The meaning of evolution.

All scholarly subjects seem to go through cycles, from periods when most of the answers seem to be known to periods when no one is sure that even the questions are right. Such is the case for evolutionary biology. Twenty years ago Mayr, in his Animal Species and Evolution (2), seemed to have shown that if evolution is a jigsaw puzzle, then at least all the edge pieces were in place. But today we are less confident and the whole subject is in the most exciting ferment. Evolution is both troubled from without by the nagging insistencies of antiscien tists and nagged from within by the troubling com plexities of genetic and developmental mechanisms and new questions about the central mystery?speciation itself. In looking over recent literature in and around the field of evolutionary theory, I am struck by the necessity to reexamine the simpler foundations of the subject, to distinguish carefully between what we know and what we merely think we know. The first and strongest of our critics to be answered should be ourselves. Scientists can always do themselves a great service by being scrupulously precise about the nature of their statements. Are they statements of fact, strong logical inferences from scientific methods, or hypotheses? In this essay I will attempt to show that the word evolution is currently used in at least three quite separate senses. The first way in which the term evolution has come to be used is the oldest: it is the general sense of change over time. We now have a wealth of scientific information about changes in the qualitative and quantitative di versity of organisms over space and time in the earth's history and a parallel set of data for changes in the earth itself. This information forms a linear?i.e., nonrepeat ing?and apparently progressive pattern of change. But it is composed of different types of statements. For ex ample, we know that there were once creatures of the sort we call dinosaurs. We know that their fossils occur