6 SEXUAL SELECTION. Part IL 



the temporary hook-like structure serves to strengthen 

 and protect the jaws, when one male charges another 

 with w^onderful 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 only fish in w^hicli the teeth 

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

 rays. In the thornback (Baia clavata) the adult male 

 has sharp, pointed teeth, directed backwards, whilst 

 those of the female are broad and flat, forming a pave- 

 ment ; so that tliese 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 : whilst young they are broad and 

 flat like those of the female. As so frequently occurs 

 with secondary sexual characters, both sexes of some 

 species of rays, for instance B. hatis, possess, when adult, 

 sharp, pointed teeth ; and here a character, pro]3er to 

 and primarily gained by the male, aj)pears to have been 

 transmitted to the offspring of both sexes. The teeth 

 are likewise pointed in both sexes of B. maculata, but 

 only when completely adult ; the males acquiring them 

 at an earlier age than the females. We shall hereafter 

 meet with analogous cases witli certain birds, in which 

 the male acquires the plumage 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 consequently both sexes 

 when adult are provided with broad, flat teeth like 

 those of the young, and of the mature females of 

 the above-mentioned species.^ As the rays are bold. 



^ See Yarrell's account of the Eays in liis ' Hist, of British Fishes,' 

 vol. ii. 1836, p. 416, with an excellent figure, and p. 422, 4r]2. 



