Thus when ancient Greek kings claimed to be descended from Zeus, and even to bear his name, we may reasonably suppose that they also attempted to exercise his divine functions by making thunder and rain for the good of their people or the terror and confusion of their foes. In this respect the legend of Salmoneus probably reflects the pretensions of a whole class of petty sovereigns who reigned of old, each over his little canton, in the oak-clad highlands of Greece.
Like their kinsmen the Irish kings, they were expected to be a source of fertility to the land and of fecundity to the cattle; and how could they fulfil these expectations better than by acting the part of their kinsman Zeus, the great god of the oak, the thunder, and the rain? They personified him, apparently, just as the Italian kings personified Jupiter.
In ancient Italy every oak was sacred to Jupiter, the Italian counterpart of Zeus; and on the Capitol at Rome the god was worshipped as the deity not merely of the oak, but of the rain and the thunder. Contrasting the piety of the good old times with the scepticism of an age when nobody thought that heaven was heaven, or cared a fig for Jupiter, a Roman writer tells us that in former days noble matrons used to go with bare feet, streaming hair, and pure minds, up the long Capitoline slope, praying to Jupiter for rain. And straightway, he goes on, it rained bucketsful, then or never, and everybody returned dripping like drowned rats. But nowadays, says he, we are no longer religious, so the fields lie baking.
When we pass from Southern to Central Europe we still meet with the great god of the oak and the thunder among the barbarous Aryans (Indo-Europeans) who dwelt in the vast primaeval forests. Thus among the Celts of Gaul the Druids esteemed nothing more sacred than the mistletoe and the oak on which it grew; they chose groves of oaks for the scene of their solemn service, and they performed none of their rites without oak leaves. The Celts, says a Greek writer, worship Zeus, and the Celtic image of Zeus is a tall oak. The Celtic conquerors, who settled in Asia in the third century before our era, appear to have carried the worship of the oak with them to their new home; for in the heart of Asia Minor the Galatian senate met in a place which bore the pure Celtic name of Drynemetum, the sacred oak grove or the temple of the oak. Indeed the very name of Druids is believed by good authorities to mean no more than oak men.