154 EVOLUTION AND GENETICS 



cestral dinosaur or an even more primitive form, the forerunner 

 of the birds. 



The Origin of Birds. The origin of the birds is another lost 

 step in the evolution of the vertebrates. That they came from 

 some extinct reptilian form is certain, but paleontology has no 

 definite information to offer concerning these ancestors. Some of 

 the dinosaurs show characteristics similar to those of birds in 

 skeletal structure, bipedal locomotion, and the development of a 

 beak. While these facts do not point to such dinosaurs as an- 

 cestors of the birds, they do harmonize with the paleontologists' 

 theory that both birds and dinosaurs came from the same stock. 

 According to this theory the ancestor of the birds must have 

 been either a primitive dinosaur or an unknown common an- 

 cestor. 



Flight Adaptations. The chief characteristics of the birds are 

 associated with flight. By comparing them with the reptiles we 

 find that no other distinctive characters are present. The avian 

 skeleton is highly specialized for lightness and rigidity by the 

 development of hollow bones and fusion of separate parts. The 

 scales are restricted to the legs and feet, where they are the same 

 as reptilian scales; elsewhere the l^ody is covered with feathers 

 which serve the double purpose of light planes for flight and a 

 warm covering for protection against the low temperature of the 

 upper air. Although much more complex, feathers are seen by 

 their development to be closely related to scales. The digestive 

 system is specialized to provide the al^undance of energy required 

 by flight. The lungs are so constructed that air enters the alveoli 

 through one passage and leaves through another, so that every 

 breath brings an entirely fresh supply of air into contact with the 

 respiratory epithelium. The vascular system is completely 

 divided into pulmonary and systemic circulations. High and 

 constant body temperature is maintained by a vaso-motor system. 

 It serves to promote the rapid metabolism demanded by flight 

 and to protect the bird against low temperature. 



Theories of the Origin of Flight. In view of the high degree 

 of specialization indicated by these characters, and the lack of 

 definite fossil ancestors, it is not surprising that all explanation 

 of the origin of birds should rest upon theories of the origin of 

 flight. These theories include cursorial origin, arboreal origin, and 

 diving origin. All assume the development of broadened limbs 



