Strabo Geography 11.9.3

[3] They say that the Aparnian Däae were emigrants from the Däae above Lake Maeotis, who are called Xandii or Parii. But the view is not altogether accepted that the Däae are a part of the Scythians who live about Maeotis. At any rate, some say that Arsaces derives his origin from the Scythians, whereas others say that he was a Bactrian, and that when in flight from the enlarged power of Diodotus and his followers he caused Parthia to revolt. But since I have said much about the Parthian usages in the sixth book of my Historical Sketches and in the second book of my History of events after Polybius, ¹ I shall omit discussion of that subject here, lest I may seem to be repeating what I have already said, though I shall mention this alone, that the Council of the Parthians, according to Poseidonius, consists of two groups, one that of kinsmen, ² and the other that of wise men and Magi, from both of which groups the kings were appointed. ³

[3] They say that the Dahae Parni were an emigrant tribe from the Dahae above the Maeotis, who are called 9andii and Parii. But it is not generally acknowledged that Dahae are to be found among the Scythians above the Meotis, yet from these Arsaces according to some was descended; according to others he was a Bactrian, and withdrawing himself from the increasing power of Diodotus, occasioned the revolt of Parthia.

We have enlarged on the subject of the Parthian customs in the sixth book of historical commentaries, and in the second of those, which are a sequel to Polybius: we shall omit what we said, in order to avoid repetition; adding this only, that Poseidonius affirms that the council of the Parthians is composed of two classes, one of relatives, (of the royal family,) and another of wise men and magi, by both of which kings are chosen.