AThink of the word 'evolution': the name Darwin and the term 'natural selection' come to mind. But any powerful general theory of evolution must account for social evolution, both human and nonhuman, and contemporary Darwinism has not persuasively made such an accounting. In Interactions: The Biological Context of Social Systems, Niles Eldredge and Marjorie Grene argue effectively and coherently against the reductionist tendencies in modern Darwinism, which they call ultra-Darwinism, also known as genic reduction. This book explores the biological underpinnings of social systems from invertebrates to mammals, particularly humans. These social systems, the authors argue, represent fusions between the economic and reproductive interests of organisms. Their theory moves away from the more prominent emphasis on reproductive biology at the core of sociobiology to reinstate the importance of economics in social organizations of all types.