the mysterious attraction and repulsion in the sphere of electricity and magnetism, occur universally in the vegetable as well as in the animal world. These movements are of the greatest biological importance for the obtaining of food, propagation, protection against disease, etc. Botanists have long known of the geotaxis of the roots and stems of plants, the heliotaxis of their leaves and flowers and of the thigmotaxis of their tendrils. Likewise the phototaxis of freely moving protistæ had been often observed, especially by _Ehrenberg_[10] of Berlin, well known for his researches on infusoria. Then _Engelmann_, _Pfeffer_, _Strassburger_, _Stahl_, and many others discovered and studied more carefully the facts concerning chemotaxis, thigmotaxis, rheotaxis, geotaxis, phototaxis, etc., of bacteria, motile spores, rhizopoda, and so on. The question arose if one should regard this singular behavior of the unicellular organisms as an expression of conscious sensations, discrimination or will. This view was as determinedly denied on the one hand as it was accepted on the other. Whilst even today certain scientists still consider the reactions of the unicellular organisms as a manifestation of conscious sensation, discrimination or will, others look upon them as unconscious reflex reactions of cell organism, taking place as purely mechanically as the spinal cord reflexes of vertebrates. This divergence of opinion would have practically no value for the development of our knowledge of irritability had not here, as in the case of the relations between the mental and physical processes in man, the view been entertained with more or less fervor, that at some stage or other in the chain of the purely physiological processes of responsivity, an intangible factor had been introduced which was considered as the essential “cause” of the peculiar reactions to stimuli. It is not here the place to enter into the question if, and in what degree, animal psychology may be a field of scientific research. Even if one looks upon conscious processes as effects of stimulation, in both lower animals and in man, in no case should one assume them to be factors of an essentially different nature, interrupting the chain of the mechanical reactions; neither should one consider the particular characteristic responses observed in unicellular organisms as effects of non-mechanical “causes.” As a result, a mysticism, in reality quite foreign to it, would be introduced into physiology. As a matter of fact the physiological investigations for the tropic reactions of stimuli, which have been carried out in great number since the end of the eighties, have shown more and more clearly that this peculiar behavior of unicellular organisms towards unilateral stimuli is produced by a comparatively simple mechanism. The analysis of this shows a difference in the intensity of the exciting or depressing effect produced by the stimulus.