UNIT X. LIFE ON THE EARTH 



Every object in the world can be classed as either 

 a living thing or a non-living thing. Have you ever 

 thought of the great variety of living things which in- 

 habit our earth? Thousands of different kinds of ani- 

 mals and nearly as many different kinds of plants are 

 known. There are also many kinds that have not yet 

 been named and described in scientific literature. 



The number of kinds of plants and animals living 

 on the earth today is small, however, compared to the 

 number of kinds that have lived and perished. From 

 a study of layers of rock which contain imprints and 

 remains of living things, we learn that plants and ani- 

 mals have lived on this earth for millions of years. 

 Many kinds have become extinct because they failed 

 to adjust themselves to a changing environment. Less 

 than a hundred years ago there were millions of pas- 

 senger pigeons on this continent ; today there is not a 

 single one. 



In some respects this unit about life on the earth is 

 the most important unit in the entire course. Man's 

 progress from savagery to his present form of civiliza- 

 tion has been due in large part to his ability to modify 

 his environment and improve his living conditions. 

 We are anxious that you master the scientific princi- 



ples underlying the origin, life processes, and im- 

 provement of living things, and especially the im- 

 provement of man himself. The work of the world 

 cannot be carried on by weaklings. The future prog- 

 ress of mankind depends in a large measure upon the 

 removal of unfit persons from society and the im- 

 provement of the fit. 



What do you already know about living things? 



1. Have you ever seen any two living things ex- 

 actly alike? 



2. How many different kinds of living things are 

 there in the world? 



3. Where did life come from originally? 



4. Why do you resemble your parents? 



5. Where did our domesticated animals, such as 

 horses, cows, dogs, pigs, and chickens, come from 

 originally? 



6. Where did our domesticated plants come from 

 originally? 



7. How are plants and animals improved today? 



8. Is man in need of any improvement? 



9. Can man be improved? 



10. Who was Luther Burbank? 



TOPIC 1. OUR SMALLEST LIVING THINGS 



SUGGESTED PROBLEMS AND QUESTIONS 



1. What are bacteria? How large are they? 



2. How do bacteria grow and reproduce? Under 

 what conditions do they grow and reproduce 

 best? 



3. How do bacteria aid man in industry, in food 

 manufacture, and in maintaining soil fertility? 



4. What is the relation of bacteria to disease? 



5. How do persons become immune to a disease? 



6. How do bacteria enter the human body? 



7. What can an individual do to help his body 

 resist disease ? 



SUGGESTIONS AND HELPS FOR STUDY 



1. Since bacteria are very small it is necessary to 

 look at them through a microscope. If you have a 

 microscope available, prepare some cultures of bac- 

 teria and study them. Directions for making cultures 

 are given in the experiments of this topic. 



2. Keep in mind as you study this topic that some 

 bacteria are very useful to man while some are deadly 

 enemies which cause disease. 



3. Be sure you understand problems 4, 5, 6, and 



7. Gather all the information you can find in the ref- 

 erences relating to these problems and master it. 

 Every person should know how diseases are caused 

 and how to prevent and control them. 

 4. Study the meanings of the following words: 



environment the surroundings of living things. 

 bacteriology a study of bacteria. 

 disinfectant something which kills bacteria. 

 antitoxin a substance injected into the body to over- 

 come the effects of some disease germ. 

 antiseptic capable of killing germs. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 134. What are bacteria and how do they 

 grow? 



Put some leaves and hay in a dish of water. After several 

 days, mount a drop of the liquid on a glass slide. Look at 

 it through a microscope. Look for three different types of 

 bacteria: rod-shaped or bacillus (basil'iis), spherical or 

 coccus (kok'us), and spiral or spirillum (spi-ril'um). How 

 many kinds can you distinguish ? Do any of them move 

 about? For continued study of bacteria it is desirable to 

 prepare bacteria gardens. The following is a simple method : 



1 See workbook, p. 79. 



215 



