470 



THE AMERICAN NATURALIST 



[Vol. L 



baceous forms. Since we have cause to believe that com- 

 munication between Xorth America and Europe existed 

 until well into the Tertiary, the supposition is altogether 

 reasonable that the genera in question have undergone at 

 least the greater part of their development and dispersal 

 since that time; and that in contrast to the ''relicts" and 

 the non-endemic genera they represent an element of very 

 recent development in the floras of the two regions. 



It is noteworthy that these indigenous endemic genera 

 are composed almost exclusively of herbaceous species, 

 Ccrcocarpus and its allies among the Rosacea 1 furnishing 

 practically the only exception to the rule in America. If 

 the conclusion is correct that such endemic types are the 

 most recently developed members of a flora, this domi- 

 nance of the herb among them constitutes excellent evi- 

 dence that it is the herbaceous element which has indeed 

 been undergoing the most rapid evolutionary change. 



In striking contrast to the highly endemic and local 

 character of so many of the north temperate genera of 

 herbs is the wide geographical range almost universal 

 among the woody types. Nearly all the tree genera of 

 temperate North America exist to-day on some portion of 

 the Eurasian continent or give evidence by fossils that 

 they once did exist there. The whole study of endemism 

 in its relation to growth forms presents us with the pic- 

 ture of a very slowly changing woody vegetation, one 

 which since the separation of the two northern land 

 masses has given rise to few or no generic types, but 

 which has been accompanied by a rapidly developing her- 

 baceous flora so quick to originate new forms that upon 

 isolation it has produced not only a throng of local species, 

 but even a goodly number of genera. 2 



Evidence of value in the present problem may also be 

 derived from a study of the relationship of the members 



demic element of the floras of oceanic islands and of somewhat isolated con- 

 tinental areas in the south temperate zone, is evidently due to the fact that 

 most of the herbaceous portion of the vegetation here has so recently arrived 

 from its seat of origin in the great land areas of the north, that it has not 

 yet had time to develop into endemic species and genera on a large scale. 



