THE HYPOTHESIS OF NEUTRALIZATION 245 



defend their lives are good enough to produce antitoxins which 

 compass their own destruction. 



410. The theory of neutralization, so simple at first sight but 

 so incredible when examined closely, is one that would naturally 

 occur to students of chemistry which to some extent all medical 

 men, and more especially all bacteriologists are by training. A 

 theory hardly less obvious and certainly more probable, is more 

 likely to find favour with students of evolution which no medical 

 men are as yet, at least by formal training. Men get * used ' to 

 opium and nicotine just as they get used to exertion or heat or 

 anything else. Opium and nicotine are toxins in a real sense. 

 No one has suggested that the power of tolerating increased doses 

 of them is due to the production of neutralizing substances, any 

 more than they have suggested that the power of tolerating heat 

 or fatigue is due to the formation of substances which chemically 

 neutralize heat or fatigue. In each case the growth of toleration, 

 like the growth of the limbs after birth, is plainly one of that large 

 class of acquired characters by means of which the higher animals 

 achieve part of their development and by which they are made so 

 adaptive. There does not seem to be any very valid reason why 

 we should place acquired immunity to disease in another category 

 and ascribe to it a miraculous origin. Certainly, antitoxins seem 

 to neutralize toxins, but it is possible to find an explanation which 

 savours less of the supernatural. 



411. The spinal cord of a rabbit in the later stages of virulent 

 rabies contains microbes and their toxins, but, presumably, little or 

 no antitoxin, for the animal, if not killed, invariably perishes. 

 But, if we kill the rabbit and dry the cord, antitoxins soon begin to 

 appear. Evidently, moreover, the quantity, or quality, or both, of 

 this substance alters in a peculiar way from day to day ; for, if we 

 inject an emulsion from a cord of a freshly killed rabbit into an 

 infected man, we do not cure his disease. Nor do we cure it if 

 we make the emulsion from a cord which has been dried a fort- 

 night. To achieve the desired result we must follow a certain 

 routine ; we must begin with a cord that has been thoroughly 

 desiccated and then proceed by fresher and fresher cords till at 

 last we inject one that is absolutely fresh and virulent. Here the 

 theory of neutralization clearly breaks down. The cells of the cord 

 are dead and dried and cannot produce a neutralizing substance ; the 

 microbes are dead or dying and are wildly unlikely to produce it ; 

 most decisive of all, the immunizing agent is not found in any 

 particular cord, but only in a succession of cords when these are used 



