The exhaustive comparison of the growth of crystals and monera (as the simplest forms of unnucleated cells) is important, because it shows the possibility of tracing the vital function of reproduction--which had usually been regarded as a quite special "wonder of life"--to purely physical conditions. The division of the growing individual into several young ones must necessarily take place when the natural limit of growth has been passed, and when the chemical composition of the growing body and the cohesion of its molecules allow no further enlargement by the assumption of new matter. In order to illustrate the limit of this transgressive growth by a simple physical example, Ostwald imagines a ball placed in a small flat basin, built up high on one side. The ball is in a state of equilibrium in the basin; when it is lightly pushed aside it always returns to its original position. But when the push goes beyond a certain point, and the ball is thrust over the side of the basin, the balance is lost; the ball does not return, but falls to the ground. The crystal behaves just in the same way in a supersaturated solution when it exercises its power of forming new crystals; and it is just the same with the bacterium growing in a nutritive fluid when it passes the limit of its volume of growth, and divides into two individuals. As we can find no morphological and little physiological difference between the living and non-living, we must look upon metabolism as the chief characteristic of organic life. This process causes the conversion of food into plasm; it is determined by the vital force itself, and is the formation of new living matter. It thus effects the nutrition and growth of the living being, and therefore its reproduction, which is merely transgressive growth. As I shall describe this metabolism fully in the tenth chapter, I will do no more here than emphasize the fact that this vital process also has analogies in inorganic chemistry, in the curious process of catalysis, especially that form of it which we call fermentation. The distinguished chemist Berzelius discovered in 1810 the remarkable fact that certain bodies, by their mere presence, apart from their chemical affinity, set other bodies in decomposition or composition without being themselves affected. Thus, for instance, sulphuric acid changes the starch in sugar without undergoing any alteration itself. Finely ground platinum brought in contact with hydrogen-superoxide divides it into hydrogen and oxygen. Berzelius called this process _catalysis_; Mitscherlich, who discovered the cause of it to be the peculiar surface-action of many bodies, gave it the name of "contact-action." It was afterwards discovered that catalysis of this kind is very general, and that a special form of it--fermentation--plays an important part in the life of organisms.