EVOLUTION 1023 



importance. (See S. J. Holmes, Studies in Animal Behaviour, Boston, 

 1916, pp. 219-328.) 



Since correct recognition of the one sex by the other is often of 

 essential importance to the race, it is not surprising to find Darwin 

 saying {Descent of Man, p. 324) : "But in most cases of this kind 

 it is impossible to distinguish between the effects of natural and 

 sexual selection." This part of the theory also remains valid to those 

 who believe at all in the evolutionary efhcacy of selection. 



Darwin primarily used the term sexual selection for all cases 

 where sifting occurs in relation not to ordinary food-getting and 

 self-preservation, but to pairing. It was only secondarily that he 

 laid emphasis on the "choice" that the female is supposed to 

 exercise in reference to rival suitors. An interesting confusion, 

 which has misled some biologists, has arisen by a double use of the 

 word selection. Darwin spoke of the female's selection, but it is 

 perfectly clear that he recognised a large field of sexual selection in 

 which there was no question of selection or choice on the part of 

 the female. (See Descent of Man, 2nd ed., 1888, vol. i, p. 323, 

 footnote.) Sexual selection meant, for Darwin, sifting in connection 

 with mating, whether the female held the sieve or not. 



In his next step Darwin used the word selection in a non-meta- 

 phorical sense: "Just as man can give beauty, according to his 

 standard of taste, to his male poultry, or more strictly can modify 

 the beauty originally acquired by the parent species, ... so it 

 appears that female birds in a state of nature, have by a long 

 selection of the more attractive males, added to their beauty 

 or other attractive qualities." {Descent of Man, 2nd ed., 1888, 

 vol. i, p. 326.) 



In many animals, at diverse levels of organisation, there is an 

 elaborate courtship-ceremonial, allied, according to Groos, to play. 

 It is sometimes on both sides ; it is usually for the most part on the 

 male's side. It includes a manifold display of decorations, colours, 

 agility, and vocal powers. Darwin's theory in this connection was 

 simply this: if there are rival males, and if they are unequally 

 endowed with structural and emotional equipment, or with the 

 capacity of using this to advantage, there will be preferential 

 mating on the female's part, and, other things equal, there wiU be 

 a selection of the type of male most successful as a suitor. It is the 

 female who sifts, but the logic of the process is the same as in 

 ordinary natural selection. 



It is conceivable that pronounced and persistent differential 

 mating might lead not merely to the establishment and augmenta- 

 tion of characters determining the result of the contest or the court- 

 ship, but also to a process of physiological and psychological 

 "isolation" (narrowing of the range of intercrossing), and thus to 

 an accentuation of the apartness of a species as regards crossing 



