Chap. XIL] THE GLACIAL PERIOD. 155 



Alpine plants and animals of the several great Euro- 

 pean 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 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 commence- 

 ment 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 how the same 

 species could then or previously have entered the two 

 continents. The explanation, I believe, lies in the 

 nature of the climate before the commencement of the 

 Glacial period. At this, the newer Pliocene period, the 

 majority of the inhabitants of the world were specifi- 

 cally the same as now, and we have good reason to be- 

 lieve that the climate was warmer than at the present 



