10 



SEXUAL SELECTION 



[Part IL 



strictly oomparccl with tliosc which aro so frequent with 

 galhnaceous birds."" 



> 



/y 



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



In a sihiroid fish, inhabiting the fresh waters of South 

 America, namely the Plecostomus harhatus " (Fig. 30), 

 the male has its mouth and interopcrculum 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 stiflf 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 



" Dr. Giinthcr makes this remark: 'Catalogue of Fishes in the Brit- 

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



" See Dr. Giinthcr on this genus, in ' Proc. Zoolog. Soc.' 1 868, p. 232. 



