Chap. XII.] DISPERSAL DURING THE GLACIAL PERIOD. 33.1 



further and further southward, unless they were stopped by barriers, 

 in which case they would perish. The mountains would become 

 covered with snow and ice, and their former Alpine inhabitants 

 would descend to the plains. By the time that the cold had reached 

 its maximum, we 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 likewise be covered by arctic plants and ani- 

 mals 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 north- 

 ward, 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-migratior 

 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 

 Eamond, are more especially allied to the plants of northern Scandi- 

 navia ; those of the United States to Labrador ; those of the moun- 

 tains of Siberia to the arctic regions of that country. These views, 

 grounded as they a*e 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 produc- 

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

 the same species on distant mountain-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. 



