The graph shows the number of northern-affinity mammals (nonvolant) and southern-affinity mammals in a North- South series of mountain ranges from Central Arizona and New Mexico to Northern Sonora and Chihuahua. The trends can be explained as the desert lowlands acting as a biogeographic filter for northward and southward migration. Note location of Davis' and Lowe's lines between the two mammalian provinces. Davis, R. 1996. The Pinalenos as an island in a montane archipelago. 123-134 in: Istock, C.A. and Hoffmann, R.S. Storm over a Mountain Archipelago: Conservation Biology and the Mount Graham Affair. Univ. Arizona Press, Tucson. Lomolino, M.V. and Davis, R. 1997. Biogeography scale and biodiversity of mountain forest mammals of western North America. Global Ecology & Biogeograplhy Letters 6: 57-76.