ii2 POPULAR SCIENCE MONTHLY. 



tion. Some, like the cormorants, herons and certain of the gull-tribe, 

 for example, build as occasion demands, either on the ground or in 

 trees. Now it is interesting to note that among these birds the young 

 cormorants and herons are completely nidicolous, the young gulls only 

 partially so, whilst the near and less specialized allies of the gulls, the 

 plovers, have nidifugous young. This indicates that in the gulls the 

 food-yolk is in process of reduction. To species breeding in large 

 colonies, or on ledges of precipitous cliffs, the reduction of the food- 

 yolk and helplessness of the young are obviously advantageous. 



That this is so may be seen in the case of the colony-breeding 

 species, since it would be impossible for the parents to recognize their 

 own offspring, if nidifugous, when running about amid those of their 

 neighbors. In consequence a large number would almost certainly 

 go unfed and soon starve, whilst great activity among the young of the 

 cliff-breeding species would be accompanied by an enormous mortality, 

 owing to falls from the cliff. 



It is contended that the facts so far submitted amply justify the 

 interpretation put upon them, but the following instances should carry 

 conviction. In the aberrant South American game-bird, the 

 hoatzin, we have probably a direct survival of the protoavian type of 

 nestlings. They are, of course, nidifugous, but they differ from all 

 other nidifugous young in the prehensile character of their wings, 

 which are armed with large claws borne upon the thumb and index 

 digit. Claws on the wing are common among birds, and hitherto they 

 have been regarded merely as vestiges indices of a reptilian ancestry. 

 The part which they play, however, in the life-history of the hoatzin, 

 coupled with certain correlated modifications to be discussed presently, 

 shows that they have a wider significance than this. 



The adult hoatzin is an absolutely arboreal bird, inhabiting the 

 dense scrub and trees bordering the lagoons and river banks of British 

 Guiana and the Amazon Valley. Its powers of flight are extremely 

 limited, and it has never been observed to alight upon the ground. 



The young, like those of other nidifugous birds, are clothed in 

 down, conspicuous, in the present instance, for its hair-like appearance 

 and sparse distribution. But whilst the locomotion of the nidifugous 

 young of other birds is bipedal that of the hoatzin must be described 

 as quadrupedal, the wings as well as the legs being brought into requisi- 

 tion as the birds make their way along the branches. Even the beak 

 is sometimes used, as in the parrots. 



In a wing used a*s a prehensile organ we should expect to find cer- 

 tain peculiarities which would not be observable in the normal wing. 

 These are not wanting. One of the first points which attract attention 

 in the examination of such a wing is the great length of the hand, which 



