10 



SEXUAL SELECTION. 



Part II. 



which is striped, as I hear from Dr. Giinther, with bright 

 colours. This filament does not contain any muscles, 

 and apparently cannot be of any direct use to the fish. 

 As in the case of the Callionymus. the males whilst 

 young resemble in colour and structure the adult 

 females. Sexual differences such as tliese may be 

 strictly compared with those w^iich are so frequent 

 with gallinaceous birds.^^ 



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



In a siluroid fish, inhabiting the fresh waters of South 

 America, namely the Plecostomus harhatus ^'^ (fig. 30), 

 the male has its mouth and interoperculum fringed with 

 a beard of stiff hair,«, of which the female shews hardly 

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

 another species of the same genus, soft flexible ten- 

 tacles project from the front part of the head of the 



1'' Dr. Giinther makes this remark ; ' Catalogue of Fishes in the 

 British Museum,' vol. iii. 18G1, p. 141. 



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



