2 y 2 THE EVOLUTION OF SEX. 



which would not be relevant to such a work as this, we must 

 summarily collate the more prominent opinions as to the share repro- 

 duction has in the process. To most of these we have already 

 alluded in the body of the book. 



(a) First of all, as to the origin of variations, we find that what 

 Treviranus recognized in the first years of this century, — namely, the 

 influence of fertilization in evoking change, — has been emphasized by 

 several, such as Brooks and Galton, and has been especially elaborated 

 by Weismann. As we have just seen, Weismann finds in the inter- 

 mingling of two " germ-plasmas," which is the essence of fertilization, 

 the sole origin of variations of any account in the evolution of the 

 species. Whether this be consistent with Weismann's theory of fertili- 

 zation 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 somewhat marked 

 contrast is the view recently advocated by Hatschek, who sees in the 

 intermingling essential to fertilization a counteractive of idiosyn- 

 cracies, 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 can not forget, indeed, how much Darwin insisted upon the role 

 of" sexual selection " ; yet it has been already shown that this recog- 

 nition of the reproductive 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 favor of the mechanism which he 

 emphasized, have been seriously impugned by Wallace in an attack 

 which reacts strongly upon the critic's own position. 



(V) 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 bar- 

 rier must interpose, dividing the species into two parts, free to develop 

 distinct histories,' without 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 

 progeny of a parent stock some were fertile inter se, but infertile with 

 the consistent members of the parent stock; these will be isolated by a 



