12 GENERAL, DEFINITIONS. 



ferent parts of the same bird. Three principal modi- 

 fications have led naturahsts to recognize as many 

 classes of feathers. These are : the pcnnaceous, char- 

 acterized as above ; the -plumaceoiis, or downy ; and 

 the Ji/oplumaccotcs, or hairy. Downy feathers have 

 a short weak stem with soft barbs, very slender bar- 

 bules, rudimentary barbicels, and no booklets. Hairy 

 feathers are still further reduced to thin stiff shafts, 

 barbs and barbules, lacking the other structures. A 

 feather may be partly downy, and pardy pennaceous. 

 There is a particular kind of feather found in various 

 birds, called powder-down. The great bulk of a bird's 

 plumage is made up of the more perfect kind of feath- 

 ers called contour-feathers, from the fact that they 

 largely determine the apparent shape of the bird ; but 

 among these contour-feathers nestle the down-feathers, 

 forming a more or less complete investiture of the 

 body, the thready plumes being intermixed with the 

 latter. One may readily observe these different kinds 

 of feathers on plucking a duck, for example. Although 

 the feathers of a bird usually appear to cover the whole 

 body, they are very seldom everywhere inserted in the 

 skin. They grow in special places called feather- 

 tracts, separated b}'' naked spaces, the form and dis- 

 position of these tracts and spaces, thus mutually dis- 

 tinguished, being characteristic to some extent of the 

 different groups of birds, and consequently being of 

 use for purposes of classification. Most birds possess 

 peculiar apparatus for oiling their feathers, in the form 

 of a gland situated on the rump. The development 

 of feathers is analogous to that of hair and scales, 

 though their structure is so much more complicated. 

 The plumage is renewed by the process of moulting. 



