Chapter 8: Instinct

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome back to The Deep Dive, where we take complex stacks of articles, research, and monumental texts, in this case Chapter 8 of Darwin's On the Origin of Species, and distill the absolute core insights you need to understand the big picture.

Our mission today is, well, it's arguably the most challenging one in Darwin's entire book.

He called this chapter's focus, The Evolution of Instinct, one of the most wonderful and difficult objections posed to the theory of natural selection.

I mean, if his theory is all about the gradual accumulation of tiny physical changes, how on earth do you account for mental qualities?

For behaviors that are seemingly perfect, fixed, and performed without any practice at all.

That's the perfect framing.

We need to understand how Darwin argues that these incredibly complex, innate behaviors, from say, the smallest ant carrying a pupa to the largest bird migrating thousands of miles, are just as malleable and susceptible to gradual variation as bones or fur.

Exactly.

It forces us to look at behavior, not just anatomy.

But first, it's probably vital to clarify what Darwin is not trying to do here.

Oh, absolutely.

He states it very explicitly.

He is not concerned with the origin of mental powers or the origin of life itself.

He just takes those as given.

So his focus is much narrower.

Much narrower.

It's strictly on the diversities of instinct that exist among animals of the same class.

His whole point is, if they vary, then selection can act on them.

That's the core starting point.

Okay, so let's start with the basics.

We all use the word instinct pretty loosely.

How does Darwin even try to define it?

Well, he knows a perfect definition is almost impossible, but he identifies a few key characteristics.

An instinctive action is generally one performed without any previous experience.

Often by a very young animal.

Right.

Like a newly hatched duckling that just starts swimming, or a bird attempting its first flight south.

And crucially, they do this without any knowledge of the ultimate purpose of the action.

That sounds like a purely robotic, automated response.

But Darwin almost immediately throws a wrench into that simple idea.

He does.

He quotes the great naturalist Pierre Huber, who pointed out that even in animals low on the scale of nature, a little dose of judgment or reason often comes into play.

So instincts aren't just these fixed automata.

Not at all.

They can be modified by circumstance, by experience, or even a basic level of reasoning.

And this inherent flexibility, this variability, is absolutely crucial for his theory of selection.

This brings us to his initial comparison, which is really helpful.

It connects behavior we can kind of understand, like unconscious repetition,

to the bigger mystery of instinct.

He draws the link between instinct and habit.

He does.

He argues that the internal frame of mind, or the mechanism behind an instinctive action, is very similar to a highly ingrained habit.

Something you do unconsciously.

Exactly.

An action you perform without conscious will, often rhythmically like repeating something by rote.

And you know how it is, if you're interrupted, you often need to go back and restart the whole sequence.

Which leads right into that fascinating detail from Huber's study of the caterpillar and its complicated hammock.

This anecdote, it really just highlights the mechanical, sequential nature of instinct.

It's a fantastic example.

So, Huber studied a caterpillar species that weaves a complex hammock in a fixed series of steps.

And he did these two really revealing experiments.

In the first one, he took a caterpillar that had just finished, let's say, stage six of construction.

And he placed it into a different hammock that was only completed up to stage three.

So what happened?

Did the caterpillar realize it was ahead of the game and just relax?

Not at all.

It simply re -performed stages four, five, and six all over again.

It was completely locked into this fixed sequence and it couldn't skip ahead.

It just behaved like a machine forced to run through its program from the point it thought it had started.

A completely non -cognitive rhythmic routine.

So what about the opposite scenario?

If Huber took a caterpillar out of its hammock at stage three and put it into one that was already finished up to stage six, did it take advantage of the free work?

Nope.

Huber said the caterpillar was much embarrassed.

It just didn't recognize the completed work.

It felt compelled to restart its own routine from stage three and tried to complete actions that were already done.

It just highlights that instincts are fixed, sequential, almost ritualistic.

It's so much like performing a deeply ingrained unconscious habit.

Exactly.

And this comparison raises a potential mechanism for how instincts might originate in the first place, inherited habit.

Right.

If a habitually performed action, like a dog retrieving a stick, generation after generation, if that becomes inherited, the end result would be basically indistinguishable from a true spontaneous instinct.

Darwin totally acknowledges this.

He says there is evidence that habitually acquired actions can sometimes be inherited.

He uses the analogy of a child prodigy.

Like Mozart.

Like Mozart, yeah.

If Mozart, instead of playing piano with just a little practice, had truly played complex music with no practice whatsoever, that would be a perfect instance of inherited habit turning into true instinct.

But this is where Darwin draws a really major line in the sand.

He introduces a crucial caveat that separates his theory from earlier Lamarckian ideas.

It's a critical distinction.

He warns us against believing that the majority of complex, wonderful instincts, like the incredible engineering of the hive bee or the social complexity of ants,

could have been acquired solely by habit and then inherited.

And why is he so firm on this?

Because for a habit to be acquired and then inherited, the individual has to perform the action, benefit from it, and then pass it down.

But we have all these complex instincts performed by sterile individuals, the neuter ants and bees.

Ah, right.

They can't have offspring.

Exactly.

So their habits, no matter how long their practice, simply cannot be passed down.

This means selection has to be the primary driver,

acting on spontaneous, profitable variations.

Habit is secondary or subordinate.

That distinction is just fundamental to the whole argument.

The core hypothesis, then, is all about spontaneous variation plus selection.

Right.

If instincts are crucial for a species'

welfare and conditions change, any slight advantageous modification of that instinct gets preserved and accumulated over time.

It's the exact same logic he uses for structural evolution, just applied to the mind.

But since we can't see the ancestors, he has to show that these gradations are at least possible by looking at sort of cousin species.

Precisely.

Which brings us to step one of his proof,

showing that instincts are not, in fact, fixed perfect endowments, but that they do vary and are inherited.

Because without variation, there's nothing for selection to select.

Exactly.

Variation and inheritance are indispensable, and instincts absolutely do vary.

We see it all the time in the migratory instinct, which can be variable in its extent like how far they fly its direction and even its total loss, where some populations just become sedentary.

Or even something as seemingly fixed as nest building.

Bird nests show remarkable variation.

Sometimes it's because of the site they choose or the climate, but often it's from totally unknown causes.

Audubon, the great ornithologist, documented these striking differences in the nests made by the very same species of bird in the Northern versus the Southern United States.

So behavior we just assume is hardwired turns out to be flexible.

And responsive to the environment.

Even the most ingenious instinctive behavior, the hive bee's construction, can show flexibility when circumstances demand it.

How so?

Well, Darwin noted observations of bees adapting when wax was deficient.

They'd work with wax hardened with vermilion or softened with lard.

If they couldn't find propolis, they'd use a cement made of wax and turpentine they found nearby.

They'll even use oatmeal if they can't find pollen.

So the instinct, while highly refined, isn't totally rigid.

There's enough variability for selection to work on.

Planky.

Let's look at a really deeply rooted fundamental instinct, fear.

How does Darwin use the slow evolution of fear to illustrate his point about inherited mental traits?

Fear, he argues, is largely an inherited instinctive quality.

You can see it in nestling birds that react to sudden movements, even though it's strengthened by their own experience.

But the acquisition of fear of man in the wild is just a perfect analogy for selection acting on a mental trait.

And he uses the difference between large and small birds in England to show this.

He does.

The larger birds in England are generally much wilder and wearier than the smaller ones.

And you have to ask why?

Because humans have hunted them more intensely.

That's it.

Historically, man has persecuted and hunted the large birds, the game birds, the raptors, with far more intensity than the tiny common songbirds.

So over generations, only the large birds that inherited a greater tendency to flee and be wary of humans survive to reproduce.

The wildness is an inherited response, a mental trait selected by hunting pressure.

Exactly.

And conversely, Darwin points out that on remote, uninhabited islands, large birds are often no more fearful than small birds because they've never been hunted.

He mentions the magpie, which is wary in England but tame in Norway, and the hooded crow, cautious in Europe but completely tame in Egypt.

It's a great comparison.

It shows that a mental quality -like wariness isn't some divinely fixed thing, but a variable, selected response to the environment.

And this all leads to a really essential principle guiding selection, the rule of self -interest.

Darwin is very firm on this.

Instinct is produced for the good of the species itself, never for the exclusive good of another species.

And to prove this, he uses the surprising relationship between aphids, plant lice, and ants.

Right.

The observation is that ants seem to milk the aphids, collecting a sweet, sugary excrucian, often called honeydew.

And observers, like Huber, suggested the aphids were voluntarily giving this to the ants.

Which looks like an instinct that benefits another species.

It does.

And that would contradict the entire mechanism of selection, which requires an advantage to the organism performing the action.

So Darwin, you know, being Darwin, he conducted a classic experiment.

What did he do?

He isolated a group of aphids for several hours, making sure they must have had a buildup of this sugary, extremely viscid substance.

Then he tried to mimic the ants by stroking or tickling the moment he introduced a single ant to the group, it began to stroke the aphids abdomen with its antenna.

Instantly, each aphids lifted its abdomen and extruded a clear drop of the sweet juice which the ant devoured.

Even with the very young aphids.

Even with the very young ones, showing it wasn't a learned behavior.

So it's not a voluntary altruistic gift.

It's an instinctive, tactile response to the ant's antenna.

But here's the crucial conclusion.

Right.

The excretion is viscid.

If the aphids doesn't get rid of it, it will eventually build up and foul the animal, leading to disease.

The ant attendance, therefore, is a huge convenience for the aphids.

It keeps it clean.

So even though the ant benefits, the action ultimately serves the aphids own welfare.

It's a great bit of detective work.

And it supports his strict rule that selection only favors actions that benefit the organism or its immediate family.

And that leads to one final point before we move on.

Imperfection.

Darwin notes that instincts aren't absolutely perfect.

And that fact is so important.

Why?

Because imperfection is the smoking gun against the idea of special creation.

If instincts were created perfect, they'd never make a mistake.

But if they're the result of gradual accumulated variations, they will carry imperfections.

The cuckoo sometimes laying in the wrong nest or an ant getting confused.

The mistakes support the theory.

They do.

Which sets the stage perfectly for our next segment, where Darwin uses domestic animals to bridge the gap between variation in the wild and the power of selection.

Right.

If variation happens in the wild, let's look at the clearest example of selection in action.

Our own domestic animals.

Domestic animals are just so critical because they demonstrate the possibility of inherited variations of instinct in nature.

They show how inherited habit and human selection have molded behavior really, really rapidly.

Man just acts as the selective force, amplifying quirks.

And we see this most clearly in specialized breeds of dogs.

Oh, absolutely.

The tendency of young pointers to point and back other dogs the very first time they're taken out.

No practice at all.

That's inherited.

Same with retrievers retrieving.

Or shepherd dogs running around a flock of sheep instead of straight at them.

Darwin argues these domestic actions, pointing, retrieving, circling, they are essentially true instincts.

So if they're true instincts, why do they sometimes seem less fixed, less perfect than, say, a wild wolf's hunting instinct?

Because they've been acted upon by far less rigorous selection and for a much, much shorter period of time.

I mean, if we saw a wolf naturally standing motionless like a statue when it scented prey, we'd immediately call that action instinctive.

In the domestic dog, we've simply selected and magnified an exaggeration of that natural pause before springing.

And the fact that you can blend these inherited mental qualities, that just confirms they operate like physical traits.

Yes.

Cross breeds shows their mental qualities blend.

It's well known that crossing a dog with a bulldog has affected the courage and obscenity of greyhounds for generations.

The qualities aren't just lost, they persist and blend, just like natural instincts would in the wild.

OK, let's go back to that distinction we drew earlier.

How are these domestic instincts actually acquired?

Is it selection alone or is it just from long continued compulsory habit?

Darwin gives examples showing both extremes.

We see selection acting on spontaneous variation alone in tumbler pigeons.

They do the strange, complex tumble in the air without ever having seen it done.

So how did that start?

Darwin figures a pigeon just showed a slight spontaneous tendency toward this behavior.

And man, finding it interesting, selected and bred only the best tumblers.

Now you have birds that sometimes can't fly 18 inches high without automatically going head over heels.

Which proves selection can create these complex specialized instincts from just minute accidental variations, even without habit.

Right.

But then you have the really compelling case where habit seems to be the main driver.

And that's the change from extreme wildness to tameness, especially in rabbits.

Darwin finds it hard to believe that domestic rabbits have been rigorously selected just for tameness.

He does.

The young of the wild rabbit are famously difficult to tame, but the young of the tame rabbit are tame instantly.

He concludes that this inherited change is largely due to habit and long continued close confinement over many generations.

It's a kind of compulsory habit.

The environment has removed the need for wildness.

And selection under domestication can also lead to the loss of natural instincts.

The loss of the broody instinct in domestic fowls is a great example.

Many breeds have been selected not to sit on their eggs because humans want continuous egg production, not incubation.

And there's that subtle complex change with the loss of protective fear responses.

Indeed.

Look at young chickens.

They've lost their instinctive fear of dogs and cats, which was present in their wild ancestor.

However, they haven't lost the basic fear response itself.

What do you mean?

If the mother hen gives that specific danger chuckle, the young chickens will instantly run and conceal themselves.

The response is still there.

That sounds adaptive,

but there's a maladaptive twist.

The irony is that the ingrained instinctive response cowering to let the mother escape is now totally useless because the mother hen through generations of domestication has almost entirely lost the power of flight.

It's an inherited instinct kept by the young, but it's been rendered meaningless by a change in the parent.

It's a perfect example.

And it makes the conclusion so clear.

Instincts are acquired or lost in domestication by a complex mix of habit, selection, and the accumulation of these peculiar mental actions that first appeared by accident.

And this variability is the raw material.

So let's now apply this framework to the truly wonderful and complex instincts we find in nature, starting with one of the most famous and frankly disturbing cases,

the parasitic cuckoo.

The cuckoo laying its eggs in other birds' nests seems at first glance like a huge leap in behavior.

How could natural selection possibly generate a habit that's so specialized and, you know, seemingly designed?

Darwin first identifies the immediate cause, the practical reason for this behavior.

And it's all logistical.

The European cuckoo lays her eggs at intervals of two or three days.

If she made her own nest and incubated, she'd end up with eggs and young birds of widely different ages, making feeding incredibly difficult.

And she has to migrate early.

Exactly.

The extended incubation time is just impractical, but the American cuckoo doesn't do this.

No, it makes its own nest.

It does.

And Darwin suggests the evolutionary path started there.

Suppose the ancient European ancestor occasionally, aberrantly, laid an egg in another nest, maybe just due to stress or inconvenience.

And if that occasional habit was profitable, maybe it allowed the parent to emigrate earlier or it produced more vigorous, young selection would favor it.

And crucially, the young that were reared that way would be likely to follow that same occasional aberrant habit by inheritance.

And through continuous selection, that slight deviation becomes a fixed, necessary instinct.

And there's even evidence of reversion.

Yes.

The rare instances of cuckoos being seen laying eggs on the bare ground, sitting on them and feeding their own young, it's like an echo of that long -lost Aboriginal instinct of nidification.

But this immediately raises the critical question that you, the listener, are probably asking right now.

The cuckoo strategy needs several specialized, coordinated adaptations.

The small egg size and that horrific ejection instinct of the young bird.

Doesn't that look like a massive, instantaneous leap that couldn't have been gradual?

That is the core challenge.

So let's take the egg size first.

The European cuckoo's egg is remarkably small, often the size of a skylark's egg, even though the cuckoo itself is four times larger.

And Darwin sees this as a real adaptation.

He does, especially because the non -parasitic American cuckoo lays full -sized eggs.

And he points to the Australian bronze cuckoo, a cousin, which shows great variability in egg size and color.

That confirms that the raw material for selection is there.

So the size variation exists.

Why select for smallness?

Darwin speculates it might be for deception to make them less obvious to the host parent or more likely because smaller eggs correlate with a shorter incubation period.

Which gives the young cuckoo a vital head start on its foster siblings.

Exactly.

And he notes that while many cuckoos prefer nests with eggs similar in color to their own, the European cuckoo often departs from this, laying pale eggs in the bright blue nests of the hedge warbler.

This imperfection, again, confirms the gradual imperfect process.

Okay, now we have to address the most odious instinct, the ejection.

The blind, helpless infant cuckoo has the strength and a specialized broad concave back that's basically designed to lever its foster brothers and any remaining eggs out of the nest to die.

How could something like that have been acquired gradually?

Darwin argues it was achieved through the gradual accumulation of, well, the blind desire, the strength, and the necessary structure.

If getting the maximum amount of food right after birth was crucial for survival, selection would favor anything that helped create that monopoly.

So what was the first step?

The initial step might have been just unintentional restlessness on the part of the slightly older, stronger bird.

If that restlessness sometimes led to a weaker sibling being jostled out, which meant more food and better survival, that habit would be improved and transmitted to an earlier and earlier age.

Until selection favored the blonde, unreasoning desire, and the proper structure to do the work of ejection.

Precisely.

It shows that modifications of instinct and structure can be transmitted to the young, just as they are to the adult.

He compares it to other young birds instinctively breaking through their own shells, or young snakes getting a temporary sharp tooth just to cut through the egg shell.

Those are instincts and structures that are present only briefly in the young, but they're still just as susceptible to modification.

Exactly.

And to really solidify this idea of gradualism, Darwin needs to show transitional forms in collateral species.

So he brings in the American starlings, the molothrous, which show parasitic habits in varying degrees of imperfection.

This is key evidence that the cuckoo's perfection wasn't instantaneous.

It is.

We see a clear three -stage gradation.

Stage one, molothrous bodius.

The imperfect self -builder.

Yes.

This species sometimes builds its own nest, sometimes appropriates one, and occasionally lays eggs in other nests.

They're mostly self -sufficient, but they show that occasional aberrant parasitic tendency, the necessary starting point.

And then stage two, molothrous planariensis, the highly imperfect parasite.

This species is invariably parasitic, but it is terrible at it.

They often start building an irregular, untidy nest, but never finish it.

And even more wastefully, they lay way too many eggs, sometimes 15 to 20 in the same foster nest, which means very few will hatch successfully.

And the incredible wastefulness goes even further.

It does.

They have this strange wasteful habit of pecking holes in the eggs, both their own and the foster parents, wasting a huge number.

This combination of incomplete nest building, overlaying, and destructive behavior points to an instinct that is clearly, visibly still in transition.

And finally, stage three, molothrous picortis.

This is the North American species that has achieved the parasitic perfection of the European cuckoo.

It lays only one egg per foster nest, ensuring successful rearing.

This clear, observable gradation from occasional to imperfect to perfect parasite strongly supports Darwin's path of slow modification.

And the observer of the imperfect one, Mr.

Hudson, was so struck by it.

He was.

Even though he was skeptical of evolution, the visible imperfection forced him to quote Darwin's words, asking if these habits must be viewed as small consequences of one general law, namely transition.

The imperfection itself confirms the process.

Okay, now let's move to the ultimate display of social instinct.

The Slave -Making Ants.

Part four tackles one of the most unbelievable examples of specialized complex behavior out there.

This is the instinct that Darwin and other naturalists considered one of the most wonderful of all.

And we have to start with the extreme case of dependence.

The ant species for mycorrhufosans.

Just detail the extent of their helplessness.

It's staggering.

This species, discovered by Huber, is absolutely dependent on its slaves.

The masters do no work of any kind except capturing future slaves.

They can't make nests, they can't feed their own larva, and they can't even feed themselves.

If the nest has to migrate, the slaves carry the masters.

The slaves determine the route and carry the masters in their jaws.

It's incredible.

Huber's experiment proved this absolute helplessness.

What did he do?

It's so stark.

He isolated 30 masters, gave them plenty of food, their own larvae, and pupa.

They did nothing.

They couldn't coordinate to feed themselves, and many starved to death.

But when Huber introduced a single slave, a single F.

fusca, she instantly went to work, fed the survivors, made cells, and tended the larvae.

The masters have totally lost the ability to survive without forced labor.

If I were the only example, the origin would seem just inexplicable.

But there's a transitional form.

The slave maker for Micah Singuine.

Exactly.

Half Singuine is the less perfect intermediate step.

The masters are red, the slaves are black, and half the size.

When you disturb the nest, the slaves work energetically with their masters, carrying larvae and pupa to safety.

They feel completely at home.

And their roles actually vary by location, which is strong evidence of the variability of instinct in the same species.

It is.

In England, Darwin observed that sanguineous slaves are strictly household slaves, who rarely leave the nest.

The masters do all the foraging.

But in Switzerland, Huber saw that the slaves habitually work with the masters to build the nest and tend the aphids for food.

Which suggests the instinct is adapting to local conditions, maybe the number of slaves they have.

That's Darwin's suggestion.

And the migration behavior is also reversed from the completely dependent species.

Right.

The masters carry the slaves.

Yes.

Darwin observed a migration where it was the masters carefully carrying their slaves in their jaws, which reinforces that this is a far less dependent transitional form.

They haven't lost the ability to do complex tasks.

Okay, let's talk about Darwin's amazing experiment to test the ants'

instinctual discrimination during a slave raid.

This proves a high degree of specialized, inherited behavior.

It's fantastic.

He witnessed the masters raiding an independent community of the habitual slave species, F.

fusca.

They killed the smaller ants, but were kept from the pupa.

So Darwin then placed two parcels of pupa nearby, one from F.

fusca and one from a species they rarely capture, F.

flava.

And what was their immediate instinctive reaction?

The F.

sanguineum masters eagerly and instantly seized the pupa of F.

fusca.

But when they encountered the pupa or even just the earth from the nest of F.

flava, they were initially terrified.

They ran away.

Really?

Yes.

Only after the little yellow F.

flava ants had crawled off did the masters take heart and cautiously carry off those pupa.

It shows a developed, inherited discrimination in the raiding instinct,

a mental ability to instantly recognize the proper slave species.

Which has been accumulated by selection.

So how does Darwin hypothesize this all started from something simple?

He suggests that non -slave -making ants, when they're foraging, will often carry off pupa of other species that are scattered near their nests, probably for food.

If these captured pupa were stored, and some of them developed into working adults by accident, and those adults started working for the new colony, and this turned out to be advantageous, well, then natural selection would step in.

Because it's easier to capture ready -made workers than to rear them yourself.

So selection strengthens the habit of collecting pupa, but it changes the purpose.

Exactly that.

The habit of collecting pupa, which started as a way to get food, is rendered permanent for the different purpose of raising slaves.

And this could progress gradually from the state of F.

sangrinia, where the masters still work, to the absolute helpless dependence of F.

ruficens.

So that path, starting from a small, profitable variation, unintentional servitude,

explains one of the greatest complexities in the social world.

Now we move to what is often considered the crowning glory of instinct.

The geometry of the hive bee cell.

The hive bee cum is, I mean, it's rightly celebrated.

Mathematicians have shown that the hexagonal structure perfectly solves this incredibly complex problem.

Making cells of the right shape to hold the most honey with the least amount of expensive wax, it just seems to require this innate, complex, geometric knowledge.

And yet Darwin argues this beautiful, mathematically precise structure falls from just a few simple instincts, demonstrating his principle of gradualism, even in the realm of perfect optimization.

We have to trace the steps.

We start with the humble bee.

Which just uses old cocoons and adds irregular, rounded wax cells.

Not very economical.

Right.

And our midpoint in the gradation is the Mexican melapona domestica.

The melapona forms cylindrical breeding cells, but its honey cells are nearly spherical and just aggregated into an irregular mass.

But the crucial insight comes from seeing how the cells relate to each other.

It does.

They're aggregated so closely that they would intersect if they were perfect spheres.

But the bees prevent this by building perfectly flat walls of wax along the lines of intersection.

That shared wall is the key step toward efficiency.

And one cell rests on three others.

Yes.

So the three flat surfaces built by the melapona unite into a small pyramid, which Huber noted is a gross imitation of the three -sided pyramidal base of the hive bee's cell.

The melapona is saving wax and labor just imperfectly.

Darwin's realization here is just brilliant.

He understood that if the melapona had simply made it spheres of perfectly equal sizes and arranged them symmetrically, the flat surfaces of intersection would automatically result in the perfect hexagonal shape.

Right.

He needed a mathematician to confirm this physical consequence.

So he consulted Professor Miller, a Cambridge geometer, who gave him this stunning mathematical confirmation.

What did he find?

Miller calculated that if you pack a number of equal spheres symmetrically in two parallel layers at a precise optimal distance, the radius times about 1 .41, the planes of intersection between these spheres mathematically result in the exact angles of the hive bee's hexagonal prism and its three -sided base.

So the perfection is a consequence of simple physical laws, not innate geometric knowledge.

Exactly.

The bees don't need to know geometry.

They just need three simple instincts, sweeping equally sized spheres, keeping them equidistant, and building flat walls where those spheres intersect.

And this mechanical explanation is supported by observed imperfection.

It is.

Professor Wyman found that the accuracy of the bees' workmanship had been greatly exaggerated.

They're not perfect geometers.

The angles often vary.

This imperfection supports the view that they're striving toward a mathematical ideal dictated by the pressure of neighbors, not executing a flawless divine plan.

And Darwin then devised two remarkable experiments to test whether bees build by this geometric sphere sweeping method.

Let's start with the thick wax strip experiment.

He placed a long, thick, rectangular strip of wax between two established combs.

And the bees, working from both sides, began to excavate these minute spherical pits.

Where the rims of the pits intersected, they stopped excavating and started building flat walls along the intersection lines.

So the result was a hexagonal prism built on what he called a scalloped edge.

Yes.

Which proves they used the sphere model excavating from a rough mass until they hit the plane of intersection.

Then came the second, more precise experiment using a thin, knife -edged ridge colored with vermilion wax.

This forced them to work on opposite sides of a very thin potential wall at the same time.

Right.

They excavated basins on both sides and they stopped their work just in time so that the bottoms of the basins didn't break through.

Instead, they left behind these thin little plates of the vermilion wax, the shared base located exactly along the planes of imaginary intersection.

They coordinate their work across the wall.

Stopping precisely when that optimal, ultra -thin shared base is achieved.

And sometimes they even make mistakes.

Darwin saw half -completed rhombic bases that were slightly concave on one side and convex on the other.

But when he put the comb back, the bees, using the warmth of the wax, would push and bend the walls into the proper intermediate plane, correcting their own geometric errors.

Which brings us to their remarkable building mechanism.

They build a thick, rough rim, 10 to 20 times thicker than the final wall.

And then gnaw it away from both sides.

This unique process, which he compares to masons piling up cement and cutting it away equally, ensures both great strength, that thick rim or gigantic coping, at the top, while achieving the ultimate economy of wax in the shared walls.

And the motive power for all this is simple, yet immense, economy.

Yes.

Because wax secretion is incredibly energy -intensive.

Experiments have shown that 12 to 15 pounds of dry sugar are consumed by a hive just to secrete one pound of wax.

Saving wax means saving immense quantities of honey and thyme, which is essential for supporting large colonies through the winter.

So any slight variation toward making cells closer together, maximizing shared walls, would be favored by natural selection.

Leading step -by -step from the irregular melapona model to the hive bee's perfect economical architecture.

We've covered complexity, gradualism, and selection.

But we are now facing the final, most serious objection.

The one Darwin himself admitted was initially insuperable.

The neuter and sterile insects.

This is the acme of difficulty for the entire theory.

Sterile female insects, the workers and soldiers, often differ widely in instinct and structure from their fertile parents, yet they cannot propagate their own specialized traits.

So how can natural selection operate without inheritance by the individual?

This is the big one.

Darwin has already established that specialized instinct and corresponding structure, like a beaker of foot, can evolve together.

Yes, those examples showed that structural and behavioral change can be gradual and correlated.

But even putting that aside, the sterile insects remain the true conceptual problem.

If a worker ant were an ordinary, fertile animal, Darwin could just say its profitable modifications were accumulated and transmitted.

But it's sterile.

It can't transmit anything.

So the first step is just to establish that structural differences can be correlated with sterility in the first place.

Which we see all the time.

We do.

We see differences in structure correlated with age, like the hooked jaws of the male salmon, and with sex.

So a difference in structure correlated with the sterile condition of workers isn't the difficulty.

The difficulty is accumulating those specific, profitable modifications by selection when the individual can't reproduce.

And this is where the solution finally emerges.

Selection must be applied not to the sterile individual, but to the family or the community.

Darwin uses these powerful analogies to make this clear.

Think about cattle breeders.

They select bulls and cows not based on their own immediate meat quality, but based on the quality of the sterile oxen that they're offspring that they produce.

So they're selecting the fertile parents for their tendency to produce

superior sterile offspring.

Exactly.

Or with flowers.

Certain varieties of double annual stock flowers produce sterile double flowers, which are the desired product.

But they're propagated only by the fertile single flowers.

The fertile parents are selected for their tendency to produce the advantageous sterile variety.

So applied to social insects.

It means any slight modification of structure or instinct correlated with the sterile condition that proves advantageous to the community will be favored.

The fertile males and females who produce the best workers will flourish and transmit to their fertile offspring a tendency to produce sterile members with those same modifications.

But the acme of the difficulty remains.

That neuters sometimes differ widely from each other, being divided into two or three distinct castes, soldiers, workers, honeypots, that often don't graduate into one another.

If selection is gradual, why are there no intermediates?

Darwin maintains this immense confidence in the principle, asserting that the intermediates ought to be found occasionally, and they are.

Mr.

F.

Smith showed that the neuters of several British ants very surprisingly in size and color, and the extreme forms can be linked by individuals taken out of the very same nest.

It does.

This ant has two main worker sizes, with some intermediates.

The larger workers have simple eyes, or a celly, which are clearly distinguishable.

The smaller workers have them in a rudimentary state.

And crucially, Darwin affirmed that the eyes in the workers of intermediate size are in an exactly intermediate condition.

Two bodies of sterile workers differing not only in size, but in specialized sensory organs, yet connected by members in an intermediate state.

The most dramatic illustration, though, comes from the driver ant, Anoma, of West Africa.

Darwin describes the size range as if a workforce included men 5 feet 4 inches high and men 16 feet high, with proportional differences in head and jaw structure.

But the essential fact is that these dramatically different castes

graduate insensibly into each other.

And Sergey Lubbock later confirmed this through meticulous drawings.

This gradation is the key to solving the acme of difficulty.

Darwin argues that natural selection first formed a graduated series, like the driver ant.

Then, by favoring the parents who produced the most efficient extremes, the highly specialized workers and the highly specialized soldiers, the selection process created distinct castes.

Favoring a division of labor.

Exactly.

Which led to the intermediate, less efficient forms just vanishing over time.

So the absence of intermediates in some species doesn't disprove gradualism, it proves the efficiency of selection in specializing the community.

The most efficient parents are those who produce the most distinct specialists.

Exactly.

And this demonstrates that the greatest difficulty the theory encountered, the sterile castes, is overcome, not only by selection acting on the family, but by proving that the necessary gradation did exist initially before the extremes were favored.

It shows the immense power of selection alone, even without habit playing a role in the sterile individuals.

This entire discussion about instinct serves as this colossal defense of the theory.

Let's just recap the main conclusions that Darwin pulls from this chapter.

So to summarize, instincts, just like physical structures, vary slightly and are inherited.

Natural selection accumulates these profitable variations, leading to complex instincts, with habit playing a supportive secondary role.

We saw that the difficulty of complex instincts, like the cuckoo's parasitism, is solved by finding these transitional, imperfect forms like the molothrus, which demonstrates that slow transitional path.

And the stunning geometric perfection of the hive B cell is shown to be a mathematical necessity that results from a few simple, inherited, sphere -sweeping actions driven by the immense economy of wax and honey -knot divine creation.

And finally, the most serious difficulty, the existence of sterile neuter castes, is overcome by applying selection not to the sterile individual, but to the family,

with evidence showing that the necessary gradations do exist, or at least once existed, between the specialized castes.

And this chapter just reinforces the theory by stressing that instincts are not perfectly endowed and they are liable to mistakes, a fact that's inexplicable if they were specially created, and that no instinct is produced for the exclusive good of another species.

And crucially, the law that natural non -facet saltum, nature makes no sudden leap, applies to instincts as much as it does to structure, confirming the gradual nature of evolution across all traits.

We should leave you with Darwin's final provocative thought from this chapter.

He suggests that it is far more satisfactory to view instincts, even the brutal ones, like the young cuckoo ejecting its foster brothers, ants making slaves, or the ichnumanidae feeding within live bodies, not especially endowed creations, but as consequences of one profound general law leading to the advancement of all organic beings.

Multiply.

Vary.

Let the strongest live and the weakest die.

It brings the specialized mechanics of a single ant's behavior right into the grand unified principle of life.

We hope this deep dive into the evolution of instinct helps you connect the final complex behavioral docs in Darwin's magnificent argument.

Thank you for joining us.

Until next time.