500 OPERATIONS SUBSIDIARY TO INDUCTION. 



other property or set of properties, more readily recognizable by the ob- 

 server, which co-exist with, and may be received as marks of, the proper- 

 ties which are the real groundwork of the classification. A natural ar- 

 rangement, for example, of animals, must be founded in the main on their 

 internal structure, but (as M. Comte remax'ks) it would be absurd that we 

 should not be able to determine the genus and species of an animal with- 

 out first killing it. On this ground, the preference, among zoological clas- 

 sifications, is probably due to that of M. De Blainville, founded on the dif- 

 ferences in the external integuments ; differences which correspond, much 

 more accurately than might be supposed, to the really important varieties, 

 both in the other parts of the structure, and in the habits and history of 

 the animals. 



This shows, more strongly than ever, how extensive a knowledge of the 

 properties of objects is necessary for making a good classification of them. 

 And as it is one of the uses of such a classification that by drawing atten- 

 tion to the properties on which it is founded, and which, if the classifica- 

 tion be good, are marks of many others, it facilitates the discovery of those 

 others ; we see in what manner our knowledge of things, and our classifi- 

 cation of them, tend mutually and indefinitely to the improvement of each 

 other. 



We said just now that the classification of objects should follow those 

 of their properties which indicate not only the most numerous, but also 

 the most important peculiarities. What is here meant by importance ? 

 It has reference to the particular end in view ; and the same objects, there- 

 fore, may admit with propriety of several different classifications. Each 

 science or art forms its classification of things according to the properties 

 which fall within its special cognizance, or of which it must take account 

 in order to accomplish its peculiar practical end. A farmer does not di- 

 vide plants, like a botanist, into dicotyledonous and monocotyledonous, but 

 into useful plants and weeds. A geologist divides fossils, not like a zoolo- 

 gist, into families corresponding to those of living species, but into fossils 

 of the paleozoic, mesozoic, and tertiary periods, above the coal and below 

 the coal, etc. Whales are or are not fish according to the purpose for 

 which we are considering them. " If we are speaking of the internal struc- 

 ture and physiology of the animal, we must not call them fish ; for in these 

 respects they deviate widely from fishes ; they have warm blood, and pro- 

 duce and suckle their young as land quadrupeds do. But this would not 

 prevent our speaking of the whale-Jishery, and calling such animals fish on 

 all occasions connected with this employment ; for the relations thus arising 

 depend upon the animal's living in the water, and being caught in a man- 

 ner similar to other fishes. A plea that human laws which mention fish do 

 not apply to whales, would be rejected at once by an intelligent judge."* 



These different classifications are all good, for the purposes of their own 

 particular departments of knowledge or practice. But when we are study- 

 ing objects not for any special practical end, but for the sake of extending 

 our knowledge of the whole of their properties and relations, we must con- 

 sider as the most important attributes those which contribute most, either 

 by themselves or by their effects, to render the things like one another, and 

 unlike other things ; which give to the class composed of them the most 

 marked individuality ; which fill, as it were, the largest space in their ex- 

 istence, and would most impress the attention of a spectator who knew all 



* iVof. Or^. iJenor., pp. 286, 287. 



