CHAPTER XIIL 



THE CO-OPERATION OF THE FACTORS. 



§ 169. Thus the phenomena of Organic Evolution may be 

 interpreted in the same way as the phenomena of all other 

 Evolution. Fully to see this, it will be needful for us to con- 

 template in their ensemble, the several processes separately 

 described in the four preceding chapters. 



If the forces acting on any aggregate remain the same, the 

 changes produced by them will presently reach a limit, at 

 which the outer forces are balanced by the inner forces; and 

 thereafter no further metamorphosis will take place. Hence, 

 that there may be continuous changes of structure in organ- 

 isms, there must be continuous changes in the incident 

 forces. This condition to the evolution of animal and vegetal 

 forms, we find to be fully satisfied. The astronomic, geologic, 

 and meteorologic changes that have been slowly but inces- 

 santly going on, and have been increasing in the complexity 

 of their combinations, have been perpetually altering the 

 circumstances of organisms ; and organisms, becoming more 

 numerous in theii kinds and higher in their kinds, have been 

 perpetually altering one another's circumstances. Thus, for 

 those progressive modifications upon modifications which or- 

 ganic evolution implies, we find a sufficient cause. The 

 increasing inner changes for which we thus find a cause in 

 the perpetual outer changes, conform, so far as we can trace 

 them, to the universal law of the instability of the homo- 



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