20 MANUAL OF ZOOLOGY. 



external characters, would certainly be grouped amongst the 

 fishes, though widely removed from them in all the essential 

 points of its structure. " Natural " systems of classification, 

 on the other hand, endeavour to arrange animals into divi- 

 sions founded upon a- due consideration of all the essential 

 and fundamental points of structure, wholly irrespective of 

 external similarity of form and habits. Philosophical classifi- 

 cation depends upon a due appreciation of what constitute 

 the true points of difference and likeness amongst animals; 

 and we have already seen that these are morphological type 

 and specialisation of function. Philosophical classification, 

 therefore, is a formal expression of the facts and laws of 

 Morphology and Physiology. It follows that the more fully 

 the programme of a philosophical and strictly natural classifi- 

 cation can be carried out, the more completely does it afford 

 a condensed exposition of the fundamental construction of the 

 objects classified. Thus, if the whale were placed by an arti- 

 ficial grouping amongst the fishes, this would simply express 

 the facts that its habits are aquatic and its body fish-like. 

 When, on the contrary, we obtain a natural classification, and 

 we learn that tlie whale is placed amongst the Mammalia, we 

 then know at once that the young whale is bom in a compa- 

 ratively helpless condition, and that its mother is provided 

 with special mammary glands for its support ; this expressing 

 a fundamental distinction from all fishes, and being associated 

 with other equally essential correlations of structure. 



The entire animal kingdom is pfimarily divided into some 

 half-a-dozen great plans of structure, the divisions thus formed 

 being called " sub-kingdoms." The sub-kingdoms are, in turn, 

 broken up into classes, classes into orders, orders into families, 

 families into genera, and genera into species. We shall examine 

 these successively, commencing with the consideration of a 

 species, since this is the zoological unit of which the larger 

 divisions are made up. 



Species. — No term is more difficult to define than " species," 

 md on no point are zoologists more divided than as to what 

 should be understood by this word. Naturalists, in fact, are 

 not yet agreed as to whether the term species expresses a real 

 and permanent distinction, or whether it is to be regarded 

 merely as a convenient, but not immutable, abstraction, the 

 employment of which is necessitated by the requirements of 

 classification. 



By Buffbn, " species." is defined as " a constant succession of 

 individuals* similar to and capable of reproducing each other." 



* In using the term "individual," it must be borne in mind that th« 



