he resented complainers who called the vast domain a "howling wilderness". After the purchase, Jefferson preserved the region's Spanish legal code and instituted a gradual approach to integrating settlers into American democracy. He believed that a period of the federal rule would be necessary while Louisianians adjusted to their new nation. Historians have differed in their assessments regarding the constitutional implications of the sale, but they typically hail the Louisiana acquisition as a major accomplishment. Frederick Jackson Turner called the purchase the most formative event in American history. Lewis and Clark Expedition (1803–1806) Jefferson anticipated further westward settlements due to the Louisiana Purchase and arranged for the exploration and mapping of the uncharted territory. He sought to establish a U.S. claim ahead of competing European interests and to find the rumored Northwest Passage. Jefferson and others were influenced by exploration accounts of Le Page du Pratz in Louisiana (1763) and Captain James Cook in the Pacific (1784), and they persuaded Congress in 1804 to fund an expedition to explore and map the newly acquired territory to the Pacific Ocean. Jefferson appointed Meriwether Lewis and William Clark to be leaders of the Corps of Discovery (1803–1806). In the months leading up to the expedition, Jefferson tutored Lewis in the sciences of mapping, botany, natural history, mineralogy, and astronomy and navigation, giving him unlimited access to his library at Monticello, which included the largest collection of books in the world on the subject of the geography and natural history of the North American continent, along with an impressive collection of maps. The expedition lasted from May 1804 to September 1806 (see timeline) and obtained a wealth of scientific and geographic knowledge, including knowledge of many Indian tribes. Other expeditions In addition to the Corps of Discovery, Jefferson organized three other western expeditions: the William Dunbar and George Hunter Expedition on the Ouachita River (1804–1805), the Thomas Freeman and Peter Custis Expedition (1806) on the Red River, and the Zebulon Pike Expedition (1806–1807) into the Rocky Mountains and the Southwest. All three produced valuable information about the American frontier. Native American affairs Jefferson's experiences with the American Indians began during his boyhood in Virginia and extended through his political career and into his retirement. He refuted the contemporary notion that Indians were inferior people and maintained that they were equal in body and mind to people of European descent. As governor of Virginia