CHAPTER XXI 

 THE BIRDS 



The birds are so sharply distinguished from all other 

 vertebrate animals that no one would make a mistake in 

 assigning to its proper class even the most aberrant mem- 

 ber of the group. There are other vertebrates that are 

 able to fly, such as the bats among mammals and, in 

 former periods of the world's history, some outlandish 

 looking reptiles called pterodactyls; but the resemblance 

 of these creatures to birds, aside from the possession of 

 wings, is quite remote. All birds are furnished with 

 feathers; they all have a horny bill; and they all have but 

 a single pair of legs which are used for walking, hopping 

 or running. This pair corresponds to the hind limbs of 

 other vertebrates, the fore limbs being modified to form 

 the wings. 



The aerial life of birds has been one of the chief causes of 

 their distinctive peculiarities of structure. Flight implies 

 strength of bone and muscle, and expanse of surface for 

 beating against the air, or for steering a course through it. 

 The expanse of surface is mainly afforded by the feathers. 

 While feathers are found in no creatures except birds 

 they represent highly modified scales such as cover the 

 bodies of reptiles. Nature is continually adapting old 

 organs to new functions and, in evolving the feather from 

 the scale, she has perfected a wonderfully complex and 

 beautiful structure that seems at first to have little in 

 common with the original source. Like scales, feathers 

 are derived from small papilla? in the skin. Commonly 



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