140 PATHOGENIC BACTERIA. 



stance essential to its life, then died out, leaving the soil 

 unfit for future occupation. 



Such a view could only apply to immunity succeeding 

 infection, not to passive immunity produced by the in- 

 jection of antitoxins, or to active immunity produced by 

 immunization to toxins. 



2. The Retention Theory of Wernich l and Chau- 

 VEAU, 2 also of historic interest, supposes that by their 

 growth in the body the bacteria leave behind them some 

 metabolic product which interferes with their further 

 and future development. 



As a theory this has much more to recommend it than 

 its predecessor, being much more in accord with facts as 

 we know them. Thus in our culture-media the bacteria 

 die out long before the nutriment is exhausted, and will 

 not grow when replanted even though the acidity or alka- 

 linity be brought again to the most favorable point. 



The formation of antitoxin and antilysin is not in- 

 compatible with this theory if one accepts that the phe- 

 nomenal powers result from transformation of the toxin 

 into antitoxin as held by Buchner. However, as immu- 

 nity can be established to ricin, abrin, serpent's venom, 

 arsenic, and numerous other non-micro-organismal 

 poisons, it cannot be said to depend upon the presence in 

 the blood of prejudicial metabolic products of bacteria, 

 as was expressed in the original statement. 



Kruse's 3 theory of lysins is a modification of Chau- 

 veau's theory. 



Like the original retention theory, Kruse's theory 

 applies solely to immunity to infection, not to immunity 

 to poisons. 



III. Phagocytosis. — In acquired immunity the pha- 

 gocytes are supposed by Metschnikoflf to acquire an appe- 

 tite for the bacteria or to become educated to take up bac- 

 teria of which they were originally fearful without injury. 



1 Virchow's Archiv, 78. 



8 Compte-rendus de la Soc. de Biol, de Paris, xc. and xci. 



8 See Flugge's Die Mikroorganismen, vol. i. 



