the microbes and the phagocytes is always prolonged, and may last for years. Victory is decided for or against the phagocytes accordingly as Natural Selection has developed in them or in the micro-organisms the greater personal vigour. When the personal vigour of both parties is evenly balanced the result of the combat is decided by circumstances, which raise or depress the vitality of the phagocytes, such as an improvement or deterioration in the blood plasma, their nutrient fluid. In the later stages of disease this must be so much deteriorated that the phagocytes are at the greatest disadvantage.
As regards relapsing fever, the phagocytes quickly react to the stimulation of the toxins and destroy the bacilli, but the acquired power is soon lapsed, hence the relapses that occur in this disease, and also the fact that one attack does not afford immunity against subsequent attacks. Here also we have therefore nothing to hope from protective inoculation. As, however, the disease only attacks individuals whose vitality has been reduced to a low ebb (by starvation, for instance), it appears that the untrained phagocytes of the healthy body are fully able to cope with the bacilli. It is probable that during the intervals of convalescence which alternate with the relapses, the pathogenic organisms are not entirely absent from the infected body, but that some of them at least are present as harmless but highly resistant spores, which regain their pathogenic but lose their resistant properties when the relapse occurs—i.e. when the phagocytes lapse their acquired powers of tolerating the toxins.
Of late Behring and others have advanced the theory, that acquired immunity is due to the production by the host of anti-toxins, that is, of chemical substances which neutralize or destroy the toxins, and which presumably persist in the blood and tissues of the host as long as the immunity lasts. It is, however, highly improbable, not
