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- 
Fia. 25.—Nestling Crow, showing feathered and infeathered portions of the body 
(pteryle and apteria). 1/2 natural size. 
sions, ptervlosis—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 grown 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 
