50 THE THEORY OF EVOLUTION 



wing-bones may be like those of the flying birds in 

 form and number, but are altogether too small to 

 be of any functional importance. This is the case 

 in the ostrich, for example, while in the cassowary 

 the digits are reduced to a single one, which is pre- 

 sumably the third. In the penguins, which are in- 

 capable of flight, the wing is not useless, but is 

 adapted to a new purpose and has been transformed 

 into a flipper or swimming paddle. This change of 

 function does not involve much modification of the 

 wing-bones, except that those of the hand are much 

 elongated and flattened; the transformation is most 

 marked in the feathers. It might almost be said 

 that these most curious birds have been adapted to 

 flight in a new medium, water instead of air. 



Still a third type of wing is exemplified by the 

 remarkable group of extinct flying reptiles known 

 as pterodactyls. In these creatures, some of which 

 attained immense size, the wing was most like that 

 of the bats, being a membrane or fold of skin, naked 

 and without hair, scales or feathers. Only one digit, 

 the fifth, or little finger, was elongated and strength- 

 ened to support the wing-membrane along its outer 

 border, while the other digits were free and provided 

 with claws. In all three groups, bats, birds and 

 pterodactyls, a vertical keel grows upon the breast- 

 bone (as any one may see in carving the breast of a 

 fowl), in order to furnish a sufficient bony attach- 

 ment to the powerful and greatly developed breast- 

 muscles, which are the principal muscles of flight. 



