INTERNAL FACTORS. 431 



that a liability to be unfolded arises from tbe actions and 

 reactions between organisms and their fluctuating envircii- 

 ments. And we see that the existence of such a cause of 

 development, presupposes the non-occurrence of development 

 where this fluctuation of actions and reactions does not 

 come into play. 



To show, however, that there must arise a certain general 

 tendency to the production of more heterogeneous aggregates, 

 is not sufficient. It is quite conceiyable that aggregates 

 should be rendered more heterogeneous by changing incident 

 forces, without having given to them that peculiar form of 

 heterogeneity required for carrying on the functions of life. 

 Hence it remains now to inquire, how the production and 

 iiialntenance of this peculiar form of heterogeneity is insurod. 



