Complex sentence: With each generation, the number of mixed-race slaves increased: the 1850 census classified 245,000 slaves as mulatto; by 1860 there were 411,000 slaves classified as mulatto out of a total slave population of 3,900,000.
Split into simpler sentences: With each generation, the number of mixed-race slaves increased. The 1850 census identified 245,000 slaves as mulatto; by 1860, there were 411,000 slaves classified as mulatto out of a total slave population of 3,900,000.