Consequently, if intercrossing be prevented, there is no reason why unuseful variations should not be perpetuated by heredity quite as much as useful ones when under the nursing influence of natural selection. Thus, if from any cause, a section of a species is prevented from intercrossing with the rest of its parent form, it is to be expected that new varieties--for the most part of a trivial and unuseful kind--should arise within that section, and in time pass into new species. This supposition is borne out by the nature of the flora and fauna of oceanic islands, which are particularly rich in peculiar species, and where intercrossing was, of course, prevented with the original parent forms by the action of the geographical boundaries. However, closely allied species are not always, or even generally, separated by geographical boundaries, and the cross-infertility remains to be explained. The cardinal feature of Mr. Romanes' theory is that the initial step in the origin of species is the arising of this infertility as an independent variation, by which, free intercrossing with the parent form on a common area is prevented, and specific differentiation rendered possible. Innumerable varieties are known to occur which do not pass into distinct species, the reason being that this initial variation, that is, incipient infertility whereby the swamping effects of intercrossing might be obviated, was lacking, and the variations became re-absorbed. That is, given any degree of sterility towards the parental form which does not extend to the varietal form, then a new species must take its origin. Without the bar of sterility, in Mr. Romanes' opinion, free intercrossing must render the formation of species impossible. Mutual sterility is thus the cause, not the result, of specific differentiation. As regards the occurrence of this initial variation, the reproductive system is known to be highly variable, its variability taking the form either of increased fertility, or of sterility in all degrees, and depending on either extrinsic causes (changes of food, climate, &c.), or on an intrinsic cause arising in the system itself. From the nature of this additional factor at work in the formation of species, Mr. Romanes called his theory 'physiological selection.' Physiological selection is conceived of as co-operating with natural selection, the former allowing the latter to act by interposing its law of sterility, with the result that the secondary specific characters may be either adaptive or non-adaptive in character. _To Miss C. E. Romanes._ Aix-les-Bains: May 1886.