INDICATIONS OF LIFE 15 



matically controlled by the factors of their environment. He 

 assumed that all living matter is sensitive and that it responds or 

 reacts to the forces of its environment, in very definite ways. 

 These forces we call stim'uli (sing, stimulus) ; the response to such a 

 stimulus we call a tro'pism. 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 environment. Response is e\ddently one indi- 

 cation of being alive. It is a means by which plants and animals 

 adjust themselves to the favorable or unfavorable factors of their 

 environment. 



Living Things show Activity. — Response to stimuli means 

 activity or movement. But movement in living things is brought 

 about by changes within the organism, while the movement 

 of an engine is brought about by the force of burning coal or ex- 

 ploding gasoline. Evidently there is a difference here, although 

 it is not easy to explain, in our present state of knowledge. 



Method of Growth in Living Things. — The most outstanding 

 difference between the living organism and the non-hving engine 

 lies in their methods of growth. An expert mechanic may build 

 an engine, but no one has ever made a living thing. Growth takes 

 place in a crystal or a limestone stalagmite in a caA^e or in the 

 familiar example of the icicle, but it is simply accretion or gradual 

 addition of non-living material on the outside. But in a plant or 

 animal a mysterious growth takes place through taking into the 

 body various substances quite unlike the body material. The 

 making over of materials in a living body to form the living stuff 

 is called assimilation. 



Reproduction in Living Things. — Another striking attribute 

 of living matter is its ability to reproduce its kind. Living plants, 

 by seeds, sprouts, buds, or cuttings, form new li\dng plants, and 

 we all know that some animals hatch from eggs and others are 

 born alive. This act of reproduction is another activity by means 

 of which we can tell that an organism is alive. But we can no 

 more say what assimilation or growth or reproduction is than we 

 can tell what electricity is. They are activities of living things. 



How Plants and Animals react to the Primary Factors of their 

 Environment. — Water. It is a weU-known fact that most living 



