In 1885 Firmin wrote ‘On the Equality of Human Races’ in response to a book by de Gobineau, which asserted the inferiority of black people and the superiority of the Aryan 'race’- a.k.a. the book that influenced Hitler’s ideology.
Firmin was an active member of the anthropology scene which meant he would have sat through talks, read countless essays, and heard the arguments of his contemporaries asserting that race was science, that anthropology could prove the existence of racial categories, and the inferiority of people who looked like he did.
What is remarkable about Firmin is that he refused to meet racism with racism. The final line in his book? “Love one another”.