Chapter 3: Struggle for Existence
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Welcome back to the Deep Dive.
Today, we are getting into what is, I think, one of the most foundational texts in all of science,
Charles Darwin's On the Origin of Species.
And specifically, we're tackling chapter three.
This is really the moment where, for a lot of people, the entire theory just clicks into place.
I mean, if chapter two established that variation is everywhere, it's the raw material.
Right, the clay.
Exactly, the clay.
Then chapter three introduces the force, the undeniable pressure that actually shapes that clay.
So that's our mission for this Deep Dive.
We're looking at the chapter called, very simply, Struggle for Existence.
And Darwin really needs this chapter to build a bridge, doesn't he?
A bridge from just, you know, simple individual differences to the huge question of how you get complex, perfectly adapted species.
Precisely.
How do you get from a slightly faster wolf to a whole new genus of canines?
And his answer is this struggle.
He argues that the struggle for life is so constant,
so pervasive that any tiny variation, even the slightest one that's profitable to an individual.
It gets preserved.
It gets preserved.
And not just preserved, but inherited.
And that accumulation over immense stretches of time of all these slight useful variations,
that's what he calls natural selection.
Yes.
He has this great line where he describes it as a power that is incessantly ready for action.
It's constantly scanning, constantly selecting the organisms best suited to survive, and crucially, to leave progeny.
It's a power, he says, that's immeasurably superior to man's feeble efforts.
Though we should probably mention the other term, the one that maybe became even more famous, which he actually borrowed from Herbert Spencer.
Ah, yes.
Survival of the fittest.
It really does sum it up in a very
punchy way.
It does.
It captures the outcome perfectly.
But, you know, before we even get into the mechanics, Darwin gives us this really important warning right at the start of the chapter.
It's a kind of mental hurdle he knows we all have to get over.
You're talking about how it's easy to say you accept the idea of universal competition,
but it's much, much harder to actually feel it, to really internalize it.
Exactly.
You look outside, and as Darwin puts it, you see the face of nature bright with gladness.
You know, you see the birds singing, everything looks green and abundant.
It all feels very peaceful.
But that's a complete illusion.
It's a dangerous illusion if you want to understand biology,
because we're forgetting the reality underneath.
We forget that those singing birds are constantly destroying life.
They're eating millions of insects or seeds.
They themselves are being destroyed.
They're eggs.
They're nestlings.
They're being hunted every single day.
The engine of competition is always running.
It's not always a loud, violent battle.
It's often quiet.
It's often hidden.
But the pressure is relentless, and that's the key.
You have to see past that bright, glad face to understand what's really driving evolution.
Okay, so let's unpack that term struggle for existence, because if you just think of it as, you know, two stags locking horns, you're missing almost the entire picture.
Darwin is very clear that he's using the term in a large and metaphorical sense.
And that breadth is absolutely critical to his entire argument, because if he limited it to just literal combat, the theory wouldn't work.
And the most important part of this big metaphorical definition is that the struggle includes success in leaving progeny.
So it's not just about an individual staying alive for another day.
No, not at all.
It's a long game.
The real measure of success is reproduction, heredity, raising your offsprings so they can reproduce.
It's a contest that's measured in future generations.
So let's look at the three main ways he applies this term.
The first one is the one we all get instinctively.
Right.
Individual versus individual.
This is the literal meaning.
It's easy to picture two wolves fighting over a carcass in winter, or, you know, two male birds competing for a mate.
Direct, visible, immediate struggle.
Okay.
Simple enough.
Then he moves to the second form.
The second form is individual versus physical conditions.
His example is a plant at the edge of a desert struggling against drought.
We all know what that means.
We call it a struggle.
Yeah.
And Darwin does too, though he makes a little note that it's maybe more accurate to say the plant is dependent on moisture.
But because the outcome, life or death, is still determined by in that physical context, he keeps it under the big umbrella of struggle.
It's a battle with the elements.
And then we get to the third one, which is where it gets really fascinating and complex.
This is the dependence and metaphorical struggle, which can be between organisms that aren't even touching.
This is the mistletoe example.
And if you can understand this, you can understand the scope of Darwin's thinking.
So mistletoe is a parasite.
It lives on, say, an apple tree.
It depends on the tree.
Yes.
But Darwin is very careful.
He says the mistletoe doesn't really struggle against the host tree.
Why not?
Well, because if it does too well, if too many mistletoes grow on the tree, the tree gets sick, it languishes, it dies, and then the mistletoe dies with it.
It's a self -defeating victory.
Right.
So it's more of a delicate balance, a shared fate.
So where's the struggle?
It's in two places.
First, there's a very direct local struggle.
If you have several mistletoe seedlings growing on the same branch.
Then they are competing with each other.
They are absolutely struggling with each other for sap, for space, for resources.
That's the intraspecies competition we'll get to later.
And the second, more metaphorical struggle.
This is the brilliant part.
The mistletoe is in a remote metaphorical struggle with every other fruit -bearing plant in the area.
How does that work?
Think about how it reproduces.
Its seeds are sticky.
Its whole evolutionary goal is to get birds to eat its fruit.
And then when the bird flies to another tree and wipes its beak, it plants a new mistletoe.
The birds are its postal service.
Exactly.
So the mistletoe is in a constant fierce competition with the berries, the hips, the holly.
Every other plant that's trying to get that same burn to eat its fruit and spread its seeds.
It's a marketing battle.
So it's a struggle over distribution rights, basically.
It's a struggle for the attention of the third party.
So you see, we've gone from a dogfight to a plan against the weather to a marketing campaign for birds.
And Darwin puts it all under one convenient powerful term, struggle for existence.
Because in the end, the result is the same.
Favorable traits get passed on, unfavorable ones disappear.
The competition is everywhere and it is always on.
Okay, so that establishes the arena of competition.
But now we have to get to the why.
Why is this struggle inevitable?
And Darwin's answer here is one of the most powerful pillars of his whole theory.
And it's rooted in pure mathematics.
Yes.
The geometrical ratio of increase.
This is just a cold, hard fact of life.
Every single living thing tends to increase at a high geometrical rate.
Think of it like compounding interest.
And if that growth were left unchecked, the numbers would become so enormous, so fast, that no country, no planet could possibly support them.
And he's borrowing this idea directly from the economist Thomas Malthus, but he's applying it to everything,
to all of nature.
And as he says, with manifold force.
Because Malthus was worried about humans, but he also knew that humans have things like prudential restraint, or they can use farming to artificially increase food.
Nature doesn't have that.
Nature has no prudential restraint from mating, no artificial increase of food.
It's just unchecked growth crashing into finite resources.
The collision is inevitable.
And he uses these incredible calculations to just hammer the point home.
Starting with the slowest breeders you can imagine, like us.
Yeah.
He says, you know, man can double his numbers in 25 years.
Now, a thousand years feels like an eternity to us.
But if you actually run the numbers on that slow doubling,
starting with just a single pair.
You find out that in less than a thousand years, there wouldn't even be standing room left on the planet for all their descendants.
It becomes physically impossible very, very quickly.
And that's for a slow breeder.
Then he moves to plants, even a really unproductive one.
He takes a hypothetical plant from Linnaeus that produces only two seeds a year.
That sounds like nothing, just a replacement rate.
But if those two seeds survive, and their offspring survive, and so on, in 20 years, that one plant has become a million plants.
A million from one in just two decades.
It's mind boggling.
But the one that really gets me, the one he clearly put the most work into calculating, is the elephant.
This is the one you should remember.
The elephant is the slowest breeder of all known animals.
And Darwin uses really conservative numbers.
He assumes it starts breeding at 30, stops at 90, only has about six young in its entire life, and lives to be a hundred.
That's a very slow, very low rate of return.
An incredibly slow rate.
And yet,
even with that, after just 740 to 750 years, which is less time than between the Norman conquest and his own day, that original pair of elephants would have nearly 19 million living descendants.
19 million.
From two.
In less than eight centuries.
That number is just terrifying when you think about it.
It means that whether you're a bacterium or an elephant, the math always wins.
The math is relentless.
And this isn't just a theory.
He points to real world evidence.
He talks about the authenticated,
astonishing increase of cattle and horses in South America and Australia when they were first introduced.
Conditions were good.
There were few predators.
And their numbers just exploded.
They exploded so fast that people back in Europe literally didn't believe the reports.
And the same with plants.
He mentions invasive species like a thistle from Europe that now covers huge plains in South America, choking everything else out.
And the key point he makes with these examples is really critical.
It's not that the cows or the thistles suddenly became more fertile.
No, not at all.
It's simply that the conditions were so good that there was less destruction.
The natural checks and balances were temporarily removed and you got to see the raw power of that geometrical increase firsthand.
Which means the conclusion is just unavoidable.
That tendency to increase has to be checked by destruction at some point in an organism's life.
Always.
If we don't see it with our farm animals, it's only because we are the check.
We slaughter thousands every year for food.
In nature, something else has to do that job.
So this gets to the difference between just being prolific, laying a million eggs and having a large population.
Right.
You'd think the animal that lays the most eggs would be the most numerous, but Darwin shows it's not that simple.
He has that great comparison.
The condor lays two eggs, the ostrich lays 20.
And yet in the same country, the condor can be the more numerous bird or his other example, the fulmar petrol, which lays only one egg.
Yet he thought it was one of the most numerous birds on the entire planet.
So what's the difference?
The rate of destruction.
A species that produces thousands of eggs is just compensating for incredibly high death rate, usually when they're very young.
So a high birth rate is just a defense against a high death rate.
That's all it is.
If you're an elephant and you can protect your one calf really, well, you don't need a lot of them to keep the population stable.
But that underlying mathematical pressure is always there, ensuring that heavy destruction will fall at some point, creating the very conditions needed for natural selection.
So we have the engine, which is this geometrical increase, and that engine demands destruction.
Let's look at the breaks.
The nature of the checks to increase.
And Darwin is pretty humble here.
He admits that we often have no idea what the exact causes are.
Yeah, he says they're often most obscure.
And that's what makes this so hard to study.
But he does give us some really clear patterns.
For plants, for example, a lot of seeds get eaten, sure.
But the real slaughter happens to the seedlings.
Right.
Especially when they're trying to grow in ground that's already crowded.
That's the kill zone.
And this is where his little backyard experiments are so brilliant.
They're so simple, but so powerful.
The one on the tiny patch of ground.
Yes.
He cleared a patch just three feet by two, and he watched and marked every single weed seedling that sprouted.
357 of them came up.
And by the end, how many were left?
Only 62.
295 were destroyed.
That's a death rate of over 80 % just in the first few weeks of life.
Just wiped out.
Mostly by slugs and insects.
But the real cause was the intense competition for light and space.
It's the first great filter that life has to pass through.
And he showed the same thing happens with mature plants, with his patch of mown turf.
Right.
He took a patch of lawn that had, I think, 20 different species of plants coexisting, and he just stopped mowing it.
He let it grow wild.
And what happened?
Chaos.
The more vigorous plants just shot up, shaded out the others, and nine of the 20 species just disappeared.
They were killed off by their neighbors.
It shows that even for adult plants, the competition never stops.
This brings up that classic tension between food and predation.
Food obviously sets the absolute limit on a population.
But Darwin argues that a lot of the time, the average number of a species isn't determined by how much food it can find.
But by how often it becomes food.
By serving as prey.
And this is where he makes that amazing,
counterintuitive argument about game animals in England.
It's one of my favorite parts of the chapter.
He says, look, if you stop shooting all the game, the grouse, the partridges, and at the same time, you stopped killing all the predators, the vermin.
You'd think the game population would skyrocket.
You'd think so.
But Darwin argues that after 20 years, you'd probably have less game than you do now.
Because the predators would multiply geometrically and just feast on the now unprotected game.
Exactly.
The human hunter is just one check in the system.
If you remove that check, but you also remove the natural checks, the system finds a new, lower balance.
It's a powerful demonstration of how important predators are.
And he notes the big exception is the elephant.
Once it's an adult, very few things can prey on it.
Right.
Which is why protecting the young is so critical for them.
It allows for that slow reproductive rate.
But then we have to talk about climate.
This is another thing we tend to get wrong.
We think of a harsh winter directly killing off the weakest birds.
And Darwin says, well, yes, but it's more complicated.
Much more complicated.
He saw this firsthand.
He estimated that the severe winter of 1854 -55 killed four fifths.
That's 80 % of the birds on his grounds.
I mean, that's a staggering mortality rate.
A severe human epidemic may be 10%.
So climate is incredibly powerful.
But his main argument is that he usually acts indirectly.
What does he mean by that?
Well, when an extreme cold snap hits, its primary effect isn't just freezing animals.
Its primary effect is reducing the food supply.
And that sudden scarcity triggers the most intense struggle imaginable between all the individuals for what little is left.
The cold starts the war for food.
So the weakest individuals, the ones that die, are probably dying of starvation and exhaustion, not just from the cold itself.
Precisely.
And he uses the idea of a species' geographical range to prove this.
You see a species become rarer as you travel south, and you assume, ah, the climate is too hot for it.
But Darwin says that's the wrong way to look at it.
He says, think about what's really happening.
As you go south, the climate changes slightly, and that change favors other species.
They become more vigorous, their numbers increase, and they simply out -compete your original species for resources and space.
So the species isn't failing because of the heat.
It's failing because the heat gave its competitors an advantage.
It's a competitive failure triggered by climate.
The only places you see a pure, direct struggle against the elements are in the most extreme environments on Earth, the high Arctic, the middle of a desert.
Everywhere else, the struggle with climate is really a proxy war fought between organisms.
The garden plant analogy he uses is perfect for this.
It is.
He says, look at all these exotic plants we can grow in our gardens in England.
They survive the winter just fine.
They're clearly happy with the climate.
But they never escape the garden.
They never become naturalized.
Never.
Because the moment they try, they have to compete with all the native plants that are perfectly adapted to that soil, and they have to resist all the native insects that are evolved to eat them.
Their failure is one of competition, not climate.
And then finally, you have diseases and epidemics.
Right, which might seem like something separate from the struggle, but Darwin links them too.
He notes that many epidemics are caused by parasites, and when a population gets too crowded, it's a paradise for parasites to spread.
So it becomes a sort of struggle between the parasite and its prey.
All of which leads to this final kind of weirdly counterintuitive point he makes about population size.
Yeah, the necessity of a large stock of individuals for a species' own preservation.
Which seems odd, but his example of farming makes it clear.
Why can we grow huge fields of wheat so easily?
Because the sheer number of seeds we plant absolutely overwhelms the local population of seed -eating birds.
There's just way too much food for them to make a real dent.
But if you try to grow just a handful of wheat stocks in your garden to collect the seeds… You'll fail every time.
Darwin says he lost every single seed in that scenario.
Because for the few birds in your garden, that small number of seeds is a feast they can easily wipe out.
So there's safety in numbers.
There's immense safety in numbers.
It explains why very rare plants are often super abundant in the few spots where they do exist.
Those spots are the only places where conditions allow them to grow in a large enough mass to overwhelm their enemies.
Now we get to the section that I think just blows people's minds.
Darwin has established that the checks are real, but often hidden.
Here he shows us that the relationships between living things are so complex and unexpected that the tiniest change can tip the entire balance.
This is where you see the genius of his observational skill.
Showing that natural selection isn't working on one thing at a time, it's working on this incredibly intricate web.
And his example of the Scotch fir and the cattle is just perfect.
So he's looking at two pieces of land, two heaths, right next to each other.
One is open and barren.
The other was enclosed and planted with fir trees about 25 years earlier.
And the difference was night and day.
The planted section was a completely different world.
Twelve new species of plants were growing there that weren't on the open heath.
Six new species of insect -eating birds had moved in.
Just because of the trees and a fence.
But Darwin wanted to know which was more important.
The trees or the fence?
So he found another spot in Surrey where there were old fir trees on an unenclosed heath.
But he noticed there were no young firs springing up.
None at all.
They weren't spreading naturally.
Right.
So he got down on his hands and knees and looked really closely among the low -growing heather.
And what did he find?
Hundreds of tiny fir tree seedlings.
Multitudes of them.
But they were all constantly being browsed down by the grazing cattle.
He found one poor little tree that had 26 rings of growth.
It was 26 years old and it was still only a few inches high.
26 years of trying and the cow just kept nipping its top off.
Exactly.
The cattle were the absolute check.
They were determining the existence of the Scotch fir on that land.
The second you build a fence and keep the cattle out, boom, a forest of young firs springs up.
A simple change in land management completely changes the ecosystem.
And from that very local observation, he jumps to a huge global one.
The case of Paraguay.
Yeah, this one is amazing.
He points out that in Paraguay, unlike the areas to the north and south, you don't find feral populations of cattle, horses, or dogs.
They just don't run wild.
And the reason is one tiny insect.
One specific fly that lays its eggs in the navels of newborn mammals, killing them almost immediately.
This one insect prevents these huge animals from taking over the country.
But as Darwin argues,
the chain doesn't stop there.
It never stops there.
That fly must be checked by something.
He speculates it's probably parasitic insects that prey on the fly.
And those parasites are probably preyed upon by insectivorous birds.
So you can trace the whole chain.
Let's say something happened and the insect -eating birds in Paraguay declined.
Then the parasites that eat the fly larvae would increase.
That would cause the fly population decrease.
If the flies decrease, then suddenly cattle and horses could survive birth in the wild and would become feral.
Their numbers would explode.
Which would completely change the vegetation of the country, which would affect other insects, which would in turn affect the insect -eating birds.
It's this incredibly complex circular web of dependencies.
Where everything is so nicely balanced, as he says, that the slightest nudge can cause massive ripples.
And that's his point.
Our ignorance of these connections is so profound that when a species goes extinct, we imagine some huge catastrophe, a volcano or an asteroid.
But often it's just the quiet, subtle victory of one organism over another.
And the most famous, most delightful example of this has to be the one connecting humblebees, mice and cats.
Oh, it's the classic.
It starts with a simple observation about pollination.
Darwin proved with experiments that humblebees are essential for fertilizing certain flowers, like red clover, because other bees literally can't reach the nectar.
Right, he covered some clover heads to keep the bees out.
And they produced zero seeds.
The unprotected ones right next to them produced thousands.
So no humblebees, no red clover.
Simple as that.
But the population of humblebees is heavily dependent on the population of field mice.
Because field mice love to raid and destroy their nests.
One expert, he cites, thought that mice destroy more than two -thirds of all humblebee nests in England.
So more mice means fewer bees, which means less clover.
But what controls the mice?
Well, especially around villages and towns.
Cats.
Cats are major predators of field mice.
So more cats means fewer mice.
Fewer mice means more bees.
More bees means more clover.
Which leads to his incredible conclusion.
That it's quite credible that the presence of a feline animal in large numbers in a district might determine the frequency of certain flowers in that district.
It sounds like a joke, but he lays out the logic step by step.
It's the ultimate demonstration of how deeply interconnected, in often hidden ways, the web of life really is.
So, okay, if this struggle is happening everywhere,
in this complex web, Darwin then has to ask, where is the pressure the most intense?
And he gives a very clear rule here.
A very clear and very important rule.
The struggle will almost invariably be most severe between the individuals of the same species and between the varieties of the same species.
So not the lion versus the zebra, but the lion versus the lion.
Exactly.
The internal competition is the most brutal.
And the reason is just, it's pure logic.
Individuals of the same species need the exact same food.
They live in the exact same places, and they face the exact same dangers.
There's a 100 % overlap in their needs.
It's a fight for the exact same resources.
It's total.
So any tiny advantage one individual has, slightly better digestion, a slightly thicker coat, a slightly better ability to hide from the same predator, that advantage becomes hugely significant.
It means the difference between life and death, and more importantly, between leaving offspring or not.
And we see this play out in agriculture all the time, which is why he goes back to it for evidence.
Right.
If you take several different varieties of wheat and plant them all mixed together in a field, and then you harvest the seed and replant it year after year, the strongest variety will win.
The one that's best suited to that specific soil and that specific climate will beat the others, as he puts it.
Within a few years, it will have completely supplanted the rest.
You're watching natural selection happen in a field.
The battle of attrition.
It is.
And that's why you can't maintain a mixed stock of, say, different breeds of mountain sheep, just by letting them run together.
The stronger, slightly better adapted breed will inevitably drive the weaker one to extinction.
You have to physically separate them to keep both.
And that same principle applies, maybe just a little less intensely,
between closely related species in the same genus.
Yes.
That's the next most severe level of competition.
Because they have a recent common ancestor, their bodies, their habits, their needs, are all still very similar.
They occupy nearly the same place in the economy of nature.
And that leads to this pattern of displacement that we see.
All the time.
He gives examples like one species of swallow driving out another in America.
Or in Scotland, the missile thrush was apparently causing the song thrush to decline.
And the really classic examples like rats or cockroaches.
Exactly.
A new, slightly more competitive rat species arrives in a port and the old one just vanishes.
Or the most dramatic one from his time.
The European hive bee being introduced to Australia.
And what did it do to the native bee?
It was exterminating it.
The small, stingless native bee just couldn't compete.
Their needs for pollen and nectar were too similar.
And the imported bee was just slightly more efficient, slightly more robust.
And that slight edge was enough to win the war.
And what's so humbling, as Darwin points out, is that we can see it happening, but we can't say for sure why the hive bee won.
The specific advantages are probably so numerous and so subtle that we'd never be able to list them all.
And all that complexity, all that competition, brings us to what he calls the grand corollary.
This is the big overarching takeaway from the entire chapter.
He says it's of the highest importance.
And the corollary is that if every organism is in this constant struggle, then the structure of every single organic being is related in a deep and often hidden way to that of all other organic beings it interacts with.
It forces us to see adaptation in a new light.
I mean, we see the obvious stuff, the teeth of a tiger, the talons of a hawk.
Those are clearly weapons for the struggle.
Sure, direct competitive tools.
But the corollary demands that we look for the hidden connections.
His example of the dandelion seed is just beautiful.
We see that fluffy parachute and we just think, ah, for floating on the wind.
But Darwin says, think deeper.
Why does it need to float so far?
The real advantage of that beautiful structure is that it allows the seed to travel far enough to find a patch of unoccupied ground because the ground is already packed with competitors.
The seed's design isn't just an adaptation against gravity, it's an adaptation against other plants.
Or his example of the water beetle with its flattened, fringed legs for swimming.
We see them and think, great for swimming.
But what does that mean in the context of struggle?
It means it can outcompete other aquatic insects for food, it can hunt more effectively, and it can escape its own predators more quickly.
Every piece of its structure is honed by competition.
Even something as simple as the food stored in a seed, like a pea or a bean.
It seems like just a little packed lunch for the embryo.
But Darwin says its main purpose is competitive.
It gives the seedling a burst of energy to grow fast and strong right at the start.
When it's in that desperate struggle with all the other seedlings around it, all fighting for the first rays of light, it's a head start in the race.
So it's an adaptation to win that seedling death match we talked about earlier.
Precisely.
And this leads him to this fantastic thought experiment for the reader, for you.
He says, imagine a common plant thriving in the middle of its range.
Why isn't it doubling its numbers every year?
We know it can handle slightly hotter or colder weather because it lives in those places at the edges of its range.
So we know the climate isn't the main thing holding it back right here, right now.
Right.
So if you wanted to magically give that plant an advantage so it could increase its numbers, what would you do?
You wouldn't just make it a little more frost resistant.
You'd have to give it an advantage over its competitors or some defense against the animals that eat it.
And we'd have no idea where to even start.
We'd be completely lost.
Because we have such a profound ignorance of the specific complex relationships between that plant and everything else around it.
And that's his point.
That realization of our own ignorance is necessary to understand just how powerful and subtle a force natural selection must be.
OK, so let's bring this all together.
Chapter 3 has built this huge logical case.
What is the one core conviction he wants us to walk away with?
The conviction is built on that simple, unassailable logic.
One, every being tries to increase at a geometrical ratio.
Two, this mathematical certainty means that every being, at some point, must face a struggle for life and suffer immense destruction.
It's not a choice.
It's a consequence.
And that constant, inevitable struggle is the engine.
It's the filter that catches and preserves every profitable variation, no matter how small and slowly, generation by generation, shapes life.
It's the mechanism.
It takes the raw material of variation and it forges it into the complex, adapted species we see all around us.
But, you know, Darwin ends the chapter on a surprisingly, I don't know, almost comforting note.
He does.
He says, the war of nature is not incessant if you think of it as constant misery.
He points out that death is usually quick.
And the grand result of this whole brutal process is that the vigorous, the healthy, and the happy survive and multiply.
It's a tough logic, but there's an elegance to it.
The destruction is what allows for the perfection.
It's a powerful and essential foundation for his entire theory.
Thank you for joining us for this deep dive into Darwin's third chapter.
Thank you.
Once you really grasp the sheer force of the struggle, you never look at a forest or a field the same way again.
You realize every single thing you see is a champion survivor.
We'll see you next time.
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