Complex sentence: The U.S. National Park Service's first director, Stephen Mather, recognized Cammerer's competence as executive secretary of the Fine Arts Commission, and appointed him as assistant director in 1919, replacing previous assistant director Horace M. Albright, who then became Director.
Split into simpler sentences: The U.S. National Park Service's first director, Stephen Mather, recognized Cammerer's competence as executive secretary of the Fine Arts Commission. He appointed Cammerer as assistant director in 1919, replacing previous assistant director Horace M. Albright, who then became Director.