Worlds lived further southwards
than at present, they must have been still more completely separated by
wider spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We have
good reason to believe that during the newer Pliocene period, before the
Glacial epoch, and whilst the majority of the inhabitants of the world were
specifically the same as now, the climate was warmer than at the present
day. Hence we may suppose that the organisms now living under the climate
of latitude 60 deg., during the Pliocene period lived further north under the
Polar Circle, in latitude 66 deg.-67 deg.; and that the strictly arctic productions
then lived on the broken land still nearer to the pole. Now if we look at a
globe, we shall see that under the Polar Circle there is almost continuous
land from western Europe, through Siberia, to eastern America. And to this
continuity of the circumpolar land, and to the consequent freedom for
intermigration under a more favourable climate, I attribute the necessary
amount of uniformity in the sub-arctic and northern 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 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 {371} 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