The Great Disaster may be natural or man-made; the new environment to which men must adjust may be created by the stilt untamed forces of gravity, of flood, fire, ice age, sunspots, or meteorites. It may be the by-product of thoughtless technology, or it may be a planned holocaust, the ultimate atomic war, bringing with it such chemical changes in atmosphere, soil, or genes as will change the face (and bowels) of the Earth itself.
But there might also be a new kind of disaster, a trouble truly alien in origin. . . .