Generic Characters in the Coccaceae 191 



each hue were nine different chromas, indicated by Arabic numerals, 

 each figure indicating the number of washes of pure color added 

 to obtain the particular chroma. 



The distribution of the organisms studied under these different 

 colors is indicated in Fig. 3, where the vertical columns indicate 

 the hues from I (white), through the yellows (II-IV), the oranges 

 (V and VI) to the reds (VII-IX), and the horizontal columns the 

 successive chromas. 



On inspection of this chart, bearing in mind the colors signified, 

 there appear at once four modes - one occurring in each chief color. 



That for the white falls at I,, for the yellows at IV,, for the oranges 

 at VIg, and for the reds at Vllg. These are not, of course, the 

 points of intcnsest color, but of the most concentrated distribution. 

 The evident clustering of the individuals around a mode, and the 

 consequent falling-away of the numbers between the modes, sug- 

 gest a variation from an ancestral center. Like most living things 

 governed by an evolutionary law of gradual change, the hues grade 

 so gently into each other that the exact placing of lines of division 

 must be arbitrary. We have, however, assumed four divisions as a 

 basis for our work, and separated them at the lowest points be- 

 tween the modes, as shown by the heavy black lines in the chart, 

 which divide the group of bacteria producing a white pigment from 

 that which produces a yellow, the yellow from the orange, and the 

 orange from the red. The striking correlations obtained between 

 chromogenesis and other properties have convinced us that this divi- 

 sion was a sound and natural one. It should be noted, however, 

 that the division of the "white" chromogens includes two sub- 

 groups — the true white pigment-formers and the forms which pro- 

 duce such a faint surface growth that no distinct color is apparent. 



We have omitted the consideration of chromogenesis from our 

 correlation tables, except that for habitat, preferring to consider 

 all chromogenic relations under one head. It will appear, on inspec- 

 tion of the following tables, that this character is really the key 

 by which most of the other correlations may be explained, and is 

 perhaps the most important single factor in the systematic grouping 

 of the Coccacese. 



It was shown under "Habitat" that the white and orange chrome- 



