40 L. L. Cavalli-Sforza 



in the distribution of the number of resistants per tube is 

 immediately apparent. Thus the last tube of the 0-0625-ml. 

 sample must have contained 8 or 9 mutants at least ; the same 

 is true of the third tube of the 0-125-ml. sample, and so on. 

 This distribution could hardly be random. Presumably, 

 resistant cells tend to form short chains which are not split 

 on dilution in broth, while they are more easily broken up in 

 agar, perhaps as a consequence of the joint action of tempera- 

 ture of the agar plus its chelating power. In such cases one 

 would tend to underestimate the number of resistant mutants 

 if the frequency of tubes showing enrichment were used for its 

 assessment, while if enrichment ratios were used one would 

 tend to overestimate it. 



Conclusions 



Tests of adaptation in bacterial and other populations are 

 available that permit the assessment of the relative import- 

 ance of genetic and non-genetic adaptation, defining the 

 former as the selection of spontaneous mutants. In the cases 

 tested so far — essentially streptomycin and chloramphenicol 

 resistance — evidence was found for the adequacy of the 

 hypothesis of genetic adaptation, and no need arose for addi- 

 tional alternative explanations. 



REFERENCES 



Cavalli-Sforza, L. L. (1952). Bull. Wld. Hlth. Org., 6, 185. 

 Cavalli-Sforza, L. L., and Lederberg, J. (1953). VI Int. Congr. 



Microbiol, p. 108. 

 Cavalli-Sforza, L. L., and Lederberg, J. (1956). Genetics, 41, 367. 

 Eagle.. H. (1955). Perspective and Horizons in Microbiology, p. 168. 



Rutgers University Press. 

 Lederberg, J., and Lederberg, E. M. (1952). J. Bad., 63, 399. 



DISCUSSION 



Hinshelwood: This is a beautiful method with streptomycin, and I 

 admire the experiment very much. But what level of chloramphenicol 

 resistance can one get by the indirect method? 



Cavalli-Sforza: The degree of resistance that was obtained in the work 

 with Lederberg was perhaps two or three times the original level. You 



