物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 4. NATURAL SELECTION. Page 2
Even without any change in the proportional numbers of the animals onwhich our wolf preyed, a cub might be born with an innate tendency topursue certain kinds of prey. Nor can this be thought very improbable;for we often observe great differences in the natural tendencies ofour domestic animals; one cat, for instance, taking to catch rats,another mice; one cat, according to Mr. St. John, bringing home wingedgame, another hares or rabbits, and another hunting on marshy groundand almost nightly catching woodcocks or snipes. The tendency to catchrats rather than mice is known to be inherited. Now, if any slightinnate change of habit or of structure benefited an individual wolf,it would have the best chance of surviving and of leaving offspring.Some of its young would probably inherit the same habits or structure,and by the repetition of this process, a new variety might be formedwhich would either supplant or coexist with the parent-form of wolf.Or, again, the wolves inhabiting a mountainous district, and thosefrequenting the lowlands, would naturally be forced to hunt differentprey; and from the continued preservation of the individuals bestfitted for the two sites, two varieties might slowly be formed. Thesevarieties would cross and blend where they met; but to this subject ofintercrossing we shall soon have to return. I may add, that, accordingto Mr. Pierce, there are two varieties of the wolf inhabiting theCatskill Mountains in the United States, one with a lightgreyhound-like form, which pursues deer, and the other more bulky,with shorter legs, which more frequently attacks the shepherd'sflocks.
Let us now take a more complex case. Certain plants excrete a sweetjuice, apparently for the sake of eliminating something injurious fromtheir sap: this is effected by glands at the base of the stipules insome Leguminosae, and at the back of the leaf of the common laurel.This juice, though small in quantity, is greedily sought by insects.Let us now suppose a little sweet juice or nectar to be excreted bythe inner bases of the petals of a flower. In this case insects inseeking the nectar would get dusted with pollen, and would certainlyoften transport the pollen from one flower to the stigma of anotherflower. The flowers of two distinct individuals of the same specieswould thus get crossed; and the act of crossing, we have good reasonto believe (as will hereafter be more fully alluded to), would producevery vigorous seedlings, which consequently would have the best chanceof flourishing and surviving. Some of these seedlings would probablyinherit the nectar-excreting power. Those individual flowers which hadthe largest glands or nectaries, and which excreted most nectar, wouldbe oftenest visited by insects, and would be oftenest crossed; and soin the long-run would gain the upper hand. Those flowers, also, whichhad their stamens and pistils placed, in relation to the size andhabits of the particular insects which visited them, so as to favourin any degree the transportal of their pollen from flower to flower,would likewise be favoured or selected. We might have taken the caseof insects visiting flowers for the sake of collecting pollen insteadof nectar; and as pollen is formed for the sole object offertilisation, its destruction appears a simple loss to the plant; yetif a little pollen were carried, at first occasionally and thenhabitually, by the pollen-devouring insects from flower to flower, anda cross thus effected, although nine-tenths of the pollen weredestroyed, it might still be a great gain to the plant; and thoseindividuals which produced more and more pollen, and had larger andlarger anthers, would be selected.
When our plant, by this process of the continued preservation ornatural selection of more and more attractive flowers, had beenrendered highly attractive to insects, they would, unintentionally ontheir part, regularly carry pollen from flower to flower; and thatthey can most effectually do this, I could easily show by manystriking instances. I will give only one--not as a very striking case,but as likewise illustrating one step in the separation of the sexesof plants, presently to be alluded to. Some holly-trees bear only maleflowers, which have four stamens producing rather a small quantity ofpollen, and a rudimentary pistil; other holly-trees bear only femaleflowers; these have a full-sized pistil, and four stamens withshrivelled anthers, in which not a grain of pollen can be detected.Having found a female tree exactly sixty yards from a male tree, I putthe stigmas of twenty flowers, taken from different branches, underthe microscope, and on all, without exception, there werepollen-grains, and on some a profusion of pollen. As the wind had setfor several days from the female to the male tree, the pollen couldnot thus have been carried. The weather had been cold and boisterous,and therefore not favourable to bees, nevertheless every female flowerwhich I examined had been effectually fertilised by the bees,accidentally dusted with pollen, having flown from tree to tree insearch of nectar. But to return to our imaginary case: as soon as theplant had been rendered so highly attractive to insects that pollenwas regularly carried from flower to flower, another process mightcommence. No naturalist doubts the advantage of what has been calledthe "physiological division of labour;" hence we may believe that itwould be advantageous to a plant to produce stamens alone in oneflower or on one whole plant, and pistils alone in another flower oron another plant. In plants under culture and placed under newconditions of life, sometimes the male organs and sometimes the femaleorgans become more or less impotent; now if we suppose this to occurin ever so slight a degree under nature, then as pollen is alreadycarried regularly from flower to flower, and as a more completeseparation of the sexes of our plant would be advantageous on theprinciple of the division of labour, individuals with this tendencymore and more increased, would be continually favoured or selected,until at last a complete separation of the sexes would be effected.
Let us now turn to the nectar-feeding insects in our imaginary case:we may suppose the plant of which we have been slowly increasing thenectar by continued selection, to be a common plant; and that certaininsects depended in main part on its nectar for food. I could givemany facts, showing how anxious bees are to save time; for instance,their habit of cutting holes and sucking the nectar at the bases ofcertain flowers, which they can, with a very little more trouble,enter by the mouth. Bearing such facts in mind, I can see no reason todoubt that an accidental deviation in the size and form of the body,or in the curvature and length of the proboscis, etc., far too slightto be appreciated by us, might profit a bee or other insect, so thatan individual so characterised would be able to obtain its food morequickly, and so have a better chance of living and leavingdescendants. Its descendants would probably inherit a tendency to asimilar slight deviation of structure. The tubes of the corollas ofthe common red and incarnate clovers (Trifolium pratense andincarnatum) do not on a hasty glance appear to differ in length; yetthe hive-bee can easily suck the nectar out of the incarnate clover,but not out of the common red clover, which is visited by humble-beesalone; so that whole fields of the red clover offer in vain anabundant supply of precious nectar to the hive-bee. Thus it might be agreat advantage to the hive-bee to have a slightly longer ordifferently constructed proboscis. On the other hand, I have found byexperiment that the fertility of clover greatly depends on beesvisiting and moving parts of the corolla, so as to push the pollen onto the stigmatic surface. Hence, again, if humble-bees were to becomerare in any country, it might be a great advantage to the red cloverto have a shorter or more deeply divided tube to its corolla, so thatthe hive-bee could visit its flowers. Thus I can understand how aflower and a bee might slowly become, either simultaneously or oneafter the other, modified and adapted in the most perfect manner toeach other, by the continued preservation of individuals presentingmutual and slightly favourable deviations of structure.
I am well aware that this doctrine of natural selection, exemplifiedin the above imaginary instances, is open to the same objections whichwere at first urged against Sir Charles Lyell's noble views on "themodern changes of the earth, as illustrative of geology;" but we nowvery seldom hear the action, for instance, of the coast-waves, calleda trifling and insignificant cause, when applied to the excavation ofgigantic valleys or to the formation of the longest lines of inlandcliffs. Natural selection can act only by the preservation andaccumulation of infinitesimally small inherited modifications, eachprofitable to the preserved being; and as modern geology has almostbanished such views as the excavation of a great valley by a singlediluvial wave, so will natural selection, if it be a true principle,banish the belief of the continued creation of new organic beings, orof any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals andplants with separated sexes, it is of course obvious that twoindividuals must always unite for each birth; but in the case ofhermaphrodites this is far from obvious. Nevertheless I am stronglyinclined to believe that with all hermaphrodites two individuals,either occasionally or habitually, concur for the reproduction oftheir kind. This view, I may add, was first suggested by AndrewKnight. We shall presently see its importance; but I must here treatthe subject with extreme brevity, though I have the materials preparedfor an ample discussion. All vertebrate animals, all insects, and someother large groups of animals, pair for each birth. Modern researchhas much diminished the number of supposed hermaphrodites, and of realhermaphrodites a large number pair; that is, two individuals regularlyunite for reproduction, which is all that concerns us. But still thereare many hermaphrodite animals which certainly do not habitually pair,and a vast majority of plants are hermaphrodites. What reason, it maybe asked, is there for supposing in these cases that two individualsever concur in reproduction? As it is impossible here to enter ondetails, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts,showing, in accordance with the almost universal belief of breeders,that with animals and plants a cross between different varieties, orbetween individuals of the same variety but of another strain, givesvigour and fertility to the offspring; and on the other hand, thatCLOSE interbreeding diminishes vigour and fertility; that these factsalone incline me to believe that it is a general law of nature(utterly ignorant though we be of the meaning of the law) that noorganic being self-fertilises itself for an eternity of generations;but that a cross with another individual is occasionally--perhaps atvery long intervals--indispensable.
On the belief that this is a law of nature, we can, I think,understand several large classes of facts, such as the following,which on any other view are inexplicable. Every hybridizer knows howunfavourable exposure to wet is to the fertilisation of a flower, yetwhat a multitude of flowers have their anthers and stigmas fullyexposed to the weather! but if an occasional cross be indispensable,the fullest freedom for the entrance of pollen from another individualwill explain this state of exposure, more especially as the plant'sown anthers and pistil generally stand so close together thatself-fertilisation seems almost inevitable. Many flowers, on the otherhand, have their organs of fructification closely enclosed, as in thegreat papilionaceous or pea-family; but in several, perhaps in all,such flowers, there is a very curious adaptation between the structureof the flower and the manner in which bees suck the nectar; for, indoing this, they either push the flower's own pollen on the stigma, orbring pollen from another flower. So necessary are the visits of beesto papilionaceous flowers, that I have found, by experiments publishedelsewhere, that their fertility is greatly diminished if these visitsbe prevented. Now, it is scarcely possible that bees should fly fromflower to flower, and not carry pollen from one to the other, to thegreat good, as I believe, of the plant. Bees will act like acamel-hair pencil, and it is quite sufficient just to touch theanthers of one flower and then the stigma of another with the samebrush to ensure fertilisation; but it must not be supposed that beeswould thus produce a multitude of hybrids between distinct species;for if you bring on the same brush a plant's own pollen and pollenfrom another species, the former will have such a prepotent effect,that it will invariably and completely destroy, as has been shown byGartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, orslowly move one after the other towards it, the contrivance seemsadapted solely to ensure self-fertilisation; and no doubt it is usefulfor this end: but, the agency of insects is often required to causethe stamens to spring forward, as Kolreuter has shown to be the casewith the barberry; and curiously in this very genus, which seems tohave a special contrivance for self-fertilisation, it is well knownthat if very closely-allied forms or varieties are planted near eachother, it is hardly possible to raise pure seedlings, so largely dothey naturally cross. In many other cases, far from there being anyaids for self-fertilisation, there are special contrivances, as Icould show from the writings of C. C. Sprengel and from my ownobservations, which effectually prevent the stigma receiving pollenfrom its own flower: for instance, in Lobelia fulgens, there is areally beautiful and elaborate contrivance by which every one of theinfinitely numerous pollen-granules are swept out of the conjoinedanthers of each flower, before the stigma of that individual flower isready to receive them; and as this flower is never visited, at leastin my garden, by insects, it never sets a seed, though by placingpollen from one flower on the stigma of another, I raised plenty ofseedlings; and whilst another species of Lobelia growing close by,which is visited by bees, seeds freely. In very many other cases,though there be no special mechanical contrivance to prevent thestigma of a flower receiving its own pollen, yet, as C. C. Sprengelhas shown, and as I can confirm, either the anthers burst before thestigma is ready for fertilisation, or the stigma is ready before thepollen of that flower is ready, so that these plants have in factseparated sexes, and must habitually be crossed. How strange are thesefacts! How strange that the pollen and stigmatic surface of the sameflower, though placed so close together, as if for the very purpose ofself-fertilisation, should in so many cases be mutually useless toeach other! How simply are these facts explained on the view of anoccasional cross with a distinct individual being advantageous orindispensable!
If several varieties of the cabbage, radish, onion, and of some otherplants, be allowed to seed near each other, a large majority, as Ihave found, of the seedlings thus raised will turn out mongrels: forinstance, I raised 233 seedling cabbages from some plants of differentvarieties growing near each other, and of these only 78 were true totheir kind, and some even of these were not perfectly true. Yet thepistil of each cabbage-flower is surrounded not only by its own sixstamens, but by those of the many other flowers on the same plant.How, then, comes it that such a vast number of the seedlings aremongrelized? I suspect that it must arise from the pollen of adistinct VARIETY having a prepotent effect over a flower's own pollen;and that this is part of the general law of good being derived fromthe intercrossing of distinct individuals of the same species. Whendistinct SPECIES are crossed the case is directly the reverse, for aplant's own pollen is always prepotent over foreign pollen; but tothis subject we shall return in a future chapter.
In the case of a gigantic tree covered with innumerable flowers, itmay be objected that pollen could seldom be carried from tree to tree,and at most only from flower to flower on the same tree, and thatflowers on the same tree can be considered as distinct individualsonly in a limited sense. I believe this objection to be valid, butthat nature has largely provided against it by giving to trees astrong tendency to bear flowers with separated sexes. When the sexesare separated, although the male and female flowers may be produced onthe same tree, we can see that pollen must be regularly carried fromflower to flower; and this will give a better chance of pollen beingoccasionally carried from tree to tree. That trees belonging to allOrders have their sexes more often separated than other plants, I findto be the case in this country; and at my request Dr. Hooker tabulatedthe trees of New Zealand, and Dr. Asa Gray those of the United States,and the result was as I anticipated. On the other hand, Dr. Hooker hasrecently informed me that he finds that the rule does not hold inAustralia; and I have made these few remarks on the sexes of treessimply to call attention to the subject.
Turning for a very brief space to animals: on the land there are somehermaphrodites, as land-mollusca and earth-worms; but these all pair.As yet I have not found a single case of a terrestrial animal whichfertilises itself. We can understand this remarkable fact, whichoffers so strong a contrast with terrestrial plants, on the view of anoccasional cross being indispensable, by considering the medium inwhich terrestrial animals live, and the nature of the fertilisingelement; for we know of no means, analogous to the action of insectsand of the wind in the case of plants, by which an occasional crosscould be effected with terrestrial animals without the concurrence oftwo individuals. Of aquatic animals, there are many self-fertilisinghermaphrodites; but here currents in the water offer an obvious meansfor an occasional cross. And, as in the case of flowers, I have as yetfailed, after consultation with one of the highest authorities,namely, Professor Huxley, to discover a single case of anhermaphrodite animal with the organs of reproduction so perfectlyenclosed within the body, that access from without and the occasionalinfluence of a distinct individual can be shown to be physicallyimpossible. Cirripedes long appeared to me to present a case of verygreat difficulty under this point of view; but I have been enabled, bya fortunate chance, elsewhere to prove that two individuals, thoughboth are self-fertilising hermaphrodites, do sometimes cross.