Stephen Jay Gould dedicates this latest collection of essays drawn from his columns in Natural History to three teachers from his Queens, N.Y., elementary school. And well he might. For in spite of Gould's grace as a writer and intelligence as a scholar, this particular collection seems to be aimed more at teaching than entertaining. The essays are grouped by twos or threes around issues in evolution. The title essay, for example, deals with evolutionary opportunism. The panda's thumb is not a true finger, but the elongation of a wrist bone which, along with appropriated muscle tissue, allows the panda to hold its beloved bamboo while eating. The point of that essay and the next two--on turtle migration and animal lures--is that nature is a tinkerer, making the most of what's available in the course of adapting to the environment. Other groups add new information on Darwin and his contemporaries (Wallace, the fierce defender of natural selection, is seen to have made an exception for the human species--which was God's handiwork) or on racism and cultural relativism (Broca and Down--of Down's syndrome--emerge as outstanding defenders of male and white supremacy). In some instances, Gould sets forth his particular point of view--his idea that evolutionary species are mostly stable and that evolution occurs as sudden rapid change, or his recent conjecture that Teilhard de Chardin was a co-conspirator in the Piltdown hoax. Still others essays center on current controversies, such as the origin of birds or the warm-bloodedness of dinosaurs. Though the argument sometimes resembles what one would expect to find in a professional journal, Gould makes clear throughout just where he stands. So, for students wanting a dynamic view of contemporary paleontology, the book may satisfy a need. Amateur naturalists, however, might prefer somewhat less energetic exercises--and more of Gould's felicitous light touches.