ment would flight be possible, for on the upstroke of the wing 
through the air the quills act like the shutters of the sails of a 
windmill, allowing the wind to pass between them and so relieving 
pressure on the uplifting wing-stroke. On the down-stroke, the 
opposite effect is produced. The full force of the stroke is 
conserved, because, owing to the overlap, the several feathers 
are now pressed closely together to form an impervious sheet. 
How are they fixed to the skeleton? To see this all the 
smaller feathers and the muscles, or “‘ flesh ” of the wing must be 
removed. It will then be found that the flight-feathers 
are divisible into two series. One, widely spaced, runs 
along the upper surface of the fore-arm: the other, closely 
packed, along what answers to the back of the hand. In 
effect this is but a single rod of bone, but it is composed of three 
elements, answering to three of the digits of the human hand— 
the thumb and the first and second fingers. But they are 
scarcely recognizable as such, for the thumb is reduced to a mere 
stump, while the two fingers have become welded together. 
The third finger, indeed, has become reduced to the palm-bone, 
and a short stump answering to the first finger joint. To this 
frame-work, which can be folded up into the shape of a Z when 
the bird is at rest, the quills are fixed by their base by means of 
slender, but very strong elastic tendons. In birds which have 
a long upper arm bone, like the Albatross, Gull, or Heron, there 
