If human nature is liable to degenerate, it is capable of proportionable improvement from the collected wisdom of ages. It is pleasant to infer from the actual progress of society, the glorious possibilities of human excellence. And, if the principles can be assembled into view, which most directly tend to diversify the genius and character of nations, some theory may be raised on these foundations that shall account more systematically for past occurrences and afford some openings and anticipations into the eventual history of the world.] The problem of dark ages, which an advocate of Progress must explain, was waved away by Priestley in his Lectures on History with the observation that they help the subsequent advance of knowledge by "breaking the progress of authority." [Footnote: This was doubtless suggested to him by some remarks of Hume in The Rise of Arts and Sciences.] This is not much of a plea for such periods viewed as machinery in a Providential plan. The great history of the Middle Ages, which in the words of its author describes "the triumph of barbarism and religion," had been completed before Priestley's Lectures appeared, and it is remarkable that he takes no account of it, though it might seem to be a work with which a theory of Progress must come to terms. Yet the sceptical historian of the Decline and Fall of the Roman Empire, who was more at home in French literature than any of his fellow-countrymen, was not opposed to the theory of Progress, and he even states it in a moderate form. Having given reasons for believing that civilised society will never again be threatened by such an irruption of barbarians as that which oppressed the arms and institutions of Rome, he allows us to "acquiesce in the pleasing conclusion that every age of the world has increased, and still increases, the real wealth, the happiness, the knowledge and perhaps the virtue of the human race."