i. 10 GEOLOGICAL PERIODS 21 



It is usual to divide the last main geological period, the Tertiary, 1 

 into epochs, Paleocene, Eocene, Oligocene, Miocene, Pliocene, and 

 Pleistocene, the names originally referring to the percentage of fossil 

 genera surviving to the present day (see p. 571). Probably the whole 

 time since the end of the Cretaceous has been about 70 million years. 

 During the early part of the Tertiary period the climate was cold, but 

 as erosion of the mountains that had been produced at the end of the 

 Cretaceous proceeded the conditions became warmer, and throughout 

 the Eocene and Oligocene there were large forests and humid con- 

 ditions. Then during the Miocene there were marked earth move- 

 ments, leading to elevation of the land and accompanied by more arid 

 conditions, with wide areas of prairie and the widespread appearance 

 of important new food plants — the grasses. The weather probably 

 became gradually colder through the Pliocene, no doubt with many 

 fluctuations, culminating in the ice ages of the Pleistocene. Here we 

 come back to the period of which we have more detailed knowledge, 

 and are reminded that the ice age was not continuous, but interrupted 

 by many w r armer periods. 



This very brief survey of geological history in the northern hemi- 

 sphere can hardly do more than remind us of the depths of our ignor- 

 ance. We see enough to be sure that climatic conditions have varied 

 throughout the millions of years, but we cannot yet see sufficient 

 details to allow us to discover whether there is any rhythm of major 

 cycles. It is easy to talk glibly of 'Carboniferous forests' or 'arid con- 

 ditions of the Permian', forgetting that these periods lasted for a time 

 that we can only roughly record in numbers and not properly imagine 

 in terms of our experience, although we are among the longest lived 

 of animals. The evidence suggests that conditions did not remain 

 stable for such a vast length of time as a whole geological period, but 

 fluctuated markedly, either irregularly or with complicated rhythms 

 of greater and lesser magnitude. We must not forget that very pro- 

 found 'climatic' changes occur every day, others every year, and some 

 every eleven years. It is not impossible that these shorter-period 

 changes, necessitating continual readjustment of animal and plant life, 

 have been as important as the slower changes in producing evolution. 



10. Summary 



To reduce to order our knowledge of vertebrate life we shall try 

 to discover its general organization and then examine the factors that 



1 This word is a survival from an old-fashioned classification of rocks, the Tertiary heing 

 the period since the Cretaceous. 



