evaluation in synthesis; thirdly, by securing the direct criticism of specialists and utilizing their judgments in the preparation of the book. Practically every chapter of the book has been examined and criticized by two or more specialists in the field treated in that chapter. In this way the author has hoped to safeguard himself and his readers from the errors of purely personal opinion and bias.
The mere listing of the names of those men and women who have assisted in the preparation of this volume would take more space than is here available. The author takes this opportunity to express to those persons collectively his deep appreciation of their assistance. Here also he wishes to acknowledge the courtesy of publishers and authors who have permitted the use of quotations for the clearer presentation of typical opinions on important points.