NATURAL IMMUNITY. 453 



organisms had especially a power of destroying the normal 

 preventive power resident in the blood and tissues. There 

 is, however, no such thing known as an organism multiply- 

 ing in the living tissues without producing local or general 

 changes, though, theoretically, there might be. We may 

 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 parti- 

 cular 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, and there is no evidence that for each case there 

 is an antitoxic body which protects till the poisonous dose 

 is reached. The serum of the fowl does not protect a 

 susceptible animal from the tetanus toxine, though the 

 serum of a naturally less susceptible animal in which a 

 resistance equal to that of the fowl has been artificially 

 developed, does possess antitoxic powers. The resistance 



