Earth-Science Reviews - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands EVOLUTIONARY PALEONTOLOGY AND THE SCIENCE OF FORM STEPHEN JAY GOULD Mttseum of Comparative Zoology, Harvard University, Cambridge, Mass. (U.S.A.) SUMMARY A science of form is now being forged within evolutionary theory. It studies adaptation by quantitative methods, using the organism-machine analogy as a guide; it seeks to reduce complex form to fewer generating factors and causal influences. If a function can be postulated for a structure, then its optimum form, or paradigm (RuDwtCK, 1961), can be specified on mechanical grounds. The approach of a structure to its paradigm provides the elusive criterion of relative efficiency that any science of adaptation requires. Physical laws and forces also specify that form be adapted to the requirements of size (surface/volume relation- ships) and space (close packing criteria). When we cannot establish paradigms on deductive criteria, an experimental approach to form is appropriate. Idealized models are favored over actual specimens because they can be built to test pre- determined factors. Paleontology need not remain solely a descriptive science based on observational methods, but may adopt the experimental techniques of explanatory procedures. It is inconceivable that each aspect of a complex form is the direct product of an individual genetic instruction. We can simplify, and thereby understand, the generation of apparent complexity by recognizing that physical forces directly influence shape and that a few simple rules can fashion some very intricate final products. These rules can be programmed; computers have simulated structures that bear remarkable correspondence to actual forms; the geometry of genetic instruction need be no more complex. The rules can be used to generate a range of potential form available to such structures as the coiled shell (RAuP, 1966). Actual forms fill only a part of the total spectrum; their basic adaptation may be grasped when we realize why unoccupied areas are not utilized. Among inductive studies of ontogeny and phylogeny, univariate techniques display trends and rates of change for single characters; they have been applied recently to the periodic growth lines of fossil shells, providing thereby a paleon- tological input to geophysics. Bivariate procedures, as the inevitable Gryphaea story illustrates, have been plagued by errors of method. When properly applied, they serve well in the separation of species and sexual dimorphs; they are the standard tool of quantitative description. Multivariate methods are based on the Earth-Sci. Rev., 6 (1970) 77-119 78 s.J. GOULD more satisfactory premise that an organism grows and evolves as a set of inter- acting parts; interactions should be considered together, not abstracted as pairs. In the R-mode, these methods may detect interrelated character clusters, reduce the high dimensionality of a system to few interpretable directions of variation, and eliminate redundant variables. In the Q-mode, they provide an objective picture of phenetic differences among samples and specify how the measured characters produce these differences. The importance of a new methodology can be gauged by its impact on ideas of life's history. A quantitative and functional science of form suggests that parallelism and convergence are dominant phenomena, not mere taxonomic nuisances. Early in their history, most phyla display great diversity at high taxonomic levels. These are not classic adaptive radiations, but sets of competing experiments in basic design. Early experimentation is followed by standardization of the best mechanical designs. These are often improved in similar ways by many independent lineages. Standardization and improvement provide invertebrate life with a history; the Phanerozoic has not been a time of endless ecological variation on a static set of basic structures. THE SHAPE OF THINGS TO COME Form and diversity are the two great subjects of natural history. Although we now think of speciation and adaptation where we spoke once of plenitude and design, it is only the terminology and not our concerns that have changed. Sys- tematics, the study of diversity, has been advanced by such thinkers as SIMPSON (1961) and MAYR (1963) to a level at which SYLVESTER-BRADLEY (1968) can justly christen a "science of diversity". Until recently, the study of form could make no such claim. It had, to be sure, its locus classicus -- D'ARcY THOMPSON (1917, second ed. 1942) -- but Thompson's treatise stood more as an awesome monument than a living work, its insights unnoted and its implications undeveloped. If paleontologists can now, as I believe, baptize a "science of form", it is because two approaches are beginning to unite under a common concept. The approaches are functional and quantitative; the concept is a mild mechanistic reductionism: an organism is a physical object subject to the laws of mechanics; its complexity can often be generated by a few, simple geometric instructions; its adaptation can be analyzed mechanically, often as an engineer would judge the efficiency of a machine built to perform a specific task i. In defending a concept that would seem crude or outdated in many physical sciences, I assert that alternative proposals offer no comparable access to the 1 I shall, if I may be permitted a literary barbarism, refer to this way of studying form as the quantifunctional approach. That it is, indeed, emerging as a fruitful strategy can be seen in the numerous papers of the recent Paleontological Society symposium entitled "Paleobiologieal Aspects of Growth and Development" (MACURDA, 1968). Earth-Sci. Rev., 6 (1970) 77-119 EVOLUTIONARY PALEONTOLOGY AND THE SCIENCE OF FORM 79 evolutionary problem of form, to the question of adaptation. Just as experimental physiology once needed Claude Bernard's determinism as a "specific conceptual tool" (COLEMAN, 1967, p.23), paleontology now requires a new strategy to found a science of adaptation. RUDWlCK understood this when he wrote (1961, p.450): "Although adaptation stands at the center of the modern debate on the mechanisms of evolutionary change, the problem of the recognition of adaptation in fossils, or the inference of function from structure has received surprisingly little attention." And later (1964b, pp.34-35): "The detection of any adaptation in a fossil organism must be based on a perception of the machine-like character of its parts and on an appreciation of their mechanical fitness to perform some function in the presumed interest of the organism." It is one of the ironies of history that the impact of evolutionary theory upon paleontology tended first to discourage the approach to adaptation we now advocate. Cuvier's functional morphology was the first triumph of biological paleontology, but it "suffered a spectacular eclipse" with the acceptance of evolu- tion (RuDWlCK, 1964b, p.38). Speculative phylogeny, based on pure morphology (PANTIN, 1951) and the supposed "laws" of its evolutionary modification, marked the paleontological approach to the history of life (GouLD, 1969b). HIS (1888, in COLEMAN, 1967; 1894), a great experimental embryologist, lamented the decline of functional studies and their replacement with a game of lineage building by "rigid morphological diagrams, abstracted by merely logical operations" (1888, in COLEMAN, 1967, p.175). Despite its later disenchantment with phylogenetic laws and parlor-room phylogenizing, paleontology has never dealt successfully with adaptation. Isolated men have had great insight -- Kovalevsky (STRELNIKOV and HECKER, 1968) and Dollo (GouLD, 1970) in particular. Neo-Lamarckists exploited the machine analogy (COPE, 1896, on kinetogenesis and JACKSON, 1891), but erred in assuming that mechanical optima implied direct mechanical pro- duction. The German school of experimental functionalists, particularly RICHTER (1929) and his followers (e.g., ZEUNER, 1933) studded the pages of Senckenbergiana and Paleobiologica with their work, but it had little outside impact and established no successful synthesis with modern evolutionary theory. Moreover, the worst features of speculative phylogeny are still with us, however reduced in frequency and impact, e.g., HOEEHAUS' suggested homology of comatulid pinnules with eurypterid walking legs (1963, p.460) -- as if a flexible structure built of hard parts could be constructed without jointing! But the conflict of functional and phylogenetic schools had no basis in necessity; any evolutionary theory must, in fact, deal with both questions. Yet since functional morphology had been the historical bailiwick of anti-evolutionists, it was degraded in the formulation of evolutionary theory. "The synthetic theory, despite its great verbal emphasis on function, tends to dissolve genuine adaptation into the non-morphological concepts of gene-pool, genetica[ fitness, adaptive zone, etc." (RuDWICK, 1964b, p.39). Our science of form must analyze adaptation Earth-Sci. Rev., 6 (1970) 77-119 80 S.J. GOULD without contradicting modern views of ontogeny and phylogeny. Moreover, it should provide new insights into paleontology's unique domain, i.e., transspecific evolution and major patterns in the history of life. This paper is written in the belief that such a science of form is now being forged. THE MECHANICS AND EXPERIMENTAL STUDY OF ADAPTATION IN FOSSILS All changes in the extremities [of fossil horses] are of course occasioned by mechanical conditions of movement and we really see that the