418 THE EVOLUTION OF LIFE. 



on the average, the organic environments of organisms have 

 been increasing 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 corre- 

 sponding reactions affect larger Floras and Faunas, in ways 

 increasingly complex and varied. 



§ 152. When the astronomic, geologic, meteorologic, and 

 organic agencies that are at work on each species of organ- 

 ism, are contemplated as becoming severally more compli- 

 cated in themselves, and at the same time as co-operating in 

 ways that are always more or less 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 that is scarcely conceivable. Every 

 kind of plant and animal may be regarded as for ever pass- 

 ing 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 



