1. The whole mechanical theory is based upon a law which is not strictly biological but belongs to science in general—the law of the conservation of matter and energy. This was first recognised by Kant as a general rational concept in his “Critique” and in the “Grundlegung der Metaphysik der Naturwissenschaft,” and was transferred by Robert Mayer and Helmholtz(59) to the domain of natural science. Just as no particle of matter can come from nothing or become nothing, so no quantum of energy can come from nothing or become nothing. It must come from somewhere and must remain somewhere. The form of energy is continually changing, but the sum of energy in the universe remains invariable and constant. Therefore, it seems to follow, there can be no specific vital phenomena. The energies concerned in the up-building, growth, and decay of the organism, and the sum of the functions performed by it, must be the exact resultant and equivalent of the potential energies stored in its material substance and the co-operative energies of its environment. The particular course of transformations they follow must have its sufficient reason in the configuration of the parts of the organism, in its relations to the environment, and the like. An intervention of “vitalistic” principles, directions and so forth, would, we are told, involve a sudden obtrusion and disappearance again of energy-effects which had no efficient cause in the previous phenomena. From any point of view it would be a miracle, and in particular it would be doing violence to the law of the constancy of the sum of energy. Apart from the inherent general “instinct”—_sit venia verbo_, for no more definite word is available—which is the quiet Socius, the concealed but powerful spring of the mechanistic convictions, as of most others, this law of the conservation of energy is probably the really central argument, and it meets us again more or less disguised in what follows. The Organic and the Inorganic. 2. What is on _à priori_ grounds demanded as a necessity, or set aside as impossible, on the strength of the axiom of the conservation of energy, must be proved _à posteriori_ by investigation. It must be shown in detail that the difference between the organic and the inorganic is only apparent. And it is here that the mechanical view of life celebrates its greatest triumph.