Studies on the Stability of the Na^ Requirement 489 



The findings reported here indicate that the Na' requirement 

 of the marine bacteria examined is a very stable one. Mutants, 

 however, have been derived from one culture, though with diffi- 

 culty, which have a capacity to grow either in the absence of Na"^ 

 or at least at very low levels of Na*. This finding provides some 

 support for the hypothesis that land and fresh-water bacteria 

 could have evolved from marine bacteria through mutations 

 giving rise to organisms with a capacity to grow either in the 

 absence of Na^ or at the low levels of Na' which would be ex- 

 pected to be present as a contaminant in most terrestrial environ- 

 ments. 



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