332 DISPERSAL DURING THE GLACIAL PERIOD. [Chap Xii. 



As the arctic forms moved first southward and afterwards back- 

 wards 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 

 together, 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 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 ex- 

 posed to somewhat different climatal influences. Their mutual rela- 

 tions will thus have been in some degree disturbed; consequently 

 they will have been liable to modification ; and they have been 

 modified ; for if we compare the present Alpine plants and animals 

 of the several great European mountain-ranges one with another, 

 though many of the species remain identically the same, some 

 exist as varieties, some as doubtful forms or sub-species, and 

 some as distinct yet closely allied species representing each other 

 on the several ranges. 



In the foregoing illustration I have assumed that at the com- 

 mencement of our imaginary Glacial period, the arctic productions 

 were as uniform round the polar regions as they are at the present 

 day. But it is also necessary to assume that many sub-arctic and 

 some few temperate forms were the same round the world, for 

 some of the species which now exist on the lower mountain-slopes 

 and on the plains of North America and Europe are the same; 

 and it may be ashed how I account for this degree of uniformity 

 in the sub-arctic and temperate forms round the world, at the 

 commencement of the real Glacial period. At the present day, 

 the sub-arctic and northern temperate productions of the Old 

 and New "Worlds are separated from each other by the whole 

 Atlantic Ocean and by the northern part of the Pacific. During 

 the Glacial period, when the inhabitants of the Old and New 

 Worlds lived farther southwards than they do at present, they 

 must have been still more completely separated from each other 

 by wider spaces of ocean ; so that it may well be asked now tn& 



