Darwin's Theory of Sexual Selection 219 
This is explained as possibly merely the result of “continued 
growth.” The improbability of selection is manifest in these 
cases, but if “continued growth ” can accomplish this much, 
why may not the whole process be also the outcome of such 
growth? At any rate, whatever the explanation is, it is im- 
portant to find a case of a secondary sexual character that the 
hypothesis obviously is insufficient to explain. 
15. It is admitted in a number of cases, as in the stag for 
instance, that, although the larynx of the male is enlarged, 
this is not, in all probability, the outcome of sexual selection, 
but in other forms this same enlargement is ascribed to the 
selection process. . 
16. It is admitted that in none of the highly colored 
British moths is there much difference according to sex, 
although when a difference of color is found in butterflies 
this is put down to the action of sexual selection. If such 
wonderful colors as those of moths can arise without the 
action of selection, why make a special explanation for those 
cases in which this difference is associated with sex? 
17. It is well known that birds sing at other times of the 
year than at the breeding season, and an attempt is made to 
account for this in that birds take pleasure in practising those 
instincts that they make use of at other times, as the cat 
plays with the captive mouse. Does not this suggest that, 
if they had certain instincts, they would be more likely to 
employ them at the times when their vitality or excitement 
is at its highest without regard to the way in which they have 
come by them ? 
18. The color of the iris of the eyes of many species of 
hornbills is said to be an intense crimson in the males, and 
white in the females. In the male condor the eye is yellowish 
brown, and in the female a bright red. Darwin admits that 
it is doubtful if this difference is the result of sexual selec- 
tion, since in the latter case the lining of the mouth is black 
in the males, and flesh-colored in the females, which does not 
