In Sir John Rhys's theory, the Iberians of the Later Stone Age are succeeded by the Goidels or Gaels, of Celtic origin, who introduced the Bronze Age in Britain and also in Ireland. Many centuries after these came the Brythons, who introduced the Iron Age, and drove the Gaels out of the greater part of England. Dr. Sympson says that the Brythons of that invasion drove the Gaels out of Wales also, but for this he has no warrant from Sir John Rhys. According to Rhys, the Gaels continued to occupy the more westerly parts of the island, even after the Roman occupation. Rhys's theory is still more elaborate. The three divisions of Gaul with which Cæsar begins the account of his Gallic war are familiar to students of Latin. Rhys equates his Neolithic Iberians of Britain and Ireland with the Iberian element in Aquitanian Gaul and Spain, his Bronze-Age Goidels or Gaels with the Celtae of Cæsar's Gallia Celtica, and his Iron-Age Brythons of England with the Belgae of Cæsar's Gallia Belgica. He goes still farther with this process of equation. Finding that the consonant Q, where it occurs in the most ancient forms of the Irish language, is replaced by P in the corresponding forms of the British or ancient Welsh language, he divides the Celts into two linguistic groups which he labels the Q-Celts and the P-Celts, and this division he makes to correspond to the other classification into Celtae and Belgae. In this way, he produces a most interesting and symmetrical set of equations showing the successive stages of population-change in Britain. First, there are the people of the Early Stone Age, not named. Secondly, the people of the Later Stone Age, Iberians. Thirdly, the people of the Bronze Age, Goidels or Gaels, or Celtae, or Q-Celts. Fourthly, the people of the Iron Age, Brythons or Britons, or Belgae, or P-Celts. For the present, let us pass away from the Iberians, and consider the theory as it concerns the Celtic migrations to Britain and Ireland. The earliest known habitat of the Celts is the region to the north of the Alps. The earliest definitely known migration of the Celts is their southward movement into Northern Italy. For this migration no earlier date than 600 B.C. is assigned.