SUSCEPTIBILITY AND IMMUNITY. 251 



tation by their accumulation in the fermenting liquid, such as alco- 

 hol, lactic acid, phenol, etc. , would not be so retained. But we can- 

 not speak so positively with reference to the toxic albuminous 

 substances which recent researches have demonstrated to be present 

 in cultures of some of the best known pathogenic bacteria. It is 

 difficult, however, to believe that an individual who has passed 

 through attacks of half a dozen different infectious diseases carries 

 about with him a store of as many different chemical substances pro- 

 duced during these attacks, and sufficient in quantity to prevent the 

 development of the several germs of these diseases. Nor does the 

 experimental evidence relating to the action of germicide and germ- 

 restraining agents justify the view that a substance capable of 

 preventing the development of one microorganism should be with- 

 out effect upon others of the same class ; but if we accept the re- 

 tention hypothesis we must admit that the inhibiting substance 

 produced by each particular pathogenic germ is effective only in 

 restraining the development of the microbe which produced it in the 

 first instance. 



Pasteur discusses this hypothesis in his paper from which we 

 have already quoted, as follows : 



4 ' We may admit the possibility that the development of the microbe, in 

 place of removing or destroying certain matters in the bodies of the fowls, 

 adds, on the contrary, something which is an obstacle to the future develop- 

 ment of this microbe. The history of the life of inferior beings authorizes 

 such a supposition. The excretions resulting from vital processes may arrest 

 vital processes of the same nature. In certain fermentations we see anti- 

 septic products make their appearance during, and as a result of, the fer- 

 mentation, which put an end to the active life of the ferments and arrest 

 the fermentations long before they are completed. In the cultivation of our 

 microbe, products may have been formed the presence of which, possibly, 

 may explain the protection following inoculation. 



"Our artificial cultures permit us to test the truth of this hypothesis. 

 Let us prepare an artificial culture of the microbe, and after having evapo- 

 rated it, in vacuo, without heat, let us bring it back to its original volume 

 by means of fresh chicken bouillon. If the extract contains a poison for 

 the life of the microbe, and if this is the cause of its failure to multiply in the 

 filtered liquid, the new liquid should remain sterile. Now, this is not the case. 

 We cannot, then, believe that during the life of the parasite certain substances 

 are produced which are capable of arresting its ulterior development." 



This experiment of Pasteur appears to be conclusive so far as the 

 particular pathogenic microorganism referred to is concerned ; and 

 we may say, in brief, that more recent investigations do not sustain 

 the view that acquired immunity is due to the retention of products 

 such as are formed by pathogenic bacteria in artificial culture media, 

 and which act by destroying these bacteria or restraining their devel- 

 opment when they are introduced into the bodies of immune animals. 



Moreover, if we suppose that the toxic substances which give 

 pathogenic power to a particular microorganism are retained in the 



