During the Paleocene and Eocene, climates were characterized by a low mean annual range of temperature (a maximum of 10-15??C), a moderate to high mean annual temperature (10-20??C), and abundant precipitation; strong broad-leaved evergreen vegetation extended to almost lat. 60??N during the Paleocene and to well above 61??N during the Eocene. Poleward of the broad-leaved evergreen forests were forests that were broad-leaved deciduous; these deciduous forests, however, were unlike extant broad-leaved deciduous forests in general floristic composition and physiognomy. Coniferous forests probably occupied the northernmost latitudes. At the end of the Eocene, a major climatic deterioration resulted in a high (> 30??C) mean annual range of temperature and a low mean annual temperature (< 10??C). Vegetation represented temperate broad-leaved deciduous and coniferous forests. The Oligocene and Neogene climatic trends represent a decrease in both mean annual range of temperature and mean annual temperature. Tundra vegetation did not appear until late in the Neogene. The present distribution of broad-leaved evergreens concomitant with the principles of plant physiology indicates that present winter light conditions at high latitudes could not support broad-leaved evergreen forest. A possible solution to the problem is to increase winter light by lessening the inclination of the earth's rotational axis. ?? 1980.