Biology 1859

On the Origin of Species

Charles Darwin

Life's vast variety arose from common ancestors through the slow sieve of natural selection.

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In depth · the introduction

Living things vary, and the ones that happen to fit their world best leave the most offspring — repeat for a few million years, and you get the whole tree of life.

The big idea

Within any group of plants or animals, no two individuals are exactly alike, and some of those differences pass to their young. Because far more young are born than can ever survive, there is a constant, quiet contest for food, space and safety. The individuals whose traits fit their surroundings a little better tend to live longer and breed more — and they hand those helpful traits on. Darwin called this sieve natural selection.

Run it for thousands upon thousands of generations and the small changes pile up into something profound. A population can drift into a new species; one ancestral form can branch into many. Follow every branch back far enough and they meet — all life on Earth is related, like the twigs of one enormous tree.

How it came about

As a young naturalist Darwin spent nearly five years circling the globe aboard HMS Beagle, and the creatures he saw — especially on the Galápagos Islands, where each island carried its own variation on a theme — planted a doubt that species were fixed and separately created. Back home he spent over twenty years quietly amassing evidence, breeding pigeons and corresponding with naturalists, reluctant to publish so explosive an idea.

In 1858 a letter from Alfred Russel Wallace, who had independently hit on the very same mechanism, forced his hand. Their work was presented jointly, and in 1859 Darwin published On the Origin of Species. The first printing sold out at once, and the argument reshaped not just biology but how humans understood their own place in nature.

Why it mattered

Before Darwin, the dazzling variety of life was usually explained as fixed and separately designed. He showed it could instead emerge from a single, blind, natural process running over immense time — and, quietly but unmistakably, that humans are one branch of that same tree, not standing outside it. It is among the most consequential ideas anyone has ever had.

A way to picture it

Think of a breeder choosing which dogs or pigeons to mate, slowly exaggerating a trait over generations until a wolf's descendants become a chihuahua. Darwin's insight was that nature does exactly this — except the “breeder” is simply the environment, quietly letting the better-suited individuals leave more offspring. No one is choosing; the choosing is just what survival does on its own, over far more time than any breeder ever has.

Where it sits

A few years after the Origin, an unknown monk named Gregor Mendel worked out how traits are actually passed on — in discrete units we now call genes — but his work sat ignored for decades. When it was rediscovered, it supplied exactly the heredity Darwin's theory had been missing, and the two ideas fused into modern evolutionary biology — the framework behind everything from vaccines to the conservation of endangered species.

The original document

Original source text

From the Introduction

Charles Darwin · On the Origin of Species by Means of Natural Selection · 1859 · Introduction

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.

The Struggle for Existence

Chapter III · The Struggle for Existence

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.

Natural Selection

Chapter IV · Natural Selection

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.

Difficulties on Theory

Chapter VI · Difficulties on Theory — organs of extreme perfection

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.

Recapitulation & Conclusion

Chapter XIV · Recapitulation and Conclusion

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