626 



ORGANIC EVOLUTION 



Fig. 25-2. This is a restoration of a herd of American mammoths that lived in Michigan as recently as 15,000 years 

 ago. To the right of the large boulder in the right foreground can be seen a giant beaver that was abundant also 

 during this period. Both species are now extinct. 



ture that they had ever hved. There are 

 many ways in which fossils are formed, 

 some of which we shall consider. 



The ideal fossil is the whole animal pre- 

 served intact so that its entire anatomy can 

 be studied in detail. This has occurred in 

 the case of insects of the Oligocene, when 

 they became embedded in the sticky pitch 



of the coniferous trees of that period. These 

 animals show bodily structures in the finest 

 detail. Even scales on the wings of butter- 

 flies are as perfectly preserved as if they 

 had lived only yesterday. In certain cold 

 regions such as Siberia the woolly mam- 

 moths (Fig. 25-2) often fell into crevasses 

 in the ice where they were quickly frozen 



Fig. 25-3. A reconstructed skeleton of a saber-tooth tiger taken from the La Brea tar pits In Los Angeles. These are 

 the actual bones which have been preserved by the action of tar. Note the large canine teeth which undoubtedly 

 made this big cat one of the more formidable animals of its day. 



