778 EDMUND W. SINNOTT 



a herbaceous species will pass through a much larger number of 

 generations than a woody one, and will consequently tend, other 

 factors being equal, to become changed in type much more rapidly. 

 We should thus expect the herbaceous element in the vegetation 

 to have been evolved at a much faster rate than the woody element. 

 The establishment of this as a fact, taken with what we know as 

 to the history and present numerical status of herbs and woody 

 plants, will provide us with a valuable clue as to the antiquity 

 of the angiosperms. 



That herbs are indeed subject to more rapid changes than any 

 other plant type is indicated by the fact that the first local species 

 and genera to develop in a region subsequent to its isolation have 

 apparently almost always been herbs. This is well illustrated by a 

 comparison of the floras of temperate North America and of Europe. 

 On these continents today there are many local or "endemic" 

 genera which are limited in their distribution to one or to the other. 

 Certain of these are evidently "relict" endemics, isolated survivors 

 of types once much more widely disseminated. They may be 

 recognized from the fact that they stand without near relatives 

 in the floras; and many of them, such as sassafras and hickory, 

 occur as fossils on both sides of the Atlantic. These relicts doubt- 

 less constitute a very ancient floral element, and it is significant 

 that among them are practically all the genera of trees and shrubs 

 which are local to either North America or Europe. The majority 

 of the endemic genera, however, seem to belong to quite a different 

 category, for they occur in groups of from three to twenty genera, 

 the members in each of which are closely related to one another, 

 each group apparently to be looked upon as a separate center of 

 evolution and the nucleus of a new family. The genera centering 

 around Lesquerella in the Cruciferae, around Eriogonum in the 

 Polygonaceae, around Godetia in the Onagraceae, around Pent- 

 stemon in the Scrophulariaceae, and around Solidago in the 

 Compositae, are a few of the sixty or more such groups in the dicoty- 

 ledonous flora of North America, and there are as many in Europe. 

 These "indigenous" endemic genera most probably had their 

 origin on their respective continents, since a free interchange of 

 plants between America and Europe was interrupted, presumably 



