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 intermigration 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 {372} 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 other marine animals, in the Mediterranean and in
the seas of Japan,--areas now separated by a continent and by nearly a
hemisphere of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the