Like other nations, too, whether Aryan or non-Aryan, the Celts had, besides their mythology, a religion. It is not enough to tell tales of shadowy gods; they must be made visible by sculpture, housed in groves or temples, served with ritual, and propitiated with sacrifices, if one is to hope for their favours. Every cult must have its priests living by the altar. The priests of the Celts are well-known to us by name as the “Druids”—a word derived from a root DR which signifies a tree, and especially the oak, in several Aryan languages.[31] This is generally—though not by all scholars—taken as proving that they paid an especial veneration to the king of trees. It is true that the mistletoe—that strange parasite upon the oak—was prominent among their “herbs of power”, and played a part in their ritual;[32] but this is equally true of other Aryan nations. By the Norse it was held sacred to the god Balder, while the Romans believed it to be the “golden bough” that gave access to Hades.[33] The accounts both of the Latin and Gaelic writers give us a fairly complete idea of the nature of the Druids, and especially of the high estimation in which they were held. They were at once the priests, the physicians, the wizards, the diviners, the theologians, the scientists, and the historians of their tribes. All spiritual power and all human knowledge were vested in them, and they ranked second only to the kings and chiefs. They were freed from all contribution to the State, whether by tribute or service in war, so that they might the better apply themselves to their divine offices. Their decisions were absolutely final, and those who disobeyed them were laid under a terrible excommunication or “boycott”.[34] Classic writers tell us how they lorded it in Gaul, where, no doubt, they borrowed splendour by imitating their more civilized neighbours. Men of the highest rank were proud to cast aside the insignia of mere mortal honour to join the company of those who claimed to be the direct mediators with the sky-god and the thunder-god, and who must have resembled the ecclesiastics of mediæval Europe in the days of their greatest power, combining, like them, spiritual and temporal dignities, and possessing the highest culture of their age. Yet it was not among these Druids of Gaul, with their splendid temples and vestments and their elaborate rituals, that the metropolis of Druidism was to be sought. We learn from Caesar that the Gallic Druids believed their religion to have come to them, originally, from Britain, and that it was their practice to send their “theological students” across the Channel to learn its doctrines at their purest source.[35] To trace a cult backwards is often to take a retrograde course in culture, and it was no doubt in Britain—which Pliny the Elder tells us “might have taught magic to Persia”[36]—that the sufficiently primitive and savage rites of the Druids of Gaul were preserved in their still more savage and primitive forms.