14 EVOLUTION OF LIVING OKGANISMS. 



they run the risk of starvation if no proteins are avail- 

 able, yet by feeding on other organisms they make 

 use of the best fuel, with the greatest amount of poten- 

 tial energy, and may have large quantities of surplus 

 energy to dispose of for the amount of energy freed 

 by the breaking down of the food material much exceeds 

 that consumed in the anabolic processes in the ordinary 

 course of life. 



All animals depend ultimately on plants for their food. 

 Carnivorous live on herbivorous animals, and these in 

 turn on plants, whose powers of synthesis are more 

 complete. 



Even the ordinary green plants can only build up pro- 

 toplasm from inorganic material in the presence of sun- 

 light. This they accomplish with the help of the green 

 substance, chlorophyll, which decomposes the carbon 

 dioxide of the atmosphere, freeing the oxygen, and com- 

 bining the carbon with water to form starch, Cell^O^ 

 thus storing up energy from the sun. With the help 

 of inorganic nitrogen compounds derived from the soil, 

 protein can then be formed. 



Many lower plants have the power of building up pro- 

 toplasm without chlorophyll and sunlight. Fungi can 

 synthesise protein from carbohydrates and inorganic 

 salts of nitrogen, but depend on organic compounds 

 for their supply of carbon. Among the multitude of 

 bacteria which abound in the soil there are many which 

 can build up proteins, and therefore protoplasm, from 

 inorganic compounds alone ; and some indeed which can 

 make use for this purpose of the free nitrogen of the 

 air. 



All the phenomena of life are associated with the 

 physico-chemical processes of metabolism taking place 

 in protoplasm alone. The three most characteristic 

 properties of living matter : irritability, or the power 

 to respond to stimuli, growth, and reproduction, all 

 depend on metabolism. An excess of anabolic over 



