Origin of Species Variorum

in doubt. For when it is stated, for instance, that the German Spitz dog crosses more easily with the fox than do other dogs, or that certain South American indigenous domestic dogs do not readily unite with European dogs, the explanation which will occur to every one, and probably the true one, is that these dogs have descended from .. aboriginally distinct species. Nevertheless the perfect fertility of so many domestic varieties, differing widely from each other in appearance, for instance those of the pigeon or .. the cabbage, is a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. Several considerations, however, render this fertility of domestic varieties less remarkable. .. .. .. .. It can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties. ↑ ↑ In the second place, we must remember how ignorant we are on the precise causes of sterility, both when species are crossed, and when species are removed from their natural conditions. On this latter head I have not had space to give the many remarkable facts which could have been adduced. With respect to sterility from crossing, it is good to reflect on the difference in the result of reciprocal crosses, and on those singular cases in which a plant can be more easily fertilised by pollen from a distinct species than by its own. When we think over such cases, and on that of the differently coloured varieties of Verbascum presently to be given, we must feel how ignorant we are, and how little likely it is that we should understand why certain forms are fertile and other forms are sterile when crossed. In the third place, there is good evidence for believing