There are now 5.6 billion people on the earth. If population growth continues at today's pace, there will be 7.6 billion inhabitants by 2015. Many observers say the planet's resources are being overrun and that growth must be stabilized. Others challenge the pessimists, pointing to man's proven ability to triumph over changing circumstances. Current trends suggest that heavy population growth in the developing world will prompt even more immigration to the United States and other prosperous countries, as poor people seek better lives. Meanwhile, resistance to further immigration is mounting in many parts of Europe and the United States. And some population experts are calling on the industrial countries to bolster foreign aid for Third World family planning and economic development to discourage further immigration.