As aPreface is in reality a Postscriptum, the author may be permitted to open it by mentioning omissions. The chief sin of omission he committed, is evidently the insuffident justice he did to the writings of Anton Kerner von Marilaun, who was - he wants to state this explicitly - the first to recognize fully the significance of crossing as the underlying cause of the origin of spedes. What else should apreface say? If the work is as condensed as the present one, it may perhaps suffise to repeat what Linnaeus said to Haller: Si quos in me vidisti errores, Tu sapientior haec ignoscas...Quos plures apud me detegere potes, eo gratior ero, tum possum omnia corrigere vivus; post mortem non licet emendare propria opuscula. By which however the author does not consider himself bound to gratefulness for every kind of critidsm. He is f. i. very little impressed by the kind of criticism which calls it "inconceivable" "verging on the absurd" etc., to believe that crossing can ever have been the underlying cause of the origin of new spedes, from authors who firmly believe that the origin of new spedes should be ascribed to some kind of variability; because it seems to him "absurd" that those who advocate the origin of new spedes from a single ances- tral one, should reproach an author who defends such an origin from two ancestral species, of stating an "inconceivable" opinion.