Pre-Christian Irish chronology has been arranged, like most ancient national chronologies, on the basis of the length of reign of certain kings. As we do not trace our descent from the "sun and moon" we are not necessitated to give our kings "a gross of centuries apiece," or to divide the assumed period of a reign between half-a-dozen monarchs;[67] and the difficulties are merely such as might be expected before chronology had become a science. The Four Masters have adopted the chronology of the Septuagint; but O'Flaherty took the system of Scaliger, and thus reduced the dates by many hundred years. The objection of hostile critics has been to the history rather than to the chronology of the history; but these objections are a mere _petitio principii_. They cannot understand how Ireland could have had a succession of kings and comparative civilization,--in fact, a national existence,--from 260 years before the building of Rome, when the Milesian colony arrived, according to the author of the _Ogygia_, at least a thousand years before the arrival of Cæsar in Britain, and his discovery that its inhabitants were half-naked savages. The real question is not what Cæsar said of the Britons, nor whether they had an ancient history before their subjugation by the victorious cohorts of Rome; but whether the annals which contained the pre-Christian history of Ireland may be accepted as, in the main, authentic. We have already given some account of the principal works from which our annals may be compiled. Before we proceed to that portion of our history the authenticity of which cannot be questioned, it may, perhaps, be useful to give an idea of the authorities for the minor details of social life, the individual incidents of a nation's being, which, in fact, make up the harmonious whole. We shall find a remarkable coincidence between the materials for early Roman history, and those for the early history of that portion of the Celtic race which colonized Ireland.