Biology and Philosophy 18: 387–400, 2003. 2003 Kluwer Academic Publishers. Printed in the Netherlands. Non-essentialist methods in pre-Darwinian taxonomy MARY P. WINSOR Institute for the History and Philosophy of Science and Technology, University of Toronto, Toronto, Ontario M5S 1A1, Canada (e-mail: [email protected]) Received 19 July 2002; accepted in revised form 19 July 2002 Key words: Essentialism, History, Linnaeus, Mayr, Method of exemplars, Polythetic groups, Popper, Typology, Whewell Abstract. The current widespread belief that taxonomic methods used before Darwin were essentialist is ill-founded. The essentialist method developed by followers of Plato and Aristotle required definitions to state properties that are always present. Polythetic groups do not obey that requirement, whatever may have been the ontological beliefs of the taxonomist recognizing such groups. Two distinct methods of forming higher taxa, by chaining and by examplar, were widely used in the period between Linnaeus and Darwin, and both generated polythetic groups. Philosopher William Whewell congratulated pre-Darwi- nian taxonomists for not adhering to the rigid ideal of definition used in the mathematical sciences. What he called the ‘‘method of types’’ is here called the ‘‘method of exemplars’’ because typology has been equated with essentialism, whereas the use of a type species as the reference point or prototype for a higher category was a practice inconsistent with essentialism. The story that the essentialism of philosophers dominated the development of systematics may prove to be a myth. The received view goes something like this: ‘‘Prior to the acceptance of evolution- ary theory, essentialism was the standard mode of classification in biological taxonomy.’’ (Ereshefsky 2001: 95). Here we have the gist of a well-known story about the development of systematics, prominent in many standard sources (Mayr 1982). Leaving aside its first and last chapters, we may summarize the central chapters of the story thus: 2.) Linnaeus followed the method of logical division developed by neoPlatonic and Aristotelian philosophers, 3.) taxonomists after Linnaeus continued to deal with species as Platonic types, 4.) Darwin achieved, or at least initiated, a revolutionary change by thinking in terms of populations. The received view, which promotes the importance of philosophy for the history of science, is widely repeated, especially in introductory courses, but its foundation in historical evidence is remarkably poor. It is inconsistent with another narrative that is better substantiated, a version of history in which experience plays a dominant role. In the ‘‘empiricism story,’’ 2’a .) taxonomists from the Renaissance onwards adhered to a wide variety of world views, but usually chose practical considerations over theoretical purity, 2’b .) Linnaeus gave them a flexible framework that proved useful for managing a vast and expanding body of data, 3’ .) Experience forced naturalists in the early 19th century gradually to relinquish belief in continuity, replacing it with belief in the naturalness of higher taxa, 4’ .) Darwins’s theory provided an explanation for the hierarchical shape of nature that taxonomists had discovered. 388 Future historians will have their hands full when they try to explicate the importance and many meanings of the word ‘‘essentialism’’ in the 20th century. What we may call the classical sense of the word refers to the misguided notion that biological species ‘‘must possess definitional essences that define them in terms of necessary and sufficient, intrinsic, unchanging, ahistorical properties,’’ but it is now proposed that the word be applied to the sophisticated notion that a natural kind, including a biological species, exhibits a cluster of properties ‘‘homeostatically’’ caused (Boyd 1999: 146). How very different the past will look when we recognize that the pre-Darwinians were essentialists in the homeostatic property cluster sense rather than the classical one. Instead of a picture of long stasis in taxonomy, caused by the stranglehold of an ancient idealism, the history of systematics will become a scene of progressive change, fueled by collection, preservation, exchange, and observation of organisms. History has the same responsibility as does science to be based on good evidence and to be open to testing, but for reasons stemming from interdisciplinary dynamics, the received view continues to elude critical evaluation. Fresh historical research is needed, but just reviewing material already published is enough to raise serious doubts about the essentialism story. Mayr (1968, 1982) has described the years between Linnaeus and Darwin as a time of ‘‘empiricism’’ and Panchen (1992) points out that ‘‘the acceptance of polythetic taxa’’ in the last half of the 18th and first half of the 19th centuries ‘‘negates essentialism.’’ In that hundred-year period, between the publication of the 10th edition of Linnaeus’s Systema Naturae in 1758 and Darwin’s (1859) publication of the Origin of Species, natural history was progressing dramatically, with museum and herbarium collections growing, workers better trained, and the number of named taxa at all levels increasing at an explosive rate. The notion that botanists and zoologists, during this busy period of achieve- ment, were frozen in the grip of ideas derived from Plato, Aristotle, and medieval scholasticism makes Darwin’s breakthrough nearly miraculous. The story of the dominance of essentialism is as dramatic, in its way, as the myth that has Columbus’s crew fearing they would fall off the edge of the earth, and I believe it is equally fictitious. The voyages of Columbus took place in the late 15th century, but the tale about sailors who believed in a flat earth was concocted long afterwards (Russell 1991). Likewise the essentialism of pre-Darwinian naturalists was not apparent to 18th or 19th century eye-witnesses but came to light only in the mid-20th century. The essentialism story was the creation of two biologists and a philosopher. In 1958 Arthur Cain, zoologist at Oxford University, hit upon the idea that Linnaeus had been following the rules of Aristotelian logical division. The distinguished Harvard zool ogist Ernst Mayr proposed in 1959 that what he called Darwin’s ‘‘population thinking’’ had replaced ‘‘typological thinking,’’ a world view derived from Plato, and Mayr repeated this idea emphatically in 1964. In 1963 David Hull, then a graduate student in the history and philosophy of science at Indiana University, made use of Karl Popper’s critique of ‘‘methodological essentialism’’ to argue that biologists should stop expecting a simple definition of the species category. Although Hull’s analysis was almost entirely philosophical, the title of the ensuing 389 article (Hull 1965), ‘‘The effect of essentialism on taxonomy – two thousand years of stasis,’’ constituted an historical claim. In 1968 Mayr took the critical step of accepting the word ‘‘essentialism,’’ which Popper (1944: 94) had coined, as a synonym for typological thinking (1968, 1969, 1976, 1982).1 Although a number of authors over the years have expressed various reservations, corrections, and doubts about parts of the Cain-Mayr-Hull story (Sokal 1962; Farber 1976; Winsor 1976, 1979, 2001, 2003; Sober 1980; Pratt 1981, 1985; Van der Hammen 1981; Stevens 1984, 1994; Atran 1990; Greene 1992; McOuat 1996; Muller-Wille¨ 1999, 2001; Camardi 2001), it is still the established view. In truth, Columbus’s crew, after many weeks at sea, did fear for their lives and begged him to turn back, because they thought the distance around the globe was so great that food and water would run out before they hit land. Likewise, beliefs allied to neoPlatonism, including morphologists’ ideal archetypes, did make it hard for some biologists to accept evolution, though not so hard as is often said (Amundson 1998). Yet just as we can better understand the westward expansion of Europeans when we know what was shared by Columbus and his men, which included both academic geography and practical sailing skills, likewise we will better understand the dynamic connection between taxonomy and evolution when we consider what Darwin shared with his fellow naturalists, which was the perception that living things ‘‘resemble each other in descending degrees, so that they can be classed in groups under groups,’’ (Darwin 1859: 411), a perception allied with a set of practices for describing and naming these groups. That perception and those practices were firmly rooted in the work of Linnaeus. Much of the literature relating essentialism to systematics is seriously flawed by the failure to separate ontology and epistemology. For the history of systematics, special effort is needed to give due weight to practice as well as theory, because many of the workers whose contributions to taxonomy have been influential have not been articulate about whatever principles were guiding them, as Peter Stevens (1994: xxi) reminds us. Above all, we must treat it as an empirical question whether various naturalists in the past were essentialists ontologically (in their world view), and as a separate question, requiring separate evidence, whether they were essential- ists epistemologically (in their method). Although we may think that people’s beliefs about the nature of reality should be tightly correlated with their research procedure, we ought not to prejudge the connection. If we assume, for example, that a person who believed in the existence of essences must have used the essentialist method, we run the risk of distorting the past through the lens of our expectation, thereby missing the opportunity to learn anything from history. We are not free to concoct out of our own reason or imagination what we think should count as the essentialist method. The warp cords around which Mayr, Cain 1 Ernst Mayr emphasized to me, after reading a draft of this paper, that his writings on typological thinking and essentialism were all aimed squarely at the level of species, which is certainly true. He holds that it is improper to apply these ideas to the higher categories, but other writers, I believe, have not respected, nor even understood, this stricture.