Up to our own day American history has been in a large degree the history of the colonization of the Great West. The existence of an area of free land, its continuous recession, and the advance of American settlement westward, explain American development... The peculiarity of American institutions is, the fact that they have been compelled to adapt themselves to the changes of an expanding people—to the changes involved in crossing a continent, in winning a wilderness, and in developing at each area of this progress out of the primitive economic and political conditions of the frontier into the complexity of city life... Thus American development has exhibited not merely advance along a single line, but a return to primitive conditions on a continually advancing frontier line, and a new development for that area.

—Frederick Jackson Turner, 1893

Frederick Jackson Turner was a historian from the late 1800s and early 1900s. The quotation above is from his most famous essay. Turner's opinion in this passage is that