If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe
to have been the case, chiefly from the distribution of the fossil
Gnathodon), then the arctic and temperate productions will at a very
late period have marched a little further north, and subsequently have
retreated to their present homes; but I have met with no satisfactory
evidence with respect to this intercalated slightly warmer period,
since the Glacial period.
The arctic forms, 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,
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
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