NATURE OF MUTATIONS 33 



and dissociating the unusual food-stuff faute de mieux, that, in 

 short, we have here a repetition of Pasteur's original observa- 

 tion that when the accustomed food-stuff is used up, the un- 

 accustomed will be attacked, just as we to-day are making a 

 virtue of necessity and are thriving upon potato — and wheat — 

 substitutes. As a matter of fact, if the experiment be modified 

 so that all the multiplying individuals are subjected to the same 

 environment — if, -that is, instead of employing a solid medium 

 a fluid medium containing the new glucoside be taken and the 

 bacilli be grown on this, then the conditions of experiment can 

 be so arranged that not some but all the bacilli acquire the 

 property of fermenting the unusual glucoside. All, that is, 

 acquire the new property. 



On Impressed as distinct from Chance Variations 



If I remember aright, this was first demonstrated in 1913 or 

 1914. Unfortunately I am away from my library and notes, 

 and have been unable to find the reference. In my dilemma — 

 for I did not wish to make the vague statement that this was 

 so without affording a reference — my colleague Major F. B. 

 Bowman, Officer in Command of No. 2 Canadian Mobile Labora- 

 tory * at Folkestone, with great kindness came to my rescue and 

 willingly conducted the very simple experiment — an experiment 

 so simple that it can easily be repeated. The underlying principle 

 of it is that the bacteria must not be supplied with an abundance 

 of alternative food-stuff. They are accustomed to obtain their 

 carbon from amines (proteins) when carbohydrates are not present. 

 Do not, therefore, use a peptone broth of the usual concentration. 

 Some years ago I pointed out that with members of the typho- 

 coli group 10 ccm. of peptone broth added to a Winchester quart 

 of sterilized water affords upon inoculation a pronounced tur- 

 bidity in twenty-four hours, in other words, an active growth 

 of the bacilli. 



Major Bowman, therefore, made up such a diluted peptone 

 bouillon — or " Soupe maigre " — with 1 per cent added of the 

 unusual glucoside in this case Isodulcite, or Rhamnose, 2 along 



1 Now No. 1 Canadian Central Laboratory. 



a Por which I am indebted to Professor Rosenheim of King's College and 

 (through Captain Tulloch) to Professor Mackenzie of Aberdeen. 



D 



