If, then, the cells of the body-republic retain so much of their independence and individuality in health, does it not seem highly probable that they do also in disease? This is known to be the case already in many morbid processes, and their number is being added to every day. The normal activities of any cell carried to excess may constitute disease, by disturbing the balance of the organism. Nay, most disease-processes on careful examination are found to be at bottom vital, often normal to the cells concerned in them. The great normal divisions of labor are paralleled by the great processes of degeneration into fat, fibrous tissue, and bone or chalk. A vital chemical change which would be perfectly healthy in one tissue or organ, in another may be fatal. Ninety-nine times out of a hundred any group of cells acts loyally in the interests of the body; once in a hundred some group acts against them, and for its own, and disease is the result. There is a perpetual struggle for survival going on between the different tissues and organs of the body. Like all other free competition, as a rule, it inures enormously to the benefit of the body-whole. Exceptionally, however, it fails to do so, and behold disease. This struggle and turmoil is not only necessary to life--it is life. Out of the varying chances of its warfare is born that incessant ebb and flow of chemical change, that inability to reach an equilibrium, which we term "vitality." The course of life, like that of a flying express train, is not a perfectly straight line, but an oscillating series of concentric curves. Without these oscillations movement could not be. Exaggerate one of them unduly, or fail to rectify it by a rebound oscillation, and you have disease. Or it is like the children's game of shuttlecock. So long as the flying shuttle keeps moving in its restless course to and fro, life is. A single stop is death. The very same blow which, rightly placed, sends it like an arrow to the safe centre of the opposing racket, if it fall obliquely, or even with too great or too little force, drives it perilously wide of its mark. It can recover the safe track only by a sudden and often violent lunge of the opposing racket. The straight course is life, the tangent disease, the saving lunge recovery. One and the same force produces all. In the millions of tiny blows dealt every minute in our body-battle, what wonder if some go wide of the mark! CHAPTER II OUR LEGACY OF HEALTH: THE POWER OF HEREDITY IN THE PREVENTION OF DISEASE