物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 8. HYBRIDISM. Page 1
Distinction between the sterility of first crosses and of hybrids.Sterility various in degree, not universal, affected by closeinterbreeding, removed by domestication.Laws governing the sterility of hybrids.Sterility not a special endowment, but incidental on otherdifferences.Causes of the sterility of first crosses and of hybrids.Parallelism between the effects of changed conditions of life andcrossing.Fertility of varieties when crossed and of their mongrel offspring notuniversal.Hybrids and mongrels compared independently of their fertility.Summary.
The view generally entertained by naturalists is that species, whenintercrossed, have been specially endowed with the quality ofsterility, in order to prevent the confusion of all organic forms.This view certainly seems at first probable, for species within thesame country could hardly have kept distinct had they been capable ofcrossing freely. The importance of the fact that hybrids are verygenerally sterile, has, I think, been much underrated by some latewriters. On the theory of natural selection the case is especiallyimportant, inasmuch as the sterility of hybrids could not possibly beof any advantage to them, and therefore could not have been acquiredby the continued preservation of successive profitable degrees ofsterility. I hope, however, to be able to show that sterility is not aspecially acquired or endowed quality, but is incidental on otheracquired differences.
In treating this subject, two classes of facts, to a large extentfundamentally different, have generally been confounded together;namely, the sterility of two species when first crossed, and thesterility of the hybrids produced from them.
carefully triedthan with plants. If our systematic arrangements can be trusted, thatis if the genera of animals are as distinct from each other,
Pure species have of course their organs of reproduction in a perfectcondition, yet when intercrossed they produce either few or nooffspring. Hybrids, on the other hand, have their reproductive organsfunctionally impotent, as may be clearly seen in the state of the maleelement in both plants and animals; though the organs themselves areperfect in structure, as far as the microscope reveals. In the firstcase the two sexual elements which go to form the embryo are perfect;in the second case they are either not at all developed, or areimperfectly developed. This distinction is important, when the causeof the sterility, which is common to the two cases, has to beconsidered. The distinction has probably been slurred over, owing tothe sterility in both cases being looked on as a special endowment,beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed tohave descended from common parents, when intercrossed, and likewisethe fertility of their mongrel offspring, is, on my theory, of equalimportance with the sterility of species; for it seems to make a broadand clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybridoffspring. It is impossible to study the several memoirs and works ofthose two conscientious and admirable observers, Kolreuter andGartner, who almost devoted their lives to this subject, without beingdeeply impressed with the high generality of some degree of sterility.Kolreuter makes the rule universal; but then he cuts the knot, for inten cases in which he found two forms, considered by most authors asdistinct species, quite fertile together, he unhesitatingly ranks themas varieties. Gartner, also, makes the rule equally universal; and hedisputes the entire fertility of Kolreuter's ten cases. But in theseand in many other cases, Gartner is obliged carefully to count theseeds, in order to show that there is any degree of sterility. Healways compares the maximum number of seeds produced by two specieswhen crossed and by their hybrid offspring, with the average numberproduced by both pure parent-species in a state of nature. But aserious cause of error seems to me to be here introduced: a plant tobe hybridised must be castrated, and, what is often more important,must be secluded in order to prevent pollen being brought to it byinsects from other plants. Nearly all the plants experimentised on byGartner were potted, and apparently were kept in a chamber in hishouse. That these processes are often injurious to the fertility of aplant cannot be doubted; for Gartner gives in his table about a scoreof cases of plants which he castrated, and artificially fertilisedwith their own pollen, and (excluding all cases such as theLeguminosae, in which there is an acknowledged difficulty in themanipulation) half of these twenty plants had their fertility in somedegree impaired. Moreover, as Gartner during several years repeatedlycrossed the primrose and cowslip, which we have such good reason tobelieve to be varieties, and only once or twice succeeded in gettingfertile seed; as he found the common red and blue pimpernels(Anagallis arvensis and coerulea), which the best botanists rank asvarieties, absolutely sterile together; and as he came to the sameconclusion in several other analogous cases; it seems to me that wemay well be permitted to doubt whether many other species are reallyso sterile, when intercrossed, as Gartner believes.
It is certain, on the one hand, that the sterility of various specieswhen crossed is so different in degree and graduates away soinsensibly, and, on the other hand, that the fertility of pure speciesis so easily affected by various circumstances, that for all practicalpurposes it is most difficult to say where perfect fertility ends andsterility begins. I think no better evidence of this can be requiredthan that the two most experienced observers who have ever lived,namely, Kolreuter and Gartner, should have arrived at diametricallyopposite conclusions in regard to the very same species. It is alsomost instructive to compare--but I have not space here to enter ondetails--the evidence advanced by our best botanists on the questionwhether certain doubtful forms should be ranked as species orvarieties, with the evidence from fertility adduced by differenthybridisers, or by the same author, from experiments made duringdifferent years. It can thus be shown that neither sterility norfertility affords any clear distinction between species and varieties;but that the evidence from this source graduates away, and is doubtfulin the same degree as is the evidence derived from otherconstitutional and structural differences.
In regard to the sterility of hybrids in successive generations;though Gartner was enabled to rear some hybrids, carefully guardingthem from a cross with either pure parent, for six or seven, and inone case for ten generations, yet he asserts positively that theirfertility never increased, but generally greatly decreased. I do notdoubt that this is usually the case, and that the fertility oftensuddenly decreases in the first few generations. Nevertheless Ibelieve that in all these experiments the fertility has beendiminished by an independent cause, namely, from close interbreeding.I have collected so large a body of facts, showing that closeinterbreeding lessens fertility, and, on the other hand, that anoccasional cross with a distinct individual or variety increasesfertility, that I cannot doubt the correctness of this almostuniversal belief amongst breeders. Hybrids are seldom raised byexperimentalists in great numbers; and as the parent-species, or otherallied hybrids, generally grow in the same garden, the visits ofinsects must be carefully prevented during the flowering season: hencehybrids will generally be fertilised during each generation by theirown individual pollen; and I am convinced that this would be injuriousto their fertility, already lessened by their hybrid origin. I amstrengthened in this conviction by a remarkable statement repeatedlymade by Gartner, namely, that if even the less fertile hybrids beartificially fertilised with hybrid pollen of the same kind, theirfertility, notwithstanding the frequent ill effects of manipulation,sometimes decidedly increases, and goes on increasing. Now, inartificial fertilisation pollen is as often taken by chance (as I knowfrom my own experience) from the anthers of another flower, as fromthe anthers of the flower itself which is to be fertilised; so that across between two flowers, though probably on the same plant, would bethus effected. Moreover, whenever complicated experiments are inprogress, so careful an observer as Gartner would have castrated hishybrids, and this would have insured in each generation a cross withthe pollen from a distinct flower, either from the same plant or fromanother plant of the same hybrid nature. And thus, the strange fact ofthe increase of fertility in the successive generations ofARTIFICIALLY FERTILISED hybrids may, I believe, be accounted for byclose interbreeding having been avoided.
Now let us turn to the results arrived at by the third mostexperienced hybridiser, namely, the Honourable and Reverend W.Herbert. He is as emphatic in his conclusion that some hybrids areperfectly fertile--as fertile as the pure parent-species--as areKolreuter and Gartner that some degree of sterility between distinctspecies is a universal law of nature. He experimentised on some of thevery same species as did Gartner. The difference in their results may,I think, be in part accounted for by Herbert's great horticulturalskill, and by his having hothouses at his command. Of his manyimportant statements I will here give only a single one as an example,namely, that "every ovule in a pod of Crinum capense fertilised by C.revolutum produced a plant, which (he says) I never saw to occur in acase of its natural fecundation." So that we here have perfect, oreven more than commonly perfect, fertility in a first cross betweentwo distinct species.
This case of the Crinum leads me to refer to a most singular fact,namely, that there are individual plants, as with certain species ofLobelia, and with all the species of the genus Hippeastrum, which canbe far more easily fertilised by the pollen of another and distinctspecies, than by their own pollen. For these plants have been found toyield seed to the pollen of a distinct species, though quite sterilewith their own pollen, notwithstanding that their own pollen was foundto be perfectly good, for it fertilised distinct species. So thatcertain individual plants and all the individuals of certain speciescan actually be hybridised much more readily than they can beself-fertilised! For instance, a bulb of Hippeastrum aulicum producedfour flowers; three were fertilised by Herbert with their own pollen,and the fourth was subsequently fertilised by the pollen of a compoundhybrid descended from three other and distinct species: the result wasthat "the ovaries of the three first flowers soon ceased to grow, andafter a few days perished entirely, whereas the pod impregnated by thepollen of the hybrid made vigorous growth and rapid progress tomaturity, and bore good seed, which vegetated freely." In a letter tome, in 1839, Mr. Herbert told me that he had then tried the experimentduring five years, and he continued to try it during severalsubsequent years, and always with the same result. This result has,also, been confirmed by other observers in the case of Hippeastrumwith its sub-genera, and in the case of some other genera, as Lobelia,Passiflora and Verbascum. Although the plants in these experimentsappeared perfectly healthy, and although both the ovules and pollen ofthe same flower were perfectly good with respect to other species, yetas they were functionally imperfect in their mutual self-action, wemust infer that the plants were in an unnatural state. Neverthelessthese facts show on what slight and mysterious causes the lesser orgreater fertility of species when crossed, in comparison with the samespecies when self-fertilised, sometimes depends.
The practical experiments of horticulturists, though not made withscientific precision, deserve some notice. It is notorious in howcomplicated a manner the species of Pelargonium, Fuchsia, Calceolaria,Petunia, Rhododendron, etc., have been crossed, yet many of thesehybrids seed freely. For instance, Herbert asserts that a hybrid fromCalceolaria integrifolia and plantaginea, species most widelydissimilar in general habit, "reproduced itself as perfectly as if ithad been a natural species from the mountains of Chile." I have takensome pains to ascertain the degree of fertility of some of the complexcrosses of Rhododendrons, and I am assured that many of them areperfectly fertile. Mr. C. Noble, for instance, informs me that heraises stocks for grafting from a hybrid between Rhododendron Ponticumand Catawbiense, and that this hybrid "seeds as freely as it ispossible to imagine." Had hybrids, when fairly treated, gone ondecreasing in fertility in each successive generation, as Gartnerbelieves to be the case, the fact would have been notorious tonurserymen. Horticulturists raise large beds of the same hybrids, andsuch alone are fairly treated, for by insect agency the severalindividuals of the same hybrid variety are allowed to freely crosswith each other, and the injurious influence of close interbreeding isthus prevented. Any one may readily convince himself of the efficiencyof insect-agency by examining the flowers of the more sterile kinds ofhybrid rhododendrons, which produce no pollen, for he will find ontheir stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully triedthan with plants. If our systematic arrangements can be trusted, thatis if the genera of animals are as distinct from each other, as arethe genera of plants, then we may infer that animals more widelyseparated in the scale of nature can be more easily crossed than inthe case of plants; but the hybrids themselves are, I think, moresterile. I doubt whether any case of a perfectly fertile hybrid animalcan be considered as thoroughly well authenticated. It should,however, be borne in mind that, owing to few animals breeding freelyunder confinement, few experiments have been fairly tried: forinstance, the canary-bird has been crossed with nine other finches,but as not one of these nine species breeds freely in confinement, wehave no right to expect that the first crosses between them and thecanary, or that their hybrids, should be perfectly fertile. Again,with respect to the fertility in successive generations of the morefertile hybrid animals, I hardly know of an instance in which twofamilies of the same hybrid have been raised at the same time fromdifferent parents, so as to avoid the ill effects of closeinterbreeding. On the contrary, brothers and sisters have usually beencrossed in each successive generation, in opposition to the constantlyrepeated admonition of every breeder. And in this case, it is not atall surprising that the inherent sterility in the hybrids should havegone on increasing. If we were to act thus, and pair brothers andsisters in the case of any pure animal, which from any cause had theleast tendency to sterility, the breed would assuredly be lost in avery few generations.
of any thoroughly well-authenticated cases ofperfectly fertile hybrid animals, I have some reason to believe thatthe hybrids from Cervulus vaginalis and Reevesii, and from Phasianuscolchicus with P. torquatus and with P. versicolor are perfectlyfertile. The hybrids from the common and Chinese geese (A. cygnoides),species which?
Although I do not know of any thoroughly well-authenticated cases ofperfectly fertile hybrid animals, I have some reason to believe thatthe hybrids from Cervulus vaginalis and Reevesii, and from Phasianuscolchicus with P. torquatus and with P. versicolor are perfectlyfertile. The hybrids from the common and Chinese geese (A. cygnoides),species which are so different that they are generally ranked indistinct genera, have often bred in this country with either pureparent, and in one single instance they have bred inter se. This waseffected by Mr. Eyton, who raised two hybrids from the same parentsbut from different hatches; and from these two birds he raised no lessthan eight hybrids (grandchildren of the pure geese) from one nest. InIndia, however, these cross-bred geese must be far more fertile; for Iam assured by two eminently capable judges, namely Mr. Blyth and Capt.Hutton, that whole flocks of these crossed geese are kept in variousparts of the country; and as they are kept for profit, where neitherpure parent-species exists, they must certainly be highly fertile.
A doctrine which originated with Pallas, has been largely accepted bymodern naturalists; namely, that most of our domestic animals havedescended from two or more aboriginal species, since commingled byintercrossing. On this view, the aboriginal species must either atfirst have produced quite fertile hybrids, or the hybrids must havebecome in subsequent generations quite fertile under domestication.This latter alternative seems to me the most probable, and I aminclined to believe in its truth, although it rests on no directevidence. I believe, for instance, that our dogs have descended fromseveral wild stocks; yet, with perhaps the exception of certainindigenous domestic dogs of South America, all are quite fertiletogether; and analogy makes me greatly doubt, whether the severalaboriginal species would at first have freely bred together and haveproduced quite fertile hybrids. So again there is reason to believethat our European and the humped Indian cattle are quite fertiletogether; but from facts communicated to me by Mr. Blyth, I think theymust be considered as distinct species. On this view of the origin ofmany of our domestic animals, we must either give up the belief of thealmost universal sterility of distinct species of animals whencrossed; or we must look at sterility, not as an indeliblecharacteristic, but as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing ofplants and animals, it may be concluded that some degree of sterility,both in first crosses and in hybrids, is an extremely general result;but that it cannot, under our present state of knowledge, beconsidered as absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the circumstances andrules governing the sterility of first crosses and of hybrids. Ourchief object will be to see whether or not the rules indicate thatspecies have specially been endowed with this quality, in order toprevent their crossing and blending together in utter confusion. Thefollowing rules and conclusions are chiefly drawn up from Gartner'sadmirable work on the hybridisation of plants. I have taken much painsto ascertain how far the rules apply to animals, and considering howscanty our knowledge is in regard to hybrid animals, I have beensurprised to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both offirst crosses and of hybrids, graduates from zero to perfectfertility. It is surprising in how many curious ways this gradationcan be shown to exist; but only the barest outline of the facts canhere be given. When pollen from a plant of one family is placed on thestigma of a plant of a distinct family, it exerts no more influencethan so much inorganic dust. From this absolute zero of fertility, thepollen of different species of the same genus applied to the stigma ofsome one species, yields a perfect gradation in the number of seedsproduced, up to nearly complete or even quite complete fertility; and,as we have seen, in certain abnormal cases, even to an excess offertility, beyond that which the plant's own pollen will produce. Soin hybrids themselves, there are some which never have produced, andprobably never would produce, even with the pollen of either pureparent, a single fertile seed: but in some of these cases a firsttrace of fertility may be detected, by the pollen of one of the pureparent-species causing the flower of the hybrid to wither earlier thanit otherwise would have done; and the early withering of the flower iswell known to be a sign of incipient fertilisation. From this extremedegree of sterility we have self-fertilised hybrids producing agreater and greater number of seeds up to perfect fertility.