8 OUTLINES OF EVOLUTIONARY BIOLOGY 



which it contains, has the power of absorbing energy directly 

 from the sun's rays and using this to build up the complex 

 proteids from very simple constituents. The feeding of the 

 organism, whether plant or animal, is comparable to the stoking 

 of the engine, but with this difference, that the food material, 

 unlike the fuel in the engine furnace, has usually to undergo 

 complex chemical processes, which may actually result in the 

 formation of new protoplasm, before it is available as a source 

 of energy. 



Supplies of energy are, of course, useless unless they can 

 be liberated when required. The fuel must be burnt, and 

 for this purpose, as we have already said, a supply of oxygen 

 gas is necessary, and the function which is concerned in pro- 

 viding this supply we call respiration. The term respiration, 

 however, is one in the employment of which we shall have to 

 exercise a certain amount of care. It is naturally associated 

 in our minds with the mechanical act of breathing which we 

 ourselves perform. We can see a man breathing, but we cannot 

 see an oak tree breathing ; nevertheless the oak tree performs 

 the function of respiration just as efficiently as the man. "We 

 have to learn to dissociate the essential part of this function, 

 which is common to all living things, from the subsidiary com- 

 plications which have been introduced in the case of the higher 

 animals during the course of their evolution from lower forms. 



Respiration, in the scientific acceptance of the term, is simply 

 the exchange by the organism of the carbon dioxide gas which 

 has been formed in the body in the process of combustion for 

 the oxygen gas which is required for that combustion. It is 

 therefore a double function oxygen being taken in and carbon 

 dioxide got rid of by one and the same process. This process 

 is, in its essential features, an extremely simple one, depending 

 upon the physical principle of osmosis or diffusion, in accordance 

 with which two gases of different densities tend to change places, 

 until equilibrium is established, whenever they are placed in 

 the necessary relations with one another, as when they are 

 separated only by some membrane through which both can 

 pass. 



Carbon dioxide, however, is not the only waste product 

 resulting from the breaking up of the complex proteids, for 

 these also contain hydrogen, nitrogen, sulphur and phosphorus, 

 and other products of decomposition are accordingly formed 



