No sooner has the study of evolution arrived at a ""modern synthesis""--of molecular genetics, population studies, and paleontology--than rumbles are heard in the distance. Stanley (Paleobiology, Johns Hopkins) is a major rumbler. The argument concerns the origin of species. Not that natural selection is in doubt: random changes in the genes and ongoing changes in the environment will favor some individuals over others; they will leave more progeny and point to future changes in the group. But does the principle explain the rapid appearance of many species that the geological record reveals from time to time? Think of the burst of multicellular life following billions of years of bacteria or algae. Think of the proliferation of mammals after the demise of the dinosaurs. Think of Homo sapiens within a geological wink of Homo erectus. For years the received wisdom has been gradualism, as Darwin proposed: little by little, step by step, minor changes lead to new species over the long haul. Stanley, for one, and Stephen Jay Gould, for another, dispute, the cherished notion and substitute ""punctuated evolution"" instead. In their theory, populous species, demonstratively successful, slog along with very little change (actually evening out changes) over the eons. The key word is ""populous."" Suppose opportunity knocks, allowing some small groups to become isolated. That way the odd progeny--still a random event--may have a change to stand out in the now-small crowd. Perhaps the odd ones will survive because of better fitness. Such small group phenomena, occurring at specified times and places in the planet's past, could account for the rapid mushrooming of new species. Stanley and Gould now feel that the fossil record is complete enough to bolster their claims. Stanley marshals arguments from primitive sea forms--stable over eons--in contrast to the rapid transit from rodent-like creatures to such diverse species as bats and whales over a few million years. Stanley is persuasive, though obfuscating in style at times. Enjoy the excitement of the fresh approach, and expect to see the debate continued.