6 SEXUAL SELECTION. [Part II. 



one male charges another with wonderful violence ; but 

 the greatly developed teeth of the male American salmon 

 may be compared with the tusks of many male mammals, 

 and they indicate an offensive rather than a protective 

 purpose. 



The salmon is not the onlv fish in which the teeth 

 differ in the two sexes. This is the case with many raj s. 

 In the thornback (JRaia clavata) the adult male has sharp, 

 pointed teeth, directed backward, while those of the fe- 

 male are broad and flat, forming a pavement; so that 

 these teeth differ in the two sexes of the same species 

 more than is usual in distinct genera of the same family. 

 The teeth of the male become sharp only when he is 

 adult : while young they are broad and flat like those of 

 the female*. As so frequently occurs with secondary sex- 

 ual characters, both sexes of some species of rays, for in- 

 stance R. batis, possess, when adult, sharp, pointed teeth ; 

 and here a character, proper to and primarily gained by 

 the male, appears to have been transmitted *to the off- 

 spring of both sexes. The teeth are likewise pointed in 

 both sexes of M. maculata, but only when completely 

 adult ; the males acquiring them at an earlier age than the 

 females. We shall hereafter meet with analogous cases 

 with certain birds, in which the male acquires the plu- 

 mage common to both adult sexes, at a somewhat earlier 

 age than the female. With other species of rays the 

 males even when old never possess sharp teeth, and con- 

 sequently both sexes when adult are provided with broad, 

 flat teeth, like those of the young, and of the mature fe- 

 males of the above-mentioned species. 9 As the" rays arc 

 bold, strong, and voracious fishes, we may suspect that the 

 males require their sharp teeth for fighting with their 

 rivals; but as they possess many parts modified and 



9 See Yarrell's account of the Rays in his ' Hist, of British Fishes,' 

 vol. ii. 1836, p. 416, with an excellent figure, and pp. 422, 432. 



