物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 5. LAWS OF VARIATION. Page 4
In the north-west part of India the Kattywar breed of horses is sogenerally striped, that, as I hear from Colonel Poole, who examinedthe breed for the Indian Government, a horse without stripes is notconsidered as purely-bred. The spine is always striped; the legs aregenerally barred; and the shoulder-stripe, which is sometimes doubleand sometimes treble, is common; the side of the face, moreover, issometimes striped. The stripes are plainest in the foal; and sometimesquite disappear in old horses. Colonel Poole has seen both gray andbay Kattywar horses striped when first foaled. I have, also, reason tosuspect, from information given me by Mr. W. W. Edwards, that with theEnglish race-horse the spinal stripe is much commoner in the foal thanin the full-grown animal. Without here entering on further details, Imay state that I have collected cases of leg and shoulder stripes inhorses of very different breeds, in various countries from Britain toEastern China; and from Norway in the north to the Malay Archipelagoin the south. In all parts of the world these stripes occur faroftenest in duns and mouse-duns; by the term dun a large range ofcolour is included, from one between brown and black to a closeapproach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on thissubject, believes that the several breeds of the horse have descendedfrom several aboriginal species--one of which, the dun, was striped;and that the above-described appearances are all due to ancientcrosses with the dun stock. But I am not at all satisfied with thistheory, and should be loth to apply it to breeds so distinct as theheavy Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race,etc., inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of thehorse-genus. Rollin asserts, that the common mule from the ass andhorse is particularly apt to have bars on its legs. I once saw a mulewith its legs so much striped that any one at first would have thoughtthat it must have been the product of a zebra; and Mr. W. C. Martin,in his excellent treatise on the horse, has given a figure of asimilar mule. In four coloured drawings, which I have seen, of hybridsbetween the ass and zebra, the legs were much more plainly barred thanthe rest of the body; and in one of them there was a doubleshoulder-stripe. In Lord Moreton's famous hybrid from a chestnut mareand male quagga, the hybrid, and even the pure offspring subsequentlyproduced from the mare by a black Arabian sire, were much more plainlybarred across the legs than is even the pure quagga. Lastly, and thisis another most remarkable case, a hybrid has been figured by Dr. Gray(and he informs me that he knows of a second case) from the ass andthe hemionus; and this hybrid, though the ass seldom has stripes onits legs and the hemionus has none and has not even a shoulder-stripe,nevertheless had all four legs barred, and had three shortshoulder-stripes, like those on the dun Welch pony, and even had somezebra-like stripes on the sides of its face. With respect to this lastfact, I was so convinced that not even a stripe of colour appears fromwhat would commonly be called an accident, that I was led solely fromthe occurrence of the face-stripes on this hybrid from the ass andhemionus, to ask Colonel Poole whether such face-stripes ever occur inthe eminently striped Kattywar breed of horses, and was, as we haveseen, answered in the affirmative.
What now are we to say to these several facts? We see several verydistinct species of the horse-genus becoming, by simple variation,striped on the legs like a zebra, or striped on the shoulders like anass. In the horse we see this tendency strong whenever a dun tintappears--a tint which approaches to that of the general colouring ofthe other species of the genus. The appearance of the stripes is notaccompanied by any change of form or by any other new character. Wesee this tendency to become striped most strongly displayed in hybridsfrom between several of the most distinct species. Now observe thecase of the several breeds of pigeons: they are descended from apigeon (including two or three sub-species or geographical races) of abluish colour, with certain bars and other marks; and when any breedassumes by simple variation a bluish tint, these bars and other marksinvariably reappear; but without any other change of form orcharacter. When the oldest and truest breeds of various colours arecrossed, we see a strong tendency for the blue tint and bars and marksto reappear in the mongrels. I have stated that the most probablehypothesis to account for the reappearance of very ancient characters,is--that there is a TENDENCY in the young of each successivegeneration to produce the long-lost character, and that this tendency,from unknown causes, sometimes prevails. And we have just seen that inseveral species of the horse-genus the stripes are either plainer orappear more commonly in the young than in the old. Call the breeds ofpigeons, some of which have bred true for centuries, species; and howexactly parallel is the case with that of the species of thehorse-genus! For myself, I venture confidently to look back thousandson thousands of generations, and I see an animal striped like a zebra,but perhaps otherwise very differently constructed, the common parentof our domestic horse, whether or not it be descended from one or morewild stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,will, I presume, assert that each species has been created with atendency to vary, both under nature and under domestication, in thisparticular manner, so as often to become striped like other species ofthe genus; and that each has been created with a strong tendency, whencrossed with species inhabiting distant quarters of the world, toproduce hybrids resembling in their stripes, not their own parents,but other species of the genus. To admit this view is, as it seems tome, to reject a real for an unreal, or at least for an unknown, cause.It makes the works of God a mere mockery and deception; I would almostas soon believe with the old and ignorant cosmogonists, that fossilshells had never lived, but had been created in stone so as to mockthe shells now living on the sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one caseout of a hundred can we pretend to assign any reason why this or thatpart differs, more or less, from the same part in the parents. Butwhenever we have the means of instituting a comparison, the same lawsappear to have acted in producing the lesser differences betweenvarieties of the same species, and the greater differences betweenspecies of the same genus. The external conditions of life, as climateand food, etc., seem to have induced some slight modifications. Habitin producing constitutional differences, and use in strengthening, anddisuse in weakening and diminishing organs, seem to have been morepotent in their effects. Homologous parts tend to vary in the sameway, and homologous parts tend to cohere. Modifications in hard partsand in external parts sometimes affect softer and internal parts. Whenone part is largely developed, perhaps it tends to draw nourishmentfrom the adjoining parts; and every part of the structure which can besaved without detriment to the individual, will be saved. Changes ofstructure at an early age will generally affect parts subsequentlydeveloped; and there are very many other correlations of growth, thenature of which we are utterly unable to understand. Multiple partsare variable in number and in structure, perhaps arising from suchparts not having been closely specialised to any particular function,so that their modifications have not been closely checked by naturalselection. It is probably from this same cause that organic beings lowin the scale of nature are more variable than those which have theirwhole organisation more specialised, and are higher in the scale.Rudimentary organs, from being useless, will be disregarded by naturalselection, and hence probably are variable. Specific characters--thatis, the characters which have come to differ since the several speciesof the same genus branched off from a common parent--are more variablethan generic characters, or those which have long been inherited, andhave not differed within this same period. In these remarks we havereferred to special parts or organs being still variable, because theyhave recently varied and thus come to differ; but we have also seen inthe second Chapter that the same principle applies to the wholeindividual; for in a district where many species of any genus arefound--that is, where there has been much former variation anddifferentiation, or where the manufactory of new specific forms hasbeen actively at work--there, on an average, we now find mostvarieties or incipient species. Secondary sexual characters are highlyvariable, and such characters differ much in the species of the samegroup. Variability in the same parts of the organisation has generallybeen taken advantage of in giving secondary sexual differences to thesexes of the same species, and specific differences to the severalspecies of the same genus. Any part or organ developed to anextraordinary size or in an extraordinary manner, in comparison withthe same part or organ in the allied species, must have gone throughan extraordinary amount of modification since the genus arose; andthus we can understand why it should often still be variable in a muchhigher degree than other parts; for variation is a long-continued andslow process, and natural selection will in such cases not as yet havehad time to overcome the tendency to further variability and toreversion to a less modified state. But when a species with anyextraordinarily-developed organ has become the parent of many modifieddescendants--which on my view must be a very slow process, requiring along lapse of time--in this case, natural selection may readily havesucceeded in giving a fixed character to the organ, in howeverextraordinary a manner it may be developed. Species inheriting nearlythe same constitution from a common parent and exposed to similarinfluences will naturally tend to present analogous variations, andthese same species may occasionally revert to some of the charactersof their ancient progenitors. Although new and important modificationsmay not arise from reversion and analogous variation, suchmodifications will add to the beautiful and harmonious diversity ofnature.
Whatever the cause may be of each slight difference in the offspringfrom their parents--and a cause for each must exist--it is the steadyaccumulation, through natural selection, of such differences, whenbeneficial to the individual, that gives rise to all the moreimportant modifications of structure, by which the innumerable beingson the face of this earth are enabled to struggle with each other, andthe best adapted to survive.