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物种起源|The Origin of Species

第八章　杂种性质|CHAPTER VIII　HYBRIDISM

属类： 双语小说 【分类】其他读物 阅读：[7135]
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第一次杂交不育性和杂种不育性的区别——不同程度的不育性，不是普遍的，受近亲交配的影响，因家养而消除——支配杂种不育性的法则——不育性不是特别的禀赋，而是伴随其他差异而起——第一次杂交不育性和杂种不育性的原因——变化了的生活条件的效果和杂交的效果之间的平行现象——变种杂交的能育性及混种后代的能育性不是普遍的——不考虑能育性，杂种和混种的比较——提要

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Distinction between the Sterility of first crosses and of hybrids— Sterility various in degree, not universal, affected by close interbreeding, removed by domestication—Laws governing the sterility of hybrids—Sterility not a special endowment, but incidental on other differences—Causes of the sterility of first crosses and of hybrids—Parallelism between the effects of changed conditions of life and crossing—Fertility of varieties when crossed and of their mongrel offspring not universal—Hybrids and mongrels compared independently of their fertility—Summary

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博物学者们一般抱有一种观点，认为一些物种互相杂交，被特别地赋予了不育性，借以阻止生物的混杂。这一观点乍看对，因为物种生活在同区域，如果可以自由杂交，很少能够保持不混杂的。杂种普遍都不育，我看这一点的重要性被最近一些作者低估了。根据自然选择理论，这个个案尤其重要，因为杂种的不育性不可能对它们有利，从而并不能由各种不同程度的、连续的、有利的不育性的持续保存而获得。不过，我希望能够阐明，不育性并不是特别获得或者禀赋的品质，而是伴随其他获得差异而产生的。

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The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms. This view certainly seems at first probable, for species within the same country could hardly have kept distinct had they been capable of crossing freely. The importance of the fact that hybrids are very generally sterile, has, I think, been much underrated by some late writers. On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences.

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讨论这一主题时，有两类截然不同的事实，一般却被混淆在一起；即：两个物种在第一次杂交时的不育性，以及由它们产生出来的杂种的不育性。

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In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded together; namely, the sterility of two species when first crossed, and the sterility of the hybrids produced from them.

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纯粹的物种当然具有完善的生殖器官，然而互相杂交时，则很少产生后代，或者不产生后代。另一方面，从动植物的雄性生殖质都可以明显看出，杂种的生殖器官在性机能上无能，虽然生殖器官本身的构造在显微镜下看来还是完善的。在上述第一种情形里，形成胚体的雌雄性生殖质是完善的，在第二种情形里，雌雄性生殖质要么完全不发育，要么发育不完全。必须考虑两种情形所共有的不育性的原因时，这种区别是重要的。由于把两种情形下的不育性都看作是并非我们理解能力所能掌握的一种特别禀赋，这种区别大概模糊了。

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Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. This distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered. The distinction has probably been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers.

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变种——即知道或认为是从共同祖先传下来的类型——杂交时的能育性，以及它们的杂种后代的能育性，对于我的理论，与物种杂交时的不育性有同等的重要性；因为这似乎在物种和变种之间划出了明确而清楚的区别。

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The fertility of varieties, that is of the forms known or believed to have descended from common parents, when intercrossed, and likewise the fertility of their mongrel offspring, is, on my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species.

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首先，是关于物种杂交时的不育性及其杂种后代的不育性。科尔路特和盖特纳两位认真可敬的观察者几乎毕生研究这个问题，凡是读过他们若干研究报告和著作的，不可能不深深感到某种程度的不育性是非常普遍的。科尔路特把这个规律普遍化了。可是十个例子中，他发现有两个类型，虽被大多数作者看作是不同物种，在杂交时却是十分能育的，于是他快刀斩乱麻，毫不犹豫地将其列为变种。盖特纳也把这个规律同样普遍化了，而对科尔路特所举十例的完全能育性提出质疑。但是在这些和许多其他个案里，盖特纳不得不细数产籽数，以指出其中有任何程度的不育性。他总是把两物种杂交时、杂种后代所产种子的最高数目，与双方纯亲种在自然状态下种子的平均数相比。但是我看由此引入了严重的出错原因：进行杂交的植物必须去势，更必须隔离，以防止昆虫带来其他植物的花粉。盖特纳所试验的植物几乎全都是盆栽的，看样子放在他家一间屋子里。这些做法无疑常常会损害植物的能育性；他在列表中所举植物有二十个个案，都去势了，并且以自花进行人工授粉（一切豆科植物除外，公认难操纵），这二十种植物的一半，能育性受到了某种程度的损害。还有，盖特纳几年里反复杂交樱草和黄花九轮草，我们有充足理由将其当变种，所以他收获能育种子的只有一两次。他使普通的红花海绿（Anagallis arvensis）和蓝花海绿（Anagallis coerulea）进行杂交，发现是绝对不育的，而这些类型曾被最优秀的植物学家们列为变种。他在许多同类个案中都得出了同样结论。依我看，我们满可以怀疑许多物种互相杂交时是否的确这样不育，这是他所认为的。

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First, for the sterility of species when crossed and of their hybrid offspring. It is impossible to study the several memoirs and works of those two conscientious and admirable observers, K?lreuter and G?rtner, who almost devoted their lives to this subject, without being deeply impressed with the high generality of some degree of sterility. K?lreuter makes the rule universal; but then he cuts the knot, for in ten cases in which he found two forms, considered by most authors as distinct species, quite fertile together, he unhesitatingly ranks them as varieties. G?rtner, also, makes the rule equally universal; and he disputes the entire fertility of K?lreuter’s ten cases. But in these and in many other cases, G?rtner is obliged carefully to count the seeds, in order to show that there is any degree of sterility. He always compares the maximum number of seeds produced by two species when crossed and by their hybrid offspring, with the average number produced by both pure parent-species in a state of nature. But a serious cause of error seems to me to be here introduced: a plant to be hybridised must be castrated, and, what is often more important, must be secluded in order to prevent pollen being brought to it by insects from other plants. Nearly all the plants experimentised on by G?rtner were potted, and apparently were kept in a chamber in his house. That these processes are often injurious to the fertility of a plant cannot be doubted; for G?rtner gives in his table about a score of cases of plants which he castrated, and artificially fertilised with their own pollen, and (excluding all cases such as the Leguminosae, in which there is an acknowledged difficulty in the manipulation) half of these twenty plants had their fertility in some degree impaired. Moreover, as G?rtner during several years repeatedly crossed the primrose and cowslip, which we have such good reason to believe to be varieties, and only once or twice succeeded in getting fertile seed; as he found the common red and blue pimpernels (Anagallis arvensis and coerulea), which the best botanists rank as varieties, absolutely sterile together; and as he came to the same conclusion in several other analogous cases; it seems to me that we may well be permitted to doubt whether many other species are really so sterile, when intercrossed, as G?rtner believes.

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可以肯定，一方面，各个物种杂交时的不育性程度大不相同，并且不易觉察地分级消失；另一方面，纯粹物种的能育性易受各种环境条件的影响，所以从一切实用的目的看，极难说出完全的能育性在何处终，而不育性又何处始。关于这一点，我想没有比史上最有经验的观察者科尔路特和盖特纳所提出的证据更为可靠的了，他们对于一模一样的物种曾得出决然相反的结论。关于某些可疑类型究应列为物种还是变种的问题，试把最优秀的植物学家们提出的证据，与不同的杂交工作者从能育性推论出来的证据或同一作者从不同年代的试验中推论出来的证据加以比较，也是最有意义的，但是这里没有篇幅来详论。由此可见，无论不育性或能育性都不能在物种和变种之间划确定的界限。从这一来源得出的证据逐渐减弱，其可疑的程度不亚于从其他体质和构造上的差异所得出的证据。

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It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. I think no better evidence of this can be required than that the two most experienced observers who have ever lived, namely, K?lreuter and G?rtner, should have arrived at diametrically opposite conclusions in regard to the very same species. It is also most instructive to compare—but I have not space here to enter on details—the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same author, from experiments made during different years. It can thus be shown that neither sterility nor fertility affords any clear distinction between species and varieties; but that the evidence from this source graduates away, and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences.

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关于杂种在连续世代中的不育性，虽然盖特纳谨慎地防止了一些杂种和纯种的父母本相杂交，能够把它们培育到六七代，一个个案甚至到了十代，但是他肯定地说，它们的能育性从不增高，反而普遍地大幅度降低了。我不怀疑这是常见的情况，能育性往往在开始几代里突然下降。我认为所有这些实验里，能育性的减低都是由于一个独立的原因，即过于接近的近亲交配。我曾搜集到多如牛毛的事实，表明很接近的近亲交配减低能育性，另一方面，与不同的个体或变种进行偶然的杂交可增高能育性，所以这个育种者几乎普遍相信的观点的正确性，我是无可置疑的。试验者们很少种植大量的杂种；并且因为亲种，或其他近缘杂种一般都养在同一园圃内，所以在开花季节必须谨慎防止昆虫进入；所以，杂种在每一世代中一般便会由自花的花粉而受精；我确信这样会损害本已由于杂种根源而降低的能育性。盖特纳反复说过的一句重要陈述，使我的这一信念加强了，他说，哪怕能育性较低的杂种，如果用同类杂种的花粉进行人工授精，不管由操纵所常常带来的不良影响，能育性往往还是决定性增高的，而且会继续不断增高。在人工授粉的过程中，偶然从另一朵花的花药上采取花粉，犹如常常从准备受精的花本身的花药上采取花粉一样是常见的事（从我的经验知道）；所以，两朵花的杂交，就这样实现了，即使大概是同一植株的两朵花。还有，凡是进行复杂的试验，像盖特纳如此谨慎的观察者也要把杂种的雄蕊去掉，这就可以在每一世代中保证用异花花粉进行杂交，这异花要么来自同一植株，要么来自同一杂种性质的另一植株。因此，我相信，与自发的自花受精正相反，人工授精的杂种在连续世代中可以增高能育性这一奇异的事实，是可以依据避免过于接近的近亲交配来解释的。

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In regard to the sterility of hybrids in successive generations; though G?rtner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts positively that their fertility never increased, but generally greatly decreased. I do not doubt that this is usually the case, and that the fertility often suddenly decreases in the first few generations. Nevertheless I believe that in all these experiments the fertility has been diminished by an independent cause, namely, from close interbreeding. I have collected so large a body of facts, showing that close interbreeding lessens fertility, and, on the other hand, that an occasional cross with a distinct individual or variety increases fertility, that I cannot doubt the correctness of this almost universal belief amongst breeders. Hybrids are seldom raised by experimentalists in great numbers; and as the parent-species, or other allied hybrids, generally grow in the same garden, the visits of insects must be carefully prevented during the flowering season: hence hybrids will generally be fertilised during each generation by their own individual pollen; and I am convinced that this would be injurious to their fertility, already lessened by their hybrid origin. I am strengthened in this conviction by a remarkable statement repeatedly made by G?rtner, namely, that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind, their fertility, notwithstanding the frequent ill effects of manipulation, sometimes decidedly increases, and goes on increasing. Now, in artificial fertilisation pollen is as often taken by chance (as I know from my own experience) from the anthers of another flower, as from the anthers of the flower itself which is to be fertilised; so that a cross between two flowers, though probably on the same plant, would be thus effected. Moreover, whenever complicated experiments are in progress, so careful an observer as G?rtner would have castrated his hybrids, and this would have insured in each generation a cross with the pollen from a distinct flower, either from the same plant or from another plant of the same hybrid nature. And thus, the strange fact of the increase of fertility in the successive generations of artificially fertilised hybrids may, I believe, be accounted for by close interbreeding having been avoided.

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现在让我们谈一谈第三位极有经验的杂交工作者赫伯特牧师所得到的结果。他的结论中强调某些杂种是完全能育的，与纯亲种一样能育，就像科尔路特和盖特纳强调不同物种之间存在着某种程度的不育性是普遍的自然法则一样。他对于盖特纳试验过的完全同样的一些物种进行了试验。结果之所以不同，我想一方面是由于赫伯特的园艺绝技，一方面是有温室可供使用。在他的许多重要陈述中，我只举出一项作为例子，即：“在长叶文殊兰（Crinum capense）蒴中的各个胚珠上授以卷叶文殊兰（C. revolutum）的花粉，就会产生自然受精情形下（他说）我从未看见过的植株。”所以这里我们看到，两个不同物种的第一次杂交，就会得到完全的甚至超完全的能育性。

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Now let us turn to the results arrived at by the third most experienced hybridiser, namely, the Honourable and Reverend W. Herbert. He is as emphatic in his conclusion that some hybrids are perfectly fertile—as fertile as the pure parent-species—as are K?lreuter and G?rtner that some degree of sterility between distinct species is a universal law of nature. He experimentised on some of the very same species as did G?rtner. The difference in their results may, I think, be in part accounted for by Herbert’s great horticultural skill, and by his having hothouses at his command. Of his many important 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 a case of its natural fecundation.” So that we here have perfect, or even more than commonly perfect, fertility in a first cross between two distinct species.

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文殊兰属这个例子使我想起一个奇妙的事实，有个体植物，如半边莲属（Lobelia）部分物种，朱顶红属（Hippeastrum）全部物种，容易用不同物种的花粉，但不易用同种花粉来受精。已经发现这些植株对不同物种的花粉结籽，虽然对于自花花粉不育，但发现其花粉使其他物种的受精，是完全正常的。所以，对于一些个体以及某些物种的一切个体，比用自花受精，实际上更容易产生杂种！例如，朱顶红（Hippeastrum aulicum）的一个鳞茎开了四朵花，赫伯特在其中的三朵花上授以自花受精，然后在第四朵花上授以从三个不同物种传下来的复杂种（compound hybrid）的花粉受精，其结果是：“前三朵花的子房很快就停止生长，几天后完全枯萎，至于由杂种花粉受精的蒴则生长旺盛，迅速达到成熟，并且结下能够自由生长的优良种子。”赫伯特先生1839年写信给我说，他已经实验五年了，其后多年继续同一试验，结果始终如一。这还被其他观察者证实，个案有朱顶红及其亚属，还有其他属，如半边莲属、西番莲属（Passiflora）、毛蕊花属（Verbascum）。尽管实验植株看上去完全健康，尽管同一种花胚珠花粉对于其他物种完全正常，可是在相互自花授粉时却机能不全，必须推论，植株处于非自然状态之中。然而，这些事例可以证明，与自花授粉的同物种相比，决定一个物种杂交能育性的高低，其原因常常是何等的微细而不可思议。

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This case of the Crinum leads me to refer to a most singular fact, namely, that there are individual plants, as with certain species of Lobelia, and with all the species of the genus Hippeastrum, which can be far more easily fertilised by the pollen of another and distinct species, than by their own pollen. For these plants have been found to yield seed to the pollen of a distinct species, though quite sterile with their own pollen, notwithstanding that their own pollen was found to be perfectly good, for it fertilised distinct species. So that certain individual plants and all the individuals of certain species can actually be hybridised much more readily than they can be self-fertilised! For instance, a bulb of Hippeastrum aulicum produced four flowers; three were fertilised by Herbert with their own pollen, and the fourth was subsequently fertilised by the pollen of a compound hybrid descended from three other and distinct species: the result was that “the ovaries of the three first flowers soon ceased to grow, and after a few days perished entirely, whereas the pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity, and bore good seed, which vegetated freely.” In a letter to me, in 1839, Mr. Herbert told me that he had then tried the experiment during five years, and he continued to try it during several subsequent years, and always with the same result. This result has, also, been confirmed by other observers in the case of Hippeastrum with its sub-genera, and in the case of some other genera, as Lobelia, Passiflora and Verbascum. Although the plants in these experiments appeared perfectly healthy, and although both the ovules and pollen of the same flower were perfectly good with respect to other species, yet as they were functionally imperfect in their mutual self-action, we must infer that the plants were in an unnatural state. Nevertheless these facts show on what slight and mysterious causes the lesser or greater fertility of species when crossed, in comparison with the same species when self-fertilised, sometimes depends.

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园艺家的实际试验虽然缺少科学精密性，却值得留意。众所周知，天竺葵属、倒挂金钟属（Fuchsia）、蒲包花属（Calceolaria）、矮牵牛属（Petunia）、杜鹃花属等等物种之间，进行过何等复杂方式的杂交，然而许多这些杂种都能自由结籽。例如，赫伯特断言，从绉叶蒲包花（Calceolaria integrifolia）和车前叶蒲包花（Calceolaria integrifolia plantaginea）这两个习性上颇不相同的物种得到一个杂种，“自己完全能够繁殖，就像来自智利山区的自然物种”。我煞费苦心地探究过杜鹃花属复杂杂交的能育性程度，可以确定多数是完全能育的。诺布尔（C. Noble）先生告诉我，他把小亚细亚杜鹃（Rhod. ponticum）和北美山杜鹃（Rhod. catawbiense）之间的杂种嫁接在某些砧木上，这个杂种“有我们所能想象的自由结籽能力”。杂种在正当的处理下，如果能育性在每一连续世代中经常不断地减低，如盖特纳所相信的那样，那这事早已被艺园者盯上了。园艺家把同一杂种培育在广大园地上，只有这样才是正当处理，因为昆虫的媒介作用，若干个体可以彼此自由杂交，阻止了接近的近亲交配的有害影响。只要检查一下杜鹃花属杂种不育种的花，任何人都易相信昆虫媒介作用的效力，它们不产生花粉，柱头上却可见来自异花的大量花粉。

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The practical experiments of horticulturists, though not made with scientific precision, deserve some notice. It is notorious in how complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids seed freely. For instance, Herbert asserts that a hybrid from Calceolaria integrifolia and plantaginea, species most widely dissimilar in general habit, “reproduced itself as perfectly as if it had been a natural species from the mountains of Chile.” I have taken some pains to ascertain the degree of fertility of some of the complex crosses of Rhododendrons, and I am assured that many of them are perfectly fertile. Mr. C. Noble, for instance, informs me that he raises stocks for grafting from a hybrid between Rhod. Ponticum and Catawbiense, and that this hybrid “seeds as freely as it is possible to imagine.” Had hybrids, when fairly treated, gone on decreasing in fertility in each successive generation, as G?rtner believes to be the case, the fact would have been notorious to nurserymen. Horticulturists raise large beds of the same hybrids, and such alone are fairly treated, for by insect agency the several individuals of the same hybrid variety are allowed to freely cross with each other, and the injurious influence of close interbreeding is thus prevented. Any one may readily convince himself of the efficiency of insect-agency by examining the flowers of the more sterile kinds of hybrid rhododendrons, which produce no pollen, for he will find on their stigmas plenty of pollen brought from other flowers.

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对动物进行的仔细试验，远比植物为少。如果我们的分类系统是可靠的，这就是说，如果动物各属彼此间的区别程度不亚于植物，就可以推论，系统上区别较大的动物，比植物易于杂交；但是我想，杂种本身则更加不育了。我怀疑任何完全能育的杂种动物个案是否可以看作彻底鉴定下来了。然而，应当记住，由于很少有动物能够在圈养中自由生育，正规的实验做得不多。例如，金丝雀曾和九种其他雀科鸣禽杂交，由于九种鸟都不能在圈养中自由生育，我们就无权指望它们与金丝雀的第一次杂交品种或者其杂种是完全能育的。至于能育的动物杂种在连续世代中的能育性，我几乎不知道任何事例，从不同父母同时培育出同一杂种的两个家族，可以避免接近的近亲交配的恶劣影响。相反，动物的兄弟姊妹通常却在每一连续世代中进行杂交，违背了每一个饲养者反复提出的告诫。在这种情形下，杂种固有的不育性继续增高，完全不足为奇。如果我们这样做，就像纯种动物的兄弟姐妹交配那样，因为它们不论什么原因都极少有不育倾向，该品种肯定会在几代之内消失。

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In regard to animals, much fewer experiments have been carefully tried than with plants. If our systematic arrangements can be trusted, that is if the genera of animals are as distinct from each other, as are the genera of plants, then we may infer that animals more widely separated in the scale of nature can be more easily crossed than in the case of plants; but the hybrids themselves are, I think, more sterile. I doubt whether any case of a perfectly fertile hybrid animal can be considered as thoroughly well authenticated. It should, however, be borne in mind that, owing to few animals breeding freely under confinement, few experiments have been fairly tried: for instance, the canary-bird has been crossed with nine other finches, but as not one of these nine species breeds freely in confinement, we have no right to expect that the first crosses between them and the canary, or that their hybrids, should be perfectly fertile. Again, with respect to the fertility in successive generations of the more fertile hybrid animals, I hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding. On the contrary, brothers and sisters have usually been crossed in each successive generation, in opposition to the constantly repeated admonition of every breeder. And in this case, it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing. If we were to act thus, and pair brothers and sisters in the case of any pure animal, which from any cause had the least tendency to sterility, the breed would assuredly be lost in a very few generations.

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虽然我不能举出彻底可靠的例子，说明动物的杂种是完全能育的，但有理由相信凡季那利斯羌鹿（Cervulus vaginalis）和列外西羌鹿（Reevesii）间的杂种以及东亚雉（Phasianus colchicus）和环雉（P. torquatus）间的杂种是完全能育的。欧洲的普通鹅和中国鹅（A. cygnoides）是截然不同的物种，一般都列为不同的属，但它们的杂种在我国与任一纯粹亲种杂交，常常是能育的，并且在一个仅有的例子里，杂种互相交配，也是能育的。这是艾顿先生的成就，他从同一对父母培育出两只杂种鹅，但不是同时孵抱的；从这两只杂种鹅又育成一窝八个杂种（是当初两只纯种鹅的孙代）。然而，在印度这些杂种鹅一定更是能育的；因为两位异常能干的法官布莱斯先生和赫顿大尉告诉我，印度到处饲育着大群这样的杂种鹅群；因为在纯粹的亲种已不存在的地方，饲养是为了养家糊口，所以它们必定是高度能育的。

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Although I do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals, I have some reason to believe that the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P. torquatus and with P. versicolor are perfectly fertile. The hybrids from the common and Chinese geese (A. cygnoides), species which are so different that they are generally ranked in distinct genera, have often bred in this country with either pure parent, and in one single instance they have bred inter se. This was effected by Mr. Eyton, who raised two hybrids from the same parents but from different hatches; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India, however, these cross-bred geese must be far more fertile; for I am assured by two eminently capable judges, namely Mr. Blyth and Capt. Hutton, that whole flocks of these crossed geese are kept in various parts of the country; and as they are kept for profit, where neither pure parent-species exists, they must certainly be highly fertile.

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由帕拉斯最初提出的学说，基本上被现代博物学接受了，那就是，大部分家养动物是从两个以上的野生物种传下来的，后来杂交混合。根据这一观点，原始的亲种要么一开头就产生了完全能育的杂种，要么就是杂种在此后的家养状况下变为能育的。后一种情形我看可能性似乎最大，我愿意相信它，尽管没有直接证据。例如，我相信家狗是从几种野生祖先传下来的，大概除了南美洲某些原产家狗，所有的家狗互相杂交，都是十分能育的；但类推起来使我大大怀疑，这几个原始物种是否在最初曾经互相杂交，而且产生了能育的杂种。因此有理由相信，普通欧洲牛与印度瘤牛互相交配是能育的；而根据布莱斯先生给我的材料，我想它们必须认作不同的物种。根据关于许多家畜起源的这个观点，我们必须要么放弃不同物种杂交时普遍不育性的信念，要么承认动物的不育性不是恒久的性状，可以在家养状况下消除。

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A doctrine which originated with Pallas, has been largely accepted by modern naturalists; namely, that most of our domestic animals have descended from two or more aboriginal species, since commingled by intercrossing. On this view, the aboriginal species must either at first have produced quite fertile hybrids, or the hybrids must have become in subsequent generations quite fertile under domestication. This latter alternative seems to me the most probable, and I am inclined to believe in its truth, although it rests on no direct evidence. I believe, for instance, that our dogs have descended from several wild stocks; yet, with perhaps the exception of certain indigenous domestic dogs of South America, all are quite fertile together; and analogy makes me greatly doubt, whether the several aboriginal species would at first have freely bred together and have produced quite fertile hybrids. So again there is reason to believe that our European and the humped Indian cattle are quite fertile together; but from facts communicated to me by Mr. Blyth, I think they must be considered as distinct species. On this view of the origin of many of our domestic animals, we must either give up the belief of the almost universal sterility of distinct species of animals when crossed; or we must look at sterility, not as an indelible characteristic, but as one capable of being removed by domestication.

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最后，根据动植物互相杂交的一切确定事实，可以得出结论，第一次杂交及其杂种具有某种程度的不育性，乃是极其一般的结果；但根据我们目前的知识而言，却不能认为这是绝对普遍的。

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Finally, looking to all the ascertained facts on the intercrossing of plants 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, be considered as absolutely universal.

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支配第一次杂交不育性和杂种不育性的法则。——关于支配第一次杂交和杂种不育性的情况与法则，现在要讨论得详细一些。主要目的在于看一看，这些法则是否表示物种被专门赋予了这种不育的性质，以阻止它们的杂交混合，一片混乱。下面的法则和结论主要是从盖特纳令人称赞的植物杂交工作中得出来的。我曾煞费苦心地确定这些法则在动物方面究竟能应用到什么程度，鉴于我们关于杂种动物的知识极其贫乏，我惊奇地发现这些同样的法则如此普遍地适用于动植物界。

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Laws governing the sterility of first Crosses and of Hybrids.—We will now consider a little more in detail the circumstances and rules governing the sterility of first crosses and of hybrids. Our chief object will be to see whether or not the rules indicate that species have specially been endowed with this quality, in order to prevent their crossing and blending together in utter confusion. The following rules and conclusions are chiefly drawn up from G?rtner’s admirable work on the hybridisation of plants. I have taken much pains to ascertain how far the rules apply to animals, and considering how scanty our knowledge is in regard to hybrid animals, I have been surprised to find how generally the same rules apply to both kingdoms.

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前面已经指出，第一次杂交能育性和杂种能育性程度，是从零能育逐渐级进到完全能育。令人惊奇的是，这种级进可由很多奇妙的方式表现出来，但这里只能提出事实的最简略概要。如果把某一科植物的花粉放在另一科植物的柱头上，其所能产生的影响并不比无机的灰尘大。从这种绝对零能育起，把不同物种的花粉放在同属某物种的柱头上，可以产生数量不同的种子，而形成一个完全系列的级进，直到几乎完全能育，甚至十分完全能育；我们知道，在某些异常情形下，甚至有过度的能育性，超过用自己花粉的能育性。杂种也是如此，有些杂种，甚至用纯粹亲种的花粉受精，也从来没有产生过、大概永远也不会产生出一粒能育的种子；但在某些这等例子里，可以看出能育性的最初痕迹，即以纯粹亲种的花粉受精，可以致使杂种的花比不如此受粉的花凋谢较早；而花的早谢为初期受精的一种征兆，是众所熟知的。从这种极度的不育性起，我们有自交能育的杂种，可以产生越来越多的种子，直到具有完全的能育性为止。

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It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to perfect fertility. It is surprising in how many curious ways this gradation can be shown to exist; but only the barest outline of the facts can here be given. When pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. From this absolute zero of fertility, the pollen of different species of the same genus applied to the stigma of some one species, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the plant’s own pollen will produce. So in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of either pure parent, a single fertile seed: but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent-species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a sign of incipient fertilisation. From this extreme degree of sterility we have self-fertilised hybrids producing a greater and greater number of seeds up to perfect fertility.

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从很难杂交的和杂交后很少产生后代的两个物种产生出来的杂种，一般是很不育的；但是第一次杂交的困难和这样产生出来的杂种的不育性——这两类事实常被混淆在一起——之间并不严格平行。在许多情形里，两个纯粹物种异常易于杂交，并产生无数的杂种后代，然而这些杂种是显著不育的。另一方面，有一些物种很少能够杂交或者极难杂交，但是终于产生出来的杂种却很能育。甚至在同一个属的范围内，例如在石竹属（Dianthus）里，也有这两种相反的情形存在。

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Hybrids from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced—two classes of facts which are generally confounded together—is by no means strict. There are many cases, in which two pure species can be united with unusual facility, and produce numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even within the limits of the same genus, for instance in Dianthus, these two opposite cases occur.

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第一次杂交的能育性和杂种的能育性比起纯粹物种的能育性，更易受不良条件的影响。不过，能育程度也内在地易于变异，因为同样的两个物种在同样环境条件下进行杂交，能育程度并不永远一样，而是部分地决定于碰巧选作试验之用的个体的体质。杂种也是如此，因为在同一个蒴里的种子培育出来的并处于同样条件下的若干个体，其能育程度常有很大差异。

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The fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is the fertility of pure species. But the degree of fertility is likewise innately variable; for it is not always the same when the same two species are crossed under the same circumstances, but depends in part upon the constitution of the individuals which happen to have been chosen for the experiment. So it is with hybrids, for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to exactly the same conditions.

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分类系统上的亲缘关系（systematic affinity）这一术语，是指物种之间在构造体质上的相似性而言，特别是生理重要性很大、亲缘物种之间差别很小的部分的构造。物种第一次杂交的能育性以及由此产生的杂种的能育性，主要是受分类系统的亲缘关系所支配的。被分类学家列为不同科的物种之间从没产生过杂种；而密切近似的物种一般容易杂交，这就阐明了这一点。但是分类系统上的亲缘关系和杂交难易之间的对应并不严格。无数的例子证明，极其密切近似的物种并不能杂交，或者极难杂交；另一方面，很不同的物种却极其容易杂交。同一个科里也许有一个属，如石竹属有许多物种极易杂交；而另一个属，如麦瓶草属（Silene），却功败垂成，极其接近的物种不能产生一个杂种。哪怕同一个属的范围内也同样会千差万别。例如，烟草属（Nicotiana）的许多物种几乎比任何其他属的物种更容易杂交，但是盖特纳发现并非特别不同的一个物种——智利尖叶烟草（N. acuminata）曾和不下八个烟草属的其他物种进行过杂交，它顽固地不能受精，也不能使其他物种受精。如此等等不一而足。

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By the term systematic affinity is meant, the resemblance between species in structure and in constitution, more especially in the structure of parts which are of high physiological importance and which differ little in the allied species. Now the fertility of first crosses between species, and of the hybrids produced from them, is largely governed by their systematic affinity. This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. But the correspondence between systematic affinity and the facility of crossing is by no means strict. A multitude of cases could be given of very closely allied species which will not unite, or only with extreme difficulty; and on the other hand of very distinct species which unite with the utmost facility. In the same family there may be a genus, as Dianthus, in which very many species can most readily be crossed; and another genus, as Silene, in which the most persevering efforts have failed to produce between extremely close species a single hybrid. Even within the limits of the same genus, we meet with this same difference; for instance, the many species of Nicotiana have been more largely crossed than the species of almost any other genus; but G?rtner found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised by, no less than eight other species of Nicotiana. Very many analogous facts could be given.

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没有人能够指出，就可辨识的性状而言，究竟是什么种类或什么数量的差异足以阻止两个物种杂交。可以证明，习性和一般外形极其明显不同的，而且花的每一部分，甚至花粉、果实，以及子叶有着极显著差异的植物也能杂交。一年生植物和多年生植物，落叶树和常绿树，生长在不同地点且适应极其不同气候的植物，也常常容易杂交。

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No one has been able to point out what kind, or what amount, of difference in any recognisable character is sufficient to prevent two species crossing. It can be shown that plants most widely different in habit and general appearance, and having strongly marked differences in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. Annual and perennial plants, deciduous and evergreen trees, plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease.

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所谓两个物种的互交（reciprocal cross），是指这样的个案：例如，先以母驴和公马杂交，然后再以母马和公驴杂交；如此，这两个物种就是互交了。在互交的难易上，常大相径庭。这种个案极重要，证明了任何两个物种的杂交能力，常和分类亲缘关系完全无关，和两者的整个体制上任何可辨别的差异无关。另一方面，它们清楚地表明，杂交能力与我们无法识别的体质差别相关，且仅限于生殖系统。科尔路特很早以前就观察到相同的两个物种之间互交结果的这种差别。兹举一例，紫茉莉（Mirabilis jalapa）容易由长筒紫茉莉（M. longiflora）的花粉来受精，且其杂种是充分能育的；但是科尔路特曾经试图以紫茉莉的花粉使长筒紫茉莉受精，接连八年试验二百多次，完全失败。还可以举若干同样显著的例子。特莱（Thuret）在某些海藻即墨角藻属（Fuci）里观察到同样的事实。另外盖特纳发现，互交难易度的小幅度差别是极普通的。他甚至在许多植物学家仅仅列为变种的亲缘接近的类型，如一年生紫罗兰（Matthiola annua）和无毛紫罗兰（Matthiola glabra）之间，观察到了这种情形。还有一个值得注意的事实，即互交产生的杂种，当然是完全相同的两个物种混合而来，不过一个物种先用作父本然后用作母本，一般在能育性上却略有不同，有时还表现了高度的差异。

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By a reciprocal cross between two species, I mean the case, for instance, 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 to have been reciprocally crossed. There is often the widest possible difference in the facility of making reciprocal crosses. Such cases are highly important, for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity, or of any recognisable difference in their whole organisation. On the other hand, these cases clearly show that the capacity for crossing is connected with constitutional differences imperceptible by us, and confined to the reproductive system. This difference in the result of reciprocal crosses between the same two species was long ago observed by K?lreuter. 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 K?lreuter tried more than two hundred times, during eight following years, to fertilise reciprocally M. longiflora with the pollen of M. jalappa, and utterly failed. Several other equally striking cases could be given. Thuret has observed the same fact with certain sea-weeds or Fuci. G?rtner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree. He has observed it even between forms so closely related (as Matthiola annua and glabra) that many botanists rank them only as varieties. It is also a remarkable fact, that hybrids raised from reciprocal crosses, though of course compounded of the very same two species, the one species having first been used as the father and then as the mother, generally differ in fertility in a small, and occasionally in a high degree.

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还可举出盖特纳若干其他的奇妙规律：例如，某些物种特别能和其他物种杂交；同属的其他物种特别能使其杂种后代类似自己；但是这两种能力不一定相辅相成。有一些杂种，不像通常那样具有双亲之间的中间性状，却总是与某一方密切相似；这等杂种虽然外观很像纯粹亲种的一方，但都是极端不育的，极少例外。还有，通常具有双亲间中间构造的杂种里，有时会出现例外异常的个体，与纯粹亲种的一方密切相似；这些杂种几乎总是极端不育，哪怕同一个蒴里的种子培育出来的其他杂种相当能育。这些事实表明，杂种的能育性根本不取决于外观上与一纯粹亲种相似。

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Several other singular rules could be given from G?rtner: for instance, some species have a remarkable power of crossing with other species; other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring; but these two powers do not at all necessarily go together. There are certain hybrids which instead of having, as is usual, an intermediate character between their two parents, always closely resemble one of them; and such hybrids, though externally so like one of their pure parent-species, are with rare exceptions extremely sterile. So again amongst hybrids which are usually intermediate in structure between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility. These facts show how completely fertility in the hybrid is independent of its external resemblance to either pure parent.

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从支配第一次杂交和杂种能育性的上述若干规律，可见必须看作是真正不同物种的类型杂交时，其能育性是从零能育逐渐到完全能育，某些条件下甚至可以过分地能育；除了显著易受有利和不利条件影响外，能育性是内在可变异的；第一次杂交的能育性以及由此产生的杂种的能育性在程度上并非一模一样；杂种的能育性和它与一亲种外观的相似性无关；最后，两个物种之间第一次杂交的难易，并不总是受制于分类的亲缘关系，即彼此相似的程度。最后这一点，已由同样两个物种之间的互交结果中表现的差异所明确证实了，其中某一物种用作父本或母本时，杂交的难易一般有某些差异，有时有极大的差异。而且，互交产生的杂种往往能育性有差异。

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Considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that when forms, which must be considered as good and distinct species, are united, their fertility graduates from zero to perfect fertility, or even to fertility under certain conditions in excess. That their fertility, besides being eminently susceptible to favourable and unfavourable conditions, is innately variable. That it is by no means always the same in degree in the first cross and in the hybrids produced from this cross. That the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent. And lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other. This latter statement is clearly proved by reciprocal crosses between the same two species, for according as the one species or the other is used as the father or the mother, there is generally some difference, and occasionally the widest possible difference, in the facility of effecting an union. The hybrids, moreover, produced from reciprocal crosses often differ in fertility.

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那么，这些复杂奇妙的规律，是否表明仅仅为着阻止自然状况中的混淆，物种才被赋予了不育性呢？我想未必。必须假定避免混淆对于各不同物种都是同等重要的，而为什么当各物种进行杂交时，不育性的程度会有如此极端的差异呢？为什么同一物种的个体中不育程度会内在地易于变异呢？为什么某些物种易于杂交，却产生很不育的杂种；而其他物种极难杂交，却产生很能育的杂种呢？为什么同样两个物种的互交结果中常常会有如此巨大的差异呢？甚至可以问，为什么会允许杂种的产生呢？既然赋予物种以产生杂种的特别能力，然后又以不同程度的不育性来阻止其进一步繁殖，而这又和亲种第一次结合的难易并无严格关联。这似乎是一种奇怪的安排。

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Now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely different in degree, when various species are crossed, all of which we must suppose it would be equally important to keep from blending together? Why should the degree of sterility be innately variable in the individuals of the same species? Why should some species cross with facility, and yet produce very sterile hybrids; and other species cross with extreme difficulty, and yet produce fairly fertile hybrids? Why should there often be so great a difference in the result of a reciprocal cross between the same two species? Why, it may even be asked, has the production of hybrids been permitted? to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems to be a strange arrangement.

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相反，上述规律和事实，依我看清楚地表明了第一次杂交的和杂种的不育性，仅仅是伴随于或者是决定于杂交物种生殖系统为主的未知差异。差异是奇特的有限的，两物种的互交中，一个物种的雄性生殖质虽然常常能自由作用于另一物种的雌性生殖质，但不能反过来起作用。最好举例来充分解释我所谓的不育性是伴随其他差异而发生的，而不是特别赋予的一种性质。由于一种植物嫁接或芽接在其他植物之上的能力，对于它们在自然状态下的利益来说并不重要，所以我设想没有人会假定这种能力是特别赋予的性质，而承认这是伴随两种植物生长法则的差异而发生的。我们有时可以从树木生长速度的差异、木质硬度的差异、树液流动期间和树液性质的差异等等看出某一种树不能嫁接另一种树的理由；但是在很多情形下，却完全看不出任何理由来。无论两种植物大小差异巨大，无论木本草本，无论常绿落叶，也无论对于广泛不同气候的适应性，都不会总是阻止它们嫁接在一起。杂交的能力受分类系统的亲缘关系所限，嫁接也是如此，还没人能把属于不同科的树嫁接在一起；相反，密切近似的物种以及同一物种的变种，虽不是一律，却通常容易嫁接。但是这种能力和杂交中一样，并不是绝对受分类系统的亲缘关系所支配。虽然同一科许多不同的属可以嫁接，但是在另一些情形里，同属物种却不能彼此嫁接。梨和（quince）列为不同的属，梨和苹果列为同属，但是把梨嫁接在上远比嫁接在苹果上来得容易。甚至不同的梨变种在上的嫁接，其难易程度也有所不同；不同杏、桃变种在某些李变种上的嫁接，也是如此。

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The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences, chiefly in the reproductive systems, of the species which are crossed. The differences being of so peculiar and limited a nature, that, in reciprocal crosses between two species the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. It will be advisable to explain a little more fully by an example what I mean by sterility being incidental on other differences, and not a specially endowed quality. As the capacity of one plant to be grafted or budded on another is so entirely unimportant for its welfare in a state of nature, I presume that no one will suppose that this capacity is a specially endowed quality, but will admit that it is incidental on differences in the laws of growth of the two plants. We can sometimes see the reason why one tree will not take on another, from differences in their rate of growth, in the hardness of their wood, in the period of the flow or nature of their sap, etc.; but in a multitude of cases we can assign no reason whatever. Great diversity in the size of two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely different climates, does not always prevent the two grafting together. As in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft trees together belonging to quite distinct families; and, on the other hand, closely allied species, and varieties of the same species, can usually, but not invariably, be grafted with ease. But this capacity, as in hybridisation, is by no means absolutely governed by systematic affinity. Although many distinct genera within the same family have been grafted together, in other cases species of the same genus will not take on each other. The pear can be grafted far more readily on the quince, which is ranked as a distinct genus, than on the apple, which is a member of the same genus. Even different varieties of the pear take with different degrees of facility on the quince; so do different varieties of the apricot and peach on certain varieties of the plum.

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盖特纳发现同样两个物种的不同个体往往在杂交中会有内在的差异，萨哥瑞特（Sagaret）认为同样两个物种的不同个体在嫁接中也是如此。在互交中，结合的难易常常是很不相等的，在嫁接中也往往如此。例如，普通醋栗不能嫁接在黑穗醋栗（currant）上，然而黑穗醋栗却能嫁接在普通醋栗上，虽然难一些。

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As G?rtner found that there was sometimes an innate difference in different individuals of the same two species in crossing; so Sagaret believes this to be the case with different individuals of the same two species in being grafted together. As in reciprocal crosses, the facility of effecting an union is often very far from equal, so it sometimes is in grafting; the common gooseberry, for instance, cannot be grafted on the currant, whereas the currant will take, though with difficulty, on the gooseberry.

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我们已经看到，具有不完全生殖器官的杂种的不育性和具有完全生殖器官的两个纯粹物种难于结合，是两回事，然而这两类不同情形在一定程度上是平行的。嫁接的情况类似；杜因（Thouin）发现刺槐属（Robinia）三个物种在本根上可以自由结籽，嫁接在其他物种上也不难，但嫁接后就不结实了。另一方面，花楸属（Sorbus）某些物种嫁接在其他物种上所结的果实，则比在本根上多一倍。后面这一点使我们想起朱顶红属、西番莲属等等的特别情形，由不同物种的花粉比由本株的花粉来受精，能够产生更多的种子。

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We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a very different case from the difficulty of uniting two pure species, which have their reproductive organs perfect; yet these two distinct cases run to a certain extent parallel. Something analogous occurs in grafting; for Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on another species, when thus grafted were rendered barren. On the other hand, certain species of Sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots. We are reminded by this latter fact of the extraordinary case of Hippeastrum, Lobelia, etc., which seeded much more freely when fertilised with the pollen of distinct species, than when self-fertilised with their own pollen.

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因此，我们看出，虽然嫁接植物的单纯愈合和雌雄性生殖质在生殖中的结合之间有着明确的根本性区别，但是不同物种的嫁接和杂交的结果，还存在着大致的平行现象。正如我们必须把支配树木嫁接难易的奇异而复杂的法则，看作是伴随营养系统为主的一些未知差异而发生的一样，我相信支配第一次杂交难易的更为复杂的法则，伴随着生殖系统中一些未知差异而发生。这两方面的差异，如我们预料到的，在某种范围内是遵循着分类系统的亲缘关系的，所谓分类系统的亲缘关系，就是试图用以说明生物间的各种相似和相异的情况。这些事实似乎绝没有指明各不同物种在嫁接或杂交上难度大小是一种特别的禀赋；虽然在杂交的场合，这种困难对于物种类型的存续和稳定是重要的，而在嫁接的场合，这种困难对于植物的利益并不重要。

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We thus see, that although there is a clear and fundamental difference between the mere adhesion of grafted stocks, and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species. And as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first crosses, are incidental on unknown differences, chiefly in their reproductive systems. These differences, in both cases, follow to a certain extent, as might have been expected, systematic affinity, by which every kind of resemblance and dissimilarity between organic beings is attempted to be expressed. The facts by no means seem to me to indicate that the greater or lesser difficulty of either grafting or crossing together various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their welfare.

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第一次杂交不育性和杂种不育性的原因。——现在可以细看一下第一次杂交和杂种的不育性的可能原因。这两者截然不同，刚刚说过，两个纯粹物种结合，具有完全生殖器官，而杂种的生殖器官不完全。即使第一次杂交，对于实现结合的困难程度，显然决定于几种不同的原因。有时雄性生殖质由于生理的关系，不可能到达胚珠，例如雌蕊过长以致花粉管不能到达子房的植物，就是如此。也有人观察过，把一个物种的花粉放在另一个远缘物种的柱头上时，虽然花粉管伸出来了，但并不能穿入柱头的表面。再者，雄性生殖质虽然可以到达雌性生殖质，但不能引起胚胎的发育，特莱对于墨角藻所做的一些试验，似乎就是如此。对于这些事实还无法解释，正如某些树为什么不能嫁接在其他树上一样。最后，也许胚胎可以发育，但早期即行死去。最后这一选项还没有得到充分的注意；但是在山鸡和家鸡的杂交工作上经验丰富的休伊特（Hewitt）先生曾给我转述过他的观察，我相信胚胎的早期死亡是第一次杂交不育性的最常见原因。一开始我不愿相信这种观点；因为杂种一旦产生，如我们所看到的骡的情形，一般是健康而长命的。然而，杂种在降生前后，是处于不同的环境条件之下的：如果杂种产生和生活在双亲所生活的地方，一般是处于适宜的生活条件之下的。但是，杂种只继承了母体的本性和体质的一半；所以产生之前，还在母体的子宫内或在由母体所产生的蛋或种子内养育的时候，可能已处于某种程度的不适宜条件之下了，因此就容易在早期死去；特别是一切极其幼小的生物，对于有害或不自然的生活条件是显著敏感的。

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Causes of the sterility of first Crosses and of Hybrids.—We may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. These two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are imperfect. Even in first crosses, the greater or lesser difficulty in effecting a union apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for the pollen-tubes to reach the ovarium. It has also been observed that when pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. Again, the male element may reach the female element, but be incapable of causing an embryo to be developed, as seems to have been the case with some of Thuret’s experiments on Fuci. No explanation can be given of these facts, any more than why certain trees cannot be grafted on others. Lastly, an embryo may be developed, and then perish at an early period. This latter alternative has not been sufficiently attended to; but I believe, from observations communicated to me by Mr. Hewitt, who has had great experience in hybridising gallinaceous birds, that the early death of the embryo is a very frequent cause of sterility in first crosses. I was at first very unwilling to believe in this view; as hybrids, when once born, are generally healthy and long-lived, as we see in the case of the common mule. Hybrids, however, are differently circumstanced before and after birth: when born and living in a country where their two parents can live, they are generally placed under suitable conditions of life. But a hybrid partakes of only half of the nature and constitution of its mother, and therefore before birth, as long as it is nourished within its mother’s womb or within the egg or seed produced by the mother, it may be exposed to conditions in some degree unsuitable, and consequently be liable to perish at an early period; more especially as all very young beings seem eminently sensitive to injurious or unnatural conditions of life.

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关于两性生殖质发育不全的杂种的不育性，情形很不相同。我已经不止一次提出过自己收集的大量事实，说明动植物离开其自然条件，生殖系统就极易受到严重的影响。事实上这是动物驯化的重大障碍。如此诱发的不育性和杂种的不育性之间，有许多相似之点。两者的不育性和一般的健康无关，且不育的个体往往身体肥大或异常茂盛。而且，不育性以不同的程度出现；雄性生殖质最易受影响，但是有时雌性比雄性受影响更厉害。两者不育的倾向在某种程度上和分类系统的亲缘关系是一致的，因为动植物的全群都是由于同样的不自然条件而招致不育的，并且全群的物种都有产生不育杂种的倾向。另一方面，一群中的一个物种时常会抵抗环境条件的巨变，而能育性无所损伤；而某些物种会产生异常能育的杂种。不试验，没有人能说，任何动物是否能够在圈养中生育，任何外来植物是否能够在栽培下自由地结籽；也不能说，一属中的任何两个物种究竟能否产生好歹不育的杂种。最后，如果生物在几个世代内都处在不是它们的自然条件下，就极易变异，我认为变异的原因是生殖系统受到特别的影响，虽然比引起不育性发生的那种影响为小。杂种也是如此，因为正如每一个试验者所观察到的，杂种的后代在连续的世代中也是极易变异的。

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In regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is very different. I have more than once alluded to a large body of facts, which I have collected, showing that when animals and plants are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously affected. This, in fact, is the great bar to the domestication of animals. Between the sterility thus superinduced and that of hybrids, there are many points of similarity. In both cases the sterility is independent of general health, and is often accompanied by excess of size or great luxuriance. In both cases, the sterility occurs in various degrees; in both, the male element is the most liable to be affected; but sometimes the female more than the male. In both, the tendency goes to a certain extent with systematic affinity, for whole groups of animals and plants are rendered impotent by the same unnatural conditions; and whole groups of species tend to produce sterile hybrids. On the other hand, one species in a group will sometimes resist great changes of conditions with unimpaired fertility; and certain species in a group will produce unusually fertile hybrids. No one can tell, till he tries, whether any particular animal will breed under confinement or any plant seed freely under culture; nor can he tell, till he tries, whether any two species of a genus will produce more or less sterile hybrids. Lastly, when organic beings are placed during several generations under conditions not natural to them, they are extremely liable to vary, which is due, as I believe, to their reproductive systems having been specially affected, though in a lesser degree than when sterility ensues. So it is with hybrids, for hybrids in successive generations are eminently liable to vary, as every experimentalist has observed.

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因此，我们可以看出，当生物处于新的不自然的条件之下时，以及当杂种从两个物种的不自然杂交中产生出来时，生殖系统都以相似方式蒙受不育影响，而与一般健康状态无关。在前一种情形下，生活条件受扰乱，虽然程度很轻微，以致觉察不到；在后一种情形下，也就是杂种，外界条件虽然保持一样，但是由于两种不同的构造和体质合为一体，体制便受到扰乱。两种体制混为一种，在发育上，周期性的活动上，不同部分和器官的相互关联上，以及其对生活条件的相互关系上，没有某种扰乱发生，几乎是不可能的。如果杂种能够互相杂交而生育，就会把同样的混成体制一代一代地传递给后代，因此毫不奇怪，不育性虽有某种程度的变异，但不致减弱。

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Thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system, independently of the general state of health, is affected by sterility in a very similar manner. In the one case, the conditions of life have been disturbed, though often in so slight a degree as to be inappreciable by us; in the other case, or that of hybrids, the external conditions have remained the same, but the organisation has been disturbed by two different structures and constitutions having been blended into one. For it is scarcely possible that two organisations should be compounded into one, without some disturbance occurring in the development, or periodical action, or mutual relation of the different parts and organs one to another, or to the conditions of life. When hybrids are able to breed inter se, they transmit to their offspring from generation to generation the same compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable, rarely diminishes.

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必须承认，除非做一些模糊的假设，我们无法理解有关杂种不育性的若干事实。例如，互交产生的杂种，其能育性并不相等；再如，与一纯粹亲种偶然地、例外地密切类似的杂种，其不育性有所增强。我不敢说上述论点已经切中事物的根源；为什么生物置于不自然条件下就会变为不育，对此还不能解释。我试图阐明的仅仅是，在某些方面有相似之处的两种情形，同样造成不育的结果，一是生活条件受扰乱，一是体制因两种体制合二为一而受到了扰乱。

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It must, however, be confessed that we cannot understand, excepting on vague hypotheses, several facts with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. Nor do I pretend that the foregoing remarks go to the root of the matter: no explanation is offered why an organism, when placed under unnatural conditions, is rendered sterile. All that I have attempted to show, is that in two cases, in some respects allied, sterility is the common result,—in the one case from the conditions of life having been disturbed, in the other case from the organisation having been disturbed by two organisations having been compounded into one.

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听起来好笑，我怀疑同样的平行现象也适用于类似的但很不相同的一些事实。生活条件的微小变化对于所有生物都是有利的，这是一个古老的几乎普遍的信念，而且建立在大量证据之上。我看到农民和园艺者就这样做，他们常常从不同土壤和气候的地方交换种子、块茎等等，然后再换回来。在动物病后复原的期间，显而易见生活习性上的几乎任何变化，都是有很大好处的。还有，关于动植物，充分证据证实，同一物种非常不同的个体之间杂交，也就是不同品系、亚种的杂交，会增强后代的生活力和能育性。根据第四章提到的事实，我认为，哪怕是雌雄同体，一定量的杂交是不可或缺的；而且最近亲属之间的近亲交配，若连续经过几代，而且生活条件保持不变，总要招致后代的衰弱不育。

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It may seem fanciful, but I suspect that a similar parallelism extends to an allied yet very different class of facts. It is an old and almost universal belief, founded, I think, on a considerable body of evidence, that slight changes in the conditions of life are beneficial to all living things. We see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, etc., from one soil or climate to another, and back again. During the convalescence of animals, we plainly see that great benefit is derived from almost any change in the habits of life. Again, both with plants and animals, there is abundant evidence, that a cross between very distinct individuals of the same species, that is between members of different strains or sub-breeds, gives vigour and fertility to the offspring. I believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; and that close interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, always induces weakness and sterility in the progeny.

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因此，一方面，生活条件的微小变化对于所有生物都有利；另一方面，轻微程度的杂交，即已有微小变异的同一物种雌雄之间的杂交，似乎会增强后代的生活力和能育性。但是，我们看到，大变化，或者特定性质的变化，往往使生物变为多少不育；且大杂交，即大不同的生物，或者不同的物种雌雄杂交，会产生某种程度不育的杂种。我很难确信，这种平行现象是偶然还是错觉。上述两组事实似乎被某个共同的、不明的纽带联结在一起了，它在本质上和生命的原则相关。

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Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is crosses between the males and females of the same species which have varied and become slightly different, give vigour and fertility to the offspring. But we have seen that greater changes, or changes of a particular nature, often render organic beings in some degree sterile; and that greater crosses, that is crosses between males and females which have become widely or specifically different, produce hybrids which are generally sterile in some degree. I cannot persuade myself that this parallelism is an accident or an illusion. Both series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life.

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变种杂交的能育性及其混种后代的能育性。——一个极有力的论点主张，物种和变种之间一定存在着某种本质区别，而且以前所有的话肯定有错误，因为变种彼此在外观上无论有多大差异，却十分容易杂交，且产生完全能育的后代。我充分承认这几乎完全属实。但观察自然状况下产生的变种时，就立刻困难重重；如果有两个向来认定的变种，杂交中有任何程度的不育性，大多数学者就会立刻把它们列为物种。例如，被大多数优秀植物学者认为是变种的蓝海绿和红海绿、报春花属和樱草，据盖特纳说在杂交中是颇为不育的，因此他便把它们列为无疑的物种了。如果我们这样循环论证下去，就必然要认可在自然状况下产生的一切变种都是能育的了。

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Fertility of Varieties when crossed, and of their Mongrel offspring.—It may be urged, as a most forcible argument, that there must be some essential distinction between species and varieties, and that there must be some error in all the foregoing remarks, inasmuch as varieties, however much they may differ from each other in external appearance, cross with perfect facility, and yield perfectly fertile offspring. I fully admit that this is almost invariably the case. But if we look to varieties produced under nature, we are immediately involved in hopeless difficulties; for if two hitherto reputed varieties be found in any degree sterile together, they are at once ranked by most naturalists as species. For instance, the blue and red pimpernel, the primrose and cowslip, which are considered by many of our best botanists as varieties, are said by G?rtner not to be quite fertile when crossed, and he consequently ranks them as undoubted species. If we thus argue in a circle, the fertility of all varieties produced under nature will assuredly have to be granted.

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如果转过来看一看家养状况下产生或者假定产生的变种，我们更要疑惑不解了。例如说德国狐狸犬比其他狗更容易与狐狸结合，某些南美洲的土著家狗和欧洲狗不能轻易杂交时，每个人心目中都会有一种解释，而且大概是正确的，即这些狗本来是从不同物种传下来的。但是，外观上有着广泛差异的很多家养变种，例如鸽子或圆白菜都有完全的能育性，是值得注意的事实，特别是当我们想起有何等众多的物种，虽然彼此极其密切近似，但杂交时却极端不育。然而，考虑到以下几点，可知家养变种的能育性并不那么引人注目。第一，可以阐明，两物种之间的区区外在差异并不能确定相互杂交的不育性程度，所以同样的规则适用于家养变种；第二，某些知名学者认为，长期的驯化过程倾向于在连续的杂种世代中消除不育性，因为一开始程度就轻。如果这一点属实，我们当然不应该指望发现在相近的生活条件下不育性出现又消失了。最后，我觉得这是最重要的一点，动植物新族是通过人类按部就班的无意识选择力量在家养条件下培育出来的，为了人类的使用和愉悦而生；既不想，也不能选择生殖系统的轻微变化，或者与生殖系统相关的其他体质差异。给几个变种提供同样的食物，一视同仁地对待，并不希望改变其一般生活习性。大自然在恒久的时代里，对于整个体制的作用是均匀和缓慢的，反正是为了各个生物本身的利益；于是可能直接或者更可能间接地通过相关生长，修改任何一个物种若干后代的生殖系统。有鉴于人类和大自然所进行的选择过程存在这种差别，结果若有差异，也就不足为奇了。

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If we turn to varieties, produced, or supposed to have been produced, under domestication, we are still involved in doubt. For when it is stated, for instance, that the German Spitz dog unites more easily than other dogs with foxes, or that certain South American indigenous domestic dogs do not readily cross with European dogs, the explanation which will occur to every one, and probably the true one, is that these dogs 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 the pigeon or of the cabbage, is a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. Several considerations, however, render the fertility of domestic varieties less remarkable than at first appears. It can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties. In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile; and if this be so, we surely ought not to expect to find sterility both appearing 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 by man’s methodical and unconscious power of selection, for his own use and pleasure: he neither wishes to select, nor could select, slight differences in the reproductive system, or other constitutional differences correlated with the reproductive system. He supplies his several varieties with the same food; treats them in nearly the same manner, and does not wish to alter their general habits of life. Nature acts uniformly and slowly during vast periods of time on the whole organisation, in any way which may be for each creature’s own good; and thus she may, either directly, or more probably indirectly, through correlation, modify the reproductive system in the several descendants from any one species. Seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result.

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我一直以来说起同一物种的变种进行杂交，好像都是恒定能育的。但是，下面将扼要叙述的少数事例，就是存在一定程度不育性的证据，这似乎是无可辩驳的。这一证据，和我们相信无数物种的不育性的证据，至少是有同等价值的。这一证据也是从反对说证人那里得来的，他们把能育性和不育性千篇一律地作为区别物种的稳妥标准。盖特纳在自家花园培育了一个矮型黄籽的玉米品种，同时在近旁培育了一个高型红籽的品种，这一工作进行了数年之久；这两个品种虽然是雌雄异花的，但绝没有自然杂交。于是他用一类玉米的花粉在另一类的十三个花穗上进行受精，仅有一个花穗结了一些籽，也不过结了五粒种子，因为这些植物是雌雄异花的，所以人工授精的操作在这里不会发生有害的作用。我相信没有人会怀疑这些玉米变种是不同物种；有必要注意这样育成的杂种植物本身是完全能育的；所以，连盖特纳也不敢承认这两个变种是不同的物种了。

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I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. But it seems to me impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which I will briefly abstract. The evidence is at least as good as that from which we believe in the sterility of a multitude of species. The evidence is, also, derived from hostile witnesses, who in all other cases consider fertility and sterility as safe criterions of specific distinction. G?rtner kept during several years a dwarf kind of maize with yellow seeds, and a tall variety with red seeds, growing near each other in his garden; and although these plants have separated sexes, they never naturally crossed. He then fertilised thirteen flowers of the one with the pollen of the other; but only a single head produced any seed, and this one head produced only five grains. Manipulation in this case could not have been injurious, as the plants have separated sexes. No one, I believe, has suspected that these varieties of maize are distinct species; and it is important to notice that the hybrid plants thus raised were themselves perfectly fertile; so that even G?rtner did not venture to consider the two varieties as specifically distinct.

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吉鲁·德·别沙连格（Girou de Buzareingues）杂交了三个葫芦变种，和玉米一样是雌雄异花的，他断言之间的差异愈大，相互受精就愈不容易。这些试验有多大的可靠性我不知道，但是萨格瑞特把试验的类型列为变种，他分类的主要根据是不育性的试验。

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Girou de Buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and he asserts that their mutual fertilisation is by so much the less easy as their differences are greater. How far these experiments may be trusted, I know not; but the forms experimentised on, are ranked by Sagaret, who mainly founds his classification by the test of infertility, as varieties.

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下面的情形就更值得注意了，乍看似乎是难以相信的，但这是如此优秀的观察者和反对说证人盖特纳在许多年内，对于毛蕊花属的九个物种所进行的无数试验的结果，即同种黄色变种和白色变种的杂交，比其同色变种的花粉授精，产生较少的种子。进而他断言，一个物种的黄色变种和白色变种与另一物种的黄色变种和白色变种杂交时，则同色变种之间的杂交比异色变种之间的杂交，能产生较多的种子。然而这些变种除了花的颜色以外，并没有任何不同之处，有时这一个变种还可从另一个变种的种子培育出来。

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The following case is far more remarkable, and seems at first quite incredible; but it is the result of an astonishing number of experiments made during many years on nine species of Verbascum, by so good an observer and so hostile a witness, as G?rtner: namely, that yellow and white varieties of the same species of Verbascum when intercrossed produce less seed, than do either coloured varieties when fertilised with pollen from their own coloured flowers. Moreover, he asserts that when yellow and white varieties of one species are crossed with yellow and white varieties of a distinct species, more seed is produced by the crosses between the same coloured flowers, than between those which are differently coloured. Yet these varieties of Verbascum present no other difference besides the mere colour of the flower; and one variety can sometimes be raised from the seed of the other.

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从我对某些蜀葵变种的观察，倾向于认为它们有类似的情况。

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From observations which I have made on certain varieties of hollyhock, I am inclined to suspect that they present analogous facts.

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科尔路特工作的准确性已被其后的每一位观察者证实了，他证明了一项值得注意的事实，即普通烟草的一个变种，如与一个大不相同的物种进行杂交，比其他变种更能育。他对普通被称作变种的五个类型进行了试验，而且是极严格的试验，即互交试验，发现它们的杂种后代是完全能育的。但是这五个变种中的一个，无论用作父本或母本与黏性烟草（Nicotiana glutinosa）进行杂交，所产生的杂种，总是不像其他四个变种与黏性烟草杂交的杂种那样不育。因此，这个变种的生殖系统必定以某种方式某种程度上变异了。

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K?lreuter, whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact, that one variety of the common tobacco is more fertile, when crossed with a widely distinct species, than are the other varieties. He experimentised on five forms, which are commonly reputed to be varieties, and which he tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspring perfectly fertile. But one of these five varieties, when used either as father or mother, and crossed with the Nicotiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with N. glutinosa. Hence the reproductive system of this one variety must have been in some manner and in some degree modified.

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有鉴于此；由于很难确定自然状态下的变种不育性，假定的变种若有任何不育性一般列为物种；由于人在生产最明确的家养变种时只选择外在性状，而并不想或者不能在生殖系统里产生隐秘的机能差异；从这几个考虑和事实看来，我想变种并不能证明其一般能育性是普遍出现的，它不能作为变种和物种之间的根本区别。依我看变种的一般能育性，不足以推翻我就第一次杂交和杂种一般不育性而非永远不育所取的观点，也就是，那不是一种特别禀赋，而是缓慢得到的变异的连带现象，特别是发生于杂交类型的生殖系统的变异。

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From these facts; from the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety if infertile in any degree would generally be ranked as species; from man selecting only external characters in the production of the most distinct domestic varieties, and from not wishing or being able to produce recondite and functional differences in the reproductive system; from these several considerations and facts, I do not think that the very general fertility of varieties can be proved to be of universal occurrence, or to form a fundamental distinction between varieties and species. The general fertility of varieties does not seem to me sufficient to overthrow the view which I have taken with respect to the very general, but not invariable, sterility of first crosses and of hybrids, namely, that it is not a special endowment, but is incidental on slowly acquired modifications, more especially in the reproductive systems of the forms which are crossed.

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除了能育性之外，杂种与混种的比较。——杂交物种的后代和杂交变种的后代，除了能育性以外，还可以在其他几方面进行比较。曾强烈希望在物种和变种之间划出一条明确界限的盖特纳，在种间杂种后代和变种间混种后代之间只能找出很少的而且依我看来是十分不重要的差异。另一方面，它们在许多重要之点上却是极其密切一致的。

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Hybrids and Mongrels compared, Independently of their fertility.— Independently of the question of fertility, the offspring of species when crossed and of varieties when crossed may be compared in several other respects. G?rtner, whose strong wish was to draw a marked line of distinction between species and varieties, could find very few and, as it seems to me, quite unimportant differences between the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties. And, on the other hand, they agree most closely in very many important respects.

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这里将极其简略地讨论这一问题。最重要的区别是，在第一代里混种较杂种易于变异，但是盖特纳却认为经长期培育的物种所产生的杂种在第一代里是常常易于变异的；我本人也曾见过这一事实的显著例子。盖特纳进而认为极其密切近似物种之间的杂种，较极其不同物种之间的杂种易于变异；这表明变异性的差异程度是分级消失的。众所熟知，当混种和较为能育的杂种繁殖到几代时，后代的变异性都大极了；但是，还能举出少数例子，表明杂种或混种长久保持着一致的性状。然而混种在连续世代里的变异性也许较杂种为大。

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I shall here discuss this subject with extreme brevity. The most important distinction is, that in the first generation mongrels are more variable than hybrids; but G?rtner admits that hybrids from species which have long been cultivated are often variable in the first generation; and I have myself seen striking instances of this fact. G?rtner further admits that hybrids between very closely allied species are more variable than those from very distinct species; and this shows that the difference in the degree of variability graduates away. When mongrels and the more fertile hybrids are propagated for several generations an extreme amount of variability in their offspring is notorious; but some few cases both of hybrids and mongrels long retaining uniformity of character could be given. The variability, however, in the successive generations of mongrels is, perhaps, greater than in hybrids.

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混种的变异性较杂种大，我看完全不足为奇。因为混种的双亲是变种，而且大都是家养变种（关于自然变种只做过很少的试验），这意味着变异性大都是新近出现的，因此我们可以期待这种变异性常常会继续，而且叠加于光由杂交行为产生的变异性上面。首次杂交或者杂种在第一代的变异性相对于连续世代的极端变异来说微不足道，这是奇事，值得注意，因其涉及并且加强了我所提出的关于普通变异性的原因的观点：由于生殖系统对于变化了的生活条件是显著敏感的，所以生殖系统往往就无能，起码无法发挥正常机能来产生和双亲类型相同的后代。第一代杂种是从生殖系统未曾受到任何影响的物种传下来的（经过长久培育的物种除外），所以不易变异；但是杂种本身的生殖系统却已受到了严重的影响，所以其后代是高度变异的。

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This greater variability of mongrels than of hybrids does not seem to me at all surprising. For the parents of mongrels are varieties, and mostly domestic varieties (very few experiments having been tried on natural varieties), and this implies in most cases that there has been recent variability; and therefore we might expect that such variability would often continue and be super-added to that arising from the mere act of crossing. The slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention. For it bears on and corroborates the view which I have taken on the cause of ordinary variability; namely, that it is due to the reproductive system being eminently sensitive to any change in the conditions of life, being thus often rendered either impotent or at least incapable of its proper function of producing offspring identical with the parent-form. Now hybrids in the first generation are descended from species (excluding those long cultivated) which have not had their reproductive systems in any way affected, and they are not variable; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable.

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还是回转来谈谈混种和杂种的比较：盖特纳说，混种比杂种更易重现任一亲类型的性状；但是，如果属实，这也肯定不过是程度差别而已。盖特纳还坚持说，任何两个物种虽然彼此密切近似，但与第三个物种杂交，其杂种彼此差异很大，然而一个物种的两个很不相同的变种，如与另一物种进行杂交，其杂种彼此差异并不大。但是据我所知，这个结论是建立在一次试验上的，并且似乎和科尔路特所做的几个试验的结果正相反。

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But to return to our comparison of mongrels and hybrids: G?rtner states that mongrels are more liable than hybrids to revert to either parent-form; but this, if it be true, is certainly only a difference in degree. G?rtner further insists that when any two species, although most closely allied to each other, are crossed with a third species, the hybrids are widely different from each other; whereas if two very distinct varieties of one species are crossed with another species, the hybrids do not differ much. But this conclusion, as far as I can make out, is founded on a single experiment; and seems directly opposed to the results of several experiments made by K?lreuter.

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盖特纳所能指出的杂种植物和混种植物之间的不重要差异，也就是这个了。另一方面，混种和杂种形似各自的亲本，特别是从近缘物种产生出来的那些杂种，按照盖特纳的说法，也是依据同一法则的。两个物种杂交时，其中一个有时具有优势的遗传力迫使杂种像自己。我相信关于植物的变种也是如此；并且关于动物，肯定也是一个变种常常较另一变种具有这种优势的遗传力。从互交中产生出来的杂种植物，一般是彼此密切相似的；从互交中产生出来的混种植物也是如此。无论杂种或混种，如果在连续世代里反复地和任何一个亲本进行杂交，都会重现任一纯粹亲类型的性状。

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These alone are the unimportant differences, which G?rtner is able to point out, between hybrid and mongrel plants. On the other hand, the resemblance in mongrels and in hybrids to their respective parents, more especially in hybrids produced from nearly related species, follows according to G?rtner the same laws. When two species are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid; and so I believe it to be with varieties of plants. With animals one variety certainly often has this prepotent power over another variety. Hybrid plants produced from a reciprocal cross, generally resemble each other closely; and so it is with mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced to either pure parent-form, by repeated crosses in successive generations with either parent.

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这几点显然也适用于动物；但是部分地由于第二性征的存在，使得上述问题过于复杂，特别是由于物种间杂交和变种间杂交里某一性较另一性强烈地具有优势的遗传力。例如，我想那些主张驴较马具有优势遗传力的作者们说得对，无论骡或驴骡都更像驴而少像马；但是，公驴较母驴更强烈地具有优势的遗传力，所以由公驴和母马所产生的后代骡，比由母驴和公马所产生的后代驴骡更像驴。

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These several remarks are apparently applicable to animals; but the subject is here excessively complicated, partly owing to the existence of secondary sexual characters; but more especially owing to prepotency in transmitting likeness running more strongly in one sex than in the other, both when one species is crossed with another, and when one variety is crossed with another variety. For instance, I think those authors are right, who maintain that the ass has a prepotent power over the horse, so that both the mule and the hinny more resemble the ass than the horse; but that the prepotency runs more 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 and stallion.

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某些作者特别着重只有混种后代密切相似于一方亲本的假设事实；但这种情形有时候杂种里也发生，我承认比混种里少得多。看一看我所搜集的事实，由杂交育成的动物，凡与一方亲本密切相似的，其相似之点似乎主要局限于性质上近于畸形和突然出现的那些性状——如皮肤白变症、黑变症（melanism）、无尾无角、多指多趾，而与通过选择慢慢获得的那些性状无关。于是，突然重现双亲任一方的完全性状的倾向，也是混种远比杂种更易发生。混种是由变种传下来的，常常是突然产生的，性状上是半畸形的；杂种是由物种传下来的，而物种则是慢慢而自然地产生的。总的来说，我完全同意普罗斯珀·卢卡斯博士的见解，他搜集了有关动物的大量事实后，得出如下的结论：不论双亲彼此的差异有多少，就是说，在同一变种个体的结合中，在不同变种个体的结合中，或在不同物种个体的结合中，子代类似亲代的法则都是一样的。

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Much stress has been laid by some authors on the supposed fact, that mongrel animals alone are born closely like one of their parents; but it can be shown that this does sometimes occur with hybrids; yet I grant much less frequently with hybrids than with mongrels. Looking to the cases which I have collected of cross-bred animals closely resembling one parent, the resemblances seem chiefly confined to characters almost monstrous in their nature, and which have suddenly appeared—such as albinism, melanism, deficiency of tail or horns, or additional fingers and toes; and do not relate to characters which have been slowly acquired by selection. Consequently, sudden reversions to the perfect character of either parent would be more likely to occur with mongrels, which are descended from varieties often suddenly produced and semi-monstrous in character, than with hybrids, which are descended from species slowly and naturally produced. 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 to its parents are the same, whether the two parents differ much or little from each other, namely in the union of individuals of the same variety, or of different varieties, or of distinct species.

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除了能育性和不育性的问题以外，物种杂交的后代和变种杂交的后代，在一切方面似乎都有普遍和密切的相似性。如果把物种看作是特别创造出来的，并且把变种看作是根据次级法则（secondary laws）产生的，这种相似性便会令人瞠目结舌。但这完全符合物种与变种之间无本质区别的观点。

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Laying aside the question of fertility and sterility, in all other respects there seems to be a general and close similarity in the offspring of crossed species, and of crossed varieties. If we look at species as having been specially created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. But it harmonises perfectly with the view that there is no essential distinction between species and varieties.

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本章提要。——差异足可列为物种的类型之间的第一次杂交以及它们的杂种，很普遍地但并非一律不育。不育性的程度不一，而且往往相差极微小，以致史上两位最谨慎的试验者根据这一标准也会在类型的排列上得出完全相反的结论。不育性在同一物种的个体里是内在地易于变异的，并且对于适宜和不适宜的生活条件是显著敏感的。不育性的程度并不严格遵循分类系统的亲缘关系，但由若干奇妙而复杂的法则支配。在同样两个物种的互交里不育性一般是不同的，有时是大为不同的。第一次杂交以及由此产生出来的杂种里，不育性的程度并非总是相等的。

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Summary of Chapter.—First crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. The sterility is of all degrees, and is often so slight that the two most careful experimentalists who have ever lived, have come to 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 unfavourable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes widely different, in reciprocal crosses between the same two species. It is not always equal in degree in a first cross and in the hybrid produced from this cross.

53
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在树的嫁接中，某一物种或变种嫁接在其他树上的能力，取决于植物生长系统的一般未知的差异；同样，在杂交中，一个物种和另一物种在结合上的难易，取决于生殖系统里的未知差异。之所以没有理由认为，物种被特别赋予了各种程度的不育性，以便防止自然状况下的杂交和混淆，是因为没有理由认为，树木被特别赋予了各种差不多的难嫁接性，以便防止树木在森林中接合。

53
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In the same manner as in grafting trees, the capacity of one species or variety to take on another, is incidental on generally unknown differences in their vegetative systems, so in crossing, the greater or less facility of one species to unite with another, is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent them crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent them becoming inarched in our forests.

54
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纯粹物种的生殖系统是完善的，其第一次杂交的不育性似乎决定于几种条件，有时候主要决定于胚胎的早期死亡。杂种的生殖系统不完善，生殖系统乃至整个体制因两个不同物种的混合而扰乱，其不育性和自然的生活条件受到扰乱的纯粹物种所屡屡发生的不育性，似乎是密切近似的。这一观点有另一种平行现象的支持：只有微弱差别的类型之间的杂交，有利于后代的生活力和能育性；生活条件的微小变化有利于一切生物的生活力和能育性。两个物种的难以杂交及其杂种后代的不育性，纵然起因不同，其程度一般是相应的，这并不奇怪；因为两者都决定于杂交物种间的某种差异量。第一次杂交的容易和如此产生的杂种的能育，以及嫁接的能力——虽然嫁接能力显然决定于广泛不同的条件——在一定程度上统统与被试验类型的分类系统亲缘关系相平行，这也不奇怪。因为分类系统的亲缘关系试图表达一切物种的相似性。

54
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The sterility of first crosses between pure species, which have their reproductive systems perfect, seems to depend on several circumstances; in some cases largely on the early death of the embryo. The sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their whole organisation disturbed by being compounded of two distinct species, seems closely allied to that sterility which so frequently affects pure species, when their natural conditions of life have been disturbed. This view is supported by a parallelism of another kind;—namely, that the crossing of forms only slightly different is favourable to the vigour and fertility of their offspring; and that slight changes in the conditions of life are apparently favourable to the vigour and fertility of all organic beings. It is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, the fertility of the hybrids produced, and the capacity of being grafted together—though this latter capacity evidently depends on widely different circumstances—should all run, to a certain extent, parallel with the systematic affinity of the forms which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species.

55
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公认为是变种，或者充分相似到足以被认为是变种的类型之间的第一次杂交，以及它们的混种后代，一般都是能育的，但不一定普遍如此。如果我们记得，我们是多么易于用循环法来辩论自然状态下的变种；如果我们记得，大多数变种是在家养状况下仅仅根据对外在差异的选择而产生出来的，而不是根据生殖系统的差异，则变种的几乎普遍而完全的能育性，就不值得奇怪了。除了能育性的问题之外，其他一切方面杂种和混种之间还有最密切而一般的相似性。最后，本章简单举出的一些事实，依我看似乎与物种与变种没有根本区别这一观点并不矛盾，甚至是支持这个观点呢。

55
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First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and not of differences in the reproductive system. In all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels. Finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties.

序号 英文/音标 中文解释 更多操作

sterility

[stə’rɪləti]

n.不生育;贫瘠

hybrid

[’haɪbrɪd]

n.混合物;杂种;混血儿

Sterility

[stə’rɪləti]

n.不生育;贫瘠

interbreed

[ˌɪntə’briːd]

v.使异种交配;使混种;使杂种繁殖

domestication

[dəˌmestɪ’keɪʃn]

n.驯服;教化

endowment

[ɪn’daʊmənt]

n.【U】捐助(奖金)

incidental

[ˌɪnsɪ’dentl]

adj.附带的;非主要的;偶然的;容易发生的

offspring

[’ɒfsprɪŋ]

n.后代;子孙;产物

mongrel

[’mʌŋɡrəl]

n.杂种狗;杂交动植物;混血儿

independently

[ˌɪndɪ’pendəntli]

adv.独立地;自立地

entertain

[ˌentə’teɪn]

v.娱乐;使有兴趣;招待;考虑;抱有;容纳

naturalist

[’nætʃrəlɪst]

n.自然主义者;博物学者

endow

[ɪn’daʊ]

vt.捐助;赋予;使具有某种品质

underrate

[ˌʌndə’reɪt]

v.低估;看轻

preservation

[ˌprezə’veɪʃn]

n.保存

confound

[kən’faʊnd]

vt.使困惑;混淆;挫败;诅咒

reproduction

[ˌriːprə’dʌkʃn]

n.再现;生殖

reproductive

[ˌriːprə’dʌktɪv]

adj.复制的;生殖的

microscope

[’maɪkrəskəʊp]

n.显微镜

embryo

[’embriəʊ]

n.胚胎;萌芽

imperfect

[ɪm’pɜːfɪkt]

adj.不完美的;有缺点的;未完善的

slur

[slɜː(r)]

vt. 忽视，略过;

reasoning

[’riːzənɪŋ]

n.推论;推理;论证

memoir

[’memwɑː(r)]

n.传记;实录(复数)memoirs: 回忆录;自传;学术论文集.

knot

[nɒt]

n.结;节

fertile

[’fɜːtaɪl]

adj.肥沃的;富饶的;能繁殖的;多产的;(创造力)丰富的

oblige

[ə’blaɪdʒ]

vt.迫使;责成;使感激;施恩于;帮 ... 的忙;使…成为必要

castrate

[kæ’streɪt]

v.阉割;删除

seclude

[sɪ’kluːd]

vt.隔离;隔绝

pollen

[’pɒlən]

n.花粉

manipulation

[məˌnɪpju’leɪʃn]

n.操纵;控制;篡改

impair

[ɪm’peə(r)]

vt.损害;削弱【计算机】 损伤.

botanist

[’bɒtənɪst]

n.植物学家

diametrical

[ˌdaɪə’metrɪkl]

adj.直径的;对立的

instructive

[ɪn’strʌktɪv]

adj.教育性的;有启发的;有益的

adduce

[ə’djuːs]

v.引证;举出

lessen

[’lesn]

v.减少;减轻

breeder

[’briːdə(r)]

n.饲养动物的人;培育植物的人;【核】增殖(反应)堆

experimentalist

[eksˌperɪ’mentəlɪst]

n.经验主义者(实验者)

anther

[’ænθə(r)]

n.花药;花粉囊

honorable

[’ɒnərəbl]

adj.光荣的;可敬的;体面的.

Herbert

[hə:bət]

n.赫伯特(姓氏;男子名)

hothouse

[’hɒthaʊs]

n.温室;温床

singular

[’sɪŋɡjələ(r)]

a. 非凡的；卓越的;

genus

[’dʒiːnəs]

n.【生物学】属;类;种;型

ovary

[’əʊvəri]

n.卵巢;【植】子房

perish

[’perɪʃ]

vi.毁灭;消失;腐烂

whereas

[ˌweər’æz]

conj.然而;鉴于

impregnate

[’ɪmpreɡneɪt]

v.注入;使充满;使饱和;使怀孕

ovule

[’ɒvjuːl]

n.胚珠;卵子

unnatural

[ʌn’nætʃrəl]

adj.不自然的;反常的;不近人情的

lesser

[’lesə(r)]

adj.较少的;较小的;次要的

horticulturist

[hɔːtɪˈkʌlʧərɪst]

n.园艺家

precision

[prɪ’sɪʒn]

n.精确;精密度

dissimilar

[dɪ’sɪmɪlə(r)]

adj.不同的

rhododendron

[ˌrəʊdə’dendrən]

n.杜鹃花

graft

[ɡrɑːft]

n.嫁接;移植

systematic

[ˌsɪstə’mætɪk]

adj.有系统的

thorough

[’θʌrə]

adj.彻底的;完全的;详尽的;细致深入的

authenticate

[ɔː’θentɪkeɪt]

v.证明...为真;验证

confinement

[kən’faɪnmənt]

n.拘禁;限制;分娩

finch

[fɪntʃ]

n.雀类

inter

[ɪn’tɜː(r)]

vt.埋葬

hatch

[hætʃ]

n.孵化;舱口，舱口盖

eminent

[’emɪnənt]

adj.著名的;卓越的

intercross

[ˌɪntɜː’krɒs]

vt.使相互交叉

hump

[hʌmp]

n.忧郁；烦恼;

ascertain

[ˌæsə’teɪn]

vt.确定;查明;探知

utter

[’ʌtə(r)]

adj.完全的;全然的;绝对的

chiefly

[’tʃiːfli]

adv.主要地

gradation

[ɡrə’deɪʃn]

n.渐变;阶段;等级;刻度;元音交替

inorganic

[ˌɪnɔː’ɡænɪk]

adj.无机的;非自然生长的

abnormal

[æb’nɔːml]

adj.反常的;不正常的

excess

[ɪk’ses]

n.超过;过量;过剩

earlier

[’ɜːlɪə]

adj.早的;初期的

wither

[’wɪðə(r)]

vi.枯萎，干枯;凋谢

incipient

[ɪn’sɪpiənt]

adj.起初的;初期的

innate

[ɪ’neɪt]

adj.天生的;固有的

capsule

[’kæpsjuːl]

n.胶囊

physiological

[ˌfɪziə’lɒdʒɪkl]

adj.生理的;生理学的

correspondence

[ˌkɒrə’spɒndəns]

n.通信;信件;相符，相似;一致，相当

utmost

[’ʌtməʊst]

adj.极度的;最大限度的

persevere

[ˌpɜːsɪ’vɪə(r)]

v.坚持;不屈不挠

obstinate

[’ɒbstɪnət]

adj.固执的;倔强的;不易屈服的;(病)难治的

cotyledon

[ˌkɒtɪ’liːdən]

n.〈植〉子叶;〈解〉绒毛叶

perennial

[pə’reniəl]

adj.永久的;反复出现的;多年生的

deciduous

[dɪ’sɪdʒuəs]

a. 凋落的;

evergreen

[’evəɡriːn]

adj.常绿的

inhabit

[ɪn’hæbɪt]

v.居住于;占据;栖息

reciprocal

[rɪ’sɪprəkl]

adj.互惠的;相互的;互补的

utterly

[’ʌtəli:]

adv.完全;全然;绝对

likeness

[’laɪknəs]

n.相像;相似物;样子

susceptible

[sə’septəbl]

adj.易受外界影响的;易受感染的;容许…的

propagation

[ˌprɒpə’ɡeɪʃn]

n.增殖;繁殖

unimportant

[ˌʌnɪm’pɔːtnt]

adj.不重要的

hardness

[hɑːdnəs]

n.坚硬;困难;严厉

adaptation

[ˌædæp’teɪʃn]

n.改编;改编成的作品;适应;【心】(感官)适应，适应性变化

invariably

[ɪn’veəriəbli]

adv.不变地;总是;一贯地

parallel

[’pærəlel]

adj.平行的

barren

[’bærən]

adj.贫瘠的;无效的;不育的

adhesion

[əd’hiːʒn]

n.粘合

rude

[ruːd]

adj.粗鲁无礼的;原始的;未加工的;粗糙的;猛烈的

impossibility

[ɪmˌpɒsə’bɪləti]

n.不可能之事;不可能

incapable

[ɪn’keɪpəbl]

adj.无能力的;不胜任的

unwilling

[ʌn’wɪlɪŋ]

adj.不愿意的

mule

[mjuːl]

n.骡子;固执的人;走私毒品者

nourish

[’nʌrɪʃ]

v.滋养;给营养;培育;怀有

liable

[’laɪəbl]

a. 有(法律)责任的；有义务的;

allude

[ə’luːd]

v.暗示;暗指

superinduce

[ˌsjuːpərɪn’djuːs]

vt.再加(添加)

ensue

[ɪn’sjuː]

v.跟着发生;继起;因而产生

scarcely

[’skeəsli]

adv.几乎不;简直不;刚刚;决不

disturbance

[dɪ’stɜːbəns]

n.扰乱;骚动

unequal

[ʌn’iːkwəl]

adj.不相等的;不平等的;不胜任的

exceptional

[ɪk’sepʃənl]

adj.异常的;例外的;特别的;杰出的

founding

[’faʊndɪŋ]

n.建立;成立,

tuber

[’tjuːbə(r)]

n.块茎;球根;【解】结节;隆起;肿胀

hermaphrodite

[hɜː’mæfrədaɪt]

n.具有两性者;阴阳人;两性体

forcible

[’fɔːsəbl]

adj.强制的;强迫的;有说服力的;用力的

hopeless

[’həʊpləs]

adj.没有希望的;绝望的;无可救药的

hitherto

[ˌhɪðə’tuː]

adv.到目前为止;迄今

repute

[rɪ’pjuːt]

n.名声;名气;声望

undoubted

[ʌn’daʊtɪd]

adj.无疑的

aboriginal

[ˌæbə’rɪdʒənl]

adj.原始的;土著的

correlate

[’kɒrəleɪt]

vt.使相互关联

indirect

[ˌɪndə’rekt]

adj.间接的;迂回的

descendant

[dɪ’sendənt]

n.后裔;子孙;后代

maize

[meɪz]

n.玉米

Manipulation

[məˌnɪpju’leɪʃn]

n.操纵;控制;篡改

gourd

[ɡʊəd]

n. 葫芦; 脑瓜

analogous

[ə’næləɡəs]

adj.类似的

occurrence

[ə’kʌrəns]

n.发生

invariable

[ɪn’veəriəbl]

adj.不变的;恒定的

modification

[ˌmɒdɪfɪ’keɪʃn]

n.修改;修正;修饰

Independently

[ˌɪndɪ’pendəntli]

adv.独立地;自立地

propagate

[’prɒpəɡeɪt]

v.繁殖;增殖;传播;传送

corroborate

[kə’rɒbəreɪt]

v.确证;证实

Hybrid

[’haɪbrɪd]

n.混合物;杂种;混血儿

applicable

[ə’plɪkəbl]

adj.合适的

resemblance

[rɪ’zembləns]

n.相像;相似

deficiency

[dɪ’fɪʃnsi]

n.缺乏;不足;缺陷;

mongrel

[’mʌŋɡrəl]

n.杂种狗;杂交动植物;混血儿

resemblance

[rɪ’zembləns]

n.相像;相似

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