156 SUMMER 



weight by a physically practicable wing surface ; expert 

 opinion holds that the problem ought theoretically to be 

 easier with a larger bird than with a small one, since in the 

 smaller bird's wing there is so much more waste margin to 

 the area. One plausible explanation is that since the power 

 of the living engine must be supplied by food, it would be 

 impossible for an ostrich to eat enough to make good the 

 waste of the extremely powerful muscles which depress the 

 wings of birds in flight. The failure is thus one of the 

 digestive processes, in spite of the fact that the ostrich's 

 digestion is not upset by bits of metal. But the reluctance 

 of the young vulture, and even of the young choughs, to 

 trust themselves to the air suggests that the real reason of 

 the flightlessness of the largest birds may be found in the 

 familiar perils of the law of gravity. The danger of falling 

 is greater for big birds than small ones ; while the newly- 

 hatched duckling can drop safely from a tree, such a fall kills 

 the young rook. Possibly there came a point in the develop- 

 ment of the larger species when they feared to practise flight 

 at all, or when all those who did try it in their unskilful youth 

 perished, and the remainder became hereditary pedestrians or 

 divers. It may be some consolation for the loss of human 

 life in learning to fly that it is not always a safe or simple 

 process even for birds. Birds, indeed, are too intelligent and 

 adaptive a race to possess the sort of security that comes of 

 living by instinct 



