Consequently we have here everything favourable for much modification--for
far more modification than with the Alpine productions, left isolated,
within a much more recent period, on the several mountain ranges and on the
arctic lands of Europe and North America. Hence, it has come, that when we
compare the now living productions of the temperate regions of the New and
Old Worlds, we find very few identical species (though Asa Gray has lately
shown that more plants are identical than was formerly supposed), but we
find in every great class many forms, which some naturalists rank as
geographical races, and others as distinct species; and a host of closely
allied or representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which, during the Pliocene or even a somewhat earlier period,
was nearly uniform along the continuous shores of the Polar Circle, will
account, on the theory of modification, for many closely allied forms now
living in marine areas completely sundered. Thus, I think, we can
understand the presence of some closely allied, still existing and extinct
tertiary forms, on the eastern and western shores of temperate North
America; and the still more striking fact of many closely allied
crustaceans (as described in Dana's admirable work), some fish and other
marine animals, inhabiting the Mediterranean and the seas of Japan--these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.
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