The year I900 A.D. need not have been remarkable. Centuries are man-made conventions after all, and although people may think in terms of tens and hundreds and thousands, nature doesn’t. She surrenders her secrets piecemeal and, so far as we know, at random. Moreover, for many people around the world, the year 1900 A.D. meant little. It was a Christian date and therefore not strictly relevant to any of the inhabitants of Africa, the Americas, Asia, or the Middle East. Nevertheless, the year that the West chose to call 1900 was an unusual year by any standard. So far as intellectual developments--the subject of this book--were concerned, four very different kinds of breakthrough were reported, each one offering a startling reappraisal of the world and man’s place within it. And these new ideas were fundamental, changing the landscape dramatically.