§ 159. Every cliange is of necessity 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 11., Chap, xvi.), we saw that in every aggregate having compound movements, there tends continually to be established a moving equilibrium; since any unequilibrated force to which such an aggregate is subject, if not of a kind to overthrow the aggregate altogether, must continue modifying its state im.til 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 the 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 understood; and to the end of insuring a 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 DIRECT EQUILliniATION. 433 which has its involved perturbations, that now increase and now decrease, is here presented to us. Suppose a new force were brought to bear on this moving equilibrium — sayby 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 force were very large, it might so derange the entire system as to cause its collapse: by overthrow of its rhythmical movements, the moving equilibrium might rapidly be changed into a complete equilibrium. But what if the incident force, falling on the system from mthout, proved insufficient to overthrow it?
There woidd 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 soon become complicated wdth the secondary effects impressed by the disturbed masses on one another; and these again with tertiary effects. Waves of perturbation woidd continue to be propagated throughout the entire system; until, around a new centre of gravity, there had been established a set of planetary motions more or less different from the preceding ones. All this would necessarily follow from the truths that any new force brought to bear on a moving equilibrium, must gradually be used up in overcoming the forces that resist the divergence it generates: which antagonizing forces, being then no longer opposed, set up a counter- action, ending in a compensating divergence in the opposite direction, that is followed by a re- compensating divergence; and so on, until there is either established some additional rhythmical movement, or some equivalent modification of the pre-existing rhythmical movements. N^ow though instead of being, like the Solar Sy^stem, in a state of independent moving equilibrium, an organism is in a state of dependent moving equilibrium {First Principles, § 130); yet this does not prevent the manifestation of the same law. Every animal daily obtains from without, a supply of force to replace the force which 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, which we call organs, having their definitely-established actions and ^ reactions, which we call functions. These rhythmical actions or functions, and the various compound rhythms resulting from their combinations, are in such adjustment as to balance the actions to which the organism is subject: there is a constant or periodic genesis of forces, which, in their kinds, amounts, and directions, suffice to antagonize the forces which the organism has constantly or periodically to bear. If then there exists this state of m.oving equilibrium among a definite set of internal actions, exposed to a definite set of external actions; what must result if any of the external actions are changed? Of course there is no longer an equilibrium. Some force which the organism habitually generates, is too great or too small to balance some incident force; and there arises a residuary force exerted by the environment on the organism, or by the organism on the environment. '• This residuary force — this unbalanced force, 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 force is permanent, its effects must be gradually diffused throughout the entire system; until it has come to be equilibrated in working those structural rearrangements which produce an exactly counterbalancing force.
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, DIRECT EQUILIBRATION. l'3-3 have been in each case modificatioiLS involved by the cstablisliment of a new balance with the new combination of conditions. In every species througliout all geologic time, there has been perpetually going on a rectification of the equilibrium, that has been perpetually disturbed by the alteration of surrounding 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. Any fresh force brought to bear on an aggregate in a state of moving equilibrium, must do one of two things: it must either overthrow the moving equilibrium altogether, or it must alter without overthrowing it; and the alteration must end in the establishment of a new moving equilibrium. Hence in organisms, death or restoration of the physiological balance, are the only alternatives.
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 knoAVn 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 t^e evidence, Ave 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 436 THE EVOLUTION OF LIFE, constituting a species, there were a continuously- existing individual, any such functional and structural divergence as we see produced by a new incident force, would necessarily go on increasing until the new incident force was counterpoised; and that the rejplacing 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 — the persistence of force negativing any other inference. 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?
Manifestly^ for any moving equilibrium to be gradually altered, it is needftd, first, that some force shall operate upon it; and, second, that the force shall not be such as to overthrow it. If in the environment there exists some agency that would act advantageously on an organism were the organism a little modified, but which does not act on it in the absence of the required modification; it is clear that this agency cannot itself tend to produce the modification. On the other hand, if the external agency be of such kind, that individuals of the species whenever affected by it, are either killed or so injured that the production of vigorous offspringis much interfered with, there cannot be directly wrought in the species, any such alteration as will fit it to cope with this external agency. 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 DIRECT EQUlIiTlJ RATION. 437 of appreciably clianging 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.
§ IGl. 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 milike 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 difibrence in the quantity of light or heat, afiects, 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, w^e have, in plants, changes tending to bring about in them, modified arrangements of functions and structures, in equilibrium with modified sets of external forces.
But now let us turn to other classes of organs possessed by plants — organs which are not at once affected in their actions by the variations of incident forces. Take first the organs of defence. Many plants are shielded against animals that Avould else devour them, by formidable thorns; and others, like the nettle, by stinging hairs. These must be counted among the appliances by which equilibriimi is maintained between the actions in the organism and the actions in its environment; seeing that all other things remaining the same, if these defences were absent, the destruction by herbivorous animals would be so increased, that the number of young plants annually produced would not suffice, 'as it now does, to balance the mortality, and the species would therefore 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 agency 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 them; and since the individuals that perpetuated themselves, would be those on which the new incident force had not fallen. Another class of organs 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, is sure to come in contact with the pollen, and to fertilize with it other flowers of the same species; and if this insect, dwindling away or disappearing from the locality, leaves behind no insects that have such shapes and habits as cause them to do the same DIRECT EQUILIBRATION. 439 tiling- efficiently, but only some wliicli do it inefficiently; it is clear tliat tlie cliange of its conditions, has no immediate tendency to work in tlie 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 of the species, that some other insect could serve them as pollen-carrier, would be sufficiently prolific to have good chances of perpetuating themselves. And on their progeny, inheriting the deviation, there would act no external force directly calculated to make the deviation greater, and the adaptation more complete; 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 or lightening of the skin, that follows exposure to greater or less heat, 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 and manipulation. 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 440 THE EVOIATTTOIV' OF TJFE.
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, perhaps, exists for the conclusion that the greater or less development of hairs, is in part immediately due to increase or decrease of demand on their passive function, as non-conductors of heat; 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 the Lejndosteus, 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 q^^^ 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 of a kind which cannot react on the maternal organism, in such way as to cause the secretion of thicker shells for the future: to suppose that they can, is to suppose that the bird imder stands the cause of the evil, and that the secretion of thicker or thinner shells can be controlled bv its DIRECT EQUILliniATlON. 441 will. In tlio second place, such developing chicks as aro contained in the shells which crack or break, arc. almost certain to die; and cannot, therefore, acquire any appropriately-modified constitutions: even supposing any conceivable relation could be shown, 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 forces into correspondence with such incident forces, there are some re- adjustments which cannot be effected by the direct balancing of inner and outer actions. It is thus with the bones. The majority of the bones have to resist muscular strains; and it is a familiar fact that 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 a counter- acting force evoked by a force 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, difier 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 these 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.
-'Yr^ 442 THE EVOLUTION OF IJFE.
§ 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 on examination, that many of the environing changes to which organisms have to be adjusted, are not of these kinds: being changes 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 that act on them; and • as their environments change, the changes which plants and animals undergo, must necessarily be changes towards a 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.
§ 164. Besides those perturbations produced in the moving equilibrium of any organism by special disturbing forces, there are ever going on many other perturbations — some which are the still-reverberating effects of disturbing forces previously experienced by the individual, and others which are the still- reverberating effects of disturbing forces experienced by ancestral individuals; 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 fimction thrown on any organ, and extra growth produced in such organ^ without there being entailed correlative changes throughout all other functions, and eventually throughout all other organs. And when treating of Yariation (§ 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 compoimd deviations of structure, endlessly varied in their kinds and amounts. That is to say, besides the primary perturbations and deviations directly caused in organisms by altered actions in their environments, there are ever being indirectly caused, secondary and tertiary perturbations and deviations, which, when compounded with one another from generation to generation, work innumerable slight modifications in the moving equilibria and correlative structures throughout the species.
Now if the individuals of a species are thus necessarily made unlike, in countless ways and degrees — if the complicated sets of rhythms which we call their functions, though similar in their general characters^ are dissimilar in their details — if in one individual the amount of action in a particular direction is greater than in any other individual, or if here a peculiar combination gives a resulting force 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 of so violent a kind 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 sj)ecies present. It cannot but happen that some will be more stable than others, when exposed to this new or altered factor. That is to say, it cannot but happen that those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die; and that 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, 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 that are the least capable of maintaining their equilibria in presence of this new incident force, there must eventually be arrived at an altered t} pc completely in equilibrium with the altered conditions.
§ 165. This survival of the fittest, v/hich I have here IXDIRECr EQUILIIiRATION. 445 souglit to express in mcclianical terms, ia tliat whicli Mr Darwin has called " natural selection, or the preservation of favoured races in the struggle for life." That there is going on a process of this kind throughout the organic world, Mr Darwin's great work on the Origui of Species has shown to the satisfaction of nearly all naturalists. Indeed, when once enunciated, the truth of his hypothesis is so obvious as scarcely to need proof. Though evidence may be required to show that natural selection accounts for everything ascribed to it, yet no evidence is required to show that natural selection has always been going on, is going on now, and must ever continue to go on. Recognizing this as an a priori certainty, let us contemplate it under its two distinct aspects.
That organisms which live, thereby prove themselves fit to live, in so far as they have been tried; while organisms which die, thereby prove themselves in some respects unfitted for living; are facts no less manifest, than is the fact that this self-acting purification of a species, must tend ever to insure adaptation between it and its environment. This adaptation may be either so maintained or so produced. Doubtless many who have looked at Nature with philosophic eyes, have observed that death of the worst and multiplication of the best, must result in the maintenance of a constitution in harmony with surrounding circumstances. That the average vigour of any race would be diminished, did the diseased and feeble habitually survive and propagate; and that the destruction of such, through failure to fulfil some of the conditions to life, leaves behind those which are able to fulfil the conditions to life, and thus keeps up the average fitness to the conditions of life; are almost self-evident truths. But to recognize '' natural selection " as a means of preserving an already- established balance between the powers of a species and the forces to which it is subject, is to recognize it only in its simplest and most general mode of action. It is the more special mo^e of action with which we are here concerned. This more special mode of action, Mr Darwin has been the first to perceive. To him we owe the discovery that natural selection is capable of producing fitness between organisms and their circmnstances; and he, too, has the merit of appreciating the immensely-important consequences that follow from this. He has w^orked up an enormous mass of evidence into an elaborate demonstration, that this " preservation of favoured races in the struggle for life," is an ever-acting cause of divergence among organic forms.
He has traced out the involved results of the process with marvellous subtlety. He has show^n how hosts of otherwise inexplicable facts, are fully accounted for by it. In brief, he has proved that the cause he alleges is a true cause; that it is a cause which we see habitually in action; and that the results to be inferred from it, are in harmony with the phenomena which the Organic Creation presents, both as a whole and in its details. Let us glance at a few of the more important interpretations which the hypothesis furnishes.
A soil possessing some ingredient in unusual quantity, may supply to a plant an excess of the matter required for a certain class of its tissues; and may cause all the parts formed of such tissues to be abnormally developed. Suppose that among these are the hairs clothing its surfaces, including those which grow on its seeds. Thus furnished Avith somewhat longer fibres, its seeds, when shed, are carried a little further by the wind before they fall to the groimd. The young plants growdng up from them, being rather more widely dispersed than those produced by other individuals of the same species, will be less liable to smother one another; and a greater number may therefore reach maturity and fructify. Supposing the next generation subject to the same peculiarity of nutrition, some of the seeds borne by its members will not simply inherit this increased development of hairs, but will carry it further; and these, still more advantaged in the same way as before, will, on the average, have still more numerous chances of continuing the race. Thus, by the survival, generation after generation, of those possess- INDIRECT EQUIUHRATION. 447 ing these longer hairs, and the inheritance of successive increments of growth in the hairs, there may result a seed deviating greatly from the original. Other individuals of the same species, subject to the different physical conditions of other localities, may develop somewhat thicker or harder coatings to their seeds: so rendering their seeds less digestible by the birds that devour them. Such thicker-coated seeds, by escaping undigested more frequently than thinnercoated ones, will have additional chances of growing up and leaving offspring; and this process, acting in a cumulative manner through successive years, will produce a seed diverging in another direction from the ancestral type. Again, elsewhere, some modification in the physiologic actions of the plant, may lead to an unusual secretion of an essential oil in the seeds; which rendering them unpalatable to creatures that would otherwise feed on them, may diminish the destruction of the seeds, so giving an advantage to the variety in its rate of multiplication; and this incidental peculiarity proving a preservative, will, as before, be gradually increased by natural selection, until it constitutes another divergence.