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 attract 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 considerable 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 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 deepseated 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 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. Thus then the arrested development of the quills begins to look as if it might have a definite meaning, and this becomes a certainty 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
