64 VARIATIONS IN FERMENTING POWER [ch. v 



presence of one sugar the fermenters of that sugar invariably 

 gain complete ascendancy in the course of a few days, while in 

 the presence of another sugar the fermenters of it invariably 

 require many months to do so. 



1 0. The shortening of the incubation period on suhcidture 

 is more easily explained. When a few bacteria are inoculated 

 into a tube of broth they multiply with amazing rapidity. 

 There is a limit, however, to the number of organisms a certain 

 volume of the medium will support — owing not only to the 

 using up of the food but also to the accumulation of waste 

 products — so that after a time multiplication takes place much 

 more slowly. Subculture into fresh medium gives a new 

 impetus to reproduction. The "fermenters" in a mixed strain 

 gain the ascendancy by virtue of their capacity to utilise the 

 sugar, which enables them to multiply more rapidly than the 

 "non-fermenters." Any factor therefore which accelerates 

 the rate of increase of both, hastens the ultimate mastery of 

 the more rapidly multiplying, that is to say, the "fermenters." 



11. "Artificial selection" appears to be an even more 

 powerful factor In developing a particular fermenting power 

 than "natural selection." For example, Goodman obtained 

 by artificial selection a strain of B. diphtheriae which had 

 practically lost its power to ferment dextrose, although it 

 had been subcultured from one dextrose media to another 

 repeatedly over a long period. 



12. When an organism has been deprived by passage or 

 by other means, of its power to ferment a particular sugar, 

 "reversion" in character occurs subsequently on ordinary 

 media, that is to say in the absence of the particular sugar in 

 question. (The addition of the latter hastens "reversion," as 

 Klotz has shown, but its presence plays only a subordinate 

 part in the process). The sequence of events in such cases 

 suggests that the enzyme is temporarily inhibited in its action 

 and not destroyed. 



13. The sudden acquisition on the part of a strain of bac- 

 teria of power to ferment a certain sugar with v)hich it has not 

 been in contact, is more difficult to explain — ^particularly so 

 when such a variation occurs after long periods, even years, of 



