IMPROVEMENT OF LIVING THINGS 



233 



Make a study of the heights and weights of the members 

 of your class. How do they vary? 



Activity 144. What indications of heredity can you 

 find in your own family? 



Study the members of your family. In what ways do they 

 resemble each other? How do they differ? How do your own 

 characteristics compare with those of your parents? Record 

 your observations. 



Activity 145. What were the original sources of our 

 cultivated plants and domesticated animals? 



Answer the question in your notebook. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



Are any two living things exactly alike? The liv- 

 ing matter of all plants and animals is protoplasm. 

 Protoplasm from all sources is similar in composition, 

 yet there are probably as many different kinds of 

 protoplasm as there are living things. The protoplasm 

 of a hen's egg develops into a chicken, never into a 

 rose bush or a robin. The protoplasm of a grain of 

 wheat develops only into a wheat plant, never into 

 an elephant or an apple tree. 



Offspring resemble their parents in structure, form, 

 and appearance. They are born with characteristics or 

 qualities which a parent or some more distant ances- 

 tors had. The tendency of living things to resemble 



their ancestors is called heredity; heredity may be de- 

 nned as the transmission of traits or characteristics 

 from parent to offspring. 



All the individuals of a particular group of living 

 things are much alike. Dogs resemble dogs. Potato 

 plants resemble potato plants. There is no difficulty 

 in recognizing human beings because all human be- 

 ings resemble each other. Yet no two individuals are 

 ever exactly alike. Every human being resembles 

 other human beings more than he resembles any other 

 animal, but each human being is different in some 

 way from all the others. It is believed that no two 

 living things are ever exactly alike. Thus we dis- 

 cover that offspring generally resemble their parents, 

 but they are also different from their parents. Why 

 is this? 



Environment is one cause of variation. It is perhaps 

 the most common and obvious cause, especially in 

 plant life. Differences in temperature, weather con- 

 ditions, water supply, and soil influence the growth of 

 common plants. Plants that grow at the timber line 

 near mountain tops are stunted and slow to flower 

 compared with similar plants at lower elevations. 

 Seeds from the same parent plants sometimes produce 

 better crops on one field than on another because of 

 different soil conditions. We are familiar with the 

 effects of too much or too little water on plants. 



The development of animals also is influenced by 

 conditions of temperature, food, light, and water. Cer- 



FIG. 3/1. IXFLUKNCE OF HEREDITY 



The photographs in the upper row show three generations of a defective strain. The lower photographs show the desirable inheri- 

 tance of another family. 



