366 MERIDIONAL ZONES CHAP. xvm. 



various phases and oscillations of temperature; so that, 

 although the chief polishing and furrowing of the rocks 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 

 contrary, the one may have preceded or followed the other by 

 a thousand or more than a thousand centuries. 



It is probable that the greatest refrigeration of Norway, 

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

 cided very nearly in time ; but when the Scandinavian and 

 Scotch mountains were encrusted with a general covering of 

 ice, similar to that now enveloping Greenland, this last country 

 may not have been in nearly 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 disappeared, precisely 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 different conditions of tem 

 perature, may co-exist, 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, during 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 temperate 



