Since our opinion as to what constitutes a higher and a lower organism is doubtless arbitrary, let us ask whether it is not possible to find an _objective_ standard by which to measure the relative perfection of the different organisms. The question must be answered in the affirmative when we take into consideration the following. Since the quantity of available free energy upon the earth is limited, the organism which transforms the energy at its disposal more completely and with the least loss into the forms of energy necessary for the function of life, must be regarded as the more perfect organism. In fact, we observe that with increasing complexity of the organisms there is for the most part also an increasing improvement in that direction, and we can therefore speak of some beings as more perfect than others. This view-point is especially significant in the evaluation of _human_ progress, appearing, as it does, as the general standard of all civilization. The perfection of the organism shows itself in relation to the outer world in the development of the _sense organs_. While a single-celled animal reacts almost exclusively to chemical, sometimes also to optical, stimuli, and receives these with the entire surface of its body, special parts of the body develop more and more toward perfection. These are the parts that respond with special ease to the appropriate stimuli, that is, react to them with an increasingly smaller expenditure of energy. Then the points at which the stimuli are received are separated from those in which the reaction occurs, and the two are connected by _conducting paths_, the nerves, in which an energy process takes place. Our present knowledge of this process still leaves much to be desired. It is a process which moves with fairly great but by no means extraordinary rapidity (about ten to thirty meters per second) along the conducting paths. At the one end of this path it is caused by actions of various kinds, chiefly that of the specific energy, for which the sense organ is developed. At the other end it discharges specific effects. There is no doubt that here we have in each instance a case of energy transformation connected with a _discharge_, that is, with the action of other energies which lie at the ends ready for change. Hence there is no equivalence between the different kinds of energy, the discharging and the discharged, mostly not even a proportional relation, although both increase and decrease simultaneously. What the form of the energy is that is propagated in the nerves is unknown. It can be either a special form which arises only under the conditions here present (just as, for example, a galvanic stream develops only under definite chemical and spacial conditions), or a special combination of known energies, as in sound and probably in light. Some day, it is likely, we shall have a more accurate knowledge of the nerve process which will solve the question.