20 THE-STORY OF THE BIRDS: 
no more the cause of feathers being plumous, than it 
is of hair being furry. 
In this connection also, though not exactly in 
order, it may be said that there is much doubt if any 
of the spaces about the body or in the bones of birds 
are filled with air for the purpose of buoyancy. 
It is not known that a bird inflates itself during 
flight, and the plumage is certainly very compact at 
this time. 
All bones that are tubular are surely so for the 
sake of putting the material into the best shape for 
strength combined with lightness ; and in the birds 
they are devoid of marrow, often doubtless to admit 
air, since they have openings connected with ducts 
leading to the lungs and other air spaces. But if 
mere lightness were the only object their interiors 
should be vacuums, since air is heavier than nothing 
at all. 
Air cavities in bones and _ tissues, especially of 
birds, are probable aids or supplements to scant lung 
surface, demanded by a very active life ; for they are 
found in the non-flying ostrich forms, and even in 
many heavy but active fossil reptiles. Some air 
spaces and sacs in birds, reptiles, and amphibians, 
and even mammals, have other uses not discussable 
here. To return to our topic, it is likely that, if we 
knew how a bird ‘first put on feathers, we should 
know more of the biography of its fore leg. 
