424 THE ICE AGE IN NORTE AMERICA. 



and the corresponding region of North America, only one 

 third is represented in western North America. 



The key applied by Professor Gray for the solution of 

 this problem was suggested by the investigations of Heer and 

 others, which had brought out the fact that, during the Ter- 

 tiary period, just before the beginning of the Ice age, a tem- 

 perate climate, corresponding to that of latitude 35° on the 

 Atlantic coast, extended far up toward the north pole, per- 

 mitting Greenland and Spitzbergen to be covered with trees 

 and plants similar, in most respects, to those found at the 

 present time in Virginia and North Carolina. Here, indeed, 

 in close proximity to the north pole, were then residing, in 

 harmony and contentment, the ancestors of nearly all the 

 plants and animals which are now found in the north tem- 

 perate zone, and here they would have continued to stay but 

 for the cold breath of the approaching Ice age, which drove 

 them from their homes, and compelled them to migrate to 

 more hospitable latitudes. 



The picture of the flight and dispersal of these forests, 

 and of their struggle to find, and adjust themselves to, other 

 homes, is second in interest to that of no other migration. 

 A single tree is helpless before such a force as an advancing 

 glacier, since a tree alone can not migrate. But a forest of 

 trees can. Trees can " take to the woods " when they can 

 do nothing else, arid so escape unfavorable conditions. 

 There is a natural climatic belt to which the life of a forest 

 is adjusted. In the present instance, as the favorable con- 

 ditions near the poles were disturbed by the cooling influ- 

 ences of the glacier approaching from the north, the indi- 

 vidual trees on that side of the forest-belt gradually perished ; 

 but, at the same time that the favorable conditions of life 

 were contracting on the north, they were expanding on the 

 south, so that along the southern belt the trees could gradu- 

 ally advance into new territory, and so the whole forest-belt 

 move southward, following the conditions favorable to its 

 existence. It is therefore easy to conceive how, with the 

 slow advance of the glacial conditions from the north, the 



