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物种起源|The Origin of Species

第四章　自然选择|CHAPTER IV　NATURAL SELECTION

属类： 双语小说 【分类】其他读物 阅读：[12196]
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自然选择——其力量和人工选择的比较——对于不重要性状的力量——对于各年龄和雌雄两性的力量——性选择——论同种个体间杂交的普遍性——对自然选择有利和不利的条件，即杂交、隔离、个体数目——作用缓慢——自然选择所引起的灭绝——性状的分歧，与任何小地区生物多样性的关联以及与归化的关联——自然选择，通过性状的分歧和灭绝，对于共同祖先的后代的作用——解释一切生物分类

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上一章一笔带过的生存斗争，究竟如何对变异发生作用的呢？人类手里证明是威力巨大的选择原则，在自然界适用吗？我想我们将会看到，它是能够极其有效地发生作用的。请记住，家养生物有无数奇特变异，尽管自然状况下变异程度差一些；而且遗传倾向如此强烈。在家养状况下，可以说生物的整个体制好歹呈现可塑性了。请记住，一切生物的相互关系及其对于生活的物理条件的关系是何等复杂而密切。既然对于人类有用的变异毫无疑问地发生过，那在广大而复杂的生存斗争中，对于各个生物好歹有用的其他变异，难道在连续的成千上万世代中就判定不可能偶尔发生吗？如果确能发生，那么我们能怀疑（必须记住产生的个体超过可能生存的个体）较其他个体具有任何优越性（即使微不足道）的个体具有最好的生存和繁育后代的机会吗？相反，我们可以确定，任何有害的变异，即使微不足道，也会遭到严格消灭。我把这种有利变异的保存和有害变异的毁灭，叫作“自然选择”。无用也无害的变异则不受自然选择的影响，留作彷徨变异要素，如我们在所谓多态种里所看到的。

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以经历某些物理变化如气候变化的一个地方为例，就可以深入理解自然选择的大致过程。当地生物比例数几乎即刻就发生变化，有些物种会灭绝。从我们所知道的各地生物密切而复杂的关系来看，可以得出结论，即使撇开气候变化不谈，某些生物的比例数发生任何变化，也会严重影响许多其他生物。如果该地区的边界是开放的，则新类型势必要迁移进去，这也会严重扰乱某些原有生物间的关系。请记住，引进一种树木或哺乳动物的影响，已经证明是何等有力。但是，对于一个岛，或障碍物部分环绕的地方，如果善于适应的新类型不能自由移入，则自然结构中就会腾出一些地位，这时如果某些原有生物好歹发生了改变，肯定会更好地加以填充；因为如果那区域允许自由移入，则外来生物早就占领那里的地位了。在这种孤岛，茫茫岁月机缘凑巧，凡有轻微的变异多少对任何物种的个体有利，使之更好地适应多变的外界条件，就有保存下来的倾向；于是，自然选择在改进生物上就有余地了。

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正如第一章所阐明的，我们有理由相信，生活条件的变化通过特别影响生殖系统的作用而引起或者增加变异性；上述的个案中，假定外界条件已变，改善了有利变异发生的机会，这对自然选择显然大大有利；不发生有利变异，自然选择便无能为力。依我看，倒不需要极端数量的变异性，人类当然可以把细小的个体差异按照任何既定的方向积累起来，产生巨大的结果，自然也做得到，而且容易得多，它有无比长久的时间可以支配嘛。我也不认为真的需要任何巨大的物理变化，例如气候的变化，不需要异乎寻常的隔离以阻碍移入，来腾出新的空位，让自然选择改进某些变异着的生物，填充进去。由于各地区的全部生物都以微妙平衡的力量斗争在一起，一个物种的构造或习性发生极细微的变异，往往会因此占优势；同样的变异进一步发展，往往会使其优势进一步扩大。还没有一个地方，所有的土著生物现已完全相互适应，而且对于生活的物理条件也完全适应，以致其中没有一种能够少许改进。因为在一切地方，本地生物往往被归化生物压得抬不起头，最终听任外来者牢牢占据全境。外来生物既能这样在各地战胜某些本地生物，我们就可以稳妥地下结论：本地生物也可能已经发生有利的变异，以便更好地抵抗这种侵入者。

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人类用按部就班而无意识的选择手段，能够产生出，而且确已产生了伟大的结果，那么大自然何所不能呢？人类只能作用于外在的可见性状，而大自然并不关心外貌，除非外貌对于生物是有用的。自然能对各种内部器官、各种微细的体质差异以及整个生命机器发生作用。人类只为自己的利益而进行选择，自然则只为她所照拂的生物的利益而进行选择。各种被选择的性状，都充分地受着自然的锻炼，而生物被置于合适的生活条件之下。人类把多种生长在不同气候下的生物养在同一处；很少用某种特有的适宜方法来锻炼各个被选择的性状；用同样的食物饲养长喙和短喙的鸽；不用特有的方法去训练长背的或长腿的四足兽；把长毛的和短毛的绵羊养在同一种气候里。人类不允许最强壮的雄性为占有雌性而斗争；并不严格地把所有劣质动物都消灭掉，而是在力所能及的范围内，在各个不同季节里良莠不分，保护所有生物。人类往往以某半畸形的类型开始选择；或者至少以足够引起自己注意的某显著变异，明显对自己有用的变异，才开始选择。在自然界，构造上或体质上的极微细差异，便能打破生活斗争的微妙平衡，得以保存下来。人类是多么反复无常，朝三暮四啊！寿命又是何等短暂啊！因而，与大自然在整个地质时代的累积结果相比较，人类所得的结果是何等贫乏啊！所以，大自然的产物远比人类的产物在性状上更“真”，更无限地适应极其复杂的生活条件，并且明显地标有更高级技巧的烙印，这又有什么值得大惊小怪的呢？

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可以说，自然选择在世界各地，每日每时都在仔细检查着每一个变异，哪怕其微细无比，并且去芜存菁，加以积累；无论何时何地，只要有机会，就不声不响、不知不觉地进行工作，针对有机的和无机的生活条件改良各种生物。这种缓慢变化的进行，我们一无所知，直到时间之手标出时代的长久流逝。而我们对于早已过去的地质时代所知有限，能看出的充其量也只是现在的生物类型和先前的并不相同罢了。

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虽然自然选择只能通过各个生物而发生作用，并且要符合各个生物的利益，然而对于我们往往认为微不足道的性状和构造，也可以这样发生作用。我们看见吃叶子的昆虫是绿色的，吃树皮的昆虫是斑灰色的；高山雷鸟（alpine ptarmigan）在冬季呈白色，而苏格兰雷鸟（red-grouse）是石南花颜色的，黑琴鸡（black-grouse）是泥灰色的，就必须相信这种颜色对这些鸟和昆虫有用是为了保身避险。松鸡（grouse）如果不在一生的某一时期被杀死，必然会增殖到无数；我们知道它们主要受猛禽的侵害；鹰依靠目力捕猎——问题严重到欧洲大陆某些地方的人被告诫不养白鸽子，因其极易受害。因此，没有理由怀疑，自然选择非常有效地给予各种松鸡以适当的颜色，并且让颜色在获得之后纯正而稳定地保存下来。我们不要以为，偶然除掉一只任何颜色的动物所产生的作用很小；应当记住，在白色绵羊群里，除掉一只略见黑色的羔羊是何等重要。至于植物，植物学者们把果实的茸毛和果肉的颜色看作是微不足道的性状，然而优秀的园艺家唐宁（Downing）说过，在美国，梅锥象甲（curculio）对光皮果实的危害远甚于茸毛果实；某种疾病对紫色梅的危害远甚于黄色梅；而黄色果肉的桃比别种果肉颜色的桃更易染上某种病害。如果广泛借助人工方法就使若干变种在栽培时见微知著，那么，在大自然里，果树势必同其他树木和大量敌害作斗争，这种差异肯定会一锤定音，哪一变种得以千秋万代——光果皮还是毛果皮，黄果肉还是紫果肉。

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观察物种间的许多细小差异（以我们的一管之见，这些差异无关紧要），我们不可忘记气候、食物等等也许能产生某种微小的直接效果。然而，更有必要记住，存在着众多不为人知的相关生长定律，如果一部分发生变异，并且变异有利于生物通过自然选择而累积起来，其他变异将会随之发生，并且常常具有意料不到的性质。

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我们知道，在家养状况下，在生命的任何期间出现的那些变异，后代往往于相同期间重现。例如，蔬菜和农作物许多变种的种子，家蚕变种的幼虫期和蛹期，鸡蛋和雏鸡的绒毛颜色，绵羊和牛快成年时的角。同样，在自然状况下，自然选择也能在任何年龄时期激活，对生物发生作用，并使其改变，只需把这一时期的有利变异累积起来，并在相应年龄时期加以遗传。如果植物得益于种子吹送得越来越远，那么依我看通过自然选择就可轻易实现，难度不会大于植棉者用选择的方法来增长和改进棉桃内的棉绒。自然选择能使昆虫的幼虫发生变异，以便适应跟成虫所遇大相径庭的许多不测。通过相关生长定律，这些变异无疑可以影响到成虫的构造。对于寿命只有几小时、一辈子不进食的昆虫，也许其大部分构造仅仅是幼虫构造连续改变的关联物。反过来也是这样，成虫的变异可能也常常影响幼虫的构造；但在所有情况下，自然选择将保证因生命的其他时期变异而派生的变异一定不能丝毫有害，因为如果有害，物种就要灭绝了。

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自然选择能使子体的构造根据亲体发生变异，也能使亲体的构造根据子体发生变异。在社会性动物里，自然选择能使各个体的构造适应群体的利益，如果各自最终得益于所选的变异。自然选择所不能做的是，改变一个物种的构造，而不给它一点好处，却是为了另一物种的利益。虽然博物学著作中找到过这种说法，但我还没有拿到过一个经得起调查的个案。动物毕生仅用过一次的构造，如果是极重要的，那么自然选择就能使之发生任何程度的变异。例如某些昆虫专门用以破茧的大颚，或者孵化的雏鸟用以啄破蛋壳的坚硬喙端等皆是。有人断言，最好的短嘴翻飞鸽死在蛋壳里的比能够破壳而出的要多，所以养鸽者在孵化时要给予协助。再说，大自然若是为了鸽子自身的利益，不得不使成年鸽子生有极短的嘴，变异过程就是极缓慢的，同时蛋内的雏鸽要受到严格选择，就要那些具有最坚硬鸽喙的雏鸽，所有弱喙的雏鸽必死无疑；或者，选择脆弱易破的蛋壳，我们知道，蛋壳的厚度也像其他各种构造一样是变异的。

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性选择。——鉴于在家养状况下，有些特性常常只见于一性，而且只遗传给同性，自然状况下大概也是如此，那么，自然选择能够改变一性对异性的功能关系，或者涉及两性完全不同的生活习性，昆虫有时就是这样。为此对于我称为“性选择”的概念要说明一下。这并不取决于生存斗争，而取决于雄性之间为了占有雌性而做的斗争。其结果并不是竞争失败者死，而是少留或者不留后代。所以性选择不如自然选择来得剧烈。一般来说，最强壮的雄性，最适于其在自然界中的位置，留下的后代也最多。但在许多情况下，胜利并不靠精力旺盛，而是靠雄性独有的特种武器。无角的雄鹿或无距的公鸡鲜有机会留下后代。性选择总是允许胜利者繁殖，确能激发不屈不挠的勇气，增加距铁的长度、翅膀拍击距脚的力量，它不亚于残酷的斗鸡者，总是知道把最会斗的公鸡仔细选择下来，以便改良品种。这种战斗定律在自然界中下降到哪一等级，我不知道；有人描述雄性鳄鱼（alligators）要占有雌性的时候，诉诸打斗、吼叫、打转，就像印第安人的战争舞蹈一样；有人观察雄性鲑鱼（salmons）整日在战斗；雄性锹形虫（stag-beetles）常常带着同性用巨型大颚咬伤的伤痕。多妻动物的雄性之间的战争大概最为剧烈，似乎总是生有特种武器。雄性食肉动物本已武装精良；但它们和别的动物，通过性选择的途径还可以生出特别的防御武器来，如狮子的鬃毛、野猪的垫肩和雄性鲑鱼的钩曲颚；为了取胜，盾和矛一样重要。

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在鸟类中间，斗争的性质常常比较平和。所有关注此问题的人都认为，许多种类的雄鸟之间最剧烈的竞争是用歌喉引诱雌鸟。圭亚那的矶鸫（rock-thrush）、极乐鸟（birds of paradise）等等鸟类，聚集在一处，雄鸟依次展开美丽的羽毛，还在雌鸟面前做出奇怪滑稽的动作，而雌鸟作为观众站在一边，最后选择最有吸引力的配偶。密切观察过笼中鸟的人们都知道，鸟儿往往怀有个体的好恶，例如赫伦爵士（R. Heron）描述过一只杂色孔雀（pied peacock）鹤立鸡群，吸引了全部雌孔雀。将任何效果都归因于这种貌似无力的手段，未免显得幼稚可笑，这里无法讨论支持这种观点所必要的细节。但是，既然人类能在短时期内，依照自己的审美标准，使矮脚鸡获得美丽优雅的姿态，我实在没有充分的理由来怀疑雌鸟依照其审美标准，在成千上万的世代中，选择鸣声最好的或开屏最美的雄鸟，由此而产生了显著的效果。我强烈猜疑，关于雄鸟和雌鸟的羽毛不同于雏鸟的某些著名定律，可用性选择对于进入育龄或者交配季节的鸟类起作用，主要改变羽毛的观点来做解释；并且这种变异在相应的年龄或者季节要么单独遗传给雄性，要么两性均遗传。但这里没有篇幅来讨论这个问题了。

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就这样，任何动物的雌雄两者如果具有相同的一般生活习性，但在构造、颜色或装饰上有所不同，我认为，这种差异主要是由性选择所引起；就是说，雄性个体在连续世代中在武器、防御手段或者魅力方面，比别的雄性略占优势，而这些优越性状又遗传给了雄性后代。然而，我不愿把所有这种性别差异都归因于这种动因，因为家养动物身上看到有一些特性出现并为雄性所专有（例如雄信鸽的垂肉、某些雄家禽的角状瘤，等等），不能认为这些特性有利于战斗或者吸引异性。自然状况下也有类似的个案，例如野生雄火鸡（turkey-cock）胸前的毛丛，既没有任何用处，也没有装饰性；——其实，假如在家养状况下出现此种毛丛，是会称为畸形的。

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自然选择作用的事例。——为了弄清自然选择如何起作用，请允许我举出一两个虚拟事例。以狼为例，捕食动物为生，有些是智取，有些是强攻，也有些是捷足先登。我们假设：最敏捷的猎物，例如鹿，由于那个地区的变迁而增殖了，或者在狼最缺粮的季节里，其他猎物减少了数量。在这样的情况下，我看不出有任何理由可以怀疑，只有最敏捷最苗条的狼才有最好的生存机会，因而被保存或被选择下来，——只要在这个或那个不得不捕食其他动物的季节里，仍能保持制服猎物的力量就行。这就像人类通过仔细的按部就班选择，或者通过无意识的选择（人人试图保存最优良的狗但根本没有想到改变这个品种），就能够改进长驱跑狗的敏捷性是一样的不容置疑。

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即使狼所捕食的动物不改变比例数，也有可能生下天性喜欢抓某种猎物的崽子。而且这种可能性还不小，我们常常看到家畜的天性千差万别。例如，一只猫喜欢逮大鼠，另一只喜欢逮家鼠。圣约翰先生说，有一只猫逮飞禽回家，另一只逮兔子回家，还有一只去沼泽地捕食，几乎天天晚上抓回来丘鹬啊，半蹼鹬什么的。众所周知，喜欢大鼠不喜欢家鼠的倾向可以遗传。假如习性、构造出现轻微的内在变化，有利于一头狼个体，它就最有机会生存并留下后代。某些狼崽子也许能继承同样的习性、构造，如此循环往复，可形成新变种，淘汰亲代类型，或者与之和平共处。再说，山区的狼群和低地的狼群自然被迫捕猎不同的动物，持续保存最适合两处的个体，可能缓慢地形成两个变体。变体相遇会杂交混合，不过杂交的题目下文再谈。我补充一下，据皮尔斯（Pierce）先生说，美国的卡茨基尔山（Catskill Mountains）栖息着狼的两个变种，一种类型追捕鹿群，像轻快的长驱跑狗那样；另一种身体庞大，腿短，常常袭击牧人的羊群。

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下面举一个复杂的个案。有些植物分泌甜液，分明是为了从体液里排除有害的物质。例如，某些豆科（Leguminosae）植物托叶基部的腺就分泌这种汁液，月桂树（laurel）叶背上的腺也是。这种甜液分量虽少，却让昆虫孜孜以求。现在让我们假设，花瓣从其基部分泌一点点甜汁液即花蜜。这样，寻找花蜜的昆虫就会沾上花粉，当然常常把它从这一朵花带到另一朵的柱头上去。同种植物的两个不同个体的花因此而杂交；我们有理由相信（容后详述），这种杂交动作能够产生强壮的幼苗，因此得到繁盛和生存的最好机会。某些幼苗也许会继承分泌花蜜的能力。凡是个体的花具有最大的腺体即蜜腺，分泌最多的蜜汁，也就会最常受到昆虫的光顾，并且最常进行杂交；长此以往，它就占上风。如果花的雄蕊和雌蕊的位置同前来光顾的那种昆虫的身体大小和习性相适合，而好歹有利于花粉的输送，那么这种花也同样会得到青睐或者选择。不妨用不是吸取花蜜而是采集花粉而往来花间的昆虫为例：花粉形成的唯一目的是为了授精，所以毁坏它对于植物来说显然是纯粹的损失；然而如果有少许花粉被吃花粉的昆虫从这朵花带到那朵花去，最初是偶然的，后来成为惯常，因此而达到杂交，虽然十分之九的花粉毁坏了，但对于植物还是大有益处的，于是那些产生越来越多花粉、具有越来越大花粉囊的个体就会被选择下来。

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长久保护或者自然选择越来越有吸引力的花朵，植物通过这种过程，就变得能够高度吸引昆虫，昆虫便会在无意中定期在花与花之间传带花粉；而且昆虫这样做非常有效，我能随便举出许多触目惊心的事例，阐明这一点。我只举一个例子，不是什么突出的个案，但同样可以说明后文将讨论的植物雌雄分化的一个步骤。有些冬青树（holly-tree）只生雄花，有四枚雄蕊只产生很少量的花粉，同时还有一个发育不全的雌蕊；有些冬青树只生雌花，具有充分大小的雌蕊，但四枚雄蕊上的花粉囊都萎缩了，找不出一粒花粉。在距离一株雄树刚刚六十码远的地方，我找到一株雌树，从不同的枝条上采选了二十朵花，柱头放在显微镜下观察，没有例外，所有柱头都有花粉，而且几个柱头有大量花粉。几天以来，风都是从雌树吹向雄树，花粉不可能由风传带过来。天气很冷且风暴雨狂，所以对于蜂是不利的。不过，我检查过的每一朵雌花，都由于往来树间找寻花蜜的蜂偶然沾上花粉而有效地受精了。现在回到虚拟的个案：一旦植物变得高度吸引昆虫，花粉便会定时在花间传播，另一个过程就可以开始了。没有一个学者会怀疑所谓“生理分工”的好处，所以可以相信，一朵花或全株植物只生雄蕊，而另一朵花或另一植株只生雌蕊，对于植物是有利的。植物栽培时放在新的生活条件下，有时候雄性器官，有时候雌性器官，好歹会变为不育。如果假定自然状况下也有这种情况发生，不论其程度多么轻微，那么，由于花粉已经定时在花间传播，按照分工原则植物较为完全的雌雄分化是有利的，有这种倾向的个体会越来越多，就会连续得到青睐而选择下来，最终达到两性的完全分化。

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现在让我们谈谈虚拟个案里吃花蜜的昆虫：假定由于连续选择使得花蜜慢慢增多的植物是一种普通植物，而某些昆虫主要是依靠其花蜜为食。可以举出许多事实，来说明蜂为了节省时间是多么急不可耐。例如，它们有在某些花的基部咬一个洞来吸食花蜜的习性，虽然只要稍微麻烦一点就能从口部进去。记住这些事实，就没有理由怀疑，虫体大小体型、喙曲度和喙长的偶然偏差等等个体差异，固然微细难察，但是对于蜂等昆虫可能是有利的，这种性状的个体能够更快得到食物，有更好的机会生存和繁衍后代。其后代也许会继承构造有类似微小偏差的倾向。红三叶草和绛三叶草（incarnatum）管形花冠的长度，粗看起来并没有什么差异，但蜜蜂能够容易地吸取绛三叶草的花蜜，却不能吸红三叶草，只有大黄蜂才来光顾它；所以红三叶草虽漫山遍野，却不能把大量供应的珍贵花蜜供给蜜蜂。因此，蜜蜂喙略长些或者构造略有差异，会大有利。相反，我做实验发现，三叶草的能育性绝对要依靠蜂类来光顾它的花，并且移动部分花管，以便把花粉摁到柱头表面。于是，如果哪个地区大黄蜂稀少起来，红三叶草花管较短或花管裂较深就大大有利，这样蜜蜂就能够光顾它的花了。这样，我就能理解，通过连续保存呈现双向的轻微有利构造偏差的个体，花和蜂如何慢慢地同时或先后发生了变异，并且以最完善的方式来互相适应。

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我深知，以上述虚拟例子来说明自然选择的学说，会遭到反对，正如当初赖尔爵士的“地球近代的变迁，地质学的例证”这种高见遭到反对是一样的；不过，运用海岸波浪等等的作用，来解说深谷的凿成或内陆的长线崖壁的形成时，现在很少听到有人说这是小儿科不重要的事情了。自然选择的作用，只能是把每一个有利于生物的微小遗传变异保存累积起来；正如近代地质学差不多摒弃了一次洪水能凿成大山谷的观点那样，自然选择如果是正确的原则，也将摒弃连续创造新生物的观点，摒弃生物的构造发生巨大突变的观点。

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论个体的杂交。——这里必须稍微讲些题外话。雌雄异体的动物和植物每次生育，其两个个体都必须交配，这当然是很明显的事；但在雌雄同体的情况下，这一点并不明显。然而我强烈倾向于认为，一切雌雄同体的两个个体或偶然地或习惯地亦要接合以繁殖它们的种类。补充一句，这种观点是安德鲁·奈特最早提出的。不久就可以看到此论的重要性；但这里必须把这个问题略提一下，虽然我有材料可做充分的讨论。所有脊椎动物、所有昆虫以及其他某些大类的动物，每次的生育都交配。近代的研究已经把曾认为雌雄同体的数目大大减少了；大多数真的雌雄同体的生物也交配。这就是说，两个个体定时进行交配以便生殖，这就是我们所要讨论的全部，但是依然有许多雌雄同体的动物肯定不经常进行交配，并且大多数植物是雌雄同株的。于是可以问：有什么理由可以假定在这种个案里，两个个体为了生殖而进行交配呢？这里详细讨论这一问题是不可能的，所以只能做一般的考察。

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首先，我曾搜集过大量事实，表明动植物变种间的杂交，或者同变种而不同品系的个体间的杂交，可以提高后代的强壮性和能育性；相反，近亲交配可以减小其强壮性和能育性，这和饲养家们的近乎普遍的信念是一致的。仅仅这些事实就使我相信，没有一种生物世世代代永远自营受精，这是自然界的一般法则（其意义我们却一无所知）；和另一个体偶然地进行交配——也许相隔较长的期间，是必不可少的。

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我想，相信了这是自然法则，就能理解几大类事实，比如以下事实如用任何其他观点都不可解释。培养杂种的人都知道：暴露在雨下，对于花的受精是何等不利，然而花粉囊和柱头完全暴露的花是何等之多！可是，如果偶然的杂交是不可缺少的，那么从他花个体来的花粉可以充分自由地进入，就可以解释这种暴露状态了，特别是植物自己的花粉囊和雌蕊一般靠得这么近，自花受精看上去简直不可避免。另一方面，有许多花却将结籽器官紧闭，如蝶形花科（papilionaceous）即豆科这一大科便是如此；但若干乃至全部这种花朵的构造与蜂吸花蜜的方式之间具有很奇妙的适应。这样做，要么将自身的花粉推向柱头，要么将其他花粉带过来。蜂的光顾对于蝶形花十分必要，我从其他地方发表的实验结果发现，蜂如遭挡驾，该花的能育性就会大大降低。你看，蜂在花间飞来飞去，很少不将花粉带来带去的，我看这就对植物大大有利。蜂的作用有如驼毛刷，只要先接触一花的花粉囊，再刷另一花的柱头，就足以确保受精的完成了。但不能假定，蜂能就此产生出大量的种间杂种来；假如同一把刷子把植物自己的花粉和外种花粉带来，前者的优势很大，不可避免地要完全毁灭外来花粉的影响，盖特纳就曾指出过这一点。

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当花的雄蕊突然向雌蕊弹跳，或者一枝一枝慢慢地向其弯曲，这种装置好像专门适应于自花受精；毫无疑问，它有用于这个目的。不过要使雄蕊向前弹跳，常常需要昆虫的助力。如科尔路特（K?lreuter）所阐明的小蘖（barberry）情形便是这样；在小蘖属里，似乎有这种特别的装置以便利自花受精。奇怪的是，众所周知，假如把密切近似的类型或变种栽培在近处，就很难得到纯种的幼苗，因为它们是大量进行自然杂交的。在许多其他个案里，不但没有自花受精的辅助手段，还有特别的装置能够有效地阻止柱头接受自花的花粉，斯普伦格尔（C. C. Sprengel）的著作以及我自己的观察可以阐明这一点。例如，亮毛半边莲确有美丽而精巧的装置，能够把花中相连的花粉囊里的无数花粉粒，在本花柱头还不能接受之前就统统扫除出去；因为从来没有昆虫来光顾这种花，至少在我的花园是如此，所以从不结籽。然而我把一花的花粉放在另一花的柱头上，却培育成了许多幼苗。我的花园还有另一种半边莲，却有蜂来光顾，能自由结籽。在很多其他个案里，虽然没有专门的机械装置去阻止柱头接受自花的花粉，然而如斯普伦格尔指出的，我也能证实，要么花粉囊在柱头能受精以前便已裂开，要么柱头在花粉未成熟以前已经成熟，事实上是雌雄分化的，必定习惯性地进行杂交。这些事实是何等奇异啊！同一花中的花粉位置和柱头平面是如此接近，好像为了自花受精专用似的，但在许多个案中，彼此并无用处，这又是何等奇异啊！如果用不同个体的偶然杂交是有利的或必需的观点，来解释此等事实，是何等简单啊！

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假如让圆白菜、萝卜、洋葱等植物的若干变种在相互接近的地方进行结籽，我发现由此培育出来的大多数实生苗都是杂种。例如，我把几个圆白菜的变种栽培在一起，由此培育出233株实生苗，其中只有78株保持了原有种类的性状，甚至其中还有若干不是完全纯粹的。然而，每一菜花的雌蕊不但有自己的六个雄蕊所围绕，同时还有同株上的许多花的雄蕊所围绕。那么，这许多的幼苗是怎么变为杂种的呢？我看，必定是因为其他变种的花粉比自己的花粉更占优势的缘故；属于同种的不同个体互相杂交有好处的一般法则。如果不同的物种进行杂交，其情形正相反，因为这时植物自己的花粉总是要比外来的花粉占优势；这一问题在以后一章里还要讲到。

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在一株大树开满无数花的个案里，我们可以反对说，花粉很少能进行树间传送，充其量只能在同树上进行花间传送而已；而且同树上的花，只有从狭义来说，才可看作不同的个体。我认为这种反对有效，但是大自然对此已大致有所防范，给予树以开出雌雄分化的花的强烈倾向。雌雄分化了，虽然雄花和雌花仍然生在同树上，可以看到花粉必须定时在花间传送；这样花粉就有更好的机会，会偶然出现树间传送。属于所有“目”（Orders）的树，在雌雄分化上较其他植物更为常见，我在英国所看到的情形就是这样；根据我的请求，胡克博士把新西兰的树列成了表，阿萨·格雷（Asa Gray）博士把美国的树列成了表，其结果都不出我之所料。另一方面，胡克博士最近告诉我说，他发现这一规律不适用于澳洲；我对于树的性别所说的这几句话，仅仅为了引起对这一问题的注意而已。

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现在略为谈谈动物方面：陆栖种有一些雌雄同体的，例如陆栖的软体动物和蚯蚓，但它们都需要交配。我还没有发现过一种陆栖动物能够自营受精。根据偶然杂交必不可少的观点，考虑一下陆栖动物的生活环境，以及精子的性质，就可以理解这种显著的事实了，它与陆栖植物对照强烈。陆栖动物无法类似于植物那样依靠昆虫或风做媒介，如果没有两个个体交配，不知道偶然的杂交有什么完成的途径。水栖动物有许多种类是能自营受精的雌雄同体，但水的流动显然可以做偶然杂交的媒介。我咨询过最高权威之一，即赫胥黎教授，我希望能找到一种雌雄同体的动物个案，生殖器官完全封闭在体内，可以证明外界进入和不同个体的偶然影响在物质上不可能发生，结果至今没有成功。在这种观点下，我早就觉得蔓足类（cirripedes）是很难解释的个例；但我有幸在他处证明了它们的两个个体，虽然都是自营受精的雌雄同体，确也有时进行杂交。

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无论在动物或植物里，同科中甚至同属中的物种，虽然整个体制上大同小异，却不时有雌雄同体和雌雄异体之分，这想必使大多数学者觉得奇哉怪也。但是如果一切雌雄同体的生物事实上也偶然杂交，那么它们与雌雄异体的物种之间的差异，从机能上来讲是很小的。

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从这几项内容以及从我搜集的但不能在这里举出的许多特别事实看来，我强烈倾向于认为，动植物界内部，与不同个体的偶然杂交是自然法则。我清楚，根据这种观点，存在不少难解的个案，我正在对其中一些进行调查。最后，我们总结如下，在许多生物中，两个个体之间杂交对于每一次生殖显然是必需的，在不少生物中，也许杂交间隔很久才进行，但我认为，没有一种生物可以永久保持自营受精。

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有利于自然选择的条件。——这是极为错综复杂的问题。大量的可遗传、多样性变异是有利的，但我看个体差异就足以起作用了。个体数量大，可增加一定时期内出现有利变异的机会，以补偿各个个体较少的变异量；所以我相信，这是成功的极重要因素。虽然大自然可以给予长久的时间让自然选择运作，却并不能给予无限的时间；一切生物都可以说努力在自然结构中占地盘，如果没有随着竞争者发生相应程度的变异和改进，任何物种都很快会灭绝。

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在人类按部就班的选择中，饲养家为了一定的目的进行选择，如果出现个体自由杂交，他的工作就要全盘终止。但是，有许多人没有改变品种的意图，却有一个近乎共同追求完美的标准，都试图用最优良的动物繁殖后代，虽然劣种动物参与大量的杂交；这种无意识的选择，肯定也会步步为营地使品种得到改进和变异。在自然状况下也是这样；在局限的区域内，自然版图中还有地盘未被完全占据，自然选择总是倾向于保存一切多多少少向正确方向变异的个体，以便更好地填充空地。但如果地区辽阔，其中的各个区域几乎必然要呈现不同的生活条件；如果自然选择在若干区域内使一个物种变异改良，那就要在各个区域的边界上与同种其他个体进行杂交。在这种情况下，杂交的效果很难被自然选择所抵消，尽管自然始终在让各个区域的全部个体按照其条件进行完全相同的变异。在连续的地域，区域间的条件一般不知不觉地渐变过渡。凡是每次生育都交配的、游动性大且繁育不十分快的动物，特别会受到杂交的影响。所以具有这种本性的动物，例如鸟，其变种一般仅局限于隔离的地区内，我认为情况正是如此。仅仅偶然进行杂交的雌雄同体的生物，还有每次生育都交配但很少迁移而增殖很快的动物，就能在任何一处迅速形成新的改良变种，并且常能在那里聚集成群，使杂交主要在同一个新变种的个体间进行。这样形成地方变种，然后可能慢慢散布到其他区域。根据这一原则，苗圃园工常常喜欢从大群的同变种植物中留存种子，以便减少其与其他变种杂交的机会。

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甚至在每次生育都交配而繁殖不快的动物里，我们也不能过高估计杂交延缓自然选择的效果。我可以举出一大堆事实来说明，在同一地区内，同种动物的各变种可以长久保持区别，这是由于栖息地不同，由于繁殖的季节略有不同，由于同一变种的个体喜欢聚头进行交配。

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杂交在自然界中起着很重要的作用，使同一物种或变种的个体在性状上保持纯粹和一致。对于每次生育都交配的动物，作用显然更为有效；但前文说过，有理由相信，一切动植物都会偶然进行杂交。即使只在间隔长时间后才进行杂交，我坚信这样生下来的幼体在强壮和能育性方面都远胜于长期连续自营受精生下来的后代，会有更好的生存并繁殖其种类的机会。这样，即使间隔的时期很长，杂交的影响归根到底还是很大的。如果果真存在从不杂交的生物，只要生活条件不变，就能使性状保持一致，但只有通过遗传的原理以及通过自然选择，把那些离开固有模式的个体消灭掉。如果生活条件改变了，也发生变异了，那只有依靠自然选择对于相似有利变异的保存，变异了的后代才能获得性状的一致性。

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自然选择过程中，隔离也是一种重要因素。在有限或者隔离的地区内，如果不很大，则有机和无机的生活条件一般是十分一致的；所以自然选择会使整个地区同种的所有个体按照同样方式针对同样的条件进行变异。而与周围不同环境地区内本来会居住的同种生物的杂交也将遭阻止。但隔离也许能更加有效地遏制气候、海拔高度等发生了物理变化之后适应性较好的生物的移入；因此地方自然生态体系就空出新场所来了，供旧有生物去争夺，并且通过构造和体质的变异而加以适应。最后，隔离阻止移入因而阻止竞争，能为新变种的缓慢改进提供时间，这对于产生新物种有时是重要的。但是，如果隔离的地区很小，要么靠周围障碍物形成，要么靠很特别的物理条件，那么其支撑的个体总数势必很少；个体少会大大延缓通过自然选择产生新种，因为减少了有利变异出现的机会。

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如果依靠自然界来验证这话是否正确，观察任何一处隔离小区域，例如海岛，虽然生活在那里的总物种数目很少，如“地理分布”一章所见，但是这些物种的极大部分是本地特产——就是说，产地在那里，世界别处是没有的。所以乍一看，好像海岛对于产生新种是大为有利的。但这样我们可能欺骗了自己，因为如果要确定究竟是隔离的小地区，还是开放的大地区如一片大陆，最有利于产生生物新类型，应当在相等的时间内来做比较，然而这是我们不可能做到的。

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虽然我并不怀疑隔离对于新种的产生相当重要，但总的说，我倾向于相信区域的广大更为重要，在产生能够天长地久而且能够广为分布的物种上尤其如此。在广大而开放的地区内，不仅可以维持同种的大量个体生存，因而有较好的机会发生有利变异，而且已经存在的物种有许多，因而生活条件极其复杂；如果众多物种中有些已经变异或改进了，那么其他物种势必也要相应程度地来改进，否则就要遭消灭。每一新类型，一旦得到大的改进，就能够向开放的毗邻地区扩展，并与许多其他类型发生竞争。因此，更多的新场所会形成，而要填补那里空缺的竞争，大地方比孤立小地方更加剧烈。还有，广大的地区虽然现在是连续的，却因为地面的变动，最近往往呈现着断裂的状态；所以隔离的好效果，在一定范围内是普遍发生的。最后，我下结论，虽然小的隔离地区在某些方面对于新种的产生是高度有利的，然而变异的过程一般在大地区内要快得多，更有甚者，大地区内产生出来而且已经战胜过许多竞争者的新类型，是那些分布得最广而且产生出最多新变种和物种的类型。因此它们在生物界的变迁史中便占有重要的位置。

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根据这种观点，我们对于“地理分布”一章里还要讲到的某些事实，大概就可以理解了。例如，澳洲这样的小大陆，现在和大幅员欧亚地区的生物比较起来，就是逊色的。正是为此，大陆生物在岛屿上到处归化。小岛上，生活竞争就不那么剧烈，变异少，灭绝也少。据希尔（Oswald Heer）说，马德拉的植物区系很像欧洲已经灭绝的第三纪植物区系，也许就因为此。所有的淡水盆地加起来，与海洋或陆地相比只是小地区。因此，淡水生物间的竞争将不像他处剧烈，新类型产生慢，旧类型灭亡也慢。硬鳞鱼类（Ganoid fishes）曾经是举足轻重的目，淡水盆地还可以找到它遗留下来的七个属；淡水里还能找到现在世界上几种最奇形怪状的动物，鸭嘴兽（Ornithorhynchus）和肺鱼（Lepidosiren），就像化石那样将自然等级上相离很远的某些目联系起来。这种动物简直可以称为活化石；由于居住在局限的地区内，竞争不剧烈，得以存留到今天。

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尽管问题错综复杂，还是要总结一下对自然选择的有利条件和不利条件。我的结论是，面向未来，对陆栖生物来说，地面经过多次沉浮的广大大陆地区，因而以断层状态长期存在，最有利于产生许多新生物类型，既可长期生存，也可广泛分布。那地区起先是一片大陆，生物的种类和个体都很多，因而陷入激烈竞争。如果地面下陷，变为分离的大岛，每个岛上还会有许多同种的个体生存：各物种分布的边界上，杂交就受到抑制；在任何种类的物理变化之后，迁入受到遏制，所以各岛的自然组成中的新场所，势必由旧有生物的变异种所填充；时间也允许各岛的变种充分地变异完善。如果地面重新抬高，岛屿再变为大陆，那里就会再发生剧烈的竞争；最有利的或改进最多的变种，就能够分布开去，改进较少的类型就会大都灭绝，而新大陆各种生物的相对比例数又要发生变化；还有，这里又成为自然选择的好场所，更进一步地来改进生物，产生出新种来。

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我完全承认，自然选择的作用始终是极其缓慢的。只有在区域的自然组成中留有一些地位，能由当地现存生物在经历某种变异后而较好占有，自然选择才可发生作用。这种地位的存在常决定于物理变化，而这种变化一般是很缓慢的；此外还决定于较适应类型的迁入受阻。可是自然选择的作用往往更取决于某些旧有生物发生变异慢，许多其他生物的互相关系就此打乱。除非出现有利的变异，什么都无法实现，而变异本身显然一贯是极其缓慢的过程，又往往被自由杂交所显著延滞。许多人会说，这若干原因总体上已足够抵消自然选择的作用了。我不这样看。我反而认为，自然选择的作用将永远是极其缓慢的，往往间隔长久的时间，并且一般只能同时作用于同一地方的极少数生物。我进一步认为，此等缓慢的、断续的自然选择，和地质学告诉我们的这世界生物变化的速度和方式丝丝入扣。

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选择的过程虽然是缓慢的，如果力量薄弱的人类尚能在人工选择方面多有作为，那么，在很长的时间里，通过自然力量的选择，我觉得生物的变异量是没有止境的，一切生物彼此之间以及与它们的生活条件之间互相适应的美和复杂关系，也是没有止境的。

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灭绝。——这个主题“地质学”一章里还要详论；但由于和自然选择密不可分，这里必须谈一下。自然选择的作用仅在于保存在某些方面有利的变异，因而使之存续。由于所有生物都按照几何级数高速增加，每一地区都已挤满了生物；于是，随着获选得宠类型数量增加，失宠的类型便减少，变得稀少了。地质学告诉我们，稀少是灭绝的前奏。我们还知道，只剩下少数个体的任何类型，遇到季节波动，或者敌害数目波动，就很有可能彻底灭绝。可以更进一步说，随着新类型持续而缓慢地产生出来，除非我们认为具有物种性质的类型可以永远无限增加，许多类型势必灭绝。地质学明白告诉我们，具有物种性质的类型的数目并没有无限增加；我们明白其没有无限增加的理由，因为自然系统中的地域数目不是无限的——倒不是我们有办法知道任何一个地区已经达到了最多物种数。也许没有一个地区已经达到了充分的居民，例如在好望角，那里的植物物种比世界任何地方都拥挤，也有某些外来植物归化，据我们所知，却并没有引起任何本土植物的灭绝。

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另外，个体数目最多的物种，在任何一定期间内，有产生有利变异的最好机会。这一点已经得到证明，第二章所讲的事实指出，普通物种拥有见于记载的变种或初始物种最多。所以，数目稀少的物种在任何一定期间内的变异或改进比较迟缓；结果，在生存斗争中，就要被普通物种变异了的后代打败。

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根据这些论点，我想如下结果顺理成章：随着新物种在时间的推移中通过自然选择而形成，其他物种就会越来越稀少，而终于灭绝。那些同正在变异和改进中的类型斗争最激烈的，当然首当其冲。我们在“生存斗争”一章里已经看到，密切近似的类型——即同种的一些变种，以及同属或近属的一些物种——由于具有近乎相同的构造、体质、习性，一般彼此竞争也最剧烈。结果，每一新变种或新种在形成的过程中，一般对于最接近的近亲压迫得也最狠，并且还倾向于消灭之。在家养生物里，人类对于改良类型的选择，也可看到同样的消灭过程。可以举出许多奇异的例子，表明牛、羊等动物的新品种，花卉的变种，是何等迅速地代替了那些古老低劣的种类。在约克郡，有历史记载，古代的黑牛被长角牛所代替，长角牛“又被短角牛所扫除，好像有某种致命的瘟疫一样”（某农业作者语）。

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性状的分歧。——我用此术语所表示的原理是极其重要的，相信可以用来解释若干重要的事实。第一，变种，即使是特征显著的变种，虽然多少带有物种的性状——如在许多场合里，它们如何分类，常令人莫衷一是——彼此之间的差异，却远比那些纯粹而明确的物种之间的差异为小。依我看，变种是形成过程中的物种，我称为初始的物种。那么，变种间的小差异如何扩大为物种间的大差异呢？这一过程经常发生，这一点必须从自然界无数的物种大都呈现显著的差异而推论出；而变种，未来的显著物种的假想原型和亲体，却呈现微细的不明确的差异。仅仅是偶然凑巧（姑且这样叫）可能致使变种在某些性状上与亲体有所差异，以后变种的后代在同一性状上又与亲体有更大程度的差异；但是仅此一点，绝没有说明同种变种、同属异种间所表现的差异何以如此常见和巨大。

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我的一向作风是从家养生物那里去探索此事的真相。这里会看到相似的情形。一羽喙稍短的鸽子引起了一个养鸽者的注意；而另一羽喙略长的鸽子却引起了另一个养鸽者的注意；在“养鸽者不要中间标准，只喜欢极端类型”这一公认原则下，他们就都选择和养育那些喙愈来愈长或愈来愈短的鸽子（翻飞鸽的亚品种实际就是这样产生的）。还有，我们设想，古代一个人喜欢快捷的马，而另一个人却需要强壮高大的马。最初的差异可能是极微细的；但是随着时间的推移，一方面连续选择快捷的马，另一方面却连续选择强壮的马，差异就增大起来，因而便会形成两个亚品种。最后，经过若干世纪，亚品种就变为两个稳定的不同品种了。等到差异慢慢扩大，具有中间性状的劣等马，即不甚快捷也不甚强壮的马，将遭到冷落，从此就逐渐消失了。这样，我们从人类的产物中看到了所谓分歧原理的作用，它引起了差异，最初仅仅是微小的，后来逐渐增大，于是品种之间及其与共同亲体之间，在性状上便有所分歧了。

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试问，类似的原理怎么能应用于自然界呢？我相信能应用而且应用得很有效，因为原委很简单，任何物种的后代，如果在构造、体质、习性上越有多样性，那么在自然组成中，就越能占有各种不同的地方，而且在数量上也就越能增加。

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在习性简单的动物里可以清楚地看到这种情形。以食肉的四足兽为例，它在任何能够维持生活的地方，早已达到饱和的平均数。如果允许自然增殖力起作用的话（区域条件没有任何变化的情形下），只有依靠变异的后代去取得其他动物目前所占据的地方，才能成功地增殖。例如，其中有些变为能吃新种类的猎物，无论死活，有些能栖息新地方，爬树、涉水，有些或者可以减少肉食习性。食肉动物的后代，在习性和构造方面越是多样性，所能占据的地方就越多。适用于一种动物的原理，也能应用于一切时间内的所有动物——如果发生变异的话；否则自然选择便无能为力。关于植物也是如此。试验证明，如果一块土地上仅播种一个草种，同时另一块类似土地上播种若干不同属的草种，就能生长更多的植物，收获更重的干草。如在同样大小的土地上先播种一个小麦变种，再混播几个小麦变种，情况也同样。所以，如果任何一个草种继续进行着变异，并且连续选择各变种，就将像异种异属的草那样彼此相区别，虽然区别很小，那么这个草种的更多个体，包括变异了的后代在内，就能成功地在同一块土地上生活。我们知道每一物种和每一变种的草年年都散播无数种子；可以说，都在竭力增殖。结果，数千代以后，不能怀疑任何一个草种的最显著变种都会有成功以及增殖的最好机会，这样就能淘汰较不显著的变种；变种到了彼此截然分明的时候，便取得物种的等级了。

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构造的巨大多样性，可以维持最大量的生物，这一原理的正确性已在许多自然情况下看到。在一块极小的地区内，特别是对自由迁入开放时，个体之间的斗争必定是剧烈的，总是可以看到巨大的生物多样性。例如，我看见一片草地，面积为三英尺乘四英尺，多年来都暴露在完全同样的条件下，那里生长着二十个物种的植物，属于十八个属和八个目，可见这些植物的差异是何等巨大。在一成不变的小岛上，植物和昆虫也是这样的；淡水池塘中也是如此。农人知道，用决然不同“目”的植物进行轮种，收获的粮食更多：自然界所进行的可以叫作同期轮种。密集生活在任何一片小土地上的动植物，大多能够在那里生活（假定这片土地没有任何特别的性质），可以说，它们都百倍努力地在那里生活；但是，可以看到，在斗争最尖锐的地方，构造多样性的优势，伴随着习性和体质的差异，决定了彼此争夺得最厉害的生物，一般是那些属于我们叫作异属和异目的生物。

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植物通过人类的作用在异地归化这一方面，同样的原理也有表现。可以料想，任何土地上能够归化的植物，一般都是那些和土著植物在亲缘上密切接近的种类；因为土著植物一般被看作是特别创造出来而适应于本土的。也许还可以料想，归化的植物大概只属于少数类群，特别适应新乡土的一定地点。但实际情形却很不同；德康多尔在他的力作里说得好，归化的植物如与土著的属和物种的数目相比，则其新属要远比新种为多。举一个例子，阿萨·格雷博士的《美国北部植物志》的最后一版里，举出260种归化的植物，属于162属。由此可见，这些归化的植物具有高度多样性。而且，它们与土著植物大不相同，因为在162个归化的属中，非土生的不下100个属，这样，现今生存于美国的属，就大大增加了。

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对于任何地区内与土著生物斗争而获胜，并且就地归化的动植物的本性加以考察，就可以大体认识到，某些土著生物必须怎样发生变异，才能胜过其他土著；我们至少可以推论出，构造的多样性达到新属差异的，于它们是有利的。

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事实上，同一地方生物多样性的好处，与某个体各器官的生理分工所产生的好处是相同的——米尔恩·爱德华兹（Milne Edwards）已经明白讨论过这一主题了。没有一个生理学家会怀疑专门消化植物性物质的胃，或专门消化肉类的胃，能够从这些物质中吸收最多的养料。所以在任何土地的总体系统中，动植物对于不同生活习性的多样性越是广泛而完善，能够在那里维持的个体数量就越大。一组体制很少多样化的动物很难与一组构造完善多样化的动物相竞争。例如，澳洲的有袋目动物可以分成若干群，但彼此差异不大，正如沃特豪斯（Waterhouse）先生等人所指出的，它们隐约代表着食肉的、反刍的、啮齿的哺乳类，但能否成功地与这些发育良好的目相竞争，是存疑的。澳洲的哺乳动物里，我们看到多样化过程处于早期的不完全发展阶段中。

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根据上面有待大大充实的讨论，我们可以假定，任何一个物种的变异后代，在构造上越多样化，便越能成功，并且能侵入其他生物占据的地方。现在我们看一看，从性状分歧大有益的这个原理，结合自然选择的原理和灭绝原理之后，能起怎样的作用。

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本书所附的图表，有助于理解这个扑朔迷离的主题。设A到L代表某地一个大属的诸物种；假定它们的相似程度并不相等，正如自然界的一般情形那样，图表里用不同距离的字母表示。我说的是一个大属，第二章说过，大属比小属平均有更多的物种发生变异，并且发生变异的物种有更多数目的变种。我们还可看到，最普通的和分布最广的物种，比罕见的和分布狭小的物种变异更多。设A是普通的、分布广的、变异的物种，并且属于本地的一个大属。从A发出的不等长、分歧扇形散开的虚线代表其变异的后代。假定变异极其微细，但性质极其多样化；假定不同时发生，而常常间隔一个长时间才发生；假定其存续期也各不相等。只有那些好歹具有利益的变异才会保存下来，或被自然选择。这里性状分歧受益原理的重要性便出现了；因为，一般这就会导致最差异的或最分歧的变异（由外侧虚线表示）受到自然选择的保存和累积。虚线遇到横线，就用小数目字标出，那是假定变异量已充分积累，因而形成一个很显著的变种，并认为在分类上有记载价值。

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图表中横线之间的距离，代表一千世代，代表一万世代则更好。千代以后，设物种A产生了两个很显著的变种，a1和m1。而变种所处的条件一般还和亲代发生变异时相同，且变异性本身是遗传的；结果它们同样具有变异的倾向，并且一般差不多像亲代那样发生变异。还有，两个变种只是轻微变异了的类型，所以倾向于遗传共同亲代A的优点，因为亲代比本地大多数生物在数量上更多；它们还要遗传亲种所隶属的那一属的更为一般的优点，在自己的地区内成为一个大属。我们知道所有这些条件对于新变种的产生都是有利的。

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这时，如果这两个变种仍能变异，那最分歧的变异在此后的千代中，一般都会保存下来。经过这段期间，设图表中的变种a1产生了变种a2，根据分歧的原理，a2和A之间的差异要比a1为大。设m1产生两个变种，即m2和s2，彼此不同，而和共同亲代A之间的差异更大。我们可以用同样的步骤把这一过程延长到任何久远的期间；有些变种，在每千代之后，只产生一个变种，但在变异越来越大的条件下，有些会产生两三个变种，有些则没有产生变种。因此变种，即共同亲代A的变异后代，一般会继续增加数量，继续在性状上进行分歧。图表中，这个过程表示到万代为止，在压缩和简单化的形式下，则到一万四千代为止。

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但这里必须说明：我并非假定这种过程会像图表中那样有规则地进行（虽然图表本身已多少搞得不规则）。我不认为，最分歧的变种战无不胜攻无不克，得到增殖：往往是一个中间类型长期存续下来，可能产生或者不产生一个以上的变异后代。因为自然选择总是按照地位的性质而起作用，该地位未被其他生物占据，或未被完全占据；而这一点又取决于无限复杂的关系。但是，一般来说，任何一个物种的后代，在构造上越多样化，就越能占据更多的地方，它们的变异后代也就越能增殖。在上面图表里，继承线在一定的间隔中断了，标以编号的字母，标志着继承的类型已充分独立，足以列为变种。但这样的中断是虚拟的，任何地方都可以插入，只要间隔的长度允许大量分歧变异量得以积累就行。

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从一个分布广、属于一个大属的普通物种产生出来的一切变异后代，常常会继承亲代在生活中得以成功的那些相同优点，所以一般既能继续增殖，又能在性状上进行分歧：这一点在图表中由A分出的数条分枝虚线表示。继承世系上后出现的更高度改进分枝的变异后代，往往会取代，也就是毁灭较早的改进较少的分枝；这在图表中表现为几条较低的分枝没有达到上面横线。有时候，变异过程无疑只限于一支世系，这样虽然在连续的世代中后代分歧变异在分量上扩大了，但变异后代在数量上并未增加。这种情形在图表中表示为，假设从A出发的各线都去掉，只留a1到a10的那一支。同样，举例说，英国赛马和英国指示犬，它们的性状显然从原种缓慢地分歧，既没有分出任何新分枝，也没有分出任何新品种。

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经过万代后，设A种产生了a10、f10和m10三个类型，由于经过历代性状的分歧，相互之间及与共同祖代之间的区别将会很大，但可能变化并不相等。如果假定图表中两条横线间的变化量极其微小，那这三个类型也许还只是十分显著的变种，或者达到了亚种的可疑范畴；但只消假定这变化过程在步骤上较多或在量上较大，就可以把这三个类型变为明确的物种。因此，图表表明了由区别变种的较小差异，升至区别物种的较大差异的各个步骤。把同样过程延续更多世代（如压缩简化了的图表所示），便得到了八个物种，用字母a14到m14表示，都是从A传衍下来的。因而我相信，物种增多了，属便形成了。

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大属里，发生变异的物种可能在一个以上。我假定图表里第二个物种I以相似的步骤，经过万代以后，产生了两个显著的变种（w10和z10）或两个物种，依据横线间所表示的假定变化量而定。一万四千世代后，假定六个新物种n14到z14产生了。在各个属里，性状已彼此极不相同的物种，一般会产生出最大数量的变异后代；因为它们在自然组成中拥有最好的机会来占有新的和广大不同的地方，所以在图表里，我选取极端物种A与近极端物种I，作为变异大和已经产生了新变种和新物种的物种。原属里的其他九个物种（用大写字母表示），会长久地继续传下不变的后代；由于篇幅有限，图表用不很长的向上虚线来表示。

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但在变异过程中，如图表所示，起了重要作用的还有另一原理，即灭绝的原理。因为在每一处充满生物的地方，自然选择的作用必然在于选取生活斗争中比其他类型更为有利的类型，任何一个物种的改进后代经常有一种倾向：在每一世系阶段中，把前辈以及原始祖代淘汰消灭掉。必须记住，在习性、体质和构造方面彼此最相近的那些类型之间，斗争一般最为剧烈。因此，介于较早的和较晚的状态之间的中间类型（即介于同种中改进较少的和改良较多的状态之间的）以及原始亲种本身，一般都有灭绝的倾向。世系上许多整个的旁支会这样灭绝，被后来的改进支系所征服。但是，如果一个物种的变异后代进入某一不同的地区，或者很快地适应于一个全新的地方，亲子间就没有竞争，两者就都可以继续生存下去。

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假定图表中所表示的变异量相当大，则物种A及全部较早的变种皆灭亡，被八个新物种a14到m14所代替；而物种I将被六个新物种（n14到z14）所代替。

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还可以进一步论述。假定该属的那些原种彼此相似的程度并不相等，自然界的情况一般就是如此；物种A和B、C及D的关系比和其他物种的关系近；物种I和G、H、K、L的关系比和其他物种的关系近，又假定A和I都是很普通而且分布很广的物种，因而比同属中的大多数其他物种本来就占有若干优势。它们的变异后代一万四千世代中共有十四个物种，也许遗传了一部分同样的优点：在世系的每一阶段还以多样化的方式进行变异改进，便在居住地区的自然组成中适应了许多相关地位。因此，它们似乎极有可能，不但会取代亲种A和I而消灭之，而且还会消灭与亲种最接近的某些原种。所以，能够传到第一万四千世代的原种是极其稀少的。可以假定与其他九个原种关系最疏远的两个物种（E与F）中，只有一个物种F把后代传到这一世系晚近阶段。

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图表里，从十一个原种传下来的新物种数目现在是十五。由于自然选择造成分歧的倾向，a14与z14之间在性状方面的极端差异量远比十一个原种之间的最大差异量大。而且，新种间的亲缘远近也很不相同。A传下来的八个后代中，a14、q14、p14三者由于都是新近从a10分出来的，亲缘比较近；b14和f14是在较早的时期从a5分出来的，故与上述三个物种在某种程度上有差别；最后o14、e14、m14彼此在亲缘上是相近的，但是在变异过程的开端便有了分歧，所以与前面的五个物种大有差别，它们可以成为亚属或者明确的属。I传下来的六个后代将形成两个亚属或两个属。但是原种I与A大不相同，在原属里差不多属于极端，所以I分出来的六个后代由于遗传的缘故，就与A的八个后代大不相同；而且，假定这两组生物向不同的方向继续分歧。而连接在原种A和I之间的中间种（这是很重要的论点），除F外也灭绝了，并且没有遗留下后代。因此，I的六个新种，以及A的八个新种，势必被列为很不同的属，甚至可以被列为不同的亚科。

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所以我认为，两个或两个以上的属，是经过变异传衍从同一属中两个以上的物种产生的。这两个以上的亲种假定是从早期一属里某一物种传下来的。图表里是用大写字母下方的虚线来表示的，其分枝向下收敛，会聚一点；这一点代表一个物种，它就是几个新亚属或属的假定单一亲种。

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新物种F14的性状值得稍加考虑，其性状假定未曾大分歧，仍然保存F的体型，无改变或少改变。这样，它和其他十四个新种的亲缘关系，属于奇怪的迂回曲折性质。由于是现在假定已经灭绝而不为人知的A和I两个亲种之间的类型传下来的，其性状应该介于这两个物种的两群后代之间。但这两群的性状已经和亲种类型有了分歧，所以新物种F14并不直接介于亲种之间，而是介于两群的亲种类型之间。每个学者都能想见这种情形的。

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图表里，各条横线都设定代表一千代，但也可以代表百万代或亿代，还可以代表包含有灭绝生物遗骸的连续地层的一部分。“地质学”一章还要讨论这一主题，我想，届时将看到图表会对灭绝生物的亲缘关系有所启示。这些生物虽然一般与现今生存的生物同目、同科、同属，但常常在性状上多少介于现存的各群之间；这种事实容易理解，因为灭绝的物种生存在远古时代，那时系统线上的分枝线还只有较小的分歧而已。

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我看没有理由把现在所解说的变异过程只限于属的形成。图表中，如果假定分歧虚线上的各个连续的群所代表的变异量是巨大的，则标着a14到p14、b14和f14，以及o14到m14的类型，将形成三个极不相同的属。还会有I传下来的两个极不相同的属，由于持续的性状分歧和不同祖先的遗传，与A的后代大不相同。该属的两个群，按图表所示的分歧变异量，形成了两个不同的科或目。这两个新科或新目，是从原属的两个物种传下来的，而这两个物种又假定是从某个更古老的、不为人知的属的一个物种传下来的。

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我们已经看到，各地最常出现变种即初始物种的，是较大属的物种。这确实是预料之中的。自然选择是通过一种类型在生存斗争中比其他类型占有优势而起作用的，主要作用于已经具有某种优势的类型。而任何一群之为大，就表明其物种从共同祖先那里遗传了共通的优点。因此，产生新的变异后代的斗争，主要发生在努力增加数目的大群之间。一个大群将慢慢征服另一个大群，减少其数量，从而减少其继续变异改进的机会。在同一大群里，后起的更完善的亚群，由于在自然组成中分歧出来并且占有许多新的地位，就经常倾向于淘汰消灭较早的、改进较少的亚群。小的破碎群及亚群终究灭亡。展望未来，我们可以预言：现在巨大的而且胜利的、最少破碎的即最少受灭绝之祸的生物群，将长此以往继续增加。但是哪几个群将最后胜利却无法预料，因为我们知道有许多从前极发达的群，现在都灭绝了。展望更远的未来，还可预言，由于大群继续不断增多，大量的小群终究要趋于灭绝，不会留下变异后代；结果，生活在任何一个时期内的物种，能把后代传到遥远未来的只是极少数。“分类”一章还要讨论这一问题，但我可以补充一句，按照这种观点，由于只有极少数古远的物种能把后代传到今日，而且由于同一物种的一切后代形成一个纲，我们就能理解，为什么动物界和植物界的每一主要大类里，现今存在的纲是如此之少。虽然极古远的物种只有少数留下变异后代，但在最遥远的地质时代里，地球上也有许多属、科、目及纲的物种分布着，其繁盛差不多就和今天一样。

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本章提要。——古往今来，在变化着的生活条件下，生物构造的各个部分如果出现变异，我想这是无可争议的；由于各个物种按几何级数增加，而在某年龄、某季节或某年代发生激烈的生存斗争，这也确是无可争议的；那么，考虑到一切生物相互之间及其与生活条件之间的无限复杂关系，会引起构造上、体质上及习性上发生对它们有利的无限多样化。如果从来没有发生过任何有益于每一生物本身繁荣的变异，就像发生的许多有益于人类的变异那样，我想是一件非常离奇的事。但是，如果有益于任何生物的变异真的发生，那么具有这种性状的个体在生活斗争中当然会有最好的机会得到保存；根据强势的遗传原理，将会产生具有同样性状的后代。我把这种保存原理简单地叫作“自然选择”。自然选择根据品质在相应龄期的遗传原理，能够改变卵、种子、幼体，就像改变成体一样容易。在许多动物里，性选择有助于普通选择，保证最强健的、最适应的雄体产生最多的后代。性选择又可使雄体独享有利的性状，以与其他雄体进行斗争。

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自然选择是否真的如此发生作用，使各种生物类型变异适应于各种条件和生活处所，这必须根据以下各章所举证的一般性质和平衡来判断。但是我们已经看到自然选择怎样引起生物的灭绝；而世界史上灭绝的作用是何等巨大，地质学已说明白了。自然选择还能引起性状的分歧；因为生物的构造、习性及体质越分歧，这个地区所能维持的生物就越多。只要对任何一处小地方的生物以及外地归化的生物加以考察，便可证明这一点。所以，任何一个物种的后代的变异过程中，一切物种增加个体数目的不断斗争中，后代如果越分歧，在生活斗争中就越有成功的好机会。这样，同一物种中不同变种间的微小差异，就有逐渐增大的倾向，一直增大为同属物种间的较大差异，甚至增大为异属间的较大差异。

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我们已经看到，变异最大的，是大属的那些普通的、广为分散的、分布范围广的物种；而且这些物种倾向于把现今在本土成为优势种的优越性传给变异后代。如前所述，自然选择引起性状的分歧，并能使改进较少的和中间类型的生物大量灭绝。我认为，根据这些原理，可以解释全部生物间亲缘关系的本质。这真是奇妙，只是我们熟视无睹而已，即全部时间和空间内的一切动植物，都可各分为群，而彼此从属关联，如我们到处看到的那样——即同种的变种间的关系最密切，同属的物种间的关系不那么密切且不均等，形成区（sections）及亚属；异属的物种间关系更疏远，并且属间关系远近程度不同，形成亚科、科、目、亚纲及纲。任何一个纲中的几个次级类群都不能列入单一行列，然皆环绕数点，这些点又环绕着另外一些点，循环往复，以至无穷。有人说物种是独立创造的，全部生物的分类便不能解释这一重大事实；但是，以我的判断，可根据遗传，以及引起灭绝和性状分歧的自然选择的复杂作用，如图表所见，这一点便可以解释。

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同一纲中一切生物的亲缘关系常用一棵大树来表示。我看这种比喻在很大程度上表达了真实情况。绿色的、生芽的小枝可以代表现存的物种；以往年代生长出来的枝条可以代表长期连续的灭绝物种。在每一生长期中，一切生长着的小枝都试图向各方分枝，并且试图遮盖和弄死周围的枝条，就像物种和种群在伟大的生存斗争中试图压倒其他物种一样。巨枝分为大枝，再逐步分为越来越小的枝，它们本身就是树幼小时生芽的小枝；这种旧芽新芽由分枝来联结的情形，正好代表一切灭绝物种和现存物种的分类，群之下又分为群。当树还仅仅是树苗时，在许多茂盛的小枝中，只有两三枝现在成长为大枝了，生存至今，并且负荷着其他的枝条；生存在久远地质时代的物种也是这样，只有很少数遗下现存的变异后代。从这树开始生长以来，许多巨枝和大枝都枯萎而且脱落了，这些枯落了的、大小不等的枝条，可以代表那些没有留下生存的后代而仅处于化石状态的全目、全科及全属。正如这里或那里看到一个细枝从树的下部分杈处开枝散叶，并且碰巧受惠，至今还在旺盛地生长着，有时我们看到鸭嘴兽或肺鱼之类的动物，它们由疏远的亲缘关系把生物的两条大枝连接起来，因生活在有庇护的地点，而从致命的竞争里得到幸免。芽生长而出新芽，新芽如果健壮，就会分出枝条，遮盖四周许多弱枝条，所以我相信，伟大的生命之树（Tree of Life）的生长也是这样，用枯落枝条填充了地壳，用生生不息的美丽枝条遮盖了地面。

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Natural Selection—its power compared with man’s selection—its power on characters of trifling importance—its power at all ages and on both sexes—Sexual Selection—On the generality of intercrosses between individuals of the same species— Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals—Slow action—Extinction caused by Natural Selection—Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation—Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent—Explains the Grouping of all organic beings

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How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation? Can the principle of selection, which we have seen is so potent in the hands of man, apply in nature? I think we shall see that it can act most effectually. Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.

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We shall best understand the probable course of natural selection by taking the case of a country undergoing some physical change, for instance, of climate. The proportional numbers of its inhabitants would almost immediately undergo a change, and some species might become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of some of the inhabitants, independently of the change of climate itself, would most seriously affect many of the others. If the country were open on its borders, new forms would certainly immigrate, and this also would seriously disturb the relations of some of the former inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the area been open to immigration, these same places would have been seized on by intruders. In such case, every slight modification, which in the course of ages chanced to arise, and which in any way favoured the individuals of any of the species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would thus have free scope for the work of improvement.

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We have reason to believe, as stated in the first chapter, that a change in the conditions of life, by specially acting on the reproductive system, causes or increases variability; and in the foregoing case the conditions of life are supposed to have undergone a change, and this would manifestly be favourable to natural selection, by giving a better chance of profitable variations occurring; and unless profitable variations do occur, natural selection can do nothing. Not that, as I believe, any extreme amount of variability is necessary; as man can certainly produce great results by adding up in any given direction mere individual differences, so could Nature, but far more easily, from having incomparably longer time at her disposal. Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, is actually necessary to produce new and unoccupied places for natural selection to fill up by modifying and improving some of the varying inhabitants. For as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one inhabitant would often give it an advantage over others; and still further modifications of the same kind would often still further increase the advantage. No country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could anyhow be improved; for in all countries, the natives have been so far conquered by naturalised productions, that they have allowed foreigners to take firm possession of the land. And as foreigners have thus everywhere beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted such intruders.

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As man can produce and certainly has produced a great result by his methodical and unconscious means of selection, what may not nature effect? Man can act only on external and visible characters: nature cares nothing for appearances, except in so far as they may be useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that of the being which she tends. Every selected character is fully exercised by her; and the being is placed under well-suited conditions of life. Man keeps the natives of many climates in the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long and a short beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. He does not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. He often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch his eye, or to be plainly useful to him. Under nature, the slightest difference of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will his products be, compared with those accumulated by nature during whole geological periods. Can we wonder, then, that nature’s productions should be far “truer” in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?

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It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

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Although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey,—so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence I can see no reason to doubt that natural selection might be most effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we should remember how essential it is in a flock of white sheep to destroy every lamb with the faintest trace of black. In plants the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh. If, with all the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such differences would effectually settle which variety, whether a smooth or downy, a yellow or purple fleshed fruit, should succeed.

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In looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem to be quite unimportant, we must not forget that climate, food, etc., probably produce some slight and direct effect. It is, however, far more necessary to bear in mind that there are many unknown laws of correlation of growth, which, when one part of the organisation is modified through variation, and the modifications are accumulated by natural selection for the good of the being, will cause other modifications, often of the most unexpected nature.

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As we see that those variations which under domestication appear at any particular period of life, tend to reappear in the offspring at the same period;—for instance, in the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult;—so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of profitable variations at that age, and by their inheritance at a corresponding age. If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees. Natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect. These modifications will no doubt affect, through the laws of correlation, the structure of the adult; and probably in the case of those insects which live only for a few hours, and which never feed, a large part of their structure is merely the correlated result of successive changes in the structure of their larvae. So, conversely, modifications in the adult will probably often affect the structure of the larva; but in all cases natural selection will ensure that modifications consequent on other modifications at a different period of life, shall not be in the least degree injurious: for if they became so, they would cause the extinction of the species.

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Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change. What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal’s whole life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, and used exclusively for opening the cocoon—or the hard tip to the beak of nestling birds, used for breaking the egg. It has been asserted, that of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. Now, if nature had to make the beak of a full-grown pigeon very short for the bird’s own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure.

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Sexual Selection.—Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex in its functional relations to the other sex, or in relation to wholly different habits of life in the two sexes, as is sometimes the case with insects. And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving offspring. Sexual selection by always allowing the victor to breed might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, as well as the brutal cock-fighter, who knows well that he can improve his breed by careful selection of the best cocks. How low in the scale of nature this law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male salmons have been seen fighting all day long; male stag- beetles often bear wounds from the huge mandibles of other males. The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. The males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear.

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Amongst birds, the contest is often of a more peaceful character. All those who have attended to the subject, believe that there is the severest rivalry between the males of many species to attract by singing the females. The rock-thrush of Guiana, birds of Paradise, and some others, congregate; and successive males display their gorgeous plumage and perform strange antics before the females, which standing by as spectators, at last choose the most attractive partner. Those who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus Sir R. Heron has described how one pied peacock was eminently attractive to all his hen birds. It may appear childish to attribute any effect to such apparently weak means: I cannot here enter on the details necessary to support this view; but if man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. I strongly suspect that some well-known laws with respect to the plumage of male and female birds, in comparison with the plumage of the young, can be explained on the view of plumage having been chiefly modified by sexual selection, acting when the birds have come to the breeding age or during the breeding season; the modifications thus produced being inherited at corresponding ages or seasons, either by the males alone, or by the males and females; but I have not space here to enter on this subject.

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Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males have had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; and have transmitted these advantages to their male offspring. Yet, I would not wish to attribute all such sexual differences to this agency: for we see peculiarities arising and becoming attached to the male sex in our domestic animals (as the wattle in male carriers, horn-like protuberances in the cocks of certain fowls, etc.), which we cannot believe to be either useful to the males in battle, or attractive to the females. We see analogous cases under nature, for instance, the tuft of hair on the breast of the turkey-cock, which can hardly be either useful or ornamental to this bird;—indeed, had the tuft appeared under domestication, it would have been called a monstrosity.

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Illustrations of the action of Natural Selection.—In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. I can under such circumstances see no reason to doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected,—provided always that they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals. I can see no more reason to doubt this, than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious selection which results from each man trying to keep the best dogs without any thought of modifying the breed.

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Even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might be born with an innate tendency to pursue certain kinds of prey. Nor can this be thought very improbable; for we often observe great differences in the natural tendencies of our domestic animals; one cat, for instance, taking to catch rats, another mice; one cat, according to Mr. St. John, bringing home winged game, another hares or rabbits, and another hunting on marshy ground and almost nightly catching woodcocks or snipes. The tendency to catch rats rather than mice is known to be inherited. Now, if any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring. Some of its young would probably inherit the same habits or structure, and by the repetition of this process, a new variety might be formed which would either supplant or coexist with the parent-form of wolf. Or, again, the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey; and from the continued preservation of the individuals best fitted for the two sites, two varieties might slowly be formed. These varieties would cross and blend where they met; but to this subject of intercrossing we shall soon have to return. I may add, that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains in the United States, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd’s flocks.

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Let us now take a more complex case. Certain plants excrete a sweet juice, apparently for the sake of eliminating something injurious from their sap: this is effected by glands at the base of the stipules in some Leguminosae, and at the back of the leaf of the common laurel. This juice, though small in quantity, is greedily sought by insects. Let us now suppose a little sweet juice or nectar to be excreted by the inner bases of the petals of a flower. In this case insects in seeking the nectar would get dusted with pollen, and would certainly often transport the pollen from one flower to the stigma of another flower. The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason to believe (as will hereafter be more fully alluded to), would produce very vigorous seedlings, which consequently would have the best chance of flourishing and surviving. Some of these seedlings would probably inherit the nectar-excreting power. Those individual flowers which had the largest glands or nectaries, and which excreted most nectar, would be oftenest visited by insects, and would be oftenest crossed; and so in the long-run would gain the upper hand. Those flowers, also, which had their stamens and pistils placed, in relation to the size and habits of the particular insects which visited them, so as to favour in any degree the transportal of their pollen from flower to flower, would likewise be favoured or selected. We might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as pollen is formed for the sole object of fertilisation, its destruction appears a simple loss to the plant; yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed, it might still be a great gain to the plant; and those individuals which produced more and more pollen, and had larger and larger anthers, would be selected.

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When our plant, by this process of the continued preservation or natural selection of more and more attractive flowers, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly carry pollen from flower to flower; and that they can most effectually do this, I could easily show by many striking instances. I will give only one—not as a very striking case, but as likewise illustrating one step in the separation of the sexes of plants, presently to be alluded to. Some holly-trees bear only male flowers, which have four stamens producing rather a small quantity of pollen, and a rudimentary pistil; other holly-trees bear only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception, there were pollen-grains, and on some a profusion of pollen. As the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. The weather had been cold and boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined had been effectually fertilised by the bees, accidentally dusted with pollen, having flown from tree to tree in search of nectar. But to return to our imaginary case: as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. No naturalist doubts the advantage of what has been called the “physiological division of labour;” hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. In plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent; now if we suppose this to occur in ever so slight a degree under nature, then as pollen is already carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be advantageous on the principle of the division of labour, individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes would be effected.

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Let us now turn to the nectar-feeding insects in our imaginary case: we may suppose the plant of which we have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects depended in main part on its nectar for food. I could give many facts, showing how anxious bees are to save time; for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which they can, with a very little more trouble, enter by the mouth. Bearing such facts in mind, I can see no reason to doubt that an accidental deviation in the size and form of the body, or in the curvature and length of the proboscis, etc., far too slight to be appreciated by us, might profit a bee or other insect, so that an individual so characterised would be able to obtain its food more quickly, and so have a better chance of living and leaving descendants. Its descendants would probably inherit a tendency to a similar slight deviation of structure. The tubes of the corollas of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humble-bees alone; so that whole fields of the red clover offer in vain an abundant supply of precious nectar to the hive-bee. Thus it might be a great advantage to the hive-bee to have a slightly longer or differently constructed proboscis. On the other hand, I have found by experiment that the fertility of clover greatly depends on bees visiting and moving parts of the corolla, so as to push the pollen on to the stigmatic surface. Hence, again, if humble-bees were to become rare in any country, it might be a great advantage to the red clover to have a shorter or more deeply divided tube to its corolla, so that the hive-bee could visit its flowers. Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by the continued preservation of individuals presenting mutual and slightly favourable deviations of structure.

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I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were at first urged against Sir Charles Lyell’s noble views on “the modern changes of the earth, as illustrative of geology;” but we now very seldom hear the action, for instance, of the coast-waves, called a trifling and insignificant cause, when applied to the excavation of gigantic valleys or to the formation of the longest lines of inland cliffs. Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.

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On the Intercrossing of Individuals.—I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course obvious that two individuals must always unite for each birth; but in the case of hermaphrodites this is far from obvious. Nevertheless I am strongly inclined to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind. This view, I may add, was first suggested by Andrew Knight. We shall presently see its importance; but I must here treat the subject with extreme brevity, though I have the materials prepared for an ample discussion. All vertebrate animals, all insects, and some other large groups of animals, pair for each birth. Modern research has much diminished the number of supposed hermaphrodites, and of real hermaphrodites a large number pair; that is, two individuals regularly unite for reproduction, which is all that concerns us. But still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of plants are hermaphrodites. What reason, it may be asked, is there for supposing in these cases that two individuals ever concur in reproduction? As it is impossible here to enter on details, I must trust to some general considerations alone.

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In the first place, I have collected so large a body of facts, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature (utterly ignorant though we be of the meaning of the law) that no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally—perhaps at very long intervals—indispensable.

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On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the following, which on any other view are inexplicable. Every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather! but if an occasional cross be indispensable, the fullest freedom for the entrance of pollen from another individual will explain this state of exposure, more especially as the plant’s own anthers and pistil generally stand so close together that self-fertilisation seems almost inevitable. Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family; but in several, perhaps in all, such flowers, there is a very curious adaptation between the structure of the flower and the manner in which bees suck the nectar; for, in doing this, they either push the flower’s own pollen on the stigma, or bring pollen from another flower. So necessary are the visits of bees to papilionaceous flowers, that I have found, by experiments published elsewhere, that their fertility is greatly diminished if these visits be prevented. Now, it is scarcely possible that bees should fly from flower to flower, and not carry pollen from one to the other, to the great good, as I believe, of the plant. Bees will act like a camel-hair pencil, and it is quite sufficient just to touch the anthers of one flower and then the stigma of another with the same brush to ensure fertilisation; but it must not be supposed that bees would thus produce a multitude of hybrids between distinct species; for if you bring on the same brush a plant’s own pollen and pollen from another species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has been shown by G?rtner, any influence from the foreign pollen.

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When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but, the agency of insects is often required to cause the stamens to spring forward, as K?lreuter has shown to be the case with the barberry; and curiously in this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that if very closely-allied forms or varieties are planted near each other, it is hardly possible to raise pure seedlings, so largely do they naturally cross. In many other cases, far from there being any aids for self-fertilisation, there are special contrivances, as I could show from the writings of C. C. Sprengel and from my own observations, which effectually prevent the stigma receiving pollen from its own flower: for instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which every one of the infinitely numerous pollen-granules are swept out of the conjoined anthers of each flower, before the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, I raised plenty of seedlings; and whilst another species of Lobelia growing close by, which is visited by bees, seeds freely. In very many other cases, though there be no special mechanical contrivance to prevent the stigma of a flower receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these plants have in fact separated sexes, and must habitually be crossed. How strange are these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should in so many cases be mutually useless to each other! How simply are these facts explained on the view of an occasional cross with a distinct individual being advantageous or indispensable!

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If several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other, a large majority, as I have found, of the seedlings thus raised will turn out mongrels: for instance, I raised 233 seedling cabbages from some plants of different varieties growing near each other, and of these only 78 were true to their kind, and some even of these were not perfectly true. Yet the pistil of each cabbage-flower is surrounded not only by its own six stamens, but by those of the many other flowers on the same plant. How, then, comes it that such a vast number of the seedlings are mongrelized? I suspect that it must arise from the pollen of a distinct variety having a prepotent effect over a flower’s own pollen; and that this is part of the general law of good being derived from the intercrossing of distinct individuals of the same species. When distinct species are crossed the case is directly the reverse, for a plant’s own pollen is always prepotent over foreign pollen; but to this subject we shall return in a future chapter.

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In the case of a gigantic tree covered with innumerable flowers, it may be objected that pollen could seldom be carried from tree to tree, and at most only from flower to flower on the same tree, and that flowers on the same tree can be considered as distinct individuals only in a limited sense. I believe this objection to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with separated sexes. When the sexes are separated, although the male and female flowers may be produced on the same tree, we can see that pollen must be regularly carried from flower to flower; and this will give a better chance of pollen being occasionally carried from tree to tree. That trees belonging to all Orders have their sexes more often separated than other plants, I find to be the case in this country; and at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as I anticipated. On the other hand, Dr. Hooker has recently informed me that he finds that the rule does not hold in Australia; and I have made these few remarks on the sexes of trees simply to call attention to the subject.

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Turning for a very brief space to animals: on the land there are some hermaphrodites, as land-mollusca and earth-worms; but these all pair. As yet I have not found a single case of a terrestrial animal which fertilises itself. We can understand this remarkable fact, which offers so strong a contrast with terrestrial plants, on the view of an occasional cross being indispensable, by considering the medium in which terrestrial animals live, and the nature of the fertilising element; for we know of no means, analogous to the action of insects and of the wind in the case of plants, by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals. Of aquatic animals, there are many self-fertilising hermaphrodites; but here currents in the water offer an obvious means for an occasional cross. And, as in the case of flowers, I have as yet failed, after consultation with one of the highest authorities, namely, Professor Huxley, to discover a single case of an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the body, that access from without and the occasional influence of a distinct individual can be shown to be physically impossible. Cirripedes long appeared to me to present a case of very great difficulty under this point of view; but I have been enabled, by a fortunate chance, elsewhere to prove that two individuals, though both are self-fertilising hermaphrodites, do sometimes cross.

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It must have struck most naturalists as a strange anomaly that, in the case of both animals and plants, species of the same family and even of the same genus, though agreeing closely with each other in almost their whole organisation, yet are not rarely, some of them hermaphrodites, and some of them unisexual. But if, in fact, all hermaphrodites do occasionally intercross with other individuals, the difference between hermaphrodites and unisexual species, as far as function is concerned, becomes very small.

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From these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature. I am well aware that there are, on this view, many cases of difficulty, some of which I am trying to investigate. Finally then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.

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Circumstances favourable to Natural Selection.—This is an extremely intricate subject. A large amount of inheritable and diversified variability is favourable, but I believe mere individual differences suffice for the work. A large number of individuals, by giving a better chance for the appearance within any given period of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success. Though nature grants vast periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving, it may be said, to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated.

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In man’s methodical selection, a breeder selects for some definite object, and free intercrossing will wholly stop his work. But when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to get and breed from the best animals, much improvement and modification surely but slowly follow from this unconscious process of selection, notwithstanding a large amount of crossing with inferior animals. Thus it will be in nature; for within a confined area, with some place in its polity not so perfectly occupied as might be, natural selection will always tend to preserve all the individuals varying in the right direction, though in different degrees, so as better to fill up the unoccupied place. But if the area be large, its several districts will almost certainly present different conditions of life; and then if natural selection be modifying and improving a species in the several districts, there will be intercrossing with the other individuals of the same species on the confines of each. And in this case the effects of intercrossing can hardly be counterbalanced by natural selection always tending to modify all the individuals in each district in exactly the same manner to the conditions of each; for in a continuous area, the conditions will generally graduate away insensibly from one district to another. The intercrossing will most affect those animals which unite for each birth, which wander much, and which do not breed at a very quick rate. Hence in animals of this nature, for instance in birds, varieties will generally be confined to separated countries; and this I believe to be the case. In hermaphrodite organisms which cross only occasionally, and likewise in animals which unite for each birth, but which wander little and which can increase at a very rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body, so that whatever intercrossing took place would be chiefly between the individuals of the same new variety. A local variety when once thus formed might subsequently slowly spread to other districts. On the above principle, nurserymen always prefer getting seed from a large body of plants of the same variety, as the chance of intercrossing with other varieties is thus lessened.

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Even in the case of slow-breeding animals, which unite for each birth, we must not overrate the effects of intercrosses in retarding natural selection; for I can bring a considerable catalogue of facts, showing that within the same area, varieties of the same animal can long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from varieties of the same kind preferring to pair together.

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Intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the same variety, true and uniform in character. It will obviously thus act far more efficiently with those animals which unite for each birth; but I have already attempted to show that we have reason to believe that occasional intercrosses take place with all animals and with all plants. Even if these take place only at long intervals, I am convinced that the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus, in the long run, the influence of intercrosses, even at rare intervals, will be great. If there exist organic beings which never intercross, uniformity of character can be retained amongst them, as long as their conditions of life remain the same, only through the principle of inheritance, and through natural selection destroying any which depart from the proper type; but if their conditions of life change and they undergo modification, uniformity of character can be given to their modified offspring, solely by natural selection preserving the same favourable variations.

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Isolation, also, is an important element in the process of natural selection. In a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be in a great degree uniform; so that natural selection will tend to modify all the individuals of a varying species throughout the area in the same manner in relation to the same conditions. Intercrosses, also, with the individuals of the same species, which otherwise would have inhabited the surrounding and differently circumstanced districts, will be prevented. But isolation probably acts more efficiently in checking the immigration of better adapted organisms, after any physical change, such as of climate or elevation of the land, etc.; and thus new places in the natural economy of the country are left open for the old inhabitants to struggle for, and become adapted to, through modifications in their structure and constitution. Lastly, isolation, by checking immigration and consequently competition, will give time for any new variety to be slowly improved; and this may sometimes be of importance in the production of new species. If, however, an isolated area be very small, either from being surrounded by barriers, or from having very peculiar physical conditions, the total number of the individuals supported on it will necessarily be very small; and fewness of individuals will greatly retard the production of new species through natural selection, by decreasing the chance of the appearance of favourable variations.

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If we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an oceanic island, although the total number of the species inhabiting it, will be found to be small, as we shall see in our chapter on geographical distribution; yet of these species a very large proportion are endemic,—that is, have been produced there, and nowhere else. Hence an oceanic island at first sight seems to have been highly favourable for the production of new species. But we may thus greatly deceive ourselves, for to ascertain whether a small isolated area, or a large open area like a continent, has been most favourable for the production of new organic forms, we ought to make the comparison within equal times; and this we are incapable of doing.

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Although I do not doubt that isolation is of considerable importance in the production of new species, on the whole I am inclined to believe that largeness of area is of more importance, more especially in the production of species, which will prove capable of enduring for a long period, and of spreading widely. Throughout a great and open area, not only will there be a better chance of favourable variations arising from the large number of individuals of the same species there supported, but the conditions of life are infinitely complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree or they will be exterminated. Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many others. Hence more new places will be formed, and the competition to fill them will be more severe, on a large than on a small and isolated area. Moreover, great areas, though now continuous, owing to oscillations of level, will often have recently existed in a broken condition, so that the good effects of isolation will generally, to a certain extent, have concurred. Finally, I conclude that, although small isolated areas probably have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas; and what is more important, that the new forms produced on large areas, which already have been victorious over many competitors, will be those that will spread most widely, will give rise to most new varieties and species, and will thus play an important part in the changing history of the organic world.

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We can, perhaps, on these views, understand some facts which will be again alluded to in our chapter on geographical distribution; for instance, that the productions of the smaller continent of Australia have formerly yielded, and apparently are now yielding, before those of the larger Europaeo-Asiatic area. Thus, also, it is that continental productions have everywhere become so largely naturalised on islands. On a small island, the race for life will have been less severe, and there will have been less modification and less extermination. Hence, perhaps, it comes that the flora of Madeira, according to Oswald Heer, resembles the extinct tertiary flora of Europe. All fresh- water basins, taken together, make a small area compared with that of the sea or of the land; and, consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been more slowly formed, and old forms more slowly exterminated. And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain extent orders now widely separated in the natural scale. These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.

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To sum up the circumstances favourable and unfavourable to natural selection, as far as the extreme intricacy of the subject permits. I conclude, looking to the future, that for terrestrial productions a large continental area, which will probably undergo many oscillations of level, and which consequently will exist for long periods in a broken condition, will be the most favourable for the production of many new forms of life, likely to endure long and to spread widely. For the area will first have existed as a continent, and the inhabitants, at this period numerous in individuals and kinds, will have been subjected to very severe competition. When converted by subsidence into large separate islands, there will still exist many individuals of the same species on each island: intercrossing on the confines of the range of each species will thus be checked: after physical changes of any kind, immigration will be prevented, so that new places in the polity of each island will have to be filled up by modifications of the old inhabitants; and time will be allowed for the varieties in each to become well modified and perfected. When, by renewed elevation, the islands shall be re-converted into a continental area, there will again be severe competition: the most favoured or improved varieties will be enabled to spread: there will be much extinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the renewed continent will again be changed; and again there will be a fair field for natural selection to improve still further the inhabitants, and thus produce new species.

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That natural selection will always act with extreme slowness, I fully admit. Its action depends on there being places in the polity of nature, which can be better occupied by some of the inhabitants of the country undergoing modification of some kind. The existence of such places will often depend on physical changes, which are generally very slow, and on the immigration of better adapted forms having been checked. But the action of natural selection will probably still oftener depend on some of the inhabitants becoming slowly modified; the mutual relations of many of the other inhabitants being thus disturbed. Nothing can be effected, unless favourable variations occur, and variation itself is apparently always a very slow process. The process will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. I do not believe so. On the other hand, I do believe that natural selection will always act very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time. I further believe, that this very slow, intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed.

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Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature’s power of selection.

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Extinction.—This subject will be more fully discussed in our chapter on Geology; but it must be here alluded to from being intimately connected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. But as from the high geometrical powers of increase of all organic beings, each area is already fully stocked with inhabitants, it follows that as each selected and favoured form increases in number, so will the less favoured forms decrease and become rare. Rarity, as geology tells us, is the precursor to extinction. We can, also, see that any form represented by few individuals will, during fluctuations in the seasons or in the number of its enemies, run a good chance of utter extinction. But we may go further than this; for as new forms are continually and slowly being produced, unless we believe that the number of specific forms goes on perpetually and almost indefinitely increasing, numbers inevitably must become extinct. That the number of specific forms has not indefinitely increased, geology shows us plainly; and indeed we can see reason why they should not have thus increased, for the number of places in the polity of nature is not indefinitely great,—not that we have any means of knowing that any one region has as yet got its maximum of species. Probably no region is as yet fully stocked, for at the Cape of Good Hope, where more species of plants are crowded together than in any other quarter of the world, some foreign plants have become naturalised, without causing, as far as we know, the extinction of any natives.

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Furthermore, the species which are most numerous in individuals will have the best chance of producing within any given period favourable variations. We have evidence of this, in the facts given in the second chapter, showing that it is the common species which afford the greatest number of recorded varieties, or incipient species. Hence, rare species will be less quickly modified or improved within any given period, and they will consequently be beaten in the race for life by the modified descendants of the commoner species.

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From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. The forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most. And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,—varieties of the same species, and species of the same genus or of related genera,—which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other. Consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. We see the same process of extermination amongst our domesticated productions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these “were swept away by the short-horns” (I quote the words of an agricultural writer) “as if by some murderous pestilence.”

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Divergence of Character.—The principle, which I have designated by this term, is of high importance on my theory, and explains, as I believe, several important facts. In the first place, varieties, even strongly-marked ones, though having somewhat of the character of species—as is shown by the hopeless doubts in many cases how to rank them—yet certainly differ from each other far less than do good and distinct species. Nevertheless, according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species. How, then, does the lesser difference between varieties become augmented into the greater difference between species? That this does habitually happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences; whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences. Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large an amount of difference as that between varieties of the same species and species of the same genus.

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As has always been my practice, let us seek light on this head from our domestic productions. We shall here find something analogous. A fancier is struck by a pigeon having a slightly shorter beak; another fancier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that “fanciers do not and will not admire a medium standard, but like extremes,” they both go on (as has actually occurred with tumbler-pigeons) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. Again, we may suppose that at an early period one man preferred swifter horses; another stronger and more bulky horses. The early differences would be very slight; in the course of time, from the continued selection of swifter horses by some breeders, and of stronger ones by others, the differences would become greater, and would be noted as forming two sub-breeds; finally, after the lapse of centuries, the sub-breeds would become converted into two well-established and distinct breeds. As the differences slowly become greater, the inferior animals with intermediate characters, being neither very swift nor very strong, will have been neglected, and will have tended to disappear. Here, then, we see in man’s productions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character both from each other and from their common parent.

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But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently, from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.

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We can clearly see this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural powers of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous. The more diversified in habits and structure the descendants of our carnivorous animal became, the more places they would be enabled to occupy. What applies to one animal will apply throughout all time to all animals—that is, if they vary—for otherwise natural selection can do nothing. So it will be with plants. It has been experimentally proved, that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can thus be raised. The same has been found to hold good when first one variety and then several mixed varieties of wheat have been sown on equal spaces of ground. Hence, if any one species of grass were to go on varying, and those varieties were continually selected which differed from each other in at all the same manner as distinct species and genera of grasses differ from each other, a greater number of individual plants of this species of grass, including its modified descendants, would succeed in living on the same piece of ground. And we well know that each species and each variety of grass is annually sowing almost countless seeds; and thus, as it may be said, is striving its utmost to increase its numbers. Consequently, I cannot doubt that in the course of many thousands of generations, the most distinct varieties of any one species of grass would always have the best chance of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and varieties, when rendered very distinct from each other, take the rank of species.

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The truth of the principle, that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances. In an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be severe, we always find great diversity in its inhabitants. For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera and to eight orders, which shows how much these plants differed from each other. So it is with the plants and insects on small and uniform islets; and so in small ponds of fresh water. Farmers find that they can raise most food by a rotation of plants belonging to the most different orders: nature follows what may be called a simultaneous rotation. Most of the animals and plants which live close round any small piece of ground, could live on it (supposing it not to be in any way peculiar in its nature), and may be said to be striving to the utmost to live there; but, it is seen, that where they come into the closest competition with each other, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders.

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The same principle is seen in the naturalisation of plants through man’s agency in foreign lands. It might have been expected that the plants which have succeeded in becoming naturalised in any land would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country. It might, also, perhaps have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes. But the case is very different; and Alph. De Candolle has well remarked in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species. To give a single instance: in the last edition of Dr. Asa Gray’s “Manual of the Flora of the Northern United States,” 260 naturalised plants are enumerated, and these belong to 162 genera. We thus see that these naturalised plants are of a highly diversified nature. They differ, moreover, to a large extent from the indigenes, for out of the 162 genera, no less than 100 genera are not there indigenous, and thus a large proportional addition is made to the genera of these States.

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By considering the nature of the plants or animals which have struggled successfully with the indigenes of any country, and have there become naturalised, we can gain some crude idea in what manner some of the natives would have had to be modified, in order to have gained an advantage over the other natives; and we may, I think, at least safely infer that diversification of structure, amounting to new generic differences, would have been profitable to them.

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The advantage of diversification in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same individual body—a subject so well elucidated by Milne Edwards. No physiologist doubts that a stomach by being adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these substances. So in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves. A set of animals, with their organisation but little diversified, could hardly compete with a set more perfectly diversified in structure. It may be doubted, for instance, whether the Australian marsupials, which are divided into groups differing but little from each other, and feebly representing, as Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and rodent mammals, could successfully compete with these well-pronounced orders. In the Australian mammals, we see the process of diversification in an early and incomplete stage of development.

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After the foregoing discussion, which ought to have been much amplified, we may, I think, assume that the modified descendants of any one species will succeed by so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of great benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, will tend to act.

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The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances. I have said a large genus, because we have seen in the second chapter, that on an average more of the species of large genera vary than of small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most widely-diffused, vary more than rare species with restricted ranges. Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The little fan of diverging dotted lines of unequal lengths proceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods. Only those variations which are in some way profitable will be preserved or naturally selected. And here the importance of the principle of benefit being derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to have formed a fairly well-marked variety, such as would be thought worthy of record in a systematic work.

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The intervals between the horizontal lines in the diagram, may represent each a thousand generations; but it would have been better if each had represented ten thousand generations. After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a1 and m1. These two varieties will generally continue to be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself hereditary, consequently they will tend to vary, and generally to vary in nearly the same manner as their parents varied. Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their common parent (A) more numerous than most of the other inhabitants of the same country; they will likewise partake of those more general advantages which made the genus to which the parent- species belonged, a large genus in its own country. And these circumstances we know to be favourable to the production of new varieties.

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If, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations. And after this interval, variety a1 is supposed in the diagram to have produced variety a2, which will, owing to the principle of divergence, differ more from (A) than did variety a1. Variety m1 is supposed to have produced two varieties, namely m2 and s2 , differing from each other, and more considerably from their common parent (A). We may continue the process by similar steps for any length of time; some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition, some producing two or three varieties, and some failing to produce any. Thus the varieties or modified descendants, proceeding from the common parent (A), will generally go on increasing in number and diverging in character. In the diagram the process is represented up to the ten-thousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation.

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But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular. I am far from thinking that the most divergent varieties will invariably prevail and multiply: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on infinitely complex relations. But as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will be increased. In our diagram the line of succession is broken at regular intervals by small numbered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to have allowed the accumulation of a considerable amount of divergent variation.

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As all the modified descendants from a common and widely-diffused species, belonging to a large genus, will tend to partake of the same advantages which made their parent successful in life, they will generally go on multiplying in number as well as diverging in character: this is represented in the diagram by the several divergent branches proceeding from (A). The modified offspring from the later and more highly improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less improved branches: this is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines. In some cases I do not doubt that the process of modification will be confined to a single line of descent, and the number of the descendants will not be increased; although the amount of divergent modification may have been increased in the successive generations. This case would be represented in the diagram, if all the lines proceeding from (A) were removed, excepting that from a1 to a10. In the same way, for instance, the English race-horse and English pointer have apparently both gone on slowly diverging in character from their original stocks, without either having given off any fresh branches or races.

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After ten thousand generations, species (A) is supposed to have produced three forms, a10, f10, and m10, which, from having diverged in character during the successive generations, will have come to differ largely, but perhaps unequally, from each other and from their common parent. If we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties; or they may have arrived at the doubtful category of sub-species; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into well-defined species: thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species. By continuing the same process for a greater number of generations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a14 and m14, all descended from (A). Thus, as I believe, species are multiplied and genera are formed.

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In a large genus it is probable that more than one species would vary. In the diagram I have assumed that a second species (I) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties (w10 and z10) or two species, according to the amount of change supposed to be represented between the horizontal lines. After fourteen thousand generations, six new species, marked by the letters n14 to z14, are supposed to have been produced. In each genus, the species, which are already extremely different in character, will generally tend to produce the greatest number of modified descendants; for these will have the best chance of filling new and widely different places in the polity of nature: hence in the diagram I have chosen the extreme species (A), and the nearly extreme species (I), as those which have largely varied, and have given rise to new varieties and species. The other nine species (marked by capital letters) of our original genus, may for a long period continue transmitting unaltered descendants; and this is shown in the diagram by the dotted lines not prolonged far upwards from want of space.

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But during the process of modification, represented in the diagram, another of our principles, namely that of extinction, will have played an important part. As in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original parent. For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as well as the original parent-species itself, will generally tend to become extinct. So it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines of descent. If, however, the modified offspring of a species get into some distinct country, or become quickly adapted to some quite new station, in which child and parent do not come into competition, both may continue to exist.

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If then our diagram be assumed to represent a considerable amount of modification, species (A) and all the earlier varieties will have become extinct, having been replaced by eight new species (a14 to m14); and (I) will have been replaced by six (n14 to z14) new species.

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But we may go further than this. The original species of our genus were supposed to resemble each other in unequal degrees, as is so generally the case in nature; species (A) being more nearly related to B, C, and D, than to the other species; and species (I) more to G, H, K, L, than to the others. These two species (A) and (I), were also supposed to be very common and widely diffused species, so that they must originally have had some advantage over most of the other species of the genus. Their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of the same advantages: they have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy of their country. It seems, therefore, to me extremely probable that they will have taken the places of, and thus exterminated, not only their parents (A) and (I), but likewise some of the original species which were most nearly related to their parents. Hence very few of the original species will have transmitted offspring to the fourteen-thousandth generation. We may suppose that only one (F), of the two species which were least closely related to the other nine original species, has transmitted descendants to this late stage of descent.

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The new species in our diagram descended from the original eleven species, will now be fifteen in number. Owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a14 and z14 will be much greater than that between the most different of the original eleven species. The new species, moreover, will be allied to each other in a widely different manner. Of the eight descendants from (A) the three marked a14, q14, p14, will be nearly related from having recently branched off from a10; b14 and f14, from having diverged at an earlier period from a5, will be in some degree distinct from the three first-named species; and lastly, o14, e14, and m14, will be nearly related one to the other, but from having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a sub-genus or even a distinct genus.

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The six descendants from (I) will form two sub-genera or even genera. But as the original species (I) differed largely from (A), standing nearly at the extreme points of the original genus, the six descendants from (I) will, owing to inheritance, differ considerably from the eight descendants from (A); the two groups, moreover, are supposed to have gone on diverging in different directions. The intermediate species, also (and this is a very important consideration), which connected the original species (A) and (I), have all become, excepting (F), extinct, and have left no descendants. Hence the six new species descended from (I), and the eight descended from (A), will have to be ranked as very distinct genera, or even as distinct sub-families.

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Thus it is, as I believe, that two or more genera are produced by descent, with modification, from two or more species of the same genus. And the two or more parent-species are supposed to have descended from some one species of an earlier genus. In our diagram, this is indicated by the broken lines, beneath the capital letters, converging in sub-branches downwards towards a single point; this point representing a single species, the supposed single parent of our several new sub-genera and genera.

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It is worth while to reflect for a moment on the character of the new species F14, which is supposed not to have diverged much in character, but to have retained the form of (F), either unaltered or altered only in a slight degree. In this case, its affinities to the other fourteen new species will be of a curious and circuitous nature. Having descended from a form which stood between the two parent-species (A) and (I), now supposed to be extinct and unknown, it will be in some degree intermediate in character between the two groups descended from these species. But as these two groups have gone on diverging in character from the type of their parents, the new species (F14) will not be directly intermediate between them, but rather between types of the two groups; and every naturalist will be able to bring some such case before his mind.

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In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or hundred million generations, and likewise a section of the successive strata of the earth’s crust including extinct remains. We shall, when we come to our chapter on Geology, have to refer again to this subject, and I think we shall then see that the diagram throws light on the affinities of extinct beings, which, though generally belonging to the same orders, or families, or genera, with those now living, yet are often, in some degree, intermediate in character between existing groups; and we can understand this fact, for the extinct species lived at very ancient epochs when the branching lines of descent had diverged less.

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I see no reason to limit the process of modification, as now explained, to the formation of genera alone. If, in our diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be very great, the forms marked a14 to p14, those marked b14 and f14, and those marked o14 to m14, will form three very distinct genera. We shall also have two very distinct genera descended from (I); and as these latter two genera, both from continued divergence of character and from inheritance from a different parent, will differ widely from the three genera descended from (A), the two little groups of genera will form two distinct families, or even orders, according to the amount of divergent modification supposed to be represented in the diagram. And the two new families, or orders, will have descended from two species of the original genus; and these two species are supposed to have descended from one species of a still more ancient and unknown genus.

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We have seen that in each country it is the species of the larger genera which oftenest present varieties or incipient species. This, indeed, might have been expected; for as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common. Hence, the struggle for the production of new and modified descendants, will mainly lie between the larger groups, which are all trying to increase in number. One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement. Within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally tend to disappear. Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which as yet have suffered least extinction, will for a long period continue to increase. But which groups will ultimately prevail, no man can predict; for we well know that many groups, formerly most extensively developed, have now become extinct. Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that of the species living at any one period, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classification, but I may add that on this view of extremely few of the more ancient species having transmitted descendants, and on the view of all the descendants of the same species making a class, we can understand how it is that there exist but very few classes in each main division of the animal and vegetable kingdoms. Although extremely few of the most ancient species may now have living and modified descendants, yet at the most remote geological period, the earth may have been as well peopled with many species of many genera, families, orders, and classes, as at the present day.

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Summary of Chapter.—If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being’s own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection. Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily as the adult. Amongst many animals, sexual selection will give its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles with other males.

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Whether natural selection has really thus acted in nature, in modifying and adapting the various forms of life to their several conditions and stations, must be judged of by the general tenour and balance of evidence given in the following chapters. But we already see how it entails extinction; and how largely extinction has acted in the world’s history, geology plainly declares. Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking at the inhabitants of any small spot or at naturalised productions. Therefore during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified these descendants become, the better will be their chance of succeeding in the battle of life. Thus the small differences distinguishing varieties of the same species, will steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera.

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We have seen that it is the common, the widely-diffused, and widely-ranging species, belonging to the larger genera, which vary most; and these will tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. On these principles, I believe, the nature of the affinities of all organic beings may be explained. It is a truly wonderful fact—the wonder of which we are apt to overlook from familiarity—that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold—namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single file, but seem rather to be clustered round points, and these round other points, and so on in almost endless cycles. On the view that each species has been independently created, I can see no explanation of this great fact in the classification of all organic beings; but, to the best of my judgment, it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram.

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The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

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序号 英文/音标 中文解释 更多操作

intercross

[ˌɪntɜː'krɒs]

vt.使相互交叉

descendant

[dɪ'sendənt]

n.后裔;子孙;后代

peculiarity

[pɪˌkjuːli'ærəti]

n.特质;特性;怪癖;古怪

lesser

['lesə(r)]

adj.较少的;较小的;次要的

domestication

[dəˌmestɪ'keɪʃn]

n.驯服;教化

infinite

['ɪnfɪnət]

adj.无穷的;无限的

undoubted

[ʌn'daʊtɪd]

adj.无疑的

rigid

['rɪdʒɪd]

adj.坚硬的;严格的;固执的

preservation

[ˌprezə'veɪʃn]

n.保存

rejection

[rɪ'dʒekʃn]

n.拒绝;被弃;【医】排异

fluctuate

['flʌktʃueɪt]

vi.波动;动摇;起伏

proportional

[prə'pɔːʃənl]

adj.成比例的;相称的

extinct

[ɪk'stɪŋkt]

adj.灭绝的;熄灭的;消亡的

numerical

[njuː'merɪkl]

adj.数字的;用数字表示的

independently

[ˌɪndɪ'pendəntli]

adv.独立地;自立地

intruder

[ɪn'truːdə(r)]

n.入侵者;干扰者;妨碍者

modification

[ˌmɒdɪfɪ'keɪʃn]

n.修改;修正;修饰

reproductive

[ˌriːprə'dʌktɪv]

adj.复制的;生殖的

manifest

['mænɪfest]

vt.显示;证实;表露

disposal

[dɪ'spəʊzl]

n.处理

conquer

['kɒŋkə(r)]

vt.征服;克服;战胜

machinery

[mə'ʃiːnəri]

n.机械

wool

[wʊl]

n.羊毛;绒线

inferior

[ɪn'fɪəriə(r)]

adj.下级的

stamp

[stæmp]

【1】 n.邮票; 图章【2】v.盖邮戳,盖印章

inorganic

[ˌɪnɔː'ɡænɪk]

adj.无机的;非自然生长的

lapse

[læps]

n.过失;流逝;间隔;堕落;失效

imperfect

[ɪm'pɜːfɪkt]

adj.不完美的;有缺点的;未完善的

past

[pɑːst]

a. 过去的;

tint

[tɪnt]

n. 色彩; 浅色; 染发剂

prey

[preɪ]

n.牺牲者;被掠食者，猎物

hawk

[hɔːk]

vt. 叫卖；吆喝;

liable

['laɪəbl]

a. 有(法律)责任的；有义务的;

botanist

['bɒtənɪst]

n.植物学家

whereas

[ˌweər'æz]

conj.然而;鉴于

ignorance

['ɪɡnərəns]

n.无知;愚昧

unimportant

[ˌʌnɪm'pɔːtnt]

adj.不重要的

offspring

['ɒfsprɪŋ]

n.后代;子孙;产物

caterpillar

['kætəpɪlə(r)]

n. 【定语】履带式...;

silkworm

['sɪlkwɜːm]

n.蚕

inheritance

[ɪn'herɪtəns]

n.遗传;遗产;继承;继承物

disseminate

[dɪ'semɪneɪt]

vt.散播;公开;宣传

pod

[pɒd]

n.豆荚;分离舱

larva

['lɑːvə]

n.幼虫;幼体

wholly

['həʊlli]

adv.完全地;全部地;一概

correlate

['kɒrəleɪt]

vt.使相互关联

converse

[kən'vɜːs]

n.相反的事物;逆命题

hatching

['hætʃɪŋ]

n.阴影;影线

simultaneous

[ˌsɪml'teɪniəs]

adj.同时发生的;同步的

beak

[biːk]

n.鸟嘴;鸟嘴状物体;鹰钩鼻

thickness

['θɪknəs]

n.厚度;浓度;层

hereditarily

[hɪredɪt'rɪlɪ]

adv.世袭地;遗传地

unsuccessful

[ˌʌnsək'sesfl]

adj.不成功的;失败的

cock

[kɒk]

n.公鸡

spur

[spɜː(r)]

n.支脉;刺激;马刺;鞭策

alligator

['ælɪɡeɪtə(r)]

n.短吻鳄

bellow

['beləʊ]

vi. & vt. 咆哮；怒号；(痛苦地)嚎叫;

whirl

[wɜːl]

vt. 使旋转;

mandible

['mændɪbl]

上颚;下颌骨;(昆虫等的)大颚

shield

[ʃiːld]

n.盾

spear

[spɪə(r)]

n.矛;标枪

antic

['æntɪk]

n. 【常用复】滑稽动作，怪相，丑相;

confinement

[kən'faɪnmənt]

n.拘禁;限制;分娩

peacock

['piːkɒk]

n.(雄)孔雀;爱炫耀的人

eminent

['emɪnənt]

adj.著名的;卓越的

childish

['tʃaɪldɪʃ]

adj.孩子气的;幼稚的

carriage

['kærɪdʒ]

n.四轮马车

bantam

['bæntəm]

n.矮脚鸡;矮小好斗者

chiefly

['tʃiːfli]

adv.主要地

protuberance

[prə'tjuːbərəns]

n.瘤;突起;结节

fowl

[faʊl]

n.禽;禽肉

ornamental

[ˌɔːnə'mentl]

adj.装饰的

imaginary

[ɪ'mædʒɪnəri]

adj.想象的;虚构的

swift

[swɪft]

adj.快的;迅速的;敏捷的

hare

[heə(r)]

n.野兔

hunting

['hʌntɪŋ]

n.打猎;搜寻【电子学】速度偏差.

inhabit

[ɪn'hæbɪt]

v.居住于;占据;栖息

mountainous

['maʊntənəs]

adj.多山的;如山一般的

lowland

['ləʊlənd]

n.低地

Pierce

[pɪəs]

v.刺穿;穿透;洞悉

petal

['petl]

n.花瓣

pollen

['pɒlən]

n.花粉

allude

[ə'luːd]

v.暗示;暗指

seedling

['siːdlɪŋ]

n.幼苗

excrete

[ɪk'skriːt]

v.排泄;分泌

stamen

['steɪmən]

n.【植】雄蕊

pistil

['pɪstɪl]

n.【植】雌蕊

habitual

[hə'bɪtʃuəl]

adj.惯常的;习惯的

anther

['ænθə(r)]

n.花药;花粉囊

unintentional

[ˌʌnɪn'tenʃənl]

adj.非故意的;无心的

shrivel

['ʃrɪvl]

v.(使)枯萎;(使)干枯;(使)皱缩

microscope

['maɪkrəskəʊp]

n.显微镜

physiological

[ˌfɪziə'lɒdʒɪkl]

adj.生理的;生理学的

labour

[ˈleɪbə]

n. 劳动;劳动力

advantageous

[ˌædvən'teɪdʒəs]

adj.有利的;有助的;有益的

continual

[kən'tɪnjuəl]

adj.不断的;频繁的;连续的

deviation

[ˌdiːvi'eɪʃn]

n.偏离;出轨;背弃

curvature

['kɜːvətʃə(r)]

n.弯曲;(几何)曲率

corolla

[kə'rɒlə]

n.花冠

exemplify

[ɪɡ'zemplɪfaɪ]

vt.例证;示范

geology

[dʒi'ɒlədʒi]

n.地质学;(某地区的)地质情况;地质学的著作

insignificant

[ˌɪnsɪɡ'nɪfɪkənt]

adj.无关紧要的;可忽略的;不重要的;无用的

excavation

[ˌekskə'veɪʃn]

n.挖掘;发掘;挖掘的洞;发掘物

inland

[ˈɪnlənd]

adj.内陆的, 国内的

infinitesimal

[ˌɪnfɪnɪ'tesɪml]

adj.极微的;无限小的

banish

['bænɪʃ]

vt.驱逐;流放;消除

hermaphrodite

[hɜː'mæfrədaɪt]

n.具有两性者;阴阳人;两性体

reproduction

[ˌriːprə'dʌkʃn]

n.再现;生殖

Knight

[naɪt]

n.骑士;爵士;武士

ample

['æmpl]

adj.丰富的;宽敞的;充足的

accordance

[ə'kɔːdns]

n.一致;符合

breeder

['briːdə(r)]

n.饲养动物的人;培育植物的人;【核】增殖(反应)堆

interbreed

[ˌɪntə'briːd]

v.使异种交配;使混种;使杂种繁殖

utterly

['ʌtəli:]

adv.完全;全然;绝对

adaptation

[ˌædæp'teɪʃn]

n.改编;改编成的作品;适应;【心】(感官)适应，适应性变化

scarcely

['skeəsli]

adv.几乎不;简直不;刚刚;决不

invariably

[ɪn'veəriəbli]

adv.不变地;总是;一贯地

genus

['dʒiːnəs]

n.【生物学】属;类;种;型

contrivance

[kən'traɪvəns]

n.机械装置;发明的才能;想出的办法;人工痕迹

mongrel

['mʌŋɡrəl]

n.杂种狗;杂交动植物;混血儿

Zealand

[ˈziːlənd]

n.西兰岛(丹麦最大的岛)

terrestrial

[tə'restriəl]

adj.陆地的;陆生的;地球的

aquatic

[ə'kwætɪk]

adj.水生的;水中的;水上的

consultation

[ˌkɒnsl'teɪʃn]

n.咨询

enclose

[ɪn'kləʊz]

vt.圈起;围住;附上;封入

fortunate

['fɔːtʃənət]

adj.幸运的;侥幸的;带来幸运的

naturalist

['nætʃrəlɪst]

n.自然主义者;博物学者

perpetuity

[ˌpɜːpə'tjuːəti]

n.永恒;永久;【经】永久年金;【法】永久所有权

indefinite

[ɪn'defɪnət]

adj.模糊的;不确定的;无限期的

exterminate

[ɪk'stɜːmɪneɪt]

vt.消灭;扑灭;根绝

counterbalance

[ˌkaʊntə'bæləns]

n.平衡;平衡力

lessen

['lesn]

v.减少;减轻

overrate

[ˌəʊvə'reɪt]

vt.评价过高;高估;估价过高

retard

['riːtɑːd]

vt.妨碍;迟延;拖住

haunt

[hɔːnt]

vt. 常去，常到(某地);

propagate

['prɒpəɡeɪt]

v.繁殖;增殖;传播;传送

elevation

[ˌelɪ'veɪʃn]

n.提升

endemic

[en'demɪk]

adj.风土的;地方的

incapable

[ɪn'keɪpəbl]

adj.无能力的;不胜任的

concur

[kən'kɜː(r)]

v.意见相同;同时发生

continental

[ˌkɒntɪ'nentl]

adj.大陆的;欧洲大陆的

flora

['flɔːrə]

n.植物;群落(大写)Flora：【罗神】花神弗洛拉; 弗洛拉(女名).

basin

['beɪsn]

n.脸盆;水池

renew

[rɪ'njuː]

v.重新开始;更新

intermittent

[ˌɪntə'mɪtənt]

adj.间歇的;断断续续的

feeble

['fiːbl]

adj.虚弱的;无力的

Geology

[dʒi'ɒlədʒi]

n.地质学;(某地区的)地质情况;地质学的著作

geometric

[ˌdʒiːə'metrɪk]

adj.几何学的;几何学图形的

precursor

[priː'kɜːsə(r)]

n.先驱;前任;前兆;【生物】前体，前驱体

utter

['ʌtə(r)]

adj.完全的;全然的;绝对的

perpetual

[pə'petʃuəl]

adj.永久的;永恒的;一再往复的

Cape

[keɪp]

n.岬;海角

incipient

[ɪn'sɪpiənt]

adj.起初的;初期的

domesticate

[də'mestɪkeɪt]

vt.驯养;使专注于家务;教化

displace

[dɪs'pleɪs]

v.取代;移置;替换

designate

['dezɪɡneɪt]

vt.指定;标明;把...定名为;选派

hopeless

['həʊpləs]

adj.没有希望的;绝望的;无可救药的

augment

[ɔːɡ'ment]

vt.增加

prototype

['prəʊtətaɪp]

n.原型;范例;雏形

diversify

[daɪ'vɜːsɪfaɪ]

vt.使多样化;使变化;使不同

grind

[ɡraɪnd]

v.磨;压迫;碾碎;磨得吱吱响;逐渐停顿

sow

[saʊ]

v.播种;散布

utmost

['ʌtməʊst]

adj.极度的;最大限度的

supplant

[sə'plɑːnt]

v. 取代; 排挤根除

diversification

[daɪˌvɜːsɪfɪ'keɪʃn]

n.多元化

turf

[tɜːf]

n.草皮;泥炭;赛马场;赛马

islet

['aɪlət]

n.小岛

rotation

[rəʊ'teɪʃn]

n.旋转;循环

Flora

['flɔːrə]

n.植物;群落(大写)Flora：【罗神】花神弗洛拉; 弗洛拉(女名).

enumerate

[ɪ'njuːməreɪt]

vt.数;列举;枚举

crude

[kruːd]

adj.天然的;未加工的;粗糙的

generic

[dʒə'nerɪk]

adj.一般的;普通的;共有的;没有商标的

elucidate

[i'luːsɪdeɪt]

v.阐明;说明

marsupial

[mɑː'suːpiəl]

n.有袋动物

incomplete

[ˌɪnkəm'pliːt]

adj.不完全的;不完整的

amplify

['æmplɪfaɪ]

v.扩大;详述;增强

diagram

['daɪəɡræm]

n.图解;图表

perplex

[pə'pleks]

v.使困惑;使糊涂;使复杂化

unequal

[ʌn'iːkwəl]

adj.不相等的;不平等的;不胜任的

diverge

[daɪ'vɜːdʒ]

v.分歧;分叉;【数】发散

horizontal

[ˌhɒrɪ'zɒntl]

adj.水平的;横的

systematic

[ˌsɪstə'mætɪk]

adj.有系统的

simplify

['sɪmplɪfaɪ]

vt.简化;使简单

irregular

[ɪ'reɡjələ(r)]

adj.不规则的;不整齐的;不合法的;不合规矩的

multiply

['mʌltɪplaɪ]

vt.乘;增加

succession

[sək'seʃn]

n.连续;继承权;继位

descent

[dɪ'sent]

n.下降;下坡;家世;血统;侵袭;衰落;继承

earlier

['ɜːlɪə]

adj.早的;初期的

pointer

['pɔɪntə(r)]

n.指针;教鞭;(能指出猎物位置的)猎犬;暗示;主意

prolong

[prə'lɒŋ]

vt.延长;拖延

collateral

[kə'lætərəl]

adj.旁系的;并行的;附属的

diffuse

[dɪ'fjuːz]

vt.扩散

commencement

[kə'mensmənt]

n.开始;毕业典礼

converge

[kən'vɜːdʒ]

vi.聚合;集中;会聚

affinity

[ə'fɪnəti]

n.姻亲;密切关系;吸引力

hitherto

[ˌhɪðə'tuː]

adv.到目前为止;迄今

crust

[krʌst]

n.外壳;坚硬的外壳;面包皮

epoch

['iːpɒk]

n. 时期; 时代; 新纪元; 划时代的大事; 世

superiority

[suːˌpɪəri'ɒrəti]

n.优越性;优势

familiarity

[fəˌmɪli'ærəti]

n.亲密;熟悉;精通;不拘礼节

twig

[twɪɡ]

n.小枝;嫩枝

ramify

['ræmɪfaɪ]

v.分枝;(使)分枝;(使)分叉

decay

[dɪ'keɪ]

v.(使)衰退

straggle

['stræɡl]

v.分散;蔓延;散乱分布;掉队

ramification

[ˌræmɪfɪ'keɪʃn]

n.分叉;衍生物，后果;支流;结果

simile

['sɪməli]

n.直喻;明喻

overtop

['əʊvə'tɒp]

vt. 高耸 ... 之上; 凌驾; 超出

overmaster

[ˌəʊvə'mɑːstə]

vt.压倒;征服;打胜

subordinate

[sə'bɔːdɪnət]

adj.下级的;次要的

Lepidosiren

[ˌlepɪdəʊ'saɪərən]

n.【动】 南美肺鱼

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