reasonably asked how I account for the necessary degree of uniformity
of the sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the Atlantic Ocean and by the extreme
northern part of the Pacific. During the Glacial period, when the
inhabitants
of the Old and New 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 , during the Pliocene period lived further
north under the Polar Circle, in latitude 66 -67 ; 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 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
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