Some observers (Mr. Buckley cites a recent article by Nicholas Eberstadt) believe that world population growth is on the edge of a drastic--perhaps even dangerous--decline. Our guests don't buy it. WGH: "1950 was when world population finally reached 2.5 billion people. In the fifty years--well, less than fifty years--since then ... world population has grown by almost 3.5 billion. Now that means all of human history and prehistory to 2.5, and then less than fifty years to add 3.5. Now as to the future ... if the median projection proves correct ... in about fifty years or so world population will again add almost 3.5 billion people." Mr. Buckley asks, "Has either of you calculated to what extent is the huge population increase that you've just mentioned owing to longevity?" VA: "Oh, that's a factor, but it's not as large a factor at the older ages as it is because of reduction of infant mortality...." WGH: "But the driving force is fertility." And we're off on an absorbing discussion of the psychology and ethics of Third World population-control programs.