This is a revised edition of a standard work for comparative linguists. A new introduction dealing mainly with a semiotic basis of change, and a final chapter on aspects of explanation, particularly in historical and human disciplines, have been added and minor changes have been made in the bulk of the text. Chapter 11 now includes a section on comparative syntax and Chapter 13 includes a brief addition on the semiotic status of the comparative method. The biography has been updated with a selection from the overwhelming amount of material which has appeared in the intervening years. The Index has been adapted to include the new material.