BIRDS' FLIGHT, AND BIRDS' WINGS. 65 



nestling is remarkable in one or two respects. 

 When newly hatched the thumb and first finger 

 are armed with long claws, by the aid of which 

 it climbs out of its nest, — which is built in dense 

 shrubs and trees overhanging the water, — to meet 

 its parents coming with food. Day by day certain 

 of the quill or flight-feathers of the wing grow 

 longer and longer, but only about half of the 

 proper number are represented in the hand. The 

 development of those near the tip of the finger 

 is held in abeyance for a very considerable time, 

 until the inner ones have grown long enough to 

 break the force of a fall should such an accident 

 take place. So soon as this assurance against 

 accident has come about the outer quills grow 

 rapidly, and the claw of this finger drops off, 

 there being no further use for it. The benefit 

 of this law of growth is obvious. If the outer 

 quills grew at the same rate as the inner ones, 

 the wing would in a few hours be useful neither as 

 an organ of flight, nor as an organ of prehension, 

 for the wing area would not be large enough to 

 support the body, nor w^ould the claw, on account 

 of the premature development of the quills, be 

 enabled to grasp objects. Yet another change 

 follows the loss of this claw. In the nestling 

 the hand is longer than the forearm, in the 

 <adult the hand is shorter than the forearm. 

 The whole wing, in short, in the adult bird, is 

 of a degenerate character. 



The hoatzin builds in trees, and passes its 



whole life therein. One has never been known 



to alight on the ground. If a nestling should 



happen by any chance to fall into the water, 



E 



