Classifications of Chinese varieties in the late 19th century and early 20th century were based on impressionistic criteria. They often followed river systems, which were historically the main routes of migration and communication in southern China. The first scientific classifications, based primarily on the evolution of Middle Chinese voiced initials, were produced by Wang Li in 1936 and Li Fang-Kuei in 1937, with minor modifications by other linguists since. The conventionally accepted set of seven dialect groups first appeared in the second edition of Yuan Jiahua's dialectology handbook (1961):