There is only one class of physiological processes in which the type of the real secondary regulation occurs. The discoveries of the last twenty years have proved beyond all doubt, and future discoveries will probably prove even more conclusively, that the so-called *immunity* against diseases is but one case out of numerous biological phenomena in which there is an adaptive correspondence between abnormal chemical stimuli and active chemical reactions on the part of the organism and in its interior, exceeding by far everything that was formerly supposed to be possible in organic regulation. The adaptive faculty of the organism against inorganic poisonous substances[110] is but small comparatively, and is almost always due not to a real process of active regulation but to the action of substances pre-existing in the organism--that is, to a sort of adaptiveness but not adaptation. Metallic poisons, for instance, may be transformed into harmless compounds by being combined with albumen or sulphuric acid and thus becoming insoluble, or free acids may be neutralised, and so on; but all these processes go on to a certain extent only, and, as was mentioned already, are almost always the result of reactions with pre-existing materials. Only in a few cases is there any sort of true adaptation to metallic substances, such as sublimate and, in a very small degree, arsenic, comparable in some respects with the adaptation to abnormally high temperatures. The organism which has been accustomed to receive at first very small amounts, say, of sublimate, and then receives greater and greater amounts of this substance by degrees, will at the end of this treatment be able to stand a quantity of the poison that would have been instantly fatal if administered at the first dose.[111] But the explanation of this adaptation is not known in any case; there seems to be some similarity between it and the so-called histogenetic immunity against organic poisons. [110] A good review is given by E. Fromm, *Die chemischen Schutzmittel des Tierkörpers bei Vergiftungen*, Strassburg, 1903. [111] Davenport, *Arch. Entw. Mech.* 2, 1895-1896, and Hausmann, *Pflüger’s Arch.* 113, 1906.