122 THE INFANCY OF ANIMALS 



to a mere vestige, though occasionally, in the embryo 

 ostrich, this digit bears the vestige of a claw. 



So, then, the wing of the hoatzin does repeat, as a 

 passing phase, the characters which were a permanent 

 feature in this ancient bird. And from this we may 

 gather that the characters they share in common are in- 

 timately connected with the struggle for existence in both 

 cases. Now as to the evidence conveyed by the fossil. 



In the first place, from the structure of the feet we know 

 that this ancient bird was arboreal ; and we may infer 

 therefore that its young were hatched in trees, as in the 

 hoatzin to-day. This being so, they would need precisely 

 similar aids to locomotion and safeguards against acci- 

 dent : further, on the attainment of adult life there would 

 be the critical period of the annual moult to be faced. 

 Now at this early stage of avian evolution it is probable 

 that all the quills would be moulted at once, as in the 

 ducks, geese, rails, and many other birds to-day. This 

 would render any assistance from the wings, during the 

 work of climbing about in the search for food, extremely 

 valuable, and hence what is but a passing phase in the 

 development of the wing in the hoatzin was a permanent 

 condition in archaeopteryx. 



But we have not yet surveyed all the evidence that the 

 wings of nestling birds can afford us in this connection ; 

 and the facts now to be marshalled lend no Httle weight 

 to what has already been said, while they further show 

 how features, apparently inexplicable, may become sud- 

 denly invested with a meaning and importance hitherto 

 undreamed of. 



The further evidence, then, which we are now to take, 

 is furnished by the nestlings of the galHnaceous birds. If 

 the wings of a young fowl, or a young pheasant, or rather 



