We may safely infer that with marine animals of all kinds there has been a
large amount of migration due to climatal and other changes; and when we
see a species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for instance,
that several species appear somewhat earlier in the palaeozoic beds of
North America than in those of Europe; time having apparently been required
for their migration from the American to the European seas. In examining
the latest deposits, in various quarters of the world, it has everywhere
been noted, that some few still existing species are common in the deposit,
but have become extinct in the immediately surrounding sea; or, conversely,
that some are now abundant in the neighbouring sea, but are rare or absent
in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the
glacial epoch, which forms only a part of one whole geological period; and
likewise to reflect on the changes of level, on the extreme change of
climate, and on the great lapse of time, all included within this same
glacial period. Yet it may be doubted whether, in any quarter of the
world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have gone on
accumulating within the same area during the whole of this period. It is
not, for instance, probable that sediment was deposited during the whole of
the glacial period near the mouth of the Mississippi, within that limit of
depth at which marine animals can best flourish: for we know that great
geographical changes occurred in other parts of America during this space
of time.
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