August Bebel
After he had finished school, Ploetz at first studied political economy in Breslau, whete he joined the "Freie Wissenschaftliche Vereinigung" (Free Scientific Union). Among his friends were his brother, his former school friend Ferdinand Simon (later son-in-law of August Bebel), the brothers Carl and Gerhart Hauptmann, Heinrich Laux and Charles Proteus Steinmetz.