374 



PAST succession: the cenbosere. 



the coniferous belts of the north and of the mountains were even then more or 

 less dijEferentiated into two climaxes as at present. The advance of the ice 

 was doubtless more or less pulsatory, but we get perhaps the clearest picture 

 of the course of the clisere if we assiune that the ice gradually overwhelmed 

 the timdra zone, and at the same time changed the conditions in the bog-scrub 

 zone in such a way that they became unfavorable for shrubs and favorable to 

 the herbs, mosses, and lichens of the tundra. At the same time, perhaps 



(i;^ Tundra 



Iggl Scrub 

 Evva Deciduous Foresi 

 [ inrai Conifers(Northcrn) 

 tv?^ ConifersCSouthem) 



Fig. 



36. — Ice-mass and cliseral zones in front of it during a 

 glacial epoch. Slightly modified from Transeau. 



because of the general deformational sequence as much as the influence of 

 the ice, the factors of the coniferous belt became less favorable to trees and 

 more favorable to shrubs. How soon this influence was felt in the deciduous 

 climax is largely a matter of conjecture, but sooner or later colder or drier 

 conditions appeared there as well. In consequence, migration to theXsouth- 

 ward alone was capable of securing the preservation of each zone. The parent 

 individuals and the migrants in all other directions were eliminated by compe- 

 tition with the newcomers from the north, by the direct action of the colder 

 climate, and ultimately by the destructive action of the ice itself. As a con- 



