SEXUAL CHARACTERS OF SOME DOMESTIC BIRDS. 49 



DARWIN'S AND WALLACE'S THEORIES OF THE ORIGIN OF 

 SECONDARY SEXUAL CHARACTERS. 



Both of these theories assume that the difference between the sexes 

 has arisen through selection — natiiral selection according to Wallace, 

 sexual selection according to Darwin. Birds, in particular, have fur- 

 nished illustrations in support of each theory. Natiiral selection is 

 assumed to have operated through the survival of those females that 

 were more protectively colored than their mates. The theory of sexual 

 selection assumes, on the other hand, that the males acquired their 

 present-day colors in response to selection on the part of the females, 

 which chose the more highly colored males. According to Darwin's 

 theory the start was made from a dull-colored monomorphic species, 

 an assumption that is not in accord with the nature of the female as 

 shown by castration, since the brilhant male colors are merely sup- 

 pressed in her. The only possible effect of selection, then, would be 

 the uncovering of a condition aheady present. But, by hypothesis, 

 this condition did not pre-exist. 



Wallace's theory, starting also with a probably dull-colored mono- 

 morphic species, is in partial agreement with the results of castration. 

 It must be modified by assuming that natural selection acted indirectly 

 through the development of an internal secretion of the ovary pari 

 passu with increased brilliancy of the male rather than on the charac- 

 ters themselves. Since, however, the female possesses the same poten- 

 tiaUties as the male, the start may have been from a highly colored 

 form rather than the reverse. 



NATURE AND MODE OF THE OVARIAN SECRETION. 



The adjustment of the ovarian secretion to the characters it modi- 

 fies is very close, as shown by the fact that the male characters produced 

 in a given female are Hke those of the corresponding male. This must 

 mean that the large element in determining the result is the heredity 

 basis and not the secretions. From this we may conclude that the 

 secretion on the whole is relatively simple and probably of uniform 

 nature. If the secretion were composed of many substances, one to 

 produce each effect involved, such as the change from a vermiculated 

 feather to penciled, from a gray and white to a black and brown, the 

 resulting complexity would be so great that one would not anticipate 

 any such close coordination as actually results. For purposes of illus- 

 tration we may assume that the ovarian secretion is simple, producing 

 its effect by oxidation or some other simple process. The sort of result 

 produced by oxidation, of course, depends upon the substance that is 

 oxidized. 



From another standpoint, the question may be raised as to whether 

 the secretion should be considered as a modifier or an inhibitor. If it 

 be assumed that it is a modifier, only one genetic basis need be assumed 



