ly
they will have been liable to modification; and this we find has been
the case; for if we compare the present Alpine plants and animals of the
several great European mountain-ranges, though very many of the species
are identically the same, some present varieties, some are ranked
as doubtful forms, and some few are distinct yet closely allied or
representative species.

In illustrating what, as I believe, actually took place during
the Glacial period, I assumed that at its commencement the arctic
productions were as uniform round the polar regions as they are at the
present day. But the foregoing remarks on distribution apply not only
to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower
mountains and on the plains of North America and Europe; and it may be
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 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