THE GLACIAL PERIOD. 367 



• As the warmth returned, the arctic forms would retreat 

 northward, closely followed up in their retreat by the pro- 

 ductions 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 breth- 

 ren 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 Amer- 

 ican 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 return- 

 ing 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 of a 

 former Glacial period, seem to me to explain in so satis- 

 factory a manner the present distribution of the alpine 

 and arctic productions of Europe and America, that when 

 in other regions we find the same species on distant moun- 

 tain summits, we may almost conclude, without other evidence, 

 that a colder climate formerly permitted their migration 

 across the intervening lowlands, now become too warm for 

 their existence. 



As the arctic forms moved first southward and after- 

 ward backward to the north, in unison with the changing 

 climate, they will not have been exposed during their long 

 migrations to any great diversity of temperature ; and as 

 they all migrated in a body together, their mutual rela- 

 tions will not have been much disturbed. Hence, in 

 accordance with the principles inculcated in this volume, 

 fchese forms will not have been liable to much modification, 



