GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 237 
are either identical with or closely related to eastern 
American species. Such characteristic plants as the 
fox-grape, poison ivy, and other sumachs, bitter-sweet 
(Celastrus), the sensitive fern ( Onoclea sensibdilis), elms, 
maples, beeches, oaks, and magnolias, very close to their 
American relatives, as well as others familiar to the 
botanist, were the predominant features of the vegeta- 
tion. Were these forms also common to Pacific North 
America and continuous across the continent, there 
would be nothing remarkable in their occurrence in 
Japan; but most of them are entirely absent from our 
Pacific coast, and from all the intermediate country. 
The list of forms common to the Mantchurian-Japan- 
ese region and Atlantic North America is very large, 
and at first seems impossible to explain; but when we 
consider them, as they doubtless are, remnants of a once 
continuous northern flora, which have survived in these 
two widely separated areas owing to very similar cli- 
matic conditions, the wonder ceases. 
The southern United States illustrate very clearly 
the very different character of plants in the same lati- 
tude, and over a continuous area, due to different condi- 
tions of topography and rainfall. The southwestern 
United States — southern California and Arizona —show 
genuine desert conditions with an extremely character- 
istic flora, of which cacti, agaves, yuccas, sage-brush, etc., 
are the conspicuous features. This flora is closely re- 
lated to that of Mexico, and to some extent to that of 
Pacific South America. As we pass eastward, the lofty 
ridge of the Rocky Mountains forms an effective barrier 
against the passage of some forms, and the heavier rain- 
fall on the eastern slopes of the mountains, increasing 
