CONSIDERATIONS INFLUENCING CLASSIFICATION 9 



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 relation- 

 ships 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 undif- 

 ferentiated 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 view- 

 points. 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 ade- 

 quately 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 classifi- 

 cation 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 (Lehr- 

 buch der Botanik, 4 Aufl., 926 pp., Wilhelm Engelmann, Leipzig). Recently 

 Sachs' concept of this group has been developed further by a Russian system- 

 atist, 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 iiber der Verhandlungen der bot. Section 

 der 33 Versammlung deutscher Naturforscher und Xrzter. 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 



