Evidence o f the fact o f evolution can be found in most fields o f biological study. But facts which give some indication o f the genetic mechanisms involved are much fewer. Most o f them fall into one o f two classes, which demonstrate two important theses. Firstly, those fossils whose evolution can be followed in a continuous series through some considerable period o f time give conclusive evidence that evolu­ tionary change can be by gradual transitions, which, moreover, progress in a single direction. Secondly, there is evidence, perhaps not quite so strong, but nevertheless very cogent, that some evolutionary changes can be by sudden and discontinuous jumps. This has emerged most clearly from studies on the distribution of plants.