PROSPECTS FOR A NATURAL SYSTEM OF CLASSIFICATION OF BACTERIA 



for a special group of substrates - carbohydrates, protein decomposi- 

 tion products, or organates - involves an adaptation to life in an envi- 

 ronment of a special range of hydrogen ion concentration. 



It must be pointed out, however, that a judicious selection of the 

 criteria on which the above mentioned subdivision should be founded, 

 is most essential. In this connexion it cannot be sufficiently stressed 

 that one should not confuse the physiological significance of the ability 

 to derive energy from the desmolytic transformation of a special group 

 of substrates with the ability to hydrolyze compounds of a complex 

 nature, like proteins, polysaccharides, fats etc. For it must be clear that 

 there exists a far more fundamental difference between an organism 

 which splits glucose into lactic acid and a second organism which prod- 

 uces out of the same substrate butyric and acetic acid, carbon dioxide 

 and hydrogen, than between two lactic acid bacteria only one of 

 which attacks maltose. In the latter case both organisms derive their 

 energy from the same conversion of glucose into lactic acid, the hydrol- 

 ysis of the maltose into glucose being only an introductory act devoid 

 of any energetic significance. Differences in the hydrolytic capacity 

 should never be applied for distinguishing systematic groups, they can 

 merely be taken into account for the differentiation of species. 



The neglect of this point of view is undoubtedly responsible for the 

 dislike with which morphologically inclined taxonomists view the 

 application of physiological characters for the delimitation of system- 

 atic units larger than species. 



Finally some remarks should be made regarding a physiological 

 character of rather wide application in present day taxonomy, viz., 

 pathogenicity. This appears to be a character of very doubtful value. 

 For the case is not rare that a pathogenic organism is so closely related 

 to a non-pathogenic one that the two are undistinguishable except 

 with the aid of infection experiments. The creation of separate genera 

 on the basis of such a character is objectionable, because this implies 

 that even the generic nature of an organism cannot be decided upon 

 independently of a knowledge of its previous history. The same diffi- 

 culty holds, albeit to a lesser extent, for a differentiation of species op 

 the basis of pathogenicity.* 



* Cf. particularly the valuable remarks made by O. Rahn. Zentralbl. Bakt. Para- 

 sitenk. Abt. II. y8, i, 1929; 79, 321, 1929; and Stapp. C, Schizomycetes (Spalt- 

 pilze oder Bakterien ) in: Sorauer, Handb. d. Pflanzenkrankh. Bd. 2. 1928. S. 1-295. 



289 



