RELATION TO ENVIRONMENT CLASSIFICATION 39 



seems to indicate a fundamental chemical difference in the constitution 

 of bacteria often morphologically very much alike. It is certainly a 

 remarkable fact that organisms such as those belonging to the colon- 

 typhoid-dysentery group, though morphologically not differentiable, may 

 still retain differences both in pathogenicity and in fermentation powers 

 after being kept for ten or more years in laboratory media, and the same 

 experience we have had with organisms belonging to the diphtheria 

 group. The virulence of plague and anthrax bacilli may be retained 

 for years in storage, and such evidence shows pretty definitely that 

 fundamental constant differences between organisms exist. 



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 consti- 

 tute permanent inherited characteristics. Of recent years much work 

 has been done on this question, which has been reviewed very thoroughly 

 by Eisenberg 1 and by Vaughan. 2 Systematic cultivation of colon and 

 typhoid bacilli in the hands of Twort, Penfold and others seems to have 

 shown that agglutination as well as fermentation 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, pneumococ- 

 cus mucosus, and pneiimococcus-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 representative 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 muta- 

 tion is possible, we do not ourselves believe that the evidence is suffi- 

 ciently 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. 



1 Eisenberg, Weichhardt's Ergebnisse, 1914. 



2 Vaughan, Jour, of Lab. & Clin. Med., 1915, vol. i, 145. 



3 Rosenow, Jour. Infect. Dis., xiv, 1914, 1. 



