392 DISPERSAL DURING 



migration across the intervening lowlands, now become 

 too warm for their existence. 



As the arctic forms moved first southward and after- 

 ward backward 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 rela- 

 tions 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 2^1'obability, 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 influences. Their mutual relations 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 m.any of the species 

 remain identically the same, some exist as varieties, some 

 as doubtful forms or sub-species and some as distinct 3^et 

 closely allied species representing each other on the several 

 ranges. 



In the foregoing illustration I have assumed that at the 

 commencement of our imaginary Glacial period, tlie 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 asked how I account for this degree of uniformity 

 in the sub-arctic and temperate forms round the worlds at 



