326 Mutation in Micro- Organisms 



matter. By mutation, accordingly, I mean a permanent change — 

 however small it may be — which takes place in a bacterium and is 

 then transmitted to subsequent generations. The word does not imply 

 anything concerning the magnitude of the change, its suddenness, or 

 the manner of its acquisition. The term denotes a change in genetic 

 constitution. All other changes which are impermanent — depending 

 generally upon changes of the environment — and not hereditarily fixed, 

 are called modifications. The word " mutation " has been used with 

 such different meanings by so many bacteriologists and others, that 

 the foregoing statement seems called for. Indeed, discussions as to 

 whether such and such a change is or is not a " mutation " might 

 have been avoided in many cases if the opponents had defined their 

 use of the word precisely. I do not wish to assert, however, that my 

 usage is the correct one : I wish merely to state what I mean when 

 I use the word in the following article. 



It is not appropriate to discuss here, I think, the applicability of the 

 word " mutation " to the Bacteria. I am well aware of the difficulties 

 involved in applying the word — generally applied to certain changes 

 in sexual multicellular organisms — to the Protista. I am also well 

 aware of the difficulties involved in the above definition. Perhaps 

 an imaginary concrete instance will serve to make its meaning clear. 

 Let us suppose that a given Bacillus is coloured red under normal 

 conditions. By growing it and its offspring upon a new medium 

 they become — let us suppose — colourless. If the organisms and their 

 descendants when transplanted again into the original medium are 

 again found to be red, then the change (loss of colour) is a modifi- 

 cation : if, on the other hand, they are found to be now permanently 

 colourless, then the change is a mutation. By far the greater number 

 of variations described in Bacteria are of the former type. 



The mutations observed in Bacteria may be conveniently grouped 

 into two classes — those in which the change is functional (e.g. changes 

 in the power of producing ferments or pigments) and those in which 

 the change is structural. Most of the mutations about to be recorded 

 are of the former type — or physiological mutations, as I shall call 

 them. I will therefore begin with a description of these, and con- 

 sider some recent work on morphological mutations in a later section 

 (p. 344). 



