CLASSIFICATION BY SERIES. 285 



of which is likely to throw most light upon the particular 

 subject. That subject being the laws of some phenomenon or 

 some set of connected phenomena; the very phenomenon or 

 set of phenomena in question must be chosen as the ground- 

 work 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 forms or degrees ; and secondly, to arrange 

 those Kinds in a series according to the degree in which they 

 exhibit it, beginning with those which exhibit most of it, and 

 terminating with those which exhibit least The principal 

 example, as yet, of such a classification, is afforded by com- 

 parative anatomy and physiology, from which, therefore, our 

 illustrations shall be taken. 



2. The object being supposed to be, the investigation 

 of the laws of animal life ; the first step, after forming the 

 most distinct conception of the phenomenon itself, possible in 

 the existing state of our knowledge, is to erect into one great 

 class (that of animals) all the known Kinds of beings where 

 that phenomenon presents itself; in however various combina- 

 tions with other properties, and in however different degrees. 

 As some of these Kinds manifest the general phenomenon of 

 animal life in a very high degree, and others in an insignifi- 

 cant degree, barely sufficient for recognition ; we must, in the 

 next place, arrange the various Kinds in a series, following 

 one another according 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 Expe- 

 rimental Inquiry discussed in the preceding Book ; the fourth 

 Method, that of Concomitant Variations. As formerly re- 

 marked, this is often the only method to which recourse can 

 be had, with assurance of a true conclusion, in cases in which 

 we have but limited means of effecting, by artificial experi- 



