242 A HISTORY OF BIRDS 



the tip of the third digit. Both thumb and finger are armed 

 with large claws. The index finger is furthermore remarkable 

 in that it is produced beyond the fold of skin which runs along 

 the hinder or post-axial border of the wing for the support of 

 the quill feathers. Examined further, the palmar surface of 

 the thumb and second finger are found to be swollen into little 

 cushions, resembling the cushion-like under-surface of the finger- 

 tips of the human hand. Next the budding quill feathers 

 attract attention, and if a series of young is being examined, 

 probably the first point to be noted is the fact that the develop- 

 ment of the quill feathers of the hand is peculiar, inasmuch 

 as in the older forms whilst the inner quills are found to have 

 pushed their way out some considerable distance the outer 

 quills are only represented by simple down feathers (111. 28). 

 Thus, a long, free finger-tip is left beyond the quills. The 

 thumb also, as yet, bears only down feathers, the future quills 

 being conspicuous by their absence. On a little reflection the 

 meaning of this becomes clear. 



The arrested development of the quills of the thumb and 

 the tip of the finger is an adaptation to the bird's peculiar needs, 

 albeit a deep-seated character, dating probably from the very 

 dawn of avian development. If all the quills were to grow at 

 an equal rate a stage would soon arrive when the wing would 

 be useless as a climbing organ by reason of the developing 

 feathers, and so expose the bird to constant danger of falling 

 before the quills had sufficiently developed to break the force 

 of such a fall. This hypothesis of the signification of the 

 arrested development of the quills receives the strongest testi- 

 mony when still older specimens are examined. In them we 

 find that as soon as the inner primaries have grown sufficiently 

 long to enable the bird to recover itself in falling, the hand 

 begins to shorten, and the claw to diminish, till at the time 

 of puberty the hand has become shorter than the forearm, the 

 claws, both of the thumb and the finger, have disappeared, the 

 thumb no longer extends to the level of the third digit, and 

 the second finger no longer projects beyond the hinder wing 

 fold (post-patagium). 



That the structural peculiarities observable in the wing of 

 the Hoatzin are not recently acquired characters cannot be 

 doubted. The presence of the claws is almost sufficient to 



