Tempo and Mode in Evolution

On two topics, in particular, the paleontologist enjoys special advantages.... The first of these general topics has to do with evolutionary rates under natural conditions, the measurement and interpretation of rates, their acceleration and deceleration, the conditions of exceptionally slow or rapid evolutions, and phenomena suggestive of inertia and momentum.... [T]hese problems are meant to be suggested by the word “tempo.” The group of related problems implied by the word “mode” involves the study of the way, manner, or pattern of evolution, a study in which tempo is a basic factor, but which embraces considerably more than tempo.... The purpose is to examine the figurative outline of the stream of life and…