Chap. XI. 
DURING THE GLACIAL PERIOD. 
367 
its iRaximum, 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 south¬ 
ward, 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 re¬ 
treat 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 
