By continued merging the centrifugal forces continually increase, but are continually repressed by crossing, until finally varieties break away to form species. Now it is easy to see, from this point of view, why artificial varieties are cross-fertile. It is because in artificial breeding we are intent only on making varieties in form, size, color, etc., and not at all on making any characterized by cross-sterility with the parent stock. Cross-sterility with the parent stock, or with other varieties, would be of no advantage, because we control the breeding, and can breed true if we desire. Sexual isolation is not necessary, because we can use physical isolation. On the contrary, such cross-sterility would be a positive disadvantage to the breeder, by limiting the range of his experiments just where they would be most prolific in making new varieties. Hence, as might be expected, all domestic varieties are cross-fertile, unless it be the extreme varieties, which may, in some instances, have passed the limit of greatest fertility. If this idea be true, then species which have originated in the same locality ought to be always cross-sterile, but species which have grown up apart, in widely separated geographical regions, ought to be sometimes cross-fertile, because they were isolated by physical not by sexual barriers. Such, Dr. Romanes thinks, is a fact. It is, however, a very important point, which ought to be carefully investigated. We say “_sometimes_.” It is probable that most geographical species also are cross-sterile; for, although the isolation by cross-sterility of slight varieties be the main cause of the origin of species, yet a species formed by isolation of any other kind will gradually become cross-sterile with other species. Although cross-sterility be the main cause of divergence, yet divergence beyond a certain limit, however caused, will bring about cross-sterility, because the reproductive organs will partake of the general change going on in every part. =Application.=--Suppose, then, a species breeding naturally in a wild state. Individual varieties are constantly being formed and again funded back into the common stock by cross-breeding. If the varieties thus formed be decided, the cross-breeding will strengthen the stock, and especially will preserve and increase its plasticity or tendency to variation. Finally, among the widely divergent varieties there is one affecting the reproductive organs of several individuals in such wise that they are infertile, or imperfectly fertile, with the parent stock, though perfectly fertile among themselves. These form a new species, which continue to increase indefinitely.