440 THE DESCENT OF MAN 



of the anal fin is also a little produced and dark colored. 

 In the male of an allied form, the Xiphophorus Hellerii 

 (Fig. 80), the inferior margin of the caudal fin is devel- 

 oped into a long filament, which, as -I hear from Dr. Giin- 

 ther, is striped with bright colors. 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 while young resemble the adult females in color 

 and structure. Sexual differences such as these may be 

 strictly compared with those which are so frequent with 

 gallinaceous birds." 



In a siluroid fish, inhabiting the fresh waters of South 

 America, the Plecoslomus harhatus" (Fig. 31), the male has 

 its mouth and inter-operculum 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 there- 

 fore 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 that stiff hairs and flexible filaments can 

 be useful in any ordinary way to the males alone. In that 

 strange monster, the Chimcera monstrosa, the male has a 

 hook-shaped bone on the top of the head, directed forward, 

 with its end rounded and covered with sharp spines ; in the 

 female "this crown is altogether absent," but what its use 

 may be to the male is utterly unknown." 



The structures as yet referred to are permanent in the 

 male after he has arrived at maturity ; but with some Blen- 



" Dr. Giintlier makes this remark, "Catalogue of Fishes in the British 

 Museum," vol. iii., 1861, p. 141. 



" See Dr. Giinther on this genus, in "Proc. Zoolog. See," 1868, p. 232. 



" P. Buokland, in "Land and Water," July, 1868, p. 311, with a figure. 

 Many other cases, could be added of structures peculiar to the male, of which 

 the uses are not known. 



