BIRDS. 325 



are probably all that can be said to be accurately known.* 

 This number is divided into about a thousand genera, and 

 the names and limits of these genera have, from time to time, 

 undergone considerable modification. This will not seem 

 surprising when it is borne in mind, that yenera are merely 

 contrivances adopted by writers for the purpose of conveniently 

 grouping together those species which most nearly resemble 

 each other. The word "species" is applied "to such indi- 

 viduals as are supposed to be descended from a common stock, 

 or which might have so descended, "f A species has a real 

 existence in nature. A genus is an abstract idea, a creation 

 of the mind, liable to be overthrown or upreared, contracted, 

 or expanded, according to the mutability of human knowledge. 

 In this little book we do not purpose entering upon the com- 

 parative merits of different systems of classification. That 

 system is the best which is founded, not upon any one set of 

 characters, but upon an intimate knowledge of all. The only 

 true foundation on which it can be reared is that which is 

 afforded by the anatomical structure. Each change of external 

 character is accompanied by a corresponding change of inter- 

 nal organization. "The external parts afford an index to the 

 internal, "f The shape of the organs by which the food is 

 taken indicate the form and structure of those by which it 

 is swallowed and digested. Hence, "if we find a bird 

 having a short-beaked bill and curved claws, we shall not 

 be wrong in inferring that it has a wide oesophagus (gullet) 

 and a large membranous stomach. " But our informa- 

 tion is incomplete, and our classification imperfect, unless 

 to a knowledge both of external and internal structure, we 

 add that which is to be acquired by the study of the living 

 objects seen in their native haunts. Thus only can we 

 ascertain to what extent each modification of structure is 

 accompanied by a corresponding change of habit; and until 

 this be done, with regard to foreign as well as to native 

 species, we must not suppose that our classification is perfect 

 and unchangeable. 



* Vide his excellent Report on "the Progress and Present State of 

 Ornithology," Report of British Association, 1844. 



f Archbishop Whately's Logic, book iv. chap. v. 



J Macgillivray's British Birds. 



Idem. This work contains an instructive and interesting series of 

 plates, exhibiting the modifications of the several parts of the alimentary 

 canal in a large number of native birds. 



F2 - 



