The Evolutionists sketches the internal conflicts that define and enliven Darwin's legacy today. After providing a cursory introduction to evolution and its history, Morris proceeds to outline the five basic tenets of Darwin's theory, as characterized by evolutionary biologist Ernst Mayr: (1) Evolution occurs; (2) there is a common ancestor; (3) species multiply; (4) evolution is incremental; and (5) the primary force behind speciation is natural selection. Scientists in the mainstream generally accept the first four tenets. It is the fifth that evokes the loudest arguments. Indeed, every generation since the publication of Darwin's Origin of Species in 1859 has pondered the role of natural selection in evolution. Even "Darwin's bulldog," the pugnacious Thomas H. Huxley, wasn't entirely sold on the idea.

Other contentious issues today include the pace of evolution, the influence of genetics, and the relatively new field of behavioral evolution. In contemporary discussions and debates, the one common thread that runs through arguments spanning such diverse disciplines as paleontology, genetics, and psychology is the voice of Harvard paleontologist and historian of science Stephen Jay Gould.

Gould became a key figure in the controversy when he and paleontologist Niles Eldredge, a curator at the American Museum of Natural History, presented their theory of punctuated equilibrium. In examining the fossil record, they saw an evolutionary pattern of long periods of relative stasis "punctuated" by sudden bursts of speciation (though what a geologist/paleontologist defines as sudden can be rather different from what a biologist considers sudden). This, among other things, helps explain some of the presumed gaps in the evolutionary record. These "gaps" were thought to be the result of a simple lack of fossilization, or perhaps of the more prosaic possibility that the relatively young science of paleontology simply hadn't yet found the intermediate fossils.

For Gould, much of the debate concerning the interpretation of punctuated equilibrium can be traced to the reductionist approach to evolution taken by opponents such as zoologist Richard Dawkins and philosopher Daniel Dennett. Gould sees them as simplifying speciation or genetics by limiting them to individuals, or to individual genes, rather than applying them to groups of individuals. On the other hand, Morris notes, the reductionists see the pluralistic views of Gould's camp as "a relatively unimportant addition to the orthodox [Darwinian] theory." Speciation may, they admit, occur rapidly at times, but so what? Moreover, Gould's rebuttals of his critics, some have complained, "may have succeeded better as a literary essay than . . . as a statement of his scientific beliefs."

In spite of the gallons of ink spilled in the battles between the two sides, Morris rightly observes, "the two camps are arguing, for the most part, not about interpretations of scientific fact but rather different ways of seeing patterns in evolution."

As for evolutionary psychology, empirical testing is always much more difficult than interpreting the fossil record. We can document behavior in nature, and we can ask human subjects why they behave the way they do, but can we consider behavior an adaptive trait under the sway of natural selection? Orthodox Darwinians maintain that natural selection is the only mechanism required, and they cite indirect evidence from genetics and neurobiology to support their view. But Gould and company believe it is empirically impossible to prove the role of natural selection in these terms. Furthermore, they insist that "spandrels" - a term borrowed from architecture to suggest "traits that were not molded by natural selection" - play a significant role in the evolution of behavior. And, they add, it is necessary to consider the impact of cultural evolution - for example, language may be simply "a byproduct of the way the brain is constructed."

The final quarter of The Evolutionists trims away much of the rhetorical excess - and there is a lot of it in the literature - surrounding many of these ideas. It predicts where, with continued research, solutions may be found in the future. For example, using computer programs to model evolution may help researchers determine the pace of speciation, as well as the factors that influence the process. Of course, Morris notes, in some cases there may be no resolution to debated issues.

To his credit, Morris does not single out or lambaste any individual for promoting a bad or awkward idea. Gould's name comes up more often than others' names do only because his views come up more often in the debate. The author clearly defines the players in this noisy, ongoing disagreement. He shows how close they often really are on some issues, and how far apart they will remain on others. Whether you are sitting on the sidelines watching the struggle or deeply engaged in the battle, Morris reminds, "Scientific controversy, though . . . bitter at times, is a healthy thing. It is a sign that scientists are questioning old ideas and looking for ways to better understand existing evidence."