The genetic and the operative evidence 62 A. Evidence from mammals 64 B. Evidence from birds 73 C. Evidence from amphibia 86 D. Evidence from crustaceans 89 E. Evidence from insects 92 PART IV. Summary and conclusions 95 Bibliography 100 Description of plates 106 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.