Darwin's On the Origin of Species. Daniel Duzdevich
Читать онлайн книгу.
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 or fancy rather than the animal or plant’s own good. Some of these useful variations probably appeared suddenly. According to many botanists, the fuller’s teazle, with its hooks unrivaled by any mechanical device, is a variety of the wild Dipsacus that arose suddenly in a seedling. The same is probably true of the turnspit dog and known to be true of the ancon sheep. But on comparing the dray horse and racehorse; the dromedary and camel; the various sheep breeds fit for cultivated land or mountainous pasture and each with wool for a different purpose; the various dog breeds each uniquely useful to humans; the gamecock (so pertinacious in battle) with breeds that are not quarrelsome, with “everlasting layers” (which never sit), with the bantam (so small and elegant); and the host of agricultural, culinary, orchard, and flower-garden breeds of plants useful to humans at different seasons and for different purposes, or so beautiful in their eyes, mere variability does not suffice. Every breed could not have been suddenly produced perfectly useful; in some instances this is historically known not to be the case. The key is the human power of cumulative selection: nature provides successive variations and humans add them up in useful directions, thus producing different breeds.
The great power of selection is not hypothetical. Several eminent breeders have drastically modified some sheep and cattle breeds even within a single lifetime. To fully appreciate what they have done, it is necessary to read some of the many treatises on the subject and actually inspect the animals. Breeders habitually speak of animals’ organization as plastic, something they can mold almost as they please. (If I had space I could quote numerous passages to this effect from expert authorities.) Youatt was a good judge of animals and probably more knowledgeable about the work of agriculturalists than anyone; he describes selection as “that which enables the agriculturalist, not only to modify the character of his flock, but to change it altogether. It is the magician’s wand, by means of which he may summon into life whatever form and mold he pleases.” Speaking of breeders’ feats with sheep, Lord Somerville says, “It would seem as if they had chalked out upon a wall a form perfect in itself, and then had given it existence.” The skillful breeder Sir John Sebright used to say about pigeons that “he would produce any given feather in three years, but it would take him six years to obtain head and beak.” The importance of selection in breeding merino sheep is so fully recognized in Saxony that the use of the principle has become a trade. The sheep are placed on a table and studied like a picture by a connoisseur three times over the course of several months. The sheep are marked and classed so that the very best can ultimately be selected for breeding.
What English breeders have effected is proven by the high prices given for animals with a good pedigree, which have been exported almost everywhere in the world. The improvement is not generally a result of crossing different breeds – a practice opposed by the best breeders, with the exception of occasional crosses among closely related sub-breeds. When a cross is made, careful selection is even more important than usual.