UNIT THREE 



How Do Living Things Keep Alive? 



1 How can living things without mouths get what they need? 



2 How can the same food produce such different results in a calf and a 



baby? 



3 What happens to food after it is swallowed? 



4 How is it that our stomachs digest tripe but do not digest themselves? 



5 What makes sawdust food for termites but not for horses? 



6 What is it that makes one breathe faster at some times than at others? 



7 What keeps the heart beating when other muscles get tired and quit? 



8 Why are some animals warm-blooded and others cold-blooded? 



9 How con animals tell what is injurious to them and what Is useful? 



The conditions for living are fundamentally the same for all species, and 

 they are essentially the same for plants as for animals. We are impressed 

 by the great variety of living forms that keep going, and under such wonder- 

 fully diverse conditions. The whale and the jellyfish live in the same ocean. 

 The eagle and the lichen make their homes on the same bare rock. 



How does any particular organism actually keep alive? How can two 

 or more totally different species keep alive in the same surroundings ? How 

 can a similar animal manage in what appears to be quite a different set- 

 ting? We know that every living cell depends upon a supply of food and 

 oxygen. How, then, do the cells in the innermost parts of a person's body, 

 or at the tips of the limbs, get the needed supplies? 



Not only do these many different kinds of plants and animals keep alive, 

 but many withstand the most extreme physical conditions. Their ways ap- 

 pear in each case to fit the special conditions, as well as the seasonal changes 

 of their habitation. They are fitted to using a wide variety of foods. They 

 are able also to adjust themselves to scarcity as well as to abundance. 



Living protoplasm produces more and more of itself out of food that 

 is quite unlike it. But out of the same kind of food an ox makes beef, a 

 sheep mutton, a horse horseflesh, and a grasshopper something entirely dif- 

 ferent. We call it "assimilation" in every case, but what happens between 

 the arrival of food in an animal and its becoming beef or mutton or human 

 flesh? 



As life goes on, wastes are produced. The simplest organisms move 

 along, leaving their wastes behind them, just as primitive people move away 

 when their camp sites become too littered and offensive. How do larger 

 plants and animals dispose of the wastes their bodies produce? Do these 



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