$$8 The Descent of Man. Tart IT. 



with a row of large, round, ocellated, bright-coloured spots; 

 whilst the same fin in the female is smaller, of a different shape, 

 and marked only with irregularly curved brown spots. In the 

 male the basal margin of the anal fin is also a little produced 

 and dark coloured. In the male of an allied form, the Xipho- 

 phorus Hellerii (fig. 30), the inferior margin of the caudal fin is 

 developed into a long filament, which, as I hear from Dr. Gunther, 

 is striped 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 the adult females in colour and structure. Sexual 

 differences such as these may be strictly compared Avith those 

 which are so frequent with gallinaceous birds. 17 



In a siluroid fish, inhabiting the fresh waters of South America, 

 the Plecostomus barbatus 18 (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 ten- 

 tacles project from the front part of the head of the male, which 

 are absent in the female. These tentacles are prolongations oi 

 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 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 forwards, 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. 1 " 



The structures as yet referred to are permanent in the male 

 after he has arrived at maturity ; but with some Blennies, and in 

 another allied genus, 20 a crest is developed on the head of the 

 male only during the breeding-season, and the body at the same 

 time becomes more brightly-coloured. There can be little doubt 

 that this crest serves as a temporary sexual ornament, for the 

 female does not exhibit a trace of it. In other species of the 

 same genus both sexes possess a crest, and in at least one species 



17 Dr. Gunther makes this re- Water,' July 1368, p. 377, with a 

 mark ; ' Catalogue of Fishes iu the figure. Many other cases could be 

 British Museum,' vol. iii. 1861, p. added of structures peculiar to the 

 141. male, of which the uses are not 



18 See Dr. Gunther on this genus, known. 



in ' Proc. Zoolog. Soc' 1868, p. 232. 20 Dr. Giinther, < Catalogue of 



" F. Buckland, in ' Land and Fishes,' vol. iii. pp. 221 and 240. 



