FOSSILS 381 



years. Recently, an earlier Egyptian civilization has been 

 unearthed that dates back more than ten thousand years. The 

 carvings left by prehistoric man in the caves of the Pyrenees are 

 perhaps twice as old as the Egyptian culture just mentioned. 

 According to the evidence that is now available, man has prob- 

 ably existed only a few hundred thousand years. Beyond man 

 stretches a chain of varied forms which can be arranged in pro- 

 gression from the highly complex ones of to-day through simpler 

 and simpler ones back to the simplest of all the one-celled plants 

 and animals. 



Fossils. In the strata of rock are found fossils in all degrees of 

 their original state of perfection, varying from trails, tracks, or 

 imprints to perfectly preserved shells, wood, bones, and complete 

 skeletons. As a rule, it is only the hard parts of the animals and 

 plants which have been preserved from decomposition. Some- 

 times the original organic substance is preserved, but more often 

 it has been replaced by mineral matter. This slow process of 

 replacement is known as petrification and the organism or the 

 part of the organism is said to be petrified. In a skeleton of a 

 prehistoric animal, particle by particle the lime was replaced by 

 indestructible silica, the element found in stone, sand, and glass. 

 Probably the most common forms of fossils are molds or casts. 

 The dead animal or plant fell into soft mud in which it made an 

 impression which was preserved long after the organic matter 

 had been destroyed. Sometimes these molds harden or became 

 filled with a substance which formed stone. 



Evidences of evolution. The organized data that support the 

 development of the higher organisms from the lower ones are based 

 on the following evidences : (1) geological evidences, (2) geo- 

 graphical evidences, (3) morphological evidences, (4) vestigial 

 evidences, and (5) embryological evidences. 



Geological evidences. The oldest known rocks appear to have 

 no fossils in them. The next layer shows traces of life of the 



