Psychologists suggest that to cure cruelty

The writer accuses

1. 
   some alien species of having exterminated certain creatures and disturbing the balance of nature
   

1. 
   ancient people of having started wars and killed each other
   
2. 
   human beings of having used the earth's natural resources without thinking of the future
   
3. 
   the young of being ignorant of the earth's problems
   

2. 
   The writer believes that
   

1. 
   the most damage to the earth has been done by human beings
   
2. 
   it is already too late to rescue the earth
   
3. 
   the earth's natural resources won't be sufficient for future generations
   

1. 
   the ecology of the earth can only be balanced by exterminating the alien species introduced deliberately
   

3.