent, sudden coming in of whole groups of species. He
may ask where are the remains of those infinitely numerous organisms which
must have existed long before the Cambrian system was deposited? We now
know that at least one animal did then exist; but I can answer this last ques-
tion only by supposing that where our oceans now extend they have extended
for an enormous period, and where our oscillating continents now stand they
have stood since the commencement of the Cambrian system; but that, long
before that epoch, the world presented a widely different aspect; and that the
older continents, formed of formations older than any known to us, exist
now only as remnants in a metamorphosed condition, or lie still buried under
the ocean.

Passing from these difficulties, the other great leading facts in palaeontology
agree admirably with the theory of descent with modification through varia-
tion and natural selection. We can thus understand how it is that new species
come in slowly and successively; how species of different classes do not neces-
sarily 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
belonging to large and dominant groups tend to leave many modified de-
scendants, which form new sub-groups and groups. As these are formed, the
species of the less vigorous groups, from their inferiority inherited from a
common progenitor, tend to become extinct together, and to leave no modi-
fied offspring on the face of the earth. But the utter extinction of a whole
group of species has sometimes been a slow process, from the survival of a
few 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 it is that dominant forms which spread widely
and yield the greatest number of varieties tend to people the world with

Online LibraryCharles DarwinThe origin of species → online text (page 35 of 50)