Evolution does not occur in a vacuum. It takes place against a background of changing conditions, some of a climatic or tectonic nature, some created by organisms themselves. The extent to which the rate and direction of evolution are controlled by organisms is the central question in a debate that has been raging for as long as evolution has been part of the intellectual vocabulary of scientists. In an effort to stimulate empirical work on the subject, we organized a symposium on the contributions that functional morphology and comparative physiology are making to paleobiology. The symposium was held as part of the North American Paleontological Convention on July 1, 1992, at the Field Museum of Natural History in Chicago. This issue of Paleobiology contains all the submitted and accepted papers presented at the symposium.