VARIATIONS IN SUSCEPTIBILITY TO TOXINS. 499 



in vitro, therefore, do not explain the presence or absence of 

 natural immunity against a living bacterium. 



2. Variations in Natural Susceptibility to Toxins. We must 

 here start with the fundamental fact, incapable of explanation, 

 that toxicity is a relative thing, or in other words, that different 

 animals have different degrees of resistance or non-suscepti- 

 bility to toxic bodies. In every case a certain dose must be 

 reached before effects can be observed, and up to that point 

 the animal has resistance. This natural resistance is found to 

 present very remarkable degrees of variation in different ani- 

 mals. The great resistance of the common fowl to the toxin 

 of the tetanus bacillus may be here mentioned; the high re- 

 sistance of the pigeon to morphia is a striking example in the 

 case of vegetable poisons. This variation in resistance to toxins 

 applies also to those which produce local effects, as well as to 

 those which cause symptoms of general poisoning. Instances 

 of this are furnished, for example, by the vegetable poisons 

 ricin and abrin, by the snake poisons, and by bacterial toxins 

 such as that of diphtheria. We must take this natural resist- 

 ance for granted, though it is possible that ere long it will be 

 explained. 



According to Ehrlich's view of the constitution of toxins, it 

 might be due to the want of combining affinity between the 

 tissue cells and the haptophorous group of the toxin ; or, on the 

 other hand, supposing this affinity to exist, it might be due to 

 an innate non-susceptibility to the action of the toxophorous 

 group. Certain investigations have been made in order to de- 

 termine the combining affinity of the nervous system of the 

 fowl with tetanus toxin, as compared with that obtaining in a 

 susceptible animal, but the results have been somewhat contra- 

 dictory. Accordingly, a general statement on this point cannot 

 at present be made. 



At present, therefore, the facts of natural immunity cannot 

 be fully explained. In some cases the insusceptibility to toxic 

 substances may explain the degrees of immunity possessed by 

 different animals, whilst in others immunity is due to special 

 bactericidal powers possessed by them. What these bacteri- 

 cidal powers really are cannot be explained on any single the- 

 ory. A vital activity of the tissues and fluids is, no doubt, 

 brought about by the presence of the bacteria, and this cannot 



