1 8 MANUAL OF ZOOLOGY. 



2. It is rarely possible to assign any reason for correlations 

 of structure, though they are certainly in no case accidental. 



3. The law is a purely empirical one, and expresses nothing 

 more than the result of experience ; so that structures which 

 we now only know as occurring in association, may ultimately 

 be found dissociated, and conjoined with other structures of a 

 different character. 



9. CLASSIFICATION. 



Classification is the arrangement of a number of diverse 

 objects into larger or smaller groups, according as they ex- 

 hibit more or less likeness to one another. The excellence of 

 any given classification will depend upon the nature of the 

 points which are taken as determining the resemblance. Sys- 

 tems of classification, in which the groups are founded upon 

 mere external and superficial points of similarity, though 

 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 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 the whale is placed amongst the Mammalia, we 

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



