144 MENDEL'S LAWS 



tion may more often alternate with cross-fertilization. Some 

 plants, and some plant-like marine animals are normally self- 

 fertilized. 1 Thus there are connecting links between constant 

 cross- and constant self-fertilization. 



236. The normal effect of conjugation, then, is to mix, with 

 a degree of intimacy which is in dispute, two germ-plasms which 

 on the whole resemble one another somewhat closely, but which 

 differ in details. The difference is greatest when individuals of 

 different species cross, and least when there is self-fertilization. 

 Indeed, self-fertilization is an approach to parthenogenesis in 

 which there is no mixture. Another approach is the alternation 

 of parthenogenesis with conjugation as in aphides. Plainly, then, 

 conjugation is not an essential part of reproduction, which may 

 and does occur without it. Indeed it is precisely the partheno- 

 genetic types that reproduce most rapidly. But it is an accom- 

 paniment of reproduction so nearly universal that we have reason 

 to believe that it must possess some extremely important function. 

 Presumably it is an adaptation which, like other adaptations, has 

 been created and maintained by Natural Selection an adaptation 

 so vastly useful that, to attain it, most multicellular species have 

 become dimorphic and have evolved all the manifold physical and 

 mental peculiarities of sex, including the keenest and fiercest of 

 instincts, so useful that to possess it the possible number of 

 offspring has been halved, since two individuals instead of one 

 are necessary to the production of each child. 2 Thus great advan- 

 tages are lost for the sake of what is probably a greater. 



237. The function of sex (of conjugation, of the mixing of 

 germ-plasms) has been, especially of recent years, one of the most 

 widely and acrimoniously debated of all the problems of heredity. 

 Too often in these discussions has the great probability that sex 

 is an adaptation, and, therefore, like other adaptations, a product 

 of Natural Selection, been ignored. Sex is so widespread and is 

 served by so many and such diverse physical and mental characters, 

 that to doubt that it has utility, to suppose it is a mere chance 

 accompaniment of life, is the very madness of scepticism. No 

 theory of sex is likely to be true that assigns to it a non-adaptive 

 function. What, then, is the function of sex ? In what way is it 

 useful? How does it serve to adapt species to their environ- 

 ments ? Moreover, how is it that some types are cross-fertilized, 

 others self-fertilized, and others parthenogenetic, while yet others 

 exhibit gradations between cross-fertilization, occurring regu- 



1 Origin of Species, p. 123. * See 336. 



