CONSIDERATIONS INFLUENCING THE CLASSIFICA- 

 TION USED IN THIS EDITION OF THE MANUAL 



Robert S. Breed 



Cornell University, Geneva, New York 



The development of the classification systems used in the various editions of 

 Bergey's Manual has caused those of us responsible for this work to give 

 considerable thought to the probable evolutionary development of the living 

 things that are included under the general terms bacteria and, more recently, 

 viruses. 



For those who are not familiar with the principles of evolution, it might be 

 well to bear in mind that all living things, including bacteria and viruses, do but 

 represent the present form of a long line of ancestral forms. Customarily these 

 lines of development are thought of as being not lineal but like the twigs and 

 branches of a tree which trace their origin back to the trunk of the tree, living 

 species being regarded as the separate and distinct tips of the twigs. 



Bacteria and viruses, endowed as they are with a simple morphology, are 

 naturally thought of as being primordial or primitive in nature. This concept 

 is fundamental in all systems of classification that have been developed for these 

 organisms. Nevertheless it should not be forgotten that the different species or 

 kinds of these morphologically simple living things now extant may have under- 

 gone many types of changes during the course of their evolutionary development. 

 However, because bacteria and viruses do not have hard parts that fossilize, 

 there is little that can be learned about their evolution directly from historical 

 geology (paleontology) . 



It is difficult to picture the environment under which the undifferentiated, 

 unicellular organisms lived when they first appeared on the earth, but it is 

 certain that this environment was quite different from the environment in which 

 similar organisms live today. One important feature of the present-day envi- 

 ronment that would have been lacking in the earliest periods would be the 

 association of unicellular organisms with more highly developed types of living 

 plants and animals and with the resultant accumulation of organic materials 

 that must take place as the natural processes of life and death go forward. 

 Organisms which are saprophytic and, still less, those which are parasitic would 

 not have had conditions favorable for their existence in the earliest periods in 

 which life developed on this planet. This makes it necessary to assume that the 

 earliest living things must have existed on comparatively simple, largely inorganic 

 food materials. With this thought in mind, some students of the systematic 

 relationships of living things have thought of the chemoautotrophic bacteria 

 that still exist as being more like primordial living things than are other types of 

 bacteria. 



