n, and
then pass away, as formerly occurred. As the cold came on, and as each
more southern zone became fitted for arctic beings and ill-fitted for
their former more temperate inhabitants, the latter would be supplanted
and arctic productions would take their places. The inhabitants of the
more temperate regions would at the same time travel southward, unless
they were stopped by barriers, in which case they would perish. The
mountains would become covered with snow and ice, and their former
Alpine inhabitants would descend to the plains. By the time that the
cold had reached its maximum, we should have a uniform arctic fauna and
flora, covering the central parts of Europe, as far south as the Alps
and Pyrenees, and even stretching into Spain. The now temperate regions
of the United States would likewise be covered by arctic plants and
animals, and these would be nearly the same with those of Europe; for
the present circumpolar inhabitants, which we suppose to have everywhere
travelled southward, are remarkably uniform round the world. We may
suppose that the Glacial period came on a little earlier or later in
North America than in Europe, so will the southern migration there have
been a little earlier or later; but this will make no difference in the
final result.

As the warmth returned, the 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
have been due south and north. The Alpine plants, for example, of