Origin of Species Variorum

Comparison with 1872

races, differing widely from each other in appearance, for instance those of the pigeon, or of 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 the fertility of domestic varieties less remarkable than at first appears. It can, in the first place, it may be observed that the amount of external difference between two species is no sure guide to their degree of mutual sterility, so that similar differences in the case of varieties would be no sure guide.

Text in this page (from paragraph 3500, sentence 510 to paragraph 3500, sentence 700, word 26) is not present in 1872

varieties, differing widely from each other in appearance, for instance of the pigeon or of 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. .. .. .. .. In the first place we must remember how ignorant we are regarding the precise cause 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 adduce the many remarkable facts which could have been given; with respect to sterility from crossing, reflect on the difference in the result of reciprocal crosses,— reflect on the singular cases in which a plant can be more easily fertilised by foreign pollen 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. It can, in the second 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 third place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids which were at first only slightly sterile; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same domestic conditions of life. ↑ ↑ Lastly, and this seems to me .. the most important consideration, new races of animals and plants are produced under domestication chiefly by man's methodical and