§ 166. But now, though it seems to me that we are thus supplied with a key to phenomena which are multitudinous and varied beyond all conception; it also seems to me that there is a moiety of the phenomena which this key wdll not unlock. Mr Darwin himself recognizes use and disuse of parts, as causes of modifications in organisms; and does this, indeed, to a greater extent than do some who accept his general conclusion. But I conceive that he does not re- cognize them to a sufficient extent. While he conclusively shows that the inheritance of changes of structure, caused by changes of function, is utterly insufficient to explain a great mass — probably the greater mass — of morphological pheno- mena; I think he leaves unconsidered a mass of morphological phenomena that are explicable as results of functionally- acquired modifications, transmitted and increased, and which are not explicable as results of natural selection.
By induction, as well as by inference from the hypothesis 29 450 THE EVOLUTION OF LIFE.
of natural selection, we know that there exists a balance among the powers of organs which habitually act together — such proportions among them, that no one has any consider- able excess of efficiency. We see, for example, that through- out the vascular sj^stem, there is maintained an equilibrium between the powers, that is, the developments, of the com- ponent parts: in some cases, under excessive exertion, the heart gives way, and we have enlargement; in other cases the large arteries give way, and we have aneurisms; in other cases the minute blood-vessels give way — now bursting, now becoming chronically congested. That is to say, in the average constitution, no superfluous strength is possessed by any of the appliances for circulating the blood. Take, again, a set of motor organs. Great strain here causes the fibres of a muscle to tear. There the muscle does not yield but the tendon snaps. Elsewhere neither muscle nor tendon is damaged, but the bone breaks. Joining with these instances the general fact, that under the same adverse conditions, difierent individuals show their slight differences of consti- tution by going wrong some in one way and some in an- other; and that even in the same individual, similar adverse conditions will now affect one viscus and now another; it becomes manifest that though there cannot be maintained an accurate or absolute balance among the powers of the organs composing an organism, yet the excesses and de- ficiencies of power are extremely slight. That they must be extremely slight, is, as before said, a deduction from the hypothesis of natural selection. Mr Darwin himself argues " that natural selection is continually trying to economize in every part of the organization. If imder changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless struc- ture." In other words, if any muscle has more fibres than can be utilized, or if a bone be stronger than needful, no ad- INDIRECT EQUILIBRATION. 451 vantage results, but rather a disadvantage — a disadvantage wliich will decrease the chances of survival. Hence it becomes a cgrollary, that among any organs which habit- ually act in concert, an increase of one can be of no service unless there is a concomitant increase of the rest. The co- operative parts must vary together; otherwise variation will be detrimental. A stronger muscle must have a stronger bone to resist its contractions; must have stronger correlated mus- cles and ligaments to secure the neighbouring articulations; must have larger blood-vessels to bring it supplies; must have a more massive nerve to bring it stimulus, and some extra development of a nervous centre to supply this extra stimulus. The question arises, then, — does spontaneous variation occur simultaneously in all these co-operative parts? Have we any reason to think that they spontaneously increase or decrease together? The assumption that they do, seems to me untenable; and its untenability will, I think, become conspicuous if we take a case, and observe how ex- tremely numerous and involved are the variations which must be supposed to occur together. In illustration of another point, we have already considered the modifica- tion required to accompany increased weight of the head. Instead of the bison, however, the moose deer, or the extinct Irish elk, will here best serve our purpose. In this species the male has enormously-developed horns, which are used for purposes of offence and defence. These horns, weighing up- wards of a hundred- weight, are carried at great mechanical disadvantage — supported as they are along with the massive skull which bears them, at the extremity of the outstretched neck. Further, that these heavy horns may be of use in fighting, the supporting bones and muscles must be strong enough, not simply to carry them, but to put them in •motion with the rapidity required for giving blows. Let us, then, ask how, by natural selection, this complex apparatus of bones and muscles can have been developed, pari j^nssu with the horns? If we suppose the horns to have originally 452 THE EVOLUTION OF LIFE.
been of Kke size with those borne by other kinds of deer; and if we suppose that in certain individuals, they became larger by spontaneous variation; what would be the con- comitant changes required to render their greater size useful? Other things equal,, the blow given by a larger horn would be a blow given by a heavier mass moving at a smaller velocity: the momentum would be the same as before; and the area of contact with the body struck being somewhat increased, while the velocity was decreased, the injury done would be less. That the horns may become better weapons, the whole apparatus which moves them must be so strength- ened as to impress more force on them, and to bear the more violent reactions of the blows given. The bones of the skull on which the horns are seated must be thickened; otherwise they will break. To render the thickening of these bones advantageous, the vertebraB of the neck must be further de- veloped; and without the ligaments that hold together these vertebrae, and the muscles which move them, are also enlarged, nothing will be gained. Such modifications of the neck will be useless, or rather will be detrimental, if its fulcrum be not made capable of resisting intenser strains: the upper dorsal vertebrae and their spines must be strengthened, that they may withstand the more violent contractions of the neck- muscles; and like changes must be made on the scapular arch. Still more must there be required a simultaneous de- velopment of the bones and muscles of the fore-legs; since each of these extra growths in the horns, in the skull, in the neck, in the shoulders, adds to the burden which the fore- legs have to bear; unless this deer with its heavier horns, head, neck, and shoulders^ had stronger fore-legs, it would not only suffer from loss of speed but would even fail in fight. Hence, to make larger horns of use, additipnal sizes must be acquired by numerous bones, muscles, and ligaments, as welL as by the blood-vessels and nerves on which their actions depend. On calling to mind how the spraining of a single small muscle in the foot, incapacitates for walking, or how a permanent weakness in one of its ligaments will greatly diminish the power of a limb, it will be seen that unless all these many changes are simultaneously made, they may as well be none of them made — or rather, they had better be none of them made; since, the enlargements of some parts, by putting greater strains on connected parts, would render them relatively weaker if they remained unenlarged. Thus, then, to account by the hypothesis of natural selection, for such a structure as that of the moose deer, or the extinct Irish elk, we must suppose a spontaneous increase in the size of the horns, to be accompanied by a spontaneous increase in each of these numerous bones and muscles and ligaments directly and indirectly implicated in the use of the horns. Can we with any propriety do this? I think not. It would be a strong supposition that the vertebrae and muscles of the neck, spontaneously enlarged at the same time as the horns. It would be a still stronger supposition that the upper dorsal vertebrae not only at the same time spontaneously became more massive, but also spontaneously altered their pro- portions in appropriate ways, by the development of their immense neural spines. And it would be an assumption still more straining our powers of belief, that along with heavier horns there should spontaneously take place the re- quired strengthenings of the scapular arch and the fore-legs. Besides the multiplication of directly-cooperative organs, the multiplication of organs that do not cooperate, save in the degree implied by their combination in the same organ- ism, seems to me a further hindrance to the development of special structures by natural selection alone. Where the life is comparatively simple, or where surrounding circumstances render some one function supremely important, the survival of the fittest may readily bring about the appropriate struc- tural change, without any aid from the transmission of func- tionally-acquired modifications. But in proportion as the life grows complex — in proportion as a healthy existence cannot be secured by a large endow^ment of some one power, 454 THE EVOLUTION OF LIFE.
but demands many powers; in the same proportion do there arise obstacles to the increase of any particular power, by "the preservation of favoured races in the struggle for life." As fast as the faculties are multiplied, so fast does it become possible for the several members of a species to have various kinds of superiorities over one another. While one saves its life by higher speed, another does the like by clearer vision, another by keener scent, another by quicker hearing, another by greater strength, another by unusual power of enduring cold or hunger, another by special sagacity, another by special timidity, another by special courage; and others by other bodily and mental attributes. Now it is unquestionably true that, other things equal, each of these attributes, giving its possessor an extra chance of life, is likely to be transmitted to posterity. But there seems no reason to suppose that it will be increased in subsequent generations by natural selection. That it may be thus increased, the individuals not possess- ing more than average endow^ments of it, must be more fre- quently killed off than individuals highly endowed with it; and this can happen only when the attribute is one of greater importance, for the time being, than most of the other attributes. If those members of the species which have but ordinary shares of it, nevertheless survive by virtue of other superiorities which they severally possess; then it is not easy to see how this particular attribute can be developed by natural selection in subsequent generations. The probability seems rather to be, that by gamogenesis, this extra endow- ment will, on the average, be diminished in posterity — just serving in the long run to compensate the deficient endow- ments of other individuals, whose special powers lie in other directions; and so to keep up the normal structure of the species. The working out of the process is here somewhat difficult to follow; but it appears to me that as fast as the number of bodily and mental faculties increases, and as fast as the maintenance of life comes to depend less on the amount of any one, and more on the combined action of all; so INDIRECT EQUILIBRATION. 455 fast does the production of specialities of character by natural selection alone, become difficult. Particularly docs this seem to be so with a species so multitudinous in its powers as mankind; and above all does it seem to be so with such of the human powers as have but minor shares in aiding the struggle for life — the aesthetic faculties, for example.
It by no means follows, however, that in cases of this kind, and cases of the preceding kind, natural selection plays no part. Wherever it is not the chief agent in working organic changes, it is still, very generally, a secondary agent. The survival of the fittest must nearly always further the produc- tion of modifications which produce fitness; whether they be modifications that have arisen incidentally, or modifications that have been caused by direct adaptation. Evidently, those individuals whose constitutions or circumstances have facili- tated the production in them of any structural change con- sequent on any functional change demanded by some new external condition, will be the individuals most likely to live and to leave descendants. There must be a natural selection of functionally-acquired peculiarities, as well as of incidental peculiarities; and hence such structural changes in a species as result from changes of habit necessitated by changed cir- cumstances, natural selection will render more rapid than they would otherwise be.
There are, however, some modifications in the sizes and forms of parts, which cannot have been aided by natural selection; but which must have resulted wholly from the inheritance of functionally-produced alterations. The dwind- ling away of organs of which the undue sizes entail no appreciable evils, furnishes the best evidence of this. Take, for an example, that diminution of the jaws and teeth which characterizes the civilized races, as contrasted with the savage races.* How can the civilized races have been bene- * I am indebted to Mv Flower for the opportunity of examining the collection of skulls in the Museum of the College of Surgeons for verification of this. Un- 456 THE EVOLUTION OF LIFE.
fited in the struggle for life, by the slight decrease in these comparatively- small bones? No functional superiority possessed by a small jaw over a large jaw, in ci^dlized life, can be named as having caused the more frequent survival of small-jaw^ed individuals. The only advantage which smallness of jaw might be supposed to give, is the advantage of economized nutrition; and this could not be great enough to further the preservation of men possessing it. The de- crease of weight in the jaw and co-operative parts, that has arisen in the course of many thousands of years, does not amount to more than a few ounces. This decrease has to be divided among the many generations that have lived and died in the interval. Let us admit that the weight of these parts diminished to the extent of an ounce in a single gener- ation (which is a large admission); it still cannot be con- tended that the having to carry an ounce less in weight, or the having to keep in repair an ounce less of tissue, could sensibly affect any man^s fate. And if it never did this — nay, if it did not cause O: frequent survival of small-jawed in- dividuals where large-jawed individuals died; natural selec- tion could neither cause nor aid diminution of the jaw and fortunately the absence, in most cases, of some or many teeth, prevented me from arriving at that specific result which would have been given by weighing a number of the under jaws in each race. Simple inspection, however, disclosed a sufficiently-conspicuous difference. The under jaws of Australians and Negroes, when placed side by side with those of Englishmen, were visibly larger, not only relatively but absolutely. One Australian jaw only, did I observe, that was about of the same actual size as an average English jaw; and this (probably the jaw of a woman) belonging as it did to a much smaller skull, bore a much greater ratio to the whole body of which it formed part, than did an English jaw of the same actual size. In all the other cases, the under jaws of these inferior races (con- taining larger teeth than our own) were absolutehj more massive than our own — often exceeding them in all dimensions; and relatively to the smaller skeletons of these inferior races, they were very much more massive. Let me add that the Australian and Negro jaws are thus strongly contrasted, not with all British jaws, but only with the jaws of the civilized British. An ancient British skull in the collection, possesses a jaw almost or quite as massive as those of the Australian skulls. And this is in harmony with the alleged relation between greater size of jaws and greater action of jaws, involved by the habits of savages.
INDIRECT EQTTILIBRATION. 457 its appendages. Here, therefore, the decreased action of these parts which has accom2)anied the growth of civilized habits (the use of tools and the disuse of coarse food), must have been the sole cause at work. During civilization this decrease of function has affected, more or less, all individuals. Through direct equilibration, diminished external stress on these parts, has resulted in diminution of the internal forces by which this stress is met. From generation to generation, this lessening of the parts consequent on functional decline has been inherited. And since the survival of individuals must always have been determined by more important struc- tural traits, this trait can have neither been facilitated nor retarded by natural selection.
§ 167. Returning from these extensive classes of facts for ■which Mr Darwin's hypothesis does not accoimt, 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 that are in equilibrium with their environments; and at the outset of this chapter, some- thing was done tow^ards showing how this continual survival of the fittest, may be understood as the progressive estab- lishment of a balance between inner and outer forces. Here, however, we must consider the matter more closely. It re- mains to be shown that this process conforms to the same general mechanical principles as do all other equilibrations.
On previous occasions we have contemplated the assem- blage 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 force which is antagonized by other forces; and that through the rhyth- 458 THE EVOLUTION OF LIFE.
mical variations in these two sets of forces, there is main- tained 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 direc- tions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhythmical functions thus maintained, con- stitutes the life of the organism; so, in a species, there is through gamogenesis a perpetual neutralization of those con- trary 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 the way in which a species is affected by causes which destroy some of its units and favour the multi- plication of others, we must consider it as a whole whose units are held together by complex forces that are ever balancing themselves and ever being disturbed — a whole whose moving equilibrium is continually being modified, and through which waves of perturbation are continually being pro- pagated. Thus much premised, let us next call to mind in what way moving equilibria in general are changed. In the first place, the necessary effect wrought by a new in- cident force falling on any part of an aggregate with balanced motions, is to produce 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 that comiter-balances the original de- viation. Consequently, to consider whether the moving equi- librium of a species is modified in the same way as moving INDIllKCrr EQUILIBRATION. 459 equilibria in general, is to consider whether, when exposed to a new force, a species yields in the direction of least resist- ance; and whether, by its thus yielding, there is generated in the species a compensating change in the opposite direc- tion. We shall find tliat 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 them- selves? The disappearance of those individuals which meet the destroying forces by the smallest defensive forces, is tan- tamount 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 any increase of certain external actions that are ever tending to overthrow their equilibria, and which they are ever counter-balancing by the absoi'p- tion of nutriment, which are the first to die? Those that are least able to generate the internal actions which antagon- ize these external actions. 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, fur- nished 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 its over- throw by the modified aggregate of external forces. Con- versely, the members that die, are, other things equal, those deficient in the power of meeting the new action by an equi- valent counter-action. Thus, in all cases, a species con- sidered 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 the relation to the special forces acting on it. The conclusion is, indeed, a truism. Eut now, what must follow from the de- 460 THE EVOLUTION OF LIFE.
struction 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 gener- ation, the effect of this deviation from the mean state is to produce a compensating deviation. For if all such as are de- ficient of power in a certain direction are destroyed, what must be the influence on posterity? Had those which are destroj^ed lived and left offspring, the next generation would have had the same average balance of powers as preceding generations: there would have been a like proportion of in- dividuals less endowed with this power, and individuals more endowed with this power. 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 con- stituted by 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 altera- tion of some external factor is so great that it leaves alive only a few individuals, characterized by extreme endowments of the power required to antagonize it; then, in succeeding generations, there is a rapid multiplication of individuals similarly characterized by 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 agency which proves fatal to them has not continued in action, such less-endowed individuals will multiply; and the species, after sundry oscillations, will return to its pre\dous mean state. But if this agency be a persistent one, such less