334 THE DESCENT OF MAN. 



The male of the Cottus scorpius, or sea-scorpion, is slenderer 

 and smaller than the female. There is also a great difference 

 In color between them. It is difficult, as Mr. Lloyd'^ remarks, 

 "for any one, who has not seen this fish during the spawning- 

 "season, when its hues are brightest, to conceive the admixture 

 "of brilliant colors with which it, in other respects so ill-favored, 

 "is at that time adorned." Both sexes of the Labrus mixtus, 

 although very different in color, are beautiful; the male being 

 orange with bright blue stripes, and the female bright red with 

 some black spots on the back. 



In the very distinct family of the Cyprinodontids— inhabitants 

 of the fresh waters of foreign lands — the sexes sometimes differ 

 much in various characters. In the male of the Mollienesia peten- 

 ensis," the dorsal fin is greatly developed and is marked with a 

 row of large, round, ocellated, bright-colored 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 

 colored. In the male of an allied form, the Xiphophorus Hellerii 

 (fig. 30), the inferior margin of the caudal fin is developed into 

 a long filament, which, as I hear from Dr. Giinther, 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 whilst young resemble the 

 adult females in color and structure. Sexual differenoes such as 

 these may be strictly compared with those v/hich are so frequent 

 with gallinaceous birds." 



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

 the Plecostomus barbatus" (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 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 that stiff hairs and flexible filaments 

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



« 'Game Birds of Sweden,' &c., 1867, p. 466. 



"With respect to this and the following- species I am indebted to 

 Dr. Gunther for information: see, also, his paper on the 'Pishes of 

 Central America,' in 'Transact. Zoolog. Soc' vol. vl. 1868, p. 485. 



" Dr. Gunther makes this remark; 'Catalogue of Pishes in the British 

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



"8 See Dr. Gunther on this genus, in 'Proo. Zoolog. Soc' 1868, p. 232. 



