ance 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
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