物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 8. HYBRIDISM. Page 3
It may seem fanciful, but I suspect that a similar parallelism extendsto an allied yet very different class of facts. It is an old andalmost universal belief, founded, I think, on a considerable body ofevidence, that slight changes in the conditions of life are beneficialto all living things. We see this acted on by farmers and gardeners intheir frequent exchanges of seed, tubers, etc., from one soil orclimate to another, and back again. During the convalescence ofanimals, we plainly see that great benefit is derived from almost anychange in the habits of life. Again, both with plants and animals,there is abundant evidence, that a cross between very distinctindividuals of the same species, that is between members of differentstrains or sub-breeds, gives vigour and fertility to the offspring. Ibelieve, indeed, from the facts alluded to in our fourth chapter, thata certain amount of crossing is indispensable even withhermaphrodites; and that close interbreeding continued during severalgenerations between the nearest relations, especially if these be keptunder the same conditions of life, always induces weakness andsterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditionsof life benefit all organic beings, and on the other hand, that slightcrosses, that is crosses between the males and females of the samespecies which have varied and become slightly different, give vigourand fertility to the offspring. But we have seen that greater changes,or changes of a particular nature, often render organic beings in somedegree sterile; and that greater crosses, that is crosses betweenmales and females which have become widely or specifically different,produce hybrids which are generally sterile in some degree. I cannotpersuade myself that this parallelism is an accident or an illusion.Both series of facts seem to be connected together by some common butunknown bond, which is essentially related to the principle of life.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.
It may be urged, as a most forcible argument, that there must be someessential distinction between species and varieties, and that theremust be some error in all the foregoing remarks, inasmuch asvarieties, however much they may differ from each other in externalappearance, cross with perfect facility, and yield perfectly fertileoffspring. I fully admit that this is almost invariably the case. Butif we look to varieties produced under nature, we are immediatelyinvolved in hopeless difficulties; for if two hitherto reputedvarieties be found in any degree sterile together, they are at onceranked by most naturalists as species. For instance, the blue and redpimpernel, the primrose and cowslip, which are considered by many ofour best botanists as varieties, are said by Gartner not to be quitefertile when crossed, and he consequently ranks them as undoubtedspecies. If we thus argue in a circle, the fertility of all varietiesproduced under nature will assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been produced,under domestication, we are still involved in doubt. For when it isstated, for instance, that the German Spitz dog unites more easilythan other dogs with foxes, or that certain South American indigenousdomestic dogs do not readily cross with European dogs, the explanationwhich will occur to everyone, and probably the true one, is that thesedogs have descended from several aboriginally distinct species.Nevertheless the perfect fertility of so many domestic varieties,differing widely from each other in appearance, for instance of thepigeon or of the cabbage, is a remarkable fact; more especially whenwe reflect how many species there are, which, though resembling eachother most closely, are utterly sterile when intercrossed. Severalconsiderations, however, render the fertility of domestic varietiesless remarkable than at first appears. It can, in the first place, beclearly shown that mere external dissimilarity between two speciesdoes not determine their greater or lesser degree of sterility whencrossed; and we may apply the same rule to domestic varieties. In thesecond place, some eminent naturalists believe that a long course ofdomestication tends to eliminate sterility in the successivegenerations of hybrids, which were at first only slightly sterile; andif this be so, we surely ought not to expect to find sterility bothappearing and disappearing under nearly the same conditions of life.Lastly, and this seems to me by far the most important consideration,new races of animals and plants are produced under domestication byman's methodical and unconscious power of selection, for his own useand pleasure: he neither wishes to select, nor could select, slightdifferences in the reproductive system, or other constitutionaldifferences correlated with the reproductive system. He supplies hisseveral varieties with the same food; treats them in nearly the samemanner, and does not wish to alter their general habits of life.Nature acts uniformly and slowly during vast periods of time on thewhole organisation, in any way which may be for each creature's owngood; and thus she may, either directly, or more probably indirectly,through correlation, modify the reproductive system in the severaldescendants from any one species. Seeing this difference in theprocess of selection, as carried on by man and nature, we need not besurprised at some difference in the result.
I have as yet spoken as if the varieties of the same species wereinvariably fertile when intercrossed. But it seems to me impossible toresist the evidence of the existence of a certain amount of sterilityin the few following cases, which I will briefly abstract. Theevidence is at least as good as that from which we believe in thesterility of a multitude of species. The evidence is, also, derivedfrom hostile witnesses, who in all other cases consider fertility andsterility as safe criterions of specific distinction. Gartner keptduring several years a dwarf kind of maize with yellow seeds, and atall variety with red seeds, growing near each other in his garden;and although these plants have separated sexes, they never naturallycrossed. He then fertilised thirteen flowers of the one with thepollen of the other; but only a single head produced any seed, andthis one head produced only five grains. Manipulation in this casecould not have been injurious, as the plants have separated sexes. Noone, I believe, has suspected that these varieties of maize aredistinct species; and it is important to notice that the hybrid plantsthus raised were themselves PERFECTLY fertile; so that even Gartnerdid not venture to consider the two varieties as specificallydistinct.
Girou de Buzareingues crossed three varieties of gourd, which like themaize has separated sexes, and he asserts that their mutualfertilisation is by so much the less easy as their differences aregreater. How far these experiments may be trusted, I know not; but theforms experimentised on, are ranked by Sagaret, who mainly founds hisclassification by the test of infertility, as varieties.
The following case is far more remarkable, and seems at first quiteincredible; but it is the result of an astonishing number ofexperiments made during many years on nine species of Verbascum, by sogood an observer and so hostile a witness, as Gartner: namely, thatyellow and white varieties of the same species of Verbascum whenintercrossed produce less seed, than do either coloured varieties whenfertilised with pollen from their own coloured flowers. Moreover, heasserts that when yellow and white varieties of one species arecrossed with yellow and white varieties of a DISTINCT species, moreseed is produced by the crosses between the same coloured flowers,than between those which are differently coloured. Yet these varietiesof Verbascum present no other difference besides the mere colour ofthe flower; and one variety can sometimes be raised from the seed ofthe other.
From observations which I have made on certain varieties of hollyhock,I am inclined to suspect that they present analogous facts.
Kolreuter, whose accuracy has been confirmed by every subsequentobserver, has proved the remarkable fact, that one variety of thecommon tobacco is more fertile, when crossed with a widely distinctspecies, than are the other varieties. He experimentised on fiveforms, which are commonly reputed to be varieties, and which he testedby the severest trial, namely, by reciprocal crosses, and he foundtheir mongrel offspring perfectly fertile. But one of these fivevarieties, when used either as father or mother, and crossed with theNicotiana glutinosa, always yielded hybrids not so sterile as thosewhich were produced from the four other varieties when crossed with N.glutinosa. Hence the reproductive system of this one variety must havebeen in some manner and in some degree modified.
From these facts; from the great difficulty of ascertaining theinfertility of varieties in a state of nature, for a supposed varietyif infertile in any degree would generally be ranked as species; fromman selecting only external characters in the production of the mostdistinct domestic varieties, and from not wishing or being able toproduce recondite and functional differences in the reproductivesystem; from these several considerations and facts, I do not thinkthat the very general fertility of varieties can be proved to be ofuniversal occurrence, or to form a fundamental distinction betweenvarieties and species. The general fertility of varieties does notseem to me sufficient to overthrow the view which I have taken withrespect to the very general, but not invariable, sterility of firstcrosses and of hybrids, namely, that it is not a special endowment,but is incidental on slowly acquired modifications, more especially inthe reproductive systems of the forms which are crossed.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.
Independently of the question of fertility, the offspring of specieswhen crossed and of varieties when crossed may be compared in severalother respects. Gartner, whose strong wish was to draw a marked lineof distinction between species and varieties, could find very few and,as it seems to me, quite unimportant differences between the so-calledhybrid offspring of species, and the so-called mongrel offspring ofvarieties. And, on the other hand, they agree most closely in verymany important respects.
I shall here discuss this subject with extreme brevity. The mostimportant distinction is, that in the first generation mongrels aremore variable than hybrids; but Gartner admits that hybrids fromspecies which have long been cultivated are often variable in thefirst generation; and I have myself seen striking instances of thisfact. Gartner further admits that hybrids between very closely alliedspecies are more variable than those from very distinct species; andthis shows that the difference in the degree of variability graduatesaway. When mongrels and the more fertile hybrids are propagated forseveral generations an extreme amount of variability in theiroffspring is notorious; but some few cases both of hybrids andmongrels long retaining uniformity of character could be given. Thevariability, however, in the successive generations of mongrels is,perhaps, greater than in hybrids.
This greater variability of mongrels than of hybrids does not seem tome at all surprising. For the parents of mongrels are varieties, andmostly domestic varieties (very few experiments having been tried onnatural varieties), and this implies in most cases that there has beenrecent variability; and therefore we might expect that suchvariability would often continue and be super-added to that arisingfrom the mere act of crossing. The slight degree of variability inhybrids from the first cross or in the first generation, in contrastwith their extreme variability in the succeeding generations, is acurious fact and deserves attention. For it bears on and corroboratesthe view which I have taken on the cause of ordinary variability;namely, that it is due to the reproductive system being eminentlysensitive to any change in the conditions of life, being thus oftenrendered either impotent or at least incapable of its proper functionof producing offspring identical with the parent-form. Now hybrids inthe first generation are descended from species (excluding those longcultivated) which have not had their reproductive systems in any wayaffected, and they are not variable; but hybrids themselves have theirreproductive systems seriously affected, and their descendants arehighly variable.
But to return to our comparison of mongrels and hybrids: Gartnerstates that mongrels are more liable than hybrids to revert to eitherparent-form; but this, if it be true, is certainly only a differencein degree. Gartner further insists that when any two species, althoughmost closely allied to each other, are crossed with a third species,the hybrids are widely different from each other; whereas if two verydistinct varieties of one species are crossed with another species,the hybrids do not differ much. But this conclusion, as far as I canmake out, is founded on a single experiment; and seems directlyopposed to the results of several experiments made by Kolreuter.
These alone are the unimportant differences, which Gartner is able topoint out, between hybrid and mongrel plants. On the other hand, theresemblance in mongrels and in hybrids to their respective parents,more especially in hybrids produced from nearly related species,follows according to Gartner the same laws. When two species arecrossed, one has sometimes a prepotent power of impressing itslikeness on the hybrid; and so I believe it to be with varieties ofplants. With animals one variety certainly often has this prepotentpower over another variety. Hybrid plants produced from a reciprocalcross, generally resemble each other closely; and so it is withmongrels from a reciprocal cross. Both hybrids and mongrels can bereduced to either pure parent-form, by repeated crosses in successivegenerations with either parent.
These several remarks are apparently applicable to animals; but thesubject is here excessively complicated, partly owing to the existenceof secondary sexual characters; but more especially owing toprepotency in transmitting likeness running more strongly in one sexthan in the other, both when one species is crossed with another, andwhen one variety is crossed with another variety. For instance, Ithink those authors are right, who maintain that the ass has aprepotent power over the horse, so that both the mule and the hinnymore resemble the ass than the horse; but that the prepotency runsmore strongly in the male-ass than in the female, so that the mule,which is the offspring of the male-ass and mare, is more like an ass,than is the hinny, which is the offspring of the female-ass andstallion.
Much stress has been laid by some authors on the supposed fact, thatmongrel animals alone are born closely like one of their parents; butit can be shown that this does sometimes occur with hybrids; yet Igrant much less frequently with hybrids than with mongrels. Looking tothe cases which I have collected of cross-bred animals closelyresembling one parent, the resemblances seem chiefly confined tocharacters almost monstrous in their nature, and which have suddenlyappeared--such as albinism, melanism, deficiency of tail or horns, oradditional fingers and toes; and do not relate to characters whichhave been slowly acquired by selection. Consequently, suddenreversions to the perfect character of either parent would be morelikely to occur with mongrels, which are descended from varietiesoften suddenly produced and semi-monstrous in character, than withhybrids, which are descended from species slowly and naturallyproduced. On the whole I entirely agree with Dr. Prosper Lucas, who,after arranging an enormous body of facts with respect to animals,comes to the conclusion, that the laws of resemblance of the child toits parents are the same, whether the two parents differ much orlittle from each other, namely in the union of individuals of the samevariety, or of different varieties, or of distinct species.
Laying aside the question of fertility and sterility, in all otherrespects there seems to be a general and close similarity in theoffspring of crossed species, and of crossed varieties. If we look atspecies as having been specially created, and at varieties as havingbeen produced by secondary laws, this similarity would be anastonishing fact. But it harmonises perfectly with the view that thereis no essential distinction between species and varieties.
SUMMARY OF CHAPTER.
First crosses between forms sufficiently distinct to be ranked asspecies, and their hybrids, are very generally, but not universally,sterile. The sterility is of all degrees, and is often so slight thatthe two most careful experimentalists who have ever lived, have cometo diametrically opposite conclusions in ranking forms by this test.The sterility is innately variable in individuals of the same species,and is eminently susceptible of favourable and unfavourableconditions. The degree of sterility does not strictly followsystematic affinity, but is governed by several curious and complexlaws. It is generally different, and sometimes widely different, inreciprocal crosses between the same two species. It is not alwaysequal in degree in a first cross and in the hybrid produced from thiscross.
In the same manner as in grafting trees, the capacity of one speciesor variety to take on another, is incidental on generally unknowndifferences in their vegetative systems, so in crossing, the greateror less facility of one species to unite with another, is incidentalon unknown differences in their reproductive systems. There is no morereason to think that species have been specially endowed with variousdegrees of sterility to prevent them crossing and blending in nature,than to think that trees have been specially endowed with various andsomewhat analogous degrees of difficulty in being grafted together inorder to prevent them becoming inarched in our forests.
The sterility of first crosses between pure species, which have theirreproductive systems perfect, seems to depend on severalcircumstances; in some cases largely on the early death of the embryo.The sterility of hybrids, which have their reproductive systemsimperfect, and which have had this system and their whole organisationdisturbed by being compounded of two distinct species, seems closelyallied to that sterility which so frequently affects pure species,when their natural conditions of life have been disturbed. This viewis supported by a parallelism of another kind;--namely, that thecrossing of forms only slightly different is favourable to the vigourand fertility of their offspring; and that slight changes in theconditions of life are apparently favourable to the vigour andfertility of all organic beings. It is not surprising that the degreeof difficulty in uniting two species, and the degree of sterility oftheir hybrid-offspring should generally correspond, though due todistinct causes; for both depend on the amount of difference of somekind between the species which are crossed. Nor is it surprising thatthe facility of effecting a first cross, the fertility of the hybridsproduced, and the capacity of being grafted together--though thislatter capacity evidently depends on widely differentcircumstances--should all run, to a certain extent, parallel with thesystematic affinity of the forms which are subjected to experiment;for systematic affinity attempts to express all kinds of resemblancebetween all species.
First crosses between forms known to be varieties, or sufficientlyalike to be considered as varieties, and their mongrel offspring, arevery generally, but not quite universally, fertile. Nor is this nearlygeneral and perfect fertility surprising, when we remember how liablewe are to argue in a circle with respect to varieties in a state ofnature; and when we remember that the greater number of varieties havebeen produced under domestication by the selection of mere externaldifferences, and not of differences in the reproductive system. In allother respects, excluding fertility, there is a close generalresemblance between hybrids and mongrels. Finally, then, the factsbriefly given in this chapter do not seem to me opposed to, but evenrather to support the view, that there is no fundamental distinctionbetween species and varieties.