ADAPTATION.
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 of 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.* We have also to note that the limited increase of size pro- duced 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 acquir- ing extra size and power in these members, begins to lose such extra size and power on ceasing to exert these members; and eventually lapses more or less nearly into the original state. Legs strengthened by a pedestrian tour, become 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 is given up. For compara- tive failure in executing a piece of music, in playing a game at chess, or in anything requiring special culture, the being out of practice is a reason of which every one recognizes the validity. It is observable, too, that the rapidity and com- pleteness 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 * Here, as in sundry places throughout this chapter, the necessities of the argu- ment have obliged me to forestall myself, by assuming the conclusion reached in a Bubsequent chapter, that modifications of structure produced by modifications of function, are transmitted to ofi"spring.
II 190 THE INDUCTIONS OF BIOLOGY.
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 mea- sure, such peculiarities. Though it is not true, as alleged, that it resumes completely the structure it had before domes- tication; 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 Austra- lian 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 compara- tive 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 re-action of function on structure, is another factor. Hence, it is well worth while inquiring how far these induc- tions are deductively interprc table.
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 sup- ply, is not obvious. We know, indeed, {First Principles, %% 96, 133,) that of necessity, the rhythmical changes pro- ADAPTATION. 191 ADAPTATION. 191 duced by antagonist organic actions, cannot any of thorn 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 ex- cess 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 equiva- lent extent, the mean state of the organ should remain con- stant. 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 ad- ditional 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 the absorbents which carry off 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 nutrition and excitation of an organ are carried on, must also be influenced by this rhythm of action and re-action; 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 ap- 192 THE INDUCTIONS OF BIOLOGY.
pliances 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 in- crease, 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 con- nected functions, allow the specially- afiPected 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 princi- ples. 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.
§ 6?. 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 condi- tioned, 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 much more manifest.
Since the function of any organ is dependent on the func- tions of the organs which supply it with materials and forces; and since the functions of these subsidiary organs are de- pendent on the functions of organs which supply them with materials and forces; it follows that before any great extra power of discharging its function, can be gained by a ADAriATION. 11) '3 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-sub- servient organs. Thus there are required numerous and wide-spread modifications. Before the artery which feeds a hard-worked muscle, can permanently furnish a large ad- ditional quantity of blood, it must increase in diameter and contractile power; and that its increase of diameter and con- tractile power may be of use, the main artery from which it diverges, must also be so far modified as to bring this addi- tional quantity of blood to the branch artery. Similarly with the veins; similarly with the absorbents; 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 prima- rily 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 in- volves 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, absorbents, and other agencies, it is manifest that a large amount of re- building 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 afiected, must take place but slowly.
We have before found our conceptions of vital processes made clearer by studying analogous social processes. In societies there is a mutual dependence of functions, essentially like that which exists in organisms; and there is also an 13 194 THE INDUCTIONS OF BIOLOGY.
essentially like re-action of functions on structures. From the laws of adaptive modification in societies, we may therefore hope to get a clue to the laws of adaptive modification in organisms. Let us suppose, then, that a society has arrived at a state of equilibrium like that of a mature animal — a state not like our own, in which growth and structural de- velopment are rapidly going on; but a state of settled balance among the functional powers of the various classes and industrial bodies, and a consequent fixity in the relative sizes of such classes and bodies. Further, let us suppose that in a society thus balanced, there occurs something which throws an unusual demand on some one industry — say an unusual demand for ships (which we will assume to be built of iron) in consequence of a competing mercantile nation having been prostrated by famine or pestilence. The imme- diate result of this additional demand for iron ships, is the employment of more workmen, and the purchase of more iron, by the ship-builders; and when, presently, the demand con- tinmng, the builders find their premises and machinery in- sufficient, they enlarge them. If the extra requirement persists, the high interest and high wages bring such extra capital and labour into the business, as are needed for new ship-building establishments. But such extra capital and labour do not come quickly; since, in a balanced community, not increasing in population and wealth, labour and capital have to be drawn from other industries, where they are already yielding the ordinary returns. Let us now go a step further. Suppose that this iron-ship-building industry, having enlarged as much as the available capital and labour permit, is still unequal to the demand; what limits its im- mediate further growth? The lack of iron. By the h3^po- thesis, the iron-producing industry, like all the other indus- tries throughout the community, yields only as much iron as is habitually required for all the purposes to which. iron is applied: ship-building being only one. If, then, extra iron is required for ship-building, the first effect is to withdraw ADAPTATION. 195 part of the iron habitually consumed for other purposes, and to raise the price of iron. Presently, the iron-makers feel this change, and their stocks dwindle. As, however, the quantity of iron required for ship-building, forms but a small part of the total quantity required for all purposes; the ex- tra demand on the iron-makers, can be nothing like so great in proportion as is the extra demand on the ship- builders. Whence it follows, that there will be much less tendency to an immediate enlargement of the iron-producing industry — the extra quantity will for some time be obtained by working extra hours. Nevertheless, if, as fast as more iron can be thus supplied, the ship- building industry goes on growing — if, consequently, the iron-makers experience a permanently- increased demand, and out of their greater profits get higher interest on capital, as well as pay higher wages; there will eventually be an abstraction of capital and labour from other industries, to enlarge the iron-producing industry: new blast- furnaces, new rolling-mills, new cottages for workmen, will be erected. But obviously, the inertia of capital and labour to be overcome, before the iron-producing industry can grow by a decrease of some other industries, will prevent its growth from taking place until long after the increased ship-build- ing industry has demanded it; and meanwhile, the growth of the ship-building industry must be limited by the deficiency of iron. A remoter restraint of the same nature, meets us if we go a step further — a restraint which can be overcome, only in a still longer time. For the manu- facture of iron depends on the supply of coal. The pro- duction of coal being previously in equilibrium with the consumption; and the consumption of coal for the manu- facture of iron, being but a small part of the total con- sumption; it follows that a considerable extension of the iron manufacture, when it at length takes place, will cause but a comparatively small additional demand on the coal-owners and coal-miners— a demand which will not, for a long period, suf- fice to cause enlargement of the coal- trade, by drawing capital 196 THE INDUCTIONS OF BIOLOGY.
and labour from otlier investments and occupations. And until the permanent extra demand for coal, has become great enough to draw from other investments and occupations, suf- ficient capital and labour to sink new mines, the increasing production of iron must be restricted by the scarcity of coal; and the multiplication of ship-yards and ship-builders, must be checked by the want of iron. Thus, in a com- munity which has reached a state of moving equilibrium, though any one industry directly affected by an additional demand, may rapidly undergo a small extra growth; yet a growth beyond this, requiring, as it does, the build- ing-up of subservient industries, less directly and strongly affected, as well as the partial 2/wbuilding of other industries, can take place only with comparative slowness. And a still further growth, requiring structural modifications of industries still more distantly affected, must take place still more slowly.
Returning from this analogy, we realize more clearly the truth, that any considerable member of an animal organism, cannot be greatly enlarged without some general re-organiza- tion. Besides a building-up of the primary, secondary, and tertiary groups of subservient parts, there must be an im- building of sundry non-subservient parts; — or at any rate, there must be permanently established, a lower nutrition of such non- subservient parts. For it must be remembered that in a mature animal, or one which has reached ja balance between assimilation and expenditure, there cannot be an in- crease in the nutrition of some organs, without a decrease in the nutrition of others; and an organic establishment of the increase, implies an organic establishment of the decrease — implies more or less change in the processes and structures throughout the entire system. And here, indeed, is disclosed one reason why growing animals under- go adaptations so much more readily than adult ones. For while there is surplus nutrition, it is possible for specially- ex- ercised parts to be specially enlarged, without any positive ADAPTATION. 197 deduction from other parts. There is required only that negative deduction, shown in the diminished growth of other parts.
§ 70. Pursuing the argument further, we reach an ex- planation of the third general truth; namely, that organisms, and species of organisms, which, under new conditions, have undergone adaptive modifications, soon return to something like their original structures, when restored to their original conditions. Seeing, as we have done, how excess of action and excess of nutrition in any part of an organism, must affect action and nutrition in subservient parts, and these again in other parts, until the re-action has divided and subdivided itself throughout the organism, affecting in decreasing degrees the more and more numerous parts more and more remotely implicated; we see that the consequent changes in the parts remotely implicated, consti- tuting the great mass of the organism, must be extremely slow. Hence, if the need for the adaptive modification ceases, before the great mass of the organism has been much altered in its structure by these ramified but minute re- ac- tions; we shall have a condition in which the specially- modified part, is not in equilibrium with the rest. All the remotely- affected organs, as yet but little changed, will, in the absence of the perturbing cause, resume very nearly their previous actions. The parts that depend on them, will consequently by and by do the same. Until at length, by a reversal of the adaptive process, the organ at first affected will be brought back almost to its original state. Reconsidering the above-drawn analogy between an organism and society, will enable us better to realize this necessity. If, in the case supposed, the extra demand for iron ships, after causing the erection of some additional ship-yards and the drawing of iron from other manufactures, were to cease;^ the old dimensions of the ship-building trade would be quickly returned to: discharged workmen would seek fresh 198 THE INDUCTIONS OF BIOLOGY.
occupations, and the new yards would be devoted to other uses. But if the increased need for ships lasted long enough, and became great enough, to cause a flow of capital and labour from other industries into the iron-manufacture, a falling off in the demand for ships, would much less rapidly entail a dwindling of the ship-building industry. For iron being now produced in greater quantity, a diminished con- sumption of it for ships, would cause a fall in its price, and a consequent fall in the cost of ships: thus enabling the ship-builders to meet the competition which we may sup- pose led to a decrease in the orders they received. And since, when new blast-furnaces and rolling-mills, &c., had been built with capital drawn from other industries, its transference back into other industries, would involve great loss; the owners, rather than transfer it, would accept unusually low in- terest; and an excess of iron would continue to be produced; resulting in an undue cheapness of ships, and a maintenance of the ship-building industry at a size beyond the need. Eventually, however, if the number of ships required still diminished, the production of iron in excess would become very unremunerative: some of the blast-furnaces would be blown out; and as much of the capital and labour as remained available, would be re-distributed among other occupations. Without repeating the steps of the argument, it will be clear that were the enlargement of the ship-building industry great enough, and did it last long enough, to cause an in- crease in the number of coal-mines; the ship-building in- dustry would be still better able to maintain itself under adverse circumstances; but that it would, though at a more distant period, end by sinking down to the needful dimensions. Thus our conclusions are: — First, that if the extra activity and growth of a particular industry, has lasted long enough only to remodel the proximately- affected industries; it will dwindle away again after a moderate period, if the need for it disappears. Second, that an enormous period must be re- quired before the re-actions produced by an enlarged industry, ADAPTATION. 199 can cause a re-construction of the whole society, and before the countless re-distributions of capital and labour, can again reach a state of equilibrium. And third, that only when such a new state of equilibrium is eventually reached, can the adaptive modification become a permanent one. How, in animal organisms, the like argument will hold, needs not be pointed out. The reader will readily follow the parallel.
That organic types should be comparatively stable, might be anticipated on the hypothesis of Evolution. If we assume, as we must according to this hypothesis, that the structure of any organism is a product of the almost infinite series of actions and re-actions to which all ancestral organisms have been exposed; we shall see that any unusual actions and re- actions brought to bear on an individual, can have but an infinitesimal effect in permanently changing the structure of the organism as a whole. The new set of forces, com- pounded with all the antecedent sets of forces, can but inap- preciably modify that moving equilibrium of functions which all these antecedent sets of forces have established. Though there may result a considerable perturbation of certain func- tions— a considerable divergence from their ordinary rhythms; yet the general centre of equilibrium cannot be sensibly changed. On the removal of the perturbing cause, the pre- vious balance will be quickly restored: the effect of the new forces being almost obliterated by the enormous aggregate of forces which the previous balance expresses.
§ 71. As thus understood, the phenomena of adaptation fall into harmony with first principles. The inference that organic types are fixed, because the deviations from them w^hich can be produced within assignable periods, are relatively small; and because, when a force producing deviation ceases, there is a return to something like the original state; proves to be an invalid inference. Without assuming fixity of species, we find good reasons for anticipating that kind and degree of stability which is observed. We find grounds for concluding, 200 THE INDUCTIONS OF BIOLOGY.
a pi^iori, that an adaptive change of structure, will soon reach a point beyond which further adaptation will be slow; for concluding that when the modifying cause has been but a short time in action, the modification generated, will be evanescent; for concluding that a modifying cause acting even for many generations, will do but little towards per- manently altering the organic equilibrium of a race; and for concluding that on the cessations of such cause, its effects will become unapparent in the course of a few gener- ations.
CHAPTER VI. INDIVIDUALITY.
§ 72. What is an individual? is a question wliich many readers will think it easy to answer. Yet it is a question that has led to much controversy among Zoologists and Botanists; and no quite satisfactory reply to it seems possi- ble. As applied to a man, or to any one of the higher animals, which are all sharply- defined and independent, the word individual has a clear meaning; though even here, when we turn from average cases to exceptional cases — as a calf with two heads and two pairs of fore-limbs — we find ourselves in doubt whether to predicate one individuality or two. But when we extend our range of observation to the organic world at large, we find that difiiculties allied to this exceptional one, meet us everywhere under every variety of form.
Each uniaxial plant may perhaps fairly be regarded as a distinct individual; though there are botanists w^ho do not make even this admission. What, however, are we to say of a multiaxial plant? It is, indeed, usual to speak of a tree with its many branches and shoots, as singular; but strong reasons may be urged for considering it as plural. Every one of its axes has a more or less independent life, and when cut ofi" and planted, may grow into the likeness of its parent; or by grafting and budding, parts of this tree may be developed upon another tree, and there manifest their 202 THE INDUCTIONS OF BIOLOGY.