Complex sentence: On his return to the United States in 1885, he was appointed assistant to his father in Philadelphia, which position he voluntarily resigned after one year in order to devote himself entirely to linguistic and archaeological studies.
Split into simpler sentences: On his return to the United States in 1885, he was appointed assistant to his father in Philadelphia, which position he voluntarily resigned after one year. He went on to devote himself entirely to linguistic and archaeological studies.