Our monistic system (whether it be taken as energism or materialism, or more correctly as hylozoism) regards all substance as having "soul"--that is to say, endowed with energy. In the chemical analysis of organisms we do not find any elements that are not found in inorganic nature; we find that the movements in organisms obey the same laws of mechanics as the latter; we believe that the conversion of energy in the living matter occurs in the same way, and is provoked by the same stimuli, as in inorganic matter. We are forced to conclude from. these experiences that the perception of stimuli--sensation in the objective and feeling in the subjective sense--is also generally present in the two. All bodies are in a certain sense "sensitive." It is just in this dynamic conception of substance that monism differs essentially from the materialistic system, which regards one part of matter as "dead" and insensitive. In this we have the best means of joining consistent materialism or realism with consistent spiritualism or idealism. But, as a first condition of such a union, we must demand a recognition that organic life is subject to the same general laws as inorganic nature. In both cases the outer world acts alike as a stimulus on the inner world of the body. We can easily see this if we glance at the various kinds of sensation which correspond to the various kinds of stimuli. Light and heat, external and internal chemical stimuli, pressure and electricity, cause analogous sensations and modifications in their effect on organic and inorganic bodies. The effect which the light-stimulus has on living matter, the sensation of light that results, and the chemical changes of energy that follow, are of great physiological importance in all organisms. We might even say that sunlight is the first, oldest, and chief source of organic life; all other exertions of force depend in the long run on the radiant energy of sunlight. The oldest and most important function of plasm--one which is at the same time a cause of its formation--is carbon-assimilation; and this plasmodomism is directly dependent on sunlight. If it acts in a one-sided way, it causes the particular form of stimulation which we call phototaxis or heliotropism. This is of a positive character--that is to say, they turn towards the source of the light--in the great majority of organisms, both protists and histona. Everybody knows that flowers that are growing in the window of a room turn to the light. However, many organisms which have grown accustomed to living in the dark are heliotropically negative; they shun the light and seek darkness, such as the fungi, many lucifugous mosses and ferns, and many deep-sea animals.