Complex sentence: The theory was widely promoted in the 1870s as a justification for the settlement of the Great Plains, a region previously known as the "Great American Desert", and the expansion of wheat growing on very marginal land in South Australia during the same period.
Split into simpler sentences: The theory was widely promoted in the 1870s as a justification for the settlement of the Great Plains, a region previously known as the "Great American Desert". It was also used to justify the expansion of wheat growing on marginal land in South Australia during the same period.