BIRDS 



473 



wing. 



The nestling, unless very recently hatched, crawls off 

 on all -fours, making much use of not only his enormous feet, 

 but of the great claws that grow at the ends of the two first 

 digits of his hand (fig. 994). If you try to pull him from the 

 nest by the legs, he holds fast by means of these wing-claws 

 and his beak. Often a young bird may be found clawing his 

 way onward some distance from the nest. If he tumbles into 

 the water he proves to be a born swimmer and diver. ... In 

 the nestling Hoatzin the hand is longer than the forearm; 

 gradually it grows shorter, while the other parts of the wing 



Fig. 994. Wings of Young (A) and Adult (B) Hoatzin (Opisthocomus}. The quills of the latter have been cut short. 



I, u, in, digits. 



lengthen, till, in the fledged bird, the forearm surpasses it (fig. 

 994). The feathers, too, adapt themselves to changing circum- 

 stances ; in the nestling the growth of the two outermost primaries 

 is completely arrested, so that the use of the claws may not be 

 impeded; when it is fledged and can fly, they begin again to 

 grow and attain their full length. With maturity, too, the claw 

 on digit No. i grows small, while that on No. 2 is lost altogether." 

 Before leaving Birds it may be well to remark that in the 

 early history of the group precocious young were no doubt the 

 rule, and it is interesting to note that they are characteristic 

 of many species in which the organization is relatively low. 

 The associated evolution of greater intelligence and increased 

 parental affection is apparently responsible for the appearance 

 of helpless nestlings. And we can only suppose that these 

 have, on the whole, a better chance of surviving the struggle 

 for existence than young birds which are soon able to take care 



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