David McCullough, the historian, has said that he chooses the topics for his books he does, because those are the topics he himself would like to read. I wanted to know more about George Gaylord Simpson (1902-1984). As a paleontologist I had read most of his books, many of his articles, and a few of his monographs. My curiosity was thus piqued to learn more about how his theories developed, how his arguments were constructed, and from what specific knowledge they proceeded. At the time, there was very little literature analyzing Simpson's work and, as I came to discover, most was superficial and concerned with just a few aspects of his voluminous body of work.

Initially I thought of writing a biography, but several circumstances soon dissuaded me. In 1978, Simpson published his autobiography, Concession to the Improbable, which provided most of the descriptive facts of his life. Also as background for the book still forming in my mind, I interviewed some three dozen persons who knew Simpson, members of his family, former students, and professional colleagues. Despite their varying views about him, they led me to the conclusion that a biography would be of much less interest than a closer examination of Simpson's major areas of research. To be sure, such discussion would need relevant biographical background, but conventional full-scale biographical treatment was less urgent.

The concept of the book thus evolved from conventional biography to a case of "reverse engineering" where one dismantles an object of interest into its constituent parts to understand how it is constructed and the way it works.The book, therefore, is not biographical in the ordinary and strict sense. However, it does progress more or less chronologically through Simpson's life, focusing on major aspects of his research (fossil mammals, evolutionary theory, biogeography, taxonomy and systematics), his ways of approaching scientific problems (continental drift, visual way of thinking), and his impact on related disciplines (biology and physical anthropology). When particularly relevant, biographical material is included, such as his early education as a paleontologist (especially the influence of Darwin and W. D. Matthew), his often difficult relations with his museum colleagues, and the apparent melancholia toward the end of his life.

Chapter One summarizes Simpson's life, thereby providing a biographic context within which his scientific work can be viewed. Chapter Two gives a chronological overview of Simpson's scientific accomplishments that indicates their significance and how it is that Simpson became the most important paleontologist of the middle part of the twentieth century as well as one of its leading evolutionary theorists. Chapter Three describes how Simpson, a young graduate student at Yale, got jump-started on his career and the special importance of the mentorship provided by William Diller Matthew, vertebrate paleontologist at the American Museum of Natural History in New York.

Chapter Four reaches back into Simpson's late teenage years, showing the impact of his reading of Charles Darwin's On the Origin of Species and how it reinforced his turn away from formal religion. Darwin's influence was so strong that Simpson, over his long career, regularly celebrated what Darwin has to teach us, philosophically as well as biologically. Chapter Five follows the various threads of Simpson's scientific development, especially as brought together in his several-year-long study of early mammals, that led to his most important single scientific contribution, which was the validation of the new genetics and population biology of the 1930s by their corroboration and further elucidation from the fossil record. In Chapter Six we see how Simpson's concept of species moved from species-as-types, embodying some invariable ideal abstract quality, to species-as-populations, composed of variable organisms upon which Darwinian natural selection operated. Chapter Seven discusses in detail Simpson's Tempo and Mode in Evolution, which represents the culmination of his ideas on evolutionary rates and patterns and was his contribution to the evolutionary synthesis that was consolidating between 1936-1947.

Chapter Eight describes how the evolutionary synthesis was incorporated into the understanding of human evolution and the special role Simpson played in first educating the younger generation of physical anthropologists returning from World War II, and then subsequently supporting their interpretation of the human fossil record as another instance of Darwinian evolution. Chapter Nine examines the reasons for Simpson’'s strong opposition to continental drift, based as it was upon his independently developed principles of historical biogeography. Until the development of Plate Tectonics theory of the 1960s, Simpson's explanation for similar fossils on widely separated continents by the natural dispersal of organisms during their lifetime seemed more credible than appeal to Wegener's wandering continents carrying their post-mortem remains by as yet unknown mechanisms.

Chapter Ten steps back and considers Simpson's use of visual imagery in formulating and expressing theoretical concepts, a style of reasoning that he particularly found most suitable. Chapter Eleven describes Simpson's relations with his institutional employers’ the American Museum and Harvard's Museum of Comparative Zoology that were often made more difficult by the hypersensitivity regarding what he thought was due him as one of the world's leading scientists of his day. Chapter Twelve concludes the book by reviewing a posthumously published novella that Simpson wrote toward the end of his life. It reflects a melancholy view of life that contrasts with the more optimistic attitudes found in his earlier work as, for example, in his Darwin writings.

It will be obvious even to the casual reader that this book is merely a first attempt at examining, interpreting, and evaluating exactly what George Gaylord Simpson accomplished professionally in his life as paleontologist and evolutionist. I hope, indeed expect, others will be stimulated by this initial effort to take up from where I have left off, whether by addition, expansion, or correction.