44 BIOLOGY AND TECHNIQUE 



In judging of mutation we must differentiate between temporary 

 changes of secondary characteristics which revert to the type rapidly 

 when brought back to the normal environment and those which con- 

 stitute permanent inherited characteristics. Of recent years much 

 work has been done on this question, which has been revived very 

 thoroughly by Eisenberg 33 and by Vaughan. 34 Systematic cultiva- 

 tion of colon and typhoid bacilli in the hands of Twort, Penfold and 

 others seems to have shown that agglutination as well as fermenta- 

 tion characteristics can be artificially changed. Furthermore, color- 

 producing organisms like the prodigiosus can be artificially changed 

 to colorless strains, and it is well known that certain microorganisms 

 rapidly lose their virulence when cultivated, and that the virulence 

 can only be brought back by passage through animals. Rosenow 3; 

 claims recently to have converted hemolytic streptococci into typical 

 streptococcus viridans, pneumococcus mucosus, and pneumococcus- 

 like organisms. In just how far these observations will be shown 

 to represent true permanent mutations we are not at present ready 

 to determine. If it will be found that organisms typically repre- 

 sentative of a well-known species can be changed in the animal body 

 or in culture into forms recognizedly typical of another species, we 

 will have to revise our classifications, and we can look upon the 

 classes as now established merely as convenient methods of making 

 discussion possible, but not as representing botanically constant 

 types. 



While we must therefore admit that a considerable degree of 

 mutation is possible, we do not ourselves believe that the evidence 

 is sufficiently strong to undermine the prevailing ideas as to the 

 constancy of species. Most mutations so far produced have readily 

 reverted to type when subjected to proper conditions. 



Eisenberg, Weichhardt's Ergebnesse, 1914. 



M Vaughan, Jour, of Lab. & Clin. Met!., 1915, vol. 1, 145. 



K Kosenow, Jour. Infect. Dis., xiv, 1914, 1. 



