Animal intelligence: Experimental studies

by Edward L. Thorndike

The evolution of the intellectual and moral nature which a higher animal really possesses from the sort of a nature which the real activities of the protozoa manifest, is far less difficult to explain.

In so far as the higher animal is a collection of original tendencies to respond to physical events without and within the body, subject to modification by the laws of exercise and effect and by these alone, and in so far as the protozoan is already possessed of a well-defined repertory of responses connected with physical events without and within the body in substantially the manner of the higher animal’s original tendencies, the problems of the evolution of behavior are definite and in the way of solution.

The previous sections gave reason for the belief that the higher animals, including man, manifest no behavior beyond expectation from the laws of instinct, exercise and effect. The human mind was seen to do no more than connect in accord with original bonds, use and disuse, and the satisfaction and discomfort resulting to the neurones. The work of Jennings has shown that the protozoa already possess full-fledged instincts, homologous with the instincts of man. They too may have specialized receptors, an action-system with a well-defined repertory and a connecting system or means of influencing the bonds between the stimuli received and the motor reactions made. The difficulties of tracing the possible development of a super-man from an infra-animal thus disappear.

There is, of course, an abundance of bona fide difficulty in discovering the unlearned behavior of each group of animals and in tracing, throughout the animal series, changes in the physical events to which animals are sensitive so that to each a different response may be attached, changes in the movements of which animals are capable, and changes in the bonds by which particular movements follow particular physical events. To find when and how animals whose natures remained nearly or quite unchanged by the satisfying and annoying effects of their behavior, gave birth to animals that could learn, is perhaps a still harder task. But these tasks concern problems that are intelligible matters of fact. They do not require a student to get out of matter something defined as beyond matter, or to get volition out of tropisms, or to get ideas of space and time out of swimming and sleeping.

The evolution of the sensitivities and of the action-systems of animals has already been subjected to matter-of-fact study by naturalists. The evolution of the connection-system will soon be. Each reflex, instinct or capacity, each bond between a given situation presented to a given physiological state and a given response, has its ancestral tree. Scratching at an irritated spot on the skin is older than arms. Following an object that is moving slowly does not have to be explained separately, as a ‘chance’ variation in dogs, sheep and babies. The mechanical trades of man are related to the miscellaneous manipulations of the apes. Little as we know of the connection-systems possessed by animals, we know enough to be sure that a bond between situation and response has ancestors and children as truly as does any bodily organ. Professor Whitman a decade ago showed the possibility of phylogenetic investigation of instinctive connections in a study which should be a stimulus and model for many others. In place of any further general account of the study of the phylogeny of the connection-system, I shall quote from his account of the concrete phylogeny of the instinct of incubation.

“b. The Incubation Instinct

1. Meaning to be Sought in Phyletic Roots.—It seems quite natural to think of incubation merely as a means of providing the heat needed for the development of the egg, and to assume that the need was felt before the means was found to meet it. Birds and eggs are thus presupposed, and as the birds could not have foreseen the need, they could not have hit upon the means except by accident. Then, what an infinite amount of chancing must have followed before the first ‘cuddling’ became a habit, and the habit a perfect instinct! We are driven to such preposterous extremities as the result of taking a purely casual feature to start with. Incubation supplies the needed heat, but that is an incidental utility that has nothing to do with the nature and origin of the instinct. It enables us to see how natural selection has added some minor adjustments, but explains nothing more. For the real meaning of the instinct we must look to its phyletic roots.

If we go back to animals standing near the remote ancestors of birds, to the amphibia and fishes, we find the same instinct stripped of its later disguises. Here one or both parents simply remain over or near the eggs and keep a watchful guard against enemies. Sometimes the movements of the parent serve to keep the eggs supplied with fresh water, but aëration is not the purpose for which the instinct exists.