26 MANUAL OF ZOOLOGY. 



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. 



10. CLASSIFICATION. 



Classification is the arrangement of a number of diverse 

 objects into larger or smaller groups, according as they exhibit 

 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. Systems of 

 classification, in which the groups are founded upon mere ex- 

 ternal 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 adap- 

 tive external resemblance to one another, that they would be 

 grouped together in any " artificial " classification. Thus, to 

 take a single example, the whale, by its 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, endea- 

 vour to arrange animals into divisions founded upon a due 

 consideration of all the essential and fundamental points of 

 structure, wholly irrespective of external similarity of form and 

 habits. Philosophical classification depends upon a due ap- 

 preciation 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. Philo- 

 sophical 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 classification can be carried out, the more completely 

 does it afford a condensed exposition of the fundamental con- 

 struction 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 classi- 

 fication, and we learn that the whale is placed amongst the 

 Mammalia, we then know at once that the young whale is 

 born in a comparatively helpless condition, and that its 

 mother is provided with special mammary glands for its sup- 



