distance of the two areas, and their separation by the Atlantic Ocean.
We can further understand the singular fact remarked on by several
observers, that the productions of Europe and America during the later
tertiary stages were more closely related to each other than they are
at the present time; for during these warmer periods the northern parts
of the Old and New Worlds will have been almost continuously united by
land, serving as a bridge, since rendered impassable by cold, for the
inter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they must have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great
region with the native American productions, and have had to compete
with them; and in the other great region, with those of the Old World.
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 the two Worlds.
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 areas completely sundered. Thus, I think, we
can understand the presence of many existing and tertiary
representative forms on the eastern and western shores of temperate
North America; and the still more striking case of many closely allied
crustaceans (as described in Dana s admirable work), of some fish and
279