It only remains to refer to several other recent researches. The most important of many are those published by M. Chauveau, on the reciprocal transformation of the immediate principles in the organism according to the conditions of its functioning and the circumstances of its activity. To deal with these researches with as much detail as they deserve, we must study the physiology of muscular contraction and of movement—that is to say, of muscular energetics. BOOK III. THE CHARACTERS COMMON TO LIVING BEINGS. Chapter I. Summary: The doctrine of vital unity.—Chapter II. The morphological unity of living beings.—Chapter III. The chemical unity of living beings.—Chapter IV. The mutability of living beings.—Chapter V. The specific form, its acquisition, and reparation.—Chapter VI. Nutrition. CHAPTER I. THE DOCTRINE OF VITAL UNITY. Phenomena common to all living beings—Theory of vital duality—Unity in the formation of immediate principles—Unity in the digestive acts—The common vital fund. When we ask the various philosophical schools what life is, some show us a chemical retort, and others show us a soul. Whether vitalists or of the mechanical school, these are the adversaries who since philosophy began have vainly contested the possession of the secret of life. We need not concern ourselves with this eternal quarrel. We need not ask Pythagoras, Plato, Aristotle, Hippocrates, Paracelsus, Van Helmont, and Stahl what idea they formed of the vital principle; nor need we probe to the depths the ideas of living nature held by Epicurus, Democritus, Boerhaave, Willis, and Lamettrie; nor need we apply to the iatromechanicians nor to the chemists. We may do better than that. We may ask nature itself.