"In the case of Indo-Iranian, there may have been early differentiation between the Indo-Aryan and Iranian branches, especially if we assume that the Iranian loss of aspiration in voiced aspirated stops was a dialectal feature which Iranian shared with Balto-Slavic and Germanic (cf. Kortlandt 1978:115). Nevertheless, Proto-Indo-Iranian for a long time remained a dialectal unity, possibly even up to the moment when the Indo-Aryans crossed the Hindu Kush mountain range and lost contact with the Iranians."