250 A HISTORY OF BIRDS 



natural incubator, the parent being unable to undertake this 

 duty. 



That the extremes of precocious development on the one 

 hand, and of helplessness on the other, are traceable to a 

 common origin represented to-day in the young Hoatzin there 

 can be no doubt. 



That the peculiar features which distinguish this bird during 

 the nestling period are distinct survivals from the very remotest 

 times may be regarded as proved by the evidence afforded by 

 the remains of the earliest known bird Archseopteryx (p. 266). 



The long hand and claws of this bird, when compared with 

 the hand and claws of the wing of the Hoatzin, display a 

 resemblance too close to be regarded as the result of conver- 

 gence (p. 240). Having regard to all the factors in the case, 

 they leave little room for doubt but that the two wings are 

 genetically connected — that of the modern bird is a direct 

 survival from the most ancient times. 



According to the views hitherto expressed the claws, both 

 in this fossil and in living birds, were simply vestiges — indices 

 of the reptilian stock from which the Class Aves was derived. 

 The correct interpretation appears to be, however, that these 

 structures are something more than this — that they were re- 

 tained by the developing birds because they v/ere inseparably 

 linked with a number of other intimately associated adaptations 

 of vital importance to the preservation of the species, at least 

 during the nestling period ; for there can be no doubt but that 

 Archaeopteryx was as absolutely pledged to an arboreal life 

 as in the Hoatzin to-day, and stood in the same need of special 

 modification of structure to make this possible. 



Among living birds the majority have adopted other modes 

 of life, or other means of accomplishing the same end, and have 

 in consequence lost the peculiar characters preserved in their 

 integrity only in the Hoatzin, and leaving more or less blurred 

 records in some other species, the distinctness of the record 

 being the rough measure of the time which has elapsed since 

 the old habits were forsaken. 



These conclusions at which we have arrived seem almost 

 irresistible; but they would never have been reached but for 

 the combined study both of fossil and recent forms. Each 

 helps to explain the other. 



