Origin Ch.13: Darwin the experimentalist

This post is part of a series on The Origin Of Species.  It was originally posted on the old blog in feb 2009, during the Darwin 200 celebrations.

Two pages into chapter thirteen we get a fantastic image of Darwin pottering in his conservatory, dipping his duck in various fish tanks. Later on the same page he’s keeping a mug of mud on his desk, pulling out the weeds as they come up. At other points in the book we find him cutting out pictures and counting the plants in his lawn. The Origin is not just Darwin writing down his thoughts after 25 years of thinking about the Galapagos. Darwin made sure his argument was solid, and in science, you find the solid ideas by giving them all a good shake, and seeing which ones fall down under the stress. Darwin made hypotheses about how the world might look if his theory is true, and then took a look to see whether the world cooperates.

Darwin’s theory dictated that all individuals within a species are related by descent; species within a genus similarly related, but more distantly; and so on for all life. Darwin observed that there are many fresh-water aquatic species whose individuals occupy many different ponds or streams. He reasoned that if all individuals of a species are related, there must be some mechanism by which individuals from those fresh-water aquatic species have spread from one pond to another. He made a hypothesis: that eggs from these species might stick to the feet of ducks, and thus spread. And so he found himself dipping ducks feet in tanks, and seeing how long the duck’s feet could remain dry while maintaining their ability to seed a sterile tank with pond life. Nowadays he would probably need institutional ethics review board approval for the use of fowl in laboratory research.

Other great practical work from Darwin includes determining the relative areas of igneous, metamorphic, and sedimentary rocks in the United States by cutting up a geological map and weighing the parts; and the time he spent several weeks observing slugs eating weeds (described here by Mike Dunford). And there is one more great experiment that I have heard attributed to Darwin, but for which I can sadly find no credible evidence, and not even at hint in Voyage. The story, told by m’colleague, who in turn has attributed it to a friendly archivist at the Grant Museum of Zoology (UCL), features a young Darwin in the Galapagos spending an afternoon tossing land and marine iguanas into the sea to see how long each could survive in the water. The naive iguanas would inevitably swim straight back to their spot on the shore, never learning to avoid the bastard with the mean overarm who kept throwing them back in. It would be a good experiment to show whether both species could have immigrated to the islands, or only the marine variety.

It’s probably not true. But Darwin was still an ingenious experimentalist.

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