THE ORIGIN OF LIFE 23 



necessary for the plant as the existence of the latter 

 is for the animal. Plants could exist perfectly well 

 without animals, since their own respiration and the 

 decay of their own bodies would furnish them with 

 the necessary materials of life ; but if all the plants 

 were destroyed and only animals left existing, they 

 might subsist on one another for a time, but their 

 substance would rapidly leak away as inorganic 

 matter, which they would have no power to reclaim 

 and assimilate. 



In the process of the circulation of the chemical 

 elements necessary for life, an essential rdle is played 

 by the organisms known as Bacteria, without whose 

 activity life as at present constituted would very 

 rapidly come to a standstill and soon perish alto- 

 gether. We have seen that the carbon necessary for 

 life is reclaimed from the dead world by the green 

 plant, which through the aid of its chlorophyll in 

 the presence of sunlight is able to abstract the 

 carbon from the carbon dioxide gas of the atmo- 

 sphere and incorporate it with its own substance. 

 In the case of nitrogen, the green plant is able to 

 abstract this element from one source and from one 

 source only, namely, the inorganic mineral nitrates 

 in the soil. 



Now, the nitrates in the soil are almost entirely 

 derived from the decay of organic matter the 

 dead bodies and excreta of animals and plants but 



