368 DISPERSAL DURING 



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 ; the}^ 

 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 many of the species 

 remain identically the same, some exist as varieties, some 

 as doubtful forms of 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 

 commencement 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 asked 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 pro- 

 ductions of the Old and New Worlds are separated from 

 each other by the whole Atlantic Ocean and by the north- 

 ern part of the Pacific. During the Glacial period, when 

 the inhabitants of the Old and New Worlds lived further 

 southward 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 how the same 

 species could then or previously have entered the two con- 

 tinents. The explanation, I believe, lies in the nature of 

 the climate, fceiore the c^mnience^nt; of \$$ glacial period 



