Lamarckism

Based on Wikipedia: Lamarckism

In 1889, a German biologist named August Weismann chopped the tails off sixty-eight white mice. Then he bred them. Then he chopped the tails off their offspring. He did this for five generations—901 mice in total—and not a single one was born with a shorter tail.

Weismann declared victory. He had disproven Lamarckism, the idea that organisms can pass on traits they acquire during their lifetimes. Case closed.

Except he hadn't proven anything at all.

The Blacksmith's Sons

The theory Weismann was attacking belongs to Jean-Baptiste Lamarck, a French zoologist who lived from 1744 to 1829. Lamarck proposed something that sounds almost reasonable at first: if you use a part of your body a lot, it gets stronger. If you never use it, it withers away. And—here's the crucial part—these changes get passed down to your children.

His most famous example involves giraffes. Imagine an ancient, short-necked ancestor of the giraffe stretching to reach leaves high in the trees. According to Lamarck, all that stretching would gradually lengthen its neck, and its offspring would be born with slightly longer necks. Generation after generation, the necks would grow, until you get the improbable creatures we see today.

He also imagined a blacksmith. Through years of hammering iron, the blacksmith develops powerful arms. Lamarck believed the blacksmith's sons would inherit this muscular development—not because they'd also worked at the forge, but simply because their father had.

This is called "soft inheritance" or "the inheritance of acquired characteristics." Your body changes during your life, and those changes somehow get written into whatever you pass on to your children.

Why Weismann's Mice Don't Matter

Here's why chopping off mouse tails proves nothing about Lamarckism: Lamarck wasn't talking about injuries. He was talking about changes that come from an organism's own efforts and behaviors in response to its environment.

A mouse doesn't use its tail to overcome environmental obstacles. It doesn't stretch its tail toward food or strengthen its tail through exercise. When you cut off a mouse's tail, you're not testing anything Lamarck actually proposed. You're testing whether random mutilation gets inherited, which is a completely different question.

The biologist Peter Gauthier put it bluntly in 1990: the experiment "lacks a key factor, namely the willful exertion of the animal in overcoming environmental obstacles." Another scientist, Michael Ghiselin, was even more dismissive, writing that "nothing that Lamarck had set forth was tested or 'disproven' by the Weismann tail-chopping experiment."

And yet this experiment appears in textbook after textbook as the definitive refutation of Lamarckism. It's one of the most persistent myths in the history of biology.

An Idea Older Than Lamarck

Lamarck didn't invent the inheritance of acquired characteristics. He just gave it a prominent place in his theory of evolution. The idea itself is ancient—at least 2,200 years old.

Hippocrates believed in it. So did Aristotle, though Aristotle noted the obvious problem that children don't always resemble their parents. Galen believed it. So did Roger Bacon, John Ray, and many others. The concept shows up repeatedly in ancient mythology and even in the Bible.

You can find it in the eighteenth century, in Denis Diderot's philosophical dialogue "D'Alembert's Dream." Charles Darwin's grandfather, Erasmus Darwin, wrote in the 1790s about warm-blooded animals developing from "one living filament" with "the power of acquiring new parts" in response to stimuli, with each improvement inherited by the next generation.

Even Rudyard Kipling's "Just So Stories"—the ones about how the leopard got its spots and the elephant got its trunk—play on this intuition. It feels right that what you do should shape what your children become.

Darwin Believed It Too

Here's something the textbooks often get wrong: Charles Darwin also believed in the inheritance of acquired characteristics.

In "On the Origin of Species," Darwin proposed natural selection as the main mechanism of evolution, but he also gave credence to the effects of use and disuse. He thought both mechanisms operated together.

Darwin went further. In 1868, he published a theory called pangenesis. He imagined that cells throughout your body throw off tiny particles called "gemmules" or "pangenes." These microscopic messengers supposedly contain information about what their parent cells have experienced. They travel through your body—not necessarily in the bloodstream—and eventually collect in your reproductive cells. There, they carry the story of your life's experiences to your offspring.

Darwin's half-cousin, Francis Galton, tried to test this theory by transfusing blood between different varieties of rabbits, expecting the offspring to show mixed characteristics. They didn't. Galton claimed he had disproved pangenesis.

Darwin fired back in a letter to the journal Nature. He pointed out that he had never mentioned blood in his writings. Pangenesis, he argued, must occur in protozoa and plants too, and they don't have blood. Galton's experiment, like Weismann's mice, had missed the point.

What Lamarck Actually Proposed

Lamarck's theory of evolution was broader than just the inheritance of acquired characteristics. Between 1800 and 1830, he developed a systematic framework built on several ideas.

First, he believed in orthogenesis—a drive toward complexity. Life, he thought, tends to become more elaborate over time. This was his primary mechanism of evolution.

Second, he believed the environment shapes organisms. When the environment changes, it creates new needs. New needs lead to new behaviors. New behaviors lead to changed patterns of organ use. Changed organ use leads to physical changes. And those changes get inherited.

Lamarck stated two specific laws. The first, the Law of Use and Disuse, said that organs grow stronger with use and weaker with disuse. An organ you exercise constantly becomes more developed; one you never use gradually disappears. The second, the Law of Soft Inheritance, said that these changes pass to offspring—provided both parents share them, or at least the parents who produce the young.

The historian Stephen Jay Gould complained that reducing Lamarckism to just the inheritance of acquired characteristics does Lamarck a disservice. It takes "one aspect of the mechanics" and elevates it "to a central focus it never had for Lamarck himself." Gould called this "more than a misnomer, and truly a discredit to the memory of a man and his much more comprehensive system."

The Eclipse of Darwin

There was a strange period in the history of evolutionary thought, from Darwin's death in the 1880s until the 1920s and 1930s, that historians call "the eclipse of Darwinism." During these decades, most scientists accepted that evolution happened, but many doubted that natural selection was the main driver.

Lamarckian ideas enjoyed a revival. Scientists who championed them were called neo-Lamarckians.

The British botanist George Henslow studied how environmental stress affects plant growth. He thought these environmentally-induced variations might explain much of plant evolution. The American entomologist Alpheus Spring Packard Jr. studied blind animals living in caves and wrote a book about Lamarck in 1901. Paleontologists like Edward Drinker Cope and Alpheus Hyatt noticed that the fossil record often showed orderly, almost linear patterns of development. These patterns seemed to them more consistent with Lamarckian inheritance than with the randomness of natural selection.

Some thinkers found Lamarckism philosophically appealing. Samuel Butler, a fierce critic of Darwin, liked the idea that organisms could shape their own evolution. If you acquire new habits, you change how you use your organs, and that kicks off evolutionary change. You become the author of your descendants' bodies. This felt more dignified than Darwin's mechanism, where random variations get filtered by the blind indifference of survival.

The Weismann Barrier

August Weismann, the mouse-tail chopper, contributed something more important than his flawed experiment. He developed the germ plasm theory.

The idea is this: your body has two kinds of cells. Somatic cells make up your body—your muscles, your skin, your brain. Germ cells are the ones in your reproductive organs that create eggs and sperm.

Weismann proposed that information flows in only one direction. Germ cells pass their information to the next generation. But somatic cells cannot write back to the germ cells. There's a barrier between them.

If this barrier exists, Lamarckian inheritance becomes impossible. Your muscles can't tell your reproductive cells about all the exercise you've been doing. Your brain can't inform your eggs or sperm about everything you've learned. The germ line is sealed off, carrying its information forward unchanged by your life's experiences.

This concept—the Weismann barrier—became foundational to modern genetics. When Gregor Mendel's work on heredity was rediscovered around 1900, it fit neatly with Weismann's framework. Genes were discrete units that passed from parent to offspring according to mathematical rules. They didn't care what you did with your life.

The Modern Synthesis

By the 1930s and 1940s, evolutionary biology had crystallized into what's called the modern synthesis. It combined Darwin's natural selection with Mendelian genetics and population mathematics. In this framework, evolution works like this: random mutations create genetic variation; natural selection filters that variation based on survival and reproduction; over time, populations change.

There was no room for Lamarckism. Acquired characteristics couldn't be inherited because the Weismann barrier prevented it. Genes changed only through random mutation, not through use or disuse. The blacksmith's arms, no matter how powerful, would leave no trace in his sons' chromosomes.

Lamarckism became a byword for bad science, a cautionary tale about intuitive ideas that turn out to be wrong.

The Twist

And then, in the twenty-first century, something strange started happening.

Researchers began discovering cases where acquired characteristics did seem to pass to offspring. Not through changes to the DNA sequence itself, but through something called epigenetics.

Here's how it works. Your DNA is like a vast library of instructions. But not all instructions are read all the time. Some books are on the shelf, gathering dust. Others are open on the table, being actively consulted. What determines which genes get expressed and which stay silent?

Part of the answer involves chemical tags attached to DNA and to the proteins that DNA wraps around. These tags can turn genes on or off. And here's the interesting part: some of these tags can be influenced by your environment and your behavior. Stress, diet, exposure to toxins—these experiences can add or remove tags.

Even more remarkably, some of these tags can be passed to offspring. This is called transgenerational epigenetic inheritance. Your life experiences can, in certain cases, mark your DNA in ways that affect your children and even your grandchildren.

This isn't exactly what Lamarck proposed. The DNA sequence itself doesn't change. But the effect is similar: something acquired during your lifetime influences the traits of your descendants.

The Hologenome

There's another way Lamarckism has made a partial comeback, and it involves the trillions of microbes living inside you.

Your body hosts a vast community of bacteria, viruses, and other microorganisms—collectively called your microbiome. These microbes have their own genomes. Together, your genome and all the genomes of your microbial passengers form what's called the hologenome.

Here's what's interesting: you acquire your microbiome during your lifetime. You get microbes from your mother during birth, from the food you eat, from the environment around you. Your diet and lifestyle can reshape your microbial community. And when you have children, you pass many of these microbes on to them.

This means that changes you make to your microbial community during your life can be inherited by your offspring. Not through your DNA, but through the living organisms you carry.

The mechanisms are entirely Darwinian—microbes evolve through mutation and selection like everything else. But the effect is somewhat Lamarckian. Your acquired microbial characteristics influence what your children inherit.

A More Complicated Picture

So was Lamarck right?

Not in the way he imagined. Giraffes didn't get long necks because their ancestors stretched toward high leaves. Blacksmiths' sons don't inherit muscular arms. The core mechanism Lamarck proposed—where use and disuse directly reshape heritable traits—doesn't work the way he thought.

But the absolute rejection of any inheritance of acquired characteristics has also proven too simple. The Weismann barrier isn't as impermeable as once believed. Epigenetic marks can sometimes cross it. Microbial passengers can carry information between generations. The clean separation between what you are born with and what you acquire during life has blurred at the edges.

The textbook story—Lamarck was wrong, Darwin and Mendel were right, case closed—turns out to be a caricature of a much messier reality. As the historian of science Rasmus Winther has noted, even Weismann himself had nuanced views about how the environment might affect heredity. The simple version we tell students leaves out most of the interesting parts.

Why It Matters

This isn't just a historical curiosity. Understanding how traits pass between generations has profound implications.

If your diet can affect your grandchildren's health through epigenetic inheritance, that changes how we think about public health. If trauma can leave heritable marks, that changes how we understand the long-term effects of war and violence. If the microbes you pass to your children shape their development, that changes how we think about antibiotics and hygiene.

The inheritance of acquired characteristics also raises philosophical questions about responsibility and fairness. If the choices you make ripple forward to affect people who haven't been born yet, what obligations does that create? If disadvantages accumulate across generations through biological as well as social mechanisms, what does that mean for equality?

Lamarck was wrong about many things. But the question he was trying to answer—how do organisms change across generations, and what role does experience play?—remains one of the deepest in biology. We're still working out the answer, and it's more complicated than anyone in the nineteenth century imagined.

The mice that Weismann mutilated taught us nothing about this question. But the question itself refuses to go away.