8 A BOOK OF BIRDS / 



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

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



This very remarkable wrist and hand have reached this unique 

 stage of development as a response to the peculiar needs of the birds' 

 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, furnishes convincing 

 evidence to show that the bird, as we see it to-day, has acquired this 

 form by a slow process of 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 last 

 remaining vestige of a claw that was once useful. In some other 

 birds, as in the Barn-door Fowl, for example, similar claws will be 

 found only in the embryo — i.e. the developing chick before it leaves 

 the egg — though quite commonly the thumb-claws persist through- 

 out life. Examine the wing of the next fowl you come across, and 

 likely enough you will find it. Now, if we go back to the Archae- 

 opteryx, we shall find that the wing possessed a large claw on each 

 of its three fingers ; and, more than this, 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 the Archaeop- 

 teryx is discovered, it is safe to predict that a yet more complex series 

 of wrist-bones will be found. We shall, in fact, have all the links 

 from the reptile to the present-day bird. 



In one particular the bird and the reptile are very different, in- 

 asmuch as while the reptiles are " cold-blooded," the birds are hot- 

 blooded creatures ; indeed, in this matter they outdo the mam- 

 malia, the temperature ranging from ioo° to 112°. The highest 

 figure is attained by the smaller perching birds, such as Finches ; 

 Hawks do not maintain a temperature above 109°, and Gulls only 

 a little above 104°. 



This rise in the temperature of the blood, which, as we have said, 

 we met with first in the birds, is due to several causes, which are too 

 technical to be discussed here in detail. But chiefest among them 



