422 



NA TURA L HIS TOR Y. 



BIRDS. 



No one needs a definition of a bird, for there is not the slightest chance 

 for mistaking one of these feathered forms for any other animal ; nowhere 

 else in the whole animal kingdom do we find feathers, and, on the other 

 hand, no bird lacks these structures. Not only the feathers mark off the 

 birds from all other groups ; there are many other features which at once 

 pronounce them distinct from all other forms which live to-day. Still 

 birds were not always so isolated as they now appear. The birds of 

 to-day have many points in common with the reptiles ; but in the ages of 

 the past there were forms — true birds — which approximated fai more 

 closely to the group which we have just dismissed. Some of these points 

 of resemblance are too abstruse for consideration in a popular work ; some 

 will appear in the following pages. 



Fio. 361. — Feather-tracts of a swift, showing the regions (dotted) where the larger 



feathers of the body are inserted. 



Since feathers are so characteristic of birds, we may be pardoned a 

 few remarks concerning them, especially since the different kinds of 

 feathers are so much used in classifying birds. In the first place, feathers 

 are not evenly distributed over the surface of a bird ; there are regions on 

 almost every bird which is without plumage ; but the position and extent 

 of these naked spaces (apteria, they are called) varies with the different 

 groups. In the adjacent cut of the body of a swift, the dots show the 

 points of insertion of the feathers of the body, while on the wings and tail 

 the basis of the feathers themselves are seen. All the white portions are 

 without feathers, although in life this nakedness does not appear, owing to 

 the fact that the feathers of the other parts spread over them. 



In the feather-tracts the feathers arise, each from a small papilla. The 

 typical feather consists of a shaft or quill, from either side of which the 





