CH. V] VARIATIONS IN FERMENTING POWER 65 



cultivation in one medium, during which repeated examina- 

 tions revealed no change in fermenting properties. 



The possibility must always be considered that the strain 

 may have been subcultured into fresh media in which the new 

 sugar was accidentally present as an unrecognised impurity, 

 and that the bacteria "learnt" to ferment this impurity after 

 a preliminary "training." They would be more likely to do 

 this if the process of subculturing were only carried out at 

 long intervals (as might easily happen in the case of a stock 

 culture) for this would afford time for the bacteria to exhaust 

 the normal sugar of the medium and their survival would then 

 depend upon their power to utilise traces of any other sugar 

 that might be present. 



The possibility also suggests itself that in a medium con- 

 taining a di-saccharide (lactose, maltose, saccharose) inversion 

 might occur to a slight extent, with the formation of traces of 

 a simpler mono-saccharide (dextrose, galactose) which the 

 bacteria growing in the medium would then, in the same way, 

 "learn" to ferment. 



Yet a third possibility is that the particular specimen of 

 the "sugar" used to test the fermenting properties of a strain 

 of bacteria may not be pure. This possible source of error is, 

 however, more easily guarded against. 



Finally — even if it be admitted that a variation in ferment- 

 ing power represents an adaptation to different foodstuffs — we 

 are forced to conclude that different members of a strain differ 

 from one another in their powers of adaptation ; for, when an 

 apparently spontaneous variation in fermenting power occurs 

 during cultivation, in a strain derived originally from a single 

 bacterium, fermenting and non-fermenting individuals may 

 be found side by side on the medium. This variation between 

 the organisms of one and the same strain we are quite unable 

 to explain. 



The Value op the Sugar Reactions. 



The unsatisfactory nature of the "sugar reactions," both 

 as a means of identification and as a basis for classification, 

 will be apparent from the following considerations. 



