INTRODUCTION 3 



most plants have the power of locomotion; the dodder and 

 most bacteria have no chlorophyl. Among animals the pro- 

 tozoon, Euglena, has chlorophyl, while cellulose is present 

 in some Protozoa and is abundant even in the ascidians of the 

 Chordata. 



FOSSILS 



Conditions of their preservation. — i. That they soon be em- 

 bedded in some protective material. 



In order that a plant or an animal may leave a record of its 

 existence, that is, become a fossil, a speedy entombment within 

 some protective material is necessary. If an organism is left 

 exposed after death, it quickly becomes disintegrated either 

 through decay or through the attacks of living animals. In 

 other words it at once becomes food to living animals and plants 

 ranging in size from bacteria and protozoons to beasts and birds 

 of prey. This disintegration is aided likewise by chemical and 

 mechanical agencies so that the dead organism passes rapidly 

 back into its inorganic elements. The undertow of waves is 

 the strongest mechanical agency in the disintegration of marine 

 organic remains, such as corals, shells and bones. 



The habitat of the animal controls the kind of protective 

 material in which it may become embedded after death. As 

 protective materials may be noted sedimentary deposits due to 

 water or wind, ashes from volcanic explosions, bog waters, resin, 

 ice, and incrustations from mineral solutions. 



Occasionally the entire animal or plant is encased by the 

 rapid deposition of silica or calcium carbonate. Such incrus- 

 tations may occur through immersion in springs, as in the 

 geyser area of the Yellowstone National Park. Much more 

 common is preservation by means of water which has perco- 

 lated through or over liniestone beds ; upon the evaporation of 

 the water the lime which it has dissolved is again deposited. 

 This is especially apt to occur in caves or at the margins of 

 limestone ledges. Many remains of Pleistocene man and 



