EXTERNAL FACTORS. 419 



upon tilings. Doubling a creature's activity, quadruples the 

 area that comes within the range of its excursions : thus 

 augmenting in number and heterogeneity, the external 

 agencies which act on it during any given interval. 



By compounding the actions of these several orders of 

 factors, there is produced a geometric progression of changes, 

 increasing with immense rapidity. And there goes on an 

 equally rapid increase in the frequency with which the com- 

 binations of the actions are altered, and the intricacies of 

 their co-operations enhanced. 



