112 SEXUAL DIMORPHISM 



classification, and I have nothing to gain by strictly following 

 the order of affinity in considering various cases, unless the 

 affinity is so close that it shows itself in similar habits and 

 similar degrees of dimorphism. This is generally the case 

 within the same family. For the definitions and limits of 

 the families I have followed the system employed by Mr. A. 

 H. Evans in the " Birds " of the Cambridge Natural History, 

 a system which, with very slight modifications, is that laic 

 down by Dr. Gadow. 1 



Phasianidw. The cock of our domestic poultry offers an 

 excellent example of the unisexual peculiarities occurring in 

 this family. 



The ancestral species, Gallus bankiva, exhibits the original 

 habits and peculiarities from which those of domestic poultry 

 are derived. The neck feathers are longer and more brightly 

 coloured in the cock than in the hen ; the tail feathers, both 

 the rectrices and the tail coverts, are elongated ; the comb 

 and wattles are larger than in the hen, and there is a spur 

 on the inside of each leg. The cock lives permanently with 

 several females, and takes no part in incubation or the care 

 of the young. He fights frequently with other cocks, and 

 makes gestures of courtship to the hens. In fighting, the 

 neck hackles are erected, which accounts for their elongation, 

 and the spur is the result of striking with the legs against 

 the legs or head of the opponent. The tail feathers, on the 

 other hand, are chiefly moved in courtship. The comb and 

 wattles are special vascular developments of the skin on the 

 crest of the head and about the cheeks. I see no reason why 

 these should not have been originally due to pecking by the 

 beaks of other birds. This influence would have come into 

 play chiefly in the fights between cocks, and the comb and 

 wattles are developed also in hens by inheritance, but to a 

 lower degree. 



1 Bronn's Thier- Reich, Aves, Syst. Theil, 1893. 



