[T]he great tragedy of Science—the slaying of a beautiful hypothesis by an ugly fact…
Thomas Henry Huxley
Biogenesis and Abiogenesis (1870)
Back in 2002, I wrote an essay entitled (in tribute to Huxley) …So Let's All Be Scientists! It was my contribution to Darwin Day Collection One: The Single Best Idea Ever [ISBN: 0972384405, Amazon.com], a collection of articles, reviews and cartoons in celebration of Darwin and science. In the essay, I observed:
…for all the thousands of slugs I have found, I have never come across a single snail in my garden. Why is that? Is it too cold (I live in the Pennines)? Do the slugs eat them […], or out-compete them? Am I just not looking hard enough? Or is there simply not enough calcium in the area to allow snails to make shells (possibly because the soil is too acidic)?
Well, it would appear that the not looking hard enough hypothesis had some merit. The other weekend, I was moving a rockery in my garden (don't ask), when I came across this:
OK, as snails go, it wasn't exactly the biggest (or, indeed, the alivest), but a snail is a snail, and an ugly fact (no matter how small) is an ugly fact: my no snails in my garden hypothesis is well and truly falsified. So now I have had to modify it slightly:
There are no big snails in my garden.
Actually, I have been keeping an active look-out for snails ever since I wrote my essay, and I can confidently say that I believe my modified hypothesis is correct. Which is odd, because I have observed large snails a couple of hundred yards away from my garden, albeit at a significantly lower altitude (I live on a very steep hill).
But, despite this stark evidence to the contrary, I still like my acidic soil explanation for the (near) absence of snails in my garden. In fact, until last week, I was growing increasingly confident that it was the correct explanation, having first confirmed that the soil in my garden is indeed acidic, and having recently come across the following in the New Scientist subscribers' archive:
Acid rain has progressively thinned the shells of eggs laid by British thrushes over the past 150 years, a new study suggests. Ornithologists fear that the trend could make thrush eggs less likely to hatch…
[Rhys Green of the Royal Society for the Protection of Birds] thinks that acid rain, caused by sulphur emissions from the burning of fossil fuels, is the most likely cause. This would reduce both the calcium content of leaf litter consumed by worms and the abundance of snails, which together make up a large part of the birds' diets.
So maybe there is still enough calcium in my garden for some very small snails, but not enough for any snails bigger than a couple of millimetres across.
I will get to the bottom of this one! Eventually.
Postscript [18-Jun-2007]: Hypothesis well and truly falsified!