What is the theme in Two Years Before the Mast by Richard Henry Dana?

Hierarchy is a theme in this book. Much of Two Years before the Mast consists of Richard Henry Dana, Jr.'s observations about the nature of society, both aboard ship and on land in still-desolate California in the 1830s. With the mind of a lawyer-in-training, Dana accepts the unwritten law of the sea, which makes the captain of a vessel, commercial as well as naval, the absolute monarch, the only person whose word matters.