n6 POPULAR SCIENCE MONTHLY. 



old enough to eliminate the traces of nestling-life on reaching puberty, 

 when the quills rendered claws, and climbing, not only needless, but 

 impossible; hence then the retention by the adult of both elongated 

 manus, free finger-tips and claws. It has been suggested, however, 

 that the claws and elongated manus of the adult archasopteryx were 

 periodically functional, the periods being the moulting seasons. It 

 is well known that many existing birds, the anseres to wit, when 

 moulting, shed all the quills at once, and in consequence are for a 

 season flightless. It may well be that this system of losing the quill 

 is a primitive one and obtained in archgeopteryx, in which case it is 

 obvious that in a bird so strictly arboreal, the climbing hand of infancy 

 would be of some service. The more usual method of moulting the 

 quills in couples prevailing amongst modern birds so as not to impair 

 the power of flight has probably come about by selection, and hence 

 the reduced and clawless hand of the adult hoatzin, fowl and turkey 

 for example; birds in which, for reasons already explained, the primi- 

 tive form of manus is still temporarily exhibited by the nestling. 



Thus then, through the wing of the hoatzin we have a revelation 

 of a phase of bird-life hitherto unsuspected; inasmuch as its peculiar 

 developmental stages, each with its period of functional activity, enable 

 us to interpret the hitherto meaningless and puzzling characters seen 

 in the wing of the fowl and turkey, and their allies. These constitute 

 wellnigh invincible proofs of an earlier and universal arboreal ex- 

 istence, extending back to the time of the earliest known bird archre- 

 opteryx. Certainly the skeleton, especially the wing, lends the 

 strongest support to this view. This carries us further back still, 

 and suggests the conclusion that the reptile stock from which the aves 

 are descended was probably also arboreal. 



That too much stress has been laid by systematists on the condition 

 of the young birds at birth is admitted. It is further maintained 

 here that its significance has been misunderstood, and that the facts 

 now brought forward are strong enough, on the one hand, to refute 

 the older views, and on the other, to justify the theory, firstly, that birds 

 were originally arboreal and their young nidifugous; secondly, that 

 nidicolous habits and helplessness of young birds are specialized adapta- 

 tions to an arboreal or gregarious mode of life; and, thirdly, that the 

 young of gallinaceous birds form a link in the chain of the evolution 

 of nidifugous habits. The free finger-tip and arrested development 

 of the outer quill-feathers point to a prior arboreal habit; whilst the 

 accelerated development of the inner quill-feathers indicates an adapta- 

 tion to enable the young to escape from the enemies surrounding 

 a terrestrial nursery. The third and last stage is represented by the 

 protective coloration, a device which has been almost universally 

 adopted by nidifugous birds, owing to its greater effectiveness. 



