物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 6. DIFFICULTIES ON THEORY. Page 1
Difficulties on the theory of descent with modification.Transitions.Absence or rarity of transitional varieties.Transitions in habits of life.Diversified habits in the same species.Species with habits widely different from those of their allies.Organs of extreme perfection.Means of transition.Cases of difficulty.Natura non facit saltum.Organs of small importance.Organs not in all cases absolutely perfect.The law of Unity of Type and of the Conditions of Existence embracedby the theory of Natural Selection.
Long before having arrived at this part of my work, a crowd ofdifficulties will have occurred to the reader. Some of them are sograve that to this day I can never reflect on them without beingstaggered; but, to the best of my judgment, the greater number areonly apparent, and those that are real are not, I think, fatal to mytheory.
These difficulties and objections may be classed under the followingheads:--
Firstly, why, if species have descended from other species byinsensibly fine gradations, do we not everywhere see innumerabletransitional forms? Why is not all nature in confusion instead of thespecies being, as we see them, well defined?
each kind of squirrel in its own country,by enabling it to escape birds or beasts of prey, or to collect foodmore quickly, or, as there is reason to believe, by lessening thedanger from occasional falls. But it does not follow!
Secondly, is it possible that an animal having, for instance, thestructure and habits of a bat, could have been formed by themodification of some animal with wholly different habits? Can webelieve that natural selection could produce, on the one hand, organsof trifling importance, such as the tail of a giraffe, which serves asa fly-flapper, and, on the other hand, organs of such wonderfulstructure, as the eye, of which we hardly as yet fully understand theinimitable perfection?
Thirdly, can instincts be acquired and modified through naturalselection? What shall we say to so marvellous an instinct as thatwhich leads the bee to make cells, which have practically anticipatedthe discoveries of profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterileand producing sterile offspring, whereas, when varieties are crossed,their fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism inseparate chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitablemodifications, each new form will tend in a fully-stocked country totake the place of, and finally to exterminate, its own less improvedparent or other less-favoured forms with which it comes intocompetition. Thus extinction and natural selection will, as we haveseen, go hand in hand. Hence, if we look at each species as descendedfrom some other unknown form, both the parent and all the transitionalvarieties will generally have been exterminated by the very process offormation and perfection of the new form.
But, as by this theory innumerable transitional forms must haveexisted, why do we not find them embedded in countless numbers in thecrust of the earth? It will be much more convenient to discuss thisquestion in the chapter on the Imperfection of the geological record;and I will here only state that I believe the answer mainly lies inthe record being incomparably less perfect than is generally supposed;the imperfection of the record being chiefly due to organic beings notinhabiting profound depths of the sea, and to their remains beingembedded and preserved to a future age only in masses of sedimentsufficiently thick and extensive to withstand an enormous amount offuture degradation; and such fossiliferous masses can be accumulatedonly where much sediment is deposited on the shallow bed of the sea,whilst it slowly subsides. These contingencies will concur onlyrarely, and after enormously long intervals. Whilst the bed of the seais stationary or is rising, or when very little sediment is beingdeposited, there will be blanks in our geological history. The crustof the earth is a vast museum; but the natural collections have beenmade only at intervals of time immensely remote.
But it may be urged that when several closely-allied species inhabitthe same territory we surely ought to find at the present time manytransitional forms. Let us take a simple case: in travelling fromnorth to south over a continent, we generally meet at successiveintervals with closely allied or representative species, evidentlyfilling nearly the same place in the natural economy of the land.These representative species often meet and interlock; and as the onebecomes rarer and rarer, the other becomes more and more frequent,till the one replaces the other. But if we compare these species wherethey intermingle, they are generally as absolutely distinct from eachother in every detail of structure as are specimens taken from themetropolis inhabited by each. By my theory these allied species havedescended from a common parent; and during the process ofmodification, each has become adapted to the conditions of life of itsown region, and has supplanted and exterminated its original parentand all the transitional varieties between its past and presentstates. Hence we ought not to expect at the present time to meet withnumerous transitional varieties in each region, though they must haveexisted there, and may be embedded there in a fossil condition. But inthe intermediate region, having intermediate conditions of life, whydo we not now find closely-linking intermediate varieties? Thisdifficulty for a long time quite confounded me. But I think it can bein large part explained.
In the first place we should be extremely cautious in inferring,because an area is now continuous, that it has been continuous duringa long period. Geology would lead us to believe that almost everycontinent has been broken up into islands even during the latertertiary periods; and in such islands distinct species might have beenseparately formed without the possibility of intermediate varietiesexisting in the intermediate zones. By changes in the form of the landand of climate, marine areas now continuous must often have existedwithin recent times in a far less continuous and uniform conditionthan at present. But I will pass over this way of escaping from thedifficulty; for I believe that many perfectly defined species havebeen formed on strictly continuous areas; though I do not doubt thatthe formerly broken condition of areas now continuous has played animportant part in the formation of new species, more especially withfreely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, wegenerally find them tolerably numerous over a large territory, thenbecoming somewhat abruptly rarer and rarer on the confines, andfinally disappearing. Hence the neutral territory between tworepresentative species is generally narrow in comparison with theterritory proper to each. We see the same fact in ascending mountains,and sometimes it is quite remarkable how abruptly, as Alph. DeCandolle has observed, a common alpine species disappears. The samefact has been noticed by Forbes in sounding the depths of the sea withthe dredge. To those who look at climate and the physical conditionsof life as the all-important elements of distribution, these factsought to cause surprise, as climate and height or depth graduate awayinsensibly. But when we bear in mind that almost every species, evenin its metropolis, would increase immensely in numbers, were it notfor other competing species; that nearly all either prey on or serveas prey for others; in short, that each organic being is eitherdirectly or indirectly related in the most important manner to otherorganic beings, we must see that the range of the inhabitants of anycountry by no means exclusively depends on insensibly changingphysical conditions, but in large part on the presence of otherspecies, on which it depends, or by which it is destroyed, or withwhich it comes into competition; and as these species are alreadydefined objects (however they may have become so), not blending oneinto another by insensible gradations, the range of any one species,depending as it does on the range of others, will tend to be sharplydefined. Moreover, each species on the confines of its range, where itexists in lessened numbers, will, during fluctuations in the number ofits enemies or of its prey, or in the seasons, be extremely liable toutter extermination; and thus its geographical range will come to bestill more sharply defined.
If I am right in believing that allied or representative species, wheninhabiting a continuous area, are generally so distributed that eachhas a wide range, with a comparatively narrow neutral territorybetween them, in which they become rather suddenly rarer and rarer;then, as varieties do not essentially differ from species, the samerule will probably apply to both; and if we in imagination adapt avarying species to a very large area, we shall have to adapt twovarieties to two large areas, and a third variety to a narrowintermediate zone. The intermediate variety, consequently, will existin lesser numbers from inhabiting a narrow and lesser area; andpractically, as far as I can make out, this rule holds good withvarieties in a state of nature. I have met with striking instances ofthe rule in the case of varieties intermediate between well-markedvarieties in the genus Balanus. And it would appear from informationgiven me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, thatgenerally when varieties intermediate between two other forms occur,they are much rarer numerically than the forms which they connect.Now, if we may trust these facts and inferences, and thereforeconclude that varieties linking two other varieties together havegenerally existed in lesser numbers than the forms which they connect,then, I think, we can understand why intermediate varieties should notendure for very long periods;--why as a general rule they should beexterminated and disappear, sooner than the forms which theyoriginally linked together.
For any form existing in lesser numbers would, as already remarked,run a greater chance of being exterminated than one existing in largenumbers; and in this particular case the intermediate form would beeminently liable to the inroads of closely allied forms existing onboth sides of it. But a far more important consideration, as Ibelieve, is that, during the process of further modification, by whichtwo varieties are supposed on my theory to be converted and perfectedinto two distinct species, the two which exist in larger numbers frominhabiting larger areas, will have a great advantage over theintermediate variety, which exists in smaller numbers in a narrow andintermediate zone. For forms existing in larger numbers will alwayshave a better chance, within any given period, of presenting furtherfavourable variations for natural selection to seize on, than will therarer forms which exist in lesser numbers. Hence, the more commonforms, in the race for life, will tend to beat and supplant the lesscommon forms, for these will be more slowly modified and improved. Itis the same principle which, as I believe, accounts for the commonspecies in each country, as shown in the second chapter, presenting onan average a greater number of well-marked varieties than do the rarerspecies. I may illustrate what I mean by supposing three varieties ofsheep to be kept, one adapted to an extensive mountainous region; asecond to a comparatively narrow, hilly tract; and a third to wideplains at the base; and that the inhabitants are all trying with equalsteadiness and skill to improve their stocks by selection; the chancesin this case will be strongly in favour of the great holders on themountains or on the plains improving their breeds more quickly thanthe small holders on the intermediate narrow, hilly tract; andconsequently the improved mountain or plain breed will soon take theplace of the less improved hill breed; and thus the two breeds, whichoriginally existed in greater numbers, will come into close contactwith each other, without the interposition of the supplanted,intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-definedobjects, and do not at any one period present an inextricable chaos ofvarying and intermediate links: firstly, because new varieties arevery slowly formed, for variation is a very slow process, and naturalselection can do nothing until favourable variations chance to occur,and until a place in the natural polity of the country can be betterfilled by some modification of some one or more of its inhabitants.And such new places will depend on slow changes of climate, or on theoccasional immigration of new inhabitants, and, probably, in a stillmore important degree, on some of the old inhabitants becoming slowlymodified, with the new forms thus produced and the old ones acting andreacting on each other. So that, in any one region and at any onetime, we ought only to see a few species presenting slightmodifications of structure in some degree permanent; and thisassuredly we do see.
Secondly, areas now continuous must often have existed within therecent period in isolated portions, in which many forms, moreespecially amongst the classes which unite for each birth and wandermuch, may have separately been rendered sufficiently distinct to rankas representative species. In this case, intermediate varietiesbetween the several representative species and their common parent,must formerly have existed in each broken portion of the land, butthese links will have been supplanted and exterminated during theprocess of natural selection, so that they will no longer exist in aliving state.
Thirdly, when two or more varieties have been formed in differentportions of a strictly continuous area, intermediate varieties will,it is probable, at first have been formed in the intermediate zones,but they will generally have had a short duration. For theseintermediate varieties will, from reasons already assigned (namelyfrom what we know of the actual distribution of closely allied orrepresentative species, and likewise of acknowledged varieties), existin the intermediate zones in lesser numbers than the varieties whichthey tend to connect. From this cause alone the intermediate varietieswill be liable to accidental extermination; and during the process offurther modification through natural selection, they will almostcertainly be beaten and supplanted by the forms which they connect;for these from existing in greater numbers will, in the aggregate,present more variation, and thus be further improved through naturalselection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory betrue, numberless intermediate varieties, linking most closely all thespecies of the same group together, must assuredly have existed; butthe very process of natural selection constantly tends, as has been sooften remarked, to exterminate the parent forms and the intermediatelinks. Consequently evidence of their former existence could be foundonly amongst fossil remains, which are preserved, as we shall in afuture chapter attempt to show, in an extremely imperfect andintermittent record.
ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITSAND STRUCTURE.
It has been asked by the opponents of such views as I hold, how, forinstance, a land carnivorous animal could have been converted into onewith aquatic habits; for how could the animal in its transitionalstate have subsisted? It would be easy to show that within the samegroup carnivorous animals exist having every intermediate gradebetween truly aquatic and strictly terrestrial habits; and as eachexists by a struggle for life, it is clear that each is well adaptedin its habits to its place in nature. Look at the Mustela vison ofNorth America, which has webbed feet and which resembles an otter inits fur, short legs, and form of tail; during summer this animal divesfor and preys on fish, but during the long winter it leaves the frozenwaters, and preys like other polecats on mice and land animals. If adifferent case had been taken, and it had been asked how aninsectivorous quadruped could possibly have been converted into aflying bat, the question would have been far more difficult, and Icould have given no answer. Yet I think such difficulties have verylittle weight.
geological record;and I will here only state that I believe the answer mainly lies inthe record being incomparably.
Here, as on other occasions, I lie under a heavy disadvantage, for outof the many striking cases which I have collected, I can give only oneor two instances of transitional habits and structures in closelyallied species of the same genus; and of diversified habits, eitherconstant or occasional, in the same species. And it seems to me thatnothing less than a long list of such cases is sufficient to lessenthe difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradationfrom animals with their tails only slightly flattened, and fromothers, as Sir J. Richardson has remarked, with the posterior part oftheir bodies rather wide and with the skin on their flanks ratherfull, to the so-called flying squirrels; and flying squirrels havetheir limbs and even the base of the tail united by a broad expanse ofskin, which serves as a parachute and allows them to glide through theair to an astonishing distance from tree to tree. We cannot doubt thateach structure is of use to each kind of squirrel in its own country,by enabling it to escape birds or beasts of prey, or to collect foodmore quickly, or, as there is reason to believe, by lessening thedanger from occasional falls. But it does not follow from this factthat the structure of each squirrel is the best that it is possible toconceive under all natural conditions. Let the climate and vegetationchange, let other competing rodents or new beasts of prey immigrate,or old ones become modified, and all analogy would lead us to believethat some at least of the squirrels would decrease in numbers orbecome exterminated, unless they also became modified and improved instructure in a corresponding manner. Therefore, I can see nodifficulty, more especially under changing conditions of life, in thecontinued preservation of individuals with fuller and fullerflank-membranes, each modification being useful, each beingpropagated, until by the accumulated effects of this process ofnatural selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly wasfalsely ranked amongst bats. It has an extremely wide flank-membrane,stretching from the corners of the jaw to the tail, and including thelimbs and the elongated fingers: the flank membrane is, also,furnished with an extensor muscle. Although no graduated links ofstructure, fitted for gliding through the air, now connect theGaleopithecus with the other Lemuridae, yet I can see no difficulty insupposing that such links formerly existed, and that each had beenformed by the same steps as in the case of the less perfectly glidingsquirrels; and that each grade of structure had been useful to itspossessor. Nor can I see any insuperable difficulty in furtherbelieving it possible that the membrane-connected fingers and fore-armof the Galeopithecus might be greatly lengthened by natural selection;and this, as far as the organs of flight are concerned, would convertit into a bat. In bats which have the wing-membrane extended from thetop of the shoulder to the tail, including the hind-legs, we perhapssee traces of an apparatus originally constructed for gliding throughthe air rather than for flight.