28 RELATIONSHIPS OF THE COCCACE^ 



heat and dryness. There is as wide a difference in 

 metabolism between the pneumococcus and a nitrifying 

 bacterium as there is in morphology between the 

 kangaroo and man. Physiological differences in one 

 case are quite as significant as morphological differences 

 in the other. 



Migula (1897) strongly maintained this ground with 

 regard to bacterial species and further pointed out that 

 the distinction between physiology and morphology is 

 merely an artificial one, since any physiological differ- 

 ence must have a morphological basis, however far below 

 the limit of our vision the difference may lie. Curiously 

 enough, he, and practically all other systematic bacteri- 

 ologists, have submitted to the arbitrary morphological 

 criterion for genera tho repudiating it for species. It 

 seems clear, however, that the same reasoning should hold 

 in both cases. If certain differences mark species, more 

 pronounced differences of the same general nature, which 

 characterize certain groups of species, may properly be 

 recognized as of generic importance. 



Secondly, an absolute requisite in regard to bacterial 

 characters, morphological or physiological, is that they 

 should be definite, and capable of so clear a description 

 that they can be identified without difficulty by other 

 observers. Vague descriptions of large size, slow lique- 

 faction of gelatin, yellow color, etc., are of little service 

 in a group where so many minute variations exist, 

 unless accompanied by definite statements as to exactly 



