338 DISPERSAL Cuat, Xi. 



lion of the inliabitiints of Europe, as cxj)lained hy Edward 

 Forbes, is substantially as follows. But we shall follow the 

 changes more readil}', by supposing a new glacial period slowly 

 to come on, and then pass away, as formerly occurred. As the 

 cold came on, and as each more southern zone became fitted 

 for the inhabitants of the north, they would take the places of 

 the former inhabitants of the temperate regions. The latter, 

 at the same time, would travel farther and farther southward, 

 unless thoy were stopped by bairiers, 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 

 animals, and these would be nearly the same with those of 

 Europe ; for the present cireumpolar inhabitants, which we sup- 

 pose 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 produc- 

 tions of the more temperate regions. And as the snow melted 

 from the bases of the mountains, the arctic forms Avould seize 

 on the cleared and thawed ground, always ascending, as the 

 warmth increased and the snow still further disappeared, higher 

 and higher, while their brethren were pursuing their northern 

 journey. Hence, when the warmth had fully returned, the 

 same species, which had lately lived together in a body on the 

 European and North American lowlands, Avould 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 understalid the identity of many plants at 

 points so immensely remote as on the mountains of the United 

 States and of Europe. We can thus also understand the fact 

 that the Alpine plants of each mountain-range are more espe- 

 cially 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 remigration on the returning warmth, would generally 

 have been due south and north. The Alpine plants, for exam- 

 ple, of Scotland, as remarked by Mr. II. C. Watson, and those 

 of the Pyrenees, as remarked by Ramond, are more especially 

 allied to the plants of northern Scandinavia ; those of the 



