Most approaches to the problem of bacterial taxonomy have arisen from either of two viewpoints, one derived from phylogenetic and the other from practical considerations. The former viewpoint too frequently arises from some major premise which has little practical connotation. The latter viewpoint often leads to the submergence of large groups of bacteria, not known to be of economic importance, because of an attitude of impatience toward any system which does not reflect the methods used in the specialized laboratory where steps in the identification of an unknown organism must be measured in terms of utility and speed. Regardless of personal viewpoints, it must be realized that precise delineation of species cannot be the primary aim of bacterial taxonomy at present. It is seldom possible and often it may not even be desirable. We must compromise by recognizing the necessity for the organization within a taxonomic system of a selected body of knowledge of important differential characters which may be applied when practical considerations demand that phylogenetically related organisms be distinguished, one from another. In addition, the taxonomic system should suggest, by implication if not by statement, the fields and directions in which further investigation is necessary or may prove fruitful. This implies that taxonomic systems must undergo periodic revision with the advent of new knowledge. This discussion will be concerned with the taxonomic system followed for the Enterobacteriaceae in the fifth edition of "Bergey's Manual of Determinative Bacteriology" (Bergey, Breed, Murray and Hitchens, 1939). THE FAMILY Enterobacteriaceae Rahn (Zentr. Bakt. Parasitenk., II, 96, 280, 1937) The difficulties besetting the taxonomist in devising and revising a classification are well illustrated by the family Erterobacteriaceae. New knowledge of this group has accumulated to such an extent that incorporation of it into our present system produces a confusingly top-heavy struc'ture. Furthermore, satisfactory labeling of many forms with intermediate characteristics is not now possible even though those forms are commonly encountered in practical work. Not only is simplication needed but also reconstruction of the general framework of-the system so that it will provide more effectively than it now does for the absorption of newly described types. This family, while not discontinuous with other bacterial families, is so well delineated by tradition and usage that no radical change in its definition is 351

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