The Animal Mind: A Textbook of Comparative Psychology

homing flight by the new and not the old landmarks. The 7 learning is essentially rapid and temporary. Where, as for instance with the honey bee, the nest remains permanently fixed in one locality, guidance by visual landmarks does not differ from the ordinary types of learning where the process is gradual, where useless movements are eliminated and useful movements organized into systems. We are still in possession of too few detailed observations on the homing flights of the wasp to draw positive conclusions as to the nature of the learning process here.

learning, in the sense of a power of reacting differently t a present stimulus because of their past experience with i Probably not a single animal form is so low that it lacks this power. But there is another type of learning, of which human beings make much use, whose existence in animals we have yet to investigate; namely, the ability to recall a mental image of an absent stimulus, a memory idea. A dog shows clearly that he remembers his master, in the sense of modifying his behavior in his master’s presence because of his previous experience. Can we be sure that he has remembered him in his absence; that he has had a ‘memory image of his master?

Most people, following the tendency to humanize animals and ignoring Lloyd Morgan’s canon, interpret as evidence of memory ideas certain features of animal behavior which are susceptible of much simpler interpretations. Dogs and cats, for instance, are supposed to dream because they snarl and twitch their muscles in sleep; but as Thorndike (704) has pointed out, such movements may be purely reflex and unaccompanied by any consciousness whatever. A dog shows depression during his master’s absence, but his state of mind may be merely vague discomfort at the lack of an accustomed set of stimuli, not a clear idea of what he wants; as when we feel that we have forgotten something or that something in our environment has been altered.

We shall first consider certain pieces of evidence which indicate that in many of the lower animals the existence of memory ideas is highly doubtful. Later, we shall note certain testimony in favor of their existence in the minds of some animals, although probably with a very restricted function. One argument from which we may conclude that animals do not make use of memory ideas where human beings would, is derived from the gradual character of the dropping off of useless movements in experiments of the puzzle-box type. A human being who had once hit by accident on the right way to open a lock could hardly fail, on being confronted with the lock a second time, to recall an idea of the successful movement, and to perform it at once, without wasting time and effort on unnecessary movements; but a dog or a cat makes almost as many random clawings and pawings the second time as the first, and only gradually omits the irrelevant motions.

In the next place, animals very generally show a lack of ability to imitate other animals when the “‘imitatee”’ is not actually present before them; they cannot imitate by remembering another animal’s movements. Imitation may be, as various authors have pointed out, of at least two different types. The first may be called instinctive imitation, and is widespread throughout the animal kingdom. It occurs when the sight or sound of one animal’s performing

a certain act operates as a direct stimulus, apparently through an inborn nervous connection, to the performance of a similar act by another animal. ‘‘TIf,”’ says Lloyd Morgan, ‘‘one of a group of chicks learns by casual experience to drink from a tin of water, others will run up and peck at the water and will themselves drink. A hen teaches her little ones to pick up grain or other food by pecking on the ground and dropping suitable materials before them, the chicks seeming to imitate her actions. ... Instinctive actions, such as scratching the ground, are performed earlier if imitation be not excluded” (507, pp. 166-167). Imitation in this sense is hardly so much a method of learning by experience as a method of supplying experience. An animal may perform an act the first time because, through inherited nervous connections, the sight of another animal’s performing it acts as a stimulus. But it will continue to perform the act, in the absence of any copy to imitate, only if the act is itself an instinctive one, like drinking in birds, or becomes permanent by reason of its consequences, just as would be the case if its first performance had been accidental rather than imitative. As a matter of fact, instinctive imitation seems usually to be concerned with actions themselves instinctive.

Inferential imitation, or what Morgan calls reflective tmitation, is a different affair. It is the case where an animal, watching another one go through an action and observing the consequences, is led to perform a similar act from a desire to bring about the same result. Such behavior naturally suggests that it is accompanied by some kind of memory idea of the action that is imitated. Now Thorndike, in his experiments on chicks, cats, and dogs, found no evidence of this type of imitation. A cat put in a puzzlebox did not learn the way out any sooner for watching,

even repeatedly, the performances of a cat that knew how to get out (704). With monkeys, Thorndike’s most extensive tests were made to find whether the animal would learn to open a box from seeing the experimenter himself do it, and his results were again, on the whole, negative (708). Small’s white rats also showed no ability to profit by each other’s experience in this way. One of each of the pairs first experimented on solved the problems presented ; the other, instead of either attacking them for itself or learning by watching the successful one, contented itself with stealing the food secured by the latter (685). Imitation, according to Yerkes, plays no considerable réle in the learning processes of the dancing mouse (820). Where an animal is not at all helped to the solution of a problem by watching another animal solve it, we are justified in concluding that if it can recall memory ideas at all, it does not make use of them in a situation where a human being would certainly do so. The lack of ability on an animal’s part to postpone reacting to a stimulus is another evidence of inability to make use of memory ideas. The very ingenious method by which such ability may be studied was the device of Hunter (350). It has been termed the Delayed Reaction Method, and its general plan is as follows. A light is shown for a few seconds in any one of three directions from the animal, which is restrained from reacting. After the light is turned off and a certain time interval has elapsed, the animal is released, and if it goes in the direction in which the light appeared, it receives food. Now white rats could succeed in running in the proper direction when the delay between the disappearance of the light and their release was not more than ten seconds, but only in case they pointed their noses at the light when it appeared and

remained motionless in this position during the interval. Clearly we get no indication from such behavior that the rat is able to recall a memory image of the light. His failure to run in the right direction when he did not keep his nose pointed in the right direction plainly suggests the absence of such ideas as influences on his behavior. Again, the nature of the errors which animals occasionally make in experiments strongly suggests the absence of memory ideas. Thus the two rats which learned the Hampton Court maze under Small’s (685) tuition both continued, after they had reached their highest point of excellence in running the maze, to take the wrong turning at the outset. Precisely this error would have been, probably, the first one eliminated in the learning of a human being, who would be able to recall some kind of memory idea of the first turning owing to its especial hold upon attention. Further, the way in which instinctive actions are often performed by animals indicates that ideas are not present as they would be toa human being’s consciousness. Human beings do some things from instinct, but the doing of them may be accompanied by ideas; a mother’s care for her child involves ideas of the child’s happiness or suffering, and of its future. Enteman’s account of the worker wasp which, lacking other food to present to a larva, bit off a portion of one end of the larva’s body and offered it to the other end to be eaten, suggests a peculiar limitation of ideas in the wasp’s mind, at least while this particular function was being performed (206). The cow, which had lamented at being deprived of her calf, and on having the stuffed skin of her offspring given to her, licked it with maternal devotion until the hay stuffing protruded, when she calmly devoured the hay (504, p.

334), had perhaps experienced some dim ideas connected with her loss, but certainly her consciousness was more absorbed by the effects of present stimulation and less occupied with ideas than a human mother’s would have been. Thorndike (704) was the first to point out how scanty is the evidence in favor of the possession of ideas by the lower animals. In addition to the fact that his dogs and cats dropped off their useless movements so slowly, he adduced the observation that while after a time the cats which had been caused to enter a puzzle-box and let themselves out before being fed would of their own accord go into the box, cats that had been from the first dropped into the box at the top never learned to go in of their own accord. He argued that if a cat had been able to have the idea of being in the box, as a necessary prelude to food, it would have been able to pass from the idea of being dropped in to that of going initself. This argument, however, is not fully convincing. The experience of being picked up and dropped into a box is very different from that of walking through a door. To the human mind, accustomed to more refined analysis of its experiences, one of these would suggest the other, but we cannot argue’ that because such a connection is not made in the animal’s mind, therefore the latter is incapable of ideas, any more than we could conclude a total absence of ideas from the consciousness of a man to whom a primrose by the river’s brim does not suggest thoughts of the moral government of the universe. Moreover, several observers have reported precisely this ability to get the habit of jumping into a box from being dropped in; our rabbits (756), which were put into a box for safe keeping between experiments, within two days acquired the trick of running to the box and scrambling into it, the whole experience being a prelude to food.

observation that his dogs and cats were not helped to learn a puzzle-box mechanism by being put through the movements. The absence of ability to pass from the experience of being put through a movement to the idea of performing the movement is no proof of incapacity to form ideas; moreover Cole (134) found that the raccoon did learn to work a fastening by being put through the movements. Hunter (351) made a similar observation on the rat, and the method seems to meet with success in the hands of animal trainers.

In general, however, we must admit, the facts point to the conclusion that ideas are very rare in the animal mind. We can in some cases, however, present positive evidence of their occurrence. One attempt to demonstrate them, that of Cole (134), it is true, seems hardly conclusive. Cole trained raccoons to discriminate between various stimuli. Cards were placed on levers so that by a touch they could be pushed up and down. The animals learned to climb up for food when one of two differently colored cards was shown, and to stay down when the other one appeared; to distinguish in a similar way between a high and a low tone, between a round and a square card, and between a card 64 X 63 inches and one 43 X 4% inches square. Of course the action of climbing up was not itself purely instinctive, but had become associated with the food instinct. The raccoons also hit upon the trick of clawing up the cards themselves, and if the one that appeared was the “‘no-food” card, they would either claw it down again and pull up the other, or proceed at once to pull up the other, leaving the “‘no-food” one also up. Since the cards were shown successively, Cole concludes that “‘remembrance of the card just shown was required for a successful response.” ‘‘Why,” he asks,

“should the animal put the red card down if it did not fail to correspond with some image he had in mind, and why when he put the green up should he leave it up and go up on the high box for food if the green did not correspond with some image he had in mind?” It seems to the writer that the supposition of an image is unnecessary, except possibly in the experiments requiring discrimination of sizes. It is perfectly possible, as we know from our own experience, to react to one stimulus and not to another without going through a comparison of the two, unless the difference between them is merely one of degree. It might have been possible for a human being to discriminate between the larger and the smaller cards only by calling up a memory image of the card not shown and comparing it with the one before him; it surely would not have been necessary for him to use images in the reactions to colors, forms, and tones. And if a human being, accustomed to much dependence on memory ideas, could get on without them here, surely a raccoon could. Even in judgments of degree, all laboratory psychologists know that human beings have a strong tendency to make absolute rather than comparative judgments, and use memory ideas but little. Better, though still unsatisfactory, evidence of the use of images is furnished by the following method: ‘Three levers were placed on the displayer. One, on being raised, displayed white, another orange, another blue. The plan was to display white, orange, and blue consecutively, then to display the same blue three times. I fed the animal if he climbed upon the high box on being shown the series white, orange, blue, and did not feed him after the series blue, blue, blue.” That is, the stimulus immediately preceding the reaction was the same in both cases. The difference lay in the foregoing stimuli. The series

“‘white, blue, red, food” and “‘red, red, red, no food” was also used. The raccoons learned to respond properly, “though,” Cole continues, ‘I never completely inhibited the animals’ tendency to start up on seeing white or blue, which were precursors of the red which meant food. Thus the animals all anticipated red on seeing its precursors, which in itself seems good evidence of ideation. Many times, however, they turned back after starting at blue or white and looked for the red, then climbed up once more, thus showing that the red was not a neglected element of the situation, but an expected color which they generally waited to see, but sometimes were too eager to wait for.” Certain details of the raccoons’ behavior are significant. “‘Each one, on seeing the first red, would drop down from a position with both front paws on the front board to stand on all fours in front of it, and merely glance up at the succeeding reds. As soon as the white appeared, however, the animal would lean up against the front board, claw down the white and blue, but never the final red.”

Now Cole thinks that the learning of this trick by the raccoons proved that “‘the animal retains an image of the cards which just preceded red.” The only alternate supposition seems to him to be that they always reacted to the number of the card in the series, which, if the series were irregularly given, would not have been the same in successive trials. To suggest one’s own interpretation of animal behavior that one has not seen, in the place of the experimenter’s interpretation, requires some temerity, but to the present writer the most natural way of accounting for the raccoon’s performances would be the supposition that in the series white, blue, red, for instance, at the end of which they were fed, the occurrence of white threw them into a state of expectancy, of readiness to climb up on the

box; this was heightened by the blue, and finally ‘‘discharged” into action by the red. During this process they may have had an anticipatory image of the blue and of the red, although there is no evidence that they did. But when the red came they did not stop to call up memory images of the preceding colors, and decline to act until they had assured themselves that those were blue and white instead of red. Preparedness to act was probably already secured by the actual occurrence of the white card at the beginning of the series. In other words, while images may have been present, they were images with a future, not a past reference. A human being reacting to a series of stimuli in this fashion would but rarely, in case his attention had wandered during the giving of the first two stimuli, have to recall them as memory images before reaction, but he might very likely have anticipatory images of the stimuli to come while waiting for them. These criticisms, which appeared in the first edition of the present work, were later repeated by Gregg and Mc- Pheeters (268 a), who made experiments similar to Cole’s.

In favor of the functioning of ideas in monkeys and raccoons is the fact that in learning to open puzzle-boxes, they drop off useless movements with great speed. And monkeys have given clear evidence of inferential imitation. Kinnaman (401) reports that in one of his experiments, where the box had to be opened by pulling out a plug, a monkey failed to work the mechanism and gave up in despair. Another monkey then came out of the cage, the first one following. Number two went to the box, seized the end of the plug with his teeth, and pulled it out. The box was set again, and monkey number one rushed to it, seized the plug as number two had done, and got the food. She immediately repeated the act eight times. A second

and similar observation was made where the mechanism was a lever. Haggerty (281), as the result of long observation and experimenting on the monkeys in the Bronx | Zoo, got some excellent instances of inferential imitation, of which one may be quoted. The act to be performed was that of climbing up the side of the cage, thrusting the | arm up inside a wooden chute, and pulling a string inside © it, as a result of which food came tumbling down. Monkey number 13 was allowed to watch monkey number 4 go through this process four times. ‘‘Number 4 was now removed and Number 13 was released in the cage. At first he looked about over the floor for food and then climbed the front wire, stopping on the brace opposite the chute. He leaned over to the chute and while still standing on the brace with his feet, tried to thrust a hand into the bottom of the chute. Failing in this, he ran along the brace ...and back again to opposite the chute; catching the rung of the chute in his hands he drew himself over to it; finding himself above the end of the chute he tried to let his body down, first on one side and then on the other, until in the most awkward manner he managed to get near enough to the end to thrust a hand up the inside far enough to reach the string. At once he pulled and the food came tumbling down on his chest and to the floor. Dropping to the floor he picked up the food and ate it” (281, pp. 360-361). Such persistence of endeavor to carry out a definite act would certainly in a human being be guided by ideas.

Again, in Hunter’s (350) work by the Delayed Reaction Method, the raccoons showed behavior which would seem to indicate the presence of a memory idea. Although they could not go in the right direction if more than twentyfive seconds had elapsed since the light was turned off, they succeeded within this interval whether they did or did not change the position of their bodies. ‘‘Each of these animals could react successfully when the wrong orientation was held at the moment of release, and when, so far as the experimenter could detect, no part of the animal’s body remained constant during the interval of delay” (p. 43). Thus, after the light was turned off, and they had moved about during the period of delay, when they were released they could move in the direction where they had seen the light. The same type of behavior, but extending over much longer periods of delay, was characteristic of children in similar tests, and would seem to be naturally accompanied by memory ideas, although Hunter prefers to speak of the re-arousal of ‘‘intra-organic cues.” In the present writer’s opinion, all ideas are accompanied by “intra-organic”’ or kinesthetic cues. We shall refer later to this point.t . Another experimental method which, like the Delayed Reaction Method, has been devised to study the possible functioning of ideas in various animals is the Multiple Choice Method. Its beginnings are to be found in the work of Hamilton (283). As he used it, the essential features were as follows. The animal was placed in a compartment with four exit doors. All of these doors were locked except one, and that one might be any one of the four except the door that was open in the previous experiment. The object of the test was to see whether or not the animal approached comprehension of this prin- . ciple. The subjects were a normal man, a defective man,

1A curious type of delayed reaction, which must await further investigation, is reported by Mast (471) of the firefly Photinus pyralis. The flash of a female firefly causes the male to move in her direction. The turning of the male occurs after the female has flashed. six boys of varying ages, one defective boy, five monkeys, sixteen dogs, seven cats, and a horse. Only the human subjects reached a stage of learning where they showed by their behavior that they realized the impossibility of opening a door that had been open in the preceding trial. The mionkeys always tried all four doors, but did not often push repeatedly at the same door or persistently neglect a door ; this lowest type of behavior was more frequent in the horse. The fact may be noted for future reference that the behavior of the horse in this situation was “stupider” than that of any of the other subjects.

Yerkes (826) developed the principle of this method: and generalized it as follows. The animal is offered the - choice among a number of compartments. The number can be varied, and their position in space can be varied. Thus, if there are ten compartments in the apparatus, only three of them may be used in a certain experiment, and these three may be situated in the middle or towards either end, so that no associations will be formed with position in space. Or in another experiment five of the compartments, in any part of the series, may be used. The compartments used in a given experiment have their entrance doors open. The problem may be varied in complexity by making the “‘right”’ compartment, the one whose entrance gives food, bear different relations to the rest. It may be the first compartment on the left, the first compartment on the right, the second on the left, the second on the right, the middle compartment, and so on. After an animal has proved its ability to learn a simple problem, such as “‘first on the right,” it may be advanced to a more complex one, such as “second on the left.”” The method has been applied to crows (129), rats (113), pigs (826), monkeys, and apes (824). The crow mastered the ‘first at

the right” and “‘first at the left” problems, but failed in five hundred trials to master the “‘second at the left”’ problem. The white rat succeeded with the “first at the right,” but failed with the “‘second from the left” problem. The pig distinguished itself by mastering “‘first at right,” “second from left,” “alternately first at left and first at right,” failing only to grasp the ‘‘middle compartment” problem. The two monkeys tested by Yerkes (824) showed improvement in dealing with the problems ‘“‘first at left,” “second from right,” “alternately first at left and first at right,” and ‘‘middle,” but appeared to owe many of their. successes to their acquired preferences and aversions for particular compartments. The “alternating” problem proved to be especially easy. An orang-utan, who showed himself in other tests the most intelligent of Yerkes’s subjects, failed to improve in solving the problems of the Multiple Choice Method. His wrong choices were so persistent, and so independent of the usual tendency to drop off useless movements, that Yerkes concluded him to be really acting on the basis of wrong ideas as to the correct solution of the problem. It is clear that a human being who had formed an incorrect theory as to the proper way to work out a problem would take longer to solve it than an animal who learned merely by the dropping off of useless movements, provided that the animal could solve it at all.

We may now examine the relation of the Multiple Choice Method to the question of the existence of memory ideas in animals. In the first place, if the “right” compartment always occupied the same position in space, clearly an animal might learn to go to it without the use of memory ideas. Kinesthetic memory, the formation of a habit of turning in a certain direction, would suffice. Next, if the correct compartment is not always in the same absolute position

in space, but is always the furthest to the right or left of all the compartments used in the experiment, the learning is still easy. The animal has only to combine the habit of turning to the right or left with the observation as to what compartments have their entrance doors raised : a compartment with closed doors offers no stimulus. Thirdly, even the problem “‘second from the left,” or right, might, it would appear, be solved without the use of a memory idea. The learning need involve only (a) the habit of turning to the left or right, and (6) the habit of reacting negatively to the open door furthest in this direction. The natural result of such a combined habit would be entering the door next to the end door. The problem of entering always the middle door of those open brings us closer to the use of memory ideas. An animal that had solved this problem would, on being confronted with the series of doors, find itself in an attitude representing a balance between the impulse of turning to the right and that of turning to the left. ‘‘Middleness” means a slight impulse to turn in one direction, offset by an equal impulse to turn in the other direction. Now the characteristic by which this situation differs from the other situations, involved in the simpler problems, is that the animal must not move at once, but must wait and assume the balanced attitude before moving. In the case of the other problems, he can start off immediately. Here an attitude must be revived before there is any actual movement. Just as in the Delayed Reaction Method success means, if the animal moves during the interval of delay, that it is able to revive an inner attitude which means motion towards the light, so here success means ability to revive an inner attitude which means movement towards the middle, a balance beween right and left movements.

Further, what is the difference between reviving such a motor attitude at the sight of a stimulus, and making an ordinary response to a stimulus, such as any animal may learn? The difference is that in the latter case an actual, visible movement is made, while in the former case the movement is internally anticipated and not externally visible. Such an internally anticipated movement is probably always present when in the human consciousness we have a memory idea: when I recall a mental image of an object such as a fork, I ‘‘internally anticipate” the movements of handling the fork. Whether the converse of this proposition is also true, and we invariably have memory ideas whenever we internally anticipate movements, is highly doubtful, but at least it may safely be said that an animal which gives evidence of being able to anticipate its own movements has the possibility of memory ideas in its consciousness. (For reasons which have been elsewhere ? stated, the present writer is inclined to think that this internal anticipation of movements means actual slight contractions of the muscles involved in performing the movements.) Whenever, then, as in the case of success in the De- layed Reaction Method where the bodily position is varied, in that of inferential imitation, and in that of choosing always the middle stimulus, the behavior seems to demand that the movements shall be anticipated by the animal which performs them, we have evidence in favor of the memory idea.

An important condition of an animal’s ability to anticipate its movements, to ‘‘know beforehand” what it Modification by Experience 303 is going to do, is obviously the ability to keep from actually reacting on the instant when the stimulus acts. To recall a memory idea, to anticipate by slight and invisible movements the response one is going to make, implies waiting a brief interval at least before making it in full. Now the development of sense-organs which can receive stimuli coming from a distance is an absolutely necessary prerequisite for the safety of delaying reaction. An important difference exists between the stimuli from objects directly in contact with an organism’s body, such as in our own experience give rise to touch, temperature, pain and taste sensations, and those which proceed from objects at a distance, such as light, sound, and odors. This difference consists in the fact that the former have a more direct and instant effect upon the organism’s welfare, and in consequence demand more rapid reaction than the latter. A stimulus in immediate contact with an animal’s body may have a harmful or beneficial influence at the moment of its impact; it may be food to be seized or an enemy to be escaped, and the seizing or escaping must be done on the instant; on the other hand, if an animal possesses the power, belonging in an increasing degree to animals as we go up the scale, of reacting to influences proceeding from objects still at a distance, it may safely delay its reaction when the stimulus is given. The danger is not so imminent, the food is not yet within reach; the full motor response to stimulation may be suspended for a short interval without imperiling the life interests of the animal. Thus one condition for the development and use of memory. ideas is the evolution of sense-organs for the reception of stimuli at a distance. "This idea-was first suggested by the writer in 1904 (755); a similar conception, developed from the neurological standpoint, appears in Sherrington’s

“The Integrative Action of the Nervous System” (681, pp. 324 ff.). Sherrington proposes the term ‘‘distance receptors” for those receptive organs ‘“‘which react to objects at a distance,” and declares that “the distance receptors contribute most to the uprearing of the cerebrum.” The most important significance of the power to act in response to distant objects Sherrington finds to be that it allows an interval for preparatory adjustment, ‘‘for preparatory reactive steps which can go far to influence the success of attempts either to obtain actual contact or to avoid actual contact with the object.”” That these preparatory steps may also involve the germ of the memory image ‘ clearly suggested by Sherrington. ‘‘We may suppose,” he says, ‘‘that in the time run through by a course of action focussed upon a final consummatory event, opportunity is given for instinct, with its germ of memory, however rudimentary, and its germ of anticipation, however slight, to evolve under selection that mental extension of the present backward into the past and forward into the future which in the highest animals forms the prerogative of more developed mind. Nothing, it would seem, could better insure the course of action taken in that interval being the right one than memory and anticipatory forecast” (p. 332).

Secondly, if memory ideas depend on the anticipation of movements, during the delay between stimulus and full response, an important condition of their variety and free use is the ability of the animal to perform a great variety of movements, and especially of movements other than those ~ of locomotion. Locomotion gets an animal into difficulties and rescues it; movements of locomotion are of the first practical importance. But they have not a great deal of variety, It is not merely a coincidence that the best

evidences of memory ideas should appear in animals which like the raccoon and the monkey are dexterous, able to use their paws for movements more complex and refined than those of locomotion. The supreme development of ideas comes in the mind of the animal which has not merely hands, but vocal organs, so that an infinite variety of delicate and complicated movements can be anticipated, and can form the basis of memory ideas. Thirdly, one of the conditions of the anticipation of a movement appears to be attention to it when it is originally performed. In order to remember a movement, we must have paid attention to the sensations which its performance occasions, to the way it feels to make the movement. And one condition for attention to the way a movement feels is being comparatively safe from external dangers when the movement is made. An animal under ordinary conditions of wild life has very little attention to spare for his own movements. It would thus seem as though one requirement which must be fulfilled if anticipated movements are to play an important part in a creature’s experience were that the animal should, for a time at least, be set free from the pressure of the practical hand-to-hand struggle for the means of existence, and thus enabled in safety to attend to its own movement sensations. Animal play, at first thought, offers an instance of such liberation from practical necessities. But as Groos has shown, animal play is not so unpractical as it looks (270). It is simply the exercise of the same instincts upon which in other circumstances the animal’s welfare depends. The attention is absorbed in external objects quite as much in play as in the actual chase or warfare. The kitten watches the string, for which she has no practical use,

have a practical use; the dogs rolling over and over each other are nearly as absorbed in each other’s movements as if they were in deadly combat. That relief from practical necessity which will serve the purpose we are considering is to be found not in play, but in infancy. If a creature spends the period during which its nervous system is undergoing most rapid development in a state of complete shelter and protection from external danger, with all its vital needs supplied, then the nervous energy which under other conditions would be expended in the processes underlying attention to external stimuli is free to be so devoted that attention will be directed toward the creature’s inner experiences. The human baby, while he may be interested in lights and sounds, in external impressions, does not need to be alert and watchful lest he miss his dinner or be dined on himself; his attention is free to be expended on his own movement experiences as well as on anything else. That young children do go through a stage of intense interest in the sensations resulting from their own movements is a fact made clear from many observations. The curious period of ‘‘self-imitation” in the child when it repeats for an indefinite period the same movement or sound, over and over again (14), is very likely a period of vivid attention to movement sensations.