330 ON THE ORIGIN OF SPECIES 



by the productions of the more temperate regions. 

 And as the snow melted from the bases of the moun- 

 tains, the arctic forms would seize on the cleared and 

 thawed ground, always ascending higher and higher, 

 as the warmth increased, whilst their brethren were 

 pursuing their northern journey. Hence, when the 

 warmth had fully returned, the same arctic species, 

 which had lately lived in a body together on the 

 lowlands of the Old and New Worlds, would be left 

 isolated on distant mountain-summits (having been ex- 

 terminated on all lesser heights) and in the arctic 

 regions of both hemispheres. 



Thus we can understand 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 especially related to the 

 arctic forms living due north or nearly due north of 

 them : for the migration as the cold came on, and the 

 re-migration on the returning warmth, will 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 satisfactory 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 mountain-summits, we may almost conclude 

 without other evidence, that a colder climate permitted 

 their former migration across the low intervening tracts, 

 since become too warm for their existence. 



If the climate, since the Glacial period, has evei 

 been in any degree warmer than at present (as some 

 geologists in the United States believe to have been 

 the case, chiefly from the distribution of the fossil 



