368 GEOGRAPHICAL DISTRIBUTION, Chap. XI. 



and those of the Pyrenees, as remarked by Eamond, are 

 more especially allied to the plants of northern Scan- 

 dinavia ; 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 

 j)erfectly 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 moun- 

 tain-summits, we may almost conclude without other 

 evidence, that a colder climate permitted their former 

 migration across the low intervening tracts, since be- 

 come too warm for their existence. 



If the climate, since the Glacial period, has ever been 

 in any degree warmer than at present (as some geo- 

 logists in the United States believe to have been the 

 case, chiefly from the distribution of the fossil Gnatho- 

 don), then the arctic and temperate productions will at 

 a very late period have marched a little further north, 

 and subsequently have retreated to their present homes ; 

 but I have met with no satisfactory evidence with respect 

 to this intercalated slightly warmer period, since the 

 Glacial period. 



The arctic forms, during their long southern migra- 

 tion and re-migration northward, will have been exposed 

 to nearly the same climate, and, as is especially to be 

 noticed, they will have kept in a body together ; con- 

 sequently their mutual relations will not have been 

 much disturbed, and, in accordance with the principles 

 inculcated in this volume, they will not have been liable 

 to much modification. But with our Alpine productions, 

 left isolated from the moment of the returning warmth, 

 first at the bases and ultimately on the summits of 

 the mountains, the case will have been somewhat dif- 



