The Animal Mind: A Textbook of Comparative Psychology

If it is true that an Amceba which had not just “‘had the experience of possessing 5’? would not have reversed its movement and gone after 6 when the latter escaped, still we cannot think it possible that c’s movements in so doing were guided by a memory image of b. It may be supposed that the recent stimulation of contact with 5 had left a part of c’s protoplasm in a condition of heightened excitability, so that the weak stimulus offered perhaps by slight water disturbances due to b’s movements after escaping produced a positive reaction, although under other circumstances no reaction would have been possible. (Compare the observation of Schaeffer, just quoted, on Amceba’s ability to react to objects not in contact with it.) In any case, there is no

\ action; no proof of the revival of a process whose original \ effects have had time to die out; and it is upon such revival ‘that the memory images which play so much part in our own conscious life depend. absence of this kind of material in the possible mental processes of Amceba. In the first place, such a lack profoundly affects the character of the experiences which the animal might be supposed to receive through external stimulation. If we call the possible conscious effect of a mechanical stimulus upon the Amceba a touch sensation, the term suggests, naturally, such sensations as we ourselves experience them. In normal human beings touch sensations are accompanied by visual suggestions, more or less clear, of course, according to the visualizing powers of the individual, but always present in some degree. Fancy, for example, one of us entering a room in the dark and groping about among the furniture. How constantly visual associations are brought into play! Not once is a mere touch impression apprehended without being translated into visual terms; the forms and positions of the articles encountered are thought of immediately as they would appear if the room were lighted. The difficulty we have in thinking of a touch sensation with no visual associations illustrates the difference between our sense experience and that of an animal incapable of recalling images of past sensations.

It is equally obvious that in the absence of memory ideas, not only must the Ameeba lack processes of imagination and reasoning, but there can be nothing like the continuous selfconsciousness of a human being, the ‘‘sense”’ of personal identity, which depends upon the power to revive past experiences. It is even possible that the ‘‘stream of consciousness” for an Amoeba may not be a continuous stream at all. Since its sensitiveness to changes in its environment is less developed than that of a human being, and there are no trains of ideas to fill up possible intervals between the occurrences of outside stimulation, the Amceba’s conscious experience may be rather a series of

“flashes” than a steady stream. And for the Ameba, again, we must remember that even such a series would not exist as such; the perception of a series would involve the revival of its past members. Each moment of consciousness is as if there were no world beyond, before, and - after it. Another consequence of that simplicity of structure which results both from the rudimentary powers of sensory discrimination and from the absence of memory ideas in the Amceba’s mind is that there can be no distinction, within a given mental process, between that which is attended to. and that which is not attended to, between the focus and the margin of consciousness. Given a consciousness which at a certain moment is composed of the qualitatively different elements A, B, C, and D, we can understand what is meant by saying that A is attended to, is in the foreground of attention, while B, C, and D remain in the background. But given, on the other hand, a creature whose conscious content at a certain time consists wholly of the qualitatively simple experience A, it is evident that attention and inattention are meaningless terms. Different moments of its consciousness may differ in intensity; but attention, involving, as it does, clearness rather than intensity, arises only when mental states have become complex and possess detail and variety within their structure.

One of the most important points in which the human -@ mind differs from the mind of the lowest animal forms consists, we have seen, in the enormously greater number of different sensations which enter into human experience, as compared with the small number of sensory discriminations ‘possible to the simpler animals. Much of the experimental work that has been done on animals has been directed toward discovering what discriminations they make among the stimuli acting upon them, and to the results of this work we shall give our attention in the next chapters. But first we ought to get a clearer idea of just what kind of evidence is needed to indicate the existence of a variety of sensations in an animal’s mind.

At the outset, we must remind ourselves that, in the absence of any satisfactory proof that the lower animal forms have minds at all, and the equal absence of any proof that they have not, all our conclusions about the number and kind of their possible sensations must remain subject to the proviso that they possess consciousness. Further, a point that was mentioned in Chapter I must again be emphasized. No evidence of discrimination between two stimuli on an animal’s part can do more than show us that for the animal they are different; just what the quality of the sensation resulting from each may be, whether it

also differ from each other, and yet if our experience could be exchanged for the animal’s, we might find in the latter othing like red and blue as we know them. Thus much being premised, what sort of evidence can be obtained that an animal does discriminate between two stimuli? Again, as in considering the evidence for the existence of consciousness in general, there is an argument from structure and an argument from behavior. The argument from structure consists primarily in the fact that an animal possesses sense organs recognizably like our own. If a creature has an organ suggesting strongly the construction of the human cochlea, or an organ with a lens and a membrane composed of rods and cones, it is highly probable that auditory stimuli in the one case and light in the other produce specific sensations. This argument from the morphology of sense organs is, however, limited in two ways. First, it is only a small part of the animal world whose sense organs resemble ours closely enough to make the analogy safe. And secondly, we do not after all know very much about the relation of our own sense-organ structure to function. We know, for example, that our own organ with a lens and retina gives us visual sensations, but we cannot say with certainty which structures in the retina furnish brightness sensations and which color sensations, nor do we know anything about the retinal structures that underlie different qualities of color

from the ear, but no one can tell us how to judge from the structure of the ear what range and fineness of pitch discriminations exist in its possessor’s mind. No investigator has yet succeeded in relating the different qualities of smell and taste to differences in the end organs. The argument from behavior is as follows: If an animal reacts in a different way to two qualitatively unlike stimuli, then, providing that it is conscious at all, it may be supposed to receive qualitatively unlike sensations from them. If it always reacts in the same way to both, then both may be supposed to be accompanied by the same sensation quality. Obviously these statements need further discussion. For one thing, it may be urged that in our own case the same external reaction is often made to stimuli that are nevertheless consciously discriminated. A man may eat with relish and without observable difference in behavior, for example, foods that yet give him perfectly distinguishable smell and taste sensations. Precisely this objection holds against a method of experimentation, formerly a good deal used, which may be called the Preference Method of testing discrimination. Vitus Graber, for instance, attempted to find whether animals belonging to a variety of species could discriminate colors, by offering them the choice of two compartments illuminated each with a different color. Clearly, if the animals chose one compartment as often as the other, it would be rash to conclude that the two lights produced for them indistinguishable sensation qualities. There might simply be the absence of any preference, along with perfect discrimination. The fact is that in all experiments upon animals, whether

to determine their power of distinguishing stimuli or their power of learning by experience, the first requisite is to give the animal what we commonly call a motive. That is, the conditions of the experiment must be so arranged that some already present tendency to act, whether inborn in the animal or acquired by previous experience, shall be appealed to. This is increasingly the case, the higher the animal worked with stands in the scale. The higher animals have what might be called a large reserve fund of discriminations. That is, they are capable of making many more selective reactions to stimuli than they need at a given moment actually to use. Hence in their case the experimenter must make a careful adjustment of conditions to bring out exactly the discrimination wanted. He must either make the performance of the reaction pleasant or its non-performance unpleasant to the animal. A monkey, for example, confronted by a set of glass tumblers covered each with a differently colored paper, may behave toward them all in precisely the same way; yet if food be put regularly in the blue tumbler, whose position in the row is varied, it becomes worth the monkey’s while to make use of his discriminative powers, and he may show by his different behavior toward the blue tumbler that it produces on him a different impression from the others.

With simpler animals the problem is less difficult. If an animal is capable only of a half dozen different ways of responding to stimulation, we may with comparative safety assume that it has less opportunity to hold them in reserve; and if such an animal invariably reacts in the same way to two different forms of stimulus, or if the variations in its response are not correlated with differences in the stimulation, it becomes probable that the two

stimuli produce in its assumed consciousness identical sensation qualities. Thus it is not the number of stimuli to which an animal reacts that can be taken as evidence of the qualitative variety of its sensations, but the number of stimuli.to.which it gives different, reactions. When. ennings, for instance, says that Amceba “reacts to all classes of stimuli to which higher animals react” (378, Pp. 19), we cannot conclude that it possesses all classes of sensations that higher animals possess, for its reactions to these different stimuli are but little varied according to the kind of stimulus. _ An ingenious way of getting evidence from behavior is the salivary reflex method devised by the Russian physiologist Pawlow (830). The salivary ducts of the dog, which lie near the surface, are operated on so that the saliva can be discharged into a graduated tube. As is well known, the sight or smell of food increases the flow of saliva. Now when any other stimulus, such as a sound, regularly accompanies the sight or smell of food, this stimulus, originally without effect on the salivary flow, comes to increase the flow even in the absence of food. If, now, the stimulus that has thus acquired the power to affect the salivary flow is given in irregular alternation with another stimulus differing slightly from it, and the other stimulus is found not to affect the flow of saliva, then the inference can be drawn that sensory discrimination between these two stimuli is possible for the animal. It is maintained by some investigators that when sensory discrimination can be studied through such simple types of behavior

1Qne of many reasons for the unsatisfactoriness of an article by A. Olzelt-Newin, entitled “Beobachtungen tiber das Leben der Protozoen” (529), lies in the author’s uncritical acceptance of the hypothesis that reaction to a special kind of stimulus means a special kind of sensation. as the salivary and other reflexes, there is less chance of misinterpretation than when more complicated choice processes are involved. In all experiments where behavior alone is the basis of inference regarding sensory discrimination, we need to take the utmost care that the animal is really responding to the stimuli, and not to some other accidental cue. Thus a dog in the Harvard laboratory was apparently discriminating accurately between two lighted areas of different size, but events proved that he was actually responding to slight pulls given by the experimenter on the leash that held him. He failed wholly when he was taken off the leash. Nowadays the careful experimenter always remains out of sight and hearing of the animal tested, and is not in contact with it in any way.

As a matter of fact, the argument from structure needs confirmatory evidence from behavior. For clearly the mere presence of a sense organ bearing sufficient likeness to our own to admit of conjecturing its function would be of no value as proof unless it were shown that the sense organ actually functioned. In order to do this, it would be necessary to show that the animal reacted to the stimulus conjectured as appropriate to the sense organ, and that removal of the organ profoundly modified the reaction. Thus we shall find that many experiments to test sensory discrimination have been made by the method of extirpating a sense organ and studying the effect on behavior. The method has many disadvantages, the chief of which lies in the fact that it is hard to say which disturbances in behavior are due actually to the loss of the organ and

which to the more widespread effects of the operation. Yet this much may be said for the combination of proof from structure and behavior involved in the Method of Extirpation, if we may so call it: where an animal reacts to a certain stimulus, for instance light, when a sense organ is intact, and fails to react to light, though otherwise normal, when the organ is removed, there arises a possibility that light may produce in the animal’s consciousness a specific sensation quality, even although the animal ordinarily reacts to light in a manner indistinguishable from that of its responses to other stimuli. Though light and mechanical stimulation, for example, both ordinarily produce a negative reaction, yet if light brings about its effect only through the medium of a specialized structure with which mechanical stimuli are not concerned, then along with the probable unpleasantness accompanying the negative reaction there may go a quality peculiar to the functioning of that special structure.

Another mode of combining evidence from structure with evidence from behavior is by the use of localized stimult. If an animal gives a response, which in itself may have nothing to mark it off from responses to other stimuli, when a special kind of stimulation is applied to certain regions of the body, and only then, while the other stimuli produce better reactions when applied elsewhere, then the suggestion is given that different sense organs are involved, and the same possibility arises of different sensation qualities.

Two other forms of evidence whereby from behavior a differentiation of sensory structures can be argued, and from differentiation of sensory structures possible differences of sensation quality, may be mentioned. The first of these consists in showing that reactions to different stimuli may be independently fatigued. The natural inference is that a specific nervous apparatus belongs to each stimulus. The second lies in demonstrating that the reactions to different stimuli occur with different degrees of rapidity. If there is a marked difference in the reaction times of an animal to different forms of stimulation, each, again, may be supposed to affect its own nervous pathway. A modification of this method consists in noting the influence of a stimulus upon the time of reaction to another nearly simultaneous stimulus. If such an influence can be shown, it is evident that the force producing it has some effect on the nervous system. By combining this method with that of extirpating a sensory structure, indications may be obtained that the nervous effect of the auxiliary stimulus is dependent on a definite receptive apparatus, and hence is probably accompanied by a special sensation. This method was used by Yerkes to demonstrate hearing in frogs (813).

One further consideration offers itself to the student of animal responses to stimulation. It has been the special endeavor of Jennings to point out the fact that these responses, instead of being wholly accounted for by the characteristics of the stimulus, are determined in part by the internal, physiological condition of the animal (378). We shall therefore note often in the course of the following pages cases where difference of reaction is due to internal rather than to external causes.

§ 14. Evidence for Discrimination of Certain “Lower” Sensation Classes Bearing all these points in mind, let us proceed to survey the evidence for variety in the sensations of animals. In the lowest forms, such evidence must be derived entirely from behavior. That from the presence of a sense organ is almost wholly lacking. And although various stimuli, as we have seen, produce reactions in Amceba, yet there is only one case where these reactions are strikingly different according to the quality of the stimulus applied. This instance consists in the distinction between foodtaking reactions, given to edible substances, and the responses to mechanical stimulation. The sense of touch,-@ : undoubtedly, must play a part in the mental life of the lowest animals that have consciousness at all. But the earliest distinction between a touch quality and a quality that is other than touch seems to occur when food sensation and contact sensation are differentiated. It is possiblew that warmth and cold also appear as distinct sensation qualities in the experience of low forms of animals, but we have little real evidence of the fact. No organs of temperature sensation are definitely known even in human beings. And the responses of low animals to thermal stimulation are not specialized. They consist usually of negative reactions, given when the animal is subjected to a temperature either above or below, but especially above, the “optimum”; and these reactions are not different from the ordinary negative type, suggesting unpleasantness rather than a specific sensation quality. In some cases the sensibility to thermal stimulation has been found to be differently distributed from that to other classes of stimuli. But in any case, sensations of warmth and cold are probably in no member of the animal kingdom differentiated into any greater number of qualitatively distinct sensations.

The sense of touch, also, shows but little internal differentiation. Its importance, so far as we can judge, is rather on the spatial than on: the qualitative side. The sense quality of pain we naturally think of as the accompaniment of the negative reaction in its more violent forms, given to a stimulus that is injuring the organism. Organic and kinesthetic sensations are hard to trace in the lower animals; for animals whose structure differs widely from our own, the qualities of these two classes must remain beyond the power of our imagination. That differences in physiological condition such as are produced by hunger, satiety, or fatigue involve differences of accompanying organic sensation in the consciousness of the animal manifesting them is possible. Kinesthetic sensations, as we shall see, are apparently concerned in the processes whereby many animals have learned to traverse a labyrinth path.

The three classes of sensation whose existence in the animal mind can be most satisfactorily traced are the chemical sense, under which s d_ taste belong, the sensé-of hearing, and the sense of sight. To the study of these the following chapters will be devoted. Since the manifestations of the chemical sense in the lowest forms of animals consist chiefly in a differentiation of response to food and to mechanical stimulation, the contact sense or sense of touch will, in discussing these forms, be considered along with the chemical sense.

WE have already seen that the most primitive type of protozoén, Ameba proteus, discriminates between edible and inedible substances. While it will sometimes ‘swallow’ inedible particles such as grains of carmine, it takes immediate measures to get rid of them, measures too prompt to be the result of an actual attempt at digestion, and» hence properly to be regarded as the effect of a chemical or food sense. Many other members of the lowest division of the animal kingdom, the Protozoa, have a structure and behavior decidedly more complicated than those of Amceba. There is a large group of single-celled animals called Ciliata, from the fact that their bodies are covered with little hairlike protoplasmic filaments or cilia which serve as organs of locomotion by acting like tiny oars. A common representative of the group is Paramecium. The structure of this animal is distinctly more specialized than that of Amceba. Not only are the cilia modified locomotory structures, but there is a definite region for food-taking. A groove extends obliquely down one side of the body, terminating at its lower end in a mouth. The cilia along this oral groove beat with especial vigor and create currents which sweep food particles to the mouth. Paramecium swims rapidly through the water with a spiral motion of its body, due to the facts that the aboral cilia beat more

strongly than the rest, and that the animal compensates for the turning thus occasioned by turning on its long axis. Its reactions to stimulation Jennings has shown to be only two in number. First, there is a very definite avoiding or negative reaction. This is given in response to decided mechanical stimulation at the anterior end, as when the animal swims rapidly against an obstacle, and also in response to chemical stimulation, to strong ultra-violet rays (299), and to temperatures above or below a certain middle region called in this case, as in analogous cases with other animals, the optimum. For Paramecium it lies between 24° and 28° C. The negative reaction consists, according to Jennings, of the following process: the animal darts backward, reversing the beat of its cilia, turns toward the aboral side (that opposite to the oral groove) by increasing the beat of the oral cilia and lessening the compensating rotation, and continues on a forward course that is now at an angle with its former line of motion. Tf this new course carries it clear of the stimulus, it continues on its way; if not, repeated contact with the stimulus causes a second reaction, the Paramecium always turning in the same direction, so that ultimately it avoids the source of stimulation (361, 378) (Fig. 4). Differing strengths of stimulus produce the reaction with different degrees of violence. When a very strong stimulus is encountered, the animals “respond first by swimming a long way backward, thus removing themselves as far as possible from the source of stimulation. Then they turn directly toward the aboral side, — the rotation on the long axis’ completely ceasing. In this way the animal may turn directly away from the drop [the stimulus] and retrace its course” (378, p. 50). On the other hand, when the stimulus is very weak the reaction may be reduced to the

following form: the Paramecium ‘“‘merely stops, or progresses more slowly, and begins to swing its anterior end about in a circle.” As long as it does not thus get out of range of the stimulus, the movement is continued. ‘‘When the anterior end is finally pointed in a direction from which no more of the stimulating agent comes, the Paramecium swims forward” (378, p. 51). Evidently, however, these are but differing degrees of a reaction whose essential features are the same.

Fic. 4.— Negative reaction of Paramecium. 4 is the source of stimulation. 1-6 are the successive positions of the animal. After Jennings (378). negative reaction certain chemicals introduced into the water, it shows a tendency to collect in the neighborhood of others. Such is the case with weak acids, with a bubble of oxygen if air has been long excluded from the slide, and with carbon dioxide, which in water of course produces acid (378). Jennings pointed out that the inclination of Paramecium to gather in groups is very likely due to the attraction for them of the carbon dioxide. which they excrete. But he has also shown that this “attraction” to certain chemicals does not mean the presence of a special

collect in a drop of weak acid, for example, they are not > ; drawn toward the acid. They simply happen, ar ordinary movements, to swim into it, and on entering it show no disturbance whatever. But when they come to the edge of the drop on their way out, they give the negative reaction to the surrounding water. In this way they are, as it were, trapped within the drop. The nearest analogue to a positive reaction in Paramecium consists in the fact that sometimes, when they come into contact with a solid, instead of darting backward, the animals merely cease moving, and extending stiffly the cilia which touch the object, remain at rest

Positive (Fig. 5). The utility of this behavior is that eee around decaying vegetable matter, the kind of cium. After SOlid oftenest found in the animal’s ordinary 8). : z Perna a are) food in the way of bacteria; it is a good anchorage. What characteristics of the stimulus determine that this ‘‘contact reaction,” rather than the negative reaction, shall be given? Does weak mechanical stimulation occasion it, as happens with Amceba’s positive reaction? Evidence in favor of this is offered by the fact that the contact reaction is more likely to occur if the animal comes against thesolid whenswimming ratherslowly. Jennings reports also that individuals vary. ‘‘Often all the individuals in a culture are thus inclined to come to rest, while in another culture all remain free-swimming, and give the avoiding reaction whenever they come in contact with a solid” (378, p. 60). This would suggest that some individuals are in a state of greater excitability than others, so that a

given stimulus acts more strongly upon them. On the other hand, there is a possibility that qualitative as well as intensive differences in the stimulus are responsible for the contrasting reactions. ‘In general,” says Jennings, Paramecium ‘‘shows a tendency to come to rest against loose or fibrous material; in other words, it reacts thus to material with which it can come in’ contact at two or more parts of the body at once. Tosmooth, hard materials, such as glass, it is much less likely to react in this manner” (378, p. 61). Perhaps, then, the spatial distribution of the stimulus over several points of the body surface increases the probability of a contact rather than an avoiding reaction.

_z_~ What, now, of the food-taking reaction in Paramecium : does it show evidence of the existence of chemical discrimination? When the animal finds itself in surroundings where certain presumably injurious chemicals, especially alkalis, are present, it gives its typical negative reaction. If this should be called evidence of a special chemical sense, we should be forgetting our general principle that only unlike reactions constitute behavior indicating sensory discrimination. Since Paramecium reacts in the same way to strong mechanical stimulation and to certain chemical stimulations, there is no reason for assum-— ‘ng a discrimination between chemical and mechanical stimuli. If it can be shown that the reaction is a localized one, that the cilia which surround the mouth reverse the direction of their beat when certain kinds of particles strike upon them, with the result that these particles are thrown out, then the question as to the existence of a chemical discrimination would depend on whether the rejected particles are chemically unlike those which are accepted, or different only in size or mechanical consist-

ency. Jennings (378) reports no such rejection of unsuitable particles in the case of Paramecium, but Metalnikow (485, 486) says that when Paramecia have been kept for some time in water containing carmine grains they cease to swallow them; the evidence being that fewer and fewer grains are found in the animals. Schaeffer (656) thinks this result is due to the mechanical change in carmine grains that have been long in the water, which become stuck together in the mucus excreted by the Paramecia. Metalnikow (487) however finds that when fresh carmine is used the Paramecia avoid it apparently as a result of their previous surfeit, and that when particles of aluminum are used instead of carmine they acquire a discrimination against these even more quickly. He therefore feels convinced that the discrimination is a chemical one.

~ Stentor is a ciliate protozoén which spends a part of its existence anchored by a long extension of its body, like the stem of a flower: at times it pulls this up and swims off. Food is taken in by the whirl of cilia around the mouth, and may be rejected. by a reversal of the direction of this whirl. Schaeffer (656) says that Stentor discriminates not only between organisms and inedible particles, but between different. kinds of organisms; he thinks, however, that the basis of discrimination is not chemical, because food soaked in a variety of chemicals is readily taken, while jelly made of food organisms is rejected. He believes the discrimination rests probably on several mechanical factors in combination, for example, size, weight, form, and surface texture, no one of which is alone sufficient to determine the choice. On the other hand Lund (446), observing another ciliate named Bursaria, finds that this organism will reject yolk of egg particles if they have been treated with certain dyes, and concludes that the basis