The inherent difficulties in a mechanistic theory of life have been perceived ever since it was formulated, even vaguely. Physiology took its origin in practical medicine, and in connexion with the healing art of the physician or surgeon it became evident in early times, and was clearly realized by Hippocrates, that diseases and injuries tend to be recovered from by an active process apart from either volition or artificial interference, and that artificial interference, where it is efficacious, is either of the nature of an aid to an active natural process or the removal of something which causes abnormal disturbance of natural processes. The mechanistic theory of life gives no coherent account of the natural tendency of the body to maintain actively and reproduce its normal structure and activities and to restore them after disturbance. In mechanisms there is no such tendency, and for this reason the mechanistic theory of life has never appealed to those engaged in the practice of medicine.

On the other hand, it became equally evident as physical science developed that mechanical interpretations of phenomena observed outside living organisms are extremely satisfactory, and can with much success be applied to a good deal of what is observed within the body. In order to give due weight to both points of view it seemed natural, and only common sense, to assume that though the body, as being material, is in itself of the same nature as other kinds of matter, the living body is nevertheless subject to a guiding interference which accounts for its differences from a machine. As we are not directly conscious of this guiding influence, its action was usually not attributed to the soul, but to a “vital principle” or “vital force,” or else to the soul acting unconsciously. The word “vitalism” may be regarded as covering every form of this belief, up to the present time.

Before Descartes put forward his mechanistic conception of life, vitalism was hardly a distinctly defined form of belief. Such expressions as “animal spirit” or “vital spirit” were, indeed, in use, but in a sense which was not by any means clear. Descartes, for instance, adopted the conception of “animal spirit”; but for him, as we have seen, this was only an attenuated form of ordinary matter. The formulation by Descartes of a definitely mechanistic theory of life led immediately to the definite formulation of a vitalistic theory by Stahl, who was also the author of the phlogiston theory which dominated chemistry and the physics of heat for so long. Stahl maintained that bodily processes are guided unconsciously, as well as consciously, by the soul, and that it is the presence of this guidance that distinguishes living from non-living bodies. He strongly contested the arguments of Descartes, and maintained that a mechanistic theory of life does not correspond with the facts.

Stahl's criticisms appealed particularly to those engaged in medicine, and we find that for the next two centuries the teaching of physiology in connexion with medicine was more or less vitalistic. The mechanistic theory might appeal to philosophers or theologians, but it certainly did not appeal to the ablest men engaged in the medical profession. Nor did it appeal much to physicists or chemists, since they were well aware of the contrast between life and ordinary physical and chemical processes. It has sometimes been assumed that Wöhler's discovery of the synthesis of urea was fatal to vitalism. All that it was fatal to was the quite artificial distinction which chemists had made between organic and inorganic chemical substances. Wöhler, like Liebig, was and remained a vitalist.

Stahl had assumed that it is the soul, acting unconsciously, which guides the bodily processes; but this hypothesis did not seem necessary, and seemed to involve, for instance, the conclusion that plants have souls. This part of Stahl's theory was therefore soon dropped, a “vital principle,” operative in all living organisms, including plants, being substituted for the soul in connexion with all unconscious bodily processes which seemed to present evidence of co-ordinated guidance. The “excitability” of muscular and nervous tissues was also attributed to the vital principle or force, which was thus apparently a source of energy. In the teachings of the Montpellier school of medicine in the eighteenth century, we find a specially detailed development of vitalistic doctrine. In the writings of Hunter in England, Bichat in Paris, and nearly all the most eminent physiologists, medical men, chemists, and biologists up to near the middle of last century, we find vitalism of some kind to be an accepted doctrine, though among phiolosophers, theologians, and certain popular writers, living organisms were frequently regarded as being, apart from the exercise of conscious control, mere machines. Others went further, and concluded that all the phenomena observed in living organisms, whether conscious or unconscious, are mechanically determined.

Vitalism remained on the whole dominant up to about the middle of last century. Johannes Müller's famous text-book of physiology, and Liebig's numerous writings on physiological chemistry, or as it is now usually called, bio-chemistry, afford a good idea of vitalism as it was then taught. The following are passages from Müller's introductory chapter:

“There is in living organic matter a principle constantly in action, the operations of which are in accordance with a rational plan, so that the individual parts which it creates in the body are adapted to the design of the whole; and this it is which distinguishes organism. Kant says, ‘The cause of the particular mode of existence of each part resides in the whole, while in nonliving masses each part contains this cause within itself.’ …

“… Stahl's contemporaries and followers have partly misunderstand this great man, in believing that, according to his view, the soul, which forms mental conceptions, also conducts with consciousness and designedly, the organization of the body. The soul (anima) spoken of by Stahl is the organizing power or principle which manifests itself in conformity with a rational law. But Stahl went too far in placing the manifestations of soul, combined with consciousness, on a level with the organizing principle, the operations of which, though in accordance with a design, obey a blind necessity. The organizing principle which, according to an eternal law, creates the different essential organs of the body, and animates them, is not itself seated in one particular organ: and it continues up to the date of birth in anencephalous monsters…. This principle, thus acting conformably to design, but without consciousness, is also manifested in the phenomena of instinct. There is great beauty and truth in the saying of Cuvier that animals acting from instinct are, as it were, possessed by an innate idea, by a dream. But that which excites the dream can be nothing less than the organizing principle, the “final cause” of the being.

“The existence of the organic principle in the germ, and its apparent independence of any special organ in the adult, as well as the fact that it is manifested in plants, in which both nervous system and consciousness are wanting, prove that this principle cannot be compared with mental consciousness, which is an after-product of development, and has its seat in one particular organ. Mind can generate no organic products: it can merely form conceptions. Our ideas of the organized being are mere conscious perceptions of the mind. The formative or organizing principle, on the other hand, is a creative power modifying matter blindly and unconsciously, according to the law of adaptation.

“Organism or the organized state is the result of the union of the organic creative power and organic matter. Whether the two have ever been separate, whether the creative archetypal ideal, the eternal ideas of Plato, as he taught in his Timaeus, have at some former period been infused into matter, and from that time forward been perpetuated in each animal and plant, is not an object of science, but of the fables and traditions which cannot be proved, and which indicate to us the limitations of our mere consciousness.”

Liebig regarded the vital principle or force as an influence which guides molecules into the positions normally occupied by them in the living tissues, and which holds them there or replaces them, in spite of physical and chemical forces tending to scatter or disintegrate them. He also believed that the reason why the consumption of oxygen and formation of carbon dioxide are so much increased during muscular exertion is that the vital force is becoming used up and thus enfeebled, so that it can no longer fully protect the living tissues against oxidation. Increased oxidation is thus a natural consequence. Outside the living tissues in the blood and other liquids, oxidation has free play to go on, and this produces the ordinary resting supply of animal heat.

Since Liebig regarded the vital principle as a source of energy which could be exhausted, but could, apparently, renew itself spontaneously and from no known source, his teaching was inherently very liable to attack. In the previous lecture we have seen how the advances made by Mayer and Helmholtz threw a flood of new light on the sources of muscular energy and the relation of oxidation to muscular contraction and other manifestations of excitability in living tissues. This meant a complete break with the doctrine that the vital principle is a source of the ordinary energy released in vital processes. Liebig's teaching also suffered a further eclipse. He thought, as we have seen, that in muscular work it is previously living tissue that is oxidized; and as living tissue is albuminous, and albuminous material is, when oxidized in the body, converted into urea, carbon dioxide, and water, a greatly increased excretion of urea ought therefore to occur after muscular work. This, however, was not found to occur under ordinary conditions. Thus muscular work must, in the main, be carried out at the expense of oxygen and carbohydrate or fat which has been brought to the muscles by the blood.

We can see how severely shaken were current vitalistic doctrines by the new conceptions of Mayer and Helmholtz. Vitalism could never afterwards be taught in the form which it had assumed in Liebig's writings. If, however, we bear this in mind, we must also bear in mind that what may be called the mechanistic part of Liebig's teaching received also a severe shock when it was gradually shown, as pointed out in the second lecture, that oxidation is not a process occurring in the blood under conditions similar to those in ordinary liquids, but in the living tissues, and in a very definitely co-ordinated manner, as Rubner showed quantitatively.

Vitalism nearly disappeared during the development of the mechanistic movement dating from the middle of last century. It is easy to see, however, that physiologists and other biologists were simply swept off their legs by that movement. The real difficulties in the way of a mechanistic theory of life still remained, though they were for the time partially concealed owing to the rudimentary applications of physical and chemical measurement to physiological phenomena, and equally rudimentary understanding of the principles of physical chemistry. Liebig's conception of the conditions under which physiological oxidation occurs, or Ludwig's of renal secretion, may be taken as typical of mechanistic conceptions which still seemed possible till after the middle of last century. A popular and very readable representation of the mechanistic physiology of last century was contained in Huxley's Elementary Physiology, which had a deservedly large circulation. This book contains an admirably clear account of most of what can be interpreted as mere machinery in the bodies of the higher animals and man. Other phenomena indicative of co-ordination everywhere in physiological processes are either passed over very lightly, or not referred to, since they were then unknown. Huxley's attitude in this respect was typical of that of the leading physiologists of his time; and we can see the same ideas reflected in a great mass of the popular literature of present times.

In the previous lecture an endeavour was made to follow the recent development of the mechanistic conception of life, and we must now endeavour to do the same for viatlistic conception. Vitalism was by no means dead at the end of last century. Such wellknown books as Bunge's Physiological Chemistry and the papers and books of Driesch and other well-known morphologists are sufficient evidence of this. At the present time vitalism may not be popular, but it is still alive in scientific or philosophical writings. A notable recent example of very empathic vitalistic interpretation, and in the form adopted by Stahl, is afforded by Professor MacDougall's book onThe Principles of Psychology. This book is specially noteworthy, since MacDougall himself has studied physiology deeply.

The real strength of vitalism depends now, just as it did in the seventeenth century, on the fact that it gives recognition to the co-ordination which can be traced in the activities of living organisms from the earliest to the latest stages in the life-history of any individual organism. This co-ordination is of such a nature that the organism's normal structure and activities are actively maintained or reproduced. We find in the living body definite mechanical structures normal vital activities or to protect normal living such as bones, skin, lungs, etc., are found to be themselves alive, either partly or wholly—that is to say, they are constantly being actively maintained by co-ordinated activity; and in individual history they have all been originally formed by co-ordinated activity for which there is no mechanical explanation, and are so formed again and again, indefinitely often, in successive generations.

The fact of the co-ordinated maintenance and reproduction may not be evident to philosophers and theologians, but is forced on the attention of medical men. Co-ordinated activity is thus a fundamental characteristic of living organisms, and is not explicable in terms of special mechanical structure. To account for the co-ordination it therefore seemed necessary to assume the action of a factor wholly different in kind from matter and energy, but capable of guiding them in a co-ordinated manner, just as men guide the unco-ordinated operations of the inorganic world. The energy expended in guidance may be quite inappreciable in ordinary measurements, just as the energy expended in guiding a heat-engine or other powerful machine is inappreciable. Hence vitalism cannot be refuted by measurements of the total intake and output of energy in living organisms. The discovery of chemical sources of the heat and power developed in living organisms was thus no real refutation of vitalism. One might as well try to prove from measurements of the intake and output of energy by a locomotive that the driver does not exist.

Whether the assumed factor is called the “vital principle,” “entelechy,” or “élan vital” does not seem to matter much, though “vital force” is an objectionable expression, as suggesting that the vital principle is only a special form of physical energy, and is an appreciable source of the energy liberated in physiological activity.

However fully physiologists are now becoming aware of the fundamental significance of co-ordination in the phenomena of life, and however conscious they may be of the defects in the mechanistic physiology of last century, they show but little tendency to return to vitalism, and for very good reasons. The more deeply we probe into the conditions which determine any physiological phenomenon, the more clearly does it appear that it is dependent on what are generally interpreted as physical and chemical conditions. This is so, however strikingly the phenomenon may illustrate the co-ordination which is so characteristic of life, and however obscure may be the actual causal connexion between the phenomenon and the material change or “stimulus” which initiates it. Thus the prick of a needle or other kinds of disturbance may, as Loeb has shown, initiate the process of division and subsequent further normal development in an unfertilized ovum which would not otherwise divide and develop. We know nothing as to any chain of physico-chemical events connecting the prick with the complex and marvellously co-ordinated developmental changes; but the prick is nevertheless what we interpret as in itself only a physical event. A suitable supply of oxygen, or moisture, or of some salt or other substance, or of heat or light, may similarly stimulate various kinds of vital activity; and we can discover no sort of vital activity which cannot be regarded as being not only excited, but also maintained, by what we regard as physical and chemical stimuli.

If there be a vital principle, it is thus impossible to demonstrate its influence apart from that of physical and chemical influences. In any case, it seems more profitable to go on discovering the actual connexions between physical and chemical influences and vital changes than to speculate as to an influence of which the action can never be separated from physical and chemical action. There remains, however, the outstanding fact that, be the physical and chemical influences or stimuli what they may, the characteristic co-ordination of physiological activity is evident unless the influences are such as to bring about what we easily recognize as pathological or abnormal changes, or to cause what we are familiar with under the name of death.

Driesch has brought forward an instance in which he argues that he has practically isolated the action of the vital principle, or “entelechy,” as, following Aristotle, he calls it. He discovered that if, in the early stage of development of certain organisms, the cells of the developing organism are separated from one another by mechanical means, each of the separated cells is still capable of initiating the whole development again from the beginning. If we assume that the orderly changes occurring in the development of a fertilized ovum depend on some physico-chemical structure present in the ovum, we are also compelled to think that this structure must divide itself up among the developing cells, so that after division their intimate structure is different, and each individual cell cannot, therefore, again develop in the same way. But as a matter of fact each cell can again develop in the same way if placed in the same medium. Driesch considers this as evidence that cells can develop in a normal manner independently of any mechanism contained within them, or of changes in their environment.

When we examine this argument we find that it contains excellent reasons for concluding that the development from an ovum cannot be regarded as simply guided by some mechanism within the original ovum. In this regard the whole of the facts regarding hereditary transmission of structure and activity have always pointed clearly in the same direction, as has already been remarked. When, however, it is assumed by Driesch that the development must occur independently of stimuli from environment, his evidence is quite inconclusive. When the cells of the developing organism are separated from one another the immediate environment of each of them is at once altered, since, as was pointed out in the last lecture, that environment is not simply the general medium in which the developing organism is floating, but an environment altered by the presence of the other cells. When the cells are separated their primitive environment is restored, and they naturally return to their primitive physiological state. We can regard their characteristic normal behaviour when cell-division again progresses as the natural response to the same series of changes in environment as occurred in the original development.

The point we are thus brought to is that while we can always discover or indicate conditions in the environment which determine both organic structure and co-ordinated physiological activity, we can never demonstrate the existence of any factor other than what may be regarded as physico-chemical which determines this action. What still remains mysterious is the specific co-ordination of activity, and corresponding organization of structure. These are characteristic of all life, however lowly an organism may be. The idea that in lowly organisms or “simple” protoplasm we are getting beyond these characters has been a veritable ignis fatuus to biologists. To realize this we have only to refer back to some of the writings of Huxley.

In a recent book, entitled The Organization of Life, Professor Seba Eldridge has discussed this subject very fully. He states clearly the difficulties of a mechanistic theory of life, and also recognizes the difficulty of directly demonstrating the influence of any factor not of a physical or chemical nature. He concludes, however, that it is necessary to assume that such a factor exists, since it seems otherwise impossible to account for the co-ordination which is so characteristic of physiological activity. He is thus driven, like Professor MacDougall, to an acceptance of the vitalistic position. In explanation of the impossibility of directly isolating by experiment the operation of the vital principle he argues that it is only in conjunction with physical and chemical causes that the vital principle can act. It is, in fact, a regulative principle which requires for its manifestation a suitable physical or chemical process which it can regulate.

Now if we admit that processes in which matter and energy are concerned are by themselves purely mechanical in their ultimate nature, it must lead to great scientific confusion if an invisible and intangible something which interferes with these mechanical processes is assumed to exist within the bodies of organisms. With such an assumption we never know “where we are.” The assumption will “explain” anything and everything which occurs in a living organism; but in practice it cannot be definitely tested in the investigation of individual phenomena, and is thus practically useless in detail as a working hypothesis.

It is not only, however, this practical uselessness that has prevented physiologists from returning to vitalism, inadequate as mechanistic conceptions have shown themselves to be in the light of accurate measurement and fuller understanding of physical and chemical principles. What appeals to them still more strongly is that however unintelligible physically the phenomena of life may be, yet these phenomena can be shown by experiment to depend on what are admitted by the vitalists to be physical conditions in the environment. It is therefore these physical conditions which determine the phenomena of life, though how they do so is totally obscure for the present. Actual experience also shows that there is no end to the further light which experimental investigation of the physical and chemical environment may throw on any physiological phenomenon.

Vitalism is thus a quite unsatisfactory hypothesis, both ultimately and from the standpoint of scientific advance. The vitalists can, however, retort, and on conclusive grounds, as we have already seen, that the mechanistic theory of life is equally unsatisfactory, since it gives no account of the co-ordination which is characteristic of all vital activity, and leads investigators to ignore the co-ordination. So serious is this defect in the physiological teaching which is at present customary that physiological teaching in connexion with medical education does not occupy the important place which it certainly ought to occupy. The so-called elementary physiology taught to students has lost connexion with the co-ordinated living activity on the existence of which sound medical practice is based, and which old-fashioned language and ideas attributed to a vis medicatrix.

The mechanistic conception of life is still reckoned orthodox among physiologists and biologists generally. This is, I think, not because it is regarded as satisfactory, but because the only alternative has seemed to be the very unsatisfactory one of vitalism. The leaders in biology became definitely mechanistic about the middle of last century, and no sufficient reason exists for now going back to vitalism, though their expectations of what the mechanistic theory would accomplish were unjustified, and at any rate have not been fulfilled. Scientific history shows, moreover, that the inertia of scientific beliefs is a very powerful influence. It is hard to realize that what we have learnt from men whom we greatly respected may in reality be far from being true.

In the next lecture I shall endeavour to make clear the necessity for a far more radical treatment of the controversy between mechanists and vitalists than that of adopting one side in the controversy between them.