Here is one biologist"s interpretation of the chronology of life during the last six hundred million years of earth history: an extended essay that draws on the author"s own data and a wide-ranging literature survey to discuss the nature and dynamics of evolutionary change in organisms and their biological surroundings. Geerat Vermeij demonstrates that escalation--the process by which species adapt to, or are limited by, their enemies as the latter increase in ability to acquire and retain resources--has been a dominant theme in the history of life despite frequent episodes of extinction.