258 Plant Life and Evolution 



flower, of our Eastern woods, has its mate in a 

 second species growing in Japan, while elsewhere 

 the genus is quite unknown. 



As might be expected in the ages that have elapsed 

 since the redistribution of the Tertiary flora took 

 place, most of the forms have changed to some ex- 

 tent, so that it is rare to meet identical species in 

 such widely separated regions as Japan and New 

 England. The change has gone so far in some 

 cases that a genus of one district is represented by 

 a different but closely allied one in the other. Thus, 

 for example, the flowering dogwood of Eastern 

 America is represented in Japan by a closely allied 

 genus, Benthamia. Identical species, however, may 

 occur. The poison-ivy and the fox-grape of At- 

 lantic North America are represented in Japan by 

 what are usually considered to be identical species, 

 and the sensitive-fern and the beautiful little 

 orchid, Pogonia, are the same in Japan and Massa- 

 chusetts. 



Our knowledge of the geological history of the 

 flora of the tropics is still incomplete, and as these 

 regions were not influenced materially by the great 

 Glacial epoch, and as the conditions in the tropical 

 regions are conducive to rapid evolution of new 

 forms, it is not remarkable that the tropical floras 

 should differ very widely from the temperate ones. 

 Moreover, as the tropics of the Old and New World 

 must have been completely isolated from very re- 

 mote times, migration from one to the other, ex- 



