Later, as it seems, the theology and worship of the neighboring nomadic Arya penetrated to these nations, and caused a religious conflict which ended with the migration of Arya to the south. At this period Zarathustra[2] (Zoroaster) came forward under the Bactrian priest and King Kava Vistaspa, as defender and reformer of the religion of the fathers against the encroachments of a strange doctrine. The Devas (Zend, Dews) or the gods of the Indian Veda appear with Zarathustra as evil spirits. Not Indra, but the hero Traetona, wages war with Ahi (Zend, Azhi), while the kavis, or priests, are attacked by him as deceivers and liars. From the belief in good spirits (Ahuras, i.e., the living, and Mazdas, i.e., the wise), the ancient genii of the country, Zarathustra developed the belief of one highest God, Ahura-Mazda (Ormuzd, Greek, [Greek: Osompzes]), a doctrine which he received by divine inspiration through the mediation of the spirit Srasha. Ahura-Mazda, surrounded by the Amesha-Spenta (Amshaspands), or the holy immortals, not until later reduced to seven, is the creator of light and life. The hurtful and evil, on the contrary, is non-existence (akem), and in the oldest parts of the Avesta, the Gathas, which go back to Zarathustra and his first followers, is not yet conceived as a personal being. First in the Vendidad, written after Zarathustra, does Angro-mainyus (Ahriman), or the evil one, with his Dews, although subordinated to Ahura-Mazda, gain a place in the Iranian conception of the universe, as the adversary of Ahura-Mazda, and as the cause of evil in the natural and spiritual world. From these conceptions there was developed in the later Parsism the system of the four periods of the world, each of three thousand years, in the book "Bundehesh." In the first period, Ahura-Mazda appears as creator of the world and as the source of good. The creation, completed by Ahura-Mazda in six days by means of the word (Honover), is in the second period destroyed by Angro-mainyus, who, appearing upon the earth in the form of a serpent, seduces the first human pair, created by Ahura-Mazda. In the third period, which begins with the revelation given to Zarathustra, Ahura-mazda and Angro-mainyus strive together for man. After this follows, in the fourth period, the victory gained by Ahura-Mazda. Sosiosh (Saoshyas), the deliverer already foretold in the Vendidad, appears. The resurrection of the dead, not taught by Zarathustra or in the Vendidad, takes place. The judgment of the world begins; the good are received into paradise and the sinners banished to hell. At last, all is purified, and Angro-mainyus himself and his Dews submit themselves to Ahura-Mazda, whose victory is celebrated in heaven with songs of praise.