CLASS AVES 



397 



hummingbird found in Central America is only 1}^ inches long from 

 the base of the bill to the base of the tail. 



426. Origin of Birds and of Flight. — A former theory of the origin 

 of birds was that they were derived from the flying reptiles, or ptero- 

 dactyls. These reptiles, however, do not resemble birds structurally 

 in the degree that some of the bipedal dinosaurs (Fig. 273) do. At the 

 present time, therefore, the latter are usually looked upon as the ancestors 

 of birds. 



Fig. 272. — One conception of the appearance of Archaeopteryx macntra Owen, based 

 upon a specimen in the British Museum. {From Wieman, "General Zoology," after Romanes, 

 by the courtesy of McGraw-Hill Book Company, Inc.) X xi- 



Different theories of the origin of fliglit have been proposed but 

 the most probable theory seems to be the one which traces the develop- 

 ment of wings to the broadening of the limbs and tail due to the increase 

 in length of scales and their modification to form feathers. Apparently 

 early birds had flight feathers both on their fore- and on their hind limbs 

 and on both sides of the tail. Such a bird was capable of gliding through 

 the air from a tree or elevated point, perhaps to a considerable distance. 

 Gradually the forelimbs developed into wings, the feathers disappeared 

 from the hind limbs, and the tail shortened and became modified into 

 such a tail as birds possess today. 



427. Flight. — In sustained flying the wings strike downward and 

 forward and the bird rides over the air, which serves to buoy it up. 



