Darwin's hypothesis sought to explain the origin of animal species by survival under competitive conditions of existence through the possession of a structure suited to the environment. Only the most elementary organism need be presupposed, together with slight variations in the course of subsequent generations, and both may be conceived to arise mechanically. There will then result in surviving organisms a gradual accumulation of such variations as promote survival under the special conditions of the environment. Such a principle had been suggested as early as the time of Empedocles, but it remained for Darwin to establish it with an unanswerable array of observation and experimentation. If any organism whatsoever endowed with the power of generation be allowed to have somehow come to be, naturalism now promises to account for the whole subsequent history of organic phenomena and the origin of any known species. [Sidenote: Mechanical Physiology.] § 113. But what of life itself? The question of the derivation of organic from inorganic matter has proved insoluble by direct means, and the case of naturalism must here rest upon such facts as the chemical homogeneity of these two kinds of matter, and the conformity of physiological processes to more general physical laws. Organic matter differs from inorganic only through the presence of proteid, a peculiar product of known elements, which cannot be artificially produced, but which is by natural means perpetually dissolved into these elements without any discoverable residuum. Respiration may be studied as a case of aerodynamics, the circulation of the blood as a case of hydrodynamics, and the heat given off in the course of work done by the body as a case of thermodynamics. And although vitalistic theories still retain a place in physiology, as do teleological theories in biology, on the whole the naturalistic programme of a reduction of organic processes to the type of the inorganic tends to prevail. [Sidenote: Mind. The Reduction to Sensation.]