means a new problem. In this lies the inexhaustible charm of our scientific research. CHAPTER VIII INTERFERENCE OF EXCITATIONS _Contents_: Examples of effects of interference of stimuli in unicellular organisms. Interference of galvanic and thermic stimuli in Paramecia. Interference of galvanic and thermic stimuli and narcotics. Interference of galvanic and mechanical stimuli. Interference of galvanotaxis and thigmotaxis in Paramecia and hypotrii infusoria. Real or homotop interference, apparent or heterotop interference. The two effects of homotop interference of excitations: Summation and inhibition of excitations. Theory of the processes of inhibition. _Hering-Gaskell_ theory. Inhibition as an expression of the refractory period. Individual possibilities of interference of two stimuli. Interference of an excitating and a depressing stimulus. Interference of two depressing stimuli. Interference of two excitating stimuli. Analysis of the interference of two excitations. Interference of two single stimuli. Conditions upon which the result of interference is dependent. Heterobole and isobole living systems. Intensity of the two stimuli. Interval between the stimuli. Specific irritability and rapidity of reaction of the living system. Latent period. Interference of single stimuli in a series. General scheme of the development of the effect of interference. Summation and inhibition. Apparent increase of irritability. Conditions of summation. Tonic excitations. Conditions of inhibition. Various types of inhibition. Interference of two series of stimuli. Relations in the nervous system. Peculiarities of the nerve fibers. Conversion of the nerve by relative fatigue from an isobolic into a heterobolic system. Until now the mechanism of the single excitation has received the major portion of our attention. It was not until we reached the subject of the origin of fatigue that we became acquainted with the effects of repeated stimulation. Here we found a case of interference of individual excitations. But fatigue is simply a special instance of such interference, for the subject of interference action occupies a much greater field. Every cell of the larger organisms, and more especially the single celled organisms, is subjected to manifold stimuli. It is indeed, quite common that two stimuli interfere with each other and manifold effects follow, depending upon the specific reaction of the cell and the quality, intensity and duration of the interfering stimuli. Sometimes the interference effect is readily understandable from a knowledge of the specific effect of the individual stimuli concerned. At other times, however, the specific reaction seems entirely different in nature than would be expected from a study of the effects of the individual stimuli. [Illustration: Fig. 39. Galvanotaxis of Paramaecium aurelia.]