 In the south , the retreat of the Song dynasty led to major demographic changes . The population of refugees from the north that <unk> in Hangzhou and Jiankang ( modern <unk> ) eventually grew greater than the population of original residents , whose numbers had dwindled from repeated Jurchen raids . The government encouraged the <unk> of peasant migrants from the southern provinces of the Song to the <unk> territories between the Yangtze and the Huai rivers . 
