190 THE STRUCTURE AND LIFE OF BIRDS  cnap. 
But it is not only when we compare base and 
extremity that the division of labour is unequal. If 
we divide the wing in imagination by a line drawn 
down its middle from base to tip, then the front half 
will do far more work than the hinder half, when the 
bird is gliding at great speed or moving his wings 
rapidly through the air. A boat sailing at an angle 
to the wind with a sail slung obliquely across it 
supplies an illustration of this. If she moves rapidly, 
only the forepart of the sail will do much work. The 
wind is blowing, say, at right angles to the boat. It 
will strike the forepart of the sail, over all the rest of 
which there will only be a backward current of air, 
which has been turned from its course by the forepart. 
The faster the boat sails, and, also, the nearer to the 
wind she sails, the truer this will be; the narrower 
will be the margin of sail that really works. This 
is called the law of Avanzini. It holds with regard 
to the bird’s wing, which during the down stroke moves 
rapidly forward as well as downward, and, of course, 
shares the onward movement of the whole bird. It is 
truer of the swiftly moving extremity than of the 
slower inner part, and this accounts for the remarkable 
way in which long wings, notably those of the Swift 
and Gannet, narrow towards their ends. Professor 
Pettigrew made some experiments which illustrate 
this. He cut away the hinder part of a Bluebottle’s 
wings and apparently it could fly equally well. In 
the same way with Sparrows, the removal of the same 
part of the wing seemed to do little damage. Still to 
describe the flight after these mutilations as “ perfect ” 
is to go too far. No failing may strike the eye. A. 
