[Sidenote: The imperial period.] Under the empire the influence of literature upon politics disappeared. Oratory no longer led to the highest power, poetry must, under some emperors at least, be careful not to overstep prescribed limits, and history could not safely record all facts with their causes and results. Even philosophical speculation was not safe if it led to practical conclusions adverse to the government. It was precisely those branches of literature which might be used for political purposes that the imperial government could hardly fail to discourage directly or indirectly, and those were the branches in which the practical Romans naturally excelled. There were, to be sure, emperors who encouraged literature, but their encouragement, leading to flattery and artificial eloquence, was little likely to raise the quality, even though it increased the quantity, of literary production. With its practical importance Roman literature loses its vigor. Aside from Tacitus and Juvenal, hardly a single powerful and vigorous author appears in the imperial period until, with the growth of Christianity, literature again acquires practical importance. That literature maintained for so many years a relatively high degree of excellence is due to the constant influence of Greece, which counteracted to some extent the forces that tended to destroy all literary life. Thus Roman literature lingered on until after the breaking up of the Roman empire. Only a small part of the great bulk of Roman literature is preserved to us, but that part includes the greatest works of the best period. Those are worthy subjects of study for their beauty of form, their clearness of thought, their power, their vigor, and their ethical qualities. The productions of the imperial period are inferior in quality to those of the republican and the Augustan times, though their quantity is proportionate to the duration of the empire; but these works also are proper subjects of study, for they also express the character of the Romans.