366 MERIDIONAL ZONES chap, xviif. 



through various phases and oscilkitions of temperature; so 

 tliat, although the chief polishing and furrowing of the roekw 

 and transportation of erratics in Europe and North America 

 may have taken place contemporaneously, according to the 

 ordinary language of geology, or when the same testacea and 

 the same post-pliocene assemblage of mammalia flourished, 

 yet the extreme development of cold on the opposite sides 

 of the ocean may not have been strictly simultaneous, but, 

 on the contrar}^, the one may have preceded or followed the 

 other b}" a thousand or more than a thousand centuries. 



It is probable that the greatest refrigeration of Xorwa}', 

 Sweden, Scotland, Wales, the Vosges, and the Alps coincided 

 very nearly in time; but when the Scandinavian and Scotch 

 mountains were incrusted with a general covering of ice, 

 similar to that now enveloping Greenland, this last country 

 may not have been in nearlj^ so glacial a condition as now, 

 just as we find that the old icy crust and great glaciers, 

 which have left their mark on the mountains of Norway and 

 Sweden, have now disajipeared, pi*ecisely at a time when the 

 accumulation of ice in Greenland is so excessive. In other 

 words, we see that in the present state of the northern hemi- 

 sphere, at the distance of about fifteen hundred- miles, two 

 meridional zones, enjoying very difterent conditions of tem- 

 perature, may coexist, and we are, therefore, at liberty to 

 imagine some former alternations of colder and milder 

 climates on the opposite sides of the ocean throughout the 

 post-pliocene era of a compensating kind, the cold on the one 

 side balancing the milder temperature on the other. By 

 assuming such a succession of events we can more easily 

 explain why there has not been a greater extermination of 

 species, both terrestrial and aquatic, in polar and temperate 

 regions, dui-ing the glacial epoch, and why so many species 

 are common to pre-glacial and post-glacial times. 



The numerous plants which are common to the temj)erate 



