GEOGRAPHICAL DISTRIBUTION 163 



to the plains; they will, also, have been subsequently 

 exposed to somewhat different climatal influences. Their 

 mutual relations will thus have been in some degree dis- 

 turbed; consequently they will have been liable to modi- 

 fication; 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 represent- 

 ing 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 re- 

 gions as they are at the present day. But it is also nec- 

 essary to assume that many sub-arctic and some few tem- 

 perate 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 productions of the Old and New 

 Worlds are separated from each other by the whole At- 

 lantic Ocean and by the northern 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 continents. The 



