474 THE AMERICAN NATURALIST [Vol.L 



forms. The greater plasticity, probably belongs to the 

 herb, owing to the larger independence of environment 

 which its discontinuous existence allows. Brevity of life- 

 cycle, however, rather than higher variability, is probably 

 the cause of the more rapid rate of change exhibited by 

 members of this type. 



The conclusion that herbs have been evolved much more 

 rapidly than trees or shrubs has certain important cor- 

 ollaries. There are at present approximately 2,600 gen- 

 era of dicotyledonous herbs and 4,000 genera of woody 

 plants. The much wider taxonomic range which we have 

 noted in the latter type makes it probable that in extinct 

 species and genera their numerical superiority is still 

 greater. If herbs are thus decidedly fewer than woody 

 plants in number of species and genera, but are neverthe- 

 less being produced at a much more rapid rate, it is highly 

 probable that the herbaceous element in the flora of the 

 world must have had a shorter evolutionary history than 

 the woody one. This is in harmony with a considerable 

 body of evidence derived from a study of the history, 

 structure and distribution of the Angiosperms, which in- 

 dicates that the most ancient members of the group were 

 woody and that herbaceous vegetation has made its ap- 

 pearance in comparatively recent geological time. 6 The 

 great steps in the evolution of the higher plants seem to 

 have taken place while trees and shrubs were the domi- 

 nant growth types, for all the important orders and the 

 great majority of families are still composed wholly or m 

 part of such plants. Herbs occur to-day in less than half 

 the families of the dicotyledons, and although such a 

 dominant portion of the vegetation in many regions, they 

 seem to be of relatively recent appearance. 



eSinnott, E. W., an<l Bailev, I. W., "The Origin and Dispersal of Her- 

 baceous Angiosperms," Annals of Botany, XXVIII, 1914, pp. 547-600. 



