物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 6. DIFFICULTIES ON THEORY. Page 3
Although we must be extremely cautious in concluding that any organcould not possibly have been produced by successive transitionalgradations, yet, undoubtedly, grave cases of difficulty occur, some ofwhich will be discussed in my future work.
One of the gravest is that of neuter insects, which are often verydifferently constructed from either the males or fertile females; butthis case will be treated of in the next chapter. The electric organsof fishes offer another case of special difficulty; it is impossibleto conceive by what steps these wondrous organs have been produced;but, as Owen and others have remarked, their intimate structureclosely resembles that of common muscle; and as it has lately beenshown that Rays have an organ closely analogous to the electricapparatus, and yet do not, as Matteuchi asserts, discharge anyelectricity, we must own that we are far too ignorant to argue that notransition of any kind is possible.
The electric organs offer another and even more serious difficulty;for they occur in only about a dozen fishes, of which several arewidely remote in their affinities. Generally when the same organappears in several members of the same class, especially if in membershaving very different habits of life, we may attribute its presence toinheritance from a common ancestor; and its absence in some of themembers to its loss through disuse or natural selection. But if theelectric organs had been inherited from one ancient progenitor thusprovided, we might have expected that all electric fishes would havebeen specially related to each other. Nor does geology at all lead tothe belief that formerly most fishes had electric organs, which mostof their modified descendants have lost. The presence of luminousorgans in a few insects, belonging to different families and orders,offers a parallel case of difficulty. Other cases could be given; forinstance in plants, the very curious contrivance of a mass ofpollen-grains, borne on a foot-stalk with a sticky gland at the end,is the same in Orchis and Asclepias,--genera almost as remote aspossible amongst flowering plants. In all these cases of two verydistinct species furnished with apparently the same anomalous organ,it should be observed that, although the general appearance andfunction of the organ may be the same, yet some fundamental differencecan generally be detected. I am inclined to believe that in nearly thesame way as two men have sometimes independently hit on the very sameinvention, so natural selection, working for the good of each beingand taking advantage of analogous variations, has sometimes modifiedin very nearly the same manner two parts in two organic beings, whichowe but little of their structure in common to inheritance from thesame ancestor.
Although in many cases it is most difficult to conjecture by whattransitions an organ could have arrived at its present state; yet,considering that the proportion of living and known forms to theextinct and unknown is very small, I have been astonished how rarelyan organ can be named, towards which no transitional grade is known tolead. The truth of this remark is indeed shown by that old canon innatural history of "Natura non facit saltum." We meet with thisadmission in the writings of almost every experienced naturalist; or,as Milne Edwards has well expressed it, nature is prodigal in variety,but niggard in innovation. Why, on the theory of Creation, should thisbe so? Why should all the parts and organs of many independent beings,each supposed to have been separately created for its proper place innature, be so invariably linked together by graduated steps? Whyshould not Nature have taken a leap from structure to structure? Onthe theory of natural selection, we can clearly understand why sheshould not; for natural selection can act only by taking advantage ofslight successive variations; she can never take a leap, but mustadvance by the shortest and slowest steps.
ORGANS OF LITTLE APPARENT IMPORTANCE.
produce absolute perfection, nor do we always meet,as far as we can judge, with this high standard under nature. Thecorrection for the aberration of light is said, on high authority, notto be perfect even in that most perfect organ, the eye. If our reasonleads us to admire with enthusiasm a multitude of inimitablecontrivances in nature.
As natural selection acts by life and death,--by the preservation ofindividuals with any favourable variation, and by the destruction ofthose with any unfavourable deviation of structure,--I have sometimesfelt much difficulty in understanding the origin of simple parts, ofwhich the importance does not seem sufficient to cause thepreservation of successively varying individuals. I have sometimesfelt as much difficulty, though of a very different kind, on thishead, as in the case of an organ as perfect and complex as the eye.
In the first place, we are much too ignorant in regard to the wholeeconomy of any one organic being, to say what slight modificationswould be of importance or not. In a former chapter I have giveninstances of most trifling characters, such as the down on fruit andthe colour of the flesh, which, from determining the attacks ofinsects or from being correlated with constitutional differences,might assuredly be acted on by natural selection. The tail of thegiraffe looks like an artificially constructed fly-flapper; and itseems at first incredible that this could have been adapted for itspresent purpose by successive slight modifications, each better andbetter, for so trifling an object as driving away flies; yet we shouldpause before being too positive even in this case, for we know thatthe distribution and existence of cattle and other animals in SouthAmerica absolutely depends on their power of resisting the attacks ofinsects: so that individuals which could by any means defendthemselves from these small enemies, would be able to range into newpastures and thus gain a great advantage. It is not that the largerquadrupeds are actually destroyed (except in some rare cases) by theflies, but they are incessantly harassed and their strength reduced,so that they are more subject to disease, or not so well enabled in acoming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been ofhigh importance to an early progenitor, and, after having been slowlyperfected at a former period, have been transmitted in nearly the samestate, although now become of very slight use; and any actuallyinjurious deviations in their structure will always have been checkedby natural selection. Seeing how important an organ of locomotion thetail is in most aquatic animals, its general presence and use for manypurposes in so many land animals, which in their lungs or modifiedswim-bladders betray their aquatic origin, may perhaps be thusaccounted for. A well-developed tail having been formed in an aquaticanimal, it might subsequently come to be worked in for all sorts ofpurposes, as a fly-flapper, an organ of prehension, or as an aid inturning, as with the dog, though the aid must be slight, for the hare,with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance tocharacters which are really of very little importance, and which haveoriginated from quite secondary causes, independently of naturalselection. We should remember that climate, food, etc., probably havesome little direct influence on the organisation; that charactersreappear from the law of reversion; that correlation of growth willhave had a most important influence in modifying various structures;and finally, that sexual selection will often have largely modifiedthe external characters of animals having a will, to give one male anadvantage in fighting with another or in charming the females.Moreover when a modification of structure has primarily arisen fromthe above or other unknown causes, it may at first have been of noadvantage to the species, but may subsequently have been takenadvantage of by the descendants of the species under new conditions oflife and with newly acquired habits.
To give a few instances to illustrate these latter remarks. If greenwoodpeckers alone had existed, and we did not know that there weremany black and pied kinds, I dare say that we should have thought thatthe green colour was a beautiful adaptation to hide thistree-frequenting bird from its enemies; and consequently that it was acharacter of importance and might have been acquired through naturalselection; as it is, I have no doubt that the colour is due to somequite distinct cause, probably to sexual selection. A trailing bambooin the Malay Archipelago climbs the loftiest trees by the aid ofexquisitely constructed hooks clustered around the ends of thebranches, and this contrivance, no doubt, is of the highest service tothe plant; but as we see nearly similar hooks on many trees which arenot climbers, the hooks on the bamboo may have arisen from unknownlaws of growth, and have been subsequently taken advantage of by theplant undergoing further modification and becoming a climber. Thenaked skin on the head of a vulture is generally looked at as a directadaptation for wallowing in putridity; and so it may be, or it maypossibly be due to the direct action of putrid matter; but we shouldbe very cautious in drawing any such inference, when we see that theskin on the head of the clean-feeding male turkey is likewise naked.The sutures in the skulls of young mammals have been advanced as abeautiful adaptation for aiding parturition, and no doubt theyfacilitate, or may be indispensable for this act; but as sutures occurin the skulls of young birds and reptiles, which have only to escapefrom a broken egg, we may infer that this structure has arisen fromthe laws of growth, and has been taken advantage of in the parturitionof the higher animals.
We are profoundly ignorant of the causes producing slight andunimportant variations; and we are immediately made conscious of thisby reflecting on the differences in the breeds of our domesticatedanimals in different countries,--more especially in the less civilizedcountries where there has been but little artificial selection.Careful observers are convinced that a damp climate affects the growthof the hair, and that with the hair the horns are correlated. Mountainbreeds always differ from lowland breeds; and a mountainous countrywould probably affect the hind limbs from exercising them more, andpossibly even the form of the pelvis; and then by the law ofhomologous variation, the front limbs and even the head would probablybe affected. The shape, also, of the pelvis might affect by pressurethe shape of the head of the young in the womb. The laboriousbreathing necessary in high regions would, we have some reason tobelieve, increase the size of the chest; and again correlation wouldcome into play. Animals kept by savages in different countries oftenhave to struggle for their own subsistence, and would be exposed to acertain extent to natural selection, and individuals with slightlydifferent constitutions would succeed best under different climates;and there is reason to believe that constitution and colour arecorrelated. A good observer, also, states that in cattlesusceptibility to the attacks of flies is correlated with colour, asis the liability to be poisoned by certain plants; so that colourwould be thus subjected to the action of natural selection. But we arefar too ignorant to speculate on the relative importance of theseveral known and unknown laws of variation; and I have here alludedto them only to show that, if we are unable to account for thecharacteristic differences of our domestic breeds, which neverthelesswe generally admit to have arisen through ordinary generation, weought not to lay too much stress on our ignorance of the precise causeof the slight analogous differences between species. I might haveadduced for this same purpose the differences between the races ofman, which are so strongly marked; I may add that some little lightcan apparently be thrown on the origin of these differences, chieflythrough sexual selection of a particular kind, but without hereentering on copious details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the protest latelymade by some naturalists, against the utilitarian doctrine that everydetail of structure has been produced for the good of its possessor.They believe that very many structures have been created for beauty inthe eyes of man, or for mere variety. This doctrine, if true, would beabsolutely fatal to my theory. Yet I fully admit that many structuresare of no direct use to their possessors. Physical conditions probablyhave had some little effect on structure, quite independently of anygood thus gained. Correlation of growth has no doubt played a mostimportant part, and a useful modification of one part will often haveentailed on other parts diversified changes of no direct use. So againcharacters which formerly were useful, or which formerly had arisenfrom correlation of growth, or from other unknown cause, may reappearfrom the law of reversion, though now of no direct use. The effects ofsexual selection, when displayed in beauty to charm the females, canbe called useful only in rather a forced sense. But by far the mostimportant consideration is that the chief part of the organisation ofevery being is simply due to inheritance; and consequently, thougheach being assuredly is well fitted for its place in nature, manystructures now have no direct relation to the habits of life of eachspecies. Thus, we can hardly believe that the webbed feet of theupland goose or of the frigate-bird are of special use to these birds;we cannot believe that the same bones in the arm of the monkey, in thefore leg of the horse, in the wing of the bat, and in the flipper ofthe seal, are of special use to these animals. We may safely attributethese structures to inheritance. But to the progenitor of the uplandgoose and of the frigate-bird, webbed feet no doubt were as useful asthey now are to the most aquatic of existing birds. So we may believethat the progenitor of the seal had not a flipper, but a foot withfive toes fitted for walking or grasping; and we may further ventureto believe that the several bones in the limbs of the monkey, horse,and bat, which have been inherited from a common progenitor, wereformerly of more special use to that progenitor, or its progenitors,than they now are to these animals having such widely diversifiedhabits. Therefore we may infer that these several bones might havebeen acquired through natural selection, subjected formerly, as now,to the several laws of inheritance, reversion, correlation of growth,etc. Hence every detail of structure in every living creature (makingsome little allowance for the direct action of physical conditions)may be viewed, either as having been of special use to some ancestralform, or as being now of special use to the descendants of thisform--either directly, or indirectly through the complex laws ofgrowth.
Natural selection cannot possibly produce any modification in any onespecies exclusively for the good of another species; though throughoutnature one species incessantly takes advantage of, and profits by, thestructure of another. But natural selection can and does often producestructures for the direct injury of other species, as we see in thefang of the adder, and in the ovipositor of the ichneumon, by whichits eggs are deposited in the living bodies of other insects. If itcould be proved that any part of the structure of any one species hadbeen formed for the exclusive good of another species, it wouldannihilate my theory, for such could not have been produced throughnatural selection. Although many statements may be found in works onnatural history to this effect, I cannot find even one which seems tome of any weight. It is admitted that the rattlesnake has apoison-fang for its own defence and for the destruction of its prey;but some authors suppose that at the same time this snake is furnishedwith a rattle for its own injury, namely, to warn its prey to escape.I would almost as soon believe that the cat curls the end of its tailwhen preparing to spring, in order to warn the doomed mouse. But Ihave not space here to enter on this and other such cases.
Natural selection will never produce in a being anything injurious toitself, for natural selection acts solely by and for the good of each.No organ will be formed, as Paley has remarked, for the purpose ofcausing pain or for doing an injury to its possessor. If a fairbalance be struck between the good and evil caused by each part, eachwill be found on the whole advantageous. After the lapse of time,under changing conditions of life, if any part comes to be injurious,it will be modified; or if it be not so, the being will becomeextinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect as,or slightly more perfect than, the other inhabitants of the samecountry with which it has to struggle for existence. And we see thatthis is the degree of perfection attained under nature. The endemicproductions of New Zealand, for instance, are perfect one comparedwith another; but they are now rapidly yielding before the advancinglegions of plants and animals introduced from Europe. Naturalselection will not produce absolute perfection, nor do we always meet,as far as we can judge, with this high standard under nature. Thecorrection for the aberration of light is said, on high authority, notto be perfect even in that most perfect organ, the eye. If our reasonleads us to admire with enthusiasm a multitude of inimitablecontrivances in nature, this same reason tells us, though we mayeasily err on both sides, that some other contrivances are lessperfect. Can we consider the sting of the wasp or of the bee asperfect, which, when used against many attacking animals, cannot bewithdrawn, owing to the backward serratures, and so inevitably causesthe death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in aremote progenitor as a boring and serrated instrument, like that in somany members of the same great order, and which has been modified butnot perfected for its present purpose, with the poison originallyadapted to cause galls subsequently intensified, we can perhapsunderstand how it is that the use of the sting should so often causethe insect's own death: for if on the whole the power of stinging beuseful to the community, it will fulfil all the requirements ofnatural selection, though it may cause the death of some few members.If we admire the truly wonderful power of scent by which the males ofmany insects find their females, can we admire the production for thissingle purpose of thousands of drones, which are utterly useless tothe community for any other end, and which are ultimately slaughteredby their industrious and sterile sisters? It may be difficult, but weought to admire the savage instinctive hatred of the queen-bee, whichurges her instantly to destroy the young queens her daughters as soonas born, or to perish herself in the combat; for undoubtedly this isfor the good of the community; and maternal love or maternal hatred,though the latter fortunately is most rare, is all the same to theinexorable principle of natural selection. If we admire the severalingenious contrivances, by which the flowers of the orchis and of manyother plants are fertilised through insect agency, can we consider asequally perfect the elaboration by our fir-trees of dense clouds ofpollen, in order that a few granules may be wafted by a chance breezeon to the ovules?