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 defi- nite. 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 WASTE AND REPAIR. 177 nervous discharges from distant organs; it is clear that spe- cial 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 Re- pair, 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 princi- ples; 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, 12 178 THE INDUCTIONS OF BIOLOGY.
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." * * * << The change once effected, may be maintained through life. And herein seems to be a proof of the assimil- ative 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 w^hich they absorb as nutriment; and if, as Mr Paget points out, they have the power when their type has been changed by disease, of moulding all materials afterwards received into the modified type; may w^e 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 con- ceded 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 units are capable 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 special- ized 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.
The repair of a wasted tissue may therefore be considered WASTE AND REPAIR. 179 as due to forces analogous to those by which a crystal repro- duces its lost apex, when placed in a solution like that from which it was formed. In either case, a mass of units of a given kind, shows a power of integrating with itself diffused units of the same kind: the only difference being, that the organic mass of units arranges the diffused units into special compound forms, before integrating them with itself. In the case of the crystal, this reintegration is ascribed to polarity — a power of whose nature we know nothing. What- ever be its nature, however, it appears probable that the power by which organs repair themselves from the nutritive matters circulating through them, is of the same order.
§ 65. That other kind of repair which shows itself in the regeneration of lost members, is comprehensible only as an effect of actions like those just referred to. The ability of an organism to recomplete itself when one of its parts has been cut off, is of the same order as the ability of an injured crystal to recomplete itself. In either case, the newly- assimi- lated 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 atoms to take a certain definite form; we must in the case of the organism, assume an analogous force. This is, in truth, not an hypothesis: it is nothing more than a generalized expression of the facts. 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 nothing more than what we see, when we assert that the organism as a whole exercises such power over the newly- 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; we have no alternative but to conclude that the aggregate forces of the body, con- trol the formative processes going on in each part. And on 180 THE INDUCTIONS OF BIOLOGY.
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 con- stitution 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 evi- dences 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 Bego- nia; and so small is the fragment which is thus capable of originating a complete plant, that something like a hundred plants might 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 exper- iments 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, bisection and tri- section effected a like result. And so with their segments, similarly produced, until as many as fifty pol^^pes 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 practica- ble, 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 now abandoned 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 obviously nega- tives the other. We have therefore no alternative but to say, that the living particles composing one of these frag- ments, have an innate tendency to arrange themselves into WASTE AND RRPATll. 181 the shape of the organism to which tlicy belong. We must infer that a plant or animal of any species, is made up of special units, in all of which there dwells the intrinsic apti- tude to aggregate into the form of that species: just as in the atoms of a salt, there dwells the intrinsic aptitude to crystal- lize in a particular way. It seems diihcult to conceive that this can be so; but we see that it is so. Groups of units taken from an organism (providing they are of a certain bulk and not much differentiated into special structures) have this power of re-arranging themselves; and we are thus compelled to recognize the tendency to assume the specific form, as inherent in all parts of the organism. 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 from the same spot a cellular mass, which, while increasing in bulk, assumes the form and structure of the original claw; we can have no hesitation in 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. In the one case as in the other, the vitalized molecules composing the tissues, show their proclivity towards a particular arrange- ment; and whether such proclivity is exhibited in repro- ducing the entire form, or in completing it when rendered imperfect, matters not.
For this property there is no fit term. If we accept the word polarity, as a name for the force by which inorganic units are aggregated into a form peculiar to them; we may apply this word to the analogous force displayed by organic units. But, as above admitted, polarity, as ascribed to atoms, is but a name for something of which we are ignorant — a name for a hypothetical property which as much needs ex- planation as that which it is used to explain. Nevertheless, in default of another word, we must employ this: taking 182 THE INDUCTIONS OF BIOLOGY.
care, however, to restrict its meaning. If we simply substi- tute the term polarity, for the circuitous expression — the power which certain units have of arranging themselves into a special form, we may, without assuming anything more than is proved, use the term organic polarity wr po- larity of the organic units, to signify the proximate cause of the ability which organisms disjDlay of reproducing lost parts.
§ 66. As we shall have frequent occasion hereafter to refer to 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 what these units are^ and by what name they may be most fitly called.
On the one hand, it cannot be in those proximate chemical compounds composing organic bodies, that this specific polar- ity dwells. It cannot be that the atoms of albmnen, or fibrine, or gelatine, or the hypothetical protein- substance, possess this power of aggregating into specific shapes; for in such case, there would be nothing to account for the unlike- nesses of different organisms. Millions of species of plants and animals, more or less contrasted in their structures, are all mainly built up of these complex atoms. But if the polarities of these atoms determined the forms of the or- ganisms they composed, the occurrence of such endlessly varied forms would be inexplicable. Hence, what we may call the chemical units, are clearly not the possessors of this property.
On the other hand, this property cannot reside in what may be roughly distinguished as the ino)yhological rmits. The germ of every organism is a microscopic 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, through- WASTE AND REPAIR.
WASTE AND REPAIR.
out 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, as they are not uni- versal, we cannot say that this tendency to aggregate into specified forms dwells in them. Finding that in many cases a fibrous tissue arises out of a structureless blastema, without cell- formation; and finding that there are creatures, such as E-hizopods, which are not cellular, but nevertheless exliibit vital activities, and perpetuate in their progeny certain specific distinctions; we are forbidden to ascribe to cells this peculiar power of arrangement. Nor, indeed, were cells universal, would such an hypothesis be acceptable; since the formation of a cell is, to some extent a manifesta- tion of this same peculiar power.
If, then, this organic polarity 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 term, jjliysiological. 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 atoms, have a more or less distinctive character. We must conclude that in each case, some slight difference of com- position in these imits, leading to some slight difierence in their mutual play of forces, produces a difierence 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.
CHAPTER y.
ADAPTATION.
§ 67. In plants, waste and repair being scarcely appre- ciable, there are not likely to arise appreciable changes in the proportions of already- formed parts. The only divergences from the average structure 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 its branches then take a diiferent inclin- ation. 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 grow- ing parts. In animals, however, besides analogous structural changes wrought during the period of growth, by subjection to circumstances unlike the ordinary circum- stances; there are structural changes similarly wrought, after maturity has been reached. Organs that. have arrived at their full size, possess a certain modifiability; so that while the organism as a whole, retains pretty ADAnATION. 185 nearly the same bulk, tlie proportions of its parts may be considerably varied. Their variations, here treated of under the title Adaptation, depend on specialities of individual action. We saw in the last chapter, that the actions of or- ganisms entail re-actions on them; and that specialities of action entail specialities of re-action. Here it remains to be pointed out, that the 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 assimi- lation: 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 each other. The illustrations fall into several groups. The growth of muscles exercised to an unusual degree, is a matter of com- mon 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 multi- ply proofs. The occurrence of changes in the struc- ture of the skin, where the skin is exposed to a stress of function, is also familiar. That thickening of the epidermis on a labourer's palm, results from continual pressure and friction, is certain: those who have not before exerted their 186 THE INDUCTIONS OF BIOLOGY.
hands, find that such an exercise as rowing, soon begins to produce a like thickening. This relation of cause and efi*ect 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 into the arteries, and when there re- sults 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 thick- ening of the heart's muscular walls, being an adaptation of it to the additional eflbrt required. Again, when an aneurism in some considerable artery has been obliterated, either arti- ficially 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 quantit}^ 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 efiiciency that fol- lows greater activity. This is manifested alike in the senses and the intellect. The palate may be cultivated in- to extreme sensitiveness, as in professional tea-tasters. An orchestral conductor gains by continual practice, an unusually great ability to discriminate differences of sound. And 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 is ordinary. The increase of power which ADAPTATION. 187 habitual exertion gives to mental faculties, needs no illustra- tion: 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 mus- cular 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 afibrd support enough to permit a halting walk. But the most remarkable modification of this order occurs in ununited fractures. '^ False joints '^ are often formed — joints which rudely simulate the hinge structure or the ball- and-socket structure, according as the muscles tend to pro- duce a motion of flexion and extension or a motion of rota- tion. In the one case, according to Rokitansky, the two ends of the broken bone become smooth and covered with perios- teum 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 from which we draw the one induction thrust the other upon us. After a time, no training makes the pugiKst or the athlete any stronger. The adult gymnast at last acquires the power 188 THE INDUCTIONS OF BIOLOGY.
to perform certain difficult feats; but certain more difficult feats, no additional practice enables bim to perform. Years of discipline give the singer a particular loudness and range of voice, beyond wbich further discipline does not give greater loudness or wider range: on tbe contrary, increased vocal ex- ercise, causing a waste in excess of repair, is often followed by decrease of power. In the perceptions we see similar limits. Tbe culture wbich exalts tbe susceptibility of tbe ear to tbe intervals and harmonies of notes, will not turn a bad ear into a good one. Life-long effiort fails to make this artist a correct draftsman, or that a fine colourist: each does better than be 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. Each man has a mathematical facult}^, a poetical faculty, or an oratorical faculty, which special educa- tion improves to a certain extent. But unless be is unusually endowed in one of these directions, no amount of education will make him a first-rate mathematician, a first-rate poet, or a first-rate orator. Thus tbe general fact appears to be, that while in each individual, certain changes in the proportions of parts, may be caused by variations of function, tbe 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 time, indefinite modi- fication may not be produced; experience proves that within assigned times, the changes wrought in races, of organisms by changes of conditions fall within narrow limits. We see, for instance, that though by discipline, aided by selective breeding, one variety of horse has had its locomotive power increased considerably beyond the locomotive powers of other varieties; yet that 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 show aptitudes for diverging in the same directions at ADAPTATION.