The exact numbers of migrants cannot be counted, because of the variety of ways of travel (some walked), and the underdeveloped government statistics apparatus. Nonetheless, based on the reports of the Chinese Maritime Customs Service and, later, the South Manchurian Railway, modern historians Thomas Gottschang and Diana Lary estimate that, during the period 1891–1942, some 25.4 million migrants arrived to Manchuria from China south of the Great Wall, and 16.7 million went back. This gives the total positive migration balance of 8.7 million people over this half a century period. This makes the scale of the migration comparable to the American westward expansion, the Russian advance into Siberia, or, on a smaller scale, the Japanese immigration to Hokkaido.