THE CONDITION OF YOUNG BIRDS AT BIRTH. 113 



is considerably longer than the forearm. The thumb is found to be 

 unusually long, and to extend beyond the level of 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 undersurface of the 

 finger-tips of the human hand. Next the budding quill-feathers at- 

 tract attention, and if a series of young is being examined, probably 

 the first point to be noted is the fact that the development of the quill 

 feathers of the hand were peculiar inasmuch as in the older forms 

 whilst the inner quills are found to have pushed their way some con- 

 siderable distance beyond the post-axial border of the wing, the outer 

 quills are only represented by simple down-feathers. 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 ab- 

 sence. 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 from the 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 de- 

 veloping 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. Thus then the arrested development of the quills begins to 

 look as if it might have a definite meaning, and this becomes a cer- 

 tainty when still older specimens are examined. In them we find 

 that as soon as the inner quills of the second digit 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 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 can not be doubted. The 

 presence of the claws is almost sufficient to prove this, for having once 

 become vestigial it is unlikely they would reacquire their primitive size. 



But we have other evidence affording the strongest confirmation of 

 the contention that the wing of the hoatzin represents an ancient 

 order of things once common to all birds. This evidence is 'writ large' 

 upon the wing of those allies of the hoatzin, the common fowl, the 



VOL. lxii. 8. 



