VARIATION. 271 



brium constituted by the vital actions in each member of 

 this family, must remain constant so long as the external ac- 

 tions to which they correspond remain constant ; and that if 

 the external actions are changed, the disturbed balance of 

 internal changes, if not overthrown, cannot cease undergoing 

 modification until the internal changes are again in equili- 

 brium with the external actions : corresponding structural 

 alterations having arisen. 



Or passing from these derivative laws to the ultimate law, 

 we see that Variation is necessitated by the persistence of force. 

 The members of a species inhabiting any area, cannot be subject 

 to like aggregates of forces over the whole of that area. And 

 if, in different parts of the area, different kinds or amounts or 

 combinations of forces act on them, they cannot but become 

 different in themselves and in their progeny. To say otherwise, 

 is to say that differences in the forces will not produce differ- 

 ences in the effects ; which is to deny the persistence of force. 



Whence it is also manifest, that there can be no variation 

 of structure, but what is directly or indirectly consequent on 

 variation of function. On the one hand, organisms in com- 

 plete equilibrium with their conditions, cannot be changed 

 except by change in their conditions ; since, to assert other- 

 wise, is to assert that there can be an effect without a cause ; 

 which is to deny the persistence of force. On the other hand, 

 any change of conditions can affect an organism only by 

 changing the actions going on in it only by altering its func- 

 tions. The alterations of functions being necessarily towards 

 a re-establishment of the equilibrium, (for if not, the equili- 

 brium must be destroyed and the life cease, either in the in- 

 dividual or in descendants,) it follows that the structural alter- 

 ations directly caused, are adaptations ; and that the correlated 

 structural alterations indirectly caused, are the concomitants of 

 adaptations. Hence, though, by the intercourse of organisms 

 that have been functionally and structurally modified in dif- 

 ferent directions, there may result organisms that deviate in 

 compound ways which appear unrelated to external condi- 



