Kingdoms, Empires, & Domains The History of High-Level Biological Classification

Kingdoms, Empires, & Domains by molecular biologist Mark A. Ragan is an encyclopaedic overview of our thinking about high-level biological classification. It combines a deeply researched history that draws on primary literature and authoritative translations with a tightly focused and well-structured text. The result is a scholarly monograph that is remarkably engaging. Its massive scope makes it one of those once-in-a-generation books that will be indispensable for science historians and academic libraries.

Two introductory digressions are necessary. First, this book is big: narrative (449 pp.), endnotes (182 pp.), ~3500 references (139 pp.), and two indices (54 pp.). Writing such a deeply researched book took time: this passion project was in gestation for 30 years, with some 8 years of solid writing interrupted by an 18-year career move in which Ragan continued to read and translate background literature. Second, Ragan is an Emeritus Professor in molecular biology rather than a (science) historian. Upon close reading, however, I found that this combination of personal interest and professional training works. In the final chapters, he reveals continuities between history and recent technologically-driven advances in ways that a historian might struggle with.

Kingdoms, Empires, & Domains covers 26 centuries of philosophical, religious, and scientific traditions in Europe and the Mediterranean and what these had to say about how life is organized. This includes, very roughly, classic Greco-Roman philosophy; Asian and Middle Eastern traditions; early Christianity, Islam, and Judaism; philosophical, religious, mystical, and popular traditions in mediaeval Europe; the rise of natural history and biology through the 15th–19th centuries in France, Germany, and Britain; and 20th–21st-century developments in genetics, molecular biology, and phylogenetics. Given the book's vast scope, this review can only characterise it. That said, I want to highlight two intertwined themes running through it: typology and topology.

First, typology: many of the topics discussed here concern the question of how, and using what characteristics, organisms were classified into different groups. The phrase "animal, vegetable, mineral" might ring a bell and for centuries was a widely accepted division. However, these three kingdoms of nature were neither rooted in prehistory nor universal. The world at this time was "a fluid conceptual landscape in which every boundary could be, and often was, transgressed" (p. 23). Also, the first mention of these groups as Kingdoms was in 17th-century alchemical tracts; botanists and zoologists later adopted this terminology. Furthermore, from the start, there was an interest in sponges, corals, and other marine invertebrates that seemed both animal and plant in nature and were soon called zoophyta. Contrary to widespread opinion, Aristotle did not coin this term; Ragan suggests later translations might have led to this misattribution. It is instead first encountered in the Neoplatonic writings of Dexippus and Themistius. Zoophyta was a moving target though, its membership a rotating cast, and opinions remained divided on what exactly they were: plant, animal, both, or rather a third intermediate group?

The 17th-century invention of the microscope introduced a new world of animalcules. Soon, people were letting all sorts of semi-liquid substances ferment on their benches, creating infusions full of "little animals" (Infusoria) that swarmed and multiplied in a drop of water. Much more recently, molecular biology, genetics, and the electron microscope completely upended our understanding. These revealed differences between microorganisms that led to proposals for three, four, five, or more kingdoms, including top-level groups that excluded plants and animals. There are fascinating discussions here on endosymbiosis, horizontal gene transfer, and metagenomics. This is where Ragan shows that he is as much at home in science history as he is at ease explaining modern science.

Second, topology: how are all these groups connected? What shape does nature take: a chain, a tree, a network? The idea of continuity in nature goes back to Plato and Aristotle and would later form the backbone of the Great Chain of Being or Scala Naturae. This metaphor became increasingly problematic, though, and fell out of favour during the 18th century. Ragan traces the first explicit use of the tree metaphor to Peter Simon Pallas in the 1760s, while the first use of a web/net metaphor is traced to a 1635 rhetorical flourish by Juan Eusebio Nieremberg with Vitaliano Donati reviving it in 1750. They were far from the only metaphors on offer though, and one chapter deals with some rather imaginative depictions of nature as a map, polygon, spiral, circle, or groups of circles. In his concluding discussion, he refers to Doolittle & Bapteste's call for pattern pluralism, i.e. the recognition that different metaphors are appropriate for different parts of the living world. Mammals diversified in a tree-like manner, while bacteria diversified in a network-like manner due to the prevalence of horizontal gene transfer.

On the whole, the level of research that went into this book is occasionally off the charts. Ragan's endnotes regularly mention how he has compared different translations of classic texts. As historians know, their interpretation is complicated by appropriations, insertions, changes, deletions, and plagiarism, so I came away with a renewed respect for their craft. Even for modern works, Ragan regularly compares different editions or goes the full monty and tracks certain concepts or words in tens if not hundreds of books or dictionaries. He quotes long passages from primary literature, gratefully relying on expert translations from the expansive Loeb Classical Library, those provided by colleagues, or his own careful translations. For all its historical sleuthing, the book is well structured and relies on subheadings to break up the text, making it a readable, even compelling work. What helps is that Ragan remains focused. More than once he acknowledges that he could say much more about certain scholars but will focus on the bits relevant to high-level biological classification.

Who is the target audience? For science historians or historically-inclined taxonomists and evolutionary biologists, this book is a goldmine of information. For academic libraries, particularly those specializing in history and the life sciences, it is an obligatory acquisition. I praised the readability, so serious lay readers are also welcome. Keep in mind, though, that this book is an investment in both time (I spent 23 days with it) and money (the price tag is high though justified given the size).