物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 1. VARIATION UNDER DOMESTICATION. Page 3
I have discussed the probable origin of domestic pigeons at some, yetquite insufficient, length; because when I first kept pigeons andwatched the several kinds, knowing well how true they bred, I feltfully as much difficulty in believing that they could ever havedescended from a common parent, as any naturalist could in coming to asimilar conclusion in regard to the many species of finches, or otherlarge groups of birds, in nature. One circumstance has struck me much;namely, that all the breeders of the various domestic animals and thecultivators of plants, with whom I have ever conversed, or whosetreatises I have read, are firmly convinced that the several breeds towhich each has attended, are descended from so many aboriginallydistinct species. Ask, as I have asked, a celebrated raiser ofHereford cattle, whether his cattle might not have descended from longhorns, and he will laugh you to scorn. I have never met a pigeon, orpoultry, or duck, or rabbit fancier, who was not fully convinced thateach main breed was descended from a distinct species. Van Mons, inhis treatise on pears and apples, shows how utterly he disbelievesthat the several sorts, for instance a Ribston-pippin or Codlin-apple,could ever have proceeded from the seeds of the same tree. Innumerableother examples could be given. The explanation, I think, is simple:from long-continued study they are strongly impressed with thedifferences between the several races; and though they well know thateach race varies slightly, for they win their prizes by selecting suchslight differences, yet they ignore all general arguments, and refuseto sum up in their minds slight differences accumulated during manysuccessive generations. May not those naturalists who, knowing farless of the laws of inheritance than does the breeder, and knowing nomore than he does of the intermediate links in the long lines ofdescent, yet admit that many of our domestic races have descended fromthe same parents--may they not learn a lesson of caution, when theyderide the idea of species in a state of nature being linealdescendants of other species?
SELECTION.
ancientlycivilised.mountainpasture, with the wool of one breed good for one purpose, and that ofanother breed for another purpose; when we compare the many breeds ofdogs, each good for man in very different ways; when we compare thegame-cock, so pertinacious in battle, with other breeds?
Let us now briefly consider the steps by which domestic races havebeen produced, either from one or from several allied species. Somelittle effect may, perhaps, be attributed to the direct action of theexternal conditions of life, and some little to habit; but he would bea bold man who would account by such agencies for the differences of adray and race horse, a greyhound and bloodhound, a carrier and tumblerpigeon. One of the most remarkable features in our domesticated racesis that we see in them adaptation, not indeed to the animal's orplant's own good, but to man's use or fancy. Some variations useful tohim have probably arisen suddenly, or by one step; many botanists, forinstance, believe that the fuller's teazle, with its hooks, whichcannot be rivalled by any mechanical contrivance, is only a variety ofthe wild Dipsacus; and this amount of change may have suddenly arisenin a seedling. So it has probably been with the turnspit dog; and thisis known to have been the case with the ancon sheep. But when wecompare the dray-horse and race-horse, the dromedary and camel, thevarious breeds of sheep fitted either for cultivated land or mountainpasture, with the wool of one breed good for one purpose, and that ofanother breed for another purpose; when we compare the many breeds ofdogs, each good for man in very different ways; when we compare thegame-cock, so pertinacious in battle, with other breeds so littlequarrelsome, with "everlasting layers" which never desire to sit, andwith the bantam so small and elegant; when we compare the host ofagricultural, culinary, orchard, and flower-garden races of plants,most useful to man at different seasons and for different purposes, orso beautiful in his eyes, we must, I think, look further than to merevariability. We cannot suppose that all the breeds were suddenlyproduced as perfect and as useful as we now see them; indeed, inseveral cases, we know that this has not been their history. The keyis man's power of accumulative selection: nature gives successivevariations; man adds them up in certain directions useful to him. Inthis sense he may be said to make for himself useful breeds.
The great power of this principle of selection is not hypothetical. Itis certain that several of our eminent breeders have, even within asingle lifetime, modified to a large extent some breeds of cattle andsheep. In order fully to realise what they have done, it is almostnecessary to read several of the many treatises devoted to thissubject, and to inspect the animals. Breeders habitually speak of ananimal's organisation as something quite plastic, which they can modelalmost as they please. If I had space I could quote numerous passagesto this effect from highly competent authorities. Youatt, who wasprobably better acquainted with the works of agriculturalists thanalmost any other individual, and who was himself a very good judge ofan animal, speaks of the principle of selection as "that which enablesthe agriculturist, not only to modify the character of his flock, butto change it altogether. It is the magician's wand, by means of whichhe may summon into life whatever form and mould he pleases." LordSomerville, speaking of what breeders have done for sheep, says:--"Itwould seem as if they had chalked out upon a wall a form perfect initself, and then had given it existence." That most skilful breeder,Sir John Sebright, used to say, with respect to pigeons, that "hewould produce any given feather in three years, but it would take himsix years to obtain head and beak." In Saxony the importance of theprinciple of selection in regard to merino sheep is so fullyrecognised, that men follow it as a trade: the sheep are placed on atable and are studied, like a picture by a connoisseur; this is donethree times at intervals of months, and the sheep are each time markedand classed, so that the very best may ultimately be selected forbreeding.
What English breeders have actually effected is proved by the enormousprices given for animals with a good pedigree; and these have now beenexported to almost every quarter of the world. The improvement is byno means generally due to crossing different breeds; all the bestbreeders are strongly opposed to this practice, except sometimesamongst closely allied sub-breeds. And when a cross has been made, theclosest selection is far more indispensable even than in ordinarycases. If selection consisted merely in separating some very distinctvariety, and breeding from it, the principle would be so obvious ashardly to be worth notice; but its importance consists in the greateffect produced by the accumulation in one direction, duringsuccessive generations, of differences absolutely inappreciable by anuneducated eye--differences which I for one have vainly attempted toappreciate. Not one man in a thousand has accuracy of eye and judgmentsufficient to become an eminent breeder. If gifted with thesequalities, and he studies his subject for years, and devotes hislifetime to it with indomitable perseverance, he will succeed, and maymake great improvements; if he wants any of these qualities, he willassuredly fail. Few would readily believe in the natural capacity andyears of practice requisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists; but thevariations are here often more abrupt. No one supposes that ourchoicest productions have been produced by a single variation from theaboriginal stock. We have proofs that this is not so in some cases, inwhich exact records have been kept; thus, to give a very triflinginstance, the steadily-increasing size of the common gooseberry may bequoted. We see an astonishing improvement in many florists' flowers,when the flowers of the present day are compared with drawings madeonly twenty or thirty years ago. When a race of plants is once prettywell established, the seed-raisers do not pick out the best plants,but merely go over their seed-beds, and pull up the "rogues," as theycall the plants that deviate from the proper standard. With animalsthis kind of selection is, in fact, also followed; for hardly any oneis so careless as to allow his worst animals to breed.
In regard to plants, there is another means of observing theaccumulated effects of selection--namely, by comparing the diversityof flowers in the different varieties of the same species in theflower-garden; the diversity of leaves, pods, or tubers, or whateverpart is valued, in the kitchen-garden, in comparison with the flowersof the same varieties; and the diversity of fruit of the same speciesin the orchard, in comparison with the leaves and flowers of the sameset of varieties. See how different the leaves of the cabbage are, andhow extremely alike the flowers; how unlike the flowers of theheartsease are, and how alike the leaves; how much the fruit of thedifferent kinds of gooseberries differ in size, colour, shape, andhairiness, and yet the flowers present very slight differences. It isnot that the varieties which differ largely in some one point do notdiffer at all in other points; this is hardly ever, perhaps never, thecase. The laws of correlation of growth, the importance of whichshould never be overlooked, will ensure some differences; but, as ageneral rule, I cannot doubt that the continued selection of slightvariations, either in the leaves, the flowers, or the fruit, willproduce races differing from each other chiefly in these characters.
It may be objected that the principle of selection has been reduced tomethodical practice for scarcely more than three-quarters of acentury; it has certainly been more attended to of late years, andmany treatises have been published on the subject; and the result, Imay add, has been, in a corresponding degree, rapid and important. Butit is very far from true that the principle is a modern discovery. Icould give several references to the full acknowledgment of theimportance of the principle in works of high antiquity. In rude andbarbarous periods of English history choice animals were oftenimported, and laws were passed to prevent their exportation: thedestruction of horses under a certain size was ordered, and this maybe compared to the "roguing" of plants by nurserymen. The principle ofselection I find distinctly given in an ancient Chinese encyclopaedia.Explicit rules are laid down by some of the Roman classical writers.From passages in Genesis, it is clear that the colour of domesticanimals was at that early period attended to. Savages now sometimescross their dogs with wild canine animals, to improve the breed, andthey formerly did so, as is attested by passages in Pliny. The savagesin South Africa match their draught cattle by colour, as do some ofthe Esquimaux their teams of dogs. Livingstone shows how much gooddomestic breeds are valued by the negroes of the interior of Africawho have not associated with Europeans. Some of these facts do notshow actual selection, but they show that the breeding of domesticanimals was carefully attended to in ancient times, and is nowattended to by the lowest savages. It would, indeed, have been astrange fact, had attention not been paid to breeding, for theinheritance of good and bad qualities is so obvious.
At the present time, eminent breeders try by methodical selection,with a distinct object in view, to make a new strain or sub-breed,superior to anything existing in the country. But, for our purpose, akind of Selection, which may be called Unconscious, and which resultsfrom every one trying to possess and breed from the best individualanimals, is more important. Thus, a man who intends keeping pointersnaturally tries to get as good dogs as he can, and afterwards breedsfrom his own best dogs, but he has no wish or expectation ofpermanently altering the breed. Nevertheless I cannot doubt that thisprocess, continued during centuries, would improve and modify anybreed, in the same way as Bakewell, Collins, etc., by this very sameprocess, only carried on more methodically, did greatly modify, evenduring their own lifetimes, the forms and qualities of their cattle.Slow and insensible changes of this kind could never be recognisedunless actual measurements or careful drawings of the breeds inquestion had been made long ago, which might serve for comparison. Insome cases, however, unchanged or but little changed individuals ofthe same breed may be found in less civilised districts, where thebreed has been less improved. There is reason to believe that KingCharles's spaniel has been unconsciously modified to a large extentsince the time of that monarch. Some highly competent authorities areconvinced that the setter is directly derived from the spaniel, andhas probably been slowly altered from it. It is known that the Englishpointer has been greatly changed within the last century, and in thiscase the change has, it is believed, been chiefly effected by crosseswith the fox-hound; but what concerns us is, that the change has beeneffected unconsciously and gradually, and yet so effectually, that,though the old Spanish pointer certainly came from Spain, Mr. Borrowhas not seen, as I am informed by him, any native dog in Spain likeour pointer.
By a similar process of selection, and by careful training, the wholebody of English racehorses have come to surpass in fleetness and sizethe parent Arab stock, so that the latter, by the regulations for theGoodwood Races, are favoured in the weights they carry. Lord Spencerand others have shown how the cattle of England have increased inweight and in early maturity, compared with the stock formerly kept inthis country. By comparing the accounts given in old pigeon treatisesof carriers and tumblers with these breeds as now existing in Britain,India, and Persia, we can, I think, clearly trace the stages throughwhich they have insensibly passed, and come to differ so greatly fromthe rock-pigeon.
Youatt gives an excellent illustration of the effects of a course ofselection, which may be considered as unconsciously followed, in sofar that the breeders could never have expected or even have wished tohave produced the result which ensued--namely, the production of twodistinct strains. The two flocks of Leicester sheep kept by Mr.Buckley and Mr. Burgess, as Mr. Youatt remarks, "have been purely bredfrom the original stock of Mr. Bakewell for upwards of fifty years.There is not a suspicion existing in the mind of any one at allacquainted with the subject that the owner of either of them hasdeviated in any one instance from the pure blood of Mr. Bakewell'sflock, and yet the difference between the sheep possessed by these twogentlemen is so great that they have the appearance of being quitedifferent varieties."
If there exist savages so barbarous as never to think of the inheritedcharacter of the offspring of their domestic animals, yet any oneanimal particularly useful to them, for any special purpose, would becarefully preserved during famines and other accidents, to whichsavages are so liable, and such choice animals would thus generallyleave more offspring than the inferior ones; so that in this casethere would be a kind of unconscious selection going on. We see thevalue set on animals even by the barbarians of Tierra del Fuego, bytheir killing and devouring their old women, in times of dearth, as ofless value than their dogs.
In plants the same gradual process of improvement, through theoccasional preservation of the best individuals, whether or notsufficiently distinct to be ranked at their first appearance asdistinct varieties, and whether or not two or more species or raceshave become blended together by crossing, may plainly be recognised inthe increased size and beauty which we now see in the varieties of theheartsease, rose, pelargonium, dahlia, and other plants, when comparedwith the older varieties or with their parent-stocks. No one wouldever expect to get a first-rate heartsease or dahlia from the seed ofa wild plant. No one would expect to raise a first-rate melting pearfrom the seed of a wild pear, though he might succeed from a poorseedling growing wild, if it had come from a garden-stock. The pear,though cultivated in classical times, appears, from Pliny'sdescription, to have been a fruit of very inferior quality. I haveseen great surprise expressed in horticultural works at the wonderfulskill of gardeners, in having produced such splendid results from suchpoor materials; but the art, I cannot doubt, has been simple, and, asfar as the final result is concerned, has been followed almostunconsciously. It has consisted in always cultivating the best knownvariety, sowing its seeds, and, when a slightly better variety haschanced to appear, selecting it, and so onwards. But the gardeners ofthe classical period, who cultivated the best pear they could procure,never thought what splendid fruit we should eat; though we owe ourexcellent fruit, in some small degree, to their having naturallychosen and preserved the best varieties they could anywhere find.
A large amount of change in our cultivated plants, thus slowly andunconsciously accumulated, explains, as I believe, the well-knownfact, that in a vast number of cases we cannot recognise, andtherefore do not know, the wild parent-stocks of the plants which havebeen longest cultivated in our flower and kitchen gardens. If it hastaken centuries or thousands of years to improve or modify most of ourplants up to their present standard of usefulness to man, we canunderstand how it is that neither Australia, the Cape of Good Hope,nor any other region inhabited by quite uncivilised man, has affordedus a single plant worth culture. It is not that these countries, sorich in species, do not by a strange chance possess the aboriginalstocks of any useful plants, but that the native plants have not beenimproved by continued selection up to a standard of perfectioncomparable with that given to the plants in countries ancientlycivilised.
In regard to the domestic animals kept by uncivilised man, it shouldnot be overlooked that they almost always have to struggle for theirown food, at least during certain seasons. And in two countries verydifferently circumstanced, individuals of the same species, havingslightly different constitutions or structure, would often succeedbetter in the one country than in the other, and thus by a process of"natural selection," as will hereafter be more fully explained, twosub-breeds might be formed. This, perhaps, partly explains what hasbeen remarked by some authors, namely, that the varieties kept bysavages have more of the character of species than the varieties keptin civilised countries.
hereafter be more fully explained, twosub-breeds might be formed. This, perhaps, partly explains what hasbeen remarked by some authors, namely, that the varieties kept bysavages have more of the character of species?
On the view here given of the all-important part which selection byman has played, it becomes at once obvious, how it is that ourdomestic races show adaptation in their structure or in their habitsto man's wants or fancies. We can, I think, further understand thefrequently abnormal character of our domestic races, and likewisetheir differences being so great in external characters and relativelyso slight in internal parts or organs. Man can hardly select, or onlywith much difficulty, any deviation of structure excepting such as isexternally visible; and indeed he rarely cares for what is internal.He can never act by selection, excepting on variations which are firstgiven to him in some slight degree by nature. No man would ever try tomake a fantail, till he saw a pigeon with a tail developed in someslight degree in an unusual manner, or a pouter till he saw a pigeonwith a crop of somewhat unusual size; and the more abnormal or unusualany character was when it first appeared, the more likely it would beto catch his attention. But to use such an expression as trying tomake a fantail, is, I have no doubt, in most cases, utterly incorrect.The man who first selected a pigeon with a slightly larger tail, neverdreamed what the descendants of that pigeon would become throughlong-continued, partly unconscious and partly methodical selection.Perhaps the parent bird of all fantails had only fourteentail-feathers somewhat expanded, like the present Java fantail, orlike individuals of other and distinct breeds, in which as many asseventeen tail-feathers have been counted. Perhaps the firstpouter-pigeon did not inflate its crop much more than the turbit nowdoes the upper part of its oesophagus,--a habit which is disregardedby all fanciers, as it is not one of the points of the breed.