At the present day, for example, many identical
species of animals are found living on the western coasts of
Britain and the eastern coasts of North America, and beds now
in course of deposition off the shores of Ireland and the seaboard
of the state of New York would necessarily contain many of the
same fossils. Such beds would be both literally and geologically
contemporaneous; but the case is different if the distance between
the areas where the strata occur be greatly increased. We find,
for example, beds containing identical fossils (the Quebec or
Skiddaw beds) in Sweden, in the north of England, in Canada,
and in Australia. Now, if all these beds were contemporaneous,
in the literal sense of the term, we should have to suppose that
the ocean at one time extended uninterruptedly between all these
points, and was peopled throughout the vast area thus indicated
by many of the same animals. Nothing, however, that we see at
the present day would justify us in imagining an ocean of such
enormous extent, and at the same time so uniform in its depth,
temperature, and other conditions of marine life, as to allow the
same animals to flourish in it from end to end; and the example
chosen is only one of a long and ever-recurring series. It is
therefore much more reasonable to explain this, and all similar
cases, as owing to the _migration_ of the fauna, in whole or in
part, from one marine area to another.
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