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CHAPTER V.: INSTINCT. - Herbert Spencer, The Principles of Psychology 
The Principles of Psychology (London: Longman, Brown, Green and Longmans, 1855).
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§ 185. Using the word, not as the vulgar do to designate all other kinds of intelligence than the human, but restricting it to its proper signification, Instinct may be defined as—compound reflex action. Strictly speaking, no line of demarcation can be drawn between it and simple reflex action, out of which it arises by successive complications. Though the two have been distinguished as sensori-motor and excito-motor, the distinction seems to me to be one that cannot be maintained as in any way definite. Sensation proper implies consciousness; consciousness, as we understand it, can come into existence only when the chief nervous centre becomes the seat of a varied succession of changes of state; and as the sensory ganglia in their lowest forms, are scarcely at all more subject to such succession than are those ganglia producing the unconscious reflex actions, there is no reason to assume that the impressions they receive are known as sensations. Whether certain muscular contractions are produced by the stimulation of an insect's optic nerve, or by the stimulation of a nerve of touch supplying one of its legs, matters not in so far as the psychological meaning of the phenonenon is concerned. In either case, by a purely automatic process, a certain change in the peripheral extremity of a nerve has produced certain motions: the relation is, for aught that appears, as direct in the one case as in the other: and there is no more reason to assume that the visual impression produces consciousness than that the tactual one does. The only scientific distinctions that can here be drawn, are those based upon the degrees of complexity in the stimuli, and in the consequent actions; and these are simply distinctions of degree, and not of kind. In so far as an instinctive action involves the co-ordination of many impressions; and in so far as the chief ganglion consequently undergoes complicated changes; in so far may there be incipient sensations—a dawning consciousness; and in so far an instinctive action may be sensori-motor or consensual. But it is clear that the consciousness is a function of the complexity; and arises only as the complexity increases. The complexity, therefore, is the thing with which we are essentially concerned.
That the validity of this definition of Instinct, as distinguished from the primitive kind of reflex action, may be clearly seen, let us, before going further, take an example. “A fly-catcher,” says Carpenter, “immediately after its exit from the egg, has been known to peck at and capture an insect—an action which requires a very exact appreciation of distance, as well as a power of precisely regulating the muscular movements in accordance with it.” Now this action, which is distinctly proved by the circumstances to be purely automatic, necessarily implies the combination of a number of separate stimuli. The excitation of a certain group of the retinal nerve-fibres must be one; and this excitation must really be in itself a complex one; seeing, that as the same effect is not produced by casting an image of any size upon the retina; and as the different effects that result from the casting of different images on the retina, must result from differences in the number or combination of the nerve-fibres affected; the retinal stimulus must really be a certain combination of stimuli. Another necessary component in the general stimulus, must be that proceeding from the muscles by which the foci of the eyes are adjusted. And yet another component must be that proceeding from the muscles by which the axes of eyes are directed to a special point. Without impressions proceeding from both these sets of muscles, it would be impossible for the head to be guided in the right direction, or for the beak to be closed at the right moment. Thus then, the action implies the excitation of two groups of retinal nerves, two groups of nerves proceeding from the muscles which adjust the foci, and two groups of nerves proceeding from the muscles which move the eyes—implies that all these nerves are excited simultaneously in special ways and degrees; and that the special co-ordination of muscular contractions by which the fly is caught, is the result of this special co-ordination of stimuli. Of such complex co-ordination directly resulting from a complex stimulus, we have abundant illustration in ourselves. All our ordinary movements, though originating in volition, are performed in a mode exactly like that described. When putting out the hand to grasp an object before us, we are wholly unconscious of the particular muscular adjustments required. We see the object, and we desire to lay hold of it; and in response to the desire the arm is put out in a special way. But were the various nervous stimuli involved in the visual impression, absent, the muscles of the arm could not be guided aright. That is to say, the special muscular co-ordination is due to the special co-ordination of sensations received from the eye and its adjusting apparatus—the volition being concerned merely in setting these processes going. The difference between one of these actions of our own, and that of the newly-hatched fly-catcher, consists in this; that whereas, in ourselves, the combined impressions and motions being almost infinitely varied and severally repeated with comparative infrequency, are not born with us, but are developed in the course of our first years, in the fly-catcher, by whose race a special combination is perpetually repeated by every individual throughout life, such combination is ready-organized.
But, returning from this illustrative comparison, and considering by themselves such cases as this of the young fly-catcher, it is unquestionable that the process is one of compound reflex action. While in simple reflex action a single impression is followed by a single contraction; while in the more developed forms of reflex action a single impression is followed by a combination of contractions; in this, that we distinguish as instinct, a combination of impressions produces a combination of contractions: and the higher the instinct the more complex are both the directive and executive co-ordinations. Let us now, however, contemplate the facts in connection with the general laws we are tracing out.
§ 186. Instinct is very obviously further removed from the purely physical life, than is simple reflex action. While simple reflex action is common to the internal visceral processes and to the external processes of animal life; instinct, properly so-called, is not. There are no instincts displayed by the kidneys, the lungs, the liver: they are confined to the actions of the nervo-muscular apparatus, which is the especial agent of the psychical life.
Again, the instinctive actions exhibit much less simultaneity—are in a great degree successive only. The co-ordination of many stimuli into one stimulus, itself involves a diminution of the many separate nervous actions going on simultaneously; and a merging of them into some combined, and therefore serial, process. Whether the various co-ordinated nervous changes which take place when the fly-catcher seizes an insect, are regarded as a series passing through its sensorium in rapid succession, or as consolidated into two successive states of its sensorium, it is equally clear that the changes in its sensorium have a much more decided linear arrangement than the changes going on in all the scattered ganglia of a centipede.
Moreover, it is not improbable that, in its higher forms, instinct is accompanied by some approach to what we understand as consciousness. There cannot be a co-ordination of many stimuli, without some centre of communication through which they are all brought into relation. In the process of bringing them into relation, this centre must be subject to the influence of each—must undergo many changes. And the quick succession of changes in a sentient centre, constitutes the raw material of consciousness. The implication is, therefore, that in proportion as instinct is developed, some kind of consciousness becomes nascent.
Yet further, the instinctive actions are more removed from the purely physical actions in this, that they answer to external phenomena that are more complex and more special. While the purely physical actions respond to those most general relations common to the environment as a whole; while the simple reflex actions respond to some of the most general relations common to the individual objects it contains; these compound reflex actions which we class as instincts, respond to those more involved relations by which certain orders of objects and actions are distinguished from others.
Thus, in the phenomena of instinct, a greater differentiation of the psychical from the physical life is seen; alike in the growing distinction between the vegetative and animal systems; in the increasing seriality of the changes in the animal system; in the consequent rise of incipient consciousness; and in the higher complexity of the outer relations to which inner relations are adjusted: which last is indeed the essence of the advance, to which the others are necessary accompaniments.
§ 187. But now let us consider how, by accumulated experiences, the compound reflex actions may be developed out of the simple ones.
For our example we may fitly take some low aquatic creature endowed with rudimentary eyes. As was before remarked, eyes of this character, sensitive as they are only to the strongest changes in the quantity of light, can be affected by opaque bodies moving in the surrounding water, only when such bodies approach so close as almost to touch the surface. Only then can the transit of such bodies produce a sufficiently marked change to be appreciated by nascent vision. But almost always the bodies that are carried by their motion quite close to the organism, will, by their further motion, be brought in contact with it. The cases in which the movement of an external body is such as to carry it by, almost at a tangent to that part of the organism where the rudimentary eye is placed, so as nearly to touch the surface in passing, but not quite, must be exceptional. Evidently, therefore, in its earliest forms, sight is, as before said, little more than anticipatory touch: visual impressions are habitually followed by tactual ones. But tactual impressions are, in all these creatures, habitually followed by contractions—contractions which, as pointed out in another place (§ 134), are in all probability the necessary effects produced by mechanical disturbance upon the vital activities—contractions which, under like stimuli, are seen even in certain plants, and are so shown to be producible by alterations in the processes of the purely physical life. Result as they may, however, it is beyond question that from the zoophytes upwards, touch and contraction form an habitual sequence; and hence, in creatures in whom the incipient vision amounts to little more than anticipatory touch, there constantly occurs the succession—a visual impression, a tactual impression, a contraction. Now the evolution of a nervous system, is a necessary concomitant of that specialization which originates the senses. Until the general sensitiveness is in some degree localized, the internuncial function of the nervous system, cannot exist: and there can be no such localized sensitiveness without there being something in the shape of nerves. A nascent sense of sight, therefore, implies a nascent nervous communication.∗ And along with a nascent nervous communication we may see the first illustration of the law of developing intelligence. If psychical states (using the term in its widest sense) which constantly follow one another in a certain order, become ever more closely connected in that order, so as eventually to become inseparable; then it must follow that if, in the experience of any race of organisms, a visual impression, a tactual impression, and a contraction, are continually repeated in this succession, the several nervous states produced will become so consolidated that the first cannot be caused without the others necessarily following—the visual impression will be instantly succeeded by a nervous excitation like that which a tactual impression produces; and this by a contraction. Thus there will arise a contraction in anticipation of touch: and when more perfect vision is acquired, there will result those convulsive movements which low organisms display when any large moving object comes into their neighbourhood.
Thus far, however, the phenomena are those of simple reflex action; or rather, reflex action that is incipiently compound. Let us now consider what must result from a further development of vision. Such further development of vision we know from positive evidence takes place under continued exercise. The Bosjesman, ever on the look-out for distant enemies and prey, has eyes very far exceeding those of the European in acuteness; and it is a legitimate inference that, with rudimentary eyes as with developed ones, increased activity will entail increased power. Assuming such increased power, what must be its consequences? The simultaneous consequences must be, that the same bodies will be discerned at a greater distance, and smaller bodies will be discerned when close to. Both of these will produce obscurations that are faint, in comparison with that complete obscuration produced by some large moving body that is about to strike the surface. But from the time when they first become appreciable, such faint obscurations will not, like the extreme ones, be habitually followed by strong tactual impressions and subsequent contractions. If produced by a large object passing at some distance, there will probably be no collision—no tactual impression at all. If produced by a small object close to, the collision that follows will be comparatively slight—so slight as not to induce a violent contraction, but simply sufficient to produce an incipient tension in the muscular apparatus—a tension such as that seen in any creature about to seize upon prey. This is by no means an assumption. It is an established fact, that among animals in general, ourselves included, a sensation or nervous stimulus, which, if slight, simply rouses attention and produces some slight muscular action, will, if it becomes intense, cause convulsive contractions of the muscles in general. It is therefore a deduction from a well-established law of the nervo-muscular system, that a creature possessed of this somewhat improved vision, will, by a partial obscuration of light, have its muscles brought into a state of partial tension—a state fitting them either for the seizure of a small animal, should the partial obscuration be caused by the impending collision of one, or for sudden retreat into a shell or convulsive movements of escape, should the obscuration be increased by the near approach of a larger animal. Thus, even by this simple advance there must necessarily be produced a somewhat greater speciality and complexity in the inner relalations answering to outer relations.
But now let us go a step further. Let us suppose the creature to be one that habitually moves about in the water; and let us suppose a somewhat further development of the faculty of sight—a development consisting in such enlargement of the retina, and such subdivision of it into separate sensitive agents, as shall admit of its different parts being independently affected. In such a creature, the eyes are subject to frequent change of impressions produced by the objects amid which it swims. These impressions fall upon different parts of its retinas, according to the positions of the objects making them. Those on one side of the creature either affect one retina only, or one much more than the other. Those above it have their images cast on the lower parts of the retinas. Those below it, if visible at all, cast images on their upper parts. Of all the impressions thus made, however, few, if any, are directly followed by any tactual impression: the creature's forward movement carries it away from the objects making them. Only when these lateral impressions made by moving objects are very strong—only when they are the impressions produced by larger animals, will there result any excitation of the motor powers. Faint lateral impressions, not being habitually followed by any tactual impressions, will have no effect upon the actions. But now mark that there are certain visual impressions, which, though not strong, are constantly followed by tactual ones; and by tactual ones of a particular kind: those impressions, namely, which are made by small objects in front. When, during its passage through the water, certain parts of the creature's two retinas are simultaneously affected by impressions of moderate strength; it very generally happens, that immediately afterwards, the feelers and head come in contact with some small body serving for food. A visual impression of a special kind, is habitually followed by a tactual impression on the prehensile organs; and, consequently, by all those muscular actions which the presentation of food to the prehensile organs calls forth. In the nature of things, this sequence must continually occur. The excitation of a particular group of retinal nerves; the excitation of the nerves of the prehensile organs; and the excitation of a special set of muscles; must become an established succession. In the creature's experience, these three psychical states are habitually connected; and must, by repetition in countless generations, become so coherent that the special visual impression will directly call forth the muscular actions by which prey is seized. Eventually, the sight of a small object in front, will, of itself, set a-going the various motions requisite for the capture of prey.
Here then, we have one of the simpler forms of instinct, which, under the requisite conditions, must necessarily be established by accumulated experiences. Let it be granted that in all creatures, as in ourselves, the law is and ever has been, that the more frequently psychical states occur in a certain order, the stronger becomes their tendency to cohere in that order, until they at last become inseparable; let it be granted that this tendency is, in however slight a degree, inherited, so that if the experiences remain the same, each successive generation bequeathes a somewhat increased tendency; and it follows, that, in cases like the one described, there must inevitably be established an automatic connection of nervous actions, corresponding to the external relations perpetually experienced. If, from some change in the environment of any species, its members are frequently brought in contact with a new relation; if the organization of the species is so far developed as to be impressible by the terms of the new relation, in close succession; then, an inner relation corresponding to this new outer relation, will gradually be formed; and will in the end become organic. The organized relations previously existing in the species will be further complicated by a superinduced relation. As in the case described, where the simultaneous excitation of two groups of nerve-fibres proceeding from special parts of two retinas is the stimulus, a compound reflex action will arise out of simple ones. An outer relation one stage more complex than before, will be responded to by an inner relation one stage more complex than before. And so on in subsequent stages of progress.
Of course this is not meant as anything more than a rough indication of the mode in which the general principles that have been enunciated, explain the development of instincts. The abstract law of intelligence being, that the strengths of the inner cohesions between psychical states must be proportionate to the persistencies of the outer relations to which they answer; and the development of intelligence into conformity with the law, being, in all cases of which we have positive knowledge, secured by the one simple principle that the outer relations produce the inner relations, and make the inner relations strong in proportion as they are themselves persistent; it was requisite to inquire whether there is reason to think that the intelligence concerning whose genesis we have no positive knowledge, had a like origin. And all that it is above proposed to show, is, that reasoning deductively from the conditions of the case, this same one simple principle appears sufficient to account for the facts—or rather, for a type of them. To trace out the actual development of instincts, in their infinite varieties and complications, must ever remain impossible. The data are inaccessible; and were they accessible, could not be adequately grasped. The foregoing is to be taken merely as an adumbration of the probable mode of development.
§ 188. And now let us consider what must be the ulterior results of this mode of development. Assuming some such process as that above suggested, to be the one by which the instincts in general are evolved; let us inquire what must must be the general characteristics of the evolution regarded in its ensemble; and observe how far they agree with the actual ones.
Without referring back to the argument elaborated in the General Synthesis, it will be clear that the progression from the lower to the higher instincts, is, throughout, a progression towards greater speciality and complexity of correspondence. The simple contraction exhibited by some creature having a rudimentary eye, when an opaque object is suddenly passed before that eye, is a much more general and more simple response than that witnessed in the creature which grasps the prey passing before it. In the first case, the effect is produced whatever the relative position of the object, providing the obscuration be considerable: in the second, it is produced only when the object is just in front. To the outer relation between a moving opacity and a living solid body, is now added a relation of position: and not only a relation of position, but one of magnitude; seeing that the effect is not the same when a large as when a small object is presented. That is to say, the external phenomenon responded to, is a co-ordinated group of relations; and internally, there is a co-ordinated group of changes—not a single impression and a single motion, but at least a pair of impressions and a considerable complication of motions. The correspondence is alike more complex and more special.
Now, that the evolution of intelligence by the multiplication of experiences, must necessarily follow this order, is demonstrable à priori. Were there no other proof, there would be the all-sufficient one, that as, in the environment, the phenomena that are the most complex and the most special are the least frequent, the experiences of them can never be so numerous as are the experiences of the simple and more general phenomena. In the daily life of every organism, the relation between a passing obscuration and a living body, is more general than the relation between one degree of obscuration and danger, or between another degree of obscuration and food; and each of these relations is more general than the relation between a particular size and form of visual impression and a particular class of objects; and this relation is more general than that between a particular size, form, and colour of visual impression, and a certain species of that class; and this again is more general than the joint impressions of form, size, colour, and motions, ma de by a member of such species when adopting a peculiar mode of defence. And as, in ascending from those simple relations exhibited by all bodies in common, the more complex the relations become, the more infrequent is their occurrence; it is an inevitable corollary, that if inner relations are moulded to outer relations by the accumulation of experiences, the simpler must be established before the more complex.
Still more clearly will the necessity of this order of progression be perceived, when it is remembered that, both externally and internally, the complex relations are composed of the simple ones; and must, therefore, come after them. Before there can be the relations presented by matter in motion, there must be those general relations of resistance and extension exhibited by the matter that moves. Before there can exist the relations implied in the action of one body on another, there must first exist the relations implied in the existence of each body. Before there can arise all those involved relations displayed in the movements of a living creature, there must first exist those chemical relations among its elements, and those structural relations among its organs, by which these involved relations are made possible. And manifestly, if the organization of inner relations in correspondence with outer relations, results from a continual registration of experiences, it is similarly impossible that the complex relations should be established before there have been established the simpler relations they involve.
Duly observing that this corollary from the experience-hypothesis is in conformity with the facts, so far as they are accessible to us, let us go on to observe some important inferences that are deducible from it.
§ 189. If, looking at the progress in its general aspect, we see that simple and general relations in the environment must be those most frequently experienced, those first responded to, and those to which the response becomes most decided; if external relations a grade less simple and general are thus rendered appreciable, and by a repeated, though a less frequently repeated, experience, also establish answering internal relations; and if this process goes on slowly extending to relations successively more complex and special, and less frequent; then it must happen, that there will ultimately be established in the organism, a great number and variety of psychical relations having different degrees of coherency. While an infinity of experiences will have rendered the first and simplest of these psychical relations absolutely indissoluble; while experiences, which, if not actually so great in number as the first, have yet been practically infinite in number, may have given indissolubleness to psychical relations that are a degree more complex; while relations, even of several succeeding degrees of complexity, though successively less frequent in experience, may yet have been so frequent as to have become psychically organic; yet it is manifest, that with relations increasingly complex and decreasingly frequent, there must come a point at which the answering psychical relations will no longer be absolutely coherent. That this may be thoroughly understood, let us illustrate it by symbols.
Suppose A and B to represent two attributes of matter in general—say extension and resistance—to the constant relation between which, a responsive relation has been established in the organism. Suppose C and D to be two extremely general attributes of animal matter—say motion and life—to which also there is a responsive internal relation. It is quite comprehensible that experiences of the united group of attributes A, B, C, D, recurring as they do in every creature met with, may eventually establish an answering connection of internal relations that is practically as absolute as the original ones. It is also comprehensible that if the creatures commonly serving for prey are of one size, L, while those found to be enemies are in most cases of another size, M; continued experience may establish different organic responses to the different groups of coexistent attributes, A, B, C, D, L, and A, B, C, D, M. And it is comprehensible, too, that when each of these large classes comes to be distinguishable into sub-classes—say by means of differences of colour—the experiences of the two groups A, B, C, D, L, S, and A, B, C, D, L, T, and of the two groups A, B, C, D, M, P, and A, B, C, D, M, Q, may still be so numerous, that the answering psychical changes are indissolubly united. But clearly, as, in course of further progress, the groups of attributes and relations that are distinguished from each other and separately responded to, become more numerous; as, by successive additions of further distinctive attributes and relations, such groups become more complex; and as each more specific kind of group is, by consequence, less frequently repeated in experience; it follows, of necessity, that the answering psychical changes must become less coherent. Not only must the group of internal states by which the group of external phenomena are symbolized, be less definitely aggregated—or at any rate the more recently added constituents of it—but the entire group, considered as a composite impression, must have a smaller power of producing the special set of actions by which the appropriate adjustment is made. This is an inevitable corollary.
And now observe the implication. If, as the instincts become higher and higher, the various psychical changes of which they are severally composed become less and less definitely co-ordinated; there must come a time when the co-ordination of them will no longer be perfectly regular. If these compound reflex actions, as they grow more compound, also become less decided; it follows that they will eventually become comparatively undecided. The actions will begin to lose their distinctly automatic character. And that which we call instinct will gradually merge into something higher.
Thus, then, we see that the conclusions deducible from the experience-hypothesis, are in harmony with such facts as we possess. We see that the evolution of instincts, as resulting from experience, is quite comprehensible. We see that, if produced by experience, this evolution must proceed from the simple to the complex; which is the indication of positive evidence so far as it is attainable. And we see that by a progression thus wrought out, instinct must in the end insensibly pass into a higher order of psychical action; which is just what we find it to do in the higher animals.
[∗]How nervous communications are established, both primarily and in all after stages of evolution, it would be going too much out of the way here to inquire. It may, and I think not improbably will, turn out, that they are produced by the very actions which they have to co-ordinate. There is evidence pointing to the inference, that the law in virtue of which all psychical states that occur together tend to cohere, and cohere the more the more they are repeated together, until they become indissoluble—the law in virtue of which many of our own acquired actions become reflex by constant repetition—is the law in virtue of which nervous connections are formed. When a change made in one part of an organism is habitually followed by a change in another; and when the electrical disturbance thus produced in one part, comes to be in constant relation to that in another; the frequent restoration of electrical equilibrium between these two parts, being always effected through the same route, may tend to establish a permanent line of conduction—a nerve. On a future occasion I hope to say something in justification of this hypothesis.