The cotton gin, developed in the 1790s, ensured the growth of southern agriculture into the 1840s (see chapter 8). As the cotton kingdom spread west, planters forged a distinctive culture around the institution of slavery. But slavery limited the development of cities, technology, and educational institutions, leaving the South increasingly dependent on the North and West for food, industrial goods, commercial resources, books and magazines, and even higher education. In addition, westward expansion extended the trade in slaves within the South, shattering black families. Still, southern planters viewed themselves as national leaders, both the repository of traditional American values and the engine of economic progress.