The arrangement of living organisms in groups according to their resemblances and the adoption of _fixed names_ is of the greatest advantage in their scientific study. For animal forms and for the higher plants this classification is gradually becoming standardized through the International Congress of Zoölogists and of Botanists respectively. Unfortunately, the naming of the bacteria has not as yet been taken up by the latter body, though announced as one of the subjects for the Congress of 1916 (postponed on account of the war). Hence there is at present no system which can be regarded as either fixed or official. [Illustration: FIG. 49.--Illustrates the genus Streptococcus. Typical chains, no staphylococcus grouping, no sarcina grouping, no flagella.] [Illustration: FIG. 50.--Illustrates the genus Micrococcus. Diplococcus, tetrads short chains and staphylococcus; no sarcina, no flagella.] [Illustration: FIG. 51.--Illustrates the genus Sarcina. Sarcina grouping, no flagella.] [Illustration: FIG. 52.--Illustrates the genus Bacillus. A bacillus with peritrichic flagella. (Student preparation.)] Since Müller’s first classification of “animalcules” in 1786 numerous attempts have been made to solve the problem. Only those beginning with Ferdinand Cohn (1872-75) are of any real value. As long as bacteria are regarded as plants it appears that the logical method is to follow the well-established botanical principles in any system for naming them. Botanists depend on morphological features almost entirely in making their distinctions. The preceding chapters have shown that the minute plants which are discussed have very few such features. They are, to recapitulate, _cell wall_, _protoplasm_, _vacuoles_, _metachromatic granules_, _capsules_, _flagella_, _spores_, _cell forms_ and _cell groupings_. Most bacteria show not more than three or four of these features, so that it is impossible by the aid of morphology alone to distinguish from each other the large number of different kinds which certainly exist. In the various systems which are conceded to be the best these characteristics do serve to classify them down to genera, leaving the “species” to be determined from their _physiological_ activities. One of these systems was adopted by the laboratory section of the American Public Health Association and by the Society of American Bacteriologists and was practically the standard in this country until superseded by the Society’s own classification. It is that of the German Bacteriologist Migula and is given below for comparison. Since practically the entire discussion in this book is concerned with the first three families the generic characteristics in these only will be given. The full classification as well as a thorough discussion of this subject is given in Lafar’s _Handbuch_, whence the following is adopted: [Illustration: FIG. 53.--Illustrates the genus Pseudomonas. A bacillus with flagella at the end only.]