The Principles of Biology, Volume 1 · Herbert Spencer
INDIRECT EQUILIBRATION.
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.
Text from Project Gutenberg, public domain in the United States.