THE MECHANISM OF INSTINCT.
Were we dealing with an ordinary mechanism, and the disturbances produced in its actions by the removal of any part, we should attribute all observed effects to interference with the conditions of dependent sequence: we should infer that the actions were imperfectly performed, or wholly abolished, because their requisite mechanical conditions were disturbed. Let us be equally precise in dealing with the physiological mechanism. If we have deprived it of an organ in which certain combinations are effected, we must expect to find all actions which were dependent on such combinations to be now impossible; but all the actions which are not directly dependent on these combinations may still be possible. The actions of feeding, for example, are determined by certain sensations, when these are present in a particular sequence, but not otherwise; the sensation of sight does not suffice, because the animal must not only see the food, he must perceive it. The action of defence and flight are also determined by certain sensations, but only when these are connected in a certain sequence: the brainless animal will defend itself, or move out of the way, under the stimulus of unpleasant sensation; but will not be moved by a prospective injury, because he fails to associate it with the sight of the threatening object. In the same way a blind man shrinks at the actual contact of the heated poker, but does not shrink at the approach of that poker which he does not see. We do not deny him the possession of the so-called instinct of Self-preservation on this ground; why deny it to the brainless animal? The brainless fish or frog swims when placed in the water, because the sensation from the moving water[280] sets going the swimming mechanism. To call this a “swimming instinct” may seem extravagant; yet it is as fully entitled to the name as Self-defence is, or the Alimentary Instinct. In all three cases there is a connate mechanism set going by appropriate feelings.
70. Since all admit that there is an Alimentary Instinct, let us see what kind of mechanism it implies. There must be a state of feeling called Hunger, which—combined with other feelings—determines certain muscular adjustments in the search, recognition, capture, and finally the swallowing of the food:—a very complex series of actions, which lead to and sustain one another until the desire is gratified. On the mental side there are three constituents, all indispensable: the hunger must be felt, the food must be discriminated, the desire must be gratified; on the physical side there are also the indispensable arrangements of the motor mechanism. Now it is obvious that the entire mechanism of this instinct cannot be localized in the brain, even if its mental elements are localized there; and there is reason to believe that even the mental elements—the feelings of hunger, discrimination, and gratification—are not exclusively localized there. The brainless animal manifests if not the feeling of Hunger, at any rate that feeling of discomfort which is the basis of Hunger. The restlessness is that of a hungry animal. Now we know that some of the Systemic Sensibility is preserved, for we see the animal breathing, swallowing, urinating, sleeping, preening its feathers, changing its attitude, resting on one leg after the fatigue of the other, etc. We may therefore infer that other systemic sensations, such as Hunger and Thirst, arise under the usual conditions.
71. We have noted an indication of Hunger; but on further observation we discover that although the food is eaten, if brought within reach of that portion of the feeding mechanism which is still intact, yet the second step—the feeling of recognition—is wanting. The animal fails to perceive the food brought under his eyes, or even placed in his mouth; unless the back part of the mouth be touched, no swallowing takes place. Hence the animal can no longer feed himself, and is therefore said to have lost his instinct. But although the mechanism of the instinct has been disturbed, its action is not wholly abolished. The brain is necessary for that combination of adjustments which normally accompany the perception of food through sight and scent; and its absence of course frustrates such combination; but we shall presently see that although certain sensible marks by which a perception is guided are absent, others may still be present, and suffice.
72. Before adducing examples let me say that we cannot legitimately attribute the abeyance of an instinct solely to the absence of the brain, 1°, because we observe a similar abeyance of the instinct and frustration of perception, even when the brain is present, and the animal is in its normal state. 2°. On the other hand, some instincts are unmistakably manifested, and some perceptions excited, after the brain has been removed. In fact, all that is needful is that some of the mental elements of such perception and such instinct be preserved; and this is the case so long as the leading element is present.
73. On the first point consider this unequivocal example. A healthy, hungry frog may be placed in a vessel in which lie a quantity of dead flies. He sees these flies, but sight is not enough; to him they are only so many black spots, in which he does not recognize his food, because the flies do not move, and the leading element in his perception of food is not a colored form, but a moving form. Hence this frog, in spite of brain and an intact organism, will starve amidst appropriate food. Whereas the frog that will not snap at motionless flies snaps at any other small moving object, though it be not his food. Goltz observed one incessantly snapping at the moving tentacles of a slug which was in the vessel—as if that were possible food! Not only the stupid frog, but the more intelligent carnivora will starve in the presence of appropriate food which is unrecognized, because the leading element in the recognition is absent. The cat will not eat a dead mouse, unless she has killed it herself. Predatory animals must capture their food—unless the scent of blood excites their alimentary instinct. So intimately is this sensation of a moving object connected with the predatory impulse, that the cat which is unexcited by the dead mouse cannot resist springing on a moving ball. We need not suppose the cat to mistake this ball for food; but we must suppose that, accustomed to pounce upon moving food, it is unable to resist the impulse of this leading sensation.
74. The presence of the brain not sufficing, in the absence of the leading sensation, we shall now see that the absence of the brain will not prevent the execution of the instinctive action, if the leading sensation be present. The brainless bird sees a heap of grain, or a pan of water, but no more recognizes them by sight alone than the frog recognizes the dead flies; yet if the bird’s feet be placed in the water, this sensation will suffice to make him drink; if placed amid the grain, this sensation will (sometimes) suffice to make him feed. Lussana and Lemoigne state that their brainless pigeons ate and drank with avidity when their feet were placed in grain and water.[281] M. Krishaber removed the hemispheres from a pigeon, and observed that when his beak was thrust into a heap of hempseed the head was quickly withdrawn, whereas when the beak was plunged into water the bird drank eagerly. Every day he was forced to feed the bird by pouring the seed into its throat, but every day it drank when the beak was thrust into the pan of water.[282] Brücke noticed that his brainless hen, which made no attempt to peck at the grain under her very eyes, began pecking if the grain were thrown on the ground with force, so as to produce a rattling sound. The sensation of hearing was here more perfect than that of vision, and sufficed to awaken the state of feeling necessary to initiate the pecking movement.[283]
75. Somewhat analogous phenomena are observed in Aphasia. The patient can see printed or written letters, and even copy them; but he cannot read, i. e. interpret, these symbols; as the birds see the grain, but cannot connect this sensation with others. These letters and words, which the patient cannot interpret when seen, he can interpret when heard; he can not only understand them when spoken, but write them if they are dictated to him. The birds recognize the grain and water (or act as if they did) when other sensations than those of sight are excited. Sound is the leading element in Language, both spoken and written. We hear the words even when we see them, but we do not see them when we hear them. The visible symbols are accessory and subordinate. But to the born deaf the visible symbols dominate. How one sensation will determine a particular group of movements which cannot be effected by any other stimulus is abundantly illustrated in disease no less than in experiment. Here is a very luminous example: Gratiolet had a patient for six months under his eye incapable of articulating a single word, owing to the incoherence of her incessant utterance—she babbled sounds, but could not group the syllables into a recognizable word. Yet she could sing the words of any song she knew, the musical sensations being sufficient to guide her vocal organs. “Ainsi la mémoire, infidèle dans le cas où les mots étaient des idées, devenait claire et précise quand les mots étaient des chansons.”[284]
76. These illustrations plainly tell how the brainless animal may starve amid his food, failing to perceive it because the leading sensation is not excited; and how the same animal may manifest his feeding instinct if the mechanism be set going by a leading sensation. We are told, indeed, that in the absence of the brain the actions are mechanical reflexes from impressions, and not comparable with the complex processes determined by perception. I think, however, that the only difference is in degree of complexity: a combination of touch, temperature, and muscular movement will be simpler than one which also combines sight, smell, and the revived images of associated sensations. The sight of a sheep affects the instinctive mechanism of a wolf only when combined with the leading element of smell. Place a stuffed sheep in a field, and no wolf will approach and spring on it, whereas the blind wolf will find and capture the real sheep; and I believe that were it practicable to remove the brain without injury to the organ of scent and the powers of locomotion, the wolf would track and capture the living sheep.
77. The outcome of this discussion is that the mechanism of each instinct is the adjustment of the organs which effect the instinctive action; and this adjustment is not simply a cerebral process, but a complex of many sensorial processes; consequently the instinct cannot be exclusively localized in the brain, although the cerebral process may be a very important element in the adjustment. This is true even on the supposition that in speaking of Instinct we refer only to the state of feeling which originates the action—separating the psychological from the physiological aspect of the phenomenon. For the brain minus the organism is obviously incapable of feelings; whereas the organism minus the brain is obviously capable of sensibilities adequate to determine the actions. Thus the feeling of hunger which prompts the alimentary actions does not arise if the animal is satiated, nor does the sexual feeling which prompts generative actions arise when the animal is castrated; but each arises when the organism is in a particular state. In vain will food be placed before the satiated animal, or a female before the castrated male; food and female are seen and recognized, but no desires are excited, in spite of the brain and its supposed instincts. On the contrary, when the brain is removed, the need of the organism for food is felt, and this need determines restless movements, which are directed by certain other sensations, and the instinctive action of feeding is finally effected; although, of course, the removal of the brain has so disturbed the normal mechanism of the instinct that the action is imperfect. Renzi says that an animal deprived of its brain has lost the intelligence which enables it to seek and seize its food, but not the instinct, since it still has the desire for food. The following experiment may illustrate this. Renzi wounded superficially one optic thalamus of a frog without injuring the external margin, or optic tract. The frog showed no appreciable loss of sight, but hopped timidly away whenever approached. Then both thalami were divided transversely, the optic tract still being spared. This frog remained motionless under every threat. It manifested no alarm, and even when directly irritated, only crawled or hopped away like a brainless frog. Sight still so far remained that obstacles were avoided.[285] Now since this animal’s brain was intact, and its organs of movement were capable of responding to stimulation, how are we to explain the loss of its instinct of self-preservation? The frog perceived no danger in a threatening approach, yet perceived an obstacle and avoided it, getting under it if there were room enough, crawling beside it if that was the easier escape. Why did one vision prompt the movements of escape, and another fail? Was it not that in the one case the normal pathway was still open, in the other closed? We know that one injury will destroy the perception of color without destroying that of light and shadow; so one injury may destroy the combination of neural processes necessary for the perception of a danger, without destroying those necessary for the perception of a hindrance. If all actions depend on their mechanical conditions, they must be disturbed according to the disturbance of the conditions. Nothnagel found that after removing the nucleus lentiformis on both sides of a rabbit, leaving all the rest of the encephalon intact, the rabbit hopped when its tail was pinched; yet although starting at the sound when hands were loudly clapped, did not hop as a normal rabbit does; nor although closing his eyes when a light was brought near them, did he ever move aside. No feeling of danger was excited by sound or sight. In striking contrast are the phenomena manifested by a rabbit whose corpora striata have been removed: it is with difficulty made to hop by pinching its skin, whereas noises and sights cause it to make terrified bounds.[286]
78. No sooner do we analyze the conditions of an instinct than we see the error of regarding instincts as localized in the brain. The cerebral process is only one factor in the product—an important factor, no doubt, since the cerebrum is the supreme centre of incitation and regulation; but its absence does not wholly carry away the activity of the mechanism, sentient and motor, on which the instincts depend, it only carries away one source of stimulation and regulation.
79. An instinct depends on a connate mechanism. Let us glance for a moment at a parallel case of an ordinary reflex action, also dependent on a connate mechanism, say that of sneezing. When the inner surface of the nose is stimulated by snuff, or other irritant, the nasal branch of the trigeminus is excited, and the effects are first a deep inspiration, then a closure of the respiratory orifices by the tongue, which in turn excites a spasmodic expiration. But the same effects are producible from quite different stimulations—namely, that of the ciliary nerves on sudden exposure to a glare of sunshine—or of the skin nerves on a sudden draught of cold air. Brücke remarks that there is perhaps no spot on the surface of the body from which this reflex may not be excited in very sensitive people. He knew a gentleman who always sneezed when in winter he laid hold of a cold door-bell; and the fit of sneezing was only arrested by giving him a crust of bread or something hard to gnaw. Now just as the connate mechanism of sneezing may be set in action by a variety of stimulations, so may the connate mechanism of an instinct.