Professor de Vries begins by reference to a remarkable memoir by Gregor Mendel[8], giving the results of his experiments in crossing varieties of _Pisum sativum_. These experiments of Mendel’s were carried out on a large scale, his account of them is excellent and complete, and the principles which he was able to deduce from them will certainly play a conspicuous part in all future discussions of evolutionary problems. It is not a little remarkable that Mendel’s work should have escaped notice, and been so long forgotten. [8] ‘Versuche üb. Pflanzenhybriden’ in the _Verh. d. Naturf. Ver. Brünn_, iv. 1865. For the purposes of his experiments Mendel selected seven pairs of characters as follows:-- 1. Shape of ripe seed, whether round; or angular and wrinkled. 2. Colour of “endosperm” (cotyledons), whether some shade of yellow; or a more or less intense green. 3. Colour of the seed-skin, whether various shades of grey and grey-brown; or white. 4. Shape of seed-pod, whether simply inflated; or deeply constricted between the seeds. 5. Colour of unripe pod, whether a shade of green; or bright yellow. 6. Nature of inflorescence, whether the flowers are arranged along the axis of the plant; or are terminal and form a kind of umbel. 7. Length of stem, whether about 6 or 7 ft. long, or about 3/4 to 1-1/2 ft. Large numbers of crosses were made between Peas differing in respect of _one_ of each of these pairs of characters. It was found that in each case the offspring of the cross exhibited the character of one of the parents in almost undiminished intensity, and intermediates which could not be at once referred to one or other of the parental forms were not found. In the case of each pair of characters there is thus one which in the first cross prevails to the exclusion of the other. This prevailing character Mendel calls the _dominant_ character, the other being the _recessive_ character[9]. [9] Note that by these novel terms the complications involved by use of the expression “prepotent” are avoided. That the existence of such “dominant” and “recessive” characters is a frequent phenomenon in cross-breeding, is well known to all who have attended to these subjects. By letting the cross-breds fertilise themselves Mendel next raised another generation. In this generation were individuals which showed the dominant character, but also individuals which presented the recessive character. Such a fact also was known in a good many instances. But Mendel discovered that in this generation the numerical proportion of dominants to recessives is on an average of cases approximately constant, being in fact _as three to one_. With very considerable regularity these numbers were approached in the case of each of his pairs of characters. There are thus in the first generation raised from the cross-breds 75 per cent. dominants and 25 per cent. recessives.