definition of birds proves defective. Furthermore, bats fly as 

 truly as birds, and some fishes and reptiles possess imperfect 

 flight, while the pterodactyles — an extinct order of reptiles — were 

 as much flying vertebrates as either bats or birds. But the 

 mechanism of flight in birds is strictly distinctive, the apparatus 

 of flight consisting of a special modification of certain feathers 

 growing upon the fore-limbs. If we compare the wing of a bat or 

 pterodactyle with the wing of a bird, we find that all the bones of 

 the arm and hand are lengthened, to give support to the parts con- 

 cerned in flight, but that here the resemblance ceases ; the wing 

 of the bat, as was the case with the pterodactyle, consisting of a 

 naked leathery membrane joined to the sides of the body and to 

 the arm, and supported by the long slender fingers of the hand ; 

 while in the bird the means of support consists of a series of long, 

 stiff, broad feathers, attached to the arm and hand, which latter 

 consists practically of a single finger, giving support to the primary 

 wing feathers. 



In respect to internal structure, birds possess many characters 

 common to mammals, and others which they share only with 

 reptiles; in other words, most of the characters which distinguish 

 birds from mammals are equally common to reptiles. Birds may, 

 however, be distinguished from mammals by the absence of milk 

 glands for the nourishment of their young ; and from reptiles by a 

 complete double circulation of the blood. 



As already said, the only feature serving to distinguish birds 

 from all other vertebrates is their covering — feathers. This 

 seemingly superficial and unimportant character is a modification 

 of the skin appendages covering the body in strict correlation 

 with the adaptation of the whole organism to aerial locomotion, 

 the whole structure being specialized to this end. The skeleton, 

 the muscles, the position of the internal organs, the manner of 

 reproduction, the covering — all are profoundly specialized to 

 enable birds to fly. The form of the body is adapted to easy 

 passage through the air, having, so to speak, the form of two cones 

 joined base to base, the neck and head forming the apex of one 

 and the tail the apex of the other. The covering of feathers gives 

 everywhere a smoothly rounded contour, effectually concealing 

 all angularities of the body. The feathers themselves are directed 



