The substance of this book was presented as the Trevelyan Lectures in the University of Cambridge in May 1973 and in a revised form as the A. Flexner Lectures at Bryn Mawr College in February–March 1974. I have retained the lecture form and only added a bibliography to each chapter. My aim was to stimulate discussion on an important subject without indulging in speculations.

I owe much gratitude to the two institutions which invited and received me so generously. In Cambridge I found myself among old friends: at Bryn Mawr I gained new ones. It was a happy time in both places. I should like to thank especially Professors Owen Chadwick and M. I. Finley of Cambridge, President Wofford, Professor Agnes Michels and Professor Russell Scott of Bryn Mawr.