THE EVOLUTION OF THE BIRDS 447 



a constant and high internal temperature. Birds are homo- 

 thermous. This condition with its consequent increased 

 efficiency of muscular and nervous processes is necessitated 

 by the great exertion required to maintain the weight of the 

 body in the air during flight. 



This was accomplished with the aid of the feathers by the 

 perfection of the vascular system (abolition of the left systemic 

 arch) and the development of the air-sacs. The latter ensure 

 a complete " blow- through " of the lungs and avoid the 

 inefficiency due to the presence of stagnant residual air in 

 blind-ended sac-shaped lungs. Further, the air-sacs assist in 

 regulation of the internal temperature by varying the amount 

 of heat lost. 



The characteristic development of the keel (carina) on 

 the sternum in flying birds is a direct adaptation to the need 

 for a firm origin for the large muscles concerned with 

 flight. 



As to the evolution of the method of flight itself, there is 

 not yet any certainty, and several theories have been pro- 

 pounded. 



The avine method of flight differs from that of all other 

 forms in that no use is made of stretched membranes. From 

 a study of the feathers and scales on the leg of the ostrich, 

 there is reason to believe that feathers were evolved in con- 

 nexion with scales, but not from them. Be that as it may, 

 the development of feathers must have resulted in an increase 

 in the surface of the animal without any material increase 

 in its weight. It is easy to see how this would have benefited 

 the ancestral birds if they inhabited trees and were in the 

 habit of leaping or parachuting from one branch to another. 

 At the same time it must be remembered that the structure of 

 the pelvic girdle and limbs of the bird are adapted to a cursorial 

 mode of life with a bipedal method of walking or running. 

 The biological success of birds is probably due in no small 

 measure to the fact that the adaptations for flight in no way 

 interfere with or involve the hind pair of limbs. The ancestors 

 of the birds must therefore have been terrestrial and cursorial 

 in habits before becoming arboreal. 



