This book is now thirty years old. Published in 1957, it was as the original preface shows completed in Dunedin during 1954 and 1955, and the doctoral dissertation of which it is an outgrowth was written between 1948 and 1951, and accepted in 1952. A great deal has happened since then to enlarge our understanding of the history which it contains or implies, but the book has continued to enjoy readers and a certain standing. The present reissue has seemed worth while, both as a means of keeping the original before the public, and as an occasion of presenting it for inspection in the context of research and interpretation carried out since it was first published. In this preface, therefore, I have attempted to place it in the context of work being done at the time when it was written, and in the retrospective essay which follows to consider it in the context of work published since that time. Some of the latter calls, more or less pressingly, for modification of the premises and conclusions which the book originally contained, and I have attempted to consider some of the questions thus raised and at the same time to review the present state of the relevant historical knowledge.