UNIT X 

 LIVING THINGS IN THEIR ENVIRONMENT 



PREVIEW 



What is being alive ? You know what your dog or cat 

 does and what you do every day. You eat, sleep, move 

 about, and play. But when it comes to really knowing 

 what life is, we cannot tell very much about it. You know 

 that you and your pet eat food, digest it, use it somehow 

 to release energy and to grow, and that you are able to 

 get rid of harmful wastes. You know that animals and 

 plants are able to form new living things like themselves. 

 But it is hard for a beginner in science to know much 

 about what life really is, for that is a problem that has 

 been troubling scientists for a good many years. 



Scientists say that everything living or nonliving is in 

 the long run a manifestation of electricity ; that the matter 

 out of which all living or nonliving stuff is composed is 

 made up of unlike units, electrons and protons ; and that 

 every change in nature is due to the action and inter- 

 action of these particles. But when you have heard this 

 statement and even seen demonstrations which show it 

 to be true, you still cannot understand what it means. 



So it is with life. We may talk very learnedly about it 

 and have all kinds of theories concerning it, but we really 

 have to go back to its manifestations to what it does 

 rather than to what it is if we are to try to understand 

 much about it. About all we can say is this : that living 

 things show their aliveness, first, by being sensitive. They 

 respond or react to the stimuli of their environment. You 



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