448 CHORD ATA. 



one large united wing. Hence the bird's wing of modern 

 times has the area of resistance formed by the vanes of the 

 feathers, the primary axis by the Hmb and the secondary 

 axes by the shafts of the feathers. Involved in flight is the 

 enormous hypertrophy of the pectoral muscles and the 

 preponderant size of the sternum with its keel. 



Again, the elongated tail of the reptile becomes in birds 

 shortened into a few free vertebrae and the rest fused into a 

 compound pygostyle. Here, again, we believe, from the 

 evidence of Archceopteryx, that the earliest birds had an 

 elongated tail, each vertebra bearing a pair of quill feathers, 

 but in the course of evolution this has become shortened 

 into the more efficient modern tail with the quill feathers 

 radiating like a fan. 



The ankylosis of the dorsal vertebrae may also be con- 

 nected largely with the mode of locomotion. 



Lastly, to the same cause we may trace the hypertrophy 

 of the sense of sight and reduction in the sense of smell. 



Feathers themselves are organs confined to birds, though 

 they may possibly have been derived by modification from 

 epidermic scales. 



Probably the great development of air-sinuses in the 

 body and in the bones is indirectly connected with the 

 aerial habit, though not merely for reducing the weight of 

 the body. The same remark applies to the very high tem- 

 perature of the body. One of the great means of reducing 

 temperature in the mammals is the system of skin-glands. 

 These, we have noticed, are absent in the birds. 



But there are other structural features of birds which 

 indicate a specialisation from the reptilian type, though not 

 apparently traceable to the peculiar mode of locomotion. 

 Of these we may merely mention the most important. 



The entire loss of teeth, functionally replaced by horny 

 jaws, as in the Chelonia amongst reptiles. 



The ankylosis of the distal carpal bones and all the 

 tarsals with neighbouring bones. (It may be noted that 

 the foot-joint of the birds is intertarsal, or between the two 

 rows of tarsals, as in reptiles.) 



The loss of the left systemic arch and the formation of 

 a four-chambered heart. 



The loss of the right ovary and oviduct. 



