During the process of studying the abundant evidence of life's progress from the simple to the complex, it becomes also apparent that it is affected by forces other than heredity. Recognition of the ever-present influence of these potent but often disregarded forces makes for harmonious living, whereas their neglect is associated with heavy penalties. I refer to the capacity for individual variation from the racial type; to the modification of each individual by his or her surroundings; and to the personal predisposition, technically described as diathesis, which influences the reaction made to every form of stimulus. Of these three forces, the first is the result of an inborn tendency to deviate from the ancestral type; an orderly process with a definite intention, by no means a mere chance fluctuation. This certainly makes for progress as well as for interest in life, though it enhances the difficulties of education, because it demands the adaptation of conditions to each individual's requirements. The second, the law of modification, takes into account the influence of environment upon inherited nature; the effects of climate, and food, for example, or of forms of occupation. Predisposition is, of course, a personal quality--a factor of primary importance in our susceptibility to or power to resist disease or in our capacity to withstand adverse conditions. This property is responsible for the greater or less degree of adaptability to new conditions possessed by each of us, and is concerned with our power to live in tune or at discord with our surroundings.