CHAPTER 26 • THE LIMITATIONS OF LIFE 



1 What things must organisms ha\e to hve? 



2 Do all living organisms ha\e to have the same thmgs? 



3 Why do organisms get old? 



4 Why do some species live so much longer than others? Why do 



some individuals of the same species live so much longer than 

 others? 



5 Is it conceivable that man may sometime be able to live forever? 



6 What environmental factors limit life? 



7 Why can some animals live only in the tropics, while others live 



only in the arctic? 



8 Do desert plants grow better if kept dry? 



9 How do organisms spread from place to place? 



10 Why is it that we do not find two species of large cats living in 

 the same region? 



Living things act as if they were driven from within to keep on living. 

 The drive for food, with its thousands of marvelous adjustments, often in- 

 volves violence or stealth. But these are matched by the violence and stealth 

 through which organisms protect themselves against the food-seekers. Both 

 food-getting and resistance to food-getting — by others — are essential parts 

 of that self-preservation which has been called "nature's first law". 



This drive to live encounters continuous changes in conditions — night and 

 day, hot and cold, changing moisture and minerals and air. It also pushes off 

 the inevitable end of individual life. The drive to live involves reproduction 

 and replacement. And life moves through space, pushing outward in all 

 directions, from every established individual plant, from every group of 

 animals. 



What factors or native qualities favor particular species? What are the 

 factors which limit the increase and spread of a species? Why is the total life 

 in a place greater at one time than at another? What part has man played in 

 modifying the distribution of life on the earth? 



Is Death a Natural Process? 



Life Is Self-limiting In all plants and animals metabolism depends 

 upon certain external conditions. The intensity of light, for example, in- 

 fluences the rate of photosynthesis or the rate of growth. At one temperature 

 metabolism in a particular kind of organism proceeds at the highest rate; at 

 another temperature it ceases altogether. But even if each special condition 

 were at some point most favorable to absorption, assimilation, oxidation, 



527 



