xxiv INTRODUCTION 



the Swifts encloses a bare space over the middle of 

 the back ; while in the Swallows it divides into a fork 

 in this region, leaving the hinder portion of the tract 

 in the form of the usual straight band. In the Finch 

 tribe the middle region of this tract is diamond- 

 shaped. The head tract, again, often encloses a 

 space; as, for example, in the Himnuning-birds and 

 Mouse-birds of Africa. The ventral tract similarly 

 presents very marked differences when a number of 

 different kinds of birds come to be examined. 



Those who may be interested in this question should 

 take, say, a Sparrow, Starling, Thrush, Pigeon, and 

 Fowl, cut off the feathers with a pair of scissors, close 

 to the body, and compare the differences between 

 them. Since these differences are constant, and since 

 each group presents a type of its own, it has been 

 found that the "pterylosis," as this arrangement of 

 the feathers is called, affords a valuable aid to the 

 classification of birds. For example, the very wide 

 difference in the pterylosis of the Swifts and Swallows 

 was the first indication of the fact that these birds 

 were not related, as they had always been supposed to 

 be, and later anatomical investigations have given 

 further proof that these birds belong to quite different 

 groups. 



In the Penguins and the Ostriches the bare spaces 

 found in other birds are hardly traceable, in so far as 

 the trunk is concerned, the feathers covering almost 

 every inch of the body. * 



In describing the external appearance of a bird 

 these tracts are commonly ignored, the body being 



