Wulff, E.V. (1943). An Introduction to Historical Plant Geography; translated by Elizabeth Brissenden. A new series of plant science books. Vol.10. Waltham, MA: Chronica Botanica Co. p.223. Republished as Chapter 18 in Foundations of biogeography: classic papers with commentaries edited by Mark V. Lomolino, Dov F. Sax and James H. Brown Google Books