THE INTERPRETATION OF EVOLUTION. 41 1 



units are those constituting molecular changes, including 

 what we call chemical composition and decomposition; and 

 it is these which we recognize as the qualitative differences 

 that arise in an aggregate. The sensible transpositions are 

 such as result when certain of the units, instead of being 

 put into different relations with their immediate neighbours, 

 are carried away from them and deposited elsewhere. 



Concerning these divisions and sub-divisions of any force 

 affecting an aggregate, the fact which it chiefly concerns us 

 to observe is, that they are complementary to each other. 

 Of the whole incident force, the effective must be that which 

 remains after deducting the non-effective. The two parts of 

 the effective force must vary inversely as each other : where 

 much of it is temporarily effective, little of it can be perma- 

 nently effective; and vice versa. Lastly, the permanently 

 effective force, being expended in working both the insen- 

 sible re-arrangements which constitute molecular modifica- 

 tion, and the sensible re-arrangements which result in struc- 

 ture, must generate of either kind an amount that is great 

 or small in proportion as it has generated a small or great 

 amount of the other. 



