The only persons in Britain who possessed any knowledge before the Roman invasion, and even for some considerable time after it, were the Druids: the real extent of their attainments is, however, doubtful and superficial, from the fact that, though they were acquainted with the Greek letters, they taught almost entirely by memory, and committed little or nothing to writing. A summary of what is known concerning Druidical knowledge is contained in the following particulars:--Concerning the universe, they believed that it should never be entirely destroyed or annihilated, though it was expected to suffer a succession of violent changes and revolutions, by the predominating powers of fire and water. They professed to have great knowledge of the movements of the heavens and stars; indeed, their religion required some attention to astronomy, since they paid considerable regard to the changes of the moon. Their time was computed by nights, according to very ancient practice, by moons or months; and by years, when the planet had gone the revolutions of the seasons. That at least they knew the reversion of the seasons, as adapted to agricultural purposes, is evident from the fact, that Cæsar landed in Britain on the 26th day of August, when he states that the harvest was all completed, excepting one field, which was more backward than the rest of the country. The sacred animal of the Druids' religion was the milk-white bull; the sacred bird, the wren; the sacred tree, the oak; the sacred plant, the mistletoe; the sacred herbs, the trefoil and the vervain; the sacred form, that of three divine letters or rays, in the shape of a cross, symbolizing the triple aspect of God. The sacred herbs and plant, with another plant, hyssop, the emblem of fortitude in adversity, were gathered on the sixth day of the moon. The great festivals of Druidism were three: the solstitial festivals of the rise and fall of the year, and the winter festival. At the spring festival, the bâltân, or sacred fire, was brought down by means of a burning-glass from the sun. No hearth in the island was held sacred till the fire on it had been relit from the bâltân. The bâltân became the Easter festival of Christianity, as the mid-winter festival, in which the mistletoe was cut with the golden sickle from the sacred oak, became Christmas. The mistletoe, with its three berries, was the symbol of the Deity in his triple aspect--its growth on the oak, of the incarnation of the Deity in man.