Complex sentence: Aptheker challenged racist writings, most notably those of Georgia-born historian Ulrich Bonnell Phillips, who cast African Americans as child-like, inferior, and uncivilized; argued that slavery was a benign institution; and defended the preservation of the Southern plantation system.
Split into simpler sentences: Aptheker challenged racist writings, most notably those of Georgia-born historian Ulrich Bonnell Phillips. The latter had characterized enslaved African Americans as child-like, inferior, and uncivilized; argued that slavery was a benign institution; and defended the preservation of the Southern plantation system.