THE FIRST PROBLEM OF BIRD LIFE. 139 



exertion by swimming, and will find it to his advantage to 

 seek his prey in or near the water. The bird with strong legs 

 and short wings will cover ground more easily by running than 

 by flying, so he will naturally look for his food on the ground. 

 Each one would employ his natural advantage or accomplish- 

 ment as the easiest way of getting his food. 



Now hunting for something to eat takes up nearly all the 

 time of these plain birds, so that the one that is fond of swim- 

 ming spends most of his time in the water ; the one that likes 

 to ran trots about so steadily that he flies very little unless 

 frightened or in danger ; wdiile the one that is light and swift 

 of wing spends his day in the air. You can guess the result. 

 Each one grows more and more adept in his own favorite 

 mode of hunting and less and less adapted to following any 

 other method. As the swimmer neither flies nor walks much, 

 he may at last become incapable of doing either with any ease : 

 the penguins that cannot fly and the grebes that cannot walk 

 are such birds. The runner may, like the ostrich, become as 

 swift of foot as a horse, yet lose his power of flight. The 

 strong-winged flying bird may, like the swallow, be tireless on 

 the wing and yet scarcely able to walk. Not only do they 

 grow unlike in the parts they exercise constantly, but also in 

 the parts they neglect to use. The limbs and muscles in con- 

 stant use grow large and strong ; those that are disused become 

 feeble and pine away, or else stay undeveloped. 



But some one asks if I mean to say that a penguin, an 

 ostrich, and a swallow were ever one bird. No, I do not mean 

 to say that. But I wished you to notice that if there ever 

 was a time — as many believe — when all the birds were just 

 plain birds (undifferentiated, a scientist would say), they could 

 not have remained so. They were bound to change, and 

 they were bound to grow unlike. 



