In contrast to those reactions to stimuli, which consist merely in the changes of a rapidity of the specific vital process, are another group of reactions in which the influence of stimuli leads to qualitative alterations in the specific vital process. In these instances, the influence of the stimulus directs the metabolism of rest into new channels, so that chemical processes occur in the cell, which under ordinary circumstances do not take place. This group of reactions, which I wish to term “metamorphic stimulation and response,” are chiefly observed where weak stimuli act continuously upon the living substance. These are essentially weak chemical stimuli, which last for a prolonged period or frequently reoccur in the life of the cell community. Examples of this are found in the continual ingestion of alcohol and other poisons by the human being, or in the formation of metabolic products of bacteria, etc. The majority of _chronic_ diseases belong to this group of reactions; disease being simply response to stimulation. Disease is life under altered vital conditions and altered vital conditions are stimuli. This simple and self-evident fact shows the immense importance which the knowledge of the general laws of the physiology of stimulation has for pathology. The pathologist, who does not wish to confine his observations to a purely superficial symptomatology or a merely histological morphology, must seek above all to penetrate as deeply as possible into the nature of the general reactions to stimulation in the living organism. It is the essential point which meets him everywhere. In spite of their great interest for pathology, however, it is just these qualitative alterations of the normal vital process produced by continuous stimulation which have up to now been least analyzed. In this field we expect much from pathological investigation which alone has the immense amount of material at its command. This will take place only when pathology adds to the almost exclusively histological direction of investigation, that also of experimental physiology. It is true that the problems of the qualitative alterations of a vital process by chronic stimulation are much more complicated than those of the rapid responses to temporary stimuli, consisting simply in mere alterations of rapidity of the specific vital process. An understanding of the nature of the former can only be expected when a deeper knowledge of the latter is gained, for, as will be seen presently, there is the closest relation between the two groups.