Sources|Discussion Questions|Experiment
Observations aboard the Beagle
During his five year journey around the world on HMS Beagle, Charles Darwin encountered many different landscapes and an enormous variety of flora and fauna. Some of his most vivid experiences were on the islands of the coast of South America, including the now-famous Galapagos archipelago. Darwin was fascinated by the geographic relationship between the South American coast and its nearby islands; he puzzled over why plants and animals on nearby islands were similar to those on the mainland. Darwin was not satisfied by the traditional explanation provided by many naturalists of his day (including the famous Harvard naturalist Louis Agassiz), that each organism was specially created for its geographic location. After his return from the Beagle, he started to challenge the tenets of special creation theory. Eventually, Darwin suggested that species were transported from the mainland to the islands, where they continued to evolve.
Seeds in salt water
For his theory to work, Darwin had to prove that it was possible for mainland species to be transported across oceans and to survive on nearby islands. In the 1850s he embarked on a series of experiments to demonstrate that a variety of cross-oceanic dispersal methods were possible. He conducted several experiments to test whether the seeds of common garden plants could be soaked in salt water, survive, and germinate. Many of Darwin's seeds survived this salt-water test, which suggested to Darwin that his theory of seed dispersal and subsequent evolution was plausible. Darwin also hypothesized that snails could be transported from mainlands to islands via birds. To test this theory Darwin tested whether pond snails would adhere to ducks' feet that were dangled into ponds. Although these experiments were simple, the results gave Darwin substantial evidence for the role of evolution in the history and distribution of life on earth.
You can download the entire packet of reading materials here: Biogeography Source Pack
SOURCES
Books
Darwin, Charles. On the Origin of Species, chapters 11 and 12
Papers
Darwin, Charles. 1855. "Does sea-water kill seeds?" Gardners' Chronicle and Agricultural Gazette no. 15: 242 & Darwin, Charles. 1855. "Does sea-water kill seeds" Gardners' Chronicle and Agricultural Gazette no. 21: 356-357.
Darwin, Charles. 1857. "On the action of sea-water on the germination of seeds. Journal of Proceedings of the Linnean Society of London (Botany) 1:130-140.
Darwin, Charles. 1882. "On the dispersal of freshwater bivalves." Nature 25:529-530.
Letters
Letter 1661—Charles Darwin to JD Hooker, 7 April 1855
Darwin writes to his good friend, Joseph Dalton Hooker (botanist and soon to be assistant director of Kew Botanic Gardens), to tell him that he has begun his seed-salting experiments. He asks Hooker to predict which seeds will be easily killed in salt water.
Letter 1669—Charles Darwin to JD Hooker, 19 April 1855
Darwin rejects Hooker’s suggestion that the seed-salting experiments should be conducted on a huge scale. Darwin is only interested in establishing the possibility that seeds could be dispersed by sea transport, he does not want to detail the particular make-up of a specific plant community.
Letter 1680—Charles Darwin to JD Hooker, 11 May 1855
Darwin congratulates Hooker on his upcoming appointment to Assistant Director at Kew. He ruminates on the most appropriate place to publish the results of his seed-salting experiment, but concludes that he should give first priority to the Gardner’s Chronicle, since he sent the first notice of the experiment to that journal. Darwin contemplates whether the floating is more important in the seed experiment than the question of germination.
Letter 1681—Charles Darwin to JD Hooker 15 May 1855
Darwin is upset with the experiment because the seeds he used did not float. He is also frustrated because his attempt to get fish to swallow seeds (to show that seeds could be transported long distances inside animals such as fish and herons) did not go well. The fish spat out the seeds after a few minutes.
DISCUSSION QUESTIONS
1. Why did Darwin undertake soak common garden seeds in salt water?
2. Why do you think Darwin did not publish the larger implications of his work in the Gardner’s Chronicle?
3. Why did Darwin seek the advice of Joseph Hooker in putting together his experiment soaking seeds in salt water? What does their exchange tell you about Darwin’s network of scientific exchange?
4. Why is Darwin interested to know whether fish will swallow common garden seeds? How does this relate to the salt water experiments?
5. How do Darwin’s experimental results relate to his theories of geographical distribution in Chapter 11 of On the Origin of Species? What types of evidence does Darwin present in the chapter to support his claims?
6. Do you think that Darwin’s work in this area was novel or challenging for his time? Why or why not?
EXPERIMENT
Recreate Charles Darwin's seeds in salt-water experiment!
What you will need:
- 5 different kinds of seeds. Our recommendations: common oats, corn, broccoli, radishes, falx seeds, spinach, common peas, capsicum and lettuce. These seed varieties were used by Darwin. Most of these seeds can be obtained from a neighborhood hardware store or seed catalogue.
- small vials of salt water. Average salt content of sea water is 3%, but you can expand this lesson by varying the concentrations in the vials, and testing those as well
- small pots to plant the seeds in
- Have the class break up into several groups
- Give one type of seed to each group
- Have each group describe its seeds: colour, texture, size, etc. Have each group make predictions about how the salt water will affect the seeds. Have a class discussion about these predictions.
- Place the seeds in small vials filled with salt water. Leave them for a week and have the students make observations at regular intervals
- Plant the seeds in compost after one week. Have the students observe the planted seeds at regular intervals for 2 weeks. At the end of the 2 weeks, have your students record the total number of seeds that germinated
Results and Discussion:
- Have each group share the total number of seeds that germinated
- Discuss why certain seeds might have germinated more than others
- Have the students reflect on their initial predictions
- Discuss why this experiment was significant for Darwin. Make the point that experiments in the present can provide empirical suggestions for events in the past
While studying biogeography, the class conducted two experiments that provided first-hand knowledge of Darwin’s work. The first experiment mimicked Darwin’s 1855 work on seeds and salt-water. The experiment is delightfully simple: each student took four jars, filled them with salt water, added one species of seed to each jar, and labelled the jars with the seed type. The students then left the seeds in their jars for four weeks. At the end of these four weeks, the students rinsed off their seeds and planted them in soil with labels to indicate where they had planted each species. Each week the class met, the students were able to check on their seeds to see whether the plants had germinated. The results aligned very well with Charles Darwin’s: those seeds that he had found to sprout also sprouted, while those seeds that didn’t germinate in Darwin’s experiment also failed to germinate in our own time. This experiment is a beautifully simple illustration of biogeography: many seeds survive weeks in salt-water and are still capable of germinating. Darwin saw this experiment as an explanation of the means by which plants appear in disparate locales.
The other experiment the class conducted also investigated the method by which species appear in different geographic locations. To conduct this experiment, each student was given a duck’s foot, string, and had access to a pond. The ducks’ feet were obtained from a local butcher. Each student tied a string around their duck’s foot and dangled it into the pond. The class left the feet in the pond for two hours. After two hours, each student should pull their duck’s foot out of the pond and see if any living creatures have attached to it. This experiment demonstrates the manner in which specimens can be transported by birds across both water and land.
