Chap. XII.] THE GLACIAL PEEIOD. 153 



inhabitants, which we suppose to have everywhere tra- 

 velled 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 moun- 

 tains, the arctic forms would seize on the cleared and 

 thawed ground, always ascending, as the warmth in- 

 creased 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 low- 

 lands, 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. "VVe 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. The Alpine plants, 

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

 Watson, and those of the Pyrenees, as remarked by 

 Bamond, are more especially allied to the plants of 

 northern Scandinavia; those of the United States to 

 Labrador ; those of the mountains of Siberia to the 

 arctic regions of that country. These views, grounded 

 as they are on the perfectly well-ascertained occurrence 



