244 A HISTORY OF BIRDS 



conspicuously from that of the young Hoatzin. This is in the 

 remarkably rapid development of the flight feathers or quills. 

 The growth of these in the Hoatzin is a comparatively slow 

 process, occupying many days, but in the Fowl and its allies 

 they are beginning to unfold when the chick is but sixteen 

 hours old, and in three days they form an efficient organ of 

 flight. The explanation of this accelerated development of the 

 quills is not far to seek — it is the result of adaptation to the 

 changed environment. The descent from the trees to the 

 ground was a descent from comparative security from enemies 

 into a world where enemies were numerous. Precocious flight 

 was the method of escape adopted, though, as we shall see 

 presently, not the only method. 



But how comes it, some may ask, that the arboreal nursery 

 was forsaken, if it afforded such security from enemies ? And, 

 further, how is it that the young of all birds hatched in trees 

 at the present time — save only the Hoatzin — are so singularly 

 helpless at birth ? What is the origin of the altricial or helpless 

 type of young ? 



As touching the migration from the forest to more open 

 ground, we may surmise that this probably took place as a 

 result of overcrowding. The old habitat left behind, the 

 young in response to the new environment underwent modifica- 

 tion, now in one direction, now in another, to bring them into 

 harmony with their particular environment. 



Among the forms known as Game-birds, and the Tinamous, 

 the most striking of these changes is that affecting the wings, 

 the nature of which we have just described in the wing of the 

 Common Fowl. But we would draw special attention to one 

 further point concerning the accelerated development of the 

 quills. This forcing, it is instructive to note, affected only those 

 quills originally concerned in the precocious flight ; those at 

 the tip of the wing, whose development was retarded so as to 

 leave the claw free for climbing purposes, still remaining un- 

 affected. This is as we might expect, for just as these inner 

 quills were sufficient during the arboreal phase, they remained, 

 and still remain, equally so for all the early demands of the 

 terrestrial life. 



The fact that similar traces of an arboreal life are rarely to 

 be found in the precocious young of birds other than Game- 



