Two main conclusions emerge from the discussions of the last three chapters: (1) that physiology encourages no notions as to a “vital principle” or force, or form of energy peculiar to the organism; and (2) that although physiological analysis resolves the metabolism of the plant and animal body into physico-chemical reactions, yet the direction taken by these is not that taken by corresponding reactions occurring in inorganic materials. From these two main conclusions we have, therefore, to construct a conception of the organism which shall be other than that of a physico-chemical mechanism.

The ordinary person, unacquainted with the results of physiological analysis, and knowing only the general modes of functioning of the human organism, has, probably, no doubt at all that it is “animated” by a principle or agency which has no counterpart in the inorganic world. This is the “natural” conclusion, and the other one, that life is only an affair of physics and chemistry, must appear altogether fanciful to anyone who knows no more than that the heart propels the blood, that the latter is “purified” in the lungs, that the stomach and liver secrete substances which digest the food, and so on. It is difficult for the modern student of biology, saturated with notions of bio-chemical activities, gels and sols and colloids and reversible enzymes and kinases and the like, to realise that the belief in a vital agency is an intuitive one, and that the mechanistic conception of life is only the result of the extension to biology of methods of investigation, and not a legitimate conclusion from their results.

To the anatomist, the embryologist, and the naturalist, as well as to the physicist unacquainted with the details of physiology, no less than to the ordinary person this is perhaps by far the most general attitude of mind. It would probably be impossible for anyone to study only organic form and habits and come to any other conclusion than that there was something immanent in the organism entirely different from the agencies which, for instance, shape continents, or deltas, or river valleys. And this conclusion would probably come with still greater force to the embryologist, even though he still possessed a general knowledge of physiological science.

The mechanistic conception of life has, without doubt, been the result of the success of a method of analysis. One sees clearly that just in proportion as physical and chemical sciences have been most prolific of discovery, so physiology, leaning upon them and borrowing their methods, has been most progressive and mechanistic.

Mechanistic hypotheses of the organism may all be traced back to Descartes, who built upon the work of Galileo and Harvey. The anatomy of Vesalius and his successors would have led to no such notions, had not the discoveries of Copernicus, Tycho, and Kepler shown men an universe actuated by mechanical law. To a thinker like Descartes, at once the very type of philosopher and man of science, Harvey’s discovery of the circulation of the blood must have suggested irresistibly the extension of mechanical law to the functioning of the human organism, and it is significant that he made this extension without including a single chemical idea, and yet produced a logical hypothesis of life as satisfactory and complete in its day as, for instance, the Weismannian hypothesis of heredity has been in ours.

His hypothesis of the organism was purely mechanical. It has been said that his organism was an automaton, like the mechanical Diana of the palace gardens which hid among the rose-bushes when the foot of a prying stranger pressed upon the springs hidden in the ground. Its functions were matters of hydraulics: of heat, and fluids, and valves. His physiology was Galenic, apart from Harvey’s discovery of the motion of the blood in a circuit, for he did not accept the notion of the heart as a propulsive apparatus. The food of the intestine was absorbed as chyle by the blood and carried to the liver, where it became endued with the “natural spirits,” and then passing to the heart it became charged with the “vital spirits” by virtue of the flame, or innate heat, of the heart, and the action of the lungs. This flame of the heart, fed by the natural spirits, expanded and rarefied the blood, and the expansion of the fluid produced a motion, which, directed by the valves of the heart and great vessels, became the circulation. The more rarefied parts of the blood ascended to the brain, and there, in the ventricles, became the “animal spirits.”

Subtle and rarefied though they were, these animal spirits were a fluid, amenable to all the laws of hydro-dynamics. This was contained in the cerebral ventricles, and its flow was regulated just like the water in the pipes and fountains of the garden mechanisms. From the brain it flowed through the nerves, which were delicate tubes in communication with the ventricles, and which were provided with valves; and this outward flow corresponds to our modern efferent nervous impulse. The afferent impulse was represented by the action of the axial threads contained in the nerve tubuli. When a sensory surface was stimulated, these threads became pulled, and the pull, acting on the wall of the cerebral ventricle, caused a valve to open and allowed animal spirits to flow along the nerve to all the parts of the body supplied by the latter. In the effector organs, muscles or glands, this influx of animal spirits produced motion or other effects. This, in brief, was the physiology of Descartes.

He spoiled it, says Huxley, by his conception of the “rational soul.” Fearing the fate of Galileo, he introduced the soul into his philosophy of the organism as a sop to the Cerberus of the Church. It was unworthy: a sacrifice of the truth which he saw clearly. Is it likely that Descartes deliberately made part of his philosophy antagonistic to the rest with the object of averting the censure of the Church? He was not a man likely to rush upon disaster, but the conviction that what he wrote had in it something great and lasting must have made it hardly possible that he should traffic with what he held to be the truth.

The rational soul was something superadded to the bodily mechanism. It was not a part of the body though it was placed in the pineal gland; a part of the brain, which by its sequestered situation and rich blood supply suggested itself as the seat of some important and mysterious function. Its existence was bound up with the integrity of the body, and on the death of the latter the soul departed. But the body did not die because the soul quitted it, it had rather become an unfit habitation for the soul. Without the latter the functions of the healthy body might still proceed automatically, and if the soul influenced action it actuated an existing mechanism, and without that mechanism it could not act, though the mechanism might act without the soul. Thought, understanding, feeling, will, imagination, memory, these were the prerogatives of the soul, and not those of the automatic body. But the movements of the latter, even voluntary movements, depended on a proper disposition of organs, and without this they were wanting or imperfect.

Thus to a thoroughgoing mechanism Descartes joined a spiritualistic and immortal entity; and this, to the materialism of the middle of the nineteenth century, was the blemish on his philosophy. Now of all men who have ever lived he is probably the one who has most profoundly influenced modern thought and investigation: to us what he wrote seems strangely modern, and this apparently arbitrary association of spiritualistic and materialistic elements in life seems almost the most modern thing in his writings. Being, he said, was indeed thought, but how could he derive thought from his clockwork body, with its valves and conduits and wires? No more can we derive consciousness from the wave of molecular disturbance passing through afferent nerve and cerebral tracts. We must account for all the energy of this disturbance, from its origin in the receptor organ to its transformation into the wave of chemical reaction in the muscle, and we must regard its transmission as a conservative process. But how does the state of consciousness accompanying the passage through the cortex of this molecular disturbance come into existence? None of the energy of the nerve disturbance has been transformed into consciousness: the latter is not energy nor anything physical. It is something concomitant with the physico-chemical events involved in a nervous process, an “epiphenomenon.” We have to imagine a “parallelism” between the mechanistic body and the mind. But if we admit that consciousness may be an effective agency in our behaviour, what is the difference between modern theories of physico-psychic parallelism and the Cartesian theory of a rational soul in association with an automatic body? Descartes denied the existence in animals other than man of the rational soul; the latter was not necessary. But he, like us, must have been familiar with reflex actions and must have seen that consciousness was not invariably associated, even in himself, with bodily activity. And he must have recognised the great distinction between the intelligent acting of man and the instinctive behaviour of the lower animals. There was something in man that was not in the brute.

Thus the first physiology, borrowing its ideas and methods from the first physics, was, like the latter, a mechanical science. After Galileo and Torricelli came Borelli with his purely mechanical conceptions of animal movement, and of the blood circulation, introducing even then mathematics into biology. There was no chemistry in these speculations, though Basil Valentine and Paracelsus and Van Helmont had preceded Descartes and Borelli. This chemistry was mystical, and though chemical reactions had been studied in the organism, they were supposed to be controlled by spiritual agencies, the “archei” of the first bio-chemists. But that notion was to disappear, and with Sylvius the conception of the animal body as a chemical mechanism arose. All that was valuable in Van Helmont’s chemistry was taken up by Sylvius, but in his mind the fermentations of the older chemists were sufficient in themselves without the mystical “sensitive soul” and “archei.” With Sylvius and Mayow physiology became based upon chemical discovery and again became mechanistic, and remained so until the time of Stahl, when chemical discovery attained for the time its greatest development.

The seventeenth century ended with the work of Stahl. It is well known to students of science how the views of this great chemist sterilised chemical investigation almost until the time of Lavoisier. The notion of phlogiston as an active constituent of material bodies entering and leaving them in their reactions with each other was a clear and simple one, and it served as a working hypothesis for the chemists who immediately followed Stahl. It was, of course, a false hypothesis, and retarded discovery to the extent that the greater part of the eighteenth century is a blank for chemistry, when compared with the seventeenth and nineteenth centuries. Deprived therefore of the stimulus afforded by new physico-chemical methods of investigation, physiology ceased to maintain the progress it had made during the previous century, and the only great name of this period is that of von Haller. Comparative anatomy, and zoological exploration, on the other hand, made enormous advances, and for these branches of biology the eighteenth century was the great period. It was the period of the historic vitalistic views—vital principles, and vital and formative forces. Stahl’s teaching dominated physiology just as it did chemistry. Chemical and physical reactions occurred in the living body just as they did in non-living matter, but they were controlled and modified by the soul, or vital principle. It has been said that Stahl’s vitalistic teaching retarded the progress of physiology, but it does not seem clear that this was the case. What did retard physiological discovery was the lack of progress made by chemistry and physics, and this may have been the result of the Stahlian phlogistic hypothesis.

However this may be, it seems clear that it was the discoveries of the great chemists of the close of the eighteenth century that again introduced mechanistic views into physiology. With the discoveries of Lavoisier and his successors the latter science acquired new methods of research and the older working hypotheses were re-introduced. There has been no recession from this position during the nineteenth century. Mechanistic biology culminated in the writings of Huxley and Max Verworn and received a new accession of strength almost in our own day in the modern discoveries of physical chemistry; and when physiology became truly a comparative science, and embraced the lower invertebrates, it became perhaps most mechanistic—witness the writings of Jacques Loeb.

Of far greater philosophical importance than the physico-chemical investigation of the functioning of individual organisms has been the essentially modern experimental study of embryological processes. The former deals essentially with the means of growth, reproduction, and so on. We can no longer doubt that the changes which we can observe taking place in the organism, either the developing embryo or the fully formed animal, are, in the long run, physico-chemical changes; and in ultimate analysis we cannot expect to find anything else than processes of this nature.

But physiological investigation has failed to provide anything more than this analysis. If we watch the development of the egg of an animal into a larval form, and continue to trace the metamorphosis of the larva into the perfect animal, we cannot fail to conclude that, beside the individual physico-chemical reactions which proceed, there is also organisation. The elementary processes must be integrated. There must be a due order and succession in them. In studying developmental processes, in considering the developing organism as a whole, we are impressed above all else with the notion that not only do physico-chemical reactions occur, but that these are marshalled into place, so to speak. When we attempt to make a description of this integration of those ultimate processes which we can describe in terms of physical chemistry, physiology fails us. “At present,” says Morgan, “we cannot see how any known principles of chemistry or of physics can explain the development of a definite form by the organism or by a piece of the organism.” It is true that we can attempt to imagine a physico-chemical mechanism which is the organisation of the developing embryo; but this must be a logically constructed mechanism, not only incapable of experimental verification, but which can also be demonstrated, purely by physical arguments, to be false. This conclusion may, without exaggeration, be said to be that of modern experimental embryology.

There have always been (in modern times) two views as to the nature of the embryological process: (1) that the egg contained the fully formed organism in a kind of rolled-up condition, and that the process of development consisted merely in the unfolding (evolution) of this embryonic organism, and in the increase in volume of its parts. This was the hypothesis of preformation held in the beginning of embryological science. It involved various consequences: the limitation, for instance, of the duration of a species, since each generation of female organisms contained in their ovaries all the future generations; with other consequences which the preformationists did not hesitate to accept. (2) The other view was the later one of epigenesis: the egg was truly homogeneous and the embryo grew from it. Obviously the acceptance of this hypothesis led to vitalism, and we find that it was abandoned just as soon as the embryologists recognised that physics provided a corpuscular theory of matter, when a return was made to the preformation views of earlier times; views which lent themselves to the construction of a mechanistic hypothesis of development.

We may state very briefly the main facts of the development of a typical animal ovum, such as that of the sea-urchin.

The fertilised ovum divides into two (2), and then each of these blastomeres divides again in a plane perpendicular to the first division plane (3). The third division plane is at right angles to the first two, and it cuts off a tier of smaller blastomeres from the tops of the first four. There are now (4) two tiers of blastomeres, a lower tier of large blastomeres and an upper tier of smaller ones. This is the 8-cell stage. Next, each of these blastomeres divides in two simultaneously so that the embryo now consists of sixteen cells. After this the divisions proceed with less regularity, but after about ten divisions the embryo consists of about 1000 cells (2¹⁰), and these are arranged to form a hollow sphere consisting of a single layer of cells. The latter are furnished with cilia, and the whole embryo, now known as the blastula, can swim about by the movements of these cilia. Further development results in another larval form—the gastrula, and yet another, the pluteus larva. After this the transformation into the fully formed sea-urchin occurs.

With various modifications this scheme represents the early development of a very large number of animals belonging to most groups.

If we study the process of cell-division we shall find it very complicated. The ovum, immediately after fertilisation, consists of two main parts, the nucleus and the cytoplasm.

Within the nucleus is a substance distinguishable from the rest; it is distributed in granules and is called the chromatin (1). When the cell is about to divide this chromatin becomes arranged in a long coiled thread (2), and then (3) this chromatic thread breaks into short rods called chromosomes. Two little granules now appear, one at each end of the nucleus, and very delicate threads, the asters, appear to pass from each of these bodies towards the chromosomes (4). Each of the latter then splits lengthways into two, and a half chromosome appears to be drawn by the asters towards the poles of the nucleus. The latter then divides (5) and then the whole cell divides. What thus, in essence, happens in nuclear divisions is that the chromatin of the nucleus is more or less accurately halved. Apparently this substance consists of very minute granules and the whole process is directed towards the splitting of each of these granules into two. A half-granule then goes to each of the daughter nuclei. Every time the embryo divides this process is repeated. Thus each of the (theoretically) 1028 cells of the blastula contains 1/1028th of the substance of each chromatic granule in the fertilised ovum.

Pfluger and Roux (in 1883 and 1888 respectively) were the pioneers in the experimental study of the development of the ovum, and the results of their work and that of their successors has, more than anything else in biology, modified and shaped our notions of the activities of the organism. Roux found, or thought so at least, that the first division of the frog’s egg marked out the right and left halves of the body, the one blastomere giving rise to the right half, the other to the left half. The next division, which separates each of these blastomeres, marked out the anterior and posterior parts of the embryo. Thus:—

Now in an experiment which has become classical Roux succeeded in killing one of the blastomeres in the 2-cell stage, while the other remained alive. The uninjured blastomere then continued to develop, but it gave rise to a half-embryo only.

Upon these experiments the Roux-Weismann hypothesis of development—the “Mosaik-Theorie”—was developed. The lay reader will see how obviously the facts of nuclear division and the experimental results indicated above lend themselves to a mechanistic hypothesis. Notice that but for the physical conception of matter as made up of molecules and atoms the mosaic-theory would hardly have shaped itself in the minds of biologists. But this notion of matter consisting of corpuscles must have suggested that the essential “living material” of the organism consisted also of corpuscles, as soon as a microscope powerful enough to see the chromatic granules was turned on a dividing cell prepared so as to render these bodies visible. Obviously the primordial ovum contained all the elements of the organisms into which it was going to develop. But then in the process of division of the ovum all these chromatic granules are shared out among the cells, and a really very pretty mechanism comes into existence for this purpose of distribution.

Weismann built up his hypothesis of the germ-plasm upon the observations we have outlined. The chromatic matter of the nucleus consists of elements called determinants, the determinants themselves being composed of ultimate bodies called biophors. Each determinant possesses all the mechanism, or factors, necessary for the development of a part of the body: there are determinants for muscles, nerves, connective tissues, for the retina of the eye, for hairs of each colour, for the nails, and so on. All these determinants are contained in the chromatin of the nucleus of the egg, and in the divisions of the latter they are gradually separated so that ultimately each cell of the larva contains the determinants for one individual part, or organ, or organ-system of the adult body. The right blastomere, for instance, contains all the determinants for the right side of the frog’s body, those for the left side being contained in the left half. The process of cell-division involved in the segmentation of the egg consists then in the orderly disintegration of this complex of determinants, and in the marshalling into place of the isolated elements. The cell body—the cytoplasm—carried out a very subordinate rÔle, mainly that of nourishing the essential chromatic substance. Such was the Roux-Weismann Mosaic-theory of development in its pristine form.

It is clearly a preformation hypothesis. It is true that the actual organism is not contained in the germ, but all the parts of the latter, even the colours of the eyes or hair, are present in it in the form of the determinants. Obviously it involves a mechanism of almost incredible complexity. But if we regard it as a working hypothesis of development this complexity of detail does not matter; its truth would be indicated by the fact that all analysis of the processes involved would tend to simplify it and to smooth out the complexity. But this is exactly what has not happened, for all subsequent investigation has necessitated subsidiary hypothesis after hypothesis. As a theory of development it has failed entirely.

If, after one of the blastomeres in the frog’s egg at the 2-cell stage be killed, the egg is then turned upside down, the results of the experiment become totally different; the uninjured blastomere develops into a whole embryo, differing from the normal one chiefly in that it is smaller. If the uninjured egg in the 2-cell stage be turned upside down two whole embryos, connected together in various ways, develop. In the frog’s egg the two first blastomeres cannot be separated from each other without rupturing them, but in the egg of the salamander they can be separated. After this separation two perfect, but small, embryos develop. In the egg of the newt a fine thread can be tied round the furrow formed by the first division. If this ligature be tied loosely it does not affect development, and then it can be seen that the median longitudinal plane of the embryo does not correspond, except by chance, with the first division plane. If the ligature be tied tightly, then each of the blastomeres gives rise to an entire embryo. If it is tied in various places monsters of various types are produced. Therefore there is no segregation of the determinants in the first two blastomeres. These results, moreover, are not exceptional, for similar ones have been obtained with other animal embryos, in fishes, Amphioxus, ascidians, medusÆ, and hydrozoa, and in some cases even each of the first four blastomeres develops into an entire embryo when it is separated from the rest. In the sea-urchin embryo the blastomeres can be shaken apart; or by removing the calcium which is contained in sea water the blastomeres can easily be separated from each other. It was then found by Driesch that each of the blastomeres in the 16-cell stage could develop into an entire embryo. It is plain, then, that up to this stage at least there has been no segregation of the determinants.

Upon the results of these experiments Driesch based his first proof of vitalism. Let us suppose that there is a mechanism in the developing egg. Now the embryo which results from the latter sooner or later acquires a three-dimensional arrangement of parts: head-end differs from tail-end, dorsal surface differs from ventral surface, and the parts differ on either side of the median plane. The mechanism must, therefore, be one which acts in three dimensions, anterior and posterior, laterally, and dorso-ventrally. Fig. 15. We may represent it by a diagram of three co-ordinate axes, x, y, z; x and y being in the plane of the paper, and z at right angles to the plane of the paper. Now in the 2-cell stage the same mechanism must be present, for this stage develops normally into one entire embryo. But since either of the blastomeres may develop into an entire embryo, the mechanism must also be present in each of them, and since in the 16-cell stage each blastomere may develop an entire embryo, it must be present in each of the sixteen blastomeres. A three-dimensional mechanism is therefore capable of division down to certain limits.

Suppose now that we allow the sea-urchin egg to develop normally up to the blastula stage. In this stage it is a hollow sphere, the wall of which is a single layer of cells. It is similar all round, that is, we cannot distinguish between top and bottom, right and left, anterior and posterior regions; but since it develops into a larva in which all these distinctions become apparent very soon, it must possess the three-dimensional mechanism, since the activity of the developmental process is going to produce different structures in each direction. Fig. 16. Now the blastula, by very careful manipulation can be divided, cut into parts with a sharp knife. Since it is similar all round the direction of the cut is purely a matter of chance. It can be cut through along the planes 1 2, 3 4, 5 6, 7 8, for instance; really there are an infinite number of planes along which the blastula can be cut into two separate parts, and the direction of the plane is not a matter of choice, but purely a matter of chance. Nevertheless, each of the parts into which the larva is cut becomes an entire embryo. For a time the partial blastula—approximately a hollow hemisphere in form—goes on developing as if it were going to become a partial embryo, but soon the opening closes up and development becomes normal. It does not matter even if the two parts into which it is divided are not alike in size; provided that a part is not too small, it will follow the ordinary course of development.

Suppose the blastula opened out on the flat, like the Mercator projection of a globe on a flat map. Suppose that a is a small element of it. Suppose that the rectangles b c d e, F G H e, I J c L, M N O e, and as many more as we care to make, represent the pieces of the blastular wall separated by our operation—they all contain the element a, but this is in a different position in each case. There are really an infinite number of such parts of the blastula and a occupies an infinitely variable position in each of them.

This demonstration is very important, so let us make it as clear as possible: Driesch’s logical proof of vitalism may be stated as follows:—

The different parts of the blastula are going to become different parts of an embryo.

The part a, occupying a definite position in the entire blastula, is going to become a definite part, having a definite position, in the embryo;

But each partial blastula becomes an entire embryo and the same part a occupies a different position in each.

Therefore any part of the blastula may become any part of the embryo.

Now if a mechanism is involved, it must, according to our ideas of mechanism, be one which is different in its parts, for each part of it produces a different result from the others;

But since any part of the mechanism may produce any of the different results contained in the embryo, every one of its parts must be similar to every other one.

That is, all the parts of the mechanism are the same, though the hypothesis requires that they should be different.

We conclude, then, that a mechanism such as we understand a mechanism to be in the physical sciences cannot be present in the developing ovum.

Nevertheless, an organisation, using this term as an ill-defined one for the present, must exist in the ovum, or the system of undifferentiated cells into which the ovum divides, during the first stages of segmentation. In certain animals, Ctenophores (Chun, Driesch, and Morgan), and Mollusca (Crampton), for instance, separation of the blastomeres in the first stages of segmentation produces different results from those mentioned above. In these cases the isolated blastomeres develop as partial embryos, that is, the latter are incomplete in certain respects, and this incompleteness corresponds, in a general way, to the incompleteness of the part of the ovum undergoing development. We have thus the apparently contradictory results: (1) each of the first few blastomeres resulting from the first divisions of the ovum is similar to the entire ovum, and develops like it; and (2) each of the first few blastomeres is different from the others, and from the entire ovum, and develops differently from the others, and from the entire ovum.

Let us try to construct a notion of what this organisation in the developing ovum must be. In the 16-blastomere stage of the sea-urchin egg we have a “system” of parts. In the case of normal development each of these parts has a certain actual fate—it will form a part of the larva into which the embryo is going to develop: It has, as Driesch says, a prospective value. But let the normal process be interfered with, and then each of these parts does something else. In the extreme case of interference, when the blastomeres are separated from each other, each blastomere, instead of forming only a part of a larva, forms a whole larva. The prospective potency of the part, that is its possible fate, is greater than its prospective value. Normally it has a limited, definite function in development, but if necessary it may greatly exceed this function.

What any one blastomere in the system will become depends upon its position with regard to the other blastomeres. When the egg of the frog is floating freely in water it lies in a certain position with the lighter part uppermost, and then development is normal, each of the two first blastomeres giving rise to a particular part of the body of the larva; that is, each of them is affected by the contact of the other and develops into whatever part of the normal embryo the other does not. But let the egg in the 2-cell stage be turned over and held so that the heavy part is uppermost: the protoplasm then begins to rotate so as to bring the lighter part uppermost; but the two blastomeres do not, as a rule, adjust themselves to the same extent, and at the same rate, and corresponding parts may fail to come into contact with each other. Lacking, then, the normal stimulus of the other part, each blastomere begins to develop by itself, and a double embryo is produced. It is clear, then, both from this case and the last one, that the actual fate of any one part of the system of blastomeres is a function of its position. What it will become depends precisely on where it is situated with respect to the other parts.

Driesch, then, calls the system of parts in such cases as the 2-cell frog embryo, or the 16-cell sea-urchin embryo, an equipotential system, since each part is potentially able to do what any other part may do, and what the whole system may do. But in normal development each part has a definite fate and its activity is co-ordinated with that of all the other parts. It is, therefore, an harmonious equipotential system, each part acting in harmony, and towards a definite result, with all the others; although if necessary it can take the place of any or all of the others.

Such an harmonious equipotential system exists only at the beginning of the development of the egg. It is represented by the 8-cell stage of Echinus but not by the 16-cell stage, since, though the 1/16-blastomeres produce gastrulÆ (the first larval stage), they do not produce plutei (the second stage). It is represented by the 4-cell stage of Amphioxus but not by the 8-cell stage. It is not exhibited even by the 2-cell stage of the Ctenophore egg. What does this mean? It means that the further development proceeds, the less complete does the “organisation” inherent in any one part of the system become. “The ontogeny assumes more and more the character of a mosaic work as it proceeds” (Wilson).

Or perhaps it means, and this is the better way of putting it, that the “organisation,” whatever it may be, depends on size. We see this very clearly in the experiment of cutting in two the blastula of the sea-urchin. If the pieces are of approximately equal size each will form an entire Pluteus larva, but if one of them is below a certain limit of size it will not continue to develop. The “organisation,” therefore, has a certain volume, and this volume is much greater than that of any one of the cells of which the fragment exhibiting it is composed. It is enormously greater than the volume of any group of determinants which we can imagine to represent the different kinds of cells composing the body of the Pluteus larva, and still more enormously greater than the volume of a “molecule” of protoplasm. Now this association of “organisation” and size is of immense philosophical importance, for it does away, once and for all, with the idea that the “organisation” is solely a series of chemical reactions. If it were, one cell of the blastula would contain it, for on the mechanistic hypothesis one cell, the egg-cell, contains it, and this cell can be divided innumerable times and still contain it. The egg is a complex equipotential system (Driesch), which divides again and again throughout innumerable generations, and still contains the “organisation.”

It is in vain that we attempt the misleading analogy of the “mass action” of physical chemistry, to show that volume may influence chemical action. In such a mass action what we have is this:—

A_a+B_b C_c+D_d

the letters A, B and C standing for chemical substances present, and the letters a and b, etc., representing the active masses of these substances. But variations in this active mass affect only the velocity of the reaction. What we have to account for in our blastula experiments is the nature of the reaction, and how can velocity or even nature of reaction affect form? If we could show that the form of the crystals deposited from a solution in some reaction depended on the volume of the solution, the analogy would be closer, though even then the difficulties in pressing it would be so enormous as to render it futile to attempt to entertain it.

A chemical mechanism cannot, then, be imagined, much less described, and the only other mechanism so far suggested is the Roux-Weismann one, involving the disintegration of the determinants supposed to be present in the egg nucleus. Let us suppose (in spite of the incredible difficulty in so doing) that there is such a mechanism. It must usher the nuclei containing the determinants of the embryonic structure into their places: those for the formation of the nerve-centre go forward; those for the mouth, gut, and anus go backwards and downwards; those for the arms go forwards, ventrally, and posteriorly, in a very definite way; and those for the complicated skeleton are distributed in a variety of directions which defy description. These nuclei are, in short, moved up and down, right and left, backwards and forwards, and become built up into a complicated architecture. Suppose we prevent this. Suppose we compress the segmenting egg between glass plates so that the nuclei are compelled to distribute themselves in one plane only: to form a flattened disc in which the only directions are right and left and anterior and posterior. This has been done by Driesch and others. On the Roux-Weismann original hypothesis a monstrous larva ought to result, for the first nuclei separated from each other have been forced into positions altogether different from those which they should have occupied had they developed normally. Yet on releasing the pressure readjustment takes place. New divisions occur so as to restore the normal form of larva. The Roux-Weismann subsidiary hypothesis is that the stimulus of the pressure has compelled the nuclei to divide at first in such a way as to compensate for the disturbance.

Let us remove some of the blastomeres. On the original hypothesis the determinants for the structures which the nuclei of these blastomeres contained have been lost. These structures should, therefore, be missing in the embryo. But nothing of the sort is the result. Other nuclei divide and replace the lost ones, and the embryo develops as in the normal mode. The reply is that in addition to the determinants which were necessary for their own peculiar function, these nuclei contained a reserve of all others. On disturbance these determinants, “latent” in all other conditions, became active and restituted the lost parts.

Let us remove some organ from an adult organism. The most remarkable experiment of this kind is the removal of the crystalline lens from the eye of the salamander. Now the lens of the eye develops from the primitive integument (ectoderm) of the head, but the iris of the eye develops mainly from a part of the primitive brain. After the operation a new lens is formed from the iris and not from the cornea. Therefore the highly specialised iris contains also determinants of other kinds. Does it contain those for itself and lens only, or others? If it contains many kinds, then we conclude that even the definite adult structures contain determinants of many other kinds than their own, that is, reserve determinants are handed down in all cells capable of restitutive processes, practically all the cells of the body. Or does it contain only its own and those of the lens? Then this highly artificial operation was anticipated, an absurd hypothesis which need not be considered.

This particular mechanistic process (and no other one is nearly so plausible) crumbles away before attempts at verification, and it survives only by the addition of subsidiary hypothesis after hypothesis. In itself this demonstrates that it is an explanation incompetent to describe the facts.

What, then, is the “organisation”? It is something elemental, and we may just as well ask what is gravity, or chemical energy, or electric energy. It cannot be said to be any of these things or any combination of them. “At present,” says a skilful and distinguished experimenter, T.H. Morgan, “we cannot see how any known principle of chemistry or of physics can explain the development of a definite form by the organism or a piece of the organism.” “Probably we shall never be able,” concludes Morgan, who is anything but a vitalist. But does not this mean just that in biology we observe the working of factors which are not physico-chemical ones?

We have seen that the physiologist studies something very different from that which the embryologist or naturalist studies. The former investigates a part of the animal, arbitrarily detached from the whole because the complexity of the functions of the simplest organism is such that all of them cannot be examined at once. He adopts the methods of physical chemistry in his investigation and whatever results he obtains are necessarily of the same order. Inevitably, from the mere nature of his method, he can see, in the organism, only physico-chemical phenomena. The embryologist, on the other hand, studies the organism as a whole and seeks to determine how definite forms are produced, and how a change in the external conditions affects the assumption of these forms. We have seen with what little success the attempts to relate embryological processes with physico-chemical ones alone have met. In all studies of organic form mechanism has failed. It is useless to attempt to press the analogies of crystalline form, and the forms assumed in nature by dynamical geological agencies. If the reader examines these analogies critically he will see that they are superficial only.

We seem, however, to see in those actions of the organism which are called “tropistic” or “tactic,” reactions of a purely physico-chemical nature, and starting with these as a basis a plausible theory of organic movements on a strictly mechanistic basis might be built up.21 A “tropism” is the movement of a fixed organism with respect to a definitely directed external stimulus. This movement may be that produced by growth of its parts, or by the differential contraction or expansion of its parts. A “taxis” we may call the motion of a freely-moving organism in response to the same directed stimuli. The movements whereby a green plant turns towards the light are called heliotropic, and those of its roots in the perpendicular direction are called geotropic. The motion of the freely-moving larva of a barnacle, for instance, in swimming towards a source of light are called “phototactic.”

In all these cases we have to think of the stimulus as a “field of energy” in the sense in which physicists speak of electric, or magnetic, or electromagnetic, or thermal, or gravity fields. In all these cases the factors affecting the movements of the organism are directed ones.

An electric field, for instance, (1), is produced by placing the electrodes of a galvanic cell at opposite extremities of a water-trough: we imagine the electrons moving from one side of the trough to the other in parallel lines, and in a certain direction. A light field (2) would be produced by the radiation of light travelling in straight lines through the water.

The movements of the organism displaying a tropism or a taxis are not caused by the stimuli of the field, but are only directed by it. In the absence of these stimuli it would swim at random. In a field, however, it will orientate itself in some direction with reference to the lines of force. A “positively phototactic” animal swims towards the focus from which the light radiation emanates, and a “negatively phototactic” one swims in the other direction. On the theory of tropistic and tactic movements this orientation is produced by the differential stimulation of the opposite sides of the organism. Let us take as a concrete example the case of a caterpillar which creeps up the stem of a plant to feed on the tender shoots near the apex. The animal possesses an elongated body, with muscles beneath the integument, and sensory nerve-endings in the latter. Its muscles are in a state of “tone,” that is, they are normally always slightly tense. The incident rays of light affect the dermal sense-organs, stimulating ganglionic centres and setting up efferent impulses which descend to the muscles. Let us suppose the animal is moving so that the longitudinal axis of its body is at an angle, say of 45°, to the direction of the incident light: one side of the body is therefore stimulated and the other is not. The stimulation of the lighted side sets up efferent nerve impulses which descend to the muscles of this side and increase their tone (or else the lack of stimulation of the other side produces impulses which inhibit the muscular tone, or impulses which would otherwise preserve the tone cease in the absence of light stimulation). In any case the muscles of the lighted side contract, and the body of the caterpillar moves so that it sets itself parallel to the direction of the radiation. Both sides of the body are then equally stimulated and the animal moves towards the light.

The animal feeds and it then creeps back down the plant. Why does it do this? Because, says Loeb, the act of feeding has reserved the “sign” of the taxis. Before, when it was hungry, it was positively phototactic, but the act of feeding (all at once, it would appear, before digestion and assimilation of the food itself) has produced chemical substances in the muscles which cause the latter to relax in response to an impulse which previously produced contraction.

The nervous link is not, of course, a necessary one. The stimulation by the energy of the field may affect the muscle substance directly, or it may, as in the case of a protozoan animal, affect the general body protoplasm in the same way. In the majority of cases, however, the orientation would be affected through the chain of sense-organ, afferent nerve, nerve centre, efferent nerve, and effector organ. This is the chain of events which on this hypothesis causes a moth to fly into a flame, or a sea-bird to dash itself against the lantern of a lighthouse.

A taxis is, then, an inevitable response by movement in a definite direction, to a directed stimulus. Including also tropisms it may be admitted that the movement is a purposeful, or at least, a useful one in some cases, as for instance the heliotropism and geotropism of the green plant. If we admit that Loeb’s description of the feeding of the caterpillar, as a tactic act, is true, we may also call this a useful act. But in the majority of cases tropisms and tactes are acts which appear to be of no use to the organism. The invasion of a part of the body which is irritated by a poison (as in inflammation) by leucocytes, is useful to the body itself, but we must regard the leucocytes as organisms, and their tactic motion leads to their destruction, and so also with other analogous acts. Just because of this we find difficulty in accounting for their origin in terms of natural selection.

This does not matter so much, since it can hardly be maintained now that the tropistic or tactic act has any reality except in a very few cases—the motions of plants, galvano-taxis, the chemico-taxic movements of bacteria and leucocytes, and some other analogous cases, perhaps, are these exceptions. It can hardly be doubted that the extension of the concept to cover the motions of many invertebrates, and even some vertebrate actions, by Loeb and his school is a straining after generality which has not been justified. The hypothesis, as Loeb has stated it, is evidently almost certainly a logical one and was obviously elaborated as a protest against the anthropomorphism which saw in the flying of a moth into a flame the expression of an emotion; or in the movements of a caterpillar on a green shrub the expression of hunger and satiety and of the inherited experience of the animal; or in the avoidance by a Paramoecium of a drop of acid the emotion of dislike of the feeling of pain. Well, let it be granted that this is so, and that the protest was a useful one, for it is obviously impossible that these notions as to the causes of the movements can be verified: does it improve matters to take refuge in an hypothesis which is just as purely physico-chemical dogmatism as the other is anthropomorphism? But the former hypothesis is at all events one which is susceptible of experimental verification and in this lies its usefulness, inasmuch as it has stimulated investigation. It is evident, however, that this verification has not yet been made. The differential afferent impulses set up by the energy-field; the increases or inhibition of muscular tone; the presence of photo-sensitive substances in the tissues of tactically acting lower animals; the change of velocity of chemical reaction, in these cases, which ought to follow stimulation—all these things could be verified if they possess reality. Yet it is only indirect proofs, capable perhaps of other interpretations, and not direct experimental ones, which have so far been adduced in favour of a general theory of tropisms.

Moreover, the close analysis of the actions of some of the lower organisms by Jennings has shown that the tactic hypothesis is probably false in the majority of cases. This observer studied the acting of the organisms themselves and not the beginning and end of the series, and he shows that the behaviour of the organisms is far more obviously described by saying that it adopts a method of “trial and error.” Let us suppose a number of infusoria (Paramoecium) in a film of water, at one part of which is a drop of acetic acid slowly diffusing out into the surrounding medium. There is a zone of changing concentrations round the drop: if we draw imaginary contours through the points where the concentration is approximately the same (the concentric rings in the diagram), and then draw straight lines normal to these rings (the radial lines) we can construct a “field” analogous to an electric or magnetic field. Fig. 19. The animal on approaching the field ought to orientate itself and take the direction of the “lines of force.” It does not, however, behave in this way, but only enters the field at random. Having entered, it remains within a part where the concentration is within certain limits. If it approaches the margin of this limited field it stops, swims backwards, revolves round its own axis, and then turns to the aboral side; and it repeats this series of movements whenever it approaches (by random) a region where the concentration is too high, or one where it is too low. In this, and other organisms we see then what Jennings has called a typical “avoiding reaction,” the precise nature of which depends on the “motor-system” of the animal. Its general movements are random ones, but having found a region of “optimum conditions” (conditions which are most suitable in its particular physiological state), it remains there.

Suppose (what indeed repeatedly happens) that an extensive “bed” of young mussels forms on a part of the sea bottom. In a short time the bed becomes populated by a shoal of small plaice feeding greedily on the little shellfish. In their peregrinations the fishes must repeatedly pass out beyond the borders of this feeding-ground. Usually, however, they will return, for failing to find the food they like they swim about in variable directions and so re-enter the shellfish bed.

Suppose (this was really a fine experiment made by Yerkes) a crab is confined in a box from which two paths lead out but only one of which leads to the water. The animal runs about at random, finds the wrong path, retraces it, tries again and again, and then finds the right path and gets back to the water. If the experiment is repeated the animal finds the right path again with rather less trouble, and after many trials it ends by finding it at once on every repetition of the experiment.

All this discussion of concrete cases leads up to our consideration of the modes of acting in the higher organisms. On the strictly mechanistic manner of thinking the actions of the organism in general are based on reactions of the tactic kind—inevitable reactions the nature of which is determined, and which follow a stimulus with a certainty often fatal to the organism displaying them. Accepting these tactic reactions as, in general, truly descriptive of the behaviour of the organism, we can build up a theory of instincts. In their simplest form instincts are reflexes—tactic movements. In their more complex forms they are concatenated reflexes, or tactes. A complicated instinctive action is one consisting of many individual actions, each of which is the stimulus for the next one; or, of course, it may also be complex in the sense that several simple reactions proceed simultaneously, upon simultaneous stimulation of different receptors. Now the extension of all this to movements of a “higher” grade is obvious.

Let us note in the first place, that the stimuli so far considered in all the examples quoted are simple elemental ones. There are, of course, relatively few such stimuli: gravity, conducted heat (the kinetic energy of material bodies), radiated heat (the energy of the ether), electric energy, chemical energy, and mechanical contact or pressure (including atmospheric vibrations). In all these cases we have a definite, measurable, physical quantity, with which we must relate a definite response in the form of a definite measurable, physico-chemical reaction. There should be a functionality between the stimulus and response, a definite, quantitative energy-transformation. To take a concrete example, a certain quantity of light energy falling upon the receptor organs of Loeb’s caterpillar ought to transform into another quantity of “nervous energy,” and this travelling in an analogous way to a “wave of explosion” ought to transform into an energy quantity of some kind, which initiates another “wave of explosion” in the muscle substance. All these transformations must be quantitative ones, and the energy of the individual light must be traced from the receptor organ to the points in the muscle where it disturbs a condition of false equilibrium in the substance of the latter. Nothing less than this is required to demonstrate the purely physical nature of a reaction, on the part of the organism, to an external stimulus. It may safely be said that physiological investigation has not yielded anything even approximating to such an experimental demonstration.

What are the stimuli to the actions of a higher organism? It is true that their elements are energies such as we have indicated, but these energies are integrated to form individualised stimuli (Driesch). The stimulus in an experimentally studied taxis is, perhaps, a field of parallel pencils of light rays of definite wave length; but in the action of a man, or a dog say, the stimulus is an immensely complicated disturbance of the ether, producing an image upon the retina of the animal. A sound stimulus employed in an investigation may be the relatively simple atmospheric disturbance produced by the sustained note of a syren or violin-string; but the stimulus in listening to an orchestra may consist of dozens of notes, with all their harmonies, sounding simultaneously at the rate perhaps of some hundred or two in the minute. All these are integrated by the trained listener, and one or two false ones among the multitude may entirely spoil the effect of the execution. Surely there is here something more than a mere difference in degree.

More important still is the strict functionality between stimulus and action that the theory of tactic responses imposes on itself. Putting this very precisely (but no more precisely than the theory demands), we say that SA = f(x, y, z), that is, the series of actions SA (the dependent variable) is a mathematical function of the independent variables x, y, z. Now is there anything like this functionality between the acting of the higher animal and the stimulus? Evidently there is not. We recognise someone whom we know very well by any one of a hundred different characters, mannerisms of walk, speech, dress, etc. He or she is the same person, whether seen close at hand, or afar off, or sideways, or in any one of almost infinitely different attitudes, and we respond to each of these very different physical stimuli by the same reaction of recognition: pleasure, dislike, avoidance, greeting, or whatever it may be. To a sportsman shooting wild game the stimulus may be some almost imperceptible tint or shading in cover of some kind, differing so little from its environment as hardly at all to be seen, yet, to his experience, upon this almost infinitesimal variation of stimulus depends his action with all its consequences. In Driesch’s example two polyglot friends met and one says to the other, “My brother is seriously ill,” or “Mon frÈre est sÉverÈment malade,” or “mein Bruder ist ernstlich erkrankt.” Here the physical stimulus is fundamentally different in each case, but the reaction—the expressions of sympathy and concern, the discussions of mutual arrangements, etc., are absolutely the same. Or let the one friend say to the other, “My mother is seriously ill,” and in spite of the very insignificant difference between the consonantal sound br in this sentence and the corresponding sound m in the other English sentence, the reaction, that is, the subsequent conversation, and the arrangements between the two friends may be entirely different.

Putting this argument in abstract form we may say, generally, that two stimuli, which are, in the physical sense, entirely different from each other, may produce absolutely the same series of reactions; and conversely two stimuli differing from each other in quite an insignificant degree may produce entirely different reactions. It is also easy to see, by analysis of the antecedents to the actions of the intelligent animal, that these stimuli are, in the majority of cases, not elemental physical agencies, but individualised and integrated groupings of these agencies; and that the animal reacts, not to their mathematical sum, as it should do on a purely mechanistic hypothesis of action, but to the typical wholes which are expressed in these groupings.22

It is no answer to this argument to say that it is not the actual atmospheric vibrations (in the case of the conversation), nor the optical image (in the case of the recognition of a friend), which are the true stimuli, but rather the mental conditions, or states of consciousness, aroused by these physical agencies. If we are to adopt a strictly mechanistic method of explaining actions, such a method as that indicated by Loeb’s hypothesis of the purely tactic behaviour of his caterpillars, then these atmospheric vibrations and optical images are most undoubtedly the true stimuli, and the reactions must be functions of them in the mathematical sense. But since this strict functionality does not exist in any behaviour-reaction closely analysed, we must grant at once that it is, indeed, not the physical series of events that determines the actual response, but truly the conscious state immediately succeeding to these physical sense-impressions. Now let us see to what conclusions this admission leads us.

Between the external stimulus (the atmospheric undulations impinging on the auditory membranes, or the light radiations impinging on the retinÆ) and the behaviour-reaction something intervenes. This is the individual history of the organism, the “associative memory” of Jacques Loeb, the “physiological state” of Jennings, the “historical basis of reacting” (historische Reaktionsbasis) of Driesch, or the “duration” of Bergson. The last concept is the most subtle and adequate one and we shall adopt it. The physical stimulus, then, leads to a state of consciousness, a perception, and this is succeeded by the action. What is the perception? There may be no perception in a reflex action; there is none in a taxis.23 These kinds of reaction follow inevitably from the nature of the stimulus—depend upon the latter, in fact; but we cannot fail to observe that the intelligent behaviour of the higher animal involves choice between alternative kinds of action. The perception, then, is this choice, or it is intimately associated with it. But it is something more than the choice of one among many kinds of response. The whole past experience of the animal enters into the perception, or at least all that part of the past experience which illuminates, in any way, the present situation. What the intelligent animal does in response to a stimulus depends not only on the stimulus but on all the stimuli that it has received in its past, and on all the effects of all those stimuli. Into the perception that intervenes between the external stimulus, then, and the action by which the animal responds what we usually call its memory enters. Its duration is really the something which is changed by the stimulus, and which then leads to the behaviour-reaction.

Duration, then, is memory, but it is more than memory as we usually think of this quality. The past endures in us in the form of “motor habits,” and when we recall it we may act over again those motor events. Careful introspection will readily convince the reader that in recalling a conversation he is really speaking inaudibly, setting in motion the nerves and muscles of his vocal mechanisms. Actions that have been learned endure; in some way cerebral and spinal tracts and connections become established and persist: undoubtedly when a cerebral lesion destroys or impairs memory it is these physical nerve tracts and cells that become affected. But in addition to this we have pure memory (Bergson’s “souvenir pur”). What, for instance, is the visual image of some thing seen in the past, which most people can form, but pure recollection?24

All the past experience of the organism—all its perceptions, and all the actions it has performed—endures, either as motor habits or mechanisms, or as pure memories. All this need not be present in its consciousness; the motor habits would not, of course, and only so much of the past would be recalled as would be relevant to the choice which the organism was about to make of the many kinds of responses possible to its motor organisations. Out of this past it would select all that was connected in any way with the actions which were possible to it in the present. It would recall all actions previously performed which resembled the one provisionally decided upon; but recalling also the other circumstances associated with those past actions, it would discover something which would lead it to modify that provisional action. Now in describing the whole behaviour of the acting organism in this way are we doing any more than simply expressing in more precise terms the “commonsense” notions of the ordinary person? The latter would sum up all this discussion by saying that what he would do in any set of circumstance depended not only on the circumstances themselves but upon his experience. Physiology shows us as clearly as possible that in the stimulation of a receptor organ, the propagation of a nervous impulse along an afferent nerve, the transmission of this impulse through the cord or brain, or both—in the propagation again of the impulse through an efferent nerve and the transformation of this impulse into a releasing agency, setting free the energy potential in the muscle substance—that in all this there can be nothing more than physico-chemical energy-transformations. All this is clear and certain. But why should the same afferent stimuli, entering the central nervous system at different times by the same avenues, and in the same manner, traverse different tracts, and issue along different efferent nerves, producing different results? Or why should different stimuli entering the central nervous system take the same intra-cerebral paths and then affect the same efferent nerves and effector organs? It is because these stimuli lead to perceptions which fuse with, and become part of the duration of, the organism. And the response then becomes a response not to the physical stimulus, but to the duration modified in this way.

Can we conceive of any physical mechanism in which the duration of the organism accumulates? Can we think of any way in which memories are stored in the central nervous system? When we say “stored,” it is our ingrained habit of thinking in terms of space and number that makes us regard memories as laid by somewhere, in the way we file papers in a cabinet, or store specimens in a museum. Supposing perceptions are stored in this way, we think of them as stored or recorded in the same way as a conversation is recorded and stored in a phonograph. The phonograph can reproduce the conversation just as it was received, but what we make use of when we utilise our experience is obviously the elements of that experience, selected and re-integrated as we require them. There must, then, be something like an analysis of our perceptions, a dissociation of these into simple constituents, and a means of restoring and recording these constituents in such a way that they can be recombined in any order, and again made to enter into our consciousness.

It is quite possible to imagine such a mechanism. Let us suppose that an efferent impulse enters the cerebral cortex via any one axon: there is a perfect labyrinth of paths along which the impulse may travel. Everywhere in the central nervous system we come upon interruptions of nervous paths formed by inter-digitating arborescent formations. The twigs of these arborescences do not, apparently, come into actual contact with each other and the impulse leaps across the gap between them. This gap is, of course, exceedingly narrow, and one can almost speak of it as a membrane, since it must be occupied by some organised substance. It has been called the synaptic membrane. Let us suppose that a stimulus of a certain nature passes through the synapse, modifying it physico-chemically as it passes. Thereafter a stimulus of similar nature will tend to pass across this particular synapse, the resistance of the latter having been decreased. It will thus tend to travel by a definite tract through the central nervous system. Now the latter we may regard in a kind of way as a very complicated switchboard, the function of which is to place any one stimulus (or series of stimuli) out of many in connection with any one motor25 mechanism (or series of mechanisms) out of many. A motor habit, or path, is then established and will persist.

Such a conception is clear and reasonable in principle, and all work on nervous physiology tends to show that it is a good working hypothesis. We cannot read modern books without feeling that immense advances will be made by its aid. But the complexity of the brain of the higher vertebrate is so incredibly great, and the difficulties of imagining the nature of the necessary physico-chemical reactions in the synapses, and elsewhere, are so immense that experimental verification may be impossible. And all that we have said applies to a single elemental stimulus, yet in any common action the stimulus is a synthesis of almost innumerable simple ones, while the response is also a synthesis. The optical image of almost any object contains a very great number of tints and colours differing almost imperceptibly: there must at least be as many simple stimuli as there are rod or cone elements in the part of the retina covered by the image. The motor responses consist of a multitude of delicately adjusted and co-ordinated muscular contractions and relaxations. If we are to accept a mechanistic hypothesis of action, of this kind, and which includes only such processes as are suggested above, it is not enough that a logical description, consistent in itself, and consistent with physico-chemical knowledge, should be formulated. The mere statement of such an hypothesis does not carry us far. If it is, in essence, mechanistic, it must be capable of experimental verification in detail.

Even if it were verified experimentally it would still leave untouched the problem of consciousness. All that we have considered are series of physico-chemical energy-transformations. How, then, does consciousness arise? We cannot even imagine its association in a functional sense with the train of events forming an afferent impulse. In some form or other mechanism must assume a dualism—a parallelism of physical and psychical processes. Physical events in the central nervous system are associated with psychical ones—when the former occur so do the latter—yet the former are not “causes” in any physical sense of the latter. Consciousness follows cerebral energy-transformations as a parallel “epiphenomenon.” At once we leave the province of mechanism, and how can we remain content with such a limitation of our descriptions? And if we conclude, as we seem obliged to do, that consciousness is an affective agency in modifying our responses to external stimuli, does not this in itself show that our concept of behaviour as a purely physico-chemical process is insufficient in its exclusiveness?

We return to a consideration of the main results of experimental embryology in a later chapter, but let us notice here what is the direction in which these results, and those of the analysis of instinctive and intelligent action, carry us. It is towards the conclusion that physico-chemical processes in the organism are only the means whereby the latter develops, and grows, and functions, and acts. In the analysis of these processes we see nothing but the reactions studied in physical chemistry; but whenever we consider the organism as a whole we seem to see a co-ordination, or a control or a direction of these physico-chemical processes. NÄgeli has said that in the development of the embryo every cell acts as it if knew what every other cell were doing. There is a kind of autonomy in the developing embryo, or regenerating organism, such that the normal, typical form and structure comes into existence even when unforeseen interference with the usual course of development has been attempted: in this case the physico-chemical reactions which proceed in the normal train of events proceed in some other way, and the new direction is imposed on the developing embryo by the organisation which we have to regard as inherent in it. This same direction and autonomy must be recognised in the behaviour of the adult organism as a whole. What is it? We attempt to think of it as an impetus which is conferred upon the physico-chemical reactions which are the manifestations of the life of the organism. It is the Élan vital of Bergson, or the entelechy of Driesch. What is included in these concepts we consider in the last chapter of this book; and before so doing it will be necessary to consider the organism from another point of view, that of its mutability when it is regarded as one member of a series of generations.