NESTLING BIRDS AND WHAT THEY TEACH 237 



the shape of a few tufts of filamentous and extremely delicate 

 down, which serves rather to exaggerate the nakedness of the 

 body than as a covering. 



It is difficult to account for this variability, which is often 

 made the more striking by reason of the fact that closely allied 

 birds like the Gannet and the Cormorant, the Heron and the 

 Stork, though living in the same environment, yet differ ex- 

 tremely in this particular. Exceptions to the rule indeed turn 

 up in the most unexpected places. Thus, though the Passeres 

 may be said never to develop more than a vestigial downy 

 covering, yet the young Lyre-bird {MeJium) is abundantly pro- 

 vided for in this matter. 



That the more loose and fragile down of the nidicolous 

 types is intimately connected with a sedentary habit there can 

 be little doubt. The Pigeon and the Sand-grouse illustrate 

 this point extremely well. These two forms are undoubtedly 

 closely allied. Yet the young of the Sand-grouse is precocious 

 and clad in a dense and very thick coat of down ; while the 

 young Pigeon is helpless and but scantily invested in down 

 feathers of a degenerate and filamentous character. 



The fact that the young of indubitably close allies — like the 

 Sand-grouse and Pigeon — may differ very conspicuously, and 

 that birds, in no way related to one another — as in the case of 

 the Owls and Hawks — may in their nestling stages bear a very 

 striking resemblance, shows at once the unreliability of this char- 

 acter for systematic purposes. At the same time it raises the 

 question — what is the significance of these two conditions ? 



There can be no doubt but that the nidifugous or precoci- 

 ous type of nestling is the more primitive, since it is nearer to 

 the reptilian condition out of which the birds have risen. Yet 

 it is not the apparently most active which are nearest to this 

 primitive stock. 



The correct interpretation of this problem is to be found, 

 probably, in the early stages of the life-history of an extremely 

 aberrant type, whose affinities are still a matter for conjecture. 

 This bird is the Hoatzin {Opisthocomns cnstatus), a native of 

 the Amazon valley. Perhaps more exclusively arboreal than 

 any other bird (p. 50), both in its adult and nestling life, this 

 species may be regarded— in so far as its habits are concerned, 

 as well as in some structural features to be discussed immedi- 



