244 THE HAUNTS OF LIFE 
their flying powers. Just as whales are the 
descendants of land mammals which went 
back to the sea (“‘secondarily aquatic”), so the 
Running Birds, with no keel on their breast- 
bone and no vane in their feathers, may be the 
descendants of flying birds which went back 
to the ground (“secondarily terrestrial”). 
There is a deep difference between the wing 
of a bird and the wing of a Flying Dragon 
or the wing of a bat—a deep difference in 
spite of the fact that all three are transformed 
fore-limbs. In the Flying Dragon and the 
bat the wing is what is called a patagial wing 
or web-wing, for what strikes the air is a 
drawn-out sheet of skin. But although the 
bird shows a little patagium or web stretched 
in front of its wing, the whole secret of the 
bird’s wing is in the feathers, borne by the 
arm and hand. In a ship the air strikes the 
sails, in a bird the sails strike the air, and in 
the bird the sails are the feathers. What 
made the bird’s flight possible was the growth 
of feathers—feathers with the barbs united 
together to form a stiff, but elastic, coherent 
vane which does not let the air through when 
they press against the air. How feathers 
began—perhaps it took a million years to per- 
