Mark A. Ragan, <i>Kingdoms, Empires, and Domains: The History of High-Level Biological Classification</i> Oxford: Oxford University Press, 2023. Pp. 816. ISBN 978-0-19-764303-7. £107.50 (hardback).

This is a remarkable book.It is a superb resource for biologists, philosophers and historians of biology, providing a treasure trove of historical perspectives on the history of biological classification at its highest levels.Numerous gaps in that history are now filled and it becomes possible to understand how and why the big groups that are the book's focuskingdoms, empires, domainscame into being and have been at least partly vindicated by the relentless molecularization of classificatory practice.Most historical and philosophical overviews of classification focus on the bottom of the hierarchy, on species and how real they seem to be (and perhaps on how such claims are unjustified).In fact, species have frequently been proclaimed to be the only real level of classification, with no theoretical justification existing for higher levels (genus, family, order, class, phylum, kingdom) and considerable worries for anything below (subspecies, strains and so on).Twentieth-century pundits on systematics such as Ernst Mayr promoted this line of thinking.Perhaps only zoologists continued to cherish the realness of groupings such as phyla due to the existence of what look like body plans.But any reality to these upper levels was anchored firmly in the theoretical justifications for species as the units of evolution.Everything higher was just how species ended up clumping together, even when schemes for the revision of kingdoms were outlined from the 1930s onwards.In the 1970s, however, a singular event in the history of classification occurred.An upstart biophysicist developed methods of molecular analysis that started at the top of the classification hierarchy.Molecular methods were already in vogue and were transforming evolutionary classification across the tree of life.These methods worked in the usual ways, with species, their connections and how these might be organized in larger groups.But then, in a much more fundamental move, Carl Woese and collaborators found molecular and organismal differences that carved nature at her most basal joints into three domains of life.Mark Ragan, a bioinformatician and historical sleuth par excellence, has revealed the lead-up to this extraordinary moment, with a gesture toward what has happened since (spoiler: it looks like there are only two domains and that these are the fundamental divisions of life).The book moves chronologically through a series of episodes that piece together key moments in the history of classification, starting with hypothetical musings on the taxonomies that earliest humans might have used and then patchily through antiquity in several cultural systems to conclude that higher-level classification is in fact fairly modern (500 BCE), or at least that it left few traces in the earlier historical