What conclusions can be drawn from these instances of response to stimuli, of which any number could still be quoted? They show us, first of all, that a state or process existing under given conditions, is altered by the influence of the stimulus. This is a fact, however, which could be expected from the beginning and is self-evident, for stimuli are alterations in the vital conditions, and when these are altered the state of the system or the happenings thereof must also alter. The question with which we are here more closely concerned, however, is a somewhat more detailed characterization of the state or process itself, as well as that of alterations produced by the influence of the stimulus. The instances of response to stimuli already cited furnish us with information in both kinds. In all these examples, the living processes occur with equal constancy and unaltered rapidity, provided a stimulus is not operative. Here, however, the gradual alterations, the result of development, must not be overlooked. An excellent example of this is seen in the eggs of sea urchin, where the development is readily perceptible. In all these instances, however, the condition is immediately changed by the influence of the stimulus. The previous state of constancy in the vital process is disturbed. The rapidity of its course is changed, being either increased or decreased, and the specific vital manifestations concerned are, therefore, augmented or diminished. We will now study the vital process with the methods of chemical investigation and consider the problem from the standpoint of metabolism. It may be noted here, that other methods, such as the transformation of energy or changes of form of the living system, would serve equally well as indicators for this purpose. In every instance there is a uniformity of the processes; the difference, however, is in the nature of the indicators and the terms used. The methods and the terms used in chemical investigation and description reach proportionately much deeper than those employed when the transformation, energy or the variations of form of the organisms are studied, and permit of the finest differentiation of the processes. The atomistic terminology is, for this reason, preëminently fitted for the description of vital processes. When we study the vital process metabolically, we can, as shown in the above-mentioned instance, divide the processes into a _metabolism of stimulation_ in contradistinction to a _metabolism of rest_.