436 THE DESCENT OF MAN 



male mammals, and they indicate an offensive rather than 

 a protective purpose. 



The salmon is not the only fish in which the teeth differ 

 in the two sexes, as this is the case with many rays. In the 

 thornback {Baia clavat) the adult male has sharp-pointed 

 teeth, directed backward, while those of the female are 

 broad and fiat, and form 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 sexual characters, both 

 sexes of some species of rays (for instance B. bads), when 

 adult, possess sharp-pointed teeth; and here a character, 

 proper to and primarily gained by the male, appears to have 

 been transmitted to the offspring of both sexes. The teeth 

 are likewise pointed in both sexes of B. maculafa, but only 

 when quite adult: the males acquiring them at an earlier 

 age than the females. We shall hereafter meet with analo- 

 gous cases in certain birds, in which the male acquires the 

 plumage common to both sexes when adult, at a somewhat 

 earlier age than does the female. "With other species of 

 rays the males, even when old, never possess sharp teeth, 

 and consequently the adults of both sexes are provided with 

 broad, flat teeth like those of the young, and like those of 

 the mature females of the above-mentioned species." As the 

 itijs are bold, strong, and voracious fish, we may suspect 

 that the males require their sharp teeth for fighting with 

 their rivals; but as they possess many parts modified and 

 adapted for the prehension of the female, it is possible that 

 their teeth may be used for this purpose. 



In regard to size, M. Carbonnier" maintains that the 

 female of almost all fishes is larger than the male; and 

 Dr. Gunther does not know of a single instance in which 



'* See Tarrell's account of the rays in his "Hist, of British Pishes," 

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

 " As quoted in "The Farmer," 1868, p. 369. 



