there occur some affecting the reproductive organs in such wise as to produce partial or complete cross-sterility with the parent form. This is the beginning of a new species. It breeds true with its own kind, and therefore all the associated variations external and visible, and therefore constituting species, although trivial and of no use in the struggle for life, are preserved. This view completely accounts for the cross-fertility of artificial breeds equivalent in other respects to species; for cross-sterility is not an end aimed at by the breeder, it being easy to prevent cross-breeding, if desired, by artificial isolation. But, if this view be true, species from widely-different geographical regions ought also to be often cross-fertile, because, having been formed by geographical isolation, sexual isolation was not a necessary factor in their formation. This point deserves testing by careful observation. It may be, and has been, objected to Dr. Romanes’s claims, that this is no new factor; that physiological selection is only a form of natural selection. This objection, it seems to us, is little more than a play upon words. It certainly is selection, and by a _natural_ process, and therefore in some sense a natural selection, but not in the sense of Darwin. It is not a selection of individuals _fittest to survive_; for cross-fertile individuals are as fit to survive as individuals, though not as species, as are cross-sterile. Natural selection is intent only on preserving the best individuals; physiological selection on preserving the kind. Natural selection continues the direction of progress unchanged; physiological makes new directions. In addition to all these factors of _organic_ evolution, there is still another far higher factor characteristic of man alone. This is the _conscious, voluntary co-operation of the thing evolving--the spirit of man--in the work of its own evolution_. This may be called the _rational factor_. This, the most important factor of human evolution, is usually ignored by writers on evolution--either as non-existent, or else as lying beyond the domain of science. We will emphasize its importance by taking it up more fully in the next chapter.