YOUNG BIRDS AND RECORDS OF THE PAST 117 



say a'ggressively naked ; others develop an apology for a 

 covering in the shape of a few tufts of filamentous and 

 extremely delicate down, which serve rather to exaggerate 

 the nakedness of the body. On the other hand, some, 

 like our hawfinch, have a comparatively thick crop of 

 down. In the Australian Lyre-bird alone among the 

 great host of species which make up the Passerine group, 

 is there what we may call an ample downy covering. 

 Herein it is of great length, and covers every part of the 

 broad tracts which later will bear the feathers. 



It is difficult to account for these differences in the 

 clothing of the nidicolous young, but that it is intimately 

 associated with sedentary habits there can be no doubt. 

 The pigeons and the sand-grouse surely demonstrate 

 this point. The former, as we have remarked, are nidi- 

 colous, and their downy vestment is reduced to a few 

 hair-like threads ; the sand-grouse are nidifugous, or 

 precocious, and are densely clad. 



The fact that the young of indubitably close allies — 

 like the pigeons and sand-grouse — may differ one from 

 another very conspicuously, while birds in no way related 

 — ^like hawks and owls — may in their nestling stages be 

 almost indistinguishable, shows at once the unreliability 

 of the condition of young birds during the early post- 

 embryonic stages of development as an index of relation- 

 ship, and therefore as a basis of classification. At the 

 same time it raises the question — What is the significance 

 of such conditions ? 



There can be no doubt but that the nidifugous or 

 precocious type of nestling is the more primitive, for 

 birds, we know, are but glorified reptiles, are divergent 

 branches of a common stock ; and among the reptiles 

 the young are always active at birth, or, to speak more 



