One of the first things to be proved by the law of substance is the basic fact that any natural force can be directly or indirectly converted into any other. Mechanical and chemical energy, sound and heat, light and electricity, are mutually convertible; they seem to be but different modes of one and the same fundamental force or _energy_. Thence follows the important thesis of the unity of all natural forces, or, as it may also be expressed, the "monism of energy." This fundamental principle is now generally recognized in the entire province of physics and chemistry, as far as it applies to inorganic substances. It seems to be otherwise with the organic world and its wealth of color and form. It is, of course, obvious that a great part of the phenomena of life may be immediately traced to mechanical and chemical energy, and to the effects of electricity and light. For other vital processes, however, especially for psychic activity and consciousness, such an interpretation is vigorously contested. Yet the modern science of evolution has achieved the task of constructing a bridge between these two apparently irreconcilable provinces. We are now certain that all the phenomena of organic life are subject to the universal law of substance no less than the phenomena of the inorganic universe. The unity of nature which necessarily follows, and the demolition of the earlier dualism, are certainly among the most valuable results of modern evolution. Thirty-three years ago I made an exhaustive effort to establish this "monism of the cosmos" and the essential unity of organic and inorganic nature by a thorough, critical demonstration, and a comparison of the accordance of these two great divisions of nature with regard to matter, form, and force.[29] A short epitome of the result is given in the fifteenth chapter of my _Natural History of Creation_. The views I put forward are accepted by the majority of modern scientists, but an attempt has been made in many quarters lately to dispute them and to maintain the old antithesis of the two divisions of nature. The ablest of these is to be found in the recent _Welt als That_ of the botanist Reinke. It defends _pure cosmological_ dualism with admirable lucidity and consistency, and only goes to prove how utterly untenable the teleological system is that is connected therewith. According to the author, physical and chemical forces alone are at work in the entire field of inorganic nature, while in the organic world we find "intelligent forces," regulative or dominant forces. The law of substance is supposed to apply to the one, but not to the other. On the whole, it is a question of the old antithesis of a mechanical and a teleological system. But before we go more fully into it, let us glance briefly at two other theories, which seem to me to be of great importance in the decision of that controversy--the carbon-theory and the theory of spontaneous generation.