XV.] gray's conclusions. 273 



have an explanation of the remarkable fact, long ago 

 noticed by Bentham, that American species have 

 reached Europe through Asia. 



" Under the light which these geological consider- 

 ations throw upon the question, I cannot resist the 

 conclusion," writes Gray, "that the extant vegetable 

 kingdom has a long and eventful history, and that 

 the explanation of apparent anomalies in the geo- 

 graphical distribution of species may be found in the 

 various and prolonged climatic or other physical 

 vicissitudes to which they have been subject in ear- 

 lier times." 



A certain flora "established itself in Greenland," 

 says 8ir J. W. Dawson, " and probably all around 

 the arctic circle, in the warm period of the earliest 

 eocene, and, as the climate of the northern hemi- 

 sphere became gradually reduced from that time till 

 the end of the pliocene, it marched on over both con- 

 tinents to the southward, chased behind by the mod- 

 ern arctic flora, and eventually by the frost and snow 

 of the glacial age." "If, however, our modern flora 

 is thus one that has returned from the south," says 

 Dawson, again, "this would account for its poverty 

 in species as compared with those of the early terti- 

 ary. Groups of plants descending from the north 

 have been rich and varied. Returning from the 

 south they are like the shattered remains of a beaten 

 army. * * * It is, indeed, not impossible that in 

 the plans of the Creator the continuous summer sun 

 of the arctic regions may have been made the means 

 for the introduction, or at least for the rapid growth 

 and multiplication, of new and more varied types of 

 plants. ***** What we have learned 



18 SUB. 



