CHAPTER XVI 



MUTATION AND PHENOTYPIC VARIATION 

 IN PHAGES 



D'Herelle (1926) was so much impressed with the capacity for 

 variation to be observed in phage that he beUeved all phages to 

 be variants of a single species. Other workers, struck by the 

 constancy of phage types, strongly opposed d'Herelle's view. 

 Some, though certainly aware that not all phages were alike, 

 wrote (and thought) in terms of "the bacteriophage." In part, 

 these different attitudes merely reflected different interests. In 

 effect, however, they produced a confusion of ideas exactly 

 parallel to that afflicting an earlier generation of microbiologists 

 who were trying to sort out the meaning of variability and con- 

 stancy met with among bacteria. 



Today, as a result of systematic study, we know that phages 

 belong to many well-separated types and that limited variation 

 of several kinds is permissible within each type. The present 

 chapter summarizes the observed range of variation. 



Many of the early observations on variation in phages have 

 been confirmed by more thorough study and many are probably 

 forever unconfirmable. Many observations of the latter type 

 can doubtless be ascribed to failure to distinguish between vari- 

 ation and simple contamination. Less obvious is the necessity 

 to distinguish between heritable and nonheritable variations. 

 Only in recent times has it become evident that certain types of 

 nonheritable variation in phages are very common. 



The material treated in this chapter will be discussed at some 

 length because much of it has not been presented elsewhere as a 

 coherent topic, and because it is the conviction of the author that 

 most of the phenomena mentioned deserve more thorough study 

 than is being devoted to them at present. 



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