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 (Onocleasensibilis), 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 



