THE FUNCTION OF CONJUGATION 147 



same hackles, saddle, and stickle feathers as the female. " Conse- 

 quently the tail is short and truncate as in a hen." 



242. Manifestly the differentiation and the alternative develop- 

 ment of the sexual characters is not in itself an end. A male has 

 patent one set of characters and a female another, and males and 

 females are reproduced alternatively, not because the individual is 

 thereby brought into closer adaptation to the general environment 

 and his or her life more surely preserved, but to afford facilities for 

 mixing the germ-plasm, a process which, for some reason, we have 

 yet to discuss and if possible discover is of advantage to the 

 species. Therefore it is likely that the sexual characters have 

 come into existence because the non-sexual characters are, in some 

 way, beneficially affected by the intermixture of the parental 

 germ-plasms. When, therefore, we have discovered how the non- 

 sexual traits are thus usefully affected by conjugation, we shall 

 have discovered the function of sex. 



243. Formerly, because, in the higher animals, young individuals 

 come into existence only after acts of conjugation, the 'obvious 

 inference' was drawn that the function of sex is to invigorate, 

 revitalize, or rejuvenate aged and debilitated germ-plasms. But, 

 as indicated by Weismann, it is difficult to understand how the 

 union of two outworn and enfeebled things can result in rejuve- 

 nescence. Moreover, parthenogenetic species, and plants which are 

 propagated by slips, retain their vitality perfectly. 



244. Next Weismann, followed by the majority of biologists, 

 believed that the special function of the intermixture of somewhat 

 dissimilar germ-plasms which occurs in conjugation is to provide 

 the spontaneous variations which are the materials on which 

 Natural Selection works, He concluded consequently that 

 parthenogenetic species vary little if at all, and, therefore, are 

 incapable of adapting themselves to changes in the environment. 

 He supposed that they secured survival, not by close adaptation 

 but only by a very rapid rate of reproduction. But rapid repro- 

 duction is an adaptation which can have arisen only through the 

 Natural Selection of favourable variations. Therefore variations 

 must have occurred even after conjugation was abandoned. Besides, 

 it has been proved directly that he was mistaken. Variations 

 occur in abundance when reproduction is parthenogenetic. Thus 

 Weismann himself discovered evidence of variations occurring 

 in the absence of sexual reproduction, 1 and as a result of a 

 biometric inquiry conducted on the parthenogenetic Daphnia 



1 The Germ-Plasm, Eng. Trans., pp. 344-6. 



