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Du Bois showcases his studies on socialism and social Darwinism as well in this work , a continuation on his examination of the genealogy of blacks outlined in The Philadelphia Negro ( 1899 ) and The Souls of Black Folk ( 1903 ) that refutes the biological differences between blacks and whites .
 Du Bois
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Du Bois