Though our knowledge of the subject is still incomplete and often tentative, much progress has been made, for instance, in a correct conception of the means by which the physiological balance of human life is adjusted, since Metchnikoff[115] drew attention to the interference brought about in man's normal development by certain fundamental disharmonies in his constitution, of which the end is premature death, if not a pathological old age. It is quite evident that unjustifiable encroachments upon the reserve powers of the human body have been commonly permitted hitherto, and though each year brings fresh proof of the extraordinary endowment which it possesses to respond to the demands made upon it, yet each year also confirms the conviction that this reserve fund must in future be husbanded and used with economy. When these powers are constantly drawn upon the body is necessarily reduced to a lower level of health. If the metaphor be employed of the body as a building in course of erection, it becomes obvious that if one of a group of converging thrusts be much weakened or withdrawn, a skilful rearrangement of forces may meet the strain, but the total strength of the structure is reduced. In how many cases has the temple of a child's body been permanently damaged by such withdrawals, or how many adolescents are launched into life with their capital of health seriously diminished by premature calls upon its resources. The duty to maintain so far as possible a condition of physiological equilibrium in ourselves and in our children amounts to an obligation; for which reason health promotion during the plastic period of early life assumes a new importance. Of course, a certain capacity for vicarious activity is associated with the various organs of the body in order to maintain their functions against temporary failure. Healthy tissues are furnished with power to respond to increased call for exertion. How often are they most sorely abused and unwisely taxed? Even now, when made aware of these facts, we are slow to apply to the conduct of life the lessons thus taught us, and continue to be filled with self-commiseration for the results to our bodies of overtaxing their capacity for accurate readjustment. It is not possible, much less desirable, that the whole population should plunge into amateur studies of recent physiological advance, nor even that it should dabble, as its units are too much disposed to do, in pseudo-scientific pathological publications. But it is both possible and desirable for all who assume the direction of their own lives or those of children to "read, mark, learn, and inwardly digest" some fruits of the labours of others in the garden of health.