Chap. XII.] 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 aje 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 moun- 

 tain-slopes and on the plains of North America and 

 Europe are the same; and it may be asked how I ac- 

 count 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 in- 

 habitants 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 



