The earlier conception of Varuna is singularly similar to that of Ahuramazda of the Avesta.
The Avesta is, indeed, our principal source for the doctrine of Zoroaster; on the subject of his person and his life it is comparatively reticent; with regard to his date it is, naturally enough, absolutely silent.
Darmesteter has failed to realize sufficiently the distinction between the Zoroaster of the later Avesta and the Zoroaster of the Gathas.
It cannot be denied that in the later Avesta, and still more in writings of more recent date, he is presented in a legendary light and endowed with superhuman powers.
The Gathas alone within the Avesta make claim to be the ipsissima verba of the prophet; in the rest of that work they are put into Zoroaster's own mouth (Yasna, 9, 1) and are expressly called "the Gathas of the holy Zoroaster" (Yasna, 57, 8).