ABSTRACT
Investigations into the processes of conditioning have mainly concentrated on mammals, notably the dog, the laboratory rat, monkeys, apes, and man himself. However, experiments have been done with other animals and the result has been that conditional behaviour has been found in all groups of animals which have been investigated. The picture which emerges from studies of different levels of animal life is one in which the capacity of conditioning seems to reflect levels of organization of the central nervous system. The more complex the nervous system, the more complex the processes of conditioning, i.e. the more can be learned. Such a view of conditioning stresses the essential similarity with regard to the mechanisms of learning throughout the animal kingdom. In this view there is no fundamental difference at the physiological level between man and the other animals in so far as conditioning phenomena are concerned. This is a view which coincides closely with the picture which is emerging from other sciences: the essential difference between man and the other animals is social, not physiological. This we consider later; now let us consider briefly how the level of complexity of the central nervous system is reflected in the complexity of the conditional behaviour which may be set up.
