The third form of equilibration displayed by organic bodies, is a sequenceof that just illustrated. When, through a change of habit or circumstance,an organism is permanently subject to some new influence, or different amountof an old influence, there arises, after more or less disturbance of theorganic rhythms, a balancing of them around the new average condition producedby this additional influence. if the quantity of motion to be habituallygenerated by a muscle becomes greater than before, its nutrition becomesgreater than before. if the expenditure of the muscle bears to its nutrition,a greater ratio than expenditure bears to nutrition in other parts of thesystem, the excess of nutrition becomes such that the muscle grows. And thecessation of its growth is the establishment of a balance between the dailywaste and the daily repair. The like is manifestly the case with all organicmodifications consequent on changes of climate or food. If we see that adifferent mode of life is followed, after a period of derangement, by somealtered condition of the system -- if we see that this altered condition,becoming by-and-by established, continues without further change; we haveno alternative but to say that the new forces brought to bear on the system,have been compensated by the opposing forces they have evoked. And this isthe interpretation of the process called adaptation. Finally, each organismillustrates the law in the ensemble of its life. At the outset it daily absorbsunder the form of food, an amount of force greater than it daily expends;and the surplus is daily equilibrated by growth. As maturity is approachedthis surplus diminishes; and in the perfect organism the day's absorptionof latent energy balances the day's expenditure of actual energy. That isto say, during adult life there is continuously exhibited an equilibriumof the third order. Eventually, the daily loss begins to outbalance the dailygain, and there results a diminishing amount of functional action; the organicrhythms extend less and less widely on each side of the medium state; andthere finally comes that complete equilibrium we call death.

The ultimate structural state accompanying that ultimate functional statetowards which an organism tends, may be deduced from one of the propositionsset down in the opening section of this chapter. We saw that the limit ofheterogeneity is reached when the equilibration of any aggregate becomescomplete -- that the re-distribution of matter can continue so long onlyas there continues some motion unbalanced. What is the implication in thecase of organic aggregates? We have seen that to maintain the moving equilibriumof one, requires the habitual genesis of internal forces corresponding innumber, directions, and amounts to the external incident forces -- as manyinner functions, single or combined, as there are single or combined outeractions to be met. But functions are the correlatives of organs; amountsof functions are, other things equal, the correlatives of sizes of organs;and combinations of functions the correlatives of connexions of organs. Hencethe structural complexity accompanying functional equilibrium, is definableas one in which there are as many specialized parts as are capable, separatelyand jointly, of counteracting the separate and joint forces amid which theorganism exists. And this is the limit of organic heterogeneity. to whichMan has approached more nearly than any other creature.

Groups of organisms display this universal tendency towards a balancevery obviously. in §85, every species of plant and animal was shownto be perpetually undergoing a rhythmical variation in number -- now fromabundance of food or absence of enemies rising above its average; and then,by a consequent scarcity of food or abundance of enemies, being depressedbelow its average. And here we have to observe that there is thus maintainedan equilibrium between the sum of those forces which result in the increaseof each race, and the sum of those forces which result in its decrease. Eitherlimit of variation is a point at which the one set of forces, before in excessof the other, is counterbalanced by it. And amid these oscillations producedby their conflict, lies that average number of the species at which its expansivetendency is in equilibrium with surrounding repressive tendencies. Nor canit be questioned that this balancing of the preservative and destructiveforces which we see going on in every race, must necessarily go on. Increaseof number cannot but continue until increase of mortality stops it; and decreaseof number cannot but continue until it is either arrested by fertility orextinguishes the race entirely. §174. The equilibrations of those nervous actions which constitutethe obverse face of mental life, may be classified in like manner with thosewhich constitute what we distinguish as bodily life. We may deal with themin the same order.

Each pulse of nerve force from moment to moment generated, (and it wasexplained in §86 that nerve currents are not continuous but rhythmical,)is met by counteracting forces, in overcoming which it is dispersed and equilibrated.