506 THE EVOLUTION OF LIFE. 



than those it was before placed in, yet it is manifest that, 

 on the average, the organic environments of organisms have 

 been advancing in heterogeneity. As the number of species 

 with which each species is directly or indirectly implicated, 

 multiplies, each species is oftener subject to changes in the 

 organic actions which influence it. These more frequent 

 changes severally grow more involved. And the correspond- 

 ing reactions affect larger Floras and Faunas, in ways increas- 

 ingly complex and varied. 



152. When the astronomic, geologic, meteorologic, and 

 organic agencies which are at work on each species of plant 

 and animal are contemplated as becoming severally more 

 complicated in themselves, and as co-operating in ways that 

 are always partially new; it will be seen that throughout 

 all time there has been an exposure of organisms to endless 

 successions of modifying causes which gradually acquire an 

 intricacy scarcely conceivable. Every kind of plant and 

 animal may be regarded as for ever passing into a new 

 environment as perpetually having its relations to external 

 circumstances altered, either by their changes with respect 

 to it when it remains stationary, or by its changes with respect 

 to them when it migrates, or by both. 



Yet a further cause of progressive alteration and compli- 

 cation in the incident forces, exists. All other things con- 

 tinuing the same, every additional faculty by which an 

 organism is brought into relation with external objects, as 

 well as every improvement in such faculty, becomes a means 

 of subjecting the organism to a greater number and variety 

 of external stimuli, and to new combinations of external 

 stimuli. So that each advance in complexity of organization, 

 itself becomes an added source of complexity in the incidence 

 of external forces. 



Once more, every increase in the locomotive powers of 

 animals, increases both the multiplicity and the multiformity 

 of the actions of things upon them, and of their reactions 



