restored the faith to its "orthodox" form after the Hellenistic Arsacids had allowed Zoroastrianism to become corrupt). Among migrations were those to cities in (or on the margins of) the great salt deserts, in particular to Yazd and Kerman, which remain centers of Iranian Zoroastrianism to this day. Yazd became the seat of the Iranian high priests during Mongol Il-Khanate rule, when the "best hope for survival [for a non-Muslim] was to be inconspicuous." Crucial to the present-day survival of Zoroastrianism was a migration from the northeastern Iranian town of "Sanjan in south-western Khorasan", to Gujarat, in western India. The descendants of that group are today known as the Parsis—"as the Gujaratis, from long tradition, called anyone from Iran"—who today represent the larger of the two groups of Zoroastrians in India. The struggle between Zoroastrianism and Islam declined in the 10th and 11th centuries. Local Iranian dynasties, "all vigorously Muslim," had emerged as largely independent vassals of the Caliphs. In the 16th century, in one of the early letters between Iranian Zoroastrians and their co-religionists in India, the priests of Yazd lamented that "no period [in human history], not even that of Alexander, had been more grievous or troublesome for the faithful than 'this millennium of the demon of Wrath'." Modern Zoroastrianism has survived into the modern period, particularly in India, where the Parsis are thought to have been present since about the 9th century. Today Zoroastrianism can be divided in two main schools of thought: reformists and traditionalists. Traditionalists are mostly Parsis and accept, beside the Gathas and Avesta, also the Middle Persian literature and like the reformists mostly developed in their modern form from 19th century developments. They generally do not allow conversion to the faith and, as such, for someone to be a Zoroastrian they must be born of Zoroastrian parents. Some traditionalists recognize the children of mixed marriages as Zoroastrians, though usually only if the father is a born Zoroastrian. Reformists tend to advocate a "return" to the Gathas, the universal nature of the faith, a decrease in ritualization, and an emphasis on the faith as philosophy rather than religion. Not all Zoroastrians identify with either school and notable examples are getting traction including Neo-Zoroastrians/Revivalists, which are usually reinterpretations of Zoroastrianism appealing towards Western concerns, and centering the idea of Zoroastrianism as a living religion and advocate the revival and maintenance of old rituals and prayers