Every one of these forward steps is due to inventors. I say inventors, in the plural, because it is proven that every change in the means of exchange has been imagined several times, in several ages--though in the same way--on the surface of our earth. Summing up--the inventive labor of this period is reduced to creating increasingly more simple and more rapid methods of _substitution_ in the commercial mechanism. The appearance of commerce on a large scale has depended on the state of agriculture, industry, ways of communication, social and economic conditions and political extension. It came into being toward the end of the Roman Republic. After the interruption of the Middle Ages the activity is taken up again by the Italian cities, the Hanseatic League, etc.; in the fifteenth century with the great maritime discoveries; in the sixteenth century by the _Conquistadores_, hungering for adventure and wealth; later on, by the mixed expeditions, whose expenses are defrayed by merchants in common, and which are often accompanied by armed bands that fight for them; lastly comes the incorporation of great companies that have been wittily dubbed "_Conquistadores_ of the counting-house." We now come to the moment when commercial invention attains its complex form and must move great masses. Taken as a whole, its psychological mechanism is the same as that of any other creative work. In the first instance, the idea arises, from inspiration, from reflection, or by chance. Then comes a period of fermenting during which the inventor sketches his construction in images, represents to himself the material to be worked upon, the grouping of stockholders, the making up of a capital, the mechanism of buying and selling, etc. All this differs from the genesis of an esthetic or mechanical work only in the end, or in the nature of the images. In the second phase it is necessary to proceed to execution--a castle in the air must be made a solid structure. Then appear a thousand obstructions in the details that must be overcome. As everywhere else, minor inventions become grafted on the principal invention; the author lets us see the poverty or richness in resource of his mind. Finally, the work is triumphant, fails, or is only half-successful. Did it keep only to these general traits, commercial imagination would be merely the reiteration, with slight changes, of forms already studied; but it has characteristics all its own that must be distinguished.