By the time that the cold had reached its maximum, we should have
an 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.
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, as the warmth increased and the snow still further disappeared,
higher and higher, whilst their brethren were pursuing their northern
journey. Hence, when the warmth had fully returned, the same species,
which had lately lived together on the European and North American
lowlands, 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.
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