As part of the Harvard Alumni Association’s “Classes Without Quizzes” program for Harvard alumni and community friends, Arnold Arboretum Director William (Ned) Friedman presented a two-session class in spring 2012 entitled Getting to Know Darwin. The class read selections from Darwin’s prodigious writings and private correspondence to gain insight into the man behind the legend, and reproduced two of the classic Down House experiments that provided the foundation for his grand synthesis of evolutionary pattern and process.
One of the experiments recreated by the class focused on a question that had dogged Darwin during his travels around the world on the HMS Beagle: why are plants found on remote islands similar to those that grow on the mainland? Darwin was not satisfied with the explanation offered by some naturalists of his day, which was that organisms were specially created for their geographic location.
Seeking to challenge this assumption, Darwin started a series of experiments in the 1850s to test whether seeds could survive being soaked in salt water. If they remained viable, it would suggest that plants could be transported by the ocean from the mainland to an island, and there continue to evolve. This would also explain why island species were similar to mainland species, even though the island environments were often quite different from those found on the mainland.
At our first session on May 1, we started our own exploration of Darwin’s salt water experiments by sealing seeds in specimen jars filled with salt water. Five weeks later, on June 5, the jars were opened and the seeds were planted. Here are some of the results from this experiment: