In the fifth book Lucretius deals with origins--of the sun, the moon, the earth (which he held to be flat, denying the existence of the antipodes); of life and its development; and of civilization. In all this he excludes design, explaining everything as produced and maintained by natural agents, "the masses, suddenly brought together, became the rudiments of earth, sea, and heaven, and the race of living things." He believed in the successive appearance of plants and animals, but in their arising separately and directly out of the earth, "under the influence of rain and the heat of the sun," thus repeating the old speculations of the emergence of life from slime, "wherefore the earth with good title has gotten and keeps the name of mother." He did not adopt Empedocles's theory of the "four roots of all things," and he will have none of the monsters--the hippogriffs, chimeras, and centaurs--which form a part of the scheme of that philosopher. These, he says, "have never existed," thus showing himself far in advance of ages when unicorns, dragons, and such-like fabled beasts were seriously believed to exist. In one respect, more discerning than Aristotle, he accepts the doctrine of the survival of the fittest as taught by the sage of Agrigentum. For he argues that since upon "the increase of some Nature set a ban, so that they could not reach the coveted flower of age, nor find food, nor be united in marriage," ... "many races of living things have died out, and been unable to beget and continue their breed." Lucretius speaks of Empedocles in terms scarcely less exaggerated than those which he applied to Epicurus. The latter is "a god" "who first found out that plan of life which is now termed wisdom, and who by tried skill rescued life from such great billows and such thick darkness and moored it in so perfect a calm and in so brilliant a light, ... he cleared men's breasts with truth-telling precepts, and fixed a limit to lust and fear, and explained what was the chief good which we all strive to reach." As to Empedocles, "that great country (Sicily) seems to have held within it nothing more glorious than this man, nothing more holy, marvellous, and dear. The verses, too, of this godlike genius cry with a loud voice, and make known his great discoveries, so that he seems scarcely born of a mortal stock."