86 THE BOOK OF NATURE STUDY 



reduced to a minimum, while its resistance on the down-stroke has 

 as much force as in the wing of the bat. 



It is suggested that those who desire to demonstrate this point 

 should procure, say, a sparrow or pigeon recently killed, when the 

 foregoing remarks can be followed out and tested. It is im- 

 possible, indeed, to really grasp the full meaning of this mechanism 

 from mere description, however carefully they may be written or 

 read. 



So much, then, for the wing as a part of the machinery of 

 flight. The driving power is furnished by the breast muscles, 

 which are relatively of enormous size, covering the whole of the 

 breast-bone, and being further provided with an additional 

 surface for attachment in the shape of a median " keel " the bony 

 plate running down the middle of the breast-bone. 



Birds which live in areas free from enemies, and containing 

 an abundant and constant supply of food which can be picked 

 up off the ground without the need of foraging over extensive 

 tracts of country, gradually cease to fly, and in consequence 

 these muscles and the supporting keel of the breast-bone slowly 

 undergo degeneration, until its flight is lost completely, as, 

 for example, in the owl-parrot, where but a vestige of the keel 

 remains; or in the ostrich tribe, where even this has 

 vanished. 



It should be pointed out, then, that a bird's standard of effi- 

 ciency in the matter of flight is determined, more or less exactly, 

 by the stimulus of external circumstances, the necessity of 

 taking long flights to obtain food or escape the rigour of climate 

 or the attacks of enemies. 



Attention may further be drawn to the fact that birds which 

 spend much of their time on the wing have long pointed wings, 

 as in the swifts, swallows, gannets and albatrosses ; while, on the 

 other hand, birds which fly but little, or live in forests or amid 

 thick undergrowth, have short rounded wings, such as in the 

 tawny owl, wren and game-birds. 



The penguins occupy a unique position among birds, their wings 

 having become transformed into swimming paddles answering 

 to those of whales and turtles, in so far as their shape is con- 

 cerned. But whereas in the penguins and turtles these paddles 



