Gärtner, especially in his work, "Die Bastarderzeugung im Pflanzenreiche," [Footnote 17] has recorded very valuable observations; and quite recently Wichura published the results of some profound observations on the hybrids of the willow. That so far no generally applicable law governing the formation and development of hybrids has been successfully formulated can hardly be wondered at by anyone who is acquainted with the extent of the task and can appreciate the difficulties with which experiments of this class have to contend. A final decision can only be arrived at when we shall have before us the results of the changed detailed experiments made on plants belonging to the most diverse orders. It requires some courage indeed to undertake a labor of such far-reaching extent; it appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which can not be overestimated in connexion with the history of the evolution of organic forms. The paper now presented records the results of such a detailed experiment. This experiment was practically confined to a small plant group, and is now after eight years' pursuit concluded in all essentials. Whether the plan upon which the separate experiments were conducted and carried out was the best suited to attain the desired end is left to the friendly decision of the reader. [Footnote 17: The Production of Hybrids in the Vegetable Kingdom.] {208}