60 ANIMAL LIFE 



which, moving freely at the shoulder, sweeps the tip 

 of the wings in spirals. Length of wing is given in 

 two ways : by elongation of the arm and hand, as in 

 storks, or by increase of the feathers in length, as in 

 swallows ; and as we saw that in the fleetest mammals 

 length of limb and reduction in the number of fingers 

 or toes went together, so in birds we find that only 

 two of the usual five lingers are used for the support 

 of the greater part of the wing. The skin between 

 these fingers and at each side of the wing grows into 

 flaps, which, being covered with feathers, form an 

 aeroplane triangular in surface-outline, and yet 

 curved so as to furnish a concavity below and a con- 

 vexity above. Upon such a membrane, only flying 

 leaps like those of squirrels could be carried out. To 

 convert the feathered membrane into a true wing it 

 must be provided with muscles sufficiently powerful 

 to lift the bird into the air ; and since not only upward 

 but also forward movement is required, the direction 

 of the wing has to be so inclined to that of the air- 

 current as to convert the oblique thrust into a large, 

 forwardly-acting component and a small, inactive one 

 at right angles to this, just as a boat's sail plays off the 

 force of wind into a propelling and a drifting force, 

 or as an insect's wing or fish's tail solves the same 

 problem of resolution. Hence the need for the powerful 

 breast muscles, mainly for depressing the wing not 

 in one vertical plane but obliquely downwards and 

 backwards. And since the efficiency of stroke is 

 guided by the speed of the bird relative to that of the 



