The basic principles of inheritance were not established as a consequence of an immense accumulation of descriptive evidence. In this respect, the history of genetics differs from that of many other branches of biology. Before Mendel's conceptual tour de force and indeed, until his rules of diploid inheritance were rediscovered at the turn of the century, the phenomenon of heredity remained a mystery. A number of plant hybridists grappled with the apparently inscrutable properties of variation and even Darwin, the most incisive and comprehensive thinker among nineteenth-century biologists, remained baffled. Nevertheless, in the space of some 80 years, a staggering explosion in our understanding of heredity and in the development of analytical techniques has propelled the science of genetics into the middle of the biological stage where it plays a role rather analogous to that of atomic physics vis-à-vis the physical sciences.