ica was favored by having nearly longitudinal 

 ranges, permitting the free passage of plants to 

 and fro along unobstructed plateaus. 



The two long American watersheds the Rocky 

 Mountains and the Alleghanies deliminate three 

 plateaus, Atlantic Slope, Mississippi Valley, and 

 Pacific Slope. We have seen that the distribution 

 of plants was not equal the Pacific Slope having 

 the lion's share, and largest growths. This is due 

 principally to four potential agencies. 



I. CONDUCTED NORTHWEST BY HIGH 

 PLATEAUS. 



The Rocky Mountains, lying near the western 

 side of the continent, extend southeasterly, upon 

 the eastern side of the high plateau of Mexico, to 

 Central America. The plants returning from the 

 south at the beginning of the Thermal Age, 80,000 

 years ago, were divided at the outset in southern 

 Mexico, and a large part barred out of the Missis- 

 sippi Valley by the Cordilleries, that killed off un- 

 fortunate individuals or families which ascended 

 their foot-hills and spurs as described while 

 other families or other members of the same fami- 

 lies were attracted to pursue the broad, free, and, 

 at the time, cool and inviting pathway along the 

 high plateau of Mexico and Arizona, diminished 

 at every league by the many spurred Rocky 

 Mountains; the survivors escaping to pass into 

 the valleys and along the sides of the mountain ' 

 ranges, some of them particularly directed north- 

 westward again by the favoring coast plateau, deci- 

 mated the while, by the high, similarly inclined 



