he arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the arctic
forms would seize on the cleared and thawed ground, always ascending higher
and higher, as the warmth increased, whilst their brethren were pursuing
their northern journey. Hence, when the warmth had fully returned, the same
arctic species, which had lately lived in a body together on the lowlands
of the Old and New Worlds, would be left isolated on distant
mountain-summits (having been exterminated on all lesser heights) and in
the arctic regions of both hemispheres.

Thus we can understand the identity of many plants at points so immensely
remote as on the mountains of the United States and 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 migration as the cold came on, and the re-migration
on the returning warmth, will generally {368} 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 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,