WHAT IS A BIRD ? 21 



unique structure, it never has been, nor is now, 

 grown by any creature under heaven other than a 

 bird, just as hairs, for instance, are found only 

 within the class mammalia — animals that suckle 

 their young. But the bird's covering is more 

 peculiarly exclusive, since no bird is clothed, 

 in any way other than with feathers, whereas 

 mammals, or more popularly " animals " may 

 have the skin covered with scales, as in the 

 manis, or horny plates, as in the armadillo, or 

 curious modifications of hairs in the shape of 

 spines, as in the porcupine, or wool as in the 

 sheep, or bristles as in the pig, or may even be 

 entirely naked, as in the whales and porpoises. 



What is a feather 1 Feathers are to the bird 

 what scales are to the fish and reptile, and hairs 

 to the mammal — a covering to protect the surface 

 of the body, and to regulate its temperature. We 

 cannot tell much about their structure without 

 a microscope. We have seen already that there 

 are different kinds of feathers. There are those 

 outside that form the general contour of the 

 body, some of which grow very large to form the 

 "quills" with which the bird flies, and from 

 which our pens and toothpicks are made, and 

 there are those below the surface, the chief of 

 which we call down, eider and swan's-down 

 being familiar to most. 



We can learn something about them however 

 without a microscope. A " quill " feather will 

 serve us best. Take one, and note first of all the 

 strong central axis or shaft ; along each side of 

 this you will see hundreds of little straight 

 branches set very close together, and held in 



