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 intermingling 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. 



(/k) 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 ro/e 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 sterility 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, with- 

 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 



