462 OEIGIN OF CULTIVATED PLANTS. 



perhaps as far as America, do not seem to have trans- 

 ported a single cultivated species. Neither has the Gulf 

 Stream produced any effect. Between America and 

 Asia two transports of useful plants perhaps took place, 

 the one by man (the Batata, or sweet potato) the other 

 by the agency of man or of the sea (the cocoa-nut palm). 



