The main reason for the large population growth in Texas between 1850 and 1860 was

an enormous number of southerners, many of them enslaved, moved west to expand the cotton belt. The 1860 census counted 169,000 enslaved persons, roughly 30% of the state population. After the civil war, whites continued to arrive from nearby southern states, causing the population to double by 1880 and double again in the following twenty years. African Americans contributed little to this late 19th century migration.