THEORY OF SEX-DIMORPHISM 73 



relative numbers of the sexes, and many of 

 the alleged cases of great numerical disparity 

 have been shown to be fallacious. 



The late Alfred Russel Wallace was a per- 

 sistent critic of the theory of sexual selection, 

 pointing out, for instance, that there is little 

 convincing evidence that the female bird 

 really chooses her partner, or chooses him 

 because of any particular excellence in colour 

 or plumage, song or dance. Contrariwise, 

 some ornithologists maintain that in some 

 species certain males are left definitely un- 

 mated, and that they are inferior in attractive- 

 ness. In the mysterious case of spiders, where 

 the courtship is often elaborate, the fastidious 

 female sometimes kills a suitor who does not 

 adequately please her ; as well as afterwards, 

 it may be, the one who does. By that time, 

 however, the latter has had some measure, 

 at least, of success in reproduction and in 

 thus handing on to his progeny his meritorious 

 qualities. So that his death is of less moment 

 to the race. Many of the illustrations of 

 sexual selection which Darwin gave remain 

 very convincing, but the theory requires to 

 be strengthened by more precise evidence 

 that considerable numbers of the less attractive 

 or less well-equipped males are either left 

 unmated, or have less numerous and less suc- 

 cessful families as the result of their matings. 

 The theory may be consistently improved 

 by recognising what some students of com- 

 parative psychology have indicated, that the 

 female probably surrenders herself not to a 

 male selected because of some particular 

 excellence, but rather to the one whose 



