98 THE GENETIC AND THE OPERATIVE EVIDENCE 



of the Sebright. Castration of ordinary males does not affect deleter- 

 iously the secondary sexual plumage (although it does the comb, be- 

 havior, etc.), in fact may even enhance their effects. But, while in 

 the mammal a secretion is necessary for the full development of the 

 secondary sexual characters, in the Sebright a secretion inhibits certain 

 of them. What element in the ordinary bird and in the Sebright 

 causes the full development of the comb, wattles, sexual behavior, etc., 

 is not known. Possibly it is the sexual elements themselves, but 

 possibly it is a secondary influence of the luteal cells producing a con- 

 trary effect on these parts from its effects on the feathers; but possibly 

 more than one kind of secretory cell is present in the testis of the cock. 



18. The causes of the development of the secondary sexual characters 

 are seen to be of such diverse physiological kinds that one may well 

 hesitate to apply the same explanation as to their evolution. In fact, 

 it is pointed out that several of the theories that have been suggested 

 run counter to the conditions that bring about the development of the 

 secondary sexual characters. 



19. An attempt is made to give a critical review of Darwin's theory 

 of sexual selection in the light of the modern genetic and operative 

 results on the secondary sexual characters of the vertebrates. It is 

 pointed out that far from extending the general theory in its applica- 

 tions, the modern work has shown in the first place that the underlying 

 conditions that call forth the development of the secondary sexual 

 differences are so diverse in the different groups of animals that it is 

 a priori very unlikely that this evolution can have been directed by the 

 same external agent, such as the choice of the female, for such an 

 assumption carries with it in several cases other implications concerning 

 the causes of the suppression of these same characters in the female 

 herself, etc. In the second place, it is pointed out that the problem of 

 the excessive development of certain characters in the male whose 

 genes are present in both sexes no longer oppresses us as it did Darwin, 

 for it has been shown both by the genetic and by the operative work 

 that a single factorial difference may be at the root of exceedingly 

 great differences in the individual. Such results, while they admittedly 

 do not in most cases tell us that the differences involved have arisen 

 at a single progressive step, show us nevertheless that such differences 

 may depend on very simple initial differences, and if so, the entire 

 problem becomes enormously simplified. To Darwin the excessive 

 development of color and ornamentation appeared due to a long, slow 

 process of evolution laboriously brought about by the female through 

 selection of those males a little more ornamented than their fellows. 

 To-day we have found out that in many cases the genetic composition 

 of a male with such ornamentation and of a female without it may be 

 almost identical, except that the genes in one chromosome are duplex 

 in one sex and simplex in the other. Owing to this initial difference, the 



