PHYSIOLOGICAL 441 



necessitatem ; but if D'Herelle has not discovered an organism, he 

 has certainly discovered a remarkable fact. 



We must now return to an alHed puzzle of great biological 

 interest — immunity. 



IMMUNITY.— It is a well-known fact of Natural History that a 

 hedgehog may be bitten by an adder without suffering any evil 

 effects so far as can be seen. This has been corroborated experimen- 

 tally, and it appears that a hedgehog does not suffer from an injection 

 of poison many times stronger than the fatal dose for a rabbit. 

 The hedgehog has natural immunity to the venom of vipers. 



But what does this immunity mean? The first step towards an 

 answer is not difficult, and has been well worked out in connection 

 with the attractive carnivores known as the Ichneumon {Herpestes 

 ichneumon) and the Mongoose {Herpestes mungos), both of which 

 are inveterate enemies of snakes, and enjoy the same natural 

 immunity as the hedgehog. If some cobra's poison be mixed with 

 mongoose's blood, and if the fluid or serum be injected under a 

 rabbit's skin, nothing happens; but an injection of the same amount 

 of poison without mongoose's serum is immediately fatal. The same 

 is true for the hedgehog and adder's poison. Evidently, then, there 

 is some substance or quality naturally present in the blood of 

 mongoose and hedgehog, which is able to counteract, neutrahse, or 

 somehow take the edge off snake toxins. 



The point may be made clearer by taking the case of the pig, 

 which is not affected by the bite of several different kinds of veno- 

 mous snake, or by subcutaneous injection of prepared snake toxin. 

 The pig is not affected, for instance, by cobra poison; yet if the 

 blood-fluid or serum of an uninjured pig be mixed with cobra 

 poison and then injected into a rabbit, there is an immediately 

 fatal result. This obviously suggests that the pig's blood does not 

 contain any "anti-toxin" that can neutrahse snake toxin. What 

 makes the difference? It is believed that the pig's "immunity" is 

 more apparent than real, that it is due to the protection afforded 

 by the thick layer of fat below the skin. This familiar fat includes 

 very few blood-vessels, and it probably serves to stop the poison 

 from getting further in. 



The hedgehog, the Ichneumon, and the mongoose can withstand 

 large injections of poison; thus the Ichneumon can successfully 

 counteract six times the dose that is fatal to a rabbit; but in all 

 cases there is, naturally enough, a limit beyond which the poison 

 is lethal. A few other mammals are known to be immune to snake 

 poison ; thus the American opossum is not aflected by the bite of a 

 rattlesnake, and the cat has a high degree of immunity to the poison 

 of vipers of various kinds. A few mammals have a distinct but 

 slight insusceptibility to snake poison ; most have none at all. 



