A DEFINITION OF EVOLUTION 



petroleum spring should occur, evaporation of the more volatile oils will 

 produce first a pool of sticky tar and then one of viscous asphalt. This has 

 happened dining the Pleistocene epoch at Rancho La Brea in southern 

 California. Many Pleistocene and Recent mammals and birds have been 

 trapped in this asphalt, and they are among the best preserved of fossils. 

 It appears to have worked in the following ways. Small mammals, her- 

 bivores, and birds try to reach the rain pools which occiu' on the surface 

 of the asphalt. In so doing, they become stuck in the soft asphalt, and 

 predators are then ensnared while attempting to catch the former. Water 

 birds may alight on the water pools and then become entrapped in the 

 asphalt around the edges. Thus Rancho La Brea is one of the richest 

 sources known for well preserved fossils of recent mammals and birds. 

 Because the city of Los Angeles has grown up around it, it no longer 

 entraps the wild fauna of the region, but the Los Angeles fire department 

 is still occasionallv called out to rescue a child who has gotten his feet 

 stuck in the asphalt. 



Another unusual method of l:)urial is the entrapment of insects in amber 

 ( Figure 24 ) . Such fossils are sometimes preserved almost perfectly, so that 

 even histological details are comparable to those of freshly fixed speci- 

 mens. 



Fir.unE 24. Tehmites in Min- 

 Cenozok: Amueh. (Biichs- 

 haiim, R., "Animals Witliout 

 Harkl)()iu"s,"' 2iul Kd., Uni- 

 \cr.sil\- of Cliicago Press, 

 1948. Spctiuicn lent bv A. E. 

 I'^inerson, pholo^rapli 1)\' P. 

 S. Ticc. ) 



68 



