INTRODUCTION. 19 



Thus, we have a general idea of a bird ; but this idea does 

 not correspond to any particular bird, or any particular 

 character of a bird. It is not precisely an ostrich, an owl, 

 a hen, or a sparrow ; it is not because it has wings, or 

 feathers, or two legs ; or because it has the power of flight, 

 or builds nests. Any, or all, of these characters would not 

 fully represent our idea of a bird ; and yet every one has a 

 distinct ideal notion of a bird, a fish, a quadruped, &c. It is 

 common, however, to speak of the animal which embodies 

 most fully the characters of a group, as the type of that 

 group. Thus we might, perhaps, regard an eagle as the 

 type of a bird, the duck as the type of a swimming-bird, and 

 the mallard as the type of a duck, and so on. 



As we must necessarily make frequent allusions to ani- 

 mals, with reference to their systematic arrangement, it seems 

 requisite to give a sketch of their classification in as popular 

 terms as may be, before entering fully upon that subject, and 

 with particular reference to the diagram fronting the title- 

 page. 



The Animal Kingdom consists of four great divisions, 

 which we call DEPARTMENTS, namely : 



I. The department of Vertebrates. 

 II. The department of Articulates. 



III. The department of Mollusks. 



IV. The department of Radiates. 



I. The department of VERTEBRATES includes all animals 

 which have an internal skeleton, with a back-bone for its 

 axis. It is divided into four classes : 



1. Mammals, (animals which nurse their young.) 



2. Birds. 



