This is an informational passage.
P1: Porter first scholar to ID African influence in African American art. Broad summary of his research.
P2: Example of Porter's work - identifying certain artists as African American. Distinguishing Porter's work from that of others - Porter focuses on African precursors.
P3: Other examples of Porter's work.
There's no scale, instead we've learned that Porter was an important figure in researching African influences on American art, including some of the specific types of research he conducted.