572 PLACENTAL MAMMALS xxi. 3- 



as humbler ranges such as the English chalkdowns, were raised into 

 their present form. This revolution marks the subdivision of the 

 Cenozoic into two periods. After it there was a further denudation 

 during the Miocene and Pliocene, with some renewed sea invasion. 

 The uplifting of the land during the Miocene probably produced arid 

 conditions unfavourable to the growth of forests, and at this time there 

 emerged several types of animal suitable for life on open prairies. 

 Conditions probably became gradually colder throughout the Plio- 

 cene, and the subsequent Pleistocene epoch was characterized by the 

 extensive glaciations of the 'Ice Ages', occurring in at least four peaks 

 of maximum cold, during each of which the ice caps advanced over 

 large parts of the continents (p. 648). 



Study of the history of the Cenozoic in the palaearctic region there- 

 fore gives us some idea of the climatic changes that have taken place, 

 and we can try to correlate these with the succession of types of animal 

 life. On the other hand, it is important not to be over-impressed with 

 any simple account of climatic changes. The periods of time involved 

 are enormously long and it is unsafe to assume that conditions 

 remained constant for any length of time that can be easily imagined, 

 or even that conditions varied at a constant rate. For example, in 

 Yellowstone Park, U.S.A., there are exposures of the remains of 

 Eocene tree-trunks and these are arranged in layers, showing that at 

 least twenty forests grew up and were covered by volcanic ash one 

 after the other. Each of these eruptions presumably produced a major 

 revolution for the animals and plants in the area concerned; we do not 

 know how wide that area may have been. Obviously no broad 

 generalization about the presence of 'humid conditions and forests' 

 throughout the Eocene can give us any clear picture of the ecological 

 conditions even in one area. The geological history shows us that 

 conditions were continually changing, though perhaps often at a rate 

 very slow in comparison with the duration of animal lives. We can 

 well imagine that these slow changes were responsible for producing 

 new conditions and hence new types of life, but the data of the rocks 

 are too obscure to show us the detailed circumstances of the emergence 

 of any particular type. 



4. Geographical regions 



Although the main land-masses have varied little during the Ter- 

 tiary period, there have been considerable changes in the opportuni- 

 ties for communication between them, both by the making and breaking 

 of narrow land-bridges and by the development of sharp tempera- 



