374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



regarded as simplified or degenerate types — a reversal of their true 

 relationship. 



The largely speculative conclusion sketched above derives a con- 

 siderable measure of probability from the recent discovery of fossil 

 bacteria, as well as evidences of their activities in some of the oldest 

 sedimentary rocks, and it is legitimate to consider that the denudation 

 of the most ancient rock surfaces was accelerated by the action of 

 bacteria in facilitating rock weathering exactly as they have been 

 shown to do at the present time. The next stage in bacterial history, 

 represented at the present time by the Metatrophic forms, was that 

 in which the necessary carbon for their activities was derived from 

 the complex nitrogenous compounds of other bacteria, or of algae as 

 the latter came into existence instead of from inorganic sources. The 

 third stage was one in which the habit of true parasitism became 

 developed, represented at the present time by the Paratrophic bac- 

 teria. The evolution of Metatrophic bacteria appears to have brought 

 to a close that period of earth history during which the process of 

 organizing complex organic molecules from inorganic materials was 

 possible, since bacterial activity would inhibit such processes, and 

 this explains in a measure why the origin of life was an evolution 

 that could not take place in later geologic times. 



Pre-Cambrian limestones, as well as those of later ages, which are 

 without traces of discrete organisms, are now considered by geologists 

 as indicating the presence of bacteria as the active agents of deposi- 

 tion, in the same manner as Drew and others have shown the recent 

 calcareous oozes to have been precipitated. This also furnishes stu- 

 dents with some measure of the probable temperatures of the waters. 

 Similarly iron bacteria are probably responsible for the bedded iron 

 ores of the primitive rocks. 



The presence of Proterozoic bacteria has been demonstrated 

 recently by Walcott. Paleozoic bacteria were first discovered by Van 

 Tiegham in 1879, indicated by the evidences of butyric fermentation 

 in the cellulose membranes of silicified plants from Saint Etienne. 

 Subsequently a considerable number of Paleozoic bacteria were de- 

 scribed by Renault and others by a microscopic study of sections of 

 petrified plants and coprolites. Evidences of bacterial activity are 

 obvious in petrified-plant material of all geological ages, but this 

 field of study is not an especially fruitful one, since these organisms 

 are so small, so ancient, and unchanging, and hence offer but few 

 reliable characters of systematic value. 



It might perhaps be legitimate to divide plant evolution into two 

 major stages — the prechlorophyllic and the chlorophyllic — and to 

 consider that the eophytic stage leading to the formation of the 

 simpler algae required as long a time as all the subsequent evolution 

 of the vegetable kingdom. When, however, it is recalled that the 



