Darwin's On the Origin of Species. Daniel Duzdevich
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to struggle for their own food, at least during certain seasons. Individuals of a single species with slightly different constitutions or structure often succeed better in one environment than another; by a process of “natural selection,” as will be explained, two sub-breeds might form. This may partly explain the observation of some authors that varieties kept by natives are more like well-defined species than the varieties of civilized countries.
Acknowledging the important role played by human selection, it becomes obvious how domestic organisms display adaptations of structure or habit conforming to human want or fancy. It also explains the frequently abnormal traits of domestic varieties and why their external characteristics vary greatly but their internal organs only slightly. Only with great difficulty can man select for structural deviations that cannot be externally observed, and he usually does not care for internal variations anyway. He can select only variations first provided by nature. He could not make a fantail, or think to or even try, until he found a pigeon with an unusually developed tail, or a pouter until he found a pigeon with an unusually large crop. The more abnormal a characteristic upon its first appearance, the more likely it is to catch his attention. But the phrase “to make a fantail” is not correct. The person who first selected a slightly larger tailed pigeon never imagined what its descendants would become through long-term, partly unconscious and partly methodical selection. Maybe the parent of all fantails had only fourteen slightly expanded tail feathers, like the Java fantail, or seventeen tail feathers like individuals of other breeds. Maybe the first pouter did not inflate its crop much more than the modern turbit inflates its upper esophagus, a habit disregarded by breeders because it is not one of the points of the breed.
A major structural deviation is not necessary to catch the breeder’s eye, which perceives extremely small differences. It is human nature to value even a slight novelty in one’s possession. The former value of slight differences cannot be judged by the value that such slight differences might have today after several breeds have been well established. Many slight deviations still arise among pigeons, but they are rejected as faults in the breed’s perfection. The common goose has not given rise to any marked varieties, so it has recently been exhibited at poultry shows as distinct from the Thoulouse goose, from which it differs only in the most fleeting characteristic: color.
I think these ideas further explain why we know nothing about the origin or history of domestic breeds. In fact, a breed, like a dialect of a language, cannot be said to have a definite origin. A man preserves and breeds an individual with some slight structural deviation, or takes more care than usual in matching his best animals, thereby improving them, and the improved offspring slowly spread into the immediate neighborhood; they are not yet separately named, and their history is disregarded because they are only slightly valued. With further improvement by the same slow and gradual process, they spread more widely and are recognized as distinct and valuable, deserving of a local name. (In semicivilized regions with limited free communication, a new sub-breed will spread slowly.) As soon as the new sub-breed’s valuable characteristics are fully acknowledged, unconscious selection always enhances the breed. Unconscious selection is perhaps more influential at one period than another, subject to the breed’s popularity, and in one region than another, according to the state of civilization of the inhabitants. The chances of any record describing such slow and immediately unobservable changes are infinitely small.
A high degree of variability is obviously favorable to the human power of selection, because it provides the raw materials for selection to work on; not that mere individual differences are insufficient to allow for the accumulation of extensive modification in almost any desired direction. Manifestly useful or pleasing variations appear only occasionally, but the odds can be increased by keeping a large number of individuals, an important tool for success. Marshall has remarked about this principle with respect to sheep in parts of Yorkshire that “as they generally belong to poor people, and are mostly in small lots, they never can be improved.” Professional plant breeders are generally more successful than amateurs in getting new and valuable varieties, because they raise large stocks of the same plant. When only a few individuals are kept, they are all allowed to breed, effectively preventing selection. Keeping large numbers of individuals naturally requires the creation of conditions favorable to that species to ensure proper breeding. But the most important point is that the organism should be so useful to or valued by humans that very close attention will be paid to even minor deviations in the quality or structure of each individual; without this, nothing can be effected. I have seen it gravely remarked how fortunate we are that the strawberry began to vary exactly when gardeners began to attend to it closely. Surely the strawberry had always varied, but slight variations were neglected; as soon as individual plants with slightly larger, earlier, or better fruit were selected for propagation over several generations (aided by some crosses with distinct species), those many admirable strawberry varieties appeared that have been raised for the past thirty or forty years.
Preventing crosses is an important element of success with animals that have separate sexes – at least in a region already stocked with other varieties. Enclosure of land plays a part in this. Wandering peoples or the inhabitants of open plains rarely have more than one breed per species. It is a huge convenience to the breeder and favorable to the formation of new breeds that pigeons mate for life, so many varieties can mingle in one aviary but keep true. Additionally, pigeons can be quickly propagated in large numbers and inferior birds easily rejected because they can serve as food when killed. Conversely, cats cannot be matched because of their nocturnal rambling habits, and though much loved by women and children, distinct breeds can hardly ever be maintained. The breeds that occasionally are observed are usually imported, often from islands. Although I do not doubt that some domestic animals intrinsically vary less than others, the rarity or absence of cat, donkey, peacock, goose, and other breeds is due mainly to selection not having been employed: in cats because they are difficult to pair, in donkeys because only a few are kept by the poor and little attention is paid to their breeding, in peacocks because they are difficult to rear and keep in large numbers, and in geese because they are valuable only for food and feathers, and because no one has taken pleasure in the display of distinct breeds.
To sum up the origin of domestic plant and animal varieties, environmental influence on the reproductive system is the most important cause of variability. I do not believe, as some authors do, that variability is an inherent and necessary contingency under all circumstances with all organic beings. The effects of variability are modified by inheritance and reversion. Variability is governed by many unknown rules, especially correlated growth. The direct action of the environment, and use and disuse, may have some effect. The final result is thus infinitely complex. In some cases intercrossing of originally distinct species played an important part in the origin of domestic breeds. The occasional crossing of established domestic breeds contributes to the creation of sub-breeds with the aid of selection, but the importance of crossing varieties has been greatly exaggerated both in animals and plants propagated by seed. (The importance of crossing distinct species and varieties is immensely important for plants temporarily propagated by cuttings, buds, etc., because the cultivator disregards the extreme variability of hybrids and mongrels and the sterility of hybrids. Plants not propagated by seed are temporary, so they are of little importance to this discussion.)7 Over all these causes of change, the cumulative action of selection, whether applied methodically and relatively quickly or unconsciously and relatively slowly but more efficiently, is by far the most predominant power.
1. St. Hilaire gives many examples in his great work on this subject, Histories des Anomalies.
2. [These phenomena are now well understood through genetics. – D.D.]
3. [“Generic characteristics” refer to those that are relevant at the genus level. – D.D.]
4. [In fact, they have; all dogs belong to the same species, descended from the wolf. – D.D.]
5.