17 



reported, under the title " The Bacteriophage (Bacteriolysin)," 

 in the British Medical Journal for August 19th, 1922. More 

 recently (November, 1922), Medical Science Abstracts and Reviews 

 has summarised the literature on this subject under the title 

 of " The d'Herelle Phenomenon." 



I think it will be agreed that the whole subject is at present 

 in a state of much uncertainty. There is a multitude of observa- 

 tions on bacterial variants, but no general explanation of the way 

 in which they arise ; and it is often impossible to find any cor- 

 relation between a particular type of variant and a particular 

 kind of disturbing influence. Work on the " lytic principle " is 

 particularly difficult to follow, because there are so many con- 

 flicting theories as to the nature of this principle. As the whole 

 subject is very far from being thoroughly worked out, this 

 confusion is only what was to be expected at the present stage. 



In the hope that further discussion may help to clear the 

 ground, I put forward the following hypothesis (pp. 17-23) as a 

 possible explanation of some of the phenomena associated with 

 bacterial variation. It seems to me that the essential problem 

 concerns the physiology of the bacterial protoplasm. After 

 developing my hypothesis on this basis, I proceed to illustrate it 

 by recent laboratory data. 



Facts to be Correlated. 

 The main facts to be correlated are the following : — 



(1) Bacteria "breed true" to their species; any varia- 

 tions which occur are within the limits of the species. 

 There is practically no evidence which would make it 

 worth one's while to give serious consideration to the 

 possibility of transmutation of species.* It need not be 

 assumed that, when the living bacterial cell divides into 

 two, each portion necessarily retains the species characters ; 

 but it does appear to be a tenable hypothesis that any new 

 individual which does not retain characters requisite for 

 identification of its species is not viable, the limits of 

 variation being thus determined by the limits of viability. 



(2) The variations which are found are not, as a rule, 

 irregular or haphazard but tend to some one definite 

 direction. This is particularly the case when the change 

 is«due to a single influence and not to a variety of factors. 

 The first stage in the process is generally the production 

 of a mixture. The " normal " strain (a) produces a mixture 

 of (a) forms, which all resemble the original, and of (b) forms, 

 which are alike among themselves and possess one or more 

 attributes differentiating them from (a). In imagining 

 how this result comes about, there are two possibilities to 

 consider. An (a) form may subdrv ide into an (a) and a (6) ; 



* I do not propose to enter here into academical questions about the 

 precise definition of " species " as applied to bacteria. 



