物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 4. NATURAL SELECTION. Page 1
Natural Selection: its power compared with man's selection, its poweron characters of trifling importance, its power at all ages and onboth sexes.Sexual Selection.On the generality of intercrosses between individuals of the samespecies.Circumstances favourable and unfavourable to Natural Selection,namely, intercrossing, isolation, number of individuals.Slow action.Extinction caused by Natural Selection.Divergence of Character, related to the diversity of inhabitants ofany small area, and to naturalisation.Action of Natural Selection, through Divergence of Character andExtinction, on the descendants from a common parent.Explains the Grouping of all organic beings.
How will the struggle for existence, discussed too briefly in the lastchapter, act in regard to variation? Can the principle of selection,which we have seen is so potent in the hands of man, apply in nature?I think we shall see that it can act most effectually. Let it be bornein mind in what an endless number of strange peculiarities ourdomestic productions, and, in a lesser degree, those under nature,vary; and how strong the hereditary tendency is. Under domestication,it may be truly said that the whole organisation becomes in somedegree plastic. Let it be borne in mind how infinitely complex andclose-fitting are the mutual relations of all organic beings to eachother and to their physical conditions of life. Can it, then, bethought improbable, seeing that variations useful to man haveundoubtedly occurred, that other variations useful in some way to eachbeing in the great and complex battle of life, should sometimes occurin the course of thousands of generations? If such do occur, can wedoubt (remembering that many more individuals are born than canpossibly survive) that individuals having any advantage, howeverslight, over others, would have the best chance of surviving and ofprocreating their kind? On the other hand, we may feel sure that anyvariation in the least degree injurious would be rigidly destroyed.This preservation of favourable variations and the rejection ofinjurious variations, I call Natural Selection. Variations neitheruseful nor injurious would not be affected by natural selection, andwould be left a fluctuating element, as perhaps we see in the speciescalled polymorphic.
We shall best understand the probable course of natural selection bytaking the case of a country undergoing some physical change, forinstance, of climate. The proportional numbers of its inhabitantswould almost immediately undergo a change, and some species mightbecome extinct. We may conclude, from what we have seen of theintimate and complex manner in which the inhabitants of each countryare bound together, that any change in the numerical proportions ofsome of the inhabitants, independently of the change of climateitself, would most seriously affect many of the others. If the countrywere open on its borders, new forms would certainly immigrate, andthis also would seriously disturb the relations of some of the formerinhabitants. Let it be remembered how powerful the influence of asingle introduced tree or mammal has been shown to be. But in the caseof an island, or of a country partly surrounded by barriers, intowhich new and better adapted forms could not freely enter, we shouldthen have places in the economy of nature which would assuredly bebetter filled up, if some of the original inhabitants were in somemanner modified; for, had the area been open to immigration, thesesame places would have been seized on by intruders. In such case,every slight modification, which in the course of ages chanced toarise, and which in any way favoured the individuals of any of thespecies, by better adapting them to their altered conditions, wouldtend to be preserved; and natural selection would thus have free scopefor the work of improvement.
We have reason to believe, as stated in the first chapter, that achange in the conditions of life, by specially acting on thereproductive system, causes or increases variability; and in theforegoing case the conditions of life are supposed to have undergone achange, and this would manifestly be favourable to natural selection,by giving a better chance of profitable variations occurring; andunless profitable variations do occur, natural selection can donothing. Not that, as I believe, any extreme amount of variability isnecessary; as man can certainly produce great results by adding up inany given direction mere individual differences, so could Nature, butfar more easily, from having incomparably longer time at her disposal.Nor do I believe that any great physical change, as of climate, or anyunusual degree of isolation to check immigration, is actuallynecessary to produce new and unoccupied places for natural selectionto fill up by modifying and improving some of the varying inhabitants.For as all the inhabitants of each country are struggling togetherwith nicely balanced forces, extremely slight modifications in thestructure or habits of one inhabitant would often give it an advantageover others; and still further modifications of the same kind wouldoften still further increase the advantage. No country can be named inwhich all the native inhabitants are now so perfectly adapted to eachother and to the physical conditions under which they live, that noneof them could anyhow be improved; for in all countries, the nativeshave been so far conquered by naturalised productions, that they haveallowed foreigners to take firm possession of the land. And asforeigners have thus everywhere beaten some of the natives, we maysafely conclude that the natives might have been modified withadvantage, so as to have better resisted such intruders.
It may be said that natural selection is daily and hourlyscrutinising, throughout the world, every variation, even theslightest; rejecting that which is bad, preserving and adding up allthat is good; silently and insensibly working, whenever and whereveropportunity offers, at the improvement of each organic being inrelation to its organic and inorganic conditions of life. We seenothing of these slow changes in progress, until the hand of time hasmarked the long lapse of ages, and then so imperfect is our view intolong past geological ages, that we only see that the forms of life arenow different from what they formerly were.
Although natural selection can act only through and for the good ofeach being, yet characters and structures, which we are apt toconsider as of very trifling importance, may thus be acted on. When wesee leaf-eating insects green, and bark-feeders mottled-grey; thealpine ptarmigan white in winter, the red-grouse the colour ofheather, and the black-grouse that of peaty earth, we must believethat these tints are of service to these birds and insects inpreserving them from danger. Grouse, if not destroyed at some periodof their lives, would increase in countless numbers; they are known tosuffer largely from birds of prey; and hawks are guided by eyesight totheir prey,--so much so, that on parts of the Continent persons arewarned not to keep white pigeons, as being the most liable todestruction. Hence I can see no reason to doubt that natural selectionmight be most effective in giving the proper colour to each kind ofgrouse, and in keeping that colour, when once acquired, true andconstant. Nor ought we to think that the occasional destruction of ananimal of any particular colour would produce little effect: we shouldremember how essential it is in a flock of white sheep to destroyevery lamb with the faintest trace of black. In plants the down on thefruit and the colour of the flesh are considered by botanists ascharacters of the most trifling importance: yet we hear from anexcellent horticulturist, Downing, that in the United Statessmooth-skinned fruits suffer far more from a beetle, a curculio, thanthose with down; that purple plums suffer far more from a certaindisease than yellow plums; whereas another disease attacksyellow-fleshed peaches far more than those with other coloured flesh.If, with all the aids of art, these slight differences make a greatdifference in cultivating the several varieties, assuredly, in a stateof nature, where the trees would have to struggle with other trees andwith a host of enemies, such differences would effectually settlewhich variety, whether a smooth or downy, a yellow or purple fleshedfruit, should succeed.
In looking at many small points of difference between species, which,as far as our ignorance permits us to judge, seem to be quiteunimportant, we must not forget that climate, food, etc., probablyproduce some slight and direct effect. It is, however, far morenecessary to bear in mind that there are many unknown laws ofcorrelation of growth, which, when one part of the organisation ismodified through variation, and the modifications are accumulated bynatural selection for the good of the being, will cause othermodifications, often of the most unexpected nature.
As we see that those variations which under domestication appear atany particular period of life, tend to reappear in the offspring atthe same period;--for instance, in the seeds of the many varieties ofour culinary and agricultural plants; in the caterpillar and cocoonstages of the varieties of the silkworm; in the eggs of poultry, andin the colour of the down of their chickens; in the horns of our sheepand cattle when nearly adult;--so in a state of nature, naturalselection will be enabled to act on and modify organic beings at anyage, by the accumulation of profitable variations at that age, and bytheir inheritance at a corresponding age. If it profit a plant to haveits seeds more and more widely disseminated by the wind, I can see nogreater difficulty in this being effected through natural selection,than in the cotton-planter increasing and improving by selection thedown in the pods on his cotton-trees. Natural selection may modify andadapt the larva of an insect to a score of contingencies, whollydifferent from those which concern the mature insect. Thesemodifications will no doubt affect, through the laws of correlation,the structure of the adult; and probably in the case of those insectswhich live only for a few hours, and which never feed, a large part oftheir structure is merely the correlated result of successive changesin the structure of their larvae. So, conversely, modifications in theadult will probably often affect the structure of the larva; but inall cases natural selection will ensure that modifications consequenton other modifications at a different period of life, shall not be inthe least degree injurious: for if they became so, they would causethe extinction of the species.
Natural selection will modify the structure of the young in relationto the parent, and of the parent in relation to the young. In socialanimals it will adapt the structure of each individual for the benefitof the community; if each in consequence profits by the selectedchange. What natural selection cannot do, is to modify the structureof one species, without giving it any advantage, for the good ofanother species; and though statements to this effect may be found inworks of natural history, I cannot find one case which will bearinvestigation. A structure used only once in an animal's whole life,if of high importance to it, might be modified to any extent bynatural selection; for instance, the great jaws possessed by certaininsects, and used exclusively for opening the cocoon--or the hard tipto the beak of nestling birds, used for breaking the egg. It has beenasserted, that of the best short-beaked tumbler-pigeons more perish inthe egg than are able to get out of it; so that fanciers assist in theact of hatching. Now, if nature had to make the beak of a full-grownpigeon very short for the bird's own advantage, the process ofmodification would be very slow, and there would be simultaneously themost rigorous selection of the young birds within the egg, which hadthe most powerful and hardest beaks, for all with weak beaks wouldinevitably perish: or, more delicate and more easily broken shellsmight be selected, the thickness of the shell being known to vary likeevery other structure.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sexand become hereditarily attached to that sex, the same fact probablyoccurs under nature, and if so, natural selection will be able tomodify one sex in its functional relations to the other sex, or inrelation to wholly different habits of life in the two sexes, as issometimes the case with insects. And this leads me to say a few wordson what I call Sexual Selection. This depends, not on a struggle forexistence, but on a struggle between the males for possession of thefemales; the result is not death to the unsuccessful competitor, butfew or no offspring. Sexual selection is, therefore, less rigorousthan natural selection. Generally, the most vigorous males, thosewhich are best fitted for their places in nature, will leave mostprogeny. But in many cases, victory will depend not on general vigour,but on having special weapons, confined to the male sex. A hornlessstag or spurless cock would have a poor chance of leaving offspring.Sexual selection by always allowing the victor to breed might surelygive indomitable courage, length to the spur, and strength to the wingto strike in the spurred leg, as well as the brutal cock-fighter, whoknows well that he can improve his breed by careful selection of thebest cocks. How low in the scale of nature this law of battledescends, I know not; male alligators have been described as fighting,bellowing, and whirling round, like Indians in a war-dance, for thepossession of the females; male salmons have been seen fighting allday long; male stag-beetles often bear wounds from the huge mandiblesof other males. The war is, perhaps, severest between the males ofpolygamous animals, and these seem oftenest provided with specialweapons. The males of carnivorous animals are already well armed;though to them and to others, special means of defence may be giventhrough means of sexual selection, as the mane to the lion, theshoulder-pad to the boar, and the hooked jaw to the male salmon; forthe shield may be as important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character. Allthose who have attended to the subject, believe that there is theseverest rivalry between the males of many species to attract bysinging the females. The rock-thrush of Guiana, birds of Paradise, andsome others, congregate; and successive males display their gorgeousplumage and perform strange antics before the females, which standingby as spectators, at last choose the most attractive partner. Thosewho have closely attended to birds in confinement well know that theyoften take individual preferences and dislikes: thus Sir R. Heron hasdescribed how one pied peacock was eminently attractive to all his henbirds. It may appear childish to attribute any effect to suchapparently weak means: I cannot here enter on the details necessary tosupport this view; but if man can in a short time give elegantcarriage and beauty to his bantams, according to his standard ofbeauty, I can see no good reason to doubt that female birds, byselecting, during thousands of generations, the most melodious orbeautiful males, according to their standard of beauty, might producea marked effect. I strongly suspect that some well-known laws withrespect to the plumage of male and female birds, in comparison withthe plumage of the young, can be explained on the view of plumagehaving been chiefly modified by sexual selection, acting when thebirds have come to the breeding age or during the breeding season; themodifications thus produced being inherited at corresponding ages orseasons, either by the males alone, or by the males and females; but Ihave not space here to enter on this subject.
Thosewho have closely attended to birds in confinement well know that theyoften take individual preferences and dislikes: thus Sir R. Heron hasdescribed how one pied peacock was eminently attractive to all his henbirds. It !
Thus it is, as I believe, that when the males and females of anyanimal have the same general habits of life, but differ in structure,colour, or ornament, such differences have been mainly caused bysexual selection; that is, individual males have had, in successivegenerations, some slight advantage over other males, in their weapons,means of defence, or charms; and have transmitted these advantages totheir male offspring. Yet, I would not wish to attribute all suchsexual differences to this agency: for we see peculiarities arisingand becoming attached to the male sex in our domestic animals (as thewattle in male carriers, horn-like protuberances in the cocks ofcertain fowls, etc.), which we cannot believe to be either useful tothe males in battle, or attractive to the females. We see analogouscases under nature, for instance, the tuft of hair on the breast ofthe turkey-cock, which can hardly be either useful or ornamental tothis bird;--indeed, had the tuft appeared under domestication, itwould have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.
In order to make it clear how, as I believe, natural selection acts, Imust beg permission to give one or two imaginary illustrations. Let ustake the case of a wolf, which preys on various animals, securing someby craft, some by strength, and some by fleetness; and let us supposethat the fleetest prey, a deer for instance, had from any change inthe country increased in numbers, or that other prey had decreased innumbers, during that season of the year when the wolf is hardestpressed for food. I can under such circumstances see no reason todoubt that the swiftest and slimmest wolves would have the best chanceof surviving, and so be preserved or selected,--provided always thatthey retained strength to master their prey at this or at some otherperiod of the year, when they might be compelled to prey on otheranimals. I can see no more reason to doubt this, than that man canimprove the fleetness of his greyhounds by careful and methodicalselection, or by that unconscious selection which results from eachman trying to keep the best dogs without any thought of modifying thebreed.