Given Wilkerson's introductory chapters on knowing and understanding history, I'm heavily disappointed this is the singular reference in the book to "The Great Chain of Being". It's not even indexed in the end.
The underlying idea of the scala naturae across a huge swath of western history is really the thing that gives rise to the problems of American history which she is attempting to bring to life here. Missing this part of the bigger picture is a serious flaw.
Does she skirt it rhetorically so as not to give fuel to the idea that "this is how things have always been?" arguments?
see also: https://en.wikipedia.org/wiki/Great_chain_of_being