668 HISTORICAL GEOLOGY 



expanded mountain glaciers from the east-west ranges (Pyrenees 

 and Alps) at the south. As a result, the less hardy plants were killed, 

 leaving a flora rather poor in species. In eastern North America, on 

 the other hand, the mountain ranges have a north-south direction; 

 and the broad plains offered few obstacles to the migration, back 

 and forth, of plants and animals. In western North America where 

 north and south migration was more difficult, 31 genera of trees are 

 found as contrasted with 66 genera in the east. 



Gn the summits of some mountains arctic plants and insects are 

 found whose presence is difficult to explain except on the assumption 

 that, as the ice retreated northward, they followed the front closely 

 moving up the sides of the mountains as the ice retreated ; and that 

 they were stranded there when the ice disappeared from the region. 



Attention has been called (p. 644) to the fact that, apparently as a 

 result of the oscillations of climate which marked the Pleistocene, 

 plants were forced to special adaptations and habitats, with the result 

 that there is now little mingling of tropical and subtropical types such 

 as was the case in the Tertiary. 



REFERENCE ON VEGETATION 

 Wright, G. F., — The Ice Age in North America, 4th ed., pp. 372-391. 



Mammoths and Mastodons. — Perhaps the most characteristic 

 Pleistocene mammals were the mammoth and the mastodon (Fig. 

 577). The mammoths (Columbian and Imperial) are true elephants, 

 and the mastodon is closely related. They have the same general 

 appearance; the most conspicuous differences being (1) in the teeth, 

 which in the mastodon have large, transverse ridges, but in the 

 mammoth (as in the living elephant) are made up of many plates 

 of enamel, alternating with cement and dentine (Fig. 548, p. 613) ; 

 (2) in the forehead, which is low in the mastodon and high and bulging 

 in the mammoth; and (3) in the shorter and more massive legs of 

 the mastodon. 



The largest specimens of the mammoth (Elephas primigenius) ex- 

 ceed in size that of any elephant now living, but their average height 

 was probably not much greater. The mastodon was somewhat smaller 

 than the mammoth. Both were covered with long hair, with prob- 

 ably an undercoating of fine wool. It is known that the mammoth 

 had this additional protection against the cold, but it is not definitely 

 known that the mastodon was thus protected. 



