WHAT IS A BIRD 1 17 



foundation, for comparatively little has been 

 added since his death. But this by the way. 

 The study of Pterylography tells us then, not 

 only that the feathers are restricted to certain 

 areas of the bird's skin, but that the size 

 and form of these areas vary. Thus we 

 may have one strongly defined tract running 

 down the backbone from the nape of the neck 

 to the tail ; this is called the spinal tract. An- 

 other runs from the throat down the front of 

 the neck and dividing at its base, passes down 

 on each side of the breast and abdomen to the 

 inner side of the thighs quite to the end 

 of the body; this is called the ventral tract. 

 Then we have a pair of tracts running across 

 the upper arm, these are the humeral tracts ; — 

 the feathers forming these tracts are known 

 to the field ornithologist as the "Scapulars" — ; 

 and a pair of tracts over the thighs; these are 

 the femoral tracts. Besides these we have the 

 feathers covering the wings, the head, and the 

 tail which are generally dealt with separately 

 in special treatises, and called each by a special 

 name. The form of these tracts is so constant 

 that experts can tell at once to what group any 

 particular bird belongs^ and often even to what 

 species. 



A little reflection will carry us a step further, 

 and suggest to us that, if the feathers are re- 

 stricted in their distribution they must, to conceal 

 the skin, be (1) very long and broad, and (2) 

 directed so as to slope away from a line drawn 

 through the middle of any given tract. And this 

 actually obtains. 



