CHAPTER XXI. 



SEGREGATION. 



§ 163. The general interpretation of Evolution is far 

 from being completed in the preceding chapters. We must 

 contemplate its changes under yet another aspect, before we 

 can form a definite conception of the process constituted by 

 them. Though the laws already set forth, furnish a key to 

 the re-arrangement of parts which Evolution exhibits, in so 

 far as it is an advance from the uniform to the multiform; 

 they furnish no key to this re-arrangement in so far as it is 

 an advance from the indefinite to the definite. On studying 

 the actions and re-actions everywhere going on, we have 

 found it to follow inevitably from a certain primordial truth, 

 that the homogeneous must lapse into the heterogeneous, 

 and that the heterogeneous must become more heterogene- 

 ous ; but we have not discovered why the differently-affected 

 parts of any simple whole, become clearly marked off from 

 each other, at the same time that they become unlike. Thus 

 far no reason has been assigned why there should not ordi- 

 narily arise a vague chaotic heterogeneity, in place of that 

 orderly heterogeneity displayed in Evolution. It still re- 

 mains to find out the cause of that local integration which 

 accompanies local differentiation — that gradually-completed 

 segregation of like units into a group, distinctly separated 

 from neighbouring groups which are severally made up of 

 other kinds of units. The rationale will be conveniently in- 

 troduced by a few instances in which we may watch this 

 segregative process taking place. 



471 



