Complex sentence: During the 1850s, the state's population grew to exceed one million, and the ambitious program of its founders was realized as Indiana became the fourth-largest state in terms of population, as measured by the 1860 census.
Split into simpler sentences: During the 1850s, the state's population grew to exceed one million. The ambitious program of its founders was realized as Indiana became the fourth-largest state in terms of population, as measured by the 1860 census.