ABSTRACT
Although it is extremely difficult to alter significantly the genetic endowment of the human species, it is easy to shape the physical and psychological expressions of each individual person. Irrespective of color and size, all human beings possess a biological plasticity which makes it possible to modify their appearance, physiological functions, and ways of life without affecting their genetic constitution. The experience of everyday life shows that environmental conditions determine to a large extent the way the genes are expressed in children, adults, and the aged. Working with experimental animals, it is easy to demonstrate that the size of the adult, its weight, its resistance to disease, as well as its longevity and certain aspects of its behavior, are profoundly influenced by the breeding conditions. From a purely deterministic point of view, the adult can be defined in terms of both his genetic heritage and his cultural heritage.
