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- 



