In 1841 and 1842 small groups of emigrants tried to discover a route to California passable by wagons. Without reliable maps or guides, they pushed ahead, retreated, detoured, split up, and regrouped, reaching their destination only at great cost of property and life. But they had found a trail, or cleared one, and by their mistakes had shown others how to take wagon trains across half a continent. By 1844 a great migration was in progress. Each successive...