Chap. XII. GENERAL RESULTS. 467 



offspring from the union of two distinct individuals, 

 especially if their progenitors have been subjected to 

 very different conditions, have an immense advantage 

 in height, weight, constitutional vigour and fertility 

 over the self-fertilised offspring from one of the same 

 parents. And this fact is amply sufficient to account 

 for the development of the sexual elements, that is, for 

 the genesis of the two sexes. 



It is a different question why the two sexes are 

 sometimes combined in the same individual and are 

 sometimes separated. As with many of the lowest 

 plants and animals the conjugation of two individuals 

 which are either quite similar or in some degree dif- 

 ferent, is a common phenomenon, it seems probable, 

 as remarked in the last chapter, that the sexes were 

 primordially separate. The individual which receives 

 the contents of the other, may be called the female ; 

 and the other, Avhich is often smaller and more loco- 

 motive, may be called the male ; though these sexual 

 names ought hardly to be applied as long as the 

 whole contents of the two forms are blended into one. 

 The object gained by the two sexes becoming united 

 in the same hermaphrodite form probably is to allow 

 of occasional or frequent self-fertilisation, so as to 

 ensure the propagation of the species, more especially 

 in the case of organisms affixed for life to the same 

 spot. There does not seem to be any great difficulty 

 in understanding how an organism, formed by the 

 conjugation of two individuals which represented the 

 two incipient sexes, might give rise by budding first 

 to a moncecious and then to an hermaphrodite form ; 

 and in the case of animals even without budding to 

 an hermaphrodite form, for the bilateral structure of 

 animals perhaps indicates that they were aboriginally 

 formed by the fusion of two individuals. 



2 H 2 



