GEOGRAPHICAL DISTRIBUTION 339 



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-migra- 

 tion 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 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 th^ 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 mounS;ain 

 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. 



As the arctic forms moved first southward and afterward back- 

 ward to the north, in unison with the changing climate, they will 

 not have been exposed during their long migrations to any great 

 diversity of temperature; and as they all migrated in a body to- 

 gether, their mutual relations will not have been much disturbed. 

 Hence, in accordance with the principles inculcated in this volume, 

 these forms will not have been liable to much modification. But 

 with the 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 

 different; for it is not likely that all the same arctic species will 

 have been left on mountain ranges far distant from each other, 

 and have survived there ever since; they will also, in all prob- 

 ability, have become mingled with ancient Alpine species, which 

 must have existed on the mountains before the commencement of 

 the Glacial epoch, and which during the coldest period will have 

 been temporarily driven down to the plains; they will, also, have 

 been subsequently exposed to somewhat different climatical in- 

 fluences. Their mutual relations will thus have been in some de- 

 gree disturbed; consequently they will have been liable to modi- 



