物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 1. VARIATION UNDER DOMESTICATION. Page 4
Nor let it be thought that some great deviation of structure would benecessary to catch the fancier's eye: he perceives extremely smalldifferences, and it is in human nature to value any novelty, howeverslight, in one's own possession. Nor must the value which wouldformerly be set on any slight differences in the individuals of thesame species, be judged of by the value which would now be set onthem, after several breeds have once fairly been established. Manyslight differences might, and indeed do now, arise amongst pigeons,which are rejected as faults or deviations from the standard ofperfection of each breed. The common goose has not given rise to anymarked varieties; hence the Thoulouse and the common breed, whichdiffer only in colour, that most fleeting of characters, have latelybeen exhibited as distinct at our poultry-shows.
I think these views further explain what has sometimes beennoticed--namely that we know nothing about the origin or history ofany of our domestic breeds. But, in fact, a breed, like a dialect of alanguage, can hardly be said to have had a definite origin. A manpreserves and breeds from an individual with some slight deviation ofstructure, or takes more care than usual in matching his best animalsand thus improves them, and the improved individuals slowly spread inthe immediate neighbourhood. But as yet they will hardly have adistinct name, and from being only slightly valued, their history willbe disregarded. When further improved by the same slow and gradualprocess, they will spread more widely, and will get recognised assomething distinct and valuable, and will then probably first receivea provincial name. In semi-civilised countries, with little freecommunication, the spreading and knowledge of any new sub-breed willbe a slow process. As soon as the points of value of the new sub-breedare once fully acknowledged, the principle, as I have called it, ofunconscious selection will always tend,--perhaps more at one periodthan at another, as the breed rises or falls in fashion,--perhaps morein one district than in another, according to the state ofcivilisation of the inhabitants--slowly to add to the characteristicfeatures of the breed, whatever they may be. But the chance will beinfinitely small of any record having been preserved of such slow,varying, and insensible changes.
I must now say a few words on the circumstances, favourable, or thereverse, to man's power of selection. A high degree of variability isobviously favourable, as freely giving the materials for selection towork on; not that mere individual differences are not amplysufficient, with extreme care, to allow of the accumulation of a largeamount of modification in almost any desired direction. But asvariations manifestly useful or pleasing to man appear onlyoccasionally, the chance of their appearance will be much increased bya large number of individuals being kept; and hence this comes to beof the highest importance to success. On this principle Marshall hasremarked, with respect to the sheep of parts of Yorkshire, that "asthey generally belong to poor people, and are mostly IN SMALL LOTS,they never can be improved." On the other hand, nurserymen, fromraising large stocks of the same plants, are generally far moresuccessful than amateurs in getting new and valuable varieties. Thekeeping of a large number of individuals of a species in any countryrequires that the species should be placed under favourable conditionsof life, so as to breed freely in that country. When the individualsof any species are scanty, all the individuals, whatever their qualitymay be, will generally be allowed to breed, and this will effectuallyprevent selection. But probably the most important point of all, is,that the animal or plant should be so highly useful to man, or so muchvalued by him, that the closest attention should be paid to even theslightest deviation in the qualities or structure of each individual.Unless such attention be paid nothing can be effected. I have seen itgravely remarked, that it was most fortunate that the strawberry beganto vary just when gardeners began to attend closely to this plant. Nodoubt the strawberry had always varied since it was cultivated, butthe slight varieties had been neglected. As soon, however, asgardeners picked out individual plants with slightly larger, earlier,or better fruit, and raised seedlings from them, and again picked outthe best seedlings and bred from them, then, there appeared (aided bysome crossing with distinct species) those many admirable varieties ofthe strawberry which have been raised during the last thirty or fortyyears.
In the case of animals with separate sexes, facility in preventingcrosses is an important element of success in the formation of newraces,--at least, in a country which is already stocked with otherraces. In this respect enclosure of the land plays a part. Wanderingsavages or the inhabitants of open plains rarely possess more than onebreed of the same species. Pigeons can be mated for life, and this isa great convenience to the fancier, for thus many races may be kepttrue, though mingled in the same aviary; and this circumstance musthave largely favoured the improvement and formation of new breeds.Pigeons, I may add, can be propagated in great numbers and at a veryquick rate, and inferior birds may be freely rejected, as when killedthey serve for food. On the other hand, cats, from their nocturnalrambling habits, cannot be matched, and, although so much valued bywomen and children, we hardly ever see a distinct breed kept up; suchbreeds as we do sometimes see are almost always imported from someother country, often from islands. Although I do not doubt that somedomestic animals vary less than others, yet the rarity or absence ofdistinct breeds of the cat, the donkey, peacock, goose, etc., may beattributed in main part to selection not having been brought intoplay: in cats, from the difficulty in pairing them; in donkeys, fromonly a few being kept by poor people, and little attention paid totheir breeding; in peacocks, from not being very easily reared and alarge stock not kept; in geese, from being valuable only for twopurposes, food and feathers, and more especially from no pleasurehaving been felt in the display of distinct breeds.
To sum up on the origin of our Domestic Races of animals and plants. Ibelieve that the conditions of life, from their action on thereproductive system, are so far of the highest importance as causingvariability. I do not believe that variability is an inherent andnecessary contingency, under all circumstances, with all organicbeings, as some authors have thought. The effects of variability aremodified by various degrees of inheritance and of reversion.Variability is governed by many unknown laws, more especially by thatof correlation of growth. Something may be attributed to the directaction of the conditions of life. Something must be attributed to useand disuse. The final result is thus rendered infinitely complex. Insome cases, I do not doubt that the intercrossing of species,aboriginally distinct, has played an important part in the origin ofour domestic productions. When in any country several domestic breedshave once been established, their occasional intercrossing, with theaid of selection, has, no doubt, largely aided in the formation of newsub-breeds; but the importance of the crossing of varieties has, Ibelieve, been greatly exaggerated, both in regard to animals and tothose plants which are propagated by seed. In plants which aretemporarily propagated by cuttings, buds, etc., the importance of thecrossing both of distinct species and of varieties is immense; for thecultivator here quite disregards the extreme variability both ofhybrids and mongrels, and the frequent sterility of hybrids; but thecases of plants not propagated by seed are of little importance to us,for their endurance is only temporary. Over all these causes of ChangeI am convinced that the accumulative action of Selection, whetherapplied methodically and more quickly, or unconsciously and moreslowly, but more efficiently, is by far the predominant Power.