When the student of Nature quits the narrow workshop of his own particular inquiry, and sets out upon an excursion into the vast kingdom of philosophical investigation, he does so, doubtless, in the hope of finding the answer to that great riddle, to the solution of a small part of which he devotes his life. Those, however, whom he leaves behind him still working at their own special branch of inquiry, regard his departure with secret misgivings on his behalf, while the born citizens of the kingdom of speculation among whom he would naturalise himself, receive him with well-authorised distrust. He is likely, therefore, to lose ground with the first, while not gaining it with the second.
The subject to the consideration of which I would now solicit your attention does certainly appear likely to lure us on towards the flattering land of speculation, but bearing in mind what I have just said, I will beware of quitting the department of natural science to which I have devoted myself hitherto. I shall, however, endeavour to attain its highest point, so as to take a freer view of the surrounding territory.
It will soon appear that I should fail in this purpose if my remarks were to confine themselves solely to physiology. I hope to show how far psychological investigations also afford not only permissible, but indispensable, aid to physiological inquiries.
Consciousness is an accompaniment of that animal and human organisation and of that material mechanism which it is the province of physiology to explore; and as long as the atoms of the brain follow their due course according to certain definite laws, there arises an inner life which springs from sensation and idea, from feeling and will.
We feel this in our own cases; it strikes us in our converse with other people; we can see it plainly in the more highly organised animals; even the lowest forms of life bear traces of it; and who can draw a line in the kingdom of organic life, and say that it is here the soul ceases?
With what eyes, then, is physiology to regard this two-fold life of the organised world? Shall she close them entirely to one whole side of it, that she may fix them more intently on the other?
So long as the physiologist is content to be a physicist, and nothing more - using the word “physicist” in its widest signification - his position in regard to the organic world is one of extreme but legitimate one-sidedness. As the crystal to the mineralogist or the vibrating string to the acoustician, so from this point of view both man and the lower animals are to the physiologist neither more nor less than the matter of which they consist. That animals feel desire and repugnance, that the material mechanism of the human frame is in chose connection with emotions of pleasure or pain, and with the active idea-life of consciousness - this cannot, in the eyes of the physicist, make the animal or human body into anything more than what it actually is. To him it is a combination of matter, subjected to the same inflexible laws as stones and plants - a material combination, the outward and inward movements of which interact as cause and effect, and are in as close connection with each other and with their surroundings as the working of a machine with the revolutions of the wheels that compose it.
Neither sensation, nor idea, nor yet conscious will, can form a link in this chain of material occurrences which make up the physical life of an organism. If I am asked a question and reply to it, the material process which the nerve fibre conveys from the organ of hearing to the brain must travel through my brain as an actual and material process before it can reach the nerves which will act upon my organs of speech. It cannot, on reaching a given place in the brain, change then and there into an immaterial something, and turn up again some time afterwards in another part of the brain as a material process. The traveller in the desert might as well hope, before he again goes forth into the wilderness of reality, to take rest and refreshment in the oasis with which the Fata Morgana illudes him; or as well might a prisoner hope to escape from his prison through a door reflected in a mirror.
So much for the physiologist in his capacity of pure physicist. As long as he remains behind the scenes in painful exploration of the details of the machinery - as long as he only observes the action of the players from behind the stage - so long will he miss the spirit of the performance, which is, nevertheless, caught easily by one who sees it from the front. May he not, then, for once in a way, be allowed to change his standpoint? True, he came not to see the representation of an imaginary world; he is in search of the actual; but surely it must help him to a comprehension of the dramatic apparatus itself, and of the manner in which it is worked, if he were to view its action from in front as well as from behind, or at least allow himself to hear what sober-minded spectators can tell him upon the subject.
There can be no question as to the answer; and hence it comes that psychology is such an indispensable help to physiology, whose fault it only in small part is that she has hitherto made such little use of this assistance; for psychology has been late in beginning to till her fertile field with the plough of the inductive method, and it is only from ground so tilled that fruits can spring which can be of service to physiology.
If, then, the student of nervous physiology takes his stand between the physicist and the psychologist, and if the first of these rightly makes the unbroken causative continuity of all material processes an axiom of his system of investigation, the prudent psychologist, on the other hand, will investigate the laws of conscious life according to the inductive method, and will hence, as much as the physicist, make the existence of fixed laws his initial assumption. If, again, the most superficial introspection teaches the physiologist that his conscious life is dependent upon the mechanical adjustments of his body, and that inversely his body is subjected with certain limitations to his will, then it only remains for him to make one assumption more, namely, that this mutual interdependence between the spiritual and the material is itself also dependent on law, and he has discovered the bond by which the science of matter and the science of consciousness are united into a single whole.
Thus regarded, the phenomena of consciousness become functions of the material changes of organised substance, and inversely - though this is involved in the use of the word “function” - the material processes of brain substance become functions of the phenomena of consciousness. For when two variables are so dependent upon one another in the changes they undergo in accordance with fixed laws that a change in either involves simultaneous and corresponding change in the other, the one is called a function of the other.
This, then, by no means implies that the two variables above-named - matter and consciousness - stand in the relation of cause and effect, antecedent and consequence, to one another. For on this subject we know nothing.
The materialist regards consciousness as a product or result of matter, while the idealist holds matter to be a result of consciousness, and a third maintains that matter and spirit are identical; with all this the physiologist, as such, has nothing whatever to do; his sole concern is with the fact that matter and consciousness are functions one of the other.
By the help of this hypothesis of the functional interdependence of matter and spirit, modern physiology is enabled to bring the phenomena of consciousness within the domain of her investigations without leaving the terra firma of scientific methods. The physiologist, as physicist, can follow the ray of light and the wave of sound or heat till they reach the organ of sense. He can watch them entering upon the ends of the nerves, and finding their way to the cells of the brain by means of the series of undulations or vibrations which they establish in the nerve filaments. Here, however, he loses all trace of them. On the other hand, still looking with the eyes of a pure physicist, he sees sound waves of speech issue from the mouth of a speaker; he observes the motion of his own limbs, and finds how this is conditional upon muscular contractions occasioned by the motor nerves, and how these nerves are in their turn excited by the cells of the central organ. But here again his knowledge comes to an end. True, he sees indications of the bridge which is to carry him from excitation of the sensory to that of the motor nerves in the labyrinth of intricately interwoven nerve cells, but he knows nothing of the inconceivably complex process which is introduced at this stage. Here the physiologist will change his standpoint; what matter will not reveal to his inquiry, he will find in the mirror, as it were, of consciousness; by way of a reflection, indeed, only, but a reflection, nevertheless, which stands in intimate relation to the object of his inquiry. When at this point he observes how one idea gives rise to another, how closely idea is connected with sensation and sensation with will, and how thought, again, and feeling are inseparable from one another, he will be compelled to suppose corresponding successions of material processes, which generate and are closely connected with one another, and which attend the whole machinery of conscious life, according to the law of the functional interdependence of matter and consciousness.
After this explanation I shall venture to regard under a single aspect a great series of phenomena which apparently have nothing to do with one another, and which belong partly to the conscious and partly to the unconscious life of organised beings. I shall regard them as the outcome of one and the same primary force of organised matter - namely, its memory or power of reproduction.
The word “memory” is often understood as though it meant nothing more than our faculty of intentionally reproducing ideas or series of ideas. But when the figures and events of bygone days rise up again unbidden in our minds, is not this also an act of recollection or memory? We have a perfect right to extend our conception of memory so as to make it embrace involuntary reproductions, of sensations, ideas, perceptions, and efforts; but we find, on having done so, that we have so far enlarged her boundaries that she proves to be an ultimate and original power, the source, and at the same time the unifying bond, of our whole conscious life.
We know that when an impression, or a series of impressions, has been made upon our senses for a long time, and always in the same way, it may come to impress itself in such a manner upon the so-called sense-memory that hours afterwards, and though a hundred other things have occupied our attention meanwhile, it will yet return suddenly to our consciousness with all the force and freshness of the original sensation. A whole group of sensations is sometimes reproduced in its due sequence as regards time and space, with so much reality that it illudes us, as though things were actually present which have long ceased to be so. We have here a striking proof of the fact that after both conscious sensation and perception have been extinguished, their material vestiges yet remain in our nervous system by way of a change in its molecular or atomic disposition,  that enables the nerve substance to reproduce all the physical processes of the original sensation, and with these the corresponding psychical processes of sensation and perception.
Every hour the phenomena of sense-memory are present with each one of us, but in a less degree than this. We are all at times aware of a host of more or less faded recollections of earlier impressions, which we either summon intentionally or which come upon us involuntarily. Visions of absent people come and go before us as faint and fleeting shadows, and the notes of long-forgotten melodies float around us, not actually heard, but yet perceptible.
Some things and occurrences, especially if they have happened to us only once and hurriedly, will be reproducible by the memory in respect only of a few conspicuous qualities; in other cases those details alone will recur to us which we have met with elsewhere, and for the reception of which the brain is, so to speak, attuned. These last recollections find themselves in fuller accord with our consciousness, and enter upon it more easily and energetically; hence also their aptitude for reproduction is enhanced; so that what is common to many things, and is therefore felt and perceived with exceptional frequency, becomes reproduced so easily that eventually the actual presence of the corresponding external stimuli is no longer necessary, and it will recur on the vibrations set up by faint stimuli from within.  Sensations arising in this way from within, as, for example, an idea of whiteness, are not, indeed, perceived with the full freshness of those raised by the actual presence of white light without us, but they are of the same kind; they are feeble repetitions of one and the same material brain process - of one and the same conscious sensation. Thus the idea of whiteness arises in our mind as a faint, almost extinct, sensation.
In this way those qualities which are common to many things become separated, as it were, in our memory from the objects with which they were originally associated, and attain an independent existence in our consciousness as ideas and conceptions, and thus the whole rich superstructure of our ideas and conceptions is built up from materials supplied by memory.
On examining more closely, we see plainly that memory is a faculty not only of our conscious states, but also, and much more so, of our unconscious ones. I was conscious of this or that yesterday, and am again conscious of it to-day. Where has it been meanwhile? It does not remain continuously within my consciousness, nevertheless it returns after having quitted it. Our ideas tread but for a moment upon the stage of consciousness, and then go back again behind the scenes, to make way for others in their place. As the player is only a king when he is on the stage, so they too exist as ideas so long only as they are recognised. How do they live when they are off the stage? For we know that they are living somewhere; give them their cue and they reappear immediately. They do not exist continuously as ideas; what is continuous is the special disposition of nerve substance in virtue of which this substance gives out to-day the same sound which it gave yesterday if it is rightly struck.  Countless reproductions of organic processes of our brain connect themselves orderly together, so that one acts as a stimulus to the next, but a phenomenon of consciousness is not necessarily attached to every link in the chain. From this it arises that a series of ideas may appear to disregard the order that would be observed in purely material processes of brain substance unaccompanied by consciousness; but on the other hand it becomes possible for a long chain of recollections to have its due development without each link in the chain being necessarily perceived by ourselves. One may emerge from the bosom of our unconscious thoughts without fully entering upon the stage of conscious perception; another dies away in unconsciousness, leaving no successor to take its place. Between the “me” of to-day and the “me” of yesterday lie night and sleep, abysses of unconsciousness; nor is there any bridge but memory with which to span them. Who can hope after this to disentangle the infinite intricacy of our inner life? For we can only follow its threads so far as they have strayed over within the bounds of consciousness. We might as well hope to familiarise ourselves with the world of forms that teem within the bosom of the sea by observing the few that now and again come to the surface and soon return into the deep.
The bond of union, therefore, which connects the individual phenomena of our consciousness lies in our unconscious world; and as we know nothing of this but what investigation into the laws of matter teach us - as, in fact, for purely experimental purposes, “matter” and the “unconscious” must be one and the same thing - so the physiologist has a full right to denote memory as, in the wider sense of the word, a function of brain substance, whose results, it is true, fall, as regards one part of them, into the domain of consciousness, while another and not less essential part escapes unperceived as purely material processes.
The perception of a body in space is a very complicated process. I see suddenly before me, for example, a white ball. This has the effect of conveying to me more than a mere sensation of whiteness. I deduce the spherical character of the ball from the gradations of light and shade upon its surface. I form a correct appreciation of its distance from my eye, and hence again I deduce an inference as to the size of the ball. What an expenditure of sensations, ideas, and inferences is found to be necessary before all this can be brought about; yet the production of a correct perception of the ball was the work only of a few seconds, and I was unconscious of the individual processes by means of which it was effected, the result as a whole being alone present in my consciousness.
The nerve substance preserves faithfully the memory of habitual actions.  Perceptions which were once long and difficult, requiring constant and conscious attention, come to reproduce themselves in transient and abridged guise, without such duration and intensity that each link has to pass over the threshold of our consciousness.
We have chains of material nerve processes to which eventually a link becomes attached that is attended with conscious perception. This is sufficiently established from the standpoint of the physiologist, and is also proved by our unconsciousness of many whole series of ideas and of the inferences we draw from them. If the soul is not to ship through the fingers of physiology, she must hold fast to the considerations suggested by our unconscious states. As far, however, as the investigations of the pure physicist are concerned, the unconscious and matter are one and the same thing, and the physiology of the unconscious is no “philosophy of the unconscious.”
By far the greater number of our movements are the result of long and arduous practice. The harmonious cooperation of the separate muscles, the finely adjusted measure of participation which each contributes to the working of the whole, must, as a rule, have been laboriously acquired, in respect of most of the movements that are necessary in order to effect it. How long does it not take each note to find its way from the eyes to the fingers of one who is beginning to learn the pianoforte; and, on the other hand, what an astonishing performance is the playing of the professional pianist. The sight of each note occasions the corresponding movement of the fingers with the speed of thought - a hurried glance at the page of music before him suffices to give rise to a whole series of harmonies; nay, when a melody has been long practised, it can be played even while the player’s attention is being given to something of a perfectly different character over and above his music.
The will need now no longer wend its way to each individual finger before the desired movements can be extorted from it; no longer now does a sustained attention keep watch over the movements of each limb; the will need exercise a supervising control only. At the word of command the muscles become active, with a due regard to time and proportion, and go on working, so long as they are bidden to keep in their accustomed groove, while a slight hint on the part of the will, will indicate to them their further journey. How could all this be if every part of the central nerve system, by means of which movement is effected, were not able [74a] to reproduce whole series of vibrations, which at an earlier date required the constant and continuous participation of consciousness, but which are now set in motion automatically on a mere touch, as it were, from consciousness - if it were not able to reproduce them the more quickly and easily in proportion to the frequency of the repetitions - if, in fact, there was no power of recollecting earlier performances? Our perceptive faculties must have remained always at their lowest stage if we had been compelled to build up consciously every process from the details of the sensation-causing materials tendered to us by our senses; nor could our voluntary movements have got beyond the helplessness of the child, if the necessary impulses could only be imparted to every movement through effort of the will and conscious reproduction of all the corresponding ideas - if, in a word, the motor nerve system had not also its memory, [74b] though that memory is unperceived by ourselves. The power of this memory is what is called “the force of habit.”
It seems, then, that we owe to memory almost all that we either have or are; that our ideas and conceptions are its work, and that our every perception, thought, and movement is derived from this source. Memory collects the countless phenomena of our existence into a single whole; and as our bodies would be scattered into the dust of their component atoms if they were not held together by the attraction of matter, so our consciousness would be broken up into as many fragments as we had lived seconds but for the binding and unifying force of memory.
We have already repeatedly seen that the reproductions of organic processes, brought about by means of the memory of the nervous system, enter but partly within the domain of consciousness, remaining unperceived in other and not less important respects. This is also confirmed by numerous facts in the life of that part of the nervous system which ministers almost exclusively to our unconscious life processes. For the memory of the so-called sympathetic ganglionic system is no less rich than that of the brain and spinal marrow, and a great part of the medical art consists in making wise use of the assistance thus afforded us.
To bring, however, this part of my observations to a close, I will take leave of the nervous system, and glance hurriedly at other phases of organised matter, where we meet with the same powers of reproduction, but in simpler guise.
Daily experience teaches us that a muscle becomes the stronger the more we use it. The muscular fibre, which in the first instance may have answered but feebly to the stimulus conducted to it by the motor nerve, does so with the greater energy the more often it is stimulated, provided, of course, that reasonable times are allowed for repose. After each individual action it becomes more capable, more disposed towards the same kind of work, and has a greater aptitude for repetition of the same organic processes. It gains also in weight, for it assimilates more matter than when constantly at rest. We have here, in its simplest form, and in a phase which comes home most closely to the comprehension of the physicist, the same power of reproduction which we encountered when we were dealing with nerve substance, but under such far more complicated conditions. And what is known thus certainly from muscle substance holds good with greater or less plainness for all our organs. More especially may we note the fact, that after increased use, alternated with times of repose, there accrues to the organ in all animal economy an increased power of execution with an increased power of assimilation and a gain in size.
This gain in size consists not only in the enlargement of the individual cells or fibres of which the organ is composed, but in the multiplication of their number; for when cells have grown to a certain size they give rise to others, which inherit more or less completely the qualities of those from which they came, and therefore appear to be repetitions of the same cell. This growth, and multiplication of cells is only a special phase of those manifold functions which characterise organised matter, and which consist not only in what goes on within the cell substance as alterations or undulatory movement of the molecular disposition, but also in that which becomes visible outside the cells as change of shape, enlargement, or subdivision. Reproduction of performance, therefore, manifests itself to us as reproduction of the cells themselves, as may be seen most plainly in the case of plants, whose chief work consists in growth, whereas with animal organism other faculties greatly preponderate.
Let us now take a brief survey of a class of facts in the case of which we may most abundantly observe the power of memory in organised matter. We have ample evidence of the fact that characteristics of an organism may descend to offspring which the organism did not inherit, but which it acquired owing to the special circumstances under which it lived; and that, in consequence, every organism imparts to the germ that issues from it a small heritage of acquisitions which it has added during its own lifetime to the gross inheritance of its race.
When we reflect that we are dealing with the heredity of acquired qualities which came to development in the most diverse parts of the parent organism, it must seem in a high degree mysterious how those parts can have any kind of influence upon a germ which develops itself in an entirely different place. Many mystical theories have been propounded for the elucidation of this question, but the following reflections may serve to bring the cause nearer to the comprehension of the physiologist.
The nerve substance, in spite of its thousandfold subdivision as cells and fibres, forms, nevertheless, a united whole, which is present directly in all organs - nay, as more recent histology conjectures, in each cell of the more important organs - or is at least in ready communication with them by means of the living, irritable, and therefore highly conductive substance of other cells. Through the connection thus established all organs find themselves in such a condition of more or less mutual interdependence upon one another, that events which happen to one are repeated in others, and a notification, however slight, of a vibration set up  in one quarter is at once conveyed even to the farthest parts of the body. With this easy and rapid intercourse between all parts is associated the more difficult communication that goes on by way of the circulation of sap or blood.
We see, further, that the process of the development of all germs that are marked out for independent existence causes a powerful reaction, even from the very beginning of that existence, on both the conscious and unconscious life of the whole organism. We may see this from the fact that the organ of reproduction stands in closer and more important relation to the remaining parts, and especially to the nervous system, than do the other organs; and, inversely, that both the perceived and unperceived events affecting the whole organism find a more marked response in the reproductive system than elsewhere.
We can now see with sufficient plainness in what the material connection is established between the acquired peculiarities of an organism, and the proclivity on the part of the germ in virtue of which it develops the special characteristics of its parent.
The microscope teaches us that no difference can be perceived between one germ and another; it cannot, however, be objected on this account that the determining cause of its ulterior development must be something immaterial, rather than the specific kind of its material constitution.
The curves and surfaces which the mathematician conceives, or finds conceivable, are more varied and infinite than the forms of animal life. Let us suppose an infinitely small segment to be taken from every possible curve; each one of these will appear as like every other as one germ is to another, yet the whole of every curve lies dormant, as it were, in each of them, and if the mathematician chooses to develop it, it will take the path indicated by the elements of each segment.
It is an error, therefore, to suppose that such fine distinctions as physiology must assume lie beyond the limits of what is conceivable by the human mind. An infinitely small change of position on the part of a point, or in the relations of the parts of a segment of a curve to one another, suffices to alter the law of its whole path, and so in like manner an infinitely small influence exercised by the parent organism on the molecular disposition of the germ  may suffice to produce a determining effect upon its whole farther development.
What is the descent of special peculiarities but a reproduction on the part of organised matter of processes in which it once took part as a germ in the germ-containing organs of its parent, and of which it seems still to retain a recollection that reappears when time and the occasion serve, inasmuch as it responds to the same or like stimuli in a like way to that in which the parent organism responded, of which it was once part, and in the events of whose history it was itself also an accomplice?  When an action through long habit or continual practice has become so much a second nature to any organisation that its effects will penetrate, though ever so faintly, into the germ that lies within it, and when this last comes to find itself in a new sphere, to extend itself, and develop into a new creature - (the individual parts of which are still always the creature itself and flesh of its flesh, so that what is reproduced is the same being as that in company with which the germ once lived, and of which it was once actually a part) - all this is as wonderful as when a grey-haired man remembers the events of his own childhood; but it is not more so. Whether we say that the same organised substance is again reproducing its past experience, or whether we prefer to hold that an offshoot or part of the original substance has waxed and developed itself since separation from the parent stock, it is plain that this will constitute a difference of degree, not kind.
When we reflect upon the fact that unimportant acquired characteristics can be reproduced in offspring, we are apt to forget that offspring is only a full-sized reproduction of the parent - a reproduction, moreover, that goes as far as possible into detail. We are so accustomed to consider family resemblance a matter of course, that we are sometimes surprised when a child is in some respect unlike its parent; surely, however, the infinite number of points in respect of which parents and children resemble one another is a more reasonable ground for our surprise.
But if the substance of the germ can reproduce characteristics acquired by the parent during its single life, how much more will it not be able to reproduce those that were congenital to the parent, and which have happened through countless generations to the organised matter of which the germ of to-day is a fragment? We cannot wonder that action already taken on innumerable past occasions by organised matter is more deeply impressed upon the recollection of the germ to which it gives rise than action taken once only during a single lifetime. [80a]
We must bear in mind that every organised being now in existence represents the last link of an inconceivably long series of organisms, which come down in a direct line of descent, and of which each has inherited a part of the acquired characteristics of its predecessor. Everything, furthermore, points in the direction of our believing that at the beginning of this chain there existed an organism of the very simplest kind, something, in fact, like those which we call organised germs. The chain of living beings thus appears to be the magnificent achievement of the reproductive power of the original organic structure from which they have all descended. As this subdivided itself and transmitted its characteristics [80b] to its descendants, these acquired new ones, and in their turn transmitted them - all new germs transmitting the chief part of what had happened to their predecessors, while the remaining part lapsed out of their memory, circumstances not stimulating it to reproduce itself.
An organised being, therefore, stands before us a product of the unconscious memory of organised matter, which, ever increasing and ever dividing itself, ever assimilating new matter and returning it in changed shape to the inorganic world, ever receiving some new thing into its memory, and transmitting its acquisitions by the way of reproduction, grows continually richer and richer the longer it lives.
Thus regarded, the development of one of the more highly organised animals represents a continuous series of organised recollections concerning the past development of the great chain of living forms, the last link of which stands before us in the particular animal we may be considering. As a complicated perception may arise by means of a rapid and superficial reproduction of long and laboriously practised brain processes, so a germ in the course of its development hurries through a series of phases, hinting at them only. Often and long foreshadowed in theories of varied characters, this conception has only now found correct exposition from a naturalist of our own time.  For Truth hides herself under many disguises from those who seek her, but in the end stands unveiled before the eyes of him whom she has chosen.
Not only is there a reproduction of form, outward and inner conformation of body, organs, and cells, but the habitual actions of the parent are also reproduced. The chicken on emerging from the eggshell runs off as its mother ran off before it; yet what an extraordinary complication of emotions and sensations is necessary in order to preserve equilibrium in running. Surely the supposition of an inborn capacity for the reproduction of these intricate actions can alone explain the facts. As habitual practice becomes a second nature to the individual during his single lifetime, so the often-repeated action of each generation becomes a second nature to the race.
The chicken not only displays great dexterity in the performance of movements for the effecting of which it has an innate capacity, but it exhibits also a tolerably high perceptive power. It immediately picks up any grain that may be thrown to it. Yet, in order to do this, more is wanted than a mere visual perception of the grains; there must be an accurate apprehension of the direction and distance of the precise spot in which each grain is lying, and there must be no less accuracy in the adjustment of the movements of the head and of the whole body. The chicken cannot have gained experience in these respects while it was still in the egg. It gained it rather from the thousands of thousands of beings that have lived before it, and from which it is directly descended.
The memory of organised substance displays itself here in the most surprising fashion. The gentle stimulus of the light proceeding from the grain that affects the retina of the chicken,  gives occasion for the reproduction of a many-linked chain of sensations, perceptions, and emotions, which were never yet brought together in the case of the individual before us. We are accustomed to regard these surprising performances of animals as manifestations of what we call instinct, and the mysticism of natural philosophy has ever shown a predilection for this theme; but if we regard instinct as the outcome of the memory or reproductive power of organised substance, and if we ascribe a memory to the race as we already ascribe it to the individual, then instinct becomes at once intelligible, and the physiologist at the same time finds a point of contact which will bring it into connection with the great series of facts indicated above as phenomena of the reproductive faculty. Here, then, we have a physical explanation which has not, indeed, been given yet, but the time for which appears to be rapidly approaching.
When, in accordance with its instinct, the caterpillar becomes a chrysalis, or the bird builds its nest, or the bee its cell, these creatures act consciously and not as blind machines. They know how to vary their proceedings within certain limits in conformity with altered circumstances, and they are thus liable to make mistakes. They feel pleasure when their work advances and pain if it is hindered; they learn by the experience thus acquired, and build on a second occasion better than on the first; but that even in the outset they hit so readily upon the most judicious way of achieving their purpose, and that their movements adapt themselves so admirably and automatically to the end they have in view - surely this is owing to the inherited acquisitions of the memory of their nerve substance, which requires but a touch and it will fall at once to the most appropriate kind of activity, thinking always, and directly, of whatever it is that may be wanted.
Man can readily acquire surprising kinds of dexterity if he confines his attention to their acquisition. Specialisation is the mother of proficiency. He who marvels at the skill with which the spider weaves her web should bear in mind that she did not learn her art all on a sudden, but that innumerable generations of spiders acquired it toilsomely and step by step - this being about all that, as a general rule, they did acquire. Man took to bows and arrows if his nets failed him - the spider starved. Thus we see the body and - what most concerns us - the whole nervous system of the new-born animal constructed beforehand, and, as it were, ready attuned for intercourse with the outside world in which it is about to play its part, by means of its tendency to respond to external stimuli in the same manner as it has often heretofore responded in the persons of its ancestors.
We naturally ask whether the brain and nervous system of the human infant are subjected to the principles we have laid down above? Man certainly finds it difficult to acquire arts of which the lower animals are born masters; but the brain of man at birth is much farther from its highest development than is the brain of an animal. It not only grows for a longer time, but it becomes stronger than that of other living beings. The brain of man may be said to be exceptionally young at birth. The lower animal is born precocious, and acts precociously; it resembles those infant prodigies whose brain, as it were, is born old into the world, but who, in spite of, or rather in addition to, their rich endowment at birth, in after life develop as much mental power as others who were less splendidly furnished to start with, but born with greater freshness of youth. Man’s brain, and indeed his whole body, affords greater scope for individuality, inasmuch as a relatively greater part of it is of post-natal growth. It develops under the influence of impressions made by the environment upon its senses, and thus makes its acquisitions in a more special and individual manner, whereas the animal receives them ready made, and of a more final, stereotyped character.
Nevertheless, it is plain we must ascribe both to the brain and body of the new-born infant a far-reaching power of remembering or reproducing things which have already come to their development thousands of times over in the persons of its ancestors. It is in virtue of this that it acquires proficiency in the actions necessary for its existence - so far as it was not already at birth proficient in them - much more quickly and easily than would be otherwise possible; but what we call instinct in the case of animals takes in man the looser form of aptitude, talent, and genius.  Granted that certain ideas are not innate, yet the fact of their taking form so easily and certainly from out of the chaos of his sensations, is due not to his own labour, but to that of the brain substance of the thousands of thousands of generations from whom he is descended. Theories concerning the development of individual consciousness which deny heredity or the power of transmission, and insist upon an entirely fresh start for every human soul, as though the infinite number of generations that have gone before us might as well have never lived for all the effect they have had upon ourselves, - such theories will contradict the facts of our daily experience at every touch and turn.
The brain processes and phenomena of consciousness which ennoble man in the eyes of his fellows have had a less ancient history than those connected with his physical needs. Hunger and the reproductive instinct affected the oldest and simplest forms of the organic world. It is in respect of these instincts, therefore, and of the means to gratify them, that the memory of organised substance is strongest - the impulses and instincts that arise hence having still paramount power over the minds of men. The spiritual life has been superadded slowly; its most splendid outcome belongs to the latest epoch in the history of organised matter, nor has any very great length of time elapsed since the nervous system was first crowned with the glory of a large and well-developed brain.
Oral tradition and written history have been called the memory of man, and this is not without its truth. But there is another and a living memory in the innate reproductive power of brain substance, and without this both writings and oral tradition would be without significance to posterity. The most sublime ideas, though never so immortalised in speech or letters, are yet nothing for heads that are out of harmony with them; they must be not only heard, but reproduced; and both speech and writing would be in vain were there not an inheritance of inward and outward brain development, growing in correspondence with the inheritance of ideas that are handed down from age to age, and did not an enhanced capacity for their reproduction on the part of each succeeding generation accompany the thoughts that have been preserved in writing. Man’s conscious memory comes to an end at death, but the unconscious memory of Nature is true and ineradicable: whoever succeeds in stamping upon her the impress of his work, she will remember him to the end of time.
Footnotes by Samuel Butler, Unconscious Memory, wherin this text represents ch 6.
 See quotation from Bonnet, p. 54 of this volume: “The soul never has a new sensation but by the inter position of the senses. This sensation has been originally attached to the motion of certain fibres. Its reproduction or recollection by the senses will then be likewise connected with these same fibres.” “Contemplation of Nature,” Engl. trans., Lond. 1776
“It appeared to me that since this memory is connected with the body, it must depend upon some change which must happen to the primitive state of the sensible fibres by the action of objects. I have, therefore, admitted as probable that the state of the fibres on which an object has acted is not precisely the same after this action as it was before I have conjectured that the sensible fibres experience more or less durable modifications, which constitute the physics of memory and recollection.”
 Professor Hering is not clear here. Vibrations (if I understand his theory rightly) should not be set up by faint stimuli from within. Whence and what are these stimuli? The vibrations within are already existing, and it is they which are the stimuli to action. On having been once set up, they either continue in sufficient force to maintain action, or they die down, and become too weak to cause further action, and perhaps even to be perceived within the mind, until they receive an accession of vibration from without. The only “stimulus from within” that should be able to generate action is that which may follow when a vibration already established in the body runs into another similar vibration already so established.
On this consciousness, and even action, might be supposed to follow without the presence of an external stimulus.
 This expression seems hardly applicable to the overtaking of an internal by an external vibration, but it is not inconsistent with it. Here, however, as frequently elsewhere, I doubt how far Professor Hering has fully realised his conception, beyond being, like myself, convinced that the phenomena of memory and of heredity have a common source.
 See quotation from Bonnet, p. 54 of this volume. By “preserving the memory of habitual actions” Professor Hering probably means, retains for a long while and repeats motion of a certain character when such motion has been once communicated to it.
[74a] It should not be “if the central nerve system were not able to reproduce whole series of vibrations,” but “if whole series of vibrations do not persist though unperceived,” if Professor Hering intends what I suppose him to intend.
[74b] Memory was in full operation for so long a time before anything like what we call a nervous system can be detected, that Professor Hering must not be supposed to be intending to confine memory to a motor nerve system. His words do not even imply that he does, but it is as well to be on one’s guard.
 It is from such passages as this, and those that follow on the next few pages, that I collect the impression of Professor Hering’s meaning which I have endeavoured to convey in the preceding chapter.
 That is to say, “an infinitely small change in the kind of vibration communicated from the parent to the germ.”
 It may be asked what is meant by responding. I may repeat that I understand Professor Hering to mean that there exists in the offspring certain vibrations, which are many of them too faint to upset equilibrium and thus generate action, until they receive an accession of force from without by the running into them of vibrations of similar characteristics to their own, which last vibrations have been set up by exterior objects. On this they become strong enough to generate that corporeal earthquake which we call action.
This may be true or not, but it is at any rate intelligible; whereas much that is written about “fraying channels” raises no definite ideas in the mind.
[80a] I interpret this, “We cannot wonder if often-repeated vibrations gather strength, and become at once more lasting and requiring less accession of vibration from without, in order to become strong enough to generate action.”
[80b] “Characteristics” must, I imagine, according to Professor Hering, resolve themselves ultimately into “vibrations,” for the characteristics depend upon the character of the vibrations.
 Professor Hartog tells me that this probably refers to Fritz Müller’s formulation of the “recapitulation process” in “Facts for Darwin,” English edition (1869), p. 114. - R.A.S.
 This is the passage which makes me suppose Professor Hering to mean that vibrations from exterior objects run into vibrations already existing within the living body, and that the accession to power thus derived is his key to an explanation of the physical basis of action.
 I interpret this: “There are fewer vibrations persistent within the bodies of the lower animals; those that there are, therefore, are stronger and more capable of generating action or upsetting the status in quo. Hence also they require less accession of vibration from without. Man is agitated by more and more varied vibrations; these, interfering, as to some extent they must, with one another, are weaker, and therefore require more accession from without before they can set the mechanical adjustments of the body in motion.”