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 palaeontologists, 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
palaeozoic 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
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 i
n 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
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 h
ave 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
palaeontology 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