keep the mass of a species comparatively homogeneous over any area in which it abounds in individuals.  Starting from a suggestion of the late Mr. Knight, now so familiar, that close interbreeding diminishes vigor and fertility; [I-8] and perceiving that bisexuality is ever aimed at in Nature—­being attained physiologically in numerous cases where it is not structurally—­Mr. Darwin has worked out the subject in detail, and shown how general is the concurrence, either habitual or occasional, of two hermaphrodite individuals in the reproduction of their kind; and has drawn the philosophical inference that probably no organic being self-fertilizes indefinitely; but that a cross with another individual is occasionally—­perhaps at very long intervals—­indispensable.  We refer the reader to the section on the intercrossing of individuals (pp. 96—­101), and also to an article in the Gardeners’ Chronicle a year and a half ago, for the details of a very interesting contribution to science, irrespective of theory.  In domestication, this intercrossing may be prevented; and in this prevention lies the art of producing varieties.  But “the art itself is Nature,” since the whole art consists in allowing the most universal of all natural tendencies in organic things (inheritance) to operate uncontrolled by other and obviously incidental tendencies.  No new power, no artificial force, is brought into play either by separating the stock of a desirable variety so as to prevent mixture, or by selecting for breeders those individuals which most largely partake of the peculiarities for which the breed is valued.  {I-9]

We see everywhere around us the remarkable results which Nature may be said to have brought about under artificial selection and separation.  Could she accomplish similar results when left to herself?  Variations might begin, we know they do begin, in a wild state.  But would any of them be preserved and carried to an equal degree of deviation?  Is there anything in Nature which in the long-run may answer to artificial selection?  Mr. Darwin thinks that there is; and Natural Selection is the key-note of his discourse,

As a preliminary, he has a short chapter to show that there is variation in Nature, and therefore something for natural selection to act upon.  He readily shows that such mere variations as may be directly referred to physical conditions (like the depauperation of plants in a sterile soil, or their dwarfing as they approach an Alpine summit, the thicker fur of an animal from far northward, etc.), and also those individual differences which we everywhere recognize but do not pretend to account for, are not separable by any assignable line from more strongly-marked varieties; likewise that there is no clear demarkation between the latter and sub-species, or varieties of the highest grade (distinguished from species not by any known inconstancy, but by the supposed lower importance of their characteristics); nor between these and recognized species.  “These differences blend into each other in an insensible series, and the series impresses the mind with an idea of an actual passage.”