ii 4 POPULAR SCIENCE MONTHLY. 



turke} r or the pheasants, for example. Herein we find the same de- 

 velopmental 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 re- 

 duced 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 life- 

 history, whilst the outer quills, three in number, are still only repre- 

 sented by delicate down feathers, thus, be it noted, leaving a free finger- 

 tip 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 thp 



