The Problems of Plant Distribution 259 



cept in modern times, must have been so infrequent 

 as to be practically of no account, and this explains 

 the rarity even of generic types common to them. 

 Except where these have been introduced by man, 

 they are, as we have seen, genera which are wide- 

 spread and generally occurring also in the temperate 

 zones. 



The development of the great continental areas in 

 Asia and America caused pronounced changes in 

 the climate, and the inner areas, arid and subjected 

 to extremes of burning heat and arctic cold, could 

 no longer support the moisture-loving plants which 

 prevailed in pre-glacial times, and hence the restric- 

 tion of these to the moister and more temperate 

 regions nearer the coast. These, arid plains also 

 acted as a barrier against migration to the regions 

 of the West, which were better adapted to their 

 growth. In the United States, however, the Pacific 

 coast, owing perhaps to its long dry summers, is 

 not at present suited to the growth of many eastern 

 trees, although we know that some of these once 

 existed there. 



Influence of the Western Mountains upon the 

 Climate of the United States. In the United 

 States, the development of the great mountain 

 masses of the West, must have exerted a great 

 influence in determining the climate of the 

 great central area, by shutting off the moisture- 

 laden winds of the Pacific. The Rocky Mountains 

 were formed at the end of the Cretaceous, 



