ABSTRACT
We are not entitled to say that hereditary factors produce particular characteristics; nevertheless we do see their effects in the orderly sequence of development in the human infant — the growth of the various organs before and after birth, the emergence of reflexes, sensory and motor capacities, later the changes of puberty. While this process of maturation is generated, as it were, from within, each stage develops from the preceding ones only under appropriate stimulation from the environment, and the stimulation needs to come at the right time. For example the development of vision depends on stimulation of the retina by light: if an ape is brought up in the dark for the first year of life, its retina is permanently impaired. Much the same principles appear to operate in mental development; speech, perception, intelligent thinking grow through a series of stages, though we will have to reconsider later the question of whether, likewise, there are critical periods of optimum sensitivity (Chapter XII).
