CLASSIFICATION BY SERIES. 323 



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 ascer- 

 tained that this relation really subsists between the 

 \ 7 ariations, a connexion between the facts themselves 

 may be confidently 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 pre- 

 ceded by the formation 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 to 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 are cases in which the arrangement required 

 for the special purpose, becomes the determining prin- 

 ciple of the classification of the same objects for gene- 

 ral purposes. This will naturally and properly happen, 

 when those laws of the objects which are sought in 

 the special inquiry enact so principal a part in the 

 general character and history of those objects exer- 

 cise so much influence in determining all the pheno- 

 mena of which they are either the agents or the 

 theatre that all other differences existing among the 

 objects are fittingly regarded as mere modifications of 

 the one phenomenon sought ; effects determined by 

 the co-operation of some incidental circumstance with 

 the laws of that phenomenon. Thus in the case of 

 animated beings, the differences between one class of 



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