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 gi'oup 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 explahi- 

 ing the selection of distinctive coui'tship-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 

 M'lio 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 j 

 other form of selection. 



(yS) 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 tlic 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 imjjrcssion is beyond 

 doubt; but we can only assume that slight intensifications of them 

 give any advantage, and we 7mist assume this sijice otherwise secondary 

 sexual characters remain inexplicable. 



The same thing is true in regard to natural selection. It is not 

 possible to bring iorwurd any actual proof of the selection-value 

 D. 4 



