On the Tendency of Varieties to depart indefinitely from the Original Type

One of the strongest arguments which have been adduced to prove the original and permanent distinctness of species is, that varieties produced in a state of domesticity are more or less unstable, and often have a tendency, if left to themselves, to return to the normal form of the parent species; and this instability is considered to be a distinctive peculiarity of all varieties, even of those occurring among wild animals in a state of nature, and to constitute a provision for preserving unchanged the originally created distinct species.

为证明物种原本就各自分明、永久不变而提出的最有力的论据之一，是说：在家养状态下产生的变种多少都不稳定，若听其自然，往往有回复到亲本物种正常类型的倾向；而这种不稳定性被认为是一切变种——甚至包括那些在自然状态下的野生动物中出现的变种——的显著特性，并构成了一种保障，使原先被创造出来的各个不同物种得以保持不变。

In the absence or scarcity of facts and observations as to varieties occurring among wild animals, this argument has had great weight with naturalists, and has led to a very general and somewhat prejudiced belief in the stability of species. Equally general, however, is the belief in what are called "permanent or true varieties,"—races of animals which continually propagate their like, but which differ so slightly (although constantly) from some other race, that the one is considered to be a variety of the other. Which is the variety and which the original species, there is generally no means of determining, except in those rare cases in which the one race has been known to produce an offspring unlike itself and resembling the other. This, however, would seem quite incompatible with the "permanent invariability of species," but the difficulty is overcome by assuming that such varieties have strict limits, and can never again vary further from the original type, although they may return to it, which, from the analogy of the domesticated animals, is considered to be highly probable, if not certainly proved.

由于关于野生动物中出现变种的事实和观察付之阙如或十分稀少，这一论据在博物学家中极有分量，并使人们普遍地、多少带着成见地相信物种是稳定的。然而，同样普遍的，是对所谓“永久变种或真变种”的信念——这是一些动物族群，它们不断繁殖出与自己相似的后代，但与另一族群的差别非常细微（尽管恒定），以致其中一个被认为是另一个的变种。哪一个是变种，哪一个是原始物种，一般无从判定，只有在少数罕见的情况下例外：已知其中一个族群生出了不像自己、却像另一族群的后代。然而，这似乎与“物种的永久不变性”全然不相容；不过，这一困难被这样一个假定克服了：这样的变种有严格的界限，永远不会再离原始类型更远，尽管它们可能回复到原始类型——根据家养动物的类比，人们认为这即使没有得到确凿的证明，也是极有可能的。

It will be observed that this argument rests entirely on the assumption, that varieties occurring in a state of nature are in all respects analogous to or even identical with those of domestic animals, and are governed by the same laws as regards their permanence or further variation. But it is the object of the present paper to show that this assumption is altogether false, that there is a general principle in nature which will cause many varieties to survive the parent species, and to give rise to successive variations departing further and further from the original type, and which also produces, in domesticated animals, the tendency of varieties to return to the parent form.

我们会看到，这一论据完全建立在这样一个假定之上：自然状态下出现的变种在一切方面都与家养动物的变种类似，甚至相同，并且在其持久或进一步变异方面受同样的法则支配。但本文的目的正是要表明：这一假定完全是错误的；自然界中有一条普遍的原理，它会使许多变种比亲本物种存活得更久，并引出一个接一个的变异，离原始类型越来越远；而在家养动物中，造成变种回复到亲本类型之倾向的，也是这同一条原理。

The life of wild animals is a struggle for existence. The full exertion of all their faculties and all their energies is required to preserve their own existence and provide for that of their infant offspring. The possibility of procuring food during the least favourable seasons, and of escaping the attacks of their most dangerous enemies, are the primary conditions which determine the existence both of individuals and of entire species. These conditions will also determine the population of a species; and by a careful consideration of all the circumstances we may be enabled to comprehend, and in some degree to explain, what at first sight appears so inexplicable—the excessive abundance of some species, while others closely allied to them are very rare.

野生动物的生活是一场生存斗争。它们必须充分发挥一切官能、一切精力，才能保全自己的生存，并为幼小的后代提供生存所需。能否在最不利的季节里获得食物，能否逃过最危险的敌人的袭击，是决定个体乃至整个物种能否存在的首要条件。这些条件也决定着一个物种的数量；仔细考察一切情况，我们或许就能理解、并在某种程度上解释那乍看起来如此费解的现象：有些物种极其繁盛，而与它们亲缘很近的另一些物种却非常稀少。

Now it is clear that what takes place among the individuals of a species must also occur among the several allied species of a group,—viz. that those which are best adapted to obtain a regular supply of food, and to defend themselves against the attacks of their enemies and the vicissitudes of the seasons, must necessarily obtain and preserve a superiority in population; while those species which from some defect of power or organization are the least capable of counteracting the vicissitudes of food, supply, &c., must diminish in numbers, and, in extreme cases, become altogether extinct. Between these extremes the species will present various degrees of capacity for ensuring the means of preserving life; and it is thus we account for the abundance or rarity of species. Our ignorance will generally prevent us from accurately tracing the effects to their causes; but could we become perfectly acquainted with the organization and habits of the various species of animals, and could we measure the capacity of each for performing the different acts necessary to its safety and existence under all the varying circumstances by which it is surrounded, we might be able even to calculate the proportionate abundance of individuals which is the necessary result.

显然，在一个物种的个体之间发生的事，也必定发生在一个类群中几个近缘的物种之间——也就是说，那些最适于获得有规律的食物供应、最能抵御敌人袭击和季节变迁的物种，必然会取得并保持数量上的优势；而那些由于能力或组织上的某种缺陷、最不能对付食物供应等等变迁的物种，其数量必定减少，在极端的情况下还会完全灭绝。在这两个极端之间，各物种在保障维持生命之手段的能力上，呈现出各种不同的程度；我们正是这样来解释物种的繁多或稀少的。我们的无知通常使我们不能准确地从结果追溯到原因；但是，倘若我们能完全了解各种动物的组织和习性，倘若我们能测量出每一种动物在包围着它的一切变化着的环境中，完成为其安全和生存所必需的种种行为的能力，我们甚至也许能够计算出作为其必然结果的个体相对数量。

If now we have succeeded in establishing these two points—1st, that the animal population of a country is generally stationary, being kept down by a periodical deficiency of food, and other checks; and, 2nd, that the comparative abundance or scarcity of the individuals of the several species is entirely due to their organization and resulting habits, which, rendering it more difficult to procure a regular supply of food and to provide for their personal safety in some cases than in others, can only be balanced by a difference in the population which have to exist in a given area—we shall be in a condition to proceed to the consideration of varieties, to which the preceding remarks have a direct and very important application.

如果我们现在已经成功地确立了这两点——第一，一个地区的动物数量一般是稳定的，它受到周期性的食物短缺以及其他抑制因素的压制；第二，各个物种个体的相对繁多或稀少，完全取决于它们的组织以及由此而来的习性，在有些情况下，组织和习性使得获取有规律的食物供应、保障自身安全比在另一些情况下更为困难，而这只能由必须生存在一定区域中的数量上的差异来抵消——那么，我们就可以着手讨论变种问题了，前面这些论述对变种问题有着直接而非常重要的应用。

Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. Even a change of colour might, by rendering them more or less distinguishable, affect their safety; a greater or less development of hair might modify their habits. More important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they inhabit. It is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. An antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carnivora; the passenger pigeon with less powerful wings would sooner or later be affected in its powers of procuring a regular supply of food; and in both cases the result must necessarily be a diminution of the population of the modified species. If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers. These results must follow as surely as old age, intemperance, or scarcity of food produce an increased mortality. In both cases there may be many individual exceptions; but on the average the rule will invariably be found to hold good. All varieties will therefore fall into two classes—those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority. Now, let some alteration of physical conditions occur in the district—a long period of drought, a destruction of vegetation by locusts, the irruption of some new carnivorous animal seeking "pastures new"—any change in fact tending to render existence more difficult to the species in question, and tasking its utmost powers to avoid complete extermination; it is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organized variety would suffer first, and, were the pressure severe, must soon become extinct. The same causes continuing in action, the parent species would next suffer, would gradually diminish in numbers, and with a recurrence of similar unfavourable conditions might also become extinct. The superior variety would then alone remain, and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety.

一个物种偏离其典型形态的大多数变异，或许是全部变异，必定对个体的习性或能力有某种确定的影响，无论多么轻微。即使是颜色的变化，也可能因为使它们更易或更难被辨认而影响它们的安全；毛发的发达程度不同，也可能改变它们的习性。更重要的变化，比如四肢或任何外部器官的力量或尺寸增大，会或多或少地影响它们获取食物的方式或它们栖息的地域范围。同样明显的是，大多数变化都会有利地或不利地影响延长生存的能力。腿较短或较弱的羚羊，必然更受猫科食肉动物袭击之苦；翅膀较无力的旅鸽，迟早会在获取有规律的食物供应的能力上受到影响；在这两种情况下，结果必然是这一变化了的类型数量减少。反之，如果任何物种产生了一个保全生存的能力稍有增强的变种，那么这个变种必然迟早会取得数量上的优势。这些结果必定随之而来，就像衰老、放纵或食物短缺必定使死亡率增高一样确定。在这两种情况下，都可能有许多个别的例外；但平均说来，会发现这一规则总是成立的。因此，所有的变种都可以分为两类：一类在同样条件下永远达不到亲本物种的数量，另一类则会迟早取得并保持数量上的优势。现在，假设这个地区的自然条件发生了某种变化——长期干旱，蝗虫毁坏植被，某种寻找“新牧场”的新食肉动物闯入——事实上，任何一种使所论物种的生存变得更加困难、需要竭尽全力才能避免被彻底消灭的变化；显然，在组成这个物种的所有个体中，那些构成数量最少、组织最孱弱的变种的个体会首先受害，如果压力严重，它们必定很快灭绝。同样的原因继续起作用，接着受害的就是亲本物种，它的数量会逐渐减少，如果类似的不利条件再次出现，它也可能灭绝。那时就只剩下那个较优的变种了，一旦环境恢复有利，它就会迅速增加，占据已灭绝的物种和变种的位置。

The variety would now have replaced the species, of which it would be a more perfectly developed and more highly organized form. It would be in all respects better adapted to secure its safety, and to prolong its individual existence and that of the race. Such a variety could not return to the original form; for that form is an inferior one, and could never compete with it for existence. Granted, therefore, a "tendency" to reproduce the original type of the species, still the variety must ever remain preponderant in numbers, and under adverse physical conditions again alone survive. But this new, improved, and populous race might itself, in course of time, give rise to new varieties, exhibiting several diverging modifications of form, any of which, tending to increase the facilities for preserving existence, must, by the same general law, in their turn become predominant. Here, then, we have progression and continued divergence deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do frequently occur. It is not, however, contended that this result would be invariable; a change of physical conditions in the district might at times materially modify it, rendering the race which had been the most capable of supporting existence under the former conditions now the least so, and even causing the extinction of the newer and, for a time, superior race, while the old or parent species and its first inferior varieties continued to flourish. Variations in unimportant parts might also occur, having no perceptible effect on the life-preserving powers; and the varieties so furnished might run a course parallel with the parent species, either giving rise to further variations or returning to the former type. All we argue for is, that certain varieties have a tendency to maintain their existence longer than the original species, and this tendency must make itself felt; for though the doctrine of chances or averages can never be trusted to on a limited scale, yet, if applied to high numbers, the results come nearer to what theory demands, and, as we approach to an infinity of examples, become strictly accurate. Now the scale on which nature works is so vast—the numbers of individuals and periods of time with which she deals approach so near to infinity, that any cause, however slight, and however liable to be veiled and counteracted by accidental circumstances, must in the end produce its full legitimate results.

现在，这个变种就取代了那个物种，成为那个物种发展得更完善、组织更高级的形态。它在一切方面都更适于保障自己的安全，延长自身个体以及族群的生存。这样的变种不可能回复到原来的形态；因为那种形态是较劣的，永远不可能与它竞争生存。因此，即使承认有一种重现物种原始类型的“倾向”，这个变种也必定始终在数量上占优势，并且在不利的自然条件下，又会成为唯一的幸存者。而这个新的、改进了的、数量众多的族群，经过一段时间，本身又可能产生新的变种，表现出几种彼此分歧的形态变化，其中任何一种只要倾向于增加保全生存的便利，就必定依照同一条普遍法则轮到它来占优势。这样，我们就从支配自然状态下动物生存的普遍法则，从变种确实经常出现这一无可争辩的事实，推演出了进展和持续的分歧。不过，我们并不认为这一结果是一成不变的；地区自然条件的变化有时可能大大改变它，使在原先条件下最能维持生存的族群如今变得最不能维持，甚至造成那较新的、一度较优的族群灭绝，而旧的或亲本物种以及它最初的那些较劣的变种却继续繁盛。在不重要的部分也可能出现变异，对保全生命的能力没有可察觉的影响；这样产生的变种可能与亲本物种平行地发展，或者引出进一步的变异，或者回复到先前的类型。我们所要论证的只是：某些变种有一种比原始物种存续得更久的倾向，而这种倾向必定会让自己显现出来；因为虽然在有限的规模上，机会或平均的学说永远靠不住，但如果应用于很大的数目，结果就会更接近理论的要求，而当我们接近无穷多的例子时，就变得严格准确。而自然运作的规模是如此之大——她所处理的个体数目和时间长度如此接近于无穷——以致任何原因，无论多么轻微，无论多么容易被偶然的情况所掩盖和抵消，最终必定产生它全部应有的结果。

Let us now turn to domesticated animals, and inquire how varieties produced among them are affected by the principles here enunciated. The essential difference in the condition of wild and domestic animals is this,—that among the former, their well-being and very existence depend upon the full exercise and healthy condition of all their senses and physical powers, whereas, among the latter, these are only partially exercised, and in some cases are absolutely unused. A wild animal has to search, and often to labour, for every mouthful of food—to exercise sight, hearing, and smell in seeking it, and in avoiding dangers, in procuring shelter from the inclemency of the seasons, and in providing for the subsistence and safety of its offspring. There is no muscle of its body that is not called into daily and hourly activity; there is no sense or faculty that is not strengthened by continual exercise. The domestic animal, on the other hand, has food provided for it, is sheltered, and often confined, to guard it against the vicissitudes of the seasons, is carefully secured from the attacks of its natural enemies, and seldom even rears its young without human assistance. Half of its senses and faculties are quite useless; and the other half are but occasionally called into feeble exercise, while even its muscular system is only irregularly called into action.

现在让我们转向家养动物，考察一下在它们中间产生的变种如何受这里所阐述的原理的影响。野生动物和家养动物在境况上的本质区别是：前者的安康乃至生存，取决于它们一切感官和体力的充分运用和健康状态；而后者的感官和体力只得到部分的运用，有时则完全不用。野生动物必须为每一口食物去寻找，并常常要付出劳苦——在寻找食物、躲避危险、找地方躲避恶劣的季节、为后代的生计和安全作准备时，都要运用视觉、听觉和嗅觉。它身上没有一块肌肉不是每天每时都被调动起来的；没有一种感官或官能不是因不断的运用而得到加强的。反之，家养动物有人为它提供食物，有遮蔽，还常被关起来，以免受季节变迁之害；它被小心地保护起来，免遭天敌的袭击，甚至很少不靠人的帮助就能养育自己的幼崽。它的一半感官和官能全无用处；另一半也只是偶尔被调动起来作微弱的运用，就连它的肌肉系统也只是不规则地被调动起来。

Now when a variety of such an animal occurs, having increased power or capacity in any organ or sense, such increase is totally useless, is never called into action, and may even exist without the animal ever becoming aware of it. In the wild animal, on the contrary, all its faculties and powers being brought into full action for the necessities of existence, any increase becomes immediately available, is strengthened by exercise, and must even slightly modify the food, the habits, and the whole economy of the race. It creates as it were a new animal, one of superior powers, and which will necessarily increase in numbers and outlive those inferior to it.

那么，当这样一种动物出现一个变种，在某一器官或感官上能力增强时，这种增强是完全无用的，从来不会被调动起来，甚至可能存在着而那动物始终没有察觉。反之，在野生动物身上，由于它的一切官能和能力都为了生存的需要而被充分调动，任何增强都会立刻派上用场，因运用而得到加强，甚至必定会稍稍改变这个族群的食物、习性和整个生计。可以说，它造出了一种新的动物，一种能力更强的动物，它必然会在数量上增加，并比那些不如它的动物活得更久。

Again, in the domesticated animal all variations have an equal chance of continuance; and those which would decidedly render a wild animal unable to compete with its fellows and continue its existence are no disadvantage whatever in a state of domesticity. Our quickly fattening pigs, short-legged sheep, pouter pigeons, and poodle dogs could never have come into existence in a state of nature, because the very first step towards such inferior forms would have led to the rapid extinction of the race; still less could they now exist in competition with their wild allies. The great speed but slight endurance of the race horse, the unwieldy strength of the ploughman's team, would both be useless in a state of nature. If turned wild on the pampas, such animals would probably soon become extinct, or under favourable circumstances might each lose those extreme qualities which would never be called into action, and in a few generations would revert to a common type, which must be that in which the various powers and faculties are so proportioned to each other as to be best adapted to procure food and secure safety,—that in which by the full exercise of every part of his organization the animal can alone continue to live. Domestic varieties, when turned wild, must return to something near the type of the original wild stock, or become altogether extinct.

再者，在家养动物中，一切变异都有同等的机会延续下去；那些肯定会使野生动物无法与同伴竞争、无法继续生存的变异，在家养状态下却毫无不利之处。我们那些长膘很快的猪、短腿的羊、球胸鸽和贵宾犬，在自然状态下永远不可能产生出来，因为朝这些较劣形态迈出的第一步，就会导致这一族群迅速灭绝；更不用说它们如今与其野生近亲竞争而生存了。赛马速度极快而耐力很差，农夫的耕畜力大却笨重，在自然状态下都毫无用处。如果把这些动物放到潘帕斯草原上去任其野生，它们也许很快就会灭绝；或者在有利的环境下，它们可能各自丧失那些永远不会被调动起来的极端特质，并在几代之内回复到一种共同的类型——这种类型必定是各种能力和官能彼此比例协调、最适于获取食物和保障安全的类型，是动物唯有充分运用其机体的每一部分才能继续生存的那种类型。家养变种一旦放归野外，必定回复到接近原始野生种群的类型，否则就完全灭绝。

We see, then, that no inferences as to varieties in a state of nature can be deduced from the observation of those occurring among domestic animals. The two are so much opposed to each other in every circumstance of their existence, that what applies to the one is almost sure not to apply to the other. Domestic animals are abnormal, irregular, artificial; they are subject to varieties which never occur and never can occur in a state of nature: their very existence depends altogether on human care; so far are many of them removed from that just proportion of faculties, that true balance of organization, by means of which alone an animal left to its own resources can preserve its existence and continue its race.

由此可见，从对家养动物中出现的变种的观察，推不出任何关于自然状态下变种的结论。两者在生存的每一种情形上都如此对立，以致适用于其一的，几乎肯定不适用于其二。家养动物是反常的、不规则的、人为的；它们会出现自然状态下从未出现、也永远不可能出现的变种：它们的生存本身完全依赖于人的照料；它们中的许多，已经远远偏离了那种恰当的官能比例、那种真正的机体平衡——一只听凭自力的动物，唯有靠这种比例与平衡，才能保全自己的生存，延续自己的族群。