6 INTRODUCTION 



ducted over a long period and with every care to prevent 

 contamination, has come to the conclusion that the 

 bacteria are not an independent group of organisms, but 

 that the bacteria, yeasts, and moulds are stages in the 

 life-history of green algae. The observations were carried 

 out both by culture methods and by microscopical examina- 

 tion. A culture of a single-celled alga belonging to the 

 Palmellacea was obtained, but by modifying the culture 

 medium in which a pure culture of the alga was growing, 

 by the addition of acid, of alkali, or of traces of copper 

 salts, other organisms, generally bacteria, occasionally 

 moulds and yeasts, and even spirochaetes, made their 

 appearance. Granting that there is no flaw in the experi- 

 mental methods, and every care seems to have been taken 

 to exclude contamination, etc., the results are susceptible 

 of another explanation, viz. that the secondary growths 

 were derived by transformation of the algal cells, in fact 

 by the phenomenon of heterogenesis which has been 

 claimed by Bastian to occur. 



Undoubtedly bacteria exhibit variations and mutations, 

 not only in morphology (see p. 16) but also in function. 

 Thus pathogenic organisms may become non-pathogenic, 

 and Twort has succeeded in training B. typhosus to fer- 

 ment lactose, which ordinarily it does not. Some experi- 

 ments by Horrocks suggest that the B. typhosus may, by 

 symbiosis with B. coli, be converted into B. alcaligenes, 

 and Revis has found many variations occur with coliform 

 organisms as a result of cultivating in malachite green 

 media, etc. 1 



As a result of exposure of sporing anthrax to ultra-violet 

 rays, Mme. Henri 2 states that stable coccoid and Gram- 

 negative, thin filamentous forms are obtained. 



1 Proc. Roy. Soc. Lond., B, vol. 85, p. 192, and vol. 86, p. 373 ; Cenlr. 

 /. Bakt., Abt. II, ]912 and "1913. 



2 Comp. Rend. Acad. Sc., vol. 158, No. 14, 1914, p. 1032. 



