When species moved north after the glaciers began to recede, different starting points contributed to the varying arrival times.
A good example of this is represented by the picture to the left. Eastern White Pine and Eastern Hemlock are thought to have survived the Wisconsin glaciation in a refugium in the eastern foothills of the Appalachians. This is contrasted by the survival of Chestnut and Maples which are believed to have survived in a refugium around the mouth of the Mississippi River (E.C.Pielou, 1991). Therefore, these species have two totally different starting points prior to migration.