As Bryan well says, "The efficiency of a machine depends so far as we know upon the maximum force, rate, amplitude, and variety of direction of its movements and upon the exactness with which below these maxima the force, rate, amplitude, and direction of the movements can be controlled." The motor efficiency of a man depends upon his ability in all these respects. Moreover, the education of the small muscles and fine adjustments of larger ones is as near mental training as physical culture can get; for these are the thought-muscles and movements, and their perfected function is to reflect and express by slight modifications of tension and tone every psychic change. Only the brain itself is more closely and immediately an organ of thought than are these muscles and their activity, reflex, spontaneous, or imitative in origin. Whether any of them are of value, as Lindley thinks, in arousing the brain to activity, or as Müller suggests, in drawing off sensations or venting efferent impulses that would otherwise distract, we need not here discuss. If so, this is, of course, a secondary and late function--nature's way of making the best of things and utilizing remnants. With these facts and their implications in mind we can next pass to consider the conditions under which the adolescent muscles best develop. Here we confront one of the greatest and most difficult problems of our age. Changes in modern motor life have been so vast and sudden as to present some of the most comprehensive and all-conditioning dangers that threaten civilized races. Not only have the forms of labor been radically changed within a generation or two, but the basal activities that shaped the body of primitive man have been suddenly swept away by the new methods of modern industry. Even popular sports, games, and recreations, so abundant in the early life of all progressive peoples, have been reduced and transformed; and the play age, that once extended on to middle life and often old age, has been restricted. Sedentary life in schools and offices, as we have seen, is reducing the vigor and size of our lower limbs. Our industry is no longer under hygienic conditions; and instead of being out of doors, in the country, or of highly diversified kinds, it is now specialized, monotonous, carried on in closed spaces, bad air, and perhaps poor light, especially in cities. The diseases and arrest bred in the young by life in shops, offices, factories, and schools increase. Work is rigidly bound to fixed hours, uniform standards, stints and piece-products; and instead of a finished article, each individual now achieves a part of a single process and knows little of those that precede or follow. Machinery has relieved the large basal muscles and laid more stress upon fine and exact movements that involve nerve strain. The coarser forms of work that involve hard lifting, carrying, digging, etc., are themselves specialized, and skilled labor requires more and more brain-work.