304 Comparative Evolution of Biomes 



25 



20 



B 15 



10 



Cretaceous Cenozoic 



Pleistocene 



Fig. 131. Suggested temperature trends during the latter part of Cretaceous 

 and Cenozoic time, combining data for the Cretaceous from Lowenstam and 

 Epstein (1954), the Cenozoic from Durham (1950) and Dorman and Gill 

 (1959), and the Pleistocene from Emiliani (1955). These authors stress that 

 the values represent trends and should not be construed as indicating absolute 

 climatic measurements. 



tore trends from middle Cretaceous time to the present (Fig. 131) 

 in which the warmer periods have been fairly long and the cooler 

 periods fairly short. 



As was stressed by Axelrod (1952) these climatic temperature 

 bands are roughly at right angles to the three continental bands 

 running from the southern through the northern hemispheres, and 

 may have been in this relative position for a long time. On the basis 

 of the known distribution of marine fossils, Stehli (1957) believes 

 that Permian climatic zonation was parallel to the present. 



Current investigations on rock paleo-magnetism (Irving, 1959; 

 Runcorn, 1959) have resulted in the suggestion that the poles have 

 wandered considerably throughout the history of the earth (Fig. 

 11). The projected figures indicate only slight wandering since 

 the Miocene, a total of only about 15° of wandering since the 

 Cretaceous, but much greater wandering before these times. All 

 theories therefore seem in agreement that the direction of climatic 

 zonation has apparently been the same from at least late Cretaceous 

 time to the present. 



Under conditions of these hypotheses exchanges of tropical ter- 

 restrial communities from the Americas to Asia or Australia would 



