Natural Selection 49 
in the female, but that certain spots of colour are absent, and these 
have probably been acquired comparatively recently by the male and 
have not yet been transmitted to the female. 
Isolation of the group of individuals which is in process of 
varying is undoubtedly of great value in sexual selection, for even 
a solitary conspicuous variation will become dominant much sooner 
in a small isolated colony, than among a large number of members 
of a species. 
Anyone who agrees with me in deriving variations from germinal 
selection will regard that process as an essential aid towards explain- 
ing the selection of distinctive courtship-characters, such as coloured 
spots, decorative feathers, horny outgrowths in birds and reptiles, 
combs, feather-tufts, and the like, since the beginnings of these would 
be presented with relative frequency in the struggle between the 
determinants within the germ-plasm. The process of transmission of 
decorative feathers to the female results, as Darwin pointed out and 
illustrated by interesting examples, in the colour-transformation of 
a whole species, and this process, as the phyletically older colouring 
of young birds shows, must, in the course of thousands of years, 
have repeated itself several times in a line of descent. 
If we survey the wealth of phenomena presented to us by 
secondary sexual characters, we can hardly fail to be convinced of 
the truth of the principle of sexual selection. And certainly no one 
who has accepted natural selection should reject sexual selection,’ 
for, not only do the two processes rest upon the same basis, but they © 
merge into one another, so that it is often impossible to say how 
much of a particular character depends on one and how much on the 
other form of selection. 
(8) Natural Selection. 
An actual proof of the theory of sexual selection is out of the 
question, if only because we cannot tell when a variation attains to 
selection-value. It is certain that a delicate sense of smell is of value’ 
to the male moth in his search for the female, but whether the posses- 
sion of one additional olfactory hair, or of ten, or of twenty additional] 
hairs leads to the success of its possessor we are unable to tell. And 
we are groping even more in the dark when we discuss the excite- 
ment caused in the female by agreeable perfumes, or by striking 
and beautiful colours. That these do make an impression is beyond} 
doubt; but we can only assume that slight intensifications of them 
give any advantage, and we must assume this since otherwise secondary 
sexual characters remain inexplicable. - 
The same thing is true in regard to natural selection. It is not 
possible to bring forward any actual proof of the selection-value 
D. 4 
