CHAPTER XXV 



THE GEOLOGICAL BACKGROUND OF EVOLUTION 



It is one thing to understand in principle the causes and consequences 

 of evolution but quite another to know the actual evolutionary changes 

 that life has undergone. The history of life would be only vaguely known 

 by inference were it not that a record has been preserved in the rocks. 

 This record consists of the fossil remains of animals and plants that lived 

 in bygone eras. It is fragmentary in the extreme; yet surprisingly it is 

 sufficiently complete to permit the main outlines of the evolutionary 

 story to be clearly read. The fossils reveal the changes that life has under- 

 gone, and the rate at which such changes occurred. Often they permit 

 us to reconstruct the ancestral creatures as they must have been in life, 

 sometimes with a wealth of detail. Most dramatically, they tell us of the 

 many blind alleys into which life has strayed, as shown by the great 

 multitudes of fossil types that have left no descendants. We may compare 

 the paleontological panorama of evolution to an enormous picture mosaic, 

 begun long ago and still unfinished, faulty and defaced in its older parts 

 but with its composition and significance plain and many of its finer 

 details still clearly visible. 



Before we can take up the story of life as told by the fossils, we shall 

 have to know something of the geological background of evolution. The 

 earth is the stage on which the life drama has been played; what has 

 been its physical history? How were fossils formed, and what kinds of 

 information do they furnish? How do we date events of the past and 

 assemble the geological and paleontological facts into a connected his- 

 tory? In order to answer these questions it will be necessary to give some 

 account of earth features and processes. 



In Chap. XXIII we mentioned that it was formerly customary to 

 attribute many geological phenomena to great catastrophic events of 

 the past — universal floods, volcanic outbursts, or what were broadly 

 referred to as "convulsions of nature." After Hutton, however, geologists 

 came gradually to accept the idea that earth features had been molded 

 by small forces acting over very long periods. Modern geology is founded 

 on the Huttonian principle of uniformitarianism, and no one now doubts 

 that earth history has been enormously long, that most changes have 



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