Page:Popular Science Monthly Volume 62.djvu/120

114 turkey or the pheasants, for example. Herein we find the same developmental stages, but with certain modifications easy to interpret.

In the limb itself—as considered apart from the appendages, the incipient quills or flight-feathers—one of the first things to attract attention is the hand, which although relatively shorter than that of the hoatzin, is still longer than the forearm; next the cushion-like pads of the thumb and second finger are missed, as also are the claws. That of the thumb, however, is generally present though in an extremely reduced condition, and in the index finger it appears only during embryonic life.

The flight feathers again reveal some very interesting features, inasmuch as the inner quills develop at a rate relatively much greater than in the hoatzin, so that they become functional earlier in the lifehistory, whilst the outer quills, three in number, are still only represented by delicate down feathers, thus, be it noted, leaving a free fingertip as in the hoatzin. The abrupt changes from quill feathers to nestling down observable in the wing of the chick and turkey seem to show that the quills have undergone a process of forcing or accelerated development, in which the inner quills have developed at an excessively rapid rate, so as to out-distance their fellows at the distal or outer extremity of the wing, which as yet are only represented by nestling down. The rapid development of the inner quills is probably due to the fact that the terrestrial mode of life demanded the aid of the wings for the purposes of flight at an earlier period than would be the case if they dwelt, like the hoatzin, in comparative security among the trees.

The developmental history of the wings of the fowl and its allies seems to leave but little room for doubt that the ancestors of these birds, like the hoatzin, were hatched in trees and crawled about among the branches. Moreover, the change from an arboreal to a terrestrial nursery seems to have taken place comparatively recently. On no other assumption can we explain the free finger-tip and the arrested development of the outer quill feathers. Nevertheless, a sufficient time has elapsed wherein to bring about the suppression of the claws. That of the index digit, being no longer useful, appearing later and later in development, has now entirely ceased to put in an appearance save only during embryonic life; in other forms, separated by a still greater lapse of time from their tree-crawling ancestors, even the embryonic claw has ceased to be.

Of considerable importance is the fact that whilst in the hoatzin and the fowl and its allies the quill feathers appear long before the contour feathers of the body—that is to say, whilst the body is still clothed in down—in the nidifugous young of ground-breeding forms such as the plovers, for example, which seek protection by concealment alone, unaided by flight, the quill feathers appear together with the