Page:Bergey's manual of determinative bacteriology.djvu/26

 Robert S. Breed Cornell University, Geneva, New York The development of the classification systems used in the various editions of 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 undergone 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 environment 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.