To guide the student in further reading, a brief list of select references in English has been appended to each chapter. Methodological discussions and much statistical and historical material have been omitted in order to make the text as simple as possible. These can be found in the references, or the teacher can supply them at his discretion.
The many authorities to whom I am indebted for both facts and interpretations of facts cannot be mentioned individually, except that I wish to express my special indebtedness to my former teachers, Professor Willcox of Cornell and Professors Small and Henderson of the University of Chicago, to whom I am under obligation either directly or indirectly for much of the substance of this book. The list of references will also indicate in the main the sources of whatever is not my own.