80 RELATIONSHIPS OF THE COCCACE^: 



The first point made evident by this diagram is the 

 fact that the yellow and orange pigment producers are 

 not intergrading forms but separate and distinct types. 

 There is a well-marked center of variation in the lighter 

 chromas of cadmium yellow, and another in the darker 

 chromas of cadmium orange, with a much smaller number 

 of cultures showing colors of intermediate grade. The 

 second point which will be noticed is the marked corre- 

 lation between chromogenic power and habitat. It is 

 evident that four groups of cocci are represented on the 

 chart, corresponding to the four types of pigment pro- 

 duction familiar to any student of the cocci. In the 

 upper left portion of the diagram are white (or colorless) 

 parasitic forms. Both the feebly growing streptococci 

 and the more vigorous white staphylococci are included 

 under this head. In the yellow, another sharply marked 

 center of variation appears, this time including organisms 

 mainly of saprophytic origin. The orange forms are 

 sharply contrasted, being almost wholly parasitic in origin; 

 and the red pigment producers, in the last three columns 

 of the figure, are with one exception saprophytic. 



These relations may be shown in another way by divid- 

 ing the cocci studied into arbitrary groups according to 

 chromogenesis. For this purpose we have classed the 

 organisms in the White column and in the upper por- 

 tions of the Light Yellow columns as White (White, I 

 Light Lemon Yellow, I-III, Light Cadmium Yellow, 

 I-III). The deeper shades of Light Cadmium Yellow, 



