1. The scientific era of the investigation of heredity begins with Mendel’s paper on plant hybridization which was not appreciated by his contemporaries. Mendel invented a method for the quantitative study of heredity which consisted essentially in crossing two forms of peas differing only in one well-defined hereditary character; and in following statistically and separately the results of this crossing and that of the inbreeding of the second and third generations of hybrids. This led him to the recognition of one essential feature of heredity; namely, that while the hybrids of the first generation are all alike, each hybrid produces two types of sex cells in equal numbers, one for each of the pure breeds which has been used for the crossing. This takes place not only when the forms used for the crossing differ in regard to one character only but also if they differ for two or more characters. The statement made is Mendel’s law of heredity, or, more correctly, Mendel’s law of the segregation of the hereditary characters of the parents in the sex cells of the hybrids.[201] Mendel’s law allows us to tabulate and calculate beforehand the relative number of different forms which appear if the offspring of a mating of two varieties are bred among themselves. [200] For the literature on the subject the reader is referred to Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., _The Mechanism of Mendelian Heredity_. New York, 1915. [201] Mendel, G., “Experiment in Plant-Hybridization,” translated in W. Bateson’s classical book on Mendel’s _Principles of Heredity_. Cambridge, 1909. In order to do this it must be remembered also that while in some cases the hybrid is an intermediate between the two parent forms, in other cases it cannot be discriminated from one of the two parent forms. In such cases the character which appears in the hybrid was called by Mendel the dominant character and the one which disappeared the recessive character. According to Bateson, who was the first to systematize the phenomena of Mendelian heredity, recessiveness means generally the absence of a character which is present in the dominant type. When, _e. g._, the cross between a tall and a dwarf form of pea gives in the first generation only tall peas, on the basis of the presence and absence theory the dominant form contains a factor for growth which is lacking in the dwarf form. While this theory fits many cases it meets with difficulties in others. Thus the presence of a factor for pigment should be dominant over the absence of such a factor, which is usually the case, inasmuch as the cross of a coloured rat or rabbit with an albino is black or coloured. There is, however, also a case where whiteness is dominant over colour, as we shall see later. This fact does not necessarily contradict the presence and absence theory.[202]