Crossing Over, the latest of three collaborations between scholar Stephen Jay Gould and artist Rosamond Wolff Purcell, brings together thought-provoking essays and uncannily beautiful photographs to disprove the popular notion that art and science exist in an antagonistic relationship. The essays and photographs collected here present art and science in conversation, rather than in opposition. As Gould writes in his preface, although the two disciplines may usually communicate in different dialects, when juxtaposed they strikingly reflect upon and enhance one another. Working together, Purcell's photographs and Gould's scientific musings speak to us about ourselves and our world in a hybrid language richer than either could command on its own. In an essay on individuality, for instance, Gould looks through the lens of evolutionary theory to address the controversial issue of cloning and the often misguided fears it evokes. As a society that exalts the concept of the individual, Gould argues, we sometimes fail to recognize that clones walk among us. Identical twins represent "the greatest of all challenges to our concept of individuality." Rosamond Purcell's photograph depicting the famous Siamese conjoined twins Eng and Chang conveys an eerie feeling that cannot be captured in words. Through its unique combination of words and photographs, Crossing Over prompts us to ponder not only the basis of the false dichotomy between art and science, but also the distinction of mind and nature, and of all humanly imposed categories of order. Gould and Purcell's work convinces the reader that a provocative interplay between art and science is not only possible, but inevitable and necessary as well.