FACTORS AND MECHANISM OF EVOLUTION 51 



school which holds that natural selection has practically nothing 

 to do with species-forming but only, and in a large general way, 

 with the control of descent; and, third, the compromise school, 

 which attributes to natural selection an important part in both 

 species-forming and control of general descent lines, but recog- 

 nizes the simultaneous existence and the considerable im- 

 portance of several other species-forming and descent-modifying 

 factors. In addition to these three schools one must note that 

 a number of active working biologists repudiate the factor of 

 natural selection entirely, holding it to be a vagary and an 

 artifact of logic. 



Associated with natural selection in the general theory of 

 selective action is Darwin's conception of sexual selection. This 

 factor was presumed by Darwin to play a part only in the forma- 

 tion and control of those often very obvious but never well- 

 understood characteristics of a secondary sexual character 

 which distinguish the sexes in many species of animals. Let 

 one recall these characters in the pea fowl, the bird of paradise, 

 the pheasant, some of the butterflies, the lamellicorn beetles, 

 many fishes, and so on. According to the theory of sexual 

 selection the females have chosen for their consorts those males 

 best endowed by variation with these ornamental character- 

 istics, so that by this selection there has come about a gradual 

 cumulation of the characteristics culminating in such bizarrerie 

 as we are familiar with in numerous living animals. 



The word selection will certainly bring to the mind of the 

 reader also a third kind of selective process, namely, that called 

 artificial selection, and this kind of selection is, of course, a 

 factor, and an important one, and has been such for some eighty 

 centuries, in the modification of plant and animal forms. But 

 however widely differing and extraordinarily modified culti- 

 vated and domesticated kinds of animals and plants may be, 

 these different kinds are not looked on by biologists as having the 

 validity, that is, the stability and characteristics of origin, that 

 the different species of animals and plants found in nature have. 



All the different kinds of pigeons, for example, are known to 

 be due primarily to the artificial modification of a single wild 

 kind, the rock dove of Europe, and all of these different artifi- 

 cially produced kinds agree in an important physiological charac- 

 teristic, namely, that of being able to mate freely with each other 

 and with their common ancestor. As this physiological char- 



