104 ORGANIC EVOLUTION 



circumstances will secure the preservation of any animal or 

 plant as a fossil. An organism, or portion of an organism, 

 to be so preserved usually must be hard ; it must be buried 

 beneath soil of the proper kind, and when buried must be 

 impregnated with mineral salts or in some other way pre- 

 served from disintegration. When once converted into a 

 fossil it must escape destruction at the hands of those 

 agencies that are constantly destroying the rocks, heat, press- 

 ure, the disintegration that comes from exposure to the 

 atmosphere, abrasion by ice, and especially erosion by water. 

 The character of whole continents has been repeatedly 

 changed by these agencies. No wonder, then, since fossiliza- 

 tion is rare and the destruction of fossils when once formed 

 so easy, that our record of past faunas and floras is so scant. 

 It is a cause for congratulation that we have so much of 

 a record as we do possess. Thousands of species of fossil 

 plants and animals are known, and as yet but a small portion 

 of the earth has been searched. We will give attention to 

 but a few illustrations of the kind of record we find in the 

 fossil-bearing rocks, choosing naturally records that are quite 

 complete. 



Let us first look at a table showing the order of for- 

 mation of fossil-bearing rocks. At the bottom of the table 

 are named the oldest of all the rocks in which fossils are 

 known to be found, the Cambrian formation, about 24,000 

 feet, four and one-half miles, in thickness. In these rocks 

 we find fossil remains of many different types, jellyfish, 

 sponges, Polyzoa, brachiopods, echinoderms, Mollusca, and 

 annulated worms, but no vertebrates. Numerous types are 

 represented, but they were simple organisms in comparison 

 with the representatives of the same types found in the 



