8 A. C. R. Dean and Sir Cyril Hinshelwood 



in the phenomena by dividing continuous ranges of quan- 

 titative variation into two arbitrarily defined ranges such as 

 "fast" and "slow" or + at 24 hours and — at 24 hours. Add 

 to this the fact that the resulting changes are not nearly as 

 stably heritable as is sometimes implied (Dean and Hinshel- 

 wood, 1954a), and the theory of rather sluggishly reversible 

 cytoplasmic adaptations no longer looks quite so much like 

 belonging to a different ideological world. 



Stability and Reversibility 



In our experience partial reversion often occurs rather 

 quickly, slow subsequent reversion following an erratic course. 

 Sometimes almost complete reversion occurs rather quickly, 

 as wdth Bacterium coli trained to utilize D-arabinose (Cross 

 and Hinshelwood, 1956), and with some yeast strains made 

 drug-resistant (Wild and Hinshelwood, 1956). Sometimes 

 reversion is hastened by growth in media to which fresh 

 adaptation is needed, the disturbing effect of the new adjust- 

 ment relieving the metastability of the old. For example, 

 growth in the presence of phenols resulted in a loss of the 

 adaptation of Bacterium lactis aerogenes (Aerohacter aerogenes) 

 to proflavine (Davies, Hinshelwood and Pryce, 1945) and 

 adaptation to proflavine of a sulphanilamide-trained strain 

 of the same organism led to the loss of the sulphanilamide 

 adaptation (James and Hinshelwood, 1947). With Aero- 

 hacter aerogenes, adaptation to D-arabinose is gradually 

 removed in this way. That the phenomenon is not due to the 

 re-selection of a few reverse mutants has been shown by the 

 fact that deliberately added cells of the original untrained 

 strain are in fact not preferentially supported by the media 

 used (Cross and Hinshelwood, 1956; Baskett and Hinshel- 

 wood, 1951). 



In general, the more thoroughly the training to drugs has 

 been impressed on the bacterial cells the less readily is it lost. 

 For example, cells which have just acquired the ability to 

 grow in the presence of the drug readily lose it on subculture 



