首页>> 文学论坛>> 动物>> 达尔文 Charles Darwin   英国 United Kingdom   汉诺威王朝   (1809年2月12日1882年4月19日)
物种起源 On the Origin of Species
  达尔文在《物种起源》中提出两个理论。第一,他认为所有的动植物都是由较早期、较原始的形式演变而来;其次,他认为生物演化是通过自然选择而来。
  
  其理论重点如下:物种并非一成不变,而是会随环境变动而改变;生物的演化是长时间连续性的缓慢改变,不是突然性的剧变;同一类生物有着共同的祖先,例如哺乳类是由同一个祖先演变而来;生物族群会随着繁殖而扩大,并超过其生存空间与食物供应的极限,引起个体间的竞争;不适应环境的个体会被淘汰,适者才能生存,并繁衍后代。
  《物种起源》-作者简介
  
  达尔文(Charles Darwin,1809~1882):英国伟大的科学家、博物学家,进化论的奠基人。生于英国一个名医世家。儿时“除打猎、养狗、抓老鼠以外,无所事事”(达尔文父亲语)。后因结识植物学家亨斯罗后对博物学产生兴趣。1831年参加 “比格尔号”舰的全球远航,1842年首次提出 “物种进化理论”。
  《物种起源》-撰写背景
  
  在达尔文那个时代,人们普遍都接受创造论,相信上帝创造世界、并一次就创造出所有的生物,同时上帝也赋予每种生物各自的角色,而每个物种的设计都非常完美,所以物种是永恒固定不变的。地层中发现的化石是古代地球曾经遭遇大洪水的证据,那些化石就是没有登上诺亚方舟的动物。地球的历史大约只有六千年左右,这是经由从亚当和夏娃开始后的人类世代所推算的时间。
  
  达尔文1809年生于英格兰舒兹伯利 (Shrewsbury)。达尔文从小就对矿物和动物有兴趣。1831年他从剑桥大学神学院毕业,然而他还是对地质学及生物学比较有兴趣。1831年12月达尔文参加了海军舰艇小猎犬号前往南美洲从事自然调查研究工作;最初他在南美海岸调查,并多次进入南美洲西边的加拉巴哥群岛,经过太平洋到达新西兰、澳大利亚及南非,然后又回到南美洲,直到1836年10月才回到英国。
  
  1836年10月,达尔文回到英国后花了22年时间撰写《物种起源》一书。1859年11月24日,达尔文的《物种起源》一书出版。这本书极大地冲击了“神创论”,引起教会激烈反对,但也得到许多有识之士的赞赏和维护。此后,达尔文的学说在全球广泛传播,不断深入人心。19世纪70年代,达尔文的学说传入中国。
  《物种起源》-理论形成
  
  达尔文在南美洲考察时,在安第斯山山顶上发现海洋生物的化石,让他感到困惑。在小猎犬号启航时,达尔文曾带了一本英国地质学家莱尔所著的《地质学原理》;莱尔认为地球的地形、地貌是经过长时间不断的细微变化的结果,莱尔相信风力、雨滴、冰雪等微小的力量,持续千万年后就可以完全改变地表的形貌。达尔文本人也相信,只要时间足够,无法察觉的细微改变也可以造成巨大的变化。达尔文推算白垩纪中期距今约有三亿年左右的历史,持续长时间微小的地震等自然因素使得原来在海中的生物遗迹能在高山上发现。
  
  在加拉巴哥群岛考察时,达尔文发现每个岛屿上的陆龟及雀鸟并没有很大的差异,但又有些许的不同。他又发现加拉巴哥群岛的生物与南美洲大陆的种类非常相似;于是他开始怀疑岛上生物可能有共同的祖先,他们之间的差异是由于千百年来适应各个岛屿不同环境的结果。每一个物种都是一些细微的变化在无数个世代的过程中产生的结果。
  1《物种起源》
  
  生物进化在当时并不是新的概念。1809年时,法国动物学家拉马克便提出:当环境改变时,物种会调适发展自己的器官来适应环境,常用的器官会发育变大、不用的器官会逐渐退化,并且这一代获取的改变会遗传给下一代;但没有科学证据可以证明“用进废退”和“获得性特征可遗传”的假说。后来达尔文又从英国人口学者马尔萨斯所著的《人口论》得到灵感;马尔萨斯认为:人类粮食的生产永远无法赶上人口的增加,致使粮食供不应求,进而发生饥荒或战争,导致一部分人口死亡。达尔文以此联想到生物演化发生的机制:演化是生存竞争中自由淘汰的结果,食物与空间等资源有限,只有最适应环境的个体才能生存下来,延续族群。
  
  “天择”的概念逐渐在达尔文的五年环球考察过程中形成。在1836年回到英国后,达尔文慢慢将他的看法写成文章,然而没有发表。大部分科学家认为,达尔文迟了很久才发表他的作品,原因之一就是担心自己的思想对于当时的社会来说过于激进。1858年,达尔文接到在马来群岛调查的博物学者华莱士有关物种形成的文章;华莱士对于物种形成的看法与他有很多相似之处,增加了达尔文对其学说的信心。于是两人在1858年的伦敦皇家科学年会中,以两人共同具名的方式,发表有关物种形成的看法。接着达尔文在1859年发表了《物种起源》。
  《物种起源》-影响
  
  引起争议
  
  达尔文的进化论当时引起了广泛的争议,被基督教会视为异端邪说,西方社会也对达尔文冷嘲热讽。不仅上帝创造万物的说法被推翻,人类也被形容为千百年来残酷的生存竞争所形成的产物,还指出人与其他哺乳动物有着共同祖先,这在当时保守的社会是相当大的震撼。
  
  毫无疑问,它给当时的社会带来的是霹雳般的震荡,像一颗炸弹一样投到“神学阵地的心脏上”,引起了整个世界特别是英国保守势力和宗教神学势力的极端惊恐和狂怒,谴责的声浪铺天盖地而来。教会势力和老派学者攻击《物种起源》是“恶作剧”,违背了上帝的启示,是一部“魔鬼的圣经”,而达尔文本人则是“英国最危险的人”。
  
  达尔文预见到这本书的出版一定会引起激烈的争论,因此,他把样书分送给莱尔、胡克、赫胥黎及格雷等科学家,以争取他们的支持和鼓励。
  
  学术地位
  
  本书是生物学史上的经典著作。1859年11月24日《物种起源》出版,当天即被抢购一空。这主要因为《物种起源》也受到英国和其他国家一些学者的积极支持,像英国的 T.H.赫胥黎、德国的E.海克尔等。《物种起源》出版,第一次把生物学建立在完全科学的基础上,以全新的生物进化思想推翻了“神创论”和“物种不变”理论,生物普遍进化的思想以及“物竞天择,适者生存”的进化机制成为学术界、思想界的公论。在书中,达尔文提出了一个又一个令人震惊的论断:生命只有一个祖先,因为生命都起源于一个原始细胞的开端;生物是从简单到复杂、从低级到高级逐步发展而来的,生物在进化中不断进行着生存斗争,进行着自然选择。由此,达尔文的生物进化论被称为19世纪自然科学的三大发现之一,《物种起源》所提及的许多观点已成为人尽皆知的常识。
  《物种起源》-面临挑战
  
  达尔文并不认为自己的理论完美无缺,他也有很多困惑。自《物种起源》出版100年间,进化论先后面临科学发展带来的以下几个“挑战”:
  
  在《物种起源》发表6年后,德国学者海克尔依据进化论思想画出了一棵“生命之树”,用“树”来形象描绘生物进化历史。然而,由于形态和生理特征的进化极其复杂,加上化石资料不够完整,“生命之树”难以反映复杂生物进化历史的全貌。在这种情况下,现代分子生物学的重要性就显现出来。现在科研人员可以通过比较DNA序列来研究不同生物之间的进化关系,构建 “进化树”。病毒等微生物“网状”进化结构的发现对此前的进化理论提出疑问,促使其进一步完善。
  
  达尔文进化论的一个重要观点是渐变论,即物种是通过微小的优势变异逐渐改进的,突变是很少的,但最令达尔文困惑的是寒武纪物种大爆发。中国科学家通过对云南澄江动物群化石的发现和研究,揭示了寒武纪物种大爆发的整体轮廓,证实几乎所有动物的祖先都曾经站在同一起跑线上。这一发现在很大程度上说明,生物的进化并非总是渐进的,而是渐进与跃进并存的过程。
  
  达尔文在《物种起源》一书中比较多地推崇自然选择,认为生物进化中有害的突变比较多,有利的突变很少,而经过长期的进化过程,有利的突变经过自然选择终于占了上风。但后来科学研究发现,物种的有害突变和有利突变都不是很多,多的是对于自然选择来说不好也不坏的中性突变,这后一类突变与进化的关联还有待深入细致的研究。
  《物种起源》-评价
  
  如果要评选过去的一千年来最重要的著作,达尔文的《物种起源》无疑问是一个强有力的竞争者。它不仅为生物学奠定了坚实的理论基础,也影响了人类社会的几乎所有方面。
  
  生物进化论是研究生物进化、生物发展规律以及如何运用这些规律的科学,是生物学的一个重要部门。进化论一词最初是拉马克提出。达尔文的《物种起源》一书奠定了进化论的科学基础。在《物种起源》发表前的欧洲,接受进化论思想的人非常少,尽管当时人们已经不再生活于中世纪愚昧的宗教统治中,伽利略和哥白尼的太阳中心说已经确立300多年,但人们仍然相信《圣经》中的上帝创世说。人和一切生物都是上帝创造的,这是天经地义,毋庸置疑的。但是这本书,大胆地向上帝创世说进行了挑战,并从根本上撼动了《圣经》的基础。
  
  这部著作的问世,第一次把生物学建立在完全科学的基础上,确定了物种的变异性和承续性,以全新的“生物进化”思想,推翻了那种把动植物看作彼此毫无联系的、偶然的、神造的、不变的东西的观点,也就是“神创论”和物种不变的理论。达尔文以自然界的规律代替了“造物主的智慧”,并直接涉及人类自身的由来及历史,使宗教的基本信念发生了动摇,导致科学与宗教间的更深刻冲突。


  Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. For the sixth edition of 1872, the short title was changed to The Origin of Species. Darwin's book introduced the theory that populations evolve over the course of generations through a process of natural selection. It presented a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution. Darwin included evidence that he had gathered on the Beagle expedition in the 1830s and his subsequent findings from research, correspondence, and experimentation.
  
  Various evolutionary ideas had already been proposed to explain new findings in biology. There was growing support for such ideas among dissident anatomists and the general public, but during the first half of the 19th century the English scientific establishment was closely tied to the Church of England, while science was part of natural theology. Ideas about the transmutation of species were controversial as they conflicted with the beliefs that species were unchanging parts of a designed hierarchy and that humans were unique, unrelated to animals. The political and theological implications were intensely debated, but transmutation was not accepted by the scientific mainstream.
  
  The book was written for non-specialist readers and attracted widespread interest upon its publication. As Darwin was an eminent scientist, his findings were taken seriously and the evidence he presented generated scientific, philosophical, and religious discussion. The debate over the book contributed to the campaign by T.H. Huxley and his fellow members of the X Club to secularise science by promoting scientific naturalism. Within two decades there was widespread scientific agreement that evolution, with a branching pattern of common descent, had occurred, but scientists were slow to give natural selection the significance that Darwin thought appropriate. During the "eclipse of Darwinism" from the 1880s to the 1930s, various other mechanisms of evolution were given more credit. With the development of the modern evolutionary synthesis in the 1930s and 1940s, Darwin's concept of evolutionary adaptation through natural selection became central to modern evolutionary theory, now the unifying concept of the life sciences.
  When on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species--that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years' work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. I hope that I may be excused for entering on these personal details, as I give them to show that I have not been hasty in coming to a decision.
   My work is now nearly finished; but as it will take me two or three more years to complete it, and as my health is far from strong, I have been urged to publish this Abstract. I have more especially been induced to do this, as Mr. Wallace, who is now studying the natural history of the Malay archipelago, has arrived at almost exactly the same general conclusions that I have on the origin of species. Last year he sent to me a memoir on this subject, with a request that I would forward it to Sir Charles Lyell, who sent it to the Linnean Society, and it is published in the third volume of the Journal of that Society. Sir C. Lyell and Dr. Hooker, who both knew of my work--the latter having read my sketch of 1844--honoured me by thinking it advisable to publish, with Mr. Wallace's excellent memoir, some brief extracts from my manuscripts.
   This Abstract, which I now publish, must necessarily be imperfect. I cannot here give references and authorities for my several statements; and I must trust to the reader reposing some confidence in my accuracy. No doubt errors will have crept in, though I hope I have always been cautious in trusting to good authorities alone. I can here give only the general conclusions at which I have arrived, with a few facts in illustration, but which, I hope, in most cases will suffice. No one can feel more sensible than I do of the necessity of hereafter publishing in detail all the facts, with references, on which my conclusions have been grounded; and I hope in a future work to do this. For I am well aware that scarcely a single point is discussed in this volume on which facts cannot be adduced, often apparently leading to conclusions directly opposite to those at which I have arrived. A fair result can be obtained only by fully stating and balancing the facts and arguments on both sides of each question; and this cannot possibly be here done.
   I much regret that want of space prevents my having the satisfaction of acknowledging the generous assistance which I have received from very many naturalists, some of them personally unknown to me. I cannot, however, let this opportunity pass without expressing my deep obligations to Dr. Hooker, who for the last fifteen years has aided me in every possible way by his large stores of knowledge and his excellent judgment.
   In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration. Naturalists continually refer to external conditions, such as climate, food, etc., as the only possible cause of variation. In one very limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, the structure, for instance, of the woodpecker, with its feet, tail, beak, and tongue, so admirably adapted to catch insects under the bark of trees. In the case of the misseltoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself.
   The author of the 'Vestiges of Creation' would, I presume, say that, after a certain unknown number of generations, some bird had given birth to a woodpecker, and some plant to the misseltoe, and that these had been produced perfect as we now see them; but this assumption seems to me to be no explanation, for it leaves the case of the coadaptations of organic beings to each other and to their physical conditions of life, untouched and unexplained.
   It is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation. At the commencement of my observations it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem. Nor have I been disappointed; in this and in all other perplexing cases I have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue. I may venture to express my conviction of the high value of such studies, although they have been very commonly neglected by naturalists.
   From these considerations, I shall devote the first chapter of this Abstract to Variation under Domestication. We shall thus see that a large amount of hereditary modification is at least possible, and, what is equally or more important, we shall see how great is the power of man in accumulating by his Selection successive slight variations. I will then pass on to the variability of species in a state of nature; but I shall, unfortunately, be compelled to treat this subject far too briefly, as it can be treated properly only by giving long catalogues of facts. We shall, however, be enabled to discuss what circumstances are most favourable to variation. In the next chapter the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from their high geometrical powers of increase, will be treated of. This is the doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be NATURALLY SELECTED. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.
   This fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life and induces what I have called Divergence of Character. In the next chapter I shall discuss the complex and little known laws of variation and of correlation of growth. In the four succeeding chapters, the most apparent and gravest difficulties on the theory will be given: namely, first, the difficulties of transitions, or in understanding how a simple being or a simple organ can be changed and perfected into a highly developed being or elaborately constructed organ; secondly the subject of Instinct, or the mental powers of animals, thirdly, Hybridism, or the infertility of species and the fertility of varieties when intercrossed; and fourthly, the imperfection of the Geological Record. In the next chapter I shall consider the geological succession of organic beings throughout time; in the eleventh and twelfth, their geographical distribution throughout space; in the thirteenth, their classification or mutual affinities, both when mature and in an embryonic condition. In the last chapter I shall give a brief recapitulation of the whole work, and a few concluding remarks.
   No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he makes due allowance for our profound ignorance in regard to the mutual relations of all the beings which live around us. Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they determine the present welfare, and, as I believe, the future success and modification of every inhabitant of this world. Still less do we know of the mutual relations of the innumerable inhabitants of the world during the many past geological epochs in its history. Although much remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists entertain, and which I formerly entertained--namely, that each species has been independently created--is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.
  Causes of Variability. Effects of Habit. Correlation of Growth. Inheritance. Character of Domestic Varieties. Difficulty of distinguishing between Varieties and Species. Origin of Domestic Varieties from one or more Species. Domestic Pigeons, their Differences and Origin. Principle of Selection anciently followed, its Effects. Methodical and Unconscious Selection. Unknown Origin of our Domestic Productions. Circumstances favourable to Man's power of Selection.
   When we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us, is, that they generally differ much more from each other, than do the individuals of any one species or variety in a state of nature. When we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, I think we are driven to conclude that this greater variability is simply due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species have been exposed under nature. There is, also, I think, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food. It seems pretty clear that organic beings must be exposed during several generations to the new conditions of life to cause any appreciable amount of variation; and that when the organisation has once begun to vary, it generally continues to vary for many generations. No case is on record of a variable being ceasing to be variable under cultivation. Our oldest cultivated plants, such as wheat, still often yield new varieties: our oldest domesticated animals are still capable of rapid improvement or modification.
   It has been disputed at what period of life the causes of variability, whatever they may be, generally act; whether during the early or late period of development of the embryo, or at the instant of conception. Geoffroy St. Hilaire's experiments show that unnatural treatment of the embryo causes monstrosities; and monstrosities cannot be separated by any clear line of distinction from mere variations. But I am strongly inclined to suspect that the most frequent cause of variability may be attributed to the male and female reproductive elements having been affected prior to the act of conception. Several reasons make me believe in this; but the chief one is the remarkable effect which confinement or cultivation has on the functions of the reproductive system; this system appearing to be far more susceptible than any other part of the organisation, to the action of any change in the conditions of life. Nothing is more easy than to tame an animal, and few things more difficult than to get it to breed freely under confinement, even in the many cases when the male and female unite. How many animals there are which will not breed, though living long under not very close confinement in their native country! This is generally attributed to vitiated instincts; but how many cultivated plants display the utmost vigour, and yet rarely or never seed! In some few such cases it has been found out that very trifling changes, such as a little more or less water at some particular period of growth, will determine whether or not the plant sets a seed. I cannot here enter on the copious details which I have collected on this curious subject; but to show how singular the laws are which determine the reproduction of animals under confinement, I may just mention that carnivorous animals, even from the tropics, breed in this country pretty freely under confinement, with the exception of the plantigrades or bear family; whereas, carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. Many exotic plants have pollen utterly worthless, in the same exact condition as in the most sterile hybrids. When, on the one hand, we see domesticated animals and plants, though often weak and sickly, yet breeding quite freely under confinement; and when, on the other hand, we see individuals, though taken young from a state of nature, perfectly tamed, long-lived, and healthy (of which I could give numerous instances), yet having their reproductive system so seriously affected by unperceived causes as to fail in acting, we need not be surprised at this system, when it does act under confinement, acting not quite regularly, and producing offspring not perfectly like their parents or variable.
   Sterility has been said to be the bane of horticulture; but on this view we owe variability to the same cause which produces sterility; and variability is the source of all the choicest productions of the garden. I may add, that as some organisms will breed most freely under the most unnatural conditions (for instance, the rabbit and ferret kept in hutches), showing that their reproductive system has not been thus affected; so will some animals and plants withstand domestication or cultivation, and vary very slightly--perhaps hardly more than in a state of nature.
   A long list could easily be given of "sporting plants;" by this term gardeners mean a single bud or offset, which suddenly assumes a new and sometimes very different character from that of the rest of the plant. Such buds can be propagated by grafting, etc., and sometimes by seed. These "sports" are extremely rare under nature, but far from rare under cultivation; and in this case we see that the treatment of the parent has affected a bud or offset, and not the ovules or pollen. But it is the opinion of most physiologists that there is no essential difference between a bud and an ovule in their earliest stages of formation; so that, in fact, "sports" support my view, that variability may be largely attributed to the ovules or pollen, or to both, having been affected by the treatment of the parent prior to the act of conception. These cases anyhow show that variation is not necessarily connected, as some authors have supposed, with the act of generation.
   Seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each other, though both the young and the parents, as Muller has remarked, have apparently been exposed to exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life are in comparison with the laws of reproduction, and of growth, and of inheritance; for had the action of the conditions been direct, if any of the young had varied, all would probably have varied in the same manner. To judge how much, in the case of any variation, we should attribute to the direct action of heat, moisture, light, food, etc., is most difficult: my impression is, that with animals such agencies have produced very little direct effect, though apparently more in the case of plants. Under this point of view, Mr. Buckman's recent experiments on plants seem extremely valuable. When all or nearly all the individuals exposed to certain conditions are affected in the same way, the change at first appears to be directly due to such conditions; but in some cases it can be shown that quite opposite conditions produce similar changes of structure. Nevertheless some slight amount of change may, I think, be attributed to the direct action of the conditions of life--as, in some cases, increased size from amount of food, colour from particular kinds of food and from light, and perhaps the thickness of fur from climate.
   Habit also has a decided influence, as in the period of flowering with plants when transported from one climate to another. In animals it has a more marked effect; for instance, I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck; and I presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use. Not a single domestic animal can be named which has not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable.
   There are many laws regulating variation, some few of which can be dimly seen, and will be hereafter briefly mentioned. I will here only allude to what may be called correlation of growth. Any change in the embryo or larva will almost certainly entail changes in the mature animal. In monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St. Hilaire's great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are quite whimsical; thus cats with blue eyes are invariably deaf; colour and constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and plants. From the facts collected by Heusinger, it appears that white sheep and pigs are differently affected from coloured individuals by certain vegetable poisons. Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet. Hence, if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly unconsciously modify other parts of the structure, owing to the mysterious laws of the correlation of growth.
   The result of the various, quite unknown, or dimly seen laws of variation is infinitely complex and diversified. It is well worth while carefully to study the several treatises published on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, etc.; and it is really surprising to note the endless points in structure and constitution in which the varieties and sub-varieties differ slightly from each other. The whole organisation seems to have become plastic, and tends to depart in some small degree from that of the parental type.
   Any variation which is not inherited is unimportant for us. But the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological importance, is endless. Dr. Prosper Lucas's treatise, in two large volumes, is the fullest and the best on this subject. No breeder doubts how strong is the tendency to inheritance: like produces like is his fundamental belief: doubts have been thrown on this principle by theoretical writers alone. When a deviation appears not unfrequently, and we see it in the father and child, we cannot tell whether it may not be due to the same original cause acting on both; but when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in the parent--say, once amongst several million individuals--and it reappears in the child, the mere doctrine of chances almost compels us to attribute its reappearance to inheritance. Every one must have heard of cases of albinism, prickly skin, hairy bodies, etc., appearing in several members of the same family. If strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inheritable. Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly.
   The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes or to one sex alone, more commonly but not exclusively to the like sex. It is a fact of some little importance to us, that peculiarities appearing in the males of our domestic breeds are often transmitted either exclusively, or in a much greater degree, to males alone. A much more important rule, which I think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to appear in the offspring at a corresponding age, though sometimes earlier. In many cases this could not be otherwise: thus the inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature; peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage. But hereditary diseases and some other facts make me believe that the rule has a wider extension, and that when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent. I believe this rule to be of the highest importance in explaining the laws of embryology. These remarks are of course confined to the first APPEARANCE of the peculiarity, and not to its primary cause, which may have acted on the ovules or male element; in nearly the same manner as in the crossed offspring from a short-horned cow by a long-horned bull, the greater length of horn, though appearing late in life, is clearly due to the male element.
   Having alluded to the subject of reversion, I may here refer to a statement often made by naturalists--namely, that our domestic varieties, when run wild, gradually but certainly revert in character to their aboriginal stocks. Hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature. I have in vain endeavoured to discover on what decisive facts the above statement has so often and so boldly been made. There would be great difficulty in proving its truth: we may safely conclude that very many of the most strongly-marked domestic varieties could not possibly live in a wild state. In many cases we do not know what the aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. It would be quite necessary, in order to prevent the effects of intercrossing, that only a single variety should be turned loose in its new home. Nevertheless, as our varieties certainly do occasionally revert in some of their characters to ancestral forms, it seems to me not improbable, that if we could succeed in naturalising, or were to cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil (in which case, however, some effect would have to be attributed to the direct action of the poor soil), that they would to a large extent, or even wholly, revert to the wild aboriginal stock. Whether or not the experiment would succeed, is not of great importance for our line of argument; for by the experiment itself the conditions of life are changed. If it could be shown that our domestic varieties manifested a strong tendency to reversion,--that is, to lose their acquired characters, whilst kept under unchanged conditions, and whilst kept in a considerable body, so that free intercrossing might check, by blending together, any slight deviations of structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species. But there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an almost infinite number of generations, would be opposed to all experience. I may add, that when under nature the conditions of life do change, variations and reversions of character probably do occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved.
   When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with species closely allied together, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. Domestic races of the same species, also, often have a somewhat monstrous character; by which I mean, that, although differing from each other, and from the other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with all the species in nature to which they are nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,--a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as, only in most cases in a lesser degree than, do closely-allied species of the same genus in a state of nature. I think this must be admitted, when we find that there are hardly any domestic races, either amongst animals or plants, which have not been ranked by some competent judges as mere varieties, and by other competent judges as the descendants of aboriginally distinct species. If any marked distinction existed between domestic races and species, this source of doubt could not so perpetually recur. It has often been stated that domestic races do not differ from each other in characters of generic value. I think it could be shown that this statement is hardly correct; but naturalists differ most widely in determining what characters are of generic value; all such valuations being at present empirical. Moreover, on the view of the origin of genera which I shall presently give, we have no right to expect often to meet with generic differences in our domesticated productions.
   When we attempt to estimate the amount of structural difference between the domestic races of the same species, we are soon involved in doubt, from not knowing whether they have descended from one or several parent-species. This point, if it could be cleared up, would be interesting; if, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their kind so truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many very closely allied and natural species--for instance, of the many foxes--inhabiting different quarters of the world. I do not believe, as we shall presently see, that all our dogs have descended from any one wild species; but, in the case of some other domestic races, there is presumptive, or even strong, evidence in favour of this view.
   It has often been assumed that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute that these capacities have added largely to the value of most of our domesticated productions; but how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations, and whether it would endure other climates? Has the little variability of the ass or guinea-fowl, or the small power of endurance of warmth by the rein-deer, or of cold by the common camel, prevented their domestication? I cannot doubt that if other animals and plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would vary on an average as largely as the parent species of our existing domesticated productions have varied.
首页>> 文学论坛>> 动物>> 达尔文 Charles Darwin   英国 United Kingdom   汉诺威王朝   (1809年2月12日1882年4月19日)