124 THE PRINCIPLES OF HEREDITY 



Up that graduated scale the cells of the animal react till 

 complete immunity is attained. The serum treatment artifi- 

 cially supplies digestive substances, and, what is even more 

 important, a scale of attenuated toxins. 



204. Various other considerations lend strength to the 

 belief that acquired immunity against any disease depends 

 essentially on a gradual habituation to its toxins, on an 

 increased power of physiological resistance, as a result of 

 which cells are enabled to perform their functions in spite 

 of the presence of toxins (which indeed do then but stimu- 

 late the cells especially concerned those which produce the 

 enzymes), not on the production of chemically antagonistic 

 substances, whether chemical (in the narrow sense) or 

 digestive. Thus it was found by Buckner that a mixture of 

 toxin and antitoxin e.g. that of tetanus in a proportion 

 harmless to an animal of one susceptible species e. g. a 

 mouse is still toxic to a weaker animal of the same species 

 or to an animal of a more susceptible species e.g. guinea-pig 

 which could not be the case did the toxin contain substances 

 which neutralized the toxin. Whereas if immunity depends 

 immediately on a physiological, not a chemical reaction, even 

 an attenuated toxin may be fatal to a very susceptible 

 animal ; an inference corroborated by the fact that a non- 

 susceptible animal may be rendered susceptible by influences 

 e. g. cold and starvation which lower the vitality. Again, 

 there is a sharp distinction between the " active " immunity 

 which is procured by experience of actual disease, and " passive" 

 immunity which results from the injection of antitoxic sera. 

 The former is enduring, the latter is fleeting. If the sera con- 

 tain substances which chemically neutralize the toxins it is 

 impossible to explain the difference ; for the diseased person 

 can do no more than produce these substances himself. But, 

 if acquired immunity be a vital reaction, the difference is 

 easily explained on the hypothesis that the attenuated 

 toxins in the sera are so mild that they cannot cause a per- 

 manent reaction, though they do cause a temporary reaction 

 which enables the person to resist immediate infection. The 

 case is exactly parallelled by the comparatively fleeting 

 immunity to small-pox conferred by one vaccination mark, 

 and by the fleeting immunity to yellow fever which residents 

 in countries where it is prevalent acquire without actual 

 illness presumably because they are frequently infected by 

 small doses of parasites. Yet again, Roux found it possible 

 to obtain from a prepared horse two lots of sera of equal 

 antitoxic value, even though no intermediate injection of 



