628 THE POPULAR SCIENCE MONTHLY. 



ing to elucidate is that, during a non-fatal attack of one of the 

 specific diseases, the cellular elements implicated which do not 

 succumb to the destructive influence of the poison acquire a tol- 

 erance to this poison which is transmissible to their progeny, and 

 which is the reason of the exemption which the individual enjoys 

 from future attacks of the same disease." 



In my chapter on Bacteria in Infectious Diseases, in Bacte- 

 ria, published in the spring of 1884, but placed in the hands of 

 the publishers in 1883, I say : " It may be that the true explana- 

 tion of the immunity afforded by a mild attack of an infectious 

 germ disease is to be found in an acquired tolerance to the action 

 of a chemical poison produced by the micro-organism, and conse- 

 quent ability to bring the resources of Nature to bear to restrict 

 invasion by the parasite." This theory of immunity has received 

 considerable support from investigations made since that date, 

 and especially from the experimental demonstration by Salmon, 

 Roux, and others that, as suggested in the work from which I 

 have quoted, immunity may result from the introduction into the 

 body of a susceptible animal of the soluble products of bacterial 

 growth filtered cultures. 



The theory of vital resistance to the toxic products evolved by 

 pathogenic bacteria is also supported by numerous experiments 

 which show that natural or acquired immunity may be over- 

 come when these toxic products are introduced in excess, or when 

 the vital resisting power of the animal has been reduced by vari- 

 ous agencies. Thus Roger has shown that the rabbit, which has 

 a natural immunity against symptomatic anthrax, succumbs to 

 infection when inoculated with a culture of the bacillus of this 

 disease, if at the same time it receives an injection of a sterilized 

 or non-sterilized culture of Bacillus prodigiosus. Monti has suc- 

 ceeded in killing animals with old and attenuated cultures of the 

 Streptococcus pyogenes or of Staphylococcus pyogenes aureus, by 

 injecting at the same time a culture of Proteus vulgaris. A simi- 

 lar result may be obtained by subjecting animals to physical 

 agencies which reduce the vital resisting power of the tissues. 

 Thus, Nocard and Roux found by experiment that an attenuated 

 culture of the anthrax bacillus, which was not fatal to guinea- 

 pigs, killed these animals when injected into the muscles of the 

 thigh after they had been bruised by mechanical violence. Char- 

 rin and Roger found that white rats, which are insusceptible to 

 anthrax, became infected and frequently died if they were ex- 

 hausted, previous to inoculation, by being compelled to turn a re- 

 volving wheel for a considerable time. Pasteur found by experi- 

 ment that fowls, which have a natural immunity against anthrax, 

 become infected and perish if they are subjected to artificial re- 

 frigeration after inoculation. This has been confirmed by the 



