Complex sentence: Historically, infant mortality claimed a considerable percentage of children born, in the 1850s in America it was estimated to be 216.8 per 1,000 for whites and 340.0 for African Americans but rates have significantly declined in the West in modern times.
Split into simpler sentences: Historically, infant mortality claimed a considerable percentage of children born. In the 1850s in America it was estimated to be 216.8 per 1,000 for whites and 340.0 for African Americans, but rates have significantly declined in the West in modern times.