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 
PEATEAUS. 
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 
