This book describes the experimental study of evolution and adaptation, carried out by means of combined field-work and laboratory genetics. That technique has been developed during the last forty years or so by my colleagues and myself, and by a small but increasing number of geneticists throughout the world. In discussing what has been achieved by these means many relevant pieces of work familiar to me have been omitted, while doubtless there are others that have escaped my attention. To those who have thus laboured without recognition here, I offer my apologies. Yet I would not include further examples were I writing again, and this for two reasons. First, my aim is not to produce a com pendium in the German fashion, for I have endeavoured to develop principles with enough instances to illustrate them but no more. Secondly, this book is in danger of becoming too long as it is: one which is in general consulted only in libraries, not read familiarly by students.