168 



SP]XUAL DIMORPHISM 



corresponds to the development of the crest. The pouch is 

 distended and expanded during fighting. We have no 

 evidence that either the pouch or the crest are of any use 

 in fighting, or are subject to selection by the female, but on 

 the other hand we are justified in supposing that the move- 

 ment of the skin and scales preceded the evolution of the 

 crest and the pouch. Here also we find that special develop- 

 ments correspond to special movements, special stimulations, 

 a correspondence which has no significance on the theory of 

 selection. 



The distensible throat-pouch is in some species of lizard 

 equally, or nearly equally developed in both sexes, in others 

 is larger in the males than in the females, in others again 

 is confined entirely to the males. This character thus 

 resembles a large number of other secondary sexual char- 

 acters in these respects. 



Amphibia 



Urodela. It seems very remarkable, considering the mode 

 of fertilisation in these animals, that sexual dimorphism should 

 be developed in them at all, while as a matter of fact it 

 is in many species developed to a high degree. There is no 

 union between the sexes. The male deposits small masses 

 of semen, or spermatophores, on the ground, in front of the 

 female ; the latter grasps them with the lips of her generative 

 opening, and they are then, as it were, swallowed by the 

 oviducts. 1 The case of fishes, however, shows that internal 

 fertilisation and copulation are by no means necessary con- 

 ditions of the existence of secondary sexual characters. We 

 have to consider whether the differentiation of males and 

 females in external characters is related in these Amphibia 



1 See Gasco, Ann. Mas. Storia Naturale di Genova, vol. xvi. 1880-81, 

 pp. 1-54. 



