A property of this system should receive our consideration at this point. It is a characteristic of every system in the world, namely, the fact that a system _is not isolated_ from its surroundings. It is a deception resulting from the selective action of our sensory organs, if we consider the bodies as separated and isolated from their environment. This deception disappears upon further analysis and when we assist our organs of sense, which only respond to certain parts of the whole process, by experimental methods of investigation. Our experience then shows us that an isolated system does not exist, but that there are instead everywhere connections which extend further and further into the infinity of the world. An organism is consequently no delimitated system and the vital process cannot, therefore, be sharply separated from the processes in the medium. We cannot draw a sharp line between vital processes and say: on the right we have factors which are necessary for the maintenance of life, and on the left factors which are not necessary. The conditional connection between individual processes extends to the entire world, and likewise a great series of constituents, each influencing the others, extend from the medium into the organism. The nature of our sense perception, and consequently the knowledge derived therefrom, is such that we are obliged to arbitrarily take into consideration merely _fragments_ from the endless interdependence of all things in the world, and so we separate the vital conditions of the organisms from their surrounding factors, as though they were independent. A conscientious theoretical analysis requires that we should never forget that in reality such an isolation does not exist. Only with the recognition of this can we distinguish for practical purposes between _internal_ and _external_ vital conditions. In such a differentiation the _internal vital conditions_ which compose the living system conceived to be isolated, are the organs, the tissues, the cells, the protoplasm and the cell nucleus, and within the protoplasm and the nucleus the arrangement and quantitative relations of certain substances, such as proteins, salts, water and the thousands of special components with their interactions and continued alterations. On the other hand, the _external vital conditions_, which act on the periphery, are the conditions of the surrounding medium, as foodstuffs, water, oxygen, static and osmotic pressure, temperature, light, etc. But this distinction has only a _practical_ value for the study of the organism as an _independent_ system. Theoretically it is as impossible to make a sharp distinction between internal and external vital conditions, as to distinguish between the vital conditions generally and the more remote conditions of the environment. All these conditions form a widely branching system of factors of which one is conditioned by the other reaching