300 SIMPLE AND COMPOUND EVOLUTION 



of these conditions, however caused, entails opposite results. 

 Be it that the components of the aggregate have special 

 aptitudes to integrate quickly, or be it that the smallness of 

 the aggregate formed of them permits the easy escape of 

 their motion, or be it that they receive little or no motion 

 in exchange for that which they part with; it alike holds 

 that but little secondary re-distribution can accompany the 

 primary re-distribution constituting their integration. 



These abstract propositions will not be fully understood 

 without illustrations. Let us, before studying simple and 

 compound Evolution as thus determined, contemplate a few 

 cases in which the quantity of internal motion is artificially 

 changed, and note the effects on the re-arrangement of 

 parts. 



§ 100. We may fitly begin with a familiar experience, 

 introducing the general principle under a rude but easily 

 comprehensible form. AY hen a vessel has been filled to the 

 brim with loose fragments, shaking the vessel causes them 

 to settle down into less space, so that more may be put in. 

 And when among the fragments there are some of much 

 greater specific gravity than the rest, these, in the course of 

 a prolonged shaking, find their way to the bottom. What 

 now is the meaning of such results, when expressed in 

 general terms? "We have a group of units acted on by an 

 incident force — the attraction of the Earth. So long as 

 these units are not agitated, this incident force produces no 

 changes in their relative positions; agitate them, and im- 

 mediately their loose arrangement passes into a more com- 

 pact arrangement. Again, so long as they are not agitated, 

 the incident force cannot separate the heavier units from 

 the lighter; agitate them, and immediately the heavier units 

 begin to segregate. Mechanical disturbances of 



more minute kinds, acting on the parts of much denser ag- 

 gregates, produce analogous effects. A piece of iron which, 

 when it leaves the workshop, is fibrous in structure, be- 



