This book is about a surprise: a biological idea which is in the air, for it has come to many minds during the last fifteen years, but is still so new and strange and so little understood that few have recognized its far-reaching importance. It may prove to be one of the most fertile scientific ideas of this century, yet it is still unnoticed except by a few who are nursing, almost unawares, the first stages of a profound revolution in biological thought. The idea is that in addition to Darwinian selection another selective process has also played an important role in determining the evolution of the species. It is now being suggested that beside the well-established external competitive selection of the "synthetic" theory of evolution, an internal selection process acts directly on mutations, mainly at the molecular, chromosomal, and cellular levels, in terms not of struggle and competition, but of the system"s capacity for coordinated activity. The purpose of this book is to discuss this idea from a general point of view, to describe its history as a collective product of many minds, and to consider some of its implications. No attempt is made to cover the many special aspects which require consideration by biochemists, geneticists, embryologists, and others.