168 THE INDUCTIONS OF BIOLOGY. 



manently established : if it is a structural' modification that 

 happens to facilitate the vital activities, "natural selection ' 

 retains and increases it ; but if not, it disappears. 



The connexion which we noted between heterogeneity 

 of structure and heterogeneity of function a connexion 

 made so familiar by experience as to appear scarcely worth 

 specifying is clearly a necessary one. It follows from the 

 general truth that in proportion to the heterogeneity of any 

 aggregate, is the heterogeneity it will produce in any inci- 

 dent force (First Principles, 116). The force continually 

 liberated in the organism by decomposition, is here the inci- 

 dent force; the functions are the variously modified forms 

 produced in its divisions by the organs they pass through ; 

 and the more multiform the organs the more multiform must 

 be the differentiations of the force passing through them. 



It follows obviously from this, that if structure progresses 

 from the homogeneous, indefinite, and incoherent, to the 

 heterogeneous, definite, and coherent, so too must function. 

 If the number of different parts in an aggregate must deter- 

 mine the number of differentiations produced in the forces 

 passing through it if the distinctness of these parts from each 

 other, must involve distinctness in their reactions, and there- 

 fore distinctness between the divisions of the differentiated 

 force ; there cannot but be a complete parallelism between 

 the development of structure and the development of func- 

 tion. If structure advances from the simple and general to 

 the complex and special, function must do the same. 



