YOUNG BIRDS AND RECORDS OF THE PAST 123 



a series of wings of such birds, be carefully studied, it 

 will be found that, in such as have begun to develop the 

 primary quills, there is an arrested development of the 

 outermost quills exactly as is the case in the hoatzin. 

 But the wings differ in this — that in the first place the 

 finger-like tip is wanting, and in the second there is only 

 a small claw on the thumb : to find the claw on the finger 

 we should have to examine the embryo. 



As we pass these wings in review it will be found that 

 the development of the outermost quills remains in 

 abeyance until the inner have acquired a length sufficient 

 to form a moderately large fan-shaped area capable of 

 effecting a short flight. So soon as this stage is reached 

 the arrested quiUs begin to grow apace, and the secondaries 

 similarly grow rapidly. The development of the primaries, 

 however, is much more rapid than is the case with the 

 hoatzin, and this acceleration, this speeding up, is evi- 

 dently a direct response to the needs of the environment, 

 for more dangers lurk on the ground than in the trees. 



But what is the meaning of the temporarily free finger- 

 tip in the young of birds which pass the first few weeks 

 of life upon the ground ? Just this : the arrested develop- 

 ment of these feathers is a reUc of earlier days, when 

 young game-birds, like young hoatzins, were hatched in 

 arboreal nurseries, and similarly needed a free finger-tip 

 to the wing to serve as a climbing organ. That this 

 change from an arboreal to a terrestrial nursery took 

 place at a very remote period is shown by the fact that 

 the claw of the finger has disappeared, and that the finger 

 itself has shortened and lost its ball, or pad-like under- 

 surface. But the hand is still, for a time, longer than 

 the fore-arm : in the adult, as in the hoatzin, the fore-arm 

 is longer than the hand. 



