A statistical criterion for species and genera among the bacteria" 



C.'E. A. WiNsr.ow 



The existence of an almost infinite number of minute vari- 

 ations in many groups of bacteria has so far almost nullified any 

 attempt at a natural classification. Isolated bacterial cells, lying 

 free in a liquid medium, are exposed in a high degree to the direct 

 effect of the environment ; and the extent to which virulence 

 may be exalted or attenuated shows how easily they respond to 

 such influences. The absence of the sex process probably magni- 

 fies any tendencies to variability. Amphimixis may well serve, 

 as Weissmann holds, to promote minor variations ; but on the 

 other hand it is reasonable to believe that it checks more extreme 

 deviations from the mean. Among asexual organisms, every 

 variation that arises is preserved intact until acted on by some 

 modifying cause. Finally, the rapid multiplication of the bacteria 

 offers an exceptional opportunity for the action of selective influ- 

 ences. The immense number o( generations which succeed each 

 other in a short space of time makes boundary lines as shifting as 

 they would become among the higher plants if a dozen geological 

 periods were considered all at once. 



In certain groups of the bacteria, among the Coccaceae for 

 example, and in the group of Bacillus Diphtheriae, the incon- 

 stancy of characters is specially marked and it is possible by ap- 

 propriate laboratory experiments to modify profoundly such 

 important metabolic properties as acid-production and chromo- 

 genesis. Other groups, like that of the aerobic spore-formers, 

 appear to be in relatively stable equilibrium. Even where the 

 characters of any given strain are fairly constant, classification is 

 often complicated by the fact that an almost infinite series of 

 minutely differing strains may be found, each apparently constant 

 under existing conditions but together forming a continuous series 

 connecting widely separated types. 



*Read before Section G of the American Association for the Advancement of Sci- 

 ence at the Baltimore Meeting, December 31, 1908. 



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