26 PRINCIPLES OF CLASSIFICATION. 



of the body is always high during life ; that the mouth is desti- 

 tute of teetli, but that its jaws are furnished with a horny 

 bill ; that the body is supported on the ground upon two legs 

 only ; and that, if its wings are sufficiently developed, the animal 

 can raise itself into the air by their movement. All these facts, 

 and many more, are included in the general idea of the structure 

 of the Bird which the Zoologist possesses ; but only a few of 

 them are included in the common idea. For a person ignorant 

 of Zoology thinks of little but the external characters ; and 

 attaches little else to the name of Bird, than the idea of an 

 animal covered with feathers, possessing one pair of wings and 

 one pair of legs, and having a horny bill. Or, if he thought of 

 the internal structure at all, the distinction upon which his mind 

 would naturally fix, would be the gizzard, or strong muscular 

 stomach possessed by most of the birds used as food ; but this 

 would not be a proper zoological character of the class, because it 

 is confined to a certain part of it only, and is, moreover, possessed 

 by animals of other tribes. 



3. Thus we see how much our labour is simplified, by 

 the union into one group, of all the animals which agree in the 

 most important or essential characters ; since, by the knowledge 

 of these characters, we are at once put in possession of a great 

 amount of important information respecting every one of the ani- 

 mals included in the group. Thus, among the many thousand 

 species which belong to the class of Birds, there is not a single 

 one that does not agree in all the characters just named ; and, 

 if we should find a new species, which no Naturalist had ever 

 met with, no Anatomist had ever examined, we might antici- 

 pate with certainty (so far, at least, as we have a right to feel 

 certain of anything of this kind), that its internal structure would 

 correspond with the description already given. For not only do 

 the animals included in the class of Birds agree amongst them- 

 selves in all these characters ; but they also differ from all others 

 in having them thus combined. Thus, if we compare Birds with 

 Insects, to which, in their mode of life and possession of the 

 powers of flight, they bear the greatest external resemblance ; we 

 shall find the points of agreement really much fewer than the 



