INTRODUCTION xix 



on to consider one or two other peculiarities of birds — 

 peculiarities which have gone on developing and per- 

 fecting since the time when birds branched off from 

 the reptile stock. 



Surely the most important of these is to be found 

 in the fore-limb. This we know as the "wing." When 

 stripped of its feathers, we could trace the wing- 

 structure of the bird in the fore-leg of any other 

 animal. Yet it cannot be used as an absolutely dis- 

 tinctive character, since in some of the Ostrich tribe, 

 for example, it has become degenerate and so reduced 

 in size as to be hardly recognised; while, if we take 

 fossil forms into consideration, we shall find that it 

 becomes still more dwarfed, until, as in the Moas, it 

 is lost altogether. 



The principal features in which the wing differs 

 from the fore-limb of other animals are found in the 

 bones of the "wrist" and "hand." In the wrist only 

 two separate bones appear, though in the embryo the 

 rudiments of several can be made out ; these disappear, 

 however, before hatching. The bones of the hand and 

 fingers are reduced to three in number — the thumb 

 and first and second fingers. The first portion of 

 these finger-bones, which answer to the bones that ex- 

 tend between the wrist and the bases of the fingers 

 and make up the palms of our hands, are firmly 

 welded together, the base of the thumb being hardly 

 traceable. The second and third are welded together 

 at each end, enclosing a space, while the finger- joints 

 are represented, in the second finger by two or some- 

 times three bones, and the third by one bone only. 



The remarkable wrist and hand have reached this 



