118 THE PRINCIPLES OF HEREDITY 



goes when, after being susceptible to disease, he becomes, 

 through illness and recovery, highly resistant to it ? Pasteur 

 supposed that the parasites of each disease against which 

 immunity may be acquired find, every species of them, some 

 special pabulum in the body on which they subsist, but on 

 which no other species can subsist, and that when this 

 special pabulum is exhausted they perish from starvation. 

 In diseases against which acquired immunity is, as a rule, 

 permanent e. g. small-pox this pabulum, he supposed, is, 

 as a rule, not renewed ; but when immunity is not per- 

 manent e. g. in diphtheria it is renewed, whereby the 

 individual again becomes susceptible. Chauveau, on the 

 other hand, supposed that acquired immunity arises because 

 the waste products of the microbes are inimical to their own 

 existence, just as alcohol, for instance, is inimical to the yeast 

 microbes that produce it from solutions of sugar ; and, there- 

 fore, that when the waste products reach a certain percentage 

 in the fluids of the infected person, the microbes perish, as 

 yeast perishes when alcohol reaches a certain percentage in a 

 fermenting fluid. In diseases in which acquired immunity is 

 permanent he supposed that the waste products are bottled 

 up within the infected person, but that they are eliminated 

 after a time in diseases against which acquired immunity is 

 not permanent. Both these theories are negatived by the 

 fact that the parasites of certain diseases e.g. anthrax 

 are able to flourish in blood drawn from animals that have 

 acquired immunity against them. 



191. Pasteur's theory is, probably, abandoned by every one. 

 Chauveau's theory is still supposed by some bacteriologists 

 to offer a partial explanation of acquired immunity. But 

 probably even this modified belief should be abandoned. 

 Waste products are much less inimical to the organisms 

 which produce, and are therefore inured to them, than they 

 are to the cells of the body. Thus alcohol may attain a 

 percentage of fourteen in a fermenting fluid, but no man 

 could exist if alcohol formed anything like that proportion of 

 his total volume. It is only reasonable to suppose that his 

 resisting powers are lowered by the waste products of patho- 

 genetic germs more than the vital powers of the parasites 

 themselves. 



192. A doctrine widely held, even at the present day, is to 

 the effect that acquired immunity is due to the production 

 within the infected person of substances which chemically 

 antagonize or otherwise neutralize the toxins, and which, for 

 that reason, have been termed antitoxins. By one set of 



