The more complex the environment is in which the individual must live, the longer is the period of infancy needed in which the necessary habits and capacities may be acquired. In the human being the period of infancy extends in a literal sense through the first five years of the individual's life. But in civilized societies it extends factually much longer. By the end of the first five years the child's physical infancy is over. It can take care of itself so far as actually feeding itself, moving about, and communicating with others is concerned. But so complex are the habits to which it must become accustomed in our civilization that it is dependent for a much longer period. The whole duration of the child's education is a prolongation of the period of infancy. In most civilized countries, until at least the age of twelve, the child is literally dependent on its parents. And with every advance in civilization has come a lengthening in the period of education, or learning. Intellectually, the period of infancy might be said not really to be over before the age of twenty-five, by which time habits of mind have become fairly well fixed. The brain and the nervous system remain fairly plastic up to that time, and if inquiry and learning have themselves become habitual, plasticity may last even longer. In the cases of the greatest intellects, of a Darwin, or a Newton, one might almost say the period of infancy lasts to old age. To be still learning at sixty is to be still a child in the best sense of the word. It is still to be open rather than rigid, still to be profiting by experience. The great social advantages of the prolonged period of infancy lie in the fact that there is a unique opportunity both for the acquisition by individuals and for the imposition on the part of society of a large number of habits of great social value. The human being, born into a world where there are many things to be learned both of natural law and human relations, is, as it were, fortunately born ignorant. He has instincts which are pliable enough to be modified into habits, and in consequence socially useful habits can be deliberately inculcated in the immature members of a society by their elders. The whole process of education is a utilization of man's prolonged period of infancy, for the deliberate acquisition of habits. This is all the more important since only by such habit formation during the long period of human infancy can the achievements of civilization be handed down from generation to generation. Art, science, industrial methods, social customs, these are not inherited by the individual as are the instincts of sex, pugnacity, etc. They are preserved only because they can be taught as habits to those beings who come into the world with a plastic equipment of instincts which lend themselves for a long time to modification.