Complex sentence: The theory arose in the late 1860s and 1870s during the westward expansion of white settlement west of the Missouri River and across the 100th meridian west, the traditional boundary line between the humid and semi-arid portions of central North America.
Split into simpler sentences: The theory arose in the late 1860s and 1870s during the westward expansion of white settlement west of the Missouri River and across the 100th meridian west. This was the traditional boundary line between the humid and semi-arid portions of central North America.