Darwin s Theory of Sexzial Selection 191 



The last sentence implies that this particular case is to 

 be explained by the females becoming larger on account of 

 the number of eggs that they are to produce. But why was 

 not the same explanation offered in the case of the spiders ? 

 It is this uncertain way of applying any explanation that sug- 

 gests itself, that puts the whole method in an unfortunate 

 light. 



In many species of fish the males are brighter in color 

 than the females. In the case of Callionymus lyra, Darwin 

 states : — 



" When fresh caught from the sea the body is yellow of 

 various shades, striped and spotted with vivid blue on the 

 head ; the dorsal fins are pale brown with dark longitudinal 

 bands, the ventral, caudal, and anal fins being bluish black. 

 The female, or sordid dragonet, was considered by Linnaeus, 

 and by many subsequent naturalists, as a distinct species ; it 

 is of a dingy reddish brown, with the dorsal fin brown and 

 the other fins white. The sexes differ also in the propor- 

 tional size of the head and mouth, and in the position of the 

 eyes ; but the most striking difference is the extraordinary 

 elongation in the male of the dorsal fin. Mr. W. Saville 

 Kent remarks that this 'singular appendage appears from 

 my observations of the species in confinement, to be subser- 

 vient to the same end as the wattles, crests, and other abnor- 

 mal adjuncts of the male in gallinaceous birds, for the purpose 

 of fascinating their mates.' " 



In the case of another fish, Cottus scorpius, there is also a 

 great difference between the sexes, and here the males be- 

 come very brilliant only at the breeding season. In other 

 fishes, in which the sexes are colored alike, the males may 

 become more brilliant during the breeding season. This, 

 too, is explained by Darwin on the assumption that those 

 males that have varied at the breeding season, so as to be- 

 come more brightly colored, have been chosen in preference 

 to the other males. 



