CLASSIFICATION. 57 



ence 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 Physio- 

 logy. It follows that the more fully the programme of a 

 philosophical and strictly natural classification can be car- 

 ried 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 artificial 

 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 the whale is placed amongst the Mammalia, we 

 then know at once that the young whale is born in a com- 

 paratively helpless condition, and that its mother is provided 

 with special mammary glands for its support ; this express- 

 ing a fundamental distinction from all fishes, and being 

 associated with other equally essential correlations of struc- 

 ture. 



The entire animal kingdom is primarily divided into 

 some half-a-dozen great plans of structure, the divisions 

 thus formed being called " sub-kingdoms." The sub-king- 

 doms 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 zoolo- 

 gical unit of which the larger divisions are made up. 



Species. No term is more difficult to define than " spe- 

 cies/' and 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 Buffon "species" is defined as "a constant succession 



