物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 5. LAWS OF VARIATION. Page 1
Effects of external conditions.Use and disuse, combined with natural selection; organs of flight andof vision.Acclimatisation.Correlation of growth.Compensation and economy of growth.False correlations.Multiple, rudimentary, and lowly organised structures variable.Parts developed in an unusual manner are highly variable: specificcharacters more variable than generic: secondary sexual charactersvariable.Species of the same genus vary in an analogous manner.Reversions to long lost characters.Summary.
I have hitherto sometimes spoken as if the variations--so common andmultiform in organic beings under domestication, and in a lesserdegree in those in a state of nature--had been due to chance. This, ofcourse, is a wholly incorrect expression, but it serves to acknowledgeplainly our ignorance of the cause of each particular variation. Someauthors believe it to be as much the function of the reproductivesystem to produce individual differences, or very slight deviations ofstructure, as to make the child like its parents. But the much greatervariability, as well as the greater frequency of monstrosities, underdomestication or cultivation, than under nature, leads me to believethat deviations of structure are in some way due to the nature of theconditions of life, to which the parents and their more remoteancestors have been exposed during several generations. I haveremarked in the first chapter--but a long catalogue of facts whichcannot be here given would be necessary to show the truth of theremark--that the reproductive system is eminently susceptible tochanges in the conditions of life; and to this system beingfunctionally disturbed in the parents, I chiefly attribute the varyingor plastic condition of the offspring. The male and female sexualelements seem to be affected before that union takes place which is toform a new being. In the case of "sporting" plants, the bud, which inits earliest condition does not apparently differ essentially from anovule, is alone affected. But why, because the reproductive system isdisturbed, this or that part should vary more or less, we areprofoundly ignorant. Nevertheless, we can here and there dimly catch afaint ray of light, and we may feel sure that there must be some causefor each deviation of structure, however slight.
How much direct effect difference of climate, food, etc., produces onany being is extremely doubtful. My impression is, that the effect isextremely small in the case of animals, but perhaps rather more inthat of plants. We may, at least, safely conclude that such influencescannot have produced the many striking and complex co-adaptations ofstructure between one organic being and another, which we seeeverywhere throughout nature. Some little influence may be attributedto climate, food, etc.: thus, E. Forbes speaks confidently that shellsat their southern limit, and when living in shallow water, are morebrightly coloured than those of the same species further north or fromgreater depths. Gould believes that birds of the same species are morebrightly coloured under a clear atmosphere, than when living onislands or near the coast. So with insects, Wollaston is convincedthat residence near the sea affects their colours. Moquin-Tandon givesa list of plants which when growing near the sea-shore have theirleaves in some degree fleshy, though not elsewhere fleshy. Severalother such cases could be given.
The fact of varieties of one species, when they range into the zone ofhabitation of other species, often acquiring in a very slight degreesome of the characters of such species, accords with our view thatspecies of all kinds are only well-marked and permanent varieties.Thus the species of shells which are confined to tropical and shallowseas are generally brighter-coloured than those confined to cold anddeeper seas. The birds which are confined to continents are, accordingto Mr. Gould, brighter-coloured than those of islands. Theinsect-species confined to sea-coasts, as every collector knows, areoften brassy or lurid. Plants which live exclusively on the sea-sideare very apt to have fleshy leaves. He who believes in the creation ofeach species, will have to say that this shell, for instance, wascreated with bright colours for a warm sea; but that this other shellbecame bright-coloured by variation when it ranged into warmer orshallower waters.
When a variation is of the slightest use to a being, we cannot tellhow much of it to attribute to the accumulative action of naturalselection, and how much to the conditions of life. Thus, it is wellknown to furriers that animals of the same species have thicker andbetter fur the more severe the climate is under which they have lived;but who can tell how much of this difference may be due to thewarmest-clad individuals having been favoured and preserved duringmany generations, and how much to the direct action of the severeclimate? for it would appear that climate has some direct action onthe hair of our domestic quadrupeds.
Instances could be given of the same variety being produced underconditions of life as different as can well be conceived; and, on theother hand, of different varieties being produced from the samespecies under the same conditions. Such facts show how indirectly theconditions of life must act. Again, innumerable instances are known toevery naturalist of species keeping true, or not varying at all,although living under the most opposite climates. Such considerationsas these incline me to lay very little weight on the direct action ofthe conditions of life. Indirectly, as already remarked, they seem toplay an important part in affecting the reproductive system, and inthus inducing variability; and natural selection will then accumulateall profitable variations, however slight, until they become plainlydeveloped and appreciable by us.
EFFECTS OF USE AND DISUSE.
From the facts alluded to in the first chapter, I think there can belittle doubt that use in our domestic animals strengthens and enlargescertain parts, and disuse diminishes them; and that such modificationsare inherited. Under free nature, we can have no standard ofcomparison, by which to judge of the effects of long-continued use ordisuse, for we know not the parent-forms; but many animals havestructures which can be explained by the effects of disuse. AsProfessor Owen has remarked, there is no greater anomaly in naturethan a bird that cannot fly; yet there are several in this state. Thelogger-headed duck of South America can only flap along the surface ofthe water, and has its wings in nearly the same condition as thedomestic Aylesbury duck. As the larger ground-feeding birds seldomtake flight except to escape danger, I believe that the nearlywingless condition of several birds, which now inhabit or have latelyinhabited several oceanic islands, tenanted by no beast of prey, hasbeen caused by disuse. The ostrich indeed inhabits continents and isexposed to danger from which it cannot escape by flight, but bykicking it can defend itself from enemies, as well as any of thesmaller quadrupeds. We may imagine that the early progenitor of theostrich had habits like those of a bustard, and that as naturalselection increased in successive generations the size and weight ofits body, its legs were used more, and its wings less, until theybecame incapable of flight.
In some cases we might easily put down to disuse modifications ofstructure which are wholly, or mainly, due to natural selection. Mr.Wollaston has discovered the remarkable fact that 200 beetles, out ofthe 550 species inhabiting Madeira, are so far deficient in wings thatthey cannot fly; and that of the twenty-nine endemic genera, no lessthan twenty-three genera have all their species in this condition!Several facts, namely, that beetles in many parts of the world arevery frequently blown to sea and perish; that the beetles in Madeira,as observed by Mr. Wollaston, lie much concealed, until the wind lullsand the sun shines; that the proportion of wingless beetles is largeron the exposed Dezertas than in Madeira itself; and especially theextraordinary fact, so strongly insisted on by Mr. Wollaston, of thealmost entire absence of certain large groups of beetles, elsewhereexcessively numerous, and which groups have habits of life almostnecessitating frequent flight;--these several considerations have mademe believe that the wingless condition of so many Madeira beetles ismainly due to the action of natural selection, but combined probablywith disuse. For during thousands of successive generations eachindividual beetle which flew least, either from its wings having beenever so little less perfectly developed or from indolent habit, willhave had the best chance of surviving from not being blown out to sea;and, on the other hand, those beetles which most readily took toflight will oftenest have been blown to sea and thus have beendestroyed.
The insects in Madeira which are not ground-feeders, and which, as theflower-feeding coleoptera and lepidoptera, must habitually use theirwings to gain their subsistence, have, as Mr. Wollaston suspects,their wings not at all reduced, but even enlarged. This is quitecompatible with the action of natural selection. For when a new insectfirst arrived on the island, the tendency of natural selection toenlarge or to reduce the wings, would depend on whether a greaternumber of individuals were saved by successfully battling with thewinds, or by giving up the attempt and rarely or never flying. As withmariners shipwrecked near a coast, it would have been better for thegood swimmers if they had been able to swim still further, whereas itwould have been better for the bad swimmers if they had not been ableto swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary insize, and in some cases are quite covered up by skin and fur. Thisstate of the eyes is probably due to gradual reduction from disuse,but aided perhaps by natural selection. In South America, a burrowingrodent, the tuco-tuco, or Ctenomys, is even more subterranean in itshabits than the mole; and I was assured by a Spaniard, who had oftencaught them, that they were frequently blind; one which I kept alivewas certainly in this condition, the cause, as appeared on dissection,having been inflammation of the nictitating membrane. As frequentinflammation of the eyes must be injurious to any animal, and as eyesare certainly not indispensable to animals with subterranean habits, areduction in their size with the adhesion of the eyelids and growth offur over them, might in such case be an advantage; and if so, naturalselection would constantly aid the effects of disuse.
It is well known that several animals, belonging to the most differentclasses, which inhabit the caves of Styria and of Kentucky, are blind.In some of the crabs the foot-stalk for the eye remains, though theeye is gone; the stand for the telescope is there, though thetelescope with its glasses has been lost. As it is difficult toimagine that eyes, though useless, could be in any way injurious toanimals living in darkness, I attribute their loss wholly to disuse.In one of the blind animals, namely, the cave-rat, the eyes are ofimmense size; and Professor Silliman thought that it regained, afterliving some days in the light, some slight power of vision. In thesame manner as in Madeira the wings of some of the insects have beenenlarged, and the wings of others have been reduced by naturalselection aided by use and disuse, so in the case of the cave-ratnatural selection seems to have struggled with the loss of light andto have increased the size of the eyes; whereas with all the otherinhabitants of the caves, disuse by itself seems to have done itswork.
It is difficult to imagine conditions of life more similar than deeplimestone caverns under a nearly similar climate; so that on thecommon view of the blind animals having been separately created forthe American and European caverns, close similarity in theirorganisation and affinities might have been expected; but, as Schiodteand others have remarked, this is not the case, and the cave-insectsof the two continents are not more closely allied than might have beenanticipated from the general resemblance of the other inhabitants ofNorth America and Europe. On my view we must suppose that Americananimals, having ordinary powers of vision, slowly migrated bysuccessive generations from the outer world into the deeper and deeperrecesses of the Kentucky caves, as did European animals into the cavesof Europe. We have some evidence of this gradation of habit; for, asSchiodte remarks, "animals not far remote from ordinary forms, preparethe transition from light to darkness. Next follow those that areconstructed for twilight; and, last of all, those destined for totaldarkness." By the time that an animal had reached, after numberlessgenerations, the deepest recesses, disuse will on this view have moreor less perfectly obliterated its eyes, and natural selection willoften have effected other changes, such as an increase in the lengthof the antennae or palpi, as a compensation for blindness.Notwithstanding such modifications, we might expect still to see inthe cave-animals of America, affinities to the other inhabitants ofthat continent, and in those of Europe, to the inhabitants of theEuropean continent. And this is the case with some of the Americancave-animals, as I hear from Professor Dana; and some of the Europeancave-insects are very closely allied to those of the surroundingcountry. It would be most difficult to give any rational explanationof the affinities of the blind cave-animals to the other inhabitantsof the two continents on the ordinary view of their independentcreation. That several of the inhabitants of the caves of the Old andNew Worlds should be closely related, we might expect from thewell-known relationship of most of their other productions. Far fromfeeling any surprise that some of the cave-animals should be veryanomalous, as Agassiz has remarked in regard to the blind fish, theAmblyopsis, and as is the case with the blind Proteus with referenceto the reptiles of Europe, I am only surprised that more wrecks ofancient life have not been preserved, owing to the less severecompetition to which the inhabitants of these dark abodes willprobably have been exposed.
ACCLIMATISATION.
Habit is hereditary with plants, as in the period of flowering, in theamount of rain requisite for seeds to germinate, in the time of sleep,etc., and this leads me to say a few words on acclimatisation. As itis extremely common for species of the same genus to inhabit very hotand very cold countries, and as I believe that all the species of thesame genus have descended from a single parent, if this view becorrect, acclimatisation must be readily effected duringlong-continued descent. It is notorious that each species is adaptedto the climate of its own home: species from an arctic or even from atemperate region cannot endure a tropical climate, or conversely. Soagain, many succulent plants cannot endure a damp climate. But thedegree of adaptation of species to the climates under which they liveis often overrated. We may infer this from our frequent inability topredict whether or not an imported plant will endure our climate, andfrom the number of plants and animals brought from warmer countrieswhich here enjoy good health. We have reason to believe that speciesin a state of nature are limited in their ranges by the competition ofother organic beings quite as much as, or more than, by adaptation toparticular climates. But whether or not the adaptation be generallyvery close, we have evidence, in the case of some few plants, of theirbecoming, to a certain extent, naturally habituated to differenttemperatures, or becoming acclimatised: thus the pines andrhododendrons, raised from seed collected by Dr. Hooker from treesgrowing at different heights on the Himalaya, were found in thiscountry to possess different constitutional powers of resisting cold.Mr. Thwaites informs me that he has observed similar facts in Ceylon,and analogous observations have been made by Mr. H. C. Watson onEuropean species of plants brought from the Azores to England. Inregard to animals, several authentic cases could be given of specieswithin historical times having largely extended their range fromwarmer to cooler latitudes, and conversely; but we do not positivelyknow that these animals were strictly adapted to their native climate,but in all ordinary cases we assume such to be the case; nor do weknow that they have subsequently become acclimatised to their newhomes.
As I believe that our domestic animals were originally chosen byuncivilised man because they were useful and bred readily underconfinement, and not because they were subsequently found capable offar-extended transportation, I think the common and extraordinarycapacity in our domestic animals of not only withstanding the mostdifferent climates but of being perfectly fertile (a far severer test)under them, may be used as an argument that a large proportion ofother animals, now in a state of nature, could easily be brought tobear widely different climates. We must not, however, push theforegoing argument too far, on account of the probable origin of someof our domestic animals from several wild stocks: the blood, forinstance, of a tropical and arctic wolf or wild dog may perhaps bemingled in our domestic breeds. The rat and mouse cannot be consideredas domestic animals, but they have been transported by man to manyparts of the world, and now have a far wider range than any otherrodent, living free under the cold climate of Faroe in the north andof the Falklands in the south, and on many islands in the torridzones. Hence I am inclined to look at adaptation to any specialclimate as a quality readily grafted on an innate wide flexibility ofconstitution, which is common to most animals. On this view, thecapacity of enduring the most different climates by man himself and byhis domestic animals, and such facts as that former species of theelephant and rhinoceros were capable of enduring a glacial climate,whereas the living species are now all tropical or sub-tropical intheir habits, ought not to be looked at as anomalies, but merely asexamples of a very common flexibility of constitution, brought, underpeculiar circumstances, into play.