6 RELATIONSHIPS OF THE COCCACE^E 



of such study must be a clearer comprehension of phy- 

 logeny than can be reached by any other means. 



Before attempting to discover natural groups among 

 the bacteria it is important to distinguish the different 

 kinds of variability which contribute to the complexity 

 of the problem. At least three sorts of variations, so 

 called, may easily be differentiated. 



In the first place, many of the characters commonly 

 observed are directly dependent upon immediate environ- 

 mental conditions. Thus B. prodigiosus produces red 

 pigment at 20 degrees and not at 37 degrees. This of 

 course is not real variation, altho in many cases it is 

 mistaken for it. The minute differences in the structure 

 of colonies on gelatin, to which so much space is often 

 devoted, are, for example, mainly due to slight variations 

 in temperature, moisture, spissitude of the medium, etc. 

 (Dunham, 1903). A second class of variations is due 

 to reactions between the organism and its environment 

 in the past. These variations are the impressions left 

 upon the organism by its recent history, such as the 

 differences in virulence due to repeated animal inocu- 

 lations, or the converse adaptations of pathogenic forms 

 to culture media. To such differences it may be con- 

 venient to apply the term " impressed variations." 



Finally there is a third class of variations, the true 

 racial variations, which differentiate related bacterial 

 strains under identical environmental conditions. In 

 some groups of bacteria these racial variations are rare; 



