80 THE MECHANISM OF LIFE 



dead animal and plant bodies, " offal," or remains. All organic 

 substances whatever are susceptible to bacterial action, and 

 since micro-organisms are universally distributed in nature, in 

 the air, soil, and in fresh and salt water, organic matter thus 

 suffers resolution into innocuous mineral substances of very 

 simple chemical constitution. There are certain conditions in 

 which this bacterial decomposition of organic matter is delayed. 

 At the bottoms of deep oceans the temperature of the sea is very 

 little above freezing-point (or is even below the freezing-point of 

 fresh- water), and there putrefactive action goes on very slowly. 

 In extreme northern climates the air temperature is also very 

 low, and dead bodies of animals are sometimes frozen in ice or 

 in the soil, and so escape decay. In very dry climates organic 

 matter also remains in a relatively stable condition, since the 

 putrefactive bacteria are unable to function in the absence of 

 water. Such is the mode of origin of many forms of guano. 



But, in general, all organic matter is ultimately resolved into 

 carbonic acid, water, and nitrate, and these substances tend to be 

 distributed all over the earth. Carbonic acid is contained to the 

 extent of about 0-04 per cent, in the atmosphere, and it is present 

 in rather larger proportions in fresh and salt water. "Water itself 

 is distributed everywhere except over desert land areas. Mineral 

 nitrogenous substances, such as nitrites, nitrates, and salts of 

 ammonia, are also contained everywhere in the soil and in fresh 

 and salt water, and oxides of nitrogen, capable of forming nitric 

 acid, are formed in the atmosphere by the combination of 

 nitrogen and oxygen that occurs whenever there are lightning 

 discharges. But mineral nitrogenous compounds — which are 

 the indispensable materials of life — occur everywhere in exceed- 

 ingly small quantities,* and the abundance of life depends on the 

 quantity of these substances that is available. 



It is therefore convenient roughly to classify all living things 

 into animals, bacteria, and plants. The animals consume 

 proteids, fats, and carbohydrates, oxidising these substances 

 into water, carbonic acid, and certain nitrogenous residues such 

 as urea, uric acid, hippuric acid, etc. The bacteria act upon the 

 nitrogenous residues, converting them into nitrate, and they also 

 act upon dead animal and plant tissues, and upon the excreta 



* Except under quite special conditions, as when deposits of " Chili 

 saltpetre " and analogous substances are formed. Here the dry atmo- 

 sphere and soil inhibit plant growth, with the ^result that nitrate 

 accumulates. 



