372 BIOLOGY: GENERAL AND MEDICAL 



Taking a preview of the field of natural immunity, 

 we find that there are many instances of tolerance to 

 poisons. Thus in addition to such cases as the tobacco 

 worm and its vegetable food, we find the mongoose and 

 hedgehog fairly immune to the venoms of the serpents 

 they kill and eat. If we take care for study the toxins 

 separated by filtration from cultures of certain bacteria, 

 we find that the rat is immune against diphtheria toxin 

 and the hen against tetanus toxin. 



Here the limits of the habituation theory are exceeded, 

 for it is as impossible to connect the rat with diphtheria 

 and the hen with tetanus as it is easy to connect the 

 mongoose and hedgehogs with venom. 



In regard to infection, the same general facts are true. 

 There are certain infections of plants not known among 

 animals; there are certain infections peculiar to the 

 lower animals hog cholera, swine-plague, chicken 

 cholera, mouse septicaemia, quarter evil, etc. against 

 all of which human beings are immune; there are certain 

 diseases of man scarlatina, varicella, whooping-cough, 

 yellow fever, etc. against all of which the lower animals 

 are immune; and there are certain diseases anthrax, 

 tuberculosis, glanders, actinomycosis, etc. to which 

 both man and the lower animals are susceptible. 



We are in the habit of speaking of certain diseases 

 as peculiar to certain animals, which of course means 

 that other animals are immune. Thus if one speaks 

 of glanders, the horse and ass are at once thought of 

 and the possibility of human infection may be considered, 

 but no one thinks of cattle, sheep, dogs, or fowls because 

 it is well known that they are immune. 



In all cases of such natural immunity, whether it be 

 shown by exemption from the ill effects of toxins or by 

 exemption from invasion by microparasites, we find it 

 common to all of the animals of the kind. It is an ex- 

 emption of a whole group, not of an individual, and it 

 bespeaks some kind of physiologico-chemical peculiarity 

 antagonistic to the particular microparasites against 

 which they are immune. 



