On the Différences between Nestling Birds. 455 



with feet and wings, and using the beak as an auxiliary when 

 greater security is needed. 



The significance and importance of the claws at this 

 juncture is attested by the quite peculiar method of develop- 

 ment which obtains in the remiges or quill-feathers of the 

 hand ; inasmuch as the growth of the quills near the tip of 

 the index digit is arrested until the inner quills have 

 developed sufficiently to serve the purpose of flight. As soon 

 as this takes place, the distal quills make their appearance, 

 when the claws, being no longer needed, disappear. The 

 reason for this peculiar succession of quill-feathers is obvious. 

 If all the quills developed at the same rate the hand would 

 speedily become useless either as an organ of prehension or 

 an organ of flight. 



The presence of claws, however, and the arrested develop- 

 ment of the distal quill-feathers are not the only peculiarities 

 of the maims of this bird during the early nestling period ; 

 for at this time the index finger is longer relatively than in 

 the adult stage, whilst the palmar surface of the tip is bare 

 and provided with a small cushion-like pad resembling that 

 of the human finger. This pad doubtless serves to increase 

 the grasping power of the digit. The great length of this 

 digit makes the manus, at this period, actually longer than 

 the fore-arm, whilst the reverse proportions obtain in the 

 adult. 



That the peculiarities of the wing of the young Opis- 

 thocomus represent an ancestral condition there can be but 

 little doubt, inasmuch as the wings of many other birds pass, 

 more or less completely, through similar phases during 

 development. This is especially well seen in the wing of the 

 nestling of the Gallinai, of which we may take the Common 

 Fowl as an example. Breeding, however, no longer in trees, 

 but upon the ground, certain curtailments have taken place 

 in the development of the wings of these birds. Thus, the 

 claw of the pollex is greatly reduced, whilst the claw of the 

 index digit is present only during embryonic life. But the 

 arrested development of the distal quills still persists to 

 attest the original function of the wing during- the nestling 

 period. The development of the inner quills of the hand, 

 however, has become greatly accelerated, apparently in order 



