Simple and Compound Evolution §98. Where the only forces at work are those directly tending toproduce aggregation or diffusion, the whole history of an aggregate willcomprise no more than the approaches of its components towards their commoncentre and their recessions from their common centre. The process of evolution,including nothing beyond what was described at the outset of the last chapter,will be simple.
Again, where the forces which cause movements towards a common centregreatly exceed all other forces, any changes additional to those of aggregationwill be comparatively insignificant: there will be integration slightly modifiedby further kinds of redistribution.
Or if because of the smallness of the mass, or because of the little motionit receives from without in return for the motion it loses, the integrationproceeds rapidly; there will similarly be wrought but insignificant effectsby secondary forces, even though these are considerable.
But when, conversely, the integration is slow. either, because the quantityof motion contained in the aggregate is relatively great; or because, thoughthe quantity of motion which each part possesses is not relatively great,the large size of the aggregate prevents easy dissipation of the motion. or because, though motion is rapidly lost more motion is rapidly received;then, other forces will cause in the aggregate sensible modifications. Alongwith the change constituting integration, there will take place further changes.
The Evolution, instead of being simple, will be compound.
These several propositions require some explanation. §99. So long as a body moves freely through space, every force whichacts on it produces an equivalent in the shape of some change in its motion.
No matter how high its velocity the slightest lateral traction or resistancecauses it to deviate from its line of movement; and the effect of the perturbinginfluence goes on accumulating in the ratio of the squares of the times duringwhich its action continues uniform. But when this same body is held fastby gravitation or cohesion, small incident forces, instead of giving it somerelative motion through space, are otherwise dissipated.
What thus holds of masses holds, in a qualified way, of the sensible partsof masses, and of molecules. As the sensible parts of a mass, and the moleculesof a mass, are, by virtue of their aggregation, not perfectly free, it isnot true of each of them, as of a body moving through space, that every incidentforce produces an equivalent change of position: part of the force goes inworking other changes. But in proportion as the parts of the molecules arefreely bound together, incident forces effect marked re-arrangements amongthem. Where the integration is so slight that the parts, sensible or insensible,are almost independent, they are almost completely amenable to every additionalaction; and along with the concentration going on there go on other re-distributions.
Contrariwise, where the parts are so close that what we call the attractionof cohesion is great, additional actions, unless intense, have little powerto cause secondary re-arrangements. The firmly-united parts do not changetheir relative positions in obedience to small perturbing forces; but eachsmall perturbing force usually does nothing more than temporarily modifythe insensible molecular motions.
How may we best express this difference in general terms? An aggregatethat is widely diffused, or but little integrated, is an aggregate containinga large quantity of motion -- actual or potential or both. An aggregate thathas become completely integrated or dense, is one containing comparativelylittle motion: most of the motion its parts once had has been lost duringthe integration that has rendered it dense. Hence, other things equal, inproportion to the quantity of motion an aggregate contains will be the quantityof secondary change in the arrangement of its parts that accompanies theprimary change in their arrangements. Hence also other things equal, in proportionto the time during which the internal motion is retained, will be the quantityof this secondary re-distribution. It matters not how these conditions arefulfilled. Whether the internal motion continues great because the componentsare of a kind that will not readily aggregate, or because surrounding conditionsprevent them from parting with their motion, or because the loss of theirmotion is impeded by the size of the aggregate they form, or because theydirectly or indirectly obtain more motion in place of that which they lose;it throughout remains true that much retained internal motion renders secondaryre-distributions facile, and that long retention of it makes possible anaccumulation of such secondary re-distributions. Conversely, non-fulfilmentof these conditions, however caused, entails opposite results. Be it thatthe components of the aggregate have special aptitudes to integrate quickly,or be it that the smallness of the aggregate permits easy escape of theirmotion, or be it that they receive little or no motion in exchange for thatwhich they lose; it alike holds that but little secondary re-distributioncan accompany the primary re-distribution constituting their integration.
Let us, before studying simple and compound Evolution as thus determined,contemplate a few cases in which the quantity of internal motion is artificiallychanged, and note the effects on the re-arrangement of parts. §100. When a vessel has been filled to the brim with loose fragments,shaking it causes them to settle down into less space, so that more may beput in. And when among th e fragments there are some of much greater specificgravity than the rest, these, in the course of a prolonged shaking, findtheir way to the bottom. What are these results, expressed in general terms?
