A large part of the education of humans as well as of animals consists precisely in the modification of our original responses to situations by a trial-and-error discovery of ways of attaining satisfactory and avoiding annoying situations. Both animals and humans, when they have several times performed a certain act that brings satisfaction, tend, on the recurrence of a similar situation, to repeat that action immediately and to eliminate with successive repetitions almost all the other responses which are possible, but which are ineffective in the attainment of some specific satisfaction. The whole training imposed by civilization on the individual is based ultimately on this fundamental fact that human beings can be taught to modify their behavior, to change their original response to a situation in the light of the consequences that follow it. This means that while man's nature remains on the whole constant, its operations may be indefinitely varied by the results which follow the operation of any given instinct. The child has its original tendency to reach toward bright objects checked by the experience of putting its hand in the flame. Later his tendency to take all the food within reach may be checked by the looks of scorn which follow that manifestation of man's original greed, or the punishment and privation which are correlated with it. Through experience with punishment and reward, humans may be taught to do precisely the opposite of what would have been their original impulse in any given situation, just as the monkey reported by one experimenter may be taught to go to the top of his cage whenever a banana has been placed at the bottom.
THE PROLONGED PERIOD OF INFANCY. Probably the most significant and unique fact of human behavior is the period of "prolonged infancy" which is characteristic of human beings alone. Fiske and Butler in particular have stressed the importance of this human trait. In the lower animals the period of infancy--that is, the period during which the young are dependent upon their parents for food, care, and training--is very short, extending even in the highest form of ape to not more than three months. This would appear, at first blush, to be a great advantage possessed by the lower animals. They come into the world equipped with a variety of tendencies to act which, within a week, or, as in the case of chickens, almost immediately after birth, are perfectly adapted to secure for them food, shelter, and protection. They are mechanisms from the beginning perfectly adjusted to their environment.