70 THE STORY OF BIRD-LIFE. 



SO long as it gets its proper amount of exercise 

 and nourishment — and the latter largely depends 

 upon the former — ^jiist so long will it remain 

 vigorous. More than this, it will respond within 

 certain limits to new demands upon it, and 

 change its form or function accordingly. But, 

 so soon as its usefulness becomes lessened, and 

 its exercise diminishes, its nourishment is slowly 

 withdrawn, and it gradually but surely starts on 

 its downward path. Growing smaller and weaker, 

 and losing more and more its characteristic shape 

 and beauty in each generation, it soon is reduced 

 to a mere vestige, and even this is sometimes 

 lost. We may then remain in entire ignorance 

 of its ever having existed, or may have our 

 attention directed to its loss by reason of its 

 presence in its nearly allied species. 



The following brief review of flightless birds 

 will afford us an excellent study of the results, 

 more or less direct, of use and disuse. 



The power of a bird's flight is probably in pro- 

 portion to its need in procuring food and escap- 

 ing enemies. The one plentiful and near at 

 hand, and the other removed, it would seem that 

 each of the birds, about which we are to speak 

 presently, insensibly settled down to a comfort- 

 able, easy existence, seldom or never exercising 

 its wings. The flight necessary to procure the 

 daily bread was also often a means of enjoyment 

 pure and simple, without any such gross motives. 

 But this source of pleasure was smothered for 

 ever in that evil day which placed food within 

 too easy reach and banished enemies to the land 

 of oblivion. As a result, the children of the 



