GEOGRAPHICAL DISTRIBUTION 161 



Europe, as far soutli 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. 



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, as the warmth increased and 

 the snow still further disappeared, higher and higher, 

 while their brethren were pursuing their northern jour- 

 ney. 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 tho 

 remigration on the returning warmth, would generally 

 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 



