Public approval of a book is indicated by its sale. A second edition of this having been exhausted, my publishers deem it advisable to issue a third edition revised to date. In assenting, I have had to examine it thoroughly in order to see whether the work required correction, and, if so, to what extent. After careful perusal I find no reason to modify any sentence, withdraw any expression, or correct any statement of fact in its pages. Much has happened since it was written in 1890, the proofs finally revised in January 1891, but in all respects my views are unchanged 3 nor have the last ten...