Scotland, as remarked by Mr. H. C. Watson, and those of the Pyrenees, as
remarked by Ramond, are more especially allied to the plants of northern
Scandinavia; those of the United States to Labrador; those of the
mountains of Siberia to the arctic regions of that country. These views,
grounded as they are on the perfectly well-ascertained occurrence of a
former Glacial period, seem to me to explain in so satisfactory a manner
the present distribution of the Alpine and Arctic productions of Europe
and America, that when in other regions we find the same species on
distant mountain-summits, we may almost conclude without other evidence,
that a colder climate permitted their former migration across the low
intervening tracts, since become too warm for their existence.

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; consequent