Why not earlier? I think we have conclusive grounds for believing that the Celtic migrations to Ireland cannot have begun very much, if at all, sooner than the fourth century B.C. Before stating these grounds, let us ask is there any discoverable reason for supposing that the Gaels inhabited Ireland from a time many centuries farther back. I think it possible that those who in modern times have entertained this view have been influenced by the dates assigned to the Gaelic immigration by Irish writers like the Four Masters and Keating. These dates may be taken to correspond closely enough with the estimates of archæological authorities for the commencement of the insular Bronze Age, and, in the absence of evidence to the contrary, it might be imagined that they were founded on some basis of tradition. It is not the habit of popular tradition to encumber itself with chronology. There is no known instance of ancient reckoning in years and periods of years that is not based on some era, on the accepted date of some real or supposed event or events. Nowhere in Irish tradition has any trace been found of the existence of a native system of chronology before the introduction of Christian learning. In a paper published in the Proceedings of the Royal Irish Academy (July, 1910), I have shown how the extant written chronology of the Irish Invasions was first originated. The method was not unlike Sir John Rhys's series of equations. The Irish historian found in Latin histories a set of definite epochs by which antiquity was divided: the beginning of the Assyrian empire, the beginning of the Median empire, the beginning of the Persian empire, the usurpation of the Magi in Persia, and the beginning of Alexander's empire. The chronology of the Irish Invasions was settled by the easy process of making each invasion coincide exactly in time with each of these epochs. It is evident that no traditional value can be attached to a chronological system of this kind. But, it may be objected, the very remoteness of the time assigned to the Gaelic invasion by Irish historians may reflect the popular belief in its remoteness. If that be so, then the earlier the historian is the more near he is to the popular tradition. In the paper just cited, I have shown that, in the earliest known version of this chronology of the Invasions, the Gaelic migration to Ireland coincides with the date of Alexander's empire, 331 B.C. That is not very far from the date assigned by Coffey for the end of the Bronze Age in Ireland, about 350 B.C. For my own part, I attach no traditional value to this coincidence, but if it pleases anyone to insist that Irish prehistoric chronology has a traditional value, then it must be conceded that tradition, as far as it is valid, is altogether favourable to the view that the Gaelic occupation of Ireland belongs to the end, and not to the beginning, of the Bronze Age.