MOLLUSCA 287 



dimorphism, as understood in the present work, includes all 

 differences in the structure or characters of males and females 

 of the same species, except the original differences between 

 the ovaries and testes. 



Darwin, with his usual candour, admits freely that there is 

 as much beauty of colour and form in Mollusca as in any 

 other sub-kingdom. This is a sufficient proof that the theory 

 of sexual selection is by no means necessary to explain what 

 to us is beautiful in animals, and also that beauty of form or 

 colour confined to one sex is not completely explained on the 

 hypothesis that it is pleasing to the other sex. The colours 

 in Mollusca, according to Darwin, are probably the direct 

 result of the nature of the tissues and the manner of growth. 

 It is strange that he did not see that this view would har- 

 monise with all the facts of sexual dimorphism, and a great 

 many others, provided it be added that the "manner of 

 growth" is affected in determinate ways by determinate 

 conditions. Whereas on the selection theory we must explain 

 the brilliant colours of a cock pheasant's plumage by the taste 

 of the female, and the equally beautiful colours and markings 

 of a Nudibranch mollusc by the nature of the tissues and 

 the manner of growth, a more rational view regards both as 

 due to the physiological processes of growth and secretion 

 modified by external forms of energy. 



Of the Gastropoda some are hermaphrodite, some of 

 separate sexes. The Pulmonata, or air-breathing terrestrial 

 snails, and the Opisthobranchia, are all hermaphrodite, and 

 among these therefore all individuals are alike. The Proso- 

 branchia, on the other hand, of which the Whelk and Peri- 

 winkle are typical examples, are of separate sexes. The 

 sexual dimorphism is, however, never very pronounced, and 

 in some species scarcely visible. In some cases the shell is a 

 little smaller or narrower in the males than in the females, in 

 certain species even the teeth of the radula are different in 



