It follows obviously from this, that if structure progresses from the homogeneous, indefinite, and incoherent, to the heterogeneous, definite, and coherent, so too must function. If the number of different parts in an aggregate must determine the number of differentiations produced in the energies passing through it—if the distinctness of these parts from one another, must involve distinctness in their reactions, and therefore distinctness between the divisions of the differentiated energy; there cannot but be a complete parallelism between the development of structure and the development of function. If structure advances from the simple and general to the complex and special, function must do the same.
WASTE AND REPAIR.
§ 62. Throughout the vegetal kingdom, the processes of Waste and Repair are comparatively insignificant in their amounts. Though all parts of plants save the leaves, or other parts which are green, give out carbonic acid; yet this carbonic acid, assuming it to indicate consumption of tissue, or rather of the protoplasm contained in the tissue, indicates but a small consumption. Of course if there is little waste there can be but little repair—that is, little of the interstitial repair which restores the integrity of parts worn by functional activity. Nor, indeed, is there displayed by plants in any considerable degree, if at all, that other species of repair which consists in the restoration of lost or injured organs. Torn leaves and the shoots that are shortened by the pruner, do not reproduce their missing parts; and though when the branch of a tree is cut off close to the trunk, the place is in course of years covered over, it is not by any reparative action in the wounded surface but by the lateral growth of the adjacent bark. Hence, without saying that Waste and Repair do not go on at all in plants, we may fitly pass them over as of no importance.
There are but slight indications of waste in those lower orders of animals which, by their comparative inactivity, show themselves least removed from vegetal life. Actiniæ kept in an aquarium, do not appreciably diminish in bulkfrom prolonged abstinence. Even fish, though much more active than most other aquatic creatures, appear to undergo but little loss of substance when kept unfed during considerable periods. Reptiles, too, maintaining no great temperature, and passing their lives mostly in a state of torpor, suffer but little diminution of mass by waste. When, however, we turn to those higher orders of animals which are active and hot-blooded, we see that waste is rapid: producing, when unchecked, a notable decrease in bulk and weight, ending very shortly in death. Besides finding that waste is inconsiderable in creatures which produce but little insensible and sensible motion, and that it becomes conspicuous in creatures which produce much insensible and sensible motion; we find that in the same creatures there is most waste when most motion is generated. This is clearly proved by hybernating animals. "Valentin found that the waking marmot excreted in the average 75 times more carbonic acid, and inhaled 41 times more oxygen than the same animal in the most complete state of hybernation. The stages between waking and most profound hybernation yielded intermediate figures. A waking hedgehog yielded about 20.5 times more carbonic acid, and consumed 18.4 times more oxygen than one in the state of hybernation."[22]If we take these quantities of absorbed oxygen and excreted carbonic acid, as indicating something like the relative amounts of consumed organic substance, we see that there is a striking contrast between the waste accompanying the ordinary state of activity, and the waste accompanying complete quiescence and reduced temperature. This difference is still more definitely shown by the fact, that the mean daily loss from starvation in rabbits and guinea-pigs, bears to that fromhybernation, the proportion of 18.3:1. Among men and domestic animals, the relation between degree of waste and amount of expended energy, though one respecting which there is little doubt, is less distinctly demonstrable; since waste is not allowed to go on uninterfered with. We have, however, in the lingering lives of invalids who are able to take scarcely any nutriment but are kept warm and still, an illustration of the extent to which waste diminishes as the expenditure of energy declines.
Besides the connexion between the waste of the organism as a whole and the production of sensible and insensible motion by the organism as a whole, there is a traceable connexion between the waste of special parts and the activities of such special parts. Experiments have shown that "the starving pigeon daily consumes in the average 40 times more muscular substance that the marmot in the state of torpor, and only 11 times more fat, 33 times more of the tissue of the alimentary canal, 18.3 times more liver, 15 times more lung, 5 times more skin." That is to say, in the hybernating animal the parts least consumed are the almost totally quiescent motor-organs, and the part most consumed is the hydro-carbonaceous deposit serving as a store of energy; whereas in the pigeon, similarly unsupplied with food but awake and active, the greatest loss takes place in the motor-organs. The relation between special activity and special waste, is illustrated, too, in the daily experiences of all: not indeed in the amount of decrease of the active parts in bulk or weight, for this we have no means of ascertaining; but in the diminished ability of such parts to perform their functions. That legs exerted for many hours in walking and arms long strained in rowing, lose their powers—that eyes become enfeebled by reading or writing without intermission—that concentrated attention, unbroken by rest, so prostrates the brain as to incapacitate it for thinking; are familiar truths. And though we have no direct evidence to this effect, there is little danger in concluding that muscles exerciseduntil they ache or become stiff, and nerves of sense rendered weary or obtuse by work, are organs so much wasted by action as to be partially incompetent.
Repair is everywhere and always making up for waste. Though the two processes vary in their relative rates both are constantly going on. Though during the active, waking state of an animal waste is in excess of repair, yet repair is in progress; and though during sleep repair is in excess of waste, yet some waste is necessitated by the carrying on of certain never-ceasing functions. The organs of these never-ceasing functions furnish, indeed, the most conclusive proofs of the simultaneity of repair and waste. Day and night the heart never stops beating, but only varies in the rapidity and vigour of its beats; and hence the loss of substance which its contractions from moment to moment entail, must from moment to moment be made good. Day and night the lungs dilate and collapse; and the muscles which make them do this must therefore be kept in a state of integrity by a repair which keeps pace with waste, or which alternately falls behind and gets in advance of it to a very slight extent.
On a survey of the facts we see, as we might expect to see, that the progress of repair is most rapid when activity is most reduced. Assuming that the organs which absorb and circulate nutriment are in proper order, the restoration of the body to a state of integrity, after the disintegration consequent on expenditure of energy, is proportionate to the diminution in expenditure of energy. Thus we all know that those who are in health, feel the greatest return of vigour after profound sleep—after complete cessation of motion. We know that a night during which the quiescence, bodily and mental, has been less decided, is usually not followed by that spontaneous overflow of energy which indicates a high state of efficiency throughout the organism. We know, again, that long-continued recumbency, even with wakefulness (providing the wakefulness is not the result of disorder), is followed by a certain renewal of strength; though arenewal less than that which would have followed the greater inactivity of slumber. We know, too, that when exhausted by labour, sitting brings a partial return of vigour. And we also know that after the violent exertion of running, a lapse into the less violent exertion of walking, results in a gradual disappearance of that prostration which the running produced. This series of illustrations conclusively proves that the rebuilding of the organism is ever making up for the pulling down of it caused by action; and that the effect of this rebuilding becomes more manifest, in proportion as the pulling down is less rapid. From each digested meal there is every few hours absorbed into the mass of prepared nutriment circulating through the body, a fresh supply of the needful organic compounds; and from the blood, thus occasionally re-enriched, the organs through which it passes are ever taking up materials to replace the materials used up in the discharge of functions. During activity the reintegration falls in arrear of the disintegration; until, as a consequence, there presently comes a general state of functional languor; ending, at length, in a quiescence which permits the reintegration to exceed the disintegration, and restore the parts to their state of integrity. Here, as wherever there are antagonistic actions, we see rhythmical divergences on opposite sides of the medium state—changes which equilibrate each other by their alternate excesses. (First Principles, §§ 85, 173.)
Illustrations are not wanting of special repair that is similarly ever in progress, and similarly has intervals during which it falls below waste and rises above it. Every one knows that a muscle, or a set of muscles, continuously strained, as by holding out a weight at arm's length, soon loses its power; and that it recovers its power more or less fully after a short rest. The several organs of the special sensations yield us like experiences. Strong tastes, powerful odours, loud sounds, temporarily unfit the nerves impressed by them for appreciating faint tastes, odours, or sounds; but theseincapacities are remedied by brief intervals of repose. Vision still better illustrates this simultaneity of waste and repair. Looking at the Sun so affects the eyes that, for a short time, they cannot perceive the things around with the usual clearness. After gazing at a bright light of a particular colour, we see, on turning the eyes to adjacent objects, an image of the complementary colour; showing that the retina has, for the moment, lost the power to feel small amounts of those rays which have strongly affected it. Such inabilities disappear in a few seconds or a few minutes, according to circumstances. And here, indeed, we are introduced to a conclusive proof that special repair is ever neutralizing special waste. For the rapidity with which the eyes recover their sensitiveness, varies with the reparative power of the individual. In youth the visual apparatus is so quickly restored to its state of integrity, that many of thesephotogenes, as they are called, cannot be perceived. When sitting on the far side of a room, and gazing out of the window against a light sky, a person who is debilitated by disease or advancing years, perceives, on transferring the gaze to the adjacent wall, a momentary negative image of the window—the sash-bars appearing light and the squares dark; but a young and healthy person has no such experience. With a rich blood and vigorous circulation, the repair of the visual nerves after impressions of moderate intensity, is nearly instantaneous.
Function carried to excess may produce waste so great that repair cannot make up for it during the ordinary daily periods of rest; and there may result incapacities of the over-taxed organs, lasting for considerable periods. We know that eyes strained by long-continued minute work lose their power for months or years: perhaps suffering an injury from which they never wholly recover. Brains, too, are often so unduly worked that permanent relaxation fails to restore them to vigour. Even of the motor organs the like holds. The most frequent cause of what is called "wasting palsy," or atrophy of the muscles, is habitual excess of exertion: theproof being that the disease occurs most frequently among those engaged in laborious handicrafts, and usually attacks first the muscles which have been most worked.
There has yet to be noticed another kind of repair—that, namely, by which injured or lost parts are restored. Among theHydrozoait is common for any portion of the body to reproduce the rest; even though the rest to be so reproduced is the greater part of the whole. In the more highly-organizedActinozoathe half of an individual will grow into a complete individual. Some of the lower Annelids, as theNais, may be cut into thirty or forty pieces and each piece will eventually become a perfect animal. As we ascend to higher forms we find this reparative power much diminished, though still considerable. The reproduction of a lost claw by a lobster or crab, is a familiar instance. Some of the inferiorVertebrataalso, as lizards, can develop new limbs or new tails, in place of those which have been cut off; and can even do this several times over, though with decreasing completeness. The highest animals, however, thus repair themselves to but a very small extent. Mammals and birds do it only in the healing of wounds; and very often but imperfectly even in this. For in muscular and glandular organs the tissues destroyed are not properly reproduced, but are replaced by tissue of an irregular kind which serves to hold the parts together. So that the power of reproducing lost parts is greatest where the organization is lowest; and almost disappears where the organization is highest. And though we cannot say that in the intermediate stages there is a constant inverse relation between reparative power and degree of organization; yet we may say that there is some approach to such a relation.
§ 63. There is an obvious and complete harmony between the first of the above inductions and the deduction which follows immediately from first principles. We have already seen (§ 23) "that whatever amount of power an organismexpends in any shape, is the correlate and equivalent of a power that was taken into it from without." Motion, sensible or insensible, generated by an organism, is insensible motion which was absorbed in producing certain chemical compounds appropriated by the organism under the form of food. As much energy as was required to raise the elements of these complex atoms to their state of unstable equilibrium, is given out in their falls to a state of stable equilibrium; and having fallen to a state of stable equilibrium they can give out no further energy, but have to be got rid of as inert and useless. It is an inevitable corollary "from the persistence of force, that each portion of mechanical or other energy which an organism exerts, implies the transformation of as much organic matter as contained this energy in a latent state;" and that this organic matter in yielding up its latent energy, loses its value for the purposes of life, and becomes waste matter needing to be excreted. The loss of these complex unstable substances must hence be proportionate to the quantity of expended force. Here, then, is the rationale of certain general facts lately indicated. Plants do not waste to any considerable degree, for the obvious reason that the sensible and insensible motions they generate are inconsiderable. Between the small waste, small activity, and low temperature of the inferior animals, the relation is similarly one admitting ofa prioriestablishment. Conversely, the rapid waste of energetic, hot-blooded animals might be foreseen with equal certainty. And not less manifestly necessary is the variation in waste which, in the same organism, attends the variation in the heat and mechanical motion produced.
Between the activity of a special part and the waste of that part, a like relation may be deductively inferred; though it cannot be inferred that this relation is equally definite. Were the activity of every organ quite independent of the activities of other organs, we might expect to trace out this relation distinctly; but since increased activity in any organor group of organs, as the muscles, necessarily entails increased activity in other organs, as in the heart, lungs, and nervous system, it is clear that special waste and general waste are too much entangled to admit of a definite relation being established between special waste and special activity. We may fairly say, however, that this relation is quite as manifest as we can reasonably anticipate.
§ 64. Deductive interpretation of the phenomena of Repair, is by no means so easy. The tendency displayed by an animal organism, as well as by each of its organs, to return to a state of integrity by the assimilation of new matter, when it has undergone the waste consequent on activity, is a tendency which is not manifestly deducible from first principles; though it appears to be in harmony with them. If in the blood there existed ready-formed units exactly like in kind to those of which each organ consists, the sorting of these units, ending in the union of each kind with already existing groups of the same kind, would be merely a good example of Segregation (First Principles, § 163). It would be analogous to the process by which, from a mixed solution of salts, there are, after an interval, deposited separate masses of these salts in the shape of different crystals. But as already said (§ 54), though the selective assimilation by which the repair of organs is effected, may result in part from an action of this kind, the facts cannot be thus wholly accounted for; since organs are in part made up of units which do not exist as such in the circulating fluids. We must suppose that, as suggested in§ 54, groups of compound units have a certain power of moulding adjacent fit materials into units of their own form. Let us see whether there is not reason to think such a power exists.
"The poison of small-pox or of scarlatina," remarks Mr. (now Sir James) Paget, "being once added to the blood, presently affects the composition of the whole: the disease pursues its course, and, if recovery ensue, the blood will seem to havereturned to its previous condition: yet it is not as it was before; for now the same poison may be added to it with impunity." ... "The change once effected, may be maintained through life. And herein seems to be a proof of the assimilative force in the blood: for there seems no other mode of explaining these cases than by admitting that the altered particles have the power of assimilating to themselves all those by which they are being replaced: in other words, all the blood that is formed after such a disease deviates from the natural composition, so far as to acquire the peculiarity engendered by the disease: it is formed according to the altered model." Now if the compound molecules of the blood, or of an organism considered in the aggregate, have the power of moulding into their own type the matters which they absorb as nutriment; and if they have the power when their type has been changed by disease, of moulding materials afterwards received into the modified type; may we not reasonably suspect that the more or less specialized molecules of each organ have, in like manner, the power of moulding the materials which the blood brings to them into similarly specialized molecules? The one conclusion seems to be a corollary from the other. Such a power cannot be claimed for the component units of the blood without being conceded to the component units of every tissue. Indeed the assertion of this power is little more than an assertion of the fact that organs composed of specialized unitsarecapable of resuming their structural integrity after they have been wasted by function. For if they do this, they must do it by forming from the materials brought to them, certain specialized units like in kind to those of which they are composed; and to say that they do this, is to say that their component units have the power of moulding fit materials into other units of the same order.
§ 65. What must we say of the ability an organism has to re-complete itself when one of its parts has been cut off?Is it of the same order as the ability of an injured crystal to re-complete itself. In either case new matter is so deposited as to restore the original outline. And if in the case of the crystal we say that the whole aggregate exerts over its parts a force which constrains the newly-integrated molecules to take a certain definite form, we seem obliged, in the case of the organism, to assume an analogous force. If when the leg of a lizard has been amputated there presently buds out the germ of a new one, which, passing through phases of development like those of the original leg, eventually assumes a like shape and structure, we assert only what we see, when we assert that the entire organism, or the adjacent part of it, exercises such power over the forming limb as makes it a repetition of its predecessor. If a leg is reproduced, where there was a leg, and a tail where there was a tail, there seems no alternative but to conclude that the forces around it control the formative processes going on in each part. And on contemplating these facts in connexion with various kindred ones, there is suggested the hypothesis, that the form of each species of organism is determined by a peculiarity in the constitution of its units—that these have a special structure in which they tend to arrange themselves; just as have the simpler units of inorganic matter. Let us glance at the evidences which more especially thrust this conclusion upon us.
A fragment of a Begonia-leaf imbedded in fit soil and kept at an appropriate temperature, will develop a young Begonia; and so small is the fragment which is thus capable of originating a complete plant, that something like a hundred plants may be produced from a single leaf. The friend to whom I owe this observation, tells me that various succulent plants have like powers of multiplication. Illustrating a similar power among animals, we have the often-cited experiments of Trembley on the common polype. Each of the four pieces into which one of these creatures was cut, grew into a perfect individual. In each of these, again, bisectionand tri-section were followed by like results. And so with their segments, similarly produced, until as many as fifty polypes had resulted from the original one. Bodies when cut off regenerated heads; heads regenerated bodies; and when a polype had been divided into as many pieces as was practicable, nearly every piece survived and became a complete animal. What, now, is the implication? We cannot say that in each portion of a Begonia-leaf, and in every fragment of a Hydra's body, there exists a ready-formed model of the entire organism. Even were there warrant for the doctrine that the germ of every organism contains the perfect organism in miniature, it still could not be contended that each considerable part of the perfect organism resulting from such a germ, contains another such miniature. Indeed the one hypothesis negatives the other. The implication seems, therefore, to be that the living particles composing one of these fragments, have an innate tendency to arrange themselves into the shape of the organism to which they belong. We must infer that the active units composing a plant or animal of any species have an intrinsic aptitude to aggregate into the form of that species. It seems difficult to conceive that this can be so; but we see that itisso. Groups of units taken from an organism (providing they are of a certain bulk and not much differentiated into special structures)havethis power of re-arranging themselves. Manifestly, too, if we are thus to interpret the reproduction of an organism from one of its amorphous fragments, we must thus interpret the reproduction of any minor portion of an organism by the remainder. When in place of its lost claw a lobster puts forth a cellular mass which, while increasing in bulk, assumes the form and structure of the original claw, we cannot avoid ascribing this result to a play of forces like that which moulds the materials contained in a piece of Begonia-leaf into the shape of a young Begonia.
§ 66. As we shall have frequent occasion hereafter to referto these units which possess the property of arranging themselves into the special structures of the organisms to which they belong; it will be well here to ask by what name they may be most fitly called.
On the one hand, it cannot be in those chemical compounds characterizing organic bodies that this specific property dwells. It cannot be that the molecules of albumin, or fibrin, or gelatine, or other proteid, possess this power of aggregating into these specific shapes; for in such case there would be nothing to account for the unlikenesses of different organisms. If the proclivities of proteid molecules determined the forms of the organisms built up of them or by them, the occurrence of such endlessly varied forms would be inexplicable. Hence what we may call thechemical unitsare clearly not the possessors of this property.
On the other hand, this property cannot reside in what may be roughly distinguished as themorphological units. The germ of every organism is a minute portion of encased protoplasm commonly called a cell. It is by multiplication of cells that all the early developmental changes are effected. The various tissues which successively arise in the unfolding organism, are primarily cellular; and in many of them the formation of cells continues to be, throughout life, the process by which repair is carried on. But though cells are so generally the ultimate visible components of organisms, that they may with some show of reason be called the morphological units; yet we cannot say that this tendency to aggregate into special forms dwells in them. In many cases a fibrous tissue arises out of a nucleated blastema, without cell-formation; and in such cases cells cannot be regarded as units possessing the structural proclivity. But the conclusive proof that the morphological units are not the building factors in an organism composed of them, is yielded by their independent homologues the so-called unicellular organisms. For each of these displays the power to assume its specific structure. Clearly, if the ability of a multicellular organismto assume its specific structure resulted from the cooperation of its component cells, then a single cell, or the independent homologue of a single cell, having no other to cooperate with, could exhibit no structural traits. Not only, however, do single-celled organisms exhibit structural traits, but these, even among the simplest, are so distinct as to originate classification into orders, genera, and species; and they are so constant as to remain the same from generation to generation.
If, then, this organic polarity (as we might figuratively call this proclivity towards a specific structural arrangement) can be possessed neither by the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may termphysiological. There seems no alternative but to suppose that the chemical units combine into units immensely more complex than themselves, complex as they are; and that in each organism the physiological units produced by this further compounding of highly compound molecules, have a more or less distinctive character. We must conclude that in each case some difference of composition in the units, or of arrangement in their components, leading to some difference in their mutual play of forces, produces a difference in the form which the aggregate of them assumes.
The facts contained in this chapter form but a small part of the evidence which thrusts this assumption upon us. We shall hereafter find various reasons for inferring that such physiological units exist, and that to their specific properties, more or less unlike in each plant and animal, various organic phenomena are due.
ADAPTATION.
§ 67. In plants waste and repair being scarcely appreciable, there are not likely to arise appreciable changes in the proportions of already-formed parts. The only divergences from the average structures of a species, which we may expect particular conditions to produce, are those producible by the action of these conditions on parts in course of formation; and such divergences we do find. We know that a tree which, standing alone in an exposed position, has a short and thick stem, has a tall and slender stem when it grows in a wood; and that also its branches then take a different inclination. We know that potato-sprouts which, on reaching the light, develop into foliage, will, in the absence of light, grow to a length of several feet without foliage. And every in-door plant furnishes proof that shoots and leaves, by habitually turning themselves to the light, exhibit a certain adaptation—an adaptation due, as we must suppose; to the special effects of the special conditions on the still growing parts. In animals, however, besides analogous structural changes wrought during the period of growth, by subjection to circumstances unlike the ordinary circumstances, there are structural changes similarly wrought after maturity has been reached. Organs that have arrived at their full sizes possess a certain modifiability; so that while theorganism as a whole retains pretty nearly the same bulk, the proportions of its parts may be considerably varied. Their variations, here treated of under the title Adaptation, depend on specialities of individual action. In the last chapter we saw that the actions of organisms entail re-actions on them; and that specialities of action entail specialities of re-action. Here it remains to be pointed out that these special actions and re-actions do not end with temporary changes, but work permanent changes.
If, in an adult animal, the waste and repair in all parts were exactly balanced—if each organ daily gained by nutrition exactly as much as it lost daily by the discharge of its function—if excess of function were followed only by such excess of nutrition as balanced the extra waste; it is clear that there would occur no change in the relative sizes of organs. But there is no such exact balance. If the excess of function, and consequent excess of waste, is moderate, it is not simply compensated by repair but more than compensated—there is a certain increase of bulk. This is true to some degree of the organism as a whole, when the organism is framed for activity. A considerable waste giving considerable power of assimilation, is more favourable to accumulation of tissue than is quiescence with its comparatively feeble assimilation: whence results a certain adaptation of the whole organism to its requirements. But it is more especially true of the parts of an organism in relation to one another. The illustrations fall into several groups. The growth of muscles exercised to an unusual degree is a matter of common observation. In the often-cited blacksmith's arm, the dancer's legs and the jockey's crural adductors, we have marked examples of a modifiability which almost every one has to some extent experienced. It is needless to multiply proofs. The occurrence of changes in the structure of the skin, where the skin is exposed to unusual stress of function, is also familiar. That thickening of the epidermis on a labourer's palm results from continual pressure andfriction, is certain. Those who have not before exerted their hands, find that such an exercise as rowing soon begins to produce a like thickening. This relation of cause and effect is still better shown by the marked indurations at the ends of a violinist's fingers. Even in mucous membrane, which ordinarily is not subject to mechanical forces of any intensity, similar modifications are possible: witness the callosity of the gums which arises in those who have lost their teeth, and have to masticate without teeth. The vascular system furnishes good instances of the increased growth that follows increased function. When, because of some permanent obstruction to the circulation, the heart has to exert a greater contractile force on the mass of blood which it propels at each pulsation, and when there results the laboured action known as palpitation, there usually occurs dilatation, or hypertrophy, or a mixture of the two: the dilatation, which is a yielding of the heart's structure under the increased strain, implying a failure to meet the emergency; but the hypertrophy, which consists in a thickening of the heart's muscular walls, being an adaptation of it to the additional effort required. Again, when an aneurism in some considerable artery has been obliterated, either artifically or by a natural inflammatory process; and when this artery has consequently ceased to be a channel for the blood; some of the adjacent arteries which anastomose with it become enlarged, so as to carry the needful quantity of blood to the parts supplied. Though we have no direct proof of analogous modifications in nervous structures, yet indirect proof is given by the greater efficiency that follows greater activity. This is manifested alike in the senses and the intellect. The palate may be cultivated into extreme sensitiveness, as in professional tea-tasters. An orchestral conductor gains, by continual practice, an unusually great ability to discriminate differences of sound. In the finger-reading of the blind we have evidence that the sense of touch may be brought by exercise to a far higher capability than isordinary.[23]The increase of power which habitual exertion gives to mental faculties needs no illustration: every person of education has personal experience of it. Even from the osseous structures evidence may be drawn. The bones of men accustomed to great muscular action are more massive, and have more strongly marked processes for the attachment of muscles, than the bones of men who lead sedentary lives; and a like contrast holds between the bones of wild and tame animals of the same species. Adaptations of another order, in which there is a qualitative rather than a quantitative modification, arise after certain accidents to which the skeleton is liable. When the hip-joint has been dislocated, and long delay has made it impossible to restore the parts to their proper places, the head of the thigh-bone, imbedded in the surrounding muscles, becomes fixed in its new position by attachments of fibrous tissue, which afford support enough to permit a halting walk. But the most remarkable modification of this order occurs in united ends of fractured bones. "False joints" are often formed—joints which rudely simulate the hinge structure or the ball-and-socket structure, according as the muscles tend to produce a motion of flexion and extension or a motion of rotation. In the one case, according to Rokitansky, the two ends of the broken bone become smooth and covered with periosteum and fibrous tissue, and are attached by ligaments that allow a certain backward and forward motion; and in the other case the ends, similarly clothed with the appropriate membranes, become the one convex and the other concave, are inclosed in a capsule, and are even occasionally supplied with synovial fluid!
The general truth that extra function is followed by extra growth, must be supplemented by the equally general truth,that beyond a limit, usually soon reached, very little, if any, further modification can be produced. The experiences which we colligate into the one induction thrust the other upon us. After a time no training makes the pugilist or the athlete any stronger. The adult gymnast at last acquires the power to perform certain difficult feats; but certain more difficult feats no additional practice enables him to perform. Years of discipline give the singer a particular loudness and range of voice, beyond which further discipline does not give greater loudness or wider range: on the contrary, increased vocal exercise, causing a waste in excess of repair, is often followed by decrease of power. In the exaltation of the perceptions we see similar limits. The culture which raises the susceptibility of the ear to the intervals and harmonies of notes, will not turn a bad ear into a good one. Lifelong effort fails to make this artist a correct draftsman or that a fine colourist: each does better than he did at first, but each falls short of the power attained by some other artists. Nor is this truth less clearly illustrated among the more complex mental powers. A man may have a mathematical faculty, a poetical faculty, or an oratorical faculty, which special education improves to a certain extent. But unless he is unusually endowed in one of those directions, no amount of education will make him a first-rate mathematician, a first-rate poet, or a first-rate orator. Thus the general fact appears to be that while in each individual certain changes in the proportions of parts may be caused by variations of functions, the congenital structure of each individual puts a limit to the modifiability of every part. Nor is this true of individuals only: it holds, in a sense, of species. Leaving open the question whether, in indefinite times, indefinite modifications may not be produced by inheritance of functionally wrought adaptations; experience proves that within assigned times, the changes wrought in races of organisms by changes of conditions fall within narrow limits. Though by discipline, aided byselective breeding, one variety of horse has had its locomotive power increased considerably beyond the locomotive powers of other varieties; yet further increase takes place, if at all, at an inappreciable rate. The different kinds of dogs, too, in which different forms and capacities have been established, do not now show aptitudes for diverging in the same directions at considerable rates. In domestic animals generally, certain accessions of intelligence have been produced by culture; but accessions beyond these are inconspicuous. It seems that in each species of organism there is a margin for functional oscillations on all sides of a mean state, and a consequent margin for structural variations; that it is possible rapidly to push functional and structural changes towards the extreme of this margin in any direction, both in an individual and in a race; but that to push these changes further in any direction, and so to alter the organism as to bring its mean state up to the extreme of the margin in that direction, is a comparatively slow process.[24]
We also have to note that the limited increase of size produced in any organ by a limited increase of its function, is not maintained unless the increase of function is permanent. A mature man or other animal, led by circumstances into exerting particular members in unusual degrees, and acquiring extra sizes in these members, begins to lose such extra sizes on ceasing to exert the members; and eventually lapses more or less nearly into the original state. Legs strengthened by a pedestrian tour, become relatively weak again after a prolonged return to sedentary life. The acquired ability to perform feats of skill disappears in course of time, if the performance of them be given up. For comparative failure in executing a piece of music, in playing a game at chess, or in anything requiring special culture, the being out of practiceis a reason which every one recognizes as valid. It is observable, too, that the rapidity and completeness with which an artificial power is lost, is proportionate to the shortness of the cultivation which evoked it. One who has for many years persevered in habits which exercise special muscles or special faculties of mind, retains the extra capacity produced, to a very considerable degree, even after a long period of desistance; but one who has persevered in such habits for but a short time has, at the end of a like period, scarcely any of the facility he had gained. Here too, as before, successions of organisms present an analogous fact. A species in which domestication continued through many generations, has organized certain peculiarities; and which afterwards, escaping domestic discipline, returns to something like its original habits; soon loses, in great measure, such peculiarities. Though it is not true, as alleged, that it resumes completely the structure it had before domestication, yet it approximates to that structure. The Dingo, or wild dog of Australia, is one of the instances given of this; and the wild horse of South America is another. Mankind, too, supplies us with instances. In the Australian bush and in the backwoods of America, the Anglo-Saxon race, in which civilization has developed the higher feelings to a considerable degree, rapidly lapses into comparative barbarism: adopting the moral code, and sometimes the habits, of savages.
§ 68. It is important to reach, if possible, some rationale of these general truths—especially of the last two. A right understanding of these laws of organic modification underlies a right understanding of the great question of species. While, as before hinted (§ 40), the action of structure on function is one of the factors in that process of differentiation by which unlike forms of plants and animals are produced, the reaction of function on structure is another factor. Hence, it is well worth while inquiring how far these inductions are deductively interpretable.
The first of them is the most difficult to deal with. Why an organ exerted somewhat beyond its wont should presently grow, and thus meet increase of demand by increase of supply, is not obvious. We know, indeed, (First Principles, §§ 85, 173,) that of necessity, the rhythmical changes produced by antagonistic organic actions cannot any of them be carried to an excess in one direction, without there being produced an equivalent excess in the opposite direction. It is a corollary from the persistence of force, that any deviation effected by a disturbing cause, acting on some member of a moving equilibrium, must (unless it altogether destroys the moving equilibrium) be eventually followed by a compensating deviation. Hence, that excess of repair should succeed excess of waste, is to be expected. But how happens the mean state of the organ to be changed? If daily extra waste naturally brings about daily extra repair only to an equivalent extent, the mean state of the organ should remain constant. How then comes the organ to augment in size and power?
Such answer to this question as we may hope to find, must be looked for in the effects wrought on the organism as a whole by increased function in one of its parts. For since the discharge of its function by any part is possible only on condition that those various other functions on which its own is immediately dependent are also discharged, it follows that excess in its function presupposes some excess in their functions. Additional work given to a muscle implies additional work given to the branch arteries which bring it blood, and additional work, smaller in proportion, to the arteries from which these branch arteries come. Similarly, the smaller and larger veins which take away the blood, as well as those structures which deal with effete products, must have more to do. And yet further, on the nervous centres which excite the muscle a certain extra duty must fall. But excess of waste will entail excess of repair, in these parts as well as in the muscle. The several appliances by which the nutritionand excitation of an organ are carried on, must also be influenced by this rhythm of action and reaction; and therefore, after losing more than usual by the destructive process they must gain more than usual by the constructive process. But temporarily-increased efficiency in these appliances by which blood and nervous force are brought to an organ, will cause extra assimilation in the organ, beyond that required to balance its extra expenditure. Regarding the functions as constituting a moving equilibrium, we may say that divergence of any function in the direction of increase, causes the functions with which it is bound up to diverge in the same direction; that these, again, cause the functions which they are bound up with, also to diverge in the same direction; and that these divergences of the connected functions allow the specially-affected function to be carried further in this direction than it could otherwise be—further than the perturbing force could carry it if it had a fixed basis.
It must be admitted that this is but a vague explanation. Among actions so involved as these, we can scarcely expect to do more than dimly discern a harmony with first principles. That the facts are to be interpreted in some such way, may, however, be inferred from the circumstance that an extra supply of blood continues for some time to be sent to an organ that has been unusually exercised; and that when unusual exercise is long continued a permanent increase of vascularity results.
§ 69. Answers to the questions—Why do these adaptive modifications in an individual animal soon reach a limit? and why, in the descendants of such animal, similarly conditioned, is this limit very slowly extended?—are to be found in the same direction as was the answer to the last question. And here the connexion of cause and consequence is more manifest.
Since the function of any organ is dependent on the functions of the organs which supply it with materials andstimuli; and since the functions of these subsidiary organs are dependent on the functions of organs which supply them with materials and stimuli; it follows that before any great extra power of discharging its function can be gained by a specially-exercised organ, a considerable extra power must be gained by a series of immediately-subservient organs, and some extra power by a secondary series of remotely-subservient organs. Thus there are required numerous and wide-spread modifications. Before the artery which feeds a hard-worked muscle can permanently furnish a large additional quantity of blood, it must increase in diameter; and that its increase of diameter may be of use, the main artery from which it diverges must also be so far modified as to bring this additional quantity of blood to the branch artery. Similarly with the veins; similarly with the structures which remove waste-products; similarly with the nerves. And when we ask what these subsidiary changes imply, we are forced to conclude that there must be an analogous group of more numerous changes ramifying throughout the system. The growth of the arteries primarily and secondarily implicated, cannot go to any extent without growth in the minor blood-vessels on which their nutrition depends; while their greater contractile power involves enlargement of the nerves which excite them, and some modification of that part of the spinal cord whence these nerves proceed. Thus, without tracing the like remote alterations implied by extra growth of the veins, lymphatics, glandular organs, and other agencies, it is manifest that a large amount of rebuilding must be done throughout the organism, before any organ of importance can be permanently increased in size and power to a great extent. Hence, though such extra growth in any part as does not necessitate considerable changes throughout the rest of the organism, may rapidly take place; a further growth in this part, requiring a re-modelling of numerous parts remotely and slightly affected, must take place but slowly.