The forces employed in the elaboration of the human being, differ in their manifestation at various stages of its growth. There are two marked forces to be noted, often confounded together, but important to distinguish--viz., the power of growth and the power of development, the former possessed throughout life, the latter at certain epochs only. The capacity for _growth_ and nutrition, by means of which the human frame is built up and maintained out of the forces derived from food and other agents, is shown until the last breath of life, by the power of repair, which continues as long as the human being lives. All action of the organism, every employment of muscular or nervous tissue, uses up such tissue. The body is wasted by its own activities, and it is only by the exact counterpoise of these two forces--disintegration and repair--that health and life itself are maintained. In youth, in connection with very rapid waste of tissue, exists a great excess of formative power, which excess enables each complete organ to enlarge and consolidate itself. The reduction of this excess of formative power to a balance with the waste of tissue, marks the strength of adult life. Its diminution below the power of repair marks the decline of life. The force of _development_, however, is shown, not in the enlargement and maintenance of existing parts, but in the creation of new tissues or organs or parts of organs, so that quite new powers are added to the individual. After birth these remarkable efforts of creative force belong exclusively to the youth of the individual. They are chiefly marked by dentition, by growth of the skeleton and the brain, and still more by the addition of the generative powers. With this work of development the adult has nothing to do; it is a burden laid especially upon the young: it is a work as important and exclusively theirs, as child-bearing is the exclusive work of the mother. One of the first lessons, then, that Physiology teaches us in relation to the healthy growth of the human being, is the slow and successive development of the various faculties. Although the complete type of the future man exists potentially in the infant, long time and varying conditions are essential to its establishment, and the type will never be attained, if the necessary time and conditions are not provided.