eastern America.
And this continuity of the circumpolar land, with the consequent freedom
under a more favourable climate for intermigration, will account for the
supposed uniformity of the sub-arctic and temperate productions of the
Old and New Worlds, at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view,
and to infer that during some earlier and still warmer period, such as
the older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants
and animals, both in the Old and New Worlds, began slowly to migrate
southwards as the climate became less warm, long before the commencement
of the Glacial period. We now see, as I believe, their descendants,
mostly in a modified condition, in the central parts of Europe and the
United States. On this view we can understand the relationship with very
little identity, between the productions of North America and Europe--a
relationship which is highly remarkable, considering the distance of
the two areas, and their separation by the whole 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
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 will have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, must have taken place long ages ago. As the plants and
animals migrated southward, they will have become mingled in the one
great region with the native American productions, and would 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