xxvn] SPECIFIC DIFFERENCES 329 



are uniform in the extreme. " By no stretch of imagination can 

 the variety in the conditions of life be made to fit one quarter 

 of the variety of structure 1 ." Even the dorsiventrality, which 

 is obviously associated with the mode of growth, must not, 

 according to Willis, be interpreted as an advantageous adapta- 

 tion, for he points out that the least modified species, in which 

 dorsiventrality hardly occurs at all, can and do live in nearly 

 all the places occupied by the family. As a conspicuous ex- 

 ample of the lack of adaptation among these plants, Willis 1 

 instances the fact that, in the great majority of species, there is 

 no device to enable the seeds to cling to the rocks upon which 

 they find themselves shed; he thinks it probable that it often 

 takes from five hundred to one thousand seeds to produce three 

 or four seedlings. 



The Podostemaceae thus exhibit great variety and marked 

 specific differentiation, but the features in which the genera 

 and the species differ from one another cannot, according to 

 Willis, be explained as adaptational. Further, the particular 

 situations in which they thrive are such as almost to preclude 

 competition with other plant forms, and there is also relatively 

 little struggle for existence even between members of the same 

 species. On these grounds Willis concludes that the evolution 

 of the group cannot be explained as due to the natural selection 

 of infinitesimal variations. 



In scrutinising Willis's criticism of selectionist views, no 

 progress can be made unless Natural Selection be analysed in 

 accordance with the two distinct claims which have been made 

 on its behalf firstly, that it is the cause of the origin of species, 

 and secondly, that it is one of the factors conditioning adapta- 

 tion. Unfortunately the distinctness of these two functions is 

 not clearly recognised in Darwin's own work, and the con- 

 fusion thus initiated has given rise to much obscurity in later 

 writings. Willis's observations certainly strike a severe blow 

 at Natural Selection considered from the first point of view, 

 i.e. as the originator of specific types. In the Podostemaceae we 

 i Willis,J. C. (I9H 1 ). 



