These three last-named peculiarities are therefore to be regarded as the special characteristics of the organisms with which we are acquainted on the surface of the earth in the conditions there prevailing. We need not regard them conceptually as unchangeable or irreplaceable. But the first three characteristics, namely, the stationary nature, self-supply of nutrition, and reproduction, we may regard as the _essential characteristics of organisms_. They constitute the frame within which everything must be found which we should recognize as living in the widest sense. =55. The Storehouse of Free Energy.= If we ask whence the organisms obtain the free energy which they require for the maintenance of their stationary existence, the answer is that _solar radiation_ alone furnishes this supply. Without this permanent supply the free energies upon the earth, so far as our knowledge goes, would long ago have reached a state of equilibrium, and the earth's bodies would be stable, that is, dead and not stationary and living. It is comprehensible, therefore, that machines should have evolved in the organism for _transforming the radiant energy of the sun into a permanent form_, and, as we have already learned, chemical energy is permanent, while radiant energy is an extremely transitory form of energy, that is, it changes very readily. The very fact that, owing to the change from day to night, the supply of radiant energy periodically ceases, makes the storing-up of energy for the night necessary to the existence of a form dependent upon it. Thus, we recognize in the _photochemical_ processes, that is, in the transformation of radiant energy into chemical energy, the foundation of life on earth. This work is done by the plants, which thus provide a store of free energy not only for their own needs but also for all the other organisms which possess themselves directly or indirectly of the plant-chemical supplies in order to utilize them for their individual purposes. In this manner nourishment in the widest sense is secured for all organisms, being based upon the regular supply of free energy derived from the sun. This also explains the great chemical similarity of all organisms, which could not subsist if they were not so constructed as to be able to utilize the chemical energy in the form in which it is provided by the plants.