ii4 THE STRUCTURE AND LIFE OF BIRDS chap. 



the greater the pace at which its extremity will move ; 

 if the velocity is doubled, it is well known that the re- 

 sistance of the air is far more than doubled, so that an 

 increase of strength is required that is altogether out 

 of proportion to the increase of length. This will be 

 made clearer, when we come to the subject of flight 

 (see p. 175). 



The Hornbills are a puzzle. The extreme short- 

 ness of the hand bones, a ridiculous anticlimax follow- 

 ing upon so grand an ulna and so portentous a 

 humerus, might suggest that they were once better 

 flyers, and that the wing is slowly undergoing reduc- 

 tion. But the mountainous beak seems to show that 

 colossal bones are an ancient heritage of the family, 

 that even feeble flight might have been difficult had 

 they not become hollow, and that existing Hornbills 

 fly quite as well as their ancestors. In cither case 

 they have been great gainers by aeration. 



The Ostrich and its allies present another difficulty. 

 But here too it may be said that by means of pneu- 

 maticity great strength has been combined with 

 lightness in a way that with solid bones would have 

 been impossible. 



Any one who wishes to realise the relation, in birds' 

 bones, of slimness to solidity, and of large girth to 

 aeration, should inspect collections such as those at 

 the Royal College of Surgeons, or at the Natural 

 History Museum at South Kensington, where a large 

 number representing different families may be seen side 

 by side. It is easy then to see that big long-winged 

 birds have wing bones thicker in proportion to their 

 length in order to bear the far greater strain upon them, 



