WHEN I had the honour to be invited by the Society of Antiquaries of Scotland in rg6z to deliver the Rhind Lectures on which this book is based, I had to decide, it seemed to me, between two alternatives. I might offer a rigorous and technical course, addressed only to my fellow prehistorians and dealing with those archaeological minutiae which have formed the subject of much of my research over the past years. On the other hand, I might take a risk, and offer instead a broad synthesis of material widely distributed in time and space, necessarily lacking in profundity, but presenting some general observations on the wider problems of prehistory. This would be appropriate to put before an audience not wholly of archaeologists, but of scholars in other disciplines, and those with a general interest in human antiquity.