200 THE BIOLOGY OF BIRDS 



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 Darv\in, sifting in 

 connection 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 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 will 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. 



(E) It is conceivable that pronounced and persistent pre- 

 ferential mating might lead not merely to the establishment 

 and augmentation of characters determining the result of the 

 contest or the courtship, 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 with related neighbour- 



