J. BRONFENBRENNER 551 



occur as a direct result of the development of active immunity by bacteria. More- 

 over, similar transformations of normal cultures, including the development of re- 

 sistance to phage, have been observed in instances where bacteria were not exposed 

 to phage at aU. Thus, for example, Hoder' has been able to produce such a transfor- 

 mation by the use of various chemicals, and D'Herelle's own experiments show that 

 the appearance of resistants is greatly influenced by the hydrogen-ion concentration 

 of the medium or by temperature, even in the presence of phage. ^ Furthermore, 

 similar aberrant types of bacteria have been isolated directly from normal cultures 

 without the intervention of any agent, thus indicating that bacteria potentially re- 

 sistant to phage exist normally in bacterial cultures.^ 



All these findings may be explained by considering a bacterial culture (as indeed 

 any other "population") as composed of individuals approaching a certain type, but 

 occasionally lacking or possessing exaggerated characteristics. Depending upon the 

 conditions, some of the variants may find themselves favored by the environment 

 and may become quantitatively dominant. On the contrary, if the environment is 

 changed so that it becomes incompatible with normal development of certain types 

 of variants, the latter are eliminated more or less completely, and the predominant 

 character of the bacterial population in the culture deviates accordingly, with corre- 

 sponding changes in the biological activity of the culture as a whole. That the variants 

 resistant to phage may be obtained from normal cultures by causing them to disso- 

 ciate under the influence of heat'' or of chemicals'' seems to suggest that these variants 

 represent the individuals in the population of the culture which survive on account 

 of their relative tolerance to the deleterious influences of the environment in general, 

 and are not the product of specific adaptation to phage. 



This simple explanation of the phenomenon of resistance does not, however, ex- 

 plain the variations in susceptibility of different strains of resistants to different close- 

 ly related phages. This phenomenon cannot be explained until it is known what 

 forces or properties are directly responsible for the higher degree of resistance of 

 certain individuals in a bacterial population of a culture to the injurious agents in 

 general and to phage in particular, and until the specific differences in the phages 

 themselves can be detected by methods other than the lysis of bacteria. The explana- 

 tions offered thus far are either highly hypotheticaP and not susceptible to experi- 

 mental inquiry, or opposed by the existing experimental evidence. For instance, al- 

 though it is true that old bacteria are generally more resistant to phage than younger 

 bacteria of the same species, Marshall's^ explanation of specific resistance of second- 



' Hoder, F.: loc cil.; Hoder, F.; and Suzuki, K.: loc cit. 



'd'Herelle, F.: The Bacteriophage and Its Behavior, pp. 182-83, 1926. These findings are of 

 particular interest in connection with the conclusion of Gratia concerning the effects of hydrogen-ion 

 concentration on the rate of bacterial dissociation (Gratia, A.: op. cit., 84, 275. 1921). 



3Gratia,A.: ibid., ig,'&2i. 1923; Arkwright, J. A.: /. Pa//z. &" Sac/., 24, 36. igii; Brit. J.Exper. 

 Path., 5, 23. 1924; Gratia, A.: op. cit., 84, 750. 192 1. 



* Hoder, F.: op. cit., Orig., 92, 197. 1925. 



sHoder, F.: ibid., 42, 197. 1925; Hoder, F., and Suzuki, K.: loc. cit.; Gratia, A.: op. cit., 84, 

 275. 1921. 



^Bail, O.: loc. cit. ? Marshall, M. S.: loc. cit. 



