GENERAL LAWS OF ORIGIN AND MIGRATION. 255 



the views of those extreme glacialists who suppose conti- 

 nental ice-caps reaching half way to the equator are borne 

 out by facts. In truth, the ice accumulating round the 

 pole must have been surrounded by water, and there must 

 have been tree-clad islands in the midst of the icy seas, 

 even in the time of greatest refrigeration. This is proved 

 by the fact that, in the Leda clay of eastern Canada, 

 which belongs to the time of greatest submergence, and 

 whose fossil shells show sea-water almost at the freezing- 

 point, there are leaves of poplars and other plants which 

 must have been drifted from neighbouring shores. Simi- 

 lar remains occur in clays of like origin in the basin of 

 the great lakes and in the West. These have been called 

 " interglacial," but there is no evidence to prove that they 

 are not truly glacial. Thus, while we need not suppose 

 that plants existed within the Arctic circle in the Glacial 

 age, we have evidence that those of the cold temperate 

 and sub-arctic zones continued to exist pretty far north. 

 At the same time the warm temperate flora would be 

 driven to the south, except where sustained in insular 

 spots warmed by the equatorial currents. It would return 

 northward on the re-elevation of the land and the re- 

 newal of warmth. 



If, however, our modern flora is thus one that has re- 

 turned from the south, this would account for its poverty 

 in species as compared with those of the early Tertiary. 

 Groups of plants descending from the north have been 

 rich and varied. Eeturning from the south they are like 

 the shattered remains of a beaten army. This, at least, 

 has been the case with such retreating floras as those of 

 the Lower Carboniferous, the Permian, and the Jurassic, 

 and possibly that of the Lower Eocene of Europe. 



The question of the supply of light to an arctic flora 

 is much less difficult than some have imagined. The 

 long summer day is in this respect a good substitute for 

 a longer season of growth, while a copious covering of 



