THE FALL OF THE LEAF. 215 



aspect. But in the miocene age this tem- 

 perate arctic flora was driven southward by 

 the advancing cold, while a more strictly 

 northern type of vegetation began to show 

 itself among the hardy survivors which could 

 accommodate themselves to the chillier win- 

 ters of the new epoch. In the pliocene period, 

 once more, the arctic miocene trees invaded 

 northern and central Europe, and a still 

 colder type appeared around the Poles. 

 Finally, with the pleistocene age, masses of 

 ice began to occupy the North Pole itself, 

 and drove even the hardiest and most arctic 

 vegetation down to the Mediterranean basin, 

 while England and half Germany were 

 covered by the enormous sheet of permanent 



glaciers. . ■- ''•'■:,.•. :;.-,^:vr'-"..-'-v 



Now though the conifers, with their tough 

 capillary leaves, did not suffer largely from 

 the change, the evergreen tropical trees were 

 clearly quite unfitted for conditions such as 

 these. Their big leaves could do no serious 

 work in the way of assimilating carbon from 



