PREVIEW 37 



of dynamite. Or electricity, in the form of a flash of lightning, 

 may destroy a tree. 



It is not easy to tell exactly what it is to be alive, any more than 

 it is easy to tell what electricity is or what radioactivity is. Elec- 

 tricity is a servant of man, but the greatest expert cannot tell 

 what the force actually is. Life is a manifestation of forces, like 

 a flame or electricity. Every living thing, as we shall see later, 

 is like a steam engine or any other machine, in that it is a medium 

 used for the transformation of energy. So to understand the 

 meaning of life we had better start by trying to see how living 

 things act in their normal environment when outside forces influ- 

 ence them. 



One of the world's great biologists, Jacques Loeb (zhak lob), 

 some years before his death attempted to prove that all living 

 things are more or less automatically controlled by the factors 

 of their environment. He assumed that all living matter is sensi- 

 tive and that it responds or reacts to the forces of its environment, 

 in very definite ways. These forces we call stimuli (sing, stimulus) ; 

 the response which is made to such a stimulus we call a tropism. 

 Loeb and his followers have shown quite conclusively that living 

 matter responds very definitely to temperature, touch, chemical 

 substances, electricity, and various other factors of the environ- 

 ment. The behavior of plants and animals in response to these 

 various stimuli is one indication of being alive. 



Response to stimuli is evidenced by activity or movement. 

 Movement in living things is brought about by changes within 

 the living material of which the organism is composed, while the 

 movement of non-living things, as an engine, is brought about by 

 the force of burning coal or exploding gasoline. This activity 

 is due to the fact that living things are like engines in another 

 respect : while the engine oxidizes fuel to release energy, they 

 oxidize the food taken into their bodies and release energy in the 

 form of motion or other kinds of work. 



Any living thing, plant or animal, must get food and digest it, 



must circulate this digested food to various parts of its body, must 



assimilate or make the prepared food into a part of itself, must 



excrete or get rid of wastes, and must reproduce or form new 



h. bio — 4 



