successor.[668] Hence the oak or a human being representing the spirit of vegetation, or both together, were burned in the Midsummer fires. How, then, did the oak come to symbolise a god equated with Zeus. Though the equation may be worthless, it is possible that the connection lay in the fact that Zeus and Juppiter had agricultural functions, or that, when the equation was made, the earlier spirit of vegetation had become a divinity with functions resembling those of Zeus. The fires were kindled to recruit the sun's life; they were fed with oak-wood, and in them an oak or a human victim representing the spirit embodied in the oak was burned. Hence it may have been thought that the sun was strengthened by the fire residing in the sacred oak; it was thus "the original storehouse or reservoir of the fire which was from time to time drawn out to feed the sun."[669] The oak thus became the symbol of a bright god also connected with growth. But, to judge by folk survivals, the older conception still remained potent, and tree or human victim affected for good all vegetable growth as well as man's life, while at the same time the fire strengthened the sun. Dr. Evans argues that "the original holy object within the central triliths of Stonehenge was a sacred tree," an oak, image of the Celtic Zeus. The tree and the stones, once associated with ancestor worship, had become symbols of "a more celestial Spirit or Spirits than those of departed human beings."[670] But Stonehenge has now been proved to have been in existence before the arrival of the Celts, hence such a cult must have been pre-Celtic, though it may quite well have been adopted by the Celts. Whether this hypothetical cult was practised by a tribe, a group of tribes, or by the whole people, must remain obscure, and, indeed, it may well be questioned whether Stonehenge was ever more than the scene of some ancestral rites. Other trees--the yew, the cypress, the alder, and the ash, were venerated, to judge by what Lucan relates of the sacred grove at Marseilles. The Irish Druids attributed special virtues to the hazel, rowan, and yew, the wood of which was used in magical ceremonies described in Irish texts.[671] Fires of rowan were lit by the Druids of rival armies, and incantations said over them in order to discomfit the opposing host,[672] and the wood of all these trees is still believed to be efficacious against fairies and witches.