By the middle of the 19th century, the population of Taiwan had reached 2.5 million. From 1905 to 1943, it grew from 3.6 million to 6.6 million. The rapid increase in population and development of cities in Taiwan has begun in the 1960s and continued through the present time. In 1986, the population of Taiwan was three times that of 1946, and had a population density of 540 persons per square kilometer. In the 1930's, there were only two cities with the population over 100,000; namely, Taipei and Tainan. But in 1960 there were nine cities. By 1985 the number of the cities with the population over 100,000 became twenty-five, the population of those cities was then 51 per cent of that of the Taiwan area. This increase was due to industrialization and urbanization.