Slavery in the South involved only those of African descent, often becoming a permanent condition. It resulted from the buying and selling of stolen Africans. The demand for cotton increased the need for slavery and the number of slaves grew dramatically between 1792 and 1860. The Underground Railroad was a network that aided runaway slaves by hiding them during the day and helping them travel at night using code words and marked trees. Frederick Douglass was a former slave and leading abolitionist who published an antislavery newspaper, while Harriet Tubman repeatedly risked her freedom to lead others to escape slavery via the Underground Railroad.