Darmesteter has failed to realize sufficiently the distinction between the Zoroaster of the later Avesta and the Zoroaster of the Gathas.
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).
The Gathas alone claim to be authentic utterances of Zoroaster, his actual expressions in presence of the assembled congregation.
He whom we hear in the Gathas has had to face, not merely all forms of outward opposition and the unbelief and lukewarmness of adherents, but also the inward misgivings of his own heart as to the truth and final victory of his cause.
So soon as the point of view is clear - that in the Gathas we have firm historical ground on which Zoroaster and his surroundings may rest, that here we have the beginnings of the Zoroastrian religion - then it becomes impossible to answer otherwise than affirmatively every general question as to the historical character of Zoroaster.