The Alpine plants, for example, of 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 formerly permitted
their migration across the intervening lowlands, now become too warm for
their existence.
As the arctic forms moved first southward and afterwards backward to the
north, in unison with the changing climate, they will not have been exposed
during their long migrations to any great diversity of temperature; and as
they all migrated in a body together, their mutual relations will not have
been much disturbed. Hence, in accordance with the principles inculcated
in this volume, these forms will not have been liable to much modification.
But with the 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 far
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 the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences.
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