Complex sentence: Fairchild's major teaching appointment was at New York University where it extended over a period of 26 yeas, from 1919 until his retirement in 1945, and where he became chairman of the Department of Sociology in the Graduate School.
Split into simpler sentences: Fairchild's major teaching appointment was at New York University. He served for 26 yeas, from 1919 until his retirement in 1945, and became chairman of the Department of Sociology in the Graduate School.