ESSENTIAL CHARACTERS OF BIRDS 69 



and hip-girdlesthe supports for the limbs. The hind-limb, 

 moreover, is especially reptilian, and this because of the forma- 

 tion of the ankle-joint, which is formed between two rows of 

 ankle bones traceable only in the very early stages of develop- 

 ment, and not between the uppermost row of ankle bones and 

 the shank of the leg, as in mammals (see also p. 74) . 



But in the structure of the fore-limb, no less than in the 

 peculiar character of the covering of the body, birds stand alone 

 among the vertebrates. This limb has become transformed 

 into a wing, so that when on the ground the body is supported 

 entirely by the hind-limbs. 



Although the jaws of birds, as is shown by fossil remains, 

 were originally implanted with teeth, they are, in modern birds, 

 toothless and encased in horn. 



Such, in brief, are the essential characters of birds, the char- 

 acters which go to distinguish birds from all other vertebrates. 

 But a better grip will be gained of the deeper meaning of these 

 characters, and of the bird's place in Nature, if these and some 

 other structural features are examined a little more in detail. 



By way of a commencement, it will perhaps be best to begin 

 with the more familiar : 



Feathers.- Feathers, then, although they completely clothe 

 the head, neck and trunk, and may even extend down to the 

 very tips of the toes, are not distributed evenly over the surface 

 of the body, as is, say the hair of a dog, or as are the scales of 

 reptiles. But they are arranged along definite areas or tracts, 

 known as pterylae, leaving more or less extensive bare spaces, 

 the " apteria." In such birds as the thrush or sparrow the 

 tracts are narrow and the space wide, but in the ducks, for in- 

 stance, on the other hand, the tracts are of enormous width. 

 The general arrangement of these tracts will be seen in the 

 accompanying photograph. What their purpose may be no one 

 has yet discovered. 



As touching the different kinds of feathers it will suffice to 

 say, that the most important are the " contour "-feathers, or 

 those which form the outline of the body; the down- feathers, 

 which answer to the under-fur of mammals, and which occur 

 only in birds such as hawks and eagles, gulls, ducks and swans. 



