THE REPRODUCTIVE FACTOR IN EVOLUTION. 307 



{a.) First of all, as to the origin of variations, we find that 

 what Treviranus recognised in the first years of this century, — 

 viz., the influence of fertilisation in evoking change, — has 

 been emphasised by several, such as Brooks and Galton, and 

 has been especially elaborated by Weismann. As we have 

 just seen, Weismann finds in the interminghng of two " germ- 

 plasmas," which is the essence of fertilisation, the sole origin of 

 variations of any account in the evolution of the species. 

 Whether this be consistent with Weismann's theory of fertilisa- 

 tion or not is matter for debate, but there is no doubt that his 

 emphasis on the evolutionary value of sexual reproduction is a 

 most important contribution to the general theory. In some- 

 what marked contrast is the view recently advocated by Hat- 

 schek, who sees in the intermingling essential to fertilisation a 

 counteractive of idiosyncracies, a means of controlling and 

 checking disadvantageous individual peculiarities. The two 

 positions are not antagonistic, but rather complementary to one 

 another. 



{b.) No impartial student of Darwinism can fail to admit, 

 that in the " struggle for existence " stress is laid upon the 

 nutritive and self-maintaining functions and strivings, while the 

 reproductive and species-maintaining activities are regarded as 

 of secondary importance. One cannot forget, indeed, how 

 much Darwin insisted upon the role of " sexual selection ; " yet 

 it has been already shown that this recognition of the repro- 

 ductive factor was, after all, very external ; that sexual selection 

 is only a special case of natural selection; that it seeks to 

 explain the elaboration, not the origin of sexual peculiarities ; 

 and lastly, that Darwin's arguments in favour of the mecha- 

 nism which he emphasised, have been seriously impugned by 

 Wallace in an attack which reacts strongly upon the critic's 

 own position. 



{c.) Romanes has recently elaborated, what others seem also 

 to have suggested, the importance of mutual sterihty in splitting 

 up one species into several. " Whenever any variation in the 

 highly variable reproductive system occurs, tending to sterility 

 with the parent form without impairing fertility with the varietal 

 form, a physiological barrier must interpose, dividing the 

 species into two parts, free to develop distinct histories, w^ith- 

 out mutual intercrossing, or by independent variation." The 

 reproductive system is very apt to vary, — why, he does not 

 say ; the consequence might readily be, that among the 



