ORIGIN AND CHARACTERISTICS 19 



Passeres, almost absent in the Pigeons, Wood- 

 peckers, Owls and so on, but is as large as the 

 main rhachis in the Cassowary and the Emu. 

 The aftershaft is generally concealed by the 

 contour feathers, but in some cases it extends 

 beyond them in the form of long slender filaments 

 or hairs on the neck or back. The Blackbird 

 furnishes a familiar example of a species pos- 

 sessing these nuchal hairs, whilst some of the 

 Bulbuls in the genus Tricholestes may be cited 

 as notable instances of the latter. Some of 

 the Warblers and Finches also exhibit them ; 

 whilst the white thread-like filaments displayed 

 by Cormorants are apparently another form of 

 filoplume. It may be mentioned that the soft 

 downy covering of so many young birds re- 

 sembles the down-plumes of adults, although it 

 differs in certain respects, notably in the abortive 

 or even absent shaft, the absence of cilia, the 

 filamentous rami, and with one exception the 

 absence of an aftershaft. But these are details 

 of far too technical a character to be dealt with 

 in a work aiming above all things to be popular, 

 and must be left to the more advanced student of 

 birds. 



Having thus briefly dealt with the distribution, 

 structure, and growth of feathers, it now becomes 



