If finally we compare the described result, still very uncertain, with those obtained by crosses made between forms of _Pisum_, which I had the honour of communicating in the year 1865, we find a very real distinction. In _Pisum_ the hybrids, obtained from the immediate crossing of two forms, have in all cases the same type, but their posterity, on the contrary, are variable and follow a definite law in their variations. In _Hieracium_ according to the present experiments the exactly opposite phenomenon seems to be exhibited. Already in describing the _Pisum_ experiments it was remarked that there are also hybrids whose posterity do not vary, and that, for example, according to Wichura the hybrids of _Salix_ reproduce themselves like pure species. In _Hieracium_ we may take it we have a similar case. Whether from this circumstance we may venture to draw the conclusion that the polymorphism of the genera _Salix_ and _Hieracium_ is connected with the special condition of their hybrids is still an open question, which may well be raised but not as yet answered. A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY. “_The most fertile men of science have made blunders, and their consciousness of such slips has been retribution enough; it is only their more sterile critics who delight to dwell too often and too long on such mistakes._” BIOMETRIKA, 1901. INTRODUCTORY. On the rediscovery and confirmation of Mendel’s Law by de Vries, Correns, and Tschermak two years ago, it became clear to many naturalists, as it certainly is to me, that we had found a principle which is destined to play a part in the Study of Evolution comparable only with the achievement of Darwin--that after the weary halt of forty years we have at last begun to march. If we look back on the post-Darwinian period we recognize one notable effort to advance. This effort--fruitful as it proved, memorable as it must ever be--was that made by Galton when he enuntiated his Law of Ancestral Heredity, subsequently modified and restated by Karl Pearson. Formulated after long and laborious inquiry, this principle beyond question gives us an expression including and denoting many phenomena in which previously no regularity had been detected. But to practical naturalists it was evident from the first that there are great groups of facts which could not on any interpretation be brought within the scope of Galton’s Law, and that by no emendation could that Law be extended to reach them. The existence of these phenomena pointed to a different physiological conception of heredity. Now it is precisely this conception that Mendel’s Law enables us to form. Whether the Mendelian principle can be extended so as to include some apparently Galtonian cases is another question, respecting which we have as yet no facts to guide us, but we have certainly no warrant for declaring such an extension to be impossible.