Williams accepts the so-called "adaptationist program" of "plan and purpose" in biology: that is, the idea that each attribute of an organism relates in some way to its efforts to survive and pass on its genes. The fish referred to in the book's title possesses light-generating cells that glow through its belly. The point of this uncanny quality, Williams suggests, has to do with the fish's habitat: It lives in deep ocean waters, and the light cells in its belly will match whatever faint sunlight penetrates the water, rendering the fish invisible to potential predators lurking below. There are also some teacherly essays on Darwinism in nature, rehearsing the old vitalism versus mechanism debates, describing with clarity and skill how natural selection operates to keep what has proven to be adaptive and cull the extremes. Williams uses as his examples such disparate events as the long evolution leading to the right size egg for a given species or the process leading to establishing the right number in a litter of young. He considers such essential matters as sex, pregnancy, aging, and death in a series of chapters exhibiting a fascination with the art of conflict and compromise in nature. Among the topics: why we have sex, why sperm are so small and eggs, in comparison, so big, why women get morning sickness and sometimes develop high blood pressure or diabetes while pregnant. Considering the evolution of body parts, Williams makes clear that we are flawed creations, demonstrating both "the power and the limitations of the evolutionary process."
In sum, some old, some new variations on the question of design (or the lack of it) in nature, by an old hand, who, if he hasn't quite the style of Stephen Jay Gould, is nonetheless well worth reading.