118 . HABIT AND INTELLIGENCE. [cHAP. 



mental mental characters of the things classified. But no definition 

 c aiac ers. ^^ fundamental characters is possible: or, in other words, 



XI rule 



possible, there is no possible rule or formula, applicable to all cases 

 alike, for determining which of the characteristics of a thing 

 are the really fundamental ones. The only approximation 

 to such a rule is, that the fundamental properties must be 

 those on which the other properties depend : this is true, 

 and indeed self-evident; but our knowledge of the way, 

 in which the various properties of the same thing are re- 

 lated to each other is too limited for us to be able to apply 

 this principle as a rule. The manner in which the colours 

 of substances depend on their chemical constitution, for 

 instance, is a subject of which we know nothing, and 

 probably shall never discover anything. We can only lay 

 down this negative rule, which is however of great im- 

 portance, that the obvious characteristics are not necessarily, 

 nor generally, the fundamental ones. 



But though it is impossible to lay down general rules 

 which shall be applicable to all sciences alike, for the 

 ascertaining of the fundamental characters, and the con- 

 sequent establishment of a true classification ; yet, in the 

 case of every particidar science, these are ascertained as a 

 necessary result of the progress of the science ; and when 

 the fundamental characters have been ascertained, and 

 when the true principles of classification, applicable to 

 that science, have been thus established, this reacts most 

 favourably on the further progress of the science. 

 Funda- In chemistry, the most obvious properties of substances 



characters ^'^^ those dependent on their usual state of molecular 

 in che- aggregation, solid, liquid, or gaseous. But the funda- 

 ^^ ^^' mental properties are those of chemical composition ; or, 

 in simple substances, those of chemical relation to other 

 substances. 

 crystaUo- In Crystallography, the fundamental characteristic is not 

 graphy, ^^lb form, but those relations between the axes (technically 

 called the crystallographic elements) on which the form 

 depends, 

 and In biological classification, the fundamental character- 



biology, istics on which the true classification of a species or of 



