It has been proved by experiment that, against many of those chemical substances, an artificial immunity may be given to the protozoa by accustoming them gradually. Very diluted solutions are added at first to the medium in which they live and, by gradually concentrating those solutions, an artificial immunity is conferred; the negative chimiotaxis becomes positive, allowing the protozoa to absorb and digest the poison, now become a food. _Habit_ is therefore the fundamental condition of artificial immunity; it must be that also of immunity naturally acquired. Having accidentally digested enfeebled microbes or having suffered an attack of disease, the unicellular being becomes accustomed to a stronger virus and becomes immune against it. The fact that so many unicellular beings have become thus accustomed is therefore connected with their sensitiveness and their digestion. Accordingly, sensitiveness, habit, and digestion are the fundamental factors of the mechanism of immunity in protozoa; this immunity thus indisputably belongs to the category of purely _cellular_ phenomena. Having arrived at this conclusion, Metchnikoff thought that the same mechanism of immunity must be found in other primitive and analogous cells, such as the phagocytes of multicellular beings. This was proved by a whole series of observations and by the fact that the immunity of higher animals is connected with an intense phagocytosis. In fact, as he ascended the scale of beings and studied their natural and artificial immunity, he ascertained that, in all of them, the essence of immunity, masked by the complexity of the organism, reduced itself to the _phagocytes becoming accustomed_ to noxious agents. The mechanism of immunity in protozoa could therefore really be compared with that of immunity in multicellular beings. Becoming accustomed and becoming immune are phenomena of a general order, for they can be manifested not only by animals, but also by plants. They, too, have to defend themselves against numerous diseases. Lower vegetables, such as myxomycetes (beings which stand on the limit between the animal and vegetable kingdoms), have an amoeboid phase, in which they are but a simple heap of formless protoplasm. During that stage of their life, myxomycete behave towards noxious agents exactly in the same way as unicellular beings and, like them, acquire immunity by becoming gradually accustomed.