物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 8. HYBRIDISM. Page 2
Hybrids from two species which are very difficult to cross, and whichrarely produce any offspring, are generally very sterile; but theparallelism between the difficulty of making a first cross, and thesterility of the hybrids thus produced--two classes of facts which aregenerally confounded together--is by no means strict. There are manycases, in which two pure species can be united with unusual facility,and produce numerous hybrid-offspring, yet these hybrids areremarkably sterile. On the other hand, there are species which can becrossed very rarely, or with extreme difficulty, but the hybrids, whenat last produced, are very fertile. Even within the limits of the samegenus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easilyaffected by unfavourable conditions, than is the fertility of purespecies. But the degree of fertility is likewise innately variable;for it is not always the same when the same two species are crossedunder the same circumstances, but depends in part upon theconstitution of the individuals which happen to have been chosen forthe experiment. So it is with hybrids, for their degree of fertilityis often found to differ greatly in the several individuals raisedfrom seed out of the same capsule and exposed to exactly the sameconditions.
By the term systematic affinity is meant, the resemblance betweenspecies in structure and in constitution, more especially in thestructure of parts which are of high physiological importance andwhich differ little in the allied species. Now the fertility of firstcrosses between species, and of the hybrids produced from them, islargely governed by their systematic affinity. This is clearly shownby hybrids never having been raised between species ranked bysystematists in distinct families; and on the other hand, by veryclosely allied species generally uniting with facility. But thecorrespondence between systematic affinity and the facility ofcrossing is by no means strict. A multitude of cases could be given ofvery closely allied species which will not unite, or only with extremedifficulty; and on the other hand of very distinct species which unitewith the utmost facility. In the same family there may be a genus, asDianthus, in which very many species can most readily be crossed; andanother genus, as Silene, in which the most persevering efforts havefailed to produce between extremely close species a single hybrid.Even within the limits of the same genus, we meet with this samedifference; for instance, the many species of Nicotiana have been morelargely crossed than the species of almost any other genus; butGartner found that N. acuminata, which is not a particularly distinctspecies, obstinately failed to fertilise, or to be fertilised by, noless than eight other species of Nicotiana. Very many analogous factscould be given.
No one has been able to point out what kind, or what amount, ofdifference in any recognisable character is sufficient to prevent twospecies crossing. It can be shown that plants most widely different inhabit and general appearance, and having strongly marked differencesin every part of the flower, even in the pollen, in the fruit, and inthe cotyledons, can be crossed. Annual and perennial plants, deciduousand evergreen trees, plants inhabiting different stations and fittedfor extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, forinstance, of a stallion-horse being first crossed with a female-ass,and then a male-ass with a mare: these two species may then be said tohave been reciprocally crossed. There is often the widest possibledifference in the facility of making reciprocal crosses. Such casesare highly important, for they prove that the capacity in any twospecies to cross is often completely independent of their systematicaffinity, or of any recognisable difference in their wholeorganisation. On the other hand, these cases clearly show that thecapacity for crossing is connected with constitutional differencesimperceptible by us, and confined to the reproductive system. Thisdifference in the result of reciprocal crosses between the same twospecies was long ago observed by Kolreuter. To give an instance:Mirabilis jalappa can easily be fertilised by the pollen of M.longiflora, and the hybrids thus produced are sufficiently fertile;but Kolreuter tried more than two hundred times, during eightfollowing years, to fertilise reciprocally M. longiflora with thepollen of M. jalappa, and utterly failed. Several other equallystriking cases could be given. Thuret has observed the same fact withcertain sea-weeds or Fuci. Gartner, moreover, found that thisdifference of facility in making reciprocal crosses is extremelycommon in a lesser degree. He has observed it even between forms soclosely related (as Matthiola annua and glabra) that many botanistsrank them only as varieties. It is also a remarkable fact, thathybrids raised from reciprocal crosses, though of course compounded ofthe very same two species, the one species having first been used asthe father and then as the mother, generally differ in fertility in asmall, and occasionally in a high degree.
Several other singular rules could be given from Gartner: forinstance, some species have a remarkable power of crossing with otherspecies; other species of the same genus have a remarkable power ofimpressing their likeness on their hybrid offspring; but these twopowers do not at all necessarily go together. There are certainhybrids which instead of having, as is usual, an intermediatecharacter between their two parents, always closely resemble one ofthem; and such hybrids, though externally so like one of their pureparent-species, are with rare exceptions extremely sterile. So againamongst hybrids which are usually intermediate in structure betweentheir parents, exceptional and abnormal individuals sometimes areborn, which closely resemble one of their pure parents; and thesehybrids are almost always utterly sterile, even when the other hybridsraised from seed from the same capsule have a considerable degree offertility. These facts show how completely fertility in the hybrid isindependent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility offirst crosses and of hybrids, we see that when forms, which must beconsidered as good and distinct species, are united, their fertilitygraduates from zero to perfect fertility, or even to fertility undercertain conditions in excess. That their fertility, besides beingeminently susceptible to favourable and unfavourable conditions, isinnately variable. That it is by no means always the same in degree inthe first cross and in the hybrids produced from this cross. That thefertility of hybrids is not related to the degree in which theyresemble in external appearance either parent. And lastly, that thefacility of making a first cross between any two species is not alwaysgoverned by their systematic affinity or degree of resemblance to eachother. This latter statement is clearly proved by reciprocal crossesbetween the same two species, for according as the one species or theother is used as the father or the mother, there is generally somedifference, and occasionally the widest possible difference, in thefacility of effecting an union. The hybrids, moreover, produced fromreciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species havebeen endowed with sterility simply to prevent their becomingconfounded in nature? I think not. For why should the sterility be soextremely different in degree, when various species are crossed, allof which we must suppose it would be equally important to keep fromblending together? Why should the degree of sterility be innatelyvariable in the individuals of the same species? Why should somespecies cross with facility, and yet produce very sterile hybrids; andother species cross with extreme difficulty, and yet produce fairlyfertile hybrids? Why should there often be so great a difference inthe result of a reciprocal cross between the same two species? Why, itmay even be asked, has the production of hybrids been permitted? togrant to species the special power of producing hybrids, and then tostop their further propagation by different degrees of sterility, notstrictly related to the facility of the first union between theirparents, seems to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearlyto indicate that the sterility both of first crosses and of hybrids issimply incidental or dependent on unknown differences, chiefly in thereproductive systems, of the species which are crossed. Thedifferences being of so peculiar and limited a nature, that, inreciprocal crosses between two species the male sexual element of theone will often freely act on the female sexual element of the other,but not in a reversed direction. It will be advisable to explain alittle more fully by an example what I mean by sterility beingincidental on other differences, and not a specially endowed quality.As the capacity of one plant to be grafted or budded on another is soentirely unimportant for its welfare in a state of nature, I presumethat no one will suppose that this capacity is a SPECIALLY endowedquality, but will admit that it is incidental on differences in thelaws of growth of the two plants. We can sometimes see the reason whyone tree will not take on another, from differences in their rate ofgrowth, in the hardness of their wood, in the period of the flow ornature of their sap, etc.; but in a multitude of cases we can assignno reason whatever. Great diversity in the size of two plants, onebeing woody and the other herbaceous, one being evergreen and theother deciduous, and adaptation to widely different climates, does notalways prevent the two grafting together. As in hybridisation, so withgrafting, the capacity is limited by systematic affinity, for no onehas been able to graft trees together belonging to quite distinctfamilies; and, on the other hand, closely allied species, andvarieties of the same species, can usually, but not invariably, begrafted with ease. But this capacity, as in hybridisation, is by nomeans absolutely governed by systematic affinity. Although manydistinct genera within the same family have been grafted together, inother cases species of the same genus will not take on each other. Thepear can be grafted far more readily on the quince, which is ranked asa distinct genus, than on the apple, which is a member of the samegenus. Even different varieties of the pear take with differentdegrees of facility on the quince; so do different varieties of theapricot and peach on certain varieties of the plum.
As Gartner found that there was sometimes an innate difference indifferent INDIVIDUALS of the same two species in crossing; so Sagaretbelieves this to be the case with different individuals of the sametwo species in being grafted together. As in reciprocal crosses, thefacility of effecting an union is often very far from equal, so itsometimes is in grafting; the common gooseberry, for instance, cannotbe grafted on the currant, whereas the currant will take, though withdifficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have theirreproductive organs in an imperfect condition, is a very differentcase from the difficulty of uniting two pure species, which have theirreproductive organs perfect; yet these two distinct cases run to acertain extent parallel. Something analogous occurs in grafting; forThouin found that three species of Robinia, which seeded freely ontheir own roots, and which could be grafted with no great difficultyon another species, when thus grafted were rendered barren. On theother hand, certain species of Sorbus, when grafted on other species,yielded twice as much fruit as when on their own roots. We arereminded by this latter fact of the extraordinary case of Hippeastrum,Lobelia, etc., which seeded much more freely when fertilised with thepollen of distinct species, than when self-fertilised with their ownpollen.
We thus see, that although there is a clear and fundamental differencebetween the mere adhesion of grafted stocks, and the union of the maleand female elements in the act of reproduction, yet that there is arude degree of parallelism in the results of grafting and of crossingdistinct species. And as we must look at the curious and complex lawsgoverning the facility with which trees can be grafted on each otheras incidental on unknown differences in their vegetative systems, so Ibelieve that the still more complex laws governing the facility offirst crosses, are incidental on unknown differences, chiefly in theirreproductive systems. These differences, in both cases, follow to acertain extent, as might have been expected, systematic affinity, bywhich every kind of resemblance and dissimilarity between organicbeings is attempted to be expressed. The facts by no means seem to meto indicate that the greater or lesser difficulty of either graftingor crossing together various species has been a special endowment;although in the case of crossing, the difficulty is as important forthe endurance and stability of specific forms, as in the case ofgrafting it is unimportant for their welfare.
CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We may now look a little closer at the probable causes of thesterility of first crosses and of hybrids. These two cases arefundamentally different, for, as just remarked, in the union of twopure species the male and female sexual elements are perfect, whereasin hybrids they are imperfect. Even in first crosses, the greater orlesser difficulty in effecting a union apparently depends on severaldistinct causes. There must sometimes be a physical impossibility inthe male element reaching the ovule, as would be the case with a planthaving a pistil too long for the pollen-tubes to reach the ovarium. Ithas also been observed that when pollen of one species is placed onthe stigma of a distantly allied species, though the pollen-tubesprotrude, they do not penetrate the stigmatic surface. Again, the maleelement may reach the female element, but be incapable of causing anembryo to be developed, as seems to have been the case with some ofThuret's experiments on Fuci. No explanation can be given of thesefacts, any more than why certain trees cannot be grafted on others.Lastly, an embryo may be developed, and then perish at an earlyperiod. This latter alternative has not been sufficiently attended to;but I believe, from observations communicated to me by Mr. Hewitt, whohas had great experience in hybridising gallinaceous birds, that theearly death of the embryo is a very frequent cause of sterility infirst crosses. I was at first very unwilling to believe in this view;as hybrids, when once born, are generally healthy and long-lived, aswe see in the case of the common mule. Hybrids, however, aredifferently circumstanced before and after birth: when born and livingin a country where their two parents can live, they are generallyplaced under suitable conditions of life. But a hybrid partakes ofonly half of the nature and constitution of its mother, and thereforebefore birth, as long as it is nourished within its mother's womb orwithin the egg or seed produced by the mother, it may be exposed toconditions in some degree unsuitable, and consequently be liable toperish at an early period; more especially as all very young beingsseem eminently sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual elementsare imperfectly developed, the case is very different. I have morethan once alluded to a large body of facts, which I have collected,showing that when animals and plants are removed from their naturalconditions, they are extremely liable to have their reproductivesystems seriously affected. This, in fact, is the great bar to thedomestication of animals. Between the sterility thus superinduced andthat of hybrids, there are many points of similarity. In both casesthe sterility is independent of general health, and is oftenaccompanied by excess of size or great luxuriance. In both cases, thesterility occurs in various degrees; in both, the male element is themost liable to be affected; but sometimes the female more than themale. In both, the tendency goes to a certain extent with systematicaffinity, for whole groups of animals and plants are rendered impotentby the same unnatural conditions; and whole groups of species tend toproduce sterile hybrids. On the other hand, one species in a groupwill sometimes resist great changes of conditions with unimpairedfertility; and certain species in a group will produce unusuallyfertile hybrids. No one can tell, till he tries, whether anyparticular animal will breed under confinement or any plant seedfreely under culture; nor can he tell, till he tries, whether any twospecies of a genus will produce more or less sterile hybrids. Lastly,when organic beings are placed during several generations underconditions not natural to them, they are extremely liable to vary,which is due, as I believe, to their reproductive systems having beenspecially affected, though in a lesser degree than when sterilityensues. So it is with hybrids, for hybrids in successive generationsare eminently liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new andunnatural conditions, and when hybrids are produced by the unnaturalcrossing of two species, the reproductive system, independently of thegeneral state of health, is affected by sterility in a very similarmanner. In the one case, the conditions of life have been disturbed,though often in so slight a degree as to be inappreciable by us; inthe other case, or that of hybrids, the external conditions haveremained the same, but the organisation has been disturbed by twodifferent structures and constitutions having been blended into one.For it is scarcely possible that two organisations should becompounded into one, without some disturbance occurring in thedevelopment, or periodical action, or mutual relation of the differentparts and organs one to another, or to the conditions of life. Whenhybrids are able to breed inter se, they transmit to their offspringfrom generation to generation the same compounded organisation, andhence we need not be surprised that their sterility, though in somedegree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting onvague hypotheses, several facts with respect to the sterility ofhybrids; for instance, the unequal fertility of hybrids produced fromreciprocal crosses; or the increased sterility in those hybrids whichoccasionally and exceptionally resemble closely either pure parent.Nor do I pretend that the foregoing remarks go to the root of thematter: no explanation is offered why an organism, when placed underunnatural conditions, is rendered sterile. All that I have attemptedto show, is that in two cases, in some respects allied, sterility isthe common result,--in the one case from the conditions of life havingbeen disturbed, in the other case from the organisation having beendisturbed by two organisations having been compounded into one.