PHENOMENA AND CONDITIONS OF LIFE 49 



Law is the best proof that this method is right. Everyone 

 knows that persons who have had the measles, scarlet-fever, 

 whooping-cough, &c., are almost perfectly immune against 

 renewed infection, a phenomenon which can only be explained 

 by the formation in the body of a protective substance which 

 remains effective for some time. 



This immunity which, as we have seen, may be acquired 

 artificially by the living organism against various toxins is a 

 natural property of many animals. We find that many infec- 

 tious diseases fatal to man are not transmissible to animals, and 

 vice versa. Further, while among closely related animals one 

 species is highly susceptible to a certain disease, another species 

 is absolutely immune. It is still more remarkable that the 

 same phenomenon is observed in individuals of the same species. 

 Why that should be so remains for the present obscure. 



Many serpent-hunters, among them the useful hedgehog, are 

 almost immune against the effects of snake poisons. One may 

 often see the hedgehog devour, with the greatest relish and 

 without the least discomfort to himself, a viper, poison-glands 

 and all. That the bite of the viper does not injure the hedgehog 

 I have been able to ascertain on various occasions. 



A still more remarkable instance of immunity is presented by 

 a tiny beetle. The fresh juicy leaves of the deadly nightshade, 

 Belladonna atropa, almost seem to provoke the appetite of all 

 plant-eaters. Yet, no matter how scarce the food supply may be, 

 the nightshade is carefully avoided by all the large grazing 

 animals, for its leaf and juice contain a deadly poison. Without 

 this terrible weapon of defence this plant would long ago have 

 been defeated in the struggle for existence. But even the most 

 powerful protection can only grant a qualified security, for the 

 poison which frightens away the largest animals has no terrors 

 for the little Haltica atropce, for the leaves and juices of the 

 deadly nightshade constitute its sole food. 



