t to explain, at least to
give a general formulation of these facts, which will serve as a framework
and guiding clue, as a "working hypothesis" for the further progress of
investigation.

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
i