Mithridates, King of Pontus, was, according to ancient legend, in consequence of his studies and experiments, soaked with all kinds of poisons to which he had become habituated by gradually increasing doses, and he had at last reached a condition in which no poison could harm him, so that when he was captured by the Romans and wished to kill himself (which was the correct thing in those days for a fallen king to do), he wept because he was unable to get any poisons which would act upon him. He was "immune" to all poisons. This real or supposed immunity resulting from the introduction into the living body at intervals of a series of doses of a poison gradually increasing strength has been called "Mithridatism," and animals and men so treated have been said to be "mithradatized." The toleration of poisonous drugs--such as tobacco and alcohol, and even of mineral poisons, such as arsenic--was, until lately, regarded as merely a special exhibition of that habituation of "adaptation by use" which living things often show in regard to some of the conditions of their life. Unusual cold, unusual heat, unusual moisture, salinity or the reverse, unusual deprivation of food, unusual muscular effort may be tolerated by animals without injury provided that they have been "gradually accustomed" to the unusual thing, or, in other words, that the unusual has been gradually made the usual; so that there is a saying that eels after a time even get used to being skinned. There was no attempt to explain the details of this process of habituation; it was assumed to be a part of the general "educability" of living matter. The study of the education of living matter, in regard to various conditions which can act upon it, has yet to be further carried out, but the way in which the poisons made by disease germs and the like, and the disease germs themselves, are dealt with in the blood and tissues has, on account of its urgent importance, from a medical point of view, been already profoundly studied by experimental and microscopic methods of late years. The old notion as to "mithridatism" was that an animal or a man would have to be separately prepared and "immunised" by habituation for every distinct kind of poison. We now know that this is not the usual way in which Nature confers immunity to poisons. Most astonishing, and at first sight magical or mysterious, powers exist in the living protoplasmic cells in and around the blood of man and higher animals, which enable their possessors to resist and combat the poison-producing microbes, and also the poison itself, of all kinds, by which the race is liable to be attacked.