THE GLACIAL PERIOD, 391 



the central parts of Europe, as far south as the Alps and 

 Pyrenees, and even stretching into Spain. 'IMie now 

 temperate regions of the United States would likewise bo 

 covered by arctic plants aud animals and these would be 

 nearly the same with those of Europe; for the present 

 eircumpolar ijihabitants, which we suppose to have every- 

 where traveled 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 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 

 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 return- 

 ing warmth, would generally have been due south and 

 north. The Alpine plants, for example, of Scotland, as 

 remarked by Mr. H. 0. Watson, and those of the Pyrenees, 

 as remarked by Eamond, are more especially allied t^o tiio 

 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, tluit wiien 

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

 ain-summits, we may almost conclude, without other 

 evidence, that a colder climate formerly permitted their 



