物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 1. VARIATION UNDER DOMESTICATION. Page 2
In the case of most of our anciently domesticated animals and plants,I do not think it is possible to come to any definite conclusion,whether they have descended from one or several species. The argumentmainly relied on by those who believe in the multiple origin of ourdomestic animals is, that we find in the most ancient records, moreespecially on the monuments of Egypt, much diversity in the breeds;and that some of the breeds closely resemble, perhaps are identicalwith, those still existing. Even if this latter fact were found morestrictly and generally true than seems to me to be the case, what doesit show, but that some of our breeds originated there, four or fivethousand years ago? But Mr. Horner's researches have rendered it insome degree probable that man sufficiently civilized to havemanufactured pottery existed in the valley of the Nile thirteen orfourteen thousand years ago; and who will pretend to say how longbefore these ancient periods, savages, like those of Tierra del Fuegoor Australia, who possess a semi-domestic dog, may not have existed inEgypt?
The whole subject must, I think, remain vague; nevertheless, I may,without here entering on any details, state that, from geographicaland other considerations, I think it highly probable that our domesticdogs have descended from several wild species. In regard to sheep andgoats I can form no opinion. I should think, from facts communicatedto me by Mr. Blyth, on the habits, voice, and constitution, etc., ofthe humped Indian cattle, that these had descended from a differentaboriginal stock from our European cattle; and several competentjudges believe that these latter have had more than one wild parent.With respect to horses, from reasons which I cannot give here, I amdoubtfully inclined to believe, in opposition to several authors, thatall the races have descended from one wild stock. Mr. Blyth, whoseopinion, from his large and varied stores of knowledge, I should valuemore than that of almost any one, thinks that all the breeds ofpoultry have proceeded from the common wild Indian fowl (Gallusbankiva). In regard to ducks and rabbits, the breeds of which differconsiderably from each other in structure, I do not doubt that theyall have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from severalaboriginal stocks, has been carried to an absurd extreme by someauthors. They believe that every race which breeds true, let thedistinctive characters be ever so slight, has had its wild prototype.At this rate there must have existed at least a score of species ofwild cattle, as many sheep, and several goats in Europe alone, andseveral even within Great Britain. One author believes that thereformerly existed in Great Britain eleven wild species of sheeppeculiar to it! When we bear in mind that Britain has now hardly onepeculiar mammal, and France but few distinct from those of Germany andconversely, and so with Hungary, Spain, etc., but that each of thesekingdoms possesses several peculiar breeds of cattle, sheep, etc., wemust admit that many domestic breeds have originated in Europe; forwhence could they have been derived, as these several countries do notpossess a number of peculiar species as distinct parent-stocks? So itis in India. Even in the case of the domestic dogs of the whole world,which I fully admit have probably descended from several wild species,I cannot doubt that there has been an immense amount of inheritedvariation. Who can believe that animals closely resembling the Italiangreyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.--sounlike all wild Canidae--ever existed freely in a state of nature? Ithas often been loosely said that all our races of dogs have beenproduced by the crossing of a few aboriginal species; but by crossingwe can get only forms in some degree intermediate between theirparents; and if we account for our several domestic races by thisprocess, we must admit the former existence of the most extreme forms,as the Italian greyhound, bloodhound, bull-dog, etc., in the wildstate. Moreover, the possibility of making distinct races by crossinghas been greatly exaggerated. There can be no doubt that a race may bemodified by occasional crosses, if aided by the careful selection ofthose individual mongrels, which present any desired character; butthat a race could be obtained nearly intermediate between twoextremely different races or species, I can hardly believe. Sir J.Sebright expressly experimentised for this object, and failed. Theoffspring from the first cross between two pure breeds is tolerablyand sometimes (as I have found with pigeons) extremely uniform, andeverything seems simple enough; but when these mongrels are crossedone with another for several generations, hardly two of them will bealike, and then the extreme difficulty, or rather utter hopelessness,of the task becomes apparent. Certainly, a breed intermediate betweenTWO VERY DISTINCT breeds could not be got without extreme care andlong-continued selection; nor can I find a single case on record of apermanent race having been thus formed.
ON THE BREEDS OF THE DOMESTIC PIGEON.
importanceof these considerations in explaining the immense amount of variationwhich pigeons have undergone, will be obvious when we treat ofSelection. We shall then, also, see how it is that the breeds so oftenhave a !
Believing that it is always best to study some special group, I have,after deliberation, taken up domestic pigeons. I have kept every breedwhich I could purchase or obtain, and have been most kindly favouredwith skins from several quarters of the world, more especially by theHonourable W. Elliot from India, and by the Honourable C. Murray fromPersia. Many treatises in different languages have been published onpigeons, and some of them are very important, as being of considerableantiquity. I have associated with several eminent fanciers, and havebeen permitted to join two of the London Pigeon Clubs. The diversityof the breeds is something astonishing. Compare the English carrierand the short-faced tumbler, and see the wonderful difference in theirbeaks, entailing corresponding differences in their skulls. Thecarrier, more especially the male bird, is also remarkable from thewonderful development of the carunculated skin about the head, andthis is accompanied by greatly elongated eyelids, very large externalorifices to the nostrils, and a wide gape of mouth. The short-facedtumbler has a beak in outline almost like that of a finch; and thecommon tumbler has the singular and strictly inherited habit of flyingat a great height in a compact flock, and tumbling in the air headover heels. The runt is a bird of great size, with long, massive beakand large feet; some of the sub-breeds of runts have very long necks,others very long wings and tails, others singularly short tails. Thebarb is allied to the carrier, but, instead of a very long beak, has avery short and very broad one. The pouter has a much elongated body,wings, and legs; and its enormously developed crop, which it gloriesin inflating, may well excite astonishment and even laughter. Theturbit has a very short and conical beak, with a line of reversedfeathers down the breast; and it has the habit of continuallyexpanding slightly the upper part of the oesophagus. The Jacobin hasthe feathers so much reversed along the back of the neck that theyform a hood, and it has, proportionally to its size, much elongatedwing and tail feathers. The trumpeter and laugher, as their namesexpress, utter a very different coo from the other breeds. The fantailhas thirty or even forty tail-feathers, instead of twelve or fourteen,the normal number in all members of the great pigeon family; and thesefeathers are kept expanded, and are carried so erect that in goodbirds the head and tail touch; the oil-gland is quite aborted. Severalother less distinct breeds might have been specified.
-stocks? So itis in India. Even in the case of the domestic dogs of the whole world,which I fully admit have probably descended from several wild species,I cannot doubt that there has been an immense amount of inheritedvariation. Who can believe that animals closely resembling the Italiangreyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.--sounlike all wild Canidae--ever existed freely in a state of nature? Ithas often!
In the skeletons of the several breeds, the development of the bonesof the face in length and breadth and curvature differs enormously.The shape, as well as the breadth and length of the ramus of the lowerjaw, varies in a highly remarkable manner. The number of the caudaland sacral vertebrae vary; as does the number of the ribs, togetherwith their relative breadth and the presence of processes. The sizeand shape of the apertures in the sternum are highly variable; so isthe degree of divergence and relative size of the two arms of thefurcula. The proportional width of the gape of mouth, the proportionallength of the eyelids, of the orifice of the nostrils, of the tongue(not always in strict correlation with the length of beak), the sizeof the crop and of the upper part of the oesophagus; the developmentand abortion of the oil-gland; the number of the primary wing andcaudal feathers; the relative length of wing and tail to each otherand to the body; the relative length of leg and of the feet; thenumber of scutellae on the toes, the development of skin between thetoes, are all points of structure which are variable. The period atwhich the perfect plumage is acquired varies, as does the state of thedown with which the nestling birds are clothed when hatched. The shapeand size of the eggs vary. The manner of flight differs remarkably; asdoes in some breeds the voice and disposition. Lastly, in certainbreeds, the males and females have come to differ to a slight degreefrom each other.
Altogether at least a score of pigeons might be chosen, which if shownto an ornithologist, and he were told that they were wild birds, wouldcertainly, I think, be ranked by him as well-defined species.Moreover, I do not believe that any ornithologist would place theEnglish carrier, the short-faced tumbler, the runt, the barb, pouter,and fantail in the same genus; more especially as in each of thesebreeds several truly-inherited sub-breeds, or species as he might havecalled them, could be shown him.
Great as the differences are between the breeds of pigeons, I am fullyconvinced that the common opinion of naturalists is correct, namely,that all have descended from the rock-pigeon (Columba livia),including under this term several geographical races or sub-species,which differ from each other in the most trifling respects. As severalof the reasons which have led me to this belief are in some degreeapplicable in other cases, I will here briefly give them. If theseveral breeds are not varieties, and have not proceeded from therock-pigeon, they must have descended from at least seven or eightaboriginal stocks; for it is impossible to make the present domesticbreeds by the crossing of any lesser number: how, for instance, coulda pouter be produced by crossing two breeds unless one of theparent-stocks possessed the characteristic enormous crop? The supposedaboriginal stocks must all have been rock-pigeons, that is, notbreeding or willingly perching on trees. But besides C. livia, withits geographical sub-species, only two or three other species ofrock-pigeons are known; and these have not any of the characters ofthe domestic breeds. Hence the supposed aboriginal stocks must eitherstill exist in the countries where they were originally domesticated,and yet be unknown to ornithologists; and this, considering theirsize, habits, and remarkable characters, seems very improbable; orthey must have become extinct in the wild state. But birds breeding onprecipices, and good fliers, are unlikely to be exterminated; and thecommon rock-pigeon, which has the same habits with the domesticbreeds, has not been exterminated even on several of the smallerBritish islets, or on the shores of the Mediterranean. Hence thesupposed extermination of so many species having similar habits withthe rock-pigeon seems to me a very rash assumption. Moreover, theseveral above-named domesticated breeds have been transported to allparts of the world, and, therefore, some of them must have beencarried back again into their native country; but not one has everbecome wild or feral, though the dovecot-pigeon, which is therock-pigeon in a very slightly altered state, has become feral inseveral places. Again, all recent experience shows that it is mostdifficult to get any wild animal to breed freely under domestication;yet on the hypothesis of the multiple origin of our pigeons, it mustbe assumed that at least seven or eight species were so thoroughlydomesticated in ancient times by half-civilized man, as to be quiteprolific under confinement.
colouring, and inmost parts of their structure, with the wild rock-pigeon, yet arecertainly highly abnormal in other parts of their structure: we maylook in vain throughout the whole great family of Columbidae for abeak like that of the English carrier, or that of the short-facedtumbler, or barb; for reversed feathers like those.
An argument, as it seems to me, of great weight, and applicable inseveral other cases, is, that the above-specified breeds, thoughagreeing generally in constitution, habits, voice, colouring, and inmost parts of their structure, with the wild rock-pigeon, yet arecertainly highly abnormal in other parts of their structure: we maylook in vain throughout the whole great family of Columbidae for abeak like that of the English carrier, or that of the short-facedtumbler, or barb; for reversed feathers like those of the jacobin; fora crop like that of the pouter; for tail-feathers like those of thefantail. Hence it must be assumed not only that half-civilized mansucceeded in thoroughly domesticating several species, but that heintentionally or by chance picked out extraordinarily abnormalspecies; and further, that these very species have since all becomeextinct or unknown. So many strange contingencies seem to meimprobable in the highest degree.
Some facts in regard to the colouring of pigeons well deserveconsideration. The rock-pigeon is of a slaty-blue, and has a whiterump (the Indian sub-species, C. intermedia of Strickland, having itbluish); the tail has a terminal dark bar, with the bases of the outerfeathers externally edged with white; the wings have two black bars;some semi-domestic breeds and some apparently truly wild breeds have,besides the two black bars, the wings chequered with black. Theseseveral marks do not occur together in any other species of the wholefamily. Now, in every one of the domestic breeds, taking thoroughlywell-bred birds, all the above marks, even to the white edging of theouter tail-feathers, sometimes concur perfectly developed. Moreover,when two birds belonging to two distinct breeds are crossed, neitherof which is blue or has any of the above-specified marks, the mongreloffspring are very apt suddenly to acquire these characters; forinstance, I crossed some uniformly white fantails with some uniformlyblack barbs, and they produced mottled brown and black birds; these Iagain crossed together, and one grandchild of the pure white fantailand pure black barb was of as beautiful a blue colour, with the whiterump, double black wing-bar, and barred and white-edged tail-feathers,as any wild rock-pigeon! We can understand these facts, on thewell-known principle of reversion to ancestral characters, if all thedomestic breeds have descended from the rock-pigeon. But if we denythis, we must make one of the two following highly improbablesuppositions. Either, firstly, that all the several imaginedaboriginal stocks were coloured and marked like the rock-pigeon,although no other existing species is thus coloured and marked, sothat in each separate breed there might be a tendency to revert to thevery same colours and markings. Or, secondly, that each breed, eventhe purest, has within a dozen or, at most, within a score ofgenerations, been crossed by the rock-pigeon: I say within a dozen ortwenty generations, for we know of no fact countenancing the beliefthat the child ever reverts to some one ancestor, removed by a greaternumber of generations. In a breed which has been crossed only oncewith some distinct breed, the tendency to reversion to any characterderived from such cross will naturally become less and less, as ineach succeeding generation there will be less of the foreign blood;but when there has been no cross with a distinct breed, and there is atendency in both parents to revert to a character, which has been lostduring some former generation, this tendency, for all that we can seeto the contrary, may be transmitted undiminished for an indefinitenumber of generations. These two distinct cases are often confoundedin treatises on inheritance.
Lastly, the hybrids or mongrels from between all the domestic breedsof pigeons are perfectly fertile. I can state this from my ownobservations, purposely made on the most distinct breeds. Now, it isdifficult, perhaps impossible, to bring forward one case of the hybridoffspring of two animals CLEARLY DISTINCT being themselves perfectlyfertile. Some authors believe that long-continued domesticationeliminates this strong tendency to sterility: from the history of thedog I think there is some probability in this hypothesis, if appliedto species closely related together, though it is unsupported by asingle experiment. But to extend the hypothesis so far as to supposethat species, aboriginally as distinct as carriers, tumblers, pouters,and fantails now are, should yield offspring perfectly fertile, interse, seems to me rash in the extreme.
From these several reasons, namely, the improbability of man havingformerly got seven or eight supposed species of pigeons to breedfreely under domestication; these supposed species being quite unknownin a wild state, and their becoming nowhere feral; these specieshaving very abnormal characters in certain respects, as compared withall other Columbidae, though so like in most other respects to therock-pigeon; the blue colour and various marks occasionally appearingin all the breeds, both when kept pure and when crossed; the mongreloffspring being perfectly fertile;--from these several reasons, takentogether, I can feel no doubt that all our domestic breeds havedescended from the Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that C. livia, or therock-pigeon, has been found capable of domestication in Europe and inIndia; and that it agrees in habits and in a great number of points ofstructure with all the domestic breeds. Secondly, although an Englishcarrier or short-faced tumbler differs immensely in certain charactersfrom the rock-pigeon, yet by comparing the several sub-breeds of thesebreeds, more especially those brought from distant countries, we canmake an almost perfect series between the extremes of structure.Thirdly, those characters which are mainly distinctive of each breed,for instance the wattle and length of beak of the carrier, theshortness of that of the tumbler, and the number of tail-feathers inthe fantail, are in each breed eminently variable; and the explanationof this fact will be obvious when we come to treat of selection.Fourthly, pigeons have been watched, and tended with the utmost care,and loved by many people. They have been domesticated for thousands ofyears in several quarters of the world; the earliest known record ofpigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as waspointed out to me by Professor Lepsius; but Mr. Birch informs me thatpigeons are given in a bill of fare in the previous dynasty. In thetime of the Romans, as we hear from Pliny, immense prices were givenfor pigeons; "nay, they are come to this pass, that they can reckon uptheir pedigree and race." Pigeons were much valued by Akber Khan inIndia, about the year 1600; never less than 20,000 pigeons were takenwith the court. "The monarchs of Iran and Turan sent him some veryrare birds;" and, continues the courtly historian, "His Majesty bycrossing the breeds, which method was never practised before, hasimproved them astonishingly." About this same period the Dutch were aseager about pigeons as were the old Romans. The paramount importanceof these considerations in explaining the immense amount of variationwhich pigeons have undergone, will be obvious when we treat ofSelection. We shall then, also, see how it is that the breeds so oftenhave a somewhat monstrous character. It is also a most favourablecircumstance for the production of distinct breeds, that male andfemale pigeons can be easily mated for life; and thus different breedscan be kept together in the same aviary.