物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 5. LAWS OF VARIATION. Page 3
Now let us turn to nature. When a part has been developed in anextraordinary manner in any one species, compared with the otherspecies of the same genus, we may conclude that this part hasundergone an extraordinary amount of modification, since the periodwhen the species branched off from the common progenitor of the genus.This period will seldom be remote in any extreme degree, as speciesvery rarely endure for more than one geological period. Anextraordinary amount of modification implies an unusually large andlong-continued amount of variability, which has continually beenaccumulated by natural selection for the benefit of the species. Butas the variability of the extraordinarily-developed part or organ hasbeen so great and long-continued within a period not excessivelyremote, we might, as a general rule, expect still to find morevariability in such parts than in other parts of the organisation,which have remained for a much longer period nearly constant. Andthis, I am convinced, is the case. That the struggle between naturalselection on the one hand, and the tendency to reversion andvariability on the other hand, will in the course of time cease; andthat the most abnormally developed organs may be made constant, I cansee no reason to doubt. Hence when an organ, however abnormal it maybe, has been transmitted in approximately the same condition to manymodified descendants, as in the case of the wing of the bat, it musthave existed, according to my theory, for an immense period in nearlythe same state; and thus it comes to be no more variable than anyother structure. It is only in those cases in which the modificationhas been comparatively recent and extraordinarily great that we oughtto find the GENERATIVE VARIABILITY, as it may be called, still presentin a high degree. For in this case the variability will seldom as yethave been fixed by the continued selection of the individuals varyingin the required manner and degree, and by the continued rejection ofthose tending to revert to a former and less modified condition.
occasionalappearance in all the breeds, of slaty-blue birds with two black barson the wings, a white rump, a bar at the end of the tail, with theouter feathers externally edged near their bases with white. As allthese marks.
The principle included in these remarks may be extended. It isnotorious that specific characters are more variable than generic. Toexplain by a simple example what is meant. If some species in a largegenus of plants had blue flowers and some had red, the colour would beonly a specific character, and no one would be surprised at one of theblue species varying into red, or conversely; but if all the specieshad blue flowers, the colour would become a generic character, and itsvariation would be a more unusual circumstance. I have chosen thisexample because an explanation is not in this case applicable, whichmost naturalists would advance, namely, that specific characters aremore variable than generic, because they are taken from parts of lessphysiological importance than those commonly used for classing genera.I believe this explanation is partly, yet only indirectly, true; Ishall, however, have to return to this subject in our chapter onClassification. It would be almost superfluous to adduce evidence insupport of the above statement, that specific characters are morevariable than generic; but I have repeatedly noticed in works onnatural history, that when an author has remarked with surprise thatsome IMPORTANT organ or part, which is generally very constantthroughout large groups of species, has DIFFERED considerably inclosely-allied species, that it has, also, been VARIABLE in theindividuals of some of the species. And this fact shows that acharacter, which is generally of generic value, when it sinks in valueand becomes only of specific value, often becomes variable, though itsphysiological importance may remain the same. Something of the samekind applies to monstrosities: at least Is. Geoffroy St. Hilaire seemsto entertain no doubt, that the more an organ normally differs in thedifferent species of the same group, the more subject it is toindividual anomalies.
On the ordinary view of each species having been independentlycreated, why should that part of the structure, which differs from thesame part in other independently-created species of the same genus, bemore variable than those parts which are closely alike in the severalspecies? I do not see that any explanation can be given. But on theview of species being only strongly marked and fixed varieties, wemight surely expect to find them still often continuing to vary inthose parts of their structure which have varied within a moderatelyrecent period, and which have thus come to differ. Or to state thecase in another manner:--the points in which all the species of agenus resemble each other, and in which they differ from the speciesof some other genus, are called generic characters; and thesecharacters in common I attribute to inheritance from a commonprogenitor, for it can rarely have happened that natural selectionwill have modified several species, fitted to more or lesswidely-different habits, in exactly the same manner: and as theseso-called generic characters have been inherited from a remote period,since that period when the species first branched off from theircommon progenitor, and subsequently have not varied or come to differin any degree, or only in a slight degree, it is not probable thatthey should vary at the present day. On the other hand, the points inwhich species differ from other species of the same genus, are calledspecific characters; and as these specific characters have varied andcome to differ within the period of the branching off of the speciesfrom a common progenitor, it is probable that they should still oftenbe in some degree variable,--at least more variable than those partsof the organisation which have for a very long period remainedconstant.
In connexion with the present subject, I will make only two otherremarks. I think it will be admitted, without my entering on details,that secondary sexual characters are very variable; I think it alsowill be admitted that species of the same group differ from each othermore widely in their secondary sexual characters, than in other partsof their organisation; compare, for instance, the amount of differencebetween the males of gallinaceous birds, in which secondary sexualcharacters are strongly displayed, with the amount of differencebetween their females; and the truth of this proposition will begranted. The cause of the original variability of secondary sexualcharacters is not manifest; but we can see why these characters shouldnot have been rendered as constant and uniform as other parts of theorganisation; for secondary sexual characters have been accumulated bysexual selection, which is less rigid in its action than ordinaryselection, as it does not entail death, but only gives fewer offspringto the less favoured males. Whatever the cause may be of thevariability of secondary sexual characters, as they are highlyvariable, sexual selection will have had a wide scope for action, andmay thus readily have succeeded in giving to the species of the samegroup a greater amount of difference in their sexual characters, thanin other parts of their structure.
It is a remarkable fact, that the secondary sexual differences betweenthe two sexes of the same species are generally displayed in the verysame parts of the organisation in which the different species of thesame genus differ from each other. Of this fact I will give inillustration two instances, the first which happen to stand on mylist; and as the differences in these cases are of a very unusualnature, the relation can hardly be accidental. The same number ofjoints in the tarsi is a character generally common to very largegroups of beetles, but in the Engidae, as Westwood has remarked, thenumber varies greatly; and the number likewise differs in the twosexes of the same species: again in fossorial hymenoptera, the mannerof neuration of the wings is a character of the highest importance,because common to large groups; but in certain genera the neurationdiffers in the different species, and likewise in the two sexes of thesame species. This relation has a clear meaning on my view of thesubject: I look at all the species of the same genus as having ascertainly descended from the same progenitor, as have the two sexes ofany one of the species. Consequently, whatever part of the structureof the common progenitor, or of its early descendants, becamevariable; variations of this part would it is highly probable, betaken advantage of by natural and sexual selection, in order to fitthe several species to their several places in the economy of nature,and likewise to fit the two sexes of the same species to each other,or to fit the males and females to different habits of life, or themales to struggle with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specificcharacters, or those which distinguish species from species, than ofgeneric characters, or those which the species possess incommon;--that the frequent extreme variability of any part which isdeveloped in a species in an extraordinary manner in comparison withthe same part in its congeners; and the not great degree ofvariability in a part, however extraordinarily it may be developed, ifit be common to a whole group of species;--that the great variabilityof secondary sexual characters, and the great amount of difference inthese same characters between closely allied species;--that secondarysexual and ordinary specific differences are generally displayed inthe same parts of the organisation,--are all principles closelyconnected together. All being mainly due to the species of the samegroup having descended from a common progenitor, from whom they haveinherited much in common,--to parts which have recently and largelyvaried being more likely still to go on varying than parts which havelong been inherited and have not varied,--to natural selection havingmore or less completely, according to the lapse of time, overmasteredthe tendency to reversion and to further variability,--to sexualselection being less rigid than ordinary selection,--and to variationsin the same parts having been accumulated by natural and sexualselection, and thus adapted for secondary sexual, and for ordinaryspecific purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONESPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, ORREVERTS TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to ourdomestic races. The most distinct breeds of pigeons, in countries mostwidely apart, present sub-varieties with reversed feathers on the headand feathers on the feet,--characters not possessed by the aboriginalrock-pigeon; these then are analogous variations in two or moredistinct races. The frequent presence of fourteen or even sixteentail-feathers in the pouter, may be considered as a variationrepresenting the normal structure of another race, the fantail. Ipresume that no one will doubt that all such analogous variations aredue to the several races of the pigeon having inherited from a commonparent the same constitution and tendency to variation, when acted onby similar unknown influences. In the vegetable kingdom we have a caseof analogous variation, in the enlarged stems, or roots as commonlycalled, of the Swedish turnip and Ruta baga, plants which severalbotanists rank as varieties produced by cultivation from a commonparent: if this be not so, the case will then be one of analogousvariation in two so-called distinct species; and to these a third maybe added, namely, the common turnip. According to the ordinary view ofeach species having been independently created, we should have toattribute this similarity in the enlarged stems of these three plants,not to the vera causa of community of descent, and a consequenttendency to vary in a like manner, but to three separate yet closelyrelated acts of creation.
With pigeons, however, we have another case, namely, the occasionalappearance in all the breeds, of slaty-blue birds with two black barson the wings, a white rump, a bar at the end of the tail, with theouter feathers externally edged near their bases with white. As allthese marks are characteristic of the parent rock-pigeon, I presumethat no one will doubt that this is a case of reversion, and not of anew yet analogous variation appearing in the several breeds. We may Ithink confidently come to this conclusion, because, as we have seen,these coloured marks are eminently liable to appear in the crossedoffspring of two distinct and differently coloured breeds; and in thiscase there is nothing in the external conditions of life to cause thereappearance of the slaty-blue, with the several marks, beyond theinfluence of the mere act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappearafter having been lost for many, perhaps for hundreds of generations.But when a breed has been crossed only once by some other breed, theoffspring occasionally show a tendency to revert in character to theforeign breed for many generations--some say, for a dozen or even ascore of generations. After twelve generations, the proportion ofblood, to use a common expression, of any one ancestor, is only 1 in2048; and yet, as we see, it is generally believed that a tendency toreversion is retained by this very small proportion of foreign blood.In a breed which has not been crossed, but in which BOTH parents havelost some character which their progenitor possessed, the tendency,whether strong or weak, to reproduce the lost character might be, aswas formerly remarked, for all that we can see to the contrary,transmitted for almost any number of generations. When a characterwhich has been lost in a breed, reappears after a great number ofgenerations, the most probable hypothesis is, not that the offspringsuddenly takes after an ancestor some hundred generations distant, butthat in each successive generation there has been a tendency toreproduce the character in question, which at last, under unknownfavourable conditions, gains an ascendancy. For instance, it isprobable that in each generation of the barb-pigeon, which producesmost rarely a blue and black-barred bird, there has been a tendency ineach generation in the plumage to assume this colour. This view ishypothetical, but could be supported by some facts; and I can see nomore abstract improbability in a tendency to produce any characterbeing inherited for an endless number of generations, than in quiteuseless or rudimentary organs being, as we all know them to be, thusinherited. Indeed, we may sometimes observe a mere tendency to producea rudiment inherited: for instance, in the common snapdragon(Antirrhinum) a rudiment of a fifth stamen so often appears, that thisplant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, tohave descended from a common parent, it might be expected that theywould occasionally vary in an analogous manner; so that a variety ofone species would resemble in some of its characters another species;this other species being on my view only a well-marked and permanentvariety. But characters thus gained would probably be of anunimportant nature, for the presence of all important characters willbe governed by natural selection, in accordance with the diversehabits of the species, and will not be left to the mutual action ofthe conditions of life and of a similar inherited constitution. Itmight further be expected that the species of the same genus wouldoccasionally exhibit reversions to lost ancestral characters. As,however, we never know the exact character of the common ancestor of agroup, we could not distinguish these two cases: if, for instance, wedid not know that the rock-pigeon was not feather-footed orturn-crowned, we could not have told, whether these characters in ourdomestic breeds were reversions or only analogous variations; but wemight have inferred that the blueness was a case of reversion, fromthe number of the markings, which are correlated with the blue tint,and which it does not appear probable would all appear together fromsimple variation. More especially we might have inferred this, fromthe blue colour and marks so often appearing when distinct breeds ofdiverse colours are crossed. Hence, though under nature it mustgenerally be left doubtful, what cases are reversions to an ancientlyexisting character, and what are new but analogous variations, yet weought, on my theory, sometimes to find the varying offspring of aspecies assuming characters (either from reversion or from analogousvariation) which already occur in some other members of the samegroup. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variablespecies in our systematic works, is due to its varieties mocking, asit were, some of the other species of the same genus. A considerablecatalogue, also, could be given of forms intermediate between twoother forms, which themselves must be doubtfully ranked as eithervarieties or species; and this shows, unless all these forms beconsidered as independently created species, that the one in varyinghas assumed some of the characters of the other, so as to produce theintermediate form. But the best evidence is afforded by parts ororgans of an important and uniform nature occasionally varying so asto acquire, in some degree, the character of the same part or organ inan allied species. I have collected a long list of such cases; buthere, as before, I lie under a great disadvantage in not being able togive them. I can only repeat that such cases certainly do occur, andseem to me very remarkable.
I will, however, give one curious and complex case, not indeed asaffecting any important character, but from occurring in severalspecies of the same genus, partly under domestication and partly undernature. It is a case apparently of reversion. The ass not rarely hasvery distinct transverse bars on its legs, like those on the legs of azebra: it has been asserted that these are plainest in the foal, andfrom inquiries which I have made, I believe this to be true. It hasalso been asserted that the stripe on each shoulder is sometimesdouble. The shoulder stripe is certainly very variable in length andoutline. A white ass, but NOT an albino, has been described withouteither spinal or shoulder-stripe; and these stripes are sometimes veryobscure, or actually quite lost, in dark-coloured asses. The koulan ofPallas is said to have been seen with a double shoulder-stripe. Thehemionus has no shoulder-stripe; but traces of it, as stated by Mr.Blyth and others, occasionally appear: and I have been informed byColonel Poole that the foals of this species are generally striped onthe legs, and faintly on the shoulder. The quagga, though so plainlybarred like a zebra over the body, is without bars on the legs; butDr. Gray has figured one specimen with very distinct zebra-like barson the hocks.
With respect to the horse, I have collected cases in England of thespinal stripe in horses of the most distinct breeds, and of ALLcolours; transverse bars on the legs are not rare in duns, mouse-duns,and in one instance in a chestnut: a faint shoulder-stripe maysometimes be seen in duns, and I have seen a trace in a bay horse. Myson made a careful examination and sketch for me of a dun Belgiancart-horse with a double stripe on each shoulder and with leg-stripes;and a man, whom I can implicitly trust, has examined for me a smalldun Welch pony with THREE short parallel stripes on each shoulder.