Something has still to be added. The reader must note two leading truthsbrought out by the foregoing exposition: the one concerning the ultimate,or rather the penultimate, state of motion which the processes describedtend to bring about; the other concerning the concomitant distribution ofmatter. This penultimate state of motion is the moving equilibrium, whichtends to arise in an aggregate having compound motions, as a transitionalstate on the way towards complete equilibrium. Throughout Evolution of allkinds there is a continual approximation to, and more or less complete maintenanceof, this moving equilibrium. As in the Solar System there has been establishedan independent moving equilibrium -- an equilibrium such that the relativemotions of its members are continually so counterbalanced by opposite motions,that the mean state of the aggregate never varies; so is it, though in aless distinct manner, with each form of dependent moving equilibrium. Thestate of things exhibited in the cycles of terrestrial changes, in the balancedfunctions of organic bodies that have reached their adult forms, and in theacting and re-acting processes of fully-developed societies, is similarlyone characterized by compensating oscillations. The involved combinationof rhythms seen in each of these cases, has an average condition which remainspractically constant during the deviations ever taking place on oppositesides of it. And the fact which we have here to observe is that, as a corollaryfrom the general law of equilibrium, every evolving aggregate must go onchanging until a moving equilibrium is established; since, as we have seen,an excess of force which the aggregate possesses in any direction, must eventuallybe expended in overcoming resistances to change in that direction: leavingbehind only those movements which compensate one another, and so form a movingequilibrium. Respecting the structural state simultaneously reached, it mustobviously be one presenting an arrangement of forces that counterbalanceall the forces to which the aggregate is subject. So long as there remainsa residual force in any direction -- be it excess of a force exercised bythe aggregate on its environment, or of a force exercised by its environmenton the aggregate, equilibrium does not exist; and therefore the re-distributionof matter must continue. Whence it follows that the limit of heterogeneitytowards which every aggregate progresses, is the formation of as many specializationsand combinations of parts, as there are specialized and combined forces tobe met. §171. Those successively changed forms which, if the nebular hypothesisbe granted, must have arisen during the evolution of the Solar System, wereso many transitional kinds of moving equilibrium, severally giving placeto more enduring kinds. Thus the assumption of an oblate spheroidal figureby condensing nebulous matter, was the assumption of a temporary and partialmoving equilibrium among the component parts -- a moving equilibrium thatmust have grown more settled as local conflicting movements were dissipated.
In the formation and detachment of the nebulous rings which, according tothis hypothesis, from time to time took place, we have instances of progressiveequilibration severally ending in the establishment of a complete movingequilibrium. For the genesis of each such ring implies a balancing of thatattractive force which the whole spheroid exercises on its equatorial portion,by that centrifugal force which the equatorial portion has acquired duringprevious concentration. So long as these two forces are not equal, the equatorialportion follows the contracting mass; but as soon as the second force hasincreased up to an equality with the first, the equatorial portion can followno further and remains behind. While, however, the resulting ring, regardedas a whole, has reached a state of moving equilibrium, its parts are notbalanced with respect to one another. As we before saw (§150) the probabilitiesagainst the maintenance of an annular form by nebulous matter are great: from the instability of the homogeneous, it is inferable that nebulous matterso distributed will break up into portions, and eventually concentrate intoa single mass. That is to say, the ring will progress towards a moving equilibriumof a more complete kind, during the dissipation of that motion which maintainedits particles in a diffused form; leaving at length a planetary body attendedperhaps by a group of minor bodies similarly produced, constituting a movingequilibrium that is all but perfect.*
