CHAPTER 28 • THE INTERDEPENDENCE OF LIFE 



1 Why cannot any species of plant or animal live entirely alone? 



2 Can the individuals of a species live by themselves? 



3 Are parasites of any use? 



4 Are weeds of any use? 



5 Can a plant or animal be useful to some species and injurious to 



others? 



6 Does the number ot individuals in a species remain about the same 



year after year? 



7 Could we make all land surfaces bear only useful plants? 



8 Is the division of labor among different species the same as the 



division of labor among the members of a beehi\'e? 



9 Could we get rid of all injurious plants and animals? 



Many a poet has sung about an island on which he might be alone, or sighed 

 for the wings of a dove on which to fly to the solitude of some vast wilderness. 

 And many a hermit has actually gone off, expecting to find comfort and peace, 

 as well as abundance and elbowroom, flir from other men. 



It is easy to understand \\'hy one should want to escape from hardships and 

 annoyances that he cannot oxercome or thrust out of his life. But if one had 

 the whole world to himself, he w^ould not get very far. Each of the multitude 

 of species can continue generation after generation only because many of the 

 other species also continue to live. Through the ages life has come to be a 

 complex of many species acting upon each other in ways that are often mu- 

 tually destructive, but such a complex seems to make possible the greatest 

 total amount of living matter — in a particular region or in general. 



Cannot any species live entirely alone? What happens to the others if any 

 species dies out? What happens to repopulate a region in which all life 

 has been destroyed? 



Could Any Organism Live by Itself? 



Life and Light All organic matter seems to derive from carbohydrates, 

 which, so far as we know% arise only from the action of light on chlorophyl. 

 We should therefore expect the first forms of life in any region or in the world 

 to have been green plants. Certainly no animal of the kinds living today, and 

 no plant lacking chlorophyl, could live before other plants or animals had 

 left some of their substance that might be used as food. 



We do not know that the earliest forms of life were "green plants". It is 

 conceivable that such compounds as viruses and enzymes developed into some 

 kinds of "living" forms before chlorophyl- bearing species appeared (see page 



559 



