204 THE BIOLOGY OF BIRDS 



crediting the hen bird with a standard of taste or capacity for 

 aesthetic valuation. " The chick selects the worm that 

 excites the strongest impulse to pick it up and eat it. So, too, 

 the hen selects that mate which by his song or otherwise 

 excites in greatest degree the mating impulse. Stripped of 

 all its unnecessary aesthetic surplusage, the hypothesis of 

 sexual selection suggests that the accepted mate is the one 

 that most strongly evokes the pairing instinct " (" Habit and 

 Instinct," 1896, p. 217). 



It may be insisted, however, that if individual excellence 

 in attractive characters (such as plumes, singing power, 

 dancing agility) does not appeal to the female, it cannot be 

 determinative in preferential mating, and therefore its 

 establishment cannot be effected by any process of sexual 

 selection. Unless the female is somehow aware of the 

 individual variation in question, the theory breaks down, 

 and yet it is difficult to believe that the female is so meticulous 

 in fastidiousness, so detailed in her preferential excitability. 



The answer, probably sound, is that the details count, 

 not as such, but as contributory to a general impression. 

 Each has its effect, but synthetically, not analytically. 

 " Even when the female seems to choose some slight im- 

 provement in colour or song or dance, the probability is that 

 she is simply surrendering herself to the male whose tout 

 ensemble has most successfully excited her sexual interest " 

 (Geddes and Thomson, " Evolution," 191 1, p. 172). 



(H) If one provisionally accepts the theory that a secondary 

 sex-character may have been established and augmented 

 because it contributed to a decision in preferential mating, 

 one has to face the further question of the significance or 

 racial justification of the courtship-habits — often so pro- 

 longed, elaborate, and exhausting. The sifting probably 

 works well in keeping up a standard of racial fitness, for the 

 most persuasive male is likely to be, among animals, the 

 fittest all round. But there is surely more than this. 



To Groos and to Julian S. Huxley we owe two luminous 

 suggestions. In his " Play of Animals " (Eng. trans. 1900, 

 p. 242), Groos suggests that " in order to preserve the 



