In 1972 Stephen Jay Gould took the scientific world by storm with his paper on punctuated equilibrium. Challenging a core assumption of Darwin's theory of evolution, it launched the controversial idea that the majority of species originates in geological moments (punctuations) and persists in stasis. Now, thirty-five years later, Punctuated Equilibrium offers his only book-length testament on a theory he fiercely promoted, repeatedly refined, and tirelessly defended.