Pliny said of the Celts: "They esteem nothing more sacred than the mistletoe and the tree on which it grows. But apart from this they choose oak-woods for their sacred groves, and perform no sacred rite without using oak branches."[663] Maximus of Tyre also speaks of the Celtic (? German) image of Zeus as a lofty oak, and an old Irish glossary gives daur, "oak," as an early Irish name for "god," and glosses it by dia, "god."[664] The sacred need-fire may have been obtained by friction from oak-wood, and it is because of the old sacredness of the oak that a piece of its wood is still used as a talisman in Brittany.[665] Other Aryan folk besides the Celts regarded the oak as the symbol of a high god, of the sun or the sky,[666] but probably this was not its earliest significance. Oak forests were once more extensive over Europe than they are now, and the old tradition that men once lived on acorns has been shown to be well-founded by the witness of archæological finds, e.g. in Northern Italy.[667] A people living in an oak region and subsisting in part on acorns might easily take the oak as a representative of the spirit of vegetation or growth. It was long-lived, its foliage was a protection, it supplied food, its wood was used as fuel, and it was thus clearly the friend of man. For these reasons, and because it was the most abiding and living thing men knew, it became the embodiment of the spirits of life and growth. Folk-lore survivals show that the spirit of vegetation in the shape of his representative was annually slain while yet in full vigour, that his life might benefit all things and be passed on undiminished to his 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.
The Irish bile was a sacred tree, of great age, growing over a holy well or fort. Five of them are described in the Dindsenchas, and one was an oak, which not only yielded acorns, but nuts and apples.[673] The mythic trees of Elysium had the same varied fruitage, and the reason in both cases is perhaps the fact that when the cultivated apple took the place of acorns and nuts as a food staple, words signifying "nut" or "acorn" were transferred to the apple. A myth of trees on which all these fruits grew might then easily arise. Another Irish bile was a yew described in a poem as "a firm strong god," while such phrases in this poem as "word-pure man," "judgment of origin," "spell of knowledge," may have some reference to the custom of writing divinations in ogham on rods of yew. The other bile were ash-trees, and from one of them the Fir Bile, "men of the tree," were named—perhaps a totem-clan.[674] The lives of kings and chiefs appear to have been connected with these trees, probably as representatives of the spirit of vegetation embodied in the tree, and under their shadow they were inaugurated. But as a substitute for the king was slain, so doubtless these pre-eminent sacred trees were too sacred, too much charged with supernatural force, to be cut down and burned, and the yearly ritual would be performed with another tree. But in time of feud one tribe gloried in destroying the bile of another; and even in the tenth century, when the bile maighe Adair was destroyed by Maelocohlen the act was regarded with horror. "But, O reader, this deed did not pass unpunished."[675] Of another bile, that of Borrisokane, it was said that any house in which a fragment of it was burned would itself be destroyed by fire.[676]
Tribal and personal names point to belief in descent from tree gods or spirits and perhaps to totemism. The Eburones were the yew-tree tribe (eburos); the Bituriges perhaps had the mistletoe for their symbol, and their surname Vivisci implies that they were called "Mistletoe men."[677] If bile (tree) is connected with the name Bile, that of the ancestor of the Milesians, this may point to some myth of descent from a sacred tree, as in the case of the Fir Bile, or "men of the tree."[678] Other names like Guidgen (Viduo-genos, "son of the tree"), Dergen (Dervo-genos, "son of the oak"), Guerngen (Verno-genos, "son of the alder"), imply filiation to a tree. Though these names became conventional, they express what had once been a living belief. Names borrowed directly from trees are also found—-Eburos or Ebur, "yew," Derua or Deruacus, "oak," etc.
The veneration of trees growing beside burial mounds or megalithic monuments was probably a pre-Celtic cult continued by the Celts. The tree embodied the ghost of the person buried under it, but such a ghost could then hardly be differentiated from a tree spirit or divinity. Even now in Celtic districts extreme veneration exists for trees growing in cemeteries and in other places. It is dangerous to cut them down or to pluck a leaf or branch from them, while in Breton churchyards the yew is thought to spread a root to the mouth of each corpse.[679] The story of the grave of Cyperissa, daughter of a Celtic king in the Danube region, from which first sprang the "mournful cypress,"[680] is connected with universal legends of trees growing from the graves of lovers until their branches intertwine. These embody the belief that the spirit of the dead is in the tree, which was thus in all likelihood the object of a cult. Instances of these legends occur in Celtic story. Yew-stakes driven through the bodies of Naisi and Deirdre to keep them apart, became yew-trees the tops of which embraced over Armagh Cathedral. A yew sprang from the grave of Bailé Mac Buain, and an apple-tree from that of his lover Aillinn, and the top of each had the form of their heads.[681] The identification of tree and ghost is here complete.
The elder, rowan, and thorn are still planted round houses to keep off witches, or sprigs of rowan are placed over doorways—a survival from the time when they were believed to be tenanted by a beneficent spirit hostile to evil influences. In Ireland and the Isle of Man the thorn is thought to be the resort of fairies, and they, like the woodland fairies or "wood men" are probably representatives of the older tree spirits and gods of groves and forests.[682]
Tree-worship was rooted in the oldest nature worship, and the Church had the utmost difficulty in suppressing it. Councils fulminated against the cult of trees, against offerings to them or the placing of lights before them and before wells or stones, and against the belief that certain trees were too sacred to be cut down or burned. Heavy fines were levied against those who practised these rites, yet still they continued.[683] Amator, Bishop of Auxerre, tried to stop the worship of a large pear-tree standing in the centre of the town and on which the semi-Christian inhabitants hung animals' heads with much ribaldry. At last S. Germanus destroyed it, but at the risk of his life. S. Martin of Tours was allowed to destroy a temple, but the people would not permit him to attack a much venerated pine-tree which stood beside it—an excellent example of the way in which the more official paganism fell before Christianity, while the older religion of the soil, from which it sprang, could not be entirely eradicated.[684] The Church often effected a compromise. Images of the gods affixed to trees were replaced by those of the Virgin, but with curious results. Legends arose telling how the faithful had been led to such trees and there discovered the image of the Madonna miraculously placed among the branches.[685] These are analogous to the legends of the discovery of images of the Virgin in the earth, such images being really those of the Matres.
Representations of sacred trees are occasionally met with on coins, altars, and ex votos.[686] If the interpretation be correct which sees a representation of part of the Cúchulainn legend on the Paris and Trèves altars, the trees figured there would not necessarily be sacred. But otherwise they may depict sacred trees.