It was in 1828 that the German chemist Whoeler (1800-1882) for the first time in biological history prepared an organic compound (urea) from inorganic materials--an achievement universally recognised to be of the utmost significance. As a distinguished historian of the science of chemistry puts it: "This discovery destroyed the difference which was then considered to exist between organic and inorganic bodies, viz. that the former could only be formed under the influence of vegetable or animal vital forces, whereas the latter could be artificially produced."[32] Ten years later another German, Schleider (1804-1881) propounded the cellular theory of the structure and growth of plants, a theory which was soon extended to animal organisms by Schwann (1810-1882). The publication of this famous theory was described by a contemporary as "a burst of daylight"; it indeed illuminated what had hitherto been buried in mystery and mythology--the structure and method of growth of plants and animals. It seemed to render superfluous any form of the old conception of a "vital force" to explain the phenomena of growth, if it could now be assumed that the cells automatically absorbed outside material, increased in number by the division of individuals, and built up the organism by continual repetition of this process. Schwann was also responsible for initiating a number of minute physiological investigations which led to a far more intimate knowledge of the action of nerves and muscles, and interpreted these in mechanical terms. "Investigations which were carried on with all the resources of modern physics regarding the phenomena of animal movements, gradually substituted for the miracles of the 'vital forces' a molecular mechanism, complicated, indeed, and likely to baffle our efforts for a long time to come, but intelligible, nevertheless, as a mechanism."[33] Subsequent researches, notably of Helmholtz (1821-1895) and Meyer, lent strong support to this interpretation. The conception of the conservation of energy (an important axiom of the mechanical theory) was successfully applied by them to the economy of organisms. The organism was found not to _create_ energy, but only to contain remarkably efficient means of deriving it from materials absorbed as food. Thus animal warmth and the power of motion are originally "sunlight transformed in the organism of the plant," and afterwards appropriated by the animal. The power with which we move our limbs is as much the product of combustion as is the power of a steam engine, the only difference being that the organism is, of the two, the more efficient converter of energy.