54 Vertebrata. 



which are joined to their neighbours by numerous 

 hooklets. 



In this respect contour-feathers differ from down, 

 in which the barbs are all discontinuous. In young 

 birds the entire plumage consists of simple down- 

 feathers covering the whole surface almost uniformly, 

 and in some birds which do not possess the power of 

 flight, this condition is more or less perpetuated, and 

 thus all the feathers have discontinuous barbs, as in 

 the ostriches. In the great majority of birds, how- 

 ever, this primitive surface clothing is shed and be- 

 comes replaced by a second growth of feathers, which 

 differs from the first in that the component feathers 

 are for the most part contour- feathers, arranged in 

 definite tracts, and between these pteryla, or feather 

 tracts, there are spaces quite devoid of contour-feathers. 

 In the course of life, many birds change their feathers 

 several times, the process being called ' moulting.' 

 To defend the feathers from the influence of moisture 

 there is a gland situated on each side of the tail bone 

 which secretes an oily material of use in lubricating 

 the plumage. The largest feathers are those of the 

 wing, and they are grouped into primaries, secondaries, 

 and tertiaries, according as they are borne respectively 

 by the hand, the lower, or the upper end of the fore- 

 arm ; over these are the scapulars, and on the rudi- 

 mental thumb is the alula. The tail feathers are 

 also long, and as they are used in steering they are 

 named rectrices to distinguish them from the oar- 

 feathers of the wing. 



The papillae which secrete the feathers are long, 

 vascular, and deeply grooved on the surface ; the 



