23 



For example, the lytic action of an active principle containing 

 both A and B may be neutralised by an anti-A serum but not 

 by an anti-B serum. Is this a valid explanation ? 



Whilst it must be freely admitted that the facts about " trans- 

 missible autolysis " are very difficult to understand, it seems to 

 me that the most promising line of explanation lies in a develop- 

 ment of the suggestion that these data are merely special instances 

 of a general principle determining bacterial variation. 



This view is in harmony with experiments which show that 

 the lytic principle is not merely an agent of destruction; it is 

 also a means of increasing bacterial resistance. In the exercise 

 of this latter property its action is progressive. First it divides 

 the bacteria into some relatively resistant forms (A), which 

 survive, and into more sensitive forms (B), a small proportion of 

 which also survive. The process is repeated with the descendants 

 of A and B ; the Weaker individuals die out and the stronger 

 survive, perhaps including amongst the latter a few more hardy 

 descendants of what were originally B forms. Carried to its 

 conclusion, the process results in the production of a pure strain 

 of highly resistant A forms, the variant (B) having died out. 

 Thereupon the pure A strain behaves like a normal culture, 

 though more robust than the normal because the tendency to 

 variation has been eliminated ; it is no longer lysogenic, because 

 the sensitive variants which, on disintegration, yielded the lytic 

 substance, have disappeared. 



At this point it is useful to note that, by using the appropriate 

 laboratory method, one of two opposite results may be achieved 

 in the experimental modification of bacteria. Taking advantage 

 of the fact that a normal culture may be split up into resistant 

 and less resistant forms, it is possible to eliminate either of the 

 two and to produce at will, a pure (or relatively pure) form of 

 either the one or the other. 



The practical importance of these laboratory results is that 

 they provide information about some of the factors which deter- 

 mine susceptibility and resistance to bacterial invasion. 



They at once raise the question, Do similar influences deter- 

 mine the production of bacterial variants in the animal body ? 



Significance of the Experimental Data. . 



Examples of bacterial variation are very numerous and 

 present a wide range of differences. All that can be said about 

 many of them is that they are carefully recorded facts but their 

 significance is obscure and the causes which produce them are 

 unknown. It will be more useful, for the purpose of this report, 

 to select some examples of variants which are clearly of importr 

 ance in the study of infection and immunity, and to confine 

 one's attention to these. 



Spontaneous Variations. 



In the first place, valuable information has been obtained 

 from observations on variants which occur " spontaneously," 



