Sir James George Frazer (18541941). The Golden Bough. 1922.
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 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.
In the religion of the ancient Germans the veneration for sacred groves seems to have held the foremost place, and according to Grimm the chief of their holy trees was the oak. It appears to have been especially dedicated to the god of thunder, Donar or Thunar, the equivalent of the Norse Thor; for a sacred oak near Geismar, in Hesse, which Boniface cut down in the eighth century, went among the heathen by the name of Jupiters oak (robur Jovis), which in old German would be Donares eih, the oak of Donar. That the Teutonic thunder god Donar, Thunar, Thor was identified with the Italian thunder god Jupiter appears from our word Thursday, Thunars day, which is merely a rendering of the Latin dies Jovis. Thus among the ancient Teutons, as among the Greeks and Italians, the god of the oak was also the god of the thunder. Moreover, he was regarded as the great fertilising power, who sent rain and caused the earth to bear fruit; for Adam of Bremen tells us that Thor presides in the air; he it is who rules thunder and lightning, wind and rains, fine weather and crops. In these respects, therefore, the Teutonic