Feathers 39 



probably covered uniformly with scale-feathers, but as 

 these increased in length there was less need for an un- 

 broken covering, the feathers of one portion overlapping 

 and protecting the surrounding parts, and besides, for 

 ease in active motions, bare patches of skin were required. 

 It has been found that the arrangement of the feathers 

 on a bird's body varies in different groups, and, such 

 variation being rather characteristic of these larger divi- 



FlG. 25. — Nestling Crow, showing feathered and infeathered portions of the body 

 (pterylae and apteria). 1/2 natural size. 



sions, pterylosis — as it is called — is of some importance 

 in classification. Penguins only, of existing birds, have 

 feathers growing uniformly on all parts of the body. In 

 the ostrich, which has given up flight and taken to run- 

 ning, the body feathers have grow^n over almost all the 

 bare spaces which existed in its flying ancestors. There 

 are two marked exceptions due to the present habits of 

 these birds. Like the camel, when resting, these giant 

 birds lean upon their breasts. This portion of the body 



