The most general result of research in physiological chemistry from the time of Lavoisier down to our own day has been to teach us that _the antecedent of the vital phenomenon is always a chemical phenomenon_. The vital energies are derived from the potential chemical energy accumulated in the immediate constituent principles of the organism. In the same way _the consequent phenomenon of the vital phenomenon is in general a thermal phenomenon_. The final form of vital energy is thermal energy. These three assertions as to the nature, the origin, and the final form of vital phenomena constitute the three fundamental principles, the three laws, of biological energetics. _Food, a Source of Heat. It is not quâ source of heat that food is the source of vital energy._—The place of vital energy in the cycle of universal energy is completely determined. It lies between the chemical energy which is its generating form and the thermal energy which is its form of disappearance, of breakdown, the “degraded form,” as the physicists say. Hence we have a result which can be immediately applied in the theory of food—namely, that heat is in the dynamical order an excretum of the animal life rejected by the living being, just as in the substantial order, urea, carbonic acid and water, are the materials used up and again rejected by it. We therefore must not think of the transformation in the animal organism of heat into vital energy, as certain physiologists always do. Nor must we think, with Béclard, of its transformation into muscular movement; or, as others have maintained, into animal electricity. This is not only an error of doctrine but an error of fact. It proceeds from a false interpretation of the principle of the mechanical equivalent of heat and a misunderstanding of Carnot’s principle. Thermal energy does not repeat the course of the energetic flux in the animal organism. The heat is not transformed into anything. It is simply dissipated. _The Part played by Animal Heat as a Condition of Physiological Manifestations._—Does this mean that heat is useless to life in the very beings in which it is most abundantly produced—_i.e._, in man and in the warm-blooded vertebrates? So far from this being so, it is necessary to life. But its utility has a peculiar character which must neither be misunderstood nor exaggerated. It is not transformed into chemical or vital reactions, but merely creates for them a favourable condition.