C. DOBELL 339 



time. As some of the colonies isolated in this way were found to 

 be slightly paler or darker than -the original, attempts were made 

 to obtain new varieties by selecting these. All such attempts gave 

 negative results. It seems certain therefore that Wolf was not 

 dealing with mixed cultures or with a race which was spontaneously 

 mutating. 



By growing B. prodigiosus in media containing minute quantities 

 of various salts, Wolf succeeded in obtaining a few races with per- 

 manently altered colour. He obtained one white race and four dark 

 red races. The white race appeared after 14 successive transplant- 

 ations on a medium containing 0*01 % of corrosive sublimate (at 

 37"5° C). The dark red races arose in a similar manner, after a 

 varying number of transplantations, on media containing minute 

 quantities of potassium permanganate, cadmium nitrate, corrosive 

 sublimate, and potassium bichromate'. All these changes were abso- 

 lutely permanent — the colonies never reverting to the original colour 

 when grown for a long period in normal media'. 



In some other experiments the mutations were of a dififerent 

 type. They occurred in four cases in organisms grown upon media 

 containing potassium bichromate, copper acetate, cadmium nitrate, 

 and nickel nitrate. All these mutated races were white, but they 

 proved to be inconstant. When plated out, the white races gave rise 

 both to white colonies and to colonies of the original red colour. The 

 white colQnies behaved again in the same way — giving when plated 

 out some red and some white colonies. A white race could be con- 

 stantly maintained in this manner by selection, but it was, obviously, 

 differently constituted from the permanent white race obtained by the 

 action of HgCIj. These impermanent white forms Wolf calls "reverting 

 mutants." 



One of the most striking features in these experiments is, I think, 

 the fact that the same chemical substance may produce several quite 

 different mutations. Thus, HgClj produced a pure dark red race, and 

 a pure white race — that is, mutations in two different directions (in- 

 tensification of colour and loss of colour). Similarly, Cd(N03)2 produced 

 a constant dark red mutation and a reverting white. The action of 

 K,Cr207 is even more remarkable. Under the influence of this salt, 



1 These also were in cultures kept at 37o° C. But with KjCrjO^ the mutation was 

 obtained at room temperature also. 



^ In Wolfs experiments raising the temperature produced only a white modification 

 in colonies grown on ordinary media. But it is stated by Migula (Sy$t. d. Baet., Vol. ii., 

 p. 845) that permanent white races can be obtained by this procedure. 



