444 OPERATIONS SUBSIDIARY TO INDUCTION. 



of some phenomenon, or some set of connected phenomena ; the very 

 phenomenon or set of phenomena in question must be chosen as the' 

 groundwork of the classification. 



The requisites of a classification intended to facilitate the study of a 

 particular phenomenon, are, first, to bring into one class all Kinds of 

 things which exhibit that phenomenon, in whatever variety of fomis or 

 degi'ees,; and secondly, to arrange those Kinds in a series according 

 to the degree in which they exhibit it, beginning with those which 

 exhibit anost of it, and terminating with those which exhibit least. 

 The principal example, as yet, of such a classification, is afforded by 

 comparative anatomy and physiology, from which, therefore, our illus- 

 trations shall be taken. 



§ 2. The object being supposed to be, the investigation of the laws 

 of animal life ; the first step, after foraiing a distinct conception of the 

 phenomenon itself, is to erect into one great class (that of animals) all 

 the known Kinds of beings where that phenomenon presents itself; in 

 however various combinations with other properties, and in however 

 different degrees. As' some of these Kinds manifest the general phe- 

 nomenon of animal life in a very high degi'ee, and others in an insig- 

 nificant degree, barely sufficient for recognition ; we must, in the next 

 place, arrange the various Kinds in a series, following one another ac- 

 cording to the degrees in which they severally exhibit the phenomenon ; 

 beginning, therefore, with man, and ending with the most imperfect 

 kinds of zoophytes. 



This is merely saying that we should put the instances, from which 

 the law is to be inductively collected, into the order which is implied 

 in one of the four Methods of Experimental Inquiry discussed in the 

 preceding Book ; the fourth Method, that of Concomitant Variations. 

 As we formerly remarked, tjiis is often the only method to which re- 

 course can be had, with assurance of a true conclusion, in cases in 

 which we have but limited means of effecting, by artificial experiments, 

 a separation of circumstances usually conjoined. The principle of the 

 method is, that facts which increase or diminish together, and disappear 

 together, are either cause and effect, or effects of a common cause. 

 When it has been ascertained that this relation really subsists between 

 the variations, a connexion between the facts themselves may be con- 

 fidently laid down, either as a law of nature or only as an empirical 

 law, according to circumstances. 



That the application of this Method must be preceded by the forma- 

 tion of such a series as we have described, is too obvious to need being 

 pointed out; and the mere arrangement of a set of objects in a series, 

 according td the degrees in which they exhibit some fact of which we 

 are seeking the law, is too naturally suggested by the necessities of 

 our inductive operations, to require any lengthened illustration here. 

 But there ai'e cases in which the aiTangement required for the special 

 purpose, becomes the determining principle of the classification of the 

 same objects for general puqwses. This will naturally and pi-operly 

 happen, when those laws of the objects which are sought in the special 

 inquiry enact so principal a part in the general character and histoiy 

 of those objects — exercise so much influence in determining all the 

 phenomena of which they are either the agents or the theatre — that 

 all other differences existing among the objects are fittingly regarded as 



