Spencer, Herbert
Watts & Co., 1950
Buku Teks SO
MacIver, Robert M.
Oxford, 1926
Buku Teks SO
Chard, Chester S.
McGraw-Hill, 1975
Buku Teks
Seckler-Hudson, Catheryn
United State Information Service, 1959
Buku Teks SO
Teilhard de Chardin, Pierre
Collins, 1960
Buku Teks SO