vi FORM AND FUNCTION 161 



margins of feathers being shed or to the influx of 

 fresh pigment, I do not know. 1 



Spurs. 



Spurs are outgrowths of bone covered by a horny 

 sheath formed from the epidermis, and, thus, they re- 

 semble the horns of oxen and antelopes. The cock's 

 spur is familiar to every one. Some birds, for instance 

 the Double Spurred Peacock, have two on each foot. 

 Spurs are also found upon the wings, for example in 

 the Crested Screamer of South America which is now 

 to be seen at the Zoological Gardens. The Cassowary's 

 spurs, which are really feathers, I have already de- 

 scribed. These are found in both sexes, as ordinary 

 wing-spurs sometimes are. All spurs are used in 

 fighting, and well-developed leg-spurs are the privilege 

 of cock birds. We should expect, therefore, to see 

 them, as we do, mainly in those species which are 

 polygamous, and which consequently have an excess 

 of males among whom there is constant war in spring- 

 time. It is with his spur that the game-cock slays 

 his rival. 



The Beak. 



In the beak the horny covering which overlies the 

 bone is a growth of the epidermis just as spurs are. 



1 On moulting- see especially (i) Bronn's Thier-Reich r vol. 

 " Aves," pp. 538-542 ; (2) Seebohm's Brit. Birds, passim ; (3) 

 Gatke, Die Vogclwarte Helgoland, pp. 156-166; (4) Le Pigeon 

 Voyageur, by F. Chapuis, pp. 103-111. I am indebted to Mr. 

 C. M. Adamson's book Some more Scraps about Birds, printed 

 for private circulation 



M 



