Epicurus' Letter to Herodotus:
Furthermore, there are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number, as was proved already, are borne on far out into space. For those atoms, which are of such nature that a world could be created out of them or made by them, have not been used up either on one world or on a limited number of worlds, nor again on all the worlds which are alike, or on those which are different from these. So that there nowhere exists an obstacle to the infinite number of the worlds.
Lucretius Book 2:
(Munro) In no wise then can it be deemed probable, when space yawns illimitable towards all points and seeds in number numberless and sum unfathomable fly about in manifold ways driven on in ceaseless motion, that this single earth and heaven have been brought into being, that those bodies of matter so many in number do nothing outside them; the more so that this world has been made by nature, just as the seeds of things have chanced spontaneously to clash, after being brought together in manifold wise without purpose, without foresight, without result, and at last have filtered through such seeds as, suddenly thrown together, were fitted to become on each occasion the rudiments of great things, of earth sea and heaven and the race of living things.
Wherefore again and again I say you must admit that there are elsewhere combinations of matter like to this with ether holds in its greedy grasp. Again when much matter is at hand, when room is there and there is no thing, no cause to hinder, things sure enough must go on and be completed. Well, then, if on the one hand there is so great a store of seeds as the whole life of living creatures cannot reckon up, and if the same force and nature abide in them and have the power to throw the seeds of things together into their several places in the same way as they are thrown together into our world, you must admit that in parts of space there are earths and various of and kinds of wild beasts.
Moreover in the sum of all there is no one thing which is begotten single in its kind and grows up single and sole of its kind; but a thing always belongs to some class and there are many things in the same kind.
First, in the case of living things, most noble Memmius, you will find that in this sort has been begotten the mountain-ranging race of wild beasts, in this sort the breed of men, in this sort too the mute shoals of scaly creatures and all bodies of fowls.
Therefore on a like principle you must admit that earth, and sun, moon, sea, and all things else that are, are not single in their kind, but rather in number past numbering; since the deep-set boundary-mark of life just as much awaits these and they are just as much of a body that had birth, as any class of things which here on earth abounds in samples of its kind.
If you well apprehend and keep in mind these things, nature free at once and rid of her haughty lords is seen to do all things spontaneously of herself without the meddling of the gods.
Velleius from Cicero's "On The Nature of The Gods"
“Moreover there is the supremely potent principle of infinity, which claims the closest and most careful study; we must understand that it has in the sum of things everything has its exact match and counterpart. This property is termed by Epicurus isonomia, or the principle of uniform distribution. From this principle it follows that if the whole number of mortals be so many, there must exist no less a number of immortals, and if the causes of destruction are beyond count, the causes of conservation also are bound to be infinite.