4GH 



THE AMERICAN NATURALIST [Vol. L 



It is an analysis only of the latter type which is of sig- 

 nificance for the present problem. 



The genera of dicotyledons endemic 1 in temperate North 

 America (Canada, the United States and northern Mexico) 

 and in Europe (inclusive of the entire Mediterranean 

 floral province) were studied in this connection. 400 

 genera were recorded as being endemic or essentially so in 

 temperate North America. In this imposing array of 

 types which are limited in their distribution to this region 

 approximately 130 stand in isolated positions in the flora, 

 being quite without near relatives, and are presumably 

 * ' relicts. ' ' To this category belong Carya, Planera, Mac- 

 lura, Garrya, Sassafras, Xanthorrhiza, Baptisia, Nemo- 

 panthus, Ceanothus, Dirca, Dionaea, Hudsonia, Rhexia, 

 Ptelea, Decodon, Houstonia, Symphoricarpos and many 

 other familiar plants. That these now exclusively Amer- 

 ican genera were indeed at one time much more widely 

 distributed is indicated by the fact that many of them are 

 found as fossils in Europe and Asia to-day. They un- 

 doubtedly represent a very ancient element in the flora. 

 The most noteworthy feature of these "relicts" is that 

 they include practically all the genera of ivoody plants 

 endemic to this region, the typically American trees and 

 shrubs. 



Such ancient and isolated relicts, however, compose but 

 a minority of the endemic genera. The remainder evi- 

 dently belong to the other category and owe their ende- 

 mism rather to the fact that they have been developed in 

 America and have never spread beyond its borders. It is 

 such endemic types which are of interest to our problem. 

 That they are actually of local origin is rendered probable 

 by their occurrence in groups of closely related genera 

 (many of which are rich in species) each group presum- 

 ably representing a separate center of evolutionary de- 

 velopment and the nucleus for a new subfamily. There 



i Genera in which 90 per cent, of the species or more are confined to the 

 region in question were considered as endemic there. Genera rather than 



change; and since endemism among species ^s so nearly universal on the 



