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So we’ve been talking about Charles Darwin and his book on earthworms, the real title
of which was The Formation of Vegetable Mould through the Action of Worms. Now Darwin was
not the first to write on the subject of earthworms. What makes his work truly germinal is that
he so nicely incorporates two major themes of modern scientific inquiry—in essence
he was modeling excellent use of the scientific method, which would be emulated by generations
of biologists for decades to come. In the previous video we discussed his use of hypothetico-deductive
reasoning, first to posit that it was earthworms that cause the gradual burying of giant stones
and stone buildings with layer after layer of soft earth brought to the surface by the
tunneling worms. He supported this hypothesis in two ways: by demonstrating that the earthworms’
activity was adequate to account for the accumulation of new soil at a rate consistent with the
observed rate of objects sinking deeper and deeper into the ground. Further, he demonstrated
that the minerals present in new soil at the surface matched the physical properties of
the substratum underlying the new soil, thereby giving his earthworm hypothesis greater support
over the competing hypotheses.
Note that there is no “proof” that Darwin was right about the earthworms causing this
slow sinking of buildings and boulders into the landscape. Contrary to a belief that is
common among non-scientists, science does not seek “scientific proof”—this would
simply not be realistic in 99.9% of all the things that scientists study. What Darwin
did was about all that a scientist can ever do—that is, to establish that a model is
the best available explanation for a natural phenomenon. In this case, worm activity is
the best available explanation for the phenomenon of things getting gradually buried over time.
Now there’s a second little experiment that Darwin performed with the help of his son—this
was basically an investigation of whether we could infer any “intelligence” on the
part of the lowly worm. No one would expect that the earthworm would need intelligence
in doing what the earthworm does. But now it occurs to me that you might not know exactly
what it is an earthworm does, so I’ll take few moments to give a bit of background.
You probably know that earthworms live in the soil, but what do they do there? Well
they tunnel—a lot. They create new tunnels by pushing and eating their way through the
earth, leaving behind them a worm-sized tube that has walls that are packed and coated
with worm-slime—these tunnels are the worm’s escape route. A lot of the dirt that the worms
eat in order to create new tunnels gets deposited in little piles near the entrance to a worm’s
tunnel—these are the “castings” that we talked about in the last video that accumulate
as new soil.
At night, the worm also forages on the surface for leaves and other litter, and when it finds
a leaf it will try to drag it backwards into its tunnel. There the leaf will decay—right
in the worm’s tunnel—and the worm has a steady food supply in the form of rotting
leaf material. A foraging worm can drag several leaves into its tube in a single night.
If you think about this basic task from a worm’s angle, pulling a big, flat leaf into
a tubular tunnel is going to present a problem. In pulling the leaves into their round holes,
the worms must somehow get them to roll into a sort of conical tube which can then be pulled
into the burrow more easily.
Now the place on the leaf’s margin where the worm grabs the leaf will to a certain
extent determine the relative ease in which the leaf rolls into a tube as it is pulled
into the worm-hole. If you’re talking about a leaf shaped like this… this will be far
easier to pull into the worm’s round entrance hole if the worm grabs the leaf by its tip.
But besides the fact that worms are blind and are doing this foraging at night when
they couldn’t see anyways, they are, after all, just worms. They aren’t little spineless
engineers out with their leaf-surveying equipment and they don’t huddle together in underground
meeting rooms to brainstorm and theorize the best way to drag a flat leaf into a round
hole—they reach around blindly, and when they find a leaf they grab it—probably at
the spot where they first touch the leaf.
Or do they? This was Darwin’s question—a very modest question but one that can be addressed
in a perfectly scientific way. If it turns out that worms are displaying some selectivity
about where on the leaf they grab, this could require a greater amount of brain power relative
to just grabbing the leaf at random—nonrandom leaf-grabbing could be a sign of worm intelligence,
which is something that no one before Darwin had ever suggested or investigated.
Okay, so what did Darwin do? Well for one, he enlisted the help of his son William to
go out and collect the leaves from worm burrows and marking where on each leaf was the vertex
of the cone—unrolling each leaf this would be where the worm had grabbed the leaf to
pull it into the worm-hole.
In the first collection (they did several, but I’ll only talk about this one), out
of 277 leaves collected-- all of the same species of plant and the leaf shape was similar
to my picture—about 80% had been pulled in by the tip. The other 20% had been pulled
in by the side of the leaf or the base of the leaf. Basically, it looked like the worms
were preferring to grab a leaf at a spot where it would be easier to drag it into a hole.
This was a great starting point for the investigation, but it what I’ve described so far is not
experimental, in the sense that there was no manipulation of conditions leading to the
data that are used to evaluate the predictions. We have the observation that worms are dragging
leaves by the pointy ends preferentially, but this isn’t quite the same as saying
there was a “decision” made by the worms based on geometry and ease of rolling the
leaf into a tube. For example, one could argue that the chemical cues that worms used to
detect a leaf were different at the tips vs. other parts, and worms were using chemical
signals to find and grab leaves—if it were chemical signals that led the worms to leaf
tips, then it would not be the case that leaf geometry was part of the criteria used by
worms to decide where to grab the leaf.
Darwin’s experiment to investigate this made use of “artificial leaves” that he
and his son William made out of paper triangles that were rubbed with animal fat (to keep
them from turning to mush in the dewy and rainy English nights). The Darwins’ triangles
were isosceles and the two like sides were three inches in length, and the base of the
triangle was shorter. These triangles were marked with equally-spaced lines parallel
to the base, and this divided the equal sides into three zones “closest to the vertex,”
“middle,” and “closest to the base”—call them zones A, B, and C. The Darwins put several
hundred of these paper triangles out in the garden and collected data the next morning.
In Darwin’s reasoning, if the worms were randomly grabbing the paper triangles by their
edges, there would be a 1:1:1 ratio of triangles being grabbed in each of the three zones.
If worms were grabbing at random, but based on available surface area to grab, then zone
C—close to the base—would be grabbed at a rate five times higher than zone A, because
there is far more area in the basal trapezoid than there is in the triangle at the tip.
What the Darwins found was that out of 303 triangles pulled into burrows by the worms,
62% were pulled in by the tip—zone A. Darwin inferred that a reasonable explanation for
this result was that worms were preferentially grabbing the triangle in zone A based on geometry
alone, since there could be no other basis for the worms’ strong bias in grabbing the
triangles by their tips.
Darwin’s investigative method here illustrates exactly the kind of logic that is at the core
of scientific experimentation. A lot of people associate science with the use of highly complex
technological materials and equipment—the technology becomes falsely equated with science
itself. But no, being highly skilled at using scientific equipment does not make a person
a scientist. The fancy equipment may be required in many studies only because that’s how
the scientist is able to investigate the questions being asked. But if you strip away all of
the white lab coats and the bells and whistles of the million-dollar machinery used in modern
research labs, you still have a core question and a rational and logical process by which
the question is being investigated—that’s where the science is. That’s what Darwin
is showing us, in this little homegrown experiment from a century and a half ago, before the
methods of modern science were analyzed and codified.
In Darwin’s case he asked a very simple question: “Do earthworms display any intelligence?”
And through a series of inferences based on observations from nature—natural leaves
that had been pulled into burrows—and an experimental design using artificial leaves—the
paper triangles—in order to eliminate uncontrolled covariates that occur with natural leaves,
he was able to show with considerable confidence that worms exhibited a preference based on
the geometry of the object that they were dragging into their burrows.
Getting back to an earlier point—did Darwin prove that worms are intelligent? Well, no,
he didn’t. He did make an extensive case for worms displaying a preference based on
geometry, and this could be attributed to “intelligence” (however you decide to
define it). Generating a mountain of supporting evidence for a proposed explanation is really
the best that science ever does. The nonscientist must learn to understand and accept this as
the way that science works.