XX INTRODUCTION 



unique stage of development as a response to the 

 peculiar need of the bird's flight, the hand being 

 drawn out into a long rod, across which the bases of 

 the quill-feathers are securely lashed by ligaments. 

 The wing, no less than the rest of the skeleton, fur- 

 nishes convincing evidence to show that the bird, as 

 we see it to-day, has acquired this form by a slow 

 transformation. In many birds, as in the Duck, the 

 Water-hen, or any of the Hawks, there will be found 

 on the thumb and the tip of the second finger a small 

 claw — the vestige of a claw that once was useful. In 

 some other birds, as in the common Fowl, similar 

 claws will be found in the embryo — in the chick be- 

 fore it leaves the shell — though quite conamonly the 

 thumb-claws persist throughout life. Examine the 

 wing of the next Fowl you come across, and likely 

 enough you will find it. 



Now, if we go back to Archasopteryx, we shall find 

 that the wing possessed a large claw on each of its 

 three fingers; and also, in this ancient bird we find 

 that the second row of wrist-bones — those at the base 

 of the fingers— though welded together to form a half- 

 moon-shaped bone, yet remained distinct from the 

 fingers. They, in fact, retained throughout life the 

 condition which is met with to-day only in very young 

 birds. When a yet more ancient bird than Archse- 

 opteryx is discovered, it is safe to predict that a still 

 more complex series of wrist-bones will be found. We 

 expect, indeed, to find all the links from the reptile 

 to the bird of the present time. 



In one particular the bird and the reptile are very 

 different, inasmuch as while reptiles are cold-blooded, 



