CHAPTER XI.
DIRECT EQUILIBRATION.
§ 159. Every change is towards a balance of forces; and of necessity can never cease until a balance of forces is reached. When treating of equilibration under its general aspects (First Principles, Part II., Chap. xxii.), we saw that every aggregate having compound movements tends continually towards a moving equilibrium; since any unequilibrated force to which such an aggregate is subject, if not of a kind to overthrow it altogether, must continue modifying its state until an equilibrium is brought about. And we saw that the structure simultaneously reached must be "one presenting an arrangement of forces that counterbalance all the forces to which the aggregate is subject;" since, "so long as there remains a residual force in any direction—be it excess of a force exercised by an aggregate on its environment, or of a force exercised by its environment on the aggregate, equilibrium does not exist; and therefore the re-distribution of matter must continue."
It is essential that this truth should here be fully comprehended; and to the end of insuring clear comprehension of it, some re-illustration is desirable. The case of the Solar System will best serve our purpose. An assemblage of bodies, each of which has its simple and compound motions that severally alternate between two extremes, and the whole of which has its involved perturbations, that now increase and now decrease, is here presented to us. Suppose a new factor were brought to bear on this moving equilibrium—say by the arrival of some wandering mass, or by an additional momentum given to one of the existing masses—what would be the result? If the strange body or the extra energy were very large, it might so derange the entire system as to cause its collapse. But what if the incident energy, falling on the system from without, proved insufficient to overthrow it? There would then arise a set of perturbations which would, in the course of an enormous period, slowly work round into a modified moving equilibrium. The effects primarily impressed on the adjacent masses, and in a smaller degree on the remoter masses, would presently become complicated with the secondary effects impressed by the disturbed masses on one another; and these again with tertiary effects. Waves of perturbation would continue to be propagated throughout the entire system; until, around a new centre of gravity, there had been established a set of planetary motions different from the preceding ones. The new energy must gradually be used up in overcoming the energies resisting the divergence it generates; which antagonizing energies, when no longer opposed, set up a counter-action, ending in a compensating divergence in the opposite direction, followed by a re-compensating divergence, and so on. Now though instead of being, like the Solar System, in a state of independent moving equilibrium, an organism is in a state of dependent moving equilibrium (First Principles, § 170); yet this does not prevent the manifestation of the same law. Every animal daily obtains from without, a supply of energy to replace the energy it expends; but this continual giving to its parts a new momentum, to make up for the momentum continually lost, does not interfere with the carrying on of actions and reactions like those just described. Here, as before, we have a definitely-arranged aggregate of parts, called organs, having their definitely-established actions and reactions, called functions. These rhythmical actions or functions, and the various compound rhythms resulting from their combinations, are so adjusted as to balance the actions to which the organism is subject: there is a constant or periodic genesis of energies which, in their kinds, amounts, and directions, suffice to antagonize the energies the organism has constantly or periodically to bear. If, then, there exists this moving equilibrium among a set of internal actions, exposed to a set of external actions, what must result if any of the external actions are changed? Of course there is no longer an equilibrium. Some energy which the organism habitually generates, is too great or too small to balance some incident energy; and there arises a residual energy exerted by the environment on the organism, or by the organism on the environment. This residual or unbalanced energy, of necessity expends itself in producing some change of state in the organism. Acting directly on some organ and modifying its function, it indirectly modifies dependent functions and remotely influences all the functions. As we have already seen ([§§ 68], [69]), if this new energy is permanent, its effects must be gradually diffused throughout the entire system; until it has come to be equilibrated in producing those structural rearrangements whence result a counter-balancing energy.
The bearing of this general truth on the question we are now dealing with is obvious. Those modifications upon modifications, which the unceasing mutations of their environments have been all along generating in organisms, have been in each case modifications involved by the establishment of a new balance with the new combination of actions. In every species throughout all geologic time, there has been perpetually going on a rectification of the equilibrium, which has been perpetually disturbed by the alteration of its circumstances; and every further heterogeneity has been the addition of a structural change entailed by a new equilibration, to the structural changes entailed by previous equilibrations. There can be no other ultimate interpretation of the matter, since change can have no other goal.
This equilibration between the functions of an organism and the actions in its environment, may be either direct or indirect. The new incident force may either immediately call forth some counteracting force, and its concomitant structural change; or it may be eventually balanced by some otherwise-produced change of function and structure. These two processes of equilibration are quite distinct, and must be separately dealt with. We will devote this chapter to the first of them.
§ 160. Direct equilibration is that process currently known as adaptation. We have already seen (Part II., Chap, v.), that individual organisms become modified when placed in new conditions of life—so modified as to re-adjust the powers to the requirements; and though there is great difficulty in disentangling the evidence, we found reason for thinking ([§ 82]) that structural changes thus caused by functional changes are inherited. In the last chapter, it was argued that if, instead of the succession of individuals constituting a species, there were a continuously-existing individual, any functional and structural divergence produced by a new incident action, would increase until the new incident action was counterpoised; and that the replacing of a continuously-existing individual by a succession of individuals, each formed out of the modified substance of its predecessor, will not prevent the like effect from being produced. Here we further find that this limit towards which any such organic change advances, in the species as in the individual, is a new moving equilibrium adjusted to the new arrangement of external forces.
But now what are the conditions under which alone, direct equilibration can occur? Are all the modifications that serve to re-fit organisms to their environments, directly adaptive modifications? And if otherwise, which are the directly adaptive and which are not? How are we to distinguish between them?
There can be no direct equilibration with an external agency which, if it acts at all, acts fatally; since the organism to be adapted disappears. Conversely, some inaccessible benefit which a small modification in the organism would make accessible, cannot by its action tend to produce this modification: the modification and the benefit do not stand in dynamic relation. The only new incident forces which can work the changes of function and structure required to bring any animal or plant into equilibrium with them, are such incident forces as operate on this animal or plant, either continuously or frequently. They must be capable of appreciably changing that set of complex rhythmical actions and reactions constituting the life of the organism; and yet must not usually produce perturbations that are fatal. Let us see what are the limits to direct equilibration hence arising.
§ 161. In plants, organs engaged in nutrition, and exposed to variations in the amounts and proportions of matters and forces utilized in nutrition, may be expected to undergo corresponding variations. We find evidence that they do this. The "changes of habit" which are common in plants, when taken to places unlike in climate or soil to those before inhabited by them, are changes of parts in which the modified external actions directly produce modified internal actions. The characters of the stem and shoots as woody or succulent, erect or procumbent; of the leaves in respect of their sizes, thicknesses, and textures; of the roots in their degrees of development and modes of growth; are obviously in immediate relation to the characters of the environment. A permanent difference in the quantity of light or heat affects, day after day, the processes going on in the leaves. Habitual rain or drought alters all the assimilative actions, and appreciably influences the organs that carry them on. Some particular substance, by its presence in the soil, gives new qualities to some of the tissues; causing greater rigidity or flexibility, and so affecting the general aspect. Here then we have changes towards modified sets of functions and structures, in equilibrium with modified sets of external forces.