Chapter 9: Hybridism

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Welcome back to The Deep Dive, where we take complex texts, distill the crucial knowledge, and serve it up fast, fresh, and fascinating.

Today we're plunging into one of the most critical battles in evolutionary history, chapter nine of Darwin's On the Origin of Species, titled Hybridism.

This is really the chapter where Darwin faces his biggest theoretical challenge head on.

I mean, the entire edifice of natural selection, it rests on this idea that species arose gradually from varieties.

But historically, the very definition of a species, it hinged on reproductive isolation.

The prevailing belief of the time, and this was the core argument for special immutable creation, was that nature or, you know, a divine power had specifically endowed species with absolute sterility.

A wall, a biological wall to keep themselves.

Exactly, to prevent them from blurring their boundaries.

This chapter is Darwin's systematic dismantling of that final and what seemed like an unassailable barrier.

Okay, so let's unpack those stakes.

If Darwin is right,

varieties, which are fertile, they gradually turn into species.

So there shouldn't be a sharp line.

There can't be.

But what we actually see in the wild is, well, it's the opposite.

Varieties cross easily, and they produce fertile offspring.

We call them mongrels.

But when you cross two distinct species,

you usually get what?

Either total failure to breed, or if you do get offspring, the hybrids, they're sterile, like the famous mule.

The classic example.

That sterility, that sharp dividing line, that seems like the single most theoretical hurdle to his whole idea of gradual evolution.

It is the core test.

For the theory to hold water, that apparent sharp line of sterility between species and fertility within varieties, it has to be revealed to be, well,

fundamentally illusory.

So Darwin's mission here isn't just to describe it.

Oh no, it's to prove scientifically that this reproductive difficulty is not a special intentional preventative measure.

It has to be an incidental result of general biological loss.

Side effect.

A side effect of divergence, exactly.

Something just as likely to appear within a single species as between two.

And before we get into the wheeze of the experiments, he immediately forces us to make a crucial distinction.

Yeah, he says, you know, these two things often get confounded in biological discussions, but we're talking about two separate classes of facts.

Okay, what's the first one?

The first is the sterility of first crosses.

So this happens when you take two pure distinct species, both with perfectly formed and functionally healthy reproductive organs, and you try to unite them.

And they just fail.

They often produce few or zero offspring.

The sexual elements are structurally perfect, but they just fail to unite effectively or develop.

It's a problem of getting the cross to happen in the first place.

In the second class.

That's the sterility of hybrids themselves.

So here, the cross actually succeeded.

The offspring, the hybrids are born.

It might look structurally perfect, you know, vigorous mule, a beautiful hybrid plant, but it can't reproduce, right?

Its own reproductive organs, and Darwin specifies this often applies particularly to the male element or the pollen, are functionally impotent or imperfectly developed.

Understanding that distinction is absolutely essential, then it is because Darwin needs to demonstrate that the cause underlying both of these things, the difficulty of the initial union and the failure of the hybrid to reproduce is tied to the incredibly sensitive and easily disturbed nature of the reproductive system, not some targeted divine decree to keep species separate.

So to begin this deep dive, he turns to the foundational work of two guys who basically dedicated their lives to this.

Colerator and Gardener.

Yeah, they essentially established the experimental protocols for hybridizing plants.

And what did they find about the degree of sterility?

Well, their work certainly confirmed what he calls the high generality of some degree of sterility.

You try to cross two forms that are recognized as distinct species and, you know, chances are high you're going to run into some kind of reproductive trouble.

But their ultimate conclusions about the universality of sterility were surprisingly contradictory.

And this is where Darwin finds his first major crack in that creationist argument.

Okay, let's start with Colerator.

He was working earlier and he saw a lot of cases where supposed species were actually quite fertile when he crossed them.

So how did he deal with that?

Well, Colerator was deeply committed to this idea that absolute sterility was the rule for true species.

It was definitional for him.

So when he found 10 examples where forms that everyone recognized as distinct species turned out to be quite fertile, he didn't question the rule.

He questioned the species.

He simply cut the knot, as Darwin puts it.

He just reclassified those 10 fertile forms as varieties instead of species.

It feels a bit like cheating, doesn't it?

It's circular reasoning.

It's highly circular.

He's using sterility as his definition of a species, which makes his conclusion that all species are sterile true by definition.

It doesn't matter what the structural evidence says.

Right.

If fertility proves they aren't species, then of course all species are sterile.

It's an unbreakable loop.

Precisely.

Now Gartner, who came later, was much more meticulous.

He disputed Colerator's claim of absolute fertility in those exceptions.

He insisted that if you looked hard enough, you'd find at least a slight degree of sterility.

But the key phrase there is looked hard enough.

Yes.

The key thing to note is that to prove this slight degree of sterility, he often had to resort to painstakingly counting seeds, sometimes over thousands of attempts.

Wait, he had to count individual seeds to prove a line existed?

That in itself sounds evidence against some sharp absolute barrier.

Absolutely.

The need to resort to such minute differences just reveals how blurry the line was in practice.

If species were specially created with the sterility barrier, you shouldn't need a microscope to find it.

It should be obvious.

Darwin points out that even Gartner's meticulous methods introduced what we'd now call significant experimental confounding variables.

A huge one.

This is a major insight into the hybrid and not accidentally self -fertilized.

A botanist has to carefully manipulate the flower.

Right.

You have to castrate it, remove the anthers, keep insects away.

Exactly.

And those manipulations are inherently injurious to the plant's natural fertility.

Gartner himself, in a control experiment, found that when he artificially fertilized about 20 pure species with their own pollen, nearly half of them showed impaired fertility.

So the sterility you see in the species cross might not be because of the foreign pollen at all.

It could just be because the reproductive organs were damaged by the experiment itself.

That's a powerful implication.

If the act of artificial fertilization messes up the fertility of pure species by 50%, then how can you confidently say that the sterility in a hybrid cross is only because of the species difference?

You can't.

And the evidence against this universal rule gets even stronger when he brings up the pimpernails.

This is a critical example.

Gartner crossed two forms, the common red pimpernail and the blue pimpernail.

Now, the best botanists of the time classified these two as mere color varieties.

So same species, different color.

Structurally identical, just different petal color.

Yet when Gartner crossed them, he found them to be absolutely sterile.

Hold on.

Let me stop you there.

That is completely counterintuitive.

If forms recognized as varieties, which have to come from a common fertile parent, can be absolutely sterile.

Then sterility cannot logically be the certain distinction that defines a species.

It proves the barrier isn't some deep structural thing.

Exactly.

If two varieties can evolve to be mutually sterile, then sterility is just a physiological accident that can happen rapidly, not some fixed divinely endowed characteristic of the species level.

And that leads Darwin to the foundational conclusion of this whole section.

Sterility is not an either -or situation.

It's highly variable and it graduates insensibly.

So the evidence for sterility, just like for structural differences, is doubtful.

Neither perfect fertility nor absolute sterility gives you a clear, certain distinction.

The line is in fact permeable and fuzzy.

Okay, so he's established that the line is blurry.

Now let's look at the next big assertion from the hybridists.

Gartner insisted that hybrid fertility never increases in successive generations, but it generally decreases, often suddenly.

Right, which would imply that nature is actively trying to eliminate these hybrid forms, that they're unnatural.

Well, how does Darwin counter that?

He introduces an independent, powerful factor,

the negative effects of too close interbreeding.

Oh.

Experimentalists, in their attempts to maintain a pure hybrid line, they often have to raise them in very small numbers.

And to prevent contamination from foreign pollen, they're forced to self -fertilize them carefully, generation after generation.

And this tacked into a recurring theme in all of Darwin's work, right?

The profound negative effects of continuous self -fertilization or, you know, incestuous breeding.

That's right.

Darwin's own experiments showed that close interbreeding lessens both vigor and fertility,

while an occasional cross with a distinct individual, even from the same variety, it boosts both.

So when Gartner sees that hybrid fertility is dropping off a cliff,

Darwin is suggesting it's not just the hybridism.

It's a compounding effect.

The sterility that's already there from the hybrid origin gets massively exacerbated by the experimental procedure, which forces this close inbreeding.

It makes the hybrid seem artificially more sterile than they would be in nature.

This sets up a great paradox that Gartner himself observed over and over.

Artificial fertilization sometimes inexplicably increased fertility of less fertile hybrids.

If manipulating the plant is generally bad for it, why would careful human intervention sometimes make things better?

And Darwin uses this paradox brilliantly to prove his point.

When a careful hybridizer like Gartner artificially fertilizes, even if he means to use the plant's own pollen, he might accidentally take pollen from a different flower on the same plant.

Or from a different plant altogether.

Or, crucially, from a genetically distinct hybrid plant.

And that act, whether it's accidental or on purpose, ensures a slight cross between distinct individuals.

So by intervening, Gartner was actually preventing the worst effects of self -fertilization, or sulfing, that would happen if the plants were just left alone.

Exactly.

That strange increase of fertility is accounted for by avoiding incestuous breeding, which is a universal biological law, not some specific law of hybridism.

The observed decrease was not the inevitable fate of the hybrid state, it was an artifact of the experimental design.

This leads directly into the work of the honorable and reverend W.

Herbert, whose results just completely contradicted this rule of universal sterility.

Herbert was an incredibly skilled horticulturalist.

He often worked in hothouses so he could provide optimal stable conditions.

And he was emphatic that some hybrids are perfectly fertile.

Even on the same species that Gartner claimed were sterile.

Yes, which just highlights the incredible sensitivity of the reproductive system to environmental factors.

The specific example Darwin cites is fascinating.

Cranum capense, fertilized by crinum revolutum.

He says, every single ovule produced a plant.

Herbert stated that the fertility was equal to, or even more than, the natural fecundation of the pure species.

This is a case of perfect, even abnormal excess, fertility between two forms recognized as distinct species.

If a hybrid can be more vigorous and fertile than its pure parents, then sterility can't possibly be an inevitable rule.

Not at all.

It can't be a fixed barrier.

This whole section really hammers home the idea that fertility is highly dependent on what Darwin calls slight and mysterious causes, including whether the plant is crossed at all.

The cases of self -impotence illustrate this perfectly.

Oh, absolutely.

Certain species, like in the genera lobelia, verbascum, passiflora, they have individual plants that cannot be fertilized by their own pollen.

Even though the pollen is perfectly good.

The pollen is viable on other plants, but they are self -impotent.

Yet, and this is the crucial part, they are easily fertilized by pollen from a distinct species or even a complex hybrid.

The hippostirastrum alchemy example is just incredible.

A flower fertilized by its own pollen died down.

Perished immediately.

But when it was fertilized by the pollen of a compound hybrid from three distinct species, it grew vigorously and bore good seed.

The biological message there is amazing.

The plant is better off mating with a highly complex, impure foreign element than with itself.

It demonstrates that the ability to cross is less about species boundaries and more about avoiding some unknown internal physiological incompatibility.

The act of crossing, even with something wildly different, can sometimes rescue the plant from self -impotence.

It just further dismantles this idea of sterility as an absolute species barrier.

And Darwin reinforces this by that professional nurserymen, who raise huge beds of hybrids and just let insects cross them freely, have maintained fertile hybrid lines for years.

Yeah, if hybrid sterility were the universally decreasing thing Gartner claimed, that whole business practice would have failed notoriously.

But it didn't.

Okay, so the plant data is pretty detailed.

What about animals?

Well, the data for animals is much scantier.

And for a very practical reason that Darwin highlights,

a lot of animals, especially wild ones, just do not breed freely under confinement.

Right, the stress of the cage.

If you can't get a canary to breed with a finch, you don't know if that's species incompatibility or just, you know, profound unhappiness affecting their reproductive function.

Exactly.

And just like with plants, the animal experimenters compounded the problem.

They were often forced to cross brothers and sisters and successive generations to maintain the line.

A practice that every livestock breeder knows is a disaster.

It leads to ruin and decreased fertility, even in pure breeds.

So this ensured that any inherent sterility in the hybrids would just increase, leading observers to wrongly conclude that hybrids were inevitably and incurably sterile.

So the experimental method was unknowingly amplifying the very thing they were trying to measure.

That's a great way to put it.

But even with these limitations,

Darwin does manage to introduce some well -authenticated cases of highly fertile hybrid animals.

Yes, the evidence is compelling.

He cites some deer hybrids and certain pheasant crosses.

But the most important case, the one that's cited all the time, involves the common and Chinese geese.

Right, anisignoids.

And these two are so distinct that systematists generally rank them in distinct genera, not just different species.

Exactly.

They are very, very different.

Yet they cross successfully and critically.

Their hybrid offspring are fertile.

More than that, in parts of India, whole flocks of these crossed geese are maintained by farmers specifically for profit, where neither of the pure parent species even exists.

And that single fact maintained for profit in large numbers, without being back crossed to the pure parents, that's definitive proof of high fertility under non -experimental conditions.

It's proof of perfect fertility.

Sterility is clearly not universal, even between forms ranked in different genera.

This shifts the discussion to the profound role of domestication in eliminating sterility, a concept initially put forward by a naturalist named Pallas.

Yes, Pallas' doctrine proposed that while the original wild parent species might have produced sterile hybrids, the process of being reared under domestication subsequently removed that sterility, allowing the hybrid offspring to become quite fertile.

So domestication has this

liberalizing effect on the sexual system.

It makes it less sensitive to wide crosses, that's the idea.

This connects back to our domesticated animals that descended from multiple wild species, dogs, cattle, pigs, whose modern varieties are all perfectly fertile when intercrossed.

But the truly decisive evidence, the one that seals the argument, involves the cattle cross.

We're talking about the Indian humped cattle, Boss Indicus, and the common European cattle, Boss Taurus.

And they show profound, verifiable differences, not just on surface, but in their osteology, their bone structures, their habits, even their voices.

Many naturalists rank them as distinct species.

Yet their crossed offspring are perfectly fertile intercepts.

So if we accept the morphological and behavioral evidence that these are distinct species and the differences are substantial,

then we have to conclude that sterility is not some indelible characteristic.

Exactly.

Sterility is capable of being removed, or at least highly reduced, by the influence of domestication.

This implies a change in the physiological state of the reproductive system.

That's a crucial distinction.

It suggests that wild species living in long periods of uniform natural conditions, they have a delicate reproductive sensitivity that domestication just erodes away.

That's it.

So to summarize this whole section, Darwin has systematically shown that sterility, while extremely general, is not absolutely universal.

And crucially, it is not an indelible fixed characteristic.

It can be removed by environmental change.

And that just removes its power as the ultimate test of a species.

Okay, so we've established the variability of sterility.

Now we shift to examining the laws that govern its appearance.

If sterility were especially endowed to prevent blending, we'd expect it to be simple, predictable, consistent.

A simple mechanism.

But Darwin shows the system is complex, messy, and full of contradictions.

Which is exactly what you'd expect if the barriers are just incidental side effects, not a purposeful design.

Right.

The gradation of fertility is astounding.

From total failure pollen acting like inorganic dust, all the way up to that abnormal excess fertility we saw with the crinum.

And the first lot of challenge in tuition is the lack of strict non -parallelism between how hard it is to make a first cross and how sterile the resulting hybrid offspring are.

I would assume that if a cross is difficult to make, the hybrid must be highly sterile because the parents are so distant.

And if the cross is easy, the hybrid should be fertile.

Is that wrong?

It is often wildly wrong.

We find many cases, famously in the genus Verbascom, the Malanes, where different species cross with unusual facility.

They produce tons of healthy offspring in the first generation.

Right.

But those hybrids are remarkably sterile when they try to breed among themselves.

And the reverse is also true.

Yes.

Other species can only be crossed with extreme difficulty, requiring hundreds of tries.

But the resulting hybrids, when you finally get them, are very fertile.

That's a huge problem for the special creation view.

Why would nature make it easy for two species to unite only to block them later with sterility?

Or make the initial cross almost impossible only to reward the success with fertile offspring?

It defies logic if the goal is absolute separation.

It certainly does.

And while systematic affinity, you know, general resemblance is a big factor, you're not crossing a mammal and a plant.

Right.

Darwin stresses the correspondence is not strict.

He cites allied species that won't unite versus very distinct species that unite easily.

He basically admits no naturalist can define what amount of visible difference is enough to prevent a cross.

The cause is hidden and physiological.

This brings us to the phenomenon Darwin stresses as highly important, the reciprocal crosses.

This is where you cross species A with B and then you reverse it and cross B with A.

And the finding here is that the facility of crossing often shows the widest possible difference.

Cole Reuter, trying to fertilize Mirabilis longiflora with pollen from M.

jalapa, failed utterly after more than 200 attempts over eight years.

Wow.

But the reciprocal cross M.

jalapa female with M.

longiflora pollen was easy and yielded quite fertile hybrids.

What's the implication of that stark difference?

If the general constitution of the species mattered, the difficulty should be symmetrical, right?

It should be.

But it proves that the capacity to cross is completely independent of the species general structure or constitution.

The difficulty resides only in the internal workings of the reproductive systems.

And what's more, the hybrids from these reciprocal crosses, even though they have the exact same genetic mix, they often differ greatly in their subsequent fertility.

This suggests the mother's reproductive system plays a profoundly specific role.

And there are also those singular rules about hybrid appearance.

Darwin notes some hybrids closely resemble only one pure parent.

And this is the contradiction.

Those hybrids, despite looking almost identical to a pure parent, are almost always extremely sterile.

This final point just completely severs any link between what a species looks like and its reproductive fate.

So let's bring this back to the core question.

Given all these messy, variable, and contradictory rules,

does this support the idea that sterility was specially endowed to prevent blending?

Darwin's conclusion is a resounding no.

If it were specially endowed, why is it so variable?

Why allow hybrids to be produced at all, only to stop them later?

The difference in reciprocal crosses is the final nail.

If the goal was separation, why would the mechanism fail one way but succeed the reverse way?

It's a strange and highly inefficient arrangement if the purpose is absolute prevention.

Exactly.

So instead, Darwin proposes that sterility is simply an incidental result of unknown differences that have accumulated in the reproductive systems.

And he uses one of his most brilliant analogies to make this concept concrete.

Grafting.

Grafting is the fusion of the vegetative systems of two plants.

The capacity of one plant to be successfully grafted onto another.

A pear onto a quince versus a pear onto an apple.

It's incidental on unknown differences in their vegetative systems.

Sap flow, growth rate, cellular compatibility.

No one argues that the difficulty of grafting was specially endowed for the plant's welfare.

And the rules governing grafting mirror the rules of hybridism almost perfectly.

Exactly.

Grafting is limited by systematic affinity but it shows exceptions.

It shows reciprocal differences.

A gooseberry won't graft on a current but a current takes on a gooseberry.

And it shows individual variability.

Darwin's philosophical leap here is profound.

If we admit the complex laws of grafting are merely incidental on vegetative differences, we must admit the equally complex laws of crossing are incidental on differences in the reproductive systems.

The difficulty of crossing is vital for the stability of species, sure, but it's just a side effect of bivergence, not a targeted design That's a brilliant analogy.

But okay, he's established sterility as an incidental outcome, not a special creation.

But now he has to preempt a critique from his own supporters.

If divergence is so important, couldn't natural selection have slowly acquired mutual sterility by favoring individuals that were a little less fertile when crossed?

It sounds plausible, right?

That lessened fertility might help keep two new species separate, giving them a survival advantage.

But Darwin argues that natural selection simply cannot produce mutual sterility this way.

He outlines three main difficulties.

Let's start with the first one, separate regions.

Sterility exists between species that inhabit distinct geographical regions thousands of miles apart.

Since they never meet and never intercross, mutual sterility could offer them no advantage whatsoever.

Right, natural selection acts locally in the present.

It can't act on a quality that offers no immediate benefit.

It can't act across continents.

If species A in Europe and species B in America are sterile, that sterility is totally irrelevant to their survival in their own habitats.

Okay, that makes sense.

What's the second difficulty?

It relates back to the complexity we just discussed,

reciprocal differences.

If natural selection were a targeted mechanism trying to optimize for separation, the results should be symmetrical.

Why would male A be sterile on female B, but male B be fertile on female A?

A reproductive barrier achieved by selection should be robust in both directions.

Otherwise, its function is only partially achieved.

Exactly.

The wide difference in reciprocal crosses doesn't look like a mechanism striving for efficient separation.

It looks like a haphazard result of different systems just bumping up against each other.

And the third difficulty concerns the very mechanism of selection itself.

How do you select for greater sterility?

This is the critical, almost mathematical proof.

No advantage and higher sterility.

Once an incipient species is divergent enough that a cross produces only a few sterile offspring, what possible advantage does an individual gain from being slightly more sterile?

If a cross already yields only 10 viable seeds, what mechanism favors the individual that produces only 9?

Selection requires a benefit.

More resources, better survival, more pure offspring, and in this case, increased sterility offers none to the individual.

In fact, it would be a disadvantage.

A slightly more fertile individual would technically be favored.

The selective pressure would always favor fertility, not sterility.

And Darwin brings in conclusive evidence from plants that shows absolute sterility.

The kind that produces no offspring at all cannot possibly have been gained through selection because there's nothing to select from.

This is the German swelling proof.

Yes.

Both Gartner and Kohlreuter showed that you can have a series of crosses all the way to species that never produce a single seed.

But even in this extreme absolute sterility, the pollen of one species can still affect the other one physiologically.

It causes the German, the ovary, to swell.

So it stimulates development, but the embryo never forms.

Or it immediately perishes.

So if no seeds are produced, there are no more sterile individuals in the next generation to be selected.

The line just ends there.

Therefore,

absolute sterility cannot have been gained through accumulating variations.

Right.

And since the laws governing all grades of sterility, from slight difficulty all the way up to this German swelling, are uniform,

Darwin argues the underlying cause has to be the same throughout, which suggests that even the lower grades of sterility were not gained by selection either.

They have to be incidental.

That is a complete repudiation of the idea that sterility is an adaptive feature achieved through natural selection.

Okay.

So since we've ruled out both special endowment and natural selection, we have to look at the immediate causes of this physiological failure.

In first crosses, we mentioned the mechanical problems, like a pollen tube that's too short.

Yes.

But a key factor Darwin stresses was historically overlooked, the frequent early death of the embryo.

The union might succeed initially, but the resulting fertilized cell, the hybrid embryo, is just inherently unstable and fails to develop fully.

And this is where the animal experiments by Hewitt and Salter on hybrid pheasants and fowls come in.

Their findings were profound.

Of about 500 eggs that were successfully fertilized from species crosses, the vast majority perished early.

The chick either failed to hatch, or if it did hatch, it died within a few days from what Darwin simply calls inability to live.

Only 12 chickens were successfully reared from 500 eggs.

Darwin admitted he was reluctant to believe this until he saw the facts.

That demonstrates that first cross sterility is often less about the failure of fertilization and more about the instability of the resulting organism.

It's just two cobbled together to survive.

Exactly.

A new being compounded of two distinct constitutions.

Now we shift to the sterility of the hybrids themselves, where their sexual elements are functionally impotent.

And Darwin draws a critical parallelism here to organisms removed from their conditions.

This is the great physiological concept of the chapter.

Removing animals or plants from their natural habitat, confinement, captivity, sudden changes in climate, it often severely affects the reproductive system, even if their general health is fine.

Think of the classic examples.

Many species of monkeys were the elephant, which often fails to breed in confinement, even when it's perfectly healthy otherwise.

And the similarity between the sterility under confinement and hybrid sterility is remarkable.

In both cases, the sterility is independent of general physical health.

It occurs in various degrees.

The male element is often most affected.

This leads to the theory that hybrid sterility is caused by the disturbance of blending two distinct organizations into one, especially when those organizations have been optimized for long periods under uniform natural conditions.

Precisely.

This disturbance affects the reproductive system in a similar way that disturbed external conditions affect pure species.

The blended organization is unstable, which impairs the delicate development of the reproductive organs.

And when hybrids breed with each other, they just pass on this compounded, disturbed organization.

Which is why the sterility persists and often increases.

And this convergence leads to the articulation of the profound double parallelism, which is central to his argument.

Yes.

On one side of the parallelism, a slight change in external conditions, swapping seeds between soils.

A slight change in diet.

It boosts vigor and fertility.

Okay.

Correspondingly, a slight cross within a species that has varied a little also boosts vigor and fertility.

We call this hybrid vigor today.

And on the other side.

A considerable change in conditions, long -term confinement, captivity leads to sterility.

And correspondingly, a cross between widely different forms, like two species,

produces hybrids that are almost always in some degree sterile.

Darwin asserts this parallelism is not accidental.

No.

It suggests a common underlying cause related to the fundamental principle of life.

The sexual system is extremely sensitive.

Optimal conditions encourage fertility.

Slight disturbance boosts it.

And major, unnatural disturbance, either from the environment or from a wildly different genetic element, causes failure.

He makes the claim that the person who explains why the elephant won't breed in confinement will be able to explain the primary cause of hybrid sterility.

It really connects the mystery of the lab bench back to the mystery of the zoo.

It reframed sterility as a physiological consequence of shock and instability, whether that shock comes from the environment or from a genetic collision.

Okay.

So to finally drive home the point that lessened fertility is an unsafe criterion for a species,

Darwin introduces these highly specialized plants,

dimorphic and trimorphic plants.

Yes.

Like the genus Lethrim or loose strife.

These are plants that exist in two or three distinct forms within the same species.

They differ only in the relative lengths of their pistols and stamens and the size of their pollen grains.

And for them to be fully fertile, they have to have what Darwin calls a legitimate crossing.

Right.

Pollen has to be transferred from stamens of a specific height in one form to pistols of a corresponding height in a different form.

It's an almost mechanical requirement.

But if the plant crosses improperly, in an illegitimate union,

say long stamens, fertilizing long pistols on the same form, the resulting infertility perfectly mirrors the crossing of distinct species.

The analogy is perfect.

The sterility from an illegitimate union within a single undisputed species behaves identically to species crossing.

It varies greatly up to utter and absolute sterility.

It's highly susceptible to external conditions.

And crucially, legitimate pollen is strongly pre -potent over illegitimate pollen, preventing the improper cross.

Just like what happens when you cross pure species with a hybrid.

Exactly.

And just like reciprocal crosses between species, reciprocal illegitimate crosses often differ greatly in their results.

Wow.

In the trimorphic Lethrim Salcaria, one illegitimate cross might yield many seeds with great ease, while the completely reciprocal illegitimate cross might not yield a single seed.

And this proves that the reproductive failure resides only in the incompatibility of the sexual elements due to these positional differences.

Not in general structural differences, exactly.

And what about the offspring of these illegitimate unions?

What happens when you raise these illegitimate plants?

These illegitimate offspring are themselves infertile in various degrees, just like hybrids cross interse.

They are often weak, dwarfed, and incurably sterile, even if you try to unite them legitimately later.

They carry the physiological burden of that improper initial cross.

This leads to Darwin's powerful thought experiment.

Imagine a botanist who knows nothing about dimorphism crosses two forms of the same species illegitimately.

He'd find reduced seed, dwarfed, utterly sterile seedlings, and based on the standard fertility test, he would conclude they were distinct species.

But he would be completely mistaken.

Completely.

And this analogy provides two critical final conclusions.

First, the physiological test of less infertility is no safe criterion of specific distinction.

And second, and fundamentally, the sterility depends exclusively on differences confined to the sexual elements, not on any structural difference in the organism as a whole.

The species barrier has been reduced to a simple physiological mistake.

Okay, so the final step in the chapter is to return to the overwhelming argument that varieties are generally perfectly fertile, and that this is what supposedly creates the broad and clear distinction from sterile species.

Right, and Darwin acknowledges this is usually the case, but he immediately circles back to the problem of circular reasoning.

Here we go again.

He notes, If two forms that we thought were varieties are found to be in any degree sterile together, well, they are at once ranked by most naturalists as species.

If we keep arguing in this circle, the fertility of all varieties will assuredly have to be granted.

It's a guaranteed result of a flawed system.

So we need to understand why domesticated varieties are so uniformly fertile, if not by special design.

And this is where Palace's doctrine comes in again.

It's the key physiological explanation.

Domestication and varying conditions tend to eliminate the tendency to mutual sterility.

Wild species, having been exposed to long periods of uniform natural conditions, they develop a highly sensitive and specialized reproductive system.

Which makes them susceptible to the shock of an unnatural cross.

Exactly.

Domesticated plants and animals, on the other hand, have been exposed to unstable, varying, and often unnatural conditions.

And this has made their reproductive systems robust, generalized, and less sensitive to crosses.

So in essence, domestication unintentionally acts as a selective filter, favoring those varieties whose reproductive systems are tough enough to cross successfully, no matter how different they are.

That's a great way to put it.

The uniform fertility of domestic varieties is not proof of a fundamental distinction.

It's a physiological side effect of their history under human care, which is actively selected against the delicate sensitivities of wild species.

But Darwin still needs to provide the counter -evidence.

The rare, authenticated cases where varieties do show some degree of mutual sterility.

He cites several, starting with Maize.

Gartner crossed a dwarf yellow variety with a tall red variety, and this cross only yielded five grains from thirteen attempts.

Importantly, the resulting mongrels were perfectly fertile, which is why even Gartner himself couldn't possibly classify them as distinct species, despite the difficulty of the initial cross.

Then we have the surprising results with the verbascum or melen varieties.

Gartner found that when he crossed yellow and white varieties of the same species, they produced measurably less seed than similarly colored varieties.

This shows even minor differences, like color, can be correlated with subtle, measurable differences in the sexual elements that induce a slight infertility.

And the Nicotiana, or tobacco case, shows that the reproductive system of a variety can be subtly modified in its compatibility with a different species.

Yes.

Kohlreuter proved that one particular variety of tobacco always yielded hybrids that were less sterile when crossed with the distinct species Nicotiana glutinosa, compared to four other varieties.

This shows its reproductive system had been modified in a way that made it more compatible with the other species.

It shows there are subtle yet measurable differences in sexual constitution among forms we'd otherwise consider mere varieties.

The overwhelming conclusion, therefore, has to be that fertility does not constitute a fundamental distinction.

Not at all.

The general sterility of species is, incidental, likely related to their long exposure to uniform natural conditions.

Finally, Darwin compares the offspring themselves,

hybrids from species crosses,

and mongrels from variety crosses.

And Gartner, who really wanted to draw a clear line, could find very few differences between them apart from their fertility.

The most important admitted distinction is that mongrels are often more variable in the first generation than hybrids.

Which is expected because the parents of mongrels are domestic varieties, which are already highly variable.

But the difference quickly graduates away.

Both follow the same laws of inheritance,

including prepotency of likeness.

Like the ass's characteristics being dominant over the horses and the mule.

Exactly.

And both can be absorbed by repeated back -crosses to the pure parent form, eventually just disappearing into the dominant lineage.

This close similarity between hybrids and mongrels in every respect other than fertility, it harmonizes perfectly with the view that there is no essential distinction between species and varieties.

The observed sterility is merely a physiological accident of divergence, not a proof of separate creation.

That was a masterful, dense, and systematic demolition of what was really a creationist stronghold.

Let's bring this home with a synthesis of Darwin's revolutionary argument in Chapter 9.

The conclusion is decisive.

Darwin successfully dismantled the idea of sterility as a fixed, divinely endowed barrier.

He demonstrated that sterility, whether in first crosses or in hybrids, is complex, variable, and general, but it is not universal.

And it's not a special endowment.

It's an incidental result, a physiological side effect of divergence, precisely analogous to the difficulty of grafting or the sterility induced by unnatural conditions like confinement.

We saw that the capacity for crossing is entirely independent of general structure.

That was proven by the reciprocal crosses and the insights from dimorphic plants, where absolute infertility can be induced just by an improper union within the same species.

Right.

Sterility is confined solely to the sexual elements, and we establish that profound double parallelism.

A large physiological shock, whether from a radical environmental shift or a wide genetic cross, leads to sterility.

This proves the line between a fertile variety and a sterile species is just a matter of degree accumulated over time.

The facts do not oppose the belief that species arose gradually from varieties.

The revolutionary impact of Chapter 9, then, is that it removes the last true obstacle to the theory of natural selection.

If sterility is just a physiological accident, then there's no longer a need for separate, fixed creation events for species.

Varieties are free to diverge gradually, and that divergence will eventually and incidentally lead to reproductive isolation.

A fascinating deep dive into a truly foundational text.

And while Darwin successfully proved what sterility is not, neither specially created nor naturally selected, he does leave us with a humbling final thought.

We are still profoundly ignorant of the precise cause of sterility, just as we're ignorant of why an elephant, when removed from its natural conditions, becomes sterile.

The fundamental mechanisms of life remain mysterious, but the theoretical barrier to evolution has been permanently removed.

A great way to put it.

Thank you for joining us, and we encourage you to consider how many other absolute biological rules might just turn out to be merely incidental physiological side effects.