物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 1. VARIATION UNDER DOMESTICATION. Page 1
Causes of Variability.Effects of Habit.Correlation of Growth.Inheritance.Character of Domestic Varieties.Difficulty of distinguishing between Varieties and Species.Origin of Domestic Varieties from one or more Species.Domestic Pigeons, their Differences and Origin.Principle of Selection anciently followed, its Effects.Methodical and Unconscious Selection.Unknown Origin of our Domestic Productions.Circumstances favourable to Man's power of Selection.
When we look to the individuals of the same variety or sub-variety ofour older cultivated plants and animals, one of the first points whichstrikes us, is, that they generally differ much more from each other,than do the individuals of any one species or variety in a state ofnature. When we reflect on the vast diversity of the plants andanimals which have been cultivated, and which have varied during allages under the most different climates and treatment, I think we aredriven to conclude that this greater variability is simply due to ourdomestic productions having been raised under conditions of life notso uniform as, and somewhat different from, those to which theparent-species have been exposed under nature. There is, also, Ithink, some probability in the view propounded by Andrew Knight, thatthis variability may be partly connected with excess of food. It seemspretty clear that organic beings must be exposed during severalgenerations to the new conditions of life to cause any appreciableamount of variation; and that when the organisation has once begun tovary, it generally continues to vary for many generations. No case ison record of a variable being ceasing to be variable undercultivation. Our oldest cultivated plants, such as wheat, still oftenyield new varieties: our oldest domesticated animals are still capableof rapid improvement or modification.
It has been disputed at what period of life the causes of variability,whatever they may be, generally act; whether during the early or lateperiod of development of the embryo, or at the instant of conception.Geoffroy St. Hilaire's experiments show that unnatural treatment ofthe embryo causes monstrosities; and monstrosities cannot be separatedby any clear line of distinction from mere variations. But I amstrongly inclined to suspect that the most frequent cause ofvariability may be attributed to the male and female reproductiveelements having been affected prior to the act of conception. Severalreasons make me believe in this; but the chief one is the remarkableeffect which confinement or cultivation has on the functions of thereproductive system; this system appearing to be far more susceptiblethan any other part of the organisation, to the action of any changein the conditions of life. Nothing is more easy than to tame ananimal, and few things more difficult than to get it to breed freelyunder confinement, even in the many cases when the male and femaleunite. How many animals there are which will not breed, though livinglong under not very close confinement in their native country! This isgenerally attributed to vitiated instincts; but how many cultivatedplants display the utmost vigour, and yet rarely or never seed! Insome few such cases it has been found out that very trifling changes,such as a little more or less water at some particular period ofgrowth, will determine whether or not the plant sets a seed. I cannothere enter on the copious details which I have collected on thiscurious subject; but to show how singular the laws are which determinethe reproduction of animals under confinement, I may just mention thatcarnivorous animals, even from the tropics, breed in this countrypretty freely under confinement, with the exception of theplantigrades or bear family; whereas, carnivorous birds, with therarest exceptions, hardly ever lay fertile eggs. Many exotic plantshave pollen utterly worthless, in the same exact condition as in themost sterile hybrids. When, on the one hand, we see domesticatedanimals and plants, though often weak and sickly, yet breeding quitefreely under confinement; and when, on the other hand, we seeindividuals, though taken young from a state of nature, perfectlytamed, long-lived, and healthy (of which I could give numerousinstances), yet having their reproductive system so seriously affectedby unperceived causes as to fail in acting, we need not be surprisedat this system, when it does act under confinement, acting not quiteregularly, and producing offspring not perfectly like their parents orvariable.
Sterility has been said to be the bane of horticulture; but on thisview we owe variability to the same cause which produces sterility;and variability is the source of all the choicest productions of thegarden. I may add, that as some organisms will breed most freely underthe most unnatural conditions (for instance, the rabbit and ferretkept in hutches), showing that their reproductive system has not beenthus affected; so will some animals and plants withstand domesticationor cultivation, and vary very slightly--perhaps hardly more than in astate of nature.
A long list could easily be given of "sporting plants;" by this termgardeners mean a single bud or offset, which suddenly assumes a newand sometimes very different character from that of the rest of theplant. Such buds can be propagated by grafting, etc., and sometimes byseed. These "sports" are extremely rare under nature, but far fromrare under cultivation; and in this case we see that the treatment ofthe parent has affected a bud or offset, and not the ovules or pollen.But it is the opinion of most physiologists that there is no essentialdifference between a bud and an ovule in their earliest stages offormation; so that, in fact, "sports" support my view, thatvariability may be largely attributed to the ovules or pollen, or toboth, having been affected by the treatment of the parent prior to theact of conception. These cases anyhow show that variation is notnecessarily connected, as some authors have supposed, with the act ofgeneration.
Seedlings from the same fruit, and the young of the same litter,sometimes differ considerably from each other, though both the youngand the parents, as Muller has remarked, have apparently been exposedto exactly the same conditions of life; and this shows how unimportantthe direct effects of the conditions of life are in comparison withthe laws of reproduction, and of growth, and of inheritance; for hadthe action of the conditions been direct, if any of the young hadvaried, all would probably have varied in the same manner. To judgehow much, in the case of any variation, we should attribute to thedirect action of heat, moisture, light, food, etc., is most difficult:my impression is, that with animals such agencies have produced verylittle direct effect, though apparently more in the case of plants.Under this point of view, Mr. Buckman's recent experiments on plantsseem extremely valuable. When all or nearly all the individualsexposed to certain conditions are affected in the same way, the changeat first appears to be directly due to such conditions; but in somecases it can be shown that quite opposite conditions produce similarchanges of structure. Nevertheless some slight amount of change may, Ithink, be attributed to the direct action of the conditions oflife--as, in some cases, increased size from amount of food, colourfrom particular kinds of food and from light, and perhaps thethickness of fur from climate.
Habit also has a decided influence, as in the period of flowering withplants when transported from one climate to another. In animals it hasa more marked effect; for instance, I find in the domestic duck thatthe bones of the wing weigh less and the bones of the leg more, inproportion to the whole skeleton, than do the same bones in thewild-duck; and I presume that this change may be safely attributed tothe domestic duck flying much less, and walking more, than its wildparent. The great and inherited development of the udders in cows andgoats in countries where they are habitually milked, in comparisonwith the state of these organs in other countries, is another instanceof the effect of use. Not a single domestic animal can be named whichhas not in some country drooping ears; and the view suggested by someauthors, that the drooping is due to the disuse of the muscles of theear, from the animals not being much alarmed by danger, seemsprobable.
There are many laws regulating variation, some few of which can bedimly seen, and will be hereafter briefly mentioned. I will here onlyallude to what may be called correlation of growth. Any change in theembryo or larva will almost certainly entail changes in the matureanimal. In monstrosities, the correlations between quite distinctparts are very curious; and many instances are given in IsidoreGeoffroy St. Hilaire's great work on this subject. Breeders believethat long limbs are almost always accompanied by an elongated head.Some instances of correlation are quite whimsical; thus cats with blueeyes are invariably deaf; colour and constitutional peculiarities gotogether, of which many remarkable cases could be given amongstanimals and plants. From the facts collected by Heusinger, it appearsthat white sheep and pigs are differently affected from colouredindividuals by certain vegetable poisons. Hairless dogs have imperfectteeth; long-haired and coarse-haired animals are apt to have, as isasserted, long or many horns; pigeons with feathered feet have skinbetween their outer toes; pigeons with short beaks have small feet,and those with long beaks large feet. Hence, if man goes on selecting,and thus augmenting, any peculiarity, he will almost certainlyunconsciously modify other parts of the structure, owing to themysterious laws of the correlation of growth.
The result of the various, quite unknown, or dimly seen laws ofvariation is infinitely complex and diversified. It is well worthwhile carefully to study the several treatises published on some ofour old cultivated plants, as on the hyacinth, potato, even thedahlia, etc.; and it is really surprising to note the endless pointsin structure and constitution in which the varieties and sub-varietiesdiffer slightly from each other. The whole organisation seems to havebecome plastic, and tends to depart in some small degree from that ofthe parental type.
Any variation which is not inherited is unimportant for us. But thenumber and diversity of inheritable deviations of structure, boththose of slight and those of considerable physiological importance, isendless. Dr. Prosper Lucas's treatise, in two large volumes, is thefullest and the best on this subject. No breeder doubts how strong isthe tendency to inheritance: like produces like is his fundamentalbelief: doubts have been thrown on this principle by theoreticalwriters alone. When a deviation appears not unfrequently, and we seeit in the father and child, we cannot tell whether it may not be dueto the same original cause acting on both; but when amongstindividuals, apparently exposed to the same conditions, any very raredeviation, due to some extraordinary combination of circumstances,appears in the parent--say, once amongst several millionindividuals--and it reappears in the child, the mere doctrine ofchances almost compels us to attribute its reappearance toinheritance. Every one must have heard of cases of albinism, pricklyskin, hairy bodies, etc., appearing in several members of the samefamily. If strange and rare deviations of structure are trulyinherited, less strange and commoner deviations may be freely admittedto be inheritable. Perhaps the correct way of viewing the wholesubject, would be, to look at the inheritance of every characterwhatever as the rule, and non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say whythe same peculiarity in different individuals of the same species, andin individuals of different species, is sometimes inherited andsometimes not so; why the child often reverts in certain characters toits grandfather or grandmother or other much more remote ancestor; whya peculiarity is often transmitted from one sex to both sexes or toone sex alone, more commonly but not exclusively to the like sex. Itis a fact of some little importance to us, that peculiaritiesappearing in the males of our domestic breeds are often transmittedeither exclusively, or in a much greater degree, to males alone. Amuch more important rule, which I think may be trusted, is that, atwhatever period of life a peculiarity first appears, it tends toappear in the offspring at a corresponding age, though sometimesearlier. In many cases this could not be otherwise: thus the inheritedpeculiarities in the horns of cattle could appear only in theoffspring when nearly mature; peculiarities in the silkworm are knownto appear at the corresponding caterpillar or cocoon stage. Buthereditary diseases and some other facts make me believe that the rulehas a wider extension, and that when there is no apparent reason why apeculiarity should appear at any particular age, yet that it does tendto appear in the offspring at the same period at which it firstappeared in the parent. I believe this rule to be of the highestimportance in explaining the laws of embryology. These remarks are ofcourse confined to the first APPEARANCE of the peculiarity, and not toits primary cause, which may have acted on the ovules or male element;in nearly the same manner as in the crossed offspring from ashort-horned cow by a long-horned bull, the greater length of horn,though appearing late in life, is clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to astatement often made by naturalists--namely, that our domesticvarieties, when run wild, gradually but certainly revert in characterto their aboriginal stocks. Hence it has been argued that nodeductions can be drawn from domestic races to species in a state ofnature. I have in vain endeavoured to discover on what decisive factsthe above statement has so often and so boldly been made. There wouldbe great difficulty in proving its truth: we may safely conclude thatvery many of the most strongly-marked domestic varieties could notpossibly live in a wild state. In many cases we do not know what theaboriginal stock was, and so could not tell whether or not nearlyperfect reversion had ensued. It would be quite necessary, in order toprevent the effects of intercrossing, that only a single varietyshould be turned loose in its new home. Nevertheless, as our varietiescertainly do occasionally revert in some of their characters toancestral forms, it seems to me not improbable, that if we couldsucceed in naturalising, or were to cultivate, during manygenerations, the several races, for instance, of the cabbage, in verypoor soil (in which case, however, some effect would have to beattributed to the direct action of the poor soil), that they would toa large extent, or even wholly, revert to the wild aboriginal stock.Whether or not the experiment would succeed, is not of greatimportance for our line of argument; for by the experiment itself theconditions of life are changed. If it could be shown that our domesticvarieties manifested a strong tendency to reversion,--that is, to losetheir acquired characters, whilst kept under unchanged conditions, andwhilst kept in a considerable body, so that free intercrossing mightcheck, by blending together, any slight deviations of structure, insuch case, I grant that we could deduce nothing from domesticvarieties in regard to species. But there is not a shadow of evidencein favour of this view: to assert that we could not breed our cart andrace-horses, long and short-horned cattle, and poultry of variousbreeds, and esculent vegetables, for an almost infinite number ofgenerations, would be opposed to all experience. I may add, that whenunder nature the conditions of life do change, variations andreversions of character probably do occur; but natural selection, aswill hereafter be explained, will determine how far the new charactersthus arising shall be preserved.
When we look to the hereditary varieties or races of our domesticanimals and plants, and compare them with species closely alliedtogether, we generally perceive in each domestic race, as alreadyremarked, less uniformity of character than in true species. Domesticraces of the same species, also, often have a somewhat monstrouscharacter; by which I mean, that, although differing from each other,and from the other species of the same genus, in several triflingrespects, they often differ in an extreme degree in some one part,both when compared one with another, and more especially when comparedwith all the species in nature to which they are nearest allied. Withthese exceptions (and with that of the perfect fertility of varietieswhen crossed,--a subject hereafter to be discussed), domestic races ofthe same species differ from each other in the same manner as, only inmost cases in a lesser degree than, do closely-allied species of thesame genus in a state of nature. I think this must be admitted, whenwe find that there are hardly any domestic races, either amongstanimals or plants, which have not been ranked by some competent judgesas mere varieties, and by other competent judges as the descendants ofaboriginally distinct species. If any marked distinction existedbetween domestic races and species, this source of doubt could not soperpetually recur. It has often been stated that domestic races do notdiffer from each other in characters of generic value. I think itcould be shown that this statement is hardly correct; but naturalistsdiffer most widely in determining what characters are of genericvalue; all such valuations being at present empirical. Moreover, onthe view of the origin of genera which I shall presently give, we haveno right to expect often to meet with generic differences in ourdomesticated productions.
When we attempt to estimate the amount of structural differencebetween the domestic races of the same species, we are soon involvedin doubt, from not knowing whether they have descended from one orseveral parent-species. This point, if it could be cleared up, wouldbe interesting; if, for instance, it could be shown that thegreyhound, bloodhound, terrier, spaniel, and bull-dog, which we allknow propagate their kind so truly, were the offspring of any singlespecies, then such facts would have great weight in making us doubtabout the immutability of the many very closely allied and naturalspecies--for instance, of the many foxes--inhabiting differentquarters of the world. I do not believe, as we shall presently see,that all our dogs have descended from any one wild species; but, inthe case of some other domestic races, there is presumptive, or evenstrong, evidence in favour of this view.
It has often been assumed that man has chosen for domesticationanimals and plants having an extraordinary inherent tendency to vary,and likewise to withstand diverse climates. I do not dispute thatthese capacities have added largely to the value of most of ourdomesticated productions; but how could a savage possibly know, whenhe first tamed an animal, whether it would vary in succeedinggenerations, and whether it would endure other climates? Has thelittle variability of the ass or guinea-fowl, or the small power ofendurance of warmth by the rein-deer, or of cold by the common camel,prevented their domestication? I cannot doubt that if other animalsand plants, equal in number to our domesticated productions, andbelonging to equally diverse classes and countries, were taken from astate of nature, and could be made to breed for an equal number ofgenerations under domestication, they would vary on an average aslargely as the parent species of our existing domesticated productionshave varied.