12 



A HISTORY OF BIRDS 



feathers are of the true teleoptyle type before the birds leave 

 the nest. These mesoptyles are, in such species, preceded by 

 a pecuHarly downy plumage made up of the protoptyle feathers. 

 In the young Barn Owl the protoptyles are never developed, 

 while the mesoptyles have so far degenerated as to be mdis- 

 tinguishable from protoptyles. That they are really mes- 

 optyles is shown by the fact that, like all mesoptyles, they 

 are thrust out upon the tips of the teleoptyles as soon as 

 these develop (p. 271). 



In some birds, as -in nestling Game-birds and Owls, pre- 

 pennae only are developed. In others, as in nestling Hawks, 

 the pre-plumulffi are larger, and play a more important part 

 than the pre-pennse. And from this there is but a step to the 

 suppression of the pre-pennse altogether, as in the young of the 

 Cormorant, wherein the nestling is clad only in pre-plumulse. 



II 



f,t.Sf>. 



III. 4. — Pterylosis of Acanthidositta chloris showing the Feather Tracts 



Pt. cap. = Pteryla capitis. Pt. h. = Pteryla humeralis. Pt. sp. = Pteryla 

 spinalis. Pt. f. = Pteryla femoralis. Pt. coll. v. = Pteryla colli ventralis. 

 Ft. u. = Pteryla ventralis. (By permission of the Editors of The Ibis.) 



The feathers of birds are not evenly distributed over the 

 body, as are the hairs of, say, the horse, but are gathered to- 

 gether to form more or less sharply defined tracts (pterylae), 

 leaving large bare spaces or " apteria," though in many birds 

 such apteria are covered with down feathers. The nature of 

 these tracts may be seen in 111. 4. Since they assume char- 



