A further circumstance must be taken into account. The be haviour of animals in their natural environments tends through out to be purposive and concerned with preservation of life. Now, we are accustomed to derive the purposefulness of human conduct from the action of will, which reasons and has an aim. And so we incline, on superficial reflection, to ascribe purposive animal conduct to the action of a similar will. Only a closer inspection of the nature of many actions of animals shows that the matter is not so simple as to justify such an analogy. We find a remarkable purposefulness not only in the behaviour of the animals but also in their bodily structure. The development and functions of organs are purposeful throughout. This shows that not all pur posefulness need be the result of the conscious workings of will. At the same time, this fact is a further ground why science should concern itself with the causes of animal behaviour. This new science, whether called the science of animal behaviour or animal psychology, is concerned not only with the behaviour of animals under normal circumstances, but it attempts, by experiment, to penetrate more deeply into the connections between cause and effect in animal conduct.
Animal reactions can be divided into f our groups, according to the degree of complication of the physiological processes which accompany them. These types of behaviour represent to a certain extent f our different levels at which the reactions of animals pro ceed. The behaviour of animals may take the form of tropisms or taxis, which are not very common; reflex movements or reflexes, which are universal; instinctive reactions; and lastly intelligent actions, found only in the highest animals. Although reflexes and instincts occur both in animals which, as regards their per formances, stand at the very summit of the animal series, and also in the lower animals, nevertheless within the animal kingdom a gradual increase is recognizable in the degree of accomplishment of reactions. Instinctive behaviour assumes a higher degree of complexity and at the same time a greater plasticity as we ascend the animal series. This is correlated with the higher development of bodily structure and the greater efficiency of the nervous system which goes with this.
types of stimuli do not coincide in the directions they tend to impose on the ani mal, the latter takes up a position which is the resultant of the two directions. Many lower animals show a definite phototaxis. Thus, not only small crustacea but also fishes and amphibian larvae swim towards light ; though this does not occur unconditionally. If, for instance, a number of tadpoles are placed in a round glass vessel filled with water, and the vessel is allowed to float on the surface of an aquarium in which other tadpoles are present, then the individuals in the aquarium itself are uninfluenced by the light. They swim about in all directions. But the tadpoles in the small dish all turn toward the source of light (fig. i ). Numerous small fishes and crustaceans behave in the same manner under like circumstances. It is the confined space which in this case calls forth or awakes the positive photo taxis.
Although tropisms and taxis such as we have already de scribed are widely spread throughout the animal kingdom, yet the conception of a tropism was originally much less wide. Jacques Loeb used the term in a much narrower sense, maintaining that the position assumed by an animal as a result of the direction of the stimuli affecting it is assumed in a purely automatic manner. The reaction he considered to be just as purely mechanical as the movement of a floating needle towards a magnet. It was, in particular, the way in which many animals react to light stimuli that caused Loeb to formulate his Tropism Theory. When a bi laterally symmetrical animal receives a stimulus, for example a ray of light, on one side, the stimulation of that part of the body, or of the sense organs affected, results automatically, through the intermediary of the nervous system, in setting into activity the locomotor organs of that side. By this means the body is turned and the rotation continues until the long axis of the animal is brought into the direction of the light rays. In other words, the animal turns until the two symmetrical sides of the body each receive an equal amount of stimulation. Then the locomotor organs of the other side commence to function and the whole animal moves forwards. Numerous investigations have shown, however, that such a regulation of behaviour purely through symmetrical stimulation, i.e., where the stimulus—in our instance light—directly causes the movement, is extremely rare in the animal kingdom. It is found particularly in such animals as harbour symbiotic algae in their bodies and therefore seek out the light. Examples are the flagellate protozoan Euglena and the flat-worm Convoluta roscoffensis. In most other cases the cir cumstances are not so simple. In particular, the processes . are complicated by the interaction of the central nervous system, which so largely controls behaviour. Through this the reactions lose their apparently automatic character. It is true that many animals place themselves symmetrically to the stimuli. Examples have been given above. Indeed, we frequently make use of the fact by allowing animals to carry out their tropisms and so to collect together in a certain spot when we wish to catch a con siderable number of individuals quickly and easily. In this gen eral sense we may continue to use the expression tropism, which of late has been so hotly attacked. We must realize, however, that it is not a simple mechanism that we have before us. If we scatter a number of young caterpillars of certain species of butterflies on the surface of a table standing at a window most of them will certainly crawl towards the light. But it is probable that one or two will move in another direction. This depends upon other stimuli which influence the central nervous system. The effect of these stimuli may then conflict with the tropism which would otherwise manifest itself.