BIRDS. 309 



bably 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 moditication. This will not seem suri)rising when 

 it is borne in mind that genera are merely contrivances adopted 

 by writers for the purpose of conveniently grouping togetlier 

 those species which most nearly resemble each other. Tlie 

 word " species " is applied to '" such individuals as are sup- 

 posed 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 

 comparative merits of different systems of classification. That 

 sj'stem 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 exteiTial 

 character is accomjianied by a corresponding change of inter- 

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

 internal." J The shape of the organs by which the food is 

 taken indicates 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 information 

 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 accompa- 

 nied 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 TJecent Progress and Present State 

 of Oniitholog}-," Report of British Association, 1844. 



t Archbishop AVhatcly's Logic, book iv. chap v. 



t 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. 



