464 INTERNAL SECRETIONS 



differences in the individual." Morgan proceeds as follows: 

 "Such results, while they admittedly do not in most cases tell 

 us that the differences involved have arisen at a single pro- 

 gressive 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 colour 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 female in birds produces an internal secretion 

 that suppresses in her the ornamentation shown by the male, 

 and in the mammal an internal secretion produced by the testes 

 causes the full development in the male of the secondary sexual 

 characters. If, as seems probable, these secretions are some 

 particular kind of substance, the condition that led to their 

 appearance historically need not have been very complex ; and 

 if mot, the problem appears simplified " [Morgan, p. 98). But I 

 should like to add that the problem as expressed by Darwin 

 becomes not only simplified, but possibly reversed. According 

 to Darwin the modest plumage of the female bird is phylogene- 

 tically older than the brilliant male one, and Darwin asks how 

 the brilliant male plumage developed from the female one. But 

 it is possible, or even very probable, that the brilliant male 

 plumage is phylogenetically older than the female one. There 

 is at least no better evidence for the older assumption of 

 Darwin than for the assumption expressed here. Under the 

 first alternative, phylogenetic transformation of the modest 

 plumage into the brilliant one would be caused by cessation, 

 in males, of the production of a hormone inhibiting the plumage 

 of the neutral form of the given race; under the second alter- 

 native, there would be phylogenetic transformation of the 

 brilliant plumage into a modest one by the production, in females, 

 of a hormone inhibiting the neutral form of the given race. 



To sum up our argument : There is no need to assume special 

 genetic sex factors for the male and female sex characters in 

 mammals and birds. Transmission of male sex characters by the 



