458 DIRECTIVE FACTORS IN EVOLUTION: 



instantaneously when there is chance contact in the water; 

 in some fishes, recognition depends on colour and on be- 

 haviour ; many experiments led Goltz to believe that the male 

 frog distinguishes the female by touch; in birds, visual and 

 auditory impressions count for most; in mammals, the scent 

 is often of chief importance (see S. J. Holmes, Studies in 

 Animal Behaviour, Boston, 1916, pp. 219-328). Since cor- 

 rect 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, if one believes in selection at all. 



(c) Darwin primarily used the term sexual selection for 

 all cases where sifting occurs in relation not to ordinary 

 nutrition 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 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, foot- 

 note.) Sexual selection meant, for Darwin, sifting in con- 

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



(d) In his next step Darwin used the word selection in 

 a non-metaphorical 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 



