Ix YOUTH, MATURITY, AND AGE 28¢. 
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, 6). 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 
Fic. 72.—Wing of Hoatzin—(a) young, (4) mature—after Pycraft. 
length. With maturity, too, the claw on digit No. 1 
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 
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