WHAT IS A BIRD? 16 



the skeleton of the wing, which will be considered 

 later, there is an equally unique difference in 

 the nature of the flying membranes which 

 this skeleton supports. In bats, the only real 

 flying m.embers of the class mammalia, this 

 membrane takes the form of a thin web of skin, 

 stretched between the fingers, and running down 

 the arm and side of the body ; in certain so- 

 called flying-squirrels this membrane is much 

 smaller in area and does not extend between 

 the fingers ; the result is a form of parachute. 

 In birds, the membrane plays a quite secondary 

 part ; instead, certain of the feathers clothing the 

 hinder margin of the wing become enormously 

 elongated and flattened out. These, by aid of 

 strong ligaments at their base, and a peculiar 

 method of over-lapping one another, form a 

 broad surface which proves even more efficient 

 for its work than does the skin folds of the 

 bat. The bat's wing would resemble the bird's, 

 if certain of the hairs along its hinder margin 

 became elongated and flattened out so as to 

 take the place of the membrane now existent. 

 A reference to the figures should make this clear- 

 Fig. 6, p. 227. 



The sum of the whole of this process of 

 evolution is a form specially adapted for rapid 

 progression through the air. The leading 

 characteristics of this form may be summed 

 up in the long slender and mobile neck 

 and pointed head, the perfectly rounded 

 and somewhat elongated body of extreme 

 lightness, raised and propelled by wings, and 

 clothed in the lightest and strongest possible 



