504 S. E. LURIA AND M. DELBRUCK 



were used — namely: (i) io.o cc aerated broth cultures; (2) .2 cc broth cultures; 

 (3) .2 cc synthetic medium cultures. 



The results of all tests for the number of resistant bacteria are summarized 

 in table 2 and table 3. 



Table 2 



The number of resistant bacteria in series of similar cultures. 



38 



28 



3S 



107 

 S 14 31 183 24 o 8 13 



6 



* Cultures in synthetic medium. 



It will be seen that in every experiment the fluctuation of the numbers of 

 resistant bacteria is tremendously higher than could be accounted for by the 

 sampling errors, in striking contrast to the results of plating from the same 

 culture (see table 1) and in conflict with the expectations from the hypothesis 

 of acquired immunity. 



We want to see next whether these results fit the expectations from the 

 hypothesis of mutation. We must therefore compare the experimental results 

 with the relations developed in the theoretical part, keeping in mind that the 

 theory con tains several simplifying assumptions. 



First we can compare, according to equation (12), the experimental and the 

 calculated values of the ratio between the standard deviation and the average 

 of the numbers of resistant bacteria. These ratios are included in tables 2 and 

 3. It is seen that the experimental and theoretical values are reasonably close. 



