Poised for the Leap

FROM the moment when living organisms appeared in the seas billions of years ago, they seemed driven by an instinctive urge to move beyond their own environment. Out of the dark waters they groped across aeons, toward the light and land and air. Like those remote ancestors, man, too, has striven continually to seek what he has never known before. He has ranged restlessly across the surface of his world; he has traveled back into the primordial oceans; he has learned to fly through his now familiar skies. For the past seven years,...