"Everything flows and nothing stays," said the Greek Heraclitus in 475 BCE. For him, change was fundamental truth underlying everything that is caught up in the flow of time.History is a testament to that philosopher's insight. Everyday societies and individual lives across the globe are shaped and reshaped in small and large ways. Fashions come and go, famine gives way to plenty, monarchs and government are swept away, and science provides us with...