VARIATION. 335 



this family, must remain constant so long as the external 

 actions 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 equi 

 librium with the external actions : corresponding structural 

 alterations having arisen. 



On passing from these derivative laws to the ultimate law, 

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

 The members of a species inhabiting any area cannot be sub 

 ject to like sets 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 other 

 wise, is to say that differences in the forces will not produce 

 differences in the effects ; which is to deny the persistence of 

 force. 



