vi FORM AND FUNCTION 161 
margins of feathers being shed or to the influx of 
fresh pigment, I do not know.! 
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 (1) Bronn’s 7/zer-Rezch, vol. 
“ Aves,” pp. 538-542; (2) Seebohm’s Brzt. Birds, passim ; (3) 
Gitke, Die Vogelwarte 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 
