"Most of those transacting business with foreign ports not only dispatched cargoes of their home commodities, but received, as return freights, cargoes of foreign productions. They had, therefore, both debtors and creditors in the countries with which they traded. The mode of payment by assignments enabled them to satisfy the claims of the latter by making over to them the certificates of indebtedness received from the former, thus dispensing alike with the necessity of transmitting money and of employing the exchanger, except for the purpose of collecting any balance which might remain. Assignments of this kind were drawn up in the form of letters requesting the debtor to pay over the sum specified to another party named in the letter, on account of the writer; specifying also the time within which, and the forms under which, the payment was to be made. From this system of payments there came naturally, as a part of the regular business of banking, the early and the more modern and perfected form of bills of exchange."