"Firstly, the exercise of life, and consequently of organic movement, constitutes its activity, tends, without ceasing, not only to develop and to extend the organization, but it tends besides to multiply the organs and to isolate them in special centres (_foyers_). To make sure whether the exercise of life tends to extend and develop the organization, it suffices to consider the state of the organs of any animal which has just been born, and to compare them in this condition with what they are when the animal has attained the period when its organs cease to receive any new development. Then we will see on what this organic law is based, which I have published in my _Recherches sur les Corps vivans_ (p. 8), _i.e._, that-- "'The special property of movement of fluids in the supple parts of the living body which contain them is to open (_frayer_) there routes, places of deposit and tissues; to create there canals, and consequently different organs; to cause these canals and these organs to vary there by reason of the diversity both of the movements as well as the nature of the fluids which occur there; finally to enlarge, to elongate, to divide and to gradually strengthen (_affermir_) these canals and their organs by the matters which are formed in the fluids in motion, which incessantly separate themselves, and a part of which is assimilated and united with organs while the rest is rejected.' "Secondly, the continual employment of an organ, especially if it is strongly exercised, strengthens this organ, develops it, increases its dimensions, enlarges and extends its faculties. "This second law of effects of exercise of life has been understood for a long time by those observers who have paid attention to the phenomena of organization. "Indeed, we know that all the time that an organ, or a system of organs, is rigorously exercised throughout a long time, not only its power, and the parts which form it, grow and strengthen themselves, but there are proofs that this organ, or system of organs, at that time attracts to itself the principal active forces of the life of the individual, because it becomes the cause which, under these conditions, makes the functions of other organs to be diminished in power. "Thus not only every organ or every part of the body, whether of man or of animals, being for a long period and more vigorously exercised than the others, has acquired a power and facility of action that the same organ could not have had before, and that it has never had in individuals which have exercised less, but also we consequently remark that the excessive employment of this organ diminishes the functions of the others and proportionately enfeebles them.