生物學
1859
物種起源
生命的浩瀚多樣，源自共同祖先，經由自然選擇這把緩慢的篩子塑造而成。
Choose your version
生物彼此有差異，而那些恰好最適應自己世界的，會留下最多的後代——讓它持續幾百萬年，你便得到整棵生命之樹。
核心想法
在任何一群動植物中，沒有兩個個體是完全相同的，而其中一些差異會傳給後代。由於產下的幼體遠多於能夠存活的數目，對食物、空間和安全，存在著一場無聲而持續的較量。那些性狀稍稍更適應環境的個體，往往活得更久、繁殖得更多——並把這些有用的性狀傳下去。達爾文把這把篩子，稱作自然選擇。
讓它運轉成千上萬代，微小的變化便累積成深刻的改變。一個種群可以漸漸變成一個新物種；一個祖先形態可以分枝成許多種。沿著每一根枝條往回追溯得足夠遠，牠們終將相遇——地球上所有的生命都彼此相關，就像同一棵參天大樹上的細枝。
它是如何誕生的
年輕的博物學家達爾文，曾搭乘「小獵犬號」環球航行近五年，一路所見的生物——尤其是在加拉帕戈斯群島，每座島上都有同一主題的不同變奏——在他心裡埋下了一顆懷疑的種子：物種或許並非固定不變、各自被創造。回到家中，他又用了二十多年，默默積累證據、飼養鴿子、與各地博物學家通信，遲遲不願發表這個過於驚世駭俗的想法。
1858 年，一封來自阿爾弗雷德·拉塞爾·華萊士的信逼得他不得不動——華萊士獨立地想到了完全相同的機制。二人的工作被聯合發表，而 1859 年，達爾文出版了《物種起源》。首印當天即告售罄；這套論證所重塑的，不僅是生物學，更是人類對自身在自然中位置的理解。
它為何重要
在達爾文之前，生命那令人眼花繚亂的多樣性，通常被解釋為固定不變、各自被設計出來的。而他表明，這一切其實可以源自一個單一的、盲目的、自然的過程，在極其漫長的時間裡運作——並且，含蓄卻無可辯駁地指出：人類只是這同一棵大樹上的一根枝條，而非置身其外。這是人類有史以來最具影響力的想法之一。
一個可以想像的畫面
想像一位育種者，挑選讓哪些狗或鴿子交配，一代代地慢慢把某個性狀放大，直到一頭狼的後代變成一隻吉娃娃。達爾文的洞見是：大自然做的正是這件事——只不過那位「育種者」，不過是環境本身，它只是悄悄地讓更適應的個體留下更多後代。沒有誰在挑選；那「挑選」，只是存活自己做的事，而它所用的時間，遠比任何育種者擁有的都要長。
它的位置
《物種起源》問世幾年後，一位默默無聞的修士格雷戈爾·孟德爾，弄清了性狀究竟是如何傳遞的——以一個個離散的單位，也就是我們今天所說的基因——但他的工作被冷落了數十年。當它被重新發現時，恰好補上了達爾文的理論一直缺失的那塊遺傳拼圖，兩個想法於是融合成了現代演化生物學——它正是從疫苗到瀕危物種保育、背後的那套框架。
Original source text
引言
When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species — that mystery of mysteries, as it has been called by one of our greatest philosophers.
In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species.
I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species. … Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.
生存競爭
A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product.
It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage.
As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected.
自然選擇
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?
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.
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.
理論的難點
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; … then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
複述與結論
It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
Down, Kent · 1859