274 ^^ -INTRODUCTION TO ZOOLOGY. 



these are of little xxse, and must be interpreted as 

 ancestral structures. 



The structural resemblances between birds and 

 reptiles have led to their being classed together^ as 

 before stated, under the name of Satjropsida. 



Characters of Birds. — Birds agree with reptiles 

 in having nucleated blood corpuscles, and one occi- 

 pital condyle, in laying eggs, and in the course of 

 development which the embryo in the egg under- 

 goes. But otherwise they differ from them greatly : 

 reptiles are cold-blooded animals, and usually slow- 

 moving ; birds are the most active of vertebrates, and 

 their temperature is accordingly higher than that of 

 other vertebrates. The modification of the fore-limb 

 to form the wing, and the presence of a covering of 

 feathers, many of them specially adapted to assist in 

 flight, are the most obvious distinguishing characters 

 of birds. The structure of the bones, which are filled 

 with air cavities in order to make them light, is 

 another adaptation for aerial life. The beak is a dis- 

 tinctive structure present throughout the class. 



Classification of Birds. — Birds are divided into 

 two great groups, the Ratitce, or birds with flat (raft- 

 like) breasts, and the Garinatx, or keel-breasted birds. 

 The former group, which is a very small one, includes 

 the African ostrich, the Australian ostrich, or emu, 

 the cassowary, and the American ostrich {Rhea) ; and 

 the Kiwi, or Apteryx (wingless bird) of New Zealand. 

 All these have the wings more or less reduced in size, 

 and run instead of flying. The latter group includes 



