348 C. SCHUCHERT MARINE INVERTEBRATE FOSSIL FAUNAS 



some forms of life rise quickly into the ascendancy, grow to large size and 

 prolific numbers, and dominate their environment. Then the fickle en- 

 vironment changes with the ever-changing earth's surface, and during 

 these times of marked alterations the once dominant species and genera 

 die out or evolve into new races, while some of the less significant forms 

 take the ascendancy and quickly come to be the rulers of their day and 

 domain — the guiding forms for the interpreting stratigraphor. 



