There are still other questions raised in this same chapter that demand serious consideration. Darwin writes as follows:—
“When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. Domestic races often have a somewhat monstrous character; by which I mean, that, although differing from each other, and from other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with the species under nature to which they are nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,—a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as do the closely allied species of the same genus in a state of nature, but the differences in most cases are less in degree. This must be admitted as true, for the domestic races of many animals and plants have been ranked by some competent judges as the descendants of aboriginally distinct species, and by other competent judges as mere varieties. If any well-marked distinction existed between a domestic race and a species, this source of doubt would not so perpetually recur.”
The point here raised in regard to the systematic value of the new forms is the question that first demands our attention. We must exclude all those cases in which several original species have been blended to make a new form, because the results are too complicated to make use of at present. The domesticated races of dogs appear to have had such a multiple origin, the origin of horses is in doubt; but the domesticated pigeons, ducks, rabbits, and fowls are supposed, by Darwin, to have come each from one original wild species. The great variety of the domestic pigeons gives perhaps the most striking illustration of changes that have taken place under domestication; and Darwin lays great stress on the evidence from this source.
It seems probable in this case, (1) that all the different races of pigeons have come from one original species; (2) that the structural differences are in some respects as great as those recognized by systematists as specifically distinct; (3) that the different races breed true to their kind; (4) that the result has been reached mainly by selecting and isolating variations that have appeared under domestication, and that probably some, at least, of these variations were fluctuating ones.