266 THE PHILOSOPHY OF BIOLOGY 



toward cessation. The plant requires also carbon 

 dioxide and the amount of this substance in the atmo- 

 sphere is very limited, while the only inorganic source 

 from which it can be renewed seems to be volcanic 

 activity : this substance also would tend to disappear. 

 A time would therefore come when plant life on the 

 earth would cease to be possible because of the disap- 

 pearance of the materials on which it depends ; but 

 while it did exist its result would be the accumulation 

 of chemical compounds of high potential energy. The 

 result of the metabolism of the plant is the formation 

 of such compounds as cellulose from woody tissues and 

 shed leaves, of other plant carbohydrates, of oils and 

 resins, and of proteids. In the absence of bacteria such 

 substances would persist unchanged : even in an earth 

 tenanted by bacteria such products as oils, lignite, 

 peat, coal, etc., have been able to accumulate through- 

 out geological time. The tendency of plant life is 

 therefore toward the accumulation of compounds of 

 high potential energy, and this process also is irre- 

 versible. 



Bacterial activity would, of itself, make continued 

 plant life possible on the earth. The essential characters 

 of these organisms are their ability to bring about the 

 most varied energy-transformations. From our present 

 point of view bacteria may be divided into paratrophic, 

 met atrophic, and prototrophic forms. Paratrophic 

 bacteria are those which live as parasites within the 

 living tissues of plants and animals : this mode of life 

 is obligatory, and these organisms are unable to live in 

 the open. The result of their activity is the breaking 

 down of protoplasmic substance. Met atrophic bacteria 

 are those that produce putrefaction and fermentation 

 of organic compounds. They may be parasitic in 

 their mode of life, but most of them live in soil, in water, 



