These events belong to a period for which Ireland has no contemporary documents of history, but for which, as it borders on the more strictly historical period, Irish traditions have their highest validity in evidence. The testimony of native tradition, as we might expect, is in accord with that of external history. The third and fourth centuries of the Christian era were a time in which nearly all the peoples of Europe outside of the Roman Empire were, so to speak, on the march with arms in their hands. At the beginning of the Christian era and before it, we have seen that this state of unrest already pervaded the Celts and Germans of Mid-Europe. A few centuries earlier still, the Celts almost alone are found in this condition of warlike mobility; for the radiation of the Celtic migratory movements in every direction--southward into Italy, westward into Gaul, Spain, Britain, and Ireland, northward into the Baltic basin, and eastward along the Danube valley and into Asia Minor--is evidence that, unlike the movements which led to the break-up of the Western Empire, the earlier Celtic migrations were not accompanied by pressure from other moving populations on their borders. I have ascribed the early expansion of the Celts to iron. The possession of iron had a two-fold effect. The natural condition of the greater part of Europe is forest. If man were absent or idle-handed, nearly all Europe in a few generations would revert to the forest state. To clear the land of woods, or even to prevent the fresh growth of woods after clearance, the implements of the Stone Age, Early and Late, cannot have been effective. Even let us suppose that large clearances could have been made by burning, at once the thickets would again spring up, and under their protection the forest trees. Nor can the possession of bronze have sufficed to subdue the natural tendency towards forest. Bronze, in the Bronze Age, was not the industrial material of the many; it belonged to the privileged few who were not hewers of wood. Iron, when it came, introduced an industrial revolution relatively greater than that which has been introduced in modern times by the steam-engine. Once people knew how to work it, iron was abundant enough to be in the hands of every worker. Iron became and has ever since remained the sole master of growing wood. With the conquest of the forests came a great extension of tillage. Iron not only cleared fertile tracts but tilled them more rapidly and deeply than was possible with the wooden spade which, as the old Irish copper mines have taught us, was the digging implement of the Bronze Age. Thus food became abundant, and with it a density of population which, before iron, was possible only in fertile and forestless regions like the flood areas of Egypt and Mesopotamia. Road making, too, progressed, and the use of vehicles. As iron furnished the many with better implements of work, it furnished them also with better implements of war.