ganism in the body of a susceptible animal accumulate during the attack and are subsequently retained, and being prejudicial to the growth of the particular micro-organism which produced them, a second infection can not occur. Support for this theory has been found by its advocates in the fact that various processes of fermentation are arrested after a time by the formation of substances which restrain the development of the micro-organisms to which they are due. But in the case of a living animal the conditions are very different, and it is hard to conceive that adventitious products of this kind could be retained for years, when in the normal processes of nutrition and excretion the tissues and fluids of the body are constantly undergoing change. Certainly the substances which arrest ordinary processes of fermentation by their accumulation in the fermenting liquid, such as alcohol, lactic acid, phenol, etc., would not be so retained. But we can not speak so positively with reference to the toxic albuminous substances which recent researches have demonstrated to be present in cultures of some of the best-known pathogenic bacteria. It is difficult, however, to believe that an individual who has passed through attacks of half a dozen different infectious diseases, carries about with him a store of as many different chemical substances produced during these attacks, and sufficient in quantity to prevent the development of the several germs of these diseases. Nor does the experimental evidence relating to the action of germicidal and germ-restraining agents justify the view that a substance capable of preventing the development of one micro-organism should be without effect upon others of the same class; but if we accept the retention hypothesis, we must admit that the inhibiting substance produced by each particular pathogenic germ is effective only in restraining the development of the microbe which produced it in the first instance.
Moreover, if we suppose that the toxic substances which give pathogenic power to a particular micro-organism are retained in the body of an immune animal, we must admit that the animal has acquired a tolerance to the pathogenic action of these toxic substances, for their presence no longer gives rise to any morbid phenomena. And this being the case, we are not restricted to the explanation that immunity depends upon a restraining influence exercised upon the microbe when subsequently introduced.
Another explanation offers itself, viz., that immunity depends upon an acquired tolerance to the toxic products of pathogenic bacteria. This is a view which the writer has advocated in various published papers since 1881. In a paper contributed to the American Journal of the Medical Sciences in April, 1881, it is presented in the following language: "The view that I am endeavor-
