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 which 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 difier 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, 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, mamlata, 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 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 YarrelTs account of the Lays in his ‘ Hist, of British Fishes,” 
Yoh ii. 1836, p. 416, with an excellent figure, and p. 422,. 432. 
