No. 596] 



RAPIDITY OF EVOLUTION 



471 



of the various growth forms and their distribution in the 

 modern system of plant classification. During the his- 

 tory of the vegetable kingdom, the continual evolution of 

 species, genera and families has been opposed by their 

 continual extinction, for one cause or another. One would 

 expect that those types in which evolution was proceeding 

 most rapidly would occur in groups, usually rather large, 

 of closely related species and genera, and that members 

 of such groups which subsequently came to be isolated by 

 the extinction of their allies would soon become centers of 

 development and give rise again to new groups. Mono- 

 types would consequently be rare among them. The more 

 slowly evolving forms, on the other hand, would be able 

 to repair the ravages of extinction much less rapidly and 

 easily and would therefore tend to occupy more or less 

 isolated positions in the system, frequently as monotypic 

 genera or families. 



To determine the distribution of herbs as contrasted 

 with woody plants in the present scheme of classification 

 an analysis was made of the dicotyledons in Engler and 

 Prantl's "Naturliche Pflanzenfamilien, ' ' supplemented 

 and brought up to date as far as possible by the seventh 

 edition of Engler-Gilg V Syllabus derPflanzenfamilien." 

 The figures obtained of course can not be regarded as 

 exact, but they are at least definite enough to bring out 

 certain general facts. 



One hundred and eight thousand specie of dicotyledon^ 

 were counted, grouped in 6,840 genera and 238 families ; 

 4,030 genera, comprising slightly over 5<>,i>0<) species, were 

 found to be composed entirely of woody plants, and 2,uM 

 genera, with slightlv under 40.000 species, entirely of herbs ; 

 180 genera, containing over 18,000 species, included l».tl. 

 woody and herbaceous members and were disregarded m 

 the count. 3 The average number of species in the w<...dy 

 genera is therefore 12.5, in the herbaceous ones, 15. In 

 their large genera (including 10 species or more) the two 



a If 90 per cent, of the species of a genu .were woody that 



