25 



north pole, the hardier races alone surviving ; and these were driven south- 

 ward in a veritable rout by the icy blasts. 



When these boreal times subsided in a degree, the advance of the forest 

 was as sure as before, but the battle order was somewhat changed to suit 

 the new conditions of soil and climate. Only the hardier tribes could re- 

 gain the northernmost posts, and of those who followed, many found their 

 former places of occupancy changed by fluvial and lacustrine formations 

 and by the drifts borne and deposited by icy sheets, while some, by their 

 constitution, were entirely unfitted from engaging in a northern campaign, 

 or found insurmountable barriers in the refrigerated east-west elevations, 

 as in Europe and Western Asia. 



PETRIFIED TREE 



In addition, there had come new troubles from volcanic eruptions, 

 which would again and again wrest the reconquered ground from- the per- 

 sistent advance guards of the arboreal army, annihilating them again and 

 again. 



Finally, when the more settled geologic and climatic conditions of the 

 present era arrived, and the sun arose over the world, ready for human 



