10 



SEXUAL SELECTION 



[Part II. 



strictly compared with those which are so frequent with 

 gallinaceous birds. 15 



Fig. 29.— Xiphophorus Hellerii. Upper figure, male; lower figure, female. 



In a siluroid fish, inhabiting the fresh waters of South 

 America, namely the JPlecostomus barbatus 16 (Fig. 30), 

 the male has its mouth and interoperculum fringed with a 

 beard of stiff hairs, of which the female shows hardly a 

 trace. These hairs are of the nature of scales. In another 

 species of the same genus, soft flexible tentacles project 

 from the front part of the head of the male, which are 

 absent in the female. These tentacles are prolongations 

 of the true skin, and therefore are not homologous with 

 the stiff hairs of the former species ; but it can hardly be 

 doubted that both serve the same purpose. What this 

 purpose may be it is difficult to conjecture ; ornament 

 does not here seem probable, but we can hardly suppose 



15 Dr. Giinther makes this remark : ' Catalogue of Fishes in the Brit- 

 ish Museum, 1 vol. iii. 1861, p. 141. 



16 See Dr. Giinther oivthis genus, in 'Proc. Zoolog. Soc.' 1868, p. 232. 



