18 



or it may subdivide into two portions, only one of which, 

 either (a) or (6), is viable. I think this latter possibility is 

 worth bearing in mind. Continuation of the influence 

 which produced the mixture may result in elimination of 

 the one form and survival of a pure culture of the other. 



(3) Starting with a pure strain of b as the normal," 

 it may be possible to grow it under conditions which cause 

 it to throw off variants (c), all resembling each other and 

 differing in the same respects from 6. And this may not 

 be the final limit to the possible number of variants. 

 By appropriate treatment of c, another variant may be 

 split off, and so on. But a multiplicity of variants does 

 not mean that they are produced in a haphazard sort of 

 way ; they seem rather to emerge one at a time, as a series 

 of consecutive reactions between the modified bacterium 

 and its environment. 



(4) Under special conditions .the characters of the 

 variant recovered from b may be those of a, i.e., reversion 

 to the original type may occur. Such reversion is most 

 commonly observed when the original influence which 

 converted a into b was in operation only for a short time. 



(5) In the initiation of bacterial variation, a bacterium 

 (A) may divide in such a way that one half (the variant) 

 is not viable, whilst a fellow bacterium (B) may divide 

 into halves which are each non-viable. If all the bacteria 

 behaved like B, the production of variants would mean 

 death of the culture. But suppose that, whilst a large 

 majority of the bacteria behave like B, the remaining 

 few behave like A; the result will be that disappearance 

 of the variant is associated with scanty growth of the 

 more resistant survivors of the normal individuals. This 

 appears to be what happens in the production of " autolysis " 

 associated with the growth of bacteria. It seems to be a 

 special example of bacterial variation, due to an influence 

 which is not an ordinary lysin, because it acts only on 

 bacteria in active growth; it is a selective stimulus, the 

 nature of whioh will be discussed later, causing the bacteria 

 to subdivide into non-viable elements with the survival 

 of some resistant individuals. For the sake of simplicity, 

 I have stated the case as above; but there is usually an 

 intermediate stage before the variants disappear entirely; 

 some of them are just viable, though tending to subdivide 

 into non-viable forms; if removed from the lytic stimulus 

 at this stage, they may be cultured as a variant charac- 

 terised by its high sensitiveness to lytic influence. 



Correlation of the Facts. 



Is there any interrelationship between the data set out above ? 

 Perhaps a clue may be found in considering the intimate way 

 in which two essential processes of bacterial life are dependent 



