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
the productions on the shores of a continent when first broken up into
an archipelago), and consequently during subsidence, though there will
be much extinction, fewer new varieties or species will be formed; and
it is during these very periods of subsidence, that our great deposits
rich in fossils have been accumulated. Nature may almost be said to
have guarded against the frequent discovery of her transitional or
linking forms.
From the foregoing 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 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. Some
cases are on record of the same species presenting distinct varieties
in the upper and lower parts of the same formation, but, as they are
rare, they may be here passed over. Although each formation has
indisputably required a vast number of years for its deposition, I can
see several reasons why each should not include a graduated series of
links between the species which then lived; but I can by no means
pretend to assign due proportional weight to the following
considerations.
Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite to change one
species into another. I am aware that two pal ontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us coming to any
just conclusion on this head. When we see a species first appearing in
the middle of any formation, it would be rash in the extreme to infer
that it had not elsewhere previously existed. So again when we find a
species disappearing before the uppermost layers have been deposited,
it would be equally rash to suppose that it then became wholly extinct.
We forget how small the area of Europe is compared with the rest of the
world; nor have the several stages of the same formation throughout
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