62 VARIATIONS IN FERMENTING POWER [ch. v 



example of adaptation to environment. If a slow fermenter 

 of dulcite is grown in a medium containing some other sugar, 

 such as dextrose, its power to ferment dulcite is not increased. 



7. The ability to split up the sugar is apparently an 

 advantage to the organism concerned. That this is actually 

 the case is confirmed by the observation of Penfold that the 

 appearance of acidity coincides with a very rapid and a very 

 marked increase in the number of organisms present. So 

 constant did he find this association of events that he regards 

 a count of the organisms as suflBcient by itself to indicate the 

 occurrence of the variation. He found, moreover, that the 

 addition of dulcite to peptone water containing a dulcite- 

 fermenting strain of ^. typhosus, rendered the medium capable 

 of supporting a population many times greater than it was 

 able to support alone. The addition of other sugars which the 

 organisms could not ferment did not lead to any increase in 

 their numbers. 



The ability to ferment the sugar, even if in some cases it 

 were not of actual benefit to the fermenting organism, might 

 still prove of advantage to it indirectly. Marked acidity ot 

 the medium is known to be unfavourable, as a rule, to 

 bacterial gi-owth ; but it might be expected that the acid 

 producing individuals in a strain would be unusually resistant 

 to the products of their own activity and that their growth 

 would, on this account, be inhibited to a less degree than 

 that of the non-fermenters. 



8. It is easy to understand how natural selection will 

 cause any character to predominate which gives the possessors 

 of it an advantage over their fellows. This is the obvious 

 explanation of the development by a strain of bacteria, when 

 grown on a certain sugar, of the power to ferment that sugar. 



Two phases of the phenomenon, however, call for further 

 explanation, namely the prolonged incubation period and the 

 shortening of this period by subculture. 



9. The incubation period may be explained in one 

 of three ways. 



In the first place it may be regarded as a " latent period " 

 during which changes occur in the organisms as a result of 



