Comparative Psychology

Capacity for Learning.

Although we have so far represented instincts as unalterable and inflexible inherited modes of action, this does not apply to all instincts. There exist numerous cases in which animals are able more or less to modify their inborn types of conduct in relation to prevailing circumstances, i.e., to adapt their behaviour to the given situation. This applies to such cases as web-construction by spiders or nest-building by birds. It is true that each species of spider makes its web according to a definite plan, which may be considered as a specific character. It is equally true that each kind of bird builds its nest in a very definite man ner, easily recognizable by the ornithologist. But in each different case the locality in which the web or nest is constructed is a different one. Moreover, the available material for making the bird's nest often varies, for instance, as regards the shape and length of the twigs collected, etc. The spider, too, in spin ning its web must adapt itself to the space at its disposal, and the bird must suit its behaviour to the conditions of the locality in which it is working, to the fork of the branch, the hollow in the trunk of a tree in which it makes the nest. And in other ways we see that animals, particularly those which as regards their con duct stand high in the animal series, break away from the inborn inflexibility of instincts and behave in this or that manner accord ing to the circumstances. Thus instincts are frequently plastic and can be adapted to conditions. The experiences which the particular animal has had during its lifetime here play a definite part. For this reason we must next describe two faculties on which the power to accumulate experiences depends.

Memory.

First comes the power of memory. This is present in all branches of the animal kingdom, even in the Protozoa. But this fact is not very astonishing, since one can consider "memory" in a cer tain sense as a general characteristic of matter. The properties of a colloid can depend upon the way in which it reached its present state, that is on previous history, age, temperature, duration of physical processes, and so on. Hys teresis of metals is somewhat similar. In animals, memory consists in the fact that a stimulus, when it acts on an animal causing a certain excitation in the body, leaves behind itself a residuum or trace. In conse quence of this the organism is to a certain extent physiologically different when later on the same stimulus acts upon it again. And so the possibility arises of an animal reacting first in one way and later in another, to the same stimulus. When a certain stimulus is repeated a number of times in a short interval, an animal may accustom itself to the stimulus. That is, on the first occasion the animal will answer the stimulus by the reaction which normally follows it. But later, the reactions become weaker and in the end the animal no longer reacts at all to this stimulus. This is not due to fatigue of the organs of motion, the muscles, but it is the re sult of becoming accustomed to the stimulus. Only after a period

of rest does the animal once more react normally to the stimulus. On the other hand, an animal whiclit is reacting in a certain way to a stimulus repeated at frequent definite intervals can "exercise" the movement in question. The animal gradually executes the movements more promptly and more rapidly than it did without the practice. Human beings find a similar gain in performance through frequent repetition or exercise. The paths of the nervous system which conduct the excitations become better and better "trodden," better suited to take up and transmit this excitation. In the end, through repetition, a given action can become a habit.

Association.

In so far as animals possess a true central nervous system with higher, dominant centres, we find in them a further and more important faculty, that of forming associations. If, at the same time as a stimulus which has a definite resulting reaction acts, another stimulus occurs which in itself would cause no change in the conduct of the animal, then, provided the two stimuli always act together, after a certain number of repetitions the reaction follows on the indifferent stimulus alone, whereas in the beginning it results only from the first stimulus. In the nerve paths, a coupling of the courses of excitation has occurred. An association of the two stimu lation and excitation phenomena has been formed. The number of repetitions necessary for such an association to be established differs both with the species of animal and with the nature of the stimulus. One or two examples will make this clearer. An earth-worm was placed in the middle limb of a T-shaped tube system so that it was possible for the animal to creep into the right-hand or into the left-hand portion of the cross-piece of the T. In one of the arms of the cross-piece, the electrodes of an induction coil were placed, so that when the worm touched these it received an electric shock. Five or six experiments were made daily with each animal tested. Whereas in the begin ning the animals went as often to the left as to the right, it became apparent that after about 8o or too experiments, the side with the electrodes was visited less often. After about 120 to 180 trials, in every 20 experiments there were at most 5--3 errors. The animals, then, had learnt to avoid the place associated with an "unpleasant" stimulus. In other words, they had been "trained" to go to a certain side. This habit of turning always to the right side, acquired by training, could be reversed by a further training. This was done by plac ing the electrodes on the side which previously had been the right one for the worms to turn into. Not only could normal earthworms be trained in this manner, but also animals from which the brain had been re moved, and even worms lacking the whole portion of the central nervous system contained in the first six segments.