Chapter 7: Objections to Natural Selection

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Welcome back to The Deep Dive, the show that extracts the crucial knowledge from dense research and complex texts, giving you the shortcut to being genuinely well -informed.

Today we are strapping in for a deep historical dive going straight to the source, Charles Darwin's monumental work on the origin of species.

Specifically, we're tackling chapter seven, miscellaneous objections to the theory of natural selection.

Right, and this is, you know, this is the point in the book where he really turns the corner.

He's laid out his theory and now he knows exactly where the skeptical arrows are being aimed.

This is Darwin in the intellectual combat zone.

And the mission for this deep dive is really for you, the curious learner.

We're not just going to summarize.

We want to slow down and unpack Darwin's really sophisticated counter -arguments.

We're focusing on his step -by -step logic, the incredible natural history examples he brings to the table, and really how he rebuts that most challenging criticism of all.

The problem of incipient structures.

I mean, how could something like an eye possibly begin?

Exactly.

Chapter seven is where he has to defend the perceived limitations of his theory.

He has to show that natural selection can account for all these apparent contradictions traits that seem useless.

The impossible beginnings of a giraffe's neck or a whale's filter plates.

It's a tour de force of biology and logic.

Okay, let's unpack this.

If the survival of the fittest is such a powerful, elegant process, why was it so hard for his contemporaries, and especially critics like St.

George Mifflvuf, to accept that it could build something from, well, from nothing?

What were the toughest questions Darwin really had to face in defending his greatest idea?

Darwin starts off by sort of clearing the deck.

Right.

He addresses these foundational, almost philosophical objections first.

He has to.

These are objections rooted in a basic misunderstanding of what he even means by terms like perfection or adaptation.

And one of the first ones comes from a distinguished German naturalist who basically accuses Darwin of saying that all living things are just imperfect.

Which is a huge misreading, and Darwin clarifies this immediately.

He says he never stated that organisms are absolutely imperfect.

His actual wording is that they are not as perfect as they might have been in relation to their conditions.

That's such a subtle shift in language, but it changes everything.

It changes everything.

It moves perfection from being some, you know, abstract static ideal to a messy dynamic state of being.

It's basically saying evolution is a race on a moving track.

The environment is always changing.

Exactly.

The climate, the geology, sure.

But even more important, the biological environment is in constant flux.

New competitors, new predators, new parasites arriving.

So even if an organism was 100 % perfectly adapted right now.

The second a new competitor evolves a slightly better way to find food, that perfection is gone.

It's immediately lost.

This state of non -absolute perfection is a prerequisite for evolution.

If everything was perfect, selection would have nothing to work on.

And Darwin has evidence for this.

He points to a really simple observation.

He does.

The success of intruding foreign species, it's compelling.

He argues very logically that if a native species were truly perfectly adapted for its home, it would never ever be pushed out by an invader.

So the very fact that invasive species can succeed proves the native wasn't perfect to begin with.

Precisely.

It shows that the native species had vulnerabilities, and the new invader brought some novel characteristic that could exploit them.

It demonstrates that evolutionary potential is always there.

Okay, that sets the stage for the next objection, which sounds a lot more, well, mathematical and serious.

The critique about longevity.

Oh, yes.

This critic, who Darwin says threw this objection at him with some parade of mathematical accuracy, basically made a very simple argument.

Which was?

If a long life is an advantage, more time to survive, more time to reproduce, then natural selection must ensure that all descendants live longer than their ancestors.

So if you followed his logic, every species should just be living longer and longer and longer as time goes on.

A steady, predictable increase.

It's a classic case of trying to impose a simple linear rule onto a hugely complex biological system.

And how does Darwin get around that?

His rebuttal is just brilliant.

He highlights how many different ways there are to survive.

He says, just consider a biennial plant or some lower animal living in a cold climate.

Right.

It might live for two years, but it's going to die in the winter.

Its longevity is cut short by the environment itself.

But the species survives.

How?

Through its seeds or its eggs.

The trait being selected for isn't a longer life for the individual.

It's the efficient production of offspring that can survive those harsh conditions.

Selection here is acting on the hardiness of the seed coat or the ability of the ova to resist frost.

Not on lifespan.

I see.

So longevity isn't some isolated trait that just gets ratcheted up.

Not at all.

Darwin says it's incredibly complex.

It's related to the organism's size, how much energy it spends on reproduction, its general activity level.

All these things are what selection is really fine tuning.

So a species could be incredibly fit by living a very short life, but producing a massive number of offspring.

That's the core insight.

The only thing that matters is reproductive success, not how long any one individual happens to stick around.

Okay.

Moving on.

There's another big historical objection that Darwin had to clear, and this one is about the pace of change.

The Egyptian monument argument.

Right.

Critics would point to animals and plants from ancient Egyptian monuments and tombs, some three or 4 ,000 years old, and say, look, they haven't changed at all.

They're identical to modern ones.

The implication being, if we can't see any change in 4 ,000 years, how can we believe these massive changes happen over millions?

And Darwin, citing G .H.

Lewis, has this fantastic reply.

He says the argument proves too much.

What does he mean by that?

Well, look at the domestic animals on those same monuments, the ancient dogs, the cattle.

They're also basically identical to modern breeds.

And yet everyone agreed that those breeds had been modified by human selection over time.

Exactly.

So if 4 ,000 years of stability doesn't disprove artificial selection, it certainly can't disprove natural selection.

It's about conditions.

The Nile Valley has been incredibly stable for millennia.

Incredibly stable.

And selection only preserves beneficial changes when circumstances are favorable for change.

If the environment is stable and the current form is working just fine, then no modification will occur.

Stability is the expected outcome in a stable environment.

He even says that animals unchanged since the glacial period are a much better test case.

A much stronger test.

Because those animals faced enormous changes, huge climate shifts, massive migrations, if evolution was some kind of innate unstoppable drive for progress, those animals should have changed.

But they didn't.

And their stability in the face of all that change just reinforces Darwin's point.

The lack of change simply means that beneficial variations didn't happen to arise or weren't selected for during that period.

It's all down to circumstance.

Which brings us to the last objection in this first part from the paleontologist Braun.

How can a new variety live side by side with its parent species?

Shouldn't the new, improved version just wipe out the old one?

Yeah, this assumes they're in a direct head -to -head competition for the exact same things.

But Darwin's point is that they don't have to be.

Right.

They can coexist perfectly happily if the new variety has become fitted for, as he says, slightly different habits of life or conditions.

Like what, for example?

Imagine a type of grass.

A new variety pops up that can handle soil that's a little bit saltier.

Or maybe its roots can go a little deeper to find water.

Okay, so it's not competing with the parent anymore.

It's moved into a new, slightly different niche.

The direct competition just ends.

And that's why, Darwin notes, you usually find permanent varieties in distinct stations.

One on highland, one on lowland.

Or for animals, they're often separated by geography.

Selection drives divergence not to destroy the original, but to specialize and reduce that pressure of life between them.

So we're shifting now from these sort of philosophical objections to the actual mechanics of how change happens.

And Braun is back with another tough question about simultaneous modification.

Yes, and this is a really intuitive one.

When you look at two different species, say a bird and a reptile,

they don't just differ in one thing.

They differ in everything.

Their whole organization is different.

So the question is, how on earth could selection modify so many parts all at the same time?

It seems impossible.

But Darwin's explanation is all about recognizing how inheritance stacks up changes over time.

He says there's no need for them to be modified simultaneously.

What he's saying is that it looks simultaneous, but it wasn't.

Exactly.

The process itself was sequential.

A small, useful variation pops up in one part.

Generations later, another one appears in a different part, then another.

Because they're all inherited together, when we look at the final product millions of years later, it appears as if it all happened at once.

And the domestic animal analogy is so important here because we've actually watched it happen.

It's his trump card.

Think about a massive, heavy -boned dray horse versus a slender, long -legged race horse.

Their entire bodies are different.

But if you trace their breeding history, it was all done in steps.

Readers focused on leg length for a while, then muscle mass, then something else.

It wasn't simultaneous.

Which brings us to this really fascinating concept that explains why the whole frame seems to change, even if selection is just focused on one part.

Correlated growth.

This is fundamental to his whole defense.

He says that even when you're selecting for just one single thing, say, a bigger flower on a plant, other parts of the organism get modified too, just as a consequence.

So it's not selection acting on those other parts directly.

It's like a physiological side effect.

A side effect of the internal mechanisms of life.

He talks about these dimly seen causes.

Right.

Can you unpack those?

Because they sound pretty technical.

Well, the first one is just the flow of nutrients.

If you're selecting for a huge fruit, the plant has to pour a massive amount of energy and resources into that one part.

Which means less energy for other parts, like the stem or the leaves.

Right.

The stem might end up weaker, not because selection wanted a weak stem, but because it was basically starved by the rapidly growing fruit.

The second cause is just mutual pressure.

As one part grows, it physically pushes on and constrains the parts around it.

And the third is how an early developing part can affect a part that develops later.

All these internal links mean that selecting for one change can cause a cascade of other secondary changes.

And this idea, correlated growth, is the key to tackling the next, and probably biggest, objection.

Useless structures.

The problem of characters of no service.

This was the core argument from critics like Braun and Nigelli.

If selection only preserves what is useful, why is nature filled with things that seem to be of no service whatsoever?

What kind of things were they pointing to?

Oh, all sorts.

The length of ears on a mouse, the tiny, complex folds in the enamel of a tooth, the way leaves are arranged on a plant stem, all these little details that seem to have no bearing on survival.

So Darwin has this three -part defense.

First, he says we have to be extremely cautious in even deciding what's useless.

He's basically saying, let's assume our own ignorance first.

Braun pointed to the ears of a mouse as a trifling, useless difference.

But Darwin cites the work of Dr.

Schobel.

Who discovered what?

That the external ears of the common mouse are packed with nerves and serve as incredibly sensitive, tactile organs.

Wait, so their ears are almost like whiskers for sensing the air around them?

In a way, yes.

They're crucial for navigating in the dark.

So what looked like a useless bit of morphology is actually a vital sensory organ.

The same goes for tail length, which can be for balance or for grasping.

Okay, so that's counterpoint one.

Maybe it's not useless.

We just don't understand it yet.

What's the second part?

The second is hidden utility revealed.

This is where structures that were once thought to be just arbitrary shapes are later discovered to be critically important.

And his big example here is orchids.

Orchids are the perfect taste.

Before his own work on them, the bizarre, complex shapes of orchid flowers were just seen as beautiful quirks of morphology.

But his research showed that every single twist and lever was a perfect adaptation form.

For ensuring insects cross -pollinated them, the structure wasn't arbitrary at all.

It was the absolute height of sexual selection and adaptation.

Okay, so some things aren't useless and some things have hidden uses.

But what about the things that really are useless?

Darwin does concede that these can arise, right?

He does.

And this is his most complex rebuttal.

He says unimportant modifications do often arise, but they arise from the laws of growth, completely independent of selection.

He uses those weird closed flowers on some plants to explain this, right?

The kleistogamous flowers, yes.

So many plants have two types of flowers.

Big, open ones to attract insects for cross -pollination, and then these tiny, closed, imperfect ones.

And the closed ones are definitely useful.

They guarantee seed production with almost no risk and very little energy.

So selection would favor having them.

Absolutely.

Selection favors the result, the guaranteed seeds.

But the structure of that closed flower, the fact that its petals are tiny or gone, that it has very few pollen grains, that's not because of selection.

It's because of what he calls functional inactivity.

Precisely.

Once the petals aren't needed to attract insects, they just get reduced because they aren't being used.

It's a law of growth, a consequence of disuse, not selection actively trying to make smaller petals.

That's an incredibly subtle distinction.

It is.

And he shows it also happens based on just the position of a part on the organism.

In some plants, the central flower will have five petals, but all the surrounding ones will have four, just because of developmental pressures.

Or, even more amazing, on the same flower head, you can find seeds of three totally different shapes and colors depending on whether they grew in the center or on the edge.

And these are differences that botanists would consider hugely important for classifying other species.

Hugely important.

Yet here they are, on the same plant, created simply by their position.

It proved that variation can and does arise from these internal laws of growth, completely separate from natural selection.

This all leads him to a really interesting paradox about how we classify life.

It does.

If a character, like the number of petals, can vary so much even on a single plant, you have to assume it's of very little importance to that plant's survival.

And if it's not important for survival,

natural selection can't really act on it.

It can't.

And this is a direct shot at Nigeli's idea of some internal drive toward perfection.

These changes are happening without any push towards higher organization.

But here's the paradox.

The characters that seem to be of the least importance to the organism.

Or often the most important to the scientists trying to classify them.

How does that make any sense?

Darwin's explanation is brilliant.

He suggests that these traits, things like the basic arrangement of leaves, the position of ovules, they first appeared as minor fluctuating variations way back in the ancestry of a group.

And they became fixed not because of selection, but just through the organism's own internal nature or stable conditions.

Right.

And because they weren't important for survival, selection didn't mess with them.

They weren't constantly being changed and adapted for new functions.

So they just got passed down unchanged through millions of years.

Exactly.

They are, in essence, genetic relics.

That's why they're so useful for systematists.

They mark the deep ancient branches of the family tree.

Highly adaptive traits like camouflage can evolve over and over again in different groups and really confuse things.

But these unimportant characters tell you about common descent.

It's like finding a small archaic symbol in the architecture of every house in a large ancient family line.

It has no function today, but it proves they all came from the same ancestral builder.

That's a perfect analogy.

That's exactly it.

So Darwin rejects this idea of an innate push towards perfection, but he still says that progressive development is a necessary consequence of natural selection.

How does he square that circle?

He defines high organization purely in functional terms.

An organism is more highly organized if its parts are more specialized for different jobs.

And since natural selection is always tending to make parts more efficient at what they do, it will naturally lead to more specialization and differentiation over time.

So you get progress, but the driving force isn't some mystical internal tendency.

It's just the relentless grinding process of adaptation to local conditions.

Which brings us to the absolute core of the chapter, the critique champion by St.

George Mivart.

This is the big one.

Mivart argued that natural selection is, and I'm quoting,

incompetent to account for the incipient stages of useful structures.

The classic what good is half an eye or what good is 5 % of a wing question.

Yes.

If a structure has to be nearly perfect to work, how could selection possibly preserve the useless tiny beginnings?

And Darwin starts his rebuttal with maybe the most famous example of all.

The giraffe's neck.

The giraffe is the perfect poster child for this problem.

You've got the long neck, the long forelegs, the specialized tongue, all working together for browsing on high branches.

It's a huge advantage during droughts when all the low -hanging leaves are gone.

And he uses this grim example of the Nyata cattle to show how important even a small structural difference can be.

He does.

These cattle have a projecting lower jaw, which means they can only eat grass.

So during a drought, when other cattle can switch to eating twigs and reeds, the Nyata cattle just starve.

So that shows that a seemingly minor feature can be the literal difference between life and death.

Absolutely.

So now apply that to the ancestors of the giraffe.

During a drought, any individual that could reach even an inch or two higher than the others would survive when its shorter relatives died.

And those survivors would interbreed, passing on that slight advantage.

Right.

And Darwin makes a key point here.

Nature doesn't have to carefully pick out and separate one pair like a human breeder does.

Natural selection in a drought preserves all the superior individuals in a whole area and kills off all the inferior ones.

It's ruthlessly efficient.

But Meibard has a counter argument.

Being bigger costs more.

You need more food.

So maybe the disadvantage of being bulkier outweighs the advantage of being taller during a famine.

It's a good point.

But Darwin's rebuttal is multifaceted.

First, empirically, giraffes exist.

They thrive.

So clearly the advantage is real.

But second, height gives them huge secondary advantages.

Like the neck acting as a watchtower.

A watchtower, exactly.

It makes them incredibly hard for predators to stalk.

And that long neck is also a powerful weapon for defense, swinging that horned head.

It's the sum of all the advantages that matters.

Meibard's second point is about uniqueness.

If high browsing is so great, why just the giraffe?

Why no long -necked gazelles or other animals?

And Darwin's answer here is all about specific competition.

He says to imagine a meadow where horses and cattle have already eaten all the lower branches, selection isn't going to start lengthening a sheep's neck in that meadow.

Because the horses and cattle already own that slightly higher niche.

Exactly.

So the real competition for the highest acacia leaves in Africa is between giraffe and giraffe.

Selection just needs to keep pushing the already tallest competitor a little bit higher.

And he also admits we just don't know everything.

He does.

He says maybe the right combination of variations just never appeared in other animals.

Or maybe their populations were held in check by parasites or disease so there was no strong selective pressure on neck length anyway.

It's a complex picture.

Let's move to the second big case study.

Protective resemblance in insects.

The stick insects.

The ones that look like bird droppings.

Meador of art's objection here was statistical.

He said that variation is indefinite.

It goes in all directions.

So how could these tiny unstable variations ever build up into a convincing resemblance?

Wouldn't they just cancel each other out?

He's basically arguing you need a big recognizable jump for selection to even notice it.

He is.

But Darwin's reply is so elegant.

He says the system doesn't need a perfect resemblance to start.

All it needs is a rude accidental resemblance to some common object.

So just by pure chance, some insect already looks a tiny bit like a twig.

Yes.

And given the sheer diversity of insects and the infinite number of objects in nature, he argues that this is highly probable.

And once that rude resemblance exists, variation is no longer indefinite.

Because selection acts like a filter.

A sieve.

Any variation that makes it look more like a twig gets preserved.

Any variation that makes it look less like a twig gets eaten.

The process just concentrates the useful resemblance over time.

And the unbelievable perfection you see in some of them is because their predators, like birds, have incredibly sharp eyesight.

Much sharper than ours.

So the resemblance has to be nearly perfect to work.

Selection is just relentlessly pushing it towards that high standard, preserving every tiny fraction of an advantage.

Okay, so having dealt with the giraffe and camouflage, Darwin moves into pure anatomy, showing these step -by -step functional shifts.

He has to.

He needs to prove that the initial forms were not useless, but that they performed some other intermediate serviceable function.

And the third case is one of the most amazing structures in nature.

The whale's baleen filter.

This just seems impossible to build gradually.

It does.

Middlevart asks, how could the very beginning of a filter like this be useful?

And Darwin's idea is that the ancestors of baleen whales probably had a mouth built something like a duck's beak.

A duck's beak?

Yes.

Ducks are sifters.

They provide a living functional model for how you can get from a easing organ to a filtering organ through gradual steps.

Okay, so walk us through those duck gradations.

You start with the best sifter.

The shoveler duck.

Its beak is lined with the most 200 thin elastic projections, like bristles.

It's an incredibly efficient sieve.

So that's the end point, the perfect filter.

What's the next step down?

The common duck.

Its projections, the lamellae, are coarser and fewer.

It still uses them for sifting mud, but they're also hard enough for crushing and cutting food, so it's a dual function tool.

Sifting and cutting, okay.

Then you go down to the Egyptian goose.

Islamellae are even less developed.

It mostly just crops grass, but it still has this little behavioral relic where it squirts water out the corners of its bill, a vestige of sifting.

And finally, the common goose.

The common goose's beak just has hard tooth -like knobs.

It's exclusively for tearing grass.

The sifting function is completely gone.

So the path is clear.

A primitive whale could have started with hard knobs for seizing prey, like the goose.

And through small variations, those knobs could have become a dual function seizing and sifting tool, like the common duck, and then eventually a pure filtering tool, like the shoveler, and finally, the massive plates of a modern whale.

Every single step along that path was useful.

That's incredible.

Okay, next up is a case that blends natural selection with something we don't talk about much anymore.

The inheritance of acquired characteristics.

The asymmetrical flatfish.

Yes, the pleuronectidae.

Their asymmetry is just profound.

Both eyes are on one side of the head.

Medivart argued that a tiny fractional movement of the eye would be worse than useless.

It would be harmful.

So how could selection ever get started?

The key was an observation by a naturalist named Maum in 1867.

He watched very young flatfish.

When they hatch, they're perfectly symmetrical.

But they can't swim upright.

No, they lack a swim bladder, so they fall to the bottom and lie on one side.

And once they're on the bottom, they develop a habit, a very beneficial habit.

They twist the lower eye upward to try and see.

They strain to look up, pressing that eye hard against the skull.

And at this age, their skull is still soft cartilage, and it physically yields to that pressure.

The muscular action literally starts to move the eye's position.

So the behavior, the habit, is what causes the initial physical change.

Yes.

And in Darwin's view, that physically induced change is then inherited.

And because having that eye on top is so beneficial, natural selection then amplifies that inherited tendency, favoring the individuals whose skulls are most flexible and whose upward -looking habit is strongest.

The whole thing starts with a behavior.

An astonishing chain of events.

Okay, let's quickly go through the last few examples.

The mammary glands.

Mivart asks how you can start with just a single drop of fluid.

Darwin says that's the wrong way to look at it.

He argues mammals descended from a marsupial -like ancestor where the young developed in a pouch.

And the glands were already there, just as skin glands.

Probably.

Skin glands in the pouch secreted some basic fluid.

Any individual that secreted a slightly more nutritious fluid would have better fed offspring.

Selection would instantly favor that.

Those glands then just became more and more specialized over time.

And what about those bizarre little snapping forceps on starfish,

the pedicillaria?

Mivart asked how the jaws and the stem could evolve at the same time.

Darwin shows that they didn't have to.

These things are just modified spines.

And you can find living starfish that have fixed snapping forceps with no stem at all.

So the snapping action evolved first, and the movable stem was added later by selection.

So for every one of these impossible structures, Darwin finds a plausible step -by -step pathway where every single intermediate stage was functional and useful.

That is the core of his entire defense.

Which brings us to the final showdown.

The end of his defense.

Where he takes on the whole philosophy of evolutionary change itself.

Mivart's idea of evolution by jumps or sultation.

Right.

Mivart suggested that species don't change gradually.

He thought they changed because of some unknown internal force, and that new species appeared suddenly, with major modifications all at once.

A bird's wing just appears.

Or a three -toed horse suddenly becomes a modern horse in one leap.

And for Darwin, this is just giving up.

It's entering the realm of miracle, not science.

So this is where it gets really interesting.

Because Darwin's argument against this is purely statistical.

Why are these big, sudden jumps so improbable in nature?

He gives a few devastating reasons.

First, these big, monstrous variations are just incredibly rare in the wild.

But even if one did appear, say, one animal is born with a revolutionary new trait, it would almost certainly be lost by intercrossing.

That one special individual has to meet with the rest of the population who are all normal.

The trait just gets diluted and bred back out of existence in a few generations.

So for a big jump to work, you'd need a whole bunch of these magically changed individuals to all appear at the same time, in the same place, so they could breed with each other.

Which is astronomically improbable.

Gradual evolution completely avoids that trap because it preserves a large number of individuals that are all varying just slightly in the right direction.

The trait gets fixed in the population without being diluted.

He says the evidence we see in nature just screams gradualism.

It does.

You look at big families of species and they're all so closely related, it's hard to even tell them apart.

You see these representative species changing slowly as you go from north to south or up a mountain.

It all points to fine, tiny steps, not big jumps.

His final weapon against this idea is embryology.

The embryology's protest.

This is such a powerful argument.

He points out that the limbs of a bird, a bat, a horse, they're all indistinguishable at an early embryonic stage.

They only differentiate into their specialized adult forms through insensibly fine steps.

So if a bird's wing had appeared in one great sudden leap.

Shouldn't the embryo show some sign of that?

Shouldn't there be some jarring, abrupt transformation during its development?

But there isn't.

The embryo is a smooth, gradual record of the past.

To believe in these sudden jumps is to believe that these massive transformations can happen without leaving a single trace on that developmental journey.

It's just incredible.

So Darwin really finishes the chapter convinced that all of these ingenious objections from his critics, when you really dig into them, they actually end up reinforcing his theory.

They do.

By systematically showing that so -called useless traits have hidden uses or arise from growth laws.

And by proving that a functional intermediate step always existed for complex structures, he shows that the mechanism of selection is more than robust enough to explain the whole vast panorama of life.

Wow.

So chapter seven is just essential.

It's Darwin at his most defensive, but also at his most forensic.

He really dismantled all those big conceptual roadblocks.

He did.

He proved that things that look useless aren't failures of his theory.

They're explained by ecology or by these deep laws of growth and correlation.

And the main takeaway really is that selection doesn't need any miracles.

Not at all.

Its power lies in its ability to just accumulate countless, tiny, probable, useful changes over immense stretches of time.

It uses everything, growth, correlation, even the inherited effects of use and disuse.

But it doesn't rely on these great sudden leaps that are so statistically unlikely.

So what this all means is that the creation of a complex eye or a whale's filter or a giraffe's neck,

it's not impossible at all.

It's just the result of a very, very long series of highly probable small advantages, relentlessly sifted and accumulated by nature.

And for your final thought, think about this historical context.

Darwin really leaned on the idea that the use of a part like that flatfish twisting its eye causes an inherited change that selection then helps along.

Now, how might that blend of agency's natural selection and the inheritance of acquired characteristics complicate how we think about the speed and direction of evolution today?

Thank you for joining us for this deep dive into Darwin's crucial defense of his theory.

We look forward to seeing you on the next deep dive.