The primary aim of this book is to explain the remarkable rule which regulated the succession to the priesthood of Diana at Aricia. When I first set myself to solve the problem more than thirty years ago, I thought that the solution could be propounded very briefly, but I soon found that to render it probable or even intelligible it was necessary to discuss certain more general questions, some of which had hardly been broached before. In successive editions the discussion of these and kindred topics has occupied more and more space, the enquiry has branched out in more and more directions, until the two volumes of the original work have expanded into twelve.