Animals do learn, all the vertebrates, at least, and many of the invertebrates. They often learn more slowly than men, but this is an advantage for our present purpose, since it makes the learning process easier to follow. Mere anecdotes of intelligent behavior in animals are of little value, but experimental studies, in which the animal's progress is followed, step by step, from the time when he is confronted with a perfectly novel situation till he has mastered the trick, have now been made in great numbers, and a few typical experiments will serve as a good introduction to the whole subject of learning.

The negative adaptation experiment.

Apply a harmless and meaningless stimulus time after time; at first the animal makes some instinctive exploring or defensive reaction; but with continued repetition of the stimulus, he ceases after a while to respond. The instinctive reaction has been detached from one of its natural stimuli.

Even in unicellular animals, negative adaptation can be observed, but in them is only temporary, like the "sensory adaptation" described in the chapter on sensation. Stop the stimulus and the original responsiveness returns after a short time. Nothing has been learned, for what is learned remains after an interval of rest.

In higher animals, permanent adaptation is common, as illustrated by a famous experiment on a spider. While the spider was in its web, a tuning fork was sounded, and the spider made the defensive reaction of dropping to the ground. It climbed back to its web, the fork was sounded again, the spider dropped again; but after several {303} repetitions in quick succession, the spider ceased to respond. Next day, to be sure, it responded as at first; but after the same performance had been repeated on several days, it ceased permanently to respond to this stimulus.

Negative adaptation is common in domestic animals, as well as in men. The horse "gets used" to the harness, and the dog to the presence of a cat in the house. Man grows accustomed to his surroundings, and to numerous unimportant sights and sounds.

The conditioned reflex experiment.

Put into a dog's mouth a tasting substance that arouses the flow of saliva, and at the same instant ring a bell; and repeat this combination of stimuli many times. Then ring the bell alone, and the saliva flows in response to the bell. The bell is a substitute stimulus, which has become attached to the salivary response by dint of having been often given along with the natural stimulus that arouses this response. At first thought, this is very weird, but do we not know of similar facts in every-day experience? The dinner bell makes the mouth water; the sight of food does the same, even the name of a savory dish will do the same.

Quite possibly, the learning process by which the substitute stimulus becomes attached to the salivary reaction is more complex in man's case. He may observe that the dinner bell means dinner, whereas the dog, we suppose, does not definitely observe the connection of the bell and the tasting substance. What the experiment shows is that a substitute stimulus can become attached to a reaction under very simple conditions.

A conditioned reflex experiment on a child deserves mention. A young child, confronted with a rabbit, showed no fear, but on the contrary reached out his hand to take the rabbit. At this instant a loud rasping noise was produced just behind the child, who quickly withdrew his hand with {304} signs of fear. After this had been repeated a few times, the child shrank from the rabbit and was evidently afraid of it. Probably it is in this way that many fears, likes and dislikes of children originate.

The signal experiment.

Place a white rat before two little doors, both just alike except that one has on it a yellow circle. The rat begins to explore. If he enters the door with the yellow sign, he finds himself in a passage which leads to a box of food; if he enters the other door he gets into a blind alley, which he explores, and then, coming out, continues his explorations till he reaches the food box and is rewarded. After this first trial is thus completed, place him back at the starting point, and he is very apt to go straight to the door that previously led to the food, for he learns simple locations very quickly. But meanwhile the experimenter may have shifted the yellow sign to the other door, connected the passage behind the marked door with the food box, and closed off the other passage; for the yellow disc in this experiment always marks the way to the food, and the other door always leads to a blind alley. The sign is shifted irregularly from one door to the other. Whenever the rat finds himself in a blind alley, he comes out and enters the other door, so finally getting his reward on every trial. But for a long time he seems incapable of responding to the yellow signal. However, the experimenter is patient; he gives the rat twenty trials a day, keeping count of the number of correct responses, and finds the number to increase little by little, till after some thirty days every response is correct and unhesitating. The rat has learned the trick.

He learns the trick somewhat more rapidly if punishment for incorrect responses is added to reward for correct responses. Place wires along the floor of the two passages, and switch an electric current into the blind alley, behind {305} the door that has no yellow circle on it. When the rat enters the blind alley and gets a shock, he makes a prompt avoiding reaction, scampering back to the starting point and cowering there for some time; eventually he makes a fresh start, avoids the door that led to the shock and therefore enters the other door, though apparently without paying any attention to the yellow sign, since when, on the next trial, the sign is moved, he avoids the place where he got the shock, without reference to the sign. But in a series of trials he learns to follow the sign.

Learning to respond to a signal might be classified under the head of substitute stimulus, since the rat learns to respond to a stimulus, the yellow disk, that at first left him unmoved. But more careful consideration shows this to be, rather, a case of substitute response. The natural reaction of a rat to a door is to enter it, not to look at its surface, but the experiment forces him to make the preliminary response of attending to the appearance of the door before entering it. The response of attending to the surface of the door is substituted for the instinctive response of entering. Otherwise put: the response of finding the marked door and entering that is substituted for the response of entering any door at random.

The maze experiment.

An animal is placed in an enclosure from which it can reach food by following a more or less complicated path. The rat is the favorite subject for this experiment, but it is a very adaptable type of experiment and can be tried on any animal. Fishes and even crabs have mastered simple mazes, and in fact to learn the way to a goal is probably possible for any species that has any power of learning whatever. The rat, placed in a maze, explores. He sniffs about, goes back and forth, enters every passage, and actually covers every square inch of the maze at least once; and in the course of these explorations {306} hits upon the food box. Replaced at the starting point, he proceeds as before, though with more speed and less dallying in the blind alleys. On successive trials he goes less and less deeply into a blind alley, till finally he passes the entrance to it without even turning his head. Thus eliminating the blind alleys one after another, he comes at length to run by a fixed route from start to finish.

Fig. 47.--(From Hicks.) Ground plan of a maze used in experiments on the rat. The central square enclosure is the food box. The dotted line shows the path taken by a rat on Its fourth trial, which occupied 4 minutes and 2 seconds.

At first thought, the elimination of useless moves seems to tell the whole story of the rat's learning process; but careful study of his behavior reveals another factor. When the rat approaches a turning point in the maze, his course bends so as to prepare for the turn; he does not simply advance to the turning point and then make the turn, but several steps before he reaches that point are organized or coÖrdinated into a sort of unit.{307}

Fig. 48.--(From Watson.) Learning curve for the rat in the maze. This is a composite or average, derived from the records of four animals. The height of the heavy line above the base line, for any trial, indicates the number of minutes consumed in that trial in passing through the maze and reaching the food box. The gradual descent of the curve indicates the gradual decrease in time required, and thus pictures the progress of the animals in learning the maze.

The combination of steps into larger units is shown also by certain variations of the experiment. It is known that the rat makes little use of the sense of sight in learning the maze, guiding himself mostly by the muscle sense. Now if the maze, after being well learned, is altered by shortening one of the straight passages, the rat runs full tilt against the new end of the passage, showing clearly that he was proceeding, not step by step, but by runs of some length. Another variation of the experiment is to place a rat that has learned a maze down in the midst of it, instead of at {308} the usual starting point. At first he is lost, and begins exploring, but, hitting on a section of the right path, he gets his cue from the "feel" of it, and races off at full speed to the food box. Now his cue could not have been any single step or turn, for these would all be too much alike; his cue must have been a familiar sequence of movements, and that sequence functions as a unit in calling out the rest of the habitual movement.

Fig. 49.--(From Watson.) A puzzle box. The animal must here reach his paw out between the bars and raise the latch, L. A spring then gently opens the door.

In short, the rat learns the path by elimination of false reactions and by combination of single steps and turns into larger reaction-units.

The puzzle-box experiment.

Place a hungry young cat in a strange cage, with a bit of fish lying just outside, and you are sure to get action. The cat extends his paw between the slats but cannot reach the fish; he pushes his nose between the slats but cannot get through; he bites the slats, claws at anything small, shakes anything loose, and tries every part of the cage. Coming to the button that fastens {309} the door, he attacks that also, and sooner or later turns the button, gets out, and eats the fish. The experimenter, having noted the time occupied in this first trial, replaces the cat, still hungry, in the cage, and another bit of fish outside. Same business, but perhaps somewhat quicker escape. More trials, perhaps on a series of days, give gradually decreasing times of escape. The useless reactions are gradually eliminated, till finally the cat, on being placed in the cage, goes instantly to the door, turns the button, goes out and starts to eat, requiring but a second or two for the whole complex reaction. Perhaps 15 or 20 trials have been required to reach this stage of prompt, unerring response. The course of improvement is rather irregular, with ups and downs, but with no sudden shift from the varied reaction of the first trial to the fixed reaction of the last. The learning process has been gradual.

This is the typical instance of learning by "trial and error", which can be defined as varied reaction with gradual elimination of the unsuccessful responses and fixation of the successful one. It is also a case of the substitute response. At first, the cat responds to the situation by reaching or pushing straight towards the food, but it learns to substitute for this most instinctive response the less direct response of going to another part of the cage and turning a button.

The cat in this experiment is evidently trying to get out of the cage and reach the food. The situation of being confined in a cage while hungry arouses an impulse or tendency to get out; but this tendency, unable at once to reach its goal, is dammed up, and remains as an inner directive force, facilitating reactions that are in the line of escape and inhibiting other reactions. When the successful response is hit upon, and the door opened, the dammed-up energy is discharged into this response; and, by repetition, {310} the successful response becomes closely attached to the escape-tendency, so as to occur promptly whenever the tendency is aroused.

There is no evidence that the cat reasons his way out of the cage. His behavior is impulsive, not deliberative. There is not even any evidence that the cat clearly observes how he gets out. If he made a clean-cut observation of the manner of escape, his time for escaping should thereupon take a sudden drop, instead of falling off gradually and irregularly from trial to trial, as it does fall off. Trial and error learning is learning by doing, and not by reasoning or observing. The cat learns to get out by getting out, not by seeing how to get out.