The delivery of my Tarner Lectures in Cambridge happened to coincide exactly with the two-hundredth anniversary of the first manned flight across the English Channel by Blanchard and Jeffries in January 1785. Like the intrepid balloonists, a public lecturer must carry supplies of hot air and of ballast to regulate his flight – hot air to be inserted when the text of the lecture is too short and ballast to be dropped when the text is too long. In preparing the lectures for publication I was able to retrieve some of the dropped ballast and to vent some of the inserted air. I am grateful to my hosts at Trinity College for their hospitality and to my audiences for their sharp questions and criticisms. In revising the book for this second edition in 1998, I have had the benefit of many additional criticisms from readers of the first edition. I am grateful to everyone who corrected my mistakes and told me about recent developments in evolutionary biology. I am especially grateful to Professor Cairns-Smith for reading and criticizing the new edition. The first edition was a lightly edited transcript of the lectures. The second edition is substantially enlarged and is no longer a transcript. Much has happened in the last thirteen years to deepen our understanding of early evolution. I have changed my story to take account of new discoveries. But the basic mystery of life's origin remains unsolved, and the central theme of the book remains unchanged.