No. 596] RAPIDITY OF EVOLUTION 



473 



ence between the two types is still more marked, for the 

 190 woody families include about 59,000 species, an aver- 

 age of 310 ; the 94 herbaceous families about 49,000 spe- 

 cies, an average of 510. 



It is therefore evident that herbs tend to be massed in 

 a comparatively few large genera and families. Among 

 trees and shrubs, on the other hand, although the majority 

 of species are also naturally in large, successful groups, 

 there is a very much greater proportion of small genera 

 and especially of small families, widely scattered through- 

 out the whole taxonomic range, which have been isolated 

 by the wholesale extinction of related forms and are ap- 

 parently very slow to develop into larger aggregations. 

 Such evidence as this, like that derived from a study of 

 indigenous endemism, seems to indicate that the rate of 

 evolution among herbs is decidedly higher than among 

 woody forms. 



There are doubtless numerous exceptions on both sides. 

 Crataegus, Eucalyptus, Acacia and other genera of trees 

 and shrubs appear to be rapidly developing new species, 

 whatever the cause thereof may be; and many herbs, far 

 from producing new forms, show every indication of be- 

 ing stationary or even of becoming extinct. These are 

 both evidently exceptions to the general rule, however. 



Of course these cases introduce the consideration of 

 another factor, naturally thought of first in connection 

 with rate of evolution among any organisms, namely, 

 their comparative ' 1 variability, ' ' using the term in its 

 broadest sense. To contrast members of the two .\yt h 

 types as a whole in this respect is necessarily wry dim- 

 cult, but an attempt was made to do so by comparing tli<« 

 proportion of varieties and named forms among n < 

 woody plants with that among the herbs in a number of 

 floras. The results are shown in the table (p. 474). 5 



The proportion of varieties and forms is therefore 

 practically the same among woody plants as it is among 

 herbs, and if this is to be regarded as at all a criterion ot 

 variability, there is little to choose between the two growth 



s Dicotyledons only are considered. 



