Hence, it is probable that in some parts of the world whole
formations have been completely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased and new stations will often be formed--all circumstances
favourable, as previously explained, for the formation of new varieties and
species; but during such periods there will generally be a blank in the
geological record. On the other hand, during subsidence, the inhabited
area and number of inhabitants will decrease (excepting on the shores of a
continent when first broken up into an archipelago), and consequently
during subsidence, though there will be much extinction, few new varieties
or species will be formed; and it is during these very periods of
subsidence that the deposits which are richest in fossils have been
accumulated.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION.
>From these several considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes much more difficult to
understand why we do not therein find closely graduated varieties between
the allied species which lived at its commencement and at its close.
Several cases are on record of the same species presenting varieties in the
upper and lower parts of the same formation.
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