Chap. XI. DURING THE GLACIAL PERIOD. 369 



ferent ; for it is not likely that all the same arctic spe- 

 cies will have been left on mountain ranges distant from 

 each other, and have survived there ever since ; they 

 will, also, in all probability have become mingled wdth 

 ancient Alpine species, which must have existed on 

 the mountains before the commencement of the Glacial 

 epoch, and which during its coldest period will have 

 been temporarily driven down to the plains ; they will, 

 also, have been exposed to somewhat different climatal 

 influences. Then- mutual relations will thus have been 

 in some degree disturbed ; consequently they will have 

 been liable to modification ; and this we find has been 

 the case ; for if we compare the present Alpine plants 

 and animals of the several great European mountain- 

 ranges, though very many of the species are identically 

 the same, some present varieties, some are ranked as 

 doubtful forms, and some few are distinct yet closely 

 allied or representative species. 



In illustrating what, as I believe, actually took place 

 during the Glacial period, I assumed that at its com- 

 mencement the arctic productions were as uniform 

 round the polar regions as they are at the present day. 

 But the foregoing remarks on distribution apply not 

 only to strictly arctic forms, but also to many sub-arctic 

 and to some few northern temperate forms, for some of 

 these are the same on the lower mountains and on the 

 plains of North America and Europe ; and it may be 

 reasonably asked how I account for the necessary de- 

 gree of uniformity of the sub-arctic and northern tem- 

 perate forms round the world, at the commencement of 

 the 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 

 Atlantic Ocean and by the extreme northern part of 

 the Pacific. During the Glacial period, when the in- 



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