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 large, but partial 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 most remarkable,
considering the 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 fin