Antoninus Pius was adopted by Hadrian as his successor when he was already fifty-one years old. His portraits thus represent him as a mature man in a sober but refined style that consciously echoes the imperial imagery adopted by Hadrian. At the beginning of his reign in A.D. 138, he had to impel a reluctant Senate to award Hadrian divine honors, and it is probably for this reason that he himself was given the title of Pius. Unlike his two immediate predecessors, Trajan and Hadrian, Antoninus did not embark on any major wars or travel widely through the Empire. Indeed, he was in effect the last emperor to spend most of his reign in the city of Rome itself. Regarded as a just and diligent administrator, Antoninus presided over the Empire at the height of its power—a time that the historian Edward Gibbon later famously referred to as the period when “the condition of the human race was most happy and most prosperous.”