The most recent of these hypotheses is that set forth by Verworn in his book “Die Biogenhypothese.”(60) He assumes, as the central vehicle of the vital functions, a unified living substance, the “biogen,” nearly related to the proteids which form the fundamental substance of protoplasm and of the cell-nucleus, and in contrast to which the other substances found in the living body are in part raw materials and reserves, and in part of a derivative nature, or the results of disruptive metabolism. Very complex chemically, “biogen” is able to operate upon the circulating or reserve “nutritive” materials in a way comparable, for instance, to the action of “nitric acid in the production of English sulphuric acid.” That is to say, it is able to set up processes of disruption and of recombination, apparently by its mere presence, but, in reality, by its own continual breaking down and building up again. At the same time it has the power, analogous to that of polymerisation in molecules, of increasing, of “growing.” The case is the same in regard to physical laws. They are identical in the living and the non-living. And many of the processes of life have already been analysed into a complex of simpler physical processes. The circulation of the blood is subject to the same laws of hydrostatics as are illustrated in all other fluids. Mechanical, static, and osmotic processes occur in the organism and constitute its vital phenomena. The eye is a _camera obscura_, an optical apparatus; the ear an acoustic instrument; the skeleton an ingenious system of levers, which obey the same laws as all other levers. E. du Bois-Reymond, in his lectures on “The Physics of Organic Metabolism” (“Physik des organischen Stoffwechsels”),(61) compiles a long and detailed list of the physical factors associated and intertwined in the most diverse ways with the fundamental phenomenon of life, namely, metabolism:—the capacities and effects of solution, diffusion of liquids, capillarity, surface tension, coagulation, transfusion with filtration, the capacities and effects of gases, aero-diffusion through porous walls, the absorption of gases through solid bodies and through fluids, and so on. Very impressive, too, are the manifold “mechanical” interpretations of intimate vital characteristics, such as the infinitely fine structure of protoplasm. For protoplasm does not fill the cell as a compact mass, but spreads itself out and builds itself up in the most delicate network or meshwork, of which it forms the threads and walls, enclosing innumerable vacuoles and alveoli, and Bütschli succeeded in making a surprisingly good imitation of this “structure” by mechanical means. Drops of oil intimately mixed with potash and placed between glass plates formed a very similar emulsion-like or foam-like structure with a visible network and with enclosed alveoli.(62)