The Platypus and the Mermaid: And Other Figments of the Classifying Imagination

The game of scientific classification is something like Twenty Questions. You begin by naming one of Linnaeus's three kingdoms -- Animal, Vegetable, or Mineral -- and proceed by splitting it into ever smaller subsets. An animal, you say? Well, does it have a backbone? Yes. Does it breathe water? No. Is it warmblooded? Yes. With feathers? Yes. Flying? No. Taller than a man? Yes. Alive today? Yes. Then it must be an ostrich, Struthio camelus. Next question.

In playing Twenty Questions, you need to begin with big, general categories and then narrow them down, but the number of ways you can do this is endless. In the game of classifying organisms, systematists ever since Linnaeus have been trying to convince themselves that there must be a natural classification -- the one true and proper way to carve up the map of the living world -- and that they can find out what it is and make everybody in the world use it. Nowadays, we have some excuse for this belief, because we classify on the basis of evolutionary relationships, and presumably, there is only one true evolutionary tree (even if we don't always know what it is). But systematists sought natural classifications for more than two hundred years before they came up with any clear or objective criteria for preferring one classification over another. Lacking such criteria, what made them so sure of themselves?

In The Platypus and the Mermaid, the historian Harriet Ritvo explores some of the motives, reasonable and otherwise, underlying British classifications of animals in the eighteenth and nineteenth centuries. Linnaeus's system, with its concentric, nonoverlapping groups, became compulsory in scientific circles in the 1800s. But it was modulated by a lingering suspicion that there Were linkages between animals along other dimensions -- that some animals within each Linnaean group were "higher" than others or formed connections with groups outside their own, as the platypus was variously thought to link mammals to birds, reptiles, or amphibians. These horizontal and vertical dimensions were expressed formally in the bizarre "quinary system" of classification, which Ritvo may be the first person ever to explain intelligibly.

I liked Ritvo's opening chapters on scientific classification, but I was frustrated by her casual approach to the details of competing systems and methods. My frustration gave way to bewilderment as I read on into the later chapters, in which she surveys the mostly nonscientific ways in which Britons classified animal breeds, sideshow monsters, and meat on the table. The amount of space (more than half the book) that Ritvo lavishes on these topics seemed excessive to me, like devoting equal space in a history of chemistry to valence theory and the making of biscuits. But Ritvo's unsystematic approach to systematics suits her purpose, which is "to situate the technical classification of animals in eighteenth- and nineteenth-century Britain within a larger cultural context," without conceding any special status to what biologists thought about these matters.

As Ritvo points out, scientific classification arose partly out of a desire to inject some order and system into the vast, messy aggregations of seashells, mounted birds, and pickled fetuses that populated the curio cabinets of seventeenth-century aristocrats. This book has something of the disorderly allure of those strange collections. Anybody who reads it will be fascinated by all the curiosities Ritvo has assembled -- the interspecies hybrids, giants dwarfs, bearded ladies, two-headed pigs, sea serpents, and stuffed mermaids -- to bolster her contention that scientific authority failed to impose an official picture of the animal kingdom on the many-voiced classificatory impulses of Britons in the Age of Reason and the Age of Steam.

Ritvo's book is worth reading for its erudition and charm and for her perceptive observations on the social and political motives embedded in the scientific doctrines she surveys -- especially those that drew analogies between animal breeds, social classes, and human races. But I was troubled by her apparent hostility to the notion of scientific authority. In one sense, "scientific authority" is something of a misnomer because science's experimental method is grounded in the notion that experience counts for everything and authority counts for nothing. But not all reports of experience have the same value. The lifetimes of tested experience about living things that go into biological classifications are worth a lot more than the photograph of the half-woman, half-ostrich on this week's cover of the Weekly World Enquirer -- or her Victorian equivalent, the stuffed mermaid.

Persons of a scientific turn of mind, who want to know how errors originate and persist, and how we can avoid making similar mistakes in the future, may find Ritvo's book unsatisfying because of her reluctance to judge the beliefs of the past. Despite this book's subtitle, mermaids are figments of the imagination and platypuses are not. I would have liked the book better if Ritvo had insisted on that all-important distinction.