Darwin's Theory of Sexual 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 Linnzus, 
and by many subsequent naturalists, as a distinct species; it 
is of a dingy reddish brown, with the dorsal fn 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. 
