INTRODUCTION xxiii 



rule, generally or evenly distributed over the body, 

 after the fashion of hairs on a dog, for instance, but, 

 on the contrary, are arranged in long and generally 

 narrow bands, or "tracts," separated by wide, bare, 

 or sometimes down-clad spaces. 



The fact that these bands vary greatly in shape 

 among birds was first realised by a German naturalist 

 named Nitzsch, who made a long and careful study 

 of the feather-tracts of birds. As a result of his pa- 

 tient work, he was able to show that the variations in 

 this arrangement followed certain definite lines, each 

 group of birds possessing a type peculiar to itself; 

 and, for the purposes of convenient description, he 

 gave these tracts distinctive names, which, in the main, 

 are followed to this day. 



Briefly, as a result of his work, he distinguished: 

 ( 1 ) a head tract, formed by the feathers clothing the 

 head; (2) a spinal tract, extending from the head 

 down the back of the neck, and along the back to the 

 tail ; ( 3 ) a ventral tract, running from the throat down 

 to the base of the neck, where it branches at the shoul- 

 ders, to run down over the breast and abdomen in the 

 form of two bands, a broad outer and a narrow inner 

 band; (4) a pair of humeral tracts, which, crossing 

 the upper arm, form the feathers known as the scapu- 

 lars; (5) the wing tract, including the quills and wing- 

 coverts; (6) the tail tract; (7) the femoral tracts, 

 which run across the thighs; (8) the leg tracts, which 

 cover the legs below the knee. 



The most important of the variations which these 

 tracts present are to be found in the spinal and head 

 tracts. Thus, in the first-named, the spinal tract in 



