NATURAL IMMUNITY. 493 



infer from this that if the toxines are completely neutralised 

 or rendered powerless in the case of any animal, that 

 animal will be immune against the particular organism. 

 This is also borne out by the fact that immunity against a 

 particular organism can be artificially obtained by injections 

 of the toxines of that organism. 



(a) Variations in Natural Susceptibility to Toxines. We 

 may consider, then, the question in the first instance from 

 the point of view of toxines. Now we must 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-susceptibility 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 

 animals. The great resistance of the common fowl to the 

 toxine of the tetanus bacillus may be here mentioned ; the 

 high resistance of the pigeon to morphia is a striking 

 example in the case of vegetable poisons. This variation 

 in resistance to toxines 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 toxines such as that of 

 diphtheria. We must take this natural resistance for 

 granted, though it is possible that ere long it will be ex- 

 plained. It is possible then that an animal might be 

 immune against the anthrax bacillus, for example, if the 

 toxines of the latter were simply inert towards the animal 

 tissues, or, in other words, if its tissues enjoyed a natural 

 insusceptibility to the toxines. In such a case the anthrax 

 bacillus would be in the position of the bacillus subtilis, 

 and would be destroyed in the tissues by the same means. 



(b) Natural Bactericidal Powers. The second factor 

 may now be considered, namely, the power of killing the 

 organism, though it appears to us that natural immunity 

 has been too exclusively looked at from this side. Special 



