certain other differentiated functions. Merchants, manufacturers, wholesale distributors of their several species, together with lawyers, bankers, &c., all employ clerks. In clerks we have a specialized class dispersed through various other classes; and having its function fused with the different functions of these various other classes. Similarly commercial travellers, though having in one sense a separate occupation, have in another sense an occupation forming part of each of the many occupations which it aids. As it is here with the sociological division of labour, so is it with the physiological division of labour above described. Just as we see in an advanced community, that while the magisterial, the clerical, the medical, the legal, the manufacturing, and the commercial activities, have grown distinct, they have yet their agencies mingled together in every locality; so in a developed organism, we see that while the general functions of circulation, secretion, absorption, excretion, contraction, excitation, &c., have become differentiated, yet through the ramifications of the systems apportioned to them, they are closely combined with each other in every organ.
§ 60. The physiological division of labour, is usually not carried so far as wholly to destroy the primary physiological community of labour. As in societies the adaptation of special classes to special duties, does not entirely disable these classes from performing each others' duties on an emergency; so in organisms, tissues and structures that have become fitted to the particular ofiices they have ordinarily to discharge, often remain partially able to discharge other offices. It has been pointed out by Dr Carpenter, that " in cases where the different functions are highly specialized, the general structure retains, more or less, the primitive community of function which originally characterized it." A few instances will bring home this generalization.
The roots and leaves of plants are widely differentiated in their functions: by the roots, water and mineral substances are absorbed; while the leaves take in, and decompose, carbonic acid. Nevertheless, leaves retain a considerable power of absorbing water; and in what are popularly called '' air-plants," the absorption of water is wholly carried on by them and by the stems. Conversely, the underground parts can partially assume the functions of leaves: the exposed tuber of a potato develops chlorophyll on its surface, and in other cases, roots, properly so called, do the like. In trees, the trunks, which have in great measure ceased to produce buds, recommence producing them if the branches are cut off; and under such circumstances the roots, though not in the habit of developing leaf-bearing organs, send up numerous suckers. Much more various examples of vicarious function may be found among animals. Starting with the extreme case of the common h^^dra, which can live when the duties of skin and stomach have been interchanged by turning it inside out, we find in all grades, even up to the highest, that absorbent and excreting organs can partially supply each others' places. Among well-organized animals, the taking in of nutriment is effected exclusively by an internal membrane; but the external membrane is not wholly without the power to take in nutriment: when food cannot be swallowed, life may be prolonged by immersing the body in nutritive fluids. The excretion of carbonic acid and absorption of oxygen, are mainly performed by the lungs, in creatures which have lungs; but in such creatures there continues a certain amount of cutaneous respiration, and in soft-skinned batrachians like the frog, this cutaneous respiration is important. Again, when the kidneys are not discharging their duties, a notable quantity of urea is got rid of by perspiration. Other instances are supplied by the higher functions. In man, the limbs, which among lower vertebrates are almost wholly organs of locomotion, are specialized into organs of locomotion and organs of manipulation. Nevertheless, the human arras and legs do, when needful, fulfil, to some extent, each Others' offices. JSTot only in childhood and old age are the arms used for purposes of support, but on occasions of emergency, as when mountaineering, they are so used by men in full vigour. And that legs are to a considerable degree capable of performing the duties of arms, is proved by the great amount of manipulatory skill reached by them when the arms are absent. Among the perceptions, too, there are examples of partial substitution. The deaf Dr Kitto described himself as having become excessively sensitive to vibrations propagated through the body; and as so having gained the power of perceiving, through his general sensations, those neighbouring concussions of which the ears ordinarily give notice. Blind people make hearing perform, in part, the office of vision. Instead of identifying the positions and sizes of neighbouring objects by the reflection of light from their surfaces, they do this in a rude way by the reflection of sound from their surfaces.
We see, as we might expect to see, that this power of performing more general functions, is great in proportion as the parts have been but little adapted to their special functions. In the hydra, where complete transposition of functions is possible, the histological differentiation that has been established, is extremely slight, or even inappreciable. Those parts of plants which show so considerable a power of discharging each others' offices, are not widely unlike in their minute structures. And the tissues that in animals are to some extent mutually vicarious, are tissues in which the original cellular composition is still conspicuous. But we do not find evidence that the muscular, nervous, or osseous tissues are able in any degree to perform those processes which the less differentiated tissues perform. Nor have we any proof that nerve can partially fulfil the duty of muscle, or muscle that of nerve. We must sa^^, therefore, that the ability to resume the primordial community of function, FUNCTION. 1G7 varies inversely as tlie established specialization of function; and that it disappears when the specialization of function becomes greats.
§ 61. Something approaching to a priori reasons may bo given for the conclusions thus reached a posteriori. They must be accepted for as much as they seem worth.
It may be argued that on the hypothesis of Evolution, Life necessarily comes before organization. On this hypothesis, organic matter in a state of homogeneous aggregation, must precede organic matter in a state of heterogeneous aggregation. But since the passing from a structureless state to a structured state, is itself a vital process, it follows that vital activity must have existed while there was yet no structure: structure could not else arise. That function takes precedence of structure, seems also implied in the definition of Life. If Life consists of inner actions so adjusted as to balance outer actions — if the actions are the substance of Life, while the adjustment of them constitutes its form; then, may we not say that the actions to be formed must come before that which forms them — that the continuous change which is the basis of function, must come before the structure which brings function into shape? Or again, since throughout all phases of Life up to the highest, every advance is the effecting of some better adjustment of inner to outer actions; and since the accompanying new complexity of structure is simply a means of making possible this better adjustment; it follows that function is from beginning to end the determining cause of structure. Not only is this manifestly true where the modification of structure arises by reaction from modification of function; but it is also true where a modification of structure otherwise produced, apparently initiates a modification of function. For it is only when such so-called spontaneous modification of structure subserves some advantageous action, that it is permanently established: if it is a structural modification that happens to facilitate the vital activities, '' natural selection " retains and increases it; but if not, it disappears.
The connexion which we noted between heterogeneity of structure and heterogeneity of function — a connexion made so familiar by experience as to appear scarcely worth specifying — is clearly a necessary one. It follows from the general truth that in proportion to the heterogeneity of any aggregate, is the heterogeneity it will produce in any incident force {First Principles, § 116). The force continually liberated in the organism by decomposition, is here the incident force; the functions are the variously modified forms produced in its divisions by the organs they pass through; and the more multiform the organs the more multiform must be the differentiations of the force passing through them.
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 forces passing through it — if the distinctness of these parts from each other, must involve distinctness in their reactions, and therefore distinctness between the divisions of the differentiated force; 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.
CHAPTER lY.
CHAPTER lY.
§ 62. Throughout tlie vegetal kingdom, the processes of Waste and Repair are comparatively insignificant in their amounts. Though plants, and especially certain parts of them, do, in the absence of light or under particular conditions, give out carbonic acid; yet this carbonic acid, assuming it to indicate consumption of 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 the 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. Actiniae kept in an aquarium, do not appreciably diminish in bulk from 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. E-eptiles, 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 that produce but little insensible and sensible motion, and that it becomes conspicuous in creatures that 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." 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 from hybernation, the proportion of 18 "3: 1. Among men and domestic animals, the relation between degree of waste and amount of expended force, though one respecting which there is little doubt, is less distinctly demonstrable; since waste is not allowed to go on unintcrfercd witli. Wc have however in the lingering lives of invalids who arc 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 force 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 w^hole; 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 than 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 force; 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 measurable 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 exercised until they ache or become stiff, and nerves of sense rendered weary or obtuse by work, are organs ^o much wasted by action as to be partially incompetent.
Repair is everywhere and alwaj^s 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 neverceasing 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 ever 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 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 organism to a state of integrity, after the disintegration consequent on expenditure of force, is proportionate to the diminution in expenditure of force. 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 that 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 a renlwal 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 scries 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 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^ 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 special sensation yield us like experiences: strong tastes^ powerful odours, and loud sounds, temporarily unfit the nerves impressed by them, for appreciating faint tastes, odours, or sounds; but these incapacities are remedied by brief intervals of repose. Vision still better illustrates this simultaneity of waste and repair. Looking at the sun so affects the eye that, for a short time, it cannot perceive the ordinary contrasts of light and shade.
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 these photogenes^ 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 j^ears, 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 overtaxed 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 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: the proof being, that the disease occurs most frequently among those engaged in laborious handicrafts, and usually attacks first the muscles that have been most worked.
There has yet to be noticed another kind of repair; — that WASTE AND llErAIR. 175 namely, by which injured or lost parts arc restored. Among the Ilydrozoa it 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-organized Adinozoa^ the half of an individual will grow into a complete individual. Some of the lower Annelids, as the Nais^ 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 inferior Vertehrata also, as lizards, can develop new limbs or new tails, in place of those that 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 between these extremes 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 a very obvious and complete harmony between the first of the above inductions, and the deduction that follows immediately from first principles. We have already seen (§ 23) " that whatever amount of power an organism expends 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 17G THE INDUCTIONS OF BIOLOGY.
chemical compounds appropriated by the organism under the form of food. As much power 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 power, 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 energj^ 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 comjDlex 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 of a priori establishment. 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 one part of the force which any organ expends, is derived from materials brought to it by the blood from moment to moment in quantities varying with the demand, and since another part of the force which such organ expends, comes to it in the shape of nervous discharges from distant organs; 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 Differentiation and Integration {Fii^st Principles, § 123). It would be analogous to the process by which, from a mixed solution of salts, there are deposited segregated masses of these salts, in the shape of diff'erent crystals. But as already said (§ 54), though the selective assimilation by which the repair of organs is effected, no doubt results in part from an action of this kind, which is* consequent on the persistence of force {First Principles, § 129), the facts cannot be thus wholly accounted for; since organs are in part made up of units that do not exist as such in the circulating fluids. The process becomes comprehensible however, if it be shown 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 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 have returned to its previous condition: yet it is not as it was before; for now the same poison may be added to it with impunity."