By the late 1930s and early 1940s Simpson, though relatively young, was already a distinguished paleontologist at the American Museum of Natural History in New York City. His achievements included a Yale doctorate in geology and paleontology, position as a visiting research scholar at the British Museum, leader of two year-long fossil-collecting expeditions to Patagonia, author of two books and more than one hundred scientific articles and monographs, and newly elected fellow of two of the most distinguished honorary and scholarly societies, the American Philosophical Society and the National Academy of Sciences. As new discoveries, ideas, and theories in genetics were being published, Simpson understood their importance and kept himself informed. Before leaving for military service in late 1942, Simpson completed a major revolutionary text entitled Tempo and Mode in Evolution, published two years later.

Simpson's book applied the concepts and conclusions of the new discoveries in genetics to the large body of fossil evidence of life's long history, and claimed that the "microevolution" of the geneticist could indeed be extrapolated to explain adequately the "macroevolution" of the paleontologist. That is, the mechanisms of generating and accumulating inherited variation, as described by laboratory geneticists and field naturalists, were sufficient to provide a parsimonious explanation of the adaptations, specializations, and evolutionary trends of the paleontologists, as measured in their fossils over long intervals of geologic time. In this respect Tempo and Mode became one of a half-dozen books that formed the basis for what came to be called the modern evolutionary synthesis. "Synthesis" because the new body of theory came from a variety of fields--genetics, ecology, anatomy, field biology, paleontology, botany as well as zoology, and biogeography-- which were integrated into a unified whole.

Simpson thereby single-handedly brought the discipline of paleontology into the mainstream of biological research by validating fossil evidence for solving evolutionary questions. Before Simpson, what fossils had to say to biologists as articulated by paleontologists was at best confusing, at worst contradictory, when compared to their own observations and theories. Simpson debunked, once and for all, many previous paleontological explanations of evolutionary phenomena that depended upon inherent or internally directed forces, like "momentum and inertia," which argued that once organisms began to evolve they continued to do so because of the momentum of past evolution; "racial senescence," the notion that organisms in a given line may exhaust their evolutionary reserves and become extinct; "orthogenesis," that organisms evolve toward some future goal and thus intermediate stages exist only as steps toward that goal rather than as viable ends in themselves; and "aristogenesis," that organisms are driven forward in their striving for perfection. Simpson demonstrated that such explanations were not consistent with modern genetic theory. He also provided the coup de grace to lingering claims for the inheritance of acquired characteristics through use and disuse. Thereafter, if paleontologists were to carry conviction, they had to ground their interpretations of macroevolution in terms of microevolution.

Another equally important contribution of Tempo and Mode was Simpson's identification of significantly varying rates of evolution--very fast, average, and very slow--and his explanation of how such differing rates would yield characteristic patterns of evolution within the fossil record (hence the title "Tempo and Mode in Evolution.") Simpson invoked a special importance for the environment, with all its physical, chemical, and biologic manifestations, in influencing evolutionary patterns and rate.

Simpson was thus preeminent among paleontologists and evolutionary biologists for the two decades following World War II; indeed he was a household word for these scientists. And as often happens when fame in a field is so great--even a field as arcane as paleontology--it spills over into public consciousness. Thus, in the 1950s and 1960s, Simpson appeared on the cover of the Saturday Review of Literature, was the subject of a full-page cartoon in the New Yorker, was featured in a radio broadcast by Lowell Thomas, and was periodically mentioned in the national newspapers, especially his home town papers, The New York Times, Herald-Tribune, Sun, and World-Telegram. He even appeared on early television, guiding an anchorman through the fossil displays at the American Museum.

Despite this public exposure and despite his many writings--both professional and popular--George Gaylord Simpson was a difficult man to know. To most people, even those colleagues with whom he worked closely, he seemed reserved, often aloof, extremely guarded about his private life, and capable of sharp critical comment. He did not make friends easily, and those whom he referred to as good friends in his autobiography were surprised to be so considered. Because Simpson made no special efforts to cultivate friendship among his many acquaintances, he put most people off. Clearly more brilliant and more renowned than most around him, he accentuated the distance between them by his lack of warmth. By the time he died, some of his colleagues of his own generation had long given up knowing him; others, through hearsay, wrote him off as cranky, difficult, even embittered.

Toward the end of his life, Simpson's reputation waned somewhat, in part because his contributions became so thoroughly assimilated into current theory and practice that the identity of the originator was forgotten. Moreover, the next generation of evolutionists, especially among paleontologists, began to question some of Simpson's conclusions. In particular, some current paleontologists challenge the idea that macroevolutionary events portrayed by fossils are merely long-term extrapolation of the short-term microevolutionary processes seen by the experimentalists and naturalists. Whatever history's final evaluation of Simpson's contribution may be, during much of his lifetime he was judged by many of his peers to be the leading paleontologist and one of the key founders of modern evolutionary theory.