物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 5. LAWS OF VARIATION. Page 2
How much of the acclimatisation of species to any peculiar climate isdue to mere habit, and how much to the natural selection of varietieshaving different innate constitutions, and how much to both meanscombined, is a very obscure question. That habit or custom has someinfluence I must believe, both from analogy, and from the incessantadvice given in agricultural works, even in the ancient Encyclopaediasof China, to be very cautious in transposing animals from one districtto another; for it is not likely that man should have succeeded inselecting so many breeds and sub-breeds with constitutions speciallyfitted for their own districts: the result must, I think, be due tohabit. On the other hand, I can see no reason to doubt that naturalselection will continually tend to preserve those individuals whichare born with constitutions best adapted to their native countries. Intreatises on many kinds of cultivated plants, certain varieties aresaid to withstand certain climates better than others: this is verystrikingly shown in works on fruit trees published in the UnitedStates, in which certain varieties are habitually recommended for thenorthern, and others for the southern States; and as most of thesevarieties are of recent origin, they cannot owe their constitutionaldifferences to habit. The case of the Jerusalem artichoke, which isnever propagated by seed, and of which consequently new varieties havenot been produced, has even been advanced--for it is now as tender asever it was--as proving that acclimatisation cannot be effected! Thecase, also, of the kidney-bean has been often cited for a similarpurpose, and with much greater weight; but until some one will sow,during a score of generations, his kidney-beans so early that a verylarge proportion are destroyed by frost, and then collect seed fromthe few survivors, with care to prevent accidental crosses, and thenagain get seed from these seedlings, with the same precautions, theexperiment cannot be said to have been even tried. Nor let it besupposed that no differences in the constitution of seedlingkidney-beans ever appear, for an account has been published how muchmore hardy some seedlings appeared to be than others.
On the whole, I think we may conclude that habit, use, and disuse,have, in some cases, played a considerable part in the modification ofthe constitution, and of the structure of various organs; but that theeffects of use and disuse have often been largely combined with, andsometimes overmastered by, the natural selection of innatedifferences.
CORRELATION OF GROWTH.
I mean by this expression that the whole organisation is so tiedtogether during its growth and development, that when slightvariations in any one part occur, and are accumulated through naturalselection, other parts become modified. This is a very importantsubject, most imperfectly understood. The most obvious case is, thatmodifications accumulated solely for the good of the young or larva,will, it may safely be concluded, affect the structure of the adult;in the same manner as any malconformation affecting the early embryo,seriously affects the whole organisation of the adult. The severalparts of the body which are homologous, and which, at an earlyembryonic period, are alike, seem liable to vary in an allied manner:we see this in the right and left sides of the body varying in thesame manner; in the front and hind legs, and even in the jaws andlimbs, varying together, for the lower jaw is believed to behomologous with the limbs. These tendencies, I do not doubt, may bemastered more or less completely by natural selection: thus a familyof stags once existed with an antler only on one side; and if this hadbeen of any great use to the breed it might probably have beenrendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend tocohere; this is often seen in monstrous plants; and nothing is morecommon than the union of homologous parts in normal structures, as theunion of the petals of the corolla into a tube. Hard parts seem toaffect the form of adjoining soft parts; it is believed by someauthors that the diversity in the shape of the pelvis in birds causesthe remarkable diversity in the shape of their kidneys. Others believethat the shape of the pelvis in the human mother influences bypressure the shape of the head of the child. In snakes, according toSchlegel, the shape of the body and the manner of swallowing determinethe position of several of the most important viscera.
The nature of the bond of correlation is very frequently quiteobscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, thatcertain malconformations very frequently, and that others rarelycoexist, without our being able to assign any reason. What can be moresingular than the relation between blue eyes and deafness in cats, andthe tortoise-shell colour with the female sex; the feathered feet andskin between the outer toes in pigeons, and the presence of more orless down on the young birds when first hatched, with the futurecolour of their plumage; or, again, the relation between the hair andteeth in the naked Turkish dog, though here probably homology comesinto play? With respect to this latter case of correlation, I think itcan hardly be accidental, that if we pick out the two orders ofmammalia which are most abnormal in their dermal coverings, viz.Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, etc.),that these are likewise the most abnormal in their teeth.
I know of no case better adapted to show the importance of the laws ofcorrelation in modifying important structures, independently ofutility and, therefore, of natural selection, than that of thedifference between the outer and inner flowers in some Compositous andUmbelliferous plants. Every one knows the difference in the ray andcentral florets of, for instance, the daisy, and this difference isoften accompanied with the abortion of parts of the flower. But, insome Compositous plants, the seeds also differ in shape and sculpture;and even the ovary itself, with its accessory parts, differs, as hasbeen described by Cassini. These differences have been attributed bysome authors to pressure, and the shape of the seeds in theray-florets in some Compositae countenances this idea; but, in thecase of the corolla of the Umbelliferae, it is by no means, as Dr.Hooker informs me, in species with the densest heads that the innerand outer flowers most frequently differ. It might have been thoughtthat the development of the ray-petals by drawing nourishment fromcertain other parts of the flower had caused their abortion; but insome Compositae there is a difference in the seeds of the outer andinner florets without any difference in the corolla. Possibly, theseseveral differences may be connected with some difference in the flowof nutriment towards the central and external flowers: we know, atleast, that in irregular flowers, those nearest to the axis areoftenest subject to peloria, and become regular. I may add, as aninstance of this, and of a striking case of correlation, that I haverecently observed in some garden pelargoniums, that the central flowerof the truss often loses the patches of darker colour in the two upperpetals; and that when this occurs, the adherent nectary is quiteaborted; when the colour is absent from only one of the two upperpetals, the nectary is only much shortened.
With respect to the difference in the corolla of the central andexterior flowers of a head or umbel, I do not feel at all sure that C.C. Sprengel's idea that the ray-florets serve to attract insects,whose agency is highly advantageous in the fertilisation of plants ofthese two orders, is so far-fetched, as it may at first appear: and ifit be advantageous, natural selection may have come into play. But inregard to the differences both in the internal and external structureof the seeds, which are not always correlated with any differences inthe flowers, it seems impossible that they can be in any wayadvantageous to the plant: yet in the Umbelliferae these differencesare of such apparent importance--the seeds being in some cases,according to Tausch, orthospermous in the exterior flowers andcoelospermous in the central flowers,--that the elder De Candollefounded his main divisions of the order on analogous differences.Hence we see that modifications of structure, viewed by systematistsas of high value, may be wholly due to unknown laws of correlatedgrowth, and without being, as far as we can see, of the slightestservice to the species.
We may often falsely attribute to correlation of growth, structureswhich are common to whole groups of species, and which in truth aresimply due to inheritance; for an ancient progenitor may have acquiredthrough natural selection some one modification in structure, and,after thousands of generations, some other and independentmodification; and these two modifications, having been transmitted toa whole group of descendants with diverse habits, would naturally bethought to be correlated in some necessary manner. So, again, I do notdoubt that some apparent correlations, occurring throughout wholeorders, are entirely due to the manner alone in which naturalselection can act. For instance, Alph. De Candolle has remarked thatwinged seeds are never found in fruits which do not open: I shouldexplain the rule by the fact that seeds could not gradually becomewinged through natural selection, except in fruits which opened; sothat the individual plants producing seeds which were a little betterfitted to be wafted further, might get an advantage over thoseproducing seed less fitted for dispersal; and this process could notpossibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period,their law of compensation or balancement of growth; or, as Goetheexpressed it, "in order to spend on one side, nature is forced toeconomise on the other side." I think this holds true to a certainextent with our domestic productions: if nourishment flows to one partor organ in excess, it rarely flows, at least in excess, to anotherpart; thus it is difficult to get a cow to give much milk and tofatten readily. The same varieties of the cabbage do not yieldabundant and nutritious foliage and a copious supply of oil-bearingseeds. When the seeds in our fruits become atrophied, the fruit itselfgains largely in size and quality. In our poultry, a large tuft offeathers on the head is generally accompanied by a diminished comb,and a large beard by diminished wattles. With species in a state ofnature it can hardly be maintained that the law is of universalapplication; but many good observers, more especially botanists,believe in its truth. I will not, however, here give any instances,for I see hardly any way of distinguishing between the effects, on theone hand, of a part being largely developed through natural selectionand another and adjoining part being reduced by this same process orby disuse, and, on the other hand, the actual withdrawal of nutrimentfrom one part owing to the excess of growth in another and adjoiningpart.
I suspect, also, that some of the cases of compensation which havebeen advanced, and likewise some other facts, may be merged under amore general principle, namely, that natural selection is continuallytrying to economise in every part of the organisation. If underchanged conditions of life a structure before useful becomes lessuseful, any diminution, however slight, in its development, will beseized on by natural selection, for it will profit the individual notto have its nutriment wasted in building up an useless structure. Ican thus only understand a fact with which I was much struck whenexamining cirripedes, and of which many other instances could begiven: namely, that when a cirripede is parasitic within another andis thus protected, it loses more or less completely its own shell orcarapace. This is the case with the male Ibla, and in a trulyextraordinary manner with the Proteolepas: for the carapace in allother cirripedes consists of the three highly-important anteriorsegments of the head enormously developed, and furnished with greatnerves and muscles; but in the parasitic and protected Proteolepas,the whole anterior part of the head is reduced to the merest rudimentattached to the bases of the prehensile antennae. Now the saving of alarge and complex structure, when rendered superfluous by theparasitic habits of the Proteolepas, though effected by slow steps,would be a decided advantage to each successive individual of thespecies; for in the struggle for life to which every animal isexposed, each individual Proteolepas would have a better chance ofsupporting itself, by less nutriment being wasted in developing astructure now become useless.
varieties aresaid to withstand certain climates better than others: this is verystrikingly shown in works on fruit trees published in the UnitedStates, in which certain varieties are habitually recommended for thenorthern, and others for the southern States; and as most of thesevarieties.
Thus, as I believe, natural selection will always succeed in the longrun in reducing and saving every part of the organisation, as soon asit is rendered superfluous, without by any means causing some otherpart to be largely developed in a corresponding degree. And,conversely, that natural selection may perfectly well succeed inlargely developing any organ, without requiring as a necessarycompensation the reduction of some adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, bothin varieties and in species, that when any part or organ is repeatedmany times in the structure of the same individual (as the vertebraein snakes, and the stamens in polyandrous flowers) the number isvariable; whereas the number of the same part or organ, when it occursin lesser numbers, is constant. The same author and some botanistshave further remarked that multiple parts are also very liable tovariation in structure. Inasmuch as this "vegetative repetition," touse Professor Owen's expression, seems to be a sign of loworganisation; the foregoing remark seems connected with the verygeneral opinion of naturalists, that beings low in the scale of natureare more variable than those which are higher. I presume that lownessin this case means that the several parts of the organisation havebeen but little specialised for particular functions; and as long asthe same part has to perform diversified work, we can perhaps see whyit should remain variable, that is, why natural selection should havepreserved or rejected each little deviation of form less carefullythan when the part has to serve for one special purpose alone. In thesame way that a knife which has to cut all sorts of things may be ofalmost any shape; whilst a tool for some particular object had betterbe of some particular shape. Natural selection, it should never beforgotten, can act on each part of each being, solely through and forits advantage.
Rudimentary parts, it has been stated by some authors, and I believewith truth, are apt to be highly variable. We shall have to recur tothe general subject of rudimentary and aborted organs; and I will hereonly add that their variability seems to be owing to theiruselessness, and therefore to natural selection having no power tocheck deviations in their structure. Thus rudimentary parts are leftto the free play of the various laws of growth, to the effects oflong-continued disuse, and to the tendency to reversion.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER,IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLYVARIABLE.
, and the presence of more orless down on the young birds when first hatched, with the futurecolour of their plumage; or, again, the relation between the hair andteeth in the naked Turkish dog, though here probably homology comesinto play? With.
Several years ago I was much struck with a remark, nearly to the aboveeffect, published by Mr. Waterhouse. I infer also from an observationmade by Professor Owen, with respect to the length of the arms of theourang-outang, that he has come to a nearly similar conclusion. It ishopeless to attempt to convince any one of the truth of thisproposition without giving the long array of facts which I havecollected, and which cannot possibly be here introduced. I can onlystate my conviction that it is a rule of high generality. I am awareof several causes of error, but I hope that I have made due allowancefor them. It should be understood that the rule by no means applies toany part, however unusually developed, unless it be unusuallydeveloped in comparison with the same part in closely allied species.Thus, the bat's wing is a most abnormal structure in the classmammalia; but the rule would not here apply, because there is a wholegroup of bats having wings; it would apply only if some one species ofbat had its wings developed in some remarkable manner in comparisonwith the other species of the same genus. The rule applies verystrongly in the case of secondary sexual characters, when displayed inany unusual manner. The term, secondary sexual characters, used byHunter, applies to characters which are attached to one sex, but arenot directly connected with the act of reproduction. The rule appliesto males and females; but as females more rarely offer remarkablesecondary sexual characters, it applies more rarely to them. The rulebeing so plainly applicable in the case of secondary sexualcharacters, may be due to the great variability of these characters,whether or not displayed in any unusual manner--of which fact I thinkthere can be little doubt. But that our rule is not confined tosecondary sexual characters is clearly shown in the case ofhermaphrodite cirripedes; and I may here add, that I particularlyattended to Mr. Waterhouse's remark, whilst investigating this Order,and I am fully convinced that the rule almost invariably holds goodwith cirripedes. I shall, in my future work, give a list of the moreremarkable cases; I will here only briefly give one, as it illustratesthe rule in its largest application. The opercular valves of sessilecirripedes (rock barnacles) are, in every sense of the word, veryimportant structures, and they differ extremely little even indifferent genera; but in the several species of one genus, Pyrgoma,these valves present a marvellous amount of diversification: thehomologous valves in the different species being sometimes whollyunlike in shape; and the amount of variation in the individuals ofseveral of the species is so great, that it is no exaggeration tostate that the varieties differ more from each other in the charactersof these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, Ihave particularly attended to them, and the rule seems to me certainlyto hold good in this class. I cannot make out that it applies toplants, and this would seriously have shaken my belief in its truth,had not the great variability in plants made it particularly difficultto compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree ormanner in any species, the fair presumption is that it is of highimportance to that species; nevertheless the part in this case iseminently liable to variation. Why should this be so? On the view thateach species has been independently created, with all its parts as wenow see them, I can see no explanation. But on the view that groups ofspecies have descended from other species, and have been modifiedthrough natural selection, I think we can obtain some light. In ourdomestic animals, if any part, or the whole animal, be neglected andno selection be applied, that part (for instance, the comb in theDorking fowl) or the whole breed will cease to have a nearly uniformcharacter. The breed will then be said to have degenerated. Inrudimentary organs, and in those which have been but littlespecialised for any particular purpose, and perhaps in polymorphicgroups, we see a nearly parallel natural case; for in such casesnatural selection either has not or cannot come into full play, andthus the organisation is left in a fluctuating condition. But whathere more especially concerns us is, that in our domestic animalsthose points, which at the present time are undergoing rapid change bycontinued selection, are also eminently liable to variation. Look atthe breeds of the pigeon; see what a prodigious amount of differencethere is in the beak of the different tumblers, in the beak and wattleof the different carriers, in the carriage and tail of our fantails,etc., these being the points now mainly attended to by Englishfanciers. Even in the sub-breeds, as in the short-faced tumbler, it isnotoriously difficult to breed them nearly to perfection, andfrequently individuals are born which depart widely from the standard.There may be truly said to be a constant struggle going on between, onthe one hand, the tendency to reversion to a less modified state, aswell as an innate tendency to further variability of all kinds, and,on the other hand, the power of steady selection to keep the breedtrue. In the long run selection gains the day, and we do not expect tofail so far as to breed a bird as coarse as a common tumbler from agood short-faced strain. But as long as selection is rapidly going on,there may always be expected to be much variability in the structureundergoing modification. It further deserves notice that thesevariable characters, produced by man's selection, sometimes becomeattached, from causes quite unknown to us, more to one sex than to theother, generally to the male sex, as with the wattle of carriers andthe enlarged crop of pouters.