262 I The Process of Evolution 



Fig. 1 1.4 I Survivorship curves for genera of pelecypod mollusks and 

 land carnivores. Continuous lines, survival of genera now extinct; broken 

 lines, survival of genera known as fossils that are still living. Ordinate, 

 percent of genera surviving; abscissa (continuous lines), duration of 

 extinct genera in the fossil record; abscissa ( broken lines ) , time elapsed 

 since appearance in fossil record of genera now living. ( From Simpson, 

 1953, The Major Features of Evolution, Columbia Universitij Press. ) 



Survivorship of extinct genera 



Percent of genera originating at stated 



time in the past that survive today 

 Pelecypods (Mean 78.0) 



'o~-~ 



-o--. 



150 200 250 300 



Time, millions of years 



350 400 450 



Land carnivores 

 (mammals) 

 (Mean 8.1) 



5 10 15 20 25 



Time, millions of years 



30 



