When the traveller looks at Rome for the first time he does not realize that there have been several cities on the same piece of ground, and that the churches and palaces and other great buildings he sees to-day rest on an earlier and invisible city buried in dust beneath the foundations of the Rome of the Twentieth Century. In like manner, and because all visible things on the surface of the earth have grown out of older things which have ceased to be, the world of habits, the ideas, customs, fancies, and arts, in which we live is a survival of a younger world which long ago disappeared. When we speak of Friday as an unlucky day, or touch wood after saying that we have had good luck for a long time, or take the trouble to look at the new moon over the right shoulder, or avoid crossing the street while a funeral is passing, we are recalling old superstitions or beliefs, a vanished world in which our remote forefathers lived.