would again be found in the arctic
regions of the Old and New Worlds, and on many isolated mountain-summits
far distant from each other.

Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants
of each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the first migration
when the cold came on, and the re-migration on the returning warmth,
would generally have been due south and north. 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 t