14 The Anatomy of Birds 



of the fore limb for flight and its consequent withdrawal from use in any other 

 form of locomotion, or for taking food. And in birds these last so overlay the 

 others that the classification of birds is a very difficult matter. This is well 

 shown by the long use of the divisions Ratitae and Carinatae, which depended 

 mainly on the presence or absence of a keel to the sternum, when this and a 

 number of other characters depended upon whether a bird did or did not fly. So 

 the structure of a bird's skeleton depends first upon the fact that it is a bird, 

 secondarily upon the manner in which it moves about, and thirdly, to a still less 

 degree, upon the way it gets its living. Flight in itself is not a distinctive charac- 

 ter, for mammals fly to-day, and reptiles, in the shape of pterodactyls, flew 

 ages ago, having successfully mastered the problem of flight about the time the 

 bird had taken its first lessons in the art. But the manner in which the fore 

 limb is modified, the ground plan, so to speak, on which it is built, is a primary 

 morphological character and differs in the birds, mammals, and reptiles. 



In the Bat (Fig. 6) the four fin- 

 gers of the hand are greatly length- 

 ened for the support of the membrane 

 forming the wing ; in the flying reptile 

 (Fig. 7) the membrane is supported 

 wholly by the enormously developed 

 fifth or little finger. In the bird the 

 bones of the hand are lessened in 

 number and peculiarly modified for 

 the attachment of feathers which 

 form the highest type of wing. 



The general characteristics of a bird's skeleton are lightness, length of neck, 

 very decided difference between the fore and hind limbs, and reduction in the 

 apparent number of bones of the hand and foot (metacarpals and metatarsals) 

 by their fusion with one another. The skull joins the neck by a single condyle 

 as in reptiles, and, also as in reptiles, the ankle joint is between the bones of the 

 ankle and not between the leg and ankle as in mammals. The jaw is not attached 

 to the cranium directly as in mammals, but by a free quadrate as in snakes and 

 some extinct reptiles. In existing reptiles other than snakes the quadrate is 

 fixed. Many ribs of the chest cavity bear little processes directed upwards and 

 backwards, termed uncinate processes, and these are found in all birds save the 

 Screamers, although almost wanting in the Secretary bird. Outside the class of 

 birds such processes occur only in that curious New Zealand reptile, the Hatteria, 

 and, in cartilage only, in Crocodiles. So, in many important particulars the 

 skeleton of a bird resembles that of a reptile, and this led Huxley to unite the two 

 in a common superclass, Sauropsida. 



This possession of characters in common with reptiles is why it is con- 

 sidered that if birds have not been directly derived from reptiles, they have had 

 a common origin. From the strong resemblances between birds and dinosaurs 

 it was long thought that these reptiles were the parent stock of birds, but this 

 theory has been practically abandoned. 



FIG. 8. Wing bones of a Pigeon. 



