INTRODUCTORY 



1 1 



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 Humming-birds and Mouse-birds 

 of Africa. The ventral tract similarly presents very marked differ- 

 ences 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, 



Beak 



Crown Bar- coverts 



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 con- 

 stant, and since each group 

 presents a type of its own, it 

 has been found that the ''ptery- 

 losis," as this arrangement of 

 the feathers is called, affords 

 a valuable aid to the classifica- 

 tion of birds. For example, the 

 very wide difference in the 

 pterylosis of the Swifts and 

 Swallows was the first indica- 

 tion 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 so conspicuous 

 in other birds are hardly traceable, the feathers covering almost 

 every inch of the body, at any rate in so far as the trunk is concerned. 



In describing the external appearance of a bird these tracts are 

 commonly ignored, the body being mapped out into certain areas such 

 as are indicated in fig. 4. 



So far we have spoken only of the feathers which form the outer 

 surface-covering of the bird, the '* contour " feathers, as they are 



Toes 



Tarso- meta- 

 tarsus 



Fig. 4.— Diagram of a bird showing the 

 different areas of the body. (After 

 Ogilvie Grant.) 



