But doctrines and systems have a vitality of their own which is altogether independent of the vagaries of fashion. It were better to regard their history, like all histories of ideas, as a kind of struggle for existence. At one moment conflicting doctrines seem to dwell in harmony side by side, content to divide the empire of knowledge between them. Another moment witnesses them rushing at each other with tumultuous energy. It may happen that in the course of the struggle some of the doctrines are worsted and disappear altogether. But more often than not their conflicting interests are reconciled and the enmity is lost in the unity of a higher synthesis. And so it may happen that a doctrine which everybody thought was quite dead may rise with greater vigour than ever. The bibliography of the subject is colossal. In addition to the general histories, which are already plentiful, the chapters devoted to the subject in every treatise on political economy, and the numerous articles which have appeared in various reviews, there is scarcely an author, however obscure, who is not the subject of a biography. To have attempted to enumerate all these works would merely have meant increasing the bulk of the book without being able to pretend that our list was exhaustive. It is scarcely necessary to add that this meant that we had to confine ourselves to the work done by the “heroes” of this volume. Their commentators and critics only came in for our attention when we had to borrow either an expression or an idea directly from them or when we felt it necessary that the reader should fill up the gaps left by our exposition. This accounts for the number of names which had to be relegated to the foot-notes. But such deliberate excision must not prevent our recognising at the outset the debt that we owe to the many writers who have traversed the ground before us. They have facilitated our task and have a perfect right to regard themselves as our collaborators. We feel certain that they will find that their labours have not been ignored or forgotten. Although this book, so far as the general task of preparation and revision is concerned, must be regarded as the result of a collective effort on the part of the two authors whose names are subjoined, the actual work of composition was undertaken by each writer separately. The Contents will sufficiently indicate the nature of this division of labour.