A. Introductory. 



The mechanical difficulties of observing structures so minute 

 as bacteria have led to the accumulation of much differential detail, 

 and to an insistence on all points of unlikeness in physiological 

 characters or cultural reactions. Further, the lack of uniform 

 methods and standards of comparison has tended to produce an 

 overwhelming multiplicity of inadequate species-descriptions, in 

 which any slight variation from existing descriptions, e. g. as regards 

 formation of colonies on gelatin, has been erected into a character 

 of specific importance. But with the publication of Marshall 

 Ward's series of studies on Thames bacteria, of F u 1 1 e r and John- 

 ston's work on the bacteria of the Ohio River, and of Jordan's 

 paper on some 600 germs found in the Mississippi River, a check 

 has been put to indiscriminate multiplication of bacterial "species". 

 Recently , again , the demonstration of linking intermediate para- 

 typhoid and enteritidis forms in the colon-typhosus group, and of 

 the marked range of individual variation occurring in other orga- 

 nisms, has rendered especially important the questions of varia- 

 bility and of the actual lines between "species", "varieties", and 

 "races". 



The chromogenic bacteria afford, from the fact of their pig- 

 mentation, a particularly suitable field for observation; and the 

 non-parasitic nature of the special group here considered is another 

 factor favorable for comparative and for variation study. The 

 warping effect of parasitic life upon the physiological and morpho- 

 logical character of organisms is well known, and more characte- 

 ristic results are to be anticipated from the study of variation 

 in a group of saprophytic organisms than in a group of pathogenic 

 organisms. 



The agreement, among a number of bacteria, in a characte- 

 ristic so marked as is pigment production, might conceivably place 

 a series of red or yellow chromogenic germs in a category by 

 themselves, and raise the question whether this agreement in color 

 production be paralleled by agreement in biochemical and other 

 features, and whether the pigmentation be a fixed, a variable, or 

 a vital character of the organisms. So far, however, attention has 

 not been directed to a series of chromogenic germs, with the 

 exception of the comparative work done byThumm, byRuzi^ka, 

 and by Jordan on strains of B. pyocyaneus, and of the chemical 

 studies on bacterial pigments in general, referred to below. The 

 best known of the red saprophytes, B. prodigiosus, has been 



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