48 



THE THEORY OF EVOLUTION 



other digits are extremely long and slender and 

 usually the terminal joints, with their claws, have 

 been suppressed, but in the fruit-eating bats, or 

 "flying foxes," the second 

 digit retains the terminal 

 joint and claw and in cer- 

 tain other families the third 

 digit (but not the sec- 

 ond) has the terminal joint, 

 though without a claw. The 

 wing is a fold of skin, which 

 is attached along the outer 

 side of the body and the 

 under side of the arm and 

 forearm and is stretched be- 

 tween the bones of the digits, 

 which support it in much the 

 same fashion as the ribs of 

 an umbrella sustain the cloth. 

 The membrane is also at- 

 tached to the legs and ex- 

 tends between the two thighs 

 and the short tail. 



In the wing of a flying 

 bird the problem of making 

 an organ of strong and steady flight is solved in an 

 entirely different manner. The effective air-plane 

 is formed by large quill-feathers, which are inserted 

 along the outer side of the bones of the arm and 

 hand, so there is no wing-membrane and the stiffness 



FIG. 3. Skeleton of right wing 

 of Leaf-nosed Bat. Letters 

 as in Fig. 1. 



