110 THE STRUCTURE AND LIFE OF BIRDS  cuap. 
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 accompanied 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. Bones very little or not at all aerated.- 
Girth of humerus. ins. Girth of humerus. ins. 
Screamer... ... 13 Logger-headed Duck. . . . Ipy 
Rhinoceros Hornbill 1g Scoter Duck . . 13 
Golden Eagle . . ‘1 13 Nestor Parrot... .. a} 
Vulture Monachus . sai 3 2k Red-throated Diver 1 
Marabou Stork ...... 21 | Spur-winged Goose . Ios 
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 
