I SECT. 3.] Origin, or Process of Causation of the Series. 55 



I consist of large classes of objects, throughout all the indi- 

 I vidual members of which a general resemblance extends. 

 J Suppose that we were considering the length of life. The 

 objects here are the human beings, or that selected class 

 of them, whose lives we are considering. The resemblance 

 existing among them is to be found in the strength and 

 soundness of their principal vital organs, together with all 

 the circumstances which collectively make up what we call 

 the goodness of their constitutions. It is true that most of 

 these circumstances do not admit of any approach to actual 

 measurement ; but, as was pointed out in the last chapter, 

 very many of the circumstances which do admit of such 

 measurement have been measured, and found to display 

 the characteristics in question. Hence, from the known 

 analogy and correlation between our various organs, there 

 can be no reasonable doubt that if we could arrange human 

 constitutions in general, or the various elements which com 

 pose them in particular, in the order of their strength, we 

 should find just such an aggregate regularity and just such 

 groupings about the mean, as the final result (viz. in this 

 case the length of their lives) presents to our notice. 



3. It will be observed therefore that for this pur 

 pose the existence of natural kinds or groups is necessary. 

 In our games of chance of course the same die may be 

 thrown, or a card be drawn from the same pack, as often 

 as we please ; but many of the events which occur to 

 human beings either cannot be repeated at all, or not often 

 enough to secure in the case of the single individual any 

 sufficient statistical uniformity. Such regularity as we trace 

 in nature is owing, much more than is often suspected, 

 to the arrangement of things in natural kinds, each of 

 them containing a large number of individuals. Were each 

 kind of animals or vegetables limited to a single pair, or 





