But, first of all, there are three great discoveries which have caused our knowledge of the interdependence of the processes of nature to progress by leaps and bounds. In the first place, the discovery of the cell, as the unit, from the multiplication and differentiation of which, the whole of plant and animal substance develop so that not only the growth and development of all higher classes of all higher organisms is recognized as following a universal law, but the very path is shown in the capacity for differentiation in the cell, by which organisms are enabled to change their forms and make thereby a more individual development. Secondly, the metamorphosis of energy which has shown us that all the so-called real forces in inorganic nature, the mechanical forces and their complements, the so-called potential energies, heat, radiation (light, radiating heat), electricity, magnetism, chemical energy, are different forms of universal motion, which pass, under certain conditions, the one into the other, so that in place of those of the one which disappear, a certain number of the other appear, so that the whole movement of nature is reduced to this perpetual process of transformation from one into the other. Finally, the proof first developed logically by Darwin, that the organic products of nature about us, including man, are the result of a long process of evolution, from a few original single cells, and these again, by virtue of chemical processes, have proceeded from protoplasm or white of egg.