§ 167. Returning from these extensive classes of facts for which Mr. Darwin's hypothesis does not account, to the still more extensive classes of facts for which it does account, and which are unaccountable on any other hypothesis; let us consider in what way this hypothesis is expressible in terms of the general doctrine of evolution. Already it has been pointed out that the evolving of modified types by "natural selection or the preservation of favoured races in the struggle for life," must be a process of equilibration; since it results in the production of organisms which are in equilibrium with their environments. At the outset of this chapter, something was done towards showing how this continual survival of the fittest may be understood as the progressive establishment of a balance between inner and outer forces. Here, however, we must consider the matter more closely.
On previous occasions we have contemplated the assemblage of individuals composing a species, as an aggregate in a state of moving equilibrium. We have seen that its powers of multiplication give it an expansive energy which is antagonized by other energies; and that through the rhythmical variations in these two sets of energies there is maintained an oscillating limit to its habitat, and an oscillating limit to its numbers. On another occasion ([§ 96]) it was shown that the aggregate of individuals constituting a species, has a kind of general life which, "like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts." We saw that "just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhythmical functions thus maintained, constitutes the life of the organism; so, in a species there is, through gamogenesis, a perpetual neutralization of those contrary deviations from the mean state, which are caused in its different parts by different sets of incident forces; and it is similarly by the rhythmical production and compensation of these contrary deviations that the species continues to live." Hence, to understand how a species is affected by causes which destroy some of its units and favour the multiplication of others, we must consider it as a whole whose parts are held together by complex forces that are ever re-balancing themselves—a whole whose moving equilibrium is continually disturbed and continually rectified. Thus much premised, let us next call to mind how moving equilibria in general are changed. In the first place, a new incident force falling on any part of an aggregate with balanced motions, produces a new motion in the direction of least resistance. In the second place, the new incident force is gradually used up in overcoming the opposing forces, and when it is all expended the opposing forces produce a recoil—a reverse deviation which counter-balances the original deviation. Consequently, to consider whether the moving equilibrium of a species is modified in the same way as moving equilibria in general, is to consider whether, when exposed to a new force, a species yields in the direction of least resistance; and whether, by its thus yielding, there is generated in the species a compensating change in the opposite direction. We shall find that it does both these things.
For what, expressed in mechanical terms, is the effect wrought on a species by some previously-unknown enemy, that kills such of its members as fail in defending themselves? The disappearance of those individuals which meet the destroying forces by the smallest preserving forces, is tantamount to the yielding of the species as a whole at the places where the resistances are the least. Or if by some general influence, such as alteration of climate, the members of a species are subject to increase of external actions which are ever tending to overthrow their equilibria, and which they are ever counter-balancing by certain physiological actions, which are the first to die? Those least able to generate the internal energies which antagonize these external energies. If the change be an increase of the winter's cold, then such members of the species as have unusual powers of getting food or of digesting food, or such as are by their constitutional aptitude for making fat, furnished with reserve stores of force, available in times of scarcity, or such as have the thickest coats and so lose least heat by radiation, survive; and their survival implies that in each of them the moving equilibrium of functions presents such an adjustment of internal forces, as prevents overthrow by the modified aggregate of external forces. Conversely, the members which die are, other things equal, those deficient in the power of meeting the new action by an equivalent counter-action. Thus, in all cases, a species considered as an aggregate in a state of moving equilibrium, has its state changed by the yielding of its fluctuating mass wherever this mass is weakest in relation to the special forces acting on it. The conclusion is, indeed, a truism. But now what must follow from the destruction of the least-resisting individuals and survival of the most-resisting individuals? On the moving equilibrium of the species as a whole, existing from generation to generation, the effect of this deviation from the mean state is to produce a compensating deviation. For if all such as are deficient of power in a certain direction are destroyed, what must be the effect on posterity? Had they lived and left offspring, the next generation would have had the same average powers as preceding generations: there would have been a like proportion of individuals less endowed with the needful power, and individuals more endowed with it. But the more-endowed individuals being alone left to continue the race, there must result a new generation characterized by a larger average endowment of this power. That is to say, on the moving equilibrium of a species, an action producing change in a given direction is followed, in the next generation, by a reaction producing an opposite change. Observe, too, that these effects correspond in their degrees of violence. If the alteration of some external factor is so great that it leaves alive only the few individuals possessing extreme endowments of the power required to antagonize it; then, in succeeding generations, there is a rapid multiplication of individuals similarly possessing extreme endowments of this power—the force impressed calls out an equivalent conflicting force. Moreover, the change is temporary where the cause is temporary, and permanent where the cause is permanent. All that are deficient in the needful attribute having been killed off, and the survivors having the needful attribute in a comparatively high degree, there will descend from them, not only some possessing equal amounts of this attribute with themselves, but also some possessing less amounts of it. If the destructive agency has not continued in action, such less-endowed individuals will multiply; and the species, after sundry oscillations, will return to its previous mean state. But if this agency be a persistent one, such less endowed individuals will be continually killed off, and eventually none but highly-endowed individuals will be produced—a new moving equilibrium, adapted to the new environing conditions, will result.
It may be objected that this mode of expressing the facts does not include the cases in which a species becomes modified in relation to surrounding agencies of a passive kind—cases like that of a plant which acquires hooked seed-vessels, by which it lays hold of the skins of passing animals, and makes them the distributors of its seeds—cases in which the outer agency has no direct tendency at first to affect the species, but in which the species so alters itself as to take advantage of the outer agency. To cases of this kind, however, the same mode of interpretation applies on simply changing the terms. While, in the aggregate of influences amid which a species exists, there are some which tend to overthrow the moving equilibria of its members, there are others which facilitate the maintenance of their moving equilibria, and some which are capable of giving their moving equilibria increased stability: instance the spread into their habitat of some new kind of prey, which is abundant at seasons when other prey is scarce. Now what is the process by which the moving equilibrium in any species becomes adapted to some additional external factor furthering its maintenance? Instead of an increased resistance to be met and counterbalanced, there is here a diminished resistance; and the diminished resistance is equilibrated in the same way as the increased resistance. As, in the one case, there is a more frequent survival of individuals whose peculiarities enable them to resist the new adverse factor; so, in the other case, there is a more frequent survival of individuals whose peculiarities enable them to take advantage of the new favourable factor. In each member of the species, the balance of functions and correlated arrangement of structures, differ slightly from those existing in other members. To say that among all its members, one is better fitted than the rest to benefit by some before-unused agency in the environment, is to say that its moving equilibrium is, in so far, more stably adjusted to the sum of surrounding influences. And if, consequently, this individual maintains its moving equilibrium when others fail, and has offspring which do the like—that is, if individuals thus characterized multiply and supplant the rest; there is, as before, a process which effects equilibration between the organism and its environment, not immediately but mediately, through the continuous intercourse between the species as a whole and the environment.
§ 168. Thus we see that indirect equilibration does whatever direct equilibration cannot do. All these processes by which organisms are re-fitted to their ever-changing environments, must be equilibrations of one kind or other. As authority for this conclusion, we have not simply the universal truth that change of every order is towards equilibrium; but we have also the truth that life itself is a moving equilibrium between inner and outer actions—a continuous adjustment of internal relations to external relations; or the maintenance of a balance between the forces to which an organism is subject and the forces which it evolves. Hence all changes which enable a species to live under altered conditions, are changes towards equilibrium with the altered conditions; and therefore those which do not come within the class of direct equilibrations, must come within the class of indirect equilibrations.
And now we reach an interpretation of Natural Selection regarded as a part of Evolution at large. As understood in First Principles, Evolution is a continuous redistribution of matter and motion; and a process of evolution which is not expressible in terms of matter and motion has not been reduced to its ultimate form. The conception of Natural Selection is manifestly one not known to physical science: its terms are not of a kind physical science can take cognisance of. But here we have found in what manner it may be brought within the realm of physical science. Rejecting metaphor we see that the process called Natural Selection is literally a survival of the fittest; and the outcome of the above argument is that survival of the fittest is a maintenance of the moving equilibrium of the functions in presence of outer actions: implying the possession of an equilibrium which is relatively stable in contrast with the unstable equilibria of those which do not survive.
CHAPTER XIII.
THE CO-OPERATION OF THE FACTORS.
§ 169. Thus the phenomena of Organic Evolution may be interpreted in the same way as the phenomena of all other Evolution. Fully to see this, it will be needful for us to contemplate in their ensemble, the several processes separately described in the four preceding chapters.
If the forces acting on any aggregate remain the same, the changes produced by them will presently reach a limit, at which the outer forces are balanced by the inner forces; and thereafter no further metamorphosis will take place. Hence, that there may be continuous changes of structure in organisms, there must be continuous changes in the incident forces. This condition to the evolution of animal and vegetal forms, we find to be fully satisfied. The astronomic, geologic, and meteorologic changes that have been slowly but incessantly going on, and have been increasing in the complexity of their combinations, have been perpetually altering the circumstances of organisms; and organisms, becoming more numerous in their kinds and higher in their kinds, have been perpetually altering one another's circumstances. Thus, for those progressive modifications upon modifications which organic evolution implies, we find a sufficient cause. The increasing inner changes for which we thus find a cause in the perpetual outer changes, conform, so far as we can trace them, to the universal law of the instability of the homogeneous. In organisms, as in all other things, the exposure of different parts to different kinds and amounts of incident forces, has necessitated their differentiation; and, for the like reason, aggregates of individuals have been lapsing into varieties, and species, and genera, and orders. Further, in each type of organism, as in the aggregate of types, the multiplication of effects has continually aided this transition from a more homogeneous to a more heterogeneous state. And yet again, that increasing segregation and concomitant increasing definiteness, associated with the growing heterogeneity of organisms, has been aided by the continual destruction of those which expose themselves to aggregates of external actions markedly incongruous with the aggregates of their internal actions, and the survival of those subject only to comparatively small incongruities. Finally, we have found that each change of structure, superposed on preceding changes, has been a re-equilibration necessitated by the disturbance of a preceding equilibrium. The maintenance of life being the maintenance of a balanced combination of functions, it follows that individuals and species that have continued to live, are individuals and species in which the balance of functions has not been overthrown. Hence survival through successive changes of conditions, implies successive adjustments of the balance to the new conditions.