On 8 February 1865, Mendel read a paper entitled Versuche über Pflanzen-Hybriden before the Naturforschender Verein in Brünn. The audience consisted of about forty; there was no discussion. At a further meeting on the following 8 March, he discussed the interpretation of his results and their significance. It is not too much to claim that this communication, which was the foundation of the science of genetics and introduced mathematics and the theory of probability into the study of inheritance, represented an advance of knowledge in biological science of an order of magnitude comparable with that of evolution by natural selection. As will be seen below, these two discoveries had the most important bearing on each other, but this was not reahzed except by Mendel himself and by biologists generally after the year 1930. Mendel’s paper was printed and published in 1866 but remained ignored and forgotten for thirty-four years. Now, one hundred years after its announcement to an inattentive and unappreciative world of science, it is possible to appraise it at its true worth. This is largely owing to the experiments, demonstrations, and conclusions of Sir Ronald Fisher, F.R.S., who, in 1930, brought out the full significance of Mendel’s work, which will be referred to below. First, however, it is necessary to consider with as rigorous precision as possible exactly what it was that Mendel did, and how he did it. Here again, it was Fisher who undertook this investigation when he subjected Mendel’s paper to close scrutiny from the combined points of view of genetics, statistics, and the history of science.