Let us have a concrete illustration. One of the simplest cases is that of the heredity of color in the Andalusian fowl which has been so clearly described by Bateson. There are two established color varieties of this fowl, one with a great deal of black and one that is white with some black markings or "splashes"; for convenience we may refer to these as the black and white varieties respectively. Each of these breeds true by itself. Black mated with black produce none but black offspring, white mated with white produce none but white offspring. Crossing black and white, however, results in the production of fowls with a sort of grayish color, called "blue" by the fancier, though in reality it is a fine mixture of black and white. At first sight we seem to have a gray hybrid race through the mixture of the black and the white races. Not so: for if we continue to breed successive generations from these blue hybrid fowls we get three differently colored forms. Some will be blue like the parents, some black like one grandparent, some white like the other grandparent. Not only this but we get certain definite proportions among these three classes of descendants. Of the total number of the immediate offspring of the hybrid blues, approximately one half will be blue like the parents, approximately one fourth black, and one fourth white like each of the grandparents. Now comes the most important fact of all. These blacks, bred together produce only blacks, the whites similarly produce only whites; the blues, on the other hand, when bred together produce progeny sorting into the same original classes and in the same proportions as were produced by the blues of the original hybrid generation. Their blacks and whites each breed true, their blues repeat the history of the preceding blues. No race of the hybrid character can be established: blues always produce blacks and whites, as well as blues. A summary of this history in graphic and diagrammatic form is given in Fig. 7. [Illustration: FIG. 7.--Diagram showing the course of color heredity in the Andalusian fowl, in which one color does not completely dominate another. _P_, parental generation. The offspring of this cross constitute _F1_, the first filial or hybrid generation. _F2_, the second filial generation. Bottom row, third filial generation.] This law of heredity was first discovered about forty-five years ago by Gregor Mendel, working with peas in the garden of the Augustinian monastery in Brünn, Austria. His work curiously failed to arouse the interest of contemporary scientists and his results were soon completely lost sight of. The independent rediscovery of Mendel's formulas of heredity, about ten years ago, was probably the most important event in the history of biology and evolution since the publication of "The Origin of Species."