Chap. XII.] THE GLACIAL PERIOD. I53 



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. The Alpine plants, 

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

 Watson, and those of the Pyrenees, as remarked by 

 Ramond, 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 



