This molecular agitation accompanying the molecular re- arrangements that are caused by oxygen taken into the animal organism, must result both from the union of oxygen v^ith those nitrogenous matters of which the tissues are composed, and from its union with those non-nitrogenous m.atters which are diffused through the tissues. Just as much heat as would be caused by the oxidation of such matters out of the body, must be caused by their oxidation in the body. In the one case as in the other, the heat must be regarded as a concomitant. Whether the distinction, made by Liebig between nitrogenous substances as tissuefood, and non-nitrogenous substances as heat-food, be true or not in a narrower sense, it cannot be accepted in the sense that tissue-food is not also heat-food. Indeed he does not himself assert it in this sense. The ability of carnivorous THE RE-ACTIONS OF ORGANIC MAITER ON FORCES. 45 animals to live and generate heat while consuming matter that is almost exclusively nitrogenous, to say nothing of the constant relation above shown between functional activity and the evolution of heat, suffices to prove that the nitrogenous compounds forming the tissues are heat- producers, as well as the non-nitrogenous compounds circulating among and through the tissues. But it is possible that this antithesis is not true even in the more restricted sense. It seems quite an admissible hypothesis that the hydro-carbons and oxy-hydrocarbons which, in traversing the system, are transformed by communicated chemical action, evolve during their transformation, not heat alone, but also other kinds of force. It may be that as the nitrogenous matter, while falling into more stable molecular arrangements, generates both that molecular agitation called heat, and such other molecular movements as are resolved into forces expended by the organism; so, too, does the non-nitrogenous matter. Or perhaps the concomitants of this metamorphosis of non- nitrogenous matter, vary with the conditions. Heat alone may result when it is transformed while in the circulating fluids, but partly heat, and partly another force, when it is transformed in some active tissue that has absorbed it: just as coal, though producing little else but heat as ordinarily burnt, has its heat partially transformed into mechanical motion if burnt in a steam-engine furnace. In such case, the antithesis of Liebig would be reduced to this; — that whereas nitrogenous substance is tissue-food both as material for building-up tissue and as material for its function; non-nitrogenous substance is tissue- food only as material for There can be no doubt that this thermal re-action which chemical action from moment to moment produces in the body, is from moment to moment an aid to further chemical action. We before saw {First Princijoles, § 103) that a state of raised molecular vibration, is favourable to those re-distributions of matter and motion which constitute Evolution. \ye saw that in organisms distinguished by the amount and rapidity of such re-distributions, this raised state of molecular vibration is conspicuous. And we here see that this raised state of molecular vibration, is itself a continuous consequence of the continuous molecular re- distributions it facilitates. The heat generated by each increment of chemical change, makes possible the succeeding increment of chemical change. In the body this connexion of phenomena is the same as we see it to be out of the body. Just as in a burning piece of wood, the heat given out by the portion actually combining with oxygen, raises the adjacent portion to a temperature at which it also can combine with oxygen; so, in a living animal, the heat produced by oxidation of each portion of tissue, maintains the temperature at which the unoxidized portions can be readily oxidized.
§ 19. Among the forces called forth from organisms by re-action against the actions to which they are subject, is Light. Phosphorescence is in some few cases displayed by plants — especially by certain fungi. Among animals it is comparatively common. All know that there are several kinds of luminous insects; and many are familiar with the fact that luminosity is a characteristic of various marine creatures.
Most of the evidence goes to show that this evolution of light, as well as the evolution of heat, is consequent on oxidation of the tissues. Light, like heat, is the expression of a raised state of molecular vibration: the difference between them being a difference in the rates of vibration. Hence by chemical action on substances contained in the organism, heat or light may be produced, according to the character of the resulting molecular vibrations. The inference that oxidation is the cause of this luminosity, does not, however, rest only on a priori grounds. It is supported by experimental evidence. In phosphorescent insects, the continuance of the light is found to depend on the continuance of respiration; and any exertion which renders respiration more active, THE RE- ACTIONS OF ORGANIC MATTER ON FORCES. 47 increases the brilliancy of the light. Moreover, by separating the luminous matter, Prof. Matteucci has shown that its emission of light is accompanied by absorption of oxygen and escape of carbonic acid. The phosphorescence of marine animals has been referred to other causes than oxidation. In some cases, however, it is, I think, explicable without assuming any more special agency. Considering that in creatures of the genus Noctiluca^ for example, ^to which the phosphorescence most commonly seen on our own coasts is due, there is no means of keeping up a constant circulation, we may infer that the movements of aerated fluids through their tissues, must be greatly affected by impulses received from without. Hence it may be that the sparkles visible at night when the waves break gently on the beach, or when an oar is dipped into the water, are called forth from these creatures by the concussion, not because of any unknown influence it excites, but because, being propagated through their delicate tissues, it produces a sudden movement of the fluids and a sudden increase of chemical action. Nevertheless, in other phosphorescent animals inhabiting the sea, as in the Pyrosoma and in certain Annelida, light seems to be really produced, not by direct re-action on the action of oxygen, but by some indirect re-action involving a transformation of force.
§ 20. The re-distributions of matter in general, are accompanied by electrical disturbances; and there is abundant evidence that electricity is generated during those re- distributions that are ever taking place in organisms. Experiments have shown " that the skin and most of the internal membranes are in opposite electrical states; " and also that between different'internal organs, as the liver and the stomach, there are electrical contrasts — such contrasts being greatest where the processes going on in the compared parts are most unlike. It has been proved by M. du Bois-Eeymond that when any point in the longitudinal section of a muscle is connected by a conductor with any point in its transverse section, an electric current is established; and further, that like results occur when nerves are substituted for muscles. The special causes of these phenomena have not yet been determined. Considering that the electric contrasts are most marked where active secretions are going on — considering, too, that while they do not exist between external parts which are similarly related to the vascular currents, they do exist between external parts which are dissimilarly related to the vascular currents — and considering also that they are extremely difficult to detect where there are no appreciable movements of fluids; it may be that they are due simply to the friction of heterogeneous substances, which is universally a cause of electric disturbance. But whatever be the interpretation, the fact remains the same, that there is throughout the living organism, an unceasing production of differences between the electric states of different parts; and consequently an unceasing restoration of electric equilibrium by the establishment of currents among these parts.
Besides these general, and not conspicuous, electrical phenomena which appear to be common to all organisms, vegetal as well as animal, there are certain special and strongly marked ones. I refer, of course, to those which have made the Torpedo and the Gymnotiis objects of so much interest. In these creatures we have a genesis of electricity that is not incidental on the performance of their different functions by the different organs; but one which is itself a function, having an organ appropriate to it. The character of this organ in both these fishes, and its largely-developed connexions with the nervous centres, have raised the suspicion, which various experiments have thus far justified, that in it there takes place a transformation of what we call nerve- force into the force known as electricity: this conclusion being more especially supported by the fact, that substances, such as morphia and strychnia, which are known to be powerfu nervous stimulants, greatly increase the violence and rapidity of the electric discharges.
But whether general or special, and in whatever manner produced, these evolutions of electricity are among the re-actions of organic matter, called forth by the actions to which it is subject. Thougli these re-actions are not direct, but seem rather to be remote consequences of those changes wrought by external agencies on the organism, they are yet incidents in that general re-distribution of motion, which these external a2:encies initiate; and as such must here be noticed.
§ 21. To these known modes of motion, has next to bo added an unknown one. Ileat, Light, and Electricity are emitted by inorganic matter when undergoing changes, as well as by organic matter. But there is a kind of force manifested in some classes of living bodies, which we cannot identify with any of the forces manifested by bodies that are not alive, — a force which is thus unknown, in the sense that it cannot be assimilated with any otherwise-recognized class. I allude to what is called nerve- force.
This is habitually generated in all animals, save the lowest, by incident forces of every kind. The gentle and violent mechanical contacts, which in ourselves produce sensations of touch and pressure — the additions and abstractions of molecular vibration, which in ourselves produce sensations of heat and cold; produce in all creatures that have nervous systems, certain nervous disturbances — disturbances which, as in ourselves, are either communicated to the chief nervous centre, and there constitute consciousness, or else result in merely physical processes that are set going elsewhere in the organism. In special parts distinguished as organs of sense, other external actions bring about other nervous re-actions; that show themselves either as special sensations, or as excitements which, without the intermediation of consciousness, i beget actions in muscles or other organs. Besides neural discharges that follow the direct incidence of external forces, there are others ever being caused by the incidence of forces which, though originally external, have become internal by absorption into the organism of the agents exerting them.
Por thus may be classed those neural discharges that from moment to moment result from modifications of the tissues, wrought by substances carried to them in the blood. That the unceasing change of matter which oxygen and other agents produce throughout the system, is accompanied by a genesis of nerve-force, is shown by various facts; — by the fact that nerve-force is no longer generated, if oxygen be withheld, or the blood prevented from circulating; by the fact that when the chemical transformation is diminished, as during sleep \Yith. its slow respiration and circulation, there is a diminution in the quantity of nerve-force; in the fact that an excessive expenditure of nerve-force, involves excessive respiration and circulation, and excessive waste of tissue. To these proofs that nerve-force is evolved in greater or less quantity, according as the conditions to rapid molecular change throughout the body, are well or ill fulfilled; may be added proofs that certain special molecular actions, are the causes of these special re-actions. The eflects of alcohol, ether, chloroform, and the vege to -alkalies, put beyond doubt the inference, that the overthrow of molecular equilibrium by chemical afiinity, when it occurs at certain places in the body, results in the overthrow of equilibrium in the nerves proceeding from these places — results, that is, in the propagation through these nerves, of the change called a nervous discharge. Indeed, looked at from this point of view, the two classes of nervous changes — the one initiated from without and the other from within — are seen to merge into one class. Both of them may be traced to metamorphosis of tissue. There can be little doubt that the sensations of touch and pressure, are consequent on accelerated changes of matter, produced by mechanical disturbance of the mingled TJIK UK- ACTIONS OF ORGANIC MATTER ON FORCES. 51 fluids and solids composing the parts affected. There is abundant evidence that the sensation of taste, is due to the chemical actions set up by particles which find their way through the membrane covering the nerves of taste; for, as Prof. Graham points out, sapid substances all belong to the class of crystalloids, which are able rapidly to permeate animal tissue, while colloids, which cannot pass through animal tissue, are all insipid. Similarly with the sense of smell. Substances which excite this sense, are necessarily more or less volatile: and their volatility being the result of their molecular mobility, implies that they have in a high degree, the power of getting at the olfactory nerves by penetrating their mucous investment. Again, the facts which photography has familiarized us with, make it clear that those nervous impressions called colours, are primarily due to certain changes wrought by light in the substance of the retina. And though, in the case of hearing, we cannot so clearly trace the connexion of cause and effect; yet as we see that the auditory apparatus is one fitted to intensify those vibrations constituting sound, and to convey them to a receptacle containing fluid in which nerves are immersed; it can scarcely be doubted that the sensation of sound proximately results from atomic re-arrangements caused in these nerves by the vibrations of the fluid: knowing, as we do, that the re-arrangement of atoms is in all cases aided by agitation. Perhaps, however, the best proof that nerveforce, whether peripheral or central in its origin, results from chemical transformation, lies in the fact that most of the chemical agents which powerfully affect the nervous system, affect it whether applied at the centre or the periphery. Various acids, mineral and vegetal, are tonics — the stronger ones being usually the stronger tonics; and this which we call their acidity, implies a power in them of acting on the nerves of taste, while the tingling or pain that follows their absorption through the skin, implies that the nerves of touch are acted on by them. Similarly with certain vegeto-alkalics i* which are peculiarly bitter. These by their bitterness, show that they affect the extremities of the nerves; while by their tonic properties, they show that they affect the nervous centres — the most intensely bitter among them, strychnia, being the most powerful nervous stimulant. However true it may be that this relation is not a regular one, since opium, hashish, and some other drugs, which work marked effects on the brain, are not remarkably sapid — however true it may be that there are relations between particular substances and particular parts of the nervous system; yet such instances do but qualify, without negativing, the general proposition. The truth of this proposition can scarcely be doubted when, to the evidence above given, is added the fact that various condiments and aromatic drugs are given as nervous stimulants; and the fact that anaesthetics, besides the general effects they produce when inhaled or swallowed, produce local effects of like kind when absorbed through the skin; and the fact that ammonia, which in consequence of its extreme molecular mobility, so quickly and so violently excites the nerves beneath the skin, as well as those of the tongue and the nose, is a rapidly-acting stimulant when taken internally.
Whether we shall ever know anything more of this nerveforce, than that it is some species of molecular disturbance that is propagated from end to end of a nerve, it is impossible to say. Whether a nerve is merely a conductor, which delivers at one of its extremities an impulse received at the other; or whether, as some now think, it is itself a generator of force which is initiated at one extremity and accumulates in its course to the other extremity; are also questions which cannot yet be answered. All we know is, that forces capable of working molecular changes in nerves, are capable of callino- forth from them manifestations of activity — discharges of some force, which, though probably allied to electricity, is not identical with it. And our evidence that nerveforce is thus originated, consists not only of such facts as the above, but also of more conclusive facts established by direct TIIF. RE-ACTIONS OF ORGANIC MAITER ON FORCES. 53 experiments on nerves — experiments wliicli sliow that nerveforce is irenerated when the cut end of a nerve is cither mechanically irritated, or acted on by some chemical agent, or subject to the galvanic current — experiments which thus prove that nerve-force is liberated by whatever disturbs the molecular equilibrium of nerve-substance. And this is all which it is necessary for us here to understand, § 22. The most important of these re-actions called forth from organisms by surrounding actions, remains to be noticed. To the above various forms of insensible motion thus caused, we have to add sensible motion. On the production of this mode of force, more especially depends the possibility of all vital phenomena. It is, indeed, usual to regard the power of generating sensible motion, as confined to one out of the two organic sub-kingdoms; or, at any rate, as possessed by but few members of the other. On looking closer into the matter, however, we see that plant-life as well as animal-life, is universally accompanied by certain manifestations of this power; and that plant-life could not otherwise continue.
Throuo^h the humblest, as well as throuo:h the hisrhest, vegetal organisms, there are ever going on certain re-distributions of matter. In protophytes the microscope shows us an internal transposition of parts, wliicli when not active enough to be immediately visible, is proved to exist by the changes of arrangement that become manifest in the course of hours and days. In the individual cells of many higher plants, an active movement among the contained granules may be witnessed. And well- developed cryptogams in common with all phanerogams, exhibit this genesis of mechanical motion still more conspicuously in the circulation of sap. It might, indeed, be concluded a priori^ that through plants displaying much differentiation of parts, an internal movement must be going on; since, without it, the mutual dependence of organs having unlike functions would seem impossible. Besides these motions of fluids kept up internally, plants, espe- 5-1 THE DATA OF BIOLOGY.
dally of tlie lower orders, are able to move tlieir external parts in relation to each other, and also to move about from place to place. Illustrations in abundance will occur to all students of recent Natural History — such illustrations as the active locomotion of the zoospores of many Algae, the rhythmical bendings of the OscillatoricB, the rambling progression of the Diatoinacece. In fact many of these smallest vegetals, and many of the larger ones in their early stages, display a mechanical activity not distinguishable from that of the simplest animals. Among well- organized plants, which are never locomotive in their adult states, we still not unfrequently meet with relative motions of parts. To such familiar cases as those of the Sensitive plant and the Yenus' fly-trap, many others may be added. When its base is irritated, the stamen of the Berberry flower leans over and touches the pistil. If the stamens of the common wild Cistus be gently brushed with the finger, they spread themselves — bending away from the seed-vessel. And some of the orchidflowers, as Mr Darwin has recently shown, shoot out masses of pollen on to the entering bee, when its trunk is thrust down in search of honey.
Though the power of moving is not, as we see, a characteristic of animals alone, yet in them, considered as a class, it is manifested to an extent so marked, as practically to become one of their distinctive characters — indeed, we may say, their most distinctive character. For it is by their immensely greater ability to generate mechanical motion, that animals are enabled to perform those actions which constitute their visible lives; and it is by their immensely greater abilitj^ to generate mechanical motion, that the higher orders of animals are most obviously distinguished from the lower orders. Though, on remembering the seemingly active movements of infusoria, some will perhaps question this last-named contrast; yet, on comparing the quantities of matter propelled through given spaces in given times, they will see that the momentum evolved is far less in the protozoa than in the teleozoa. Those sensible motions of animals are cfFectcd by various organs under various stimuli. In the humblest forms, and even in some of the more developed ones which. • inhabit tlie water, locomotion results from the vibrations of cilia: the contractility resides in these waving hairs that grow from the surface. Some of the AcalephcB^ and their allies the Polypes, move when mechanically irritated: the long pendant tentacle of a PhysaUa is suddenly drawn up if touched; and, as well as its tentacles, the whole body of a Hydra collapses if roughly handled, or jarred by some shock in its neighbourhood. In all the higher animals however, and to a smaller degree in many of the lower, sensible motion is generated by a special tissue, under the special excitement of a neural discharge. Though it is not strictly true that such animals show no sensible motions otherwise caused; since all of them have certain ciliated membranes, and since the circulation of fluid in them is partially due to osmotic and capillary actions; yet, generally speaking, we may say that their movements are effected only by muscles that contract only through the agency of nerves.
What special transformations offeree generate these various mechanical changes, we do not, in most cases, know. Those re-distributions of fluid, with the alterations of form sometimes caused by them, that result from osmose, are not, indeed, quite incomprehensible. Certain motions of plants which, like those of the " animated oat," follow contact with water, are easily interpreted; as are also such other vegetal motions as those of the Touch-me-not, the Squirting Cucumber, and the Carpoholus. But we have as yet no clue to the mode in which molecular movement is transformed into the movement of masses, in animals. We cannot refer to known causes the rhythmical action of a Medusa's disc, or that slow decrease of bulk that spreads throughout the mass of an Alcyonium, when one of its component individuals has been irritated. 'Hov are we any better able to say how the insensible motion transmitted through a nerve, gives rise to sensible motion in a muscle. It is true tliat Science lias given to Art, several methods of changing insensible into sensible motion. By applying heat to water we vaporize it; and the movement of its expanding vapour, we transfer to solid matter; but it is clear that the genesis of muscular movement is in no way analogous to this. The force evolved during chemical transformations in a galvanic battery, w^e communicate to a soft iron magnet through a wire coiled round it; and it would be quite possible, by placing near to each other several magnets thus excited, to obtain, through the attraction of each for its neighbours, an accumulated movement made up of their separate movements, and thus to mechanically imitate a muscular contraction; but from what we know of organic matter, and the structure of muscle, there is no reason to suppose that anything analogous to this takes place in it. We can, however, through one kind of molecular change, produce sensible changes of aggregation such as possibly might, when occurring in organic substance, cause sensible motion in it: I refer to allotropic change. Sulphur, for example, assumes different crystalline and non-crystalline forms at different temperatures; and may be made to pass backwards and forwards from one form to another, by slight variations of temperature: undergoing each time an alteration of bulk. We know that this allotropisra, or rather its analogue isomerism, prevails among colloids — inorganic and organic. We also know that some of these metamorphoses among colloids, are accompanied by visible re-arrangements: instance hydrated silicic acid, which, after passing from its soluble state to the state of an insoluble jelly, begins, in a few days, to contract, and to give out part of its contained water. Now, considering that such isomeric changes of organic as well as inorganic colloids, are often very rapidly produced by very slio-ht causes, it seems not impossible that some of the colloids constituting muscle, may be thus changed by a nervous discharge— resuming their previous condition when the discharge ceases. And it is conceivable that by structural arrangements, minute sensible motions so caused, may be accumulated into largo sensible motions. There is, however, no evidence to support this supposition.
§ 23. But the truths which it is here our business especially to note, are quite independent of hypotheses or interpretations. It is sufficient for the ends we have in view, to observe that organic matter docs exhibit these several conspicuous re- actions, when acted on by incident forces: it is not requisite that we should know Jiow these re- actions originate.
In the last chapter were set forth the several modes ia whicb incident forces cause re -distributions of organic matter; and in this chapter have been set forth the several modes in which is manifested the motion accompanying this re-distribution. There we contemplated under its several aspects, the general fact, that in consequence of its extreme instability, organic matter undergoes extensive molecular re-arrangements, on very slight changes of conditions. And here we have contemplated under its several aspects, the correlative general fact, that during these extensive molecular re-arrangements, there are necessarily evolved large amounts of force. In the one case the atoms of which organic matter consists, are regarded as changing from positions of unstable equilibrium to positions of stable equilibrium; and in the other case they are regarded as giving out in their falls from unstable to stable equilibrium, certain momenta — momenta that may be manifested as heat, light, electricity, nerveforce or mechanical motion, according as the conditions determine.
I will add only that these evolutions of force are rigorously dependent on these changes of matter. It is a corollary from that primordial truth which, as we have seen, underlies all other truths, {First Principles, §§ 76, 141,) that w^hatever amount of power an organism expends in any shape, is the correlate and equivalent of a power that was taken into it from without. On the one hand, it follows 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 on the other hand, it follows from the persistence of force that no such transformation of organic matter containing this latent energy can take place, without the energy being in one shape or other manifested.
CHAPTER lY.* PROXIMATE DEFINITION OF LIFE.
§ 24. To those wlio accept the general doctrine of Evolution, it needs scarcely be pointed out that classifications are subjective conceptions, which have no absolute demarcations in Nature corresponding to them. They are applianCes by which we limit and arrange the matters under investigation; and so facilitate our thinking. Consequently, when we attempt to define anything complex, or make a generalization of facts other than the most simple, we can scarcely ever avoid including more than we intended, or leaving out something that should be taken in. Thus it happens that on seeking a definition of Life, we have great difficulty in finding one that is neither more nor less than sufficient. Let us look at a few of the most tenable definitions that have been given. While recognizing the respects in which they are defective, we shall see what requirements a more complete one must fulfil.
* This chapter and the following two chapters originally appeared in Part III. of the Principles of Psychology: forming a preliminary which, though indispensahle to the argument there developed, was somewhat parenthetical. Having now to deal with the general science of Biology before the more special one of Psychology, it becomes possible to transfer these chapters to their proper place. They have been carefully revised.
Sclielling said that Life is the tendency to individuation. This formula, until studied, convej^s little meaning. But it needs only to consider it as illustrated by the facts of development, or by the contrasts between lower and higher forms of life, to recognize its value; especially in respect of comprehensiveness. As before shown, however, {First Princij^les, § 56), it is objectionable, partly on the ground that it refers, not so much to the functional changes constituting Life, as to the structural changes of those aggregations of matter which manifest Life; and partly on the ground that it includes under the idea Life, much that we usually exclude from it: for instance — crystallization.
The definition of Richerand, — " Life is a collection of phenomena which succeed each other during a limited time in an organized body," — is liable to the fatal criticism, that it equally applies to the decay which goes on after death. For this, too, is " a collection of phenomena which succeed each other during a limited time in an organized body."