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
pal ontology 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
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