But even these inventions, great as was their influence upon the progress of civilization, can scarcely be considered, it seems to me, as taking rank with the great epochal discoveries that gave their names to the preceding ages. Nor can any invention of the sixteenth or seventeenth century be hailed as really ushering in a new era. The invention for which that honor was reserved was a development of the eighteenth century; and did not come fully to its heritage until the early days of the nineteenth century. The invention was the application of steam to the purposes of mechanics. When this application was made, as wide a gap was crossed as that which separated the Stone Age from the Age of Metal; then the epoch in which the world was living when history begins was brought to a close, and a new era, the Age of Steam, was ushered in. Scarcely had the world begun to adjust itself to the new conditions of the Age of Steam, when yet another power was made subservient to man's needs, and the Age of Steam was supplemented, not to say supplanted, by the Age of Electricity. Of course the new progressive movements did not necessarily imply elimination of old conditions; they imply merely the subordination of old powers to newer and better ones. Stone implements by no means ceased to have utility at once when metal implements came into vogue. Bronze long held its own against iron, and still has its utility. And iron itself finds but an added sphere of usefulness in the Age of Steam and Electricity. All great changes are relatively slow. It is only as we look back upon them and view them in perspective that they seem cataclysmic. Gunpowder did not at once supplant the crossbow, and the cannon was long held to be inferior to the catapult. The printed book did not instantly make its way against the work of the scribe. Neither did the steam engine immediately supplant water power and the direct application of human labor. But in each case the new invention virtually rang the death knell of the old method from the hour of its inauguration, and the end was no less sure because it was delayed. And it requires no great powers of divination to foretell that in the coming age, the electric dynamo driven by water power may take the place of the steam engine. The Age of Steam may pass, with only at most a few generations of domination. And it is within the possibilities that the Age of Electricity will scarcely come into its own before it may be displaced by an Age of Radio-Activity. To press that point, however, would be to enter the field of prophecy, which is no part of my present purpose.