(Welch's Clostridium), Salmonella pullorum (the Salmonella of chicks), Streptococcus lactis (the Streptococcus of milk), Brucella abortus (the Brucella of abortion), Clostridium tetani (the Clostridium of tetanus), Diplococcus pneumoniae (the Diplococcus of pneumonia), Salmonella anatum (the Salmonella of ducks).
(d) A noun in apposition, that is, an explanatory noun. This does not agree necessarily with the generic name in gender. This method of naming is relatively uncommon in bacteriology. Examples are Actinomyces scabies (the scurf or scab Actinomyces), Bacillus lacticola (the milk-dweller Bacillus), Rhizobium radicicola (the root-dweller Rhizobium), Salmonella london (The London Salmonella).
5. The author of the name of a taxon is often cited by having his name follow that of the species, as Bacillus subtilis Cohn. Sometimes the name of another author is indicated also in parentheses, as Micrococcus luteus (Schroeter) Cohn. This means that Schroeter first named the species, giving it the specific epithet luteum, (placing it in the genus Bacteridium) . Cohn transferred it to the genus Micrococcus. It should be noted that the name of a person following that of an organism frequently is not that of the individual who first discovered or described it, but of the person who first gave it the accepted name. For example, Clostridium welchii (Migula) Holland was first described by Dr. Wm. H. Welch, but not named by him. It was named by Migula in honor of Dr. Welch and later placed in the genus Clostridium by Holland.
6. Sometimes species of bacteria are subdivided into subspecies or varieties. These are likewise given Latin designations, and the entire name written, as: Streptococcus lactis subspecies (var.) maltigenes (the Streptococcus of milk producing malt flavor), or merely Streptococcus lactis maltigenes.
Some principles of taxonomy. The student of bacteriology should recognize the meaning of certain terms used regularly in classifications.
(1) Species (plural species). A species of plant (or animal) is assumed above to be one kind of plant. But how much difference must exist between two cultures of bacteria before one is justified in regarding the organisms in them as being of distinct kinds or species? No rule can be laid down. It depends largely upon convenience and upon more or less arbitrary but considered decision. As stated by Hitchcock (Descriptive Systematic Botany, New York, 1925, p. 8): "The unit of classification is a coherent group of like individuals, called a species. The term is difficult to define with precision because a species is not a definite entity, but a taxonomic concept." Hucker and Pederson (New York Agric. Exper. Sta. Tech. Bull. 167, 1930, p. 39) state: "The difficulty met with among these lower forms in dividing them into well-defined groups has led many to question whether these small groups of 'species' are natural groups and whether such groups can be considered to be similar to 'species' among higher forms. However this may be, it is necessary to arrange bacteria as well as possible into groups or so-called 'species' for convenience in classification," and again (Hucker, New York Agric. Exper. Sta. Tech. Bull. 100, 1924, p. 29), "characters applicable to the differentiation of species must evidence a certain amount of constancy when studied over a large series of tests. Furthermore, characters adapted
