This book is one of those which, if it lives for a number of decades, and if it requires any Preface at all, wants a new one every ten years. The first Preface to a book is apt to be explanatory, perhaps apologetic, in the expectation of attacks from various quarters. If the book is in some points in advance of public opinion, it is natural that the writer should try to smooth the way to the reception of his more or less aggressive ideas. He wishes to convince, not to offend,--to obtain a hearing for his thought, not to stir up angry opposition in those who do not accept it. There is commonly an anxious look about a first Preface. The author thinks he shall be misapprehended about this or that matter, that his well-meant expressions will probably be invidiously interpreted by those whom he looks upon as prejudiced critics, and if he deals with living questions that he will be attacked as a destructive by the conservatives and reproached for his timidity by the noisier radicals. The first Preface, therefore, is likely to be the weakest part of a work containing the thoughts of an honest writer. After a time the writer has cooled down from his excitement,--has got over his apprehensions, is pleased to find that his book is still read, and that he must write a new Preface. He comes smiling to his task. How many things have explained themselves in the ten or twenty or thirty years since he came before his untried public in those almost plaintive paragraphs in which he introduced himself to his readers,--for the Preface writer, no matter how fierce a combatant he may prove, comes on to the stage with his shield on his right arm and his sword in his left hand.