CLASSIFICATION. 1 7 



'often useful in the earlier stages of science, are always found 

 in the long run to be inaccurate. It is needless, in fact, to 

 point out that many living beings, the structure of which is 

 fundamentally different, may, nevertheless, present such an 

 amount of adaptive external resemblance to one another, that 

 they would be grouped together in any ' artificial ' classifica- 

 tion. Thus, to take a single example, the whale, by its ex- 

 ternal 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 classifi- 

 cation, 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 classi- 

 fication 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 the whale is placed amongst the Mammalia, we 

 then know at once that the young whale is born 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 primarily 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 fami- 

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

 examine these successively, commencing with the considera- 

 tion 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,' 

 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 re- 



VOL. I. C 



