IX 



YOUTH, MATURITY, AND ACE 



28c 



nestling Hoatzin the hand is longer than the forearm ; 

 gradually it grows shorter, while the other parts of the 

 wing lengthen, till, in the fledged bird, the forearm 

 surpasses it (Fig. 72 a, b). The feathers, too, adapt 

 themselves to changing circumstances ; in the nest- 

 ling 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 



^Ar^l 



Fig. 72.— Wing of Hoatzin — (a) young, (/>) mature — after Pycraft. 



length. With maturity, too, the claw on digit No. I 

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

 The pause made by the growing quills was first 

 noticed by Mr. Pycraft, and he has further pointed 

 out that in the common chick we have traces of a 

 similar development. There too we find the hand of 

 the young bird longer, though only by a little, than 

 the forearm ; the second digit has a claw only in the 

 embryo stage ; the first is much reduced, but retains 



U 



