154 DISPERSAL DURING [Chap. XII. 



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 formerly 

 permitted their migration across the intervening low- 

 lands, now become too warm for their existence. 



As the arctic forms moved first southward and after- 

 wards backwards 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 re- 

 turning warmth, first at the bases and ultimately on 

 the summits of tlie 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 probability, 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 cHmatal influences. Their mutual relations 

 will thus have been in some degree disturbed ; conse- 

 quently they will have been liable to modification ; and 

 they have been modified ; for if we compare the present 



