Polytheistic creeds are far less jealous than monotheism; and the development of Zoroastrianism had been in a polytheistic direction.

By the time that the Zoroastrians were brought into contact with Magism, the first fervor of their religious zeal had abated, and they were in that intermediate condition of religious faith which at once impresses and is impressed, acts upon other systems, and allows itself to be acted upon in return.
The result which supervened upon contact with Magism seems to have been a fusion, an absorption into Zoroastrianism of all the chief points of the Magian belief, and all the more remarkable of the Magian religious usages.

This absorption appears to have taken place in Media.

It was there that the Arian tribes first associated with themselves, and formally adopted into their body, the priest-caste of the Magi, which thenceforth was recognized as one of the six Median tribes.

It is there that Magi are first found acting in the capacity of Arian priests.
According to all the accounts which have come down to us, they soon acquired a predominating influence, which they no doubt used to impress their own religious doctrines more and more upon the nation at large, and to thrust into the background, so far as they dared, the peculiar features of the old Arian belief.