come into existence, are liable to decay, and, like all other individual elements, are exposed to a gradual increase and decrease. [1189] So we might have assumed from other positions in his system. Between the individual worlds both Democritus and Epicurus insert intermediate world-spaces, in which by the clustering of atoms from time to time new worlds come into being. [1190] [(2) Origin of the world.] The origin of our world is thus described. At a certain period of time—Lucretius [1191] believes at no very distant period—a cluster of atoms of varying shape and size was formed in this definite portion of space. These atoms meeting, there first arose from the pressure and rebound of the quickly falling particles motions of every variety in every direction. Soon the greater atoms pressing downwards, by dint of weight forced upwards the smaller and lighter atoms, the fiery ones topmost and with the greatest impetus to form the ether, and afterwards those which form the air. [1192] The upper pressure ceasing, these masses, under the pressure of particles still joining it from below, spread forth sidewards, and thus the belts of fire and air were formed. Next uprose those atoms out of which the sun and stars are formed into the heights, and at the same time the earth settled down, its inner part being partially exhausted in those places where the sea now is. By the influence of the warmth of the ether, and the sun-heat, the earth-mass was bound together more closely, the sea was pressed out of it, and the surface assumed an uneven character. [1193] The world is shut off from other worlds and from empty space by those bodies which form its external boundary. [1194] [(3) Arrangement of the universe.]