The Timetree of Life.
Stephen Jay Gould had a nice little "unpopular" essay on this topic "Cones and Ladders: Constraining Evolution by Canonical Icons." Gould mentions too the enduring influence of Ernst Haeckel's Tree of Life ---which is topped by "Menschen"--- on the common understanding of nature. Gould also held the Louis Agassiz chair at Harvard. While writing his "Mismeasure of Man," Gould found in the Agassiz archive the full text of a letter from Agassiz to his mother in which he described is first encounter with Negros. In 1846 Agassiz had arrived in the United States a noted Naturalist and needing to avoid some debts back in Europe. He immediately traveled to Philadelphia to meet Dr. Samuel G. Morton. Morton was the leading scientific proponent of polygenism and had amassed one of the largest crania collections in the world. Gould spent a great deal of time replicating Morton's experiments measuring their cranial capacity.
This is the text of Agassiz's letter to his mother as first published by Gould:
It was in Philadelphia that I first found myself in prolonged contact with Negroes; all the domestics in my hotel were men of color. I can scarcely express to you the painful impression that I received, especially since the feeling that they inspired in me is contrary to all our ideas about the confraternity of the human type [genre] and the unique origin of our species. But truth before all. Nevertheless, I experienced pity at the sight of this degraded and degenerate race, and their lot inspired compassion in me in thinking that they are really men. Nonetheless, it is impossible for me to reprocess the feeling that they are not of the same blood as us. In seeing their black faces with their thick lips and grimacing teeth, the wool on their head, their bent knees, their elongated hands, their large curved nails, and especially the livid color of the palm of their hands, I could not take my eyes off their face in order to tell them to stay far away. And when they advanced that hideous hand towards my plate in order to serve me, I wished I were able to depart in order to eat a piece of bread elsewhere, rather than dine with such service. What unhappiness for the white race---to have tied their existence so closely with that of Negroes in certain countries! God preserve us from such contact!
It was during this time that Agassiz met Samuel Morton, whom Agassiz recognized immediately as a scholar who was “after Georges Cuvier... the only zoologist who had any influence on his mind and scientific opinions.”
Haeckel's Tree of Life