CHAPTER XIII. 

 THE CO-OPERATION OF THE FACTORS. 



S 169. Thus the phenomena of organic evolution, maj be 

 interpreted in the same way as tlie phenomena of all other 

 evolution. Those universal laws of the re-distribution of 

 matter and motion, to which things in general conform, are 

 conformed to by all living things ; whether considered in 

 their individual histories, in their histories as species, or in 

 their aggregate history. However otherwise they may ordin- 

 arily be expressed, the truths of development as exhibited in 

 the animal and vegetal kingdoms, prove to be expressible an 

 manifestations of those abstract truths set forth in First 

 Principles. 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 in the aggregate will presently 

 reach a limit, at which the constant outer forces are balanced 

 by the constant inner forces ; and thereafter no further me- 

 tamorphosis will take place. Hence, that there may be 

 continuous changes of structure in organisms, 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 slowlj' but incessantly going on, and 

 have been increasing iu the complexity of their combinatioBH, 



