DISPERSAL DURING GLACIAL PERIOD 413 



had reached its maximum, wc should have an arctic fauna 

 and flora, covering the central parts of Europe, as far south 

 as the Alps and Pyrenees, and even stretching into Spain. 

 The now temperate regions of the United States would like- 

 wise 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 every- 

 where 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 produc- 

 tions 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 as- 

 cending, 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 re- 

 marked by Ramond, are more especially allied to the plants 

 of northern Scandinavia ; those of the United States to Lab- 

 rador; 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 satisfactory a manner 

 the present distribution of the Alpine and Arctic productions 

 of Europe and America, that when in other regions we find 



