Mr. Lester R. Brown has pointed out that there are four principal biological systems of the earth. They are fisheries, forests, grasslands and croplands. They are the foundation of the global economic system. They supply our food and provide raw materials for industry except minerals. But man’s demands on these systems are reaching an unsustainable level. Overfishing is very common. Local forests are disappearing to provide firewood and wood. The world is losing 40 to 50 million acres of forests a year. Similarly, grasslands and woodlands are being converted into deserts and wastelands.