no THE STRUCTURE AND LIFE OF BIRDS chap. 



hollow bones are advantageous to a bird, natural 

 selection can bring it about that they become hollow 

 and that somehow the bird is able to dispense with 

 the marrow. This would be a bold assumption, did 

 we not know it to be an accomplished fact. The 

 bones are hollow, and there is no want of life in the 

 birds. 



We shall find that pneumaticity in a bone implies 

 Greater girth in proportion to its length, and conse- 

 quently greater strength ; and that a decrease of 

 weight has accompani:d the increase of strength, 

 mainly through the drying up of the marrow, but 

 partly through a reduction, if we allow for the increased 

 size of the bones, in the thickness of the hard osseous 

 shell. I shall give first a few measurements to show 

 that in the case of birds whose skeletons have little 

 or no aeration, the girth of the bones is, relatively to 

 the bulk and weight of the body, considerably less. 



Bones highly pneumatic. 



Girth of humerus. ins. 



Screamer i| 



Rhinoceros Hornbill . . . i^ 



Golden Eagle 1 1 



Vulture Monachus 2\ 



Marabou Stork 2}| 



Bones very little or not at all aerated. 

 Girth of humerus. 



Logger-headed Duck . . 



Scoter Duck 



Nestor Parrot 



Red- throated Diver . 

 Spur-winged Goose . . 



ins. 

 irV 



l 3 



TS 



a 



4 



I 



These measurements speak for themselves, even 

 without any exact statement of the weights of the 

 birds ; but the following illustration will do more to 

 explain the problem of hollow bones. The shoulder 

 bones of a Skua Gull, which has scarcely any aeration, 

 of a vociferous Sea Eagle, and a Hornbill, both of which 

 are highly pneumatic, are placed side by side. The 



