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Okay, by now you know the basics of Darwin’s greatest and most famous contribution to our
understanding of the world—natural selection, but did you know that he is also the author
of one of the most important books on earthworms and the way they impact the world around us?
Yes, well now you do. Earthworms in the phylum Annelida are part of the bill of fare in this
unit, so now is as good a time as any for us to talk about Darwin’s worm obsession.
First off and before anyone asks, there iss a valid reason for my decision to include
this information (published in 1881) in a modern zoology class for nonmajor students.
Actually there are two reasons—the second we’ll get to in the next video, also based
on Darwin’s worm book. My two motivations here are both related to the scientific method,
and the first has the added benefit of re-emphasizing in somewhat more tangible terms the concept
of gradual changes accumulating into much larger things if given enough time.
Darwin’s fascination with worms began in 1837, when his uncle Josiah Wedgwood showed
Darwin a field where he had spread cinders and chunks of limestone some twenty years
earlier. He explained that the cinders and limestone seemed to have disappeared, but
they were actually buried under four inches of new soil. Wedgwood was actually the one
who suggested to Darwin that this was the work of earthworms.
Now Darwin had other things going on, but he did take up this study of worms on and
off throughout the rest of his life. He found several other examples—locations where there
were accurate records of how many years had passed since certain objects were put down
on top of the soil surface, and he found that on average (and depending on the location)
about one-fifth of an inch of soil per year was piling up and gradually burying the things
that were once sitting right on top of the soil.
Darwin did some pretty extensive further research, comparing notes with his contacts in other
parts of the world, and he nicely documented that this phenomenon of new soil accumulation
was pretty much the same everywhere you had earthworms. This wasn’t the same, however,
as establishing that earthworms were in fact the mechanism for this new soil accumulation.
He suspected earthworms but could not rule out other causes. Was it necessarily earthworms
doing this? Why not the accumulation of dust from the air? or maybe animal poop was mixing
with decaying vegetation and this was gradually piling up at the top, making new soil. Let’s
say for now that these are three competing explanations for this build up of new soil:
worms, airborne dust, and poop. What could be done in order to distinguish between these
three causes?
Well let’s think for a moment like the good scientist that Darwin was. If you have ever
dug a hole deep enough, for example to plant a tree, you’ll probably know that the soil
layer containing organic matter—the good soil we sometimes call topsoil—is only the
uppermost few inches. Below that you get to clay or sediments or loose rock material overlying
a layer of bedrock. If it was the accumulation of earthworm castings that caused the build-up
of soil over the long haul, then tiny bits of rock-like mineral material present in this
new soil would be same as whatever kind of earth was there underneath, in other words,
there would be a strong match between the minerals in the soil with those that are directly
below—the rock or the clay or whatever. If, however, it was airborne sediment, the
minerals being deposited would have blown in from elsewhere and would not necessarily
match the kind of dirt that was underneath. If it was animal poop and rotting vegetation,
there wouldn’t be much of any mineral material at all—it would be purely organic. Darwin
noted that when the new soil formed over a chalky substrate it was light in color because
of the chalk particles it contained. When you looked at an area where the underlying
material was iron-rich, the new soil on the surface was also iron-rich. Basically the
little particles of rock in the new soil was always a good match with the chemistry of
the parent material that was underneath, and this is enough evidence to exclude the two
competing models of dust and poop and to support Darwin’s hypothesis that worms were bringing
up stuff from the tunneling they were doing below the soil line.
Finally there’s the issue of evaluating plausibility. Darwin had documented the soil
buildup and provided further evidence in favor of earthworms as the cause, but is it really
possible for stupid little earthworms to create a fifth of an inch of new soil each year?
Darwin anticipated this type of question and made sure he had the relevant data—he is
actually well-known for being ultra-meticulous about having lots of data before ever drawing
a conclusion. Darwin had his contacts from around the world send him worm castings with
descriptions of the conditions under which they were collected, and he carried out several
measurements of his own. He estimated that, depending on a lot of things including worm
density, kind of subsoil, availability of water, amount of vegetation, you could have
between 7.5 to 18 tons of castings thrown up by worms per acre per year, and in the
areas where he had documented a fifth of an inch of new soil deposition, the estimated
dry mass of soil was almost right for a fifth of an inch. Yes, an acre’s worth of a fifth
of an inch of topsoil really does weigh several tons.
To summarize, Darwin’s proposition that worm activity causes the soil to gradually
rise up—around paving stones, buildings, and even whole cities—required evidence
on three fronts. First, establishing that this slow burial was a gradual process that
was going on at all times, including the present. Second, identifying worm activity as the most
supported of the different possible explanatory mechanisms. Third, establishing that the amount
of castings being thrown up by actual worms in their natural state is a good match for
the rate of slow burial. Darwin’s work on this began in 1837 and the book was finally
published in 1881, just a year before Darwin died.
Now in reading this particular volume of Darwin’s life work—and in truth it really amounts
to a rather dull book on worms—I was struck mostly by the tremendous care with which Darwin
presents and supports his basic proposition. While the individual effort of a single worm
or even a whole field of worms seems like so little to anyone making day-to-day observations,
the castings that they make accumulate over time, and ultimately they have a very profound
effect in shaping the terrestrial landscape. Whole Roman cities get buried completely within
800 years, and enormous rocks of ancient places of druid worship (like Stonehenge) are similarly
buried by the soft soil brought up by the worms.
To scholars of Darwin the parallels are obvious between Darwin’s worm book and his more
famous work on evolutionary change by natural selection. Gradual change that seems so insignificant
when one is looking at a population from one generation to the next can accumulate over
a thousand generations to make for really substantial amounts of evolutionary change,
What I’ll do in the next video is walk you through a worm-focused activity that Darwin
did with his sons. It nicely illustrates another facet of the scientific approach to understanding
the natural world, and by this I mean specifically: manipulating variables in an experiment.