20 



such opportunities. When a young culture is growing under 

 favourable conditions, without the production of any variants, 

 it may be presumed that the stabilising influence which promotes 

 synthesis acts at exactly the right time and in the right way. 

 But under less favourable circumstances, this influence may 

 begin to operate (a) a little too soon, or (6) a little too late, 

 or (c) much too late ; and the consequences may be, respectively 

 (a) failure of some of the bacteria to develop their full biological 

 properties, e.g., loss of some function or of some antigenic con- 

 stituent, (6) the acquisition (after full development of biological 

 potentialities) of some additional chemical groups which tempo- 

 rarily mask one or more of these potentialities, e.g., disappearance 

 and subsequent return of some biological property, (c) failure 

 to build up the new protoplasm requisite for growth, with 

 " autolysis " as the result. And one might amplify these possible 

 causes of variation by imagining that the stabilising influence 

 did not act " quite in the right way," i.e., that it caused some 

 deviation from the normal molecular or colloidal arrangements 

 for the synthesis of chemical groups into protein. 



Is it possible to explain in more definite terms the idea that 

 certain variations in bacterial life are attributable to irregularities 

 in the action of a " stabilising influence " ? I think it is; but 

 one must not be expected to perform impossibilities. No 

 physiologist can explain all the activities of living protoplasm; 

 and bacteriologists are equally unable to give the precise reasons 

 why a bacterium grows or why it grows in a particular way. 

 All that can be done is to note certain influences which probably 

 participate in the phenomena of bacterial life. 



I propose first to describe some of these suggested influences 

 in general terms and then to illustrate them by concrete examples.* 



It is convenient to consider separately — I. The initial stimulus 

 to variation and II. The propagation of the variant. 



I. The causes which may initiate variation are many and 

 diverse. Roughly and provisionally they may be divided into 

 (a) those which are obviously non-specific, i.e., where there 

 can be no structural relationship of the " lock and key " type 

 between the stimulus and the bacterial protoplasm, and (b) specific 

 influences, where the stimulus has, directly or indirectly, a special 

 selective action, due to structural relationship, upon the bacterium 

 or upon the environment of the bacterium. 



Under (a) are included a large number of purely physical 

 influences, such as a slight change in temperature, or in the 

 reaction of the medium, or in the colloidal balance between the 

 bacterial constituents and their environment. Exactly how 

 each influence operates cannot be explained, but it is known that 

 the normal arrang ement of the particles which are being synthe- 



* Perhaps readers not already familiar with the subject will find the 

 argument easier to follow if they take the concrete examples (pp. 23-33) 

 first. This, of course, is not the correct logical sequence, as the examples 

 are not the basis of the more general statement, but merely illustrations 

 of it. 



