Viewing banking theory as an element in general economic theory, the reason for the development of banking as a mechanism likewise becomes plain. It is the outcome of the experience of other methods of exchange. In the early history of civilization, there was a long period during which the principal exchanges were effected by means of barter. This period was followed by another during which money exchanges came in to supersede barter to a very considerable extent. Within comparatively modern times a credit system has succeeded the systems of ancient and mediaeval times. It should not be understood from what has been said that these periods of barter, money, and credit exchange are sharply marked off from one another, or even that they shade into one another by imperceptible degrees. On the contrary, the facts in the case seem to show that there was extensive overlapping, and that fairly advanced ideas of credit were developed quite early in the period of money exchanges, while barter, as is well known, has persisted in many parts of the world down to modern times and has even been broadened and confirmed since the close of the European war because of the inadequacy of the money and credit systems of Europe, as seen in the shipment of materials from the United States to Europe and the return of finished goods made from such materials in payment therefor. There are thus no distinct "periods," in the chronological sense, which may be marked off from one another as indicating the duration of the systems of barter, money, or credit. It is possible to speak of "periods" in this connection only in the sense that the predominant characteristic or controlling method of exchange employed at any given time may be said to have been that of either barter, money, or credit. Speaking in this restricted sense, it is fair to regard the sixteenth century as a period characterized by a wide use of money, while the nineteenth century and the beginning of the twentieth, particularly the years after 1850, was essentially a credit period, and accordingly a period in which banking was brought to a development which had previously not been known.