SUMMARY AND CONCLUSION. 561 



and that so the less heterogeneous tends continually to be- 

 come more heterogeneous. 



A further step in the inquiry disclosed a secondary cause 

 of increasing multiformity. Every differentiated part is 

 not simply a seat of further differentiations, but also a parent 

 of further differentiations; since, in growing unlike other 

 parts, it becomes a centre of unlike reactions on incident 

 forces, and by so adding to the diversity of forces at work, 

 adds to the diversity of effects produced. This multiplica- 

 tion of effects proved to be similarly traceable throughout 

 all Nature in the actions and reactions that go on through- 

 out the Solar System, in the never-ceasing geologic com- 

 plications, in the involved symptoms produced in organisms 

 by disturbing influences, in the many thoughts and feelings 

 generated by single impressions, and in the ever-ramifying 

 results of each new agency brought to bear on a society. 

 To which was added the corollary, confirmed by abundant 

 facts, that the multiplication of effects advances in a geo- 

 metrical progression along with advancing heterogeneity. 



Completely to interpret the structural changes constitut- 

 ing Evolution, there remained to assign a reason for that 

 increasingly-distinct demarcation of parts, which accompa- 

 nies the production of differences among parts. This reason 

 we discovered to be, the segregation of mixed units under 

 the action of forces capable of moving them. We saw that 

 when unlike incident forces have made the parts of an 

 aggregate unlike in the natures of their component units, 

 there necessarily arises a tendency to separation of the dis- 

 similar units from one another, and to a clustering of those 

 units which are similar. This cause of the local integra- 

 tions that accompany local differentiations, turned out to 

 be likewise exemplified by all kinds of Evolution by the 

 formation of celestial bodies, by the moulding of the Earth's 

 crust, by organic modifications, by the establishment of 

 mental distinctions, by the genesis of social divisions. 



At length, to the query whether these processes have any 



