during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as is
especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed, and,
in accordance with the principles inculcated in this volume, they will not
have been liable to much modification. But with our Alpine productions,
left isolated from the moment of the returning warmth, {369} first at the
bases and ultimately on the summits of the mountains, the case will have
been somewhat different; for it is not likely that all the same arctic
species will have been left on mountain ranges distant from each other, and
have survived there ever since; they will, also, in all probability have
become mingled with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which during
its coldest period will have been temporarily driven down to the plains;
they will, also, have been exposed to somewhat different climatal
influences. Their mutual relations will thus have been in some degree
disturbed; consequently 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 {370} Atlantic Ocean
and by the extreme northern part of the Pacific. During the Glacial period,
when the inhabitants of the Old and New