THE GENETIC AND THE OPERATIVE EVIDENCE RELATING 

 TO SECONDARY SEXUAL CHARACTERS. 



BY T. H. MORGAN. 



PART I. 



There are a few races of poultry that have two kinds of males, one 

 with the feathering of the ordinary cock, the other with the feathering 

 of the hen. The Hamburgs and the Campines are perhaps the best 

 known races of this sort. Convention amongst breeders, in certain 

 countries, has determined that the cock-feathered bird shall be the 

 standard, and at other times and places that the hen-feathered males 

 shall be the show birds. In one breed, at least, viz, the Sebright 

 bantams, the hen-feathered cock is the only known type. Cock- 

 feathered Sebrights have never been seen, so far as I know. This 

 breed is pure for hen-feathering. As shown in plate 1, figure 3, the 

 male Sebright lacks the long, pointed saddle feathers at the base of 

 the tail of the common cock, also the peculiar back and neck feathers 

 (hackles) of the cock bird, as well as the male feathering on the bow 

 of the wing. His feathers in these parts are almost exactly like those 

 of the hen (plate 4, fig. 4). The long sickle feathers covering the true 

 tail are also absent, although the two median ones sometimes occur 

 in males of this race. 



The Sebrights seemed excellent material for studying the heredity 

 of this type of plumage in the male. In 1911 I began to study this 

 problem, and crossed Sebrights to Black-Breasted Game bantams. 

 The latter race was chosen not only because the males have the typical 

 cock-feathering, but also because the coloration of these birds resembles 

 very closely that of the jungle-fowl, from which many, perhaps all, of 

 our domesticated races have sprung. 



In dissecting some of the F 2 birds from this cross I noticed that the 

 testis of the male was often more flattened than is the testis of the 

 typical male bird, that it was often somewhat pear-shaped, and that 

 frequently it was in part or entirely black. Recalling that male 

 Sebrights are said to be often partially sterile, the idea naturally 

 suggested itself that these birds are hen-feathered because the testes 

 have assumed some of the characteristics of the ovary. It had long 

 been supposed, and had been finally established by Goodale, that the 

 presence of the ovary in the female suppresses her potential develop- 

 ment of plumage, for when the ovaries of the hen are diseased or 

 removed she develops the plumage of the male. This reasoning led 

 me to try the experiment of castrating the hen-feathered males in 



