454 W. P. Pycraft: 



ON THE ORIGIN OF THE DIFFERENCES BETWEEN 

 NESTLING BIRDS. 



BY 



W. P. Pycraft, A.L.S., F.Z.S., etc. 



The very striking differences which obtain between the 

 young of different groups of birds at birth has long been a 

 subject of speculation, though but little has resulted there- 

 from. 



Some of the older naturalists seized upon the condition of 

 the young at birth as a basis of classification, either separating 

 those birds whose young were born helpless and naked, from 

 those whose offspring emerged from the shell covered with 

 down ; or dividing those birds with blind and helpless young 

 from those whose young emerged covered with down and 

 able to run about, 



These systems have, however, long since been discarded, 

 and at the present day, when the condition of the young at 

 birth is regarded at all, it is as an index of specialization ; 

 helpless and naked young being held to indicate a relatively 

 more specialized condition than active or precocial young, 

 the precocial condition being regarded as primitive. 



As to the correctness of the latter view there can be no 

 doubt, but much more underlies this than is generally sivpposed, 

 for if the early life-history of the several groups of birds be 

 carefully surveyed, it is contended here, that convincing 

 evidence is obtainable to show how the different conditions 

 of the helpless or altricial young and the active or precocial 

 young have been brought about. 



The key to the whole position is probably to be found in the 

 life-history of the young of the South American Hoatzin, 

 Opisthocomus cristatus. This remarkable bird, throughout the 

 whole of its life, is strictly arboreal. Indeed it is said never 

 to have been seen upon the ground. The young are j>recocial, 

 and differ from those of all other birds in the great size of 

 the claws in the pollex and index digits. Armed with these 

 claws, the newly-hatched young crawl out of the nest and 

 wander about among 1 the surrounding: branches holding' fast 



