30 PRINCIPLES OF CLASSIFICATION. 



assemblage of species, that possesses certain points of agreement, 

 less striking than those which characterise the class, but enough 

 to distinguish each group from others having the same general 

 structure. Thus among Birds, we have the Birds of Prey, the 

 Perching Birds, the Climbing Birds, the Running Birds, the 

 Scratchir.g Birds, the Wading Birds, and the Aquatic Birds ; 

 each of these orders being known by some peculiar form of 

 the bill and legs, which distinguishes it from the rest, and which 

 is common (though often with great modifications) to all the 

 Birds contained in it. With the peculiar forms of the bill and 

 legs, which especially distinguish the order, and which show its 

 adaptation to some particular kind of life, we have certain other 

 characters combined. Thus the Rasores, or Scratching Birds, 

 which feed upon grains or seeds, live for the most part upon the 

 ground, whence their food is obtained ; their bodies are heavy 

 and their wings short, so that they cannot rise in the air without 

 difficulty ; and they are all furnished with a gizzard, or stout 

 muscular stomach, for grinding down their food. On the other 

 hand, among the Birds of Prey, which obtain their food by 

 pursuing other birds in the air, we find the body and wings 

 adapted to active flight ; and the stomach destitute of thick 

 muscular walls, as the nature of the food introduced into it 

 renders it much less difficult of digestion, than that on which the 

 Fowl tribe is supported. 



9. In like manner, Orders are subdivided into Families, upon 

 characters of still less importance, such as minute differences in 

 the form of the feet and bill, or in the arrangement of the wing- 

 feathers ; but these differences are always connected with some- 

 thing in the internal structure, and in the habits of the tribe, 

 that shows it to be really distinct from others, which it may 

 strongly resemble in general appearance. Families, again, are 

 divided into Genera ; each of which includes a number of Species, 

 whose points of difference are very slight, whilst their points of 

 agreement include all the characters which are of any importance. 

 Thus we find that each Genus commonly includes a number of 

 Species, differing from each other (it may be) in little else than 

 size and colour, and agreeing in every other respect ; thus of the 



