In this book both the fitness effects of different breeding systems and plant outbreeding mechanisms, in particular self-incompatibility in all its variety, are reviewed for the first time. The integrating factor is Charles Darwin, who with several Central European scientists initiated the modern investigation of both topics. The historical content is designed to demonstrate the development of understanding, the lineage of concepts and lines of investigation. This presentation of the basic historical background permits the incorporation of many detailed mechanisms into this short book. The authors have condensed hundreds of references very substantially in order to make clear the strong associations between breeding system and fitness. Particular attention is focussed on major unsolved problems, especially the origin of self-incompatibility and the origin of diversity within these systems. The envisaged readership encompasses academics, research students (from the end of the first degree upwards) and research workers in the plant sciences.