NESTLING BIRDS AND WHAT THEY TEACH 243 



prove this, for having once become vestigial it is unHkely they 

 would reacquire their primitive size. 



But we have other evidence affording the strongest con- 

 firmation 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 Turkey or the Pheasants, 

 for example. 



Although these birds are no longer hatched in trees we find 

 in them the same developmental stages as those met with in 

 the Hoatzin, but with certain modifications easy to interpret. 



If the wing of a chick of, say, sixteen hours old (111. 29) be 

 compared with that of a young Hoatzin of the same age, it will 

 be found that the same relative proportion between the hand 

 and forearm exists, but that the claws are now reduced to one 

 — that on the thumb — and this is but a mere vestige. The claw 

 of the finger appears only during embryonic life, and is absorbed 

 before the chick is hatched. Passing on to an examination of 

 the developing quills in the chick with relation to the hand, we 

 find that, as in the Hoatzin, these are at this stage restricted to 

 the wristward region of the hand so as to leave a free finger- 

 tip, but this and the thumb lack the cushion-like pads of the 

 Hoatzin. Now the arrested development of these terminal 

 quill or flight feathers is absolutely inexplicable in a bird 

 hatched on the ground, and only becomes intelligible when 

 viewed in the light revealed by the Hoatzin. In other words, 

 it can only be explained on the hypothesis that at an earlier 

 period in the life-history of this bird the wing was used as a 

 climbing organ. The remoteness of this period accounts for 

 the disappearance of the claws and the relatively shorter hand,, 

 though, as we have already remarked, this is still longer thani 

 the forearm. As in the Hoatzin, moreover, by the time that 

 maturity is reached the relative lengths between hand and 

 forearm have changed, the latter being longer than the former. 

 But in one particular the wing of the young Fowl differs 



1 There is good reason to believe that the young of the Turacos, when more 

 is known of the nesting habits of these birds, will prove to follow very closely 

 the peculiarities of the young Hoatzin. The only known nestling of this bird, 

 described in the Aviculiiiral Magazine for January, 1905, when a month old. had 

 enormous wings, the quills of which had not completed their growth, while the 

 body was still invested in a short, black down, sparsely distributed over the body. 



