Apart from the distinct _characters_ that fall under these two categories there are the fluctuating quantitative _conditions_. These depend for the most part, as already pointed out, on variations in the point at which the ontogeny of a character is stopped; and the stopping-point is, in turn, often, if not usually, determined by external conditions which favor or restrict the ontogeny. Whether or not such quantitative variations are transmitted is still doubtful. Our experiment in increasing qualities, such as redness in plumage-color, by selection of quantitative fluctuations have not been successful in the sense anticipated; neither have selections of comb, polydactylism, or syndactylism. Recently, prolonged attempts at the Maine Agricultural Experiment Station to increase egg-yield of poultry by selection have been without result. Apparently, within limits, these quantitative variations have so exclusively an ontogenetic signification that they are not reproduced so long, at least, as environmental conditions are not allowed to vary widely. The conclusions which others have reached, and upon which de Vries has laid the greatest stress, that quantitative and qualitative characters differ fundamentally in their heritability is supported by our experiments. G. THE RÔLE OF HYBRIDIZATION IN EVOLUTION. The criticism has often been made of modern studies in hybridization that they are really unimportant for evolution because hybridization is uncommon in nature. Even at the beginning of the new era it could be replied that, first, we did not know how common hybridization might turn out to be in nature, and, second, that certainly in human marriage and among domesticated animals and plants, intermixing of characters played a most important part, and, finally, the laws of inheritance of characters were of such grave physiological import as to deserve study wholly apart from any question of the rôle of hybridization in evolution.