Nevertheless, although this inference is so strongly grounded, we ought to remember that it is only an inference. In order fully to verify the theory of physiological selection, we ought to prove by experiment the fact of cross-infertility between these synoical varieties, as we learn that it afterwards obtains between synoical species. It is to be regretted that the theory of physiological selection did not occur to the mind of Nägeli, because he would then, no doubt, have ascertained this by actual experiment. As it is, the great value of his observations goes no further than establishing a strong presumption, that it _must_ be selective fertility which causes the progressive differentiation of synoical varieties; and also that, if so, this _must_ be the principal factor in the differentiation of vegetable species, seeing that some ninety-five per cent. are of synoical origin. _Evidence from Experimental Research._ My paper on _Physiological Selection_ pointed out that the whole theory would have to stand or fall with the experimental proof of the presence or the absence of cross-infertility between varieties of the same species growing on common areas. From the facts and considerations which we have hitherto been dealing with, it did indeed appear to me that there was the strongest conceivable ground for inferring that cross-infertility between such varieties would be found by experiment to be a phenomenon of highly general occurrence--amply sufficient ground to prove that allied species on common areas for the most part owed their origin to this character of mutual sterility, and not vice versa as previously supposed. At that time I was not aware that any experiments had been made in this direction. Soon after the paper was published, however, my attention was directed to a laborious research which had been directed to this very point, and carried on for more than thirty years, by M. Jordan[25]. This had not attracted the general notice which it undoubtedly deserved; and I have since ascertained that even Darwin began to look into it only a few months before his death. [25] _Remarques sur le fait de l'existence en société à l'état sauvage des espèces végétales affines et sur d'autres faits relatifs à la question de l'espèce_, par Alexis Jordan; lues au congrès de l'Association Française pour l'Avancemeat des Sciences, 2^me session, Lyon, séance de 28 Août, 1873.