i8o THE STORY OF THE BIRDS 



birds indeed that cannot fly, and in all cases to the 

 contrary the wings have reached their present de- 

 graded and functionless state through disuse. It is 

 almost a demonstrable fact that the earliest bird 

 forms could fly, and that all flightless birds, no 

 matter how archaic their type (such as the Ratitae), 

 have been brought to that condition by the neglect 

 of their ancestors to make proper use of their 

 wings. Birds, then, are the only vertebrate animals 

 that can fly, with the sole exception of bats, and in 

 their case the analogy is scarcely a perfect one. 



It requires but a very elementary knowledge of 

 the anatomy of birds to convince us that these 

 creatures are in every way fitted for an aerial 

 existence. Not only is the general shape of the 

 body one best adapted for easy cleavage of the 

 air, but the bones being hollow, are exceptionally 

 light, whilst the extraordinary lightness of their 

 dermal covering has passed into a proverb, ^* As 

 light as a feather." Then again, as we have 

 already seen, the anterior limbs of a bird are 

 attached to the highest part of the body, whilst 

 all the least weighty organs are similarly situated. 

 The heavy pectoral muscles and the stomach 

 being in the lowest part of the body renders the 

 centre of gravity as advantageous as possible be- 

 neath the suspension centre of the wings. Then, 



