All cultures and societies contain beliefs or theories about the psychological development of their people. Persons in even the most primitive of cultures were exposed to development—the growth of human beings, other animals, plants, and trees. Their observations of growth prompted explanations. The growth of an organism proceeds through stages during which there is differentiation and integration at successively more complex levels of organization. This model of development describes the growth of animals from single cell to mature adult form and, as we will see, is useful in understanding the development of species and the psychological development of persons. The general model of development, as well as Darwin's theory of evolution, are now well established in biology and related areas. Theories of learning that rely on models of conditioning are not static in the way the type and psychiatric diagnostic models are; indeed psychologists within this tradition have stressed the openness of persons to change.