The Origin of SpeciesAge-adapted version

The book came out quietly, without any noise from its author. But the world made noise for him. People who usually talked about Italy and volunteer armies now talked about species. Some read it from cover to cover. Others just enjoyed having an opinion.

Some priests shook their heads gently. Others shouted without knowing the matter. Old ladies and gentlemen whispered that it was dangerous.

Learned people looked through forgotten books to say the author was probably no better than an ape himself. But those who thought calmly and philosophically called the book a new and powerful tool in the workshop of thinking. All naturalists who had been paying attention understood that it announced a new time in natural history.

Still, such a book is not easy to read. Not because the language is unclear, but because the material is heavy with facts. The author had traveled far, collected patiently, examined animals, plants, small details in bodies, layers of rock, islands and continents. He knew so much that in a short book he had to press the material together like dried travel provisions.

Each slice was thick with nourishment, and you had to chew slowly.

The lack of space forced him sometimes to take things for granted. Someone who gets scared of new words or is not used to looking at nature through a magnifying glass might think that things are just assumed here. But someone who has trained their eye a little finds more evidence than they thought, just packed tight together.

When you turn toward nature, you learn to wonder less and admire more. Among all that is remarkable, the development of a living creature from an embryo still stands as one of the most wondrous things. Imagine a newly laid salamander egg, a small, clear, round thing that shows nothing but a thin membrane and a sticky, almost transparent fluid with tiny grains inside.

There the possibilities lie, hidden and still. Then warmth comes, not too strong, just steady and mild, and the mass begins to divide, again and again, as if an invisible hand with a very fine spatula were dividing clay into smaller and smaller pieces to build something fine.

The grains become many and small enough to build the finest bridges and beams in the animal's body.

Then it looks as if an unseen finger draws a line for the backbone, pinches the mass a little and shapes a head here, a tail there, a back, a belly, a hint of arms and legs. If you watch long enough, hour after hour, it is easy to feel that an artist outside your field of vision is working according to a plan.

When the little amphibian swims in the water, something similar continues. Food particles settle where they should, so the animal grows into the shape, color and size that marked its parents. If a foot or a tail gets bitten off, it often grows back, and always in the salamander's own way. What is replaced does not suddenly become like a frog's foot by chance.

The same pattern is found in the oak that grows from an acorn into a tree of the same kind as the one the acorn fell from; in the humble lichen spore that lays a new gray-green coating just like the old one.

We hardly expect anything else, and that is why we call a child who does not look like either mother or father a strange exception.

All this points to one first, great law: Offspring resemble their parents more than anything else. That is nature's habit. We do not yet know everything about why it is so. But we know that the phenomena of life are not separate from other physical phenomena, and that matter and force work here too, no differently than in the pull of stars and the flash of lightning. So living bodies must follow the same laws as other matter. We lack the details of how. But that there is a current that drives offspring in the direction of the parents' form is safe to say.

At the same time we see something else, and it is at least as important: The resemblance is never complete. There is always a small difference. The offspring of two parents does not fall exactly in the middle between them, but lies somewhere around, a little to the side. Such small differences are not hard to understand, when you think about how many forces work together in a body, and how small the chance is that the sum will be exactly the same each time. The habit of resembling mother and father thus stands side by side with a weaker, but real, drift to vary a little.

Most often these deviations are small. But once in a while they strike with much greater force, and then we call the offspring a variation or a sport. Often we do not know exactly how such sports arose. Two stories will make it clear.

The first is about a farmer by the Charles River in Massachusetts, Seth Wright. He had a flock of fifteen ewes and one ram, all of the common type. In 1791 one of the ewes gave birth to a male lamb that was very different from both parents: The body was long compared to its height, and the legs were short and bandy. The lamb could not jump over fences like the others, which tested the farmer's patience. The little ram was called the Ancon or Otter sheep, and it would soon get a name far beyond the farm's fences.

The second story is from Malta. A married couple, Kelleia, with completely ordinary hands and feet, had a son, Gratio, with six fingers on each hand and six toes on each foot. He could use the extra fingers, but on the feet they were not quite as good. No one could point to any reason in the family or surroundings that could explain this.

Two things are worth noticing. In both cases the variation came with a leap, not slowly and gradually, but suddenly complete in one birth: The short, bandy-legged ram stood there, and Gratio stood there with six fingers. In neither case could anyone point to climate, food or other outside causes. They just appeared, as much in nature does, without us seeing the cause. And what would someone who always looks for purpose say about what higher goal the short Ancon legs or Gratio's extra fingers served?

So we know that variations can appear without us understanding why. Many probably appear this way, even if sometimes outside conditions really can affect, for example giving plants more petals at the cost of stamens, or making fur thicker. No matter the origin, this is most important: Once a variation exists, it generally follows the same law of inheritance as all other life – like gives like. Descendants usually carry the same deviation with them. Often it seems that a newly appeared variation is especially strong in inheritance, as if it has an advantage over the normal.

We see this clearly in Gratio's family. He married a woman with five fingers and five toes. They had four children. The eldest, Salvator, had six fingers and six toes like the father. The next two had five fingers and five toes, but one with slightly deformed hands and feet.

The youngest, Marie, had five fingers and five toes, but the thumbs were crooked. When these children married completely normal spouses, something strange happened. Salvator had four children, three with six fingers and toes, and one that was completely five-fingered. Here the six-fingered trait had somehow skipped one generation, but came back in full strength, even though both grandparents in that pair had been five-fingered.

With Marie, who herself only had crooked thumbs, a son appeared with six toes, while three other children were normal.

With her brother George, the one with small crookedness, came two daughters with six fingers and toes, then a daughter with six fingers on both hands and six toes only on the right foot, and finally a son with five fingers and toes. The brother who was completely ordinary, Andre, had many children, all with five fingers and toes. All in all, we see how such a variation jumps back and forth, sometimes skipping a generation, but always having enough power to show itself in full strength.

If something as deviant as six fingers can hold on like this, it is no wonder that smaller deviations hold even better. That is why the story of the Ancon sheep is especially useful. The neighbors, quick and practical, suggested that Seth Wright should slaughter the old ram and only use the short-legged one. The result was just as expected.

Lambs that were born were mostly either pure Ancons or completely ordinary. When Ancons had lambs with each other, the lambs were always pure Ancons. Only one possible exception was mentioned in writing. This is a rare but clear example of a distinct race appearing in one leap and at once breeding "true."

Also when Ancon was crossed with ordinary sheep, it did not produce blends, but either the one or the other.

By choosing two Ancon parents every time, it became easy to build up a strong race. They even kept to themselves when they ran with others. Probably this race could have continued forever, but better wool and meat from another breed, the merino, made people stop caring about the short-legged ones. Soon they were gone, and many years later it was hard to find a single specimen.

Gratio did not become the founder of a family of six-fingered men, the way Seth Wright's lamb became the founder of a whole nation of short-legged sheep. Not because the one variation was weaker in itself, but because the choice of partners was different. Seth Wright kept the Ancons together. Gratio's sons could not marry their sisters, and the grandchildren did not choose their six-fingered cousins.

In the one case, systematic, conscious selection was carried out over several generations; in the other, it was not. So no fixed race of humans with six fingers was established. A race is basically a continued variation that has been maintained through selection. When both parents show the same deviation, the chance increases that the children will pass on the same.

Is there any part of an animal that cannot vary? From what we can see, no. And when a part does vary, the variation can be inherited. If you choose who gets to have children with whom, it can become common in a family. Animal breeders have known this for a long time.

The whole English work of improving domestic animals over the last hundred years is built on it. Color, shape, size, the texture of hair and wool, the body's proportions, how easily the animal puts on weight or how hardy it is, how much milk it gives, running speed, strength, temperament, intelligence, special instincts – all this can be influenced and inherited.

People who work with horses, dogs, cows and poultry have not waited for philosophers to explain it. They have seen it happen. Even disease can follow. A skilled doctor, Brown-Séquard, showed that epilepsy that was artificially caused in guinea pigs was passed on to their offspring.

But a race, once made, is not forever fixed. New variations can arise within it. If you take them further through selection, new races can again emerge. So you can, given enough time and focused work, produce an astonishing number of races that come from the same ancestor, so different that we would almost believe they were different species if we only saw each one separately.

A wonderful example is the pigeon family. Careful studies have shown, with good reason, that the wild rock pigeon is the ancestor of all our domestic pigeons. Yet there exist more than a hundred clearly distinct domestic races. Four main types stand out: tumblers, pouters, carriers and fantails. They differ not only in size, color and habits, but in the shape of the beak and skull, the relationship between the length of the beak and the skull, the number of tail feathers, how big the feet are, whether a gland in the back exists or not, and the number of vertebrae. Exactly such traits often separate natural genera and species of birds.

The strangest thing is that all these race differences cannot be traced back to different living conditions. Pigeons are usually kept and fed quite similarly. Yet they have turned in widely different directions. Here it is especially worth mentioning that some with high authority have thought that only traits connected to bones where muscles attach really vary. The pigeon tells the opposite: The skeleton of the wings has hardly changed, while the number of tail feathers, the length of the beak and skull, and the number of vertebrae have varied greatly – traits that muscle force does not control in the same direct way.

Here we meet a tangle that has bothered many who want to define "species." If you look at appearance and build alone, you could without hesitation call the tumbler and the pouter two different species, if you had only found them as fossils or gotten their skins sent home as wild birds from a foreign land. They are "good" species in that form of definition that only uses form and build.

But if you look at kinship and origin, they are not two species. They come from the same rock pigeon.

Put another way: The physiological definition of species – "all who descend from the same original ancestor" – does not hit the same boundaries as the morphological one – "all who resemble each other thus and thus in form and build." And because races from the same ancestor can so often become so different in form that they resemble species, the question becomes pressing: Are species really nothing but races that have been allowed to live and split long enough?

Many have pointed to a possible way to separate races from "true" species: Try to cross them. If they manage to get fertile offspring together? If two clearly different forms get no offspring at all, or if the offspring is sterile, one usually says they are different species. If they get fertile offspring, they are only races of the same species.

There is some truth in this. We see that races we know are made through selection, like spaniel and greyhound, or draught horse and Arabian, or tumbler and pouter, without difficulty have offspring together, and that these mixtures, when they in turn have children with each other, are as fertile as any.

At the same time we know examples where two different natural species do not get fertile offspring. The horse and the donkey produce mules, but we have no certain documentation that two mules have produced a live foal together. The rock pigeon and the ring dove also seem to be infertile together. That is why many physiologists said: Here you have the touchstone! Find out whether two forms get fertile offspring together. If yes, they are races. If no, they are species.

Unfortunately, this test can seldom be used cleanly and clearly. Many wild animals stop breeding altogether in captivity, even with their own females. That they do not mate with another species in a cage means little if they will not mate at all. Many wild animals cannot even tolerate each other if one is tame and the other wild.

With plants it is not easier. Most are hermaphroditic; it is hard to prevent their own pollen from reaching, and just as hard to ensure that foreign pollen works when it should. And even if you succeed the first time, you must follow the offspring for a long time to see if it in turn is fertile.

And when you have managed all this, the answers can still be confusing. There are plants that are more fertile when they get pollen from another species than from their own. There are also cases, as with certain algae in the sea, where males from one species can fertilize the eggs of another, while males from the other species cannot do the opposite.

Two researchers who cross the same two species each in their own direction can thus end up with different conclusions.

Some forms that many suspect are only races behave almost sterile when crossed. At the same time we know of both animals and plants that have always been regarded as different species, but turn out to get fertile offspring if you only manage to get them to mate. There is also no safe connection between how similar species are in build and how fertile they are together.

So the picture becomes more nuanced. The author who gathered these experiences concluded roughly like this: First crosses between forms that normally get the status of species, and their hybrids, are often, but not always, sterile. The degree of sterilization varies greatly, sometimes so much that two thorough researchers who have done the same experiments still end up with quite different interpretations of the same type of forms.

Sterility is a trait that even individuals can show in slightly different degrees, depending on conditions. It does not follow neatly and politely systematic relationships, but is governed by special laws that we still do not fully understand. It can even be completely different in the two opposite crosses of the same species.

Think of the art of grafting, when you put a branch from one tree onto another. That two kinds of trees "fit" is a detail in how their green living tissue responds to each other; we do not know all the reasons. In the same way, it is perhaps accidental differences in the reproductive system itself that make it easier or harder for two species to have offspring together. It is therefore not necessary to believe that someone has stamped species with different doses of "sterility" to prevent them from mixing. Just as no one has stamped trees to prevent them from growing together in the forest.

Sterility in the first cross is caused by several things, often that the embryo dies early. Sterility in hybrids, which have a mixed nature and sometimes disturbed whole organization, resembles the sterility we see in pure species when living conditions are suddenly changed for them. Studies also show another parallel: Small differences between parent forms can give children that are healthier and more fertile than the parents, and small changes in living conditions can also give increased vigor.

That there is some correspondence between how difficult it is to get a first cross to succeed, and how sterile the subsequent hybrids become, is not surprising.

Both depend on a certain "amount of difference," but they rule at different stages of the life cycle. Nor is it strange that how easily something "takes" in grafting, how fertile a first cross is, and how fertile the hybrids are, often keep some pace with how close the forms are in classification. Classification tries to gather all kinds of similarities we see.

Varieties of the same species, whether they arise in nature or in the barn, are very often – but not absolutely always – fertile when they cross, and their children are also fertile. This is not surprising, if you remember that most variations that people pick out in animal breeding or gardening are external – color, shape – while the reproductive system itself usually remains untouched. And in everything except fertility, mongrels (offspring of race crosses) and hybrids (offspring of species crosses) resemble each other.

Nevertheless, one fact stands firm, and any explanation of the origin of species must include it: In nature there exist groups of animals and plants where the members cannot get fertile offspring with members of other groups. And there exist hybrids that are completely sterile among themselves. If such things occurred only twice in all of life, any theory would still have to explain them. If it cannot, it limps.

When we gather the threads, we can say this:

Living beings can be divided into many clearly separated kinds when we look at form and build. Those are the morphological species. They can also be divided into groups that initially get fertile offspring with each other and tend to make children that resemble themselves. Those are the physiological species. Children most often resemble parents, but slip a little, and if humans or nature choose certain of these small deviations over many generations, it becomes a race – and such a race can to a high degree look like a species in form and way of life.

But no one has yet certainly shown that a race, when it crosses with another race of the same original species, shows the same deep sterility that many species do when they cross with each other. On the other side, it has also not been proven that all species, without exception, produce completely sterile hybrids among themselves. On the contrary, much suggests that there are whole grades of fertility out there, from zero to a hundred.

From the earliest times, humans have tried to explain how everything came to be. Some pointed to the Nile's mud, others to a primeval egg, some to the will of a god that resembles humans. Today we smile at many old myths, most have long since settled into rest. But one ancient idea, written down in an old country by writers whose names and times no one knows anymore, has in a strange way persisted.

Many hold it sacred and use it as a measure for everything new, also about the origin of species. The story after Galileo tells of researchers whose lives were damaged by nose-to-the-ground readings of old texts. The world has punished them, but the cause of research was not diminished.

For again and again it has shown itself like this: When knowledge and old dogmas stand in pure conflict, the old thoughts must sooner or later give way. That does not mean that those who believe must stop believing. It means that if you want to know how nature works, you must let nature's own signs be the highest judge.

In our matter, two opposing explanations remain when we have set aside old ideas and empty speculation. One says that each species was made as it is, by a special creative act, without having come from any other form. The other says that species grew from earlier forms, which in turn came from others, in a long train, by natural reasons and forces, exactly like those that also today create variations and races.

This last thought also makes it probable, but not absolutely necessary, that all life began in one or a few original forms. Where these original forms themselves came from, this thought does not need to answer. They may have been created; they may have arisen from lifeless matter according to nature's laws.

If you want to hold on to the first, the special creation, it is tempting to use Greek and Hebrew myths as support. Strangely enough, today's "special creation" in academic dress does not fit properly with the old Hebrew story of the world's beginning. But more important than that: It fits poorly with what we see in rocks and earth.

Geology has made one clear point: The animals and plants of the past do not lie in strictly divided layers, where nature would mysteriously have stopped completely and then started all over again. There are no great "cracks" in time's layers, no sharp boundaries where everything shifts from plants to water animals to land animals in collected pulses. Every year one finds more intermediate links, new strata that bind older times together.

The layers that connect ice age deposits with old Tertiary layers; the layers at Maastricht that bind the Tertiary period to the Chalk; the rocks at St.

Cassian where animal life from the Mesozoic and Paleozoic share space as neighbors in a time period that was previously thought to be poor in life. And geologists argue annually about where a given layer belongs, whether it is younger or older, because the boundary is flowing.

An independent researcher, M. Pictet, calculated this: How large a share of the genera in a given layer do we find again in the layer below? He found that it was never below one third, 33 percent. The smallest inheritance was at the beginning of the Mesozoic, in the Triassic. Often the commonality was much higher: sixty, eighty, yes as high as ninety-four percent.

At the same time, many small parts of a layer – take the Lias, for example – have completely unique species that are only found there.

In a vertical section of about thirty meters, one can find a dozen different ammonites at specific heights, and none of them go up or down to neighboring zones. If you hold on to special creation, you must then admit that the creator intervened repeatedly, at times corresponding to the thickness of each such layer, to make a new ammonite. That becomes a heavy thought.

And you gain little from it. For if one once believed that creation went as in an old book, the teaching that today is used by some learned people to save the idea is already far from the original text. It gets no help from good science either. Often one line of reasoning is presented: "If species were not created that way, we do not understand why the eye is as it is, or how an instinct could come to be."

This is a challenge to ignorance: "Accept my explanation, or be uncertain!" But saying that something was created exactly so explains nothing really. An explanation only becomes real when you can show that the phenomenon is an example of a natural law. An "override from above" neither follows nor shows such laws. If species came to be in this way, we cannot investigate or speak sensibly about the causes.

Can we ever have so much knowledge that we can rightly say: This could not have happened by nature's causes? To be sure, we would have to know all possible combinations of forces through infinite time and all their consequences. We do not. As long as we do not know, it is better to try natural hypotheses than to resort to those that only put a stamp on ignorance.

Then the first hypothesis is not only poor in content; it also looks young and unfinished, in light of how science has always matured. In astronomy, the stars sang, and the planets were led by heavenly hands, people believed. Now they follow the law of gravity, and schoolchildren can calculate the path of a flying stone with the same rules that govern a planet's orbit.

Lightning was once God's messenger, but today it is humanity's messenger as it rushes through wires. We know that every flash in the evening sky follows specific conditions, and if we knew everything, we could have calculated it.

Large trading houses rest on statistical laws that govern what first looks random in human lives. Plague, famine and disease follow causes we largely control; they are not necessary punishments.

Everywhere science has won ground, the picture has become this: A quiet, continuous order that changes slowly, thread by thread, without jumps in the fabric of matter and force. Shall biology alone stand outside and wave in the face of this picture?

Moreover, there exist other strong signs against special creation. If you look at a map, you find the strange distribution of species. The sea on each side of Panama's narrow strip of land has completely different species; islands in the middle of the ocean have their own animals and plants, yet many resemble those found on the nearest mainland. If you look in time, you find that the mammals in the youngest Tertiary layers in the Old and New World mostly belong to the same genera as those living now in the same areas.

Crocodiles from the oldest layers of the Mesozoic look like today's, but with small differences in the backbone, in the nasal passages, and in some other details. If you look at development, all members of a large group go through similar stages in embryonic life, and when they are adults, their parts are arranged according to the same plan. If you look at the human family, humans are more like gorillas than gorillas are like lemurs.

If you look at "useless" parts, you find that guinea pigs have milk teeth in the embryo that never break through and never chew, as if they carry an old habit they do not use.

In the female dugong, the tusks lie hidden and never come out. Why do such things exist if everything was made to be useful and perfect in itself?

If you answer "because it pleased the Creator" to all this, you have not given an explanation. You have only put a label on a chest where we wish to look inside. All these patterns ask for a natural explanation that shows kinship, repetition and traces of history, not an outside will that we cannot investigate. That is what makes the explanation that species are specially made for each place and each purpose, one by one, seem hollow.

Long before our time, some thought in this direction. A Frenchman who lived as a consul in the Mediterranean, Benoît de Maillet, traveled and thought at a time when modern natural science was young. He kept much to himself until he was old. Only then did he let his thoughts slip out, that water had once covered the whole earth, laid down the rock masses on the bottom and the mountains, and then withdrawn gradually.

He imagined that when land appeared, animals that lived in the water could go onto land and slowly change, through many generations, into land animals and even air animals.

He also saw, with clear vision for his time, that when we explain the past, we must use the causes we know from the present. Currents, deposits, erosion, time – all this works on the earth today; it worked before too. He thought that life's forms also followed such laws, even though he did not know exactly how.

After him came Jean-Baptiste Lamarck. He saw a continuous scale in life, though with branches and unevenness. He thought that organs grow when they are used, and shrink when they are not used; and that such changes are inherited. If an animal's way of life changes, the body also changes through many generations, and thus new species come to be.

The surroundings then create new "needs," which in turn change which parts of the body are used. This way he wanted to start all life in water, through a kind of spontaneous generation, and from there explain everything.

But Lamarck put too much weight on use and disuse. He overlooked that plants, which do not have "will" and actions in the same way, still change; he did not fully share the thought that species could die out on a large scale; he knew nothing about the hard competition that constantly happens between individuals and species; and he did not think that natural selection could choose or "sift" small differences. His weight was wrongly distributed, and many of his ideas were later easy to strike down.

Then a new sound came. Two men, each on their side of the world, listened to nature in their own way, but heard much of the same. One, Alfred Russel Wallace, collected and looked at animals and plants in the island world between Asia and Australia. The other, Charles Darwin, had for years filled books with notes, read, collected, interviewed, bred pigeons, and thought.

When Wallace sent his idea to Darwin, Darwin found that it resembled the seed of something he himself had been working on for twenty years. He did not hesitate, but admitted the resemblance. Two friends, Lyell and Hooker, helped make it fair. In 1858, Wallace's text and a short account from Darwin were read on the same evening. A year later came Darwin's great book.

What was the core? That variations exist; that many of them are inherited; that there are far more newborns than adults that survive; that those who have small advantages in their environment more often survive and have children; that the sum of small selections over long periods of time shapes a race; and that races, after even longer time, can become so different that they are species. The process that does this in nature, he called natural selection. It resembles what breeders do when they choose parent animals, except that in nature cold and heat, lack of food and enemies and disease do the same job.

How did he investigate this? Not by picking out one bird or one plant and looking only at it, but by gathering large, secure facts, explaining them soberly, and seeing whether one and the same thought could carry them all. He used what many philosophers call a mixture of induction, deduction and testing. He first gathered facts about variation, inheritance, competition and time.

Then he reasoned: If species really arise through selection of inherited variations, what should then happen with distribution in space and time, with similarities and differences in appearance, with the riddles of embryonic development?

Finally he looked in nature's book: Do the predictions match? In many places they did. In other places we still fumble. But no one went further than him in trying to make everything fit together without calling on invisible hands.

Is everything then proven, point by point, as in mathematics? No. One weakness that the author's strictest critics have pointed out is that no one has yet shown completely clearly that two races, formed through selection from the same ancestor, have become so different that they are sterile with each other, as many species are. Races with all kinds of "species-like" characteristics have been bred, but infertility between them has not been decisively established.

That he himself pointed out this weak point says much about his honesty. Much suggests that such infertility could arise if one tried long enough, and some experiments point in the direction that reproductive ability is sensitive to small, composite differences. But we still lack the pure example.

Other objections fall more easily. Some say that natural selection is not "proven," but that it is just said that it "must" happen. But what else could one expect? No one can stand in the forest for a thousand years and watch every bird and every seed. It must be thought about from numbers and laws.

Others say that because humans in breeding choose with thought, nature cannot do the same. That is wrongly thought.

Sifting is not something that requires consciousness. If a violent rain shower falls over a salt-sand mixture, the salt is washed away and the sand remains – much more easily than a human with tweezers could have done. In a similar way, nature's hard conditions remove a little more of the form that tolerates least, and let more of the other remain. That is not thinking. It is just lawful force at work.

Some also complain that we do not find "intermediate forms" everywhere. But precisely if natural selection has worked over a long time, and if species break up into many branches that each go further and further, most intermediate forms will either have been outcompeted and died out, or mixed into something else, or hidden in layers we cannot reach. Moreover, the "ancestral form" does not have to look like something halfway between the two children. The rock pigeon that gave both tumblers and pouters is not a bird that is half tumbler, half pouter. It is itself. The two children are each their own extreme in other directions. That insight unties many knots.

That nature never leaps, people have liked to say. "Natura non facit saltum." But nature does make leaps now and then. The bandy-legged ram and Gratio with six fingers stood there, whole at once. That nature sometimes jumps does not prevent the whole from being gradual. It only reminds us that both small steps and rare jumps exist in the dance.

When all is said, the picture stands like this:

– Children resemble parents. Small deviations arise. Sometimes large ones arise.

– If you choose who gets to have children with whom, you can make small differences large, and large differences stable.

– Thus humans have made races. Prickles in pigeons' tails, long necks in pouters, small beaks in tumblers; whole rows of animals in the barn.

– In nature, a selection happens without human hand. Hard winters, competitors, enemies, lack of water and food, disease – all these press constantly.

– Over very long time, the selection of inherited variations can create differences so large that we call them species.

– Some species do not get fertile offspring when they cross with others. Such things are important and must be explained.

– The hypothesis that species were made each separately explains little. It only puts a period without a sentence.

– The hypothesis of natural origin explains much, ties together signs from rock and sea, from forest and heart, and keeps the door open for new knowledge.

Let us go back a couple of rounds to the examples often mentioned, to see how they shine in this picture.

Horse and donkey produce mules. Mules are usually sterile in both sexes. This has been used as a sign of deep species boundaries. But the same world shows us spaniel and greyhound giving mongrel dogs, healthy and fertile, and their children in turn are fertile. Pigeon breeds that we know come from the same ancestor cross freely and get rich flocks of children. Plants sometimes prefer foreign pollen over their own. In some algae, only one cross direction works, not the other. So fertility and sterility point more to hidden, often small differences in the reproductive system than to one fixed law.

In the rocks, the form of life flows from layer to layer. Some traits die out. Others live on almost unchanged. Pictet's calculation, that never less than a third of the genera in one layer are also found in the time layer below, and often almost all, is a powerful reminder. Species come and go, but life's plans glide, not jump, even though nature sometimes has taken leaps.

The ammonites in the Lias – each species tied to its zone as if an invisible line in the chalk held it fast – could be explained by countless, repeated acts of creation. But it makes the mind tired to imagine that. Slow change, selection and time are more like a steady pen that explains a line, even though the pen sometimes stops and starts suddenly.

Unused, or rather "rudimentary," organs are like forgotten notes in the body. The guinea pig's milk teeth that never see the light of day, the female dugong's tusks that never break the gum, lie there as weak shadows from a past. If everything was made in one sweep to fit exactly as it is, why do these exist? And why should all mammals, birds and reptiles be built according to roughly the same plan in arm and leg, if not because they share an old stem and many ancestors?

Look also at kinship among living animals. Humans resemble gorillas more than gorillas resemble lemurs. This does not say everything about mind and worth, but it says something about the building plan in the bones, in the hands, in the ribs.

Finally: For many naturalists today – and surely more tomorrow – Darwin's thought is not just the best we have. The way he showed how to bind together large amounts of scattered facts under a few clear lines has changed not only natural science, but also how many think about the world. Even if some parts of his path may one day have to be redrawn a little, as Kepler and Newton did for Copernicus's circles, the work remains. Many chapters in his book – on variation, on the struggle for existence, on instinct, on hybrids, on gaps in the earth's memory, on the earth's geography – have almost no equal in power.

And the way forward? Perhaps someone will show, in a laboratory or a large enclosure, that races over time can get such a deep difference that they become sterile with each other. Perhaps there are phenomena in the life of species here and there that cannot be bent under natural selection alone. Then someone will add small, new lines.

But the direction is probably right. That children resemble their mother and father, and yet never completely, that some of these small differences become favored, that the sum of many such small inclinations points the way up through time – all this are beams that bear, not only in this house, but in many other thoughts about nature.

When you close the book and look out the window, you can think of the little mass inside the salamander eggshell. The hidden artist we first think we see steps back for another picture: A lawful, seething world, where life's great builder is the sum of small actions, millions multiplied by millions, through the frame of time.

And in this frame, new pages are written every day – in gardens, in barns, in rooms where pigeons puff up and court, on pastures where lambs sprint and stumble on short or long legs, in mosses on stones along streams, and in wetlands where a young amphibian lifts itself from a grainy lump to a body plan of beautiful order.

Not one place is a magic word needed to make the piece fit together. For even where we lack a link or a piece, we know that nature does not close the door to questions. It carries its own answers, slowly, in the same forces that hold stars in place and seeds in the ground. In this light, the question of the origin of species is not a closed gate with a guard who says "it is so."

It becomes a gate standing ajar, which you can push a little more open each year. And what glimmers through becomes clearer: kinship, change, testing, selection, and the form that grows forth, not according to an outside drawing, but as a melody in nature's own language.

Therefore you still hear cooing from dovecotes all over the world, and far away, calls from mountains where goats and sheep trudge. Somewhere in a valley there once was a farmer on his knees lifting a lamb with short, crooked legs. He shook his head and had no idea that he held something that would teach the world a part of nature's secret book.

And on an island in the Mediterranean, a young boy stretched out six fingers to reach, and some years later someone wrote down what they had seen, with a touch of wonder.

In this light, no birth becomes too "strange" and no feather too "odd." They become only new syllables in a story that has flowed for a long time, which we still understand a little more of, the more we choose to look accurately, count correctly, and let small facts build big thoughts.

But thoughts alone are not enough. They must be related to what we can measure, weigh, cut, and count. That makes the difference between a fine tale and wise science. The language of old tales will always exist, but in nature's hall, daily life steps forward in another way. When we go out into the day, we know that none of this diminishes the awe for the world.

On the contrary, wonder becomes deeper, and respect greater. For what can be investigated and still be beautiful is stronger to rest in than what can only be confessed. It also makes it easier to be patient when pieces are missing. We have seen nature fill gaps before. It will do so again. And no one must or can stop that stream. It is life itself.