222 



KNOWLEDGE, 



[October, 1903. 



placed it on record that this bird is one of the most strictly 

 arboreal of all known birds, never having been seen to 

 alight on the ground. The haunt of this species being the 

 low bushes and trees fringing river banks and lagoons of 

 the regions vratered by the Amazon. 



So strictly a tree-dweller naturally, we may say indeed 

 necessarily, establishes its nursery in the tree-tops ; and 

 herein we are brought face to face with a surprise. In all 

 other instances, the young of birds reared in trees are 

 hatched in a perfectly blind, naked, and helpless condition. 

 Not so with the Hoatzin, however. The young emerge from 

 the shell endowed with a very lively disposition, wandering 

 about the branches of the tree in which the nest is placed, 

 at their own sweet will. 



These wanderings are necessarily attended with con- 

 siderable peril, but the risk of accidents is diminished by 

 the presence of the large claws already referred to. 

 Grasping the boughs with enormous feet, and aided by 

 the claws and beak, they are among the most expert of 

 climbers at a very tender age. But the wing at this time 

 differs in several other remarkable jjarticulars from that 

 of other birds, and even from that of the adult condition. 

 Closely examined, it will be found that the hand is con- 

 spicuously longer than the forearm, and that the thumb is 

 also unusually long. Furthermore, the undersurface of 

 the thumb and first finger will be found to resemble those 

 of the human finger, in that they terminate in a fleshy ball, 

 obviously useful for grasping purposes. 



At this time, then, the wings serve the very un-birdlike 

 function of forelegs, and locomotion is quadrupedal rather 

 tban bipedal, and this remains the case till the power of 

 flight is attained. The development of flight is associated 

 with another remarkable feature. The qudls of the flight 

 feathers, it will be remembered, grow from the whole border 

 of the forearm and hand. Now if these quills all grew at 

 the same rate, those of the hand would soon render 

 climbing impossible, or at least highly dangerous, by 

 impeding the hold of the claws, and at the same time 

 these feathers would not be large enough to break the 

 force of a fall, which would Ite fatal. Consequently in the 

 case of the young Hoatzin, the growth of the outermost 

 quills — those nearest the claws — is comjjletely suspended 

 till the innermost quills of the hand and those of the fore- 

 arm immediately proximate, are sufficiently large to function 

 at least as a parachute in the case of accidents. As soon 

 as this has been arrived at a complete change in the form 

 of the wing takes place. The further growth of the claws 

 is not only arrested, but the claws themselves become 

 absorbed, so that, in the adult, no trace thereof is left. 

 Simultaneously the forearm commences to grow more 

 rapidly than the hand, so that, when the bird is adult, the 

 latter is shorter instead of longer than the former ! 



There can be little doubt but that the climbing phase in 

 the life-history of this bird is a primitive character, 

 carrying us back to the very dawn of avian development. 

 It cannot be a character acquired by this siiecies in 

 adaptation to its peculiar mode of Ufe, inasmuch as this 

 would imply that the claws had been redeveloped from the 

 vestigial condition into which they had sunk, and we 

 know that resuscitation of this kind never takes place in 

 nature. They must then have been handed on in unbroken 

 descent from the very earliest birds, having remained 

 functional throughout the whole of this time. 



Fortunately, however, we have no need to depend upon 

 the sweet reasonableness of this view for its general 

 acceptance. Evidence is obtainable on the one hand 

 through an appeal to the dry bones of the past, and on 

 the other by an examination of species still living, which 

 makes its acceptance irresistible. 



We will turn first to the fossil to which reference has 



already been made — Archaeopteryx. In this ancient bird, 

 the wing, when carefully examined, will Ije found to be 

 unmistakably that of a forest-dwelling species, inasmuch 

 as in general contour it agrees precisely with that common 

 to all dwellers among trees, being rounded instead of 

 pointed in form. Birds so diverse as eagles and cuckoos, 

 owls and game-birds, foremost among which stands tlie 

 Hoatzin, when strictly forest-dwellers, or dwellers amid 

 jungles, have wings of this type, whilst birds which fly 

 much in the open have pointed wings. Furthermore, tJie 

 wing of this fossil agrees closely with that of the young 

 Hoatzin in that the hand is longer than the forearm, and 

 has the top of the index finger free and armed with a 

 claw. Only in this it differs, that whereas these proportions 

 obtain only in the nestling Hoatzin. they were retained, 

 together with the claws, throughout life in the fossil form. 

 The explanation of the difference lies probably in the fact 

 that the need for climbing was not merely confined to the 

 nestling period, but was demanded periodically throughout 

 life. And for this reason, like the ducks, and some other 

 birds of to-day, Archaeopteryx probably moulted all its 

 quill feathers at once, and thus, till the new feathers had 

 grown sufficiently long and strong, the bird was flightless. 

 At this time a reversion to the locomotion of its infancy 

 would be necessary, and hence the retention throughout 

 life of the claws and the long grasping hand. 



The wing, then, of the young Hoatzin must be regarded 

 as a unique survival of primitive times, showing that the 

 young of the earliest birds were not only precocious, but 

 hatched in an arboreal nursery. If this be so, it becomes 

 highly probable that the conditions which obtain in the 

 young Hoatzin were at one time general among birds. 

 Certainly we have the strongest evidence in support of this 

 view in the wings of 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 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 

 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 whea 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 than 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 hmger than the former. But in one 

 particular th<' wing of tln' young fowl differs conspicuously 

 from that of the yciung Hoatzin. This is in the remarkably 

 rapid development of the flight-feathers or quills. The 

 growth of these in the Hoatziu is a comparatively slow 

 process, occupying many days, but in the fowl and its 

 allies they are beginning to unfold when the chick is but 

 sixteen hours old, and in three days they form an efficient 



