These cases of reversion are enough to show us that the old unmodified ancestral determinants may persist in the germ-plasm through long series of generations. But an even deeper glimpse into the dim ancient history of our modern species of horses is afforded by the occurrence of three-toed horses, references to a small number of which the palæontologist Marsh was able to discover in literature, and one of which he was able to observe in life. Julius Caesar possessed a horse whose three-toed feet represented a reversion to the horses of Tertiary times, _Mesohippus_, _Miohippus_, and _Protohippus_ or _Hipparion_; for all these genera possessed, in addition to the strong middle toe, two weaker and shorter lateral toes. In the germ-plasm of our modern horses there must still persist in certain ids the determinants of the ancestral foot, which, after a long succession of favourable reducing divisions combined with favourable chances of fertilization, may come to be in a majority, and may thus be able to induce a reappearance of a character which has long been hidden under the surface of the present-day type of the species. I do not propose to enter further on the discussion of the phenomena of inheritance. A more detailed investigation of the phenomena of reversion is to be found in '_The Germ-plasm_' published ten years ago; and the discussion could not be resumed here without a critical consideration of a relatively large series of newly acquired facts not always harmonizing, and, as yet, not even fully available. The year 1900 has given us the investigations of three botanists, De Vries, Correns, and Tschermak, who have sought by experiments in hybridization between different sorts of peas, beans, maize, and other plants, to throw light on the phenomena of inheritance, and thus on the actual processes which occur in the germ-plasm at the reducing division. This led to the discovery that similar experiments had been published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and that these had been formulated as a law which is now called Mendel's law. Correns showed, however, that this law, though correct in certain cases, did not by any means hold good in all, and we must thus postpone the working of this new material into our theory until a very much wider basis of facts has been supplied by the botanists. There is less to be hoped for from the zoologists in regard to this problem owing to the almost insuperable difficulties in the way of a long series of experiments in hybridization in animals. I myself have repeatedly attempted experiments in this direction, and have always had to abandon them, either because the crossing succeeded too rarely, or because the hybrids did not reproduce among themselves, or did so defectively, or because the distinguishing characters of the crossed breeds proved insufficiently tenacious or diagnostic. But it would be a fine task for