328 PRINCIPLES OF ANIMAL BIOLOGY 



so that even the minute anatomy is preserved. Such objects are said 

 to be 'petrified (the process being called petrifaction). 



Large collections of fossils are sometimes found at ancient water 

 holes, where animals congregated and died in periods of drought, or in 

 asphalt pools where they were trapped and were probably attacked by 

 predators which also were caught in the mire. The great collection 

 of fossil bears, lions, saber-toothed tigers, horses, elephants, antelopes, 

 and vultures at Rancho La Brea near Los Angeles was caught in a pit 

 of tar. Caves are likewise the sites of numerous such collections. For 

 the most part, however, fossil forms occur singly or in small groups, 

 where they are discovered during excavations for buildings, by mine 

 operations, or other accidental means. 



Fig. 275. — Natural casts of dinosaur tracks and rain imprints. {From Schuchert, "Histori- 

 cal Geology," courtesy of John Wiley & Sons, Inc.) 



Paleontology Relates Two Evolutions. — Like zoogeography, paleon- 

 tology treats of the interrelations of two evolutions, the evolution of the 

 earth, and the evolution of living things. According to either of two 

 prevalent theories of the origin of the earth, this planet was in some way 

 derived from the sun, and went through a period of great heat. It is 

 only the earth's history in the later cool period that concerns us in the 

 study of fossils. Many of the superficial parts of the earth's crust are 

 in strata of different kinds of rock. Obviously, where these strata are 

 undisturbed, the lower ones were deposited first and are the oldest. In 

 many places the strata have been compressed sidewise, and forced to 

 rise in arches. With further lateral pressure, the arch may break, and 

 the strata of one slope be shifted over the strata of the other slope. At 



