THE CO-OPERATION OF THE FACTORS. 465 



have been perpetually altering the circumstances of organ- 

 isms ; and organisms, as they have become more numerous in 

 their kinds and higher in their kinds, have been perpetually 

 altering one another's circumstances. Thus, for those pro- 

 gressive modifications upon modifications which organic evo- 

 lution implies, we find a sufficient cause in the modifications 

 after modifications, which every environment over the Earth's 

 surface has been undergoing, throughout all geologic and pre- 

 geologic times. The progressive inner changes for 



which we thus find a cause in the continuous outer changes, 

 conform, so far as we can trace them, to that universal law of 

 the instability of the homogeneous, which is manifested 

 throughout evolution in general. We see that in organisms, 

 as in all other things, the exposure of different parts to 

 different kinds and amounts of incident forces, has necessi- 

 tated their differentiation ; and that for the like reason, 

 aggregates of individuals have been lapsing into varieties, 

 and species, and genera, and classes. "We also see that in 

 each type of organism, as in the aggregate of types, the mul- 

 tiplication of effects has continually aided this transition from 

 a more homogeneous to a more heterogeneous state. And 

 yet again, we see that that increasing segregation, and con- 

 comitant increasing definiteness, which characterizes the 

 growing heterogeneity of organisms, has been insured by the 

 necessary maintenance of them under combinations of forces 

 not greatly unlike preceding combinations by the continual 

 destruction of those which expose themselves to aggregates 

 of external actions markedly incongruous with the aggregates 

 of their internal actions, and the survival of those subject 

 only to comparatively small incongruities. Finally, 



we have found that each change of structure, superposed on 

 preceding changes, has been a re-equilibration necessitated by 

 the disturbance of a preceding equilibrium. The maintenance 

 of life being the maintenance of a balanced combination of 

 functions, it follows that individuals and species that have 

 continued to live, are individuals and species in which the 



