No. 596] 



IAP1DITY OF EVOLUTION 



-177 



changes in the organic world, as perhaps the time when 

 the angiospermous stock began to be differentiated from 

 its gymnospermous ancestors. 



We may point out, in conclusion, that a recognition of 

 differences in the rate of evolution between various 

 growth forms is evidently of importance in many prob- 

 lems concerned with the phylogeny, ecology or distribu- 

 tion of the higher plants. Upon the question, for ex- 

 ample, as to whether the boreal species in the floras of 

 southern South America and Australasia are of ancient or 

 recent arrival there, some light is thrown by the fact that 

 these species are almost all herbs. That so many mem- 

 bers of a growth-form which is usually subject to rapid 

 change still maintain specific identity with distant north- 

 ern types argues strongly for their comparatively recent 

 arrival in the south. A similar question is raised by 

 Willis's studies of the flora of Ceylon. 8 He regards the 

 endemic element here as one which is of local and recent 

 origin and as much younger than the non-endemic ele- 

 ment ; but the fact that of the endemic species, by impli- 

 cation the ones which are changing the fastest, less than 

 one fourth are herbs, whereas nearly one half of the non- 

 endemic species belong to this growth-form, would sug- 

 gest the opposite conclusion ; for had this large body of 

 herbs been in existence here for a very long period, as 

 Willis supposes, we should have expected it to develop a 

 relatively much greater body of endemic species. In such 

 questions as this it should be borne in mind that evolution 

 has not been a steady and uniform process among all 

 forms, but that the decided diftVivn.-,- between the vari- 



accumulate heritable variations introduces a factor which 

 it is always necessary to consider. 



SUMMABY 



1. The most recently evolved element in the floras of 

 temperate North America and of Europe, as determined 



