argely 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 {343} 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