HOW DO LIVING THINGS REPRODUCE? 611 



fish, placed in water containing large amounts of magnesium salts^ 

 may produce one-eyed fish. Pressure or other mechanical stimuli 

 may cause the developing eggs of some marine animals to develop 

 into many different sorts of abnormal forms which do not become 

 adults. William Beebe has induced great changes in the colors 

 of birds by keeping them in unusual conditions of humidity. 

 However, the greatest evidence seems to support the fact that 

 plants and animals do not in general hand down the changes 

 brought about by the environment. 



Practical Exercise 2. Can you give any examples of how environment has 

 caused changes in animals? Do you see any relation between environment 

 and adaptations ? Explain to the class. 



Practical Exercise 3. Write a 300-word paragraph on environment as a 

 factor in molding one's life. 



Self-Testing Exercise 



Roots grow (1) because they are affected by (2). 



The kinds and amounts of food of an animal or a plant will affect 



its (3) and (4) . Two rows of corn plants from the 



same ear in different environments will be (5). It has been 



shown by a few experiments that (6) caused by environment 



may sometimes be (7) . 



PROBLEM n. HOW DO LIVING THINGS REPRODUCE 

 AND DEVELOP? 



Let us now try to discover why it is that plants and animals tend 

 to hand down unchanged the characteristics that we think of 

 as inheritable. The 

 simplest one-celled 

 plants and animals 

 grow. But since 

 they take in food 

 and oxygen and give 

 off body wastes 

 through their cell 

 membranes and 

 walls, the size of the 

 cell body is limited. 

 This^ limitation is 



A 



A 1 A* 



Which has the greater surface area, A or i and 2 ? Why do 

 cells divide ? 



