ii8 THE INFANCY OF ANIMALS 



correctly, at hatching. Yet it is not the apparently 

 most active types among young birds which are to be 

 regarded as most nearly representing the ancestral con- 

 dition. 



The truth of this view at any rate seems to be demon- 

 strated by what obtains in the case of the nestling of 

 the hoatzin {Opisthocomus cristatus). This bird, which 

 represents an extremely aberrant type of doubtful 

 affinities, is a native of the Amazon vaUey. It is, perhaps, 

 the most exclusively arboreal of living birds, and hence 

 more nearly approaches Archaeopteryx — the oldest known 

 fossil bird — and the proto-avian types, than any other 

 member of the class Aves known to us. This nestling, 

 then, seems to present us with the key to the evolution 

 of the two types of nestling of which we have already 

 spoken — the nidifugous or precocious, and the nidicolous 

 or helpless types ; and further throws a quite unexpected 

 light on the meaning of the vestigial claws met with in 

 so many adult birds of diverse types. 



Briefly the story is as follows. Hatched in a rough nest 

 of sticks placed on the bough of a tree overhanging the 

 water, the young hoatzin, soon after leaving the egg-shell, 

 begins, with his nest-fellows, to make climbing expeditions 

 about the tree in the neighbourhood of the nest, using 

 not only the beak and the enormous feet, parrot-fashion, 

 but also the wings, which at this stage of growth differ 

 from the wings of all other birds, for they are now used 

 as an extra pair of feet, or let us say as a pair of 

 hands. But it is not only their use in this way which 

 is remarkable, but also their structure. 



In the first place the hand — answering to the human 

 hand, though possessing but the thumb and first finger — 

 is much longer than the fore-arm, while the thumb and 



