But now let us turn to other classes of organs possessed by plants—organs which are not at once affected in their actions by variations of incident forces. Take first the organs of defence. Many plants are shielded against animals that would else devour them, by formidable thorns; and others, like the nettle, by stinging hairs. These must be counted among the appliances by which equilibrium is maintained between the actions in the organism and the actions in its environment; seeing that were these defences absent, the destruction by herbivorous animals would be so much increased, that the number of young plants annually produced would not suffice, as it now does, to balance the mortality, and the species would disappear. But these defensive appliances, though they aid in maintaining the balance between inner and outer actions, cannot have been directly called forth by the outer actions which they serve to neutralize; for these outer actions do not continuously affect the functions of the plant even in a general way, still less in the special way required. Suppose a species of nettle bare of poison-hairs, to be habitually eaten by some mammal intruding on its habitat. The actions of this mammal would have no direct tendency to develop poison-hairs in the plant; since the individuals devoured could not bequeath changes of structure, even were the actions of a kind to produce fit ones; and since the individuals which perpetuated themselves would be those on which the new incident force had not fallen. Organs of another class, similarly circumstanced, are those of reproduction. Like the organs of defence these are not, during the life of the individual plant, variably exercised by variable external actions; and therefore do not fulfil those conditions under which structural changes may be directly caused by changes in the environment. The generative apparatus contained in every flower acts only once during its existence; and even then, the parts subserve their ends in a passive rather than an active way. Functionally-produced modifications are therefore out of the question. If a plant's anthers are so placed that the insect which most commonly frequents its flowers, must come in contact with the pollen, and fertilize with it other flowers of the same species; and if this insect, dwindling away or disappearing from the locality, leaves behind no insects having such shapes and habits as cause them to do the same thing efficiently, but only some which do it inefficiently; it is clear that this change of its conditions has no immediate tendency to work in the plant any such structural change as shall bring about a new balance with its conditions. For the anthers, which, even when they discharge their functions, do it simply by standing in the way of the insect, are, under the supposed circumstances, left untouched by the insect; and this remaining untouched cannot have the effect of so modifying the stamens as to bring the anthers into a position to be touched by some other insect. Only those individuals whose parts of fructification so far differed from the average form that some other insect could serve them as pollen-carrier, would have good chances of perpetuating themselves. And on their progeny, inheriting the deviation, there would act no external force directly tending to make the deviation greater; since the new circumstances to which re-adaptation is required, are such as do not in the least alter the equilibrium of functions constituting the life of the individual plant.
§ 162. Among animals, adaptation by direct equilibration is similarly traceable wherever, during the life of the individual, an external change generates some constant or repeated change of function. This is conspicuously the case with such parts of an animal as are immediately exposed to diffused influences, like those of climate, and with such parts of an animal as are occupied in its mechanical actions on the environment. Of the one class of cases, the darkening of the skin which follows exposure to one or other extreme of temperature, may be taken as an instance; and with the other class of cases we are made familiar by the increase and decrease which use and disuse cause in the organs of motion. It is needless here to exemplify these: they were treated of in the Second Part of this work.
But in animals, as in plants, there are many indispensable offices fulfilled by parts between which and the external conditions they respond to, there is no such action and reaction as can directly produce an equilibrium. This is especially manifest with dermal appendages. Some ground exists for the conclusion that the greater or less development of hairs, is in part immediately due to increase or decrease of demand on the passive function, as forming a non-conducting coat; but be this as it may, it is impossible that there can exist any such cause for those immense developments of hairs which we see in the quills of the porcupine, or those complex developments of them known as feathers. Such an enamelled armour as is worn by Lepidosteus, is inexplicable as a direct result of any functionally-worked change. For purposes of defence, such an armour is as needful, or more needful, for hosts of other fishes; and did it result from any direct reaction of the organism against any offensive actions it was subject to, there seems no reason why other fishes should not have developed similar protective coverings. Of sundry reproductive appliances the like may be said. The secretion of an egg-shell round the substance of an egg, in the oviduct of a bird, is quite inexplicable as a consequence of some functionally-wrought modification of structure, immediately caused by some modification of external conditions. The end fulfilled by the egg-shell, is that of protecting the contained mass against certain slight pressures and collisions, to which it is liable during incubation. How, by any process of direct equilibration, could it come to have the required thickness? or, indeed, how could it come to exist at all? Suppose this protective envelope to be too weak, so that some of the eggs a bird lays are broken or cracked. In the first place, the breakages or crackings are actions which cannot react on the maternal organism in such ways as to cause the secretion of thicker shells for the future: to suppose that they can, is to suppose that the bird understands the cause of the evil, and that the secretion of thicker shells can be effected by its will. In the second place, such developing chicks as are contained in the shells which crack or break, are almost certain to die; and cannot, therefore, acquire appropriately-modified constitutions: even supposing any relation could exist between the impression received and the change required. Meanwhile, such eggs as escape breakage are not influenced at all by the requirement; and hence, on the birds developed from them, there cannot have acted any force tending to work the needful adjustment of functions. In no way, therefore, can a direct equilibration between constitution and conditions be here produced. Even in organs that can be modified by certain incident actions into correspondence with such incident actions, there are some re-adjustments which cannot be effected by direct balancing. It is thus with the bones. The majority of the bones have to resist muscular strains; and variations in the muscular strains call forth, by reaction, variations in the strengths of the bones. Here there is direct equilibration. But though the greater massiveness acquired by bones subject to greater strains, may be ascribed to counter-acting forces evoked by forces brought into action; it is impossible that the acquirement of greater lengths by bones can be thus accounted for. It has been supposed that the elongation of the metatarsals in wading birds, has resulted from direct adaptation to conditions of life. To justify this supposition, however, it must be shown that the mechanical actions and reactions in the legs of a wading bird, differ from those in the legs of other birds; and that the differential actions are equilibrated by the extra lengths. There is not the slightest evidence of this. The metatarsals of a bird have to bear no appreciable strains but those due to the superincumbent weight. Standing in the water does not appreciably alter such strains; and even if it did, an increase in the lengths of these bones would not fit them any better to meet the altered strains.
§ 163. The conclusion at which we arrive is, then, that there go on in all organisms, certain changes of function and structure that are directly consequent on changes in the incident forces—inner changes by which the outer changes are balanced, and the equilibrium restored. Such re-equilibrations, which are often conspicuously exhibited in individuals, we have reason to believe continue in successive generations; until they are completed by the arrival at structures fitted to the modified conditions. But, at the same time, we see that the modified conditions to which organisms may be adapted by direct equilibration, are conditions of certain classes only. That a new external action may be met by a new internal action, it is needful that it shall either continuously or frequently be borne by the individuals of the species, without killing or seriously injuring them; and shall act in such way as to affect their functions. And we find that many of the environing agencies—evil or good—to which organisms have to be adjusted, are not of these kinds: being agencies which either do not immediately affect the functions at all, or else affect them in ways that prove fatal.
Hence there must be at work some other process which equilibrates the actions of organisms with the actions they are exposed to. Plants and animals that continue to exist, are necessarily plants and animals whose powers balance the powers acting on them; and as their environments change, the changes which plants and animals undergo must necessarily be changes towards re-establishment of the balance. Besides direct equilibration, there must therefore be an indirect equilibration. How this goes on we have now to inquire.
CHAPTER XII.
INDIRECT EQUILIBRATION.
§ 164. Besides those perturbations produced in any organism by special disturbing forces, there are ever going on many others—the reverberating effects of disturbing forces previously experienced by the individual, or by ancestors; and the multiplied deviations of function so caused imply multiplied deviations of structure. In [§ 155] there was re-illustrated the truth, set forth at length when treating of Adaptation ([§ 69]), that an organism in a state of moving equilibrium, cannot have extra function thrown on any organ, and extra growth produced in such organ, without correlative changes being entailed throughout all other functions, and eventually throughout all other organs. And when treating of Variation ([§ 90]), we saw that individuals which have been made, by their different circumstances, to deviate functionally and structurally from the average type in different directions, will bequeath to their joint offspring, compound perturbations of function and compound deviations of structure, endlessly varied in their kinds and amounts.
Now if the individuals of a species are thus necessarily made unlike in countless ways and degrees—if in one individual the amount of energy in a particular direction is greater than in any other individual, or if here a peculiar combination gives a resulting action which is not found elsewhere; then, among all the individuals, some will be less liable than others to have their equilibria overthrown by a particular incident force previously unexperienced. Unless the change in the environment is so violent as to be universally fatal to the species, it must affect more or less differently the slightly-different moving equilibria which the members of the species present. Inevitably some will be more stable than others when exposed to this new or altered factor. That is to say, those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die; and those will survive whose functions happen to be most nearly in equilibrium with the modified aggregate of external forces.
But this survival of the fittest[[52]] implies multiplication of the fittest. Out of the fittest thus multiplied there will, as before, be an overthrowing of the moving equilibrium wherever it presents the least opposing force to the new incident force. And by the continual destruction of the individuals least capable of maintaining their equilibria in presence of this new incident force, there must eventually be reached an altered type completely in equilibrium with the altered conditions.