breeds, voice and disposition differ remarkably. Lastly, in certain breeds the males and females have come to differ slightly from each other.
There are at least twenty pigeon breeds that an ornithologist would classify as well-defined species if he were told they were wild birds. I doubt any ornithologist would place the English carrier, short-faced tumbler, runt, barb, pouter, and fantail in the same genus, especially because for each there are several true-breeding sub-breeds, or, as he would call them, species.
Despite the great differences among pigeon breeds, I agree with the common opinion of naturalists that they have all descended from the rock pigeon (Columba livia), a category that includes several geographical varieties or sub-species differing from one another in minor respects. Some of the justifications for this conclusion are somewhat applicable in other cases, so I will briefly give them here. If pigeon breeds are not varieties and have not descended from the rock pigeon, then they must have descended from at least seven or eight original stocks, because it would be impossible to generate the present domestic breeds by the crossing of any fewer. For example, how could a pouter be produced by crossing unless one of the parental stocks possessed the characteristically enormous crop? The supposed original stocks must all have been rock pigeons – that is, not breeding or perching in trees. But besides C. livia only two or three other species of rock pigeon are known, and these lack the characteristics of domestic breeds. Therefore, the supposed original stocks would have to either (1) still exist unknown to ornithologists in the regions where they were first domesticated, or (2) have become extinct. Because of the rock pigeon’s size, habits, and remarkable characteristics, (1) is unlikely. And birds that are good fliers and breed on precipices are unlikely to become extinct. The common rock pigeon has the same habits as the domestic breeds and hasn’t been exterminated on several British islets or the shores of the Mediterranean, making (2) a rash assumption. Furthermore, the above-mentioned breeds have been introduced to all parts of the world, so some of them must have wound up in their supposed native regions, yet not one has ever became wild or feral (although the dovecot pigeon, which is the rock pigeon in a slightly altered state, has become feral in several places). All recent experience demonstrates the difficulty of breeding wild animals in confinement, but the hypothesis of plural origin for domestic pigeons implies that at least seven or eight species were so thoroughly domesticated in ancient times by half-civilized humans that they were quite prolific under confinement.
An important argument, also relevant to other cases, is that although these breeds are generally the same in constitution, habits, voice, coloring, and most structural parts as the wild rock pigeon, they are extraordinarily abnormal in other parts of structure. We could search the entire pigeon family in vain for a beak like the English carrier’s, short-faced tumbler’s, or barb’s; for reversed feathers like the Jacobin’s; for a crop like the pouter’s; or for tail feathers like the fantail’s. So it would have to be assumed not only that half-civilized humans thoroughly domesticated several species but also that they intentionally or unintentionally picked abnormal species, and that these species are now all unknown or extinct. So many strange contingencies seem very improbable.
Coloration is worth considering. The rock pigeon is slate blue with a white rump (the Indian sub-species, Strickland’s C. intermedia, has a bluish rump). The tail has a terminal dark bar with the bases of the outer feathers edged in white; the wings have two black bars; some semidomestic and apparently wild breeds also have black checkers on the wings. These marks do not occur together in any other species of the whole family. All of these marks, down to the white edging of the tail feathers, sometimes develop perfectly in thoroughly well-bred specimens from every one of the domestic breeds. And when two birds from distinct breeds lacking these marks and blue coloring are crossed, the mongrel offspring are very likely to acquire them. I crossed some white fantails with black barbs and got mottled brown and black offspring. Then I crossed these together and one of the offspring had as beautiful a blue color, white rump, black double wing bar, and barred white-edged tail feathers as any wild rock pigeon! These observations can be understood through the well-known principle of reversion to ancestral characteristics, if all domestic breeds have descended from the rock pigeon. The alternative would require making one of two very unlikely assumptions: (1) all of the supposed multiple original stocks were colored and marked like the rock pigeon, even though no such species exists today, so that each breed reverts to the same characteristics, or (2) even the purest breed has been crossed by the rock pigeon within twelve or twenty generations.5
Hybrid offspring from between all domestic pigeons are fertile. I can state this from my own crosses, intentionally made between the most distinct breeds. It is difficult, if not impossible, to suggest a single case of fertile hybrid offspring from two unambiguously distinct species. Some authors believe that long-term domestication eliminates this strong tendency for sterility. This hypothesis may be true if applied to closely related species, based on the history of the dog, but is unsupported by a single experiment. However, it would be rash to extend the hypothesis and claim that supposedly original “species” as distinct as the carrier, tumbler, pouter, and fantail could have produced fertile offspring when crossed.
I feel no doubt that all domestic pigeon breeds have descended from Columba livia and its geographical sub-species. To reiterate, the reasons are: (1) it is unlikely that primitive humans got seven or eight supposed pigeon species to breed under domestication, with none of these supposed species existing today and none of the breeds having become feral in their supposed native regions; (2) these species have certain abnormal characteristics with respect to the whole pigeon family but are like rock pigeons in other respects; (3) the blue color and marks of the rock pigeon occasionally appear in all breeds both when kept pure and when crossed; and (4) mongrel offspring are fertile.
There is even further support for my assertion. The rock pigeon has been domesticated recently in Europe and India, agreeing in habit and many structural characteristics with all domestic breeds. Furthermore, it is possible to make an almost perfect incremental series between extremes of structure using sub-breeds within any one breed, especially if we include specimens from distant regions. Also, the main distinctive feature of each breed is highly variable. These considerations will be invoked in discussing selection as explaining the immense amount of variation pigeons have undergone. The reason that the breeds often have such monstrous characteristics will also be explained.
When I first kept pigeons, I felt as much difficulty in believing that they have descended from one parent as any naturalist would about the many species of finches or other large bird groups in the wild. It was striking to me that every breeder of domestic animals and every cultivator of plants with whom I talked or whose treatises I read is convinced that each breed has descended from a distinct original species. A celebrated breeder of Hereford cattle would laugh with scorn at the suggestion that his livestock have descended from long-horns. I have never met a pigeon, poultry, duck, or rabbit breeder who was not fully convinced that each main breed has descended from a distinct species. In his treatise on pears and apples, Van Mons rejects that the several varieties (such as Ribston pippin and Codlin apple) could ever have proceeded from seeds of the same tree. There are innumerable other examples. The explanation, I think, is simple: long-term study impresses on the mind differences between breeds, and although they know that individuals of each breed vary slightly – prizes are won by the selection of such slight differences – they fail to sum up in their minds how, over many generations, slight differences can accumulate into large differences. There are naturalists who know less about inheritance and no more about intermediate links in the lines of descent than breeders but nevertheless admit that many domestic varieties have descended from common parents. Yet they deride the idea of species in nature being lineal descendants of other species. Perhaps they should be more cautious.
What are the steps by which a domestic variety arises from one or several related species? Environmental conditions and habit may play a minor role, but they cannot account for the differences between a dray and a racehorse, a greyhound and a bloodhound, or a carrier and a tumbler pigeon. One of the most remarkable features of domesticated organisms is that we see in them adaptations, but to human use
