154 TYPES OF ANIMAL LIFE 



The ear seems at the first glance to be a double 

 organ, a very small one appearing inside the larger one. 

 This appearance, however, is due merely to the very 

 great development of that small prominence (called the 

 " tragus ") which in ourselves projects backward, to cover- 

 externally and so to guard the opening of the ear. 



When treating of the opossum we spoke of flying 

 opossums and flying squirrels, but no one of these crea- 

 tures, any more than the flying fish or any existing 

 reptile, really " flies." 



The bat, however, flies as truly as the bird does, and 

 in the same way by striking the air with its fore limbs, 

 but the mechanism is very different. When consider- 

 ing the turkey \ve noted how in birds the bones of the 

 hand are reduced to a minimum, the fingers being both 

 diminished in number and greatly shortened. From the 

 brief description of the bat's wing just given we may see 

 that in it the very opposite condition obtains. 



A similar condition existed in the pterodactyls', in- 

 asmuch as they flew by means of a wing membrane 

 sustained by the elongated bones of the hand. Never- 

 theless, in those reptiles it was only one finger which 

 was thus elongated. Thus here again the similarity of 

 their wing with that of the bat must have arisen inde- 

 pendently. 



But another independent similarity of structure, one 

 which must have arisen at least four times over, may 

 be noted with respect to organs which subserve the 

 movements of the wings. 



When treating of birds, we noted their almost uni- 

 versally " keeled " breast-bone, which, by the fact of its 

 being keeled, affords sufficient scope for the implantation 

 of the powerful muscles which act on the wings. Bats 

 also require powerful muscles of the kind, and on that 



