CHAPTER FIVE 



Evidences of Evolutmi 



IV: Paleo7itology and Genetics 



The various lines of evidence discussed thus far all indicate that the 

 species now populating the earth must have been produced by evolution, 

 but generally the only possible direct evidence for a specific line of de- 

 scent is a series of fossils leading stepwise from an ancestral to a descended 

 species. Hence the science of paleontology, which deals with fossil re- 

 mains, has unique importance for evolution. 



THE GEOLOGICAL TIME SCALE 



Before beginning the discussion of paleontological evidence for evolution, 

 however, it is necessary to introduce the problem of geological time. For- 

 tunately, there are methods of determining the ages of rock deposits in the 

 earth's crust. The oldest method is applicable only to sedimentary rocks— 

 those successive layers or strata of rocks which are formed by slow settling 

 out of sediments from the oceans or other large bodies of water. The use 

 of the method for dating purposes is based upon the assumption that those 

 geological processes which arc observable in action no^^' are the same ones 

 which have determined the past history of this earth, and that they have 

 in the past acted at rates comparable to those now observable. As applied 

 to the problem at hand, this simply means that the sedimentary rocks of 

 the past were deposited at rates comparable to those which are now being 

 deposited. In a purely comparative way, dating bv this means is fairly 

 satisfactory. The deepest strata are the oldest, \\hile the most supcu'ficial 

 strata are quite recent. Thick strata represent long-continued deposition, 

 while thin strata represent short periods of deposition. Thus some relative 

 concepts of geological time are readily gained from an inspection of the 

 sedimentary rock deposits. But more exact ideas mc difficult to obtain 

 because it is clear that sedimentation is now progrtvssing at very different 

 rates in different parts of the world, and there is no reason to doubt that 

 the variation in the rate of sedimentation has been as great over much of 

 the world's history. It may well have been greater at times. Hence calcu- 

 lations of age based upon the thicknesses of stratified rocks lead to such 



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