In living aggregates, and especially in animals, these conflicting processesgo on with great activity under several forms. There is not merely what wemay call the passive integration of matter, which inanimate masses effectby simple molecular attractions, but there is an active integration of itunder the form of food. In addition to that passive superficial disintegrationwhich inanimate objects suffer from external agents, animals produce in themselvesactive internal disintegration, by absorbing such agents. While, like inorganicaggregates, they passively radiate and receive motion, they are also activeabsorbers of motion latent in food, and active expenders of that motion.

But notwithstanding this complication of the two processes, and the immenseexaltation of the conflict between them, it remains true that there is alwaysa differential progress towards either integration or disintegration. Duringthe earlier part of the cycle of changes the integration predominates --there goes on what we call growth. The middle part of the cycle is usuallycharacterized, not by equilibrium between the integrating and disintegratingprocesses, but by alternate excesses of them. And the cycle closes with aperiod in which the disintegration, beginning to predominate, eventuallyputs a stop to integration, and after death undoes what integration had originallydone. At no moment are assimilation and waste so balanced that no increaseor decrease of mass is going on. Even in cases where one part is growingwhile other parts are dwindling, and even in cases where different partsare differently exposed to external sources of motion, so that some are expandingwhile others are contracting, the truth still holds. For the chances areinfinity to one against these opposite changes balancing one another; andif they do not balance, the aggregate as a whole is integrating or disintegrating.

Hence that the changes ever going on are from a diffused imperceptiblestate to a concentrated perceptible state, and back again to a diffused inmperceptiblestate, must be that universal law of redistribution of matter and motion,which serves to unify the seemingly diverse groups of changes, as well asthe entire course of each group. §97. The processes thus everywhere in antagonism, and everywheregaining now a temporary and now an enduring predominance the one over theother, we call Evolution and Dissolution. Evolution under its most generalaspect is the integration of matter and concomitant dissipation of motion;while Dissolution is the absorption of motion and concomitant disintegrationof matter.

The last of these titles answers its purpose tolerably well, but the firstis open to grave objections. Evolution has other meanings, some of whichare incongruous with, and some even directly opposed to, the meaning heregiven to it. The evolution of a gas is literally an absorption of motionand distintegration of matter, which is exactly the reverse of that whichwe here call Evolution. As ordinarily understood, to evolve is to unfold,to open and expand, to throw out; whereas as understood here, the processof evolving, though it implies increase of a concrete aggregate, and in sofar an expansion of it, implies that its component matter has passed froma more diffused to a more concentrated state -- has contracted. The antitheticalword Involution would more truly express the nature of the change; and would,indeed, describe better those secondary characters of it which we shall haveto deal with presently. We are obliged, however, notwithstanding the liabilitiesto confusion resulting from these unlike and even contradictory meanings,to use Evolution as antithetical to Dissolution. The word is now so widelyrecognized as signifying, not, indeed, the general process above described,but sundry of its most conspicuous varieties, and certain of its secondarybut most remarkable accompaniments, that we cannot now substitute anotherword.

While, then, we shall by Dissolution everywhere mean the process tacitlyimplied by its ordinary meaning -- the absorption of motion and disintegrationof matter; we shall everywhere mean by Evolution, the process which is alwaysan integration of matter and dissipation of motion, but which, as we shallnow see, is in most cases much more than this.