物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 3. STRUGGLE FOR EXISTENCE. Page 1
Bears on natural selection.The term used in a wide sense.Geometrical powers of increase.Rapid increase of naturalised animals and plants.Nature of the checks to increase.Competition universal.Effects of climate.Protection from the number of individuals.Complex relations of all animals and plants throughout nature.Struggle for life most severe between individuals and varieties of thesame species; often severe between species of the same genus.The relation of organism to organism the most important of allrelations.
Before entering on the subject of this chapter, I must make a fewpreliminary remarks, to show how the struggle for existence bears onNatural Selection. It has been seen in the last chapter that amongstorganic beings in a state of nature there is some individualvariability; indeed I am not aware that this has ever been disputed.It is immaterial for us whether a multitude of doubtful forms becalled species or sub-species or varieties; what rank, for instance,the two or three hundred doubtful forms of British plants are entitledto hold, if the existence of any well-marked varieties be admitted.But the mere existence of individual variability and of some fewwell-marked varieties, though necessary as the foundation for thework, helps us but little in understanding how species arise innature. How have all those exquisite adaptations of one part of theorganisation to another part, and to the conditions of life, and ofone distinct organic being to another being, been perfected? We seethese beautiful co-adaptations most plainly in the woodpecker andmissletoe; and only a little less plainly in the humblest parasitewhich clings to the hairs of a quadruped or feathers of a bird; in thestructure of the beetle which dives through the water; in the plumedseed which is wafted by the gentlest breeze; in short, we seebeautiful adaptations everywhere and in every part of the organicworld.
Again, it may be asked, how is it that varieties, which I have calledincipient species, become ultimately converted into good and distinctspecies, which in most cases obviously differ from each other far morethan do the varieties of the same species? How do those groups ofspecies, which constitute what are called distinct genera, and whichdiffer from each other more than do the species of the same genus,arise? All these results, as we shall more fully see in the nextchapter, follow inevitably from the struggle for life. Owing to thisstruggle for life, any variation, however slight and from whatevercause proceeding, if it be in any degree profitable to an individualof any species, in its infinitely complex relations to other organicbeings and to external nature, will tend to the preservation of thatindividual, and will generally be inherited by its offspring. Theoffspring, also, will thus have a better chance of surviving, for, ofthe many individuals of any species which are periodically born, but asmall number can survive. I have called this principle, by which eachslight variation, if useful, is preserved, by the term of NaturalSelection, in order to mark its relation to man's power of selection.We have seen that man by selection can certainly produce greatresults, and can adapt organic beings to his own uses, through theaccumulation of slight but useful variations, given to him by the handof Nature. But Natural Selection, as we shall hereafter see, is apower incessantly ready for action, and is as immeasurably superior toman's feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle forexistence. In my future work this subject shall be treated, as it welldeserves, at much greater length. The elder De Candolle and Lyell havelargely and philosophically shown that all organic beings are exposedto severe competition. In regard to plants, no one has treated thissubject with more spirit and ability than W. Herbert, Dean ofManchester, evidently the result of his great horticultural knowledge.Nothing is easier than to admit in words the truth of the universalstruggle for life, or more difficult--at least I have found itso--than constantly to bear this conclusion in mind. Yet unless it bethoroughly engrained in the mind, I am convinced that the wholeeconomy of nature, with every fact on distribution, rarity, abundance,extinction, and variation, will be dimly seen or quite misunderstood.We behold the face of nature bright with gladness, we often seesuperabundance of food; we do not see, or we forget, that the birdswhich are idly singing round us mostly live on insects or seeds, andare thus constantly destroying life; or we forget how largely thesesongsters, or their eggs, or their nestlings, are destroyed by birdsand beasts of prey; we do not always bear in mind, that though foodmay be now superabundant, it is not so at all seasons of eachrecurring year.
I should premise that I use the term Struggle for Existence in a largeand metaphorical sense, including dependence of one being on another,and including (which is more important) not only the life of theindividual, but success in leaving progeny. Two canine animals in atime of dearth, may be truly said to struggle with each other whichshall get food and live. But a plant on the edge of a desert is saidto struggle for life against the drought, though more properly itshould be said to be dependent on the moisture. A plant which annuallyproduces a thousand seeds, of which on an average only one comes tomaturity, may be more truly said to struggle with the plants of thesame and other kinds which already clothe the ground. The missletoe isdependent on the apple and a few other trees, but can only in afar-fetched sense be said to struggle with these trees, for if toomany of these parasites grow on the same tree, it will languish anddie. But several seedling missletoes, growing close together on thesame branch, may more truly be said to struggle with each other. Asthe missletoe is disseminated by birds, its existence depends onbirds; and it may metaphorically be said to struggle with otherfruit-bearing plants, in order to tempt birds to devour and thusdisseminate its seeds rather than those of other plants. In theseseveral senses, which pass into each other, I use for convenience sakethe general term of struggle for existence.
A struggle for existence inevitably follows from the high rate atwhich all organic beings tend to increase. Every being, which duringits natural lifetime produces several eggs or seeds, must sufferdestruction during some period of its life, and during some season oroccasional year, otherwise, on the principle of geometrical increase,its numbers would quickly become so inordinately great that no countrycould support the product. Hence, as more individuals are producedthan can possibly survive, there must in every case be a struggle forexistence, either one individual with another of the same species, orwith the individuals of distinct species, or with the physicalconditions of life. It is the doctrine of Malthus applied withmanifold force to the whole animal and vegetable kingdoms; for in thiscase there can be no artificial increase of food, and no prudentialrestraint from marriage. Although some species may be now increasing,more or less rapidly, in numbers, all cannot do so, for the worldwould not hold them.
by the progeny of a single pair. Even slow-breedingman has doubled in twenty-five years, and at this rate, in a fewthousand years, there would literally not be standing room for hisprogeny. Linnaeus has.
There is no exception to the rule that every organic being naturallyincreases at so high a rate, that if not destroyed, the earth wouldsoon be covered by the progeny of a single pair. Even slow-breedingman has doubled in twenty-five years, and at this rate, in a fewthousand years, there would literally not be standing room for hisprogeny. Linnaeus has calculated that if an annual plant produced onlytwo seeds--and there is no plant so unproductive as this--and theirseedlings next year produced two, and so on, then in twenty yearsthere would be a million plants. The elephant is reckoned to be theslowest breeder of all known animals, and I have taken some pains toestimate its probable minimum rate of natural increase: it will beunder the mark to assume that it breeds when thirty years old, andgoes on breeding till ninety years old, bringing forth three pair ofyoung in this interval; if this be so, at the end of the fifth centurythere would be alive fifteen million elephants, descended from thefirst pair.
But we have better evidence on this subject than mere theoreticalcalculations, namely, the numerous recorded cases of the astonishinglyrapid increase of various animals in a state of nature, whencircumstances have been favourable to them during two or threefollowing seasons. Still more striking is the evidence from ourdomestic animals of many kinds which have run wild in several parts ofthe world: if the statements of the rate of increase of slow-breedingcattle and horses in South America, and latterly in Australia, had notbeen well authenticated, they would have been quite incredible. So itis with plants: cases could be given of introduced plants which havebecome common throughout whole islands in a period of less than tenyears. Several of the plants now most numerous over the wide plains ofLa Plata, clothing square leagues of surface almost to the exclusionof all other plants, have been introduced from Europe; and there areplants which now range in India, as I hear from Dr. Falconer, fromCape Comorin to the Himalaya, which have been imported from Americasince its discovery. In such cases, and endless instances could begiven, no one supposes that the fertility of these animals or plantshas been suddenly and temporarily increased in any sensible degree.The obvious explanation is that the conditions of life have been veryfavourable, and that there has consequently been less destruction ofthe old and young, and that nearly all the young have been enabled tobreed. In such cases the geometrical ratio of increase, the result ofwhich never fails to be surprising, simply explains theextraordinarily rapid increase and wide diffusion of naturalisedproductions in their new homes.
In a state of nature almost every plant produces seed, and amongstanimals there are very few which do not annually pair. Hence we mayconfidently assert, that all plants and animals are tending toincrease at a geometrical ratio, that all would most rapidly stockevery station in which they could any how exist, and that thegeometrical tendency to increase must be checked by destruction atsome period of life. Our familiarity with the larger domestic animalstends, I think, to mislead us: we see no great destruction falling onthem, and we forget that thousands are annually slaughtered for food,and that in a state of nature an equal number would have somehow to bedisposed of.
The only difference between organisms which annually produce eggs orseeds by the thousand, and those which produce extremely few, is, thatthe slow-breeders would require a few more years to people, underfavourable conditions, a whole district, let it be ever so large. Thecondor lays a couple of eggs and the ostrich a score, and yet in thesame country the condor may be the more numerous of the two: theFulmar petrel lays but one egg, yet it is believed to be the mostnumerous bird in the world. One fly deposits hundreds of eggs, andanother, like the hippobosca, a single one; but this difference doesnot determine how many individuals of the two species can be supportedin a district. A large number of eggs is of some importance to thosespecies, which depend on a rapidly fluctuating amount of food, for itallows them rapidly to increase in number. But the real importance ofa large number of eggs or seeds is to make up for much destruction atsome period of life; and this period in the great majority of cases isan early one. If an animal can in any way protect its own eggs oryoung, a small number may be produced, and yet the average stock befully kept up; but if many eggs or young are destroyed, many must beproduced, or the species will become extinct. It would suffice to keepup the full number of a tree, which lived on an average for a thousandyears, if a single seed were produced once in a thousand years,supposing that this seed were never destroyed, and could be ensured togerminate in a fitting place. So that in all cases, the average numberof any animal or plant depends only indirectly on the number of itseggs or seeds.
In looking at Nature, it is most necessary to keep the foregoingconsiderations always in mind--never to forget that every singleorganic being around us may be said to be striving to the utmost toincrease in numbers; that each lives by a struggle at some period ofits life; that heavy destruction inevitably falls either on the youngor old, during each generation or at recurrent intervals. Lighten anycheck, mitigate the destruction ever so little, and the number of thespecies will almost instantaneously increase to any amount. The faceof Nature may be compared to a yielding surface, with ten thousandsharp wedges packed close together and driven inwards by incessantblows, sometimes one wedge being struck, and then another with greaterforce.
What checks the natural tendency of each species to increase in numberis most obscure. Look at the most vigorous species; by as much as itswarms in numbers, by so much will its tendency to increase be stillfurther increased. We know not exactly what the checks are in even onesingle instance. Nor will this surprise any one who reflects howignorant we are on this head, even in regard to mankind, soincomparably better known than any other animal. This subject has beenably treated by several authors, and I shall, in my future work,discuss some of the checks at considerable length, more especially inregard to the feral animals of South America. Here I will make only afew remarks, just to recall to the reader's mind some of the chiefpoints. Eggs or very young animals seem generally to suffer most, butthis is not invariably the case. With plants there is a vastdestruction of seeds, but, from some observations which I have made, Ibelieve that it is the seedlings which suffer most from germinating inground already thickly stocked with other plants. Seedlings, also, aredestroyed in vast numbers by various enemies; for instance, on a pieceof ground three feet long and two wide, dug and cleared, and wherethere could be no choking from other plants, I marked all theseedlings of our native weeds as they came up, and out of the 357 noless than 295 were destroyed, chiefly by slugs and insects. If turfwhich has long been mown, and the case would be the same with turfclosely browsed by quadrupeds, be let to grow, the more vigorousplants gradually kill the less vigorous, though fully grown, plants:thus out of twenty species growing on a little plot of turf (threefeet by four) nine species perished from the other species beingallowed to grow up freely.
The amount of food for each species of course gives the extreme limitto which each can increase; but very frequently it is not theobtaining food, but the serving as prey to other animals, whichdetermines the average numbers of a species. Thus, there seems to belittle doubt that the stock of partridges, grouse, and hares on anylarge estate depends chiefly on the destruction of vermin. If not onehead of game were shot during the next twenty years in England, and,at the same time, if no vermin were destroyed, there would, in allprobability, be less game than at present, although hundreds ofthousands of game animals are now annually killed. On the other hand,in some cases, as with the elephant and rhinoceros, none are destroyedby beasts of prey: even the tiger in India most rarely dares to attacka young elephant protected by its dam.
Climate plays an important part in determining the average numbers ofa species, and periodical seasons of extreme cold or drought, Ibelieve to be the most effective of all checks. I estimated that thewinter of 1854-55 destroyed four-fifths of the birds in my owngrounds; and this is a tremendous destruction, when we remember thatten per cent. is an extraordinarily severe mortality from epidemicswith man. The action of climate seems at first sight to be quiteindependent of the struggle for existence; but in so far as climatechiefly acts in reducing food, it brings on the most severe strugglebetween the individuals, whether of the same or of distinct species,which subsist on the same kind of food. Even when climate, forinstance extreme cold, acts directly, it will be the least vigorous,or those which have got least food through the advancing winter, whichwill suffer most. When we travel from south to north, or from a dampregion to a dry, we invariably see some species gradually gettingrarer and rarer, and finally disappearing; and the change of climatebeing conspicuous, we are tempted to attribute the whole effect to itsdirect action. But this is a very false view: we forget that eachspecies, even where it most abounds, is constantly suffering enormousdestruction at some period of its life, from enemies or fromcompetitors for the same place and food; and if these enemies orcompetitors be in the least degree favoured by any slight change ofclimate, they will increase in numbers, and, as each area is alreadyfully stocked with inhabitants, the other species will decrease. Whenwe travel southward and see a species decreasing in numbers, we mayfeel sure that the cause lies quite as much in other species beingfavoured, as in this one being hurt. So it is when we travelnorthward, but in a somewhat lesser degree, for the number of speciesof all kinds, and therefore of competitors, decreases northwards;hence in going northward, or in ascending a mountain, we far oftenermeet with stunted forms, due to the DIRECTLY injurious action ofclimate, than we do in proceeding southwards or in descending amountain. When we reach the Arctic regions, or snow-capped summits, orabsolute deserts, the struggle for life is almost exclusively with theelements.
That climate acts in main part indirectly by favouring other species,we may clearly see in the prodigious number of plants in our gardenswhich can perfectly well endure our climate, but which never becomenaturalised, for they cannot compete with our native plants, norresist destruction by our native animals.
When a species, owing to highly favourable circumstances, increasesinordinately in numbers in a small tract, epidemics--at least, thisseems generally to occur with our game animals--often ensue: and herewe have a limiting check independent of the struggle for life. Buteven some of these so-called epidemics appear to be due to parasiticworms, which have from some cause, possibly in part through facilityof diffusion amongst the crowded animals, been disproportionablyfavoured: and here comes in a sort of struggle between the parasiteand its prey.
On the other hand, in many cases, a large stock of individuals of thesame species, relatively to the numbers of its enemies, is absolutelynecessary for its preservation. Thus we can easily raise plenty ofcorn and rape-seed, etc., in our fields, because the seeds are ingreat excess compared with the number of birds which feed on them; norcan the birds, though having a superabundance of food at this oneseason, increase in number proportionally to the supply of seed, astheir numbers are checked during winter: but any one who has tried,knows how troublesome it is to get seed from a few wheat or other suchplants in a garden; I have in this case lost every single seed. Thisview of the necessity of a large stock of the same species for itspreservation, explains, I believe, some singular facts in nature, suchas that of very rare plants being sometimes extremely abundant in thefew spots where they do occur; and that of some social plants beingsocial, that is, abounding in individuals, even on the extremeconfines of their range. For in such cases, we may believe, that aplant could exist only where the conditions of its life were sofavourable that many could exist together, and thus save each otherfrom utter destruction. I should add that the good effects of frequentintercrossing, and the ill effects of close interbreeding, probablycome into play in some of these cases; but on this intricate subject Iwill not here enlarge.