Scholars who support the immigrationist theory propose that the Romanians descended from the Romanized inhabitants of the provinces to the south of the Danube. Following the collapse of the empire's frontiers around 620, some of this population moved south to regions where Latin had not been widely spoken. Others took refuge in the Balkan Mountains where they adopted an itinerant form of sheep- and goat-breeding, giving rise to the modern Vlach shepherds. Their mobile lifestyle contributed to their spread in the mountainous zones. The start of their northward migration cannot exactly be dated, but they did not settle in the lands north to the Lower Danube before the end of the 10th century and they crossed the Carpathians after the mid-12th century.