Except as regards a few minor points (especially in Chapter X) he makes no claim to having based his work upon hitherto unpublished manuscript sources. The main part of the study, however (that dealing with the time of the Crusades, Chapters IV-XIII), is founded essentially upon printed editions of the primary sources for the history of civilization in the period. The first three chapters, on the other hand, relating as they do to the background of medieval geography and covering an enormous field, of necessity have to a large degree been written with the aid of the secondary works of modern scholars. The volume is an enlargement of a thesis submitted in 1922 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in history at Harvard University. Some of the research was pursued in Europe in 1919–1920, during which academic year the writer held the Woodbury Lowery Fellowship from Harvard. Subsequently the American Geographical Society has generously permitted him, while acting as Librarian of the Society, to devote much time to the revision of the manuscript. The writer owes a special debt of gratitude to Professor C. H. Haskins, largely as a result of whose advice the particular period dealt with was selected. Helpful suggestions and the occasional receipt from Professor Haskins of a photograph or transcript of a manuscript bearing upon an apposite topic have been a constant stimulus. Useful suggestions have also been made by Professor R. P. Blake of Harvard and by Monsieur Charles de La Roncière of the Bibliothèque Nationale in Paris. Dr. Charles Singer of the University of London courteously permitted the use of Plate VIII from his _Scientific Views and Visions of St. Hildegard_ (1917) as a basis for Figure 5 of the present volume. The writer is indebted to Mr. W. L. G. Joerg, the editor, from whose editorial skill, experience, and tireless care the book has greatly profited. He also wishes to thank Mr. George W. Robinson, Secretary of the Harvard Graduate School of Arts and Sciences, Miss Genevieve R. Fallon, formerly of Radcliffe College, and Mr. Arthur A. Brooks and members of the library staff of the American Geographical Society for their painstaking aid with manuscript and proof. _New York, October 7, 1924._ INTRODUCTION THE TIME OF THE CRUSADES