The questions now present themselves what is the scientific definition of a living organism? What are the scientific criteria of the distinction between an animal and a plant? Is it possible to give such definitions without the employment of psychological categories? It will I think be admitted that it is unreasonable to expect that these questions can receive answers that are more than tentative until Biological Science has reached a very advanced stage. During the earlier stages only amplifications qualifications and modifications of common-sense criteria and distinctions can be expected. In fact final answers to such questions must be regarded as goals to be attained if at all only when Biological Science has reached a very high stage of development. We must be prepared to contemplate as at least a possibility the answer to the question what is the ultimate distinction between living matter and non-living matter? to be that as long as we remain within the categories of Natural Science in the restricted meaning of the term which I have adopted in these lectures that is excluding all psychical and psychological categories and employing only those of physico-chemical schematism there is no such ultimate distinction. That in fact the difference between what is called living matter and what is called non-living material can be represented only as a difference of degree in complexity of structure and of the physico-chemical processes associated with the two types of material. In this connection it is well to remember that the range of organisms with which modern Biological Science deals has been enormously extended by the use of the microscope; and that thus the border-line cases of organisms of comparatively simple structure and functions inaccessible to ordinary observation form a most important part of the study in relation to such questions as those I have indicated.

However different the biological sciences in respect of procedure and history when regarded superficially may appear to be from the inorganic sciences the method applied in dealing with the phenomena in which living organisms are involved is fundamentally the same as that which is applicable to the inorganic domain. Based upon the observation sifting and classification of the facts of perception concepts and conceptual schemes are constructed which suffice to represent sequences of phenomena of certain classes in the domain under consideration. All that I have said in the earlier lectures as to the total inability of scientific method to attain to explanations in the full sense of the term of any happenings in the perceptual domain is as valid in Biology as in physics and Chemistry. All modern biological theories in their descriptions have reference to processes of the physico-chemical order and are consequently for this reason as well as others subject to the same limitations as Physics and Chemistry. Thus Biology as a great department of Natural Science is and from the nature of its methods must always remain unable to discover an answer to the metaphysical question what life is in what its essence consists. What it can do is to give conceptual descriptions of what living organisms do of the phenomena of which the parts of the organism are the seat and of the interactions of the organism with its environment which includes other living organisms of similar and dissimilar kinds. It can endeavour to ascertain the physical conditions subject to which what we call life manifests itself. As in the case of the inorganic Sciences the limitations imposed by the character of its methods have by no means always been fully recognized by those who have built up the edifice of Biology; and this has frequently led to the employment of language in the statement of theories and laws which taken as it stands implies that efficient causation has been discovered immanent in the processes and sequences that are described. The assumptions of physical realism have been accepted by many probably most men of Science; but of these assumptions Biological Science is really as independent as are Physics and Chemistry.

A fundamental question in relation to the character of Biological Science is that as to the nature of the concepts which it must employ in formulating its conceptual schemes and laws. In the first place it may be asked how far Biological Science can confine itself to the concepts employed in Physics and Chemistry; such concepts as are sometimes spoken of as mechanical although for historical reasons it is probably better to avoid the expression. The fruitful work done by modern Physiology has been carried out subject to the assumption that the categories of Physics and Chemistry are sufficient to form the basis of that work. The striking success of investigations of this order gives ample warrant for belief in the utility of the assumption as a working hypothesis and is such as to afford a justification for the hope that a continuation of investigations on the same lines may lead to an indefinite extension of physiological knowledge. It would however appear that these physiological investigations are confined to special processes in the organism and require integration before they can be applied to give an account of the coordinated happenings in which the organism as a whole is involved. Moreover it should be remarked that the concepts of Physics and Chemistry cannot be regarded as once for all fixed; at the present time they are very noticeably in a state of flux. Further it has to be taken into account that in accordance with the discoveries of the modern Science of Biochemistry the chemical processes in the living organism exhibit marked peculiarities which differentiate them from those which take place in non-living matter.

It may next be asked whether Biological Science requires in addition to the concepts of Physics and Chemistry further concepts of a kind not specifically psychical. Attempts have been made to supplement the physico-chemical concepts by others for the purpose of attaining a more complete and satisfactory account of what goes on in the living organism than can be provided with the help of the former concepts alone. The difficulties encountered in such attempts have been great. It has proved difficult or as some would say impossible define and delimit such concepts with a precision of the kind they must have if they are to perform a useful function in scientific theories. That the want of such concepts is widely felt is clear but however desirable their construction may be without adequate definition and circumscription and without precise postulations as regards their relations with the physico-chemical concepts they will represent little more than words employed to cloak ignorance. Besides the difficulties of precise definition and of correlation with existing concepts and assuming that these difficulties can be overcome there remains the question as to the utility of such concepts in formulating and extending physiological knowledge. It has for example been maintained by Dr J. S. Haldane that the concept of the organism as a whole as distinct from all the parts and their physico-chemical relations but having relations with those parts is a necessary one if we are to have any understanding of the living body. This concept of an entity which is concerned with the unification of the whole complex of phenomena in the living body is an exceedingly plausible one and from various points of view commends itself in a high degree to the mind. But has it been formulated and have its relations with the parts been formulated in such wise as to enable Physiologists really to increase their knowledge of the living organism? Is such a concept from the point of view of Physiology more than an aspiration an idea of great cogency? Does it assist in solving the problem of the integration of all the processes in the body? Has not all the precise knowledge which has actually been obtained by Physiologists as to these processes dispensed with this conception because it has lacked the requisite distinctness of outline? I gather that up to the present time however much its need may be felt such a purely physiological concept has not been constructed so as to satisfy the criteria I have indicated. There can of course be no a priori objection to the introduction of such a concept in Physiological Science provided it is properly defined and proves itself useful in furthering the aims of that Science. Physio logiest appear for the most part to think that the true line of progress of the knowledge of the organism which they seek to obtain must at the present time be in accordance with the methodological assumption that the living being is to be regarded for their purposes as a physico-chemical mechanism to be investigated by methods in which quantitative chemical processes and measurable physical processes are alone dealt with. It may be true and in the judgment of most of us it is true that the physico-chemical categories will always prove to be inadequate to do more than describe conceptually greater or smaller but certainly limited tracts of phenomena in the living organism. Nevertheless the Physiologist who in the light of present conditions accepts as his present policy that of restricting his investigations to the tracing out of physico-chemical and consequently measurable processes has full justification for his attitude; and he is the best judge of the prospective value of that policy. It is only when his policy is developed into a dogma that he gives an opening for legitimate criticism. It must I think be admitted that the problem of relating all the phenomena in the organism with a concept which represents the aspect of the whole organism not as a mere sum of parts but as a unified individual has not been solved. The problem of the relation of the one and the many has proved as intractable in Physiology as in the wider domain with which Philosophy concerns itself. In recent times the discovery that a living organism animal or plant contains except in the case of the simple uni-cellular beings an association of cells looser or closer according to the particular kind of organism each of which cells has its independent life but each of which also provides its specialized contribution to the life and maintenance of the organism as an individual whole has emphasized the urgent character of that special form of the problem of the relation between the one and the many to which the single organism gives rise.

Suggestions have also been made as to the introduction of concepts which should be capable of employment in those parts of Biology in which the individual organism is transcended and which should play a useful part in the formulation of racial relations. Such concepts perhaps in even greater degree than in the case of the single organism suffer from a vagueness of definition which unfits them for use in precise scientific theory however great may be the need for them felt by the speculative mind.

The question whether psychical or at least psychological concepts are to be admitted in Biological Science is one which besides being a methodological question of great importance is related to general issues of far-reaching import as regards our general views of the world. The existence of a psychical side of at least the higher organisms is now universally recognized and in the case of lower organisms some rudimentary psychical elements such as bare sensation indistinct awareness of changes in the environment and even some rudimentary form of memory of past experience are most frequently assumed to be present. In so far as such concepts of a psychical or psychological character are employed as an essential element in the conceptual descriptive schemes of Biology that Science does not wholly belong to Natural Science in accordance with the meaning to which I have restricted the term in these lectures but which restriction I have admitted to be open to very pertinent criticism and have adopted only for convenience. Biology may so far as such conceptions form part of it be described as a mixed Science partly physical and partly psychological. A survey of the history of Biological Science shows that many or most biological theories of a general kind have employed in their statement psychological concepts; but whether and how far these can be eliminated is a crucial question which has led to much diversity of opinion in recent times. In the special department of Physiology as I have already observed the recent tendency has been to employ a methodological restriction to physico-chemical concepts and thus to resume the processes within the living organism in conceptual schemes which involve these concepts only. This however does not or should not involve the dogmatic assumption that there are no limits to what can be attained by this method; and I shall presently give reasons which tend to prove that such limits must actually exist.

I have already in one of my earlier lectures observed that Psychology of which the object is to describe conceptually the mental processes of the typical individual has two methods open to it. The first is that of introspection in which the observer contemplates the processes in his own mind and accepts similar descriptions from others as regards their own minds. The second method is that of inference from the behavior of other persons that is from the physical phenomena exhibited by them and assumed to be in correlation with the psychical side of such persons. In the case of the psychology of animals the second method is the only one at our disposal. The first method is the one which affords the only basis for the interpretations of physical concomitants in man or in other animals which we make in drawing conclusions as to the psychical processes associated with them. We have direct knowledge of the psychical processes in our own minds and we have also knowledge of the perceptual or physical processes which we connect with the former. By what has been termed ejection we transfer the psychical side or at least some elements of it to other living organisms which appear to originate physical sequences of a kind similar to those which we regard ourselves as originating. Thus our knowledge of the psychical side of living organisms in general when that side is presumed to exist is essentially indirect and inferential. The question whether psychological concepts should be included or not in the conceptual laws and schemes of Biology would appear to depend largely if not wholly upon whether or not such concepts are regarded as necessary for the purpose of introducing an element of contingency into the sequences to which the laws and schemes have reference; and thus of at least to some extent impairing the value of such laws and schemes as instruments for predicting what will happen in concrete cases. If it is believed that no such element of contingency is involved in the use of psychological concepts it would appear that the psychical side of the living organism is relegated to the position of a mere epi-phenomenon which does not really affect the physical events; this is the position of the thoroughgoing psycho-physical parallelist. For those who hold this view the psychical concepts can then play no essential role in the conceptual theories and laws in question and might without real loss be eliminated from them.

I have at the beginning of these lectures emphasized the fact that it is not in the province of Science to deal with the purely individual; its role is to resume conceptually what a class of individuals have in common to extract the universal. When a conceptual scheme is applied in any individual case there is always some part greater or less in amount and importance of what happens or is observed which lies outside the scope of the scientific generalization employed. It may even be held that in general a scientific theory is essentially of a statistical character the individual peculiarities of particular percepts or trains of percepts to the description of which it is applied being left out of account but being always in some degree present. In the case of those percepts and trains of percepts with which Biological Science is concerned this residuum of an individual character is often and perhaps usually of much greater importance than in the cases in which no living organisms are involved. It is certainly true that the more highly organized a living being is the greater is the importance of the individual peculiarities which escaping all schematic description remain outside the purview of scientific schematism.

The contemplation of the problems presented by the living organism and its relations especially in the case of man and the higher animals brings us face to face with the question of how the relation between the psychical and the physical domains is to be conceived; of the relation between body and mind. It is a question which cannot be simply ignored in connection with any general view of the nature and scope of Biological Science although it may be very properly ignored by the investigators in many special departments of that Science. That the psychical side of a human being and his body which represents the construct of what we directly perceive exercise an apparent influence upon one another is a matter of common knowledge. A change in the moral character of a man is sometimes the apparent effect of a blow on his head which may be ascertained to be accompanied by a lesion in his brain. Conversely a psychical disturbance such as that produced by bad news is apparently the cause of marked physical disturbance in the body temporary or permanent sometimes even of death. One thing seems certain; that in any comprehensive view of the matter we can leave out of account neither the conceptual knowledge of the set of percepts which we call the physical side of a man or animal as resumed in its representation by physico-chemical descriptive schemes nor the direct apprehension which we have in our own cases of the psychical side of our being and which we are forced to admit by inference as being present at least to some degree of development in other animals; although its form may be of lesser complexity shading down to bare awareness or sensation as we descend the scale of animal existence. For Natural Science as applied to man and other living organisms the question takes the form what limitations if any does the presumed presence of the psychical factor introduce into the scope of that scientific method which attempts to describe in terms of physico-chemical mechanism the inner processes in living organisms and their relations with the environment? One answer to this question to which I have already referred in an earlier lecture that given by the thorough-going psycho-physical parallelist is that there is no such limitation because the psychical side is a mere Begleiterscheinung an epi-phenomenon of which the physical organism is really quite independent. Now it cannot possibly be maintained that the correctness of this view has been proved. The actual successes which have been attained in the representation of particular tracts of physiological phenomena as physico-chemical processes great as they undoubtedly have been are still at an immeasurable distance from the attainment of such proof in relation to the organism as a whole. The assertion of this view in its absolute form is then merely a dogma resting on nothing but an illegitimate extension to a whole of what may have been shown to be true of some parts. It is in direct contradiction with the immediate deliverance of our consciousness as to the real efficiency of the will the determinations of which we regard as completely dominated by nothing outside our own spiritual nature. Prima facie the validity of this primary intuitive apprehension is quite as much entitled to recognition as the more indirect conceptual knowledge provided by Natural Science. Nothing short of the most cogent evidence far greater than any that has hitherto been adduced would be necessary before we could properly admit that this intuitive apprehension is wholly illusory.

The attitude of the pure Idealist or of the Panpsychist is a quite different one. Their philosophical position is that of psychical monism just as the system of the psycho-physical parallelist amounts in practice though not necessarily in terms to materialistic monism. The view of psychical monism is that there is in reality but one system concerned in the living organism and that that one system is fundamentally psychical; that the separation into mind and body is artificial and if made cannot render possible the treatment of either part in complete independence of the other. Very much the same statement applies to the views of the neutral monist who does not attribute either to the physical or to the psychical elements a fundamental role but regards them both as aspects or modes of some reality more fundamental than either. However much there may be to be said from a philosophical point of view in favour of a monistic system of a kind which refrains from completely subordinating one side of the nature of the living organism to the other such a system has not yet been shown to be capable of being so developed in detail as to constitute an articulated monistic Science into which all the manifold particulars of organic life both on the physical and the psychical sides can be fitted. It follows that unless we are prepared to remain in a nebulous region of generalities we must necessarily for scientific purposes as in ordinary life remain content with a methodological dualism which provisionally recognizes in separation the two domains of the psychical and the physical. The investigation of these two domains are respectively the functions of Psychology and of Biology. That there are relations between the two domains is manifest and cannot be ignored on either side whenever a general view of the scope of either Science is to be formulated. The problem of formulating these relations is one of such great difficulty on account of the presumed or provisional disparateness of the two domains that no even tolerably complete solution is in sight. For those phenomena connected with living organisms in which mechanical or rather physico-chemical categories seemed to be insufficient or in which it seemed to be impossible to ignore the relations between the physical and psychical sides of the organism some method of provisionally representing those relations in a very general way has frequently been attempted in various vitalistic theories. The attempts made in the older vitalism to localize and delineate the nature of the interaction between body and mind were crude and had the character of makeshifts introduced at various points ad hoc. They frequently involved the use of such vague and hybrid expressions as “vital force”: and their inability to rid themselves of the nebulosity of view which the use of such expressions indicates has had the result that they have fallen into general discredit amongst Biologists. Renewed attempts have been made in recent times to deal with the problem of the relations between the physical and psychical sides of the living organism. On the one hand efforts have been made to conceive or rather to represent the nature of the influence which the psychical side exerts upon the physical side of the living being; and on the other hand the investigations which fall under the head of the department known as Psycho-physics or as Physiological Psychology have resulted in the construction of a mass of detailed empirical knowledge of the relations between physical sense-data and the sensations or the perceptions to which they are regarded as giving rise. I propose to give some indications of both these lines of thought.

A widely spread idea is that the action of the psychical upon the physical can be represented as of the nature of guidance which must manifest itself as physical guidance. This guidance is regarded as on the psychical side teleological in character. It is not always assumed that the consciousness of the individual is necessarily involved in it; it is often thought of as immanent in all the parts of the organism and manifesting itself in guidance of the motions of the molecular or submolecular constituents of the living body. Now it is clear that if the psychical side of a living being is to be regarded as having any intelligible relation with the physical organism and through that organism with the physical environment the organism cannot be regarded as simply and solely a machine or physico-chemical complex in which all the processes can be completely described as a dynamical system in which all the changes are completely determined by the inner relations of its parts and its relations with the external physical environment. The effect of the psychical side can only be represented as an actual interference with what would be the course of the physical system if it were independent of the psychical side. It is a matter for consideration what form this interference can take how it should be conceived as manifesting itself in the physical organism. This question can only be considered in the light of facts obtained by accurate observation of the actual working of the living body as a whole in relation to its environment and of the presumably connected processes in the various parts of the body. These facts of observation may be expected to throw light upon the points of similarity and of difference between a living body and a machine or system composed of non-living materials.

A living organism is a system of complex structure composed essentially of a mixture of substances of very complex chemical constitution which fall under the term protoplasm and also a large amount of water and some small admixture of other substances. The protein substances are relatively stable and yet in the living organism they are continually breaking down into chemical substances of simpler atomic constitution a process known as katabolism; and they are constantly being built up this latter process being known as anabolism. To the totality of physico-chemical changes in the living substance of the organism the term metabolism is applied. In an anabolic process energy is required for the purpose of transforming substances of comparatively small chemical energy into more complex substances of much higher chemical potential. On the other hand in katabolic processes energy is liberated by chemical changes of the reverse kind. In the higher animals the proteins carbohydrates and fats contained in the food are broken up into simpler constituents in the process of digestion and are then synthesized into more complex constituents by an anabolic process and become capable of being transported by the tissue fluids to all parts of the body. In the animal body an important part is played by the purely physical processes of osmosis and diffusion of liquids in the circulation of food materials secretory and excretory substances from blood to lymph and from lymph to cell-substance or to glandular cavities. All such processes anabolic katabolic and mechanical may be regarded from the point of view of Energetics of which the concept of energy and its transformations is the basis; and the question arises whether or how far the principles of Energetics apply to the living body and to its various parts? As regards the law of Conservation of Energy known also as the first law of Thermodynamics it appears to have been established as the result of various investigations conducted with great care that the law holds good for the living body of an animal. The energy of the food and oxygen absorbed by the animal is shown to have its equivalent in the mechanical work done by the animal together with the energy lost from its body by conduction and radiation. In general the metabolism of the animal as a whole and the chemico-physical changes in detail conform to the law of Conservation of Energy. The forms which energy takes in the living body are mechanical chemical thermal and electrical; in fact the same forms as are found in the inorganic domain. The existence of no form of energy peculiar to living matter has to be postulated as a condition that the law of Conservation of Energy should hold good for the organism and its parts. The idea that some such special form of energy often denoted by the term Biotic Energy is required to account for the phenomena in the living organism can thus in all probability be rejected. The whole weight of evidence goes to show that the influence of the psychical side upon the physical side of the organism cannot be conceived of as taking the form of a supply of energy and it must consequently be such as to be consistent with the law of Conservation of Energy; the various forms of the energy in the living organism being the same as in the non-living domain. But it by no means follows from this that the physical side of the living being must be regarded as independent of the psychical side. For the changes in a system are not determined by the law of Conservation of Energy alone; the nature and amounts of the transformations of one form of energy into equivalent amounts in other forms have to be known before the changes in the system are completely determinate. It is just in these transformations from one form of energy to another that striking differences are found between the cases of the living organism and of the non-living domain.

With a view to the consideration of the nature of these differences let us compare in respect of the transformations of energy the processes which go on in the working of a steam engine and in a warm-blooded animal. In a steam engine the source of the energy from which the work done by the engine is derived is chemical potential energy of the coal. When the coal is burned in the boiler-furnace this energy is transformed into heat which vaporizes water in the boiler producing steam at high temperature. By the expansion of the steam in the high-pressure cylinder mechanical work is done in driving forward the piston. The steam is then cooled in the condenser. During the whole process a certain amount of heat is converted into an equivalent amount of mechanical work. The high chemical potential energy of the coal and oxygen is transformed in large part into the energy of heat the remainder being the low chemical potential energy of the residual products. Of the heat produced a large part is dissipated or rendered unavailable for use by radiation from the boiler steam-pipes and other parts of the machine and by friction or remains as low-temperature heat in the water of the condenser. The nett result is that only a comparatively small part of the energy derived from the chemical transformation of the coal and oxygen has been made available by the engine for the performance of mechanical work. In the warm-blooded animal in which the temperature is maintained at a constant level usually higher than that of the medium in which it lives the source of energy is the chemical energy of the food and oxygen which is taken into the body. These substances undergo chemical transformations in the alimentary canal and in the tissues of the body in the processes of digestion and assimilation. A certain proportion of the food taken into the body has become a part of the muscles nerves etc. of the body. At the same time portions of the substances of high chemical potential the proteins carbohydrates and fats are transformed into water urea and carbon-dioxide which are substances of relatively low chemical potential. The energy taken into the animal reappears in large part as mechanical work that used in bodily movements in the motion of the heart lungs blood etc.; and a part appears as heat sufficient to compensate the loss of heat by radiation and conduction from the body; also a portion is employed in the formation of digestive juices and in the propagation of nerve currents A chief point in which this process is distinguished from that which goes on in the steam engine consists in the much smaller part played by the production and dissipation of heat. It appears that the chemical transformations proceed without the production of anything approaching the amount of heat developed when chemical transformations occur in connection with non-living matter. The changes from chemical to mechanical energy appear to take place more directly than in inorganic processes; a comparatively small amount of heat is produced and the dissipation of energy is accordingly greatly reduced. This feature of the transformation of chemical energy with only an insignificant production of heat is still more marked in the case of the cold blooded animals whose temperature is nearly the same as that of the medium and rapidly adjusts itself to changes in the temperature of the medium. In their case the rate of the metabolic changes is dependent on the varying temperature of the environment whereas the corresponding changes in the warm-blooded animal are within limits almost independent of the temperature of the environment. It thus appears that the living animal considered as a chemico-physical mechanism in which energy is received from outside and is converted into mechanical work done by the organism is very much more efficient than is the steam engine or any other machine that can be constructed for doing mechanical work; in the sense that the proportion of the energy supplied that is transformed into such work is much greater in the former case than in the latter.

The transformation of energy which goes on in the cells of the green leaves of plants is of a still more peculiar character. Whilst the animal receives substances of high chemical potential in its food the green plant receives carbon-dioxide and water substances of relatively low chemical potential and transforms them in the tissues of the leaves into starch a substance of much higher chemical potential. The energy required to produce this transformation is derived from the solar radiation; in some manner the details of which we are unable at present completely to describe the chlorophyll in the cells of the green plant absorbs these radiations and utilizes their energy in the anabolic process of building up starch. This starch is converted into soluble sugar which is circulated through the vessels of the plant. The plant at the same time absorbs nitrates from the soil and the soluble sugar together with the nitrogenous salts is employed in building up and accumulating proteins and other organic substances in the plant. During these processes there is no dissipation of heat and only a little mechanical work is done by the plant in connection with the circulation of protoplasm in the movements of the tendrils and in other ways. The general effect of the processes which go on in the living plant is to build up retain and accumulate a store of energy in a form in which it is available for doing mechanical and other work when it is liberated. This effect is produced by transforming energy from the sun which would otherwise be dissipated and become unavailable. In the general economy of living organisms the energy of the organic materials in vegetable foods built up in the manner I have described is essential for the existence of animal life. Moreover the stores of energy in wood and the energy derived in past ages from the solar radiation and stored up in coal form the chief sources from which we draw the thermal energy which we require for the practical purposes of human life.

The transformations of energy which occur in machines and generally in the inorganic domain are irreversible always in one direction tending to the diminution of available energy. This fact of observation has been formulated in the second law of Thermodynamics the most precise form of which is the law of Clausius that in any isolated system the entropy tends to a maximum. The question arises whether a living organism considered as a physico-chemical mechanism is such that its transformations of energy are in accordance with this law? It is probably difficult to show that there is in the organism a direct infringement of the law; that the processes are reversible; but it is clear that as a matter of degree the amount of dissipation of energy in the processes connected with living organisms is very much less than in any corresponding processes connected with a non-living mechanism alone. The second law of Thermodynamics is usually regarded as a statistical result which holds for the vast swarms of particles in motion in all directions of which material systems are conceived to consist. From this point of view either the law would not hold good or the consequences to which it leads would be modified in degree in a system in which the number of molecules involved in a particular transformation of energy was so small as to make the purely statistical method inapplicable. A similar default in or modification in the effect of the law might occur if we conceived that by some selective process applied to the individual particles without any alteration in the energy of their motions but affecting the directions of these motions the statistical result obtained by considering all directions as alike in reference to the motions were invalidated. In either of these ways it might be possible to account for the striking difference which I have described between the transformations of energy in the living organism and in the mechanisms to which the second law of Thermodynamics applies in its most precise form. These considerations give rise to the suggestion that the peculiarities of the processes which occur in the living organism may be regarded as due to some control of the transformations of energy in the organism which is not present in the case of the processes of a corresponding kind which occur in the inorganic domain. As we have already seen unless the supposed influence of the psychical side of the living organism is merely an illusion there must be somewhere or somehow what can only be represented as an interference manifesting itself within the physical processes themselves; and it would appear that some regulation or control of the transformations of energy is the way in which this interference can be best conceived; for the facts of observation appear to negative the idea that any supply of energy which would nullify the law of conservation is involved. The processes in the living organism may thus be represented as controlled by an agency of which the effect is to favour the anabolic building up of stores of chemical energy of high potential and by release of such energy to permit of sudden transformations of it into mechanical work.

A view of the character of the processes in the living organism in this order of ideas has been developed both on the biological and on the philosophical side by Prof. Driesch in his theory of entelechy. He regards the mode of operation of entelechy to consist in suspension of such transformations of energy as would be possible on the basis of pre-existing differences of intensity for example of chemical potential and further in the relaxation of such suspension. He regards entelechy as non-material and non-spatial but acting so to speak into space. Although it regulates the transformations in the organism its effects are strictly limited by the possibilities afforded by the structure of the organism; its action must be subject to given preformed conditions of a physical kind and it does not itself involve the supply to or the extraction from the organism of energy in any form. A full account has been given in his Gifford lectures by Driesch of the “proofs” and arguments upon which his vitalistic theory rests. I must rest content with having indicated in the briefest manner a possible mode in which the purposive activity of a living organism may be held to manifest itself in the physical processes in the organism and consequently indirectly in some degree in physical sequences in the physical world in general.

It is no doubt possible to exaggerate the part which the mental side of an organism should be regarded as playing in what happens in the body. In this connection the experiments of Loeb and his deductions from them leading to his theory of tropisms are of much interest-he has endeavoured with apparent success to account for such phenomena as the turning of organisms of certain species towards the light or in other cases away from the light on purely physico-chemical principles without taking account of the existence of sensations or of will as mental factors in the phenomena. He attempts to show that in such cases neither the notion of will nor that of blind instinct prompting the organisms to their actions is necessary. The following utterance shows that he is sanguine as to the extension of his substitution of physico-chemical description for psychical factors to a degree which but few will regard as warranted by any evidence which we possess at present. He writes¹:

However far Loeb's mode of viewing the responses to stimuli in the case of lower organisms may be justified in the case of man and higher animals it very soon reaches limits which cannot be passed. The lack of uniformity in the responses of men to external stimuli is no doubt in part traceable to differences in their organic constitutions dependent on differences of past history but at least in the case of the more complicated complexes of stimuli when all possible allowance has been made for individuality of physical constitution there remains a residuum which renders futile the hope or the fear that the notion of an individual psychical character exhibiting itself in the nature of the response to such stimuli can ever be dispensed with. So far as this individual psychical element is operative it escapes not only physical analysis but also schematic psychological analysis. In it consists the freedom of will of the individual not necessarily to be regarded as ultimately purely arbitrary but as not completely determined by or capable of being linked up in a determinate scheme with anything foreign to itself. The recognition of this fact does not make all phenomena in which that element is concerned wholly inaccessible to scientific description; for a determination of the will is preceded and succeeded by physical sequences of such a character that they or at least portions of them are representable by psychophysical conceptions; and on the psychical side a somewhat similar statement holds as regards psychological sequences.

The physiologists of the eighteenth century especially von Haller were the first to demonstrate the importance of the properties of irritability and sensibility in the nervous system and to emphasize the function of the central organ of the nervous system the brain in synthesizing the elements which represent sensation on the physical side. But the emergence of the Science of Psycho-physics or Physiological Psychology as a distinct department may be regarded as due to Cabanis (1757–1808) who laid its foundations in his Rapports du Physique et du Moral de l'Homme. His leading idea related with the Philosophy of Locke was that the function of the brain in relation to thought is parallel to the physical functions of other organs of the body as he expresses himself:

This amounts apparently to a practical subordination of thought to its physical concomitants.

At the end of the eighteenth and the beginning of the nineteenth centuries many vague and fanciful theories arose in the shape of speculations connected with animal electricity animal magnetism mesmerism and Phrenology which retarded the progress of the Science and for a time produced a deflection from those really fruitful lines of investigation which were approached by the school of Johannes Müller and were later put on a firm basis by Helmholtz and Emil Du Bois-Reymond. The distinction between sensory and motor nerves that the anterior nerves of the spine as efferent or motor nerves carry the nervous stimulus to the different organs whilst the posterior nerves as sensory or afferent nerves carry the peripheral stimuli of the senses to the nervous centres was established by the labours of Charles Bell Magendie and Johannes Müller. The last of these investigators in his doctrine of “specific energies” introduced another important distinction into the theory of the sensory nervous apparatus. This theory was afterwards adopted and its importance emphasized by Helmholtz who brought it into connection with the theory of colour first advanced by Thomas Young. In accordance with the doctrine of specific energies by the stimulus of any single nerve-fibre only such sensations can be produced as belong to the order of one definite sense and every stimulus which affects this nerve produces only sensations belonging to this definite order for example any effective stimulus of the optic nerve-apparatus produces always the sensation of light whereas the same stimulus would if effective produce in the auditory nerve-apparatus the sensation of sound. Thus the quality of our sensations depends not on the stimulus but on the nervous apparatus. A prodigious amount of physiological and anatomical work has been done with a view to elucidate the functions of the external or terminal organ relating to a particular sense of the connecting nerves and of the central organ situated in the brain which we must regard as related on the physical side with perception of the particular kind. Of first rate importance in this order of investigation is the work of Helmholtz in Physiological Optics and Physiological Acoustics. He showed for example that the ear when subjected to anatomical and acoustical analysis exhibits itself as a delicate resonator which absorbs the different elementary periodic movements into which musical sounds can be analysed harmonically and that different nerve-fibres carry them separately to the central organ in the brain with which we must regard perception of sound as connected. He was enabled to relate the quality of musical notes known as “timbre” which is different for the same note produced by different instruments with the physical production of the notes. Helmholtz accepted Young's hypothesis that there exist in the eye three distinct kinds of nerve-fibres related to three distinct modes of colour sensation corresponding to the three simple colours red green and violet of which all colours are compounded. As regards sound he regarded differences of pitch and character in notes as dependent upon the differences of sensitive nerve-fibres; each nerve-fibre exhibiting only difference of intensity of the stimulus. That this should be so is in accordance with the doctrine of specific energies.

The investigations of the brothers Weber and especially of E. H. Weber of Leipzig were commenced even before those of Johannes Midler. They had as their object the establishment of the relation between the subjective side of sensation and its physiological side. This experimental work was continued by Fechner to whom the term Psycho-physics is due. His treatment of the subject as represented in his Elemente der Psycho-physik published in 1860 was an investigation of the relations of mind and body involving the measurement of psychical quantities and the establishment of their correlation with physical quantities. The philosopher Herbart had already attempted to subject psychical phenomena to exact methods of calculation and Lotze had suggested the existence of a definite and constant connection between sensation and stimulus. The fundamental difficulty of such quantitative correlations consists in the fact that sensation is not an extensive magnitude consisting of equal units although a particular sensation may have intensive magnitude; and this fact has naturally led to much criticism of all attempts at correlation in a mathematical form between the psychical and physical sides of sensation. Of the investigations of Wundt and many other investigators in this important field it is impossible for me to speak. A flood of light has been thrown by these investigators upon the physical processes within the organisms which are related with sensations and perceptions. But great as is the scientific importance of the detailed investigations of Physiological Psychology and invaluable in their application to practical problems of life as the results obtained and to be obtained may prove to be there remains a gap between the physical and psychical sides of the organism unbridged and perhaps from its very nature impassable for our intelligence.

In many respects the progress of the Science of living organisms has been much slower and more difficult than in the cases of those branches of Science which deal with inorganic processes. This can be accounted for in a large measure by the fact that the extreme complexity of structure and of function of the living organism makes the process of isolating those elements which should be examined as separate objects or processes a much more intricate and difficult one than in the class of phenomena with which only Physics and Chemistry are concerned. In the latter Science a particular phenomenon which it is desired to examine can be more readily submitted to laboratory experiments in which the requisite conditions of isolation from other and disturbing elements are artificially produced than in biological Science where the phenomena to be examined occur in connection with an animal or plant simply as it is given as a whole with all its complex inter-relations and in which the complete isolation of a particular process is either impossible or can only be made with some approximation by the employment of a very high degree of technical skill. In Biology the problems of mere classification are more complex and occupy a much larger place in the history of the development of the Science than is the case in the inorganic Sciences. That stage of Biological Science which is resumed under the term Natural History is one of vastly greater extent than what corresponds to it in such sciences as Physics and Chemistry; moreover it must be regarded as still far from complete.

The method of abstraction and generalization by which scientific laws and theories descriptive of processes are set up is of slower and more difficult application in Biology than in the inorganic Sciences. Great as have been the difficulties of reaching conceptual laws which resume extensive tracts of phenomena in the inorganic Sciences and however incomplete the results of this process may still be in the case of the Biological Sciences the difficulties of such achievement have been still greater and the stages at present reached must be in all or most departments regarded as less advanced than in the sciences which are not concerned with life. The degrees of abstraction and concreteness are very various in different departments of Science but the ultimate aims of all branches of Biological Science are parallel with those of inorganic Sciences. Much of Biology remains purely descriptive with a low degree of abstractness and a large amount of detail remaining to be filled up....

Many of the great generalizations of Biological Science have been due in large measure to the breadth of view attained by those who examined living organisms their habits distribution and environment on a large scale as travellers; and who thus avoided the narrowing effect of too exclusive occupation with work in the laboratory. Such work in the laboratory is largely of an anatomical and morphological character in which the dead organism is examined; and even in the physiological work the living organism is torn asunder from the environment in which its race was developed.

In the next three lectures I propose to sketch the origin and development of some of the great general theories and special departments of Biological Science.