THE PRESENT EVOLUTION OF MAN — PHYSICAL 229 



the microbes aud 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 



