work (Professor R. S. Breed, Professor E. G. D. Murray and Dr. N. R. Smith) have developed an outline classification which expresses our ideas of the relationships of the simplest types of living things. These are represented by such common terms as true bacteria, filamentous bacteria, actinomycetes, slime bacteria, spirochetes, rickettsias and related larger viruses and the filterable viruses. This general classification also expresses our ideas of the relationships of these undifferentiated types of living things to higher plants.
This outline may not express the views of other special students of this subject adequately, as all such outlines represent compromises between differing viewpoints. One such difference of viewpoint that has been discussed among the three of us chiefly responsible for the outline given here has been the question whether a third kingdom, the Protophyta as defined below, ought not to be recognized in addition to the Plant and Animal Kingdoms. Prof. E. G. D. Murray has been the one in our group who has felt most strongly that the bacteria and related organisms are so different from plants and animals that they should be grouped in a kingdom equal in rank with these kingdoms. It is quite probable that support for this viewpoint would be stronger if early biologists had known how different these important and widely diversified microorganisms are from plants and animals. Even today it must be recognized that our knowledge of the number of kinds of bacteria is growing rapidly as habitats not previously adequately explored are studied. The human body is, as a matter of fact, practically the only habitat that has been comprehensively studied as a source of bacteria. Even in this case it is the bacteria that cause diseases that are best known.
Our knowledge of the still smaller types of parasitic and pathogenic organisms such as the numerous kinds of organisms found in the Rickettsiales and Virales is still more inadequate than our knowledge of the true bacteria. In fact our present-day knowledge of the filterable viruses could perhaps best be compared with Cohn's knowledge of the bacteria when he first drew up a system of classification for bacteria in 1872.
Three groups are included in the outline presented here: (a) the blue-green algae, (b) bacteria and related forms, and (c) the rickettsias and viruses. These are placed in a single division of the plant kingdom for which the term Protophyta has been used. This name was suggested by a botanist, Sachs (Lehrbuch der Botanik, 4 Aufl., 926 pp., Wilhelm Engelmann, Leipzig). Recently Sachs' concept of this group has been developed further by a Russian systematist, N. A. Krassilnikov (Guide to the Bacteria and Actinomycetes (Russian), Izd. Akad. Nauk, Moskau, U.S.S.R., 1949, 830 pp.), and it is developed still further in the present edition of the Manual.
Of the three names used for the different classes of Protophyta, Schizomycetes was suggested by von Naegeli (Bericht über der Verhandlungen der bot. Section der 33 Versammlung deutscher Naturforscher und Ärzter. Bot. Ztng., 15, 1857, 760) and Schizophyceae by Cohn (Jahresber. Schles. Ges. f. vaterl. Cultur f. 1879, 279-289), and these have been generally used. The development of our knowledge of the rickettsias and viruses is so recent that no truly satisfactory class name
