THE CHEMICAL BASIS OF LIFE 239 



of active growth, taking up food materials of various kinds 

 and converting them into its own substance. This 

 process which is known as assimilation is an essential at- 

 tribute of all living material. The assimilated material 

 is not added to the outside as in the growth of stones and 

 most crystals, but permeates the entire mass. An animal 

 may live upon various other kinds of protoplasm, but the 

 foreign protoplasm is broken down and absorbed, and then 

 worked over in the wonderful chemical laboratory of the 

 living tissues into the peculiar protoplasm of the devouring 

 animal. 



Along with the assimilation of food, protoplasm is 

 continually undergoing a process of breaking down or 

 waste, and the materials so formed are got rid of. This 

 process is called excretion. A living organism may thus 

 be compared to a vortex through which matter is con- 

 tinually passing; the food taken in is broken down and 

 built up into living substance which after a time is broken 

 down again and eliminated. The form of the organism, 

 like that of a vortex or a waterfall, may remain constant, 

 but the matter of which it is composed is subject to a con- 

 tinual change. 



All protoplasm requires oxygen. The oxidation of pro- 

 toplasm supplies heat and other forms of energy just as 

 the oxidation or burning of coal in a furnace supplies 

 heat for the running of an engine. Among the most 

 common products of oxidation in living matter are car- 

 bon dioxide and water which are the same compounds 

 that result from the burning of a candle or a piece of wood. 

 The union of organic substance with oxygen and the giving 

 off of the products of this union (CO2 and H^O) is called 

 respiration. The cessation of respiration results in the 

 death of an organism just as the withdrawal of oxygen 

 will quickly put out a fire. 



