物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 4. NATURAL SELECTION. Page 3
It must have struck most naturalists as a strange anomaly that, in thecase of both animals and plants, species of the same family and evenof the same genus, though agreeing closely with each other in almosttheir whole organisation, yet are not rarely, some of themhermaphrodites, and some of them unisexual. But if, in fact, allhermaphrodites do occasionally intercross with other individuals, thedifference between hermaphrodites and unisexual species, as far asfunction is concerned, becomes very small.
From these several considerations and from the many special factswhich I have collected, but which I am not here able to give, I amstrongly inclined to suspect that, both in the vegetable and animalkingdoms, an occasional intercross with a distinct individual is a lawof nature. I am well aware that there are, on this view, many cases ofdifficulty, some of which I am trying to investigate. Finally then, wemay conclude that in many organic beings, a cross between twoindividuals is an obvious necessity for each birth; in many others itoccurs perhaps only at long intervals; but in none, as I suspect, canself-fertilisation go on for perpetuity.
CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.
This is an extremely intricate subject. A large amount of inheritableand diversified variability is favourable, but I believe mereindividual differences suffice for the work. A large number ofindividuals, by giving a better chance for the appearance within anygiven period of profitable variations, will compensate for a lesseramount of variability in each individual, and is, I believe, anextremely important element of success. Though nature grants vastperiods of time for the work of natural selection, she does not grantan indefinite period; for as all organic beings are striving, it maybe said, to seize on each place in the economy of nature, if any onespecies does not become modified and improved in a correspondingdegree with its competitors, it will soon be exterminated.
In man's methodical selection, a breeder selects for some definiteobject, and free intercrossing will wholly stop his work. But whenmany men, without intending to alter the breed, have a nearly commonstandard of perfection, and all try to get and breed from the bestanimals, much improvement and modification surely but slowly followfrom this unconscious process of selection, notwithstanding a largeamount of crossing with inferior animals. Thus it will be in nature;for within a confined area, with some place in its polity not soperfectly occupied as might be, natural selection will always tend topreserve all the individuals varying in the right direction, though indifferent degrees, so as better to fill up the unoccupied place. Butif the area be large, its several districts will almost certainlypresent different conditions of life; and then if natural selection bemodifying and improving a species in the several districts, there willbe intercrossing with the other individuals of the same species on theconfines of each. And in this case the effects of intercrossing canhardly be counterbalanced by natural selection always tending tomodify all the individuals in each district in exactly the same mannerto the conditions of each; for in a continuous area, the conditionswill generally graduate away insensibly from one district to another.The intercrossing will most affect those animals which unite for eachbirth, which wander much, and which do not breed at a very quick rate.Hence in animals of this nature, for instance in birds, varieties willgenerally be confined to separated countries; and this I believe to bethe case. In hermaphrodite organisms which cross only occasionally,and likewise in animals which unite for each birth, but which wanderlittle and which can increase at a very rapid rate, a new and improvedvariety might be quickly formed on any one spot, and might theremaintain itself in a body, so that whatever intercrossing took placewould be chiefly between the individuals of the same new variety. Alocal variety when once thus formed might subsequently slowly spreadto other districts. On the above principle, nurserymen always prefergetting seed from a large body of plants of the same variety, as thechance of intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth,we must not overrate the effects of intercrosses in retarding naturalselection; for I can bring a considerable catalogue of facts, showingthat within the same area, varieties of the same animal can longremain distinct, from haunting different stations, from breeding atslightly different seasons, or from varieties of the same kindpreferring to pair together.
Intercrossing plays a very important part in nature in keeping theindividuals of the same species, or of the same variety, true anduniform in character. It will obviously thus act far more efficientlywith those animals which unite for each birth; but I have alreadyattempted to show that we have reason to believe that occasionalintercrosses take place with all animals and with all plants. Even ifthese take place only at long intervals, I am convinced that the youngthus produced will gain so much in vigour and fertility over theoffspring from long-continued self-fertilisation, that they will havea better chance of surviving and propagating their kind; and thus, inthe long run, the influence of intercrosses, even at rare intervals,will be great. If there exist organic beings which never intercross,uniformity of character can be retained amongst them, as long as theirconditions of life remain the same, only through the principle ofinheritance, and through natural selection destroying any which departfrom the proper type; but if their conditions of life change and theyundergo modification, uniformity of character can be given to theirmodified offspring, solely by natural selection preserving the samefavourable variations.
Isolation, also, is an important element in the process of naturalselection. In a confined or isolated area, if not very large, theorganic and inorganic conditions of life will generally be in a greatdegree uniform; so that natural selection will tend to modify all theindividuals of a varying species throughout the area in the samemanner in relation to the same conditions. Intercrosses, also, withthe individuals of the same species, which otherwise would haveinhabited the surrounding and differently circumstanced districts,will be prevented. But isolation probably acts more efficiently inchecking the immigration of better adapted organisms, after anyphysical change, such as of climate or elevation of the land, etc.;and thus new places in the natural economy of the country are leftopen for the old inhabitants to struggle for, and become adapted to,through modifications in their structure and constitution. Lastly,isolation, by checking immigration and consequently competition, willgive time for any new variety to be slowly improved; and this maysometimes be of importance in the production of new species. If,however, an isolated area be very small, either from being surroundedby barriers, or from having very peculiar physical conditions, thetotal number of the individuals supported on it will necessarily bevery small; and fewness of individuals will greatly retard theproduction of new species through natural selection, by decreasing thechance of the appearance of favourable variations.
If we turn to nature to test the truth of these remarks, and look atany small isolated area, such as an oceanic island, although the totalnumber of the species inhabiting it, will be found to be small, as weshall see in our chapter on geographical distribution; yet of thesespecies a very large proportion are endemic,--that is, have beenproduced there, and nowhere else. Hence an oceanic island at firstsight seems to have been highly favourable for the production of newspecies. But we may thus greatly deceive ourselves, for to ascertainwhether a small isolated area, or a large open area like a continent,has been most favourable for the production of new organic forms, weought to make the comparison within equal times; and this we areincapable of doing.
Although I do not doubt that isolation is of considerable importancein the production of new species, on the whole I am inclined tobelieve that largeness of area is of more importance, more especiallyin the production of species, which will prove capable of enduring fora long period, and of spreading widely. Throughout a great and openarea, not only will there be a better chance of favourable variationsarising from the large number of individuals of the same species theresupported, but the conditions of life are infinitely complex from thelarge number of already existing species; and if some of these manyspecies become modified and improved, others will have to be improvedin a corresponding degree or they will be exterminated. Each new form,also, as soon as it has been much improved, will be able to spreadover the open and continuous area, and will thus come into competitionwith many others. Hence more new places will be formed, and thecompetition to fill them will be more severe, on a large than on asmall and isolated area. Moreover, great areas, though now continuous,owing to oscillations of level, will often have recently existed in abroken condition, so that the good effects of isolation willgenerally, to a certain extent, have concurred. Finally, I concludethat, although small isolated areas probably have been in somerespects highly favourable for the production of new species, yet thatthe course of modification will generally have been more rapid onlarge areas; and what is more important, that the new forms producedon large areas, which already have been victorious over manycompetitors, will be those that will spread most widely, will giverise to most new varieties and species, and will thus play animportant part in the changing history of the organic world.
We can, perhaps, on these views, understand some facts which will beagain alluded to in our chapter on geographical distribution; forinstance, that the productions of the smaller continent of Australiahave formerly yielded, and apparently are now yielding, before thoseof the larger Europaeo-Asiatic area. Thus, also, it is thatcontinental productions have everywhere become so largely naturalisedon islands. On a small island, the race for life will have been lesssevere, and there will have been less modification and lessextermination. Hence, perhaps, it comes that the flora of Madeira,according to Oswald Heer, resembles the extinct tertiary flora ofEurope. All fresh-water basins, taken together, make a small areacompared with that of the sea or of the land; and, consequently, thecompetition between fresh-water productions will have been less severethan elsewhere; new forms will have been more slowly formed, and oldforms more slowly exterminated. And it is in fresh water that we findseven genera of Ganoid fishes, remnants of a once preponderant order:and in fresh water we find some of the most anomalous forms now knownin the world, as the Ornithorhynchus and Lepidosiren, which, likefossils, connect to a certain extent orders now widely separated inthe natural scale. These anomalous forms may almost be called livingfossils; they have endured to the present day, from having inhabited aconfined area, and from having thus been exposed to less severecompetition.
To sum up the circumstances favourable and unfavourable to naturalselection, as far as the extreme intricacy of the subject permits. Iconclude, looking to the future, that for terrestrial productions alarge continental area, which will probably undergo many oscillationsof level, and which consequently will exist for long periods in abroken condition, will be the most favourable for the production ofmany new forms of life, likely to endure long and to spread widely.For the area will first have existed as a continent, and theinhabitants, at this period numerous in individuals and kinds, willhave been subjected to very severe competition. When converted bysubsidence into large separate islands, there will still exist manyindividuals of the same species on each island: intercrossing on theconfines of the range of each species will thus be checked: afterphysical changes of any kind, immigration will be prevented, so thatnew places in the polity of each island will have to be filled up bymodifications of the old inhabitants; and time will be allowed for thevarieties in each to become well modified and perfected. When, byrenewed elevation, the islands shall be re-converted into acontinental area, there will again be severe competition: the mostfavoured or improved varieties will be enabled to spread: there willbe much extinction of the less improved forms, and the relativeproportional numbers of the various inhabitants of the renewedcontinent will again be changed; and again there will be a fair fieldfor natural selection to improve still further the inhabitants, andthus produce new species.
That natural selection will always act with extreme slowness, I fullyadmit. Its action depends on there being places in the polity ofnature, which can be better occupied by some of the inhabitants of thecountry undergoing modification of some kind. The existence of suchplaces will often depend on physical changes, which are generally veryslow, and on the immigration of better adapted forms having beenchecked. But the action of natural selection will probably stilloftener depend on some of the inhabitants becoming slowly modified;the mutual relations of many of the other inhabitants being thusdisturbed. Nothing can be effected, unless favourable variationsoccur, and variation itself is apparently always a very slow process.The process will often be greatly retarded by free intercrossing. Manywill exclaim that these several causes are amply sufficient wholly tostop the action of natural selection. I do not believe so. On theother hand, I do believe that natural selection will always act veryslowly, often only at long intervals of time, and generally on only avery few of the inhabitants of the same region at the same time. Ifurther believe, that this very slow, intermittent action of naturalselection accords perfectly well with what geology tells us of therate and manner at which the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble man can do muchby his powers of artificial selection, I can see no limit to theamount of change, to the beauty and infinite complexity of thecoadaptations between all organic beings, one with another and withtheir physical conditions of life, which may be effected in the longcourse of time by nature's power of selection.
EXTINCTION.
This subject will be more fully discussed in our chapter on Geology;but it must be here alluded to from being intimately connected withnatural selection. Natural selection acts solely through thepreservation of variations in some way advantageous, whichconsequently endure. But as from the high geometrical powers ofincrease of all organic beings, each area is already fully stockedwith inhabitants, it follows that as each selected and favoured formincreases in number, so will the less favoured forms decrease andbecome rare. Rarity, as geology tells us, is the precursor toextinction. We can, also, see that any form represented by fewindividuals will, during fluctuations in the seasons or in the numberof its enemies, run a good chance of utter extinction. But we may gofurther than this; for as new forms are continually and slowly beingproduced, unless we believe that the number of specific forms goes onperpetually and almost indefinitely increasing, numbers inevitablymust become extinct. That the number of specific forms has notindefinitely increased, geology shows us plainly; and indeed we cansee reason why they should not have thus increased, for the number ofplaces in the polity of nature is not indefinitely great,--not that wehave any means of knowing that any one region has as yet got itsmaximum of species. Probably no region is as yet fully stocked, for atthe Cape of Good Hope, where more species of plants are crowdedtogether than in any other quarter of the world, some foreign plantshave become naturalised, without causing, as far as we know, theextinction of any natives.
Furthermore, the species which are most numerous in individuals willhave the best chance of producing within any given period favourablevariations. We have evidence of this, in the facts given in the secondchapter, showing that it is the common species which afford thegreatest number of recorded varieties, or incipient species. Hence,rare species will be less quickly modified or improved within anygiven period, and they will consequently be beaten in the race forlife by the modified descendants of the commoner species.
From these several considerations I think it inevitably follows, thatas new species in the course of time are formed through naturalselection, others will become rarer and rarer, and finally extinct.The forms which stand in closest competition with those undergoingmodification and improvement, will naturally suffer most. And we haveseen in the chapter on the Struggle for Existence that it is the mostclosely-allied forms,--varieties of the same species, and species ofthe same genus or of related genera,--which, from having nearly thesame structure, constitution, and habits, generally come into theseverest competition with each other. Consequently, each new varietyor species, during the progress of its formation, will generally presshardest on its nearest kindred, and tend to exterminate them. We seethe same process of extermination amongst our domesticatedproductions, through the selection of improved forms by man. Manycurious instances could be given showing how quickly new breeds ofcattle, sheep, and other animals, and varieties of flowers, take theplace of older and inferior kinds. In Yorkshire, it is historicallyknown that the ancient black cattle were displaced by the long-horns,and that these "were swept away by the short-horns" (I quote the wordsof an agricultural writer) "as if by some murderous pestilence."
DIVERGENCE OF CHARACTER.
The principle, which I have designated by this term, is of highimportance on my theory, and explains, as I believe, several importantfacts. In the first place, varieties, even strongly-marked ones,though having somewhat of the character of species--as is shown by thehopeless doubts in many cases how to rank them--yet certainly differfrom each other far less than do good and distinct species.Nevertheless, according to my view, varieties are species in theprocess of formation, or are, as I have called them, incipientspecies. How, then, does the lesser difference between varietiesbecome augmented into the greater difference between species? Thatthis does habitually happen, we must infer from most of theinnumerable species throughout nature presenting well-markeddifferences; whereas varieties, the supposed prototypes and parents offuture well-marked species, present slight and ill-defineddifferences. Mere chance, as we may call it, might cause one varietyto differ in some character from its parents, and the offspring ofthis variety again to differ from its parent in the very samecharacter and in a greater degree; but this alone would never accountfor so habitual and large an amount of difference as that betweenvarieties of the same species and species of the same genus.