THE ANTIQUITY OF THE ANGIOSPERMS 779 



in the Early or Middle Tertiary; for had they existed before that 

 date in anything like their present numbers and importance, it is 

 highly unlikely that they would now be represented in the floras 

 of both hemispheres. During the time since the isolation of the 

 two continents, and while the rest of the flora have remained 

 unchanged or have been developing endemic species merely, these 

 plants have evidently undergone much wider changes, until they 

 have finally given rise to new generic types. We are thus forced 

 to conclude that the indigenous endemic genera constitute the 

 most rapidly evolving members of their flora; and it is significant 

 that they include practically nothing but herbaceous species — 

 surely excellent evidence that the herb changes in type more rapidly 

 than the tree or the shrub. 



Further evidence pointing to the same conclusion is presented 

 by a study of the distribution of herbs and of woody plants in the 

 modern scheme of botanical classification, for herbs are found to 

 occur in larger groups than woody plants, their genera containing 

 more species and their families more genera. Monotypes and 

 very small genera and families are very much less common among 

 herbs than among woody plants. These facts are what one might 

 expect on the supposition that herbs are changing faster than the 

 rest of the angiospermous vegetation, for the more rapid production 

 of new forms leads to the building up of larger aggregations, and 

 enables genera or families which have become reduced in size 

 through extinction to repair these ravages quickly. 



A study of the structure, distribution, and ancestry of herba- 

 ceous angiosperms 1 indicates that they have been evolved in com- 

 paratively recent times from a woody ancestry, and have undergone 

 practically their whole course of development since the beginning 

 of the Tertiary. As opposed to this rapid change among herbs, 

 we know from fossil evidence that very many woody genera have 

 existed with very little alteration for a much more extended period 

 than the length of the Tertiary — a convincing demonstration of 

 the slowness with which trees and shrubs undergo evolutionary 

 change. Almost all our woody genera bear evidence, in present 



1 E. W. Sinnott and I. W. Bailey, "The Origin and Dispersal of Herbaceous 

 Angiosperms," Annals of Botany, XXVIII (1914), 547-6°°- 



