A project to follow On the Origin of Species
Darwin began to observe English orchids and collect specimens from abroad in the years immediately following the publication of On the Origin of Species. Examining orchids was a welcome relief from the stress of publication and gave Darwin the chance to explore his theory of natural selection in the plant world. For this project, Darwin integrated his own experiments and observations with information and specimens from his correspondence network. Although Darwin would continue to publish large synthetic works (including The Descent of Man in 1872 and The Expression of the Emotions in Man and Animals in 1872), much of his time after 1859 was spent on concentrated, highly experimental and detailed scientific work like that on orchids.
A "flank movement against the enemy"
Darwin's first specialized monograph ought to be seen as evidence of Darwin's lifelong development of a theoretical edifice to support the idea of natural selection, his mechanism of evolutionary change. Darwin theorized that orchids and their pollinators had co-evolved, and he documented many examples to illustrate this point. Darwin referred to his work on orchids as a “flank movement against the enemy”. In his correspondence with Joseph Dalton Hooker, the director of Kew Gardens, Darwin relies on Hooker's expertise regarding the fertilisation strategies of orchids and their pollinators.
You can download the entire packet of reading materials here: Orchids Source Pack
Darwin, Charles 1862. On the various contrivances by which British and foreign orchids are fertilised by insects. London: John Murray.
Letter 3286- Charles Darwin to Joseph Dalton Hooker 15 October 1861
Darwin writes to JD Hooker, botanist and Director of Kew Gardens, about orchid anatomy. He is particularly interested in the mechanism of pollination of the orchid Catasetum, which ejects its pollinia with a sticky gland so that the pollen will stick on the head of an insect.
Letter 3421—Charles Darwin to Joseph Dalton Hooker 30 January 1862
Darwin tells Hooker about a specimen of Angraecum sequipedale which has a nectary 111 inches long. He speculates that there must be a moth with a long enough proboscis to reach the nectar at the bottom of the flower’s lengthy nectary.
Letter 3662—Charles Darwin to Asa Gray 23-4 July 1862
Darwin tells Asa Gray, a professor of Botany at Harvard University and one of the main advocates for Darwin’s theory in the United States, that Gray was the only person to realize that Darwin’s Orchid book was a “flank movement” on the “enemies” of Darwin’s theory of natural selection. The book was Darwin's first detailed demonstration of the power of natural selection; it explored how complex ecological interactions resulted in the co-evolution of orchids and their pollinating insects.
Letter 5637- Alfred Russel Wallace to Charles Darwin 1 October 1867
Alfred Russell Wallace, a naturalist who independently formulated a theory of evolution by natural selection in 1858, writes about his defense of Darwin’s theory against the criticisms of the Duke of Argyll. He mentions an illustration of Darwin’s orchid and its predicted moth.
1. Why is Darwin interested in mechanisms of fertilisation? How do you think this subject could relate to the concept of co-evolution?
2. Darwin was right in his theorization of a moth to accompany the orchid with the extraordinary long nectary. Do you think this type of speculation in evolutionary explanations was helpful to Darwin's science? Why or why not?
3. How did Darwin attempt to verify his speculations on the relationship between tropical moths and orchids? Did he attempt to journey to tropical locations himself?
4. Why do you think Darwin described his work on orchids as a "flank movement against the enemy"? Who is the enemy? What type of theory of species origins would these "enemies" be arguing for?
Complement the readings in this module with a classroom activity for your students. You may order the orchid species Darwin observed directly from garden suppliers such as this one. Have your students observe the structure of the orchids and speculate about the type of pollinator and mechanism of pollination.
Here's an example of an activity to accompany this lesson performed at Harvard University:
Although Darwin’s observations of orchids provided many useful examples of evolution by natural selection, Darwin saw this work as more of an enjoyable past-time than a rigorous academic endeavour. This attitude towards his work on orchids is shown in an 1860 letter Darwin wrote to the American botanist Asa Gray, in which he stated:
Not being able lately to work I have amused myself about Orchids. I have been struck with amazement at beauty of contrivances with respect to fertilisation by insects.
After reading a selection of letters about Darwin’s work with orchids and Chapter 1 of Darwin’s 1862 book On the various contrivances by which British and foreign orchids are fertilised by insects, the students found it useful to learn about orchids in a more experiential environment. To this end, the students visited the collection of glass flowers in the Harvard Museum of Natural History. These glass flowers were made by German glass artisans Leopold and Rudolph Blaschka from 1887-1936. The flowers were originally created for Professor George Lincoln Goodale as a means of teaching botany.
In addition to visiting this collection, the students were able to conduct the experiment Darwin describes on pages 14-16 of On the various contrivances by which British and foreign orchids are fertilised by insects. The experiment is simple – all you need is an orchid of the genera Catasetum. This genera of orchid uses a pollen release mechanism that ejects pollinia onto insects as they enter the orchid. To activate this pollen release mechanism, students need only push a finger or a pencil into the orchid. Just as Darwin describes, the pollinia will be ejected onto the students’ fingers or pencils, allowing them to appreciate, firsthand, this fascinating method of fertilization.