Chap. XI. DURING THE GLACIAL PERIOD. 367 



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 

 have been due south and north. The Alpine plants, for 

 example, of Scotland, as remarked by Mr. H. C. Watson, 



