550 THE EVOLUTION OF LIFE. 



geneous. In organisms, as in all other things, the exposure 

 of different parts to different kinds and amounts of incident 

 forces, has necessitated their differentiation; and, for the 

 like reason, aggregates of individuals have been lapsing into 

 varieties, and species, and genera, and orders. Further, 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, that increasing segregation and concomitant in- 

 creasing definiteness, associated with the growing hetero- 

 geneity of organisms, has been aided by the continual de- 

 struction 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 mainte- 

 nance of life being the maintenance of a balanced combina- 

 tion of functions, it follows that individuals and species that 

 have continued to live, are individuals and species in which 

 the balance of functions has not been overthrown. Hence 

 survival through successive changes of conditions, implies 

 successive adjustments of the balance to the new conditions. 



The actions that are here specified in succession, are in 

 reality simultaneous; and they must be so conceived before 

 organic evolution can be rightly understood. Some aid 

 towards so conceiving them will be given by the annexed 

 table, representing the co-operation of the factors. 



§ 170. Respecting this co-operation, it remains only to 

 point out the respective shares of the factors in producing 

 the total result; and the way in which the proportions of 

 their respective shares vary as evolution progresses. 



At first, changes in the amounts and combinations of inor- 

 ganic forces, astronomic, geologic, and meteorologic, were the 



