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 chang- 

 ing climate, they will not have been exposed during 

 their long migrations to any great diversity of tem- 

 perature; and as they all migrated in a body together, 

 their mutual relations will not have been much dis- 

 turbed. Hence, in accordance with the principles in- 

 culcated in this volume, these forms will not have been 

 liable to much modification. But with the Alpine pro- 

 ductions, left isolated from the moment of the return- 

 ing 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 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 some- 

 what different climatal influences. Their mutual rela- 

 tions will thus have been in some degree disturbed; con- 

 sequently they will have been liable to modification; and 

 they have been modified; for if we compare the present 



