物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 4. NATURAL SELECTION. Page 4
As has always been my practice, let us seek light on this head fromour domestic productions. We shall here find something analogous. Afancier is struck by a pigeon having a slightly shorter beak; anotherfancier is struck by a pigeon having a rather longer beak; and on theacknowledged principle that "fanciers do not and will not admire amedium standard, but like extremes," they both go on (as has actuallyoccurred with tumbler-pigeons) choosing and breeding from birds withlonger and longer beaks, or with shorter and shorter beaks. Again, wemay suppose that at an early period one man preferred swifter horses;another stronger and more bulky horses. The early differences would bevery slight; in the course of time, from the continued selection ofswifter horses by some breeders, and of stronger ones by others, thedifferences would become greater, and would be noted as forming twosub-breeds; finally, after the lapse of centuries, the sub-breedswould become converted into two well-established and distinct breeds.As the differences slowly become greater, the inferior animals withintermediate characters, being neither very swift nor very strong,will have been neglected, and will have tended to disappear. Here,then, we see in man's productions the action of what may be called theprinciple of divergence, causing differences, at first barelyappreciable, steadily to increase, and the breeds to diverge incharacter both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature?I believe it can and does apply most efficiently, from the simplecircumstance that the more diversified the descendants from any onespecies become in structure, constitution, and habits, by so much willthey be better enabled to seize on many and widely diversified placesin the polity of nature, and so be enabled to increase in numbers.
We can clearly see this in the case of animals with simple habits.Take the case of a carnivorous quadruped, of which the number that canbe supported in any country has long ago arrived at its full average.If its natural powers of increase be allowed to act, it can succeed inincreasing (the country not undergoing any change in its conditions)only by its varying descendants seizing on places at present occupiedby other animals: some of them, for instance, being enabled to feed onnew kinds of prey, either dead or alive; some inhabiting new stations,climbing trees, frequenting water, and some perhaps becoming lesscarnivorous. The more diversified in habits and structure thedescendants of our carnivorous animal became, the more places theywould be enabled to occupy. What applies to one animal will applythroughout all time to all animals--that is, if they vary--forotherwise natural selection can do nothing. So it will be with plants.It has been experimentally proved, that if a plot of ground be sownwith one species of grass, and a similar plot be sown with severaldistinct genera of grasses, a greater number of plants and a greaterweight of dry herbage can thus be raised. The same has been found tohold good when first one variety and then several mixed varieties ofwheat have been sown on equal spaces of ground. Hence, if any onespecies of grass were to go on varying, and those varieties werecontinually selected which differed from each other in at all the samemanner as distinct species and genera of grasses differ from eachother, a greater number of individual plants of this species of grass,including its modified descendants, would succeed in living on thesame piece of ground. And we well know that each species and eachvariety of grass is annually sowing almost countless seeds; and thus,as it may be said, is striving its utmost to increase its numbers.Consequently, I cannot doubt that in the course of many thousands ofgenerations, the most distinct varieties of any one species of grasswould always have the best chance of succeeding and of increasing innumbers, and thus of supplanting the less distinct varieties; andvarieties, when rendered very distinct from each other, take the rankof species.
The truth of the principle, that the greatest amount of life can besupported by great diversification of structure, is seen under manynatural circumstances. In an extremely small area, especially iffreely open to immigration, and where the contest between individualand individual must be severe, we always find great diversity in itsinhabitants. For instance, I found that a piece of turf, three feet byfour in size, which had been exposed for many years to exactly thesame conditions, supported twenty species of plants, and thesebelonged to eighteen genera and to eight orders, which shows how muchthese plants differed from each other. So it is with the plants andinsects on small and uniform islets; and so in small ponds of freshwater. Farmers find that they can raise most food by a rotation ofplants belonging to the most different orders: nature follows what maybe called a simultaneous rotation. Most of the animals and plantswhich live close round any small piece of ground, could live on it(supposing it not to be in any way peculiar in its nature), and may besaid to be striving to the utmost to live there; but, it is seen, thatwhere they come into the closest competition with each other, theadvantages of diversification of structure, with the accompanyingdifferences of habit and constitution, determine that the inhabitants,which thus jostle each other most closely, shall, as a general rule,belong to what we call different genera and orders.
The same principle is seen in the naturalisation of plants throughman's agency in foreign lands. It might have been expected that theplants which have succeeded in becoming naturalised in any land wouldgenerally have been closely allied to the indigenes; for these arecommonly looked at as specially created and adapted for their owncountry. It might, also, perhaps have been expected that naturalisedplants would have belonged to a few groups more especially adapted tocertain stations in their new homes. But the case is very different;and Alph. De Candolle has well remarked in his great and admirablework, that floras gain by naturalisation, proportionally with thenumber of the native genera and species, far more in new genera thanin new species. To give a single instance: in the last edition of Dr.Asa Gray's 'Manual of the Flora of the Northern United States,' 260naturalised plants are enumerated, and these belong to 162 genera. Wethus see that these naturalised plants are of a highly diversifiednature. They differ, moreover, to a large extent from the indigenes,for out of the 162 genera, no less than 100 genera are not thereindigenous, and thus a large proportional addition is made to thegenera of these States.
By considering the nature of the plants or animals which havestruggled successfully with the indigenes of any country, and havethere become naturalised, we can gain some crude idea in what mannersome of the natives would have had to be modified, in order to havegained an advantage over the other natives; and we may, I think, atleast safely infer that diversification of structure, amounting to newgeneric differences, would have been profitable to them.
The advantage of diversification in the inhabitants of the same regionis, in fact, the same as that of the physiological division of labourin the organs of the same individual body--a subject so wellelucidated by Milne Edwards. No physiologist doubts that a stomach bybeing adapted to digest vegetable matter alone, or flesh alone, drawsmost nutriment from these substances. So in the general economy of anyland, the more widely and perfectly the animals and plants arediversified for different habits of life, so will a greater number ofindividuals be capable of there supporting themselves. A set ofanimals, with their organisation but little diversified, could hardlycompete with a set more perfectly diversified in structure. It may bedoubted, for instance, whether the Australian marsupials, which aredivided into groups differing but little from each other, and feeblyrepresenting, as Mr. Waterhouse and others have remarked, ourcarnivorous, ruminant, and rodent mammals, could successfully competewith these well-pronounced orders. In the Australian mammals, we seethe process of diversification in an early and incomplete stage ofdevelopment. After the foregoing discussion, which ought to have beenmuch amplified, we may, I think, assume that the modified descendantsof any one species will succeed by so much the better as they becomemore diversified in structure, and are thus enabled to encroach onplaces occupied by other beings. Now let us see how this principle ofgreat benefit being derived from divergence of character, combinedwith the principles of natural selection and of extinction, will tendto act.
The accompanying diagram will aid us in understanding this ratherperplexing subject. Let A to L represent the species of a genus largein its own country; these species are supposed to resemble each otherin unequal degrees, as is so generally the case in nature, and as isrepresented in the diagram by the letters standing at unequaldistances. I have said a large genus, because we have seen in thesecond chapter, that on an average more of the species of large generavary than of small genera; and the varying species of the large generapresent a greater number of varieties. We have, also, seen that thespecies, which are the commonest and the most widely-diffused, varymore than rare species with restricted ranges. Let (A) be a common,widely-diffused, and varying species, belonging to a genus large inits own country. The little fan of diverging dotted lines of unequallengths proceeding from (A), may represent its varying offspring. Thevariations are supposed to be extremely slight, but of the mostdiversified nature; they are not supposed all to appearsimultaneously, but often after long intervals of time; nor are theyall supposed to endure for equal periods. Only those variations whichare in some way profitable will be preserved or naturally selected.And here the importance of the principle of benefit being derived fromdivergence of character comes in; for this will generally lead to themost different or divergent variations (represented by the outerdotted lines) being preserved and accumulated by natural selection.When a dotted line reaches one of the horizontal lines, and is theremarked by a small numbered letter, a sufficient amount of variation issupposed to have been accumulated to have formed a fairly well-markedvariety, such as would be thought worthy of record in a systematicwork.
The intervals between the horizontal lines in the diagram, mayrepresent each a thousand generations; but it would have been betterif each had represented ten thousand generations. After a thousandgenerations, species (A) is supposed to have produced two fairlywell-marked varieties, namely a1 and m1. These two varieties willgenerally continue to be exposed to the same conditions which madetheir parents variable, and the tendency to variability is in itselfhereditary, consequently they will tend to vary, and generally to varyin nearly the same manner as their parents varied. Moreover, these twovarieties, being only slightly modified forms, will tend to inheritthose advantages which made their common parent (A) more numerous thanmost of the other inhabitants of the same country; they will likewisepartake of those more general advantages which made the genus to whichthe parent-species belonged, a large genus in its own country. Andthese circumstances we know to be favourable to the production of newvarieties.
If, then, these two varieties be variable, the most divergent of theirvariations will generally be preserved during the next thousandgenerations. And after this interval, variety a1 is supposed in thediagram to have produced variety a2, which will, owing to theprinciple of divergence, differ more from (A) than did variety a1.Variety m1 is supposed to have produced two varieties, namely m2 ands2, differing from each other, and more considerably from their commonparent (A). We may continue the process by similar steps for anylength of time; some of the varieties, after each thousandgenerations, producing only a single variety, but in a more and moremodified condition, some producing two or three varieties, and somefailing to produce any. Thus the varieties or modified descendants,proceeding from the common parent (A), will generally go on increasingin number and diverging in character. In the diagram the process isrepresented up to the ten-thousandth generation, and under a condensedand simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process evergoes on so regularly as is represented in the diagram, though initself made somewhat irregular. I am far from thinking that the mostdivergent varieties will invariably prevail and multiply: a mediumform may often long endure, and may or may not produce more than onemodified descendant; for natural selection will always act accordingto the nature of the places which are either unoccupied or notperfectly occupied by other beings; and this will depend on infinitelycomplex relations. But as a general rule, the more diversified instructure the descendants from any one species can be rendered, themore places they will be enabled to seize on, and the more theirmodified progeny will be increased. In our diagram the line ofsuccession is broken at regular intervals by small numbered lettersmarking the successive forms which have become sufficiently distinctto be recorded as varieties. But these breaks are imaginary, and mighthave been inserted anywhere, after intervals long enough to haveallowed the accumulation of a considerable amount of divergentvariation.
As all the modified descendants from a common and widely-diffusedspecies, belonging to a large genus, will tend to partake of the sameadvantages which made their parent successful in life, they willgenerally go on multiplying in number as well as diverging incharacter: this is represented in the diagram by the several divergentbranches proceeding from (A). The modified offspring from the laterand more highly improved branches in the lines of descent, will, it isprobable, often take the place of, and so destroy, the earlier andless improved branches: this is represented in the diagram by some ofthe lower branches not reaching to the upper horizontal lines. In somecases I do not doubt that the process of modification will be confinedto a single line of descent, and the number of the descendants willnot be increased; although the amount of divergent modification mayhave been increased in the successive generations. This case would berepresented in the diagram, if all the lines proceeding from (A) wereremoved, excepting that from a1 to a10. In the same way, for instance,the English race-horse and English pointer have apparently both goneon slowly diverging in character from their original stocks, withouteither having given off any fresh branches or races.
After ten thousand generations, species (A) is supposed to haveproduced three forms, a10, f10, and m10, which, from having divergedin character during the successive generations, will have come todiffer largely, but perhaps unequally, from each other and from theircommon parent. If we suppose the amount of change between eachhorizontal line in our diagram to be excessively small, these threeforms may still be only well-marked varieties; or they may havearrived at the doubtful category of sub-species; but we have only tosuppose the steps in the process of modification to be more numerousor greater in amount, to convert these three forms into well-definedspecies: thus the diagram illustrates the steps by which the smalldifferences distinguishing varieties are increased into the largerdifferences distinguishing species. By continuing the same process fora greater number of generations (as shown in the diagram in acondensed and simplified manner), we get eight species, marked by theletters between a14 and m14, all descended from (A). Thus, as Ibelieve, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary.In the diagram I have assumed that a second species (I) has produced,by analogous steps, after ten thousand generations, either twowell-marked varieties (w10 and z10) or two species, according to theamount of change supposed to be represented between the horizontallines. After fourteen thousand generations, six new species, marked bythe letters n14 to z14, are supposed to have been produced. In eachgenus, the species, which are already extremely different incharacter, will generally tend to produce the greatest number ofmodified descendants; for these will have the best chance of fillingnew and widely different places in the polity of nature: hence in thediagram I have chosen the extreme species (A), and the nearly extremespecies (I), as those which have largely varied, and have given riseto new varieties and species. The other nine species (marked bycapital letters) of our original genus, may for a long period continuetransmitting unaltered descendants; and this is shown in the diagramby the dotted lines not prolonged far upwards from want of space.
But during the process of modification, represented in the diagram,another of our principles, namely that of extinction, will have playedan important part. As in each fully stocked country natural selectionnecessarily acts by the selected form having some advantage in thestruggle for life over other forms, there will be a constant tendencyin the improved descendants of any one species to supplant andexterminate in each stage of descent their predecessors and theiroriginal parent. For it should be remembered that the competition willgenerally be most severe between those forms which are most nearlyrelated to each other in habits, constitution, and structure. Henceall the intermediate forms between the earlier and later states, thatis between the less and more improved state of a species, as well asthe original parent-species itself, will generally tend to becomeextinct. So it probably will be with many whole collateral lines ofdescent, which will be conquered by later and improved lines ofdescent. If, however, the modified offspring of a species get intosome distinct country, or become quickly adapted to some quite newstation, in which child and parent do not come into competition, bothmay continue to exist.
If then our diagram be assumed to represent a considerable amount ofmodification, species (A) and all the earlier varieties will havebecome extinct, having been replaced by eight new species (a14 tom14); and (I) will have been replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genuswere supposed to resemble each other in unequal degrees, as is sogenerally the case in nature; species (A) being more nearly related toB, C, and D, than to the other species; and species (I) more to G, H,K, L, than to the others. These two species (A) and (I), were alsosupposed to be very common and widely diffused species, so that theymust originally have had some advantage over most of the other speciesof the genus. Their modified descendants, fourteen in number at thefourteen-thousandth generation, will probably have inherited some ofthe same advantages: they have also been modified and improved in adiversified manner at each stage of descent, so as to have becomeadapted to many related places in the natural economy of theircountry. It seems, therefore, to me extremely probable that they willhave taken the places of, and thus exterminated, not only theirparents (A) and (I), but likewise some of the original species whichwere most nearly related to their parents. Hence very few of theoriginal species will have transmitted offspring to thefourteen-thousandth generation. We may suppose that only one (F), ofthe two species which were least closely related to the other nineoriginal species, has transmitted descendants to this late stage ofdescent.